How to reduce pollutant emissions from small fluidised-bed combustors

Applied Energ.v 41 (1992) 1-94

How to Reduce Pollutant Emissions From Small Fluidised-Bed Combustors

K. Findlay* Fina plc, 1 Ashley Avenue, Epsom, Surrey KT1 9AD, UK

& S. D. Probert Department of Applied Energy, Cranfield Institute of Technology, Bedford MK43 0AL, UK

ABSTRACT As a result o f 27 series of tests, it was concluded that the maximum reduction of NOx emissions occurred when the sulphur retention was also at its highest, so emphasising the important role that CaSO 4 plays as a catalyst in pollutionreducing reactions. Although the minimal emissions of both S02 and NO x (at 85 and 45ppm, respectiveO') presently recorded occurred at a bed temperature of 800° C, under two-stage combustion and a low oxygen-in-theClue concentration ( = 2%), the combustion efficiency under these conditions was relatively low (at 72"9% without limestone and 84"5% with limestone added to the .[tuidised bed). Optimal conditions for achieving maximum combustion efficiency and minimum pollutant emission occurred at the highest bed-temperature ( = IO00°C) employed, under two-stage combustion conditions for a moderate O, high ( ~ 4%) oxygen-in-the-flue concentration. Under these conditions, the CaS, formed from the lime in the substoichiometric bed, was completely oxidised to CaSO 4 by the moderately high Oz concentrations in the freeboard, so optimising the reductions of both the NO~ and SO 2 emissions. The addition o f limestone was found to increase the combustion e~'cieno' by just under 3%, to a maximum o f 91,3%, under these conditions. Further, the presence of limestone (which gave an added Ca: S * Present address: Manufacturing and Process Industries Division, W. S. Atkins (Consultants) Ltd, Woodcote Grove, Ashley Road, Epsom, Surrey, KT18 5BW, UK. 1

Applied Energy 0306-2619/91/$03'50 (~ 1991 Elsevier Science Publishers Ltd, England. Printed in Great Britain

2

K. Findlay, S. D. Probert mole ratio of 2), resulted in reductions in the NOx emissions o f 83% ( i.e. f r o m 283 to 47 ppm) and in the SO 2 emission of 74% (i.e. f r o m 455 to l l 7 ppm ). Both the NOx and S 0 2 emissions were greatly reduced by this addition o f limestone, under most operating conditions. The magnitude of the reduction varied according to the bed's temperature, e.g. at a bed temperature o f 800°C, under two-stage conditions, the NOx emissions were reduced by 71% and the S02 emission by 76%, provided sufficient limestone was added to the bed to give a 2:1 Ca:S ratio. Similarly, the use o f recycled gas, to achieve bed attemperation during these tests, led not only to a reduction in the NOx emissions of 33%o, compared with only 15% achieved in previous experiments, but also to a 26%0 reduction in the S 0 2 emission. The latter was a direct result of the increased residence time for S 0 2 gas in the contact with the limestone~ash particles within the combustor. When burning S.A. Duff with: ( i ) the exhaust-gas re(3'cled back to the bed; (ii) l#nestone added to the bed ( io give a Ca: S mole ratio of 2); and (iii) the ftuMised bed operated at a relatit~elv high bed-temperature ( ~ IO00'~C) under two-stage combustion conditions with a 4% concentration o f Oz-in-the-flue: then a 90%o overall reduction in NO~ emissions (compared with those occurring under oxidising conditions with no limestone added) and a sulphur retention of 74% were achieved. Larger sulphur retentions ensued by reducing the bed temperature to 800°C and using lower oxygen-in-the-[tue concentrations ( ~ 2%), but this occurred to the detriment o f the combustion efficiency, Nevertheless, the lower bedtemperature o f ~ 800°C was needed to avoid the.formation o f clinker when burning a low-ash fusion coal, such as Maryport smalls. By contrast, the use o f a high bed-temperature ( ~ IO00:C) with low values o f the oxygen-in-the-flue concentration, resulted in no sulphur retention; all the CaS being partially oxidised to SOz under these operating conditions, with or without limestone present.

NOTATION

C CO CO2 N NO NO2 NO*

NO~ 02 S S-

Carbon, e.g. in the char Carbon monoxide Carbon dioxide Nitrogen Nitrogen monoxide Nitrogen dioxide NO 2 in its electrically excited state Nitrogen oxides Oxygen Sulphur Sulphide

Pollutant emissions from fb combustors

SO2 SO3 SOx

3

Sulphur dioxide Sulphur trioxide Sulphur oxides

Abbreviations BT Po fbc 02 (flue) pf ppm vppm

Bed temperature Fluidised bed Fluidised-bed combustion Oxygen in the flue Pulverised fuel Parts per million Volumetric parts per million

Unless stated otherwise, the term 'oxygen' (e.g. as used for the legend of the abscissae of various graphs) implies oxygen-in-the-flue concentration.

Glossary Bed's stoichiometric ratio: Free-board height:

Fuel nitrogen: Gas-start tubes:

Lock hopper:

Over-firing: Oxidising bed:

Oxygen-in-the-flue concentration:

The quantity of oxygen actually supplied divided by the minimum quantity of oxygen required to ensure that complete combustion ensues. This defines the region where secondary combustion occurs, i.e. immediately above the fluidising bed, within the combustion chamber--see (4), (5) and (6) in Fig. 2. Nitrogen present within the fuel itself. Tubes via which liquefied petroleum-gas is injected into the fluidising air within the spargepipes, for ignition within the fb combustor at start-up. A system of coal hoppers, separated by air-tight valves. The latter prevent gases escaping from the slightly over-pressurised combustion-chamber, yet allow the required quantity of coal to enter it. Continued attempts by the burner to spark ignite the fuel, despite it already being lit. In the present text, this description implies that all the combustion air necessary enters the first-stage primary-combustion zone (i.e. is introduced into the fb itself). The concentration of oxygen within the flue gases.

4

K. Findlay, S. D. Probert

S. A. Duff, Gedling Blanzy and Maryport coals:

Sparge pipe:

Total ash:

Two-stage bed:

Two-stage combustion:

These originate from various collieries in different parts of the world. They differ considerably in composition, e.g. Maryport has high sulphur and low nitrogen contents, whereas South African Duff has low sulphur yet a moderately high nitrogen content. Unless stated otherwise, the coal used in the present tests was S. A. Duff. A horizontal pipe, with two rows of aligned radial holes located symmetrically along its length, and through which the fluidising (i.e. primary) air passes from the plenum chamber to the fluidised bed. The total quantity of ash present within the coal. Some remains as an inert residue following combustion and is collected in the fluid-bed or within the cyclone, or unfortunately part of it may be emitted via the stack. In this system, only part of the combustion air enters via the first stage (i.e. the primary combustion zone). For complete combustion to ensue, extra air is added to the second-stage combustion zone. This process consists of the partial combustion of the fuel to form carbon monoxide, under substoichiometric conditions within the first stage. During the second stage, the CO is combusted completely to form CO 2 as a result of the addition of 'secondary' air.

SCOPE OF THIS I N V E S T I G A T I O N With respect to the performances of fluidised beds, many conflicting research results exist concerning (i) the use of two-stage combustion and flue-gas recirculation for reducing NO Xemissions, and (ii) the introduction of limestone into the bed in order to reduce SO2 emissions. Also, surprisingly little has been accomplished to determine the effects on (i) SOa emissions, of employing two-stage combustion, and (ii) NOx emissions, of limestone addition to the bed. Further, there have been relatively few detailed investigations concerning the effect of flue-gas recirculation on the rate of SO 2 emission. However, such knowledge, not only for each particular

Pollutant emissions from fb eombustors

5

pollutant-emission, but for all the pollutants produced, is desirable when designing a commercial combustion-plant. The effects of design changes to the fb on its combustion efficiency are usually of paramount importance, especially for large plant: economic considerations often take a high priority, e.g. for power-station boilers. Therefore the objectives of fbc design must be to maximise combustion efficiency, whilst simultaneously reducing both the NOx and SOz emissions. Thus, in the present project, both the NO~ and SO2 emissions from a fbc have been monitored over a range of bed temperatures, using various O z (flue) concentrations, utilising both two-stage and oxidising conditions, with and without limestone present in the bed. Analyses of samples of the bed and cyclone ash permitted the combustion efficiency achieved to be determined.

NEED FOR REDUCED POLLUTANT-EMISSIONS Increasing concerns about acid rain and also the recent alleged excessive 'greenhouse effect' phenomena are imposing political pressures on all governments to limit and reduce pollutant emissions ejected into the atmosphere. Although the mechanisms by which 'acid rain' are produced are not fully understood, SO2, SO3 and NO Xare known to be major contributors and so stricter restrictions on the emissions of these pollutants are being implemented for instance by the European Community. Legislation, introduced on 22 March 1985, has already led to the combined emissions of S O 2 and SO3 being restricted, in some parts of Europe, to < 400 mg m - 3 of outflowing gases (which is equivalent to < 148 ppm) from all new fb plants built after 1 January 1985 irrespective of their thermal capacities. 1 By AD 1995, this upper limit will be reduced even further to 2 5 0 m g m -3 (i.e. 92ppm). With regard to NO X, the emissions are now restricted to 650 mg m - 3 (i.e. 317 ppm) for all new fb plants installed after 1985 and will be reduced, even more dramatically, to 200 mg m - 3 (i.e. 98 ppm), by AD 1995. Clearly, as a result of the introduction of these increasingly rigorous restrictions, a positive effort now has to be made to monitor accurately and reduce the quantities of SO2, SO3 and NO Xemitted from UK fbc plants. Then British manufacturers of such equipment will be more likely to achieve a greater market-penetration worldwide. In addition, Britain is under threat to make its environmental-emissions standards more stringent in order to comply with those of the best of the European Community. However, because of the enormous costs which would be involved, e.g. for installing desulphurisation plant at each coal-fired and heavy-fuel-oil-fired power-

6

K. Findlay, S. D. Probert

station, the imposition of more severe requirements is something which successive British Governments have been reluctant to adopt. Nevertheless, a U K directive (see for instance Ref. 2) indicates that stricter restrictions on pollutant emissions are to be applied to high-power plant: SO/emissions are to be limited to < 2000 mg (N m)- 3, or 700 ppm, for coal-fired plant with a capacity in excess of 100 MW, or 400 mg (N m)-3, i.e. 140 ppm, for plant exceeding 500MW capacity. Although these requirements are not as stringent as those already enforced in most EC countries, they will go some way towards helping the UK Government achieve its aim of reducing national emissions of SO2 by 30% from their 1980 levels. Undoubtedly restrictions applying to the emissions of NO x and SO Xfrom smaller boilerplant in the U K will soon follow. The use of fbc plant has advantages over conventional fixed-bed or pffired plant, designed for the same purpose, due to the relatively low combustion temperatures that occur in the former, 3 In addition, where recycled exhaust-gas is used for bed attemperation, the temperature of the fb can be controlled very precisely, regardless of the rate of energy output. Therefore, in the case of the maximum thermal-load imposed, this type of fb can operate at much higher temperatures than those ensuing in other tb's, and for which in-bed cooling tubes are usually employed. However, concern has been expressed that the NO x emissions from some fbc's, 4 with flue-gas concentrations of 400-500ppm, far from being lower than those from conventional plant (as was expected owing to the lower combustortemperatures of the fb's), were in fact comparable with those from highertemperature coal combustors, such as pf burners. Such levels of NOx emissions would not comply with the environmental standards set by the European Community, and additional methods of NO Xremoval, such as the injection of ammonia into the flue-gas, would then be required and lead to considerable additional expense. Similarly, although some high-ash, low-sulphur coals (i.e. usually those with a high CaO content in the ash), can achieve SO 2 emissions within the stipulated standards set by the UK Government without the need for sorbent addition, 5 the use of higher-sulphur coals meant that emission standards could not be met without the addition of supplemental alkali to the bed. 6 This however may be unsatisfactory because, apart from the additional cost of adding the sorbent, the subsequent disposal of the sulphated sorbent, following its removal from the fb, presents a problem in itself. Calcium sulphite in particular is very unstable and breaks down when wetted to form CaO and sulphuric acid, so making it unacceptable for disposal by landfill. Therefore, the aim is to achieve maximum effective utilisation of the limestone in order to minimise the amount required wherever possible.

Pollutant emissions from fb combustors

7

T H E E X P E R I M E N T A L R I G (see Figs 1 and 2) The experimental fb consists of a refractory-lined combustion chamber, of square cross-section. The coal can be fed on to the bed by means of two chutes: the higher one was chosen for the presently reported experiments because of the more-even distribution of the coal then achieved compared with that when the coal was fed from the chute just above the bed's surface. However the use of a much finer, and hence more easily elutriated coal, would have necessitated the use of the lower chute so as to introduce the coal nearer to the bed's surface and thereby reduce the carbon loss to the cyclone. F r o m the combustion chamber, the gases were passed through a flue-gas heat-exchanger, in order to cool them prior to their entry to the recycled-gas filter system and blower. The lower the temperature of the recycled gas, the greater its attemperation capability and hence the smaller the load on the recycled-gas blower. The remaining combustion-gases having passed through the cyclone separator (in order to remove fly-ash particles and unburnt carbon) were ejected via the stack. To avoid condensation of the wet gases in the cyclone (caused by the relatively low temperatues of the gases leaving the water-cooled heat-exchanger) and hence the occurrence of low-temperature acid-corrosion, steam tracing was applied to the gas pipework leading to and from the cyclone, as well as to the cyclone itself.

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LEGEND I. 'LIVE' COAL-FEED HOPPER AND METERING SCREW 2. COAL HOPPER 3. GRAVITY COAL-CHUTE 3n. ALTERNATIVE COAL- FEED POINT L,. COMBUSTION CHAMBER 5a FLUE-GAS COOLER : Isf SECTION 5b. FLUE-GAS COOLER : 2nd SECTION 6. CYCLONE - GAS CLEANER 7. RECYCLED-GAS FILTERS

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8. RECYCLED-GAS BLOWER 9. PRIMARY AIR 10. SECONDARY- AIR ENTRY POINTS 11. INSTRUMENT CONNECTION 12. ASH - REMOVAL POINT 13 COOLING WATER I~. RECYCLED-GAS LINE 15. FLUE GAS REMOVAL TO CHIMNEY

Schematic vertical section through the small fbc test rig used in the investigation.

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Fig. 2. Mechanical details of this small fbc test rig (l), Bed removal hopper; (2) the fluidisedbcd containment chamber; (3), abovc-bed combustion zone; (4)-{6), freeboard zone divided into 3; sections (4) and (5) being the lower, (6) the highest section. R, Inlet for Coal-Feed; S, Inlet for Sand Feed; Y, Instrument Connection; Z, Secondary Air Inlet.

D~T FILTER

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VALVE FOR EN PURGE

S -LINE

Fig. 3.

A side view of the bed-removal hopper showing the Valtac Valve, which controls the flow of nitrogen purge-gas to the slumped bed.

IR

SECONDARY AIR' INJECTION PIPES

Fig. 4.

A horizontal cross-sectional view of the dismantled fbc, taken from above indicating tile locations of the four horizontal sparge-pipes across the bed.

Pollutant emissionsfrom Jb combustors

9

Samples of the combustion gas were drawn off after the cyclone and carried, by steam-traced pipework, to the gas-sampling equipment for analysis. All of the components of the test apparatus are contained within a single steel structure: this is sectionalised in an attempt to provide ease of access for maintenance and modification purposes.

THE FLUIDISED-BED COMBUSTOR The combustion chamber, as shown in Fig. 2, consists of an outer steel shell of 300mm square cross-section and approximately 3.35m long. It was designed as a series of flanged sections for ease of assembly, separation, transportation and re-assembly (should the unit require to be moved to a different site). Additionally, had it proved necessary, the freeboard height could have been changed by removing one or more of these sections. The various sections of the combustion chamber are numbered (1)-{6) as in Fig. 2, with (1), the bed-removal hopper, being the lowest. The latter was flanged at its top to facilitate removal in order to allow ready access to the fb for cleaning and clinker evacuation, when necessary. There was also a flanged connection at the base of the hopper, to which was attached a 25 mm cast-steel lubricated plug-valve. This could be opened during the operation of the combustor in order to remove small quantities of 'below-bed' material, although the removal of larger pieces of ash from the fluid-bed itself could usually be achieved only once the fb and its immersed sparge pipes had been allowed to cool sufficiently. In addition, when attempting to conserve the unstable combustion products following a test run, nitrogen was injected into the side of the bed-removal hopper, via an on-off Valtac valve shown in Fig. 3. Section (2) (Fig. 2) comprised the 330 mm high fb containment chamber. It had openings for the sparge pipes (see Fig. 4), instruments and if necessary an in-bed fuel-feed. Figure 5 shows the location of the plenum, where primary air and the recycled gas were mixed prior to their entry to the sparge-pipes. The fluid bed was brought up to the coal's ignition temperature by means of a propane-gas ignitor, located in section (3) as designated in Fig. 2 (see Fig. 6). In addition, propane gas was introduced into the inlet of each spargepipe. Figure 7 illustrates the arrangement of the 'gas start' tubes within the sparge-pipe plenum. As the propane gas entered the fb via the sparge-pipe distributor deck, it was lit by an overhead ignitor. So long as the fluidising velocity was: (a)

sufficient to allow the bed to expand to its full height (i.e. to reach approximately the level of the ignitor), and

10

K. Findlay, S. D. Probert

(b) not excessive, which would result in the ignitor flame being deflected away from the bed itself, then the propane-gas flame from the ignitor would be redirected by the fb particles back into the bed. Thus the propane gas, introduced from below, would burn within the fluid-bed so serving to increase the bed's temperature by 10-40°C per minute, depending on the air and propane-gas flows. SPARGE-PIPEEARRYING PIPELINECARRYING THE FLUIDISING GASES RECYCLEDEXHAt,15T INTO THE FBC GAS BACK TO THE PLENUH

BLANKEDFLANGE-FOR EASYACCESSTO THE FLUID BED DURING SHUTDOWN

Fig. 5. A side viewof the fbc combustorshowingthe variousthermocouplessituated below and above the bed.

Pollutant emissions from fb combustors

!1

The flows of air and propane to the ignitor were regulated using a control unit (see Fig. 8). The propane and air were mixed prior to their entry into the fuel pipe leading to the base of the Peabody 675 ignitor-control box, type KB 588 (see Fig. 9). Electrical terminals on opposite sides of the ignitor provide the spark for initiating ignition and a flame detector was available, for safety reasons, in order to prevent over-firing. Apart from the over-bed ignitor, section (3) of the combustion chamber also included two 50-ram angled-connections to the above-bed coal- and

SPARSE-PIPES

ELECTRICAL CONNECTIONS TO SPARK PLUGS

BLANKED ENDS OF

Fig. 6.

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PROPANE GAS IGNITOR

\

GAS LINE CARRYING THE PROPANE/GAS

,,GNITO TTURETOTNE.

A side view of the fbc showing the position of the propane-gas ignitor.

12

K. Findlay, S. D. Probert

sand-feeding chutes and eighteen 25mm connections for the supply of secondary air, that is, six secondary air inlets at each of three levels marked Z on Fig. 2: there were two inlets on each of three sides of the combustion chamber facing the coal- and sand-feed inlets. The lower feed-chute was used for sand, whereas the upper feed-chute was for coal.

C O A L - H A N D L I N G SYSTEM Due to the absence of an induced-draught fan or blower, the coal had to be fed into the fb against the over-pressure of gases emanating from within the furnace. A furnace pressure of not more than 4kPa was anticipated: in practice, the pressure was well below this value, namely between 1"0 and 1.5 kPa. In order to try to prevent the combustion gases escaping from the combustor through the coal-feed, three lock-hoppers were installed. This system allowed the continuous operation of the test rig, so long as the top hopper was kept full of coal. Because the rate of coal feed employed during the experiments was at the low end of the available range, the control regulators (i) were adjusted to provide the maximum degree of control at the lower settings, and (ii) remained at the same position throughout a whole test run. This allowed constant coal-feed rates to be achieved and so permitted comparative tests to be carried out.

FB TEST P R O C E D U R E The experimental programme is illustrated in Fig. 10 and involved 24 combustion tests, using S. A. Duff, with and without limestone, in the fbc. Wherever possible, the fb operating conditions were maintained invariant during each test except for a single parameter at a time deliberately being varied. For example, operating under two-stage combustion conditions at a bed temperature of say 800°C, the 02 (flue) concentration was adjusted in runs 6, 7 and 10 to be approximately 6, 4 and 2%, respectively, whilst keeping all other influential parameters constant. Gaseous emissions were then monitored for each test in order to ascertain the effect of the Oz (flue) concentration on the rates of emissions of pollutant gases, such as SO z and NO x. In some tests, e.g. runs 8, 10 and 13, the effect of changing the ratio of the rates of primary-to-secondary air being fed to the bed was determined. Also runs 24, 25 and 26, involved the combustion of three different coaltypes (namely Gedling, Maryport and Blanzy, respectively) under similar conditions of around 2% O z (flue) concentration and a bed temperature of

~AS START-TUBES

PRIMARY AIR INLET

RECYCLED -GAS INLET

Fig. 7. A view of the plenum arrangement, taken from below, showing the position of the recycled-gas and primary-air inlets.

AIR VALVE

Fig. 8.

FLOWMETER

A side view of the fbc showing the positions of the pressure tappings for the base and middle of the fluid bed, respectively.

PROPANE GAS INLET TUBE

AIR INLET TUBE

Fig. 9.

AIR / PROPANE GAS MIX OUTLET TUBE TO IGNITOR

A side view showing the propane-gas and air inlet-tubes, and the outlet pipe, carrying the propane/air mixture to the ignitor.

Pollutant emissions from fb combustors

13

900°C, in order to investigate the effect of coal type on the NO, and SO2 emissions. Tests were carried out using oxidising, as well as two-stage, combustion conditions, both with and without limestone present in the bed. Thus, for each test, the coal-feed rate and air flow were adjusted accordingly in order to provide the required bed temperature and O 2 concentration in the flue. The bottom coal-feed hopper was maintained full throughout each trial, in order to ensure a constant flow of dry, sieved coal. The coal feed-rate was normally set at a high value ( ~, 7.8 kg h - t) for the conditions of low 02 (flue) concentration and high bed temperature, and the fluidising velocity was maintained relatively invariant. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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14

K. Findlay, S. D. Probert

The use of two-stage combustion meant that, under the conditions of a relatively high 02 (flue) concentration, the recycled gas supplied a large percentage of the required O z for the bed, so reducing the primary, fluidising-air flow requirement. Under these conditions, fluctuations in the Oz content of the flue gas were observed prior to achieving fluid-bed stabilisation. Large increases in the 02 (flue) concentration were counteracted by reducing the secondary air-flow (and hence the Oz in the recycled gas). When eventually the necessary ratio of recycled gas to primary air was achieved so as to provide the required constant Oz (flue) concentration, then the experimental results, including the gaseous emissions, were recorded on a chart. As limestone was added to the coal, it was carefully measured by weight in order to maintain, within experimental error, a Ca:S ratio of 2. This ratio was chosen because previous experience 7 suggested that an in-bed sulphur capture of 85% can then be achieved. However the optimal Ca:S ratio depends on the particular coal employed, its overall sulphur content and the CaO content in the ash. Nevertheless the use of a Ca :S ratio of 2 is generally the aim for most large fbc plants operating within the USA. 8

RESULTS A N D DISCUSSION

NO~ emissions All the measurements of NO x emissions versus 0 2(flue) concentration obtained from the present series of tests (i) confirmed that the formation of NO x was favoured by high oxygen-concentrations, and (ii) corroborated approximately the data obtained by C U R L and Grimethorpe (see Ref. 9), but disagreed with Martens e t al. 1° However, the much lower values of NO X emissions that occurred when two-stage combustion was employed (see Table 1), clearly indicate that achieving a low in-bed oxygen concentration was an important factor in limiting the NO x emissions. This it did, particularly at high bed temperatures, by enhancing the catalytic conversion of fuel nitrogen, by char, to inert nitrogen (rather than NOx). Under these two-stage combustion conditions, the reaction of NH3, released from the coal volatiles, also increased at higher bed temperatures (e.g. 1000°C). This led to the NH 3 reaction with NO in the oxygen-deficient conditions of the two-stage fb, releasing inert nitrogen rather than NO x, which would otherwise be formed in the freeboard. When using low bed temperatures, relatively low emission rates were also achieved under conditions favourable to the formation of CaSO4, which acted as a catalyst for NOx-reducing reactions.

Pollutant emissions from fb combustors

15

TABLE 1

Average NOx Emissions (in vppm) Bed temperature 800°C

900°C

lO00° C

Oxidising Two-stage

Oxidising Two-stage

Oxidising Two-stage

2% O2 in No lime flue With lime

211 48

167 49

333 176

4% 02 in No lime flue With lime

379

150 80

467

6% Oz in No lime flue With lime

533 242

200 107

280 92

352 241

217 153

438 160

283 47a

Relatively low concentrations of NOx emissions would appear to occur under two-stage combustion conditions, with Ca:S= 2'0. a Lowest concentration of NOx emission achieved.

Results taken from runs 10 and 13, as presented in Figs 11 and 12 respectively, show that for significant reductions in N O X emissions to be achieved, the primary to secondary air-ratio should preferably be 30/70 rather than 70/30, i.e. 30% of the total combustion air should pass through the fb. Lyngfelt and Leckner ~1 have already concluded that under such intermediate levels o f in-bed 0 2 concentration and at high bed temperatures, C a O becomes stable as the CaSO4 is decomposed. Further, the range of in-bed 0 2 concentrations, at which the C a O was stable, increased as the bed temperature rose. The C a O was a major catalyst for the NOx-reducing reaction by limestone. Also, at low values o f the in-bed oxygen concentration, which occur when using a primary-to-secondary air ratio o f 30/70, CaS would have been formed in preference to CaO. U n d e r the low 02 (flue) concentrations prevalent during run 13, this CaS would have been oxidised only partially to SO2, in the secondary combustion-zone, so leading to the formation o f the C a O catalyst within the freeboard. Thus, there was a decrease in the rate o f N O x emissions when using the lower primary-to-secondary air ratio, with limestone present. Use of the recycled gas reduced both the N O x and SO2 emissions, presumably by increasing the residence times o f the combustion gases within the bed. Thus the rate of SO2 removal by the bed-ash and limestone was enhanced, and the NOx emissions exposed to further NO~-reducing reactions, e.g. by being catalysed by the high concentration of char found within the lb. Interestingly, recycling the flue gas produced a similar effect to

25(

7

PRIMA~-TO-SECONOART 7--~T~--

201

&

.

.

70:30

/"

30:70

i°

i

.

i _

~

.

t " •

150

z

100

eee

•

50

0

. . . . . . . . .

0

• ........

i

05

1

~

J.

.

.

.

.

.

1.5

.

~

..........

2

J 25

0XYOEN,(%)

Fig. 11.

The effect of the ratio o f the mass flows o f primary-to-secondary air on N O x

emissions as observed during run 10, without limestone added to the bed, whose temperature was maintained at 800°C. 400

OXIDISING TWO-STAGE 70],30 TWO-STAGE 30]70

• o 0

J

Q. O_ 200

0 z

o

oo

,d.og., "°

~

o

%

0 00

0

0 0

00

I

I

2

4 OXYGEN,(%}

Fig. 12. The effect of the primary-to-secondary air-ratio on the rate of NO, emissions as observed during run 13, with limestone added, at a bed temperature of 1000°C. Results under oxidising conditions were compared with those taken during two-stage combustion, using air ratios of 30/70 and 70/30, respectively.

Pollutant emissions from j'b combustors

17

the back-diffusion of NO into the fuel-rich part of the reaction zone, which was found by Simmons and Crowhurst 12 also to lead to an enhanced rate of removal of NO. Reduction o f N O x emissions

From the present measurements, it can be concluded that relatively low rates of NO x emissions can be achieved under the following operating conditions. I Low 02-in-the-flue concentration When recycled gas was employed for bed attemperation (see Figs 13-25), the NO x emission and the O 2 (flue) concentration were found to fluctuate in phase with each other, periodically and persistently throughout each of the test runs. The presence of recycled gas, by increasing the total O2 concentration within the bed, increased the rate of oxidation of fuel N to NO x, under sub-stoichiometric conditions. In the presence of lime and char, when using a relatively low bed temperature of around 800°C, if the in-bed O2 concentration was also sufficiently low (as dictated by two-stage combustion with low 02 (flue) concentrations of between 1 and 2%), then some of the fuel N was reduced preferentially t o N 2. The rate of reaction would be slow at this temperature: a higher bed temperature would have led to a greater reduction. In addition, 35(

~_ zsc z

200

150 0

0'.S

1

115 ½ OXYfiEN IN FLUE , (%)

215

Fig. 13. NO x flue-gas concentration plotted against 0 2 (flue) concentration under oxidising conditions, when burning SA Duff without limestone in the bed, which is at 900°C.

18

K. Findlay, S. D. Probert

I i i

~'X

I % I

k z

2

,~

. / i /

I

I ~

~

I

l-

--,.

I

/ ~/

Aa"//"

% %

/%

A "

v V//-v '~

/X

" v' "

[~')X IX

"

~/v',, ~ o V '-'v

~1 I ~

~3 I

'

:[1~

A/x~-

~,,, ,# I

I

TIM(M E,N I UTES) Fig. 14.

Time-plots: these measurements were taken at l-min time intervals, under the conditions as described for Fig. 13.

500

/*5(

3E

&'7"."" 300 250 2C

o

,~%

•

~" i

~

~

OXYGEN IN FLUE, (%)

Fig. 15. NO, flue-gas concentration plotted against 02 (flue) concentration, under oxidising conditions, when burning SA Duff without limestone in the bed, which is at 1000°C.

Pollutant emissions from fb combustors

19

15

s

/.,% jI

l

'

/~ ~

It I t

t ''

t I

NOx~

1._ i

I

"

"

~t

I

~'~

t,i It

'

' ~l_g

II

J3

I

t% z c~

ss% I Qz t uJ I -1

3 h g 6 */ 8 9 10 I'I I'2 I13 IZ ll5 ll6 17 18 1~) 20 il 22 2J3 2• 25 16 217 28 ~ 3i TIME, (MINUTES)

Fig. 16.

Time plots: for the experiment conducted under the conditions given for Fig. 15.

3O0

ill

Ip=

OXYGEN

Fig. 17.

IN F L U E ,

(%)

NOx-emission concentrations, when burning SA Duff under oxidising conditions with limestone in the bed, which is at 900cC.

20

K. Findlay, S. D. Probert

I I I

v"" \' " " ~ --' " 02

"""

'~'i ' " ff~ I I I I

22 z~ z~. 2~ 2~ ~ z~ 2~ ~ • 3~ 31 3'~ 3~ ~ ~ 3'8 ~ ~o ~1 ~o TIME, (MINUTES)

Fig. 18.

Time-plots for the system whose behaviour is also described in Fig. 17.

2OO

o= 150

z

10(

OXYGEN IN THE FLUE, (%)

Fig. 19. The relationship between NO, concentration in the flue-gas and the 0 2 (flue) concentration, when burning SA Duff under oxidising conditions, with limestone present in the bed, which is at 1000' C.

Pollutant emissions from fb combustors

21 ~6

Oz

z

e

2

s

s%

I q

i/

_o

~t

I

~s

~

I

i

%

NOx

26 27 28 29 30 31 32 33 ~ ~

=

:i

36 37 38 39 ~ /.1 /*2 43 /*/* /*S /*6 /,7 /*8 /*9 SO TIME, {MINUTES)

Fig. 20. T i m e - p l o t s for the system, w h o s e b e h a v i o u r is also d e s c r i b e d in Fig. 19.

./

"E

oo

LLJ

20

;o

~o

~o

1~o

REEYELED GAS FLOW.(kg/hr)

Fig. 21.

T h e r e c y c l e d - g a s flow i n c r e a s e d t h e total o x y g e n - c o n c e n t r a t i o n w i t h i n t h e fbc.

K. Findlay, S. D. Probert

22 8

78

+

0.,. BE0

/ .

--

>...

~•

i II i I

,: 02 IN REEYELEDGAS +

~

,

]

i

,

,

,

,

J

,

~

]

,

,

,

~

*

,

,

i

i

i

I

Z

-

I

J

i

1

i

I0

2 3 ~, 5 6 7 8 9 10 11 12 13 I~,151617 18 19 20 21 22 23 2/.25262728293031 32 TIHE, (MINUTES) F i l l . 22.

The in-phase variations

o f t h e O= c o n c e n t r a t i o n

o f t h e r e c y c l e d gas a n d t h e i n - b e d

oxygen concentration during the early part (i.e. for the first 32 l-min time intervals following the commencement of the test) of run 16.

710

,-4

o

L

+

i i I I I I

IN FLUIDISED BED

~ I

I iI|

|l ~l

I-- -%

Ii I IN RECYCLED GAS

| +"

i

I

\

I

I

I I

i

+!

I x

. . . . . . . . . . . . . . . . ~ .... , , , ,',J- , , , L "~i0 33 3~ 35 36 37 38 39 4.0/+1 ~2 4.3 /,4/,5/,6 ~7/,8 /,9 50 51 52 53 5/, 55 56 57 58 59 60 61 62 63 6/, TIME. (MINUTES)

Fig. 23. As for Fig. 22, but for the subsequent part of run 16 (i.e. during the period from 33 to 64minutes following the commencement of the test).

Pollutant emissions from fb combustors

23

50£

R30(

,.=, ×

20£

10(

TOTAL RATE OF 0z FLOW THROUSH BED, (kg/hr)

Fig. 24.

NO, emissions from the fb, which was at 800°C, without limestone added to it.

•

,

-....

d=

•

•my'i'm /,~

/:

• /

ms

• •

• •

•

-4,,

z

0z - IN - RECYCLED GAS, (kg/hr)

Fig. 25. How the recycled exhaust-gas influenced the NO x emissions during run 16, at a bed temperature of 800°C, under two-stage combustion conditions, with limestone added to the bed.

K. Findlay, S. D. Probert

24

e-,

2 E

e-, o I=-

,.Q

E

o~o~o~

8 0

>0

q~

0 e.-.

0

8

0 0

0 0 0

0

o O0

,,%

Z

s

00 0

0 0

O0 ~ 0

° o~

~'~ -6

0

0

10 o

I o

o

(Wdd se)'XoN

e-,

t~

Pollutant emissions from fb combustors

25

some fuel N was also reduced to N 2 by the presence of CaSO 4, which is known to be stable at these low bed temperatures. Reduced formation of thermal NO X plus the simultaneous combination of both these reactions gives rise to low values for the NOx emissions (see Fig. 26). This figure also shows how, when the in-bed oxygen concentration was high, the NO~ emissions also increased. Where recycled gas was not used, as in run 27, no correlation between the 0 2 (flue) concentration and NO~ emission was evident. When high bed temperatures (,-~ 1000°C) were employed, in the presence of limestone, the use of two-stage combustion and moderately high 02 (flue) concentrations led to intermediate levels of in-bed 02 concentration, so favouring the formation of CaO rather than CaS (though some CaS would still be produced), CaO being a main catalyst for the NO~-reducing reactions. The moderately high 02 concentrations would ensure the complete oxidation of any CaS from the bed to form CaSO4 in the freeboard, CaSO4 also acting as a catalyst for the NOx-reducing reactions. Simultaneously, the use of a sub-stoichiometric fb, with a high bed temperature, enhanced the rate of reaction of fuel-nitrogen and ammonia to form nitrogen (rather than NO~), as catalysed by the char present in the bed. 600

40C

~

mm •

mn

x

a.. o..

x

X~

x o z

20C x •

0

TWO-STAfiE OXIDISING

'2 OXYGEN,(%)

Fig. 27. Employing two-stage combustion and low 0 2 (flue) concentrations reduced the NO x emissions for a bed temperature of 1000°C, without limestone being present in the bed.

K. Findlay, S. D. Probert

26

The 0 2 (flue) concentration is one of the main parameters dictating the rate of NO-reducing reactions occurring in the freeboard, low 02 (flue) concentrations tending to favour the reduction of NO Xto N 2 (see Fig. 27).

2 Two-stage combustion This process, by reducing the in-bed 02 concentration, led to significantly less NO X emissions occurring during all the test runs, with or without limestone present in the fb, when the bed temperature exceeded 850°C. For instance, without limestone present, the NO x emissions were reduced by ---41% at a bed temperature of 1000°C (see Fig. 27) and by ,--25% at a bed temperature of 900°C (see Fig. 28). When limestone was present in the fb, the reduction in NOx emissions achieved by using two-stage combustion, at high bed temperatures, was even more spectacular: for example, ,,~ 70 and -,~50% reductions at bed temperatures of 1000 and 900°C respectively (see Figs 29 and 30). It can be concluded from the present observations that two-stage combustion is an effective means for reducing NO, emissions, under operating conditions of high bed temperature, with a bed stoichiometry of between 0-3 and unity. Lower levels of bed stoichiometry were difficult to achieve in the experimental fb without loss of bed fluidisation. In addition, 600 x •

TWO-STAlE OXIDISIN6

4

500 I1 _ . . l ~

Q.

II

•

X X

~oo

o 3o0

200

X

100

I

1

Z

3

~

OXY6EN,(%)

Fig.

28.

As for Fig. 27, but with a bed temperature of 900°C.

Pollutant emissions from fb combustors

27

25(

"

OXIDISING

•

--x--TWO-STAGE

20(

•1 •

I

I I • •

•

•

•

•

•

•

/ /

•

I

150 ~

~

o_

•

ii

mid

•

•

i1~

••

•

c~ 100

z

x

x x

xxx

.Xxx~

~

X

XXXAX X XXX

x X

X~

x " ~ X XX

~x

"x

~----

"

X

XXX

OXYGEN,l%]

Fig. 29.

As for Fig. 27, but with limestone a d d e d to the bed: hence less N O x emissions ensued.

30C

25C

20¢

a.

1SC

x

/

Ip= 10C

•

0x.0,S,.G

x

TWO-STAGE

xkx

50

I

i

i

1

2

3

OXYGEN,("/.,}

Fig. 30.

As for Fig. 28 but with limestone a d d e d to the bed: hence less N O x emissions ensued.

K. Findlay, S. D. Probert

28 40C

~X~TWO-STAGE

- -'N--OXIOISING

•/m m /

300

/

m" x

xx x mlm_~"-'~ "/x 200 Z

100

o OXYGEN,l%)

Fig. 31. This comparison between NO x emissions, taken under oxidising and two-stage combustion conditions, shows that little or no benefit was achieved by the use of two-stage combustion at a bed temperature of 800~'C without limestone present in the bed.

the use of lower bed temperatures ( ~ 800°C) led to no significant effect for two-stage combustion, with or without lime present (see Figs 31 and 32), except for high values of O 2 (flue) concentration (Fig. 33).

3 By the addition of limestone In almost all cases, the addition of limestone to the fb led to reductions in NO X emissions, the magnitude of the reduction depending on the bed temperature and the Oz (flue) concentration.

100

---N-OXIDISING - -x-TWO-STAGE XX xxX:x

o. ~-

X

~so

--

X

X X

",

x

z

xX Xx,X 0

x.x

•

X

x

x

X

x

~~-

;.'" " ~

•

;

m u

•

•

•

=.

xNN

:'k

x x

I

I

I

I

1

2

3

4

OXYGEN, ( % )

Fig. 32. The use of two-stage combustion had little effect when using a relatively low O2-influe concentration at a bed temperature of 800°C with limestone added to the bed.

Pollutant emissions from jib combustors

29

600

•

OXIDISING

x

TWO-STAGE

40C

d o

200

I

I

EXCESS OXYGEN.(%)

Fig. 33. As for Fig. 32, but for relatively high O2-in-flue concentrations: the use of two-stage combustion was beneficial. 350

L M IE S T O N E • W T iHL M I ESTONE J -

• 300

WITHOUT

250 ,

,

20C O.

150

100

50

1

2

3

OXYGEN, (%)

Fig. 34. The beneficial effect of the presence of limestone on reducing the N O x emissions, under oxidising conditions at a bed temperature of 900°C and low 0 2 (flue) concentrations.

30

K. Findlay, S. D. Probert

At a bed temperature of 900°C, the addition of limestone resulted in a 25% reduction in NO x emissions, under oxidising conditions with a 1% 02 (flue) concentration (see Fig. 34). As the 02 concentration increased to 3%, the reduction in NO x also rose, to 30%, possibly due to the formation of CaSO 4 as a result of the oxidation of CaS. Under two-stage combustion conditions, a 58% reduction in the NO x emissions was obtained when limestone was added (see Fig. 35), hence indicating that the char reaction taking place within the fb favoured the use of low in-bed 02 concentrations at this temperature. When the bed's temperature was increased to 1000°C, under oxidising conditions, a significant change of behaviour occurred: the reduction in the NOx emissions achieved by adding limestone became ,-~33% , under oxidising conditions, with an 0 2 (flue) concentration of between 1 and 3% (see Fig. 36), and 72% for an 0 2 (flue) concentration of 2"5% in a later test (see Fig. 37). This was reduced to 63% for an 02 (flue)concentration of 6%. Under two-stage combustion conditions, the reduction in NO Xemissions achieved by adding limestone remained unchanged at ~ 3 3 % when using an O z (flue) concentration of 2% (see Fig. 38), but when the 02 (flue) concentration was approximately 4%, it was much higher (,-~ 83%) during other tests (e.g. see Fig. 39). Under two-stage combustion conditions and high bed temperatures, the reduction in NOx emissions increased from a maximum of 72% under oxidising conditions, to 83% when using two-stage combustion. However, 600

•

WITHOUTLIMESTONE

•

WITH LIMESTONE

z

200

Fig. 35.

~

mm

i

I

i

2

4

6

OXYGEN,(*/*1 As for Fig. 34, but under two-stage combustion conditions.

/+50

~,00

•

WITHOUTLIMESTONE

•

WITH LIMESTONE

=;,== ~== " • .==~'J'~"==

350

= _ ===~=_ =

2= Q.

¢~ 300 -%

•

/

Z

25C

. . 2 "

20(~

15C

I

I

I

I

2

3

OXYGEN, (%)

Fig. 36.

As for F i g 34, but at a bed temperature o f 1000°C

600

WITHOUT LIMESTONE WITH LIMESTONE

i~

~ l

400

Qct.

N z

0_~0

200

•

I

I

I

2

4

6

OXYGEN, (%)

Fig. 37.

As for Fig. 36, but for moderately high 0 2 (flue) concentrations.

32

K. Findlay, S. D. Probert

~00

t~ t~.

•

WITHOUT LIMESTONE

•

WITH LIMESTONE

200

I" z

°

o

QQ

•

OXYGEN, (%)

Fig. 38.

As for Fig. 36, but under two-stage combustion conditions.

600

• WITHOUT LIMESTONE • WITH LIMESTONE

400 z

Q.

~

x z

200

0

I

0

2

r

I

•

[

I

I

¢

6

8

OXYGEN. [%)

Fig. 39.

As for Fig. 38, but for moderately high 02 (flue) concentrations.

33

Pollutant emissions from fb combustors 350

300

250

%" Q,.

200 g ~

"-~ 150

•

WITHOUT LIHESTONE

•

WITH LIHESTONE

100

•

• I

1

eo

~

%

I

I

I

2

3

l,

OXYGEN, (%)

Fig. 40. The beneficial effect of limestone in the bed on reducing the NO, emissions under oxidising conditions, at a bed temperature of 800°C, and low O2 (flue) concentrations. as Fig. 36 shows, the reduction of 0 2 (flue) concentration from 2-5 to 1-0% led to a decrease in the percentage of N O x reduction achieved. This indicates that it is the use o f a low in-bed oxygen concentration which leads to the low N O x emissions in the presence of limestone rather than the low value of the 0 2 in the flue. The latter, as it rose up to 5 - 6 % , under both oxidising and two-stage combustion conditions, increased the effect of the limestone (see Figs 36, 38 and 39). Qualitatively similar results were obtained for a bed temperature of 800°C: as the O 2 (flue) concentration increased from i to 3%, the percentage reduction in N O x achieved by adding limestone rose significantly (see Fig. 40). However, when higher values of the 0 2 (flue) concentration ensued, the percentage reduction was less (see Fig. 41). This suggests that, at a bed temperature of 800°C, although moderate levels o f 0 2 are required for the limestone to reduce effectively the N O x emissions, too high an oxygen concentration tends to affect adversely this trend. This is p r o b a b l y due to the rapid blockage of the limestone's pores by the large molecules o f the CaSO 4 reactant. Thus less complete sulphation o f the limestone would ensue, which would, in turn, lead to less C a S O 4 being present as a catalyst. This would not apply at a bed temperature o f 1000°C, at which C a S O 4 would not be stable

34

°°i f

K. Findlay, S. D. Probert

~ L'°°/ ~• - 0

• • WITHLIMESTONE OXYGEN,{%)

Fig. 41. As for Fig. 40, but under high 02 (flue)concentrations. under the reducing conditions in the bed. However, the formation of CaS under two-stage combustion conditions, together with its complete oxidation to CaSO4 at the moderately high O2 concentrations ( ~ 4%) in the freeboard would also lead to a near maximum reduction in NO~ emissions under the prevailing operating conditions. At a bed temperature of 800°C, under two-stage combustion conditions, the reduction in NO x emissions tended to increase as the Oz(flue) concentration increased (see Figs 4244). However, at a very low O2 (flue) level, of 0"5%, the reduction in NO x emissions was less. This confirms the observations made by Williams et al., 13 who also noted that, if the first-stage bed's stoichiometric ratio was too low, the NO~ levels increased as more unreduced oxidisable N species passed on to the second stage, and the ratio of the amounts of CaO to CaSO4 present was likely to be low. 4 By adjusting the bed's temperature The use of a low bed temperature of around 800°C led to reduced NO x emissions, with or without limestone added, under both two-stage and oxidising conditions (see Figs 26, 4547). However, under two-stage conditions without limestone present in the bed, the NOx emissions were actually lower for a bed temperature of 1000°C than 900°C. This indicated that NOx-reducing reactions were favoured at the higher bed temperature, when the in-bed O2 concentration was low (see Fig. 46). This comparative

Pollutant emissions from fb combustors

35

350

300

•

WITHOUT LIHESTONE

•

WITH LIHESTONE

•

"

250

200

w

~- 150

100

50

i

i

i

i

5

6

7

8

OXYGEN, (%)

Fig. 42.

As for Fig. 41, but under two-stage combustion conditions.

200

Xx~b~(~

x~ ~ ~ ~ x .

--x-

-

WITHOUT LIHESTONE

1S0 WITH LIHESTONE

Xx~)O~,.x.-~s Q.

x~X~x X

I0(~

°°

OO0

o

•

oo

•

°S

•

~o

L

•

OO

eeo 8 • •

•

•

OXYGEN (%)

Fig. 43.

As for Fig. 42, but using moderate-to-high 0 2 (flue) concentrations.

K. Findlay, S. D. Probert

36 40(] •

WITHOUTLIMESTONE

•

WITH LIMESTONE

I1•

200

i#

I I] Oo

•

O 000

,Dee

0

• •

== e

'1

•

9g ~.

•

1 2

OXYGEN, (%)

Fig. 44.

As for Fig. 42, but using low 0 2 (flue) concentrations.

reduction in NO~ was also apparent when limestone was present in the bed, but only at moderately high values of the O2 (flue) concentration--see Fig. 26--so indicating that some other mechanism for NO~ reduction was taking place, in addition to catalysis by the char. This additional NO~-reducing reaction occurred in the presence of either CaO or CaSO4.

5 By selecting an appropriate quality of the coal Although the NO x emissions were generally dependent on the percentage nitrogen content in the coal, some anomalies did occur, as shown in Fig. 48, where the use of SA Duff coal led to higher NO Xemissions than occurred with Gedling coal, even though the Duff had a lower nitrogen content: Blanzy coal gave higher NOx emissions than Maryport coal, with a nitrogen content that was 0"2% lower. A comparison of the NO x emissions from the four different coals, under similar operating conditions, is presented in Figs 49 and 50. No direct relationship could be identified between the NO~ emissions and the volatile content of the coal. Whereas Blanzy did have a higher volatile

600 SYMBOL

BED TEMPERATURE

x o

IO00*C 900"C 800%

•

XxX

40C

°

.,,.

xx~e;o~ ox 20(

0

I 2

0

I t,

OXYGEN, (%1

Fig. 45.

The effect of bed temperature on the NO x emissions under oxidising conditions, without limestone being present in the bed.

I,OC

0

o .x

O0 0

O X oX

a.

x~

,.o.g.,.., "I

=o

~oo

't.xx x/~.. x

Xx.,.M"

x.~%

xx

xX x

•

•

_ ,,..-

•

20(

BED TEMPERATURE

li

I000"C 900"6 800"C

0

I

0

2

L

',

OXYGEN, (%)

Fig. 46.

As for Fig. 45, but under two-stage combustion conditions.

K. Findlay, S. D. Probert

38

oj

300

25( O OO

•

•

20C •

1SC •

o

1000°C

•

900oc

~--800"CTWO

RUNS

IP"

z

10C

SC

I 1

i 2

I 3

I

OXYGEN(%)

Fig. 47. As for Fig. 45, but with limestone added to the bed.

/~00

® 300 •

G

L COAL COMBUSTION QUALITY MODE

o. a.

200

J 100

~

L~ m

~o

•

•

6EOLIN6 OXIOISIN6 GEl)LING TWO-STAGE MARYPORT OXIDISING MARYPORT TWO-STAGE BLANZY OXIDISING S.A.DUFF OXIDISING S.A.DUFF TWO-STAGE

/

0

I

12

I

1~

46

.

I

i

°A, NITROGENiN COAL

Fig. 48.

The effect o f nitrogen content on NO~ emissions when burning various coals under similar conditions.

Pollutant emissions from fb combustors

39

/,00 0

007,"

0/0

0

X

o

350

30C

2sc (~

o/

•

J"

20C

x 150 J:rdr' ~ e~

100

•

o x

!LA.DUFF 5EDLING

o

BLANZY

•

MARYPORT

50

0

1 OXYGEN,(%)

Fig. 49. NO Xemissions dependence on the 0 2 (flue) concentration, when burning different coals separately under oxidising conditions, at a bed temperature of 900°C.

content than Maryport coal (i.e. 31.6% compared with 30.4%, respectively), which might explain its higher NO x emission, South African Duff had a volatile content lower than that of Gedling coal. It would appear, therefore, that the selection of a suitable coal (to give low NO Xemissions) cannot be predicted easily and that a low NOx-producing coal can most confidently be selected, at present, only as a result of the observed NO x emissions achieved in practice. Increased levels of CaO and/or Na20 in the coal, as in the case of Gedling coal, did lead during combustion to lower emissions of NO Xthan might be expected as a result of a particular coal's nitrogen-content. If the CaO and Na20 reacted with the NOx, then the absorption of the NOx on to the limestone surface would inevitably interfere with its adsorption of SO2; the two reactions acting competitively with each other. It follows therefore that any reduction in the NO~, as a result of the absorption by the lime, would lead to a consequent increase in the SO2 emission if the limestone concentrations were low and hence rate-limiting. However, this did not

K. Findlay, S. D. Probert

40

/,50

~00

0 0

0

0

O

0 O0

0

O

0

35(

O

300

o. 250 z

2OO

° °o~O°oo

/

KE_._Y

f 150

o x •

x x x~ x¢"~

S.A.DUFF GEDLING MARYPORT

100

/

•

OXYGEN,(%)

Fig. 50. As for Fig. 49, but under two-stagecombustion conditions. occur, and the greatest sulphation by the limestone coincided with the m a x i m u m reduction o f the N O x emissions, so indicating that the C a O and CaSO4 present within the limestone acted as catalysts for the NOx-reducing reactions.

6 By using recycled gas for bed attemperation Significant reductions (e,g, by ~ 33 %) in the N O x emissions were achieved by recycling the exhaust gases to the fb. Cooling these recycled gases prior to their re-entry to the fb also provided an effective means o f bed attemperation. This was concluded by comparing the N O Xemissions with and without recycled gas to the fb, see runs 22 and 27. Both tests otherwise occurred under similar operating conditions, i.e. a bed temperature of 800°C and a high O2 (flue) concentration (of 6-8%). The NOx emissions during run 27, i.e. without recycled gas, were 321 ppm (see Table 1) and were reduced when recycled gas was added (as during run 22). The reinjection of the NO~-

Pollutant emissions from fb combustors

41

rich exhaust gases into the fb thus led to the enhanced removal of NO x by (i) NOx-reducing reactions taking place within the bed, as well as (ii) catalysis by char and limestone. SO 2 emissions

There was a greater tendency for reduced S O 2 emissions to occur as the O2 (flue) concentration increased, i.e. the opposite trend to that which ensued for NO X. For the majority of tests without limestone present in the bed, under both oxidising and two-stage combustion conditions the SO 2 emission increased as the O2(flue ) concentration declined (see Figs 51-53). Qualitatively similar behaviour also ensued when LG8 was added, but only for the selected bed temperatures of 900 and 1000°C and low values of 02 (flue), e.g. as in Fig. 54 (for 900°C) and Fig. 55 (for 1000°C). At higher values of 0 2 (flue), a decrease in the SO2 emission was found to occur as the 0 2 (flue) concentration was reduced from 4 to 2.7% and increased from 4 to 5.6%, e.g. see Fig. 56 (for 1000°C, without limestone) and Figs 57 and 58 (for 1000°C, with limestone). Increased SO 2 emission at an O2 (flue) concentration of 4% could well have coincided with increased fluidising velocity due to the maximum requirement for recycled gas occurring under these stoichiometric conditions. As O2 (flue) concentration was increased beyond the stoichiometric 7O0

!

•

•

•

".K .. m

m

%

•

nm

mm

mm.mh Tm'-

• •

OXYfiEN IN FLUE. (%)

Fig. 51. The relationship between SO 2 flue-gas and 0 2 (flue) concentrations, when burning SA Duffwithout limestone under two-stage combustion conditions, at a bed temperature of 900°C.

K. Findlay, S. D. Probert

42 6~

550

50C

il

•• •

i=~=n =min.• • -

•

" l- - " ~ , ..•" == •

~m_. _ •

350 300

o

Fig. 52.

;

~

~

OXYGEN INFLUE,(%)

As for Fig. 51, but under oxidising conditions, at a bed temperature of IO00°C.

requirement, the additional air would act as a heat diluent, and cooler combustion air is a much more efficient means of bad attemperation than recycled gas. Thus volumetric gas flow, and hence fluidising velocity, would fall, and the residence time for sulphur retention would increase. At lower values of 02 (flue) concentration, e.g. than 4% only substoichiometric combustion would take place within the fb, and reduced heat release within the bed would lead to a reduced recycled gas requirement, and lower fluidising velocities, hence providing increased residence time for sulphur retention by the coal ash or lime. This could explain the occurrence of a maximum SO2 emission at 0 2 (flue) concentrations of around 4-4-5% , as shown in Figs 56-57 inclusive. Use of staged combustion (see Fig. 58) means that only a proportion of the air is supplied to the bed and greater concentrations of 02 (flue) concentration can be achieved without stoichiometric combustion within the bed. Timeplots--see Figs 59-61 inclusive--show how the SO 2 emission variations matched the fluctuations in the 02 (flue) concentration. Under conditions of low bed temperature (i.e. 800°C) with limestone present in the bed, a slight decline in the SO2 emission upon reducing the O 2 (flue) concentration was observed--see Fig. 62. This confirms that the decomposition of CaSO4 did not occur at this bed temperature, in the presence of limestone, so corroborating what would be expected according to the equilibrium diagram shown in Fig. 6314 and that the CaS was completely oxidised to CaSO 4 at 800°C, the amount of CaSO4 formed reduc-

Pollutant emissions from fb coynbustors

43

350 o. o. m

z°

300

25C

1

I

OXYfEN IN FLUE,l%)

(a) ~.5C

/I

•

•

35C

~

•

•

•

•

•

•

• •

--

m "~-~--J----...~

ll

•

i

30C

OXYGEN IN FLUE, (%)

(b) Fig. 53. (a) As for Fig. 52, but at a bed temperature of 800°C. (b) As for (a) but under twostage c o m b u s t i o n conditions: larger rates o f SO2 were produced with this system.

ing as the 0 2 (flue) concentration increased. By comparison, at the higher bed temperatures and low 02 (flue) concentrations, insufficient 02 was available to oxidise completely the CaS to CaS04, and so rejection of S02 took place. Only by ensuring that the 02 (flue) concentration rose to 4% was sufficient 02 available to oxidise completely the CaS to CaS04. These conclusions agree with those ofJonke et al. ~5 (who also observed a reduction in sulphur retention by the limestone for 02 (flue) concentrations below 3%,

~

(a)

SSO

l

• WITHOUTLIHESTONE

• ~

~m

• WITH LIHESTONE

SO0

t,S(~

o. Q.

\

i° .\.

40C

•

• m~,..." .

•

• •

•

IN ..

.J

3S0 ql~lo'kqt%

•

oeeo~.

mmmm •

30(1

250

I

i OXYGEN,(%1

L,50

(b)

t~

o_

35C

• •

25(

• •

•

•

m

•

t, OXYfiEN IN FLUE, (%) Fig. 54. (a) The effect of limestone on reducing the S O 2 emissions under two-stage combustion conditions, at a bed temperature of 9 ~ C . This comparison of results, obtained d u r i n g runs 23 a n d 8, shows a ~ 3 0 % reduction in 5 0 2 emissions when limestone was present. (b) T h e relationship between SO 2 flue-gas a n d O 2 (flue) concentrations, when b u r n i n g SA Duff u n d e r oxidising conditions, with limestone present in the bed at 900°C.

Pollutant emissions from f b combustors

45

t,oo

IK

•

Z

•

250

•

mm m m•

• mm •

•

~m •

2o0

•

mmm

~

mmm

~

OXYfiEN (N FLUE, (%)

Fig. 55. The relationship between SO2 flue-gasand 02 (flue)concentrations, when burning SA Duff with limestone present in the fb under oxidising conditions, at a bed temperature of 1000~C.

at a bed temperature of 840°C), but disagree with the theoretical predictions by Burdett.l 6 Therefore under low bed temperature conditions, the sulphation of limestone by SO2 was enhanced, the degree of sulphation increasing as the in-bed 02 concentration was reduced, by using two-stage combustion, according to Burdett's sulphate-shell hypothesis. 16 Under these conditions of low 0 2

mm

mmmm m • •

mu

•

350

Y:

300

250 OXYGEN IN FLUE, (%)

Fig. 56. As for Fig. 55, but without limestone in the bed. Thus, at moderately high 02 concentrations of 4% SO2 emission appears to reach a maximum and then falls back at higher 02 (flue) concentrations.

46

K. Findlay, S. D. Probert 2oc

c~ 1SO •

~U

i

•

mm

100

•

•

•

•

m

I

i

i

5

6

OXY6EN IN FLUE, (%)

Fig. 57. Even though qualitatively under nominally similar conditions to those applying for Fig. 55, at higher values of 02 (flue) concentration the trend is quite different with respect to O 2 concentration. This is partly due to the difficulty of achieving exact reproducibility of conditions when using the fb. Nevertheless, the data corroborate that the presence of limestone is beneficial, and that maximum SO2 emissions tend to occur, in this instance, at an 02 (flue) concentration of 4.5%.

concentration, ,-~40% of the retained sulphur was in the form of sulphide rather than sulphate. At higher temperatures, the limestone tended firstly to crystallise (see Fig. 64) at ~ 900°C, and then to fuse into dense agglomerates (see Fig. 65) at ,,~ 1000°C. The limestone showed similar eutectic properties to the coal ash at high temperatures (,-~900°C): Maryport-smalls ash also agglomerated to form clinker (see Fig. 66). 150

•

mmm

& • m z o

100

m ~/j~if

~

~

m

~

•

m-mm •

•

I= •

•

•

•

• •

&.,,

50 3

i /*

i 5

I 6

OXYOEN IN FLUE, (%)

Fig. 58. The relationship between SO 2 flue-gas and 0 2 (flue) concentrations, when burning S. A. Duff under conditions of two-stage combustion for a primary-to-secondary air ratio of 30/70 with limestone, at a bed temperature of 1000°C. Again, it can be seen that the presence of limestone in the bed at moderately-low 0 2 (flue) concentrations is beneficial in reducing the SO 2 pollution.

Pollutant emissions from fb combustors

47 q5

q3

z

,,

'V"

-"

,,

~

~1

q v

t~

~

SO~

=

I ' I

I I I I I

~ ~. ~ ,; ÷ ~ ~) ~b ~'~ ~'2 ~'~ ~. ~'~ ~'~ ~'7 ~ ~'9 2'o fi z'2 2'~ ~ z's 2~° TIME, (MINUTES)

Fig. 59.

Time-plots showing the out-of-phase variations of the SO2 flue-gas with respect to the O~ (flue) concentrations. These measurements were taken at l-min time intervals, when burning S. A. Duff wHhout limestone, under oxidising conditions, at a bed temperature of 800°C. -1s t

~0~

r

.-e

/I a I

.-r' z

II ~

I

iil'~t

I

t~

I --

Ill

•%

Ii

II ~',ll SO z

~

q3

I

I

I

t~

J2

t.U

I I I

]1 I I I I

TIME, ( MINUTES )

Fig. 60.

As for Fig. 59, but for l-min intervals from 26 to 50 minutes after the commencement of the test.

48

K. Findlay, S. D. Probert

AA

,,_

o

v

t

",,I

~-'-/i l t ¢

I@

°

I

I I

/ #

z

:E2~

-13

I

i---~

i

l

~%

I

//I

I

I

lI

z

/ I i I~

I %

II ~O

I

/

II

I

~l

II

[

I I

I

I

~t

SO~

I

12

l

I

JlI I I I I I

sl s'2 s~ s'~ sg s'6 5'7 ~ sb 6'0 6'1 6~ 6'3 g~ 6's-g6 6'7 d8 6'9 7'0 7q 7'2 7~ 7'~ 7S TIME, (MINUTES)

Fig. 61.

As for Fig. 59, but over the period of 51-75 minutes from the commencement of the test.

The results of the present investigation show that, by ensuring substoichiometric conditions apply within the two-stage fb, the molecules of CaS, formed in preference to the larger, bulkier CaSO4 molecules, caused a more complete sulphation of the limestone to occur within the fb. A ,-, 12% reduction in SO z emissions is shown in Fig. 67: this was achieved by using two-stage combustion at a bed temperature of 800°C. This result is comparable to that obtained by Burdett: he obtained a 10% reduction in SO 2 156

•

10(3 i m--

so

•

L---

~

• •

•

mmm

•

m, ~ . j m m ~

•

~

~

OXYGEN IN FLUE. (%)

Fig. 62. The relationshipbetweenSO2 flue-gasand 0

2 (flue)

concentrationswhen burning

SA Duff under oxidising conditions with limestone, at a bed temperature of 800°C.

Pollutant emissions from jib combustors

49

CaS CoSO~

/,

R~:~ u _ . - - I C=I

~a

Ca0 -5

I

h

I -15

I

I

i

I

-I0

I

I

-5

LOG1o{PARTIALPRESSUREOFOz} Fig. 63.

The equilibrium diagram for the CaS, CaO, CaSO4, H2S , 02 and H20 systems: the partial pressure of H20 is assumed to be 0.5.11

emisions when using a sub-stoichiometric fb, also at a bed temperature of 800°C. However, at higher bed temperatures, decomposition of the CaSO4 under the reducing conditions in the bed, and incomplete oxidation of the CaS to SO z, meant that moderate-to-high 02 (flue) concentrations were favoured (see Fig. 68). Under the conditions of low 0 2 (flue) concentration and high bed temperature (,-~ 1000°C), the SO2 emissions were greatly increased (see Fig. 69). Thus the SO2 emission reached its largest value, as the O z concentration in the bed was reduced to 30% of that experienced under oxidising conditions: this corroborates the results obtained by Shimuzu e t al. 17 at a bed temperature of 900°C and Petrill e t al. ~8 at a bed temperature of 840°C. Although, for the range of bed temperatures employed, no absorption of SO 2 by char would be expected because any sulphates formed would decompose quickly at these high temperatures, there would still have been some absorption of SO z by the unburnt carbon (i.e. char) present in the cyclone ash. In fact, at cyclone temperatures below 330°C, (depending on the efficiency of the flue-gas heat-exchanger in reducing the flue-gas temperatures), activated char is an effective means for absorbing both SO 2 and NO x. Also, the HzSO 4 formed below --~300°C would react with any residual ammonia present in the flue-gases to form (NH4)/SO 4. Thus the emissions of SO z and NOx could both be further reduced by using ceramic or bag

50

K. Findlay, S. D. Probert

20 mm

Fig. 64.

Samples of the crystalline limestone agglomerate, as formed in the fbc at a bed temperature of 900'~C.

filters for particulate collection, especially if, as under two-stage combustion conditions, the unburnt carbon content of the cyclone ash was high. The addition of FeO to the bed would lead to increased sulphur retention for high O2 (flue) concentrations. Certainly during runs 1, 22 and 27, under oxidising conditions, when the O2 (flue) concentrations were relatively high, the large sulphur retentions by the bed ash could be attributed to the unusually high levels of Fe20 3 detected in the ash samples. Thus the use of a coal with a high inherent iron-content, could also enhance the sulphur retention by the bed ash, when high Oa (flue) concentrations ensued.

Pollutant emissions from fb combustors

51

20ram

Fig. 65. Samples of the fused limestone agglomerate, formed within the fbc at a bed temperature of 1000°C. Reduction of S O 2 emissions

Lower rates of SO2 emissions were achieved in the present tests by the use of the following operating conditions.

l ( a ) L o w in-bed o.vygen concentrations When limestone was added to the bed, at 800°C, reduced SO2 emissions occurred at low Oz (flue) concentrations and low in-bed 02 concentrations. This was because both CaS and CaSO4 are stable over the full range o f in-

52

K. Findlay, S. D. Probert

Fig. 66. Clinker produced during run 25, when burning Maryport smalls. bed 02 concentrations, at this relatively low bed temperature. Thus low oxygen-concentrations did not affect the stability of the CaSO4, which was also formed as a result of the oxidation of the CaS in the freeboard. Under these conditions, the reaction was essentially diffusion-controlled, a more complete sulphation of the limestone being achieved under low oxygenconcentrations, when the relatively small CaS molecule would be formed. l ( b ) Moderate-to-high oxygen-in-the-flue concentration Even if limestone was not added to the bed, reduced SO/emissions could be achieved at the moderate-to-high O 2 (flue) concentrations required for the sulphation of, for example, the FeO in the coal ash. This reduction tended to rise rapidly as the 02 (flue) concentration was increased above 9% (see Fig. 70) presumably because then there was sufficient O z available for complete sulphation of the coal ash. Conversely, as the O z (flue) concentration was reduced below about 1%, the SO2 emissions increased significantly (see Fig.

Pollutant emissions from fb combustors

53

400 rn o

O

a. Q.

OXIDISING TWO-STAGE

[]

o

~ E h

zoo

g

0

i

I

i

6

8

10

OXYfiEN,(%.)

Fig. 67. The beneficial effect of using two-stage combustion to reduce SO 2 emissions, at a bed temperature of 800°C, using relatively high 02(flue) concentrations, with limestone added to the bed.

~x ~"~ X~",..x.~

z~

COMBUSTIONCONDfflONS[ DATA

]

TWO-STAGE;30:70 TWO-STAGE;70:30

l I

,N0 0x,0,

d-s - - - - ~

--

es

}

i ]

x o

I'I

z!s

OXYGEN,(*) Fig. 68. The effects of using different values of the primary-to-secondary air ratio, namely 30/70 and 70/30, respectively, on the SO2 emissions, under two-stage combustion conditions. These results were compared with SO2emissionsunder o×idising conditions when using a bed temperature of 800°C, without limestone added to the bed.

K. Findlay, S. D. Probert

54 800 xX I

•

i!

OXIDISING

x TWO-STAGE 30/70

x x

x

o TWO-STAGE 70/30

xV 600

XI

X ~x

x

a. o_

x

600

x

00

g,,

x

x

~X""'~Xx~ x~'~"----~"~'X~-I XX

X

200

OXYGEN,(%)

Fig. 69.

As for Fig. 68, except that the temperature was 1000°C, and limestone was added to the bed.

71). However, at an O2 (flue) concentration of 1"56%, maximum sulphurretention by the coal ash (of 31.8 %; see Tables 2 and 3) was achieved under oxidising conditions.

l(c) General conditions Achieving a significant sulphur capture, at a bed temperature of 800°C, required using an 02 (flue) concentration of approximately 2-6% when limestone was added, and approximately 2-4% when limestone was not employed. Figure 72 shows the marked reduction in sulphur retention occurring as the O z concentration is increased, particularly under conditions of two-stage combustion, at 800"C, with limestone added. At higher bed-temperatures of 900 and 1000°C, both CaS and CaSO4 are unstable at intermediate in-bed oxygen concentrations. Under these, CaO tends to be stable, its range of occurrence increasing with bed temperature. Thus no sulphation of the limestone in the bed would be expected under reducing conditions at these high bed-temperatures. However, the introduction of sufficient secondary-air into the freeboard

Pollutant emissions from fb combustors 600

55

•

FOR OXIDISING CONDITIONS

x

FOR TWO - STAGE COMBUSTION

•Xxx

E

•

~

~(x~

.

R. ~00

X XX X

~

2O0

1'0 OXYGEN (%)

Fig. 70.

This comparison between the SO 2 emissions, taken under oxidising and two-stage combustion conditions, shows that there is little or no effect of two-stage combustion under high 02 (flue) concentrations at a bed temperature of 800°C.

450

600 X

•

OXIOISING

x

TWO-STAGE

X xx x

a. ck

350

300

25(

20C 0

~

2

3

j 4

OXYGEN,(=/.)

Fig. 71. The effect of using a low 02 (flue) concentration upon increasing the SO 2 emissions under both oxidising and two-stage combustion conditions, at a bed temperature of 900°C, with limestone added to the bed.

K. Findlay, S. D. Probert

56

100 WITHOUT LIMESTONE OXIDISING BED TEMPERATURE o []

80

900% 8000C

TWO-STAGE

1000"C

t,., 60

800%

WITH LIMESTONE OXIDISING BED TEMPERATURE

40

.-e

....

20

800"C

..[3._

o

~ ..---.....~.~ V

2

t,

~

&~• j~

"13. V

"-

6

B

TWO-STAGE 1000"C 8oo'c

10

OXYGEN, (*/,} Fig. 72. The effect of 0 2(flue) concentration on the sulphur retention under various operating conditions, when burning SA Duff, with and without ]imestone present in the bed.

may lead to complete oxidation of any CaS, formed in the substoichiometric bed, to C a S O 4, a s occurred in runs 18 and 19. When limestone was not added, the greatest sulphur retention was achieved under oxidising rather than two-stage combustion conditions, so that higher in-bed 02 concentrations were required for the sulphation of the coal ash at all bed temperatures (see Fig. 73). The largest sulphur retention (=31.8%, as shown in Table 3) for the present tests occurred at a bed temperature of 800°C, but was also high (namely 28.4%) at a bed temperature of 900°C using almost similar, low values of O2(flue) concentration, i.e. 1.53 and 1.56%, respectively. The sulphur retention at a bed temperature of 1000°C, and a low 0 2 (flue) concentration (of 1.7%), as was achieved during run 5 under oxidising conditions, was much smaller (i.e. at 3"6%) than that at lower bed-temperatures, although the use of a higher value of 0 2 (flue) concentration of 4% as in run 11 resulted in much higher sulphur retentions (see Fig. 74). Because limestone was present, only moderate O2(flue) concentrations were required in order to oxidise completely the CaS to CaSO4 at these high bed temperatures. The use of lower 0 2 (flue) concentrations and two-stage combustion led to partial oxidation of the CaS to SO2, and no sulphur retention (as in run 12).

2 Two-stage combustion Both Fig. 75 and Table 4 show that, when no limestone was added to the bed, the SO 2 emissions were considerably higher if two-stage combustion was

Pollutant emissions from fb combustors

57

TABLE 2 Percentage Overall Sulphur Retention in Bed

Run

Ia 2 3 4 5b 6" 7 8a 9" 10 11 12 b 13 c 14 15 16 17 c 18 ~ 19 20 a 21 22 23 c 27 d

% Total retained sulphur found in bed 2.0 0"6 0.6 0"9 1-7 1"9 0-6 5"3 5"2 0"5 1"1 9"2 6"5 47"2 51" 1 60"8 32-5 58'4 29"2 50"5 30"6 37"9 58'7 56"6

Average % sulphur retention (actual) 22"9 28'4 19"1 15"1 3"6 15-7 13-1 10-6 31"8 21"3 9"9 0 18"6 56'2 44.2 58"0 45'6 65"6 74-0 80"0 81"0 14'9 23-8 74-2

% Sulphur retained in bed, as % of total sulphur input 0"5 0"2 0-1 0'1 0-06 0-3 0"08 0-6 1"6 0'1 0'11 0 1'2 26'5 22-6 35"0 14-8 38'3 21 "6 40-4 24'8 5"7 14-0 42"0

a The highest sulphur retention in the bed ash occurred at a bed temperature of 800°C in oxidising conditions or high 02 (flue) concentration, two-stage combustion conditions. At a bed temperature of ~ 900°C the employment of a low 0 2 (flue) concentration and two-stage combustion was favoured. b The lowest sulphur retention in the bed ash occurred at a bed temperature of 1000°C and a low 02 (flue) concentration, under either oxidising or two-stage conditions. c The lowest sulphur retention in the bed, by the lime, also occurred at high bed temperatures (of ~ 9 0 0 ° C or ~1000°C) and low values (i.e. 1-2%) o f 02 (flue) concentration. a The highest values of the sulphur retention, by the lime in the bed, occurred under oxidising conditions at a bed temperature of 800°C, at low or high 02 (flue) concentrations, and at moderately high 02 (flue) concentrations at a bed temperature o f ~ 1000°C.

K. Findlay, S. D. Probert

58

employed, even at a bed temperature of 800°C, i.e. when CaSO 4 was stable. This indicates that the CaS, formed in the sub-stoichiometric two-stage bed, was not completely oxidised to CaSO4 at a bed temperature of 800°C, some of the CaS forming SO2 even under moderate-to-high O2(flue) concentrations. The amount of sulphur retention lost as a result of using twostage combustion was small at moderate and high values of O2(flue) concentration. However, when using low Oz(flue) concentrations of approximately 2%, then a ~ 15% increase in SO2 emission resulted under TABLE 3

Sulphur Retention (Without Limestone) When Burning SA Duff

Run

1 B T = 800°C; oxidising 2 BT = 900°C; oxidising 3 BT = 900°C; oxidising 4 B T = 800°C; oxidising 5 B T = 1000°C; oxidising 6 oxidising; BT = 800°C; two-stage 7 B T = 800°C; two-stage 8 oxidising; BT = 900°C; two-stage 9 B T = 800°C; oxidising 10 BT = 800°C; two-stage 11 oxidising; B T = 1000°C; two-stage 12 B T = 1000°C; two-stage

Average 02-in-flue concen lration

Sulphur retention (%)

(%)

Average S02 emission (ppm)

6"24

347

22'9

1.53

322

28-4

3-85

364

19.1

4-22

382

15.1

1.70

434

3"6

7-33

374

16.9

5.52 4.29

385 391

14.5 13.1

2.04

373

17"1

2.56 1"56

432 307

4.0 31.8"

0.99

354

21.3

3-87

361

19"8

4.40 2-26

455 500

0 0

Maximum sulphur retention in the present tests, when limestone was not added to the bed, tended to occur at a bed temperature of 800°C, under oxidising conditions and low values of the 02 (flue) concentration. "Highest value of sulphur retention.

Pollutant emissions from Jb combustors

59

600

~, 5O0

x

•

\x

x

•

TWO-STAGE o×,o,~,NO

X\x

.xZ x

..-,, •

..',~

I.,,,,

• .,'."4.

,

,,.'=

x

•

•".= . ~ ~ . .

~

30C

OXYGEN,(%) Fig. 73. The effect of two-stage combustion on increasing the S02 emissions at a bed temperature of lO00°C and low 0 2 (flue) concentrations, without limestone added to the bed x

55(

x

x x

x ×

×

~ 451

.%ram . •

OXYGEN.(%)

Fig. 74. The effect of two-stage combustion on increasing the SO2 emissions, at a bed temperature of 1000°C, without limestone added, using moderately high values of the 0 2 (flue) concentration.

K. Findlay, S. D. Probert

60 t,50

COMBUSTIONCONDITIONS~DATA I SYMBOL x x

TWO- STAGE OXIDISING

~° x~, x

......

X 350

I

X x

x x x

x,

250 __

~

_

_

, xv x

x

l"

x

J

.

J 2

1

0

x •

i

3

OXYGEN,C%)

Fig. 75. A comparison between SOs emissions, taken under oxidising conditions during run 9, and under two-stage combustion conditions during run 10. This shows that there is little or no effect of two-stage combustion when using the lower 0 2 (flue) concentrations (i.e. below 1%) at a bed temperature of 800°C, without limestone added. However, at O2(ftue ) concentrations of 1% and above, a 10-13% increase in SOz emissions ensued when two-stage combustion conditions were employed.

TABLE 4 AverageSO2 E m i s s i o n s ( i n v p p m )

Bed temperature 800°C

900~C

Oxidising ~vo-stage

IO00°C

Oxidising Two-stage Oxidising Two-stage

2% 02 in N o lime flue With lime

307 90 a

354 85

373 251

4 % 02 in No lime flue With lime

382

391 189

364

6% O2 in N o lime flue With lime

374 302

385 197

432 343

434 313

500 336

361 155

455 117

Relatively low concentrations of SO 2 emissions would usually appear to occur at a low bed temperature, under oxidising conditions. a Lowest SO2 emission concentration achieved.

Pollutant emissions from fb combustors

61

two-stage combustion conditions. This compares with a ,-~ 16% increase at a bed temperature of 900°C (see Fig. 76). The reduction in sulphur retention under two-stage combustion conditions (without lime present) could be partly the result of incomplete oxidation of the CaS (formed by the sulphidation of the CaO present within the bed ash) to SO2 in the freeboard, even at a low bed temperature of 800°C. It is most probably also due to the reduced sulphation of the FeO in the coalash, at relatively low in-bed 02 concentrations. The formation of Fe2(SO4) 3

X

xXXx X

v

x

x

x

"1"

•

:

~

•

)~XxcXXX

XX

FOR OXIDISING CONDITIONS

20O

----X---

FOR TWO-STAGE COMBUSTION

0

OXYSEN. (%)

Fig. 76.

The effect of two-stage combustion on increasing the SO2 emissions, at a bed temperature of 900°C, without limestone being added to the bed.

62

K. Findlay, S. D. Probert

was thought to be increased at the high 02 (flue) concentrations encountered during runs 1 and 6 (as indicated by the abnormally high concentrations of iron oxide measured in the cyclone ash during these test runs). However, when limestone was added to the fb, at a bed temperature of 800°C, the use of two-stage combustion did lead to a small reduction in the SO 2 emission, thereby giving the lowest value of SO 2 emissions attained in all of the tests, at 85 ppm, and the best achieved sulphur retention (see Fig. 77). Clearly, therefore, under two-stage combustion conditions, the limestone reacted with hydrogen sulphide, in the sub-stoichiometric reducing conditions of the bed, to produce CaS. Formation of the smaller CaS molecule led to a more complete sulphation of the limestone (compared with when oxidising conditions were employed, and yielded the larger CaSO4 molecule). This CaS was then oxidised completely to CaSO4 in the freeboard, so resulting in an overall sulphur-retention of 81%. At a bed temperature of 900°C, using a low 0 2 (flue) concentration of around 2%, the average SO 2 emission increased significantly when twostage combustion conditions were employed, compared with a smaller increase at a bed temperature of 1000°C (see Table 4). When using a slightly higher 0 2(flue) concentration of around 4%, at a bed temperature of 1000°C, this reverted to a decrease in the SO 2 emission when two-stage combustion was employed, so giving an overall sulphur retention of 74%. Under these conditions of high bed temperature and two-stage combustion, when CaS would have been formed in the bed, a 4% 02 (flue) concentration was sufficient to oxidise completely the CaS to CaSO4 in the freeboard (see 150

X

x xx

100 GL

•

x~x

- - -- ~

X

x •

•

II

--

XX

•

-- -- -i~

xXxX

-- 1--

•

• --

--

•

dJD

|

I1 m i

ilml

•

i

x X X

N

X XXXx

X

xX xX

X

X X

so

-"

OXIDISING

--x--TWO-STAGE

0

OXYGEN,(%)

Fig. 77.

The effect of two-stage c o m b u s t i o n on reducing the SO 2 emissions, at a bed

temperatureof 800°C, with limestoneadded. Two-stagecombustionled to a small reduction in the SO2 emissions (of just under 6%), thereby giving the lowest value (85 ppm) of SO2 emissions attained throughout all these tests.

Pollutant emissions from fb combustors

63

Fig. 78). The slight loss in sulphur retention (compared with that achieved when a bed temperature of 800°C was employed) was due to the lack of sulphation which would have occurred in the bed at these highertemperature reducing conditions. Similarly, when a high value ( ~ 6%) of 02 in the flue ensued, at a bed temperature of 800°C, then two-stage combustion again resulted in a reduction in the SO2 emission. From this we can conclude that the use of two-stage combustion does provide an effective means for increasing the sulphur retention by the limestone. It does this by allowing the formation of the relatively small CaS molecule, which is less effective at blocking the pores in the limestone. Thus a more complete sulphation of the limestone ensues, so corroborating Burdett's shell-formation theory. 19 However, the advantage of CaS formation in the bed can be realised only so long as moderate-to-high levels of 0 2 are provided in the freeboard, in order to facilitate the complete oxidation of CaS to CaSO4.

3 Adjustment of the bed's temperature When the 0 2 (flue) concentration was kept low (i.e. less than 4%) the greatest sulphur retention for the present tests always occurred at a bed temperature 200 E] E]~

13 E]

Q3

150

E]

C]

O~

..__ -- ,0 ~-

QD

O

D

Q 0

sr

100

OO~O

0

OO O

O

50 _

_

_ r~_ _ O X I D I S I N f i o---- TWO-STAGE

0 OXYGEN, [%)

Fig. 78. The effect of two-stage combustion in reducing SO 2 emissions, at a bed temperature of 1000°C, using moderately high 02 (flue) concentrations in the freeboard with limestone added. Under these conditions, a sulphur retention of 74% was achieved using a Ca:S ratio of ~2.

64

K. Findlay, S. D. Probert 600

D

$50

500

°O ~0

o

oo o o ° o - " ~

800

[] O ° o

°

•

[]

o

o ooO

o

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" ~ [• ]

= - , ~~ ~

[]

I!

N

~ B T []

[]

[]N

:900°C

[]

300

BT=800°(i

250

200

2[ OXYGEN,1%1

Fig. 79.

The effect of bed temperature o n S O 2 emissions under oxidising conditions, without limestone being added to the bed.

of 800°C, with or without limestone added (see Figs 79-82). The main exception, shown in Fig. 83, ensued under oxidising conditions, at high values (i.e. between 3 and 7%) of the 02 (flue) concentration. Then the bed's temperature appeared to have little effect on the SO2-emission concentration. When the O 2 (flue) concentration was low, at high bed temperatures (of 900 or 1000°C), incomplete oxidation of the CaS and decomposition of the CaSO4 under reducing conditions in the bed both led to increased SO2 emissions. At a bed temperature of 800°C, both the CaS and the CaSO4 were stable over the full range of in-bed 02 concentrations. Similarly, oxidation of the

Pollutant emissionsJrom fb combustors

65

:!

i.-

0

0

r~

I: w

g~ 0

¢ll

""O ~

c~

z

t~ >-

K= ..c.~ ~...'~ 0 "~ ~ ,--,

-

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&

&

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&

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~

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m

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(Ndd

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r~

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~,

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(Hdd so)' ZOS

~

c

._~ ~

= c5"5

¢-.

66

K. Findlay, S. D. Probert

SYMBOL BEDTEMPERATURE o 1000=E

800

oo

600]

co o )o )o

•

90OoC

•

800°C

0 0

oo o

Q" 400

o o

o o

,.,,;

o

200

oO

oOON

oQeoO o dl =Do

o

~

~

6I

OXYfEN,(%J

Fig. 82.

As for Fig. 81, but under two-stage combustion conditions.

i.e. BEST STRAIGHT- LINE FIT TO ll-lE EXPERIMENTAL DATA

60(

35(

~TA\ ~f

30(

~ m•

iw

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.;•--

- ""-~" . . . . . .

•

,

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25(

20(

OXYGEN(%) , Fig. 83.

The effect of bed temperature on SO 2 emissions under oxidising conditions, when using high O2 (flue) concentrations, without limestone added to the bed.

67

Pollutant emissions from fb combustors TABLE 5

Sulphur Retention With Limestone When Burning SA Duff Run

Conditions: oxidising (ox) or two-stage ( t - s ) ; BT (in °C)

13 13

ox; IO00°C t-s; IO00°C primary:secondary = 70/30 primary: secondary = 30/70 t-s; 800°C ox; 900°C t-s; 800°C ox; IO00°C ox; IO00°C t-s; IO00°C ox; 800°C ox; 800°C t-s; 800°C ox; 800°C t-s; 900°C ox; 800°C

14 15 16 17 18 19 20 21 21 22 23 27

Average 02-in-flue Average SO 2 concentration emission (%) (ppm)

Sulphur retention (%)

1.61

313

30.5

1"78 1.91 5"58 1"75 4.20 1"64 4"02 4.62 2-25 1-41 1'10 7-86 1.12 8-16

336 456 197 251 189 245 155 117 90 85 86 302 343 116

25'3 0 56.2 44.2 58'0 45'6 65"6 74.0 80'0 81'1a 80'9 14-9 23"8 74-2

Maximum sulphur retention occurred at low values of 02 (flue) concentration, under oxidising conditions in the bed and with limestone present there; a bed temperature of 800°C is again preferred. a Highest sulphur retention value obtained in present tests with limestone added. C a S to C a S O 4 also a p p e a r e d to be c o m p l e t e at this low bed t e m p e r a t u r e , so leading to relatively large s u l p h u r retentions. B o t h the m a x i m u m m e a s u r e d a n d p r e d i c t e d s u l p h u r - r e t e n t i o n values were o b t a i n e d for a bed t e m p e r a t u r e o f 800°C (see T a b l e s 5-7), a high value o f s u l p h u r r e t e n t i o n also being achieved at a bed t e m p e r a t u r e o f 1000°C d u r i n g r u n 19, b u t o n l y w h e n m o d e r a t e l y - h i g h O 2 (flue) c o n c e n t r a t i o n s ensued. 4 Addition o f limestone W i t h few exceptions, the a d d i t i o n o f l i m e s t o n e to the fb led to a m a r k e d r e d u c t i o n in S O z emissions, c o m p a r e d with w h e n coal ash was the sole SO2 a b s o r b e n t . H o w e v e r , the a m o u n t b y which the SOE emission was r e d u c e d by the l i m e s t o n e a d d i t i o n , did v a r y a c c o r d i n g to the bed t e m p e r a t u r e a n d 0 2 (flue) c o n c e n t r a t i o n . T h e e x c e p t i o n s o c c u r r e d d u r i n g r u n 22 (when a n air in-leakage gave suspect results) a n d in r u n 13, for t w o - s t a g e c o m b u s t i o n with a p r i m a r y - t o s e c o n d a r y air r a t i o o f 30/70, at a bed t e m p e r a t u r e o f 1000°C a n d low (i.e. 2 % ) 0 2 (flue) c o n c e n t r a t i o n . D u r i n g the t w o - s t a g e c o m b u s t i o n c o n d i t i o n s

68

K. Findlay, S. D. Probert

TABLE 6 S u l p h u r R e t e n t i o n ( W i t h o u t Limestone Present) W h e n Burning SA Duff Run

Average 02-in-flue concentration (%)

Average calculated sulphur retention according to emission

Measured sulphur retention in ash (%)

% Sulphide in total ash (remainder being sulphate

%oS in bed ash

1 BT 800°C;

6"24

22'9

39"0*

3"9

91"7

1"53

28'4

17"0

11"1

13"3

3"85

19"1

13"8

17"8

0

4"22

15"1

12'1

2"8

0

1"70

3"6

5'1

9"2

57"1

2 3 4 5 6

7 8

9 10 11

12

oxidising BT 900°C; oxidising BT 900°C; oxidising BT 800°C; oxidising BT 1000:~C: oxidising BT 800°C; oxidising two-stage BT 800°C; two-stage BT 900°C; oxidising two-stage BT 800°C; oxidising BT 800°C; two-stage BT 1000°C; oxidising two-stage BT 1000°C; two-stage

7.33) 5.52f 6'4 4"29

16.9) 14.5f 15"7 13'I

9.6

4.4

0

25'2

1"1

100

22:50~)23

17-1 4.~)10"6

7.0

3.9

72.7

1.56

31.8

7.6

11.7

66.6

0-99

21-3

9.8

0

0

3.87"1. 4.40) 4' 1 2"26

19"8"~9.9 0 ) 0

7'0

19-5

18-2

7'3

26"0

41'7

* The conditions (of those tested) to achieve the greatest sulphur retention, without limestone being present in the bed, would again appear to be at a bed temperature of ~800°C, but in this instance at relatively high values of the 02 (flue) concentration (i.e. above 6%). Under these conditions 91-7% of all the sulphur retained by the bed was in the sulphide (as opposed to the sulphate) form.

experienced in run 13, low in-bed O2 concentrations resulted in the formation of CaS which, using the low 02 (flue) concentrations employed, were oxidised only incompletely to form SO2 and C a O . 2° Thus, sulphur retention by the limestone was not expected under these circumstances. Under oxidising conditions, when CaSO4 would probably also be formed, a net reduction in the SO2 emission of around 30% was achieved by adding limestone at a bed temperature of 900°C (see Fig. 84) and 1000°C (see Fig. 85), compared with ~ 70% at a bed temperature of 800°C (see Fig. 86), all at

69

Pollutant emissions from f b combustors

TABLE 7 Sulphur Retention With Limestone Present W h e n Burning SA Duff Run

Average

Average

(see Fig. 10)

02-in-flue concentration

calculated sulphur retention in ash or limestone according to

(%)

Measured sulphur retention in ash

% Sulphide in total ash remainder being sulphate)

% S in bed ash

(%)

emission (%) 13 BT 1000°C; oxidising two-stage 70/30 two-stage 30/70 14 BT 800°C 15 BT 900°C 16 BT 800°C 17 BT 1000~C 18 BT 1000°C 19 BT 1000°C 20 BT 800°C 21 BT 800°C; oxidising two-stage 22 BT 800"C 23 BT 900°C 27 BT 800°C

1.61~ 1.78 ~1.77 1.91J 5'58 1"75 4"20 1.64 4.02 4"62 2.25

30.51 2~.3; 18"6

8"06

15.2

57.1

56.2 44.2 58-0 45.6 65"6 74-0 80"0

19.0 14.1 22"9 9'8 11 9"3 25.0*

4"0 24"6 3-0 6"4 3"9 4-3 2"9

11.1 75'0 0 33"3 0 0 62"5

1.41 1"26 1.10)

81"1 80.~)81.0

15.2

1"2

27.3

7.86 1.12 8.16

14.9 23-8 74.2

20.9 11.3 21-9

0.97 12.8 2.6

0 20.0 6.7

* The highest sulphur retention, when limestone was added, was again at a low bed temperature (of 800°C) and low values of the 02 (flue) concentration under oxidising conditions, according to both prediction and measurement. In run 20 the greatest measured sulphur retention again coincided with having the maximum percentage of sulphide in the bed ash.

low values of the 02 (flue) concentration. This serves to confirm that CaS and CaSO4 are not stable at higher bed-temperatures, under low O a (flue) concentrations, and so a lower bed temperature (e.g. ~ 800°C) would be preferred in order to increase the amount of sulphur retained by the limestone. This confirms the observations of Lyngfelt and Leckner, tt who also noted that the maximum sulphur retention occurred at a bed temperature of between 800 and 850°C under reducing conditions. At a bed temperature of 800°C, decomposition of the CaSO4 and CaS, to CaO, was not observed, tl Also employing an increased O/(flue) concentration led to a reduced sulphur retention. Here, by increasing the 0 2 (flue) concentration to around 5%, the reduction in SO/ emissions by having the lime present fell to .~ 60% (see Fig. 87). The reduction was .-~42% at an Oa (flue) concentration o f 7% (see Fig. 88), which did not compare favourably with the -.. 70% reduction achieved when using low values of 02 (flue) concentration, as shown in Fig. 86, also under oxidising conditions.

45C

D

•

40(~

3so •

o ~:~...~,t~a []

~ 0° 0

30o

0

WITHOUT LIMESTONE

o

WITH LIMESTONE

250

200

I

I

I

1

2

3

OXYGEN. 1%}

Fig. 84. The beneficial effect of limestone in reducing the SO 2 emissions under oxidising conditions at a bed temperature of 900°C. Comparison of the SO2 emissions during runs 15 and 2 (i.e. with and without limestone, respectively) shows a ~ 3 0 % reduction in SO 2 emissions at an 02 (flue) concentration of 1'5% due to the presence of the limestone.

sso 500

450

°°

cP o o C ~

o

8o~

o

°o° ~00 0

0

350 o o

WITHOUT LIMESTONE WITH LIMESTONE

300

250

200 OXYGEN, (%)

Fig. 85. As for Fig. 84, except t h a t the bed t e m p e r a t u r e was 1000°C. C o m p a r i s o n of the S O 2 emissions d u r i n g runs 17 a n d 5 (i.e. with a n d w i t h o u t limestone, respectively) shows a ~ 2 9 % reduction in SO 2 emissions at an 0 2 (flue) c o n c e n t r a t i o n o f 1"5% due to the presence o f the limestone•

Pollutant emissions from fb combustors

71

350

300

250

200

0

WITHOUT LIMESTONE

o

WITH LIMESTONE

o,.. cL

- 150

O0

10(]

0

0

0

O0

0

0

0

50

2

3

OXYGEN, (%)

Fig. 86. As for Fig. 84, except that the bed temperature was 800°C. Comparison of the S O 2 emissions during runs 20 and 9 (i.e. with and without limestone, respectively) shows a ~ 70% reduction in SO 2 emissions at an 0 2 (flue) concentration of 1.5%. 600,

• •

Wl/I-IOUTLIMESTONE WITH LIMESTC~E

6,0(

o. o_

2OO

~4t~

6

8

;o

~2

OXYGEN, ( % )

Fig. 87. As for Fig. 86, but for moderately high 0 2(flue) concentrations. Then a 60% reduction in SO 2 emissions was observed at an 0 2 (flue) concentration of 6%.

72

K. Findlay, S. D. Probert

soo[

4Sll

4O

I

0

o

ooo D

[] O

o

B

~ °o~ °

o

o

3SO

,~ 300

u~

250 O

0

O0

0 0

200 0

D WITHOUT LIMESTONE

150

o WITH LIMESTONE

100

OXYfEN, (%)

Fig. 88. As for Figs 86 and 87, under nominally similar conditions. The presence of limestone reduced the SO2 emission by some 43% (at an 0 2 (flue) concentration of 7%), and a bed temperature of 800GC. This (i) confirms that Burdett's shell-sulphation theory holds true for limestone LG8, at a bed temperature of 800°C, and (ii) suggests that the ultimate sulphation o f the limestone is diffusion-controlled, complete sulphation o f the limestone being prevented by the formation o f a CaSO4 shell, which blocks the limestone pores. Burdett proposed that an increased 02 (flue) concentration would serve to reduce the ultimate sulphation of the limestone, by causing a more rapid sulphation of the lime, to form an impermeable outer shell of CaSO4, before sulphation of the innermost part of the limestone particles could occur. At higher bed temperatures of 900 and 1000°C, however, when decompositions of the CaS and CaSO4 ensued 11, higher values of the Oz (flue) concentration led to a net increase in sulphur retention, giving a reduction in the SO2 emission of ~ 54% at a bed temperature o f 1000°C (see Fig. 89). Here, Burdett's theory no longer holds true, because CaSO4 is known not to be stable at high bed temperatures under reducing conditions

Pollutant emissions from fb combustors

73

400

350

[]

o

D

[]

30C

250

20C

o

o o

o

t~

°08°0 o

150

o

o

o Bo ° ~ ° 0 ' ~

100 0

WITHOUT LIMESTONE

o

WITH LIMESTONE

SO

I 4 OXYGEN.

I S

i 6

17

(%)

Fig. 89. As for Fig. 84, but for moderately high 0 2(flue) concentrations, a n d a bed t e m p e r a t u r e of 1000°C. The c o m p a r i s o n o f results, o b t a i n e d d u r i n g runs 18 a n d 11 (with a n d without limestone, respectively), shows that, at a n 0 2 (flue) c o n c e n t r a t i o n o f 4 % , a ~ 5 5 % reduction in SO 2 emissions occurred when limestone was added to the bed.

in the bed 11. However, where CaS is formed, its complete oxidation to CaSO4 in the oxidising conditions of the freeboard, is known to be favoured by employing high O 2 (flue) concentrations.18 Thus, under these conditions, the use of two-stage combustion is a positive advantage, leading to the formation of CaS and hence a more complete sulphation of the limestone would be achieved in the bed. The CaS is then oxidised to CaSO4 under moderate-to-high 02 concentrations in the freeboard. Use of lower 02 (flue) concentrations, as in run 13, led to little or no sulphur retention, because the CaS was then only partially oxidised to SO2. Thus, two-stage combustion, by providing low in-bed 02 concentrations for the sulphidation of the limestone, to form CaS (rather than the bulkier CaSO4 molecule), did result in a more complete sulphation of the lime in the bed. At a bed temperature of 800°C, two-stage combustion provided a 77% reduction in the SO 2 emissions by limestone (see Fig. 90), compared

K. Findlay, S. D. Probert

74

450F "x

/

x

~ ^~d,x -"~x x x '~x x

x

300

250 --×

It. ct.

200

-~-

WITHOUT LIMESTONE -

WITH LIMESTONE

150

100

50

OXYGEN,(%I

Fig. 90. Effect of limestone on reducing the SOz emissions under two-stage combustion conditions, at a bed temperature of 800'~C. This comparison of results, obtained during runs 21 and 10 (i.e. with and without limestone, respectively), shows that at an •2(flue ) concentration of 1% a ~ 76% reduction in SO2 emissions was achieved when limestone was added to the bed. with the ~ 7 1 % a c h i e v e d u n d e r oxidising c o n d i t i o n s a n d low values ( ~ 1%) o f the 0 2 (flue) c o n c e n t r a t i o n . T h i s w a s r e d u c e d to ~ 5 5 % w h e n using a n 0 2 (flue) c o n c e n t r a t i o n o f a r o u n d 4 % a n d w a s d o w n to ~ 4 6 % w h e n the c o n c e n t r a t i o n rose to a r o u n d 5 % (see Figs 91 a n d 92, respectively). It is c o n c l u d e d t h a t the use o f a higher O2 (flue) c o n c e n t r a t i o n led to a n i n c r e a s e d i n - b e d o x y g e n c o n c e n t r a t i o n s ( f r o m the recycled gas), w h i c h was n o t c o n d u c i v e to the f o r m a t i o n o f CaS. A rise in the s u l p h u r r e t e n t i o n b y the l i m e s t o n e w a s also o b s e r v e d w h e n using t w o - s t a g e c o m b u s t i o n at a higher bed t e m p e r a t u r e . T h i s a m o u n t e d to a n --, 8 % i m p r o v e m e n t in the s u l p h u r r e t e n t i o n at 900°C (see Figs 54(a) a n d 84) a n d ,,~ 3 % at 1000°C (see Figs 85 a n d 93). Similarly, a ,-~ 1 2 % r e d u c t i o n in

~S

m°

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,-'=

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K. Findlay, S. D. Probert

76

550

n

[]

50(; rl rl

-0.--.

/.5C

40C a. o_ 0

" 350

0

30(]

[]

WITHOUT LIMESTONE

o

WITH LIMESTONE

250

2oo

i

I

3I OXYGEN, (%)

Fig. 93.

As for Fig. 90, but for a bed temperature of 1000~C. This comparison of results

from runs 13 and 12 (i.e. with and without limestone, respectively)shows that, at an O2 (flue) concentration of 2%, a ~ 33% reduction in SO2 emissions ensued when limestone was added to the bed. the SO 2 emissions was observed under two-stage combustion conditions, compared with those under oxidising conditions, at a bed temperature of 800°C (see Fig. 67).

5 Selecting coal quality Comparisons of S O 2 emissions from four different qualities of coal, under similar operating conditions, are shown in Figs 94 and 95 and Tables 8 and 9. Reductions in the SO2 emission could be achieved simply by selecting a coal with a low sulphur-concentration (e.g. of around 1% or less), and high inherent concentrations of calcium and sodium oxides, e.g. Gedling coal (see Fig. 96). However, such coals are not always readily available, and those with a high inherent ash-content m a y prove to be difficult to handle. Additionally, the cost of transportation is usually taken into account when choosing the fuel. Undoubtedly, where a low-sulphur coal is available, burning this (rather than another coal) would be preferable to having to rely on any other means

TABLE $

Sulphur Retention of Various Coals, Without Lime Being Present in the Bed Run; coal type; condition

Average 02-in-flue concentration (%)

Average SOz emission (ppm)

Sulphur retention in bed (%)

24 Gedling Oxidising 24 Gedling Two-stage 25 Maryport Oxodising 25 Maryport Two-stage 26 Blanzy Oxodising 8 S. A. Duff Oxidising 8 S. A. Duff Two-stage

1"52

464

52'1

1.01

584

39.4

1.13

1 218

14-7

1"52

1 247

12"6

1-00

803

14-5

2-04

373

17.1

2-56

432

4.0

TABLE 9

Sulphur Retention of Various Types of Coal, Without Lime Being Present in the Bed Run

Coal type

Process and average O2-in-flue concentration

Average calculated sulphur retention in ash or limestone according to emission

Measured sulphur retention in ash (%)

Sulphide in bed ash

(%) 24

Gedling

Oxidising, 1"52% Two-stage, 1.01%

52"1) 39.4 45.7

2'66

25

Maryport

Oxidising, 1"13% Two-stage, 1"52%

14"7) 12"6 13"6

1"29

50% S-

26

Blanzy

Oxidising, 1-00%

14.5

4.91

38.9% S-

S.A. Duff

Oxidising, 2.04% Two-stage, 2.56%

17.1) 4.0 10.6

6.97

72.7% S-

8

0

K. Findlay, S. D. Probert

78 1400

{

1200

2033 -

723.20,(°/ol

100C

t~ 800

BLANZY

1

600

GEDLING / - S O ~ (ppm)=

~OC

o

o

~.88 - 15.98 02(%)

o

OoO / / S . A DUFF

S02 Ippm) : # + 4 3 2 - 2 8 9

021%)

200

12 0

i

i

~

6

OXYGEN. 1%)

Fig. 94.

Effect of coal quality upon SOz emissions, under oxidising conditions, at a bed temperature of 900°C.

for reducing the pollutant emission. However, if only high-sulphur coals can be obtained, careful operating practices should be adopted in order to ensure that the restrictions on pollutant emissions are met.

6 Employing recycled gas By recycling the exhaust gas back to the fb, the average residence time o f the SO2 gas within the bed can be increased, so extending the contact period between the SO2 gas and any SO2 sorbent, such as coal ash or limestone. Thus a reduction in the SO2 emission would result--see Fig. 97. During run 27 the recycled gas flow had been reduced to zero due to a blockage in the recycled-gas fan. Then there was an apparent increase in the

Pollutant emissions from fb combustors

79

1600

1600

1200

• MARYPORT 1000

o 5EDLING o S A DUFF

800

(mY')

600

l+O0

o o

E~I

*oo o

o o o o

o oo

200

OXYGEN, (%)

Fig. 95.

As for Fig. 94, except under two-stage combustion conditions.

SO 2 emission of 26%, compared with the SO 2 concentration, for similar values o f the bed temperature and 02 (flue) concentration but using recycled gas as during run 22. This increase would have been greater had it not been for the reduced fluidising velocity during run 27, as a result of there being no recycled gas flow through the fb. The increased SO 2 emission was a direct result of the reduced residence-time of the SO2 gases within the bed. Thus, the use of recycled gas led to a net decrease in SO 2 emissions of at least 20%. This was achieved despite the increased fluidising-velocity provided by the recycled gas, which would have led consequently to a reduced residence-time

80

K. Findlay, S. D. Probert

8o

[] 6C n

4c

COAL COMBUSTION OUALITY HODE

20

F,EDLIN6 OXIDISINO 6EOLIN6 TWO-STAOE RARYPORT OXIDISIN6 HARYPORT TWO STAGE BLANZY OXIDISING S.A.DUFF OXIOISIN6 S.A.DUFF TWO-'STA6E

. ~ . . . ~ ~ " •

0

ol 0.5

~

I 1.5

J2

% COMBUSTIBLE SULPHUR IN COAL

Fig. 96.

Approximate variation of the mean sulphur-retention effectiveness with the combustible sulphur content of the coal.

•

•

mmmLJ 30(

20(

02 IN RECYCLED GAS, (kg/hr)

Fig. 97. The variation of the S O 2 cmission with the amount of 02 within the recycled exhaust-gas, at a bed temperature of 800C, without limestone added to the bed.

Pollutant emissions from fb combustors

8!

for the SOz gas being in contact with the fluid bed particles, prior to the recycling of the gas. Nevertheless, an overall increase in contact time of the SO 2 gas with the sorbent particles was achieved by recycling the exhaust flue-gases to the bed.

Combustion efficiency The largest values of the combustion efficiency for the present tests occurred under conditions of high bed temperature and/or high O2 (flue) concentration (see Figs 98-100). It was confirmed by results given in Table 10, which showed that combustion efficiencies of over 90% were achieved only during: (i) run 23, two-stage (at 900°C); (ii) run 18 (at 1000°C), oxidising; and (iii) run 1, oxidising, with recycled gas and (iv) run 27, oxidising, without recycled gas (at 800cC), for a high 0 2 (flue) concentration. Thus achieving maximum combustion efficiencies required e i t h e r a high 02 (flue) concentration (of ~ 6% or above), preferably using oxidising conditions in the bed, or a high bed temperature. High in-bed 02 concentrations were required in order to provide complete oxidation of the carbon to carbon dioxide, within the well-mixed, high heat-transfer zone of the fb, thus confirming the findings of McLaren and Williamsfl t Therefore, the use of two-stage combustion invariably led to a decrease in the combustion efficiency, except under conditions of very low in-bed O z concentration, when the high temperatures in the secondary combustion-zone led to an increase in the rate of 1 O0

1000°C

>: uu t_J

90

U.l o =

I-BED

80

700

TEMPERA;URE (*C)

i

2

i

<> 800 • BOO O 900 • 900 "~ 1000 • = 1000

COMBUSTION CONOfflON OXIOISING TWO.STAGE oxInlSING TWO-STAGE OXIDISING TWO-STAGE

6 OXYGEN. ( % )

Fig. 98. Variation of the combustion efficiency with 02 (flue) concentration and bed temperature, under conditions of both oxidising and two-stage combustion, with limestone added to the bed.

82

K. Findlay, S. D. Probert 1 O0

90

80 2: p-.

BED TEMPERATURE I'C)

70

O • O IO Z~ • i

60 0

800°C 800°C 900QC 900"C 1000+C 1000"C

i

2

COMBUSTION

CONDITION 0XIDISING TWO-STAGE CIXIDISING TWO-STAGE OXIDISING TWO-STAGE

I

4

_i

6

8

OXYGEN, (%)

Fig. 99.

As for Fig. 98, but without limestone in the bed. The presence of limestone increases the combustion efficiency slightly for a bed temperature of 800C.

100

90

•

O

.

•

I000°C , , ~ . , ' 8 0 0 ° C

z

u_ u.a 80 u_

•

IED EMPERA

-

rURE('C)

I.--

o L.a

70 O

60

L

j

2

~

Q 800°£ • 800°C O 900°C • 900°C /x 1000oC • 1000°C ~ 800°C • 800"C Q 900"C • 900°C ~7 ]000"C • '~000"C i

6

EOMBUSTION CONDITION OXI01SING -I TWO-STAGE OXIDISiNG WITHOUT TWO-STAGE LIMESTONE OXll3iSING TWO-STAGE OXIDISING ] TWO-STAGE OXIDISIN6 WlIH TWO-STAGE LIMESTONE OXIOISING TW0-STAfiE L

8

OXYGEN. (°/o]

Fig. 100.

The effect of various operating variables upon the combustion efficiency when burning SA Duff coal.

83

Pollutant emissions from j b combustors TABLE 10 Combustion Efficiency

Bed temperature

2% 0 2 in flue

No lime With lime

4% 02 in flue

No lime

800 °C

900 +C

1000" C

Oxidising Two-stage

Oxidising Two-stage

Oxidising Two-stage

Run 9 85'3% Run 20 86-2%

Run 10 72.3% Run 21 84.5%

Run 2 64.9% Run 15 88'8%

Run 4 82"3%

Run 7 77.8% Run 16 85"5%

Run 3 76-6%

With lime 6 % O 2in flue

No lime With lime

10% 02 in No lime flue

Runl 91-6% Run 22 85.6%

Run 8 80.9% Run 23 93-2%

Run 5 85.1% Run 17 86.2%

Run 12 82-3% Run 13 85"3%

Run 18 91.4%

Run 11 82.8% Run 19 88'2%

Run 6 86"8% Run 14 88"6%

Run 27 94.6%

When using two-stage operating conditions without limestone present, at a bed temperature of 900 C, the combustion efficiencies when burning different coal qualities were: S. A. Duff, 80.9%; Gelding, 94"2%; Maryport, 97.4%; and Blanzy, 90"9°.

oxidation o f the carbon monoxide and elutriated carbon fines to CO 2, despite the lower concentration of 0 2 present in the bed. U n d e r such conditions of elevated freeboard-temperature, the combustion efficiency reached a m a x i m u m value of 93-2% during run 23, with limestone added to the bed, for a bed temperature of 900°C, with recycled gas. This compared with a combustion efficiency of only 88"8% under oxidising conditions, during run 15. A similar a n o m a l y occurred, without limestone addition to the bed, at the same bed temperature. For instance, the combustion efficiency of 80"9% achieved during run 8 (i.e. for two-stage combustion) greatly exceeded that obtained during run 2 (i.e. for an oxidising environment) at 64-9%. Also, during run 14, the two-stage, combustion efficiency was slightly higher at 88-6% than that during run 22, under oxidising conditions (i.e. 86"5%). However, in all other cases, the use of two-stage combustion led to a decrease in the combustion efficiency. This ranged from 12.4% under low values ( ~ 2%) of O2 (flue) concentration at a bed temperature of 800°C, to (i) only

84

K. Findlay, S. D. Probert

4"5% at moderate and high values o f O 2 (flue) concentration at the same bed temperature, without limestone, and (ii) 0"9% under low O2(flue ) concentrations ( --~2%) at a bed temperature of 1000°C. All other tested sets of conditions gave reductions in combustion efficiency of between 0"9 and 3'2% under two-stage conditions, with or without lime present in the bed. The reduction in combustion efficiency, under two-stage combustion conditions, was greater at the lower bed temperatures (i.e. when high temperatures did not occur in the oxygen-rich, freeboard zone), especially under low O2(flue) concentrations (e.g.=2%). Then low in-bed 02 concentrations would have led to only partial combustion occurring in the bed itself. At low bed temperatures, the rate of oxidation of the partially oxidised CO to CO 2 would have been much slower than at the higher bed temperatures, and the combustion of unburnt carbon fines in the freeboard would also be extremely slow, so leading to large combustible losses. The presence of limestone was found to be an effective combustion improver, especially under two-stage combustion conditions, and particularly at the lower bed temperature of 800°C. The increase in combustion efficiency achieved by adding limestone varied from approximately 24% under oxidising conditions at a bed temperature of 900°C, to 11-6% under two-stage conditions at a bed temperature of 800°C, to only 1.1% under oxidising conditions, at a bed temperature of 1000°C. The observed ability of limestone to act as a combustion improver thus confirms the conclusions of Jonke et aL lS The increase in combustion efficiency for O2(flue) concentrations in excess of 5%, is clearly shown in Figs 98-100 and summarised by plotting all

o~ z w m

•

•

•

5 ~ 8 76

OXYGEN IN FLUE,(%)

Fig. I01.

Variation of the combustion efficiencywith 02 (flue)concentration using different coal-qualities, over the full range of operating conditions.

Pollutant emissionsfrom fb combustors

85

the combustion results on a single graph, see Fig. 101. Although internally consistent, the present results contradicted those of Haque eta]., 22 who found a decrease in combustion efficiency when the excess-air levels exceeded 5%. However, the present results did show a reduction in combustion efficiency at an O2(flue) concentration o f around 3%, which could have been attributed to the effect of recycled gas: the use o f recycled gas and hence increased fluidising velocity led to a marked increase in the rate o f combustible loss from the bed. When comparing the results of run 22 (with recycled gas) with those o f run 27 (without recycled gas), both at a bed temperature o f 800°C and an O2(flue) concentration of 8%, the use of recycled gas produced a reduction in combustion efficiency o f 9"9%. In run 27, i.e. without recycled gas, the highest combustion efficiency ( = 94-6%) achieved in all the present tests occurred. As the Oz(flue) concentration was increased to above 1%, more combustion occurred within the fluid bed, so leading to a greater heat release and hence a larger recycled-gas requirement. This in turn led to an increased fluidising-velocity and a reduced combustion efficiency. However, once stoichiometric conditions were reached in the bed, apparently at an 02 (flue) c o n c e n t r a t i o n o f ~ 3 - 5 % , the m a x i m u m rate o f heat release and consequently m a x i m u m recycled gas flow would be achieved. Any increase in the 0 2 (flue) concentration above this level would act merely as a heat diluent, so causing a reduction in the recycled-gas flow. Because the a i r - being at a lower t e m p e r a t u r e - - i s a far more effective heat-diluent than the recycled gas, its use resulted in a net reduction o f the fluidising velocity, so increasing the combustion efficiency achieved over the range of 3 to 8% O2 (flue) concentration. Thus, by altering the fluidising velocity, the use of recycled gas influenced significantly the combustion efficiency. How to increase the combustion efficiency High efficiencies were achieved under the following operating conditions:1 High 0 2 (.flue) concentration The experimental observations showed that the combustion efficiency reached a minimum at moderate O z (flue) concentrations ( ~ 4%) (see Fig. 101 ) due to the effect o f the recycled gas flow in raising the fluidising velocity. However one of the highest values of combustion efficiency occurred during run 18, under moderate O z (flue) concentrations, at a bed temperature of 1000°C. With a combustion efficiency of 91.4%, this gave an increase in efficiency of over 5%, compared with when a lower Oz (flue) concentration of below 2% was employed, as in run 17. Similarly, during run 1, a combustion efficiency o f 91-3% was attained under a high O2 (flue) concentration o f ,-~6%, compared with 86.2% under similar conditions, but at low

86

K. Findlay, S. D. Probert

values of 02 (flue) concentration as in run 20. Such increases in combustion efficiency with an increasing O 2 (flue) concentration coincide with a significant decrease in the combustible loss, as observed previously, 2t -23 and decrease in particle burn-out time, as reported by Be6r. 24 2 Use o f two-stage combustion By producing only partial combustion of the coal within the bed, two-stage combustion can be used to:

(a) reduce the fluidising velocity needed; (b) increase the freeboard temperature; and (c) reduce the elutriation of unburnt carbon fines. However, the advantages of two-stage combustion can only be realised either for a high bed-temperature (i.e. > 900~C), which favours the secondary combustion of CO and volatiles in the freeboard and any elutriated carbon fines, as in run 23, or when the high O2 (flue) concentration utilised requires that the fluidising velocity is high under o×idising conditions, as in run 14. 3 By adjusting the bed's temperature As shown in Figs 98-100, the combustion efficiency increased with the bed's temperature over the range of values employed during the tests, i.e. 800-1000°C. An average increase in combustion efficiency of 5"5% per 100°C was observed during the present tests, at an Oz (flue) concentration of 2%, compared with an average rise of 7% per 100°C as observed by Rogers and Minchener, 2s and 3% per 100~'C as observed by Gulyurtlu and Cabrita. 23 The value of approximately 5-5% per 100°C was expected when burning SA Duff, because this coal has characteristics midway between those of the highly volatile semi-butiminous coal (which exhibited an increase of 2"3% per 100'~C rise in bed temperature) and anthracite (which rose by 8"3% per 100°C, as determined by Mei et al.5). 4 By the addition ojlimestone to the bed Because it acted as a combustion improver, limestone in the present tests served to increase the combustion efficiency by 6"9% on average. The greatest increase ( = 2 4 % ) in combustion efficiency by the presence of limestone occurred under oxidising conditions and low values of 02 (flue) concentration. However, substantial increases in combustion efficiency of around 12% were also obtained under two-stage combustion conditions, at both 800 and 900°C. Thus limestone LG8 acted as a catalyst for the combustion reaction. At a bed temperature of 1000°C, the effect of limestone was less significant giving a 3% rise in combustion efficiency under twostage combustion conditions and a 1"1% rise under oxidising conditions.

Pollutant emissions from fb combustors

87

5 By selecting a suitable quality o f coal A fall in combustion efficiency (by almost 10%) occurred when the volatile content of the coal was below 28%, as in the case of SA Duff. Thus the use of high-volatility coals, such as Gedling, would help increase the combustion efficiency. However, the greatest combustion efficiency was achieved using Maryport coal, which had a lower volatile content than Gedling, and a higher fixed-carbon content: for it a combustion efficiency of 97"4% occurred, i.e. similar in magnitude to that found by others. 2~ The only other noticeable features of the Maryport coal were the high sulphur and Fe20 3 contents in its ash. Thus high values of the combustion efficiency could be achieved by selecting a coal, with high concentrations of both fixed-carbon and volatiles (and a low ash-content), for combustion in the fb. 6 Limited use of recycled gas By increasing the fluidising velocity, the recycled gas served to reduce the combustion efficiency under conditions of maximum recycled gas flow, so leading to a 5% decrease in the combustion efficiency at a bed temperature of 800°C and a high 0 2 (flue) concentration. The largest recycled-gas flow occurred with the greatest rate of heat release in the bed, under stoichiometric conditions, and this in turn corresponded with the maximum fluidising velocity for the bed. In the present tests, this corresponded to ~ 4 % 02, for which the combustion efficiency was at a minimum, as shown in Fig. 101. Thus the largest combustion efficiencies occurred under conditions of low recycled gas-flow, for example, under two-stage combustion conditions, using a primary-to-secondary air-ratio of 30/70.

CONCLUSIONS The main objective of this investigation was to identify the conditions for the reduction of NO Xand SO2 emissions, whilst also seeking to maximise the combustion efficiency, which is an economic requirement for commercial fbcs. One of the main problems associated with the simultaneous reduction of NOx and SO 2 emissions is that the conditions required to reduce one, e.g. the NO~, at low 02 concentrations, can lead to an increase in the other, i.e. SO2, because higher 02 concentrations are required in order to form CaSO4. This is illustrated in Fig. 102, where the use of sub-stoichiometric quantities of air is shown to lead to reduced NO (and increased NH3) concentrations, but only at the cost of reduced sulphur retention.1 v The sulphur-retention values for the present tests show an O2 concentration of ,~ 4% to be required before

K. Findlay, S. D. Probert

88

1.0-FULLY RETAINED

FULLY [ONVERTED-1.0

<> FOR Co/S=7 o FOR [a/S=5

0 O~...----~ < > < > ~ o

N

~

o

N

N

~

,~ NH3; FOR Ea/S =5

~\

0.5

J

~ Eo/S = 5

.

'

-

110

'

-

.

.

.

A-A. . . . . .

A.~I.5_A__

0

ST01CHIOMETRIC RATIO OF AIR TO FUEL

Fig. 102.

Variations of the sulphur retention and conversion of fuel N to NO and NH 3 with the stoichiometric ratio of air.

a large sulphur capture can ensue (see Fig. 103). Thus the use of two-stage combustion, whilst providing an effective means for reducing NO x emissions, can lead to greatly reduced sulphur retentions within the bed, when using primary-to-secondary air ratios below 0.7 unless relatively high levels of O z (flue) concentration (~. 4%) ensue. Though CaS would be formed at lower values of the stoichiometric air ratio (i.e. below 0"7), the CaS would be oxidised to either SO2 or calcium sulphate in the freeboard (depending on the bed's temperature and O2 (flue) concentration, following the injection of secondary air). The coal ash alone was not completely oxidised to CaSO4, even at the low bed-temperature of 800°C. It follows that the use of two-stage combustion alone (without limestone present) would not allow the objectives identified earlier to be satisfied. The simultaneous removal of SO2 and NO x could be achieved by using active coke, at the flue-gas temperature, with the addition o f N H 3. However, this requires the use of a m o v i n g bed o f active coke, which adds to the capital cost and maintenance of the plant and will probably be employed only when environmental restrictions demand it, in conjunction with any necessary modifications to the burner itself.

Pollutant emissions from fb combustors

mmm

89

•

= 60

J OXYGEN,(%}

Fig. 103.

A compilation of the average sulphur-retention values when burning SA Dull

under a range of differentoperating conditions, e.g. bed temperature. These data, taken from test runs 1-23 and 27, show that an oxygen-in-the-flueconcentration of ~ 4% was required so that the largest sulphur-capture (up to 81%)ensued. Average values of the NO x and SO 2 emissions taken during runs 1-27 inclusive are shown in Tables 1 and 3, respectively: average values of the combustion efficiency achieved are given in Table 10. When using recycled gas, the highest-measured combustion-efficiencies (i.e. 91"4 and 93"2%; see Table 10) were achieved respectively under (i) oxidising conditions, with lime, at a bed temperature of 1000°C; and (ii) twostage combustion conditions, with lime, at a bed temperature of 900°C. However, under these conditions, both the SO2 and NO X emissions were approximately double those when the bed temperature was 800°C. However, a slightly reduced combustion efficiency of 85%, as achieved for an 800°C bed temperature, under two-stage combustion conditions, with lime, would be unavoidable in view of the environmental limitations. Because the addition o f limestone normally tended to increase the combustion efficiency by at least 1-5%, and by a m a x i m u m o f 11"5% under two-stage combustion conditions, at a bed temperature o f 800°C, limestone would appear to act as

90

K. Findlay, S. D. Probert

a combustion improver under most conditions of operation. Its ability to increase the combustion efficiency, whilst also acting to reduce both the NO x and SO 2 emissions, means that limestone addition is desirable in order to achieve high fb performances coupled with reduced pollutant emissions. Table 1 shows a reduced NOx emission of 47 ppm under conditions of high bed temperature ( = 1000°C) and two-stage combustion with limestone added to the bed, using a moderately high 0 2 (flue) concentration of around 4%. Similarly low emissions of NO x, of 48 and 49 ppm, were also achieved at a bed temperature of 80qC, with lime, under both oxidising and two-stage combustion conditions, and ~ 2% 02 (flue) concentrations. Interestingly, these conditions for reduced NOx emissions also coincided with those for reduced SO 2 emissions. As Table 5 shows, the lowest values of the latter, at 90 and 85ppm, again occurred at a bed temperature of 800°C, under oxidising and two-stage combustion conditions, respectively, using a low 0 2 (flue) concentration, and also a low value, of 117 ppm, was achieved at a high bed temperature (= 1000~C) for two-stage combustion using moderately high O2 (flue) concentrations. Thus it is apparent that, where limestone has been added to the fb, optimal conditions for the sulphation of lime do lead to optimal conditions for the reduction of NO x emissions, and that CaSO4 acts as the catalyst for the NOx-reducing reactions. When using the highest bed temperature employed, i.e. 1000°C, coupled with two-stage combustion, low in-bed O 2 concentrations lead to little or no formation of CaSO4 in the bed, the CaSO~ decomposing at these high bedtemperatures and reducing conditions, whereas the CaS and/or CaO are stable. However, the high bed-temperature would result in a reduced NOx emission from the bed, and increased rates of reaction of the fuel-nitrogen with char and ammonia, the CaO acting as a catalyst. Further, this reaction would occur whilst still in the oxygen-deficient conditions of the two-stage bed, leading to the formation of inert nitrogen (rather than NO 0 prior to the secondary-air injection into the freeboard. Thus the use of two-stage combustion and high bed temperatures did enhance the NO~-reducing reactions in the bed, so leading to less NO~ emissions. However, the addition of moderately high quantities of secondary air to the freeboard, would provide sufficient oxygen to oxidise completely the CaS, formed in the substoichiometric fb, to CaSO4, so resulting in (i) maximum sulphur retention, and (ii) lower values of the NO~ emissions, with the CaSO4 possibly acting as a catalyst for even more NO~-reducing reactions. CaO was a catalyst for one of the above-mentioned reactions, as confirmed by the NO~ emissions being reduced under two-stage combustion conditions when the CaO was known to be stable, at the higher bedtemperatures (whereas CaS and C a S O 4 w e r e unstable). As Fig. 12 shows, during run 13, at a bed temperature of 1000°C, the NO~ emissions actually

Pollutant emissions from fb combustors

91

increased with 0 2 (flue) concentration at the lowest primary-to-secondary air ratio employed (=30:70), because CaS would have been formed in preference to CaSO4. At these low 02 (flue) concentrations ( -~ 2%), all of the CaS was oxidised partially to SO2 and CaO: none was completely oxidised to CaSO 4. Thus, under these conditions, the sulphur retention was zero (see Table 7), so indicating that no CaSO4 was present, although the NOx emissions were still much lower (by --~30%) than prevailed using the same conditions, but without limestone present. Thus, under these conditions, CaO must have been acting as the catalyst for at least one of the NOxreducing reactions. When using the lower bed temperature (of 800°C), CaO was not stable (see the equilibrium diagram presented as Fig. 63) in the bed itself. The formation of CaS, under the reducing conditions predominating in the twostage fluid bed, led to slightly greater sulphur retentions by the lime, compared with when oxidising conditions applied. This is shown by the slightly lower SO2 emission ( - 8 5 p p m ) under two-stage combustion conditions, compared with 90 p p m under oxidising conditions (see Table 5); the increased size of the CaSO4 molecule leading to premature blockage of the limestone pores prior to sulphation being completed (according to Burdett's shell-sulphation theory). Nevertheless, sulphation of the limestone to form CaSO4 was favoured at these lower bed temperatures ( ~ 800°C), when decomposition of the CaSO4 under reducing conditions, did not occur. The results also indicated that complete oxidation of CaS to CaSO4 occurred at a bed temperature of 800°C when using LG8 limestone in the bed. The rate of formation of NOx was reduced at these lower bed temperatures, with or without the addition of limestone to the bed (see Figs 26 and 46, respectively). However, the NO x emissions at the high bed temperature of 1000°C--without limestone present, so favouring the NOx reduction by char--were lower than those occurring when using a bed temperature of 900°C. Presumably, at these lower bed-temperatures, the CaSO4 and/or CaO would exert greater catalytic effects as the reaction between the NOx and char is very slow at such temperatures, 27 the maximum overall concentration of CaSO4 occurring (according to the SO2 emissions listed in Table 3), under two-stage combustion conditions, at an 0 2 (flue) concentration of 2% with lime present in the bed. Thus, both CaO and CaSO4 act as catalysts for the NOx-reducing reactions occurring in the fb and freeboard. As a result, significant reductions in both SO2 and NO x emissions could be achieved simultaneously, in line with the objectives of this investigation. Further, the combustion efficiency was found to be greatest, for the present tests, at high bed temperatures ( ~ 1000°C), with high 02 (flue) concentrations. Therefore,

92

K. Findlay, S. D. Probert

when burning SA Duff, in order to achieve optimal conditions for reducing both SO2 and NO x emissions, and increased combustion-efficiencies, the higher bed temperature is preferred with limestone present under two-stage combustion conditions and moderately-high (,-~4%) O2 (flue) concentrations. When the initial ash-fusion temperature of the coal is around or below 1000°C, as in the case of Maryport smalls, the lower bed temperature (,-~800°C) should be selected, with lime present in the bed. Significant reductions in both the SO2 and NO x emissions were achieved at the cost of a slightly reduced combustion efficiency, i.e. 85% compared with 88% at the higher bed temperature. When limestone was n o t added to the fb, the use of two-stage combustion at a high bed temperature still led to reductions ( ~ 3 5 % ) in the NO x emissions but not to the extent ( ~ 71%) as observed when limestone was added. This implies that, although the char reduction of NO x was still occurring, the absence of added lime means that the limestone catalysed NOx-reducing reaction no longer ensued in the presence of coal-ash alone. Here the greatest NO Xreduction occurred at a bed temperature of 800°C and an O2 (flue) concentration of 4%, for two-stage combustion, giving NO~ emissions of 150 ppm without lime present in the bed. Again, the maximum reduction in SO2 emission to 307 ppm also occurred at this bed temperature, but, in this case, at slightly lower values of the O z(flue) concentration (,-~ 2%), under oxidising conditions (see Table 3). The present results compare favourably with those obtained by others, z8 where NO x reductions of 20-40% have been achieved when using two-stage combustion, without the presence of lime. However, when the results, concerning the effect of flue-gas recirculation (i.e. the use of recycled exhaust gas for bed attemperation), are compared with the 15% achieved by others z6 the present 33% reduction in NO x emissions appears encouraging.

ACKNOWLEDGEMENTS The authors are grateful to the Science and Engineering Research Council and to Fina (UK) for partial sponsorship of this project.

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