The Rocket stove requires forced draft to create sufficient mixing in the flame to achieve Tier 4 for PM2.5. In addition, when sticks are pushed too fast into the Rocket, too much wood gas enters the combustion zone and a lot of smoke is emitted. Metering of the fuel is necessary to keep the Rocket stove even moderately clean burning. Unfortunately, that is unlikely to happen in real life.
On the other hand, the TLUD supplies a constant amount of wood gas into the combustion zone. A five inch in diameter reaction vessel produces around 4kW of energy (5 liters of water boils in about 20 minutes using a pot skirt with a 6mm gap).
The emissions of CO and PM2.5 can be very low in a natural draft TLUD when:
- The primary air controls the rate of reactions (how fast the wood pellets are turned into wood gas).
- The jets of pre-heated secondary air provide sufficient mixing of wood gas, flame, and air by completely covering the top of the fuel bed.
- The burner section allows sufficient Temperature, Time, and Turbulence to complete the combustion process.
Nothing is perfect. The TLUD, whether forced draft or natural draft, has trouble achieving enough Turn Down Ratio to simmer food efficiently. It’s also generally necessary to burn pellets to lower emissions to Tier 4 levels. At the same time, we love to cook on TLUDs and it is great not having to constantly adjust the fuel!
As Dr. Tom Reed said, “Now you’re cooking with the real natural gas.”