Generic approach for estimating ﬁnal feed water temperature and extraction pressures in pulverised coal power plants
An important choice that Coal ﬁred power plant (CFPPs) designer needs to make is the ﬁnal feed water tem-perature (FFWT) as well as extraction pressures from the turbines which supplies heating steam to the heaters. Diﬀerent methods have been reported in literature for determining the extraction pressures. The optimal cycle eﬃciency method (OCEM) appears to be the most common approach reported. It adopts an equal partitioning of feed water temperature (or enthalpy) rise across all the feed water heaters including the economizer which is situated in the boiler. However, it seems that most power plant designers deviate from this approach for eco-nomic and/or operational reasons. Diﬀerent OEMs have developed thermo-economically optimized methods for determining the FWH extraction pressures. However, these methods are not disclosed in open literature. In this research, the normalized feed water temperature rise across the FWHs and economizers of various CFPPs were examined. A modiﬁcation of the OCEM approach which better correlates to what is observed in current plants is proposed. The key modiﬁcation is in incorporating a suitable FFWT. Furthermore, the feed water temperature rise between the condenser exit and the FFWT is equally partitioned across all the feed heaters (excluding the economizers). The method was tested on the case study plants, and a maximum error of 7% was found for the ﬁnal feed water temperatures while the maximum error of the saturated vapour enthalpies that were calculated using the predicted extraction pressures was 2.2%.