A recent white paper by Leslie Bromberg of MIT’s Plasma Science and Fusion Center and Wai K. Cheng of the Sloan Automotive Laboratory assessing the prospects for methanol as an alternative transportation fuel in the US concludes that methanol is a safe and viable transportation fuel, although it not as good as ethanol in terms of energy density and ease of handling. However, while significant investment would need to be made for large-scale methanol deployment in the transportation sector, there is no technical hurdle both in terms of vehicle application and of distribution infrastructure, they noted.
While methanol has not become a substantial transportation fuel in US despite earlier trials, its present large industrial scale use and the former availability of production methanol flex-fuel vehicles (FFV)—developed as part of the earlier methanol programs—have demonstrated that it is a viable fuel and technology exists for both vehicle application and fuel distribution.
Large scale production of methanol from natural gas and coal is a well-developed technology and there is progress on the economic conversion of biomass to methanol using thermo-chemical processes. In comparison, they noted, the technology for bio-chemical ethanol production from cellulosic biomass is not sufficiently developed yet. Sufficient feedstock of natural gas and coal exists to enable the use of non-renewable methanol as a transition fuel to renewable methanol from biomass, they suggested.
Methanol first surfaced as a potentially interesting transportation fuel in the wake of the oil crisis in 1973. The early interest resulted in several programs, mainly based in California, for its use. Bromberg and Cheng attributed the failure of methanol in becoming a substantial transportation fuel component in US earlier to several factors:
- Methanol was introduced in a period of rapidly falling petroleum fuel prices; there was no economic incentive for continuing the methanol program.
- There is no strong advocacy for methanol (unlike ethanol) as a transportation fuel; therefore, they said, it has been displaced by ethanol as oxygenate of choice in gasoline blends. Furthermore, while generating methanol from biomass thermochemically is a well developed technology, there is little advocacy for that as a pathway towards replacing petroleum fuel with renewables. Instead, crop-based ethanol has been promoted by the federal government (through tax incentives) as the transition fuel towards cellulosic bio-fuel production.
Among methanol’s attractive attributes for transportation are:
- It is a liquid fuel which can be blended with gasoline and ethanol and can be used with today’s vehicle technology at minimal incremental costs.
- It is a high octane fuel with combustion characteristics that allow engines specifically designed for methanol fuel to match the best efficiencies of diesels while meeting current pollutant emission regulations.
- It is a safe fuel. The toxicity (mortality) is comparable to or better than gasoline. It also biodegrades quickly (compared to petroleum fuels) in case of a spill.
- There is a very large potential supply of methanol since it can be made from natural gas, coal and biomass feedstocks.
- Produced from renewable biomass, methanol is an attractive green house gas reduction transportation fuel option in the longer term. A bridging option is to use methanol derived from natural gas, with a CO2 intensity that is no worse than conventional fuels. There is also the possibility of achieving greenhouse gas reduction by CO2 sequestration in the methanol generation process.
- Multiple ways exist for introduction of methanol into the fuel infrastructure (light blends or heavy blends) and into vehicles (light duty or heavy duty applications). The optimal approaches are different in different countries and in different markets.
Methanol from non-renewable coal or natural gas could be used as a bridging option towards transition to renewable methanol for sustainable transportation. Methanol can readily be made from natural gas or coal (there is plentiful supply in the US of both) so that large scale domestic production, infrastructure, and vehicle use could be developed. The resulting transportation system could then be transitioned to the renewable methanol. It should be further noted that such system is also amenable to the use of renewable ethanol, should large scale bio-production of cellulosic ethanol be realized in the future.—Bromberg and Cheng
To introduce methanol significantly into the market place, Bromberg and Cheng wrote, both methanol vehicles and fuel infrastructure have to be deployed simultaneously.
Vehicle use and optimization. Performance of methanol FFVs—i.e., a vehicle that can run on M85 (85% methanol) as well as gasoline—has been comparable or slightly better than conventional gasoline. However, they noted, options exist for better performance/efficiency using the excellent combustion properties of methanol:
Small displacement, stoichiometric light-duty engines;
Direct Injection Alcohol Boosted (DIAB), a two-tank system that uses direct injection (DI) of methanol when the engine is prone to knock (usually at conditions of high torque). In the DIAB concept, DI of the knock suppressing fuel is used only in the amount required to prevent knock and gasoline is supplied to the cylinder by conventional PFI (port fuel injection).
Since the engine operates at stoichiometry (using a typical oxygen feedback), a very high specific torque output can be produced while emissions can be maintained at low levels through the well-proven and relatively simple three-way catalyst system without the use of EGR as a major diluent.
The technology opens the possibility of a spark-ignited gasoline engine operating at high compression ratio (12 - 14) and high boost ratios of 2.0- 2.5 times ambient pressure, which is sufficient to produce a torque output equivalent to or greater than more highly turbocharged heavy duty diesel engines operating lean with significant EGR. The methanol-boosted DIAB engine can be almost as efficient (as measured by BTE, brake thermal efficiency) as a diesel and have comparable specific CO2 emissions as well. The concept has been demonstrated and proven in systematic dynamometer tests at Ford. In addition, Honda has independently investigated the concept.
Medium duty applications in a dedicated alcohol (methanol and ethanol) engine, using high level blends of alcohols. Researchers at the US EPA investigating this approach concluded that the M85 and E85 engines could provide efficiencies comparable or higher than the diesel, in dedicated fuel, high compression ratio turbo charged/downsized engines. Preignition was avoided using a combination of intake air temperature control and latent heat cooling of the charge air from the vaporization of the fuel. Cooled EGR was used to lower the engine-out NOx levels, reduce the need for intake throttling at low to moderate loads while maintaining stoichiometric operation, suppress the tendency for knock at higher compression ratio, and maintain reasonable turbine inlet temperatures.
Directly injected spark ignited heavy-duty engines. Small, very high power density, spark ignition engines which are fueled with ethanol, methanol or mixed alcohols can be used as a substitute of heavy duty diesel engines, with higher engine thermal efficiency and much reduced size and weight. In this manner a 4 liter engine could potentially be used to replace a diesel engine with a displacement as high as 11 liters. (Earlier post.)
The “methanol economy” in the US has the potential to substantially decrease energy dependence, providing energy security using domestic feedstocks and labor, with substantially lower footprint to the environment (GHG), with a product that seems competitive in the present markets. However, substantial obstacles exist (lack of vehicle/infrastructure/manufacturing) which can be overcome with market incentives. In the US there is not enough biomass to displace all of the transportation fuels with methanol.
Methanol can be used directly in fuel cells. Although this technology is making inroads in electronics, with low power requirements, the technology is still too early in its development, unlikely to achieve significant commercialization in the transportation sector within the next 2 decades.
A full comparison between the different options has to be done, reflecting the present conditions. Although an attractive fuel, methanol is not a silver bullet that is better than the alternatives in all categories, and it is likely that a combination of the proposed solutions (including ethanol/methanol/gasoline mixtures and NG) is better suited to solve the massive transportation problem. The preferred solution may depend on the region, the market sector and other externalities, including past and present policies.—Bromberg and Cheng
L. Bromberg and W.K. Cheng (2010) Methanol as an alternative transportation fuel in the US: Options for sustainable and/or energy-secure transportation (PSFC/RR-10-12)