Hello Rick and welcome to the DOW
I would be very careful with areas in the gasifier where a large amount of hot gas can accumulate or be stored. Wherever the gas is hot I try to keep those areas to a minimum . If hot gas and air gets mixed in a small area you will get a burp, belch or fart . If you have a large area under the same conditions you have a bomb.
Somewhere here on the DOW a member Wayne Baker writes about his gasifier ending up on the top of his house back years ago by not being aware of the above.
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Now about the FEMA
I have made a couple of fema type gasifiers work but it is very tricky. The fuel depth or surface area has to be controlled . When I was experimenting with the fema I would start with charcoal at the grate then golf ball , base ball and bigger and this wood had been in 55 gallon drums and set fire and when burning good the top was put on and the fire put out leaving partially chared wood
This has to do with the amount of surface area of the fuel the flames and heat is being exposed to and very difficult for me to explain . The best way to try to explain is if the fuel is not put in properly the process is flooded by fuel .
Also the same can be accomplished by adding in the proper amount of pellets as the gasifier is being operated . Look over this thread about the dizziler and you can see what I am trying to explain . If he puts in too much small fuel the gasification will quit . 
Thanks Wayne