A Numerical Investigation of the Effects of Fuel Composition on the Minimum Ignition Energy for Homogeneous Biogas-Air Mixtures

Vassilios Papapostolou,Nilanjan Chakraborty,Charles Turquand D’Auzay

doi:10.1007/s10494-020-00229-8

Vassilios Papapostolou, Nilanjan Chakraborty + Show 1 more

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https://doi.org/10.1007/s10494-020-00229-8

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Abstract

The minimum ignition energy (MIE) requirements for ensuring successful thermal runaway and self-sustained flame propagation have been analysed for forced ignition of homogeneous stoichiometric biogas-air mixtures for a wide range of initial turbulence intensities and CO2 dilutions using three-dimensional Direct Numerical Simulations under decaying turbulence. The biogas is represented by a CH4 + CO2 mixture and a two-step chemical mechanism involving incomplete oxidation of CH4 to CO and H2O and an equilibrium between the CO oxidation and the CO2 dissociation has been used for simulating biogas-air combustion. It has been found that the MIE increases with increasing CO2 content in the biogas due to the detrimental effect of the CO2 dilution on the burning and heat release rates. The MIE for ensuring self-sustained flame propagation has been found to be greater than the MIE for ensuring only thermal runaway irrespective of its outcome for large root-mean-square (rms) values of turbulent velocity fluctuation, and the MIE values increase with increasing rms turbulent velocity for both cases. It has been found that the MIE values increase more steeply with increasing rms turbulent velocity beyond a critical turbulence intensity than in the case of smaller turbulence intensities. The variations of the normalised MIE (MIE normalised by the value for the quiescent laminar condition) with normalised turbulence intensity for biogas-air mixtures are found to be qualitatively similar to those obtained for the undiluted mixture. However, the critical turbulence intensity has been found to decrease with increasing CO2 dilution. It has been found that the normalised MIE for self-sustained flame propagation increases with increasing rms turbulent velocity following a power-law and the power-law exponent has been found not to vary much with the level of CO2 dilution. This behaviour has been explained using a scaling analysis and flame wrinkling statistics. The stochasticity of the ignition event has been analysed by using different realisations of statistically similar turbulent flow fields for the energy inputs corresponding to the MIE and it has been demonstrated that successful outcomes are obtained in most of the instances, justifying the accuracy of the MIE values identified by this analysis.

Highlights

Localised forced ignition of homogeneous mixtures is of critical importance for the efficient use of fuel in Spark Ignition (SI) engines and industrial gas turbines
The minimum ignition energy (MIE) of stoichiometric biogas-air mixtures with varying levels of C O2 dilution have been numerically evaluated under homogeneous isotropic decaying turbulence for a wide range of initial turbulence intensities using three-dimensional Direct Numerical Simulations (DNS)
The range of the critical turbulence intensity values for the MIE transition has been found to be consistent with previous experimental (Huang et al 2007; Shy et al 2010, 2017, b; Peng et al 2013; Jiang et al 2018; Cardin et al 2013a, b) and numerical (Turquand d’Auzay et al 2019a) findings

Summary

Introduction

Localised forced ignition (e.g. spark or laser ignition) of homogeneous mixtures is of critical importance for the efficient use of fuel in Spark Ignition (SI) engines and industrial gas turbines. Fossil fuel reserves are finite and eco-friendly alternative renewable fuels are becoming ever more important. One such alternative fuel is biogas, which can be used as either a compliment or a replacement for existing fossil fuels. Depending on the production method used, it can be a carbon–neutral fuel and due to the existing natural gas infrastructure, it can be stored and transported whilst being used in conjunction with natural gas for power generation and transportation (Holm-Nielsen et al 2009). The composition of biogas is of critical importance, as large variations in CO2 content can affect the ignition of the fuel, leading to adverse effects on the subsequent flame propagation (Lieuwen et al 2008)

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