In the past several years, natural gas cost for an East Coast plant has increased from a typical value of $2.50 per million BTU to $10 per million BTU. Because the thermal oxidizer incineration temperatures must remain constant, fuel costs for air pollution cleanup have increased proportionately.
The plant has four thermal oxidizers operating at an incineration temperature of 1,500°F (815°C). In the interest of energy conservation, the company asked one oxidizer maker to look into methods of reducing the fuel usage for these thermal oxidizers. A thorough study of the application showed that there were two ways to reduce oxidizer fuel use and still meet the air pollution control requirements:
- Install a standard heat exchanger.
- Install an auxiliary catalytic assembly.
The best efficiency is obtained when the two effluent flows are equal. A typical effluent flow pattern is shown in figure 1 using values from the energy savings study. By using the 1,500°F exit temperature from the thermal oxidizer to heat the 350°F (177°C) effluent flow from the process to 1,024°F (551°C), the firing rate could be reduced by 674°F (374°C).
Because the plant’s air quality permit specified a 1,500°F incineration temperature to achieve a 98.5 percent destruction ratio, the heat exchanger specifications called for T309 stainless steel, which is rated at 2,000°F (1,093°C).
Consideration was given to using one large heat exchanger instead of four individual heat exchangers. This path was abandoned for two reasons. First, there would be times when only one thermal oxidizer would be online, and having only one oxidizer online would affect heat exchanger performance. Second, the processor decided against this arrangement because when the tubes were cleaned, all four processes would have to shut down.
Although one of the advantages of a heat exchanger is that it is a passive device with no moving parts, heat exchangers produce a measurable pressure drop. In this case, it was 5.4" w.c. This had to be overcome by an extraction fan.
Because the process under study was generating particulate matter as well as volatile organic compounds (VOCs), the style of heat exchanger considered was a shell-and-tube so that the tubes could periodically be cleaned. The heat exchanger tube-cleaning process involves removing sections of inlet and outlet duct as well as the two heat exchanger end plates. Then, using brushes or high pressure water, the buildup on the inside of the heat exchanger tubes is removed.
Another way is to leave the thermal oxidizers in place. Then, by inserting a high temperature diverter gate between the existing thermal oxidizers and their outlet stacks, the effluent is sent to an auxiliary catalytic assembly when the gates are switched to the catalytic position (figure 2). The plant’s existing thermal oxidizers still would be used to raise the temperature of the effluent, but because of catalytic action, the incineration temperature would be lowered to 625°F (330°C) while still meeting the required 98.5 percent destruction ratio. And, unlike a typical catalytic oxidizer, the auxiliary catalytic assembly does not have a burner, safety gas train, combustion chamber or mixing chamber.
In the process under study, it was recommended that on a scheduled basis, the catalytic blocks be removed for inspection and cleaning with compressed air. The catalytic blocks are monolithic, round, honeycomb construction (figure 3). With this construction, the surface area and activity level remains constant. Those catalytic assemblies, which are composed of some form of pellets, agglomerate with the system vibration and reduce total surface area and activity level.
An additional controller also is installed to perform the following duties:
- Monitor the incoming temperature to the auxiliary catalytic assembly to be sure the temperature never exceeds the thermal degradation temperature of the catalyst.
- Monitor the on/off status of each process so the extraction fan can be set for the proper volume.
- Monitor the position of the diverter gates.
- Monitor the pressure drop across the catalyst bed.
- Monitor the catalytic exit temperature so the system exotherm can be calculated.
For the application, the advantages of the auxiliary catalytic assembly vs. four heat exchanges were:
- Installed project cost recovery time was 4.6 months for the catalytic assembly vs. 28.7 months for the heat exchanger system.
- There was a major reduction in piping temperatures for the auxiliary catalytic assembly vs. the heat exchanger system.
- The piping arrangement for the auxiliary catalytic assembly is much simpler than for the four heat exchangers.
- Flame generated NOX is lower for the catalytic assembly due to the reduction in oxidation temperature from 1,500°F to 625°F.
- Total weight of four heat exchangers and fans was calculated to be 13,800 lb vs. 3,100 lb for a single auxiliary catalytic assembly and extraction fan.
- When service is needed on the catalytic assembly, the diverter gates are switched to the existing stacks and there is no downtime. When the heat exchanger tubes need to be cleaned, the thermal oxidizer must be shut down for 6 to 8 hours.
For some applications, the auxiliary catalytic assembly may provide a method of reducing energy usage of an oxidizer. Before changing from thermal incineration to catalytic incineration, it must be verified that the effluent stream does not contain any of the known precious metal catalyst poisons.
In this application, the temperature reduction was 825°F (458°C). This temperature reduction may seem on the high side. However, typically going from thermal incineration to catalytic incineration for the same destruction ratio will give temperature reductions of 600 to 800°F (330 to 440°C).