Natural Draft Top Lit Up Draft Stoves, 2021

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

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.