Condensing scrubber for wood boiler.

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

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?

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?

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.