In 2007, NREL published a report—Thermochemical Ethanol via Indirect Gasification and Mixed Alcohol Synthesis of Lignocellulosic Biomass—that assessed the potential for a gasification/mixed alcohol synthesis process to meet a cost target of $1.07/gallon by 2012. A key finding of that report was the importance of R&D for the synthesis catalysts.

Poor performance could increase MESP [minimum ethanol selling price] by 25% or more. Whether this is due to actual non-target catalyst formulations or due to poor performance in Clean-Up and Conditioning that leads to poor alcohol synthesis catalyst performance, the cost effects are major. The catalyst cost sensitivity range was extremely large, from $2.50/lb to over $2,250/lb. This was done to bracket a variety of potential catalyst systems, not just cobalt moly-sulfide. Exotic metals such as rhodium (Rh) or ruthenium (Ru) can add considerable cost to a catalyst system even at relatively low concentrations. At low catalyst costs, total CO conversion and alcohol selectivity (CO₂-free basis) have the largest impact on the overall MESP. The catalyst productivity (g/kg/hr) did not show much impact over the sensitivity range chosen. In reality, all of these catalyst performance indicators are tightly linked. It is unlikely that research could change one without affecting the others.

The thermochemical conversion process for mixed alcohol production has six basic steps:

• Feedstock handling and preparation.

• Gasification. NREL considered indirect gasification is considered in its assessment. Heat for the endothermic gasification reactions is supplied by circulating hot synthetic olivine “sand” between the gasifier and the char combustor. Conveyors and hoppers are used to feed the biomass to the low-pressure indirectly-heated entrained flow gasifier. Steam is injected into the gasifier to aid in stabilizing the entrained flow of biomass and sand through the gasifier.

  The biomass chemically converts to a mixture of syngas components (CO, H₂, CO₂, CH₄, etc.), tars, and a solid “char” that is mainly the fixed carbon residual from the biomass plus carbon (coke) deposited on the sand. Cyclones at the exit of the gasifier separate the char and sand from the syngas.

  Air is introduced to the bottom of the reactor and serves as a carrier gas for the fluidized bed plus the oxidant for burning the char and coke. The heat of combustion heats the sand to more than 1,800°F.

• Gas cleanup and conditioning. This consists of multiple operations: reforming of tars and other hydrocarbons to CO and H₂; syngas cooling/quench; and acid gas (CO₂ and H₂S) removal with subsequent reduction of H₂S to sulfur. Tar reforming is envisioned to occur in an isothermal fluidized bed reactor; de-activated reforming catalyst is separated from the effluent syngas and regenerated on-line.

  The hot syngas is cooled through heat exchange with the steam cycle and additional cooling via water scrubbing, which also removes impurities such as particulates and ammonia along with any residual tars. The cooled syngas enters an amine unit to remove the CO₂ and H₂S. The CO₂ is vented to the atmosphere in this design.

• Alcohol synthesis. The cleaned and conditioned syngas is converted to alcohols in a fixed bed reactor. The mixture of alcohol and unconverted syngas is cooled through heat exchange with the steam cycle and other process streams. The liquid alcohols are separated by condensing them away from the unconverted syngas.

• Alcohol separation. The alcohol stream from the alcohol synthesis section is depressurized and dehydrated using vapor-phase molecular sieves. The dehydrated alcohol stream is introduced to the main alcohol separation column that splits methanol and ethanol from the higher molecular weight alcohols.

• Heat and power. A conventional steam cycle produces heat (as steam) for the gasifier and reformer operations and electricity for internal power requirements (with the possibility of exporting excess electricity as a co-product). The steam cycle is integrated with the biomass conversion process.

The alcohol synthesis reactor is the key to the entire process. After researching several decades of work on alcohol synthesis catalysts, NREL selected a modified Fischer-Tropsch catalyst for the process design, specifically a molybdenum-disulfide-based (MoS₂) catalyst, originally from Dow/Union Carbide.

The former Dow/UCC catalyst was chosen as the basis because of its relatively high ethanol selectivity and because its product slate is a mixture of linear alcohols (as opposed to the branched alcohols that result from modified methanol catalysts). This particular catalyst uses high surface area MoS₂ promoted with alkali metal salts (e.g. potassium carbonate) and cobalt (CoS). These promoters shift the product slate from hydrocarbons to alcohols, and can either be supported on alumina or activated carbon, or be used unsupported.

In its design, NREL is targeting a much higher ethanol distribution (70.66 wt%) than found in pervious work (Dow, 34.5%; SRI, 446.12%), enabled by the almost complete recycle of methanol within the NREL process. In the alcohol purification section downstream, virtually all methanol is recovered via distillation and recycled back to mix with the compressed syngas. This is done in order to increase the production of ethanol and higher alcohols.

Resources

• Thermochemical Ethanol via Indirect Gasification and Mixed Alcohol Synthesis of Lignocellulosic Biomass (NREL/TP-510-41168)

• DOE Thermochemical Conversion Roadmap Workshop (January 2007)