Diesel is the cleanest fossil fuel, all things considered. The cleanest fossil fuels — and the dirtiest — all produce emissions. Determining what makes fossil fuel dirty is essentially an emissions inventory.  It requires looking at a number of fuel production and consumption variables.

First and foremost, the entire lifecycle of a fossil fuel is relevant to its environmental friendliness. To determine whether or not a fuel is clean, that fuel’s lifecycle carbon footprint must be taken into consideration as a whole. A comparison of combustion emissions is not sufficient.

Combustion emissions only reflect the environmental impact of the tail end of a fuel’s life cycle.

Fossil Fuels in the “Cleanest Fossil Fuel” Debate

Not all fossil fuels are in the “cleanest fossil fuel debate.” For example, few scientists/researchers — or even laymen for that matter — argue coal is one of the cleanest fossil fuels. The mining of coal, alone, has  the impact of coal on the environment, particularly open pit mining. And, the combustion emissions from coal are the highest of any fossil fuel.

On the other hand, proponents of natural gas label it, “the cleanest fossil fuel,” because of its low carbon dioxide combustion emissions. Also marketed with the “clean” label — though not fossil fuels — are alternative fuels like biomass and biofuel. The propane industry, as well as other gas fuels, also reaps the rewards of a “clean” label.

Combustion Emissions, the Strawman

Combustion emissions are a fallacy. A strawman fallacy used to escape the real issues of carbon emissions.  Proponents of low combustion emission fuels often argue as if combustion emissions is the only contributor — or at least the largest — to greenhouse gases and global warming.

Combustion emissions do not account for all emissions. Measuring the global warming potential of a fuel throughout its entire life cycle often tells a different story than one gets from a cursory look at combustion emissions.

Extraction and production add to a fuel’s carbon footprint. Refining/conditioning tells another part of the story. The transportation and storage of a fuel tells yet another. Combustion emissions are the last chapter of a fuel’s life cycle.

Fuel Volume to Emissions Ratios versus Energy to Emissions Ratios, the Second Strawman

Diesel and fuel oil — as the result of higher emissions on a combustion emissions per unit-of-volume scale — fall prey to the “dirty” label. However, there is a difference between emissions on a volume scale and emissions on an energy scale. For example, fuels that produce low emissions on a volume scale can produce high amounts of emissions on a net energy scale.

A certain amount of energy is necessary to perform a given amount of work. So, volume emissions are not as important as the ratio between emissions and net energy.

In other words, emissions per gallon are not as important as emissions per Btu produced. Truly, gallon (or liter) emissions per fuel type are irrelevant because each gallon of a different type of fuel produces different sums of energy.

Emissions to energy produced tell the real story.