Biomass Suspension Combustion: Effect of Two-Stage Combustionon NOx Emissions in a Laboratory-Scale Swirl Burner

Weigang Lin,* Peter A. Jensen, and Anker D. Jensen

Department of Chemical and Biochemical Engineering, Technical UniVersity of Denmark, Building 229,Lyngby DK-2800, Denmark

ReceiVed June 20, 2008. ReVised Manuscript ReceiVed September 22, 2008

A systematic study was performed in a suspension fired 20 kW laboratory-scale swirl burner test rig forcombustion of biomass and co-combustion of natural gas and biomass. The main focus is put on the effect oftwo-stage combustion on the NO emission, as well as its effect on the incomplete combustion. When two-stage combustion was applied, the NO emission level can be significantly reduced. The experimental resultsshow that an optimal first-stage combustion stoichiometry (λ1) exists, at which a minimum NO emission canbe achieved. An optimal stoichiometry of around 0.8 in the fuel-rich zone exists with respect to minimizingNO emissions. When using wood and straw as co-firing fuels, 15-25% of the fuel-N is converted to NO.Straw appears to give the lowest conversion of fuel-N to NO. The results indicate that the optimal stoichiometryin the fuel-rich (λ1) zone for gaining the lowest NO may result from the homogeneous reaction, by comparingthe NO emissions when firing natural gas with NH3 addition and co-firing natural gas and biomass. Theexperimental results also show no significant increase of incomplete combustion of gas and char by applyingoptimized two-stage combustion.

Introduction

The consequences of carbon dioxide (CO2)-induced globalwarming cause major concern worldwide. The consumption ofenergy produced with fossil fuels is the major factor thatcontributes to the global warming. Biomass is a renewableenergy resource and has a nature of CO2 neutrality. In the last2 decades, the amount of biofuels consumed for power genera-tion has increased significantly. In the United States, alreadymore than 9 GWe capacity of power generation based onbiomass has been installed.1,2 In the European Union, it ispredicted that the biomass-based power generation capacity willincrease from 7.8 GWe in 2001 to 26.5 GWe in 2010.3 InDenmark, more than 1.3 million tons of wood and 750 000 tonsof straw were used in power plants in the year 2001.4 Accordingto the Danish energy policy, the part of biomass in the energysupply pattern will continue to expand in the future. The use ofwood and straw in Denmark will reach 9 million tons (105 PJ)by the year 2030 according to the plan “Energy 21”.5 Until 2002,Denmark has 50 combined heating and power (CHP) plantsoperating on wood chips, 25 on wood pellets, and 75 straw-

fired plants.6 Many of the biomass-fired plants are grate boilers,but suspension firing of biomass have been used recently. Therehas then been an increasing demand to have a higher flexibilitywhen firing biomass fuels in terms of load flexibility whilemaintaining high power efficiency, and biomass suspensionfiring can fulfill these requirement. An example of a co-firingpower plant is the Danish Avedøreværket unit 2 (AVV2). TheAVV2 unit, with ultra-super-critical (USC) steam parametersis a multifuel power plant for co-firing natural gas, oil, andpulverized wood pellets with a net electricity efficiency of upto 47%.7 However, suspension combustion of biomass is stillin an initial stage, and problems, such as ash deposition,corrosion, emission, and ash disposal may arise. Among theproblems, this work will investigate the NO emission andcombustion efficiency when co-firing wood, straw, and naturalgas. It is known that when co-firing the woody biomass andnatural gas on the AVV2 boiler, the emission of NO is normallyat a low level. However, it occurs that the NO emission mayexceed the emission limit level occasionally when the SCR unitis not in operation.8 Also, firing or co-firing biomass may resultin a fast deactivation of SCR catalysts.9,10 The replacement orregeneration of the SCR catalyst is costly. Thus, it is desirableto apply primary methods for minimizing NO emissions.

The staged combustion has been proven an effective primarymethod to reduce the NOx emissions in the suspension combus-

* To whom correspondence should be addressed. Telephone: +45-45252835. Fax: +45-45882258. E-mail: wl@kt.dtu.dk.

(1) Bain, R. L.; Amos, W. P.; Downing, M.; Perlack, R. L. Highlightsof Biopower Technical Assessment: State of the Industry and the Technol-ogy, National Renewable Energy Laboratory, Golden, CO, April 2003; p39.

(2) Veringa, H. J. Advanced Techniques for Generation of Energy fromBiomass and Waste, Energy Research Center of the Netherlands (ECN),Petten, The Netherlands, 2005; p 24.

(3) Insights, B. The Future of Global Biomass Power Generation: TheTechnology, Economics and Impact of Biomass Power Generation, StrategicMarket Report, Research and Markets, Dublin, Ireland, 2004.

(4) Schultz, G. Securing of supply in short and longer term of woodand straw. In 2001 Task Meeting of IEA Bioenergy Task 30, Denmark,2001.

(5) Larsen, I. Renewable EnergysDanish Solutions, Danish EnergyAuthority, Copenhagen, Denmark, 2003.

(6) IEA Energy Policies of IEA CountriessDenmark 2002 Review.Organisation for Economic Co-operation and Development (OECD)/International Energy Agency (IEA), 2002; p 132.

(7) Ottosen, P.; Gullev, L. Avedøre unit 2sThe world’s largest biomass-fuelled CHP plant. News from DBDH, 2005.

(8) Jensen, J. P. Personal comunication, 2006.(9) Zheng, Y. J.; Jensen, A. D.; Johnsson, J. E. Appl. Catal., B 2005,

60 (3-4), 253–264.(10) Kling, A.; Andersson, C.; Myringer, A.; Eskilsson, D.; Jaras, S. G.

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Energy & Fuels 2009, 23, 1398–14051398

10.1021/ef8004866 CCC: $40.75 2009 American Chemical SocietyPublished on Web 02/27/2009

tion system.11-23 In addition to the aerodynamic air staging (lowNOx burner), two-stage combustion, in which fuel-rich and fuel-lean zones of combustion are segregated through externaldivided air ports,13 is widely applied. It has been reported thatthe minimum NOx level is observed with two-stage combustionfor firing gas11,12,18 and firing coal in the suspension combustionsystem.14,15,22 The degree of NOx reduction by two-stagecombustion and the optimal stoichiometry of the fuel-rich zoneconsiderably depend upon the type of fuels. However, it wasreported that the unburned carbon content in the fly ash increaseswhen measures are taken for reducing the emission of NO inpulverized coal-fired boilers.22,24-26 However, to the knowledgeof the authors, no detailed information is available with respectto the influence of two-stage combustion on the NO emissionsand unburned carbon in fly ash for suspension combustion ofbiomass and co-combustion of biomass and natural gas.Although limited investigations are published on co-firing coal

and biomass27-30 and co-firing biomass and natural gas,31 nosystematic results with respect to the effect of two-stagecombustion on the NOx emissions and burnout behaviors arereported.

The main objective of this work is to study experimentallythe emissions of NOx and CO, as well as the unburned carbonin fly ash from combustion of biomass and co-combustion ofbiomass and natural gas, with a focus on the influence of two-stage combustion and optimal first-stage stoichiometry. Experi-ments were performed in a laboratory-scale 20 kW swirl burnerfiring wood, straw, and natural gas for providing knowledge ofhow to reduce NOx emissions without the cost of combustionefficiency.

Experimental Section

Laboratory-Scale 20 kW Test Rig. Experiments were carriedout in a suspension fired 20 kW laboratory-scale swirl burner testrig, whose diagram is shown in Figure 1. The system consists of afuels and air dosing part, a combustion unit, a flue gas processingpart, and sampling, analytical, and data acquisition system. Thecombustion air is divided into three streams: primary air for feedingthe solid fuel, secondary air to form the swirling flow of the burner,and tertiary air for the two-stage combustion. Each air stream iscontrolled by a needle valve and a rotameter. Natural gas is fromthe distribution grid. Ammonia can be added to the natural gasstream to simulate fuel nitrogen. The solid fuels are stored in acontainer, below which a screw feeder is weighed continuously bya balance. The core part of the test rig is a vertical oriented furnace,with a swirl burner located at its top. The schematic of the furnaceis illustrated in Figure 2. The inner diameter of the furnace is 315mm, with a total height of 1.85 m. The furnace wall is made ofrefractory materials, which can tolerate a temperature of up to 1600°C. The fuels are fired downward. A detailed description of theswirl burner is given elsewhere.32 Eight ports are situated on oneside of the furnace, where thermocouples can be inserted to monitorthe temperature distribution in the furnace chamber. Two windowsare installed in the furnace, one at the near-burner region and theother in the bottom of the furnace, which allow us to observedirectly the flame structure. To study the effect of two-stagecombustion, the tertiary air stream can be injected at differentlocations: from the top ports, from port 7, and from port 6, wherea neck is installed to separate the two combustion zones as shownin Figure 2.

A water-cooled particle-sampling probe with a conic tip can beinserted from the furnace bottom. The probe is connected to a hotporous ceramic filter with a heated pipe line to avoid watercondensation. The amount of gas withdrawn by the sampling systemis controlled in such a way that the isokinetic sampling is obtained.

The exit of the furnace is followed by a large, low-pressure dropgas-solid separator. After the separator, the flue gas is cooled downin a heat exchanger and vented to a chimney.

The flue gas is sampled through port 8 near the furnace exit.After the gas conditioning system, the concentrations of O2, CO2,CO, NO, and SO2 in the flue gas is analyzed by a series ofcontinuous gas analyzers.

The signals of measured temperatures, gas concentrations, andweight of biofuels in the feeding container are continuouslymonitored and logged to a computer by a data acquisition system.

(11) Martinzp, F. J.; Dederick, P. K. NOx from fuel nitrogen in two-stage combustion. In the 18th International Symposium on Combustion,The Combustion Institute, Pittsburgh, PA, 1977; pp 191-198.

(12) Takagi, T.; Tatsumi, T.; Ogasawara, M. Combust. Flame 1979, 35,17–25.

(13) Wendt, J. O. L. Prog. Energy Combust. Sci. 1980, 6 (2), 201–222.(14) Chen, S. L.; Heap, M. P.; Pershing, D. W.; Martin, G. B. Influence

of coal composition on the fate of volatile and char nitrogen duringcombustion. In the 19th International Symposium on Combustion, TheCombustion Institute, Pittsburgh, PA, 1982; pp 1271-1280.

(15) Chen, S. L.; Heap, M. P.; Pershing, D. W.; Martin, G. B. Fuel1982, 61 (12), 1218–1224.

(16) Beer, J. M. Chem. Eng. Sci. 1994, 49 (24A), 4067–4083.(17) Wendt, J. O. L. Combust. Sci. Technol. 1995, 108 (4-6), 323–

344.(18) Mao, F. H.; Barat, R. B. Combust. Flame 1996, 105 (4), 557–568.(19) Li, S. C.; Williams, F. A. Combust. Flame 1999, 118 (3), 399–

414.(20) Makino, K. IFRF Combust. J. 2000, article number 200007.(21) Smith, D. J. Power Eng. 2001, 105 (10), 71–74.(22) Ikeda, M.; Makino, H.; Morinaga, H.; Higashiyama, K. JSME Int.

J., Ser. BsFluids Therm. Eng. 2004, 47 (2), 180–185.(23) Man, C. K.; Gibbins, J. R.; Witkamp, J. G.; Zhang, J. Fuel 2005,

84 (17), 2190–2195.(24) Walsh, P. M. Energy Fuels 1997, 11 (5), 965–971.(25) Ikeda, M.; Kozai, Y.; Makino, H. JSME Int. J., Ser. BsFluids

Therm. Eng. 2002, 45 (3), 506–511.(26) Huang, L. K.; Li, Z. Q.; Sun, R.; Zhou, J. Fuel Process. Technol.

2006, 87 (4), 363–371.

(27) Spliethoff, H.; Hein, K. R. G. Fuel Process. Technol. 1998, 54(1-3), 189–205.

(28) Annamalai, K.; Thien, B.; Sweeten, J. Fuel 2003, 82 (10), 1183–1193.

(29) Damstedt, B.; Pederson, J. M.; Hansen, D.; Knighton, T.; Jones,J.; Christensen, C.; Baxter, L.; Tree, D. Proc. Combust. Inst. 2007, 31 (2),2813–2820.

(30) Wu, C.; Tree, D.; Baxter, L. Proc. Combust. Inst. 2007, 31 (2),2787–2794.

(31) Casaca, C.; Costa, M. Combust. Sci. Technol. 2003, 175 (11), 1953–1977.

(32) Lin, W.; Jensen, P. A.; Jensen, A. D. Manuscript prepared.

Figure 1. Diagram of the test rig for co-combustion of natural gas andbiofuels.

Biomass Suspension Combustion Energy & Fuels, Vol. 23, 2009 1399

Fuels. The fuels used in this study are natural gas and twobiofuels, beech saw dust (wood) and pulverized wheat straw pellet(straw). The proximate and ultimate analyses of the two biofuelsand the chemical composition of natural gas are listed in Tables 1and 2, respectively. The major difference of the compositionbetween the two fuels is the content of ash and nitrogen. Strawhas a higher ash content than wood. The nitrogen content of strawis nearly 4 times of that of wood.

The particle size distributions of wood and straw, determinedby sieve, are shown in Figure 3, indicating a large particle size ofwood. From the size distribution, the median diameters of the woodand straw powder are calculated to be 0.28 and 0.16 mm,respectively.

Experimental Conditions. In this set of experiments, theemphasis is put on how two-stage combustion will affect the NO

emission and the degree of burnout of combustibles (e.g., the COemission and unburned carbon in fly ash). The experimentsperformed include firing of natural gas with NH3 addition, co-firingof natural gas and wood, co-firing of natural gas and straw, co-firing of wood and straw, and firing of wood solely. Before anexperiment is performed, the furnace is heated by firing natural gasfor at least 6 h to reach a stable condition. In varying the fraction oftertiary air, the tangential flow to the swirl burner is adjusted to keepthe swirl number approximately equal to 2. When (co)firing biomass,the oxygen level at the furnace exit is kept to about 4%, which

Figure 2. Illustration of the furnace, in which the swirl burner is located on the top, and the position of the tertiary air injection.

Table 1. Proximate and Ultimate Analyses of Biofuels Used (ona Delivered Basis)

fuel beech saw dust straw pellet

Proximate Analysismoisture wt % 9.04 8.65ash wt % 0.61 4.76volatile wt % 76.70 69.87fixed carbon (by difference) wt % 13.65 16.72lower caloric value MJ/kg 16.44 15.76

Ultimate Analysiscarbon wt % 45.05 42.88hydrogen wt % 5.76 5.65oxygen (by difference) wt % 39.41 37.51nitrogen wt % 0.13 0.49sulfur wt % 0.01 0.06

Figure 3. Particle size distribution of the beech saw dust and pulverizedstraw pellets.

1400 Energy & Fuels, Vol. 23, 2009 Lin et al.

corresponds to an air ratio of approximately 1.25. When firing naturalgas, the oxygen level in the exit is around 2%, which is in a similarrange of gas-fired power plants. The range of applied operatingparameters in the experiments is listed in Table 3.

Results

Addition of NH3 to Natural Gas Flame. The results of theinfluence of two-stage combustion on the NO emission fromNH3-dosed natural gas flames are shown in Figure 4.

The figure presents two sets of results performed at similarconditions, as well as the NO emission level of natural gas flamewithout the addition of NH3. The results illustrate the same trendwith respect to the effect of the first-stage combustion stoichi-ometry on the NO emissions when NH3 is added. A minimumNO emission is observed with an optimal value of λ1 around0.8. The NO level seems not to be affected directly by λ1 forthe case without NH3 addition, as indicated by the results inFigure 4. The NO level reaches the maximum at the λ1 close tounity, probably because of the higher temperature in the nearburner region at λ1 ≈ 1, observed in some experiments.

The results are generally in agreement with many studies ofgas-phase hydrocarbon combustion with the addition of nitrogencompounds, which showed minimum NO levels at λ1 from 0.7to 0.83.11,12,18 However, it seems that the optimal value of λ1

depends upon the type of fuel and nitrogen-containing com-pound as seen when comparing the present results to those fromMao and Barat.18 Figure 5 shows a comparison of the effect oftwo-stage firing on fuel-N conversion to NO from natural gaswith the addition of NH3 by the present work and by firingethylene with the addition of CH3NH3.18 It is shown that aminimum of the conversion of nitrogen compounds to NOoccurs for both cases, although the reactor configuration andthe gas mixing behaviors may be different. In addition, it isobserved that the optimal value of λ1 for combustion of C2H4

+ CH3NH3 is lower (around 0.7) than that for combustion ofCH4 + NH3, probably because of the different reaction routesof the conversion to NO.

Co-firing Biomass and Natural Gas. The effect of two-stage combustion was tested by co-firing natural gas and twobiofuels: beech saw dust (wood) and pulverized wheat strawpellet (straw).

Figure 6 presents the experimental results of the effect ofthe first-stage combustion stoichiometry on the NO emissionwhen co-combustion of wood/natural gas and straw/natural gas.For co-firing of wood and gas, a minimum of NO emission isobserved at a λ1 value of 0.8. It is noticed that the NO emissioncan be reduced to a level as low as about 55 ppm and, incomparison to one-stage combustion, a reduction of nearly 60%can be obtained.

For co-firing straw and gas, it appears that a NO minimumexists at a λ1 value of 0.8-0.9. The NO emission can be reducedby more than 75% compared to one-stage combustion.

Because the nitrogen contents of wood and straw are different,it is difficult to compare the results in Figure 6 directly. Thus,a comparison of the conversion of fuel-N to NO for co-firingwood/gas and straw/gas with respect to the effect of two-stagecombustion is shown in Figure 7. A similar trend is revealedfor the co-combustion of natural gas with the two biofuels.However, the conversion of the fuel-N to NO is higher for co-firing wood than for co-firing straw. It should be emphasized

Figure 4. Effect of first-stage combustion stoichiometry on the NOemission at similar conditions with and without adding NH3 to gasflames ([, λtot ) 1.1; 9 and 2, λtot ) 1.15).

Table 2. Natural Gas Composition

CH4 mol % 89.06C2H6 mol % 6.08C3H8 mol % 2.47iC4H10 mol % 0.39nC4H10 mol % 0.54C5H12 mol % 0.11nC5 mol % 0.08C5

+ mol % 0.05N2 mol % 0.29CO2 mol % 0.91lower caloric value MJ/Nm3 39.624density kg/Nm3 0.8243

Figure 5. Comparison of the effect of the first-stage stoichiometry onthe conversion of fuel-N to NO in firing natural gas with NH3 dosing(this work, NH3,in ) 325 ppm) and in firing ethylene with the additionof CH3NH3 (with inlet nitrogen concentration of 950 ppm) in a two-stage turbulent flow combustor (adopted from Mao and Barat18).

Figure 6. Effect of two-stage combustion on the NO emission levelswhen co-firing wood/gas (wood share of 50% on a thermal basis) andstraw/gas (straw share of 44% on a thermal basis), with a totalcombustion stoichiomtry of 1.25.

Biomass Suspension Combustion Energy & Fuels, Vol. 23, 2009 1401

that the contribution of NO formation from natural gas combus-tion is not excluded in the calculation of the conversion of fuel-Nto NO in Figure 7, which may give a slightly higher conversion.

Co-firing Wood and Straw. The influence of two-stage co-firing of wood and straw is presented in Figure 8. For acomparison, the results of 100% wood firing are also shown inFigure 8.

It is seen that the behavior of the NO emission when co-firing the two biofuels is somewhat different from co-firingnatural gas and wood or straw and also from firing wood alone,although an optimal λ1 for NO emission exists. Unlike the otherco-firing, in which the NO level sharply increases when λ1 ishigher than 0.9, the NO level increases progressively but nosharp increase is observed when λ1 is higher than 0.9, whenco-firing wood and straw.

It should be noticed that the NO levels from one-stagecombustion (far right point in Figure 8) are the same from co-firing wood/straw and from firing wood, although the nitrogencontent in straw is much higher than that in wood. Thus, theconversion of the fuel-N to NO is lower when co-firing wood/straw than firing wood, as shown in Figure 9. The resultsindicate that a synergistic effect for NO reduction may occur

when co-firing wood and straw, which will be discussed in alater section.

Effect of the Location Where the Tertiary Air StreamIs Injected. The effect of the location of the tertiary air injectionon the NO emission is examined by the introduction of thetertiary air stream from four locations: port 6, the default locationof the tertiary air injection and 910 mm below the burner mouth;port 7, located 225 mm downstream of port 6; and 50 and 450mm below the burner mouth by two injection probes from thetop of the furnace, parallel to the direction of gas flow. It shouldbe emphasized that the tertiary air stream is in a cross-flow tothe main gas stream in the furnace for ports 6 and 7, whichwill result in a better mixing of the two gas streams than theparallel flow from the top probes.

The experimental results of firing wood are presented inFigure 10. It is shown that the NO level decreases for λ1 < 1when shifting the tertiary air from port 6 to port 7. Both themixing behaviors between the tertiary air stream and main gasstream and the residence time in the fuel-rich zone maycontribute to the difference.22

On the basis of the geometric parameters, gas flow, and thetotal stoichiometry, the values of average gas residence time inthe fuel-rich zone at different values of λ1 are estimated andlisted in Table 4.

It should be mentioned that the tertiary air stream cannot mixwith the main flow immediately when injecting the tertiary airfrom the furnace top. The actual residence time for these twocases is too complicated to be estimated accurately in such a

Figure 7. Comparison of the conversion of fuel-N to NO for co-firingnatural gas and saw dust and co-firing natural gas and straw powder.

Figure 8. Effect of two-stage combustion on the NO emission levelwhen co-firing wood/straw (wood share of 60% on a thermal basis)and when firing wood.

Table 3. Experimental Conditions Applied in the Study onTwo-Stage Combustion

NG +NH3

NG +wood

NG +straw

wood +straw wood

fuel share (% ona thermal basis)

50/50 56/44 60/40 100

λtot 1.1-1.15 1.25 1.25 1.25 1.23-1.27λ1 0.65-1.1 0.7-1.25 0.75-1.25 0.6-1.25 0.65-1.25third air location P6 P6 P6, P7 P6, P7 P6, P7, Ptop

power (kW) 23 24 24 20 22-24

Figure 9. Effect of two-stage combustion on conversion of fuel-N toNO when co-firing wood/straw (wood share of 60% on a thermal basis)and when firing wood.

Figure 10. Effect of the location of the tertiary air injection on NOemission when firing wood (λtot ) 1.23-1.27).

1402 Energy & Fuels, Vol. 23, 2009 Lin et al.

simple way. Because of the difference in flow mixing patterns,the injection of the tertiary air from ports 6 and 7 may beconsidered as two-stage combustion with overfiring air in aboiler, while the injection of the third air from the ports in thefurnace top may be regarded to simulate the overburner air ina combustion chamber.

CO Emission Level and Unburned Carbon in the FlyAsh. The effect of two-stage combustion on the emission ofCO for co-firing wood and natural gas and firing wood is shownin Figure 11. The CO emission level in co-combustion of wood/gas is, in general, much lower (<60 ppm) than the valuesreported in the literature (from 200-1000 ppm at an excess of15% and biomass share of 20%),31 although it increases slightlywith a decrease of λ1. When co-firing straw/gas, the CO levelis lower than 10 ppm at all λ1 when λtot is 1.25. The low COemission may be caused by the fact that straw has a smallerparticle size, which agrees with the conclusion that the COemission when co-firing biomass is largely dependent upon thefineness of the fuel particles.27 In the case of firing 100% wood,the CO level is higher than co-firing with natural gas, especiallyfor a low value of λ1. When co-firing wood/straw, the CO levelis, in general, lower than firing 100% wood.

It is also shown in Figure 11 that the emission of CO andthe unburned carbon in the fly ash are correlated: A high COlevel corresponds to a high carbon content in fly ash, i.e., lowcarbon burnout. The results are in agreement with the case offiring coals33 and co-firing coal and biomass.27

The effect of the location of the tertiary air injection on theCO emission when firing wood is shown in Figure 12. Theemission level of CO is very high (>2000 ppm), when thetertiary air is injected from port 7 in the case of λ1 being lowerthan unity. In contrast, The CO level is lower than 50 ppm,when injecting the third air stream from the top ports. Animportant parameter that governs the CO emission in thesuspension combustion is the excess air level. When the tertiaryair injection is shifted downstream in two-stage combustion,the residence time in the secondary stage will be shortened.

Obviously, there is not enough time for the large amount ofCO entering the secondary stage to be burned completely.

Temperature Profiles in the Furnace. The effect of two-stage combustion on the temperature profiles in the furnace whenfiring wood is shown in Figure 13. It is indicated that the highesttemperature occurs at port 3. In the region near the burner (port1), the temperature is lower than at port 3. The temperaturesare decreased downward. Such temperature profiles are observedfor co-firing wood/gas and straw/gas. When firing natural gas,the highest temperature occurs at port 1. The difference is dueto the fact that natural gas ignites as soon as it enters the furnace,while biomass particles experience first devolatilization and thevolatile ignition near port 2, downstream of port 1. It is noticedfrom visual observation that there is no further delay of volatileignition when co-firing wood compared to co-firing straw,although the particle size of wood is larger that that of straw,suggesting a same order of magnitude of particle heating rateand devolatilization rate for wood and straw. The figure alsoshows a higher temperature at port 7 at low λ1 than at high λ1.This indicates that more combustibles burn at the secondary

(33) Chen, S. L.; Pershing, D. W.; Heap, W. P. Bench-scale Evaluationof Non-US Coals NOx Formation under Excess Air and Staged CombustionConditions, Energy and Environmental Research Corporation, Irvine, CA,1981; p 80.

Figure 11. Effect of two-stage combustion on CO emission when co-firing different fuels and on the unburned carbon in fly ash when firingwood.

Table 4. Average Gas Residence Time in the Fuel-Rich Zone inTwo-Stage Combustion

average residence time in the fuel-rich zone (s)

port\λ1 0.6 0.7 0.8 0.9 1 1.1 1.2Ptop5 0.24 0.20 0.18 0.16 0.14 0.13 0.12Ptop45 2.14 1.83 1.60 1.42 1.28 1.17 1.07P6 4.32 3.70 3.24 2.88 2.59 2.36 2.16P7 6.27 5.37 4.70 4.18 3.76 3.42 3.13

Figure 12. Effect of the tertiary air injection location on CO emissionwhen firing wood.

Figure 13. Temperature profiles in two-stage combustion of wood atdifferent first stage stoichimetry (tertiary air injection from port 6, λtot

) 1.26).

Biomass Suspension Combustion Energy & Fuels, Vol. 23, 2009 1403

stage (downstream of port 6) at lower λ1, i.e., more reducingcondition at the first stage.

Discussion

Comparison of NH3-Dosed Natural Gas Flame andCo-firing of Wood/Gas. In many studies, nitrogen-containingcompounds are added to gaseous fuel combustion to simulatethe conversion of the fuel-N to NO.11,12,18,34 By this method, itis easier to model the conversion process and understand themechanisms. However, the results obtained may be differentfrom those obtained when firing solid fuels at similar conditions.Figure 14 shows a comparison of the influence of two-stagecombustion of natural gas with NH3 dosing, co-firing wood/natural gas, and wood firing on the NO emission profile. In thefigure, the equivalent inlet concentrations of nitrogen-containingcompounds (defined as the NO concentration in the dry fluegas corrected to 6% of O2 when all inlet N compounds areconverted to NO) are 325, 175, and 340 ppm for NH3-dosednatural gas flame, co-firing wood/natural gas, and firing wood,respectively.

It is observed from Figure 14 that the NO emission profileswith respect to the fuel-rich stage stoichiometry (λ1) are similarfor co-firing wood/gas and firing wood. In contrast, the gradient

of the NO emission with respect to λ1 is higher for the NH3-dosed natural gas flame than for firing or co-firing wood.

The difference in the NO emission profiles for the two casesmay result from exclusion of the heterogeneous reaction whenfiring natural gas. The heterogeneous reactions may have twoeffects on wood co-firing. In the devolatilization stage, part ofthe volatile-N in the biofuel may be released as N2 instead ofNH3 or HCN because of the catalytic effect of alkali and alkalineearth compounds,35 which are present in biofuels. In addition,the char combustion is slower than volatile combustion, andthe biomass char has a high reactivity toward reduction ofNO.36,37 The reduction of NO by char occurs at reducingconditions as well as at oxidizing conditions. The heterogeneousreduction of NO may be the main reason for the lower NO levelat high λ1 in the co-firing experiments. It is noticed that theoptimal values of λ1 are almost the same, and the minimumvalues of NO emission are close for all three cases. This mayindicate that the homogeneous reactions play a major role inthe minimal NO level observed at two-stage combustion. Athigh λ1, NO will be formed by direct oxidation of fuel-N, whichis mostly released with volatile at high temperatures. At verylow λ1, a large fraction of N-containing compounds, such asHCN and NH3, released from devolatilazation are not oxidizedat the first stage and enter the second stage to oxidize to NOwhen mixed with the tertiary air. Both experimental work38 andmodeling work39 confirmed that HCN and NH3 levels increaseat the exit of the first stage.

Synergistic Effect for NO Reduction of Co-firing. In coalcombustion, it has been reported that a synergistic effect onblending is observed,40 which is indicated by a differencebetween the actual performance of the blend and that predictedby the addition of the performance of individual fuels and theblend ratio. In Figure 8, a lower level of NO is shown thanexpected when co-firing wood and straw, when considering theadditive property from co-firing wood/gas and straw/gas inFigure 6. The synergistic effect on NO emission is also reportedwhen blending two coals with very different characteristics insuspension combustion.41,42 With the co-firing of coal and strawin a pulverized coal boiler, the conversion of fuel-N to NOdecreased with an increasing straw share, which was explainedby the high-volatile content in straw.43 It is well-known thattwo-stage combustion is more effective in reducing NO for thehigh-volatile coal than the low-volatile coal.20

In the present work, it is not probable that the synergisticeffect on NO emission is caused by the volatile content becauseboth wood and straw have a high-volatile content with the sameorder of magnitude. One of the possible reasons may be thedifference in the reactivity of char for reduction of NO. A

(34) Sullivan, N.; Jensen, A.; Glarborg, P.; Day, M. S.; Grcar, J. F.;Bell, J. B. Combust. Flame 2002, 131 (3), 285–298.

(35) Ohtsuka, Y.; Wu, Z. H.; Furimsky, E. Fuel 1997, 76 (14-15), 1361–1367.

(36) Sorensen, C. O.; Johnsson, J. E.; Jensen, A. Energy Fuels 2001,15 (6), 1359–1368.

(37) Garijo, E. G.; Jensen, A. D.; Glarborg, P. Energy Fuels 2003, 17(6), 1429–1436.

(38) Chen, S. L.; Heap, M. P.; Pershing, D. W. Bench-scale NOemissions testing of world coals: Influence of particle size and temperature,The 1982 Joint Symposium on Stationary Combustion NO Control, PaloAlto, CA, 1982; pp 35.1-35.19.

(39) Jensen, A.; Johnsson, J. E. Chem. Eng. Sci. 1997, 52 (11), 1715–1731.

(40) Majid, A. A.; Paterson, N.; Reed, G. P.; Dugwell, D. R.; Kandiyoti,R. Energy Fuels 2005, 19 (3), 968–976.

(41) Haas, J.; Tamura, M.; Weber, R. Fuel 2001, 80 (9), 1317–1323.(42) Rubiera, F.; Arenillas, A.; Arias, B.; Pis, J. J. Fuel Process. Technol.

2002, 77, 111–117.(43) Pedersen, L. S.; Nielsen, H. P.; Kiil, S.; Hansen, L. A.; Dam-

Johanesn, K.; Kildsig, F.; Christensen, J.; Jespersen, P. Fuel 1996, 75 (13),1584–1590.

Figure 14. Comparison of the effect of two-stage combustion firingnatural gas with NH3 addition, co-firing wood/natural gas, and firingwood in the swirl burner.

Figure 15. Comparison of the effect of two-stage suspension combus-tion of different fuels on NO emissions (9, this work; [, 2, and O,Chen et al.;15 ×, Spliethoff et al.27).

1404 Energy & Fuels, Vol. 23, 2009 Lin et al.

previous investigation showed that the reactivity of the charfrom straw is higher that that from wood.37 In addition, thedifference in the mineral content in wood and straw mayinfluence the char yield at suspension combustion conditions.The difference in particle size distribution of the two biofuelsmay also affect the aerodynamics in the near burner region,which may influence the NO formation and destruction routes.In addition, the volatile-N distribution caused by the catalyticeffect of ash constituent summarized by Glarborg et al.44 mayalso contribute to the synergistic effect. More work is neededfor understanding the synergistic effect.

Effect of Fuel Type on NO Emission When UsingTwo-Stage Combustion. To understand the effect of two-stagecombustion on the NO emission using different fuels, acomparison of this work and the work in the literature15,27 isshown in Figure 15.

It is shown that a minimum of NO emission occurs for allfuels, except for the anthracite, in which the NO emissionincreases with an increasing value of λ1. This agrees with theprevious hypothesis that the NO minimum of two-stagecombustion is mainly caused by the homogeneous reactions.The figure also demonstrates that the optimal value of λ1 variesin a range of 0.7-0.85.

The main results in the present work, regarding the influenceof nitrogen content per unit power input of fuels on the NOand CO emissions, as well as on the conversion of fuel-N toNO at the optimal first stage combustion stoichiometry, aresummarized in Table 5. It can be seen that a low NO emissionis obtained by applying two-stage combustion at an optimal first-stage stoichiometry. At such conditions, the CO level is at alow level. In addition, it seems that, in general, the conversionof fuel-N to NO decreases with an increase in the input fuel-N,as reviewed by Glarborg et al.44

Conclusions

From the experimental study of two-stage suspension com-bustion of biomass and co-firing gas/biomass, the followingconclusions can be drawn:

Two-stage combustion can significantly reduce the NOemission. An optimal stoichiometry of around 0.8 in the fuel-rich zone exists with respect to minimizing NO emissions. Whenusing wood and straw as co-firing fuels, 15-25% of the fuel-Nis converted to NO. Straw appears to give the lowest conversionof fuel-N to NO. In addition, the CO emission can be kept at alow level at the optimal λ1.

The results of the influence of two-stage combustion on NOemission when firing natural gas with NH3 addition and co-firing natural gas and biomass indicate that the optimalstoichiometry in the fuel-rich (λ1) zone for gaining the lowestNO may result from the homogeneous reaction (volatilecombustion). The difference in NO profiles with respect to λ1

of NH3-dosed natural gas flame and biomass firing is probablycaused by the heterogeneous reactions involving char in NOformation and destruction.

A synergistic effect occurs when co-firing two biofuels, woodand straw, with respect to NO emission, with a lower conversionof fuel-N to NO than those of individual biofuels. Thesynergistic effect may be caused by the reactivity of the biofuelchar toward NO reduction, the catalytic effect of ash towardthe volatile-N distribution, and the difference in near burneraerodynamics because of the particle size distribution of thetwo biofuels.

The experimental results show that NO emission levels canbe significantly lowered when co-firing natural gas and biomasswithout increasing incomplete combustion of gas and solids byapplying optimized two-stage combustion.

Acknowledgment. The financial support of the present work bythe companies Energinet.dk, Dong Energy A/S, and Vattenfall A.D.(contract number 6526) is gratefully acknowledged. This work isalso a part of the Combustion and Harmful Emission Control(CHEC) Research Center funded by the Technical University ofDenmark, the Danish Technical Research Council, the EuropeanUnion, the Nordic Energy Research, Dong Energy A/S, VattenfallA.B., F L Smidth A/S, and the Public Service Obligation.

EF8004866(44) Glarborg, P.; Jensen, A. D.; Johnsson, J. E. Prog. Energy Combust.

Sci. 2003, 29 (2), 89–113.

Table 5. Effect of Inlet Fuel-N on the NO and CO Emissions at Optimal λ1

fuelinlet fuel-N

(mol N kJ-1) λ1, opt

conversion of fuel-Nto NO at optimal λ1 (%)

NO emission at optimalλ1 (ppm at 6% O2)

CO emission at optimalλ1 (ppm at 6% O2)

co-firing wood/gas (50/50%) 2.72 0.82 31 55 33gas with 325 ppm NH3 addition 4.82 0.83 26 86 0firing wood 5.43 0.85 24 81 125co-firing straw/gas (44/56%) 9.85 0.8 15 95 7co-firing straw /wood (40/60%) 12.21 0.77 17 128 94

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