6.2.1 The Renascence of an Old Chemistry for Biomass-Based Fuels?

The generation of a combustible gas, synthesis gas (“syngas”), from biomass was discussed briefly in chapter 2 (section 2.1). Technologies for the conversion of coal and natural gas to liquid fuels were also included in chapter 5 (section 5.6) as part of a survey of different strategies for adapting to potentially dwindling crude oil reserves. The chemistry of gas-to-liquid fuel transformations was developed in the first quarter of the twentieth century and utilized extensively in Germany during World War II; further evolution led to commercial production processes being initi­ated for peacetime purposes in the 1990s.84,85

The essential step, known as the Fischer-Tropsch (FT) reaction, can be written as

nCO + 2nH2 ^ [CH2]n + nH2O,

where [CH2]n represents a range of hydrocarbons, ranging from low-molecular-weight gases (n = 1, methane), by way of gasoline (n = 5-12), diesel fuel (n = 13-17), and as far as solid waxes (n > 17). The reaction requires catalysts for realistic rates to be achieved, usually iron or cobalt (although transition metals will function effectively) at high temperatures (180-350°C) and high pressures; the higher the temperature, the higher the proportion of gas and liquid hydrocarbon products.

To date, no process has been commercialized from plant biomass feedstocks, and the FT technology could be described as “radical” or “nth” generation for biofuels were it not that the key elements of the chemistry and production options are reason­ably well established in industrial processes with fossil inputs; in a climate of high crude oil prices, the environmental desirability of low-sulfur diesel, and the drive to commercialize otherwise unmarketable natural gas in remote locations are impor­tant synergies (table 6.6).86 FT biomass-to-liquid fuel (FT-BtL) from lignocellulosic sources is particularly attractive because of the high CO2 emission reduction poten­tial (up to 90% when substituting conventional gasoline and diesel) and the ability to use woody materials from low-grade land, thus avoiding the pressures on land use in OECD countries contemplating agriculture-based bioethanol or biodiesel production on a large scale.79 The principal barrier to large-scale biomass FT-BtL appears to be the suboptimal mixture of gases in syngas as prepared from plant materials: the lower the molar ratio of H2:CO, the more the proportion of high-molecular-weight products formed in the FT reaction, but biomass gasification results in a wide range of H2:CO ratios, often with an excess of CO, together with appreciable amounts of CO2, methane, and higher hydrocarbons as well as smaller amounts of condensable tars and ammonia.87

The methane can be transformed to CO and H2 by a number of different reac­tions, including the uncatalyzed (but again high-temperature and high-pressure)

processes:88

CH4 + O2 ^ CO2 + 2H2O and CH4 + H2O ^ CO + 3H2

Partial removal of CO (and formation of additional H^ is possible by the water — shift reaction:

CO + H2O ^ CO2 + H2

Finally, the physical removal (adsorption) of CO2 (an inert gas for FT reactions) is relatively straightforward, but a higher-yielding process can be devised (at least, in principle) by including a catalytic reduction of the CO2 to using multiple FT reactors in series with an intermediate water removal step:89

CO2 + 3H2 ^ [-CH2-] + 2H2O

Complete wood-based FT-BtL production involves, therefore, a multistage pro­cess, incorporating biomass pretreatment, syngas purification, and optional syngas recycling, plus gas turbine power generation for unused syngas and, for FT die­sel, a hydrocracking step to generate a mixture of diesel, naphtha, and kerosene (figure 6.8).87,88