Incineration types, figure, parts, functions

Solid waste management in chemical engineering

1. Incineration

2. Incineration • Incineration i s a controlled combustion process for burning solid, liquid, or gaseous combustible waste to gases and residue containing little or no combustible material when properly carried out. High-temperature incineration is carried out at 3000 F to 3400 F (1649 to 1871 C). It is a hygienic way of disposing solid waste. It is suitable if waste contains more hazardous material and organic content. This process is the most effective process for completely destroying plastic waste and pathogenic medical waste. It is expensive process, compared to other methods of waste disposal. • Municipal solid wastes are burnt at high temperature in big furnaces called incinerators. Combustible substances such as plastic materials, rubbish, garbage, dead organisms are separated for burning in incinerators. The non-combustible materials can be left out for recycling and reuse. About 10 % solid material and ash remains after combustion which can be disposed off by other means.

3. Contd … The heat produced in the incinerator during burning of refuse is used for generation of electricity through turbines. Municipal solid waste is generally wet and has a high calorific value. Therefore, it has to be dried first before burning. Waste is dried in a preheater from where it is taken to a large incinerating furnace called "destructor" which can incinerate about 100 to 150 tonnes per hour. Temperature normally maintained in a combustion chamber is about 700 C which may be increased to 1000 C when electricity is to be generated.

4. Incineration - Advantages It reduces the waste volume by 90 per cent Requires very little space Safest from hygienic point of view The only method available for safe and complete decomposition/destruction of plastic waste An incinerator plant of 3000 tonnes per day capacity can generate 3MW of power.

5. Incineration- Disadvantages Its capital and operating cost is very high Operation needs skilled personnel Formation of smoke, dust and ashes needs further disposal and that may cause air pollution. During incineration high levels of dioxins, furans, lead and cadmium may be emitted with the fly ash of incinerator.

6. Incineration objectives Volume reduction : Depending on its composition, incineration reduces the volume of solid wastes to be disposed of by an average of 90% Stabilisation of waste : Incinerator output (i.e., ash) is considerably more inert than incinerator input (i.e., solid wastes), mainly due to the oxidation of the organic components of the waste stream. This leads to a reduction of landfill management problems (since the organic fraction is responsible for landfill gas production) and the organic compounds present in landfill leachate Recovery of energy from waste (EFW): Energy recovered from burning the wastes is used to generate steam for use in onsite electricity generation or export to local factories or district heating schemes. Sterilization of waste : This is of primary importance in the incineration of clinical or biomedical waste. Incineration of solid wastes will also ensure destruction of pathogens prior to final disposal in a landfill

7. Schematic representation of Incineration

8. Incineration Technologies (1)Grate Firing Systems for Municipal Waste Incineration Moving Grate: A moving grate is a typical combustion design of a municipal solid waste incinerator. Waste is dropped by a crane on to the descending grate, which moves into the combustion chamber and eventually moves down to drop the burnt residuals into an ash pit at the other end of the grate . The moving grate is a metallic porous bed, allowing primary combustion air to flow through from the bottom. Secondary combustion air is supplied by nozzles from above the grate, facilitating a complete combustion by the introduction of turbulence

9. Moving Grate

10. Contd … Different types of grate firing systems for refuse incineration are shown in Figure : ( i ). Reciprocating grates, (ii). Roller grates and, (iii). Reversed feed grates. Depending on the grate type, the furnace geometry and the secondary air injection concept have to be optimized. Typically, the primary combustion air cools the grate bars. Besides the air cooling effect is limited for very high calorific values of the refuse and the bars can be damaged or destroyed by intense heating. For this reason water cooled bars have been developed and successfully applied in various plants. The primary stoichiometry can be optimized with respect to the burnout behaviour or gaseous emissions such as CO and NO. Greater complexity and a higher susceptibility to malfunctions are the main disadvantages

11. Types of grate system

12. (2)Furnace burning incineration system Mass-burning system Mass-burning systems are the predominant form of MSW incineration. A mass burn facility typically consists of a reciprocating grate combustion system and a refractory-lined, water-walled steam generator. Mass-burn systems generally consist of either two or three incineration units ranging in capacity from 50 to 1,000 tonnes per day. These facilities can accept refuse that has undergone little preprocessing other than the removal of oversized items. This versatility makes mass-burn facilities convenient and flexible, local programmes to separate household hazardous wastes (e.g., cleaners and pesticides) and recover certain materials (e.g., iron scrap) are necessary to help ensure environmentally viable incineration and resource conservation.

13. Contd … Mass-burning of waste can also be achieved by the use of a rotary kiln. Rotary kilns use a turning cylinder, either refractor or water wall design, to tumble the waste through the system. The kiln is reclined, with waste entering at the high elevation end and ash and non-combustibles leaving at the lower end.

14. Typical Mass-Burn Facility

15. Contd … (2) Refuse derived fuel (RDF) system The term RDF is commonly used to refer to solid waste that has been mechanically processed to produce a storable, transportable and more homogeneous fuel for combustion. RDF systems have two basic components: RDF production and RDF incineration RDF production facilities make RDF in various forms through material separation, size reduction and pelletising . Although RDF processing has the advantage of removing recyclables and contaminants from the combustion stream, on an average, capital costs per tonne for incineration units that use RDF are higher than for other incineration options

16. Contd … There are two primary types of systems in operation ( i ) Shred-and-burn systems: Shred-and-burn systems are the simplest form of RDF production. The process system typically consists of shredding the MSW to the desired particle size that allows effective feeding to the combustor and magnetic removal of ferrous metal, with the remaining portion delivered to the combustor. There is no attempt to remove other non-combustible materials in the MSW before combustion. This, in essence, is a system with minimal processing and removal of noncombustibles

17. Contd … (ii)Simplified process systems: This is a system that removes a significant portion of the non-combustibles. A simplified process system involves processing the MSW to produce an RDF with a significant portion of the non-combustibles removed before combustion. The MSW process removes more than 85% of the ferrous metals, a significant percentage of the remaining non-combustible (i.e., glass, nonferrous metals, dirt, sand, etc.), and shreds the material to a nominal particle top size of 10 to 15 cm to allow effective firing in the combustion unit

18. Typical Simplified RDF Facility

19. Contd … (3) Modular incineration Modular incinerator units are usually prefabricated units with relatively small capacities between 5 and 120 tonnes of solid waste per day. Typical facilities have between 1 and 4 units with a total plant capacity of about 15 to 400 tonnes per day. The majority of modular units produce steam as the sole energy product. Due to their small capacity, modular incinerators are generally used in small communities or for commercial and industrial operations.

20. Contd … ( 4)Fluidized-bed incineration Fluidized bed combustion has recently increased in application in municipal solid waste incinerators, although it is still mainly used for the combustion of hazardous waste Fluidized-bed incineration of MSW is typically medium scale, with processing capacity from 50 to 150 tonnes per day. In this system, a bed of limestone or sand that can withstand high temperatures, fed by an air distribution system, replaces the grate. The heating of the bed and an increase in the air velocities cause the bed to bubble, which gives rise to the term fluidised .

21. Contd … There are different types of fluidized bed combustors (bubbling, rotating and circulating fluidized bed), but the principle of the design remains the same. Waste particles are suspended by the upward flow of combustion air injected from beneath so that it seems like a fluid, by which the turbulence created enhances uniform mixing and heat transfer hence an increased combustion efficiency. Fluidised-bed systems are more consistent in their operation than mass burn and can be controlled more effectively to achieve higher energy conversion efficiency, less residual ash and lower air emissions. Fluidised-bed incinerators require front-end preprocessing, also called fuel preparation

22. Fluidized Bed Incinerator

23. Advantages and Disadvantages Advantages Good gas-solid contact & mixing High specific heat capacity Can accommodate variations in fuel quality Disadvantages Carbon loss with ash Feed pre-processing required

24. Contd.. (5) Rotary kiln A rotary kiln are commonly used for combusting industrial and hazardous wastes, but is also used in some municipal solid waste incinerators. The principle design consists of two thermal treatment chambers: a slightly inclined primary chamber where waste is fed in (together with inlet of hot exhaust air with oxygen), rotated and thermally decomposed by the heat radiation from the secondary chamber. The re‐combustion chamber positioned at the rear of the kiln where the decomposition air and the rest waste is completely burnt with the supply of secondary air. Rotary kiln have the advantage of producing a low level of NOx and thermal destruction of hazardous chemicals