Investigation of the Performance of Air-Steam Combined Cycle for Electric Power Plants Using Low Grade Solid Fuels

Abstract

Since the solid fuels like coal produce a lot of ash upon burning, the products of combustion can’t be expanded as it is in a Gas Turbine (GT). Hence, the operation of a combined cycle with solid fuels includes: (i) production of syngas from the coal to operate a gas turbine engine and (ii) using the leftover coal after gasification to produce steam and operate a steam turbine engine. To avoid the coal-gasification and to use the solid coal fuel as it is in a combined cycle power plant, a novel Air-Steam Combined Cycle (ASCC) is proposed in the present work. ASCC comprises a gas turbine cycle (operating by the Brayton cycle) with the air as the working fluid and a steam turbine cycle (operating by the Rankine cycle) with the steam as the working fluid. A fraction F of the air is compressed, regenerated and finally heated to an Air Turbine Inlet Temperature (ATIT) by the hot products of combustion produced upon burning of the bituminous coal in a combustor. The residual heat energy of products of combustion is then utilized in a Heat Recovery Steam Generator (HRSG) to generate the steam initially and subsequently to preheat the remaining fraction (1-F) of the air. After expansion in an air turbine, the hot air passes through a regenerator directly into a combustor along with the preheated air for burning the coal so as to utilize the energy of expanded air completely. ASCC is analyzed based on the first and second laws of thermodynamics and a computer code is developed in MATLAB to simulate the cycle performance at different compressor pressure ratios, ATITs, and HRSG pressures. The performance of ASCC is compared with that of Baseline Steam Turbine Cycle (BSTC) for the same flue gas (stack) temperature. It is found that the overall thermal efficiency of ASCC can go up to 33.0%–37.5% depending on the compressor pressure ratio, ATIT and HRSG pressure as against to 29.0%–29.5% of BSTC.