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Energy and Furnace Technology

Wlodzimierz Blasiak, Professor Royal Institute of Technology (KTH)

School of Industrial Engineering and ManagementDepartment of Materials Science and Engineering

Division of Energy and Furnace Technology

Clean Combustion Technologies

Overview

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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 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)

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)

MixingPrimary

combustion zone

Secondary combustion/mixi

ng zone

Fuel Secondary air

Primary airFlue gases

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

MixingPrimary

combustion(<1)

Secondary combustion

( > 1)

fuel Secondary air (OFA, ...)

Primary air Flue

gases

Intermediate zone

korozja

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Air Staging with external flue gas recirculation

mixingPrimary

combustion zone

Secondary combustion/mi

xing zone

Flue gases

Secondary air

fuel

Primary 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

duża prędkość powietrza

duża prędkość powietrzaWidok 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 time and reduced LOI

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Gas reburning in PC boiler

coal 100%

Conventional combustion

Gas REBURNING

coal 80%

Gas, biomass20%

OFA(overfire air)

Primary combustion zone

Reburning zone

Complete combustion zone

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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/MJSulphur 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 biofuel compounds

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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 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

• 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 (825–900oC) 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

Wood firing percentage(heat input)

Material preparationStrategy required

Firing strategy required Boiler investment required

2-5 - co-pulverize with coal- separate receiving and handling (cyclone)

- fire with coal- fire in secondary air system (cyclone)

- use existing boiler- use existing boiler

10-15 - separate receiving and handling (PC)- separate receiving, common storage (cyclone)

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

-use existing boiler

- 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)

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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