Burning NCGs in Recovery Boiler

Utilizing state of the art technology is vital to meeting all of the environmental regulations your mill is subject to both today and in the future. Valmet’s superior system and extremely high oxidation rate can burn off NCG and allow our customers to enjoy an odorless and environmentally-friendly pulp mill. NCG stands for Non-Condensable gases and that they are composed of mostly sulfur gases often referred to as TRS, and gaseous methanol. While at a low concentration they can give off a very strong rotten egg smell, at slightly higher concentrations, they can be toxic.

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NCG are collected at various points in the process such as tanks, bins, digesters and washers. They can be in one of two forms: DNCG which are below the flammability limit and will not be able to burn off on their own, and, CNCG which is above the flammability limit where they will need oxygen to burn. Now, in the past mills have used lime kiln, power boilers or a dedicated incinerator to burn off NCG. But there can be problems with these choices. Limekiln burning of NCG can lead to the formation of rings that will cause a loss of production. Burning NCG in a power boiler results in a higher SO2 emissions unless the boiler happens to have a scrubber; there could also be corrosion in the boiler SH and Tubular Air Heater which would cause slagging and plugging issues. The problem with having a dedicated incinerator is that it is both capital intensive and a huge source of maintenance.

The solution to this problem is to turn to the recovery boiler, as it is the ideal location since it is designed to handle black liquor. The SO2 produced is captured by the sodium fumes that are in the boiler and it allows the sulfur to be returned to the pulping process. The heat from the NCG will increase the steam production from the boiler and since the venting of NCG is no longer an acceptable practice, the location of the NCG disposal must be readily available. Recovery boilers are of course readily available as the mill needs to be able to make pulp.

Valmet has extensive experience with the disposal of NCG in the recovery boiler and is able to set up a collection system and have both DNCG and CNCG injected into the recovery boiler is a safe and reliable way.