Can We Burn Coal Cleanly
The World needs Low Cost Energy
To maintain a high standard of living for all the civilizations on the Earth will require abundant amounts of cheap clean energy. Coal as an energy fuel is one of the cheapest forms of fuel for energy on the planet. But coal burning does produce gases like carbon dioxide, which many believe produces an additional greenhouse effect, above that which is necessary to protect the planet.
The question is, can we burn coal cleanly, to produce the inexpensive energy we want, but not endanger the earth? Exciting research from around the world suggests that this may be so. For example, coal can be turned into gas and cleaned before burning. This type of power plant is called an IGCC plant.
Absorbing CO2 using Solvents
There are a lot of procedures to stop CO2 getting into the air, but they are expensive and currently require a lot of energy. Special chemicals called solvents, which capture and hold the gas in their structure, a bit like a sponge holding water, can absorb the gas. The carbon dioxide bonds with the solvent forming a weak intermediate compound which is then broken down by heat. The most widely used and successful solvents to date are MEA which stands for monoethylamine, and Selexol (a dimethylether of polyethylene glycol). These are man made organic solvents that can bond with carbon dioxide. A typical plant that can capture both sulphur oxides SO2 and carbon dioxide is shown left.
The gases created by burning coal (flue gas) are passed through a series of chambers firstly to exchange heat, then to remove sulphur dioxide and finally carbon dioxide. All these processes act in the same manner by first scrubbing the flue gas in absorption towers and to collect the CO2 and then regenerating the solvent and releasing the CO2. After the CO2 is released it must be disposed of, a process called sequestration. But we will cover this later.
Absorbing CO2 using Membranes
Gas separation membranes
Another method of extracting carbon dioxide is with the use of gas separation membranes. The membrane separates the gas stream coming from the flue of the power house into two - the permeate stream and the retentate stream. The amount of CO2 separated is determined by the selectivity of the membrane and by the ratio of the permeate flow to the feed flow; or the ratio of permeate pressure to the feed pressure.
It is usually necessary to recycle the feed stream because the membrane cannot remove all the CO2 in one pass. There are a number of different types of gas separation membranes currently available. Some have only been examined on the laboratory scale. In descending order as regards to commercialisation they are polymer membranes, palladium membranes, facilitated transport membranes and molecular sieves.
Gas absorption membranes
Gas absorption membranes are membranes which are used as interface devices between a gas flow and a liquid flow (see the figure left). The separation is caused by the presence of an absorption liquid on one side of the membrane which selectively removes certain gases from the gas stream on the other side of the membrane. In contrast with gas separation membranes, it is not essential that the membrane has any selectivity at all. It is merely intended to provide a contacting area without mixing gas and absorption liquid flow. The selectivity of the process is derived from the absorption liquid. )
Removal of flue gas components, like SO2 or CO2, is achieved through the use of porous, hydrophobic membranes in combination with suitable absorption liquids, such as sulphite, carbonate or amine solutions.
Microporous membranes are limited by the allowable trans-membrane pressure difference. This means that the absorption liquid and gas stream need to be at similar pressure levels. Absorption processes based on membranes are on the verge of commercial introduction. They have the advantage of high selectivity.
The CO2 is removed from each process with the aid of gas separation membranes and gas absorption membranes (in combination with MEA). The gas separation membranes used were polyphenyleneoxide and polydimethylsiloxane. The former has good CO2/N2 separation characteristics (with very low CO2 content in the gas stream) and costs about l50US$/m2. The latter at US$300/m2 is a good CO2/O2 separator. These two were selected for their properties and the requirement that commercially available technology should be applied where possible.
