A natural draft TLUD can be as clean burning as a forced draft TLUD burning wood pellets. On the other hand, a natural draft Rocket stove needs a fan to be clean burning.
Dr. Tom Reed’s forced draft (FD) Woodgas stove achieved an emissions rate of 2mg/min of PM2.5 with pellet fuel (ARC, 2015). The Kirk Harris natural draft (ND) TLUD emitted 0.7mg/min of PM2.5, when tested at Lawrence Berkeley National Lab with pellets.
WHO Intermediate Emission Rate Targets
|Unvented stove||Vented stove|
|PM2.51.75 mg/min||PM2.5 1.75 mg/min|
|CO 0.35 g/min||CO 1.45 g/min|
ND TLUDs tend to be pretty tall to generate necessary draft. A FD TLUD can be shorter since the fan creates the draft. Taller ND TLUDs can be expensive to manufacture. Precise control of primary air enabled a 3 to 1 turn down ratio in the Harris ND TLUD, difficult to achieve in a FD TLUD. (The FD Mimi-Moto, for example, has two combustion chambers, small and large, to provide cooks with high and low power).
Modern ND TLUDs
- The primary air is adjusted to control the rate at which pellets are turned into combustible gases.
- The secondary air jets cover the fuel bed.
- A hole in the concentrator plate above the secondary air jets forces the flame into a vertical cylinder.
- The cylinder of flame then enters static devices that create further mixing of air, flame, and gases.
- The flame is shortened and does not touch the bottom of the pot where gases can condense into smoke.