Operating catalytic combustion

Gaseous vent streams from the different unit operations may contain traces (or more) of HCl, CO, methane, ethylene, chlorine, and vinyl chloride. These can sometimes be treated chemically, or a specific chemical value can be recovered by scmbbing, sorption, or other method when economically justified. Eor objectionable components in the vent streams, however, the common treatment method is either incineration or catalytic combustion, followed by removal of HCl from the effluent gas. 

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... 

Hikiforov N.N. Bakham, The Effect of Aluminum Additives On the Operational Effectiveness of the Catalytic Combustion Agent Fe203 , Rept No FTD-HT-23-627-74 (1973)... 

Catalytic combustion experiments have been performed in a flow reactor operating below the lower explosion limits using HC/02/He mixtures. The product analysis was done by gas chromatography. FT-IR spectra have been recorded with a Nicolet Magna 750 instrument, using conventional IR cells connected with evacuation-gas manipulation apparatus. The powder was pressed into self-supporting disks, calcined in air at 773 K and outgassed at 773 K for 20 minutes before experiments. 

Further, the development of miniaturized devices for the generation of power and/ or heat is discussed here as it represents an emerging field of application of catalytic combustion. Due to the presence of the catalytic phase, the microcombustors have the potential to operate at significantly lower temperatures and higher surface-to-volume ratios than non-catalytic microcombustors. This makes them a viable solution for the development of miniaturized power devices as an alternative to batteries. 

J2.2 Lean Catalytic Combustion for Gas Turbines 365 Table 12.1 Design criteria and operating conditions of GT combustors. 

Different design concepts have been proposed to match the severe requirements of catalytic combustors. A main classification criterion is based on fuel/air stoichiometry in the catalyst section, which has a dominant effect on the selection of catalytic materials and on the operating characteristics of the combustor. In this section, only configurations based on lean catalytic combustion will be described. The peculiar characteristics of rich catalytic combustion will be described in a separate section. 

Early studies in this field [35, 36] indicated that a high surface-to-volume ratio, which represents a hurdle for gas-phase combustion, is instead an advantage for catalytic combustion. In fact the small scale enhances considerably the rate of gas-solid mass transfer, which favors the kinetics of the combustion process and compensates for the short residence time. Also, as is well established for large-scale systems, the presence of a catalytic phase allows for stable combustion at significantly lower temperature than traditional homogeneous burners [55, 56]. This makes the design and operation of microcombustors more fiexible. Several recent studies have explored the potential of catalytic microcombustors using H2 [37, 38, 50], methane [37], propane [52,53,57] and mixtures of H2 with propane [57], butane [38,47,52] and dimethyl ether [52]. 

The SRCO catalytic combustion unit treats volatile organic compound (VOC) laden process exhaust air. SRCO stands for self-recuperative catalytic oxidizer. The SRCO can be furnished as a complete operating vacuum extraction and catalytic oxidation system or as a stand-alone catalytic oxidizer to interface with an existing vacuum extraction and/or air stripper system. HD-SRCO stands for halogenated destruction self-recuperative catalytic oxidizer. This system is basically the same as the SRCO system, except that it remediates halogenated hydrocarbons using a different catalyst. 

Catalytic combustion for gas turbines has received much attention in recent years in view of its unique capability of simultaneous control of NOX) CO, and unbumed hydrocarbon emissions.1 One of the major challenges to be faced in the development of industrial devices is associated with the severe requirements on catalytic materials posed by extreme operating conditions of gas turbine combustors. The catalytic combustor has to ignite the mixture of fuel (typically natural gas) and air at low temperature, preferably at the compressor outlet temperature (about 350 °C), guarantee complete combustion in few milliseconds, and withstand strong thermal stresses arising from long-term operation at temperatures above 1000°C and rapid temperature transients. 

Another important aspect is optimization of the process parameters. The challenge here is to control catalytic combustion, in order to attain overlapping of the combustion and reforming reaction over a sufficient interval [46]. The operating conditions are mainly determined by the fuel composition and the heat exchange... 

At present, one catalytic combustion system has been implemented at a full scale the XONON Cool Combustion technology, developed by Catalytica Energy Systems 157,158). The system is operated as follows Fuel from a lean-mix prebumer and the main fuel stream together with compressed air pass through the catalyst module (palladium oxide catalyst deposited on corrugated metal foil) in which the gas reaches a temperature up to 1623 K. The UHC and CO are combusted to essentially full conversion, downstream of the catalyst in the homogenous combustion zone. The guaranteed emission levels are as follows NOj < 3 ppm. 

However, the catalytic combustion of methane is more difficult than the combustion of other VOCs, since this compound is very difficult to activate. This aspect leads to higher operation temperature, causing fast deactivation of conventional supported precious metal catalysts due to crystallites coalescence. 

The deep oxidation of methane was chosen as model reaction because this reaction is considered as one of the most difficult catalytic combustions, and the simultaneous presence of methane and sulphur compounds is common in many off-gases. Sulphur dioxide was chosen as the model sulphur compound because in previous works it was observed that other sulphur compounds were oxidised to SO2 at the operation temperatures. 

Because of the outstanding prospects for catalytic combustions, a lot of R D work has been earned out in recent decades on this subject. For the above-mentioned reasons, a considerable proportion of the research is dedicated to the development of novel materials for monoliths. Pilot and demonstration plants for monolithic combustion are in operation. 

An advantage offered by catalytic combustion that may be important for heat generation is the potentially improved heat transfer, since heat is generated on a solid surface. Heat generation systems operating at catalyst temperatures up to 800-1000 C may thus be controlled more efficiently through external cooling of the catalyst. 

Saracco G., Veldsink J.W., Versteeg G.F. and van Swaaij W.P.M., Catalytic combustion of propane in a membrane reactor with separate feed of reactants. I. Operation in absence of trans-membrane pressure gradients, Chem. Eng. Sci., 56 2005 (1995). 

Catalytic combustion of CO is a fast reaction. At normal operation the catalyst is in the outer mass transport controlled region, this means that the reaction rate of the catalyst is higher than the rate of transport of reactants from the gas bulk to the outer surface of the catalyst. The apparent activity is then affected by parameters like geometry of catalyst, flow rate and properties of the gas, like viscosity and density. A change in temperature effects the properties of the gas and the flow rate, it therefore effects the conversion slightly, however it is not the change in intrinsic reaction rate that can be seen. 

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