Catalytic Combustion Processes

Catalytic combustion can produce a stable surface flame at low fuel/air ratio and temperatures as low as 1300°C, and this avoids the formation of thermal NOX. Operating costs are also significantly lower.Compression np to an oper- 

In a more interesting process, all of the fuel/air mixture is added to a combustor with three sections. The first section contains an active palladium oxide catalyst that can operate up to about 800°C before being reduced to palladium metal which is less active. The palladium oxide catalyst is regenerated by reoxidation of the metal as temperature falls. A more stable catalyst in the second section continues the catalytic combustion. In the third section, combnstion is completed by thermal reaction and the gas temperature increases to 1300°-1400 C. Overall, less than 1 ppm NOX is formed. Palladium oxide is supported on a monolith coated with temperature resistant barium hexaaluminate.  

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Transition metal oxides represent a prominent class of partial oxidation catalysts [1-3]. Nevertheless, materials belonging to this class are also active in catalytic combustion. Total oxidation processes for environmental protection are mostly carried out industriaUy on the much more expensive noble metal-based catalysts [4]. Total oxidation is directly related to partial oxidation, athough opposes to it. Thus, investigations on the mechanism of catalytic combustion by transition metal oxides can be useful both to avoid it in partial oxidation and to develop new cheaper materials for catalytic combustion processes. However, although some aspects of the selective oxidation mechanisms appear to be rather established, like the involvement of lattice catalyst oxygen (nucleophilic oxygen) in Mars-van Krevelen type redox cycles [5], others are still uncompletely clarified. Even less is known on the mechanism of total oxidation over transition metal oxides [1-4,6]. 

In addition to the selective oxidation, the complete oxidation and catalytic combustion processes are also important topics in catalytic fields. 

The air pollutants of volatile organic compoimds emitted from many industrial processes and transportation activities could be abated by catalytic combustion processes. Scire et al. reported the catalytic combustion of 2-propanol, methanol, and toluene on ceria-gold catalysts. The catalysts were prepared with coprecipitation and deposition-precipitation methods. The gold significantly enhanced the catalytic activity of ceria for the oxidation of these volatile organic compounds. The supposed reason is that the gold NFs weakened the mobility/reactivity of surface lattice oxygen (Scire et al., 2003). 

The catalytic combustion processes may be used advantageously when removing smelling organic substances, for the destruction of vapours of organic substances used as solvents, and of phenol, formaldehyde, etc. 

Catalytic combustion of a carbonaceous fuel, e.g. natural gas and catalytic combustion process... 

Sulfur recovery catalysts. Sulfur reclaim is to transform the acidic gas containing H2S produced during processes into sulfate, and then sulfate is recycled. The sulfur recovery has many methods, such as iron oxide process, ADA process, G-V process, hypermanganate process, dichromate process, catalytic combustion process and Claus method etc. Herein, the Claus method will be introduced briefly. 

Oxides of nitrogen, NO, can also form. These are generally at low levels and too low an oxidation state to consider water scmbbing. A basic reagent picks up the NO2, but not the lower oxidation states the principal oxide is usually NO, not NO2. Generally, control of NO is achieved by control of the combustion process to minimize NO, ie, avoidance of high temperatures in combination with high oxidant concentrations, and if abatement is required, various approaches specific to NO have been employed. Examples are NH injection and catalytic abatement (43). 

Catalytic combustion is feasible for purification processes only when impurities are at concentrations <10% of lower flammabiUty limit and when bulk stream already consists of oxidation products, eg, airstreams, off-gases, and other inerts. 

Temperature. The temperature for combustion processes must be balanced between the minimum temperature required to combust the original contaminants and any intermediate by-products completely and the maximum temperature at which the ash becomes molten. Typical operating temperatures for thermal processes are incineration (750—1650°C), catalytic incineration (315—550°C), pyrolysis (475—815°C), and wet air oxidation (150—260°C at 10,350 kPa) (15). Pyrolysis is thermal decomposition in the absence of oxygen or with less than the stoichiometric amount of oxygen required. Because exhaust gases from pyrolytic operations are somewhat "dirty" with particulate matter and organics, pyrolysis is not often used for hazardous wastes. 

When NO destmction efficiencies approaching 90% are required, some form of post-combustion technology appHed downstream of the combustion 2one is needed to reduce the NO formed during the combustion process. Three post-combustion NO control technologies are utilized selective catalytic reduction (SCR) nonselective catalytic reduction (NSCR) and selective noncatalytic reduction (SNCR). 

Three rapid oxidation methods are typically used to destroy combustible contaminants (1) flares (direct-fiame-combnstion), (2) thermal combustors, and (3) catalytic combustors. The thermal and flare methods are characterized by the presence of a flame during combustion. The combustion process is also commonly referred to as afterburning or incineration. ... 

Catalytic combustion is a process in which a combustible compound and oxygen react on the surface of a catalyst, leading to complete oxidation of the compound. This process takes place without a flame and at much lower temperatures than those associated with conventional flame combustion. Due partly to the lower operating temperature, catalytic combustion produces lower emissions of nitrogen oxides (NOx) than conventional combustion. Catalytic combustion is now widely used to remove pollutants from... 

These two research areas share the common characteristic of involving inorganic solids in the combustion process. Catalytic combustion research focuses on using the solid to facilitate the oxidation of well-known fuels such as hydrogen and methane. Materials synthesis research focuses on using combustion as a means to react the solids either with each other or a gas, such as nitrogen (which in this case acts as an oxidizer), to make new solid materials. 

When these metal additives are delivered to the furnace, in whatever form suitable, they become oxides. In this active form, they exert a catalytic effect on the fuel combustion process. 

Selective catalytic reduction (SCR) and selective noncatalytic reduction processes (SNCR) are widely employed in large industrial and utility boiler plants, as well as in municipal waste incineration plants and other combustion processes. They are used to complement mechanical improvements (such as low NOx burners and furnace design modifications) as an aid to reducing the emission levels of NOx, S02, and other noxious gases into the atmosphere. 

Qean and efficient processes production of at) mc chemicals witiiout salt, replaconent of HE or H2SO4, catalytic combustion, hi tempoature selective oxidation firel cells ... 

Heterogeneous catalysts are the active ingredients in automobile catalytic converters. When combustion occurs in an automobile engine, side reactions generate small amounts of undesired products. Some carbon atoms end up as poisonous CO rather than CO2. Another reaction that takes place at the high temperatures and pressures in automobile engines is the conversion of N2 to NO. Furthermore, the combustion process fails to bum all the hydrocarbons. Hydrocarbons, CO, and NO all are undesirable pollutants that can be removed from exhaust gases... 

Pellistors are used to detect flammable gases like CO, NH3, CH4 or natural gas. Some flammable gases, their upper and lower explosion limits and the corresponding self-ignition temperatures are listed in Tab. 5.1. This kind of gas sensor uses the exothermicity of gas combustion on a catalytic surface. As the combustion process is activated at higher temperatures, a pellistor is equipped with a heater coil which heats up the active catalytic surface to an operative temperature of about 500 °C. Usually a Platinum coil is used as heater, embedded in an inert support structure which itself is covered by the active catalyst (see Fig. 5.33). The most frequently used catalysts are platinum, palladium, iridium and rhodium. 

As can be seen in Fig. 5.44 as well, flames in gas heaters have a similar emission spectrum. Besides the UV surveillance ionization electrodes are often used in gas burners. The method is cheap and secure but it disturbs the combustion process since the electrode has to be placed close to the flames. New developments in gas heaters focus on catalytic combustion on a metal mesh. There, an ionization electrode would fail due to the lack of a flame. However, the characteristic UV emission is still present... 

Oxycat A catalytic oxidation process for removing combustible vapors from air and industrial exhaust gases. The catalyst is platinum on alumina, supported inside a porcelain tube. 

We have, so far, not considered the combustion process route. Obviously, it is a very important process route towards electricity production, with minor adaptations of present installations. The catalytic challenge relates to the purification of the emission gasses, which will have an increased NOx level. 

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