Particulate fumes are sub-micron in size, but they were causing big headaches for Domtar Papers of Cornwall, Ont.
The fumes, created by vapour evaporation, are among the most difficult emissions to measure, control and capture. They can produce a visible haze as well as odour at extremely low mass loadings. Yet the low weight of fume emissions often allows compliance within regulatory tolerances. Their small size can confound measurement while making collection difficult.
Domtar Papers was struggling with emissions from its black liquor recovery boiler. The company’s existing wet scrubber system kept particulates within regulatory tolerance. Yet, emissions were visible and occasionally produced an objectionable odour.
In the Kraft pulping (or sulfate) processes, which accounts for about three quarters of all US pulp produced in North America, chemicals are used to break down wood pulp. This process is usually considered a good way to make strong ("Kraft" is German for "strength") and flexible paper products. Part of the process’s appeal is that the chemicals that are used in it are easily recovered, making for cost-efficiency and environmental soundness.
The Kraft process uses a sodium-based alkaline pulping solution, or liquor, consisting of sodium sulfide (Na2S) and sodium hydroxide (NaOH) in 10 percent solution. This liquor (known as "white liquor") is mixed with the wood chips in a reaction vessel. The output products are separated wood fibers, or pulp, and a liquid that contains the dissolved solids in a solution of reacted and unreacted pulping chemicals. This is what is known as "black liquor." The black liquor undergoes a chemical recovery process to regenerate white liquor for the first pulping step. Overall, the Kraft process converts approximately 50 percent of input into pulp.
To maintain its reputation as a responsible community member and employer, Domtar was on the lookout for technology that could reduce fume emissions by at least 90 percent, yet required little maintenance.
Because of the potential corrosiveness and stickiness of particles in this recovery boiler equipped with direct-contact evaporator, dry collection systems were not considered sufficiently reliable or effective. While conventional venturi scrubbers were an obvious consideration, Domtar decided that they were neither reliable or efficient enough for this application.
Domtar considered several types of wet scrubbers, and the choice was narrowed down to two contenders: one featuring a nozzle spray chamber configuration and one which previously had been proven in recovery boiler service. Both suppliers guaranteed the required particulate emissions, set by Canadian authorities, of 0.08 g/DSm3 or 0.035 gr/SCFD. Domtar chose to test both scrubbers simultaneously. Pilot units from the two vendors were installed side by side on a slip stream from the black liquor direct-contact evaporator, both in a position to achieve as identical conditions as possible.
According to the findings, the boiler service unit, which is equipped with a round throat and a dual-leaf, adjustable damper, edged out the nozzle spray chamber system. Designed like a dentist’s bowl, the gas-liquid contact system in a boiler service unit consists of a funnel though which the raw gas and scrubbing liquid flow concurrently. As the gas enters the large end of the funnel, liquid is injected tangentially against the walls of the funnel. The water swirls down the sides and toward the bottom, the small end of the funnel, covering the entire surface of the interior walls. As the liquid and gas flow into the converging section of the funnel, the increasing gas velocity atomizes the liquid and the particulates are entrapped by the liquid droplets.
The gas/liquid mixture then enters a cyclone which separates the particulate-laden liquid droplets from the gas. Clean gas exits the stack and the scrubbing liquid drains to a recycle tank for reuse in the venturi. The solids concentration in the recycle liquid is controlled by a bleed from the recycle tank. The bleed stream, high in sodium sulfate, is returned to the plant’s black liquor system and fresh water is used as make-up in the recycle tank.
Key to the effectiveness of such a system is maintaining a constant gas velocity in the throat, at stream process variations. If gas velocity through the throat drops below optimum, the scrubbing liquid will not be sufficiently atomized to capture the particulate. If gas velocity is above optimum, the high gas velocity increases pressure drop through the venturi, requiring greater fan power and higher operating costs.
The system controls gas velocity in the throat with a double leaf damper. As operating variations in the recovery boiler cause changes in gas flow, the damper automatically adjusts to maintain constant throat velocity. By maintaining constant velocity (or pressure drop) in the throat, particulate collection efficiency is maintained at its optimum level.
An ID fan downstream of the cyclonic separator, operated by a variable speed drive, maintains a constant draft on the recovery boiler and emission control equipment. Unlike a conventional venturi throat which has a rectangular cross section configuration, the throat in this system is round. The round cross section improves coverage of the throat walls with scrubbing liquid, reducing the potential for dry spots and the resultant abrasion damage. A round throat also improves the atomization efficiency by maintaining a more uniform gas velocity profile.
Since installation in April 1998, the scrubber has operated with virtually no maintenance requirements according to Domtar. Particulate discharge from the recovery boiler stack have been reduced by 95 percent, maintaining particulate emissions well below 0.08 g/DSm3 (0.035 gr/scfd). The system handles 130,000 DSm3/hr. (97,231 ACFM) of gas at a pressure drop of up to 37 in. WC and uses a liquid flow of 1500 gpm. Another benefit of the system has been the virtual elimination of a visible plume of condensed water droplets.
There are two main reasons for this benefit. First, the cyclonic separator downstream of the venture removes the water droplets in the gas stream entering the venturi. Also, as the 1200 HP ID fan decreases the gas pressure to 37 in. WC, it raises the gas temperature about 15 deg. F. This amount of the reheat is sufficient to eliminate a visible plume of condensed water at the stack. While reduced plume visibility may not be a pollution issue, it does improve community relations.
The scrubber, as well as associated ductwork, ID fan, and detailed design and engineering was provided by AirPol Inc., of Parsippany, NJ. AirPol has produced this venturi in designs with an inlet as large as 13 feet in diameter, and a throat as large as 8 ft. in diameter. Applications include particulate collection from steel furnaces, chemical and pharmaceutical plants, and hazardous waste incinerators.
Nathan Mallet is an editor with Clifford/Elliot Ltd. He prepared this article with files from Levy & Associates of New York City.