Entrained Flow Gasification of Biomass

The present Ph. D. thesis describes experimental and modeling investigations on entrained flow gasification of biomass and an experimental investigation on entrained flow cogasification of biomass and coal. A review of the current knowledge of biomass entrained flow gasification is presented. Biomass gasification experiments were performed in a laboratory-scale atmospheric pressure entrained flow reactor with the aim to investigate the effects of operating parameters and biomass types on syngas products. A wide range of operating parameters was involved: reactor temperature, steam/carbon ratio, excess air ratio, oxygen concentration, feeder gas flow, and residence time. Wood, straw, and lignin were used as biomass fuels. In general, the carbon conversion was higher than 90 % in the biomass gasification experiments conducted at high temperatures (> 1200 °C). The biomass carbon that was not converted to gas in the gasification process only appeared as soot particles in the syngas in all experiments, except for two experiments conducted at 1000 °C without steam addition where a very small amount of char was also left. The effects of reactor temperature, steam/carbon ratio, and excess air ratio on the yields of H₂ and CO were noticeable, while the effects of oxygen concentration, feeder gas flow, and residence time on the yields of H₂ and CO were negligible. The yield of soot could be reduced by a higher reactor temperature, higher steam/carbon ratio, higher excess air ratio, lower oxygen concentration, larger feeder gas flow, and longer residence time.
Wood, straw, and lignin had similar gasification behavior except with regard to soot formation. The soot yield was lowest during straw gasification possibly because of its high potassium content. The equilibrium product compositions under the experimental conditions were calculated by using the FactSage Program. At high temperature with steam addition, the experimental product compositions were close to the calculated equilibrium gas compositions. Besides a comprehensive experimental investigation on biomass gasification, a few experiments of biomass pyrolysis were also performed with the aim to obtain a better understanding of the whole gasification process. In comparison to gasification, higher yields of H₂, CO, and soot were produced during pyrolysis. During wood gasification, the major part of the filter sample was soot on the basis of simultaneous thermal analysis (STA). Soot appeared as agglomerated nano-size spherical particles (< 100 nm) which are very rich in carbon on the basis of scanning electron microscopy (SEM) images coupled with energy dispersive spectroscopy (EDS). In comparison to wood gasification, the filter sample obtained from straw gasification had quite low content of soot while high contents of volatilizable KCl and K₂SO₄, and thereby appeared as irregular crystals (> 100 nm). During lignin gasification, the filter sample mainly consisted of soot and nonvolatilizable inorganic matter. The parent wood particles and the derived wood char samples obtained from the gasification experiment conducted at 1000 °C had similar structure, size, and shape according to SEM images, but the derived wood char particle surface looked smoother indicating some degree of melting. In STA analysis, the wood char was more reactive than the wood soot with respect to both oxidation and CO₂ gasification. Besides, the wood soot produced at higher temperature was more reactive than the soot produced at lower temperature. Biomass and coal co-gasification experiments were performed in the same entrained flow reactor. The effect of mixing ratio of different fuels on syngas products was investigated at 1400 °C with steam addition. The yields of residual particulates (char and/or soot) decreased with increasing straw fraction during straw/wood co-gasification and with increasing biomass fraction (straw or wood) during biomass/coal co-gasification. Besides, their yields in the cogasification experiments were lower than the calculated values from their weighted yields in the individual fuel gasification experiments, indicating a synergistic effect on lowering the yields of residual particulates during co-gasification. The yields of H₂, CO, and CO₂ remained nearly unchanged with varying mixing ratio during straw/wood co-gasification, while
increased gradually with increasing biomass mixing ratio during biomass/coal co-gasification. A mathematic model of biomass entrained flow gasification was developed. The model included mixing, drying and pyrolysis, char-gas and soot-gas reactions, detailed gas-phase reactions, and mass and heat transfer. The model could reasonable predict the yields of syngas products obtained in the biomass gasification experiments. Moreover, the simulation results suggest that the soot can be completely converted and thereby the H₂ and CO yields can reach the maximum values if the reactor length is increased to 2.5 – 3 m under a reasonable condition (high temperature with steam addition).