One of the many conundra in transportation emissions is why the use of biofuels, promising environmentally friendly reductions in emissions especially for heavy-duty vehicles, is not taking off faster and attracting only limited policy focus. Compare that to the excited talk around battery electrification of trucks, which generously could be described as a challenging solution.
Across the range of biofuels that Emissions Analytics has tested, some patterns become clear. The first observation is that, on average, they tend to lead to very little change in emissions at the tailpipe. Second, within that average, there tends to be significant variability between vehicle models on the same fuel. Therefore, if comparing tailpipe performance, it is difficult to generalise about the benefits of a particular biofuel, and the expected benefit in any case may be small.
A corollary of the first point is that the proposed benefit of these fuels relies on emissions reduction – often but not always focusing on carbon dioxide (CO2) – located upstream in the fuel production and distribution. This part of the supply chain is often opaque, and not easily subject to independent verification or regulation, as is possible with tailpipe measurement. Therefore, even if the benefits of a biofuel sound attractive, but cannot be verified, it would be right to take a sceptical position. On top of that, there may be secondary, unintended consequences due to competition for scarce resources as biofuel production is scaled, for example by the earlier effects on food supply and prices where raw materials were crops.
Rightly, therefore, many governments have been cautious in incentivising biofuels too categorically. For example, E10 – gasoline containing up to 10% ethanol – has been long proposed in the United Kingdom (UK) and other European countries, yet its adoption has been slow. Natural (fossil) gas, familiar at the pump as either Liquefied Petroleum Gas (LPG) or Autogas, attracts a significant tax advantage in the UK and other countries, yet there is limited forward visibility because the tax advantage is always at risk of withdrawal. Fleets, consequently, have been cautious in converting to natural gas.
To illustrate the dilemma, we can consider test results from two programmes Emissions Analytics has conducted, one covering E10 gasoline, the other compressed natural gas (CNG) and liquefied natural gas (LNG).
Switching from E5 to the higher ethanol E10 gasoline in Europe is often held out as a quick way to deliver large aggregate CO2 reductions as it can be distributed at scale through the existing refuelling infrastructure, to be used both in light- and heavy-duty applications.
To investigate this, we tested E10 on 17 gasoline vehicles in Europe from a wide range of manufacturers. On average, CO2 emissions fell by 0.5%, but this varied at the model level from an increase of 7.1% to a decrease of 6.3%. All were relatively new Euro 5 or Euro 6 vehicles, tested on our standardised on-road cycle made up of urban, rural and motorway elements. Due to the lower energy content of E10, the fuel economy worsened on average by 1.2%, with a similarly wide variation. Perhaps unexpectedly, emissions of nitrogen oxides (NOx) increased by 16.6% on average, from an initial level of 59 mg/km. Carbon monoxide (CO) fell on average by 4.5%.
Therefore, at the tailpipe, the reduction in CO2 was negligible overall. The value of the reduction in CO did not outweigh the effects of the increased NOx emissions, as non-compliant urban air quality is generally caused by excessive NO2 concentrations. For the introduction of E10 to be valuable, the upstream CO2 reduction would need to be large enough to outweigh that increase in NOx emissions. Sufficient transparency around those upstream emissions and any secondary consequences, is not offered by the supply chain and therefore the overall benefit of E10 should rightly be treated with scepticism.
Turning to natural gas, we were invited to be part of the first extensive UK study to assess real-world performance of heavy freight vehicles fuelled by this alternative. The two-year test ending in 2019 was led by biomethane supplier Air Liquide, with three transport operators (Howard Tenens, Asda and Kuehne + Nagel) and three technical partners: Cenex (data monitoring), Microlise (telematics) and Emissions Analytics (real-world testing). The project was supported by Innovate UK.
The goal was to consider the overall performance of gas as a fuel for heavy freight, compared to diesel. A secondary aim was to only use 100% biomethane, which in this case was derived from food waste and supplied by Air Liquide.
Biomethane is chemically similar to its fossil gas counterpart, although the product of a different refining process that starts with biogas that is then improved to increase the methane content and purge it of impurities. Biogas is the raw product of anaerobic decomposition of waste ranging from human and livestock excreta to food waste. Whether from a renewable feedstock or not, gas can be packaged in compressed but gaseous form, or as liquid; it can also be combusted in both spark ignition and compression ignition engines.
Heavy freight vehicles have been produced by different manufacturers to run off both CNG and LNG derivatives. Why the industry has both variants reflects sector immaturity, but also different strengths and weaknesses. For example, when liquefied at minus 160 degrees Celsius, LNG has a higher calorific value for its volume and thereby requires less space for its storage, an important consideration for road freight; but refuelling with LNG requires more safety procedures.
The broader primary energy market is now over 20% gas, following a big rise in recent years, but is dominated by fossil gas drawn from deposits of natural and shale gas, or syngas and coal gas from industrial processes. Of the overall gas market, biomethane, which is renewable as it is made from waste, constitutes just 0.1%, but is growing rapidly1.
In our consortium test, three different models of gas-fuelled heavy goods vehicle were pitted against their nearest diesel counterparts: rigid/CNG/spark ignition, articulated/LNG/spark ignition and articulated/LNG/compression ignition. The vehicles were drawn from three different fleets and three different manufacturers. The test covered urban, rural and motorway driving to reflect typical duty cycles. While different payloads were tested, we will focus on the 60% payload results here.
On average the gas vehicles emitted 8% less tailpipe CO2 than their diesel pairs, but this disguises variation from a 15% reduction to a 4% increase. It was only the switch to biomethane that delivered consistent, material reductions in well-wheel (WTW) greenhouse gas (GHG) savings: of around 80%, and not less than 76% compared to the diesels. The missing 20% reflects various inefficiencies, losses and a small amount of measured methane slippage, while the CO2 benefits rest heavily on the upstream, energy grid benefits of the fuel itself.
Turning now to air quality, particle number emissions were on average 5.4 x 1010 #/km (23%) lower on the gas vehicles compared to the diesel counterparts. However, the results were sensitive to payload and duty cycle, variable between vehicles as shown in the chart below.
In contrast, NOx emissions were on average 0.02 g/km (59%) higher on the gas vehicles compared to the diesels. Yet again, however the results were variable between vehicles and conditions, as shown below.
Therefore, there is no consistent pattern in the tailpipe emissions between vehicles and fuels and, consequently, the advantage of gas-powered vehicles rests heavily on the upstream CO2 benefits. With a supplier such as Air Liquide, those characteristics are more open to scrutiny and verification, but scaling and diversification of the supply of biomethane would need to adhere to the same standards to ensure those benefits were delivered.
Recently, UK retailer the John Lewis Partnership announced that it will replace 600 of its heaviest freight trucks with biomethane-powered gas alternatives, prioritising overall GHG emissions while retaining a wide array of options for the rest of its 4,800-strong fleet where electrification may begin to play a serious role for vehicles with a gross vehicle weight (GVW) below 10 tonnes.
In this instance the retailer plans to install its own fuelling infrastructure to guarantee that the gas used is 100% renewable, whereas more broadly it is more typical that governments incentivise the addition of renewable gas to the existing gas supply, highly efficient in that it takes advantage of existing infrastructure, but also subject then to confusion about what proportion of the gas is actually renewable, the fossil and non-fossil components comingling.
The John Lewis example appears to show that the climate-advantage of 100% biogas is now strong enough for management to overcome other drawbacks associated with an immature sector where fuelling infrastructure remains patchy, and where the true business case rests on the need for long-term tax incentives that are by nature uncertain.
Another broader question with no clear answer yet is the exact scalability of renewable gas feedstocks, the typical candidates being food, municipal and sewage waste. As soon as the feedstock becomes biomass from crops or wood products there is the risk of unintended consequence, if those feedstocks might have been put to better use – the subject is hugely complex but supply-chain transparency is an imperative where the climate-benefitting value of the fuel is its primary attraction.
All in all, these complexities may lead to greater focus on synthetic liquid fuels rather than biofuels, as proposed by some academics and industry representatives, and which may have application beyond road transportation such as in aviation. This approach may prove to be more transparent and scalable, and the greater control of the manufacturing process may deliver reductions in air pollutant emissions. In both cases, the results from these test programmes show the essential need for independent real-world testing at the vehicle model level, to avoid policy being based on generalities, hunches and good marketing.
Footnotes:
- See: IEA, Outlook for biogas and biomethane: Prospects for organic growth
World Energy Outlook special report
Fuel report — March 2020