DoD Researchers Work to Increase the Production of Higher Chain Hydrocarbons from CO2 Using a Traditional Fischer-Tropsch Catalyst
Researchers at the US Naval Research Laboratory (NRL) and the Center for Applied Energy Research at the University of Kentucky are investigating the hydrogenation of CO2 using a conventional Fischer-Tropsch cobalt catalyst for the production of valuable hydrocarbon materials.
Other studies have shown the ability to convert CO2 primarily to methane with a distribution of other hydrocarbons (earlier post as one example). The focus of this work, reported online 25 June in the ACS journal Energy & Fuels, is to attempt to improve the production distribution toward higher chain hydrocarbons (HCs) and increase conversion rates using conventional Fischer-Tropsch catalysts (Co-Pt/Al2O3).
The US Department of Defense (DoD) is the single largest buyer and consumer of fuel at 12.6 million barrels per day, according to the Defense Energy Support Center.
World-wide “peak oil” production is expected to occur from 2010 to 2025+ (by some experts estimate that we have already reached peak production since 2004). This along with increasing demand can cause large swings in price and availability. Fuel independence would alleviate uncertainties in the world market supply of oil along with commercial fluctuations in price.
In addition only high energy density petroleum-derived fuel meets stringent military aviation requirements. Thus, the DOD has a vested interest in maintaining this supply by supporting the development of synthetic hydrocarbon fuel from the vast natural resources, such as coal, shale, gas hydrates, and CO2, available in the United States.—Dorner et al. (2009)
Very little research has been performed applying CO2 as the carbon source in synthetic fuel production, the authors note, because generally CO2—which is chemically very stable—has been thought of as having too high of an energy barrier for polymerization, even in the presence of a catalyst.
The problem with conventional Fischer-Tropsch processes for the production of synfuels is their carbon intensity, and their being practical only for “land-based operations.” NRL is eyeing the concentration of CO2 in the ocean (“relatively concentrated at approximately 100 mg/L of seawater“) as a potential source.
...if CO2 could economically be extracted from the ocean, then marine engineering processes could be envisioned to use this carbon source as a potential chemical feedstock. From an environmental perspective, such a process would have tremendous benefits in reducing the impact of anthropogenic CO2 on climate change and would eliminate the emission of sulfur and nitrogen compounds that are readily produced from the combustion of petroleum-derived fuels.—Dorner et al. (2009)
The researchers conducted the CO2 hydrogenation reactions in a one-liter three-phase slurry continuously stirred tank reactor. The team measured the ability to direct product distribution as a function of different feed gas ratios of H2 and CO2 (3:1, 2:1, and 1:1) as well as operating pressures ranging from 450 to 150 psig.
Under all conditions investigated, methane remains the primary product, with concentrations ranging from 97.6% of the product to 93.1%; higher concentrations of C2-C4 hydrocarbons were found at the 1:1 ratio. The researchers also found that the portion of longer chain hydrocarbons (i.e., hydrocarbons above methane) increases with increasing time on stream (TOS), irrespective of the H2/CO2 ratio.
The authors suggest that deactivation of the methane-forming active sites on the catalyst with increasing TOS may play a role in this product distribution shift toward C2-C4 HC with increasing TOS, irrespective of the feed gas ratio. They also speculate that the change in the feed gas ratio leads to a lowering of the catalyst’s methanation ability of CO2 in favor of chain growth, with two different active sites for methane and C2-C4 products present on the surface of the catalyst.
Future research will focus on determining the proposed reaction mechanism and corroborating the hypotheses.
Robert W. Dorner, Dennis R. Hardy, Frederick W. Williams, Burtron H. Davis and Heather D. Willauer (2009) Influence of Gas Feed Composition and Pressure on the Catalytic Conversion of CO2 to Hydrocarbons Using a Traditional Cobalt-Based Fischer-Tropsch Catalyst. Energy Fuels, Article ASAP doi: 10.1021/ef900275m