This is really interesting. I'm fortunate enough to live in a city that collects yard waste and composts it. I've never considered that it could be used as an energy source, much less a carbon negative one. I doubt I could produce a usable amount of electricity by myself with something like this, but I'd be curious to see how much energy a city of 900,000 could produce with yard waste.
Edit: From the feedstocks page (http://www.allpowerlabs.com/support/support-feedstock) it looks like this works best with wood and woodlike fuels. I'd still be curious if grass clippings could be processed in a way to make them work similarly.
Grass clippings and a variety of waste streams can be pelletized to produce a fuel that can be gasified. Although it's generally not recommended to leave pelletized fuel in a gasifier during shut down, as any residual moisture will cause the pellets to decompose into their powdered form and thus clog up the reactor. Solid fuel handling is more difficult than liquid or gas fuels as you have to use augers and gravity. There is a tendency for "bridging" and compacting where the space between the solid components become either too large or too small. Different reactor designs can accommodate different fuel sources but there is no one size fits all for all fuel types.
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While composting is great, it does produce a significant amount of methane, which is a much worse greenhouse gas than carbon dioxide (~84 CO2e). Ideally this would be captured and put to use.
The 'bridging' problem is one area where vehicular applications have less of a problem given the fact that the generator is constantly being shaken, especially when used on tractors or other similar equipment (e.g. old-style bone-rattler pickup trucks). A modified diesel engine in which the injectors have been swapped for long-necked spark plugs can run on gengas and has a higher efficiency and relative power output than a petrol engine which is fed with the same fuel. On this type of application the extra bulk for the generator and fuel doesn't matter as much either.
Adding a spark plug (and the timing circuitry) is definitely possible. You can often find diesel engine blocks that have been converted to propane or natural gas that are essentially this arrangement.
You can also run a diesel engine on duel fuel, where the (bio)diesel injection provides the flame front to ignite the woodgas and air mixture. In this case no modification is necessary. Only the bare minimum, equivalent to just idling on diesel is necessary. This is a robust system where the governor for the diesel engine can compensate for lower woodgas fuel content when need be.
Edit: From the feedstocks page (http://www.allpowerlabs.com/support/support-feedstock) it looks like this works best with wood and woodlike fuels. I'd still be curious if grass clippings could be processed in a way to make them work similarly.