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

Greg Manning a31ford at gmail.com
Thu Dec 26 13:22:33 CST 2013

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
> Ken
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