Inhibiting effect of CO2 on the oxidative combustion thermodynamics of coal

Study on the thermal kinetics and microscopic characteristics of oxidized coal

Revealing the characteristics of spontaneous combustion and re-combustion of oxidized coal is of great significance for the coal fire prevention and control. Synchronous Thermal Analyzer (STA) and Fourier Transform Infrared Spectrometer (FTIR) were used to measure the thermal kinetics and microscopic characteristics of coal samples with different oxidation degrees (unoxidized, 100 ℃, 200 ℃ and 300 ℃ oxidized coal). It is found that the characteristic temperatures decrease first and then increase with the increasing degree of oxidation. The ignition temperature of 100 ℃-O coal (oxidized at 100 ℃ for 6 h) is relatively the lowest at 334.1 ℃. Pyrolysis and gas-phase combustion reactions dominate the weight loss process, while solid-phase combustion reactions are relatively minor. The gas-phase combustion ratio of 100 ℃-O coal is the highest at 68.56%. With the deepening of coal oxidation degree, the relative content of aliphatic hydrocarbons and hydroxyl groups gradually decreases, while that of oxygen-containing functional groups (C-O, C = O, COOH, etc.) increases first and then decreases, reaching the highest value of 42.2% at 100 ℃. Moreover, the 100 ℃-O coal has the minimum temperature at the point of maximum exothermic power of 378.5 ℃, the highest exothermic power of -53.09 mW/mg and the maximum enthalpy of -18,579 J/g. All results show that 100 ℃-O coal has the highest risk of spontaneous combustion than the other three coal samples. This suggests that there is a maximum point of spontaneous combustion risk in the range of pre-oxidization temperatures of oxidized coal.

Investigation on the co-combustion characteristics of multiple biomass and coal under O2/CO2 condition and the interaction between different biomass

The co-combustion of coal and biomass in O₂/CO₂ conditions is a promising technology for CO₂ capture and waste disposal. Little attention has been paid to the interaction between different biomass in co-combustion process, which is of great significance to the study of the co-combustion mechanism. The co-combustion behavior of coal and multiple biomass under isothermal conditions was characterized by thermogravimetric method, and the interaction between different biomass was investigated from the perspective of thermogravimetric and proximate analysis. It found that biomass blending could remarkably improve the combustion performance of coal. Compared to the theoretical prediction, the interaction between coal and biomass showed remarkably promoting effects when the coal was blended with different biomass. While the interaction between different biomass was weak. Moreover, the influence of proximate analysis on combustion characteristic parameters was studied by establishing the linear relationship between combustion characteristic parameters and proximate analysis. The effects of proximate analysis on characteristic time/S were divided into five categories, and it were mainly controlled by the interaction both between coal with biomass and between different biomass.

Determination of Cocombustion Kinetic Parameters for Bituminous Coal and Pinus Sawdust Blends

Cocombustion of bituminous coal (HC) and Pinus sawdust (PS) was investigated in this paper with the aim of determining the kinetic parameters relevant to cocombustion reactions of their fuel blends. PS was used because it is a waste biomass product capable of generating energy. Motivated by the need to partly substitute HC used in existing boilers with PS, the optimum kinetic parameters at different blending ratios were thus investigated with the ultimate goal of diversifying the energy portfolio for these boilers. Blended samples were prepared with a PS substitution by mass ranging from 0 to 30%, thus producing five samples, namely:100HC, 90HC10PS, 80HC20PS, 70HC30PS, and 100PS. A simultaneous thermogravimetric analyzer was used to investigate the degradation of the fuel samples under a synthetic air atmosphere using 5, 12.5, and 20 °C/min heating rates. The kinetic parameters were evaluated using the distributed activation energy model (DAEM) due to its ability to evaluate complex parallel chemical mechanisms. The influential homogenous volatile combustion and heterogenous combustion stages produced an increasing trend for activation energy (E _a) with increased PS (100HC to 70HC30PS) from an average of 61.80-104.34 kJ/mol while the pre-exponential factor increased from 1.31 × 10⁵ to 6.52 × 10⁸. Generally, blending of HC with PS did not produce a linear variation of the kinetic parameters; thus, by using various plots, an optimum blending ratio of 80HC20PS was deduced.

Co-combustion of high alkali coal with municipal sludge: Thermal behaviour, kinetic analysis, and micro characteristic

Blending sludge rich in protein and aliphatic hydrocarbons into the high alkali coal (HAC) has been demonstrated to reduce the ash melting temperature of the HAC/sludge mixture, thereby increasing the effectiveness and efficiency of liquid slagging. However, whether the incorporation of sludge can affect the combustion stability of the original coal-fired boiler is still debatable. As the combustion stability of the fuel can directly affect the operational safety of the boiler, it is of great practical value for exploring the effect of sludge incorporation on the combustion performance of HAC. In this work, the thermal behaviour and microscopic properties of individual HAC, municipal sludge (MS) and HAC/MS mixtures were tested using a Thermogravimetric analyser (TGA) and a Fourier transform infrared (FTIR) spectrometer, respectively. The exothermic, thermodynamic and functional group evolution patterns during the combustion of these samples were also evaluated. Ignition temperatures (T_i) of the HAC/MS mixtures were relatively lower than that of individual HAC, and decreased with the increase in sludge mass ratio (SMR). The synergistic effect of the co-combustion of HAC and MS resulted in a slightly higher total heat release during the combustion of MS10HAC90 (i.e., the mass percentage of MS and HAC is 1:9) than HAC alone, however, the total heat release of the blend decreased progressively with increasing SMR. The experimental values of the average E_α for all four mixtures were lower than the theoretical values, indicating that the addition of MS lowered the reaction energy barriers of the mixtures. Consumption rates of the principal groups in samples during the oxidation and combustion all tended to increase progressively with increasing SMR. There are three major synergistic effects existing during co-combustion of HAC and MS: (1) the reaction of free radicals with benzene molecules; (2) the interaction of free radicals; and (3) the catalytic effect of alkali and alkaline earth metals. These findings can provide theoretical guidance for the determination of key parameters (mixing ratio) for the blending of HAC and MS, and can fill the research gap in terms of microscopic reactivity and synergistic effects during the co-combustion of HAC and MS.

Quantitative analysis of heat release during coal oxygen-lean combustion in a O2/CO2/N2 atmosphere by TG-DTG-DSC

Heat release of coal combustion in an oxygen-lean and multi-gas environment is a common phenomenon, coalfield fires caused by it can lead to serious environmental destruction and loss of coal resources. Simultaneous thermal analysis experiments for Bulianta (BLT, high-volatile bituminous coal) and Yuwu coal (YW, anthracite) in 21vol.%O₂/79vol.%N₂ and 15vol.%O₂/5vol.%CO₂/80vol.%N₂ were carried out to study the law of heat release. Based on the TG-DTG-DSC curves, the combustion characteristic parameters were analyzed. Decreasing O₂ concentration caused a significant reduction of local reactivity and further the decreasing maximum heat release rate for low-rank coal, while increasing CO₂ concentration caused a significant thermal lag effect and further the increasing maximum heat release rate for high-rank coal. The relationship between the heat release rate and the reaction rate constant was quantitatively analyzed. At the increasing stage of the heat release rate, the heat release rate of the two coals increased conforming to ExpGro1 exponential model. At the decreasing stage of the heat release rate, the heat release rate of YW coal decreased exponentially with the reaction rate constant, while the heat release rate of BLT coal decreased linearly. Regardless of the atmospheres, the conversion rates corresponding to maximum heat release rate of BLT and YW coal were about 0.80 and 0.50, respectively, indicating that the coal rank played a dominant role. The results are helpful to understand the heat release process of coal oxygen-lean combustion in O₂/CO₂/N₂.

Effect of Oxygen Concentration on the Oxidative Thermodynamics and Spontaneous Combustion of Pulverized Coal

Spontaneous combustion of pulverized coal has become a safety topic and has been extensively researched. This study using differential scanning calorimetry investigated the exothermic characteristics and spontaneous combustion risk of three metamorphic pulverized coal samples during oxidative combustion, for oxygen concentrations of 21, 19, 17, 15, 13, 11, 9, 7, and 5 vol %. Results indicated that decreased oxygen concentrations reduced exothermic intensity and substantially increased ignition temperatures. The oxidative thermal release observed during the combustion stage was conspicuously higher than during the low-temperature oxidation stage. Thermal release during low-temperature oxidation was low during low oxygen concentrations; however, when the oxygen concentration was less than 13.0 vol.%, it had a considerable influence on exothermic combustion. When the oxygen level was lowered from 21.0 to 5.0 vol %, spontaneous combustion risk indexes lessened from 2.07 (sample A), 1.85 (sample B), and 0.81 [J/(mg min °C2)] (sample C) to 1.08 (sample A), 1.13 (sample B), and 0.40 [J/(mg min °C2)] (sample C), respectively. Both apparent activation energy and spontaneous combustion risk indexes of the samples decreased saliently as oxygen concentration decreased. Thus, reducing oxygen concentration would be an effective method of inhibiting or possibly even preventing the spontaneous combustion of pulverized coal.

Experimental Study on the Inhibition Effects of Nitrogen and Carbon Dioxide on Coal Spontaneous Combustion

Inert gases can effectively inhibit coal spontaneous combustion. In this paper, the inhibition effect of inert gases (N2 and CO2) on coal spontaneous combustion was studied. In the low-temperature oxidation stage, the constant-temperature heat release and apparent activation energy of coal sample were measured and calculated by the C80 micro-calorimeter. In the high-temperature combustion stage, the critical temperature, maximum peak temperature, ignition temperature, and burn-out temperature of coal samples were analyzed by the synchronous thermal analyzer. The results demonstrate that with the decrease of O2 concentration, the oxidation heat release of coal samples drops gradually while the apparent activation energy increases gradually. In the N2 and CO2 atmospheres, as the O2 concentration is reduced to 1.5% and 3%, respectively, the value of apparent activation energy changes from negative to positive, and the spontaneous reaction transits to a nonspontaneous reaction. The TG-DTG (thermogravimetric-derivative thermogravimetric) curve of coal sample in the high-temperature combustion stage indicates that the critical temperature exhibits a W-shaped trend with the decrease of O2 concentration, which also leads to gradual increases of maximum peak temperature, ignition temperature, and burn-out temperature. The above results signify that increasing the inert gas concentration can gradually reduce the oxidation and combustion rate and improve the inhibition effect on coal spontaneous combustion. In addition, when the O2 concentration is the same, the inhibition effect of CO2 on coal spontaneous combustion is superior to that of N2.

Thermokinetic behaviour and functional group variation during spontaneous combustion of raw coal and its preoxidised form

Coal spontaneous combustion (CSC) is a major problem in coal mining. In the vicinity of underground goaf, secondary or repeated oxidation processes of the residual coal inevitably occur, increasing the risk of coal fires. In this study, the thermal reaction behaviour of two types of raw coal samples and three preoxidised coal samples with different oxidation temperatures (80, 130, and 180 °C) were investigated. The physical and chemical properties of the samples were measured using thermogravimetric analyser-Fourier transform infrared spectroscopy (TGA-FTIR) with heating rates of 1.0, 2.0, 5.0, and 10.0 °C min⁻¹. According to the characteristic temperatures in the heating processes, the entire CSC procedure can be divided into three stages: oxidation, combustion, and burnout. The results indicated that the aliphatic side chain lengths of preoxidised coal were shorter, and the number of branched aliphatic side chains was lower than that of raw coal. Furthermore, the model for the mechanism of preoxidised coal differed from that of raw coal. Average values of the apparent activation energy of the preoxidised coal samples were lower than those of the raw coal samples. Therefore, compared with raw coal, preoxidised coal requires less energy to react and more readily undergoes spontaneous combustion.

Coal spontaneous combustion (CSC) is a major problem in coal mining.