EGEE 101
Energy and the Environment

Natural Gas Alternatives

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Moon light oil rig in the ocean.
There is great expense in building oil rigs out in the ocean. The structures are massive, it is very expensive and difficult to build, and costly to operate. However, it can extract large quantities of crude oil and natural gas from deep beneath the ocean floor.

Natural gas can be a valuable fuel. However, we have seen that we do not historically have a very large supply of natural gas, or do we? The reserve and resource discussion has relevance here. As we start to run out of supply (we need more than we can produce) the price increases (laws of supply and demand). As the price increases, then those resources that had marginal economic potential now have a greater potential to produce a profit and so are moved from being a resource into being a reserve. Technology also has a role to play: we can drill deeper (in our efforts to find natural gas), or go offshore. We can also look for alternative sources of methane: shale gas, coalbed methane, gas hydrates, synthesis gas generated methane, and landfill gases (methane).

Coalbed Methane

The methane that was once the enemy of the coal miner can now be extracted either prior to mining, or from coal resources that are too deep to be economically mined. Currently, about 7% of the methane used in the U.S. is from coalbed methane. Recall that as the coal matures it is increasingly rich in carbon and towards the end of the bituminous stage there is a loss of hydrogen. Most of this hydrogen forms methane that may become trapped in the porous structure of the coal. Usually after mining coal, the methane escapes or the methane is released along with the ventilation air. We can recover probably about 100 Tcf (trillion cubic feet) or about a 5-year supply if it was to provide all the US consumption.

Plot of the rise and decline of Coalbed methane production in the US, peaking about 2009.
U.S. coalbed methane production.
Credit: EIA

We have always had methane contained within certain coal seams. But not until methane had an increased value was it economically efficient to extract. We also needed to develop the techniques to drill into the coal seams to maximize the return of the methane. With both of these prerequisites in place (high methane cost and advanced extraction technologies), the reserve of coalbed methane has grown despite the methane being extracted (more recent dip is due to the higher value of wet shale gas (wet here meaning containing liquid hydrocarbons also).

Coal Gasification

Synthesis gas (also known as syn gas): CO and H2 is produced by the gasification reaction:

C + H20 ------> CO + H2

The carbon is generic (could be natural gas, coal, char, or any source of carbon in a carbonaceous material). Mostly, however, it will be coal, a fossil fuel that we have a lot of! Steam (or oxygen) is passed over the hot coal and gaseous products form, known as "water gas." The process is endothermic (requires heat) so energy is required to heat the coal, this could be done by coal combustion but this uses a portion of the carbon. Instead, a balance is achieved between the exothermic (heat producing) reaction of carbon and sub-stoichiometric quantity of oxygen (not enough oxygen to produce CO2):

2C + O2 -------> 2CO

and the endothermic reaction gasification reaction.

If air is used the gas has a low calorific value (100-125 Btu/scf) (SCF is standard cubic foot) and can be used as a fuel. Do you remember that nitrogen in the air dilutes the energy of combustion? If oxygen, instead of air, is used to gasify the carbonaceous material, the gas has a medium calorific value (approximately 300 Btu/scf). Or by not adding nitrogen as a dilutant we get more energy out of the synthesis gas.

The water gas is subjected to the water-gas shift reaction:

CO + H2O <-----> CO2 + H2

Which converts some of the CO to CO2 and hydrogen or vice versa. This is done to change the ratio of CO and H2, and through removal or addition of components, changes in pressure and temperature, the equilibrium can be manipulated and the ratio of water-gas components shifted to the desired ratio depending on the required products, which include substitute natural gas (SNG -methane), methanol, or gasoline.

If methane is the desired product, the cleaned gases (to avoid poisoning the catalyst) undergo the water–gas shift to change the H2 to CO ratio to 3:1 prior to the methanation step:

3 H2 + CO ----------> CH4 + H2O

and

C + 2 H2 ----> CH4

Methane Hydrates

 blue flame
A burning lump of methane hydrate.
Credit: NETL

Methane hydrates are important for one reason: there is so much methane in the form of methane hydrates that it dwarfs our traditional supply. If we can only reach a small percentage of the methane hydrates we will have a vast energy source. They are found both on land (in some of the permafrost areas) and in the ocean on the seafloor.

The methane flame shown to the right is blue in color because of the CH radicals within the flame. Here a solid, ice-like hydrate is on fire; the hydrate melts, releasing more methane.

 Permafrost in the artic
Methane hydrates in the arctic
Credit: NETL
 map of known locations of methane hydrates
Location of methane hydrates.
Credit: Oak Ridge National Laboratory

As you can see, we will be increasing the use of these non-traditional sources of natural gas, notably shale gas, tight gas, and coalbed methane (but not methane hydrates, yet!). Tight natural gas is methane from low permeability sources.

Methane growth is predicted to be mostly shale gas with coalbed methane only making a slight contribution
Current and future methane sources for US production.
Credit: EIA