Wood powered Gasser??

[desperate]

As some of you know we installed a Woodwarm 16kW "Dragonstove" at Cactusville nearly two years ago, and rapidly fell in love with it, we're kind of weird that way, especially as our gas bills have plummeted to about 20% of the previous level. But there has always been a little niggling problem, the boiler. If you are familiar with my CRAC thread you will remember that we shove all the heat up into the thermal store via a direct circuit and then use a heat exchanger in the thermal store as a heat source for the central heating. When I designed the system my fear was that we wouldn't be able to draw heat through the thermal store heat exchanger into the central heating quick enough for the demand. To my surprise the bottleneck in the system is getting heat from the stove into the store. the store is 7metres directly above the stove all plumbed in 28mm with almost no restrictions, it also has a pump which is switched if the stove gets above a set temperature. It doesn't seem to matter wheather the pump is running or not the heat up time for the store is quite slow.

When the store is above 60 degrees or so it is easily capable of running the central heating as well as the Gasser can, so getting heat out of the store is not the problem, my suspicion is that the stove boiler is not up to the job, in fact I am convinced it is little more than a piece of junk. When you think about it a back boiler at the bottom of the combustion area is all wrong according to the laws of physics in our universe. It should I think be right at the top of the stove extracting heat after the combustion of all the gas has taken place, that is how all gas boilers work, flame impingement is to be avoided at all costs, so why not in a wood burner?

Why not have a proper heat exchanger at the top of the stove? It could incorporate a damper so fine adjustment could regulate how much heat were extracted from the burnt gasses before going up the flue, this way one could allow enough heat up the flue to drive the draught but no more. I envisage a slab type heat exchanger that is a bit smaller than the plan size of the stove, say 40cm*60cm by perhaps 10cm thick, in the centre lined up with the flue outlet would be a clear hole the size of the outlet, all around the centre hole would be passages with fins or similar to provide a large heat exchange area, the large central hole would allow one to sweep the chimbley, but could be closed off with a sliding baffle to force more gas through the exchanger.

If you look inside a Gasser with a cast iron exchanger it is obvious that the combustion takes place well before the heat exchange happens, and also that very intricate shapes can be cast in iron. This set me thinking of some sperryments to be done 1 how much heat can we extract and still have a decent draught? 2Will the exchanger get bunged up too quickly? 3 Can we burn all the tar so the exchanger and flue do stay clean. Theres probably loads more questions to be answered, but now I need another glass of vino and a rummage in my playpen AKA... garage.


Any thoughts? I would really like to get something sorted before the burning season starts again as judging by the temperature of the chimney breast we are loosing loads of heat unnecessarily.

More soon.....................

Sainte


Desperate

[billi]

Sláinte


Vino rosso/tinto here ...

But agree it  is very interesting to  think about boiler type wood burners

my folks had this one and  we meet the builder/designer   and it took him a while to get it right ...

still i am amazed how he managed to get the draught  strong enough to allow the burn downwards without any electric vent
http://www.youtube.com/watch?v=SkuYEch1s7E&feature=youtu.be

Billi

[desperate]

Cheers billi, your very best of health, we need a bottoms up smiley.......

thanks for that link, that looks like a properly thought out design, I like the idea of lining the combustion chamber with fire brick, it should easily be possible to reach a thousand degrees in there, then anything that doesn't burn will only be ash, as for the down draught at the bottom, i'm don't know why they feel the need to do that, it certainly looks like a good piece of kit though.


Deshp

[Ivan]

I think you've hit the nail on the head. Woodstoves with boilers are counterproductive, though most purchasers aren't aware of that (I wasn't when I bought one). Hot water from your stove is a great idea, but not so great when you see a 10-15% reduction in efficiency, meaning more wood is required for the same amount of heat, and more pollution is created.

The big problem is that a conventional stove is attempting to radiate heat from the combustion area, whereas for high efficiency we want this area to be as hot as possible. The alternative is to have a very tall stove, so that you can have combustion at the bottom and radiative heat (or heat exchangers for water) at the top, or alternatively a deep stove, where flue gases are passed up and down alongside heat exchangers at the rear. This requires a far bigger stove than would fit in most UK fireplaces, so the UK market for a stove that could do this would be very limited, even if it were 90% instead of 70% efficient.

Another issue is that combustion would need to be hot and clean, otherwise the heatexchangers will tar-up quickly, causing a)flue blockage and b)reduced heat exchanger efficiency. I think the gassifier stoves wait until they are up to a preset temperature before passing flue gases through the heatexchangers.

[Brandon]

ivan, they do indeed, usually by means of a bypass that goes from the top of the burn chamber to the flue way, when open the fire heats the chimney to get the draw going (and for refuelling) and when closed, the burn heads down into the lower chamber where the real heat is produced (heated air is added as it passes through the [often ceramic] throat), then into the h/e / boiler.

Desp, I think it was rogeriko who has a cast h/e on the flue outlet of his stove.

here

[desperate]

Brandon, it was rodgerikos brilliant bodgineering that first inspired me along this train of thought.

Ivan, as you say we need the room to separate the combustion and heat exchange functions in a stove, my feeling is that it is mainly down to control of the air input.
Last winter when the dragon stove was burning full chat I bodged my old gas manometer onto the air intake at the base of the stove, when the airwash control at the top of the stove was nearly closed we could get over 30mbar pressure drop, all the water was blown out.................. .. My feeling is that with this pressure to play with we ought to be able to get the air to do pretty much what we want.

If you consider a standard type (not blown) gasser burner, primary air is supplied into the venturi to mix before ignition, and then when the mixture issues from the burner head there is an excess of secondary air present to ensure combustion is complete. Now why cant we do something similar with a WBS? In the structions for our Dragonstove they say to treat the lower air control as a sort of "choke valve" to open while starting and establishing the fire, then as the stove warms up to a normalish temp to open up the upper airwash valve and shut the lower control. This doesn't make a lot of sense to me, why not let a small amount of air to enter through the fuel bed to provide enough heat to release the flammable gas, and then duct air through the fuel bed to preheat it and then mix it with the released gas to completely burn it. If there were several ducts coming up through the base of the fire it should be possible to preheat the air to quite a high temperature 500-750 degrees c? Once that gets mixed with the gasses coming from the fuel bed it should rapidly burn any flammable vapours completely provided there is an excess of oxygen present.

Once all that has taken place then extract as much heat as possible, commensurate with keeping a decent flow of gasses to clear the P'sOC from the room/stove, personally I don't think this needs a lot of energy. Last winter we were burning about 3-6Kg of wood an hour, or releasing 12-24kW at an overall efficiency of what, 50-60% ish, bit of guesswork going on here, but it would seem as if we are loosing at least 7or 8kW up the chimbley, ok so I know that the breast gets pretty hot after several hours which we recover overnight, but when you think about it if we had a 6kW hoover in the chimbley half our furniture and the cat would get sucked up, not to mention MicroDesp.

I am convinced if we can get the combustion bit sorted before the heat exchanger we get rid of all those flue/exchanger tarring problemsand then minimise the energy being lost.

I have a cunning plan My Lord, to try and answer these questions I will be offering some piccys soon for your feedback.


Desperate

[Brandon]

Desp,

I do love your imagery, a 6kW hoover!

You are describing pretty much what is done in a gassifying boiler, the tricky bit as was alluded to earlier, is the physical separation (appliance size) that is needed to achieve this.  Billi's parents appliance is a case in point, larger than your average stove.

This is of course what dunsely claim they are achieving with their yorkshire boiler stove.  I believe renewable john has one, and likes it.  The only dealings I had with one were in a house that despite having a LARGE living room, did not require the amount of heat to room that it was giving (resulting in the owner running the appliance far too close to slumber for it to really "do its thing".

Alas, if we are trying to attain high burn temps in a living room appliance (bear in mind that we are aiming for 900-100^oC we need to radically rethink the basic design to keep all that heat from making the rooms occupants turn crispy.

[Ivan]

Putting more air in the bottom of a stove certainly increases temperature, but it increases the rate of fuel consumption, and that means increasing the rate of flue gas production, and therefore, although hotter, it moves much faster through the fire. The key to separating combustion from heat-extraction, is in retention-time. It's a bit easier to understand in oil or gas burners, where the flame zones are very easily distinguished. With a much slower burning fuel, such as wood, achieving sufficient combustion-chamber retention time at the right temperature is really quite tricky, unless you build a large (usually tall) well-insulated box for burning, but not giving up any heat, and then pass the flue gasses through a heat exchanger only after they have been fully burnt. Even modern woodstoves send a significant percentage of their fuel, unburnt up the flue (and even more so for water-boiler models due to their lower combustion chamber).

The only way I know of, to increase the combustion chamber retention time, without having to design a tall woodstove is to introduce a vortex inside the combustion chamber, thus increasing the mixing of air/fuel gases and increasing the retention time through lengthening the path gases take leaving the combustion chamber. This is the process used in the Burley range of woodstoves, a technology which is patented.

[knighty]

my folks had this one and  we meet the builder/designer   and it took him a while to get it right ...
still i am amazed how he managed to get the draught  strong enough to allow the burn downwards without any electric vent
http://www.youtube.com/watch?v=SkuYEch1s7E&feature=youtu.be

anyone know how these work ?  I've found a few with google but no real diagrams of how they work...

I'm guessing an air intake near the middle somewhere and a flue out of the top

while you warm up the flue heats up to temperature creating the draught

then once you're up to temp you work a valve which means the hot gas now has to go down through the fire box and then up the same (hot) flue... the heat/draught in the flue is enough to keep it going while the rest of it heats up ?

[rogeriko]

Just for everyones info my bodgineering has worked perfectly for 3 years heating 400 litres water in 2-3 hours. I have never even cleaned inside the heat exchanger but because the chimney is colder than normal it collects a lot of soot. My chimney is about 8-9 metres of vertical/horizontal pipe winding around 4 rightangle bends inside the house. Here in Greece thin galvanised chimney is only about 4 euros a meter so it is replaced every year because it goes rusty before it soots up!! If there was a vertical only chimney the soot would fall to the bottom where it could easily be removed. It is obviously only the horizontal sections that collect soot.

I am changing the fire system this year to a different design using a heat exchanger about the size of a laptop mounted flat in the top of a Prity stove with firebricks and the hot water pipe will run 1 meter up inside the chimney hopefully this will promote thermosyphoning. There will also be a circulation pump like I use now because the hot water tank is on the same level as the fire. Pictures will follow as I build it.

[billi]

Knighty  i found these drawings but unfortunately not the  description





I should have a cleaning instruction with more drawings  somewhere  on file, but cannot find it any-more   

[desperate]

Thanks for your thought everyone, Ivan you have identified what the basic problem is I think namely the combustion rate. The retention time is as you say restricted by the overall size of the stove, so I guess I will need to introduce just enough air at the base of the fuel bed to produce the gasses at the rate we need. Then if I can preheat the secondary air to a high enough temperature the flame rate will increase dramatically, but also to increase retention time I am developing the FARTARs (r)   (front and rear twin aperture retention system,) no vortex needed here just BA.
Also rattling around in my brain is the idea of catalysis, most cars have them to burn up all the smeg glopped out of the engine, why not a WBS??

Brandon, you mention temps up to 1000 deg c, I envisage lining  the stove with firebricks or even casting a fireproof shell to fit inside the base shouldn't be too difficult with some vermiculite and fire cement both would be good for 1400 degs or so, or what about that white fluffy stuff that lines gasser heat exchangers? It may be a bit fragile, but I really dont think a 1000 degrees is a problem.

Time for my medication...............

Desperate

[knighty]

Knighty  i found these drawings

thanks for those.... it's quite a simple design really... looks like the air comes in at the front middle ?

I was half expecting the handle on the side to work 2 valves.... so once you start taking the exhaust out of the bottom you could heat the air in at the top at the same time.... but I guess that would be tricky as it would be easy for the fire to start to draw up the air intake ?

[Brandon]

Not a problem, just making sure that it had been considered.

Double glazing or a full door complete with fire brick lining...

[desperate]

Yeah, it would be nice to keep the glazed door and line the other 3 sides and the base, do you reckon the glass would be up to the job? Maybe some of the secondary air heat exchange pipery could sheild the glass a bit. I am going to try and knock up a sketch or two for your perusal.

Desperate