Biogas-based fuels as renewable energy in the transport sector: an overview of the potential of using CBG, LBG and other vehicle fuels produced from biogas

Abstract

The energy supply in the world needs to change from fossil fuels to renewable alternatives. Biogas is such a renewable alternative, and there is potential to increase the biogas production in the world. In recent decades, many countries have increasingly been upgrading biogas to vehicle fuel. In the last few years, the interest has also increased in liquefying biogas for heavier transports. Biogas can also be a raw material for other fuels by gasifying the biogas, for example Fischer-Tropsch fuels, methanol, dimethyl ether and hydrogen. This study provides an overview of vehicle fuels that can be produced from biogas, their technological maturity and their respective potentials as substitutes for fossil fuels in the transport system. A common factor for all of them is that they are most often produced from fossil fuels. Compressed and liquefied methane are the only fuels being commercially produced using biogas. The other fuels all have strengths that both compressed and liquefied methane lack, for example the possibility of emission-free fuel cell vehicles. However, they are all less mature technologies than compressed and liquefied methane. The greatest short-term potential is thus for expanded use of biogas as compressed and liquefied biomethane.

Keywords:

• Biogas
• transport
• biogas-based fuels
• compressed biomethane
• liquefied biomethane

Introduction

Over 80% of the energy supply in the world in 2017 came from fossil sources [1]. To avoid undesirable global warming, this current fossil regime must be transformed into one that is based on renewable energy. However, in contrast to fossil fuels, there is not yet any easy way to produce huge amounts of storable and easily utilizable renewable fuels from a single source. In 2018, less than 20% of the energy used in the European Union (EU) came from renewable sources, and a variety of different raw materials and energy carriers had to be used to achieve even that limited share [2]. A system of different raw materials and energy solutions that work together to solve the problem must thus be developed to achieve a global, large-scale transition to renewable energy.

Biogas, which is a methane and carbon dioxide mix produced by anaerobic digestion of biomass, is one alternative that could be a part of this system. Biogas is especially interesting as an alternative fuel since it is not only a renewable fuel, but because biogas production also can help solve other societal problems – mostly by improving waste treatment, but also through positive secondary effects like improvement of soil structures [3,4] and reduced use of mineral fertilizers [5,6]. Many different biomass sources can be used to produce renewable methane, and all countries have at least some potential to produce biogas, but only 0.2% of the total primary world energy supply came from biogas during 2014 [7]. However, the biogas potential in the world from just waste (i.e. urban waste, agro-industry and sewage sludge) has been estimated to around 3% of the fossil fuel use in 2017 [1, combined with 8].

In most countries, biogas is primarily used for producing electricity and heat. However, biogas can also be used for other purposes, such as for producing vehicle fuels. Biogas is in many countries increasingly being upgraded to compressed biomethane (CBG), as a renewable version of compressed natural gas (CNG) for use in especially cars and buses. During the last few years, there has been an increased interest in liquefied biomethane (LBG) for use in heavier transports instead of liquefied natural gas (LNG). There are also other options available. Apart from CBG and LBG, biogas can also be used to produce syngas, which in turn can be used to produce renewable versions of fuels such as hydrogen, methanol or DME. These fuels have different characteristics and potentials than CBG and LBG, which might make them suitable for other parts of the renewable energy system that needs to be developed. However, like compressed and liquefied methane, they are currently all produced from primarily fossil fuels.

The purpose of this study is to contribute with an overview and an increased understanding of fuels that can be produced from biogas and their potential to be used as a substitution for fossil energy in the transport sector. The study answers the following research questions:

• For what purpose have the different fuels been developed?

• What strengths and weaknesses do these fuels have?

• How far have the fuels come in their development, and how are they produced and used today?

Method

A literature inventory was made to be able to identify what possible fuels can be produced from biogas and their potentials. This inventory was wide and included both academic literature and grey literature, newspaper articles and reports from companies based on previously found newspaper articles, depending on where information could be found. In some cases, especially with the alternative fuels that are less mature and used, information is scarce and hard to access. Some journal articles, such as one by Ahmadi Moghaddam et al. [9], had previously studied either one or several of the possible biogas-based fuels. Such overview articles were used to create an overview of the alternatives and their strengths and weaknesses. Journal articles were also used to show the historical context of the fuels [10–14] and what the current research is focused on e.g. [12,15–17]. Grey literature, such as governmental or organizational reports, e.g. [18,19], were valuable sources for statistics on the different fuels and applications as well as on new or planned developments. Newspaper articles were used to find information on recent developments, such as decisions to build new production sites. This information were then corroborated by the company’s annual report or their homepage, e.g. [20,21].

The author also had continuous access to a biogas-related network of people, companies and authorities, through a competence center for biogas based at Linköping University. The findings were presented and discussed several times within this network.

A bibliometric search for each fuel was conducted to complement the inventory. This was done to get an indication of how the interest in the fuels developed throughout the years. The bibliometric search was performed using the Scopus database, and the search looked in the titles, keywords and abstracts for the terms presented in Table 1. Several of the fuels, for example hydrogen, are common in chemical production and the distinction of adding the word ‘fuel’ to the search terms was considered needed. However, it was assumed that there was no need to add the word ‘fuel’ to the search terms when doing the bibliometric search of CBG and LBG since methane gas used for other purposes is not usually compressed or liquefied. In most cases, the figures produced from the bibliometric search were cut off around the year 2000, since after then, the number of hits rapidly increased for all search terms, which made it hard to distinguish the earlier interest.

Table 1. The search terms used for the bibliometric search.

Fuel	Search terms
Compressed biomethane (CBG)	compressed AND "natural gas" OR biogas OR biomethane
Liquefied biomethane (LBG)	liquefied AND "natural gas" OR biogas OR biomethane
Hydrogen	hydrogen AND fuel
Methanol	methanol AND fuel
Dimetyl ether (DME)	‘dimethyl ether’ AND fuel
Fischer-Tropsch fuels (FT-fuels)	‘Fischer Tropsch’ AND fuel

Fuels produced from biogas

Biogas can, via different processes, be used to produce several different transportation fuels, where CBG, LBG, hydrogen, methanol, dimethyl ether and Fischer-Tropsch (FT) fuels are the most likely options. This section gives a short introduction to how these six fuels are produced by using biogas as raw material.

Raw biogas contains mostly methane gas and carbon dioxide, usually around 50–70% and 30–50%, respectively. There are a couple of different main processes in which this raw biogas can be used in the transportation system: combusting it to produce electric power, upgrading it to a higher methane content, gasifying it to syngas, or methanotrophy or partial oxidation to methanol. These processes are shown in Figure 1, along with the subsequent fuels that are produced.

Figure 1. Biogas can be used to produce several possible fuels, either by upgrading it to a higher methane content, by combusting it to produce electric power, by gasifying it to syngas or by methanotrophy or partial oxidation.

By using a generator and a turbine when combusting raw biogas, electricity and heat can be produced. The electricity can be used in the common power system and thus be used for charging electric vehicles [22]. Electricity can also be converted to hydrogen via electrolysis, and the hydrogen can be used in fuel cell vehicles [23].

Raw biogas can also be upgraded to a higher purity – biomethane, with a methane content of at least 97%. The three most common methods to achieve this purity are water scrubbing, pressure swing adsorption and chemical (amine) absorption [17]. The methods differ in, for example, how much methane losses there are, how pure the end product/s is/are, how much energy was used in the upgrading and if it required any chemicals or special equipment [17,24]. Biomethane produced from upgrading can be used directly as fuel in two ways – either by compressing it to 200 bar (compressed biogas, CBG) or by cooling it down to a liquid at −162 °C (liquefied biogas, LBG). To liquefy biomethane, an extra polishing step is often needed where the biomethane is purified even further. Gustafsson et al. [24] compared different upgrading methods for CBG and LBG as well as ways to distribute the gas, and found that LBG requires more energy than CBG (0.03-0.04 kWh/MJ LBG instead of 0.02-0.03 kWh/MJ CBG), but LBG often seems to have less global warming impact than CBG due to reduced methane slip [24].

Apart from using the biomethane directly as fuel, it can also be used to produce other fuels via gasification of the biomethane. Gasification of biomethane produces syngas, a mix between hydrogen and carbon monoxide (CO). The syngas can then be used to produce other fuels. By shifting the CO, hydrogen can be produced out of the syngas. The hydrogen in turn can produce electricity [25]. It also works the other way around – producing hydrogen via using electricity and water, which can then be shifted to syngas. The syngas can also be used in fuel synthesis to produce methanol, which in turn can be used to produce dimethyl ether (DME) [25]. DME can also be produced directly from syngas [26], but the majority of the DME production existing today uses methanol as feedstock. Syngas can also be used in a Fischer Tropsch (FT) synthesis to create Fischer Tropsch fuels, such as synthetic diesel.

It is also possible to produce methanol directly from biogas via methanotrophy or partial oxidation [17].

Biogas-based fuels

Although CBG, LBG, hydrogen, methanol, DME and FT fuels can all be produced from the same resource – biogas – they are all different in their strengths, weaknesses and maturity. This section presents, for each fuel, their respective backgrounds, why they were developed, their strengths and weaknesses and how far they have come in their development.

CBG

CBG is a gaseous fuel that consists of at least 97% methane and is compressed to around 200 bar. It is interchangeable with compressed natural gas (CNG) – methane gas produced from fossil natural gas instead of renewable biogas – which has been used as a transportation fuel since at least the 1930s [13,14]. However, the first spiked interest in CNG came in the 1970s due to the two oil crises [12]. From around 1955 and onward, cheap oil and petroleum were abundant, but these two oil crises made countries look for domestic energy resources and how these could be transformed into fuel for decreasing their import-dependency. One of these resources was natural gas and its subsequent use as CNG, which gained a particular interest in, for example, Canada, Brazil, Argentina and New Zeeland [13]. The vehicles that existed at that time were conversions of gasoline or diesel vehicles. In 1986, after seven years with a CNG program in New Zeeland, over 10% of all cars there were NGVs – over 100,000 vehicles [13]. However, the market in New Zeeland collapsed soon afterward due to rescinded policies [14].

Later, in the 1980s and 1990s, the interest in CNG increased (Figure 2). Local air pollution was a hot issue, and due to lower emissions of air pollutants, natural gas vehicles were viewed as a preferable option. Furthermore, natural gas vehicles emit less particulate matter than petrol and diesel vehicles, and their emissions have a lower tropospheric ozone formation potential [27]. Several countries had a growth of NGVs in the 1990s due to the combination of reduced air pollution and increased energy independence; Argentina, Brazil and China are some examples [14].

Figure 2. A bibliometric search of journal articles and books indicates an increased interest since the 1970s.

During these first decades of increased CNG interest, there was also some focus on using biogas rather than natural gas. In some places, biogas was already being produced as a by-product from treatment of waste, such as wastewater sludge, manure and plant wastes [10,11,28–30]. In other places, there were interest in starting biogas production from those substrates [30] or substrates like organic waste [28] and algae [30]. Interest in collecting methane from landfills also developed [30]. The use of biogas was at this time seldom connected to transport fuels. Instead, raw biogas was used as fuel for heating [28,30], electricity production [29] or cooking [10,11]. However, there was some focus on raw biogas or CBG as transport fuel [28,31–34].

Finally, a third benefit was added when global warming became a hot topic. The possibility in using biogas, which reduces the emissions of greenhouse gases compared to fossil fuels, increased the interest in biogas, e.g. [32,35].

Today, the research on CBG often focuses on available upgrading technologies and their comparative performance, e.g. [16,24,36–38] and exhaust emissions from gas vehicles, e.g. [15,39]. There are also studies on local potentials of biogas, e.g. [40,41]. Khan et al. [12] reviewed how CNG can be used as a transportation fuel. An additional research focus is the technological, economic and environmental aspects for producing compressed methane from electricity, e.g. [42–44].

In 2019, there were over 27 million NGVs in the world, with over 32,000 refilling stations [45]. China, Iran, India, Pakistan, Argentina, Brazil and Italy all had more than 1,000,000 NGVs each [45]. Several large auto manufacturers, such as Audi, Fiat, Ford, Iveco, Opel, Seat, Skoda, Suzuki and Volkswagen, have at least one car model that can use CBG [46]. The models are a mix for different purposes, like small city cars, executive coupe cars, SUVs, light commercial vans and trucks [46]. However, compressed methane is a gas fuel and requires large volumes to transport, and CNG/CBG is thus more appropriate for smaller sized vehicles than heavy duty vehicles [47]. Regarding heavy vehicles, it has primarily been used for driving shorter distances, such as public transport and garbage removal [47,48]. Due to its need to refuel within 500 km [48,49], it is not considered a good alternative for heavy duty vehicles that need to drive longer distances [47–49].

The methane gas vehicles existing today almost exclusively use natural gas rather than biogas, but in some countries, there is a trend of increased CBG production. One of those countries is Sweden, where CBG has been used since the 1990s and where 60% (1.3 TWh) of the biogas produced was upgraded to CBG in 2018 [18]. Denmark, which has earlier focused on biogas for power and heat, was expected to have a 50% upgrading of the biogas produced in 2018, a dramatic increase from the non-existing upgrading in 2012 [50]. Other examples of countries with rapid growth of biomethane production during the last decade are France [51, combined with 52], Finland [53] and Switzerland [54, combined with 55].

LBG

A major problem with compressed methane is that it requires a large space to store the energy. However, if the methane is liquefied – by cooling it to −162° C – the needed storage space will be much smaller. Liquefied natural gas has been used as a way to transport natural gas since the 1960s [56], but in the 2000s, the interest grew dramatically, as indicated by Figure 3, and in the 2010s, commercial use in vehicles began.

Figure 3. A bibliometric search of journal articles and books indicates that there has been an interest in LNG since the 1960s – especially during the 1970s and 1980s – but that the real growth of interest started in the 2000s.

Today, LBG research mainly focuses on upgrading and liquefication technologies [24,57–59]. There are also studies on the feasibility of using LBG as a road transportation fuel [60, 61], as well as a marine transportation fuel [62,63]. There are also ongoing studies on the possibility of using the cold energy in liquefied methane, e.g. [64–67].

LNG requires less volume than CNG for storing the same amount of energy (1 liter LNG instead of 2.4 liter CNG), which makes it a better choice for heavy duty transport [48]. During 2017, there were around 150,000 LNG heavy trucks in use in China alone [68]. A few of the largest auto producers, such as Volvo, Scania, Iveco [69] and Dongfeng [70], recently started to produce heavy vehicles running on LNG. Hagos and Ahlgren [47] found that the preliminary results from an LNG project in Europe confirmed that LNG was a suitable alternative for medium and long distance trucks in Europe. However, the economic viability of LNG use in heavy trucks is often dependent upon the price of the fuel, since the vehicle itself is more expensive to buy than a diesel truck [48].

A difficulty for using LNG for heavy transport is that the refueling infrastructure is still limited [49]. In 2017, there were only 100 refueling stations for LNG in Europe [71]. However, one company is planning to have built 50 more refueling stations by 2020 in the Nordic countries [20].

Apart from being used in heavy road vehicles, LNG is also of increasing interest to the shipping industry due to air pollution regulations [72]. In 2017, there were 100 ships worldwide fueled by LNG, but over 100 new ships were confirmed to be built [71]. In 2016, 85% of the LNG ships (excluding LNG tankers) were active in Norway, and the majority of them were either car-/passenger ferries or platform supply vessels [73]. The ships that were being built in 2016 had more diverse purposes – everything from container ships to cruises [73]. However, an LNG cistern has to be 3-4 times bigger than a cistern for common marine oils [74], and thus it is difficult to retrofit old ships to use LNG.

Like compressed methane, liquefied methane is almost exclusively produced from natural gas rather than biogas. There were only seven active LBG plants in the world in 2017: one in Sweden, one in Norway, one in the Netherlands, two in the United Kingdom and two in the United States [75]. Since then, several more have been built or have been started to be built, for example in Sweden [19,21,76], Norway [21, 77], the United Kingdom [78], and the Netherlands/Belgium [79]. The LBG is used for heavy road vehicles, ships and for industrial usage in manufacturing [19,21,76,79].

Hydrogen

Already in the late nineteenth century, there were ideas about replacing fossil-based combustion engines with a more efficient and pollution-free alternative [80]. The alternative in question was hydrogen fuel cells, which has the advantage to produce power without combustion – and thus can be used without any local air pollution from the motor. Hydrogen was first produced from electricity and water in the 1800s, and almost 40 years later, the first hydrogen fuel cell was created [80]. However, the technique was still extremely immature, and it would take almost 100 more years until the first hydrogen/oxygen fuel cell for practical use was created [80]. The interest in hydrogen started to increase in the 1960s (Figure 4), and the first idea of a hydrogen economy was published a year before the oil crisis in 1973 [81]. The idea was to use hydrogen produced from, for example, solar power and nuclear power for transportation, storage and fuel [82]. Hydrogen vehicles had the advantages of no tailpipe emissions and providing a possibility to store and transport fuel directly produced from electricity. The added benefit of this being possible by using renewable sources and not affecting the climate has increased the interest in hydrogen, and in 2007, the first mass production of fuel cell cars began [80].

Figure 4. A bibliometric search of journal articles and books indicates that there has been an increased interest in hydrogen since the 1960s, which gained speed after 1973.

Today, the research on biogas-based hydrogen focuses mainly on different technologies that can produce hydrogen from biogas, e.g. [83–89]. A common research theme for hydrogen is otherwise different techniques for producing hydrogen from water and sunlight, e.g. [90–93]. Other important focus areas include fuel cells, e.g. [94–96] and hydrogen storage, e.g. [97–99].

In 2018, there were around 13,000 fuel cell vehicles worldwide, and many auto manufacturers are working on developing hydrogen cars, buses, trucks, etc. [100]. Some of them, e.g. Toyota and Hyundai, have already managed to create commercial models [100]. However, the purchase price of hydrogen fuel cell vehicles is currently high in comparison to other alternatives [23,101]. The interest in fuel cell vehicles is most pronounced in China, Japan and South Korea. The three countries respectively have targets of 1 million, 800,000 and 630,000 fuel cell vehicles by 2030 [102]. However, there is also an interest in other countries like US (especially California), Norway, Germany and France [100].

Over 400 billion m³ of hydrogen are used worldwide today [83], although not necessarily as transport fuel. Instead, almost all hydrogen is used for various industrial purposes, for example, within chemical industries. However, like all the other potential fuels, the hydrogen used today is most commonly produced from fossil sources, especially natural gas [83,102]. The hydrogen that is produced using renewable sources often uses electricity and water. In Norway, a test project that used biogas to produce hydrogen has been carried out [103] and there are also other examples of companies and countries working with hydrogen from biogas [104,105].

Methanol

Methanol is another fuel that reaped benefits from the oil crises in the 1970s, that increased the interest in many alternative fuels (Figure 5). Methanol vehicles already existed in small numbers since at least the 1930s [106], but there was no sizable spiked interest until the oil crisis made countries aware of their vulnerability in being import-dependent. Several countries launched programs experimenting with methanol as a fuel, with large fleet trials in the 1980s and 1990s [107].

Figure 5. A bibliometric search of journal articles and books indicates that there has been an increased interest in methanol since the first oil crisis in 1973.

Today, a common theme in the research on renewable methanol is how it can be produced, either from biogas or from other renewable sources, e.g. [108–113]. There have also been studies on the use of methanol from renewable sources in shipping [63,114] and in light-duty vehicles [115,116]. Regarding the current research on methanol in general, there are also several studies done on methanol fuel cells, e.g. [109,117–119].

The largest user of methanol in vehicles today is China, with methanol being used both as different blends with gasoline and as a pure M100 fuel [107]. There have been pilot programs in China with methanol vehicles where many million kilometers have been driven [120]. A few other countries, such as Israel and Australia, use methanol to blend with gasoline [107]. Methanol can also be used in fuel cells [120,121], for example as range extenders [122], or as methanol fuel cell powered trucks, vans and cars [120]. In the EU, methanol is allowed in gasoline at low blends up to 3% [123].

Methanol is also a possible ship fuel [120,121], but it is not used to the same degree as LNG. A difference between these two fuels is that, in contrast to LNG, methanol can be used more easily in retrofitted ships [124]. In 2018, there were eight ships in use (one roll-on/roll-off passenger vessel and seven tankers), and there were at least four more ships planned for 2019 [120].

Around 400 TWh methanol is produced each year, but large parts of the production are used in the petrochemical industry [107]. Like all the other potential fuels, the methanol used today is most commonly produced from fossil sources, especially natural gas and coal [121]. Only one methanol plant could be found that uses biogas as a resource to produce the methanol – in the Netherlands, with a yearly production of above 300 GWh [120]. Apart from that plant, there are several actors that work with production of methanol from different kinds of renewable sources – some with commercial plants and some that are in research and development [120].

DME

With the growing concern with local air pollution in the 1990s, a new fuel was promoted as a lesser-polluting alternative to diesel – DME [26]. Compared to diesel, DME produces less NOx, carbon monoxide and hydrocarbons, and the combustion does not produce any soot at all [26]. However, even though there has been an increased interest in DME since the 1990s (Figure 6), the interest is still low in relation to, for example, liquefied methane or methanol.

Figure 6. A bibliometric search of journal articles and books indicates that there has been an increased interest in DME since the end of the 1990s.

Today, there are many studies about how DME can be produced, either from biogas or other renewable sources, e.g. [125–127]. In 2006, Semelsberger et al. made a thorough review of DME as an alternative fuel – including aspects like fuel properties, production and infrastructure. Apart from this, there have also been studies about the use of DME in compression ignition engines [128–130] and DME in comparison with other vehicle fuels [9,131]. There has also been at least one study about the regional development of DME [132].

There are a few auto manufacturers that have worked with developing heavy vehicles compatible with DME, including AB Volvo, Isuzu Trucks, Nissan Diesel and Shanghai Diesel Co [133]. Volvo, together with other actors, performed a field test in Sweden with biomass-based DME [134]. According to an interview with a representative from Volvo, the field test in Sweden was successful but ended due to insufficient availability of the fuel and lack of customer interest [135].

However, like all the other potential fuels, the DME used today is almost exclusively produced from fossil sources. China is the major market for DME, but not necessarily for transport – most of the DME is blended with LPG. DME is also, for example, used as an aerosol propellant [136]. In other countries where DME is used, such as India, other purposes than transport fuel also dominate [136]. The DME produced today is primarily from natural gas and coal [136]. The only found plant for renewable DME was a demonstration plant in Sweden, which produced biomass-based DME from black liquor [137].

FT fuels

In 1925, the chemists Franz Fischer and Hans Tropsch discovered that it was possible to produce liquid hydrocarbons from carbon monoxide and hydrogen at certain temperatures and with certain catalysts [138]. This Fischer Tropsch (FT) process had the potential for creating liquid fuels from especially the more unmanageable coal, thus allowing the energy to be used in vehicles. The process was continuously studied, especially in Germany, for the next two decades, with a total production of 600 000 tons/year as maximum, partly due to a political push for fuel independence [139]. After World War II, the interest spread to other parts of the world [140]. However, when large oil fields were found in several places in the world in the 1950s, the interest waned since there were no longer any economic incentives due to the abundance of petroleum [140]. Since then, there has only been one large-scale production site that has continuously been producing FT fuels – Sasol in South Africa, which uses cheap coal as raw material [139]. By the end of the 1970s, the interest in Fischer-Tropsch fuels started to increase (Figure 7), most likely due to the oil crises and the subsequently increased interest in alternatives to petrol.

Figure 7. A bibliometric search of journal articles and books indicates that there has been an increased interest in Fischer-Tropsch fuels in the 1980s and from the end of the 1990s.

Today, there are many studies about the Fischer-Tropsch process and the catalysts that can be used, e.g. [141–143]. Apart from this, there have also been studies about areas such as existing Fischer-Tropsch diesel production in different existing plants [144], the possibility of using the Fischer-Tropsch process to produce fuel for aviation [145], Fischer-Tropsch diesel in comparison with other vehicle fuels [9, 131] and air pollution from Fischer-Tropsch diesel [146].

The Fischer-Tropsch process can be used to produce several different kinds of liquid fuels. One major alternative is synthetic diesel, as it is compatible with diesel infrastructure and vehicles. Diesel is the most common oil product, with an equivalent use of around 200 million tons of oil per year in Europe alone [147]. There is diesel on the market produced from natural gas through the Fischer-Tropsch process, but no producer uses biogas.

Analysis and discussion

The scale of biogas production

Due to the nature of the raw material, there is a possibility to produce biogas from domestic resources in every country in the world. This is thus an alternative for increasing national energy security, an ambition which was important for increasing the interest in many alternative fuels in the 1970s. However, the best raw material to use from a resource efficiency perspective, as well as the raw material that is certain to exist in every country, is waste streams like wastewater treatment plants or farms with manure – which, however, will always be limited. The raw materials for biogas are spread out with smaller volumes over a larger area, which will limit the scale of biogas production plants.

This creates difficulties for some of the alternatives since they require a certain volume to be economically feasible. In 2015 in Sweden, CBG production was done in scales between 0 and 108 GWh/year per plant (the mean was 17 GWh/year) [148]. LBG requires larger production plants – a study done on different alternatives for LBG production showed that production plants require at least 50 GWh of yearly production to be economically feasible [149]. The existing commercial LBG plants are also most often on that scale, with a yearly production of 65-125 GWh [19,21,77]. There is, however, an exception with the farm-based commercial LBG production in the United Kingdom that started in 2018, which only has a yearly production of around 15 GWh of LBG [78]. The other biogas-based fuel alternatives require even larger scales, according to a study made by Zinoviev et al. [25] (Figure 8). The one plant that was found using biogas to produce methanol is also larger than any existing LBG plant, with a yearly production of over 300 GWh [120]. However, research has been done on small-scale production of liquid fuels from biogas [150].

Figure 8. The size of a typical, expected plant, based on Zinoviev et al. [25]. In the study, FT fuels, DME, methanol and hydrogen are all based on the gasification of other kinds of biomass than biogas.

Gasification and syngas production appear more flexible than biomethane. Gasification can use several different biomass sources – including biomass that might otherwise be hard to produce liquid fuels from, like forest residues. Another aspect is that the majority of the gasification facilities is the same for all the different syngas fuels, and only a small part is dedicated to producing the particular fuel that is the end product. It would require an investment to change from producing one fuel for another, but it might still be more flexible concerning output than upgrading to biomethane.

The limited size of biogas production and biogas production plants also causes other concerns. If we take Sweden as an example, which accounted for 75% of all the biomethane used as a transport fuel in the EU during 2015 [151], the total biogas potential from waste is somewhere around 15 TWh [152]. Cars in Sweden use 50 TWh of fuel yearly [153], which is more than three times the production potential. That number is not including heavy vehicles, which in itself is also larger than the biogas production potential from waste streams. The largest biogas plant produces around 100-120 GWh yearly, but over 90% of the plants produce less than 20 GWh [148]. There are already a few LNG ships bunkering in Sweden, and one of them is a large ferry that bunkers daily in Sweden. This single ship uses twice as much fuel than the largest biogas plant produces, and over 10% of the entire current national biogas production [149]. Biogas thus has its limitations in how much of the energy demand of the transport sector that can be substituted.

Energy

The more energy that is used in the production, the less efficient the fuel is. Upgrading the biogas to biomethane requires cleaning of the biogas to remove carbon dioxide and other impurities. To create vehicle fuel requires either compressing to high pressure (200 bar) or cooling to low temperatures (-162° C) – processes which both require relatively large amounts of energy. The syngas track, on the other hand, requires energy for the gasification and fuel synthesis processes. Apart from the energy input, the actual amount of fuel yielded from the production process of the different types of biogas-based fuels will also differ.

However, the energy use in the production and the fuel yield is not always the same in connection with the different fuels. Instead, there are slight differences depending on particular circumstances in each case, such as what catalysts are used, process temperatures, production scales, type of reaction used and type of technology. An example of this is found in Ghosh et al. [108], where five potential cases of methanol production processes were studied, which all differed in how much methanol could be yielded from the biogas (between 1912 and 2100 tons per day) and how much energy the processes required. Or Zain and Mohamed [154], who give an overview of previous studies on methane to syngas conversion and the conversion of CO₂ to methanol, focusing on the parameters they used and what conversion rates they got. Another example is provided by Gustafsson et al. [24], who studied energy use in different upgrading methods for CBG and LBG.

To compare the different biogas-based fuels with each other, one previous study, Ahmadi Moghaddam et al. [9], was used in particular. This study was chosen since it looked at most of the biogas-based fuel alternatives (hydrogen was not included). Since the data is from a single source, the assumptions are the same, and it should thus be easier to compare the fuel alternatives than if different sources were used with different assumptions. However, this should primarily be considered as rough estimations. As explained previously, in reality, there will be differences within each type of fuel, depending on the circumstances.

These differences in the energy needed to produce the different fuels show that LBG has the most energy-intensive production phase, while methanol and CBG have the least energy-intensive production phases (Figure 9).

Figure 9. The primary energy input from the production phase of some of the biogas-based alternatives, based on Ahmadi Moghaddam et al. [9].

According to Ahmadi Moghaddam et al. [9] (Figure 10), Fischer Tropsch will have a much lower fuel yield than the other alternatives. However, apart from fuel, there will also be steam and heat produced from FTD, methanol and DME production.

Figure 10. The fuel yield from producing some of the alternatives from biogas, based on Ahmadi Moghaddam et al. [9].

Another aspect that will affect the outcome is the volumetric energy density since the fuels in many situations will be transported on trucks or other vehicles with a certain maximum volume. The vehicles using the fuels will also require a larger tank or have to refuel more often. This relates to whether the fuel is a liquid or not, which is also the reason for the interest in both Fischer-Tropsch diesel and liquefying natural gas. CBG and hydrogen are both gaseous fuels, which require larger volumes and are uneconomic to transport. As can be seen in Figure 11, CBG and hydrogen have the lowest energy densities. However, all the fuels other than synthetic diesel have energy densities that are significantly lower than gasoline or diesel. There is the possibility of requiring less energy in dedicated vehicles due to, for example, a higher octane number in methanol, but the effect would not be great enough to completely counter the disparity between the energy densities.

Figure 11. The volumetric energy density of the biogas-based fuels.

As previously explained, the volumetric energy density makes a large difference. If less fuel can be transported in a single truck, more energy is required to transport fuels with low volumetric energy density. This was another part of the study made by Ahmadi Moghaddam et al. [9] – how much energy can be yielded from the different biogas-based fuels depending on how far they have to be distributed. As Figure 12 shows, CBG might be the best at shorter distances, but at longer distances, it will be more efficient to use liquid fuel, even if it uses more energy in its production phase. CBG, which had the largest fuel yield and best efficiency at a 100 km distribution distance, is even the fuel with the worst efficiency if the distribution distance was scaled up to 1000 km – simply due to the amount of energy needed to transport it for the longer distance.

Figure 12. The energy yield for fueling a bus with some of the different alternatives at two different distances, based on Ahmadi Moghaddam et al. [9].

Technical challenges

Apart from energy, fuels can also have other characteristics that need to be considered. The temperature needs to stay at below −162 °C to keep biomethane liquid, meaning that there is a need for constant cooling of the system. LBG will thus be most suited for vehicles that are used continuously without long breaks in which the liquid can have the chance to evaporate.

An important part of the spiked interests of the potential biogas-based fuels is that they would have less emissions of both air pollutants and greenhouse gases. However, less air pollution and global warming do not mean that the fuel is good in other ways. Methanol, which can be used in fuel cells to completely take away the local air pollution from combustion engines, is toxic and burns with an invisible flame [107], which makes accidents more hazardous. Another problem is that even though the burning of CBG and LBG will not add to global warming, small leaks of it will. The methane that leaks is a powerful greenhouse gas and has 72 times the effect of carbon dioxide from a 20-year perspective, or 25 times the effect of carbon dioxide from a 100-year perspective [155]. Large spills of methane can thus quite easily counteract the climate benefits of using a renewable fuel instead of a fossil fuel.

There are also some other technical difficulties to consider. Methanol is corrosive [121], which makes the requirements on the engine higher. DME also requires more from the engine since the viscosity is lower and there are problems with lubrication [26].

These kinds of technical challenges are not a problem per se – for example, gasoline is also toxic, and ethanol is also corrosive. However, the difficulty can lie in if the technology is less mature and has not had the opportunity to develop the best way to deal with the difficulties. This is especially the case for methanol and DME, which are both the least mature technologies at the same time as they both have technical difficulties that increase the requirements for the engine.

Technological maturity and current use of the fuel

The alternatives differ a lot in how mature the technologies are, which in many cases depends upon what investments in the fossil versions have been made previously since the biogas versions are all interchangeable with their fossil counterparts. In the same way, the actual potential of the fuels can, in the end, largely depend upon what investments will be done by the actors producing and using the interchangeable fossil versions, as actors producing and using the biogas-based versions do not at the moment have the resources to make such large investments.

CBG has come furthest in its technological development, with millions of vehicles worldwide that could run on it and experiences with biogas versions since the 1990s. LBG has also come quite far in its technological development, and there is a trend towards more heavy road transports and ships running on liquefied methane as well as increased production of LBG.

Since a few years back, there exist commercial hydrogen fuel cell vehicles, but they are still rare and expensive. Hydrogen is also, together with electricity, almost always part of long-term goals for the transportation sector since fuel cells can be more efficient than combustion engines, do not have any emissions of air pollutants and can be produced from excess electricity.

Methanol is used for blends in gasoline in some parts of the world, like China. However, the use of it as fuel is still rare outside the few countries that have invested in it, and the interest is very local. There are, however, some ships that use methanol as fuel, and there is a theoretical possibility to use it in fuel cells. DME is the least technologically mature fuel and has until now only been used in vehicles for field tests and demonstrations. There is no infrastructure that can be used and adding the technological difficulties it is unlikely that dimethyl ether will be used in large scales in the near future.

The production of synthetic diesel from biogas via the Fischer-Tropsch process is non-existent, but synthetic diesel can be used in all existing diesel vehicles and diesel infrastructure without any need to update the fleet. FT diesel is interchangeable with diesel and can thus be directly used in a large part of the current world fleet of road vehicles, which is also a large part of the explanation why there historically has been an interest in this fuel. This alternative would be resource efficient since it would decrease the amount of fossil fuel without requiring any new vehicles. However, due to the low yield of FT diesel from biogas that Ahmadi Moghaddam et al. [9] found, it would not be the most resource-efficient way of using biogas. The discussions on banning diesel vehicles in some capacities that have emerged in some countries [156] may also influence the interest in this alternative negatively.

Conclusions

The production of fuels from biogas mainly follows two different tracks – either upgrading to biomethane and then compressing it (CBG) or liquefying it (LBG), or gasifying it to syngas to use for further fuel synthesis (hydrogen, methanol, DME and FT diesel).

CBG and LBG are the only two alternatives that are currently being produced commercially from biogas, and are the most likely options for biogas use in the transport sector in the near future. CBG has come further in its technological maturity with millions of vehicles worldwide using CNG. CBG can also be economically viable in smaller scales than the other alternatives, which can be an important factor as the raw material for producing biogas is often spread out with smaller volumes over a larger area. However, the use of LNG is increasing in heavy duty vehicles and shipping, which enables the production and use of LBG.

Hydrogen, methanol, DME and FT diesel are only extremely rarely produced from biogas, if at all. Among these four fuels, hydrogen is the most likely option for further developments, as it will likely be a part of the future transport system due to its use in pollution-free fuel cell vehicles. However, both renewable methanol and FT diesel can be used in common gasoline or diesel engines, with either low blends (methanol) or high blends (FT diesel).

Abbreviations
CBG	=	compressed biomethane, compressed biogas, renewable natural gas, CBM, bio-CNG, RNG
CNG	=	compressed natural gas
DME	=	dimethyl ether
FT diesel	=	diesel produced via the Fischer-Tropsch process
FT fuels	=	fuels via the Fischer-Tropsch process
LBG	=	liquefied biomethane, liquefied biogas, liquefied renewable natural gas, LBM, bio-LNG
LNG	=	liquefied natural gas
NGVs	=	natural gas vehicles, vehicles compatible with methane

Acknowledgements

The author is grateful for financial support for writing this article from the Environmental Bus Project and from the Biogas Research Center (BRC) under research area 3. The Environmental Bus Project was a project funded by Vinnova, where one part focused on alternative usage areas for biogas. The BRC is a transdisciplinary center of excellence with the overall goal of promoting resource-efficient biogas solutions.

The author would also like to thank Stefan Anderberg for comments that have greatly improved the manuscript, and for acting as a sounding board throughout the entire writing process.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The BRC is funded by the Swedish Energy Agency, Linköping University, SLU and other partners from industry, municipalities, and several public and private organizations.