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Coal Combustion Residuals

Edition: January - March 2018

Electricity generation has largely been powered by coal as the raw material, for last 200 years and it is expected to be the major source of electricity for the next few decades. However, coal impacts the environment more than any other source of electricity generation such as natural gas, oil, and renewable energy sources. 

As we all know coal is burnt to produce electricity, this creates residues such as ash and minor quantities of stony waste matter or what is technically called slag and gypsum (synthetic). The latter quantities are more to do with additional process-related residues. These are together known as Coal Combustion Residuals (CCRs). These residuals pollute the atmosphere and surrounding areas and hence if timely action is not taken to dispose of the CCR, it will impact the environment. 

The process of CCR production is as follows: when coal is burned at high temperature it uses its heat to convert water into steam for the generation of electricity; ash is the noncombustible material that gets left behind. Ash is the most predominant form of CCR. This is then categorized into fly ash, a very fine, powdery material composed mostly of silica, carbon, sulphur, iron, and aluminium which get carried by the exhaust flue gas to the stack; bottom ash, a coarse ash particle that is too large to be carried up into the smoke stacks, settles at the bottom of the boiler; boiler slag which is the molten bottom ash at the bottom of a boiler that turns into pellets which have a smooth glassy appearance after it is cooled with water. Gypsum (synthetic) is a by-product formed from removal of sulphur through a process known as flue gas desulfurization or scrubber - emission control technology. Gypsum is different from coal ash and quite similar to mined gypsum. 

The challenge for policymakers and scientists is to manage CCRs, which are responsible for environmental pollution. As a result, policymakers have given rules and regulations for safe utilization and disposal for these CCRs. In the Indian context, the Ministry of Environment and Forests issued Notification, 1999, which focuses on 100% utilization of fly ash in a phased manner. It recommends that fly ash be supplied as raw material to manufacturers in a closed tank at no cost. The Central Electricity Authority has been working for effective utilization of CCR in India. 

Technological advancement has allowed us to utilise the residue, especially fly ash, for production purposes such as i) cement manufacturing, ii) brick/tile/concrete manufacturing, iii) making of wallboard/plaster of Paris, and iv) soil conditioner for agriculture. 

Cement/brick/tile manufacturing: In these industries, ash increases the strength, workability, durability of cement and concrete. Therefore, fly ash partly replaces the raw materials such as iron, limestone and sand for manufacturing of cement, especially Portland Pozzolana Cement (permitted by IS: 1489  from 15% to 35%). This makes fly ash an economical substitute. Further, a major portion of fly ash is used in concrete industries for making roads, embankments, sidewalks, foundations etc. replacing cement with fly ash. Concrete block manufacturing industry uses bottom ash as lightweight aggregate to replace natural aggregate which reduces the pressure on natural resources. 

Wallboard/plaster of Paris Gypsum (synthetic) can replace naturally mined gypsum. Gypsum contributes about 90-95% in the making of wallboard and Plaster of Paris. This helps reduction of carbon dioxide emission due to natural mining and minimises the other costs incurred due to storage and transport.

Agriculture: Gypsum (synthetic) can be used as soil conditioner in the agricultural industry. Gypsum contains enough sulfur and calcium which can be used by plants as secondary nutrients. Calcium provides primary structural support for cell growth whereas sulfur increases chlorophyll development and protein synthesis in plants. Gypsum also helps in increasing water infiltration capacity of soil and reduces soil erosion. All this subsequently increases the crop yield. 

The Government of India has put in place a robust framework for reducing ash content in coal which eventually will reduce the CCR generation and increase its utilization. However, the actual utilization of fly ash is still far below production levels, which in turn impacts the environment. This indicates that implementation of fly ash utilization rules require greater additional investigation and follow up.






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