Air quality implications of transport biofuel consumption

Many countries have initiated policy support for biofuel consumption to reduce transport sector CO2 emissions. However, transportation is also a key contributor to urban air pollution, which is a pressing issue in many large cities. How biofuels compare with fossil fuels in terms of air pollutant emissions that negatively affect human health is therefore a key consideration in their use.

To understand the impact of biofuels on air pollutant emissions from road vehicles, there are two determining factors to consider: first, the combination of the vehicle’s engine and exhaust aftertreatment technology;[1] and second, the characteristics of the biofuel compared with the fossil fuel being replaced. Taken together, these two components provide an indication of the performance of biofuels relative to fossil fuels in terms of air pollutant emissions and impact on human health.

The IEA’s ‘Advanced Motor Fuels’ technology collaboration programme (TCP) compared the emissions of various fuels by category of air pollutant. For cars with less sophisticated engines, air pollutant emissions are higher from diesel engines than from spark-ignition (gasoline) engines (Table 3.8). With cars that meet the equivalent of a Euro 3 emissions standard, e.g. manufactured around the year 2000 in Europe, biomethane has the most potential to deliver low air pollutant emissions.

E85 blends can deliver lower PM emissions than fossil gasoline in older cars, but the reduction is relatively minor. For more widely consumed low-ethanol blends, e.g. E10, the effects on air pollutant emissions are less pronounced than for higher blends, although in older engines without catalysts, E10 can reduce CO and VOC emissions to a small extent. In catalyst-equipped cars the impact of ethanol blending on emissions is minor.

In modern cars, biomethane again delivers the lowest emissions, while ethanol in high blends can reduce PM emissions compared with gasoline for cars with direct injection engines. For older diesel engine cars, FAME biodiesel and HVO generally improve air quality compared with fossil diesel. FAME biodiesel results in higher NO_Xemissions than HVO, however. In modern diesel cars, fuel type has a minimal effect on air pollutant emissions.