Feedstock-To-Tailpipe Analysis of Soybean-Derived Biodiesel and Glycerol Valorization in Kansas

Abstract

Biodiesel has become a popular substitute to ultra-low sulfur diesel due to its production from renewable or waste feedstocks and direct use in compression-ignition engines. Biodiesel is currently fabricated via oil transesterification which also produces crude glycerol as a byproduct. The crude glycerol has low economic value and is often disposed of directly and causes environmental harm. However, glycerol is rich in carbon, hydrogen, and oxygen. Therefore, it is necessary to find ways to valorize glycerol into useful products such as soap, hydrogen, methanol, dimethyl ether (DME), and ammonia to economize biodiesel production and increase profitability. However, the conversion of glycerol into useful products requires material and energy inputs that carry a significant environmental footprint which must be evaluated with the help of a comprehensive life cycle analysis (LCA). Past LCAs of biodiesel have differed in system boundaries, data sources, and co-product allocations resulting in a wide range of energy and emissions results. Therefore, this research reviews the literature and proposes a robust LCA methodology in the form of a Well-to-Exhaust (WtE) analysis, and conducts a comprehensive biodiesel LCA with Kansas as the control volume. The WtE methodology is applied to both renewable and waste feedstocks to determine their renewability.The results show that biodiesel production from soybeans in Kansas has a fossil energy ratio (FER) of 4.64 compared to the national average of 5.54. Moreover, waste cooking oil biodiesel has a higher FER of 6.25, mainly because of the waste feedstock assumption. These values indicate greater scope for biodiesel production in Kansas. Besides, glycerol use for soap and hydrogen production also holds commercial value. Specifically, glycerol (FER = 0.55) can replace methane (FER = 0.59) as the major feedstock for hydrogen production via steam reforming. This could be instrumental in minimizing glycerol waste while offsetting methane use for hydrogen production. Other than these findings, methanol recovery from glycerol and reuse in the biodiesel production loop offsets significant energy (consumed in methanol production) and pathway emissions. Furthermore, low FERs were noted for methanol, dimethyl ether, and ammonia production from glycerol, suggesting that these processes consume more fossil energy than the amount of renewable energy recovered. However, these results will likely improve over time with improvements in soybean production and glycerol valorization processes. Nonetheless, endeavors to fabricate dimethyl ether from glycerol could reap benefits from the standpoint of assisted fuel combustion in internal combustion engines. The PtE results from ULSD-DME and biodiesel-DME show that at high engine loads, the high reactivity of dimethyl ether under low temperatures can help beat the NOx-PM tradeoff usually seen in compression-ignition engines. This opens up avenues for further experimental research to explore the possibility of utilizing DME-assisted combustion to beat the tradeoff over a wider range of loads. Finally, this effort finds that combustion models can be used to supplement experimental findings for use in WtE framework. However, caution is advised when using these models as the underlying simplifications can sometimes lead to erroneous results.