Refining Biomass: More Big Players Emerge

Dow Chemical is getting on board the US National Renewable Energy Lab's thermochemical biomass to energy conversion plan. As more large oil, chemical, and energy companies join in the hunt for affordable bioenergy, the chances for a breakthrough bioenergy technology get better.

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

1. Feedstock handling and preparation.
   
   
2. Gasification. Heat for the endothermic gasification reactions is supplied by circulating hot synthetic olivine “sand” between the gasifier and the char combustor...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.


3. Gas cleanup and conditioning. 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.
4. 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.
5. 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.
6. 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.
   
   ...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. _GCC

The emphasis on ethanol production reflects nearer term priorities. The ethanol economy has been put in place by US government mandate, reflecting special interest "corn state" industries and politics. An emphasis on ethanol almost guarantees a massive shift to "flex-fuel" automobiles within North America. In terms of motorist choice, the flex-fuel approach may work out quite well.