Here in Vermont it is a good idea to have a backup source of heat in the winter. We have an 1800's parlor stove in the living room. Dealing with several cords of firewood, especially in the city (Montpelier) is a real pain ... buying it, getting it dumped in your driveway, loading it through the basement window and stacking it ... you know.
This year we decided to try to find a way of using wood pellets to cut down on the carrying and storage and mess. To burn them in a wood stove I bought an expanded stainless steel basket.
While this works, that is to say the pellets burn, it is at best like putting a big log on the fire ... a fair amount of smoke and it is finicky about air supply. I thought there must be a better way.
I found myself looking into the engineering and physics of wood fires and stumbled upon a goldmine of information in the web concerning efforts to create efficient, low pollution, simple and inexpensive to build biomass burning cooking stoves for use in the 'developing' world ... places where people cook indoors on open fires and where indoor air pollution and the consequent bad health effects are a real problem. While there are myriad sources of information I found the Aprovecho Research Center web site discussing mainly the 'rocket stove' and the blogs of Dr. Sai Bhaskar Reddy Nakka discussing various wood gasifiers most inspiring. From Aprovecho's tutorials I noted two desired separate components of stove building ... (1) efficient clean burning and (2) efficient heat extraction.
December 1-15, 2008:
After reading up a bit I blundered upon a fireplace pellet burning device from Finland called the Tulipiippu. This device is put inside of your fireplace or wood stove so the chimney takes away the fumes in a safe manner. The nice thing about these is that they operate on natural draft ... no electricity or fans required. Based on photos of that device I tried to make one to try out before I tried to buy one. This burner combines elements of a rocket stove and a smoke burner or 'tlud' gasifier. A really cool design, but a bit difficult to understand in terms of the physics of burning.
This burner takes perhaps 3 liters of pellets and burns with a nice flame for about 1.5 hours before the gasification of the fuel is complete, the flame extinguishes, and the charcoal begins to smolder. Once it is in the charcoal stage there is little you can do to make it work right again except for dumping the charcoal in an ash can outdoors and starting over. The other down side is that it tends to leave about 50% by volume of charcoal in the burner - that's a lot of unused thermal energy! Smoldering charcoal produces volumes of carbon monoxide too. So back to the drawing board to find a way to: (1) keep the stove lit for a day's worth of heating and (2) burn the charcoal naturally and safely in the process of stove use. I also mustn't forget ... I haven't dealt with the heat extraction problem yet either.
I guessed that charcoal burning was largely a matter of air supply ... the natural draft of the 'Finnish' burner stops working efficiently when the gasification is nearly complete. Blowing on or through the coals with a bellows, as a blacksmith would, shows that there is still a great potential heat source there.
December 16-25, 2008
Although I like the idea of a natural draft stove, I decided to try a version powered with a low voltage fan which could be run from a battery if necessary. I had purchased a small gasifier camping cook stove last summer which looked to be a promising design, although not robust enough as it stands as a heating unit.
Here is what I came up with as a workable design. I bought a military 50 caliber ammo can to be used as the main enclosure, three 2 quart stainless steel 'bain marie' inserts used in commercial restaurants for fuel canisters and receptacle, two 24 volt DC 80mm brush-less computer fans and grills, two old DC cell-phone chargers (wall-warts) to power the fans, and a piece of 2.5 inch U-bend stainless steel muffler pipe to be used as a heat exchanger. Although I could have scrounged more for freebies and did this for almost nothing ... I just went ahead and bought most of this on-line for a total of about $75 including shipping.
I used an electric jig saw with a metal-cutting blade to cut holes in the ammo can, one on top to accept the fuel canister, and one on the side for one of the fans. I cut about 2 inches off of the top of one bain marie to create a collar to accept the fuel canister, and using some cobalt drill bits I drilled a series of holes in the tops and bottoms of the other two bain marie's for the primary and secondary air supplies required for the 'gasifier' burn. The top holes are 0.25 inch diameter spaced 0.5 inch apart, the bottom holes are 0.125 inch diameter spaced 1.0 inch apart.
The ammo can is airtight, so the fan mounted on the side provides the primary air through the bottom holes and the secondary air (for smoke burning) through the top holes. The collar and the fuel canisters have matching sets of holes, so that the canister can be rotated in the collar to adjust the secondary air flow relative to the primary air flow ... to allow the most effective burn for a particular fuel. Another useful feature providing further adjustment is that the fans can operate over a wide supply voltage range from less than 5 volts to 24 volts with a proportional increase in speed over this range. I put a metal baffle between the fan and the fuel canister ... to keep the radiant heat from the charcoal in the fuel canister from melting the plastic fan!
The entire unit is placed in our old wood stove as a safe enclosure which vents the fumes. The U-shaped pipe with a fan attached to one end is placed on top of the fuel canister. When the fire is burning, the stainless pipe and its fan act as a heat exchanger to bring the heat into the room. Of course we have active smoke and CO detectors in the room!
The stove burns for 1-1.5 hours on a 1.5 liter canister full of fuel, with 200 degree Fahrenheit air exiting the heat exchanger pipe throughout that time. When about 0.5-1 inch of hot coals are left in the bottom, the fuel canister is lifted out of the unit with a pair of pliers, the other fuel canister is filled and put in the receptacle, and the remaining hot coals are dumped on top to immediately re-ignite the burner without delay in heating.
We found that a slow fan speed (with 5 volts driving it) on the heat exchanger is best and it also helps to make sure that the hot end of the pipe is long enough to extend past the inlet (fan) side so that the hot air is not just fed back into the heat exchanger. We also put an old dinner fork in U-tube to break up laminar flow and help promote more efficient air heating in the pipe. To keep the heat output high throughout the burn and to use all the charcoal, we found that the burner fan velocity needs to be increased (driving volts increased from 5 volts to somewhere between 7-9 volts) when the the transition from gasification to charcoal burning takes place. At these low voltages, the fans are almost silent! With this stove, 10 or so hours of burning time and something like 10-15 liters of fuel produces a total for the entire day of 200-250 ml of ash and no charcoal. This ash is spread on our slippery sidewalk outdoors. A very efficient, low pollution, batch loading pellet stove ... and tidy too!
February 2, 2009
I've designed a new heat exchanger with much better effect! With this unit I have increased the surface area to air-volume ratio of the heated part. This design uses five 1" diameter stainless steel tubes for air flow, which each have 2.5 times the surface area to volume ratio as the single U-tube. The new heat exchanger has a baffle at the back end to prevent hot gases from circumventing the heat absorbing parts. This results in air exiting the heat exchanger at 420 degrees Fahrenheit at the lowest fan speed and 350 degrees at the highest fan speed ... quite a bit higher than the 200 degrees seen at low fan speed with the other heat exchanger. While this comparison of old to new is not too precise now, I will do some more careful tests in time to come.