China-US team concludes duckweed biorefineries can be cost-competitive with petroleum-based processes
Researchers from the US and China have determined that a duckweed biorefinery producing a range of gasoline, diesel and kerosene products can be economically competitive with petroleum-based processes, even in some cases without environmental legislation that penalizes greenhouse gas emissions. A paper describing their analysis of four different scenarios for duckweed biorefineries is published in the ACS journal Industrial & Engineering Chemistry Research.
Duckweed, an aquatic plant that floats on or near the surface of still or slow-moving freshwater, is attractive as a raw material for biofuel production. It grows fast, thrives in wastewater that has no other use, does not impact the food supply and can be harvested more easily than algae and other aquatic plants. However, few studies have been done on the use of duckweed as a raw material for biofuel production.
The team, comprising researchers from Princeton University; Peking University; Institute of Process Engineering, Chinese Academy of Sciences; and PetroChina company, investigated four different thermochemical pathways for the production of gasoline, diesel, and kerosene from gasified duckweed synthesis gas as the intermediate:
Low-temperature and high-temperature Fischer−Tropsch processes (LTFT and HTFT) using both iron and cobalt based catalysts. Clean syngas is converted to hydrocarbons via cobalt or iron-based catalysts operating at either low or high temperature. The residue/wax produced from FT synthesis is directed to a hydrocracker, and the vapor phase C3−C22 hydrocarbons are sent for further upgrading.
Methanol to hydrocarbons via the methanol-to-gasoline (MTG) or methanol-to-olefins (MTO) processes. The hydrocarbons are refined into the final liquid products using ZSM-5 catalytic conversion, oligomerization, alkylation, isomerization, hydrotreating, reforming, and hydrocracking.
|Fischer−Tropsch (FT) synthesis flowsheet. Credit: ACS, Baliban et al. Click to enlarge.||Methanol synthesis and upgrading flowsheet. Credit: ACS, Baliban et al. Click to enlarge.|
The team developed a process synthesis framework to select the refining pathway that will produce the liquid fuels at the lowest possible cost. The used the synthesis framework to determine the effect of refinery capacity and liquid fuel composition on the overall system cost, the refinery topological design, the process material/energy balances, and the lifecycle greenhouse gas emissions.
The researchers used four case studies focused on two target capacities (i.e., 1,000 and 5,000 bpd) and two product compositions (i.e., unrestricted and US demand ratios of gasoline, diesel, and kerosene) to demonstrate the capability of the process synthesis framework and determine the process design that has the lowest overall cost.
The price of crude oil for which the duckweed BTL refineries will be competitive is $100/bbl for the 1 kBD unrestricted study, $69/bbl for the 5 kBD unrestricted study, $105/bbl for the 1 kBD US ratio study, and $72/bbl for the 5 kBD US ratio study. An important highlight for these four studies is the strong use of methanol synthesis opposed to FT synthesis. The lack of inert production during methanol synthesis allows for the use of a large internal synthesis gas loop and less complex synthesis gas conversion design within the refinery. The methanol can be readily converted to gasoline, diesel, and kerosene using a ZSM- 5 catalyst.
A parametric analysis on the duckweed purchase price indicates that there exists a threshold price of duckweed above which the refinery will no longer be economically competitive with crude oil refining. This threshold level for duckweed purchase depends on the desired refinery capacity and will decrease as the capacity decreases.
If crude oil was priced around $105/bbl, then the 1 kBD refineries would be economically competitive with a duckweed purchase price of $50/dry metric ton. A reduction in the duckweed purchase price to $30/dry metric ton will make the 1 kBD duckweed refineries competitive at crude prices above $95/bbl. For the 5 kBD refineries, the process synthesis framework demonstrates the economic viability at a crude price above $72/bbl for duckweed purchase prices at $50/dry metric ton. If this purchase price was raised to $70/dry metric ton, the refineries would remain competitive at crude priced above $82/bbl.—Baliban et al.
The US National Science Foundation and the Chinese Academy of Sciences provided funding for the research.
Richard C. Baliban, Josephine A. Elia, Christodoulos A. Floudas, Xin Xiao, Zhijian Zhang, Jie Li, Hongbin Cao, Jiong Ma, Yong Qiao, and Xuteng Hu (2013) Thermochemical Conversion of Duckweed Biomass to Gasoline, Diesel, and Jet Fuel: Process Synthesis and Global Optimization. Industrial & Engineering Chemistry Research doi: 10.1021/ie3034703