How to Calculate the Volume of Landfill Gas

Due to changes in the way landfill sites are licensed and controlled, the U.S. Environmental Agency requests regular measurements of the gas flow rate, which is a measure of how much gas is extracted from the landfill in Newton meters cubed per hour (Nm^3/hr). Using calculations and, to some extent, estimations based on the values you have, is cheaper and less disruptive than digging into the landfill and installing monitoring equipment, but there will be occasions when you need to measure the volume of gas using a wet test meter.

Things You'll Need

• Flame ionization detector
• Calculator
• Gas flow meter
• Barometer
• Tecjet valve

Show More

Instructions

• 1

  Measure the percentage of methane present in your landfill using a flame ionization detector. Expect the landfill gas methane percentage value to be between 45 and 55 percent for the average American landfill. Multiply the percentage value of landfill gas methane by 1000. Record the value obtained, which represents British Thermal Units (BTUs) per cubic foot, a measure of the heat required to raise the temperature of 1 pound of water by 1 degree Fahrenheit, according to SI Metric.

• 2

  Multiply the "running load", or power output of the landfill machine, which should be written in the machine's documentation, by 1.34 to give the kilowatt to horsepower equivalent per hour. Expect the "running load" to be in the region of 700 or 800 kW. Multiply the resulting value by the value of BTUs per cubic foot to find the required BTU's per hour. Divide by 60 to convert this value to BTUs per minute.

• 3

  Divide the horsepower equivalent by the number of BTUs per cubic foot of landfill gas to give a landfill gas flow in cubic feet per minute at the required percentage methane. An example flow for a system running at 800 kilowatts and with a 45 percent methane concentration would be around 300 cubic feet of gas flowing through the landfill system every minute.

• 4

  Calculate the maximum possible volume of gas for each engine if you have more than one. Add together the resulting values to give an estimation of the total site's gas volume and flow of gas per minute, which changes according to changes in the type and composition of gas present, the engine load, the temperature, the moisture level in the "soil" and the moisture levels in the gas itself.

• 5

  Make the calculation more specific if possible. Define some basic variables,. Measure the methane generation rate using a choice of methods. Either estimate the methane generation rate by comparing your landfill with the average landfill site using a comparative amount of energy and general correlations based on landfill mass and age, or use empirical well testing, which involves field testing at the landfill using a barometer. Measure the methane generation capacity (L0) using a standard comparison chart. According to the Environmental Protection Agency, the value depends on the type of waste deposited in the landfill, but a conventional setup has a default L0 value of 170 m2/mg.

• 6

  Install an independent meter to monitor the gas flow and measure its duration of flow. Use a Tecjet valve with a power range of 20 to 20,000 kilowatts, according to Shindaea, for fuel metering and measurement of low pressure fuel systems, which accurately calculates fuel temperature, pressure upstream and the valve position to determine gas flow, and therefore volume in a given, defined period of time.