Researchers from three US Department of Energy (DOE) labs—Argonne National Laboratory, Pacific Northwest National Laboratory (PNNL) and the National Renewable Energy Laboratory (NREL)—have investigated the economic and environmental implications of producing bioblendstocks for diesel fuels from two wet waste-to-fuel pathways: hydroprocessed esters and fatty acids (HEFA) from yellow grease and swine manure hydrothermal liquefaction (HTL) followed by biocrude upgrading. An open-access paper on their results is published in the journal ACS Sustainable Chemistry & Engineering.

Heavy-duty diesel vehicles using mixing controlled compression ignition (MCCI), the most common ignition and combustion strategies for heavy-duty diesel engines, contribute significantly to the emissions of GHG, oxides of nitrogen (NO_x), and particular matter (PM) and are subject to stringent emission standards. Producing MCCI fuel blendstocks from terrestrial biomass holds promise to make alternative fuels with favorable MCCI fuel properties, such as high cetane number and low aromatics, that could help curb engine-out NO_x and PM emissions, which could help to meet increasingly stringent emission regulations and bring about reduction in emission aftertreatment costs and eventually vehicle ownership costs.

Meanwhile, wet waste feedstocks, such as animal manure and fats, oils, and greases (FOG), represent another important category of resources that could be utilized to produce MCCI bioblendstocks due to its abundant availability. Skaggs et al. estimated that with conversion by hydrothermal liquefaction (HTL) and upgrading, the wet waste resource availability in the United States could be converted to jet fuel that is equivalent to about 24% of the U.S. demand in 2016.

Given the nature of wet waste resources that requires dedicated waste management practices to collect, store, treat, and dispose of the waste, shifting the waste resources from going through conventional waste management practices to utilization as feedstocks for energy production may represent an avoided cost of waste management and disposal. About 61% of the total sewage sludge, 27% of the total manure, and 7% of the total food waste may be available at negative prices, while FOG is more commoditized with its price determined by market demand. Thus, the use of wet waste resources as feedstocks in advanced bioenergy processes represents an opportunity to recycle organic waste material into renewable energy and at the same time offset the cost and environmental impact of their disposal via more conventional practices, such as landfilling, anaerobic digestion (AD), or incineration.

—Ou et al.

Among their findings:

• The estimated minimum fuel selling prices were $1.22/gasoline liter equivalent (GLE) and $0.94/GLE for the yellow grease to HEFA and swine manure HTL pathways, respectively.

• The life-cycle greenhouse gas (GHG) emissions of the two pathways were 11.2 and −33.3 g of CO₂e/MJ, respectively, for the yellow grease to HEFA and swine manure HTL pathways. The credits of avoided emissions from conventional swine manure management were the main reason for the negative GHG emissions of the swine manure HTL pathway.

• The marginal GHG emissions abatement costs were estimated to be $116–$270/tonne CO2e and $5–$103/tonne CO₂e for the yellow grease HEFA and swine manure HTL pathways, respectively, for a diesel price ranging between $0.5/GLE and $0.9/GLE.

• Since the yellow grease HEFA pathway is already commercialized, it can benefit from the $200/tonne carbon credit in the California Low Carbon Fuel Standard market, which could help the yellow grease HEFA pathway to achieve near-zero marginal GHG emissions abatement cost.

Cost breakdown of the minimum fuel selling price for MCCI fuel produced from (a) yellow grease and (b) swine manure HTL, in comparison to the diesel price of $0.79/GLE. OU et al.

Life-cycle GHG emissions results for MCCI bioblendstocks produced from wet waste feedstocks, in comparison to 91 g of CO₂e/MJ of petroleum diesel. OU et al.

TEA/LCA results of the two waste feedstock pathways highlight their economic and environmental performances and opportunities for improvement. From an economic perspective, the price of waste feedstocks is low and even negative when the cost impact of fuel carbon regulation is considered. Meanwhile, the plant size has a great impact on the MFSP. It is thus critical to develop a robust supply chain for a large-scale waste-to-fuel facility so that production costs could be reduced with economies of scale.

On the environmental side, using waste feedstocks for energy production could offer great GHG emission reduction potentials relative to petroleum diesel fuels. Avoided manure management emissions due to shifting the waste feedstocks to MCCI bioblendstock production could generate great GHG credits for a waste-to-fuel pathway like in the case of the swine manure HTL pathway. No BAU scenario was considered for the yellow grease to HEFA diesel pathway because yellow grease has been utilized for fuel production in an established market. Should the market dynamics of yellow grease as a feedstock for biofuel production change, the cost and environmental impacts of treating yellow grease at wastewater treatment plants should be considered in future analysis.

—Ou et al.

Resources

• Longwen Ou, Shuyun Li, Ling Tao, Steven Phillips, Troy Hawkins, Avantika Singh, Lesley Snowden-Swan, and Hao Cai (2021) “Techno-economic Analysis and Life-Cycle Analysis of Renewable Diesel Fuels Produced with Waste Feedstocks” ACS Sustainable Chemistry & Engineering doi: 10.1021/acssuschemeng.1c06561