A novel combined cooling-heating and power (CCHP) system integrated organic Rankine cycle for waste heat recovery of bottom slag in coal-fired plants
Introduction
The trends in enhancing the thermal efficiency and generating efficiency for coal-fired power plants have reached a new high as the air pollutants emission regulation becomes stricter. The most promising technology to improving the efficiency of power plants is turned out to be waste heat recovery (WHR) presented in many researches [1], [2], [3]. Taking advantage of the WHR technology could increase the power output or improve the energy utilization efficiency by exporting thermal water and chilled water.
In a coal-fired power plant, lower heating value (LHV) implied in the fuel consisting of coal and oxygen could be transformed into the electric power with a conversion ratio of 38–45, while 8–12 percentage points of total energy discharge into the environment in the kinds of bottom slag and flue gas. What’s more, the emission temperature for bottom slag and flue gas varying with the seasonal variation has great difference, 850–950 °C and 100–150 °C respectively. The traditional WHR technologies utilized in power plants include the economizers heating the condensate and feed water, preheaters heating the primary-air flow and secondary-air flow, and slag cooler recovering the waste heat of the bottom slag [4], [5]. Nevertheless, the traditional methods focus on the 1st Law efficiency to mitigate the energy loss. It means that these methods consider excessively the amount of energy consumption instead of the cascade utilization, leading to the increase of the exergy loss.
Organic Rankine Cycle (ORC) systems based on the typical Rankine cycle using water as its working fluid have been widely employed in cascade utilization of waste heat on account of high overall efficiency and compact structure [6], [7], [8], [9], [10]. What’s more, ORC technology is prominent for its effective utilization of low-temperature thermal source dismissed into the environment in conventional energy exploitation. Compared with Steam Rankine Cycle (SRC) and Ammonia-water Kalina Cycle (AWKC), ORC could recovery waste heat less than 250 °C considered generally to be moderate-to-low grade.
The particularity of working fluids equipped with low specific vaporization heat enables the ORC system to produce power by using low-temperature thermal source [6]. As many researches proposed, the selection of a suitable working fluid throwing effects on the cycle performance and system compactness is of great importance for the design of ORC systems [11], [12], [13], [14], [15], [16]. Bao and Zhao [17] have presented a review on the influence of working fluids' category and their thermodynamic and physical properties on ORC's performance. The working fluids were collapsed into three categories on the basis of vapor saturation curves in the temperature-entropy diagram: dry fluids, isentropic fluids and wet fluids. According to the thermodynamic performance, four dry working fluids were evaluated as the most suitable fluid candidates [18]. The results suggested that siloxanes MDM could yield the most suitable fluid in the preliminary phase of the project. Jang et al. [19] screened eight candidate ORC's fluids classified into three groups according to the magnitude of latent heat. Fluids with great latent heat were in favor of improving ORC system performance. Yang et al. [20], [21] presented experimental and numerical researches on the alternative to R245fa. The results indicated that R1233zd(E) has been proven as an appropriate alternative to R245fa. Besides the thermal physical properties, the safety and environmental protection for working fluids, such as the flammability, toxicity, Ozone Depletion Potential (ODP) and Global Warming Potential (GWP), should also be considered comprehensively as mentioned in [22], [23].
A typical ORC system consists of an evaporator used to absorb waste heat, an expander producing shaft power, a condenser where the organic working fluid transforms into saturated liquid, a pump applied for pressurization of working fluid. As a critical component in ORC system, the aerodynamic performance and geometric dimension of the expander affect the system performance. Bao et al. [17] introduced a comprehensive review of the expander selection for ORC system. Expanders were categorized into two types: the velocity type and the volume type. Compared to the volume type ones, turbine expanders as the velocity type, especially radial-inflow turbines, are frequently selected for their high efficiency and compactness. Xia et al. [24] investigated the influence of radial-inflow turbine on ORC system with methods of one-dimensional design and three-dimensional CFD analysis. They concluded that the addition of splitter blade could improve the performance of the ORC radial-inflow turbine. In general, the expansion ratio of a single-stage radial turbine, exerting significant influence on the ORC system performance, was restricted to be about 4 [25], [26]. Kang [27] designed a two-stage radial turbine consisted of high-pressure turbine and low-pressure turbine to improve the system performance by increasing the expansion ratio. With the application of real Equation of State (EOS), Fiaschi et al. [28] developed a zero-dimensional model for the design of radial turbo-expanders for ORC system. The total to total efficiency of the designed machines ranged between 0.72 and 0.80, which provided a good agreement with other researches in [6], [29], [30], [31].
Compared with the researches on working fluids and expanders, the investigation on ORC system is closer to practical application. Vast works on ORC systems have been done to improve the waste heat recovery from the internal combustion engine, geothermal-solar power plant, truck diesel engines and gas turbine exhaust gas. To enhance the geothermal energy efficiency and improve the performance of the geothermal ORC system, Bassetti et al. [32] proposed a new hybrid Geothermal-Concentrating Solar Power (GEO-CSP) plant, in which solar energy was used to heat up the geothermal fluid entering the heat exchanger of ORC system. Compared to the geothermal-only plant, the new plant produced higher efficiency and gained more solar energy raises from 5.3% to 6.3% with the integration of thermal energy storage. Since the first application of ORC system in recovering engine waste heat in a vehicle was presented by Patel and Doye [33], many studies focused on the cascade utilization of exhaust gas and jacket water waste heat from engines by ORC system [34], [35], [36], [37]. The combination of ORC and gas turbine in cogeneration system has attracted researchers' attention, getting the utmost out of the gas turbine exhaust [38], [39], [40], [41], [42].
As mentioned above, the working fluids and application of ORC system in waste heat recovery have been analyzed in detail. However, there are few researches on the utilization of ORC for waste heat recovery in coal-fired plant. Liu et al. [1], [2] discussed the application of ORC system to recover the blowdown waste heat and exhaust enthalpy, which boosted the generating efficiency by increasing the power output. Most researches on the working fluids investigated the influence of their thermophysical properties on the cycle efficiency, by modeling the ORC system with kinds of working fluids. It seems that there is not a comprehensive and statistical screening for ORC working fluids, considering the fluid properties such as the flammability, toxicity, ODP and GWP.
In the present work, a novel combined cooling-heating and power (CCHP) system integrated Organic Rankine Cycle (CCHP-ORC) for recovering the waste heat of bottom slag in coal-fired plant is proposed for the first time. Firstly, the CCHP-ORC configuration and thermodynamic models for critical components are described in detail. Secondly, Analytic Hierarchy Process (AHP) is devoted to select optimal working fluids with the consideration of thermophysical properties, safety and environmental protection. Finally, the influences of superheat degree, chilled water mass flow rate and condenser temperature on the performances of the proposed system are discussed.
Section snippets
CCHP-ORC system description
In the present study, a novel CCHP-ORC system is employed to exploit the waste heat of bottom slag in a 300 MW coal-fired plant. Fig. 2 depictures a schematic diagram of CCHP-ORC system, consisting of three cycles named Cycle1 ∼ Cycle3 respectively. In case of Cycle1, the working fluid with high temperature and pressure flows into the turbine(I) to generate electricity (1–2). Then the low pressure vapor transforms into saturation liquid (2–3) in the heat sink by rejecting heat to the heat
Mathematical modeling
A MATLAB procedure is developed for the proposed system based on the mass, energy and exergy balances of each component [46]. The performance parameters are established in the viewpoints of the first and second laws of thermodynamics. REFPROP database is linked to obtain the thermo-dynamic properties of the working fluids of interest [47]. To simplify the CCHP-ORC system model simulations, the general assumptions are listed as follows:
- (1)
The system operates at steady state.
- (2)
The variations of
Candidates for working fluid
Dry fluids, of which the saturated vapor curves have a positive slope and liquid droplets are not formed during the expansion process, are chosen as the working fluids in this paper [43], [44], [45]. Fig. 4 depicts the temperature-entropy (T-s) diagrams for the candidates, including R227ea, R1234ze(E), R601a, R600, R123, R245fa, Hexane, Heptane and Cyclohexane. The physical, environmental and safety properties for the nine candidates are listed in Table 2. It can be shown that all the chosen
Results and discussion
The effects of superheat degree, condenser temperature, heat consumer temperature and chilled water mass flow rate on the performance of the proposed systems are conducted. R1234ze(E) and heptane/R601a identified as the optimal candidates with AHP method are applied to the SF-CCHP-ORC and DF-CCHP-ORC system, respectively.
The exergy destruction distributions in SF-CCHP-ORC and DF-CCHP-ORC system are shown in Fig. 6. The exergy destructions of H-HE and L-HE are absent on account of the
Conclusion and future directions
In the present study, a novel combined cooling-heating and power (CCHP) system integrated Organic Rankine Cycle (CCHP-ORC) for recovering the waste heat of bottom slag in coal-fired plant is proposed for the first time. SF-CCHP-ORC and DF-CCHP-ORC systems are classified according to whether the working fluid in Cycle1 is identical as that in Cycle2 and Cycle3. The CCHP-ORC configuration and thermodynamic models for critical components are described in detail. Considering the thermophysical
Acknowledgements
The authors would like to thank the Fundamental Research Funds for the Central Universities of China (No. 531107051214) for the support.
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