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Study shows biodiesel blends in buses reduce PM, other harmful exhaust elements, EC and CO

A new study on the combustion properties of biodiesel for use in urban transit buses found that using biodiesel can effectively reduce the mass of particulate matter released in both hot and cold idle modes. The study, published by the Mineta National Transit Research Consortium (MNTRC), observed a reduction in amount of particulate matter, number of elements, and elemental carbon; the reduction is considered beneficial to promoting the clean air and human health.

The researchers found that biodiesel has many advantages over regular diesel even in a very low blend percentage, including low emissions of particulate matter, combustion elements (mainly sulfur), elemental carbon, and carbon monoxide. In sum, they recommended that governments consider using blends of biodiesel in urban and commercial vehicles to enhance air quality.

Biofuels, such as biodiesel, offer benefits as a possible alternative to conventional fuels due to their fuel source sustainability and reduced environmental impact. Before they can be used, however, it is essential to understand their physical properties, combustion chemistry, and characterization of the exhaust due to a number of issues associated with fuel properties—for example, a lower heating value and higher cloud point than regular diesel. High viscosity of biodiesel may lead to poor atomization of the fuel spray and inaccurate operation of the fuel injectors, so, it may cause fuel injector problems. Biodiesel may produce high NOx emissions. Depending on the feedstocks and blending ratios used to produce the fuel, variations in chemical properties may also be an issue.

—Kumar et al.

In the study, the team investigated the combustion of biodiesel from various types of feedstocks—soybean methyl ester (SME); tallow oil (TO); and waste cooking oil (WCO)—in a variety of volume percent blends (B00, B20, B50, and B100) using a bench-top combustion chamber in a laboratory setting. Ultra-low-sulfur diesel (ULSD) was used as base fuel. Different combinations of combustion temperature and pressure were applied to investigate their effects on emissions. In addition, physical properties (flash point, cloud point, and kinematic viscosity) of all biodiesel blends were measured following the American Society for Testing and Materials (ASTM) standard methods.

Particulate matter (PM) samples were collected through field tests of 10 different transit buses to investigate the source of elements in the emission gases. A similar procedure was followed to collect and analyze PM from the laboratory combustion experiments to determine precisely which elements are from biodiesel fuels. A total of eleven inorganic and metal elements were detected in the laboratory experiments, while fifteen elements were observed in field experiments. Calcium (Ca), sodium (Na), and iron (Fe) were the major elements found in the PM emissions in both the field experiments (77 to 85 wt %) and the lab experiments (up to 90 wt%).

Based on gravimetric analysis, PM emissions significantly decreased by less than 17% on average when using B20, and newer transit buses showed a greater PM reduction (more than 98% on average) than old buses when using ULSD. For both hot and cold idle tests, a substantially high reduction in total particulate matter (TPM) was observed, and the maximum PM concentrations for ten different buses under hot and cold idle conditions were 2.77 and 5.59 µg/m3, respectively.

Elemental carbon (EC) and organic carbon (OC) analyses of the collected PM from field tests were carried out by an accredited analytical laboratory. OC/EC analyses showed that more OC was emitted during cold idling (>80%) than in hot idling (>65%). Furthermore, the OC/EC ratio was found to be greater for new buses with catalytic convertors (9.57 – 13.37) than for old buses without converters (1.85 – 4.55). Positive matrix factorization (PMF) determined that four sources—oil (including fuel and engine oil), lubricant, engine parts, and ambient conditions—contributed heavily to the generation of PM in the exhaust.

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