After numerous tests to improve the pellet burning woodgas stoves so that I could use one to add supplemental heat to my greenhouse, I find that all such stoves give out some smoke after the open flames have died out and the stoves have switched to the charcoal burning stage. But since there is not much charcoal left to burn after the smoke dies out, the heat generated from the charcoal burning stage is not long.
This made me to switch from wood pellet burning to charcoal burning, meaning that I started using charcoal as the fuel right in the beginning. Anyone having made BBQ using charcoal knows that the red hot charcoal lasts a long time. So this should serve the purpose of adding supplemental heat to my greenhouse.
I also find that it is better to place the charcoal containing chamber on top of a J-type rocket stove so that charcoal burning could reach a smokeless burning stage quicker than lighting newspaper underneath a chimney charcoal starter shown below:
Note that charcoal was burning so hot that it melt the Al foil tape that fell into the stove and got stuck on top of the central riser of this stove below: