Alternative liquid fuels for cars and planes: biodiesel, ethanol, liquid gas, hydrogen?

Oil is the lifeblood of modern world economies, especially as liquid fuel resource for transport. In 2009 the world consumed an estimated 84 to 85 million barrels of oil.

There is growing demand for liquid fuel for the growing economies of India and China. At this rate, how long can we go on pumping fossil fuels out of the ground without exhausting our supplies?

There are a number of issues related to this:

US Naval Officer compares sample bottles of traditional diesel fuel marine and an algae-derived alternative fuel
US Naval Officer compares sample bottles of traditional diesel fuel marine and an algae-derived alternative fuel. Source: Pubic Domain
Refueling a fuel cell car
Refueling a fuel cell car. Source: Pubic Domain

Alternative Liquid Fuels

Figure 1. Predicted world oil production with 4 models. (Ref A)
Figure 1. Predicted world oil production with 4 models. (Ref A). Source: Pubic Domain
Figure 2. Predicted and Actual American Oil Production (Ref B)
Figure 2. Predicted and Actual American Oil Production (Ref B). Source: Pubic Domain
Source: Pubic Domain

Oil Production is Set to Peak and Decline

The production from a single oil well has been modelled and generally follows a bell-shaped production curve, with output growing, steadying and then declining as the reserves are depleted. This is known as the Hubbert Curve, developed by Shell geologist M. King Hubbert in 1995. These curves can be developed and extrapolated for the production of individual countries and for global oil production.

The world figures are based on the concept that the major oil companies develop the large, easily to extract and shallow oil fields first, and then move onto smaller, deeper oil fields when the large ones decline. New technology tends to allow previously unexploited oil deposits viable which then come into production. Eventually total global production starts to fall as the limited resources of oil are depleted. Before this downfall in production begins, it reaches a point known as peak oil, which marks the maximum production rate before the final decline begins (Fig 1, Ref A) . Individual nations have already reached peak oil. U.S. production peaked in 1971 as was predicted by Hubbert's models (Fig 2, Ref B).

Figure 2 shows that the United States crude oil production, as predicted by the models, peaked in 1970, after which it started to decline. This decline was reversed for several years, because of the development and production from the Alaskan fields. Once again the models showed a good fit supporting the reliability of the extrapolations.

Peak Production Models

Source: Pubic Domain
Source: Pubic Domain

When Will Global Oil Production Peak and Start to Decline?

There is a lot of debate about when the peak global oil production will, or has occurred, various estimated ranging from 2005 (it has already peaked) to sometime between now and 2018. The Hubbert Peak, generally corresponds with the point when 50% of all possible recoverable oil on earth has been used. A recent study conducted by Dr. C. J. Campbell is shown in the figure with various extrapolations (Ref A). The 4 different lines on the graph (Fig 1) correspond to the 4 possible scenarios taking place from1996 onward. It can be seen that no matter which model actually occurs, the outcome is reasonably fixed. This is because the Ultimate size of the resource ( the cumulative total production) is a constant value. Whilst it may be possible to alter the shape of the curve, one cannot alter the area beneath it which is the accumulated oil reserve. The ‘premature peak’ in the early 1970s corresponds to the oil crisis of 1973. Also see the graphs Figures 2 and 3 for World Oil Production modelling (Ref B).

As shown in Figure 3, the models suggests that world production will peak about 2014, and then it will decline to about 40% of current production in 2050.

The world crude oil reserves are declining at an annual rate of about 2%.

Figure 4 shows the model for the cumulative world oil production and actual data. Once again there is a good match with the data.

World Oil Production Outlook

Figure 3. Predicted and Actual World Oil Production (Ref B)
Figure 3. Predicted and Actual World Oil Production (Ref B). Source: Pubic Domain
Figure 4. Model of World Cumulative Oil Production (Ref B)
Figure 4. Model of World Cumulative Oil Production (Ref B). Source: Pubic Domain

Clearly there are ominous signs that the world's oil reserves are close to the peak of production and are likely to decrease in the near future, despite the glut in 2020. The modelling also shows that we have depleted close to half the total oil reserves on the planet, and that oil will be virtually depleted (down to 20% of current production) in the next 50 years.

It is time to be looking at replacements for liquid fuels - firstly for road transport and secondly for air transport.

Alternative Fuels for Road Transport

Air engine - The air engine is a piston engine driven by compressed air as a no-waste source of energy. The expansion of the highly compressed air, stored in high-pressure tanks, is used to drive the pistons and move the vehicle. The only exhaust is cool air, which could be used for air conditioning the car. The energy to compress the air has to come from conventional sources (electricity).

Battery Electric Vehicles - are electric vehicles driven by batteries in which energy is stored chemically - lead-acid, nickel metal hydride, NiCd, absorbed glass mat, Li-poly, Li-ion and zinc-air batteries. The energy stored in the batteries is derived from conventional sources. Electric cares were first developed in the 1890's, and were recently re-developed.
Solar - A solar car is an electric vehicle powered by solar panels on the car that charge batteries. The propulsion system is similar to battery electric vehicles.

Ammonia Fueled Vehicles - Ammonia GreenNH3 has been successfully trialled in Canada. It can be run in conventional spark ignition and diesel engines with some engines with minor engine modifications. It can also be used in jet engines and can be made from renewable electricity. It has only about half the density of petrol or diesel can be stored in tanks. Its toxicity can be managed and the emission are only nitrogen and water.

Biofuels – Ethanol, Methanol and Butanol – The use of these fuels is not new, as the first commercial vehicle that used ethanol was the famous Ford Model T, produced from 1908 - 1927. Its carburetor could be adjusted for use of ethanol or gasoline or ethanol, or some combination of both. While ethanol, methanol butanol have been use as an automotive fuel, they have traditionally been produced chemically from petroleum or natural gas. Ethanol (and butanol) can be derived organically as renewable resources and can be easily produced from fermentation of sugar or starch in various crops and organic waste including grain, sugar beets, sugarcane, or even milk lactose. Howvee there has been heated debate about the merits of using food products for fuel production (2008 food vs fuel debate). Most modern cars, without modification, can be run with up to 10% - 15% ethanol mixed with gasoline (E10-E15) and with minor upgrades can be run with ethanol concentrations as high as 85% (E85), and up to 100% (E100) in warmer climates such as Brazil. Ethanol has about a third lower less energy per volume than gasoline.

Biodiesel – The efficiency of Diesel engines is about 45% compared with just 25-30% in gasoline engines and diesel has a slightly higher energy density per volume.. Biodiesel is produced organically from vegetable and animal by-products. Many oil-seed farmers use a biodiesel blend in their tractors and other equipment to promote a 'grow your own' campaign. Many Diesel-powered cars can run easily, or with minor engine modifications on 100% pure vegetable oils. 

Biogas - Compressed Biogas can be used for spark-plug engines after purification of the raw gas to remove water, hydrogen sulphide and other impurities and particles.
Charcoal - In the 1930s Chinese cars were run on charcoal. 

Compressed Natural Gas - High pressure compressed natural gas, which is mostly methane, can be used in normal combustion engines and the burning of methane produces the lowest amount CO2 of all fossil fuels. Gasoline cars can be retrofitted with gas cylinders and the cars can be switched between gas and petroleum.

Unconventional Gas (Coal Seam Gas) is gas (mostly methane) that is trapped in impermeable hard rock or sandstone, in coal seams and in shale deposits.

Hydrogen/Fuel Cell - Hydrogen can be used as a fuel either via combustion or via fuel-cell conversion into electricity for powers electric motors. With both methods, the only emission is water from the combination of oxygen and hydrogen. 

Oxyhydrogen – This involves using oxygen and hydrogen made by electrolysis of water. It can be used in internal combustion engines. The stored gases can be dangerous and the energy to produce the gas has to come from other sources.

Liquid Nitrogen - Liquid nitrogen works in a similar way to compressed air, but the nitrogen gas can e stored in a liquid form. When the liquid nitrogen is heated the pressurized nitrogen gas can be used to power a piston or turbine engine. 

LPG or Autogas LPG - is a low pressure liquefied petroleum gas mixture mostly of propane and butane which burns in conventional piston engines with less CO2 emissions than gasoline. Cars and trucks cars can be retrofitted with gas tanks and become dual fuel vehicles.

Steam - Steam cars, which were developed in the early 1900s use a steam engine, burning wood, ethanol, coal, charcoal or other materials. The fuel is burned in a boiler and the heat is used to convert water into pressurised steam. When the water turns to steam, it expands. The expansion creates pressure. The pressure pushes the pistons to drive the wheels. be vaporized into steam, taking advantage of the heat that would otherwise be wasted.

Wood gas - Wood gas can be used to run spark-plug engine using a wood gasifier. This method was commonly used during World War II when conventional fuels were scarce. 

Multiple Fuel Sources / Hybrid – This involves using one of more combinations of the methods listed above. A hybrid vehicle uses multiple propulsion systems to drive the car including recovering energy during braking. The most common type is the gasoline-electric hybrid vehicle type, which uses energy stored in batteries and gasoline (petrol) tanks.

Alternative Liquid Jet Aviation Fuels

Aviation consumes about 2% of all fossil fuels burnt - about 11 % of the fuel consumed by the transportation sector, compared with about 80% used for road transport. Most of the fuel is derived from oil as a kerosene/paraffin fuel known as JET A-1. The use of alternative fuels for aviation is not a new:

Alternative fuel options and requirements

The main criteria for developing alternative aviation fuels are related to maintaining the efficiency of the plane design and the engines. Planes may have to be redesigned to carry extra weight, to store the fuel and to burn the alternative fuels. Aircraft need to be lightweight with efficient engines and wings. Aviation fuels needs to have a high energy content per unit volume and weight and to be from sustainable sources. Some of the alternatives are listed below:

Synthetic liquid fuels (Syn-Jet) - Synthetic fuel or synfuel is any liquid fuel derived from natural gas, coal, or biomass and possibly from other sources such as tar sand, oil shale, waste plastics. The Synthetic liquid fuel produced is almost identical to kerosene, but extra processing may required to deal with particulates and the effects of low temperatures on the fuels. These fuels produce equivalent levels of carbon dioxide to petroleum kerosene and may not have global warming benefits for fossil based source materials.

Bio-jet - fuel can bemade from agricultural oil crops like canola and soya. The synthetic or biojet fuels of the future will have to be processed to meet jet fuel specifications.

Ethanol is not a good option for long-haul aircraft as ethanol fuelled aircraft would need much larger wings and engines reducing fuel efficiency (Fig 5, Ref C).

Hydrogen use in plane has been proved and may be a very long-term option but will require resigned aircraft (Fig 6, Ref C) and major ground infrastructure changes.

Aviation Fuel Options

Figure 5. The Ethanol powered plane (Ref C)
Figure 5. The Ethanol powered plane (Ref C). Source: Pubic Domain
Figure 6. The Hydrogen powered Plane (Ref C)
Figure 6. The Hydrogen powered Plane (Ref C). Source: Pubic Domain
Figure 7. Best Fuel options in terms of weight and volume (Ref C)
Figure 7. Best Fuel options in terms of weight and volume (Ref C). Source: Pubic Domain

Conclusion: What are the Best Alternative Fuels for Planes?

The requirements for aviation fuel depend on the weight/energy ratio and the volume/energy ratio for the fuel. Figure 7, (Ref C) shows that Syn-Jet /Bio-Jet fuels are the best in terms of Volume/Energy content and Liquid hydrogen is best in terms of Weight/ Energy Content.

Aircraft Design - because synthetic bio-jet fuel and synthetic jet fuel made from coal and natural gas have about the same volume, weight and performance characteristics of current oil-derived jet fuel, they would be relatively easy to use and not affect the design of the plane.

Ethanol-powered airplanes - would have to be specifically designed. Figure 5 shows a modified design with bigger wings. Ethanol requires about 65 % more storage volume for the same amount of energy as kerosene fuel, and Ethanol also weighs more, requiring larger wings and less efficient aircraft.

Hydrogen (and methane) powered airplanes - Because Hydrogen must be stored in its liquid cryogenic form there are insulation and pressurization issues. This means that liquid hydrogen and other liquefied gas fuels cannot be stored in the wings and planes would need to be enlarged to allow for tanks in the fuselage (Fig5, Ref C)

Sustainability and Carbon Trading Implications - For a long-term energy solutions, the liquid fuel should be sustainable and there are carbon trading issues. Synthetic fuels derived from coal or natural gas, are not sustainable. Biofuels are derived from plants and are sustainable, so that Bio-Jet fuel becomes an option, but the competition with food production also becomes an issue.


Further research is needed to identify the sustainable alternatives and the strategy to make the transition perhaps using Syn-Jet made from coal and gas as an interim solution. Aviation Biofuel needs to be developed especially when carbon trading starts in earnest, worldwide.

Given that world oil production is close to its peak the need to develop a strategy is urgent.


Reference A The Hubbert Peak for World Oil

Reference B Forecasting World Crude Oil Production Using Multicyclic Hubbert Model

Reference C: Alternative Fuels and Their Potential Impact on Aviation