Selection of Thermal Oxidisers R. Jones, Gasco Pty Ltd SELECTION OF THERMAL OXIDISERS RICHARD JONES SALES AND MARKETING MANAGER GASCO PTY LTD 8/981 MOUNTAIN HWY, BORONIA 3155 VICTORIA, AUSTRALIA Phone: +61 3 9720 8577 Fax: +61 3 9720 4240 Email r.jones gasco.net.au ABSTRACT Gaseous and liquid waste streams are often incinerated to destroy VOC's, hydrocarbons and other compounds (especially sulphur bearing), to eliminate odour and sometimes to remove visible smoke plumes. Incinerators for the destruction of gaseous and liquid streams are generally referred to as Thermal Oxidisers and while the principal of operation is simple (generally the oxidation of various compounds to carbon dioxide and water vapour) special attention must be paid to the full chemical composition, the potential for particulates and the operating duty before deciding on the most appropriate form of Thermal Oxidiser (TO). The classification of waste streams is discussed and the application and advantages of simple Afterburners, Recuperative TO's, Catalytic Thermal Oxidisers (CTO's), Regenerative Thermal Oxidisers (RTO's) and Regenerative Catalytic Oxidisers (RCO's) is reviewed.
Selection of Thermal Oxidisers R. Jones Gasco Pty Ltd CONTENTS 1.0 WASTE STREAM CLASSIFICATION 2.0 HEAT RECOVERY SYSTEMS 3.0 CATALYTIC THERMAL OXIDISERS (CTO) 4.0 REGENERATIVE THERMAL OXIDISER (RTO) 5.0 REGENERATIVE CATALYTIC OXIDISER (RCO) SUMMARY
Selection of Thermal Oxidisers R. Jones Gasco Pty Ltd INTRODUCTION In evaluating the options for Thermal Oxidisers in the destruction of HC's and odour, it is often the case that the destruction of additional compounds within the waste stream is either desirable or essential. Hydrocarbons, VOC's, H 2 S and associated sulphur compounds, CHC's, CI, etc may be present individually or collectively. We have seen many cases where customers have focussed only on capital cost, energy cost and destruction efficiency. Such a simplistic approach has resulted in the selection and operation of highly thermal efficient options only to have them removed because of poisoning or persistent blockages, over temperature excursions, periodic emissions exceeding odour and VOC limits, high maintenance and high downtime. "Low running cost solutions" have sometimes had to be replaced by simpler but less efficient alternatives. With the most appropriate Thermal Oxidation solution, highly effective destruction of HC's and/or reduction in odour will be assured. A classification of the different types of waste stream is essential and their implications to the equipment design operation and destruction efficiency considered before the secondary issues of capital cost and operating cost are reviewed.
Selection of Thermal Oxidisers R. Jones Gasco Pty Ltd 1.0 WASTE STREAM CLASSIFICATION Thermal oxidation technology is highly effective in the destruction of odour in waste streams arising from a multitude of sources and processes. The selection of the most appropriate type of TO however is not always straightforward and requires the consideration of many factors. The composition of waste gas stream can have a very significant affect on the final design and as a consequence many TO systems are custom designed for a given application. 1.1 Waste Stream Flowrate Variations Often TO vendors are presented with a waste stream specification with a single flow rate and single composition. Very few processes produce waste streams that do not have some variability. TO's must be designed for extremes as well as for the average or 'normal' operation. All designs and types of TO's will have turn down limitations for waste stream flow, ranging from 10:1 or more for simple TO to 2 to 3:1 for TO with heat recovery. Start up may require much higher support fuel because VOC s in waste stream have not been established. 1.2 VOC/HC Concentration(LEL) Variations Spikes in VOC's/HC's concentrations in waste stream may elevate chamber temperatures and shorten life of refractories, heat exchanger bundles, catalysts and regenerative beds. In some cases these can be catastrophic within very short time frames if other control strategies have not been anticipated and allowance mode for materials compatibility, bypass, dilution, process interlock or other methods of accommodation. If the aggregate of the flammable constituents of a waste gas exceeds 25% of the Lower Explosive Limit (LEL) in the air stream, then special design and approval issues are required. Generally approval to operate gas fired appliances requires that a waste gas stream be always less than 25% of LEL in air. This approval is not limited to the vendor's equipment design, but must take into account the client's process upstream of the TO and it often becomes the client's ultimate responsibility to demonstrate that the process is intrinsically safe, ie that the LEL cannot exceed 25% of LEL through a HAZOP analysis. The authorities under special circumstances, may issue a dispensation to operate with higher levels of LEL. If it is considered that >25% of LEL could arise, then it is normal practice to install detonation and flame arrestors, however authorities usually consider these as last resorts and will often insist on further control strategies including continuous LEL monitoring (with multiple instruments) and control using air dilution.
Selection of Thermal Oxidisers R. Jones Gasco Pty Ltd 1.3 Minor Contaminants Major Problems The presence of unanticipated components of a waste stream can create major problems in TOs. It is therefore very important to identify and anticipate as far as is practical, all the constituents likely to be present and to quantify their concentration. 1.3.1 Inorganic Solid Particulates Inorganic particulates, usually in the form of Si02, or metal oxides can cause issues with RTO's, CTO's and CRO's through plugging of heat exchanger media, catalyst beds and valves leading to increasing pressure drop and increased maintenance. This can occur quite quickly. The TO with shell and tube recuperator may also be affected through some build up on heat exchanger tubing, however the affect is limited and periodic cleaning without total disassembly is practical. RTO's can accommodate moderate particulates level if anticipated and design of beds adjusted. 1.3.2 Salts Salts in the waste stream can have a particularly harmful affect if not anticipated. Many salts will melt at combustion chamber temperatures and most will melt at flame envelope temperatures. If several types of salt are present then a low melting point eutectic can form. These salts can attack and flux refractories causing shortened life and they can attack heat exchange media and can adhere to and corrode heat exchanger tube bundles. These comments apply to all forms of TO. Special designs to accommodate salts and to mitigate heat exchanger attack and build up are available. Ensure the equipment supplier is informed of salts potential. 1.3.3 Silicones and Siloxanes These burn to form a Si02 ash and even at relatively low concentrations can cause blockages in RTO's and CTO's if not anticipated. Designs will permit RTO's to accommodate these up to appoint. CTO's however are not likely to have a useful working life. 1.3.4 Heavy Metals Heavy metals can 'poison' CTO's catalyst beds and render them ineffective. This can occur over hours to years, dependant upon concentration. They can also form complex salts with silicon and other refractory components and cause fluxing. Heavy metals present in 'trace' quantities however are often accommodated in standard TO designs.
Selection of Thermal Oxidisers R. Jones Gasco Pty Ltd 1.3.5 Suspended or Condensable Organics, Tars, Oils and Fats Tars, oils and fats, if suspended in air stream, can cause major problems through the build-up of carbonaceous layers in heat exchangers. Industry examples include rendering works and coffee roasting. Unless anticipated this can be a particular issue with RTO's where, because of the temperature profile between the bed entry and exits, there will also be a range of temperatures where these materials are not burnt, but baked onto the media, causing progressive build-up and pressure drop, and when the RTO cycle is reversed a proportion will be partially burnt, but because the exit gas is not hot enough (because of the excellent thermal efficiency), these tars and fats will produce a smoky, odorous exhaust and render the solution ineffective. Burnout cycles and preheat of waste stream will reduce or remove the problem with RTO s; however these functions should be included at beginning, not retrofitted. CTO's will also suffer from fouling of catalyst beds. TO's (Afterburners) need not be affected and if used with recuperators, an equilibrium condition on the heat exchanger is usually reached with any smoke swept into the combustion chamber and destroyed. The TO however requires much more fuel than say an RTO. Sometimes fire suppression in the ducting leading to the oxidiser may be required. 1.3.6 Halogenated Compounds Of the halogens, the most commonly encountered is chlorine as chlorinated hydrocarbons, or from the incineration of some plastics, and less occasionally fluorine (often in alumina refineries.) Higher temperatures are required to destroy these compounds compared to VOC's and this, together with the acid gas created, means that subsequent scrubbing is likely required. The higher the temperature the greater the tendency to form HCI as opposed to Cl. Materials selection for refractories, steel shells and heat recovery equipment become critical in the presence of CI and other halogens. Equipment may be quickly rendered inoperative if halogens are not anticipated. 1.4 Classification of Waste Streams Most gaseous waste streams can be grouped into one of three main classifications: 1. Contaminated air streams 2. Contaminated inert gas streams 3. Rich gas streams The process design strategy for each gaseous waste classification is as follows:
Selection of Thermal Oxidisers R. Jones Gasco Pty Ltd 1.4.1 Contaminated Air Streams The first category of gaseous waste streams is an air stream contaminated with low levels of combustible compounds. Smoke and odours are typical examples of this type of waste gas stream. These gas streams have a minimum of 18% (volume) oxygen (02); have no possibility of an explosion hazard the combustible compound concentration (loading) is well below the lower flammable limit (less than 25% of the LEL). Gases with lower oxygen levels, higher organic compound concentrations and/or corrosive compounds and particulates fall into the other two (2) classifications. Examples of industries and processes which generate contaminated air streams are: Coating operations Pulp and paper Printing lines Computer chip manufacturing (semi Paint booths conductor) Converting Drying Textile finishing Ventilation hoods Solvent cleaning Certain chemical processes Packaging Rendering plants Food processing and baking Sewage treatment plants (a) Contaminated Air Streams In Line Burner System The initial process design uses the contaminated air stream as the combustion air source for the burner. For applications with low to moderate flow rates no corrosive compounds present; the gas stream is clean with few particulates; and with a gaseous fuel such as natural gas, propane or liquefied petroleum gas (LPG), the most economical (equipment cost) design option uses an induct burner (also known as a raw gas burner, duct burner or in-line burner). The contaminated air stream is used as the combustion air source. It passes through the burner mixing plates and is thoroughly mixed with the fuel and ignited (See Figure 1). FIGURE 1: Contaminated Air Stream In-line Burner
Selection of Thermal Oxidisers R. Jones Gasco Pty Ltd The resulting products of combustion are raised to the required operating temperature and held at that temperature for a sufficient amount of time. The burner design and a 'profile plate' create the necessary turbulence. This design is called 'direct flame incineration'. The contaminants pass directly through a flame front. It is a highly effective form of destruction for volatile organic compounds (VOC's) and their associated odour. It is also thermally efficient in that no outside air is introduced into the system. The typical requirements and features for this type of process (system) include: The contaminated air stream has very low or zero sulphur compounds, halogenated compounds, and/or particulates. The contaminated concentration is less than 25% of the LEL. The oxygen concentration is greater than or equal to 18% (volume) at ambient conditions. Gaseous fuels only (natural gas, propane, LPG, etc). The pressure of the contaminated air stream at the burner must be between 761 and 766 mm Hg (0.5 and 3 inches w.c.). A blower can provide the motive force. The maximum inlet gas temperature is 540 C. This temperature is limited by the burner materials of construction. The maximum operating temperature (POC outlet temperature) is 930 C. This temperature is also limited by the burner materials of construction. The maximum fuel turndown is 20 to 1. The maximum contaminated air flow turndown is 3 to 1. (b) Contaminated Air Stream Nozzle Mix Burner When liquid fuels (fuel oil, Bunker C, kerosene, waste liquids, etc) or dual fuels are used, or when the oxygen content of the contaminated air stream is between 16% and 18% (volume) at ambient conditions, a nozzle mix burner must be used in lieu of the duct burner. Depending upon the contaminated air flow rate and the burner design, all or part of the air stream is used as combustion air and passes through the burner. In most instances, if the entire contaminated air stream is introduced through the burner, low flame temperatures, unstable combustion, high carbon monoxide (CO) emissions high levels of products of incomplete combustion and nuisance shut downs would occur. For most nozzle mix burner applications, the air stream should be 'split' into a primary air stream and a secondary air stream. The primary air stream is introduced into the burner as the combustion air. The secondary air stream enters the system downstream of the burner and acts as quench air or dilution air (See Figure 2).
Selection of Thermal Oxidisers R. Jones Gasco Pty Ltd FIGURE 2: Contaminated Air Stream Nozzle Mix Burner The efficiency is equal to that of the induct burner system discussed above. No outside air is added to the system. However, only a portion of the contaminated air stream, the primary air stream, passes through the flame front (direct flame incineration). The secondary air stream does not pass through the flame front. It must be properly mixed with the burner products of combustion. Suitable turbulence is necessary to oxidise the contaminants in the secondary air stream. Achieving proper mixing and turbulence are not as easy as they seem. Injecting or adding the secondary air stream into the burner flame can quench the flame too quickly. This can lead to high CO emissions, high levels of products of incomplete combustion, low destruction efficiencies and unstable combustion. A number of designs and configurations are acceptable for introducing secondary air into the system. One of the best is to 'jet' the secondary air into the burner products of combustion well downstream of the flame root. Multiple jets or nozzles with sufficient velocity to penetrate the hot burner POC's is the preferred approach. The process requirements for this system are as follows: The contaminated air stream has very low or zero sulphur compounds, halogenated compounds, and/or particulates. The contaminated concentration is less than 25% of the LEL. The oxygen concentration is greater than or equal to 18% (volume) at ambient conditions. Gaseous fuels only (natural gas, propane, LPG, etc) The pressure of the contaminated air stream at the burner must be between 761 and 766 mm Hg (0.5 and 3 inches w.c.). A blower can provide the motive force. The maximum inlet gas temperature is 540 C. This temperature is limited by the burner materials of construction. The maximum operating temperature (POC outlet temperature) is 930 C. This temperature is also limited by the burner materials of construction. The maximum fuel turndown is 20 to 1. The maximum contaminated air flow turndown is 3 to 1.
Selection of Thermal Oxidisers R. Jones Gasco Pty Ltd (c) Contaminated Air Stream Auxiliary Burner/Dedicated Combustion Air When the contaminated air stream contains particulates, condensable gases, corrosive compounds and/or acids; when little or no air pressure drop can be allowed; or when the air temperature is higher than the burner materials of construction can withstand, a nozzle mix burner is used with ambient (outside) combustion air. The contaminated air is introduced downstream of the burner and mixed with the burner products of combustion. In order to conserve fuel, the burner should be operated with very low excess air or a specially designed sub-stoichiometric air/fuel burner should be supplied. As mentioned above, mixing (turbulence) is the key to maximising the destruction of the contaminants, reducing CO emissions, minimising PICs and maintain reliable operation. The optimum design depends upon the fume composition, fume temperature and turndown requirements. The most economical and straightforward design is to mount the burner radially or at an angle to the contaminated air flow. The burner fires across the contaminated air stream. The air stream passes through the burner flame. (See Figure 3). FIGURE 3: Contaminated Air Stream Auxiliary Burner With this design, the contaminated airstream can have inlet temperatures up to 1200 C, or higher, and operating temperatures up to 1200 C, or higher. These maximum temperatures are determined by the desired destruction level, materials of construction, upstream process conditions, blower design and/or heat recovery requirements. "Jetting" the contaminated airstream into the burner POC's can result in higher destruction efficiencies. However, the airstream must be under a sufficient positive pressure and free of particulates. Sticky particulates are of most concern. They prevent the "jetting" design from being utilised.
Selection of Thermal Oxidisers R. Jones Gasco Pty Ltd 1.4.2 Contaminated Inert Gas Streams These streams have less than 8% (volume) oxygen with low concentrations of organic compounds. The gas heating value less than 1,180 kj./nm 3. Since oxygen level is below 8%, flash back and explosions are not a concern. The contaminant concentration and the oxygen concentration are not sufficient to develop and sustain a flame front. In addition, the energy release is not large enough for an explosion. Typical examples of industries and processes which generate this type of waste gas stream include: Pulp and paper Inert drying or curing operations Scrubber off-gases Ceramic industry Manmade fibre manufacturing Absorber off-gases Resin manufacturing (interim state) Diesel/engine exhaust Heat treating furnaces Asphalt manufacturing Chemical industry Petrochemical industry The process design for this type of waste gas stream uses a conventional nozzle mix burner firing a liquid or gaseous fuel. Ambient air is used for combustion air. The contaminated inert gas stream is introduced into the combustion chamber downstream of the burner flame root. Depending upon the inert gas characteristics (composition, flow and temperature), the gas can enter the combustion chamber through a single tangential nozzle, a single radial nozzle, a single axial nozzle or through a series of nozzles. Relatively clean gas streams, free of particulates, sticky tars, oil mists and/or smokes can be introduced through multiple nozzles. The inert gas is "jetted" into the burner products of combustion. (See Figure 4). FIGURE 4: Contaminated Inert Gas Stream
Selection of Thermal Oxidisers R. Jones Gasco Pty Ltd The recommended method for introducing a dirty gas with particulates, sticky tars, oil mists and/or smokes is a single nozzle. If the fume contains between 3% and 8% (volume) oxygen, a special sub stoichiometric burner should be utilised. The burner operates with less than the stoichiometric air requirement (fuel rich) to conserve fuel). The oxygen in the inert gas stream provides the balance of the oxygen required for complete combustion. Please note that the ambient air blower must be sized for the maximum burner firing rate and to add enough ambient air to maintain a minimum of 3% (volume, wet) excess oxygen in the POC's for all operating cases. These include the start up, warm up and shut down cases. The process requirements for this system are as follows: The contaminant concentration is low. The heating value of the gas is less than 1,180 kj/nm 3. The oxygen concentration is less than 8% (volume). Liquid and/or gaseous fuels. The pressure of the contaminated inert gas stream at the inlet nozzle must be between 768 and 775mm Hg (4 to 8 inches w.c.). A blower can provide the motive force. The maximum inlet gas temperature is only limited by the inlet nozzle(s) materials of construction. The maximum operating temperature (POC outlet temperature) is only limited by the combustion chamber materials of construction. A maximum operating temperature of 1200 C is practical. The maximum fuel turndown is 20 to 1 for gaseous fuel and 6 to 1 for liquid fuels. The inert gas flow turndown is very high. With certain designs, the inert gas flow can be zero
Selection of Thermal Oxidisers R. Jones Gasco Pty Ltd 1.4.3 Rich Gas Streams These streams have very low oxygen content and a high percentage of combustible compounds and a heating value greater than 1,968 kj/nm 3 (50 Btu/std.cu.ft). Since the oxygen concentration is very low (less than 3% by volume), flash back is not a concern. The oxygen concentration is not sufficient to develop and sustain a flame front. Any energy release prior to the combustion zone is not large enough for an explosion. Typical examples of industries and processes which generate this type of waste gas stream include: Pulp and paper Tank vents Scrubber off-gases CO gas Reactor exhaust gases Process upset gases Resin kettle off-gases Blast furnace gases Heat treating furnaces Stripper off-gases Chemical industry Petrochemical industry Landfill gas Gasco Thermal Oxidiser Treating Rich Gas Stream at Shipping Terminal
Selection of Thermal Oxidisers R. Jones Gasco Pty Ltd The design approach for this type of stream is to use the waste gas as a fuel in a burner. A specially designed Low Calorific Gas burner is used. In certain cases, a dual or multi-fuel burner is used. The waste gas is ducted to the burner and 'fired' through a central nozzle or injector assembly. Combustion air is introduced around the waste nozzle. The burner design, especially the waste injector assembly, is critical to achieving proper and stable combustion and nuisance free operation. The burner design is also crucial to achieving high levels of destruction. In most of the applications involving rich gas streams, the composition and flow rate of the waste gas can change significantly. During process start up, shut down and upset conditions, the waste stream may become 'inert' or the flow may decrease below the design turndown rate. Unstable combustion can occur or the flame can be extinguished (snuffed out). In order to prevent this from occurring, a secondary (or auxiliary) burner is provided (See Figure 5). This burner uses a conventional fuel and fires during all operating conditions. This burner provides a positive, stable and robust ignition source. This burner also maintains the necessary operating temperature when the 'rich' waste stream becomes 'inert', or when the waste flow is shut off or decreases below the design turndown limitations. FIGURE 5: Rich Gas Stream
Selection of Thermal Oxidisers R. Jones Gasco Pty Ltd 2.0 HEAT RECOVERY SYSTEMS In certain waste combustion/waste destruction systems, heat recovery requirements are the primary factors in establishing the overall combustion process configuration. However the user and supplier should never forget that safety and contaminant destruction are the primary objectives for a waste combustion/destruction system. Fuel conservation, although an important economic consideration is a secondary design consideration. For contaminated airstreams, the most common form of heat recovery is preheating the airstream itself. Two types of heat recovery (preheat) methods can be provided. The first type is called recuperative heat recovery. Recuperative heat exchangers are also used with Catalytic Thermal Oxidisers. See Section 3. The second method of preheat utilises re g enerative heat recovery. These systems are called Regenerative Thermal Oxidisers, or RTO's. An RTO uses a ceramic bed to preheat the contaminated airstream. These are discussed further in Section 4. Heat recovery boilers (steam or hot water), process gas heat exchangers, hot oil heaters and/or hot air heaters can be added to a thermal oxidiser treating contaminated airstream. Direct recycle of the POC's back into a process is another form of heat recovery. However, preheating the contaminated airstream in order to minimise fuel consumption is the most common heat recovery option. Gasco Thermal Oxidiser with Heat Recovery to Hot Oil System at Gas Processing Plant
Selection of Thermal Oxidisers R. Jones Gasco Pty Ltd 2.1 RECUPERATIVE SYSTEM CONFIGURATIONS Waste gas preheat is shown in Figure 6. In this case, the waste gas contains sufficient 0 2 to supply 0 2 needed for burner. Typically 50-75% of the heat may be recovered with this configuration. Beyond this the c law of diminishing returns' takes over. FIGURE 6: Waste Gas Preheat Heat Recovery For streams that are 0 2 deficient, the arrangement in Figure 7 may be used. The positions of the heat exchangers can be reversed, however usually the highest flow rate is heated first or the stream requiring the highest preheat. If cracking in the waste stream is an issue, then it would be heated second, achieving lower preheat. FIGURE 7: Waste Gas and Combustion Air Preheat
Selection of Thermal Oxidisers R. Jones Gasco Pty Ltd Gasco Thermal Oxidiser with Preheating of Waste Gas and Combustion Air at LNG Plant
Selection of Thermal Oxidisers R. Jones Gasco Pty Ltd 3.0 CATALYTIC THERMAL OXIDISERS (CTO) In this form of oxidiser, a catalyst is used to accelerate the rate of oxidation reaction whilst remaining unaffected itself. The catalyst enables the destruction of organic compounds at up to 10 times the rate, and at much lower temperatures than a conventional oxidiser. Depending upon the compounds being treated, temperatures of 200-300 C are common. It is the ability of the catalytic oxidisers to achieve a high level of destruction at a significantly lower temperature that reduces the operating cost compared to a conventional oxidiser even operating with heat recovery. The most common arrangement of a catalytic oxidiser is shown in Figure 8. Here a heat exchanger is used to recover heat from the exhaust gas and to preheat the incoming foul air stream. A burner is still required for start up and to maintain the reaction temperature under fluctuating concentrations of VOC's in the foul air system. Often the operation can be self-sustaining, depending upon VOC concentration with the burner remaining on pilot only. FIGURE 8: Catalytic Thermal Oxidiser with Waste Gas Preheat 3.1 CATALYSTS Catalyst materials include platinum, platinum group metals, alloys of these or base metals including oxides of chromium and manganese. Platinum group catalysts are typically used with aromatic compounds, benzene, toluene, xylene, etc; whole oxygenated compounds, alcohols, acetates, ketones often use base metal catalyst.
Selection of Thermal Oxidisers R. Jones Gasco Pty Ltd The catalyst is available in honeycomb form where the substrate, either ceramic or metallic, is covered with the catalyst material. It is also available as coated ceramic beads where a bed is employed. 3.2 POISONING Some inorganic compounds, heavy metals plus sulphur, can 'poison' the catalysts and depending on concentration of these, the poisoning process can occur over hours or years. 3.3 MASKING The accumulation of non-combusted organic material or solid inorganic material accumulating on the catalyst surface will prevent gas reaching the catalyst sites, reducing reactivity, increasing pressure drop and reducing destruction efficiency. 3.4 SINTERING Sintering may occur through spikes of high HC or VOC in the waste stream. Most catalysts cannot tolerate temperatures above 650 C for extended periods. A loss of surface area on activity and destruction efficiency will result. The possibility of spikes must be anticipated so that control strategies are built in the process design. Generally, a catalytic oxidiser will be physically smaller than a conventional TO, however, the cost of precious metal catalysts generally will push the capital cost of a catalytic unit above the conventional unit. 3.5 DEACTIVATION Sometimes provision needs to be made for a gradual decline in the conversion rates of catalytic oxidisers by incorporating room for extra banks of catalyst in the original design. In bed catalysts using coated beads, an extra layer of beads sometimes needs to be retrofitted. Also fritting of beads through thermal cycling leads to a gradual loss in efficiency over time, although in some circumstances this may be beneficial in reducing masking. 3.6 CATALYST REGENERATION Deactivated catalysts may be regenerated by one of three techniques; thermal, physical or chemical cleaning. With thermal, activity is often restored if catalyst is masked by organic compounds. Operating at 50-100 C above the normal operating temperature is often sufficient to restore normal operation through oxidation of deposits before returning to original temperature.
Selection of Thermal Oxidisers R. Jones Gasco Pty Ltd Physical cleaning may involve air blast or vacuuming to remove dust accumulation causing masking and pressure drop. Chemical treatment with acid or alkaline cleaning solutions or combinations may be used to remove accretions of masked material without damaging catalyst or substrate. 3.7 COMPARISON OF CTOs WITH TOs In comparison to TO's, CTO's have lower supplementary fuel costs (sometimes no supplementary fuel is required apart from start up), however other factors which must be considered include: Destruction efficiency versus flow rate Overall capital cost Catalyst life and replacement or reactivation costs Destruction efficiency with time Variability of VOC concentration Presence or potential for catalyst poisons and inhibitors Waste stream temperature and variability 3.8 CONSTRUCTION CTO s, because of the lower operating temperatures and reduced residence time, requirements are physically smaller than a TO with heat recovery. The lower temperatures also generally mean they do not need refractory lining, instead stainless steel chambers, externally lagged with mineral wool and clad with sheet metal is the norm. CTO equipment is also much lighter than alternatives and they are sometimes mounted above other process equipment (ovens, etc). 3.9 CTO DESTRUCTION EFFICIENCIES The destruction efficiency (DRE) for a given VOC is dependant upon flow rate, temperature and catalyst type. Many CTO vendors quote destruction efficiencies between 90 and 95%. If a DRE of say 99% is required, then the size of the catalyst bed must often be doubled to achieve this. For this reason, where very high destruction efficiencies are required, and where a broad spectrum of compounds may be present, a TO is often preferred. Where high flow rates are concerned, the RTO becomes increasingly justified.
Selection of Thermal Oxidisers R. Jones Gasco Pty Ltd 4.0 REGENERATIVE THERMAL OXIDISERS (RTO) RTO's employ the same heat recovery concept as employed by open hearth steel making and glass furnaces over 100 years ago, whereby chambers filled with checkerwork brick were heated by the furnace exhaust and then the flow reversed and the air flow picked up heat from the regenerator chamber before firing took place in the furnace. In the RTO the checkerwork brick is replaced by ceramic saddles or 'honeycomb' ceramic media, known as structured packing. The latter is now used wherever practical because of the combination of higher surface area to volume ratio and low pressure drop. 4.1 MULTIPLE BED RTO's The first RTO's had three regenerator chambers connected to a common combustion chamber, see Figure 9. Standard gas burners are used to preheat the chamber before introduction of the process air and to provide the supplementary energy required to bring the hot air to operating (destruction) temperature. Gasco RTO Treating VOC s from Paint Line Curing Ovens at Car Manufacturer
Selection of Thermal Oxidisers R. Jones Gasco Pty Ltd Figure 9: Multiple Bed RTO In operation, one chamber would be preheating the incoming waste stream, a second chamber would be exhausting and media being heated, and the third chamber which was previously exhausting is purged back into the chamber along with the waste stream that remained within the ducting and plenum at the previous bed cycle. With a three-bed system, the overall destruction efficiency is generally 99%+. To reduce capital costs, RTO's are sometimes supplied with two beds. in this case, without a purge cycle, a small amount of unreacted waste stream is exhausted with each cycle. The spike of VOC's or other compounds in the stack each cycle means that the average destruction efficiency over time is usually about 98% maximum. This 1% difference in destruction efficiency could make the difference between Ground Level Concentration compliance or failure if not anticipated. If, however, a site has multiple sources of emissions and the oxidation and disposal via stack is but one of a series of measures to reduce emissions, it may be that 98% DRE is more than adequate. 4.2 SINGLE CHAMBER RTO's This concept employs a single vertical cylindrical chamber and cylindrical structural packing. The chamber is divided into segments. Waste gas enters the chamber vertically upwards, as shown in Figure 10. A burner in the chamber above the media provides supplementary heating as required.
Selection of Thermal Oxidisers R. Jones Gasco Pty Ltd FIGURE 10: Single Chamber RTO The bed may be either continuously rotated or indexed with exhaust gases flowing downwards and exiting at the base. A small purge segment prevents unreacted gases from being exhausted. 4.3 THERMAL EFFICIENCY The RTO is very thermally efficient and is usually chosen when air streams are low in VOC and high in volume. Thermal efficiencies of 92 to 95%+ are common. In many cases no auxiliary fuel at all may be necessary because of VOC's in the waste stream. VOC levels as low as 2-3% of LEL may be self sustaining. A waste stream entering at ambient will usually have an exhaust temperature of 50-60 C above ambient. 4.4 AUXILIARY FUEL INJECTION Although RTOs generally have one or more burners to get the system up to operating temperature, in some designs where auxiliary fuel is required to reach destruction temperature the fuel is mixed in the waste stream to provide the necessary heating value. The advantages of this method are the minimising of NOx formation since the reaction takes place within the bed (sometimes called flameless oxidation) and the peak temperatures within flame envelope, the source of most thermal NO is avoided. This technique does however require special precautions to meet gas regulations or underwriters requirements.
Selection of Thermal Oxidisers R. Jones Gasco Pty Ltd 4.5 VARIATIONS IN VOC CONTENT Because of their very high thermal efficiency, RTO's generally operate with waste streams containing less than 15% of LEL. Concentrations above this figure can dramatically increase the temperatures within the chambers causing melting of refractories and media, and distortion of casing. If a waste stream can experience such wide fluctuations then control strategies must be built in at the outset to avoid serious damage. These include hot bypass where a proportion of the hot gases from the common combustion chamber are bypassed to the stack via hot ducting and damper. Materials of construction are important and the solution expensive. A cold bypass is another strategy where a proportion of the cold waste stream bypasses the preheating bed and is injected directly into the chamber. Without the preheat, the final temperature can be more easily controlled. Without careful planning however, the destruction efficiency may be compromised as the total residence time at temperature is reduced. If the likelihood of VOC peaks is very infrequent and or their duration is very short, then diversion of some of the waste stream directly to the stack may be an option with dilution air added to the remaining waste stream continuing to the RTO. 4.6 PARTICULATES As previously discussed organic particulates can deposit within the media beds and in time plugging and flow reduction becomes a problem. It is a case of prevention being better than cure, and pre-filtration may be justified as essential. By selecting larger channel sizes in structural media RTO blockage will be minimised. Organic particulates will require a bake out cycle, and if not planned for may not be possible if valves and ducting cannot take the increased temperatures of around 450 C. 4.7 LOW TEMPERATURE CORROSION Because of the low exhaust temperatures, any acid gases arising from the oxidation of halogenated compounds or sulphuric compounds will more likely present as C1 2 and S0 3. For this reason particular attention must be paid to materials of construction of chamber casings, valves, plenums and ductwork and stainless steel are most commonly employed if these gases are present. 4.8 ALKALIS Some waste streams including those from the wood products industries may contain alkali metals such as sodium or potassium. These attack many ceramic media and refractories and require the use of high grade high alumina content material usually in excess of 75% alumina.
Selection of Thermal Oxidisers R. Jones Gasco Pty Ltd 5.0 REGENERATIVE CATALYTIC OXIDISERS (RCO) This is a fairly recent development and employs the addition of a layer of catalyst to the top of the regenerator beds. See Figure 11. Waste Stream FIGURE 11: Regenerative Catalytic Oxidiser (RCO) Because of the catalyst, the temperature required for a given level of destruction is reduced and therefore the amount of preheat, the size of the regenerator chambers and the size of connecting chamber are all reduced. The pressure drop through the system will also be reduced. The amount of auxiliary fuel is similarly reduced and the opportunity to operate without auxiliary fuel is expanded. Previous comments regarding CTO's and RTO's also apply to RCO's with respect to poisoning, masking, sintering, plugging, etc, plus the special consideration required for halogenated compounds and sulphur compounds. Control of VOC spikes needs careful consideration to avoid damage and loss of destruction efficiency. If all the above are satisfied then an RCO may become the optimum solution with respect to lowest energy consumption. It is sometimes possible to retrofit a layer of catalyst material to existing RTO.
Selection of Thermal Oxidisers R. Jones Gasco Pty Ltd SUMMARY In choosing the most appropriate TO type, special consideration must be given to the classification of the waste stream. If the stream treatment is only for destruction of low level VOC's or odour and gaseous composition and particulate concentration permits, the decision making favours the lowest running cost; RTO or RCO options. If flow rates are very low and VOC's high, then lowest capital cost option may override and favour a simple TO. The presence however of high particulates, condensable hydrocarbons, halogenated or sulphonated compounds, salts and alkalis makes the decision more complex and will sometimes favour TO's with or without heat recovery. The opportunity to recover waste heat for process heating or steam raising may also decide the route chosen. Notwithstanding above qualifications the application of RTO's continues to expand and is now generally the first selection whenever process conditions permit.