When considering future energy systems, it is very important to separate energy from fuel.
One of the characteristics of petroleum (oil) that makes it a perfect product is the
fact that it is both a “Primary Energy Source” and a “Fuel.” While in the context of
petroleum, these can be considered the same thing, future energy production
and fuel systems require that primary energy sources and fuels be understood as
two different things. This is due to the fact that there are no replacements waiting in the
wings that offer both properties.
Industrial mankind’s largest challenge at this point is replacing the large quantity of energy that
is currently provided by petroleum. Replacing oil as a fuel is also a serious problem, in large
part because the primary energy problem is so serious. If the primary energy problem is “solved,” many fuel solutions are available.
Combining carbon dioxide removed from air and hydrogen split from water using a Fischer–Tropsch like process could produce high quality diesel fuel essentially from air and water. Unfortunately, this process is so energy intense that it is not really considered a viable solution. And it is considered unlikely that we will come up with a primary source of energy so plentiful to make it viable.
So although this system is not viable, it does demonstrate that the most difficult challenge facing Industrial Man is replacing the energy that currently is obtained from fossil fuels.
Because the energy sources available to replace fossil fuel are of lower density and higher cost than fossil fuel, the energy efficiency of the fuel is also very important. Energy efficiency of fuel in this context refers to how much additional energy must be used to obtain, manufacture, use, and dispose of the fuel and its ingredients.
The most useful fuels are hydrocarbon fuel or fuels that are made from hydrogen and carbon. The hydrogen provides the large quantities of energy that are desirable in a fuel, while the carbon provides the storage and handling characteristics that make a fuel like diesel, jet, gasoline, and alcohols so desirable.
Carbon represents two serious challenges in regard to its future use as an ingredient in fuels.
1. Obtaining it:
Obtaining the clean carbon that is necessary to manufacture large quantities
of hydrocarbon fuels is not a trivial task. This is often overlooked as
carbon in the form of CO2 (carbon dioxide) is considered a pollutant at
nearly all points that it is emitted. But closer examination reveals some
- Location: Is the carbon source stationary or moving? Since we are usually talking about a gas, capturing and transporting it is expensive.
- Cleanliness: Most processes require clean carbon. If the source produces dirty carbon, cleaning it up can be a very energy intensive and financially expensive proposition.
- Timeliness: Is the primary energy available at the same times as all of the other ingredients of the fuel? If not, what is the energy and financial costs of the storage?
2. Disposing of it:
What is the cost of disposing of it? Carbon is increasingly being implicated as a pollutant. This status means that the cost of using it as a component of fuel is likely to increase. It remains to be seen what the carbon taxes of the future will look like, but it looks increasingly likely that there will be clear penalties for emitting carbon into the atmosphere.
Hydrogen is often seen as the fuel that will be used to replace hydrocarbons. Hydrogen has some very serious limitations, including:
- Very low density: The density of hydrogen is so low that shipping it as a gas using tanker type equipment that is similar in design to the equipment that is now used to transport petroleum is out of the question. Shipping it as a refrigerated liquid improves this substantially but is energy intense, and creates another challenging set of storage issues.
- Flammability: Hydrogen is extremely flammable, it takes very little energy to ignite, and can self ignite under certain circumstances.
- Leakage and embrittlement: Hydrogen can leak through many container and plumbing materials, and can make them brittle and dangerous. The most common and cost-effective materials for tanks and plumbing cannot be used.
There is one fuel that can replace hydrocarbons in many of the applications where
the density of hydrogen is too low. This fuel is ammonia. Ammonia is similar in
handling characteristics to propane and it is already commonly used in
industry and agriculture.
The advantages of ammonia in comparison to hydrogen are:
- Higher Storage Density
- Reduced Flammability
- No Carbon.
Ammonia is composed from nitrogen and hydrogen, and therefore can be made using several
known methods from air, water, and large quantities of energy. Since we currently don’t
know how to capture large quantities of renewable energy, most ammonia today is made from natural gas.
Ammonia has some challenges that often disqualify it from consideration as a fuel.
- Toxicity: Ammonia is toxic enough that is will likely not be approved for common, untrained use.
- Odor: Ammonia has a very strong odor, although this is desirable as it is nature’s defense against the toxicity.
- Storage Considerations: Ammonia is a gas at standard atmospheric conditions. Its storage characteristics are similar to propane, with about half the energy density. This disqualifies ammonia for some uses, such as aviation.
While clearly ammonia is not the magic bullet that some seek, a survey of the realities of carbon capture and disposal, photosynthetic efficiency, battery efficiency, and storage density indicate that there are no other fuels that are as ready to be pressed into service as ammonia.
Ammonia is already one of the most commonly used and synthesized chemicals. The most common use is fertilizer, and without ammonia fertilizer there would be large scale starvation. Although ammonia may be too difficult and toxic to handle by untrained operators, it is ready today for professional use.
Now we just need the energy.