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.
Here is an example of a small scale pelletizer. I do not know these folks or their product, but use it as an example of basic cost for a small unit for small farm settings. http://pelletpros.com/id68.html It would take a dedicated amount of time and energy to harvest, dry, and pelletize duckweed on this micro scale. You probably wouldn't break even on costs unless you had at least five water acres of duckweed and a method to dry it cheap or free.
The briquette method just might work. Interesting concept. There is a very thin waxy coating on duckweed fronds that lends to pelletizing. That process produces heat which helps the wax bind the duckweed material together. From the looks of a small scale briquette model, there isn't going to be much heat generated. Maybe it doesn't matter. I've got some dried duckweed. Will see how it holds its shape under slight to moderate pressure. Keep on innovating This is great.