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Heat and Power > Emission control > Flashcards

Emission control Flashcards

1
Q

Pollution Prevention and Control

A

‘best available techniques’ means the most effective and advanced stage in the development of activities which indicate the practical suitability of particular techniques for providing in principle the basis for emission limit values designed to prevent and, where that is not practicable, generally to reduce emissions and the impact on the environment as a whole

‘available techniques’ means those developed on a scale which allows implementation in the relevant industrial sector, under economically and technicallyviable conditions

best available techniques will change with time, particularly in the light of technical advances, the competent authorities should monitor or be informed of such progress

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2
Q

Particulate and gaseous emissions - the amount emitted depends on

A

•The amount emitted depends on
–Overall efficiency of the power plant (fuel consumption)
–Fuel composition (C, H, N, O, S)
–Combustion device
–Combustion conditions (primary reduction)
–Efficiency cleaning device (secondary reduction)

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3
Q

Products of combustion - incomplete and complete

A
  • Carbondioxide(CO2)
  • Water(H2O)•Sulphurdioxide(SO2,SO3)
  • Ashes
  • Nitrogenoxides(NO,NO2,N2O)
  • Particles
  • Carbonmonoxide(CO)
  • Methane(CH4)
  • Hydrocarbons(HC,PM,PAH,VOC)
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4
Q

Combustion - stages

A

Drying devolatisation oxidation

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5
Q

Composition analysis of fuels

A
  • Proximate analysis Combustibles (fixed& volatile matter) + ash+ moisture
  • Ultimate analysis of combustibles C+H+N+S+O
  • Complete content C+H+N+S+O + ash+ moisture= 1
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6
Q

Particulate emissions

A

•Fly ash needs to be separated from the flue gas in a cleaning device
•The amount of fly ash (mfa) produced during combustion is
mfa= Amf(1-Abottom) [kg]
mf = mass of fuel burned [kg]
Abottom= mass fraction ash collected in the bottom of the boiler [kg/kg]
A = total ash mass fraction [kg/kg]

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7
Q

Fly ash removal devices

A

Cyclone (high temperature)
Filters
Electrostatic precipitator, ESP

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8
Q

SulfurOxides, SOx

A
  • SOx= SO2and SO3
  • Global reactionS + O2→ SO2with S from the fuel and O2from the oxidant (air)
  • SOxemission thus depends on the fuel-S content
  • The emission also depends on the conversion of fuel-S to SOx(sulfur may be retained in the ashes)
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9
Q

Sorbents for In-situ and Desulfurizationprocesses

A
  • Calcium-based sorbents are common due to low cost and high capture efficiency
  • The sorbents used–Calcium hydroxide (hydrated lime), Ca(OH)2–Calcium carbonate (limestone), CaCO3–CaO+ 1/2O2+ SO2→ CaSO4
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10
Q

Sulfur removal

A

•Fluidized bed (see figure)–Limestone addedin the furnace, in a fluidized bed of sand–CaCO3+ heat → CaO+ CO2–SO2+ CaO+1/2 O2 → CaSO4

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11
Q

Furnace sorbent injection/humidification system

A
  • Limestone/hydrated limeinjected into furnace(~1000-1200ºC)
  • Simple injection system
  • SO2capture with ash
  • Reduction: 50-60%
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12
Q

DesulfurizationProcesses

A
  • Desulfurization in flue gases after combustion
  • The two main categories common in industry–Wet system:
  • water-based sorbent slurries used in scrubbers
  • Lime/limestone scrubbing most common process with gypsum as by-product–Dry system
  • Sorbent is directly mixed with SO2
  • No commercial by-product
  • Removal efficiency –wide range, but lower than wet system
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13
Q

Wet desulfurisation

A

•Typical wet scrubber
–Water-based sorbent sprayed into reactor
–Flue gas enters in the bottom -counter flow
–SO2captured and removed as a solution (slurry)

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14
Q

Dry desulfurization process

A
  • Sorbent is directly mixed with SO2
  • Compared to wet scrubbing:
  • lower investment cost
  • dry waste products
  • lower operational cost
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15
Q

Nitrogen Chemistry

A

see diagram

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16
Q

The Three formation routes of NO:

A
  • Fuel-N mechanism–Dominating formation route in coal flames–Evolves from nitrogen contained in char and volatiles, which are formed during combustion
  • Thermal NO mechanism–Free nitrogen (air borne) that reacts mainly with oxygen (to some extent also OH) during high temperatures–Main reaction path in natural gas and liquid fuel combustion
  • Prompt NO mechanism–Hydrocarbons first react with nitrogen under fuel-rich conditions–The nitrogen containing hydrocarbon can then react to NOx–In general of little importance
17
Q

NO2 reduction

A
  • Primary measures (during combustion)–Air staging (also termed staged combustion)–Fuel staging (also termed reburning)–Flue gas recirculation: exhaust gases are recirculated to decrease combustion temperatures and to utilize reburning–Low excess air ratio: limit available oxygen that can react with nitrogen to NOx
  • Secondary measures (after combustion)–Selective catalytic reduction (SCR)–Selective non-catalytic reduction (SNCR)
18
Q

Air Staging

A

•Staged oxidizer → initial oxygen lean zone (air-fuel ratio: λ< 1)followed by an oxygen rich zone
•Performed either in the furnace and/or in a burner
•Purpose of the primary zone:
–reduce amount of available oxygen in areas that are critical to NO formation
–reduce the peak temperature
•The secondary zone is used to ensure complete burnout of the fuel
•Air staging used in low NOx burners–up to ~50% reduction in coal burners

19
Q

FuelStaging(or Reburning)

A
  • Secondary fuel injection creates sub-stoichiometric flame downstream of the primary combustion zone •Hydrocarbon radicals (CHi) are formed which reduce the NO produced in the primary zone
  • After the reburningzone, additional oxidizer is introduced to ensure sufficient burnout
  • Up to 50-70% removal efficiency
20
Q

Secondarymeasures:

A

SCR and SNCR
•SNCR (selective non-catalytic reduction)
–NO reduced via agent to N2
–Certain temperatures and residence times required
–Removal efficiency varies in wide range (~30-70%)
•SCR (selective catalytic reduction)
–Uses ammonia (NH3) together with catalyst to reduce NO
–High investment cost
–High NOxremoval efficiency (70-90% ) independent of fuel
- Important to monitor NH3 slip

21
Q

Main ccs

A

Pre-combustion
Post-combustion
Oxy-fuel combustion
Chemical looping combustion

22
Q

Ideal sorbent:

A
  • Low regeneration heat duty
  • High rate of reaction
  • Low material cost•Low degradation
  • Non toxic
  • Non hazardous
23
Q

Oxy-fuel

A

•Combustion without N2
•Air separation unit
•High CO2concentration
•Gas or solid fuels
•Various combustion systems
•New flue gas cleaning required
CO2compression and transportation
•Fuelcombustedin a mixture of oxygen and recycledfluegas
•CO2 concentrations above 90% are obtained in the flue gas
•This enablesefficientCO2 capture on largescale

24
Q

Chemical looping

A
  • Combustion without N2
  • Air/ fuel reactor system
  • High CO2concentration
  • Gas or solid fuels•Fluidized beds
  • New flue gas cleaning required
  • CO2compression and transportation

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