11. solid waste management - incineration

Incineration of Solid Waste

Definition of Incineration. Incineration is a controlled combustion process for reducing solid, liquid, or gaseous combustible wastes primarily to carbon dioxide, water vapor, other gases, and a relatively small, noncombustible residue that can be further processed or land-filled in an environmentally acceptable manner.

Incineration

It is the process of direct burning of wastes in the presence of excess air (oxygen) at temperatures of about 8000C and above, liberating heat energy, inert gases and ash.

Incineration • Net energy yield depends upon the

density and composition of the waste; relative percentage of moisture and inert materials, which add to the heat loss; ignition temperature; size and shape of the constituents; design of the combustion system (fixed bed/ fluidised bed ), etc.

Incineration

In practice, about 65 to 80 % of the energy content of the organic matter can be recovered as heat energy, which can be utilised either for direct thermal applications, or for producing power via steam turbine generators (with typical conversion efficiency of about 30%).

The combustion temperatures of conventional incinerators fuelled only by wastes are about 760o C in the furnace, , and in excess of 870oC in the secondary combustion chamber.

These temperatures are needed to avoid odour from incomplete combustion but are insufficient to burn or even melt glass.

To avoid the deficiencies of conventional incinerators, some modern incinerators utilise higher temperatures of up to 1650oC using supplementary fuel.

These reduce waste volume by 97% and convert metal and glass to ash.

Wastes burned solely for volume reduction may not need any auxiliary fuel except for start-up.

When the objective is steam production, supplementary fuel may have to be used with the pulverized refuse, because of the variable energy content of the waste or in the event that the quantity of waste available is insufficient.

While Incineration is extensively used as an important method of waste disposal, it is associated with some polluting discharges which are of environmental concern, although in varying degrees of severity. These can fortunately be effectively controlled by installing suitable pollution control devices and by suitable furnace construction and control of the combustion process.

Combustion

Combustion may be defined as the rapid chemical combination of oxygen with the combustible elements of a fuel.

Oxidation

Incineration is an oxidation process, where organic constituents react with oxygen and release heat during the process.

END

Description

The incineration of solid waste involves a sequence of steps in the primary process, which includes drying, volatilization, combustion of fixed carbon, and burnout of char of the solids, which is followed by a secondary process, the combustion of the vapors, gases, and particulates driven off during the primary process.

Waste Characterization.

For design purposes, the most important characteristics are the

• Higher heating value, • Moisture content, and • Percent of inert material in the waste.

Incineration Advantages

• Incineration is an efficient way to reduce the waste volume and demand for landfill space. • Incineration plants can be located close to the center of gravity of waste generation, thus reducing the cost of waste transportation.• Incineration provides the best way to eliminate methane gas emissions from waste management processes.

Incineration Advantages

• Furthermore, energy from waste projects provides a substitute for fossil fuel combustion.• One of the most attractive features of the incineration process is that it can be used to reduce the original volume of combustibles by 80 to 95 percent.

Incineration Advantages

• Waste incineration may be advantageous when a landfill cannot be sited because of a lack of suitable sites or long haulage distances, which result in high costs.

Incineration Disadvantages

• An incineration plant involves heavy investments and high operating costs.• Furthermore, waste incineration is only applicable if certain requirementsare met.• The complexity of an incineration plant requires skilled staff.

Incineration Disadvantages

•The residues from the flue gas cleaning can contaminate theenvironment if not handled appropriately, and must bedisposed of in controlled and well-operated landfills to prevent ground and surface water pollution.

Objectives of Combustion

The objectives of combustion in an incinerator are • Complete destruction of the organic constituents to form harmless gases and the prevention of the release of any harmful material to the environment.

Objectives of Combustion

• Efficient conversion of the heat released, into useful energy, is secondary to safe and efficient destruction of the waste.

3 Ts for Efficient Oxidation

The oxidation of the combustible elements requires a temperature high enough to ignite the constituents, mixing of the material with oxygen, or turbulence and sufficient time for complete combustion, (i.e., the three "Ts" of combustion).

3 Ts for Efficient Oxidation

Proper attention to these three factors can produce destruction / conversion efficiencies of 99.9%-99.95% in well-operated incinerators.

Major Combustible Chemical Elements

The two major combustible chemical elements of significance are Carbon and Hydrogen.

MECHANISM OF COMBUSTION

Mechanism of combustion consists of two stage process

1. Primary Combustion Process

2. Secondary Combustion Process

MECHANISM OF COMBUSTION.

1. Primary Combustion Process. The thermal destruction of waste

(or any other solid fuel with significant moisture content) is accomplished in four phases as described below:

Phase One.

• The first phase is the drying phase that occurs in the initial heating of the heterogeneous material. Moisture is driven off as the material is heated past the vaporization temperature of water. Drying is usually complete by the time the material has reached 300oF.

Phase Two.

The second phase is the volatilization of vapors and gases which occurs as the temperature of the waste continues to rise.

Phase Three.

The third phase in the burn down of solids is the in-place oxidation of the burnable solids left after the vapors and gases have been volatilized.

Phase FourThe fourth phase in the process involves the final burn down of char and the consolidation and cooling of the inert residues, known as bottom ash. This material is the end product, which, after a short period of cooling on the hearth/grate, is dumped into the ash-receiving system.

Secondary Combustion.

The secondary combustionzone / secondary combustion chamber must provide the desiredtemperature, turbulence, and excess air required to achieve complete destruction of all theunburned gases, vapors, and particulates released from the primary combustion process.

Secondary Combustion.

The complete destruction of high-flash-point, low-heat-content vapors and particulates requires more time and greater turbulence than does the complete destruction of themore easily burned materials.

Basic Types of Incineration Plants

Both Stoker and Fluidised Bed type furnaces are used in incinerators.

The modern municipal incinerators are usually of the continuously burning type, and may have “water wall” construction in the combustion chamber in place of the older, more common refractory lining. Corrosion of “water wall” units can, however, be a problem. Recent advancements include Twin Interchanging Fluidised Bed Combustor developed by a company in Japan, which is claimed to be capable of completely combusting wastes of low to high calorific values at very high overall efficiency

Types of Incineration Plants

• Some basic types of Incineration Plants are

1. Mass Burn:

2. Modular Combustion Units:

3. Refuse-Derived Fuel (RDF) based Power Plants:

Modular Combustion Units:

Modular incinerators are simply small ‘mass burn’ plants with capacity ranging from 25 to 300 tonnes per day. The boilers are built in a factory and shipped to the plant site, rather than being erected on the site, as is the case with larger plants. These facilities are often used in small communities.

Refuse-Derived Fuel (RDF) based Power Plants:

In an RDF plant, waste is processed before burning. Typically, the noncombustible items are removed, separating glass and metals for recycling.

Refuse-Derived Fuel (RDF) based Power Plants:

• The combustible waste is shredded into a smaller, more uniform particle size for burning. The RDF thus produced may be burned in boilers on-site, or it may be shipped to off-site boilers for energy conversion. If the RDF is to be used off-site, it is usually densified into pellets through the process of pelletisation.

Pyrolysis/ GasificationPyrolysis is also refered to as destructive distillation or carbonization. It is the process of thermal decomposition of organic matter at high temperature (about 9000C) in an inert (oxygen deficient) atmosphere or vacuum, producing a mixture of combustible Carbon Monoxide, Methane, Hydrogen, Ethane [CO, CH4, H2, C2H6] and non-combustible Carbon Dioxide, water, Nitrogen [CO2, H2O, N2] gases, pyroligenous liquid, chemicals and charcoal..

Pyrolysis/ GasificationThe pyroligenous liquid has high heat

value and is a feasible substitute of industrial fuel oil. Amount of each end-product depends on the chemical composition of the organic matter and operatingconditions. Quantity and chemical composition of each product changes with pyrolysis temperature, residence time, pressure, feed stock and other variables

GasificationGasification involves thermal decomposition of organic matter at high temperatures in presence of limited amounts of air/ oxygen, producing mainly a mixture of combustible and non-combustible gas (carbon Monoxide, Hydrogen and Carbon Dioxide).

This process is similar to Pyrolysis, involving some secondary/ different high temperature (>1000oC) chemistry which improves the heating value of gaseous output and increases the gaseous yield (mainly combustible gases CO+H2) and lesser quantity of other residues. The gas can be cooled, cleaned and then utilized in IC engines to generate electricity.

Pyrolysis/ Gasification is already a proven method for homogenous organic matter like wood, pulp etc. and is now being recognised as an attractive option for MSW also. In these processes, besides net energy recovery, proper destruction of the waste is also ensured. The products are easy to store and handle. These processes are therefore being increasingly favoured in place of incineration.

Disadvantages

Municipal solid waste (MSW) incineration

plants tend to be among the most expensive

solid waste management options, and they

require highly skilled personnel and careful

maintenance. For these reasons, incineration

tends to be a good choice only when other,

simpler, and less expensive choices are not

available.