Combustion characteristics of low-rank coal chars in O2/CO2, O2/N2 and O2/Ar by TGA
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摘要: 利用热重研究了两种中国西北典型低阶煤半焦的燃烧特性。探究了不同气氛（O2/CO2、O2/N2和O2/Ar）和不同氧气浓度对其燃烧特性的影响。实验结果表明, 无论是反应气氛还是氧气浓度都会对低阶煤半焦的燃烧产生影响。相比于N2和Ar, CO2明显有利于燃烧反应进行: 当反应气氛由O2/CO2变为O2/Ar时, 两种不同低阶煤半焦的燃尽温度分别升高了63.7和68.8℃; 而当反应气氛由O2/CO2变为O2/N2时, 两种不同低阶煤半焦的燃尽温度分别升高了135.9和129.6℃。在研究范围内, 氧气浓度的提高也能明显提高半焦的燃烧性能。与此同时, 半焦燃烧特性的动力学分析表明, 随着氧气浓度提高, 两种半焦燃烧反应的表观活化能E和指前因子A均呈增大趋势。通过对E和A两者关系的分析结果表明, 半焦富氧燃烧的活化能和指前因子存在动力学补偿效应。Abstract: The combustion reactivity of two chars prepared from two low-rank coals in Northwest China were studied using a thermogravimetric analyzer (TGA). The effects of different atmospheres (O2/CO2, O2/N2 and O2/Ar) and different oxygen concentrations on the combustion characteristics were investigated. The results indicate that both atmosphere and oxygen concentration show effectiveness on combustion of char. Compared with N2 and Ar, CO2 could significantly promote the reaction. When combustion atmosphere changes from O2/CO2 to O2/Ar, the burnout temperature increases by 63.7 and 68.8℃ for the two chars respectively. Meanwhile, when the combustion atmosphere changes from O2/CO2 to O2/N2, that is 135.9 and 129.6℃, respectively. An increase in concentration of oxygen can also improve the combustion performance of chars in the test. At the same time, kinetic analysis of the combustion profiles of the chars reveals that both the apparent activation energy E and the pre-exponential factor A increased with increasing oxygen concentration and the compensation effect exists between activation energy E and pre-exponential factor A of chars' combustion.
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Figure 1 Schematic of defined method of ignition temperature (ti) and burnout temperature (tb)
Figure 2 TG and DTG curves of chars' combustion in three different atmospheres of 20% O2 concentration
(a): L-char; (b): B-char □: O2/CO2; ○: O2/Ar; △: O2/N2; ■: O2/CO2; ●: O2/Ar; ▲: O2/N2
Figure 3 Comparison of the ratio for physical properties of different atmospheres at 600 ℃ and 0.1 MPa
Figure 4 TG and DTG curves of L-char and B-char combustion in different O2 concentrations
(a): L-char; (b): B-char □: 20% O2; ○: 40% O2; △: 60% O2; ■: 20% O2; ●: 40% O2; ▲: 60% O2
Figure 5 Relationship between oxygen concentration and the kinetics parameters for combustion of L-char and B-char
Table 1 Proximate analysis and ultimate analysis of the coal and char samples
Sample Proximate analysis w/% Ultimate analysis w/% M V A FC N C H S O L-coal 12.61 41.33 6.98 39.08 0.36 61.93 5.29 1.20 31.50 L-char 0.00 4.90 12.11 83.00 0.48 83.73 1.31 0.82 13.65 B-coal 5.36 27.68 7.18 59.78 1.04 77.96 2.47 1.00 17.42 B-char 0.00 3.46 10.54 86.00 0.79 83.11 0.97 1.48 13.95 L-coal: Shenmu lignite; B-coal: Shenmu bituminous coal; B-char: Shenmu bituminous coal char; L-char: Shenmu lignite char 
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Table 2 Combustion characteristic temperatures of chars in different atmospheres of 20% O2 concentration
Sample Characteristic
parameterO2/CO2 O2/Ar O2/N2 L-char ti/℃ 381.4 380.1 379.8 tb/℃ 509.5 573.2 645.4 B-char ti/℃ 462.9 483.2 487.3 tb/℃ 588.8 657.6 718.4
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Table 3 Combustion characteristic parameters of chars in different O2 concentrations of O2/Ar atmosphere
Sample Characteristic parameter 20% 40% 60% L-char ti /℃ 380.1 369.8 362.2 tb /℃ 573.2 548.4 480.1 E/(kJ·mol-1) 128.84 174.54 219.56 A/(1·s-1) 2.56×106 4.36×1010 3.79×1014 B-char ti/℃ 483.2 460.1 440.6 tb /℃ 657.6 632.7 559.7 E/(kJ·mol-1) 221.59 245.99 286.62 A/(1·s-1) 8.33×1011 1.74×1014 4.35×1017
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A.1 Linear correlation coefficient of different models of L-char combustion in 40% O2/Ar
No. Symbol Function Reaction mechanism R2 1 D1 parabolic law 1-D diffusion 0.943 7 2 D2 valensi (barrer) equation 2-D diffusion (sylindrical symmetry) 0.967 9 3 D3 jander equation 3-D diffusion (globular symmetry) 0.986 0 4 D4 ginstling-brounshtein equation 3-D diffusion (globular symmetry) 0.975 3 5 D5 inverse jander equation 3-D diffusion 0.933 4 6 D6 Z-L-T equation 3-D diffusion 0.996 1 7 A1 aurami-erofeev equation nucleation growth (n=1) 0.993 6 8 A1.5 aurami-erofeev equation nucleation growth (n=1, 5) 0.992 0 9 A2 aurami-erofeev equation nucleation growth (n=2) 0.989 8 10 A3 aurami-erofeev equation nucleation growth (n=3) 0.982 1 11 A4 aurami-erofeev equation nucleation growth (n=4) 0.964 6
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