Energy Vol. 16, No. 5, pp. 849-858, 1991 0360-5442/91 $3.00 + 0.00 Printed in Great Britain. All rights reserved Copyright @ 1991 Pergamon Press plc

STUDY ON REDUCTION OF SO, AND NO, EMISSIONS IN A PULSATING COMBUSTOR BURNING PETROLEUM

COKE

M. R. WANG and K. C. CHANGt Institute of Aeronautics and Astronautics, National Cheng-Kung University, Tainan, Taiwan, R.O.C.

(Received 12 February 1990)

Abstract-We have investigated the SO, and NO, emissions of petroleum-coke burning in a Rijke-type pulsating combustor. Petroleum coke has a higher heating value and lower ash content, as well as a much higher sulfur content, than coal. Significant SO, reduction has been achieved by adding limestone or dolomite particles to the pulsating combustor. Control of NO, emissions can be obtained by using the low-excess-air-firing and staged-combustion techniques.

INTRODUCTION

In recent years, there has been an increased interest in pulsating combustion systems which provide repeated explosions. This system offers the potential for higher efficiency of heat transfer and lower pollutant emissions than conventional combustion systems. Zinn and co-workers’92 developed a Rijke-tube-type pulsating combustor to burn coal. We3v4 have achieved reductions of SO2 emissions in this type of coal combustor by injecting limestone or dolomite particles into the combustor bed. The success of our previous study has led to this application of the pulsating combustor to burning petroleum coke.

Petroleum coke is a solid fuel produced from the bottom residual in the petroleum refining process. In Table 1, we show comparisons of the properties of petroleum coke and local coal. The comparison reveals that petroleum coke has a higher heating value and sulfur content than coal. It is accordingly speculated that the previously developed sulfur-removal technology using limestone/dolomite additives would be required in the burning of the petroleum coke to control SO2 emissions in the flue gas. In addition to SO2 emissions, nitrogen oxide (NO,) pollutants have long been of concern in burning solid fossil fuels. Nitrogen oxide pollutants emanating from combustion systems arise from two principal sources, viz. thermal NO, and fuel NO,. Thermal NO, is formed as the result of reaction of nitrogen and oxygen in the combustion air at elevated temperatures, while fuel NO, is formed when nitrogen that is chemically bound in the fuel reacts with oxygen. Unlike sulfur in the fuel molecule, nitrogen is very tightly bound (in the form of an aromatic ring) in the molecule. For this reason, the conversion of nitrogen from gas, oil and coal is highly variable, ranging from 50 to 100% conversion with gas and light oils to 1540% with coals. Surveys of approaches for the control of NO, from combustion systems5*6 have indicated that the most cost-effective technologies are based on changing the combustion process to minimize the formation of NO,. Other technologies, such as denitrification of fuels and the use of post-combustion methods for NO, control,7 have also been considered. These latter technologies have not yet received wide acceptance because of their higher costs. A modification of the Rijke-tube-type, pulsating combustor, based on the concept of staged combustion, has been employed in our experimental work.

EXPERIMENTAL FACILITY

A schematic of the experimental setup is shown in Fig. 1, The cylindrical combustion chamber has a diameter of 0.15 m and a height of 3 m. Two decoupling chambers, each with

tTo whom all correspondence should be addressed.

849

850 M. R. WANG and K. C. CHANG

Table 1. Comparisons of properties of petroleum-coke and coal. Local coal and petroleum-coke were supplied by the Chinese Petroleum Corp.

in Taiwan.

Proximate Analysis I Petroleum Coke (wt%) 1 Coal (wt%)

Moisture 0.39 0.77

Volatile Matter a.22 32.90

Fixed Carbon 91.34 53.68

Ash 0.05 12.65

Heating Value (kJ/kg) 34,310 29,750

Ultimate Analysis (dry basis) I Petroleum Coke (wt%)

C 67.34 70.78

H 3.12 4.54

N 1.18 1.33

0 0.36 9.56

S 7.95 1.14

Ash 0.05 12.65

dimensions of 0.45 x 0.45 x 0.6 m3, are linked to the two ends of the combustion chamber to satisfy the required boundary conditions of acoustically open ends for generating pulsed flow of combustion air. The combustion bed is located at a quarter of the tube length from the lower end of the combustor. Figure 2 presents a schematic of the combustor around the combustion zone. The combustor is not insulated, and a water-coil cooling system, which is placed above the combustor bed, is used to control the combustion temperature. Secondary air is introduced

decoupler

sampling probe

fan

J----c

rET

1 fuel inlet

r I I

Isecondary air\

water separator

9 vi,mw~ compressor

.observat witidow

Fig. 1. Schematic of the experimental facility.

Reductions of SO2 and NO, emissions in a pulsating combusto1 851

Fig. 2. Schematic of the combustor around the combustion zone.

into the combustion chamber via an injection port at a height of 600 mm above the combustor bed (see Fig. 2) to perform the two-stage combustion designed to reduce NO, emissions. The solid fuels and the additives are conveyed into the combustion chamber via two variable-speed, motor-driven augers. As indicated in Fig. 2, the solid fuels are injected into the combustor at a height of 11OOmm above the combustor bed, while the fine additives are injected directly into the combustion zone at a height of 25.4 mm.

The instrumentation for continuous gas analyses of CO, COZ, 02, SOS, and NO, in the flue gas consists of a CO and CO* analyzer (Beckman Model 864), an O2 analyzer (Beckman Model 755), an SOZ analyzer (Beckman Model 865), and a NO, analyzer (Beckman Model 950A). All of the measured concentrations obtained with this gas-analysis system refer to a dry basis. Since the sampling probe for the gas-analysis system is made of a pipe consisting of 16 uniformly-distributed holes and inserted horizontally across the combustor at the position near the top (2150 mm above the combustor bed), the collected gas samples should represent the mean properties across the sampling section adequately.

Temperature measurements at five positions on the axis above the combustor bed are made by using uncoated, 3.175 mm dia, chromel-aiumel (K type) thermocouples. Figure 3 shows a typical temperature distribution above the combustor bed. As observed from the quartz window, which is installed on the wall in the combustion zone, the pulsating flow leads to spouting-type particle motion in this region. The combustion zone extends about 120 mm from the combustor bed, depending on the combustion conditions employed (fuel-feeding rate, excess-air percentage, etc.). The first thermocouple placed in the lowest position (100 mm above the combustor bed, cf. Fig. 3) is immersed in the combustion zone and its readings indicate the combustion temperatures. Since the fine additives are injected directly into the combustion zone where strong spouting particle motion is present, the temperatures measured with the first thermocouple represent the reaction temperatures for the calcium carbonate particles and the emitted SOZ. It is clearly shown in Fig. 3 that the injection location of the secondary air (6OOmm above the combustor bed) is beyond the primary combustion zone. Acoustic oscillations inside the pulsating combustor are induced as soon as the fuel particles are ignited.

852 M. R. WANG and K. C. CHANG

Excess air = 1 %

dB levet = 162

y 1000 c

600-

0

400 I III I I II I II

0 0.4 0.6 1.2 1.6 2.0

Distance above the combustor bed (ml

Fig. 3. Temperature distribution in the pulsating combustor during a typical test.

The amplitude of the acoustic oscillation is measured with a microphone inserted into the combustion chamber. Figure 4 depicts the time variation of the oscillation amplitude during a typical test. The oscillation amplitude depends on the difference between the acoustic gain (attributed to the heat released from combustion) and the acoustic loss (attributed to friction and acoustic radiation from both open ends of the combustor).

FUEL AND ADDITIVE PROPERTIES

Local coal and calcium carbonate rocks (limestone and dolomite) are used in the present study. Petroleum coke is obtained from the bottom residual provided by the Chinese Petroleum Corp. and is made by the use of the delayed coking method. The results of proximate and ultimate analyses of the petroleum-coke and coal compositions are summarized in Table 1. Particle sizes between 7 and 8 mm of this coal or petroleum coke are burned in the experiments. Analyses of the limestone and dolomite compositions are summarized in Table 2. The CaO content of the limestone particles is 53.33 wt%, while the CaO and MgO contents of the dolomite particles are 33.05 and 19.16%, respectively. The CaO and MgO are the two principal constituents of the carbonate rocks that react with the emitted SOZ from the

200 r

- 190- m Excess air = 1% 0 180-

r 150-

i : 140 -

k 130 -

z 120-

s cl7 110 -

100 1 I I I I I 1

0 5 10 15 20 25 30

Time (min)

Fig. 4. Time dependence of the oscillation amplitude during a typical test.

Reductions of SO, and NO, emissions in a pulsating combustor

Table 2. Analyses of limestone and dolomite compositions.

853

Constituent

CaO

MgO

.U20::

St02

P

KzO

Limestone (WC%)

53.33

0.46

0.72

2.3.:

0.015

0.13

Dolomite (wt%)

33.05

19.16

0.14

0.26

0.018

trace

combustor. The available surface areas of the limestone and dolomite additives depend on the sizes and pore structures of the particles. According to our experience,3*4 the sulfur-removal efficiency can be increased by using smaller particles; however, the use of excessively small particles will lead to a serious dust-collection problem in the present cyclone. Limestone or dolomite particles about 38 pm in size are therefore selected for our tests. The fine additives injected into the combustion zone essentially follow the streamlines of gas flow and are blown out of the combustor along with the exhaust gas.

RESULTS AND DISCUSSION

The test cases consist of coal or petroleum-coke combustion in a 15 f 3% excess air environment with a fuel feeding rate of 0.066 kg/set-m2 (based on the bed area). Calcium carbonate is used as an additive in the study of sulfur retention. Our previous study3 has shown that the optimal operating temperature for sulfur retention in burning coal under pulsating conditions is about 1150 K. This result is also consistent with that obtained by using the conventional fluidized-bed combustor to burn coa1.8*9 The temperatures in the combustion zone are controlled within 1150 f 50 K by adjusting the water-flow rates in the cooling system. Additives of both limestone and dolomite particles are used to control SO2 emissions in the flue gas. The test-case burnings with coal provide baseline data. These data are then used to assess our results for burning petroleum coke.

In coal burning, coal ash gradually accumulates in the combustor bed and finally causes choking which, in turn, stops burning. Nevertheless, our burning experiments run for approximately 30 min. In contrast to coal, petroleum coke is almost ash-free, as is shown in Table 1. Thus, the tests associated with petroleum-coke burning could be continued longer than those for coal burning. In our study, each test burn of petroleum coke lasts 2-3 h. Figures 5(a), (b) and (c) are three typical plots showing the time dependences of the C02, SO2 and NO, concentrations, respectively, during the test (see Fig. 3). The times shown in these plots indicate the duration since ignition. The plot of CO2 concentration vs time reveals that it takes about 5 min to reach steady combustion. It is known that NO, formation is sensitive to the O2 concentration in the combustor, while SO2 formation is not. The observations explain why the time variation of the NO, concentration recorded in Fig. 5(c) is similiar to that of Fig. 5(a). On the other hand, the very short time required to reach a steady value for the SO2 concentration [see Fig. 5(b)] is attributed to acoustic oscillations occurring immediately after ignition.

We show in Fig. 6 a comparison of SO2 reductions for coal and petroleum-coke burnings resulting from the addition of limestone. The limited feeding capability for the additives in our experimental facility has restricted the burning of petroleum coke to the range of low Ca/S mole ratios due to the fact that the sulfur contents of the petroleum coke are about seven times those of the coal (cf. Table 1). Because of this limitation, we find small reductions in the percentages of SO* when burning petroleum coke. Two measured data points obtained with the coal-burning fluidized-bed combustors are included in Fig. 6 for comparison with the results obtained in our pulsating combustor. The experiments conducted by Ehrlich et al9 were

854

25

M. R. WANG and K. C. CHANG

I I I I 1 0 5 10 15 20 25 0

Excess air = 1 % dB level = 162

Time (min)

(b)

I I I I I 5 10 15 20 25

Time (min)

0 5 10 15 20 25

Time (min)

Fig. 5. Time dependences of (a) COz, (b) SO, and (c) NO, concentrations during a tYpical test.

+

0

0 +

0 0 Cool, present study

0 A Petroleum coke,

0 present study 0

fA 0 Coal, Ehrllch et 01’

8

2 %,

+ Coal, Beittcl et cl”

I I I I I

1 2 3 4 5 6

Co/S mol ratio

Fig. 6. Comparison of SO2 reduction for coal and petroleum-coke burnings with limestone (38 pm in size) additive.

Reductions of SO, and NO, emissions in a pulsating combustor 855

operated in the temperature range of 1090-1150 K with limestone additive of 40 pm particle

size, while the experiments conducted by Beittel et al” were operated at temperatures around 1340 K with limestone additive of 13 pm particle size. Thus, the operating conditions used by Ehrlich et al9 are close to ours and show that the present SO,-reduction efficiency is lower than that obtained with the fluidized-bed combustor. Since our combustor is not insulated, heat loss through the combustor wall is significant. As a result, we operate essentially with a short reaction zone of about 120 mm length (compare Fig. 3) for reaction of emitted SO2 and calcium carbonate particles. Ehrlich et al9 observed a significant effect of the length of the reaction zone on SOrreduction efficiency. Their experimental data are shown in Fig. 6 and were obtained with a bed depth of 457.2 mm. The relatively short bed depth may be a primary factor for the low SO,-reduction efficiency observed with our combustor. The high mixing rate is attributed to strong pulse motion in the combustion zone, which permits us to operate this combustor with high Ca/S ratios. Similar results for burning with dolomite additive are presented in Fig. 7.

Comparisons of SOZ reductions in tests with limestone and dolomite (see Figs. 6 and 7) show that dolomite is more effective than limestone in reducing SO*. Reid” reported that MgO did not react efficiently with SOZ above 970 K, which is a lower temperature than our operating temperatures at the combustor bed. In a previous study,3 we corroborated this argument. Accordingly, the SO* emissions captured by MgO are negligibly small in our burns. Borgwardt and Harvey” found that the porosities of calcined dolomite particles were greater than those of calcined limestone particles. Furthermore, the required mass of dolomite is greater than that of limestone for the same Ca/S mol ratio since the weight percentages of dolomite and limestone are 33.05 and 53.33, respectively (see Table 2). The better performance of dolomite additive is attributed to the relatively greater exposed surfaces on which the reaction of CaO and SO,? occurs. Our previous work4 and that of Ehrlich et al9 showed that there were no significant differences in the SO,-reduction efficiency for limestone and dolomite when measured for the same mass of additive. The comparison shown in Fig. 7 indicates that the SO,-reduction efficiency in coal burnings is somewhat greater than for burning petroleum coke. Table 1 shows that the weight percentage of ash from coal (12.65%) is much greater than that for petroleum coke (0.05%). We, therefore, speculate that the alkaline material of coal ash may help in the capture of SOZ.

It is customary to use the generic term NO, for NO and NO*. NO is the major constituent of NO,, particularly at high temperatures. It is generally believed (Ref. 6) that NOz is the product of rapid quenching reactions which convert NO to NO*. NO, formation depends mainly on the temperature and O2 concentration in the combustion zone. Since we are interested in simultaneous reductions of SO2 and NO, emissions, the temperatures in the combustor bed have been restricted to values that are suitable for SO:! reduction. i.e., 1150 f 50 K. Two test

COOL

Petroleum

0 A

I I I I I I 1 0 1 2 3 4 5 6 7

Co/S maC ratio

Fig. 7. Comparison of SO, reduction for coal and petroleum-coke burnings with dolomite (38 pm in size) additive.

856 M. R. WANG and K. C. CHANG

+ The concentrations hove been corrected

to the basis of 0% except air

01 I I I I I I -10 0 10 20 30 40 50

Excess olr (vol %)

Fig. 8. Dependence of CO, emissions on excess air percentage without secondary air injection.

bums have been performed for petroleum coke with the fuel feeding rate set at 0.0472 kg/sec- m* in order to investigate the reduction of NO, emissions in the present pulsating combustor. The first test case is run without secondary air injection. Figure 8 shows the dependence of CO2 emissions on the excess air percentage. Using measured concentrations of 02, CO, CO*, NO,, and CO*, the excess air percentage is calculated.13 Here, the NO, concentration is assumed to be represented by the concentration of the major constituent NO. Since the NO, concentration is small compared with the concentrations of other combustion products, this simplification does not cause any significant errors. It may be seen from Fig. 8 that the CO2 concentrations reach constant values when the excess air is above 5%, which implies that complete burnout is accomplished with 5% of excess air. It is well known that both thermal and fuel NO, formations are effectively suppressed by reducing the local flame-zone oxygen concentrations

800 c + The concentrotlons have been corrected

700 to the bosls of 0% excess 011

*. . . *. .-_. .;

. .- *.. *..“..:. *. es..:..

. . .: . .

. . . :*..*:. -. c : . . . ..*. . *-. ‘.. .

:.:

0 I I I I I J -10 0 10 20 30 40 50

Excess air (vol %)

Fig. 9. Dependence of NO, emissions on excess air percentage without secondary air injection.

Reductions of SO, and NO, emissions in a pulsating combustor

800 r

857

+ The concentrotlons have been corrected

01 I I I I 1 I -10 0 10 20 30 40 50

Excess air (vol %I

Fig. 10. Dependence of NO, emissions on excess air percentage with secondary air injection.

(low excess-air firing-technique). Pershing and Wendt14 showed that the formation of thermal NO, could be ignored at temperatures <19OOK. Based on this argument, NO, emissions measured in the combustion system are primarily fuel NO,. In Fig. 9, we display the dependence of NO, emissions on the excess air percentage and find a trend which is consistent with that predicted for the low excess-air firing-technique.

Another available de-NO, technique is staged combustion, which requires that a portion of the combustion air is introduced with the fuel so that combustion occurs initially in an oxygen-deficient mode. The remaining air for complete combustion is introduced in the second stage. The second test case was run with secondary air injection of 1.18 l/set and a variable primary air-flow rate. As noted (compare Fig. 4), the analytical systems for CO2 and NO, require about 5 min to reach a new steady-state after a change of operating conditions. Therefore, the time for recording the measured data is set at not less than 10 min after a change of the primary air-flow rate. In Fig. 10, we present the measured NO, emissions vs excess air percentage. The NO, emissions can clearly be lowered by using staged combustion. A comparison of Figs. 5 and 6 indicates that a reduction of approximately 150 ppm of NO, could be achieved by using this de-NO, technique for burning petroleum coke.

CONCLUDING REMARKS

An experimental study has been performed to investigate the reductions of SO2 and NO, emissions in a Rijke-type pulsating combustor. It is found that both limestone and dolomite additives react with the SO2 during the combustion of petroleum coke. The S02-reduction capability in the burning of petroleum coke is limited by insufficient feeding rates of additives in our experimental facility. NO, reduction in petroleum-coke burning is achieved by reducing the amount of excess air in the combustion chamber. Further reductions of NO, emissions (about 150 ppm) may be made by using staged combustion with lo-40% excess air.

Acknowledgement-This work has been sponsored by the National Science Council of the Republic of China under Contract No. NSC77-O410-EOO6-50Z.

858 M. R. WANG and K. C. CHANG

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