Subscriber access provided by University of Winnipeg Library
Critical Review
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
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 53
Environmental Science & Technology
Graphical Abstract 85x48mm (96 x 96 DPI)
ACS Paragon Plus Environment
Environmental Science & Technology
Page 2 of 53
1 2
Mitigating methane: emerging technologies to combat climate
3
change’s second leading contributor
4 ±Chris Pratta, Kevin Tateb
5 6 7 8 9 10
a
School of Environment and Science/Australian Rivers Institute, Griffith University, 170
Kessels Road, Nathan, Brisbane, Australia, 4111
11 12
b
13
New Zealand, 4442
Landcare Research – Manaaki Whenua, Riddet Road, Massey University Palmerston North,
14 15
Corresponding author: email,
[email protected], phone +737353605
16 17
Dedication – Sadly, Dr Kevin Tate passed away during the final stages of producing this
18
manuscript. Kevin will be remembered as an outstanding scientist, inspiring others with his
19
unwavering commitment to tackling climate change.
20 21
1 ACS Paragon Plus Environment
Page 3 of 53
Environmental Science & Technology
22
Abstract
23
Methane (CH4) is the second greatest contributor to anthropogenic climate change. Emissions have
24
tripled since pre-industrial times and continue to rise rapidly, given the fact that the key sources –
25
food production, energy generation and waste management – are inexorably tied to population
26
growth. Until recently, the pursuit of CH4 mitigation approaches has tended to align with
27
opportunities for easy energy recovery through gas capture and flaring. Consequently, effective
28
abatement has been largely restricted to confined high-concentration sources such as landfills and
29
anaerobic digesters, which do not represent a major share of CH4’s emission profile. However, in
30
more recent years we have witnessed a quantum leap in the sophistication, diversity and affordability
31
of CH4 mitigation technologies on the back of rapid advances in molecular analytical techniques,
32
developments in material sciences and increasingly efficient engineering processes. Here, we present
33
some of the latest concepts, designs and applications in CH4 mitigation, identifying a number of
34
abatement synergies across multiple industries and sectors. We also propose novel ways to manipulate
35
cutting-edge technology approaches for even more effective mitigation potential. The goal of this
36
review is to stimulate the ongoing quest for and uptake of practicable CH4 mitigation options;
37
supplementing established and proven approaches with immature yet potentially high-impact
38
technologies. There has arguably never been, and if we don’t act soon nor will there be, a better
39
opportunity to combat climate change’s second most significant greenhouse gas.
40 41
Key words
42
Climate change, cross-sector, methane, mitigation
43
2 ACS Paragon Plus Environment
Environmental Science & Technology
Page 4 of 53
44
Introduction
45
Anthropogenic emissions of methane (CH4) are second only to carbon dioxide (CO2).
46
According to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change
47
1
48
dioxide equivalent (CO2-e) basis. Based on the IPCC’s recently revised Global Warming
49
Potential (GWP) factor of 28 for CH4 1, CH4 emissions likely contribute more than 20%
50
towards total anthropogenic greenhouse gas (GHG) emissions, mainly from biomass burning,
51
wetlands, rice paddies, enteric and waste emissions, and fossil fuel use 2. There is also
52
increasing evidence that the vast store of CH4 in frozen soils in the Arctic is now being
53
released as these soils thaw 3. Moreover, other stores of previously untapped CH4, such as
54
deep ocean sediment hydrates and clathrates, are currently being targeted as potential
55
alternative energy sources
56
activities has triggered concern 6. This contribution to climate change from direct and indirect
57
anthropogenic CH4 emissions is clearly being recognised within the science and engineering
58
spheres, as evidenced by the recent development of organisations such as the Global Methane
59
Initiative and the Global Research Alliance.
, annual anthropogenic CH4 emissions total approximately 8 gigaton (Gt) on a carbon
4, 5
and the risk of substantial indirect CH4 emissions from these
60 61
Anthropogenic CH4 sources vary in magnitude and are produced across a diverse range of
62
industries and sectors. Yet, despite CH4’s importance to climate change, few articles exist
63
regarding technical mitigation opportunities across the key emission sectors. In 2012 there
64
was a concentrated effort in this area with four overview papers published reporting emission
65
estimates and mitigation options for the major CH4 sources
66
discussed in the reviews by Höglund-Isaksson 7, Karakurt, et al. 8, and Yusuf, et al.
67
presented from a high-level working principle perspective – i.e., logistics of gas recovery,
68
collection and potential use as well as land use management practises to decrease emissions.
7-10
. The mitigation options 10
were
3 ACS Paragon Plus Environment
Page 5 of 53
Environmental Science & Technology
69
Moreover, these reviews focused on mitigation potential for individual sources. By contrast,
70
Stolaroff, et al.
71
potential and presented a range of CH4 mitigation options from a technology perspective.
72
Since then, new scientific and engineering advances have paved the way for enhanced CH4
73
mitigation technology development across several key contributing sectors. These
74
technologies range in maturity from fledgling concepts, to field-tested systems that are now
75
ready for full-scale deployment and validation, right through to commercially-viable
76
practices.
9
in their review recognised the potential for cross-sectoral CH4 mitigation
77 78
In this review we springboard off the work of Stolaroff, et al. 9 and other earlier reviews to
79
highlight the latest developments and opportunities in CH4 mitigation technologies. Our
80
review focuses on advances in this field over the past five years which have progressed on the
81
back of three rapidly evolving disciplines: 1) material sciences; 2) microbial ecology; and 3)
82
design and engineering. We largely avoid focusing on recent developments in enteric CH4
83
mitigation using feed modifications, vaccines and animal selection – a wealth of thorough
84
reviews evaluating the progress of these approaches can be readily found in the literature
85
14
86
which innovative options are also critically needed. As an integrated aspect of our review we
87
also appraise the latest developments in emission estimates for the key sources.
11-
. Rather, our emphasis is on emerging mitigation approaches for other key sectors, for
88 89
Through presenting the underpinning concepts and practical considerations associated with
90
emergent technologies we strive to stimulate opportunities for CH4 mitigation on a cross-
91
sectoral scale. Building on the effectiveness of existing conventional technologies and
92
established management practises that have been demonstrated to abate CH4 emissions, we
93
envisage that the promotion of cross-sectoral emergent technologies will foster a more
4 ACS Paragon Plus Environment
Environmental Science & Technology
Page 6 of 53
94
efficient approach for tackling global CH4 emissions. Given the recent call for ‘aggressive
95
CH4 mitigation’ measures issued by Kim, et al.
96
pressing opportunity for appraising the state technology development in this field.
15
, there has arguably never been a more
97 98
Contributions and trends for key emission sources
99
Current emission estimates for anthropogenic CH4 sources are presented in Fig. 1, illustrating
100
how CH4 emissions compare with those of the other main GHGs (Fig. 1a). The magnitudes of
101
emissions from the key anthropogenic CH4 sources have previously been well-documented
102
and are summarised in Fig. 1b, using a combination of literature values and IPCC estimates.
103
It can be seen that total estimates vary between documented sources, resulting from
104
uncertainties in emission estimates for each sector. The emission estimates in Fig.1b
105
incorporate the IPCC’s Second Assessment Report GWP of 21 for CH4 which is currently
106
adopted in most national inventories, although there is increasing momentum for adoption of
107
the GWP of 28 proposed by the IPCC’s AR5 metrics 16.
108 109
Two central concepts from Fig. 1b are pertinent to this review: 1) large-scale hydroelectric
110
dams are a significant CH4 source, despite not been accounted for as direct CH4 emission
111
sources by the IPCC or by most other inventory documents
112
emissions are reasonably consistent across the key emission sources. From a mitigation
113
perspective, the recognition of large-scale hydroelectric dams as a primary emission source is
114
important because there are a number of inventive and practical approaches for abating these
115
emissions, some of which might hold relevance to mitigating emissions from other sources.
17
, and 2) the magnitudes of
116 117 118
5 ACS Paragon Plus Environment
Page 7 of 53
Environmental Science & Technology
119 120
The fact that emission magnitudes are reasonably evenly distributed across the main sources
121
is also telling, because it highlights the need for a cross-sectoral approach to achieve
122
comprehensive CH4 abatement. In Fig. 2 we project how the balance of direct CH4 emissions
123
from the major sectors will look in the coming decades. We emphasise that these projections
124
are broad estimates based on current trends in the key CH4-producing sectors and that several
125
unavoidable caveats are associated with this type of exercise. This is particularly true for the
126
energy sector where forecasting trends can be problematic. For example, Utgikar and Scott 56
127
note that accurate estimates and forecasts of energy demand are hampered by barriers
128
associated with technology adoption; socio-economic factors; and volatile economic policies.
129
Moreover, as noted by Feng and Zhang
130
may conceivably change drastically in coming years as some countries adopt widespread
131
energy saving measures whereas other developing countries surge in energy demand to meet
132
growing economies.
57
and Aydin, et al.
58
current trends in energy use
133 134
The agricultural and waste sectors, by comparison, are projected to continue to grow rapidly
135
in the coming decades, potentially even to the point of where they could exceed sustainable
136
boundaries 59, 60. Yet, considering all of the factors above, our 2050 projections indicate that
137
the relative contributions of the direct anthropogenic CH4 emission sectors will remain
138
reasonably consistent into the latter part of this century, albeit with a slight increase in the
139
proportion of agricultural emissions resulting from a surge in food demand and a
140
corresponding proportional decrease in energy-driven emissions (Fig. 2).
141 142
Recognition of a strong diversity of key CH4 emission sources well into the 21st Century is
143
important if we are to limit global mean temperature increases to 2⁰C or less by 2100, a target
6 ACS Paragon Plus Environment
Environmental Science & Technology
Page 8 of 53
144
agreed to at the Paris summit in 2015. Sixteen of the first twenty publications generated by a
145
Google Scholar search employing the terms ‘methane mitigation’ pertain to enteric
146
abatement. While large strides are being made in providing solutions for decreasing ruminant
147
CH4, clearly these emissions are just one component of the total anthropogenic CH4 budget.
148
There is a real need for giving equal focus to the pursuance of mitigation strategies for other
149
key CH4 sources.
150 151
In the following sections we present some of the emerging and novel technologies in the field
152
of anthropogenic CH4 abatement. Rather than present the technologies on a source-by-source
153
basis we discuss the processes and mechanisms involved in the technology and then elaborate
154
where we see potential for cross-sectoral application.
155 156
Breaking the bond – advances in catalysts for low concentration and low temperature
157
methane oxidation
158
Breaking the carbon-hydrogen bond in the CH4 molecule is the first obstacle faced by all CH4
159
mitigation approaches that aren’t based on avoiding CH4 production. Although this reaction
160
is lucrative from an energy production perspective with a standard enthalpy of combustion of
161
–882 kJ/mol, a substantial amount of activation energy is required for its initiation. In CH4-
162
rich gas such as biogas produced at landfills and in wastewater treatment systems energy
163
recovery from gas combustion is technically feasible and widely-practised 68-70. However, in
164
gas streams that are
