Practical solution for environmental problems at Trombay Power Station

Fly ash aggregate plant turns disposal problems into construction materials

Mr. R. Chandramouli and Mr. A.V. Dagaonkar

Tata Electric Cos.

Mr. G. Motza and Mr. D. Mahadew

Hoogovens Technical Services Energy & Environment BV

Due to relatively low calorific value and high ash content of Indian coals, the fly ash produced during power generation in India is approximately 50 million tons annually, based on consumption of approximately 160 million tons of coal. The disposal of ash has become a matter of great concern, as the conventional ash disposal methods require construction of ash ponds on large areas of land, very often fertile agricultural land. Based on the actual power generation level, 30,000 hectares of land is required for ash disposal.

Dumping the fly ash into the sea creates large pollution problems for the marine flora and fauna, which, in the long term, becomes a real danger for public health.

High population growth in India and increasing industrialization have already put tremendous pressure on the land and environment. Therefore, further use of agricultural land for disposal of ash will be extremely difficult. On the other hand, in the future, coal consumption for power generation is expected to increase from 160 million tons to 330 million tons in the next few years, proportionately increasing the fly ash produced.

Use constraints

The application of fly ash in India is affected by specific traditions and local circumstances, such as:

– Indian consumers have a strong preference for traditional building materials such as clay bricks and cement products based on natural aggregates.

– Most power stations have a wet ash handling system, where fly ash is transported as slurry to ash ponds. A dry collection system has to be installed before the ash can be used on a commercial scale.

– Many thermal power stations are located on the coal deposits and are therefore far away from populated areas.

– The price of naturally available products, such as bricks and gravel, is generally very low and therefore less than the price of the products which can be made of fly ash.

– A large-scale application of fly ash products is not promoted for large projects.

– Initial investments for fly ash utilization plants are relatively high and are not considered an integral part of the power station.

– For an appropriate market penetration, more promotional efforts for use of fly ash products are required.

However, despite these constraints, several applications of fly ash can be considered, such as environmentally safe landfill material through stabilization with lime, gypsum or cement; the fine fraction of the fly ash can be used for the production of the cement clinker; as a replacement for cement and as filling material in concrete application; as lightweight gravel; as a ceramic material with improved thermal properties for insulation; or for fabrication of bricks or tiles.

Lightweight aggregates

Worldwide, many technologies have been developed for the production of artificial aggregates from fly ash. Only two of them have reached the commercial status–the sintering process Lytag and the cold-bonded process Aardelite.

Comparing the economics and the energy consumption, Aardelite offers a promising alternative for fly ash use, especially for the Indian market.

The Aardelite process is free of emissions and the gravel produced can be used for a variety of applications in the construction industry, including masonry elements, precast concrete elements, ready-mix concrete for buildings up to five floors and bituminous concrete for road foundation.

Aardelite is a Dutch process, mainly developed for the production of aggregates from coal fly ash. The process uses fly ash, lime and water and operates at atmospheric pressure at temperatures of around 90 C. The process is based on the hardening of a fly ash/lime mixture due to reaction of calcium with water and silica, and alumina which are already available in the fly ash. During these reactions, calcium silicate/aluminate minerals are formed through a reaction mechanism similar to that of portland cement. These minerals bind the fly ash particles tightly, resulting in strong, hard stones.

Specific characteristics of the Aardelite process are:

– Lime must be hydrated to calcium hydroxide prior to pellet production. This is required because the volume increase which occurs during hydration of lime may cause cracking of pellets.

– An important process parameter is the quality of mixing. Intensive mixing can minimize the use of lime which is the most expensive feedstock.

– The mixing temperature may affect the initial rate of the bonding reactions, depending on the quality of the fly ash. Part of the process water is used for the chemical reactions, however the main portion is used for the pelletizing process.

Selection criteria

While looking for a solution to ash disposal problems at Trombay Power Station, Tata Electric Cos. (TEC) and Tata Consulting Engineers (TCE), conducted a detailed study of fly ash processing technologies available in the world. After comparison of several options, the Aardelite technology was found to be the best solution for Trombay.

Major advantages of the Aardelite process are:

– It has a low energy consumption because of its +/-90 C curing temperature.

– The process is emission free and the small quantities of dust generated during processing are collected in dust filters and recycled.

– Compared to the sintering process, the investment and production costs are relatively low.

– The process can be operated by average semi-skilled personnel and does not require complicated upkeep.

From the application and marketing perspective, there are also several advantages. The spherical shape of the fly ash pellets result in an improved workability of the concrete, requiring less vibration energy for shaping and compacting. Lower specific density of the fly ash pellets results in savings when used in building materials, especially where weight is a critical aspect as in foundations, building elements, transportation costs or handling of the building products. Final size distribution of the product can be tailored to meet the customer`s requirements at the same production cost.

Trombay Power Station is located near Mumbai, which is a highly industrialized city in India with a well-developed building industry. Due to the new regulations regarding environmental protection, the cost for disposal of fly ash is increasing.

Production

Dry fly ash is extracted from the hopper of the 500 MW Unit 5 and transported to a dry fly ash storage silo with a storage capacity of 280 tons, located in the vicinity of the Aardelite plant, with a storage capacity of 280 tons. Transport of the fly ash takes place via a dense phase pneumatic conveying system, covering a distance of about 450 m at the rate of 50 tons per hour. This is one of the longest fly ash conveying dense-phase systems in India. The major advantage of the system is the reduced abrasive wear on the pipelines, particularly at bends where the material moves at lower speeds. Fly ash from the silo and hydrated lime from lime storage are conveyed to the main plant using this same method.

Once the fly ash and lime are transported to the production plant, the fly ash is divided into two streams. One stream is fed into the mixer for pellet production, and the other stream is used for embedding the green pellets. The embedding material prevents the green pellets from sticking together during the curing process. After curing, the embedding material is separated from the cured pellets and used as feedstock for pellet production. The green pellets are produced by mixing ash, lime and water in a high-intensity mixer to obtain a homogenous mass.

This mixture is fed into the pelletizer, where pellets of desired size are produced through rotation under an adjustable angle while spraying water. The green pellets are embedded in fly ash and fed through the rotary preheater in the curing silos where hardening takes place. The curing process takes 16 to 18 hours. When cured, the pellets are separated from the embedding material in a rotary screen and sorted by size through a vibration screen. Particles smaller than 2 mm return as feedstock in the process.

In the Trombay situation, the pellets are screened in three fractions–2 to 8 mm, 8 to 16 mm and 16 to 32 mm–for use as a gravel substitute in the production of concrete products.

The manufacturing process is monitored and controlled via a digital control system TDC3000, manufactured by Tata-Honeywell, located in the control room. Variable speed drives and the MCC are also located in the same building. The control room has two universal stations on which all equipment and parameters can be monitored and controlled. Signals coming from plant instruments, pressure, temperature, zero speed switches, control valves, flow switches, level probes and limit switches enable the operators to closely monitor the entire process. Any variation from the set values can be detected immediately and rectified through control valves and variable speed drives. The system also has the facility to monitor and record various parameters for 96 hours, which can be recalled on screen or printed. All equipment can be started or stopped from the control room.

Aggregate plant construction

TCE undertook the detailed engineering, plant layout and design of electrical and instrumentation systems for the whole plant. As this plant was the first of its kind in India, there were no established suppliers for the major process equipment. After delivery of the basic engineering package from Aardelite, including process specifications, requests for bids were issued for local as well as foreign manufacturers. Extensive efforts were made in locating Indian suppliers who could manufacture the process equipment to the specifications of the Aardelite technology (see table).

The mixers and pelletizers, the most important equipment of the plant, required locating Indian manufacturers with experience in the design and manufacture of similar equipment. A few local manufacturers produce various types of mixers for the chemical industry. Based on the parameters and specification supplied by Aardelite engineers, a local manufacturer designed and guaranteed the mixer. The expertise for special linings for the mixer`s shell was available in India. To ensure that the mixer would meet the requirements of the process, a model was manufactured for tests on laboratory scale prior to approval of the final design.

Pelletizer manufacturers and suppliers for the rotary preheater, the rotary screen, impact scales and vibrating screen were also found locally and selected jointly with the Aardelite engineers.

Applications

Mumbai is India`s commercial capital, attracting an increasing population and thereby increasing construction activities in and around the city. The demand for natural aggregates and sand is increasing. It is estimated that nearly 4 million tons of aggregates and sand were required during 1995 in the Bombay area, and the amount is still increasing. The fly ash pellets offer a solution by minimizing the environmental degradation caused by quarrying. Some of the favorable characteristics, such as improved thermal properties, low specific density compared with natural gravel, and round shape, make it eminently suitable for use in many construction areas.

When using these lightweight pellets for the manufacture of concrete and concrete products, the unit weight decreased by approximately 25 percent. Further, the fly ash pellets result in better cementing of concrete due to their absorption capacity, which results in an excellent bond between the gravel and the cement matrix.

Promotional efforts

An assortment of visitors from India and abroad have visited the fly ash aggregates plant at Trombay, including power and steel plant representatives, showing keen interest in the process as a solution to their ash disposal problems. Representatives from cement plants and the building construction industry have also visited. As a part of forward integration, a fly ash block-making plant will be built in the near future. Various applications in road making may promote direct use of aggregates by various civil contractors.

In developing countries, the production of low-cost building material faces problems due to the cost and availability of cement. Cement remains the most expensive building material in these countries.

Conclusion

There is a tendency to view the capital cost of the fly ash pellets plant as a cost to be recovered by sale of the fly ash pellets for its products, but this may not be practical. What has to be realized is that, as an electrostatic precipitator is regarded as part of the capital cost of the power plant and not separately evaluated for its economics, the plant for fly ash handling and its conversion should also be treated as part of the power plant capital cost. Initiative must be taken by environmental bodies to promote the use of fly ash products, which is happening in several countries. There are also suggestions that the production of conventional clay bricks, which degrade the environment due to removing the top soil should not be permitted to continue in areas near thermal power plants.

Click here to enlarge image

Authors:

Mr. R. Chandramouli is TEC vice president in Bombay, India. Mr. A.V. Dagaonkar, is site manager for TEC, also in Bombay.

Mr. G. Motza is project manager for Hoogovens Technical Services Energy & Environment BV in IJmuiden, The Netherlands; and Mr. D. Mahadew is a process engineer for Hoogovens, also in IJmuiden, The Nether