Sufficiently cleansing and efficiently reforming syngas are complicated elements of gasification for biofuels production and continued research seeks to optimize both.
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.
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.
Engineering the cost-effective integration of the gasifier, cleansing technologies and FT system is key, according to Stephen Piccot, director of advanced energy and transportation technologies for Southern Research in North Carolina. Piccot's team has developed an integrated 4-ton-per-day pilot-scale biorefinery that cleans syngas using a "biosolvent" and scrubber system, he says. The system is attached to an FT system, converting the syngas to diesel fuel and FT wax. "It has a conventional syngas cleaning system that we're commissioning and trying to optimize at the moment," Piccot says. "It's gotten to the point where it looks pretty clean and it's working well." Piccot and his team haven't analyzed the wax yet, but it looks "stark white," he says, and any impurities would probably show. "We'll integrate a more advanced system this year," he says.
Also in North Carolina, Southern Research and its partners are using U.S. DOE funding to develop another system quite similar to the MCST that would bypass scrubbers, instead using injection of advanced sorbents, a candle filter for solids removal and a tar cracking catalyst to finish the job, according to Piccot. "Dealing with tars is tough to do and the only way to do it now is with these scrubbing approaches, but everyone is trying to move away from them," he says. The process will keep the syngas hot and avoid using liquids, thereby avoiding liquid waste. As in Alabama, the project's progress has been slow, but Piccot expects to have the system built and ready for experimenting this year. The North Carolina location has a bit more funding than Alabama, Piccot says, so the system might be up and running there first.
Along with syngas cleansing, several projects are underway to optimize the biomass feeder system. According to Piccot, feeders and syngas cleanup are the two biggest issues facing the industry. "And we're wrestling with both of them here," he says, adding that the organization is looking to create a win-win situation in syngas cleanup: avoiding the generation of liquids, and turning the tar into something useful. "If we can crack these long-chain tars down to syngas, we just made our system more efficient," he says. "We can produce more fuel now."
Bob Dahlin of Southern Research has studied rare earth metals and found that they function well as low-temperature catalysts for cracking tars, Gale says. That process needs further research, however, to determine the best composition of the metals, how much is needed, optimal temperature range, the lowest level of toxin concentration they can achieve, and what will deactivate them, among other questions, Gale adds. "There are a lot of details on the development of those low-temperature catalysts that still need to be addressed," he says. Other cleaning processes are in the works, too, such as passing the syngas through plasma, effectively killing everything but the gas.
A lot of work has already been done on high-temperature catalysts and it's been determined that a nickel-based dolomite substrate might be the best because it is resistant to sulfur deactivation. Its nickel composition, however, makes it costly. "You definitely need to protect it if you're going to use that type of catalyst because it is very expensive," Gale says.
In addition, Emery Energy Co. is working with partners to develop a proprietary means to mitigate tars and oils in biomass syngas, joining several other companies conducting research projects in the field. Numerous projects in the U.S. dealing with catalyst research have received federal funding. U.S. DOE Secretary Steven Chu appropriated a portion of the $1.2 billion for science from the American Recovery and Reinvestment Act to catalysis for energy research at Energy Frontier Research Centers, according to the DOE.
An Uncommon Practice
While gasification with subsequent cleansing and conversion to liquid is the subject of plenty of research, the practice is not all that common on a commercial scale, Gale says. A South African plant that gasifies coal, cleans the syngas and uses it for fuels is the only current commercial project both Piccot and Gale can identify. But even that system does not clean the syngas to levels ideal for Southern Research's processes. While several companies are poised to develop commercial tar-cracking catalysts the market does not yet exist, Gale says. "There's not too much in the way of commercial catalysts at this time," he adds. But research into tar-cracking catalysts is common among those developing syngas cleanup systems.
Peter Flynn, a University of Alberta professor and biomass expert, agrees that gasification and post-processing are rare and says oxygen gasification has been used only a few times in history-including at the South African plant-to produce syngas. "I don't think it makes sense, personally," he says of gasification. Flynn was a corresponding author on a study conducted in 2006-'07 by Erin Searcy, who was then a student of his, on the end uses for corn stover and straw, finding that gasification and FT processes were significantly less economical than other processes. "If you want electricity, burn [biomass], if you want biofuel, do lignocellulosic fermentation," he says, adding that air gasification is much less economical for electricity production than simply burning the biomass. Flynn says he and Searcy expect the results would be similar using woody biomass. The study has not yet been published.
The problem with FT conversion is that it's expensive, Flynn says. "The catalyst works; you can make this work," he says. "It's just much more expensive than other things you can do with biomass."
Even so, Flynn and Piccot agree that FT conversion catalysts are well-developed and established. "I wouldn't list that as a big area of uncertainty and research need," Piccot says, adding that there is much to do in the area of cleaning and reforming. "The general trend is to ditch the scrubbers," he says. "They're just a big and inefficient mess."
Lisa Gibson is a Biomass Magazine associate editor. Reach her at firstname.lastname@example.org or (701) 738-4952.