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Assessment and Modelling of the Waste Heat Availability from Gas Turbine Based CHP Systems for ORC Systems

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Assessment and Modelling of the Waste Heat Availability from Gas Turbine Based CHP Systems for ORC Systems European Association for the International Conference on Renewable Energies and Power Quality Development of Renewable Energies, Environment (ICREPQ’12) and Power Quality (EA4EPQ) Santiago de Compostela (Spain), 28th to 30th March, 2012 Assessment and modelling of the waste heat availability from gas turbine based CHP systems for ORC systems E. Firdaus1, K.Saaed1, D.Bryant2, M.Jones1, S.Biggs3 and B.Bahawodin1 1 Computing Engineering and Mathematics University of Brighton BN2 4GJ (United Kingdom) Phone/Fax number: +0044 203 0060166/65, e-mail: [email protected], [email protected], [email protected], [email protected] 2 Heatcatcher Ltd. The Roller Mill Mill Lane Uckfield TN22 5AA (United Kingdom) Phone/Fax number: +0044 203 0060166/65, e-mail: [email protected] 3 Efficient Air Ltd. The Roller Mill Mill Lane Uckfield TN22 5AA (United Kingdom) Phone/Fax number: +0044 182 5748150/51, e-mail: [email protected] Abstract. This paper presents the findings of modelling of Interest in capturing low-grade (low temperature) heat waste heat availability from a Combined Heat and Power (CHP) has grown dramatically in past decades [1]. Important system with a rated electrical output of 4.35 MW and steam alternatives have been proposed to generate electricity production of 8,165 kg/hr at 16 barg. The model has been from low temperature heat sources such as solar thermal developed using HYSYS DynamicsTM. The amount of waste heat power, industrial waste heat, engine exhaust gases and available from the CHP system is dependent on the ambient air, domestic boilers [1]-[2]. The potential for utilising waste steam production, and gas turbine power output. A Pitot traverse heat from industrial applications is particularly promising measurement across the duct was undertaken to determine the [3] because of the large amount of waste heat. Statistical actual amount of waste heat available from the CHP system. The measurements were conducted in accordance to BS EN investigations indicate that low-grade waste heat account 15259:2007 standards. The simulation results of waste heat for 50% or more of the total heat generated in industry availability have been compared to experimental data at various [4]. CHP power outputs. The HYSYS DynamicsTM model waste heat calculation was shown to be 5.3% lower than experimental waste Using conventional methods such as steam rankine cycle heat measurements. An analysis of the waste heat availability by to recover energy from low-grade heat is economically both modelling and experiment was done which shows that the infeasible [5]-[6]. However, the Organic Rankine Cycle CHP system waste heat available between 3.82 MW and (ORC) system [1],[6],[4],[7] uses a high molecular 5.09 MW. Recovering this low grade heat from the CHP system working fluid which boils at a lower temperature than using Clean CycleTM 125 ORC systems generates between 217 kW and 344 kW of electricity, respectively. Increases of water and is thus more efficient than water with low 2.3% in electrical efficiency of the CHP system are predicted. grade waste heat. Working fluids used in the ORC system have been studied, such as R-113 [8]-[11], R-245fa Key words [5],[12],[13], R-245ca [9]-[11], Toluene [7],[11] and Ammonia [10],[14]. waste heat recovery; gas turbine; modelling; Hysys; CHP; A typical ORC system basically comprises a pump, a ORC turbine, an evaporator and a condenser. The working fluid is vaporised by a heat source in the evaporator. The 1. Introduction superheated vapour of the working fluid expands in the turbine to generate electricity and is then condensed. The condensed working fluid is finally sent via a pump into https://doi.org/10.24084/repqj10.714 1391 RE&PQJ, Vol.1, No.10, April 2012 the evaporator thus closing the cycle. An economiser can • investigate the feasibility of integrating an ORC increase the efficiency of the system by pre-heating the system with a CHP system. working fluid entering the evaporator with the heat from the working fluid leaving the evaporator (see figure 1). Natural Gas The biggest influences on the performance of the ORC system are the amount and temperature of the waste heat. Feed Exhaust Combustion Water An accurate assessment of waste heat availability is Chamber Gas Waste Heat needed for optimisation of waste heat recovery. Turbine Compressor Generator Steam Boiler Steam Air Inlet Condenser Fig. 2. Simplified process flow diagram of CHP systems based Turbine Generator on gas turbine in simple-cycle (Brayton cycle) mode integrated with waste heat steam boiler. Evaporator Economiser Table I. Summary of different combined cycles integrated with gas turbine. Combined Gas turbine model New Pump cycle/Technology overall Fig. 1. Process flow diagram of ORC system integrated with an efficiency economiser. Gas to gas 1.4 MW Heron [22] 39-43% recuperation 21 MW Rolls-Royce [23] 42% This paper focuses on low-grade heat recovery from gas cycle turbine based systems. Most of the modern gas turbines Brayton-rankine W501G cycle Siemens/Westinghouse [24] 58% available in the market offer efficiencies up to 42% [17]. GT24 and GT26 ABB- One important disadvantage of a gas turbine is that it does Alstom[24] 58.5% not perform well in part-load operation [17]-[19]. For General Electric 60% instance, at 50% load, a gas turbine achieves around 75% Brayton-brayton Allison 571 K [17] 43.2% of the full-load efficiency, and at 30% this drops to 50% of cycle LM2500 General Electric the nominal efficiency [17]. Technology has combined Brayton-Stirling RB211 Rolls Royce 47.7% with gas turbines to boost the overall efficiency to more cycle than 50% [20]-[21]. Most of the technologies utilise the Chemical LM5000PC General Electric 45% high-grade heat exhaust from the gas turbine. Table I recuperation summarises these different technologies. The low-grade cycle Cheng cycle M1A-13CC KAWASAKI heat exhausted from combined cycle can be further Heavy Industries 33.7% utilised. For instance, the temperature of exhaust gas 501-KII Allison Engine 39.9% leaving Brayton-Brayton cycle is between 200°C and Company 250°C [17]. LM5000 General Electric 43.8% This paper focuses on low-grade heat recovery from 2. Methodology industrial Combined Heat and Power (CHP) systems, based on gas turbines in a simple-cycle (Brayton cycle) A CHP system is modelled based around the following mode integrated with a waste heat steam boiler as shown subsystems: a combustion chamber, a gas turbine and a in figure 2. A significant amount of heat is lost through waste heat boiler. The simplified flow diagram of the flue gases as all the heat produced from the combustion CHP system is presented in figure 2. A software based process of the fuel is not transferred to the water/ steam in model of the CHP system was developed using HYSYS the boiler [25]. Evans reported about 10-20% energy used DynamicsTM integrates all concepts and considerations. in a CHP system is lost through the flue and casing [15]. The actual measurements of the exhaust gas were There is significant benefit in utilising the low-grade waste conducted using a Pitot tube in order to determine the heat from a CHP system. actual temperature and flow rate of the exhaust gas. Historic performance records of the CHP system from Recovering waste heat from a CHP system is challenging 2007 to 2009 were used in order to investigate the waste since the amount of waste heat exhausted from the system heat availability from the CHP system. is dependent on the upstream process. Thus, it is important to investigate the energy and amount of waste heat. The A. Process simulation energy in waste heat is a function of mass flow rate, chemical composition, and temperature. The amount of The gas turbine engine under consideration in this study waste heat available from a CHP system is dependent on is an industrial, simple cycle gas turbine engine the load of the plant. consisting of an axial flow compressor, combustion chamber and axial flow turbine. The simulation model of The main objectives of this study are to: the gas turbine system, integrated with waste heat steam boiler was developed using HYSYS DynamicsTM • establish the waste heat availability from a CHP software. The CHP system is composed of a 4.35 MW system at various ambient temperatures and outputs. gas turbine (model SGT-100-1S) and a waste heat steam boiler with a rated capacity of 8,165 kg/hr at 16 barg. The https://doi.org/10.24084/repqj10.714 1392 RE&PQJ, Vol.1, No.10, April 2012 process flow drawing in the HYSYS DynamicsTM fuel consumption, gas turbine power output, simulator is shown in figure 3. exhaust gas temperature from gas turbine and steam production were also monitored. 3 6 9 2 5 8 1.26 m Tapping Point Fig. 3. HYSYS Process Flow Diagram for CHP systems 1 4 7 1) Simulation Data. The technical simulation data of 1.26 m the 4.35 MW CHP systems is shown in table II. Fig. 4(a). Measurement grid according to BS EN 15259:2007 Standard Table II. Gas Turbine (Model SGT-100-1S) and waste heat steam boiler principal data at ambient temperature 5ºC. Gas Turbine Heat Input kW 15,213 Heat Rate kJ/kW.h 11,810 Generator Output kW 4,637 Speed of Gas Turbine Rpm 16,500 Pressure Ratio 13.0 Turbine Inlet Temperature oC 1,054 Turbine Outlet Temperature oC 522.6 Air Flow kg/s 18.38 Compressor Exit Pressure bar(a) 13.29 Steam Boiler Fig. 4(b).
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