Sustainability Metrics for Biomass-Based Carbon Chemicals - ACS

Research Article pubs.acs.org/journal/ascecg

Sustainability Metrics for Biomass-Based Carbon Chemicals István T. Horváth,*,† Edit Cséfalvay,†,‡ László T. Mika,†,§ and Máté Debreczeni†,§ †

Department of Biology and Chemistry, City University of Hong Kong, Kowloon, Hong Kong Department of Energy Engineering, Budapest University of Technology and Economics, H-1111 Budapest, Hungary § Department of Chemical and Environmental Process Engineering, Budapest University of Technology and Economics, H-1111 Budapest, Hungary ‡

S Supporting Information *

ABSTRACT: The application of biomass-based resources for the production of chemicals could slow the depletion rate of fossil reserves and enable the development of a sustainable chemical industry. Three sustainability metrics, the sustainability value of resource replacement (SVrep), the sustainability value of the fate of waste (SVwaste), and the sustainability indicator (SUSind), were defined for biomass-based carbon chemicals by using the ethanol equivalent (EE) as a common currency. These sustainability metrics were calculated for ethylene, propylene, toluene, p-xylene, styrene, and ethylene oxide in the U.S.A. for 2008 and 2014. Our calculations are based on the initial chemical dehydration of corn-ethanol to ethylene followed by its conversion by existing chemical processes. These basic chemicals cannot be produced sustainably at this time primarily due to the limited availability of bioethanol. Consequently, bioethanol-based carbon products should only be labeled “sustainable” when the necessary biomass is available to produce the required bioethanol, independently of social and economic changes. The waste management of the processes shows much better sustainability values than the resource management, due to the successful greening of petrochemical processes. KEYWORDS: Sustainability, Ethanol equivalent, Metrics, Biomass-based basic chemicals volution combines the selection of the fittest with reproduction and variation to diversify the biological portfolio for successful survival even under unexpected environmental conditions.1 A paradigm changing contribution of the evolving human race occurred when the selection was assisted by early mankind’s conscious choices to increase the probability of survival. The rapid broadening of the scope of choices has led to many suitable developments,2 which have increased significantly the survival rate of mankind and contributed to the development of an increasingly sophisticated society enjoying a higher and higher quality of life. It was recognized by Thomas Malthus in 1798 that the exponential growth of population could surpass adequate food production, which could lead to famine, plagues, and even war.3 To address the unpredictable dynamic interactions between the growing population, food consumption, industrial production, the use of natural resources, and environmental damages,4 the term sustainable development was introduced, though it was poorly defined by the World Commission on Environment and Development in 1987. Their report entitled “Our Common Future”, stated that sustainable development “should meet the needs of the present without compromising the ability of f uture generations to meet their own needs”.5 Unfortunately, the key requisite to accurately predict the needs of future generations

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has been impossible to meet,6 due to the end of history illusion7 and the extremely fast rate of scientific and technical advances.8 Consequently, “sustainability” has frequently been replaced with “suitability” by many stake holders, as they have vested or even conflict of interests to call a “suitable development” erroneously a “sustainable development” to generate profits for businesses, secure funding for NGOs (nongovernmental organizations) and environmentalists, or get elected or re-elected as politicians at the expense of the environment. Unfortunately, the definition of sustainable development by the World Commission on Environment and Development5 was easily interpreted differently and used to label suitable developments as sustainable. Consequently, it has been difficult to define proper metrics to its measurement and evaluation. Addressing the issue as to how to define sustainability, a group of professionals from scientific, engineering, economic, and ecology background at the U.S. EPA formulated a new definition: “Sustainability occurs when we maintain or improve the material and social conditions for human health and the Received: December 16, 2016 Revised: January 31, 2017 Published: February 21, 2017 2734

DOI: 10.1021/acssuschemeng.6b03074 ACS Sustainable Chem. Eng. 2017, 5, 2734−2740

Research Article

ACS Sustainable Chemistry & Engineering

simplest metrics are the atom economy by Trost15 and the Efactor by Sheldon.16 An advanced method, using the E-factor, total energy efficiency, land use, and capital and raw material costs, was developed for the comparison of chemicals produced by petrochemical- and biomass-based routes.17 The incorporation of costs analysis could lead to subjective and therefore unexpectedly different results. We have recently reported a refined definition of sustainability: the Earth’s natural resources, including energy, should be used at a rate at which they can be replaced naturally, and the generation of waste cannot be faster than the rate of their remediation.18 Thus, sustainability is an intrinsic property of a molecule, a material, an energy source, a reaction, a process, a building, a village, a town, or a country and is independent of social and economic issues. While appropriate economic and social activities could help to expand sustainability, premature and/or poor understanding of the issues or the vested and sometimes even conflicts of interests for the stakeholders could lead to suitable developments with minimal or not even identifiable sustainable components. We now report a novel set of metrics to evaluate the sustainability of carbon-based chemicals by addressing and merging the resource and waste issues. We have defined the sustainability values of resource replacement (SVrep) and the sustainability values of the fate of waste (SVwaste), which were used to establish a sustainability indicator (SUSind) to assess the sustainability of fossil- and biomass-based carbon chemicals. It is important to emphasize that the metrics are based on the ethanol equivalent as a common currency to bring all types of carbon chemicals to a comparable domain by using molar equivalency.18 All calculations are reported in the Supporting Information (SI) in detail with appropriate references.

environment over time without exceeding the ecological capabilities that support them”.9 Although the accountability has improved, the incorporation of costs could lead to subjective and therefore vastly different evaluations. Considering sustainability, over 500 different sustainability indices have been developed in the last 25 years.10 They can be classified according to three aspects of sustainability leading to one-dimensional ecological, economic, and sociological metrics. Two-dimensional metrics were also identified such as ecoefficiency, socio-ecological, and socio-economic metrics. Finally, three-dimensional metrics have been developed by intersecting different aspects of sustainability. Four sustainable development indicators were suggested for a wide range of process systems by Sikdar including material intensity, energy intensity, potential chemical risk, and potential environmental impact.9 The first two indicators are directly correlating to the process itself, and the latter two provide information on risk to human health and the environmental impact. Energy intensity is defined as the amount of nonrenewable energy used during the production of a unit mass of product.11 Although energy intensity represents a popular indicator, it hardly meets the requirements of sustainability as the use of nonrenewable energy is far from sustainable. A technology which extracts resources f rom the ecosphere has to consume the energy at the same or lower rate than the rate of the resource reproduction.12 This approach states that renewable resources shall be included in the production technology. A renewability factor was introduced to reflect the fraction of renewable exergy in total exergy consumption. Exergy analyses can be the answer for the need for quantitative data dealing with processes consuming natural resources. According to the thermodynamic concept of sustainability during the conversion processes, the natural resources either in the form of material or energy are converted to consumer materials and heat. The energy represents a loss that results in a given extent of loss of work-potential. The theoretical work-potential is a universal measure in exergy analyses.13 Exergy of a system is the maximum useful work possible during a process that brings the system into equilibrium with a heat reservoir. Exergy analysis calculates with lost-work; it determines production efficiency and environmental efficiency, which then is multiplied to give the overall exergetic efficiency. The sustainability coefficient is derived by the arithmetic mean of the renewability factor and the overall exergetic efficiency. Their work represented a breakthrough in illustration and quantification of the sustainability of a technology. Sustainability indicators are often defined as performance factors taking into account economic and environmental performances of processes, which later can be implemented into life-cycle assessment analysis.14 It is important to emphasize that life-cycle analysis has not been the purpose of our work. Focusing on another important aspect of sustainability, the fate of wastes has to be considered. Materials used from the ecosphere can be converted to useful products, and the wastes can be emitted back to the ecosphere. It was emphasized by de Swaan Arons that a sustainable technology should not emit harmful products at all.12 Because we cannot completely avoid the formation of wastes, their treatment either by the natural decomposition or manmade technologies have to be considered during the evaluation of the sustainability of a technology. Several studies have formulated green chemistry metrics for the description of chemical reactions and technologies. The two

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SUSTAINABILITY VALUE OF RESOURCE REPLACEMENT (SVREP) The sustainability value of resource replacement (SVrep) is defined to provide a method to calculate how much of the necessary resources (EEnecessary resource) required during a given time (tconsumption) can be replaced in a given time (treplacement), by using the available biomass-based resources (EEavailable resource) and the best replacement technologies, which have measurable effectiveness (ERoE) (eq 1a). While the EEnecessary resource is the sum of EE based on carbon-atom equivalency according to the overall yield of the production from bioethanol and the EE of standard enthalpy of the reaction (EEoverall standard enthalpy of reaction), the EEavailable resource is the total amount of bioethanol available on the market. In order to ensure reliable reference data, the initial calculations were based on the first-generation corn-based bioethanol technology practiced in the U.S. in 2008.19 The measurable effectiveness of the reference technology is characterized by its ethanol return of ethanol or ERoE (the same as EROI20), which is 2.3 indicating that out of the total of 3.3 units of bioethanol, 1 unit is required for the production of 2.3 units of bioethanol.20 Thus, the total EEavailable resource has to be multiplied by 2.3/(1 + 2.3) = 0.7 to secure the sustainable production of the resource. ERoE ERoE · ·EEavailable resource 1 + ERoE 2.3

SVrep =

EEnecessary resource tconsumption

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+ EEsecondary resource

t replacement

(1a)


Research Article

ACS Sustainable Chemistry & Engineering Table 1. Scenarios for SVrep, Selected Cases case

EEavailable resource

EEsecondary resource

treplacement [year]

ERoE

SVrep

1 2 3 4 5 6 7 8 9

0

0 0 0

1 1 1 1 10 10 1 1 1

2.3 2.3 2.3 2.3 2.3 2.3 4.1 4.1 58

0 0.7 ≥1 ≥1 0.7 ≥1 1.43 1 24.79

= EEnecessary resource ≥1.43 × EEnecessary resource 0 10 × EEnecessary resource ≥1.43 × 10 × EEnecessary resource = EEnecessary resource 0 = EEnecessary resource

≥EEnecessary resource 0 0 0 = EEnecessary resource 0

production technology, which is expressed by (ERoE/ 1+ERoE). For example, the best second- and third-generation biomass technologies could increase the ERoE to 6.5 (e.g., 1 unit of bioethanol can produce 5.5 units of bioethanol).24 Even more dramatic development was reported using algae with an ERoE = 23 or an improvement by a factor of 1025 of the reference technology.19 The possible values of SVrep for selected scenarios are shown in Table 1. For simplification, EEnecessary resource and tconsumption were assumed to be unity (i.e., 1 million tons and 1 year, respectively). When all the bioethanol is used as fuel component in gasoline blends, no available secondary resources remain for the production of chemicals, and therefore, it is a nonsustainable situation, which means that SVrep equals zero (see case 1). When the available resource equals the quantity of necessary resource, it is still not sustainable because the ethanol production needs 1 unit of energy input, which should be covered from ethanol as well and results in an SVrep value of 0.7 (case 2). The available resource shall be at least 1.43-times higher than the necessary resource to reach sustainable resource replacement: SVrep = 1 (case 3). Cases 1−3 relate to the unity required resource and assumes that no secondary resource is available and that one year is the time of consumption and replacement (e.g., under continental climate, the replacement for corn-ethanol is 1 year). As the quantity of the available resource overcomes this critical value of 1.43 (which is the efficiency of the current ethanol production technology: (1 + 2.3)/2.3 = 1.43), the replacement becomes sustainable (SVrep ≥ 1). Considering the effect of secondary resource, it can be stated that in spite of nonavailable resource from primary source, the replacement can be treated as sustainable when the secondary resource originates from another technology at equal or greater volume as the necessary demand (case 4). A key factor in sustainable replacement is the time: if it is much longer than the time of the consumption, the replacement cannot be sustainable unless an enormous volume of primary or secondary resources are available for utilization (case 5). However, if the time of replacement takes 10 years and the available resource is 10 × 1.43 times more than the necessary resource simultaneously, the replacement is sustainable (case 6). By the improvement of the bioethanol production technology (ERoE > 2.3), the value of SVrep can definitely be increased (cases 7−9), but the input energy no doubt should be covered by bioethanol. To achieve sustainable replacement, the (ERoE+1)/ERoE-times higher EEavailable resource should be accessed as standing facilities, or EEsecondary resource shall cover the whole EEnecessary resource.

Ideally, the available resource should be at least equal with the necessary resource and the amount required to produce the necessary resource during the same time period to avoid either resource shortage or overproduction leading to storage issues. In general, the resource pool could be increased by using the same chemical as a secondary resource (EEsecondary resource) from another biomass-based technology. For example, nitro-benzene is produced by the reaction of benzene with nitric acid in the presence of sulfuric acid.21 The starting feedstock benzene could be produced directly from ethanol or by the Selective Toluene Disproportionation Process (STDP) as one of the side products.22 In the STDP process, toluene is catalytically converted to the mixture of o-, m-, and p-xylenes and benzene in the presence of ZSM-5 zeolite. Because the p-xylene and benzene exit the cages of the zeolite 1000-times faster than the o- and m-xylenes, the latter two must isomerize to p-xylene to maintain the thermodynamic equilibria resulting in selectivities of 87% p-xylene, 6.5% o-xylenes, 6.5% m-xylenes, and 100% benzene. Because the desired product of the process is p-xylene, the o- and m-xylenes as well as the benzene could be considered as side products or wastes. Because two molecules of toluene are converted to the mixture of xylenes and benzene in a ratio of 1:1, the overall yield of p-xylene is 43.5%. However, if one these chemicals is used as a resource in another technology, it will increase the bioethanol-based available resources. Of course, this type of waste/resource integrations have been mastered by today’s petrochemical refineries and will be a great challenge for future biorefineries. The rearrangement of eq 1a to eq 1b reveals that SVrep has two factors: EE of the resource produced by a given effectiveness of the replacement technology and a time ratio. SVrep =

ERoE ERoE · ·EEavailable resource 1 + ERoE 2.3

+ EEsecondary resource tconsumption · EEnecessary resource treplacement

(1b)

Although these factors are dimensionless, we shall emphasize that all parameters have to be substituted in the same dimension: million tons in the case of EE, and year in the case of time. Thus, the SVrep must be zero, or the replacement is not sustainable at all, when there is no available resource or the replacement takes an extremely long time. The latter is especially true for all fossil resources as their formation took several hundred million years23 and will be used up in a few hundred years. The minimal sustainability requires that SVrep is equal to 1, which could be achieved when (ERoE)2· EE available resource /2.3·(1+ERoE) + EE secondary resource = (EEnecessary resource) and treplacement = tconsumption. The sustainability increases when plenty of resources are available, and the replacement time is less than the consumption time. The estimated SVrep also depends on the effectiveness of the 2736



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SUSTAINABILITY VALUE OF THE FATE OF WASTE The sustainability value of the fate of waste (SVwaste) is defined to be equal to one, when the continuously produced generated waste (EEgenerated waste) is equal to the continuously treated wastes (EEtreated waste) in the same time period (twaste generation = twaste treatment) and no wastes are released to the environment (EEuntreated waste = 0 and twaste natural decomposition = 0) (eq 2). The treatment methods could include incineration, chemical and biological treatment, and disposal to official waste storage sites including landfills. All wastes released to the environment must be considered as untreated waste. SVwaste =

EEgenerated waste

(2)

It should be noted that the following assumptions have been made: (a) if no waste treatment method is applied, the associated EE and time must be equal to zero; (b) the time of natural decomposition of untreated waste should include 1 year (if waste generation time is assumed to be 1 year) and the time needed to reach the local governments’ regulation level in the environment based on its natural half-life; (c) in the case of multistep technologies with the potential to generate wastes in each step, the longest time of natural decomposition should be used for the overall process; and (d) in the case of contamination of air, water, or soil, the longest time of natural decomposition should be used. If a side or minor product can be utilized in another technology, it should be considered as a secondary resource and not as a waste. Because the sum of EEtreated waste and EEuntreated waste must be equal to EEgenerated waste, all waste streams are accounted for. By selecting the longest time for the natural decomposition of untreated waste, the persistent component of the waste with the longest half-life will significantly lower SVwaste. In other words, prevention of persistent waste formation by the development of efficient processes or integrated valorization of the wastes are the preferred path to reach sustainability.

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SUSTAINABILITY INDICATOR The sustainability indicator (SUS ind) is based on the sustainability values of resource replacement (SVrep) and the sustainability values of fate of the waste (SVwaste) by merging a two-body issue to a single one (eq 3).26 SUSind =

SUSTAINABILITY OF CARBON-BASED CHEMICALS

Today, almost all of the carbon-based chemicals are produced from fossil resources with very high carbon-efficiency in oil refineries developed in the last 60 years.28 Unfortunately, fossil resources are not sustainable according to our definition,18 as the time of their replacement (hundreds of millions of years) is much longer than the rate of their consumption (several hundred years).23 Furthermore, their use for combustion to produce energy is also not sustainable, as the generation of carbon dioxide is much faster than the rate of remediation of carbon dioxide via photosynthesis. Consequently, the carbon dioxide level increased from 317 ppm in 1959 to 407 ppm by the summer of 2016,29 and there is no indication that the trend will slow in the near future. In addition, the generation of energy from biomass is already beyond reality as we do not have enough farmable land to produce food and energy crops simultaneously to satisfy both the food and energy demand of the rapidly growing population. Although the development of biomass-based carbon chemicals has begun some time ago,30 besides bioethanol19 and biodiesel31 only a few processes have been commercialized.32,33 In addition, the unpredictable price of fossil resources has a major impact on the profitability of carbon-based chemicals produced from biomass. However, when the fossil resources will be depleted to the level that their price will become prohibitively high for use, carbon-based chemicals have to be made either from carbon dioxide or biomass. Therefore, we should focus on the development of biomass-based carbon chemicals and the transformation of petrochemical to biochemical refineries. It should be noted that only bioethanol is produced in volumes large enough to be considered as the feedstock for the production of basic carbon chemicals. We have designed a bioethanol-based production scheme of ethylene, propylene, toluene, p-xylene, styrene, ethylene oxide, syngas, and hydrogen involving intermediates 1- and 2butenes, hexane, 1-hexenes, heptane, 1-heptenes, 1-heptanal, and 1-heptanol (Scheme 1.) utilizing demonstrated reactions and processes (see SI for details and references). In order to demonstrate the applicability of the sustainability metrics, we have calculated the sustainability values of resource replacement (SVrep), the sustainability values of the fate of the

EE treated waste + EE untreated waste t waste treatment + t waste natural decomposition t waste generation

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Scheme 1. Ethanol-Based Production of Ethylene, Propylene, Toluene, Xylenes, Styrene, and Ethylene Oxide (See SI for Details)

SVrep·SVwaste SVrep + SVwaste

(3)

Consequently, sustainability requires that all resources must be replaced (SVrep = 1) and that all wastes can be recycled or the remaining parts treated (SVwaste = 1) in time. While SUSind ≥ 0.5 is indicating equal or better than required sustainability, SUSind < 0.5 shows unsustainable situations. It should be emphasized that sustainability can be significantly improved by appropriate social and economic activities. However, we could not define meaningful sustainability values of social activities (SVsoc) and/or economic activities (SVecon) due to the unpredictable impact of vested or conflicts of interests for some of the stakeholders. However, if reliable definitions can be developed for SVsoc and SVecon, the sustainability indicator (SUSind) could be extended to define a three- or four-body system.27 2737


0.96 0.113

2008 22.5 37.47 1.206 38.68 25.46 0.00 0.00 27.8 7.08 0.128 0.37 0.00 0.00 0.37 1.044 0.96 0.166

2014 24.7 40.91 1.319 42.23 28.1 0.00 0.00 42.8 12.03 0.200 0.41 0.00 0.00 0.41

ethylene

0.48 0.101

2008 14.8 47.26 0.177 47.43 31.22 0.00 0.00 27.8 8.68 0.128 7.62 0.00 2.70 4.92 1.099 0.48 0.141

2014 13.3 42.39 0.159 42.55 28.31 0.00 0.00 42.8 12.13 0.200 6.83 0.00 2.42 4.41

propylene

0.49 0.101

2008 3.13 8.76 −0.078 8.68 5.71 0.00 0.00 27.8 1.59 0.128 2.08 0.00 2.02 0.06 1.044 0.49 0.142

2014 4.99 13.94 −0.124 13.82 9.20 0.00 0.00 42.8 3.94 0.200 3.31 0.00 3.21 0.09

toluene

0.49 0.101

2008 5.41 49.37 −1.287 48.08 31.65 0.00 0.00 27.8 8.80 0.128 12.02 0.00 11.61 0.41 1.044 0.49 0.142

2014 4.80 43.70 −1.141 42.56 28.32 0.00 0.00 42.8 12.13 0.200 10.64 0.00 10.27 0.36

p-xylene

0.88 0.365

2008 4.1 5.62 0.191 5.81 3.82 16.21 2.89 27.8 1.06 0.625 0.17 0.00 0.00 0.17 1.137 0.88 0.429

2014 4.40 6.05 0.205 6.25 4.16 14.35 3.99 42.8 1.78 0.837 0.19 0.00 0.00 0.19

styrene

0.62 0.106

2008 2.9 3.40 −0.151 3.25 2.14 0.00 0.00 27.8 0.59 0.128 0.37 0.00 0.00 0.37 1.601 0.62 0.151

2014 2.54 2.97 −0.132 2.84 1.89 0.00 0.00 42.8 0.81 0.200 0.32 0.00 0.00 0.32

ethylene oxide

2014 54.65 149.97 0.285 150.26 100 14.35 3.99 42.8 0.226 21.70 0.00 15.91 5.79 n.a. n.a. n.a.

2008 52.84 151.87 0.058 151.93 100 16.21 2.89 27.8 0.147 22.63 0.00 16.33 6.30 n.a. n.a. n.a

Σ chemicals

a

mt = million tonnes. bThe chemical is completely consumed within 1 year. cBased on carbon-atom molar equivalency of the chemicals and the combined yields (conversion × selectivity) of all reactions from ethanol to the chemical. dOverall standard enthalpy of reaction: the sum of each reaction’s enthalpy. e(EEnecessary resource/EEΣChemicals)·100 fThe amount of benzene available from the xylene and styrene production. gBioethanol production in the U.S.A. in 2008 and 2014. h(EEnecessary resource/EEΣChemicals)·EEavailable resource including the proportional EE of recycled benzene originating from xylene and styrene production.

produced amount [mt/year]a,b EErequired resource [mt]c EEoverall standard enthalpy of reaction [mt]d EEnecessary resource [mt] EEproportional necessary resource [%]e EEsecondary resource of benzenef [mt] EEproportional secondary resource [mt] EEavailable resourceg [mt/year] EEproportional available resource [mt/year]h SVrep2.3 EEgenerated waste [mt/year] EErecycled waste [mt/year] EEtreated waste [mt/year] EEuntreated waste [mt] 1 + twaste natural decomposition [year] SVwaste SUSind

basic chemicals

Table 2. Sustainability Analysis of Basic Chemicals in the U.S.A. in 2008 and 2014

ACS Sustainable Chemistry & Engineering Research Article

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Figure 1. Sustainability metrics of basic chemicals in the U.S.A. in 2008 and 2014.

waste (SVwaste), and the sustainability indicators (SUSind) for six basic chemicals including ethylene, propylene, toluene, pxylene, styrene, and ethylene oxide if they were produced from bioethanol by known reactions and processes (Scheme 1) for the consumed quantities in the U.S.A. in 2008 and 2014 (Table 2 and Figure 1). If all these basic chemicals have to be produced from biomass, the total amount of required bioethanol would have been 151.93 mt in 2008 and 150.26 mt in 2014, which were significantly more than the total bioethanol production of 27.8 mt in 2008 and 42.8 mt in 2014. Because we could not decide how to distribute the limited amount of bioethanol, we have used it proportionally for each chemical. The SVrep for these chemicals were between 0.128−0.625 in 2008 and 0.200−0.837 in 2014, and therefore, none of them were sustainable in both years. It should be noted, that the SVrep of styrene is much higher than the others due to the utilization of benzene, produced as a side product in the production of p-xylenes, as a secondary resource. This result indicates that integration of side and waste products could significantly lower resource demand and increase sustainability. The SVrep for these chemicals would be much smaller, if we would have taken out all the bioethanol burned as gasoline additive. In order to achieve sustainability, the total available bioethanol should be equal or higher than 151.93 mt. This would require the production of 515 million tons of corn, which would be 1.67 times the 308 million tonnes of total corn produced in the U.S. in 2008. Subsequently, 53 million hectare land or 40.2% of the total of 132 million hectares of farmed land should be used for corn production. Of course, the demand for bioethanol, corn, and arable land would be significantly higher if bioethanol should be used as fuel in the same time (as the calculations in this paper used every drop of bioethanol for the production of carbon-based chemicals). One possible approach to secure the required 151.93 mt bioethanol is to improve the ERoE from 2.3 to at least 18, a formidable challenge on production technologies. Alternatively, the valorization of agricultural residues and food wastes could also contribute significant amount of bioethanol,34 not to mention the beneficial effect on waste management of highly populated areas. The EE of the generated wastes (EEgenerated waste) for each chemical is based on the EE of wastes in each step shown in Scheme 1 and detailed in the Supporting Information. The SVwaste values for these chemicals were between 0.48−0.96 in both 2008 and 2014, and therefore, none of them were sustainable in both years. Although waste prevention was the best (SVwaste = 0.96) for the dehydration of

ethanol to ethylene, all the other technologies involve processes with the formation of significant amounts of wastes (see SI for detailed information). For example, the production of propylene from ethanol characterized by SVwaste value of 0.48 due to the low yield of the isomerization of butene-1 to butene2. Because both the SVrep and SVwaste values are below 1, the sustainability indicators for all these chemicals are between 0.1−0.429, indicating that none of them could be produced by sustainable processes. Therefore, bioethanol-based carbon products should be labeled “sustainable” only when the necessary land is available to produce the biomass and bioethanol independently of social and economic changes. In conclusion, we have shown that the sustainability values of resource replacement (SVrep), the sustainability values of the fate of waste (SVwaste), and the sustainability indicator (SUSind) can be used to measure the sustainability of carbon chemicals produced from bioethanol. All the six basic chemicals analyzed in this paper cannot be produced sustainably at this time because of the limited availability of bioethanol. However, a 10fold increase of the volume of bioethanol could easily result in a completely biomass-based production of carbon-based chemicals. The waste management of the processes shows higher sustainability values than the resource management, due to the mostly successful waste prevention and process integration of contemporary petrochemical technologies. Our results suggest that chemical companies could easily achieve sustainability based on the molar equivalency of the total carbon atoms used by simply owning enough land to produce their renewable resources. Of course, the position and size of the lands and the crops produced should be carefully selected to secure sustainable farming and production technologies. Although sustainability may be reached on a molar basis, an additional challenge is to cover the associated energy demand of production processes, storage, and transportation by renewable energy. The sustainability of biomassbased energy systems will be addressed in details in a forthcoming article. Finally, we hope that our definition18 and metrics described in this paper have addressed most of the suggestions and concerns with respect to sustainability.35

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b03074. 2739


Research Article


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Additional data and references, as noted in the text (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

István T. Horváth: 0000-0002-6324-186X László T. Mika: 0000-0002-8520-0065 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Some of this work was funded by the Innovation and Technology Support Program of the Innovation and Technology Fund of the Government of the Hong Kong SAR (ITS/079/13). Any opinions, findings, conclusions, or recommendations expressed in this material (or by members of the project team) do not reflect the views of the Government of the Hong Kong SAR, the Innovation and Technology Commission or the Panel of Assessors for the Innovation and Technology Support Program of the Innovation and Technology Fund. We also thank the Environment and Conservation Fund (ECF/31/2014) for partial financial support. L.T. Mika is grateful to the support of János Bolyai Research Scholarship of the Hungarian Academy of Sciences. E. Cséfalvay thanks the support of “BME R+D+I project”, sponsored by the grant TÁ MOP 4.2.1/B-09/1/KMR-20100002, Budapest University of Technology and Economics.

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Sustainability Metrics for Biomass-Based Carbon Chemicals - ACS

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