HVLC or dilute noncondensible gas (NCG) sources are those that have typical NCG concentrations less then the lower explosion limit (LEL). These are sources that have a large amount of air infiltration due to process or operational concerns. In this category are the brown stock washers, filtrate tanks, oxygen delignification systems, liquor storage tanks, screens, deckers and knotters. Continuous chip bin gases also fall into this category; however these gases present a unique problem in that under certain upset conditions their composition can exceed the LEL. Dilute NCG sources can total tens of thousands of ACFM of NCG that require incinertion. Therefore it is important to do what ever is possible to minimize the flow.

New tanks should be designed for limited pressure and vacuum conditions so that they can be sealed. New washer hoods should be installed on vacuum drum washers to limit air infiltration. Ambient air doctor fans on washers should be converted to recirculation systems and exhaust from the filtrate tank should be recycled back to the respective washer hood. All of these items will help reduce the volume of dilute gas in addition to gas coolers to reduce the water vapor volume. While the dilute gas may be burned in the lime kiln, in most instances the dilute NCG volume is so large this is not feasible.

The volume of dilute gas requires a fan as the motive force to send the gases to the point of incineration be it the recovery boiler, power boiler, regenerative thermal oxidizer (RTO) or dedicated incinerator. The fan needs to be spark resistant and designed to minimize any condensate build up in the casing. Mist eliminators are installed upstream and downstream of the fan to minimize erosion of the impeller. Depending on where the gases are burned a steam heater may be employed to reheat the NCG prior to combustion. Rupture discs are provided as well to protect against overpressure conditions. Even though the gases are presumed to be below the LEL we typically include a flame arrester as one more safety feature.

All the appropriate safety interlocks and safe guards are included in the system. The system is designed to vent on low NCG flow, high NCG temperature at the point of incineration, loss of flame or steam generation in the boiler or incinerator, low fan speed, and loss of electrical power and instrument air.

A. H. Lundberg Associates, Inc. can provide dilute NCG systems with thermal oxidation in all accustomed locations including a regenerative thermal oxidizer.