Applying thermodynamics to digestion/gasification processes: the Attainable Region approach

The research shows theoretical calculations on the thermodynamics of digestion/gasification processes where glucose is used as a surrogate for biomass. The change in Enthalpy (∆H) and Gibbs Free Energy (∆G) is used to obtain the Attainable Region (AR) that shows the overall thermodynamic limits for digestion/gasification from 1 mol of glucose. Gibbs Free Energy and Enthalpy (G–H) plots were calculated for the temperature range 25–1500 °C. The results show the effect of temperature on the AR for the processes when water is in both liquid and gas states using 25 °C, 1 bar as the reference state. The AR results show that the production of CO, H2, CH4 and CO2 are feasible at all temperatures studied. The minimum Gibbs Free Energy becomes more negative from −418.68 kJ mol−1 at 25 °C to −3024.34 kJ mol−1 at 1500 °C while the process shifts from exothermic (−141.90 kJ mol−1) to endothermic (1161.80 kJ mol−1) for the respective temperatures. Methane and carbon dioxide are favoured products (minimum Gibbs Free Energy) for temperatures up to about 600 °C, and this therefore includes Anaerobic Digestion. The process is exothermic below 500 °C, and thus Anaerobic Digestion requires heat removal. As the temperature continues to increase, hydrogen production becomes more favourable than methane production. The production of gas is endothermic above 500 °C, and it needs a supply of heat that could be done, either by combustion or by electricity (plasma gasification). The calculations show that glucose conversion at temperatures around 700 °C favours the production of carbon dioxide and hydrogen at minimum G. Generally, the results show that the gas from high-temperature gasification (>~800 °C) typically carries the energy mainly in syngas components CO and H2, whereas at low-temperature gasification (<500 °C) the energy is carried in CH4. The overall analysis for the temperature range (25–1500 °C) also suggests a close relationship between biogas production/digestion and gasification as biogas production can be referred to as a form of low-temperature gasification.