Turbine-driven compressors for noncondensible gas removal at geothermal steam power plants

Abstract

An innovative concept for an advanced turbinedriven compressor system is investigated as an alternative to conventional steam-jet ejector or hybrid liquid-ring vacuum pump systems for extraction of noncondensible gases at geothermal power plants. The new design eliminates the need for a motorlgearbox drive, lubrication system and high speed shaft seal normally associated with centrifugal compressor units. The result is a high efficiency, reliable, low maintenance system that has significant advantages over the conventionalvacuum systems. The turbinedriven compressor system has a higher capital cost than the conventional systems, but savings from reduced operating costs pay back the difference in less than a year. INTRODUCTION The steam flow in geothermal power plants has a much higher concentrationof noncondensiblegas (NCG) than conventional power plants. 4n conventional power plants, NCG is introduced through vacuum leaks and dissolved gas in boiler make-up water and representative concentrations are 10-50 ppm (by volume). In geothermal plants, NCG flows from the geothermal reservoir and concentrations can be two orders of magnitude higher than for a conventional plant. The NCG must be removed from the condenser and compressed from condenser pressure to ambient pressure. The NCG extraction systems have a high capital cost and require a significant amount of power. Consequently, overall plant performance and cost are sensitive to the type of NCG extraction equipment that is used. Most geothermal power plants currently use a train of steam-jet ejectors to compress the NCG to ambient pressure. Steam-jet ejectors have a relatively low capital cost and high reliability. However, they are low efficiency devices and require a high flow rate of motive steam. To reduce the power required by the NCG extraction system, some geothermal power plants utilize a higher performance hybrid system. These systems use steam-jet ejectors for high vacuum compression and liquid-ring vacuum pumps for low vacuum compression. Since the efficiency of a liquid-ring vacuum pump is superior to a steam-jet ejector, this hybrid approach reduces the power required by the NCG extraction system. However, liquid-ring pumps have a high capital cost, and a number of performance restrictions that limit their effectiveness. Very high performance NCG extraction can be achieved by using centrifugal compressors. Centrifugal compressors have higher efficiencies than ejectors or liquid-ring pumps and they are not subject to the performance restrictions that limit liquid-ring pumps. However, these compressors have seen limited use in NCG extraction systems because of high capital cost, and maintenance and reliability concerns about the high speed rotating assemblies. This paper will discuss a new turbine-driven compressor design that incorporates a number of novel features. With this design, it is possible to utilize the performance advantages offered by Copyright c 1994 by Barber-Nichols Inc. Published by the American Institute of Aeronautics and Astronautics, Inc with permission