4.4.1 BIOMASS TO LIQUIDS (BTL)

Biomass to liquids can be described as a renewable version of fossil fuel — based technologies like coal to liquids (CTL) and gas to liquids (GTL), involving the integration of two different processes: biomass gasification to syngas (H2/CO) and F-T synthesis. Even though both technologies are well known and relatively mature, integration remains a challenge in BTL, because the utilization of lignocellulosic biomass as a feedstock (in substi­tution for classical carbon sources such as coal and natural gas) introduces new difficulties in the overall process. Biomass gasification is achieved by treatment at high temperatures (e. g., 1100-1500 K) under a well-controlled oxidizing atmosphere (e. g., air, steam, oxygen). Control over the composi­tion of the outlet gaseous stream is difficult and depends on a variety of factors including the oxidizing agent, biomass particle size and gasifier design. [52] In this respect, research indicates that utilization of pure oxy­gen atmosphere, small particle sizes (lower than 1 mm diameter), and a combination of high temperatures, high pressures and low residence times favors the production of syngas versus producer gas (a mixture of CO, H2, CO2, CH4, and N2 used for heat and electricity production). [53-55]

The direct integration of biomass gasification and F-T synthesis re­quires an intermediate gas-cleaning system, because the gaseous stream delivered from the gasifier typically contains a number of contaminants that need to be removed before the F-T unit, which is highly sensitive to impurities. Thus, tars (condensable high molecular weight hydrocarbons produced by incomplete biomass gasification), volatile species such as NH3, HCl, and sulfur compounds (produced by gasification of lignocellu — lose impurity components), fine particles, and ashes typically accompany CO and H2 in the outlet gaseous stream. The high number of contami­nants, along with the strict cleaning standards imposed by the F-T unit, [56] require the use of multiple steps and advanced technologies [52] that contribute significantly to the complexity and cost of the BTL plant. Ad­ditionally, because biomass contains higher amounts of oxygen compared to coal, the syngas delivered from lignocellulosic sources is typically en­riched in CO (H2/CO % 0.5), and F-T synthesis requires syngas with a H2/ CO ratio closer to 2.57, [58]. By providing sufficient water co-feeding, theH2/CO ratio can be adjusted by means of an intermediate water gas — shift (WGS, CO + H2O / CO2 + H2) reactor situated between the gasifier and the F-T unit.

The F-T reactor, the last unit of the BTL plant, achieves conversion of syngas to a distribution of alkanes over Co-, Fe-, or Ru-based catalysts in a well-developed industrial process. [58] However, the hydrocarbons produced by the direct route range from Cj to C50, and neither gasoline nor diesel fuels can be produced selectively without generating a large amount of undesired products. Indirect approaches involve initial produc­tion of heavy hydrocarbons (waxes), followed by controlled cracking of the heavy compounds to diesel and gasoline components to overcome this limitation. [59]

The cost of producing the biofuel, negatively affected by the complex­ity of the process, is the main factor limiting the commercialization of BTL technologies. Application of economies — of-scale allows for improvements in the economics of the process at the expense of having large centralized facilities that, as indicated in a previous section, lead to higher costs for transporting the low energy density biomass. BTL profit margins can be in­creased by co-producing, along with liquid hydrocarbon fuels, higher-value chemicals such as methanol [60] and hydrogen [6j,62] from lignocellulose — derived syngas. Another positive aspect of BTL is its versatility. Thus, since any source of lignocellulose can be potentially gasified, BTL technologies are not constrained to a particular biomass feedstock or fraction.