Clean Energy Application Preliminary Development
Creation of Methane
Within municipal solid waste landfills, methane is formed as a by-product of decomposition of organic material disposed within it. Simply put, complex organic compounds such as carbohydrates will break down into less complex components ultimately leading to the formation of methane and carbon dioxide (Figure 1). However, to accomplish this breakdown, microbes feed on the landfilled material. Facultative and anaerobic bacteria form simpler organic fatty acids from the complex materials. Special methanogenic bacteria consume the organic fatty acids and create methane and CO2. Despite this seemingly simple process, there are many factors that affect each stage of decomposition each impacting the development of a successful potential energy recovery project.
Figure 1 – Example of Biochemical Reactions
- Carbohydrates ⇒ C6H12O6 (simple sugar)
- C6H12O6 ⇒ by fermentation ⇒ 2 CO2 + 2C2H5OH (ethyl alcohol)
- 2C2H5OH + H20 ⇒ by hydrolysis ⇒ CH3COOH (acetate) + H2
- CH3COOH ⇒ by decarboxylation ⇒ CH4 + CO2
The competition of microbial species coupled with the type of organic substrate, moisture, nutrients and resultant chemical and physical environment derived from these interactions establish methane formation quality and quantity; two important considerations for project development.
To further complicate the decomposition process, there are progressive stages from aerobic (in the abundance of air) to anaerobic (in the absence of air) that influence the formation of methane. Oxygen is TOXIC to methanogenic bacteria. Any amount of oxygen, no matter how small, will impact the bacterial population and subsequently the formation of methane.
When waste is first placed in a landfill, air fills the spaces within it. Over time, as little as several days to several months, the existing microbial population on the waste material consumes the oxygen in the first stage of decomposition (aerobic). Microbes use organic material and oxygen to metabolize the waste with the by-product being CO2 and water vapor.
When oxygen is depleted, the aerobic microbial population will decline to be replaced by anaerobic facultative microbes. As these microbes produce organic acids, methanogenic bacteria begin to prosper. However, if the methanogenic bacteria are not present in numbers cable of metabolizing all of the organic acids that are being produced or are slowed by unfavorable physical and chemical conditions within the landfill, then the volatile organic acids build up which, if left unchecked, results in a drop in pH. Significant drops in pH will cause the methanogenic bacterial population to decline which in turn results in a further decline in methane production.
Most factors influencing methane formation are not within the control of owner and operators once the waste material has been landfilled with the exception of moisture and oxygen. Therefore, for successful project development, it is important to understand the ramifications of factors that you or the landfill owner/operator has no control over and those factors that you do.
Waste types landfilled, age of the material, and quantities are factors that can not be controlled after the waste has been placed yet impact long term viability of energy recovery projects. Moisture and oxygen are controllable, but system design may limit such control. Oxygen enters landfills initially through the process of waste placement and on a secondary basis through the process of removing gas from the landfill through vacuum induced collection. Of note is that oxygen is consumed by microbes within the waste mass, so what is measured at common combustion devices such as a flare prior to energy recovery is not what is entering the landfill. As such, a large influx of oxygen into the landfill, but not out would indicate conditions that are potentially toxic to methanogens which consequently would result in a decline in both concentration and quantity of methane (Figure 2).
Available Fuel Gas Volume
How much gas should you expect from a landfill?
No two landfills are the same. In very simplistic terms, the amount of gas you can expect from a landfill is dependent on the organic material present. Laboratory and lysimeter tests have shown that between 2 and 7 cubic feet of gas can be expected from every pound of generic waste. At any given time, however, the rate of its generation is dependent on many factors.
Modeling versus Actual Measured Rates
Extreme caution is advised when comparing a model to actual measured rates. Often, the basis for each is different leading to an “apple to oranges” comparison. First, actual measured rates typically include methane, CO2, and air. All models only include methane and CO2. Second, a model establishes the theoretical maximum gas production where actual rates are the result of a certain collection efficiency which is not directly measurable. It is these two differences that create confusion and often inaccurate representations of gas production at landfills.
The typical model is based on a first order kinetic model used to describe the oxygen uptake by bacteria and other similar biological systems. For landfills, it is used to describe organic material decay and the methane that is produced in proportion to the amount of decomposable material present. The model includes four variables: waste mass (R), decay constant (k), time (t,c) and theoretical methane yield (Lo).
Qm = 2Lo R (ekc – ekt)
Qm = maximum expected gas generation flow rate, cubic meters per year
Lo = methane generation potential, cubic meters per megagram solid waste
R = average annual acceptance rate, megagrams per year
k = methane generation rate constant, year-1
c = time since closure, years
t = age of the landfill
There are two time components in the model: 1) time since closure and; 2) age of the landfill. For the purpose of design, each must be minimized to establish the highest methane potential. Over time, less organic material is available and subsequently less methane is generated. The model also assumes a negligible lag prior to the onset of methanogenesis. Therefore, the model, by its very structure, assumes that peak methane potential occurs at time zero and decays exponentially as organic material is decomposed. Therefore, to establish the maximum methane generation potential the time since closure value should be zero.
The second time factor, age of the landfill, must be minimized in the context of the permitted capacity and the rate at which that capacity is filled. Landfills accumulate organic material over time. Under the model, the maximum methane generation potential occurs when the accumulated organic material (less the amount decomposed during that period) is greatest, i.e. typically when waste acceptance ceases or the rate of waste acceptance is less than the amount of organic material being decomposed. Thus, the sooner the landfill reaches its capacity the greater the methane generation potential. The age of the landfill that establishes the maximum methane generation potential is defined as the design capacity divided by the maximum daily rate. Given that waste acceptance less than this rate would extend the operational time period that subsequently reduces methane potential, the maximum generation rate will occur at the maximum disposal rate.
The total methane potential is predicated on the total amount of organic material landfilled and MSW contains the organic material, assuming that all material is MSW establishes the maximum organic content. This is what is generally assumed, but not all landfills accept just MSW. Therefore, models based on this may show higher gas rates than actual conditions.
The Lo factor is a representation of the theoretical amount of methane that is generated from a unit mass of waste over the life of the landfill. US EPA recommends, in the AP-42 emissions factors for landfills, that a “Lo” of 100 cubic meters per megagram be used for all purposes other than for applicability of the Landfill NSPS. For purposes of the applicability of the NSPS, US EPA requires a Lo of 169.9 cubic meters per megagram. It is not common to test for Lo and therefore a certain amount of uncertainty is present in models.
K – Factor
Lastly, the “k” factor represents the decay constant (how quickly methane generation declines in proportion to the amount of organic material present). US EPA recommends 0.04/yr for purposes other than NSPS applicability, except where site-specific values are warranted.
Because of the limited variables, all landfill gas models are impacted by simplification despite what anyone may claim. There are many factors that affect gas generation with many of them unknown or impossible to obtain reliably or economically.
Application Sensitivity to Future Gas Rate Variability
Since models are predicated on so few variables, they are susceptible to future changes at the landfill; changes in waste volume, type, and environmental conditions all impact energy recovery. Successful projects all hinge on recognizing the model limitations and planning for that variability.
Many factors influence gas generation with precipitation being one of the most important. Optimum moisture translates into maximum gas production, but care must be used when designing extraction systems because collection will suffer.
Fuel Pressures Available
As one would expect, a certain amount of pressure is generated by landfill gas formation, but it is generally no greater than 5 – 10 inches water column. The reason for this low pressure is the porosity of the landfill and that although all currently operating landfills utilize impermeable boundaries, they are not gas tight.
Clean Energy Quality
Landfill gas will contain the primary decomposition by-products, methane and carbon dioxide. It will also contain certain amounts of air (oxygen, nitrogen and argon) that is introduced into the landfill by the application of a vacuum to the extraction wells. In addition, landfill gas will contain trace corrosive elements that come from volatilization of compounds carried in with typical solid waste with the exception of sulfur compounds (most commonly hydrogen sulfide, methyl mercaptans, and dimethyl sulfide) released by specific types of microbes. It is these trace compounds that can be most harmful to end use equipment depending on their concentration causing metal corrosion and fatigue. Note also, that high oxygen will also detrimentally impact engine performance by causing detonation problems and burned valves. Understanding each of these elements and monitoring them is important to equipment performance and longevity.
Application sensitivity to variable gas composition – Application critical compounds found in LFG
Analysis – Sulfur analysis of fuel gases are common, but because of hydrogen sulfide, the hold time is short. It is imperative that sulfur samples be analyzed within 72 hours. Further, the sample container is important for sulfur analysis. Glass lined, specially treated metal canisters or Teflon sample bags (provided that the sample time is less than 24 hours) can be used.
Variability – Sulfur concentrations at landfills will vary depending on waste types. A substantial increase in wall board or other construction type debris will increase sulfur content. Similarly, the introduction of waste water sludge’s that are undigested or only partially so with no lime stabilization will introduce sulfur reducing microbes into the landfill. These microbes will compete for organics with methanogens subsequently reducing methane production. At most landfills, sulfur can be expected to be less than 200 ppm, however, levels as high as several thousand have been found.
Application Limitations – During combustion, water vapor is formed and sulfur compounds break down into sulfur ions. These compounds then reform as sulfuric acid that can attack valve guides, liners, and other metal components in the fuel system. It is also not uncommon to find iron sulfide deposits accumulating in the fuel gas pressure regulator or other fuel system components. Although high TBN oils can aide in reducing the affects of sulfuric acid attack, hydrogen sulfide can directly attack metals causing them to become brittle.
Landfill gas contains trace amounts of volatile and semi-volatile compounds. These compounds are stripped from the landfilled waste by the organic material decomposition gases (primarily methane and CO2). Many of these compounds contain chlorinated and fluorinated elements that come from household cleaning products and other similar landfilled items.
Analysis – Detection of halogenated compounds is very common with many standard methods used to determine individual concentrations. However, it is also useful to obtain a single measurement of total halogen concentration since it is difficult to establish a fuel limit for each halogenated compound in landfill gas. A laboratory method utilizing oxidation of the sample in an oxygen and CO2 rich atmosphere yields a hydrogen halide by-product that can be measured using microcoulometric titration. Samples can be collected in a Suma treated canister or Tedlar bag with no significant hold time limit for analysis.
Variability – Because halogenated compounds come from the landfilled waste, they will vary as the waste stream varies. Note that research has found that concentrations of halogenated compounds decrease over time. This is not unexpected given that most halogenated compounds are very volatile and once removed from the landfilled waste, the concentration in the gas stream will decline. However, for landfills that still accept waste, the reduction in halogenated compounds in placed waste would be offset by the introduction of new waste. Nationwide testing has established that, although variable, the concentrations at most landfills are less than 1000 ppm.
Application Limitations – During the combustion process, halogenated compounds break down into chlorine and fluorine ions. These ions reform with the water vapor generated during combustion to form HCl and HF acids that result in accelerated wear of piston rings, cylinder liners, valve stems and guides.
Siloxanes are man-made cyclic and linear organic compounds that contain silicon. They are used in personal hygiene, health care and industrial products. As such, they can be found in landfills as used product is disposed of. Siloxanes, similar to halogenated compounds are volatilized and become part of the gas stream that is combusted. These siloxanes are converted to silicon dioxide (SiO2) during combustion which can form deposits. Most notably, they collect on pistons and valves. At landfills, D3, D4, D5, L2 and L3 are the only siloxanes found above detection limits.
Analysis – Different laboratories use different sampling and analytical techniques and do not use a consistent set of target compounds. In addition, the limits of detection vary at an individual laboratory over time and between laboratories. The most common sampling methods are methanol impinger, SUMA treated canister and Tedlar bags.