8-Clean Combustion Technologies

7/31/2019 8-Clean Combustion Technologies

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Energy and Furnace Technology

W lodz im ierz B las iak , Pr o fessor

Roya l I ns t i t u t e o f Techno logy ( KTH)Schoo l o f I ndus t r ia l Eng ineer ing and Managem entDepar t m en t o f Ma t e r ia l s Sc ience and Eng ineer ing

Div is ion o f Ener gy an d Fur nace Techno log y

Clean Combustion Technologies

Overview


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Legislation in Sweden


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Carbon monoxide

It is the product ofi n com p le te com bus t i on and is:

- Flammable (from 12,5 % up...)

- Colorless,

- Odorless gas,

- Easy to mix with air,

- Extremelly toxic (from 50 ppm can produce symptoms ofpoisoning),

- 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 istermed after-burning and depends on process design:

- cooling of flue gases,

- oxygen availability,- residence time,

- water content.


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Carbon monoxide destruction isa 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 asecond.

It has been reported that the time required for the oxidation of CO is

about 10 times the time needed for oxidation of hydrocarbons toCO. (slow reaction !)

In the absence of water CO is extremely difficult to burn. Incineratorexperience shows that temperatures of 750-800 C are requiredwith an actual residence time at this temperature of 0.2 0.4seconds and 4 5 % O2 as a minimum to achieve nearlycomplete oxidation of CO to CO2.

Units with poor mixing patterns exhibit outlet CO concentrationshigher 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 isdependent on;

the reaction temperature,

the local stoichiometry,

the residence time.


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Su m m at io n o n N Ox f o r m at io n

1. The NOx formation is depending on combustion

conditions.

2. As with all chemical processes, the rate of formation ofNOx is, among other things, a function of temperatureand residence time.

- NOx formation is reduced by both lowering the flametemperature and shortening the residence time of thecombustion 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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Ava i lab le Tech n o log ies

1 . Rem ova l o f t h e sou r ce of pollution (sulphur,

nitrogen, ..) from fuel,Pre-combustion approach removes impurities such as sulphur,

from the coal before it is burnt. Among possible methodsone may distinguish coal cleaning and upgrading, coalblending, coal switching and bioprocesses.

2. Avo id ing t he p roduc t i on of the pollutants duringcombustion (so called primary measures or in-furnace measures),

3. Re m o v i n g t h e p o l l u t a n t s from the flue gases byend of pipe technologies prior to emission.


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Primary measures of NOx reduction

strategy of NOxreduction during formation/combustion

Control of concentration of oxygen

contacting with fuel (air excesscontrol) through air staging andmixing of fuel and air.

- Control of oxygen concentration

distribution in whole volume ofcombustion,

- Low but high enough (to completecombustion) oxygen concentration

Control of combustion temperature(flame) through increase ofcombustion zone as result flue gasrecirculation (Dilution).


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NO species versus stochiometry(pulverised coal combustion)


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Why control of temperature,

oxygen concentration and time isso important ?

Th e r m a l NO - st r o n g l y d e pe n ds on t em p er a t u r e ) ,less dependen t on O2 .

- reduction at first through limitation of temperature andoxygen avialbaility as well as residence time).

Fue l NO st ron g l y depends on O2 and m uch l ess ont e m p e r a t u r e .

- reduction through limitation of oxygen during first stageof combustion (during devolatilisation),

- and through monitoring/control of coke residuecombustion it means through control of oxygenconcentration, temperature and residence time along thecoke residue particles way.


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Methods to limit formation of NO during combustionprocess (primary methods)

A. Combustion air staging through:

- Air staging (basic method),

- Fuel staging,

- Flue gas recirculation (internal, external). Does notreduce very much efficiency (change of relation betweenconvection and radiation) but may create operationalproblems,

- Injection of water/steam (risk of efficiency drop andcorrosion).


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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) introductionof ammonia chemicals (ammonia, trona) into combustionchamber,

- 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 boileroutlet (outside combustion chamber and process)

C. Reduction performed at the outlet of flue gases:

- SCR (Selective Catalytic Reduction) introduction ofammonia chemicals into low temperature flue gasesbetween economiser and air heater.

- SCR disadvantages:- high cost of investment dependent on NOxreduction 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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Se lect i ve Ca t a ly t i c Redu ct ion - SCR


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Selective Catalytic Reduction


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Ai r St ag in g , Ov er Fi r e Ai r ( OFA)

MixingPrimary

combustionzone

Secondarycombustion/mix

ing zone

Fuel Secondaryair

Primary airFluegases


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New look at Air Staging process (air staging withextensive internal recirculation-mixing)

MixingPrimary

combustion

( 1)

fuel Secondaryair (OFA, ...)

Primaryair Flue

gases

Intermediate zone

korozja


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Air St ag ing w i th ex t e rna l f l ue gas reci r cu la t i on

mixingPrimary

combustion

zone

Secondarycombustion/mi

xing zone

Fluegases

Secondaryair

fuel

Primaryair


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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 % oftheoretical air required with high risk ofcorrosion, CO emission and LOI increase

2. Advanced Rotating OFA system system of high pressure air nozzlesasymetricaly 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 oxygenconcentration by OFA


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System of many low pressure air nozzles, OFA

Problem seen low oxygen content, high temperaturecorrosion of walls


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Rotating OFA

Widok z gry

dua prdkopowietrza

dua prdkopowietrza

dua prdkopowietrza

dua prdkopowietrza

Widok z boku

Paliwo/powietrzePaliwo/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 timeand reduced LOI


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Gas reburning in PC boiler

coal

100%

Conventionalcombustion

Gas REBURNING

coal80%

Gas,biomass20%

OFA(overfire air)

Primarycombustionzone

Reburningzone

Complete

combustionzone


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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 neutralwhen grown and converted in a closed-loop

production scheme

NOx may be reduced by extended fuel staging orreburning (high volatile and low N content inbiomass)

NO + CHi HCN NCO NH N N2

SOx reduced by decreased sulphur content in thebiofuel

(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/MJSulphur content in oil (average): 72 mg S/MJ

SOx reduced by sulphur retention in alkali biofuelcompounds


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NOx reduction by the in-furnace measures


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Selective Non-Catalytic Reduction - SNCR

1. SNCR technique employs direct injection of a nitrogenousreagent (normally ammonia NH3) into the flue gas stream.

NOx is reduced by gas-phase, free radical reactions. Processis however effective over a realtively narrow temperaturerange.

- 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 passesunreacted into the back end of the plant, where it formscorrosive 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 injectionlevel of NH3,

2. Too short residence time. Optimum about 1 sek butnot shorter then 0.3 sek

3. Not good mixing because of:- NOx concentration is not unform and not stableat 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 qualitymixing


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Rebu r n in g com b ined w i t h SNCR

( f o r d eep N Ox r ed u ct i on )


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Reburning and SNCR


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Reburning combined with SNCR


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Location of various sorbent inputsin 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 removaltechnology 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 theupper part of the f u r nace cham ber , the sorbent is decomposed,i.e. decarbonised or dehydrated in accordance withthe 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 + O2 CaSO4 + heat

Furnace sorbent injection providesthe additional benefit of removing SO3, chlorides, and fluoridefrom 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

SO d ti d b t


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SOx reduction dry sorbent

injection


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SO2 removal at different temperaturewindows for sorbent injection

SO d ti d b t i j ti


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SOx reduction dry sorbent injection

Wet de SOx methods


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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 scrubbingprocess (in situ oxidation) or in separate vessel, can convert thesulphite residue to calcium sulphate:

CaSO3 + O2 + 2 H2O CaSO4 2 H2O

Overall reaction can be written as follows:

CaCO3 + SO2 + O2 + 2 H2O CaSO4 2 H2O + CO2

After precipitation from the solution calcium sulphate, is a subject tofurther treatment (washing and dehydration) and eventually producea usable gypsum rest product.


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Wet de-SOx methods


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CO2 reduction

C fi i t t i d th i i t


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Cofiring strategies and their requirements

- new boiler designed with

biomass parameters (e.g.fluidized bed)

- fire separately in common,

multifuel boiler

- separate receiving and handling of

alternative fuels

25-50

- use existing boiler heavilymodified, overfire air

- fire above coal burners orcyclone barrels

- reburning strategy: separate receivingand preparation

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-use existing boiler

- use existing boiler

- separate burners (PC)- fire with coal (cyclone)

- separate receiving and handling (PC)- separate receiving, common storage(cyclone)

10-15



- fire with coal

- fire in secondary air system(cyclone)

- co-pulverize with coal

- separate receiving and handling(cyclone)

2-5

Boiler investment requiredFiring strategy requiredMaterial preparationStrategy required

Wood firingpercentage(heat input)


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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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8-Clean Combustion Technologies