Process Simulation of Using Coal Pyrolysis Gas to Control NO and

Article pubs.acs.org/EF

Process Simulation of Using Coal Pyrolysis Gas to Control NO and N2O Emissions during Coal Decoupling Combustion in a Circulating Fluidized Bed Combustor Based on Aspen Plus Xuemin Yang,*,† Bing Liu,†,

‡

and Weigang Lin†

†

State Key Laboratory of Multiphase Complex Systems, 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 of CFB coal decoupling combustion process has been developed based on Aspen Plus and verified by comparing with the reported NO and N2O emissions from experiments. The detailed information on formation and decomposition of NO and N2O along the height of CFB riser column can be quantitatively simulated by the developed process simulation model. The simulated results show that about 99.9% of the emitted NO in flue gas is controlled by fuel−N combustion; however, the introduced NH3 in coal pyrolysis gaseous products as an NO precursor can play an important role on NO formation from about 39.7% to 97.6% in the dense phase region at the lower CFB riser column. About 98.9%−99.8% of NO decomposition along the height of CFB riser column is dominated by char particles reduction. About 91.8%−95.8% of the emitted N2O in flue gas is controlled by fuel−N combustion; however, the introduced HCN in coal pyrolysis gaseous products as an N2O precursor has an obvious effect on N2O formation from 74.2% to 94.6% in the dense phase region at the lower CFB riser column. The contribution of char particles reduction on N2O decomposition can be found as 1.8%−3.6% along the height of CFB riser column. A competitive relationship between O2 oxidization and CO reduction on N2O decomposition along the height of CFB riser column during the CFB coal decoupling combustion process has been revealed. and coal types, as reported elsewhere.6,7,15,16 The basic configuration and operation parameters of the traditional 30 kW CFB combustor have been described in previous publications.6,7,15,16 The CFB coal decoupling combustion experiments were carried out in a modified 30 kW CFB combustor, as shown in Figure 1.7,17 The essentials of the CFB coal decoupling combustion process can be summarized as follows: (1) coal combustion is compulsorily separated into two subprocesses as pyrolysis and char combustion; (2) the pulverized coal powders are pyrolyzed in the CFB downer column/reactor, while char is combusted in the CFB riser column/reactor; (3) the pyrolysis gaseous products are introduced into the CFB riser column at a reasonable height of CFB riser column to decrease the formed NO and N2O from char combustion. The objective of the developed CFB coal decoupling combustion process is to decrease NO and N2O emissions as a new de−NOx technology or a new CFB combustion process. Obviously, two methods can be used to introduce the pyrolysis gaseous products into the CFB riser column7,17 as shown in Figure 1: one is introducing the pyrolysis gaseous products with the pyrolyzed solid products into the CFB riser column7,17−19 from the U-type valve installed at the bottom of the CFB downer column; the other is introducing the pyrolysis gaseous products obtained by a gas−solid separator into the CFB riser column by a specially designed connection tube at a reasonable height of CFB riser column, while the separated pyrolysis solid products are introduced into the bottom of the CFB riser column through the U-type valve.

1. INTRODUCTION Tremendous focus has been attracted to the circulating fluidized bed (CFB) combustion process with its wide applications since the 1980s because of high combustion efficiency and low emissions of gaseous pollutes. Compared with other combustion technologies, such as grate boiler, pulverized coal fired boiler, and cyclone boiler, the CFB combustor has many advantages from its industrial applications1−3 such as easy operation, lower operation temperature, wide adaptability for fuels, large load adjustment range from 30% to 100% of full load, and discharging of high activity solid ash as raw material for high quality cements. However, higher N2O emission from industrial CFB coal combustion boilers is a remarkable defect although CFB combustor emits lower NO than other coal combustion boilers. To deplete this disadvantage, some traditional de−NOx technologies4−10 had been applied; meanwhile, some newer technologies such as reverse air stage combustion11−14 and coal decoupling combustion15 have been developed to decrease N2O emission and control NO emission during CFB coal combustion process. As a new CFB combustion process, the CFB coal decoupling combustion process has been developed at the Institute of Process Engineering, Chinese Academy of Sciences (IPE, CAS) for decreasing NO emission, especially decreasing N2O emission during coal CFB combustion process. The emission behavior of NO and N2O during the traditional CFB coal combustion process, as well as the newly developed CFB coal decoupling combustion process, have been experimentally investigated in a 30 kW CFB combustor with changing the excess air percentage, the first stage stoichiometry, introduction position of secondary air, inletting position of coal powders, © 2012 American Chemical Society

Received: May 7, 2012 Revised: July 9, 2012 Published: July 10, 2012 5210

dx.doi.org/10.1021/ef300777x | Energy Fuels 2012, 26, 5210−5225

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simulate the emission behavior of NO and N2O under the condition of introducing pyrolysis gaseous products into the CFB riser column through a connection tube between the CFB downer column and CFB riser column based on the Aspen Plus software. The simulated results of formation and emissions of NO and N2O have been compared with the reported15 results during a CFB coal decoupling combustion process in a 30 kW CFB combustor. The quantitative information of formation and decomposition for NO and N2O along the height of CFB riser column have been revealed from the developed process simulation model because the information is difficult to be measured by experiments. The ultimate aim of this study is to provide the optimal configuration and operation parameters for the newly developed CFB coal decoupling combustion process, as well as the emission mechanisms of NO and N2O in the CFB coal decoupling combustor.

2. DESCRIPTION OF THE SIMULATED CFB COAL DECOUPLING COMBUSTION PROCESS As described in Section 1, two methods can be applied to implement the newly developed CFB coal decoupling combustion process. The simulated CFB coal decoupling combustion process in this study is the second alternative, that is, introducing the pyrolysis gaseous products into the CFB riser column at a reasonable height through a specially designed connection tube between the CFB downer column and CFB riser column, as shown in Figure 1.7,17 The specially designed connection tube must be heated at about 773 K to avoid block from the condensed phases such as char and tar from the gas−solid separator installed in the CFB downer column. The operation process of the simulated CFB coal decoupling combustion process (i.e., the second method of the CFB coal decoupling combustion process) can be described as follows: (1) The silica sands with an average diameter of 0.255 mm were charged into a particles storage hopper in the CFB downer column as solid heat carrier. (2) The pulverized coal particles were charged in the CFB downer column using nitrogen gas as carrying gas from a port at the upper zone of a solid−solid mixer installed beneath the solid particles storage hopper. (3) The pyrolysis products including pyrolysis gas, char particles, and tar entered into a gas−solid separator in the CFB downer column and were separated. The separated pyrolysis gaseous products were sucked into the CFB riser column at a reasonable height above a gas distributor by a specially designed connection tube according to pressure difference; the separated solid particles including char particles, tar, and silica sands flowed into the bottom of the CFB riser column via U-type valve installed at the bottom of the CFB downer column, and char particles were combusted by the introduced primary air as well as secondary air along the CFB riser column. (4) The unburned char, gases, and silica sands at the top of the CFB riser column were separated by a cyclone at the top of the CFB downer column. The separated gas was emitted to a vent as flue gas; the separated solid particles containing unburned char particles and silica sands were stored in a solid particles storage hopper. (5) The solid particles flowed down into a solid−solid mixer at reasonable flow rate by controlling the opening of a butterfly valve installed in the CFB downer column between the solid particles storage hopper and the solid−solid mixer. The primary air was electrically preheated at 473 K in order to easily ignite fuel particles. The heated primary air was introduced into the CFB riser column through a gas distribution chamber at the bottom of the CFB riser column. The secondary

Figure 1. Schematic illustration of a 30 kW CFB coal decoupling combustion process.

It had been pointed out20−22 that the process simulation is a low cost tool to realize the process optimization. Therefore, the process simulation on formation and decomposition of NO and N2O along the height of CFB riser column can provide the valuable information not only for a traditional CFB combustion process but also for the newly developed CFB coal decoupling combustion process. As a sophisticated software package of process simulation, the Aspen Plus software has been successfully used to simulate the steady state processes containing solids, electrolytes, coal, and biomass based on the sequential modular method, as well as the equation-oriented method for sensitivity analysis, design optimization, and case study,10,23−27 after Aspen Plus has been developed since the 1980s because the Aspen Plus software can include the chemical thermodynamics, chemical reaction kinetics, transfer phenomena, fluid dynamics or hydrodynamics, and so on. A process simulation model of the traditional CFB coal combustion in a 30 kW CFB combustor for three coals had been developed18 in Aspen Plus environment23,25,28 based on a coal traditional combustion model,28 gas−solid hydrodynamics,28 coal combustion kinetics,11,29 as well as macrokinetics of char−N transformation to NO and N2O.30 The comparison31 between the simulated and reported6,7,16,17 results of formation and emissions for NO and N2O during the traditional CFB coal combustion process in a 30 kW CFB combustor shows that the developed process simulation model18,19,31 can be successfully used to predict the coal combustion behavior, especially the emissions of NO and N2O during the traditional CFB coal combustion process. To provide valuable information for the newly developed CFB coal decoupling combustion process, a process simulation model for CFB coal decoupling combustion in a 30 kW CFB combustor had been developed in previous publication19 to mainly simulate the emission behavior of NO and N2O under the condition of introducing pyrolysis gaseous products into the CFB riser column through a U-type valve and verified by comparison the simulated and the reported15 results. To further provide the useful information for the newly developed CFB coal decoupling combustion process, a process simulation model of the CFB coal decoupling combustion in a 30 kW CFB combustor has been developed in this study to 5211


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air at ambient temperature was tangentially introduced into the CFB riser column through one of three ports located at 1.66 m (H1), 2.80 m (H2), and 4.00 m (H3) above the gas distributor, respectively.

3. MATHEMATICAL MODELING OF THE SIMULATED CFB COAL DECOUPLING COMBUSTION PROCESS It has been described in Section 1 and Section 2 that pyrolysis of the pulverized coal particles in the CFB downer column, high efficiency separation of the pyrolysis gaseous products and solid materials including char in the CFB downer column, introduction position of the separated pyrolysis gaseous products from the CFB downer column into the CFB riser column, and combustion of the generated char in the CFB riser column are very important to realize the simulated or the second method of CFB coal decoupling combustion process. The most important configuration is the CFB riser column, where the combustion rate of fuel particles is affected by the gas−solid hydrodynamics and multiphase macrocombustion reaction kinetics. The gas−solid hydrodynamics affects the distribution of solid particles in the CFB riser column, and furthermore influences the contact area between solid particles and gas phase. The multiphase macrocombustion reaction kinetics in the CFB riser column includes the intrinsic kinetics of the related reactions combined with heat and mass transfer between solid particles and gas phase. Hence, the multiphase macrocombustion reaction kinetics will affect the consumption rate of fuel particles and formation rate of gaseous pollutants. Therefore, it is of great importance to accurately describe the gas−solid hydrodynamics and multiphase macrocombustion reaction kinetics in the CFB riser column for the exact simulation of CFB coal decoupling combustion process with introducing the pyrolysis gaseous products to control NO and N2O emissions. 3.1. Modeling of Gas−Solid Hydrodynamics in CFB Riser Column. To simplify the description of gas−solid hydrodynamics modeling in a CFB riser column, some assumptions have been adopted in this study and can be summarized as follows: (1) The process of CFB coal decoupling combustion is considered as a steady state process. (2) The coal particles are burned turbulently and have a very good internal backmixing at each location in a CFB riser column. (3) The unburned coal particles separated from flue gas by a cyclone are well mixed with the newly charged coal particles and will return to the bottom of the CFB riser column for further combustion. Thus, all solid particles can be well circulated and mixed by an ideal external recirculation. (4) The size variation of solid particles by friction in the CFB reactor is neglected. As the most important reactor, the CFB riser column can be divided into several reaction subunits along the height of CFB riser column according to the characteristics of gas−solid hydrodynamics. These sequentially divided subunits along the height of CFB riser column are closely dependent on each other through exchanges of materials and heat streams. The relationship between the voidage εi and the height z of CFB riser column in the divided five subunits along the height of CFB riser column during CFB coal combustion process in a 30 kW CFB combustor is schematically illustrated in Figure 2. The difference of gas superficial velocity Ug caused by the introduced secondary air can divide the CFB riser column into two regions as the dense phase region and the dilute phase region. In order to describe the voidage distribution along the height of CFB riser column, the dense phase region can be further divided into two reaction regions as turbulent region 1 and turbulent region 2. The solid particles in the dense phase

Figure 2. Schematic illustration of relationship between voidage and height of CFB riser column in five different subunits during a 30 kW CFB coal decoupling combustion process.

region are only fluidized by the sum of primary air and relaxation air from a U-type valve installed at the bottom of the CFB downer column. Therefore, the gas phases and solid particles become turbulent and mixed perfectly because the gas superficial velocity Ug is usually greater than the critical velocity Uc.23 Thus, a constant mean voidage is assumed in the dense phase region as ε1̅ ≈ ε2̅ = ε(z 2)

(1)

The solid particles in the dilute phase region are fluidized by the sum of primary air, relaxation air, and secondary air. Thus, the dilute phase region can be divided into three reaction regions as the lower acceleration region 3, upper acceleration region 4, and completely fluidized region 5. The distribution of solid particles in the above-mentioned three subregions of the dilute phase regions has an exponential decay relationship with the height z of CFB riser column as23 ε* − ε(z) = e−az ε* − ε−∞

(2)

Therefore, the average voidage in the lower acceleration region 3, upper acceleration region 4, and completely fluidized region 5 in the upper dilute phase region can be also calculated as follows zi zi 1 εi̅ = ∫ ε(z)dz = [ε* − (ε* − ε−∞)e−az]dz ∫ zi − 1 zi − zi − 1 zi − 1 1 = ε* − (ε−∞ − ε*)(e−azi − e−azi − 1) a(zi − zi − 1) i = 3, 4, and 5

(3)

The relationship among the decay index a, average particle diameters de, and gas superficial velocity Ug plotted by Kunii and Levenspiel32 based on data from different researchers shows that the product of decay index a and gas superficial velocity Ug is a constant (i.e., aUg = C).23,32 The average particle size de in this study is larger than 8 × 10−5 m; the calculated Ug is in a range 4−7 m·s−1. The constant C is taken as 4.0−5.0 s−1.32 The symbol ε* represents the voidage at top of the CFB riser column in the dilute phase region under the saturated condition and can be calculated by23 1 ε* = ΦG 1 + U ρs g p

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(4)


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Table 1. Chemical Composition and Partial Size Distribution of Applied Coal Particles partial size distribution (mass %) proximate analysisa (mass %)

ultimate analysis (mass %)

size range (mm)

volatile matter

ash

fixed carbon

C

H

N

S

O

0.60−0.45

0.45−0.355

0.355−0.28

0.28−0.20

0.20−0.16

0.16−0.154

26.4

13.1

60.5

73.0

3.7

1.2

0.6

8.4

49.7

13.5

16.5

11.2

4.4

4.7

a

b

Based on dry basis. bBy difference.

where the crushing index Φ is related with the Froud number, Fr, as32,33 5.6 Φ=1+ + 0.47Fr 0.41 , Fr Umf =

de2(ρp − ρg )g 1650μ

wH2O = 0.409 − 2.389(VM/100) + 4.554(VM/100)2 (8e)

U2 Fr = mf , gde

wtar = −0.325 + 7.279(VM/100) − 12.880(VM/100)2 (8f)

,

de =

1

∑ dx

The precursors of the emitted NO and N2O (i.e., NH3 and HCN) are considered to be contained in the generated tar from coal pyrolysis process. The synthesis reaction formulas of pyrolysis gaseous products, key stable elementary substances, and conversion ratios of the investigated coal for forming CH4, CO2, CO, H2O, NH3, HCN, and H2S have been calculated at 1023 K and are summarized in Table 2.

i

i

i

(5)

3.2. Modeling of Equivalent Pyrolysis Process of Coal. It is recommended by the Aspen Plus software20−22 that coal can be treated as a mixture composed of a series of stable elementary substances as carbon, hydrogen, oxygen, nitrogen, sulfur, and ash-forming elements because no molecular formula can be applied to describe the coal chemical composition due to its structural complexity and composition diversity.22 The chemical composition and particle size distribution of the applied coal particles are listed in Table 1. Therefore, the ultimate and proximate analyses of the applied coal have been applied to describe the coal pyrolysis process using the equivalent pyrolysis model.22 During the equivalent pyrolysis modeling, the rapid pyrolysis process of the studied coal in the CFB downer column can be treated as two cascade steps in this study: the first step is the equivalent DECOMPOSITION of coal to the above-mentioned stable elementary substances; the second step is the SYNTHESIS of coal pyrolysis products from the above-mentioned stable elementary substances according to the principle of mass conservation. It is well-known that the gaseous volatile matters and char are two major pyrolysis products of coal. The main components of gaseous volatile matters are CH4, H2, CO2, CO, H2O, and tar. The yield of gaseous volatile matters for the studied coal can be calculated from the correlation developed from the experimental data of Loison and Chauvin by Rajan and Wen29 as Vyield = VM − α − β

Table 2. Conversion Ratios of Key Components in Each Synthesis Reaction of Pyrolysis Gaseous Products from Stable Elementary Substances of the Applied Coal at 1023 K

(7a)

β = 0.2(VM − 10.9)

(7b)

key component

conversion ratio (%)

C(graphite) C(graphite) C(graphite) H2 H2 H2 H2

2.199 0.257 0.873 6.546 0.476 0.125 1.011

3.3. Modeling of Multiphase Macrocombustion Reaction Kinetics of Pyrolysis Products. To simplify the multiphase macrocombustion reaction kinetics model of pyrolysis products in the CFB riser column, the basic assumptions proposed in this study are summarized as follows: (1) the surface temperature of char particles Tp is higher than gas temperature Tg and can be calculated by TP = Tg + 6.6 × 104 Co2;34 (2) the combustion of char particles in the cyclone as well as in the CFB downer column is negligible because the residence time of char particles and silica sands as heat carrier in the cyclone is very short and oxygen concentration in the CFB downer column is very low;23 (3) the decrease of char particle diameter by friction during combustion process in the CFB reactor is also omitted; (4) the primary crack of coal particles and the secondary crack of char particles are neglected23 because the average diameter of the pulverized coal particles is very small. The chemical reaction formulas, expressions of chemical reaction rates, and corresponding reaction rate constants for describing the combustion of pyrolysis products in the CFB riser column are summarized in Table 3.18 The number of char particles Nchar, i in the ith subunit defined as the ratio of char particles volume Vchar,i in ith subunit in CFB riser column to the average volume of single char particle Ve,char can be calculated by

(6)

α = exp(26.41 − 3.961 ln T + 1.15VM)

combination reaction C + 2H2 → CH4 C + O2 → CO2 C + 0.5O2 → CO H2 + 0.5O2 → H2O N2 + 3H2 → 2NH3 0.5N2 + C + 0.5H2 → HCN H2 + S → H2S

The mass fraction wi of each component in gaseous volatile matters from coal rapid pyrolysis can be calculated by29 wCH4 = 0.201 − 0.469(VM/100) + 0.241(VM/100)2 (8a)

wH2 = 0.157 − 0.868(VM/100) + 1.388(VM/100)2 (8b)

wCO2 = 0.135 − 0.900(VM/100) + 1.906(VM/100)2 (8c) 2

Nchar, i =

wCO = 0.428 − 2.653(VM/100) + 4.845(VM/100)

(8d) 5213

Vchar, i Ve,char

=

Vi (1 − εi̅ )ηi 1 3 πde,char 6

(9a)


5214

R(14)

R(13)

R(12)

1 1 O2 → N2O 4 2

1 N2 2

N2O + C → CO2 + N2

NO + CO → CO2 +

1 NO + C → CO + N2 2

[N]fuel +

R(11)

1 O2 → NO 2

3 1 1 O2 → H 2 + CO + N2O 4 2 2

1 1 N2 + O2 → NOthermal 2 2

[N]fuel +

HCN +

R(10)

R(9)

R(8)

char 1 3 3 O2 ⎯⎯⎯→ N2 + H 2O 4 2 2

NH3 +

R(7)

1 O2 → H 2O 2

3 O2 → CO + 2H 2O 2

char 5 3 NH3 + O2 ⎯⎯⎯→ NO + H 2O 4 2

H2 +

CH4 +

C + CO2 → 2CO2

1 O2 → CO2 2

⎛ ⎛2 ⎞ 1 2⎞ O2 → ⎜2 − ⎟CO + ⎜ − 1⎟CO2 φ φ⎠ ⎠ ⎝ ⎝φ

CO +

C+

chemical reaction formula

R(6)

R(5)

R(4)

R(3)

R(2)

R(1)

reaction no.

dt

dC NH3

dt

0.7 = k4CCH C 0.8 4 O2

2 = Nchar, iπde,char k 3CCO2

=

=

CO2 + k′7

k 7C NH3CO2

CO2 + k′6

k6C NH3CO2

= k5CO2C H1.52

dC NH3

dt

dC H2

dt

dCCH4

dt

dnCO2

dCCO 1 = k 2CCO CO0.32 C H0.52O dt

dnC 2 = Nchar, iπde,char k1CO2 dt

dt

=

nchar ‐ N ⎛ dnC ⎞⎛ k11C NO ⎞ ⎜− ⎟⎜ ⎟ nchar ‐ C ⎝ dt ⎠⎝ 1 + k11C NO ⎠

= k10C N2CO0.52

dn[N]fuel → 1/2N2O

dt

dn NOthermal

dn[N]fuel → NO nchar − N ⎛ dnC ⎞⎛ 1 − k 9C NO ⎞ ⎜− ⎟⎜ ⎟ dt nchar − C ⎝ dt ⎠⎝ 1 + k 9C NO ⎠

−

−

dt

dn N2O

2 = Nchar, iπde,char k14C N2O

dC NO = f (C NO , CCO) dt

dn 2 − C = Nchar, iπde,char k12C NO dt

−

−

−

dC HCN = k 8C HCNCO2 dt

−

−

−

−

−

−

−

reaction rate

1 k1,r

+

1 k1,d

constant of reaction rate

k′7 = 01.054

⎛ 10000 ⎞ ⎟, k 7 = 3.38 × 107 exp⎜− ⎝ T ⎠

(kIC NO(kIICCO + kIII)) (kIC NO + kIICCO + kIII)

⎛ 16983 ⎞ ⎟ k14 = 2.9 × 109 exp⎜− ⎝ T ⎠

kI = 0.1826, kII = 0.00786, kIII = 0.002531

⎛ 19000 ⎞ ⎟ k13 = 1.952 × 1010 exp⎜− ⎝ T ⎠

f (C NO , CCO) = k13

⎛ 12000 ⎞ ⎟ k12 = 5.85 × 107 exp⎜− ⎝ T ⎠

⎛ 3551 ⎞ ⎟ k11 = k 9 = 900 exp⎜− ⎝ T ⎠

⎛ 65300 ⎞ ⎟ k10 = 3 × 1014 exp⎜− ⎝ T ⎠

⎛ 3551 ⎞ ⎟ k 9 = 900 exp⎜− ⎝ T ⎠

⎛ 10000 ⎞ ⎟ k 8 = 2.14 × 105 exp⎜− ⎝ T ⎠

k′6 = 0.054

⎛ 10000 ⎞ ⎟, k6 = 3.38 × 107 exp⎜− ⎝ T ⎠

⎛ 3240 ⎞ ⎟ k5 = 1.63 × 109T1.5 exp⎜− ⎝ T ⎠

⎛ 24157 ⎞ ⎟ k 3 = 4.1 × 106 exp⎜ ⎝ T ⎠

⎛ 29787 ⎞ ⎟ k 3 = 4.1 × 106 exp⎜− ⎝ T ⎠

⎛ 8056 ⎞ ⎟ k 2 = 1.9 × 106 exp⎜ ⎝ T ⎠

⎛ − 1.4947 × 108 ⎞ MCShpφDg k1,r = 8710 exp⎜ ⎟, k1,d = R T de,charR uT ⎠ ⎝ u

k1 =

R uT MC

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

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The volume fraction of char particles ηi in the ith subunit can be calculated by qm,char, i / ρchar, i Ve,char

(

qm,char, i / ρchar, i Ve,char

) + ⎛⎝ ⎜

qm,SiO / ρSiO 2

Ve,SiO2

2

⎞ ⎟ ⎠

(9b)

⎛ 10000 ⎞ 43.5 ⎟ exp⎜− 0.74 ⎝ T ⎠ de,char

The Sherwood number, Shp, of reaction R(1) in Table 3 for describing the reaction rate constant k1,d can be calculated as follows:35 Shp = 2ε + 0.69

Rep ε

3

Sc

(10a)

where the Reynolds number, Rep, and the Schmidt number, Sc, are defined as μg Arε 4.75 Rep = , Sc = 4.75 ρg Dg 18 + 0.61 Arε (10b)

k16 =

⎛ 5292 ⎞ ⎟ k15 = 5.01 × 1013 exp⎜− ⎝ T ⎠

constant of reaction rate

ηi =

The Archimedes number, Ar, and diffusivity coefficient of O2, Dg, in nitrogen34 in eq 10b can be calculated by Ar =

gde3(ρp − ρg ) μg2

,

⎛ T ⎞1.73 ⎟ Dg = 3.13 × 10−4⎜ ⎝ 1500 ⎠

= εcharρchar k16C N2O dt

dC N2O

⎧ 2ξ + 2 dchar < 0.05mm ⎪ ⎪ξ+2 ⎪ ξ φ = ⎨ 2ξ + 2 − 0.95 (dchar) 0.05mm ≤ dchar ≤ 1.0mm ⎪ ξ+2 ⎪ ⎪ dchar > 1.0mm ⎩1.0

−

= k15C N2OCCO dt

−

dC N2O

reaction rate

(10c)

The mechanism factor φ for describing char combustion in the definition of reaction rate constant k1,d in reaction R(1) in Table 3 can be calculated by29

(11)

1 O2 → N2 + O2 2 R(16)

(12)

Therefore, the above-mentioned multiphase macrocombustion reaction kinetics model for describing combustion reactions of 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 coal decoupling combustor. 3.4. Development of Process Simulation Model for the Simulated CFB Coal Decoupling Combustion Process Based on Aspen Plus. The commercial software of Aspen Plus has been used in this study to develop the process simulation model for simulating the behavior of formation and emissions for NO and N2O during CFB coal decoupling combustion process in a 30 kW CFB combustor including gas−solid hydrodynamics model,23,29 equivalent pyrolysis model of coal,29 and multiphase macrocombustion reaction kinetics model3,4,30,36,37 of coal pyrolysis products described in detail elsewhere.18,19 The developed flow sheet of a CFB coal decoupling combustion process with introducing the coal pyrolysis gaseous products to control NO and N2O

N2O +

N2O + CO → CO2 + N2

ξ = 2500 exp( −51900/R uT )

R(15)

reaction no.

Table 3. continued


where ξ is the concentration ratio of CO to CO 2 generated during char combustion process and can be calculated by29

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CO, H2O, NH3, HCN, and H2S, and solid particles containing char particles, liquid tar, and silica sands as heat carrier in the CFB downer column is represented by a module of separation (Sep1). (5) The ash discharging operation from the bottom of the CFB riser column is represented by a module of separation (Sep2). (6) The combustion process of solid pyrolysis products along the height of CFB riser column is represented by five modules of continuous stirred tank reactor RCSTRi with i = 1, 2, 3, 4, and 5, respectively. (7) The mixing of the heated primary air and solid pyrolysis products is represented by a mixer module (Mixer1); the mixing process between the introduced coal pyrolysis gaseous products from the CFB downer column into the CFB riser column and the combustion products from RCSTR1 in the CFB riser column is represented by a mixer module (Mixer3); and the mixing of secondary air at ambient temperature and substances from RCSTR2 is represented by a mixer module (Mixer2). (8) The separation of the unburned char particles and silica sands from flue gas in a cyclone at the top of the CFB downer column is represented by a module of separator (Sep3). (9) The heating process of primary air is represented by a module of heater (Heater1). To describe the influence of gas superficial velocity Ug on the voidage profile ε(z) along the height of CFB riser column in the developed gas−solid hydrodynamics model labeled as EPSS, the gas superficial velocity Ug must be recalculated at each iteration to get a new value as initial value for the gas superficial velocity Ug at the next iteration. The calculation module labeled as EPSS was developed to implement this function. The information streams in Figure 3 show that the calculated gas superficial velocity Ug at one iteration for describing ε(z) in five modules of RCSTRi with i = 1, 2, 3, 4, and 5 are the initial values of Ug at the next iteration for the purpose of decreasing the iteration number in this study. It should be emphasized that the developed multiphase macrocombustion reaction kinetics model of pyrolysis products in the CFB combustor requires the mole ratio of char−N to carbon in char particles (i.e., nchar−N/nchar−C); it can be calculated by a developed calculator module assigned as NDC in Fortran codes, as reported elsewhere.18,19 The calculated nchar−N/nchar−C in five cascade modules of RCSTRi with i = 1, 2, 3, 4, and 5 will be used to simulate the gas−solid heterogeneous combustion reaction rate of char−N in each module of RCSTRi with i = 1, 2, 3, 4, and 5, as reported elsewhere.18,19 Meanwhile, the mole ratio of unburned char−N to total char−N in char particles in the cyclone represented in a module of RCSTR5 is also needed in the developed multiphase macrocombustion reaction kinetics model of pyrolysis products, it can be calculated by a developed labeled as COALN in Fortran codes according to the mole ratio of N to C in coal particles from chemical composition of the investigated coal particles in Table 1. The other function of the developed calculator module COALN was used to make N from the unburned char−N in coal particles into the module of Hopper and let N from the introduced air into flue gas through a module of Sep3. The main initial conditions of the applied built-in seven modules in this study are almost the same as the previous publications18,19 for the same module. The accuracy of the developed process simulation model for describing the CFB coal decoupling combustion process has been verified by comparing with the reported15 results of CFB coal decoupling combustion in a 30 kW CFB combustor. The developed process simulation model of the CFB coal decoupling combustion process is also used to predict the results of other cases without experimental results as experimental

emissions in a 30 kW CFB combustor based on Aspen Plus is shown in Figure 3. The developed process simulation model for CFB coal decoupling combustion in a 30 kW CFB combustor can be summarized in four steps as follows:18,19,31 (1) The CFB coal decoupling combustion process in a 30 kW CFB combustor is described by the related built-in modules in Aspen Plus environment, as shown in Figure 3. (2) The gas−solid

Figure 3. Constructed flow sheet of a 30 kW CFB coal decoupling combustion process based on Aspen Plus.

hydrodynamics in the CFB riser column is described by a calculator assigned as EPSS with in-line Fortran codes, as in Figure 3. (3) The equivalent pyrolysis process of coal is described by a module of Ryield and a module of Stoic1 in Figure 3, while the oxidation of the generated H2S from coal pyrolysis as well as the residual sulfur element into SO2 by oxygen element in coal particles is represented by a module of RStoic2. (4) The multiphase macrocombustion reaction kinetics of pyrolysis products in the CFB combustor is described by a developed external Fortran subroutine, as illustrated in Figure 3. It can be obtained from Figure 3 that the fully steady state of the simulated CFB coal decoupling combustion process in a 30 kW CFB combustor can be represented by seven modules, as follows: (1) The pyrolysis process of the charged coal particles in the CFB downer column is represented by a built-in module of yield reactor (RYield) with the chemical composition and size distribution in Table 1 and a module of stoichiometry reactor (RStoic1) with the specified conversion ratio in Aspen Plus environment. (2) The mixing process of silica sands and pyrolysis products is represented by a module of hopper-type mixer (Hopper). (3) The heat exchange between the existing silica sands and coal pyrolysis products in the CFB downer column is represented by a module of heater (Heater2). (4) The separation between coal pyrolysis gaseous products including CH4, H2, CO2, 5216


Energy & Fuels

Article

Figure 4. Simulated concentration profile of O2, CO2, NO, N2O, and CO along height of CFB riser column with introducing secondary air at 1.66, 2.80, and 4.00 m above a gas distributor during a 30 kW CFB coal decoupling combustion process, respectively.

illustrated in Figure 4. The other operation conditions of the CFB coal decoupling combustion process in a 30 kW CFB combustor can be summarized as follows: the coal feed rate is 2.83 kg·h−1, the flow rate of primary air is 20.0 m3·h−1, and the flow rate of secondary air is 5.0 m3·h−1. It can be observed from Figure 4a and b that changing the introduction position of secondary air from 1.66 to 4.00 m via 2.80 m above a gas distributor can make obvious variations of concentration profiles for O2 and CO2 along the height of CFB riser column, although there was no change in the concentrations for O2 and CO2 in flue gas at three different introduction positions of secondary air. This means that changing the introduction position of secondary air cannot essentially affect the coal combustion efficiency during CFB coal decoupling combustion process in a 30 kW CFB combustor. Introducing the coal pyrolysis gaseous products from the CFB downer column into the upper dense phase region at the lower CFB riser column can obviously decrease O2 concentration, as shown in Figure 4a, and largely increase CO2 concentration, as shown in Figure 4b, because the charged coal pyrolysis gaseous products into the CFB riser column contains CO and CO2 species, rather than O2 species. It can be observed from Figure 4c and d that improving the introduction position of secondary air from 1.66 to 4.00 m via 2.80 m can result in a decrease of formation and emissions of

difficulty. The converged solutions of the developed process simulation model can be obtained after 30 to 80 iterations in about 3.5 min on a personal computer with a 1.25 G RAM and 3.0 GHz CPU.

4. PROCESS SIMULATION RESULTS AND DISCUSSION 4.1. Prediction of Introduction Position of Secondary Air on Concentration Profiles of Gaseous Pollutants. The reasonable introduction position of secondary air is a key configuration parameter for the newly developed CFB coal decoupling combustion process6,7,15−17 because the optimal value of the first stage stoichiometry can effectively affect NO and N2O emissions in flue gas. Finding the optimal introduction position of secondary air by tremendous experiments is not an ideal choice because changing the introduction positions of secondary air cannot be easily realized in a CFB combustor with a fixed CFB configuration. The process simulation is considered as a cost-saving method to determine the optimal introduction position of secondary air. The simulated concentration profile of O2, CO2, NO, N2O, and CO along the height of CFB riser column with the introduction position of secondary air at 1.66 m (H1), 2.80 m (H2), and 4.00 m (H3) above a gas distributor during the CFB coal decoupling combustion process for the investigated coal particles in a 30 kW CFB combustor under conditions of excess air percentage as 11.40% is 5217


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Article

Figure 5. Comparison of simulated and reported15 NO and N2O emissions in flue gas under condition of various excess air percentages with introducing secondary air at 1.66, 2.80, and 4.00 m above a gas distributor during a 30 kW CFB coal decoupling combustion process.

secondary air cannot change the emitted CO concentration in flue gas, that is, no negative effect on coal combustion efficiency. 4.2. Comprehensive Effect of Introduction Position of Secondary Air and Excess Air Percentage on NO and N2O Emissions. The comparison of the simulated and reported15 NO and N2O emissions in flue gas with changing the introduction position of secondary air at 1.66 m (H1), 2.80 m (H2), and 4.00 m (H3) above a gas distributor during CFB coal decoupling combustion process in a 30 kW CFB combustor under the condition of changing the excess air percentage from 5% to 35% is illustrated in Figure 5. The other operation conditions of the CFB coal decoupling combustion process can be summarized as follows: the coal feed rate is 2.83 kg·h−1; the flow rate of secondary air is 5.61 m3·h−1, which accounts for 25% of the theoretical air amount. Although there is a small difference between the simulated and reported15 results of NO and N2O emissions, the same change tendency between the excess air percentage and the emitted concentration of NO or N2O in flue gas can be observed from Figure 5; that is, increasing the excess air percentage can result in an obvious increase of NO emission and lead to a stable variation of N2O emission for the investigated coal particles during CFB coal decoupling combustion process in a 30 kW CFB combustor at three different introduction positions of secondary air. It can be observed from Figure 5a that changing the excess air percentage from 5% to 12% can lead to a lower NO emission under the condition of introduction position of secondary air at 1.66 m above a gas distributor; however, changing the excess air percentage from 12% to 35%, the introduction position of secondary air at 4.00 m along the height of CFB riser column can result in a lower NO emission. The comprehensive effect of introduction position of secondary air and excess air percentage on N2O emission shown in Figure 5b is different from that in NO emission, shown in Figure 5a. Changing the excess air percentage from 5% to 22% corresponds to a lower N2O emission under the condition of introduction position of secondary air at 4.00 m above a gas distributor, while when changing the excess air percentage from 22% to 35%, the introduction position of secondary air at 1.66 m above a gas distributor can lead to a lower N2O emission. This result implies that not the single influence but the comprehensive effect of the introduction position of secondary air and excess air percentage can largely affect NO and N2O emissions during CFB coal decoupling combustion process in a 30 kW CFB combustor. 4.3. Comprehensive Effect of Introduction Position of Secondary Air and First Stage Stoichiometry on NO and N2O Emissions. The comparison of the simulated and reported15

NO and N2O during CFB coal decoupling combustion process. The reasons for the decreasing NO profile can be summarized as follows: (1) Improving the introduction position of secondary air can expand the O2-lean zone in the dense phase region at the lower CFB riser column, while the residence time of solid particles in the dense phase region at the lower CFB riser column is long enough. Therefore, a stronger reducing atmosphere can be formed to benefit NO decomposition via reactions R(12) and R(13), as listed in Table 3. (2) The formed N2O in the expanded O2-lean zone in the dense phase region at the lower CFB riser column can be reduced6 by the accumulated char particles to form N2O via reaction R(14) in Table 3. The lower temperature profile is maybe the major reason for the simulated decrease of N2O emission38 by improving the introduction position of secondary air, as shown in Figure 4d. Therefore, improving the introduction position of secondary air is a valid measurement to decrease NO and N2O emissions without the negative influence on the coal combustion efficiency during CFB coal decoupling combustion process. The introduction position of coal pyrolysis gaseous products into the CFB riser column is a critical position along the height of CFB riser column for CO concentration. Below this critical position, the CO concentration shows a large increase tendency with an increase of the height of CFB riser column; above this critical position, the CO concentration demonstrates an obvious decrease tendency with an increase of the height of CFB riser column. This result can be explained as follows: (1) The incomplete combustion of char particles with primary air will generate tremendous CO species, and make CO concentration increase sharply in the lower dense phase region at the lower CFB riser column. (2) The generated CO from both incomplete combustion of char particles and the introduced coal pyrolysis gaseous products can be rapidly oxidized or burned into CO2 with primary air in the upper dense phase region at the lower CFB riser column. However, below the critical position, that is, the position of introducing coal pyrolysis gaseous products at 1.66 m above a gas distributor, improving the introduction position of secondary air from 1.66 to 2.80 m can decrease CO concentration; further improving the introduction position of secondary air from 2.80 to 4.00 m cannot affect CO concentration. Above the critical position, improving the introduction position of secondary air from 1.6 to 2.80 m cannot lead to a large change of CO concentration profile along the height of CFB riser column; further improving the introduction position of secondary air from 2.80 to 4.00 m can result in an obvious increase of CO concentration profile. It is very important that improving the introduction position of 5218


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Figure 6. Comparison of simulated and reported15 NO and N2O emissions in flue gas under condition of various values of the first stage stoichiometry with introducing secondary air at 1.66, 2.80, and 4.00 m above a gas distributor during a 30 kW CFB coal decoupling combustion process.

NO and N2O emissions in flue gas with changing the introduction position of secondary air at 1.66 m (H1), 2.80 m (H2), and 4.00 m (H3) above a gas distributor during a 30 kW CFB coal decoupling combustion process under the condition of changing the first stage stoichiometry from 0.8 to 1.1 is illustrated in Figure 6. The other operation conditions of the CFB coal decoupling combustion process can be summarized as follows: the coal feed rate of is 2.83 kg·h−1, the flow rate of total air is 28.05 m3·h−1, and the excess air percentage is about 25%. Although there is a small difference between the simulated and reported15 NO and N2O emissions, the similar change tendency between the first stage stoichiometry and the emitted concentration of NO or N2O in flue gas can be observed; that is, increasing the first stage stoichiometry can result in an obvious increase of NO emission and a stable change of N2O emission for the investigated coal during a 30 kW CFB coal decoupling combustion process at three different introduction positions of secondary air. It can be observed from Figure 6a that changing the first stage stoichiometry from 0.8 to 0.9 can lead to a lower NO emission under the condition of introduction position of secondary air at 1.66 m above a gas distributor along the height of CFB riser column; however, changing the first stage stoichiometry from 0.9 to 1.1, the introduction position of secondary air at 4.00 m above a gas distributor can result in a lower NO emission. Changing the first stage stoichiometry from 0.8 to 0.9 corresponds a lower N2O emission under the condition of introduction position of secondary air at 4.00 m above a gas distributor, while upon changing the first stage stoichiometry from 0.9 to 1.1, a lower N2O emission can be obtained with the introduction position of secondary air at 1.66 m above a gas distributor. This result suggests that not the single influence but the comprehensive effect of the introduction position of secondary air and the first stage stoichiometry can largely influent NO and N2O emissions during CFB coal decoupling combustion process in a 30 kW CFB combustor. 4.4. Quantitative Analysis of Contribution Ratios of Related Reactions on Formation and Decomposition of NO and N2O. Not only the concentrations of gaseous pollutants in flue gas but also the concentration profiles of gaseous components along the height of CFB riser column can be quantitatively simulated by the developed process simulation model for CFB coal decoupling combustion process in a 30 kW CFB combustor. The case described in Section 4.1 is chosen to investigate the contribution ratios of related reactions on the formation and decomposition of NO and N2O along the height of CFB riser column with three introduction positions of secondary

air at 1.66 m (H1), 2.80 m (H2), and 4.00 m (H3) above a gas distributor. This means that the simulated case of the CFB coal decoupling combustion in a 30 kW CFB combustor is assumed to be operated under conditions of the coal feed rate as 2.83 kg·h−1, the flow rate of primary air as 20.0 m3·h−1, the flow rate of secondary air as 5.0 m3·hr−1, and excess air percentage as 11.40%. The calculated reaction rates of the applied 16 reactions along the height of CFB riser column during CFB coal decoupling combustion process in a 30 kW CFB combustor under the above-mentioned CFB operation conditions with three introduction positions of secondary air at 1.66 m (H1), 2.80 m (H2), and 4.00 m (H3) above the gas distributor are summarized in Table 4. It should be emphasized that the calculated reaction rates of the applied 16 reactions in Table 4 can be applied to represent the reaction rates of the related reactions in the central position of the five modules of RCSTRi with i = 1, 2, 3, 4, and 5, which has a very close relationship with the height of CFB riser column, as described in Section 3.4. Because the three introduction positions of secondary air at 1.66 m (H1), 2.80 m (H2), and 4.00 m (H3) above a gas distributor have been taken in this study, the height of the divided dense phase region and dilute phase region along the height of CFB riser column should be variable. When the introduction position of secondary air is at 1.66 m (H1), the module of RCSTR1 is used to represent the turbulent region 1, which corresponds the height of CFB riser column from 0 to 1.50 m above a gas distributor; the module of RCSTR2 is applied to represent the turbulent region 2, which corresponds the height of CFB riser column from 1.50 to 1.66 m; the module of RCSTR3 is designed to represent the lower acceleration region 3, which corresponds the height of CFB riser column from 1.66 to 3.32 m; the module of RCSTR4 is assigned to represent the upper acceleration region 4, which corresponds the height of CFB riser column from 3.32 to 4.97 m; the module of RCSTR5 is employed to represent the completely fluidized region 5, which corresponds the height of CFB riser column from 4.97 to 6.63 m. When the introduction position of secondary air is at 2.80 m (H2), the module of RCSTR1 is used to represent the turbulent region 1, which is from 0 to 1.66 m above a gas distributor; the module of RCSTR2 is applied to represent the turbulent region 2, which is from 1.66 to 2.80 m; the module of RCSTR3 is designed to represent the lower acceleration region 3, which is from 2.80 to 4.08 m; the module of RCSTR4 is assigned to represent the upper acceleration region 4, which is from 4.08 to 5.36 m; the module of RCSTR5 is employed to represent the completely fluidized region 5, which is from 5.36 to 6.63 m. 5219


0.00

0.00

5220

N2O + C → CO + N2

N2O + CO → CO2 + N2

1 N2O + O2 → N2 + O2 2

R(16)

1 N2 2

R(15)

NO + CO → CO2 +

1 N2 2

1 1 O2 → N2O 2 4

NO + C → CO +

[N]fuel +


1 O2 → NO 2

1 1 3 O2 → H2 + CO + N2O 2 2 4

char 1 3 3 O2 ⎯⎯⎯⎯⎯→ N2 + H2O 2 2 4

char 3 5 O2 ⎯⎯⎯⎯⎯→ NO + H2O 2 4

[N]fuel +

HCN +

NH3 +

NH3 +

1 O2 → H2O 2

R(14)

R(13)

R(12)

R(11)

R(10)

R(9)

R(8)

R(7)

R(6)

H2 +

R(4)

R(5)

3 CH 4 + O2 → CO + 2H2O 2

7.44 × 10−9 2.31 × 10−11 4.84 × 10−11 1.65 × 10−9 6.06 × 10−10

8.94 × 10−9

2.45 × 10−11

5.32 × 10−11

1.69 × 10−9

5.85 × 10−10

1.58 × 10−16

1.58 × 10−16 1.29 × 10−8

3.97 × 10−8

3.79 × 10−8

1.46 × 10−8

0.00

0.00

0.00

0.00

0.00

0.00

0.00

2.78 × 10−8

3.21 × 10−8

CO2 + C → 2CO

0.00

1.69 × 10−5

1 O2 → CO2 2

1.75 × 10−5

CO +

z = 0.83 m 2.29 × 10−5

z = 0.75 m

2.37 × 10−5


RCSTR1

H2 = 2.80 m

⎛ ⎛2 ⎞ 1 2⎞ C + O2 → ⎜2 − ⎟CO + ⎜ − 1⎟CO2 φ φ⎠ ⎝ ⎝φ ⎠

R(3)

R(2)

R(1)

reaction no.

H1 = 1.66 m

5.65 × 10−10

1.54 × 10−9

4.35 × 10−11

2.01 × 10−11

6.31 × 10−9

1.13 × 10−8

1.47 × 10−16

4.26 × 10−8

0.00

0.00

0.00

0.00

0.00

2.57 × 10−8

1.66 × 10−5

2.25 × 10−5

z = 0.83 m

H3 = 4.00 m

2.31 × 10−8 3.13 × 10−8

2.29 × 10−8 1.13 × 10−8

1.85 × 10−11

9.49 × 10−11

1.91 × 10−12

1.85 × 10−12

6.18 × 10−10

6.48 × 10−10

1.63 × 10−17

6.86 × 10−10

1.19 × 10−9

6.99 × 10−11

4.37 × 10−11

1.13 × 10−8

6.57 × 10−9

7.39 × 10−16

5.62 × 10−10

2.31 × 10−8

2.29 × 10−8

3.47 × 10−8

1.12 × 10−5

1.11 × 10−5

1.02 × 10

−6

4.00 × 10

2.90 × 10−8

−7

7.85 × 10−6

8.19 × 10−6

z = 2.23 m

RCSTR2

H2 = 2.80 m

2.26 × 10−11

1.08 × 10−6

7.68 × 10−7

z = 1.58 m

H1 = 1.66 m

8.16 × 10−10

2.05 × 10−9

9.04 × 10−11

6.09 × 10−11

2.93 × 10−8

1.13 × 10−8

1.50 × 10−16

5.80 × 10−10

3.26 × 10−8

2.31 × 10−8

2.31 × 10−8

1.12 × 10−5

1.01 × 10

−6

8.70 × 10−8

1.47 × 10−5

1.17 × 10−5

z = 2.83 m

H3 = 4.00 m

9.03 × 10−10

9.18 × 10−10

3.06 × 10−11

5.36 × 10−11

5.00 × 10−9

5.09 × 10−9

9.01 × 10−16

2.52 × 10−9

2.30 × 10−8

1.80 × 10−10

1.12 × 10−10

1.15 × 10−8

6.45 × 10

−7

3.08 × 10−8

6.82 × 10−6

7.43 × 10−6

z = 2.49 m

H1 = 1.66 m

7.80 × 10−10

5.10 × 10−10

2.44 × 10−11

4.36 × 10−11

3.31 × 10−9

2.79 × 10−9

1.08 × 10−15

7.83 × 10−10

4.45 × 10−9

8.38 × 10−12

5.24 × 10−12

2.23 × 10−9

3.59 × 10

−8

2.90 × 10−8

3.56 × 10−6

4.41 × 10−6

z = 3.44 m

RCSTR3

H2 = 2.80 m

5.19 × 10−10

3.01 × 10−10

1.62 × 10−11

2.77 × 10−11

2.01 × 10−9

1.51 × 10−9

8.34 × 10−16

3.36 × 10−10

2.77 × 10−9

3.93 × 10−12

2.46 × 10−12

1.39 × 10−9

1.53 × 10

−8

2.21 × 10−8

2.17 × 10−6

2.70 × 10−6

5z = 4.44 m

H3 = 4.00 m

chemical reaction rate (kmol·s−1)

1.03 × 10−9

4.25 × 10−10

2.05 × 10−11

5.91 × 10−11

2.93 × 10−9

2.43 × 10−9

1.30 × 10−15

9.40 × 10−10

1.83 × 10−9

4.07 × 10−12

2.55 × 10−12

9.17 × 10−10

9.49 × 10

−9

2.36 × 10−8

2.91 × 10−6

3.68 × 10−6

z = 4.15 m

H1 = 1.66 m

4.66 × 10−10

2.26 × 10−10

9.36 × 10−12

3.23 × 10−11

2.78 × 10−9

1.84 × 10−9

1.01 × 10−16

4.45 × 10−10

6.80 × 10−10

4.76 × 10−13

2.98 × 10−13

3.40 × 10−10

3.20 × 10

−9

8.31 × 10−9

1.71 × 10−6

2.18 × 10−6

z = 4.72 m

RCSTR4

H2 = 2.80 m

5.42 × 10−10

1.63 × 10−10

9.60 × 10−12

2.88 × 10−11

1.14 × 10−9

7.82 × 10−10

9.44 × 10−16

1.30 × 10−10

4.49 × 10−10

1.93 × 10−13

1.21 × 10−13

2.24 × 10−10

1.08 × 10

−10

1.38 × 10−8

1.15 × 10−6

1.42 × 10−6

z = 5.32 m

H3 = 4.00 m

1.12 × 10−9

5.00 × 10−10

3.28 × 10−11

6.25 × 10−11

4.00 × 10−9

2.55 × 10−9

1.89 × 10−15

7.13 × 10−11

1.57 × 10−10

9.98 × 10−14

6.24 × 10−14

7.94 × 10−11

2.12 × 10

−11

4.51 × 10−8

3.19 × 10−6

4.20 × 10−6

z = 5.80 m

H1 = 1.66 m

5.13 × 10−10

2.82 × 10−10

1.48 × 10−11

3.41 × 10−11

3.83 × 10−9

2.15 × 10−9

1.38 × 10−16

4.38 × 10−11

9.35 × 10−11

1.95 × 10−14

1.22 × 10−14

4.98 × 10−11

2.02 × 10

−11

1.45 × 10−8

1.99 × 10−6

2.66 × 10−6

z = 5.99 m

RCSTR5

H2 = 2.80 m

3.41 × 10−10

1.79 × 10−10

9.53 × 10−12

2.13 × 10−11

2.30 × 10−9

1.28 × 10−9

9.12 × 10−17

1.41 × 10−11

1.14 × 10−10

2.01 × 10−14

1.25 × 10−14

5.80 × 10−11

1.23 × 10−11

9.55 × 10−9

1.29 × 10−6

1.73 × 10−6

z = 6.19 m

H3 = 4.00 m

Table 4. Calculated Reaction Rates of the Applied 16 Chemical Reactions in 5 RCSTRi Modules during a 30 kW CFB Coal Decoupling Combustion Process for the Applied Coal with Three Introduction Positions of Secondary Air (1.66 m (H1), 2.80 m (H2), and 4.00 m (H3) above a Gas Distributor)



category

N2O decomposition

N2O formation

NO decomposition

1 O2 → NO 2

N2O + C → CO + N2

N2O + CO → CO2 + N2

1 N2O + O2 → N2 + O2 2

R(14)

R(16)

1 1 O2 → N2O 4 2 2.10 71.64 26.26

72.62 25.10

100

2.28

100

0.00

0.31

0.27

3.78 × 10−11

4.19 × 10−7 99.69

100

100

99.73

z = 0.83 m 0

z = 0.75 m

RCSTR1

0

0.00 3 1 1 O2 → H2 + CO + N2O 4 2 2

[N]fuel +

HCN +

1 N2 2

1 N2 2

NO + CO → CO2 +

NO + C → CO +


[N]fuel +

char 5 3 NH3 + O2 ⎯⎯⎯⎯→ NO + H2O 4 2


R(15)

R(11)

R(8)

R(13)

R(12)

R(10)

R(9)

NO formation R(6)

reaction no. z = 0.83 m

26.24

71.73

2.02

100

0.00

0.32

99.68

3.31 × 10−7

100

0

z = 1.58 m

16.06

82.28

1.65

5.41

94.59

0.30

99.70

2.82 × 10−8

60.26

39.74

35.19

61.23

3.58

17.33

82.67

0.39

99.61

3.13 × 10−6

2.38

97.62

z = 2.23 m

RCSTR2 z = 2.83 m

27.57

69.38

3.06

25.76

74.24

0.21

99.79

6.36 × 10−7

2.45

97.55

z = 2.49 m

48.78

49.57

1.65

18.15

81.85

1.06

98.94

3.42 × 10−5

95.74

4.26

59.35

38.79

1.86

38.56

61.44

1.30

98.70

1.37 × 10−5

99.34

0.66

z = 3.44 m

RCSTR3 z = 4.44 m

62.09

35.98

1.94

35.32

64.68

1.36

98.64

2.47 × 10−5

99.27

0.73

z = 4.15 m

69.73

28.87

1.39

56.94

43.06

1.98

98.02

1.38 × 10−5

99.73

0.27

66.47

32.20

1.34

72.99

27.01

1.15

98.85

2.28 × 10−5

99.93

0.07

z = 4.72 m

RCSTR4 z = 5.32 m

75.86

22.79

1.34

63.56

36.44

2.47

97.53

7.26 × 10−4

99.91

0.09

z = 5.80 m

67.84

30.18

1.98

94.21

5.79

1.54

98.46

2.65 × 10−3

99.90997

0.08738

63.31

34.86

1.83

95.83

4.17

0.88

99.12

3.14 × 10−4

99.97179

0.0279

z = 5.99 m

RCSTR5

64.41

33.79

1.80

91.82

8.18

0.92

99.08

6.46 × 10−4

99.91045

0.0889

z = 6.19 m

H1 = 1.66 m H2 = 2.80 m H3 = 4.00 m H1 = 1.66 m H2 = 2.80 m H3 = 4.00 m H1 = 1.66 m H2 = 2.80 m H3 = 4.00 m H1 = 1.66 m H2 = 2.80 m H3 = 4.00 m H1 = 1.66 m H2 = 2.80 m H3 = 4.00 m

contribution ratio of chemical reaction ( × 10−2)

Table 5. Calculated Contribution Ratios of Related Reactions for Formation and Decomposition of NO and N2O in Five RCSTRi Modules during a 30 kW CFB Coal Decoupling Combustion Process for the Applied Coal with Three Introduction Positions of Secondary Air (1.66 m (H1), 2.80 m (H2), and 4.00 m (H3) above a Gas Distributor)


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Figure 7. Simulated contribution ratio of related reactions on formation (a) and decomposition (b) of NO along height of CFB riser column with introducing secondary air at 1.66, 2.80, and 4.00 m above a gas distributor during a 30 kW CFB coal decoupling combustion process.

In the same way, when the introduction position of secondary air is at 4.00 m (H3), the module of RCSTR1 is used to represent the turbulent region 1, which is from 0 to 1.66 m above a gas distributor; the module of RCSTR2 is applied to represent the turbulent region 2, which is from 1.66 to 4.00 m above a gas distributor; the module of RCSTR3 is designed to represent the lower acceleration region 3, which is from 4.00 to 4.88 m above a gas distributor; the module of RCSTR4 is assigned to represent the upper acceleration region 4, which is from 4.88 to 5.75 m above a gas distributor; the module of RCSTR5 is employed to represent the completely fluidized region 5, which is from 5.75 to 6.63 m above a gas distributor. The height of middle position for the applied five modules of RCSTRi with i = 1, 2, 3, 4, and 5 above a gas distributor have been listed in Table 4 as z value with three introduction positions of secondary air at 1.66 m (H1), 2.80 m (H2), and 4.00 m (H3). 4.4.1. Quantitative Analysis of Contribution Ratios of Related Reactions on Formation and Decomposition of NO. The contribution ratios of reactions R(6), R(9), and R(10) on NO formation, as well as reactions R(12) and R(13) on NO decomposition in five modules of RCSTRi with i = 1, 2, 3, 4, and 5 for representing the height of CFB riser column during CFB coal decoupling combustion process in a 30 kW CFB combustor with three introduction positions of secondary air at 1.66 m (H1), 2.80 m (H2), and 4.00 m (H3) above a gas

distributor can be calculated from data in Table 4 and summarized in Table 5. To describe directly, the calculated contribution ratio of the related reactions on formation and decomposition of NO along the height of CFB riser column during CFB coal decoupling combustion process in a 30 kW CFB combustor is shown in Figure 7. It can be observed from Figure 7a1 that reaction R(9) can play an important role on NO formation along the height of CFB riser column corresponding to the modules of RCSTR1, RCSTR3, RCSTR4, and RCSTR5, but not for RCSTR2 because the introduced coal pyrolysis gaseous products containing NH3 can promote reaction R(6) on NO formation. It can be obtained from Figure 7a2−a3 and Table 5 that (1) NO formation at the position corresponding to the module of RCSTR1 is absolutely controlled by reaction R(9); (2) NO formation at the position corresponding to the module of RCSTR2 is predominately controlled by reaction R(6) because the introduced coal pyrolysis gaseous products containing NH3 has an important role on the formation of NO in this region; (3) The NO formation at the position corresponding to the modules of RCSTR3, RCSTR4, and RCSTR5 is thoroughly controlled by reaction R(9); (4) changing the introduction position of secondary air from 2.80 to 4.00 m cannot largely affect the variation tendency of contribution ratio of reaction R(9) or R(6) on NO formation along the height of CFB riser column. 5222


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Figure 8. Simulated contribution ratio of related reactions on formation (a) and decomposition (b) of N2O along height of CFB riser column with introducing secondary air at 1.66, 2.80, and 4.00 m above a gas distributor during a 30 kW CFB coal decoupling combustion process.

4.4.2. Quantitative Analysis of Contribution Ratios of Related Reactions on Formation and Decomposition of N2O. The contribution ratios of reaction R(8) and R(11) on N2O formation as well as reaction R(14), R(15), and R(16) on N2O decomposition in five modules of RCSTRi with i = 1, 2, 3, 4, and 5 for representing the height of CFB riser column during CFB coal decoupling combustion process in a 30 kW CFB combustor with three introduction positions of secondary air at 1.66 m (H1), 2.80 m (H2), and 4.00 m (H3) above a gas distributor can be calculated from data in Table 4 and summarized in Table 5. To represent clearly, the calculated contribution ratio of the related reactions on formation and decomposition of N2O along the height of CFB riser column during CFB coal decoupling combustion process in a 30 kW CFB combustor is illustrated in Figure 8. It can be observed from Figure 8a1−a3 and Table 5 that N2O formation is thoroughly controlled by reaction R(11) at positions corresponding to the modules of RCSTR1 and RCSTR5 and comprehensively dominated by reaction R(8) and R(11) at positions corresponding to the modules of RCSTR2, RCSTR3, and RCSTR4 during CFB coal decoupling combustion process. The contribution ratio of reaction R(11) shows an increasing tendency with an increase of the height of CFB riser column corresponding to RCSTR2−RCSTR4 via RCSTR3. Oppositely, the contribution ratio of reaction R(8) indicates a decreasing trend with an increase of the height of

These results mean that fuel−N is the main resource of NO formation during char particles combustion process along the height of CFB riser column; however, the introduced NH3 in coal pyrolysis gas as an NO precursor only plays an important role on NO formation at the position corresponding to the module of RCSTR2 because the coal pyrolysis gaseous products from the CFB downer column are introduced into the CFB riser column corresponding to the position of a module of RCSTR2. No quantifiable thermal NO can be formed through reaction R(10) during CFB coal decoupling combustion process. This finding is in good agreement with that in traditional CFB coal combustion boiler reported elsewhere.39,40 It can be observed from Figure 7b1−b3 and Table 5 that the NO decomposition is controlled by reaction R(12) in the five modules of RCSTRi with i = 1, 2, 3, 4, and 5 along the height of CFB riser column with three introduction positions of secondary air at 1.66 m (H1), 2.80 m (H2), and 4.00 m (H3) above a gas distributor. Char particles account for almost the whole contribution of NO decomposition along the height of CFB riser column during CFB coal decoupling combustion process; however, a very small contribution of CO to NO decomposition can be quantitatively simulated. In addition, changing the introduction position of secondary air from 1.66 to 4.00 m via 2.80 m cannot largely affect the variation tendency of contribution ratio of reaction R(12) or R(13) on N2O formation along the height of CFB riser column. 5223


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CFB riser column corresponding to RCSTR2−RCSTR4 via RCSTR3. Changing the introduction position of secondary air from 1.66 m (H1) to 4.00 m (H2) through 2.80 m (H3) above a gas distributor cannot lead to an obvious variation of the contribution ratio for reaction R(11) or R(8) on N2O formation along the height of CFB riser column. This result means that HCN combustion by O2 via reaction R(8) can dominate N2O formation at positions corresponding to modules of RCSTR2 and RCSTR3 because the introduced HCN in coal pyrolysis gaseous products is the N2O precursor; meanwhile, combustion of fuel−N or char−N via reaction R(11) only accounts for a dominating contribution on N2O formation in the dense phase region at the lower CFB riser column as well as in the dilute phase region at the upper CFB riser column, that is, in RCSTR1 and RCSTR5 during the CFB coal decoupling combustion process in a 30 kW CFB combustor. It can be observed from Figure 8b1−b3 and Table 5 that N2O decomposition is comprehensively controlled by CO via reaction R(15) and O2 via reaction R(16) during CFB coal decoupling combustion process in a 30 kW CFB combustor. The contribution ratio of reaction R(15) via CO reduction on N2O decomposition shows a decreasing tendency with an increase of the height of CFB riser column; on the contrary, the contribution ratio of reaction R(16) via O2 oxidization on N2O decomposition shows an increasing tendency with an increase of the height of CFB riser column. The contribution of reaction R(14) via char reduction on N2O decomposition is very small, less than 4%.

39.7% to 97.6% in the dense phase region at the lower CFB riser column. About 98.9%−99.8% of NO decomposition along the height of CFB riser column is dominated by char particles reduction. (5) About 91.8%−95.8% of the emitted N2O in flue gas is controlled by fuel−N combustion; however, the introduced HCN in coal pyrolysis gaseous products as an N2O precursor has an obvious effect on N2O formation from 74.2% to 94.6% in the dense phase region at the lower CFB riser column. The importance of the introduced HCN in coal pyrolysis gaseous products on N2O formation shows a decay tendency from 81.9% to 27.0% above the dense phase region along the height of CFB riser column and further decreases to 8.2% at the outlet of the CFB coal decoupling combustor. The contribution of char particles reduction on N2O decomposition can be found as 1.8%−3.6% along the height of CFB riser column. A competitive relationship between O2 oxidization and CO reduction on N2O decomposition has been revealed along the height of CFB riser column during the CFB coal decoupling combustion process.

■

AUTHOR INFORMATION

Corresponding Author

*Tel./fax: +86−10−82622893. E-mail: [email protected]. ac.cn. Notes

The authors declare no competing financial interest.

■ ■

5. CONCLUSIONS A process simulation model of CFB coal decoupling combustion process in a 30 kW CFB combustor has been developed based on Aspen Plus by considering the gas−solid hydrodynamics via Aspen Plus in-line Fortran codes, the equivalent coal pyrolysis model via Aspen Plus in−line modules, and the multiphase macrocombustion reaction kinetics of coal pyrolysis products via external Fortran subroutines. The simulated results of NO and N2O emissions have been compared with the reported15 ones from experiments to verify the accuracy of the developed process simulation model of CFB coal decoupling combustion process. The main summary remarks can be summarized as follows: (1) The NO and N2O emissions in flue gas can be reliably simulated by the developed process simulation model of CFB coal decoupling combustion process in a 30 kW CFB combustor with various CFB configuration and operation parameters. (2) Not only NO and N2O emissions in flue gas but also the detailed information on formation and decomposition of NO and N2O along the height of CFB riser column can be quantitatively simulated by the developed process simulation model for CFB coal decoupling combustion process. (3) Changing the introduction position of secondary air can be recommended as a main measurement to ideally realize the CFB coal decoupling combustion in a 30 kW CFB combustor with the fixed configuration parameters. (4) About 99.9% of the emitted NO in flue gas is controlled by fuel−N combustion; however, the introduced NH3 in coal pyrolysis gaseous products as an NO precursor can play an important role on NO formation from about

ACKNOWLEDGMENTS The financial support of this work by the Natural Sciences Foundation of China (Project No.50576101) is kindly appreciated. NOMENCLATURE

Roman Letters

a = decay index of cluster in free board of CFB riser column, m−1 Ar = Archimedes number, dimensionless C = constant, 4−5 s−1 Ci = volume concentration of component i in gas, mol·m−3 CO2 = volume molar concentration of O2 on surface of char particles, kmol·m−3 de, char = equivalent or average diameter of char particles, m Dg = diffusivity coefficient for oxygen in nitrogen, m2·s−1 Fr = Froude number, dimensionless g = gravitational acceleration, 9.81 m·s−2 Gs = mass flux of solid particles, kg·s−1·m−2 hi = height of the ith reaction subunit in CFB riser column, m MC = relative atom weight of carbon element, 12.0, dimensionless Nchar, i = number of char particles in the ith reaction subunit in CFB riser column, dimensionless qm, char, i = mass flow of char into the ith reaction subunit in CFB riser column, kg·s−1 qm,SiO2 = mass flow of silica sands into reaction subunits in CFB riser column, kg·s−1 Q = heat duty, J Ru = Uuniversal gas constant, 8.314 J·mol−1 K−1 Rep = Reynolds number of particles, dimensionless Sc = Schmidt number, dimensionless Shp = Sherwood number of particles, dimensionless t = time, s

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T = absolute temperature, K Tp = absolute temperature of particles, K Tg = absolute temperature of gas, K Ug = gas superficial velocity, m·s−1 Umf = critical fluidization velocity, m·s−1 Vi = volume of ith reaction subunit in CFB riser column, m3 Vchar, i = volume of char particles in ith subunit in CFB riser column, m3 Ve, char = equivalent volume of single char particles in CFB riser column, m3 Vyield = yield of volatile matters during coal pyrolysis process, kg·(100 kg coal)−1 Ve,SiO2 = equivalent volume of single silica particle, m3 VM = content of volatile matters in coal, kg·(100 kg coal)−1 wi = mass fraction of component i in pyrolysis gaseous products, dimensionless xi = weight fraction of particles at di interval, dimensionless z = height of CFB riser column, m Greek Letters

α = coefficient for describing yield of volatile matters, kg·(100 kg coal)−1 β = coefficient for describing yield of volatile matters, kg·(100 kg coal)−1 ε = voidage, dimensionless ε(z) = voidage at height of z in CFB riser column, dimensionless ε* = voidage under saturated conditions, dimensionless ε−∞ = voidage equivalent to the value at the height as −∞ in CFB riser column, dimensionless εi̅ = average voidage in ith reaction subunit in a CFB riser column, dimensionless ηi = volume fraction of char particles in ith reaction subunit, dimensionless μg = viscosity of gas, Pa·s ξ = ratio of CO concentration to CO2 concentration formed during char combustion process, dimensionless ρp = density of solid particles, kg·m−3 ρchar, i = density of char particles in the ith reaction subunit, kg·m−3 ρSiO2 = density of silica sands, kg·m−3 ρg = density of gas, kg·m−3 φ = mechanism factor of char during combustion process, dimensionless Φ = crushing index, dimensionless Subscripts

i = ith reaction subunit or component i, dimensionless

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Process Simulation of Using Coal Pyrolysis Gas to Control NO and

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