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Indirect coal combustion can solve some problems such as low efficiency and air pollution during traditional coal combustion, this research investigates energy release characteristics of indirect combustion and aims to achieve maximum energy utilisation. Firstly, Energy Utilization Diagrams Methodology was used to explore exergy cascade release pathways and exergy losses for typical indirect combustion such as supercritical water gasification re-combustion, steam reforming re-combustion, and chemical-looping combustion, which are compared with direct combustion. It is pointed out that indirect combustion has changed release pathways of chemical exergy, and decreased energy level difference between chemical exergy in fuel and thermal energy in combustion, then exergy losses is reduced and exergy in fuel is released step by step. Secondly, to prove that indirect combustion can indeed reduce exergy losses and improve exergy efficiency in the combustion process, supercritical water gasification re-combustion with least exergy losses was selected and exergy analysis model was established by energy quality coefficient. Results show that thermal efficiency and exergy efficiency of supercritical water gasification re-combustion reaches as high as 94.8% and 79.5%, respectively, 2.23% and 19.61% higher than those of traditional coal-fired boilers. As a result, energy matching and conversion are more reasonable in indirect coal combustion processes. [ABSTRACT FROM AUTHOR]

The oxidation environment has important influence on the transformation of the energy contained in fuel and generation of pollutants. To the problem of nearly 50% exergy losses in coal oxidation at air atmosphere, this research intends to change oxidation atmosphere from air to supercritical water/oxidant and achieve efficient release of exergy in coal at about 650 °C with the aid of a high solubility and unique performance of heat and mass transfer of supercritical water. Therefore, firstly, based on the exergy analysis theory and the energy-utilization diagrams, the release mechanism of exergy of coal in supercritical water oxidation process is revealed. It is pointed out that supercritical water oxidation has changed the release pathways of chemical exergy, and decreased the level difference between chemical exergy and thermal energy, and more exergy is released. Meanwhile, there is also no exergy loss of physical heat transfer. As a result, supercritical water oxidation has higher exergy efficiency than conventional oxidation. Secondly, the exergy losses, level difference between chemical exergy and thermal energy as well as exergy efficiency, are quantitatively investigated. Results show that the exergy efficiency of supercritical water oxidation reactor may be as high as 80.1% and has increased by 27% relative to conventional boilers. Thirdly, based on supercritical water oxidation of coal, a concept power generation system is constructed. Exergy efficiency is calculated and exergy analysis is provided for the supercritical water oxidation power plant. Compared with conventional power plant, exergy efficiency in supercritical water oxidation plant reaches as high as 61.3% and 21% higher than that in conventional power plant. Finally, from the results obtained, it is believed that the commercial breakthrough of the supercritical water oxidation process will be possible when the corrosion and salt deposition in the reactor are solved. [ABSTRACT FROM AUTHOR]

Abstract: Biomass gasification in supercritical water is a promising technology for hydrogen production by utilizing wet biomass. A new experimental system of biomass gasification in supercritical water was built in SKLMF. In this paper, a comprehensive thermodynamic analysis, including chemical equilibrium in the reactor, gas–liquid equilibrium in the high-pressure separator, exergy and energy analysis of the whole system, was conducted. Chemical equilibrium model is based on minimizing Gibbs free energy. By chemical equilibrium analysis in the reactor, rules of the main parametric effects on biomass gasification in supercritical water are obtained. Simultaneously, a high-pressure gas–liquid equilibrium model was proposed based on modified universal functional activity coefficient (UNIFAC) model, Soave–Redlich–Kwong (SRK) equation of state and modified Huron–Vidal second-order (MHV2) mixing rule. Effects of pressure, temperature and water recycled ratio on gas–liquid equilibrium in high-pressure separation were discussed. Finally, results from energy and exergy analysis show that energy and exergy efficiencies of the whole system are in excess of 40% and increase with increasing heat transfer efficiencies. Energy loss of the system is caused mainly by heat transfer and exergy loss is mainly caused by heat transfer and chemical reaction. Our research provided a thermodynamic tool for improvement of design and operation optimization of biomass gasification system in SKLMF, which may be also applicable to other biomass gasification system. [Copyright &y& Elsevier]