FROM THE EDITORIAL DESK

Thermal Power stations using pulverized coal or lignite as fuel generate large quantities of ash as a by-product. There are about 82 power plants in India, which form the major source of flyash in the country. With the commissioning of super thermal power plants and with the increasing use of low grade coal of high ash content, the current production of ash is about 85 million tonnes per year. This figure is likely to go upto 100 million tonnes per year by the year 2000 AD and pose serious ecological problems.

Appreciating the overall concern for the environment and the need for the safe disposal and effective utilisation of flyash, Department of Science & Technology (DST) as the nodal agency and Technology Information and Assessment Council (TIFAC) as the implementing agency commissioned a Flyash Mission in 1994. Flyash Mission: a Technology Project in Mission Mode (TPMM) is being implemented with participation of Ministry of Environment & Forest, Ministry of Power, Thermal Power Stations, R&D institutions and Industry. The focus is on demonstration of coal ash related technologies for developing confidence and thus ensuring large scale adoption. A number of projects taken up by the industry, R&D institutions and academia have shown encouraging results. Some of these projects have been completed, multiplier effects have started and other projects are heading towards meaningful completion. A judicious mix of select thrust areas have been taken up for concerted efforts. These include Flyash Characterisation, Hydraulic Structures, Handling and Transportation, Agriculture related studies and Application, Ash Ponds and Dams, Reclamation of Ash Ponds for Human Settlement, Roads and Embankments, Underground Mine Fills and related research projects.

Although the scope for use of ash in concrete, brick making, soil-stabilization treatment and other applications has been well recognized, only a small quantity of the total ash produced in India is currently utilized in such applications. Most of the ash generated from the power plants is disposed off in the vicinity of the plant as a waste material covering several hectares of valuable land. The bulk utilization of ash is possible in two areas, namely, ash dyke construction and filling of low-lying areas. Coal ash has been successfully used as structural fills in many developed countries. However, this particular bulk utilization of ash is yet to be implemented in India. Since most of the thermal power plants in India are located in areas where natural materials are either scarce or expensive, the availability of flyash is bound to provide an economic alternative to natural soils.

The properties of ash are a function of several variables such as coal source, degree of pulverization, design of boiler unit, loading and firing conditions, handling and storage methods. Thus, it is not surprising that a higher degree of variation can occur in ash, not only between power plants but within a single power plant also. A change in any of the above factors can result in detectable changes in the properties of the ash produced. The degree to which any change affects the utilization potential of ash is a function of the nature and degree of the change and the particular application for which the ash might be used. The physical, geotechnical and chemical parameters to characterize flyash are the same as those for natural soils, e.g., specific gravity, grain size, Atterberg limits, compaction characteristics, permeability coefficient, shear strength parameters and consolidation parameters. The procedures for determination of these parameters are also similar to those for soils.

Primarily, the flyash is disposed off using either dry or wet disposal scheme. In dry disposal, the flyash is transported by truck, chute or conveyor at the site and disposed off by constructing a dry embankment (dyke). In wet disposal, the flyash is transported as slurry through pipe and disposed off in impoundment called "ash pond". Most of the power plants in India use wet disposal system, and when the lagoons are full, four basic options are available: (a) constructing new lagoons using conventional constructional material, (b) hauling of flyash from the existing lagoons to another disposal site, (c) raising the existing dyke using conventional constructional material, and (d) raising the dyke using flyash excavated from the lagoon ("ash dyke"). The option of raising the existing dyke is very cost effective because any fly ash used for constructing dyke would, in addition to saving the earth filling cost, enhance disposal capacity of the lagoon. The constructional methods for an ash dyke can be grouped into three broad categories: (a) Upstream method, (b) Downstream method and (c) Centerline method. Fig.1 shows typical configurations of embankments constructed using the different methods. The construction procedure of an ash dyke includes surface treatment of lagoon ash, spreading and compaction, benching and soil cover.

Upstream Method

Downstream Method

Centreline Method

An important aspect of design of ash dykes is the internal drainage system. The seepage discharge from internal surfaces must be controlled with filters that permit water to escape freely and also to hold particles in place and the piezometric surface on the downstream of the dyke. The internal drainage system consists of construction of rock toe, 0.5m thick sand blanket and sand chimney. After completion of the final section including earth cover the turfing is developed from sod on the downstream slope.

Large scale use of ash as a fill material can be applied where (a) flyash replaces another material and is therefore in direct competition with that material, (b) flyash itself is used by the power generating company producing the flyash to improve the economics of the overall disposal of surplus flyash; and (c) at some additional cost, flyash disposal is combined with the rehabilitation and reclamation of land areas desecrated by other operations.

Fills can be constructed as structural fills where the flyash is placed in thin lifts and compacted. Structural flyash fills are relatively incompressible and are suitable for the support of buildings and other structures. Non-structural flyash fill can be used for the development of parks, parking lots, playgrounds and other similar lightly loaded facilities. One of the most significant characteristics of flyash in its use as a fill material is its strength. Well-compacted flyash has strength comparable to or greater than soils normally used in earth fill operations. In addition, lignite flyash possesses self-hardening properties which can result in the development of shear strengths. The addition of illite or cement can induce hardening in bituminous flyash which may not self-harden alone. Significant increases in shear strength can be realized in relatively short periods of time and it can be very useful in the design of embankments.

The environmental aspects of ash disposal aim at minimizing air and water pollution. Directly related to these concerns is the additional environmental goal of aesthetically enhancing ash disposal facilities. The ash produced in thermal power plants can cause all three environmental risks - air, surface water and groundwater pollution. The pathways of pollutant movement through all these modes are schematically represented in Fig.2.

Fig.2 Pathways of Pollutant Movement around Ash Disposal Facility

Air pollution is caused by direct emissions of toxic gases from the power plants as well as wind-blown ash dust from ash mound/pond. The air-borne dust can fall in surface water system or soil and may contaminate the water/soil system. The wet system of disposal in most power plants causes discharge of particulate ash directly into the nearby surface water system. The long storage of ash in ponds under wet condition and humid climate can cause leaching of toxic metals from ash and contaminate the underlying soil and ultimately the groundwater system. However, most of these environmental problems can be minimised by incorporating engineering measures in the design of ash ponds and continuous monitoring of surface and groundwater water systems.

IIT Kanpur has pioneered the development of the ash dyke design in India. A number of ash disposal facilities across the country have been designed at this Institute. Currently, a major technology demonstration project on Ash Dyke Design: Raising, Maintenance and Monitoring sponsored by Flyash Mission with active support from NTPC is being executed at the Department of Civil Engineering. As a part of this project, a detailed manual on ash disposal practice has been prepared to standardize the procedures for ash disposal across the country. Another Flyash Mission sponsored project involving detailed geotechnical and environmental investigation of an ashfill site near Badarpur, New Delhi, is nearing completion. Based on field and laboratory investigations, detailed guidelines have been prepared for constructing ashfills and possibility of groundwater pollution through leaching of toxic metals examined. A national conference, Flyash Disposal and Deposition: Beyond 2000 AD, organised at the Institute in February 1999, looked into areas such as design of ash disposal facility, reclamation of ash ponds, structural filling of low-lying areas by flyash and the environmental aspects of ash disposal.

The problems related to flyash are complex and huge, requiring a national effort and commitment. The expertise available at different institutions has to be fully harnessed to develop technologies for large scale eco-friendly utilization of flyash.