Case C was defined considering a natural gas combined cycle plant based on two PG6101(FA) units, one unfired HRSG for each gas turbine and just one steam turbine. The total capacity of the NG-CC is 187.2 MW, with an additional contri­bution of 53.2 MW from biomass. Thus, the total plant capacity is 240.4 MW. The steam cycle parameters and the overall cycle performance are the same as presented in Table 9.2. The biomass contribution to the cofiring plant occurs in a conventional steam power cycle in which steam is generated at the same live-steam parameters as in the HRSGs, producing 53.2 MW as well. Main simulation results are presented in Table 9.4.

The feasibility assessment was accomplished based on the assumptions described earlier for a NG-CC power plant. The initial capital cost for a 185 MW class was estimated as 480US$/kW. For the steam cycle plant based on biomass, the unit capital cost is estimated at 1110 US$/kW. This evaluation is based on cost functions for the main plant components and results were checked with published data. Annual operation and maintenance costs (excluding fuel) were considered as equi­valent to 5 per cent of the initial capital cost.

The main results of the feasibility analysis are presented in Table 9.5. The addition of a biomass module represents an increase in the plant capacity of about 28 per cent vis-a-vis the natural gas combined cycle plant. As the cost of producing electricity

through the biomass plant running all over the year (capacity factor 0.85, as well as for the NG-CC) is higher than the cost estimated for electricity produced in the NG-CC plant (40.0 x 33.5 US$/MWh), there is a rise in the cost of electricity produced by the cofired plant.

For full-year operation of the biomass-based power plant, the IDR is significantly reduced vis-a-vis the NG-CC case. The investment would be more feasible if carbon credits were available but these should be as high as 40 $/t C02 to make the cost of electricity of the cofired plant equivalent to the NG-CC.

However, if the biomass-based power plant operates for a shorter period during the year (e. g. just during the 6-7 months of the sugarcane harvest season), the final cost of electricity rises. For instance, for 4080 h of operation per year (200 days of harvesting and a capacity factor of 0.85 during this period), the cost of electricity produced by the biomass power plant rises to 61.4US$/MWh and the cost of electricity of the cofired unit increases to 37.7 US$/MWh. Simplifying the analysis, it was considered that the average biomass cost is 8 US$/t in both cases. The IDR is estimated as 19.9, 20.6 and 21.4 per cent for the three cases considered (depending on the value of the carbon credits).

The Case C was also evaluated considering a capital cost for the steam cycle plant based on biomass at 500 US$/kW. This is a reasonable estimate for equipment built in Brazil as the cost in US dollars has dropped significantly due to the devaluation of Brazilian currency since early 1999. As can be seen in Table 9.5, in this case, the cost of electricity generated in the cofired unit could be lower than in the natural gas combined cycle unit, even without carbon credits.