Terminology used for renewable liquid and gaseous fuels based on the conversion of electricity: a review

Accepted Manuscript Terminology used for renewable liquid and gaseous fuels based on the conversion of electricity: a review Iva Ridjan, M.Eng, PhD Fellow, Brian Vad Mathiesen, PhD, M.Sc, Professor with Specific Responsibilities, David Connolly, PhD, B.Eng, Assistant Professor PII:

S0959-6526(15)00696-4

DOI:

10.1016/j.jclepro.2015.05.117

Reference:

JCLP 5631

To appear in:

Journal of Cleaner Production

Received Date: 27 January 2015 Revised Date:

13 May 2015

Accepted Date: 27 May 2015

Please cite this article as: Ridjan I, Mathiesen BV, Connolly D, Terminology used for renewable liquid and gaseous fuels based on the conversion of electricity: a review, Journal of Cleaner Production (2015), doi: 10.1016/j.jclepro.2015.05.117. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Iva Ridjan, M.Eng 1 PhD Fellow Phone: +45 9940 2950 [email protected]

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Brian Vad Mathiesen, PhD, M.Sc.1 Professor with Specific Responsibilities Phone: +45 9940 7218 [email protected]

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Terminology used for renewable liquid and gaseous fuels based on the conversion of electricity: a review

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David Connolly, PhD, B.Eng 1 Assistant Professor Phone: +45 9940 2483 [email protected]

Abstract

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As the transport sector transitions away from fossil fuels and renewable fuels shift into focus, it is important that the terminology around renewable fuels is clarified. A number of terms such as synthetic fuel and electrofuel are used to describe both renewable and alternative fuels. The aim of this article is to identify and review these terms to avoid any potential misuse. An integrative review of terminology has been made. This review did not differentiate the articles in terms of the methodologies applied, but had the main objective to identify the terminology used and its definition. The results confirm that the term synthetic fuel is used generically in the majority of articles, without providing information about the production process of the fuel or differentiating between fossil-based and renewable-based synthetic fuels. The majority of the articles use the term synthetic fuel to describe fuels produced with coal-, gas- and biomass-to-liquid (xTL) technologies. However, a number of articles use the term beyond this definition. Results for the term electrofuel gave a similar outcome, as it was not clear which processes were used for the fuel production. In some cases, both synthetic and electrofuel referred to fuels produced through the same process, even though in reality the two processes are distinctly different. This could lead to a misinterpretation, especially if the terminology is utilized by policymakers. To prevent this, the article ends with a preliminary proposal for how to differentiate synthetic fuels from electrofuels based on the production process. Keywords: electrofuel, synthetic fuel, renewable fuels

1. Introduction

Transport is the backbone of our society and a true indicator of a country’s economy, development and sustainability [1]. In a transport sector with growing demand, renewable fuels are increasingly becoming vital for a sustainable future. The transport system is very complex with different needs and modes; hence, the conversion to a renewable, low-carbon fuel system is very slow. The existing transport infrastructure is well developed for petroleum derived liquid fuels, supplying more than 90% of the transport demand [2]. 1

All authors are affiliated to: Sustainable Energy Planning Research Group, Department of Development and Planning, Aalborg University, A.C.Meyers Vænge 15, DK-2450 Copenhagen

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Changing the infrastructure is a very cost-intensive and time-consuming process [3]. The latter should not be disregarded when introducing different fuel alternatives that achieve a reduction in emissions, increased sustainability, and a shift towards carbon free transport. Many proposed fuel alternatives require a significant alteration of the transport infrastructure, since their technical properties are not compatible with the existing infrastructure [4]. To avoid a transformation of the whole transport infrastructure, fuels that can be utilized in the existing infrastructure would be preferable. It is also important to consider the consumer behaviour and the willingness of the consumers to adapt to and pay for the new transport options [5,6].

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Renewable and alternative fuels are very well reported in research. However, renewable and alternative should not be used interchangeably as the terms do not necessarily refer to the same fuels. Renewable fuels utilise renewable resources for fuel production, including a variety of fuels that use biomass and other renewable energy processes [7]. Alternative fuels, on the other hand, are defined as any alternative to gasoline that can be produced without restrictions on the type of feedstock resource, meaning that they can either be derived from renewable or fossil resources [8]. A variety of production processes are available for renewable and alternative liquid or gaseous fuels, either on a small or on a large scale.

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The conversion of the transport sector to renewable energy is a long-term goal for many countries worldwide. Therefore, only fuels that are based on renewable sources and can be utilised in systems with a high share of renewable sources are of interest in this article. Favourable processes are processes without the direct usage of fossil fuels and in some cases without the use of biomass resources. By generating hydrogen through water electrolysis using electrolysers, hydrogen can be bound to a carbon source in a process called hydrogenation. This process produces synthetic gas and was introduced in 1977 by Steinberg [9].The carbon source can be derived from recycling CO2 from stationary [10] or non-stationary sources [11]. The carbon source can also be derived from biomass through a gasification process, if the produced gas is further upgraded with hydrogen. When the synthetic gas is produced, it can be converted through a certain type of chemical synthesis that will define the fuel output. There are various fuel outputs which should be defined based on both the needs of the demand side and the infrastructure available. In addition to this process, there are other renewable fuel production processes which exclude fossil fuel input and convert electricity into fuel, such as microbial electrosynthesis [12]. These processes, however, are not related to the specific focus of this article and the definition of the terminology.

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In the ideal concept, fuel is carbon neutral, since all parts of the production cycle are carbon neutral; the electrolysers are powered by a renewable electricity source and the carbon is derived from recycling CO2 emissions from biomass combustion. The integration of electrolysis into the production of transport fuel is beneficial from a system point of view, since electrolysers can balance intermittent renewable sources by consuming surplus electricity [13]. The production of hydrocarbons by recycling carbon emissions from energy and industrial processes is prioritised over biomass carbon, due to constraints on biomass resources, land use conflicts, issues raised around biofuel sustainability, and the climate effect [14,15]. The fuels that are produced from CO2 emissions and water resources and not directly tied to biomass resources and are theoretically capable of meeting the world’s fuel demand. The primary objectives of this article are to investigate the terminology which is mostly used for fuels produced through the conversion of electricity through electrolysis and the relation between the terminology used and the production process itself. Two terms have been identified which define these types of fuels: synthetic fuel and electrofuel. In the literature, there seems to be no clear definition of any of these terms; the terms are used for a variety of different processes and utilised resources. This was confirmed by the review process which showed that, even though there are general definitions of synthetic

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ACCEPTED MANUSCRIPT and electrofuels, these terms are not used accordingly nor is it possible to precisely define the production process to which the terms refer.

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Differentiating between synthetic fuels and electrofuels is not a key concern today, since electrofuel plants are still only being developed as demonstration plants [16,17]. However, in the future, electrofuels are expected to play a key role in low-carbon energy systems. This has been quantified in [18,19] using the Smart Energy System concept [20,21] which indicates that electrofuels could represent over 50% of the electricity demand in future low-carbon energy systems. In these systems, it is essential to differentiate between synthetic and electrofuels, since the two production processes have very different impacts on the surrounding energy system. The aim in this article is to ensure that there is a clear distinction in the terminology that reflects the key differences in these production processes. This will ensure that scientists and policymakers have the terminology necessary to distinguish between these two very different fuel processes, which will be essential in the discussion about the future low-carbon energy system.

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An understanding of the current use of these terms in the literature will potentially contribute to better transparency and future use patterns of the terminology. Even though there is no present need for clarifying the use of this terminology, in the future there will be a need to support the right fuel technology development. The fuel terminology will play an important role in the future smart energy systems where fuel production will be associated with renewable energy and cross-sector integration will be necessary in order to accommodate a high share of intermittent renewable resources. The standardization of the terminology is therefore important in order to avoid a potential misunderstanding when discussing the regulatory perspective and technological development.

2. Methodology

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In order to identify the terminology used for targeted production cycles, a review process was carried out. This method was chosen because it makes it possible to gather an overview of the terms used in different literature. The search was primarily conducted using Scopus. The literature review was performed in different steps in order to exclude all non-relevant literature. The search terms were synthetic fuels “OR” electrofuels with the following criteria: only English written articles, book chapters and reviews published between 2006 and 2014 (April). Limits on subject area or source title were not applied.

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The search process and the number of articles that have met the search criteria are shown in Figure 1. The first step of the search resulted in 3,634 articles, which were narrowed down by focusing on the fuel production process. In order to narrow down the search with the exact focus, a new set of terms was included in the review process. The following terms have a strong connection with the production process of interest: CO2 capture, carbon dioxide, hydrogen, alternative fuels, hydrocarbon, biomass and transport. In the second step, by adding the extra terms the search narrowed down to 607 articles and 3 book series. The term “electrofuel” was included in the search; however, articles with this terminology did not appear. Therefore, a separate search including only the term electrofuels was carried out, which resulted in 7 articles. These articles were added to the previous results, making a total of 617 articles going to the third step of the review. In the third step, the abstracts were screened and the articles that described a production process or presented a definition of synthetic or electrofuel were retained. Many articles contained information beyond the scope of this specific review, such as specific catalysts, fuel cells, and biochemical processes. Articles that were considered relevant after the review process were included in the further analysis. After the third step, 147 articles had been retained.

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The articles chosen in the previous step were assessed in order to identify the most commonly used terms and to group the terminology according to its definition and production process. An in-depth review of all the abstracts was conducted in order to perform the categorization. After a thorough review, 115 articles were identified which used the searched terminology, where the definition of the term and the production process could be identified from the abstract. When the terminology was used, but not explained in the abstract, a review of the article was carried out to find the definition or a description of the process used. The 115 articles were categorized according to the term used and subcategorized according to the main resources and conversion processes used. As a result, 7 terms with 21 subdivisions were reported for “synthetic fuels” and 9 terms with 13 subdivisions were reported for gaseous fuels (see Table 1 and Table 2).

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Note that the methodology of the articles was not evaluated, since the objective was to identify the utilised terms and the description of the production process.

3. Results

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The results from the literature review for synthetic fuel and electrofuel are shown in Table 1. Few variants of the term have been reported: synthetic fuel, synthetic liquid fuel, synthetic jet fuel, synthetic diesel and gasoline, and synthetic biofuels. All the grouped articles in which the term and definition are determined add up to 77 articles. From the total number of articles, if sub-grouped according to variants of the term, 21 different processes can be identified. The resources utilised for the production of fuel differ from a fossil input such as coal to a non-fossil input such as carbon dioxide and water. This literature review has found that the term synthetic fuel is most frequently used for coal-, gas-, and biomass-to-liquid (xTL) technologies, which represent a thermochemical conversion. More precisely, 53 articles in all sub-groups refer to xTL processes, corresponding to almost 70 per cent of the total number of articles. However, the remaining 30 per cent of the reviewed articles still refer to different processes which need to be taken into account. The term electrofuel has been identified in five articles in total, all of which use carbon dioxide as a main source for the fuel production, but with different conversion processes. Note that both synthetic fuels and electrofuel are used for defining the production process which we focus on here.

Electrofuel as a term

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The results for gaseous fuels that are related to the term synthetic fuel and that could be used as a mediator for further conversion to liquid fuels are shown in Table 2. There are in total 33 articles which use 9 variations of the term of which some terms have different resource origins or conversion processes.

Searching for the term electrofuel does not deliver many results due to the relative novelty of the term. However, even with a preliminary review of the articles that use this terminology, it is indicated that the term has different meanings, or more precisely refer to different fuel production processes. Overall, the term can be perceived as referring to a means for electricity storage in the form of liquid fuels. There are two main ways of using the term; one which refers to the fuel produced through the biological conversion of carbon dioxide and one which refers to fuel production through the combination of CO2 and hydrogen. The biological conversion of carbon dioxide involves different microorganisms. This process produces energy-dense fuels, where the conversion of electricity into biochemical energy is combined with powering carbon metabolism by converting it to fuel [22-24]. A programme of developing electrofuels was established in 2009 by the US Department of Energy (DOE). The aim of the programme is to research the non-photosynthetic autotrophic organisms for conversion of the electricity from intermittent renewable energy to liquid fuels. In some cases, electrofuels are referred to as advanced biofuels [25].

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ACCEPTED MANUSCRIPT The second definition of electrofuel is given by Pearson et al in [26]. Referring to Olah et al [27], the term electrofuel is used for the production of fuel through the combination of carbon dioxide and hydrogen, in particular with CO2 recycling and hydrogen derived from water electrolysis. The terminology is used in the same way as in Pearson et al [11]. It is noticeable that the same term is used for different types of fuel productions. The overall context is aligned, because all the presented concepts can be perceived as electricity storage, but the production cycles vary significantly.

Synthetic fuel as a term

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By conducting the review, 110 articles were identified that use the term synthetic fuel or that refer to the gaseous phase of a fuel such as syngas, synthetic natural gas, synthetic biogas, and power gas. By comparing 110 articles, it was concluded that this terminology does not necessarily refer to the same fuel production process or source of fuel, which confirms our primary assumption.

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The correlation between the term synthetic fuel and synthetic gas or synthetic natural gas was noticed in most of the reviewed articles. On the condition that synthetic gas or synthetic natural gas is a mediator in the production process of synthetic fuel, the term synthetic is used. Interestingly, when it comes to the resources used for producing synthetic fuels, these can vary from fossil fuels such as coal or natural gas to a carbon source and hydrogen, which is produced with renewable energy through water electrolysis. The conversion of the produced gas to fuel is mainly done by Fischer-Tropsch, but in some cases other types of fuel syntheses are used.

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The term synthetic natural gas or synthetic gas mostly refers to the gasification of biomass or coal. It is important to note the difference between synthetic gas (synthesis gas, syngas) and synthetic natural gas (SNG), as the latter is methanated synthetic gas and has a higher energy density than syngas. In comparison with syngas, which cannot be transported through the natural gas network due to its explosive properties and high concentration of hydrogen, synthetic natural gas (SNG) can be transported through the existing natural gas network. Syngas, synthetic gas and synthesis gas usually refer to a 2:1 mixture of H2 and CO [28]. It is primarily a mixture of hydrogen (H2) and carbon monoxide (CO), but can also contain significant, although lower, concentrations of methane (CH4) and carbon dioxide (CO2), along with smaller amounts of impurities such as chlorides, sulphur compounds, and heavier hydrocarbons [29]. The ratio of hydrogen and carbon monoxide in syngas can vary, depending on the processes used.

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In some cases, the term synthetic natural gas refers to bio-SNG, since biomass has been used to produce the synthetic natural gas [30,31]. In other cases, the word natural is taken out from the term [32], which can be misleading because the composition of synthetic natural gas and synthetic gas is not the same. In some cases, synthesis gas is produced by dry reforming of methane, which is then converted to synthetic gasoline or synthetic diesel [33]. A term like producer gas can also be found in the literature and this has a different meaning depending on the country perspective. In the United States, for example, producer gas is a generic term that among other meanings stands for syngas produced through the gasification of wood, charcoal or coal. In the United Kingdom, the term refers to a suction/producer gas, as a mixture of carbon dioxide and non-flammable gases resulting from the partial combustion of coal or other sources. In most cases, synthetic fuel is defined as a fuel produced by xTL processes, coal-to-liquid (CTL), gas-toliquid (GTL) and biomass-to-liquid (BTL) that all end with Fischer-Tropsch synthesis [34]. From a comparison of these processes, it is obvious that if the term synthetic fuel is used, it cannot be assumed which are the main resources for the production. The term synthetic fuel, referring to the biomass-to-liquid production process, is also defined as biofuel [35]. The classification of biomass-to-liquid as biofuels was seen in all publications by Demirbas included in the review [36-38]. Dahmen et al [39] and Chiche et al [40] use the

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ACCEPTED MANUSCRIPT term synthetic biofuels as a blend term in order to clarify a resource from which fuels are made. The same terminology – synthetic fuels – is also used for fuel produced through co-electrolysis of CO2 and water or by pairing CO2 with hydrogen from water electrolysis, the so-called CO2 hydrogenation (see Table 1). The only term that continually refers to fuels produced with Fischer-Tropsch synthesis by using either coal or natural gas as a resource is synthetic jet fuel. However, there is still no obvious differentiation between the resources.

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These results offer indisputable evidence that the term synthetic fuels, and the associated sub-terms, is being used as a generic term that does not reflect the production process. It is also clear that for some production processes and in some cases, the term electrofuel corresponds to the term synthetic fuel. This confirms that it is not possible to know exactly which resources, conversion processes, and fuels are being referred to by the terms synthetic fuel or electrofuel. Correspondingly, this hinders the identification of the term to be used for renewable fuels produced without the use of fossil sources and through the conversion of electricity to fuel in future energy systems. If there is no clear reference point for this terminology, there is a space for potential misunderstanding and promotion of undesirable technologies and fuels. Standardization is needed for regulatory purposes, for renewable fuel tracking and reporting, and for the account of emissions.

4. Discussion

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The objective of the article was to profile the use of the term synthetic fuels for renewable energy systems based on the conversion of electricity to fuel. The production process does not involve a fossil resource input, but utilises recycled CO2 emissions in combination with an electrolysis process powered by renewable electricity. The overall results show the wide spectrum in which this terminology is used. However, it is important that researchers become more specific when using the terminology in the future. The results clearly indicate that there are flaws in the current definition of both synthetic fuel and electrofuel, as both terms are used beyond their current definition. Synthetic fuel is used as a generic term that is not associated with a specific production process or specific resources.

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The well-known definition of synthetic fuel provided by the Energy Information Administration (EIA) [41] defines it as a fuel produced by coal, natural gas and biomass through a chemical conversion. A similar definition is given by the International Energy Agency (IEA) [42] with the exclusion of biomass as a source of fuel. These definitions refer to synthetic fuels as xTL processes derived from coal, natural gas and in some cases biomass. The xTL processes finish with a Fischer-Tropsch synthesis and have a long tradition in fuel production starting in Germany just before and during World War II [43]. The majority of the articles from the literature review refer to this definition of synthetic fuels as fuels produced by xTL processes. Han and Chan [44] provide a very wide definition of synthetic fuels as being fuels produced by resources other than petroleum, thus also from coal and natural gas. The wide definition of synthetic fuels seems to be accepted in research as the term is used very generically in the published literature. In the literature dating from the 1970s, hydrogen is defined as a synthetic fuel along with a variety of other fuels produced through different processes such as water electrolysis, thermochemical and biological processes [45,46]. This leads to a conclusion that there seems to be no clear definition of synthetic fuel and that the definition has changed during the years. According to the Oxford dictionary [47], the first definition of synthetic is: “made by chemical synthesis, especially to imitate a natural product”. According to this, the use of the term synthetic for synthetic fuels is correct, as they are made from chemical synthesis and they imitate the fuels that already exist. However,

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ACCEPTED MANUSCRIPT the definition is too broad as it does not exclude the processes that are different but still end with chemical synthesis. Another interpretation of the term synthetic fuel is that it refers to all fuels produced through the conversion of synthesis gas as an intermediate product into another form of fuel. This definition has the same broad scope and it does not differentiate between the varieties of fuels produced through different types of processes.

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It is important to have a parent term for fuels to distinguish between different types of fuels and their connection with the end use. The use of a parent term for fuels is not a novelty (see Figure 2). For example, oil is a parent term for many different end products that are directly connected to the types of oil. The terms synthetic fuel and electrofuel should also be considered as parent terms for different end products. The main difference is of course the production process which forms the basis of these end products.

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There is indeed a requirement to distinguish between the terms synthetic fuels and electrofuels as they are different from the point of view of the production process. The fundamental idea is that both production processes provide new fuel options that could replace fossil fuels in the transport sector. As the majority of the articles refer to synthetic fuels as xTL processed fuels, the synthetic fuels should be kept within the scope of the Fischer-Tropsch fuels that are produced by gasification of either coal, natural gas or biomass in so-called xTL processes (see Figure 3). When it comes to synthetic fuels produced from coal or natural gas, they are alternative options to fossil fuels and cannot be referred to as renewable synthetic fuels, but rather fossil synthetic fuels. The only renewable path of synthetic fuels is the biomass-to-liquid process. To differentiate between the initial resources, the use of the abbreviations CTL, GTL and BTL should be encouraged.

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Electrofuels are fuels produced by storing electricity as chemical energy in the form of liquid or gas fuels in so-called xTE processes (coal-, biomass- and emission(CO2)-to-electrofuel, see Figure 3); correspondingly, coal-to-electrofuels (CTE), biomass-to-electrofuels (BTE) and emission-to-electrofuels (ETE). The main characteristic of these fuels is a significant use of electricity in the production process. For example, in the BTE process, 39% of the input is electricity in the case of methanol production and, in the case of methane production, 57% of the input is electricity, while in the ETE process, all of the initial energy is based on electricity [48]. The idea of binding carbon with electricity through the electrolysis of water can lead to carbon neutral fuels [26], for example in the case of emission-to-electrofuels. The benefit of producing fuel through this process is that, in future energy systems with a high share of excess electricity production and intermittent electricity sources, the possibility of storing electricity in the form of fuel would give an opportunity to balance the system. This is the key distinguishing feature which can be lost if electrofuels are branded as another form of synthetic fuels. A renewable energy system with electrofuels can connect intermittent renewable energy to relatively cheap fuel storage (see Figure 4). This connection can enable intermittent renewable energy penetrations of over 80% in the electricity system [18], since it creates a very large flexibility to balance resources such as wind and solar power. If a synthetic fuel, such as CTL, GTL, or BTL, was implemented instead of an electrofuel, this connection would not be possible and maximum renewable penetrations would be approximately 50-60% [18,49]. It is important to emphasise once again that, in a low-carbon energy system, these electrofuels could account for more than 50% of the total final electricity demand, demonstrating the major role that they could play in the future. As alternative transport fuels are promoted by the researches and policymakers, it is essential to distinguish between the two very different production processes associated with synthetic fuels and electrofuels, since they both have extremely different impacts on the surrounding energy system. The parent terminology promoted here is xTL for conventional synthetic fuels and xTE for electrofuels (see Figure 3).

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Another advantage of electrofuels is that the production of these finishes with chemical synthesis (see Figure 3), meaning that the fuel output can be determined by using different types of catalysis depending on the requirements from the demand side. When the fuel is produced from the recycling of CO2 emissions, a certain amount of electricity is needed for the extraction of the emissions, either from the stationary sources or through air capturing, and these fuels represent the closed-carbon-loop. Only in cases when both the carbon and electricity come from renewable sources can the fuels be referred to as renewable electrofuels.

5. Limitations of the review

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This review has only included articles published between 2006 and the beginning of 2014. A search within a broader time frame could provide a more detailed overview, but according to the terms used in articles dating from the 1970s [45,46], it is probable that the result would be very similar to the ones presented in this article. In this review, a refining search was carried out in order to limit the search results to the articles that have described the production process of the fuel related to the term used. A search without this requirement may result in an even broader and more inconsistent use of the terminology as there would be no transparency in terms of what the authors refer to with these terms.

6. Conclusion

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Our findings suggest that a more consistent use of the terms synthetic fuel and electrofuel is needed for two key reasons. Firstly, their role as transport fuels can become much more significant in the following years and especially in the longer term as more intermittent renewable energy is utilised. Secondly, the production process of conventional synthetic fuels has a much different impact on the rest of the energy system compared to electrofuels. It is important that the terminology is standardised due to the potential danger that the terminology could be incorrectly interpreted, thus leading to misunderstandings in both the technological development, academic discussion, and the implementation of policies.

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The review showed that there is no exact definition of synthetic fuel or electrofuel to which we can refer, as they are used for a range of production processes and feedstock for fuel production. Based on the results, it is suggested that synthetic fuels and electrofuels should be used as separate ‘parent’ terms rather than under the common term of synthetic fuels. For the fuels with a high share of electricity in the production process, the term electrofuel should be used together with the corresponding abbreviation xTE. For conventional synthetic fuels which are based on Fischer-Tropsch synthesis, the current xTL terminology is sufficient. The terms should be further divided into renewable synthetic fuel for fuels produced from renewable feedstock and fossil synthetic fuels for fuels produced from fossil feedstock. The term renewable electrofuel should be used only in cases when both electricity and carbon are obtained from renewable energy sources. The process of converting emissions to fuel is considered as renewable no matter where the emissions are obtained, since this process is a closed carbon loop.

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ACCEPTED MANUSCRIPT [51] Abanades S, Villafan-Vidales HI. CO2 and H2O conversion to solar fuels via two-step solar thermochemical looping using iron oxide redox pair. Chem.Eng.J. 2011;175(1):368-375. [52] Abanades S. CO2 and H2O reduction by solar thermochemical looping using SnO2/SnO redox reactions: Thermogravimetric analysis. Int J Hydrogen Energy 2012;37(10):8223-8231.

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[62] Graves C, Ebbesen SD, Mogensen M, Lackner KS. Sustainable hydrocarbon fuels by recycling CO2 and H2O with renewable or nuclear energy. Renewable and Sustainable Energy Reviews 2011;15(1):1-23.

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ACCEPTED MANUSCRIPT [68] Kim YH, Jun K-, Joo H, Han C, Song IK. A simulation study on gas-to-liquid (natural gas to FischerTropsch synthetic fuel) process optimization. Chem.Eng.J. 2009;155(1-2):427-432. [69] Heikkilä J, Virtanen A, Rönkkö T, Keskinen J, Aakko-Saksa P, Murtonen T. Nanoparticle Emissions from a Heavy-Duty Engine Running on Alternative Diesel Fuels. Environmental Science and Technology 2009;43(24):9501-9506.

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[72] Liu G, Larson ED, Williams RH, Kreutz TG, Guo X. Making Fischer-Tropsch fuels and electricity from coal and biomass: Performance and cost analysis. Energy and Fuels 2011;25(1):415-437. [73] Williams RH, Liu G, Kreutz TG, Larson ED. Coal and biomass to fuels and power. Annual Review of Chemical and Biomolecular Engineering 2011;2:529-553.

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[76] Sudiro M, Bertucco A. Synthetic fuels by a limited CO2 emission process which uses both fossil and solar energy. Energy and Fuels 2007;21(6):3668-3675. [77] Mushrush GW, Willauer HD, Bauserman JW, Williams FW. Incompatibility of Fischer-Tropsch diesel with petroleum and soybean biodiesel blends. Industrial and Engineering Chemistry Research 2009;48(15):7364-7367.

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[78] Link DD, Gormley RJ, Baltrus JP, Anderson RR, Zandhuis PH. Potential additives to promote seal swell in synthetic fuels and their effect on thermal stability. Energy and Fuels 2008;22(2):1115-1120.

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[79] Lilik GK, Boehman AL. Advanced diesel combustion of a high cetane number fuel with low hydrocarbon and carbon monoxide emissions. Energy and Fuels 2011;25(4):1444-1456. [80] Law CK. Fuel options for next-generation chemical propulsion. AIAA J. 2012;50(1):19-36. [81] Nylund N, Aakko-Saksa P, Sipilä K. Status and outlook for biofuels, other alternative fuels and new vehicles. VTT Tiedotteita - Valtion Teknillinen Tutkimuskeskus 2008;(2426):3-161. [82] Goto S, Oguma M, Seto T. Research and development trends of DME vehicles. Review of Automotive Engineering 2006;27(1):029-037. [83] Erturk M. Economic analysis of unconventional liquid fuel sources. Renewable and Sustainable Energy Reviews 2011;15(6):2766-2771. [84] Kler AM, Tyurina EA, Mednikov AS, Stepanov VV. The combined technology for production of synthetic fuels and electricity with reduced CO2 emissions. International Journal of Low-Carbon Technologies 2010;5(4):264-272.

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ACCEPTED MANUSCRIPT [85] Kim H, Han K, Yoon ES. Development of dimethyl ether production process based on biomass gasification by using mixed-integer nonlinear programming. J.Chem.Eng.Japan 2010;43(8):671-681. [86] Nzihou A, Flamant G, Stanmore B. Synthetic fuels from biomass using concentrated solar energy - A review. Energy 2012;42(1):121-131.

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[87] Apaydin-Varol E, Uzun BB, Önal E, Pütün AE. Synthetic fuel production from cottonseed: Fast pyrolysis and a TGA/FT-IR/MS study. Journal of Analytical and Applied Pyrolysis 2014;105:83-90. [88] Tret'yakov VF, Makarfi YI, Tret'yakov KV, Frantsuzova NA, Talyshinskii RM. The catalytic conversion of bioethanol to hydrocarbon fuel: A review and study. Catalysis in Industry 2010;2(4):402-420. [89] Zeman F, Castaldi M. An investigation of synthetic fuel production via chemical looping. Environmental Science and Technology 2008;42(8):2723-2727.

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[90] Ermanoski I, Siegel NP, Stechel EB. A new reactor concept for efficient solar-thermochemical fuel production. Journal of Solar Energy Engineering, Transactions of the ASME 2013;135(3).

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[91] Ridjan I, Mathiesen BV, Connolly D, Duić N. The feasibility of synthetic fuels in renewable energy systems. Energy 2013;57:76-84. [92] Stoots C, Shunn L, O'Brien J. Integrated operation of the INL HYTEST system and high-temperature steam electrolysis for synthetic natural gas production. Nuclear Technology 2012;178(1):83-93. [93] Olah GA, Prakash GKS, Goeppert A. Anthropogenic chemical carbon cycle for a sustainable future. J.Am.Chem.Soc. 2011;133(33):12881-12898.

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[94] Lund H, Münster E. Integrated transportation and energy sector CO2 emission control strategies. Transp.Policy 2006;13(5):426-433. [95] Van-Dal ÉS, Bouallou C. Design and simulation of a methanol production plant from CO2 hydrogenation. J.Clean.Prod. 2013;57:38-45.

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[96] Vibhatavata P, Borgard J-, Tabarant M, Bianchi D, Mansilla C. Chemical recycling of carbon dioxide emissions from a cement plant into dimethyl ether, a case study of an integrated process in France using a Reverse Water Gas Shift (RWGS) step. Int J Hydrogen Energy 2013;38(15):6397-6405.

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[97] Doss B, Ramose C, Atkins S. Optimization of methanol synthesis from carbon dioxide and hydrogen: Demonstration of a pilot-scale carbon-neutral synthetic fuels process. Energy and Fuels 2009;23(9):46474650. [98] Xie K, Zhang Y, Meng G, Irvine JTS. Direct synthesis of methane from CO2/H2O in an oxygen-ion conducting solid oxide electrolyser. Energy and Environmental Science 2011;4(6):2218-2222. [99] Sun X, Chen M, Jensen SH, Ebbesen SD, Graves C, Mogensen M. Thermodynamic analysis of synthetic hydrocarbon fuel production in pressurized solid oxide electrolysis cells. Int J Hydrogen Energy 2012;37(22):17101-17110. [100] Ni M. 2D thermal modeling of a solid oxide electrolyzer cell (SOEC) for syngas production by H2O/CO2 co-electrolysis. Int J Hydrogen Energy 2012;37(8):6389-6399.

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ACCEPTED MANUSCRIPT [101] Uryga-Bugajska I, Pourkashanian M, Borman D, Catalanotti E, Wilson CW. Theoretical investigation of the performance of alternative aviation fuels in an aero-engine combustion chamber. Proceedings of the Institution of Mechanical Engineers, Part G: Journal of Aerospace Engineering 2011;225(8):874-885. [102] Wong SS, Thomas A, Barbaris B, Lantz RC, Witten ML. Pulmonary evaluation of permissible exposure limit of syntroleum S-8 synthetic jet fuel in mice. Toxicological Sciences 2009;109(2):312-320.

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[103] Tremblay RT, Martin SA, Fisher JW. Metabolites from inhalation of aerosolized S-8 synthetic jet fuel in rats. Inhal.Toxicol. 2011;23(1):11-16. [104] Robb TM, Rogers MJ, Woodward SS, Wong SS, Witten ML. In vitro time- and dose-effect response of JP-8 and S-8 jet fuel on alveolar type II epithelial cells of rats. Toxicol.Ind.Health 2010;26(6):367-374.

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[105] Hui X, Kumar K, Sung C-, Edwards T, Gardner D. Experimental studies on the combustion characteristics of alternative jet fuels. Fuel 2012;98:176-182. [106] Kick T, Herbst J, Kathrotia T, Marquetand J, Braun-Unkhoff M, Naumann C, Riedel U. An experimental and modeling study of burning velocities of possible future synthetic jet fuels. Energy 2012;43(1):111-123.

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[107] Christie S, Raper D, Lee DS, Williams PI, Rye L, Blakey S, Wilson CW, Lobo P, Hagen D, Whitefield PD. Polycyclic aromatic hydrocarbon emissions from the combustion of alternative fuels in a gas turbine engine. Environmental Science and Technology 2012;46(11):6393-6400. [108] Scenna R, DuBois TG, Nieh S. Autothermal reforming of synthetic JP-8 derived from a coal syngas stream. Fuel 2013;108:731-739.

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[109] van der Westhuizen R, Ajam M, De Coning P, Beens J, de Villiers A, Sandra P. Comprehensive twodimensional gas chromatography for the analysis of synthetic and crude-derived jet fuels. Journal of Chromatography A 2011;1218(28):4478-4486. [110] Fritzer J, Giuliani F, Strzelecki A, Bodoc V. Validation of an infrared extinction method for fuel vapor concentration measurements towards the systematic comparison between alternative and conventional fuels for aviation. Journal of Engineering for Gas Turbines and Power 2012;134(1).

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[111] Kahandawala MSP, DeWitt MJ, Corporan E, Sidhu SS. Ignition and emission characteristics of surrogate and practical jet fuels. Energy and Fuels 2008;22(6):3673-3679.

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[112] Corporan E, DeWitt MJ, Klingshirn CD, Striebich R, Cheng MD. Emissions characteristics of military helicopter engines with JP-8 and Fischer-Tropsch fuels. J.Propul.Power 2010;26(2):317-324. [113] Bruno TJ, Smith BL. Improvements in the measurement of distillation curves. 2. Application to aerospace/aviation fuels RP-1 and S-8. Industrial and Engineering Chemistry Research 2006;45(12):43814388. [114] Pucher G, Allan W, Poitras P. Characteristics of deposits in gas turbine combustion chambers using synthetic and conventional jet fuels. Journal of Engineering for Gas Turbines and Power 2013;135(7). [115] Bezaire N, Wadumesthrige K, Simon Ng KY, Salley SO. Limitations of the use of cetane index for alternative compression ignition engine fuels. Fuel 2010;89(12):3807-3813. [116] Murtonen T, Aakko-Saksa P, Kuronen M, Mikkonen S, Lehtoranta K. Emissions with heavy-duty diesel engines and vehicles using FAME, HVO and GTL fuels with and without DOC+POC aftertreatment. SAE International Journal of Fuels and Lubricants 2010;2(2):147-166.

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ACCEPTED MANUSCRIPT [117] Kurevija T, Kukulj N, Rajković D. Global prospects of synthetic diesel fuel produced from hydrocarbon resources in oil & gas exporting countries. Rudarsko Geolosko Naftni Zbornik 2007;19:79-86. [118] Kang I, Yoon S, Bae G, Kim J, Baek S, Bae J. The tests of 1 kW e diesel reformer and solid oxide fuel cell system. Journal of Fuel Cell Science and Technology 2010;7(3):0310121-0310125.

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[119] Horel A, Schiewer S. Investigation of the physical and chemical parameters affecting biodegradation of diesel and synthetic diesel fuel contaminating Alaskan soils. Cold Reg.Sci.Technol. 2009;58(3):113-119. [120] Nabi MN, Hustad JE. Investigation of engine performance and emissions of a diesel engine with a blend of marine gas oil and synthetic diesel fuel. Environ.Technol. 2012;33(1):9-15. [121] Papalexandrou MA, Pilavachi PA, Chatzimouratidis AI. Evaluation of liquid bio-fuels using the Analytic Hierarchy Process. Process Saf.Environ.Prot. 2008;86(5):360-374.

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[122] Sakanishi K. Production of synthetic diesel fuel from the gasification of woody biomass. AIST Today (International Edition) 2006(21):4-5.

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[123] Gassner M, Maréchal F. Thermo-economic process model for thermochemical production of Synthetic Natural Gas (SNG) from lignocellulosic biomass. Biomass Bioenergy 2009;33(11):1587-1604. [124] Gassner M, Baciocchi R, Maréchal F, Mazzotti M. Integrated design of a gas separation system for the upgrade of crude SNG with membranes. Chemical Engineering and Processing: Process Intensification 2009;48(9):1391-1404. [125] Kalt G, Kranzl L. Assessing the economic efficiency of bioenergy technologies in climate mitigation and fossil fuel replacement in Austria using a techno-economic approach. Appl.Energy 2011;88(11):3665-3684.

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[126] Gassner M, Maréchal F. Thermo-economic optimisation of the integration of electrolysis in synthetic natural gas production from wood. Energy 2008;33(2):189-198. [127] Bernier E, Maréchal F, Samson R. Optimal greenhouse gas emissions in NGCC plants integrating life cycle assessment. Energy 2012;37(1):639-648.

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[128] Koppatz S, Pfeifer C, Rauch R, Hofbauer H, Marquard-Moellenstedt T, Specht M. H2 rich product gas by steam gasification of biomass with in situ CO2 absorption in a dual fluidized bed system of 8 MW fuel input. Fuel Process Technol 2009;90(7-8):914-921.

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[129] van der Meijden CM, Veringa HJ, Rabou LPLM. The production of synthetic natural gas (SNG): A comparison of three wood gasification systems for energy balance and overall efficiency. Biomass Bioenergy 2010;34(3):302-311. [130] Toonssen R, Woudstra N, Verkooijen AHM. Decentralized generation of electricity with solid oxide fuel cells from centrally converted biomass. Int J Hydrogen Energy 2010;35(14):7594-7607. [131] Difs K, Wetterlund E, Trygg L, Söderström M. Biomass gasification opportunities in a district heating system. Biomass Bioenergy 2010;34(5):637-651. [132] Meyer B, Ogriseck K. Polygeneration-IGCC concepts for the production of hydrogen rich fuels based on lignite. International Journal of Energy Technology and Policy 2007;5(3):280-289. [133] Lee D, Kim Y, Kim W, Park S. A new clean power generation process integrating molten carbonate fuel cells and coal-based synthetic natural gas production. J.Chem.Eng.Japan 2013;46(1):70-78.

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ACCEPTED MANUSCRIPT [134] Ding Y, Han W, Chai Q, Yang S, Shen W. Coal-based synthetic natural gas (SNG): A solution to China's energy security and CO2 reduction? Energy Policy 2013;55:445-453. [135] Mian A, Ensinas AA, Ambrosetti G, Maréchal F. Optimal design of solar assisted hydrothermal gasification for microalgae to synthetic natural gas conversion. Chemical Engineering Transactions 2013;35:1009-1014.

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[136] Demirbas A. Emission characteristics of gasohol and diesohol. Energy Sources, Part A: Recovery, Utilization and Environmental Effects 2009;31(13):1099-1104. [137] Hellsmark H, Jacobsson S. Opportunities for and limits to Academics as System builders-The case of realizing the potential of gasified biomass in Austria. Energy Policy 2009;37(12):5597-5611.

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[138] Aigner I, Pfeifer C, Hofbauer H. Co-gasification of coal and wood in a dual fluidized bed gasifier. Fuel 2011;90(7):2404-2412.

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[139] Dunst KM, Karczewski J, Miruszewski T, Kusz B, Gazda M, Molin S, Jasinski P. Investigation of functional layers of solid oxide fuel cell anodes for synthetic biogas reforming. Solid State Ionics 2013;251:70-77. [140] Mohseni F, Magnusson M, Görling M, Alvfors P. Biogas from renewable electricity - Increasing a climate neutral fuel supply. Appl.Energy 2012;90(1):11-16. [141] Subramanian P, Sampathrajan A, Venkatachalam P. Fluidized bed gasification of select granular biomaterials. Bioresour.Technol. 2011;102(2):1914-1920.

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[142] Yoon SJ, Lee J-. Hydrogen-rich syngas production through coal and charcoal gasification using microwave steam and air plasma torch. Int J Hydrogen Energy 2012;37(22):17093-17100. [143] Martinez-Frias J, Aceves SM, Smith JR, Brandt H. A coal-fired power plant with zero-atmospheric emissions. Journal of Engineering for Gas Turbines and Power 2008;130(2). [144] Leyko AB, Gupta AK. Temperature and pressure effects on hydrogen separation from syngas. Journal of Energy Resources Technology, Transactions of the ASME 2013;135(3).

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[145] Delattin F, Lorenzo GD, Rizzo S, Bram S, Ruyck JD. Combustion of syngas in a pressurized microturbine-like combustor: Experimental results. Appl.Energy 2010;87(4):1441-1452.

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[146] Ebbesen SD, Høgh J, Nielsen KA, Nielsen JU, Mogensen M. Durable SOC stacks for production of hydrogen and synthesis gas by high temperature electrolysis. Int J Hydrogen Energy 2011;36(13):73637373. [147] Kirillov VA, Kuzin NA, Amosov YI, Kireenkov VV, Sobyanin VA. Catalysts for the conversion of hydrocarbon and synthetic fuels for onboard syngas generators. Catalysis in Industry 2011;3(2):176-182. [148] Yanovskiy LS, Baykov AV. New prospects for ecologically clean power and pure water generation units with SOFC. Renewable Energy 2013;56:72-76. [149] Mustafi NN, Miraglia YC, Raine RR, Bansal PK, Elder ST. Spark-ignition engine performance with 'Powergas' fuel (mixture of CO/H2): A comparison with gasoline and natural gas. Fuel 2006;85(12-13):16051612.

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ACCEPTED MANUSCRIPT [150] Yan Q, Yu F, Liu J, Street J, Gao J, Cai Z, Zhang J. Catalytic conversion wood syngas to synthetic aviation turbine fuels over a multifunctional catalyst. Bioresour.Technol. 2013;127:281-290.

Figure 1. Illustration of a search and selection process Figure 2. Oil, electrofuel and synthetic fuel as parent terms

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Figure 3. Synthetic and electrofuel with related production processes. Dotted line is used only in the case of CO2 based electrofuels Figure 4. Interaction between sectors and technologies in a future smart energy system. The green lines and boxes outline how surplus electricity from intermittent renewable energy sources, such as wind power, can be used to meet the transport demand or stored as fuel

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of related processes used for gaseous fuels

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synthetic natural gas bio-synthetic natural gas coal-, gas-, biomass-to-liquid coal-to-liquid gas-to-liquid biomass-to-liquid coal-, biomass-, emission(CO2)-to-electrofuel coal-to-electrofuels biomass-to-electrofuels emission(CO2)-to-electrofuels

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SNG bioSNG xTL CTL GTL BTL xTE CTE BTE ETE

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Abbreviation list

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Table 1. Terminology and description of related processes for liquid fuels

Carbon dioxide Electrofuel Carbon dioxide

Process description

Context

Reference

Biological conversion of carbon dioxide with different microorganisms by use of renewable electricity

Biotechnology

[22-25]

CO2 recycling paired with water electrolysis

Energy

[11]

Energy/Chemical

[61,62]

Energy/Chemical

[63,64]

Chemical

[65]

Solar thermochemical dissociation of CO2 and H2O with metal oxides for production of CO and H2 ( syngas) that is processed to synthetic fuels Direct liquefaction of biomass / Biomass hydrogenation – upgrading biomass with hydrogen Catalytic oxidation of natural gas for producing synthesis gas for processing into fuels

Thermochemical dissociation

Biomass waste

Liquefaction/ Biomass hydrogenation

Natural gas

Catalytic oxidation

CO2 and water Coal, biomass, residual oil etc. Coal, natural gas, other forms of fossil fuels, biomass

Co-electrolysis xTL (Thermochemical conversion)

Co-electrolysis of steam and CO2 and Fischer-Tropsch synthesis

Energy

[44]

Fischer-Tropsch synthesis from coal, biomass, residual oil, etc

Chemical

[66]

xTL (Thermochemical conversion)

Coal-to-liquid (CTL), Gas-to-liquid (GTL), Biomass-to-liquid (BTL), Coal and biomass to liquid (CBTL) with the Fischer–Tropsch process

Automotive

[67-92]

Gasification of coal or biomass with different fuel synthesis

Energy

[93,94]

Biomass gasification, catalytic conversion of syngas to liquids

Energy

[95]

Chemical

[96]

Hydrogenating a liquid fraction derived from bioethanol and containing aromatic hydrocarbons to produce hydrocarbon fuels

Chemical

[97]

Chemical looping of CO2 with methanol synthesis

Chemical

[98]

Solar thermochemical process from H2O and CO2 with Fischer-Tropsch

Energy

[99]

Biomass Synthetic fuels Biomass Bioethanol

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Coal or biomass

Gasification with methanol or DME synthesis (Thermochemical conversion) Gasification (Thermochemical conversion) Pyrolysis (Thermochemical conversion)

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Carbon dioxide

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Synthetic liquid fuel

Main conversion process Electrobioreactor/Biological CO2 fixation CO2 hydrogenation

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Main resources

Fast pyrolysis for bio-oil production

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Terminology used

Hydrogenation

CO2 and water

Chemical looping combustion (CLC) Solar-thermochemical cycles

CO2 and water

CO2 hydrogenation

Chemical recycling of carbon dioxide and hydrogen into fuels (CO2 hydrogenation with water electrolysis)

Energy/Chemical

[47-51,100,101]

CO2 and water

Co-electrolysis

Co-electrolysis of H2O and CO2 with fuel synthesis

Energy

[45,46,102]

Carbon dioxide

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Lignocellulosic biomass Biomass

Dry and steam reforming xTL (co-processing with petcoke, coal or vacuum residues) xTL (fast pyrolysis and gasification)

Fuel/Automotive

[36,40,118-124]

Synthesis gas produced by dry and steam reforming of methane with Fischer-Tropsch

Chemical

[34]

Fischer-Tropsch based xTL processes (converting lignocellulosic biomass with co-processing with petcoke, coal or vacuum residues)

Fuel

[42]

Biomass-to-liquids (BTL) with fast pyrolysis and gasification

Fuel

[41]

RI PT

Synthetic biofuels

Methane

[103-117]

SC

Synthetic gasoline and diesel

Aviation fuel

M AN U

Coal, natural gas, biomass

Coal-to-liquid (CTL), Gas-to-liquid (GTL) and Biomass-to-liquid (BTL) with the Fischer–Tropsch process Coal-to-liquid (CTL), Gas-to-liquid (GTL) and Biomass-to-liquid (BTL) with the Fischer–Tropsch process

TE D

Synthetic diesel

xTL (Thermochemical conversion) xTL (Thermochemical conversion)

EP

Coal, natural gas

AC C

Synthetic jet fuel

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Table 2. Terminology and description of related processes used for gaseous fuels Process description

Context

Reference

Coal, lignin, biomass, lignocellulosic biomass, dry black liquor

Gasification (thermochemical conversion

Gasification of coal or biomass for synthetic natural gas production

Energy/Chemical

[33,125-136]

Catalytic hydrothermal gasification

Chemical

[137]

Energy

[31,32]

Energy

[37-39]

Co-gasification of coal and wood, or gasification of biomass

Energy

[138,139]

Biogas derived by anaerobic digestion

Chemical

[140]

Biomass gasification or digestion for biogas production. Produced CO2 is paired with hydrogen from water electrolysis for Sabatier reaction rd for extra methane production (3 generation biofuels – fully synthetic fuels)

Energy

[141]

Gasification of different resources for syngas production

Energy/Chemical

[142-146]

High temperature co-electrolysis of CO2 and H2O

Chemical

[43]

Conversion of hydrocarbons to syngas by catalysts

Chemical

[147]

Bio-syngas

Biomass

Synthetic natural gas (bioSNG)

Coal and wood, biomass

Synthetic biogas Synthetic methane (biogas)

Biomass

Biomass and water

Coal, biomass, low grade fuel, wastes Synthetic gas (syngas)

Power gas

Wood syngas

Gasification of biomass

SC

Biomass

Gasification of biomass with Fischer-Tropsch for liquid fuel production

Co-gasification, gasification Anaerobic digestion (Biochemical conversion) Gasification or digestion (Thermochemical conversion) Gasification (Thermochemical conversion)

M AN U

Bio-synthetic natural gas (bioSNG)

Hydrothermal gasification Gasification (thermochemical conversion) Gasification (thermochemical conversion)

TE D

Microalgae

RI PT

Main conversion process

EP

Synthetic natural gas (SNG)

Main resources

CO2 and water

Co-electrolysis

Hydrocarbons

Catalysis

Hydrocarbons

Water-vapour or carbon dioxide conversion

Conversion of hydrocarbons by water-vapor or carbon-dioxide conversion

Chemical

[148]

Water

AquaFuel process

Synthetic gas derived during the processing of Aqua-fuel (encompasses an electric discharge on carbon rods submerged in water)

Automotive

[149]

Wood chips

Gasification (Thermochemical conversion)

Gasification of wood chips

Energy

[150]

AC C

Terminology used

AC C

EP

TE D

M AN U

SC

RI PT

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AC C

EP

TE D

M AN U

SC

RI PT

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AC C

EP

TE D

M AN U

SC

RI PT

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AC C

EP

TE D

M AN U

SC

RI PT

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Highlights

AC C

EP

TE D

M AN U

SC

RI PT

A need for clear terminology within renewable and alternative fuel More consistent use of the term synthetic fuel and electrofuel is necessary Synthetic fuels and electrofuels should be used as separate ‘parent’ terms