Abstract
To maximize the system-level heat integration, three retrofit concepts of waste heat recovery via organic Rankine cycle (ORC), in-depth boiler-turbine integration, and coupling of both are proposed, analyzed and comprehensively compared in terms of thermodynamic and economic performances. For thermodynamic analysis, exergy analysis is employed with grand composite curves illustrated to identify how the systems are fundamentally and quantitatively improved, and to highlight key processes for system improvement. For economic analysis, annual revenue and investment payback period are calculated based on the estimation of capital investment of each component to identify the economic feasibility and competitiveness of each retrofit concept proposed. The results show that the in-depth boiler-turbine integration achieves a better temperature match of heat flows involved for different fluids and multi-stage air preheating, thus a significant improvement of power output (23.99 MW), which is much larger than that of the system with only ORC (6.49 MW). This is mainly due to the limitation of the ultra-low temperature (from 135 to 75 °C) heat available from the flue gas for ORC. The thermodynamic improvement is mostly contributed by the reduction of exergy destruction within the boiler subsystem, which is eventually converted to mechanical power; while the exergy destruction within the turbine system is almost not changed for the three concepts. The selection of ORC working fluids is performed to maximize the power output. Due to the low-grade heat source, the cycle with R11 offers the largest additional net power generation but is not significantly better than the other preselected working fluids. Economically, the in-depth boiler-turbine integration is the most economic completive solution with a payback period of only 0.78 year. The ORC concept is less attractive for a sole application due to a long payback time (2.26 years). However, by coupling both concepts, a net power output of 26.51 MW and a payback time of almost one year are achieved, which may promote the large-scale production and deployment of ORC with a cost reduction and competitiveness enhancement.
Highlights
Coal-fired power generation plays a decisive role in the power generation sector in China and is significant to ensure secure electricity supply
Since the performance of the involved organic Rankine cycle (ORC) is largely affected by the selected organic working fluid for given heat source, the ORC working fluid and the corresponding evaporating and condensing pressures are first selected with genetic algorithm (GA) to maximize the power generation of ORC
Eight working fluids are screened in terms of the net additional power output with respect to two operating variables: the pressures of evaporation and condensation
Summary
Coal-fired power generation plays a decisive role in the power generation sector in China and is significant to ensure secure electricity supply. China’s government has imposed a policy of replacing small, non-efficient subcritical power plants with large, highly-efficient supercritical or even ultra-supercritical power plants, which employ very high steam pressure and temperature (over 25 MPa and 600 ◦ C). With such an effort, the specific coal consumption of coal-fired power plants in China has been reduced significantly, for example, 321 g/kWh for plants over 600 MW [1]. The system design of thermal power plants faces a severe requirement of theory and technology innovation to achieve deep energy conservation, considering varying coal quality, complex operating boundary, peak-shaving responsibility and the requirement of ultra-low pollutant emissions [9,10,11,12,13]
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