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Regenerative thermal oxidation

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Regenerative thermal oxidation systems (also called thermoreactors) are based, as in recuperative systems, on subjecting the gases to a sufficiently high temperature to produce the oxidation of the organic compounds. But they are characterized by including some devices, named regenerators, that recover the heat of the cleaned gases. These regenerators are elements of ceramic material that accumulate the heat of the gases that leave the oxidation chamber. Through a system of valves, consecutive operating cycles are established by which the cleaned gases, which are at an elevated temperature (about 800 ºC), transfer their heat to the ceramic meterial so that the contaminated gases, which enter the equipment cold, take from them this heat in the next cycle.

The main features of these equipment are:

  • Minimum fuel consumption, since it allows very high heat recovery efficiencies (greater than 95%)

  • Very low operating and maintenance costs

  • High cleaning efficiency

  • Long-lasting equipment

  • Great reliability, due to well-proven equipment



Regenerative thermal oxidation plants with ceramic material are widely proven in cleaning waste gases pulluted with organic compounds.

Operation

The thermoreactor consists of three canisters filled with ceramic elements communicated by its upper part through the oxidation chamber. The
three recovery canisters work in a cyclic way to achieve heating and subsequent cooling with an energy recovery efficiency of 95% ± 2%.

The gaseous effluent to be cleaned is soaked up by means of a centrifugal fan that can be at the inlet or outlet of the equipment. When placed at the outlet, the equipment works in depression, eliminating the potential danger of hot gas leaks. These gases will pass alternatively through the three canisters filled with heat accumulator ceramic material.

The gases to be cleaned enter through the first tower whose ceramic material has been heated in a previous cycle. The heat accumulated in the material is transferred to the gases so that they increase their temperature until, when they reach the top of the canister, they have reached a temperature close to that necessary to achieve their complete oxidation (about 800 ºC).

Operating scheme of a thermoreactor.

In the oxidation chamber, one or two burners are installed (depending on the size of the equipment) that will be responsible, when necessary, for providing the necessary energy for the gases to reach the oxidation temperature. The oxidation of the organic compounds is exothermic, and the energy consumption in the burners will depend, therefore, on the concentration of pollutants in the gases to be purified: For a higher concentration of organic compounds, the energy consumption in the burners will be lower, being able to be null when the concentration is above 1.7 g/Nm³.

To achieve the complete thermal oxidation of the compounds contained in the gases to be cleaned, they remain at least for 0.6 seconds in the oxidation chamber, which is at a high enough temperature (between 760 and 820 ° C).

Operating scheme of a thermoreactor. Click here to enlarge.

 

The hot gases leaving the oxidation chamber passe through the second ceramic canister. Initially, this canister will be cold and will trap the heat contained in the purified gases that, in turn, will cool before leaving the bottom of the canister and will be sent to the stack.

While canisters 1 and 2 work in the cleaning of the waste gas, a third canister is in purge mode, sweeping with clean gas the waste gase that has not reached the oxidation chamber in a previous cycle. This way, emission peaks of pollutants are avoided at each cycle change.

When the first canister is cold and the second is hot, a change of valves is carried out so that the gas is entered through the hot canister (2), which will transfer the accumulated heat to the gas to be cleaned, leaving through the cold canister that has been purged in the previous cycle (3) and canister 1 will enter in "purge" mode to drag the untreated gases that have remained in the holes of the ceramic filling and in the lower part of the canister.

The time that elapses in each change of valves oscillates between 1,5 and 2,5 minutes. The less time it takes to change the position of the valves, the greater the energy recovery, but the more frequent changes in the direction of the gas will have to be and the greater the wear on the valves. Normally, the temperature of the gases at the thermoreactor outlet is about 40 to 50 ° C higher than the inlet temperature.

The last position of the cycle would be the entry of gases through canister 3, exit through 1 and the tower 2 would be in purge mode.

The ceramic material used as a filler has undergone an evolution in recent years seeking, above all, to reduce the pressure loss of the gases as it passes through the thermoreactor. In the first thermoreactors, the canisters were filled with ceramic elements in the form of "saddle" and the pressure drop was around 5,000 Pa. Subsequently, fillings called "Honeycomb" were used, which have the disadvantage of being rigid and with little capacity to expand. Lately, fillings called "MLM" ("Multi Layer Media") are used, which are formed by plates without joining each other (without problems of dilation) that significantly reduce the pressure drop (decreasing about 2,500 Pa) and facilitate the distribution of the gas throughout the volume of the bed. This reduces the operating cost, because the necessary power in the fan is reduced.

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Saddle

Honeycomb

Multi Layer Media

 
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