Optimizing a Pressurized Fluidized Bed Combustion Combined Cycle With Gas Turbine Topping Cycle
Authors: Bohn D. , Dibelius G. H. , Pitt R. U. , Faatz R. , Cerri G. , Salvini C. 
Published: February 2015

Combined Cycles for the generation of electricity or co-generation of heat and power with Pressurized Fluidized Bed Combustion (PFBC) of coal and a gas turbine topping cycle fired with a fuel suitable for gas turbines have been studied to set up optimum process parameters with respect to net efficiency, emissions including CO2, and keeping in mind the feasibility of the plant components. As a basic approach, natural gas has been considered as a fuel for the topping gas turbine. Net efficiencies up to 50% (LHV) are acheived.

For the calculations, a Recursive Equality Constraint Quadratic Programming Method (RECQPM) is applied. The method is semi-implicit, i. e. the equations describing the process are solved using a non-linear equations system solver; the modular structure of the cycle is, however, made up for by programme modules set up for the relevant components/units.

With the process layouts studied, the PFBC should be operated at a pressure level to allow for a compact design of the PFBC steam generator and the Hot Gas Clean-Up Unit (HGCU), and to take advantage of the pressure with respect to combustion efficiency, in-bed sulfur retention and NOx-reduction. The overall pressure ratio of the topping gas turbine, e. g. consisting of an LP-compressor plus a free running HP spool, and exhausting to the PFB combustor, should be in the range of 30. Further developments of gas turbine technology with respect to pressure ratio and turbine inlet temperature can be incorporated into the process and will be associated with an increase of overall efficiency.

The heat to be extracted from the coal fired PFBC at the typical combustion temperature of 850°C allows for steam generation at conventional live steam and reheat temperatures (and pressures). The incorporation of advanced steam cycle parameters, as actually considered for pulverized coal fired boilers, would again increase the overall net efficiency of the cycle by some 5% with an increase of both the live steam and the reheat temperature from 540°C to 600°C.

In contrast to conventional combined cycles with an unfired waste heat boiler for steam generation, the overall efficiency of the PFBC combined cycle with gas turbine topping cycle is only marginally affected by dual pressure steam cycle arrangements, except for very sophisticated and costly designs.

To use gas from an integrated coal gasification unit rather than natural gas as a fuel for the topping gas turbine would result in an entirely coal based process. Due to the capability of the PFBC to burn residues of coal gasification and gas purification, this process, compared to pure Integrated Gasification Combined Cycles (IGCC), is less sensitive to the carbon conversion acheived. Even more, the raw gas purification might be simplified, and the process efficiency might be increased as a result of the sulphur removal to be acheived in the PFBC rather than in a raw gas sweetening process. Some preliminary findings for a process with an integrated partial gasification unit are discussed.