Monday, April 30, 2007

A techno-economic analysis of a syngas biorefinery producing hydrogen and PHA

Scott C. Bents and Robert C. Brown. Center for Sustainable Environmental Technologies, Iowa State University, 286 Metals Development Bldg., Ames, IA 50011

Recent fluctuations in crude oil prices have increased the interest in biorenewable resources.  Since crude oil is used as an energy source as well as a feedstock for many products, biorenewable resources must fulfill both these roles.  One alternative method for processing biomass is gasification followed by syngas fermentation.  This poster is an exploration into the technical and economic issues related to a syngas fermenting biorefinery producing 50 metric tons per day of hydrogen gas and 13 metric tons per day of PHA, a biodegradable plastic.

Gasification is a thermochemical conversion process which offers an alternative to other biorefinery conversion methods such as enzymatic processes.  Rather than using complex molecules to assist in the breakdown of resilient compounds found in biomass, thermochemical conversion uses heat to break biomass into syngas, which consists mainly of carbon monoxide and hydrogen.

Syngas fermentation uses bacteria to consume the carbon monoxide portion of syngas in order to produce a variety of chemicals.  These bacteria can be considered biological catalysts, which offer several advantages over traditional metal-based catalysts.  These advantages include operating temperatures and pressures at standard conditions, lower sensitivity to the CO:H2 ratio in syngas, and greater resistance to contaminants in the syngas such as char, tar, ash, chlorine and sulfur.

This analysis used switchgrass costing $55 per metric ton as a feedstock, and the bacterium Rhodospirillum rubrum to ferment the syngas.   PHA at 95% purity was produced for approximately $0.90 per kilogram.  A hydrogen market value of $1.90 per kilogram was assumed.