The enormous increase in the quantum and diversity of waste materials has led to an increasing awareness about the need to adopt scientific methods for their safe disposal. While there is an obvious need to minimize the generation of wastes and to reuse and recycle them, the technologies for recovery of energy can play a vital role. These technologies can lead to a substantial reduction in the overall waste quantities requiring final disposal, which can be better managed for safe disposal while meeting pollution control standards. As is well known, conventional stand-alone waste-to-energy (WTE) plants utilize municipal solid waste as fuel rather than materials such as coal, oil or natural gas. Waste pelletization offers an interesting flexible alternative since pellets can be sent to a variety of approved industrial applications (e. g., greenhouses). (See article, page 19.)
Pelletization of municipal solid waste involves the processes of segregating, crushing, mixing high-and low-heat value organic waste material and solidifying it to produce fuel pellets or briquettes, also referred to as refuse derived fuel (RDF). The process condenses the waste or changes its physical form and enriches its organic content through removal of inorganic materials and moisture. The calorific value of RDF pellets can be around 4000 kcal/ kg depending upon the percentage of organic matter in the waste, plus additives and binder materials used in the pelletization process. (For instance, a producer could add post-industrial plastic to the “recipe” to boost BTU value.)
Since pelletization enriches the organic content of the waste through removal of inorganic materials and moisture, it can be very effective method for preparing an enriched fuel feed for other thermo-chemical processes like pyrolysis/gasification, apart from incineration. Pellets can be used for heating plant boilers and for the generation of electricity. The important applications of RDF are found in the following spheres:
• Cement kilns
• RDF power plants
• Coal-fired power plants
• Industrial steam/heat boilers
• Pellet stoves
The conversion of solid waste into briquettes provides an alternative means for disposal of garbage which is currently disposed in landfills. There are several advantage associated with pelletization, such as:
• High calorific value fuel
• Uniform physical and chemical composition • Low moisture content
• Ease of storage, handling and transportation
• Lower pollutant emissions
• Reduction of excess air requirements during combustion
• Use of the main by-product, ash, in brick-kilns
• Improved energy conversion efficiency
• Can be used in a variety of stoker boilers and standalone facilities.
Production of fuel pellets
An RDF production line consists of several unit operations in series in order to separate unwanted components and condition the combustible matter to obtain the required characteristics. The main unit operations are screening, shredding, size reduction, classification, separation (either metal, glass or wet organic materials), drying and densification. These unit operations can be arranged in different sequences depending on the raw waste composition and the required RDF quality.
Manual separation: In mixed waste, bulky items such as appliances, furniture, etc. and specified contaminants (e. g. hazardous waste) can be removed manually before mechanical processing. Manual sorting also serves as a recycling process for paper, glass/plastic containers and aluminum cans. Equipment involved in manual separation usually includes a sorting belt or table.
Size reduction: Size reduction is an important unit operation in waste processing facilities since it gives a good degree of size uniformity to the raw waste. Sometimes, secondary or tertiary shredding is required for production of the desired quality of RDF. A hammer mill is commonly used for mixed wastes while shear shredder is employed for materials that are difficult to shred such as tires, aluminum and plastic.
Screening: Screening results in size separation by dividing the waste into two streams called oversize (retained on the screen) and undersize (passed through the screen) fractions. Trommel screens are commonly used due to their effectiveness and efficiency. Disc screens are used for separation of inorganic fractions from waste streams.
Air classification: Air classification is a separation process governed by the difference in aerodynamic characteristics of waste. The process involves an interaction among moving streams of air, shredded wastes and gravitational force. In air classification of shredded mixed waste, paper and plastic materials tend to be concentrated in the light fraction and metals and glasses are the main components of the heavy fraction.
Magnetic separation: Magnetic separation is used to segregate ferrous metals. The magnetic metal recovery per unit weight of total magnetic metal in mixed municipal waste is about 80 per cent for single stage magnets. Higher recovery rates can be achieved by using multi-stage magnetic separation (as high as 85-90 per cent when used after the air classifier).
Drying and densification: The moisture content in municipal waste can be as high as 55 per cent even during non-rainy days and requires drying to produce the pellets with reasonable heating value. The moisture level is brought down to around 15 per cent through a multistage drying system. Drying can be done by solar or by hot air drying or by a combination of both.
Based on the requirement, about 5 to 10 per cent binder/additives are mixed with the fluff to produce good quality fuel pellets. The pellets coming out of the densification unit are cooled, bagged and stored in the pellet storage yard for dispatch. Municipal waste requires pre-processing to prepare fuel pellets to improve its consist-consistency, storage and handling characteristics, combustion behaviour and calorific value. Technological im provements are taking place in the realms of advanced source separation, resource recovery and production/utilisation of recovered fuel in both existing and new plants for this purpose. There has been an increase in global interest in the preparation of RDF containing a blend of preprocessed MSW with coal suitable for combustion in pulverised coal and fluidised bed boilers.
Salman Zafar manages the renewable energy advisory firm BioEnergy Consult in Aligarh, India. Contact Salman email@example.com