Arnaldo Walter, Monica R. Souza[10] and Andre Faaij

9.1. WHY COFIRING?

The term cofiring has been often applied to designate the combined use of fuels in power plants as well as in industrial steam boilers. A special case is the combined use of biomass and fossil fuels, the most acknowledged idea being the mix of biomass and coal in power plants. In some countries, such as the United States, The Nether­lands, Austria and Finland, cofiring biomass and fossil fuel has been commercially practiced for power production since the mid-1990s.

Environmental issues, mainly those concerned with mitigation of airborne emis­sions (carbon dioxide and other gaseous pollutants, especially sulfur oxides), are the main reasons for pursuing efforts on coftring. Owing to substantial reduction of technical and economic risks, cofiring has also been considered as the first step in enhancing biomass utilization for power generation in some countries. With cofiring, for instance, it is possible to take advantage of the relatively high efficiency of large coal boilers without incurring a large investment (Sondreal et al., 2001).

Cofiring biomass and natural gas has been considered to a less extent so far, and no significant commercial experience has been identified. Recently, a report on cofiring biomass-derived fuels and natural gas in gas turbines has been released in The Netherlands (De Kant and Bodegom, 2000). The research has focused on the technical feasibility and the potential of cofiring low-heating-content fuels and natural gas over different power configurations. Gas turbine constraints and required adaptations have been inventoried with the gas turbine suppliers. Likewise, a similar study was developed some years ago at the National Renewable Energy Laboratory — NREL in the United States, but only a preliminary analysis of technical options was conducted at that time (Spath, 1995).

Considering the biomass use, the term cofiring has been applied in a widespread sense. Strictly, cofiring corresponds to burning a mix of fuels in the same thermal device. However, cofiring has also been understood as: (i) biomass or fossil fuel use to complement the main fuel supply; (ii) biomass use to increase plant capacity, burning fuels without mixing; and even (iii) when biomass is used to full substitution of a fossil fuel in an existing power plant.

This chapter describes a research focused on opportunities for developing power production from sugarcane residues (sugarcane bagasse and sugarcane trash, i. e. leaves and tops of the plant) based on cofiring with natural gas. The three technical alternatives presented are based on the wide definition of cofiring mentioned earlier.