The kraft pulping process involves cooking wood chips in a chemical pulping solution under high pressure and temperature. The combination of pulping chemicals is called
white liquor and contains primarily sodium hydroxide and sodium sulphide.
The use of
sulphur means that the by-products of the pulping process will contain sulphur-laden gases, referred to as
Total Reduced Sulphur (TRS). There are also other gases such as methanol (wood alcohol) and
terpenes generated from the spent cooking chemicals and wood.
These off-gases, often referred to as non-condensable gases, are broken down into two groups: Concentrated Non-Condensable Gases (
CNCG) and Dilute Non-Condensable Gases (
DNCG).
The CNCG group is at very high concentrations and is very flammable. These gases, due to their nature are collected in a sophisticated system, and are fed directly into a furnace as a fuel under controlled conditions.
The DNCG group, as the name implies, contains diluted concentrations of gases, which are usually released directly to the atmosphere.
There are two kraft pulp mills located at the Thunder Bay Operation. The mills, ‘A’ and ‘B’, were built in 1965 and 1975 respectively. A CNCG system has been in operation since the start up of these mills. The CNCGs are used as fuel in either #6 Power Boiler or our
Lime Kilns.
The DNCGs, which consist primarily of various tank and process vent emissions are released directly at the roof level to the atmosphere. While each vent on its own is not necessary cause for concern, as a group they contribute significantly to the
odour coming from the mill, specifically in the near-mill vicinity.
To address this issue, a new collection system is in the process of being designed and installed. Expertise was acquired from Scandinavia, where this type of collection has been in practice for many years.
The new DNCG system will collect exhaust gases from roof top vents from all over the kraft mill, and pipe them to a “concentrator” system. Because the various compounds from these emission sources are at such low concentrations they cannot be burned efficiently. They must first be concentrated after which they’ll be burned in the new ‘C’ Recovery Boiler.
After completion, this system will be the first of its kind in Eastern Canada—and will reduce odour-causing emissions by rooftop vents significantly. The system will be fully operational by the end of 2004.