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.
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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.
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