Making Industries Accountable
Industry is responsible for emitting air pollutants into the atmosphere. Fortunately, many governments have realized this and brought about legislation which limits the amount of pollution these industries can emit.
Industries such as Municipal Solid Waste incinerators, power stations and chemical plants, have instruments positioned at strategic locations in the flue treatment system which continually monitor stack emissions, testing the fumes for gases, particulates and vapors.
Government Environmental Departments make their own unannounced random checks on stack emissions from industries as well as inspecting records of past emission testing.
They can recommend solutions and have the power to shut down any plant whose pollutant levels do not meet the regulations.
This is an article on industrial stack emissions where we will examine current methods of fume treatment and the testing techniques using a coal-fired power plant as an example. We will also look into the methods in use today along with suggested solutions to the pollutants which are still being emitted into the atmosphere.
We begin then, by examining the fume treatment process in a typical coal-fired power plant.
Fume Treatment and Testing of Fumes from a Coal-Fired Power Plant
With the current world recession and the high price of oil and gas supplies, many countries are building more coal-fired power plants. Unfortunately, out of all the fossil fuels, coal produces the greatest amount of CO2 emissions when combusted. This is an area where we will investigate a few possible solutions.
Coal is imported by ship from countries as far away as Russia, Australia and South Africa, usually being transported to the power stations by rail and held in a large stockpile.
The coal is conveyed from here to pulverizers which crush it to coal dust and blow it into boiler furnaces. The heat from this is used to raise steam and run steam turbines which drive electrical generators, supplying power to the national grids.
When the coal is burned in the boiler furnace, gases, particulates and vapors are produced and most of these are removed in the fume treatment plant process.
There are various stages in this type of plant and each stage is carefully monitored, the constituents of the combustion fume being tested before and after treatment, with the final test being carried out on the stack emissions.
The following few sections will explain how some of the pollutants are tested before and after treatment, which pollutants escape into the atmosphere, and what possible solutions there are to mitigate these emissions.
These make up 20% of the total ashes produced from burning coal. They are removed from the bottom of the furnaces after being quenched with water by a mechanical means. The solution is sold to the construction industry where it is used as an additive to road surfacing materials.
Furnace ash is tested for pyrites (iron) and these are removed from the ash by electromagnets and sent to landfills or are stored in ponds, where further treatment may be used as embankment construction.
Furnace ash is also tested for corrosiveness through checking the chloride and sulphate content. This is important as the ash will be used as an additive to road surfacing materials, and could cause acid run-off.
Dust and Particulates in Fly Ash
The furnace ash is about 20% of the ash produced by burning coal, the other 80% is made up of fly ash suspended in the fumes, some being microscopic and must be removed. This is carried out by passing the fumes through a bag filter, or an electrostatic precipitator. Either method efficiently removes a very high percentage of the fine dust and dangerous particulates.
Testing of Particulates
This is carried out using optical particulate analyzers, which are incorporated in numerous probes inserted into the fume extraction ductwork before and after the particulate treatment process.
Tests can also be carried out using a centrifuge. This is a machine in which a bowl spins at thousands of revs/min, the fumes being directed into the bowl. This results in the larger particles being thrown to the outside due to centrifugal force, the smaller particulates being retained near the center. Subsequent measurement reveals the actual sizes of the particulates before and after treatment.
The optical test is said to be inaccurate, hence the number of probes used, where as the centrifuge would seem to separate the different sizes of particulates giving a more accurate measurement.
The accuracy of the test equipment is pivotal to the control of the particulates, many of the smaller ones such as PM10 and PM 2.5 being carcinogenic.
Oxides of Sulphur (SOx) and Nitrogen (NOx)
SOx and NOx are responsible for the acid rain which ruins forests and causes acidification of rivers and lakes, effectively killing off aquatic flora and fauna.
Sulphur Oxides Removal
Oxides of sulphur are formed in the exhaust gases due to the sulphur content in the coal. The sulphur is currently removed by cooling the gases then passing them through a scrubber. A scrubber is a large vertical vessel, the fumes entering at the bottom. As they pass upwards they are sprayed with a solution of lime which removes the sulphur content from the fumes.
Nitrogen Oxides Removal
The nitrogen is formed in the exhaust gases due to the high temperatures produced during the combustion of the coal. NOx can be controlled by lowering the temperature of the gasses at the combustion stage by spraying water or steam into the combustion zone. Another more expensive method is known as selective catalytic removal. This is where the gases are sprayed with ammonia while passing over a catalyst (a catalyst is a substance that speeds up a process), the ammonia reacting with the nitrogen oxides, effectively removing them from the gases.
Testing for SOx and NOx content in exhaust gases
This is carried out using solid electrode sensors placed in the exhaust gas ducting before and after the relevant treatment components, and before releasing stack emissions into our atmosphere.
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:
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.
- Control Air Pollution, http://www.controlairpollution.com/apc.htm
- Air Quality and Pollution: Measurement Instrumentation for Air Quality Monitoring, http://www.campbellsci.com/air-quality