Thermal Treatment of Waste

Material from archived Dublin Waste-to-Energy project

What is Thermal Treatment?

Thermal Treatment is a process by which heat is applied to waste, in order to sanitise it and reduce its bulk, prior to final disposal. While thermal treatment plants can operate without generating energy, Irish waste management policy requires thermal treatment facilities to incorporate energy recovery capacity; the energy generated can, in turn, be used for example to power the plant and for district heating systems.

Thermal treatment can involve a number of processes most of which are commonly referred to as incineration. According to the EU Directive on Incineration of Waste 2000, 'Incineration plant'

"means any stationary or mobile technical unit and equipment dedicated to the thermal treatment of wastes with or without energy recovery. This includes the incineration by oxidation of waste, as well as the other thermal processes such as pyrolysis, gasification or plasma processes, in so far as the substances resulting from the treatment are subsequently incinerated."

EU Directive on Incineration of Waste (Approved by EU Parliament and Council November 2000)

Purpose of Thermal Treatment

The primary function of thermal treatment is to convert the waste to a stable and usable end product and reduce the amount that requires final disposal in landfills. Thermal treatment processes recover the energy in municipal solid waste (MSW) and convert it to electricity and/or useful heat, gas etc. Because a high proportion of MSW can produce methane on landfilling, recovering energy from the waste will help reduce the 'greenhouse effect'.

Types of Thermal Treatment

While the terms thermal treatment and incineration are often considered synonymous, it is important to understand that in the context of the Dublin Waste Management Plan the phrase thermal treatment refers to a number of processes, of which incineration in one such process. Other thermal treatment processes include Gasification and Pyrolysis.

Incineration

Incineration is the most common technology used for converting municipal solid waste into energy. Pre-sorted (segregated) waste or unsorted waste is the fuel in a combustion process that uses excess air to ensure complete combustion. Waste is delivered to the plant, where it is stored in a large enclosed bunker that serves as waste buffer capacity. The bunker area is kept under negative pressure thus preventing odours from escaping the building. Overhead cranes feed the waste to the incineration unit, where it is converted into energy. This usually takes place on a moving grate in the bottom of the combustion chamber.

Pyrolysis

Pyrolysis is a thermal pre-treatment method, which transforms pre-shredded waste into a gas and a residue consisting partly of inert materials, and partly of a char. The process takes place in an externally heated reactor in which the pyrolysis gas is formed. The gas may be used partly to heat the reactor and partly for generating stream in a boiler. The char may be used as a low-grade fuel in a large power plant, if available or it may be disposed of in a landfill.

Gasification

This process is similar to pyrolysis in some ways. With gasification, the pre-treated waste is fed into an externally heated reactor where the carbonaceous material in the waste stream reacts with a gasifying agent (e.g. air, oxygen or steam) at temperatures of 800-1100°C or higher. Chemical reactions form a combustible gas (with traces of tar) which is stored or burned to generate heat. The process is very inefficient in terms of energy recovery and therefore has a higher cost of processing.

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The basics: terminology (source)

Combustion:

CxHyOz + Excess Air (O2+N2) → CO2, H2O, O2, N2, Heat

Gasification:

CxHyOz + Air, O2 and/or H2O → CO2, CO, H2, CH4, CnHm

Pyrolysis:

CxHyOz + Heat → Char + Liquid + Gas + H2O

Combustion:

– Reaction of a substance at stoichiometric* conditions or greater with air/O2 [l ≥ 1 ]

Gasification:

– Conversion of a carbonaceous feedstock into a gas [“producer gas’’] under sub-stoichiometric conditions [l< 1]

Pyrolysis:

– Thermal degradation of a material in the absence of oxygen [l= 0]

Producer gas or syngas:

– Gas consisting of CO2, CO, CH4, H2, CxHy and H2O

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* Stoichiometric or 'Theoretical Combustion' is the ideal combustion process, where fuel is burned completely.

A complete combustion is a process burning all the carbon (C) to (CO2), all the hydrogen (H) to (H2O) and all the sulphur (S) to (SO2).

With unburned components in the exhaust gas, such as C, H2, CO, the combustion process is uncompleted and not stoichiometric. (source)