Numerical investigation of biogas flameless combustion Seyed Ehsan Hosseini ⇑ , Ghobad Bagheri, 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 27 September 2013 Accepted 3 February 2014 Keywords: Biogas Flameless combustion Simulation Entropy Fuel consumption Pollutant formation abstract The purpose of this investigation is to analyze combustion characteristics of biogas flameless mode based on clean technology development strategies. A three dimensional (3D) computational fluid dynamic (CFD) study has been performed to illustrate various priorities of biogas flameless combustion compared to the conventional mode. The effects of preheated temperature and wall temperature, reaction zone and pollutant formation are observed and the impacts of combustion and turbulence models on numerical results are discussed. Although preheated conventional combustion could be effective in terms of fuel consumption reduction, NO x formation increases. It has been found that biogas is not eligible to be applied in furnace heat up due to its low calorific value (LCV) and it is necessary to utilize a high calorific value fuel to preheat the furnace. The required enthalpy for biogas auto-ignition temperature is supplied by enthalpy of preheated oxidizer. In biogas flameless combustion, the mean temperature of the furnace is lower than traditional combustion throughout the chamber. Compared to the biogas flameless combus- tion with uniform temperature, very high and fluctuated temperatures are recorded in conventional com- bustion. Since high entropy generation intensifies irreversibility, exergy loss is higher in biogas conventional combustion compared to the biogas flameless regime. Entropy generation minimization in flameless mode is attributed to the uniform temperature inside the chamber. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Industrial development is in debt of energy consumption and more than 80% of the world energy demand is supplied by different kinds of fossil fuels. However, the resources of fossil fuels are de- pleted day by day and the future energy scenario of the world has become one of the main concerns. Indeed, environmental con- cerns have increased due to raising rate of emissions released from fossil fuel combustion. Global warming (GW) has become one of the most important environmental issues due to increasing rate of greenhouse gases (GHGs) generation. Therefore, the request for clean alternative fuel and efficient combustion technology has become more important [1–3]. In recent decades, utilization of renewable and sustainable energy such as biomass, solar energy, wind energy, hydropower and geothermal has been developed properly. Furthermore, biogas from wastewater effluent, municipal solid wastes (MSW), animal waste and agricultural by-products have been employed for combined heat and power (CHP) genera- tion purposes [4–8]. In the other hand, necessity of biogas collec- tion from aforementioned resources is unavoidable because CH 4 and CO 2 as the main components of biogas participate in the GW constitution actively [9]. Since the negative effect of CH 4 on the GW is 23 times more than CO 2 , biogas collection from anaerobic digestion (AD) has become more highlighted. In most of the AD, the percentage of CH 4 is enough to be considered as a clean fuel. The amount of CH 4 in biogas components depends on the feedstock is various (from 40% up to 60%) [10]. In biogas conventional com- bustion, pollutant formation mitigates compared to traditional combustion of pure CH 4 . However, biogas traditional combustion encounters some problems due to LCV of biogas. Therefore, biogas should be upgraded to remove its non-combustible CO 2 impurity [11]. Based on the application of biogas, pure biogas can be cleaned and upgraded by some technologies such as water scrubbing, cryo- genic process and membrane. In order to prevent implementation of upgrading equipment, application of flameless combustion was proposed for pure biogas combustion. Since combustion is still the most important technique for energy generation, flameless com- bustion was introduced to improve the combustion efficiency and decrease pollutant formation concomitantly [12,13]. The importance of flameless combustion technology has become more highlighted when the inability of other combustion technologies in terms of simultaneous pollutant reduction and thermal efficiency enhancement was proven. Low emission formation in flameless combustion is obtained due to dilution of the air combustion by inert gases such as nitrogen (N 2 ) and CO 2 [14–16]. Therefore, http://dx.doi.org/10.1016/j.enconman.2014.02.006 0196-8904/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +60 1112600959. E-mail address: seyed.ehsan.hosseini@gmail.com (S.E. Hosseini). Energy Conversion and Management 81 (2014) 41–50 Contents lists available at ScienceDirect Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman