PROBLEMS WITH KRAFT PULP MILL NON-CONDENSIBLE GAS INCINERATORS
Environmental concerns and pressure from the authorities have forced the U.S. pulp mills to continuously reduce the emissions to the atmosphere of particulates and acid gases. At Kraft mills specific compounds that have received attention lately are Non-Condensible Gases (NCG) and Total Reduced Sulfur (TRS). The first attempts to dispose of NCG were to incinerate these gases in the lime kiln or in the power boiler. There are several disadvantages with this approach, such as long transportation distance from source to disposal point and possible detrimental effects on the combustion source and the pulping process.
The incineration process converts the TRS compounds in the NCG to sulfur dioxide (SO2) and a small amount of sulfur trioxide (SO3). In those cases where the amount of SO2 generated exceeds the total allowable amount for the mill, an SO2 scrubber is required. This scrubber will have to be sized for the total flue gas volume from the combustion device. The gas volume from a dedicated incinerator is several orders of magnitude smaller than that from a lime kiln or a boiler, and will thus require a smaller scrubber.
The dedicated NCG incineration system offers operating personnel a way to isolate by-product disposal from the primary functions of the mill. The unsteady nature of NCG streams may cause problems when the lime kiln or the power boiler are the primary disposal devices. But does a dedicated incinerator mean smooth sailing? According to reports from the field, problems still exist. This paper includes a brief overview of the different options, followed by a sampling of field problems and solutions from various installations.
When it first became desirable or necessary to destroy NCG and TRS from an environmental standpoint, the most obvious solution was to thermally oxidize these gases in one of the existing combustion sources, available in the pulp mill. The recovery furnace, the power boiler and the lime re-burning kiln were primary sources of high temperatures, where the NCG could be converted to sulfur dioxide (SO2), which gas in many cases can be released to the atmosphere without objectionable odor, or alternately be absorbed in a downstream scrubber.
Although this approach appears to be a natural and simple way to deal with the disposal of the NCG, the use of an existing combustion unit in the mill can have its drawbacks. The combustion unit is often located far away from the NCG sources and long runs of ductwork can present problems in logistics and cost.
A greater problem from a cost standpoint arises when, as a result of incineration of the NCG, the amount of SO2 emitted from the combustion unit, causes the total amount of SO2 from the mill to exceed the maximum allowed in the environmental permit. That will necessitate the installation of a scrubber for absorption of the SO2. This scrubber has to be designed for the total flue gas volume from the combustion unit, and will thus be proportionately very large for disposal of a small volume of NCG.
The impact of the NCG disposal on the process in the combustion unit must also be considered. The chemicals added by thermal oxidation of NCG in the lime kiln may adversely affect the causticizing or the liquor composition. The combustion of NCG may also increase corrosion in the recovery furnace or power boiler. Many mills find that operating problems at the lime kiln increase in form of clinker and ring formation when NCG is introduced.
WHAT IS NCG ?
NCG is what remains after the gases, generated during the pulping process, have been cooled and the heavier components have condensed to liquid. The malodorous gases typically found in a kraft pulp mill are:
There are many sources of NCG in the pulp mill. Some sources emit a small gas volume with high concentrations of NCG, while others have large volumes with low concentrations. A typical volume vs. concentration comparison may be as follows:
High-Volume/Low Concentration(HVLC) - Volume =30
Low-Volume/High Concentration (LVHC) - Volume =1
|CH3SH||15,000 - 50,000|
NCG AND TRS SOURCES
The non-condensible gas streams are normally collected in two separate systems and directed to two different disposal units. HVLC gases from brown stock washers etc. are directed to the recovery furnace or to the power boiler, where they replace part of the combustion air. LVHC gases from digesters, blow tanks, evaporators, black liquor storage and oxidation tanks are conveyed to the lime kiln for destruction, where their heating value allows reduction of the normal kiln fuel rate. Type of wood being pulped and type of digester can have radical effect on the amount and concentration of the NCG generated. It will subsequently have an impact on gathering system design, combustor sizing and air/fuel control logic.
A dedicated incinerator represents additional cost to the mill, but allows separation of the NCG treating requirements from the main business of the mill - producing pulp.
In mills where other methods are currently working on part of the NCG streams, an incinerator may serve as the "back-up" device and be pressed into service only when the primary combustion device must be taken out of service. This type of incinerator is subject to very rapid startups and must be designed to take abuse. For instance, hard refractory lining the combustion chamber (brick or castable) is replaced by ceramic fiber blanket products, or else stainless steel construction and external cooling are used to eliminate all refractory requirements. In some mills the use of a simple enclosed flare to handle temporary NCG flow has been very successful. Normally back-up incinerators or flares are not equipped with SO2 scrubbers and the temporary flow of SO2 to atmosphere must be factored into the mills overall emission inventory.
In mills where NCG control has not been implemented previously, or where the use of existing combustion devices has proven troublesome, a dedicated incinerator system may be used as the primary treatment device, leaving the lime kiln, power boiler or stand-by flare to serve as the back-up devices. A full time incinerator can use the more durable hard refractories and is equipped with a scrubber for SO2 removal. It is also usually capable of a greater "up-time" percentage than the traditional combustion devices, since disposing of NCG is its only function.
Incinerator System Components.
A dedicated incinerator must completely destroy the combustibles in the NCG streams in a safe manner (no flashbacks) and thoroughly treat the resulting flue gas to remove SO2 before discharge to atmosphere. For clarity each system can be separated into the following components:
Theoretically the HVLC and LVHC streams are too lean or too rich, respectively, to sustain combustion by themselves (that is, they are collected so that their ratio of combustibles to air is not in the explosive range.) Variations in the NCG sources or leaks in the collection system can change this from time to time, so steps must be taken to prevent static electricity in the ducting or the flame present in the incinerator from causing a flashback or explosion. For this reason collection system design must be handled carefully. Detonation arrestors and high velocity passages built into the incinerator system are normally required.
The burner always contains an ignition device (usually a natural gas or propane fired pilot burner) as well as an auxiliary fuel gun for initial heating of the furnace and maintenance of furnace temperature when the NCG streams are too lean to provide self-sustained combustion. The burner distributes and mixes the NCG streams and any required outside air so that stable combustion is achieved under all NCG flow and composition conditions. Since the burner flame is the primary source of any NOx formed, control of fuel/air mixing may be important in certain areas of the country. The fuel can be natural gas, fuel oil, turpentine or any similar stream. Natural gas is the most common fuel.
Any combustion reaction involves the reaction of oxygen with combustible molecules. The combustion reactions need some time to approach completion - and higher temperatures speed up the reactions, reducing the time requirement. Today the generally accepted furnace conditions are 1600oF with 0.75 second retention time for full TRS combustion. The furnace is simply a chamber in which the reacting gases are held while combustion occurs. Furnace design must accommodate thermal expansion and refractory placement for thermal protection of the structural shell without creating corrosion problems. Refractory life must be long enough to allow maintenance only at the regular mill turnarounds. NCG incinerators typically handle gaseous wastes, but various waste liquids may also be fed.
Flue gas quench.
If the incinerator flue gas must be scrubbed (SO2 levels usually force this) then some method of cooling the gas prior to scrubbing is necessary. A boiler could be used for part of the cooling, but usually water sprays are used to cool the flue gas. The rapid temperature change at this stage can cause corrosive and thermal expansion problems.
Passing the cooled flue gas through a wet scrubber will remove part of the SO2 generated in the furnace. Removal of about 99% of the SO2 is usually required to meet permit requirements and this means the recirculated scrubber water has to be kept alkaline. Most systems inject caustic (NaOH) for this reason. Minerals in the scrubber feed water plus sodium sulfite and sodium bisulfite formed by the NaOH can build up during operation. Part of the scrubber liquid is therefore discharged and replaced by fresh feed water in order to avoid mineral deposits within the scrubber.
The cleaned incinerator exhaust is discharged at a point above adjacent structures, since traces of TRS and SO2 will still be present. In addition the flue gas is usually saturated with water vapor and will form a steam plume in most weather conditions. The actual discharge elevation could also be set by the air permit to achieve dispersion targets. The stack is also where provided with sample ports for use during the trial burn required by the state air board.
The control system is designed primarily for safety. Since the incinerator is a combustion device there is the potential for excessive temperature, loss of combustion (discharge of untreated TRS) and flashback into the NCG ducts under certain conditions. Thus flame scanners and temperature switches are interlocked with the fuel and NCG feed valves. Certain conditions of temperature and oxygen content are needed for minimum TRS destruction and are carefully controlled. Likewise, proper water flow and scrubber pH are needed for good SO2 removal, adding to the system control package. Currently the safety related controls are normally hard-wired in a panel near the burner, while performance controls are routed through the mills Distributed Control System (DCS). Since NCG flow and composition may vary quite a bit, the control hardware and tuning must be pretty nimble in order to maintain the best furnace conditions at all times.
SO3 removal (optional).
As mentioned above, most of the sulfur atoms in the NCG pick up two oxygen atoms, forming SO2. A small fraction, as much as 5-10% - depending on furnace conditions [1,2] of the SO2 molecules further oxidize to SO3. While SO2 is easily removed by packed bed scrubbers, the SO3 tends to form aerosol droplets with water vapor in the flue gas. These droplets are so small they act like particulate and largely are unaffected by packed bed treatment. In flue gas they may be visible as a grey-blue haze leaving the stack. If elimination of this part of the "SOx" produced by combustion is desired, a high efficiency mist eliminator (sulfuric acid plant type "candles"), wet electrostatic precipitator or high energy venturi scrubber is required, adding both cost and energy consumption to the incinerator system.
A number of dedicated NCG incinerators are currently in service. Most systems have some history of problems (and solutions), beginning with the installation of equipment and continuing during day-to-day operation as the hardware ages. Since each mill generates NCG streams with unique properties, depending on wood supply, pulping method and other factors, there is really no such thing as a "standard" NCG incinerator. The user should not expect the same reliability found with, say, a good quality centrifugal pump.
These problems normally consist of misalignment at equipment flanges or anchor bolts, and are solved after a certain amount of grumbling by all parties. Both project cost and schedule may be affected, but in the end a functioning system normally results .
Capacity and control problems.
Variations in the NCG flow and composition are to be expected. Even if the NCG streams are already being collected, careful monitoring of the flows and compositions is necessary in order to gather accurate data for system sizing .
Capacity problems can be dealt with through burner changes, blower modifications, furnace extensions and other measures - all of which are cheaper than putting in a whole new system (as finally was done at the Michigan mill.) Selecting a site for the system occurs early in the project and should be approached with an eye to future capacity increases. For instance, although uncommon, a vertical up-flow furnace allows easy addition of extra volume (furnace length) while minimizing plot space requirements. Blowers can be selected such that the housing will accommodate a larger wheel. Combined with a larger motor, quite a bit of capacity and pressure increase is possible. Fortunately scrubber packings are available in many formats and the ones typically used by equipment vendors can be replaced with higher capacity types (more expensive, though.)
Typically refractory materials are used to line the interior of the incinerator furnace and flame impingement areas of the burner, as well as parts of the flue gas quench system in some designs.
Burner refractory is normally the roughest service and routine replacement should be expected - maybe every year or so. Loss of this refractory can lead to poor flame shape, unstable combustion and damage to the underlying metal of the burner.
Furnace refractory protects the gas tight furnace shell from the high flue gas temperatures. This refractory is usually brick or a hard castable held in place on the shell with specialized stainless steel anchors. To handle quick startups a ceramic fiber type refractory (blanket or board) can be used.
Refractory problems - Flue gas quench system.
Quench refractory must tolerate hot dry conditions (1600oF) where the flue gas enters, and wet cool conditions (180oF) at the outlet end. The key is proper selection, installation and steady operation. Failure of the water supply hardware can result in early damage to the quench refractory, so the use of low flow and low pressure trips is recommended.
Better yet would be a quench device incorporating no refractory at all - some all stainless (alloy 316) designs are available which seem to have good operating histories.
SO2 scrubber problems.
Most SO2 scrubbers use a packed bed for gas / liquid contact. Venturis, spray towers and other types may also be used, although energy requirements (pressure drop) or capital cost may be excessive.
SO3 removal problems.
Despite the best efforts of mill personnel, system designers and equipment suppliers, problems with dedicated NCG incineration and scrubbing systems can occur. Certain problem areas seem to occur from time to time and are worth close examination during the project design phase. Implementation of the Cluster Rules will result in the use of an increased number of these systems, which should lead to improved results as standard methods develop.
Reed, Robert D., Furnace Operations, Gulf Publishing Company, Houston, TX, p. 56.
Ward, JoAnn E. and Andrew P. Ting, "Waste Incineration", Environmental Technology, 1(1):35 (1982).
Banks, Dan, Hazel Ladner, Jim Howe and Carl Connally, "Evolutionary NCG Incineration System Installed at Mead Coated Board, Mahrt, Alabama", TAPPI 1996 Engineering Conference and Trade Fair.
Lloyd, Gary, Arnold Farr and Roger Tembreull, "Design and Installation of a Replacement Thermal Oxidizer for Odor Abatement", TAPPI 1997 International Environmental Conference & Exhibit Proceedings, TAPPI Press, Atlanta, p. 667.
Ohman, Heikki, Jukka Kivivasara, Tom Macnpaa, Reino Lammi and Mika Jarvensivu, "Diagnosis of the Non-Condensible Gas Collection System Operation", TAPPI 1998 International Environmental Conference & Exhibit Proceedings, TAPPI Press, Atlanta, p.137.
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