Production of bio-oil

Dr. Neil Canter, Contributing Editor | TLT Tech Beat April 2013

A new pyrolysis process is under development to increase liquid production for fuel use.

KEY CONCEPTS
• Fast pyrolysis is a procedure used to extract bio-oil from a variety of different biofeedstocks such as switchgrass, camelina, canola and white oak wood.
• The bio-oil produced is a complex mixture that is combustible with further research ongoing to convert it to a usable alternative to diesel and gasoline.
• The ultimate goal is to enable farmers to convert biomass generated during a harvest into fuel to be used for their tractors.

RESEARCH CONTINUES TO FIND A COST-EFFICIENT PROCESS to develop renewable oils and fuels from sources other than petroleum. In the U.S., this movement is driven in part by a proposed standard that requires increasing the supply of renewable fuels to 36 billion gallons by 2022.

One challenge in the development of alternative fuels is to use feedstocks that are not linked to the food supply. In a previous TLT article, a new approach for potentially preparing hydrocarbons suitable for use as fuel is to emulate photosynthesis (1). A process has been developed to efficiently convert carbon dioxide, one of the raw materials used in photosynthesis, into carbon monoxide that can then be a precursor for higher molecular hydrocarbons suitable as liquid fuels. The key element in the process is the use of an ionic liquid that forms a complex with carbon dioxide.

A technique that has drawn renewed interest in the development of renewable fuels is pyrolysis of biofeedstocks derived from nonfood crops. Dr. Akwasi Boateng, lead scientist/chemical engineer at the Agricultural Research Service’s Eastern Regional Research Center of the U.S. Department of Agriculture in Wyndmoor, Pa., says, “Pyrolysis involves the direct heating of a substrate to elevated temperatures in the absence of oxygen/air. This is in direct contrast to combustion, which requires the use of air or gasification where a small amount of air is used.”

Pyrolysis is a technique that dates back to the ancient Egyptians who used it to produce tars for embalming. This type of pyrolysis is known as slow pyrolysis based on the rate of heating and the time involved. Boateng explains, “In slow pyrolysis, the long residence time leads to the formation of a large percentage of charcoal (also known as biochar).”

Research is now underway to develop a new pyrolysis process that significantly increases the production of liquid and can conceivably be used as a fuel or even possibly as a lubricant basestock.

FAST PYROLYSIS
Boateng and his fellow researchers are scaling up a process known as fast pyrolysis that maximizes the formation of a liquid known as bio-oil. He says, “The procedure to use in achieving a high percentage of liquids is to rapidly heat the biofeedstock at a rate of 1,000 C/second and limit the residence time in the reactor to only a few seconds.”

The researchers also grind the biofeedstocks so that small particles with high surface areas are used. Boateng uses a continuous process to move the biofeedstock from a hopper into the reactor that contains a heterogeneous catalyst in a bubbling fluidized bed. The pyrolysis takes place at 500 C under a nitrogen sparge needed to maintain an inert atmosphere. Once completed, the product mixture is quickly moved to two cyclone separators where the biochar is collected.

At this point, the temperature of the gas stream drops to about 325 C, according to Boateng. Condensation of the gas stream then takes place in a series of four quench chillers, which enables liquids to be fractionated out through a drop in the temperature gradient. Non-condensed gas then proceeds into an electrostatic precipitator that acts as a final liquid collection point and is run at 30 kV. The remaining gas then is recycled or is analyzed in a gas chromatograph. Figure 1 shows the reactor setup behind the main control panel.


Figure 1. Development of fast pyrolysis using the setup shown hopes to lead to the production of a direct alternative for crude petroleum that can be refined into gasoline and diesel from a variety of biofeedstocks. (Courtesy of the U.S. Department of Agriculture)

Boateng says, “We typically obtain 60% to 70% bio-oil (also known as pyrolysis oil), 20% to 25% biochar and 10% to 15% synthesis gas from this process. The bio-oil is combustible and can be used as a fuel in boiler applications. The synthesis gas consists of a mixture of carbon dioxide, carbon monoxide, hydrogen and a small amount of methane.

Bio-oil is a complex mixture that makes analysis very difficult. Dr. Charles Mullen, a research chemist in Boateng’s group, adds, “We believe there are at least 400 compounds in the bio-oil, but have only been able to quantify about 10% of these materials.”

The goal for the researchers is to develop a process that would enable the bio-oil to be used as a direct alternative for crude petroleum for subsequent refining to green gasoline and diesel. The main difference between bio-oil and a mineral oil feedstock is the presence of oxygen.

Boateng indicates that the oxygen content of the product is not that different from the raw material. Oxygenated materials derived from high-acid content feedstocks such as cellulosics are very reactive and can oligomerize to form undesirable side products. They can also be corrosive. For this reason, research is continuing to find the right catalyst to be used in fast pyrolysis to deoxygenate the biofeedstock. Examples of oxygenated species present in bio-oil are carboxylic acids, aldehydes, ketones, cyclic ethers and phenolics. Water is also a side product that can be produced in concentrations as high as 30%.

A hydrogen deficit in the reaction mixture leads to the formation of mostly aromatic compounds. Mullen says, “There is just not enough hydrogen present to form aliphatic and cyclic hydrocarbons.”

The researchers are working on a follow-up hydrogenation/hydrodesulfurization process to reduce aromatics and remove sulfurized species. Boateng says, “We are now able to make hydrocarbons using this process in combination with the fast pyrolysis.”

Any nonfood, biofeedstock can be used in this process. The researchers have worked with such raw materials as switchgrass, camelina, canola and white oak wood.

Ideally, the researchers envision fast pyrolysis as a technique that farmers can use in their facilities to generate fuel for their tractors and other equipment. Boateng says, “We believe that farmers can use the nonfood biomass they generate during harvest to be the fuel source.

Neil Goldberg, the group’s mechanical engineer, adds, “We currently work with a pilot reactor that has a capacity to convert 5 kilograms of biofeedstock per hour. A bigger unit is under construction that will handle 80 kilograms of raw material per hour. This unit should be operational in the 3rd quarter of 2013.

Further information on fast pyrolysis can be found in a recent article (2) and patent (3) or by contacting Boateng at akwasi.boateng@ars.usda.gov.

REFERENCES
1. Canter, N. (2012), “Artificial Photosynthesis: Conversion of Carbon Dioxide to Carbon Monoxide,” TLT, 68 (2), pp. 10-11.
2. Mihalcik, D., Boateng, A., Mullen, C. and Goldberg, N. (2011), “Packed-bed Catalytic Cracking of Oak-Derived Pyrolytic Vapors,” Industrial & Engineering Chemical Research, 50 (23) pp. 13304-13312.
3. Boateng, A., Goldberg, N., Johnson, P., Seth, S., Mullen, C. and Rao, S. (2012), “Production of Stable Pyrolysis Bio-Oil from Mustard Family Seeds, Mustard Family Seed Presscake, and Defatted Mustard Family Seed Presscake,” U.S. Patent 8,317,883 B1.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.