Thermodynamic and Economic Analyses of Reformative Design for High Back-Pressure Heating in Coal-Fueled Cogeneration Units.

Heng Chen,Gangye Shi,Gang Xu,Yongping Yang,Jidong Xu,Yunyun Wu,Wenyi Liu

doi:10.3390/e21040342

Abstract

High back-pressure (HBP) heating technology has been identified as an effective approach to improve the efficiency of combined heat and power (CHP). In this study, the novel concept of a HBP heating system with energy cascade utilization is developed and its probability examined. In the reformative design, the extracted heating steam from the intermediate-pressure turbine (IPT) is first drawn to an additional turbine where its excess pressure can be converted into electricity, then steam with a lower pressure can be employed to heat the supply water. As a consequence, the exergy destruction in the supply water heating process can be reduced and the efficiency of the cogeneration unit raised. A detailed thermodynamic investigation was performed based on a typical coal-fired HBP–CHP unit incorporating the proposed configuration. The results show that the artificial thermal efficiency (ATE) promotion was as much as 2.01 percentage points, with an additional net power output of 8.4 MW compared to the reference unit. This was attributed to a 14.65 percentage-point increment in the exergy efficiency of the supply water heating process caused by the suggested retrofitting. The influences of the unit power output, unit heat output, supply water and return water temperatures and turbine back pressure on the thermal performance of the modified system are discussed as well. In addition, the economic performance of the new design is assessed, indicating that the proposed concept is financially feasible.

Highlights

Combined heat and power (CHP) cogeneration is a method of generating power and heat in parallel from one energy source at or near the site of consumption, and its potential to provide benefits comes from making the best possible utilization of fuel energy [1,2] which can convert the chemically bound fuel energy into electricity and heat at an overall efficiency above 90% [3,4].Cogeneration is capable of reducing fuel consumption by 20–30% as compared to the decoupled production in conventional power and heating stations [5,6]
A modified High back-pressure (HBP) heating design with energy cascade utilization was proposed to promote the efficiency of CHP and its practicability
The results of the thermodynamic evaluation indicate that the proposed unit can import 8.4 matter flow (MW) extra electricity with a corresponding energy utilization factor improvement of 1.08 percentage points and artificial thermal efficiency improvement of 2.01 percentage points

Summary

Introduction

Cogeneration is capable of reducing fuel consumption by 20–30% as compared to the decoupled production in conventional power and heating stations [5,6]. By recovering and using heat, cogeneration units can diminish environmental emissions in the generation sector by 13–18% [7], which has been recognized as playing a crucial role in energy sustainability and the attempt to achieve climate change response objectives [8,9]. Since cogeneration can alleviate the need for increased power generation capacity and reduce the environmental impact associated with energy consumption, it has Entropy 2019, 21, 342; doi:10.3390/e21040342 www.mdpi.com/journal/entropy. In China, the operational CHP capacity has been over 300 million kW since 2016, and 350 million kW of coal-fired power units will be retrofitted into cogeneration units for district heating before 2020 [13]. It is obvious that cogeneration will become more significant for district heating globally

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