Measurement OF Non-Methane Hydrocarbons (NMHC)

Presently, the emission standards are specified in terms of non-methane hydrocarbons. Methane content of HC emissions is determined by one of the following methods:

• Gas chromatographic (GC) method or
• Non-methane cutter (NMC) method

In the GC method, sample is injected into GC column which separates the sample into two parts:

(i) CH ₄ -air-CO, and

(ii) NMHC–CO₂ –H₂O.

A molecular sieve column separates methane from air and CO before passing it to FID. Thus methane content is measured that is deducted from the total hydrocarbon content.

In the NMC method, all hydrocarbons except CH₄ are oxidized to CO₂and water on a catalyst, so that when the gas sample is passed through NMC only CH₄ is detected by HFID. The NMC cutter is calibrated for catalytic effect on CH₄ and higher hydrocarbon (ethane) mixtures in presence of water vapours with values typical of exhaust gas at or above 600 K. The sample can be alternatively passed through NMC or bypasses the NMC. In this manner, the total HC and methane alone present in the exhaust gas sample are determined.