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

Integrating second generation bioethanol production in combined heat and power units is expected to increase system energy efficiencies while producing sustainable fuel for the transportation sector at a competitive price. By applying exergy analysis, this study assessed the efficiency of an integrated system in which steam extracted from an existing combined heat and power unit is used for covering the heating demand of a lignocellulosic ethanol production facility. The integration solution was designed and optimized using already existing steam extraction points in the combined heat and power unit solely. The exergy flows inside the ethanol facility were determined, and the exergy in steam flows into and out of the system components were determined by combining exergy analysis with pinch analysis and setting a constant heat exchange temperature difference of 10K. The ethanol facility produces ethanol, solid biofuel, molasses, and is able to produce district heating hot water. Considering all products equally valuable, the exergy efficiency of the ethanol facility was found to be 0.790 during integrated operation with zero district heating production, and 0.852 during integrated operation with full district heating production. During separate operation, the exergy efficiency dropped to 0.564 with zero district heating production and 0.583 with full district heating production. The ratio of exergy losses and destruction in the heat integration network to the total exergy destruction and losses in the system was in the range of 0.46-0.87 depending on the system operation. This study suggests that a well-designed heat integration network can increase the exergy efficiency of the integrated system markedly.