If I had to guess as to why use coffee beans as fuel, I understand that when roasted, they are actually subjected to a process called torrefaction. In this way, moisture and other undesirable compounds in the raw biomass are boiled off. What remains has a Btu content just under that of coal, burns more consistently, and is resistant to moisture. Even if the coffee was first used to make the tasty beverage, I'm sure the used grounds would still have plenty of use as a gasification fuel.
The costs of processing the 'fuel' is actually paid for by the first use of coffee: drinking. To get a ready-to-use gasification fuel as a by-product sounds like a great way to extend its uses.
p.s., sorry, I think I duped this reply