Environmental Aspects Of Generation of Electrical Energy:
Conversion of one form of energy or another to electrical form has unwanted side effects and the pollutants generated in the process have to be disposed off. Pollutants know no geographical boundary, as result the pollution issue has become a nightmarish problem and strong national and international pressure groups have sprung up and they are having a definite impact on the development of Environmental Aspects Of Generation of Electrical Energy.
Governmental awareness has created numerous legislation at national and international levels, which power engineers have to be fully conversant with in practice of their profession and survey and planning of large power projects. Lengthy, time consuming procedures at government level, PIL (public interest litigation) and demonstrative protests have delayed several projects in several countries.
This has led to favouring of small-size projects and redevelopment of existing sites. But with the increasing gap in electric demand and production, our country has to move forward for several large thermal, hydro and nuclear power projects. Emphasis is being laid on conservation issues, curtailment of transmission losses, theft, subsidized power supplies and above all on sustainable development with appropriate technology wherever feasible.
It has to be particularly assured that no irreversible damage is caused to environment which would affect the living conditions of the future Generation of Electrical Energy. Irreversible damages like ozone layer holes and global warming caused by increase in CO2 in the atmosphere are already showing up.
We shall treat here only pollution as caused by thermal plants using coal as feedstock. Certain issues concerning this have already been highlighted already. The fossil fuel based generating plants form the backbone of power Generation of Electrical Energy in our country and also globally as other options (like nuclear and even hydro) have even stronger hazards associated with them.
Also it should be understood that pollution in large cities like Delhi is caused more by vehicular traffic and their emission. In Delhi of course Inderprastha and Badarpur power stations contribute their share in certain areas.
Problematic pollutants in emission of coal-based generating plants are.
Though the account that follows will be general, it needs to be mentioned here that Indian coal has comparatively low sulphur content but a very high ash content which in some coals may be as high as 53%. A brief account of various pollutants, their likely impact and methods of abatements are presented as follows.
Oxides of Sulphur (SO2):
Most of the sulphur present in the fossil fuel is oxidized to SO2 in the combustion chamber before being emitted by the chimney. In atmosphere it gets further oxidized to H2SO4 and metallic sulphates which are the major source of concern as these can cause acid rain, impaired visibility, damage to buildings and vegetation. Sulphate concentrations of 9-10 μg/m3 of air aggravate asthma, lung and heart disease. It may also be noted that although sulphur does not accumulate in air, it does so in soil.
Sulphur emission can be controlled by:
- Use of fuel with less than 1% sulphur; generally not a feasible solution.
- Use of chemical reaction to remove sulphur in the form of sulphuric acid, from combustion products by limestone scrubbers or fluidized bed combustion.
- Removing sulphur from the coal by gasification or floatation processes.
It has been noticed that the byproduct sulphur could off-set the cost of sulphur recovery plant.
Oxides of Nitrogen (NOx):
Of these NO2, nitrogen oxides, is a major concern as a pollutant. It is soluble in water and so has adverse affect on human health as it enters the lungs on inhaling and combining with moisture converts to nitrous and nitric acids, which damage the lungs. At levels of 25-100 parts per million NOx can cause acute bronchitis and pneumonia.
Emission of NOx can be controlled by fitting advanced technology burners which can assure more complete combustion, thereby reducing these oxides from being emitted. These can also be removed from the combustion products by absorption process by certain solvents going on to the stock.
Oxides of Carbon (CO, CO2):
CO is a very toxic pollutant but it gets converted to CO2 in the open atmosphere (if available) surrounding the plant. On the other hand CO2 has been identified as a major cause of global warming. It is not yet a serious problem in developing countries.
During the oxidation process in combustion chamber certain light weight hydrocarbon may be formed. The compounds are a major source of photochemical reaction that adds to depletion of ozone layer.
Particulates (fly ash):
Dust content is particularly high in the Indian coal. Particulates come out of the stack in the form of fly ash. It comprises fine particles of carbon, ash and other inert materials. In high concentrations, these cause poor visibility and respiratory diseases.
Concentration of pollutants can be reduced by dispersal over a wider area by use of high stacks. Precipitators can be used to remove particles as the flue gases rise up the stack. If in the stack a vertical wire is strung in the middle and charged to a high negative potential, it emits electrons. These electrons are captured by the gas molecules thereby becoming negative ions. These ions accelerate towards the walls, get neutralized on hitting the walls and the particles drop down the walls. Precipitators have high efficiency up to 99% for large particles, but they have poor performance for particles of size less than 0.1 um in diameter. The efficiency of precipitators is high with reasonable sulphur content in flue gases but drops for low sulphur content coals; 99% for 3% sulphur and 83% for 0.5% sulphur.
Fabric filters in form of hag houses have also been employed and are located before the flue gases enter the stack.
Steam from low-pressure turbine has to be liquefied in a condenser and reduced to lowest possible temperature to maximize the thermodynamic efficiency. The best efficiency of steam-cycle practically achievable is about 40%. It means that 60% of the heat in steam at the cycle end must be removed. This is achieved by following two methods.
- Once through circulation through condenser cooling tubes of sea or river water where available. This raises the temperature of water in these two sources and threatens sea and river life around in sea and downstream in river. These are serious environmental objections and many times cannot be overruled and also there may be legislation against it.
- Cooling towers Cool water is circulated round the condenser tube to remove heat from the exhaust steam in order to condense it.