Landfills are commonly formed by depositing municipal solid waste and many other types of trash in a canyon or pit (or even on flat ground) and depositing soil on top of the solid waste and trash. It is common for there to be alternating layers of trash and soil, one atop another in the landfill. The waste and soil layers are individually and collectively porous media through which gas may readily flow. Once municipal waste is disposed of at a landfill, the organic portion of the waste begins to decompose. This decomposition initially proceeds through an aerobic biodegradation process wherein much of the available oxygen in the buried waste is consumed. This decomposition produces end products which are primarily carbon dioxide and water. After a while, usually ranging from a few weeks to several months, the waste consumes essentially all the free oxygen in the landfill. The decomposition of the waste then proceeds through an anaerobic biodegradation process. During the anaerobic decomposition of the waste, microbes break down the cellulose and other organic wastes to produce methane (CH4) and carbon dioxide (CO2). The landfill gas (LFG) that is formed typically includes about 55% methane, 44% carbon dioxide and less than 1% trace gas. The trace gases consist of a wide variety of volatile compounds, which vary depending on the particular landfill.
As anaerobic gas production proceeds, the methane and carbon dioxide concentration in the landfill increases. The mixture of methane and carbon dioxide eventually begins to migrate within the landfill toward the surface of the landfill and into the atmosphere. Surface emissions of landfill gas are not desirable since the primary constituents of the landfill gas are known green house gases. In addition, the trace gases in the landfill gas can also lead to the formation of ozone, and/or result in undesirable odors. Furthermore, the landfill gas may migrate laterally in the subsurface of the landfill and accumulate in nearby buildings or other structures, thereby creating potentially dangerous conditions due to the methane content of landfill gas. Also, the landfill gas may move to regions containing ground water, thereby potentially resulting in the contamination of the ground water. As such, it is desirable to collect landfill gas to prevent these negative environmental effects. Also, it is desirable to collect landfill gas for energy recovery purposes since the methane content of landfill gas can be used as a source of fuel.
Landfill gas well extraction systems are commonly used to control landfill gas surface emissions, control landfill gas subsurface migration away from the landfill, and often to collect landfill gas for energy recovery. These extraction systems typically include one or more vertical and/or horizontal landfill gas extraction wells that are in fluid communication with one or more header piping systems. The header piping system is, in turn, fluidly connected to a vacuum source (e.g., centrifugal blower, etc.).
When the methane concentration is relatively high and nitrogen is relatively low in the landfill gas, for example, little or no air may be penetrating the landfill, thus the extraction rate of the landfill gas can be increased. When the extracted landfill gas is nitrogen rich and methane poor, and/or when oxygen is in the landfill gas, or when the molecular ratio of carbon dioxide to methane is high signaling substantial amounts of aerobic decomposition, the extraction rate of the landfill gas is reduced.
The process of controlling flow of the landfill gas into the landfill well is known as “tuning.” Various techniques have been used to “tune” the flowrate of landfill gas into the well. One technique is to adjust the wellhead valve position. If the wellhead valve position is not calibrated for a given flow rate, this method of operation is not very reliable. The position of the valve handle typically does not provide sufficient information about the well to control it. Another technique is to control the flowrate by controlling the wellhead vacuum. This technique relies on the relationship of well pressure/vacuum to flow for a given well. Reliance upon this method of operation is difficult since the relationship between flow and pressure is difficult to affect while performing day-to-day well field adjustments. As decomposition, moisture, and other conditions change, this method can also become unreliable.
Another technique is to control the flowrate by using a fixed or portable flow measurement device at each wellhead to obtain data needed to calculate volumetric (or mass) flow rates of the landfill gas being extracted from the landfill. This method of control is the most accurate and reliable of the various techniques used to control landfill gas flowrate into the well. The measurements can also be used to monitor the quantity of methane extracted (Landfill gas flowrate times percent methane in landfill gas) and/or the quantity of BTUs recovered per hour (landfill gas flowrate times percent methane in landfill gas times BTUs per cubic foot of Methane times 60 minutes per hour). Measuring the flowrate of landfill gas from the landfill is an essential part of monitoring and adjusting a well in a landfill. The well should be adjusted until the amount of methane recovered is maximized for the long term. A greater amount of methane or energy can usually be recovered over the short term; however, this ultimately leads to diminishing returns. This is seen in stages as increased carbon dioxide and gas temperature, and later as increased oxygen from well over-pull. In time, the methane content in the landfill gas will decline, resulting in a portion of the landfill, usually at the surface, being driven aerobic. The frequency of landfill well monitoring can vary. Normal monitoring frequency for a complete field monitoring session with full field readings will vary from typically once a month to once a week. Well field monitoring should not normally be extended beyond one month. Typical field readings for each well includes a) name of field tester, b) location of landfill well, c) date/time of readings of landfill well, d) landfill gas composition (e.g., methane, oxygen, carbon dioxide, nitrogen, etc.), e) wellhead gas temperature, f) ambient air temperature, g) static pressure of wellhead, h) applied vacuum pressure in wellhead, i) wellhead gas flow, j) wellhead adjustment valve position, k) new wellhead vacuum and flow information after any flowrate adjustment, l) calculation of landfill gas flowrate and methane flowrate; and m) comments and/or notes regarding well, landfill, testing procedure, etc. Other types of gasses in the landfill gas may be tested (e.g., carbon monoxide, hydrogen sulphide, etc.) if problems are suspected in the landfill.
A portable gas monitor is commonly used to “tune” landfill gas flowrate into the well. The equipment cost, equipment maintenance, and personnel costs for constantly monitoring a gas well is generally too expensive and unnecessary to properly monitor a landfill well. The composition of the extracted landfill gas and the pressure in the well is measured periodically (e.g., daily, weekly, monthly, etc.). Generally, a landfill well is monitored every month, three months, six months or twelve months depending on the size of the well, the location of the well and the gas volume flowing from the well. One type of prior art portable gas monitor currently used is a monitor offered by Landtec, a division of CES, Inc. Landtec currently offers several models of portable gas monitors, namely SEM-500, GEM 500, GEM 2000 and GEM 2000 PLUS. These portable monitors are carried to a landfill or landfill well, temporarily connected to the landfill well, measure information from the connected landfill well, disconnected from the landfill well, carried to another landfill well on a same or different landfill, temporarily connected to the new landfill well, measure information from the connected to new landfill well, disconnected from the new landfill well, etc. This process is repeated for each different landfill being measured by the portable monitor. These portable monitors are designed to only monitor and measure one landfill well at a time. These devices can be used to measure the landfill gas composition being drawn from the landfill well, the temperature of the landfill gas, and the vacuum being drawn on the landfill, etc. The testing procedure at each well can take about 3-60 minutes, depending on the type of well and number of measurements taken from the well. These readings are then used to “tune” the flowrate of landfill gas being drawn from the landfill. After the desired readings are obtained, the portable gas monitor is disconnected from the landfill well and then subsequently reconnected to a different landfill well to obtain readings from such different landfill well.
Although these portable monitors enable the testing of wells, these portable monitors have several drawbacks. During the fall, winter and springs months, the outside temperature can drop to below 40° F. in various regions of the world. Operation of these prior art portable monitors can begins to slow down in colder temperatures, and in some situations, the portable monitor will malfunction or altogether stop functioning in colder temperatures. These monitors include internal analytical components and LCD screens that tend to malfunction or fail in colder weather. As such, when the internal analytical components and/or LCD screen does not properly work, the operator cannot obtain and/or take readings from the portable monitor or operate the functions of the portable monitor. As such, during the testing of a landfill well, the testing period may be significantly extended due to slow operation of the portable monitor or be interrupted when the portable monitor fails to properly operate. The only recourse by an operator when a monitor fails is to use a new monitor for testing, detach the monitor from the well and bring the monitor into a warm environment to “thaw out” the monitor, or delay testing of the well until there is a warmer day.
Another problem with these prior art portable monitors is that the monitor cannot simultaneously measure the applied vacuum on the well, the differential vacuum on the well, and the available vacuum that can be applied to the well. These different vacuum values are obtained on different regions of the wellhead. Static pressure or applied vacuum represents the actual pressure applied to the well. Differential vacuum or pressure is the pressure drop across an orifice plate and is used to measure fluid flow through the wellhead. Available vacuum or header pressure is the pressure in the header to the wellhead that can be applied to the landfill well. These prior art portable monitors only have two pressure testing ports that are designed to only determine the vacuum being applied to the landfill. As such, if an operator wanted to determine the available vacuum that can be applied to the well, the operator would be required to reconnect the monitor to obtain such information. Such a procedure is inconvenient, time consuming, and can be difficult and undesirable in inclement weather.
Another problem with these prior art portable monitors is that the portable monitor must be placed on or near the well during testing. As such, the individual using the portable monitor must be constantly near the portable monitor in order to operate the portable monitor, to obtain readings from the monitor, and to determine whether the portable monitor is properly operating. Having to be in close proximity to the portable monitor during testing is inconvenient and can also be uncomfortable during inclement weather.
Another problem with these prior art portable monitors is that the portable monitor is sometimes improperly connected to the wrong well. On large landfill sites, the landfill wells are not always properly marked or easily located. As such, when a tester locates a landfill well and begins the testing of the well, the identity of the well recorded by the tester may be incorrect, thus providing incorrect information about the performance of the well and landfill over a certain time period. In view of the deficiencies that exist in prior art portable monitors for landfill wells, there is a need for a portable monitor that simplifies the testing of landfill wells and which overcomes the past deficiencies of prior art portable monitors.