What term describes the amount of unburned hydrocarbons in flue gas?

The term that describes the amount of unburned hydrocarbons in flue gas is smoke density. Smoke density refers to the visible emission from a chimney, which is a result of incomplete combustion of fuel. When hydrocarbons are not fully burned during the combustion process, they are released as particulate matter, contributing to smoke density. High smoke density indicates a higher level of unburned hydrocarbons being emitted, which can signify inefficiencies in the combustion process.

In contrast, flue pressure relates to the pressure within the flue or chimney system and does not directly measure unburned hydrocarbons. Combustion efficiency refers to how effectively a fuel is converted to heat during combustion, which includes factors beyond just unburned hydrocarbons, as it looks at the overall performance of the heating appliance. Fuel quality pertains to the characteristics of the fuel itself, such as its composition and properties, which influence the combustion process but do not specifically measure unburned hydrocarbons in the flue gas.