Tag: incinerator

Waste Burning and Incineration Combustion Process

The combustion, or burning, of solid waste proceeds through a series of stages. Water is first driven from the unburned waste by heat produced from material burning nearby or from an auxiliary burner. As the waste heats up, carbon and other substances are released and converted into burnable gases. This is referred to as gasification. These gases are then able to mix with oxygen. If the temperature inside the burn chamber is high enough and maintained for a long enough period of time, the hot gases are completely converted into water vapour and carbon dioxide, which is then released into the air. If the temperature inside the burn chamber is not high enough and the burn time is too short, complete conversion of the burnable gases does not occur and visible smoke is released into the air.  Another result of burning at low temperatures is the creation of pollutants that were not originally present in the waste. This process is known as de novo synthesis. Dioxins, furans and other complex chemical pollutants can be formed through this process.

Ash produced from combustion takes the form of either fly ash or bottom ash.  Fly ash is the fine particles carried away in the form of smoke while bottom ash is the course non-combustible and unburned material that remains after the burn is complete. The type and amount of pollutants in the fly and bottom ash depend upon what waste is burned and completeness of the combustion process.

The completeness of combustion is determined by all of the following factors:

Temperature

The temperature generated is a function of the heating value of the waste and auxiliary fuel, incinerator or burn unit design, air supply and combustion control.  Complete combustion requires high temperatures. Generally, temperatures that exceed 650oC with a holding time of 1-2 seconds will cause complete combustion of most food and other common household waste.  Segregation of waste is required when using methods that don’t routinely achieve these temperatures. Dual chamber incinerators, which are designed to burn complex mixtures of waste, hazardous waste and biomedical waste, must provide a temperature higher than 1000oC and a holding time of at least one second to ensure complete combustion and minimize dioxin and furan emissions.  When these high temperatures and holding times are achieved, waste will be completely burned and ash, smoke and pollutant concentrations will be minimized.

Because exhaust gas temperatures vary from ambient to greater than 1000°C each time a batch waste incinerator is used, optional air pollution control systems with evaporative cooling towers and scrubbers are seldom recommended. However, it may be necessary to employ these systems with large continuous feed incinerators if additional cleaning of exhaust gas is required by regulatory authorities.

Holding Time

Complete combustion takes time.  Holding time, otherwise known as retention or residence time, is the length of time available to ensure the complete mixing of air and fuel, and thus the complete burning of waste. Low temperatures, low heating values of the waste and reduced turbulence require that the holding time be increased to complete the combustion process.

Turbulence

The turbulent mixing of burnable gases with sufficient oxygen is needed to promote good contact between the burning waste and incoming air. This will help in achieving the high temperatures at which waste can be completely burned. The amount of mixing is influenced by the shape and size of the burn chamber and how the air is injected. Passive under-fire ventilation achieved during open burning does not result in sufficient turbulence for the burning of a wide variety of waste.  Also, it is important not to overfill the burn chamber as airflow may be blocked and the amount of turbulence further reduced.  The more advanced incineration designs provide effective turbulence through the forced introduction of air directly into hot zones.

Composition of the Waste

The heating value, wetness and chemical properties of the waste affect the combustion process and the pollutants that are contained in the resulting smoke and ash. The higher the burn temperature, holding time and turbulence that are achieved, the less effect the composition of the waste has on completeness of the burn.

Environmental Guideline for the Burning and Incineration of Solid Waste

People living and working in Nunavut often have limited options available for cost effective and environmentally sound

management of household and other solid waste. The widespread presence of permafrost, lack of adequate cover material and

remote locations make open burning and incineration a common and widespread practice to reduce the volume of solid waste and

make it less of an attractant to wildlife.  A wide variety of combustion methods are used ranging from open burning on the

ground to high temperature dual-chamber commercial incinerators.  Generally, high temperature incinerators are more expensive

to purchase and operate and cause less pollution than do the less expensive and lower temperature methods.  However, high

temperature incinerators can safely dispose of a wider variety of waste than can the lower temperature open burning methods.

The Guideline for the Burning and Incineration of Solid Waste (the Guideline) is not intended to promote or endorse the

burning and incineration of solid waste. It is intended to be a resource for traditional, field and commercial camp

operators, communities and others considering burning and incineration as an element of their solid waste management program.

It examines waste burning and incineration methods that are used in Nunavut, their hazards and risks and outlines best

management practices that can reduce impacts on the environment, reduce human-wildlife interactions and ensure worker and

public health and safety. This Guideline does not address incineration of biomedical waste, hazardous waste and sewage

sludge. The management of these wastes requires specific equipment, operational controls and training that are beyond the

scope of the current document.

The Environmental Protection Act enables the Government of Nunavut to implement measures to preserve, protect and enhance the

quality of the environment. Section 2.2 of the Act provides the Minister with authority to develop, coordinate, and

administer the Guideline.

The Guideline is not an official statement of the law. For further information and guidance, the owner or person in charge,

management or control of a solid waste is encouraged to review all applicable legislation and consult the Department of

Environment, other regulatory agencies or qualified persons with expertise in the management of solid waste.

Incinerators differ from the simpler methods of open burning

Incinerators differ from the simpler methods of open burning as the operator has a higher degree of control over the burning process. The resulting higher temperatures, longer holding times and greater turbulence lead to more complete combustion of the waste. Although a wider range of wastes can be destroyed using high temperature single or dual-chambered incinerators, determined efforts should still be taken to reduce the quantity and type of waste generated and to implement other changes which would result in reductions in air emissions. Refer to section 3 for further information proper waste management practices and a listing of what waste can and cannot be incinerated.

The incinerator manufacturer’s operating instructions must be followed at all times to ensure designed temperature, holding time and turbulence conditions are achieved and to avoid damage to the facility. When operating during winter months, additional care must be taken because cold air introduced into the primary and secondary chambers may make it difficult for normal operating temperatures to be achieved. Operators must be properly trained and qualified to operate the equipment under both normal and emergency conditions. Owners are strongly encouraged to consult system manufacturers or other qualified persons with expertise before purchasing an incinerator. Additional guidance on the selection of incinerator technologies and their operational requirements can be obtained by referring to Environment Canada’s Technical Document for Batch Waste Incineration.

The installation and operation of monitoring and control systems is critical for the proper and safe operation of any incinerator.  The design, installation, certification and operation of continuous emissions monitoring systems (CEMS) should comply with the principles described in Environment Canada’s Protocols and Performance Specifications for Continuous Monitoring of Gaseous Emissions  from Thermal Power Generation. While the document is written for power generation facilities, the principles apply equally well to other types of facilities and continuous emissions monitoring systems. For incinerators operating in Nunavut, key operational parameters must be monitored at all times using on-line instruments capable of continuously measuring the combustion process and stack emissions quality. These instruments should be equipped with visible and audible alarms and be on-line whenever the incinerator is in operation, including ‘start-up’ and ‘cool down’ phases. Table 3 lists the monitoring and control system requirements.