Study On Underground Coal Fire Control And Application Based On Heat Transfer Using A Gravity Assisted Heat Pipe

Coal fires,as one of the widespread geologic hazards around the world,are the uncontrolled exposed or subsurface coal seam burning or smoldering,which result in direct or indirect huge loses in coal resources and serious damage to ecological environment,and thread human beings' health and safety.Existing coal fire control is composed of preparation,trench excavation and stripping,water penetration,borehole drilling,water and grout(or gel)injection,and loess cover,causing ecological destruction.Coal fires are not completely extinguished because of the limited cooling effect.At the same time,a lot of water is wasted during the process of coal fire prevention,and the groundwater is easy to be polluted.Some coal fires are even expanding because of excessive mining of small collieries and lacking mine-closing related technical requirements for abandoned collieries.In this paper,both the prevention and recycling use of coal fires were considered synergistically in case of the angle of heat energy utilization.The coal combustion characteristics were analyzed.The effects of heat transfer using a gravity assisted heat pipe on controlling coal fires were studied.A technique of distributed heat extraction from existing water injection boreholes for thermoelectric power generation was developed.The heat from underground coal fires could be extracted in order to decrease the temperatures in coal fire areas,prevent the spread or reignition of coal fires,and save the water for firefighting,and be converted directly to clean electricity,realizing a synergy between control and utilization of coal fires.The main research contents and achievements are as follows:1)Experiments of low-temperature coal oxidation and coal combustion with different supplying oxygen concentrations was carried out to study the kinetic characteristics of coal combustion during the low-temperature oxidation stage,newly ignited stage(NIS),accelerating stage(AS),consistently burning stage(CBS),slowly burning-out stage(SBS)and extinct stage(ES)as well as the changes in temperature and gas products.The heat levels were determinated.Results showed that in the process of low-temperature coal oxidation,the slow-oxidation regenerative period gradually extended with decreasing supplying oxygen concentration,but the apparent activation energy at the rapid-oxidation stage changed slightly.Some indicators in a certain temperature range under different supplying oxygen concentrations were the same,such as the CO/CO2 ratio(50-190 ?)and the C3H8/C2H6 ratio(130-190 ?).A minimal oxygen mass flux was obtained to make sure that a constant first-order Arrhenius equation could be applied for describing the oxygen consumption rate from CBS to ES.The equations of oxygen consumption rate and combustion chemical reaction were simplified.Furthermore,the CO/CO2 ratio and SO2 concentration were proposed to judge the development stages of coal fires.Heat levels in the process of coal combustion were divided into three grades.Based on the three-grade heat release of coal combustion in oxygen-enriched environment,the heat release from coal combustion with conventional unknown air leakage supply could be divided into high-level heat,middle-level heat,and low-level heat.In addition,the oxygen-poor or oxygen-rich combustion of coal could be distinguished according to CO/CO2.2)The thermoelectric power generation using heat from coal fires was proposed.Experiments of heat utilization based on thermoelectric power generation were conducted.Four types of common thermoelectric power generators were tested to analyze their main thermoelectric properties such as the thermoelectromotive force of a thermoelectric couple,generator electrical resistance,maximum power output per unit cross-sectional area of thermoelements,maximum power output per unit contact area,and maximum thermoelectric conversion efficiency,as well as the cost-effectiveness of different generators.An optimal generator was selected after comprehensive analysis.Furthermore,the thermoelectric conversion efficiency could be improved by decreasing the cold side temperature of the thermoelectric module.3)In order to investigate the effects of heat transfer on the evolution of coal fires,the numerical simulation of multiphysics coupling which involves the chemical reaction of coal combustion,heat transfer in porous media,fume seepage and transfer of flue gas components based on Darcy's law,and equivalent solid heat transfer simulating a gravity assisted heat pipe,was conducted.It showed that compared with the temperature of coal fire area without heat transfer,the coal fire temperature near the evaporator of the gravity assisted heat pipe gradually decreased with decreasing temperature on the condenser.The influence range on the regional temperature increased.When the temperature on the condenser was 20 ?,the radius of the influence range was approximately 1 m.The effects of heat transfer on coal seam combustion mainly manifested as lower coal consumption in the radius of 0.2 m to the evaporator of the gravity assisted heat pipe.The low-temperature oxidation time of coal was prolonged.4)For the sake of optimizing the heat transfer and thermoelectric conversion technology,the matching of heat transfer and thermoelectric conversion,thermoelectric module structure,and heat dissipation at cold side were analyzed,respectively.It showed that a gravity assisted heat pipe with water(a filling ratio of 40%)as the working medium had a higher heat transfer.When the temperature on the evaporator was more than 100 ? and less than 300 ?,the heat pipe had a relatively outstanding heat transfer performance.In addition,the power generation efficiency increased in Hill function with increasing heat input.When the total number of thermoelectric assemblies,the contact temperature of hot and cold sides and the thermal resistance of all parts were constant,larger power output can be obtained by increasing the series number of each thermoelectric module with the increase of load resistance.At a specific temperature difference between cold and hot side,the series and parallel mode of power generation module had little effect on the maximum output power.However,the increasing thickness of the multilayer thermoelectric power generation module would result in more serious heat waste.The effects of heat dissipation modes on cold side,such as air cooling,circulating water cooling and flowing water cooling,of thermoelectric power generation on maximum output power were analyzed,respectively.Heat dissipation through circulating water cooling without water input in case of water shortage and heat dissipation using flow water cooling in case of sufficient water source were proposed to improve the efficiency of thermoelectric power generation.5)A system of heat extraction from coal fires using a gravity assisted heat pipe for thermoelectric power generation was designed,and then was tested in situ.The power generation performance,thermoelectric conversion stability,and the effects of environmental temperature on the thermoelectric conversion efficiency were analyzed.It showed that the system could significantly reduce the temperature in the borehole,the maximum reduction being approximately 154 ?.In addition,a "three-zone linkage" mode of control and utilization of coal fires was proposed to make full use of heat and reduce engineering quantities.Three zones include a preparation zone,a control zone and a recovery zone.At the same time,jointing power using multiple systems in multiple boreholes could achieve the control and utilization of coal fires well.