The simulation results of a preliminary analysis of Case В are presented in Figure 9.1 for different rates of natural gas replacement. More details of the simulation proce­dure adopted are presented in Walter et al., 1999. The far left points correspond to gas turbine operation with natural gas only, while the far right points correspond to gas turbine operation just with biomass-derived gas. Gas turbine derating influences combined cycle performance. The gradual switch from natural gas to syngas, as far as GT derating is not necessary, increases power production due to (i) a larger gas

mass flow and (ii) an increase in compressor pressure ratio. When a larger amount of syngas is used and GT derating is imposed, net power production drops due to derating itself and due to the increasing power consumption of auxiliaries (mainly syngas compressor and air compressor).

Based on the same economic assumptions previously presented for Case A, a feasibility analysis of partial switch from natural gas to syngas was developed. For all intermediary cases between the two limits (GT operation on natural gas and on syngas only), the installed capital cost was estimated as the cost of a conventional natural gas combined cycle plus the capital cost of a biomass gasifier and gas­cleaning unit. The additional investment on syngas production was calculated according to the amount of biomass required (as received, with 50 per cent mois­ture), assuming a reference value (Faaij et al., 1997) and a scaling factor of 0.70. Finally, a 1.6 factor was applied over the estimated capital cost to take into account the overall set of extra costs regarding GT adaptation and equipment installation.

The cost of electricity as a function of the share of natural gas replaced is presented in Figure 9.2. The same figure includes results that correspond to the impact of credits due to carbon dioxide abatement. As can be seen, carbon credits equivalent to 10US$/t of C02 avoided could make the cofiring option feasible vis-a-vis electricity generation from natural gas in a large range of mixtures between natural gas and biomass-derived gas. At a credit price equivalent to 20 US$/t C02 avoided, electricity generation from cofiring natural gas and biomass is even cheaper than some conventional alternatives.

Finally, Figure 9.3 shows the evolution of investment IDR as a function of the share of natural gas replaced and carbon credits as well. The decline of IDR as far as gas turbine operation is switched from natural gas to syngas is due to the higher

cost of electricity (0 $/t C02). Carbons credits make IDR reduction smoother or can counterbalance this tendency at higher carbon credit prices.