Just thinking about building my first gasifier. Fuel is to be wood chip but would like to build it so it can be run on a mixture of wood chip and sawdust eventually. I plan to use automation to overcome some of the difficulties of using a fuel mix like this. I have a tube 130 mm internal diameter and 400 mm long. I can add extra length if needed. I intend to use thermocouples and a map sensor to control temp and airflow.
Intend to use a flat plate that rotates with a scraper bar to hold the char bed at about 900 degrees. If the temp drops below this the plate will rotate and the scraper bar will empty the plate allowing fresh char into this zone. It may only move 1/8 th of a turn depending on temperature. The map sensor will maintain a .5 hg pressure drop within the system by utilising a servo to open or close a tap that alters airflow into the gasifier depending on Fuel load requirements. I intend to use a pipe with a wide slot pointing down as an air injector. I believe this can be oversized as air velocity is dependent on pressure drop and with pressure drop being maintained by opening or restricting air flow set by the map sensor
To put it simply if the woodgas required is 10 cfm the velocity of the air into the Gasifier is the same as if the woodgas required Is only 1 cfm per minute. The rotating plate maintains the movement of wood through the system while ensuring a high temp to crack tars. There will be no restriction in flow other then the rotating plate to time fuel flow through the gasifier. Fuel flow is governed by temperature in the char bed this way.
For a start if I can get a unit producing woodgas to flare off will be good then move onto running a lawnmower engine eventually.
My questions are is maintaining 900 degrees in the char bed enough or should it be higher. How much distance is required from air inlet to char bed. Any other thoughts are welcome.