handbook of combustion (online) || staged combustion and exhaust gas recirculation in fluidized beds

7Staged Combustion and Exhaust Gas Recirculationin Fluidized BedsMarkus Bolh�ar-Nordenkampf

7.1Introduction

Increasing concern over atmospheric pollutants [1] is changing the focus of boilerand combustion-system design, as well as broadening the fuels used [2]. Moreover,the demand for boilers that can convert solid fuels with greater combustionefficiencies is increasing [3]. In the last few years various measures have beendeveloped to reduce emissions during combustion were these originate, in thefurnace. The combustion of fossil fuels produces emissions that have been linked tothe formation of acid rain, smog, changes to the ozone layer, and the so-called�greenhouse effect.� However, the new biomass-derived fuels also have to meetstronger emission regulations and achieve high steam parameters in combinationwith high fuel alkali content [4]. To mitigate these problems, federal and localregulations are currently in place that limit emissions of nitrogen oxides, sulfuroxides, and particulatematter [1].While emission limits vary between countries, stateand local regulations, a trend can be seen towards more stringent emission controlsdepending on the input fuel.

Manycombustion-control techniqueshaveemergedtoreducefossil fuelemissions.These techniques generally focus on the reduction of nitrogen oxides, by influencingthe combustionprocess,which cangreatly influence the formation anddestructionofnitrogen oxides. The developed techniques for fossil-fuel applications have beenapplied to new fuels like biomass, to reduce emissions from these fuels as well.

In recent years, staged combustion and flue-gas recirculation have gained evermore importance due to the tightening of emission regulations, as well as the use ofnew fuels. This chapter will focus on the state of the art of flue-gas recirculation andstaged combustion in modern combustors, such as gas and oil-fired applications,modern grate-fired applications, and bubbling fluidized beds. First, principles ofstaged combustion and flue-gas recirculation are discussed in general. To increasethe efficiency of the combustion process, the requirements on the maximum excessair in the flue gas are increased steadily. This means that demand is increasing forcombustion technologies that can achieve low carbonmonoxide emissions as well as

Handbook of Combustion Vol.5: New TechnologiesEdited by Maximilian Lackner, Franz Winter, and Avinash K. AgarwalCopyright � 2010 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimISBN: 978-3-527-32449-1

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low nitrogen oxides emissions, combined with a low excess air at the outlet. Theseissues are examined by reviewing the combustor design inmodern oil- and gas-fired,grate-fired, and bubbling fluidized bed applications. In future, staged combustionand flue-gas recirculation will gain in importance to actively control the combustionprocess for even more difficult fuels, with higher pollutants and low melting ashcomponents, as well as to further optimize the flue-gas emissions from the variouscombustion processes.

7.2General Aspects of Combustion

7.2.1Excess Air

For commercial applications,more than the theoretical necessary air (stoichiometric)isneeded to assure complete combustionof the fuel. This excess air isneededbecausemixing of the air and fuel in a boiler is not ideal. Since the excess air that is not used forthe combustion process leaves the unit at stack temperature, the amount of excess airshould be minimized. The energy required to heat this air from ambient to stacktemperatureusuallyservesnopurposeandisconsideredas lostheat,andhencetohavereduced the efficiency of the combustion process. Typical values of excess air requiredfor different equipment are shown inTable 7.1 for various fuels andmethodsoffiring.When substoichiometric firing is employed in the combustion zone, less than thetheoretically needed air is used; therefore the values shown inTable 7.1would apply tothe furnace zonewhere thefinal air is admitted to complete combustion. The amountofexcessairat theexitof thesteamgeneratorequipment(whereit isusuallymonitored)must be greater than the air required for stoichiometric combustion. Inmodernunitswith membrane construction the excess air is usually only approx 2% at full load.

7.2.2Formation of Nitrogen Oxides

Nitrogen oxides in the form of nitrogen oxide, nitrogen dioxide, and nitrous oxide areproduced during combustion by two primary mechanisms: thermal nitrogen oxideformation and by the formation from fuel nitrogen. Thermal nitrogen oxides resultfrom the dissociation and oxidation of nitrogen in the combustion air. The rate anddegree of thermal nitrogen oxide formation depends upon oxygen availability duringthe combustion process and is exponentially dependent upon combustion temper-ature. Thermal nitrogen oxide reactions occur rapidly at combustion temperatures inexcess of 1538 �C. Thermal nitrogen oxide is the primary source of nitrogen oxideformation fromnatural gas, distillate oils, andmost biomass fuels because these fuelsare generally low in, or devoid of, nitrogen. Fuel nitrogen oxides, in contrast, resultsfrom oxidation of nitrogen organically bound in the fuel. Fuel-bound nitrogen in theform of volatile compounds is intimately tied to the fuel hydrocarbon chains. For this

162j 7 Staged Combustion and Exhaust Gas Recirculation in Fluidized Beds

reason, the formation of fuel nitrogen oxides is linked to both fuel nitrogen contentand fuel volatility. Inhibiting oxygen availability during the early stages of combus-tion, during fuel devolatilization, is the most effective means of controlling fuelnitrogen oxides formation [5–9].

In numerous combustion processes nitrogen oxides control techniques arecommonly used. These vary in effectiveness and cost. In all cases, control methodsare aimed at reducing either thermal nitrogen oxides or fuel nitrogen oxides, ora combination of both.

7.3Staged Combustion and Flue-Gas Recirculation in Gas- and Oil-Fired Applications

7.3.1Low Excess Air

Low excess air effectively reduces nitrogen oxides emissions with little, if any, capitalexpenditure. Low excess air is a desirable method of increasing thermal efficiency

Table 7.1 Typical excess air requirements for different fuels.

Fuel Type of furnace of burners Excess air (wt%)

Pulverized coal Completely water cooled furnace –

wet or dry ash removal15–20

Partly water cooled furnace 15–40Crushed coal Cyclone furnace – pressure or suction 13–20Coal Fluidized bed combustor 15–20

Spreader stoker 25–35Water cooled vibrating grate stoker 25–35Chain grate and traveling grate 25–35Underfeed stoker 25–40

Fuel oil Register type burners 3–15Natural, coke oven andrefinery gas

Register type burners 3–15

Blast furnace Register type burners 15–30Wood/bark Traveling grate, water-cooled

vibrating grate20–25

Fluidized bed combustion 4–15Refuse-derived fuels (RDF) Complete water cooled furnace –

traveling grate40–60

Municipal solid waste Water-cooled/refractory covered furnace –

reciprocating grate70–100

Rotary kiln 60–100Bagasse All furnaces 25–35Black liquor Recovery furnaces for Kraft and soda

pulping process15–20

7.3 Staged Combustion and Flue-Gas Recirculation in Gas- and Oil-Fired Applications j163

and has the added benefit of inhibiting thermal nitrogen oxide formation. If burnerstability and combustion efficiency are maintained at acceptable levels, lowering theexcess air may reduce nitrogen oxides by as much as 10–20%. The success of thismethod depends largely upon fuel properties and the ability to carefully control fueland air distribution to the burners. Operation may require more sophisticatedmethods of measuring and regulating fuel and air flow to the burners and modifica-tions to the air delivery system to ensure equal distribution of combustion air to allburners.

7.3.2Staged Combustion

Two-stage combustion is a relatively long-standing and acceptedmethod of achievingsignificant reduction of nitrogen oxides. Combustion air is directed to the burnerzone in quantities less than that theoretically required to burn the fuel, with theremainder of the air introduced through overfire air ports. By diverting combustionair away from the burners, oxygen concentration in the lower furnace is reduced,thereby limiting the oxidation of chemically bound nitrogen in the fuel. By intro-ducing the total combustion air over a larger portion of the furnace, peak flametemperatures are also lowered.

Appropriate design of a two-stage combustion system can reduce emissionsof nitrogen oxides by as much as 50% and simultaneously maintain acceptablecombustion performance.

The following factors must be considered in the overall design of the system:

1) Burner zone stoichiometry: The fraction of theoretical air directed to the burners ispredetermined to allow proper sizing of the burners and overfire air ports.Normally a burner zone stoichiometry in the range of 0.85 to 0.90 will resultin desired levels of nitrogen oxide reduction without notable adverse effects oncombustion stability and turndown.

2) Overfire air-port design: Overfire air-ports must be designed for thorough mixingof air and combustion gases in the second stage of combustion. Ports must havethe flexibility to regulate flow and air penetration to promote mixing both nearthe furnacewalls and toward the centre of the furnace.Mixing efficiencymust bemaintained over the anticipated boiler load range and the range in burner zonestoichiometries.

3) Burner design: Burners must be able to operate at lower air-flow rates andvelocities without affecting the combustion stability. In a two-stage combustionsystem, burner zone stoichiometry is typically increased with decreasing load toensure that burner air velocities are maintained above minimum limits. Thisfurther ensures positive windbox-to-furnace differential pressures at reducedloads.

4) Overfire air-port location: Sufficient residence time from the burner zone to theoverfire air-ports and from the ports to the furnace exit is critical to proper systemdesign. Overfire air-ports must be located to optimize nitrogen oxides reduction


and combustion efficiency and to limit the change to furnace exit gastemperatures.

5) Furnace geometry: Furnace geometry influences burner arrangement and flamepatterns, residence time, and thermal environment during the first and secondstages of combustion. Liberal furnace sizing is generally favorable for lowernitrogen oxides as combustion temperatures are lower and residence times areincreased.

6) Airflow control: Ideally, overfire air-ports are housed in a dedicated wind-boxcompartment. In thismanner, air to the nitrogen oxides ports can bemetered andcontrolled separately from air to the burners. This permits operation at desiredstoichiometric levels in the lower furnace and allows for compensation to the flowsplitasaresultofairflowadjustmentstoindividualburnersornitrogenoxidesports.

Additional flexibility in controlling burner fuel and air-flow characteristics isrequired to optimize combustion under two-staged systems. Consequently, im-proved burner designs are emerging to address the demand for tighter control ofair flow and fuel firing patterns to individual burners.

In the reducing gas of the lower furnace, sulfur is converted into hydrogen sulfiderather than sulfur dioxide and sulfur trioxide. The corrosiveness of reducing gas andthe potential for increased corrosion of lower furnace wall tubes is highly dependentupon hydrogen sulfide and the carbon monoxide concentration. Two-stage combus-tion is therefore not recommended when firing high sulfur residual fuel oils.

7.3.3Flue-Gas Recirculation

Flue-gas recirculation to theburners iswell suited toreducenitrogenoxidesemissionswhenthecontributionoffuelnitrogentototalnitrogenoxidesformationis low.For thisreason, theuseofgasrecirculationisgenerally limitedto thecombustionofnaturalgasand fuel oils, as well as biomass. By introducing flue gas from the back of the boiler(economizer outlet, after the flue gas cleaning or after the induced draft fan) into thecombustion air stream burner, peak flame temperatures are lowered and nitrogenoxides emissions are significantly reduced (Figure 7.1).

Airfoils are commonly used to mix recirculated flue gas with the combustion air.Flue gas is introduced in the sides of the secondary air-measuring foils and exitsthrough slots downstream of the air-measurement taps. This method ensuresthorough mixing of flue gas and combustion air before reaching the burners anddoes not affect the air-flow metering capability of the foils.

In general, increasing the rate of flue-gas recirculation to the burners results in anincreasingly significant reduction of nitrogen oxides. Target nitrogen oxides emis-sion levels and limitations on equipment size and boiler components dictate thepractical limit of recirculated flue gas for control of nitrogen oxides. Other limitingfactors include burner stability and oxygen concentration of the combustion air.Oxygen content must be maintained at or above 17% on a dry basis for safe andreliable operation of the combustion equipment.


The expense of a flue-gas recirculation system can be significant. Gas recirculationfans are required for the desired flow quantities at static pressures, capable ofovercoming losses through the flues, ducts, mixing devices, and the burnersthemselves. Additional controls and instruments are also necessary to regulate thegas recirculation flow to the wind-box at desired levels over the load range. In retrofitapplications, significant cost is associated with routing of flues and ducts to permitmixing of the flue gas with combustion air. Also, the accompanying increase infurnace gas weight at full-load operation may require modifications to convectionpass surfaces or dictate changes to standard operating procedures.

From an operational standpoint, the introduction of flue-gas recirculation asa retrofit nitrogen oxides control technique must, in virtually all cases, be accom-panied by the installation of overfire air-ports. Oil and gas burners, initially designedwithout future consideration to flue-gas recirculation, are not properly sized toaccommodate the increase in burner mass flow as a result of recirculated flue gas.The quantity of flue gas necessary to significantly reduce emissions of nitrogenoxides will, in all likelihood, result in burner-throat velocities that exceed standarddesign practices. This, in turn, may cause burner instability, prohibitive burnerdifferentials and, in the case of gas firing, undesirable pulsation. Therefore, theinstallation of overfire air-ports in conjunction with flue-gas recirculation serves twouseful purposes: first lower nitrogen oxides emissions through two-stage combus-tion and second a decrease in mass flow of air to the burners to accommodate theincreased burden of recirculated flue gas.

Figure 7.1 Flue-gas recirculation, low nitrogen oxides system for oil and gas firing.


When employing flue-gas recirculation in combination with overfire air, it isdesirable to house the overfire air-ports in a dedicated wind-box compartmentseparate from the burners. In this manner it is possible to introduce recirculatedflue gas to the burners only. This permits more efficient use of the gas-recirculationfans and overall system design as only that portion of flue gas introduced through theburners is considered effective in controlling emissions of nitrogen oxides.

7.3.4Reburning

By effective staging, both fuel and combustion air, nitrogen oxides emissions as lowas 40 to 60 ppm (corrected to 3% oxygen) are possible when firing residual oil. Evenlower nitrogen oxides emissions are possible when firing natural gas. Heat input isspread over a larger portion of the furnace, with combustion air carefully regulated tovarious zones to achieve optimum nitrogen oxides reduction (Figure 7.2).

In reburning, the lower furnace ormain burner zone provides themajor portion ofthe total heat input to the furnace. Similar to two-stage combustion, air less than thattheoretically required to burn the fuel is introduced into this zone. Combustion gasesfrom themain burner zone then pass through a second combustion zone termed thereburning zone. Here, burners provide the remaining heat input to the furnace to

Figure 7.2 Boiler side view showing reburn principle and combustion zones for a utility boiler.


achieve full load operation but at a significantly lower stoichiometry. By injectingreburn fuel above the main burner zone, a nitrogen oxides reducing region isproduced in the furnace where hydrocarbon radicals from the partially oxidizedreburn fuel strip oxygen from the nitrogen oxide molecules, leaving elementalnitrogen to ultimately form molecular nitrogen.

Overfire air-ports are installed above the reburning zone where the rest of the air isintroduced to complete combustion in an environment both chemically and ther-mally inconducive to nitrogen oxides formation.

Application of this technologymust consider several variables. System parametersrequiring definition include: fuel split between the main combustion zone and thereburn zone, stoichiometry to the main burners, stoichiometry to the reburnburners, overall stoichiometry in the reburn zone of the furnace, residence timein the reburn zone, and residence time required above the overfire air-ports tocomplete combustion. An optimum range of values has been defined for each ofthese parameters through laboratory tests and field applications and is largelydependent upon the type of fuel being fired.

Implementation of reburning technology adds considerable complexity to oper-ation and maintenance of the overall combustion system. Initial costs may beprohibitive, particularly for retrofits. Economically, the potential benefits and tech-nicalmerit of the reburning processmust be commensurate with long-term goals forabatement of nitrogen oxides.

7.4Stage Combustion in Modern Grate-Fired Applications

In grate-fired applications the design of air supply system (primary air and secondaryair) plays a very important role in the efficient and complete combustion of the fuel,especially in biomass applications. For grate firing, the overall excess air for mostbiomass fuels is normally set to 25% or above. The split ratio of primary air tosecondary air tends to be 40/60 in modern grate-fired boilers burning biomass,instead of 80/20 in older units, which leaves much more freedom to advancedsecondary air supply.

The primary air distribution, together with the movement of the grate, affectssignificantly the mixing and the fuel conversion in the fuel bed. Though some grate-fired boilers burning biomass may have a low buildup of materials on the bed, mostbiomass-fired grate furnaces in the literature [10] may be interpreted as a cross-flowreactor, where biomass is fed in a thick layer perpendicular to the primary air flow.The bottom of the biomass bed is exposed to the preheated primary air while the topof the bed is within the furnace. The fuel bed consists of a huge number of solidparticles that are piled up on the grate with a characteristic porosity. The fuel bed isheated by over-bed radiation from flames and refractory furnace walls until it igniteson the top surface of the fuel bed. Propagation of the ignition front in the bed is ofpractical interest, as it determines the releases of volatiles, and affects the heat outputfrom a given grate area and the stability of combustion. It is also directly related to the


release of volatile nitrogen species (ammonia and hydrogen cyanide) and nitrogenoxide formed from volatiles.Hence knowledge of the factors influencing the speed ofthe ignition front is important for optimizing gas-phase combustion in the freeboard.

The generally accepted combustion mechanism of crosscurrent units may bedescribed as follows [11–15]. After ignition, a reaction front propagates from thesurface of the bed downwards to the grate against the direction of the primary air.

The heat, generated in the reaction front, is transported against the combustionair flow and dries and devolatilizes the raw fuel. This allows the reaction front topropagate. Owing to the opposing directions of the heat flow and the air flow, the heatis not transported downwards far from the position where it is released, and thereaction front is narrow. The heat generated in the reaction front originates fromoxidation of fuel and, if not all oxygen is consumed in the narrow reaction front, a charlayer will be formed above the reaction front.

When the reaction front reaches the surface of the grate, a secondary reaction front(e.g., a char burnout front), propagating upwards to the surface of the bed, burns thepreviously formed char layer.

This traditional combustion behavior in the fuel bed on the grate may not alwaysbe observed. Fuel properties (e.g., moisture, heating value, and particle size) andoperating conditions (e.g., primary air flow-rate) have a significant influence on thecombustion behavior in the fuel bed [16, 17].

Depending on the primary air supply rate, three modes of combustion in thebiomass bed have been identified: oxygen-limited combustion under low air supplyrate, reaction-limited combustionwhen increasing primary air supply, and extinctionby convection if the primary air flow-rate is increased further. In modern grate-firedboilers burning biomass, the biomass combustion in the fuel bed ismore likely undersubstoichiometric conditions (e.g., fuel-rich), because biomass fuels typically havea higher volatiles content on a dry basis. Moreover, themultiple zones of under-grateprimary air are often used, which can help to achieve a more favorable temperaturedistribution, high ash burnout, and low emissions.

7.4.1Advanced Secondary Air Supply

The advanced secondary air supply system is one of the most important elements inthe optimization of gas combustion in the freeboard, for complete burnout and loweremissions, for example, by forming local recirculation zones or rotating flows and byforming different local combustion environments (e.g., fuel-rich or oxygen-rich). It isprobably the most flexible way to retrofit old grate-fired boilers for a better burnoutand lower pollutant emissions. The gases released from biomass conversion on thegrate and a small amount of entrained fuel particles continue to combust in thefreeboard, in which the secondary air supply plays a significant role in the mixing,burnout, and emissions. Advanced secondary air-staging is also often used inmodern biomass-fired grate boilers. The basic idea of air-staging is to reducethe formation of nitrogen oxides by reducing oxygen availability in the flame andby lowering flame-temperature peaks. In air-staged combustion process, the first

7.4 Stage Combustion in Modern Grate-Fired Applications j169

air-deficient (e.g., fuel-rich) zone reduces nitrogen oxides formation, and completecombustion is achieved only after the addition of over fire air in the second zone (e.g.,the burnout zone).

For example, in an advanced secondary air supply in a straw-fired vibrating-grateboiler [18], the secondary air nozzles have different diameters, spacing, and orienta-tions, and are staggered. The staggered arrangement of the over-fire air jets canprovide an effective curtain of combustion air, and can also form a double rotatingflow on the horizontal cross-sections in the burnout zone, which prolongs theresidence time of the combustibles, distributes the temperature more evenly, andleads to a better burnout. The characteristics of the combustion air supply and thecombustion zone in such a grate-fired boiler are sketched in Figure 7.3. Most of thecombustibles are released into the freeboard from the first half of the grate and theenhanced air-staging forms a local fuel-rich combustion environment in the front-bottom part. The air jets, located on the rear wall in the lower furnace, form a localair-rich environment and a stable recirculation zone in the rear-bottomcorner, both ofwhich help stabilize the combustion on the last section of the grate and reduce theincompletely burned char in the bottom ash.

During the advanced air-staged combustion process, the mixing, temperature,residence time, and local stoichiometry play key roles. Further, optimization of thesecondary air jets in terms of amount, momentum, diameter, location, spacing, andorientation are vital for a well-functioning, staged combustion system.

Figure 7.3 Sketch of the air supply and the resultant different zones in a grate-fired boiler burningbiomass.


7.5Stage Combustion and Flue-Gas Recirculation in Modern Bubbling Fluidized-BedBoiler Applications

In modern bubbling fluidizing bed boilers the combustion chamber is an integratedpart of the boiler and is formed by water-cooled membrane walls that are part of theboiler evaporator. Depending upon the lowest heating value of the solid fuels fired,the main part of combustion chamber is covered with heat-insulating refractorylining to secure the desired combustion conditions within the operating range of thesolid fuel. The other purpose of the refractory lining is to protect the heating surfacesfrom erosion by the fluidized bed and from corrosion due to the substoichiometriccombustion conditions [19, 20].

The general characteristic of the bubbling fluidizing bed boiler technology is thesolid-fuel combustion within the fluidized bed consisting of bed material and fuel.The amount of inert bed material, consisting in most cases of quartz sand and bedash, is higher than 95 wt%. Consequently, the main part of the fluidized bed isactually hot sand with a minor portion of fuel. The bed material has very high heatstorage capacity andhigh particle surface, which enables a very intense heat exchangebetween sand and fuel, enhanced by the fluidization.

Hence, fluidized beds have the following important advantages:

. The combustion process is accelerated.

. The fuel-drying phase is short, since the fuel warms-up very quickly after beingfed to the furnace.

. The subsequent gasification phase is accelerated, since the fuel is heated up to thereaction temperature very quickly.

. The burning process is accelerated, since heavy abrasion by the bedmaterial takesplace on the surface of the combusted fuel particle.

. The high heat-storage capacity of the fluidized bed stabilizes the combustionreactions, which enables the combustion of a broad fuel range.

. Owing to the high surface area of the bed particles and the fuel ash (calciumoxide,magnesium oxide) as well as the appropriate temperature, contaminants likesulfur dioxide, hydrochloric acid, and hydrogen fluoride can be partially removedfrom theflue gas inside the furnace. This effect can be enhanced in terms of sulfurdioxide by adding limestone to the fluidized bed.

7.5.1Staged Combustion

Modern fluidized bed boilers are constructed so as to allow a staged combustion ofthe fuel; hence the combustion air is fed to the boiler on two ormore different levels.Normally, primary air is fed trough the air distributor at the fluidized bed bottom andthe secondary air at the freeboard outlet. Further, recirculated flue gas can be used asadditional flow to the primary air or as separate flow to the post-combustion chamber(Figure 7.4).

7.5 Stage Combustion and Flue-Gas Recirculation inModern Bubbling Fluidized-Bed Boiler Applications j171

Combustion conditions within the bed and freeboard area are predominantlysubstoichiometric. This means that in this area there is too little oxygen available fora complete combustion of the fuel. At the beginning of the post-combustion chamberthe necessary air excess must be reached by adding secondary air to enable thecomplete conversion of fuel and secure adequate emissions. This causes a rise intemperature and oxygen content (Figure 7.5). The turbulence in this area of the firstpass results in very low carbon monoxide emission values in the fuel gas.

For high calorific fuels, recirculation gas is injected above the secondary air to�cool� the flue gases in the post-combustion chamber. This staged combustionconcept results in a homogenous and moderate temperature profile in the furnaceand the first pass of the boiler and thus low nitrogen oxides emission.

Safe operation is possible at an oxygen content of approximately 2.0 vol.wet%. Thenormal operation range is in the area of 3.0–5.5 vol.wet%. A lower oxygen content has

Figure 7.4 Sketch of a modern fluidized bed boiler with air staging and flue-gas recirculation.


a positive influence on the nitrogen oxides emissions and on the boiler efficiency;however, the lower limit of the oxygen-content is determined by the increase incarbon monoxide emissions, due to incomplete combustion.

Flue-gas recirculation into the fluidized bed and the freeboard supports the stagedcombustion at different boiler loads and fuels. Additionally, it can help to preventhigh local temperatures and hence high emissions at high support firing levels usingfuel oil gas. The flue-gas recirculation gas can be fed normally at two stages into theboiler, depending on the operation:

. as primary air/recirculated flue-gas mixture through the air distributor;

. to the freeboard outlet above the fluidized bed to reduce temperatures in this area.

The staged combustion concept within the designed solid fuel operation range aswell as the boiler load range allows very effective temperature control within theimportant combustion sections. The results are:

. low nitrogen oxides emissions

. optimum retention of contaminants (sulfur dioxide, hydrochloric acid) within theash;

. optimum temperature profile in the furnace (mainly fluidized bed temperature)in terms of ashes with a low softening point temperature [20–22].

Figure 7.5 Temperature profile and oxygen concentration in a modern fluidized bed boiler.


7.5.2Bed Fluidization

The primary air has two important functions in the combustion process:

. control of bed temperature by controlling heat release within the bed area;

. it acts as fluidization agent for the bed.

During substoichiometric operation of the fluidized bed the primary air flow isreduced relative to the fuel flow, if a constant bed temperature is kept. Hence, thefluidization gas flow, and therefore also the fluidization velocity, would alsodecrease. This would lead to areas in the bed that are defluidized and hence tothe possibilities of hot spots and possible agglomeration of melted ash particles andthe bed material.

To secure the necessary fluidization of the bed, the missing volume flow issubstituted by recirculated flue gas. By this operation, the fluidization gas flow canbe kept constant over the total operation range for different heating values and fordifferent boiler operation loads.

7.5.3Bed Temperature

A suitable, stable, and uniform temperature (D20 �C) within the fluidized bed area isimportant for a stable fuel combustion and retention of contaminants (sulfur oxide,hydrochloric acid, andhydrogenfluoride)within the ash. The equilibriumconditionsof the chemical reactions that take place in the bed area strongly depend on the bedtemperature. The bed temperature is controlled by adjusting the oxygen level in thefluidized bed, and hence in the fluidization gas. This controls the heat release inthe fluidized bed and thereby the temperature of the bed. The oxygen content in thefluidization gas can be controlled by mixing primary air (approximate 21% oxygen)with re-circulated flue gas (approx. 4% oxygen) as described above.

The optimal bed temperature for low sulfur oxide emissions is 750–800 �C; outsidethis temperature range sulfur oxide emission will increase.

This substoichiometric bed operation allows control of the bed temperature in therange 650–820 �C. Therefore, fuel with low ash melting temperature can also beburned without any sintering problems in the bed. The standard operation temper-ature of the fluidized bed is approximately 760 �C.

Figure 7.6 shows the correlation between the lower heating value of the fuel andlambda (air-to-fuel ratio referred to stoichiometric combustion) obtained frommeasurements from 15 biomass-fired fluidized bed plants [21], necessary to main-tain the desired 760 �C bed temperature.

It can be seen that fuels with lower heating values between 2.5 and 6MJ kg�1

require high lambda values to operate the bed within the desired temperature range.With lower heating values of 2.5MJ kg�1 the lambda value approaches unity, whichmeans that stoichiometric combustion conditions are necessary to maintain thedesired temperature.


The higher lambda values also indicate higher water content of the fuel, since thiswater has to be evaporated in the bed by the heat yielded from combustion.

Clearly, the correlation between the necessary lambda and the lower heating valueflattens at 8 up to 18MJ kg�1. For standard biomass fuels such aswood chips, lambdaranges between 0.35 and 0.45.

The strong influence of water content on boiler efficiency can be seen inFigure 7.7 with a strong variation between lower heating values (LHVs) 5 and10MJ kg�1.

Figure 7.8 shows the flue gas and air flows at different heating values referred toa base case of 7MJ kg�1. In the direction of LHVs, both the flue gas flows and thenecessary air flows strongly increase. The flue gas flow is affected by the higher watercontent of the fuel that leaves the boiler with the flue gas. The higher specific airflowsare necessary to maintain the staged combustion principle.

Implementation of staged combustion technology with flue-gas recirculation influidized-bed applications adds considerable complexity to operation and mainte-nance of the overall combustion system. Furthermore, the investment cost andadditional operation cost must be considered. Economically, the potential benefitsand technical merits of lowering the emissions and reducing the risks of agglom-erations across a broad fuel range, as well as increasing part-load behavior and fuelflexibility, are higher than the additional investment costs.

Figure 7.6 Correlation between the lower heating value and lambda (air-to-fuel ratio) in thefluidized bed to maintain a constant bed temperature of 760 �C.


7.6Outlook

Emission limits within the European Union and in the rest of the world will becomemore stringent in the future, whichwill in turn increase the pressure to develop cleancombustion technologies. In the past, emphasis was placed on end-of-pipe technol-ogies intended to clean the polluted flue gas. However, today the focus is onpreventing the forming of emissions at their origin.

Future development of the combustion process will clearly be directed towardscoping with more difficult fuels, with higher pollutant contents and low-melting ashfractions, aswell as towards further optimizing the combustion process. Future plantswill focus even more on the control of the combustion process by staging the air to agreater extent over the complete path to achieve an even flatter temperature profile.Modern process control systems, combined with fast measuring devices, will make itpossible to actively control the combustion process in a large power plant in the sameway as the combustion process is currently controlled in a modern car engine.

In future, fuel flexibility will gain ever greater importance since the fuel marketshows a tendency to higher volatility in terms of quality and prices. This will increasethe demand for combustion technologies that can cope with a broad range of fuels,

Figure 7.7 Correlation between lower heating value (LHV) and boiler efficiency.


without the need of revamping the whole boiler. Stage combustion technologies withflue-gas recirculation offer these advantages and can fulfill this demand.

Biomass power plants, which have previously not had to conform to pressingemission limits, will be forced in the near future to maintain the same emissionstandards as fossil fuel plants. Thiswill give additional impetus to the development ofadvanced combustion systems within this field.

7.7Summary

In the last few years staged combustion and flue-gas recirculation have gained evergreater importance due to the tightening of emission regulations, aswell as the use ofnew fuels.

To increase the efficiency of the combustion process, the requirements for themaximum excess air in the flue gas are increasing steadily. This means that demandis increasing for combustion technologies that can achieve low carbon monoxide

Figure 7.8 Correlation between lower heating value and flue gas, with specific air flows to theboiler.

7.7 Summary j177

emissions, as well as lownitrogen oxides emissions combinedwith a low excess air atthe outlet.

In gas- and oil-fired applications, low excess air in the burners in combinationwithstaged combustion has become state of the art for new power plants. A furtherdecrease in nitrogen oxides emission could be achieved by using recirculatedflue gasas a mixing agent with the combustion air stream, lowering the burner peak flametemperature. This has led to the design of boilers with three distinct zones to reduceemissions: a main combustion zone with low nitrogen oxides burners, a reburningzonewith reburning burners, and a zonewhere the complete combustion takes placewith overfire air-ports.

Staged combustion is also frequently used today in modern grate-fired applica-tions. The primary air is fed through the grate to the boiler and the secondary air isdivided to create different combustion zones over the grate, producing a fuel-richzone, an oxygen-rich zone, and a zone were the originating gases are burned-out.Using this arrangement,modern grate-fired applications can achieve lower excess airrates, combined with low emissions.

In fluidized-bed technology, staged combustion is frequently used in combinationwith new fuels, such as biomass. In this case, a constant fluidization of the bed, incombination with the possibility to control the bed temperature, is vital for safecombustion of these new low-emission fuels.Here the recirculatedflue gas is used asa mixing agent with the primary air to enable substoichiometric combustion in thebed and freeboard area. The secondary air is then used as overfire air.

In future, staged combustion andflue-gas recirculationwill gain importance in theactive control of the combustion process, even in the case of more difficult fuels withhigher pollutant contents and lowmelting ash fractions, as well as helping to furtheroptimize the flue-gas emissions from the combustion process.

References

1 EU (2001) Directive 2001/80/EC of theEuropean Parliament and of the Councilof 23 October 2001 on the limitation ofemissions of certain pollutants into the airfrom large combustion plants.

2 Kolb, T., Bleckwehl, S., Gehrmann, H.J.,and Seifert, H. (2008) Characterisation ofcombustion behaviour of refuse derivedfuel. Journal of the Energy Institute,81 (1), 1–6.

3 Qiurong, C. and Scheffknecht, G. (2002)Boiler design and materials aspects foradvanced steam power plants.Forschungszentrums Juelich, EnergyTechnology, 21 (Pt. 2 Materials forAdvanced Powder Engineering Part 2),1019–1033.

4 Ayhan, D. (2005) Potential applications ofrenewable energy sources, biomasscombustion problems in boiler powersystems and combustion relatedenvironmental issues. Progress in Energyand Combustion Science, 31 (2), 171–192.

5 L€offler, G., Sieber, R., Harasek, M.,and Hofbauer, H. (2005) Nitrogen oxidesformation in natural gas combustion -evaluation of simplified reaction schemesfor CFD calculations. Industrial &Engineering Chemistry Research, 44,6622–6633.

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7 Tourunen, A. (2009) Experimental trendsof nitrogen oxide in circulating fluidizedbed combustion. Fuel, 88, 1333–1341.

8 Winter, F., Wartha, C., and Hofbauer, H.(1999) Nitrogen oxide and nitrous oxideformation during the combustion ofwood, straw, malt waste and peat.Bioresource Technology, 70 (1), 39–49.

9 Li, H., Lu, G.Q., and Rudolph, V. (1998)The kinetics of nitrogen oxide and nitrousoxide reduction over coal chars influidized-bed combustion. ChemicalEngineering Science, 53 (1), 1–26.

10 Yin, C., Rosendahl, L.A., and Kær, S.K.(2008)Grate-firing of biomass for heat andpower production. Progress in Energy andCombustion Science, 34, 725–754.

11 van der Lans, R., Pedersen, L., Jensen, A.,Glarborg, P., and Dam-Johansen, K.(2000)Modeling and experiments of strawcombustion in a grate furnace. Biomass &Bioenergy, 19, 199–208.

12 Saastamoinen, J., Taipale, R.,Horttanainen, M., and Sarkomaa, P.(2000) Propagation of the ignition frontin beds of wood particles. Combustionand Flame, 123, 214–226.

13 Thunman, H. and Leckner, B. (2001)Ignition and propagation of a reactionfront in crosscurrent bed combustion ofwet biofuels. Fuel, 80, 473–481.

14 Zhou,H., Jensen, A., Glarborg, P., Jensen,P., and Kavaliauskas, A. (2005) Numericalmodeling of straw combustion in a fixedbed. Fuel, 84, 389–403.

15 Yang, Y.B., Sharifi, V.N., and Swithenbank,J. (2004) Effect of air flow rate and fuelmoisture on the burning behaviors ofbiomass and simulated municipal solidwastes inpackedbeds.Fuel, 83, 1553–1562.

16 Yang, Y.B., Ryu, C., Khor, A., Yates, N.E.,Sharifi, V.N., and Swithenbank, J.

(2005) Effect of fuel properties onbiomass combustion. Part II. Modelingapproach - identification of thecontrolling factors. Fuel, 84, 2116–2130.

17 Yin, C., Rosendahl, L., Kær, S.K., Clausen,S., Hvid, S.L., and Hille, T. (2008)Mathematical modeling and experimentalstudy of biomass combustion in a thermal108 MW grate-fired boiler. Energy & Fuels,22, 1380–1390.

18 Tschanun, I. and Mineur, M. (2003)Biomass combustion with state of the artbubbling bed steam generators.Proceedings of the Power Gen Europe2003, 6–8 May 2003, D€usseldorf,Germany.

19 Bolh�ar-Nordenkampf, M., Gartnar, F.,Tschanun, I., and Kaiser, S. (2006)Combustion of biomass in bubblingfluidized beds - operation experiences.Proceedings of the 19th Conference onFluidized Bed Combustion, 21–24 May2006, Vienna, Austria.

20 Tschanun, I.,Holarek,M., Gartnar, F., andGlatzer, A. (2003)Westfield, Fife/Scotlandbecomes world�s first to burn poultry litterin FBC. Proceedings of the Power GenEurope 2001, 29–31 May 2001, Brussels,Belgium.

21 Bolh�ar-Nordenkampf, M., Gartnar, F.,Tschanun, I., and Kaiser, S. (2005)Operating experiences from FBC-plantsusing various biomass fuels.Proceedings of the 14th BioenergyConference, 17–21 October 2005,Paris, France.

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References j179

handbook of combustion (online) || staged combustion and exhaust gas recirculation in fluidized beds

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