LVHC or strong NCG is gas from the pulping process that is normally above the upper explosion limit (UEL). These gases include those from the multiple effect evaporator vacuum system, the batch digester blow heat system and the digester flash/relief system including the turpentine recovery system. Disposal of the strong gases can take place in the recovery or power boiler, lime kiln and dedicated incinerator. An additional disposal location is a flare, though this is typically only allowed as a backup incineration point.

Each source is a separate collection point protected by a flame arrester and rupture disc plus vessel pressure protection as necessary. A steam ejector is used to draw the gas from the source to the point of incineration.

The steam ejector has no moving or rotating parts and the motive steam acts as a highly efficient flame arrester adding to the system safety. From the ejector the gases pass through a mist eliminator and flame arrester prior to the point of incineration.

Many mills have difficulty with complete combustion of the TRS in lime kilns and with sulfur dioxide emissions from various combustion locations including lime kilns and boilers. A caustic scrubber using white liquor or sodium hydroxide is frequently used to remove ionizable sulfur compounds from the NCG. This translates to about a 50% reduction in sulfur. Depending upon the installation, the gases from the scrubber may be cooled in a shell and tube condenser.

The NCG collection system is designed with all of the appropriate safety features including safe velocities within the collection header, proper pipe sloping and drainage, and caution in the method of injection at the point of incineration. In addition, the system is designed to shut down into a fail safe mode based upon a number of conditions, including low NCG flow, high temperature in the NCG pipe at the point of incineration, loss of flame at the point of combustion plus others.