“Just as the biochem needs better enzymes, thermochem needs better catalysts,” says Steve Kelley, professor and department head of wood and paper science at North Carolina State University. A research and development partnership among NCSU, the University of Utah and Research Triangle Institute, a nonprofit research organization in North Carolina, was one of four alliances that received grants from a U.S. DOE solicitation to improve thermochemical processes. “The catalysts developed so far work well on natural gas reforming, but with natural gas there aren’t the tars, ammonia and chlorine there is with the biomass,” Kelley says. According to Brian Kneale of Albemarle Corp., catalysis of syngas from coal or natural gas is rather simple. What’s much more difficult is producing a catalyst system capable of “thermal and mechanical stability,” with the greatest challenge being engineering a reactor system to “maximize thermal efficiency in a compact design.”
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And just as the need for better enzymes and C-5 ethanologens is sometimes seen as a “package deal” for the biochem guys, Kelley says, the same can be said about synthesis gas cleanup and selecting optimal gas-to-liquid catalysts in thermochemical reforming. “They go hand-in-glove,” he says. “If you’ve got a more robust gas-to-liquid catalyst then you don’t need the same high-quality gas cleanup catalyst. But if you’ve got a really outstanding high-quality gas cleanup [catalyst], then you can really take anything off the shelf for a gas-to-liquid catalyst—but to test them they need to be run together.” That’s the trick.
Despite the parallel challenges in both routes to cellulosic ethanol, thermochemical holds at least one big advantage over and above its yield and conversion efficiencies—feedstock quality and consistency is of little concern. “I don’t care if there’s a little bit of bark residue or treated wood going into my thermochem process because there are sorbents and other ways of handling that,” Kelley explains. “It’s a much more robust process.” Sorbents are material substrates used to absorb and contain other substances. In some fluidized bed boiler operations, for example, calcium stone is used to trap sulfur.
The thermochemical process may be robust but the life of some cleansing and reforming catalysts is fragile: Killing the catalyst in hours due to the presence of contaminants has been a shared experience for researchers in this field, experts say. Syngas from wood has been known to foul up catalysts in less than 24 hours, and corn stover, rich in minerals and ash, can nullify a catalyst system in minutes.
Kelley says RTI has intellectual property on sorbent-based and fluid cracking catalysts for the tars gained from gasifying biomass, but the NCSU, UU and RTI project will also use other available sorbent technologies to get rid of chlorine, nitrogen and sulfur. “It will be a combination,” he says. Tars, ammonia, chlorine, heavy metals and sometimes sulfur are the major contaminants in untreated biomass syngas, which, if left untreated before entering the gas-to-liquid reformer, would mean certain catalyst death by poisoning.
Experience in tar and ammonia destruction gained at Southern Research Institute, a not-for-profit organization that conducts scientific research at facilities in Alabama, Maryland and North Carolina, include high-temperature cracking at 1,650 to 1,750 degrees Fahrenheit with nickel-based catalysts, in addition to lower temperature tar cracking at 800 degrees F using modified fluid cracking catalysts. Southern Research is also working on another ammonia destruction process that uses reverse selective catalytic reduction at relatively low temperatures: between 700 and 800 degrees F. “In [reverse SCR (selective catalytic reduction)], nitrogen oxides are injected or generated in situ to react with the ammonia and convert it to nitrogen,” according to Southern Research.
“There are a lot of claims out there about magic sorbents, guard columns or pretreatments that work,” Kelley says. Guard columns help trap contaminants from the syngas prior to entering the main reactor columns, and are dispensable whereas main columns are not. “But no one’s got published data out there,” he says. “The real data will come out of our work and that of the other award recipients and DOE will be able to use it.”
Expanded Efforts
In addition to the NCSU, UU and RTI consortium to improve syngas cleanup and catalyst selection, DOE funded three more projects with similar interests. ConocoPhillips Co. and Iowa State University are partnering to test an integrated biomass-to-liquids system whose process, as described by the energy department, uses “gas cooling through oil scrubbing rather than water scrubbing in order to minimize wastewater treatment.” The intended biomass for gasification is switchgrass. The DOE’s description of the ConocoPhilips and Iowa State University process continues: “The gas-oil scrubbing liquid will then be sent to a coker in existing petroleum refining operations to be used as a feedstock.” The team was awarded $2 million toward the $3.1 million project.
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