Gasifying Biomass with Sunlight
Sundrop Fuels, a startup based in Louisville, CO, says it has developed a cleaner and more efficient way to turn biomass into synthetic fuels by harnessing the intense heat of the sun to vaporize wood and crop waste. Its process can produce twice the amount of gasoline or diesel per ton of biomass compared to conventional biomass gasification systems, the company claims.
Gasification occurs when dry biomass or other carbon-based materials are heated to above 700 ºC in the presence of steam. At those temperatures, most of the biomass is converted to a synthetic gas. This “syngas” is made up of hydrogen and carbon monoxide, which are the chemical building blocks for higher-value fuels such as methanol, ethanol, and gasoline.
But the heat required for this process usually comes from a portion of the biomass being gasified. “You end up burning 30 to 35 percent of the biomass,” says Alan Weimer, a chemical engineering professor at the University of Colorado, Boulder.
A few years ago, Weimer and his research team began looking at ways of using concentrated solar heat to drive the gasification process. It worked so well that Weimer and Chris Perkins, the graduate student who came up with the idea, went on to cofound Copernican Energy to commercialize the approach. Copernican was acquired by Sundrop Fuels in 2008, and its solar-reactor technology is now at the heart of a 1.5-megawatt thermal solar gasification demonstration facility in Colorado.
The gasifier system consists of ceramic tubes that pass through a furnace. The gasifier is mounted atop a tower surrounded by a field of solar concentrating mirrors that reflect sunlight back to the furnace. As the biomass is dropped through the intensely hot ceramic tubes, it is vaporized into syngas.
Weimer, a former Dow Chemical engineer, says the system is “agnostic” to the types of biomass it can process. “It’s like a sledgehammer because of the (1,200 to 1,300 ºC) temperatures it operates at,” he says, explaining that conventional gasification uses lower temperatures to try to minimize the volume of biomass used to fuel the process. But keeping the temperature lower poses another problem. Gasification at temperatures below 1,000 ºC leaves behind tar. “And that tar is expensive to get rid of,” says Weimer. “If you leave it in there, it will end up killing your catalysts downstream when you try to reform your product into (liquid) fuel.”
The higher temperatures also make for a better quality syngas. Conventional gasification typically produces a syngas mixture that’s half hydrogen and half carbon monoxide. Sundrop’s process achieves a hydrogen-to-CO ratio of two to one.
“I can tell you, the economics have been looked at quite extensively, and the idea of being able to produce gasoline at less than $2 a gallon without subsidies, we believe that’s a real number,” Weimer says. The financial upside is even greater if carbon pricing becomes a reality, because the solar-driven process results in a reduction in greenhouse-gas emissions compared to conventional fuel production. “The key now is to design a scalable solar reactor.”
Ajay Dalai, an associate professor of chemical engineering at the University of Saskatchewan, says powering gasification with solar energy has merit but could prove tricky. “When you’re transferring the heat into the pipe, how do you make sure that it is distributed thoroughly through the biomass?” Controlling heat transfer and temperature levels will be key, he says.
Wayne Simmons, chief executive officer of Sundrop Fuels,
doesn’t underestimate the challenges to commercializing the technology. He acknowledges, for example, that the greatest biomass resources aren’t located where the greatest solar resources are. Still, some woody biomass does exist in the U.S. Southwest, where Sundrop plans to build its first commercial plant. States such as New Mexico and Arizona, for example, regularly thin their forests to lower the risk of forest fires. But to access more feedstock, Sundrop is also looking at transporting energy crops, such as switchgrass, by rail from as far north as Kansas and as far east as Texas.
Construction on Sundrop’s first commercial facility is expected to begin this year. The company plans to couple its solar gasification plant with a pilot-scale biorefinery that can produce up to eight million gallons of transportation fuel annually. It’s targeting 2015 for a full-scale biorefinery that can produce 100 million gallons a year.
The company has attracted some key investors, including venture-capital firm Kleiner Perkins Caufield & Byers.
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