Pipe2016:Gas provides an excellent modeling tool for designing and analyzing a landfill gas extraction system.
There are several approaches to modeling an extraction system: a simple-yet-accurate approach, using Pipe2016:Gas, is illustrated below.
In this example a compressor (fan or blower) at the collection point creates a vacuum within the piping system. The vacuum is usually about -15 to -30 inches of water (around 1 psi or less). The compressor must create enough suction at each wellhead, or collection node, to draw the required amount of gas. For this example it is assumed that each of the nodes will collect 60 standard cubic feet/minute (SCFM). Thirteen collection nodes are modeled at the end of 4-inch pvc pipes (yellow), producing 780 SCFM. The rest of the collection system is 8-inch pvc (black).
Control valves are positioned at the collection nodes. The piping system and compressor must be sized so a sufficient vacuum is pulled at each node. The required analysis is easily done by designating the desired inflow at each wellhead and calculating the vacuum pressure required. If this pressure is sufficient to allow the control valve to operate at the desired flow using the available pressure drop then the collection system will work. If the pressure is insufficient then either larger pipes or a larger compressor are needed. Conversely if the pressure differences are significantly larger than required, then smaller pipes or a smaller compressor may be used.
Figure CS1-1 Landfill Gas Extraction System
Model Data – The Gas System Data Menu is shown in Figure CS1-2. For this example methane properties were used. The Gas model provides a Lookup Properties Tool which gives the required properties of methane at 86 degrees F as shown in Figure CS1-2 (specific gravity, viscosity etc.). However, landfill gas is not pure methane so site-specific gas properties should be used instead. In this figure the flow units are SCFM and the pressure units are inches of water, which are commonly-used units for landfill gas extraction studies.
Figure CS1-2 System Data for Landfill Gas (Methane)
Elevation data may be entered for each node. Node elevations for this example are shown in Figure CS1-3.
As illustrated in Figure CS1-4, the pipes are all pvc with a roughness of 0.1 millifeet. The reservoir pressure represents the suction pressure for the compressor. For this example, -15 inches of water was used, representing a compressor which produces a vacuum pressure of 15 inches of water. Running the model calculates the pressure drop at each wellhead control valve for a total flow of 780 SCFM at this vacuum pressure.
Figure CS1-3 Elevation Data
Figure CS1-4 Pipe, Reservoir and Junction Data
Results – The pressures and flows for the simulation are shown in Figure CS1-5. The analysis shows that the pressure drop across the control valves at the wellheads varies from 6.7 to 4.9 inches of water (assuming the well head pressure is atmospheric). This should be sufficient to overcome losses through the control valves and control the flows. Required valve settings can also be determined. For example, the valve setting for the smallest pressure differential (4.9 inches) means that the control valve will be set such that 60 SCFM will cause a pressure drop of 4.9 inches of water. This datum allows the selection of an appropriate valve. It should be noted that identical results will be obtained if the reservoir representing the compressor suction is replaced by a compressor (pump) which operates at 15 inches and 780 SCFM while discharging the gas at atmospheric pressure.
Figure CS1-5 Pressures (inches of water) and Flows (SCFM) in the Collection System