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Engineering Analysis of Indirect Biomass Liquefaction

By By John Hurley
Hydrogen-powered fuel cell vehicles may be several years away with respect to viable market penetration, but other fuel cell applications are more viable in the near-term. In fact, fuel cells may be a near-term solution to power production in remote areas. Most remote power systems or generators run on diesel fuel or gasoline. Future remote power systems may run on indigenous biomass residues such as forest or agricultural wastes that power fuel cells with a completely "green" footprint. As oil prices continue to climb, advanced biomass-powered energy systems will continue to gain market acceptance.

The Energy & Environmental Research Center has partnered with IdaTech LLC and completed a study to determine the feasibility and process economics for production and operation of a truck-mounted biomass gasification plant that would produce synthetic gas, or syngas. The syngas would be converted to methanol as a liquid hydrogen carrier for use in remote fuel cell power systems. Currently, most hydrogen is produced from fossil sources, but this project touts renewable biomass-derived hydrogen using wood residue as the primary feedstock. Methanol would be produced and converted to hydrogen at remote sites. The hydrogen would then be used to make electricity in a proton exchange membrane fuel cell system. A detailed study was completed to identify the most appropriate method for biomass gasification relative to methanol production and to identify potential methanol production systems that can handle various compositions of syngas. The goal is to demonstrate wood-based gasification and methanol production at approximately 200,000 gallons per year.

The EERC investigated several air, oxygen, steam and thermally integrated gasification systems and decided a thermally integrated downdraft gasification system is the most economical system for methanol synthesis in a portable system. Key features include its ease of operation and efficient use of heat. A significant advantage is that the biomass does not need to be predried because the heat integration assists in maintaining the uniform gasification temperatures required for complete conversion of green biomass. Moisture is utilized in the simplified air gasification process. Also, a comparison of oxygen gasification versus air gasification (oxygen blown versus air blown) showed that the complexity of oxygen gasification is not warranted.

Methanol synthesis options were also analyzed. A literature review indicated that the integrated gasifier should be able to meet the requirements for methanol synthesis without significant gas conditioning or drying of the feedstock.

The economic analysis reveals that the cost to produce methanol using thermally integrated gasification is 79 cents per gallon, assuming a 20-year life with an estimated capital cost of $665,844 financed at 6.5 percent. The economics assume an 85 percent annual availability for a plant powered with grid electricity producing 176,967 gallons of methanol per year. The performance is favorable compared to other gasification approaches and minimizes equipment components and labor. Methanol pricing, provided by Methanex, was $1.80 per gallon at the time of the January 2007 study.

John Hurley is a senior research advisor at the EERC in Grand Forks, N.D. He can be reached at jhurley@undeerc.org or (701) 777-5159.
 

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