The Advantages of Catalytic Incineration
GENERAL INFORMATION
Destruction of violatile organic compounds and particulate matter can be accomplished by incinerating them either with a conventional thermal oxidizer, or when applicable, with a catalytic oxidizer.
In either case, the factors that have to be taken into account when designing a system are known as the three "T"'s. They are: TEMPERATURE, TIME, and TURBULENCE.
The first step in the process of incineration consists of raising the temperature of the pollutants to that temperature at which the heat driven chemical reaction will convert hydrocarbons into carbon dioxide and water vapor.
The general equation is: 4H + 1C + 30 ---> 2H2O + 1CO2
For this reaction to go to completion in a thermal oxidizer, the temperature must be in the range of 1,000°F to 2,100°F.
In the presence of a suitable catalyst, the temperature range is usually reduced to between 500°F and 1,000°F.
The second factor in straight thermal oxidation is the residence time at the elevated temperature. Again the advantage lies with the catalyst process.
Turbulence is needed for mixing the cold air containing the pollutant with the heated air from the burner. The turbulence factor is the same for thermal and catalytic incineration.
CATALYTIC INCINERATION TEMPERATURES
Tests run on several common industrial chemical compounds showed a significant temperature advantage for the catalytic process over the straight thermal process as shown in the following comparison with all temperature in Farenheit:
| CHEMICAL COMPOUND | CATALYTIC INCINERATION | THERMAL INCINERATION |
|---|---|---|
| Benzene | 440° | 1,460° |
| Carbon Tetrachloride | 610° | 1,430° |
| Methy Ethyl Ketone | 600° | 1,780° |
| Cyanide | 480° | 1,800° |
| Data courtesy of Manufacturers of Emission Controls Association | ||
THE LIGHT OFF CURVE
A plot of destruction efficiency vs. temperature generates a curve which is asymptotic to the 100% line. In general, the higher the temperature the better the destruction efficiency up to this point. After the light-off curve exceeds the 98% value, raising the temperature produces smaller and smaller improvement in the destruction of efficiency.
Bureaucrats being bureaucrats, local Air Quality Districts will frequently try to impose an incineration temperature in a catalytic oxidizer permit. Determination of the best catalytic incineration temperature is a complex chemical analysis generally beyond the capabilities of any Air Quality District. Our principal is to follow the temperature value as supplied to us by our catalyst supplier.
Whenever we are asked to design a catalytic oxidizer for an effluent stream with chemical compounds which we have not processed before, we send the information to the technical personnel at Johnson Matthey, who then make the temperature determination for us.
Below is a typical light-off curve:
THE TURBULENCE FACTOR
The better the turbulence, the better the mixing of the pollutant laden effluent with the products of combustion. Here, there is no advantage of catalytic incineration over straight thermal incineration.
Turbulence is a matter of mechanical design. Conversion Products has spent considerable time and experimentation to achieve the best possible mixing of the two streams.
The easiest way of testing the effectiveness of the mixing design is to take a temperature traverse which should yield a dispersion of not more than + / - 3% of the target incineration temperature.
THE CATALYST BED
Prior to 1990, the product line at Conversion Products had been limited to straight thermal oxidizers with refractory linings. In the early 1990's, we investigated the catalysts available to us for use in what was then our primary customer base- food and chemical processing.
For the catalyst bed, we standardized on the catalyst manufactured by Johnson Mattthey for the following reasons:
- A very high ratio of surface area to catalyst volume which gives us the highest destruction ratios with the minimum of catalyst space.
- Rigid monolithic construction which means that the amount of catalyst surface does not change due to vibration or other mechanical actions which can occur with any pellet type catalyst.
- Since a significant number of our applications are based on a round construction, Johnson Matthey is capable of fabricating round catalyst blocks up to our maximum round size.
- Excellent support help in designing actual applications.
- Johnson Matthey also supplies small test plugs coated under the same conditions as the main catalyst bed. We have always been sucessful in having local Air Quality Administrations accept a Johnson Matthey laboratory report on the activity level of the test plugs in place of an expensive source test to prove that the catalyst is still performing at an acceptable level.
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SYSTEM DESIGN
Before designing our first catalytic oxidizer, the principals of Conversion Products spent a week with our catalyst supplier's technical personnel to determine the best design practices. We felt that this was necessary because there had been so many failures of catalytic installations in the past.
The principal factors leading to catalytic failure are:
- Insufficient knowledge of the make up of the effluent stream to be processed. This led to installations where the effluent stream contained chemicals which were incompatable with catalytic incineration. This invariably led to catalytic poisoning with its attendant catalytic failure.
- Poor mechanical design which usually resulted in incomplete mixing and a temperature gradient entering the catalyst that was so large that portions of the entering stream were below the point on the light off curve where effective destruction could take place. Therefore, the mixture leaving the oxidizer had an average destruction ratio which was below specifications.
- A major factor which still leads to catalytic failure is poorly designed control systems. Fortunately, one of the principals of Conversion Products had come from one of the high-tech companies in Silicon Valley, who, in addition to having experience in the design of control systems, had a background in applied mathematics. This permits Conversion Products to include statistical quality control equations in the software for excellent control.
On the hardware side, our control systems include measurements of temperature rise and pressure drop across the catalyst bed. This information is used in monitoring catalyst performance as well as predicting when a catalyst bed need's service.
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MECHANICAL DESIGN FEATURES
In the 1990's, in addition to researching catalyst suppliers, we also investigated other methods of construction. This research led to a change from carbon steel bodies with refractory linings to maintenance free stainless steel bodies with ceramic lining.
The change from refractory to ceramic had three advantages. First, there was a major reduction in weight when we changed from 120 pound/cubic foot refractory to 8 pound/cubic foot ceramics.
The second advantage was that we could now guarantee a surface temperature that would not exceed 125° F when not in direct sunlight. To make this temperature guarantee using refractory would have resulted in monstrous sizes when dealing with those applications requiring high incineration temperatures. The alternative was to use a refractory lining with an insulating layer between the lining and the outside shell. This accomplished the temperature goal but significantly raised the manufacturing cost.
Now we can offer a safe surface temperature at a much lower cost compared to the refractory/insulation method.
The third advantage is a reduction in maintenance. Eventually, all refractories will crack and crumble and have to be replaced. If the oxidizer uses steel anchors, the cracking is accelerated because the coefficient of expansion of the steel is greater than that of refractory. Consequently, every cycle of heat up generates stress in the refractory leading to its eventual destruction.
Our ceramic insulation has a zero coefficient of thermal expansion.
With our current ceramic insulation, we have installations with over 15 years of service and no indication of failure.
With the lower temperature due to ceramics, our oxidizers can be installed anywhere, including for instance, coffee shops where the proprieter roasts coffee on site, and because of environmental regulations, needs to abate the smoke, odor, particulate matter, and/or violatile organic compounds. We have many installations where our polished stainless steel oxidizer stands right beside the coffee shop roaster.
We have developed several proprietary software programs which permit us to design a complete oxidizer in a matter of minutes. This coupled with our basic flexible design and manufacturing method allows us to design every oxidizer to meet the exact requirements of each installation. We do not force the user to take a standard design which may be either too large or too small for the user's space requirements and the destruction standards to be met.
Because of our proprietary design of the combustion/mixing chamber, we have a large latitude in choosing chamber velocity, This gives us the freedom to vary the inside diameter of the oxidizer and it's length so that while still achieving good mixing, we can size the oxidizer so it will fit in the user's available space.
BURNER SYSTEM & COMBUSTION SAFEGUARDS
To maintain the design light-off temperature, a good burner system is required. Also, in todays environment, the burner system needs to be low in NOx and CO.
We supply burners manufactured by Eclipse, Maxon, and North American.
These burner systems are available for natural gas, propane, propane/butane mixtures, and where necessary, oil.These burner manufacturers have standard burners which meet most local district NOx and CO limits. However, in those instances where local conditions require extemely low emission burners, all three of these manufacturers have burners avaliable in a wide range of BTU ratings, with NOx in the 5 ppm to 8 ppm range, and CO in the 2 ppm to 5 ppm range.
Since we are usually dealing with burner systems that are in the multi-million BTU category, combustion safety is madatory.
There are two prevalent standards for combustion safety: NFPA (National Fire Protection Association) and IRI (Industrial Risk Insurers). In following these standards, we supply the follwing combustion safety equipment with our burner systems. The failure of anyone of them will shut down the burner system:
- Low fuel pressure switch.
- High fuel pressure switch.
- Combustion air pressure switch.
- Double block valves.
- Proof of low fire start-meaning that the combustion process cannot start unless the system reports that the valves are guaranteeing low fuel flow on start-up.
- Eclipse/Dungs leak test system placed between two block valves (when required). The leak test system checks both up-stream and down-stream block valves for leakage. This system also eliminates the need for running the vent line from the conventional vent valve to the atmosphere,
- Flame surveillance system.
OPERATOR TRAINING
All of our oxidizers are run at the specified temperature for a minimum of 4 continuous hours. During this test, we record the burner performance by measuring CO, NOx, sulphur compounds, and oxygen content. We also take surface temperature readings so that we have evidence that our surface temperatures are meeting our 125°F target.
We always invite the user to witness these tests and whenever possible, that they bring some of the operating personnel for hands-on training.
Even though our control system has essentially de-skilled the task of running a catalytic oxidizer, it is still necessary for the operating personnel to know how the system works and how to use the information which appears on our touch screen operator interface.
PARTICULATE MATTER
Particulate matter cannot be destroyed by catalytic oxidization. Therefore, if there is particulate matter in an effluent stream which is being destroyed with a catalyst, the particulate matter has to be destroyed before reaching the catalyst bed.
One cause of failure of catayltic oxidizers is a buildup of unburned particulate matter on the face of the catalyst bed which goes undetected. If a lump of particulate matter catches on fire, it will burn a hole in the catalyst bed. This obviously reduces the catalyst effectiveness.
To meet today's much tighter particulate destruction standards, the Conversion Products oxidizers provide a proprietary combustion/mixing chamber which injects the particulate matter into the flame envelope at a high velocity. This velocity is well above the maximum "cross velocity" recommended by the burner manafacturers. However, with our proprietary combustion/mixing chamber design, the flame envelope maintains the standard configuration that it normally has when there is no impingement of the external pollutant stream.
FUEL COST SAVINGS
Because of the lower incineration temperature with a catalytic oxidizer, the burner is smaller and the actual cubic feet per minute will be less than with straight thermal. This means that the body and the burner system are lower in cost with a catalytic than with a thermal oxidizer.
However, most precious metals catalysts have platinum as their major constituent. Platinim is expensive. Presently, platinum is about 25% higher than the cost of gold on the precious metals market. Although the body and burner system are less expensive, the added cost of the platinum catalyst results in a catalytic oxidzer having a higher total capital cost than a thermal oxidizer.
If the destruction ratio is the same, why go to a more expensive catalytic oxidizer?
THE SAVINGS IN FUEL COST.
We have recently completed an oxidizer project for a chemical company which needed to destroy Ethyl Acetate. An analysis was prepared comparing the project cost with a thermal oxidizer vs. the cost with a catalytic oxidizer.
The specifics were:
| Effluent flow rate | 1,200 scfm | |
| Effluent entering temperature | 100°F | |
| Required Destruction Ratio | 99.0% | |
| Fuel cost | $8.50 / MMBTU | |
| Thermal incineration temperature | 1,400°F | |
| Catalytic incineration temperature | 625°F | |
| Increased capital cost for catalytic | $44,500 | |
| Firing rate for thermal | 3.0 MBTU/HR | |
| Firing rate for catalytic | 1.3 MMBTU/HR | |
| Annual hours worked | 3,700 | |
| Thermal Annual cost = 3,700 x 3.0 x 8.50 = $94,350 | ||
| Catalytic Annual cost = 3,700 x 1.3 x 8.50 = $40,885 | ||
| Annual savings in fuel cost = $53,465 | ||
| The recovery period = 44,500 / 53,465 x 12 = 10 months | ||
The decision was made to go with a catalytic oxidizer.