Feasibility study for a 300kW pilot scale hydrogen production process from wood biomass

The case of Sandviksverket, Växjö

Thesis

Author: Derick Samjeila Ewang Supervisor: Professor Michael Strand Examiner: Professor Michael Strand Term: 20VT Subject: Bioenergy Technology Level: Masters Course code: 4BT04E

Abstract Historically, fossil fuel sources (coal, oil and natural gas) have played significant role in meet global energy demand. As global population continue to rise, more and more energy resources are needed and there is a continuous depletion of these energy resources. The prices of these fuels have often fluctuated over the years and this has greatly affected the economy of several nations. The use of these fossil-based fuels has had enormous impact on the environment. This is as a result carbon dioxide and other greenhouse gases emissions. Many nations are continuously showing great interest and efforts to work together to address some of these issues, for example, the global response to combat climate change (Paris Agreement), which has been ratified by over 188 states. There is a growing interest in the technological development of renewable sources like biomass, in the production of heat, electricity and synthetic fuels. The renewable energy consumption target by the European union, stands at 32%, with a minimum of 14% in the road and transport sector. Sweden as a member state, has made tremendous progress in reducing its dependence on fossil fuels and increase its use of renewables (particularly biomass) over the years. Sweden has a target to reduce its CO2 emission in the transport sector by 79% and an independence of fossil fuels in its vehicle fleets by 2030. The thermochemical conversion of biomass to hydrogen, to power hydrogen fuel cell vehicles, has been suggested as one route to achieve this target. Biomass has high volatile content and the kinetics and stoichiometry of thermochemical conversion is very complex. This study evaluates the feasibility of a 300 kW hydrogen pilot scale production unit from biomass (wood pellet) in Växjö. Mass and energy balance calculations were performed, mainly in the first two reactors, in the envisage design and the lower heating value of the product gas was determined. Data that were used for this study was gotten from the plant in Växjö, literature survey of related systems and tools that were used to facilitate the calculations include Microsoft Excel and HSC chemistry for equilibrium calculations. The result of mass and energy balance analysis on the 1MW fuel feeding system, showed that, the flow rate of sand required for the pyrolysis process (700°) is 2610 kg/h, and the energy of pyrolysis equals 217 MJ/h. Partial burning of the pyrolysis gas in the secondary reactor produces product gas consisting of mainly CO, CO2 H2 and H2O with volume percent equal 47.08%, 6.94%, 30.29% and 15.69% respectively. The calculated lower heating value of this gas is 9.04 MJ/Nm3 when pure oxygen was used in partial burning of pyrolysis gas and approximately 6.0 MJ/Nm3 when air was used for burning. The 1MW fuel (wood pellet) feeding via thermochemical conversion has a production potential of 229 kW of hydrogen gas. Based on the parameters considered in this thermochemical process, the calculated Cold Gas Efficiency equal 59.7% and a Carbon Conversion Efficiency equal to 70.3%. Key woods: Hydrogen, Biomass, pyrolysis, gasification, combustion.

Acknowledgments First and foremost, I wish to express my most sincere gratitude to my supervisor Professor Michael Strand for his continues guidance and availability throughout this project work. Your sincere and timely response to all my questions facilitated the progress of this work which I deeply appreciate. I am forever grateful for being a recipient of Linneaus University Scholarship Scheme. I wouldn’t have been able to pay the tuition fees Linneaus University. Thank you very much for the tuition fee reduction scholarship. I will do my best to be a good ambassador of Linneaus University. Am forever indebted to every member of my beloved family (The Ewang’s and Nyambi’s). Thank you for your prayer, love, moral and financial support. I could come this far because you guys believe in me. To my late grand mom I know you are with your creator. Thank for the legacy you left behind. To my best friend and prayer partner (Prudencia Makebe), I love you very much. To the all my classmates, especially Alice and Nasrin. I will forever cherish the time we spent together. Thank you, guys, for the corporation. To all the teachers and staffs of the Department of Build Environment and Energy Technology, I say thank you for you for the sacrifices. You made learning easy and my stay in Sweden memorable.

Table of contents 1 Introduction 1

1.1 Why hydrogen as a transport fuel in Sweden 2 1.2 Location of project 2

1.2.1 Motivations and challenges faced by the municipality of Växjö 2

1.3 Objectives 3

2 Bioenergy energy use in Sweden 4 2.1 An overview of emissions by sector in Sweden 4 2.2 Bioenergy potential in Sweden 5 2.3 Environmental concerns and carbon tax 6 2.4 Experience with wood for heating in Sweden 8 2.5 A broad political support 9 2.6 Share of energy use in Sweden 9 2.7 Swedish Forest Industry 10 2.8 Uses of hydrogen 11 2.9 Fuel cell electric vehicles (FCEV) – a long term solution 12 2.10 Vehicles fuelled by hydrogen 12 2.11 Fuel cell vehicles and hydrogen fueling stations as of the end of 2018 13 2.12 Projections of Freight activity or heavyduty transport 15

2.12.1 Volvo Group and Daimler Truck AG joint venture for large-scale fuel cells production 15

3 Methods of Hydrogen production 16 3.1 Thermochemical conversion of biomass 16 3.2 Biomass gasification 17

3.2.1 Drying 17 3.2.2 Pyrolysis 18 3.2.3 Combustion 19 3.2.4 Gasification 19

3.3 Technologies of biomass gasification 20 3.3.1 Moving bed gasification 20 3.3.2 Fluid bed gasification 21 3.3.3 Entrained flow gasification 21 3.3.4 Dual fluidized bed(DFB) gasification 21

3.4 Product gas upgrading and cleaning process 23 3.4.1 Water gas shift 23 3.4.2 Technologies for hydrogen separation 24 3.4.3 Dust filters 25

4 METHODOLOGY 26 4.1 Process overview 26 4.2 Fuel conversion rate 27

4.3 Drying and devolatilization 28 4.3.1 Drying 28 4.3.2 Devolatilisation 28

4.4 Mass and energy balance of the pyrolysis reactor 29 4.4.1 Chemical formula of biomass 29 Molar fraction of water in fuel 30

4.5 Equivalence Ratio 30 4.6 Mass balance of the secondary tar cracking reactor 31 4.7 Overall mass balance summary 31 Figure 19. Mass balance of inflows and outflows streams produced from the gasification process. 32 4.8 Energy balance calculations 32

4.8.1 Determination of the flow rate of sand required for the pyrolysis process 33 4.8.2 Energy balance of pyrolysis reactor 34 4.8.3 Energy balance of tar cracking reactor 35

4.9 Thermodynamic equilibrium 37 4.10 Determination of the heating value of the product gas from the secondary reactor. 38

4.10.1 Temperature influence of the process equilibrium 38 4.10.2 Heating value of product gas 38

4.11 Energy efficiency calculations 39 4.11.1 Cold Gas Efficiency (CGE %) 39 4.11.2 Carbon Conversion Efficiency (CCE %) 39

5 RESULTS AND DISCUSSION 41 5.1 Flow rate of bed material required for the pyrolysis process 41 5.2 Result of mass balance of the pyrolysis process 42 5.3 Results of equilibrium calculations 44

5.3.1 The influence on temperature on the pyrolysis gas composition 44 5.3.2 Fuel moisture content (MC) influence on gas composition 45 5.3.3 The influence of oxygen concentration on the composition of product gas in the secondary tar cracking reactor. 46 5.3.4 Estimation of lower heating value of the product gas from secondary reactor. 47

5.4 Mass balance of the secondary tar cracking reactor 48 5.5 Result summary of overall mass balance 49 5.6 Energy balance calculation results 49 5.7 Heating value of the product gas 51 5.8 Efficiency parameters of the pyrolysis and tar cracking reactors 51

6 CONCLUSIONS 53

7 Reference 54

8 Appendix 59

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1 Introduction

Fuels consumption over the years has evolved from solids such as wood to coal, an then a parallel use of crude oil, which is preceded by today’s increasing use of natural gas. The state of science and technology has often guided the usage and requirement of energy carriers needed for industrial development. Fossil fuels have been the main energy source used by mankind to meet the growing global energy demand [1]. It should be highlighted that, the primary energy consumption according to the International Energy Agency (IEA), is expected to increase by 1.6% per year and the fossil fuels (oil, natural gas and coal) reserves supplying a greater part of the world energy needs are declining. Studies from the Energy Research Centre of the Netherlands (ECN) has shown that the global oil production might decline within 30 years [2]. Mounting to the problem of securing energy supply is the CO

2 emissions resulting from fossil fuels combustion, which causes global warming. The need to address climate change issues and reduce fossil fuel use and its related carbon dioxide emission has been the driving force of the Paris Agreement. Despite withdrawal by the United States of America from this agreement, this United Nations Framework Convention on Climate Change (UNFCCC), which brought all nations with a common cause of setting ambitious goals, with the aim to keep global temperature rise below 2 °C, is however the strongest international framework for the development of sustainable alternative technologies and promotion of the use of renewable energy sources across the globe [3]. The substitution of fossil fuels with hydrogen, could be one of the ideal methods to enhance the decarbonization of global economy [1]. Hydrogen is an essential intermediate in the chemical industry and as of 2019, annual global production of hydrogen stands at about 70 million tonnes, with a market valued of USD 117.49 billion. According to the International Energy Agency (IEA) statistics, this is equivalent to 14.4 exajoules (EJ), around 4% of global final energy. Hydrogen production is anticipated to be 122.58 million tonnes and its market value is expected to reach US$191.8 billion by 2024 [4]. About 95% of all production of hydrogen is from fossil fuels (natural gas and coal) and these processes are often associated with huge greenhouse gases (GHGs) emissions. While the other close to 5% is hydrogen which is a by-product from chlorine production through electrolysis and a small fraction of H2 is generated from the electrolysis of water, a process which uses electricity to split water [5]. Currently hydrogen production from renewable sources such as wind, solar or through thermochemical and biological routes are not significant. However, hydrogen from renewable energy sources is a potential efficient and environmentally friendly secondary energy carrier to meet the

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world's growing energy demand. The use of hydrogen as fuel is a vital step of not only decarbonizing our present energy systems (i.e. reducing GHG emissions, particularly CO2) but hydrogen could also be use directly as feedstock for further syntheses and in electricity generation and storage [6].

1.1 Why hydrogen as a transport fuel in Sweden In Sweden, municipalities and industries have been showing strong interest in increasing the market share of hydrogen and the use of the fuel cell technology. Their aim is to substitute fossil fuels with suitable sustainable alternatives. Sweden as member of the European Union is actively working to meet the EU climate change plans of a 20% cut in emissions of GHG by 2020 (compared to the 1990 levels), an introduction of an average 20% share of renewable in the European energy mix, and an energy consumption cut of 20% by it member states [7]. To meet EU’s ambitions to increase the share of renewables, reduce emissions and increase energy efficiency, the Swedish government together with its opposition parties jointly work to ensure a smooth and feasible transition its energy system. The expected outcome is to reduce emissions and ensure economic and environmental sustainability. The Swedish national energy strategy has an environmental target in the transport sector to reduce its carbon dioxide emissions by 70% and an independence of fossil fuels of the vehicle fleet by 2030 [8]. Sweden is more ambitious and has as long-term target a net zero emissions of GHG by 2045 and a negative emission thereafter. In addition, the nation strives for a 100% renewable electricity production by 2040. To achieve these targets and to domestically secure for supply of alternative fuels, one of the suggested routes is through hydrogen production from thermochemical conversion of biomass.

1.2 Location of project This study entails the feasibility of a 300 kW hydrogen production test facility from biomass, in Växjö. Växjö is one of Swedish municipalities with much experience in the field of renewable energy and very sound environmental management policies. The municipality of Växjö has been internationally recognized for its “Environmental Programme”, and the city is known for its slogan " The Greenest city in Europe". This has attracted visitors from all over the world, with many wanting to learn and emulate the city’s sustainable development strategies [9].

1.2.1 Motivations and challenges faced by the municipality of Växjö Political commitment and unity The unanimous agreement by the local politicians to stop the use of fossil fuels in 1996 has led to some of the municipality success. The municipality is working on its very challenging environmental goal of becoming “Fossil fuel free Växjö” by 2030 [9].

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Tremendous success has been recorded so far, for example the city cogeneration plant (Sandvik) as of December 2019 is proud of its 100% fossil fuel free district heating, district cooling and electricity. Figure 1 presents a comparison of CO2 emission from fossil-based sources per resident in Växjö, used in transport, heating and in electricity. Emissions have decreased by about 58% since 1993(compared to 2017 data) and this is mainly due to the use of biomass for heating and electricity production and other renewables. Nonetheless, comparing the 1993 and 2017 data, the total carbon dioxide emissions attributed to transport is still very significant and shown in figure 1. There is a need to address this issue and the production of hydrogen by gasification of biomass is a very important area to exploit if Växjö is to meet its target of becoming fossil fuel free by 2030 [10].

Figure1. Carbon dioxide emissions per resident in Växjö (tons)

1.3 Objectives

This study focuses on the thermochemical routes (pyrolysis and partial burning of the pyrolysis gas) for hydrogen production from woody biomass. The aim of this study is to provide details of the technical aspects required for the design of a 300 kW, hydrogen pilot scale test facility, via thermochemical conversion processes (of woody biomass), gas upgrading and cleaning in the Sandvik cogeneration plant in Växjö. The task performed and study limit, in order to achieve the objectives, include the following:

§ Performing a mass and energy balance of the pyrolysis and the secondary tar cracking reactors, based on calculated fuel feeding rates and selected process parameters;

§ Determining the heating value of the product gas and;

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100000120000140000160000180000

Transport Heating Electricity

Tonnes of fossil CO2

1993 2017

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§ Calculating the Cold Gas Efficiency and Carbon Conversion Efficiency of the process.

2 Bioenergy energy use in Sweden Sweden desire to ensure a long-term energy security has been at the forefront on the country’s push for its bioenergy expansion. Before the first oil crisis in 1973, Sweden had an 80% dependence on imported fossil fuels (mainly oil). Oil rationing as a result of the crisis and the sharp increase in oil prices throughout the decade, resulted in significant economic toll. This cause the oil-importing countries to switch from oil to other sources of energy. During this period, the strong political push for nuclear power was also questioned. This led to the development of the bioenergy sector as an alternative to enhance energy security i.e., the switching of oil heating plants with wood-based fuels for district heating. Today, the Swedish energy sector (particularly the transport sector), still largely depend on oil imports coming from the North Sea (Norway) and Russia. Oil imports nevertheless still entail energy dependency and is still a strain on the Swedish economy [11].

2.1 An overview of emissions by sector in Sweden Figure 2 shows the historical trend of emissions calculated in carbon dioxide equivalence from various sectors in Sweden between the 1990-2017. It is observed that, emissions from the transport sector dominates and the emission in 2017 is lower than that in 1990. This decrease from 1990 to 2017 is as a result of the development and use of more energy efficient cars, and also the increasing use of biofuels in the transport sector.

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Figure 2. Evolution of greenhouse gas emissions in various sector in Sweden from 1990-2017 The emissions from energy industries is mainly from electricity and heat and heat production and emission fluctuations over the years are due to weather conditions, which influences the need for heating. Emissions from manufacturing industries and construction have decreased since from the 90’s and this decrease is due to the switch from oil to biomass. An important example in this sector is the pulp and paper industry. The slight decrease in emissions in the agriculture sector during this period is as a result of less livestock and smaller amounts of fertilizers being used. Emissions from Industrial processes and product use is from mainly fluorinated GHGs (e.g. in cooling systems). Emissions in this category fluctuates with level of production and there has been a decrease in the emissions in the chemical industry thanks to enhanced production technologies [12].

2.2 Bioenergy potential in Sweden

Swedens quest for an alternatives to imported oil and nuclear power in the 1970s and 1980s, has promoted research on renewable energy and energy efficiency. A broad research programme was initiated around the 1980’s involving the state energy company, new energy agency and research institutes. Bioenergy was one of the major research areas and study focus on investigating the potential of biomass, its methods of harvest and sustainable use. The scientific basis of the political decisions to promote bioenergy use was fostered by the results of the special expert commission appointed by the Swedish government in 1990. This expert commission investigated the

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potential of biomass from forestry and confirmed large potential in their report. Also, previous research had shown the large energy potentials of residues from final felling and from thinning operations. This is thanks to the availability of technology to harvest these residues, and a clear positive synergy between harvesting of industrial wood and increased use of biomass for energy purpose [11].

2.3 Environmental concerns and carbon tax

Even though energy security concerns motivated Sweden’s bioenergy development, another vital push has been environmental issues. Since from the 1960s, environmental policy had been a growing subject in Swedish politics. This was further highlighted when Sweden hosted the 1972 United Nations environmental summit in Stockholm. This summit raised issues such as air and water pollution, acidification from emissions of sulphur and nitrous oxide, and concerns on the limited supplies of food and materials. The mounting pressures due to climate issue moving to the forefront further led to the Rio summit in 1992 and the negotiated Kyoto Protocol in 1997. The implementation of carbon tax in 1990 by the Swedish parliament was part of a big tax reform. Carbon tax went into effect in 1991 and the industry sector had a lower tax rate than those of households and the service sector. Over the years, it has been increased many times, up till 2018 where the tax rate is the same for all sectors of the Swedish economy outside the European Union Emission Trading Scheme. Sweden has the highest carbon tax globally. Swedish carbon tax is charged or levied per tonne of CO2 emissions on the different fossil fuels (heating oil, propane, fossil gas and coal) as in figure 3.

Source: Swedish government, Ministry of Finance, and Svebio, 2018

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Figure 3. Carbon tax in Sweden, from 1991-2018 (EUR per tonne of CO2)

The most important factor that has promoted bioenergy and made it more competitive with fossil fuels in Sweden has been carbon tax. This continue to be important as the prices of oil have weakened and might remain weak over the longer period. Meanwhile, there has been increasing volumes, improvement of harvesting techniques and conversion technology. These has greatly reduced biomass cost, and making it competitive with fossil fuels, even without incentives in many applications [11]. The prices of heating oil and pellets for residential use in Sweden from 2006 to 2016 is shown in figure 4.

Figure 4. The Influence of carbon tax on heating oil versus pellet prices in Sweden (Swedish krona-SEK/ Megawatt hour-MWh) [13].

The green line shows the price for pellets, while the darker bars shows market price for oil excluding taxes, and lighter bars show the additional oil price as a result of carbon tax. Carbon tax has nearly doubled the price of heating oil for this period, making wood pellets competitive. From figure it is also observe that the pellets cost is less than that of heating oil during the period considered (excluding carbon tax). This difference in prices was very small in some years. However, in other to motivate the switching of fuel from heating oil to biomass, the price difference needs to be significant enough, so as to recover the investment cost of new boilers, fuel storage systems and equipment handling [11].

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2.4 Experience with wood for heating in Sweden

Most district heating systems in Sweden are run by municipalities and the country has had a well-developed district heating system since in the 1970s. The rebuilding or installation of most combined heat and power (CHP) plants within municipalities were boosted by government incentives aimed at enabling the switch from fossil fuels to renewable fuels. These investment grants and carbon taxation facilitated the rebuilding of heat plants or installation of CHP plants in large as well as in smaller cities and towns. Figure shows the Swedish Bioheat Map i.e. heat plants and grids that uses biomass and biogenic waste for district heating purposes. Larger cities, usually have more than one heat plant to supplying the grid and wood is mostly used as fuel in these plants. However, waste plants are used in biger cities with capacities large enough to support the high cost of advanced flue gas cleaning and need for the cost effectiveness of all year-round operation. Also, there is market for the hot water generated in summertime, as well as for district cooling by these utilities [11]. In recent years, biomass and municipal waste has substituted fossil fuels in almost all district heating in Sweden. Considering 2015 for example as shown in Figure 5, biomass accounted for roughly 60% of the fuel for district heating, while about 15% was from the use of municipal waste. This combined accounts for about three-quarters of Sweden’s district heating needs.

Source:SwedishEnergyAgency(2017)

Figure 5. Fuels used for district heating in Sweden, 1970-2015 (TWh)

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2.5 A broad political support

Bioenergy use and development in Sweden has gain strong public support, and is being promoted, thanks to a combination of factors already discussed (Sweden’s energy security concerns, environmental pressures, CO2 tax, renewable electricity certificate requirement system and the nation’s experience in the efficient use of wood resources from forests). At the national scale, this support is being manifested through effective forest governance and long-term general incentives such as carbon tax. In addition to this, there is also support at local levels, for example some municipalities have implemented measures to encourage the use renewable energy technologies and energy efficiency [11].

2.6 Share of energy use in Sweden

Sweden has achieved much success as a result of the policies it has implemented so far to meet the country energy needs. Bioenergy, mainly woody biomass (wood chips, firewood, waste wood, pellets, briquettes and Logging residues i.e. branches, tops, stems) is used to meet much of these needs. Wood has a number of advantages over fossil fuels: its renewable resource; readily available in a country like Sweden with a well-structured and sustainable managed forests; it is carbon neutral; relatively lesser amount of sulfur and heavy metals emissions.

The advantage of using biomass as fuel is that it is considered a carbon dioxide neutral fuel. This is because the amount of CO2 that is released during burning of biomass, is equals to the amount that is taken from the atmosphere during growth of the biomass. Furthermore, fuels obtained from biomass, for example hydrogen, methane, Fischer Tropsch (FT) diesel and methanol, have the potential to become a CO2 negative fuel. This is because a share of the biomass carbon is separated as CO2 during the process of production and can therefore be sequestrated. Such process are very attractive methods for reducing the level of greenhouse gases emissions in the atmosphere.

Sweden leads the EU and is one of the world leaders in the deployment of renewable energy. These measures have not only reduced the entire nation’s greenhouse gas emissions and but also a significant reduction of the nation’s dependence on imported oil. As presented in figure 6, as of data from 2016, bioenergy accounted for 37% of final energy use in Sweden, while renewable energy (mainly hydro power, wind power and ambient air to heat pumps) provided a share total of 54%. Hence, bioenergy use meets more than one-third of Sweden energy needs [11].

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Source: Svebio, based on data from Statistics Sweden and Swedish Energy Authority, 2017.

Figure 6. Energy use in Sweden, 2016

2.7 Swedish Forest Industry Approximately 67% of land surface of Sweden is covered with forest, making Sweden one of the richest European countries in forests. This is equivalent to 28.4 million hectares of forest of which about 22 million hectares actively managed for different uses. Close to half of this forest area is owned by 300,000 private owners, normally with some relatively small holdings. Products from the forest includes timber, wood fuel and felling residues. The Currently mean growth rate of Swedish forests stands at 5.1 cubic meters per hectares per year. All forest owners in Sweden are obliged to sustain wood production taking into consideration: biodiversity; recreational needs enhancement; protection soil and waters and mitigation of climate change. Figure 7 shows a representation of the share of different wood found in the Swedish forest sector while figure 8 shows million cubic meter of wood flow in the Swedish market in 2011[14,15].

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Source: Swedish Forest Agency

Figure 7. Share of the various types of wood found in the Swedish forest sector [15]

Figure 8. Million m3 of wood flow in the Swedish market [15]

The forest industry in Sweden and its by- products provide the major source of bioenergy used in the various municipalities. It should be noted that another important driver for wood products is the construction sector. The attributes that favors the use of wood in the construction sector is thanks to the fact that wood is renewable, ecological, environmentally friendly and climate-smart material and it is also durable. The increasing demand of wood for energy purposes have however strengthens the supply side and increase the profitability of forest industry.

2.8 Uses of hydrogen

Four main uses of hydrogen production worldwide is for ammonia production 50%, refinery applications 22%, methanol production 14%, reduction

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processes 7% and the remaining 7% spread to other consumers. Some reasons accounting for the worlds increasing demand for hydrogen production include: the need for more ammonia and methanol; more hydrogen is needed for hydro-desulfurization processes due to more stringent environmental regulations promoting the production sulfur free products and the growing interest of the use of hydrogen as an energy carrier[5,16]. There has been increasing demand in the use of hydrogen as fuel in transport sector. Thanks to breakthroughs in fuel cell technology and the quest of many nations to develop economies which are independent of fossil fuels.

2.9 Fuel cell electric vehicles (FCEV) – a long term solution To secure our mobile lifestyle, fuel cell electric vehicles (FCEV) are a long term solution to the growing shortage and the rising cost of energy resources. Automotive industries considers FCEV as one major approach towards a sustainable and affordable mobility for the general public. This has been a driving for development of hydrogen powered cars by companies such as Daimler, Ford, GM/Opel, Honda, Hyundai/ Kia, Renault, Nissan and Toyota. The development of these cars has motivated other stakeholders to start the building of commercial network of hydrogen refuelling stations. Advantages of such initiatives include:

i. Reduction of GHGs emissions ii. Energy storage

One major challenge faced by the global energy systems is the availability of economic energy storage technologies. By comparing to other energy storage solutions, hydrogen has larger scale energy storage capacity and over longer periods of time. This makes it storage to be a valuable complementary solution to expensive investments in new grids systems.

iii. Job opportunities Since the use of hydrogen in the transport sector is very promising, it is inevitable that there will be an increase in the labor force at local, nation and international levels. This will be as a result of investments made by organizations and nations in the development of hydrogen and fuel cells technology.

2.10 Vehicles fuelled by hydrogen Hydrogen can be used to power FCEVs and with high energy efficiency. This fuel is environmentally friendly as water is the only local emissions. The operating principle of the FCEV involves a principal component i.e. the fuel cell which converts hydrogen and oxygen in an electrochemical process to electricity. The electricity produced by the fuel cell is used in an electric engine to drive the power train or propels the vehicle. In order to control the fuel cell and manage the power supply to the engine, a small battery is used. Comparing to vehicle designs using battery (battery-powered electric vehicles) as the only mode of energy storage, compressed hydrogen achieves more energy storage

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and thus a longer driving range. In addition to these, there is no lengthy recharging time as with battery-powered electric vehicles. The fuel cell continues to work as long as it has hydrogen and oxygen flowing into it. Fuel cell electric powertrain is suitable for various types of vehicles. Present limitations include energy storage and power density in the fuel cells, hence limiting use to lighter vehicles rather than heavy long-distance trucks. FCEVs present in today’s market can travel about 700km on a single tank while refueling in minutes as opposed to being recharged in several hours. Furthermore, adding to the environmental advantages of hydrogen fuel cell technology over conventional fossil fuel engines, the energy efficiency of fuel cell in the range of 40 %-60 %, compared with the average energy efficiency of 25 % for petrol engine. Hyundai ix35 and Toyota Mirai were the first FCEVs that was recently manufactured, with handling and performance similar to that of a conventional vehicle. The range of this vehicle varies between 500-594 km and have a refuelling time of three minutes [17,19]. The price of ix35 in Sweden is SEK 629 000 (excl. VAT), while that of Toyota Mirai is USD 56 934 (SEK 486 000) (market not specified) [18]. Three Hyundai ix35 FCEVs are currently used in Sweden. There is a possibility of a significantly reduction of the prices of these vehicles as a result of improvement in research and development of the manufacturing processes of these vehicles as well as with economies of scale. However, most of the large car manufacturers plan to release FCEV models in the near future. In Sweden, the Nordic Hydrogen Corridor (NHC) initiative which has as aim of providing zero emission transport solutions based on hydrogen, is now managed by Hydrogen Sweden (Vätgas Sverige) and includes vehicles, hydrogen production process and distribution. Two new partners in addition to the car producers Hyundai and Toyota are Statkraft (largest producer of renewable energy in Europe) and Everfuel (green hydrogen fuel distributor). In Sweden, about 40 different cities and municipalities are interested in hydrogen-based zero emission transport and the NHC-partners are working with the most promising locations in order to ensure sufficient demand before taking investment decisions and moving to execution phase. An increase in the use of FCEVs within the European Commission has been projected as an important step towards reaching an emission free transport sector. This is because hydrogen will cause no GHG emissions as it is produced from renewable energy [20].

2.11 Fuel cell vehicles and hydrogen fueling stations as of the end of 2018

Globally, the number of FCEVs exceeded 12,900 by the end of 2018. This represents an 80% increase in 2018. Approximately 46% of these vehicles are in the United States, followed by Japan with 23% and China with around 14%. A greater majority of the vehicles are passenger cars in most parts of the world

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while the Chinese stock predominatly commercial vehicles. A more detailed picture is presented in figure 9.

Figure 9. Global proportion of FCEVs, 2018 There are 376 refueling stations in operation worldwide. The three countries with the highest Hydrogen Refueling Stations (HRS) included: Japan with 100 refueling; Germany having 60; and the U.S with 44 refueling stations [22].

Figure 10. Number of hydrogen refueling stations by country

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The EU targets a minimum of 747 hydrogen refueling stations by 2025. Also, the Hydrogen Mobility Europe project plans to place more than 1,400 fuel cell cars to customers and deploy 49 hydrogen refueling stations across Europe by the end of 2020. By the year 2030, the Hydrogen Council [22] has projected that 1 in 12 cars in Germany, Japan, South Korea and California (USA) would be powered by hydrogen. This globally accounts for about 10-15 million cars and 500,000 trucks. While by 2050, hydrogen will power more than 400 million cars, 15-around 20 million trucks, and approximately 5 million buses globally.

2.12 Projections of Freight activity or heavyduty transport Heavy duty vehicles accounts for about 47% of CO2 emissions from land based mobility and approximately 8% of the total global CO2 emissions. Also, it has been projected that freight activity (ton-km) will double by 2050 and renewable hydrogen is the most promising zero-emission fuel for heavy vehicles in the nearest future [23].

2.12.1 Volvo Group and Daimler Truck AG joint venture for large-scale fuel cells production

These companies aim to lead in the development of sustainable transportation. To achieve this, they intent to develop, produce and commercialize fuel cell systems for heavy-duty vehicle applications and other related automotive and non-automotive uses. This initiative aligns with the Green Deal vision of sustainable transport and a carbon neutral Europe by the year 2050. According to Martin Daum, Chairman of the Board of Management Daimler Truck AG, the use of fuel cells are one important way of meeting the growing need of a CO2-neutral transport fuel. In the last two decades, Daimler through its Mercedes-Benz fuel cell unit has built significant expertise and combining both company’ experience will be a turning point in bringing hydrogen fuel cell powered trucks and buses onto our roads. The chemical energy of hydrogen is converted to green electricity which is used to power electric trucks used in long-haul operations, and this complement battery electric vehicles and renewable fuels. According to Martin Lundsted, CEO and president of the Volvo Group the realization of this vision requires support and contributions of other companies and institutions also need to support and contribute companies and institutions to develop and establish such needed fuel infrastructure. Even though both companies continue to compete in all other areas of business and operate as an independent and autonomous entity, the advantages of this 50/50 partnership between Daimler Truck AG and the Volvo Group include: reduction of development costs for both companies; speed up market introduction of fuel cell systems for heavy-duty transport and demanding long distance applications [24].

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3 Methods of Hydrogen production

Projected H2 demands require looking for alternative routes for future hydrogen production, currently dominated by non-renewable sources. Generally, renewable hydrogen can be produced through electrochemical, biological and thermochemical processes. Electrolysis of water is the most important electrochemical method which attracts a lot of interest and the electricity must be generated by a sustainable energy source to split water molecules into hydrogen and oxygen. However, fluctuations in the output of renewable electricity from wind power and photovoltaic energy systems have created a growing need for energy storage systems. Hence, converting electricity into chemical energy (hydrogen) by means of electrolysis represents a promising complementing technology as the H2 generated can be either stored or reconverted into electricity during times of an undersupply [25]. Biologically or photo-biologically methods for hydrogen production employs different microorganisms over a series of metabolisms. Examples of hydrogen producing metabolic pathways include: Biophotolysis of water using green algae, photo-fermentation, dark fermentation, biological water gas shift reaction, and hybrid systems. The hydrogen- producing enzymes catalyzing such biological hydrogen production processes include; hydrogenase and nitrogenase. These methods have the advantage of operating under ambient temperature and pressure, in addition to the use of renewable feedstock or solar energy. It should be noted however that, the state-of-the-art of these technologies is at laboratory scale and practical industrial applications still need to be demonstrated [26, 27]. The state-of-the-art for industrial scale H2 production is through thermochemical routes, based mainly on fossil fuels. However, renewable hydrogen through thermochemical processes can be achieved using biomass as feedstock. Conversion technologies used for the generation of hydrogen from hydrocarbons such as biomass and fossil fuels involve pyrolysis, gasification and reforming processes. The products of these thermochemical processes is synthesis gas, consisting mainly of hydrogen and carbon monoxide. Synthesis gas obtained may then be subjected to other downstream processes in order to produce pure hydrogen.

3.1 Thermochemical conversion of biomass

Biomass, a product of the process of photosynthesis, is a major form of solar energy conversion and accumulation. It is a clean and renewable energ source. Energy production from biomass through thermochemical conversion processes, will not produce lots of harmful Sulphur dioxides (SO2), oxides of nitrogen, and other pollutants, in addition to the fact that CO2 emissions is almost zero. Globally, it has been estimated that, biomass energy will account

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for about 50 % of the global energy consumption by 2050 [28]. Biomass as a fuel contains up approximately 70 % or more volatile raw material properties, making it very suitable for application in thermochemical technologies such as gasification [29].

3.2 Biomass gasification

The technology of biomass gasification is a thermo-chemical process which convert solid biomass fuels such as wood, crop residues, sewage sludge and municipal wastes into a fuel gas. This fuel gas or product gas could be used could be used for applications such as electricity power generation, heating purposes and in the synthesis of other fuels or chemical products. The chemical reaction in the gasification process involves a partial oxidation reaction, which converts the solid biomass to gaseous fuel, using an air-fuel ratio less than 1 at specific temperature and pressure. It should be noted that pure oxygen or steam could also be used as gasifying agent in this process. The chemistry of this process is very complex. The final product obtained depend on the process parameters and many different chemical reactions occur at high temperature and other process conditions like the specific equivalence ratio (air fuel ratio), pressure and catalyst used in the process [30]. The process of biomass gasification can be categorised into three steps.

§ Upstream processing steps: This includes, biomass reduction size, drying and preparation of gasifying agents);

§ Gasification process steps: Involving mainly the pyrolysis or devolatilization and gasification reactions, and;

§ Downstream process steps: Includes the various technologies used in producer gas clean-up, upgrading, reforming and gas utilisation [31].

The biomass gasification process step is the heart of the process. Biomass is thermo-chemically converted to energy rich combustible gaseous product in controlled conditions, using different gasifying agents, for example air, O2 steam and CO2 or different proportions of a combination of these gasifying agents. Unlike the process of combustion in which the biomass oxidation is completed in one-step, the process of solid biomass gasification undergoes a series of physical transformation and chemical reactions within the gasifying reactor our gasification units. An illustration of theses processes are shown in Figure [32].

3.2.1 Drying

In the drying process, the moisture content of the solid biomass evaporates, and theses leaves a dry biomass. Drying occurs in the temperature range between 100–200 oC and the solid fuel in this drying process doesn’t

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decomposed, since temperature is not high enough to cause chemical reactions. The steam releases in these processes may later contribute in chemical reactions such as the WGS.

3.2.2 Pyrolysis

Pyrolysis process is simply defined as the thermal decomposition of biomass in an inert or oxygen-free atmosphere. Pyrolysis is an endothermic process and solid biomass when exposed to elevated temperature in the gasifier, and in the absence of a gasifying agent yields mainly three types of products: volatiles, which include mainly permanent gas, condensable gas and solid residue or char [33].

Figure 11. A schematic presentation of the pyrolysis process of biomass and it products

In thermochemical conversion of biomass, pyrolysis precedes the gasification process and the temperature range for the release of these volatiles can vary between 350-700oC. In this first step, there is devolatization of the volatile materials and thermal breakdown of weaker chemical bonds found within the large hydrocarbon molecules in the solid biomass. The composition of the low temperature volatile vapours consist of gaseous species such as carbon dioxide, carbon monoxide, hydrogen , methane and other lighter hydrocarbons. Also present are large condensable molecules (comprising of phenol and acids) called primary tars. These primary tars are highly oxygenated compounds, a characteristic which makes it highly reactivity. Finally, there is also the solid residue or chars a material consisting of mainly carbon and ash. It should also be noted that if gasifying agent is present and at relatively elevated temperatures i.e. 700-850 oC, secondary gas-phase reactions such a cracking, reforming, combustion, and CO shift, of primary tars occurs and these reactions produce combustible gases and secondary tars consisting of phenolic compounds and olefins. Furthermore, at even higher temperatures (850-1000oC), tertiary conversion of secondary tars, producing

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polyaromatic hydrocarbons (PAHs) occurs and also some soot formation soot observed at this reaction conditions [34, 35].

3.2.3 Combustion

Combustion within the gasification environment involves partial oxidation of a portion of the char and/or volatile products with oxygen (limited amount) to produce CO2, CO, H2O and the heat required to sustain the various gasification reactions.

3.2.4 Gasification

Gasification process involve the partial oxidation by the gasifying agent at high temperatures (600-1500oC) of the char residues, pyrolysis tars (primary, secondary and tertiary tars) and pyrolysis gases to produce a producer gas. The composition of the producer gas consists of mainly hydrogen (H2), carbon monoxide (CO), carbon dioxide (CO2), methane (CH4) and traces of lighter hydrocarbons (ethylene, ethane and propane). In the presence of incomplete reactions of the product gas may contain char and tar [36].

Figure 12. Schematic representation of the three processes in thermochemical biomass conversion [32].

It should be noted as earlier stated that during the gasification process step, several different thermal processes are taking place. However, depending on process conditions, different endothermic and exothermic chemical reactions take place in the gasification units(s). Given that gasification reactions are

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reversible, establishing the direction of these reaction and its conversion requires knowledge reaction kinetics thermodynamics. Thermodynamic equilibrium of gasification reactions imposes a high effect on the thermal efficiency and the composition of the producer gas.

3.3 Technologies of biomass gasification

The technology of Biomass gasification has become one of the most efficient and practical methods of biomass energy conversion process and has attracted lots of research interest. There are basically three types of well-developed and commercially available gasifiers. They include fixed bed, entrained flow and fluidized bed (FBG) gasifiers. In recent years, research scholars has paid much attention on the development dual fluidized bed biomass gasification technology. This is a very promising technology with potentials to significantly contribute in meeting the global increase in energy demand and climate change challenges.

These gasifiers differ in their design details and this may depend on the gasification agent (air, oxygen, steam or mixture), pressure which may be either atmospheric or pressurized, and the mode of heat supplied (autothermal or allothermal). The product gas composition and its heating value is directly affected by the type of gasification agent that is being used. For example, the LHV of product gas air-blown gasification of biomass is in the range of 4-7 MJ/Nm3 (contains approximately 50% nitrogen), while that of gasification of oxygen and steam have LHV of 10-15 and 13-20 MJ/Nm3 respectively [37,38].

3.3.1 Moving bed gasification Moving or fixed bed gasifiers are design base on two type of concepts i.e. updraft and downdraft. In the case of updraft gasifier, the fuel (biomass) is feed from the top of the reactor and it moves downwards under gravity via the drying, devolatilization, gasification and oxidation zones. In this design, the gasification agent is supplied from the bottom, while the product gas is collected at the top. Thus, it is an autothermal gasification process involving heat generated by chemical reactions during gasification. Advantages of the updraft gasifier includes it simplicity, high char conversion, high overall efficiency and flexibility to size of fuel particles and the moisture content. A major disadvantage of this design is the high tar content of product gas since it leaves the reactor near the pyrolysis zone. Gasification medium is not always, in counter-current flow with the fuel as in downdraft gasifiers. Biomass and gasifying agent flow in same direction(co-current) via the drying, devolatilization, oxidation and gasification zones. Tar concentration is much lower compared to updraft gasifier as the gas passes through the hottest oxidation zone before leaving the reactor. The disadvantage of this design include, lower energy efficiency, greater amount of particulate matter

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entrained in product gas, and fuel used for this design requires that its moisture content be less than 25% This technology is limited to smaller scales and with fuel feed size ranging from 6-50 mm [39].

3.3.2 Fluid bed gasification This is an autothermal gasification process which occurs inside a fluidized bed reactor, with either silica sand or catalytic particles being used as bed material. In the design of this reactor, size distribution of bed material is very important as too large particles i.e. >10 mm are not fluidized, whereas too small or fine particles are lifted out of the bed. Pure O2 and steam are often used as oxidizers in order to avoid the dilution of syngas by N2, in case where air was used as oxidizing agent. Fluidization enable proper mixing between bed materials, fuel particles and oxidizing agent and this promotes heat and mass transfer. Due to the proper mixing in the fluidized bed, there is an even distribution of the inert bed material, ash, unconverted char and fuel particles, inside the bed. A disadvantage with this is the reduction carbon conversion efficiency as a result of unconverted char and fuel particles removed together with ash from the gasifier [36]. It should be noted the fluidization regime changes with increasing fluidization velocity and there are three different process regimes for fluid bed gasifiers. This consist of stationary or bubbling fluidized bed (BFB) with velocity in the of 0.5-2 m/s, circulating fluidized bed (CFB), velocity in the of 2-5 m/s or transport reactor. Also, FBG are very sensitive to sintering and agglomeration of the ash/bed materials, hence they are generally operated below 1000 °C i.e. below the softening temperature of fuel ash so as to avoid bed agglomeration problems. This low gasification temperature results to syngas having large amount of higher hydrocarbons and tars [39].

3.3.3 Entrained flow gasification In entrained flow gasifiers (EFG) the fuel being feed and the oxidant are in co-current flow. Residence time within EFG is in the order of a few seconds and for this reason EFG requires much smaller particles, higher gasification temperature to attain complete fuel conversion, compared to the other type of gasifiers. Oxygen is generally used as the oxidizing agent in other to achieve the high temperatures desired. A benefits with EFG is the conversion of CH4 and higher hydrocarbons such as tars, due to the high process temperature in the gasifier. Hence, EFG produces syngas with the highest quality [39]. Besides, this technology is fuel flexible as most gasifiers operate in the slagging range, implying the removal of fuel ash from the gasifier as a smelt.

3.3.4 Dual fluidized bed(DFB) gasification

DFB gasification is based on the use of two fluidized reactors i.e. one for gasification and the other for combustion [40]. The forms of the gasifier includes a bubbling fluidized bed, a circulating fluidized bed, a two-stage

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fluidized bed, a U-shaped fluidized bed, moving bed or down-bed etc.[41]. The two interconnected reactors are separated with loop seals, which allows the transport of bed material between the reactors and also preventing cross-contamination of gases between the two reactors. Steam is used as the fluidization and gasification agent in the gasifier. The principle of the dual fluidized bed gasification process is shown in Figure 13. The system reactor consists of mainly two parts: an endothermic gasifier and an exothermic combustion furnace, where the processes of drying, pyrolysis, gasification and combustion of the biomass takes place. The first part is a bubbling fluidized bed gasification reactor, and the other consist of a circulating fluidized bed combustion reactor. Since the gasification reactions are endothermic, the heat required for these reactions are supplied by circulating hot bed materials i.e. either sand or olivine particles from the combustion reactor [42]. The bed materials transfer heat from the combustion unit or reactor to the gasification units where the volatiles are released. Air is usually used as oxidizer in combustion reactor and steam in the gasification reactor resulting in high heating value of the syngas due to avoidance of syngas by N

2 dilution. The residual char after the biomass gasification process is transported from the gasification unit to the combustion unit along with the bed materials which is further heated in the combustion unit by the burning of char particles. This design improves the rate of carbon conversion and the system thermal efficiency [43]. The main disadvantage of DFB is the high yield of larger hydrocarbons and tars, produced as a result low gasification temperature (around 850 °C). Compared to O

2 blown entrained flow gasification, the investment cost of DFB gasifiers is relatively lower since there is no O

2 production plant.

Figure 13 The principle of dual fluidized bed (DFBG) gasification process.

Heat

Char + sand

Combustion uint

Gasification unit

Fuel +Air Fuel

Flue gases (CO2 , N2 ,O2, H2O)

Water vapour

Circulation of bed material

Flue gases (CO2 , CO , H2 CH4 H2O)

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3.4 Product gas upgrading and cleaning process The state of the art unit operations used for upgrading and cleaning product gas in order to improve hydrogen yield include the following

3.4.1 Water gas shift To increase H2 yield and lower the content of CO generated in product gas, a WGS unit is used. The WGS reaction shown below is a well-established technology used industrially to produce hydrogen or setting CO/H2 ratio of synthesis gas. CO+H2O↔H2 +CO2 ∆H= −41.2kJ/mol WGS reaction converts carbon monoxide and steam into hydrogen and carbon dioxide. The equilibrium equilibrium constant of this reaction decreases with temperature. This implies high conversions are favored by low temperatures, as presented in Figure 14 [5].

Figure 14. The Variation of equilibrium constant (Kp) for WGS reaction with temperature [5]. At industrial scale, WGS unit usually consist of one or more fixed bed reactors. For relatively low CO conversion in order to adjustment CO/H2 ratio for synthesis a by-pass, high temperature (HT) WGS stage is used, while in the case of H2 production, the product gas stream is treated in 2 or 3 steps at gradually lower inlet and outlet temperature, this is to attain a high CO conversion. Many different catalyst are used in order to attain sufficient economic reaction rates. The use of Fe-Cr-based catalysts are suitable for HT WGS steps. High temperature stages operate adiabatically and the inlet gas temperature from 350 to 550 °C, with space velocities ranging from 400 to 1 200 per hour. Operating pressure will depends on the requirement of the plant. (Liu et al., 2010) The advantages of Fe-Cr-based catalysts is that they are robust against

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sulfur poisoning. H2S is often observed in the product gas of steam biomass gasification[44]. Low temperature (LT) WGS stage (around 200 °C) uses Co-Mo or Cu-Zn-based catalysts. It should be noted that Co-Mo catalyst are resistant to the presence of sulfur components, while Cu-Zn-based catalyst is very sensitive to sulfur poisoning [5], hence, requires sulfur removal.

3.4.2 Technologies for hydrogen separation Pressure swing adsorption (PSA) and membrane based processes are used the separation technologies used. However, PSA is used when high purity (> 99%) is required. In PSA process the gas molecules are physically bound onto a solid adsorbent material. Interaction between gas and adsorbent depends on the component of the gas component, its partial pressure, adsorbent type, and temperature. This state-of-the-art gas separation process is widely use in different commercial scale applications, i.e. for example in hydrogen production, upgrading of biogas and in air separation [45]. A simplified flow chat for the PAS process is shown in figure 15.

Figure 15. Simplified flowchart of a PSA process. In this process, the compressed feed is successively fed into various adsorber vessels. The regeneration of the of the vessels are carried out by lowering the pressure, while flushing with the product (raffinate) at high-pressure. Low-pressure product (adsorbate) containing the contaminants of the feed may be reused in other upstream or downstream processes. The main components of product gas and their adsorption strength on activated carbon is given by the following relation: CO2 > CH4 > CO > H2. (Liu et al., 2010) This implies

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CO2 is preferably adsorbed on activated carbon and is better removed from feed gas stream compared to for example H2. Activated carbon is a very good adsorbent for pure hydrogen production. Methods aiming to reach the fuel cell grade H2 production through PSA of product gas from DFB biomass steam gasification plants has been carried out in DFB plants in Güssing and Oberwart, Austria. In their experiments with lab-scale PSA unit (using activated carbon as adsorbents) a H2 recovery of about 80% was achieved [44,46].

3.4.3 Dust filters Dust separation in from the product gas in commercial gasifiers is achieved by the use of bag house filters. In order to ensure the durability of the bag house filter product gas needs to be cooled down below 200 °C. Char residue particles which are entrained from gasification reactor together with product gas are collected in the filter. Fly char may be returned internally (e.g. into the combustion reactor in a DFB system) or it may be discharged from the system as waste stream. Dust particles from the product gas may also be separated using electrostatic precipitators (ESP). ESPs are more advantageous in the cases of the removal of smaller dust particles. Particles separation from product gas stream using the force of an induced electrostatic charge. Certain commercial scale fluidized bed air gasification reactors employed the use of ESPs. It should be noted that, the high carbon content of fly coke from gasification processes, makes ESPs problematic as downstream cleaning. Wet ESPs on the other hand are suitable alternative for cooling and condensing, in order to remove aerosols of solids or liquids[47].

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4 METHODOLOGY

The areas that were covered in this methodology section include: a presentation of the design layout of the gasification process for hydrogen production which is to be installed in the Sandvik plant; the investigated system boundary of study; data collections from literature and assumptions made in the mass and energy balance calculations; some simulation calculations that were performed; The tools that were used for the calculations include Microsoft excel and HSC Chemistry for equilibrium calculations.

4.1 Process overview

The entire process flow and layout is presented in figure16 and figure respectively. The investigated system boundary for the study focus, on the pyrolysis and tar cracking reactors as shown in figure16.

Figure 16. overview of the hydrogen production process in Sandvik plant

For the pilot test facility to be installed in Sandvik, wood pellets are to be used as fuel and commercial wood pellets is assumed to contain about 8% moisture in this study. For a 1MW feeding system, the wood pellets are to come in

gas

Pyrolysis/gasification

Tar cracking

Fuel

dust

S-hydrolysis

H2S HCL NH3 HC N

Reforming

HT WGS

LT WGS

Metanisation

PSA

Compresion H2

Carbon residue

Drying

O2/air

Temp. oC

1500

1000

500

0

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contact with the bed material (sand) at 700 °C. In the pyrolysis reactor, there is simultaneous drying and devolatilization of the wood pellets. The sand and the and the char that are produced from this process, are carried back into the combustion or the boiler unit. The pyrolysis vapor which consist of the condensable and non-condensable gases which are considered in this study, flows into the secondary unit or tar cracking reactor. In this reactor, part of the pyrolysis vapor is burn by controlling the inflow of oxygen. This process increases the reactor temperature. The high temperature in this reactor enables the conversion of both the larger and the light organic fractions in the pyrolysis vapor and hence improves the quality of the product gas. The final product gas is pass through several upgrading and cleaning operation to obtain pure hydrogen. A summary of the operating principle of the two gasifiers is presented in figure 17.

Figure 17. Operation principle of envisaged H2 production test facility to be installed in Växjö.

4.2 Fuel conversion rate Wood pellets are to be used as fuel for the gasification process. They are commercially available. The value of the moisture content and lower heating value LHVfuel of the wood pellets used in this study are approximated 8% and 18.5 MJ/kg respectively. To obtain a gasifier capacity Pgasifier of 1MW, the fuel conversion rate ṁfuel (kg/h) was calculated using equation (1).

ṁfuel = !!"#$%$&'"#$%(&)

(1)

Char + sand

Combustion unit

Pyrolysis reactor

Fuel +Air

Fuel +

moisture

Flue gases (CO , N

Circulation of bed material

Secondary tar cracking

reactor

Pyrolysis vapor

Condensable +

Non-condensable

gases

Product gas cleaning & upgrading

O2/air

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The lower heating value of biomass (LHVfuel MJ/kg) was calculated according to Boie formula

LHVbiomass =34.835xC +93,870xH -10.800xO +6280xN + 10.465xS (2)

Where xC, xH, xO, xN, and xs are mass fraction of the elements C, H, O, N, and S that make up the composition of the fuel considered in this study [48].

4.3 Drying and devolatilization

4.3.1 Drying

The moisture content of the wood pellet considered in this study is 8%. This moisture content is very good in order to achieve generate a high heating value product gas. We assume in this study as the wood pellets enter the primary pyrolysis reactor, the water bound (moisture) in the wood pellets irreversibly and instantaneously changes to the gas phase due to the high temperature of the incoming bed material(sand). Given that biomass is a very complex mixture of organic materials, with major elements being carbon, hydrogen, oxygen, nitrogen and Sulphur, data from ultimate analysis of biomass that was used to determine the chemical formula of wood and the heating value of the gas did not consider the impact of the nitrogen and Sulphur content of the fuel.

4.3.2 Devolatilisation

The devolatilization process is normal a very complex phenomenon as a result of the numerous chemical and physical transformation occurring rapidly and simultaneously. It should be noted that the production of primary tar from the pyrolysis reactor is measurable and this study relied on experimental measured values at the required pyrolysis temperature. Data from proximate and ultimate analysis were vital in order to calculate the yield and the composition of the devolatilization products. Figure 18 shows the 3 major competing reactions of the devolatilization process.

Figure 18. simplified representation of the devolatilization process

Wood Tar (oil)

Gas

Volatiles

Char Fixed carbon

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It was assumed that the char yield (in weight %) produced in the pyrolysis step, takes the value of the value of the fixed carbon from the proximate analysis of wood pellets. Furthermore, it is also assumed that the total hydrogen and oxygen content of the wood pellet is released in this devolatilization step and the volatiles that was considered in this study include a mixture of CO, CO2, H2, H2O, CH4, C2H4 and primary tar.

4.4 Mass and energy balance of the pyrolysis reactor

Mass and energy balance calculations were performed on the pyrolysis and tar cracking steps, based on data collected from literature studies and assumptions considered in the gasification process design. Parameters that were taken into consideration in order to develop the process mass balance include: Proximate and ultimate analysis data (table 1 ) of wood pellets which is to be used as input fuel. Data from this fuel analyses provided the necessary information for calculating the input stream mass balance on molar basis. Also, experimental data of the measured gases from similar studies were used to ease the calculations.

Table 1. Wood pellets analysis taken into consideration for mass and energy balance [50].

Proximate analysis Value Moisture (% wt.) 8 Ash (% wt.) 0.50 Volatiles (% wt.) 80.57 Fixed Carbon (% wt.) 18.94 Ultimate analysis C (wt %) 49.8 H (wt %) 6.1 O (wt %) 43.6 N (wt %) 0.16 S (wt %) 0.005 Lower heating value (MJ/kg) 18.5

4.4.1 Chemical formula of biomass

Despite the complexity to determine the chemical formula of biomass, several approximations have been imposed to generalize its chemical formula. The method used to determine the chemical formula of the fuel is based on the approximations of the generalized chemical of biomass. This method utilizes

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elemental composition from ultimate analysis of dry biomass to calculate the chemical formula, based on a single atom of carbon as expressed in equation.

Typical chemical formula of biomass = CcHhOo

c=1; h = #%∗&+'%∗&,

; o = (%∗&+'%∗&-

(3)

The values of C%, H% and O% are the percent composition of carbon, hydrogen and oxygen taken from ultimate analysis of dry biomass. Table gives a summary of the properties wood pellets based on ultimate analysis that is to be used in this thermochemical conversion process.

The molecular mass of wood pellets is estimated as:

Mbiomass =MC *c+&,)

*h+&.)

*o (4)

where MC, MH and MO are the molecular mass in g/mol of carbon, hydrogen and oxygen respectively.

Molar fraction of water in fuel The mole fraction of the water present in the wood pellets is calculated as ! =

*∗&/$-0"##&,1.(,-*)%(&)

(5)

"!2O is the molecular weight of water and Mbiomass is the mass of biomass feed. where h is the moisture content of biomass[49]. Base on the above formula, and the proximate and ultimate analysis data, the molar flow of the pyrolysis products are estimated.

4.5 Equivalence Ratio

ER is an important gasifier design parameter. It is simply a ratio of actual air-to-fuel ratio to the stoichiometric air-to-fuel ratio is given in equation. The term ER is generally used for air-deficient conditions, which are practical in many gasification systems.

ER (<1.0)gasification = /012346789:;/3=>

?16=0*=6@:1>=06789:;/3=>= EA (>1.0)combustion (5)

where EA is excess air coefficient.

This design parameter is useful in the secondary tar cracking reactor where only a limited fraction of the stoichiometric amount amount of oxygen is used to burn the incoming pyrolysis gases and hence increase the reactor temperature which enable the cracking of tar. The stoichiometric amount of

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oxygen required for the oxidation of the pyrolysis gas was calculated based on the molar composition of the pyrolysis gas as given in appendix E. In the case of tar, the calculations were done base on its calculated molecular formula from data collected from ultimate analysis results. ER is used to dictates gasifier performance i.e., for pyrolysis which occurs in the absence of oxygen/air ER is zero while in the case of biomass gasification, ER lies between 0.2 and 0.3[51].

4.6 Mass balance of the secondary tar cracking reactor

In the secondary tar cracking reactor, input oxygen flow is very important as it plays the role of raising the temperature of the secondary reactor, which enables cracking of tar and other hydrocarbons that is coming from the pyrolysis reactor. This also has an influence in the total volume of the product gas. In the secondary tar cracking reactor, the temperature of the reactor needs to be increased in order to allow for the oxidation of the higher hydrocarbon fractions which are contained in the pyrolysis vapor coming from the primary reactor. The objective of this step in to increase the mole fraction of the incondensable gases in the final product gas at the expense of the higher organic fractions. To achieve this, required oxygen flow rate into the secondary reactor was determined. It was assumed that since the temperature of the pyrolysis gas is very high (700°C), the introduction of oxygen will result in partial oxidation of not only the tar content of the pyrolysis gas but also of species such H2, CH4 C2H4, present in the pyrolysis gas. The molar flow of oxygen required for the partial oxidation of the combustible gases was determined based on the assumption that the combustible organic fractions were partially oxidized mainly to hydrogen and carbon monoxide. Knowing the molar flow of the pyrolysis gas, the final product gas composition was determined based on the stochiometric ratios of the partial oxidation steps.

4.7 Overall mass balance summary

In order to ease the comprehension of the mass balance of the entire process, the various streams that enter and leave the process were calculated and displayed is as in figure19, which shows the various inflows and outflows streams that are produced from the gasification process.

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Figure 19. Mass balance of inflows and outflows streams produced from the gasification process.

4.8 Energy balance calculations

In order to calculate the energy balance, the following were taken into consideration:

§ Chemical energy content of the fuel § The amount of heat or thermal energy that is stored in the bed material

(sand) based on its specific heat, and which is used to provide the energy needed for the to pyrolyze the supplied fuel (wood pellets)

The char in this process design represents the energy which is not given to the system, that is, it is considered the carbon not converted to product gas. This is to be supplied back to the combustion unit in order to improve the efficiency of this design.

Tars present in the vapor phase contains some energy. The sensible heat was calculated using data for benzene, while its chemical energy content was bases on its calculated molar and mass flow and its LHV that was gotten from literature

The chemical and thermal energy of the product gas or syngas stream is of great importance as it is in this study. Chemical energy is contained in compounds of the product gases that can be burned and the compounds

Boiler Combustion

unit

Pyrolysis reactor

Tar cracking reactor

Raw product gas upgrading and cleaning operations

Wood pellets(kg/h)

Sand (kg/h)

Sand +char + ash(kg/h)

Permanent gases (kg/h) Tar (kg/h)

Product gas (kg/h)

O2 (kg/h)

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considered in this case include; CH4, H2, CO and C2H4. While thermal energy on the other hand is as a result of the heat absorbed and contained in the product gas due to its increased temperature. These entities enable the calculation of the final lower heating value of the product gas.

4.8.1 Determination of the flow rate of sand required for the pyrolysis process

In this process, the heat for the pyrolysis reaction taking place in the primary gasifier (Qpyrolysis) is provided by the sand coming from the boiler unit. It is assumed in this study that the heat absorbed is released in the pyrolysis of the wood pellets and the sand particles absorb all the heat released from the combustion unit. The heat required for pyrolysis in the primary gasifier is ex- pressed as:

Qpyrolysis = Qmoist + Qvap + Qsteam + Qpellet (6)

Where

Qmoist is the heat required to convert the moisture in the wood pellets to vapor phase and is calculated according to equation

Qmoist = ṁdrywood*Ymoist*CpH2O*(Tevap -Tref) (7) Qvap is the heat of vaporization of water Calculated according to equation 8 Qvap = ṁdrywood*Ymoist*∆Hvap (8) Qsteam is the heat used to heat the water vapour produced to the pyrolysis temperature and is calculated according to equation Qsteam = ṁdrywood*Ymoist*Cpsteam*(Tpyro -Tevap) (9)

And finally Qpellets is the heat required to heat the wood pellets to pyrolysis temperature

Qpellets = ṁdrypellets *Cpwood*(Tpyro -Tref) (10)

Qpellets is the heat for the wood pellets to be heated to the pyrolysis temperature

The overall heat that is transferred into the pyrolysis reactor by bed material is given equation

Qpyrolysis = ṁsand *Cpsand*(Tpyrolysis -Texit) (111) Qmoist, Qvap, Qsteam, and Qpellets was determine and base on using the data presented in table. From the value of Qpyrolysis that was determine, using the

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above equations, the mass flow of sand that is required for the pyrolysis process was determine according to equation [52].

ṁsand =A23'-)3#$#

'B#"45∗(C23'-)3#$#-C&7$8) (12)

Table 2. Input parameters for energy calculations

Input parameters Values Feed (Fuel) input (MW) 1 Lower heating value (LHV) of fuel MJ/kg 18.5 Specific heat capacity of sand, (Cpsand, kJ/kg/K) 0,83 Pyrolysis temperature (Tpyro, K) 973.15 Temperature of sand exiting prolysis reactor (Texit, K) 873.15 Moisture of wood pellet (Ymoist) 8% Specific heat capacity of water (CpH2OkJ/kg/K) 4.187

Evaporation temperature of water Tevap(K) 373.15 Reference temperature (Tref, K) 298.25

Heat of vaporization of water (∆Hvap, kJ/kg) 2260

Specific heat capacity of steam (Cpsteam, kJ/kg/K) 1,996

Specific heat capacity of wood (Cpwoodpellet, kJ/kg/K) 1.1635

4.8.2 Energy balance of pyrolysis reactor

Energy balance of pyrolysis reactor is based on the consideration that the heat provided by the sand (Qpyrolysis) is equal to the heat gained by the biomass for heat-up and for water vaporization.

The influence of the moisture content of the fuel (wood pellet) on the flow rate of bed material, flow rate for fuel and the energy required for the pyrolysis process was determined by a variation of the moisture content of the fuel.

Also, the thermal input of the fuel is given is given by the equation

Pfuel = ṁfuel*LHVfuel (13)

Where:

Pfuel is the power potential of the input fuel

35(60)

ṁfuel is the calculated flow rate of the fuel

LHVfuel is the lower heating value of the fuel

The energy demand (Hdemand) of the pyrolysis reactor may be expressed as:

Hdemand = Hout - Hin (14)

Where Hin is a summation of the sensible energy of bed material and chemical energy of the fuel entering the reactor and Hout is the total energy leaving the reactor.

The total energy entering the pyrolysis reactor which is equal to the sum of the thermal input of the fuel and the sensible heat supplied by the bed material is given by.

Hin=ṁfuel*LHVfuel + #D66EB:44:1F:;F + #@6=F1.F:;F (15)

The total outlet heat (Hout, MJ/h) was obtained as follows:

Hout=!"#$%&'(.*%+, +!"#$%&'(.(-.( +ṁtar*LHVtar+!/'$.(-.(+ṁchar*LHVchar+!*0'$(-.(+Hloss (16)

Where:

LHVchar and LHVtar (MJ/kg) are the lower heating value of char and tar

Mchar (kg/h) and Mtar (kg/h) are the mass flow rate of the unreacted char and tar respectively.

$B8>693F.06@H is the heat of combustion of the pyrolysis products

$B8>693F.F:;F , $0*3>.F:;F , and $13>F:;F,are the sensible heat of the product gas, the unreacted char and tar respectively [53].

4.8.3 Energy balance of tar cracking reactor

The product gas thermal content is affected by two parameters:

• The chemical energy contained in possible fuel compounds such as: CO, CH4, and H2.

36(60)

• The thermal energy which is calculated via thermodynamics i.e. the potential energy created as a result of high Temperature.

The total energy entering into the secondary tar cracking reactor is the energy out of the pyrolysis reactor calculated using equation (17) minus the sensible and chemical energy content of char. This because the leaves the pyrolysis reactor back to the combustion unit.

The final energy out of the secondary tar cracking reactor is calculated using equation

Hpdtgas =$BE193F.06@H +$BE193F.F:;F (17)

It is assume that the system is well insulated and the heat loss is the reactors is negligible.

The gases phase of the pyrolysis process consists of CO, CO2, H2, CH4 C2H4, H2O and tar.

Sensible energy calculation

The sensible heat for both the pyrolysis vapor and product gas from the secondary tar cracking reactor were calculated using the thermodynamic relation given in equation.

Hsens = m*Cp*ΔT (18)

where, m , is mass; Cp its specific heat capacity and ΔT the change in temperature.

However, since the pyrolysis vapor and product gas from the secondary tar cracking reactors consist of different species, equation 18 is modified to equation 19.

Hsens = ṁi*Cpi*(T-Tref) (19)

Where: ṁi is the mass flow of species i, in pyrolysis gas or the product gas.

Cpi [kJ/(kg·K)] is the specific heat capacity of species i, in pyrolysis gas or the product gas, which is a function of temperature.

For the two separate reactors, T is their respective final temperature while reference temperature, Tref was considered to be 298K or 25 oC.

37(60)

The chemical energy contained in the pyrolysis and product gases was calculated using similar approach by multiplying the molar flow, ni of the energy containing species and the heat of combustion, $=' of the various gases as in equation [54].

$0*:@' =*nH2*DHH2 + *nCO*DHCO+*nCH4*DHCH4+*nC2H4*DHC2H4 (20)

Where $0*:@' is the chemical energy of the gaseous species

The equation 20 may also be written as

$0*:@' =∑ ') ∗ $=' (21)

Where: mi is the mass flow energy containing species(kg/h) & HIJ,heat of combustion of species i(MJ/kg).

The above results of the above calculations are summarized as in figure 20.

Figure 20. Energy balance flow chat

4.9 Thermodynamic equilibrium

In order to increase the understanding of the thermochemical process and to find the optimal operation conditions for the reactors, thermodynamic equilibrium calculations were performed using the software HSC chemistry 6.0. Base on the elemental composition of the fuel that was chosen, equilibrium calculations were done and important operating parameters of the such a temperature, moisture content and equivalence ratio was varied. The

Bioler Combustion

unit

Pyrolysis reactor

Tar cracking reactor

Raw product gas upgrading and cleaning operations

Wood pellets(MJ/h)

Sand (MJ/h)ssss

Sand (MJ/h) char + ash(MJ/h)

Permanent gases (MJ/m3h) Tar (MJ/h)

Product gas (MJ/m3)

O2

38(60)

HSC Chemistry software performs the thermodynamic simulation of the pyrolysis process of the fuel using the values from ultimate analysis presented in Table 1. With HSC chemistry calculations, it is possible to simulate the various chemical reactions and processes on thermochemical basis. It uses the principle that the total Gibbs energy of the system at equilibrium is at minimum in order to predict the state of equilibrium [55]. For simplicity in this in this study bas on the data of ultimate analysis, only the C, H, O bearing species was investigated.

4.10 Determination of the heating value of the product gas from the secondary reactor.

4.10.1 Temperature influence of the process equilibrium

Temperature is a very important parameter in the entire gasification process. This is because temperature affects the equilibrium reactions which are involved in the process of gasification. However, temperature is a parameter which cannot be controlled directly in real systems [56]. It depends on the equivalence ratio i.e. the higher the ER, the higher the temperature. The advantage of higher temperatures in this system will be that the tar in yield of the pyrolysis vapor coming from the tar cracking reactor will be significantly reduced.

4.10.2 Heating value of product gas The heating value or calorific value of a fuel is simply defined as the amount of heat produced by the complete combustion of a specified amount of the fuel. It unit of measurement is energy per unit of mass or volume. Heating value is classified in two categories i.e. the higher heating value (HHV) which is the heat produced when a fuel is combusted, and products have re- turned to a temperature of 25 °C(water in the liquid phase) and lower heating value (LHV) on the other hand is determined by the subtraction of the heat of vaporization of the water vapor (during combustion of the fuel) from the higher heating value. It treats the H2O that is formed as a vapor [57] i.e. the energy which is needed to vaporize the H2O is therefore not released as heat.

The lower heating value was considered in this study since the water in the thermochemical conversion processes is in the vapor phase. The heating value of the product gas was calculated by considering the contribution of each chemical species i.e. multiplying the volume or mole fraction by the heating value of each constituent as in Equation [51].

HV = xjHV isyngas (22)

39(60)

Where: xj represents the mole fraction of each chemical species that is con- tained in the product gas and HV isyngas represents the heating values of each chemical species contained in product gas.

By assuming that the product gas was composed of mainly H2, CO and CH4 the lower heating of the of the product was calculated using equation 23.

LHV =10.79x H2 + 12.26x CO + 35.81 xCH4 (23)

Assuming that equilibrium is achieve in the process of burning the pyrolysis product, the LHV of the product gas was determined at different equivalent ratios.

4.11 Energy efficiency calculations Two factors are considered in the efficiency calculation of these gasification process. They include the following:

4.11.1 Cold Gas Efficiency (CGE %) The Cold Gas Efficiency represents chemical energy that is contained in the product gas, to the total chemical energy contained in the fuel which is being fed into the pyrolysis reactor. In practical terms, it is the ratio of the calorific value of the product gas to the calorific value of the feedstock (wood pellets). Fuel (wood pellet ) thermal is calculated according to equation 13

Pfuel = ṁfuel*LHVfuel (13)

Thermal output of the fuel is calculated similarly as considered in the energy balance i.e.

Pproduct gas = ṁproduct gas *LHVproduct gas (24)

Where:

Pproduct gas is the power potential of fuel output ṁfuel is the calculated flow rate of the product gas , in kg/h

LHVfuel is the lower heating value of the product gas Finally CGE is calculated as:

CGE% = !2'-5(98!"#

!%(&) (25)

4.11.2 Carbon Conversion Efficiency (CCE %) Carbon conversion on the other hand is an indicator of carbon left unreacted during gasification process. It is express as the ratio of the sum of mass of carbon compounds in the product gas (CO, CO2, CH4) to the total carbon mass inserted to the gasifier(i.e. the pyrolysis or primary reactor).

40(60)

The mass flow of carbon entering the primary pyrolysis reactor is calculated as: -= ṁfuel *[C%]fuel (26)

Where, - is the mass flow of carbon into the pyrolysis reactor ṁfuel is the mass flow of fuel fed in to the pyrolysis reactor [C%]fuel is the carbon content of the fuel In a similar manner, carbon compounds present in the product gas are calculated as: [-i] = .prdtgas *[Ci,syngas %] (27) Where, [-i] is the concentration of compound i in the product gas .prdtgas is the product gas production rate [Ci,syngas %] is the percentage of compound I in the product gas

After the calculation of the concentration of each compound in the product gas, final carbon mass is calculated by multiplying with carbon molar weight (12 g/mol).

Finally:

CCE%=['=]

' (28)

41(60)

5 RESULTS AND DISCUSSION 5.1 Flow rate of bed material required for the pyrolysis process Using the input parameters stated in table 1 and 2, the calculated fuel (wood pellets) flow rate into the pyrolysis reactor, heat required for the pyrolysis and flow of sand are presented in table 3. The operating temperature of the of the pyrolysis reactor in this this study is 700°C and based on the mass and energy balance calculations that was developed in the excel model, a circulation rate of at least 2610.22kg/h i.e approximately 0.725kg/s of bed material is required for the operation of the pyrolysis reactor at the conditions that were considered. The mass flow is for the full load of the 1 MW fuel feeding system, which corresponds to 211.52 kg wood pellets feed per hour. Table 3. fuel flow, energy required for pyrolysis and sand flow from combustion unit Quantity of fuel and heat flow Value Wood pellet flow rate (moist free) (kg/h) 194.59 Wood pellet flow rate (Fuel + Moisture) (kg/h) 211.52 Qmoist (MJ/h) 5.31 Qvap (MJ/h) 38.24 Qsteam (MJ/h) 20.26 Qpellets (MJ/h) 152.83 Qpyrolysis (MJ/h) 216.65 ṁsand (kg/h) 2610.22

It should be noted that, fuel moisture content and temperature are two vital parameters that will influence the pyrolysis process. For this pilot hydrogen facility that is to be installed in Sandvik, we assume a constant pyrolysis temperature (700oC) provided by the sand from the bubbling fluidized combustion unit. In the process model that was developed in Microsoft Excel, the fuel moisture was varied and the influence of this variation on the required fuel flow rate, heat required for the pyrolysis process and the flow rate of sand is shown in figure.

The bed material (sand) plays an important role in heat transfer. It serves as the heat carrier, providing the heat required to the endothermic pyrolysis reactions. Considering the moisture content, which is a very important fuel property for example, if the MC increases, the heat demand for the pyrolysis of the wood pellets increases. This causes also an increase in the rate of circulation rate of the bed material needed to meet the high temperature

Sand mass

42(60)

pyrolysis process. Thus, this variation of the MC and its influence on the energy required for pyrolysis (Qsand), bed material and fuel flow (ṁsand and ṁwood pellet), is shown in figure 21.

Figure 21. The influence of the fuel moisture on the fuel flow rate, heat or energy needed for pyrolysis and flow rate of sand

5.2 Result of mass balance of the pyrolysis process

The mass balance of the pyrolysis reactor uses the input data from Table 1, in order to predict the composition of the devolatilization products. From the results of the data of proximate and ultimate analysis presented in table, the calculated mass and molar flow of the devolatilization products are given in table. This result is based on the calculated fuel mass flow of 211.52 kg/h and the yield of the volatiles and char.

The number of moles of the pyrolysis products was predicted from the molecular weight of the various species that made up the pyrolysis products. The mean molecular formula for the fuel (wood pellet), char and tar that was calculated from the data of ultimate analysis (appendix) are CH

1.7O

0.6

C1H0.14O0.037 and C1H1.431O0.537 respectively. However, in this study, char was considered to be entirely carbon.

0

1

2

3

4

5

6

7

8

9

0

500

1000

1500

2000

2500

0 10 20 30 40 50 60 70 80

Fuel flow rate kg/h Qpyro MJ/h Flow rate of sand kg/s

Energy for pyro. (MJ/h) & Fuel mass flow (kg/h)

% Moisture

43(60)

Table 4. Result of mass and molar flow of pyrolysis process based on yield at pyrolysis temperature.

Pyrolysis products

%Yield T=700℃

Yield fraction

Mass flow (kg/h)

Molar flow Pdt(mol/h)

H2O 8 0.08 16.92 1021.82

Tar 19 0.19 40.19 1825.21 Non-condensable gases (NCG)

54.06 0.5406 114.35 4716.75

Char 18.94 0.1894 40.06 3338.43

The mass flow of the char produced is deducted from the total product flow since it conveyed back to the combustion unit. In this process, the volatiles consist of the condensable and non-condensable gases. The condensable consist of water in the form of steam and tar is a mixture of various higher organic fractions. However, due to the high process temperature considered in this design, the condensable are all in the vapor phase in both the pyrolysis and secondary tar conversion reactors.

The non-condensable considered in this study include; CO, CO2, H2, H2O, CH4 and C2H4. The yield of these non-condensable gases varies significantly with temperature [59,60]. The molar flow was estimated base on measured yield of these gases from experimental studies. This calculation uses the measured yield in volume percent and using the ideal gas assumption that volume percent is equal to mole percent. The determined total molar flow of the non-condensable gases is 4716.75mol/h.

Table 5. The estimated molar and mass flow of the non-condensable gases in the pyrolysis reactor.

Non-condensable gases

Molecular weight (g/mol)

%Yield, T=700℃

Yield faction[58]

Molar flow (mol/h)

Mass flow (kg/h)

CO 28 44.3 0.443 2089.52 58.51 CO2 44 16.7 0.167 787.70 34.66

CH4 16 15.5 0.155 731.10 11.70

H2 2 16.1 0.161 759.40 1.52 C2H4 28 7.4 0.074 349.04 9.77

44(60)

5.3 Results of equilibrium calculations

5.3.1 The influence on temperature on the pyrolysis gas composition Using typical elemental composition of dry wood from ultimate analysis which is given in table 1 and varying the pyrolysis temperature from 600°C to 1400°C, the result of the predicted composition of the pyrolysis products is given in figure 22. By increasing the temperature, the carbon monoxide and hydrogen formation in the pyrolysis gas increases steadily up to about a temperature of about 900°C for the two gases. Above 900°C an increase in temperature has no significant influence on the formation of both species. At 900°C, the pyrolysis reaction reaches the end and almost no reaction reactions occur at this temperature. The content of H2 (approximately 41.3 mol%) and CO (approximately 37.6 mol%) remain high and stable. The carbon residue is considered as charcoal, which is formed as a result of carbonization. From the equilibrium calculation, it is observed that the formation methane and carbon dioxide during pyrolysis is maximum at lower temperature. For the chosen temperature range, both species formation is maximum at 600°C and decreases steadily to negligible amount at about 900°C. The decrease in the concentration of CO

2 with temperature is accompanied with a simultaneous increase in H2 and CO concentration. High gasification temperatures are well known to favour hydrogen production at the detriment of the higher hydrocarbons examples which are dehydrogenated as a result of thermal cracking process.

Figure 22. Influence of pyrolysis temperature on the on the composition of pyrolysis product Note: C2H4 is included in calculation but to low conc. for display.

600 700 800 900 1000 1100 1200 1300 14000.00

0.05

0.10

0.15

0.20

0.25

0.30

File: C:\HSC6\Gibbs\Syngas.OGI

C

kmol

Temperature

H2(g)

CO(g)

C

H2O(g)CO2(g)

CH4(g)

45(60)

It should be noted however that the HSC calculations don’t take into consideration the chemical reaction kinetics of the chemical reactions and calculations doesn’t give the required time that is needed for the reaction to reach theoretical equilibrium state.

5.3.2 Fuel moisture content (MC) influence on gas composition Increasing the molar flow of water in the form of steam while the reactor temperature is maintained at 700°C, at this temperature, water–gas shift reaction proceeds in the forward direction. The outcome is an increase in the formation of H2 and CO2 and a decrease in the formation of CO as depicted in figure 23. These observations are inline other studies [61,62].

Figure 23. Influence on water addition of water on product gas composition at constant pyrolysis temperature (700 °C) up to about 100% of wood d.s.

As the moisture content increases, more amount of heat energy is being consume as latent and sensible heat and thereby increasing the total energy which is needed for the for the pyrolysis process. In addition to this, increase in moisture content increases gas yield as a result of an increase in the availability of moles of water in the form of steam in the system and this is link to a decrease in the LHV of the product gas. [63,64]. It should be noted however that at very high levels of moisture, it will be very difficult to attain the required pyrolysis temperature and as a result this will lead to a much inferior gas quality and hence and overall lower efficiency of the gasification process.

Because HSC calculations represents the compositions of the pyrolysis product in enable us to understand that at the pyrolysis temperature of 700 °C

0.0 0.2 0.4 0.6 0.8 1.00.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

File: C:\HSC6\Gibbs\Syngash2o.OGI

kmol

kmol

H2O(g)

H2O(g)

H2(g)

CO(g)CO2(g)

C

CH4(g)

46(60)

which is considered in this design, the products species that was considered in this study (H

2, CO, CO

2, CH

4, C2H4 and H2O) are not yet at equilibrium hence,

increase the temperature by partial burning of the pyrolysis gas will try drive the reaction to the equilibrium state and enable a significant reduction in the tar concentration of the product gas. This combustion reactions increases the temperature of the secondary reactor with enable the cracking of the tar contained in the pyrolysis vapor.

5.3.3 The influence of oxygen concentration on the composition of product gas in the secondary tar cracking reactor.

The amount of oxygen added has no effect on the pyrolysis reactor since no air or oxygen was introduced in this reactor. However, ER has been observed to alter the composition of the pyrolysis vapor in the secondary tar conversion reactor. Considering the fact that the secondary tar cracking reactor is autothermal, the temperature of this reactor strongly depends on the amount of oxygen that is fed into the the secondary reactor. This process control parameter can be expressed as ER. This implies a variation of the ER or gasification temperature, will have a significant effect on both the product gas composition and the heating value. The total amount of oxygen that is required for stoichiometric combustion of the pyrolysis products based on the amount of combustible products that is given in appendix is 5.93 kmol.

The effect of oxygen concentration on the calculated pyrolysis gas composition entering the tar cracking reactor given in figure 24, at constant temperature (700 °C)

Figure 24. The effect of increasing amount of oxygen on the in the secondary reactor.

0.5 1.0 1.5 2.0 2.5 3.00.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

File: C:\HSC6\Gibbs\deick\New folder\GibbsIn.OGI

kmol

kmol

O2(g)

CO2(g)CO(g)H2(g)

H2O(g)

CH4(g)

47(60)

The concentration of CH4 remains very low and decreases with and increase in the amount oxygen. At about 1.5 kmol which corresponds to ER equal 0.25, the amount of methane in the product gas is negligible. The concentration of hydrogen H

2 and CO decreases with an increase in amount of oxygen. This observe decrease in the amount of H2 and CO is obviously as a result of more and more oxidation of these species in the pyrolysis. From figure 23, as the amount of H2, and CO decreases, the yields of CO2 and H2O keep increasing. Furthermore, the concentration of H2 reduces at a slower rate compared to the concentration of H2O with increasing amount of oxygen. This can be explained by the fact that CO and H2O are involved in water–gas shift equilibrium and are hence consumed, producing H2 and CO2 [65]. Base on the above observations the secondary tar cracking reactor should be operated lower concentration of oxygen or ER in order to have a better gas quality is desired.

5.3.4 Estimation of lower heating value of the product gas from secondary reactor.

Normally, the gas yield is supposed to increase after the oxidation reactions in the secondary tar cracking reactor due to the increasing moles of intake oxygen/air. Figure 25 shows the result of the influence of varying the ER (varied from 0.1 to 0.42) on the heating value of the pyrolysis vapor entering the, the secondary reactor. The lower heating value of the gas decreases sharply with increasing equivalence ratio. The main reason for this is the oxidation of the energy containing gas phase constituents i.e. primarily CO, H2, CH4 and tar [63,65].

Figure 25. LHV of product gas as a function of variation of the equivalence ratio

10,16

8,26

6,89

5,45

3,95

0

2

4

6

8

10

12

0 0,05 0,1 0,15 0,2 0,25 0,3 0,35 0,4 0,45

Equivalence ratio

LHV MJ/m3

48(60)

The above observations have demonstrated that the overall impact of increasing ER, reduces the concentration of CO, H2 and CH4 in the product gas and hence reduce the quality of the product gas. Higher ER will continue to favor the exothermic gas phase combustion reactions, which result to an increase in temperature in the secondary tar conversion reactor. These observations are consistent with experimental studies as well as other predictions methodology using kinetics and thermodynamic equilibrium models [66].

5.4 Mass balance of the secondary tar cracking reactor The total molar flow of oxygen that is required for stoichiometric combustion of the combustible faction pyrolysis gases that was calculated is 5.93 kmol/h. Based on the assumption that tar (C1H1.4O0.5) , CH4 and C2H4 in the pyrolysis gas were partially oxidized into H2 and CO and H2 in to H2O, the moles of oxygen required based on this assumption equals 1.55 kmol/h. The calculated equivalence ration based on oxygen requirement for stoichiometric and partial burning of the combustion gases is 0.26. This ER fall in the range of 0.2 and 0.3 which is applicable in many gasification systems design. It should be noted that the quality of the product gas from the gasifier strongly depends on ER. ER must be significantly below 1.0 to ensure that the product gas is being gasified rather than being combusted. However, if ER is to low the tar in the pyrolysis gas will persist due to low temperature of the reactor [51]. Based on the above mention assumptions, the estimated product gas that will be exiting the secondary tar reactor is presented in table 6. Table 6. Molar and mass flow of product gas Composition product gas

Molar flow (mol/h) Mass flow(kg/h) Volume %

CO 5343.90 149.63 47.08 CO2 787.69 34.66 6.94 CH4 0 0 0 H2 3437.92 6.87 30.29 H2O 1781.22 32.06 15.69

Total mass flow of product gas (kg/h)

223.22

From the above considerations, a summary of both the mass balance for both the pyrolysis and tar cracking reactors is presented in figure 26.

49(60)

5.5 Result summary of overall mass balance

Figure 26. Overall mass balance for both pyrolysis and tar cracking when operating with wood pellets as fuel (1MW) with moisture content of 8%

5.6 Energy balance calculation results From the energy balance calculations, we obtain that the Qpyrolysis is equal 216.65 MJ/h (60.18kW). Given that the fuel flow rate is 211.52 kg/h the heat absorbed for the pyrolysis process neglecting heat loss is 1.02 MJ/kg. This represents about 5.5% of the fuel (wood pellet) enthalpy input (3913,04 MJ/h) on a LHV basis. Considering the total chemical energy of wood pellets into the pyrolysis reactor (3913.04 MJ/h) and the sensible heat (216.65 MJ/h) provided by the sand from the combustion unit, the total chemical and sensible energy of the pyrolysis products are 3913 and 207 MJ/h respectively. Pyrolysis The percentage share of the chemical energy content of the pyrolysis product is

Boiler Combustion

unit

Pyrolysis reactor 700oC

Tar cracking reactor

Raw product gas upgrading and cleaning operations

Wood pellets 211.52kg/h

Sand 2610,22kg/h

Sand 2610.22kg/h char 40.06kg/h

Non-condensable gases 116.15kg/h H2O 18.39kg/h Tar 40.19kg/h

Product gas 223.22 kg/h

O2 49.62 kg/h

50(60)

approximately 46.6% for permanent gases or non-condensable gases, 19.8% for tar related compounds and 33.6% for char which is conveyed back into the boiler unit. The total chemical energy flow of the pyrolysis vapor (supplied mainly by CO, CH4, C2H4, H2 and tar) into the secondary tar cracking reactor is 2599 MJ/h, and the chemical energy flow of product gas is 2335 MJ/h (648.6 kW). The energy difference of 264 MJ/h between the inlet and outlet chemical energy flow in the secondary tar cracking reactor is as a result of the fact that, some of the pyrolysis vapor or gas entering the secondary tar cracking reactor is being partially burned, so as to increases the reactor temperature, which will enable the cracking of the tar components and the higher organic fractions in pyrolysis vapor. In this gasification process for hydrogen production, the calculated sensible energy (Qpyrolysis) supplied by the bed material (at 700 oC) coming from the combustion unit equal 216.65 MJ/h. The calculated sensible energy of the of the pyrolysis gas and product gas from the secondary tar cracking reactors are 207 MJ/h and 444.3 MJ/h respectively. The relatively high sensible heat or energy of the secondary tar cracking reactor is explained by the fact that, partial burning of the pyrolysis gas results to an increase in the reactor temperature and hence an increase in enthalpy of the product gas. A summary of the energy balanced i.e. the sensible heat and the chemical energy for both reactors are present in figure 27.

Bioler Combustion

unit

Pyrolysis reactor 700oC

Tar cracking reactor

Raw product gas upgrading and cleaning operations

$N2:40 = 3913.04 MJ/h

/OPQROSQO = 216.65 MJ/h

"12341567.= 1314 MJ/h

"8945:9;<;1567. =

Permanent gases (CO, CH4, C2H4 &H2) 1823 MJ/h + Tar 776 MJ/h 0TUVWXUOYOOSQO =

207 MJ/h

"845=>1?A3;1567. =2335 MJ/h (648.6 kW)

0TVWRZ[\]PO^SQO =444.3 MJ/h

O2 /Air 25 oC

Tsand, in =700 oC

Tsand, out =600 oC

51(60)

Figure 27. Overall energy balance for both the pyrolysis and tar cracking when operating with wood pellets as fuel (1MW) with assumed moisture content of 8%

5.7 Heating value of the product gas

Based on the results of the calculations of the molar flow of the product gases from the tar cracking reactor present in table 6, the LHV of the product gases determined using equation 23 and the volume fraction of the product gases equals 9.04 MJ/m3. The methane content of the product gas negligible (zero) based on the assumptions considered in the study. This value is for ER equal 0.26. This is equivalent to oxygen mass flow of 49.62 kg/h (i.e. the case where the pure oxygen is used for burning of the pyrolysis gas). If pure oxygen is replaced with air assume to be consist of 21% oxygen and 79% nitrogen for burning of the pyrolysis gas, the calculated LHV of the product gas in this case is approximately 6.0 MJ/m3. The mass and energy balance calculations shows that the thermochemical conversion of 1MW of the (wood pellets) yield or produces 6.87 kg/h of hydrogen and knowing that the lower heating value of hydrogen, from literature is 120 MJ/kg, gives a total of 824.4 MJ/h, which is equivalent to 229 kW. This fall short of the intended 300 kW pilot scale hydrogen production test facility. That notwithstanding, since the product gas composition is rich in CO and steam or water vapor, water-gas-shift unit operations have been incorporated in the product gas upgrading steps, which will in further increase the volume fraction of hydrogen and its overall yield.

5.8 Efficiency parameters of the pyrolysis and tar cracking reactors

It should be highlighted that the efficiency of biomass conversions is into fuel gas strongly depends on the operation conditions of the gasifier(s) and fuel properties. Higher efficiencies are often achieved at higher temperatures [67,68] and this study, based on the assumptions that were made so far, the results of the gasifier efficiency factors can be summarized in table7. Table 7. Results of the gasifier efficiency parameters Cold Gas Efficiency Parameter Value Pfuel 3913,04 MJ/h (1MW) Pproduct gas 2335 MJ/h (648.6 kW) CGE% 59.7 Carbon Conversion Efficiency Parameter Value

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Carbon fuel input 104.70 kg/h C in CO 5343.90 mol/h C in CO2 787.69 mol/h C in CH4 0 Mass flow of carbon in syngas 73.58 kg/h CCE% 70.3

The efficiency indicators(of the primary pyrolysis and the secondary tar cracking reactor) of this hydrogen production pilot scale test facility presents Cold Gas Efficiency equal to 59.7% and a Carbon Conversion Efficiency equal to 70.3%. It should be noted that CGE decreases once the ER is large to a certain extent. So in order to achieve product gas with high CGE, the ER should be properly controlled, so that not more than required combustible species of the product gas will be burn.

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6 CONCLUSIONS The objective of this study has been to evaluate the possibility of producing hydrogen from woody biomass (wood pellet) via thermochemical routes. The approach used involve mass and energy balance calculations and from the results obtained the LHV of the product gas was determined. From this study, the following conclusions are drawn: To produce hydrogen from thermochemical routes, it is beneficial to operate the reactors are high temperatures. Higher temperatures increase the molar concentration of H

2 and CO in the product gas. The is at the expense of methane and other higher hydrocarbons or tar found in the pyrolysis gas. For an adiabatic process, increase in ER also implies increase in the gasification temperature and corresponding decrease in the product gas LHV. Hence the ER should be proper controlled in order to achieve a gas of good quality. Product gas dilution by nitrogen in air oxidation is detrimental to the LHV and application of the product gas. However, the use of pure oxygen as an oxidant adds extra capital cost on the process. Increase in fuels moisture content reduces the product gas LHV. This because the small increase in the concentration of H

2 is overshadowed by the rapid

decrease in the molar concentration of CO. Since the ultimate goal is to produce hydrogen, promoting conditions for water gas shift reactions in the reactors will maximize the production of H

2 at the expense of CO. Reactor operation conditions (temperature, equivalence ratio etc.) and fuel properties and very important in parameter that will affects the efficiency of the overall thermochemical process. Simple calculations which assumes thermodynamic equilibrium, may be used to approximately predict the general behavior of a gasification process and the yield of the major gas components. This study has quantitatively evaluated the different pyrolysis and tar cracking product products from the thermochemical conversion of wood pellets. The final product gas will be upgraded in downstream process such reforming, WGS reactions and PSA since the final goal is to produce hydrogen from this biomass feed stock. The project work will contribute in the design knowledge required for the installation, operation and evaluation of the 300 kW pilot scale hydrogen production (from biomass) test facility plant in Växjö.

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8 Appendix

Appendix A

Table The influence of the fuel moisture on the fuel flow rate, heat or energy needed for pyrolysis and flow rate of sand.

%Moisture content

Fuel flow rate kg/h

Qpyro MJ/h

Flow rate of sand kg/s

Flow rate of sand kg/h

0 194,59 152,83 0,5114 1841,29 5 204,84 191,46 0,6408 2306,69 8 211,516 216,65 0,7251 2610,22 15 228,94 282,35 0,9449 3401,76 25 259,46 397,47 1,3302 4788,83 50 389,19 886,77 2,967 10683,92 75 778,38 2354,64 7,8803 28369,17

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Appendix B Determination of molecular formula of wood pellet, char and tar from ultimate analysis data

Emental analysis Wood pellets

Content Atomic wt(g/mol) Moles Mean composition

C 49.5 12 4.125 1 H 6.2 1 6.2 1.50 O 44.3 16 2.77 0.67 Mean composition formula CH

1.7O

0.6 Molecular

weight(g/mol) 24.24

Char C 93.22 12 7.768 1 H 1.09 1 1.09 0.14

O 4.6 16 0.2875 0.037 Mean formula: C1H0.14O0.037

Molecular weight(g/mol)

12.73

Tar

C 54.5 12 4.542 1

H 6.5 1 6.5 1.431

O 39 16 2.44 0.537 Mean formula: C1H1.431O0.537

Molecular weight(g/mol)

22.018

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Appendix C Influence of equivalence ration on heating value of pyrolysis gas.

ER=0.1

ER=0,17

ER=0,26

ER=0,34

ER=0,42

Gases moles mole fraction

moles mole fraction

Moles mole fraction

moles mole fraction

moles mole fraction

CO 3,2 0,395061 2,7 0,33333 2,25 0,277778 1,75 0,214724 1,2 0,148148 H2 3 0,370370 2,8 0,345679 2,45 0,302469 2,1 0,257668 1,6 0,197531 CO2 1 0,123456 1,5 0,18518 2 0,24691 2,5 0,30674 3 0,370370 H20 0,6 0,074074 1 0,12346 1,35 0,166667 1,8 0,22086 2,3 0,28395 CH4 0,3 0,037037 0,1 0,01235 0,05 0,00617 0 0 0 0 SUM 8,1

8,1

8,1

8,15

8,1

LHV 10,1599

8,2564

6,894

5,45

3,95

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APPENDIX D

Excel Model for sensible energy calculation for primary pyrolysis reactor

non/condensable gases

Mwt g/mol

%Yield, T=700℃

Yield faction

Molar flow mol/h

Mass flow(kg/h)

Cp(kJ/(kg K))

Tpyro-Tref(K)

Qsense

CO 28 44,3 0,443 2089,52064 58,5065779 1,174 575 39494,8654 CO2 44 16,7 0,167 787,697397 34,6586855 1,22 575 24313,0679 CH4 16 15,5 0,155 731,096387 11,6975422 4,348 575 29245,0252

H2 2 16,1 0,161 759,396892 1,51879378 14,9 575 13012,2657 C2H4 28 7,4 0,074 349,039565 9,77310783 3,18 575 17870,1277 H2O 18 - - 1021,81577 18,3926838 2,2252 575 23533,255 Tar

1825,20564 40,1880141 2,571 575 59410,9459

Total sensible heat

206879,553

MJ/h 206,879553

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Excel Model for sensible energy calculation for primary pyrolysis reactor

non/condensable gases

molar flow mol/h

mass flow(kg/h)

Cp(kJ/(kgmol)) Tpyro-Tref(K) Qsense

CO 5343,9018 149,62925 1,227 1075 197364,7219 CO2 787,697397 34,6586855 1,29 1075 48062,93208 CH4 0 0 4,348 1075 0 H2 3437,91585 6,8758317 15,44 1075 114125,0546 H2O 1781,21266 32,0618279 2,458 1075 84718,57083

Total sensible heat 444271,2795 MJ/h 444,2712795

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APPENDIX E Stoichiometric oxygen calculation

Calculation of required oxygen for partial burning of pyrolysis gas

Tar cracking reaction and partial oxidation CH4 + 0,5O2 CO + 2H2

C1H1,4O0,5 + 0,25 O2 CO + 0,7 H2

H2 + 0,5O2 H2O

C2H4 + O2 2CO + 2H2

Product gas composition moles product gas CO 3254,381158

H2 3437,915852

H2O 759,3968919

Required oxygen 1550,587615

mass flow of oxygen 49,61880367

mass flow of nitrigen 183,5895736 Stoichiometric oxygen for combustion of pyrolysis gas 0,73CH4 + 1,46O2 0,73 CO2 + 1,46 H2O 1,83C1H1,4O0,5 + 3,99/2 O2 1,89CO2 + 2,62/2 H2O 0,35C2H4 + 2,1/2O2 0,7CO2 + 0,76 H2O 0,76H2 + 0,76/2O2 0,76 H2O 2,09CO + 2,09/2O2 2,09 CO2

The wood pellet pyrolysis devolatization process producing gas, tar and char is

dominated by endothermic reactions whereas the secondary tar cracking treactions

which produces gas and char (carbon residues) is considered exothermic.

In this study, the pyrolysis of the wood pellets is assumed to decomposes into light

gases, tar including heavy gases, and char

R1: Biomass → Light gases

R2: Biomass → Tar

R3: Biomass → Char

Homogenous Secondary Reactions

The possible gas phase reactions in the secondary tar cracking reactor include:

R4: CH4 + 1.5O2 → CO + 2H2O

R5: H2 + 0.5O2 → H2O

R6: CO + 0.5O2 → CO2

C2H4 + O2 → 2CO + 2H2

R7: C1H1,4O0,5 + 0,25 O2 → CO + 0,7 H2

R8: CO+H2O↔CO2 +H2

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APPENDIX F Excel Model for heat of combustion calculation

Pyrolysis gas

Mass flow Heat of combustion, MJ/kg

Chemical energy flow of pyrolysis product, MJ/kg

CO 58,5065779 10,1 590,916437 CH4 11,6975422 49,85 583,122478 H2 1,51879378 119,96 182,194502 C2H4 9,77310783 47,74 466,568168 TAR 40,1880141 19,3 775,628672

2598,43026

2,34E+03 Product gas

Mass flow Heat of combustion, MJ/kg

Chemical energy flow of product gas , MJ/kg

CO 149,63 10,1 1511,263 H2 6,87 119,96 824,1252

2335,3882 C to CO2 40,0611046 32,8 1314,00423

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Figure. Technical drawing of the bottom part of the boiler in Sandvik. Source Växjö Energi AB

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