Effect of Stoichiometric Ratio on Char-Nitrogen Conversion under

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Effect of Stoichiometric Ratio on Char-Nitrogen Conversion under High-Temperature Entrained Flow Combustion Conditions Yupeng Li, Rui Sun, Min Wang, Zhuozhi Wang, Jie Xu, and Xiaohan Ren Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00685 • Publication Date (Web): 24 Apr 2018 Downloaded from http://pubs.acs.org on April 25, 2018

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Energy & Fuels

4

Effect of Stoichiometric Ratio on Char-Nitrogen Conversion under High-Temperature Entrained Flow Combustion Conditions

5 6 7 8 9

Yupeng Li,† Rui Sun,*,† Min Wang, † Zhuozhi Wang,† Jie Xu,† Xiaohan Ren‡ †School of Energy Science and Engineering, Harbin Institute of Technology, Harbin, Heilongjiang 150001, People’s Republic of China ‡Institute of Thermal Science and Technology, Shandong University, Jinan, Shandong 250061, People’s Republic of China

1 2 3

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

ABSTRACT: The char-O2 reaction under high-temperature entrained flow conditions was investigated experimentally and numerically to study the effect of the stoichiometric ratio (SR) on char-nitrogen conversion. A numerical model was established based on the unsteady convection-diffusion and reaction equations, in which Stefan flow was considered. The change of specific surface area during char combustion was considered by locally using the Random Pore Model (RPM). The intrinsic reactivity parameters of char-O2 and char-NO reactions were measured using a thermal gravimetric analyser (TGA) and drop tube furnace (DTF), respectively. The numerical results indicate that the predictions of carbon conversion and NO release agreed well with experimental data. Based on numerical results, for a given carbon conversion, with the decrease of SR, the kinetics-diffusion controlled char-O2 reaction is pushed toward the kinetics-controlled regime, leading to more significant local carbon conversion and thus more developed pore structure inside char particles, including higher porosity and larger specific surface area. Therefore, the oxidation of char-nitrogen occurs more deeply inside char particles and NO release prefers to diffuse towards the particle centre, rather than outward. The larger specific surface area deep inside the char particle also gives rise to the total char-NO reaction rate at a lower SR. As a result, the total fractional conversion of char-N to NO decreases as SR decreases under high-temperature entrained flow combustion conditions. KEYWORDS: char combustion; stoichiometric ratio; pore structure; mass transfer; char-nitrogen release

32

1 Introduction

33 34 35 36 37

A portion of fuel-nitrogen (fuel-N) is released as nitrogen oxides (NOx, primarily as NO) during coal combustion, which can cause serious environment pollution problems,1 such as acid rain, photochemical smog, and the ‘greenhouse effect’. Fuel-N consists of volatile nitrogen (volatile-N) and char-nitrogen (char-N). In fact, the releases of volatile-N and char-N are supposed to behave independently due to the significant difference between the oxidation rates of volatile-N

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38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81

and char-N.2,3 The NOx produced from the homogeneous oxidation of volatile-N has been effectively controlled by modifying the SR of O2/fuel in the combustion chamber; however, the NOx produced from the heterogeneous oxidation of char-N is less controllable.4 NO formed by char-N oxidation can be adsorbed on surface active sites, C(), of char to form nitrogen-containing surface functionalities, C(N), which can be subsequently attacked by NO-forming N2.5-7 Moreover, NO can also be reduced by CO on the char surface.8 Therefore, the ultimate yield of NO relies on the oxidation of char-N and the reduction of NO, which may play important roles in final pulverized-coal (PC) combustion NOx emissions levels. The effect of SR on the fate of char-N during combustion has been widely investigated experimentally. Song et al.9 performed combustion experiments of lignite char in an entrained flow reactor (EFR) at high temperatures (1250 and 1750 K) and observed that the fractional conversion of char-N to NOx increases as SR increases. Nelson et al.10 obtained a similar trend in the fractional conversion of char-N to NO for four chars, but their results are more scattered. However, an inverse trend was observed in the particle fluidized bed experiments of Tullin et al.11 and Goel et al.12. Interestingly, Spinti and Pershing13 found an increase and then a decrease of the fractional conversion of char-N to NO as SR increased for pulverized coal and explained that N2O yields rose and were rapidly destroyed to form N2 at high O2 concentrations. Nevertheless, De Soete14 and Ashman et al.15,16 found that O2 has little effect on the fractional conversion of char-N to NO using a fixed bed reactor. In summary, because of the different conditions, as well as coal ranks, it is difficult to determine a clear relationship between SR and the ultimate yield of NO and, especially, to understand the effect of SR on the fate of char-N among different studies. Furthermore, mass transfer becomes dominant in industrial combustion furnaces, especially for porous char particles.17 The mass transfer of O2 affects the depth that O2 penetrates into the char particle and thus affects where the oxidation product NO is formed. Analogously, the mass transfer of the char-N oxidation product (NO) affects the reduction characteristics of NO during char combustion. Therefore, mass transfer can be a major factor that affects the ultimate yield of NO at high temperatures. The classical Thiele analysis18 proposed a concise relationship to determine the effect of reactant diffusion on the actual reaction rate for a uniform diffusion coefficient throughout the solid particle. However, the pore structure of char always changes during combustion,19 and more importantly, in a diffusion controlled regime, the non-uniform distribution of O2 concentration makes a non-uniform change of char structure, as well as a non-uniform diffusion coefficient.20 The random pore model (RPM)21 depicts the variation trend of pore surface area with the mass conversion of porous solids based on the initial pore structural properties in a kinetics-controlled regime. In addition, in diffusion controlled regimes, several researchers have applied RPM locally to predict the carbon conversion of char combustion.20,22 However, most of the investigations ignored the total mass convection of the gas mixture during combustion which affects gas diffusion, called Stefan flow.23 Sadhukhan et al.24 simulated the combustion dynamics of a porous coal char in a fluidized bed based on the unsteady convection-diffusion equations which take Stefan flow into consideration. In this study, the conversion of char-N to NO during char combustion reactions under high-temperature entrained flow conditions was investigated experimentally and numerically. A char combustion model was established to predict carbon conversion and char-N release during heterogeneous combustion reactions under high-temperature entrained flow conditions. Stefan flow was considered based on the unsteady convection-diffusion equation. RPM was applied


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locally to consider the changes in pore structure. Due to the significant difference of kinetics parameters in different studies, the kinetics parameters of char-O2 and char-NO were measured using a thermal gravimetric analyser (TGA) and EFR, respectively. The numerical results were compared with the experimental results to determine the effect of SR on the pore structure change and the char-N conversion under high-temperature entrained flow combustion conditions.

87

2 Materials and Methods

88

2.1 Char sample preparations

89 90 91 92 93 94 95 96

The char sample used in this study was prepared by pyrolysis of Zhundong (ZD) coal, a typical Chinese bituminous coal. The raw coal was crushed and later sieved to obtain 53-75-µm coal particles. To remove the volatile components thoroughly, the coal particles were pyrolysed in an Ar atmosphere at 1273 K for 30 min using a horizontal tube reactor to obtain the ZD char samples. Next, the ZD char samples were collected and sealed in a sample bottle. The ultimate and proximate analyses of ZD coal and ZD char samples are given in Table 1. Table 1. Ultimate analysis and proximate analysis of ZD coal and ZD char samples. Ultimate analysis (dry and ash free, %)

Proximate analysis (as received, %)

Sample Cdaf

Hdaf

Ndaf

Sdaf

Odaf

Moisture

Volatile

Fixed Carbon

Ash

ZD coal

73.85

3.97

0.68

0.25

21.26

5.53

32.05

59.56

2.88

ZD char

94.80

0.37

1.14

0.28

3.41

0.43

1.63

93.55

4.48

97 98

2.2 Isothermal TG experiments

99 100 101 102 103 104 105 106 107

To obtain the intrinsic kinetics parameter of the char-O2 reaction for ZD char, experiments for the char-O2 reaction were conducted by means of a thermal gravimetric analyser (TGA/SDTA851e, METTLER-TOLEDO) operating at atmospheric pressure.25 Approximately 5 mg ZD char were carefully placed in an alumina crucible to ensure that the particles were in the form of a monolayer with a uniform distribution. The sample was first heated to the reaction temperature (T = 673, 723, 773, and 823 K) at a heating rate of 20 K/min in an inert protective atmosphere (N2) and then maintained at the reaction temperature. Next, the inert protective atmosphere was switched to the reaction atmosphere, high purity O2, until the sample burned out. The gas flow rate was set at 60 mL/min.

108

2.3 Entrained flow reactor experiments

109 110 111 112 113 114 115

The high-temperature char-O2 and char-NO reactions were performed in DTF. The schematic of the DTF system is shown in Figure 1. The EFR is an alumina tube with an inner diameter of 40 mm heated by six MoSi2 heating elements wrapped around the tube. Before the experiments, the EFR was heated electrically to the reaction temperature. ZD char particles were fed by a vibratory powder feeder at a feeding rate of 4.2 g/h and were then entrained by Ar (1 L/min) into the EFR. The reaction gas flow (5 L/min) was preheated to 600 K before being introduced into the EFR. The reaction gas flow was straightened by a plate of porous ceramic on the top of the EFR and


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later mixed with the char entrained by Ar in the reaction zone. For the char-O2 reaction, the mixed reaction gas consisted of O2 (SR = 0.25, 0.6, 1, 1.5, 2.5, that is 0.5, 1.2, 2, 3, 5% vol. of O2) and Ar, and the reaction temperatures were set to 1473 and 1673 K. For the char-NO reaction, the mixed reaction gas consisted of NO (833 ppmv) and Ar, and the reaction temperatures were set to 1273, 1373, 1473, 1573, and 1673 K. The reaction products were quickly cooled to terminate the reaction with the oil-cooled tube at the bottom of the EFR. The partially reacted char particles were collected with a metal filter screen for each experiment, and the composition of exhaust gases was measured by an online Fourier transform infrared (FTIR) gas analyser (GASMET-DX4000, Finland), of which the measurement upper limit and accuracy of the main possible gaseous products are listed in Table 2. The temperature field of the EFR was measured by a thermocouple, and a 400 mm-long constant temperature zone was maintained, as shown in Figure 1. Therefore, assuming the difference in relative velocities between the gas and char particles is zero, the residence time can be controlled between 0.22 and 1.17 s, depending on the reaction temperature, by adjusting the position of a removable oil-cooled feed tube. For this study, the residence times of the tested char are 0.255, 0.51, 0.765, 1.02 s for T = 1473 K, and 0.223, 0.445, 0.668, 0.89 s for T = 1673 K. Table 2. Measurement upper limit and accuracy of the FTIR gas analyser. Gaseous products

Upper limit

Unit

Accuracy

CO2

30

% vol.

±2%

CO

10000

ppmv

±2%

N2 O

500

ppmv

±2%

NO

1500

ppmv

±2%

NO2

300

ppmv

±2%

NH3

2000

ppmv

±2%

HCN

500

ppmv

±2%

O2

25

% vol.

±2%

134 Powder feeder Entrained flow Porous ceramic Thermal insulation material

Removable oil-cooled feed tube Gas preheater

Alumina tube （EFR）

Reaction gas

Constant temperature zone 400 mm

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Heating elements

Thermocouple

Exhaust gas emission

Oil-cooled tube Outgoing nozzle

Vacuum pump

135

Metal filter screen FTIR


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Figure 1. Schematic of the EFR system.

137

2.4 Char characterization measurements

138 139 140 141 142 143 144

Because of the deformation and collapse of pore surfaces under high pressure, mercury porosimetry was used to estimate the structure of the large pores. Additionally, nitrogen adsorption analysis was used to estimate the structure of the small pores. The entire pore structure of the ZD char sample was measured by combining the mercury porosimetry (Autopore II 9220, Micrometritics) and nitrogen adsorption analyses (AUTOSORB-1-C, Quantachrome). The mass fraction of carbon and nitrogen left in the partially reacted char was measured by an elemental analyser.

145

3 Numerical Model

146

3.1 Basic assumptions

147 148 149 150 151 152

The char particle is regarded as a central symmetry porous sphere in relative static gaseous flow, where the Sherwood number equals 2,26 therefore, the thickness of gaseous boundary layer around the char particle is estimated to equal the particle radius R0.26 Due to the low sample loads and high heating rate of char particles, the reactions in EFR were approximately considered to be isothermal. 27. Therefore, the particle temperature Tp is assumed to be constant and equal to the reaction temperature (T = 1473 or 1673 K).

153

3.2 Chemical reaction model

154 155 156 157

The heterogeneous reactions that occur on pore surfaces are considered as follows: (1) The overall reaction of carbon oxidation η+2 η 1 C+ O2 → CO + CO 2 (η + 1) η +1 η +1 2 where η is the ratio of CO to CO2 produced by the oxidation of char, expressed as:20

12400   R T   g 

η=2500exp

158

159

162 163 164 165 166 167 168 169

(1)

(2) Reaction of CO2 with char

160 161

(R1)

C + CO2 → 2CO

(R2)

char-N + O2 → NO

(R3)

(3) Oxidation of char-N

assuming that the primary product of char-N oxidation is NO and the molar ratio of char-N release to carbon conversion is equal the molar ratio of nitrogen to carbon in the initial char.28 (4) Reduction of NO by char

1 (R4) NO+ C → N2 +CO 2 The homogeneous reaction that only occurs in the pore inside char particle is considered as follows: (5) Reduction of NO by CO catalysed by char surface


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1 char (R5) NO+ CO → N2 +CO2 2 The homogeneous reaction that occurs both in the pore inside the char particle and in the gaseous boundary layer is considered as follows: (6) Oxidation of CO catalysed by H2O 1 H2O (R6) CO+ O2   →CO2 2 assuming that the H2O concentration is constant in the gas phase and equals 1.16×10-4 vol. according to the moisture of the char sample. The kinetics parameters of the reactions considered in numerical model are summarized in Table 3. Table 3. Kinetics parameters of reactions considered in the numerical model. Reaction

Kinetics equation

Constants

Ref.

1

R1=A1exp E1 / ( RgT )

A1 = 185461 g/(m2·s·atm)

Measured by TG

E1 = 129102 J/mol

in this study

2

R2 =A2exp E2 / ( RgT ) ( pCO2 / T )

4

R4 =A4exp E4 / ( RgT )

No.

R5 = kNO,CO 5

k1cNO (k2cCO + k3 ) k1cNO + k2cCO + k3

kNO,CO =1.952×10 exp( −19000/ T ) 10

6

0.5 0.5 R6 =A6exp E6 / ( RgT )  cCO2cO2 cH2O

A2 = 5.301×107 mol·K/(m2·s·atm) E2 = 248120 J/mol

29,30

A4 = 0.1588 mol/(m2·s·Pa)

Measured by DTF

E4 = 152869 J/mol

in this study

k1 = 0.1826 k2 = 0.00786

31

k3 = 0.002531 A6 = 1.3×108 m3/(mol·s) E6 = 125400 J/mol

32

180 181

3.3 Single-particle model

182 183 184 185 186 187 188

The effect of mass transfer is significant at high reaction temperatures; therefore, both of the intraparticle region and the gaseous boundary layer region are divided into 20 concentric annular volume elements (the subscript k labelled from 1 to 20) with constant volume. The chemical reaction and mass transfer in each volume is governed by local conditions. It is assumed that the ash does not fall off; therefore, the particle radius is constant, and the particle density changes during combustion, which is appropriate to study the mass transfer in porous particle. In intraparticle region, the local carbon conversion in each element XC,k is governed by dXC,k (2) =RCSM,k dt where RC is the overall local reactivity of carbon, which depends upon the reactions that occur on the char surfaces including R1, R2, and R4. SM,k is the local specific surface area based on mass, which changes with the loss of carbon during combustion. Therefore, RPM was used locally to estimate the change of local specific surface area based on mass and volume with respect to local

189 190 191 192 193


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carbon conversion: SM,k ( X C,k )

= 1 −ψ ln(1 − X C,k )

(3)

= (1 − X C,k ) 1 −ψ ln(1 − X C,k )

(4)

SM0

196

SV,k ( X C,k ) SV0

197 198 199 200 201

where ψ is the structural parameter of ZD char, which was calculated from the overall pore-size distribution based on Bhatia and Perlmutter21. The local porosity θk and local apparent density ρA,k of the char particle also change with the loss of carbon: ρA,k (5) = X C,k (1 − Fash ) + Fash ρA0 θk =

202

ρA,k ρT

(6)

204

The local pore size is determined according to the empirical value proposed by Satterfield33: 2r θ (7) rp,k = f k SM,k ρA,k

205 206 207 208 209

where rf is the surface roughness factor and is taken as 2 for the carbon surface34. In both of the intraparticle region and the gaseous boundary layer region, the mass transfer of each gas phase is controlled by a spherical, one-dimensional, convection-diffusion equation which consists of an unsteady term, a convection term, a diffusion term, and a source term as shown consecutively below:24

203

210 211 212 213 214

∂ (θρg mi ) + R12 ∂∂R ( R2 N g M g mi ) =De,i ρg R12  ∂∂R  R2 ∂∂mRi   + ∑ Ri ∂t   

where mi is the mass fraction of a given gaseous phase, ΣRi represents the formation rate of a given gaseous phase, and ρg is the density of the gas mixture. Mg is the molecular weight of gas mixture. Note that θ equals 1 for the gaseous boundary layer region. Ng is the total molar flux of the gas mixture, expressed as: N g =∑ Ni

215 216 217 218 219 220

(10)

N i =xi N g − De,i c g grad(xi )

where xi is the molar fraction of a given gaseous phase cg is the total molar concentration of gas mixture, as governed by the total molar balance equation of the gas mixture:24 ∂ (θ cg )

+

1 ∂ ( R2 N g ) =∑ Ri,g R2 ∂R

(11)

where ΣRi,g represents the total molar formation rate of the gas mixture. De,i is the effective diffusion coefficient of a given gaseous phase determined from De,i = Dbulk,i

223 224

(9)

where Ni is the molar flux of a given gas phase, which is given by:24

∂t

221 222

(8)

(12)

for the gaseous boundary layer region, and −1

225

θ 1 1  De,i =  +   τ  Dbulk,i Dknud,i 


(13)

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226 227 228 229 230 231 232 233

for the intraparticle region. The tortuosity τ is estimated to be:35 1 (14) θ In a multi-component system, the molecular diffusion coefficient Dbulk,i can be estimated by Stefan-Maxwell equation.24 Due to the high mole fraction of argon (≥ 95%) in reaction zone, for simplicity, Dbulk,i is estimated to be the binary diffusion coefficient in argon determined from the Fuller-Schettler-Giddings equation36 as Yu et al.37 and Dknud,i is the Knudsen diffusion coefficient that is given by

τ=

Dknud,i = 3.07rp

234

235 236

Ni t =0 = 0

Ng R=0 = 0 −De,i ρg

∂mi ∂R

(18)

+ Ng Mg mi

R=0

R=0

=0

(19)

At the interface of intraparticle region and gaseous boundary layer region (R = R0)

(m )

245

i R= R0

intraparticle

(N )

246

g R= R 0

248

(17)

Boundary conditions include the following: At char particle centre (R = 0)

243

247

(16)

The molar flux of each gas gaseous phase is zero:

242

244

(15)

Mi

mi t =0 = 0

239 240 241

Tp

Initial conditions include the following: The mass fraction of each gaseous phase is zero except Ar:

237 238

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 ∂mi −D ρ  e,i g ∂R 

+ Ng Mg mi R=R0

(

= mi R=R

intraparticle

0

)

(

= Ng R=R

0

(20) gaseous

)

(21) gaseous

  ∂m =  −De,i ρg i  R=R0   ∂R  intraparticle 

+ Ng Mg mi R=R0

  R=R0   gaseous

(22)

At the interface of gaseous boundary layer region and ambient gas flow (R = 2R0) mi R=2R = mi ,∞

249

(23)

0

250

3.4 Entrained flow reactor model

251 252 253 254 255 256

The char particles burn in an EFR, where the concentrations of different gaseous phases in ambient flow always change along the length of the reactor during char combustion. Thus, based on the mass balance equation, the change of gaseous concentrations in ambient flow during char combustion was considered. By experimental measurement, the maximum mole ratio of CO to CO2 in exhaust gases among all cases is lower than 0.05, which indicates that most of CO is oxidized to CO2 in the


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257 258 259 260

ambient gas flow during char combustion. In this instance, CO is assumed to be oxidized to CO2 by O2 in the ambient flow. Moreover, the change of carbon conversion caused by C-NO reaction (R4) is considered to be negligible for the relative minimum compared with the reaction rate of R1. Therefore,

261

dYO2 dY dX = − CO2 = −α C dt dt dt

262 263 264

(24)

where α depends on the ratio of char feed rate to the ambient gas flow rate. For NO and N2, dYNO FN / MN dYCO2 = ⋅ ⋅ BNO dt FC / MC dt

(25)

dYN2 FN / MN dYCO2 = ⋅ ⋅ (1− BNO ) dt FC / MC dt

265

(26)

where BNO is the fractional conversion of char-N to NO.

267

3.5 Solution algorithms

268 269 270 271 272 273 274

The partial differential governing equations (Equations 8 and 11) are discretized in time (t) and space (R) using an implicit finite volume method (FVM)24, which generates a set of linear equations. The nonlinear source terms are linearized by the Taylor series approximation. The resultant equations are solved using a tridiagonal matrix algorithm. A MATLAB code was developed to solve the numerical model equations. The convergence criterion was selected to be 10-4 for all normalized principal variables. The calculated residence time for char reactions is 1.05 s.

275

4. Results and Discussion

276

4.1 Pore structure characteristics of initial ZD char

2

277 278 279 280 281 282 283 284

a)

Differential pore volume Vdif [cm3/(g·nm)]

266

Differential specific surface area Adif [m /(g·nm)]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

101 N2 adsorption

100

Mercury porosimetry

10-1 10

-2

10

-3

10-4 10-5 10-6 10-7 10-8 10-9 0 10

101

102

103

104

N2 adsorption Mercury porosimetry

10-2 10-3 10-4 10-5 10-6 10-7

100

105

Pore diameter dp [nm]

10-1

b)

101

102

103

104

105


Figure 2. Pore distribution of initial ZD char: a) differential specific surface area and b) differential pore volume.

Based on the Barrett-Joyner-Halenda (BJH) method, the initial char pore-size distribution can be measured and calculated from its nitrogen adsorption results, as shown in Figure 2. It is clear that both the differential specific surface area and the differential pore volume decrease monotonously with the increase of the pore diameter. Figure 2 also presents the pore structure


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measured by the mercury porosimetry analysis. Note that because of the packing state of the char particles during measurement, a portion of the pores with a large diameter measured by mercury porosimetry represent the inter-particle pores. Both the differential specific surface area and the differential pore volume obtained from both measurements match perfectly near a pore diameter of 100 nm, which confirms the reliability of the results. However, clear drops in the differential specific surface area and differential pore volume from mercury porosimetry were observed for pore diameters smaller than 100 nm, which may indicate the collapse of pore surfaces under high pressure. Therefore, we combined the results of the nitrogen adsorption analysis for pore diameters smaller than 100 nm and the results of the mercury porosimetry analysis for pore diameters larger than 100 nm. Next, by integrating the combined results, the pore-size distribution of the cumulative specific surface area and cumulative pore volume of overall pores were obtained, as shown in Figure 3. ZD char consists of pores with a wide range of different sizes, specifically from approximately 2 nm to dozens of micrometres. The pores with diameters smaller than 1000 nm contribute to approximately 95% of the total specific surface area, but only contribute approximately 8.5% to the total pore volume, which indicates a negative factor against mass transfer. Hence, in order to obtain a reliable initial intraparticle porosity and an apparent density of ZD char for the numerical model, the Knudsen number is used to distinguish the intraparticle pores. The Knudsen number is given by λ (27) Kn = dp where dp is the pore diameter and λ is the mean free path of gas molecules of O2, which is given by kT (28) λ= B 2 2π dmP 10 9

1.0 Cumulative surface area Cumulative pore volume

0.9

8

0.8

7

0.7

6

0.6

5

0.5

4

0.4

3

0.3

2

0.2

1

0.1

0 100

101

102

103

104

105

Cumulative pore volume Vcum [cm3/g]

285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303

Cumulative surface area Acum [m2/g]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 24

0.0

308


309 310

Figure 3. Cumulative surface area and cumulative pore volume of initial ZD char.

311 312 313 314 315 316 317

If the diameters of pores satisfies Kn ≤ 0.1 (Fick diffusion or molecular diffusion38), the diffusion of O2 is primarily controlled by the collision of gas molecules, the same as the diffusing pattern in gaseous boundary layers. However, if the diameters of pores satisfies Kn > 0.1 (transition diffusion and Knudsen diffusion38), the diffusion of O2 is partly or mainly controlled by the collision between gas molecules and pore surfaces. Therefore, the pores with diameters satisfying Kn > 0.1 (dp < 4600 nm) are regarded as the intraparticle pores to calculate initial intraparticle porosity and apparent density of ZD char.


Page 11 of 24

318

4.2 Intrinsic reactivity of char-O2 and char-NO reaction

319 320 321 322 323

The isothermal TG experiments were performed at low temperatures (673, 723, 773, and 823 K); therefore, it is assumed that the char-O2 reaction is an isothermal kinetics-controlled reaction, of which all diffusion resistances are negligible. Based on RPM and the half-conversion reaction time method39, the intrinsic kinetics parameters of the ZD char-O2 reaction in isothermal TG were derived. The expressions of RPM are as follows:

324

dXC = (1− XC ) 1−ψ ln(1− X C ) dκ

(29)

325

  ψκ  XC = 1− exp −κ 1+  4   

(30)

326

where X is the burn-off ratio and κ is the dimensionless reaction time, which is given by

327

κ = k0t

328 329

where k0 is the initial reaction rate with respect to mass. Therefore, for the kinetics-controlled reaction, assuming the intrinsic reaction order is 1, we have

(31)

k0 = R1SV0 pO2

330

(32)

T [K] 850

800

750

700

-7

ln R1 [g/(m2·s·atm)]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Experimental data Linear fitted curve

-8 -9 -10 -11 -12

0.0012

0.0013

0.0014

0.0015

1/T [1/K]

331 332 333

650

Figure 4. Arrhenius plot of the ZD char-O2 reaction.

334

The integration of Equation 24 from XC = 0 to XC = 0.5 yields the following:

335

k0t0.5 =

336 337 338 339 340 341 342 343

(

)

2 1+ψ ln2 −1

ψ

(33)

Thus, R1 can be determined by solving Equations 32 and 33. Next, the Arrhenius plot of the intrinsic ZD char-O2 reaction were determined by linear fitting. Figure 4 shows the relationship between reaction temperature and the intrinsic reaction rate measured by TG. For T ≤ 773 K, there is an excellent linear correlation between ln R1 and 1/T. When the reaction temperature reaches 823 K, because of the mass transfer limitation, an evident drop of ln R1 is observed. Therefore, an Arrhenius plot of the char-O2 intrinsic reaction was obtained through a linear fitting of the points in Figure 4 for T ≤ 773 K, as shown in Equation 34. The activation energy of the ZD char-O2 reaction (E1) is 129.1 kJ/mol, which is close to that of


Energy & Fuels

344

bituminous-coal char (134 kJ/mol) reported in the literature17,40.

345

 129102  R1 = 185461exp −  R T  g  

(34)

346 347 348 349 350 351 352 353 354 355

By manipulating the experimental results of the char-NO reaction in the EFR, the kinetics parameters of the ZD char-NO reaction were obtained for the numerical model. Due to the low concentration of NO, the carbon conversion is very low such that the change of pore structure is neglected during char-NO reaction. According to a previous study41, the char-NO reaction is found to be first-order with respect to NO. The calculation method of intrinsic reactivity of char-NO reaction R4 can be found elsewhere42 by using the classical Thiele analysis. The Arrhenius plot of R4 is obtained by linear fitting as shown in Figure 5. In the reaction, temperature ranges from 1273 to 1673 K, and the Arrhenius expression of the intrinsic reaction of ZD char-NO reaction is shown in Equation 35. The activation energy of ZD char-NO reaction (E4) is 152.9 kJ/mol, which is close to that measured in the literature43 (147 kJ/mol):

356

 152869  R4 = 0.1588exp  −  R T  g  

(35)

T [K] 1800 1700 1600 -12

2

ln R4 [mol/(m ·s·Pa)]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 24

1500

1400

1300

Experimental data Linear fitted curve

-13 -14 -15 -16

-17 0.00055 0.00060 0.00065 0.00070 0.00075 0.00080

357 358 359

1/T [1/K]

Figure 5. Arrhenius plot of the ZD char-NO reaction.

360

4.3 Char conversion and changes in pore structure in EFR

361 362 363 364 365 366 367

Figure 6 shows carbon conversion along the EFR obtained by numerical modelling and experimental measurements during char combustion under high-temperature entrained flow conditions. The numerical results agree well with the experimental data for all cases. At high SR, the carbon conversion increases rapidly at the initial stage, and consequently, the rising trend slows due to the lower char-O2 reaction rates at high carbon conversion. At low SR, the carbon conversion increases more gently during the residence time due to the low char-O2 reaction rate under the low O2 concentration.


Page 13 of 24

XC

0.8

0.6

0.4

Experimental SR = 0.25 SR = 0.6 SR = 1 SR = 1.5 SR = 2.5 Numerical SR = 0.25 SR = 0.6 SR = 1 SR = 1.5 SR = 2.5

0.8

0.2

0.0

0.0

0.2

0.4

0.6

0.8

a)

0.0

1.0

t [s]

0.0

0.2

0.4

0.6

1.0

Figure 6. Experimental and numerical results of carbon conversion during char combustion in EFR: a) T = 1473 K and b) T = 1673 K.

0.6 0.4

XC = 0.1

0.8 0.6 0.4

0.2 0.0 0.0

0.2 0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

1.0

XC,k

0.8

0.0 1.0 1.0

XC = 0.2

XC = 0.4

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0 0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

1.0 0.8

0.0 1.0 1.0

XC = 0.6

XC = 0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0 0.0

XC,k

XC,k

1.0

SR = 0.25 SR = 0.6 SR = 1 SR = 1.5 SR = 2.5 Maximum SV,k

XC = 0.05

XC,k

1.0 0.8

372 373 374

0.8

t [s]

b)

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

XC,k

369 370 371

0.6

0.4

0.2

368

Experimental SR = 0.25 SR = 0.6 SR = 1 SR = 1.5 SR = 2.5 Numerical SR = 0.25 SR = 0.6 SR = 1 SR = 1.5 SR = 2.5

1.0

XC

1.0

XC,k

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

0.0 1.0

R/R0

R/R0

Figure 7. Local carbon conversion along the radius at different degrees of carbon conversion for T = 1673 K. (Numerical)


Energy & Fuels

1.0

1.0 SR = 0.25 SR = 0.6 SR = 1 SR = 1.5 SR = 2.5

XC = 0.05

θk

0.6

XC = 0.1

0.8 0.6

θk

0.8

0.4

0.4

0.2

0.2

0.0 0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

1.0 0.8

0.0 1.0 1.0

XC = 0.2

XC = 0.4

0.8

0.4

θk

0.6

0.4

θk

0.6

0.2

0.2

0.0 0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

1.0 0.8

0.0 1.0 1.0

XC = 0.6

XC = 0.8

0.8 0.6

0.4

0.4

0.2

0.2

θk

0.6

θk

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 24

0.0 0.0

375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

R/R0

0.6

0.8

0.0 1.0

R/R0

Figure 8. Local porosity along the radius at different degrees of carbon conversion for T = 1673 K. (Numerical)

As shown in Figure 7 and Figure 8, the local carbon conversion and the local porosity decreases as SR increases at a position deep inside the char particles, but increases as SR increases at a position close to the external surface of char particles. In other words, at a lower SR, the local carbon conversion and the local porosity appear to increase more uniformly throughout the char particle during combustion. In addition, at a lower SR, the higher local carbon conversion deep inside char particle leads to a higher local porosity and thus a stronger mass transfer, which in turn enhances O2 penetration to deeper inside the char particle. As shown in Figure 9, for a given carbon conversion, the ratio of local O2 concentration to ambient O2 concentration is higher throughout char particle at a lower SR, indicating that O2 tends to penetrate more deeply inside char particle. Additionally, for a given SR, the ratio of local O2 concentration to ambient O2 concentration increases with an increase in carbon conversion, indicating that O2 tends to penetrate more deeply as char combustion proceeds due to the growing porosity (Figure 8). To consider the change of local specific area with carbon consumption, the carbon conversion for maximum local specific surface area is also shown in Figure 7 by the dashed line at XC,k = 0.387, which is calculated from Equation 4. The local carbon conversion of lower SR is closer to the dashed lines at the position deep inside the char particle, indicating a larger local specific area deep inside the char particle.


Page 15 of 24

1.0

1.0 SR = 0.25 SR = 0.6 SR = 1 SR = 1.5 SR = 2.5

0.6

0.4

0.4

0.2

0.2

cO2,k/cO2,∞

XC = 0.05

XC = 0.2

0.0

0.0

1.0

1.0

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2 XC = 0.4

0.0 0.0

396 397 398

cO2,k/cO2,∞

0.6

0.8

cO2,k/cO2,∞

cO2,k/cO2,∞

0.8

0.2

0.4

0.6

0.8

XC = 0.8 1.0

0.0

0.2

R/R0

0.4

0.6

0.0 1.0

0.8

R/R0

Figure 9. Ratio of local O2 concentration to ambient O2 concentration along the radius direction at different degrees of carbon conversion for T = 1673 K. (Numerical)

399 3.0

3.0 SR = 0.25 SR = 0.6 SR = 1 SR = 1.5 SR = 2.5

1.0

XC = 0.05

SV,k/SV0

0.0 0.0

0.2

0.5

XC = 0.1 0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

0.0 1.0

3.0

3.0

2.5

2.5

2.0

2.0

1.5

1.5

1.0 0.5

1.0

XC = 0.2

0.0 0.0

0.5

XC = 0.4 0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

0.0 1.0

3.0

3.0

2.5

2.5

2.0

2.0

1.5

1.5

1.0 0.5

1.0

XC = 0.6

0.0 0.0

403 404 405 406 407 408 409 410

1.5

1.0 0.5

400 401 402

2.0

0.5

XC = 0.8 0.2

0.4

0.6

0.8

1.0

0.0

SV,k/SV0

1.5

2.5

SV,k/SV0

2.0

SV,k/SV0

SV,k/SV0

2.5

SV,k/SV0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

0.2

0.4

0.6

0.8

0.0 1.0

R/R0

R/R0

Figure 10. Ratio of local specific surface area of partial reacted char to the specific surface area of initial char along radius at different degrees of carbon conversion for T = 1673 K. (Numerical)

Specifically, Figure 10 shows the ratio of local specific surface area of partially reacted char to that of the initial char along the particle radius for different degrees of carbon conversion at a reaction temperature of 1673 K. The local specific surface area of lower SR is almost always larger throughout the char particle during combustion, except at the position close to the external surface of the char particle for fairly low carbon conversion (i.e., XC,k = 0.05). As a result, the total specific surface area increases as SR decreases for a given carbon conversion, leading to a more developed pore structure, especially at positions deep inside the char particle.


3.0

3.0

2.5

2.5

2.0

2.0

SV/SV0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

SV/SV0

Energy & Fuels

1.5 1.0 0.5

Kinetics controlled Numerical SR = 0.25 SR = 0.6 SR = 1 SR = 1.5 SR = 2.5

0.0 0.0

411

a)

0.2

1.5 1.0 0.5

0.4

0.6

XC

0.8

0.0 0.0

1.0

Page 16 of 24

Kinetics controlled Numerical Experimental SR = 0.25 SR = 0.25 SR = 0.6 SR = 1 SR = 1 SR = 1.5 SR = 2.5

0.2

b)

0.4

0.6

0.8

1.0

XC

412 413 414

Figure 11. Ratio of total specific surface area of partially reacted char to the specific surface area of initial char

415 416 417 418 419 420 421 422 423 424 425 426 427

Figure 11 shows the ratio of the total specific surface area of partially reacted char to that of initial char with respect to total carbon conversion under different SR, and in contrast, the dashed line represents the case of a kinetics-controlled reaction predicted by RPM (Equation 4). Compared to the experimental data (Figure 11b), the numerical prediction of specific surface area is reliable. Based on numerical profiles, for a given reaction temperature, the change in total specific surface area during char combustion approaches the case of a kinetics-controlled reaction as SR decreases. In addition, for a given SR, the change in total specific surface area is closer to the case of a kinetics-controlled reaction at a lower temperature. For the char-O2 reaction, a lower reaction temperature or a lower SR indicates lower chemical reaction rates of the char-O2 reaction, which pushes the kinetics-diffusion-controlled reaction toward the kinetics-controlled regime. Consequently, at a lower reaction temperature or a lower SR, O2 penetrates the char particle more deeply during combustion (Figure 9), so that the char-O2 reaction is able to occur more deeply inside the char particle, leading to a more developed pore structure.

428

4.4 Char-N conversion during char combustion

429 430 431 432 433 434 435 436 437 438 439

Figure 12 shows the experimental and numerical results of NO concentration during char combustion in residence time. The NO concentration predicted by numerical modelling matches the experimental results well. Based on the numerical profiles, the NO concentration always increases as reaction time increases at a reaction temperature of 1473 K for all cases. However, at 1673 K, the NO concentration increases first and then decreases during combustion under fuel-rich conditions (SR < 1). The formation and reduction of NO are competitive during char combustion, and therefore the increase in reaction temperature has a larger influence on the char-NO reduction reaction rate than the char-O2 reaction rate due to the higher activation energy associated with char-NO reduction. Consequently, the reduction of NO is dominant at 1673 K when the char-O2 reaction becomes slower as the ambient O2 concentration decreases during combustion, especially in fuel-rich conditions.

with respect to carbon conversion: a) T = 1473 K and b) T = 1673 K. (Numerical)


Page 17 of 24

100 80 60 40

120 Experimental SR = 0.25 SR = 0.6 SR = 1 SR = 1.5 SR = 2.5 Numerical SR = 0.25 SR = 0.6 SR = 1 SR = 1.5 SR = 2.5

NO concentration [ppm]


120

0

Experimental SR = 0.25 SR = 0.6 SR = 1 SR = 1.5 80 SR = 2.5 Numerical SR = 0.25 60 SR = 0.6 SR = 1 40 SR = 1.5 SR = 2.5

100

20

20

0.0

0.2

0.4

0.6

0.8

0

1.0

t [s]

0.0

0.2

0.4

0.6

0.8

1.0

t [s]

440

a)

441 442

Figure 12. Experimental and numerical results of NO concentrations during char combustion in EFR: a) T =

b)

1473 K and b) T = 1673 K.

443 1.00

1.00 Experimental Numerical SR = 0.25 SR = 0.25 SR = 0.6 SR = 0.6 SR = 1 SR = 1 SR = 1.5 SR = 1.5 SR = 2.5 SR = 2.5

0.95 0.90 0.85 0.80

Fractional conversion of char-N to NO


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

0.75 0.70 0.65 0.60 0.55 0.50 0.45 0.40 0.35 0.30 0.0

444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466

a)

Experimental Numerical SR = 0.25 SR = 0.25 SR = 0.6 SR = 0.6 SR = 1 SR = 1 SR = 1.5 SR = 1.5 SR = 2.5 SR = 2.5

0.95 0.90 0.85 0.80 0.75 0.70 0.65 0.60 0.55 0.50 0.45 0.40 0.35

0.2

0.4

0.6

t [s]

0.8

0.30 0.0

1.0

0.2

b)

0.4

0.6

0.8

1.0

t [s]

Figure 13. Experimental and numerical results of the fractional conversion of char-N to NO during char combustion in EFR: a) T = 1473 K and b) T = 1673 K.

Figure 13. shows the fractional conversion of char-N to NO obtained by numerical modelling and experimental measurements. Both the experimental and numerical results show that the fractional conversion of char-N to NO increases as SR increases for a given reaction time during char combustion, which is consistent with the results obtained by Song et al.9 and Nelson et al.10. The fractional conversion of char-N to NO rises first and then drops for all cases, indicating that the NO reduction becomes more dominant than the primary NO formation as char combustion proceeds. The fractional conversion of char-N to NO is influenced by two competing processes: the primary NO formation amount, depending on the oxidation rate of char-N, and NO reduction, depending on the reduction rate of NO. Figure 14 shows the primary NO formation rate, NO reduction rate, and NO net formation rate of a single char particle during combustion at different SR. At fuel rich conditions (SR = 0.25), the NO reduction rate is lower than the primary NO formation rate at the initial stage during char combustion, resulting in a positive net formation rate of NO. However, as char combustion proceeds, the NO reduction rate surpasses the primary NO formation rate due to the low O2 concentration, resulting in a negative NO net formation, which indicates that NO reduction becomes dominant. As SR increases, the ratio of the NO reduction rate to the primary NO formation rate decreases, indicating that primary NO formation is dominant. Furthermore, at SR = 1 or 1.5, the NO net formation rate is always positive almost for the entire residence time. As a result, the fractional conversion of char-N to NO increases as SR


Energy & Fuels

467

increases.

NO formation or reduction rate [×10-12 kg/s]

2.5 2.0 1.5 1.0 0.5 0.0 -0.5

SR = 0.25

0.0

0.4

0.6

0.8

1.0

0.6

0.8

1.0

0.6

0.8

1.0

SR = 1

10 5 0 -5 0.0

0.2

0.4

25 20 15 10 5 0 -5

SR = 1.5

0.0

468 469 470

0.2

Primary NO formation rate NO reduction rate NO net formation rate

15

0.2

0.4

t [s] Figure 14. Primary NO formation rate, NO reduction rate, and NO net formation rate of single char particle during combustion at T = 1673 K. (Numerical)

471 Amount of NO formation or reduction [×10-13kg]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 24

472 473 474 475 476 477 478 479 480 481 482 483 484 485 486

50 Amount of primary NO fomation Amount of NO reduction Amount of net NO formation

45 40 35 30 25 20 15 10 5 0 0.0

0.5

1.0

1.5

2.0

2.5

SR

Figure 15. Amount of primary NO formation, NO reduction, and net NO formation of a single char particle during a reaction time of 0.89 s at T = 1673 K. (Numerical)

By integrating the results that are shown in Figure 14 over the entire residence time of 0.89 s in EFR at T = 1673 K, Figure 15 shows the amount of primary NO formation, NO reduction, and net NO formation of a single char particle for different SR. During the residence time, the amount of primary NO formation depends on carbon conversion, such that it first increases as SR increases and later levels off at high SR, where carbon almost burns out. Under fuel-rich conditions, the amount of NO reduction increases as SR increases, which might be affected by the higher NO concentration inside the char particle due to the higher amount of primary NO formation. However, the slope of primary NO formation is larger than that of NO reduction, resulting in an increasing net NO formation. However, under fuel-lean conditions, the amount of NO reduction decreases gently as NO formation increases, even if the amount of primary NO formation increases slightly. As a result, the amount of net NO formation increases gently under


Page 19 of 24

487 488 489 490 491 492 493 494 495 496 497 498 499 500 501

fuel-lean conditions. Therefore, there must be other factors that affect NO reduction, except for the NO concentration inside char particle, which is discussed below. Moreover, the dependence of the fractional conversion of char-N to NO on carbon conversion obtained by numerical modelling is shown in Figure 16. For a given carbon conversion, the fractional conversion of char-N to NO increases as SR increases. An obvious drop is found under fuel-rich conditions at 1673 K, which is caused by the dominant effect of NO reduction due to the nearly zero O2 concentration at the final stage of fuel-rich combustion (Figure 14). In the numerical model, both the heterogeneous char-NO reaction and homogeneous NO-CO reaction were considered inside char particles during combustion. Based on the numerical results, at low SR, almost all of the NO reduction was contributed to by the char-NO reaction. At a higher SR or a higher reaction temperature, the NO reduction contribution of NO-CO reaction becomes higher, which might be caused by a higher CO concentration inside the char particles due to a higher char-O2 reaction rate inside. However, the NO reduction contribution of the NO-CO reaction is lower than 10%, even at SR = 2.5 for T = 1673 K. Therefore, the char-NO reaction is the main source of NO reduction in the studied SR range. 0.60


0.70


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

0.65

0.60

0.55

SR = 0.25 SR = 0.6 SR = 1 SR = 1.5 SR = 2.5

0.50

0.45 0.0

0.2

0.4

0.6

XC

0.8

0.55 0.50 0.45 0.40

0.30 0.25 0.0

1.0

SR = 0.25 SR = 0.6 SR = 1 SR = 1.5 SR = 2.5

0.35

0.2

0.4

0.6

0.8

1.0

XC

502 503 504 505

a)

506 507 508 509 510 511 512 513 514 515 516 517 518

Figure 17 shows the distribution of NO concentration along the particle radius direction during char combustion at a reaction temperature of 1673 K. The NO concentration first increases and then decreases along the radius direction for all cases. The oxidation of char-N continuously produces NO inside the char particle during char combustion, generating a high NO concentration area inside the char particle. According to the mass transfer equation, NO can be transferred from the high NO concentration area to the low NO concentration area. For a kinetics-diffusion controlled char-O2 reaction, two areas of low NO concentration exist: the ambient gas flow and the centre area of char particle where char-O2 reaction is weak (Figure 7). Therefore, a peak value of NO concentration occurs inside the char particle as shown in Figure 17. For a given carbon conversion, because O2 penetrates more deeply inside the char particle at a lower SR, the peak is closer to the centre of the char particle. As char combustion proceeds, the char-O2 reaction occurs deeper inside the char particles, pushing the peak toward the centres of the char particles.

b)

Figure 16. Dependence of the fractional conversion of char-N to NO on total carbon conversion during char combustion in EFR: a) T = 1473 K and b) T = 1673 K. (Numerical)


Energy & Fuels

800

500 400 300

XC = 0.2

700 600

SR = 0.25 SR = 0.6 SR = 1 SR = 1.5 SR = 2.5

500 400 300

200

200

100

100

0 0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

800 700

0 1.0 800

XC = 0.4

XC = 0.8

700

600

600

500

500

400

400

300

300

200

200

100

100

0 0.0


600

800

XC = 0.05

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8



700


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0 1.0

R/R0

R/R0

519 520 521

Figure 17. Distribution of NO concentration along the radius during char combustion at T = 1673 K. (Numerical)

522 523

Figure 18 shows the local cumulative NO reduced by the char-NO reaction of a single char particle at different degrees of carbon conversion, which is expressed below:

524

Ck ( XC ) = ∫

t ( XC )

0

525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544

R4,k SV,kVk pNO,k MNOdt

(31)

The curve first climbs and then drops in the radius direction due to the unimodal NO concentration distribution, as shown in Figure 17. At a lower SR, the peak value of local cumulative NO reduced by the char-NO reaction is closer to the particle centre for a given carbon conversion, which is also affected by the NO concentration distribution. A higher porosity represents a higher diffusion coefficient, which is in favour of the penetration of NO and vice versa. As shown in Figure 8, at a lower SR, the local porosity is higher at the position close to particle centre but is lower at the position close to the external particle surface. Therefore, the NO that forms inside the char particle is easier to diffuse towards the particle centre but is more difficult to diffuse towards the particle external surface to be released, which favours the reduction of NO inside the char particle. In addition, at a lower SR, the larger specific area (Figures 10 and 11) accelerates the overall char-NO reaction rate. All of these give rise to the cumulative local NO reduced by the char-NO reaction deeper inside the char particle at a lower SR, as shown in Figure 18. As a result, for a given carbon conversion, the total NO reduced by the char-NO reaction decreases as SR increases, therefore the fractional conversion of char-N to NO increases as SR increases (Figure 16). Moreover, under fuel-lean conditions, carbon almost burns out at T = 1673 K during the residence time of 0.89 s; therefore at a lower SR, the total NO reduced by the char-NO reaction is higher, even if the NO concentration inside the char particle is lower. Hence, during the residence time, the amount of NO reduction decreases as SR increases in a fuel-lean region (Figure 15). Thus, the fractional conversion of char-N to NO increases as SR increases (as shown in Figures 13 and 16).


545 546 547 548

NO reduced by char-NO reaction [10-14 kg]

1.8 1.6 1.4 1.2

SR = 0.25 SR = 0.6 SR = 1 SR = 1.5 SR = 2.5

1.0 0.8 0.6 0.4

XC=0.05

0.0 0.0

0.2

0.4

0.6

0.8

1.0


0.2


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Energy & Fuels


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8

6

4

2

XC=0.4 0 0.0

0.2

0.4

0.6

0.8

1.0

5

4

3

2

1

XC=0.2 0 0.0

0.2

0.4

0.2

0.4

0.6

0.8

1.0

0.6

0.8

1.0

12 10 8 6 4 2

XC=0.8 0 0.0

R/R0

R/R0

Figure 18. Local cumulative NO reduced by the char-NO reaction of a single char particle with respect to radius during char combustion at T = 1673 K. (Numerical)

549 550 551 552 553 554 555 556 557 558 559 560

In addition, the distribution of local cumulative NO reduced by the char-NO reaction also changes with the degree of carbon conversion. For low carbon conversion, char-O2 reaction mainly occurs close to the external surface of the char particle (Figure 7) and the porosity near the char centre is low (Figure 8). Therefore, the oxidation product, NO, is apt to release out of the char particle and be reduced closer to the external surface of the char particle as shown in Figure 18 for XC = 0.05. As char combustion proceeds, the char-O2 reaction occurs deeper inside the char particle, generating a higher porosity near the char centre. Therefore, NO is apt to penetrate towards the particle centre and be reduced deeper inside the char particle, as shown in Figure 18, which gives rise to NO reduction. Therefore, the NO reduction rate approaches the primary NO formation rate, and even surpasses it as char combustion proceeds as shown in Figure 14. As a result, the fractional conversion of char-N to NO tends to decrease as char combustion proceeds as shown in Figures 13 and 16.

561

Conclusions

562 563 564 565 566 567

A model of the char-O2 reaction under high-temperature entrained flow conditions was established based on the unsteady convection-diffusion and reaction equations. The carbon conversion and NO release predicted by numerical modelling match experimental data well. Conclusions are drawn as follows: (1) As SR decreased, the kinetics-diffusion controlled char-O2 reaction was pushed towards a kinetics-controlled regime due to a decrease of the char oxidation rate. For a given carbon conversion, O2 was able to penetrate and react more deeply inside the char particle at


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

568 569 570 571 572 573 574 575 576 577

lower SR, resulting in a larger specific surface area and higher porosity deep inside the char particle but a lower porosity close to the external surface of the char particle. (2) For a given reaction time or a given carbon conversion, the fractional conversion of char-N to NO increased as SR increased. (3) For a given carbon conversion, at a lower SR, the NO formed inside the char particle prefers to penetrate towards the char particle centre rather than be released outward, which enhances the reduction of NO, especially at positions deep inside the char particle. The larger specific surface area also accelerated the overall char-NO reaction rate. (4) For a given SR, NO is apt to penetrate towards the particle centre and be reduced deeper inside the char particle as char combustion proceeds.

η A Adif Acum BNO c cg De dm dp E F kB Kn m M Mg Ng p P R rf Rg rp SM SV T t t0.5 Tp Vk Vcum Vdif

Nomenclature mole ratio of CO to CO2 pre-exponential factor differential specific surface area [m2/(g·nm)] cumulative surface area [m2/g] fractional conversion of char-N to NO mole concentration [mol/m3] mole concentration of gas mixture [mol/m3] effectiveness diffusion coefficient [m2/s] effective molecular diameter [m] pore diameter [m or nm] activation energy [J/mol] mass fraction of element in char Boltzmann constant [J/K] Knudsen number mass fraction in gas mixture molar mass [kg/mol] molar mass of gas mixture [kg/mol] molar flux of gas mixture [mol/(m2·s)] partial pressure [atm] total pressure [atm] char particle radius [m] surface roughness factor gas constant [J/(mol·K)] pore radius [m] specific surface area based on mass [m2/g] specific surface area based on volume [m2/m3] reaction temperature [K] reaction time [s] reaction time of XC = 0.5 [s] particle temperature [K] volume (of each element in char) cumulative pore volume [cm3/g] differential pore volume [cm3/(g·nm)]


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Energy & Fuels

x XC Y θ λ ρA ρg ρT τ ψ Subscripts: 0 ∞ C CO CO2 gaseous i intraparticle k N N2 NO O2

mole fraction in gas mixture carbon conversion volume fraction of gas in gas mixture porosity mean free path of molecules [m] apparent density of char [kg/m3] density of gas mixture [kg/m3] true density of char [kg/m3] tortuosity structural parameter initial condition ambient flow carbon element carbon monoxide carbon dioxide gaseous boundary layer region gaseous component intraparticle region local nitrogen element nitrogen nitrogen monoxide oxygen

578 579

Acknowledgements

580 581

Financial support from the National Natural Science Foundation of China (Grant Nos. 51476046 and 51536002) is gratefully acknowledged.

582

Reference

583 584 585 586 587 588 589 590 591 592 593

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Effect of Stoichiometric Ratio on Char-Nitrogen Conversion under

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