Regenerative thermal oxidizers, also known as RTOs, are used to control volatile organic compounds (VOCs) emitted by industrial processes. As a general rule, RTO technology has been successful with most installations, operating trouble-free for extended periods. In some cases, however, operation has been troublesome, and a good portion of these problem applications has been on biomass dryers such as dryers used in ethanol production, wood dryers and sewage sludge dryers. This article addresses why some regenerative thermal oxidizers have problems and how to avoid them.
Regenerative thermal oxidation technology is a simple way of preserving the temperature needed to oxidize VOCs. In an RTO, VOC-laden gas is routed into a heat recovery chamber that is filled with ceramic media (figure 1). By passing through the inlet heat recovery chamber, the emission stream is preheated to a temperature very near the combustion chamber temperature. In the combustion chamber, a natural gas burner maintains the temperature to approximately 1,500°F (~815°C), the temperature required for complete thermal oxidation.
After a prescribed period of time -- typically, two to six minutes -- the gas stream is reversed. This back-and-forth, regenerative operation allows the RTO to recover up to 95 percent of the heat generated in the combustion chamber, which minimizes fuel costs.
As a general rule, a properly designed RTO unit can operate continuously for extremely long periods of time without undue downtime or significant maintenance. While there are many regenerative thermal oxidizers operating in this manner well into their second decade, some cannot. The question is, why?
The Influence of InputsWith any regenerative thermal oxidizer system, remember this: System inputs define RTO operation.
In the design of a regenerative thermal oxidizer, there are three inputs to consider: fuel, the VOC-laden gas stream, and particulate. Some may say that in many applications, there is no particulate, but that is not entirely correct. There is always some particulate matter in an emission stream; the quantity may be negligible -- as in ambient air -- but it is always present.
Of the three RTO inputs, the first two do not vary enough to affect operation unpredictably. Specifically, the normal fuels -- natural gas or propane -- do not vary enough to affect operation. Likewise, while the VOC concentration in the gas stream does vary, process upsets due to excessive VOCs can be accounted for in the oxidizer design. The oxidizer builder simply allows for the necessary operating flexibility in the design of the RTO system with regard to dilution air, hot or cold side bypass, process monitoring, and other tools at his disposal.
The presence of particulate is another matter. Particles in the gas stream are the biggest threat to efficient regenerative thermal oxidizer operation as they can lead to bed fouling and even bed degradation, as well as fires. Among all of the VOC-emitting processes, biomass dryers are particularly prone to such problems because of the many ways biomass drying can generate particles.
Particle Sources and EffectsTo deal with particulate effectively, one must understand the various sources of particulate, the problems associated with each source, and potential approaches to minimizing or eliminating problems caused by them. These concepts will be demonstrated using a biomass drying model, but the principles hold true for all regenerative thermal oxidizers.
Coarse Particles.Identified as those particles greater than 5 micron, coarse particles originate exclusively from mechanical means such as the tumbling or pneumatic action of a dryer. Examples are dust from a fiberboard dryer or liquid droplets downstream of a scrubber. Typically, particles of this origin impact on the cold face of a regenerative thermal oxidizer’s media bed and cause media bed plugging (figure 2). If left unabated, this buildup can become a fire hazard.
Fine Particles.Identified as those with a diameter less than 1 micron, fine particles are almost exclusively caused by thermal processes. In other words, particles in this size range are formed when a vapor cools and condenses into a particle. The resulting particle can be either solid or liquid, depending on its chemical makeup. Common examples of liquid fine particles are condensable organic compounds such as oils or resins. Examples of thermally generated solid fine particles are metal fumes such as iron or potassium oxide.
Fine particles can be chemically inert or reactive. If they are chemically inert, the chief problem in regenerative thermal oxidizers is the potential to plug the heat exchange media. An example of a chemically inert fine particle that can plug an RTO is the silicon dioxide that comes from burning VOCs that contain silicon such as silanes or chlorosilanes (figure 3).
Chemically reactive fine particles also cause plugging. However, they have an additional deleterious effect in that they tend to attack the heat exchange media in the oxidizer. Examples of chemically active fine particles are sodium and potassium oxides. These react with the internals (stoneware) of the regenerative thermal oxidizer at high temperatures and cause embrittlement of the media with attendant crumbling and bed plugging.
Liquid fine particles generally evaporate as they penetrate deep into the oxidizer media bed. There, the organic matter will return to the vapor state, where it can be burned in the regenerative thermal oxidizer. Liquid fine particles found in RTO applications normally are not chemically active.
Solutions to Particulate ThreatsThe most important parts of an RTO design effort in applications with significant amounts of particulate matter is first recognizing the threat and then characterizing the type and concentration of the particulate matter. Once these steps are taken, picking the right solution to the problem is relatively easy, if not inexpensive. The following are some general guidelines for the particulate threats you may face.
Coarse Particulate.Low efficiency upstream collectors such as low energy wet scrubbers or properly designed centrifugal collectors (cyclones or multiclones) can greatly reduce or eliminate problems that may be caused by coarse particulate. However, if wet scrubbing is selected, designers must make sure efficient mist eliminators are used; otherwise, one coarse particulate problem could be replaced by another.
Another approach that may be tried is the use of alternative heat exchange media. If the coarse particulate is combustible, as in many biomass drying situations, then the use of open-cell structured media at the bottom of the media bed can be employed to allow the particulate to penetrate deep into the bed, where it will burn.
Chemically resistant media such as high alumina may be appropriate for situations where the particulate is reactive such as with ash from direct-fired biomass dryers. Care should be taken in selecting this option because even the most expensive chemically resistant ceramics may have limited life if the particulate loading is too high.
If the fine particulate matter is condensable, heating the gas stream to re-volatilize the condensable matter so that it enters the regenerative thermal oxidizer as a vapor also can control the problem. Alternatively, if the condensable matter tends to leave a residue on the RTO cold face as it evaporates on the warm surface, then regenerative thermal oxidizer bake-out protocols can be employed.
In conclusion, know your enemy! Defining the particulate is the first step in ensuring trouble-free, long-term RTO operation. Once the input particulate is characterized and quantified, oxidizer users can develop the upstream gas-cleaning strategy that provides the optimum level of cleanliness.