Cleansing and Reforming Syngas
Four research alliances received grants from the U.S. DOE to optimize thermochemical biofuels production. Biomass Magazine offers an overview of those projects.
"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."
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."
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.
The energy department also awarded $1.7 million to a project run by Emery Energy Co., Ceramatec Inc. and Western Research Institute, a nonprofit research organization in Laramie, Wyo., which plans to demonstrate new, low-cost means to mitigate tars and oils in biomass syngas including high-impact corn stover. "In the case of conventional biomass syngas cleaning, it is done using either quench methods or tar crackers, and additional downstream equipment," says Ben Phillips, president of Emery Energy. "We're not disclosing our process, but novel methods will focus on the conversion and reforming of tars and oils that won't use the processes I just mentioned. Also we use additional subsequent unit operations in the gas flow line to condition the syngas in order to get to the purity levels required for Fischer Tropsch catalysis and ethanol catalysis. We have a very holistic and integrated view between the gasifier and synthesis gas cleaning steps, and that integration will give us a technological advantage over other processes to produce the high-purity syngas necessary for downstream applications."
WRI and Cerametic are both subcontractors for Emery Energy: Ceramatec is contributing technology and doing some co-engineering work with Emery Energy; and WRI is hosting the gasifier and syngas cleaning facility, also providing technology and operational services, personnel and resources to execute the project, Phillips says. Emery Energy is making modifications to its existing pilot gasifier system in Salt Lake City, and will relocate to WRI along with the synthesis gas cleaning train. Emery Energy is adding $1.2 million to DOE's grant for a project total of nearly $3 million. "For the sake of this DOE program, we are going to be modifying and mitigating tar and oil species, i.e., converting those to additional syngas downstream of the gasifier," he says.
Another syngas cleansing project receiving DOE grant funding includes Southern Research in partnership with Pall Corp., Thermochem Recovery International Inc. and Rentech Inc. The project will test a 1 megawatt gasifier for syngas generation with ceramic filter technology and a proven sorbent/catalyst system for syngas decontamination. Stephen Piccot, director of advanced energy and transportation technologies with Southern Research, says its project is just beginning as the last round of required paperwork is finalized. "So far we've assembled the team and completed a group design for a syngas cleanup system, which is part of Phase I," Piccot says. In its entirety, Phase I consists of design, fabrication and testing of the gas cleanup system on Thermochem Recovery International's biomass gasifier, installation of which is underway at Southern Research as part of a separate contract. This is a three-year project. By the end of 2008, fabrication of the syngas cleanup system is expected to have begun, with test runs and optimizing strategies to start sometime in 2009, along with Phase II.
"Phase II will be linking up all that stuff with a Fischer-Tropsch line and converting the clean syngas into FT wax," Piccot tells Biomass Magazine. "Our proposal added on a refinery pilot step where we take the FT wax and convert it into clean diesel, and the final step for us is to evaluate the performance of the clean diesel in a passenger truck."
Due to secrecy agreements, Piccot says he couldn't discuss specifics about the catalysts, scrubbers or sorbent injection systems to be used in the design, but he did reveal what he sees as challenges ahead. "The technology to convert clean syngas into liquid products is catalyst based, and the requirements of the cleanliness of the syngas is rather fixed and we know what those are, but getting those proper technologies matched to meet those targets-that's the challenge," Piccot says. "The gas needs to be pretty clean to avoid poisoning those FT catalysts. I think technically it's not as big a challenge as [figuring out] how to do it cost-effectively. That's where the challenge is." In a technical paper, Kneale writes: "There is much to be gained by tailoring the catalysis to maximize yield in the product state of choice and this continues to be a major subject of research by Albemarle and others."
Assuming the engineers pulling all these pieces together find no mechanical issues getting everything to heat up at the same time without blowing a pump or burning out a heater, Kelley says the real test is demonstrating successful interaction between the operation of the gasifier, the amount of tars captured in the first catalyst and the productivity of the second catalytic bed.
Thus, the ultimate value of these four research projects-and others out there that may not have received DOE grants-is real time spent on-stream with these systems working simultaneously; gaining actual data on yield from biomass synthesis gas along with productivity, efficiency and lifetime of the catalysts.
Ron Kotrba is a Biomass Magazine senior writer. Reach him at firstname.lastname@example.org or (701) 738-4942.