To continue the current blog series, this week will be exploring the alternative energy potential of capturing methane gas. Similar to carbon dioxide, methane is a naturally occurring greenhouse gas; however, methane is 21 times more capable of trapping heat within the atmosphere than carbon dioxide, the CO2 equivalent (CO2e or CDE) is 21.
[i] For at least this reason, it is important to control the amounts of methane being released into the atmosphere. The processed food output from animals (and humans) decompose to form methane, as does organic-laden municipal solid waste with kitchen scraps and yard recycling. The main factors that affect this form of methane production are: the waste’s environment, length of decomposition, and packing density. Some landfills lack an abundance of organic materials and instead have large amounts of construction and demolition debris making them poor methane producers. Methane is odorless; the noxious smell experienced at landfills and farms is actually caused by hydrogen sulfide, which is another byproduct of the breakdown of organic matter. Another possible source which could yield a large supply of methane is the gas trapped in permafrost, and in the crust of the Earth. When permafrost melts, then methane is slowly released; it is not currently captured. As for the methane in the crust, it is believed to be at depths of 100 to 200 km and exists from the inorganic reactions between water and rock, instead of from decomposing organic matter. The challenge harvesting these reserves is that even the deepest of oil and gas wells are only between 5 and 10 km deep, and the safety and viability of a 200 km well is unknown. [ii] One benefit to collecting methane for use as an alternative energy source is that it burns cleanly, as opposed to fossil fuels, and therefore has a very minute impact on the environment.
The Energy Generation Process Using Methane
Currently there is not a highly effective method of retrieving pure methane from the landfill. It is extremely difficult to separate methane from the hydrogen sulfide and the carbon dioxide present. The current most commonly used process of capturing methane gas begins when anaerobic bacteria digests organic waste. This releases methane, carbon dioxide, hydrogen sulfide and small amounts of nitrogen. In order to capture the gas before it is released into the air, engineers drill a series of wells in the ground. These wells connect via a system of channels that connect laterally to a large vacuum pump. This vacuum pump draws in the methane gas, pressurizes it, and forces it through another series of pipes into the compression facility. Blowers force the gas through a series of heating and cooling chambers, ranging from 99 ° F to 30 ° F. During the final phase the gaseous mixture passes through a fine filter, approximately 1 micro-meter, in an attempt to remove the maximum amount of impurities. In order to generate power at the compression facility, the gas is typically pumped into six 20-cylinder combustion engines, which power several generators. In one scenario, each engine can produce 1.6 megawatts of electricity, totaling 9.6 megawatts for the engines combined. [iii]
The Next Step in Production and Storage
In order to discover a more successful method of isolating pure methane, researchers use computational fluid dynamics (CFD) simulations to test thousands of materials for their methane absorption rates. This is an application of mechanical engineering. One material that shows good potential is zeolite, a porous mineral commonly used as an absorber. This is an application of materials science engineering. The pattern of the zeolite’s pore structure varies depending on the concentration of methane in the air. At very low levels the material must have a high affinity for methane because the gas only becomes flammable at 5% concentration. At higher concentrations, methane molecules interact with each other and are more easily absorbed. Once the gas is at 60% concentration it becomes very easy to transport and liquefy.[iv] Using the zeolite material, a refinery can repeat this cycle and create an even higher concentration.
A separate development involves using methane as a storage system for other forms of alternative energy. Scientists at Pennsylvania State University have discovered that a certain bacteria, when combined with carbon dioxide, can convert electricity to methane. This means that any surplus power generated from alternative energy sources (solar, wind, tidal etc.) can be stored as methane at about an 80% efficiency rate. [v]
[i] Dulalab Environmental News and Information “Methane” 2011
[ii] Dulalab Environmental News and Information “Methane” 2011
[iii] CPS Energy “Landfill Gas—Turning Waste into Usable Energy” 2013
[iv] Irfan, Umair Scientific American “Methane Proves Hard to Capture” April 23, 2013
[v] Felsinger, Alex Clean Technical “Bacteria Turns Excess Clean Energy Into Methane” April 5, 2009