The combustion mitigation of methane as a non-CO2 greenhouse gas

TL;DR: A review of fugitive methane combustion mitigation and utilisation technologies, which are primarily aimed at methane emissions from coal mining activities, with a focus on modelling and simulation of ultra-lean methane oxidation/combustion is presented in this paper.

About: This article is published in Progress in Energy and Combustion Science.The article was published on 2016-08-12 and is currently open access. It has received 53 citations till now. The article focuses on the topics: Methane & Combustion.

Summary

Introduction

• Anthropogenic emissions of non-CO2 greenhouse gases such as fugitive methane contribute significantly to global warming.
• The challenges associated with ultra-lean methane oxidation are on the ignition of the ultra-lean mixture and sustainability of the combustion process.
• Anthropogenic emissions of non-CO2 greenhouse gases [1], such as methane (CH4), nitrous oxide (N2O), ozone-depleting substances (ODSs), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), sulphur hexafluoride (SF6) and nitrogen trifluoride (NF3) also contribute significantly to warming.
• The extractive activities in the oil and gas industry including shale gas generate about three times CH4 emissions of that from coal mines [7] (more than 20% of the total anthropogenic CH4 emissions), although the question of how much methane escapes from natural gas and oil wells into the atmosphere remains unknown and challengeable.
• When the mixtures ignite near the burn-off limits, the effects of heat and mass transfer also play an important role and must be taken into account.

2.1.1 Overview of ultra-lean methane combustion

• For oxidation/combustion of ultra-lean methane/air mixtures, technologies are available for mitigation only or for combined mitigation and utilisation.
• Porous burners operate on the principle that the solid porous matrix serves as a means of recirculating heat from the hot combustion products to the incoming reactants, resulting in high burning velocities and extended lean flammability limits.
• This encourages the continuation of research on oxidation of ultra-lean methane mixtures with the use of effective heat recovery systems that can deliver appropriate preheating conditions for the incoming mixture.
• For the applications of plasma, new observations of plasma assisted ignition enhancement, ultra-lean combustion, cool flames, flameless combustion, and controllability of plasma discharge have been reported [25].
• MILD combustion refers to conditions where temperature of the reactant mixture is higher than the autoignition temperature, while the temperature rise during combustion is relatively low [37, 38].

2.1.2 Numerical modelling approach for methane oxidation/combustion

• For the mitigation and utilisation of ultra-lean methane mixtures, optimisation of practical systems for lean-burn applications is an essential step in the technological developments towards the reduction of greenhouse gases.
• Equations (1)-(4) constitute the basic governing equations for turbulent reacting flows.
• In practical CFD simulations, the calculations associated with the chemistry can be very time consuming, especially when one uses a detailed mechanism considering a large number of species.
• Modelling MILD combustion is very likely to require accurate descriptions of turbulence-chemistry interactions.
• In the following sub-section, the chemical kinetics for CH4 oxidation/combustion especially for ultra-lean conditions is discussed.

2.2.1 General background on combustion chemistry for methane oxidation

• This sub-section provides some basic background on methane oxidation kinetics, which serves as the basis for kinetic studies at ultra-lean conditions.
• The reaction mechanism of methane follows a complex scheme and usually can be decomposed into two main pathways depending on whether the fuel burns in lean or rich conditions [56, 57, 59].
• The reaction of the methyl radical with hydroxyl radicals is also a main chemical pathway for the oxidation of CH3, and several kinetic paths are possible depending on temperature and pressure.
• The elementary steps are gathered together forming detailed chemical kinetics mechanisms that represent the combustion chemistry of any hydrocarbon fuel.
• These schemes can be used for general use under a wide range of operating conditions, but users should carefully check the actual conditions for which they are designed before applying them for particular applications.

2.2.2 Chemical description of ultra-lean methane oxidation

• While the chemical kinetics of most practical fuels under standard burning conditions is relatively well understood, as described before, the study of chemical kinetics of mixtures in unconventional burning conditions (e.g. highly diluted mixtures or ultra-lean conditions, low temperature and high pressure among others) still requires further research.
• Their work showed different chemical kinetic pathways for methane autoignition in which the reactivity of the oxidation process changes.
• At the intermediate temperature range (T = 975K), the reaction rates increase substantially with respect to the low temperature ignition and similar chemical pathways are observed, although alternative recombination routes to ethane and the formation of formaldehyde and OH radical also occur.
• C2 hydrocarbons are formed at such conditions.
• An important aspect of MILD combustion, which also relates to the oxidation of highly diluted mixtures, is that combustion can only be possible after preheating the reactants [37, 38].

2.2.3 Effects of catalysts

• Catalytic combustion of hydrocarbon fuels has been proven to be a promising approach to oxidise ultra-lean methane mixtures by extending the flammability limits and enhancing the kinetics associated to the oxidation process [15, 16, 19].
• There are many types of catalysts used for combustion applications, but noble metal catalysts (Pt, Pd, Rh or combinations) are usually preferred over metal-oxide catalyst due to their enhanced conversion properties [16, 95].
• There are several comprehensive review efforts in the literature summarising the state-of-the-art in the use of catalysts to enhance methane combustion [16, 95, 96], but none of them directly address their performance on the oxidation of ultra-lean methane mixtures.
• These equations include mass and energy balances over the catalytic surface and should include heterogeneous chemical reactions, diffusion, as well as convective, conductive and radiative energy transport with the resistive heating of the catalyst and the existence of chemical source terms on the surface [107].
• The results indicated a certain level of correlation with the experiments and encouraged developing more sophisticated models.

2.2.4 Reduced chemical kinetics for ultra-lean methane oxidation

• The oxidation process of highly diluted methane becomes complex not only from the chemical point of view, but also from the practical point of view when it comes to modelling the combustion process in practical burners.
• The elaboration of an accurate predictive reaction mechanism must surmount two major challenges: the detailed reaction mechanisms composed of numerous elementary reactions and the estimation of the kinetic and thermodynamic parameters of these elementary reactions.
• Firstly, the combustion of ultra-lean methane is highly sensitive to variations in concentration due to the previously described reaction sustainability issues; therefore, a combustion mitigation system has to be flexible in dealing with CH4 concentration variations, as the CH4 concentration often changes from case to case and also changes with the operating conditions for the same case [15].
• It provides a great potential for the use of VAM as a principal fuel source because the technology is able to oxidise diluted methane in coal mine ventilation air and produce useable energy from heat exchangers operating at an optimal temperature.
• Reducing or mitigating CH4 emissions while exploiting the potential utilisation of the thermal energy have brought up a wide range of scientific, technological and financial challenges.

Figures

Fig. 2. Reaction pathways for the homogenous combustion of methane.

Fig. 3. A possible chemical mechanism for methane catalytic oxidation [16].

Frequently asked questions

Q1. What have the authors contributed in "The combustion mitigation of methane as a non-co2 greenhouse gas" ?

A review of fugitive methane combustion mitigation and utilisation technologies, which are primarily aimed at methane emissions from coal mining activities, with a focus on modelling and simulation of ultra-lean methane oxidation/combustion is presented. Recuperative combustion provides a promising means for mitigating ultra-lean methane emissions. Further technological developments may be focussed on developing innovative capturing technology as well as technological innovations to achieve effective ignition and sustainable oxidation/combustion. 

Q2. What are some examples of technical limitations using experimental work?

Some examples of technical limitations using experimental work are associatedto the minimization of heat losses from the system and maximization of fuel-air mixing for oxidation. 

Q3. What are the prerequisites for the development of these mechanisms?

understandings on detailed chemical pathways and reaction rates are the prerequisites for the development of these mechanisms. 

Q4. What is the need for robust technologies for the treatment of ultra-lean air mixtures?

The need for robust technologies for the treatment of ultra-lean CH4 air mixtures calls for further investigation and optimisation of recuperative combustion, involving flow and combustion control for sustainable oxidation/combustion. 

Q5. What can be the useful method for analyzing the control mechanisms of burners?

For the design of burners for practical applications, numerical methods can be particularly useful in understanding the controlling mechanisms and for optimising the design parameters. 

Q6. What are the main reasons why fundamental theory is an indispensable partner in reaction kinetics?

Although improvements on theoretical kinetics and quantum chemistry have made fundamental theory an indispensable partner in reaction kinetics [113], chemical kineticschemes cannot be predicted at the fundamental level without empiricisms and simplifications at the present time. 

Q7. What is the effect of the addition of cavities and fuel segmentation in catalytic reactors?

The addition of cavities and fuel segmentation in catalytic reactors has also been investigated leading to enhanced conversion rates for certain fuels under particular conditions [103, 104]. 

Q8. What technologies can be used for the ignition of ultra-lean mixtures?

For the ignition of ultra-lean mixtures, new technologies such as plasma-assisted ignition/combustion can be potentially utilised. 

Q9. What can be used to complement experimental studies?

In this regard, numerical simulations can be used to complement experimental studies by providing insight and identifying key physicochemical processes. 

Q10. What are the important aspects to be determined by these schemes?

Some important aspects to be determined are the ignition delay time and flame propagation predicted by these schemes, since the reaction sets might not be appropriate when the radical pool of some species is rather low or the dilution overcomes certain values. 

Q11. What is the role of catalysts in the mitigation of ultra-lean methane?

As a promising new technology, the application of plasma-assisted combustion to ultra-lean methane mitigation needs to be further investigated, both experimentally and theoretically/numerically. 

Q12. What is the argument that developing reduced schemes for ultra-lean methane is premature?

It is arguable that developing reduced schemes for ultra-lean CH4 oxidation/combustion is somewhat premature, considering that chemical kinetics needs to be better understood.