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Configuration of materially retained carbon in our society: A WIO-MFA-based approach for Japan Hajime Ohno, Hirokazu Sato, and Yasuhiro Fukushima Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b06412 • Publication Date (Web): 16 Mar 2018 Downloaded from http://pubs.acs.org on March 17, 2018

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Environmental Science & Technology

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Configuration of materially retained carbon in our

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society: A WIO-MFA-based approach for Japan

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Hajime Ohno1*, Hirokazu Sato1, and Yasuhiro Fukushima1

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1

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6-07 Aramaki Aza Aoba, Aoba-ku, Sendai, Miyagi 980-8579, Japan

Department of Chemical Engineering, Graduate School of Engineering, Tohoku University, 6-

6

ACS Paragon Plus Environment

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ABSTRACT: To achieve the goals of Paris Agreement, global society is directing much effort in

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substantially reducing greenhouse gas (GHG) emissions. In addition to energy-related efforts,

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prevention of carbon release into the atmosphere with carbon capture and storage (CCS) and/or

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utilization of biomass resources is considered indispensable to achieving the global objective. In

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this study, considering carbon-containing goods as carbon reservoirs in our society similar to

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forests and reservoirs enabling CCS, the flow of materially utilized carbon was quantified by

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input-output-based material flow analysis (IO-MFA). Consequently, in 2011, 6.3 Mt-C of

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petroleum-derived carbon and 7.9 Mt-C of wood-derived carbon were introduced to the Japanese

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society as end-use products (e.g., automobiles and constructions) in various forms (e.g., plastics

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and synthetic rubbers). The total amount (14.2 Mt-C) corresponded to 4.1% (52.1 Mt-CO2) of

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annual CO2 emission in Japan in 2011. Subsequently, by referring to the technology that can

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treat carbon in the target forms in end-of-life products, the recoverability of carbon as a material

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has been discussed with respect to each form and end-use of carbon. By numerically showing the

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necessity and potential of implementing appropriate technologies, this study provides scientific

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direction for policymakers to establish a quality carbon cycle in our society.


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TOC art

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1. INTRODUCTION

26

An ambitious goal toward the reduction of greenhouse gas (GHG) emissions was adopted as

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Paris Agreement for the mitigation of global warming.1 To achieve the goal, every participating

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nation is committed to make substantial efforts in reducing GHG emissions, and many nations

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have included in their agenda actions related to emission reduction in the energy sector.2

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Particularly, significant contributions are expected from the improvements in conversion

31

efficiency and the shifting of energy composition toward a renewable energy-oriented mix.3 In

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addition to energy-related efforts, prevention of carbon release into the atmosphere is recognized

33

as indispensable to achieving the global objective. As a decarbonization strategy, Rockström et

34

al.4 highlighted the importance of carbon capture and storage (CCS) technologies, which would

35

capture and store CO2 generated by fossil fuel-based power plants underground and/or under the

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ocean bottom, thereby preventing CO2 emissions into the atmosphere.5 To capture more CO2

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from atmosphere through improved management of land use, land-use change and forestry

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(LULUCF) is also important; trees absorb CO2 through photosynthesis and fix about 4 Gt/yr of

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carbon in their body globally.6 Together with such efforts, enhanced utilization of woody

40

materials and products can prevent carbon release into the atmosphere for a certain period.7

41

Similar to forests and reservoirs enabling CCS, goods in our society store carbon as well. In

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addition to the abovementioned wood products, petroleum-derived products such as plastics and

43

synthetic rubbers materially retain carbon. At the end of product lifetime, the carbon is released

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as CO2 when the products are incinerated, regardless of with/without energy recovery. In Japan,

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for example, GHG emission from the waste treatment sector accounts for 2.3% of the annual

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GHG emissions.8 Among the wastes, incineration of waste plastics contribute to approximately

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half of the emissions.8 On the other hand, mechanical and/or chemical recycling of products


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would allow carbon reuse in some other product without contributing to climate change.

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Therefore, appropriate recovery of carbon from wastes can contribute to create a cycle with

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enhanced retention of carbon in our society that will also reduce the rate of consumption of

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virgin carbon sources.

52

The amount of carbon stored in wooden products has been quantified by each country

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according to the IPCC guideline 9 in addition to the carbon cycle in ecosystems.6, 10 On the other

54

hand, those stored in petroleum-derived products are less addressed. The flow and stock of such

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products are rarely analyzed, whereas the flows of other major elements have been actively

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quantified by material/substance flow analysis (MFA/SFA)11 in the last two decades.12, 13 The

57

exceptions are the research on global carbon flow by Fujimori and Matsuoka,14 network analysis

58

of carbon metabolism by Chen and Chen,15, 16 MFA studies by Gielen,17-20 and dynamic analysis

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by Lauk et al.13 Fujimori and Matsuoka, and Chen and Chen demonstrated a global/local flow

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analysis for carbon, including both emission and material use of petrochemical- and wood-

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derived carbons.14-16 Further, studies by Gielen17 and others18-20 precisely quantified the national

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flow of petroleum-derived carbon in petrochemical production and regarded the products as a

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potential source of carbon emission. Lauk et al.13 conducted dynamic quantification of

64

socioeconomic carbon stocks from 1900 to 2008 in terms of wood-derived and petroleum-

65

derived carbons.

66

However, in these pioneering works, the end-uses of carbon were not clarified in detail. Both

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the type of end-uses (e.g., automobiles and constructions) and forms (e.g., plastic resins and

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synthetic rubbers) significantly affect the fate of carbon; however, the former has never been

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considered in these pioneering studies. For each type of end-uses and forms, the product lifetime

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and the reuse, refurbish, remanufacturing and/or recycling (hereafter referred to as "recovery")


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systems (with consideration of their ready availability) vary. Therefore, clarification of both the

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end-uses and forms of carbon is crucial for understanding the current flow and stock of

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materially utilized carbon in our society, which can then serve as a platform for subsequent

74

discussion on the development of sustainable carbon cycle by coupling the societal and global

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carbon cycles.21

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This study quantifies the flow of materially utilized petroleum-derived and wood-derived

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carbons toward their end-uses by using the waste input-output MFA (WIO-MFA) model.22, 23

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This model is a useful top-down approach for estimating the flows of target materials among the

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economic activities described in an input-output (IO) table. The advantage of the model over

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conventional MFA approaches is that it can comprehensively trace the flow-paths of multiple

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target materials in inter-industrial transactions to final destinations described in an IO table

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simultaneously with considerations on generation of losses and the need for elimination of

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immaterial transactions from the original monetary IO table. In addition, material compositions

84

of products present in the IO table are obtained, which represent the national averages of the

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material composition without preparing the composition data of products with enough

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representativeness prior to analysis such as conventional MFA. These features are suitable for

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the analysis of materially utilized carbon flow because the carbon contents in highly fabricated

88

products can hardly be obtained from statistics and other sources and may be a source of high

89

uncertainty, as Chen and Chen noted.15 Furthermore, since the model analyzes the flow of

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materials based on national monetary IO table with unit conversions for some inputs of key

91

sectors from monetary to physical, it can provide a higher resolution of IO-based material flow

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rather than other IO-based MFA using physical IO tables (PIOTs)24 or IO table with limited

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number of sectors,15, 16 as proposed by Nakamura et al.23 The target materials in previous studies


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using WIO-MFA were base metals,22, 23, 25, 26 polyvinyl chloride,27 steel alloying elements,28-31

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and alloy steel materials.32 In this study, carbon flow is traced by regarding basic petrochemicals

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and timber as the target materials for petroleum-derived and wood-derived carbons, respectively,

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through various sectors toward end-uses (e.g., automobiles and constructions). Each of the

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sectors was studied in detail to determine the forms of carbon (e.g., plastic resins and synthetic

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rubbers) utilized and produced by them. Subsequently, we discuss the recoverability of carbon in

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each form and/or end-use present in Japanese IO table by considering the availability of recovery

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systems.

102 103 104

2. METHODOLOGY AND DATA 2.1. WIO-MFA. WIO-MFA enables us to estimate the compositions of materials in products

105

by tracing the direct and indirect flow of materials in a supply-chain.22,

106

conducted based on a national IO table compiling inter-industrial transactions with some rows

107

representing the sector inputs in physical unit (i.e., tonne) together with the rows representing the

108

sector inputs in monetary value (i.e., 106 JPY). The composition of materials in product is

109

derived as follows:

( − ) =

= ⊗ ( ⊗ ).

23

WIO-MFA is

(1) (2)

110

and represent the “filtered” input coefficient matrices corresponding to Here,

111

the inputs of materials (M) to products (P) and inputs of products (P) to products (P),

112

respectively. The sets P and M refer to the products included in the IO table and the target

113

materials, respectively. The ingredients of the sets are listed in the Supporting Information

114

(SI).The operator ⊗ represents Hadamard product (i.e., element-wise product). “Filtered”


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implies that the inputs do not form the mass of a product (e.g., the inputs of services, electricity,

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and process losses of materials) are removed from the general input coefficient matrix by

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multiplying the filter matrices for non-physical inputs () and process losses () as formulated

118

in equation (2).22, 23 When input to sector is a physical input, the (, )-elements in matrix ,

119 120 121

= 1; otherwise = 0. The (, )-elements in matrix , , range between 0 and 1, which

gives the ratio of input that becomes the weight of product . The (, )-elements in matrix ,

, is calculated as follows:

= 122 123 124

,

(3)

where is the (, )-element of the IO table for WIO-MFA and is the th row of a vector

for domestic production. When sectors and have the same unit (i.e., both have physical

unit or monetary unit), has no unit. On the other hand, when the units of and are

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different, i.e., has a physical unit and has a monetary unit or vice versa, the units are t/106

126

JPY or 10 6 JPY/t in this study. Consequently, from equation (1), the units of the elements in

127

matrix are a mix of no unit and t/106 JPY for the physically indicated sectors and others,

128 129 130 131

respectively. The (, )-element of matrix represents the mass of material directly and

indirectly introduced in a single unit (i.e., 1 t or 106 JPY) of product .

The physical flows of materials in a supply-chain can be described in the form of an IO table by applying matrix to the IO table as follows: !

132 133

where

!

#! (, ∈ P, ∈ M) =" ( = , ∈ P, ∈ M),

(4)

is an (, )-element of the physical IO table for material , % ! ; #! is an (, )-

element of matrix representing the content of material in product ; (, ∈ P) is an (, )-


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element of ; ( = , ∈ P) is an (, )-element of , and is the th row of vector &

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representing the domestic productions of products. Furthermore, the contents of materials in the

136

final demands of products can be calculated as follows:

'(! = #! '( ( ∈ P, ) ∈ K, ∈ M),

137

(5)

where '(! and '( represent the (, ))-elements of the matrix representing the final demands for

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products in terms of content of material , + ! , and monetary basis, +, respectively. Set K

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includes export, import, and domestic final demands. Here, the basic formula of IO analysis,

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& = ( − ) ', is completed, where ' = ∑( '( . Notably, when ) is import, we are assuming

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that the material compositions of the imported products are the same as that of the domestically

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produced products. In addition to final demands, the process losses in each product-producing

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sector are quantified as follows:

-! = .(1 − ) /∈0

144 145 146

148

∈ P, ∈ M),

(6)

where -! represents the th row of vector 1! for mass of loss of material during the

production of product , and and

!

correspond to the (, )-element of matrix and % ! ,

respectively. Summarizing them, the total flow of material can be compiled as 2 ! as follows:

2! = 3 147

! (

%! 1! 4

+! 5, 0

(7)

where superscript ⊤ refers to the transpose of a matrix or vector. The material balance between input and output can be described as follows:


9


.

!

.

!

∈

∈

+

. '(! (∈8

=

!

+

= #! =

!

-!

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(8)

(9)

149

Here, ! is the content of in product . Equation (8) represents the balance between demands

150

and productions. The balance between inputs and outputs as well as losses is given in equation

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(9).

152

Further details on WIO-MFA can be found in previous works.22, 23, 28, 33

153 154 155

2.2. Sector Definition and Data. The WIO-MFA table for carbon flow analysis was

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constructed based on Japan’s national IO table for 2011. We chose basic petrochemicals

157

(ethylene, propylene, etc.), aromatic petrochemicals (benzene, toluene, xylene, etc.), recycled

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resins (recycled thermoplastic resin and high function resin), domestic and imported timbers, and

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recycled paper as target materials (i.e., the set of M in WIO-MFA) for carbon flow according to

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the sector definition in the original IO table for Japan. Carbon derived from basic petrochemicals

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(hereafter BPCs) as well as aromatic petrochemicals (hereafter APCs), and domestic and

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imported timbers are referred to as primary petro-carbon and primary woody-carbon,

163

respectively. The recycled resins and recycled paper are regarded as secondary petro-carbon and

164

secondary woody-carbon, respectively. As described in equation (1), the calculation of WIO-

165

MFA includes the flows starting from “M” (i.e., BPCs, APCs, and timbers) and spreading to “P.”

166

This implies that sectors with lower degree of fabrication compared to “M” sectors such as

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naphtha, petroleum, natural gas, and coal, which were categorized as resources “R” providing


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carbon for “M”, were excluded from the scope of carbon flow in this study. By defining BPCs

169

and APCs as M, the amount of petro-carbon can be obtained by applying carbon-containing ratio

170

(C-ratio) based on their stoichiometric coefficients rather than “R” sectors, which are the raw

171

materials of BPCs and APCs. The C-ratio is defined as the weight ratio of carbon in the

172

molecular weight of a material. For example, the C-ratio of ethylene (C2H4) can be calculated as

173

(12*2) / (12*2+1*4) ≈ 0.86. The C-ratios in “R” sectors are uncertain because of their varying

174

lengths of carbon chains. In case of “P” sectors, all sectors having a higher degree of fabrication

175

than that of the “M” sectors in the IO table were categorized. The details of sector classification

176

and categorization into “R,”, “M,” and “P” are provided in SI.

177

The original IO table for Japan has compiled the transactions within the Japanese economy in a

178

monetary unit (i.e., 1 million JPY). Therefore, the values of target sectors as well as intermediate

179

products sectors (hereafter key sectors) in the rows needed to be described with respect to the

180

mass of carbon for carbon flow analysis by WIO-MFA. The sectors which are converted their

181

values in the rows in the IO table into carbon mass basis were listed in Table S2 in SI. To obtain

182

the flows as carbon mass basis, the monetary values were converted into physical values (i.e.,

183

tonne) as a first step.

184

The IO table for Japan has a physical-value table indicating physical inputs as an appendix for a

185

limited number of sectors.34 Thus, we applied the physical values to all the sectors listed in Table

186

S2 for which physical inputs were available; otherwise, unit price (i.e., t/million JPY) was

187

applied to convert the monetary unit to physical unit. As the second step, the physical values of

188

the key sectors were converted into carbon mass basis by applying C-ratio. For petrochemicals,

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the C-ratio can be obtained based on their stoichiometric coefficients and molecular weights. The

190

amount of carbon in timbers was determined by tree species based on their bulk density and


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carbon contents.8 It should be noted that the other BPCs, other APCs, and petrochemical

192

intermediate product sectors in Table S2 (i.e., aliphatic intermediates, cyclic intermediates,

193

thermo-setting resins, thermoplastic resins, high-function resins, other resins, and synthetic

194

rubber) are aggregated sectors consisting of subsectors. Thus, the C-ratios for these aggregated

195

sectors cannot be simply obtained from the stoichiometric coefficient of a single product. The

196

details of the calculation of weighted average C-ratios for each of these sectors presented is

197

described in the SI. We introduced the values of key sectors in carbon mass basis for not only

198

“M,” but also “P” to maintain the mass balance of carbon at least among the key sectors. For

199

example, aliphatic intermediates are produced mainly by the inputs of BPCs. Thus, the inputs of

200

carbon associated with BPCs to aliphatic intermediates were already quantified by the unit

201

conversion procedure for inputs of BPCs from monetary to physical and from physical to carbon

202

mass basis, as mentioned above. Besides, the produced masses of aliphatic intermediates can be

203

obtained from statistics,34 and their carbon contents can be calculated by the C-ratio based on

204

stoichiometric coefficient. Consequently, we can determine the yield ratio of carbon in the

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production of aliphatic intermediates based on the mass balance of carbon between the inputs

206

and production. The obtained yield ratios of carbon for key sectors categorized in “P” were

207

utilized as the filter in the calculation of to separate the loss of carbon during productions

208

from the subsequent flow of carbon. The balances between timbers and wooden material (i.e.,

209

lumber, plywood, and wooden tip) were also maintained in the same way. For paper production

210

including pulping, the yield ratios were referred to Van Ewijk et al.35 For other fabrication

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processes of carbon-containing products, the yield ratio was homogeneously set as 0.9 due to

212

limited data availability. Further details on the unit conversion steps and the yield ratios are

213

presented in SI.


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In addition to sector categorization into “R,” “M,” and “P,” we defined several sectors in “P”

215

sectors as form-determining sectors, which represent the forms of carbon in our society. The

216

form-determining sectors are listed in Table S8. Using equation (1) by re-categorizing these

217

form-determining sectors and sectors with lower degree of fabrication compared to them (e.g.,

218

BPCs, APCs, and intermediates) as “M” and “R,” respectively, carbon flows starting from the

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form-determining sector could be drawn.23, 31 Thus, we obtained the forms of carbon in final

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demands for end-uses. This procedure is explained in more detail in the SI.

221

As a limitation of this method, we cannot trace carbon flows originating from materials other

222

than the target materials defined in this study such as carbon black and carbon fiber. Although

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they are included in the original IO table for Japan,34 they are aggregated into sectors comprising

224

various subsectors. Due to limited data on such relatively minor materials, it is difficult to assign

225

inputs to carbon materials from the aggregated sector to specific sectors in the IO table.

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Moreover, the classifications of the aggregated sectors in the IO table may also introduce some

227

uncertainties to the flow and composition of carbon although the IO table for Japan is one of the

228

highest-resolution tables among the national IO tables. To deal with this limitation, sector

229

disaggregation with extrapolation of data from process-based bottom-up MFA has often been

230

applied in previous studies.27-30 However, it is not the case in this study because the focus of this

231

study is the quantification and visualization of overall trend of carbon flow in Japanese economy

232

in 2011. Due to the abovementioned issues in sector definition and classification, the current

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model can only be applied to Japanese IO table having around 400 sectors including sectors of

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individual petrochemicals and intermediates. Another candidate IO table to which the method

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can be applied is the IO table for the U.S.,36 which has large numbers of sectors similar to the

236

Japanese table. However, the data-intensive disaggregation of sectors is necessary because of its


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aggregated sector classification on petrochemicals. Therefore, the results in this study represent a

238

benchmark for the case of a developed country to quantify the importance for consideration of

239

the carbon retention in our society.

240 241

3. RESULTS AND DISCUSSIONS

242

3.1. Carbon Flow in Japanese Economy. In the case study on Japanese economy in 2011, it

243

was found that carbon was diversely utilized as a part of material, which forms the weight of

244

products originating from petrochemicals and timbers. Figure 1 illustrates the Japanese carbon

245

flows with 23 nodes, which is an aggregated representation of sectors in the IO table, net trade,

246

and losses. The details of sector aggregation in Figure 1 and carbon contents of products in the

247

IO table are provided in the SI. In addition, the accuracy of the model was tested in the SI. In

248

terms of woody-carbon, the imports of wooden intermediates such as lumbers, plywood, and

249

wooden chips contributed to enlarge the inflow of primary woody-carbon in addition to domestic

250

timbers. Consequently, 16.5 Mt of primary woody-carbon and 21.6 Mt of primary petro-carbon

251

were introduced to Japanese economy in 2011. In addition to primary carbons, recycled paper

252

added 6.4 Mt of secondary woody-carbon into paper products, whereas recycled resins provided

253

tiny flows of secondary petro-carbon into resins and plastic products. Petro-carbon was widely

254

spread in various sectors mainly in the form of plastic products, rubber products, and other

255

chemical products. Eventually, 29% (6.3 Mt-C) of the initial input of petro-carbon remained in

256

domestic final demands. On the other hand, woody-carbon was intensively consumed in limited

257

sectors such as construction and paper products. In domestic final demands, 53% (7.9 Mt-C) of

258

primary woody-carbon was introduced to the society. Consequently, total 14.2 Mt-C was newly

259

added to carbon retention in Japanese society, which corresponded to about 4.1% (52.1 Mt-CO2)


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of annual carbon emissions as CO2 in Japan in 2011.8 Besides the retained carbon in domestic

261

final demands, considerable amounts of carbon flowed into the node of service activities

262

accounting for 5.0 Mt and 8.1 Mt of petro- and woody-carbons, respectively. However, service

263

activities do not provide any physical product but immaterial products to customers. Therefore,

264

the carbon-entered sectors belonging to service activities were regarded as non-recoverable

265

carbon in further discussions due to the difficulty in determining the fates of carbons introduced

266

into the sectors. The flows from the service sectors were filtered out by matrix Φ in equation (2).

267 268

Figure 1. Comprehensive flow of petro-carbon and woody-carbon in Japanese economy in 2011.

269

The details of sector aggregation and the carbon contents of products of the sectors in IO table

270

are given in the SI.

271

Among the flows of carbons, some left the system as losses. The loss of petro-carbon during

272

the production of aliphatic and cyclic intermediates was estimated at approximately 3.9 Mt by


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balancing the carbon input from BPCs and APCs and the output of the intermediates. In addition,

274

6.4 Mt of woody-carbon was eliminated from the flow during pulping and paper production as

275

lignin and/or paper sludge. A large part of the losses of woody-carbon from paper production

276

have been recovered as an energy source,35 which implies that it led to the release of carbon into

277

atmosphere as CO2 although it could be regarded as carbon-neutral. Furthermore, packaging

278

materials for intermediate products left the system as loss. In total, 7.4 Mt and 12.3 Mt of petro-

279

and woody-carbon, respectively, were lost through economic activities.

280 281

3.2. Forms of Carbon. In addition to the amount of carbon introduced to the society,

282

information on the form of carbon is indispensable to the development of a strategy to enhance

283

carbon retention because the forms of carbon highly affect the recoverability of carbon from end-

284

of-life (EoL) products. Carbon retained as a component in final demands as the forms listed in

285

Table S7 was calculated as 13.3 Mt-C, which is 94% of the total retained carbon (14.2 Mt-C).

286

Other portions went to the products with lower degree of fabrication without passing through the

287

focal sectors.

288

Resins share was 22% (3.1 Mt) of carbon retained in the final demands (Figure 2)

289

corresponding to 11.5 Mt-CO2. Compared to CO2 emission from waste plastic incineration (7.1

290

Mt-CO2) in Japan in 2011,8 the net increase in carbon retention in the form of resin can be

291

estimated as around 1.2 Mt-C. Among the resins, thermoplastic resin dominated the share

292

followed by high-function resin (e.g., polyethylene terephthalate (PET); for more details on

293

categorization, see SI). Recycling technologies have been developed for the wastes of these two

294

types of resins37,

295

resins such as thermosetting resins.45 However, till date, the main treatment of thermoplastic and

38

evaluated by LCA,39-43 and practically implemented44 compared to other


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high-function resins is heat recovery by incineration.44, 46 Synthetic rubber with 2% share of

297

carbon retained in the final demands is also similarly recycled.47 Innovations in recycling of

298

resins and rubber are required to keep the retained carbon as functional as possible.48, 49 Further,

299

chemical fibers have 4% share of retained carbon. Because the main user of chemical fibers is

300

the apparel industry, reuse of second-hand clothing would contribute to carbon preservation in

301

products.50 Although the share of other chemical products is 11% of carbon retained in the final

302

demands in various forms, recovering carbon from them appears difficult. For example, paints

303

with a large share of the category “other chemical products” emit some carbon after painting

304

with the evaporation of solvent.51 Carbon emissions resulting from the usage of products in this

305

category are quantified in the National Greenhouse Gas Emission Inventory.8

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Figure 2. Shares of forms of carbon in final demands. The inner pie chart represents share in

308

large-scale categories of forms, and the outer pie chart shows the breakdowns in each large-scale

309

category. Percentages for the outer pie chart are provided in SI.

310

Wooden intermediates, which are consumed mainly in constructions (Figure 1), has 35% share

311

of retained carbon in the final demands. This implies that a large part of carbon entering our

312

society in this form may get retained for a relatively long time according to the lifetime of

313

constructions.52 In addition, as Sathre and Gustavsson demonstrated,53 cascade utilizations of

314

recovered wood for materials would contribute to the reduction of carbon emission although it

315

depends on the maturity of forests and its availability as a biomass resource. Paper products have

316

20% share of retained carbon in the final demands mainly as paper and cardboard boxes. More

317

than half of paper products were produced by utilizing recycled paper in Japan in 2011 (63%).54

318

Consequently, paper products are highly contributing to retained carbon in our society. However,

319

certain types of paper such as sanitary paper (sharing 0.6%) are difficult to recover as paper

320

materials.

321

Likewise, we can discuss the recoverability of carbon retained in various forms by referring to

322

the technology that can treat carbon in the target forms in EoL products. A review of the

323

recoverability of carbon in various forms revealed that a large portion of carbon (approximately

324

81%) are theoretically recoverable and can be retained in products again, except for carbon

325

utilized in forms that are hard to recover such as cosmetics, paints, and sanitary paper. Although

326

LCA works have proved the benefits of recycling in the reduction of CO2 emission and/or energy

327

conservation, many of the proposed recycling technologies have not been practically applied yet

328

in the current society. In the extreme, it implies that if we could recover all of the theoretically

329

recoverable carbons introduced to the Japanese society in 2011 at their EoL stage with


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330

appropriate technologies, approximately 42.2 Mt of CO2 emission corresponding to 3.3% of CO2

331

emission in 2011 would be avoided. However, we must also consider the energy consumption

332

and GHG emission during the recovery processes in reality. The emission reduction by carbon

333

recovery as material would substantially contribute to decarbonization toward the achievement

334

of Japanese target of reducing to -26% GHG emission by 2030 from the level in 2013, as stated

335

in the agenda for the Paris agreement.2

336 337

3.3. Final Destination of Carbon. Multiple forms of carbon are jointly utilized in products.

338

Therefore, the mode and time of EoL carbon generation should be considered based on the types

339

of products (i.e., end-uses) as well as forms. Figure 3 shows the top 10 products with the highest

340

contents of petro-carbon in their final demands with the breakdown into forms of carbon. Petro-

341

carbon retention in the top 10 products shared 41% of petro-carbon introduced in Japanese

342

society in 2011. Carbon contents derived from wood-originated forms were also observed in

343

residential constructions due to the use of coated paper and miscellaneous wooden products

344

containing small amounts of petro-carbon in addition to woody-carbon. The result of woody-

345

carbon is presented in the SI.

346

Passenger motor cars were the most petro-carbon-demanding products, followed by cosmetics

347

and dentifrices. Although a large carbon flow as electric and electronic products into the

348

domestic final demand is observed in Figure 1, they did not appear in the top 10 products due to

349

their relatively small final demands as individual products. For example, household electric

350

appliances (except for air-conditioner) and cellular phones ranked 11th and 12th, respectively.

351

Carbon forms in passenger motor cars were mainly dominated by resins used in various parts

352

such as bumpers, cases for lamps, and dashboards.55 Although some of them are removed for


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353

reuse in the EoL treatment of automobiles, majority of them are shredded together with other

354

materials (e.g., metals, rubbers, and textiles) and collected as automobile shredder residue

355

(ASR).55, 56 Once various kinds of resins are mixed in ASR, it is almost impossible to separate

356

them from each other. The mechanical recycling of recovered resins wherein they are

357

horizontally utilized in the forms is difficult unless a sensor-based sophisticated sorting

358

technique for resins that is under development57 becomes available. To date, the difficulty in re-

359

sorting the resins of ASR demands the adoption of energy recovery technologies as the main

360

treatment method.56 Instead of horizontal mechanical recycling of resins from ASR,

361

gasification56,

362

investigated as an alternative to recover carbon rather than its utilization as a heat source. In

363

addition to increasing the removal of resin parts from EoL automobiles for reuse, practical

364

implementations of these ASR treatment technologies in EoL automobile recycling will

365

contribute to the preservation of carbon in our society.

58

and/or thermal conversion of ASR into value-added products59 have been


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366 367

Figure 3. Top 10 largest petro-carbon-containing products (∈ P) in their final demands with

368

breakdown into forms of carbon. Top 10 products share 41% of petro-carbon introduced to

369

Japanese society in 2011. Cosmetics and dentifrices appear in the top 10 as a product as well as a

370

form due to its large domestic final demand as itself rather than as endogenous demands. In

371

addition, small amounts of petro-C accompanied with wood-originated forms such as coated

372

paper and miscellaneous wooden products are observed particularly in residential constructions.

373

Percentage breakdowns are provided in SI.

374

As mentioned in Section 3.2, carbon consumed in the forms of other chemical products would

375

be hardly recovered from the EoL products. Cosmetics and dentifrices have large demands for

376

carbon in their own forms, which are likely to dissipate immediately after use. In the final

377

demands for cosmetics and dentifrices, some resin-derived carbons were observed. The resins are

378

mainly for packaging materials, while some of them are microplastics (from facial cleansers and


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379

dentifrices), which have recently been regarded as a problematic pollutant in the marine

380

environment.60 Carbon dissipation as microplastics is undesirable due to carbon loss into

381

environment in harmful forms. Besides, carbon in the forms of paint and varnish appeared in

382

many kinds of products. As mentioned earlier, some portions of carbon in paints and coatings

383

diffuse into air with solvent evaporation after painting.51 However, as long as paint and varnish

384

are included in long-lifetime products such as automobiles and constructions, the remained

385

portion of retained carbon in paint and varnish can be preserved in products during their lifetime.

386

This implies that the extension of product lifetime will also contribute to carbon preservation in

387

society as well as reducing frequent use of carbon in dissipative forms.

388 389

3.4. Implications and Future Direction of the Research. This study quantified and

390

visualized the flow of carbon in Japanese economy in 2011. Furthermore, the recoverability of

391

carbon in EoL stage of carbon end-uses was discussed referring to related recycling technologies.

392

While EoL carbon end-uses involve incineration aiming at energy recovery in many current

393

cases, emerging technologies leading to preservation of carbon in society are under development.

394

At the same time, in the policy side, the quality of recycling and the subsequent preservation of

395

materials are indicated as two of the essential components of the ongoing discussion on the

396

concept and definition of circular economy.61-63 Japan, who has 3R (reduce, reuse, and recycle)

397

laws, is also investigating the need for updating the laws with advanced recycling system.49, 64 As

398

shown in Figure 3, passenger motor cars and plastic products are the main resin-using products,

399

as observed by analyzing the carbon flows starting from the form-determining sectors. Under the

400

current law in Japan, for example, material recovery of resins from ASR generated during EoL

401

automobile treatment is not expressly required, and ASR is mainly treated by incineration with


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402

energy recovery.56 In addition, small plastic products other than packaging and/or container are

403

not separately collected as plastic waste in the current system.64 To establish a quality carbon

404

cycle in our society as well as a global cycle, interaction between technologists and policymakers

405

is highly expected. This study can facilitate such an interaction by numerically showing the

406

necessity and potential of implementing appropriate technologies and policies.

407

To provide more practical suggestions, dynamic MFA considering product lifetimes will be a

408

future direction of the study to determine the release of carbon or its return to the society under

409

the current or advanced recycling system by applying MaTrace model.65-67 In addition,

410

optimization of selected technologies among multiple recycling options for satisfying the

411

objectives (e.g., minimization of GHG emission and/or cost) can be conducted by applying IO-

412

based optimization methods.68-70 The issue of contamination by plasticizers for specific types of

413

plastic resins in recycling as well as the sanitary issue for food-contacting use of recycled resins

414

should also be considered as constraints in the optimization by tracing flows passing through

415

sectors in which such problems may occur.71-73 Furthermore, the contribution of demand side

416

such as household consumption74-77 to the carbon cycle will be evaluated as a future direction

417

because behaviors of the demand side may directly affect the carbon release especially from

418

relatively short-lifetime products such as cosmetics and beverage bottles. Even if the government

419

dispenses the law for waste separation and collection toward recycling enhancement, the effect

420

of the law would depend on the behavior of people. In this regard, a decomposition analysis for

421

household consumption categorized into several groups by ages or incomes74-77 will be effective

422

for determining the groups that own much carbon in their households, and thus, can mainly serve

423

toward the development of carbon-cycling society. In the decomposition analysis, the future

424

trend of carbon retention will also be estimated by adjusting to the population change


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425

prediction.74 This series of studies will provide some directions toward the development of low-

426

carbon circular economy.

427 428

ASSOCIATED CONTENT

429

Supporting Information. This Supporting Information is available free of charge via the

430

Internet at http://pubs.acs.org.

431

Detailed list of sets, sector classification and categorization, calculation of C-ratio of aggregated

432

sectors, flowchart of calculation procedure, determination of yield ratio for inputs of packaging

433

materials, method to determine material composition of products by forms of carbon, details of

434

sector aggregation, carbon contents of products of the sectors in IO table, the test for the

435

accuracy of the model, supplemental data for Figure 2 and Figure 3, and graph of top 10 woody-

436

carbon-demanding sectors (PDF).

437

Matrices and vectors used in the calculation (Microsoft Excel spreadsheet).

438

AUTHOR INFORMATION

439

Corresponding Author

440

*Tel.: +81-22-795-5869; E-mail: [email protected].

441

Notes

442

The authors declare no competing financial interests.

443

ACKNOWLEDGMENT

444

This study was supported by the Japan Society for the Promotion of Science (JSPS) (KAKENHI

445

16K20914).


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446

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