(10a) Kinetic Study of Biomass Gasification at High Pressures

Authors: 
Iisa, K., National Renewable Energy Laboratory


A major advantage of biomass gasification in thermochemical platform for biomass utilization is that all parts of any type of biomass can be converted to syngas, making it attractive for unmerchantable wood, harvest residue, and waste from forest and paper industry. During gasification (using steam and/or CO2), biomass is converted to syngas (desired components CO and H2). There are two major constraints in the development of gasification protocols: (1) besides the desired CO and H2, a number of contaminants including hydrocarbons and tars are also formed during biomass gasification which adds significantly to the costs of downstream processing and gas-conditioning, and (2) carbon gasification reaction is inhibited by CO and H2. The mechanism of biomass gasification is complicated and depends on the gasification agent (steam, air, or CO2). Langmuir-Hinshelwood type kinetic models are widely used to describe these gasification processes. Reducing the amount of these contaminants in the raw syngas will decrease overall costs.

This study has two goals: (i) to study biomass gasification rates at high pressures as well as measure the rates of formation of hydrocarbons and tar contaminants, and (ii) to develop kinetic models that quantitatively predict the rates of biomass gasification and the rates of contaminant formation.

Two different reactor types are being utilized in this study: pressurized entrained flow reactor (PEFR) and pressurized thermogravimetric analyzer (PTGA). These two reactors complement each other in terms of the biomass heating rates and residence times. In addition, experiments conducted at atmospheric pressure will provide a broader range of model applicability and enable a better mechanistic understanding of the gasification process. The three biomass candidates (loblolly pine, switch grass, and corn stover) would differ in their elemental composition and ash content which will likely affect the gasification behavior. The biomass gasification variables include pressure, temperature, gas-phase composition, and particle size.