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Kraft pulp mill
The use of sodium sulfide in the kraft pulping
process causes the release of fumes whose unpleasant characteristic
odour is well known. It is also recognized that in the digester,
the sulfide ion combines with various chemical compounds derived
from the wood to form organic sulfides such as methyl mercaptan
(CH3SH), dimethyl sulfide (CH32S), dimethyl bisulfide (CH32S2)
and hydrogen sulfide (H2S), as well as turpentine and methanol
vapors. Together, these gases are called non-condensable gases
(NCG). New environmental standards for the control of NCG emissions
require kraft pulp mills to ensure a large reduction in these
emissions, in Quebec, as in Ontario and in the United States
(20 ppm and less). Until recently, NCGs were destroyed in the
following manner:
- Low volume high concentration (LVHC) gases were incinerated
in a lime kiln;
- High volume low concentration (HVLC) gases were incinerated
in a black liquor boiler.
This approach resulted in substantial fuel consumption
savings and partial absorption of the SO2. However, the disadvantages
of this solution were numerous:
- Interference by the NCGs in the optimal operation of the
lime kiln and the black liquor recovery boiler with the resultant
exceeding of increasingly strict norms for releasing TRSs into
the atmosphere;
- Operational problems with the lime kiln and boiler, such
as fused salt accumulations, various organic and ash deposits.
The main advantage of a dedicated incinerator
lies in the absence of interference with plant operations and
the achievement of a high level of TRS destruction. Used for
this application since 1996, the BIOTOX® process can be considered
the Best Available Control Technology (BACT) at this time for
meeting the industry’s strict standards for atmospheric
releases. The application of BIOTOX® technology in the treatment
of pulp and paper industry emissions is very recent. Biothermica
has taken on several challenges in applying BIOTOX® regenerative
thermal oxidation technology in the treatment of TRSs, such as:
- Adapting non-condensable gases to the production cycle;
- Simultaneous treatment of LVHC (low volume high concentration)
gases and HVLC (high volume low concentration) gases;
- Controlling
the mixture’s oxygen rate
in a manner that keeps the mixture of gases below the lower
explosive limit (LEL). (especially where LVHC gases are involved);
- Long-term durability of the device where corrosive gases
are in contact with cold parts (entry pipes, inside shell of
the combustion chamber, bed bottoms, packing supports, inside
parts of dampers) etc…
Biothermica has succeeded in adapting BIOTOX® technology
to pulp and paper emissions. The following improvements have been
made:
- Development of methods that ensure the non-corrosiveness of
incinerator equipment parts in contact with corrosive gases;
- Development of a regenerative incinerator preventive maintenance
program.
In 1997, Biothermica established a Canadian premiere
with the installation of a BIOTOX® unit at a kraft pulp mill.
The unit reduced TRSs by 99.6% with a thermal recuperation rate
of 88.5%.
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