53


intensively and determined to be a reliable fuel alternative. The captured carbon dioxide is processed to methanol through the process of direct carbon dioxide hydrogenation. Direct carbon dioxide hydrogenation has been shown to carry out this process with promising results due to the reduced heat necessary to perform the reaction as opposed to the process of carbon dioxide hydrogenation via a reverse-water-gas-shift reaction. This shift reaction, from CO2


to CO, has been utilized due to being able


to use the desirable copper catalyst. Although, a different metal catalyst would have to be used for the reaction between CO2


H2 and


gases because this reaction produces water, which causes the copper catalyst to shut down. In Figure 3, the reaction equations for each process are shown.


Figure 4. Electricity requirements in formulating certain e-fuel bases. [11]


emissions than both fossil fuels and the German electricity mix. Therefore, it is important to use renewable sources of energy when designing e-fuels [14]. Figure A1 also breaks down where most of the carbon dioxide emissions are coming from. The electrolysis is what makes up most of the CO2-eq electricity mix while there are no CO2-eq


emissions produced by German emissions produced by


electrolysis using wind energy. Figure 3. Hydrogenation reaction equations of CO2 /CO and H2 [9]


A study from Nankai University concludes that the precursor of CO2


This environmentally friendly trait that e-fuels have is further backed up by the Greenhouse Gas Emissions (GHG) that are produced by the fuel. Figure A2 illustrates the German Electricity Mix being substituted by renewable energy in a stepwise manner. As depicted, the use of wind energy for electrolysis has a major reduction in CO2-eq


emissions by about 85%. As additional


can yield a methanol product with the use of a Ruthenium (Ru) catalyst. The metric used to measure the effectiveness of a catalyst is the turnover number. The turnover number is defi ned as the number of substrate molecules transformed per minute by a single enzyme molecule [10]. Catalyst Ru3


(CO)12


renewable energy sources were substituted in, the fewer CO2-eq emissions were produced. We can conclude that it is essential that


the electrolysis is powered by renewable electricity for e-fuels to achieve low CO2-eq


emissions [14]. was


experimentally tested and was determined to have a turnover number of 94.5 with the potential of achieving a turnover number of up to 221 in the presence of an acidic additive [9]. The main concern of the use of the Ru catalyst is the fact that the system would often suffer from harsh reaction conditions such as high temperatures and pressures along with the presence of co- products. In contrast, a Nickel (Ni) catalyst has also been explored due to how inexpensive Ni catalysts tend to be along with the lower reaction temperatures observed during methanol synthesis and CO2


high yield of CH3


[9]. Another plus to the reaction with the Ni catalyst is that the reaction is reversible. So, why are we evaluating methanol as opposed to other possible e-fuels?


As seen in Figure 4, methanol-based e-fuels are among the lowest in energy required to produce the fuel. The storage and transport of methanol e-fuels is also a massive advantage. If there was to be a methanol spill in the ocean the methanol would dissolve within 24 to 48 hours with virtually no negative environmental impacts [12]. In addition, methanol has a density comparable to a typical petroleum-based fuel which means no additional pressure is required in the storage of the methanol e-fuels unlike in the storage of hydrogen-based e-fuels. With petroleum-based fuels expected to be maintained as the dominant fuel source for the foreseeable future, methanol fuels have been experimentally determined to be an effective carbon neutral additive to diesel fuels to reduce the CO2


emissions of the current most important


popular backup fuel in the power generation sector [11]. Ultimately, the conversion from excess CO2


to a reliable e-fuel


has been successfully achieved. As the combination of these technologies is still relatively new, it’s just a matter of time until these processes become sustainable and the potential rise of a new dominant fuel source is seen.


With the rise of every new fuel source, the pros and cons must be weighed. The effi ciency and effectiveness of the resulting e-fuel along with the environmental benefi ts has made e-fuels a viable option for the future. The fi rst major benefi t of e-fuels to be discussed is how e-fuels are environmentally and climate-friendly fuels. E-fuels produced using renewable electricity do not produce any carbon emissions in the process. Since renewable electricity is preferred, this would provide a platform for growth in many different types of renewable energy sources such as solar and wind power. The potential of e-fuels to be utilized in daily processes, such as in an engine of a vehicle, allows for the incorporation of sustainability into existing infrastructure in both the transportation and heating industry sectors [13].


emissions from wind energy, fossil fuels, and a German electricity mix for electrolysis. Both wind energy and the German electricity mix can be used as sources for the e-fuels. Figure A1 shows that wind energy as the source produces signifi cantly fewer carbon


In order to determine how environmentally friendly e-fuels are, we must talk about the source from which e-fuels are made. This source is renewable energy. A study was done comparing the CO2- eq


Figure A1. Emissions with renewable electricity (wind) and German electricity mix (DE grid mix) [14]. WWW.ENVIROTECH-ONLINE.COM


elimination [9]. The Ni catalyst was shown to produce a OBcat which can be used as a methanol precursor


Another essential benefi t of e-fuels is how economically favorable e-fuels are to the fuel industry. In the analysis of economic benefi ts, it is important to fi rst look at the production costs currently for e-fuels. The biggest contributing factor to the cost of e-fuels comes from the production of hydrogen. The hydrogen can come from both renewable and non-renewable sources. However, e-fuels require hydrogen from renewable processes in order to maintain net-zero carbon emissions produced. The most used of these processes is electrolysis. There are multiple types of electrolyzers and, thus, electrolysis processes that are performed through these different types of electrolyzers which ultimately result in different compositions of an e-fuel that each has its own associated cost. This is depicted in Figure A3. The low and high values in Figure A3 depict the most optimistic and pessimistic values in the literature, these values are used for the effi ciencies and costs of electrolyzers and fuel synthesis, respectively and the average data is used as the base [2].


We can conclude from Figure A3 that e-fuel production costs can be kept as low as less than 100 /MWh which is comparable to typical petroleum-based fuels. It is also observed that over the fi ve years of this study the production costs of e-fuels have been signifi cantly lowered, some by over 50%.


The most unique benefi t of e-fuels is the high energy density found within them. High energy density is related to the relative easiness of the ability to store fuels and thus helps maintain a low storage cost [15]. This can be compared to other types of fuels as well. A study from 2021 shows a comparison of e-fuels being used in fuel cells and comparing them to alcohol fuel cells. The results of e-fuels being used as the anode are shown in Figure A4. As can


be seen from Figure A4, the cell achieves an energy effi ciency of 41.8% at 200 mA/cm2


[16] which is much higher effi ciency than


conventional alcohol fuel cells. Conventional alcohol fuel cells have about a 7% energy effi ciency [17].


E-fuels have numerous benefi ts to be considered. These advantages are reasons why e-fuels are looked at as a possible solution to reducing carbon emissions in the future. Analyzing how e-fuels are currently being used can get us an idea of how they may advance in the future.


Projects to commercialize e-fuels are already underway. In specifi c, the company Carbon Recycling International (CRI) has been a pioneer in the development of CO2 current projects include the Shuni CO2


to methanol plants. Their Olah renewable methanol plant, and MEFCO2


-to-methanol plant, George . These facilities have


received millions of dollars in funding and for good reason as the George Olah plant has the capacity to recycle 5500 tons of carbon dioxide emissions [18] and the yearly production rate reaches 5 million liters of methanol fuel, which amounts to about 2.5% of the Icelandic fuel consumption [3]. Another company making a major impact in the e-fuels industry is Velocys’. Velocys’ is focused on the Fischer-Tropsch (FT) process in creating an e-fuel meant for jets. The company’s most recent accomplishment, in 2021, was the second successful woody biomass to jet fuel demonstration in Japan. The fl ight successfully traveled from Tokyo to Sapporo on June 17th, 2021. The fuel assembled for the fl ight was confi rmed to fully conform to ASTM D7566 and the full amount of the FT synthesis technology neat biojet fuel produced since then (2,366L) were also confi rmed its conformity [19]. As more companies embark on their adventures into experimenting with e-fuels we should see a sharp rise in production and usage of e-fuels.


The future of e-fuels is bright. It is imperative that explorations on


developing more effi cient and cost-effective ways to produce H2 from water electrolysis and implementation of renewable energy for CO2


renewable energy and H2


hydrogenation are researched [20]. New technologies for production will only continue to grow.


Many nations will revert to renewable energy rather than fossil fuels in the future. One such country is Denmark which aims to become fossil fuel independent within its transport sector. An analysis of an anticipated 2050 Danish Energy System report shows that e-fuels that use biomass would thrive within this system. It’s stated that e-biofuels would make up 43% of the transport sector of the base model utilized. In all of the scenarios, the total production of e-biofuels and e-fuels is at least 26% [21].


Germany has also mapped out a scheme to reduce current CO2 emissions by an additional 50% to total a 95% change from 1990 by 2050. Their plan includes 80% of the nationwide electricity


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