Biofuels: Many possibilities to decarbonise transport

Posted in: biofuels

Biofuels provide a wide variety of decarbonisation options for transport.

Since the turn of the millennium, virtually all of the UK’s industrial sectors have been on a consistent trajectory of reducing emissions, all aside from transport, whose emissions reduction has flatlined over the last 10 years. This is not through a lack of available technology: options certainly exist that could play big roles in decarbonising the transport sector, but many have yet to be widely deployed.

Electrification is clearly the eventual destination for the domestic fleet: policies in place to ban the sale of petrol, diesel, and hybrid engine vehicles by 2035 will help catalyse this change, but electrification will be a slow process requiring large changes in generation capacity, grid capacity and charging infrastructure. These changes will take time, and in the meantime the only viable decarbonisation option for transport is to use biofuels.

From field to fuel tank

Strictly, a biofuel is any combustible fuel (liquid or solid) derived from biological material, and the forms most familiar to the layman are likely to be those known as first generation biofuels: those where the biomass has come directly from crops.

Every petrol user in the UK is already using biofuel in their vehicle, perhaps without even realising, as all petrol blends in the UK include up to 5% bioethanol. This fuel is produced by the fermentation of high-starch crops such as wheat, corn or rice, or of sucrose extracted from plants such as sugarcane. Higher percentages of bioethanol are legal to use in the UK, and the bioethanol industry is advocating for a 10% blend to become more widely available. However, the government is currently reluctant to implement this, with their main concern being how ethanol may cause damage to older car engines. Higher percentages are seen elsewhere globally, with countries such as Brazil (which has a large bioethanol industry) having blends of up to 85%.

However, the most widely-known form of biofuel is likely to be biodiesel. As its name suggests, it is biofuel that can be blended with fossil diesel and run in diesel engines. It is most commonly produced from vegetable oils such as rapeseed, soybean or palm oil, which are processed in order to be able to mix with fossil diesel. In the UK, commercially available diesel can currently only contain up to 7% biodiesel, but hauliers with their own fuel pool can use anything up to and including 100% biodiesel. However, these higher blends require engine conversions in order to run safely and reliably. Vegetable oils can also be hydrogenated, producing what is known as hydrogenated vegetable oil (HVO), which can be used as a jet fuel. Jet fuel is in a sense the “holy grail” of the biofuels industry, as the aviation sector has significant emission levels, and electrification isn’t currently a realistic decarbonisation solution.

However, as is the case across the bioeconomy, utilising crops as biomass conjures the spectre of Indirect Land-Use Change (ILUC). All of the crops listed above are also used for food as well as biofuel feedstock. Thus, there are concerns that utilising them as biofuel feedstock creates a “deficit” in food production, as demand for both uses has to be met by the same crop. In theory this will result in the clearing of more virgin land to meet this demand, offsetting the environmental benefits offered by using biomass in the first place. It is very difficult to prove when ILUC has happened, but it is taken very seriously by policymakers. Where biodiesel is concerned, the problem is heightened, as the deficit will most likely be made up with palm oil. Palm oil is the most efficient (in terms of yield per hectare) vegetable oil to cultivate, which makes it an obvious choice for either food or biofuel producers. However, this comes at the expense of highly diverse rainforest, which must be cleared in order to grow palm oil, which in turn releases huge quantities of methane into the atmosphere, offsetting any greenhouse gas savings from utilising biofuels in transport. This heightened ILUC problem means that in virtually all cases the use of palm oil should be discouraged as a biofuel feedstock. This creates an additional problem for HVO jet fuel, as palm oil requires the lowest amount of hydrogen to produce HVO, making it attractive as a feedstock, but with undeniable environmental implications.

Beyond crops

Naturally, the ILUC problem has stimulated interest in producing biofuels from other sources of biomass – namely non-edible plants. Here, bioethanol moves back into the spotlight, as non-edible portions of plants can be used to produce ethanol, termed in the US as “cellulosic ethanol”. The key advantage is that the cellulose cannot be used in food, and as it is created as a residue of food production in very large quantities, it is therefore available in realistic volumes for biofuel production – it is, in effect, a crop waste. However, this is not without its drawbacks: producing cellulosic ethanol at high volumes is an expensive task, and few large-scale plants are currently running. This is owing to the increased difficulty of breaking down cellulose into sugars that can be fermented. This is compounded by a market reality: the ethanol produced is functionally the same as that from fermenting food crops, but costs 3-5 times more to produce, meaning cellulosic ethanol is always fighting a steep uphill commercial battle.

Utilising waste

If waste is available as a feedstock, it is always going to be preferred to virgin biomass. The rationale is obvious: it doesn’t impinge on food production, but also redirects waste streams that would otherwise be destined for landfill or incineration. Cellulosic crop residues are just one waste that can be used for biofuel production, but any biofuel that is derived from waste will be given preferential treatment in most legislation. These biofuels are called “advanced biofuels”, and in the EU they count twice as much as first generation biofuels when tallying against targets and mandates, to incentivise their production.

Most prominent among these waste-based advanced biofuels is the use of used cooking oil (UCO) as an alternative to vegetable oils for producing biodiesel. However, the sourcing of reliable UCO is highly problematic: as UCO’s popularity as a feedstock has grown, so too has its price, and it is now more expensive than palm oil. NNFCC has previously reported that this creates a risk of fraud when UCO is imported, as UCO exporters can profit from mixing palm oil into UCO which  is undetectable without specialist equipment. This, of course, presents the risk of UCO biodiesel having the same ILUC problems as vegetable oil biodiesel, making it potentially environmentally unviable.

However, waste is rarely as homogenous as used cooking oil, and so its use as a feedstock typically tends towards more specialist fuels: either those that are easier to produce from waste, or fuels that command high enough market prices to justify the increased processing costs.

In densely populated areas, air quality is becoming a significant health issue, and so there is increased interest in using biofuel equivalents of compressed natural gas (CNG), as burning it releases much less particulate matter. Biomethane can be produced from biogenic waste through anaerobic digestion and then used for the same purpose, reducing carbon emissions as well. These fuels offer similar performance and fuel efficiency as diesel, making them a good choice for heavy goods vehicles (HGVs), and this has resulted in commercial fleets adopting biomethane as a greener, “cleaner burning” fuel. The same can be said of BioLPG: a natural by-product of HVO production that is chemically identical to fossil LPG (liquefied propane gas, sometimes called petroleum gas), meaning it can be used to fuel the same kinds of vehicles that already run on LPG. The main downside to BioPLG and biomethane is the scarcity of filling stations in the UK, and most biomethane vehicles will have a “reserve” fuel tank in order to keep the vehicle running in case of an emergency.

Even waste as heterogeneous as municipal solid waste (MSW) can be processed into biofuel through gasification: this process produces carbon monoxide and hydrogen, which can then be synthesised into fuels using a Fischer-Tropsch process. This is currently of interest for aviation biofuel, as it is not reliant on palm oil in the way HVO is.

Away from biomass completely

The UK also offers regulatory support for biofuels that have no biological content at all, though it may seem to be a contradiction. Hydrogen can be used as a fuel, and is desirable due to its high energy combustion and zero carbon emissions. Hydrogen is produced by electrolysis, which requires large amounts of electricity. Ordinarily this would carry with it the burden of carbon emissions from the generation of that energy, but if zero-emission sources are used to produce that electricity, such as solar, wind, or hydroelectricity, then the resulting hydrogen is considered a renewable fuel of non-biological origin (RFNBO) and is classed as an advanced biofuel. Other fuels such as methanol can also potentially benefit from this classification, if their production process doesn’t produce any CO2.

The options for decarbonising transport in the short term are myriad, but currently more needs to be done to bring them into the mainstream, to provide a solid foundation on which electrification can build.

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This article was written by Bob Horton, Research Analyst at NNFCC.

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