Fly ash : properties, ecological concerns & sustainable use
- Prabhat Kumar Rai -
Fly ash is non-combusted byproduct of pulverized coal combustion of coal-fired Thermal Power Plants (TPP). It consists of fine particles that rise with the flue gas. Generally, fly ash comprises silicon dioxide (SiO2), calcium oxide (CaO), ferrous dioxide (FeO2) and aluminium trioxide (Al2O3). In addition, it may include traces of arsenic beryllium, boron, cadmium, chromium, cobalt, lead magnese, mercury, molybdenum, selenium, strontitum, thallium and vanadium. It also contains some organic materials in the form of un-burnt carbon. Constituents vary depending on specific coal-bed configuration.
Fly ash has been defined in Cement & Concrete terminology (ACI Committee) as ‘the finely divided residue resulting from the combustion of ground or powdered coal, which is transported from fire box through the boiler by flue gases.’
It possesses pozzolanic characteristics. It is a silicaceous material which in presence of water reacts with calcium hydroxide at ordinary temperature to produce cementitious compounds. The micron-sized fly ash particles consisting primarily of silica, alumina & iron which are spherical in shape allow them to flow and blend freely in mixture. The fluidity increases as the particle size distribution becomes wider. This property makes fly ash a desirable admixture of concrete. The fly ash lends lubricating action when concrete is in plastic stage. Because of high density and pozzolanic action, fewer bleed channels accrue and permeability is decreased. Reduction in bleeding decreases porosity and chemical attack. It also results in improved bond strength due to improved paste. Fly ash addresses the problems related to durability of concrete mass effectively. It combines with free line and imparts high structural strength to concrete over time. The lubricating action of fly ash reduces water content and drying shrinkage. The pozzolanic reaction between fly ash and lime generates less heat, resulting in reduced thermal cracking when fly ash is used to replace Portland cement. It forms C-S-H gel having similar bonding properties in the concrete as those produced by hydration of cement paste. Additional comentitious material is produced from reaction of liberated surplus lime with fly ash which blocks the capillary voids and reduces the risk of leaching out of surplus free lime thereby reducing the permeability of concrete. Blending of cement with fly ash helps reduce the amount of CO2 generation. It serves as a performance improver in Ordinary Portland Cement. Fly ash when treated with sodium hydrate appears to function well as a catalyst for converting polyethylene into substance similar to crude oil in a high temperature process called pyrolysis.
Due to its strength & lower water content, cracking is reduced. It acts as ball bearings thus filling small voids and produces denser concrete which requires less water for installation resulting in water savings. Owing to more resistant to sulphate attack and alkali-aggregate reactivity, the demand for natural resources is reduced with concomitant reduction in associated environmental impact of extracting and processing them. It has higher ultimate strength then conventional concrete, so in specific applications less material is required to accomplish a given structural need. Fly ash can also be added to plaster, reducing or eliminating crazing, drying shrinkage cracks, de-bonding expansion & other defects. It reaches its maximum strength slowly than concrete made with only Portland cement.
3. ECOLOGICAL ASPECTS:
Since coal contains trace levels of arsenic, barium, beryllium, boron, cadmium, chromium, thallium, selenium, molybdenum, nickel, radium, thorium, copper, lead, vanadium, zinc and mercury, its ash is likely to contain these traces and therefore cannot be dumped or stored where rain water can leach metals & make them to aquifers. The presence of high contaminant level in many Coal Contaminant Residue (CCR) leachates may create human health & ecological concerns. Exposure of fly ash through skin contact, inhalation of fine particles, fugitive dust & drinking water may well present health risks. Fine crystalline silica present in fly ash has been linked with lung damage, in particular silicosis. CaO reacts with H2O to form Ca(OH)2 giving fly ash a ph value somewhere between 10 to 12, a medium to strong base. This can cause lung damage if present in sufficient quantity. Chromium is of concern because in its toxic form - hexavalent chromium- it is a known carcinogen.
In worst case, traces of arsenic, vanadium and mercury may lead to cancer, neurological problems and developmental defects warranting safeguards for storage, handling, transport and disposal. Unlined coal ash landfills & fill sites cause widespread groundwater contamination from toxic chemicals in coal ash. Although coal ash constitutes numerous toxic constituents, arsenic is often the primary threat to human health and the environment. Arsenic is acutely toxic at high concentrations (greater than 300 ppb). Exposure to vanadium in the air has been associated with the lung damage at high concentrations and vanadium pentaoxide is treated as possible human carcinogen. Ingestion of vanadium reduces red blood cells, increases blood pressure and causes neurological problems. Mercury is a patent neurotoxin known to cause a number of adverse health effects on the brain and central nervous system. It is known mutagen and teratogen (an agent that disrupts embryonic development) apart from being suspected carcinogen. When elemental mercury makes ingress into soil and water, microscopic organisms may convert it into methyl mercury that builds up in the bodies of fish and other marine mammals and finds way to food web.
4. MANDATE IN MoEF’s NOTIFICATION:
MoEF (Ministry of Environment and Forest) has legislated that every construction agency engaged in construction of buildings within a specified radius for coal-fired Thermal Power Plant to use only fly ash based products for construction. They have also prescribed minimum fly ash content for Fly Ash bricks and blocks. There are specific instructions about reclamation; compaction of low-lying areas with soil only by fly ash has been prescribed. It has been mandated that at least 20% of dry Electro Static Precipitator (ESP) fly ash shall be available free of charge for the units manufacturing fly ash or clay fly ash bricks, blocks or tiles on priority over other users. MoEF has also mandated cent per cent fly ash utilization in five years for old plants from the date of issue of notification (2009). However, for new Thermal Power Plants, 100% utilization of fly ash within 3 years from date of commissioning has been stipulated. They have further directed that the amount collected from sale of fly ash by TPP should be kept in separate accounts head and shall be utilized only for development of infrastructure for use of fly ash until 100% fly ash utilization level is achieved. Similarly, for building construction agencies as well as agencies undertaking construction of roads or flyovers and reclamation and compaction of low-lying areas, specific provision has to be made in the tender documents & schedule of approved materials etc. MoEF has also issued an advisory dated 13.4.2015 for use of coal with ash content not exceeding 34% for certain Thermal Power Plants. The National Green Tribunal has already banned digging of earth for brick kiln & road construction etc. and made it mandatory to obtain clearance from Environment Ministry as per directive dated 24.6.2013.
5. PROCESSES INVOLVED:
The fly ash evacuation is generally done pneumatically and stored in silos. From silos, it can be loaded on road vehicles/tankers. It is removed from plant exhaust gases primarily by ESP or bag-houses and secondarily by scrubber system. Nearly 85% of fly ash is disposed of using the lean slurry disposal system where one part of ash & 8-10 parts of water is pumped through pipes to an ash pond. It entails huge requirement of water apart from creation of waste land, health and environmental concerns associated with probability of leaching of heavy metals as well as air-borne fugitive ash.
Various methods adopted by the Thermal Power Plants to reduce sulphur, nitrogen oxides (NOx) and other impurities from coal include effective ways of cleaning coal after it is mined, use of flue gas desulphurization equipments (Scrubbers) to clean sulphur from the smoke before it leaves smokestacks, use of catalytic converter for NOx reduction and ESP and bag-houses for the particulate matters reduction, The emphasis is on technologies which would make coal more energy-efficient so that less quantity needs to be burned.
6. ENVIRONMENT-FRIENDLY USES:
Fly ash's environment - friendly uses by way of diverting the material from waste stream, reducing the energy investment in procuring virgin materials, conserving virgin materials and alleviating pollution, deserve special mention.
It serves as a good mineral fill in hot mix asphalt applications. Apart from improving the fluidity of road bases and sub-bases, embankments and other structural fills, fertilizer, ice control on roads & parking lots, it finds use in cosmetics, kitchen counter tops, bowling balls, mails boxes, metal castings, binding materials, filler in wood and plastic products, fire place, utility poles and cross arms, railway sleepers, sign posts, roofing tiles, paving stones, park benches etc. Bottom ash is used in sintered and geotechnical applications, structural fills, clay-ash fire bricks and agricultural applications etc.
7. BENEFICIAL ADMIXTURE WITH CONCRETE MIXES :
It reduces corrosion of reinforced steel by eventual reduction in alkali-silica reaction. It conserves water by reducing water demand in concrete mixes. It can be mixed with molten metal and cast to reduce overall weight and density, due to low density of fly ash. In view of its alkalinity and water absorption capacity, it may be used in combination with other alkaline materials to transform sewage sludge in organic fertilizers or bio fuel.
During hydration process of cement, lime releases out and is rendered surplus in the hydrated cement. Leached out lime has deleterious effect on concrete as it makes it porous, thus weakening its bond with aggregates and making it vulnerable to cracks. If fly ash is mixed, the surplus lime gives rise to pozzolanic reaction with attendant benefits as described above.
For every one ton of cement produce approximately 6.5 million BTUs of energy is consumed and one ton of CO2 is released. Replacing that one ton of cement with fly ash saves enough energy with eventual ecological benefits. Fly ash is used as mineral admixture in concrete. Fly ash adds value to cement industry as it replaces clinker. Its impact can be well assessed as India's cement industry is reckoned as 3rd largest in the world. Substitution of a waste product for large portions of Portland cement significantly improves concrete's environments characteristics. According to a conservative estimate, use of fly ash would save approximately 100 MT of limestone and 20 MT of coal and would add up to 55 MT of annual cement production with 70% less expenditure.
Setting up of cement grinding units near power plants may be an economical proposition particularly for Thermal Power plants which are not strategically located with respect to proximity of cement plants. The grinding unit for the cement plant Wadi has been built in the premises of Raichur Power Plant. The clinker produced is transported to the grinding unit at Raichur, where it is mixed with fly ash generated by the Power Plant. This solution has got its own appeal as cement consumption centres are more likely to be located near Power Plants than cement plants which may be located near limestone deposits.. Being geo-polymeric binder, it has got tremendous potential for concrete and construction materials.
8. FLY ASH BRICKS :
Fly ash bricks are very useful & environment friendly building product. It is uniform in shape & size, consumes less mortar in brick work, plaster thickness is less as compared to clay bricks, thus saves cement mortar. It is less energy intensive as compared to clay bricks. It can be manufactured at consumption sites also. If can be produced with or without frog. These are basically machine-made bricks manufactured by hydraulic or vibratory process. Raw materials include hydrated lime, gypsum, locally available sand/stone dust and water. Minimum requirement for good quality fly ash brick making is fly ash 30%, cement 5-7%, gypsum 1% and balance sand or crushed dust. If 60% fly ash is added, quality of fly ash brick will be very good. Owing to high concentration of CaO in fly ash, the brick is described as self-cementing. The manufacturing methods & processes save energy, reduces mercury pollution and costs 20% less than the conventional clay brick manufacturing. It is lighter than clay brick. Its high strength preclude chances of breakage during transportation. Due to uniform sizes, the mortar requirement for joints & plasters reduces by almost 50%. Due to low water penetration, seepage of water through bricks is considerably reduced. Gypsum plaster (Plaster of Paris) can be directly applied on these bricks without a backing coat of lime plaster. These bricks do not require soaking in water; sprinkling of water before use is enough. The raw material is mixed in pan mixer to have semi-dry uniform mix. Moulded bricks are dried for at least 2 days in a shed depending on weather conditions. After that, it is cured for 2 to 3 weeks. These bricks are sound, compact and uniform in shape. BIS 12894-2002 for ash bricks has been specified by CPWD & other construction agencies for use in their construction works.
On the flip side, the mechanical bonding strength is weak. But it can be improved by adding marble dust. Only modular sizes of fly ash brick can be produced. Air exchange is little poor. According to a conservative estimate, around 100 billion bricks (300 MT of top soil) are needed in our country every year. Erosion of top soil for clay bricks may render 3000 hectares of fertile land barren each year. Clay-fly ash bricks which is a compromise between traditional clay bricks with due admixture of fly ash are also being used. The fuel requirement in making these bricks is considerably reduced as fly ash containing some un-burnt carbon particles also.
American Coal Ash Association, Coal Ash Institute of India &TERI etc. have recommended a host of measures to promote widespread and intensive use of fly ash products. Some of the recommendations are cited below:
· A critical study of each Thermal Power Plant to be carried out with a view to estimating its fly ash potential in the cement/concrete industries or back fill or brick industry etc. A cess may be levied on unutilized fly ash, the difference between assessed potential and actual utilization.
· Making it mandatory for coal mining companies to adopt selective mining techniques with washing limited to the extent that ash cannot be further reduced by selective mining. Selective mining the coal without impurities may prove economical than washing. Beneficiation involves large requirement of land and water coupled with problem of disposal of washery rejects.
· Transportation of coal one way and fly ash on the return trip on routes where power plants and cement plants are located. Techno-economic consideration to be addressed as the cost of transportation of fly ash is high.
· Provision of closed tank wagons for transportation of dry ash in an economical and environment - friendly manner.
· 100% accelerated depreciation on grinding units set up near Thermal Power Plants, provided certain proportion of the cement production is PPC. Similar incentives for special purpose mixers used in manufacturing of fly ash bricks.
· Increasing royalty on soil, especially for use in brick making. It will make the soil more expensive than fly ash and encourage brick industry to replace soil with fly ash to the greatest extent possible.
· Provide lower income tax rebate on housing loans availed for the projects that do not use fly ash based produces.
· Levy differential excise duties on OPC & PPC (by increasing duty on OPC or reducing or removing that on PPC). Misconception that PPC is equivalent to inferior grade of OPC to be debunked.
· Waiver of sale tax on fly ash products and machineries utilized for production of fly ash - based products.
· Making it mandatory for Thermal Power Plants to supply fly ash to cement producers consistent with the BIS specification for cement grade.
· 100% accelerated depreciation on investments by Thermal Power Plants in collection of fly ash, its proper storage, handling and transportation. Presently, there is little focus on dry ash collection and much of the ash continues to be pumped into ash dykes.
· Levy an environmental cess on existing fly ash ponds.
· On lines of CSR, focus on Corporate Environmental Responsibility (CER).
· Provide 100% accelerated depreciation and zero import duty (currently at 51%) on surface miners used for selective mining.
· While establishing coal linkage for Thermal Power Plants, also establish fly ash linkages with a network of coal mines, cement producers, brick manufactures and others users.
· Mobilization of financial resources, joint effort of coal & power sectors, for establishing desired infrastructure for high quality dry fly ash, silos for ash storage & transfer and wagons for transportation etc.
· Certain percentage of coal mined, collected under Coal Conservation & Development Act (CCDA) to be diverted for establishing transportation infrastructure for use of fly ash by coal mines.
10. IN LIEU OF CONCLUSION:
There is a growing need to provide adequate incentives for innovative uses and development of more efficient coal combustion technologies like Fluidized Bed Combustion, gasification and the hybrid system.
The two-pronged approach i.e. limiting fly ash generation by reducing ash content of coal used for Thermal Power Plants and increasing fly ash utilization and exploring its further uses needs to be followed vigorously by multiple stake-holders like coal mining companies, Thermal Power Plants, cement industries, brick manufactures, road and housing sectors, etc. It is essential to establish commercial scale technology demonstration centres for various applications to dispel misconceptions about its negative impact and increase public acceptance.
The Centre for Fly Ash Research & Management (CFARM) has rightly recognized it as a wealth generating segment. It is high time instead of looking down upon the fly ash as a waste, we should realize the tremendous potential of the useful by- product and treat it as an opportunity to convert 'waste' into ‘wealth’ by enhancing its productive uses.
Former Chairman of Bihar State Electricity Board & Former Chairman-cum-Managing Director Bihar State Power (Holding) Co. Ltd. . Former IRSEE ( Indian Railway Service of Electrical Engineers ) . Presently Energy Adviser to Hon’ble C.M. Govt. of Bihar . Distinguished Alumni of Bihar College of Engineering ( Now NIT , Patna ) Patna University. First Class First with Distinction in B.Sc.(Electrical Engineering). Alumni Association GOLD MEDALIST from IIT, Kharagpur , adjudged as the BEST M.TECH. STUDENT.
Administrator and Technocrat of International repute and a prolific writer . His writings depicts vivid pictures of socio-economic scenario of developing & changing India , projects inherent values of the society and re-narrates the concept of modernization . Writing has always been one of his forte, alongside his ability for sharp, critical analysis and conceptual thinking. It was this foresight and his sharp and apt analysis of developmental processes gives him an edge over others.
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