Mr Switchgrass is an inverted downdraft gasifier design that burns/gasifiers chopped switchgrass or weeds with little to no smoke problems. A rule of thumb is that each section will burn for a solid ten minutes at full flame. I measured 66 lbs for one load and it ran for about 40 to 50 minutes.
Experimental investigations of long stick wood gasification in a bottom lit updraft fixed bed gasifier
A. A. Saravanakumar, Spray Engineering Devices Limited, Noida, Uttar Pradesh, India, July 17, 2008