CO2 ABATEMENT BY CO-FIRING OF NATURAL GAS AND BIOMASS DERIVED GAS IN A GAS TURBINE

In this work, a possible way for partial CO2 emissions reduction from gas turbine exhausts by co-firing with biomass is investigated. The basic principle is the recirculation of a fraction of the exhausts (still rich in oxygen) to a gasifier, in order to produce syngas to mix with natural gas fuel. As biomass is a CO2 neutral fuel, the fraction of replaced natural gas is a measure of CO2 removal potential of the powerplant. The investigated solution considers the conversion of solid fuel to a gaseous fuel into an atmospheric gasifier, which is blown with a recirculated fraction of hot gas turbine exhausts, typically still rich in air. In this way, the heat content of the exhausts may be exploited to partially sustain the gasification section. The produced syngas, after the tar removal into the high temperature cracker, is thus sent to the cooling section, consisting of three main components: (I) gas turbine recuperator, (II) heat recovery steam generator and (III) condensing heat exchanger to cool down the syngas close to the environmental temperature before the subsequent recompression and mixing with natural gas fuel into the combustion chamber. The water stream produced within the condensing heat exchanger upstream the syngas compression is vaporised and sent back to the gasifier. If very limited modification to the existing gas turbine has to be applied in order to keep the additional costs limited, only a relatively reduced fraction of the low calorific value syngas may be mixed with natural gas. The analysis at different levels of co-firing has shown that no appreciable redesign has to be applied to the target GE5 machine up to 25–30% (heat rate based) renewable fraction. With an accurate heat recovery from the cooling/cleaning system of the syngas, the same levels of efficiency of the original machine have been achieved, in spite of the relatively large power consumption of the syngas recompression. Very interesting results have been obtained within the 10–30% range of biomass co-firing, with CO2 removal levels between 30% and 50% with reference to the values of the base GE5 gas turbine powerplant. The economic analysis has shown that, in spite of the high investment required for the syngas fuel production chain (gasifier, coolers, cleaners and fuel compressor), approximately at the same level of gas turbine itself, there is an interesting attractiveness due to the possibility of selling high-value green certificates and CO2 allowances, which reduce the payback time to 2–4 years. The uncertainty on the calculated economic parameters are greatly influenced by the uncertainty on actual biomass availability and yearly working time of powerplant, whereas off design operation, which affects mainly the uncertainty of compressor and turbine efficiency, is mainly reflected on the uncertainty of electric power output and efficiency.