[Gasification] Wood heating in the UK - whole log gasification

energiesnaturals energiesnaturals at gmx.de
Thu Dec 26 17:16:25 CST 2013

Merry Christmas Ken and list ( I am aware that I am a tad late)

One way to burn whole logs like we do (45cm across x 55 cm long) in an efficient way is to build a well closed ,dome shaped burning chamber out of 15 cm fire brick and have individually regulated, preheated primary and 2,dary air , at least 2 pipes either side.
You build up afire with kindling as you describe it and after 15 min you can add  an entire log of pitchy pine and it will first gasify very nicely and than burn the charcoal if you want.
The secret is to keep the  walls of the combustion chamber warm and do not use them as heat xchangers!
You build a convenient xchanger above it and use the hot exhaust gas. Build it large enough to reduce the exhaust temp to 90 deg C or less and you will be way above 50 % eff. Ours has been working for 20+ years and is still doing fine with 2 mm black steel pipes.

We never cut anything below 55 cm long and never split anything below 45 cm, believe me or come and see!

Cheers and a happy new year


Enviado desde Samsung tabletKen Boak <ken.boak at gmail.com> escribió:Greg

Thanks for sharing.

I want to try a few ideas for myself to see if this is indeed possible on the small split logs I can produce locally.

Our heating needs, and indeed modest on account of the mild climate here, but I would like to find an efficient solution for all the thousands of acres of neglected coppiced hardwood.  Cast iron victorian stoves may be quaint, but I am sure there are ways to improve the overall efficiency with radical redesign.

The main burner/heat exchanger on our 24kW gas boiler is no bigger than a gallon paint tin.  Perhaps there is design lesson to be learned here

Anything to reduce mechanical handling and processing of wood fuels has to be a step in the right direction

Happy New Year 


On 26 December 2013 19:22, Greg Manning <a31ford at gmail.com> wrote:
Greetings Ken, and list members.

Ken, I'm going to point you to a video of the "underside" of a whole log (or split) "cordwood as we call it here" stove that is a downdraft gasifier.

I can speak at length privately, however only somewhat on list, as this is a proprietary design. 

Here's the link to the video:

Greg Manning

On Thu, Dec 26, 2013 at 12:59 PM, Ken Boak <ken.boak at gmail.com> wrote:

Happy Christmas to the gasification list.

At this time of year, during the festive holiday season, I get a bit more time to manage the running of our woodstove, as it rapidly warms the room and produces a cheery effect.

Our property is fairly conventionally heated by natural gas, but a few years back, I took the decision to invest in a woodstove with back-boiler, to provide an alternative or back-up to the gas fired system.

The woodstove has a flat steel tank at the rear, the "back boilerr", in place of a couple of the firebricks lining.  This circulates heated water entirely by the thermosyphon principle to a radiator located in the bedroom/workroom directly above the stove. So in effect the stove heats the living room/kitchen area  directly, and the room upstairs by circulating hot water.

No electricity is required for circulation, and if worse-case we had an extended power outage, this stove would provide heat and comfort in the two main occupied areas of the house. Stoking it and attending it is often more interesting than what is being shown on TV!

With a few days off work, I have had time to monitor the stove and make some assessments of its overall performance. Its a fairly traditional stove,  a rectangular box,, made from bolted together cast iron panels and partly lined with firebrick. It's described as a multifuel stove - having been supplied with a cast iron removable grate for burning coal - which is not used when burning wood. It's approximately 24" wide, 12" deep and 18" tall.

In the UK, a common size for firewood logs, intended for the domestic woodstove is about 10" (254mm) long, and equal to a 1/4 round taken from a limb that may have been 5 or 6" in diameter.  The reason for this is that there is a lot of coppiced hardwood, which has become mis-managed in the last 20 years, so there are a lot of trees with 6" diameter shoots.  The popularity of the "firewood processor" machine, means that a lot of this wood is now coming on the market as domestic firewood, and sold to suburbanites at vastly inflated prices (about $0.50 per kilo).

I am burning a mixture of kiln dried Silver Birch, and air dried other species which includes ash, oak and sweet chestnut. The silver birch splits well and makes excellent kindling.  One log is split into 8 or 10 kindling sticks and these are built into a pyramid around 2 or 3 sheets of scrunched up newspaper. Lighting is quick and easy - as the birch is kiln dried, and within 5 minutes you will have a roaring fire and the larger logs can be added.

The logs have an average weight of approximately 1 kg.  I have found that a normal burn rate of these is two per hour.  I burn two at a time, and each hour, add a further two to the burning char bed from the previous logs.  With firewood having a calorific value of approximately 4kWh/kg - I estimate the fuel input is in the order of 8 to 10kW.

Of course, with a traditional stove, so much of the heat energy is lost up the chimney, and goes to create the draft.  The efficiency of the stove, might be in the region of 50% - somewhat better than the open wood fire.  It occurred to me that by way of a 2 stage gasification process, it would be possible to increase the overall system efficiency, resulting in less wood consumption, or more heat output per log.

This leads to a question - is it possible to design a gasifier aimed at handling whole log gasification - where a log is 10" long and no more than 6" across?  Can you recreate the temperatures, turbulence and reactants, found within the combustion zone of the woodstove, and use this to thermally process a single firewood log, at the rate of one every 30 minutes or so?

I've had some ideas on how this can be done, effectively using a length of 6" diameter stovepipe/fluepipe to make a compact gasifier.  Logs would be loaded in from the top, and the length of the pipe chosen to perhaps hold 4 logs at any time - about 1m  (40") tall.  The bottom log would sit in the combustion zone - so the end of this log is constantly under the action of the air nozzles. The logs above are subjected to the elevated temperatures and begin to pyrolise, char and split on their descent down the tube.

Beneath the combustion zone would be a fairly conventional hearth, and reduction zone, with the char supported by a grate below that. For an overall idea of the system - think of HS Mukundas open top gasifier.

Use of twin-wall stainless flue pipe would allow the air to be pre-heated in the outer annulus - adding to the overall efficiency.  The hot syn-gas could be burned in whatever appropriate burner geometry deemed necessary for either radiant space heating or water heating with a suitable heat exchanger coil.

I hope to try to build a prototype of this over the next 10 days (a working gasification holiday?)  and to see whether a log can be reduced in this manner.  If all that is needed is heat, then the restrictions to produce a tar free gas need not apply. If one can use whole logs, without having to resort to woodchip - then this will be a considerable saving in mechanical handling and wood processing.

At the end of the day - this gasifying stove needs to be as simple to operate as the existing woodstove.  Reloading with a couple of logs each hour, and no sophisticated need for fan- forced draft or electricity to operate. Draft would come from the chimney as per now - about 25 to 30 feet, 6" diameter.

If anyone has experience of something similar - please let me know.

Happy Holidays


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