Under pressure and in hot water: biomass to gas
As members of a carbon resource dependent culture gazing into our uncertain energy future, we are daily inundated with stories on topics of high fuel prices, pollution, global warming and energy alternatives that are less than economically feasible or that might even be socially irresponsible.
But William “Bill” Jacoby, an associate professor of biological engineering with a joint appointment in chemical engineering at the University of Missouri, has a simple, one-word answer to questions concerning renewable, clean and affordable energy alternatives: biomass.
Jacoby and his research team are exploring the process of turning biomass into vapor fuels using super critical water gasification (SCWG), with promising results.
“Fossilized resources are finite. We need sustainable alternatives,” Jacoby said. “This process has been around awhile, but it hasn’t been investigated thoroughly. Several years ago, I began to study supercritical water gasification because I thought the process had great potential, and I still do. We gasify biomass at a higher rate and with higher efficiency than previously reported in the literature.”
Doug Hendry, a chemical engineering doctoral student working in Jacoby’s lab — known as the Carbon Recycling Center — explained the lab’s biomass gasification apparatus operates with water heated to 750 degrees Celsius at 200 atmospheres of pressure, or approximately 3,000 pounds per square inch (psi).
“The water becomes a supercritical fluid, somewhere between liquid and steam,” Hendry said.
To begin the process, water is preheated within the continuous reactor apparatus. The biomass is streamed in and passes through the reactor’s furnace for only a few seconds, where the supercritical water serves as both a solvent and a reactant as a small proportion of the water splits to contribute hydrogen and oxygen to the reaction.
Biomass decomposes in this high-temperature, high-pressure process. There is very little oxygen, so it doesn’t burn. Instead it gasifies, forming hydrogen, methane and carbon dioxide. This vapor mixture is similar to natural gas produced at high pressure. Water is efficiently removed in a high-pressure equilibrium phase separator.
Gasification requires heat, so energy must be added to the reactor. However, supercritical water is also infinitely miscible in air and oxygen, so a portion of the biomass can be burned to provide the energy required to gasify the remainder. Between 10 and 25 percent, depending on type of biomass, must be combusted to yield an energy-neutral conversion process. Other attractive aspects of the approach are very low formation of char and tar, catalysts are not required, and nitrogen and other minerals can be recovered from the water stream.
The group is using algae in their current experiments, but Jacoby said any type of biomass could be utilized, from agricultural waste to sewage sludge. He envisions SCWG as an “end-of-pipe” technology at the biomass refinery. “After valuable products are extracted, the wet sludge that’s left over can be converted to fuel gas via SCWG,” he said.
Andy Miller, a chemical engineering master’s candidate working in the lab, said that the difficulty of pumping solids into the high-pressure reactor has been the greatest challenge, but that they are making progress with that upstream process.
“Trying to feed solid slurries into a high pressure environment continuously, is a significant engineering challenge” Hendry said.
“We use a hydraulic cylinder with a piston. Water comes in on one side and algae on the other,” Miller said. “We were mixing dry, powdered algae suspended in water in a five percent solution but it would clog. Recently, preliminary tests using higher concentrations worked better. It’s counterintuitive.”
Miller said that the group might consider using sawdust or fruits just to develop the solids feeding technology, but right now, they’re not trying to identify what gasifies best, but rather working at maximizing the process.
Hendry has been concentrating on the downstream end of the process and said he has been making progress. “We use high pressure equilibrium phase separators and one of the things that I’m trying to do is add additional phase separators to separate the hydrogen and methane from the carbon dioxide,” he said.
“I think it’s a good way to incorporate basic engineering principles to help solve the energy crisis in this country, Hendry said of the work. “It’s challenging, but at the same time, this is a very viable solution.”
Because the products of the process are vapor fuels rather than liquid “drop in” fuels and there is an existing emphasis on the latter, this technology does not fit the immediate strategy of the Department of Energy. However, high reaction rate (small footprint) and a positive energy balance may create a market pull.
“It has great potential,” said Jacoby. “We’re not quite ready. We have more work to do.”
And thanks to a recent grant from Mizzou Advantage, he has funding to continue work on the biomass refinery research.
“I enjoy both the teaching and the research aspects of this job,” Jacoby said. “The impact of our research product may be significant, and I hope it is. However, I know that the impact of training students to embrace and implement the biomass refinery concept will be positive and long-lasting.”
Another application of supercritical fluids
Nick Wilkinson, a senior biological engineering major working in the Carbon Recycling Center, is using supercritical carbon dioxide to remove oil from soybeans.
Currently, the prevailing technology to extract vegetable oil from plants’ seeds uses hexane, a solvent made from crude oil. It is more efficient and less expensive than squeezing the oil from seeds, but there are disadvantages to the hexane crush process.
Wilkinson’s research project has shown that using supercritical CO2 to remove the oil from soybeans is simple, efficient and effective — and the beans do not need to be dried or de-hulled as in the hexane process, further reducing the energy and time for extraction.
“We raise the temperature in the reactor to 80 degrees Celsius and increase the pressure to 7,000 psi and the CO2 pulls out the oil. For every gram of soybeans, we use 10 grams of carbon dioxide,” said Wilkinson, describing the process. “We are working to reduce that number to create a process that is commercially viable.”
As with SCWG, the process is rapid. Wilkinson said it can take less than a minute to completely extract the meal. He projected the process might become portable and potentially occur at the harvest site. Residual meal could be used as animal feed.
Or it could be turned to high-quality fuel gases using the supercritical water gasification process.
“I really like the idea that what I am doing goes directly to the heart of green energy and sustainability. And I get to work with machinery.” Wilkinson said.