We are publishing a series of posts to do with evaluating alternative sources of energy. This article is based on the post Energy Evaluation: Properties describes a method by which different fuels and energy sources can be compared.
The Properties category summarizes the technical features of each energy source. They are summarized in the structure shown in the Table, which provides values for four of the energy supplies derived from fossil fuels. All values are in SI/metric units. (A description of the units of measurement is provided at the end of this chapter.) Boiling and melting point values have been rounded to the nearest degree.
Many of the values provided are approximate depending on the source and composition of the energy source. In particular, the composition of coal varies substantially. The highest quality of coal is anthracite (shown in the Table); the lowest quality coal is lignite. Their energy properties differ significantly from one another.
It must be stressed that the values provided are approximate depending on the source and composition of that type of energy. In particular, the composition of coal varies markedly. The highest quality of coal is anthracite and is shown in the Table (the lowest quality coal is lignite).
- Energy Density refers to the amount of energy provided on either a weight or volumetric basis. One of the most important features of fossil fuels is that their high energy density. An alternative measure for the same property is ‘Gasoline Gallon Equivalency’ — the volume of fuel that has the same energy content of one gallon of gasoline.
- Specific Gravity is the density of the item (for liquids at their melting point) relative to water at ambient conditions. A value of 1.0 is equivalent to a density of 1000 kg/m3.
- Melting Point is the temperature at which the solid form of the fuel (if there one) melts to form a liquid.
- Boiling Point is the temperature at which the fuel boils at atmospheric pressure. (100°C is the temperature at which water boils.)
- CO2 Emission provides an approximate value for the amount of CO2 that is emitted when that fuel is burned.
- Flash Point (closed cup) is the lowest temperature at which its vapors from that material ignite in the presence of an ignition source. The flash point is not the same as the autoignition temperature, which is the point at which a material will spontaneously ignite in the presence of air, even if there is no source of ignition.
The following observations can be made about the values shown in the Table.
- All the fossil fuels have a high energy density (firewood is not a fossil fuel; it has a high water content which reduces its energy density).
- Melting and boiling points are not an important parameter for the fuels shown. However, they are important for some of the alternatives energy sources, such as ammonia.
- Methane has a high energy density on a weight basis, but the fact that it is a gas at ambient conditions makes it less attractive than oil products.
- All the fossil fuels generate large amounts of CO2 (coal in particular) for the energy that they provide. This is why they need to be replaced.
- The low flash point of gasoline creates a safety concern. This will be discussed in future posts.
As various alternative fuels are evaluated their energy properties will be added to this Table. In general, it will become clear that none of the alternatives provide the unique combination of properties of fossil fuels. Hence, there will be a thermodynamic penalty to pay when fossil fuels are replaced.
This last point is crucially important.
Alternative energy sources do not provide the same quality of energy as hydrocarbon fuels. Therefore, there will be an incremental cost associated with their use. This is a thermodynamic reality.