High Temperature Steam Production And Studies Of Hydrogen-rich Gas From High Temperature Steam Gasification And Reforming Of Biomass

Biomass gasification process is one of promising ways of producing hydrogen-rich gas, and it is still in the primary exploring stage.Although many researchers had performed experimental study in hydrogen production by biomass gasification and the reforming of product gas and tars,these gasification studies were conducted on low temperature gasification agents,low concentration of H₂ and high content of tars in producing gas.Thus,a new treatment process which integrated the high temperature steam generation and biomass gasification process was presented for hydrogen-rich production.In this paper,a high temperature steam generator was invented for high temperature steam and air production,and the thermal efficiency of high temperature steam generator was analyzed.Numerical simulation on the process of high temperature steam generator was carried out.The kinetic characteristics of isothermal and non-isothermal thermo-gravimetric analysis were studied on different operation conditions.The type of sequence batch and continuous batch biomass high temperature gasification were researched on fixed-bed gasifier combined with a porous ceramic reformer.Finally,a pyrolysis and reduction zone models was established to simulate the global process of biomass gasification.The following works are carried out main experimental results and conclusions are as follows in this dissertation:(1) This paper introduced the operational principle of the high temperature pressured steam generator.The stove structure and insulating layer were designed and computed.Based on the features and control requirements,the running and controlling programme was proposed.The hot experiments of high temperature steam and air were performed after finishing the installation of high temperature steam generator.And then,the variety of temperature for high temperature agents and exhaust gas in different switching periods and one switching period were studied.The efficiency of temperature and heat recycle was also investigated.It was found that the steam efficiency of heat recycle reach maximum value in 60s switching period,however,that of air in the optimum value is 30s.The heat balance and efficiency of system were calculated through the analysis of generator energy input and output.It is concluded that the efficiency of whole system obtained 73.56%and the error of heat balance is 5.03%. (2) The heat exchange of half cycle of combustion and steam preheating were investigated with numerical simulation method.The validity of model was verified through compared the values of experiment and calculation.Several operation parameters such as switching period,fuel inlet velocity,equivalent ratio and preheat agents were studied with numerical simulation.The results show that more heat was carried away by exhaust gas with longer switching period.It is not the optimum option to choose too long and too short switching period for high temperature steam generation,and 60s is the fine value by calculation for system.The release heat increases with the increasing fuel inlet velocity,and it is unfavorable for too fast inlet velocity due to incomplete combustion.The porous ceramic is in high temperature state in the first part 0.2-0.35m as equivalent ratio is 1.5.For the centre line of regenerative chamber,the temperature distribution of air preheating was below steam one,the curves of preheating temperature was affected greatly by their physical properties.(3) Thermogravimetry of biomass pyrolysis,gasification and combustion were investigated in thermo-balance and a small batch laboratory-scale externally heated fixed-bed. Different operation conditions(heating rate,particle sizes and flow rates of carrier gas) were investigated.The kinetic characteristics of isothermal thermo-gravimetric analysis were studied on different operation conditions.Comparison the two different modes of weight loss from TG and DTG,it was found that the behavior of thermogravimetric process of small and large amount samples are different obviously.(4) The type of sequence batch biomass high steam temperature gasification was studied on fixed-bed gasifier combined with a porous ceramic reformer.Reaction temperature,steam flow rate and particle sizes as different effect factors were studied on producer gas composition.The experimental results show that H₂ production increases from 21.91g/kg biomass to 71.63g/kg biomass as reaction temperature increases from 800 to 950℃.With increase of steam flow rate,the concentration of carbon monoxide increase slightly and low heating value(LHV) changed between 11.87 and 12.04kJ/m³,H₂ production reach maximum value as steam flow rate is 20.2g/min.With particle sizes of biomass decreased,producer gas production declined(5) Hydrogen-rich gas produced from biomass employed an updraft gasifier with a continuous biomass feeder were studied in this paper.A porous ceramic reformer was combined with the gasifier for producer gas reforming.The effects of gasifier temperature, equivalence ratio(ER),steam to biomass ratio(S/B),reforming temperature and the effect of porous ceramic reforming on the gas characteristic parameters(composition,density,yield, low heating value,and residence time,etc.) were investigated.The results show that a higher temperature favors the hydrogen production.With the increasing gasifier temperature varying from 800℃to 950℃,the hydrogen yield increased from 74.84 to 135.4g H₂/kg biomass, irrespectively.The low heating values vary first increased and then decreased with the increased ER from 0 to 0.3.A steam/biomass ratio of 2.05 was found as the optimum in the all steam gasification runs.H₂ production reaches maximum at the reforming temperature is 800^C.The effect of porous ceramic reforming showed the water soluble tar produced in the porous ceramic reforming,the conversion ratio of total organic carbon(TOC) contents show is between 29.93%and 50.31%,and the hydrogen concentration obviously higher than that of without porous ceramic reforming.With reaction temperature increases,CO concentration accounts for a large proportion in producer gas as air is agents,H₂ production and LHV increases with temperature rises.However,CO concentration decreases and CO₂ increase with ER increase,as a result,LHV of producer gas declines.(6) The behavior of a globe fixed bed biomass gasification reactor was simulated with a self-programming software.Pyrolysis zone and reduction zone models are combined to simulate the global process of biomass gasification.The volatile and gases broken up from pyrolysis zone were assumed to crack into an equivalent amount of CO,CH₄ and H₂O.It is considered that the volatile and gas leave pyrolysis zone instantaneously and then entering reduction zone as initial gas concentrations.The numerical method applied is Runge-Kutta forth order for the solution of pyrolysis zone model and finite difference for the reduction zone model to solve numerically the coupled ordinary differential equations.Simulations are carried out for the varying pyrolysis temperature with heating rate of 25K/min and constant temperature 1400K as the initial reduction zone temperature at the same time.The simulation results for temperature and concentration of gaseous species are in good agreement with published experimental data.