Transcript PowerPoint-presentation

Clean Combustion Technologies
Overview
Wlodzimierz Blasiak, Professor
Royal Institute of Technology (KTH)
School of Industrial Engineering and Management
Department of Materials Science and Engineering
Division of Energy and Furnace Technology
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
1. The NOx formation is depending on combustion
conditions.
2. 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
1. 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 infurnace measures),
3. Removing the pollutants from the flue gases by
“end of pipe“ technologies prior to emission.
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– strategy of NOx
reduction during formation/combustion
Primary measures of NOx reduction
• 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)
A. 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)
Primary air
Mixing
Fuel
Primary
combustion
zone
Secondary
combustion/mixi
ng zone
Flue
gases
Secondary
air
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New look at Air Staging process (air staging with
extensive internal recirculation-mixing)
Intermediate zone
Primary
air
Mixing
Primary
combustion
(l<1)
fuel
Secondary
combustion
korozja
(l > 1)
Flue
gases
Secondary air
(OFA, ...)
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Air Staging with external flue gas recirculation
Primary
air
mixing
fuel
Primary
combustion
zone
Secondary
combustion/mi
xing zone
Flue
gases
Secondary
air
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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
duża prędkość powietrza
duża prędkość powietrza
Widok z boku
duża prędkość powietrza
Paliwo/powietrze
duża prędkość powietrza
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
80%
coal
100%
Conventional
combustion
Primary
combustion
zone
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 Reb+SNCR
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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 infurnace measures
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Selective Non-Catalytic Reduction - SNCR
1.
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
2.
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.
3.
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:
1. 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
3. 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
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•
•
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
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•
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•
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 (825–900oC)  CaO + CO2
Ca(OH)2 + heat  CaO + H2O
and then, lime reacts with SO2 in accordance with the belowdescribed 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
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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:
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CaSO3 + ½ O2 + 2 H2O  CaSO4  2 H2O
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•
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
Wood firing
percentage
(heat input)
Material preparation
Strategy required
Firing strategy required
Boiler investment required
- co-pulverize with coal
- separate receiving and handling
(cyclone)
- fire with coal
- fire in secondary air system
(cyclone)
- use existing boiler
- use existing boiler
- separate receiving and handling (PC)
- separate receiving, common storage
(cyclone)
- separate burners (PC)
- fire with coal (cyclone)
-use existing boiler
15-35
- reburning strategy: separate receiving
and preparation
- fire above coal burners or
cyclone barrels
- use existing boiler heavily
modified, overfire air
25-50
- separate receiving and handling of
alternative fuels
- fire separately in common,
multifuel boiler
- new boiler designed with
biomass parameters (e.g.
fluidized bed)
2-5
10-15
- use existing boiler
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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
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