Methanol is the simplest alcohol. It has the chemical formula (CH3OH) and is more volatile and more toxic than ethanol. Traditionally, methanol has been considered wood alcohol, because it was produced by pyrolysis of wood. In theory, pyrolysis of wood could be considered biomethanol, because it is producing the alcohol-based fuel from a biological source (wood). However, the term biomethanol is typically used to describe methanol produced from one of two methods: Fischer Tropsch reaction of syn gas or biomethane. A common source of biomethane is landfills. The process of making biomethanol from landfill gas and syn gas is a cost-intensive chemical process. Although in theory this is a large source of energy, because current residues/waste by-products from agricultural and forest products amount to approximately one-third of the total commercial energy use.6

In general, we would break biomethanol production into three methods: syn­gas, bio-gas/bio-methane, and carbohydrates. Syn-gas contains carbon monoxide (~30vol%), hydrogen (25-30vol%), carbon dioxide (20-30vol%), methane (~10vol.%), and ethane (~3vol.%). Typically, purification and/or gas conditioning are needed before syn-gas can be catalytically converted to biomethanol. The catalyst used in the reactor for methanol synthesis is typically copper oxide, zinc oxide, or chromium oxide.7 The two standard chemical reactions for methanol synthesis at these catalysts are shown below.

CO + 2H2 « CH3OH

CO2 + 3H2 « CH3OH + H2O

Both of these reactions are exothermic and result in a loss of moles of gas, so Le Chatelier’s principle would dictate that the reaction is favored by high pressure and low temperature. Side products can be produced and need to be considered if depending on the purification needed of the methanol product. Side products could include dimethyl ether, formaldehyde, or more complex alcohols.7 These side products may decrease the energy density of the fuel as well as the toxicity.

Biogas or biomethane that is captured from landfills is often times called landfill gas. Landfill gas is typically considered to be the same as natural gas, but it is not. Natural gas is more than 80% methane and landfill gas is typically 40-60% methanol with the remaining gas being mostly carbon dioxide. The EPA predicts that each pound of biodegradeable waste in the landfill will produce 10-12 standard cubic feet of gas over a 25-year period.8 This landfill gas accounts for 34% of the methane emissions,8 so it is clearly a large source of methane that could be tapped for fuel purposes. The overall reaction for methanol production from landfill gas is:

CH4 + H2O « CH2OH + H2O

However, this process is not direct. The landfill gas is reformed to syn gas after a pretreatment to remove sulfur compounds and a compression to 400 psi and then the syn gas is reacted to methanol and purified.8

The general method for biomethanol production from carbohydrates is shown in Fig. 11.1 where carbohydrates are gasified and partially oxidized to hydrogen and carbon monoxide which is used to catalytically produce methanol via the same catalytic methanol synthesis method described above for syn gas.7 Clearly, biomass is more complex than syn gas, so more pretreatment, gas cleaning, and gas conditioning is needed. In general, pretreatment involves chipping to a size below 5 cm and drying, whereas gas cleaning involves removing tars, soot, alkali metals, BTX (benzene, toluene, and xylenes), and inorganic impurities (HCl, ammonia, HCN, H2S, and COS).7 Gas conditioning is involved in getting rid of the methane and other hydrocarbons in the gas as well as altering the ratios of CO:CO2:H2 if necessary via the water gas shift reaction, amine stripping of carbon dioxide, or other CO2 scrubbing methods. Steam reforming of methane (and other light hydrocarbons) over nickel catalysts will form carbon monoxide and hydrogen,7 which then can be used directly for producing methanol. Overall, this is an area of research that appears to be the future of considering methanol as a biofuel, but currently researchers in the United States are more focused on ethanol as a fuel due to volatility and toxicity issues. Methanol also has lower volumetric and gravimetric energy density, which limits its usefulness for portable or transportation power.