Enhancement of exergy efficiency in combustion systems using flameless mode Seyed Ehsan Hosseini ⇑ , Mazlan Abdul Wahid High-Speed Reacting Flow Laboratory, Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor, Malaysia article info Article history: Received 7 April 2014 Accepted 10 June 2014 Available online 18 July 2014 Keywords: Exergy efficiency Conventional combustion Flameless mode Dilution Preheated air abstract An exergitic-based analysis of methane (CH 4 ) conventional and flameless combustion in a lab-scale fur- nace is performed to determine the rate of pollutant formation and the effective potential of a given amount of fuel in the various combustion modes. The effects of inlet air temperature on exergy efficiency and pollutant formation of conventional combustion in various equivalence ratios are analyzed. The rate of exergy destruction in different conditions of flameless combustion (various equivalence ratios, oxygen concentration in the oxidizer and the effects of diluent) are computed using three-dimensional (3D) com- putational fluid dynamic (CFD). Fuel consumption reduction and exergy efficiency augmentation are the main positive consequences of using preheated air temperature in conventional combustion, however pollutants especially NO x formation increases dramatically. Low and moderate temperature inside the chamber conducts the flameless combustion system to low level pollutant formation. Fuel consumption and exergy destruction reduce drastically in flameless mode in comparison with conventional combus- tion. Exergy efficiency of conventional and flameless mode is 75% and 88% respectively in stoichiometric combustion. When CO 2 is used for dilution of oxidizer, chemical exergy increases due to high CO 2 con- centration in the combustion products and exergy efficiency reduces around 2% compared to dilution with nitrogen (N 2 ). Since the rate of irreversibilities in combustion systems is very high in combined heat and power (CHP) generation and other industries, application of flameless combustion could be effective in terms of pollutant formation mitigation and exergy efficiency augmentation. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Today around 80% of electricity of the world is provided by com- bustion process of which coal, natural gas and nuclear energy are the main sources of power generation by 45%, 20% and 15% respec- tively [1–3]. The chemical potential energy of the fuel is usually con- verted to the electricity by combustion phenomena in boiler steam turbine systems. Not only power generation but also development of industrial sectors is in debt of combustion systems. Steel facto- ries, glass making, cement production, petroleum refining, food pro- duction are the main fossil fuel consumers through combustion systems in the world [4–7]. Therefore, combustion efficiency plays crucial role in heating- related energy savings. Heat is lost from combustors via various methods such as radiation losses and hot flue gas losses. Thus, it is vital to identify the energy wastage factors in the combustion system to optimize the combustion operation. It has been pointed out that if the temperature of exhaust gases in a boiler is about 550 K, around 30% of the boiler’s energy is wasted. Hence, substantial part of energy could be returned by the heat recovery from flue gases [8]. Although the first law of thermody- namics (FLT) is applied to analyze the energy consumption, the quality aspect of energy is not taken into consideration. Therefore, exergy analyses based on second law of thermodynamic (SLT) should be done to determine the effective work potential of a given amount of energy at reference state. Energy and exergitic analyses have been performed for various industrial systems in different countries [9–13]. In addition, exergy-based analysis is usually done to evaluate the performance of thermochemical or thermal systems [14,15]. Exergy expresses the right efficiency of the engineering systems through linkage between the physical, engineering and environment aspects. Thus, exergy computation can provide a deeper insight into the system and new projections for process improvement could be feasible [16]. Consequently, both energy and exergy analyses of a process is vital to optimize and improve the performance of the system through taking appropriate energy strategies [17,18]. Furthermore, exergy analysis plays crucial role for global environmental, national pollution reduction, energy plan- ning, resource optimization and regional. http://dx.doi.org/10.1016/j.enconman.2014.06.065 0196-8904/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Address: NO. 801, U8C, Perdana, Universiti Teknologi Malaysia (UTM), 81310 Skudai, Johor, Malaysia. Tel.: +60 1112600959. E-mail address: Seyed.ehsan.hosseini@gmail.com (S.E. Hosseini). Energy Conversion and Management 86 (2014) 1154–1163 Contents lists available at ScienceDirect Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman