Can you burn duckweed in a rocket heater? Yes and no. First, duckweed contains approx. 94% water. This requires drying energy. If not a renewable form, like solar, then the most efficient method I've seen is a drying process by Compact Contractors of America. They use a spray dryer with wet biomass and achieve a fine, high Btu power that can power gas turbine power systems. This would work in a rocket heater. I'm looking into the mass balances of this method currently.
OR you could solar dry duckweed to get a large percentage of moisture out and then pelletize it. Duckweed contains a waxy coating on the fronds that make a great binding agent in pellets.
Tamra at www.DuckweedGardening.com