The Landfill gas tool is used to define the "Landfill gas" fuel selected by the user from the "Fuel type" list in the other worksheets. The landfill gas (LFG) may be collected from a nearby landfill via an existing LFG collection system at the landfill, or through the construction of a new LFG collection system. The user enters the cost for the new LFG collection system in the Cost Analysis worksheet.

Landfill gas is generated by the biological decomposition of wastes placed in a landfill. The composition of landfill gas is highly variable and depends on a number of site-specific conditions including solid waste composition, density, moisture content, and age. The specific composition of landfill gas varies significantly from landfill to landfill and even from place to place within a single landfill. However, landfill gas is typically comprised of methane and carbon dioxide, approximately 50 percent each by volume, with trace quantities of other compounds. Methane is the primary component of landfill gas that contributes to the gas's heating value. The heating value of methane is typically defined on a volume basis.

There are numerous models available for estimating rates of landfill gas generation, however accepted industry standard models are generally first order kinetic models that rely on a number of basic assumptions. These models are used to predict the variation of landfill gas generation rates with time for a typical unit mass of solid waste. This generation rate curve is then applied to records, or projections, of solid waste filling at a site to produce an estimate of the site's landfill gas generation over time.

RETScreen uses the Scholl Canyon Model. This model, with defined default parameters, is the empirical, first-order decay model most widely accepted and used by industry and regulatory agencies, including Environment Canada and the United States Environmental Protection Agency (USEPA). There are many more detailed models available for the estimation of landfill gas generation rates, however, these models require more specific knowledge of the waste quantities, waste composition, and land-filling practices associated with the site than is normally available, especially for older landfill sites where such records were not required. The Scholl Canyon Model is based on the assumption that there is a constant fraction of biodegradable material in the landfill per unit of time, and is an estimate of the generation of methane from this biodegradable material [The World Bank, 2004].