CGB and HGB also are used to keep the furnace from reaching an excessive temperature, which could cause permanent refractory damage or require specification of a different refractory grade. A typical high-temperature-shutdown set point might be 1,800°F.
HGB gives a higher stack temperature than that for most RTO designs. Special stack construction (stainless or refractory-lined carbon steel) may be required to avoid damage.
During normal operation RTO furnace and stack temperatures vary over a narrow range. This is because when the beds switch the waste gas entering the furnace (and the furnace gas entering the stack), they now are flowing through the alternate bed. The waste gas suddenly is hotter because it’s flowing through the bed recently in contact with the hot flue gas. The flue gas suddenly is colder because it’s flowing through the bed recently cooled by the incoming waste gas. If the beds switch every 5 minutes, the flue gas and combustion chamber reach their “average” temperatures approximately 2.5 minutes after the switch.
Different dynamics In a direct fired incinerator increasing the fuel flow results in hotter stack gas within 5 seconds to 10 seconds. In an RTO increasing the fuel flow to the burner immediately heats the furnace — but the stack gas temperature rise is delayed by the heat absorbed in the packed beds. The higher RTO furnace temperature puts more heat in the packed bed receiving the flue gas. At the end of the cycle that bed is hotter than at the end of the previous cycle; when the waste gas is switched back into it the waste gas temperature entering the furnace will be higher than in the previous cycle. The stack temperature as well as the furnace temperature will swing around an average value as the bed switching proceeds. In fact, a step change in fuel flow may require several cycles of bed switching to reach stable average stack and furnace temperatures.
This type of delayed response happens with any variation in RTO operation, including changes to waste gas flow, waste gas hydrocarbon content, burner fuel or air flow, and CGB or HGB flow.
As we all know, every pound of waste gas, air or fuel gas entering must be matched by a pound of flue gas out the stack and every Btu entering the RTO (as sensible heat due to a heated waste gas or as hydrocarbon heat release) must show up as a Btu in the stack gas or as heat loss through the vessel shell. What happens in the packed beds or the switching valves is important for saving fuel but, taken as a whole, “what goes in has to equal what comes out.” So, for instance:
• If it’s impossible to feed enough air to produce around 3% oxygen in the stack gas, then the RTO can’t operate as intended.
• If the measured stack temperature is higher or lower than predicted by the Btu balance, then some other input isn’t being considered — maybe the waste gas is richer or leaner than expected.
Given a specific waste gas, air flow, fuel gas flow and heat loss through the vessel refractory and insulation, you can calculate the stack gas flow, composition and temperature even if nothing is known about the bed packing, switching times, bypass flow or any other detail.
RTO design involves eight steps:
1. Perform a heat and material balance on the waste stream, including minimum/maximum flow, minimum/maximum hydrocarbon load, etc. Determine if any of the cases excludes use of an RTO — for instance, is the waste-gas hydrocarbon load so high that a different type of incinerator would make more sense?
2. Specify the packing types and amounts, along with the bed switching times to achieve the heat recovery efficiency needed for all operating cases. Packing vendors can provide these calculations for their products.
3. Size the combustion chamber, stack, inlet ducting, any bypass ducting, etc.
4. Size and specify the waste gas blower, fuel gas burner and combustion air blower.
5. Specify the type and amounts of refractory lining and external insulation.
6. Prepare the process and instrumentation diagram and process flow diagram(s).
7. Put together specification sheets for purchase of blowers, burners, instruments, etc.
8. Prepare fabrication drawings, parts lists, operating instructions and other documentation.
An Attractive Alternative
An RTO provides the highest fuel efficiency of any type of waste-gas thermal oxidizer and, thus, may allow you to cut costs for incineration. However, it isn’t best for all services, so you must understand its limitations. When an RTO is the right choice, you must then consider its particular design and operational issues.
Avoid Common RTO Mistakes
Most RTO problems stem from incorrect application of the technology:
• Waste-gas heating value is higher than expected. The RTO overheats without major changes to the system (e.g., removing packing or installing hot or cold bypass). This may cause premature ignition of waste or even flashbacks to the process.
• Waste-gas heating value is lower than expected. RTO fuel usage will be high. The fix requires addition of heat exchange packing, which may be limited by chamber dimensions.
• Waste gas has unexpected particulate matter. Simple dusts will block gas passage through the heat exchange medium but can be vacuumed off. Reactive particles can bond to the medium, ruining it and, thus, requiring bed replacement. Combustible particles may collect in the medium and light off, causing thermal damage. Waste gas filters might be needed.
• Waste gas flow is greater than expected. Pressure drops through the RTO system may require larger blowers or different heat exchange medium.
• Waste gas flow is intermittent, with rapid startups required. From a standing start an RTO system can require several hours of heat-up. Abrupt flow or heating value changes cause temperature excursions and higher stack emissions.
Other RTO problems involve operating and maintenance practices:
• Nuisance shutdowns tempt operations to use automatic restart logic. Stay with manual restarts to catch potential safety problems and avoid disaster.
• Maintenance takes shortcuts. Using the wrong type of replacement thermocouple, leaving access doors loose (leaky) and other casual mistakes can result in elevated emissions, thermal damage or worse.
• Staff doesn’t pay due care with the waste-gas gathering system. Spilling solvent under a process vent collection hood can convert the low-Btu waste gas into a high Btu hazard for the RTO.
• Seasonal plant operating changes are ignored. Fuel usage can creep up. So, review RTO bed switching times. They affect heat recovery and fuel usage.
• The refractory lining is neglected. It doesn’t last forever. Look for developing hot spots so you can schedule repairs to avoid unplanned shutdowns.
Dan Banks is principal consultant for Banks Engineering, Inc., Tulsa, Okla. You can e-mail him firstname.lastname@example.org.