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2-way VOC Oxidation Catalyst for control of CO and HC-based Air Toxics (VOC/HAP/Odour)

Catalytic Incineration
The most practical and effective technique for destroying VOC is incineration or oxidation, a process in which VOC captured from an industrial process are oxidised at elevated temperatures and converted to carbon dioxide (CO2) and water (H2O) as shown below.

Several other VOC capturing techniques also exist (e.g. biological fermentation, adsorption, condensation) but by far the most commonly specified VOC control methods are thermal or catalytic oxidation.

VOC Oxidation Catalyst
Conventional oxidation catalysts (COC) are designed for 2-way control of CO and HC but are also effective in controlling VOC/HAP emissions. COC can remove benzene, aromatic hydrocarbons, acrolein, formaldehyde and other toxic compounds at temperatures typically found in industrial and commercial process streams and engine exhaust. COC are also routinely employed in combination with SCR catalysts to control HC (VOC/HAP) emissions and ammonia slip.

Different catalysts are needed for different hydrocarbon species, unsaturated hydrocarbons typically easier to oxidise than saturated hydrocarbons. Methane is the most difficult, followed by ethane and propane.

Around the world, hydrocarbon emissions are regulated differently depending on geographical location. Some regulations or permits stipulate total hydrocarbon (THC) conversion, sometimes written as unburned HC (UHC). Others seek to limit non-methane hydrocarbon (nmHC) conversion or even non-methane, non-ethane (nmneHC) conversion, perhaps the most commonly recognised definitions of VOC.

Where the hydrocarbons are mainly unsaturated and VOC control is required, as is the case in natural gas reciprocating engine, then Johnson Matthey would typically specify a platinum-based Honeycat® catalyst from its Concat® range.

Where the hydrocarbons are mainly saturated, and there is a nmHC requirement, then Johnson Matthey typically specifies a relatively high-loaded palladium or platinum/palladium-based Honeycat® catalyst from its MHC® line-up.

In situations where the is a requirement for methane conversion, as is often the case in Europe, then provided there is sufficient temperature for the catalyst to light-off and the exhaust stream is relatively free of catalyst contaminants, Johnson Matthey would typically specify a high-loaded palladium-based Honeycat® catalyst from its LHC® range.

A typical VOC catalyst comprises PGM (platinum group metals) mixed with a high surface area alumina-based washcoat and applied to the surface of a honeycomb support structure or substrate of fabricated stainless steel foils (metal monolith) or extruded cordierite (ceramic monolith) or ceramic pellets or beads. The precious metals are applied in such a manner to provide as many reaction sites as possible in a stable configuration to ensure high performance and long life.

Catalytic oxidation is a proven, cost-effective technology capable of controlling up to 99% of VOC emissions from industrial and commercial sources and over 150 of the 189 air toxics identified by the US CAA. Over the past thirty years, several thousands of catalytic oxidisers have been installed worldwide in a diverse array of industrial and commercial applications from surface coating, printing and solvent use operations, as well as chemical and petroleum manufacturing.

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Catalytic Oxidiser
In a typical catalytic oxidiser (catox) unit, a blower drives the exhaust fumes into the catalytic incineration unit, where a burner preheats the gas to operating temperature (monitored and maintained by thermocouples and a temperature controller). Preheated gas is then passed across a VOC catalyst to combust the VOCs. Oxidation of the VOCs produces additional heat (exotherm) and this may be used to heat incoming fumes via a heat exchanger.

Recuperative Catalytic Oxidiser (RCO)
Recuperative heat exchangers can recover between 50% and 75% of the heat released during oxidation and catalytic oxidisers employing this technique are referred to as recuperative catalytic oxidisers (RCO) units.

Regenerative thermal oxidisers (RTO) units can be even more efficient, with multiple ceramic chambers (often bare substrates) used to recover as much as 90% to 95% of the heat from oxidation. In both cases, the result is additional fuel savings. These operate by passing fumes over a hot surface (typically an array of ceramic blocks) in order to combust or oxidise the VOC. Some thermal oxidisers now incorporate a catalyst layer within each combustion chamber to reduce the fuel penalty and ensure optimum emissions performance.

In contrast to thermal oxidation techniques, which require high temperatures and hence large amounts of fuel to maintain the temperatures, a catalyst will react and oxidise VOC at much lower temperatures, meaning less fuel and lower costs. Catalytic oxidation of benzene, for example, requires a temperature of 227°C (440°F) and a residence time of 0.24s for 99% destruction in a catalytic oxidiser but 793°C (1460°F) and a residence time of 1.0s for 99% destruction in a thermal oxidiser.

The lower temperature required for catalytic oxidation also extends mechanical durability of key components such as the heat exchanger resulting in lower maintenance costs.

Over the past thirty years, catalytic oxidisers have been installed and operated successfully in a wide variety of VOC destruction applications with a proven track record of high efficiencies and long life. Ultimately, selection of the right VOC destruction technique will depends on a series of factors including solvent concentration, flow rate, and explosive limit.

With the threat of global warming and increased public and political awareness of environmental issues and sources of air pollution, catalytic oxidisers will continue to play a major role in reducing VOC, HAP, CO and air toxic emissions from stationary sources. Catalytic oxidation techniques offer a number of practical and economic advantages and benefits over other established VOC control technologies as a consequence of the ability to function at lower operating temperatures, with lighter weights and with greater compactness (system footprint) with negligible impact on overall NOx and CO2 emissions benefiting the user through reduced fuel consumption and lower material/maintenance costs and, for chlorinated streams, no concerns over possible dioxin/furan formation.

Honeycat® Products for VOC Oxidation
Johnson Matthey’s Honeycat® range of stationary products is manufactured in the same plants and to the same exacting quality standards as those used for the automotive industry and can be purchased either as stand-alone elements, housed units, or designed to specific requirements.

Honeycat® VOC Oxidation Catalyst technology can be purchased either directly from Johnson Matthey or through specialised plant engineering and equipment suppliers working in partnership with Johnson Matthey who will incorporate the catalyst in a purposed designed system to ensure regulatory emissions compliance with minimum pressure drop.

Metallic or Ceramic Honeycomb Monolith Block
Johnson Matthey’s Honeycat® range of VOC Oxidation Catalysts is available on metallic and ceramic honeycomb substrate monoliths in a variety of configurations, types and sizes depending on the application.

Images of a typical metallic “radiator” and ceramic monolithic honeycomb substrate media

Honeycat® VOC catalysts supplied on ceramic substrate are supplied on standard 150mm ceramic cubes in a range of cell densities from 100 to 400cpsi depending on application.

Product available on metallic substrate

Mantle and matrix components of a typical metallic block (separated for schematic purposes only).

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