2.2.4. Biomass

Biomass power plants in California use a combination of raw biomass residues (including forest and agricultural residues from trees, foliage, roots and chips from wood processing residues) and, secondarily, municipal solid waste residues (mostly cellulose) [42]. A total installed capacity of 1325 MW is reported [36], which includes biogas (*cf.* Section 2.2.5). The WECC heat and power co-generation model from the Ecoinvent database [38] was selected to represent biomass electricity generation, and all energy and environmental impacts of this multi-output process were allocated on an energy content basis. This model is limited to the use of woodchips as feedstock; however, given that

biomass-fired plants contribute just 2% to the total electricity generated in-state (see Section 2.3), such simplification was deemed acceptable.

#### 2.2.5. Biogas

Biogas is a mixture of gases produced from the anaerobic decomposition of municipal solid waste (kitchen waste, garden waste), livestock manure, food processing waste, agricultural wastes and industrial wastewater. The process occurs in a controlled environment such as in airtight containers, in floating covers on lagoons or directly in landfills [43].

The resulting biogas is mostly composed of methane (CH4) and carbon dioxide (CO2), but also contains other hydrocarbons and significant traces of ammonia (NH3) and hydrogen sulfide (H2S).

The Ecoinvent WECC biogas-fired heat and power co-generation model [38] was selected to assess biogas electricity generation, and all energy and environmental impacts of this multi-output process were allocated on an energy content basis.
