Parallel cofiring involves installation of an independent separate 100% biomass-fired boiler to produce low-grade (pressure and temperature) steam. The steam generated in the biomass boiler is utilized in meeting the process demand of the coal-fired power plant instead of using high-pressure steam from the main boiler. This option has the lowest risk with highest reliability, as it runs independently in parallel to the existing boiler unit.
This option avoids to a great extent the potential for biomass generated fouling or corrosion of boiler tubes as the flue gas from biomass does not contact any heating surface of the boiler. It is, however, expensive, though some cost can be offset by allowing the boiler to vent its flue gas into upstream of the existing particulate separator and fan system of the main boiler avoiding the cost of a dust separator, induced fan, and stack.
If one does not account for reliability or technological maturity, direct firing option of cofiring offers the highest return on investment. If reliability is an overriding issue, option 3 of parallel cofiring, though most expensive, should be the best choice.