MIDGE stove improvements

Today was spent modifying the gasifier. After the air intake had been improved it was necessary to seal the rest of the gasifier to prevent leakage of air to where is wasn't wanted.

Previously, the combustion chamber rested precariously on four bolts attached to the outer chamber.

The weight of the combustion chamber pushed down on the bolts and allowed a sizeable air gap to appear between the top of the combustion chamber and the top lid. This gap disrupted the flow of air through the secondary air holes in the combustion chamber.

To remedy this problem the hole at the top of the cowling was widened by cutting small tabs around the circumference of the hole so that the hole the combustion chamber could just pass through. The tabs were then folded back onto combustion chamber to lock it in place.

Not only did this reduce the air gap but also meant that the supporting bolts could be removed from under the combustion chamber as the cowling now supported the combustion chamber.

A new ring of secondary air hole were drilled into the combustion chamber and the gasifier reassembled.

Another test firing was performed. The air throughput was much enhanced resulting in much higher temperatures being read.

The draught was such that red hot particles of ash were ejected from the combustion chamber. A draught control that changes the size of the primary air intake may be needed. The very bright colour of the combustion shows great heat being generated.

Future Experiments

Aluminium Foundry - Because of the immense heat being generated an experiment to melt aluminium in a crucible will be carried out. The crucible will require a lid so that the combustion chamber can be charged with additional fuel during the melt.

Automated Refuelling - During today's burn additional fuel was dropped by hand into the combustion chamber via the stove pipe. It was necessary to drop small amounts often rather than one large charge of fuel otherwise the combustion of wood gas was terminated and smoke was generated in its stead. This was remedied with a single drop of paraffin but was not ideal. An auger dropping pellets into the stove pipe would be ideal. Knowing the rate of refuelling required depends on many variables that will need to be discovered through experiment.

Warning!

Make sure you take all necessary precautions. Especially when you smell a new odour. A new smell in your nostrils means something new has happened. When I returned to the house to wash my hands I noticed in the mirror that I had melted some of my hair!

MIDGE stove - test runs #3 & #4

As suspected, it was a lack of air that was causing the smoking and the extinction of wood gas.

The existing primary air holes were enlarged but the wood gas was again extinguished after a quarter of the fuel had been burnt.

For test run #4, more air holes were added to the bottom of the outer container.

When the stove pipe was put on top of the stove a forceful draught could be heard, which resulted in better combustion. No smoke was seen to be generated.

The ignited gas jets from the secondary holes at the top of the combustion chamber could clearly be seen.

The fuel used consisted of wood chippings from a garden shredder. This appears to be a better fuel than the wood "pellets" with regards to starting a fire. However, another test with the updated stove will be carried out tomorrow as the pellets are denser than the chippings and so can give off more heat energy per burn.

The pellets could also be kept back and used in some automated refuelling operation for a stove that is already in operation. The reasoning being that the pellets are of uniform size and an auger or similar could administer a given amount of pellets every so often. The stove could then sustain heat for as long as required.

Though the stove pipe improved the draught, the length of the pipe makes the stove less useful. Future experiments will involve finding the minimum pipe length that provides an adequate draught, allows access to the combustion chamber for refuelling and permits a water coil to be so positioned that water can be heated.

MIDGE stove - test run #2

Another test of the wood gasifier was performed today. The results were completely different to yesterday. The hole at the top of the cowling was widened and the fuel had wood chips added on top instead of cloth.

Almost immediately wood gas was generated. It was ignited intermittently and created small explosions that almost blew the flame out. It was like listening to a steam engine leaving a station with the sound of the steam slowly puffing. Five minutes later a larger explosion blew the cover off the top. The top was replaced. After 10 minutes the flame went out and a lot of smoke billowed out until the fuel had been consumed.

Tomorrow, wood chips will be used instead of pellets and the primary air holes will be made larger.

MIDGE stove - test run #1

The stormy weather had calmed today and so we were presented with an opportunity to give the wood gasifier stove a test run. Plans for this can be found simply by performing a search for "midge stove". We used a 20 litre vegetable oil can picked up from a restaurant for the outer container and a 1 kilogramme coffee can for the inner combustion chamber.

The top cowling is the bottom off another 20 litre oil can. These cans are very plentiful and we hope to use another one for the construction of an aluminium foundry. The hole at the top is a little narrow and will be widened for the next firing so that is corresponds with the diameter of the combustion chamber.

The fuel used was rhododendron "pellets". Whips of willow, birch and rhododendron are coppiced on the land here and cut into 25 mm lengths and range in width from 8 to 12 mm.

For this test run, 500 g of pellets were used.

The top layer of pellets were doused with paraffin oil and on top of that was placed a piece of cotton cloth, also doused in oil. The cotton was then lit.

After 10 minutes, the flame died down and a pyrolysis layer could be seen quite clearly beneath a layer of charcoal.

By using an infra-red thermometer we noted that the sides of the combustion chamber were at almost 600 C. The pyrolysis layer was much higher and was off the scale. The outer chamber temperature was over 100 C.

There was a period when a large amount of smoke was generated. Some stirring of the fuel with a metal rod stopped this and combustion continued as before. It is believed that a "bridge" had formed and prevented air from reaching the pyrolysis zone.

The total burn time was approximately 1 hour. The remaining ash was weighed and found to be 7 g or 1.4% of the initial mass of fuel. This means that 98.6% of the fuel was combusted.

With the outer can reaching over 100 C we wonder if a water jacket could be used to gather this heat for heating a hot water cylinder. This would be in addition to a coil in the much hotter combustion chamber.

Would the removal of heat affect the combustion? Further experiments will be needed to check this.

The next firing will be carried out after the cowling hole has been widened and the outer container air holes have been widened too.

A different fuel will also be tried in a later test, namely fine wood chips from a garden shredding machine.