“Dirty Coal”: The Problem Is Nitrogen
The phrase “dirty coal” seems to have become one word. For example, the reports on the recent unsuccessful COP26 conference mostly dwelled on the fact that “phase out of coal” was change to “phase down of coal”.
So why does coal have such a bad reputation? There are two ways at looking at its “dirtiness”. The first is that, being a solid, it leaves behind ash and slag that has to be disposed of. This is an environmental challenge, but it is a manageable challenge. The second reason for coal’s bad reputation is that it contains a higher carbon to hydrogen ratio than other fuels, particularly natural gas — which is primarily methane (CH4 — four hydrogen atoms for each carbon atom). Therefore, for a given energy output, coal emits more carbon dioxide (CO2) than other fuels.
But why does it emit more CO2? One reason is that the air that is used in combustion is about 79% nitrogen. It takes a “free ride” through the combustion process and is emitted directly to the atmosphere. Hence the CO2 in the effluent is of low concentration. Hence it is difficult to extract and sequester that CO2.
One way around this problem could be to burn coal in an oxy-combustion process. CO2 is still created, and it still has to be captured and sequestered. But, once the water content is condensed, the flue gas is almost pure CO2. Hence it is much easier and more economical to dispose of the CO2.