Sulphur pollutant formation during coal combustion

Sulphur pollutant formation during coal combustion

Sulphur pollutant combustion Steven D. Zaugg, Angus L. Douglas Smoot formation U. Blackham, during Paul 0. Hedman coal and Departments of Chemi...

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Sulphur pollutant combustion Steven D. Zaugg, Angus L. Douglas Smoot

formation

U. Blackham,

during

Paul 0. Hedman

coal

and

Departments of Chemistry and Chemical Engineering, Brigham Young University, Provo, Utah 84602, USA (Received 12 February 1988; revised 22 August 1988)

A laboratory-scale

pulverized coal combustor was used to determine the effects of secondary air swirl, stoichiometric ratio (O,/fuel), and coal type on the formation and reaction of sulphur pollutants (SO,, H,S, COS and CS,). Detailed local measurements within the reactor were obtained by analysing solid-liquid-gas samples collected with a water-quenched probe. Increasing the stoichiometric ratio increased sulphur conversion and SO, levels, and decreased H,S, COS, and CS, levels. Swirl of secondary combustion air had a pronounced effect on the distribution of sulphur species formed at an O,-coal stoichiometric ratio of 0.8, but had very little effect at stoichiometric ratios of0.57 and 1.17. Combustion of a bituminous coal produced more SO, and less H,S, COS, and CS, compared with a subbituminous coal. (Keywords: combustion of coal; sulphur dioxide; sulphur)

The use of pulverized coal in electric utility power plants has stimulated considerable research to minimize emissions of pollutants. Methods include cleaning the coal prior to combustion and downstream scrubbing of sulphur and nitrogen gases. Further reduction of pollutant gases may also be accomplished by altering parameters which affect the combustion environment. Staging of combustion air’*2, control of the air/fuel mixture ratio through burner modifications3, and secondary stream swirl4 have been used to control NO, formation processes. Some success in controlling the SO, formed in the combustion of coal has been achieved5 by adding pulverized limestone to the fuel/air stream. Further work6 has shown that pressure-hydrated dolomitic limes are more effective than limestone. While the NO, emissions can be reduced to N, through proper control of reaction conditions (fuel staging) there are no analogous chemical reaction sequences for SO, reduction. Consequently, the effect of reactor parameters on sulphur pollutant formation has received much less attention, even though capture of fuel-sulphur by limestone and the retention of sulphur in the char are somewhat dependent on reactor conditions and coal type. Therefore, the object of this study was to determine the effects of stoichiometric ratio, secondary air swirl, and forms of fuel-sulphur on the distribution of gas and solid phase sulphur species within the flame zone of a pulverized coal combustor. Asay et al.’ have reported the effects of these variables on the overall combustion process, which in turn has a significant impact on the fate of fuel-sulphur. Most of the work to date involving sulphur chemistry has been obtained from measurements of reactor effluents. A better understanding of the controlling processes in a reactor is gained if detailed local samples are analysed. Further, the acquisition of detailed local data is essential for evaluating model predictions. During this test programme, samples were removed from a laboratory-scale combustor with the use of a water00162361/89/03034608$3.00 0 1989 Butterworth & Co. (Publishers)

346

Ltd.

FUEL, 1989, Vol 68, March

quenched probe. Measurements of SO,, COS, H,S, CS, and solid sulphur remaining in the char were made on the solid, liquid and gas samples. While SO, gas was not measured directly, most of it dissolved in the quench water and was reported as SO,. The corresponding measurement of nitrogen pollutants and coal burnout values have been reported elsewhere7. TEST FACILITY AND PROGRAM Reactor

The combustor and water-quenched probe of Figure 1 have been described by Burkinshaw et a/.’ The reactor is 150 cm in length and has an inside diameter of 20 cm. It consists of five interchangeable sections, allowing the probe to be positioned at various axial locations. The probe (Figure 2) was mounted on a traversing mechanism that permitted sample collection at various radial locations. Coal from a rotary disc feeder was entrained in preheated air and introduced into the top of the reactor through a primary inlet tube. Preheated secondary air was brought into the reactor through a concentric annulus around the primary stream. The secondary air passed through an adjustable swirl block used to impart various degrees of tangential momentum to control the mixing rates of the two streams. The calculated swirl numbers (Sg) were determined from the geometry of the swirl block and ranged from 0 to 6.2. Calibration of the swirl block by Harding et ~1.~showed experimental swirl numbers to be somewhat less than computed values. The stoichiometric ratio (SR = inlet air/theoretical air required for complete coal combustion) was varied by changing the secondary air flow rate. All combustion tests were conducted with the temperature of the primary air at 300 K and the secondary air at 590 K. The coal feed rate was fixed at 10.2 kg/l-’ (dry basis). A Wyoming subbituminous coal from the Belle

Sulphur

pollutant

formation Table 1

Secondary + Air

during Proximate

coal combustion: and ultimate

analyses

S. 0. Zaugg

et al.

of test coals

Utah bituminous

Wyoming subbituminous

2.4 8.5 45.4 43.1 0.54 29 540

21.8 5.0 32.9 34.3 0.38 20 040

7.8 70.9 5.1 1.4 14.3 0.55 0.08 0.47 49.9

5.1 66.2 4.7 0.97 22.6 0.51 0.22 0.29 -43

Proximate (wet basis, wt “i:) Moisture Ash Volatiles Fixed carbon Sulphur Heating value (kJ/kg)

Swirl Methane -

Ultimate Ash C H N

Wall Temperature

0, S (total) S (pyritic)* S (organicy Mass mean particle

Probe-

152 cm

Casing v

A-l-

Ceramic Insulation

Exhaust w4*-

Figure 1

(dry basis, wt%)

Schematic

diagram

Quench Water

of laboratory

coal combustor

1.l1 cm O.D. Stainless Steel Tubing /-

0.476 cm I.D. Stainless Steel Tubing

a) Probe Tip

diameter

“Ash is determined by a different b By ASTM D 2492 ’ By difference

(pm)

procedure

for the two methods

Ayre Mine was used when determining the effect of Skand SR on sulphur pollutant formation and reaction. A high volatile Utah bituminous coal from the Deseret Mine in Utah was also tested to provide comparisons between the two coal types. The proximate and ultimate analyses of the two coals are given in Table I. The Wyoming coal was pulverized to 80% through 200 mesh and the Utah coal was pulverized to 70% through 200 mesh. The particle size distributions of the two coals are reported by Asay9~‘o and Harding”, respectively. A set of nine tests were conducted with the Wyoming coal at three swirl numbers (0.0, 2.2, 6.2) and three stoichiometric ratios (0.57, 0.87 and 1.17). Five samples from each test were obtained near the reactor exit at radial distances of 0, 3, 5,7 and 1Ocm from the reactor centreline. Average properties at the reactor exit were determined by mass flow-weighted integration over the radial cross section. Measured sulphur species concentrations were referenced to 07; excess oxygen. Ten additional tests with the Wyoming coal at an SR = 1.05 and SE,= 3.0 were conducted to measure sulphur species at 0,3, 5,7 and 1Ocm radial positions for each of 10 axial locations. Isoconcentration maps were constructed from measured values to give a general description of sulphur species reactions occurring in the reactor. Four more reactor tests with the Utah high volatile bitumious coal were conducted at an SR = 1.0 and Si = 2.2. Samples were obtained at four axial locations and radial profiles of SO, and H,S were compared with radial profiles for the Wyoming subbituminous coal tests. The differences in reactor conditions were considered when making comparisons between the two coal types. Sample analysis Samples containing gas, liquid and solid phases were removed from the reactor with a traversing, stainless steel, water-quenched probe. Several techniques were used to analyse the reactor effluents” and brief descriptions of the methods are given below.

b) Probe

Figure 2

Schematic

diagram

of the sample

probe

and probe

tip

Gaseous species. An HP-5735A g.c. with a flame photometric detector was used to measure SO,, COS, CS,, and H,S. A 2.7m x 3.2 mm teflon Chromosil 310

FUEL, 1989, Vol 68, March

347

Sulphur pollutant formation during coal combustion: S. 0. Zaugg

column with nitrogen as carrier gas was used to make the separation. H,, 0, and other permanent gases were measured using an HP-5832A chromatcgraph equipped with a thermal conductivity detector. Helium was used as the carrier gas at a flow rate of 25 ml min- ‘. A 1.8 m x 3.2 mm stainless steel Poropack Q SO/l00 column, and a 4.9 m x 3.2 mm stainless steel molecular sieve 5A 60/80 column were used in a series/by-pass contiguration to make the separation. The oven temperature was initially set at 0°C by use of a cryogenic valve and liquid CO, to facilitate the separation of Ar and 0,. Liquid

species. An Orion model 94-16 Ag+/S= ion selective electrode was used to analyse any H,S which dissolved in the probe quench-water. Sulphite ion resulting from dissolved SO, along with the sulphide ion was oxidized to sulphate ion and analysed by ion chromatography with a Dionex Model 10 Ion Chromatograph.

et al.

600 500 400 300 200 100 0 250 200 150 100

50

Solids. A Perkin Elmer 240 elemental analyser was used

to determine the weight fraction of carbon, hydrogen, nitrogen, and ash in the virgin coal and char samples. A LECO Corp. Model 634-000 Sulphur Analyser was used to measure total sulphur and, in a few samples, the organic sulphur after the inorganic sulphur had been removed. The analysis of pyritic sulphur in virgin coal samples was accomplished using the ASTM D 2492 procedure, in which the Fe,O, is first removed with HCl, followed by the conversion of the FeS, to Fe+ + + with HNO, and the Fe+ + + determined by atomic absorption. The organic sulphur is calculated as the difference between the total and pyritic sulphur. The procedure used for the collected char samples, in which the inorganic sulphur may be FeS as well as FeS,, was the leaching of the samples with HNO, to remove the resulting Fe+ + + and SOT and determining the organic sulphur remaining in the solid with the LECO Sulphur Analyser. The inorganic sulphur was then calculated by difference.

8: 60

'

0

1

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5

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7

8

9

10

Radial location(cm)

Reproducibility of radial concentration profiles for sulphur gases taken at the reactor exit for the Wyoming subbituminous coal (SR=0.94; $=4.5; 0, run I; 0, run 2; 0, run 3; A, run 4) Figure 3

Error analysis

Preliminary tests were conducted to determine the errors associated with sampling, analysis, and combustion test reproducibility. The coal feed rate was reproducible to f5%. Analytical errors for sulphur gases were within f4% and errors for permanent gases were approximately + 1.5%. Measurements of sulphide and sulphate in the quench-water were reproducible to within + 3% and &-1.8%, respectively. Elemental concentrations of C, H, N and S in the char were generally within &3%. Several combustion tests were repeated with Wyoming subbituminous coal to determine the reproducibility of the entire test procedure. The radial distribution of sulphur gases collected near the exit of the reactor for four reproduced tests at an SR = 0.94 and Si = 4.5 is illustrated in Figure 3. For these reactor conditions, the local value of SO, agreed to within + 8%. Corresponding results for the smaller concentrations of H,S, COS, and CS, were between + 10 and *20%. The precision of sulphur gas measurements was closely related to the gas concentration. At higher stoichiometric ratios, when the concentrations of H,S, COS, or CS, were much below 20 ppm, the relative errors were often between f 10 to + 50%. While values for the amount of sulphur remaining in the char are not shown, they were reproducible to _+5 to *lo%.

348

FUEL, 1989, Vol 68, March

RESULTS Effects of stoichiometric

ratio and swirl number

The effects of secondary air swirl and stoichiometric ratio on SO,, H,S and COS concentrations at the exit of the reactor are shown in Figure 4. Increasing the swirl had a pronounced effect on the effluent concentration of H,S and SO, at an SR=0.87, but less effect at SR values of 0.57 and 1.17. The effect of swirl at an SR=0.87 is attributed to its influence on flow stability and flame location. Without swirl (S: = 0), ignition was delayed until nearly 60 cm from the primary inlet. When ignition finally occurred, the coal and air were well-mixed and resulted in high SO, concentrations. As swirl was increased, the flame was observed to move upward and became attached to the primary inlet at an Si value of Z.2. The reaction of coal volatiles with oxygen from the primary inlet, and slow mixing of oxygen from the secondary air resulted in the formation of a fuel-rich core. Sulphur devolatilization in this fuel-rich zone increased the H,S concentration with a subsequent reduction in the level of SO,. For the fuel-lean condition (SR = 1.17), a change in swirl from 0.0 to 2.2 caused a significant increase in the SO, concentration, with little effect on the H,S concentration. Apparently any increase in the amount of H,S formed in the fuel-rich core at a swirl number of 2.2 is

Sulphur pollutant

formation

during coal combustion:

600

O-2 0-v A

cos

S. 0. Zaugg et al.

Dried Coal (4.5% Moisture)

500

0 - soz 0 -

H2S

400

A

-

COS

0.6

0.7

300 100 z .f 8 E I 9 & k a s L

0

fQq----i@,

500

I

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I

200

PI I

100

SR = 0.67 4oo

o\.

300

-

l-l -0.5

lo200

-

0.8

Stoichiometric

u5 2

/O-

s

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1.0

1.1

1.2

Ratio

Figure 5 Effect of stoichiometric ratio on etlluent SO,, H,S, and COS concentrations at a swirl number = 2.2, for the Wyoming subbituminous coal; 0 SO,, 0, H,S; A. COS

A 400

0.9

1

SR=O57 300

0

0

-----0

:,a_,~ , : P

1

2

3

4

Secondary SwirlNumber

!

,h

5

6

1

1

(S:)

Figure 4 Effect of secondary swirl number on ellluent SO,, COS, and H,S concentrations at three stoichiometric ratios for the Wyoming subbituminous coal: 0, SO,; 0, H,S; A, COS

converted to SO, as mixing with excess oxygen becomes more complete near the reactor exit. At an SR = 1.17, oxygen is no longer a limiting reactant and an increase in swirl resulted in an increase in coal burnout. Thus, a greater amount of fuel-sulphur was converted to gas phase species, which were oxidized to SO,. Even with substantial excess air, Figure 4 shows detectable quantities of COS and H,S, well above equilibrium values. This results from lack of completion of the mixing and combustion process as shown by Asay et al.’ For example, exit plane CO concentrations for this case varied radically from 1 to 5% even in the presence of molecular oxygen. At an SR value of 0.57, the condition was so fuel-rich that any change in the mixing rates had no measurable effect on the distribution of sulphur species formed. The effect of SR on the distribution of H,S, SO, and COS (Figure 5) showed that SO, was the predominant species at SR values of 1.17 and 0.87. However, at an SR value of 0.57, SO, concentrations were much less than H,S concentrations, and even COS concentrations were comparable to SO, values. The integrated CS, concentrations (not shown) for all these conditions were less than lOppm, even though much higher concentrations were measured at the centreline for the fuel-rich conditions. As expected, increasing the stoichiometric ratio increases the overall gas phase concentration of sulphur species.

Reactor

pollutant

maps

Reactor iso-concentration maps for SO,, H,S, COS, CS,, 0, and H, (Figure 6) were constructed by radial and axial interpolation of data obtained from measurements at the 0, 3, 5,7 and 10 cm radial positions for each of ten axial locations for the subbituminous coal. These maps indicate trends in species concentrations and relative rates of various chemical reactions. In the forward regions of the reactor, coal burnout values were low and early devolatilization occurred in an oxygen-rich environment. Reactive sulphur intermediates were released from the coal and rapidly oxidized to form SO,. The formation of other sulphur pollutants was minimal in this region of early devolatilization. Midway down the reactor (70 cm), mixing effects began to reduce the large SO, concentration gradients between the walls and the centre of the reactor. No effort was made to determine SO, directly. Others12-‘4 have indicated that SO, is between 1 and 4% of the total SO, concentration from combustion effluents. As oxygen was depleted near the centreline from reactions with coal volatiles, the formation of H,S, COS and CS, was observed. The H,S concentration continued to increase on the centreline in an increasingly fuel-rich zone, and large radial gradients existed near the reactor exit. The H,S concentration gradients correlated directly with the trend observed for the H, profiles, and inversely with the 0, profiles. The formation of H,S mostly occurred during later coal devolatilization and heterogeneous char reactions. It is conceivable that H,S may also have formed at the boundary of the fuel-rich core from the conversion of SO, to H,S due to mixing effects. CS, and COS, like H,S, appear to form only in the fuel-rich core. Trends for the CS, concentration map are remarkably similar to those of H,. CS, reaches peak concentrations midway through the reactor, whereas COS concentrations continue to increase along the centreline. This may occur because the stability of COS is greater than CS, with respect to oxidation based on thermochemical equilibrium considerations.

FUEL,

1989,

Vol 68, March

349

Sulphur pollutant formation during coal combustion. S. 0. Zaugg et al.

6

I

I

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I

I rro

I

I ,

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I

H2S (wm)

-

a.7

02

/7?



0

20

40

60

80

100

120

\ I

140

Iso-concentration

I 80

1

I 100

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1 120

I 140

maps of SO,, H,S, COS, CS,, 0, and H, for the Wyoming subbituminous coal; St,= 3.0, SR = 1.06

The mass percentage conversion of nitrogen, carbon, sulphur, and hydrogen from the solid coal to the gas phase (Figure 7) was determined by elemental analysis of the char using a forced ash balance. The elemental conversion rates are likely related to the form and distribution of the elements throughout the coal matrix. In the upper part of the reactor and toward the centreline, early coal devolatilization released hydrogen more rapidly than carbon. Sulphur was initially released at a faster rate than carbon or nitrogen, probably due to release of loosely bound aliphatic sulphur in C-S, S-H and S-S bonds. However, in the lower regions of the reactor, the conversion of carbon and nitrogen surpassed sulphur conversion at z 72% conversion. This indicates the existence of tightly bound sulphur, possibly in the form of FeS and polyaromatic thiophenes. Absorption of sulphur into the ash may also account in part for the lower sulphur conversion. No noticeable differences were observed in the release rates of nitrogen and carbon in either the forward or aft regions of the reactor. A few centreline samples provided the required amount of material to determine the concentration of organic sulphur in the sample by first removing the inorganic sulphur (FeS and FeS,) with HNO,. The inorganic sulphur was then calculated as the difference between total and organic sulphur. Although the resulting data were somewhat limited, a general trend in the relative conversion rates for the two forms of sulphur was observed, as shown in Figure 8. The ratio of percent total

350

60

Axial Location (cm)

Axial Location (cm)

Figure 6

I 40

(mole percent)

FUEL, 1989, Vol 68, March

sulphur conversion to percent carbon conversion is greater than unity in the early stages of reaction. Thus, the rate of release of sulphur based on the original sulphur present in the coal is greater than the rate of release of carbon based on original carbon present in the coal. About halfway down the reactor this ratio changes to less than unity. The other curves show that more of the sulphur released in the early stages is organic rather than inorganic; about three to four times at the start of the reaction. However, not all of the organic sulphur is released in the early stages of reaction. As the release of inorganic sulphur increases in the later stage of reaction, there is also continued release of organic sulphur. This supports the suggestion that part of the organic sulphur is present in a tightly bound form, such as polyaromatic thiophe’ne-like heterocycles. The release of sulphur from FeS is also quite s10w’~ and probably contributes signilicantly to the portion of inorganic sulphur remaining in the char samples near the reactor exit. Effect of coal type

The effect of coal type on forms of sulphur in coal was examined by comparing results from the combustion of a bituminous coal with those of the subbituminous coal. The percentage of the total sulphur in the two coals on a dry basis was similar (Table I), while the distribution between organic and inorganic sulphur forms was quite different for the two coals. The radial concentration profiles of SO, and H,S at four axial locations have been

Sulphur

pollutant

compared in Figures 9 and IO, respectively. The higher swirl number for the subbituminous coal (Sg= 3.0) resulted in flatter SO, profiles than for the bituminous coal (Sg=2.2), which had significant radial gradients in the SO, profile near the reactor exit (Figure 9). Sulphur conversion (Table 2) in the forward regions of the reactor was much higher for the bituminous coal 10

“i’

a

1’ ‘\’

formation

during coal combustion:

S. 0. Zaugg et al.

resulting in higher SO, and H,S concentrations at the 41 cm axial location (Figures 9 and IO). The bituminous coal contained a greater amount of organic sulphur (Table I), having a loosely bound portion that is released at a faster rate than the inorganic sulphur (Figure 8). Also, Attar and Dupius l6 found that high-volatile coals generally contain a much greater amount of loosely bound mercaptan (-SH) sulphur than low-volatile coals. Continuing down the reactor, the concentration of H,S decreased for the bituminous coal and increased for the subbituminous coal. Because the initial release of sulphur was slower for the subbituminous coal, a greater reaction of the sulphur was retained in the char to be released in the lower regions of the reactor (Tub/e 2). Thus, a greater amount of sulphur was released into the fuel-rich core for the subbituminous coal, resulting in greater concentrations of H,S near the exit. The fate of the fuel-sulphur at the 132 cm axial position (Table 3) demonstrates how differences in coal type and sulphur forms may affect the distribution of sulphur pollutants. For example, H,S, COS and CS, were formed in greater quantities for the subbituminous coal. A greater amount of sulphur was retained in the char of the subbituminous coal even though carbon conversion was higher than for bituminous coal. Thus, sulphur conversion is not necessarily a strict function ofcarbon conversion, but is also dependent on the forms of sulphur in the virgin coal. Greater retention of sulphur by the subbituminous coal could result from the higher calcium content of the ash in the subbituminous coal (17.5”,, compared with 8.0”;,) as indicated in Table 4. However it was not determined if any of the sulphate observed in the probe quench water was due to the leaching of calcium sulphate from the ash and char samples.

” “_I (Hydrogen)

CONCLUSIONS 0

40

20

60

80

120

100

The conclusions are based on observations from a small laboratory-scale reactor with short residence times, close to 300 ms (Ref. 4). Neither coal burnout nor gas mixing were complete at the exit plane of the reactor. No attempt

140

Axial Locamn (cm)

Figure 7 Maps of mass percentage of elemental conversion of H, S. C, N with the Wyoming subbituminous coal; St =3.0, SR = I .06

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0

Ratio of % Sulfur / % Carbon Conversion

A

Ratio of % Organic S / % Total S Conversion

0

Ratio of % Inorganic S / % Total S Conversion

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0.0 0

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Axial Location (cm) Figure 8

Comparison

of inorganic

sulphur

and organic

sulphur

conversion

for centreline

samples

of the Wyoming

subbituminous

FUEL, 1989, Vol 68, March

coal

351

Sulphur

pollutant

formation

Axial Location 41 cm

8oo 600 -

during coal combustion:

increased until the flame became attached at the burner, the formation of a fuel-rich core caused an increase in the H,S concentration with a subsequent decrease in the SOZ concentration. In the forward regions of the reactor, rapid devolatilization of fuel-sulphur formed reactive intermediates, which were oxidized to SO,. A major portion of the SO, appeared to form early in the reactor, probably from cleavage of aliphatic sulphur functional groups, and to

(’

d 0

400

-

200 00

/

J\

0 /

(Wyoming (Utah bituminous coal)

“\

subbiiuminous

A

et al.

S. 0. Zaugg

6

coal)

1000 800 - ”

cm

250 600 -

I

I

I

I-

I

f( O-O

400 r

2 d

/

cf

do\

D

iii g

OLo

(Wyoming

(Utah bituminous)

subbituminous)

_

2000 1000 6oo _ 102cm

I g 0 s ‘Z I!

605 -

71 cm

/(

I

/0-o 40°

5

sF

& w

-00

200 -

-9

0

woo

~___-”

*OO _ 132cm

102 cm

600 400 -a/

/

o/0-

+“c”\

O-0

200 0’

10



8

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6

4

2

0

1

1

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2

4

6

8

10 0

Radial Location (cm) Figure

250

9 Comparison of radial proliles of SO, concentration for the

Utah bituminous coal (SR = 1.O,5:=2.2) and Wyoming subbituminous coal (SR = 1.05, 9. = 3.0) at four axial locations

Table 2 Comparison of sulphur conversion for the bituminous and subbituminous coals

Axial location (cm)

Sulphur conversion’ (Gas, wt%) bituminous coal

41 71 102 132

19 92 95 95

Sulphur conversion” (Gas, wt%) subbituminous coal

200 150 100 50

0

L \

132cm

1 10

0

6

4

2

0

2

4

6

0

10

Radial Location (cm)

60 82 87 91

4 Integrated sulphur conversion as determined by gas phase analysis and the feed rate of sulphur in the raw coal

Figure 10

Comparison of radial profiles of H,S concentration for the Utah bituminous coal (SR = 1.O,$ = 2.2) and Wyoming subbituminous coal (SR = I LX, Z$=3.0) at four axial locations Table 3

Fate of fuel sulphur for bituminous and subbituminous coals (132 cm axial position) %

was made to generalize these observations for larger reactors or longer residence times. Secondary air swirl had very little effect on sulphur pollutant formation for fuel-rich (SR = 0.57) and fuel-lean (SR= 1.17) conditions. At an SR value of 0.87, the concentrations of H,S and SO, were quite dependent on swirl number for a subbituminous coal. The primary effect of swirl was to control the location of ignition. As swirl

352

FUEL, 1989, Vol 68. March

Carbon Sulphuc conversion’ mnversion” H,S Bituminous 89 Subbituminous 96

95 91

1.0 7.4

SO,

COS CS,

SVtirb

93 77

0.6 4.8

5 9

0.8 1.7

“Conversion based on gas phase analysis and elemental composition of the coal bThe amount of sulphur reported in the char was calculated from a forced sulphur balance

Sulphur pollutant Table 4

Compound SiO, A&O3 FerO, TiO Mn?l Cl CaO sro K,O Na,O MgO P,O,

Analysis

of coal ash (wt%) Utah” bituminous

Wyomingb subbituminous

46 18 4.2 0.93 0.05 0.2 8.0 0.18 0.6 4.6 1.1 1.0

NA’ NA’ 4.4 1.23 0.13 0.35 17.5 0.9 0.3 1.4 3.6 0.5

’ Analysed by the US Geological Survey, Denver, Colorado *Sodium and magnesium analysed by atomic absorption (a.a) with the remainder of the elements analysed by proton induced X-ray emission (PIXE) ‘Data not available because of problems with the proton induced X-ray emission (PIXE) method for analysis of these elements

some extent from decomposition of FeS, to form FeS. On the reactor centreline, coal reactions with primary oxygen and slow mixing of oxygen from the secondary air resulted in the formation of a fuel-rich core. H,S, COS, and CS, were formed primarily in this fuel-rich region, most likely from further decomposition of FeS, and FeS as well as devolatilization of tightly bound organic sulphur. The elemental conversion rate for early coal devolatilization near the reactor inlet is in the order: H > S > C, N. A portion of the organic sulphur was released from the coal at a faster rate than either carbon or pyritic sulphur. Another portion of the organic sulphur was tightly bound, apparently in condensed aromatic thiophenes, and was released at a slower rate. The fate of fuel-sulphur was dependent upon the distribution of organic and inorganic sulphur in the raw coal. A greater fraction of fuel-sulphur was converted to SO, during combustion of a bituminous coal than while firing with a subbituminous coal. This was because the bituminous coal contained a greater amount of organic sulphur, which was released early in the combustion zone into a oxygen-rich environment. The subbituminous coal contained a greater fraction of pyritic sulphur, which was released later into a fuel-rich zone near the reactor centreline, forming greater concentrations of H,S, COS and CS,. No attempt was made to predict the fate of fuel-sulphur in this study, although a model for sulphur

formation

during coal combustion:

S. D. Zaugg et al.

behaviour during combustion17 is being developed, in conjunction with an available comprehensive combustion model18. ACKNOWLEDGEMENTS This work was sponsored by the Electric Power Research Institute, Palo Alto, California (Contract No. RP364l-3). Additional support was also provided by Brigham Young University, Foster Wheeler Corp. (Livingston, NJ) and Babcock and Wilcox Corp. (Alliance, Ohio). Thanks are given to B. Asay, R. LaFollette, R. Pace and N. Hsia for providing reactor samples. REFERENCES 1

6 7 8 9 10 11 12 13

14

15 16 17 18

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