Article pubs.acs.org/EF
Combustion Characteristics of Coal Gangue under an Atmosphere of Coal Mine Methane Jun Ren, Chuanjin Xie, Xuan Guo, Zhifeng Qin, Jian-Ying Lin,* and Zhong Li Key Laboratory of Coal Science and Technology, Taiyuan University of Technology, Ministry of Education and Shanxi Province, 030024 Taiyuan, China ABSTRACT: Combustion characteristics of coal gangue under O2/N2 mixtures with different O2 concentrations and coal mine methane (CMM) conditions were investigated by using thermogravimetric analysis (TGA) and tube reactor tests. Calculation results for the combustion characteristic parameters show that ignition temperature, burnout temperature, and peak temperature increase, while the maximum and average combustion rates decrease with decreasing O2 concentrations. The mass loss decreases and comprehensive combustion characteristic index increases slightly when CH4 is present in the combustion atmosphere with equivalent O2 concentration. TGA and tube reactor tests indicate that the mass loss of coal gangue is mainly attributed to the combustion of volatile matter and of fixed carbon. A temperature rise to ∼650 °C markedly inhibits carbon combustion because of O2 consumption caused by combustion of CH4 and H2 produced from CH4 cracking. Kinetic parameters (apparent activation energy (E), reaction order, and frequency factor) were calculated for various atmospheres by direct linear regression of the kinetic equation from a single TGA curve. The E value of coal gangue combustion varies from 117.6 to 137.4 kJ/mol under different atmospheres, and is slightly higher in CMM than in the O2/N2 mixture with equivalent O2 concentration.
1. INTRODUCTION Coal mine methane (CMM) is an unconventional gas that is stored in coal seams and surrounding rocks during the formation of coal. For safety reasons, CMM should be diluted and removed from coal mines through a ventilation system. When released CMM mixes with air, the CH4 concentration in the mixture is approximately 30−50 mol %, that of O2 is ∼10 mol %, and N2 comprises the remainder.1 The explosion limit of methane in the air changes from 5% to 15% at ordinary temperature and pressure and expands rapidly as the temperature or pressure increases, severely limiting utilization of this gas. In China, about 30% of CMM is utilized, but the quality of the remaining 70% is too poor for use and is therefore released to the atmosphere.2 CH4 is the third most important greenhouse gas (GHG) after H2O vapor and CO2, and has a global warming potential that is 21 times that of CO2.3,4 Moreover, it destroys the ozone layer seven times faster than does CO2.4 The amount of methane emitted from coal mines comprises ∼8% of the world’s anthropogenic methane emissions, which contributes 17% to the total anthropogenic GHG emissions.5 To recycle methane from CMM, it is necessary to remove contaminants, including hydrogen sulfide (occasionally found in CMM), water vapor, oxygen, carbon dioxide, and nitrogen through a series of connected processes. Deoxygenation is the most technically challenging and expensive process as most pipelines have very strict oxygen limits (typically 0.1% or 1000 ppm).6 Catalytic and noncatalytic deoxygenation are the two main methods used in CMM deoxygenation. Catalytic deoxygenation uses methane catalytic combustion to remove oxygen, leading to the consumption of some methane.7 The choice of catalyst is critical as the catalyst is easily deactivated. Dong et al.8 performed deoxygenation of CMM by using coke, but it is a relatively expensive process. Coal gangue remains as a residue in coal mining and washing, and it occupies land. Spontaneous © 2014 American Chemical Society
combustion and leaching of this residue result in serious pollution of air, water, and soil.9,10 In a previous study, we reported the removal of oxygen in CMM by using coal gangue combustion. In this process, two kinds of byproducts of coal mines are simultaneously utilized through the reaction between oxygen and carbon in coal gangue. This process may be used as a novel solution for recycling these byproducts.11 It is necessary to investigate the combustion characteristics of coal gangue under CMM atmospheres; such study can provide important data necessary for industrialized utilization of coal gangue and CMM. Previous studies have reported the combustion characteristics of coal, coal gangue, sludge, and biomass. Meng et al.12 found that the reactivity of coal gangue under oxy-fuel conditions differed from that under air combustion conditions. Xiao et al.13 studied the cocombustion behavior of sewage sludge with coal gangue or coal. Muhammad et al.14 found that apparent activation energy (E) decreases with increasing palm shell composition in coal as well as increasing O2 concentration in the oxy-fuel. Ran et al.15 concluded that higher volatile matter content of coal gangue contributes to its overall combustion performance. Nevertheless, studies on combustion characteristics of coal gangue under CMM atmospheres are few. In the present study, thermogravimetric analysis (TGA) was applied to investigate the combustion characteristics of coal and coal gangue under various O2/N2 mixtures as well as under a CMM atmosphere with different CH4 concentrations. The effects of concentrations of O2 and CH4 and of the properties of coal gangue on the combustion process of coal gangue were investigated. A method based on direct linear regression of the Received: February 22, 2014 Revised: May 16, 2014 Published: May 16, 2014 3688
dx.doi.org/10.1021/ef500446j | Energy Fuels 2014, 28, 3688−3695
Energy & Fuels
Article
Table 1. Ultimate and Proximate Analytical Results for Coal Slime and Coal Gangue Samples Ultimate analyses (wt %, ad)
Proximate analyses (wt %)
Sample
C
H
O
N
S
Mar
Vd
Ad
FCd
coal coal gangue
76.71 26.78
4.99 2.07
6.08 20.5
1.65 0.25
0.87 0.70
1.05 1.03
23.10 15.65
8.65 48.65
67.20 34.67
matter during combustion and identify the end of combustion or burnout. Ti, which was defined as the temperature at which samples started burning,12 may be determined through several methods. Wang et al.16,17 defined Ti as the temperature at which the combustion rate is raised to 1 wt %/min at the start of a major combustion process. Li et al.18 reported a TGA−DTG tangent method. As shown in Figure 1, a vertical line passing through
kinetic equation was used to simultaneously calculate kinetic parameters for various atmospheres by using a single TGA curve. To investigate the combustion behaviors of coal gangue, tube reactor tests were done in a CMM atmosphere in a fixedbed reactor under optimized reaction conditions.
2. EXPERIMENTAL SECTION 2.1. Sample Preparation. 2.1.1. Coal and Coal Gangue Samples. Coal and coal gangue samples were collected from a coal seam at Malan Mine of the Xishan Coal Electricity Group Co. Ltd. (Shanxi Province, China). Samples were milled and sieved into the particle size of ∼74 μm and then stored in a sealed plastic bag for the experiments. Results of proximate and ultimate analyses of the samples are given in Table 1. 2.1.2. Experimental Atmospheres. In this experiment, five different atmospheres were used. The atmospheres were prepared by mixing O2/N2 or CH4/O2/N2 under laboratory conditions and stored in cylinders. These various experimental atmospheres were designated as air, Nit85, Nit88, CMM-1, and CMM-2. Corresponding compositions of all atmospheres are presented in Table 2. Nit85 and CMM-1 atmospheres, as well as Nit88 and CMM-2 atmospheres, have equivalent O2 concentrations. Table 2. Compositions of the of Various Experimental Atmospheres
Figure 1. Determination of the ignition temperature (Ti) through the tangent method.
atmosphere composition (vol %) atmosphere
O2
CH4
N2
air Nit85 Nit88 CMM-1 CMM-2
21 15 12 15 12
0 0 0 28 43
79 85 88 57 45
the DTG peak point intersecting the TGA curve (point A) was constructed. A line tangent to the TGA curve at point A, which met the extended TGA initial level line at point B, was drawn. The temperature corresponding to point B was defined as Ti. In our study, Ti was calculated by using the method of Li et al.18 Tmax was the temperature corresponding to the peak of the DTG curve.16 Tb was defined as the temperature at which sample oxidation was completed; it was taken as the point immediately before reaction ceased, when the rate of mass loss was 1 wt %/min.19,20 2.4. Tube Reactor Tests. Tube reactor tests were performed on coal gangue under the CMM-2 atmosphere in a fixed-bed reactor with a continuous flow system at atmospheric pressure. The schematic of the combustion experimental setup is shown in Figure 2. The reactor was made of stainless steel tube having an inner diameter of 8 mm and a length of 60 cm. About 4.0 g of coal gangue sample was preloaded in the tubular reactor. CMM-2 was introduced into the reactor after the reactor was heated to the specified temperature (550, 600, 650, 700, or 750 °C) at a heating rate of 5 °C/min (650 °C. This result is due to the marked increase in the rate of methane decomposition above 650 °C. Moreover, the H2 concentration also rises with the decrease in flow rate at the same temperature, this change also becoming significant when the temperature is >650 °C. Figure 8 shows the change in the rate of CH4 loss with reaction temperature at different flow rates of the feed gas. The rate of CH4 loss increases with increasing temperature at the same flow rate. There is no marked change in the rate of CH4 loss at 550−650 °C, whereas the rate of CH4 loss increases drastically when the temperature is higher than 650 °C. This result shows that higher temperature or lower flow rate can lead to greater loss of methane. The gas flow rate is an important factor that affects the combustion process, as it varies the residence time of oxygen in the reactor. At a low flow rate (50 mL/min), the concentrations
TGA profile, the mass loss of coal gangue starts at 460 °C in the CMM-2 atmosphere. After stabilization of the sample weight, the atmosphere was switched from CMM-2 to Nit88. A further slight mass loss of ∼2.5 wt %, which is expected, is apparent in the TGA curve. This loss indicates that there are still combustible residues after coal gangue combustion in CMM-2 atmosphere. These residues are mainly attributed to the presence of CH4. Table 3 shows the combustion characteristic parameters of coal gangue under CMM atmosphere. It is evident from Table 3 that with the increase in CH4 concentrations and the decrease in O2 concentrations in the combustion atmospheres, all Ti, Tb, and Tmax values slightly increase. This phenomenon is consistent with the conditions in the O2/N2 atmosphere. Comparison of the combustion performance of coal gangue in 3692
dx.doi.org/10.1021/ef500446j | Energy Fuels 2014, 28, 3688−3695
Energy & Fuels
Article
Figure 8. Rate of methane loss of CMM-2 under different reaction conditions.
Figure 9. Variations in CO and CO2 concentrations in the outlet gas as a function of reaction temperature (flow rate: 100 mL/min).
of H2 and CH4 in the outlet gas showed marked changes compared with those at higher flow rates at the same temperature. Although the combustion reaction rate is considerably high, a certain residence time is required to complete the combustion reaction. However, extended residence times lead to significant loss of methane.11 Figures 7 and 8 show that the mathematical relation between H2 production and CH4 loss, as shown by eq 2, is not exact. This result suggests that CH4 loss is not only attributed to CH4 cracking, but also to CH4 combustion, as shown in eq 2. The change in flow rate of feed gas leads to varying contact times between CMM and coal gangue. The contact time shortens at higher flow rate of feed gas, thereby decreasing the rates of CH4 cracking and of CH4 combustion. Although longer contact time facilitates the reaction between O2 in CMM with volatile and fixed carbon in coal gangue, it also enhances CH4 loss.11
temperature increase. However, they changed markedly when the temperature reached 650 °C. This change further confirms the combustion of CH4 or H2 with some of the O2 at higher temperatures. In our previous work, we explored suitable conditions for CMM deoxygenation.11 Analysis in this work also confirms that when deoxygenation is performed with the appropriate flow rate of feed gas at
