Study Finds Environmental and Tailpipe Pollutants Benefits in Using Biodiesel in Construction Vehicles
Using biodiesel in construction vehicles offers promising environmental benefits in terms of reduced tailpipe emissions as well as reductions in fuel cycle emissions of selected pollutants, according to a new study by researchers at North Carolina State University. A paper on their work was published 16 July in the ACS journal Environmental Science & Technology.
The researchers developed an updated and modified life cycle inventory (LCI) to estimate fuel cycle energy consumption and emissions of selected pollutants and greenhouse gases. Key improvements in the LCI included an update of combustion emission factors based on 2006 US national average emission rates; comparison of pre-NSPS (New Source Performance Standards) and NSPS-compliant soyoil plants; and the use of portable emission measurement system (PEMS) data for real-world tailpipe emissions factors based on 15 nonroad diesel vehicles: five backhoes, four front-end loaders, and six motor graders.
Vehicle emissions scenarios included: baseline petroleum diesel (PD) in-use measurement data based on PEMS; B20 (20% biodiesel) in-use measurement data based on PEMS ; estimated B20 vehicle emissions based on EPA’s engine dynamometer data; and estimated B20 vehicle emissions based on NREL’s chassis dynamometer data.
Life cycle fossil energy reductions are estimated at 9% for B20 and 42% for B100 versus petroleum diesel based on the current national energy mix. Fuel cycle emissions will contribute a larger share of total life cycle emissions as new engines enter the in-use fleet.
Seven scenarios were used in the LCI analysis. The baseline scenario is for PD with in-use PEMS measurement data for tailpipe emissions. For the B20 LCI, each vehicle has two fuel cycle scenarios and three vehicle emissions scenarios, which includes six scenarios in total. In order to represent real-world activities, two or three duty cycles were observed for each type of vehicle during PEMS measurements. The tailpipe emissions used in the LCI model were averaged over those duty cycles.
The researchers found that the average differences in life cycle emissions for B20 versus diesel are:
- 3.5% higher for NOx
- 11.8% lower for PM
- 1.6% higher for HC
- 4.1% lower for CO
Local urban tailpipe emissions are estimated to be:
- 24% lower for HC
- 20% lower for CO
- 17% lower for PM
- 0.9% lower for NOx
The largest portion of biodiesel fuel cycle HC emissions are from soyoil plants. The new NSPS standard is expected to significantly reduce these HC emissions. If biodiesel fuel production occurs in the same airsheds as tailpipe emissions, local urban HC, CO, and PM emissions would be decreased, but there may be an increase or a decrease in NOx depending on vehicle tailpipe NOx emissions.
The air quality benefits of using biodiesel will depend on the geographic locations of the fuel production and of the vehicles consuming the fuel and on prevailing baseline air quality conditions. It is likely that the reduction in tailpipe emissions from use of B20 will occur in airsheds where air quality is a significant problem, whereas the emissions associated with the fuel cycle may occur in rural areas where air quality (e.g., ozone, PM) problems may be less pressing. However, highly localized air quality problems in rural area can be severe in some cases.
.Biodiesel is a promising alternative to diesel, but there are environmental trade-offs. Nonetheless, soy-based B20 offers promising environmental benefits in terms of reduced tailpipe emissions as well as reductions in fuel cycle emissions of selected pollutants. Further studies are needed to improve confidence in vehicle emission factors, which were a key source of uncertainty in LCI emissions, are needed. Further study to characterize the speciation of HC and PM will also be useful.—Pang et al. (2009)
Shih-Hao Pang, H. Christopher Frey and William J. Rasdorf (2009) Life Cycle Inventory Energy Consumption and Emissions for Biodiesel versus Petroleum Diesel Fueled Construction Vehicles. Environ. Sci. Technol., Article ASAP doi: 10.1021/es802916u