| Coal fires are mainly induced by mining activities. China is the country that produces and consumes the largest amount of coal in the world. Thus, it is also the country suffering from the most serious coal fires globally. With a rapid economic development, China is stepping into the post-industrialization era. The government and society expect high standards for resources utilization, environment, ecology, and life quality. It challenges the traditional, extensive and highly polluted coal industry, and forces the coal industry to reform. However, in the future, coal will still be the predominant energy in China. Negative effects on environments like PM 2.5 resulting from emissions of poisonous gases, smoke and soot by electric power plants are the major concerns for people health and environment. However, people ignore that coal fires significantly threaten coal resources, environments and ecologies. By statistics, coal fires devour thousands tons of coal resources per year and release approximately 0.57 million tons of greenhouse gases(such as CO2 and CH4), which accounts for around 0.1%-0.22% of total CO2 emissions by fossil fuels. In addition, they exhaust a large amount of poisonous gases(such as CO, SO2, H2 S, NxO, etc.) and result in subsidence and collapse, and pollute environments. Hence, in order to develop healthy ecology and friend environment, extinguishments and preventions of coal fires are significant for reducing emissions of greenhouse gases and protecting coal resources and environment as well as reforming of coal industry. Coal fires are mainly distributed in Inner Mongolia, Xinjiang, Ningxia, Qinghai, Shanxi, Shaanxi, Heilongjiang, etc. National government, local administration and mining companies invested enormous funds to combat coal fires. After fire-fighting for over one decade, coal fires do not expand and a number of large coal fires were extinguished. But there are still a large number of coal fires in China. Thus, coal industry still faces a serious situation arising from unscientific and unsustainable mining techniques, complicity of coal-fire hazard attributed to complicated couplings of multiphysics such as chemical reaction(oxidation, self-ignition and combustion), hydraulic field, thermal field, mechanical failure, and multi-phases of gas, solid and liquid, and coarse investigation and limited understanding due to their complicity. Therefore, studies on couplings of chemical reaction, hydraulic field and thermal field can help to shed lights on understanding multi-physic couplings and mechanisms of coal fire development and to extinguish and prevent coal fires.This dissertation comprehensively employs experimental, theoretical, field work and numerical approaches to investigate on couplings of chemical reaction, hydraulic field and thermal field of coal fires. Non-isothermal TGA experimental method was initially used to study on chemical reaction, flow field and thermal field of small-scale pulverized coal combustion. Specifically, at different heating rates(thermal field), the dissertation experimentally analyzed effects of oxygen diffusion(flow field) on the rate of chemical reaction of coal samples. The TGA experiments include five steps of heat and mass transfer and three various diffusion approaches, i.e. intraparticle or pore diffusion, interparticle diffusion and external diffusion. In order to analyze the influences of intraparticle diffusion, interparticle diffusion and external diffusion on the rates of chemical reaction of coal samples, three TGA groups were designed. The first group was that five various particle sizes(<74 ?m, 86.5 ?m, 111 ?m, 160.5 ?m and 550 ?m) of coal samples were put on a thin round Al2O3 plate as a single layer. Coal samples with the sameparticle size(<74 ?m) but different masses(36 mg, 44 mg and 62 mg) were fitted into a cylinder Al2O3 crucible, for 36 mg and 44 mg coal samples Al2O3 plate with different heights were also fitted into the bottom of the crucible to exclude external diffusion, which was considered as the second group. Compared to the second group, the last group was that coal samples with the same particle size(<74 ?m) but different masses(36 mg and 44 mg) without Al2O3 plate were fitted into the cylinder Al2O3 crucible so that the external diffusion was involved. Heating at 2K/min, 5K/min and 10K/min rates and purging oxygen and nitrogen at 20 ml/min and 80 ml/min flow velocities respectively, two coal samples(a bituminous and subbituminous collected from Changcun Coal Mine and Xin'an Coal Mine of Henan Energy and Chemical Industry Group Co., LTD) were tested according to designed TGA experiments. Then effectiveness factor was used to analyze the limitations of chemical reaction rates caused by oxygen diffusions. Experimental studied results indicated that intra- and interparticle diffusion had significant limitations for chemical reaction rate. For instance, while heating rate was 5K/min and coal sample was the bituminous with 550 ?m particle size, due to intraparticle diffusion, the rate of chemical reaction decreased to 44% of intrinsic reaction rate; owing to interparticle diffusion, the rate of chemical reaction of 62 mg coal samples with 4.2 mm diffusional length dropped 25.5% compared to the intrinsic reaction rate. However, external diffusion posed slight limitations on the chemical reaction rate. It should be stressed that heating rates, thermal conductivity of containers and calorific values of coal can influence experimental results. Hence it is important to set optimal heating rates based on thermal conductivity of container and calorific values of coal. Activation energy, Thiele Modulus and kinetic models were analyzed to study the diffusive related reaction regime of subbituminous collected from Xin'an Coal Mine. The results showed that all chemical reaction of coal almost belonged to the reaction-diffusion regime.In the lights of TGA experimental research, time-scale and Air-Fuel Coefficient analyses, theory of heat and mass transfer in porous media, and the single-particle reaction-diffusion model proposed by Krishnaswamy, a formula estimating the rate of chemical reaction of coal was proposed in this dissertation. It involved kinetic reaction(Arrhenius Equation) and factors influencing oxygen transport, such as particle size, the representative length of oxygen concentration drop, diffusion coefficient and effectiveness factor, which illustrates that the rate of chemical reaction dominated by the reaction-diffusion regime resulted from the balance of dynamics of both kinetic reaction and oxygen transport.Field work was conducted in the Wuda coal-fire region of Inner Mongolia(one of the largest coal fires in China and the most sufficiently studied region) and hill-side coal-fire region located in Zhangcun Town of Mentougou District of Beijing. Characteristics of chemical reaction, hydraulic field and thermal field were summarized according to two field campaigns.? Characteristics of thermal field. Convection, conduction and radiation were three approaches to dissipate thermal energy, whereas convection and radiation were the major ways to release thermal energy. Thermal convection was mainly attributed to convective gas flow emitted from cracks and fissures induced by coal fires and thermal radiation was the sole signal for remote sensing.? Characteristics of hydraulic field. Fracture field created by coal fires involved fissures, cracks, funnels, vents and sponges. Ambient air and emissions released by coal fires were ventilated in the fracture field, which formed the hydraulic field of coal fires. Transport of species such as air, smoke and vapor came with heat convective transfer. Thus, fracture field was the main pathway for heat and mass transfer. Besides, air leakage from the abandoned galleries was another way to transfer heat and mass and it greatly accelerated development and propagation of coal fires. Convection, diffusion and dispersion were three approaches of mass transfer. Driven force of mass transfer included thermal buoyancy, wind-driven force, atmospheric pressure difference between gas flow inlet(s) and outlet(s), mechanical ventilation(like air leakage from the underground mining work location), and concentration gradient.? Characteristics of chemical reaction. The characteristics of chemical reaction observed at ground were colorful smokes, minerals and tars, which could be the indicators to identify the status of underground coal fires.Couplings of chemical reaction, hydraulic field and thermal field of coal fires were analyzed in-depth:(?)Afterwards, a mathematical model was established in the lights of the couplings. Advantages and processes of the solution using COMSOL Multiphysics commercial software to simulate chemically, hydraulically and thermally coupled coal fires were illustrated. Based on in-situ surveys and previous studies, a two-dimensional, large-scale, unsteady and heterogeneous model was developed and appropriate boundary and initial conditions were defined. Furthermore, this model involved air leakage from an abandoned gallery, pressure difference between flow inlet and outlets and heterogeneous thermal conductivity, porosity and permeability of various zones such as combustion zone, rubble zone and cracks induced by coal fires. Incorporating parameters obtained from experiments(kinetic parameters, thermal conductivity, permeability, etc.), hill-side coal fires were simulated using COMSOL Multiphysics software. Effects of air leakage from the abandoned gallery and of fluctuant atmospheric pressure on hill-side coal fires were analyzed. Results indicated that air leakage from the abandoned gallery greatly enhanced initiation, development and propagation of coal fires. Thus, sealing abandoned galleries to cut off pathways of oxygen transport can effectively prevent and extinguish coal fires. Additionally, if pressure difference between gas flow inlet and outlet was near to or larger than the amplitude of pressure fluctuation, fluctuated pressure affected velocities and directions of gas flow emitted from cracks, which explained the “inhale-exhale” phenomenon occurred at cracks. Furthermore, comparison of numerical studies on effects of air leakage and field work conducted in Wuda coal-fire zone validated simulation results and the proposed formula estimating the rate of chemical reaction influenced by oxygen transport.For coal stockpiles with coarse particles, due to large porosity and fast gas flow, it is not reasonable to utilize Darcy's law, but Brinkman law. Owing to self-ignition of coal stockpiles arising from the couplings of gas flow field, temperature field and oxidation reaction of coal, variations from gas flow field can lead to significant effects on temperature field and oxidaiont reaction, which are different from those caused by self-ignition of coal piles with fine particles. The studied results show that wind-driven forced convection played a predominant role in the convective transport as long as wind velocity was not quite low(<0.001 m/s); the increase of wind velocity had the positive or the negative influence on both natural convection and the rate of oxidation reaction, depending on the critical wind velocity value(0.05 m/s) to sustain balances of both the heat and the availability of oxygen in the coarse coal pile; with wind velocity increase, the location of spontaneous ignition in the pile migrated from the lower part close to the surface of the leeward side and of the upper part. |
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