Experimental Study Of Biomass Gasification With High Temperature Steam For Producing Hydrogen

It has been urgent to find renewable alternative energy sources due to the shortage offossil fuel and its increasing harm to the environment during utilization. The approachof producing hydrogen from biomass can not only mitigate the energy crisis and reduceemission of pollutant but also benefit to realize a smooth transition from a fossil energysystem to a renewable one. Steam gasification of biomass is regarded as a potential wayto produce hydrogen under atmospheric pressure, which is worth to be investigated.We have developed an experimental system for high-temperature steam gasification ofbiomass, where compressed wood cylinder pellet with20mm diameter and30mm high(the mass is about10g) is used to study the effects of steam flow rates and temperatures,especially the temperature over900℃, on the synthesis gas composition. Hydrogenproduction ratio(B), which is defined as the ratio of hydrogen production per unit massof product(Q) to the potential one(C), is introduced, combining with the Q and C, toevaluate how do they affect the gasification. Parallel with the experimental study, wehave also performed the numerical simulation of gasification using ASPEN software toinvestigate the changing trend of gasification products, assuming the gasification is atthermodynamic equilibrium.In the gasification experiments with steam flow rates from3.9g/min through17.3g/min,the amount of hydrogen produced, C, Q and B all reach their optimal at9.0g/min. Underthis condition, the retention time of the synthesis gas in the high temperature zone of thereaction vessel is2.2s. In the experiments of gasification with steam flow rate9.0g/minand temperatures ranging from617℃to1435℃, we find that the effect of temperatureon the components of synthesis gas is different in different temperature ranges. Whenthe temperature is lower than917℃as temperature increases, the amount of allcomponents changes significantly, yield of H2increases, while yields of CH4, CO andCO2decrease. And the rate of changing becomes slow when the temperature exceeds917℃. The amount of H2increases with the increase of temperature beginning with617℃, decreases after917℃and increases again after1018℃. C and Q increase all thetime, while B decreases after1018℃. During the experiments of biomass pyrolysis with the temperature increasing from634℃to930℃, C、Q and B increase all the time, while CH4production increases first andthen decreases and reaches its peak at730°C. When comparing the components ofsynthesis gas from pyrolysis and steam gasification, we notice that only slightdifference at about634℃，while the difference becomes more and more evident as thetemperature increasing. The addition of steam at high temperatures can promote thereaction of reforming of hydrogen and significantly increase hydrogen production. Atabout930℃，the percentages of hydrogen reach only33.2vol.%under pyrolysis andeven59.8vol.%under gasification, respectively.The resulting trend of H2and CH4from ASPEN simulation coincides quite well withthat from experiments with the variation of temperature and steam flow rates. Howeverthe amount of CO and CO2obtained from experiments are lower, and higher thansimulation results, respectively.In this paper, we obtain the preliminary effects of temperature and steam flow rates onthe synthesis gas compositions of gasification according to both experiments andASPEN equilibrium calculation. In the experiments the hydrogen-rich gas of59.8vol.%H2was successfully prepared under the conditions of atmospheric pressure. The resultsof this paper can provide some reference for the follow-up high temperature steamgasification of biomass.