| China’s characteristics energy reserves which is rich coal, meager oil and little gas determines the coal is still one of China’s main energy. China is one of the largest coal production and consumption countries. At the last count, although the raw coal production decreased for the past two years, the output of raw coal increased in the past decade. Coal as the main strategic position for industrial raw materials and energy will for a long time not be moved. Meanwhile, China is one of the most serious coalmine fire countries, coalmine fire can greatly endanger the human life safety and living environment. 90% of coalmine fire is caused by coal seam spontaneous combustion, so the early prediction of coal spontaneous combustion is of great significance. The most widely used method of forecasting coal spontaneous combustion is the index gas method at home and abroad. Because of the high sensitivity, mass generation and easy to detect, CO is the common and effectively used coal spontaneous combustion predict gas. However, field practice showed that coal spontaneous combustion is not the only source of CO, primal CO occurrence in folding and fracture can desorbed while mining, the explosion and burning of explosives can produce CO, mining explosive proof cart will exhaust CO gases in high output and high efficiency mine. The multi-source of underground CO interferes the prediction of coal spontaneous combustion, and brings a new tast to establish the early warning system of CO. The research has great significance on the investigation of underground gas migration law and the improvement of fire prediction accuracy.Through reading many documents, literature on CO as the index gas of coal seam spontaneous combustion and multi-source of underground CO were organized and combined. The main source of underground CO including: CO produced by oxidation of left coal in goaf, primal CO emission from coal seam, mining explosive proof cart exhaust CO, CO generating from blasting of mining process. In order to identify the CO sources in underground coal mine,generation mechanism and dissipation regularity of coal seam primal CO have been studied, CO generation regularity and mechanism have been investigated, generation and dissipation regularity of CO when blasing have been studied, theoretically analysis and numerical simulation have been done on explosive proof cart generating and dissipate CO.The origin of coal seam primal CO includes two types: organic origin and abiogenic. Organic origin can also be divided into thermogenic and biogenic. CO generated by abiogenic mainly caused by the change of geological conditions. Thermogenic means the fracture of oxygen containing functional groups generates a large number of gas in the coal forming thermal degradation process. Biogenic means organic matter converted to gas by microorganisms in the process of coal forming. The experiment was done on 2m deep coal core, grinded the coal samples in best rotation rate in ball mill, four groups of experiments have been done: With the gas flow rate of 60 ml/min. Break the 1# coal sample in 25℃ for 3 hours, and then do the desorption experiment charging with pure nitrogen. Break the 2# coal sample in 30℃ for 3 hours, and then do the desorption experiment charging with pure nitrogen. Break the 3# coal sample in 30℃ for 4 hours, and then do the desorption experiment charging with pure nitrogen. Break the 4# coal sample in 30℃ for 4 hours, and then do the desorption experiment charging with standard air. Collect desorption gas samples every 1 hour after grinding. By comparing the 1# and 2# coal samples, the first released CO concentration is greater when the temperature is higher, by comparing the 2# and 3# coal samples, the first released CO concentration is greater when the grinding time is longer, the variation trends of CO concentration desorpted by 1#~3# coal samples with time are basically the same. 4# coal sample reacts with oxygen in air which generates CO, CO generation decreases as coal sample being oxidized. The mathematical model of coal seam primal CO emission has been confirmed by fitting the relation of CO concentration and time.It can not be denied that left coal can be oxidized and generating CO. Microcosmic mechanism of coal and oxygen reaction has been studied by infrared spectrum experiment and thermogravimetric experiment. Using electronic scales weigh 0.0005 g pulverized coal and 0.0795 g potassium bromide powder, grinding and mixing evenly, tabletting the powder and put the tablet into tensor27 fourier transform infrared spectrometer sample room, and scan the tablet. Get the peak value of functional groups by analyzing the spectrogram of coal sample in different degree of oxidation. Thermogravimetric experiment has done on STA449F3 simultaneous thermal analyzer, set the heating rate of 2 ℃/min, 5 ℃/min, 10 ℃/min, 20 ℃/min and 30 ℃/min. Take 10 mg coal sample each time, set the temperature range from room temperature to 1000℃, with the flow rate of 100 mL/min. By analyzing the TG-DTG curves, coal sample experiences 3 stages in the coal sample heating up process, the corresponding functional group changes in each of the stages. To ensure the oxygen consumption rate and CO generation rate during coal and oxygen reaction in low temperature, temperature programming experiment has done to investigate the coal oxidation characteristic in low temperature. The low temperature coal oxidation device is made by safety research lab in China university of mining&technology(Beijing), the device is mainly composed of hot gas path system, coal sample cans, program temperature control box, gas gathering and analysis system. Take 250 g coal sample every experiment, number the different grain size of coal sample from XT-1#~XT-5#, number the compound grain size coal of XT~6#. Heating temperature range is from 30 to 210℃, collect gas production every 15℃. The change regularity of CO concentration and generation rate with temperature of different grain size have been confirmed, the change regularity of CO concentration and generation rate with oxygen concentration of different grain size combine coal sample have been confirmed. The correlation of CO concentration and generation change rate and temperature has been fitted, mathematical model has been confirmed.Blasting is the main way of underground roadway construction. Through reading many documents, gas production and concentration generated by the commonly used permissible explosives explode have been summarized, the explosion reaction formula has been confirmed. The explosives mainly constitute of C, H, N and O, the general equation is C H N Oa b c d, O is oxidizing agent, C and H is combustible obtained, O can oxidize C and H in reaction process, oxygen balance Qb would be reached. Oxygen balance can be divided into excess oxygen balance(Qb>0), zero-oxygen balance(Qb=0) and negative oxygen balance(Qb<0). CO is generated in negative oxygen balance explosives, because of the lack of oxidizing agent, combustible agent can’t be oxidized completely. Blasting operation can be regarded as instantaneous fierce detonation reaction, gun smoke thrown distance and CO concentration of instantaneous blast can be calculated by empirical formula, based on Champman-Enskog equation, time that CO discharge from roadway, control body volume, initial concentration of CO in control body, diffusion coefficient and ventilation quantity can be concluded.Underground mining explosive proof cart is auxiliary transportation equipment, powered by diesel engines. Diesel engines can generate CO in runing state. By analyzing the four stages(ignition delay and combustion period, fast burning period, slow burning period and combustion period), the main stage of CO generation has been confirmed, CO generation mechanism of diesel engines has been concluded, which mainly include Westbrook fuel combustion model and Hantman four steps reaction model. Exhaust gas diffusion model is used to describe the process from exhaustion to atmosphere dispersion, which can predict the pollutant concentration at different time and different place, the commonly used dispersion models includes Gaussian model, empirical model, numerical model and box model at present. Due to the specificity of underground roadway, numerical model has been chosen to investigate the dispersion of explosive proof cart exhaust gas.Fluid dynamics numerical simulation software FLUENT is used to simulate underground multi-source CO gas migration. Simulation of gas migration in the roadway must satisfy three equations(mass conservation equation, also known as continuity equation, momentum equation and energy conservation equation), at the same time,as the research object is mixture of CO and air, so the component conservation equation should also be satisfied. This paper simulates CO migration which were produced by mine flame-proof vehicle, residual coal oxidation, blasting operations and original occurrence CO, respectively, and explores the effect of every source CO on CO concentration at return airflow corner. Simulation results shows that CO produced by single mine flame-proof vehicle almost has no effects on CO concentration at return airflow corner. The CO concentration of tail gas more than 24 ppm is only within the scope of 0.5 m at the end of the vehicle, other areas have few CO. It is obvious the inlet airflow of auxiliary transportation has great dilution effect. Oxidation and spontaneous combustion of residual coal in goaf is a long and slow process, because of the mine pressure when mining, different position in goaf has different void fraction. So the mined-out area can be regarded as a porous medium, and when the wind flows through working face, air leakage into goaf happens through seepage effect. CO generated by oxidation of heritage coal and oxygen gas in air pours out under the action of seepage effect. Simulation results show that CO concentration at working face return airflow corner is about 12~17 ppm, beyond the maximum allowable concentration ruled by "Coalmine Safety Regulations". When considering original occurrence CO comes out from working face and supposing CO spewing continuously under the limit condition, the numerical simulation results show that the CO concentration of working face return airflow corner does not change with time, and it is visible original occurrence CO gas does not lead to CO gas concentration of return airflow corner overrun. Blasting operation at heading face will produce a large amount of CO, CO migration and concentration distribution at different time after blasting with the velocity 12 m/s of ventilation duct are simulated in this paper. It is found thatthe maximum CO concentration gradually moves to the entrance of heading tunnel until there was no CO left. This process takes about 3 mints. Monitoring CO concentration at a specific point of return airflow corner after blasting, it is found that large amount of transfinite CO concentration has a short duration, namely falls to the maximum allowable concentration range within 5 mints, and CO concentration accords with exponential function.By comparing the results of numerical simulation and field monitoring results, the downhole CO sources are summarized and identifications of each CO source are obtained. CO concentration of different sources variation law with time is concluded and the return air corner CO concentration forecast mathematical model is set up. According to the mathematical model, multi-source CO accumulation and disaster-causing situation have been analyzed. pre-warning CO index gas system of underground spontaneous combustion has been built, which can provide an important basis for coal mine fire prediction and management caused by coal spontaneous combustion. |
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