Energy & Fuels, Vol.27, No.12, 7460-7469, 2013
Updraft Gasification of Mesquite Fuel Using Air/Steam and CO2/O-2 Mixtures
Gasification of woody biomass is an environmentally promising technology that provides an alternative to the use of fossil fuel. Typically, partial air oxidation is used to gasify biomass fuels, resulting in low-quality product gases due to dilution by nitrogen (N-2) in air, which limits the application of the product gases for thermal power generation. The objective of this study was to identify optimum conditions for producing gases from woody biomass with enhanced heating values. Air, air/steam, and a carbon dioxide/oxygen (CO2/O-2) mixture were used as gasification media for the gasification of pure mesquite (low-quality fuel) and a blend of mesquite and coal (high-quality fuel) using a 10 kW adiabatic fixed-bed reactor. Thermogravimetric analysis (TGA) along with limited emission studies using Fourier transform infrared (FTIR) spectrometry were performed to obtain basic kinetic data on raw mesquite pyrolysis. The effects of the steam-to-fuel ratio (S/F) and equivalence ratio (ER) on the peak temperature (T-peak) in the gasifier, product gas composition, and higher heating value (HHV) of the product gases were investigated. It was found that T-peak decreased from 1050 to 850 C as S/F was increased from 0 to 0.45. However, the carbon dioxide (CO2) and hydrogen (H-2) concentrations increased and the carbon monoxide (CO) percentage decreased with the introduction of larger amounts of steam. For air/steam gasification, the HHV of the producer gas was estimated to be in the range of 2800-3800 kJ/Nm(3). The highest HHV was obtained at S/F = 0.3 and ER = 2.7 (optimum). The HHV of the gas from air/steam gasification was found to be almost the same as that produced from pure-air gasification (2800-4000 kJ), except for an increased H-2 yield for air/steam. However, the HHVs of the gases obtained from CO2/O-2 (79:21 on a volume basis) gasification (4000-6000 kJ/Nm(3)) were generally higher. Separation of CO2 from the product gases obtained using CO2/O-2 gasification can further enhance the heating value of the product gas such that the HHV of the resulting gas mixture is 40% of the HHV of natural gas. Hence, the use of a CO2/O-2 mixture as the gasification medium and of coal blended with raw biomass as the fuel is the best method for upgrading the quality of product gases from the gasification of woody biomass.