A method to convert biomass to gas is the natural process of anaerobic digestion/decomposition in a device called an anaerobic digester. Anaerobic digestion is a process by which micro-organisms break down biomass in the absence of oxygen. 
In a large oxygen-free vessel called a digester, or in a covered pool or lagoon, the biomass is mixed in a series of stages with organisms such as acetogens and methanogens. The organisms chemically interact with the biomass resulting in biogas consisting primarily of methane and carbon dioxide. This gas can then be used as a substitute for natural gas to power reciprocating engines, gas turbines, and fuel cells.
Typical biomass used in anaerobic digesters includes animal manure, organic residues from food processing and breweries, other organic industry residues, and energy crops grown specifically for this purpose.
In a related process, anaerobic digestion occurs naturally in landfills. Under the right circumstances, landfill gas can be captured and used for power generation. In these applications, a reciprocating engine or small gas turbine is typically used for power generation since it is able to use the low-quality gas that is common in landfills.