Investigation of the effect of air coefficient on the combustion characteristics of pulverized coal pre-pyrolysis

doi:10.11949/0438-1157.20240879

Abstract

Abstract:

Pulverized coal pre-pyrolysis combustion method is an effective way to reduce NO _x and stabilize combustion. To investigate the pre-pyrolysis combustion characteristics of pulverized coal, the RPM-MSRM model (random pore model-multiphase surface reaction model) for pulverized coal pre-pyrolysis combustion was established, and the effects of air coefficient on generation of pyrolysis gases and conversion of fuel nitrogen were investigated. The results show that after pulverized coal pre-pyrolysis, the volatile matter is pyrolyzed and part of the coke is gasified to generate a large amount of pyrolysis gas [23%—38%(vol)] and high-temperature coke (>800℃), and the flue gas jet rigidity is enhanced, which helps to improve the combustion stability. The rigidity of the flue gas jet is enhanced, which helps to improve the combustion stability. The air coefficient is the main factor affecting the characteristics of pulverized coal pre-pyrolysis. The air coefficient is positively correlated with outlet temperature and negatively correlated with pyrolysis gas concentration. The optimum pre-pyrolysis air coefficient for the device in this paper is 0.3. The lowest NO _x concentration at the unit outlet is 26.82 mg/m³ (@6%O₂,standard operating conditions), and the highest reduction efficiency of fuel nitrogen is 99.51%. Therefore, pulverized coal pre-pyrolysis combustion should be maintained at the optimum air coefficient conditions to achieve the best nitrogen reduction.

Key words: coal combustion, pyrolysis, computational fluid dynamics, fuel nitrogen, char model

摘要：

煤粉预热解燃烧方法是降低氮氧化物和稳定燃烧的有效途径。为了探究煤粉预热解燃烧特性，建立了适用于煤粉预热解燃烧的RPM-MSRM（random pore model-multiphase surface reaction model）模型，并探究了空气系数对热解气生成和燃料氮转化过程的影响。研究结果表明：煤粉预热解后挥发分热解和部分焦炭气化生成大量热解气［23%~38%（体积分数）］和高温焦炭（>800℃），烟气射流刚性增强，有助于提高燃烧稳定性；空气系数是影响煤粉预热解特征的主要因素，空气系数与出口温度呈正相关关系，与热解气浓度呈负相关关系；本装置的最佳预热解空气系数为0.3，此时装置出口NO _x 浓度最低为26.82 mg/m³（@6%O₂，标准工况），燃料氮的还原效率最高为99.51%。因此，煤粉预热解燃烧应尽量保持在最佳空气系数下进行以达到最佳的降氮效果。

关键词: 煤燃烧, 热解, 计算流体力学, 燃料氮, 焦炭模型

CLC Number:

TK 16

Guojia YAO, Zhi WANG, Ang SU, Dongge FENG, Hong TANG, Lingfang SUN. Investigation of the effect of air coefficient on the combustion characteristics of pulverized coal pre-pyrolysis[J]. CIESC Journal, 2025, 76(3): 1243-1252.

姚国家, 王志, 苏昂, 冯东阁, 唐宏, 孙灵芳. 空气系数对煤粉预热解燃烧特性的影响分析[J]. 化工学报, 2025, 76(3): 1243-1252.

Figures/Tables 14

Fig.1 Schematic of pulverized coal pre-pyrolysis combustion

Table 1 Coal proximate and ultimate analyses data

煤	工业分析/%				元素分析/%					Q_ar,net/(kJ/kg)
煤	M_ar	A_ar	V_ar	FC_ar	C_ar	H_ar	O_ar	N_ar	S_ar	Q_ar,net/(kJ/kg)
神华烟煤	14.50	7.37	28.67	49.46	65.10	3.25	8.08	0.66	0.71	23790

Fig.2 Pre-pyrolysis device schematic and grid: (a) system diagram； (b) structural diagram； (c) inlet and outlet grid,； (d) sectional grid

Table 2 Mathematical model parameters

项目	模型	参数
湍流模型	Realizable k-ɛ模型
辐射模型	离散坐标模型	方位4×4；像素2×2
烟气吸收系数	灰色气体加权和模型
湍流反应速率模型	涡耗散概念模型
拉格朗日随机追踪	颗粒随机轨道模型	迭代次数：10
颗粒辐射参数		发射率：0.9；散射系数：0.6
气固耦合模型	particle-source-in-cell方法
热力型NO _x 模型	extended Zeldovich机理
燃料型NO _x 模型	de Soete机理
焦炭表面还原模型	$\frac{d [N O]}{d t} = 230 \times e^{\frac{- 142737}{R T}} A_{E} P_{N O}$	A_E：焦炭比表面积，m²/kg；P_NO：NO分压，Pa
再燃模型	$\begin{array}{l} \frac{d [N O]}{d t} = - k_{1} [C H] [N O] - \\ k_{2} [C H_{2}] [N O] - k_{3} [C H_{3}] [N O] \end{array}$	k₁=10⁸ m³/(mol·s)；k₂=1.4×10⁶e^-550/^T m³/(mol·s)；k₃=2×10⁵ m³/(mol·s)

Table 2 Mathematical model parameters

项目	模型	参数
湍流模型	Realizable k-ɛ模型
辐射模型	离散坐标模型	方位4×4；像素2×2
烟气吸收系数	灰色气体加权和模型
湍流反应速率模型	涡耗散概念模型
拉格朗日随机追踪	颗粒随机轨道模型	迭代次数：10
颗粒辐射参数		发射率：0.9；散射系数：0.6
气固耦合模型	particle-source-in-cell方法
热力型NO _x 模型	extended Zeldovich机理
燃料型NO _x 模型	de Soete机理
焦炭表面还原模型	$\frac{d [N O]}{d t} = 230 \times e^{\frac{- 142737}{R T}} A_{E} P_{N O}$	A_E：焦炭比表面积，m²/kg；P_NO：NO分压，Pa
再燃模型	$\begin{array}{l} \frac{d [N O]}{d t} = - k_{1} [C H] [N O] - \\ k_{2} [C H_{2}] [N O] - k_{3} [C H_{3}] [N O] \end{array}$	k₁=10⁸ m³/(mol·s)；k₂=1.4×10⁶e^-550/^T m³/(mol·s)；k₃=2×10⁵ m³/(mol·s)

Table 3 Gas-phase reactions and related kinetic parameters

反应	化学反应方程式	A_r	E_r /(J/kmol)	m	a	b	c	文献
R1	Vol→x₁CH₄+x₂CO+x₃H₂+x₄SO₂+x₅N₂	10¹⁸	0	0	0	0	0	—
R2	CO+0.5O₂→CO₂	2.239×10¹²	1.674×10⁸	0	1	0.25	0.5[H₂O]	[23]
R3	H₂+0.5O₂→H₂O	6.8×10¹⁵	1.67×10⁸	-1	0.25	1.5	0	[23]
R4	CH₄+2O₂→CO₂+2H₂O	2.119×10¹¹	2.052×10⁸	0	0.2	1.3	0	[24]
R5	CO+H₂O→CO₂+H₂	2.34×10¹⁰	2.883×10⁸	0	0.5	1	0	[25]
R6	CO₂+H₂→CO+H₂O	2.2×10⁷	1.9×10⁸	0	0.5	1	0	[26]
R7	CH₄+H₂O→CO+3H₂	8.0×10⁷	2.51×10⁸	0	0.5	1	0	[26]

Table 4 Char gasification reactions and related kinetic parameters

反应	反应方程式	A_r	E_r /(J/kmol)	n	文献
R8	C(s)+0.5O₂→CO	113	1.3×10⁸	0.68	[31]
R9	C(s)+CO₂→2CO	62.3	2.531×10⁸	1	[16]
R10	C(s)+H₂O→CO+H₂	0.465	1.905×10⁸	1	[16]

Table 5 Numerical simulation cases setup

工况	煤粉质量流量/(kg/s)	一次风质量流量/(kg/s)	空气系数
1	1.24	1.93	0.2
2	1.24	2.89	0.3
3	1.24	3.85	0.4
4	1.24	4.82	0.5

Table 6 Comparison between the CFD results and experimental data of case 2

项目	实验结果	计算结果		误差分析
项目	实验结果	RPM-MSRM模型	MSRM模型	RPM-MSRM模型	MSRM模型
CH₄体积分数/%	2.93	2.81	1.91	-4.09%	-34.81%
H₂体积分数/%	10.26	10.61	8.46	＋3.41%	-17.54%
CO体积分数/%	16.83	17.88	12.97	＋6.24%	-22.94%
CO₂体积分数/%	9.48	8.95	12.56	-5.59%	＋32.49%
出口温度/K	1257	1313	1125	＋4.46%	-10.50%

Fig.3 Velocity distribution of pre-pyrolysis device: (a) along-travel velocity; (b) outlet velocity; (c) velocity distribution contours; (d) cross-sectional velocity distribution contours (Y=4 m)

Table 7 Summary of particles conversion ratio and residue time

工况	空气系数	颗粒转化率/%		颗粒平均停留时间/s
工况	空气系数	挥发分	焦炭	颗粒平均停留时间/s
1	0.2	100	31.53	2.68
2	0.3	100	50.97	1.87
3	0.4	100	65.23	1.18
4	0.5	100	74.78	0.82

Fig.4 Temperature and pyrolytic gas axial distribution

Fig.5 Outlet pyrolytic gas concentration

Fig.6 Outlet NO x concentration and HCN mole fraction

Fig.7 Fuel nitrogen conversion and reduction efficiency

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