Metering, Mixing, Temperature, and Time
The Mimi Moto forced draft TLUD achieves around 1-2mg/min PM2.5 at high power without an appreciable amount of residence time, as seen below. The jets of forced air create a downward flow of flame but there is only 7cm between the top of the fuel bed and the bottom of the pot when starting the stove.
Experiments have shown that elevated temperatures shorten the combustion time for CO and PM 2.5. At 900°C the combustion time required for complete combustion is less than half that at 700°C for all studied biomass particles. (Li, 2016) At 900°C, a residence period of between 0.6 to 1 second resulted in close to complete combustion of well mixed CO and PM 2.5. (Lu, 2008, Yang, 2008, Grieco, 2011). Boman (2005) reports that high temperature (>850°C) in a 5kW combustion zone combined with air rich and well mixed conditions for 0.5-1.0 second in the post combustion zone resulted in an almost complete depletion of particulate matter. Interestingly, when temperatures are around 900°C the near complete combustion of CO and PM requires only short residence times of 0.5 second. During such conditions, the residence time in the post-combustion zone is of minor importance for minimizing the emissions of products of incomplete combustion. For optimal results, a residence time of 0.5 seconds is suggested.
The Mimi Moto forced draft TLUD is clean burning at 950°C with very limited combustion time. Perhaps the combination of 1.) Metering the right amount of wood-gas into the combustion zone 2.) Coupled with molecular mixing 3.) At around 950°C reduces the need for 4.) Longer combustion times?
References
C. Boman, A. Nordin, R. Westerholm, M. Öhman, D. Boström.
“Emissions from small-scale combustion of biomass fuels- extensive quantification and characterization.” Umeå University, 2005.
H. Lu, W. Robert, G. Peirce, B. Ripa, and L. L. Baxter. “Comprehensive study of biomass particle combustion.” Energy Fuels, 22, pp. 2826-2839, 2008. doi:10.1021/ef800006z
Y. B. Yang, V. N. Sharifi, J. Swithenbank, L. Ma, L. I. Darvell, J. M. Jones, et al.
“Combustion of a single particle of biomass.” Energy Fuels, 22, pp. 306-316, 2007. doi:10.1021/ef700305r
E. Grieco, G. Baldi. “Analysis and modelling of wood pyrolysis.” Chemical Engineering Science, 66 (2011), pp. 650-660
E. Hroncova, J. Ladomersky, j. Valicek, L. Dzurenda. “Combustion of Biomass Fuels and Residues: Emissions Production Perspective.” Developments in Combustion Technology, 2016 DOI: 10.5772/63793
J. Li, M. C. Paul, P. L. Younger, I. Watson, M. Hossain, S. Welch. “Prediction of high-temperature rapid combustion behavior of woody biomass particles.” Fuel, Vol. 165, (1 February): 205-214, 2016.
doi:10.1016/j.fuel.2015.10.061
This all looks very familiar, a Tom Reed style top lit gasifier look-a-like. “Metering” in the title is described as metering of the wood-gas fuel, but you are also metering air. Do you have any control over the secondary air? It would be good to know what the excess air factor is. With these devices you can find a sweet spot for combustion but what I have always wondered is how quickly the PM 2.5 emissions curve rises either side of the sweet spot. For example; if your lowest PM 2.5 rate were at an excess air factor of 1.4 then what is the PM 2.5 rate at and excess air factor of 1.1, 1.2, 1.3, 1.5, 1.6, and 1.7. Its not simple to keep mixing velocity and total output constant but it would still be of interest. If you can’t control the secondary air then one option is to insert less or more volatile fuel pellets into layers of the fuel bed and see if that shifts the excess air range.
Also, is that thermocouple shielded?
Thanks for sharing your results so far.
Alex English
Enterprise, Ontario, Canada
Thank you for sharing these results.
For what I see at the pictures, the temperature of the flames at 950°C are measured without the pot? Higher temperatures and cleaner combustion is achieved when the pot is placed on the stove, because the pot reflects more heat back into the combustion chamber and it extends the mixing/combustion time.
The ratio between primary and secondary air is very important to control the right amount of wood gas for complete combustion.
Too much wood-gas > flames too high > flames hit pot before fully mixed > bad emissions (while combustion temperatures are still high)
Too little wood-gas > flames too weak > temperatures too low for full combustion.
The Mimi Moto stove has control over the combined primary and secondary air to maintain the optimal spot for combustion. Depending on the fuel (pellet) type we tune the chambers primary to secondary air-ratio for optimal performance.
Aart de Heer
Mimi Moto