Estimates of landfill gas (LFG) emissions made by the EPA and the Intergovernmental Panel on Climate Change have suggested that landfills are a significant source of man-made methane emissions, ranking 3rd in the US behind natural gas systems and ruminant digestion (interpreted as cow flatulence). Given methane is 25 times more potent as a greenhouse gas than carbon dioxide, its more familiar accomplice in global warming, determining how much methane comes from landfills is important in demonstrating how much of an impact they may have on measured warming trends.
FIGURE 2. TUNABLE DIODE LASER AND MIRROR ARRAY FIELD SET-UP FOR VRPM METHOD
This information, coupled with recent greenhouse gas mandatory and voluntary reporting frameworks (EPA, SWICS) for landfills and the desire to capture as much LFG as possible for landfill-gas-to-energy systems, makes accurate measurement of LFG emissions a critical component in LFG and carbon accounting efforts. The Environmental Research and Education Foundation (EREF), the largest source of funding for solid waste related research in the US, has led the way in funding initiatives to better understand and measure LFG. Significant work has also been done by EREF corporate stakeholders Waste Management Inc. and Veolia Environmental Services Inc., which collectively have advanced this knowledge significantly in recent years. To understand the nuances behind fugitive gas emissions, it is important first to understand both how LFG is formed and how collection systems may influence the release of fugitive emissions.
LFG Formation Within the Landfill
When refuse is initially placed in a landfill it undergoes four distinct stages of decomposition:
- The aerobic phase
- The acid phase
- The accelerated methane phase
- The well-decomposed phase
To demonstrate what is happening in each phase, consider the following example of a 1-yard-by-1-yard-by-1-yard cube of refuse after it is compacted at the working face. Initially, after a few hours or days, this compacted refuse is buried with more refuse being placed on top of it. When this happens, the oxygen in the nooks and crannies of this compacted refuse cube is quickly consumed by microorganisms (e.g., bacteria) that hitched a ride with the refuse, trickled into the waste from daily cover soil, or migrated from adjacent refuse.
Once all the oxygen is consumed in the aerobic phase, these same bacteria break down the organic materials in the waste to cellulosic compounds and fermentable sugars, and these sugars are further transformed into acids (primarily acetic, butyric, and propionic acids), hence the name “acid phase.” Although the acids result in a drop in pH locally within the waste, this conversion also creates an environment that is ripe for methane, or CH4, formation. Enter the methanogens, a family of microbes with the capability to convert the acids that are formed to equal parts carbon dioxide and methane. As the methanogens begin to make methane, the spatially localized pH returns to neutral, and the acids that have built up are consumed. At this point, which can take anywhere from a few days to months, depending on moisture content, our hypothetical cube of waste enters the accelerated methane phase and methane production increases exponentially as the methanogen population increases. Once the majority of degradable materials in the waste have been broken down, the waste enters the well-decomposed phase, and gas production decreases to very low levels.
Why is the above important, and how does this relate to gas production and emission for the entire landfill? This basic description of landfill microbiology is the foundation for how methane is formed in a landfill and is the basis for estimating LFG production within a landfill, which also predicates, from a mass-balance perspective, how much gas can be collected and how much may be lost in fugitive emissions. If you are familiar with EPA’s LandGEM model, the primary method used to estimate LFG production from landfills, then the above description is essentially summarized by the two primary variables within the model, Lo, the methane potential, describes how much methane can be formed and is an intrinsic property of the composition of a given ton of waste. Considered another way, this is analogous to how much “food” the microbes have to eat, since the ultimate methane yield is fixed based on what’s in the waste. The decay rate, k, describes how quickly the bulk waste goes through the four phases of decomposition described above. The decay rate correlates to both microbial population and metabolism, which is in large part dictated by the environmental conditions in the waste (e.g., moisture content, temperature, pH, etc.). The higher the decay rate, the more quickly the decomposable organic fraction of the waste is consumed and converted to biogas.
Current research by Dr. Mort Barlaz and his research team at North Carolina State University, via funding from EREF, aims to revisit and update the two major inputs to the LandGEM model, k and Lo. The original inputs to LandGEM were developed based on data from the 1990s, and there have been changes in both landfill practice and waste composition over the past 10 to 15 years. The value of this work is becomes rather clear, given the decay rate will vary in regions of the US with markedly different climates and that waste decomposition in the field rarely occurs under optimal conditions. This work will also explore uncertainty in LandGEM predictions and help owners to better estimate the size of gas collection systems and LFG to energy systems.
LFG’s Significant Role in Fugitive Emissions
Ideally, all LFG would be collected and used beneficially. However, rarely does anything occur with 100% efficiency. Recent work funded by EREF and performed by the University of Delaware (Dr. Paul Imhoff) in the field has demonstrated that LFG collection efficiency is dependent on, in addition to collection well number and spacing, such other factors as gas extraction rate, well depth, the presence of perched water adjacent to a collection well, and how well the cover soil seals (i.e., the extent of cracks in the cover soil). Research conducted by North Carolina State University and partially funded by EREF has shown that LFG collection efficiency averaged over the lifetime of the landfill is affected by the decay rate (k-value) and how the landfill is managed. For example, a landfill that phases in installation of its LFG collection system or uses a state-of-the-art cover can achieve average collection efficiencies of 70% to 85%, assuming that gas collected throughout a 100-year landfill life and decay rate of 0.04 (which is the typical for a traditional landfill).
Microbial Methane Oxidation: the Last Line of Defense
In looking at this, it may be tempting to assume that the 15% to 30% of LFG that was not collected winds up as fugitive emissions. This is not the case. One more barrier exists before the uncollected gas is emitted from the surface of the landfill into the atmosphere. While methanogens make the methane within the landfill, another class of microbes dwells in landfill cover soils. These methane-oxidizing bacteria consume methane that seeps through the cover soil, converting it to carbon dioxide. For landfill sections without a final impermeable cover, these methane oxidizers are the last line of defense in reducing fugitive methane emissions. Thus, the higher the methane oxidation the better, since this means more methane is being destroyed and converted to carbon dioxide rather than being released to the atmosphere.
How much methane can be oxidized? EPA guidelines state that 10% of methane passing through a soil cover is oxidized. However, recent research funded in part by EREF and conducted by Florida State University is challenging this. The study, conducted by Dr. Jeff Chanton, examined methane oxidation rates in five different climates and in different seasons across the US. Nearly 1,400 measurements were performed at 20 sites, using two independent techniques to estimate methane oxidation: (1) flux chambers and (2) optical remote-sensing devices to quantify methane emitted directly from the landfill surface. Figure 1 depicts the results from this work using both methods as a function of climate type.
Aggregating these measurements, a minimum methane oxidation of 25% is obtained, which is 2.5 times higher than the EPA guideline. However, in many cases methane oxidation was higher and the average measured was 36 ± 7%. This suggests that the EPA guideline for methane oxidation is overly conservative, but the other important implication is that estimates of fugitive emissions are likely inflated, since the EPA guideline is typically the assumption used to compute fugitive emissions.
Estimating Fugitive Methane Emissions Has Its Limitations
The reason values used for methane oxidation in landfill cover soils affect emissions estimates is that fugitive emissions have historically been computed indirectly because they have, until recently, been extremely difficult to accurately measure directly. Without direct measurements, fugitive emissions from a landfill must be indirectly computed using the following conceptual equation:
Fugitive CH4 Emissions equals
CH4 Produced
minus CH4 Collected
minus CH4 Oxidized in the Cover Soil
FIGURE 6. TRUCK-MOUNTED MOBILE SPECTROSCOPY UNIT USED FOR THE TRACER GAS CORRELATION TECHNIQUE.
Looking at this equation, the CH4 produced is estimated using the EPA LandGEM model and the CH4 collected is quantified either based on site-specific data from the collection system, or, in the absence of this data, an estimate is used for average gas collection efficiency. The CH4 oxidized is typically based on the EPA guideline of 10%; however, recent estimates of fugitive emissions have used some of the estimates based on Dr. Chanton’s work at Florida State. Given this, one can see that there are a number of assumptions made, which creates uncertainty associated with exactly how much methane is released from a landfill as a fugitive emission. For example, using the LandGEM model assumes the landfill is behaving the way the model predicts in a given year, which may reflect actual gas production onsite. Although the model can in part be validated using collected gas data, this can be challenging and the data can be difficult to obtain if one is trying to obtain estimates from multiple sites. The other key assumption made, as alluded to earlier, is specifically how much methane oxidation is occurring for a given site. Current research suggests using EPA guidelines for methane oxidation may be off by as much as 15% to 33%, which will skew estimate of fugitive emissions. Thus, the multiple implicit assumptions made by indirectly computing fugitive emissions can substantially increase the level of error.
Direct Measurements Reduce Uncertainty
Researchers have been aware of the above limitations for a while now, and one of the most obvious ways to reduce uncertainty in quantifying fugitive emissions is to directly measure them. Early efforts in 2006 were led by the EPA and Arcadis, which resulted in the development of what is now called the EPA OTM-10 Method for quantifying fugitive emissions. OTM-10 is uses a measurement technique called vertical radial plume mapping (VRPM) that employs a tunable diode laser and an array of mirrors called retroreflectors (Figure 2). Passing the laser beam through this field and reflecting it back allows for the amount of methane that passes through a known vertical plane, or flux, to be computed, as shown in Figure 3. However, these initial efforts showed direct measurement is more complex than one might think and can be time consuming and costly. Other difficulties with OTM-10 are that the equipment, once set up, cannot be moved quickly if wind conditions change. Additionally, estimates of fugitive emissions from the entire landfill had to be extrapolated based on multiple measurements over multiple days using multiple setup locations.
These limitations resulted in the desire to determine if easier measurement methods were available. This led to a $600,000 study funded by EREF in cooperation with Waste Management, Veolia Environmental Services, the EPA, and SCS Engineers to compare 5 technologies to measure fugitive emissions:
- Vertical radial plume mapping (EPA’s OTM-10 Method)
- Tracer gas correlation using mobile spectroscopy
- Differential absorption LiDAR
- Micrometeorological method
- Flux chambers
This was a landmark study in a number of ways: All testing was conducted on the same landfill at the same time, which meant wind and weather conditions were similar, and the landfill emissions were identical, so the primary differences between measurements were attributed to the measurement techniques themselves. In addition to measuring emissions from the landfill, another component of the project was added that allowed direct comparison of technique accuracy. A controlled release of methane was made from a simulated diffusive area (Figure 4), and three of the five technologies were evaluated. Because of equipment limitations, the two methods not included (the micrometeorological and flux chamber methods) could not be used. Since the mass of methane released was known, the accuracy of the individual methods could be compared. This component of the study was a huge stride forward because it showed, for the first time, the relative accuracy of multiple measurement techniques assembled at the same location under field conditions.
The results, shown in Figure 5, suggested that the tracer gas correlation using mobile spectroscopy was one of the most accurate techniques while vertical radial plume mapping (EPA’s OTM-10 technique) had significant loss in accuracy the farther away the measurement equipment was from the point of release. Error for the VRPM method varied between 3% and 48% and was less for the tracer gas method, ranging from 4% to 20%. Variability was also lower for the tracer gas method (7%) compared to the VRPM method (25%). The tracer gas mobile spectroscopy correlation technique also has some distinct advantages over the other methods. It is mobile and can be more easily used to measure methane flux over large areas without multiple setups, as is usually required for the OTM-10 technique. In addition, the spectroscopy equipment used for the tracer gas technique can be mounted in a pickup truck relatively easily, making it mobile (Figure 6).
Since this study was completed, significant effort has been made to further develop the tracer gas correlation technique by identifying the best conditions to obtain consistent and accurate measurements. Measurements using this technique are made by placing a tracer gas such as acetylene on the landfill surface (e.g., near the working face) and releasing the gas at a known rate. The mobile spectroscopy unit then takes measurements some distance downwind of the landfill (typically a quarter-mile away or more). Since the mass per unit time of the tracer is known, then the concentration of the tracer and methane measured downwind of the landfill can be used to estimate the methane emissions from the entire landfill, as depicted in Figure 7.
Recent research, conducted by Florida State University and Cygnus Environmental via a grant from EREF and in conjunction with Waste Management Inc. and EPA efforts, has focused on a variant of the tracer gas correlation method that uses a technology called cavity ring-down spectroscopy to measure whole landfill emissions. Similar to the technology used in the comparative study, the technique allows for the tracking of emission plumes from the entire landfill and the research is identifying which environmental conditions yield the most accurate measurements of fugitive emissions. Measurements taken at a landfill in the Midwest (Figure 8) demonstrate how well fugitive emissions can be characterized using this technique.
Collectively, this body of research has advanced field-scale fugitive emissions quantitation; nevertheless, there is still more to do to bring the technology to a point where widespread use in the field can be considered.
