Yes, the tubes are small, 5" standard, 4" smallest.
Depending the load , engine size, i can make it hot on the outside, i have some heat pictures on another threat.
Using the heat to pre heat air or fuel does benefit the gas efficiency or at least the fuel consumption.
For heat transfer and materials there is a logical path to follow… where is the heat flowing to ?
( Logical from hot to cold )
The small size tube is creating a kind of balance between descending (cold) fuel with the ascending (hot) gas
Carbon / charcoal however is also a great insulator…
If your nozzle protrude far enough inside the drum, the outer walls will stay cool.
Practical, i could build a charcoal gasifier from or within a plastic drum… just need to know where as i am going to bring the heat from the nozzle…
To give a number; for each pound of carbon gasified to CO you generate about 4000 BTU of heat
The more heat you get in the fuel (preheating) the more efficient your gasifier will be ( higher CO % in the gas )
Thats why i keep the vertical shape, hot gas passing thru the charcoal, gas exitting cold ( ambient)