Regenerative Thermal Oxidizers - RTOs
Vapor Combustion Equipment
Process Description
For the control of low concentration hydrocarbon contaminated air streams, RTO’s offer unmatched fuel efficiency. The energy of the hot combustion products is used to preheat the incoming waste gas. Unlike Recuperative Thermal Oxidizers that perform this function using an indirect contact heat exchanger (shell-and-tube or plate-plate type) to preheat waste gases or combustion air, the RTO uses a direct contact heat exchanger. These direct contact heat exchangers consist of a bed of porous ceramic packing or other structured, high heat capacity media.
An RTO uses at least two media beds and a common flue gas furnace (residence time chamber). Prior to receiving vapors, a small burner in the furnace section is fired to heat the start-up air. The start-up air flows through Bed No. 1 into the furnace and is then to discharged through Bed No. 2 and onto the atmospheric stack. During this process, Bed No. 2 becomes hot, eventually reaching the desired outlet temperature. The inlet side of Bed No. 2 normally reaches temperatures in the range 1600ºF (900°C). When the desired temperature is reached, automated valves switch, and the start-up air is replaced with the contaminated air stream taking the reverse path through the two beds and furnace. The automated valves direct the treated gas out of Bed No. 1 to the common stack. As the contaminated air passes through Bed No. 2, it is heated as it flows across the hot packing media. The furnace burner is now firing at very low rates. The furnace burner may be unnecessary if the hydrocarbon content in the vapor stream is sufficient to provide heat of combustion adequate to make up for heat losses of the equipment. Once the outlet temperature of Bed No. 1 reaches a desired level, the valves will switch again and directs the contaminated air in the original direction. Cyclical operation proceeds automatically as long as contaminated air is present.
Valve switching may occur every few minutes, depending on the bed size, flow rate and concentration of the contaminated air stream. With careful design, the destruction efficiency of the hydrocarbons in the contaminated stream can be in excess of 99%. Since the thermal efficiency of this design can easily exceed 90%, supplemental fuel consumption can be extremely low. The thermal efficiency of the recuperative design is in the 70% range. If the hydrocarbon concentration of the contaminated air increases, dilution air must be added to the stream before it enters the RTO to avoid excessive temperatures in the furnace. Due to bed volume requirements, an RTO is likely to need more plot space than a traditional Enclosed Thermal Oxidizer.