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

[Ken Boak]

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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