Study On The Gas-solid Reaction Mechanism Of Coal Particle Combustion Under Concentrating Photothermal Heating

Coal is still playing a leading role in China's primary energy production and consumption.“Carbon peak”by 2030 and“carbon neutrality”by 2060 targets have been added to China's overall plan for ecological conservation,improving the efficient and clean coal utilization standards is one important measure for promoting the sustainable development of energy industry.Experimental study on combustion mechanism of single coal particle is the basis of developing combustion theory and technology.In this study,the concentrating photothermal heating method was proposed,a room temperature chamber type experimental apparatus with multi-information synchronous acquisition was constructed.The effects of volumetric reaction,heating rate,coal rank and oxygen partial pressure on the combustion characteristics of coal were studied,which mainly included the following:Firstly,the experimental apparatus for coal combustion under concentrating photothermal heating was developed,which realized the synchronous acquisition of combustion image,temperature,mass loss and gas generation rate.The average heating rate of coal particle reached 69.4?/s and the temperature of coal particle could reach above1100?,indicating the experimental parameters could be adjusted in a wide range.Repeated experiments showed that,over 80%of the relative standard deviation of CO generation rate,mass loss and particle temperature was within 5%,verifing the reliability of the experimental apparatus.Secondly,the decoupling control of solid phase temperature and gas phase temperature in the reaction chamber was realized by adding the reaction gas preheating device to the concentrating photothermal experimental apparatus.During the combustion process,the ignition delay of coal particle decreased by 50%-60%,and the flame duration increased by1.1-4 times with the increase of gas phase temperature for the significantly enhanced the volumetric reactions.Based on the experimental data,it was found that the ignition delay time?_i and the solid phase temperature T should satisfy the equation?_i=a×(f(T)-b)^-c,where a was related to the gas phase temperature,b was determined by the coal rank,and c was the effective reaction order related to the oxygen partial pressure.Then,the effect of volumetric reactions on the combustion characteristics was analyzed in the concentrating photothermal experimental apparatus with limited volumetric reactions.With the increase of heating rate,the ignition mechanism of coal particle changed from"heterogeneous"to"joint homo-heterogeneous",and further changed to"homogeneous"when the heating rate reached 52.4?/s.NO was generated only at the"sub-fast"(52.4?/s)and"fast"(69.4?/s)heating rates,and conformed to the"fuel NO"mechanism.Kinetics studies of the reactions around the maximum mass loss rate were carried out.With the increase of heating rate,the apparent activation energy slightly decreased from 84.45 and 66.18 to 62.28k J/mol.The fitting curve of reactions at“fast”heating rate was two-stage-style and the activation energy further decreased to 33.74 and11.54k J/mol.Next,the combustion characteristics of 3 typical ranks of coals(bituminous coal,sub-bituminous coal and lignite)were studied at fast heating rate.In the initial stage,the temperature of coal particle was mainly affected by the content of moisture and volatile,while the peak temperature was determined by the volatile content.Through combustion process,the density of bituminous coal and sub-bituminous coal decreased,increased and then decreased,while the density of lignite continued to decrease.In the devolatilization stage,the generation rates of combustible components in flue gas decreased with the decrease of coal rank.The specific reaction rates of bituminous coal,sub-bituminous coal and lignite in char combustion stage were 0.0076,0.0045 and 0.0099 s^-1,respectively.The formation of NO conformed to the"fuel NO"mechanism,and bituminous coal generated the highest amount of NO during combustion.Subsequently,the effect of oxygen partial pressure on the combustion characteristics of coal particles at fast heating rate was further studied.Obvious flames were observed only in the atmosphere of?40%oxygen partial pressure.The increase of oxygen partial pressure shortened the combustion time of coal particles in the form of exponential function,and increased the maximum combustion temperature in the form of power function,but showed little effect on the variation rules of density and volume.With the increase of oxygen partial pressure,the specific reaction rates of coal char increased exponentially,the maximum generation rates of CO₂ increased linearly,the generation of CO₂ also increased in the form of power function.The generation of CO and CH₄ was inhibited while the generation of"fuel NO"was promoted by the increase of oxygen partial pressure.Finally,through the establishment of drying model,devolatilization model,volatile combustion model and char combustion model,the temperature change process of coal particle during combustion at fast heating rate was accurately predicted.95.0%of the experimental data was in±50?of the predicted data by the gas-solid reaction model,which also showed good generality for typical bituminous coal,sub-bituminous coal and lignite coal of China.The prediction accuracy decreased slightly with the decrease of coal rank because of the effect of ash.The gas-solid reaction model was also verified to be applicable for the combustion in different oxygen partial pressures.