ANALYSIS OF A RECIPROCATE ENGINE-BASED COGENERATION PLANT WITH HIGH TEMPERATURE HEAT RECOVERY FOR INDUSTRIAL USES

摘要

In consequence of the increasing awareness on the future scarcity of fossil energy sources and the global warming impact of energy conversion processes, the European Union has been planning several actions to enhance the efficiency of energy use and reduce the environmental impact. The declared goals of EU actions are synthetized in the 20-20-20 formula, consisting of an expected 20% increase of energy efficiency, a 20% contribution to the total energy supply by renewable sources and a 20% abatement of pollutant emissions. Applications of cogeneration in process industry can significantly contribute to achieve these targets. In this paper a reciprocate engine-based Combined Heat and Power (CHP) plant is presented, serving a pasta factory located in Sicily and installed by an Energy Service COmpany (ESCO) within the context of a national implementation scheme of Energy Saving Certificates (or "white certificates"). The CHP plant, with a 650 kWe capacity, currently covers a relevant fraction of the electric and high-temperature heat loads during peak hours, while it is switched off during off-peak hours because of the much lower electricity price. Heat content of flue gases is recovered by two cascaded gas-diathermic oil and diathermic oil-water heat exchangers; the superheated water obtained is then supplied to the pasta dryers. The first part of the paper provides a detailed plant description and an energetic analysis of historical performance data collected along the last two years of operation. Both the critical analysis of the lay-out and the evaluation of energy saving indicators reveal the current scheme to represent a sub-optimal solution for the particular application. In the second part of the paper a modified solution is simulated, consisting of the same CHP unit equipped with additional heat exchangers for heat recovery from the cooling water jacket circuit. The marginal energetic and economic benefits compared to the current plant setup are calculated; the results are presented in analytic and graphical form, coherently with the provisions of Directive 2004/8/EC and accounting separately for the different cost and revenues (fuel for the CHP unit and the supplementary boilers, electricity purchased from or supplied to the grid, taxes, etc.). The improved solution, designed to increase the thermal efficiency of the CHP unit by allowing a full exploitation of heat cascades, resulted to provide evident benefits and to make the CHP unit to comply with all the current legislative provisions for the assessment of highly efficient CHP plants. Margins for further improvements are also briefly discussed.