Energy, exergy and emergy analysis of using straw as fuel in district heating plants

Straw is a renewable biomass with considerable potential as a fuel in most countries with cereal production. The harvesting, handling and conversion systems, however, require inputs of fossil fuels and other natural resources. In this study, straw is evaluated as a fuel in district heating plants with respect to energy requirements, exergy consumption and from an emergy point of view. Assuming straw to be a by-product of cultivation of cereals, the calculations show that the energy balance is favourable (12:1) when direct and indirect energy requirements are taken into account (nitrogen replacement not considered). The exergy analysis, however, shows that the conversion step is ineffective in the sense that energy quality is lost. The emergy analysis, which is a method that tries to integrate the human economy with the ecological system, indicates that large amounts of energy have been used in the past to form the straw fuel (the net emergy yield ratio is 1.1).