The measurement of hydrocarbons in flue gas or vehicle exhausts is now standard practice. Hydrocarbons are recognised as important greenhouse gases in their own right, having considerably more warming effect per molecule than carbon dioxide. The length of time that hydrocarbons exist in the atmosphere is limited, however. They oxidise in time to carbon dioxide under the influence of oxygen, ozone and ultra-violet radiation.
Nevertheless, hydrocarbons in flue gas are a contribution to global warming, as well as being pure waste and signs of an incorrectly adjusted combustion process.
The type and quantity of hydrocarbons produced will depend very much on the combustion conditions. The general type of industrial combustion process involves atmospheric burning of a fuel, without any extremes of temperature and pressure. Despite most fuels being long-chain hydrocarbons, the only measurable hydrocarbon component in flue gases is methane. This may, at first sight, seem strange, since methane is a highly combustible gas, but it is a measured and proven fact that only traces of ethane and higher hydrocarbons are ever found. The main reason for this is that it is relatively easy to break the carbon - carbon bond to leave carbon atoms attached to hydrogen atoms. Once split, the carbons will preferentially join with oxygen or free hydrogen atoms, rather than reforming the more instable chain structure.
The only exception to this is the internal combustion engine, which subjects the fuel air mixture to extremes of temperature and pressure during the combustion process. Here, there is production of the higher hydrocarbons, notably hexane or n-hexane. These hydrocarbons are much easier to measure than the shorter chains or methane.
An infrared sensor set to measure n-hexane will react to the presence of methane, and vice-versa. The main trouble is the difference in signal produced. A methane sensor will report 30 ppm of n-hexane as roughly 900 ppm!
Clearly, different measuring techniques are needed for measuring the gases from different processes in this case, but for flue gases, the question of total hydrocarbons resolves simply to a measurement of methane.