Capturing Biogas from Landfills and Using it for Cogeneration
In a process quite similar to the one that takes place in digester tanks in a wastewater treatment plant, bacteria in a landfill breaks down the trash in the absence of oxygen (anaerobic digestion). A by-product of this anaerobic breakdown is landfill gas, which contains approximately 50% methane and 50% carbon dioxide with small amounts of nitrogen and oxygen. Today’s companies must measure the amount of landfill gas produced to sell it, mix it, get emissions credits and meet reporting requirements. To do this, a series of pipes are embedded within the landfill to collect the gas. A “flare” is used to burn off the collected gas. For cogeneration, this gas is compressed and mixed with natural gas, which in turn can be used to run a generator.
The Sierra Solution
Landfill gas is very humid, at low pressure (20 - 40 mBarg) and about 40-60°C (100F) when it is produced by the landfill. Most companies are interested in measuring the biogas as it leaves the landfill, but this is difficult. Low pressure makes differential pressure (dP) devices like orifice plates unsuitable since dP devices require a fairly large differential pressure to operate. Also, the landfill gas is often very dirty, with a high moisture and particulate content. This can clog up devices like annubars and orifice plates, and gum up meters with moving parts like turbine meters. The flagship or North America’s best-selling (1) mass flow meter family of products, Sierra’s MultiTrak 640S, provides the solution for all of these problems. The insertion design eliminates pressure drop, has no moving parts, and can measure both high and low flows with a 100:1 turndown.
(1) 2009 Flow Reasearch Sudy, Yoder