Ultra-high temperature steam gasification of biomass

• Main Author: Waheed, Qari Muhammad Khalid
• Other Authors: Williams, Paul
• Published: University of Leeds 2013
• Subjects: 662
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.605282

In this research, hydrogen production from conventional slow pyrolysis, flash pyrolysis, steam gasification and catalytic steam gasification of various biomass samples including rice husk, wood pellets, wheat straw and sugarcane bagasse was investigated at ultra-high temperature (~1000 °C). During flash pyrolysis of the waste wood, the gas yield was improved to ~78 wt.% as compared to ~25 wt.% obtained during slow pyrolysis. The addition of steam enhanced the hydrogen concentration from 26.91 vol.% for pyrolysis to 44.13 vol.% for steam gasification. The comparison of pyrolysis, steam gasification and catalytic steam gasification in a down-draft gasification reactor at 950 °C using rice husk, bagasse and wheat straw showed a significant increase in gas yield as well as hydrogen yield. The hydrogen yield was enhanced from ~2 mmoles g-1 for pyrolysis to ~25 mmoles g-1 during steam gasification using a 10 wt.% Ni-dolomite catalyst. The higher hydrogen yield was due to the enhanced steam reforming of hydrocarbons and thermal cracking of tar compounds at higher temperature. When compared with the other catalysts such as 10 wt.% Ni-dolomite, 10 wt.% Ni-MgO, and 10 wt.% Ni-SiO2, the 10 wt.% Ni-Al2O3 catalyst showed the highest hydrogen yield of 29.62 mmoles g-1. The investigation on gasification temperature showed that the hydrogen yield was significantly improved from 21.17 mmoles g-1 at 800 °C to 35.65 mmoles g-1 at 1050 °C. The hydrogen concentration in the product gas mixture was increased from 50.32 vol.% at 800 °C to 67.41 vol.% at 1050 °C. The increase in steam injection rate from 6 to 35 ml hr-1 enhanced the hydrogen yield from 29.93 mmoles g-1 to 44.47 mmoles g-1. The hydrogen concentration increased from 60.73 to 72.92 vol.%. The increase was mainly due to the shift in the equilibrium of the water gas shift reaction as H2:CO ratio increased from 2.97 to 7.78. The other process variables such as catalyst to sample ratio, carrier gas flow rate showed little or no influence on the gas yield and hydrogen yield. The steam gasification of residual biomass char was performed at 950 °C to recover extra hydrogen. The presence of 10 wt.% Ni-Al2O3 in the gasifier improved the hydrogen yield to ~47 mmoles per gram of biomass as compared to the other catalysts such as 10 wt.% Ni-dolomite and 10 wt.% Ni-MgO. The gasification temperature showed a positive influence on hydrogen yield from 750 °C to 950 °C. The increase in steam injection rate from 6 ml hr-1 to 15 ml hr-1 enhanced the hydrogen yield from 46.81 to 52.10 mmoles g-1 of biomass.