Pyrolysis of forest residues: An approach to techno-economics for bio-fuel production

The techno-economics for producing liquid fuels from Maine forest residues were determined from a combination of: (1) laboratory experiments at USDA-ARS's Eastern Regional Research Center using hog fuel (a secondary woody residue produced from mill byproducts such as sawdust, bark and shavings) as a feedstock for pyrolysis to establish product yields and composition, and (2) Aspen Plus® process simulation for a feed rate of 2000 dry metric tons per day to estimate energy requirements and equipment sizes.The simulated plant includes feedstock sizing and drying, pyrolysis, hydrogen production and hydrotreatment of pyrolysis oils. The biomass is converted into bio-oil (61% yield), char (24%) and gases (15%) in the pyrolysis reactor, with an energy demand of 17%. The bio-oil is then hydrotreated to remove oxygen, thereby producing hydrocarbon fuels. The final mass yield of gasoline/diesel hydrocarbons is 16% with a 40% energy yield based on the dry biomass fed, this yield represents a fuel production of 51.9 gallons per dry metric ton of feedstock. A unique aspect of the process simulated herein is that pyrolysis char and gases are used as sources for both thermal energy and hydrogen, greatly decreasing the need to input fossil energy. The total capital investment for a grass-roots plant was estimated to be US$427 million with an annual operational cost of US$154 million. With a 30 year project life, a minimum fuel selling price was determined to be US$6.25 per gallon. The economic concerns are related to high capital costs, high feedstock costs and short hydrotreating catalyst lifetimes.