An Improved System for Utilizing Low-Temperature Waste Heat of Flue Gas from Coal-Fired Power Plants

Abstract

Abstract:In this paper, an improved system to efficiently utilize the low-temperature waste heat from the flue gas of coal-fired power plants is proposed based on heat cascade theory. The essence of the proposed system is that the waste heat of exhausted flue gas is not only used to preheat air for assisting coal combustion as usual but also to heat up feedwater and for low-pressure steam extraction. Air preheating is performed by both the exhaust flue gas in the boiler island and the low-pressure steam extraction in the turbine island; thereby part of the flue gas heat originally exchanged in the air preheater can be saved and introduced to heat the feedwater and the high-temperature condensed water. Consequently, part of the high-pressure steam is saved for further expansion in the steam turbine, which results in additional net power output. Based on the design data of a typical 1000 MW ultra-supercritical coal-fired power plant in China, an in-depth analysis of the energy-saving characteristics of the improved waste heat utilization system (WHUS) and the conventional WHUS is conducted. When the improved WHUS is adopted in a typical 1000 MW unit, net power output increases by 19.51 MW, exergy efficiency improves to 45.46%, and net annual revenue reaches USD 4.741 million while for the conventional WHUS, these performance parameters are 5.83 MW, 44.80% and USD 1.244 million, respectively. The research described in this paper provides a feasible energy-saving option for coal-fired power plants.

New Criteria to Characterize the Waste Heat Recovery

“…Recent papers [1][2][3][4] prove that this topic is a well-studied one by the analysis of thermodynamic models for the simulation and optimization of power systems [1,2], as well as energy and exergy analysis of waste heat recovery systems [3,4]. For instance, an assessment with the citation indexing service Web of Science, that provides a comprehensive citation search through multiple databases, gives for "waste heat" a total of 23,717 references within its core collection over the period 1950-2017, of which 313 are highly cited.…”

“…The paper topic is mainly concerned by fundamental aspects and methodology than by aspects developed regarding process sites, even if these developments become more and more explored [2,3,[21][22][23][24].…”

Node Temperature of the Coupled High-Low Energy Grade Flus Gas Waste Heat Recovery System

“…Xu et al [9] suggested use two-stage air preheater and LTE to heat condensed water and inlet air, which saved more high-grade extraction steam for power generation. Huang et al [10] proposed an improved waste heat use system for power plant boilers based on the traditional bypass flue heat recovery system, in which low-pressure steam is extracted from the low-pressure cylinder to preheat the air, replacing high-temperature flue gas to heat the feed water of the boiler, and the calculation showed the unit's exergy efficiency was increased to 45.46% on a 1000 MW unit. Liu et al [11] introduced a coupled high-low energy grade flus gas waste heat recovery system (CWHRS) applied to a 1000 MW double-reheat unit, which has produced good energy-saving effects.…”

Work Availability and Exergy Analysis

“…An Improved System for utilizing low-temperature waste heat of flue gas from coal-fired power plants was carried out by Huang et al [8]. In this research article, an improved system to efficiently utilize the low-temperature waste heat from the flue gas of coal-fired power plants was proposed based on heat cascade theory.…”