Page 1 of 53 ACS Paragon Plus Environment ... - ACS Publications

Hydroelectric dams: an overlooked methane source with significant mitigation potential. 470. 471. Hydroelectric power generation has long been regarde...
1 downloads 4 Views 1MB Size
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