Database of Waste Management Technologies Life

Thermal Treatment Technologies

 

Generic information

 

Waste Thermal Treatment Technologies

Thermal treatment technologies of MSW (Municipal Solid Waste) and of RDF/SRF (Refuse-Derived Fuel/Solid Recovered Fuel) come in a range of designs. All systems are designed and engineered to control and optimise the incineration process and maximize the recovery of energy and heat.

The main types of incineration plants, that have been developed, are two:

  • Plants that require little pretreatment of the waste (mass-fired),
  • Plants operating with treated waste (SRF/RDF) as fuel.

The most known advanced thermal treatment technologies of MSW and RDF/SRF are the followings:

However the most mature and well developed waste thermal technology is incineration.

 

Waste Incineration Process

Incineration of waste is essentially a rapid oxidation process of the combustible materials of the waste that generates heat and converts the waste to the gaseous products of combustion (flue gases), namely carbon dioxide and water vapour, which are released to the atmosphere. At the end of the burning process, there may be residual materials and ash that cannot burn.

Figure 1

Figure 1: Waste incineration process

The main stages of incineration process are:

  • drying and degassing – here, volatile content is evolved (e.g. hydrocarbons and water) at temperatures generally between 100 and 300 °C. The drying and degassing process do not require any oxidising agent and are only dependent on the supplied heat
  • pyrolysis and gasification - pyrolysis is the further decomposition of organic substances in the absence of an oxidising agent at approx. 250 – 700 °C. Gasification of the carbonaceous residues is the reaction of the residues with water vapour and CO2 at temperatures, typically between 500 and 1000 °C, but can occur at temperatures up to 1600 °C. Thus, solid organic matter is transferred to the gaseous phase. In addition to the temperature, water, steam and oxygen support this reaction
  • oxidation - the combustible gases created in the previous stages are oxidised, depending on the selected incineration method, at flue-gas temperatures generally between 800 and 1450 °C.

In fully oxidative incineration the main constituents of the flue-gas are: water vapour, nitrogen, carbon dioxide and oxygen. Depending on the composition of the material incinerated and on the operating conditions, smaller amounts of CO, HCl, HF, HBr, HI, NOX SO2, VOCs, PCDD/F, PCBs and heavy metal compounds (among others) are formed or remain.

Depending on the combustion temperatures during the main stages of incineration, volatile heavy metals and inorganic compounds (e.g. salts) are totally or partly evaporated. These substances are transferred from the input waste to both the flue-gas and the fly ash it contains. A mineral residue fly ash (dust) and heavier solid ash (bottom ash) are created.

The proportions of solid residue vary greatly according to the waste type and detailed process design.

To ensure complete combustion of the waste, the incineration process must meet the following conditions:

  • sufficient quantity of combustible material and oxygen (O2) in the furnace
  • desirable ignition temperature,
  • correct ratio of fuel – oxygen
  • continuous removal of flue gas produced during the combustion
  • continuous removal of the combustion residues.

The combustion stage is only one stage of the overall incineration installation. Incinerators usually comprise a complex set of interacting technical components which, when considered together, effect an overall treatment of the waste.

In general, the basic components of a waste incineration plant are:

 

Objectives of Waste Incineration Treatment

The main objective of waste incineration is to treat wastes so as to reduce their volume and hazard, whilst capturing (and thus concentrating) or destroying potentially harmful substances that are, or may be, released during incineration. In Waste to Energy Plants the recovery of energy and heat is another major objective. The main objectives of a waste incineration plant and components that are responsible to fulfill each objective are presented in the following table.

Objective Responsibility of
  • Destruction of organic substances
  • Evaporation of water
  • Evaporation of volatile heavy metals and inorganic salts
  • Production of potentially exploitable slag
  • Volume reduction of residues
 Furnace
  • Recovery of useable energy (and) heat
 Energy/heat recovery System
  • Removal and concentration of volatile heavy metals and inorganic matter into solid residues
  • Minimising emissions to all media
 Air pollution Control System

There are two major incineration technologies that can be employed to burn MSW or RDF/SRF:

  • Grate technologies
    • Moving grate
    • Fixed grate
  • Fluidized bed technologies
    • Bubbling FB
    • Circulating FB

In general a Waste to Energy Incineration plant may include the following operations:

  • incoming waste reception
  • storage of waste and raw materials
  • pretreatment of waste (where required, on-site or off-site)
  • loading of waste into the process
  • thermal treatment of the waste
  • energy recovery (e.g. boiler) and conversion
  • flue-gas cleaning
  • flue-gas cleaning residue management (from flue-gas treatment)
  • flue-gas discharge
  • emissions monitoring and control
  • waste water control and treatment (e.g. from site drainage, flue-gas treatment, storage)
  • ash/bottom ash management and treatment (arising from the combustion stage)
  • solid residue discharge/disposal.

Each of these stages is generally adapted in terms of design, for the type(s) of waste that are treated at the installation.

Figure 2

Figure 2: Example layout of Municipal Solid Waste Incineration Plant

The installation must comply with the regulations regarding:

 

Energy Recovery

The recovery of energy from waste incineration plants is mandatory according the EU Waste Incineration Directive 2000/76/EC and a modern waste incinerator has a high potential for energy recovery as is depicted in Figure 3.

The energy efficiency of energy recovery installations for MSW can be distinguished in electrical and thermal efficiency. Its primary or boiler efficiency is in the order of 80 % and more, the power efficiency amounts to 20 – 25 %, in modern plants with boilers made from high corrosion resistant alloys even more to than 30 %. The best strategy, however, is combined heat and power (CHP). In such configurations the overall energy efficiency can reach more than 60 %. The average electrical efficiency of combined heat power plants is 14,2% and the thermal efficiency is 45,9%. The combined efficiency is required to be at least 65% according to the Waste Framework Directive 98/2008/EC.

Figure 3

Figure 3: Energy flow in a waste incinerator