6.3.1 Methanol: Thermochemical and Biological Routes

The versatility of the FT process is that almost any hydrocarbon produced that can be derived from petroleum can be made from syngas, not only alkanes and alkenes but oxygenated compounds; the exact mixture of products obtained can be varied by choices of catalyst, pressure, and temperature, and straight-chain alcohols are pro­duced in the “synthol” reaction at 400-450°C and 14-MPa pressure in the presence of an iron catalyst.94 Industrial production from natural gas has, however, been dom­inated since the 1960s by a lower temperature and pressure process invented by Imperial Chemical Industries in which CO, CO2, and H2, derived by steam reform­ing, are reacted over a mixed Cu/ZnO/Al2O3 catalyst at 250°C and 50-10 MPa when two reactions occur:95

CO + 2H2 ^ CH3OH
CO2 + 3H2 ^ CH3OH + H2O

A recent development has been to combine syngas production from methane with the reduction of ZnO to metallic zinc in a metallurgical plant; the syngas has a H2:CO ratio of approximately 2:1, highly suitable for methanol production.96 A renewable-resource route for methanol (one of the largest bulk chemicals in the contemporary world) is, however, entirely feasible via biomass gasification as an intermediate step, and this would be entirely appropriate given methanol’s older name of “wood alcohol,” indicative of its historical provenance by incomplete combustion.

As an “energy carrier,” methanol is inferior to ethanol, with an energy content of only 75% (on either a weight or a volume basis) compared with ethanol and approximately 50% compared with conventional gasoline.97 Blends of methanol with conventional gasoline up to 20% can be tolerated without the need for engine modifications, that is, as a fuel extender; the corrosive effect of methanol on some engine materials limits the extent of this substitution.98 Methanol would have been an excellent replacement for MTBE as a gasoline oxygenate additive (chapter 1, section 1.4), but its acute neurotoxicity is well known and a barrier to several poten­tial uses. One notable exception, however, is as an at-site (or on-board) source of hydrogen for fuel cells (chapter 7, section 7.1): between 1983 and 2000, nearly 50 patents were granted for catalytic methanol “reforming” systems to automobile producers (such as General Motors, Daimler-Benz, DaimlerChrysler, and Honda), chemical multinationals (DuPont, BASF, etc.), and major oil companies (CON­OCO, Standard Oil Company, etc.).95 Combined reforming with liquid water and gaseous oxygen has been intensively investigated for use in mobile applications for transportation:

(s+p)CH3OH (l) + sH2O (l) + 0.5pO2 ^ (s+p)CO2 + (3s+2p)H2

since the composition of the reactant feed can be varied and the process carried out under a wide range of operating conditions.[61]

The first pilot plant for testing and evaluating the production process for “bio­methanol” was established in a program that commenced in 2000 between the Minis­try of Agriculture, Forestry, and Fisheries of Japan and Mitsubishi Heavy Industries at Nagasaki (Japan); various feedstocks have been investigated, including wood, rice husks, rice bran, and rice straw.99 The test plant consisted of

• A drier and grinder for the biomass input (crushed waste wood)

• A syngas generator

• A gas purifier

• A methanol synthesis vessel (with an unspecified catalyst)

The pilot plant was designed for a capacity of 240 kg/day, with a methanol yield (weight of methanol produced per unit dry weight of material) of 9-13%. A larger plant (100-tonne daily capacity) is predicted to have a much higher methanol yield (38-50%).

No economic analysis of the Japanese pilot facility has been published, but a the­oretical study of methanol production via the syngas route suggested that methanol from biomass (by 2002) had production costs approximately twice those of conven­tional gasoline on an equal energy basis.100 The surge in crude oil and gasoline refin­ery gate costs subsequently (figure 5.1) implies that methanol production costs from biomass sources would now (mid-2008) be competitive. This encouraging result is very timely because direct methanol fuel cell (DMFC) technology has reached the stage where Toshiba in Japan has announced the development of a micro-DMFC suitable for powering MP3 players.101 A U. S. patent covering aspects of DMFC con­struction was also issued in March 2007 to Creare (Hanover, New Hampshire).102 The Japanese invention utilizes a polymer electrolyte membrane device with the electrochemical reactions:

CH3OH + H2O ^ CO2 + 6H+ + 6e — (anode)

1.5O2 + 6H+ + 6e — ^ 3H2O (cathode)

The inputs are concentrated methanol and air (O2); the only outputs are water and CO2 and electricity (100 mW) sufficient to power a portable device for 20 hours on a 2-cm3 charge of solvent. The prospects for large DMFCs for heavier duty use are presently unclear.

Another thermochemical route has been explored to convert methanol to another biofuel, dimethylether (DME), (CH3)2O, a highly volatile liquid that is a suitable fuel for diesel engines because of its low self-ignition temperature and high cetane num — ber.103 Although bio-DME has only half the energy content of conventional diesel, diesel engines can easily be retrofitted for bio-DME use. Well-to-wheel analyses showed that bio-DME was a little inferior to FT diesel for total fossil fuel substitu­tion and pollutant emissions.9192