Supply Chain Sustainability Analysis of Whole Algae Hydrothermal Liquefaction and Upgrading

The Department of Energy’s Bioenergy Technology Office (BETO) collaborates with a wide range of institutions towards the development and deployment of biofuels and bioproducts. To facilitate this effort, BETO and its partner national laboratories develop detailed techno-economic assessments (TEA) of biofuel production technologies as part of the development of design cases and state of technology (SOT) analyses. A design case is a TEA that outlines a target case for a particular biofuel pathway. It enables preliminary identification of data gaps and research and development needs and provides goals and targets against which technology progress is assessed. On the other hand, an SOT analysis assesses progress within and across relevant technology areas based on actual experimental results relative to technical targets and cost goals from design cases and includes technical, economic, and environmental criteria as available.

BETO also develops supply chain sustainability analyses (SCSA) for key biofuel production technologies that are the subject of design case or SOT analyses (Dunn et al. 2013). The SCSA utilizes a life-cycle analysis to estimate the energy use and greenhouse gas (GHG) emissions associated with biofuel production and assists in comparing several biofuel pathways. This report documents an SCSA of whole algae hydrothermal liquefaction (AHTL) as the conversion technology to produce renewable diesel (RD). Jones et al. (2014) developed the design case process model that provides the material and energy intensity of the feedstock conversion step in the SCSA.

The SCSA production stages for microalgae-derived RD are presented in Figure 1. Various inputs (red boxes) can be considered for each supply chain step (green boxes). These inputs can include energy, fertilizers for biomass growth, and any materials that may be needed during the conversion process. The major environmental output from the system is GHG emissions, which come from direct sources like fuel combustion during a processing step or indirect sources like fertilizer production. Another common output is coproducts, which can be used to displace materials or energy from other production processes. There can be difficulties in allocating emissions to these co-products (Wang et al., 2011), so care is needed during their consideration.

The SCSA for RD produced via AHTL starts with feedstock production, which requires nutrients (fertilizers), water (not considered in this study), and energy in the form of electricity and other fuels, e.g., natural gas. After production, the feedstock is transported to the conversion facility, or biorefinery, using energy in the form of a transportation fuel. In the case of microalgae, cultivation ponds are assumed to be co-located with the conversion facility (Davis et al., 2012; Frank et al., 2011) meaning a transportation fuel is not required. However, energy is needed for pumping the biomass from the harvesting units to the biorefinery. For the algae-to-RD production reported here, the harvested feedstock goes to a thermal conversion process, which includes material inputs like catalysts and sulfuric acid. A small amount of naphtha, which was treated as a liquid fuel, is produced along with RD in the AHTL pathway. No other co-products are produced in the fully integrated AHTL algae-to- RD pathway. The total supply chain emissions burdens were allocated to total fuel produced, including naphtha and RD.

The renewable fuel, after the conversion process, is transported to a fueling station by train, barge, and truck. The biogenic CO2 released when the fuel is combusted balance out with the atmospheric CO2 that the algae incorporated when it was growing (Frank et al., 2011). The emissions described above are the so-called, “fuel cycle” emissions. Emissions are also associated with the construction of the plant (Canter et al., 2014). These “infrastructure cycle” emissions were estimated in this study.