Process Simulation Development of Coal Combustion in a Circulating

ARTICLE pubs.acs.org/EF

Process Simulation Development of Coal Combustion in a Circulating Fluidized Bed Combustor Based on Aspen Plus Bing Liu,†,‡ Xuemin Yang,,† Wenli Song,† and Weigang Lin† †

State Key Lab of Multiphase Complex System, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, P. R. China ‡ Graduate University of Chinese Academy of Sciences, Beijing 100039, P. R. China ABSTRACT: A process simulation model has been developed to simulate and predict the condition of three kinds of coal combustion in a 30 kW circulating fluidized bed combustor based on Aspen Plus with considering gas-solid hydrodynamics via Aspen Plus in-line FORTRAN codes and combustion reaction kinetics via some external FORTRAN subroutines simultaneously. The related mathematical models in the process simulation, such as a gas-solid hydrodynamics model in the CFB riser, equivalent pyrolysis model of coal, and combustion kinetics model of coal pyrolysis products, have been presented in detail. The profiles of voidage and gas pressure along the height of riser are discussed and preliminarily predicted by the developed model in this paper. It can be concluded from the results of the process simulation model that increasing gas superficial velocity can lead to a decrease of voidage in the dense region and lower acceleration region, and a slight increase of voidage in the upper acceleration region and completely fluidized region. Furthermore, increasing gas superficial velocity can also effectively increase the total pressure drop of the riser. Therefore, a larger inlet pressure is required when more primary air is introduced into the riser bottom.

1. INTRODUCTION Circulating fluidized bed (CFB) combustion technology has been rapidly developed and widely applied since the 1980s because of its high combustion efficiency and low pollution emissions. The main advantages of CFB combustion technology for industrial applications can be summarized as follows:1-3 easy operation, lower operation temperature, wide adaptability of fuels, wide load adjustment range from 30% to 100% of full load, and discharged solid ash with high activity as raw material for high quality cements. However, high N2O emission from CFB coal combustion is an obvious obstacle under conditions of increased environmental protection consciousness from society as well as the more strict legislation of limiting greenhouse gases emissions, although it emits lower NO compared with other combustion technologies. Besides traditional coal combustion technology, some new technologies, such as reverse air stage combustion4-6 and decoupling combustion,7,8 have been proposed and developed in recent years. As a new CFB combustion technology, the coal decoupling combustion process has been developed at the Institute of Process Engineering, Chinese Academy of Sciences (IPE, CAS),9,10 for decreasing NOx emission, especially reducing N2O emission. The emission behavior of NO and N2O from traditional CFB combustors and the newly developed decoupling CFB combustor have been investigated through changing excess air number, first stage stoichiometry, position of introducing the secondary air, position of charging coal, and coal types.11-13 To further provide detailed information of releasing mechanism for NO and N2O during coal decoupling combustion in a CFB combustor, simulating the formation and emission of NO and N2O along the height of the CFB riser is of importance using the process simulation method.11-13 Therefore, a modeling of coal combustion in a CFB combustor is developed using process system simulation. r 2011 American Chemical Society

The process system simulation is a newly developed study method for process engineering based on the system engineering sciences, covering chemical thermodynamics, chemical reaction kinetics, transfer phenomena, and gas-solid hydrodynamics using computer technology as the basic simulation tool. The sophisticated commercial software packages for the process system simulation include Hysys, Chem CAD, Pro II, Aspen, and so on.17 The software package of Aspen Plus has been successfully applied to simulate some steady processes containing solids, electrolytes, coals, and biomass samples based on the sequential modular method and the equation-oriented method for sensitivity analysis, design optimization, and case study. Besides, it has also been successfully applied to simulate coal gasification and a combustion process with near zero emission,18 a process of coal gasification in an entrained-bed gasifier,19 a biomass gasification process in a fluidized bed,12,20 and a process of two-stage coal gasification.21 The above-mentioned applications have proven that Aspen Plus can make a process simulation with reasonable reliability and adaptability from the viewpoint of computable simulation. However, some limitations in the above-mentioned simulation examples based on Aspen Plus have been exposed and can be summarized as follows: the chemical reaction equilibrium method22 with a minimization of Gibbs free energy for the studied system has been applied to calculate the production yield during simulation of coal or biomass gasification or combustion without taking gas-solid hydrodynamics and chemical reaction kinetics into consideration. Obviously, the short enough residence time of biomass or coal cannot make gasification or combustion reactions reach real thermodynamic equilibrium Received: October 21, 2010 Revised: January 19, 2011 Published: March 08, 2011 1721

dx.doi.org/10.1021/ef101439s | Energy Fuels 2011, 25, 1721–1730

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Table 1. Chemical Compositions and Size Distributions of Three Applied Coals proximate analysisa (mass %)

ultimate analysis (mass %)

size distribution (mass %) size range (mm)

coal type volatile matter ash fixed carbon

a

C

H

N

S

Ob

0.60-0.45 0.45-0.355 0.355-0.28 0.28-0.20 0.20-0.16 0.16-0.154

coal A

36.2

5.5

58.3

72.4

4.6

1.3

0.5

15.7

40.6

9.4

16.6

13.8

coal B

30.7

18.4

50.9

65.8

4.2

1.2

1.6

8.8

32.1

9.8

18.7

9.9

coal C

26.4

13.1

60.5

73.0

3.7

1.2

0.6

8.4

49.7

13.5

16.5

11.2

4.7 15 4.4

14.9 14.5 4.7

On a dry basis. b By difference.

for these systems. Sotudeh-Gharebaagh et al.11 have wonderfully simulated the results of a 0.8 MW CFB coal combustion boiler by considering gas-solid hydrodynamics and combustion reaction kinetics using Aspen Plus. And their study has shown that it is very necessary to consider gas-solid hydrodynamics and chemical reaction kinetics when the process of coal combustion in a CFB combustor is simulated. In this part of our research, a process simulation model has been developed to simulate three kinds of coal combustion in a 30 kW CFB combustor based on Aspen Plus software with considering gas-solid hydrodynamics via Aspen Plus in-line FORTRAN codes and combustion reaction kinetics via some external FORTRAN subroutines, simultaneously. The related mathematical models in the process simulation model, such as the gas-solid hydrodynamics model in the CFB riser, equivalent pyrolysis model of coal, and combustion kinetics model of coal pyrolysis products, have been presented in this study; meanwhile, the profiles of voidage and gas pressure along the height of CFB riser are simulated to verify accuracy of the process model. The aim in the next step is to simulate the effect of operating conditions of the CFB combustor on the formation and emission of gas components, especially NO and N2O, during coal combustion in CFB combustor. The ultimate destination of this study is to develop a universal simulation method to predict pollution formation and emission of NO and N2O during coal combustion in the CFB combustor and, then, to provide valuable information of emission and control for NO and N2O in the newly developed coal decoupling combustion technology.23

2. OBJECTIVES AND PROCESS SIMULATION TOOL 2.1. Process of Coal Combustion in a CFB Combustor. In order to quantitatively simulate the formation and emission behaviors of NO and N2O during CFB coal combustion, a 30 kW CFB combustor was applied to carry out coal combustion experiments using three kinds of coals as coal A, coal B, and coal C reported elsewhere.8 It should be specially pointed out that coal A was mined from a coal mine in Shangwan area, Inner Mongolian Autonomous Region; coal B was from a coal mine in Datong region, Shanxi Province, while coal C was from a coal mine in Tiechanggou area, Xinjiang Uygur Autonomous Region. The chemical compositions and particle size distributions of three applied coals are summarized in Table 1.8 The detailed information of configuration and operation procedures for the 30 kW CFB combustor had been reported in previous publications elsewhere8,10 as shown in Figure 1. The 30 kW CFB combustor mainly consists of a riser with a height of 6.63 m and an inner diameter of 0.086 m and a downer with a height of 3.0 m and an inner diameter of 0.039 m. A cyclone was installed at the top of the downer to separate solid particles from flue gas, while a U-type valve was installed at the bottom of the downer. A specially designed

Figure 1. Schematic illustration of a 30 kW circulating fluidized bed combustor.

feeding system containing a screw feeder and a pneumatic feeder was installed at the position just below a particle storage hopper on the downer to charge silica sands. The silica sands with an average diameter as 0.255 mm are applied as the solid heat carrier. The pulverized coal powders were introduced from the specially designed feeding system into the CFB riser at a position of 0.020 m above the gas distributor. The primary air was preheated to 473 K and charged into the riser through the bottom of a gas distributor to ignite the charged coal powders easily. The secondary air at ambient temperature was tangentially inlet into the CFB riser through a port located at 1.66 or 2.8 m above the gas distributor. 2.2. Simulation Software. The commercial software of Aspen Plus has been applied in this study to simulate the formation and emission behaviors of NO and N2O during coal combustion in the 30 kW CFB combustor. It has been verified17 that the software package of Aspen Plus can be successfully applied to simulate a steady-state operation process for conceptual design, optimization, and performance monitoring. The built-in library of various modules for characterizing different chemical unit operations combined with a built-in database of complete physical property parameters, such as thermodynamic and physical parameters, can be invoked using a built-in system analysis strategy with the appropriate modular calculating order to simulate a process.

3. SIMULATION APPROACHES OF COAL COMBUSTION IN A CFB COMBUSTOR The coal combustion process in a CFB combustor can be separated into two major subprocesses: one is coal combustion and the other is gas-solid (unburned char particles and silica 1722

dx.doi.org/10.1021/ef101439s |Energy Fuels 2011, 25, 1721–1730

Energy & Fuels particles as the heat carrier) separation. As a key configuration of a CFB combustor, although the CFB riser is an independent configuration, it can be divided into several reaction subunits according to the characteristics of coal combustion reactions and gas-solid hydrodynamics along the height of CFB riser. These sequentially divided subunits in the CFB riser are closely dependent on each other by the exchanges of materials and heat. Therefore, simulating these sequentially divided subunits in the CFB riser can simplify the process simulation of coal combustion in the CFB riser with high precision. The coal combustion rate in the CFB riser is affected by gas-solid hydrodynamics as well as macrokinetics of all coal combustion related reactions. The gas-solid hydrodynamics affects distribution of solid particle, i.e., solid particle numbers in each divided subunits in the CFB riser; furthermore, it influences the contact area between char or coal particles and air. The macrokinetics of all coal combustion related reactions in the CFB riser covers their intrinsic kinetics combined with heat and mass transfer between char or coal particles and the gas phase. Certainly, the macrokinetics of coal combustion reactions in the CFB riser affects the mass consumption rate of coal particles and formation rate of pollutants during coal combustion in a CFB combustor. Therefore, it is of great importance to accurately describe the gas-solid hydrodynamics and macrokinetics of coal combustion reactions in subunits in the CFB riser when exactly simulating coal combustion in a CFB combustor using Aspen Plus. 3.1. Gas-Solid Hydrodynamics of Coal Combustion in CFB Riser. 3.1.1. Hypothesis of Gas-Solid Hydrodynamics. It is well-known that the difference of gas superficial velocity Ug caused by the inlet secondary air can divide the CFB riser into two regions: a dense region and dilute region. The particles of coal and silica sands are only fluidized by the sum of primary air and relaxation air from a U-type valve, where the region becomes turbulent because Ug is usually higher than the critical velocity Uc.11,24 In order to accurately describe the voidage distribution along the CFB riser height, the dense region is divided into two reaction regions: turbulent region 1 and turbulent region 2; while, the dilute region, whose solid particles are fluidized by the sum of primary air, relaxation air, and secondary air, is divided into three reaction regions: the lower acceleration region 3, upper acceleration region 4, and completely fluidized region 5. Assumptions of the developed gas-solid hydrodynamics model in the CFB riser during coal combustion are summarized as follows: (1) Coal combustion in a CFB combustor is considered as a steady state process, and each physical parameters Fi is a constant without changing over time t, i.e., ∂Fi/∂t = 0. (2) Coal or char particles are burned turbulently in the CFB riser; hence, all solid particles, such as coal or char particles, silica sand particles, and generated coal ash particles, have very good internal backmixing in each subunit of the CFB riser. The unburned char particles and silica sand particles are separated from the exhaust flue gas by a cyclone and reenter into the CFB riser bottom to combust repeatedly until completely burn out, so all solid particles have ideally external recirculation and mixing.25 Therefore, there are wonderful mixing characteristics of all solid particles in a CFB combustor, certainly in the divided five subunits in the CFB riser.25 (3) Continuous variation of voidage in the CFB riser is neglected and treated as five different constants for the divided five subunits in the CFB riser, respectively. (4) Solid particle size reduction by friction among particles and particles with the riser walls or with the downer wall is neglected.

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3.1.2. Gas-Solid Hydrodynamic Model. The volume fraction distribution of solid particles, i.e., voidage ε, in the CFB riser has an exponential decay relation with the CFB riser heights z as follows26,27 ε - εðzÞ ¼ e-az ð1Þ ε - ε-¥ The relationship among decay index a, average particle diameters, and gas superficial velocity Ug plotted by Kunii and Levenspiel26 based on data from different researchers shows that the product of a and Ug is a constant for each particle size as aUg ¼ C

ð2Þ

The average particle size of coal and silica sands as heat carrier de in this study is larger than 80 μm, the calculated Ug is in a range of 4-7 m 3 s-1, and the constant C in eq 2 is chosen as 4.0-5.0.26 Differing with dilute region, the average voidage in turbulent region 1 and turbulent region 2 in the dense region is considered as the same value,11 because flow characteristics of solid particles is the same as that in the turbulent fluidized bed. Thus, the average voidage in turbulent region 1 and turbulent region 2 in the dense region can be calculated by11 Z z2 1 ε1 ε2 ¼ εðzÞ dz z2 - z0 z0 ¼

1 z2 - z0

¼ ε -

Z

z2

½ε - ðε - ε-¥ Þe-az dz

z0

1 ðε-¥ - εÞðe-az2 - e-az0 Þ aðz2 - z0 Þ

ð3Þ

Similar to eq 3, the average voidage of acceleration region 3, acceleration region 4 and completely fluidized region 5 in the upper dilute region can also be calculated as follows ε i ¼ ε -

1 ðε-¥ - εÞðe-azi - e-azi - 1 Þ, aðzi - zi - 1 Þ i ¼ 3, 4, 5

ð4Þ

The voidage at the CFB riser top in the dilute phase under saturated condition ε in eqs 1, 3, and 4 can be determined as11,28 ε ¼ 1=ð1 þ ΦGs =Ug Fp Þ ð5Þ where the crushing index Φ can be calculated by11,28 Φ ¼ 1þ Umf ¼

5:6 þ 0:47Fr 0:41 , Fr

de 2 ðFp - Fg Þg 1650μ

,

de ¼

Fr ¼

Umf 2 , gde

1 xi i di

∑

ð6Þ

The pressure drop Δpi in the divided five subunits in the CFB riser can be calculated as Δpi ¼ hi ð1 - εi ÞðFp - Fg Þg,

i ¼ 1, 2, 3, 4, 5

ð7Þ

The calculated voidage profile along the CFB riser height in the 30 kW CFB combustor is illustrated in Figure 2. The voidage in turbulent region 1 is considered as the same as that in turbulent region 2, and the average voidage in acceleration region 3, acceleration region 4, and completely fluidized region 5 is 1723


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Table 2. Conversion Ratio of Key Components in Each Combination Reaction for Three Coals at 1023 K conversion ratio (%) combination reaction

key component coal A

coal B

coal C

C þ 2H2 f CH4

C(graphite)

1.984

2.378

2.199

C þ O2 f CO2

C(graphite)

0.677

0.416

0.257

C þ 0.5O2 f CO H2 þ 0.5O2 f H2O

C(graphite) H2

1.851 10.382

1.196 7.254

0.873 6.546

N2 þ 3H2 f 2NH3

H2

0.382

0.419

0.476

0.5N2 þ C þ 0.5H2 f HCN

H2

0.100

0.110

0.125

H2 þ S f H2S

H2

0.677

2.375

1.011

wH2 ¼ 0:157 - 0:868ðVM=100Þ þ 1:388ðVM=100Þ2 ð10bÞ wCO2 ¼ 0:135 - 0:900ðVM=100Þ þ 1:906ðVM=100Þ2

Figure 2. Schematic illustration of relationship between voidage and height in five different subunits along CFB riser height in a 30 kW CFB combustor.

assigned to be equal to that at an appropriate height hi (i = 3, 4, 5) in each region, respectively. 3.2. Equivalent Pyrolysis Model and Combustion Kinetics Model of Pyrolysis Products. The charged coal powders will be rapidly pyrolyzed in the CFB riser during coal combustion.3,29,30 Therefore, the combustion of coal powders in the CFB riser can be treated as two cascade processes as rapid pyrolysis and combustion of the generated pyrolysis products containing char. 3.2.1. Equivalent Pyrolysis Model. No molecular formula can be applied to describe chemical composition of a coal due to its structural complexity and composition diversity. It is recommended by Aspen Plus31 that coal can be considered as a mixture composed of a series of stable elementary substances, such as carbon, hydrogen, oxygen, nitrogen, sulfur, and ash-forming elements,31 i.e., C(graphite) þ H2 þ O2 þ N2 þ S(rhombic) þ ASH, during simulation of the formation and emission behaviors of NO and N2O in a coal combustion CFB combustor according to results of ultimate analysis and proximate analysis listed in Table 1. Therefore, the rapid pyrolysis process of three studied coals at the CFB riser bottom is treated as two cascade steps in this study: the first step is the equivalent decomposition of above-mentioned stable elementary substances and the second step is the formation of coal pyrolysis products by stable elementary substances. It is well-known that gaseous volatile matters and char are two major pyrolysis products of coal. The main components of gaseous volatile matter are CH4, H2, CO2, CO, H2O, and tar. The yield of volatile matters can be calculated as follows.32-34 Vyield ¼ VM - R - β

ð8Þ

R ¼ expð26:41 - 3:961 ln T þ 1:15VMÞ

ð9aÞ

β ¼ 0:2ðVM - 10:9Þ

ð9bÞ

The mass fraction wi of each component in volatile matters from coal rapid pyrolysis can be calculated by34,35 wCH4 ¼ 0:201 - 0:469ðVM=100Þ þ 0:241ðVM=100Þ2 ð10aÞ

ð10cÞ wCO ¼ 0:428 - 2:653ðVM=100Þ þ 4:845ðVM=100Þ2 ð10dÞ wH2 O ¼ 0:409 - 2:389ðVM=100Þ þ 4:554ðVM=100Þ2 ð10eÞ wTar ¼ - 0:325 þ 7:279ðVM=100Þ - 12:880ðVM=100Þ2 ð10f Þ The precursors of emitted NO and N2O, i.e., NH3 and HCN, are considered to be contained in the generated tar from pyrolysis. The formation reaction formulas, key stable elementary substances, and conversion ratios of three studied coals for forming CH4, CO2, CO, H2O, NH3, and HCN are calculated at 1023 K and summarized in Table 2. The calculation procedures of conversion ratio listed in Table 2 for three studied coals can be summarized as follows: (1) the yield of volatile matters containing pyrolysis gas and tar can be calculated by eqs 8 and 9; (2) the mass fraction of abovementioned six components, CH4, CO2, CO, H2O, NH3, and HCN, except NO and N2O, in pyrolysis gas products and tar containing NH3 and HCN, can be calculated from eq 10; (3) the generation ratios of NH3 and HCN in 1.0 g coal are chosen as 0.1% and 0.125%, respectively;23 (4) the sulfur element in coal is considered to enter into volatile matter and exist as H2S in pyrolysis products. Therefore, the conversion ratios of key stable elementary substances listed in Table 2 can be obtained to describe the result of the fast pyrolysis of three studied coals in Aspen Plus simulation under the fixed CFB operation conditions. It should be specially pointed out that the formed tar is treated as hydrocarbon compounds except for the known amount of NH3 and HCN, and all hydrocarbon compounds in tar are considered to be transformed to CO2 and H2O during combustion in the CFB riser. 3.2.2. Combustion Kinetics Model of Pyrolysis Products. To simplify the combustion kinetics model of pyrolysis products in the CFB riser, the basic assumptions proposed in this study are summarized as follows: (1) The high enough temperature at the CFB riser bottom can have the charged coal powders pyrolyzed rapidly and 1724


¼ ¼

dC - dtNH3

1725

- dndtC ¼ Nchar, i πde, char 2 k12 CNO

- dCdtNO ¼ f ðCNO , CCO Þ

-

dC - dtN2 O

NO þ CO f CO2 þ 12N2

N2 O þ C f CO þ N2

N2 O þ CO f CO2 þ N2

N2 O þ 12O2

(13)

(14)

(15)

(16)

f N2 þ O2

¼ k15 CN2 O CCO

dCN2 O dt

¼ εchar Fchar k16 CN2 O

¼ Nchar, i πde, char 2 k14 CN2 O

k11 CNO 1 þ k11 CNO

dnN2 O dt

NO þ C f CO þ 12N2

¼ nchar-N nchar-C

- dndtC

(12)

f

dn - ½Nfueldtf 1=2N2 O

¼ k10 CN2 CO2

(11)

1 2N2 O

f NOthermal

½Nfuel þ 14O2

(10)

0:5

f NO

þ 12O2

1 2N2

(9) dCNOthermal dt

f

- dCdtHCN ¼ k8 CHCN CO2 dn 1 - k9 CNO dnC - ½Nfueldt f NO ¼ nnchar-N dt 1 þ k C char-C 9 NO

½Nfuel þ 12O2

þ CO þ 12N2 O

(8)

1 2H2

1:5

HCN þ 34O2

(7)

(6)

k6 CNH3 CO2 0 CO2 þ k6 k7 CNH3 CO2 0 CO2 þ k7

¼ k5 CO2 CH2

dC - dtNH3

f H2 O

char NH3 þ 54O2 sf NO þ 32H2 O char NH3 þ 34O2 sf 12N2 þ 32H2 O

(5)

f CO þ 2H2 O

¼

H2 þ 12O2

(4) dC - dtH2

CH4 þ 32O2 0:8 k4 C0:7 CH4 CO2

¼ Nchar, i πde, char k3 CCO2

C þ CO2 f 2CO

(3) dC - dtCH4

dn - dtCO2 2

- dCdtCO ¼ k2 CCO 1 CO2 0:3 CH2 O 0:5

CO þ 12O2 f CO2

(1)

(2)

reaction rate - dndtC ¼ Nchar, i πde, char 2 k1 CO2

chemical reaction equation

C þ j1 O2 f 2 - j2 CO þ j2 - 1 CO2

reaction no.

ðkI CNO ðkII CCO þ kIII ÞÞ ðkI CNO þ kII CCO þ kIII Þ

k15 ¼ 5:01 1013 exp - 5292 T 10000 k16 ¼ de,43:5 0:74 exp T char

kI ¼ 0:1826, kII ¼ 0:00786, kIII ¼ 0:002531 k14 ¼ 2:9 109 exp - 16983 T

19000 k13 ¼ 1:952 1010 exp T

f ðCNO , CCO Þ ¼ k13

15, 32, 38

15, 32, 38

15, 32, 38

15, 32, 38

16, 38

16, 38

15

15

15

15

15

1, 14, 29

15, 32, 38

15

k12 ¼ 5:85 107 exp - 12000 T

16, 38

MC Shp jDg de, char Ru T

- 1:4947108 Ru T

14

ref

k2 ¼ 1:9 106 exp - 8056 T k3 ¼ 4:1 106 exp - 29787 T k4 ¼ 1:585 1010 exp - 24157 T k5 ¼ 1:63 109 T 1:5 exp - 3240 T 0 k6 ¼ 3:38 107 exp - 10000 , k6 ¼ 0:054 T 10000 0 7 k7 ¼ 3:38 10 exp - T , k7 ¼ 0:054 k8 ¼ 2:14 105 exp - 10000 T k9 ¼ 900 exp - 3551 T k10 ¼ 3 1014 exp - 65300 T k11 ¼ k9 ¼ 900 exp - 3551 T

k1, d ¼

k1, r ¼ 8710 exp

k1 ¼ 1 1 þ k1, r k1, d

Ru T MC

constant of reaction rate

Table 3. Chemical Reaction Equations, Expressions of Chemical Reaction Rate, and Corresponding Reaction Rate Constants for Describing Combustion of Pyrolysis Products in the CFB Riser

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the pyrolysis products will be immediately burned by air at the same time. (2) The surface temperature of char particles Tp is higher than gas temperature Tg, and Tp can be calculated by36 Tp ¼ Tg þ 66CO2

ð11Þ

(3) The residence time of char particles and silica sands in the cyclone is very short, and oxygen concentration in the downer is very low. Thus, combustion of char particles in a cyclone as well as in the CFB downer is negligible.11,37 (4) The decay of char particles’ size caused by friction among char particles and silica sands, and collision between solid particles and CFB rsier wall is neglected.11,37 Therefore, the reduction of char particle diameter during combustion process is also omitted. (5) The primary crack of coal particles and the secondary crack of char particles are neglected.11,37,38 (6) All sulfur elements in coal particles is considered to transfer to H2S at pyrolysis stage and then convert to SO2 completely at combustion stage of pyrolysis products. The chemical reaction equations, expressions of chemical reaction rate, and corresponding reaction rate constants for describing combustion of pyrolysis products in the CFB riser are summarized in Table 3. It can be observed from Table 3 that reactions 1, 2, 9, 11, 12, and 14 present gas-solid heterogeneous reactions. The numbers of char particles in the ith subunit Nchar,i, defined as the ratio of char particles volume Vchar,i to average volume of single char particle Ve,char, can be calculated by Nchar, i ¼

Vchar, i Vi ð1 - εi Þηi ¼ 1 Ve, char πd3 6 e, char

Figure 3. Constructed flow sheet of CFB coal combustion process based on Aspen Plus.

ð12aÞ

Ar ¼

The volume fraction of char particles in the ith subunit ηi can be calculated by qm, char, i =Ve, char Fchar, i ηi ¼ qm, char, i qm, SiO2 =Ve, char þ =Ve, SiO2 Fchar, i FSiO2

ð12bÞ

The Sherwood number Shp of reaction 1 in Table 3 for describing k1,d can be calculated as follows39 rffiffiffiffiffiffiffi ffiffiffiffi Rep p 3 Shp ¼ 2ε þ 0:69 ð13aÞ Sc ε where the Reynolds number Rep and the Schmidt number Sc are defined as follows Rep ¼

Arε

4:75

pffiffiffiffiffiffiffiffiffiffiffiffiffi, 18 þ 0:61 Arε4:75

Sc ¼

μg F g Dg

ð13bÞ

The Archimedes number Ar and diffusivity coefficient of O2 in nitrogen Dg in eq 13b can be calculated by36

gde 3 Fg ðFp - Fg Þ μg 2

Dg ¼ 3:13 10

-4

,

T 1500

1:73 ð13cÞ

The mechanism factor j for describing char combustion in the definition of k1,d in reaction 1 in Table 3 can be calculated by14

8 2ξ þ 2 > > dchar < 0:05 mm > > ξþ2 > > < ξ ðdchar - 0:05Þ j ¼ 2ξ þ 2 0:95 > 0:05 mm e dchar e 1:0 mm > > > ξþ2 > > : 1:0 dchar > 1:0 mm

ð14Þ where ξ is the concentration ratio of CO to CO2 generated during char combustion and can be calculated by14 ξ ¼ 2500 expð - 51900=Ru Tp Þ

ð15Þ

Therefore, the above-mentioned kinetics model of combustion reactions for pyrolysis products can be developed by combining eqs 11-15 with all intrinsic reaction rates listed in Table 3 to simulate the coal combustion in the 30 kW CFB combustor. 1726


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Table 4. Initial Values of Related Parameters in Each Module for Simulation of CFB Coal Combustion by Aspen Plus subprocess

applied module

decomposition

Ryield

input variables (1) temperature T = 298.15 K (2) pressure p = 101.325 kPa (3) component yield (see coal ultimate analysis in Table 1) (4) chemical composition of ash

pyrolysis

(1) temperature T = 298.15 K

Rstoic

(2) pressure p = 101.325 kPa (3) chemical stoichiometry (See Table 2) combustion

(4) conversion ratio of key component (See Table 2) (1) heat duty Q = 0 J

RCSTR

(2) pressure p = 101.325 kPa (3) available chemical reaction CSTRi (i = 1, 2, 3, 4, 5) heat exchange

(1) temperature Theater1 = 473 K, Theater2 = 1023 K

Heater

(2) pressure p = 101.325 kPa separation

Sep

4. CONSTRUCTION OF SIMULATION MODEL FOR CFB COAL COMBUSTION BASED ON ASPEN PLUS 4.1. Description of Coal Combustion in CFB Combustor Using Built-in Modules in Aspen Plus. It is necessary to divide

a simulated process into several subprocesses, in which the calculation methods have similar properties with the built-in modules in Aspen Plus for the purpose of accurately simulating the process using Aspen Plus. The flow sheet of the coal combustion process in the CFB combustor has been described by seven related built-in modules in the Aspen Plus as shown in Figure 3. It can be observed from Figure 3 that the fully steady coal combustion process in the CFB combustor can be presented by the following seven kinds of modules: (1) The pyrolysis process of charged coal powders at the CFB riser bottom is presented by the built-in module of reactor yield (RYield) and stoichiometry reactor (RStoic) with specified reaction contents or conversion ratios in Aspen Plus, simultaneously. The built-in RYield module presents the equivalent coal decomposition process into the stable elementary substances as described in section 3.2.1, and the built-in RStoic module presents the equivalent combination of pyrolysis products from the stable elementary substances. (2) The combustion process of pyrolysis products along the height of the CFB riser is presented by five modules of continuous stirred tank reactors (RCSTRi, i = 1, 2, 3, 4, 5), respectively. Turbulent regions 1 and 2 in the dense region are presented as RCSTR1 and RCSTR2; while acceleration regions 3 and 4 and completely fluidized region 5 in the dilute region are presented as RCSTR3, RCSTR4 and RCSTR5, respectively. (3) The cyclone separation of unburned char particles and silica sands from flue gas is presented by a module of a separator (Sep 1). For simplifying the simulation, it is assumed that 1% of the formed ash is discharged from the cyclone with flue gas. (4) The mixing between silica sands and unburned char particles from the flue gas is presented by a module of a hopper-type mixer (Hopper). (5) The heat exchange among the existing silica sands, unburned char particles, and the newly charged silica sands at ambient temperature in the CFB downer is represented by a module of a heater (Heater 2). (6) The ash discharge operation from the U-type value at the CFB downer bottom is presented by a module of a seperator (Sep 2). It is assumed that 50% of the formed ash, 20% of the inlet silica sands, and 0.5% of the unburned char particles will be discharged

split fraction of all components in one outlet

from U-type value at the downer bottom. Other solid particles will enter into the CFB rsier bottom and be further burned along the CFB riser height. (7) The heating process of primary air is presented by a module of a heater (Heater 1). The mixing of the heated primary air with solid substances from the CFB downer is presented by a module of a mixer (Mixer 1) in RCSTR 1. (8) The mixing of secondary air at ambient temperature and substances from RCSTR2 is presented by a mixer module (Mixer 2) in RCSTR 3. It should be specially pointed out that about 5 kg of silica sands with an average diameter as 0.255 mm was added into the CFB combustor through a particle storage hopper located at the upper position of the CFB downer while starting the 30 kW CFB combustor. This means that when the added silica sands are acting as a heat carrier, this is an intermittent, not a continuous, i. e., not a steady, process. Therefore, the operation of adding silica sands into the CFB combustor must be converted into a steady feeding process when correctly applying Aspen Plus. Because the circulating rate of solid substances can be easily controlled by adjusting the opening of the butterfly valve located under the particle storage hopper at the CFB downer,8 a circulating rate of solid substances of 250 kg 3 h-1 is applied in the simulation by keeping the circulating rate of solid substances at 250 kg 3 h-1 as that controlled in most experiment runs. 4.2. Choosing Physical Properties of Related Substances and Initial Conditions for Chosen Built-in Modules in Aspen Plus. The physical properties of the related gas substances in 15 built-in modules are calculated by the built-in IDEAL method in Aspen Plus at high temperature under atmospheric pressure,31 and physical properties of the related solid substances are determined by the built-in method in Aspen Plus for conventional substances at high temperature. Thus, the decided physical properties of gas and solid substances can be applied to calculate the parameters of combustion reactions such as enthalpies and densities by Aspen Plus. The major initial conditions of six applied modules in this study are summarized in Table 4. 4.3. Description of Gas-Solid Hydrodynamics in CFB Riser Using Module CALCULATOR with In-line FORTRAN. To describe the influence of gas superficial velocity Ug on the voidage profile along the CFB riser height ε(z) using Aspen Plus, the gas superficial velocity Ug must be recalculated at each iteration to get a new value as initial value of Ug at the next iteration. The module CALCULATOR with in-line FORTRAN 1727


Energy & Fuels code is chosen to implement this function. The information streams illustrated in Figure 3 show that the calculated Ug at one iteration for describing ε(z) in five modules of RCSTRi (i = 1, 2, 3, 4, 5) will be chosen as the initial value of Ug at next iteration for the purpose of reducing the iteration number in this study. 4.4. Description of Combustion Reaction Kinetics for Pyrolysis Products Using External FORTRAN Subroutine. To accurately describe the combustion process of pyrolysis products along the CFB riser height, it is necessary to give expressions of reaction rate for all indispensable reactions in the divided five subunits presented by five cascade modules of RCSTRi (i = 1, 2, 3, 4, 5). The applied expressions of reaction rates for all indispensable reactions in five cascade modules of RCSTRi (i = 1, 2, 3, 4, 5) are summarized in Table 3. However, it is difficult to directly apply the standard data interface in Aspen Plus to describe reaction rates for all indispensable reactions in five cascade modules of RCSTRi (i = 1, 2, 3, 4, 5) listed in Table 3 and make use of the gas-solid hydrodynamics model in section 3 as their complexity. An external FORTRAN subroutine code has been developed to link the applied five cascade modules of RCSTRi (i = 1, 2, 3, 4, 5) in Aspen Plus for simulating CFB coal combustion in this study, respectively. This means five external FORTRAN subroutine codes have been developed for the five cascade modules of RCSTRi (i = 1, 2, 3, 4, 5) to exchange data information with the main simulation program of Aspen Plus.

5. SIMULATION RESULTS AND DISCUSSION To verify the developed process simulation model, the profiles of voidage and gas pressure along the CFB riser height at various gas superficial velocities are preliminarily calculated under

Figure 4. Calculated voidage profiles along CFB riser height in a superficial velocity range from 4.69 to 6.58 m 3 s-1 in a 30 kW CFB combustor for coal A at 1153 K.

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specified CFB operation conditions in this study. The converged simulation solutions can be obtained after 30 to 80 iterations in about 2 min using a personal computer with a 1.25 G RAM and a 3.0 GHz CPU. 5.1. Prediction of Voidage Profile along CFB Riser Height. Simulation is a more effective method to determine the voidage profile along the CFB riser height at coal combustion temperature compared with the direct experimental measurement. The simulated voidage profile under condition of changing gas superficial velocity Ug in a range of 4.69-6.58 m 3 s-1 at 1153 K for coal A as an example is shown in Figure 4. The average voidage in the dense region, i.e., at position corresponding to module RCSTR1 and module RCSTR2, is calculated from eq 3, while the average voidages in the dilute region, i.e., at position corresponding to modules of RCSTR3, RCSTR4, and RCSTR5, are calculated from eq 4. It can be observed from Figure 4 that there is a sharp increase of voidage at the position of inletting the secondary air, i.e., interface between outlet of module RCSTR2 and inlet of module RCSTR3 as the charging port of the secondary air. Improving Ug from 4.69 to 6.58 m 3 s-1 can lead to a decrease of voidage at position for modules of RCSTR1, RCSTR2, and RCSTR3 at a fixed circulating rate of solid particles, simultaneously; however, a slight increase of voidage can be observed at position for modules of RCSTR4 and RCSTR5 with an increase of Ug from 4.69 to 6.58 m 3 s-1. It can be explained that further combustion of char particles at the upper dilute region of the CFB riser can result in reducing amount of char particles in the circulating solid particles, and finally reducing the circulating rate of solid particles. Taking module RCSTR5 as an example, the circulating rate of solid particles can reduce from 14.93 to 13.25 kg 3 m-2 3 s-1 with an increase of Ug from 4.69 to 6.58 m 3 s-1 at 1153 K. Undoubtedly, increasing temperature along the CFB riser height can also generate a small contribution to the increase of voidage with increasing Ug as the gas volume expansion. Increasing Ug from 4.69 to 6.58 m 3 s-1 at 1153 K can make the voidage profile along the CFB riser height to some degree flat because greater Ug can lead to a smaller decay index a as shown in eq 2. 5.2. Prediction of Pressure Profile along CFB Riser Height. The effects of gas superficial velocity Ug on the calculated total pressure drop and the decay index a are illustrated in Figure 5a in a Ug range from 4.69 to 6.58 m 3 s-1 in the 30 kW coal combustion CFB combustor for coal A at 1153 K, respectively. Although the voidage has a complex relation with variation of Ug shown in Figure 4, i.e., the greater Ug can lead to smaller voidage at a position below module RCSTR3 and greater voidage at

Figure 5. Effect of gas superficial velocity on calculated pressure drop along CFB riser height as well as decay index and calculated pressure profiles along CFB riser height in a superficial velocity range of 4.69-6.58 m 3 s-1 in a 30 kW CFB combustor for coal A at 1153 K, respectively. 1728


Energy & Fuels position above module RCSTR4, increasing Ug from 4.69 to 6.58 m 3 s-1 can effectively decrease the total voidage of the CFB riser because increasing Ug can largely increase the total pressure drop of the CFB riser. Certainly, the decay index a shows an obvious reducing tendency with an increase of Ug from eq 2. The calculated pressure profile along the CFB riser during coal A combustion at 1153 K is illustrated in Figure 5b, where the outlet pressure of the CFB riser is nearly equal to the ambient pressure. As obtained from in the practical experiments, a larger inlet pressure is required when more primary air is introduced into the CFB riser bottom.

6. CONCLUSIONS A process simulation model has been developed to simulate coal combustion in a 30 kW CFB combustor based on Aspen Plus software coupled with the gas-solid hydrodynamics via Aspen Plus in-line FORTRAN codes and the combustion reaction kinetics via some external FORTRAN subroutines simultaneously. The process simulation development of coal combustion in CFB combustor, as the main results of this study, is reported in detail. The preliminary calculation results of the profiles of voidage and gas pressure along the CFB riser height are illustrated as a verification of the developed model. The calculated results of the process simulation model show that improving Ug from 4.69 to 6.58 m 3 s-1 can lead to a decrease of voidage at positions for modules of RCSTR1, RCSTR2, and RCSTR3 at a fixed circulating rate of solid particles, simultaneously; however, a slight increase of voidage can be observed at positions for modules of RCSTR4 and RCSTR5 with an increase of Ug from 4.69 to 6.58 m 3 s-1. Increasing Ug from 4.69 to 6.58 m 3 s-1 can effectively increase the total pressure drop of the CFB riser, Therefore, a larger inlet pressure is required when more primary air is introduced into the CFB riser bottom. ’ AUTHOR INFORMATION Corresponding Author

Mailing address: State Key Lab of Multiphase Complex System, Institute of Process Engineering, Chinese Academy of Sciences, P.O. Box 353, No. 1 Bei-er-tiao Street, Haidian District, Beijing, 100190, P. R. China. Tel: 86-10-82622893. Fax: 86-10-82622893. E-mail: [email protected] or [email protected].

’ ACKNOWLEDGMENT The finical support of this work by the Natural Sciences Foundation of China (Project No.50576101) is kindly appreciated. ’ NOMENCLATURE Roman Letter

a = decay index of cluster in free board, m-1 Ar = Archimedes number, C = constant 4-5, s-1 Ci = concentration of component i in gas, mol 3 m-3 dchar = char particle diameter, m de = equivalent diameter, m de,char = equivalent or average diameter of char particles, m di = particle dimension interval, m Dg = diffusivity coefficient of oxygen in nitrogen, m2 3 s-1 Fi = relevant physical quantity Fr = Froude number, -

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g = gravitational acceleration, m 3 s-2 Gs = mass flux of solid particles, kg 3 s-1 3 m-2 hi = height of the ith reaction subunit in CFB riser, m ki = constant of reaction rate for reaction i without uniform unit Mc = relative atom weight of carbon element, Nchar,i = number of char particles in the ith reaction subunit in CFB riser p = pressure, kPa Δpi = pressure drop in the ith reaction subunit in CFB riser, Pa qm,char,i = mass flow of char to the ith reaction subunit in CFB riser, kg 3 s-1 qm,SiO2 = mass flow of silica sands to reaction subunits in CFB riser, kg 3 s-1 Q = heat duty, J Ru = universal gas constant, 8.314 J 3 mol-1 3 K-1 Rep = Reynolds number of particles, Sc = Schmidt number, Shp = Sherwood number of particles, t = time, s T = absolute temperature, K Tp = absolute temperature of particles, K Tg = absolute temperature of gas, K Ug = gas superficial velocity, m 3 s-1 Umf = critical fluidization velocity, m 3 s1 Vi = volume of ith reaction subunit in CFB riser, m3 Vchar,i = volume of char particles in ith subunit in CFB riser, m3 Ve,char = equivalent volume of single char particles in CFB riser, m3 Vyield = yield of volatile matter during pyrolysis of coal, kg 3 (100 kg coal)-1 Ve,SiO2 = average volume of single silica particle, m VM = content of volatile matter in coal, kg 3 (100 kg coal)-1 wi = mass fraction of component i in pyrolysis gas, xi = weight fraction of particles at di interval, yi = volume fraction of component i in gas, vol % or ppmv z = height of CFB riser, m Greek Letter

R = coefficient for describing yield of volatile matter from coal, kg 3 (100 kg coal)-1 β = coefficient for describing yield of volatile matter from coal, kg 3 (100 kg coal)-1 ε = voidage, ε(z) = voidage at height of z in CFB riser, ε = voidage under saturated conditions, ε-¥ = voidage equivalent to the value at the height -¥ in the CFB riser, hεi = average voidage in ith reaction subunit, ηi = volume fraction of char particles in ith reaction subunit, λ = excess air number, μg = viscosity of gas, Pa 3 s ξ = ratio of CO concentration to CO2 concentration formed during char combustion, Fp = density of solid particles, kg 3 m-3 Fchar,i = density of char particles in the ith reaction subunit, kg 3 m-3 FSiO2 = density of silica sands in reaction subunits, kg 3 m-3 Fg = density of gas, kg 3 m-3 j = mechanism factor of char combustion, Φ = crushing index, Subscripts i = ith reaction subunit or compobnent i, 1729


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