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Journal of Analytical and Applied Pyrolysis 97 (2012) 1–10

Contents lists available at SciVerse ScienceDirect

Journal of Analytical and Applied Pyrolysis

journa l h o me page: www.elsev ier .com/ locate / jaap

otary kiln pyrolysis of straw and fermentation residues in a 3 MW pilot plant –nfluence of pyrolysis temperature on pyrolysis product performance

tefan Kerna,∗, Michael Halwachsb, Gerhard Kampichlerc, Christoph Pfeifera, Tobias Pröll a,ermann Hofbauera

Vienna University of Technology, Institute of Chemical Engineering, Getreidemarkt 9/166, 1060 Vienna, AustriaBioenergy2020+ GmbH, Inffeldgasse 21b, 8010 Graz, AustriaEnergieversorgung Niederösterreich EVN AG, 2344 Maria Enzersdorf, Austria

r t i c l e i n f o

rticle history:eceived 13 October 2011ccepted 13 May 2012vailable online 22 May 2012

eywords:yrolysiso-firingtraw

a b s t r a c t

The idea of co-firing biomass in an already existing coal-fired power plant could play a major contributionin the reduction of carbon dioxide emissions. Huge amounts of unused biomass in terms of agriculturalresidues such as straw, which is a cheap and local feedstock, are often available. But due to the highamount of corrosive ash elements (K, Cl, etc.), the residues are usually not suitable for co-firing in athermal power plant. Therefore, the feedstock is converted by low temperature pyrolysis into gaseouspyrolysis products and charcoal. A 3 MW pyrolysis pilot plant located next to a coal-fired power plantnear Vienna was set up in 2008. For the process, an externally heated rotary kiln reactor with a designfuel power of 3 MW is used which can handle about 0.6–0.8 t/h straw. The aim is to investigate the

DGSotary kiln

fundamentals for scale-up to the desired size for co-firing in a coal-fired power plant. In addition to thedesired fuel for the process, which is wheat straw, a testing series for DDGS was also performed. The highamount of pyrolysis oil in the gas had positive effects on the heating value of the pyrolysis gas. Chemicalefficiencies of this pyrolysis pilot plant of up to 67% for pyrolysis temperatures between 450 ◦C and 600 ◦Ccan be reached. The focus of this work is set on the pyrolysis products and their behavior at differentpyrolysis temperatures as well as the performance of the pyrolysis process.

© 2012 Elsevier B.V. All rights reserved.

. Introduction

The world energy demand is continuously growing. The Interna-ional Energy Agency predicts an increase in global primary energyemand of about 45% from 2008 to 2030. The world energy outlookhows that the limit of 450 ppm CO2 equivalents in the atmosphereannot be reached only by the OECD countries even if they ceasell further production of CO2 equivalents [1]. The energetic use ofiomass is basically neutral to the global carbon balance, becausehe same amount of carbon dioxide that is released during the com-ustion of biomass would be released by aerobic decompositionf biomass [2]. The difference between combustion and aerobicecomposition of biomass is the fact that for cultivating, harvest-

ng, transportation and treatment, fossil fuels are usually used [3].his means that extensively produced biomass (wood and residues

ike straw) has a much smaller CO2 footprint than those whichre produced only for combustion (maize, rape, etc.). Also in this

∗ Corresponding author. Tel.: +43 1 58801 166382; fax: +43 1 58801 16699.E-mail address: stefan.kern@tuwien.ac.at (S. Kern).

165-2370/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.jaap.2012.05.006

case extensively produced biomass is the designated fuel for thepyrolysis process.

A very effective way for utilizing biomass in power plants isco-firing in already existing coal-fired power plants. The benefit ofthis method is that large power plants operate at steam param-eters that aim at high electrical efficiencies. Another advantageof substituting coal with biomass is that a lot of carbon dioxideemissions can be saved due to the relatively high output of CO2emissions by burning coal even compared to other fossil fuels. Co-firing can be accomplished via three different modifications, whichare direct, indirect and parallel co-firing. For direct co-firing, a mix-ture of the standard fuel and the additional fuel is burned togetherin the boiler, while for parallel co-firing, a separate boiler whereonly the additional fuel is burned would be required. In this case,indirect co-firing would be the aim as only the gaseous productsmade by pyrolysis or gasification would be burned in the coal-firedboiler. Co-combustion of biomass with coal is a matter of inten-sive research for different applications, and several comprehensive

studies exist on this topic [4–9].

Co-firing of solid biomass into conventional hard coal or lignite-fired power plants is possible, but its use is limited to someboundary conditions. The fraction of biomass in the total thermal

2 S. Kern et al. / Journal of Analytical and Applied Pyrolysis 97 (2012) 1–10

the u

pietodgo

tlltncvsp(ctawtlts

scptapgbre[t

Fig. 1. Parts of the plant influenced by

ower of a conventional coal-fired thermal power plant is lim-ted by the mass flow rate of biomass that can be burned withoutxpecting significant drawbacks. Drawbacks could be corrosion ofhe boiler, higher emissions or disadvantages concerning the flowf the gases in the power plant. The possibility of co-firing is alsoependent on the resulting flow rates of mass, volume and exhaustas. Fig. 1 shows the influence on the parts of a plant in the co-firingf biomass.

To be competitive on the market, the feedstock for co-firing haso be cheap and available in huge amounts. Agricultural residues,ike straw, are in many cases unused and are therefore cheap andocally available. However, as mentioned before, the composition ofhe biomass is essential and may cause problems. Corrosive compo-ents like chlorine or potassium can be harmful to the boiler of theoal-fired power plant, because these power plants employing pul-erized coal firing are optimized for high efficiency. Therefore, theteam parameters (temperature and pressure) have to be as high asossible. At these high temperatures, the mentioned componentsCl, K, etc.) cause high temperature corrosion in the boiler espe-ially at the super heater surface [11,12]. Some tests have shownhat severe corrosion takes place at test probes with metal temper-tures above 520 ◦C [12]. At the power plant at Dürnrohr/Austria,here co-firing is planned based on the investigations in this paper,

he superheated steam has a temperature of 530 ◦C [13] at nominaload, so co-firing of straw would be dangerous for the durability ofhe parts of the power plant. Thus, there has to be an intermediatetep for the co-firing of straw in coal-fired power plants.

Via pyrolysis, the feedstock can be split into gaseous pyroly-is products and charcoal. The aim of this process is to retain theorrosive elements in the char and to produce a highly calorificyrolysis gas that can be co-fired in the power plant. Research inhe area of biomass pyrolysis has shown that the release of chlorinend alkali metals during thermal utilization into gaseous pyrolysisroducts is quite promising for this case [14–17]. These investi-ations have shown that the first amount of chlorine is releasedetween 200 ◦C and 400 ◦C, and most of the residual chlorine is

eleased when temperatures increase from 700 ◦C to 900 ◦C. Thentrainment of potassium starts at temperatures higher than 700 ◦C16,17]. These facts have been strengthened by investigations athe pyrolysis pilot plant described in a previous study [18]. With

se of co-firing biomass, based on [10].

this knowledge, the temperature boundaries for the pyrolysis pro-cess are given in this case; as temperatures above 700 ◦C shouldbe avoided, the accomplished test runs were done between 450 ◦Cand 600 ◦C. To fill the gap of the seasonal feedstock availability ofstraw, there could be used, in addition to conventional wood chips,the residues of renewable liquid fuel production to provide a signif-icant contribution, as Austria’s largest bioethanol production plantis nearby the pyrolysis pilot plant. The waste products of bioethanolproduction are dry distiller’s grains with solubles (DDGS) that areoften used pelletized as an animal feed for cattle due to the highprotein content. The production of DDGS is batch-wise and due tothe fact that it will be used as an animal feed, the requirements onsanitation are high. If a produced batch does not fulfill these regu-lations, it cannot be sold as an animal feed and has to be disposed,for example in a municipal waste combustor. Pyrolysis of DDGS hasbeen investigated also by [19,20] with a focus on the high nitrogencontent of this special fuel.

The type of reactor chosen here is an externally heated rotarykiln reactor. Basically also a fluidized bed reactor could be usedfor the operational temperature range and process parameters butrotary kiln reactors have advantages for solid fuels with differentsizes and heating values that result in a different fuel mass flow[21,22]. Large knowledge about rotary kilns exists as this type ofreactor is commercially used for incineration of fuels with a widevariety of size, heating value, water content or pollutants. Thepyrolysis of wood, plastics and scrap tires in a rotary kiln pyrol-ysis reactor has been investigated widely by Li et al. [23,24], whoreported the influence of the final pyrolysis temperature on the pro-duction of gas, oil and char. It turned out that the gas productionincreased and char production decreased by higher final pyroly-sis temperature for all feedstock types, whereas the formation ofpyrolysis oil decreased for wood and PE plastics but increased forwaste tires at higher temperatures [23].

The experimental campaign carried out at the pilot plant forthis paper sets a focus on the product yields and the pyrolysis oilcontents for different pyrolysis temperatures. The 3 MW pyrolysis

pilot plant, described later in this paper, is not connected to a coal-fired power plant yet. However, it provides information and resultsfor a scale-up that could not be achieved with a small-scale labtesting rig.

l and Applied Pyrolysis 97 (2012) 1–10 3

1

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2

2

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nat6ntf

Table 1Design data of the pilot plant for pyrolysis operation.

Unit Value

Fuel thermal input (MW) 3–5Biomass amount (kg/h) 600–1100Gaseous pyrolysis products (gas, oil, water) (kg/h) 460–800Pyrolysis char amount (kg/h) 140–300Combustion air amount (Nm3/h) 2000–3000Flue gas amount combustion total (Nm3/h) 7300–12800Heating gas amount for rotary kiln reactor

(hot flue gas)(Nm3/h) 2500–5900

Recirculation gas for combustion (Nm3/h) 1000–4200Water injection total (m3/h) 1.4–2.4Heating gas temperature (◦C) 800–1000

S. Kern et al. / Journal of Analytica

.1. Pyrolysis mechanisms and pyrolysis reactions

Biomass is a mixture of cellulose, hemicellulose, lignin andinor amounts of other organic components that each degrade at

ifferent rates and by different mechanisms and pathways. Mostnowledge exists on the thermal decomposition of cellulose as it ishe major part of the wood and as it not as complex as hemicelluloser lignin [25].

For pyrolysis of cellulose, the Broido-Shafizadeh model accord-ng to Bradburry et al. [26] has turned out to be the most adequate

odel for biomass pyrolysis. The pyrolysis of cellulose is charac-erized by a decreasing polymerization grade by splitting glycolicompounds between the glucose units by heat during two par-llel reactions investigated by Bradburry Bradburry et al. [26]. Ifhe condensable vapors produced by this reaction are not removedmmediately out of the hot reactor, they can be cracked by a sec-ndary reaction into char and gas. This reaction is undesired inhe production of liquid products by fast pyrolysis [26–30]. Thectivation energy for the depolymerization reaction at higher tem-eratures is the highest, followed by the dehydration reaction thatccurs at lower temperatures and the secondary cracking of theondensable gases which has the lowest activation energy [31].

Hemicelluloses and lignin are depolymerized by steaming atigh temperatures for a short time. The majority of the pyroly-is products from lignin are char and tar, while only about 10%f the initial mass of lignin is converted to gas [32]. Kumar andratt [33] investigated the temperature range for the initiation ofyrolysis for cellulose between 275 ◦C and 350 ◦C, for hemicelluloseetween 150 ◦C and 350 ◦C and for lignin from 250 ◦C to 500 ◦C.he main sources for gaseous products for the pyrolysis processf biomass are cellulose and hemicellulose, whereas hemicelluloseauses more non-condensable gases and less tar than are made byellulose.

. Material and methods

.1. The pyrolysis pilot plant

Fig. 2 shows a flow sheet with the most important parts of theilot plant. The delivery and handling of the unground biomass arearried out by agricultural vehicles. After cutting by a shredder, theaterial is stored temporarily in a hopper. In an indirectly heated

otary kiln reactor, pyrolysis takes place to produce combustibleases and char from biomass. The combustible gases are burnedn a burner muffle located in the freeboard of the fluidized bedombustion chamber implemented as an afterburner. The fluidizeded combustion is not operated simultaneously with pyrolysis gasombustion but is dedicated for char combustion after a pyroly-is campaign. For the indirect heating of the rotary kiln reactor,ue gas from the afterburner is used. Charcoal is cooled and storedemporarily in a charcoal hopper. The exhaust gases from the after-urner are cooled in a spray type cooler by injection of water andleaned in a spray absorber; the dust is removed in a fabric filter.yrolysis of biomass and burning of the temporarily stored charakes place periodically. If the charcoal hopper is full, pyrolysis istopped and the char is burned in the fluidized bed combustionhamber.

The pyrolysis reactor is a jacked rotary kiln reactor that is exter-ally heated. The heating medium is hot gas that is produced in thefterburner by combustion of the pyrolysis gas. The pyrolysis gasemperature at the outlet of the rotary kiln is between 450 ◦C and

30 ◦C. At this temperature, the main amount of undesired compo-ents, such as heavy metals, sodium and potassium salts, stay inhe pyrolysis char. The feedstock is fed in via a lock system at theront wall of the rotary kiln. To transport the pyrolysis gas out of the

Pyrolysis gas temperature (exit of the rotarykiln)

(◦C) 350–650

rotary kiln and to provide optimal process conditions, a fan gener-ates a pressure of a few millibar lower than the ambient pressure.To prevent the penetration of air, all locks and seals are pressurizedwith seal gas. As seal gas, nitrogen, oxygen-deficient recirculationgas or steam can be used. The heating jacket around the reactoris divided into three heating zones. This allows adjustment of thetemperature and distribution of the heat along the kiln. The tem-perature of the hot gas is greater than 800 ◦C. At the inner wall ofthe rotary kiln reactor, temperatures greater than 650 ◦C can beachieved. Under these conditions, pyrolysis of the biomass occurs.In order to avoid condensation, the gas pipes to the burner muffleare equipped with a trace heating system that uses the used heat-ing gas leaving the jacket of the rotary kiln reactor as a heatingmedium. At the discharge of the rotary kiln reactor, the pyrolysischar is transported via a lock system to an indirectly cooled screwconveyor and a water-cooled chain to cool it to storage temperatureand conveyed to the charcoal hopper for storage.

For start-up and shutdown, as well as for necessary auxiliaryfiring, a gas burner in the freeboard of the fluidized bed reactor isavailable. The pilot plant is not designed for generating energy orheat so the hot flue gas after the combustion chamber has to becooled for additional gas cleaning. Therefore, a spray-type coolerthat injects water into the hot gas stream is used. As a result, thetemperature after the spray-type cooler is below 400 ◦C. For gascleaning, a spray absorber, a selective non-catalytic de-NOx anda fabric filter are provided. As an absorbent, calcium hydroxide isused to clean the gas of sulfur dioxide and hydrochloride acid. If thecontents of HCl and SO2 in the flue gas are very low, it is often notnecessary to use calcium hydroxide. In this case, pure water can beused in the spray absorber. For the reduction of nitrogen oxides,ammonia injection in the freeboard of the fluidized bed chamberis available, a so-called selective non-catalytic de-NOx (SNCR). Thissystem is only used if the NOx emissions exceed limits. Table 1summarizes general operational parameters of the plant.

2.2. Balance of the plant

For research and optimization of the pilot plant the parametersof all streams in the plant have to be known. There are many mea-suring points for temperature, pressure and flow at the plant. Thecomposition of pyrolysis gas, feedstock and char are also measured,but there are some parameters that cannot be measured. Such avalue is for example the volumetric flow rate of the pyrolysis gas.Due to the trace heating that is implemented by the pipe-in-pipeconstruction of the pyrolysis gas pipe from the rotary kiln reac-tor to the afterburner, the construction of a measuring orifice is

not possible. In such a case, the value has to be calculated. Thisis done by a balance of the plant with the available data. For thispurpose, the balance tool IPSEpro has been used. IPSEpro is a sta-tionary, equation-oriented flow sheet simulation tool that has been

4 S. Kern et al. / Journal of Analytical and Applied Pyrolysis 97 (2012) 1–10

f the p

dab

2

2

mamg

2

amtatssitsT

aaamdtw

Fig. 2. Flow sheet o

eveloped for power generating systems [34]. Detailed informationbout IPSEpro, its mode of operation and its utilization for biomassased energy systems can be found in [35].

.3. Analytics

.3.1. Gas measurementThe pyrolysis gas mainly consists of methane (CH4), carbon

onoxide (CO), carbon dioxide (CO2), hydrogen (H2), water (H2O)nd small amounts of oxygen (O2) and nitrogen (N2). The appliedeasurement device for permanent gas measuring is an extractive

as analyzer (model S700, type S710) made by Sick–Maihak.

.3.2. Pyrolysis oil measurementThe applied method is used to measure the dust, entrained char

nd tar content in a gas stream together with the water content. Theain difference from CEN/TS 15439 which is a standard method for

ar measurements is the utilization of toluene as the solvent, whichllows for simultaneous measurement of the tar and the water con-ent in the product gas. The scheme of the tar measuring system ishown in Fig. 3. The gas enters the heated sampling line, which con-ists of a cyclone and a glass wool filled filter cartridge, where dusts deposited. Afterwards, the gas is led through six washing bot-les; five of them are filled with toluene. The washing bottles areituated in a cooling pond which is cooled to -10 ◦C by a cryostat.here, tar/pyrolysis oil and steam condense.

The liquid phases in the washing bottles are unified and thequeous phase is separated from the toluene phase. GC-MS samplesre taken from the toluene phase. The samples are analyzed by

Perkin-Elmer Autosystem XL GC with a PerkinElmer Turbomass

ass spectrometer. The toluene is evaporated from the sample to

etermine the total mass of the pyrolysis oil. To analyze the oil inhe filter cartridge, it is necessary to carry out a soxhlet extractionith isopropanol (IPA). Again, a GC-MS sample of the IPA phase is

yrolysis pilot plant.

taken and analyzed. The filter cartridge is ashed in a muffle furnacefor identification of entrained char and dust.

2.3.3. Feedstock and pyrolysis char analysisThe feedstock for the pyrolysis process, straw and DDGS, as well

as the pyrolysis char are analyzed by the “Testing Laboratory forCombustion Systems” at the Vienna University of Technology. Thesampling and preparation of the fuels is done according to DIN51701. After determination of the water content, described in DIN51718 (drying at 30 ◦C to constant mass, grinding of the dried sam-ple to a maximum particle size of 1 mm and drying of this sample at106 ± 2 ◦C in an inert atmosphere to constant mass), the ash contentis detected according to DIN 51719 by burning the sample to con-stant mass. C, H, N and S are measured by an elementary analyzerEA 1108 CHNS-O made by Carlo Erba. This is done by burning thesample in an oxygen atmosphere. To make sure that a possible COformation is avoided, the gas passes a tungsten catalyst that ensurescomplete oxidation. Afterwards the gas passes a layer of cooper at atemperature of 860 ◦C where free oxygen is bound. Here, also nitro-gen oxides are reduced to N2. As a result the gas consists only of thecomponents CO2, H2O, N2 and SO2 that can be detected. Presentchlorine in the gas is absorbed in an aqueous perhydrol solution(H2O2) which is analyzed afterwards by capillary electrophoresis.

3. Results

3.1. Operation with straw

For the balance of the operating points straw with the compo-sition listed in Table 2 was used. The values are arithmetic mean

values of four straw samples. The difference of the total sum of theelements to 100% is due to the fact that the oxygen content was notdetected here. After pyrolysis, the elemental composition of theproduced char was also detected. The values are arithmetic mean

S. Kern et al. / Journal of Analytical and Applied Pyrolysis 97 (2012) 1–10 5

Fig. 3. Tar/pyrolysis oil

Table 2Elemental analysis of straw and char from straw pyrolysis at 550 ◦C (db . . . dry basis).

Straw Char

Water content wt.% 8.92 2.54Ash content (550 ◦C) wt.%db 4.17 16.17Carbon content wt.%db 47.70 75.08Hydrogen content wt.%db 5.26 1.57Nitrogen content wt.%db 0.46 0.81Sulfur content wt.%db 0.07 0.10Chlorine content wt.%db 0.14 0.35Sodium content wt.%db 0.004 0.013

vtTeytetotattF

F

Potassium content wt.%db 0.61 2.17

Total wt.%db 58.40 96.26

alues of three char samples. By comparing fuel and char analyses,he fundamental idea for the pyrolysis plant can be strengthened.he two most critical components, chlorine and potassium, werenriched in the pyrolysis char, which is the basic idea of this pyrol-sis purpose. For the following balances, test runs at pyrolysisemperatures of 450, 500, 550 and 600 ◦C were performed. An inter-sting point for discussion is the output of gas, oil and char forhe different pyrolysis temperatures. For comparison of the fourperating points, the energy and mass contents of those three frac-ions were each converted to the reference of the sum of the energy

nd mass contents of gas, oil and char. The mass and energy con-ents were consequently calculated dry, so the water content inhe gaseous products (gas and oil vapor) was not considered. Inig. 4, it can be seen that by increasing pyrolysis temperature, the

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ig. 4. Mass fractions for the pyrolysis products of straw vs. pyrolysis temperature.

sampling system.

produced amount of pyrolysis gas increased significantly, whereasthe amount of char slightly decreased and the mass of pyrolysisoil was heavily reduced. This graph strengthens the already knownfact that a higher pyrolysis temperature forces the production ofgas at the expense of the formation of pyrolysis oil. The highertemperature caused decomposition of oil to gas as it was explainedbefore. The slightly lower mass of char results from the fact thatmore volatile compounds were stripped due to higher tempera-tures. Fig. 5 shows that increasing pyrolysis temperature causedan increase in energy delivered by pyrolysis gas and char and adecrease in the energy delivered by pyrolysis oil. Most of theseeffects were caused by the different amounts of the products thatwere generated (see Fig. 4). A further reason is that, due to differentpyrolysis temperatures, the chemical composition of the productswas slightly different. So, at lower temperatures in the rotary kilnreactor, there were more polyaromatic compounds with a highheating value formed that contributed to the pyrolysis oil yield.

At every operating point, several samplings of pyrolysis oil, par-ticulate matter and water in the pyrolysis gas have been carried out.These results were validated by the mass and energy balance of theprocess. The validated results are shown in Figs. 6 and 7. Particulatematter that has to be removed for many downstream processes isdivided here into dust and coke. Coke consists nearly completely ofcarbon that was detected by burning the dust sample and measur-

ing of the weight difference. In this case, for the pyrolysis of indoorstored straw, the dust content decreased while coke increased withhigher pyrolysis temperatures, but the total amount of particulatematter decreases slightly. As previously explained, the amount of

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Fig. 5. Energy fractions for the pyrolysis products of straw vs. pyrolysis temperature.

6 S. Kern et al. / Journal of Analytical and Applied Pyrolysis 97 (2012) 1–10

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ebahnpdpput

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Table 3Elemental analysis of DDGS and char from DDGS pyrolysis at 550 ◦C (db . . . dry basis).

DDGS DDGS char

Water content wt.% 6.5 –Ash content (550 ◦C) wt.%db 6.28 23.6Carbon content wt.%db 49.04 64.39Hydrogen content wt.%db 7.05 1.48Nitrogen content wt.%db 1.00 1.00Sulfur content wt.%db 0.392 0.291Chlorine content wt.%db 0.174 0.48Sodium content wt.%db 0.835 1.09Potassium content wt.%db 1.28 2.05

Total wt.%db 66.051 94.381

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ig. 6. Yield of water and pyrolysis oil as a function of the pyrolysis temperature.

yrolysis oil in the gas decreased with higher temperatures, andhe water content also decreased slightly, because steam reformingeactions were forced by higher temperatures (Fig. 11).

.2. Operation with DDGS

As mentioned in the introduction of this paper, the pyrolysisilot plant has also been operated with a fermentation residue thatccurs as a side product during ethanol production from wheat,orn and triticale. The production plant for bioethanol is locatedear the pyrolysis pilot, so DDGS from this plant would be also aegional available feedstock. The opportunity has been taken to testelletized DDGS as a fuel for the pyrolysis process. The analyses ofeedstock and char are pointed out in Table 3.

Also for DDGS, the amount of chlorine and potassium wasnriched in the charcoal. DDGS showed a completely differentehavior during pyrolysis than straw. With this feedstock, test runst temperatures of 450, 500 and 550 ◦C were performed. The resultsere show very interesting trends of the three produced compo-ents gas, oil and char: Fig. 8 displays the mass fractions of theyrolysis products of DDGS with temperature variation and Fig. 9emonstrates the trends of the energy fractions of the pyrolysis

roducts of DDGS with temperature variation. The mass of theyrolysis gas had a significant contribution to the gaseous prod-cts and it increased for higher temperatures. There are distinctiverends for the mass fraction of pyrolysis gas and pyrolysis oil. While

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dust coke

ig. 7. Content of dust and coke in the pyrolysis gas as a function of the pyrolysisemperature.

Pyrolysis gas Pyrolysis oil Char

Fig. 8. Mass fractions for the pyrolysis products of DDGS vs. pyrolysis temperature.

the produced amount of pyrolysis gas increased due to higherpyrolysis temperatures, the mass fraction of pyrolysis oil decreased.A special feature here is that for pyrolysis temperatures lower than500 ◦C, the major part of the gaseous products was present in theform of oil, whereas for temperatures higher than 500 ◦C, pyrolysisgas represented the bigger part. Pyrolysis oil and water contentof the pyrolysis gas were also measured for this testing period.Fig. 10 gives information about these datasets. The water content

was reduced from 62.5 to 61.2 vol.% and the pyrolysis oil amountwas reduced more significantly by an increase in the pyrolysis tem-perature.

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Fig. 9. Energy fractions for the pyrolysis products of DDGS vs. pyrolysis temperature.

S. Kern et al. / Journal of Analytical and Applied Pyrolysis 97 (2012) 1–10 7

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ig. 10. Yield of water and pyrolysis oil as a function of the pyrolysis temperature.

If the results of the pyrolysis product mass distribution areompared with the results of rotary kiln pyrolysis of scrap tires,nvestigated by Li et al. [24], there can be seen that straw behavedompletely different. The amount of pyrolysis gas made by strawanged from 49.6 wt.% at 450 ◦C to 62.7 wt.% compared to a gas yieldor pyrolysis of scrap tires of 13.1 wt.% at 450 ◦C to 18.0 wt.% at00 ◦C. DDGS behaved up to 500 ◦C like scrap tires with a gas yieldf 18.5 wt.% at 500 ◦C, but at higher temperatures the production ofyrolysis gas rapidly increased.

.3. Gas composition

Due to the different composition of the two fuels, the gas com-osition was also different for straw and DDGS. By the plot ofhe permanent gas composition in relation to the pyrolysis tem-eratures, it can be seen that the composition of the gas did nothange significantly in the operated temperature range. For strawFig. 11), CO was present at a concentration of about 34 vol.%db, CO2t 27 vol.%db, CH4 at 25.5 vol.%db and H2 at 10 vol.%db. For DDGSFig. 12), the amount of H2 was negligible, CO and CH4 were about5 vol.%db and CO2 was up to 64 vol.%db. The gas composition madey DDGS has, compared to the gas made by straw, a much lowereating value. That goes along with the distribution of the energyontents of the pyrolysis products (Figs. 5 and 9) as it was shown

hat most of the energy content of the gaseous products was pro-ided by pyrolysis oil. So, the lower heating value for dry pyrolysisas without pyrolysis oil from straw ranged between 14.44 MJ/Nm3

0

5

10

15

20

25

30

35

40

650600550500450400

pyro

lysis

ga

s y

ield

[vo

l.%

db]

pyrolysis temperature [°C]

CH4

CO

CO2

H2

Fig. 11. Pyrolysis gas composition of straw vs. pyrolysis temperature.

pyrolysis temperature [°C]

Fig. 12. Pyrolysis gas composition of DDGS vs. pyrolysis temperature.

at 450 ◦C and 14.97 MJ/Nm3 at 600 ◦C. In contrast to this, for DDGS,the lower heating value reached only 3.66 MJ/Nm3 at 450 ◦C and3.84 MJ/Nm3 at 550 ◦C due to the high CO2 and low CH4 content inthe pyrolysis gas.

3.4. Chemical efficiency

Chemical efficiency is the ratio of the chemical energy of theproduct to the chemical energy of the feedstock. The feedstock inthe pyrolysis process is the biomass (straw or DDGS). The productsare the gaseous products of pyrolysis that are used for co-firing.In this case, the gaseous products contain the pyrolysis gas andthe pyrolysis oil including the water content in the fuel and in thepyrolysis gas. Fly char and entrained dust are unaccounted for herebecause those are only minor amounts, and the loading of solidfines depends very much on the fluid dynamic design of the reac-tor. Equation 1 shows how this was calculated. The lower hatingvalue for the used straw was 17.26 MJ/kgdaf and for DDGS it was22.84 MJ/kgdaf. According to the energy balance, the lower heatingvalue of the pyrolysis oil was 38.5 MJ/kg.

Equation 1: chemical efficiency

�c = VPG × LHVPG + moil × LHVoil

mfuel × LHVfuel

As shown in Fig. 13, the chemical efficiency for the pyrolysis of

straw was up to 56%. The chemical efficiency for the pyrolysis ofDDGS was higher (up to 67%) because of the high energy densityof pyrolysis oil which was predominantly formed from DDGS. In

0.0

0.2

0.4

0.6

0.8

1.0

600550500450

chem

icaleffi

cien

cy[1]

pyrolysis temperature [°C]

stra wDDGS

Fig. 13. Chemical efficiency for straw and DDGS vs. pyrolysis temperature.

8 S. Kern et al. / Journal of Analytical and Applied Pyrolysis 97 (2012) 1–10

a pyrolysis operation of straw at a temperature of 550 ◦C.

arT

3

opfbpt4aawawgfoa(tCssp

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

620600580560540520500480460440

pyrolysisgas/fuelinpu

t[Nm³/kg]

Stra w DD GS

for DDGS as the input power also dropped from 960 kJ to 670 kJ◦

Fig. 14. Energy balance of the pyrolysis reactor for

ddition, Fig. 14 illustrates the energy balance of the rotary kilneactor at a pyrolysis temperature of 550 ◦C during straw pyrolysis.he heat loss of the reactor is included in the cold gas stream.

.5. Product yield per fuel input

A good overview of the influence of the pyrolysis temperaturen the net production of gas and oil and the required energy for therocess give the ratios of product and needed heat to the input ofuel. They show which quantity of the pyrolysis products was madey one kilogram of fuel input. As was shown before, the amount ofyrolysis oil decreased with higher pyrolysis temperatures. Duringhe pyrolysis of straw, pyrolysis oil was yielded at an amount of8.3 g at 600 ◦C to 131.2 g at 450 ◦C per kg straw. DDGS provided

higher oil yield than straw, as there were 171.0 g of oil formedt 550 ◦C and 340.2 g per kg DDGS at 450 ◦C (Fig. 15). Pyrolysis gasas produced at an amount from 0.57 Nm3 at 450 ◦C to 0.62 Nm3

t 600 ◦C per kg straw and 0.47 Nm3 at 450 ◦C to 0.71 Nm3 at 550 ◦Cere made by 1 kg of DDGS (Fig. 16). So, the net production of

aseous pyrolysis products increased (Fig. 17). This strengthens theact that higher temperatures activate secondary cracking reactionsf the pyrolysis oil to produce mainly gas and only a very smallmount of char. The gaseous products contain the pyrolysis gasincluding water) and pyrolysis oil. That is because the products ofhis process that will be used for co-firing are all gaseous products.

ooling these gases to the point where the components of pyroly-is oil condense before firing in the boiler should be avoided, so theum of the gas, oil and water amounts is crucial. In Fig. 18, the inputower for the pyrolysis process in the rotary kiln reactor is shown.

0

50

100

150

200

250

300

350

400

620600580560540520500480460440

pyrolysisoil/fuelinpu

t[g/kg]

pyro lysi s temp eratu re [°C]

Straw DDGS

Fig. 15. Yield of pyrolysis oil per fuel input vs. pyrolysis temperature.

pyrol ysis temperature [° C]

Fig. 16. Yield of pyrolysis gas per fuel input vs. pyrolysis temperature.

For the pyrolysis of straw, it can be estimated that higher temper-atures need more input power to keep the process running. Thiswas calculated from the energy balance. Here, the power increasedfrom 807 kJ at 450 ◦C to 926 kJ at 600 ◦C for an input of one kilo-gram of straw. Only for a pyrolysis temperature of 550 ◦C there wasa small decrease in the input power. This decrease is considerable

per kg DDGS at 550 C, so it can be assumed that in this range ofpyrolysis temperatures, some exothermic processes take place.

0.29

0.30

0.31

0.32

0.33

620600580560540520500480460440

char/fue

linp

ut[kg/kg]

pyrolysi s temper atu re [°C]

Stra w DDGS

Fig. 17. Yield of gaseous pyrolysis products (gas, oil, water) per fuel input vs. pyrol-ysis temperature.

S. Kern et al. / Journal of Analytical and

0

200

400

600

800

1000

1200

620600580560540520500480460440

pyrolysisinpu

tpow

er[kJ/kg]

Straw DDGS

4

odpaeiapfabgpaocwkwsvTsmfigbft0tbawip

paacftc

[

[

[

[

[

[

[

[

[

[

[

pyrolys is temperat ure [°C]

Fig. 18. Required input power per fuel input vs. pyrolysis temperature.

. Conclusion

The most important reason for the construction and operationf the indirect heated rotary kiln pilot plant was to gain fun-amental information on the initiation of a pyrolysis plant thatroduces gaseous pyrolysis products that are suitable for co-firingt coal-fired power plants. Pyrolysis of agricultural residues in anxternally heated rotary kiln pyrolysis reactor is a not extensivelynvestigated process that has potential to be optimized. Further, theim to retain as much of the undesired components, like chlorine,otassium or sodium, in the pyrolysis char to prevent the boilerrom high temperature corrosion requires intelligent process man-gement of the pyrolysis parameters. Fortunately, it has alreadyeen proven that this process technology is suitable for producing aas whose combustion products do not cause corrosion, as shown inrevious work at this pilot plant. But, with this work, there has been

focus on the energetic side of the process technology. It turnedut that the operation, handling and control of the pilot plant, espe-ially the pyrolysis reactor, can be done without serious problemsith the current state of knowledge. The ruggedly designed rotary

iln reactor is also insusceptible to contamination of the feedstockith soil or other small inorganic particles. The balance of the plant

ystem can be done in an accurate way as there are many measuredalues available that are recorded by the process control system.his offers the advantage of clearing the uncertainties of the mea-urements so problematic values can be found and checked. Theeasured and calculated values in this paper show great promise

or large-scale industrial application. The most important fact heres to bring as much energy as possible from the feedstock into theaseous products (gas, oil and water) that can be burned in theoiler of a power plant without the risk of hot corrosion. This per-ormance is pointed out by the chemical efficiency of the processhat is, depending on the feedstock, between 0.55 for straw and.67 for DDGS at 450 ◦C. This shows that between 55 and 67% ofhe energy of the feedstock is transferred to combustible gases. Aig advantage of using this system for co-firing is its flexibility withlternative fuels. The rotary kiln pyrolysis reactor is suitable for aide range of fuels, and the power plant where the pyrolysis gas

s burned is not as affected as feeding these fuels directly into theower plant.

So far, only less attention was given to the solid pyrolysisroduct, pyrolysis char or so called biochar. The pyrolysis char has

very high heating value and is nearly free of any water content. Inddition to the combustion in corrosive resistant boilers, biochar

ould also be restored to the fields where the feedstock camerom. Biochar is perfectly suitable for the improvement of soil andherefore it could be used as a soil amendment. Using the pyrolysishar as reducing agent in the non-ferrous metallurgy is also under

[

Applied Pyrolysis 97 (2012) 1–10 9

investigation [36]. The application of biochar techniques to soil hasrarely been investigated up to now. A very important point for thenear future will be the sequestration of carbon. Biochar applicationto soil places carbon originated from atmospheric CO2 into the soilto protect it from surface combustion by fires and to maintain it inrelatively stable forms for a long period of time with opportunitiesfor carbon trading. A better contact with soil minerals enhancesbiochar stability and increases its mean residence time. Seques-tration in subsoil may be an especially effective way to increasestability. A deeper application method may therefore be useful forincreasing the carbon trading value of biochar [37,38].

Acknowledgements

We gratefully acknowledge the support of EnergieversorgungNiederösterreich (EVN) AG and Bioenergy2020+ GmbH. Moreover,the authors would like to express their thanks to the “TestingLaboratory for Combustion Systems” at the Vienna University ofTechnology.

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