Impact of Gas Turbine De-Rating on the Performance and on the Economics of Big-GT Cycles

Abstract

Biomass gasification and the sequential use of the produced gas in gas turbines (the so called Biomass Integrated Gasifier - Gas Turbine cycles) is one of the technologies that can minimize carbon dioxide emissions during electricity generation. As the primer mover of such power plants, gas turbine performance is a key parameter on the economics of BIG-GT systems. At least within the first generation of this technology, gas turbines originally designed for burning conventional fuels (natural gas and light oils) will be adapted to syngas burning. At this stage some changes on combustion chamber will be required, and some modifications on operational control parameters must be done as well. Gas turbine de-rating (reduction of gas turbine firing temperature) is the simplest control alternative to avoid problems concerned with reduction of compressor surge margin and increased thermal loads. However, from the performance view point of the gas turbine (and also for the whole system), this seems to be the worst option. In this paper the de-rating alternative is analyzed both from a thermodynamic and an economic point of view. A BIG-CC system based on a nearly atmospheric air-blown gasifier is considered. De-rating is analyzed vis-à-vis moderate increases of compressor pressure ratio. A simulation code (developed by the authors) able to perform analysis of thermal power systems (and particularly BIG-GT systems) is applied on this study. Cost functions are used to estimate the total installed unit capital cost, allowing a proper evaluation of electricity generating costs. The results indicate that electricity costs are sensitive to de-rating either due to the reduction on electricity generating efficiency and to the increase on the unit capital cost.

Thermodynamic Analyses of Biomass Post-Firing and Co-Firing Combined Cycles

“…The externally fired combined cycle (EFCC) can utilize biomass only as a fuel and does not require filters, but it uses a low calorific value fuel and has a relatively low energy efficiency [10,11]. The co-firing combined cycle fires various fractions of natural gas and biomass and has a reasonable efficiency [12,13], while mitigating some of the potential challenges when turbines are fired with low calorific fuels, e.g., de-rating [14,15].…”