Biomass Conversions to Liquid Fuels via BTL Processes

[CLIVE RICHARDSON]

Biomass to Liquids (BTL) processes are being designed based on the thermo-chemical platform for converting biomass to biofuels. In an indirect liquefaction process, synthesis gas (syn-gas, CO+H2) is first produced via gasification of solid biomass or liquid bio-oil produced by the fast pyrolysis of biomass. Syn-gas can be converted to synthetic gasoline, jet-fuel or diesel using Fischer-Tropsch synthesis of hydrocarbons or synthetic alcohols, such as ethanol, through different catalytic processes. These gas to liquids (GTL) technologies, previously utilizing coal or low-cost, remote natural gas as feedstock for liquid fuels production can be adapted for the conversion of bio-mass to liquids (BTL).

In order to reduce the cost of biomass transportation, the BTL process can be decoupled into two steps. In the first step, the biomass is converted to a liquid form via fast pyrolysis at distributed facilities close to the source of biomass. The liquefied biomass, commonly called as bio-oil, is then transported to a much larger central facility where it is gasified/reformed into synthesis gas at high pressure. The syn-gas is subsequently cleaned/upgraded and then converted into liquid fuels. The advantages of this two step process are lower cost of transporting biomass feedstock and significant reduction of power requirement for producing high-pressure synthesis gas. In addition, compared to its biomass source, bio-oil contains significantly lower ash, sulfur and nitrogen compounds, which are poisonous for the GTL reaction catalysts. The synthesis gas cleaning step at the centralized facility can thus be made significantly less strenuous.

Several ongoing research activities on the BTL processes have provided CSET facilities with equipment and instrumentation necessary for demonstrating the production of liquid fuels from biomass through thermo-chemical and catalytic processes. The new bench-scale high pressure GTL reaction system is capable of converting syn-gas to liquid fuels using fixed-bed reaction mode or slurry-type reaction mode. Analysis of the synthesis gas composition is carried out using a Varian micro-GC and the liquid products composition using a Varian 3800 GC /Saturn 2200 MS.

The Co-Zr/Al2O3 catalysts that are used in Fisher-Tropsch synthesis have been successfully prepared in CSET and tested on a higher pressure slurry reactor (Figure 1). The relationships between reaction conditions and hydrocarbon product distributions have been thoroughly studied. Using Rhodium nanoparticles supported on mesoporous materials as catalysts, syn-gas can be selectively transformed to alcohol, preferably ethanol, in a high pressure fixed bed reactor (Figure 2) at a temperature ranging from 240-260˚C. These projects are providing great opportunity to develop technologies that are crucial in transforming biomass to biofuels and value added chemicals.