Firing of Non-Condensible Waste Gas Streams in Stand Alone Modular Incineration Systems

Presented at the 1992 Incineration Conference
Albuquerque, New Mexico
Session XXI

Roberto E. Santos (P.E.). Jonathan C. Backlund (General Manager), Coen Company, Inc.

Disposal of various Non-Condensible Gases (NCG’s) in Pulp and Paper mills is always a concern if present and future environmental regulations are to be met. The present technology for treating these NCG’s involves incineration in the lime kiln, in the power boiler, or even in the recovery boiler. This paper investigates the alternative means of NCG disposal in a modular fume incinerator as a stand- by or full time operation at a recent installation in the United States. Performance parameters and emissions of TRS (Total Reduced Sulfur), Sulfur Oxides, Nitrogen Oxides, and Carbon Monoxide are discussed.

Keywords: Incineration “Environmental” Control Engineering

Although about a third of the carbon monoxide (CO) and volatile organic compounds (VOC) emissions to the atmosphere are generated by transportation, much regulatory is paid to industrial emissions. Manufacturing operations account for only 20% of measurable air pollution in North America, yet this segment receives the greatest public attention due to the high visibility of industrial plants, and because of the characteristic odors and/or visible plumes associated with certain industrial processes.

In the Pulp and Paper industry, a difficult waste stream problem is the disposal of malodorous gases from the kraft process. The human detection threshold is as low as 1 part per billion (ppb) for reduced sulfur gases (1). Non-condensible gases (NCG’s) containing reduced sulfur gases are not only noxious but also very hazardous. NCG’s are highly corrosive due to the sulfur compounds and chlorides with water vapor, and volatile enough to be explosive.

Incineration (or thermal destruction) has been well recognized in the paper mill industry as the preferred method of disposal. Firing NCG’s in the power or recovery boilers is often more troublesome because of high capital and maintenance costs plus, the increased emissions of sulfur dioxide (SO2) in the stack. The lime kiln is the first choice for most plants, but mill production can be affected by any downtime in the kiln. Potlatch Corporation at their Cypress Bend facility near McGehee, Arkansas realized that an alternative stand-by system was necessary to keep production at near 100% and meet the strict environmental regulations. A stand alone incinerator was the answer.

NON-CONDENSIBLE GASES:

The primary sources of NCG’s in a kraft mill are digesters, evaporators and turpentine recovery systems. Kraft mill odor can be attributed to four reduced sulfur gases namely: Hydrogen Sulfide (H2S), Methyl Mercaptan (CH3SH), Dimethyl Sulfide (CH3SCH3), and Dimethyl Disulfide (CH3SSCH3)- Collectively they are referred to as total reduced sulfur (TRS) gases. Volatile organic compounds (VOC) other than those containing sulfur are also emitted during digester relief. Typical constituents are alcohols, terpenes, and phenols. Table I shows a typical analysis of an NCG gas stream.

NCG COLLECTION SYSTEM:

Non-condensible gases from digester relief, digester blow and liquor evaporators can be combined into one gas header for treatment. NCG’s can be air conveyed by booster fans or motivated by steam ejectors if concentrations are within the explosive limits. In an air conveyed system the mixture is diluted to below 25% of the lower explosive limit (LEL). For high concentration low volume gases with considerable amounts of methanol and turpentine vapors the mixture is often steam ejected. The steam reduces the heating value and oxygen concentration of the fuel and therefore decreases the chance for an explosion. For both dilute and concentrated NCG’s pipeflow velocities must be above the flame propagation speed to prevent possible flash back. Flame propagation or burning velocity of pure turpentine vapor can reportedly be as high as 500 ft/s under controlled conditions (2,3). Methanol and TRS gases are in the neighborhood of 1.6 ft/s. Table II shows combustion properties of NCG’S. The design and operation of an NCG incineration system must consider all potential explosion hazards. Safety devices such as flame arresters and rupture discs are commonly used for protection throughout the system.

Kraft mills with continuous digesters emit fairly steady flows of NCG’S. However when batch digesters are used some method of equalizing flows to the incinerator is necessary. Large vessels (gas holders) are typically used to provide surge capacity so that blow gases can be metered at a constant rate.

INCINERATION CONCEPTS:

The concept of destroying highly toxic, corrosive and explosive gases by incineration is fairly straightforward. The incineration process is simply rapid oxidation of organic substances by direct or indirect heat thus, reducing the volume and toxicity of the remaining residuals. In the case of NCG’S, the VOC’s are oxidized to C02 and U20 and TRS gases are converted to S02. With the right flame temperature (1500-1800 F), retention time (0.5 sec minimum) and adequate mixing with excess oxygen (4%), a destruction and removal efficiency (DRE) of 99.99% is attainable. DRE is defined by the following formula:

The concept might be straightforward but the process is quite complex. The combustion process involves complex interactions of heat and mass transfer and chemical kinetics in a two-phase system. ne type of burner design makes a tremendous difference in achieving a DRE to “four nines”. About 99% of wastes are destroyed in the primary flame zone of the burner and the remaining 1% in the post-flame zone (retention chamber). A high performance burner and a well designed furnace chamber will allow very little “slippage” of pollutants into the flue gas creating less load on the downstream emission control devices. The ultimate goal of NCG incineration is to convert the toxic, malodorous fumes into combustion products which can be easily removed by the downstream emission control devices, with the remaining inert gases released directly to the atmosphere.

QUENCHING SYSTEM:

The purpose of the quenching system is to reduce combustion flue gas temperature from 1800 to 185 F. The purpose is twofold: it minimizes gas volume flow rates for downstream processing and it provides gases at a temperature which is acceptable for scrubbing. Typically, quenching is accomplished by saturating the hot combustion products with water. During the quenching process, heat recovery can be a desirable option.

SCRUBBER SYSTEM:

The combustion products from an NCG incinerator contain a fair amount of sulfur oxides that need to go through an emission control device before exhausting to the atmosphere. There are many types of scrubbers that can be used to absorb S02′ The simplest and most cost effective type for a stand alone incineration system is the stationary wet packed bed scrubber. Packed bed scrubbers remove particulates and gaseous contaminants by a gas absorption process involving intimate gas/liquid contact. These scrubbers are vessels filled with randomly oriented packing material. The scrubbing liquid or caustic is fed to the top of the vessel, with gases flowing counter current. As the liquid flows down through the bed of packing it wets the packing material and thus provides interfacial surface area for mass transfer with the gas stream.

MILL EXPERIENCE:

The Potlatch Corporation kraft mill at Cypress Bend near McGehee, Arkansas produces 495 U.S. tons a day of pulp in four batch digesters. Each digester has a volume of 6000 ft3 and blows down at 30 minute intervals. Prior to August 1991 all non-condensible gases were incinerated in the lime kiln. Even though the mill is located in a sparsely populated area effective odor control was a priority. The state of Arkansas mandated a report of all kiln downtimes in excess of 30 minutes. It was not uncommon to vent untreated NCG’s and TRS gases to the atmosphere during kiln shutdowns but new state and federal regulations have severely limited this practice. After considering the physical location of the NCG sources it was decided that a stand-by stand alone modular incinerator would be built beside the lime kiln.

Figure 1, Simplified Flow Diagram of NCG Incineration System at Cypress Bend Pulp and Paper

A modular design was desired due to space constraints. NCG’s were the dilute type conveyed by air at varying flow rates (see Table 111). Heating value of the gas, stream also varied depending on which wood is used in the pulping process. Southern pines can more than double the concentration of combustibles compared to the northern hardwoods. The incinerator had to be designed to handle NCG’s with heating values between 27 to 45 Btu/ft3.

The NCG’s are introduced into the furnace coaxially directly into a low pressure drop high mix duct burner. The duct burner fires natural gas as a sustaining fuel to the waste stream. Thermal oxidation of the waste stream occurs in the soft lined refractory furnace. Ceramic fiber refractory modules (Z-Blok) were used due to the unique thermal properties. Thermal conductivity is much lower than comparable firebrick and castable refractories (2 Btu-ft/hr-ft2F at 2000 F). Because it has very little thermal mass, the furnace can reach incineration temperature in under 2 minutes. The high emissivity value of 0.95 allows a cold face temperature of 130 F with a lining thickness of only 6 inches. The retention chamber was sized for a 1 second residence time to ensure complete destruction of the waste gases. Computer designed mixing baffles were also used in the chamber to enhance mixing. In operation, attaining 99.99% DRE was not a problem.

Furnace flue gases enter a quench chamber at 1800 F. Plant water is used to quench gases to a fully saturated vapor at 185 F. A modular wet packed bed scrubber from Interel was specified due to its compact design. Pumps, motors, probes and controllers are all integrally mounted on the scrubber vessel using a unique shelf design. The scrubber absorbs 99.96% of all S02 from the flue gas. The plant uses 20% wt NaOH as caustic to scrub the sulfur oxides. White liquor was used initially but had to be abandoned because TRS levels at the stack increased. At equilibrium the sodium sulfide (NaS) in the white liquor liberated and formed hydrogen sulfide, H2Sv- In effect they were creating a TRS gas instead of disposing of it.

The gases are exhausted to a free standing stack motivated by a variable drive induced draft (ID) fan. Thermal oxidation in a stand alone incinerator was generally perceived as a poor choice because of the corrosion potential and the lack of absorptive chemicals that the lime kiln has. But with proper material selection along with careful considerations to the nature the NCG waste stream, that perception can easily be dispelled.

MATERIAL SELECTION:

Despite very low emissions of S02 mild steel should not be used in these systems. Sulfuric and sulfurous acids which are soluble in water and hygroscopic will always be present. Any of the austenitic stainless steels (ie. type 304) is acceptable at ambient temperatures and high acid concentrations. For dilute sulfuric acid only the molybdenum grades such as type 316 and 317 are useful, although type 304 may be used when only a trace of acid is present (4). Fiberglass reinforced polyester (FRP) materials are very popular in pulp mills because of their corrosion resistance to many acids. Attention must be given to the type of resin used to match the temperature service. This is especially critical in the scrubber vessel, induced draft fan, and exhaust stack. Type 316L stainless steel was used exclusively in the Cypress Bend incinerator.

MEETING EMISSIONS:

Air pollution control to meet present regulations was the prime directive in designing the incineration system at Cypress Bend. Emission requirements are usually dictated by the total annual emission rate allowed by the state for the system. Table IV shows typical emission rates expected from the subject NCG incinerator.

SUMMARY:

The modular incinerator concept can accommodate different high performance burners for different composition and volumes of waste streams. If there are space constraints, a modular incinerator can operate in different configurations (ie. vertically or horizontally). Modular systems allows future retrofit at minimal cost. A stand alone incinerator can reduce problems associated with incinerating NCG’s in the lime kiln, power boiler or recovery boiler. Ringing problems in lime kilns have been attributed to incineration of concentrated NCG’s. Corrosion problems in boiler tubes and reduction of boiler efficiency have been blamed on NCG firing. With more stringent environmental regulations requiring tighter closure of all industrial processes and pressure to reduce odorous emissions, stand alone incinerators become a more feasible and attractive means of NCG disposal in the pulp and paper industry.

REFERENCES:
  1. USEPA, Technology Transfer, “Environmental Pollution Control, Pulp and Paper Industry, Part 1: Air” EPA 625/7-76-001 (October 1976).
  2. Coward, H.F., and Jones, G.W., Limits of Flammability of Gases and Vapors, U.S. Dept. of the Interior Bureau of Mines, Bulletin 503, 1952.
  3. Zabetakis, M.G., Flammability Characteristics of Combustible Gases and Vapors, U.S. Dept. of the Interior Bureau of Mines, Bulletin 627, 1965.
  4. Metals Handbook, Volume 13, 9th ed., Corrosion, American Society for Metals, 1987, p 1150.
  5. Rogers, H.G., “Firing Waste Streams in Recovery Boilers,” TAPPI Journal (December 1985).
  6. Lee, C.C., Huffman, G.L., and Oberacker, D.A., “An Overview of Hazardous/Toxic Waste Incineration,” Journal of the Air Pollution Control Association (August     1986).
  7. Burgess, T.L., Kjeruif, E.B., Tann, T.I., “Design Considerations for High- Concentration, Low Volume Non-Condensible Gas Systems,” TAPPI Journal (September 1984)
  8. Brunner, C.R., “Incineration: Today’s Hot Option for Waste Disposal,” Chemical Engineering (October 12, 1987).