A New York company is building on the results of a study by Columbia University researchers to optimize solid fuel-to-energy conversions, with a new and surprising reactive medium in biomass gasification: carbon dioxide.
Conventional gasification uses steam, but this creates two issues: it uses water, and the entire reaction is endothermic, meaning it absorbs heat and requires energy input, Castaldi says. Using carbon dioxide in the process is less energy intensive, as it is already a gas and does not require the heating of water to produce steam. "The big energy savings come because I'm not using water," he says. "Any process that uses less water is better." The concept became the focus of a study by Castaldi and post-doctoral researcher Heidi Butterman called "CO2 as a Carbon-Neutral Source via Enhanced Biomass Gasification," featured on the Web site of the Journal of Environmental Science and Technology.
Energy efficiency is not the only benefit of using the common greenhouse gas during biomass gasification. "To my surprise, when I did the experiments, not only did it need less energy, but it also more efficiently converts the solid fuel," he says, adding that using steam leaves behind residual that has some carbon left. "With carbon dioxide, the only thing left is the nongasifiable minerals that are in that biomass," he explains. The use of oxygen instead of water/steam is an option, but it is highly reactive and can combust the biomass instead of gasifying it, he explains. "[Carbon dioxide] is more reactive than steam, but not as reactive as oxygen, and that's important," he says.
The How and Why
Castaldi and Butterman used a range of carbon dioxide (0 percent to 100 percent) and steam mixtures on about 50 different kinds of biomass, finding that between 25 percent and 40 percent carbon dioxide seemed optimal, depending on the process and desired end product. "Adding much more than 40 percent carbon dioxide in that process is only adding a diluent," he says. Feedstocks such as beach grass, pine needles, poplar wood and municipal solid waste, along with coal, were gasified at temperatures of 25 to 1,000 degrees Celsius (77 to 1,832 degrees Fahrenheit) at rates of 1 to 100 degrees Celsius per minute in the range of carbon dioxide/steam mixtures, according to the study.
The increased efficiency occurs for two reasons. The first is because of carbon dioxide's reactivity. "If it's not reactive enough, like the steam, you form a residual that is very, very low in surface area, that's nonporous," Castaldi says. "And what happens is, as it reacts, it becomes more and more difficult to react." He compares the reaction to a sponge, saying it's crucial to absorb the reactive medium all the way through, not just on the surface. Steam reacts mostly on the surface, densifying the biomass and preventing it from absorbing more steam. But the carbon dioxide reacts at the right amount to not only continuously react with the biomass, but to keep pores open or even open them further, he says. The carbon dioxide enables the biomass to keep its sponge-like quality, or porosity, while steam collapses those pores, he says.
Another reason that carbon dioxide increases biomass gasification efficiency is the increased occurrence of the water-gas shift reaction: water and carbon monoxide reacting to form hydrogen and carbon dioxide. It works like this: as the mixture of steam and carbon dioxide goes over the biomass and gasifies it, the carbon dioxide reacts more than the steam, which means there is steam present that is not reacting with solid biomass, Castaldi explains. It's left in the gas phase and as the carbon dioxide gasifies the biomass and makes carbon monoxide, that carbon monoxide goes into the gas phase and reacts with water via the water-gas shift reaction. The reaction is exothermic, meaning it releases heat, and the steam the carbon dioxide leaves behind increases that heat release, thereby increasing occurrence of the entire reaction, he says. "A system using carbon dioxide needs less energy because there's an exothermic reaction that's a little more engaged," he says. The process does not use all carbon dioxide, Castaldi says, but about 30 percent. "It turns out that the energy needed to create syngas from steam and biomass is nearly equal to making syngas using all carbon dioxide and biomass," he says of the reaction. But the difference is in the heat release.
In addition, some of the carbon dioxide input-between 20 percent and 50 percent of that 30 percent-is actually converted into carbon monoxide, Castaldi says. "So now I'm introducing a sufficient quantity of carbon dioxide that causes the process to actually utilize a good portion of it," he says.
In this process, the input of carbon dioxide determines the ratio of hydrogen to carbon monoxide in the syngas. With more carbon dioxide, the ratio goes down, increasing carbon monoxide and decreasing hydrogen. Tweaking input can make desirable syngas compositions for different processes, such as turbine combustion, special chemicals production, Fischer-Tropsch for diesel fuels, and others, Castaldi says.
Applications and Implications
Applied globally, this process could recycle tens of hundreds of megatons of carbon dioxide per year, Castaldi and Butterman say. The use of the greenhouse gas in the low-temperature gasification of beach grass on a global scale could create a beneficial use for 437 million metric tons (482 million tons) of carbon dioxide, based on estimated transportation needs in 2008, according to the study. That's the equivalent of taking about 308 million typical vehicles, producing 6 metric tons of carbon dioxide per year, off the road.
Using carbon dioxide for gasification in large power plants can increase efficiency by 3 percent to 4 percent, Castaldi says. He recognizes that it may seem small, but when hundreds of megawatts are being produced, that percentage adds up. "That's huge," he emphasizes. "That's a lot of power." Companies want greater efficiency to boost economic benefits. They also want to be environmentally conscious, but efficiency will increase profit and all businesses strive for that, he says.
Castaldi and Butterman studied bench-scale applications, but Castaldi has set up a larger process at Columbia and says the data and information match and still look promising. Even so, more work remains to be done. "I still need to understand the reactions that are going on and need to find if there's a better biomass to use or if there's a better coal to use," he says. "How does this work in terms of municipal solid waste?" More research in the optimal percentage of carbon dioxide in the mixture is warranted, also, he adds, as the bench-scale study was not exhaustive in that regard.
"Do I think this could be deployed on a wide scale?" he asks. "Absolutely. You could employ this technology today in existing coal-fired power plants. It really depends on how serious people are in terms of using waste streams." Streams with a wide range of carbon dioxide percentages can be used, the study showed, but the big question is where to get it. "There's definitely potential," he says. "The data is there. It's just an engineering solution."
That solution may come from the New York division of ATK, a Minnesota-based aerospace and defense company. ATK's Center for Energy and Aerospace Innovation and Columbia University have been working together on various alternative energy processes for years. "This was an idea that professor Castaldi had approached our organization with and we've been working with him and his group on developing a subscale prototype," says Dean Modroukas, ATK director for advanced programs. ATK has a small 2.5-kilowatt operational system using torrefied waste biomass and coal, and now is working on the funding for scale-up. The emphasis thus far has been on the gasification process and preprocessing upstream. "Right now, the focus of the activity has been on the heart of the system," Modroukas says. The next step will be using a solid oxide fuel cell to convert the syngas to electricity.
"Our goal is to work with professor Castaldi and his team to take it to the next level and bring it to light from the product perspective," he says. "We're hopeful that as the process goes on and as we continue to make the successes we've been making, that with all the partners involved, we'll be able to make this a product offering." Target markets would be military depots where equipment is located and hardware is built, Modroukas says. "Those types of facilities generate a lot of waste," he says, including wood, cardboard, crates and paper products. "All of that just normally gets shipped away." The system also could be used for distributed power generation for the commercial space. Modroukas declined to delve into details about the ongoing project, as more research and work is being done and no timeline has been established.
While ATK is accustomed to defense and aerospace projects, the company has found a new interest in renewable and clean energy solutions, Modroukas says. "When you look at us, you may see all the bullets and all the rockets, but a lot of the technology that goes into developing a gasifier and putting together systems that can actually work in long term and have the reliability that's necessary is perfectly suited for a defense company," he says. "It's an exciting time and we're very excited about the process. It's going quite well and it's been quite successful."
The process illuminates the capabilities and opportunities of using the gas for something, and on a broader scale, of using waste for something and gaining value. "It's not just about sequestering the carbon dioxide," Castaldi says. "It's not just about capturing it and burying it." Any good engineer would look at a system and question how the waste can be used for another process, he adds.
Lisa Gibson is a Biomass Magazine associate editor. Reach her at firstname.lastname@example.org or (701) 738-4952.