Demand for synthetic graphite, a top-priority critical material in the U.S., is increasing because of its use in lithium-ion battery anodes and in steel-making. To meet the increased demand, while also cultivating a domestic source of synthetic graphite (most of which currently comes from China), a team of researchers at the National Laboratory of the Rockies (formerly NREL; Golden, Colo.; www.nlr.gov) led by NLR chemical engineer Carrie Farberow and NLR battery scientist Bertrand Tremolet de Villers, and at North Carolina State University (NCSU; Raleigh, N.C.; www.ncsu.edu) led by professor Sunkyu Park, is developing a process for making biomass-derived graphite that uses existing petroleum-refinery equipment and works at reduced temperatures compared to conventional graphite synthesis from needle coke.
The synthetic graphite process came out of the group’s search for ways to transform forestry and agricultural waste into fuels, such as aviation fuel. The initial step involves the pyrolysis of waste biomass to produce a pyrolysis oil, which is fed into a delayed coker unit. Light fractions are volatilized and collected through condensation for aviation-fuel production, while the solid coke is subjected to subsequent thermal treatment for synthetic graphite production.
The researchers explored two pathways for making the synthetic graphite. One involves a conventional graphitization process, but required the development of a catalytic fast-pyrolysis process in the initial step to upgrade the pyrolysis oil. The upgrade is necessary to render the pyrolysis oil compatible with refinery delayed-coking processes known to produce the specific cokes that will convert to graphite. Only certain cokes have a sufficient level of aromaticity to become graphite at high temperatures (3,000°C). The catalytic fast pyrolysis also reduces the acidity and oxygen content of the pyrolysis oil.
Source: NLR
A second graphite pathway uses coke produced from fast pyrolysis oil that does not necessarily have the structure needed to make highly ordered graphite. “We found that adding iron-metal shavings to the coke during the heating not only restructures the coke into graphite, but also does so at a significantly lower temperature (<1,500˚C) — half of the traditional graphitization temperature,” explains NLR’s Farberow. The lower-temperature graphite production could result in significant cost and energy savings. This catalytic graphitization can convert various carbon feedstocks into high-quality graphite, which is not possible with high-temperature thermal graphitization.
Among the engineering challenges was tuning the delayed coking parameters to yield a bio-coke. Biomass-derived pyrolysis oil behaves differently in the delayed coker unit than crude oil. Another challenge is to remove the residual iron catalysts after catalytic graphitization. “This is because leftover iron adversely affects the graphite performance in a lithium-ion battery,” Tremolet de Villers explains. “We remove the residual iron by acid-washing the graphite, and are working on ideas to recycle the acid and the iron catalyst at low cost,” Park says.
To scale up bio-graphite production, the laboratory is building an integrated delayed coker that can convert larger quantities of pyrolysis oil into graphite and jet fuel. A technoeconomic analysis published in Bioresource Technology by the project team showed that the process can be financially viable.