I.- Ping Chung and Steve Londerville
14. Pollutant Emissions
6.2 Fundamentals
During catalytic waste gas cleaning, pollutants gen- erally react with another compound on the catalyst’s surface. This compound must either be available in the waste gas or must be added. One catalytic clean- ing method or reaction is the oxidation of hydrocarbon combustion shown in the following:
vC Hm nC Hm n+vO2O2→vCO2CO2+vH O2 H O2 (6.1)
The reduction of nitrogen oxides (e.g., in waste gas) from nitric acid plants with ammonia is a possible catalytic reaction as follows:
vNOxNOx+vNH3NH3→vN2N2+vH O2 H O2 (6.2) Another possibility is a simple decomposition of a pol- lutant such as ozone (O3):
vO3O3 vO2O2 1vO2O2
→ +2 (6.3)
Technical catalysts in catalytic oxidation systems are often bulk materials or molds. Bulk material can be balls, cylinders, or rings. The molds are in the shape of honeycombs or monoliths. The catalyst material is crossed by small and tiny capillary tubes and contains small cavities. This kind of porous catalyst has, in addi- tion to their outer geometric surface, an inner surface.
The inner surface is many times greater than the size of the outer surface and therefore is determinate of the catalyst activity. Nonporous catalysts (e.g., wires coated with precious metal) are not often used. Chemical reac- tions can occur on both the outer and the inner surfaces.
A schematic illustration of the combustion process, tak- ing place in a catalytic reactor, is shown in Figure 6.1.
The chemical conversion rate or destruction and removal efficiency (DRE) in catalytic combustion systems depends on the catalyst activity, the concentrations of the reacting partners and the reaction temperature. The activ- ity of catalyst defines the quantity of the substance, which is converted per contact volume of catalyst and time mea- sured under standard conditions. For a continued gas reaction in a reactor, the reaction partners in the gas-filled space must pass through the diffusion boundary layer
1 g = 150 m2 “HC” + O2 H2O + CO2
Figure 6.1 Catalyst’s function.
Catalytic Combustion 139
surrounding the catalyst to reach the outer surface. Gases are then diffused through the pores to the inner surface.
The reaction rate, which is exponentially dependent on the temperature, interacts with transport processes, which are generally much less dependent on the tem- perature. Isothermal methods can be differentiated into three areas (1) kinetic area, (2) pore diffusion area, and (3) substance crossover area (boundary layer diffusion area).
The term “space velocity” is used for the catalytic system design of a reactor in reference to the isothermal method.
Space velocity (RG) is defined as the ratio of gas volumet- ric flow V˙ to catalyst volume VK in a standard state:
V V
R V m
V R m
K G
N K G N
h m
h m or h
=
3 3 3
3 1
, [ ],
( ) [ ]
× −
(6.4) In practice, the h−1 unit is commonly used. The gas volu- metric flow V is commonly related to the standard state (damp).
6.2.2 Measurement and Control engineering
The measurement, recording, and control engineering of the catalytic system has to be performed so that all sub- sequent operating states can be controlled safely and reli- ably, and dangerous situations can be avoided. Specific operating conditions and various process parameters must be monitored specific to the application. Generally, minimum gas flow rate, mass flow rate, and the intake temperature of the catalyst should be monitored. The sup- ply of waste gas into the catalytic system must be inter- rupted if the minimum temperature falls below acceptable operational levels. The delay between the measurement and the shut-off device must be taken into consideration.
The oxygen content (minimum O2 content) may need to be monitored depending on waste gas conditions. The intake concentration of pollutants in the waste gas or pol- luted gas, which can be oxidized, must maintain a certain level before entry into the reaction zone.
For proper operation, monitoring of the catalyst bed temperature is necessary. Depending on the bulk height, it is important to take two or three quick temperature mea- surements (or a single measurement of the reactor emis- sions in the case of a honeycomb shape). At the start, the system will be heated with fresh air or circulated air. After a plant failure, the entire waste gas cleaning system must be cleaned using a flushing procedure. During a plant shut- down, the supply piping must be purged and the emission concentration of the organic components monitored.
Depending on the application, important safety control elements may have to be performed with integrated fault detection. In the case of a failure, the plant must not be restarted automatically. To safely operate the unit, safety control circuits are provided. Necessary control functions
are safeguarded with hardwire interlocks or program- mable fail-safe control systems. (Volume 2, Chapter 2 provides a general overview of combustion controls.) 6.2.2.1 Selection of Catalyst
The popular definition of a catalyst is that it is a sub- stance that accelerates a chemical process without being consumed during this process. A catalyst is composed of a carrier coated with an active material. The reason for the carrier is that often the active material has a very low mechanical strength and is very expensive. By plac- ing the active material on a carrier, the required strength is provided, and at the same time, the expensive active material is spread out over a large accessible area.
Catalyst structures may be either full contact or sup- ported catalysts. Full contact catalysts consist of an active phase. Various pollutants and catalyst poisons call for different types of active material, and the diverse sizes of plants and different dust loadings call for differ- ent physical shapes of the carrier. The active material is a metal oxide, a noble metal, or a combination the two.
Generally, noble metals such as palladium (Pd) and plat- inum (Pt) or transition metal oxide like oxides of titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), and copper (Cu) are used for active materials. The oxides can be used in pure form, as compounds and with various additives. Additives in low quantities often cause a significant increase in activ- ity. Full contacts are often inferior to the supported cata- lysts with regard to activity, but can in some cases provide certain advantages regarding the considerable activity reserve of the solid material. The activity reserve is useful with regard to poisoning or mechanical stress (abrasion).
Supported catalysts are all catalysts with an active phase applied on a structural material—the carrier. The active phase determines the catalytic properties in interaction with the carrier. The mass fraction of the active phase in the catalyst is low (typical values for precious metal cata- lysts lie between 0.1% and 0.5%).
The active phase is applied preferably on the surface or in the surface layer (surface impregnation) and oper- ate in the boundary layer diffusion area.
The carriers of the catalysts available today are pellets—formed as spheres (alumina) or rings (alumina or silica)—and monoliths made of alumina placed on a skeleton (see Figure 6.2). Carrier materials are metals and may be in the form of monoliths (honeycombs), formed sheets (expanded grids), turnings, wires, webbing, or nets. In many cases, metal oxides such as Al2O3, SiO2, TiO2, ZrO2, and MgO are either molded or natural, and synthetic minerals such as bims, mullite, cordierite, ste- atite, and zeolite are molded and used as carrier material.
Chemical or physical properties of the carrier can either exacerbate or even prevent the direct raising of
140 The John Zink Hamworthy Combustion Handbook
the active phase. In these cases, a thin intermediate layer (wash coat) of ceramic or synthetic fibers is applied on the substrate and then impregnated with the catalytic active component. By applying an intermediate layer, a large surface can be covered, which is important for both kinetics and the diffusion area.
The advantages of pellets are that they are less sensitive to poisoning due to more active material per air volume treated, and the cost is lower than monoliths. The great- est advantages of monoliths are a lower volume due to a higher activity per volume, a lower pressure drop, and the ability to let dust pass through the catalyst (see Table 6.1).
Table 6.1
Catalyst Types in Different Processes
Process Application in
Pollutants to
Be Converted Catalyst Form Components
Oxidative methods Refineries CO, HCs, VOC Monoliths, bulk
materials Metal oxides
Chemical industry foundries Precious metal catalyst
Waste incinerators VOC, PCDDF Metal oxides
Stationary motors CO, HCs Precious metal catalyst (three-
way catalyst, precious catalyst) Food industries (roasting facilities,
smokehouses) VOC Precious metal catalyst, metal
oxides Paint manufacturers, printing
plants VOC Precious metal catalyst, metal
oxides Reductive methods Glass/mineral oil/chemical/steel/
non-ferrous metal industrial NOx Monoliths, bulk materials (extrudates, balls, and pellets)
V2O5/TiO2/SiO2/WO3 and other metal oxides
Power plants, waste incinerators, stationary motors
Nitric acid manufacture NO, NO2, N2O Precious metal/Al2O3
Metal oxides (e.g., Fe) zeolithes
(a) (b)
Figure 6.2
(a) Bulk materials: pellets catalyst–spheres and rings (balls, rings, and cylinders) and (b) types of monolith catalyst monolith (honeycomb) material.
Catalytic Combustion 141
General requirements of catalysts include high cata- lytic activity at a low working temperature; selectivity (only desired reactions accelerated); thermal, chemical, and mechanical reliability (temperature, knocks, fric- tion, vibrations); high service life; low pressure loss;
regenerability; environmentally friendly disposal; and ease of handling.
Catalysts may undergo physical or chemical changes during operation. Reasons for these changes include diffusion and migratory processes on the surface or in the catalyst interior, phase changes, recrystallization, and formation or decomposition of surface compounds or of functional groups on the carrier surface. In prac- tice, a waste gas catalyst must be able to cope with dif- ferent operating conditions.
Depending on the area of application and the catalyst type, the space velocities must be between approxi- mately 2,000 L/h (reductive method) and approximately 300,000 L/h (CO oxidation for stationary motors) and the operating temperatures of waste gas cleaning cata- lysts range between room temperature (ozone disin- tegration) and a maximum of approximately 800°C (1500°F).
6.2.2.2 Deactivation and Reactivation of Catalysts Catalysts are deactivated and reactivated in catalytic waste gas treatment systems.1–4 Catalysts naturally age and have a limited expected service life. Under certain marginal conditions, deactivation mechanisms can also cause a premature loss of function. The term, “catalyst deactivation,” describes the reduction of operational activity compared to the initial activity.
Several parameters determine the activity of the waste gas catalyst material including kind, type, and composition of the catalytic material, distribution of active components on the catalyst, specific contact sur- face, size, and number of pores, and mass transport of the substance to and from the reaction sites.
Causes of catalyst deactivation mechanisms may be chemical, thermal, or mechanical. Chemical deactiva- tion is a result of unwanted reaction between the waste gas and the catalyst. This can result in the catalytic properties being directly impaired due to substance changes in the catalyst and the reaction sites, fouling by reaction products specifically over the active sites, blocking the catalyst surface with strong sorptive bonds and/or chemical reactions, or covering the catalyst non- specifically. Additionally, a contact reaction with the waste gas can lead to loss of mechanical stability in the catalyst, which in turn can cause deactivation due to material degradation or the loss of structure. Chemical changes in the catalytic material are often irreversible;
however, deactivation caused by deposits can be par- tially rectified. The substances responsible for chemical
deactivation include halogen compounds (HCl, HBr, and HF), compounds that contain sulfur (reversible poi- sons), elements such as phosphorus, arsenic, silicon, and lead, and alkaline earth metals (irreversible poisons).
Thermal deactivation is either caused by extreme temperature peaks or by a constant excessive operat- ing temperature. The conversion of high concentration of pollutants will occur generally at the extreme tem- perature peaks of the catalyst. Also, if the pollutants are sorptively bonded to the catalyst, sudden oxidation, when the catalytic waste gas cleaning process is started, may result. The valid temperature ranges to prevent thermal deactivation are specified by the catalyst man- ufacturer and are based on the catalyst type. Thermal deactivation is not reversible.
If subjected to temperatures between 1200°F (650°C) and 1350°F (730°C) for extended periods of time, many catalysts will begin to suffer significant damage as a result of sintering. Sintering is the melting and coales- cence of the active catalyst material, which results in a loss of available catalyst surface area and, consequently, a loss of catalytic activity. The rate of sintering increases rapidly with increasing temperature. A catalyst that shows the first signs of damage at 1200°F (650°C) will likely be severely damaged in a matter of hours at 1500°F (820°C). Therefore, for long-term operation and the best DRE, the catalyst bed needs to be maintained above the temperature at which high-rate reactions occur, but below the temperature at which significant sintering occurs. Typical catalyst outlet temperatures are in the range of 600°F (320°C)–1000°F (540°C). The reduction of activity may trigger sintering on the catalyst surface, changes in shape and structure (e.g., crystal grating, modification), and material migration on the catalyst’s surface (active sites).
Deactivation due to mechanical influences is caused by wearing of catalyst material, either due to abrasive particles in the waste gas being treated or due to fric- tional movements in the catalyst when in operation (plant vibrations), or deposits of particles on the catalyst’s surface together with impaired substance conveyance.
6.2.2.3 Criteria for Selecting a Suitable Catalyst
When selecting a suitable catalyst, resistance to chemi- cal attacks or thermal deactivation must be taken into consideration. Also, the presence of deactivating sub- stances in the waste gas should be considered from the outset. An analysis of the waste gas is generally required to determine valid concentrations. Substances and processes that influence the composition of the waste gas must be taken into account. If the operating conditions and failures are variable or unknown, pilot studies should be conducted to determine the deactivat- ing effect of the waste gas.
142 The John Zink Hamworthy Combustion Handbook
The activity of used catalysts is determined by standardized activity tests. These tests use conver- sion temperature patterns and start temperatures to determine the activity with synthetic waste gas mix- tures. The causes for the reduction in activity can be determined by studying die substance composition and morphology of the catalyst. These studies can also reveal the deactivation process conditions, the remaining lifetime of the catalyst, and the possibilities for reactivation.
6.2.2.4 Protective Measures against Catalyst Deactivation
The treatment process and the method of protecting a catalyst require a careful design with a good under- standing of the operating conditions and a thorough knowledge of the waste gas composition. Adequate or sufficient experience should be available combined with pilot test results. Only waste gases with a known composition should be cleaned in the catalyst. Catalytic waste gas cleaning must comply with legal regulations based upon technical guidelines or specified air qual- ity standards. Problems may arise during the removal of certain compounds, such as sulfur, halogen, nitrogen compounds, or carcinogenic substances. Here, it is best to use a combination of methods or use a more suitable catalyst material.
The protection against chemical deactivation requires measures for preventing catalyst poisoning due to chemical reactions and must be implemented in specific manners. Possible prevention measures are selecting stable carriers and active components, operation at high temperatures, thereby preventing the condensation of organic substances, coking and strong adsorption of reaction partners or products, regular regeneration due to combustion or curing (e.g., in the event of cok- ing, adsorption), desorption (e.g., by blowing out with air in the event of adsorption), and additional heating if necessary. The measures also include use of combined pretreatment processing stages, such as pre-separation of aerosols, catalyst poisons, and particles by prefiltra- tion and prewashing, condensing substances contained in the waste gas (e.g., low-boiling organic compounds), and preceding adsorption. Often suitable prevention can be achieved by adding guard beds or sacrificial lay- ers to protect the main catalyst, using a multi-bed cata- lyst arrangement for selective progressive reactions, and providing a suitable temperature profile to prevent foul- ing and insufficient temperatures.
An increase in structural changes (e.g., crystal for- mation, sintering, and phase change) in the catalytic material, together with the attrition of the inner sur- face and impaired distribution of active components can be caused by increasing temperatures. The use of
temperature-stable catalysts allows waste gases to be treated at high temperatures. Temperature increases that result from higher concentrations (e.g., in the event of solvent oxidation) must be prevented through the implementation of control measures.
The stability and instability of the reactor must be considered when estimating the expected tempera- ture increase in the catalyst bed. Fixed-bed catalytic reactors have a two-phase system (solid/gas). Within the system, unsteady relationships are sporadically established, influenced by current substance trans- port and heat transfer rates, heat retention and reac- tion kinetics, and where the waste gas temperature does not correspond to the temperature in the reac- tion zone. The differences may be higher or lower than the adiabatically calculated temperatures. They are also noticeable by a migration of the reaction zone within the reactor—this is the layer of the catalyst in which the main substance conversion takes place.
The site of the reaction zone can be measured using a steep temperature increase in the catalyst bed. These effects and their negative influence on the thermal demands on the catalyst can be counteracted by con- trolling the preheat temperature of the waste gas, by external heating or cooling, integrated heat exchang- ers, or a controlled operation of an additional heater (gas fired or electrical). Catalyst manufacturers allow for unsteady procedures with a short-term maximum temperature capacity.
At low temperatures, catalysts work as an adsorption material. To prevent spontaneous oxidation and related spontaneous heating of adsorbed pollutants at these temperatures, the catalyst must only be supplied with waste gas containing pollutants during normal operat- ing temperatures. This requires the catalyst to be pre- heated to the required operation temperature.
The catalyst can be protected from mechanical deacti- vation by implementing technical measures for reducing the mechanical demands on the catalyst. The protec- tion can be provided by suitable flow routing, reactor construction, monitored plant operation, operationally oriented maintenance intervals, and pre-separation of particles. Further, selecting a suitable catalyst shape protects it against damage due to mechanical loads.
6.2.2.5 Reactivation of Catalysts
After the performance of the catalyst has been reduced by deactivation, the original activity level may be partially or fully restored, depending on the condi- tion of the catalyst. This is achieved by reactivation measures selected according to the condition of the catalyst. Determining the best reactivation measure depends on the condition of the catalyst using meth- ods such as visual observations to check for fouling,
Catalytic Combustion 143
discoloration, and cracks. Also, x-ray fluorescence analysis (XRF) may be done to determine changes in the basic composition compared to a reference model and a wet chemical analysis may be done to deter- mine the concentration of the elements, which have been identified as deactivating. The most widely used technique for estimating surface area is the so-called BET method.5 The BET surface analysis to determine the amount of reduction in the catalytic surface or an activity test, to measure the activity of the used pat- tern before and after the reactivation compared to a reference model may also be used.