Gas-to-liquid conversion catalysts are fussy and the concentration levels of toxins, including hydrogen sulfide, sulfur, trace metals, ammonia and organic molecules such as tars in syngas need to be low. Limitations for Fischer-Tropsch systems can depend on the manufacturer’s specifications, but typically sulfur should be at or below 50 parts per billion, and metals and hydrogen cyanide should be at about 10 parts per billion, according to Thomas Gale, manager of Power Systems Research at the Southern Research Institute in Birmingham, Ala. “But even at 50 parts per billion, you’re still going to damage your catalyst,” he says. The contaminants react with the catalytic sites on the surface, forming a new product that is no longer a catalyst for the desired process. Different processes and catalysts demand different concentration levels, but all are strict. Any level of contaminants in syngas will damage a catalyst, but lower levels will allow it to function longer without needing a replacement.
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Typical cleansing techniques include sorbents, scrubbers, filters and reforming catalysts that crack tars into smaller molecules, or more syngas, along with guard columns that can be used to scrub syngas prior to reforming. Tars may be the most difficult contaminant to handle, as they will gum up the conversion process.
Sorbents—material substrates that absorb and contain other substances—capture halides, trace metals and sulfur, and also help crack tars. Their application depends on the type of gasifier and the other methods of cleanup being deployed. Gale has conducted a considerable amount of research using sorbents to capture trace metals before they go through syngas filters, which are also studied at Southern Research, specifically candle filters. The filters consist of a solid ceramic or metal material that covers a cage and forms a tube-like structure. The ceramic or metal has small pores that capture the char from the syngas before it continues up the tube, emerging clean on the other side. “Ultimately, we’d like to just have carbon monoxide and hydrogen in our syngas,” Gale says. Several stages of syngas cleaning using sorbents and other materials that crack tars before the filter, along with tar-cracking catalysts after the filter, could be the most effective cleaning system, and Gale’s team has made progress in developing that process.
Applying It All
Southern Research, which operates in Alabama, North Carolina and Maryland, has filed a full submission for a patent on its Multi-Component Syngas Treatment system, Gale says. “The concept is to apply all of the available syngas cleanup techniques that we have into a compact unit that can be used to connect distributed biomass gasification systems with the end component,” he says.
Because Southern Research is a nonprofit organization, development of the system has been slow. The organization is working on a laboratory-scale pressurized gasifier, but has only had the funding thus far to purchase the main reactor, Gale says. It will continue to work on building the structural supports and a thermal oxidizer, along with all the other associated equipment. Once that system is up, the first priority will be to start testing MCST configurations for different potential uses and gasifying systems, he says. “Eventually, we’d like to have a small Fischer-Tropsch system hooked up to it.” The type and extent of cleaning depend on the desired end product and type of gasifier the MCST is attached to. Additional catalysts could be incorporated after the filters to shift the syngas composition more toward hydrogen. “If we’re good at doing our cracking, we’ll have carbon monoxide and hydrogen, but typically many applications wish to have more hydrogen than you might otherwise produce,” Gale says. The system will also include catalytic candle filters. “So inside the filter, there will be catalysts,” he says. “That’s an important component we’re implementing.”
The MCST will be designed to be cheaper, more efficient and more environmentally friendly than other cleansing processes. Scrubbing, for example is an expensive form of cleanup. “We hope that we’ll be able to remove some of the metals upstream of the candle filter, so scrubbing will not be required,” Gale says, adding that it’s very possible the MCST will include scrubbers for certain applications.
Ideally, Southern Research will develop and optimize the MCST for commercial use all over the world. Because of funding hurdles, however, a timeline for that goal has not been established. The catch in the scenario is that sometimes it takes significant investment and development, with the presentation of technical findings, before commercial interest is piqued. Still, the system will address what Gale says is the biggest challenge in syngas cleanup: integrating everything into a compact unit.
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