22 Incineration and Combustion

Dr. Yogalakshmi K. N

epgp books

 

Objectives:

 

1.      To understand the concept of combustion

 

2.      To understand the processes of incineration

 

3.      To study the various incineration technologies;

 

4.      To release the advantages, disadvantages and applicability of incineration

 

5.      To estimate the energy generation potential of wastes

 

1.0 Introduction

 

Waste is an unwanted material that is of no use to the owner and hence is disposed off in open area. Disposal onto the land or water body is not the solution as they create huge problem to the environment and health of the human beings. Moreover, the polluter pay principle of environmental protection act has forced the industries to treat the waste prior to disposal. Thermal treatment uses high temperature to degrade or destruct the waste. They are designed to thermally degrade the waste and recover energy in the form of heat or electricity. Its main purpose is to reduce the impact of residual waste on the environment. Incineration is a type of combustion where waste is oxidized at high temperature of 900 – 1200 ⁰C to produce carbon dioxide, water and a residue ‘ash’. Though incineration reduces 90% of waste by volume, it generates huge amount of gaseous and particulate pollutants.

 

2.0 Process description

 

Combustion is a process by which a material is burned in the presence of oxygen. It is an exothermic reaction between the substance being burnt and oxygen. Combustion breaks down the organic and inorganic substances into stable end products such as carbon dioxide and water. The predominant reactions occur between carbon and hydrogen with oxygen in air to form carbon dioxide and water. Oxides of sulphur (SOx) and nitrogen (NOx) are other products of combustion. Insufficient oxygen can affect the combustion process and result in formation of carbon monoxide and hydrogen chloride (when hydrogen reacts with organic chlorine molecules). Carbon monoxide is harmful and hence to avoid its formation sufficient oxygen should be flushed into the chamber. All combustion furnaces should be provided with good mixing facilities to provide uniform distribution of air throughout the waste material and other gases released during the burning of waste. At the end of combustion process, an incombustible particulate matter termed ‘bottom ash’ is left behind in the combustion chamber. Some particulate matter is emitted outside the chamber along with the flue gas and they are termed as ‘fly ash’. Incineration is the best example of combustion.

 

 

Incineration is a combustion process in which the waste material is burned in the presence of oxygen by applying high temperature of 900 -1100 ⁰C. This temperature is selected to ensure good combustion, complete elimination of odours and protection of the walls of the incinerator. Incineration is referred to as thermal treatment and is considered as the best alternative route for disposal of solid waste. During the process of incineration, the waste material is oxidised to produce ash, flue gas and heat along with water vapour, nitrogen, carbon dioxide and oxygen. A variety of wastes can be thermally treated by incineration. In addition to carbon dioxide, other gases such as carbon monoxide, hydrogen chloride, hydrogen fluoride, nitrogen oxides, sulphur dioxide, volatile organic carbon, dioxins, furans, polychlorinated biphenyls are also produced depending on the nature of waste and the availability of oxygen. At the end of incineration, bottom ash remains as the inorganic constituents of combustion. Solid lumps of particulate matter are also carried by flue gas and are emitted into the atmosphere. To avoid or reduce the air pollution problem, the incinerators can be fitted with electrostatic precipitators or bag house filters to remove the particulate matter in the flue gas. This adds to the cost of the incinerators. Incineration reduces the volume of waste by 95-96% and mass by 80-85% depending on composition and degree of recovery of materials. Pathogens and harmful organisms are killed during the process and hence incineration is most suitable for management of biomedical or clinical wastes. Harmful toxins are also removed or converted during incineration process. Chemical multi-product plants with diverse toxic or very toxic wastewater streams, which cannot be routed to a conventional wastewater treatment plant can be destroyed in incinerators. In recent times, modern incinerators are used to recover energy from the heat generated during combustion of waste. Heat is converted into electric power through rotation of stream turbines.

 

3.0 Advantages of incineration

  • Less requirement of landfills: Incineration reduces almost 95% of waste volume. The ash that is left out as residue is taken to landfills for disposal. The landfill space is reduced.
  • Incineration can be carried out near the point of waste collection: Incinerators are compact units and hence can be set up at the source of waste generation or in transfer stations. They are can be used as decentralized systems.
  • Substantial reduction of weight up to 75% and volume up to 90% of solid waste.
  • Waste is reduced into biologically sterile ash product: High temperatures of 900-1100⁰C kills pathogens and other harmful organisms present in the waste, thereby sterilizing them. The residue (i.e) ash left out after incineration is biologically sterile.
  • Incineration process does not produce methane gas unlike landfill.
  • Waste-to-energy (production of electricity and heat).Waste incineration can be a source of low cost energy to produce steam for electric power generation, industrial process heating or hot water for district heating thereby conserving primary fuel resource.
  • The bottom ash residue is considered non-injurious and can be used for materials recovery or as secondary aggregates in construction.
  • It is the best practicable environmental option for many hazardous wastes such as highly inflammable, volatile, toxic and infectious wastes.

 

4.0 Disadvantages of incineration

 

  • Emissions from the incinerators: The first major concern for incineration stands against its risk of emissions. Gases such as carbon dioxide, carbon monoxide, SOx, NOx are commonly emitted during the process of oxidation of waste in the incinerators. They are serious air pollutants. Harmful emissions such as furans and dioxin are also released during the combustion of co-mingled waste. Fly ash also contributes to air pollution problems. These air pollutants endanger human health and environment and hence require air pollution control devices. However, despite the regulations, restrictions and developments, concerns about emission are still alive.
  • High investment, operation and maintenance costs: Incinerators are energy intensive process and hence consumes more amount of electricity for combustion of waste. Thus, the cost of operation remains high. Moreover, the incinerators face problems of corrosion due to formation of sulphuric acid and hydrochloric acid during combustion of chlorine containing wastes (paper, plastics etc.). The emission pipes too get abraded due to deposition of particulate matter which becomes hard later. These factors increase the cost of maintenance of incinerators.
  • Also due to high capital investment, the incinerator must be tied to long term waste disposal contracts thereby limiting the choice of waste disposal options.
  • An inorganic residue produced by the incineration process further needs safe and proper disposal.
  • Loss of organic substances such as kitchen waste or green waste from gardening: When comingled wastes are incinerated, the organic or biodegradable substances that can be degraded by simple biological routes are lost. Organic substances can be recovered and used as fertilizer Incinerating plants are producers of heavy metals, which are injurious even in minor amounts.
  • Although, modern incinerators comply with existing emission legislation, there is some public concern that the emitted levels may still have adverse effect on health
  • The incineration is designed on the basis of a certain calorific value for the waste. Removal of materials such as paper and plastics for recycling may reduce the overall calorific value of the waste and consequently may affect the incinerators performance.
  • Local communities have always opposed the presence of incinerating plant in the locality.
  • Operation requires skilled operators

 

5.0 Applicability of Incineration

  • Incineration can be applied or used only if the following criteria are fulfilled.
  • In places where waste management is being practiced for a number of years.
  • The municipal solid waste is disposed of in controlled or secured landfills.
  • Stable supply of combustible waste is required. Large scale systems require minimum of at least 50,000 metric tons/year.
  • Calorific value of waste is very important to achieve efficient incineration. Calorific value should at least be around 7 MJ/kg and should not be lower 6 MJ/kg.
  • The community where the incinerators are installed should be willing to bear the cost tipping fees, and tax-based subsidies.
  • Recruitment of skilled staff is required

 

7.0 Incineration systems

 

An incinerator is designed to maximise the combustion efficiency of waste and heat output. It should also focus on minimising emissions by balancing the oxygen (air) and time, temperature and turbulence. The three “Ts” should be maintained throughout the process. On the basis of capacity, nature of the waste to be combusted, type of system incinerators is classified into the following types:

  • Mass burn incinerators
  • Modular incinerators
  • Fluidized bed incinerators
  • rotary kiln
  • starved air or pyrolytic kiln.
  • Refuse derived

 

7.1 Mass burn incineration

 

Incineration done on a larger scale is termed as mass incineration. It is commonly used for municipal solid waste. Two, three or more incinerator units are used in mass burn incineration. The burning or combustion of waste is performed in single chamber. Mass burn incinerators can combust around 1000 tons of waste per day. It is a simple and flexible method as they accept municipal waste for combustion with minimum processing. The pre-processed waste is brought through trucks to the tipping floor. After the waste is tipped onto the floor they are pushed into the pit through bull dozers. The waste is then fed into the incineration chamber through a chute and a grate system. Since mass burn incineration combusts huge amount of waste they are built in a way to recover energy. Modern incinerators consist of a refractory lined combustion system with steam generator to recover energy in the form of heat or electricity. The steam generator is double walled and lined with refractory material. Water is circulated between the walls which is converted to steam by the heat liberated during incineration process.

 

The modern mass burn incinerator is divided into five main areas. They are

  1. Waste feeding system
  2. Furnace
  3. Heat recovery
  4. Emission control
  5. Energy recovery

 

7.1.1 Waste feeding system

 

The waste feeding system consists of three units: delivery system, bunker and feeding. The waste collection vehicle that enters the processing site are weighed at the entrance on arrival and departure to determine the charge for disposal and incinerator operation. Since mass incinerators are used for combusting municipal waste, odour problems arise due to biodegradation and incineration plants is kept under a slight negative pressure because the combustion air is taken from the waste storage area which prevents escape of odour. The waste brought to the processing site are stored in bunkers till they are fed to incinerators. A bunker can store in the range of 1000-3000 tonnes. Bunkers are divided into different sections to store wastes of different calorific values. Long time storage of waste leads to rotting and odour problems. Hazardous waste and bulky items are separated and then stored in bunkers. Feeder consists of a steal hopper through which the waste is fed into the incinerator. Through grate by hydraulic ram the waste is fed. To avoid air leakage into the furnace, the hoppers are half filled with waste. Hydraulic shutters are used to seal the hoppers at the furnace entrance to prevent the fire from burning back. Additionally, the feed chute can be water cooled or refractory lined to prevent fire.

 

7.1.2 Furnace

 

The waste is fed to the incinerator through controlled ram. The waste is fed in a way to combust

 

50 tonne/hour. Sometimes multiple furnaces are used for combusting waste. This might act as a stand during repair and maintenance. Auxillary burners are used to increase the temperature of the furnace especially during the start up. three stages of incineration occur inside the furnace: i) drying and devolatilization ii) combustion of volatiles and soot iii) combustion of the solid carbonaceous residue. However, these stages are not clearly distinguished due to other factors (moisture content, thermal degradation temperature, volatile composition and ignition temperature and carbon content).

 

7.1.2.1 Drying and Devolatilization

 

Once the waste enters the incinerator the moisture is removed due to the pre heated air or initiating temperature or heat radiated from incinerator walls. Then the organic matter undergoes thermal decomposition. Volatile organic matter (hydrogen, carbon monoxide, methane, ethane and other high molecular weight hydrocarbons), combustible gases and vapours are produced. Devolatilization occurs at temperature of 200-750 ºC and begins at the end of drying phase. The release of volatiles occurs at 425-550 ºC and depends on the nature and physical state of waste. Example: polystyrene decomposes in the temperature range of 450-500 ºC yielding 99% volatiles whereas wood decomposes in the temperature range 280-500 ºC to produce 70% volatiles. Likewise, example of physical state include paper decomposes in seconds unlike wood that takes minutes to breakdown.

 

7.1.2.2 Combustion of volatiles

 

The volatiles released are burned in the combustion chamber located above the grate. High temperature, complete mixing, excess air, long residence time are required for complete combustion of volatiles. The gases formed are subjected to 850 ºC for 2 seconds to ensure complete combustion of volatiles. Temperatures above 1200 ºC should be avoided to prevent the formation of slag by fusion of ash residue. The gases must be held in the combustion chamber for a minimum period of 2-4 seconds to achieve complete combustion. Moreover, supply of secondary air through the nozzles above the grate will minimize pyrolysis and ensure complete combustion.

 

7.1.2.3 Combustion of the solid carbonaceous residue

 

After the burning of volatile gases, the residue consists of carbon in the fixed form. The char is burnt in grate for 30 to 60 mins. The ash (i.e) bottom ash which is around 30% of solid waste is discharged into water trough for cooling. Further they are either completely burnt or disposed off in landfills. The fly ash is collected through cyclones, electrostatic precipitators and bag filters and disposed off.

Source: https://whyfiles.org/wp-content/uploads/2011/03/ecomaine_processdiagram.gif

Figure 2 Energy recovery from incinerators

 

7.1.3 Heat recovery

 

The heat generated during incineration process escapes as flue gas at a temperature of approximately 1000-1200˚C. The flue gas must be cooled before it is taken to flue gas cleaning system. The flue gas is cooled through boiler where the heat is used to boil the water and convert them to steam. The flue gases are corrosive in nature and contain high levels of particulate matter. Sometimes at temperature of 650 and 700ºC the fly ash fuses at the boiler tube surface.

 

7.1.4 Emission control

 

The emissions from incinerator include flue gases with particulate matter. Acid gases such as hydrogen chloride, hydrogen fluoride, sulphur dioxide is also released during incineration. Mercury, cadmium and lead are heavy metals that are commonly reported emissions. Cyclones, bag filters and electrostatic precipitators are used to remove particulate matter. Likewise, scrubbing, adsorption and absorption is used to remove the gaseous pollutants line SOx and NOx.

 

7.1.5 Energy recovery

 

The steam that is generated by the boilers rotates the turbine due to high pressure and temperature. Through various stages, the velocity of the turbine shaft increases and results in the generation of electricity. Around 0.3-0.7 MWh energy is produced from incinerators while combusting 1 tonne of municipal solid waste. It depends on parameters such as calorific value of waste, plant size and steam parameters.

 

The modern municipal waste incinerator relies on the production of steam for electricity generation or district heating to ensure cost effectiveness of the process. Electricity is generated from the steam produced in the boilers via a steam condensing turbine. The high pressure, high temperature steam enters the turbine and passes through the various stages of turbine and as it passes it expands and reaches high velocity, turning the blades of the turbine and hence turbine shaft generating electricity. In a municipal solid waste incinerator depending on the plant size, steam parameters, steam utilisation efficiency and the calorific value of the waste, about 0.3-0.7 MWh of electricity can be generated from one tonne of waste. In case of district heating, the high temperature, high pressure steam passes through heat exchangers which generate hot water under pressure for distribution to homes, offices and institutions. The water is often superheated.

 

7.2 Modular incinerators

 

Modular incinerators are small infact portable units with capacity varying between 5 and 120 tons of solid waste per day. They are more suitable for smaller communities, commercial and industrial operations. A sequence of incinerators of capacity as low as 15 tons per day to a total capacity of 400 tons per day are constructed as part of modular incinerators. Any number of units can be added or removed to the existing units. Addition or detachment of a unit depends on the quantity of waste generation from the service area and the anticipated maintenance cycle of the units. Modular incinerators are prefabricated units and can be constructed within short period of time. Also, the cost of construction of these modular units are very less in comparison to mass burn incinerators. A modular incinerator consists of a refractory-lined furnace and a waste heat boiler. Two combustion chambers are used in modular incinerators where gas generated in first or primary chamber is fed to the after burner or secondary chamber to achieve complete combustion and reduced emission. The majority of modular units produce steam as the sole energy product. Further, modular incinerators are operated in batch and continuous mode. Incinerators with capacity less than 50 tons per day are operated in batches while continuous incinerators are operated for 8 to 16 hours per day.

 

7.3 Fluidized-bed incinerators

 

Fluidized-bed incineration is a small-scale incineration unit which is used for burning municipal solid waste. The capacity of these incinerators varies from 50 to 150 tonns of wastes per day. It consists of a refractory lined vertical chamber with a bed of sand, silica, ceramic or limestone. Air is blown from the bottom of the chamber to fluidize the bed materials. The heat supplied to the bed and the increasing air velocities cause the bed to bubble. The turbulence generated within the chamber causes the solid material to flow above. However, to avoid elutriation of the material, a cyclone is fixed to collect and recirculate the material back to the bed. These beds operate under longer residence time and the solid particles are thereby exposed to the hot zone for loner period of time. Better combustion with minimized air emissions are advantages of these incinerators due to high burn out. Generally, to achieve better combustion the bed material is pre heated. Also, shredded solid waste and refuse derived pellets are combusted efficiently. Dispersion of incoming waste and sufficient residence time are major reasons for complete combustion in fluidized bed incinerators. Two types of fluidized-bed technologies are popular namely bubbling bed and a circulating or turbulent bed. Air flow pattern and bed material used differentiate bubbling bed from circulating bed. Type of waste also varies between bubbling bed and circulating bed fluidized incinerators. Fluidized-bed incinerators are more compatible with high-recovery recycling systems. They are popular in high-recycling cities in developing countries.

 

7.3.1 Advantages of the fluidized bed

  • Relatively low capital and maintenance costs due to a simple design concept.
  • High thermal efficiency. Overall thermal efficiency of up to 90%.
  • Flexible and consistent
  • high energy conversion efficiency less residual ash
  • lower emissions
  • Suitable for handling liquid or solid waste either separately or in combination. reduction of toxic and dangerous substances
  • Multi fuel input

 

7.3.2 Disadvantages of the fluidized bed

  • At present, it is not a common nor thoroughly tested technology for MSW incineration. Requires pre treatment
  • stringent requirements for size, calorific value and ash content
  • Relatively strict demands to size and composition of the waste

 

7.4 Refuse-derived fuel

 

Refuse-derived fuel (RDF) are refereed to solid waste that is mechanically processed so that it can be used as a fuel. RDF are easily stored and can be transported for use anywhere in the world. RDF production and RDF incineration. Waste materials are segregated, shredded and then pelletized. Briefly, the waste is dropped onto the tipping floor and through conveyor belt it is carried for segregation and sorting. While the waste is carried through the conveyor belt, strong magnet is passed over to separate the ferrous metals. More screening is done through vibrating screen, hammer mills and pelletizing units. Sometimes even manual separation is done. The screening processes are tailored according to the requirement. RDF production is complete with pellatization. RDF incineration can be performed with any type of combustor. RDF system is operated by two ways: Simplified process and shred and burn systems. In simplified system, the non-combustibles are removed. Around 85% of ferrous metals and non-combustibles such as dirt, sand, non-ferrous metals are removed. After segregation the waste is shredded to a size of 10-15 cm and fed to incinerator for combustion. Shred and burn systems does focus on removal of combustibles. The waste that is dropped on to the tipping floor is shredded and directly fed into the incinerator without any segregation. This type of RDF production is considered to be easy and simplest form. It requires minimum processing. The ash that comes out of the incinerator is subjected to magnets for removal of non – ferrous metals.

 

Depending on the combustibles, the ash content varies. Higher the combustibles, lower is the ash content. High ash content leads to low quality fuel. When pre-processing of waste is carried out before incineration, it adds to the operating and maintenance costs. It also reduces the reliability of the RDF production. Generally, RDF technology is more costly than other incineration options.

 

7.5 Starved air incinerators

 

Starved air incinerators are small systems with two stage combustion chambers: pyrolytic and combustion chamber. They are widely used for clinical wastes. They are smaller than modular incinerators with capacity ranging from 5 to 100 tonns per day. Similar to modular incinerators, additional units can be added according to the requirement. In the pyrolytic chamber the waste is chemically decomposed by the action of heat in the inert atmosphere. During heating, a gas is generated which is when ignited becomes self-supporting in air. Due to insufficient oxygen, high proportion of incomplete products are formed. Carbon and hydrogen that is formed is sent to second combustion chamber where they are subjected to 200% excess air. It increases the temperature of the chamber to 1000 1200 ⁰C and combusts the wastes. Hydrogen and carbon monoxide is converted to carbon dioxide. Dioxins and PCBs are also broken down in the second chamber. During this process NOx is not formed. In the second chamber turbulence and mixing occurs due to gas that enters from the first chamber.

 

7.5.1 Advantages:

  • It is a controlled combustion process
  • Low release of volatile organic compounds and carbon monoxide Release of low particulate matter in flue gas
  • low combustion temperature in the primary chamber aids in pollution control by minimizing the vaporization of the metallic components of the waste fast construction time relatively low construction cost flexibile

 

7.5.2 Disadvantages:

  • limited size
  • lower thermal efficiency
  • higher maintenance costs and shorter equipment life

 

7.6 Rotary kiln incinerators (RKI)

 

RKI is also a two-stage incineration similar to starved air incinerators. First stage is operated under oxidative mode where 50-200% of excess air is applied for combustion of waste. The primary chamber is inclined and rotates on rollers. The chamber is lined with ceramic and rotates at a revolution of two to six per hour. The size of the kiln is around 1-6 m in diameter and 4-20 m in length. The waste is loaded at the front end and ignited by a burner. The waste mixture is agitated through rotation and slowly the waste tumbles down the chamber and drops down as ash. Mixing expose the surface of the waste material to heat from auxiliary burner. The temperature and retention time of the kiln is around 1200 ⁰C and 30 min. At temperatures higher than 1200 ⁰C, a molten slag is formed, which generally absorbs the particulate matter. The gas generated in the primary chamber is then forced into the secondary chamber where excess air with burners are used to burn the combustible gases. The major advantage of rotary kiln is it combines high temperature and long residence time. Generally, the rotary kilns are used for municipal solid waste, clinical waste, hazardous waste, sewage sludge etc. Also, they are used to clean up contaminated sites. Liquid wastes are also burnt in rotary kilns. A secondary chamber is mostly used for hazardous waste to ensure complete degradation of waste. Rotary kilns are extremely complex process.

Source: http://www.pollutionissues.com/Ho-Li/Incineration.html

Figure Rotary kiln Incinerators

 

8.0 Energy content of waste

 

Energy content of waste is calculated using the values of ultimate analysis of waste. They are expressed as KJ/kg or BTU/1b. Dulong’s formula is used to calculate the energy value of waste.

 

Energy value (BTU/lb) = 145.4 C + 620 (H – 1/8 O) + 41S

 

where C, H, O, and S are in percent by weight (dry basis) and can be converted to KJ/kg by: BTU/lb x 2.326 = KJ/kg

Table 1 Ultimate analysis of combustible waste

 

 

Eg. Energy content of food waste calculated as per dulong’s formula is

145.4 (48) + 620 ((6.4) – 1/8 (37.6)) + 41(0.4) = 8049.6 BTU/lb

 

9.0 Summary

 

To summarize, in this module we have familiarized about

  • The concept of combustion and incineration and its process description Incineration technologies
  • Merits and demerits of incineration Applicability of incineration
  • Energy efficiency of waste

 

you can view video on Incineration and Combustion

References