Life-cycle Emissions Analysis of Alternative Fuels for Heavy Vehicles

This report examines available information on low and ultra-low sulfur diesel and alternative fuels for heavy vehicles in terms of their emissions of greenhouse gases and air pollutants. Most of this information is from overseas, and has been used to estimate emissions for heavy vehicles using such fuels in Australia. This is done within a life-cycle framework that considers both the pre-combustion emissions and the tailpipe emissions during combustion. This approach is sometimes called the full fuel-cycle or the “well-to-wheel” emissions (even though the raw materials for biofuels do not come from wells) and considers the chain of feedstock production, feedstock transportation, fuel production, fuel distribution and, finally, vehicle use. It is difficult to compare and rank “like-with-like” when examining the emissions (in grams emitted per kilometre travelled) from different fuels. In the case of LPG, few heavy vehicles use it so that data concerning its emissions are scarce. In the case of other fuels it is rare for individual studies to have examined similar engines using similar pollution control equipment. This means that there is extreme variability in the available emissions data, and it is possible to produce misleading comparisons where the best result from one fuel is compared to the worst result from another fuel. Accordingly, wherever possible, this study is based on results that comprise sufficient samples to enable statistics to be used to estimate and quantify the uncertainty in the data. The rankings that are produced are based on rank-score statistics incorporating the uncertainty. The fuels examined are low sulfur diesel (LSD), ultra-low sulfur diesel (ULS), compressed natural gas (CNG), liquefied natural gas (LNG), liquefied petroleum gas (LPG), ethanol, diesohol, canola oil, biodiesel, and waste oil. Greenhouse gas emissions Biodiesel has the lowest greenhouse gas emissions on a life-cycle basis. In fact, biodiesel emits larger quantities of CO2 than conventional fuels, but as most of this is from renewable carbon stocks, that fraction is not counted towards the greenhouse gas emissions from the fuel. Ethanol comes next and then the gaseous fuels (LPG, CNG, LNG). The life-cycle emissions of greenhouse gases from diesel are reduced if waste oil is used as a diesel extender, but the processing energy required to generate LSD and ULS in Australia increase their greenhouse gas emissions compared to diesel fuel. The extra energy required to liquefy and cool LNG means that it has the highest life-cycle greenhouse gas emissions of all the fuels that were considered. Air pollutant emissions We used a risk-weighted scoring system, based on estimates of human health risk, to rank the fuels. On a life-cycle basis, the gaseous fuels (LPG, CNG) give the lowest contribution to air pollution on this criterion. In the case of urban buses, LSD and ULS come next (though these results are based on only one UK test), then ethanol. The use of waste oil as a diesel extender increases air pollution. Biodiesel scores poorly in relation to air quality because its production and use generates considerable amounts of particulate matter.