It is not a new idea; traditionally, the seeds of grass like barley are malted and used to make beer. In that sense, my pusher mower has run on fermented grass more or less successfully; only the grass used to power the mower did not come from my own lawn but the fridge.
I admire the idea, but it cannot be worth the cost or effort. It would be so much easier to just buy fuel or an extension cord.
In my case, if I were fed my own grass I might well refuse to push my pusher mower over the lawn.
But that's me. Not everybody would turn down a batch of fresh grass.
Here's another approach to the problem.
Feed your mower grass directly - it mows by getting fed.
Free fertilizer to boot.
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2011 Honda Insight + HID, LEDs, tiny PV panel, extra brake pad return springs, neutral wheel alignment, 44/42 PSI (air), PHEV light (inop), tightened wheel nut.
lifetime FE over 0.2 Gmeter or 0.13 Mmile.
For confirmation go to people just like you.
For education go to people unlike yourself.