Thermal Incineration: Benefits and Limitations

The most effective way of ensuring essentially complete destruction of VOCs present in gaseous effluent is by incineration. Thermal incineration of VOCs is a widely used technology, and can be performed by direct-flame heating in a combustion chamber, at temperatures of 760°C or more, depending upon the nature of the VOCs.

The three principles of thermal incineration are: time, temperature and turbulence. In order to achieve a high destruction of VOCs the following criteria must be satisfied: sufficient time – of 0.5 to 2 seconds, temperature – of 760°C or greater, and turbulence – to achieve good mixing of the VOCs with air. If any one of these criteria is not achieved, then the VOC oxidation reactions will not be completed and excess emission levels will be generated.

Thermal incinerators can also suffer extreme thermal stresses, due to their high working temperatures, especially during cycles of heating to the operating temperature, and cooling during shutdown. This can result in the distortion of ductwork and heat exchange surfaces, creating the potential for cracks and leaks. Thus dirty process gases with high VOC content can contaminate the cleaned incinerator exhaust gas. Operating companies may then be faced with the prospect of a major capital outlay either to modify or replace equipment within their incinerator.

Return to Compliance with a Retrofitted Catalyst System

One technology which can help in this situation is catalytic oxidation. The technology for the successful catalytic oxidation of VOCs as an alternative to direct thermal incineration has been proven over many years through installation of catalytic oxidation systems to treat VOC emissions from a wide range of industrial processes. More recently catalysts have been brought to public awareness by fitting of catalytic converters to new cars.

The main advantage of catalytic oxidation is the ability of the catalyst to operate at significantly lower temperatures than direct thermal incineration. In many applications this can give lower capital costs and/or lower operating costs.

When the performance of a thermal incinerator is falling short of legislated requirements, either due to a design limitation or to thermal stress-induced leakage, then the addition of an oxidation catalyst into the exhaust of the thermal incinerator can bring the incineration system back into compliance. The basic layout of a thermal incineration system fitted with a catalytic reactor is shown in Figure 1. The system comprises an incineration unit operating at a minimum temperature of 760°C together with a primary heat exchanger which preheats the inlet gases from the process. A secondary heat exchanger is normally installed to recover energy for other purposes. The temperature of the exhaust gas, after passing through a secondary heat exchanger, is typically around 400°C.

The temperature needed for the catalytic oxidation reaction for the majority of VOCs is between 280 and 400°C, and for carbon monoxide which can arise from partially combusted VOCs between 200 and 250°C. Therefore, the addition of a catalytic reactor into the exhaust duct of the incinerator presents the catalyst with the optimum temperature it needs to facilitate the complete oxidation of VOC and carbon monoxide emissions. Moreover, the catalyst is capable of operating at up to a temperature of 750°C, and it is therefore unlikely to be damaged, even under emergency venting of the thermal incinerator.

Catalyst Use in a Coil Coating Process

One such retrofit of a catalytic reactor into a thermal incinerator was completed by Johnson Matthey at Alcoa Manufacturing (GB), Swansea. Alcoa is the world's oldest and largest manufacturer of aluminium sheet for the beer, beverage and general purpose can manufacturing industry. Problems had been experienced with the thermal incinerator which treats fumes from the coil coating line, causing general concern about the odour and the level of VOC and carbon monoxide emissions into the atmosphere.

In the coil coating plant continuous aluminium sheet is coated with lacquers, which produce
VOCs when passed through a high temperature treatment oven to make the finished product. The high concentration VOCs are destroyed by the thermal incinerator before exhausting to the atmosphere. However, after operating for 6 years the incinerator was found to be suffering from a leakage of VOCs across the primary heat exchanger into the cleaned exhaust gas. There was also some concern that the VOCs were not being effectively destroyed at the design incineration temperature of 720 to 760°C.

An on-site demonstration was undertaken by Johnson Matthey using a pilot-scale platinum-based oxidation catalyst connected so as to take a sidestream from the incinerator exhaust gas flow, at 380 to 425°C. During the extended trial the catalyst proved its capability not only in removing the odour from the incinerator exhaust gas, but also in achieving emissions containing very low levels of VOCs and carbon monoxide. As a result a full-scale catalytic reactor was designed for the maximum process flowrate of 70,000 Nm³/h, taking into account the pressure drop limitations of the existing process exhaust fan and the available space in and around the incinerator exhaust duct.

The platinum-based catalyst was supplied in the form of metal honeycomb monoliths which were welded together to form two large panels and then lowered into a purpose-designed reactor housing, see Figure 2. The catalytic reactor was fully insulated to ensure maximum temperature of the gases reaching the catalyst, in order to attain the highest oxidation performanee. Installation of the catalytic reactor was completed within 48 hours, requiring the minimum shutdown on the coil coating plant, see Figure 3.

As presented in the initial evaluation from the consultant of the options for Alcoa, the overall investment in the catalytic reactor installation has shown a significant saving both in capital cost and in process downtime when compared with the alternatives available.

Continuous monitoring of the stack emissions shows the catalyst to be achieving VOC and carbon monoxide emissions well within the limits set by the Environmental Protection Act.

Contuiuing Benefits from a Catalyst Retrofit

Once an oxidation catalyst has been retrofitted into a thermal incinerator exhaust duct there is the potential for significant savings in operating costs through a reduction of the combustion temperature in the incinerator. In Germany, following the retrofit of a platinum-based catalyst system by Johnson Matthey, the operator of a phenolic resin coating plant has been able to reduce his incineration temperature to less than 600°C. This has resulted in a reduced efficiency within the thermal incinerator which presents the downstream catalyst with greatly increased concentrations of VOCs and carbon monoxide.

However, the catalyst effectively oxidises these harmful gases and has brought the incinerator into compliance with the strict German air pollution control regulations (T. A. LUFT) while providing considerable savings in incinerator fuel for the operator. Moreover, operation at lower temperature has the added benefit of reducing thermal stresses in the incinerator, which would be expected to extend its operating life.

Platinum-based catalysts used for VOC control in industrial applications have been proven to have a typical lifetime of between five and seven years. Therefore, retrofitting a thermal incinerator with an oxidation catalyst represents a long-term solution to the problem of noncompliance with existing legislation, and can even be an answer to meeting future, more stringent, environmental legislation.