What is True Primary and Secondary Gasification?
Gasification is the common name for the chemical process of "Pyrolysis" that biomass fules undergo
when they are heated to different temperatures. This combustion process results in an extremely
high final temperature (over 2,000° F). This completely burns the exhause gases (smoke) to nothing,
virtually resulting in a smoke-free burn process. Click on the images below for a larger version.
Photos of Secondary Burn Temperatures Over 2000° F
Based on my mechanical engineering education/experience and having studied the pyrolysis
process
in detail, I would say that gasifiers are best NOT modeled as diffusion flame. Diffusion
flames
are
those in which the mixing of fresh air happens only by diffusion, as in a candle.
Anything that
provides mechanical mixing
of the fuel and fresh air will result in higher combustion temperatures.
One measure of combustion temperature is to observe whether "ash fusion" has been reached.
Ash fusion is the temperature at which the ash starts to melt and fuse/weld together. Ash fusion temperatures vary by species, but a reasonable range is about 2100° to 2500° F. In my EKO40
system with 1,000 gallons thermal storage (water battery), I definitively reach ash fusion
temperatures in the secondary chamber and have placed a special, high temperature
thermocouple to measure these extremely high temperatures. I have monitored these
temperatures for several months now and have measured temperatures exceeding 2,180°.
In addition to a thermocouple placed in the secondary chamber, I have also placed a
thermocouple in the primary (low temperature) chamber so that the overall system efficiency
can be measured. With secondary chamber temperatures over 2,000° F, the flue exhause
temperature range from 350-425° F. Thus, the "real life" system efficiencies are approximately
80-85% (depending on the type of moisture content of the fuel being used).
