Thermodynamics of gas-char reactions: first and second law analysis

In energy transformation processes such as combustion, gasification and reforming of fossil and renewable fuels, the conservation of energy (first law of thermodn.) as well as the quality of energy (second law of thermodn.) is important. This study focuses on the conversion of biomass with air and/or steam into gaseous components and char represented by solid carbon (graphite). Energy and exergy (available energy) losses are analyzed by calcg. the compn. of a dry, ash-free typical biomass feed represented by CH1.4O0.59N0.0017 in equil. with varying amts. of air and/or steam. The anal. is carried out for adiabatic systems at atm. pressure, with input of biomass and air at ambient conditions and steam at atm. pressure and temp. of 500 K.For air gasification, energy and exergy in the product gas have a sharp max. at the point where all carbon is consumed, the carbon boundary point. This is the optimum point for operating an air-blown biomass gasifier. For gasification with steam, operation at the carbon boundary point is also optimal, but thermodn. process losses hardly increase when adding more steam than required. The efficiency of steam and air-blown gasification was compared using the definition of rational efficiency. Although gasification by steam is more efficient (87.6% vs. 80.5%), this difference is expected to level off if exergy losses for the prodn. of steam are taken into account. The choice between steam and air as a gasifying medium therefore seems to depend more on the required gas compns. For steam gasification, the product gas contains mainly methane and carbon dioxide, while hydrogen, carbon monoxide, and (at least 38%) nitrogen are the main product gases for air gasification