Combustion and NO x emission characteristics of a retrofitted down-fired 660MW e utility boiler at different loads
Zhengqi Li ⇑,Guangkui Liu,Qunyi Zhu,Zhichao Chen,Feng Ren
School of Energy Science and Engineering,Harbin Institute of Technology,92,West Dazhi Street,Harbin 150001,China
a r t i c l e i n f o Article history:
Received 7September 2010
Received in revid form 23November 2010Accepted 20January 2011
Available online 12February 2011Keywords:
Down-fired boiler Retrofit
Carbon content in fly ash Thermal efficiency
a b s t r a c t
Industrial experiments were performed for a retrofitted 660MW e full-scale down-fired boiler.Measure-ments of ignition of the primary air/fuel mixture flow,the gas temperature distribution of the furnace and the gas components in the furnace were conducted at loads of 660,550and 330MW e .With decreasing load,the gas temperature decreas and the ignition position of the primary coal/air flow becomes farther along the axis of the fuel-rich pipe in the burner region under the arches.The furnace temperature also decreas with decreasing load,as does the difference between the temperatures in the burning region and the lower position of the burnout region.With decreasing load,the exhaust gas temperature decreas from 129.8°C to 114.3°C,while NO x emissi
ons decrea from 2448to 1610mg/m 3.All three loads result in low carbon content in fly ash and great boiler thermal efficiency higher than 92%.Com-pared with the ca of 660MW e before retrofit,the exhaust gas temperature decread from 136to 129.8°C,the carbon content in the fly ash decread from 9.55%to 2.43%and the boiler efficiency incread from 84.54%to 93.66%.
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1.Introduction
Rerves of anthracite and lean coal are abundant and globally distributed.The u of anthracite and lean coal,which have low-volatility contents,prents difficulties in ignition and burnout.More and more rearch has been conducted around the world to solve the problems.At prent,down-fired combustion is widely applied in the power industry to consume low-volatility coals,and its main merit is that it achieves a high degree of burn-out by prolonging the residence time of pulverized-coal in the fur-nace.However,a practical down-fired boiler operation still suffers from the problems of high carbon content in the fly ash and poor flame stabilization at low load without oil support firing.
Preci understanding of the behavior of char particles in pul-verized-coal combustion systems is critical in determining funda-mental process that occur during heterogeneous combustion.Combus
tion data obtained for full-scale equipment can give the combustion and NO x emission characteristics of real combustors,in particular the turbulent flow of industrial coal flames.As a re-sult,studies using full-scale equipment are highly desirable and a necessity.In the surrounds of wall-fired boilers,measurements are made of the local mean concentrations of O 2,CO,CO 2,and NO x ,gas temperatures,and char burnout through veral obrv-ing doors in the utility boilers [1–5].Experiments have also been performed for tangentially fired boilers [6–9].However,for down-fired boilers adopting Foster Wheeler technology,there has only been the work of Li et al.,who measured the gas temperature,gas species concentrations in the furnace and carbon content in the fly ash in a 300MW e boiler before and after retrofit [10–14].
In the prent study,in situ experiments were carried out for a 660MW e down-fired pulverized-coal boiler after retrofit;this unit has the maximum capacity of any currently ud worldwide [15].Measurements of ignition of the primary air/fuel mixture flow,the gas temperature distribution of the furnace and the gas compo-nents in the furnace were made for this full-scale boiler at the rated,middle and half loads.The collected data were ud to deter-mine the combustion and NO x emissions characteristics of the boi-ler at different loads.The results obtained from the experiments help solve similar problems and benefit the design and operation of 600MW e and 1000
筹谋MW e down-fired boilers,and the data can be ud to support theoretical and numerical calculations.2.The utility boiler
The investigated 660MW e boiler,having the largest capacity of any down-fired boiler in the world,was made by Foster Wheeler (FW)Corp.Fig.1a is a schematic diagram of the furnace.Arches di-vide the furnace into two:the lower furnace below the arches and the upper furnace above the arches.Originally,36cyclone burners were arranged on the arches to produce a W-shaped flame.The fuel-rich flow streaming from the cyclone nozzle is near the
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Corresponding author.Tel.:+8645186418854;fax:+8645186412528.
E-mail address:green@hit.edu (Z.Li).
water-cooled wall while the fuel-leanflow jetting from the fuel-lean pipe faces the furnace center.Under the arches,there are three tiers of condary air denoted D,E and F,all of which are fed into the furnace horizontally.Details of the specific structure of this kind of boiler can be found in the literature[10–13].
During practical operation,the boiler suffers from great resis-tance and rious abrasion of the cyclone and high carbon content infly ash up to7–10%.Fuel-richflows form after the paration of the coal/airflows in the cyclones.The swirl intensities of fuel-rich flows decrea at the exits under the effect of the adjustable vane, which provides great resistance.The fuel-richflows with residual swirl have little rigidity and abrade the nozzles of the cyclones [16].The condary air under the arches enters the furnace horizon-tally and mixes prematurely with the fuel-richflow.This reduces the down-forwardflow and space for combustion and decreas the temperature in the lower furnace,which caus a ries of problems including the delayed ignition of pulverized-coal and high carbon content in thefly ash[13,17].There is fuel-leanflow with low momentum and shallow penetration depth near the furnace center, which readily shortens theflame and reduces upflow to the burnout region quickly.This is also a reason for the poor burnout degree of coal particles and high carbon content in thefly ash[18].
In2009,Li fitted the combustion system with many items.Fig.1b is a schematic diagram of the retrofitted combustion system.A fuel louver concentrator replaced the original cyclone as it has much lower resistance than the cyclone.The fuel/airflowfirst pass through the louver concentrator and parates into fuel-rich flow and fuel-leanflow.The fuel-richflow further divides into two and is inj
ected into the furnace through the original primary air ducts facing the furnace center while the fuel-leanflow as vent air is injected into the furnace near the wall.The vent air ducts are con-trolled by valves.The horizontal condary air is modified by the F-tier condary air with an angle of declination of20°.
In this retrofitted boiler,measurements were conducted at loads of660,550and330MW e with a constant opening of the vent air valves.
丽江什么时候去最合适
3.Data acquisition methods and experimental conditions
The formal in situ experiments were carried out in the2026tph down-fired pulverized-coal boiler to investigate certain aspects of the combustion process and NO x formation in the furnace.During the experiments,soot blowing and wer bleeding were not per-mitted.The coal ud in the experiments was a mixture of anthra-cite and lean coal.Sample characteristics of coals before and after retrofit are prented in Table1.
戒烟心得体会The following parameters were measured:(1)The gas temper-ature of the furnace was measured with a leucoscope through obrving doors in the front,rear and side walls.The layout of the obrving doors is shown in Fig.2.The measurement error is 50°C.(2)The gas temperature of the fur
nace was measured with a nickel chromium–nickel silicon thermocouple through obrving doors1,2and3.As shown in Fig.2,obrving door1is in the air-flow zone of the tier D and E slots,obrving door2is in the airflow zone of the tier F slots,and obrving door3is above the arches. The end of the bare thermocouple was expod in the furnace,so the temperature measured should be higher than the local gas temperature becau of highflame radiation.However,becau of radiation between the bare thermocouple and the water-cooled wall,and the clo proximity of the two,the temperature mea-sured should be lower than the local gas temperature.The calcula-tions have indicated that in the region of highest temperature the ‘‘true’’temperature do not exceed the measured one by more than 8%[3,19].The thermocouples ud were thoroughly checked be-fore leaving the factory,giving high confidence in the temperature measurement results.To minimize errors due to ash deposition,
儿童故事寒号鸟Table1
Characteristics of the coal ud in the experiments before and after retrofit.
Quantity Before retrofit After retrofit
Proximate analysis,wt.%(as received)
Volatile10.729.29
Ash30.8431.51
Moisture0.54 2.48
Fixed carbon57.9056.72
Net heating value(kJ/kg)23,53121,250
Ultimate analysis,wt.%(as received)
红楼梦黛玉葬花Carbon59.7059.48
Hydrogen 2.95 2.52
Oxygen 3.81 3.53
Nitrogen0.820.83
Sulfur 1.340.79
Z.Li et al./Applied Energy88(2011)2400–24062401
the thermocouples were frequently retracted from the furnace and, where necessary,any deposits were carefully removed.Moreover, the probes were replaced if there was any thermal distortion ob-rved.(3)The primary air/flow temperature distribution was measured with the same thermocouple inrted parallel to the axis of the fuel-rich pipe,as shown in Fig.1.(4)Gas compositions were sampled using a water-cooled stainless steel probe2.5m in length for analysis of the local mean O2,CO and NO x concentrations.As shown in Fig.3,the probe comprid a centrally located10mm (inner diameter)tube,through which quenched samples were evacuated,surrounded by a tube for probe cooling.The probe was cleaned frequently by blowing high-pressure air through it to maintain a constant suction rate.The water-cooled probe was inrted into the furnace through obrving door
s1–3(e Fig.2).The gas withdrawn were analyzed online using a Testo 350M system.Theflue gas after the air heater was also analyzed online.Calibrations with standard mixtures including zero concen-trations were performed before each measurement ssion.The measurement error for O2and CO2concentrations was1%,while that for CO and NO x concentrations was50ppm.(5)Unburnt car-bon infly ash was determined by collectingfly ash using a particle-sampling device with constant suction speed.
Values of the main operating parameters for the three operating loads are listed in Table2.
4.Results and discussion
Fig.4shows the gas temperature distribution of the fuel-burn-ing zone along the cyclone axes of the burners;zero points were t at the tips of the burner nozzles in the furnace.With decreasing load,the gas temperature decreas and the ignition position of the primary coal/airflow becomes farther along the axis of the fuel-rich pipe in the burner region under the arches,especially as the load decreas from550to330MW e.For the rated load,the gas temperature ris rapidly as the measurement point extends to lower positions,exceeding1000°C at a position400mm from the end of the fuel-rich nozzle and exceeding1200°C at 1400mm.For the load of550MW e,the gas temperature i
s a little lower than that for the rated load in the burner region,exceeding 1000°C at a position800mm from the fuel-rich nozzle.For the half load,the gas temperature ris slowly and the temperature gradi-ent is lower than in the other two cas,barely reaching1000°C at 2400mm.
The mass ratio of coal/air decreas as the load decreas,indi-cating that the concentration of pulverized-coal falls in the primary air and the momentum of coal/airflow decreas,resulting in a
Table2
Boiler operating conditions and measurement results.
剩余劳动力
Quantity Before retrofit After retrofit
660MW e660MW e550MW e330MW e
Totalflux of the primary air
(kg/s)
121127110.574
Temperature of the primary
air(°C)
130124125114
Totalflux of the condary
air(kg/s)
657620511396
Temperature of the
condary air(°C)
398405397362 Coal feeding rate(ton/h)268.6282.7244.6147.7 O2at the furnace exit(dry
volume%)
3.35 2.58 2.02 5.31
O2influe gas(dry volume
%)
4.76 3.99 3.047.27
NO x influe gas(mg/m3at6%
O2dry)
1181244821631610
Carbon infly ash(%)9.55 2.43 6.02 3.24 Exhaust gas temperature
(°C)
136129.8119.3114.3
Thermal efficiency of the boiler(%)84.5493.6692.5992.86
2402Z.Li et al./Applied Energy88(2011)2400–2406
shallower penetration depth and shorter residence time of the pri-mary coal/airflow in the lower furnace.For the reasons,the gas temperature measured along the axis direction of the fuel-rich pipe decreas and the ignition of the primary coal/airflow in the bur-ner region under the arches is f
arther along the axis.
The mass ratio of coal/air is defined to indicate the coal concen-tration in the primary air.As the load falls from660to330MW e, the mass ratio of coal/air drops from0.618to0.554,which indi-cates an obvious decrea in the coal concentration;thus,the heat and time needed for coal ignition increa.This is one of the rea-sons for the low gas temperature in the burner region and far igni-tion position of the coal particles at half load.For the load of 550MW e,the mass ratio of coal/air is0.615,which is little different to that for the rated load.Furthermore,the quantity of primary air and coal feed rate both decrea as the load decreas,as does the momentum of coal/airflow,leading to shallow penetration of the pulverized-coal in the lower furnace.The short residence times then lower the overall temperature of the burner zone to the ex-tent that the fuel-richflow cannot obtain sufficient heat from the recirculating up-flowing gas(e Fig.1).
Fig.5prents furnace gas temperature variations as measured using the thermocouple inrted through obrving doors1–3.On the whole,the gas temperature measured through obrving doors 1–3decreas with decreasing load.In the ca of the temperature distribution measured through obrving door1,the gas tempera-turefirst increas and then decreas becau some of the high-temperature gas recirculates into the near-wall zone.As the measurement points move deeper into t
he furnace,the measured temperature begins to fall as the thermocouple enters the fuel-rich flow.At the rated load,the gas temperature is steadily around 1000°C at points further than1400mm,while for the load of 550MW e,the measured temperature is a little lower.At the same position but for a load of330MW e,the temperature falls below 700°C gradually,indicating that the fuel-richflow has not ignited. This temperature quence is the same as that for the temperature gradients of the fuel-richflow mentioned above,indicating that ignition conditions worn as the load decreas.
In the cas of measuring the temperature through obrving doors2and3,once the temperature reached1250°C,no further measurements were taken at deeper positions for the simple rea-son of protecting the thermocouple from burnout.From the rela-tively limited temperature distribution,Fig.5shows that in the zone near obrving door2,the temperature ris rapidly at loads of660and550MW e,and at400mm,temperatures already exceed 1250°C.This indicates that coal burns more intenly in the lower furnace in both cas.At the load of330MW e,the temperature ris to nearly1200°C at the position1800mm from the side wall, increasing more slowly than in the other two cas.As illustrated above for the gas temperature in the burner region,the momentum of coal/airflow at half load is low,which results in a shallow pen-etration depth and short residence time of the primary coal/air flow and low furnace temperature in the lower furnace.Further-more,the F-tier
air accounts for a large proportion of the condary air entering the furnace under the arches;therefore,much of the air does not take part in combustion immediately,which decreas the temperature in the region near obrving door2.In the zones near obrving door3above the arches,the temperature increas to more than1100°C at a load of330MW e,which is just a little lower than the temperature near obrving door2.This can be ex-plained by the delay in the fuel-richflow ignition causing much of the fuel to burn in the upper furnace.
亮粉色Fig.6prents furnace temperature variations measured through obrving doors for the three loads.The furnace tempera-tures shown in Fig.6are averages of values measured through obrving doors at the same level.In the three cas,gas tempera-ture peaks are all in the lower furnace,and the distances from the peak positions to the exit of the furnace are relatively large.Resi-dence times for coal burning in the higher-temperature zone are thus longer,which favors fuel burnout.The difference between temperatures in the burning region and the lower position of the burnout region decreas as the load decreas,and is just35°C at a load of330MW e.This is becau the momentum of coal/air flow decreas,which results in a shallower penetration depth and shorter residence time of the primary coal/airflow in the lower furnace;the ignition position becomes farther from the fuel-rich nozzle,and more pulverized-coal combusts completely in the upper furnace.Moreo
ver,the temperature in the furnace decreas with the decreasing load.The reason for this is that the total heat
Z.Li et al./Applied Energy88(2011)2400–24062403
provided to the furnace also falls with decreasing load,decreasing the furnace temperature.
Fig.7shows the variations in gas species concentrations in the zones near obrving doors1–3.Gas species concentration mea-surements begin400mm from the side wall to avoid the effect of an air leak.In all three cas,the O2concentration quence is C(door1)>C(door2)>C(door3),which describes well theflow of coal as illustrated in Fig.1b.Fuel-richflowsfirst inject down-ward into the furnace and then rever their direction upwards in the down-fired boiler.The coal/airflows pass quentially through the obrvation doors1,2,and3and O2is consumed con-tinuously along the path of theflame.Thus,in each ca,the O2 concentration in the zone near obrvation door1was the highest, that in the zone near obrvation door2was intermediate and that near obrvation door3was the lowest.
In the airflowflow zone of the D and E tiers and in the zone near the wall,a great quantity of hot gas lowers the measured O2con-centration.When the measurement points are deeper and clor to the fuel-richflow,the measured value increas.O2concentra-tions near obrving door1increa with decreasing load,espe-cially from550to330MW e.For loads of660and550MW e,at the position1800m
m from the wall,O2concentrations are nearly 14%,suggesting that the fuel-richflow is beginning to react and a certain amount of O2is being consumed.For the load of 330MW e,however,O2concentrations are higher than18%,indi-cating that the fuel-richflow has not ignited,which agrees with the conclusion drawn from the temperature analysis.In the near-wall zone of obrving doors2and3,O2concentrations at half load are higher than in the other two cas,even as high as8%in the near-wall zone of obrving door3.
钱学森电影观后感
At all three loads,CO concentrations strictly rule contrary to O2 concentrations.In the zones near obrving doors1–3,the higher the O2concentration,the lower the CO concentration.In the air-flowflow zone of the D and E tiers,CO concentrations decrea with decreasing load becau of the delayed ignition of the fuel/ airflow.In the near-wall zone of obrving doors2and3,CO con-centrations in the ca of the half load are lower than in the other two cas becau of the highly oxidizing atmosphere in the furnace.
2404Z.Li et al./Applied Energy88(2011)2400–2406
