A single flash integrated gas turbine-geothermal power plant with non condensable gas combustion

摘要

In this work the possibility of improving the performance of the 20 MW standard geothermal power plant of ENEL (Italian Electric Power Company) has been studied. The conventional geothermal cycle was modified by adding a gas turbine, an organic Rankine cycle (ORC), and a flash separator. Exhaust heat from the gas turbine is recovered in a geothermal steam superheater and by the ORC. The results of a thermodynamic analysis have been applied to a test case based on the Mt. Amiata geothermal field, where geothermal fluid has a water mass fraction ranging from 30 to 50 by wt. and about 8 by wt. of the steam fraction is non condensable gases (NCG). The non condensable gases are a mixture of CO_2 (about 95 by weight), H_2S, H_2, Hg, NH_3 and CH_4. Legislation on the emission of some of these substances into the atmosphere means that geothermal power plants are now equipped with gas clean-up devices but these increase the plant capital costs and reduce performance. Some of the com- ponents contained in the NCG, however, such as H_2S, H_2 and CH_4, have an acceptable lower heating value (LHV) and can be considered a source of energy. Hence, they could be burned to reduce their environmental impact while recovering energy that would otherwise be lost. In the power plant presented in this paper, NCG are mixed with inlet air in the gas turbine and burnt in the combustion chamber, thus helping to increase hybrid cycle performance.