The advantage of catalytic combustion, when compared to the thermal one, is to guarantee the complete oxidation of volatile organic substances at relatively low temperatures, usually 320-450°C. The low combustion temperature, as well as allowing less energy consumption, ensures a negligible production of NOx, a pollutant thet in combustors operating at higher temperatures is usually present in concentrations as high as the higher limits permitted by current regulations.
The oxidation of organic pollutants can be achieved with a yield not inferior to 98% if the contact time between effluent to purify and catalyst is adequate.
As a catalyst, the best performance in terms of kinetics, yield, selectivity, and resistance to poisoning agents is obtained with precious metals, such us Pt and Pd, finely dispersed in aluminium or magnesium oxide at medium or high microporous surface.
Alumina, or other microporous oxides, is the functional support necessary to assure the precious metal dispersion in microcrystal in a high density of active sites. Often, oxides impregnated with precious metals are supported by cordierite monoliths or metal surfaces that ensure mechanical support of the catalyst and high active surfaces exposed to the air flow.
For special applications, such as dioxin oxidation, De-NOx catalysts made up of V2O5-TiO2 oxides. For the oxidation of halogenated hydrocarbons, catalysts based upon Cr2O3 impregnated on Al2O3, TiO2, and microporous SiO2 have been successfully tested.
In the case of ceramic monoliths, the catalytic reaction is generally limited by the transport into the external gaseous film and generally requires spatial velocity at a level of 20,000-35,000 hours-1, related to a gas flow rate at 273K.
Thermal regenerative and restoring recovery
The catalytic combustion technology can be coupled with regenerative thermal recovery or traditional recovery with surface heat exchanger. Of course, it is the solvents concentration which determines the most suitable solution of thermal recovery.
Thermal recovery of the regenerative type is proposed for concentrations of organic solvent between 0.80-1.5 g/Nm3 and in this filed the regenerative catalytic combustion constitutes a viable alternative to regenerative thermal one.
The catalytic combustion plant with regenerative thermal recovery is very similar to the thermal combustion plant and therefore involves two or three towers with ceramic filling, the combustion chamber in the head with a burner for combustion of auxiliary fuel, the bottom valves for distribution of polluted air, washing air, and purified air flows.
The ceramic filling must preferably be the rectangular monoliths kind, with straight channels that can be linked atop with catalytic monoliths, in continuity of flow with the thermal monoliths.
Thermal recovery of recuperative type is applied to the catalytic combustion for concentration at level 3-6 g/Nm3, with thermal yield variation from 0.5 to 0.65 depending on the solvents concentration.
In the recuperative catalytic combustion, the polluted air current before converging into the combustion chamber, is conveyed to a heat exchanger, heated in counter-current by the purified air flow outgoing from the catalytic unit.
The combustion chamber operates with a line burner which prevents the entry of cold combusting air, and allows the combustion of a non-negligible fraction of vaporized solvents that pass through the flame. A temperature controller operates in the combustion chamber, regulating the chamber temperature by acting on the inlet valve of auxiliary fuel and ensures a minimum trigger temperature of 250°C of the inlet air flow.
Downstream of the catalytic unit operates the primary temperature controller which regulates the output temperature between 380-450°C, with direct action on the secondary temperature controller operating in the combustion chamber.
In catalytic combustion, the economic advantages resulting from lower auxiliary fuel consumption must be compared with the catalyst purchase cost, with the washing costs, and with the replacement cost of an exhausted catalyst. Economic considerations often turn in favour of the catalytic combustion as in the absence of well-known and specific hazardous substances the useful catalyst life is assured at a level of 15,000 – 30,000 hours.
It should be considered that, in normal applications, the air flow must be filtered by powders up to 0.1-0.3 g/Nm3 to avoid soiling of absorption surfaces and frequent use of suitable washing of the catalytic units.
Cematek has an extensive experience in the field of catalytic combustion of recovery type, usually proposed in combination with the solvent concentration process.
The catalytic combustion is normally operated at a concentration between 3-8 g/Nm3 depending on the operating conditions of the upstream solvent concentration. Downstream temperature of the catalytic unit is controlled between 320-450°C according to the type of the solvents to be oxidized. The catalyst is precious metals Pt or Pt+Pd based, dispersed in microporous alumina in monoliths of cordierite with straight channels.
In accordance with the temperature profiles along the catalytic surface from the triggering temperature, often below 260°C, to the output temperature, often over 450°C, there are monoliths of different composition and formulation in order to optimize the cost of the catalytic unit with the highest performance.
With spatial velocity between 20,000-25,000 hours-1 CO2 conversion yields of more than 98% are obtained.
Heat output of the recovery unit is usually expected at 50% which is optimum value for the combustor coupled with the solvent concentration unit.
Cematek proposes the catalytic combustion of regenerative type with two towers in the processes of CO2 combustion, dioxin destruction, oxidation of chlorinated hydrocarbons. Sometimes the catalytic combustion of regenerative type is proposed in the oxidation of organic solvents, if the complexity of the solvent mixture is such as to discourage any kind of adsorbent available solution.
Cematek has a pilot plant dedicated to the study of catalytic oxidation reactions, with a capacity of 50 Nm3/hour, built with the purpose of refining the procedures of catalytic surfaces sizing and experimentation of new formulations. Test are programmed on this pilot plant to verify the level of activity of catalytic monoliths installed in operative plants and for the control of monoliths reactivated after washing with basic and acid solutions. It can be installed on branches of industrial polluted air flows in order to verify the effectiveness of catalytic oxidation on gaseous effluents.