Theoretical Analysis And Experimental Research Of Stratified Downdraft Biomass Gasification

Biomass gasification is one of the clean and efficient methods to use biomass energy and stratified downdraft gasifier has been widely used in rural central gas supply and small-scale power generation. As the development of the biomass pelletizing technology which can pelletize loose material into pellets, the large-scale application of biomass gasification becomes possible. However, at present the gasifier design depends on experience data in many ways, and a large number of essentials of complex gasification reaction have not been fully revealed. Especially there are very few theoretical analysis and experimental study on the pyrolysis and gasification of large biomass pellets in stratified downdraft biomass gasifier. Theoretical analysis can reveal the gasification nature, make up the lack of experiments and it is of great significance for improving the gasifier structure, developing new gasifier type and improving the gasification efficiency. With the aid of numerical simulation and experiments, the research of theory and application of the large biomass pellets pyrolysis and gasification in stratified downdraft biomass gasifier were carried out in the paper.Because flaming pyrolysis zone and reduction zone are the most important two zones in stratified downdraft gasifier, they are simulated numerically from individual and overall perspective.A mathematical model to describe the pyrolysis of single biomass particle in flaming pyrolysis zone was developed by coupling the heat transfer equation with the semi-global chemical kinetics equations. By the model the heat transfer parameters in the flaming pyrolysis zone were calculated, the pyrolysis process inside the particles was revealed and the height of the flaming pyrolysis zone was calculated. The results showed that the convective heat transfer coefficient hs of 80.4 W/(m2-K) and char emissivity ε of 0.792 were suitable for the flaming pyrolysis zone of stratified downdraft biomass gasifier; The average heating rate inside pellet was 182.5 K/(min), which was comparatively fast; With the increase of flaming temperature, flaming pyrolysis time decreased, residual char mass fraction gradually reduced from 16.92% to 13.97%; With the increase of particle diameter, pyrolysis time and char yield increased. Under typical operating conditions,the height of naming pyrolysis zone in stratified downdraft biomass gasifier was 1.1-2.2 times of the characteristic pellet diameter.To study influence of parameters on the whole flaming pyrolysis zone,the thermodynamic eqirilibrium model was developed.In thermodynamic equilibrium model, the influence of the heat loss, tar and ash were considered. The influence of parameters on product gas composition and temperature was investigated. It provided initial parameters for the reduction zone model. The results showed that ER, moisture content and heat loss had obvious effect on the temperature and compositions of the product gas. The air preheated temperature and ash content also had obvious impact on the gas temperature, but little impact on the gas composition. With increment of ER and decreasing of moisture content,the possibility of slagging increased. As the ER value was lower, raising air preheated temperature had positive effect on gasification. However, higher heat loss and lower ER Ksulted in incompletely pyrolysis and combusiton. The contents of CO2 and H2O in the product gas of flaming prolysis zone, which had important effect on the Kaction of reduction zone,were 10%-15% and 20%-25% for com stalk gasification.To deeply understand the reduction procew in stratified downdraft gasifier,reduction model of single charcoal particle were proposed. A mathematical model to describe the reduction of single charcoal particle in reduction zone was developed by、coupling the heat and mass transfer equations with the reduction chemical kinetics equations which included surface function F(X)which reflected internal available surface area, the active carbon sites and the pore attribute value. The coefficients of the heat and mass transfer were calculated and the parameters profiles in the reduction zone were revealed. Then the height of reduction zone was calculated. The results showed that mass transfer coefficient of CO2 and H2O were 0.008?1.08m/s and 0.007?0.97m/s, heat transfer coefficient was 50?195 W/(m2·K) ; Temperature distribution inside charcoal particle varied with different diameter, and difference of concentration between CO2 and H2O inside was great. When surrounding temperature was lower the model was close to homogeneous model and when higher it was close to unreacted shrinkage kernel model, thus the gasification reaction of charcoal particles must adopted non-isothermal model which include both heat and mass transfer. Heat transfer coefficient and mass transfer coefficient of H2O had influence on full conversion of charcoal, hower mass transfer coefficient of CO2 had little. For φ10-30 mm biomass pellet, the height of reduction zone in stratified downdraft gasifier was about 200-300 mm.To study influence of parameters on the whole reduction zone, the chemical kinetic model was developed. Based on the kinetics of five main chemical reactions in the reduction zone and combined with charcoal reactive factor CRF which has important influence on the reduction reaction, the overall chemical kinetic model of reduction zone was developed. The optimal value of CRF was analyzed and the parameters profiles in reduction zone were revealed. The results showe that CRF=e30z was most suitable for model; Along the bed height, reduction temperature gradually decreased; The inlet gas temperature of reduction zone had great influence on the reduction temperature and gas concentration distribution, however the apparent gas velocity had little influence; Along the bed height of reduction zone, the gas molar concentration distribution was different, the gas superficial velocity decreased and pressure drop decreased in monotone linear. Combining thermal dynamic equilibrium model of flaming pyrolysis zone with chemical kinetics model of reduction zone constituted a complete mathematical model of the stratified downdraft gasifier. The influence of ER and moisture content of biomass on outlet gas composition and temperature were analyzed in detail. The results showed that with ER increase, exit gas temperature, gas flow rate and gas yields increased, gas calorific value slightly increased firstly and then decreased; With moisture content increase, exit gas temperature decreased, gas flow rate and gas yield increased, gas calorific value decreased; The change of ER and moisture content resulted in different trends of gas composition.The gasifier was design according to the model results and the experimental bench of stratified downdraft gasifier using biomass pellets as feed materials was established. A large gas centralized supply system with biomass pellets gasification was designed and a demonstration project was constructed. The parameters were measured and relative operating condition was analyzed. The results showed that the calculated values using model were in good agreement with the experimental results; The operating condition was stable and good fluidity. Gas flow rate was the only adjustable parameter, and was accordance with gasification intensity and ER; With increase of gas flow rate, the temperature in the gasifier, gas heating value, gasification efficiency and pressure drop increased, tar yields decreased and ash yields increased; After purification system, tar and ash content reduced below 50mg which was conform to the requirements of the internal combustion engine power; Demonstrative project of gas centralized supply system using large biomass pellet gasification which operated stability with gas calorific value of 5284 kJ/Nm3, gasification efficiency of 72.8%, ash and tar content of 32mg/Nm3 can produce good social and economic benefits.In summary, pyrolysis and gasification process of larger biomass pellets in stratified downdraft gasifier were studied systematically through theoretical analysis and experimental study. It provided important theoretical foundation and practice basis for the application of biomass pellets in the biomass gasification.