Biomethanol: a Fuel of the Future
From filling stations to engines, biomethanol can be used as a low-cost, sustainable alternative to traditional fuels. Mitch Beadie asks: Could this be the answer to tomorrow's supply and environmental problems?
With transport being the world's fastest growing energy user, countries are increasingly setting biofuel quotas to reduce dependence on fossil fuels. For example, the EU's Renewable Energy Directive has set targets of 5.75% by 2010 and 10% by 2020. The result has been the particularly rapid growth of the biodiesel market with production at around 10 million tonnes in Europe alone and growing consistently at 25% a year.
Biodiesel can be produced from animal fats or vegetable oils, with production techniques generating glycerine as a by-product (10%).
Every tonne of biodiesel roughly produces 100kg of glycerine. Glycerine is used to make a range of products including toothpaste, soaps and paints, but supply is now greatly outstripping demand, and much of the product is incinerated.
A novel process from Dutch company BioMCN uses glycerine to produce the sustainable fuel biomethanol.
Biomethanol is identical to methanol, which is the simplest (and cheapest) of the alcohols. It is a versatile chemical that can produce a range of polymers and fuels. Its most immediate fuel use is to produce bio-methyl tertiary butyl ether (MTBE) to increase octane levels in petrol to prevent 'knocking'.
Longer term, though, biomethanol is a good alternative to ethanol for replacing petrol in automotive engines. Nobel Prize winner George Olah has recommended an entire methanol economy instead of the much-promoted hydrogen economy.
Biomethanol – like biodiesel – should be produced without taking up the agricultural land that is increasingly needed to feed the world's population (a quarter of grain crops grown in the USA are already reportedly being diverted from food to produce fuel for cars, which is simply unsustainable).
The EU's Renewable Energy Directive allows energy companies to count the energy content of sustainable fuels twice. Where biomethanol is produced from waste as in BioMCN's process, it counts double towards renewable energy targets.
Biomethanol can also be produced by gasifying organic materials, and can produce a variety of fuels such as biohydrogen, bio-dimethyl ether (DME), methanol-to-gasoline (MTG), petrol blends and biodiesel. That versatility is attractive to countries looking for security of fuel supplies, and makes biomethanol a good contender for a low-cost sustainable fuel of the future.
Ideal fuel characteristics
The huge energy concentrated into fossil fuels has made them difficult to replace. However, fuels have several important characteristics and low carbon number alcohols such as methanol and ethanol outperform petrol on nearly all of them.
For a start, the low carbon number gives more power than petrol, with lower gCO2 / MJ emissions. Methanol and ethanol burn at lower temperatures than petrol, which reduces the heat lost from the exhaust.
The flame speed (speed of expansion of a flame front during combustion) is higher, so combustion efficiency is higher and with better tolerance to exhaust gas recirculation.
Methanol and ethanol have much higher octane ratings than petrol, which makes them less likely to autoignite under pressure before the spark in internal combustion engines (knocking). The higher octane rating also allows a higher compression ratio for higher thermal efficiency and significantly more power than an equivalent petrol engine.
The heat of vaporisation is high for both methanol and ethanol, which again leads to high thermal efficiency and, with a low air-fuel ratio, reduces the cycle temperature. It also gives strong charge-cooling effects in turbocharged engines to improve knock suppression.
The high heat of vaporisation makes it harder to start a methanol or ethanol engine in cold weather. That and the lower energy content worsens fuel consumption and so shortens a car's range. Those are the two major disadvantages of low carbon alcohols compared with petrol. The cold start is normally avoided by limiting flexi-fuel vehicles to 85% methanol (M85) or ethanol (E85), while methanol and ethanol's improved thermal efficiency makes up for the higher fuel consumption.
Methanol and ethanol are generally safer because they are harder to ignite and release less heat when they burn (fuels should ideally only ignite under pressure with a spark, and should burn in a controlled way rather than exploding). They are also more environmentally benign. Although toxic, methanol is less toxic than petrol, important because fuels often spill and mix with groundwater. Neither methanol nor ethanol persist in surface water, and are both biodegrade. However, methanol exhausts can contain formaldehyde while ethanol can contain aldehydes.
With fuels normally being hydrocarbons, the carbon burns in oxygen to form CO2 and / or CO with hydrogen burning to form water vapour (there can also be hydrocarbons, and NOx and SOx in emissions). If produced from a renewable resource, the fuel just returns the carbon back into the atmosphere, but methanol has until now been almost all fossil fuel generated.
Produced from waste
Methanol (CH3OH) is conventionally produced from methane (natural gas). Purified methane (CH4 ) is cracked with steam in a steam reformer using a nickel catalyst at high temperature (>500°C). The methane and steam splits into syngas, a mix of H2, CO2 and CO. The syngas is cooled and compressed to around 100 bar, with the separate components reacting in a synthesis reactor to produce methanol.
BioMCN’s process is similar, but accepts crude glycerine instead of methane. The crude glycerine is purified and gasified. It is purified using vacuum distillation, where it is evaporated to remove impurities.
The glycerine vapour is fed into the steam reformer as before, with distillation removing water, light alkanes and denser fractions. The resulting methanol is 99.85% methanol, which is the same purity as from methane. It reduces CO2 well-to-wheel emissions by 70% of each litre of petrol saved.
Although none are as advanced as BioMCN, other companies in Europe and the US are working on biomethanol processes. It has been successfully produced from feedstocks like wood waste, grass, algae, black liquor from pulping processes, and methane gas from landfills and animal waste. It has the potential to become the least expensive of the carbon neutral biofuels.
As well as bio-MTBE, biomethanol can be used to produce MTG or petrol blends to replace petrol, biodiesel or DME to replace diesel. And, with four hydrogen atoms, methanol is also a good way of storing hydrogen. It can be converted to hydrogen when filling cars, or in the car itself (direct methanol fuel cells can convert methanol straight to hydrogen). It is a liquid and can be stored at room temperature, while hydrogen has to be at a pressure near 700 bar or temperature near -270ºC and so needs a heavy and expensive tank.
A methanol economy is a much more practical alternative than a hydrogen economy. Using biomethanol would need only slight changes to filling stations and car engines – unlike hydrogen, which would need a completely new infrastructure.