TLUD Vesto Grasifier
Crispin Pemberton-Pigott, New Dawn Engineering, Swaziland, April 2, 2010
An update. We seem to have a good set of dimensions for the Vesto bodied Grasifier. The cooking is quick, the CO/CO2 ratio is low, the burn time is long and the thermal efficiency is high, the EA is low and the flame temperature is high.
Boiling, 6 litres, well, still too soon to tell because we were messing around so much but about 5-7minutes per litre. Let's say 7.
The CO/CO2 ratio is 0.2% for long periods of time. Let's say 0.3%. If floats between 0.18 and 0.3% for ages.
Thermal efficiency is quite constant 45-50%, let's say 45%. It peaks just over 50%.
Burn time is over 1 hr per load, two hrs at the lower burn rate and about 90 minutes at the higher. The volume is 2 litres max (will check it that was a full load). I wasn't very concerned about that, just about the burn.
The Excess Air about 225% with it wandering from 180% to 300%. A lot of the time it is below 250% which is a good sign.
The NO is quite high indicating a seriously high flame temperature. About 550 ppm (EF, O2 at 0%). It is comparable with a hot coal fire which was a big surprise - the biggest of all. I don't think is has to do with the fact the fuel is Switchgrass, I think the temperature of formation (700) is being reached over a large portion of surface area of the fire.
The combustion area (pyrolysation area) is 123 cm^2 and the air supply (with fuel piled against the holes) is 3.65 cm^2 for a ratio of 3%. That is, the area of the primary air supply holes is 3% of the pyrolysing area. The Primary/Secondary air comparison is 3:16 (1:5.33). As the primary air is partially blocked by fuel, the effective ratio is greater, probably closer to 1:8. I want to check the EA on high power before trying to reduce that ratio.
Peter Van Driel from Waterloo was in attendance. He provided some interesting processed wood to try as an ignition fuel but that was pretty much a failure. It was not hot enough long enough to create decent ignition of the upper surface of the pellets (this is a TLUD). He is offering to field test a Grasifier on a camping trip with his buddy from Darfur who lives around here. There is some excitement to send some 'home'. For camping it will have to be a minimalist version of the Vesto because of weight. It would be nice to preserve most of the heat transfer efficiency.
I tried a primary air feed of 2.6% a couple of days ago and it was very nice, however the power was on the lower edge of the fire being sustainable and the power level too low. I had excellent combustion readings and a thermal efficiency of 50% but it just took too long to boil 6 litres in a 7 litre pot - nearly an hour.
My guess is that a variable rate of 2.5 to 4.5% on the primary side would function well, save at the end when the area should be maximised. If it covers that range, the power level might be modified by nearly 1/2 which is .....ehh.... better than no change at all.