Investigation on Sodium Fate for High Alkali Coal during Circulating

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Investigation on sodium fate for high alkali coal during circulating fluidized bed combustion Jieqiang Ji, Leming Cheng, Yanquan Liu, Yangjun Wei, and Li Nie Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b03812 • Publication Date (Web): 09 Jan 2019 Downloaded from http://pubs.acs.org on January 9, 2019

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Energy & Fuels

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Investigation on sodium fate for high alkali coal during

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circulating fluidized bed combustion

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Jieqiang Ji1, Leming Cheng1*, Yanquan Liu1, Yangjun Wei1, Li Nie1,2

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1.State Key Laboratory of Clean Energy Utilization, Institute of Thermal Power

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Engineering, Zhejiang University, Hangzhou 310027, PR China

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2.Dongfang Boiler Group Co. Ltd., Zigong 643001, PR China

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Abstract

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For better understanding the release and transformation mechanisms of sodium

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compounds during high alkali coal combustion in circulating fluidized bed (CFB)

11

boilers, a sodium migration model of coal combustion was proposed in this work. The

12

model included sub-models of sodium species release, chemical reactions, vapor

13

condensation, particles deposition and shedding. They were coupled in a 3D CFB

14

comprehensive combustion model to predict the sodium fate. Example simulation was

15

conducted on a 30 kW CFB test rig where experimental measurement has been done.

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The distributions of sodium compounds in gas, ash and deposits were computed during

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the high alkali Zhundong coal combustion. Results show that good agreements are

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achieved between the simulations and the experiments on the hydrodynamics,

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temperature, gas species and sodium distributions.

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Keywords: Sodium fate; High alkali coal; Simulation; Circulating fluidized bed

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1. Introduction

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High contents of alkali and alkaline earth metal (AAEM) elements in coal may 1

ACS Paragon Plus Environment

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induce a series of problems on heat transfer surfaces during coal combustion like

24

fouling, deposition and slagging. A typical example is the utilization of Zhundong (ZD)

25

coal in Xinjiang, China.

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Concluded from previous researches, the ash problems are primarily caused by

27

two reasons1: (1) low-temperature eutectics containing AAEM are formed at high-

28

temperature region and are easy to be adhered on the heating surface, which cause the

29

slagging formation; (2) re-condensing of evaporated alkali metals at lower gas

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temperature in the convection heating surface will adsorb fly ash particles, which cause

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the fouling formation.

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Ash deposition and alkali metals transformation during pulverized coal (PC)

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combustion have been studied by many research groups2-6. In Wang et al.’s2, Wang et

34

al.’s3 and Wei et al.’s4 work, the slag and deposit samples were collected from various

35

heating surfaces in full-scale pulverized coal furnaces. By analyzing these samples,

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sodium distributions at different positions were obtained. Zhou et al.5 presented the

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chemical compositions of deposits on different probes in a 300 kW furnace, and pointed

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out that contents of Na and K declined with the deposit growth direction. Neville6 gave

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the relationship between particle size and vaporization/condensation of sodium for

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pulverized coal combustion. Generally, temperature, particle size and ash composition

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are main factors affecting the sodium behavior and ash deposition2.

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However, the transformation pathways of sodium in PC boilers may be different

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from those in circulating fluidized bed (CFB) boilers. This can be due to the reasons:

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(1) larger coal particle sizes in CFB boilers than those in PC boilers; (2) higher solid 2


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particles suspension density in CFB boilers than those in PC boilers; (3) particles falling

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down along the waterwall in CFB boilers; (4) lower furnace temperatures in CFB

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boilers than that in PC boilers. For the safe utilization of the high alkali coal in CFB

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boilers, the ash related problems and alkali metals transformation during CFB

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combustion should be investigated.

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Several researches studied the deposition issue of ZD coal during CFB

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gasification/combustion process by Song’s group7-10. Behavior of sodium species was

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regarded closely associated with ash problems. Song et al.8 pointed out that Na and Cl

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were highly volatile at high temperature and would condense on the heating surface to

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form an initial sticky layer. In his another work9, he reported that sodium contents in

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the bottom ash and fly ash were higher than that in the ash composition analysis, which

56

meant that more sodium retained in the ash particles due to the condensation or

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chemical reactions. Qi et al.7 did experimental research with ZD coal and found that

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Na-rich particles from fly ash may deposit on the probe. Those results gave some

59

information on the sodium distributions in ash and deposits. However, the

60

transformation mechanisms of sodium species between gas phase and solid phase in

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CFB boilers are still not clear.

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Investigating the fate of sodium can reveal its migration rule and understand the

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effect of sodium behavior on deposition. Researches about the AAEM elements

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behavior during pyrolysis process of a Victorian brown coal were carried out by Li et

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al.’s group11-15. They studied the fates and roles of AAEM13, as well as the effects of

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heating rate, elevated pressure14, CO2 gasification15 on the volatilization of AAEM. 3


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Hodges et al. investigated the sodium fate during the combustion of three UK

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bituminous coals in a laboratory-scale fluidized bed, and gave the content distributions

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of sodium in volatile components, retained in bed sand, associated with fly ash, or in

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the form of deposits16. Manzoori et al. reported the sodium fate of low rank coals in

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fluidized bed combustion and found the formation of a molten ash layer on the char

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surface17. However, the sodium fate of ZD coal in a CFB combustion system is absent

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in literature. Hence, it’s necessary to deeply study the sodium distributions (in gas, ash,

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deposits) and migration mechanisms for ZD coal in CFB boilers.

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Theory and numerical simulation may process the mechanism analysis to study

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the ash-related issues. Researchers mainly focused on the development of single

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deposition model at early stage. For slagging modelling, the sticking probability

78

methods were based on the ash softening temperature, ash melting degree or ash

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viscosity18. For fouling modelling, the sticking probability methods were determined

80

by forces balance (elastic rebound force, Van der Waals adhesion force and gravity)19,

81

or the energy balance (interfacial energy and kinetic energy)20. Recently, the deposition

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simulation was improved by combining single deposition model with other sub-models.

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Leppanen et al.21 proposed a deposit growth model including the thermophoresis,

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diffusion and condensation process. Yang et al.22 considered the inertial impaction,

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thermophoresis and condensation in his deposition simulation. In Hansen’s work23,

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models including diffusion, condensation, thermophoresis, impaction and reaction were

87

considered to simulate the ash deposition. However, the alkali metal release process

88

and gaseous reactions were treated simply by using thermodynamics software 4


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Factsage21, or using empirical formula23 in those literature. Additionally, the effect of

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condensation sodium species on the sticking probability was not considered and

91

coupled in those work. The existing models have difficulty to simulate the sodium fate.

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In this work, a detailed sodium migration model including the sodium species

93

release, chemical reactions, vapor condensation, particles deposition and shedding was

94

developed and coupled with a 3D comprehensive CFB combustion model. It was

95

applied to a 30 kW CFB test rig to investigate the sodium fate for ZD coal. Predicted

96

results, such as hydrodynamics, temperature, gas species and sodium distributions,

97

were compared with the measurement data from the 30 kW CFB test rig.

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2. Model description

99

This work focused on the sodium fate during the ZD coal combustion. Since the

100

sodium behavior was related with the ZD coal combustion process, the hydrodynamics

101

and combustion in a CFB furnace should be obtained first. In this work, they were

102

calculated based on the 3D comprehensive CFB combustion model24.

103

A sodium migration model was developed and coupled with the 3D

104

comprehensive combustion model, including sodium species release, homogeneous and

105

heterogeneous reactions, vapor condensation, particles deposition (inertial impaction

106

and thermophoresis) and shedding. Details of these models are shown in the following

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parts.

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2.1 Hydrodynamics and combustion model

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The developed 3D comprehensive CFB combustion model is the fundamentals of

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the simulation. It incorporates gas–solid hydrodynamics, coal combustion, heat transfer 5


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on heat exchange surfaces in the furnace, and heat transfer between furnace and

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working medium in the heat transfer tubes. Distributions of solid concentration, gas

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species, heat flux, and working medium temperature in a CFB boiler furnace could be

114

obtained. Detail information can be found in our previous work24.

115

2.2 Sodium species release

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The existence forms of sodium in the raw coal have impact on the release behavior

117

of sodium species. Generally, sodium compounds in coal were classified into four

118

portions, including water-soluble (H2O-soluble), ammonium acetate-soluble (NH4Ac-

119

soluble), hydrochloric acid-soluble (HCl-soluble) and insoluble25-27. Our previous work

120

indicated that contents of different sodium forms in the ZD coal were 72.1% H2O-

121

soluble, 13.3% NH4Ac-soluble, 2.8% HCl-soluble and 11.8% insoluble, respectively25.

122

At 900 oC, 26% of the H2O-soluble and 7% of the NH4Ac-soluble sodium were released

123

to the gas phase, and 20% of the H2O-soluble sodium was transformed into HCl-soluble,

124

while content of the insoluble sodium kept constant as the temperature increased25.

125

The H2O-soluble sodium was released from the coal particle during the

126

devolatilization stage, and one possible mechanism was the direct vaporization of NaCl

127

from the coal particle28. The acid-soluble sodium (NH4Ac-soluble and HCl-soluble)

128

was considered to release in the form of Na during the char combustion process28, 29.

129

Therefore, it’s assumed that 26% of the H2O-soluble sodium is released in the

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form of NaCl during devolatilization stage and 7% of the NH4Ac-soluble sodium is

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released in the form of Na during char combustion stage, as shown in equations (1~3).

132

Mass fractions of the volatile components in equation (1) are determined by Loison & 6


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Chauvin’s formula and adjusted by the mass/energy balance method24. Mass fractions

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of the N and S compounds are calculated based on our previous work24, 30.

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Volatile  0.8094CH 4  16.495H 2  0.05CO2  0.1CO  14.259 H 2 O  0.034Tar 0.0238H 2 S  0.0045 NH 3  0.0402 HCN  0.0235NaCl  0.0124HCl

(1)

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CN 0.00371 S0.00198 Na0.00015  0.50384O2  CO  0.00371NO  0.00198SO2  0.00015Na

(2)

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CN 0.00371 S0.00198 Na0.00015  1.00384O2  CO2  0.00371NO  0.00198SO2  0.00015Na

(3)

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2.3 Sodium species reactions

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Released from the coal particles, the vaporized sodium will participate in

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homogeneous and heterogeneous reactions. In this work, only NaCl and Na2SO4 are

141

considered as the stable gaseous sodium species during combustion, since other sodium

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compounds were not observed in Oleschko’s on-line measurement experiments31, 32.

143

For the homogeneous reactions, Glarborg and Marshall33, 34 presented a detailed

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mechanism (115 species, 1342 reactions) of alkali metal reactions involving

145

C/H/O/N/S/Cl/K/Na. However, it is difficult to couple all the reactions to the

146

combustion model due to the huge computation time. Based on our previous sensitivity

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analysis35 on the influences of varied reactions on NaCl/Na2SO4 formations, it is

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concluded that the reactions (R1-1) ~ (R1-5) in Table 1 are essential for the sodium

149

species migration during the operating temperature for CFB boilers. Additional, the

150

transformation reaction between NaCl(g) and Na2SO4(g) is also taken into

151

consideration36, as shown in (R1-6).

152

Regarding the heterogeneous reactions, the absorption of alkali vapor by

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aluminium silicate clay is considered in this work37. Punjak, Uberoi and Shadman38-40

154

investigated the interactions of alkali-metal vapors with various solid sorbents, 7


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including bauxite, kaolinite, emathlite, and developed a mathematical model to describe

156

the absorption process. Based on Punjak et al., Uberoi et al. and Mann et al.’s work,

157

Niksa29 presented a simplified overall reaction (R1-7). This reaction is applied in this

158

work.

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160 161 162

Table 1. Coefficients of sodium species reactions a

a.

No.

Reaction

A

n

E

R1-1

Na  HCl  NaCl  H

2.41e8

0

41800

R1-2

Na  O2  NaO2

1.38e6

-2.05

1713

R1-3

NaO2  HCl  NaCl  HO2

1.39e8

0

0

R1-4

Na  SO2  NaSO2

9.25e6

0

0

R1-5

NaSO2  NaO2  Na2 SO4

1.0e8

0

0

R1-6

2 NaCl  SO2  H 2 O  0.5O2  Na2 SO4  2 HCl

5.0e8

0

10100

R1-7

Six Al y Oz   Na( g )  Six Al y Oz  Na

7.2e9

0

63000

A, n and E refer to pre-exponential reaction constant, temperature exponent and activation energy for the Arrhenius formula k  A  T n  exp(

E ). Rg T

2.4 Condensation

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In a combustion chamber, alkali vapors will be supersaturated due to the flue gas

164

cooling or some chemical reactions. This will further induce the condensation process.

165

The formation of a sticky layer, coming from condensed vapors, is expected to be

166

crucial for particles deposition in the early stage41.

167

Normally, three types of condensation were proposed42: (1) directly condensing

168

on the heating exchanging surface - called as direct condensation, (2) condensing on 8


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the particle surface - called as heterogeneous condensation, (3) form new aerosol

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particles - called as nucleation. In this work, the nucleation condensation is neglected

171

due to the high furnace temperature22.

172

For the direct condensation, the vapor condensation mass flux is depending on

173

saturation pressure and determined by equation (4~6)21,

174

pressure of the condensing species in equation (4) may be calculated via equation (7)42.

175

Coefficients A0, B0, C0 for varied sodium species are obtained from Kleinhans’s work42. Chet  Sh 

176

( Di (Tg )  Di (Ts ))0.5 Dh  Rg

[

pi (Tg ) Tg



22.

pi , s (Ts ) Ts

The saturation vapor

]

(4)

177

Sh  0.023Re0.8  Sc 0.4

(5)

178

Sc   g / (  g  Di (Tg ))

(6)

179

where Chet is the heterogeneous condensation rate, Di(T) is the diffusion coefficient for

180

gas either at the flue gas temperature(Tg) or deposit surface temperature (Ts), pi(Tg) is

181

the partial pressure of the vapor, pi,s(Ts) is the saturation vapor pressure at the deposit

182

surface temperature.

183

log pi , s (Ts )  A0 (10000 / Ts  B0 )  C0

(7)

184

For the heterogeneous condensation, the condensation mass flux is determined by

185

equation (8~9)43. It is also dependent on the partial pressure of the condensing vapors.

186

187

Chom 

Fi 

Fi  MWi  6 i  d p 3  

2    Di (Tg )  d p  ( pi (Tg )  pi , s (Tp )) k B  Tg

(8)

(9)

188

where Chom is the homogeneous condensation rate, MWi is the molecular weight of the

189

condensing compound, ρi is the density of the condensing compound, Tp is the particle 9


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temperature, kB is the Boltzmann constant.

191

The deposit surface temperature Ts can be calculated as follows:

192

Ts 

Ldep



 qh  Tw

(10)

193

where Ldep and qh are deposit thickness and heat flux, respectively, obtained from the

194

CFD calculations. λ is the thermal conductivity of deposit, Tw is the probe surface

195

temperature.

196

2.5 Deposition

197

Sodium in solid particles will be transformed to deposits through the deposition

198

process. Several mechanisms contributed to the build-up of deposition on the heating

199

surface23, 44, including condensation (C), inertial impaction (I), thermophoresis (TH),

200

Brownian diffusion (BD) and eddy diffusion (ED), as shown in equation (11). For these

201

mechanisms, condensation contributes to the initial sticky layer and the calculation

202

method have been discussed in section 2.4. Larger particles (dp > 10 μm) are primarily

203

affected by inertial impaction, while thermophoresis is significant for particles smaller

204

than 1 μm. For Brownian and eddy diffusion, particles of sizes less than 0.1 μm are

205

influenced.

206

dm(t , )  C (t , )  TH (t , )  I (t , )  BD(t , )  ED(t , ) dt

(11)

207

According to the size distribution analysis of fly ash from our experiment (in

208

Section 4, Figure 4), particles smaller than 0.1 μm are negligible. Hence, Brownian and

209

eddy diffusion are not considered in this model.

210 211

Mechanisms of inertial impaction and thermophoresis will be discussed as follows. (1) Inertial impaction 10


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The deposition rate of particles by inertial impaction is expressed by the total

213

particle mass flux, the impact efficiency and sticking probability, as shown in equation

214

(12). A general expression (equation (13)) determining the impact efficiency is

215

presented in Huang’s work44, which is a function of Stokes number St. Here, St

216

represents the ratio of inertial and drag forces44.

217

I  qim   

218

(12)

  [1  b  ( St  a)1  c  ( St  a)2  d  ( St  a)3 ]1 , St  0.14

219

  0, St  0.14

(13)

220

St 

 p  d p2  u p 9   g  Dh

(14)

221

where I is the inertial impaction deposition rate, qim is the particle flux by impaction, η

222

is the impact efficiency, ξ is the sticking probability, a, b, c and d are modified

223

coefficients44.

224

The sticking probability depends on two factors: the impacted particles’ properties

225

and thickness of the sticky layer on a particle or heating surface. After impaction, if the

226

surface-particle interfacial energy is greater than kinetic energy of the reflected particle,

227

the particle will be captured. Lee20 established a series of expressions to describe the

228

sticking probability (equation (15-17)).

229



2    Ac 0.5  m p  (u p r ) 2

(15)

230

0.5  m p  (u p i )2  0.5  m p  (u p r / ev )2  DE

(16)

231

DE  i  (u p i )2  Ac    sin 2   (1   f / tan  )

(17)

232

where γ is the surface tension, upr is the velocity of reflected particle, upi is the velocity 11


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of impact particle, ev is the coefficient of restitution for velocity, DE is the kinetic

234

energy loss, ρc is the density of the condensed compound, δ is the thickness of the sticky

235

layer, μf is the friction coefficient between surface and particle, β is the impact angle.

236

(2) Thermophoresis

237

Thermophoresis is a phenomenon by which particles transport due to the

238

temperature gradients. The deposition rate by thermophoresis could be obtained from

239

equation (18). It’s assumed that all particles transformed to the surface will collide on

240

the surface23. The sticking probability ξ is determined by equation (15) as well.

241

However, in equation (18), the particle mass flux qth needs to be modified, since the

242

incident velocity uth is changed with the thermophoresis effect. In this work, uth is

243

considered by Fick’s law for fine particles by thermophoresis43, as shown in equation

244

(19-20).

245

TH  qth  

(18)

246

qth   p  uth  Ac

(19)

247

uth 

  K th

(20)

Tp

Tgas

248

where TH is the thermophoresis deposition rate, qth is the particle flux by

249

thermophoresis, Kth is the thermophoretic coefficient and is determined by Talbot45,

250

Tg is temperature gradient prevailing in the gas phase.

251

2.6 Shedding

252

For the presence of large amount of bed material inside a CFB boiler, the deposited

253

particles have probability to fall off from the heating surface. The shedding process is

254

mainly related to the forces acting on the deposited particles. 12


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Four forces are considered in this simulation, including drag force Fd, gravity G,

256

adhesion force Fa and contact force Fn. When the four forces reach equilibrium, it’s

257

called the critical remove velocity for the incident particles. As the incident velocity

258

exceeds the critical remove velocity, the forces equilibrium can’t be maintained and the

259

deposited particles will fall off. The adhesion force and contact force are calculated by

260

equation (21-22)46.

261

Fa  1.5    Rc  

262

Fn 

4  EY  R p 0.5   1.5 3

(21)

(22)

263

where Fa is the adhesion force, Fn is the contact force, Rc is the curvature radius, Γ is

264

the surface energy between the particles, EY is the Youngs modulus, τ is the

265

interpenetration distance46.

266 267

In general, the fate of sodium in a CFB boiler during combustion may be summarized shown in Figure 1.

268 269 270

Figure 1. Sodium fate in CFB combustion

3. Model setup

271

A finite volume method is applied to the mathematical models’ computation. All

272

convective terms are solved by first-order upwind discretization scheme. The algorithm 13


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273

of phase coupled SIMPLE is selected for pressure-velocity coupling. The

274

heterogeneous reaction rates, as well as the condensation and deposition flux are coded

275

by the User-Defined Functions (UDFs) and coupled with FLUENT. Approximately

276

1410000 grid cells are contained in the meshes.

277

4. CFB test rig and the experimental measurement

278

The computing object of the simulation is a 30 kW hot CFB test rig47. Figure 2

279

shows the schematic diagram and mesh of the CFB test rig. The cylindrical furnace is

280

4.2 m high with an inner dimeter of 0.13 m. A coal inlet is positioned at the height of

281

0.43 m. The exiting solid materials from the outlets return to the furnace through a UDF

282

program. Six secondary air inlets (S1~S6) are located along the height. The lower three

283

inlets (H(S1) = 1.02 m, H(S2) = 1.19 m, H(S3) = 1.35 m) are used during the experiment.

284

The ash deposition probe has an outer diameter of 0.03 m and a length of 0.4 m, as

285

shown in Figure 3. It is inserted into the furnace through the port in S4 (H = 2.12 m)

286

and fixed by flanges. Its insert length is 0.2 m. The probe is made of jacket tubes of

287

stainless steel. Its surface temperature is controlled by air flow inside the tube and

288

measured by a thermocouple installed inside the probe.

14


Page 14 of 34

Page 15 of 34 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

289 290

Figure 2. Geometry and mesh of the furnace

291 292

Figure 3. Schematic structure of the probe

293

In the tests, the gas temperature and pressure were measured by thermocouples

294

(T1~T9) and pressure taps (P1~P2) located along the furnace, respectively. The

295

measuring points are marked in Figure 2. Flue gas were sampled at the furnace exit and

296

analyzed. Fly ash was collected by a bag filter47. Its mass flow was determined based

297

on the weight increment of the bag filter. Deposits were sampled on the probe surface

298

after the experiments. The compositions of fly ash and deposits were determined by the

299

X-ray Fluorescence (XRF). A Malvern Mastersizer 2000 size analyzer was used to

300

obtain the particle size distributions of fly ash. The ash samples were collected when

301

the probe was not inserted into the furnace. Result was shown in Figure 4.

15


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

Page 16 of 34

302 303

Figure 4. Ash particle distribution

304

The boundary conditions used are given in Table 2. Primary and secondary air

305

velocities are determined by the total air flow and SA/(PA+SA) ratio. The total air flow

306

is calculated by the given superficial gas velocity, while SA/(PA+SA) ratio is set as 0.3

307

to reduce the NOx emission. The probe surface temperature is selected as 700 K

308

according to the typical heating surface temperature in larger scale CFB boilers48, 49.

309

The ambient temperature is set as 300 K. Insulated cotton with a thickness of 0.1 m is

310

set as the object of the furnace wall according to the test rig.

311 312 313

Table 3 and Table 4 give the proximate, ultimate and ash composition analysis of the combusted ZD coal, respectively. Table 2. Boundary conditions in the simulation Parameter

Unit

Value

Superficial gas velocity

m/s

4.0

Coal feed rate

kg/s

0.00291

Primary air velocity

m/s

0.635

16


Page 17 of 34 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

314

Energy & Fuels

Secondary air velocity

m/s

0.399

SA/(PA+SA)b

-

0.3

Primary air temperature

K

300

Secondary air temperature

K

300

Solid return temperature

K

1123

Probe surface temperature

K

700

b. PA and SA refer to primary air and secondary air, respectively.

315

Table 3. Proximate and Ultimate Analysis Proximate Analysis c,d

Ultimate Analysis

Aad

6.43 w%

Cad

62.89 w%

Vad

27.91 w%

Had

3.04 w%

Mad

11.17 w%

Nad

0.55 w%

Mt

23.42 w%

Sad

0.51 w%

Qnet,ar

19589 kJ/kg

Oad

15.41 w%

Nat

0.226 w%

Ash density

2400 kg/m3

Packing limit e

0.6

316

c.

317

d. ar, ad and t refer to as received basis, air dried basis and total content, respectively.

318

e.

319

A, V and M refer to ash, volatile and moisture, respectively.

limits the maximum volume fraction for the solid phase. Table 4. Ash composition analysis Ash fusion temperature f/ oC

Ash composition/ w%

17


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

320

SiO2

Al2O3

Fe2O3

CaO

MgO

K2O

Na2O

SO3

DT

ST

HT

FT

41.57

11.16

4.79

16.21

6.48

0.51

3.5

8.05

1134

1145

1162

1192

f.

DT-deformation temperature; ST-softening temperature; HT-hemispherical temperature; FT-

321

flowing temperature.

322

5. Result and discussion

323

5.1 Hydrodynamic and combustion

324 325

Page 18 of 34

Figure 5 shows the pressure drops along the furnace height. The computing and experimental results are in good agreement.

326

Figure 6 shows the axial distribution of gaseous temperature along the furnace

327

height. Compared with the experiment results, reasonable results are obtained for

328

temperature distribution. It shows that the temperature increases at the bottom of the

329

furnace due to the rapid oxidizing reaction of volatiles and char particles. After reaching

330

the peak point, the gaseous temperature experiences a mild decrease due to the heat

331

exchange through the furnace wall.

332 18


Page 19 of 34 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

333

Energy & Fuels

Figure 5. Pressure drop

Figure 6. Gas temperature distribution

334

Figure 7~10 give contours of O2, CO, NO and SO2 distributions. The O2

335

concentration decreases at the coal inlet due to the oxidation reactions. An increment

336

of O2 content is observed at 1m because of the secondary air injection. CO gathers at

337

the bottom of the furnace due to the inadequate oxygen in this region during staged

338

combustion. As secondary air is injected, CO concentration decreases quickly.

339

Comparing the averaged gaseous species concentrations (O2, CO, NOx, SO2) with the

340

measurement at the outlet of the furnace, they are matched reasonably well (Table 5).

341 342

Figure 7. O2 content

Figure 8. CO content

343

344

Figure 9. NO content

Figure 10. SO2 content

Table 5. Gas species contents at the outlet O2

CO

NO

SO2

(%)

(ppm)

(ppm)

(ppm)

simulation

9.0

443.4

158.4

153.6

experiment

8.4

508.9

167.3

146.4

5.2 Sodium fate 19


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

345

As shown in Figure 1, the final occurrences of sodium in a CFB boiler are

346

classified into three portions: in gas phase, in ash and in deposits. The distributions of

347

sodium in gas phase and solid phase, as well as the transformation behavior of sodium

348

species are discussed as bellow.

349

(1) In gas phase

350

Distributions of NaCl(g) and Na2SO4(g) are shown in Figure 11~12. The content

351

of NaCl(g) increases near the coal inlet port, since majority of sodium compounds are

352

released following the rapid devolatilization and char combustion process at this

353

position. Due to the conversion reaction from NaCl(g) to Na2SO4(g) (R1-6) and the

354

absorption reaction (R1-7), NaCl(g) content start to decrease soon afterwards. Figure

355

12 shows that Na2SO4(g) mainly forms above the coal inlet port. Its content increases

356

along the furnace height, which is caused by the homogeneous reactions (R1-4, 1-5, 1-

357

6). Compared with NaCl(g), concentration of Na2SO4(g) is more evenly distributed in

358

the furnace.

359

Contents of NaCl(g) and Na2SO4(g) at the outlet of the furnace are about 0.4 ppm

360

and 1.1 ppm, respectively. They are in reasonable range compared to those from

361

literatures29, 50.

20


Page 20 of 34

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Energy & Fuels

362 363 364

Figure 11. NaCl distribution

Figure 12. Na2SO4 distribution

(2) In ash

365

Generally, the sodium compounds retain in the ash through four pathways: (1)

366

inhabitation of insoluble sodium and portion of soluble sodium in coal ash, (2)

367

adsorption of sodium vapors by ash compositions, (3) condensation of sodium vapors

368

on the particle surface, (4) falling of ash particles containing sodium from the probe

369

surface. The content of ash particles containing sodium (including the ash containing

370

original sodium, ash containing adsorbed sodium and ash containing condensed sodium)

371

will increase along the furnace height, as shown in Figure 13. However, when passing

372

the probe region, ash particles containing sodium will decrease due to the inertial

373

impaction and thermophoresis process.

374

Combined the particle concentrations in Figure 13 with the sodium content in each

375

particle, the sodium distribution in ash is calculated and given in Figure 14. It shows

376

that sodium content in ash increases along the furnace height, while it decreases at the

377

probe area due to the deposition behavior. 21


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

378 379

Page 22 of 34

Mass flow rate of ash, sodium in ash at the outlet of the furnace are shown in Table 6. Simulation results agree well with the experimental measurements.

380 381

Figure 13. Content of ash containing Na

382

383

Figure 14. Na distribution in ash

Table 6. Mass flow rate at the outlet ash (kg/s)

Na in ash (kg/s)

simulation

1.47×10-4

5.01×10-6

experiment

1.90×10-4

5.64×10-6

(3) In deposits

384

Results of deposits build-up on the probe surface by mechanisms of condensation,

385

inertial impaction and thermophoresis are given. Figure 15~17 show the particles

386

deposition flux and deposit–Na flux on the probe surface according to these three

387

mechanisms.

22


Page 23 of 34 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

388 389

Figure 15 (a). Deposition flux by condensation

Figure 15 (b). Na flux by condensation

390 391

Figure 16 (a). Deposition flux by impaction

Figure 16 (b). Na flux by impaction

Figure 17 (a). Deposition flux by thermophoresis

Figure 17 (b). Na flux by thermophoresis

392 393 394

Shown on Figure 15 (a), 16(a) and 17 (a), the highest deposition rate is observed

395

at the impaction process. The deposition flux by condensation is lower than that by

396

impaction or thermophoresis. These results of variation tendency agree well with Yang

397

et al.’s22 and Hansen et al.’s23 work. Additionally, the deposition fluxes of condensation,

398

inertial impaction and thermophoresis have same order of magnitude with the

399

quantitative results from Hansen et al.’s23 experiments.

400

In Figure 15 (b), the flux of deposit–Na by condensation is distributed around the 23


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

401

probe surface. Its value at the side surface (90o) is higher than that at the windward (0o)

402

or at the leeward (180o). The reason is that sodium vapor in the gas stream follows the

403

streamlines around the probe, which leads to the lower vapor concentration on the

404

windward or leeward surface. Additionally, almost all the condensed sodium come

405

from Na2SO4(g).

406

The inertial impaction is primarily acting on larger particles. Particles with large

407

inertia in a fluid stream are no longer able to follow the streamlines around the probe

408

and then collide and deposit on the surface. Due to the higher concentration of particles

409

at the windward (0o) of the probe surface, the flux of deposit–Na by inertial impaction

410

mainly concentrates on this area, as displayed in Figure 16 (b).

411

The thermophoresis phenomenon is caused by the temperature gradient between

412

the gas phase and probe surface. This induces the thermophoresis force to push the

413

particles in the direction of the colder surface. Therefore, thermophoresis deposition on

414

a cylinder is more evenly distributed51. This is the reason that the deposit–Na flux is

415

evenly distributed on the probe surface in Figure 17 (b). However, the flux at the

416

leeward (180o) surface is lower, due to the lower gas temperature near the probe in this

417

region, which decreases the temperature gradient and reduces the incident velocity of

418

particles (equation (18-20)).

419

Considering these three mechanisms of deposits build-up, as well as the shedding

420

process, the net flux of deposit–Na can be calculated (Figure 18). By statistics on the

421

whole surface, the average flux around the probe surface is 6.14×10-9 kg/s. This value

422

agrees well with the experiment result (6.59×10-9 kg/s). 24


Page 24 of 34

Page 25 of 34 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

423 424 425

Figure 18. Net deposit–Na flux

(4) Summary

426

Table 7 gives summarization of the distributions of sodium in varied forms. Most

427

of the sodium retain in the fly ash and leave the furnace. A small portion of sodium stay

428

as deposits on the probe surface. However, it has to be noted that the deposits will

429

accumulate with the time and finally induce the ash problems. By comparing the

430

simulation results of hydrodynamics, combustion and sodium distributions with those

431

from experiment or literature, the sodium migration model is validated.

432

433

Table 7. Sodium distributions Emission as gas phase

Emission as ash

Retained in deposits

(kg/s)

(kg/s)

(kg/s)

simulation

1.52×10-7

5.01×10-6

6.14×10-9

experiment

-

5.64×10-6

6.59×10-9

6. Conclusions

434

A sodium migration model was developed. It incorporated sub-models including

435

sodium species release, chemical reactions, vapor condensation, particles deposition

436

and shedding. These models were coupled in a 3D comprehensive combustion model. 25


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

437

It predicted the release and transformation behavior of sodium compounds during a

438

high alkali coal combustion in circulating fluidized bed boilers. Distributions of sodium

439

compounds in gas phase, in ash and in deposits could be obtained.

440 441

Applying the model to a 30 kW hot CFB test rig, simulation and experiment results show good agreement.

442 443

Author information

444

Corresponding Author

445

*Email: [email protected]

446 447

Acknowledgements

448

This work is supported by the National Key Research & Development Program of

449

China (2018YFB0605403).

450 451

Symbols

452

Ac = contact area between particle and surface (m2)

453

dp = particle diameter (m)

454

Dh = hydraulic diameter of the flow channel (m)

455

Di = diffusion coefficient (m2/s)

456

H= furnace height (m)

457

k= rate constant

458

m = mass (kg) 26


Page 26 of 34

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Energy & Fuels

459

mp = particle mass (kg)

460

pi = partial pressure of the vapor (Pa)

461

pi,s = saturation vapor pressure (Pa)

462

Rg = universal gas constant (J/mol. K)

463

Rp = particle radius (m)

464

Re = Reynolds number

465

Sc = Schmidt number

466

Sh = Sherwood number

467

St = Stocks number

468

t = time (s)

469

T = temperature (K)

470

Tg = gas temperature (K)

471

Tp = particle temperature (K)

472

Ts = deposit surface temperature (K)

473

up = particle velocity (m/s)

474

μg = gas dynamic viscosity (Pa. s)

475

ν = kinematic viscosity (m2/s)

476

ρg = gas density (kg/m3)

477

ρp = particle density (kg/m3)

478

θ = angle (o)

479

η = impact efficiency

480

ξ = sticking probability 27


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

481 482

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483

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10. Song, G.; Song, W.; Qi, X.; Lu, Q., Transformation Characteristics of Sodium of

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11. Quyn, D. M.; Wu, H.; Li, C., Volatilisation and catalytic effects of alkali and

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alkaline earth metallic species during the pyrolysis and gasification of Victorian brown

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effects of alkali and alkaline earth metallic species during the pyrolysis and gasification

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Investigation on Sodium Fate for High Alkali Coal during Circulating

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