Synthetic alcohol is a pragmatic option to reduce in-use CO2 output from automotive sources while ensuring cost-effectiveness and system robustness.
Interference with the Earth’s climate system by mankind is widely accepted as something that will impact heavily on the future of the planet. The increase in the global average surface temperature since the industrial revolution is thought to be linked to the concentration levels of various so-called ‘greenhouse gases’, which have risen from an equivalent of 280ppm carbon dioxide (CO2) to around 430ppm today. CO2 has been identified as the most important greenhouse gas, contributing 60 percent of the increased global warming effect. The associated temperature rise is projected to accelerate during this century and global warming is predicted to cause increase in sea levels and precipitation leading to higher risks of flooding of low-lying countries.
Approximately 75 percent of anthropogenic CO2 emissions are due to the combustion of fossil fuels and it is virtually certain that CO2 emissions, as a result of burning fossil fuels, will be the dominant factor determining CO2 concentrations for the rest of the century. Worldwide, the emissions of CO2 due to transport is currently in the region of 15 percent of the total emissions. Transport has, therefore, become a significant target for legislators and forthcoming regulations. Given that fuel consumption is directly proportional to CO2 output, and that only improvements in powertrain fuel efficiency are reflected as pro-rata improvements in vehicle fuel consumption, there are three main routes to reduce CO2 production from any given engine and fuel combination:
1. Increased investment in engine technologies:
This is a widely-followed route at present. However, the modern engine is a highly-developed system, and hence any further improvement is likely to be very hard won, with diminishing returns for any increase in complexity and considerable on-cost being the probable outcome of this approach.
2. Increased investment in transmission technologies (including hybridisation):
Conventional mechanical transmissions are very cost-optimised, and hence investing in transmission technologies is proportionally more expensive than engine improvements. It does offer greater rewards on the drive cycle, though there are questions over real-world fuel consumption benefits. Also, as with all technologies, the subject of the environmental cost of manufacture (or ‘embedded CO2’) is becoming more of an issue.
3. Change the fuel on which such vehicles operate to one with a renewable element:
This is complementary to the above mentioned options (whereas simultaneous in both engine and transmission technology are not always fully complementary), and can offer a significant reduction in net CO2 output through capture of CO2 within the production-and-use cycle. Provided the correct fuel is decided upon, this option is also relatively simple to realise in terms of the engine and vehicle technologies required to store and use the fuel.
The molecular-hydrogen-based ‘hydrogen economy’ is often held to be the ultimate solution to reduce CO2 emissions, promising as it does that (if the hydrogen is manufactured with the use of renewable energy) there will be no emission of CO2. However, such an energy economy is widely acknowledged only to be possible some time in the future, and is often linked with the widespread adoption of the fuel cell as a transportation prime mover. A far more pragmatic and immediate approach is to move to an energy economy based on a synthetic alcohol fuel which can also offer the potential to actually reduce the amount of CO2 in the atmosphere in the long-term as is discussed later. The uptake of such a ‘synthetic alcohol economy’ can be accelerated by the fact that alcohols can be burned in internal combustion engines of the type commonly produced now in any mixture with gasoline, permitting a gradual migration to the new energy economy with no step input of capital by any of the stakeholders: OEM, fuel supplier and end customer.
Compared to liquid fuels, molecular hydrogen has significant disadvantages as a fuel, nearly all of which are concerned with its distribution and storage. Since it is a low-density energy storage medium, only about 20 percent of the energy which can be stored in a tank of gasoline can be stored in a hydrogen tank. This means that frequent refuelling will be necessary. Furthermore, by necessity, there will be a period requiring vehicles to be of dual-fuel nature in the transition to any alternative fuel economy, i.e. vehicles will have to carry two independent fuel systems. BMW and Mazda now offer dual-fuel hydrogen/gasoline vehicles for fleet trials. These share three characteristics: reduced power and range on hydrogen, the need for two separate fuel systems, and the complexity, mass and expense of the hydrogen tank. It should be noted that even at production levels of 100,000 per annum, estimates of US$ 14,250 have been made for the cost of such systems. These vehicles also highlight the fact that a dual distribution infrastructure will be necessary in any transition to a molecular hydrogen economy: this has huge cost implications for the fuel distributors and retailers.
Conversely, the low-carbon-number alcohols ethanol and methanol, while containing less energy per unit volume than gasoline, still offer practical ranges from existing tank sizes using cost-effective technologies. The fact that they are liquids at ambient temperature and pressure requires only simple distribution and storage infrastructures and low manufacturing costs. Refuelling times are also the same as for current liquid-fuelled vehicles. However, because both alcohols are miscible with gasoline, the crucial fact about their adoption as alternatives is that there is no need for a second dedicated fuel system on board for a vehicle. This limits the cost of the vehicle modifications necessary to begin the transition process to the alternative energy economy. The challenges that alcohols present in storage because of their increased corrosiveness are well understood and fundamentally trivial in comparison to the challenges of storing a gas. Crucially, such flex-fuel vehicles are sold in world markets now under existing business cases without excess subsidy from governments or manufacturers, which will not be the case for the introduction of a molecular hydrogen energy economy.
Production of ethanol makes it possible to reduce fossil-based CO2 release by using biomass as the main feedstock in its production, leading to a partially-closed CO2 production-and-use ‘cycle’. With current levels of technology using sugars as the feedstock, up to 90 percent of the CO2 can be kept in this cycle, though the savings are very process-dependent. However, it would not be possible to provide much more than 10 percent of the primary energy required for road transport from ethanol, and there are concerns about using food crops as the primary feedstock, which is ethically unacceptable and (in many cases) geopolitically impractical. These concerns have spurred the development of new production processes which break down the lignin and hemicellulose in biomass or waste to manufacture ethanol; such processes promise to further increase the proportion of CO2 capture and simultaneously to massively increase the proportion of renewable energy available to transportation without affecting food production.
Regardless of these developments, there will be an upper limit on ethanol production via biological routes. As an alternative to ethanol, the manufacture of methanol has been proposed as a fully renewable energy carrier in the long-term future, as it has the potential to remove the absolute limit on renewable alcohols as the basis for a future energy economy. Methanol can be synthesised from various feedstocks, including biogas or from hydrogen and factory flue gases which would normally be discharged directly into the atmosphere. Eventually, the necessary carbon could be taken directly from CO2 removed from the atmosphere by scrubbers (in an analogue of photosynthesis in the case of ethanol manufactured from biological sources). This approach, already proven in laboratories, results in a shorter CO2 cycle than is the case for ethanol. There are, therefore, many different feedstocks offering a range of manufacturing routes for synthetic methanol, ultimately promising the introduction of a fully renewable fuel (as promised by using atmospheric CO2 as the prime source of the necessary carbon).
Clearly a renewable (or non-fossil-fuel-based) energy source for the hydrogen still necessary in the methanol production process is required, and the most feasible candidate for this in the short-term is nuclear power: such an eventuality would have to be accepted anyway in any move to a molecular hydrogen economy. Of course, the use of synthetic methanol introduced in the manner described here could be viewed as a practical means of using hydrogen as the main energy carrier by chemically changing it into a liquid miscible with gasoline, so that hydrogen becomes a realistic proposition as a transport fuel.
As a result, with either ethanol or methanol, the rate of release of fossil-based CO2 into the atmosphere can be reduced. If the processes used to manufacture these fuels can achieve 100 percent capture, the release of fossil CO2 into the atmosphere attributable to the fuels used for transport would be completely eliminated. This leads to obvious benefits in for global warming and would drastically change energy security considerations for any country adopting the approach.
Furthermore, since ethylene and propylene are easily synthesised from methanol, it is possible to use the latter to replace oil as the primary feedstock of the petrochemical industry. Given that the methanol could be synthesised from atmospheric CO2, a route therefore exists to effectively remove CO2 from the atmosphere through its ultimate conversion into plastics, paints etc. Hence this approach offers a means of reducing the CO2 concentration in the atmosphere and, as a result, reversing the process of global warming caused by human beings, driven by the economics of the transport energy supply market. This is illustrated in Figure 1, which also shows that the approach permits the balancing of carbon removal on one side by inputting carbon into the cycle on the other side, allowing a degree of fine-tuning or the ability for some nations to release carbon from fossil fuels as others remove it.
From the foregoing, there is thus a clear route to a future in which a potentially CO2-neutral, or even CO2-negative transport energy economy could be realised. This cannot be the case with a molecular-hydrogen economy. The process starts with the widespread productionisation of alcohol flex-fuel vehicles to provide encouragement to suppliers of ethanol-blend fuels to invest in more efficient production processes and thus increase the amount of primary energy available from renewable sources. Simultaneously, practical investigation of the production of methanol from renewable sources can begin with the knowledge that the vehicles to use that fuel can be created by making minimal changes to the existing ethanol flex-fuel fleet. Because of the physical similarity of ethanol and methanol, this is expected to be principally by modifications to vehicle and powertrain software: the fuel system and engine will require no further changes.
The only impetus required to begin realisation of this vision is for manufacturers to produce such vehicles, which could either be voluntary or forced by legislation. This first step is trivial compared to taking the first steps towards a molecular hydrogen economy. The fully-connected nature of the approach for all stakeholders is shown in figure 2 together with possible dates should the political will be strong enough.
The process also ensures that cost-effective and robust technology solutions can be made available for all customers worldwide; customers in developing economies will not have to pay for fuel consumption-reduction technologies such as hybridisation, downsizing or spray-guided direct injection systems with NOx-reduction technology etc, which are currently seen as expensive in mature markets. Instead the use of alcohol fuels, being themselves superior to gasoline as fuels for the spark-ignition engine, will complement these new engine technologies whenever and wherever they are applied.
