Energy and Furnace Technology

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Clean Combustion Technologies Overview
Energy and Furnace Technology
Wlodzimierz Blasiak, Professor Royal Institute
of Technology (KTH)School of Industrial
Engineering and ManagementDepartment of
Materials Science and EngineeringDivision of
Energy and Furnace Technology
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Legislation in Sweden
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Carbon monoxide

• It is the product of incomplete combustion and
  is
• Flammable (from 12,5 up...)
• Colorless,
• Odorless gas,
• Easy to mix with air,
• Extremelly toxic (from 50 ppm can produce
  symptoms of poisoning),
• - ALWAYS BE VERY CAREFUL and do measure it if you
  want be ...

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Carbon monoxide combustion (after-burning)

• CO is subsequently slowly oxidised to CO2 by the
  reactions
• CO OH CO2 H
• H H2O H2 OH
• CO H2O CO2 H2
• Conversion of CO to CO2 in the post-flame zone
  gases is termed after-burning and depends on
  process design
• - cooling of flue gases,
• - oxygen availability,
• - residence time,
• - water content.

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Carbon monoxide destruction is a must !

• Destruction of most hydrocarbons occurs very
  rapidly at temperatures between 550 C and 650 C.
• Possible exception is methane which is stable
  molecule and require higher temperature (750 C)
  for oxidation in a few tenths of a second.
• It has been reported that the time required for
  the oxidation of CO is about 10 times the time
  needed for oxidation of hydrocarbons to CO. (slow
  reaction !)
• In the absence of water CO is extremely difficult
  to burn. Incinerator experience shows that
  temperatures of 750-800 C are required with an
  actual residence time at this temperature of 0.2
  0.4 seconds and 4 5 O2 as a minimum to
  achieve nearly complete oxidation of CO to CO2.
• Units with poor mixing patterns exhibit outlet CO
  concentrations higher than 1000 ppm though
  temperatures are at 750 800 C level.

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Thermal NO (nitric oxide) formation

• The formation rate of thermal NO is dependent on
• the reaction temperature,
• the local stoichiometry,
• the residence time.

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Summation on NOx formation

• The NOx formation is depending on combustion
  conditions.
• As with all chemical processes, the rate of
  formation of NOx is, among other things, a
  function of temperature and residence time.
• NOx formation is reduced by both lowering the
  flame temperature and shortening the residence
  time of the combustion gases,
• Lower (uniform !) flame temperature can be
  obtained by
• mixing the fuel with large excess of combustion
  air,
• Control of mixing (eliminate hot spots)

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Available Technologies

• Removal of the source of pollution (sulphur,
  nitrogen, ..) from fuel,
• Pre-combustion approach removes impurities such
  as sulphur, from the coal before it is burnt.
  Among possible methods one may distinguish coal
  cleaning and upgrading, coal blending, coal
  switching and bioprocesses.
• 2. Avoiding the production of the pollutants
  during combustion (so called primary measures or
  in-furnace measures),
• 3. Removing the pollutants from the flue gases by
  end of pipe technologies prior to emission.

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Primary measures of NOx reduction strategy of
NOx reduction during formation/combustion

• Control of concentration of oxygen contacting
  with fuel (air excess control) through air
  staging and mixing of fuel and air.
• - Control of oxygen concentration distribution
  in whole volume of combustion,
• - Low but high enough (to complete combustion)
  oxygen concentration
• Control of combustion temperature (flame) through
  increase of combustion zone as result flue gas
  recirculation (Dilution).

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NO species versus stochiometry (pulverised coal
combustion)
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Why control of temperature, oxygen concentration
and time is so important ?

• Thermal NO - strongly depends on temperature),
  less dependent on O2.
• - reduction at first through limitation of
  temperature and oxygen avialbaility as well as
  residence time).
• Fuel NO strongly depends on O2 and much less on
  temperature.
• - reduction through limitation of oxygen during
  first stage of combustion (during
  devolatilisation),
• - and through monitoring/control of coke residue
  combustion it means through control of oxygen
  concentration, temperature and residence time
  along the coke residue particles way.

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Methods to limit formation of NO during
combustion process (primary methods)

• Combustion air staging through
• - Air staging (basic method),
• - Fuel staging,
• - Flue gas recirculation (internal, external).
  Does not reduce very much efficiency (change of
  relation between convection and radiation) but
  may create operational problems,
• - Injection of water/steam (risk of efficiency
  drop and corrosion).

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Methods to reduce NO already formed during first
stages of combustion

• B. Reduction inside combustion chamber
• - SNCR (Selective Non Catalytic Reduction)
  introduction of ammonia chemicals (ammonia,
  trona) into combustion chamber,
• - Reburning introduction of secondary fuel
  (gas, coal, ) which creates CHi or/and NH3
  reducing NO.

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Methods to reduce already formed NOx at the
boiler outlet (outside combustion chamber and
process)

• C. Reduction performed at the outlet of flue
  gases
• SCR (Selective Catalytic Reduction)
  introduction of ammonia chemicals into low
  temperature flue gases between economiser and air
  heater.
• SCR disadvantages
• - high cost of investment dependent on NOx
  reduction level,
• - high operational cost ,
• - risk of ammonia slip,
• - catalyst life time,
• - storage of used catalysts.

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Selective Catalytic Reduction
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Selective Catalytic Reduction - SCR
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Selective Catalytic Reduction
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Air Staging, Over Fire Air (OFA)
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New look at Air Staging process (air staging with
extensive internal recirculation-mixing)
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Air Staging with external flue gas recirculation
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Air staging secondary air injection methods

• Direct injection of secondary air through air
  nozzles placed on walls
• 1. Conventional OFA (Over-Fire-Air) system of
  many low pressure nozzles,
• Allows primary air reduction down to 90-95 of
  theoretical air required with high risk of
  corrosion, CO emission and LOI increase
• 2. Advanced Rotating OFA system system of high
  pressure air nozzles asymetricaly placed on
  walls.
• Allows reduction of primary air down to 70-75
  of theoretical air without creating corrosion or
  CO and LOI.

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Air staging - burners
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Air staging - burners
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Air staging boilers, furnaces
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NOx versus type of combustion chamber
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System of low pressure nozzles 1 (conventional
OFA) Main disadvanatge week control of flow and
oxygen concentration by OFA
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System of many low pressure air nozzles,
OFA Problem seen low oxygen content, high
temperature corrosion of walls
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Rotating OFA
Widok z góry
duza predkosc powietrza
duza predkosc powietrza
Widok z boku
duza predkosc powietrza
duza predkosc powietrza
Paliwo/powietrze
Paliwo/powietrze
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Homogenous temperature profile in furnace
From CFD
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Baseline/ROFA comparison NOx
Baseline
ROFA
From CFD
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Increased particle residence time and reduced LOI
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Gas reburning in PC boiler
Complete combustion zone
OFA (overfire air)
Reburning zone
Gas, biomass 20
coal 100
coal 80
Primary combustion zone
Conventional combustion
Gas REBURNING
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Reburning - theoretical concept
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Retrofiting to reburning
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Retrofiting to reburning
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Reburning and RebSNCR
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NOx reduction via co-firing (reburning)

• Biomass combustion is considered CO2 neutral when
  grown and converted in a closed-loop production
  scheme
• NOx may be reduced by extended fuel staging or
  reburning (high volatile and low N content in
  biomass)
• NO CHi ? HCN ? NCO ?NH ? N ?N2
• SOx reduced by decreased sulphur content in the
  biofuel
• (often proportionally to the biofuel thermal
  load)
• Sulphur content in coal 150-235 mg S/MJ,
  average 217 mg S/MJ
• Sulphur content in peat 100-180 mg S/MJ,
  average 127 mg S/MJ
• Sulphur content in oil (average) 72 mg S/MJ
• SOx reduced by sulphur retention in alkali
  biofuel compounds

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NOx reduction by the in-furnace measures
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Selective Non-Catalytic Reduction - SNCR

• SNCR technique employs direct injection of a
  nitrogenous reagent (normally ammonia NH3) into
  the flue gas stream. NOx is reduced by gas-phase,
  free radical reactions. Process is however
  effective over a realtively narrow temperature
  range.
• - Ammonia - (NH3) (temperature 900 1000 C)
• - Urea - (NH2)2CO (temperature up to 1100 C)
• 4NO 4 NH3 O2 ?? 4N2 6 H2O
• At low temperature reaction is very slow and NH3
  passes unreacted into the back end of the plant,
  where it forms corrosive ammonium salts which can
  also cause fouling.
• At high temperature, the injected NH3 is oxidised
  to form NOx, so that NOx emission can actually
  increase.

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SNCR Temperature window for NO reduction (input
about 500 ppm NOx, NH3 molar ratio to NO 1.6)
ref.
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SNCR - Selective Non-Catalytic Reduction

• Practical problems with SNCR are results of
• Non-uniform temperature distribution at the
  injection level of NH3,
• 2. Too short residence time. Optimum about 1 sek
  but not shorter then 0.3 sek
• Not good mixing because of
• NOx concentration is not unform and not stable at
  the injection level
• mixing system does not follow the changes of flow
  with changes of load.

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Ammonia slip because of too short residence time
and low quality mixing
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Reburning combined with SNCR (for deep NOx
reduction)
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Reburning and SNCR
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Reburning combined with SNCR
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Location of various sorbent inputs in a typical
power station
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De-SOx methods

• Wet scrubber systems capable of achieving
  reduction efficiencies up to 99 percent
• Spray dry scrubbers, also known as semi dry,
  which can achieve reduction efficiencies of over
  90 percent
• Dry sorbent injection, the lowest cost SOx
  removal technology and the most appropriate
  technology if large reduction efficiencies are
  not required

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SOx reduction dry sorbent injection

• When limestone, hydrated lime or dolomite is
  introduced into the upper part of the furnace
  chamber, the sorbent is decomposed, i.e.
  decarbonised or dehydrated in accordance with
  the following reactions
• CaCO3 heat (825900oC) ? CaO CO2
• Ca(OH)2 heat ? CaO H2O
• and then, lime reacts with SO2 in accordance with
  the below-described reactions
• CaO SO2 ? CaSO3 heat
• CaO SO2 ½ O2 ? CaSO4 heat
• Furnace sorbent injection provides the
  additional benefit of removing SO3, chlorides,
  and fluoride from the flue gas as follow
• CaO SO3 ? CaSO4 heat
• CaO 2 HCl ? CaCl2 H2O heat
• CaO 2 HF ? CaF2 H2O heat

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SO2 removal reactions in furnace sorbent
injection
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SOx reduction dry sorbent injection
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SO2 removal at different temperature windows for
sorbent injection
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SOx reduction dry sorbent injection
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Wet de-SOx methods

• Fresh slurry is continuously charged into the
  absorber. Reduction of sulphur dioxide creates
  calcium sulphite according to the reaction
• SO2 H2O ? H2SO3
• CaCO3 H2SO3 ? CaSO3 CO2 H2O
• An oxidation step, either as an integrated part
  of the scrubbing process (in situ oxidation) or
  in separate vessel, can convert the sulphite
  residue to calcium sulphate
• CaSO3 ½ O2 2 H2O ? CaSO4 ? 2 H2O
• Overall reaction can be written as follows
• CaCO3 SO2 ½ O2 2 H2O ? CaSO4 ? 2 H2O CO2
• After precipitation from the solution calcium
  sulphate, is a subject to further treatment
  (washing and dehydration) and eventually produces
  a usable gypsum rest product.

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Wet de-SOx methods
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CO2 reduction
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Cofiring strategies and their requirements
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Co-firing with gasified biomass (reburning)
Introduction of chlorine and alkali compounds
into furnace is avoided
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Thank you