Thermal Treatment of Municipal Solid Waste (MSW)
-By- Dr. S. Sakthivel
Incineration or mass combustion which called as full oxidative combustion also is the thermal breakdown of waste in the presence of excess of air, producing a hot flue gas (containing CO2, O2, N2, SO2 & water vapour) and solid residue (slag, ash or char). The objective of incineration is to reduce its volume (about 90%) and destroy harmful substances that may be released during combustion process. The heat energy is carried by flue gases containing CO2, O2, N2, SO2 & water vapour and it is used for the production of steam using steam boiler and then generate power using steam turbine generator (STG). Incineration technology is in mature state and is commercially operated worldwide. The combustion of carbonaceous materials can be characterized by the following well known chemical reactions shown in below.
C + O2 → CO2
½ O2 + H2 → H2O
N+O2 → NO2
Thermal treatment of solid waste provides following advantages:
- It can reduce the volume of disposable solid wastes up to 90%. This is advantageous to the environment as well as the economy. First, a lesser quantity of waste will need to be disposed through landfill and second, it reduces cost and environmental burdens of transporting of waste for landfill at some other location.
- It can decompose toxic part of the material into CO2.
- Steam can be generated by burning the wastes which can be further used for on-site electricity generation or it can be exported to nearby local factories.
- Limited space is required for this process.
Although thermal treatment has above mentioned benefits, the disadvantages/ challenges that need to be addressed are:
- Mixed waste contains significant amount of moisture and inert. Both the elements bring down calorific value of the waste and thus reduce energy output of the process, making the system inefficient. The moisture percentage and presence of inerts, in quantity more than the threshold limits, may even destabilize the process. Also, it may call for supplementary fuel to keep it stable. These provisions will lead to increase in the operating cost.
- Emits trace amount of air pollutants like dioxins/furans. Suitable additional technologies need to be provided to treat these pollutants. Online monitoring needs to be ensured.
- High maintenance and capital cost
- Selection of an incineration technology is mainly based on several factors like desired utilization from heat recovery, quantity and characteristics of feedstock, end-use requirements and environmental standards. Typically, the following design and operational parameters like desired temperatures, residence times, minimize emissions, destroy pathogens, avoid clinker formation & slagging of the ash and minimize auxiliary fuel consumption are considered for superlative operation.
a) Temperature: The operational temperature is foremost parameter in the incineration operation. Typically, an incinerator is required a minimum temperature of 850 oC for 2 seconds to produce combustion products like CO2, O2, N2, SO2, etc. The process products are in both gas phase (CO2, H2O, O2 and N2) and solid phase (ash and slag). There are three stages that occur at various temperatures in an incinerator as follows.
- Drying and Degassing – Volatile content is released between 100 0C to 300 0C
- Gasification – Solid organic matter is transferred to the gaseous phase at 400 0C to 700 0C.
- Oxidation : The combustible gases created are oxidised between 800 0C to 1450 0C
The many guidelines are recommended the key design and operating parameter of temperature, for example, primary chamber 540 to 980 oC, secondary chamber 980 to 1200 oC (EPA 1990 recommendations); >850/1100 0C (S. African and EU standards); >1000/1100 0C (Indian and Thai standards). The reaction or incineration temperature depends on quality of waste and their characterization. The feedstock quality is a vital play role in incineration plant. As per CPHEEO manual, Incinerator design may be accepted / allowed in the range of Low Calorific Value (LCV) from 1200 kcal/kg to 2200 kcal /kg. Later, Planning Commission report 2014, reported that the average LCV of the waste required shall be at least 6 MJ/kg (1433 Kcal/kg) throughout all seasons for ‘economical operation’. If the waste has less than 1100 kcal/kg (dry basis), the system cannot attain the self-sustaining combustion and its leads and 10 % auxiliary fuel as diesel. It indicates that the waste should be minimum 1400 kcal/kg of LCV, which is suitable for economic incineration operation.
b) Dry air requirement: The actual amount of oxygen supplied is normally greater than theoretical oxygen for complete combustion. Thus, excess oxygen will fulfil the complete combustion. Practically, the presence of oxygen is air (21% wt) is not sufficient /or not fulfil the complete combustion. Hence, the excess air would be supplied to incinerator in the range of 140 to 200% for complete completion. Some of the plant is operated at requirement of specific air requirements 4500 m³ per tonne waste.
c) Flue gas: Typically, CO2, O2 and N2 are present in flue gas as 14 %, 10.3 & 75.56 respectively. Solid Waste Incineration (SWI) plants generally produce flue-gas volumes (at 11 % oxygen) of between 4500 and 6000 m³ per tonne of waste.
d) Solid residue: The combustion residues include bottom ash, fly ash, bottom ash, non-combusted organic and inorganic materials. European Commission 2006 document was reported industrial data that the solid residues waste to be in the range of 220-390 kg of bottom ash per tonne of solid waste. In the present analysis estimated that 258 kg / metric ton of MSW grate residue generated from theoretical calculation. Fly ash is collected by dry gas cleaning system, which is collecting approximately 45-52 kg of dust and residues per tonne of waste.
e) Moisture content: Typically Indian municipal solid waste (MSW) contains more than 55 % moisture in almost all the cities. High moisture content leads to poor ignition and hinder the complete combustion, which affects the quality of combustion. The MSW have maximum moisture content of 45% allows to produce flue gas temperature at min 850 oC.
f) Calorific value for MSW: Furthermore, the MSW is required minimum 1200 kcal/kg of low heating value of sample to produce flue gas temperature at ~800 oC.
There are a few information derived from TCE’s in-house study such as
- Pre-treatment of MSW is mandatory process to increase the desired calorific value
- Waste should have minimum 1200 kcal/kg of low heating value of sample to produce flue gas temperature at ~800 oC
- Waste have maximum moisture content of 45 % allows to produce desirable flue gas temperature.
TCE has experience of several projects related to the MSW management and leverages its expertise in thermal power sector to assess feasibilities for various ‘Waste to Energy’ projects. TCE conducts critical study of waste characteristics data to advice on appropriate technology for treatment and provides services in the field of environmental impact assessment (EIA), feasibility studies, detailed engineering, and project management etc. covering entire lifecycle of the project.