Numerical investigation of the effects of spatial distribution of CO2 dilution on localised forced ignition of stoichiometric biogas-air mixtures
Session chaired by Pr. Christine Rousselle
As fossil fuel reserves are finite and gradually running out, eco-friendly alternative renewable fuels are becoming ever more important. One such alternative fuel is biogas, which can be used as either a complement or a replacement for existing fossil fuels. Moreover, depending on the production method used, biogas can be a carbon neutral fuel, and due to existing natural gas infrastructure, it can be easily stored and transported whilst also being used in conjunction with natural gas for power generation and transportation [1-6]. Biogas is primarily composed of CH4 and CO2, however production of industrial quantities of biogas with a fixed composition is hard due to its potential biological origins [2]. The composition of biogas is of critical importance, as variations in CO2 content can affect its localised forced ignition (e.g. spark and laser ignition) performance, leading to adverse effects on the subsequent flame propagation [1-6]. Experimental studies of forced ignition of biogas/air mixtures revealed that CO2 acts as a heat sink leading to a higher energy requirement for ignition as well as a slower and cooler flame [1-6]. It has been found that CO2 dilution adversely affects the flame kernel formation and can potentially lead to flame extinction for biogas/air fuelled gas turbines [3,4]. In a recent DNS analysis [7] by the authors, the localised forced ignition of statistically planar mixing layers for different extents of CO2 dilution with CH4-air mixtures was investigated, which demonstrated that an increase in CO2 dilution leads to reductions of burning rate and the probability of obtaining self-sustained combustion once the ignitor is switched off. The analysis which is being proposed will concentrate on the effects of the nature of the spatial distribution of CO2 dilution (i.e. variance and integral length scale of CO2 dilution) has in the case of localised forced ignition of stoichiometric biogas (assumed to be CH4 + CO2)-air mixtures for a range of different turbulence intensities. Thus, the localised forced ignition and subsequent flame propagation for stoichiometric biogas-air mixtures with different spatial distributions of CO2 dilution (i.e. mole fraction of CO2 in CH4/CO2 blend) under different flow conditions (e.g. quiescent laminar condition and different turbulence intensities) have been analysed using three-dimensional Direct Numerical Simulations. The biogas is taken to be a mixture of CH4 and CO2, and a two-step chemical mechanism which has been demonstrated to capture the effects of CO2 dilution on laminar burning velocity with sufficient accuracy has been used for the purposes of the parametric analysis which spans the mean, standard deviation and integral length scale of the initial Gaussian distributions of spatial CO2 dilution in the unburned gas. The CO2 dilution level has not been found not to have any significant influences on the maximum values of temperature and the reaction rate magnitude of CH4. However, CO2 dilution acts to reduce the probability of finding large reaction rate magnitudes of CH4, which also leads to a decreasing trend of burned gas volume with increasing levels of mean CO2 dilution irrespective of flow conditions. Moreover, an increase in turbulence intensity acts to reduce the burned gas volume irrespective of mixture composition due to the enhancement of heat transfer from the hot gas kernel. The initial values of integral length scale and standard deviation of CO2 dilution (i.e. l_{\psi}and \sigma_{psi}) have been found to affect the burned gas volume to varying degrees for the parameter range considered here. The effects that the variation of the initial turbulence intensity, mean dilution value and l_{\psi} and \sigma_{psi} have on the ignition phenomenon and subsequent flame extinction or self-sustained propagation will be analysed in the full paper. References 1. A. Vasavan, P. de Goey, et al., Numerical study on the autoignition of biogas in moderate or intense low oxygen dilution nonpremixed combustion systems, Energy Fuels, 32 (8) (2018) 8768–8780. 2. S. Rasi, et al., Trace compounds of biogas from different biogas production plants, Energy, 32 (8) (2007) 1375-1380. 3. T. Lieuwen, et al., Fuel flexibility influences on premixed combustor blowout, flashback autoignition, and stability, J. Eng. Gas Turbines Power, 130 (1) (2008) 810. 4. Y. Lafay, et al., Experimental study of biogas combustion using a gas turbine configuration. Exp. Fluids, 43(2-3) (2007) 395-410. 5. I. A. Mulla, et al., Evolution of flame-kernel in laser-induced spark ignited mixtures: A parametric study. Combust. Flame, 164 (2016) 303-318. 6. C. Forsich, et al., Characterization of laser-induced ignition of biogas-air mixtures. Biomass Bioenergy, 27(2004) 299–312. 7. C. Turquand d’Auzay, et al. , Effects of turbulence intensity and biogas composition on the localised forced ignition of turbulent mixing layers, Combust. Sci. Technol., 191 (2019) 868-897.
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