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

Kevin C kchisholm at ca.inter.net
Thu Dec 26 15:07:33 CST 2013

Dear Ken

If indeed your efficiency is poor because of excessive "Stack Loss"  
then teh simplest thing would be to control stack loss.

The three "Stack Loss" components are:
1: Excess temperature
2: Excess air
3: Incomplete combustion

The first thing to do is check the chimney exhaust visually, to ensure  
that it is venting clear gas, or white condensate. If there is gray,  
blue, yellow or black smoke, then adjust secondary air to get rid of it.

The next thing, if there is no smoke, is to cut back on secondary air,  
to cause some, and then re-open it just enough to get back to "a "no  
smoke stack."

Then measure Stack temperature. If above 350 F, it is usually  
worthwhile adding heat capture surface to recover heat for use in the  
living space.

You probably only need about .02" of overfire draft, to get the  
products of combustion out of the fire box. If draft is excessive, you  
may wish to consider partially obstructing the stove outlet.

The above might be the cheapest and most practical way to proceed, but  
then with your site specific circumstances, and your capabilities, a  
gasification system might be do-able.

Best wishes,


Quoting Ken Boak <ken.boak at gmail.com>:

> 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
> Ken

More information about the Gasification mailing list