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Heat Recovery From Sooted Flue Gas

12/13/2011 5:46 AM

Dear all,

I have an idea to recover heat from our boiler which is used for heating oil tanks. The exhaust gas of the boiler is usually between 210-230 C and I believe there is a significant amount of heat there that can be recovered. We use heavy oil as the fuel for the boiler. The products of the combustion has a significant amount of soot - that's where the problem begins. The soot will surely drastically reduce the heat transfer which will cause inefficient heat recovery. Manual cleaning of soot will be painful. At this point, what kind of exhaust waste heat recovery along with an appropriate soot removal system would you recommend?

Any idea will be helpful.

Thanks.

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#1

Re: Heat recovery from sooted flue gas

12/13/2011 5:52 AM

Soot is unburned fuel, which indicates that there are losses in terms of thermal efficiency. So the first step is to improve the combustion efficiency as this will yield higher dividends.

Once that's done, tackle the heat recovery next. The simplest use of it is to pre-heat the boiler feed water. However, any equipment used will need to be acid-resistant, as sulphuric acid will begin to condense from the flue gases as the temperature drops below 134degC.

This boiler is operating at optimal blow-down ratio, isn't it?

And the feed water treatment regime has been optimised, hasn't it?

And all the lagging around the boiler and its steam delivery lines is in top condition, isn't it?

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#2
In reply to #1

Re: Heat recovery from sooted flue gas

12/13/2011 6:11 AM

Thanks for prompt answer.

The products of the combustion might also be something rather than soot (along with soot). Is that possible? Moreover, whatever you can do, it is still not possible to reach %100 combustion of all the fuel. Do you think that the reason why I get more soot is because I am using heavy oil (API=11-12) ??

You asked me a few questions but honestly I do not think I can accurately answer them. Could you first explain, in detail if possible, the optimal blow-down, feed water treatment regime optimization and what does top condition mean for steam delivery and lagging lines?

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#3
In reply to #2

Re: Heat recovery from sooted flue gas

12/13/2011 6:33 AM

<...optimal blow-down, feed water treatment regime optimization and.....lagging lines...>

These are the things that need attention first, prior to the heat recovery proposal, which is going to involve significant capital equipment. They are priorities.

It would appear that they are areas for significant investigation and also learning opportunities.

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#4
In reply to #3

Re: Heat recovery from sooted flue gas

12/13/2011 7:00 AM

let me do some search and learn about them. I think I already know some of them but I am not familiar to the terms in English.

then, hopefully, we can continue to discuss the main issue.

thanks.

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#5

Re: Heat Recovery From Sooted Flue Gas

12/13/2011 1:11 PM

I would recommend optimization of your burner first. In some cases this can be accomplished by adding water and an emulsifier to the fuel oil. In any case this is the first step, imo.

https://www.epa.gov/ttnchie1/ap42/ch01/final/c01s03.pdf

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#6

Re: Heat Recovery From Sooted Flue Gas

12/13/2011 1:32 PM

Aggie,

First, understand how your boiler works and understand how soot is wasted money.

Second, understand how the amount of excess oxygen is a measurement of the combustion process. Have knowlegeable people test this periodically

THEN AND ONLY THEN, consider waste heat recovery.

Your simplest and most effective "waste heat recovery device" is an economizer. Perform a system evaluation to detemine the impact of an economizer on your existing system. Consider the drop in steam pressure.

Get quotes for a new economizer from the original vendor of the boiler

T H E N.........

Report back to us...

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#8
In reply to #6

Re: Heat Recovery From Sooted Flue Gas

12/14/2011 9:10 AM

I am with you on the wasted money and the economizer. I do have something to add to the "Second, understand how the amount of excess oxygen is a measurement of the combustion process. Have knowledgeable people test this periodically." The air and atmosphere around any given boiler are constantly changing. It would be best to add a boiler combustion monitoring & oxygen trim system. These systems are similar to what is in a modern car. It isn't enough to test the excess oxygen on an occasional basis because there are too many constantly changing variables. Most companies adjust for 10% excess oxygen during these periodic checks.

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#7

Re: Heat Recovery From Sooted Flue Gas

12/14/2011 12:56 AM

There have already been some good answers from people who clearly understand boilers.

I'd suggest you also calculate how much heat you could, in principle, recover using E=mc(dT)

The exhaust gas temp is fairly low, the specific heat of air is low (~1kJ/kg) and the efficiency will be low, so the energy(=money) recovered could also be low (it will depend on your circumstances).

Subtracting the cost of maintenance may mean the potential gain is zero or less. Good luck

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#9

Re: Heat Recovery From Sooted Flue Gas

12/14/2011 12:06 PM

Assuming that you still have significant soot from burning heavy oil after following the advice you've received here, a soot removal system upstream of the waste heat recovery apparatus might be adapted from this:

http://www.freepatentsonline.com/7901485.pdf

Note that the soot would be dynamically concentrated out of the flow path of the gas by a high shear process comprising centrifugation.

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