Exergy analysis of a combined heat and power plant with integrated lignocellulosic ethanol production

Lignocellulosic ethanol production is often assumed integrated in polygeneration systems because of its energy intensive nature. The objective of this study is to investigate potential irreversibilities from such integration, and what impact it has on the efficiency of the integrated ethanol production. An exergy analysis is carried out for a modelled polygeneration system in which lignocellulosic ethanol production based on hydrothermal pretreatment is integrated in an existing combined heat and power (CHP) plant. The ethanol facility is driven by steam extracted from the CHP unit when feasible, and a gas boiler is used as back-up when integration is not possible. The system was evaluated according to six operation points that alternate on the following three different operation parameters: Load in the CHP unit, integrated versus separate operation, and inclusion of district heating production in the ethanol facility. The calculated standard exergy efficiency of the ethanol facility varied from 0.564 to 0.855, of which the highest was obtained for integrated operation at reduced CHP load and full district heating production in the ethanol facility, and the lowest for separate operation with zero district heating production in the ethanol facility. The results suggest that the efficiency of integrating lignocellulosic ethanol production in CHP plants is highly dependent on operation, and it is therefore suggested that the expected operation pattern of such polygeneration system is taken into account when evaluating the potential of the ethanol production.