MODELIRANJE I OPTIMIZACIJA PROCESA ODSUMPORAVANJA DIREKTNIM UNOŠENJEM SORBENTA U LOŽIŠTE ENERGETSKOG

KOTLA NA UGLJENI PRAH

I. D. Tomanović*, V. B. Beljanski, S. V. Belošević, M. A. Sijerčić, B. D. Stanković, N. Đ. Crnomarkovi ć i A. D. Stojanović

Institut za nuklearne nauke „Vinča“, Univerzitet u Beogradu, Mike Alasa 12 – 14 11001,

Beograd, Srbija* Apstrakt: Radi ispitivanja mogućnosti smanjenja emisije oksida sumpora, razvijen je numerički model procesa odsumporavanja dimnih gasova direktnim unošenjem u ložište sprašenog sorbenta na bazi kalcijuma (konkretno kreča, tj. CaO) i inkorporiran u prethodno razvijeni CFD kod za predviđanje procesa u ložištu kotla sa sagorevanjem ugljenog praha. Model odslikava značajan uticaj organizacije procesa sagorevanja, kao i polazne količine sumpora u gorivu na koncentraciju oksida sumpora u izlaznom preseku kotla. Radi numeričkog ispitivanja procesa odsumporavanja direktnim unošenjem sorbenta u ložište, razvijena je posebna varijanta 3D kompjuterskog koda za simulaciju ložišnih procesa, sa mogućnošću numeričkog praćenja čestica sprašenog sorbenta. Razvijena su i odgovarajuća unapređenja korisnićkog interfejsa koja olakšavaju variranje parametara procesa. Kao osnova za model reakcije sulfatizacije čestica kreča, tj. reakcije sumpor-dioksida sa CaO, uzet je Borgwardtov empirijski model zasnovan na konceptu pora, koji predstavlja zadovoljavajući kompromis između brzine proračuna i tačnosti rezultata, a prethodno je verifikovan poređenjem sa raspoloživim rezultatima u eksperimentalnom reaktoru. Izvedena je numerička studija uticaja odabranih parametara procesa, kao što su odnos Ca/S, mesto unošenja, protok i veličina čestice sorbenta, na efikasnost sniženja emisije SO2 iz ložišta kotla bloka Kostolac B. Odnos Ca/S i temperatura dimnih gasova imaju značajan uticaj na količinu apsorbovanog sumpora, odnosno na brzinu kojom se odvija reakcija sulfatizacije. Ovakva numerička ispitivanja mogu biti osnova za iznalaženje optimalnih, efikasnih i ekonomičnih rešenja za redukciju emisije SO2 iz kotlovskih ložišta konkretnih termoenergetskih postrojenja, samostalno, ili u kombinaciji sa drugim metodama redukcije emisije. Klju čne reči: parni kotao, odsumporavanje, krečnjak, model, numerička simulacija

MODELLING AND OPTIMISATION OF DESUPLHURISATION PROCESS BY DIRECT SORBENT INJECTION IN FURNACE OF

PULVERISED COAL UTILITY BOILER

I. D. Tomanovic*, V. B. Beljanski, S. V. Belosevic, M. A. Sijercic, B. D. Stankovic, N. Dj. Crnomarkovic and A. D. Stojanovic

Vinca Institute of Nuclear Sciences, University of Belgrade, Mike Alasa 12 – 14 11001,

Belgrade, Serbia* Abstract: In order to examine the possibilities for reduction of emission of the sulfur oxides, a numerical model of flue gas desulphurization by direct calcium based pulverized sorbent (specifically limestone, i.e. CaO) injection was developed and incorporated into previously developed CFD code for prediction of processes in furnace of pulverized coal utility boiler. The model reflects significant influence of combustion process organization, and the initial

amount of sulfur in the coal on sulfur oxides concentration at furnace exit. For numerical investigation of desulphurization process by direct sorbent injection into furnace, special variant of 3D computer code for simulation of furnace processes, with possibility to numerically track particles of pulverized sorbent was developed. Corresponding improvements of user interface were also developed to allow easier variation of process parameters. As base for model of limestone particle sulfation reaction, i.e. reaction of sulfur – dioxide with CaO, Borgwardt’s empirical approach based on the pore concept is adopted, which represents a satisfactory compromise between calculation speed and accuracy of the obtained results, and it was previously validated by comparison with available results from the experimental reactor. Numerical study with selected process parameters, such as Ca/S ratio, injection position, flow and size distribution of sorbent particle, was performed in order to determinate SO2 emission reduction efficiency at the end of furnace of utility boiler Kostolac B. Ratio Ca/S and temperature of flue gas has significant impact on amount of absorbed sulfur, i.e. on sulfation reaction rate. Such numerical investigation can be base for finding optimal, efficient and cost effective solutions for SO2 emission reduction form power plant boiler furnaces, using just sorbent injection, or combining it with other emission reduction methods. Key words: steam boiler, desulphurization, limestone, model, numerical simulation 1. INTRODUCTION

Growing demand for electric power production leads to further increase in use of fossil fuels. Widely used are solid fuels – coals of different type and quality. Due to combustion of coals in large power plants significant amount of pollutant gasses, such as sulfur oxides are released in atmosphere. In Serbia most of electric power comes from coal powered plants, thus they are one of largest sources of pollution, and significant effort must be done to research possible improvements concerning reduction of gaseous pollutant emission. International efforts include different laws and directives concerning emission reduction, effectively forcing plant operators to invest into development and implementation of new technologies. Of most interest for us are European directives such as 2010/75/EU which is soon to repeal 2001/80/EC considering integrated pollution prevention and control, both considering problems of air pollution from different industrial and other sources of any scale. This directive set upper limit emission values of pollutant gasses, as well plan for their reduction over given time period, which is of great importance considering global ecology problems. When considering large power plants with heat input over 300 MW this limit is set to 200 mg/Nm3, or at least 96% emission reduction. This emission or 96% reduction is hard to achieve in older power plants, and thus new technologies must be applied.

Today different technologies for desulfatisation of flue gas are present on market, varying in complexity, effectiveness and costs. After introduction of new regulations power companies must invest into new technologies and modernization of exiting power plants. Most common desulfatisation processes are wet scrubbing, dry scrubbing and dry scrubbing with spraying, sorbent injection, regenerative systems and combined deNOx/deSO2 processes. Most commonly used among these are wet scrubbing processes, with 99% efficiency in some cases. Wet processes with limestone have as final product gypsum which can be later sold to construction industry for partial return of process and investment costs [1]. On the other side we have furnace sorbent injection (FSI) as competitive technology for sulfur removal. FSI has low investment and running costs, with positive ecology effects. Compared to wet scrubbing it cost about 15% for entire installation. Wet scrubbing creates great amount of dry and liquid

waste and uses up to 4% of total plant power production. Limestone itself is widely present and has low price.

At first FSI was used on smaller scale plants powered by low sulfur coals where wet or dry scrubber methods were not cost effective. This technology, applied on large power plants has many advantages, such as low initial investment, low operation and maintenance costs, simple use, easy integration into exiting air control systems. Method itself is based on adding calcium based sorbent in furnace before or during combustion process. During process calcination of calcium carbide and bonding sulfur dioxide and trioxide with calcium oxide occurs simultaneously.

900 10003 2

800 11002 2 4

300 7003 4

1

2

C

C

C

CaCO CaO CO

CaO SO O CaSO

CaO SO CaSO

− °

− °

− °

→ +

+ + →

+ →

(1)

Process efficiency is governed by relation between Calcium and Sulfur. This relation is called Ca/S ratio, and it presents number of moles of calcium as its oxide or hydroxide per one mole of sulfur that enters bond in fuel. This ratio influences process efficiency, and by adding more or less sorbent we control reduction of sulfur oxides in flue gas.

Sorbent should not be injected in zones exceeding 1000 – 1100 °C or intensive sintering and deactivation of sorbent could occur. Desulfatisation does not increase proportionally with increase of Ca/S ratio, thus making higher ratios both technologically and economically unfavorable [2].

During furnace sorbent injection heterogeneous reduction reactions of sulfur dioxide occur. If partial pressure of carbon dioxide around particle is lower than partial pressure of carbon dioxide in particle of sorbent indirect sulfatisation takes place. This is common for pulverized coal boilers and fluidized bed combustion at normal conditions [3]. Direct sulfatisation can be split into three steps: calcination, sintering and sulfation. First step is calcination which happens almost instantly when sorbent particle enter furnace which can be seen in (1), first reaction. As the calcination and sintering reactions have both high rates it they considered to occur instantly and are not modeled, but rather we consider CaO to enter furnace directly. This approach was widely used in some simpler models [4, 5, 6, 7]. Sulfatisation is most complex and dominant process of these three. Sulfatisation reaction is described by second reaction shown in (1).

Our goal was to investigate possible reduction obtained by using FSI on Kostolac B power plant boilers. With this bearing in mind we implemented model for SO2 absorption by injected sorbent and corresponding user interface. Here we intend to demonstrate some of capabilities of developed software which is to be used for numerical examination of possible SO2 emission reduction. New module for tracking of sorbent particles, and particle reactions with gas is implemented based on Borgwardts model. User interface sustained various changes. The software makes it possible for user to control parameters such as Ca/S ratio, directly influencing mass flow rate of sorbent and secondary air used to carry it, sorbent injection port vertical position, and position of ports on the wall – above burners or middle of wall. In figure 1. is given a view of user interface used for this. Software presents good numerical investigation tool and could be later used during simulations of various operation regimes.

Figure 1. User interface form used for input of sorbent injection parameters

2. MATHEMATICAL AND NUMERICA MODEL USED

Here we discuss only the new sulfur oxides formation and absorption model used in code, since more detailed description of mathematical model of boiler is described in previous papers [8] in detail.

For absorbtion of SO2 new subroutine was written, containing sorbent particle tracking and particle reactions with SO2. As mentioned, there are few different approaches to modelling reaction between particle and gas phase, introduced by Borgwardt, Kocaefe, Ramachandran and Doraiswamy, and they can be separated into three groups [9]: unreacted shrinking core, grain model and pore model.

In this paper we use Borgwardts sulfatisation model of CaO particle in order to investigate possibility to reduce emission of sulfur oxides at boiler furnace exit. In 1970. Borgwardt introduced his model, which cover both sulfatisation and sintering [7]. All of his experiments were carried out in the reactor with simulated flue gas flow, while sorbent particles were held in a stream on a mesh placed in middle of reactor. According to his observations rate of sulfatisation show number of moles of SO2 absorbed per 1g of sorbent particle and unit of time. This rate is given as equation (2).

2

´

SO

1 d

dm

a a

nR k c

w t

ηρ

= ⋅ = ⋅ ⋅ (2)

In which the reaction rate is described by first order Arrhenius equation:

/E RTak A e−= ⋅ (3)

Influence of sintering, size of particle and reduction of reacting surface are described using effectiveness factor η, showing relation between current reaction rate and rate at zero sulfate loading. By introducing effectiveness factor, in a relatively simple way specific complexities

of sintering and sulfation are taken into account, while base attributes of process are maintained. Effectiveness factor display exponential dependence on amount of absorbed sulfur dioxide, sintering and other non-measurable reaction parameters. This effectiveness factor is given by expression

´n

weβ

η⋅−

= (4)

The reaction rate given in (2) gives good insight in amount of absorbed sulfur dioxide absorbed in sorbent particle over given period of time. This reaction should be added to source term of equations describing sulfur dioxide reaction field over entire furnace. In expression (5) term

2p,SOmS presents formation of sulfur dioxide which depends on combustion

rate of coal and is considered to be linked to it directly, while term 2

CaOp,SOS represents

apsorbtion term which depends on aR given by expression (2).

( ) ( ) ( )2 2 2

2 2 2

2 2

SO SO SO

SO SO SO CaOeff eff effp,SO p,SOm

U V Wx y z

S Sx x y y z zχ χ χ

ρ χ ρ χ ρ χ

χ χ χµ µ µσ σ σ

∂ ∂ ∂+ + =∂ ∂ ∂

∂ ∂ ∂∂ ∂ ∂= ⋅ + ⋅ + ⋅ + + ∂ ∂ ∂ ∂ ∂ ∂

(5)

Model of sulfur oxide reaction with sorbent particles is implemented in previously developed and verified model and software for simulation of combustion process in boiler furnace of power plant.

More detailed verification of combustion model can be found in [8]. Combustion model considers sulfur dioxide release rate to be proportional to release rate of carbon. This model is verified against experimental results obtained for power plant Kostolac B1 and B2, and results are presented in table 1. Agreement of experimental and simulated values was good. Table 1. Comparison of emission values obtained by numerical simulation and experimental values obtained by measurements on full scale boiler furnace

Operation regime

Sukr/Ssag (up),

% Emiss. of SO2 acc. to model,

mg/Nm3 Measured SO2 emission,

mg/Nm3 B-1-2010-8607 1.30/0.986 5434.1 5533.0 B-2-2009-8703 1.39/1.085 5520.8 5597.3

Model is further coupled with module developed for sorbent reactions and sorbent particle tracking. Borgwardts model used here was previously verified in set of numerical simulations. Simulation results were compared in detail against Borgwardts experimental results. Achieved agreement between data was satisfactory, and model is considered to be verified. This is described in more detail in our previous work describing model implementation and comparison between Borgwardts results and ours obtained by simulations. All simulation results show good agreement with his experimental data, and when used with corrections proposed by Punbusayakul et al. [6], agreement was good even for larger diameter particles. This model has good stability and convergence when implemented in to the code.

3. NUMERICAL SIMULATIONS AND DISCUSSION

In this paper we investigate furnace sorbent injection as one of possible solutions for sulfur emission reduction from power plant boiler furnace Kostolac B1 and B2. These power plant boilers are both 350 MWe at full load, with nominal steam capacity 1000 t/h each. The water-wall dry-bottom furnaces (15.1 m x 15.1 m x 43.0 m), with after combustion device-grate, are identical, and are fully modeled for use in code [8]. The furnaces are optimized for burning Serbian pulverized lignite Drmno. Furnaces are tangentially fired by eight burner tiers of which one is always off and is kept as reserve. Each burner tier consist of two main (lower) burners, where larger diameter particles are injected, and two upper-stage burners used for injection of smaller class particles. Above and below each burner are secondary air nozzles through which secondary air is introduced to ensure good combustion. Burner setup and burner tiers layout can be seen in figure 2.

Figure 2. Boiler furnace burner configuration for power plant boilers Kostolac B1 and B2 and single burner tier vertical layout

Table 2. Composition and kinetic parameters of dolomite 1343 used in this paper

Activation energy, E[cal/mol]a

Frequency factor, A[1/s] a Frequency factor at zero sulfation, A0[1/s] a

14200 1.1·106 1.888·106 CaO, [%] MgO, [%] Fe2O3, [%] SiO2, [%] Specific part.

den, [g/cm3] Part. porosity,

[g/cm3] 94 0.87 0.66 2.98 1.88 0.45

avalues obtained at 1e-3 g/mol sulfate loading, at temperature 980 °C for particles of initial diameter 0.0096 cm

In all simulations we use limestone 1343 whose composition and kinetic parameters are given in table 2. Number of simulations was performed where we varied different process parameters such as Ca/S molar ratio, amount of air used for sorbent injection and position of sorbent injection ports. In simulations as base was used nominal boiler operation regime, to demonstrate possible uses of developed software.

Table 3. Nominal operation regime for power plant boiler Kostolac B1 and B2 with distribution of mass flow of fuel and air over burner tiers.

Fuel distribution over burner tiers [%] Air-coal mixture

through main burners, [%]

Secondary air through main burners, [%]

Main burners Upper stage burners Lower stage Upper stage Lower stage Upper stage

45.5 24.5 19.5 10.5 57.0 67.8

Operation parameters for boiler in projected operation regime are given in table 3. This regime is used in all simulations as basis, while other parameters concerning emission reduction method are varied. Here we use this regime as base in order to demonstrate possibilities of software and implemented module. All created test-cases are made with varied sorbent injection parameters as the main focus here is to present possibilities of new module.

All results obtained by simulations are given in table 4. showing Ca/S relation, furnace exit gas temperature and emission of sulfur. Last column show reduction at boiler exit, since particle reaction does not end in furnace and it reacts until flue gas temperature drops to certain level. First test-case in table is Boiler projected regime, and it has no sorbent injection. Emission obtained this way is to be referred in further test-cases as nominal emission and it will be used to evaluate emission reduction and predict boiler behavior with different operation parameters. Different Ca/S ratios were used in test-cases 2 – 19. We started with ratio low as 1.0 and increased it up to 5.0. Simulations 2 – 10 were performed for sorbent injection ports placed together near middle of furnace walls, while cases 11 – 19 are used to simulate same Ca/S ratios, but with separated injection ports, placed above burner tiers. As we can see from figure 3. emission reduction depends directly on amount of sorbent injected and on position of injection ports which influences distribution of sorbent inside of furnace. We can notice considerable drop in temperature as Ca/S ratio increases, this is due to increased flow of secondary air used to carry sorbent particles, which increases together with sorbent mass flow proportionally. Selecting right

Test-cases 20 – 23, combined with test cases 5 and 14 examine influence of sorbent injection position on emission at furnace exit. Here we simulate three different vertical positions above burner tiers, with two horizontal injection port positions. In test-cases 5, 20 and 21 sorbent is injected through ports set above burner tiers on furnace wall, while in test-cases 14, 22 and 23 sorbent is injected through two ports on middle of wall close to each other. From figure 4. it is clear that position of sorbent injection port has great significance on boiler emission, as we inject sorbent at lower position we have more reduction due to longer residence time of particle inside of field. For same Ca/S ratio ports set above burner tiers show better reduction compared to those set near middle of wall. This indicates that ports should be placed as low as possible (considering temperature of flame, since intensive

sintering could occur if temperature is too high), spread along the furnace wall in order to obtain optimal sorbent distribution.

Table 4. Results of numerical simulations, and comparison of emissions with nominal operation regime

Test-case

Ca/S molar ratio,

[mol/mol]

Sorbent injection port positiona, [m]

Main sorbent transport air flowb, [%]

Sorbent injection port position on

wallc

FEGT, [°C]

Emission reduction at boiler exit,

[%] 1 - - - - 1021 - 2 1.0 0.36 100 M 1021 5.4 3 1.5 0.36 100 M 1019 7.6 4 2.0 0.36 100 M 1016 10.0 5 2.5 0.36 100 M 1013 12.3 6 3.0 0.36 100 M 1011 14.8 7 3.5 0.36 100 M 1009 17.2 8 4.0 0.36 100 M 1008 19.3 9 4.5 0.36 100 M 1007 21.5 10 5.0 0.36 100 M 1007 23.2 11 1.0 0.36 100 B 1018 7.2 12 1.5 0.36 100 B 1016 11.3 13 2.0 0.36 100 B 1015 14.5 14 2.5 0.36 100 B 1013 17.0 15 3.0 0.36 100 B 1012 19.8 16 3.5 0.36 100 B 1009 22.9 17 4.0 0.36 100 B 1007 25.5 18 4.5 0.36 100 B 1005 27.7 19 5.0 0.36 100 B 1003 29.5 20 2.5 2.71 100 M 1024 11.8 21 2.5 5.06 100 M 1026 10.2 22 2.5 2.71 100 B 1022 15.4 23 2.5 5.06 100 B 1026 12.4 24 2.5 0.36 10 B 1019 16.5 25 2.5 0.36 25 B 1016 17.7 26 2.5 0.36 50 B 1016 18.5 27 2.5 0.36 200 B 1005 15.3 28 2.5 0.36 300 B 997 12.5

aabove lowest part of wall steam superheater. bgiven in percent of mass flow of nominal air flow which is six times greater than mass flow of sorbent. cM – sorbent injection ports positioned near middle of wall, B – sorbent injection ports positioned above burner tiers.

Further test-cases 24 – 28 combined with 14 investigate influence of secondary air amount used to carry sorbent on reduction. Mass flow of air used to carry pulverized sorbent is set to be six times greater that mass flow of sorbent. Figure 5. gives us insight in emission behavior when air mass flow is varied. We can see that by reducing amount of air the reduction slightly increases, but only to certain extent, while by further reducing air flow

emission rapidly rises. It can be caused by longer particle residence time and higher temperature field around particles due to less mixing of secondary air with hot flue gases. Further decrease in air flow leads to increase of emission which is caused by poor penetration and distribution of sorbent particles. This potentially has good impact on boiler performance, however it must be taken into consideration that reducing amount of secondary air could lead to poor distribution of particles.

In order to clarify the way sorbent is injected in the furnace we give here two boiler cross sections showing furnace without sorbent injection ports and with sorbent injection ports active (test-case 19). From the figure 6. we see that SO2 concentration field around sorbent injection ports, and towards the exit of furnace changes as the sorbent is injected. At injection ports above burner we see good penetration of air carrying sorbent particles, and further on emission reduction inside of furnace can be seen. Top down view of furnace cross section placed just above sorbent injection ports gives good view of reduction of SO2 on places with high concentration of sorbent particles.

Figure 3. Influence of Ca/S molar ratio and horizontal injection ports position on emission

Figure 4. Influence of vertical and horizontal injection ports position on emission

Figure 5. Influence of secondary air flow used for sorbent injection on emission

Figure 6. SO2 concentration fields for test-cases 1 (no sorbent injected) and 19 (sorbent injection Ca/S = 5.0)

4. CONCLUSION

Displayed test-cases present good demonstration of capabilities of software with implemented Borgwardts model. Possibility to vary many relevant parameters is shown. Further variations can be performed for optimized and other operation regimes, varying all process parameters, such as coal quality, air and coal flow rates, inlet air temperatures and others.

Software is capable of carrying out simulations with various operation parameters, and as such presents a good aid in further research and optimization. It presents great tool for analysis and as such will be used to obtain preliminary, numerical results to evaluate boiler

behavior when new technology is applied, and further it can be used to predict boiler behavior in various operation regimes, before they are applied on boiler.

Borgwardts model is appropriate for use with sorbent particles of larger diameters around 100 µm, however for smaller particle sizes alternative models must be used. Implementation of alternative model and comparison of obtained results will be task of further research and development.

ACKNOWLEDGEMENT This work has been supported by the Republic of Serbia Ministry of Education, Science and Technological Development (project: “Increase in energy and ecology efficiency of processes in pulverized coal-fired furnace and optimization of utility steam boiler air preheater by using in-house developed software tools”, No. TR-33018).

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