Geothermal power plant repowering by means of biomass combustion

Abstract

The idea of increasing both the power output and the thermal efficiency of geothermal steam cycles by means of steam superheating has a long history. However, most (if not all) evaluations carried out so far, including a previous Enel study, dealt with fossil fuel superheat; moreover, no hybrid power plant of this kind is known to exist in operation. Enel Green Power is dedicated to the development of generation from renewable energy sources only; therefore, the idea to use biomass combustion to repower an existing geothermal power plant appeared very interesting. This scheme proved to be technically feasible by the fact that the fleet of geothermal power plants in operation in Italy is largely made up of standardized units with an installed capacity of 20 MWe, adaptable to steam inlet pressures from 5 to 20 bar. Of course, these units feature a power output much lower than 20 MW when the reservoir characteristics dictates a steam pressure near to the lower limit. In this case, all the equipment has the capability to increase the power output by means of steam superheating within the temperature limit of the existing turbine, after the necessary modifications to the turbine to accommodate for the new operating conditions (higher volumetric flow). This paper describes the history of the Cornia 2 project (from the name of the existing 20 MWe power plant, located in the Southern part of Larderello field), from the feasibility study of the biomass superheat, analyzing the different schemes envisaged, to the detailed engineering phase. The main technical issues that arose during the process are dealt with, as well as the main findings and solutions developed for this first-of-a-kind biomass-geothermal hybrid power plant.