FEATURE:

Regulations require noncondensible gases to be collected and destroyed as part of an overall pollution prevention strategy, but safety issues remain at forefront

By Travis Allen

NCG Handling, Incineration Concerns Drive Need for Safe System Design

Editor's note: This article is part of a more detailed and extensive discussion of the proper designs for collecting, handling, and incinerating noncondensible gases. The complete article can be found in the Technology and Mill Operations section of www.paperloop.com under Extra Edition.

The collection, transport, and incineration of noncondensible gases (NCGs) is an increasingly important portion of millwide pollution and odor control. Sulfur is released during digesting and evaporation in the form of total reduced sulfur (TRS) compounds. The collection and destruction of NCGs containing TRS will soon be required of all pulp mills.

The TRS compounds most commonly encountered include hydrogen sulfide (H2S), methyl mercaptan (CH3SH), dimethyl sulfide (CH3SCH3), and dimethyl disulfide (CH3SSCH3). These gases are noxious and have very low thresholds of odor detectibility. Pulp mill NCG sources can be divided into four categories, depending on their composition.

Concentrated NCG (or LVHC-low volume, high concentration): Consist of relatively high TRS concentrations and low oxygen contents. The design philosophy with these gases is to eliminate the explosion hazard through oxidant concentration reduction. It is critical to prevent air from entering into the system so that the TRS concentration, and in particular, turpentine vapors do not have sufficient oxygen to allow for ignition. Sources of concentrated NCG include:

   • Turpentine recovery system vent (digester relief condenser, decanter, underflow tank, and turpentine storage tank)
   • Blow heat recovery vent
   • Evaporator hotwell vent
   • Foul condensate storage tanks
   • Continuous digester relief

Dilute NCG (or HVLC-high volume, low concentration): Consist of significantly lower TRS concentrations and oxygen contents approaching that of atmospheric air. The design philosophy with these gases is to eliminate the explosion hazard through combustible concentration reduction. This is accomplished by diluting the sources with sufficient air to ensure that the TRS concentration remains well below its LEL (Lower Explosive Limit). It is also important to limit the amount of air ingress such that additional volumes of air and moisture are not introduced into the system thereby necessitating larger piping and downstream equipment. Sources of DNCG include:

   • Black liquor oxidation vent
   • Blow tanks and accumulators
   • Foam tanks
   • Soap skimming tanks
   • Weak black liquor storage tanks
   • Strong black liquor storage tanks
   • Filtrate tanks
   • Hoods
   • Knotter (screen) hoods
   • Contaminate condensate tanks
   • Air stripping equipment

Chip Bin Gases: Special consideration must be given to the source for its potential for containing significant quantities of turpentine. Overhead Vapors: From a steam stripping system for foul condensate. These vapors are a mixture of methanol, water, and TRS gases. Processes and NCG sources must be carefully examined prior to system design. A clear understanding of the source components is critical in designing a safe system. In concentrated NCGs, the TRS gases typically make up only about 10% of the volume. The largest component is air, which has been depleted of 50% or more of its oxygen through reacting with reducing agents. On the other hand, dilute NCGs are primarily air, with TRS amounts usually less than 0.1%. Combining dilute and concentrated systems can result in an explosive mixture.

FLAMMABILITY. In addition to the high toxicity of these gases, TRS, methanol (CH3OH), and turpentine (a-pinene) are flammable in the presence of sufficient oxygen. If contained in a pipeline or vessel, they can be explosive. Fans have frequently provided the ignition source for NCG system fires, including hot spots on the casing if rubbed by the impeller, a hot impeller shaft due to a bearing failure, or sparks created by foreign material hitting the impeller. Welding torches have ignited many NCG fires as well. Static discharge has also been identified as the ignition source in several NCG explosions. The combustion properties of these components are detailed in Table 1. As shown in Figure 1, the flammability of the NCG components result in a combustible range of NCG vapor from 0.8% through 44.0% by volume. Proper design should be based on the conservative assumption of a flammable mixture when NCG concentration is between 0% and 45%. The mitigation of this exposure must be the top priority in designing NCG collection systems.

FIGURE 1. Flammability limits of NCG components.

There are two recognized techniques used in the mitigation of vapor explosions. This is accomplished through either the prevention or control of deflagrations. A deflagration is defined as a propagation of a combustion zone (rapid burning) at velocity that is less than the speed of sound. A detonation occurs when the burning rate is greater than the speed of sound.

Preventing combustion: The first technique is based on preventing combustion. The two methods of achieving this are:

   • Oxidant concentration reduction- The technique of maintaining the concentration of the oxidant in a closed space below the concentration required for ignition to occur. This is the basic design methodology for a concentrated system.
   • Combustion concentration reduction-The technique of maintaining the concentration of combustible material in a closed space below the lower flammable limit. This is the basic design methodology for a dilute system.

Limiting or preventing damage: The second technique is based on limiting or preventing damage by controlling the deflagration. The four methods used to achieve this are:

   • Deflagration suppression-The technique of detecting and arresting combustion in a confined space while the combustion is still in its incipient stage, thus preventing the development of pressure that could result in an explosion.
   • Deflagration pressure containment-The technique of specifying the design pressure of a vessel and its appurtenances so they are capable of withstanding the maximum pressures resulting from an internal deflagration.
   • Spark extinguishing systems-The technique by which the radiant energy of a spark or an ember is detected and the spark or ember is quenched.
   • Isolation methods-The technique of preventing the propagation of certain stream properties (deflagration, mass flow, ignition capability) from being conveyed past a predefined point. This is typically accomplished through flame-front diversion and venting.

Figure 2 shows a flammability diagram representing a mixture of combustible gas and air at a fixed temperature and pressure. Air is made up of an inert gas (nitrogen), and an oxidant (oxygen). A mixture, whether ignitable or not, of air (79% nitrogen and 21% oxygen) and combustible gas is represented by the line CDFG. Point D is the upper explosive limit (UEL) (approximately 45% for NCG), and Point F is the LEL (approximately 1% for NCG).

Any point within the boundary of ABDEF is in the flammable range and can be ignited. Any point outside of this boundary represents a mixture that cannot be ignited. Point E is the minimum oxygen concentration required to support ignition (limiting oxidant concentration); any mixture containing less oxygen cannot be ignited. This concentration varies for different gases and mixtures. Point H represents an arbitrary mixture of flammable gas well within the flammable range.

Oxidant concentration reduction works by reducing the concentration of oxygen. As the oxygen content is reduced, the nitrogen content increases, in effect, moving point H toward the apex, point E. Once the concentration of oxygen is reduced beyond point E, the mixture cannot be ignited.

Combustible concentration reduction works by reducing the concentration of flammable gas. As the concentration of gas is reduced, point H will eventually drop below the lower flammability line, AFE. Once this happens, the mixture cannot be ignited. Inerting with steam, or another inert agent, combines both of these reduction methods by reducing the oxygen component and the fuel component, thereby moving the point H toward the apex, point E, and below the lower flammability line, AFE.

TURPENTINE REMOVAL. As previously noted, the flame propagation speed for sulfur gases is relatively slow, but the flame propagation speed for turpentine is extremely fast (greater than 500 ft/sec). While NCG systems can be designed to handle the flame propagation speed of TRS, it is not practical to design against the flame propagation speed of turpentine. For this reason, the importance of minimizing the presence of turpentine in the NCG system cannot be over-stressed.

Accordingly, a great deal of emphasis must be put on identifying potential sources of turpentine and then limiting turpentine concentrations within the collection system. Continuous digester chip bins are usually considered to be a dilute source. However, they should be considered as a separate source, due to the high turpentine levels that they contain. The highly flammable concentration of gases that can be expected from the chip bin demands thorough evaluation and consideration. Once these gases have been treated for the turpentine, they can be safely combined with other dilute NCGs.

Chip steaming can drive volatile compounds, such as terpenes, out of the chips. This situation is aggravated when flash steam is used instead of process steam and if the steam breaks through the chips due to low chip bin level (steam blow through). Flash steam is already concentrated with TRS and turpentine. In any case, large quantities of turpentine and TRS vapor may be present in chip bin gas. Because water and turpentine are immiscible, they will decant in the pipeline. Static charges sufficient to cause ignition of the turpentine can be generated by the shearing action of these two liquids entering a fan or cascading down a vertical pipe run.

System design should be based upon the assumption that turpentine will be present in saturation concentrations at the collected temperatures. This may not always be the case, but for a safe design this conservative assumption should be taken.

FIGURE 2. Flammability diagram showing a mixture of combustible gas and air at a fixed temperature and pressure.

SAFETY AND CONTROL SYSTEMS. Safety must be the prime consideration in handling NCG streams. The permanent prevention of human exposure to the immediately dangerous and highly poisonous hydrogen sulfide must be the basic design standard of a NCG collection system. The use of properly designed system interlocks, permissives, and control logic can greatly reduce the possibility of an explosion. It is imperative that the system is provided with monitoring and system interlocks to ensure that the system remains safe during upset conditions, as well as during normal operation.

The process control for a NCG system should be designed to achieve two main objectives. The primary objective of process controls should be to provide automatic isolation of the system during unsafe conditions. This is accomplished at two independent levels: isolation of gases from the point of incineration and isolation from collection at the sources. The second objective is to stabilize the system's flow, temperature, and pressure to reduce the net effect on the boiler/incinerator and also within the NCG collection system. This is usually accomplished by adding ambient air at the fan suction on dilute systems to compensate for low flow conditions. This maintains a fixed minimum volumetric flow to the boiler and thus a known dilution factor for combined sources. It also allows for purging of the boiler area piping.

The fail-safe position of the system should be to have NCG vented to atmosphere, where emission restrictions will allow. Some of the safety interlocks to accomplish this include:

   • Loss of burning permissives at the incineration point.
   • Loss of flame at the incineration point. When a boiler is the incineration point, low steam flow from that boiler should cause collected gases to vent.
   • Low purge steam pressure.
   • Low steam flow to ejector/low speed switch on dilute fans.
   • High temperature in a collection line approaching the incineration point.
   • High and low temperature switch/sensor in the combustion chamber, independent of the operating controls.
   • High pressure at ejector/fan discharge.
   • High and low pressure switches on concentrated systems.
   • High LEL in collection/transport piping.
   • A double block and bleed safety valve arrangement should be utilized on the main NCG supply near the boiler, wherever feasible.

COLLECTION CONSIDERATIONS. The key items which must be considered in any design includes the collection method based on the identification of potential sources, physical layouts, the removal of turpentine prior to collection, the dilution of TRS within the system, drainage and removal of condensates, and finally the disposal method selected.

Although field testing will reveal an expected wide range of the components for various sources, a typical mixture must be assumed for the system design. Fluctuations in operating conditions limit the accuracy of this "assumed" composition. Relying solely on vendor/supplier input does not fully address these concerns. Continuous monitoring through the use of LEL meters provides the only assurance that the NCG is within the design parameters. Monitoring operational conditions does not provide sufficient feedback to determine what the LEL is.

It is imperative that results of upset conditions are addressed during the design and HAZOP review of the system. During such events, highly combustible concentrations can be present at some sources within the system. The most notorious source is the chip bin where the design must take into account TRS accumulations due to the instabilities of pre-steaming and level control within the bin.

Chip bin: This area warrants special consideration because of the previously mentioned potential of having turpentine in the gas stream. Maintaining an accurate chip level is very important in the operation of the chip bin NCG collection system. Experience has shown that chip bin upsets can result in highly combustible mixtures in the collection header. Such upset conditions may not be picked up by process instrument readings like temperature downstream of conditioning systems such as gas coolers. Alarms and interlocks at this point are too slow and are not sufficiently sensitive to chip bin upsets that cause high LEL values. Due to this slow response, monitoring must be provided at the chip bin itself. This monitoring should include chip bin level and temperature interlocks and continuous LEL monitoring.

It is not desirable to rely solely on the LEL meter to determine if a potential TRS surge is present. The three-pronged approach of temperature, chip bin level, and LEL monitoring should be provided. Such monitoring may be interlocked to alarm at one level and divert to vent at another level. As the chip bin is the main source of turpentine in dilute systems, the LEL meter could be interlocked to divert the NCGs to atmosphere at a predetermined level such as 30% LEL; while the remainder of the system is left online. At another level, such as 60% LEL, the dilute system is diverted from the boiler to atmosphere and the pressure control valve at the fan suction is interlocked to close.

Proper conditioning of the chip bin gases includes cooling, condensing, and scrubbing to remove as much turpentine as possible. The coldest water available should be used in the cooler/scrubber. Even at temperatures as low as 87ºF, and atmospheric pressure, the vapor pressure of the a-pinene fraction of turpentine is high enough to create an explosive mixture. Once the chip bin gas has been cooled, scrubbed, and diluted, it can be treated as any other dilute gas.

Sealed chip bins require vacuum and over-pressure protection. This is best provided by a mechanical swinggate breaker design.

Heavy black liquor tank collection: A second source of concentrated vapors is in the tank headspace. In particular, the heavy black liquor tanks. During an extended shutdown, a buildup of combustible gases can occur. The use of a combustibility meter in systems containing sources of this type is highly recommended. Additional precautions that should be followed include:

   • Venting tanks to atmosphere during startup period or reducing collection to limit LEL spikes to acceptable levels
   • Continuously purge tanks to dilute system during shutdown period using either air or steam.

For gases to be adequately collected in the gaseous phase, they must be free of the following:

   • Fiber carryover: Needs to be eliminated to remove the possibility of restricting the NCG flow and to remove the possibility of providing an additional fuel source should combustion occur with significant fiber accumulation in the piping.
   • Excessive entrained moisture: Must be removed to reduce the potential for erosion of dilute fans and to minimize the need for condensate collection tanks. In addition, concentrated and dilute gases from individual sources must be fully condensed with respect to water vapor. This is particularly important on the NCG stream from the blow heat recovery system equipment.
   • Excessive flow variation: Particularly in concentrated systems makes stable operation more difficult since these flow variations could cause low flow conditions which could result in the potential of flashbacks in the system's transmission lines at the point of incineration of NCGs. Typically, the minimum flow is met with steam addition. Air should not be used to meet this requirement.
   • Turpentine carryover: Can be minimized by collecting the gases at lower temperatures and by adequately sloping and draining the piping system to avoid any turpentine buildup at low points.

The overhead vapors from steam stripping systems should typically be kept separate from other NCG streams since both types vent under pressure and both types contain a mixture of TRS, methanol, and water. The potential for condensing a portion of the methanol and water when mixing with NCG exists in both cases. However, the benefit of the steam ejector's elevated temperature help to alleviate condensation. Proper design could allow for a combined system downstream of the ejector. Control of the stripper system is required to ensure that the stripper gases are consistently supplied to the incineration location with regard to flow, pressure, and temperature of the gas.

TRANSPORT. There are many design considerations in the safe handling of NCG streams. If all of these are addressed properly, the NCG system will present no major operating problems. Basic design considerations include:

   • Since these gases are corrosive, equipment and piping are normally stainless steel, with TFE-based gasketing. Leaks must be avoided, since these gases are toxic and potentially explosive.
   • Concentrated and dilute gas streams should not be intermixed. The Dilute gases could dilute the concentrated gases into the explosive range where they become more dangerous to handle. Common foul condensate drains should also be avoided.
   • Concentration levels for dilute systems should be maintained below 25% of LEL for the combined system. Where continuous monitoring of the LEL is provided along with the recommended interlocks, the concentration may approach, but not exceed 60% of the LEL before venting.
   • Sufficient piping velocities should be maintained to ensure that gases are moving above the flame propagation speed of the TRS components. Typically, an adequate safety factor is provided when these velocities are maintained in the range of 50 to 100 ft./sec. It is possible to design an NCG system with piping velocities above the flame propagation speed of TRS compounds, but not above the flame propagation speed of turpentine (500+ ft./sec.). As such, every effort must be made to prevent any accumulation of condensate and/or turpentine throughout the system.
   • Piping systems must be designed so that condensate is not allowed to collect in the piping. If allowed to collect in the piping, two problems can occur: flow can become restricted due to a condensate "plug," and turpentine can collect on the surfaces of any accumulated condensate. All piping low points, even small piping connections at the bottom of a pipe, must be drained into a sealed condensate collection tank.
   • To prevent excessive condensate formation, collection and transmission piping should be insulated. Systems utilizing steam ejector lines should be insulated from the steam ejector to the incineration point. For stripper gases it is necessary to insulate the entire line from the stripper to the incineration point. In particularly cold climates, heat tracing of these sections of lines may be required.
   • Flow of the stripped overheads must be stable and consistent and of a methanol concentration to support combustion to be adequately burned. These gases from the stripper system should be considered as a separate fuel source. Safety considerations dictate that fuel sources be steady and consistent to any fired location.
   • The entire system must be effectively grounded.
   • A means to purge all the lines must be provided. Concentrated systems usually have automatic steam purging of gas lines prior to startup and after shutdown to remove any stagnant pockets of air or gas. Dilute systems may use a combination of air-purge and steam-purge.
   • Collection lines between the digester chip bin and fiber scrubbers should be provided with clean-outs. Low-flow alarms should be used to help identify possible pluggage in transport lines or equipment.

SWEEP VS. SEALED SYSTEM. System capacity not only impacts installation cost but can also reduce the options for incineration available at any given mill site. It is therefore important to limit design flows where possible by minimizing tramp air pulled through the system. Any atmospheric openings on sources, such as goosenecks, overflows, doors, pressure/vacuum breakers, etc., represent air entry points that add to total system volume when collected. A certain amount of leakage cannot be avoided. The design basis should take the conservative approach and assume a maximum fill rate and peak temperatures at all sources.

EJECTORS/FANS. A desirable feature for collection of sources containing turpentine is the use of steam ejectors for motivation. Steam ejectors have now been established as the standard for use in concentrated systems and their positive features of non-sparking, oxidant concentration reduction, combustion concentration reduction, and flame arresting also apply to dilute system design. Concentrated NCGs must not be diluted, and every effort must be made to design and install a system that prevents air from being inadvertently pulled into the concentrate lines.

Since the vacuum that an ejector pulls varies inversely with gas flow, it is possible to pull a high vacuum under low flow conditions. This may cause a vacuum breaker to open, allowing air into the system. To overcome this problem, a pressure or flow controller is used on the ejector suction. It is possible to size the ejector and downstream piping such that the velocity is greater than the flame propagation speed of TRS gases. However, it is not possible to design against the flame propagation velocity of turpentine. Steam has long been used as an inerting agent (preventing a combustible mixture from igniting) and a method of fire suppression. Steam-smothering systems have been used to protect evaporators, pulp dryers, and thermal oil heaters. The ejector steam provides adequate inerting when the application rate is maintained above 2.5 lb/min/100 ft3.

Based on the volume of gases to be collected, either fans or ejectors may be used to collect and transport the NCGs. Fans are typically used for motivating dilute NCG streams due to the large flows required. These fans must be of spark-proof design and heavy-walled construction. Tapered lock bearings should be specified to reduce the possibility of the fan separating from the shaft. Direct drive centrifugal fans are preferred. While variable speed fans are often specified to allow for greater flow flexibility, dilute systems are typically run at full capacity, negating this benefit. In addition, variable speed fans require greater care and maintenance to ensure proper alignment. Depending on the number and location of sources, individual source fans can be used to ensure that the source lines remain adequately diluted. Dilute systems utilizing multiple fans should be interlocked such that the loss of one fan does not result in reverse flow due to backpressure.

GAS SCRUBBERS AND GAS COOLERS. A gas scrubber is used to prevent introduction of fiber into the system. A gas cooler is often provided to remove excessive moisture and reduce the volume of gases in a dilute system, and a white liquor scrubber is used to remove a portion of the TRS from the NCG stream. The condensate collected from an indirect cooler will be relatively small in volume.

This condensate is typically added to other sources of foul condensate for further treatment. Gas coolers can reduce transmission line sizing resulting in material and installation cost savings. White liquor scrubbers are particularly effective in removing hydrogen sulfide and methyl mercaptan. Direct contact coolers have the benefit of providing an immediate heat sink upon steam breakthrough. For southern mills with relatively warm water, they provide a much lower exit gas temperature, thus reducing turpentine concentrations. Cooling water flow and outlet gas temperature monitoring should be provided.

FLAME ARRESTERS. A properly-designed dilute system will have combustible concentrations several magnitudes less than the LEL level. Installation of flame arresters for these systems is generally not justified given the extremely low fire potential. Flame arresters are designed, tested and approved as end of vent line arresters and are typically only approved for use with Group D gases (as defined by the National Electric Code).

Acetylene is considered a Group D gas; Hydrogen and Ethylene Oxide are Group B gases. Of interest to the paper industry, H2S and mercaptans would be considered Group C gases, while the majority of other common NCG components, including methanol and turpentine, are considered Group D gases. Mixed NCG gases should be treated as Group C for design purposes.

Further, caution must be exercised when using an end-of-line arrester as an inline arrester and vice versa. In certain confined geometries, such as pipes, where the length exceeds the diameter by a factor of ten or more, a conventional combustion reaction can self-accelerate to the point where a transition from deflagration to detonation occurs if the mixture is in the detonable range of compositions. This transition is a function of the stoichiometric and burning ratios of the gases, the pressure, temperature, and sources of ignition. Such a detonation will blow past a flame arrester. A misapplied flame arrester can be rendered totally ineffective.

There is an unfortunate tendency to assume that simply because a flame arrester is listed by UL or is approved by Factory Mutual Research Corp., it is suitable for any flame arresting application. In many instances, the arrester is intended to be a last defense against flame propagation should all other safety mechanisms fail. A false sense of security in an inappropriately applied arrester is a very dangerous condition.

If a flammable mixture and possible burn back is anticipated, that condition must be corrected, as opposed to relying on the flame arrester to save the day. Preventing the accumulation of a combustible mixture should be the first approach. If this cannot be achieved, diversion of the gases is needed. Flame arresters are not in themselves sufficient to limit or prevent damage.

Flame arresters at sources such as condensate and storage tanks can be effective in isolating these sources in an incident. The use of direct contact gas coolers and steam ejectors provides protection from flameback at other sources such as steamed chip bins. One of the main locations where flame arresters should be considered is upstream of the final incineration point. To be effective, the flame arrester must be located per the manufacturer's guidelines, typically within 10 pipe diameters of the incineration point. If the system is integrated into the forced draft air system of a boiler, the flame arrester will be of limited use. Any fires occurring in the system would take place at the entry of the combustion chamber, which is much too far downstream for a flame arrester to be of assistance.

Flame arresters should be installed at both ends of the system for both concentrated and stripper gases. These devices should be installed on each source and on the piping just prior to the incineration points. Particular care should be made to chose and install flame arresters so that condensate cannot collect in the flame arrester.

Flame arresters are normally installed with pressure differential gauges to check on the cleanliness of the elements. Cleaning is recommended at pressure drop measurements from 3- to 5-in. H2O.

Flame arresters should be designed with large enough air passages to provide almost no restriction to flow and to prevent pluggage. Careful attention must be given to ensure that any low points created by the installed flame arrester are sufficiently drained.

PRESSURE/VACUUM BREAKERS. Pressure/vacuum breakers protect sources from over-pressurization or excessive vacuum while eliminating or limiting air entry into the NCG system. Careful consideration of existing tanks is needed, especially for Dilute NCG sources.

The key problem is the ability of the vessels to withstand pressure and vacuum while in operation. This is a significant concern for mills where tanks were originally designed for atmosphere conditions only. It should be noted that even for an open or "sweep" collection system some vacuum would be imposed on the source. Careful consideration must be given to tank integrity and normal and upset pressure/vacuum design. If a tank cannot resist any vacuum, the dilute NCG system will need to be larger to accommodate the additional sweep air that is drawn through the tank itself, instead of past the tank. Air sweeping through liquor storage tanks will cause evaporation and liquor cooling, resulting in tank pressures changing. Depending on the response of the NCG system to pressure variations, there is the potential for localized venting of tanks.

   • Pressure/vacuum breakers protect sources from over-pressurization or excessive vacuum while limiting air entry into the NCG system. Typical vacuum settings are 3- to 6-in. H2O for concentrated NCG systems and 1- to 2-in. H2O for dilute NCG systems.
   • An isolation valve at a sealed source makes the use of a pressure/vacuum relief valve mandatory. Pressure relief devices are also important at the individual sources and near the incineration points. High pressure means that either a restriction has occurred in the piping or combustion is occurring. Devices such as pressure sensors can be effective monitors with the use of rupture discs as pressure relief safety devices.
   • Rupture discs are provided to protect the NCG piping during rapid over-pressurization. In concentrated systems, rupture discs should generally be provided at bends in the pipe. The additional criterion that must be addressed in both concentrated and dilute systems is ensuring that the discharge vents to a safe location. Position, orientation, quantity, and size of rupture discs required in dilute systems, to effectively vent deflagrations, makes them impractical and unreliable for this purpose. They should be provided wherever over-pressurization of the system may occur due to a shut valve or plugged line. The use of rupture discs in dilute system design follows the same arguments as for flame arresters-i.e., a properly designed system would not need rupture discs except where process over-pressurization may occur. Shutoff valves should not be installed on the line feeding the rupture disc. The preferred method is to use three-way valves with two rupture discs to allow for continued safe operation while changing out of a failed disc.

ENTRAINMENT SEPARATORS. Moisture separators are provided to remove large droplets and mists from the NCG stream. Entrainment separation equipment is important to prevent entrained moisture particles from blocking flame-scanning equipment within the incineration locations. They can also be excellent locations for a low point drain within the piping. Trapped drainpipes must be designed to allow for free flow as well as possible pressure buildup during upset. Entrainment separators can also prevent erosion damage particularly when installed on the suction side of the fan for the Dilute system.

Many of the flashbacks that are attributed to NCG systems may actually be fine aerosols of turpentine that ignite in the line just prior to the incineration point. The use of an entrainment separator can be particularly effective in preventing these aerosols of turpentine from reaching the incineration point.

COMBUSTIBILITY (LEL) METER. One of the main problems with combustibility measurement within NCG systems is the number of components present and their variable concentrations. Many combustibility meters are calibrated for a single component only and are not effective in NCG service. Hand-held LEL meters are calibrated against gases (typically methane) which have a different response than TRS gases. H2S may have a response rate that is only 25% that of the calibration gas, so that a 25% reading is actually at the 100% LEL value.

The FTD (Flame Temperature Detector) is one device that has successfully overcome this problem. This device calculates the % LEL based on the temperature rise of a NCG sample as compared with hydrogen. Sample lines must be heated and filtered. Continuous LEL monitoring at the chip bin and prior to the point of incineration should be provided.

Hydrogen gas used for calibration must be handled and stored in accordance with Factory Mutual's Standard for Compressed Gases and NFPA's (National Fire Protection Association) Standard for Gaseous Hydrogen Systems at Consumer Sites. Basically, storing and utilizing hydrogen must to be done in an area adequately separated from other occupancies so that it does not expose personnel. The order of preference for the location of storage is 1) outdoors, 2) in a separate building, or 3) in a special room. Separation distances, piping, and storage container requirements are outlined in the two documents mentioned above.

CONDENSATE TANKS. Dilute and concentrated drain lines should be kept separate. Combining dilute drains into a single loop with one tie-in to the common drain line will limit potential for interaction between the two systems. This will also improve isolation and maintenance capabilities.

SEALS. The collection systems will utilize seal loops to prevent air from entering the system or to prevent foul gases from entering sewer systems. Water seals for tanks must be reviewed for their location, freezing potential, provision of continuous water makeup, and provision for clean out.

Elevated seal loops should not be used since siphoning may empty them. Mechanical type seals may be a good alternative. The results of over pressurization and loss of the water seal must be evaluated, since the seals will likely fail prior to rupture disc failure.

INCINERATION OF NCGS. In order for NCGs to be properly destroyed by combustion, four conditions must be met. These are:

   • Temperature of 1400 degrees F
   • Residence time of 0.5 sec.
   • Excess oxygen content of 3% to 4%
   • Effective mixing.

These should be considered as basic conditions; if any of them are exceeded, the others can be reduced.

The three typical locations to incinerate NCGs are lime kilns, power boilers, and dedicated incinerators. All three methods of incineration require a stable operating condition prior to introduction of NCGs. Such a condition is typically determined by operating time, temperature, or steam flow.

Lime kiln: Lime kilns have several advantages for the incineration of NCGs-high temperature and long residence time, efficient use of the heat of combustion, and absorption of most of the SO2 formed during incineration. To maximize the effectiveness of lime kiln incineration and reduce the possibility of increased TRS emissions due to incomplete conversion of TRS into SO2, exit gases should contain at least 2% oxygen. A dedicated NCG nozzle mounted on the kiln hood should be used to introduce NCG gas streams into the kiln.

The noncondensible gases absorb light in the ultraviolet (UV) range, and consequently can give a false "flame out" signal if they pass in front of a UV flame detector. Therefore, the NCG nozzle should be placed well away from the flame detectors, typically above the burner. Disadvantages of using the kiln include:

The kiln may be operating close to overload condition
   • There may be increased formation of stones and rings due to excessive formation of sodium sulfate
   • There may be limited absorption capacity for SO2 in the lime in the kiln, especially when electrostatic precipitator particle separators are used (white liquor scrubbers can be effectively used to reduce sulfur under these conditions)
   • It may have a very small capacity for incineration of dilute NCG
   • Burner front space may not be available.

Power boilers: The incineration of NCG in a boiler critical to operations must be carefully considered. Incineration should only be allowed during stable operating periods, and interlocks should be provided to ensure this condition. Automatic closing emergency shutoff valves (or dampers) in the NCG piping are needed. Steam or air purging of this isolated line is needed.

Incineration of NCG that is 25% or less of the LEL and high in oxygen content, such as in dilute systems, may be accomplished by adding the stream to the secondary combustion air supply upstream of the air port plenum. Incineration of NCG that is above the upper explosive limit (UEL), which may occur in concentrated systems, can be accomplished through a separate, dedicated waste gas burner. This burner should have dedicated combustion air and be provided with flame monitoring. SO2 emissions are a limitation at many sites, such that dilute gases could only be considered for incineration.

Stand-alone incinerators: These have several advantages. Typically, they can be operated independently of the process. The resulting reduction in lost time due to maintenance and troubleshooting of kilns and boilers may pay for the installation. SO2 scrubbing is required, and caustic demand can be very high for this type of incineration.

SYSTEM MANAGEMENT. The NCG system runs through many critical areas of the mill-digesting, bleach plant, powerhouse, and kilns-the main exceptions being manufacturing and storage areas. The design of a safe and efficient NCG collection system will involve each of these areas. Several vendors may be working on the system design and the coordination between these vendors is critical to the success of the system. It is the project team's responsibility during design to ensure that all the vendors are working together as a team.

It is imperative that reliable methods are put into place to ensure that maintenance is being performed and that the equipment and design changes are being tracked for reliability. Future additions and changes to the system will continue to occur. These must be closely tracked to ensure that the original design parameters are not breached.

The mill is the final owner of this system and instituting a system to track the safety and reliability of the system is it's responsibility, from design through operation.

TRAVIS ALLEN is fire protection engineer, Weyerhaeuser Co., Federal Way, Wash.