Experimental trends of NO in circulating fluidized bed combustion

Experimental trends of NO in circulating fluidized bed combustion

Fuel 88 (2009) 1333–1341 Contents lists available at ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel Experimental trends of NO in...

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Fuel 88 (2009) 1333–1341

Contents lists available at ScienceDirect

Fuel journal homepage: www.elsevier.com/locate/fuel

Experimental trends of NO in circulating fluidized bed combustion A. Tourunen *, J. Saastamoinen, H. Nevalainen VTT Technical Research Centre of Finland, P.O. Box 1603, FI-40101, Jyväskylä, Finland

a r t i c l e

i n f o

Article history: Received 10 March 2008 Received in revised form 16 December 2008 Accepted 17 December 2008 Available online 14 January 2009 Keywords: Nitrogen emissions NO N2O Char inventory CFBC

a b s t r a c t Experimental trends for the dependence of CO, NO and N2O emissions on bed temperature and oxygen concentration in circulating fluidized bed combustion (CFB) are presented. The main focus is on the nitrogen emission formation in the lower furnace area. A test campaign including seven tests with a laboratory scale CFB test rig were carried out to produce appropriate data of the phenomena. These experiments show that NO emissions above the dense bed decrease with decreasing temperature or oxygen concentration. Instead, N2O emissions increase when the bed temperature is decreased and decrease when the oxygen concentration is decreased. These trends can partly be explained by heterogeneous reactions between NO and char, since decrease in temperature or oxygen concentration increases the bed char inventory. However, oxygen and temperature also affect directly on NO emissions. Correlations for the CO, NO, N2O, NH3 and HCN concentrations at the exit of dense bed were developed. This type of correlations can, among other things, be applied as boundary conditions to the more sophisticated CFD models that are usually applied to modelling of diluted part of the furnace. CFD modelling of the dense bed area is complicated and accuracy is not sufficient, thus simplified experimental correlations can aid in the development of furnace design towards better emission performance. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction The circulating fluidized bed (CFB) boiler technology is a relatively young compared to pulverized coal boilers or other types of conventional coal fired boilers. At present, CFB boilers represent the market for relatively small units, in terms of utility requirements. The largest CFB boilers in operation are about 300 MWe. In recent years, the once through supercritical (OTSC) CFB technology has been developed and the first boiler of its kind is under construction in Poland (460 MWe). However, further scaling up the technology to real utility scale (600–800 MWe) is needed to fulfil the future requirements of utility operators. CFB combustion has shown its environmental benefits due to its inherently low NOx emissions without selective catalytic reduction. The emissions standards, however, will tighten in the future and, therefore, new ways for efficient and cost-effective emission reduction are needed. Secondly, as the scaling up of the CFB technology is considered there is a need for better understanding and predicting the phenomena of emission formation and destruction in larger CFB furnaces both in stationary and dynamic conditions. Development and implementation of suitable emission models based on detailed CFD modelling are therefore required. Naturally, pilot scale tests as well as results from tests at commercial boilers

* Tel.: +358 20 722 2718; fax: +358 20 722 2597. E-mail address: Antti.Tourunen@vtt.fi (A. Tourunen) 0016-2361/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2008.12.020

are needed for achieving the theoretical development and verification of the emission models. The combustion process in CFBs is mainly affected by the designs and choices that are made in the lower furnace. Fuel and air feeding and penetration will be essential to provide a proper mixing of fuel and air for optimal combustion performance. Primary air flow rates vs. secondary air flow rates and locations will also have their effect on combustion performance and optimized reducing conditions for low NOx performance and conditions for sulfur capture. The energy balance of the lower furnace also needs special consideration to provide proper temperature profiles for the process. The general trends of NOx formation and destruction are well known, but in the lower furnace area more information is needed to scale up the boiler design and maintain the superior emission performance, which is typical for the CFB technology, also on a utility scale. CFD modelling related to the lower furnace and especially to the dense bed area is very complicated and the accuracy of the models is not sufficient thus detailed experimental information from pilot scale environments is needed. During the past decades, much information has been published on NOx emissions, but there is very limited information available about measured char inventory and its effect on NOx emissions. NO reduction by char possibly enhanced by CO is used in several papers as an explanation for the NO destruction, but the lack of measurement of char inventory prevents the conclusions of the actual magnitude and importance of the heterogeneous reactions between NO and char. Further, it is

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difficult to separate the effects of oxygen concentration and temperature on NO emissions without measurement of char inventory, which is highly dependent on those two parameters. Char inventory measurements have been very rarely published combined with measurements of NO concentrations. A well-known trend in power plants and other combustion devices is that when increasing CO in flue gases, NO is usually reduced (e.g. [1]). NO and N2O emissions in CFB combustion conditions have been studied experimentally [2–18] and by modelling [2,19–22]. The reduction of NO to N2 with char is known to be significant in fluidized bed combustion of coal [23]. Reviews on the kinetics of NO reduction with char particles have been presented in the literature [24,25]. The effect of coal type and the role of NO reduction by char in fluidized bed combustion have recently been studied [26]. Correlations for NO depending on O2 concentration and temperature have been presented for pressurised fluidized beds [27] but not for atmospheric CFBC. The experimental trends for the dependence of NO emissions on operational conditions in a CFBC pilot scale reactor are studied in the present paper combined with char inventory.

2. Experimental The effects of bed temperature and primary air oxygen concentration on emission formation in the lower furnace area were studied with a laboratory scale CFB reactor (see Fig. 1). The fuel power of the CFB-pilot is 40–60 kW during normal operation. The height of the riser is 8 m and the inner diameter 167 mm. The reactor is equipped with several separately controlled electrically heated and water/air-cooled zones in order to control the process conditions (for example oxygen level, temperature and load) almost independently. Several ports for gas and solid material sampling are located in the freeboard area. The main fraction of the fly ash is removed by the secondary cyclone. The bag house, which collects the finest fly ash from the flue gas, is located after the gas cooler. The reactor is controlled with a computer on which all measurement data is saved. Fuel can be fed into the reactor through two separate fuel-feeding lines. There is also a feeder for additives such as limestone. Fuel containers are mounted on the top of scales, which enables the determination of mass flow rates for solid fuels as a weight loss against time.

The combustion air can be divided into primary, secondary, and tertiary air. Primary air is fed through an air grid with 1100 evenly distributed 1 mm holes. Oxygen concentration of primary air can be controlled by mixing nitrogen gas with air. The secondary and tertiary airs can be fed to three different levels of the reactor. In this work, the lowest feeding point (1.3 m above the air grid) for the secondary air was applied. The reactor is equipped with FTIRspectrometer and traditional on-line analysers for main flue gas compounds. Gas samples can be taken to FTIR-spectrometer from different levels of the freeboard and also before and after cyclones, gas cooler and bag house filter. Traditional on-line analysers are connected to the flue gas duct between the gas cooler and bag house filter. In this work, flue gas concentrations were measured 1.1 m from the air grid (above the dense bed but before secondary air feed) by a FTIR gas analyser and an O2 analyser that was connected to the same sample line with a FTIR spectrometer. The main flue gas compounds were also measured from the outlet of the reactor (flue gas duct) including O2, CO2, CO, NO, and SO2 measurements (Servomex 4900, (Gfx) IR). In addition to this, vertical flue gas profiles along the riser height were measured in two test runs with the FTIR spectrometer combined with O2 analyser. The test campaign included seven tests in which oxygen concentration of primary air was varied between 13 and 21 vol% and bed temperature was varied from 752 to 912 °C (see Table 1). Proximate and ultimate analysis for the fuel that was used in the pilot scale CFB experiments are shown in Table 2. The main aim was to study the lower furnace effect (dense bed) on emission formation and thus process conditions (O2, T) were varied in the region that was before the secondary air feeding port (1.3 m). The work was focused on the primary zone (dense bed region), because the major part of the char (which is one focus in the paper) is located in the dense bed. Information on the whole reactor has been added (in addition to primary zone) for the reader to see the effect of primary zone to the actual emissions of the whole unit. Bed temperature was changed in tests 1 and 5–7 as seen in Fig. 2a and fluidization velocity was kept constant by adjusting primary air flow with nitrogen. Oxygen concentration in primary air was varied in tests 1–4 with the addition of nitrogen and during those tests the bed temperature was kept as constant as possible (average 866 °C). The overall stoichiometry over the reactor was maintained by increasing secondary air flow in proportion to nitrogen dilution in primary air. Test number 1 is a reference case and

FTIR sampling port Gas analysator

from separator

Bag filter

sec. gas

reactor

Observation port To stack

gas & solids sampling ports

Deposit probe port

Gas cooling

FTIR + O

Sampling port

controllable water-cooled rod

Secondary cyclone

Primary Sampling port cyclone FTIR sampling port

Zone 4

fuel feed

controllable loop seal heater

Sampling port

Zone 3 Sampling port

controllable bed cooler

Sampling port

Circulation material sample

Zone 2 Sampling port

primary gas with adjustable temperatureand composition

Fuel container 1 and 2 Additive container

Zone 1 Secondary air Nitrogen

Bed material sample M

Primary gas heating PC control and data logging system Sampling port

Fig. 1. Schematic picture of the sampling arrangements in the pilot CFB reactor.

Air

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A. Tourunen et al. / Fuel 88 (2009) 1333–1341 Table 1 Fuel: Polish bituminous coal (Ø < 2 mm). Limestone: Commercial CFB-limestone applied with Ca/S molar ratio 2. Tbed (°C)

Primary gas O2 content (%)

1 2 3 4 5 6 7

871 864 866 861 752 800 912

20 18 15 13 18 19 21

Table 2 Proximate and ultimate analysis of Polish bituminous coal. Fuel analysis

Polish bituminous coal

Moisture (wt.%) Ash, dry basis, (wt.%) 815 °C Volatile content, dry basis (wt.%) Higher heating value, dry basis (kJ/kg) Lower heating value, dry basis (kJ/kg)

4.6 12.6 30.5 29,310 28,350

1000 850 100 800 10

Test2 Test4 Test6 Average pressure profile

700

1 4

6

8

10

Pressure (Pa)

Temperature (°C)

900

Flue gas oxygen concentration (vol-%, dry)

b

10000

950

2

Test 3

Test 4

Test 5

Test 6

Test 7

ment located 1.1 m from the air grid (see Fig. 3). Fuel-N conversion to nitrogen compounds was calculated as a ratio of measured mass flow rate of specific nitrogen compound to nitrogen feed rate as fuel. The total fuel nitrogen conversion to NOx, N2O, NH3, and HCN compounds was between 15% and 20% in all tests. The same conversions are shown in Fig. 4a–b for different series in order to illustrate the effect of bed temperature and primary air oxygen concentration on nitrogen emission formation. These variables have been shown to be the most important factors affecting NO emissions (e.g. [19,14]). Fuel nitrogen conversion to NO and N2O decreases and conversion to NH3 increases when the primary air oxygen concentration was diluted with nitrogen (tests 1–4). There is also a very slight increase in HCN conversion. Hence the effect of decrease in O2 concentration on formation of nitrogen compounds is mainly related to the increase of ammonia concentration at the expense of NO and N2O. With normal O2 concentration (test 1) NO conversion is about 1.5 times higher compared to low O2 concentration (test 4). Changes in bed temperature affect mainly the ratio between NO and N2O conversions, while other measured conversions for nitrogen compounds remain almost unchanged. The NO conversion above the dense bed doubled when the bed temperature increased from 752 to 912 °C. Similar results were reported by, for example, Zhao et al., [14] who noticed that NOx emissions (in the stack gas) doubled along with temperature increase from 780 to 880 °C.

Conversion of fuel nitrogen to different nitrogen compounds (NOx, N2O, NH3, and HCN) was calculated based on FTIR measure-

0

Test 2

Fig. 3. Conversion of fuel nitrogen to nitrogen compounds in the lower part of the furnace (1.1 m).

71.1 4.4 1.22 0.79 9.72 0.17 56.9

Test1 Test3 Test5 Test7

5

Test 1

2.1. Formation of nitrogen compounds in the dense bed area

750

10

0

thus it belongs to both of these two otherwise independent series (O2 and T series). A clear negative correlation between oxygen and CO concentrations was noticed in the dense bed area in tests 1–4 (see Fig. 2b). There is a slight decrease in oxygen concentration above the dense bed in tests 5–7, while CO concentration decreases rapidly due to temperature increase from 752 to 912 °C. Flue gas oxygen concentration at the outlet of the reactor remained rather constant during all the experiments by adjusting secondary air flows between the tests. The aim was to maintain overall excess air ratio as constant (see Fig. 2b) during the modifications of lower furnace conditions. Primary gas grid velocity (fluidization velocity) was maintained as constant in each test runs corresponding to 2 m/s and 0.6 s residence time before the gas sampling probe. The average gas velocity after the secondary air feed was 3.6 m/s.

a

15

7

O2, bed O2, stack CO, bed

6

10000

8000 5 6000

4 3

4000

2 2000 1 0

Flue gas CO concentration (vol-ppm, dry)

Element analysis, dry basis C-content (wt.%) H-content (wt.%) N-content (wt.%) S-content (wt.%) O-content (wt.%) Cl-content (wt.%) Fixed carbon, dry basis (wt.%)

NH3 NOx

20 Fuel-N converted to nitrogen compounds (%)

Test no.

HCN N2O

0 Test1

Test2

Test3

Test4

Test5

Test6

Test7

Distance from the air grid (m) Fig. 2. (a) Measured temperature profiles and an average pressure profile illustrating distribution of solid material during the laboratory scale CFB experiments. (b) Flue gas oxygen and carbon monoxide concentrations measured above the dense bed and flue gas oxygen concentration in the stack.

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HCN NH3

N2O NOx

100 %

80 %

60 %

40 %

20 %

O2 20 %

O2 18 %

Test 1

Test 2

O2 15 %

O2 13 %

0% Test 3

b Fuel-N converted to nitrogen compounds

Fuel-N converted to nitrogen compounds

a

HCN NH3

N2O NOx

100 %

80 %

60 %

40 %

20 % 752 oC

800 oC

871 o C

912 oC

Test 5

Test 6

Test 1

Test 7

0%

Test 4

Fig. 4. (a) Fuel nitrogen conversions to different compounds in the lower part of the furnace scaled to 100% (NOx + N2O + NH3 + HCN = 100%) with different primary air oxygen concentrations. (b) Fuel nitrogen conversions to different compounds in the lower part of the furnace scaled to 100% with different bed temperatures.

Table 3 Release of chemical compounds of Polish bituminous coal during the pyrolysis and char combustion in laboratory oven at 915 °C. The figures are based on weight ratio of dry fuel. Chemical element

Pyrolysis (wt.%)

Char combustion (wt.%)

C H N S

27.0 88.4 31.9 25.3

73.0 11.6 68.1 74.7

NH3 and HCN are the compounds that are typically formed during the devolatilization of fuel particles [28] and later these compounds oxidize partly to NO and N2O. Release of nitrogen during pyrolysis was also studied by laboratory oven tests (see Table 3). These results show that about 32% of fuel nitrogen is released during the devolatilization of fuel in a laboratory oven at 915 °C. This means that fuel nitrogen is released roughly in the same proportion during the pyrolysis as the volatiles are released. This result is well in line with the reported observations about the correlation between NOx emission and the product of fuel volatile and nitrogen content [14]. Fig. 5 shows the calculated effects of temperature and oxygen concentration on the rates of formation of NO by oxidation of NH3 and HCN and reduction of NO by reactions with NH3 and HCN. It is seen increase in temperature increases the rates both formation and reduction rates. At low bed temperatures more of HCN and NH3 formed during pyrolysis will escape the bed without oxidation to NO, which is in line with the

a

experimental result (Fig. 4b) that NO concentration is low at low temperature. Fig. 5 also shows that the rate of oxidation of NH3 increases directly proportional to O2 concentration, but that the rate of oxidation of HCN becomes independent of O2 concentration when O2level is exceeds a critical level. This is in agreements with the experimental results presented in Figs. 3 and 4 showing NH3 concentration to be very sensitive to oxygen concentration but HCN being not so sensitive. Fig. 6 illustrates the effect of char inventory in the reduction of NO. Polish coal is fed with a continuous feed and along this feed either a small batch of coal or char is fed instantaneously into the bed. NO emission increased just after the impulse feed of coal (1.4–2 min) but it decreased after 2 min. This may be explained by the increase in NO emission through volatile-N combustion followed by NO reduction by char particles. The explanation was confirmed by the impulse feed of char, which supported the above stated assumption (see Fig. 6). 2.2. Gas profiles along the furnace height NO and N2O concentration profiles were measured along the furnace height during tests 5 and 7 (Fig. 7). The high bed temperature (test 7, 912 °C) leads to a high NO concentration level throughout the furnace height compared to low bed temperature (test 5, 752 °C). N2O concentration behaves contrary to NO, which can be explained (at least partly) by char inventory and destruction of NO to N2O with heterogeneous char reactions. An interesting detail is that with the low bed temperature (test 5) NO and N2O con-

b

0.00004

0.00002 0.00001 0 -0.00001 -0.00002

0.000001 0 -0.000001 -0.000002

-0.00003 -0.00004 750

0.000003 0.000002

Rate (kmolNO m-3s-1)

Rate (kmolNO m-3s-1)

0.00003

-0.000003

800

850

Temperature (oC)

900

950

0

0.05

0.1

0.15

0.2

Mass fraction of O2

Fig. 5. Rate of NO formation (full symbols) by oxidation of NH3 (d) and HCN (N) and rate of destruction of NO (empty symbols) due to reaction with NH3 (s) and HCN (4). In calculations mass fractions of NO, NH3, and HCN are 0.0001. (a) Effect of temperature, when mass faction of O2 is 0.05. (b) Effect of oxygen mass fraction, when temperature is 850 °C. Calculations are based on the assumption of perfect mixing of gases and global mechanisms presented in the literature (NH3-reactions [29], HCN-reactions [30]).

1337

200 9.5

NO

8.5

150

7.5 100 6.5

NO, char

O2

NO, Polish coal

50

5.5

O2, char O2, Polish coal

4.5

0 0

1

2

3

4

5

6

Oxygen concentration in flue gas (vol-%, dry)

NO concentration in flue gas (vol-ppm, dry)

A. Tourunen et al. / Fuel 88 (2009) 1333–1341

7

Time (min) Fig. 6. Flue gas concentrations during continuous combustion of coal with impulse feed of coal and char batches.

Fuel-N conversion to NO and N2O (%)

18

Test 7, NO Test 5, NO Test 5 (NO+ N2O)

16 14

Test 7, N2O Test 5, N2O Test 7 (NO+N2O)

12 10 8 6 4 2 0 0

1

2

3

4

5

6

7

8

9

Distance from the air grid [m] Fig. 7. NO and N2O conversion profiles measured during tests 5 and 7 along the furnace height.

versions are on the same level in the lower part of the furnace, while with the high bed temperature they are on the same level at the exit of the reactor. The sum (NO + N2O) is constant at low temperature (test 5) and decreases with the higher temperatures (test 7). Experimental results available in the literature showed similar profiles, e.g. De Diego et al., [5], Kilpinen et al. [31] and Zhao et al. [13].

tom region was calculated by multiplying total solid material inventory on that specific region by solid (combustible) carbon concentration in the solid material sample taken from that specific region. The total solid material inventory was calculated based on the pressure difference over the specific region. The relative char inventory means that char inventories has been scaled from 0% to 100%. Test 4 with the highest char inventory is used as the reference value (100%) corresponding 120 g in bed and 19 g/min in circulation. Measured char inventories (combustible carbon) are shown in Fig. 8. Char inventory was more sensitive for the oxygen concentration changes in primary air compared to the changes that were made for the bed temperature (see Fig. 9). When oxygen concentration in primary air was changed from 20% to 13% char inventory decreased about 50%: Char inventory increased with decreasing O2 concentration in the primary air. The char inventory varied only about 15% for the bed temperature variation over the total range (752–912 °C). There is a clear correlation between char inventory and NO if the series are studied separately (see Fig. 10a,b). NO conversion measured above the dense bed (1.1 m) has a stronger correlation with char inventory at the bed compared to NO conversion determined from the stack gas. This is obvious due to high char inventory in the dense bed where the heterogeneous reactions can take place. As seen in Fig. 9, the primary air oxygen concentration has a stronger effect, within variation range, on char inventory and thus the correlation between NO conversion and char inventory is

2.3. Effect of char inventory on CO, NO and N2O emissions 120 Normalized char loadings (%) Test Bed Circulation 1 37.0 48.2 2 44.6 29.6 3 69.3 59.5 4 100.0 100.0 5 43.2 39.6 6 37.0 34.7 7 26.7 26.3

100

Char in bed

Char loading (%)

Low bed temperature as well as oxygen concentration leads to high char inventory in the dense bed. It is well known that the reduction of NO to N2 with char can be significant in CFB combustion. Thus, it is interesting to study how a clear correlation between NO concentration and char inventory can be found. In CFB conditions, NO and N2O formation and destruction can be dominated by heterogeneous char reactions. There are several papers available in which the importance of char inventory is discussed as playing a dominant role for low NO emissions, especially in coal combustion. However, there is lack of experimental information available of measured char inventory combined with emission measurements. In many papers, char inventory is based on simplified models, and thus causes error in model prediction due to lack of experimental validation (e.g. [19,32]). During the measurement campaign solid material samples were taken in each test run from the bed and from the circulation loop. Total carbon, combustible carbon and carbonate carbon concentrations were analysed for the samples. The char inventory in the bot-

Char in circulation 80

60

40

20

0 Test 1

Test 2

Test 3

Test 4

Test 5

Test 6

Test 7

Fig. 8. Char inventory in bed and in circulation loop based on pilot scale CFB measurements (tests 1–7). Char inventories are scaled separately for the bed and the circulation loop (0–100%). Test 4 with the highest char inventory is used as the reference value (100%) corresponding 120 g in bed and 19 g/min in circulation.

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a

7

9

11

13

Temperature varied Oxygen concentration varied

80

Test 1 40

0 770

720

820

870

Average O2 concentration on the bed area (vol-%, dry)

15

7

5

Fuel-N conversion to NO (%)

Char loading in the bed (%)

b

Average O2 concentration on the bed area (vol-%, dry) 5

9

11

16

13

15

Test 1

12

8

Temperature varied

4

Oxygen concentration varied 0 720

920

770

820

870

920

Average bed temperature (oC)

Average bed temperature (oC)

Fig. 9. (a) Effect of oxygen concentration and bed temperature on char inventory based on pilot scale measurements. (b) Fuel-N conversion to NO in tests 1–7, in which test 1 is a shared point for the T and O2 series. Average O2 concentration is arithmetic average of primary gas O2 concentration and flue gas O2 concentration measured 1.1 m from the air grid.

16

NO, bed NO, stack N2O, bed

12

8

4

0 50

60

70

80

90

100

110

Char inventory (%)

Fuel-N conversion to NO and N2O (%)

b Fuel-N conversion to NO and N2O (%)

a

16

NO, bed NO, stack N2O, bed

12

8

4

0 20

40

60

80

100

Char inventory (%)

Fig. 10. (a) Fuel nitrogen conversions as a function of char inventory (T series). (b) Fuel nitrogen conversion as a function of char inventory (O2 series). Char inventory is scaled separately for both series (0–100%).

clearer in O2 series. When both series are presented in the same figure, as they are in Fig. 11, the differences in the slopes of the trend lines are clearly seen. The trend for N2O emission is different between the series, but otherwise the trends are parallel, except for the T series, which has the sharper slopes with the lower char inventories. If we assume that NO þ char  N ! N2 O, this seems to be a reasonable reaction for the T series in which N2O conversion is increasing and NO is decreasing with increasing char inventory. For the O2 series, the NO and N2O concentrations are decreasing with increasing char inventory and thus N2O formation by charN reaction with NO cannot explain the observed trends. As seen

in Fig. 4, ammonia (NH3) conversion increases with the low oxygen concentrations. This can be explained by the reduction of oxidation reaction of ammonia to NO and further to N2O. Due to this reduction, conversions of NO and N2O decrease with low oxygen concentrations (see Fig. 4). We can conclude that when temperature is varied, the NO formation increases with the bed temperature and N2O decreases that can be explained with heterogeneous char reactions. Variation of combustion stoichiometry, when the oxygen concentration is decreased, leads to a decrease of fuel nitrogen conversions to NO and N2O. 3. Correlations for emission formation in the lower furnace

Fuel-N conversion to NO and N2O (%)

20

NO, bed, O2 series NO, stack, O2 series N2O, bed, O2 series NO, bed, T series NO, stack, T series N2O, bed, T series

16

12

8

4

0 0

20

40

60

80

100

120

Char inventory (%) Fig. 11. Fuel nitrogen conversions to NO and N2O as a function of char inventory.

Experimental results related to flue gas emission have been measured in different scales and compared in the literature. Laboratory scale CFB pilots have been compared to commercial scale CFB boilers [4,33,34]. The results indicate that laboratory scale measurements can also be of value especially related to phenomena studies such as emission formation and destruction in wellcontrolled conditions. Correlations, shown by Eqs. (1)–(6), were developed to predict emission formation for CO, NO, N2O, NH3, HCN and char inventory in the lower furnace area as functions of flue gas oxygen concentration and bed temperature (see Fig. 12a–d). These correlations are based on the experiments presented in Table 1. The operational range of Table 1 is 752–912 °C and 6.7–12.1 vol% for the average temperature and the oxygen concentration in the lower part of the furnace, respectively. In addition the correlations are based on only for the specific fuel and the reactor

1339

10

a

Calculated NO conversion (%)

Calculated CO conversion (%)

A. Tourunen et al. / Fuel 88 (2009) 1333–1341

5

0 0

5

15

b 10

5

10

5

10

10

15

Measured NO conversion (%)

c

Calculated char inventory (%)

Calculated N2O conversion (%)

Measured CO conversion (%)

5

0

100

d

80 60 40 20 0

0

5

10

Measured N2O conversion

0

20

40

60

80

100

Measured char inventory (%)

Fig. 12. (a) Correlation for the CO conversion on the dense bed area as a function of oxygen concentration and bed temperature. (b) Correlation for the NO conversion on the dense bed area as a function of oxygen concentration and bed temperature. (c) Correlation for the N2O conversion on the dense bed area as a function of oxygen concentration and bed temperature. (d) Correlation for the char inventory on the dense bed area as a function of oxygen concentration and bed temperature.

 O  0:0078  T bed Þ Y CO ¼ 56100  expð0:29  X 2

ð1Þ

 O Þ þ 0:0443  T bed  38:45 Y NO ¼ 5:9  expð0:0636  X 2

ð2Þ

 7:544 Þ  0:03  T bedj þ 12:35 Y N2 O ¼ lnðX O2

ð3Þ

 1:7254  0:0858  T bed þ 72; mchar ¼ 2536:8  X O2

ð4Þ

 4:5352  0:0012  T bed þ 1:15 Y NH3 ¼ 33611  X O2

ð5Þ

 O Þ  0:0817  T bed þ 0:9 Y HCN ¼ 1514  expð0:0091  X 2

ð6Þ

 denotes average where Y is conversion of specific emission (%), X volume fraction of oxygen (%) at the bed area, T is average bed temperature (°C) and mchar is char inventory in the bed (%). Temperature dependence is linear in NO, N2O and char inventory (mchar ) correlations, which leads to a conclusion that heterogeneous char reactions are in a dominant role for NO and N2O emissions if temperature is varied over the typical operation range of CFB conditions. Lyngfelt et al. [11] showed experimentally that bed temperature has no significant effect on NO emissions in the stack gas during the combustion of wood chips in a 12 MWth CFB. Temperature was varied from 740 to 850 °C and NO remains almost constant, but CO emissions decreased radically. The result supports a conclusion that char inventory is very low in combustion of wood-chips compared to, e.g., coal combustion and due to that the effect of temperature is not significantly affecting NO emissions. The effect of primary air oxygen concentration on NO and N2O is more complex than the effect of temperature. 4. Comparison of conversions in the stack gas and in the flue gas above the dense bed The lower furnace area is in a key role when the emission performance of a boiler is predicted. However, from the operator point of view, only the overall emissions in the stack gas have importance. Overall fuel nitrogen conversion to NO is compared to con-

versions measured above the dense bed in Fig. 13a–b. Overall fuel nitrogen conversion to NO in the stack gas reaches a minimum value with 18 vol% oxygen concentration in the primary air. In contrast, above the dense bed, fuel nitrogen conversions to NO decreased slightly with decreasing O2 concentration in the primary air. When comparing the results between the lower furnace and stack gas it is worth noticing that O2 concentration in the stack gas remains almost constant in all experiments and also temperature at the exit of the reactor was rather constant (see Fig. 2a,b). It has also been reported that CO reduces NO more effectively compared to char [35]. There is also a clear positive correlation between char inventory and CO conversion above the dense bed, which strengthens the reduction of NO at the bed area. When conversion to NO reaches the minimum value in the stack gas, CO conversion obtains its maximum. Similar results have been reported [11,36]. Generally, the air staging (primary air O2 concentration) seems to be optimal (18 vol%) in test 2 from overall NO conversion point of view (stack gas). The staging is too heavy in tests 3–4, which leads to high formation of NH3. However, the appropriate air staging with low bed temperature leads to low overall conversion to NO, but, at the same time, fuel nitrogen conversion to N2O will typically obtain a maximum value. In future, there will also be an emission limit for N2O, which is a much more harmful greenhouse gas than CO2. NO conversion trend is linear for the T series, except for the highest temperature (912 °C) (see Fig. 13b). This is probably related to radical change in sulphur capture, which has been reported to have an effect on NO emissions throughout changes in SO2 and free CaO concentrations [5,37]. CO emissions in the stack gas can be correlated with the cyclone outlet temperature (see Fig. 13c) within typical operational conditions of CFB combustion. If the bed temperature is very low (752 °C), correlation fails to predict the overall CO conversion based on only the cyclone outlet temperature. Similar experimental results have been reported (e.g. [19,11,38]). If we calculate the difference between the fuel nitrogen conversions to NO above the dense bed (1.1 m) and at the stack gas, we

A. Tourunen et al. / Fuel 88 (2009) 1333–1341

10

0.2

8 6

0.1

4 2

15

16

18

20

5 20

0 700

22

750

CO, Stack CO, Stack, Test 5 low bed temperature

0.3

0.2

0.1

0.0 840

860

850

0 950

900

880

900 o

Temperature at the outlet of the primary cyclone ( C)

Fuel-C conversion to CO above the dense bed (%) Difference of Fuel-N conversion to NO (%)

CO conversion at the stack gas (%)

c

820

800

Bed temperature (oC)

0.5

800

60

40

O2 concentration in primary air (vol-%)

0.4

80

10

0.0 14

12

b

NO above the dense bed NO at the stack gas SO2 above the dense bed SO2 at the stack gas

Conversion of fuel-S to SO2 (%)

12

0.3

a

NO above the dense bed NO at the stack gas CO above the dense bed CO at the stack gas

Conversion of fuel-N to NO (%)

Conversion of fuel-N to NO and fuel-C to CO (%)

14

Conversion of fuel-C to CO at the stack gas (%)

1340

0

2

4

6

8

10

10

d

8 6 4 NO conversion difference (char loading in bed) NO conversion difference (char loading in circulation) NO conversion difference (CO conversion)

2 0 0

20

40

60

80

100

Char inventory (%)

Fig. 13. (a) Fuel-N conversion to NO measured above the dense bed and in the stack gas for O2 series. (b) Fuel-N conversion to NO measured above the dense bed and in the stack gas for T series. (c) Fuel-C conversion to CO measured in the stack gas. (d) The change of NO conversion from 1.1 m (above the bed) to stack gas as a function of char loading in bed and in circulation flow and fuel-C conversion to CO.

can also illustrate the dependence of NO conversion on char inventory and CO conversion at the upper part of the reactor (see Fig. 13d). The higher char inventory or CO conversion is in the bed area, the less is the reduction of NO conversion in upper part of the reactor (i.e. between 1.1 m and the stack gas). 5. Conclusions The effect of temperature, oxygen concentration and char inventory all seem to affect the emission of NO. However, the amount of char in the bed is not an independent parameter, but it depends on the oxygen concentration and temperature. Besides the amount of char, also its reactivity with NO depends on temperature, which leads to opposing effects; increase in temperature decreases the amount of char but increases its reaction rate with NO. Oxygen concentration has also small similar effect, since particles burn at higher temperature with increasing oxygen concentration. Therefore it is difficult to determine the relative importance of the direct effects of temperature and oxygen concentration and the effect of NO reduction by char on the resulting NO concentration. In addition, this depends much on the split between nitrogen release as volatiles and in char. NO resulting from volatiles is directly affected by temperature and oxygen concentration. (a) NO emissions above the dense bed can be correlated with average bed temperature and oxygen concentration.The effect of temperature on NO conversion is linear within typical operation conditions of CFB. If the temperature is increased above the 900 °C, the correlation changes due to changes in sulphur capture conditions, i.e., sulphur capture remarkably slows down increasing SO2 concentration and on the other hand free CaO concentration. These both have been reported to affect NO emissions (e.g. [5,37]). (b) The linear correlation between the bed temperature and NO can be linked with char inventory, which also has a linear correlation with the bed temperature. Due to that decrease of conversion to NO (along with temperature increase) can be assumed to be

strongly influenced by heterogeneous char reactions (NO + charN ? N2O). The measured flue gas profiles for NO and N2O also support the earlier conclusion due to the decrease of NO with increasing of N2O along the furnace height. (c) The dependence of fuel nitrogen conversion to NO on oxygen concentration in the bed can be described with an exponential curve. Decrease in oxygen concentration increases conversion to ammonia at the expense of NO and N2O. The conversion to ammonia increases especially in reducing conditions, which can be explained by the reduction of the rate of oxidation of ammonia to NO and further to N2O. NO conversion measured above the dense bed decreased with decreasing primary air oxygen concentration, but the overall (in the whole reactor) conversion to NO started to increase if too low oxygen concentrations were applied. Overall CO concentration reaches the maximum value at the same condition when the NO was minimized in air staging tests. (d) NO, N2O, CO and char inventory can be correlated (with reasonable accuracy) with temperature and oxygen concentration in the bed area. However, the operation range of simplified correlations is limited to typical CFB conditions. Adverse conditions create limited range of validity for the correlations, for example, very high/low air staging or temperature leads to remarkable changes in complicated chemical reactions (reducing/oxidizing conditions, sulphur capture, etc.) which affect nitrogen emissions. (e) Char inventory combined with CO concentration is a significant parameter for the NO reduction. The effect of char particles on NO reduction is clearly shown in Fig. 6 with impulse feed of fuel and char batches. Decrease in temperature or oxygen concentration increases the bed inventory and CO concentration, which both lead to reduction of NO emissions. However, it is difficult to see separate effect of char due to gas phase reactions of NO. (f) The higher char inventory in the bed or CO concentration above the dense bed is the less is the change (reduction) of fuel nitrogen conversion to NO in the upper part of the reactor. This is possibly explained by high NO reduction by char and CO already in the lower furnace area.

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Developed correlations can be applied as boundary conditions to the more sophisticated CFD models that are usually applied to modelling of the diluted part of the furnace. CFD modelling of the dense bed area is complicated and the accuracy is not sufficient, and thus simplified experimental correlations can aid in development of furnace design towards better emission performance. Acknowledgement The authors would like to acknowledge financial support from the European Union (EC contract number RFCR-CT-2005-00009). References [1] Cardu M, Baica M. Regarding the relation between the NOx content and CO content in thermo power plants flue gases. Energy Convers Manage 2005;46:1–166. [2] Basu P. Combustion of coal in circulating fluidized-bed boilers: a review. Chem Eng Sci 1999;54:5547–57. [3] Shimizu T, Toyono M. Emissions of NOx and N2O during co-combustion of dried sewage sludge with coal in a circulating fluidized-bed combustor. Fuel 2007;86:2308–15. [4] Knöbig T, Werther J, Åmand LE, Leckner B. Comparison of large- and smallscale circulating fluidized bed combustors with respect to pollutant formation and reduction for different fuels. Fuel 1998;77:1635–42. [5] De Diego LF, Londono CA, Wan XS, Gibbs BM. Influence of operating parameters on NOx and N2O axial profiles in a circulating fluidized-bed combustor. Fuel 1996;75:971–8. [6] Desroches-Ducarne E, Marty E, Martin G, Delfosse L. Co-combustion of coal and municipal solid waste in a circulating fluidized-bed. Fuel 1998;77:1311–5. [7] Hiltunen M, Kilpinen P, Hupa M, Lee YY. N2O emissions from CFB boilers: Experimental results and chemical interpretation. In: 11th International Conference on Fluidized-Bed Combustion, Montreal; 1991. [8] Li Z, Lu Q, Na Y. N2O and NO emissions from co-firing MSW with coals in pilot scale CFBC. Fuel Process Technol 2004;85:1539–49. [9] Leckner B, Karlsson M, Mjörnell M, Hagman U. Emissions from a 165 MWth circulating fluidized-bed boiler. J Inst Energy 1992;65:22–130. [10] Lekomtseva YG, Baskakov AP, Munts VA. Formation and suppression of NOx and N2O in a circulating fluidized bed combustor (Part 2). Therm Eng 1993;40:567–70. [11] Lyngfelt A, Leckner B. Combustion of wood-chips in circulating fluidized bed boilers – NO and CO emissions as functions of temperature and air-staging. Fuel 1999;78:1065–72. [12] Philippek C, Werther J. Co-combustion of wet sewage sludge in a coal-fired circulating fluidised-bed combustor. J Inst Energy 1997;70:141–50. [13] Zhao J, Brereton C, Grace JR, Lim J, Legros R. Gas concentration profiles and NOx formation in circulating fluidized-bed combustion. Fuel 1997;76:853–60. [14] Zhao J, Grace JR, Lim J, Brereton CMH, Legros R. Influence of operating parameters on NOx emissions from a circulating fluidized bed combustor. Fuel 1994;73:1650–7. [15] Åmand LE, Leckner B. Formation of N2O in a circulating fluidized-bed combustor. Energy Fuel 1993;7:1097–107. [16] Åmand LE, Leckner B. Reduction of N2O in a circulating fluidized-bed combustor. Fuel 1994;73:1389–97. [17] Åmand LE, Leckner B. Influence of air supply on the emissions of NO and N2O from a circulating fluidized-bed boiler. In: Twenty-fourth Symposium (International) on Combustion, the Combustion Institute; 1992. p. 1407–14.

1341

[18] Ogunsola OI. Investigation of the causes of seasonal variations on NOx emissions from waste-coal-fired circulating fluidized-bed utility plants. Ind Eng Chem Res 2001;40:3869–78. [19] Talukdar J, Basu P. A simplified model of nitric oxide emissions from a circulating fluidized-bed combustor. Can J Chem Eng 1995;73:635–43. [20] Goel S, Sarofim AF, Kilpinen P, Hupa M. Emissions of nitrogen oxides from circulating fluidized-bed cmbustors: modelling results using detailed chemistry, In: 26th Symposium (International) on Combustion; 1996. p. 3317–24. [21] Johnsson JE, Åmand LE, Dam-Johansen K, Leckner B. Modelling of N2O reduction and decomposition in a circulating fluidized-bed boiler. Energy Fuel 1996;10:970–9. [22] Collings ME, Mann ME. Empirical modelling of N2O emissions from circulating fluidized-bed combustion. Energy Fuel 1994;8:1083–94. [23] Kunii D, Wu KT, Furusawa T. NOx emission control from a fluidized-bed combustor of coal. Effect of in situ formed char on NO reduction. Chem Eng Sci 1980;35:170–7. [24] Aarna I, Suuberg EM. A Review of the kinetics of the nitric oxide–carbon reaction. Fuel 1997;76:475–91. [25] Li YH, Lu GQ, Rudolph V. The kinetics of NO and N2O reduction over coal chars in fluidised-bed combustion. Chem Eng Sci 1998;53:1–26. [26] Durrani AK, Akhtar NA, Suzuki Y, Hatano H. Effect of coal type on NO emissions in fluidized-bed combustion: the role of NO reduction by char. 2000 ICCS&T, Okinawa; 2005. p. 10. [27] Abe R, Sasatsu H, Harada T, Misawa N, Saitou I. Prediction of emission gas concentration from pressurized fluidized-bed combustion (PFBC) of coal under dynamic operation conditions. Fuel 2001;80:135–44. [28] Winter F, Wartha C, Löffler G, Hofbauer H. The NO and N2O formation mechanism during devolatilization and char combustion under fluidized-bed conditions. In: Twenty-Sixth Symposiums (International) on Combustion, the Combustion Institute; 1996. p. 3325–34. [29] Mitchell JW, Tarbell JM. Kinetic model of nitric oxide formation during pulverized coal combustion. AIChE Journal 1982;28:302–11. [30] DeSoete GG. Overall reaction rates of NO and N2 formation from fuel nitrogen. In: Fifteenth Symposium (International) on Combustion, the Combustion Institute; 1975. p.1093–102. [31] Kilpinen P, Kallio S, Konttinen J, Mueller C, Jungar A, Hupa M, Åmand LE, Leckner B. Towards a quantitative understanding of NOx and N2O emission formation in full-scale circulating fluidized-bed combustors. In: 16th International Conference on FBC, ASME Reno; May 2001. [32] Kilpinen P, Kallio S, Engblom M. The influence of combustion conditions and fuel type on char-carbon and char-nitrogen oxidation. In: 12th International Conference on Coal Science, Australia; 2003. [33] Nevalainen H, Jegoroff M, Saastamoinen J, Tourunen A, Jäntti T, Kettunen A, et al. Firing of coal and biomass and their mixtures in 50 kW and 12 MW circulating fluidized-beds – phenomenon study and comparison of scales. Fuel 2007;86:2043–51. [34] Häsä H, Tourunen A, Saastamoinen J, Kirkinen AP, Hyppänen T, Kettunen A. Combustion characteristics of fuels – experiment scale-up from bench scale reactors to commercial scale CFB boiler. In: Proceedings of the 18th International Conference on Fluidized Bed Combustion. Toronto: ASME; 2005. p. 391–400. [35] Johnsson JE. A kinetic model for NOx formation in fluidized-bed combustion. In: Manaker AR, editor. In: Proceedings of 10th International Conference FBC. New York: San Francisco; 1989. p. 1111–8. [36] Lyngfelt A, Åmand LE, Leckner B. Reversed air staging – a method for reduction of N2O emissions from fluidized-bed combustion of coal. Fuel 1998;77:953–9. [37] Åmand LE, Leckner B, Dam-Johansen K. Influence of SO2 on the NO/N2O chemistry in fluidized-bed combustion: 1 full-scale experiment. Fuel 1993;72:557–64. [38] Lennart G, Leckner B. Abatement of N2O emissions from circulating fluidizedbed combustion through afterburning. Ind Eng Chem Res 1995;34:1419–27.