Modified Combined Cycle With Pressure Fluidized Bed Combustion Boiler
Jacek Dzierzgowski, Stanislaw Sobkowski
1992
Volume 4: Heat Transfer; Electric Power; Industrial and Cogeneration
unpublished
The article describes conversion of conventional steam cycle with 200 MW turbine into combined steam-gas cycle with pressure fluidized bed combustion boiler. In order to raise cycle thermal efficiency an additional combustion chamber before a gas turbine was introduced. Two modifications of the combined cycle were considered. In one of them natural gas in the additional combustion chamber is burnt with the boiler flue gas only. In the other gas is burnt with additional air stream taken from
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... nd the gas turbine compressor. Optimizing calculations of the cycle thermal efficiency in function of some cycle's main parameters were carried out. NOMENCLATURE g 02 -mass fraction of free oxygen in boiler flue gas, m -mass flow, N -electric output, t -temperature, W -lower heating value, Z -number of working feedwater steam heaters, -efficiency, ii -compressor pressure ratio, c x -excess air number Subscripts A -air, C -coal, compressor, CC -combustion chamber, FB -pressure fluidized bed combustion boiler, FW -feedwater, GT -gas turbine, NG -natural gas, ST -steam turbine CONVENTIONAL STEAM CYCLE The conventional steam cycle consists of coal fired boiler and steam turbine with an electric output of 200 MW. The turbine is of superheat condensing type with eight feedwater regenerative heaters. Live steam pressure is 13 MPa, and temperature 535 C. The feedwater temperature behind the last regenerative heater has a value of t FW = 240 C. Plant thermal efficiency is equal to U = 38.9%. Apart of raising plant efficiency conversion to the PFBC cycle allows to use coal as a main fuel for combined cycle what may be important for countries with no natural resources and high costs of other fuels. With the use of sorbent material even the worst sorts of coal can be burnt with very low NO x and other emissions. MODERN PFBC CYCLE In modern PFBC cycle (see Fig.l ) the boiler flue gas enters gas turbine with temperature virtually equal to the bed temperature. The turbine exhaust gas is used for the boiler feedwater heating. Compressor of the gas turbine delivers heated up air to the boiler. Value of the air flow m A depends mainly on assumed boiler excess air number XFB. Owing to the use of a gas feedwater heater some of the steam heaters can be turned off. Number of working steam heaters depends on assumed values of gas outlet temperature t 5 and the outlet temperature difference St 5 which defines the feedwater temperature after the steam heaters t w = t 5 -St 5 (see Fig.2 ). Turning off steam heaters may cause an overloading of the steam tur-
doi:10.1115/92-gt-146
fatcat:mfc5yc7cczgszn2boqmfa5zpeq