Thermoeconomic Evaluation of Cogeneration Systems for a Chemical Plant

Abstract

This paper presents the comparative exergy and thermoeconomic analysis of three cogeneration systems designed for a chemical plant. These systems must produce steam and electricity for the processes of the plant. These comparisons are developed for two scenarios: in the first one the systems generate steam and electricity for the plant and in the second one the systems generate steam and electricity for the plant and export electricity. The cogeneration systems are: a steam cycle with condensation-extraction steam turbine, a gas turbine based system and a combined cycle based system.

The exergy analysis developed for the cogeneration systems evaluates the exergy efficiency and the exergy destroyed in each set of equipment, as well as the overall cogeneration plant performance. The overall exergy efficiency of the plants and the exergy efficiency of each set of equipment are defined as the ratio of the useful exergetic effect of the equipment/system to the consumed exergy. The importance of each set of equipment in the overall exergy efficiency is quantified by the use of the factor f, defined as the ratio of the supplied exergy in a particular set of equipment to the consumed exergy in the plant. Equality and extraction cost partition methods are utilised (in the steam and gas turbines) in order to determine the production costs of steam (at 6 and 18 bar) and electricity, for each one of the considered operating scenarios of the plants. This comparison indicates the feasibility of the cogeneration systems for each production scenario.

Keywords

• exergy analysis, thermoeconomic analysis, cogeneration in chemical plants