PyrolysisWhat is Pyrolysis? Pyrolysis of woody biomass is an endothermic thermo-chemical process, which needs heat to occur. Similar to gasification, pyrolysis causes heat and chemical reactions to take place, simultaneously changing the chemical composition and physical phase of the fuel. Pyrolysis actually takes place during the gasification process. However, pyrolysis is different from gasification and other high-temperature processes because it does not involve reactions with other reagents such as oxygen or steam, and the products formed are different.

The process of pyrolysis is commonly used in the chemical industry to produce charcoal, activated carbon, methanol, and other chemicals from wood and to convert biomass into syngas and bio-char.

How Pyrolysis Works?Pyrolysis takes place under various controlled conditions within different pyrolysis reactors (e.g., bubbling fluidized bed, circulating fluidized beds, rotating cone pyrolyzer etc.). The methods and feedstocks used in pyrolysis depend on the desired products. The process can be made quite efficient as a portion of the fuels produced can be used to provide all of the necessary energy to drive the reaction. There are two main types of pyrolysis:

Fast Pyrolysis: Utilizes woody feedstocks, which have been ground to relatively small particles >3mm and dried to a low moisture content of 10% or less. Fuel is then subjected to temperatures of 400-500°C or higher for a very short time of between 0.5 to 2 seconds. The high temperature and oxygen starved environment within the pyrolysis chamber cause the organic matter to thermally decompose. The chemical bonds of the material break down at a very fast rate changing the physical state of the fuel. Separation occurs as the solids, liquids, and gases produced separate and by-products are formed. Typical product yields from fast pyrolysis are about 60-70% bio-oil, 12-15% char, and 13-25% gas.

Slow Pyrolysis: Also known as carbonization begins between 200-400°C. Woody fuel is placed into the pyrolysis chamber where it stays for minutes, hours, or even days. Depending on the system, feedstocks used may be larger than those used in fast pyrolysis. Size, moisture content, feedstock type and length of time the fuel is left in the reactor will all depend on the desired yields. Products of slow pyrolysis differ from fast pyrolysis because the lower temperature and longer exposure time favor the formation of solids. This method creates larger quantities of bio-char (activated carbon), although gas and bio-oil are also produced.

PyrolysisBenefits of Pyrolysis• Pyrolysis uses biomass to produce bio fuels, which help to mitigate the effects of global warming by reducing use of fossil fuels.

• Promoting good forestry practices by building up new forest industry and technologies benefits communities and the environment.

• Pyrolysis facilities can achieve high efficiency rates by generating heat and power from their own processes.

• Pyrolysis plants are made up of units, which can be changed to deal with available waste streams or feedstocks (e.g., wood chips or farm waste). This flexibility allows for advantageous adaptation to industry changes.

• The pyrolysis process produces multiple useful products, much like the petroleum industry. This value added product stream maximizes the benefits of biomass use.

• Char, oil and gas are comparable to naturally occurring fossil fuels in terms of composition and formation process, and can be excellent substitutes for fossil fuels.

• Bio-char produced from pyrolysis improves soil health by increasing its ability to retain fertilizers and release them slowly. It also contains many micronutrients needed by plants. It is safer than manure and sewage since it has been disinfected at high temperatures. When used as fertilizer bio-char acts as a carbon sink, sequestering carbon in the ground and helping to mitigate climate change.

• Bio-char releases nutrients at a slow rate and greatly reduces the risk of water table contamination when used as a fertilizer.

• Liquid bio-oil produced from pyrolysis is similar to synthetic diesel fuel and can be used as a fuel.

• Bio-oil has a fuel value that is 50-70% that of petroleum based fuels making it more cost effective to transport than raw biomass. Bio-oil can also be used in boilers or upgraded to a transportation fuel.

Information Sources:

US Department of Agriculture (2010) Biomass pyrolysis research: what is pyrolysis? Retrieved May 24, 2012, from http://www.ars.usda.gov/Main/docs.htm?docid=19898

Kansas State University (2011) Sustainable Energy Research Experience for Undergraduates. Retrieved May 25, 2012, from http://sustainable-energy.ksu.edu/REU/S11/jmarkham/index_background_info.html

Wise Geek.com (n.d) What is pyrolysis? Retrieved May 25, 2012, from http://www.wisegeek.com/what-is-pyrolysis.htm

Friends of the Earth (2009) Pyrolysis, gasification and plasma. Retrieved May 25, 2012, from http://www.foe.co.uk/resource/briefings/gasification_pyrolysis.pdf