Date of Award

5-2014

Level of Access

Campus-Only Thesis

Degree Name

Master of Science (MS)

Department

Chemical Engineering

Advisor

M. Clayton Wheeler

Second Committee Member

William J. DeSisto

Third Committee Member

Adriaan R. P. van Heiningen

Abstract

The techno-economics for producing liquid fuels from forest biomass were determined from a combination of: (1) laboratory experiments to establish product yields and composition, and (2) process simulation to estimate energy requirements and equipment sizes. Petroleum and it derivatives have become one of the most important sources of energy for many countries. Because of petroleum’s price volatility and the fact that is a nonrenewable natural resource there is a special interest to find substitutes that can be used without major changes to the current fuel distribution and utilization infrastructure. Because of its simplicity, fast pyrolysis is an attractive alternative for the conversion process, which is scalable from mobile units to thousands of tons per day. However, the utility of the oils produced by fast pyrolysis is limited because of the chemical and physical properties of these highly oxygenated oils which require additional catalytic hydrotreating to make them suitable for refinery blendstocks.

Pyrolysis experiments using Maine forest residues were conducted at USDA’s Eastern Regional Agricultural Research Center to determine pyrolysis yields and product properties. The system included a continuous fluidized bed operating at approximately 500°C with a feed rate of 1 kg/hr with nitrogen as the fluidization medium. Products were collected in a cyclone char separator, cold water condensers, and an electrostatic precipitator. Aspen Plus® was used to model the material and energy balances for a 1000 dry metric ton per day commercial plant that included feedstock sizing and drying, pyrolysis, hydrogen production and hydrotreatment of pyrolysis oils. The process simulation implemented yield and composition data from the pyrolysis experiments along with recent literature data for state-of-the-art pyrolysis oil upgrading.

About 6% of the heating value of the forest residue feedstock, which contains approximately 44% water, is required for drying. The biomass is converted into bio-oil (60% yield), char (25%) and gases (15%) in the pyrolysis reactor, with an energy demand of 17%. During upgrading, the bio-oil is treated to get a final 16% gasoline/diesel mass yield and 40% energy yield based on the dry biomass fed. The pyrolysis char is gasified and the gases, in combination with other gases from upgrading and pyrolysis, are utilized to generate the hydrogen for bio-oil upgrading. The energy requirements for the entire plant are satisfied by burning part of the pyrolysis char and the off gases from the hydrogen purification step. The capital investment is estimated to be $190 MM with an annual cost of manufacturing of $75 MM. The goal of converting forest residues into an infrastructure-compatible fuel is demonstrated successfully, but there are concerns related to low yields, high capital investment and short lifetimes of expensive catalysts.

This techno-economic analysis for the production of bio-fuels from forest residues gives a feasible option to supply fuels from a renewable natural resource. It presents a process that, under sustainable forest operations, is energetically independent, and most importantly, does not require an external hydrogen supply. To compete with current fuel prices there must be improvements in process yields and catalyst robustness. Our interdisciplinary group at the University of Maine is working to achieve these goals.

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