Final answer:
Solid pollutants, such as fly ash, are primarily produced when burning coal due to its high impurity content and the tendency for incomplete combustion, which leads to more emissions and residue compared to burning natural gas, light fuel oil, or propane.
Step-by-step explanation:
Solid pollutants, such as fly ash, are produced when burning fossil fuels that have high impurity levels and when incomplete combustion occurs. Coal is a naturally occurring renewable solid fossil fuel that produces the most pollution and greenhouse gases when burned. This includes a variety of toxic substances including fly ash, which is a residue left after coal combustion. Fly ash can contain heavy metals and other hazardous compounds that pose environmental health risks.
Coal is known for having a high sulfur content, and when burned, especially without proper emission controls, it can result in various pollutants such as sulfur dioxide (SO₂) and nitrogen oxides (NOx), as well as fly ash and greenhouse gases like carbon dioxide (CO₂). On the other hand, burning natural gas, light fuel oil, or propane releases fewer pollutants and typically does not produce solid residues like fly ash due to their cleaner burn and lower sulfur content.