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

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.

A direct numerical simulation analysis of localised forced ignition in turbulent slot jets of CH4/CO2 blends

“…Pera et al 15 demonstrated that mixture homogeneity induced by residual burnt gas has a significant influence on early flame propagation and cyclic variability in the case of forced ignition. The analysis of the forced ignition of homogeneous and inhomogeneous partially-premixed biogas mixtures using DNS data by the present authors, 16,17,12,18,19 building upon previous work by Chakraborty and co-workers on methane/air mixtures, 20–26 confirmed the experimental findings of the effects of CO 2 concentration in the fuel blend on the MIE, kernel growth and ignition success. However, these studies considered homogeneous decaying turbulence such that the understanding of CO 2 concentration in the fuel blend on the forced ignition, kernel development and subsequent transition to self-sustained propagating or stabilised flame in shear-generated turbulence and inhomogeneous mixtures that leads to the co-existence of numerous flame regimes has received relatively limited attention.…”

“…spark and laser ignition) performance potentially affecting the subsequent flame propagation. It has further been reported 10,6,11,9,12 that the CO 2 acts as a heat sink, which results in an MIE increase as the amount of CO 2 concentration increases. A similar trend was highlighted by Larsson, Berg and Bonaldo 13 who experimentally investigated the MIE required for successful ignition of hydrocarbon fuels mixed with inert gas in an effort to address the industry demands to ignite gas turbines using the fuel available on offshore plants.…”

“…Therefore, any inaccuracies in the ignition delay prediction do not significantly affect the results presented in this paper because the subsequent flame kernel development is principally determined by flame propagation. Recently, it has been demonstrated by Papapostolou et al 12 that the minimum ignition energy variation with turbulence intensity for the stoichiometric homogeneous mixed biogas-air mixture using this chemical mechanism provides excellent qualitative and quantitative agreements between DNS 46,12 and experimental 53,54 results. Moreover, the flame structure resulting from localised forced ignition of turbulent gaseous mixing layer and droplet-laden mixtures using single-step Arrhenius-type chemistry 24,55,56 are qualitatively similar to those obtained from detailed chemistry simulations.…”

Effects of spatial distribution of CO2 dilution on localised forced ignition of stoichiometric biogas-air mixtures

“…Moreover, the seminal DNS analysis by Mastorakos, Baritaud, and Poinsot (1997) with single-step Arrhenius type chemistry revealed that the reaction rate magnitude of fuel and scalar dissipation rate are negatively correlated in the case of autoignition, which was found to be qualitatively similar in subsequent detailed chemistry DNS studies (Im, Chen, Law 1998). Recently, it has been demonstrated by the authors (Papapostolou, Turquand d'Auzay, Chakraborty 2020) that the MIE variation with turbulence intensity for the stoichiometric homogeneous mixed biogas-air mixture using this chemical mechanism provides excellent qualitative and quantitative agreements between DNS and experimental (Cardin et al 2013a(Cardin et al , 2013b results. Moreover, the flame structure resulting from localized forced ignition of turbulent gaseous mixing layer and droplet-laden mixtures using single step Arrhenius type chemistry Mastorakos 2006, 2008;Papapostolou et al 2019) are qualitatively similar to those obtained from detailed chemistry simulations (Ray et al, 2001;Neophytou, Mastorakos, Cant 2010).…”

Effects of the spatial distribution of CO2 dilution on localised forced ignition of stoichiometric CH4 - CO2 - air mixtures

“…Moreover, the seminal DNS analysis by Mastorakos et al (1997) with single-step Arrhenius type chemistry revealed that the reaction rate magnitude of fuel and scalar dissipation rate are negatively correlated in the case of autoignition, which was found to be qualitatively similar in subsequent detailed chemistry DNS studies (Im et al, 1998). Recently, it has been demonstrated by the authors (Papapostolou et al, 2020) that the MIE variation with turbulence intensity for the stoichiometric homogeneous mixed biogas-air mixture using this chemical mechanism provides excellent qualitative and quantitative agreements between DNS and experimental (Cardin et al, 2013a,b) results. Moreover, the flame structure resulting from localised forced ignition of turbulent gaseous mixing layer and droplet-laden mixtures using single step Arrhenius type chemistry (Chakraborty and Mastorakos, 2006, Papapostolou et al, 2019 are qualitatively similar to those obtained from detailed chemistry simulations (Ray et al, 20021;Neophytou et al, 2010).…”