Bio-refinery reduces build-up of greenhouse gases

University of Georgia scientists are developing a bio-refinery that will be an environmentally sound alternative to crude oil refineries. The bio-refinery processes such biomass as agricultural waste and biofuel crops to produce environmentally beneficial fuel.

“Obtaining our energy through a bio-refinery instead of depending on fossil fuels doesn’t just reduce carbon dioxide emissions,” said K.C. Das, a bioconversion engineer for UGA’s College of Agricultural and Environmental Sciences, “it actually sequesters carbon.”

That means a potential source of carbon dioxide is transformed into a form of carbon that’s harmless. “It could remain in the soil for a long time, away from the atmosphere where it would contribute to global warming,” Das said.

The researchers began with a simple idea: The chemical difference between hydrocarbons, such as coal and oil, and carbohydrates, found in plants, is small. “We realised that all we’re missing is a process that can mimic nature’s conversion of biomass to fossil fuel,” Das said.

Pyrolysis, an old technique that creates charcoal, can do this. Pyrolysis transforms biomass and agricultural waste products into a fuel and chemical feedstock called bio-oil. One by-product of pyrolysis is hydrogen, a much cleaner fuel and a substance used to make ammonia fertiliser.

Unfortunately most hydrogen ammonia/nitrogen fertiliser producers commonly use natural gas (a non- renewable resource), which creates large amounts of greenhouse gases. Agricultural waste is a renewable resource and is net-zero in emitting greenhouse gases.

“You basically take peanut hulls and heat them to 450-500 degrees Celsius,” Das explained. “In the absence of oxygen, the cellulose pyrolizes and forms oil. It looks a lot like engine oil, but it’s a little more viscous.”

One of the most exciting aspects of the technology is that it also generates carbon char, a solid form of carbon. Unlike carbon dioxide, the analogous by-product of crude oil refineries and a major environmental problem, the char is harmless and even beneficial.

Eprida, a private company and partner with the UGA team, has developed a process that turns char into a slow-release nitrogen fertiliser. Studies show that char in this form restores soil fertility and increases crop yields

“Char is unique because it is quiescent – it just stays there,” Das said. “With fossil fuels, carbon dioxide that has been in the soil for millions of years is used up in a few years, producing large amounts of carbon dioxide that end up in the atmosphere. This pyrolysis technology actually sequesters the carbon while adding ammonia to the char so that we can put it back in the soil as a fertiliser.”

Since the bio-refinery uses agricultural waste like peanut hulls, poultry litter and other by-products, the technology also turns an environmental obstacle into an environmental advantage.

While the technology is very promising, there are still challenges to work on. A major challenge is that bio-oil is unstable and reactive, which makes it more difficult to work with.

“There are around 300 compounds in bio-oil,” Das said. Engines must be modified or the fuel must be modified to work with a fuel that has different properties.

The team is also looking for new applications for both the technology and its by-products. Other useful by-products discovered so far are glues, “a very high-value product,” Das said. “Paper mills have a lot of wood waste and manufacture plywood, which uses a lot of glue. Setting up a wood waste bio-refinery on-site would be ideal.” Pyrolysis products can also be used to flavour food in products like liquid smoke.

Finally, they’re working to make the process more efficient, which will make it more economical.

“It is a complex process,” Das said. “The typical process is to make something and throw away the trash. The difference in a bio-refinery is that everything is used for something else, everything has value.”