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
Explain features of the Rotary kiln technology
- The mass burning incinerator based on a rotary kiln consists of a layered burning of the waste in a rotating cylinder.
- The material is transported through the furnace by the rotations of the inclined cylinder.
- The excess air ratio is well above that of the moving grate incinerator and the fluidized bed. Consequently, the energy efficiency is slightly lower and may not exceed 80 %.
- Lower maximum capacity 20 t/h
- Lower energy efficiency (max. 80 %)
- Most commonly used for burning of waste with specific characteristics:
- Hazardous/chemical waste (ensures confinement)
- Low heating value waste
Explain features of the Fluidized bed technology
- Fluidized bed incineration is based on a principle where solid particles mixed with the fuel are fluidized by air.
- By fluidization the fuel and solids are suspended in an upward air stream, thereby behaving like a fluid.
- The fluidized bed technology has a number of appealing characteristics in relation to combustion technique, reduction of dangerous substances in the fluidized bed reactor itself, flexibility regarding low quality fuels, costs etc.
- Relatively new in waste incineration
- High thermal efficiency
- Has some advantages in relation to process specific emissions
- The main disadvantage is that it has specific requirements on the waste input:
- Size
- Heating value
- Ash content
- Typical application is sewage sludge incineration, RDF
Which types of energy recovery do we find in relation to waste incineration?
- Hot water for district heating
- Process steam for various industries
- Electricity or combined heat and power
The output is dependent on: • Existing infrastructure • Connection to a power grid • District heating network • Annual energy consumption pattern • Prices of the various types of energy and possible agreements with the consumer(s).
Explain the Rankine Cycle
The energy recovery from a steam producing boiler is known from conventional power plant technology as the Rankine process.
The Rankine cycle is a process commonly found in power plants. The working fluid, in this case water, is alternately
vaporized and condensed.
- First water is pressurized adiabatically (i.e. without heat exchange with the environment) by the feed water pump.
- Then water is vaporized and superheated at constant pressure in the boiler.
- From the boiler the steam is transferred to the turbine where it expands thereby exerting work in the form of rotation energy, which is transformed into power in the generator. When the vapor expands, it cools and the pressure drops. Ideally this process is isentropic, i.e. adiabatic and reversible.
- The last step in the cycle is condensation of vapors, and the condensate is returned to the feed water tank for re-entry into the cycle.
In a combined heat and power plant, the heat released when condensing the vapor, is transferred to the district heating network.
What are the critical aspects of energy recovery with waste incineration?
- Continuous stable operation
- High content of Cl and other elements in waste determine high potential fouling and corrosion (limiting both maximum and minimum temperature of the flue gas in the boiler area)
- Steam cycle 40 bar and 400C in comparison Coal fired power plant steam: 200-300 bar, 580C
- Limited energy recovery as electricity to 25-35% of thermal input for power only
- Additional heat recovery can increase efficiency to 90-100% (less electricity)
- Heat demand is not always available
Explain the difference between gross efficiency and net efficiency.
Gross efficiency: Do not subtract the internal energy use from the produced energy.
η (gross) = (produced electricity + produced heat ) /energy content (LHV) of waste and auxiliary fuel input
Net efficiency: Subtract the internal energy use from the produced energy.
η(net) = (produced electricity + produced heat -internal use) /energy content (LHV) of waste and auxiliary fuel input
Energy efficient incineration means recovery. What should the energy efficiency be above to be considered as recovery and not disposal?
If ’Energy Efficiency’ > 0.60/0.65 –> Recovery
Why is it important to distinguish between recovery and disposal?
- Because regulation on shipment of waste depends on if the waste is shipped for recovery or disposal
- Shipment of waste for recovery could take place without any further regulation
- Shipment of waste for disposal should be approved by the country of origin as well as the country of final disposal. If any of the countries say ‘no’ the shipment could not take place
Mention types of combustion air pollutants
- CO,CO2
- SO2
- NOx (NO + NO2 + N2O)
- HCl, HF, HBr
- Particulate matter
- Trace metals (Hg, As, Cd, Cr, Pb etc.)
- Organic compounds (Unburned Hydro Carbons, UHC), PAH, Dioxins
