Mitigating methane: emerging technologies to combat climate

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Mitigating methane: emerging technologies to combat climate change’s second leading contributor Chris Pratt, and Kevin Tate Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b04711 • Publication Date (Web): 02 May 2018 Downloaded from http://pubs.acs.org on May 3, 2018

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Graphical Abstract 85x48mm (96 x 96 DPI)

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Mitigating methane: emerging technologies to combat climate

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change’s second leading contributor

4 ±Chris Pratta, Kevin Tateb

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a

School of Environment and Science/Australian Rivers Institute, Griffith University, 170

Kessels Road, Nathan, Brisbane, Australia, 4111

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b

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New Zealand, 4442

Landcare Research – Manaaki Whenua, Riddet Road, Massey University Palmerston North,

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Corresponding author: email, [email protected], phone +737353605

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Dedication – Sadly, Dr Kevin Tate passed away during the final stages of producing this

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manuscript. Kevin will be remembered as an outstanding scientist, inspiring others with his

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unwavering commitment to tackling climate change.

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Abstract

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Methane (CH4) is the second greatest contributor to anthropogenic climate change. Emissions have

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tripled since pre-industrial times and continue to rise rapidly, given the fact that the key sources –

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food production, energy generation and waste management – are inexorably tied to population

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growth. Until recently, the pursuit of CH4 mitigation approaches has tended to align with

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opportunities for easy energy recovery through gas capture and flaring. Consequently, effective

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abatement has been largely restricted to confined high-concentration sources such as landfills and

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anaerobic digesters, which do not represent a major share of CH4’s emission profile. However, in

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more recent years we have witnessed a quantum leap in the sophistication, diversity and affordability

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of CH4 mitigation technologies on the back of rapid advances in molecular analytical techniques,

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developments in material sciences and increasingly efficient engineering processes. Here, we present

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some of the latest concepts, designs and applications in CH4 mitigation, identifying a number of

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abatement synergies across multiple industries and sectors. We also propose novel ways to manipulate

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cutting-edge technology approaches for even more effective mitigation potential. The goal of this

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review is to stimulate the ongoing quest for and uptake of practicable CH4 mitigation options;

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supplementing established and proven approaches with immature yet potentially high-impact

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technologies. There has arguably never been, and if we don’t act soon nor will there be, a better

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opportunity to combat climate change’s second most significant greenhouse gas.

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Key words

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Climate change, cross-sector, methane, mitigation

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Introduction

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Anthropogenic emissions of methane (CH4) are second only to carbon dioxide (CO2).

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According to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change

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dioxide equivalent (CO2-e) basis. Based on the IPCC’s recently revised Global Warming

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Potential (GWP) factor of 28 for CH4 1, CH4 emissions likely contribute more than 20%

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towards total anthropogenic greenhouse gas (GHG) emissions, mainly from biomass burning,

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wetlands, rice paddies, enteric and waste emissions, and fossil fuel use 2. There is also

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increasing evidence that the vast store of CH4 in frozen soils in the Arctic is now being

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released as these soils thaw 3. Moreover, other stores of previously untapped CH4, such as

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deep ocean sediment hydrates and clathrates, are currently being targeted as potential

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alternative energy sources

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activities has triggered concern 6. This contribution to climate change from direct and indirect

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anthropogenic CH4 emissions is clearly being recognised within the science and engineering

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spheres, as evidenced by the recent development of organisations such as the Global Methane

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Initiative and the Global Research Alliance.

, annual anthropogenic CH4 emissions total approximately 8 gigaton (Gt) on a carbon

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and the risk of substantial indirect CH4 emissions from these

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Anthropogenic CH4 sources vary in magnitude and are produced across a diverse range of

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industries and sectors. Yet, despite CH4’s importance to climate change, few articles exist

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regarding technical mitigation opportunities across the key emission sectors. In 2012 there

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was a concentrated effort in this area with four overview papers published reporting emission

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estimates and mitigation options for the major CH4 sources

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discussed in the reviews by Höglund-Isaksson 7, Karakurt, et al. 8, and Yusuf, et al.

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presented from a high-level working principle perspective – i.e., logistics of gas recovery,

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collection and potential use as well as land use management practises to decrease emissions.

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. The mitigation options 10

were

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Moreover, these reviews focused on mitigation potential for individual sources. By contrast,

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Stolaroff, et al.

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potential and presented a range of CH4 mitigation options from a technology perspective.

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Since then, new scientific and engineering advances have paved the way for enhanced CH4

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mitigation technology development across several key contributing sectors. These

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technologies range in maturity from fledgling concepts, to field-tested systems that are now

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ready for full-scale deployment and validation, right through to commercially-viable

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practices.

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in their review recognised the potential for cross-sectoral CH4 mitigation

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In this review we springboard off the work of Stolaroff, et al. 9 and other earlier reviews to

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highlight the latest developments and opportunities in CH4 mitigation technologies. Our

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review focuses on advances in this field over the past five years which have progressed on the

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back of three rapidly evolving disciplines: 1) material sciences; 2) microbial ecology; and 3)

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design and engineering. We largely avoid focusing on recent developments in enteric CH4

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mitigation using feed modifications, vaccines and animal selection – a wealth of thorough

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reviews evaluating the progress of these approaches can be readily found in the literature

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which innovative options are also critically needed. As an integrated aspect of our review we

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also appraise the latest developments in emission estimates for the key sources.

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. Rather, our emphasis is on emerging mitigation approaches for other key sectors, for

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Through presenting the underpinning concepts and practical considerations associated with

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emergent technologies we strive to stimulate opportunities for CH4 mitigation on a cross-

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sectoral scale. Building on the effectiveness of existing conventional technologies and

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established management practises that have been demonstrated to abate CH4 emissions, we

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envisage that the promotion of cross-sectoral emergent technologies will foster a more

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efficient approach for tackling global CH4 emissions. Given the recent call for ‘aggressive

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CH4 mitigation’ measures issued by Kim, et al.

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pressing opportunity for appraising the state technology development in this field.

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, there has arguably never been a more

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Contributions and trends for key emission sources

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Current emission estimates for anthropogenic CH4 sources are presented in Fig. 1, illustrating

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how CH4 emissions compare with those of the other main GHGs (Fig. 1a). The magnitudes of

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emissions from the key anthropogenic CH4 sources have previously been well-documented

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and are summarised in Fig. 1b, using a combination of literature values and IPCC estimates.

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It can be seen that total estimates vary between documented sources, resulting from

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uncertainties in emission estimates for each sector. The emission estimates in Fig.1b

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incorporate the IPCC’s Second Assessment Report GWP of 21 for CH4 which is currently

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adopted in most national inventories, although there is increasing momentum for adoption of

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the GWP of 28 proposed by the IPCC’s AR5 metrics 16.

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Two central concepts from Fig. 1b are pertinent to this review: 1) large-scale hydroelectric

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dams are a significant CH4 source, despite not been accounted for as direct CH4 emission

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sources by the IPCC or by most other inventory documents

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emissions are reasonably consistent across the key emission sources. From a mitigation

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perspective, the recognition of large-scale hydroelectric dams as a primary emission source is

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important because there are a number of inventive and practical approaches for abating these

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emissions, some of which might hold relevance to mitigating emissions from other sources.

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, and 2) the magnitudes of

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The fact that emission magnitudes are reasonably evenly distributed across the main sources

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is also telling, because it highlights the need for a cross-sectoral approach to achieve

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comprehensive CH4 abatement. In Fig. 2 we project how the balance of direct CH4 emissions

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from the major sectors will look in the coming decades. We emphasise that these projections

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are broad estimates based on current trends in the key CH4-producing sectors and that several

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unavoidable caveats are associated with this type of exercise. This is particularly true for the

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energy sector where forecasting trends can be problematic. For example, Utgikar and Scott 56

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note that accurate estimates and forecasts of energy demand are hampered by barriers

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associated with technology adoption; socio-economic factors; and volatile economic policies.

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Moreover, as noted by Feng and Zhang

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may conceivably change drastically in coming years as some countries adopt widespread

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energy saving measures whereas other developing countries surge in energy demand to meet

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growing economies.

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and Aydin, et al.

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current trends in energy use

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The agricultural and waste sectors, by comparison, are projected to continue to grow rapidly

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in the coming decades, potentially even to the point of where they could exceed sustainable

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boundaries 59, 60. Yet, considering all of the factors above, our 2050 projections indicate that

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the relative contributions of the direct anthropogenic CH4 emission sectors will remain

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reasonably consistent into the latter part of this century, albeit with a slight increase in the

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proportion of agricultural emissions resulting from a surge in food demand and a

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corresponding proportional decrease in energy-driven emissions (Fig. 2).

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Recognition of a strong diversity of key CH4 emission sources well into the 21st Century is

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important if we are to limit global mean temperature increases to 2⁰C or less by 2100, a target

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agreed to at the Paris summit in 2015. Sixteen of the first twenty publications generated by a

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Google Scholar search employing the terms ‘methane mitigation’ pertain to enteric

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abatement. While large strides are being made in providing solutions for decreasing ruminant

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CH4, clearly these emissions are just one component of the total anthropogenic CH4 budget.

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There is a real need for giving equal focus to the pursuance of mitigation strategies for other

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key CH4 sources.

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In the following sections we present some of the emerging and novel technologies in the field

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of anthropogenic CH4 abatement. Rather than present the technologies on a source-by-source

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basis we discuss the processes and mechanisms involved in the technology and then elaborate

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where we see potential for cross-sectoral application.

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Breaking the bond – advances in catalysts for low concentration and low temperature

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methane oxidation

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Breaking the carbon-hydrogen bond in the CH4 molecule is the first obstacle faced by all CH4

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mitigation approaches that aren’t based on avoiding CH4 production. Although this reaction

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is lucrative from an energy production perspective with a standard enthalpy of combustion of

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–882 kJ/mol, a substantial amount of activation energy is required for its initiation. In CH4-

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rich gas such as biogas produced at landfills and in wastewater treatment systems energy

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recovery from gas combustion is technically feasible and widely-practised 68-70. However, in

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gas streams that are