Ejector Compression of Natural Gas for Small Turbines

Abstract

Natural-gas-fuelled total-energy systems for supplying the utility demands of residential and commercial buildings typically require 500 s.h.p. for the machinery drives. Although gas turbines are competitive prime movers in this size range, difficulties arise from the need to compress the natural gas fuel to the pressure of the turbine combustion chamber.In this study the use of non-condensing single-and two-stage steam-jet ejectors was analysed for gas compression to a pressure of 150 lb/in2. Steam-supply conditions from 5000 to 1000 lb/in2and 800 to 1200°F were considered; methane was supplied at 15 lb/in2and 60°F.The specific steam consumption (lb steam per lb gas) of a two-stage ejector was found to be much better than that of a single-stage ejector. The minimum specific steam consumption (6 lb steam per lb gas) occurred at the extreme steam conditions considered in the study.The steam-methane mixture this produced was not flammable when mixed in any proportion with air. Of several methods considered to reduce the steam content of the mixture delivered by the ejector, only one, post-ejector cooling and condensate removal, was suitable for analysis. Cooling-water flow rates in excess of 14 lb water per lb gas were necessary to produce potentially flammable mixtures.Estimates were made of the major effects on turbine engine performance arising from the use of a steam ejector for gas compression. A comparison was drawn for two idealized cycles and for two others in which realistic values of component efficiencies were included. In each series the gas was compressed by conventional means in one cycle and by means of a steam-jet ejector in the other.The steam injected with the methane in the turbine combustion chamber caused an appreciable decrease in the cycle temperatures and significant improvement in both the turbine output and thermal efficiency. When the additional heat input necessary to generate the steam for the ejector was included, the overall thermal efficiency of the steam-ejector cycle was less than that of the conventional cycle.