A split concept for HRSG (heat recovery steam generators) with simultaneous area reduction and performance improvement

A split concept for boilers and heat recovery steam generators (HRSG), where flue gas recycling is required for controlling the maximal temperature, is proposed for reducing the heat exchange area and/or the recycling power requirements. The concept is demonstrated in the context of an HRSG of a pressurized oxy-coal combustion process, where the hot flue gas entering the HRSG is diluted by recycled flue gas to comply with the temperature constraint. The split concept proposes splitting the hot flue gas prior to dilution, and introducing the splitted fraction, with or without a secondary recycling stream, at an intermediate point in the HRSG. As a result, the split allows for lower recycling power requirements (lower diluent flowrate) and a smaller heat exchange area because the average temperature difference between the hot and cold streams in the heat exchanger is increased. Multi-objective optimization, for area and power requirements, is performed and the Pareto front is constructed. Results include a reduction by 37% without a change in the compensation power requirements, or a decrease in the power requirements by 18% (corresponding to 0.15 percentage points in cycle efficiency increase) while simultaneously reducing the area by 12%.