A combined cycle plant based on a PG6101(FA) gas turbine, such as the one con­sidered in this study, and following the hypothesis assumed, consumes about 500 thousand Nm3/day of natural gas operating at full load basis (about 430 Nm3/day for an average capacity factor of 0.85). Thus to guarantee full operation of a 100 MW plant, some 540 thousand Nm3/day would be needed. Table 9.6 summarizes the amount of natural gas that can be displaced per set of 100 MW power capacity for each cofiring alternative analyzed in this study. Results for Case В correspond to the best case from the point of view of capacity, i. e. 101.7 MW of net capacity, for 61.9 per cent substitution of natural gas on mass basis.

To give a perspective of this contribution, we look into the present situation in Brazil. The Brazilian government has recently decided to support the installation of almost 17 GW of thermal power capacity, totalling 49 plants, most of them designed to fire natural gas. Of this total capacity, 6.5 GW are planned in industrialized sites of the State of Sao Paulo, not far from the regions where there is a large con­centration of sugarcane mills. The natural gas demand to allow the operation of 6500 MW new capacity in the State of Sao Paulo is estimated at 25 to 26 million Nm3/day, taking into account the predicted average annual capacity factor and actual natural gas to electricity efficiencies (in round numbers, larger than the performance figures used in this article). This natural gas volume is quite substantial as a share of the capacity of the brand new Brazil-Bolivia natural gas pipeline, the main source of natural gas supply for the years to come.

The adoption of a Case C strategy would allow a maximum displacement of natural gas of about (124.6/577.0) x 25-26 million Nm3/day = 5.4-5.6 million Nm3/day. With the strategy that corresponds to Case B, the maximum volume of natural gas that could be displaced would be (345.6/577.0) x 25-26 million Nm3/day= 15.0-15.6 million Nm3/day. On the other hand, theoretically, a Case A

strategy would allow 100 per cent natural gas displacement. It is clear from the numbers presented that cofiring biomass and natural gas could make the natural gas market more flexible, without drawbacks to electricity generation.

Sugarcane production in the State of Sao Paulo is estimated at 180 million tons per year. With this level of production, some 23.2 million tons of sugarcane trash could be recovered (50 per cent of the amount available, owing to topographic constraints; sugarcane trash availability is equivalent to approximately 25 per cent of sugarcane mass). The full availability of bagasse is estimated at 46.8 million tons/ year (13 per cent fiber content and 50 per cent moisture), the ordinary surplus being about 10-15 per cent of the total availability. Achieving 6500 MW capacity, full implementation of the Case C strategy would demand 18.7 million tons of biomass per year, while the Case В strategy would require 33.9 million tons/year. Nowadays, trash is essentially burned at the field before sugarcane harvesting. For bagasse, there is not a market able to consume all the existing surplus. Hence, from the point of view of biomass availability, there is no particular constraint regarding the imple­mentation of the cofiring alternatives analyzed here.

From the economic point of view, the preliminary results indicate that Case В can be considered as a reasonable alternative as the cost of electricity produced is kept at an acceptable level, and the investment IDR is not reduced to any large extent when biomass contribution is considered. If the biomass plant can be built mainly with equipment manufactured in Brazil (thus at lower capital cost), Case C is the best option, as it allows a reduction on the cost of electricity produced while enhancing the investment IDR. According to the results, to substitute biomass for natural gas in BIG-CC power plants (Case A) is not a good alternative as some amount of natural gas allows improvements in the system economics. Obviously, the oppor­tunity to generate carbon credits from cofired plants would make a substantial difference on the economics of such alternatives.

The feasibility of Case В and Case C could be further improved with larger power units, taking advantage of economies of scale and, consequently, reducing the capital costs per unit energy generated. Strictly speaking, the location of the power plant would be a matter of concern regarding biomass transportation costs. The same is true regarding plant size — as the scale of the plant increases, costs related to logistics also increase.