Know Your Enemy

Understanding particulates smooths the operations of regenerative thermal oxidizers

Regenerativer thermal oxidizers (RTOs) that control volatile organic compounds (VOCs) emitted by a wide variety of industrial processes are widely accepted. As a general rule, RTO technology has been very successful with most installations, operating trouble-free for extended periods. In some cases, however, operation has been troublesome, and a good proportion of these problem applications have been on biomass dryers. Biomass dryers include wood dryers, sewage sludge dryers, and dryers used in ethanol production. This article addresses why some RTOs have problems and how to avoid them.

RTO Basics 101
Regenerative thermal oxidation technology is a simple way of preserving the temperature needed to oxidize VOCs. It works like this:
As shown in the diagram below, VOC-laden gas is routed into a heat recovery chamber that is filled with ceramic media. 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 degrees Fahrenheit (the temperature required for complete thermal oxidation).

Upon exiting the combustion chamber, the emission stream enters the outlet heat recovery chamber. The gas stream passes through the outlet heat transfer media bed where the heat energy gained from the inlet heat recovery chambers and combustion chamber is transferred to the ceramic heat exchange media (heat sink). This is the final step in the regenerative process. Typical discharge temperatures from RTO systems are approximately 75 degreees Fahrenheit above the inlet temperature. Finally, the emission stream exits the RTO system through the outlet diverter valves and is transferred to the stack via the induced draft fan.

After a prescribed period of time (2 to 6 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 to greatly minimize 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 RTOs operating in this manner well into their second decade, some cannot. The question is why?

The key is to understand the importance of system inputs. Inputs define RTO operation. There are three; fuel, VOC-laden gas stream and particulate. (One 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 are not much of a factor. The normal fuels, natural gas or propane, do not vary enough to affect operation. The VOC concentration in the gas stream does vary but process upsets, due to excessive VOC, can be accounted for by allowing necessary operating flexibility in the design of the RTO system, i.e. dilution air, hot or cold side bypass, process monitoring, etc.

Particulates are another matter. Particles in the gas stream are the biggest threat to efficient RTO operation as they can lead to bed fouling and/or degradation and 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.

Using a biomass-drying model, we can explore the various sources of particulates, the problems associated with each, and potential approaches to minimizing or eliminating problems caused by them.

Coarse particles
Coarse particles are particles greater than 5 microns. Their origin is exclusively mechanical, such as in 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 an RTO media bed (bottom) and cause plugging of the bed. If left unabated, this build-up can also become a fire hazard. Photographs of build-up on RTO cold face surfaces caused by such coarse particulate are shown below.

Fine Particles
Fine particles are those with a diameter of less than 1 micron. They 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 resultant particle can be either solid or liquid, depending on its chemical make-up. Common examples of liquid fine particles are condensible organic compounds such as oils or resins. Examples of thermally generated solid fine particles are metal fumes such as iron or potassium oxide.

Particles in this size range appear as the familiar blue haze that is often seen coming out of biomass dryers. In the case of liquid fine particles, these come from the evaporation of organic material in the dryer and the resulting cooling of the exhaust. Solid fine particles have their origin in the heat source where ash in the fuel vaporizes in the flame and condenses as it leaves the flame front.

Fine particles can be chemically inert or reactive. If they are chemically inert, the chief problem in RTOs 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. A photograph showing the plugging effect of inert, fine particles is shown below.

Media Plugging by Fine Particulate
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 an RTO. Examples of chemically active fine particles are sodium and potassium oxides. These react with the internals (stoneware) of the RTO 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 RTO media bed. Thus, the organic matter will return to the vapor state where it can be burned in the RTO. Liquid fine particles found in RTO applications are normally not chemically active.

Solutions to Particulate Threats
The most important part of an RTO design effort in applications with significant amounts of particulate matter is first recognizing this threat and then characterizing the type and concentration of the particulate matter. Once this is done, picking the right solution to the problem is relatively easy, if not inexpensive. The following are some general guidelines for the differing particulate threats.

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.

Fine particulate
Fine particulate matter, whether inert or reactive, presents a more difficult problem. Because of their fine size, removing these particles requires the use of sophisticated gas-cleaning equipment. This means fabric filtration, electrostatic precipitation, or high energy wet scrubbing. The choice depends on the nature of the gas stream.

Chemically resistant media such as high alumina may also be appropriate for situations where the particulate is reactive; e.g. ash from direct-fired biomass dryers. Care should be taken in electing 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 condensible, heating the gas stream to revolitalize the condensibles so that they enter the RTO as a vapor can also control the problem. Alternatively, if the condensibles tend to leave a residue on the RTO cold face as they evaporate on this warm surface, then RTO bake out protocols may be employed.

To be successful, understand your adversary. Defining the particulate is the first step in ensuring trouble-free, long-term RTO operation. Once the input particulate is characterized and quantified then develop the upstream gas-cleaning strategy that provides the optimum level of cleanliness.

This article originally appeared in the 07/01/2007 issue of Environmental Protection.

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