The mesquite woody species that dominate uncultivated lands in the south central Great Plains, U.S.A. (Texas, Oklahoma) may have the potential for bioenergy utilization. Gasification of woody biomass is an environmentally promising technology that provides an alternative to the use of fossil fuel. The objective of this study is to study enhancement of heat values of mesquite (Prosopis glandulosa) using an adiabatic fixed bed gasifier using air-steam as gasification medium and compare these values with those from other methods which include: air (N2:O2), CO2-O2 mixture, enriched air, blending biomass with high quality coal etc. The effects of steam/fuel ratio (S:F) and equivalence ratio (ER) on the gasification peak temperature (Tpeak), gas compositions, and gas higher heating value (HHV) were investigated. It is found that the Tpeak in air-steam gasification is lower than that of air gasification and the Tpeak decreased with the increase in the steam to fuel ratio (S:F). In addition, when S: F ratio increased from 0 to 0.45, the CO2 and H2 concentration increased while the CO percentage decreased. The mole composition of the end product gas contained: 8–19% CO, 4–15% H2, 1–3% CH4, and 10–18% CO2. The HHV of the mesquite gas was estimated to be in a range of 2800 kJ/Nm3 to 3800 kJ/Nm3. The HHV increased when the S: F increased from 0.15 to 0.3, and then it decreased when the S: F was increased from 0.3 to 0.45 and the highest value was obtained at S: F = 0.3 and ER = 2.7 (optimum). The HHV of the producer gas from air-steam gasification is found to be almost same as that produced from pure air gasification (2800–4000 kJ) and lower than that obtained from CO2:O2 mixture (79:21) gasification (4000 to 6000 kJ/Nm3). However, H2 rich producer gas was obtained with air steam mixture.

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