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In the last post we looked at the basic composition of landfill gas. Now let’s use that information to calculate the density. The density of a gas is a critical factor when measuring its flow rate. Density is mass divided by volume, so let’s first calculate the volume of a quantity of landfill gas.
As you may remember from high school chemistry, the properties of a gas change dramatically with temperature and pressure, as described by the ideal gas law (P is the gas pressure, V is the volume, n is the number of moles of the gas, R is a constant called the Universal Gas Constant, and T is temperature):
So where do we start? Well, we can easily search online and find that R has a value of 8.314 J/(mole*K) and we can calculate everything assuming we are working with 1 mole of gas. Because that leaves three real variables (pressure, volume and temperature), we’ll need to fix two and solve for the third. This requires some additional information about landfill gas extraction systems.
After talking with a bunch of landfills, I’ve found that the system pressure in the gas extraction system is typically around 40-50” of water (vacuum, measured relative to the atmospheric pressure). 40” of water is equivalent to 99.6 millibar, so for simplicity I’ll assume the landfill gas is at 900 mbar (since 40” of vacuum means -99.6 mbar relative to the atmosphere, which is usually around 1 bar). In SI units, this is 90,000 Pascal.
Because landfill gas is a byproduct of anaerobic digestion, it is usually around 40° C, or 313.15 K (in SI units). Now that I’ve specified a value for P and T, I can calculate the volume of 1 mole of hypothetical gas:
There are tons of good online calculators to do basic calculations with the ideal gas law, like this one. In the next post, we’ll combine this result with the information we found last time about landfill gas compositions in order to calculate some density ranges.
I recently set about designing a flow meter for landfill gas (LFG). My search for a good online reference about LFG was futile, so I decided to create one myself. This article and the next several in this series will be dedicated to calculating some basic properties of landfill gas. The ultimate objective is to arrive at the density and the Reynolds number, which is a fundamental parameter of fluids flowing in a pipe. But first we need to establish some more basic properties.
Composition of landfill gas
The first step is to identify the major components of landfill gas. Because LFG is created as a result of anaerobic digestion, it is hot and very humid. The gas composition also varies as a function of the age of the landfill, temperature, rainfall, and vacuum pressure in the collection system, and the underlying solid waste composition.
After talking with many landfill gas to energy plants and landfill operators, it seems that the normal composition of LFG is roughly in the following ranges:
CH4 = 35-55%
CO2 = 15-35%
O2 = 0-4%
The remainder of the gas (called “balance gas”) is primarily N2 and water vapor, along with trace compounds like Hydrogen Sulfide (H2S) and other contaminants (Benzene, refrigerants, etc.) that depend highly on the composition of the solid waste. Since my primary goal is to calculate the fluid properties, the trace components do not have a significant impact.
H2S is highly toxic and has a very distinct odor, so is a major concern for landfill operators. The smell is detectable at very low concentrations (a few parts per million!), and it poses significant health and environmental problems at concentrations above 50 ppm or so. H2S seems to be created mainly by decomposing drywall, and hence is more of a problem at sites that take a lot of construction waste.
Our next post will explore some of the chemical and physical properties of landfill gas in order to help calculate the fluid properties.
Landfill Gas to Energy (LFGTE) projects seem like a very logical one to pursue..take a waste material and turn it into energy, then sell it for money. However, one of the major concerns that is holding back many landfill from rushing to support this idea is capital recovery. Installing a turbine for electricity generation (or pipes for selling methane directly) is not a small investment. If a project is not sized properly, there’s the risk that there won’t be sufficient methane and hence revenue to support the LFGTE project. A popular tool is the LandGEM model which uses a first-order decomposition rate to estimate annual emissions. It is extremely important to pick the two input parameters, methane yield and decay rate, very carefully. The methane yield is dependent on the waste composition. Decay rate is based primarily on environmental factors. Traditionally, these factors were derived based on laboratory experiments. However, it’s difficult for this kind of research to take into consideration environmental factors that could affect methane generation, such as atmosphere pressure, humidity, temperature, and etc.
There are proposals now to collect data in the field and to derive a model of landfill gas generation stochastically. Such model would create a great check and balance with the existing LandGEM model to predict landfill gas generation. Having better resolution of a landfill’s gas production may also help landfill managers better tune the wellheads for methane extraction. We’ll be blogging more about research related to this concept hereon!