A reduced order full plant model for oxyfuel combustion

The linkage of models generated in different commercial and/or proprietary software packages is becoming the state-of-the-art for the investigation of power generation applications. Here, a sophisticated process modelling software package, gPROMS, has been linked with a commercial CFD code, ANSYS FLUENT version 12, in order to create a reduced order model (ROM) for oxyfuel combustion. This is the first such application, and represents a vital step in the evaluation of the impact of oxyfuel operation on a conventional pulverised fuel plant. In order to use existing steam cycle technology, the heat duties to the furnace walls and heat exchangers must match those of conventional combustion. Evidence from experiments conducted by Smart et al. (Fuel 89 (2010) 2468-2476) at 0.5 MW scale indicate that this is possible; however there is no operating experience of full-scale technology. A full plant model of a 500 MW e sub-critical UK power station has been adapted for oxyfuel combustion. In order to relate heat transfer in the boiler to the level of oxygen enrichment and recycle ratio, a CFD-generated database of results for air and oxyfuel combustion with an excess oxygen value of 5% in both cases has been developed. Non-linear correlations of heat duties for various heat transfer sections of the plant are developed from the database, and linked to the full plant model. The plant model was then investigated over a range of oxygen concentrations from 25% to 35% by volume at the furnace inlet, which corresponds to a dry recycle ratio range of 66-77%. The aim in this case was to assess the potential for fitting oxyfuel burners to an existing plant. The optimum level of oxygen enrichment is found to be 32% and this corresponds to a recycle ratio of 70%. For these conditions the steam temperatures and heat duties closely match those of the conventional air-firing operation.