Study On Combustion Mechanism And Numerical Simulation Of New Gas-Wood Mixed Firing Kiln

Pine wood has been used as a fuel for the firing of traditional ceramic products due to its high calorific value,high volatile content and the abundance of reducing media in the flue gas after combustion,and has played a huge role in the long history of Chinese ceramic culture.Gas fuels,on the other hand,are used as the main fuel for the modern ceramic industry due to their higher calorific value,higher energy utilisation and lower pollution,and modern gas-fired kilns have the advantage of being easily automated and able to produce ceramics in large quantities.The aim of the gas-wood mixed firing is to combine the advantages of both to design a new way of firing ceramics,to produce ceramics with wood-fired characteristics in a low-carbon,environmentally friendly,energy-saving and emission-reducing way and thus to pass on an innovative ceramic culture that can increase mass production and reduce environmental pollution compared to wood firing,and to increase the value of ceramic products compared to gas firing.The combustion characteristics of gas-wood mixed firing are analyzed from the perspective of chemical kinetics in this thesis.The flame propagation velocity prediction values of gas firing and wood firing are calculated by using CHEMKIN software.By comparing with the measured values,the GRI-Mech 3.0 mechanism is determined as the detailed mechanism of two firing forms.Combined with the temperature sensitivity analysis method and the product formation rate analysis method,the mechanism was reduced under the conditions of wood firing and gas firing,respectively.A set of reduced mechanism of wood firing containing 25 components and 32 step elementary reactions and a set of reduced mechanism of gas firing containing 19 components and 25 step elementary reactions were obtained.After removing the repeated components and elementary reactions,a set of reduced mechanism of gas-wood mixed firing containing26 components and 39 step elementary reactions was obtained.The accuracy of the reduced mechanism was verified by comparing the predicted value of the reduced mechanism of gas-wood with the measured value.Then the CHEMKIN reactor group was used to compare the results of wood firing,gas firing and gas-wood mixed firing under different chemical equivalence ratios,and the influence of pinewood pyrolysis temperature on the results of gas-wood mixed firing was analyzed.The reduced mechanism of gas-wood mixed firing was coupled with FLUENT software to analyze the influence of air excess coefficient on the combustion of gas-wood mixed firing kiln.The following conclusions were obtained:（1）The difference between the gas-wood mixed firing and the wood-fired and gas-fired results is more obvious,with the combustion temperature and the molar fraction of the components in the product basically in between.The gas-fire product contains more O,H,OH and other highly reactive radicals,which are closely related to the combustion temperature,than the wood-fired product,and a greater proportion of CO and other reducing media than the gas-fired product,which facilitates the firing of porcelain with wood-fired characteristics and a significant reduction in CO₂ emissions compared to wood-fired,but after the PSR reactor is mixed again,the generation of NOx increases more significantly compared to the other two forms of firing.However,after remixing in the PSR reactor,NOx production increases significantly compared to the other two firing forms.（2）The pyrolysis temperature of pine firewood increases with the kiln firing temperature,and the proportion of highly reactive radicals and reducing media in the oxidising atmosphere of the gas-wood mixed firing products increases significantly at1100 K-1200 K.There is also an exponential increase after switching from oxidising to reducing atmosphere,whereas the traditional wood firing process usually switches from oxidising to reducing firing stage at 940°C（1213 K）or earlier,and the results of the gas-wood mixed firing are more in line with the traditional wood firing reducing atmosphere conversion conditions.（3）The FLUENT simulation results show that as the air excess coefficient increases,the gas-wood mixed firing temperature and H₂O content increase,the reducing medium content decreases,but the CO₂ content does not change much,and the remaining O₂content is negligible,indicating that an air excess coefficient greater than 1 does not guarantee that the combustion atmosphere is in an oxidising atmosphere and the air intake still needs to be controlled according to the actual situation.