ON LOW-TEMPERATURE COAL SPONTANEOUS COMBUSTION IN THE AIR OF REDUCED OXYGEN AND LOW METHANE CONCENTRATION

Original scientific paper Laboratory tests were conducted to study coal spontaneous combustion at low-temperature (30 ÷ 200 °C) in the air of reduced oxygen (10 ÷ 21 %) and low methane concentrations (0 ÷ 3 %). The production of carbon monoxide in coal spontaneous combustion was analysed. The test results show that both oxygen and methane concentrations affect coal spontaneous combustion. At given oxygen concentration the methane concentration in air had a positive impact on coal spontaneous combustion and the impact increased with methane concentration. This impact was weakened and diminished with the rise of temperature. The initial carbon dioxide temperate at which carbon monoxide started to appear increased with the reduction of oxygen concentration in the air and the increase was somewhat negated by the presence of increased methane concentration in air. However, as the oxygen concentration reduced to a certain value, methane concentration showed little or no influence on coal spontaneous combustion. The low temperature sorption characteristics of oxygen and methane by coal were applied to explain the results. The findings in this study are particularly useful in the prediction and control of coal spontaneous combustion in gassy coal mines.


Introduction
Spontaneous combustion of coal is a serious hazard in coal-related industries such as coal mining, coal transportation and coal storage.If not properly controlled, it can lead to the emission of toxic and explosive gases together with propagation to open fire, and it can become a potential ignition source for an explosion if exposed to a flammable mixture of gas.It is well known that the physical and chemical interaction between coal and molecular oxygen (O 2 ), or coal oxidation, is the major reason responsible for spontaneous combustion of coal.Coal oxidation is a complicated process involving a number of phenomena such as the release of heat and the emission of gaseous products.If the heat is retained, the coal mass will increase in temperature and the oxidation rate will increase, leading to spontaneous combustion [1÷3].
Research into coal oxidation and its application for the detection and prevention of spontaneous combustion of coal has continued for more than 150 years [4÷9].One strand of the research work is on the effect of various factors on the oxidation process and characteristics of gaseous products of coal [10÷12].The gaseous products, in particular carbon monoxide (CO) and ethene (C 2 H 4 ), are important indicators to detect the onset and development stages of spontaneous combustion in coal mines [13÷15].Coal oxidation and its gaseous products are affected by several factors such as composition and physical properties of coal, temperature, moisture content and partial pressure of O 2 in the gas medium [16÷19].
The atmosphere in underground coal mines often contains methane (CH 4 ) and has less O 2 concentration than in normal atmospheric air.In-situ measurements indicate that the atmosphere in active goafs can have a wide range of O 2 (0 ÷ 21 %) and CH 4 (0 ÷ 100 % CH 4 ), and the CH 4 concentration in roadways adjacent to goafs can have up to 3 % [20÷22].Some field observations suggest that low CH 4 concentration in mine atmosphere tends to shorten the incubation period of spontaneous combustion of coal.For example, the incubation period of spontaneous combustion of the 3 L coal seam in Tengdong coal mine, Shandong, China, was measured to be between 3 to 6 months in normal air condition and field observation indicated that the incubation period was only 29 days in the goaf with 0,5 to 4 % CH 4 concentration.
A number of investigators have observed a dependence of the rate of coal oxidation on oxygen concentration in the gas medium [23], they suggested that the rate of oxygen consumption over a wide range of oxygen concentration can be expressed as a power of the partial pressure of oxygen in the oxidation medium with the exponent varying between zero and unity.However, in these studies no CH 4 was present in the gas medium.In fact, there is little published literature available on how coal oxidation and spontaneous combustion are affected by the combined effect of the reduced partial pressure of O 2 and the existence of low CH 4 concentration in mine atmosphere.
CO is the most commonly used gas indictor in the prediction of coal spontaneous combustion in coal mines as its appearance and production rate indicate the stages of coal spontaneous combustion [24÷26].This laboratory study aims to understand the effect of the reduced partial pressure of O 2 (10 to 21 %) and low CH 4 concentration (0 ÷ 3 %) in mine atmosphere on coal spontaneous combustion in the temperature range from 30 to 200 °C.

Test method
The test apparatus consists of a gas mixing system, a temperature control system, a gas analysing system, and a coal sample cylinder, as shown in Figure 1.The apparatus is designed to simulate coal spontaneous combustion under various gas conditions and analyse the gaseous products of spontaneous combustion.The gas mixing system can make gases of any mixture of O 2 , N 2 and CH 4 and provide steady gas flow to the sample cylinder.The temperature control system can provide controllable temperature environments for coal spontaneous combustion.The gas analysing system is used to analyse the composition of the gaseous products of coal spontaneous combustion.The gas mixing system consists of an O 2 cylinder, a N 2 cylinder, a CH 4 cylinder, a cylinder for mixed gas, gas pressure regulators, gas pressure stabilisers, gas flow metres, gas flow valves and gas pipelines.A mixed gas of any combination of O 2 , N 2 and CH 4 concentrations can be obtained by adjusting flow rates from the cylinders of O 2 , N 2 and CH 4 .The cylinder for mixed gas is connected to the coal sample container and ready for use.
The temperature control system includes an enclosed tank, a temperature sensor and an electric heating unit.The inner and outer layers of the tank use stainless steel and cold-rolling carbon steel respectively and the gap between the layers is filled with high density aluminium silicate fibre for heat in simulation.A Pt100 temperature sensor is installed to monitor the tank temperature and stainless steel heating pipes are used to control the tank temperature.The heating pipe lines are installed at the bottom and on both sides of the tank.The system is designed to control the tank temperature in the range of 0 and 650 °C and the rate of temperature increase between1and 20 °C/min.The gas analysing system uses a coal mine specific gas chromatography GC-4085B.It can detect CO, O 2 , N 2 , CO 2 , CH 4 , C 2 H 4 , C 2 H 6 , C 2 H 2 , C 3 H 8 and C 4 H 10 .It takes about 4 to 8 min to analyze each sample.The minimum detection limit of CO is 0,5 ppm.A gas drying device is installed on the gas inlet side of the gas chromatography.The coal sample cylinder is shown in Fig. 2. The cylinder is 350 mm high and 100 mm in diameter.The top of the cylinder is sealed with a sealing cap and the bottom of the cylinder is welded and sealed.The gas inlet is on the left bottom of the cylinder and the gas outlet is on the right top of the cylinder.A layer of meshed copper wires is inserted 80 mm above the bottom of the cylinder to ensure even distribution of gas inflow and coal sample is placed on the copper wires.A layer of asbestos is covered on the top of coal sample to prevent the gas outlet from blockage.Three temperature sensors are installed inside the cylinder and they are labelled as 1#, 2# and 3# from the top to bottom of the cylinder.The sensors are located 230 mm, 180 mm and 130 mm respectively above the bottom of the cylinder.
Bituminous coal was sourced from the 3 L coal seam in Tengdong coal mine, Shandong, China.Lumps of coal were taken from a freshly exposed coal face of the mine with a chain saw by removing a layer of coal from the face of 25 cm thick to avoid the possibility of per oxidation.The coal was sealed in plastic cling wrap upon its retrieval from the face and filled with nitrogen.The coal was then transported to a laboratory in Shandong University of Science Technology, crushed and sieved into samples of particle sizes of 0,18 to 0,38 mm.Its proximate analysis results (air dry basis) show that it contains 1,47 % moisture, 7,58 % ash, 53,55 % fixed carbon, 37,4 % volatile matter and 0,45 % total sulphur.
For each test run, the cylinder was loaded with coal sample of 200 g and put in the temperature control tank.The inlet gas of preset compositions was fed into the cylinder at a steady flow rate of 100 ml/min.The start test temperature was set at 30 °C which was in line with the in situ temperature of the 3 L coal seam.The temperature increase rate was controlled at 1 °C/min and the final test temperature was set at 200 °C.The gaseous product at the temperature range of 30 to 200.°C was sampled and analyzed of its CO concentration with the gas chromatography.The tests were conducted for a total of 12 coal samples in the gas of various compositions of O 2 , CH 4 and N 2 .Tab. 1 lists the compositions of gas in the tests.

The Initial CO Temperature
The initial CO temperature is defined as the temperature at which CO starts to appear in coal spontaneous combustion.In this study, the initial CO temperature and production are extracted from Tabs. 2 to 4 and summarized in Tab. 5.It can be seen from Tab. 5 that the initial CO temperature and production varied considerably with O 2 and CH 4 concentrations.In case that no CH 4 was present in the gas medium, the initial CO temperature increased with reduction in O 2 concentration.For example, the initial CO temperature was 30.°C with 21 % O 2 and the temperature increased to 50 °C and 60 °C as O 2 concentration dropped to 15 % and 10 % respectively.In case that low CH 4 concentration was present in the gas medium, the initial CO temperature also increased with reduction in O 2 concentration though the increase was somewhat subdued.For example, with increase in CH 4 concentration from 0 to 3,06 % the initial CO temperature dropped from 60.°C to 50.°C with 10 % O 2 and from 50 °C to 30 °C with 15 % O 2 .
The results indicate that the O 2 and CH 4 concentrations affect coal spontaneous combustion.The increase in the initial CO temperature with the reduced O 2 concentration indicates that O 2 decrease in the gas medium has negative impact on coal spontaneous combustion.The subdued increase in the initial CO temperature with the presence of the low CH 4 concentration indicates that CH 4 in the gas medium has a positive impact on coal spontaneous combustion.

CO production
For gases of 10 % O 2 and 0 ÷ 3 % CH 4 , the CO production in the temperature range of 30 to 200.°C is shown in Fig. 3.It can be seen from Fig. 3 that at the same temperature the CO production had no obvious and consistent variation with the CH 4 concentration.For example, at 100.°C the CO production was 106, 116, 109 and 93 ppm with the CH 4 concentration of 0 %, 0,82 %, 1,94 % and 3,06 % respectively.This indicates that the presence of low CH 4 concentration in the gas medium of 10 % O 2 has no or little influence on the CO production.
For gases of 15 % O 2 and low CH 4 concentrations, the CO production in the temperature range of 30 to 200.°C is shown in Fig. 4. It can be seen from Fig. 4 that at the same temperature the CO production generally increased with the CH 4 concentration.For example, at 100.°C the CO production was 106, 113, 108 and 127 ppm with the CH 4 concentration of 0 %, 0,82 %, 1,94 % and 3,06 % respectively.This indicates that CH 4 had a positive impact on CO production.However, the positive impact gradually diminished with the rise of temperature.For example, at 200 °C the CH 4 concentration showed no or little influence on the CO production.
For gases of 21 % O 2 and low CH 4 concentrations, the CO production in the temperature range of 30 to 200.°C is shown in Fig. 5.It can be seen from Fig. 5 that at the same temperature the CO production generally increased with the CH 4 concentration.For example at 100.°C the CO production was 147 ppm with the gas of zero CH 4 concentration and the CO production increased to 156, 169 and 189 ppm as the CH 4 concentration rose to 0,82 %, 1,94 % and 3,06 % respectively.This indicates that CH 4 had a positive impact on CO production.However, the positive impacts lowly diminished with the rise of temperature.For example, at 160.°C the CH 4 concentration had no or little influence on the CO production.A comparison of results in Figs. 3, 4 and 5 shows that at the same temperature and with the low CH 4 concentration the CO production with 21 % O 2 was significantly higher than that with 10 % and 15 % O 2 and this became more obvious with the rise of temperature.For example at 200.°C the CO production with 21 % O 2 was 13000 ÷ 14000 ppm and the CO production dropped to 8000-9000ppm with 10 and 15 % O 2 .This indicates that the O 2 concentration in the gas medium had a much more significant effect than the low CH 4 concentration on coal spontaneous combustion.
Low temperature spontaneous combustion of coal is a complicated process and has not yet been fully understood.However it is generally accepted that the gaseous products of coal spontaneous combustion in low temperature such as CO and CO 2 are formed through the sorption sequence.The sorption sequence is considered to be the three-step processes, i.e. the chemisorption of O 2 on surface of coal pores and formation of unstable carbon-oxygen complexes, decomposition of the unstable solid oxygenated intermediates to gaseous products such as CO and CO 2 and stable solid complexes, and the degradation of the stable solid complexes and generation of new active sites for coal spontaneous combustion.As heat is generated mainly from the chemical adsorption of O 2 by coal and chemical reaction in coal spontaneous combustion, the O 2 concentration in the gas medium is the main reason for the heat accumulation of coal and the subsequent CO production.The reduction in O 2 concentration in the gas medium slows coal spontaneous combustion and CO production.

Figure 1
Figure 1 Test apparatus

( a )
The temperature range 30 to 100℃ (b) Temperature range of 100 to 200 °C Figure 3 CO production for gases of 10 % O 2 and 0 ÷ 3 % CH 4

( a )
The temperature range 30 to 100 °C (b) The temperature range 100 to 200 °C Figure 4 CO production for gases of 15 % O 2 and 0 ÷ 3 %CH 4In case that low CH 4 concentration is present in coal spontaneous combustion, as coal has higher geophysical adsorption capability of CH 4 than O 2[27], CH 4 will occupy part of coal surface through physical adsorption and this will reduce the area of coal surface available for O 2 adsorption.The partial occupation of coal surface by CH 4 molecules may hinder the dissipation of heat generated through the chemisorptions and chemical reaction between O 2 and coal, resulting in more heat accumulation on coal surface and accelerate coal spontaneous combustion.With the rise in temperature, more CH 4 desorbs from coal surface and its hindering effect on heat accumulation gradually diminishes and eventually disappears.

( a ) 4 Conclusion
The temperature range 30 to 100 °C (b) The temperature range 100 to 200 °C Figure 5 CO production for gases of 21 % O 2 and 0 ÷ 3 % CH 4 This paper presented the test results of lowtemperature spontaneous combustion of coal in the air of 10 ÷ 21 % O 2 and 0 ÷ 3 % CH 4 in an attempt to explain the observed shortened incubation period of coal spontaneous combustion in mine atmosphere.It is concluded that: (1) Both the initial CO temperature at which CO starts to appear in coal spontaneous combustion and the CO production at the initial CO temperature increase with reduction in O 2 concentration in air.Reducing O 2 concentration in air generates less heat accumulation of coal and slows coal spontaneous combustion.(2) The presence of low CH 4 concentration in air has a positive impact on coal spontaneous combustion.However, the impact gradually diminishes with the rise in temperature.The partial occupation of coal surface by CH 4 molecules may hinder the dissipation of heat generated through the chemisorptions and chemical reaction between O 2 and coal, resulting in more heat accumulation on coal surface and accelerate coal spontaneous combustion.With the rise in temperature more CH 4 molecules desorb from coal surface and its hindering effect on heat accumulation gradually diminishes and disappears.(3) O 2 concentration in air has a more profound effect on coal spontaneous combustion than the presence of low CH 4 concentration in air.

Table 1
The inlet gas compositions

Table 2
CO production (ppm) in gases of 10 % O 2 and 0 ÷ 3,06 % CH 4 Tab. 2 lists the CO productions of coal spontaneous combustion in gases containing10 % O 2 and 0 ÷ 3 % CH 4 in the temperature range from 30 to 200.°C.For gases containing 15 % and 21 % O 2 the CO productions were listed in Tabs. 3 and 4 respectively.

Table 5
The initial CO temperature and production