Conference review: pathways to renewable hydrocarbons

November 3, 2010

BY Ron Kotrba

An afternoon breakout session at BBI International’s Southeast Biomass Conference & Trade Show featured a panel focused exclusively on technological pathways to renewable hydrocarbons.


Phillip Steele, a professor in the College of Forest Resources at Mississippi State University discussed the hydroprocessing of bio-oil derived from fast pyrolysis of biomass. At the start of his presentation he passed around samples of bio-oil, char, feedstock and further refined hydrocarbon fuels from bio-oil. He warned the audience that, if they were to smell the bio-oil, they would realize it has a very pungent, foul-smelling odor, which it does. The samples were made from pine and Giant Miscanthus.


Fast pyrolysis employs temperatures of 400 to 550 degrees Celsius in the absence of oxygen to get a liquid bio-oil product. The bio-oil yield from fast-pyrolyzing pine was 65 percent, and the yield from Giant Miscanthus was 60 percent, Steele said. Bio-oil has high viscosity and a stability problem because of the oxygen content, and has low energy density. The oxygen content in bio-oil is around 45 to 50 percent. "It’s a recalcitrant chemical compound with a lot of water," Steele said. "We can get 1.1 barrel (about 46 gallons) of hydrocarbon per dry ton." Bio-oil contains about 25 percent water or less, and the lower the water content, the better. "It takes more hydrogen to get the water out," he noted. Its pH level is about 3. With all of the drawbacks of bio-oil, it makes for a good feedstock for further refining. But, Steele said, "If you don’t do a two-stage process, you get too much coke on the catalyst."


After the first stage of catalysis, the water content drops from 25 percent to 5 percent, and the acid value is cut in half, and oxygen content drops from 45 percent to around 19 percent, Steele explained. Then, once the second stage is completed, water and oxygen content are eliminated, the acidity is lowered to a neutral pH, and it displays the same viscosity as diesel fuel. The cetane number of the refined renewable hydrocarbon is 60. The two-stage process yields paraffin and isoparaffin wax of 35 percent, 17 percent aromatics, 37 percent naphthenes and 11 percent polynucleic aromatics.


Steele said the fuel breakdown is 37 percent gasoline compounds, 27 percent Jet-A fuel, 25 percent diesel and 11 percent heavy fuel oil.

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He then spoke about lignocellulosic boiler fuel (LBF). At MSU they’ve created two varieties, one of which they added some chemicals to in order to increase heat value, lower the acidity and water content (around 7 percent). "To use LBF you may need to harden your boilers with ceramics or high quality steel in some places," Steele said. The altered variety produces an intense flame "as result of chemical manipulations we did," he said. LBF is a good entrée into the renewable hydrocarbon sector, Steele said, because it’s relatively simple and there are much less stringent standards for boiler fuels compared to those of onroad fuels. "Work out a pyrolysis process, get a market for boiler fuel going and get cash flowing, then think about hydrocarbons for on-road fuel," he said.


MSU is building a 7,000-square-foot pilot plant to handle 4 dry tons per day auger, to be able to produce 140 gallons a day of hydrotreated fuel and 140 gallons a day of LBF. "We’re looking at mild hydrotreating followed by hydrocracking to achieve the 1.1 barrels per dry ton," Steele concluded.


Jeff Sherman, executive vice president and chief business development officer with GRT Inc., spoke about his company’s so-called BTF Process. The variability of biomass can be difficult, not to mention that its carbon is bound to oxygen, nitrogen and other elements. "Biomass’ complexity requires a combination of technologies," he said, "so finding that one ‘magic catalyst’ will be tough."


He discussed anaerobic digestion (AD) as a starting point to get a methane-rich gas. Through AD, one ton of biomass can produce 10,000 standard cubic feet of methane. gas, but it’s produced at atmospheric pressure, which doesn’t make for an ideal transportation fuel. One can clean and compress it, but our national infrastructure is not designed for compressed gas, and cleanup and compression are energy intense.


The BTF Process entails a bromine mediated light alkalane conversion. "Instead of using oxygen to activate the methane molecule, we use bromine," Sherman said. "A zeolite catalyst couples them together … the trick is taking molecules of bromine combined with hydrocarbons under mild conditions to generate liquid hydrocarbon fuels, and make a cyclical process."

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BTF is a three-step process, he said, and is simple, requiring no reformer or air separation. Existing technologies require more steps, he said. The challenge, however, comes with using bromine, which is a halogen. It’s corrosive, similar to hydrogen chloride, so one must carefully select materials for construction. Also the perception of bromine is hard to overcome, as methyl bromide is an ozone depleter. "We try to explain it’s only used as an intermediate, it’s never stored, we make and convert it to product immediately," Sherman said. For feedstock, he said the company is focused on digesting thin stillage from ethanol plants.


David Bransby, a professor of energy and fiber crops at Auburn University, gave an overview of technologies emerging today. Regarding corn ethanol production, he asked, "Why did we do such a stupid thing?" He answered his own question by saying at the time, in the late 1970s and early 1980s, the U.S. had huge surpluses of corn, so it was right for the time, but it’s not the way to deal with our energy crisis today. He said the big problem with ethanol is that it only contains 66 percent of the energy content of gasoline. "Brazil did it, but we are not Brazil," he said.


Bransby also warned the audience that we in the U.S. need to watch what other countries are doing, or we can get left behind. He said he went to Germany to visit MMEAG, a 1.3 MMgy renewable diesel plant built for only $6 million. Its production costs come in at around $2 a gallon.


"Cheap fossil fuel is a disincentive for biofuel production," Bransby said. He also noted that the involvement of Big Oil in the renewable hydrocarbons sector is "really moving things along."


David Johnson, a senior chemist with the Biosciences Center at the National Renewable Energy Lab, spoke on converting lignin to hydrocarbon fuels. Biomass is 15 to 25 percent lignin, and it consists of complex aromatic structures. But it’s very high energy content. "The catalysts must get complete deoxygenation and hydroprocessing with minimal hydrogenation," he said. Improvements needed in converting lignin to renewable hydrocarbons include getting better control of hydroprocessing catalysts, and lowering the cost of base recycling. His research is focused on using ethanol process lignin.


 


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