Applied Energy 93 (2012) 606–613

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Applied Energy

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Effects of biomass particle size during cofiring under air-fired and oxyfuel conditions

Melissa L. Holtmeyer, Benjamin M. Kumfer, Richard L. Axelbaum ⇑Consortium for Clean Coal Utilization, Energy, Environmental & Chemical Engineering Department, Washington University in St. Louis, St. Louis, MO 63130, USA

a r t i c l e i n f o a b s t r a c t

Article history:Received 20 July 2011Received in revised form 4 November 2011Accepted 12 November 2011Available online 4 January 2012

Keywords:Coal combustionNOx

BiomassCofiringOxy-fuelOxy-combustion

0306-2619/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.apenergy.2011.11.042

⇑ Corresponding author.E-mail address: axelbaum@wustl.edu (R.L. Axelbau

Carbon capture and storage (CCS), when applied to biomass cofiring systems, can remove atmo-spheric CO2 since the CO2 that is consumed by the biomass during growth is not released back intothe atmosphere. Biomass cofiring can also potentially contribute to meeting renewable portfoliostandards (RPS), and result in reduced pollutant emissions, including sulfur oxides (SOx) and mer-cury. However, biomass fuels are widely variable in composition, particle size, and nitrogen content,which can make utilization of these fuels challenging. In this work, a numerical study was con-ducted for cofiring of pulverized coal and sawdust under air-fired and oxyfuel conditions to inves-tigate the effects of cofiring on flame length and nitric oxide (NO) formation. Previous experimentshave shown an increase in nitrogen conversion to NO when cofiring under both air-fired and oxy-fuel combustion, despite the fact that the sawdust cofired had less fuel-bound nitrogen. Computa-tional fluid dynamics (CFD) is used to determine the cause of the increased NO conversion and toidentify differences between air-fired and oxyfuel cofired flames. The simulations reveal that cofiredflames have longer volatile-flame regions (the flame envelope), and this length is influenced by theincreased volatile fraction and particle size associated with the biomass. Flame length theory forturbulent, non-premixed gaseous diffusion flames was found to be useful in interpreting theobserved results in both air-fired and oxyfuel combustion. Large biomass particles that are notentrained in the near-burner region breakthrough the flame envelope, and this was shown to bedetrimental to controlling NO formation. During oxy-cofiring combustion, particle breakthroughoccurs at smaller diameter, leading to increased nitrogen conversion to NO when compared toair-fired conditions. This is a direct result of a decreased flame envelope length and elevated oxygenconcentrations.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Carbon dioxide emissions are at unprecedented levels and areputting the public welfare at risk through climate change. An ap-proach to large-scale power generation is needed to significantlylimit emissions and even remove atmospheric CO2, while attempt-ing to minimize disruption to the existing power infrastructure. Adiverse power generation portfolio including both advanced fossilfuel and renewable energies is needed to reduce atmosphericCO2 to below 1990 levels [1]. Oxyfuel combustion with carbon cap-ture and storage (CCS) and biomass cofiring, individually and incombination, are both approaches to coal-fired power that cancontribute to this plan.

Oxyfuel combustion enables CCS by burning fuels with a combi-nation of oxygen and recycled flue gas instead of air. The flue gas isprimarily composed of carbon dioxide and water vapor. The wateris condensed out to create a concentrated CO2 stream for geological

ll rights reserved.

m).

storage. Oxyfuel combustion combined with geological storage is anear-zero emission technology that can be applied to both existingand new coal-fired power plants [2].

Biomass cofiring could reduce the overall CO2 emissions of theU.S. utilities sector by 100 million tons per year by displacing fossilfuel combustion with the near carbon-neutral combustion of bio-mass [3]. Another important challenge with respect to electricitygeneration that can be addressed with biomass is renewable port-folio standards (RPS). Recognizing that many regions do not pos-sess sufficient solar or wind resources to meet RPS targets,biomass cofiring with coal may play an important role in futurepower generation. Cofiring can also reduce formation of pollutantssuch as SOx and Hg, while the impact on NOx formation is variable[4]. Furthermore, by combining oxyfuel combustion and biomasscofiring with CCS, oxy-cofiring can be a potentially carbon negativetechnology since biomass consumes CO2 as it grows. Accordingly,this approach has been identified as a technology that can be uti-lized to stabilize atmospheric CO2 emissions [5]. Biomass cofiringis presently occurring around the world, while oxyfuel combustionis being demonstrated at the pilot-scale level, with plans for

Table 1Ultimate and proximate analyses.

PRB coal Sawdust

HHV (MJ/kg) (DAF) 29.7 20.2

Proximate analysis (wt.%) (dry)Ash 7.5 0.6Volatile matter 43.4 84.5Fixed carbon 49.1 14.9

Ultimate analysis (wt.%) (dry)Carbon 69.51 49.28Hydrogen 4.61 5.79Oxygen 17.02 44.14Nitrogen 0.97 0.15Sulfur 0.4 0.05

Table 2Particle size.

PRB coal Sawdust

Sieve analysis (wt.%) retained18 Mesh 0 030 Mesh <0.1 2.340 Mesh – –50 Mesh <0.1 36.370 Mesh – –100 Mesh 1.9–3.2 81.3200 Mesh 18–29 94.7

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full-scale demonstrations. At this time, little is known about theimpacts of oxy-cofiring on combustion performance and NOx for-mation. This work provides an understanding of flame structureand NO formation during cofiring under both air-fired and oxyfuelcombustion conditions.

The chemical composition and physical appearance of biomassfuels can vary significantly from coal, and the biomass itself canconsist of several different botanic fractions with wide variationsin composition [6,7]. Furthermore, biomass fuels prepared forcofiring typically have a larger particle size than coal, which can al-ter flame structure and combustion performance when cofiring. Asa consequence, NO formation, which is a complex function of mix-ing, temperature, fluid mechanics, and chemistry [8], can also beinfluenced. The relationship between particle size and NOx forma-tion has been investigated for coal combustion [9–11], as well asfor biomass cofiring when the biomass is used as a reburn fuel[12,13]. Cofiring studies conducted with biomass fuels havingvarying nitrogen content have shown that NOx formation doesnot scale with fuel-bound nitrogen [13,14]. Damstedt et al.[15,16] first showed that large biomass particles penetrate throughthe near burner internal recirculation zone leading to volatile re-lease further downstream and a longer fuel-rich zone. The releaseof volatile-nitrogen was also delayed, which in this case, led to anin-flame reburn effect reducing NO to N2.

This work aims to understand changes in flame structure, aswell as the effects on NO formation, when cofiring biomass inair-fired and oxyfuel conditions through a systematic study fo-cused on biomass particle size, volatility and nitrogen content.Experimental results have been previously reported by this group[14]. In the present work, the dependence of flame structure andNO formation on biomass particle size and cofiring ratio is ex-plored numerically, to provide a detailed analysis of the flamestructure and a better understanding the observed experimentalresults.

2. Summary of experimental findings

In a previous work, experiments were conducted in a cylindri-cal, horizontally-fired 30 kWth combustor with a 14 cmID � 78 cm combustion section, followed by a 37 cm ID � 120 cmburnout section [14,17]. The secondary oxidizer was introducedtangentially, creating a swirling flow to assist in flame stabiliza-tion. Pulverized coal and sawdust (wood waste) were fed to thecombustor using separate volumetric screw feeders (K-Tron andSchenk AccuRate) and were entrained into the primary oxidizer(PO) stream using an eductor. The two feeder outlets and the educ-tor were contained within an enclosure maintained at atmosphericpressure to prevent air ingress, and the gas flowing into the fuel in-let of the eductor was controlled and measured. Under oxyfuelconditions, flue gases were not recycled; rather, industrial gradeCO2 was used in a once-through fashion to determine the effectsof N2 replacement with CO2. The CO2 was mixed with O2 beforeentering the burner, at a ratio of 30 vol.% O2/70 vol.% CO2.

Subbituminous Powder River Basin (PRB) coal and sawdust ob-tained from a local sawmill were utilized in this study. Sieve, prox-imate, and ultimate analyses are provided in Tables 1 and 2. Somevariation in the coal particle size distribution was observed overthe course of experiments as indicated in the table. As obtainedfrom the sawmill, the bulk sawdust contained 21 wt.% moisture,which led to feeding difficulties; however, after reducing the mois-ture content to 16 wt.% by exposing the sawdust to laboratoryroom air, successful feeding was achieved.

Skeen et al. [14] experimentally investigated the effects of vary-ing cofiring ratio on NO emissions during air-fired and oxyfuelcombustion. In all experiments, conditions were set such that the

thermal input was 30 kW and the exhaust gas oxygen concentra-tion was maintained at 3 vol.% O2. From this work, N conversionto NO increased with cofiring ratio, resulting in higher than ex-pected NO emissions based on fuel nitrogen content, during bothair-fired and oxyfuel combustion. For a 40% cofiring ratio, N con-version increased by 14% and 32% for air-fired and oxyfuel combus-tion, respectively, compared to the base case of coal only. Since thesawdust has less fuel-bound nitrogen, one might expect a decreasein NO emissions. Only when biomass particle size was reduced bysieving with a 50 mesh sieve was a reduction in NO observed underair-fired conditions. With reduced biomass particle size and 40%cofiring ratio, NO emissions decreased by 15%.

During oxyfuel conditions, NO was reduced over the air-firedcase, largely due to the elimination of the thermal-NOx mechanism.However, unlike the air-fired case, the NO emissions remainedconstant under oxyfuel even when cofiring the sieved biomasswith reduced particle size. A goal of this work is to use computa-tional fluid dynamics (CFD) to understand these trends with a fo-cus on the effects of biomass particle size and the biomass fuel-fraction on flame structure.

3. Numerical methods

ANSYS FLUENT version 13.0 was used for this study. This pro-gram allows for user-defined modifications to the sub-models toinclude both coal and biomass combustion tracking. Transportequations were solved for the continuous phase in the Eulerianframe of reference, while simulating the discrete phase in theLagrangian frame. Second-order upwind algorithms were used tosolve the continuity, momentum, and species equations. Coal andbiomass reactions were modeled using the species transport sub-model. The finite-rate/eddy dissipation model, wherein both theArrhenius and eddy-dissipation reaction rates are calculated, wasapplied for turbulent chemical reactions. The SST k–x turbulencemodel was implemented because of its abilities to capture thebehavior of swirling flows. The computational domain was atwo-dimensional axisymmetric computational mesh with 40,000quadrilateral mesh elements. Mesh independence was checked

Table 3Inlet flow rates for air-fired and oxyfuel combustion.

Inlet Air Oxyfuel: 30% O2/70% CO2

Primary (m3/h) 4.4 4.4Secondary (m3/h) 29 18.1

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by computing solutions on several meshes including coarse,adapted, and fine meshes.

All discrete particles were assumed spherical. Therefore, thedrag force term was based only on the equivalent spherical particlediameter for both coal and biomass. Intra-particle heat and masstransfer were not included in the discrete phase particle model.These effects are thought to be of secondary importance [18].

Boundary conditions were specified based on those of theexperiments. The primary oxidizer (PO) inlet was specified as amass-flow inlet with the fuel injection specified to maintain30 kW thermal input for both coal-only and cofiring cases. The sec-ondary oxidizer (SO) inlet was specified as a velocity-inlet with ax-ial and swirl components, while ensuring that the overall massflow of air or O2/CO2 was maintained. Table 3 shows the flow ratespecifications for air-fired and oxyfuel combustion. The walls of thecombustion chamber were maintained at 1000 K.

The numerical model was used to examine trends during cofir-ing in both air-fired and oxyfuel combustion conditions. Thenumerical results have shown reasonable agreement with theexperimental results for NO reported in Skeen et al. [14], whichwill be discussed later, and with centerline temperature measure-ments for coal-only flames in air-fired conditions.

3.1. Radiation model

The Discrete Ordinates radiation model was implemented forboth oxy- and air-fired combustion. When coal or biomass arecombusted in an environment of O2 and recycled flue gas, the con-tribution to the total radiation from the gases increases [19], whichcan potentially affect ignition, stability, temperature, and pollutantformation. Carbon dioxide and H2O are radiatively active, and atthe higher concentrations associated with oxyfuel conditions, theycan alter the radiative properties of the gas. Consequently, for oxy-fuel combustion, modifications to the absorption coefficient weremade using a user-defined function.

An Exponential Wide Band Model (EWBM) [20] is used as auser-defined function to the discrete ordinates radiation model inANSYS Fluent to predict the radiative properties of gas mixturescontaining: H2O, CO2, CO, CH4, NO, and SO2. The EWBM uses theintegrated ban intensity, line-width to spacing parameter, andthe bandwidth parameter to describe the total gas band absorp-tance and to capture the transitions between vibrational and rota-tional energy states, which provide the greatest contribution to theabsorption coefficient.

Particle radiation can be a major contributor to radiation heattransfer during solid fuel combustion originating from either coalparticles (coal, char, ash) or soot. However, the focus of this workis on the volatile flame envelope. In this zone, particles are under-going devolatilization, where temperatures and particle radiationare low compared to the downstream. Therefore, particle radiationeffects are neglected.

3.2. Coal devolatilization

Coal particle devolatilization rate was modeled by the ChemicalPercolation Devolatilization (CPD) model [21]. Detailed mecha-nisms of bridge breaking and rearranging with the coal latticestructure, tar evolution, light gas release, and cross-linking were

included. The input parameters for the CPD sub-model were esti-mated using a chemical structure correlation, with 13C NMRparameters, based on the ultimate and proximate analysis of theparent coal.

The chemical properties of the volatile matter species wereapproximated from the ultimate and proximate analyses [22].Three global homogeneous reactions were modeled to capturethe gaseous volatile reaction, including a global mechanism for vol-atile oxidation, the conversion of CO to CO2, and the oxidation ofhydrogen to water vapor.

volþ xO2 ! aCOþ bH2 þ cN2 ð1Þ

COþ 12

O2 ! CO2 ð2Þ

H2 þ12

O2 ! H2O ð3Þ

3.3. Coal char combustion

For solid particle (char) surface oxidation, a multiple-surface-reaction char model was employed. The following three reactionswere assumed to occur as the gas-phase oxidizer diffuses to theparticle surface and into the pores.

CðsÞ þ 12

O2 ! CO ð4Þ

CðsÞ þ CO2 ! 2CO ð5Þ

CðsÞ þH2O2 ! COþH2 ð6Þ

Once near the surface, the gas is adsorbed and reaction occurs.Global kinetics for the char reaction rate, represented as Arrheniusexpressions, were based on the activation energies, reaction orders,and pre-exponential factors assumed in [23]. Since the heteroge-neous reactions can be either controlled by chemical reaction ordiffusion, a model of diffusive transport of oxidizer through a por-ous particle is included.

3.4. Biomass devolatilization

The rate of devolatilization of biomass particles was describedusing a single-rate kinetic model,

�dmp

dt¼ k½mp � ð1� fv ;0Þð1� fw;0Þmp;0� ð7Þ

where mp, k, fv,0, fw,0, and mp,0 are the mass of the particle, the ki-netic rate, the mass fraction of volatiles initially present in the par-ticle, the mass fraction of evaporating species initially present in theparticle, and the initial mass of the particle, respectively. The kineticrate, k, was defined by an Arrhenius relationship. The kinetic rateparameters for sawdust were derived from thermogravimetric anal-ysis using a Hi-Res TGA (TA Instruments Inc.). The pre-exponentialfactor, A, and the activation energy, E, were found to be 2.0E+06 s�1

and 1.8E+07 J/kmol, respectively. Determination techniques accord-ing to [24] for the activation energy and [25] for the pre-exponentialfactor were used to develop the Arrhenius form of the equation. Al-beit this model is notably simple, an understanding and comparisonof the basic trends between air-fired and oxyfuel combustion condi-tions are desired, and the model is sufficiently detailed to addressthis.

The sawdust discrete phase fuel injection was composed of twocomponents: volatile-matter species and combusting (char) parti-cle. The composition of the sawdust volatiles was approximatedfrom the ultimate and proximate analyses, similar to the coal

M.L. Holtmeyer et al. / Applied Energy 93 (2012) 606–613 609

volatile species. The homogeneous volatile reaction was set up in amanner similar to that of the coal volatile oxidation.

3.5. Biomass char combustion

It has been shown that during sawdust char oxidation the ratesof chemical reaction and gas diffusion through pores are compara-ble [26]. Therefore, a kinetics/diffusion-limited surface combustionmodel was chosen based on Baum and Street [27] and Field [28]. Inthis model the rates of diffusion of oxidizer and the kinetics of thechemical reaction on the surface are calculated, to determinethe limiting process. The particle diameter remains constant butthe density can change based on combustion conditions.

Fig. 1. Temperature contours (K) for 30 kW (a) 20% cofiring flame with bulksawdust, (b) coal only. (c) Volatile reaction with oxygen (kg mol/m3 s) for coalflame. Arrows denote flame envelope length.

3.6. NOx model

As NOx is a trace species, its presence does not significantly af-fect the calculation of the combustion solution or evolution ofother species within the CFD model. Therefore, the post-processingNOx model in ANSYS Fluent 13.0 was used. The model for homoge-neous NOx formation is divided into two computations for fuel andthermal NO, which included turbulent interactions for both oxygenand temperature. Prompt NO was not considered in this work sinceits total contribution is expected to be small, approximately 5%[29].

Fuel-bound nitrogen can either be released during devolatiliza-tion (referred to volatile-N), or can remain the in char to react fur-ther downstream (referred to as char-N). Nitrogen partitioning incoal particles is based on the methods in [9,30], where 40% is vol-atile-N and the rest is char-N. Upon being released from the parti-cle, volatile-N reacts with the surrounding gas to form nitrogenintermediates, including NH3 and HCN. The location of releaseand the local conditions will determine which species are formed.For this work, volatile-N is assumed to evolve as 60% NH3 and 40%HCN [31]. Char particles can either produce or consume NO on thesurface. NO production from char-N is assumed to form from HCNintermediates [32]. The intermediate species can react with oxygento form NO. Some NO maybe be reduced downstream throughreactions with other intermediates or hydrocarbon species.

Biomass particle nitrogen partitioning, which is based on themethods of Jiachun et al. [33], is assumed to be 70% volatile-Nand 30% char-N. Both volatile-N and char-N is assumed to reactto form NH3 intermediates.

Thermal-NO is formed from N2 that enters with the combustionair by breaking the N2 bond, which is favored at high temperature[34]. The mechanism assumed for formation is the Zel’dovichmechanism using the partial equilibrium approach for radical Oand OH concentrations.

Fig. 2. Temperature contours (K) for 20% cofiring flames with (a) bulk sawdust and(b) sawdust with the same particle size distribution as coal. Arrows denote flameenvelope length.

4. Numerical results and discussion

4.1. Cofiring impacts on flame length

Current coal-fired systems have been optimized to produce sta-ble, low emission flames. The impacts of adding biomass are dem-onstrated in Fig. 1 for 20% cofiring of sawdust during air-firedconditions. The high temperature zone in the cofiring flame, shownin Fig. 1a, is noticeably longer than that in the coal flame, shown inFig. 1b. Burning particles were also experimentally observed fur-ther downstream during cofiring.

For this study, the flame length is defined as the high tempera-ture zone that is controlled by the volatile reaction with oxygen,which is shown in Fig. 1c. This will be referred to as the flame enve-lope. Black arrows on temperature contours denote the end of the

flame envelope and indicate the increase in size of the cofiredflame.

Damstedt et al. [16] attributed longer flame zones observedduring experiment when cofiring straw to the denser knee parti-cles that could not be entrained in the near burner zone. These par-ticles differ from coal particles in both fuel composition andparticle size. To understand what is controlling the increase inflame envelope length during cofiring, simulations were run tocompare two cofired flames with different particle size distribu-tions under air-fired conditions. Fig. 2a is a 20% bulk sawdust co-fired flame, as was previously shown, while, Fig. 2b is a 20%cofired flame with sawdust that has the same particle size distribu-tion as that of coal.

By cofiring sawdust that has the same particle size distributionas that of coal, the effects of fuel composition can be separatedfrom particle size effects. The flame envelope lengths for the twoflames shown in Fig. 2 are nearly identical. This comparison showsthat the increase in flame envelope length observed when cofiringsawdust is due to changes in the fuel composition, specifically theincrease in volatile fraction. From Table 1, sawdust has approxi-mately double the volatile matter as that of the coal. To furtherinvestigate this, the results are interpreted in light of combustiontheory for gaseous non-premixed turbulent jet flames.

According to Delichatsios [35], flame length for gaseous non-premixed turbulent jet flames can be expressed as

Lf � aðfsÞ�1 ð8Þ

where fs is stoichiometric mixture fraction and a � L�dj(qe/q1)1/2. L�

is dimensionless flame length, dj is initial jet diameter, and qe/q1 is

Fig. 4. Centerline temperatures for 20% cofiring flames with sawdust of varyingparticle sizes including the same particle size distribution as coal (solid line),600 lm monodisperse (---), and 1 mm monodisperse (-�-).

610 M.L. Holtmeyer et al. / Applied Energy 93 (2012) 606–613

the ratio of nozzle fluid to ambient gas density. For flames in themomentum-controlled regime, the value for the dimensionlessflame length is constant [35]. The stoichiometric mixture fractionis defined as

fs � 1þ YF;OWOvO

YO;OWFvF

� ��1

ð9Þ

where Yi,O is mass fraction for fuel (F) and oxidizer (O) at the inlet,Wi is molecular weight, and vi is stoichiometric coefficient. Substi-tuting Eqs. (9) into (8), flame length can be defined in terms ofthe mass fractions of fuel and oxidizer,

Lf � a bYF;O

YO;Oþ 1

� �ð10Þ

where b is (WOvO/WFvF). From this equation it is clear that flamelength is linearly dependent on fuel stream mass fraction. For coaland cofired flames, the ‘‘gaseous fuel’’ can be considered the vola-tiles that have been released, and the flame length refers to thelength of the flame envelope. For coal-only flames the volatile massfraction in the primary is 0.21 and for the 20% cofiring flames it is0.27, which is a 29% increase. Similarly, according to Eq. (10), theflame length increases by 29%. Therefore, presuming that all ofthe volatiles are released within the flame envelope, an increasein flame envelope length would be expected during cofiring basedon the increase in volatile fraction.

Since the mean size of the biomass particles is usually largerthan that of coal, the effects of particle size on the flame envelopelength were investigated by systematically varying biomass parti-cle size using monodisperse particle size distributions. As will beseen, the choice of a monodisperse size distribution allows for aconvenient interpretation of the effects of particle size.

4.2. Particle size impacts on flame envelope length

Fig. 3 compares the changes to the flame structure for three30 kW cofired flames in air-fired conditions. Fig. 3a is a flamewhere the sawdust has the same particle size distribution as coal.Fig. 3b is a flame with a monodisperse particle size distribution of600 lm sawdust particles and Fig. 3c is a flame with a monodis-perse particle size distribution of 1 mm sawdust particles. Allflames consist of 20% biomass by weight.

Locations of decreased centerline temperatures within theflame envelope in Fig. 3b and downstream of the flame envelopein Fig. 3c correspond to the endothermic devolatilization of thesawdust particles. When comparing Fig. 3a to Fig. 3b, the flameenvelope length is similar even though the biomass particle sizedistribution has changed dramatically. Fig. 3c shows that whenthe 1 mm biomass particles breakthrough the flame envelope,

Fig. 3. Temperature contours (K) for 20% cofiring flames with (a) sawdust with thesame particle size distribution as coal, (b) sawdust with a monodisperse particlesize distribution diameter of 600 lm, and (c) sawdust with a monodisperse particlesize distribution diameter of 1 mm.

the flame envelope length decreases. In this case volatiles are re-leased farther downstream, thereby decreasing the volatile fractionin the near burner region. Consistent with the theory presented inSection 4.1, a decrease in volatile fraction in the near burner regionresults in a decreased flame envelope length. This may have a sig-nificant impact on NOx formation, as will be shown later.

As shown in Fig. 4, the location of devolatilization of the mono-disperse biomass particles is very pronounced along the centerline.The 600 lm and 1 mm particles are too large to be entrained by therecirculation zone at the exit of the primary and penetrate throughalong the centerline.

Fig. 4 also demonstrates that if volatile release occurs within theflame envelope, then the location of peak temperature remainsnearly the same. This can be also seen in Fig. 3 by comparingthe centerline temperature for the case of a distributed sawdust

Fig. 5. (a) Theoretical particle size distribution of sawdust to truncate and (b)centerline temperatures for 20% cofiring flame.

Fig. 6. Contours of NO mole fraction for (a) coal flame and 20% cofiring flames with(b) sawdust with the same particle size distribution as coal, (c) sawdust with amonodisperse particle size distribution diameter of 600 lm, and (d) sawdust with amonodisperse particle size diameter of 1 mm.

Fig. 7. Temperature contours (K) for cofiring flames with bulk sawdust for varyingcofiring ratios including (a) 20% cofiring and (b) 40% cofiring.

M.L. Holtmeyer et al. / Applied Energy 93 (2012) 606–613 611

particle size distribution, with that of a uniform size of 600 lm. Onthe other hand, for the 1 mm biomass particle case, volatile releaseoccurs beyond the flame envelope, and the flame envelope lengthis shorter and the peak temperature has shifted closer to theburner.

By utilizing the endothermicity associated with volatile release,biomass particle size may be selected in order to strategically de-crease the peak temperature. For example, Fig. 5a shows how thecenterline peak temperature can be truncated by selecting the bio-mass size distribution shown in Fig. 5b. This size distribution wasselected so that the region of devolatilization associated with largebiomass particles aligned with the region of peak centerline tem-perature. In this configuration the biomass particles have largeaxial momentum, thus devolatilization occurs along the centerline.Alternative trajectories for the biomass particles are possible withproper burner design, and thus, this approach can serve as a meth-od of controlling peak temperatures in select region of the flame.

4.3. NO formation

Fig. 6 shows mole fraction contours for NO in coal-only andcofiring flames. Net NO formation during cofiring is a consequenceof the combined effects of production and destruction. The initialzone of production is due to volatile-N reaction with oxygen inthe primary, fuel-carrying stream. Downstream of this initial pro-duction zone, where the NO mole fraction is negligible, corre-sponds to NO destruction. At this location in the combustionchamber, high concentrations of volatile species are releasedresulting in a fuel-rich, oxygen-depleted zone. NO that has previ-ously formed can interact with these and other nitrogen containingspecies to reduce to N2. NO production is also occurring along theoutside of these zones near the walls due to char-N and thermal-NO formation during air-fired conditions.

Fig. 6a shows NO mole fraction contours in a coal flame. Highconcentrations of NO are produced early in the combustion cham-ber primarily due to the early formation of HCN from volatile-N.NO production also occurs outside the zone of NO destruction pri-

Table 4Summary of NO emissions from air-fired combustion conditions.

Fuel Fuel feedrate (kg/h)

N content(wt.%)

E[

Coal 4.7 0.98 910 wt.% Cofiring 4.8 0.90 840 wt.% Cofiring 6.0 0.64 8

marily due to char-N conversion and thermal-NO. Fig. 6b showsthat when 20% of the fuel is substituted with sawdust having thesame particle size distribution as coal, NO production decreasesand the zone of NO destruction enlarges leading to a 30% decreasein NO emissions when compared to the coal-only case. Whenincreasing the particle size, as in Fig. 6c, the size of the initial pro-duction zone slightly decreases, but the destruction zone is moredistributed since the biomass particles are devolatilizing later,resulting in only a 14% decrease in NO emissions over the coal-onlycase. For 1 mm size sawdust particles, NO emissions are nearly thesame as the coal-only case due to late devolatilization outside theflame envelope into regions of higher oxygen and temperature asshown in Fig. 6d. At the extremes shown in Fig. 6b and d, small bio-mass particles enhance NO destruction, while large biomass parti-cles can produce NO downstream of the regions of destruction.

4.4. Cofiring ratio

The previous analysis of biomass particle size was completedusing a 20% cofiring ratio, which is similar to that currently usedin selected industrial boilers [36], but as RPS continue to requirehigher percentages of renewable power and regulations becomemore stringent, burning larger percentages of biomass may bedesirable. In the following, the effects of increasing the cofiring ra-tio to 40% are assessed.

Fig. 7 shows the effects of the increased cofiring ratio on flameenvelope length in air-fired conditions when cofiring bulk sawdust.As shown in Fig. 7b, with the higher percentage of sawdust, theflame envelope length increases over that of the 20% cofiring caseshown in Fig. 7a. As expected from Section 4.1, the flame envelopelength increases with higher cofiring ratio due to the larger amountof volatiles. The volatile mass fraction in the primary for 40% cofir-ing is 0.38, compared to 0.27 for the 20% cofiring case, an increaseof 40%. As anticipated, from Eq. (10) the flame length increasesabout 40% as well.

With increased cofiring ratios, there is a greater fraction of bio-mass particles in the flame envelope and thus the envelope flamelength increases. Also, there are a greater number of large biomassparticles that penetrate the flame envelope and devolatilize in thehigh temperature zone. As such, a longer, extended region of lowtemperature along the centerline is maintained. These changes inflame envelope length and particle breakthrough during high cofir-

xperiment14] NO (ng/J)

PredictedNO (ng/J)

Increase in experimentalN conversion to NO (%)

2 90 –9 90 39 83 14

612 M.L. Holtmeyer et al. / Applied Energy 93 (2012) 606–613

ing ratios also impact NO formation. A summary of NO emissionsfrom air-fired conditions for both experimental [14] and predictedresults with varying cofiring ratios for bulk sawdust is shown inTable 4.

The ‘‘N Content’’ listed in Table 4 is the overall fuel-bound nitro-gen content of the fuel mixture, which decreases during cofiring. Inorder to maintain the same thermal input for all flames, the overallfuel input is higher for the cofiring cases since the heating value ofthe sawdust is lower than that of the coal. Given the much lowernitrogen content of the sawdust, one might expect that the NOconcentration in the exhaust would be less when cofiring. None-theless, the NO emissions remained nearly constant. Consequently,the conversion of N to NO is found to increase with cofiring ratio.Aside from the impacts of particle size on NO formation, the ratioof NH3/HCN released from biomass devolatilization is generallyhigher than that of coal, which may also lead to increased conver-sion [37,38].

With ANSYS Fluent one is able to evaluate the contribution ofthermal NO individually. Analysis of the numerical results indi-cates that changes in thermal-NO during cofiring do not signifi-cantly contribute to the increases in N conversion during air-fired conditions.

4.5. Oxy-cofiring combustion

The effects of biomass particle size are investigated in oxyfuelconditions with an oxidizer composition of 30 vol.% O2/70 vol.%CO2. The thermal input is maintained at 30 kW and the exhaustO2 at 3 vol.% for all flames. These constraints impact the primaryoxidizer (PO) and secondary oxidizer (SO) inlet conditions, andthus, the flame structure. Table 3 shows a 17% decrease in SO inletflow rate when the PO flow rate is maintained equal to that of air-fired conditions. The impact of the O2/CO2 environment and re-duced flow rates can be seen by comparing the temperature con-tours for air-fired conditions (Fig. 8a) with those of oxyfuelconditions (Fig. 8b). Both cases include 20% cofiring with sawdustthat consists of 600 lm monodisperse particles.

The flame envelope length in the oxy-cofiring flame is shortercompared to air-fired conditions. This is a result of the reducedSO flow rate and increased oxygen concentrations. Flame envelopelength, as calculated in Eq. (10), is inversely proportional to oxygen

Table 5Summary of NO emissions from oxyfuel combustion conditions.

Fuel Fuel feedrate (kg/h)

N content(wt.%)

Coal 4.9 0.9710 wt.% Cofiring 4.8 0.9040 wt.% Cofiring 6.0 0.64

Fig. 8. Temperature contours (K) for 20% cofiring during (a) air-fired and (b) oxyfuelcombustion conditions.

concentration. Therefore, for a similar volatile mass fraction, onewould expect a shorter flame envelope under oxyfuel combustionconditions. Other studies have also demonstrated reduced flamelengths under oxyfuel combustion conditions [39,40].

Sawdust particle breakthrough also impacts the flame envelopelength during oxyfuel combustion. Under air-fired conditions par-ticle breakthrough is not observed until the monodisperse particlesize reaches 1 mm in diameter. For oxyfuel conditions, particlebreakthrough is observed for a particle size of 600 lm. Therefore,biomass particles can more easily breakthrough the oxyfuel com-bustion flame envelope and this will lead to devolatilization in re-gions of higher temperature and oxygen concentrations, which waspreviously shown to increase net NO formation. Table 5 summa-rizes NO emissions for oxyfuel combustion for both experimentaland predicted results with varying cofiring ratios utilizing bulksawdust.

Comparing the experimental results in Tables 4 and 5, de-creased NO emissions are observed for all oxyfuel combustioncases, which can be attributed to the elimination of thermal-NOx

since N2 is eliminated from the oxidizer. The numerical resultsfor NO emissions are consistently lower than that of the experi-mental measurements; however, the trends are reproduced. Stud-ies were also performed with a higher cofiring ratio and, similar tothe air-fired cases, as the cofiring ratio increases the NO emissionsremain relatively constant. Increased nitrogen conversion to NO isnot only a result of the shorter flame envelope, but also due to in-creased particle breakthrough, when compared to air-fired condi-tions. When cofiring higher percentages of biomass, a greaterquantity of large particles devolatilizes outside of the flame enve-lope leading to greater NO conversions.

5. Conclusions

Cofiring of biomass can help regions meet RPS, while reducingemissions of CO2, SOx and Hg. However, the variability of biomassmakes these fuels a challenge to work with. Three characteristics ofbiomass were varied in this study: volatile fraction, particle sizeand nitrogen content. The effects of these variables on flame struc-ture and the resulting impacts on NO formation in air-fired andoxyfuel conditions.

The flame envelope length, which is the high temperature re-gion controlled by the reaction of volatiles with oxygen, was influ-enced by both the volatile fraction and particle size. The cofiringflames had longer flame envelope lengths when compared tocoal-only flames due to the increased volatile fraction associatedwith biomass. Shorter flames were predicted during oxyfuel com-bustion. By systematically varying biomass particle size usingmonodisperse particle size distributions, particle breakthrough oflarge biomass particles was shown to impact flame envelopelength and centerline peak temperatures. Furthermore, biomassparticle size distribution has been shown to be potentially usefulin controlling peak temperatures in selected regions by utilizingthe endothermicity associated with volatile release and the largeinertia of the particles to direct the particles to the location ofinterest. Large biomass particle breakthrough also resulted in in-creased nitrogen conversion to NO, which is consistent with earlier

Experiment[14] NO (ng/J)

PredictedNO (ng/J)

Increase in experimentalN conversion to NO (%)

59 43 –60 44 1462 46 32.5

M.L. Holtmeyer et al. / Applied Energy 93 (2012) 606–613 613

experimental results. Impacts on flame envelope length and NOformation were magnified at higher cofiring ratios.

Nitrogen oxide formation during oxyfuel combustion combinedwith biomass cofiring was also evaluated. Limits on trace speciesduring CCS have yet to be determined, so the impacts of biomasson NO formation must be understood. When oxyfuel combustionflames were compared to air-fired flames, the flame envelopelength was shorter due to elevated oxygen concentrations and de-creased flow rates. This resulting change in flame structure wasshown to be detrimental to NO formation. Due to the smaller flamelength, the breakthrough of large biomass particles occurred atsmaller particle diameters (600 lm compared to 1 mm for air-firedconditions). Therefore, a larger number of particles released theirvolatiles and volatile-N in areas of higher temperature and oxygenconcentrations, leading to increased nitrogen conversion to NO.However, overall NO formation during oxyfuel combustion is re-duced compared to air-fired conditions due to the elimination ofthe thermal-NOx mechanism.

Acknowledgments

The authors gratefully acknowledge the financial support of theConsortium for Clean Coal Utilization at Washington University inSt. Louis. M.L.H. was supported in part by a U.S. EPA STAR Fellow-ship. The authors would also like to thank the following organiza-tions: Ameren, for providing coal and fuel analyses and ANSYS forproviding the UDF for oxyfuel combustion radiation.

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