DESIGN OF A HIGH EFFICIENCY WASTE TO ENERGY PLANT WITH HYBRID COMBINED CYCLE AND CONSUMING A LIMITED AMOUNT OF NATURAL GAS

摘要

Several power plants use hybrid cycles where gas turbines or gas engines exhaust heat is used in steam bottoming cycles burning biomass or MSW [1, 2, 4, 5]. One of the largest of this kind is the Zabalgarbi plant in Bilbao, Spain, where the clean exhaust gases of a GE LM6000 gas turbine are used to superheat the steam produced in the MSW boiler. This has many advantages such as reducing corrosion in boiler superheaters and increasing thermodynamic efficiency. The drawback however is that the natural gas share, a fossil fuel, is high. In Bilbao only 22% of the exported power comes from MSW. This also poses an operational problem when the large gas turbine is down for maintenance or due to malfunction, or when the gas price is too high to allow economic operation. In this work we present a way, named Optimized Combined Cycle (OCC), to reduce the natural gas share to less than 25% by introducing a small gas turbine, or gas engine, providing approximately the plant’s own power consumption. The exhaust of such a small gas machine does not have the energy necessary to superheat all the steam produced in the MSW boiler, in general, below 420°C to avoid corrosion. To solve this problem the gas machine exhaust flow is “artificially” increased with pre-heated fresh air and using a natural gas duct burner (DB) to reach the required steam superheating. Natural gas consumption in the DB is reduced by preheating the air supply to the duct burner using the hot flue gas exiting the external steam superheater (SH). In OCC the optimum configuration is not the highest thermodynamic efficiency solution, in general leading to higher prices, but the one that gives the best economic results for a given amount of MSW to be treated. OCC is an efficient tool to improve economic feasibility of WTE plants. It has the additional advantage that we can replace natural gas with landfill gas or biogas from digestion of biomass or of the organic fraction of MSW (OFM).