At present, the CO2 produced by the burning of the coal remains in the exhaust gases and is discharged up the stack and into the atmosphere. CO2, as we know, is a greenhouse gas and when you consider the coal-fired power stations operating worldwide, a huge amount of CO2 is emitted.
There are, however, several methods developed to mitigate these stack emissions:
1. Improve the plant efficiency. This involves more energy efficient combustion methods leading to higher pressure boilers and turbines. Any increase in efficiency will reduce CO2 emissions.
2. Addition of biomass to the combustion system. There are already power plants mixing biomass with the coal. The biomass is usually from trees, grasses and crops and is chopped into small pieces before being fed into the boiler along with the coal. As the biomass is CO2 neutral, any addition to the coal will reduce CO2 emissions.
3. Carbon capture and sequestration. This technology has been developed and already applied by an oil and gas company operating in the North Sea. The gas being extracted has a high percentage of CO2 and is not suitable for distribution until processed to reduce CO2 levels. Once the CO2 has been extracted from the gas, it is pumped into an undersea aquifer for long-term storage. This system has been operating for many years and so far has proved satisfactory.
Back on dry land, the UK government has banned any new coal-fired power stations from being built without a carbon capture system in place. However, agreement on the locations for long-term storage of the captured CO2 is still to be reached. Numerous suggestions have been made, all requiring the CO2 to be liquefied. Some of the more popular suggested methods are listed below:
- Redundant coal mines
- Saline aquifers
- Depleted oil and gas reservoirs
- Injection into deep ocean
Out of the above suggestions, the depletion of oil and gas reservoirs is currently the favored method for long-term storage of CO2. Research and development into this and other methods is ongoing and hopefully will promote further stack emission testing and solutions.