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Mitigating pollution from coal based power plants

K.M. Mahbubur Rahman
AFTER Rampal, Bangladesh is going to set up more thermal power plants of 1,320 MW capacity each at Moheshkhali, Patuakhali and Matarbari under foreign collaboration based on imported coal. All these plants will emit pollution into atmosphere. The Rampal power plant is a potential threat to the Sundarbans nearby, and all of them will affect the people, livestock, crops, groves and plantations in the neighbouring areas. This article will discuss the aspects of pollution and ways and means of mitigation for safety of people.
Emission of pollutants and their impact
The flue gas with the products of combustion of coal will spread in the surrounding areas. The emission will consist mainly of carbon dioxide (CO2), sulphur dioxide (SO2), nitrogen oxide (NOx) and miniature particles. All of them are injurious to health, and will have the following effects:
Carbon dioxide will be discharged into the atmosphere at the rate of 30,000 ton/day. It will add to global warming, which is a cause of climate change.
Sulphur dioxide with moisture creates acid mist which enters the lungs, gets mixed with blood and affects blood chemistry. In extreme cases sulphur dioxide will combine with water in the atmosphere in the presence of nitrogen oxide as promoter and make sulphuric acid, which will come down as acid rain and damage the crops, vegetation and pisciculture. Further, the atmosphere will generally be acidic and will enhance corrosion rate of metallic structures and sheet metal roofing of rural homes.
Nitrogen oxide creates ozone or smog at ground level. Inhalation can burn lung tissue, and cause asthma and chronic respiratory diseases.
The particulates (fly ash) can cause severe asthma, bronchitis and premature death. Fly ash also carries uranium, a radioactive matter. With rain, fly ash will leach into the soil, and radiation will contaminate agricultural produce. Ingesting even a small amount of radiation may have fatal consequences.
These effects of pollution will be felt after prolonged exposure in the form of health problems and premature deaths in the community.
Emission control and effluent treatment
Emission will have to be strictly controlled to minimise pollution. The following equipments are compulsory requirements and should be specified in the contract document:(1) Electrostatic Precipitators/Bag filter houses are very important as they can capture 99.9% fly ash particulates. The separated fly ash can be pneumatically handled and stored in silos as a by-product for beneficial use. Output of fly ash in each plant will be 4,200 ton/day.
(2) Flue-gas Desulphurisation (FGD) is a technology that utilises chemical scrubbers to remove sulphur dioxide from exhaust flue gas. It reacts chemically with lime to form calcium sulphate (gypsum), which is a useful byproduct. If the product is wet slurry, it should be dehydrated by centrifuge. Output of gypsum in each power plant will be 620 ton/day. Application of this technology cannot be avoided as imported coals may contain high percentage of sulphur.
(3) Selective catalytic Reduction (SCR) system has to be applied to remove nitrogen oxide from boiler exhaust. The system uses ammonia to reduce nitrogen oxide with the help of a catalyst to simple nitrogen, a harmless inert gas. It is a necessary application as imported coal may have high nitrogen content.
The effluents from the water treatment plant, desalination plant and other auxiliaries should not be released into the rivers before clarification by standard effluent treatment system.
Beneficial use of residual by-products
All avenues need to be explored for "beneficial use" and recycling of the waste products.
Fly ash: It is a very light weight and fine particle like talcum powder with cement-like properties.
Fly ash can be used in: (a) the production of cement;
(b) manufacture of fly ash bricks; (c) as a component in road bases, sub bases and pavements; (d) blending with portland cement in masonry work.
Bottom ash: It is heavier, coarse and granular combustion product which collects at the bottom of the boiler. The fields of application are (a) in structural fills and land reclamation; (b) as aggregate in RCC road bases and pavements; (c) as feedstock in production of cement clinker.
Output of ash in each plant will be 1,800 ton/day
Gypsum: FGD gypsum is widely used in the production of gypsum wallboards and in cement production. Gypsum from all the power plants will suffice as raw material to make a gypsum board factory viable.
Need for more efficient boiler
Selection of highly efficient boiler is very important, because higher efficiency leads to fuel economy and less pollution. Bangladesh Power Development Board (BPDB), the joint owner of the Rampal power plant, is going for Supercritical (SC) boiler, which is two generations behind the latest development.
Ultra Supercritical (USC) and Advanced Ultra Supercritical (AUSC) boilers with improved pulverised coal combustion system and higher thermal efficiency have come in the market. BPDB may seriously consider selection of at least the next higher version USC boiler of efficiency 45% in place of SC boiler of efficiency 40%.
The efficiency improvement of 5% will be a great advantage. The coal requirement will be reduced by 235,000 ton/year, which will save Tk.125 cr. per year. The initial fixed cost may be high but will be compensated in the long run by fuel economy. Also, the emission of CO2 will be reduced by 1,600 ton/day.
Conclusion
A coal-fired power plant produces clean electrical energy but pollutes the atmosphere and environment. The pollution has to be minimised by using state-of-the-art technologies for the sake of the health of the people.
There is no denying that Bangladesh needs electricity for development and progress. The proposed power plants and those in the pipeline are going to provide the much needed electricity for rapid economic growth to reach the level of middle income country by 2040. The country has to balance eco-friendly need and nature for better economy and better life of people - the spirit should be progress to the maximum and pollution at the minimum.The writer Is a Senior Mechanical Engineer.
Email: mahbub.r_407@yahoo.com