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CPE406 Energy from Biomass and Waste > Waste Incineration and CHP > Flashcards

Waste Incineration and CHP Flashcards

1
Q

Define waste incineration

A

The oxidation of the combustible materials contained in the waste to produce heat, ash and flue gas (carbon dioxide, water vapour and nitrogen)

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2
Q

What is the objective of waste incineration?

A

To reduce the volume of the waste and reduce the hazard, whilst capturing or destroying potentially harmful subtances

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3
Q

Is oxygen needed for incineration?

A

Yes, a sufficient supply is required

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4
Q

Is energy recovery possible with waste incineration?

A

Yes

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5
Q

What types of waste can be incinerated?

A

MSW, sewage sludge, industrial toxic waste, hazardous waste, clinical waste

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6
Q

Where is waste incineration most widely used?

A

Japan and the EU

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7
Q

How much of MSW and hazardous waste was treated by incineration in the EU in 2006?

A

25% of MSW and 12% of hazardous waste

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8
Q

What are the three stages of incineration?

A

1 - Drying and devolitlisation - 200-750 degrees C, doesnt require oxidising agent, only dependent on heat
2 - Combustion of volatiles and soot - immediately above the surface of the waste on the grate and in the combustion chamber above the grate - 850 degrees C for 2 seconds to ensure complete burn out
3 - Combustion of the solid carbonaceous residue (mainly char) - takes place on the grate and mat take 30-60 min for complete burnout
These stages generally overlap

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9
Q

How much of the MSW comes out of the process as bottom ash?

A

10% by volume and 20-30% by weight of the solid weight input

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10
Q

What are the energy inputs into the incineration process?

A

Waste, support fuels (diesel/natural gas) for start up and shut down, to maintain required temperatures with lower CV waste and for flue-gas reheating before treatment, and imported electricity for start up and shut down phases when all lines are stopped and for plants without electricity generation

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11
Q

Name and describe the 5 key steps of waste incineration

A

1 - loading waste into the process (pre-treatment, input, waste bunker, feeding unit)
2 - incineration - firing grate plus some recycle loops
3 - heat recovery - boiler
4 - emissions control (flue gas cleaning) - separating particles, separating gas and vapour, bag house filter. Also bottom ash and residue management
5 - energy recovery via district heating/electricity generation - steam and hot water

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12
Q

Name the 4 types of waste incinerators and what they are most commonly used for

A

Grate incinerators - MSW, mass burn (10-50 tonnes/hr), different types of grate (moving, roller etc)
Fluidised bed incinerators - sewage sludge
Rotary kiln incinerators - hazardous waste
Starved air incinerators - clinical waste/industrial toxic waste

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13
Q

What is the purpose of the grate and where is it found?

A

Found in the heart of the incinerator, the automatic grate serves to move waste from the inlet hopper to the discharge end

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14
Q

How is the residence time of the waste inside the incinerator changed?

A

Using a variable speed drive attached to the grate, which allows for changes in composition

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15
Q

What is the typical throughput of the most commonly used moving grate?

A

10-50 tonnes/hr

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16
Q

Name 5 types of moving grate

A

Roller grate, rocker grate, stoker grate, forward reciprocating grate, reverse reciprocating grate

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17
Q

Draw the 5 types of moving grate

A

Drawing 8

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18
Q

What are the typical design parameters for a roller grate?

A

1.5 m diameter, 3-15 m/h, 30 degree incline to assist the movement of waste, directional vane underneath to guide primary air

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19
Q

Describe a rocker grate

A

Alternative rows of mechanical rockers which are pivoted or rocked to produce an upward and forward motion to advance the waste

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20
Q

Describe a stoker grate

A

Horizontal travelling grates generally arranged in drying, ignition and burn-out positions, also assist the distribution and control of primary air

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21
Q

Describe a reciprocating grate

A

3 or more sections with a step of 0.5-1m between sections - each section is a serious of fixed and movable bars in a stair-case-like arrangement that push the waste up and down to move it along

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22
Q

Describe a fluidised bed incinerator

A

A bed of sand particles are contained in a vertical refactory-lined chamber and the primary combustion air is blown through; the sand particles are kept fluidised by the velocity of the air. Drying, devolitisation, ignition and combustion all happen within the bed.

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23
Q

What are the two types of fluidised bed incinerators?

A

Bubbling fluidised bed (BFB) and circulating fluidised bed (CFB)

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24
Q

Draw a BFB

A

Drawing 9

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25
Q

Draw a CFB

A

Drawing 10

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26
Q

What are the main differences between BFBs and CFBs?

A

BFBs have a low air velocity (1-3 m/s), intense mixing and often used for sewage sludge (96% water), CFBs have a high air velocity (5-6 m/s) and a large proportion of asnd is carried over with the flue gases, so a cyclone is used to remove the sand from the flue gases so it can be reused

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27
Q

What are the advantages of fluidised bed incinerators?

A
  • Can handle high moisture and ash wastes and can be completely incinerated due to high thermal capacitance of sand and excellent thermal conductance of the fluidised bed
  • Small consumption of auxiliary fuel
  • Lower excess air
  • Lower NOx due to lower operating temp (around 850 degrees C)
  • Quick start-up and shut down
  • Low carbon-in-ash (<1%)
  • Small site requirement
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28
Q

What are the disadvantages of fluidised bed incinerators?

A
  • Much more fly-ash

- Fuel pre-conditioning is required as maximum waste size of 150 mm is necessary

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29
Q

What are the two stages inside a rotary kiln incinerator?

A

1 - oxidative mode (50-200% excess air)
2 - gases from the primary rotary kiln pass to the secondary chamber (with the excess air) to completely burn out the combustible gases, vapours, tars and soot

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30
Q

Draw a rotary kiln incinerator

A

Drawing 11

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31
Q

Which part of the two part rotary kiln incinerator is the rotary kiln and how does it work?

A

This is the primary chamber, it is an inclined cylinder lined with ceramic that rotates on rollers at different speeds (2 revs/min - 6 revs /hr depending on the type of waste and kiln). The waste goes in the front and is ignited by a burner, which is then tumbled and moved down the kiln and reached the end as ash.

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32
Q

What are the usual dimensions of a rotary kiln?

A

Diameter: 1-6 m
Length: 4-20 m

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33
Q

What is the residence time of the first and second chambers of a rotary kiln incinerator?

A

Rotary kiln: >30min

Secondary chamber: 1-3 s

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34
Q

What is the typical temperature of the first and second chambers of a rotary kiln incinerator?

A

Rotary kiln: 1200 degrees C

Secondary chamber: up to 1400 degrees C

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35
Q

What is the typical throughput of a rotary kiln incinerator?

A

4000-5000 tonnes/year

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36
Q

What are the advantages of rotary kiln incinerators?

A

Commonly used for hazardous waste, able to incinerate various types of waste (solids, liquids, sludges) at the same time, minimal waste pre-processing, direct disposal of wastes in metal drums, availability of many types of feeding mechanisms (ram, feeder, screw, direct injection), readily controlled residence time of waste to kiln, high turbulence and effective contact

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37
Q

What are the disadvantages of rotary kiln incinerators?

A

Relatively high particulate carryover to the gas stream, separate afterburner normally required for destruction of volatiles, conditions across the kiln length are hard to control, relatively high excess air (usually 100%) required, effective kiln seal difficult to obtain, significant amount of heat is lost in the ash discharge

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38
Q

What is the capacity and power output of the Fawley rotary kiln incinerator plant?

A

6000 tonnes/year (oil sludges, contaminated wood, meat and bone meal) to generate 34MWh of electricity each year (supply for 7250 UK households)

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39
Q

What makes the secondary chamber at the Fawley rotary kiln incinerator plant different?

A

It is split into sections with baffles, with the slag quench unit right at the start.

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40
Q

How does the gas cleaning plant at the Fawley rotary kiln incinerator plant operate?

A

The flue gas is quenched, then scrubbed in an obsorber with NaOH, then passed through ESPs to remove the fine ash, then blown out through a stack using an induced draft fan (air supply required). The ESPs require a water supply, and the quenching and absorber units use a cooling fluid.

41
Q

What is the geometry and operating conditions of the Fawley rotary kiln incinerator?

A

Refractory lined steel drum, 12x4.5m, liquid fed via burners and lances, solids fed via automated drum conveyor, T=900-1200 degrees C, residence time = 2-4 hours

42
Q

What is the geometry and operating conditions of the Fawley rotary kiln after burn chamber?

A

T=1120-1150 degrees C, auxiliary fuel and additional air to increase reaction temperature, residence time (gases) = 4s, destruction efficiency >99.99995%

43
Q

What are the two stages of starved air waste incineration?

A

1 - semi-pyrolysis (the chemical decomposition of the waste by the action of heat with limited air, heating the waste in an inert atmosphere produces a gas)
2 - combustion stage with a secondary burner in excess air (approx. 200% stoichiometric)

44
Q

What kind of gas velocities are used in starved air waste incineration?

A

Realtively low gas velocities ensure particulate matter stays in the first stage (reducing particulate pollutants such as heavy metal) and inhibits the formation of NOx gases.

45
Q

What temperature is used in the second stage of starved air waste incineration?

A

> 1000 degrees C to destroy any dioxins

46
Q

Draw a starved air waste incinerator

A

Drawing 12

47
Q

What temperature is used in the first stage of starved air waste incineration?

A

700-800 degrees C

48
Q

What is the typical throughput of starved air waste incineration?

A

400-25000 tonnes/year

49
Q

What types of waste is starved air waste incineration usually used for?

A

Industrial toxic solid waste or clinical waste

50
Q

What is the advantage of starved air waste incineration having a more controlled combustion?

A

Lower release of volatile organics and CO

51
Q

What are the five main considerations for boiler design?

A

1 - reasonably good heat transfer without excessive fouling
2 - allow for cleaning of the boiler surfaces
3 - allow for optimum circulation of the boiler feed water
4 - mechanically stable at the operating conditions
5 - built at the lowest cost

52
Q

What 4 things effect boiler efficiency?

A

1 - large amounts of excess air required for combustion and to cool the chamber increases losses of sensible heat in the flue gases (boiler feedstock)
2 - ash content of MSW is high which increases sensible heat losses and unburnt carbon in the residue (less feedstock)
3 - boiler is large so increases the heat losses of the plant
4 - operates with short and long term fluctuations in load

53
Q

What are the two key issues in boiler operation?

A

Tube fouling and corrosion

54
Q

What is tube fouling?

A

Flyash, soot and volatised metal compounds from the flue gases form a layer of deposits on the boiler tubes, which gradually decreases the rate of heat transfer from the hot flue gases to the water inside the steel tubes.

55
Q

What does the rate of tube fouling depend on?

A

The dust loading of the flue gases, and the stickiness of the fly ash (which depends on temperature, flue gas velocity and tube bank geometry)

56
Q

What 3 methods can be employed to remove tube fouling scale deposits and which is most common?

A

Soot blowers - using superheated steam (most common, usually carried out once per shift (every 1-2 months))
Shot cleaning - dropping a cast iron shot onto the tubes to knock off the fouling
Rapping the tubes - rapping the tube banks to knock off the deposits

57
Q

How often does a boiler have to be thoroughly mechanically and wet cleaned?

A

Every 4000 hours of operation (6 months)

58
Q

How is corrosion in a boiler avoided?

A

Critical control of temperature

59
Q

What is high temperature corrosion influenced by?

A

Temperature, presence of low melting phases such as alkali bisulphates, pyrosulphates, and acid gases such as HCl and SO3, the nature of the tube metal

60
Q

How is the corrosive gas HCl formed in an incinerator?

A

Through the combustion of plastics such as PVC, and is corrosive at low temperatures

61
Q

What gas temperature is required to avoid downstream dew-point corrosion?

A

200 degrees C, as HCl dew point is 27-60 degrees C

62
Q

Why do we need to clean flue gases?

A

Air, water and solid emissions are regulated by the EC Waste Incineration Directive, and the emissions given by an incineration plant before clean up are way above what is legally permitted

63
Q

What are the emissions of most concern in flue gases?

A

Total particulates/dust, acidic and corrosive gases (HCl HF, NOx, SO2), heavy metals (mercury, lead), dioxins and furans, wastewater, ash residue

64
Q

Name 6 pieces of flue gas cleaning equipment

A

Cyclones, ESPs, wet scrubbers, dry scrubbers, fabric filters, de-NOx systems

65
Q

What are cyclones used for in flue gas cleaning?

A

Effective at removing particles larger than 15 um so is used as a preliminary collector

66
Q

Describe the basic principles of the operation of a cyclone.

A

The dust-laden gas stream enter tangentially, forms a vortex and rotates in a helical path down the tube, the particles are flung to the inner wall by centrifugal force and then drop down to the bottom, the cleaned gas forms a second vortex which flows up the middle of the cyclone.

67
Q

Draw a cyclone

A

Drawing 13

68
Q

How effective is an ESP?

A

97-99.5% effective at removing particles down to sub-micron in size

69
Q

What is the typical voltage of an ESP?

A

50kV

70
Q

Describe the basic principles of the operation of an ESP.

A

Dust-laden gas stream enters the ESP and the particles are charged by negative ions produced by a corona discharge in an intense electrostatic field. These are attracted to the collector electrode plates and the collected particles accumulate. The plates are cleaned by rapping with a rotating hammer system which dislodges the layer of accumulated particles. The dust falls from the plates to the bottom of the unit where it collects in a hopper.

71
Q

Draw an ESP

A

Drawing 14

72
Q

How effective are fabric filters?

A

Removal of particulates down to submicron in size

73
Q

Draw a fabric filter

A

Drawing 15

74
Q

When are fabric filters generally used?

A

Following ESP

75
Q

Describe the basic principles of the operation of a fabric filter.

A

The dust laden gas flows through a series of elongated permeable fabric bags, which can have a range of pore sizes, which filters out the particles. The bags (up to 100) are housed in a casing. The operational temperature is low and acids can cause damage due to the fabric construction.

76
Q

How effective are wet scrubbers at removing HCl?

A

95%

77
Q

What are wet scrubbers used for?

A

To remove soluble acid gases such as HCl, HF and SO2 (also removes some particulates and heavy metals)

78
Q

Describe the basic principles of the operation of a wet scrubber.

A

Gases are passed counter-current through thin films or sprays of liquid (usually an alkaline solution such as lime (calcium hydroxide) or sodium hydroxide) in a tower, which provides a large surface area for reaction of the gas with the droplet.

79
Q

Draw a wet scrubber

A

Drawing 16

80
Q

What is the main disadvantage of a wet scrubber?

A

The need for treatment of a highly polluted liquid afterwards

81
Q

What is a dry scrubber used for?

A

Removal of acid gases

82
Q

Describe the basic principles of the operation of a dry scrubber.

A

Flue gases that have been cooled to about 160 degrees C are passed counter-current through a spray of dry- fine-grained power (eg. dry CaOH) on which the acid gases react. A fabric filter is used after.

83
Q

Draw a dry scrubber

A

Drawing 17

84
Q

Draw a typical de-NOx system

A

Drawing 18

85
Q

What is a de-NOx system used for?

A

Removes nitrogen oxides (typically 90% NO and 10% NO2) that contribute to acid rain that are formed in the combustion process. NOx can be reduced by combustin at lower temperature and oxygen availabilities.

86
Q

What are typical de-NOx catalysts?

A

Platinum, palladium, vanadium oxide, titanium oxide

87
Q

Why do de-NOx systems often use a catalyst?

A

Ammonia can be used to reduce NOx at 850-950 degrees C without a catalyst, but can reduce NOx at 300-400 degrees C with a catalyst (SCR = selective catalytic reduction)

88
Q

Describe and draw a typical flue gas cleaning system

A

Drawing 19
Particulates in the flue gas are removed by the ESP and pre-collector, then the acid gases are removed by a wet lime scrubber, then activated carbon adsorbs the mercury and dioxins and furans, then a fabric filer removes any fine particulates along with the activated carbon and adsorbed pollutants.
Then NOx is removed by ammonia to form inert hydrogen and and water at 850-950 degrees C (without a catalyst)

89
Q

What is CHP?

A

The simultaneous generation of usable heat and power in a single process

90
Q

What feedstock can be used for CHP?

A

Natural gas, coal, biomass and waste

91
Q

How is electricity generated through CHP?

A

Steam turbine, gas turbine, combined cycle systems and reciprocating engines

92
Q

How is heat generated through CHP?

A

Steam or hot water

93
Q

How may heat be delivered from CHP?

A

Hot air (waste or sludge drying), hot water (district heating/greenhouses) or steam (tyre manufacturing, paper/pulp manufacturing)

94
Q

What is the overall reaction for the incineration of waste?

A

Drawing 20

95
Q

What are the drivers for CHP?

A

High efficiency recover (>75%), reduced CO2 emissions (up to 30%), UK national targets, EU directives

96
Q

What are the disadvantages of CHP?

A

Difficult to find a long term buyer for heat, operating demands of heat buyer might not fit when the EfW plant is running, plant controllability - different demands on waste disposal, power generation and heat demand might not all fit together, value of hot water and steam are low compared to power

97
Q

Give an example of a CHP plant

A

Veolia Sheffield

98
Q

What are the design parameters of the Veolia Sheffield plant?

A

Two weighbridges 50 tons 18x3m and 14x3m
5 tipping bays in the tipping hall
One incineration line
28 t/h @ 9210 KJ/kg CV
225000 tons/year @ 91% availability
Martin reverse reciprocating grate, 5 rows, 13 steps
Electricity 21 MWe
Heat 45 MWth
Two pipe networks, 12km and 30km (up to 120 degrees C, 16 bar, 87MW boiler back up, computer controlled for max efficiency)
Established over 25 years (one of the oldest in the UK)

99
Q

What are the benefits of Sheffield CHP to general public/business?

A

Reduced heating, capital and maintenance costs, conservation of finite fossil fuels and large energy savings, space-saving, cleaner local environment

CPE406 Energy from Biomass and Waste (8 decks)

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