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Fuel 88 (2009) 40–45
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Fuel
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Detailed measurements in a pulverized-coal-fired large-scale laboratoryfurnace with air staging
A. Ribeirete, M. Costa *
Mechanical Engineering Department, Instituto Superior Técnico, Technical University of Lisbon, Avenida Rovisco Pais, 1049-001 Lisbon, Portugal
a r t i c l e i n f o a b s t r a c t
Article history:Received 13 April 2008Received in revised form 30 July 2008Accepted 31 July 2008Available online 24 August 2008
Keywords:Pulverized-coalDetailed measurementsAir stagingNOx emissionsParticle burnout
0016-2361/$ - see front matter � 2008 Elsevier Ltd. Adoi:10.1016/j.fuel.2008.07.033
* Corresponding author. Tel.: +351 218417378; faxE-mail address: [email protected] (M. Costa).
The aim of the present work was to evaluate the performance of a pulverized-coal-fired large-scale lab-oratory furnace with air staging. New data are reported for gas phase species concentration, temperatureand particle burnout for two primary zone stoichiometric ratios, 1.15 (unstaged flame) and 0.95 (stagedflame), other operating conditions being fixed. The results revealed that the reduction in primary zonestoichiometric ratio caused a decrease in NOx emissions from 421 to 180 mg/N m3@6%O2, an increasein CO emissions from 51 to 168 mg/N m3@6%O2 and a reduction in particle burnout from 81.8% to76.5%. It was concluded that the reduction of the O2 availability in the primary zone inhibits the NO for-mation, mainly via the fuel mechanism, but it affects negatively both the CO and the char oxidation pro-cesses because, under staging conditions, both processes tend to occur in the vicinity of the over fire airinjection region, where the temperatures are relatively low.
� 2008 Elsevier Ltd. All rights reserved.
1. Introduction
Recent years have seen a significant pressure on coal-firedpower stations to limit pollutant emissions such as NOx. Coal willcontinue to play a significant role in power generation in manyindustrial and developing countries so that technological solutionsfor NOx reduction will remain an important issue. Among the exist-ing solutions for NOx control, staged combustion with over fire airemerges as an attractive solution for older boilers, especially be-cause of its reduced costs of installation and operation. This tech-nology separates the combustion process into a primary zonewith a deficiency of air and a second burnout zone with excessair. Alone, this technique reduces NOx by some 50–70%, but onlywithin well-defined applicability limits [1].
The present work concentrates on the evaluation of the perfor-mance of a pulverized-coal-fired large-scale laboratory furnacewith air staging. Related studies include those of Spliethoff et al.[2], Bool and Kobayashi [3] and Costa and Azevedo [4], among oth-ers. Spliethoff et al. [2] evaluated the effects of stoichiometry andresidence time in the primary (fuel-rich) zone and the effects oftemperature for air staging with different coals; Bool and Kobay-ashi [3] considered the use of oxygen enhanced combustion tominimize the problems typically associated with air staged com-bustion systems, namely burner stability and particle burnoutperformance; and Costa and Azevedo [4] characterized the com-
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bustion process in a 300 MWe, front-wall-fired, pulverized-coal,utility boiler operating under deep staging conditions.
Recently, Ribeirete and Costa [5] quantified the impact of the airstaging on the overall performance (pollutant emissions and parti-cle burnout) of a laboratory furnace fired by an industry-type pul-verized-coal swirl burner. In order to enhance the understanding ofthe large number of relevant physical phenomena involved in airstaged combustion, the present study focused on detailed in-flamemeasurements of gas species concentration and gas temperaturesfor two of those furnace operating conditions studied previously[5].
2. The furnace, procedures and test conditions
The air staging trials were carried out in the Instituto SuperiorTécnico (IST) large-scale laboratory furnace. The combustionchamber is cylindrical in shape. Its axis is vertical to minimizeasymmetry due to natural convection and biased ash particle depo-sition and it is down-fired to facilitate particulate removal. The cyl-inder comprises eight water-cooled steel segments each 0.3 m inheight and 0.6 m in internal diameter. The roof section and theupper four segments are lined with a layer of refractory, and a cera-mic fiber blanket sandwiched between the refractory and thewater-cooled jacket. Each segment incorporates a pair of diametri-cally opposed 0.22 m diameter ports for probing and injection ofover fire air. A more detailed description of the experimental facil-ity can be found elsewhere [6].
Fig. 1 shows schematically the arrangement of the furnace roof,burner and air staging system. The burner geometry is typical of
Dimensions in mm
Ø 600 150 50Z
= 9
6032
8
CeramicfibreIgnitor
burner Quarl
Refractorycement
Water
Burner gun
Over fire air
60o
300
Ø 56
Ø 120
Solid fuel + primary air
Natural gas
Secondary air
3025.321.317.7
Fig. 1. Schematic of the furnace roof, burner and air staging system.
A. Ribeirete, M. Costa / Fuel 88 (2009) 40–45 41
that used in power stations for wall-fired boilers and consists of aburner gun and a secondary air supply in a conventional double-concentric configuration, terminating in an interchangeable refrac-tory quarl of half-angle 30�. In this work, the burner gun used con-sisted of two concentric tubes: the central orifice was used for theintroduction of pulverized-coal and transport (primary) air and theannular orifice was used for the introduction of a small amount ofnatural gas in order to help the flame stabilization process. The sec-ondary air entered a plenum chamber situated above the burner, inwhich it encountered a moveable block swirl generator of the typedeveloped at the International Flame Research Foundation. The airthen flowed through an interchangeable cylindrical duct and, sub-sequently, into the refractory quarl section. The primary air wassupplied by an air compressor (10 bar) and its flow rate was mea-
Table 1Coal characteristics (as fired)
Proximate analysis (wt%)Volatiles 37.00Fixed carbon 56.49Moisture 4.80Ash 2.42
Ultimate analysis (wt%)Carbon 77.78Hydrogen 4.77Oxygen 7.67Nitrogen 1.90Sulphur 0.66
High heating value (MJ/kg) 30.84Low heating value (MJ/kg) 29.72Sauter mean diameter (lm) 18.16
sured using a calibrated rotameter. The secondary air was suppliedby a fan of variable speed and its flow rate was measured with acalibrated orifice plate installed upstream of the fan. The naturalgas was supplied by the portuguese gas company via the IST grid.In this case, the gas flow was controlled with pressure regulatorsand valves and the flow rate measured using a calibrated rotame-ter. The pulverized-coal was transported to the burner via a loss-in-weight feeder and a compressed air ejector system. The solidfuels feeding system is fully described elsewhere [6].
As can be seen in Fig. 1, the over fire air was injected inside thecombustion chamber from the furnace axis. In an initial set of
Table 2Furnace operating conditions
Parameter FlameA
FlameB
Total thermal input (kW) 100Overall excess air (%) 15Coal mass flow rate (kg/h) 7.27Natural gas mass flow rate (kg/h) 3.16Primary air mass flow rate (kg/h) 7.96Primary air temperature (�C) 25Secondary air mass flow rate (kg/h) 130.5 106.5Secondary air temperature (�C) 25Secondary air swirl number (–) 1.0Over fire air mass flow rate (kg/h) 0 24Primary zone stoichiometric ratio, kpz (–) 1.15 0.95Distance from the burner exit to the over fire air injector, Z
(mm)n.a. 960
CO emissions (mg/N m3@6%O2) 51 168NOx emissions (mg/N m3@6%O2) 421 180Particle burnout (%) 81.8 76.5
42 A. Ribeirete, M. Costa / Fuel 88 (2009) 40–45
experiments, the location of the over fire air injection system wasvaried along the furnace height and the flue-gas concentrationsand particle burnout measured [5]. The outcome of this procedureallowed us to define the appropriate location for the over fire airinjection system used in the present work.
Gas sampling for the measurement of local mean O2, CO, CO2,unburnt hydrocarbons (HC) and NOx concentrations was achievedusing a stainless steel water-cooled, water-quenched probe [6].The analytical instrumentation included a magnetic pressure ana-lyzer for O2 measurements, non-dispersive infrared gas analyzersfor CO and CO2 measurements, a flame ionization detector for HCmeasurements and a chemiluminescent analyzer for NOx measure-ments. Uncertainties associated with quenching of chemical reac-
O2CO2CO
Z = 25 mm
0
4
8
12
16
20
Z = 75 mm
0
4
8
12
16
20
Z = 150 mm
Z = 300 mm
0
4
8
12
16
20
0
4
8
12
16
20
Z = 880 mm
0
4
8
250200150100500
12
16
20
O2,
CO
2,C
O(d
ryvo
lum
e%
)
HC
(dry
volu
me
%);
T/10
0(º
C)
Radial distance (mm)
Fig. 2. Radial profiles of mean gas species conc
tions within the sampling probe were insignificant andaerodynamic disturbances due to the probe suction velocity wereminimized by balancing the static pressure of the incoming flowin the vicinity of the probe inlet, measured through a static pres-sure tap near the tip, with that of the furnace wall at the same axiallocation by adjusting the probe suction [6]. No attempt was madeto quantify the probe flow disturbances. The repeatability of thedata was, on average, within 10%.
Local mean gas temperature measurements were performedwith the aid of fine wire (76 lm) platinum/platinum:13% rhodiumthermocouples. Because radiation losses represent the majorsource of uncertainty in the mean temperature measurements anattempt has been made to quantify them [6]. The calculation indi-
NOxHCT
0
100
200
300
400
500
600
0
2
4
6
8
10
12
14
250200150100500
Z = 75 mm
Z = 150 mm
Z = 300 mm
Z = 880 mm
Radial distance (mm)
Z = 25 mm
0
100
200
300
400
500
600
0
100
200
300
400
500
600
0
100
200
300
400
500
600
0
100
200
300
400
500
600
NO
x (d
ryvo
lum
epp
m)
0
2
4
6
8
10
12
14
0
2
4
6
8
10
12
14
0
2
4
6
8
10
1214
0
2
4
6
8
10
1214
entration and gas temperature for flame A.
A. Ribeirete, M. Costa / Fuel 88 (2009) 40–45 43
cated that in the regions of highest temperature the ‘‘true” temper-ature do not exceed the measured one by more than 10%.
Z = 800 mm
Z = 880 mm
Radial distance (mm)20015010050 0520
0
4
8
12
16
20
0
4
8
12
16
20
O2CO2CO
Z = 25 mm
0
4
8
12
16
20
Z = 75 mm
0
4
8
12
16
20
Z = 150 mm
0
4
8
12
16
20
Z = 300 mm
0
4
8
12
16
20
Z = 700 mm
0
4
8
12
16
20 HC
(dry
volu
me
%);
T/10
0(º
C)
O2,
CO
2,C
O(d
ryvo
lum
e%
)
O2CO2CO
Fig. 3. Radial profiles of mean gas species conc
The gases for the measurement of the flue-gas concentrationdata were withdrawn using a water-cooled stainless steel probe.
Z = 800 mm
Z = 880 mm
Radial distance (mm)20015010050 0520
0
100
200
300
400
500
600
0
100
200
300
400
500
600NOxHCT
0
2
4
6
8
10
12
14
0
2
4
6
8
10
1214
Z = 25 mm
0
2
4
6
8
10
12
14
Z = 75 mm
0
2
4
6
8
10
12
14
Z = 150 mm
0
2
4
6
8
10
12
14
Z = 300 mm
0
2
4
6
8
10
12
14
Z = 700 mm
0
2
4
6
8
10
12
140
0
100
200
300
400
500
600
100
200
300
400
500
600
0
100
200
300
400
500
600
0
100
200
300
400
500
600
0
100
200
300
400
500
600
NO
x (d
ryvo
lum
epp
m)
NOxHCT
entration and gas temperature for flame B.
44 A. Ribeirete, M. Costa / Fuel 88 (2009) 40–45
The analytical instrumentation used was that referred to above.At the furnace exit, where the gas composition was nearlyuniform, probe effects were negligible and errors arose mainlyfrom quenching of chemical reactions and sample handling.Samples were quenched near the probe tip to about 150 �Cand condensation of water within the probe was avoided by con-trolling the inlet temperature of the cooling water (typically toaround 60 �C). Repeatability of the flue-gas data was, on average,within 5%.
Solids sampling from the flue-gas was performed with the aid ofa stainless steel water-cooled, water-quenched probe [6], being theparticle burnout data obtained from the following equation:
W ¼ ½1� ðxk=xxÞ�=ð1�xkÞ; ð1Þ
where W is the particle burnout, x is the dry ash mass fraction, andthe subscripts k and x refer to the ash content in the input solid fueland in the char sample, respectively. In order to determine the accu-racy of the experimental procedure and of the measurements, car-bon and oxygen elemental mass balances have been performedfor all tested conditions. The overall mass balance discrepancieswere below 3%. Radial traverses at the exit sampling location indi-cated no spatial variation in particle burnout and repeatability was,on average, within 10%.
A UK bituminous coal was used in the present work. The coalproperties and the furnace operating conditions for the two setsof measurements reported herein, referred to as flames A (un-staged flame) and B (staged flame), are given in Tables 1 and 2,respectively.
3. Results and discussion
As a preliminary remark, it should be stressed that the second-ary air swirl number of 1.0 used in this work (Table 2) is large en-ough to generate a strong internal recirculation zone (IRZ) whichreverses the coal particle velocities. An external recirculation zone(ERZ) is also established due to flow separation at the quarl exit.The IRZ plays the crucial role in the stabilization process of thepresent flames. The primary jet and the coal particles penetratethis zone to an extent which depends on their momenta and onthe level of swirl. As the coal particles mix with the hot recirculat-ed gas, their temperatures increase sharply resulting in rapiddevolatilisation and initial burnout.
Fig. 2 shows the radial profiles of mean gas species concentra-tion (O2, CO, CO2, HC, expressed in terms of C3H8, and NOx) andgas temperature for flame A (unstaged flame) at five axial loca-tions. At Z = 25 mm (Z is the axial distance from the furnace roof)and Z = 75 mm is noticeable the influence of the central jet in theflow aerodynamics. At Z = 25 mm the maximum in O2 concentra-tion at r = 60 mm (r is the radial distance) is due to the mixingwith the secondary air flow. This fact is consistent with thelow local values of CO2 and NOx concentrations and gas temper-ature observed in this region. Around the furnace axis the O2
concentrations are relatively low and the CO2 and NOx concentra-tions as well as the mean gas temperatures are relatively highrevealing the presence of a zone of intense combustion. Awayfrom the axis, for r > 80 mm, the O2 and CO2 concentrations areuniform due to the presence of the non-reacting and product-richERZ.
At Z = 75 mm the reaction zone is slightly wider than atZ = 25 mm owing to the expansion of the central jet with the com-bustion process progressing as typified by the oxygen consumptionaround the furnace axis. At Z = 150 mm, the radial profiles still dis-close the presence of a zone of intense combustion, with the gastemperatures around the furnace axis presenting the biggest mea-sured values. For Z > 300 mm the chemical species concentration
and gas temperature profiles are relatively uniform indicating thatthe combustion process is in its final stages.
Fig. 3 shows the radial profiles of mean gas species concentra-tion and gas temperature for flame B (staged flame) at seven axiallocations. Viewed as a whole Figs. 2 and 3 show that the overallcharacters of the two flames are very similar. The lower availabilityof oxygen in the primary zone of flame B is immediately percepti-ble from the lower O2 concentration measured values. In addition,it is observed that at Z = 25, 75 and 150 mm flame B presents high-er gas temperatures, as compared with those encountered in flameA, because kpz � 1 for flame B. In spite of the highest temperaturesencountered in flame B, the lower availability of oxygen in its nearburner region inhibits the formation of NO in this flame – in fact,flame B presents significantly lower NOx concentrations in all mea-sured positions, as compared with flame A. According with Spliet-hoff et al. [2], the existence of a primary zone poor in O2 inhibitsessentially the formation of NO via the fuel mechanism, whichcan be responsible for about 80% of the total NO formed in pulver-ized-coal flames.
Note, however, that the lower O2 concentrations observed innear burner region of flame B have a negative impact in its overallcombustion efficiency, as compared with flame A, despite the high-est temperatures encountered in flame B. Indeed, because of thelack of oxygen in the primary zone, the oxidation process of thechar in flame B extends well into the secondary zone, where thetemperatures are relatively low (see Fig. 3), with particle burnoutsuffering accordingly.
The discussion above is consistent with the values of CO andNOx emissions and particle burnout measured in the flue-gasesfor flames A and B; specifically, 51 mg/N m3@6%O2, 421 mg/N m3@6%O2 and 81.8%, respectively, for the flame A and 168 mg/N m3@6%O2, 180 mg/N m3@6%O2 and 76.5%, respectively, for flameB (see Table 2). In brief, a reduction in kpz from 1.15 to 0.95 causeda significant decrease in NOx emissions, a significant increase in COemissions and a moderate reduction in particle burnout. As dis-cussed earlier, the reduction of the O2 availability in the primaryzone for kpz = 0.95 inhibits the NO formation, mainly via the fuelmechanism, which explains the decrease in the NOx emissions.On the other hand, the reduction of the O2 concentrations in theprimary zone for kpz = 0.95 affects negatively both the CO and thechar oxidation processes because both processes tend to occur inthe vicinity of the over fire air injection region, where the temper-atures are relatively low, as discussed above.
Finally, it should be stressed that the present results are consis-tent with those obtained by Spliethoff et al. [2] and Bool andKobayashi [3] as far as NOx reduction is concerned. Unfortunately,none of these works considered the impact of the air staging on theglobal combustion efficiency that the present study reveals to besignificant.
4. Conclusions
Detailed measurements have been performed in a pulverized-coal-fired large-scale laboratory furnace with air staging for twoflames with distinct primary zone stoichiometric ratios, 1.15 (un-staged flame) and 0.95 (staged flame), other operating conditionsbeing fixed.
The reduction in primary zone stoichiometric ratio caused a de-crease in NOx emissions from 421 to 180 mg/N m3@6%O2, an in-crease in CO emissions from 51 to 168 mg/N m3@6%O2 and areduction in particle burnout from 81.8% to 76.5%.
It was concluded that the reduction of the O2 availability in theprimary zone inhibits the NO formation, mainly via the fuel mech-anism, but it affects negatively both the CO and the char oxidationprocesses because, under staging conditions, both processes tend
A. Ribeirete, M. Costa / Fuel 88 (2009) 40–45 45
to occur in the vicinity of the over fire air injection region, wherethe temperatures are relatively low.
Acknowledgment
Financial support for this work was provided by the CEC/ECSCResearch Programme of the European Commission under the Con-tract No. RFC-CR-03004 and is acknowledged with gratitude.
References
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[2] Spliethoff H, Greul U, Rudiger H, Hein KRG. Basic effects on NOx emissions in airstaging and reburning at a bench-scale test facility. Fuel 1996;75:560–4.
[3] Bool L, Kobayashi H. NOx reduction from a 44-MW wall-fired boiler utilizingoxygen enhanced combustion. In: Proceedings of the 28th internationaltechnical conference on coal utilization and fuel systems, Clearwater, Florida,USA; 2003.
[4] Costa M, Azevedo JLT. Experimental characterization of an industrial pulverizedcoal fired furnace under deep staging conditions. Combust Sci Technol2007;179:1923–35.
[5] Ribeirete A, Costa M. Impact of air staging on the overall performance of apulverized coal fired furnace. Proc Combust Inst, in press.
[6] Casaca C, Costa M. Co-combustion of biomass in a natural gas fired furnace.Combust Sci Technol 2003;175:1953–77.
