Soot morphology from glycerol/biodiesel/diesel emulsion fuel combustion in an indirect-injection engine

Glycerol is a biorenewable co-product of biodiesel manufacturing which continues to receive interest as a diesel fuel additive in transportation, heating and power applications. Its unique chemical properties lead to complex combustion features resulting in unique particulate matter emissions which are not fully characterized using traditional in-pipe methods. Therefore, uncertaintly exists for the expected particular matter structure and chemical composition. This study is the first detailed examination of the particulate matter morphology and composition of glycerol emulsion fuels emitted from a four-cylinder, high-speed, indirect-injection engine. Three glycerol emulsion fuels are prepared at 10, 20 and 30% by weight into a 50/50 mixture of biodiesel and low-sulfur diesel with emissions quantitated according to a modified ISO D8178 D2 protocol. The emulsion fuels meet US EPA emissions limits for the engine class, but with an increased fuel consumption of approximately 50% by weight, due to low energy density, compared to low sulfur diesel fuel. Particulate matter is characterized and quantitated using established TGA and MCPC/SEMS methods which result in a significant discrepancy in overall mass emission rate of 0.05 and 0.01 g/kW-h, respectively, at the 30% blend level. TEM imaging is used to characterize the particles revealing a shift from chain-like to cluster-like particles with increasing levels of semi-volatile organics matter fraction at higher glycerol concentrations. Primary particle size is found to be largely unaffected by glycerol concentration with nominal particle diameters of 25 nm across the fuel set. This result suggests that semi-volatile organic material inhibits the agglomeration of primary particles resulting in increased PM 2.5 emissions and whose presence is not accurately quantitated using gravimetric and mobility methods alone.