I recently set about designing a flow meter for landfill gas (LFG). My search for a good online reference about LFG was futile, so I decided to create one myself. This article and the next several in this series will be dedicated to calculating some basic properties of landfill gas. The ultimate objective is to arrive at the density and the Reynolds number, which is a fundamental parameter of fluids flowing in a pipe. But first we need to establish some more basic properties.
Composition of landfill gas
The first step is to identify the major components of landfill gas. Because LFG is created as a result of anaerobic digestion, it is hot and very humid. The gas composition also varies as a function of the age of the landfill, temperature, rainfall, and vacuum pressure in the collection system, and the underlying solid waste composition.
After talking with many landfill gas to energy plants and landfill operators, it seems that the normal composition of LFG is roughly in the following ranges:
CH4 = 35-55%
CO2 = 15-35%
O2 = 0-4%
The remainder of the gas (called “balance gas”) is primarily N2 and water vapor, along with trace compounds like Hydrogen Sulfide (H2S) and other contaminants (Benzene, refrigerants, etc.) that depend highly on the composition of the solid waste. Since my primary goal is to calculate the fluid properties, the trace components do not have a significant impact.
H2S is highly toxic and has a very distinct odor, so is a major concern for landfill operators. The smell is detectable at very low concentrations (a few parts per million!), and it poses significant health and environmental problems at concentrations above 50 ppm or so. H2S seems to be created mainly by decomposing drywall, and hence is more of a problem at sites that take a lot of construction waste.
Our next post will explore some of the chemical and physical properties of landfill gas in order to help calculate the fluid properties.