Conversion of steam power plant into cogeneration unit - Case study

Abstract

One of possible solutions leading to the increase of steam power plant thermal efficiency is its conversion into a cogeneration unit. However, it requires the heat recipient to be on-site and, if so, it may be done by implementation of absorption heat pump (AHP) technology with low temperature waste heat recovery and utilization. The paper presents results of an analysis of benefits resulting from implementation of absorption heat pump into the steam power plant for recovery of low temperature waste heat, due to additional demand for heat delivery to dedicated recipient (DHR). The demand for heat along the whole year was assumed and the calculations were performed for two cases. In the first case, which was called the reference case, the heat was produced by dedicated heat exchanger (DHX) fed with steam taken from the pipeline between the medium and low-pressure turbine sections (MPLPP). In the second case, absorption heat pump (AHP) as the main unit for heat production and recovery was assumed. However, due to insufficient heat production in some conditions, peak heat exchanger (PHX) placed directly behind the heat pump was considered. Both heat pump and pick heat exchanger were fed with steam taken from MPLPP, however, due to the AHP requirements the stem was mixed with hot water taken from hot water tank to achieve the steam saturation state. Results of calculations performed show that implementation of a heat recuperation by absorption heat pump implemented to conventional steam power plant increases the overall efficiency of cogeneration system, but at the same time it causes the electricity production to decrease. Moreover, the production of heat by absorption heat pump has lower impact on the electricity generation compared to generation of heat by conventional heat exchanger. If reference power plant without heat production is considered, due to the heat pump implementation the electricity production decreases of about 1.32% for heat production with dedicated heat exchanger, and of about 0.89% for absorption heat pump with peek heat exchanger respectively. It means that, owing to implementation of AHP instead of DHX with the same heat production, the power plant may generate more than 0.4% of total electricity produced.