From gasoline to gasification, or why we don’t power cars with wood today (2017)

The gas going into the engine ideally is broken down to hydrogen and carbon monoxide, with some methane. This is also scrubbed of all tars and ash for the safety of your engine. Within the pistons of your generator's engine it is combusted to water and carbon dioxide. Unlike gasoline and diesel which needs to pyrolysis within the engine's cylinders or fuel rail, the syngas has already been broken down to its bare combustable components...so emissions are usually much lower.

That being said, there is a variety in the solid fuels used, and operational and engineering problems to overcome for some waste streams. There are backyard versions that use water to scrub the woodgas, leaving behind large ponds of toxic waste full of PAHs (Poly Aromatic Hydrocarbons) and other carcinogens. But the exhaust from the motors is generally better than that of a gas or diesel engine. (Source, I used to work for All Power Labs, where we did a variety of tests on our fuel, exhaust and ash production)

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.