There’s been a lot of talk about a hypothetical hydrogen economy, but what if methanol was the fuel of choice? SSCP-DTP student Alexandra Hicken discovers how methanol can be produced sustainably in the last blog in our sustainability series.
Much research has been carried out into the possibility of using hydrogen for the generation and redistribution of energy under a system called the hydrogen economy. Despite its environmentally favourable properties such as ‘clean’ combustion giving water as the only by-product, the use of hydrogen as a fuel raises safety issues due to its extreme volatility and explosive nature.
The methanol economy has been presented as a viable alternative to the hydrogen economy. Compared to hydrogen, methanol is a more practical energy source due its compatibility with much of the existing infrastructure and its already extensive use in industry.2 It’s easy to imagine a future where methanol replaces the petrol in our cars or the gas in our heating systems. But making the methanol economy a reality depends on our ability to find new and efficient ways of producing methanol from fossil fuel sources or by recycling CO2.
Methanol From Fossil Fuels
Despite much of the focus of sustainable technologies and development being on new, renewable processes, it is also imperative that existing processes are improved. Methanol is currently almost exclusively synthesised from syn-gas (produced from reacting any hydrocarbon source such as coal, natural gas or biomass with water or steam), a mixture of hydrogen, carbon monoxide and small amounts of CO2.3 Despite modern methanol plants having a high selectivity for methanol and high-energy efficiencies (99% and 70%), high pressures and temperatures (50-100 atm and 200-300 °C) are still required in order to synthesise methanol.
Producing methanol more sustainably
Synthesising methanol by hydrogenating CO2 is a comparatively sustainable way of producing methanol. In order for the methanol to be classed as renewable, the CO2 used as a starting material must be a waste product, the hydrogen utilised must not be produced from fossil fuel sources, and the energy required to break the vital chemical bonds must be from a renewable source. The Olah plant, located in Iceland, is the first example of a methanol production plant that meets these criteria. Using cheap, geothermal energy to produce hydrogen via electrolysis and the capture of waste CO2 from nearby power plants, Carbon Recycling International (CRI) are able to produce around 3500 tonnes of methanol per year. My work in the Crimmin group at Imperial focuses on the synthesis of new heterobimetallic complexes as models to probe the catalytic site for the hydrogenation of CO2 to methanol.
Methanol fuelled cars, buses and planes are a possibility, but our existing fleet of vehicles would require some serious upgrades to run on this new fuel source. Due to the higher chemical complexity of methanol gasoline compared to methanol, methanol has around half the energy density of gasoline. However, it has a higher octane rating – meaning it can withstand higher pressures before igniting –thus methanol-powered engines can run at a higher efficiency than those powered with gasoline However, methanol is incompatible with some of the materials used in gasoline distribution and storage, and has chemical composition so different to diesel that compression ignition engines must be completely adapted for use.4 Therefore, there are still many problems associated with the direct use of methanol in internal combustion and compression ignition (diesel) engines.
Methanol as a Chemical Feedstock
Methanol is currently one of the most important starting materials for the chemical industry, whereby the majority of the 40 million tonnes of methanol produced each year are used for the production of industrially important chemicals such as formaldehyde, acetic acid and methyl tert-butyl ether.
