Recent research from China suggests that partially converting both natural gas and coal into 'syngas' fuel for power generation can generate around 6 per cent more electricity than competing 'conventional' gasification methods. The savings arise from more efficient use of the chemical energy in gas and coal which is lost in a conventional burner.
Despite climate change concerns, coal and natural gas are certain to remain the primary energy source for power supply globally1 in the near future. However, their energy efficiency could be improved. Burning fuels to create high-temperature working fluid and drive turbines wastes up to 40 per cent of the chemical energy in coal and 30 per cent of that in gas and more efficient use is an important area of research for the European Commission2.
Gasification is one of several efficiency improvements in which coal is oxidised with oxygen and steam, liberating up to 95 per cent of the carbon as methane and carbon dioxide gas, as well as pollutants, such as sulfur and alkali metals, which must be removed.
The researchers modelled a new gasification system which partially converts fossil fuels into 'syngas'. Syngas is the abbreviation for 'synthesis gas' and is comprised of carbon monoxide, carbon dioxide and hydrogen. The production process is combined with a heat recovery subsystem using partial gasification (in steam and air at lower temperature) of both coal and natural gas. This oxidises only the chemically-active carbon into syngas (around 60 per cent). Inactive carbon is retained in ash and char. The char is burned outside the gasification unit to supply more heat to the methane-steam reforming process and the exhaust gases are supplied to a heat recovery steam generator, which is added to the steam generated when burning the syngas.
The study simulated this system with conventional gasification systems to compare their electrical generating efficiency. In comparison with laboratory experiments, hydrogen and carbon monoxide concentrations in the syngas are 3-6 per cent higher in the model, while CO2 and methane concentrations are 2-4 per cent lower. The models found that the partial gasification system generated 6 per cent more electricity from the same quantity of gas and coal supplied.
The efficiency is the result of improved capture of chemical energy during syngas production. As well as improved efficiency, the study suggests there may be potential financial savings from using smaller equipment, which gasifies a smaller percentage of the fuel, and lower temperatures (850°C rather than 1350°C) which need less expensive materials. However, savings of $300-400/KW may be offset by the cost of the char reformer unit which is unique to the partial gasification process.