S. K. Maheshwari
Grasim Industries Limited
August 2007
World Cement

Rapid industrialisation has put a huge burden on resources throughout the world. Energy is one such resource having a wider impact on all aspects of human life. The cement industry is a major consumer of energy and in India, which has the third largest coal reserves in the world, it consumes around 6 per cent of the total coal produced as fuel in kilns only. This provides for only around 80 per cent of its fuel requirement. It is expected that, at the present rate of coal consumption, the coal reserves in India will be exhausted in the next 70 years. It is therefore important to look for alternatives. At the same time, it is imperative for all corporations to fulfill their social commitment towards sustainability by reducing the consumption of fossil fuels and reducing greenhouse gas emissions.

Industrialisation has not only had an impact on energy demand, but also on waste generation. This has been further impacted by rises in populations and changes in population profile from rural to urban. It is estimated that 11 Indian cities generate more than 1000 tpd of waste. Mumbai alone generates more than 5000t of solid waste daily. A significant portion of waste contains organic matter (e. g. paper, cardboard, plastic, rubber, etc.). These materials have good calorific values and can therefore be used as alternative fuels. Hazardous industrial wastes such as paints, sludge and oil can also be used as alternative fuel.

Waste as fuel
The use of waste as alternative fuel may appear as something new in the Indian context, however, the concept is already in practice in several other countries. Traditionally, it started with burning waste in specifically designed incinerators, which were sometimes equipped with power generating units. Over the years, the cement kiln emerged as better alternative to the incinerator because of some of its inherent characteristics. Grasim tried the following wastes as fuel in its kilns, resulting in partial substitution:

• Sludge from effluent treatment plants
• Tyre chips
• Agricultural waste
• Refinery sludge
• Paint sludge

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Based on the use of alternate fuels, two of its units — Grasim South Cement and Andhra Pradesh Cement plant (UltraTech) — have already been issued CER (Certified Emission Reduction). There are eight other similar projects currently in the pipeline.

Technical feasibility
Several countries have been using waste as fuel for over 15 years. Based on their experience, it has been found that the cement kilns are highly suitable for burning waste. This is mainly due to its following reasons:

• High flame temperatures (2000 ^oC) ensures complete destruction of harmful pollutants
• Residence time of combustion gases above 1000 ^oC in excess of 3 to 4 seconds ensures complete destruction of pollutants
• Complete scrubbing of exhaust gases due to countercurrent flow of raw material resulting in trapping of heavy metals, sulphur and other pollutants within clinker
• Inclusion of ashes and residual metals from the waste within the clinker crystal structure
• Kiln lines are equipped with ESPs/bagfilters to ensure negligible particulate emission
• Intense contact between solid and gas phases ensures condensation of volatiles, absorbs SO₂ and neutralises acid gases.

Cement kilns present an opportunity where burning waste is a recovery operation and the term often used is valorisation. This is because of the fact that the combustible parts of the waste replace fossil fuels and the non-combustible parts replace raw materials like silica and iron. The environmental impact is negligible and the energy efficiency is the highest compared to incineration equipped with power generation. At the same time, the option offers benefits to the society in the following form:

• Conservation of non-renewable fossil fuels
• Reduction in environmental impact related to coal mining
• No need for investment in incinerators
• Overall lower CO₂ and methane emissions by replacement of fossil fuel
• Conservation of raw materials for the cement industry, as hazardous wastes partially replace some of the traditional materials like silica, iron. etc.

There could be process problems associated with burning waste in the cement kiln, which can be tackled by adopting some technological changes and through process mastery.

Grasim's experience
In India, Grasim (an Aditya Birla Group Company) has taken the initiative to burn waste as fuel in its kilns. The company has established waste handling facilities at two units, Grasim South Cement and Vikram Cement. The company has also installed a municipal solid waste processing plant. The details are presented in the following sections.

Trial with effluent treatment plant (ETP) sludge
The first trial undertaken by Grasim at its unit in Karnataka was using sludge from a pharmaceutical company. Table 1 contains the analysis of the sludge. During the trial, more than 200 t of this sludge (3 to 6 per cent of total fuel) was used along with coal. The major observations that arose from this were:

• The level of pollutants do not represent any regular trend
• The concentrations of NOx, HCI, HF and TOC showed slight increase in concentrations during the trial runs, whereas other pollutants are showing decreasing trends
• Some metals showed marginal increased concentration
• All the parameters are well below the EU norms for co-incineration in a cement kiln

Fig.3 Click on thumbnail for a larger view

• It is designed to process about 400 to 500 tpd of MSW and generates around 130 to 140 tpd of refuse derived fuel (RDF) in the form of fluff
• The technology and equipment have been supplied by M/s Humboldt Wedag India Ltd and M/s Doppstadt, Germany
• Grasim reached an agreement with Jaipur Municipal Corporation for the regular supply of waste

The composition of waste is such that it follows for evaporation of moisture (around 25 per cent), 30 per cent is inert material, 20 per cent is reused or recycled and the balance 30 per cent is RDF. A process flow diagram of the installed system is presented in Figure 3. Table 2 shows the properties of RDF derived from the municipal waste.

The company has installed a complete waste utilisation system at Vikram Cement. The system comprises an unloading, storage, dosing and feeding system for municipal waste, agricultural waste and tyre chips. To avoid environmental problems during storage and handling of municipal waste, a special storage silo with walking floor extraction and a closed belt conveyor system is employed. The feeding and substitution capacity of the facility is designed to replace 15 per cent of the total heat consumption of the system by municipal waste, tyre chips or agricultural waste. The waste derived fuel (WDF) compounds are stored and dosed according to their physical properties.

The walking floor extraction system consists of storage-cum-feed silo with an active bottom discharge designed for a continuous operation of 24 hrs/day with minimum maintenance requirements. The material transport takes place at a pre-determined volumetric or gravimetric rate and the movement is actuated by hydraulic cylinders. The sensor located in the surge hopper senses the proper filling of the surge hopper. The feeding occurs through a set of two chain conveyors that distribute the material across the silo. Discharge is through a rotary feeder. Table 3 indicates a summary of the waste fuels being handled by the system.

Conclusion
With the increasing burden of energy costs and the availability of fuels throughout the world, it is natural to look for alternatives. At the same time, the increasing awareness of greenhouse gas emission associated with the use of fossil fuels has forced businesses to re-think their energy strategies. Grasim has already taken the initiative in this direction in India and has installed municipal solid waste processing facilities and waste burning systems at two of its locations. Trials for specific wastes have already been conducted at two of the locations. However, the long-term success lies in a coherent policy on the subject, along with the creation of suitable infrastructure.