Although, as stated earlier, there are a number of gas-to-liquid processes capable of produc­ing liquid transportation fuels from biomass syngas, a mixed-alcohol synthesis process is specifically discussed here because of the synergies with the biochemical approach in pro­ducing ethanol as the primary product. The commercial success of mixed-alcohol synthesis has been limited by poor selectivity and low product yields. Single-pass yields are on the order of 10% syngas conversion (38.5% carbon monoxide conversion) to alcohols, with methanol typically being the most abundant alcohol produced (41, 42). For mixed-alcohol synthesis to become an economical commercial process, improved catalysts are needed (43). Improvements in mixed-alcohol synthesis catalysts could increase alcohol yields and the se­lectivity of ethanol production from clean syngas, as well as improve the overall economics of the process through better heat integration and control and fewer syngas recycling loops. Specific research needed to accomplish these objectives is as follows:

• Develop improved mixed-alcohol catalysts that increase the single-pass carbon monoxide conversion from 38.5 to 50% (and potentially higher) and improve the carbon monoxide selectivity to alcohols from 80 to 90%.

• Develop improved mixed-alcohol catalysts with higher activity that require a lower oper­ating pressure (1000 psia compared with 2000 psia) to decrease process-operating costs. The combination of lower syngas pressure for alcohol synthesis and less unconverted syn­gas to recompress and recycle has the added benefit of lowering the energy requirement for the improved synthesis loop.

• Explore alternative mixed-alcohol synthesis reactors and catalysts. Greatly improved tem­perature control of the exothermic synthesis reaction has been demonstrated to improve yields and product selectivity. Precise temperature control reactor designs need to be de­veloped for the mixed-alcohol synthesis reaction to improve the yields and economics of the process.

2.3.1.3 Integration/demonstration needs

For any sophisticated conversion process, combining individual unit operations into a com­plete, integrated, systematic process is a challenge. To demonstrate economic competitive­ness, individual pilot-scale operations and complete integrated pilot development runs will be required. A specific challenge is to continue to demonstrate process intensification and higher yields at pilot scale to reduce capital costs.