Characterizing Emissions from the Combustion of Biofuels September 2008

Emissions from two biofuels, a soy-based biodiesel and an animal-based biodiesel, were measured and compared to emissions from a distillate petroleum fuel oil. The three fuels were burned in a small (3.5x10 Btu/hr) firetube boiler designed for use in institutional, commercial, and light industrial applications. Emissions were measured for carbon monoxide (CO), carbon dioxide (CO2), oxides of nitrogen (NOx), and sulfur dioxide (SO2) using continuous emission monitors. Concentrations and size distributions of particulate matter (PM) were also measured. Flue gas samples were collected and analyzed to determine concentrations of aldehydes and other volatile organic compounds, polycyclic aromatic hydrocarbons, and polychlorinated biphenyls. The boiler efficiency was also determined for operation using each of the three fuels. The most significant difference was for PM, where the distillate fuel oil had emissions roughly ten times higher than for either of the two biodiesel fuels. The particle size distributions (measuring particle volume) showed a mode near 1 μm for the two biodiesels and near 2.5 μm for the distillate fuel oil. All three fuels also had a mode near 20 nm. SO2 was nearly four times higher for the distillate petroleum fuel oil than for either the soy or animal biodiesel. NOx emissions were slightly higher for the distillate fuel oil than for the two biodiesels, but all three were within 6% of one another. CO and CO2 concentrations were approximately the same for the three fuels. The differences in concentrations of the organic compounds were relatively small, with the emissions patterns being similar for all three fuels. Boiler efficiencies were also similar for the three fuels, with any difference being within the unit’s measured variability range. In general, the two biodiesel fuels emitted less pollutants than the distillate fuel oil, and the low life-cycle CO2 emissions for the biodiesels results in a net CO2 reduction of nearly 75% when using these fuels compared to the petroleum distillate fuel.