Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

Waste Management > Lecture 9 - Thermal treatment (Ch. 8.1-8.3) > Flashcards

Lecture 9 - Thermal treatment (Ch. 8.1-8.3) Flashcards

1
Q

What is the defintion on waste incineration?

A

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.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

Mention the three thermal conversion processes.

A

Pyrolysis
Gasification
Combustion-based waste-to-energy

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

Explain pyrolysis

A

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.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

Explain gasification

A

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).

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

Explain combustion-based waste-to-energy (WtE)

A

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.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

What does Tanner’s diagram tells and what are the combustion-parameters?

A

Waste within the shaded area can be combusted without additional fuel

Parameters:

  • Moisture < 50%
  • Ash content < 60%
  • Combustible (organic) solids > 25%
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

Explain Lower Heating Value

A

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.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

Explain Higher Heating Value

A

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.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

Explain how/why the heating value is important for designing the waste incineration plant.

A

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.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

What is the difference between lower and higher heating value?

A

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.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

What is the formula for calculating higher heating value?

A

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
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

The Waste Incineration Directive among other things focuses on two aspects to be optimized in incineration. Which ones?

A
  1. 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
  2. Limits on flue gas air emissions
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

Mention two methods for calculating lower heating value.

A 
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%

  1. 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)
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

Mention the three types of incineration technologies teached in the lecture.

A
  • Moving grate incineration
  • Rotary kiln incineration
  • Fluidized bed incineration
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

Explain features of the Moving grate technology

A
  • 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
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

Explain features of the Rotary kiln technology

A
  • 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
17
Q

Explain features of the Fluidized bed technology

A
  • 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
18
Q

Which types of energy recovery do we find in relation to waste incineration?

A
  • 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).
19
Q

Explain the Rankine Cycle

A

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.

  1. First water is pressurized adiabatically (i.e. without heat exchange with the environment) by the feed water pump.
  2. Then water is vaporized and superheated at constant pressure in the boiler.
  3. 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.
  4. 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.

20
Q

What are the critical aspects of energy recovery with waste incineration?

A
  • 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
21
Q

Explain the difference between gross efficiency and net efficiency.

A

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

22
Q

Energy efficient incineration means recovery. What should the energy efficiency be above to be considered as recovery and not disposal?

A

If ’Energy Efficiency’ > 0.60/0.65 –> Recovery

23
Q

Why is it important to distinguish between recovery and disposal?

A
  • 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
24
Q

Mention types of combustion air pollutants

A
  • 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
25
Q

What are the two main strategies for Air Pollution Control and what are the objectives?

A
  • Wet flue gas cleaning - consisting of up to five individual treatment steps
  • Dry or semidry flue gas cleaning - fly ash, the metals, the acid gases and PCDD/F are removed in one common treatment step.

The objectives are:

  1. Removal of dust and heavy metals
  2. Acid gas neutralization
  3. Abatement of PCDD/F and other organic micro-pollutants
  4. Reduction of nitrogen oxides
26
Q

What are the techniques of dust/fly ash removal?

A
  • Cyclones (flues gas max 850 ᵒC)
  • Electrostatic precipitators (ESPs) (flue gas max 400 ᵒC)
  • Fabric or bag house filters(flue gas max 250 ᵒC)
  • Venturi scrubbers
27
Q

In acid gas neutralization wet techniques are used for absorption of gaseous components into a liquid. Mention types of wet techniques.

A
  • Jet and spray scrubbers
  • Packed tower scrubbers
  • Plate and tray towers
28
Q

The wet techniques usually comprises of 2 stages. Which?

What are the usually outputs of the wet techniques?

A

Acidic scrubber to remove HCl, HF and HBr, as well as for Hg

Slightly acidic or neutral ‘alkaline’ scrubber for SO2

Outputs:
Wastewater discharge 100-300 l/t waste that needs treatment
Solid residues: dewatered sludge 2.5 kg/t waste, Gypsum 5 kg/t waste

29
Q

Acid Gas Neutralization can also be done with dry and semidry techniques. Explain these techniques.

A

The neutralization of acidic gases happens in the gas phase by the reagents being injected directly in the flue gas, the products of the reactions are removed in the subsequent particle removal step.

30
Q

Mention tecniques for the abatement of PCDD/F

A

Techniques:
- Activated Carbon injection in combination with a bag house filter

  • Plastic materials have been found to absorb very efficiently PCDD/Fs thus they can be added to the packing of alkaline scrubbers in wet systems
  • Ceramic filter candles covered by catalytic layers for NOx and PCDD/Fs
31
Q

Explain the two tecniques used for reduction of nitrogen oxides.

A

Selective noncatalytic reduction (SNCR).
• SNCR uses the injection of ammonia (NH3) or NH2-containing compounds into the secondary combustion chamber of the furnace for the reduction of NOx to N2
• Reduction efficiency is between 40-70 %
• Can cause surplus NH3 in the flue gas

Selective catalytic reduction (SCR).
• Reduction by NH3 is achieved at the surface of a suitable catalyst.
• The most common catalysts are based on V2O5 stabilized in TiO2 and zeolite materials.
• The typical design of catalysts is the honeycomb type.

32
Q

How much reduces incineration the volume and the weight of the waste?

A

Incineration reduces the volume of the waste by approximately 90% and its weight by 70–80 %.

33
Q

What are the outputs of incineration?

A

Superheated steam (86%)

Losses:
Flue gas (10%), Bottom ash (2%) and Radiation/Convection (2%)
34
Q

What are the disposal options for solid residues?

A

Landfilling:
o Bottom ash: Non-hazardous waste
o Fly ash/ APC residues: Hazardous waste (currently no real utilization, landfilling only after treatment/stabilization)

Reuse in road construction:
o Bottom ash as substitute for granular material (gravel etc.) in pavements, embankments, noise barriers, etc.
o Bottom ash (occasionally mixed with other ashes) as aggregate in concrete and asphalt (not commonly used –yet)

Specific utilization:
o Refilling of mines with APC ashes (Germany)
o Neutralization of waste acid with APC ashes (Norway)

35
Q

What is done in the upgrading of bottom ashes?

A
  • Removal of oversize items: metals, concrete, unburned large items
  • Removal of iron metals
    o from large items
    o from screen siftings
  • Cleaning of scrap iron
  • Removal of aluminum (eddy current separator)
  • Removal of stainless steel
  • Ageing for typically 1-3 months
36
Q

What is the nominal heating value used for?

A

The nominal heating value is used for designing the waste incineration plant.
The nominal heating value should preferably correspond to the average heating value expected over the lifetime of the plant.

37
Q

Cooling of the flue gases is obtained with a boiler where the energy released from incineration is initially recovered as hot water or steam. The end uses are power, district heating or steam, depending on the type of boiler. The boilers are divided into three categories. Mention these categories.

A
  • The hot water boiler allows the production of heat only (hot water). The boiler producing hot water is also used if no heat recovery is possible (cooling of the surplus heat).
  • The low pressure (LP) boiler allows the production of LP steam, only. This steam can be used in industry or exchanged to hot water.
  • The high pressure boiler producing steam allows power generation and combinations of power and process steam or heat.
38
Q

Mention three types of energy conversion technologies and their associated efficency in relation to heat and power.

A

Electrical Power Only –> When producing electrical power only, it is possible to recover an output of up to 35% of the available energy in the waste as power.

Combined Heat and Power –> Possible to utilize more than 90% of the energy of the waste. With a boiler designed for waste incineration (moderate steam parameters) an output of electricity of up to 27% and an output of heat of 60–65% can be achieved, depending on the flow and return temperatures of the district heating system.

Process Steam and Power –> the electrical output may be found somewhere 20% and 35 %, depending on the amount of process steam extracted from the turbine.

39
Q

The total themal efficiency can exceed 100% in cases with energy recovery from flue gas cleaning. How can this be explained?

A

A significant amount of energy, adding approximately 10% to the thermal efficiency, may further be recovered at a low temperature level through condensation of water vapor from the flue gas. Such a recovery system may be integrated in a wet scrubber (see Chapter 8.2) by circulating the scrubbing liquid through a heat exchanger.

It appears that by flue gas condensation, the total thermal efficiency may exceed 100 %, which seems to conflict with basic physical laws. This is not the case. The explanation is that the thermal efficiency is calculated on the basis of the lower heating value, for which it is a precondition that water leaves the process in its evaporated state. In other words, the flue gas condensation system exploits an energy resource not accounted for in the lower heating value.

Waste Management (13 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions