Experimental and computational studies on a gasifier based stove S. Varunkumar ⇑ , N.K.S. Rajan, H.S. Mukunda Combustion, Gasification and Propulsion Laboratory, Department of Aerospace Engineering, Indian Institute of Science, Bangalore 560 012, India article info Article history: Received 18 July 2011 Received in revised form 30 August 2011 Accepted 31 August 2011 Available online xxxx Keywords: Gasifier stove Biomass combustion Gasification efficiency abstract The work reported here is concerned with a detailed thermochemical evaluation of the flaming mode behaviour of a gasifier based stove. Determination of the gas composition over the fuel bed, surface and gas temperatures in the gasification process constitute principal experimental features. A simple atomic balance for the gasification reaction combined with the gas composition from the experiments is used to determine the CH 4 equivalent of higher hydrocarbons and the gasification efficiency (g g ). The components of utilization efficiency, namely, gasification–combustion and heat transfer are explored. Reactive flow computational studies using the measured gas composition over the fuel bed are used to simulate the thermochemical flow field and heat transfer to the vessel; hither-to-ignored vessel size effects in the extraction of heat from the stove are established clearly. The overall flaming mode efficiency of the stove is 50–54%; the convective and radiative components of heat transfer are established to be 45– 47 and 5–7% respectively. The efficiency estimates from reacting computational fluid dynamics (RCFD) compare well with experiments. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction A stove design that ensures near-stoichiometric operation to maximize efficiency and minimize emissions, limits the velocities in the fuel residing zone to limit particle carry-over with vessels of practical relevance was evolved using principles of gasifier with air supply from a fan [1–4]. Gasification is one of the predominant routes for conversion of biomass to energy (see for instance [5,6]). This stove design was engineered using the principles of gasifica- tion into a commercial product and sold commercially to half-a- million house holds along with agriculture residue based pellet fuel. This stove assures an utilization efficiency of over 50% and CO emission of 0.75 g/MJ of heat input. Compared to stoves based on air supply by natural convection, these efficiencies are 40–60% higher and CO emissions about 50–70% lower. Pushing the under- standing to the next level, where the thermochemical processes are examined in detail may allow further optimization of efficiency and emissions. One important feature guiding these developments is that efficiency and emissions have an inverse correlation – high- er efficiency obtained by better combustion allows for lowering the emissions. There are several earlier studies on packed bed gasification and combustion [5–11]. Most of these studies have been restricted to high air flow rate (superficial velocity >8 cm/s) regimes which find application in large scale combustion in power generation systems. Work of Reed and colleagues [3,12] is probably the only one which addresses the problem of thermochemical processes in the super- ficial velocity range of 3–6 cm/s, a range of particular interest to small clean combustion devices of focus in this paper. Current work goes beyond the work of Reed and colleagues by addressing the crucial gas phase processes affecting the various components of efficiency. This stove shown in Fig. 1 has a combustion space of 100 mm diameter and 130 mm depth with a grate in the bottom and wood chips or pellets of biomass can be used as fuel. The stove is a top-lit downdraft gasifier (see [13]) with air for gasification (primary air) provided from the bottom region and the gas generated in the pro- cess of gasification burnt on top with secondary air to ensure com- plete combustion and minimum emissions. Domestic cooking requires power in the range of 3–4 kWth and this corresponds to a biomass consumption of 10–15 g/min. This requires a primary air flow rate in the range of 15–25 g/min for 100 mm diameter stove; the superficial velocity, a key parameter for obtaining opti- mal gas quality [12] corresponding to this range is 3–6 cm/s. When the stove is loaded with biomass and is lit on top by sprinkling small amounts of liquid fuel (say, alcohol or kerosene), de-volatil- isation of biomass leads to ignition of particles in the top most layer. This in turn leads to similar processes in the subsequent lay- ers of biomass particles. This flame front propagates into the fuel bed against the air stream similar to a premixed flame propagation in a tube, albeit with heterogeneous local fuel source. The air that passes from the bottom through the bed aids flaming combustion of the biomass pieces and the gases move upward through the hot char bed left behind by the propagating pyrolysis front. This leads 0196-8904/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2011.08.022 ⇑ Corresponding author. Tel.: +91 80 23600536; fax: +91 80 23601692. E-mail addresses: varun@cgpl.iisc.ernet.in (S. Varunkumar), nksr@cgpl.iisc. ernet.in (N.K.S. Rajan), mukunda@cgpl.iisc.ernet.in (H.S. Mukunda). Energy Conversion and Management 53 (2011) 135–141 Contents lists available at SciVerse ScienceDirect Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman