Geothermal noncondensable gas removal from turbine inlet steam

Abstract

Noncondensable gas (NCG), which is always present in geothermal steam, causes power plant inefficiencies that result in increased steam consumption and higher plant costs. In many steam fields over the world, the NCG content exceeds 5%w in the steam, a level at which steam consumption and costs increase rapidly as a function of the NCG concentration. Steam rates increase as the NCG level rises because of two factors. First, the steam jet ejectors and vacuum pumps that evacuate NCG from the turbine-condenser require more steam and electric power for their operation. Second, higher gas levels lead to a higher condenser pressure (that is, a higher turbine outlet pressure), thus yielding a lower power output per unit of steam feeding the turbine. The costs of the cooling water system, and parasitic power losses within the circulating pump and tower fans, increase. These problems can be alleviated when a reboiler system is used for steam cleaning that is, the reboiler system removes NCG from steam upstream of the turbine. This is accomplished with a simple, standard heat exchanger (the reboiler) within which geothermal steam is condensed and its NCG components are discharged. The heat of condensation is used to produce NCG-free steam by evaporation of clean water. The source of clean water is the same condensate recovered from condensing steam. The reboiler system design and operation is described, and then applied to the Botong geothermal plant on Luzon in the Philippines. The reduction in steam rate and cooling water usage is summarized.