Burning NCGs in recovery boilers makes pulp mills odorless

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 non-condensable gases (NCGs), allowing pulp mills to be odorless and more environmentally-friendly. NCGs are composed of mostly sulfur gases, often referred to as TRS (total reduced sulphur), and gaseous methanol. While at a low concentration they give off a very strong rotten egg smell, at slightly higher concentrations, they are toxic.

Collection of NCGs

NCGs are collected at various points in the process such as tanks, bins, digesters and washers. They can be in one of two forms: DNCGs which are below the flammability limit and will not be able to burn off on their own, and, CNCGs which are above the flammability limit where they will need oxygen to burn.

Issues with burning NCGs

In the past mills have used lime kiln, power boilers or a dedicated incinerator to burn off NCGs. But there can be problems with these choices. Lime kiln burning of NCGs can lead to the formation of rings that will cause a loss of production. Burning NCGs 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.

Solution

The solution to this problem is to turn to the recovery boiler. 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 which allows the sulfur to be returned to the pulping process. The heat from the NCGs will increase the steam production from the boiler and since the venting of NCGs 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 NCGs in the recovery boiler and is able to set up a collection system to have both DNCGs and CNCGs injected into the recovery boiler. It is a safe and reliable way of disposal.

For more information about NCG disposal, contact your Valmet representative.