Lecture 9 - Thermal treatment (Ch. 8.1-8.3) Flashcards
What is the defintion on waste incineration?
Waste incineration is thermal conversion of waste with a surplus of air. This releases energy and produces solid residues as well as a flue gas emitted to the atmosphere.
Mention the three thermal conversion processes.
Pyrolysis
Gasification
Combustion-based waste-to-energy
Explain pyrolysis
The thermal breakdown of waste in the absence of air. Waste is heated to high temperatures (> 300°C) by an external energy source, without adding steam or oxygen.
The intermediate products that will be created are char, pyrolysis oil and syngas.
An example of pyrolysis is the conversion of wood into charcoal.
Explain gasification
The thermal breakdown of waste (temperatures >750 °C) under a controlled (lower than necessary for combustion) oxygen atmosphere, thus creating as an intermediate product syngas instead of direct combustion of the waste (e.g. the
conversion of coal into gas).
Explain combustion-based waste-to-energy (WtE)
Mass burn incineration based on thermal conversion processes in the presence of surplus air. It releases the energy contained in the chemical matrix of waste in the form of heat and produces solid residues as well as flue gas which is cleaned before release into the atmosphere.
What does Tanner’s diagram tells and what are the combustion-parameters?
Waste within the shaded area can be combusted without additional fuel
Parameters:
- Moisture < 50%
- Ash content < 60%
- Combustible (organic) solids > 25%
Explain Lower Heating Value
The energy released upon complete oxidation of the fuel, carbon to CO2, hydrogen to H2O and sulphur to SO2. Water leaves the process in its evaporated state.
The lower heating value of the waste (the energy content available from complete combustion when assuming no energy losses) is the most important variable for determining whether the waste can sustain the combustion process without supplementary fuel.
Explain Higher Heating Value
Higher Heating Value – the energy released upon complete oxidation of the fuel. Water leaves the process in its liquid state.
Theoretical maximum energy release of the fuel.
Explain how/why the heating value is important for designing the waste incineration plant.
The minimum lower heating value required for a controlled incineration depends on the furnace design. Low-grade fuels require a furnace design minimizing heat loss and allowing for drying of the waste prior to ignition, and the air needed for the combustion process should be preheated.
When the heating value is high, the furnace design should allow for extraction of heat from the furnace, e.g. by integrating the boiler in the furnace. The heating value is therefore an important parameter for the planning and design of a waste incineration plant.
What is the difference between lower and higher heating value?
The difference is the heat of condensation of the water vapour in flue gas (originating both from the moisture content of the waste and the oxidation of the hydrogen in the waste)
Higher heating value (HHV) is calculated with the product of water being in liquid form while lower heating value (LHV) is calculated with the product of water being in vapor form.
What is the formula for calculating higher heating value?
Hhigh = Hlow + (W + H ∗ 8.937) ∗ 2445 kJ/kg
W = moisture content (wt%) H = hydrogen content (wt%) 2445 = enthalpy of vaporization of H2O in kJ/kg
The Waste Incineration Directive among other things focuses on two aspects to be optimized in incineration. Which ones?
- Optimization of incineration process: ensure complete burnout
- minimum temperature for the combustion gases in the afterburning chamber
850 ᵒC – Municipal waste
1100 ᵒC – certain types of hazardous waste
- 2 s residence time at the above temperature
Incomplete burnout causes air emissions of CO and total organic carbon (TOC) in bottom ash to exceed limits
- 3 % is the EU limit for TOC - Limits on flue gas air emissions
Mention two methods for calculating lower heating value.
1. Based on elemental composition (C, H, O, N, S, Cl) using an empirical formula. Example Schwanecke (1976):
Hlow(kJ/kg) = 348C% + 939H% + 105S% + 63N% − 108O% − 24.5H2O%
- The heating value of a representative sample is determined in the laboratory by the so-called bomb calorimeter method => higher heating value of the dry sample(Hhigh,DS)
Hlow = Hhigh,DS ∗ DS − W∗2445 − H∗8.937 ∗2445 [kJ/kg]
W = moisture content (wt% initial sample) H = hydrogen content (wt% initial sample) DS = dry solids content (wt% initial sample)
Mention the three types of incineration technologies teached in the lecture.
- Moving grate incineration
- Rotary kiln incineration
- Fluidized bed incineration
Explain features of the Moving grate technology
- The conventional mass burn incinerator based on a moving grate consists of a layered burning of the waste on the grate that transports the waste through the furnace.
- On the grate the waste is dried and then burned at high temperature while air is supplied.
- Well proven technology
- Can accommodate large variations in waste composition and in heating values
- Can be built in very large scale (50 t/h)
- High investment and maintenance costs
- Transportation
- Agitation (movement of one or more components of a mixture to improve contact)
- Even distribution of the combustion air