A team of researchers has developed a high-temperature catalyst biomass gasification system they say can double the amount of biofuel that can be produced with an acre of biomass, and with no char byproduct.
Paul Dauenhauer, assistant professor at the University of Massachusetts Amherst Department of Chemical Engineering, says he and his team of researchers from the department and the University of Minnesota have developed that system.
It began four years ago when Dauenhauer started designing a biomass gasifier that would use a high-temperature catalyst, but without the harmful byproduct biochar. His research has evolved and the team-led by him and University of Minnesota Professor Lanny Schmidt-now says it can alter the char-free process to convert all the biomass carbon to the essential biofuel element carbon monoxide instead of a mix of carbon monoxide and carbon dioxide. "When I got started in this, I wanted to find a process that would allow me to use wood fibers and switchgrass and things like that directly with catalysts," Dauenhauer says. "Now, since I've been able to have that breakthrough, the question is, what can I do with it? What's exciting about this current research is that since now we're able to use catalysts directly with biomass, we can do all sorts of crazy things like add methane and carbon dioxide and it does all the chemistry in a single reactor."
Inside the special catalytic reactor sits a hot surface that melts the cellulose immediately upon contact into a liquid droplet that is essentially melted sugar. The heat transfer is dominantly from the surface, at least 10 times higher than transfer from the hot air in the reactor, Dauenhauer says. "It makes it really easy to heat the biomass up extremely fast," he says, adding that the reaction takes place so quickly it cannot be seen with the naked eye, so the team used a high-speed camera to capture the event. "The fact that the biomass forms this liquid is very exciting for researchers such as myself because now we know how these things convert and we can use it to make reactors," he says. The discovery has already led to more research into how wood chips are converted into fuel.
Dauenhauer says the achievement of the liquid droplet without char is all in the reactor design. "If you design your reactor with different heating rates, and basically a different geometry will do this for you, it'll make the particle heat up really slowly and instead of seeing the liquid, you'll actually see char formation and things like that," he explains. A lot of processes make biochar, but when using high-temperature catalysts, the biochar will coat the catalyst, rendering it less effective.
"So the benefit of touching particles with this high-temperature surface is you get this conversion to a liquid due to the high heating rates," Dauenhauer says. "It basically makes the particle convert down a chemical pathway that's very favorable for making gasses and volatile organics." In addition, the reactor needs to control the heat flow to keep it moving in the right direction and allow the particles to hit the hot surface while permitting the heat to get to it. The team is using a rhodium-based reforming catalyst, but has also demonstrated its process with a nickel-based catalyst and will continue research into cheaper catalysts in the future, although the main focus is the reactor design, Dauenhauer says.
With existing gasification technology, only about 50 percent of biomass carbon is converted to carbon monoxide, the remainder emitted as carbon dioxide, according to Dauenhauer. The team's design, however, recovers all the carbon in the biomass and, with the right balance of carbon dioxide and methane, converts it all to carbon monoxide.
The team used Dauenhauer's ternary diagram (see page 45) to accomplish the perfect combination of fuel, oxygen and carbon dioxide that would facilitate the reverse water-gas shift (RWGS) reaction. "If you operate the reactor within that teardrop, you can actually get all of your biomass carbon back as carbon monoxide," he says. But in order to do that, researchers had to co-feed hydrogen into the reaction to supply the energy needed to drive the RWGS reaction, they say. In lieu of a hydrogen injection, the team adds methane to create hydrogen inside the catalytic reactor. "Using this catalyst, now we can add the carbon dioxide, the biomass and, instead of adding hydrogen, we actually generate the hydrogen within the reactor and use it right away," he says. "Everything happens within a single reactor. That's why I get so excited. It does so many things at once."
The system could be set up near an existing natural gas power plant, providing ready access to methane and carbon dioxide. The team has a ways to go before its project is ready for commercialization, although Dauenhauer believes it may be ready in as few as two years. While the hot surface reaction has been tested on aspen trees, cellulose, starch, polyethanelene and municipal solid waste, the team has only used pure cellulose in experiments with carbon dioxide and methane additives.
The response from the research community to the team's findings has been mixed, Dauenhauer says, and one crucial aspect of the process has yet to be determined: economics.
"As always, there are two questions: Does the process work; and is it economic compared with other uses of biomass?" says Peter Flynn, mechanical engineering professor at the University of Alberta. Biomass gasification and subsequent Fischer-Tropsch conversion to liquid fuels are just not economic processes, Flynn says. "[The researchers are] getting a higher yield of syngas from the biomass by using methane, so in effect, they have a way of turning biomass and methane into a precursor of liquid fuels," he explains. "But today, if you just turn biomass into liquid fuel, it's not as attractive as other things you can do with the biomass and if you turn natural gas into liquid fuel, it's not as attractive as other things you can do with natural gas, unless you've got a stranded field [of natural gas]. So I don't know if putting these two things together will make favorable economics. But it is something new."
The process might modify the actual yield from the process for the same capital cost, says Doug Bull, thermochemical projects manager for Iowa State University's Center for Sustainable Environmental Technologies. "That's how it might help the economics because they might actually get more product for a similar cost of plant equipment," Bull says. Understanding both the chemistry that's happening and the reactor that can double the yield of carbon monoxide does change the economic analysis considerably, Dauenhauer says. "If you derive twice as much fuel from an acre of land as before, you're doubling your return potentially," he says.
Bull agrees with Flynn that gasification is not economic, adding that fast pyrolysis is pulling ahead in popularity with its cheaper cost and simplicity. Even the U.S. DOE is shifting more toward pyrolysis, Bull says, as investment is risky for costly gasification processes. At the end of May, the DOE allocated up to $11 million over the next three years for research and development of biomass pyrolysis for advanced biofuels.
Both Bull and Flynn are skeptical of some of the details of the team's process, having not experimented with the reactions themselves. "They're sort of just modifying the reaction chemistry to get more of the gasses they desire," Bull says, adding that complete conversion of all the biomass carbon is a difficult feat. "They might be able to increase it dramatically, but I don't think entirely all of it would be [converted]. I'm pretty sure they'd still have a little bit of carbon dioxide, even if it's only like 5 percent." The reaction can only be forced to a certain point, he explains. Dauenhauer says the amount of carbon dioxide that is converted to carbon monoxide depends on where inside the teardrop shape the reactor is operating. "The closer the reaction conditions are to the center, the higher the conversion of CO2," he says. Even some of the carbon dioxide added to the process is converted, he adds.
Since char production has been a problem in biomass gasification, a system that eliminates it would be a significant development, Flynn says, although there are other proposed solutions. "They are saying they have found a spot they can operate where they don't produce any char, and if that's true, it's a big deal." It's one of two reasons the process is intriguing, he adds, along with methane that in effect can be converted to liquid transportation fuels. "It sounds reasonable. I know enough to say to it sounds reasonable."
Dauenhauer says the ultimate goal of the research is to answer the scientific questions that will lead to a wide array of new biomass processes and reactions. "In terms of the university, we're focused on generating the intellectual property that American companies and start-up companies can use to get these new types of processes and reactors into the field as quickly as possible."
Lisa Gibson is a Biomass Magazine associate editor. Reach her at email@example.com or (701) 738-4952.