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Thermal or Catalytic VOC Oxidation?

By Reijo Lylykangas

Thermal and catalytic oxidizers are the most common methods in fulfilling the VOC emissions regulations with incineration. Catalytic oxidation offers several advantages in comparison to thermal oxidation, especially with low VOC contents. The most important are cost savings, cleanness of the process, and small size. In addition, the quality of catalysts has been significantly improved and new generation noble metal catalysts offer a service life of more than 15 years.

Introduction

Volatile Organic Compounds (VOCs) do not have a single and simple definition. Generally these are understood as a synonym to hydrocarbon solvents. VOC emissions are in many ways harmful to nature and human beings. They are about 11 times more “effective” greenhouse gases (GWP 100y) than CO2 and can together with Oxides of Nitrogen (NOx) form low atmosphere Ozone (O3) in sunshine.

The European Union has made directives to limit VOC emissions. The first one for industrial emissions was Council Directive 1999/13/EC. Five years later, it was completed by the so-called paint directive 2004/42/EY. Based on BAT (Best Available Technology) solutions, local authorities have implemented tighter limits, in many cases from 50-100 mgC/Nm3 set in the directive to 20 mgC/Nm3. The European Union is still aiming to cut VOC emissions even further.

Many technical solutions are used to abate VOC emissions. The most common methods are thermal and catalytic oxidation. The biggest problem for oxidation is the low VOC content, which is typically in the range of 0.1 to 10 g/Nm3. In order to oxidize such a small concentration of VOC, an enormous amount of air must be heated to a temperature where the reactions can happen. In many cases the energy contained in the VOCs is not high enough to maintain continuous oxidation. A lot of supporting energy is needed (Fig.1). Heating of a gas stream of 10s000 Nm3/h to the thermal oxidation temperature (ca. 800°C) requires 2.8 MW power. With a good heat exchanger, 90% of the energy can be transferred to the incoming gas stream but still a lot of supporting energy would be needed. If this energy is made by burning liquefied petroleum gas, more CO2 is created. One kilo of propane creates 3 kilos CO2.Fig.1. Heating energy in thermal vs. catalytic VOC oxidationFig.1. Heating energy in thermal vs. catalytic VOC oxidation

Catalysts lower the activation energy for oxidizing VOCs and thereby the required reaction temperatures. By using a catalyst, only one third of the energy is needed compared to thermal oxidation. The use of catalysts in the oxidation of VOCs has been restrained by the limited durability of traditional oxidation catalysts. These catalysts, which typically comprise pelletized aluminum oxide support and base metal oxide, with or without a small amount of Palladium (Pd) or Platinum (Pt) that offer only 2 to 3 years of service life. Now the situation has changed thanks to the new generation noble metal catalysts, which can tolerate high temperatures and catalyst poisons so that over 15 years of service life has been reached.

Thermal and catalytic oxidation

Oxidation of VOCs needs certain activation energy before the reaction can start. Activation energy depends on how strong the chemical bonds are between Hydrogen (H2), Carbon (C) and other possible atoms. This means the required reaction temperatures vary quite a lot case by case. For instance Xylene, Propanol and Methanol can be oxidized at lower temperatures than Acetone or Heptane (Fig.2). Often the behavior is analogical in thermal and catalytic oxidation but some catalysts are more selective for certain hydrocarbons.

Fig.2. Light off temperature of different hydrocarbonsFig.2. Light off temperature of different hydrocarbons

Thermal oxidation reactions start when the temperature gets above 600oC. Firstly, the reaction rate increases slowly. When the temperature reaches a 800oC reaction rate it speeds up fast. The higher the temperature the more effective and faster oxidation results can be achieved. In thermal VOC oxidation, the maximum temperature must be limited because of formation of Nitrogen Oxidizes (NOx). The higher the temperature the more Nitrogen and Oxygen contained in the air start to react together forming NOx. New EU standards limit NOx emissions to 100 mg/Nm3. This limit can be met by keeping the burning temperature as low as possible. Therefore, in order to achieve proper oxidation of VOCs, the residence time should be long enough, typically 1-1,5s. If there is a lack of temperature and/or residence time, residual emissions start to increase. The first sign is the increased CO emission, which is a result of incomplete oxidation of hydrocarbons. In the EU, CO emissions are limited to 100 mg/Nm3.

A catalyst significantly reduces the activation energy needed to start oxidation reactions (Fig 2). Reactions start already at 150oC and speed up fast until 300oC. After that, the reaction rate increases slower until 800oC to speed up again. In the beginning when the reactions start, their rate is controlled by the chemical kinetics. This means that the slowest step in the heterogeneous reaction chain (Fig. 3) is the reaction on the surface of the catalyst. Above 300oC the situation changes. The reaction rate on the catalyst surface accelerates exponentially, and then the slowest step in the reaction chain is the diffusion of reactants in the catalyst pores, the so-called internal mass transfer. The internal diffusion rate depends on the shape of the flow channel and of the temperature.  In the third phase (ca. 800oC), the reaction speed accelerates. This is because the reactions happen both catalytically and thermally.

Fig.3. Heterogenous catalytic reactionFig.3. Heterogenous catalytic reaction

Fig. 3 shows the whole reaction chain in a heterogeneously catalyzed reaction. The surface of the catalyst contains a porous catalyst carrier layer in which the catalytically active materials are distributed. The micropores of the carrier form a very large surface so that in a liter of a catalyst is a higher surface area than in a football field, which is about one hectare. 

Fig.4. Mass-transfer in different catalyst structuresFig.4. Mass-transfer in different catalyst structures

A new mixer type of substrate has improved the efficiency of catalytic converter when the temperature is above 300oC (Fig.4). In this structure, the Sherwood number is increased from 2.5-3 to 12. This means that the relative mass transfer of molecules from the bulk gas onto the catalyst surface (external mass transfer) increases more than fourfold. It will guarantee high performance even if the catalyst is partly deactivated by poisons like phosphor, zinc, calcium, or sulfur (Fig. 6)

Fig.6. Distribution of poisoning compoundsFig.6. Distribution of poisoning compounds

On the catalyst surface, the reactions happen very fast. Gas residence time is ca. 0.06 s. Catalytic reactions are about 20 times faster than in the case of thermal oxidation. Because of the low operation temperature, no NOx is formed. Despite the shorter residence time, catalytic reactions are more accurately controlled so that the reactions are fully completed. Intermediate products like CO are not created.

Different type of Catalysts

Nearly all cars clean their emissions with catalysts. In the late 1970s, the pellet type of catalysts was used. These catalysts used round aluminum oxide granules activated by base metal oxide or Platinum Group Metals (PGM). Very soon it was found that base metal oxide catalysts could not tolerate high exhaust gas temperatures, nor fuel and lubricant based catalyst poisons like lead, zinc, phosphor, sulfur, calcium, etc. More expensive PGMs could tolerate high temperatures much better and poisons (other than lead) somewhat better. Therefore, it was mandatory to replace lead in the fuel with other additives.

Further improvement was still needed. One reason was that the reactions happen only in the very thin top layer of the catalyzed granules; the effective diffusion depth is only about 0.08 mm. Therefore, less than 20% of the active catalyst material was available to use. The other problem was that granules were grinding in a continuous movement against each other. The solution was to replace the granules with a “honeycomb” type of ceramic or metallic substrates, which were chemically coated. The thickness of the catalytically active coating layer was not more than what could be utilized. Coatings were catalyzed with PGMs. Coating layers after that were under effective development so that this kind of catalysts can today survive in most cases the entire service life of the car and oxidize the VOCs emitted by cars.

The same trend can be seen in the industrial VOC catalysts. Similar “honeycomb” type catalysts as used in cars will replace pellets. Catalysts activated with base metal oxides (Cu-,Co-,Cr-,Mo-,Fe-, etc.) have to be changed every 2-3 years in industrial applications. They can be very active in fresh conditions in some applications but lose activity quite soon because the metal oxides can react with other materials and they do not tolerate higher temperature peaks. Pellet catalysts activated with base metal oxides have spoiled reputation of catalysts in industrial VOC applications.

In oxidation catalysts, the commonly used active metals are Platinum (Pt) and Palladium (Pd), which are dispersed in very small particles all over the huge coating surface. Today’s sophisticated coatings layers include optimized carrier structure, stabilizers, and promotors. After many decades of extensive research work, these offer, when used in the right way, a rather long service life (>15 years). The higher the PGM loading the more active and durable the catalyst is. A summary of the comparison between the base metal oxide catalysts and the PGM catalysts is shown in Fig. 5.

Fig.5. PGM vs. Base metal oxide catalystFig.5. PGM vs. Base metal oxide catalyst

Catalyst poisons vs. thermal oxidation

A catalyst can be deactivated with catalyst poisons. The strongest poisons are organic silicon, heavy metals, and phosphor compounds when they penetrate inside catalyst pores in gaseous form and react there. The reaction products such as silicon, heavy metal and phosphor compounds cannot diffuse out of the pores like gaseous compounds. This starts to deactivate the catalyst.

The poisoning of the catalyst was studied of a catalyst that was used 223,000 km on the road. The catalyst was split in 25 mm long pieces and poisons were analyzed. The results are shown in Fig.6. In the first 25 mm zone, the highest amount of poisons was found. In second zone, fewer poisons were found. The two last zones were nearly clean. We need to keep in mind that in car catalysts thermal degradation has simultaneous essential role because temperature can reach 900oC. Operation temperature in the first zone was increased to 450oC. The catalyst passed emissions tests in the car well before post mortem analyses.

The same happens in industrial VOC oxidizers. Possible poisons are collected in the frontal area. The rest remains clean. This means that catalysts tolerate a limited amount of catalyst poisons. However, in VOC catalysts the thermal deactivation does not play any role, because the temperature is below 600oC.

Even in case the catalyst of a VOC oxidizer is poisoned it can be washed and its original activity regained. Washing is made by dipping the catalyst in alkalic and acidic solutions. It takes only one day and it is relative easy to do.

In thermal burner silicon, heavy metals and phosphor cannot poison anything but these can cover and block the top layer of the heat exchanger if the content of these poisons is high. In the worst case, the top layer of the heat exchangers must be changed yearly.

Particles in thermal and catalytic oxidation

Both oxidizer types are limited by the content of incoming non-organic particles to the level of 1-3 mg/Nm3. The target is to avoid potential blocking of the heat exchangers and the catalysts. This risk is higher if particles contain glue, tar, etc. Organic residuals can be removed through bake-out in both catalytic and thermal oxidizers.

For the catalyst, a small amount of particles, which are bigger than the pore size of the catalyst support (typically 0.02 microns), is not a problem. That is because these particles cannot penetrate the active sites of the catalyst (Fig. 7). 

Fig.7. Particle blogging of a catalyst surfaceFig.7. Particle blogging of a catalyst surface

Chlorinated VOCs

Destruction of Chlorinated VOCs imposes several problems. Both thermal and catalytic oxidation are principally possible. However, oxidation must be done properly so that dangerous side products like dioxins or furans are not created. In addition, Hydrochloric acid (HCl) is formed in the oxidation process. HCl creates corrosive acid with water, which means acid proof materials and the neutralizing of the exhaust gas are required.

In order to achieve the complete abatement of VOCs, thermal oxidation should be performed at high temperatures (>900oC). In catalytic oxidation, a special type of catalyst and elevated temperature (>400oC) are needed. The oxidation behavior of different Chlorinated VOCs and VOC combinations vary. In addition, the emission requirements for different Chlorinated VOCs vary widely.

When you should replace thermal oxidation with catalytic oxidation

The major advantage of the catalytic oxidation is its approx. 500oC lower reaction temperature. It needs much less energy to heat the gas stream up to the operation temperature (Fig. 1). If we take a relatively small VOC application with a capacity of 10,000 Nm3/h, it will need nearly 2,8 MW power to heat it to thermal oxidation temperature and slightly less than 1 MW to catalytic oxidation temperature. If heat exchangers have 90% efficiency, the heat loss is accordingly 280 kW and 100 kW. The difference in cost can be estimated to be €12,6/h and €100,800/a in continuous three shift operation. This is the reason why some existing Thermal Regenerative Oxidizers (RTO) are being considered to be changed to Regenerative Catalytic Oxidizers (RCO) by installing catalysts in burning chambers.

Other advantages of catalytic oxidation are:

    - Very high (99.9%) conversion rate

    - NOx and CO emission is negligible

    - Short residence time enables small sizing

    - Catalysts are valuable after the service life, PGMs can be recycled