# Condensing scrubber for wood boiler.



## Snail (Oct 2, 2011)

Many large-scale thermal water heating furnaces recover the heat from the flue gases by means of a condensing scrubber.  In this type of system, the flue gases are saturated with moisture and the partially-cooled, wet flue gas is then passed through a condensing scrubber.  A condensing scrubber is also technically a heat-exchanger, of the direct-contact type.  There are a number of advantages of this type of system compared with more conventional plate or tube condensors:
The heat exchanger is also a very good flue gas cleaner.  It is very good at removing most pollutant gases.  It is near perfect at rmoving particles larger than about PM5.  For particles less than PM1 or so, efficiency is not good for any system, but a condensing scrubber is only surpassed by bag houses or electrostatic precipitators.

The condensing scrubber heat exchanger is cheaper than non-contact heat exchangers.

The condensing heat exchanger captures the latent heat of condensation, just as a condensing gas water heater does.  The resulting flue gases are cool yet creosote build-up is not a problem, as the creosote has been removed at the scrubber.

There are of course disadvantages.  The corrosion potential is horrendous if suitable material is not used.  Since the nasties are all collected in the cooling water, the water may require treatment before disposal.

Although this type of system appears to be gaining popularity for large-scale systems,  over several decades at least, I have never heard of it being used for domestic systems.  I wonder why.  It seems as if many of the advantages claimed for large-scale systems could well apply to very small scale systems as well.  Simplicity of construction would appear to be just as relevant on the small scale.  A much cleaner flue output is also not to be sneezed at.  

I found one paper in which a very simple bubbler type wet scrubber was used as the condensor-scrubber for a 17kw diesel burner(1).  I can't see any reason why this would not work for a wood gasifier, but that may well be because I just don't know enough.

Anyone got any comments or thoughts?

(1) Design of scrubbers for condensing boilers, F Haase and H Koehne, Progress in Energy and Combustion Science Vol 25 (1999), pp305-337


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## stee6043 (Oct 2, 2011)

My first thought is that a properly operated wood burner should not be releaseing lots of "nasties".  Coal and diesel thermal plants are very different animals.

I just don't see the need for what is likely a very expensive addition to a chimney on a high effieciency wood gasifier burning properly seasoned wood.  But I look forward to seeing other thoughts on this one!


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## tom in maine (Oct 2, 2011)

I agree with Stee, a proper operating wood gasifier would eliminate the need for a scrubber.
The scrubber is going to leave you with a lot of "polycyclic hydrocarbons", nasty stuff that would probably be considered
hazardous waste unless you can come up with a use for water laced with polycyclic hydrocarbons.


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## Frozen Canuck (Oct 2, 2011)

Snail: This has been discussed here a few times & has lead to the same conclusion; don't do it, even if you can as you will have to deal with what most juristictions would consider a hazardous waste. So you would spend a lot of $ to get a few more % eff & in the end have to dispose of expensive waste. Doubt it would ever leave you in a positive situation $ wise. You also have to factor in the extra chimney you would consume as this condensate would be very corrosive, as opposed to fossil condensate. More of a fuel situation rather than a technology one.


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## benjamin (Oct 3, 2011)

I'm considering the idea in a way.

The payback just isn't there for a small to tiny wood burner that burns a couple of cords a year, compared to a commercial system that runs 24/7/365.  Also, many woodburners, gasifiers included, have very little control over excess air.  A typical goal is 100% excess air, meaning twice as much air is fed to the fire as the amount needed to supply the required oxygen.  This excess air cuts down the efficiency of the boiler only slightly, but it would reduce the amount of heat available from the condensing portion more significantly because the excess air will hold humidity (steam) that won't condense as easily as if it were more pure steam.   

Please let us know what you discover.  Now I have to go back and find some old threads.


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## Snail (Oct 3, 2011)

Tom,

The only nasties in the condensing water will be the ones that would otherwise have gone up the stack.  Isn't it better to have them where you can control them, if indeed there are significant amounts?  I was considering having a breather for the condensation water tank that fed into the secondary air line.  It would need a valve for when the system was not running, which is a complication of course.  Had anyone got a reference for the solubility of VOC and PAH in water?

Although modern gasifiers are far cleaner than the previous generation of wood burners, "clean enough" is a movable target.  Particulates are the wood burners' particular Archilles heel at present.  I think that we need to look forward to the inevitable tightening up of requirements

I need to clarify.  It would clearly be a bad idea to add a condensing scrubber to an existing design of gasifier boiler, as these already have a conventional plate condenser.  I am talking about a new design situation and exploring the technological possibilities.  The output from the secondary burn chamber would pass immediately to the saturator and then to the condenser/heat exchanger.

The point about chimney corrosion is not valid.  The exhaust gas is only at about 60 to 80 deg C.  It is saturated by moisture, but that is distilled water.  A short plastic pipe is all that is needed for a "chimney" (No condensing system will work by natural draught anway, so a fan is needed.)  I would take the exhaust out though the wall and into an inverted U of aluminium, to hide the condensation plume.

The corrosion potential is mainly in the saturator and ducting from there to the condenser.  As wood is low in sulphur, the problem should be solvable without going to unobtainium ducting.

Someone mentioned that the subject had been discussed here before.  I didn't find it when I searched this site.  Can you point me to it please?


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## Snail (Oct 3, 2011)

Benjamin,

Point taken about the excess air.  However, isn't that possible to counter by increasing the amount of moisture added in the saturator?  Or indeed to the secondary air as well?  I must admit that I suspect that the control of the saturator may be the most technically difficult part of this system.  I haven't been able to find a lot on it by google and what I did find looks complicated.  OTH maybe if you are not fighting for the last erg of efficiency, the control might be able to be relaxed a bit?


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## benjamin (Oct 3, 2011)

Snail

I agree with you entirely.  The heat exchanger substitution instead of add on occurred to me a little late.  

As far as a small scale gasifer having cleaner exhaust than a large scale coal or diesel burner, everything I've ever read or seen would indicate that larger scale operations have FAR higher standards and far lower (percentage wise of course) emissions than even a reasonably run wood burner.   

Another possibility for dealing with the condensed "nasties" would be to dry the lowest dirtiest level of water and feed the solids back to the primary combustion chamber, which of course involves transferring a huge amount of heat, but that is exactly what this condenser would be doing very well.


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## benjamin (Oct 3, 2011)

You beat me to the post.

In my understanding, the unrecoverable heat lost from excess air is used heating the air itself and also the humidity that the air will hold at the higher temp. So it's the amount of excess air, and the final (coolest) temperature that the gasses reach that limits the amount of heat recoverable.  In addition to the amount of hydrogen vs carbon, carbon dioxide doesn't condense so easily.  

This is why methane (CH4?  ) is usually burned in condensing household appliances, oil is occasionally burned in condensing household appliances, and there are only two of us considering trying to condense wood exhaust.

Just a tinkerer here though, no engineer.

Edit:  The concept seems very appealing to me all around, I'm not sure what kind of control you are talking about for the condenser.   It seems that a simple pump would be all that would be needed?  with an overflow for the excess water.  The complicated part would be trying to remove the gunk, maybe a wood chip filter bed that gets dried and recycled to the input?


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## Frozen Canuck (Oct 3, 2011)

Snail: I don't think condensing was ever the topic (title of a thread), rather it was discussed in a thread maybe on eff or some other related topic. That will make it harder to search unfortunately. 
Hopefully someone with a better memory than I will remember. 
Don't get me wrong (someone) has to figure out condensing for wood burners may as well be you. If you have a plan for dealing with the condensate, great. 
Starting with a blank sheet designing your own system will help as you can proceed from the start with condensing in mind. That will help when dealing with the variability in the fuel.
I am sure we will all look forward to your posts on the progress. Hope all works well.


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## Chris Hoskin (Oct 3, 2011)

Cool stuff.  Certainly condensing is in the future of pellet - and probably wood boilers.  I had only thought about it as an efficiency thing - getting more heat out of the exhaust gas.  But combining it with a scrubber to improve emissions too sounds very promising.  I look forward to following this thread, thanks for posting Snail! Chris


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## peakbagger (Oct 3, 2011)

I was at a presentation by the danish district heating council a few years back and many of their installations use condensing heat exchangers to capture the energy from converting water vapor to liquid water and the temperature drop. They have use for warm water so it makes sense. 

If you see references to HHV and LHV for heating values, the difference is that the HHV includes the energy in the water vapor and the LHV  doesnt. Generally the bigger kicker is the lack of stack draft from cooling down temps too low and air emmission impacts. The lack of stack draft can be cured with a fan, either an induced draft or a forced draft fan to force exhaust up the stack. Air emmisions are a bit trickier, the wet scrubber will remove some pollutants, but not all. Normally a hot exhaust plume will rise and disperse over a wide area, a cold plume will discharge out of the stack and have minimal rise, so the pollutants will be more concentrated in the local area. 

There is a lot of liquids captured by a wet scrubber and the scrubber brine will most likely be classified as hazardous waste. Stainless works pretty well for corrosion issues although stress corrrosion cracking could be a issue. 

The actual construction of a condensing scrubber is not as simple as some think. In order to get particulate out of the exhaust stream, the individual droplets of water have to come into physical contact with the particulate. This usually means a high pressure pump and a fine spray nozzle to discharge alrges amounts of fine droplets. Generally there needs to be demister section downstream of the scrubber to keep water vapor from going up the stack as the alternative is a large block of ice accumulating on or around the stack during cold condtions.  

I expect that it is one of these projects where technically it can be done but due to economics and complexity it would not be viable for a home sized unit.


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## Snail (Oct 3, 2011)

Thank you for your reply Peakbagger.  Regarding your points:

It's my understanding that the majority of gasifiers use fans, so the main change here is the need for a more powerful fan to overcome the head loss over the scrubber, which does not seem too major an undertaking.

The volume of water likely to require disposal is that initially contained in the fuel used and generated by combustion.  Although not insignificant, it does not seem to me to be "large" compared with normal household water use. 

The toxicity of the waste-water for the Danish installations may be due to the fact that most of them burn waste.  Municipal rubbish, sewerage sludge and the like.  I am having a bit of trouble finding a resource that would give me a good handle on how much and what kind of toxicity would be generated by the burning of clean wood, of various moisture and bark contents in a full-on gasifier burn.  Does anyone have any data?

As I asked Tom, Is it not preferable to contain any toxicity and treat it, if actually required, rather than broadcast it to the neighborhood? 

Regarding the wet scrubber design, the paper that I quoted used a very simple bubbler, which would seem much easier to build than other systems.  This system may not match the efficiency of other types but it should not be compared with state-of-the-art major installations.  Instead, the true comparison is with existing domestic systems that have no flue gas treatment at all.

A demister after the scrubber is essential of course.  However this will not remove any water vapour, only water droplets.  The outgoing air stream will be at saturation for the outlet temperature, which will be a little above your storage tank temperature.  However, this vapour is a small quantity of water when condensed and will initially be hot, allowing it to be disposed of before forming a block of ice, in many climates at least.

I did not think of the concentration effect of the lower temperature exhaust.  However, the average house chimney isn't tall enough to punch though an inversion layer, even a very small and low one, so it's not much of a dispersing factor.  At best, in still air the effect would be to share pollution with a few near neighbours.  In windy conditions, as our winters usually deliver, I don't think it would make much of a difference at all.  Lower wind velocity a few dozen metres nearer the ground would at least partially be compensated for by greater turbulence?  The siting of the exhaust obviously needs a bit more thought however.


WARNING RANT AHEAD

As I noted, earlier, the pressure is on in many countries to ban domestic wood burning altogether.  pm 2.5 is the new bogeyman.  I am very suspicious of the medical profession and its tendency to go off half-cocked on a number of public health issues.  No doubt it is a tendency fostered by the notorious "God" complex of that profession.  However, the "Five-cities" research is a bit troubling, even to me.  

Wood is the best domestic heating fuel on so many levels, so it would be a pity if we wood-burners fouled our own nest and got banned as a result.  

In my country, domestic electricity use is growing fast as a result of official promotion of heat pumps over fires for heating.  Although we are about 60 percent hydro generated, the new consumption is almost all fossil fuel generated.  Pressure is on to dam our remaining scenic rivers, to extract dirty brown coal beneath prime farmland and to drill in some of the deepest, most stormy seas ever attempted.  This around a country that has less than a thousandth of the USA's resources to deal with oil spills.  The USA, found a blow-out in the calm shallow Gulf more than a bit of a handful, so what do we do when the inevitable accident occurs?  I imagine that similar issues affect other nations.


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## benjamin (Oct 4, 2011)

Snail said:
			
		

> As I asked Tom, Is it not preferable to contain any toxicity and treat it, if actually required, rather than broadcast it to the neighborhood?



Agreed in principle, but in this country anyone is allowed to smoke out the neighborhood with a wet fire in a lousy fireplace but you need a permit to dump well water from a heat pump. 

Good luck getting approval to dispose of wood boiler condensate.  Of course, most do it yourselfers in this country don't ask for approval.


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## tom in maine (Oct 4, 2011)

The theory is that the nasty material exists in large quantities in the exhaust in the first place. 
If you are asking to burn wood in something less than a gasifier design in order to keep costs down,
then I think a scrubber is not worth considering. 

A clean burning device makes more sense than trying to clean it up after the fact.

What comes out of gasifiers is a pretty miniscule compared to even ten years ago.

Concentrating exhaust into a liquid seems more of an issue than allowing that small amount of material to 
dissipate in the atmosphere.

It is likely that we will scrub it at some point, but this is an acidic nasty brew that still has to be disposed of.


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