Suitable operating conditions improving hydrogen production from pine wood sawdust in supercritical water

Abstract: Biomass is a kind of renewable raw material and it is the 4th largest energy in the world due to its abundant total resource reserve. Biomass is considered to be a future energy because it is renewable, abundant and carbon neutral. However, because of its loose structure and low energy density, biomass has low direct utilization efficiency. Technologies of hydrogen production from biomass include biomass gasification technology, biomass pyrolysis technology and biomass supercritical water gasification technology currently. Hydrogen production from biomass gasification technology is easy to produce the tarry material and has low gasification rate. Hydrogen production from biomass pyrolysis technology can not directly handle wet biomass, which needs dehydrating. However, hydrogen production from biomass by supercritical water gasification is a kind of clean and efficient hydrogen production technology, which has been developed recently. It can directly handle the biomass with high moisture content, and need not the drying process with high energy consumption; it can obtain the high gasification rate and effectively overcome the above problems of biomass gasification technology and biomass pyrolysis technology. Therefore, it has become an international hot technology for biomass utilization. The technology is based on a series of unique properties of supercritical water, such as low dielectric constant, low viscosity and high diffusion coefficient, and uses supercritical water as the reaction medium; and the conversion from biomass to H2 is completed in hot compressed water with the temperature and pressure above the critical values (647 K and 22.1 MPa). Many researches of hydrogen production from biomass with supercritical water gasification technology have focused on the effects of different operating conditions (reaction temperature, reaction pressure, reactant mass fraction, residence time, catalyst, etc.) on the biomass (mainly the compound concentration, such as cellulose and glucose) and its inherent mechanism, but the researches on real biomass (such as wood sawdust and straw) are few. In order to understand the effect of operating conditions on hydrogen production process of biomass gasification in supercritical water, the catalytic activities of hydrogen production of Fe, Na2CO3 and CuSO4 were explored. The result showed that the order of effects was Fe > Na2CO3 > CuSO4 under the conditions of 500℃ reaction temperature, 30 MPa reaction pressure, 30 min residence time, wood sawdust mass fraction of 8% and particle size from 8 to 16 mesh with pine wood sawdust as raw material. The effects of reaction pressure 30 MPa, residence time 30 min, reaction temperature (420-500℃), wood sawdust mass fraction (8%-40%) and particle size (2-1000 mesh) on the hydrogen production processes with Fe as catalyst were investigated. The results indicated that increasing the mass fraction of catalyst Fe could significantly improve the effect of hydrogen production; with the increasing of the temperature, the gasification efficiency, carbon gasification efficiency, hydrogen gasification efficiency and hydrogen yield were increased accordingly; the lower the wood sawdust mass fraction, the higher the gas gasification efficiency and hydrogen gasification efficiency; the particle size of wood sawdust had no significant effect on gas gasification; the molar fraction, gasification efficiency and yield of hydrogen reached 42.1%, 98.1% and 6.62 mol/kg under the optimum operation conditions (reaction temperature of 500℃, reaction pressure of 30 MPa, residence time of 30 min, Fe mass fraction of 2%, wood sawdust mass fraction of 8% and particle size of 8-16 mesh). The research results can be used as a reference for future industrial development and technical applications.