[Gasification] Wood heating in the UK - whole log gasification
ken.boak at gmail.com
Thu Dec 26 13:30:08 CST 2013
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
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"
>> 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
>> 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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