Thermal oxidiser efficiency 2

[TerryB]

We are running a natural gas fired thermal oxidiser to destroy solvent laden air from coating process. The solvents include methanol, acetone, THF, MEK, MIBK etc.
Gas consumption is extremely high and we are trying to optimise temperatures at flue stack and oxidation to maximise efficiency.

Has anybody done this or similar and would welcome sharing experiences ?

Thanks
Terry

 

[Fizzhead]

TerryB:

Interesting situation, though certainly not uncommon. I have experience with tuning several of these units and it's not an extremely difficult task. First, I have some questions. What do you mean by "extremely high gas consumption" (i.e., I want to understand a little about the size of the unit and it's consumption)? What kind of heat recovery do you have? AAHX? Packed bed? Type and age of packing? What's the current efficiency of that unit? How old is the unit? How large a gas stream? What destruction efficiency are you targeting? Do you preconcentrate fumes before feeding to the oxidizer? Do you have a CEM? Is it recently calibrated? How recently have you done performance testing on the unit to determine the operating regime and efficiency? As you can see, these are basic questions, but that's where the answer will be found.

 

[25362]

From reading about the subject it appears that there are at least two proven technologies to reduce the fuel consumption and cost:

1. Using catalysts in the retention chamber which would enable reducing afterburner temperatures from about 1000-1200^oF to about 600^oF. Catalysts are said to reduce the LEL of a contaminated stream, so that a stream that wouldn't burn without additional fuel in a simple combustion chamber will burn without additional fuel over a catalyst. The catalyst may be expensive but the fuel savings would be significant.

2. By exchanging heat between the incoming contaminated gas stream and the hot, "cleaned" gas effluent going to stack.

 

[yorkjim]

Hello Group,

From reading TerryB's question, it appears that the process unit is a simple oxidation step - with no heat recovery. If this is true, then high fuel use is expected.

If, as Fizzhead and 25362 imply, there is some type of heat recovery, then more information is needed before optimization steps can be discussed.

But in general - given the type of printing that TerryB describes, a typical oxidizer would be a RTO or regenerative termal oxidizer. In the past the heat recovery media used was random ceramic, such as saddles.Typical design thermal efficiency of such units was 93 - 95%. But even if the design thermal efficiency was 95%, over time random media tends to nest and settle. This both restricts air flow as well as reduces thermal efficiency. I have personally dealt with several dozen such RTOs. And the difference in fuel consumption between 93% and 95% is most likely not insignificant.

Retrofit of a older RTO unit from random to structured heat recovery media often results in:

increased air handling capacity
reduced pressure drop = $ per year power savings
improved thermal efficiency = $ per year fuel savings

and as a final kicker, the structured media will have less depth than the random, so the size of the combustion chamber increases which increases residence time of VOCs at oxidation T, so destruction efficiency actually nudges up a bit, and also longer lifetime can be expected from the structured media

If TerryB's plant simply has some type of older flare technology, then a new RTO, designed around structured heat recovery media, might be easily justified. If fuel costs are $500,000 per year, at 95% thermal efficiency this will go down to $25,000 per year. That kind of savings can easily justify a capital project.

There still tends to be a lot of resistance to RTO technology around the industry - too big, too expensive, hard to maintain - but these impressions were made 5-10 years ago when the only heat recovery medias available were random. In the past 5 years structured media has reduced the size of RTOs and greatly improved their reliability. As a result both intial capital and operating costs of a new RTO are far less than what was common just a few years ago.

Competition in the industry is also keen.

Per Fizzhead and 25362, we need a full description of the process - air flow, type of unit, etc., to go any further.