English

• 

  China has rich coal resources and depends on coal for more than 65% of its energy needs^{[1, 2]}. No matter how much gas, oil, and renewable or nuclear energy sources are utilized, coal will remain the dominant energy resources for a long time in China^[3]. However, among the total coal reserves in China, more than half are the low-rank coals^[4]. With the growing demand for energy and the gradual decrease in the availability of high-rank coals resources, utilization of low-rank coals, such as lignite and bituminous coal, is as important as that of high-rank coals, especially for China^[5] with large reserves of lignite. However, transportation of low-rank coals over long distances is generally regarded as uneconomical as their high moisture content entails high transportation costs^[6]. Moreover, their low calorific value, low thermal efficiency of combustion, and high tendency to spontaneous combustion restrict their direct and large-scale utilization^{[7, 8]}. Therefore, low-rank coals need to be upgraded not only to make their reactivity characteristics resemble those of high-rank coals but also to increase heat value of the upgraded coals. However, the question is whether and how low-rank coals can be upgraded into a useful form of energy, which is efficient usage of such coals and is also economically transportable and safe in storage. The conversion processes, such as liquefaction, gasification and pyrolysis, can be realized by upgrading the energy grade of low-rank coals^[9-11]. Considering the composition and structural characteristics of low-rank coals such as higher volatility and lower fixed carbon content as well as high chemical reactivity, a method^[12] named "the stage utilization of low-rank coal" is proposed to convert and use the low-rank coals effectively and economically. In this method, the low-rank coals are firstly pyrolyzed under moderate temperature to extract combustible gas, liquid fuel and other value-added products, and then the surplus char^[13] is utilized for combustion or gasification.

  There are oxy-fuel combustion and oxygen-enriched combustion technology in the literatures^[14-21]. Oxy-fuel combustion is considered as one of the new options to capture CO₂ from coal-fired power plants. This technology replaces air by O₂/CO₂ mixture for coal combustion, so that the combustion process produces less other gas to get high concentration of CO₂. Oxygen-enriched combustion means the O₂ concentration is higher than 21%, which can reduce ignition temperature of fuels, accelerate combustion speed, promote completely combustion, improve flame temperature, reduce amount of flue gas, and improve heat utilization.

  Many researchers have studied on coal combustion characteristics under oxy-fuel or oxy-enriched atmosphere^[22]. Liu^[23] subjected two chars to non-isothermal combustion tests in a TGA under mixtures of O₂/CO₂ and O₂/N₂ with O₂ concentrations of 3%, 6%, 10%, 21%, and 30%. The detailed comparisons of measured char combustion rates show that replacing the inert nitrogen gas in the oxidizer with CO₂ has very little influence on the measured combustion rates of coal chars at any char conversion level under the experiment conditions. However, different results have been reported by other investigators. Rathnam et al^[24] measured the reactivity of four Australian coals under simulated air (O₂/N₂) and oxy-fuel (O₂/CO₂) atmospheres using a TGA under non-isothermal conditions up to 1 473 K. They revealed that a significant increase in reactivity was observed at high temperatures in 2% O₂/CO₂ atmosphere which was not observed in the 2% O₂/N₂ case. Zhou et al^[14] studied the combustion characteristics of lignite in O₂/CO₂ atmosphere by TGA, and found that CO₂ did have a delay influence on the combustion process, especially the maximum mass-loss rate and the burnout temperature. For oxy-enriched atmosphere combustion, Fan et al^[25] investigated combustion characteristics of three coals under high oxygen concentrations (more than 21%), and showed that all the ignitability, combustion property, and burnout were greatly improved when enrichment oxygen was used, especially for small particles.

  This work aims to get more information about the oxy-fuel and oxygen-enriched combustions of low-rank coal chars and to clarify the effects of different combustion atmospheres (especially CO₂) and oxygen concentration on a typical combustion process using TGA. Inert gases nitrogen and argon were employed as comparison atmospheres with the purpose of exploring impact of CO₂ on the combustion process under oxy-fuel atmosphere. The relevant kinetics analysis of the combustion of low-rank coal chars under different oxygen concentrations was also presented.

  1   Experimental and data processing methods

  1.1   Sample preparation

  Shenmu bituminous coal and Shenmu lignite were used as raw coals for preparing the char samples. Both the two coals are representative low-rank coals in Northwest China.

  The chars were prepared by isothermal rapid pyrolysis method. Firstly, the raw coal was placed in a quartz boat, and then sealed in a quartz tube to drive air for 30 min with N₂. After the temperature of tube furnace reached the desired value (700 ℃), the quartz tube with the raw coal was quickly placed in the middle of the furnace. Coal was heated at 700 ℃ for 60 min. The quartz tube was then taken out of the furnace and cooled down to room temperature, and the char sample was obtained. The proximate and ultimate analyses of coals and chars are listed in Table 1.

  Table 1.  Proximate analysis and ultimate analysis of the coal and char samples

  表选项

  导出Excel

  下载全尺寸表图像

  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

  Table 1.  Proximate analysis and ultimate analysis of the coal and char samples

  1.2   Experimental method

  TGA were performed in a "Rubotherm-DynTHERM Magnetic Suspension Balance of High Temperature and High Pressure Thermogravimetric Analyzer" produced by Rubotherm Company. Each char sample was ground, sieved to 160-200 mesh before TGA. Generally, dosage of char sample was (25±1) mg. The container was made from Al₂O₃. Heating rate of all experiments was 5 ℃/min from room temperature to 900 ℃. The thermogravimetric analyzer was calibrated for temperature reading and for buoyancy effect before tests. The instrument is equipped with four different and individually controlled reaction gases, and can automatically control and measure the flow rates of each reaction atmosphere. The reaction gases were mixture gases of O₂/CO₂, O₂/N₂ and O₂/Ar, with oxygen concentrations of 20%, 40% and 60%. And the total flow rate of reaction gases was 100 mL/min.

  1.3   Determination of combustion characteri-stic temperatures from TGA

  There are several methods to determine the combustion characteristic temperatures such as ignition temperature and burnout temperature from TGA. In this work, the TG-DTG tangent method was adopted to determine ignition temperature (t_i) and burnout temperature (t_b) as shown in Figure 1.

  Figure 1.  Schematic of defined method of ignition temperature (t_i) and burnout temperature (t_b)

  图选项

  下载全尺寸图像

  下载幻灯片

  First, vertical line 1 was drawn through the peak point of DTG curve, which met TG curve at point A. Tangent line 2 of TG curve was then made at point A. Line 2 intersected the extended TG initial level line at point B, and met the extended TG termination level line at point C. The ignition temperature and the burnout temperature were referred as the corresponding temperature at point B and point C respectively.

  1.4   Kinetic analysis

  The combustion of chars in TGA can be described by the following equation^[26]:

  where, α is the degree of reaction; f(α) is a function called the reaction model describing the dependence of reaction rate on reaction's extent.

  The degree of reaction α can be represented as:

  where, m₀ is the initial mass of the sample, m is the mass of the reaction time t and m_∞ is the final mass of the sample.

  The Arrhenius equation is as following:

  where, k is the rate constant of a chemical reaction on the absolute temperature T (in kelvins), A is the pre-exponential factor, E is the apparent activation energy and R is the universal gas constant.

  Insertion of the Arrhenius equation for the reaction constant into Eq. (1) leads to

  In the non-isothermal thermogravimetric method, the parameter β is introduced, which is defined as

  Substituting Eq. (5) into Eq. (4) and then doing integral, the following equation is obtained:

  In Eq. (6), T₀ is the beginning temperature, which can be ignored under low temperature. Coats-Redfern method^[27] can be adapted to Eq. (6), so the Eq. (6) can be expressed as follows:

  Introducing logarithm into Eq. (7), the following equation is obtained:

  For most of the reaction region and value of apparent activation energy E, $\lg \left[{\frac{{AR}}{{\beta E}}(1-\frac{{2RT}}{E})} \right]$ is almost a constant. Then, Eq. (8) is a linear relationship in which the independent variable is 1/T and the dependent variable is lg (F(α)/T²) with the slope -E/2.3R and the intercept $\lg \left[{\frac{{AR}}{{\beta E}}(1-\frac{{2RT}}{E})} \right]$.

  Therefore, we can obtain the corresponding kinetic parameters from the data obtained from the experiment. By linearizing the curve of "lg[F(α)/T²]" versus "1/T", we can calculate E and A according to the slope and intercept. Thus, the corresponding kinetic parameter is obtained through the TGA data.

  It is worth noting that the reaction model F(α) in Eq. (8) should be determined before calculation. The above process is shown in the appendix. Finally, D₆ model i.e. $F (\alpha)={\{ {[1/(1-\alpha)]^{\frac{1}{3}}} -1\} ^2}$ was chosen.

  2   Results and discussion

  2.1   Effects of O₂/CO₂ atmosphere compared with O₂/N₂, O₂/Ar using TGA

  Figure 2 shows the comparisons among O₂/CO₂, O₂/N₂ and O₂/Ar atmospheres of 20% O₂ concentration in chars' combustion processes. Várhegyi et al^[28] compared the char reactivity under O₂/CO₂ and O₂/Ar atmospheres, while they had not observed any significant change when replacing argon by CO₂, probably due to the fact that the char + CO₂ reactions had lower rates than the char oxidation. But in this work, clear differences have been observed among O₂/CO₂, O₂/N₂ and O₂/Ar atmospheres. DTG curves of both L-char and B-char combustion in O₂/CO₂ atmosphere are sharper and higher as compared with those in O₂/Ar and O₂/N₂ atmospheres, and TG curves of chars' combustion in O₂/CO₂ atmosphere are at low-temperature areas as compared with those in O₂/Ar and O₂/N₂ atmospheres.

  Figure 2.  TG and DTG curves of chars' combustion in three different atmospheres of 20% O₂ concentration

  图选项

  下载全尺寸图像

  下载幻灯片

  Ignition temperature can be considered as the start of combustion phenomenon^[29], while burnout temperature as the termination of the process^[30]. As is listed in Table 2, the ignition temperatures of the two chars' combustion in the three atmospheres have little differences. Whereas, the burnout temperatures of both chars in O₂/CO₂ atmosphere are lower than those in O₂/Ar and O₂/N₂ atmospheres. When the combustion atmosphere was changed from O₂/CO₂ to O₂/Ar, the burnout temperature increased 63.7 and 68.8 ℃ for L-char and B-char, respectively. Meanwhile, when the atmosphere changed from O₂/CO₂ to O₂/N₂, the burnout temperature increased 135.9 and 129.6 ℃ for L-char and B-char, respectively. In other words, compared with N₂ and Ar, CO₂ could promote the char combustion reaction in 20% O₂ concentration for the low-rank coal chars tested.

  Table 2.  Combustion characteristic temperatures of chars in different atmospheres of 20% O₂ concentration

  表选项

  导出Excel

  下载全尺寸表图像

  Sample	Characteristic parameter	O2/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

  Table 2.  Combustion characteristic temperatures of chars in different atmospheres of 20% O₂ concentration

  The possible reason of different combustion performances of chars under different atmospheres is that there are differences for heat capacity and transport property among the three atmospheres. Figure 3 shows the ratio of thermal diffusivity, density, specific heat at constant pressure (C_p), thermal conductivity and viscosity for CO₂ to Ar and CO₂ to N₂ at 600 ℃ respectively. It is clear that CO₂ has higher density, C_p and thermal conductivity and lower thermal diffusivity and viscosity than N₂ and Ar. Thermal diffusivity and density, nevertheless, have the sequence of CO₂, Ar and N₂, which correspond to the order of their TG curves. Maybe the lower thermal diffusivity and higher density help CO₂ promote the char combustion reaction.

  Figure 3.  Comparison of the ratio for physical properties of different atmospheres at 600 ℃ and 0.1 MPa

  图选项

  下载全尺寸图像

  下载幻灯片

  2.2   Effect of O₂ concentration in O₂/Ar atmosphere using TGA

  From the TG and DTG curves shown in Figure 4, it can be observed that the combustion processes of both L-char and B-char are strongly affected by the O₂ concentration. The TG and DTG curves were shifted significantly to lower temperature zone and the maximum mass loss rate increased with the increase in O₂ concentration.

  Figure 4.  TG and DTG curves of L-char and B-char combustion in different O₂ concentrations

  图选项

  下载全尺寸图像

  下载幻灯片

  Table 3 lists the combustion characteristic parameters of two chars in different O₂ concentrations. It is clear that ignition temperature and burnout temperature decreased as the O₂ concentration increased. However, the decrease in burnout temperature was much obvious than that of the ignition temperature, which is the same as literatures^{[24, 25, 31]}. The ignition temperature of L-char and B-char decreased 17.9 and 42.6 ℃ respectively whereas their burnout temperature decreased 93.1 and 97.9 ℃ respectively with increase in O₂ concentrations from 20% to 60%. Hence, the increase in O₂ concentration can promote the combustion reaction of chars, especially the burnout temperature.

  Table 3.  Combustion characteristic parameters of chars in different O₂ concentrations of O₂/Ar atmosphere

  表选项

  导出Excel

  下载全尺寸表图像

  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

  Table 3.  Combustion characteristic parameters of chars in different O₂ concentrations of O₂/Ar atmosphere

  According to the reaction kinetic theory, lower activation energy means that reaction would occur easily^[32]. Since higher O₂ concentration could promote the combustion reaction, it should correspond to lower activation energy. But when it comes to kinetics of chars' combustion we studied, the experimental results revealed that the higher the O₂ concentration, the greater the activation energy E for the both studied chars.

  The relationship between the kinetics parameters (apparent activation energy E and pre-exponential factor A) and O₂ concentration for L-char and B-char combustion in O₂/Ar atmosphere is shown in Figure 5. E and lnA show a good linear relationship with the O₂ concentration (i.e. from 20% to 60%). Moreover, it is found that linear relationship also exists between E and lnA (Figure 6), which indicates that the compensation effect indwells in low-rank coal chars' combustion between apparent activation energy E and pre-exponential factor A^{[33, 34]}. Generally, lower apparent activation energy and higher pre-exponential factor correspond to more quick chemical reaction rate. However, the compensation effect makes it more complicated to analyze for char combustion in different oxygen concentrations. Hence, it is inappropriate to judge the reactivity of a chemical reaction like char combustion reaction just by using the activation energy or pre-exponential factor.

  Figure 5.  Relationship between oxygen concentration and the kinetics parameters for combustion of L-char and B-char

  图选项

  下载全尺寸图像

  下载幻灯片

  Figure 6.  Compensation effect of combustion of L-char and B-char

  图选项

  下载全尺寸图像

  下载幻灯片

  3   Conclusions

  Two typical Chinese low-rank coal chars' combustion was performed in thermogravimetric analyzer in O₂/CO₂, O₂/N₂ and O₂/Ar mixture atmospheres with different O₂ concentrations. The results show that CO₂ has a stimulating influence on the combustion process (especially on the maximum mass-loss rate and the burnout temperature) of the chars as compared with Ar and N₂. The density and thermal diffusivity of combustion atmosphere may play important roles in combustion reaction of chars using TGA. Increasing O₂ concentration can strongly improve the combustion performance of the chars. Both ignition temperature and burnout temperature decrease with the increasing O₂ concentration. The kinetics analysis results reveal that the higher the O₂ concentration employed, the greater the activation energy E and pre-exponential factor were. Hence the compensation effect indwells in low-rank coal chars' combustion between apparent activation energy E and pre-exponential factor A. It is inappropriate to judge the reactivity of char combustion reaction just by using the activation energy or pre-exponential factor.

  Appendix: Determination of F(α)

  Using the data of L-char combustion in 40% O₂/Ar atmosphere, plot curves of "lg[F(α)/T²]" versus "1/T" of some common models. Then, choose the most appropriate model according to the "linear correlation coefficient R²" of each model. From the Table A.1, we can easily obtain that the D₆ model is the wanted model.

  Table A.1.  Linear correlation coefficient of different models of L-char combustion in 40% O₂/Ar

  表选项

  导出Excel

  下载全尺寸表图像

  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

  Table A.1.  Linear correlation coefficient of different models of L-char combustion in 40% O₂/Ar
• 1. [1]

     XIE K, LI W, ZHAO W. Coal chemical industry and its sustainable development in China[J]. Energy, 2010, 35(11):  4349-4355. doi: 10.1016/j.energy.2009.05.029

  2. [2]

     BP Statistical Review of World Energy 2015[Z]. London: British Petroleum, 2015.

  3. [3]

     SHEALY M, DORIAN J P. Growing Chinese coal use: Dramatic resource and environmental implications[J]. Energy Policy, 2010, 38(5):  2116-2122. doi: 10.1016/j.enpol.2009.06.051

  4. [4]

     GE L, ZHANG Y, WANG Z, ZHOU J, CEN K. Effects of microwave irradiation treatment on physicochemical characteristics of Chinese low-rank coals[J]. Energy Conv Manag, 2013, 71(71):  84-91.

  5. [5]

     YU J, TAHMASEBI A, HAN Y, YIN F, LI X. A review on water in low rank coals: The existence, interaction with coal structure and effects on coal utilization[J]. Fuel Process Technol, 2013, 106(2):  9-20.

  6. [6]

     HOU H H, SHAO L Y, TANG Y, WANG S, WANG X T, LIU S. Study on coal bed methane genetic types and formation models of low rank coal in China[J]. China Min Mag, 2014, 23:  66-69.

  7. [7]

     KHAN M Z, CHUN D H, YOO J, KIM S D, RHIM Y J, CHOI H K, LIM J, LEE S, RIFELLA A. Evaluation of the effect of a palm acid oil coating on upgrading low rank coal[J]. RSC Adv, 2015, 5(78):  63955-63963. doi: 10.1039/C5RA08994H

  8. [8]

     AZIZ M, KANSHA Y, KISHIMOTO A, KOTANI Y, LIU Y, TSUTSUMI A. Advanced energy saving in low rank coal drying based on self-heat recuperation technology[J]. Fuel Process Technol, 2012, 104(12):  16-22.

  9. [9]

     BHOI S, BANERJEE T, MOHANTY K. Insights on the combustion and pyrolysis behavior of three different ranks of coals using reactive molecular dynamics simulation[J]. RSC Adv, 2016, 6(4):  2559-2570. doi: 10.1039/C5RA23181G

  10. [10]

      ZHANG L, LI T, QUYN D, DONG L, QIU P, LI C. Formation of nascent char structure during the fast pyrolysis of mallee wood and low-rank coals[J]. Fuel, 2015, 150:  486-492. doi: 10.1016/j.fuel.2015.02.066

  11. [11]

      周剑林, 王永刚, 黄鑫, 张书, 林雄超. 低阶煤中含氧官能团分布的研究[J]. 燃料化学学报, 2013,41,(2): 134-138. ZHOU Jian-lin, WANG Yong-gang, HUANG Xin, ZHANG Shu, LIN Xiong-chao. Determination of O-containing functional groups distribution in low-rank coals by chemical titration[J]. J Fuel Chem Technol, 2013, 41(2):  134-138.

  12. [12]

      WANG J G, ZHAO X H. Demonstration of key technologies for clean and efficient utilization of low-rank coal[J]. Bull Chin Acad Geol Sci, 2012, 27(3):  382-388.

  13. [13]

      许修强, 王永刚, 陈国鹏, 陈宗定, 秦中宇, 戴谨泽, 张书, 许德平. 水蒸气对褐煤原位气化半焦反应性能及微观结构的影响[J]. 燃料化学学报, 2015,43,(5): 546-553. XU Xiu-qiang, WANG Yong-gang, CHEN Guo-peng, CHEN Zong-ding, QIN Zhong-yu, DAI Jin-ze, ZHANG Shu, XU De-ping. Effects of steam on the reactivity and microstructure of char from in-situ gasification of brown coal[J]. J Fuel Chem Technol, 2015, 43(5):  546-553.

  14. [14]

      ZHOU Z, HU X, YOU Z, WANG Z, ZHOU J, CEN K. Oxy-fuel combustion characteristics and kinetic parameters of lignite coal from thermo-gravimetric data[J]. Thermochim Acta, 2013, 553(553):  54-59.

  15. [15]

      YUZBASI N S, SEL UK N. Air and oxy-fuel combustion characteristics of biomass/lignite blends in TGA-FTIR[J]. Fuel Process Technol, 2011, 92(5):  1101-1108. doi: 10.1016/j.fuproc.2011.01.005

  16. [16]

      SENNECA O, CORTESE L. Kinetics of coal oxy-combustion by means of different experimental techniques[J]. Fuel, 2012, 102(6):  751-759.

  17. [17]

      GIL M V, RIAZA J, ÁLVAREZ L, PEVIDA C, PIS J J, RUBIERA F. Oxy-fuel combustion kinetics and morphology of coal chars obtained in N₂ and CO₂ atmospheres in an entrained flow reactor[J]. Appl Energy, 2012, 91(1):  67-74. doi: 10.1016/j.apenergy.2011.09.017

  18. [18]

      LIU X, CHEN M, YU D. Oxygen enriched co-combustion characteristics of herbaceous biomass and bituminous coal[J]. Thermochim. Acta, 2013, 569(18):  17-24.

  19. [19]

      DAOOD S, NIMMO W, EDGE P, GIBBS B. Deep-staged, oxygen enriched combustion of coal[J]. Fuel, 2012, 101:  187-196. doi: 10.1016/j.fuel.2011.02.007

  20. [20]

      FU Z, ZHANG S, LI X, SHAO J, WANG K, CHEN H. MSW oxy-enriched incineration technology applied in China: Combustion temperature, flue gas loss and economic considerations[J]. Waste Manage, 2015, 38(12):  149-156.

  21. [21]

      PICKARD S, DAOOD S, NIMMO W, LORD R, POURKASHANIAN M. Bio-CCS: Co-firing of established greenfield and novel, brownfield biomass resources under air, oxygen-enriched air and oxy-fuel conditions[J]. Energy Procedia, 2013, 37:  6062-6069. doi: 10.1016/j.egypro.2013.06.535

  22. [22]

      DING N, ZHANG C W, LUO C, ZHENG Y, LIU Z G. Effect of hematite addition to CaSO₄ oxygen carrier in chemical looping combustion of coal char[J]. RSC Adv, 2015, 5(69):  56362-56376. doi: 10.1039/C5RA06887H

  23. [23]

      LIU H. Combustion of coal chars in O₂/CO₂ and O₂/N₂ mixtures: A comparative study with non-isothermal thermogravimetric analyzer (TGA) tests[J]. Energy Fuels, 2009, 23(9):  4278-4285. doi: 10.1021/ef9002928

  24. [24]

      RATHNAM R K, ELLIOTT L K, WALL T F, LIU Y, MOGHTADERI B. Differences in reactivity of pulverised coal in air (O₂/N₂) and oxy-fuel (O₂/CO₂) conditions[J]. Fuel Process Technol, 2009, 90(6):  797-802. doi: 10.1016/j.fuproc.2009.02.009

  25. [25]

      FAN Y S, ZOU Z, CAO Z, XU Y, JIANG X. Ignition characteristics of pulverized coal under high oxygen concentrations[J]. Energy Fuels, 2008, 22(2):  892-897. doi: 10.1021/ef7006497

  26. [26]

      OTERO M, GOMEZ X, GARCIA A I, MORÁN A. Non-isothermal thermogravimetric analysis of the combustion of two different carbonaceous materials[J]. J Therm Anal Calorim, 2008, 93(2):  619-626. doi: 10.1007/s10973-007-8415-y

  27. [27]

      EBRAHIMI-KAHRIZSANGI R, ABBASI M H. Evaluation of reliability of Coats-Redfern method for kinetic analysis of non-isothermal TGA[J]. Trans Nonferrous Met Soc China, 2008, 18(1):  217-221. doi: 10.1016/S1003-6326(08)60039-4

  28. [28]

      VÁRHEGYI G, SZABÓ P, JAKAB E, TILL F, RICHARD J R. Mathematical modeling of char reactivity in Ar-O₂ and CO₂-O₂ mixtures[J]. Energy Fuels, 1996, 10(6):  1208-1214. doi: 10.1021/ef950252z

  29. [29]

      GUPTA R P, GURURAJAN V S, LUCAS J A, WALL T F. Ignition temperature of pulverized coal particles: Experimental techniques and coal-related influences[J]. Combust Flame, 1990, 79(3/4):  333-339.

  30. [30]

      RIAZA J, ÁLVAREZ L, GIL M V, PEVIDA C, PIS J J, RUBIERA F. Effect of oxy-fuel combustion with steam addition on coal ignition and burnout in an entrained flow reactor[J]. Energy, 2011, 36(8):  5314-5319. doi: 10.1016/j.energy.2011.06.039

  31. [31]

      LI Q, ZHAO C, CHEN X, WU W, LI Y. Comparison of pulverized coal combustion in air and in O₂/CO₂ mixtures by thermo-gravimetric analysis[J]. J Anal Appl Pyrolysis, 2009, 85(1/2):  521-528.

  32. [32]

      MASEL R I. Chemical Kinetics and Catalysis[M]. New York: Wiley-Interscience, 2001.

  33. [33]

      BROWN M E, GALWEY A K. The significance of "compensation effects" appearing in data published in "computational aspects of kinetic analysis": ICTAC project, 2000[J]. Thermochim Acta, 2002, 387(2):  173-183. doi: 10.1016/S0040-6031(01)00841-3

  34. [34]

      ZSAKO J. Compensation effect in heterogeneous non-isothermal kinetics[J]. J Therm Anal, 1996, 47(6):  1679-1690. doi: 10.1007/BF01980913

• 

  Figure 1  Schematic of defined method of ignition temperature (t_i) and burnout temperature (t_b)

  下载: 全尺寸图片 幻灯片
  

  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

  下载: 全尺寸图片 幻灯片
  

  Figure 3  Comparison of the ratio for physical properties of different atmospheres at 600 ℃ and 0.1 MPa

  下载: 全尺寸图片 幻灯片
  

  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

  下载: 全尺寸图片 幻灯片
  

  Figure 5  Relationship between oxygen concentration and the kinetics parameters for combustion of L-char and B-char

  下载: 全尺寸图片 幻灯片
  

  Figure 6  Compensation effect of combustion of L-char and B-char

  下载: 全尺寸图片 幻灯片

  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

  下载: 导出CSV

  Table 2.  Combustion characteristic temperatures of chars in different atmospheres of 20% O₂ concentration

  Sample	Characteristic parameter	O2/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

  下载: 导出CSV

  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

  下载: 导出CSV

  Table 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

  下载: 导出CSV
•