Changes in Thermal Kinetics Characteristics during Low-Temperature

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Changes in thermal kinetics characteristics during low-temperature oxidation of low-ranking coals under lean-oxygen conditions Xiaoxing Zhong, Linda Li, Yun Chen, Guolan Dou, and Haihui Xin Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02197 • Publication Date (Web): 01 Dec 2016 Downloaded from http://pubs.acs.org on December 2, 2016

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Changes in thermal kinetics characteristics during

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low-temperature oxidation of low-ranking coals under

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lean-oxygen conditions Xiaoxing Zhong*,†,‡, Linda Li†,‡, Yun Chen§, Guolan Dou†,‡, Haihui Xin†,‡

4 5

†

6

Technology, Xuzhou 221116, China

7

‡

School of Safety Engineering, China University of Mining and Technology, Xuzhou 221116, China

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§

FAW-Volkswagen Automobile Co. Ltd, Changchun 130000, China

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ABSTRACT: The present work derives the heat flow and mass-loss curves for the oxidation of

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three low-ranking coals under lean-oxygen conditions using thermogravimetry and differential

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scanning calorimetry. Relationships between the exothermic character of the low-temperature

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oxidation (the stage before the volatile combustion of coal) and oxygen concentration were

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analyzed. Kinetic parameters for low-temperature coal oxidation under lean-oxygen conditions

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were obtained, based on a derived Arrhenius equation. The results demonstrate that there is a

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critical oxygen concentration, above which the full oxidative combustion reaction of low-ranking

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coal will reach under lean-oxygen conditions of continuous ventilation; if the oxygen

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concentration exceeds the critical value, the temperature at which coal oxidative combustion

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reaction is completed is retarded as the oxygen concentration decreases. The amount of heat

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released during low-temperature oxidation decreases with the decrease of oxygen concentration,

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but the influence on total quantity of heat released is small. During the low-temperature oxidation

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of low-ranking coal, three stages of kinetic parameters were defined: during Stages 1 and 2, the

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apparent activation energy and pre-exponential factor fluctuated slightly at average values as the

Key Laboratory of Coal Methane and Fire Control, Ministry of Education, China University of Mining and

ACS Paragon Plus Environment

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oxygen concentration changed; however, during Stage 3, the apparent activation energy decreased

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in a linear manner as the oxygen concentration decreased and pre-exponential factor for XJ-H and

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XJ-Z coal samples decreased in a linear manner while it decreased in an exponential manner for

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PS coal sample.

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Keywords: Low-ranking coal; Oxygen-poor; Low-temperature oxidation; Kinetic parameters

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1. INTRODUCTION

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With respect to the interaction of coal and oxygen, spontaneous combustion may occur when

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the rate of heat generation from the coal is faster than its rate of release. Spontaneous combustion

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not only wastes coal resources, but produces hazardous gases and can cause gas or coal dust

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explosions.1-2 An understanding of the thermal and kinetic parameters of the low-temperature coal

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oxidation process is necessary to forecast and avoid the risks of spontaneous combustion in coal

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mine goafs and coal fire areas.

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Adiabatic oxidation, basket heating, and thermal analysis are generally applicable methods for

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studying coal oxidation at low temperatures. Some scholars have studied the process based on the

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adiabatic oxidation technique proposed by Davis and Byrne3 in 1924. Adiabatic oxidation was

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adopted by Ren et al.4 to study the tendency for spontaneous combustion in 18 coal samples and

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the influences of moisture content, grain diameter, initial temperature, and sample pre-oxidation

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on the process. Beamish et al.5 used adiabatic oxidation experiments to determine the heating rate

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and analyze how this influences pre-oxidation. From measuring adiabatic temperature curves for

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nine coal samples, Lin et al.6 concluded that the activation energy gradually decreased with

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increasing temperature during the spontaneous combustion process. Adiabatic measurements are,

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however, very time-consuming because the experimental conditions are harsh and the amount of


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heat generated during the initial oxidation step is small. The traditional basket-heating method is

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based on Frank-Kamenetskii’s steady-state analysis, but this method requires the critical

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temperatures of many different-sized coal samples. Chen7 and Jones et al.8 later proposed a revised

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basket-heating method, which only needs to test the junction temperature of a single-sized coal in

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the critical temperature range. Although the testing time is less than that of the Frank-Kamenetskii

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method, it is necessary to change temperatures to confirm the critical temperature, so this

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approach is still time-consuming. Compared with adiabatic oxidation and basket-heating methods,

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thermal analysis has the advantages of requiring fewer test samples, a shorter experimental

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duration, and good repeatability and is now widely used in the study of spontaneous combustion

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of coal.9-17 Qingsong Wang11-12 adopted C80 microcalorimeter to test the exothermic process of

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coal spontaneous combustion and predicted the self-heating oxidation temperatures (SHOT) of

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coal stockpile. Peng Chen13 adopted TG-DSC to test thermal decomposition process at different

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heating rate for three different ranks coal, and a model-fitting method and an isoconversional

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method are adopted to analyze the kinetic parameter at low-temperature oxidation stage. Rotaru14

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adopted thermogravimetry (TG), differential scanning calorimetry (DSC), and differential thermal

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analysis (DTA) to examine the exothermic processes of Romanian and Urals oblast bituminous

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coals and compared their kinetic parameters. Benfell et al.15 used thermogravimetry to analyze

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New Zealand and Eastern Australian coals, and identified the temperatures of the various features

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of the exothermic process and the relationship between the volatile matter content of different

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grain diameters and the maximum rate of combustion. Zhang et al.16 tested Shengdong coal and

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determined the TG curves for the low-temperature oxidation step using the difference between

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experiments carried out in dry air and in pure nitrogen. Changdong Sheng, Li et al.17 used TGA


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and DSC to analyze the dynamic parameters of low-temperature oxidation of coal and compared

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these with results obtained from the basket heating method.

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Most experimental research into the low-temperature oxidation of coal has, however, been

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conducted in an air atmosphere. In actual coal mine production, spontaneous combustion disasters

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happen under lean-oxygen conditions, when the oxygen concentration is lower than that of air. For

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instance, spontaneous combustion of goaf occurs in the self-sustaining combustion zone, where

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the oxygen concentration is between 5% and 18%.18-21 Spontaneous combustion in coalfield fire

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areas also creates lean-oxygen conditions because consumption of oxygen during the burning of

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the fire lowers the oxygen concentration of those areas with poor ventilation to below that of air.

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Research into the thermal properties of the low-temperature oxidation of coal under

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lean-oxygen conditions is rare, and most is concerned with determining thermodynamic properties

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at a particular low oxygen concentration. Wang et al.22 derived kinetic data from one model coal

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and three real coal samples under conditions of 10% oxygen, based on DTG experiments. Zhan et

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al.23 analyzed the exothermic character and activation energy of coal in 10% oxygen. Changdong

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Sheng, Li et al.17 compared the kinetic data obtained at 5% and 15% oxygen concentrations using

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TGA–DSC. There is currently no systematic study concerning the effect of oxygen concentration

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on the thermal properties of low-temperature oxidation of coal under lean-oxygen conditions.

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Spontaneous combustion occurs particularly in low metamorphic coals (low-ranking coal)

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such as lignite, long flame coal and non-caking coal and so on.24-25 This type of low-ranking coal

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comprises about half of the world’s coal reserves. About 41% of global low-ranking coal reserves

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occur in China, mainly distributed in Xin Jiang municipality, Shanxi and Shanxi Province.26 This

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study measures the heat flow and mass changes during the reaction of low-ranking coal under


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lean-oxygen conditions, based on TG–DSC methods, and analyzes changes in kinetics

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characteristics of the low-temperature oxidation.

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2. EXPERIMENTAL SECTION

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2.1. Coal samples. Low-ranking coal samples were sourced from the coal area of Hoxtolgay,

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Xinjiang Province (long flame coal labeled XJ-H), the south opencast coal mine of Zhundong,

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Xinjing Province (non-caking coal XJ-Z), and the No. 3 coal mine of the China National Coal

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Group Co., Ltd., in Pingshuo, Shanxi Province (long flame coal PS). Since spontaneous

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combustion all happened to untreated raw coal in coal mine goafs, coal fire areas and ground coal

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piles, raw coal collected from the coal mine was selected as coal sample in this experiment. The

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fresh raw coal lumps were taken from the coal mine and sent to the laboratory hermetically and

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preserved in the low temperature. In the experiment, coal sample lumps were milled with their

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surfaces stripping under an inert atmosphere in a glove box and fragments which met the particle

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sizes required were used for the experimental investigations. Where, in order to analyze the

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influence of particle size to heat flow values, the experimental particle sizes of XJ-Z coal were set

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as 200-450µm, 125-200µm, 96-125µm, 74-96µm and ＜74µm respectively, from which the

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optimal particle size applicable to this experiment will be selected. The experimental particle size

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of XJ-H coal and PS coal was the optimal particle size of XJ-Z coal selected. Their properties are

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given in Table 1.

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2.2. Experimental process. Simultaneous thermal analysis was carried out in a SDT-Q600

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instrument (TA of American), which was used to conduct TG and DSC experiments on XJ-Z coal

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first. At the oxygen concentration of 21%, the change of heat flow for XJ-Z coal of five different

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particle sizes with the reaction temperature, so as to analyze the optimal particle size applicable to


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this experiment. Then, at the optimal particle sizes, the change of the heat flow and mass of the

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three low-ranking coals with the reaction temperature under different oxygen concentrations.

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Reaction gases in this experiment is composed of ultra-high-purity N2 and ultra-high-purity O2 at

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certain proportion, with the oxygen concentration being 21%, 16%, 12%, 8%, 6%, 4%, and 2%

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respectively. The purge gas flow rate was 100 ml/min and the sample was heated from 28°C to

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1000°C at a heating rate of 2°C/min.27

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3. RESULTS AND DISCUSSION

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3.1. DSC of XJ-Z coal at different particle sizes and analysis. Figure 1 shows the heat flow

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curves of XJ-Z coal of different particle sizes at the oxygen concentration of 21%. The stagnation

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temperature (Tc) of the coal in the low-temperature step is determined by extrapolation of the

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onset of the heat flow curve.28 As shown in Figure 2, To is the initial exothermic temperature and

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the range from To to Tc represents the low-temperature oxidation step. The To values for XJ-Z coal

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sample at different particle sizes are shown in Table 2. For XJ-Z coal sample at different particle

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sizes, the heat outputs of the entire exothermic process, ∆H, and the heat released by

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low-temperature oxidation, ∆HL, were obtained by integrating the heat flow curves; the results are

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given in Table 2. From Figure 1 and Table 2, it can be seen that when the particle decreased, the

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initial exothermic temperature decreased while the heat released by low-temperature oxidation

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increased and showed stronger exothermic effect. Because the reaction of coal was relatively weak

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in low-temperature oxidation and the mass of testing coal used in the TG-DSC experimental

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system was only 10mg, if coal samples with large particles were chosen as testing coal, the heat

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flow value would be greatly influenced by the response accuracy of SDT-Q600 instrument. On the

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contrary, effect of SDT-Q600 instrument’s response accuracy on test data was reduced by


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choosing coal samples with small particles and test data would be more accurate. It will also help

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to analyze the reaction characteristics of coal at low temperature oxidation. Therefore, coal sample

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with particle size < 74 microns was selected as testing sample at different oxygen concentrations.

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3.2. DSC-TG under lean-oxygen conditions and analysis. Figures 3 to 5 show the heat flow

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(DSC) and TG curves for the coal samples in an oxygen-poor atmosphere. From the heat flow

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curve, it can be seen that for three coal samples there is a distinct difference between curves when

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the oxygen concentration is 2% and curves with other oxygen concentration. The heat flow curve

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resembles that of a pyrolysis process under pure N2 conditions, where the oxidative combustion

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reaction cannot continue. For the PS sample, although there is a combustion peak for the

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high-temperature step at an oxygen concentration of 4%, there is a visible decrease in heat flow

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for oxygen concentrations of 6% to 21% and the curve remains in the low-temperature phase.

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From Figure 5(b), we can see a distinction between the TG curves at 4% oxygen for the PS sample

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and those above this value. The TG characteristics below 450 °C are similar to those measured at

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an oxygen concentration of 2%. This illustrates that the oxidative combustion reaction cannot

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continue for these samples when the oxygen concentration is 4%. For oxygen concentrations

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above 4% for XJ-H and XJ-Z and above 6% for PS, the overall trends of the heat flow curves for

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the various oxygen concentrations are similar. As the temperature increases, the heat flow

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increases gradually at first and two exothermic peaks appear during combustion; the first one is

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volatiles exothermic peak and the second one is fixed carbon exothermic peak. The first

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exothermic peak for three coal samples is lower than the second one. With lower oxygen

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concentration, the exothermic peak decreases and the exothermic process migrates to the

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high-temperature step. This illustrates that the reaction rate slows down when the oxygen


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concentration is low and higher temperature is needed for complete reaction.

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The overall trends of the TG curves resemble each other under lean-oxygen conditions. As the

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oxygen concentration is reduced, the TG curves shift to higher temperature. Until oxygen

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concentrations is 2% for XJ-H and XJ-Z and is 4% for PS, the TG curves show obvious migration.

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To analyze how the oxygen concentration influences the initial and final reaction rates, differential

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processing was applied. Figure 6(a) shows the DTG curves of XJ-H, as an example. The oxygen

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concentration has a similar influence on the initial and final reaction rates. Figure 6(b) shows a

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partially enlarged portion of the low-temperature DTG curve (where a negative DTG value

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signifies mass gain). As the oxygen concentration decreases, the maximum rate of mass gain

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decreases and the mass-gain temperature range shrinks. The mass-gain process disappears when

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the oxygen concentration is equal to or lower than 4%. This may be explained as follows: mass

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variation in the low-temperature oxidative process is caused by oxygen absorption during surface

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oxidation of the coal and the formation of intermediate products (ethers, carbonyls, and other

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stable materials belonging to the carbonyl class); when the oxygen concentration is too low, the

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oxidation rate and oxygen absorption slows down and the mass loss caused by the decomposition

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of the intermediate products becomes larger than the mass gain attributed to oxygen absorption, so

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the overall performance is a loss of mass. When the temperature rises to about 230°C, the coal

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samples rapidly lose mass; the lower the oxygen concentration, the lower the rate of mass loss.

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3.3. Relationship between thermal characteristics of the low-temperature oxidation phase

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and oxygen concentration. The To and Tc values for the three coal samples under different

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oxygen concentrations, as well as the heat outputs of the entire exothermic process, ∆H, and the

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heat released by low-temperature oxidation, ∆HL, are shown in Table 3. From Table 3, it is seen


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that when the oxygen concentration exceeds 4% for XJ-H and XJ-Z and 6% for PS, the total heat

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released during the entire exothermic process changes little and the initial exothermic temperature

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is generally the same with the variation of oxygen concentration in the poorly ventilated

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atmosphere. When the oxygen concentration is 2% for XJ-H and XJ-Z and 4% for PS, the

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exothermic character of three coal samples in low-temperature oxidation has not been detected.

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Although there is an exothermic peak in the later reaction of PS coal sample at an oxygen

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concentration of 4%, the decreasing range for heat outputs of the entire exothermic process is

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greater than that of at oxygen concentration exceeding 6%. This indicates that a critical oxygen

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concentration exists for different coals (the critical oxygen concentration is between 2% and 4%

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for XJ-H and XJ-Z and between 4% and 6% for PS), at which the oxidation combustion reaction is

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full under oxygen-poor conditions. The coal can still fully react when the oxygen concentration

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exceeds this critical value; however, the temperature required for the complete reaction is retarded,

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although both To and Tc hardly vary as the oxygen concentration is reduced. The heat output for

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the entire reaction and the ratio of this to the total heat released during the low-temperature

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oxidation decrease as the oxygen concentration decreases. This explains why the extent of

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oxidation in the low-temperature step decreased with decreasing oxygen concentration.

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4. CHANGE OF KINETIC PARAMETERS DURING LOW-TEMPERATURE

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OXIDATION OF COAL UNDER OXYGEN-LEAN CONDITIONS

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4.1. Principle of determination of kinetic parameters. Kinetic analysis can be carried out using

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either the single or multiple scan-rate methods.29-31 In the process of solving kinetic parameters

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using a single scan rate, it is necessary to first select a linear optimal equation as the most probable

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mechanism function using the “mode matching method”; however, because of the dynamic


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compensation effect, when kinetic triplets are determined from a thermal analysis curve, the

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values of the apparent activation energy (E) and the pre-exponential factor (A) can provide a good

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linear fit for all mode functions by mutual compensation. Therefore, unless either E or the most

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probable mechanism function f(α) are ascertained beforehand, the accuracy of kinetic results

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obtained from a single curve is greatly affected. Multiple scan-rate methods, such as those of

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Flynn–Wall–Ozawa (FWO), Kissinger–Akahira–Sunose (KAS), and Friedman, conduct kinetic

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analysis by measuring the exothermic peak at different temperature rise rates; however, because of

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the high temperature of the exothermic peak for coal samples, the results obtained generally reflect

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the characteristics of the coal combustion step, rather than the low-temperature oxidation step.

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In view of the defects of these traditional methods, Changdong Sheng, Li et al.17 and

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Changdong Sheng, Fan et al.32 proposed a method based on TG–DSC experiments to study the

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low-temperature oxidation of coal. According to the kinetic theory of chemical reactions and the

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Arrhenius Law, the rate of heat release (the reaction order is assumed to be 1) is given by:

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= exp − ⁄

(1)

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Where q is the rate of heat release (W); m is the mass of the coal sample (g); Q is the heat of

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reaction (J/g); A is the pre-exponential factor (s−1); is the apparent activation energy (J/mol);

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R is the universal gas constant (8.314 J/(mol·K)); and T is the reaction temperature (K). When a

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calorimeter is used to conduct kinetic analysis of the oxidation of coal, eq 1 is usually used to

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represent the rate of heat release for the exothermic low-temperature oxidation process.

218 219 220

Processing eq 1 by transposition and taking logarithms gives: ln ⁄ = ln − ⁄

(2)

Where q and m at any temperature are obtained from the TG and DSC experimental conditions.


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A scatter diagram of ln(q/m) against 1/T is plotted and linear fitting is performed; E and A are

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respectively determined from the slope and intercept.

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4.2. Relationship between kinetic parameters and oxygen concentration for the

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low-temperature oxidation step. Using eq 2, scatter diagrams of ln(q/m) against 1000/T for the

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low-temperature oxidation of coal were plotted by analyzing the TG–DSC experimental results for

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XJ-H and XJ-Z at oxygen concentrations between 4% and 21% and for PS at oxygen

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concentrations between 6% and 21%. Considering sample XJ-H with an oxygen concentration of

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21% as an example (Figure 7), ln(q/m) shows three stages of kinetic behavior as the temperature

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increases: first an increase (defined as Stage 1), then a slow increase (Stage 2), and finally a rapid

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increase (Stage 3). The scatter values exhibit good linear correlation with 1000/T. The equations of

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the linear fits to each of the three stages are shown in Figure 7. (To avoid the influence of

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transitions between stages, some points in the transition regions were discarded in the linear fitting

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process.)17 The intersection point of fitting curves are treated as the demarcation point of three

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stages.

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The values of E and A for the three stages of low-temperature oxidation of three coal samples

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were calculated from the respective slopes and intercepts; the results are shown in Table 4. For

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these three coal samples, the apparent activation energy of the process is small, especially in Stage

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2, the value of which is significantly smaller than that for Stages 1 and 3. The apparent activation

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energy under certain oxygen concentrations is even negative. The reason is that the

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low-temperature reaction of coal is very complicated which includes moisture evaporation,

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physical and chemical absorption of oxygen accompanied by change of heat flow and mass. The

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value of apparent activation energy at different stage is depended on the relation of q/m and


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temperature (T) at different stages. When the ln(q/m) is of negative temperature coefficient, the

244

apparent activation energy is negative.16,33-35 The values of A throughout the low-temperature

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oxidation step were relatively small, which is consistent with the slow reaction rates.

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Figures. 8 to 10 show the variation of the kinetic parameters of coal samples XJ-H, XJ-Z, and

247

PS, respectively, with oxygen concentration. The influence of oxygen concentration on the kinetic

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parameters in Stages 1 and 2 is small: the apparent activation energies (E) of samples XJ-H, XJ-Z,

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and PS in Stage 1 fluctuate slightly around 5.26 kJ/mol, 22.13 kJ/mol, and 3.02 kJ/mol,

250

respectively, while A averages 3.41×10−4 s−1, 0.033 s−1, and 1×10−4 s−1, respectively; for Stage 2, E

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for samples XJ-H, XJ-Z, and PS averages 0.392 kJ/mol, −0.29 kJ/mol, and −0.182 kJ/mol,

252

respectively, and A averages 6.34×10−5 s−1, 3.68×10−5 s−1, and 2.72×10−5 s−1, respectively.

253

However, the kinetic parameters of Stage 3 differ significantly, the Origin, a data processing

254

software was used in the fitting of the changing relationship between the kinetic parameters and

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oxygen concentration at the Stage 3 of low-temperature oxidation. The function which could

256

obtain the maximal value of goodness of fit R2 would be the final function of relationship between

257

the kinetic parameters and oxygen concentration in Stage 3. The result shows that E decreases

258

linearly with decreasing oxygen concentration. The relationships between the apparent activation

259

energy and oxygen concentration ( ) for XJ-H, XJ-Z, and PS in Stage 3 are given by =

260

0.44 + 7.01, E = 0.62c" + 4.65, and E = 0.85c" + 5.56, respectively. The pre-exponential

261

factor decreases either linearly or exponentially: the relationships between A and oxygen

262

concentration for XJ-H and XJ-Z during Stage 3 are given by = 1.97 × 10'( − 4.6 × 10'(

263

and = 1.04 × 10'( − 1.18 × 10'(, respectively, while that of PS is A = 0.002e*.++(,- .

264

4.3. Application and comparison. The above analyses illustrate that, for continuous ventilation


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under lean-oxygen conditions and in the oxygen concentration range required for full oxidative

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combustion, a rule—not a constant value—determines the change in apparent activation energy

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and pre-exponential factor of the low-temperature oxidation of coal as the oxygen concentration

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decreases. In calculation using Arrhenius equation, we must therefore revise the expressions for E

269

and A of the reaction rate constant, k, as given in eq 3; other research into spontaneous combustion

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processes occurring in coal piles, gob mining, and other areas36 proposes the use of eq 4. eq 5 is an

271

equation for calculating k transformed from eq 1.

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k =  g co2  exp(−  f co2  / RT )    

( )

(3)

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k =  c( o2 )  A′ exp( − E ′ / RT )

(4)

274

k=

( ) n

q

(5)

mQ

275

Where k is the reaction rate constant (s-1); refers to the oxygen concentration (%);

276

./ 0is the function that prescribes how the apparent activation energy changes with oxygen

277

concentration; g ( co ) is the function that prescribes how the pre-exponential factor changes with 2

278

oxygen concentration; n is the reaction order (generally 1)36; E ′ refers to the apparent activation

279

energy at Stage 3 when the oxygen concentration is at 21%. Where, A′ =A/0.21 (A is the

280

pre-exponential factor obtained in this paper at the third stage of low temperature oxidation at the

281

oxygen concentration of 21%).

282

Considering the low-temperature oxidation of XJ-H coal in 12% oxygen as an example, Figure

283

11 shows the k value calculated from eqs 3 and 4 compared with the experimental results

284

calculated from eq 5. The k value obtained in eq 3 closely approximates that in experimental

285

measurements, while that in eq 4 differs significantly from these.

286

Table 5 shows the extent of deviation between the calculated and experimental k values for


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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

287

XJ-H coal at oxygen concentrations of 12%, 8%, and 6% and temperatures of 50°C, 100°C, 150°C,

288

and 200°C. Table 5 indicates that, regardless of the relationship between E and A and the oxygen

289

concentration, the extent of deviation between the calculated and experimental k values at different

290

temperatures reduces as the oxygen concentration increases. According to this comparison,

291

revision of the traditional Arrhenius equation based on eq 3 can improve estimates of the kinetic

292

parameters for these reactions and contribute to increasing the accuracy of numerical calculations.

293

5. CONCLUSIONS

294

(1) Under conditions of continuous ventilation, when the oxygen concentration exceeds the

295

critical oxygen concentration，the full oxidative combustion reaction of a low-ranking coal will

296

reach. The decrease of the oxygen concentration will result in the decrease of the oxidation degree

297

at low-temperature oxidation. The rate of heat release during low-temperature oxidation reduces

298

with decreasing oxygen concentration; however, considering the entire reaction, the total quantity

299

of heat released is only slightly influenced by decreasing oxygen concentration and the

300

temperature at which the full oxidative combustion reaction of coal is retarded.

301

(2) During the low-temperature oxidation of low-ranking coal, three stages of kinetic

302

parameters are identified: in Stages 1 and 2, the kinetic parameters (E and A) are slightly

303

influenced by the oxygen concentration; however, in Stage 3, the apparent activation energy

304

decreases linearly and the pre-exponential factor decreases either linearly or exponentially as the

305

oxygen concentration decreases.

306

(3) In calculating the kinetic characteristics of coal under lean-oxygen conditions, revision of

307

the traditional Arrhenius equation, based on the relationships between the apparent activation

308

energy and the pre-exponential factor and oxygen concentration, can increase the accuracy of the


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numerical calculations.

310

■ AUTHOR INFORMATION

311

Corresponding Author

312

*Tel.: +86 13585396491. E-mail: [email protected].

313

Notes

314

The authors declare no competing financial interest.

315

■ Acknowledgments

316

The authors wish to express their gratitude for joint funding by the National Natural Science

317

Foundation of China (No.51474210), the New Century Excellent Researcher Award Program from

318

the Ministry of Education of the People’s Republic of China (No.NCET-13-1021), special funding

319

for talent cultivation project of China University of Mining and Technology (2015YC01), the

320

Projects of Scientific Research and Innovation for the Graduates in Jiangsu Colleges and

321

Universities (SJZZ16_0273), and the Priority Academic Program Development of Jiangsu Higher

322

Education Institutions.

323

■ References

324

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a sample of Ural bituminous coal. J. Thermal Analysis and Calorimetry 2012, 110(3), 1283-1291.

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injection into goaf. Heat Mass Transfer 2016, 52, 2723-2734.

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combustion in goaf. International Journal of Mining Science and Technology 2013, 23, 489-493.

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Table 1. Properties of the coal samples Properties

XJ-H

XJ-Z

PS

Mada

5.27

15.92

6.60

Aadb

Proximate analysis (%)

9.29

6.61

5.79

c

15.48

7.86

6.20

Vadd

44.74

20.68

33.21

Vdafe

48.33

26.69

37.9

40.7

56.79

54.4

C

77.04

73.97

76.88

H

5.14

3.82

3.32

N

1.30

1.37

0.61

S

1.00

0.436

0.35

O

15.52

20.38

18.84

Lower heating value (MJ/kg)

22.76

23.30

21.93

Ad

FCad

f

Ultimate analysis (%, dry ash-free basis)

398

a

Moisture of air dried basis

399

b

Ash content of air dried basis

400

c

Ash content of dried basis

401

d

Volatile of air dried basis

402

e

Volatile of dry ash-free basis

403 404

f

Fixed carbon of air dried basis


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405

Table 2. Initial exothermic temperature and heat release of XJ-Z coal of different particle sizes at oxygen

406

concentration of 21% Particle size (µm)

Toa(0C)

∆Hb (J/g)

∆HLc (J/g)

200-450

43.46

22001

1566

125-200

42.72

21951

1702

96-125

42.18

22090

1913

74-96

41.74

21803

2103

Changes in Thermal Kinetics Characteristics during Low-Temperature

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