Limiting NOx and SO2 emissions from an industrial-size fluidised-bed combustor

Applied Energy 45 (1993) 1-99

Limiting N O x and S O 2 Emissions from an Industrial-Size Fluidised-Bed C o m b u s t o r

K. Findlay* Fina plc, 1 Ashley Avenue, Epsom, Surrey, UK, KT18 5AD

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

,4 BS TRA C T For the reported series of tests, the largest reductions in NO, enlissions were achieved (together with maximunl combustion ~ciencies) hy using a prinlary-to-secondarv-air ratio of 30/70 Jor the Huidised bed (FB). Under such an operathlg condition, high freeboard-tenlperatures coukt be attained. e.g. up to 200~ C above that qflthe bed, so promoting the combustion q/'unhurnt carbon-lines in the freeboard zone, i.e. secondary conlbustion ensued. This tended to counteract the effect O['the reduced rate q['carbon conlbustion in the hed, i.e. as a result of the increased rate of oxidation occurring in the [?eehoard. Reductions in both the NO~ and SO, emissions could be achieved by using such a two-stage combustion process, without q[:[~,cting adversely the o'~erall combustion efficienc3', l f a primao,-to-secondary-air ratio q[ 70/30 (rather than 30/70) was employed, the rate of SO: emissions ,(ell, but the temperature uplift in the freeboard zone was less pronounced and a reduction in combustion ~ffl"ciencT of 2% was observed. The rates of emissions qf NOx and SO, couM also be reduced by the adoption of wise choices .[br the design and operation procedure qf the [tuidised-hed combustor (FBC). For example, a 45% decrease in the rate 0/ NO x emissions was achieved in one test mere O"by halving the bed's depth. The * Present address: W. S. Atkins (Consultants) Ltd, Woodcote Grove, Epsom, Surrey, UK, KTI8 5BW. 1

Applied Energy 0306-2619/93/$06.00 C 1993 Elsevier Science Publishers Ltd, England. Printed in Great Britain

2

K. Findlay, S. D. Probert lower the available oxygen-concentration (including that provided by re~3'cledflue-gases) within the bed, the smaller was this decrease. On the other hand, a doubling of the bed's depth led to a reduction in the SO 2 emission of approximately 4%, provided that the in-bed oxygen concentration was maintained sufficiently high, so as to promote sulphation of the ash and an)' limestone present in the bed. The location ~( the secondary-air nozzle (i.e. in the.freeboard zone) also had sign(fl'cant influences on the emission rates of the NO x and SO,.. A 13% reduction in the rate of NOx emissions was achieved by increasing the height (above the sparge pipes) of the secondao,-air's nozzle from 1"5 to 2"6 m. The larger this height, the greater the opportuni O, for char-NOx-reducing reactions ([or which high-temperature conditions are preferred within the jreeboard) to ensue, prior to the secondao'-air injection. However, the rates of SO, emission fell by ~ 10% when the height of the secondary-air's nozzle was reduced from 2"6 to 1.5 m above the sparge pipes. This was due presumab O' to the increased residence times of the ash and limestone sorbents (for facilitating sulphation) in the freeboard, under the oxidising condition ([bllowing secondao,-air injection), which favours the complete sulphation of the sulphidated ash~lime to CaSO~. In the present O' reported series of tests, the rate of NO~ emissions could be reduced by up to 83% mere O, hy adjusting the bed's depth and the secondao'air's nozzle-height, although these alterations led simultaneous O' to a 14% increase in the rate of $02 emissions, and vice versa. Thus, the emissions of NOx and SO x could be controlled by the use of an appropriate design of FBC. The choice made, in practice, usually depends on which of these emission rates is critical with respect to compO, ing with the environmental-pollution directives. Great care must be taken when comparing emission rates.from the FBs, either o f dffferent sizes or incorporating alternative methods of air distribution, in order to take account of scaling effects. It is unwise, for instance, to use observations .[rom a laboratoo'-size FBC to predict the quantitative behaviour of an industrial-size (i.e. large) FBC unit. This arises because of the variations in the oxygen concentration, resulting .from the usually non-homogeneous, relatively less eff~ctive.fluidisation achieved in the large bed; the rates of NO~ emissions from the large FBC tend to be smaller (as a result o f the existence of pockets of relative O: low concentration oxygen in the bed) and simultaneously the rates o/'SO: emission are invariably higher. In the presentO' reported tests, under otherwise nominally similar conditions, an approximate halving of the rate of NO~ emissions resulted when using the larger FBC (rather than the small one) together with a doubling of the SO,emission rates, even though the average percentages of o.Evgen present in the .flue gases remained identical Jor both the small and large FBC units. The recommended bed-depth depends upon what emission one seeks to reduce. However, where feasible,for an industrial-size combustor, it is wise to employ a shallow bed ( ~ 3 4 0 m m depth, when static) in order to incur relatively low operating costs.

L i m i t i n g N O x a n d S O 2 emissions./i'om an F B c o m h u s t o r

3

GLOSSARY

Bed depth: the depth of particulate material within a static bed, measured as the vertical distance between the bed's upper surface when static (i.e. prior to the fluidisation of the material of the bed) and the top of the sparge-pipe airdistributors. Freeboank that part of the combustion chamber which lies immediately above the bed's surface when fluidised; within this region, secomtarv combustion ensues. Nozzle height: the vertical distance (above the top of the horizontal spargepipe air-distributors) of the secondary-air-injection nozzles, which are in the freeboard. Oxygen concentration: unless stated otherwise, this refers in the present report to the O2 concentration in the flue. Percentage closure: this indicates the ratio of the output to input values. Primary air: this air is introduced into the fluidised bed through the spargepipe air-distributors. Besides stimulating fluidisation, it provides the oxygen required for the first stage of combustion, which occurs within the fluidised bed. Runs and trials: these refer to tests with the small and large rigs, respectively. Secondao' air: the air which is introduced via the nozzle(s) into the secondary combustion-zone, i.e. above the fluidising bed, in order to try to complete the combustion of the fuel. Section: in this report, the word 'section' refers to a time-interval. Sparge-pipe air-distributor(s): a pipe (or series of pipes), with holes drilled in its[their) side(s), through which air is blown into the bed of sand, in order to achieve fluidisation of these particles as well as primary combustion. NOTATION cfm CO FB FBC FG H HGG LP

Cubic feet per minute Carbon monoxide Fluidised bed Fluidised-bed combustor Flue-gas Bed depth (mm) Hot-gas generator Low pressure

4

NB Nm 3 NO~ 02 SA SO 2 SO 3

SOx

TF WG

K. Findlay, S. D. Probert

Nominal bore Normal cubic metre of gas, i.e. at 15°C and 1 atmosphere pressure Nitrogen oxides Oxygen South African Sulphur dioxide Sulphur trioxide Sulphur oxides Freeboard temperature (°C) Water gauge

1 THE POLLUTION PROBLEM The already allegedly excessive--and increasing--global-greenhouse effect, as well as the confirmed acid-rain deposition and other adverse environmental impacts, resulting from our 'effluent' society, make it increasingly urgent that each of us accepts responsibility for reducing pollution emissions, e.g. of CO, CO 2, NO x, N 2 0 , S O x and unburnt carbonparticulates (i.e. soot), into the atmosphere. In order that the European Community (EC) environmental directives can be obeyed, those pollutants which arise as a result of burning low-grade fuels (such as coal, refuse and dry sewage) usually need to be reduced. Further, these improvements should be achieved at minimal additional financial costs to the operators. The rates of emissions of CO and unburnt solids are minimised using equipment of optimal thermal-design under conditions appropriate to obtaining maximum combustion-efficiency: the energy-savings can provide financial incentives to induce one to achieve this optimisation. However, the minimisation of SO2 and NO x emissions does not necessarily guarantee obtaining energy-savings, and can result in considerable increases in costs. One of the major problems in reducing overall pollution-emissions is that, whereas the SO 2 emissions in general tend to decrease as the O2-in-the-flue concentration increases, the NOx and SO3 emissions actually rise with the inbed 02 concentration. In other words, the conditions required to reduce one emission can lead to a net increase in another, particularly when an oxygenated exhaust-gas is recycled in order to obtain bed attemperation.

2 E N V I R O N M E N T A L TESTS The large test-rig, being 1.2 m x "1.2m, had a fluid-bed horizontal free-surface area approximately 64 times that of the laboratory-size FBC test-rig

Limiting NO.~ and S O 2 emissions .[?om an FB combustor

5

employed in a previous set of experiments. 1 The large-size FBC is representative of those operating in practice as industrial units. One of the main aims of this project was to identify the differences and similarities of behaviour when comparing the NO X and SO 2 emissions from these two vastly different sizes of FBCs using nominally similar operating conditions. The basic question was 'are the results from operating the small test-rig representative of those obtained with the large test-rig?'. If so, this could save significantly on experimental costs for any proposed test, because the small rig could then be used rather than the large rig. However, should qualitative trend or quantitative discrepancies arise between the two sets of results, then those from the larger test-rig would probably be accepted as design data because they represent more closely what is likely to be encountered in industrial practice. Using the large experimental-rig also enabled a better understanding to be obtained of two-stage combustion within the FBC: the resulting pollutant emissions; and the effects of altering the magnitudes of operating parameters, such as bed depth, location of the secondary-air injection and the primary-to-secondary-air ratio. In addition, two types of limestone-either the coarse LG8 or the finer L G l l - - w e r e added to the bed, during different experiments, in order to determine their individual effects on the NOx and SO2 emissions. The deductions from this series of tests should enable more-accurate predictions of the emissions from industrial FBs, when operating under various known conditions to be obtained. Thus, the conclusions from this investigation should help designers reduce the emissions from, as well as increase the performances of, FBCs. It is hoped thereby that the increasingly stringent environmental-impact restrictions, concerning permitted levels of pollutant emissions, will be satisfied more easily (at reduced costs to the operators).

3 THE L A R G E TEST-RIG (see Fig. 1) A N D ITS OPERATION Each FBC tested consisted of a refractory-lined combustor fitted with a deck of sparge pipes (for distributing the fluidising air). The large rig also incorporated a flue-gas cooler, a waste-heat boiler and an economiser, each of which acted as a heat exchanger (thereby cooling the exhaust gasesl. In order to improve the energy efficiency of the large unit, rather than use a separate steam-generator, some of the steam from the waste-heat boiler was used to provide steam tracing for the cyclone and flue-gas pipework. This helped prevent condensation of the steam (which was present within the fluegas in the cyclone) on the dust particles; otherwise blockages, as well as corrosion of the cyclone, tended to occur.

6

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Unlike the small FBC test-rig, the large unit had a bed extraction-screw fitted; this enabled bed samples to be taken during each combustion trial. For the small rig, in order to obtain worthwhile results, the operating conditions had to be maintained relatively invariant throughout each test-run. However, in the case of the large unit, the experimental conditions could be changed significantly while the bed was operating, so reducing the number of 'startups' required in order to complete a variety of experiments with different conditions being able to be imposed during a single run. The large test-rig was equivalent to a typical full-size industrial unit, but certain modifications were made in order to increase its experimental flexibility. These enabled more detailed analyses of the two-stage combustion-process occurring within the FBC to be achieved, particularly with respect to the emissions of SO2 and NO x. 3.1 The fluidised-bed combustor unit The large FB, with a horizontal surface area of 1"44m z, had a thermal output of up to 4 MW, depending on the coal feed-rate. The FB containment chamber (see Fig. 2) consisted of a box, having a 320-mm-thick refractoryUREEBOARO THERMOCOUPLE / / //~1 / // / 1 Ca FLUE-GAS~ / / / It/ / / / A II.,,THERMOCOUPLE

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K. Findlay, S. D. Probert

brick lining, with an outlet at the top of one side and ~ 4 m of freeboard above the fluidising bed. The refractory lining was backed by a 50-mm rockwool blanket and a stainless-steel outer shell. The sparge-pipe air-distributor deck (see Figs 2 and 3) was situated approximately 300 mm above the base of the combustor and consisted of eight sparge pipes, each of 76-2 mm nominal bore. Each sparge pipe was mounted separately, by means of two-hole clamp flanges, on to 102-ramdiameter mounting spigots, through which the sparge pipes were fed. The mounting spigots were welded into a stainless-steel and refractory mounting box, which was 'sandwich' flanged to the existing frame of the vessel. The rectangular plug section, so formed, was designed to be flush with the refractory face of the FB vessel containing the FB, but could be removed easily, so giving freedom to modify the distribution system should that have proved to be necessary. Four of the sparge pipes were fabricated from Incoloy 800G Sch. 10, the sparge-pipe wall being 3mm thick. The other four sparge pipes were constructed of stainless-steel type 310 Sch. 40, with 5.55-ram-thick walls. The SPARfiE RINGSUPPORT THERM( APPRO> SPARGE +

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Limiting N O x and SO 2 emissions JJ'om an FB combustor

9

two different types of sparge pipe were positioned alternately so as to be parallel and at 150-ram centres: the ring supports, used to restrain the sparge pipes at their ends, were constructed of Incoloy 800H fitted to a support frame of stainless-steel type 316. This enabled the relative durabilities of the sparge pipes to be assessed under similar operating conditions. Primary-combustion (i.e. the fluidising) air was fed to the sparge pipes by means of a Rootes type variable-speed blower, which is capable of ejecting 0"9428m3/s of air at about 1000mm WG pressure. The air was passed around the casing of a 290-kW oil burner--used for start-up---and into a refractory-lined 'T' piece, where it was mixed with the recycled gas (see Fig. 3). This system formed the entry to a tapered, cylindrical, refractory-lined plenum, similar to that used on commercial FBC plant, the only exception being that the side exit-slot was on the centre line. The ~T' piece and plenum incorporated a two-layer cast in-situ refractory lining, which protected the outer metal-shell from the high-temperature gases within. Rather than use an over-bed burner, as employed for the small FBC unit described previously (Findlay & Probert, 1992~, the fluidising air was preheated. The main problem with using an over-bed burner was that the relatively cold fluidising air tended to deflect the flame away from the bed. The resulting rate of heat transfer to the FB during the bed's start-up was then very small. Even in the case of the small FBC test-rig, the start-up often took nearly an hour before the required bed-temperature was attained because of this deflection away from the bed of the overhead burner-flame. However, by preheating the.[tuidising air, the start-up of the large FBC could be completed in just 20 min; this was a far more rapid means of raising the bed's temperature. The main disadvantages of the preheating method were the high capital and maintenance costs associated with the refractory lining then required in order to protect the outer stainless-steel shell from the hot gases within the plenum. On the opposite side of the fluid bed (i.e. with respect to the plenum}, another refractory plug (again mounted flush with the inner surface of the refractory of the FB vessel) contained an inspection port; this permitted the fluidising bed's material to be sampled (see Fig. 4). The presence of this entry also allowed easy access to within the vessel close to the free ends of the sparge pipes. The refractory lining of the combustor wall was pierced by a coal-feed inlet at ~0.5 m above the bed's surface. Fuel was introduced to the bed via this inlet by means of a 0.22-m-diameter screw, which enabled coal to be dropped onto the surface of the bed. However, because of the large width and breadth of the bed, it is unlikely that a uniform distribution of the coal ensued. Yet this is fortunate in some ways, because with existing designs of over-bed coal-feeds in industrial FBCs such inadequacies occur. Hence, the

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present series of tests is likely to provide results representing reasonably those ensuing in practice. Four secondary-air inlets were positioned at about 1-5 m above the bed's surface for trials 1-5; in trial 6, secondary-air inlets were incorporated at three different heights, namely 2-6, 1"9 and 1.5m above the bed. In the subsequent tests, secondary air was injected into the bed through any combination of the six nozzles at 2.6 m, and four nozzles at 1"9 m, above the sparge tubes. Because of the damage sustained while in use, the refractory required complete re-lining after trial 5. The positions of the secondary-air ports, as used in trial 6, are shown in Fig. 2. The secondary-air ports had a nominal bore of 76"2 mm and were positioned approximately 30cm apart on two opposite walls of the combustor unit, with the lower secondary-air injection ports at 1465 m3/s above the sparge pipes, i.e. just over the 'above-bed' thermojunction and 555 m m above the coal-feed pprt. Secondary combustion occurred as the partially combusted gases reacted with the secondary air near where it was

Limiting NOx and S O 2 emissions,/kom an FB comhustor

II

injected, so raising the temperature of the gases. The height of freeboard available, i.e. that in which secondary combustion could take place, varied between ~ 1.4 and 2.5 m according to the location of secondary-air injector selected. The combustion flue-gases then passed, via the 550-mm-diameter outlet, through a horizontal duct to the stainless-steel water heat-exchanger. which was referred to earlier as the flue-gas cooler. From here the hot flue-gases passed to the waste-heat boiler and economiser, where they were further cooled to less than 400C-~ see Fig. 5.

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K. Findlay, S. D. Probert

The gases then flowed through a pair of parallel cyclones (for the removal of sand and ash particulates) prior to their exit to the stack. Flue-gas samples were obtained (for chemical analyses) from the emissions leaving the cyclone, and dust samples were taken from the gas stream prior to its entry to the stack.

3.2 Recycled gas Some of the clean, cooled exhaust-gas leaving the cyclone was recycled back to the FB as a means of achieving bed attemperation. This recycled gas was drawn from the cyclone outlet by two gas-recycling fans, placed in series; these provided a total flow of approximately 3400 kg/h of cooled exhaust gas at ,--1000 mm WG differential pressure with respect to atmospheric and at approximately 450°C. The fans then discharged the recycled gas to a spray chamber, where the gas temperature was reduced even further by means of a spill-back controlled water-spray. The addition of the 'wet' recycled gas to the FB acted as a means of bed attemperation for two reasons: (i) The recycled gas extracted heat from the bed. Steam not only has a high specific-heat capacity, but 'wet' recycled gas also tends to draw heat from the bed because of the water vaporising to form steam. (ii) The presence of steam and unburnt carbon within the bed could be expected to promote the endothermic water-gas reaction: C + H20 ~

CO + H 2

The degree of bed attemperation was controlled, downstream of the quench chamber, by means of a butterfly damper, which regulated the flow of recycled gas according to the bed's temperature, as indicated by a thermojunction.

3.3 Coal feed Coal was introduced to the FB by means of a chevron elevator-belt, fitted with a magnet in order to ensure the removal of any ferromagnetic contaminants which may have been present with the coal. The elevator was fed by a coal hopper, which was filled frequently by a mechanical shovel, in order to ensure a constant flow of fuel to the entry chute of the lock-hopper, coal-feed system see Fig. 6. Because the operation of the FBC relied on having a forced draught to the stack (as in an oil-fired boiler), a lock-hopper system was required in order to facilitate the entry of the coal, despite an over-pressure within the furnace. One of the problems with the previously employed small FBC test-rig was

Limiting N O x and SO 2 emLs'sions f r o m an FB combustor

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that the hot exhaust-gases tended to escape through the coal-feed screw into the coal-hopper above. Condensation of the steam present in these exhaust gases was responsible for the frequent blockages which occurred in the coal feed-pipe, because wet coal has a tendency to agglomerate and compact. The lock-hopper system employed in the large rig, with a nitrogen-purge feeding into the lower hopper, prevented any escape of hot gases through the feed screws. The coal-feed sequence was usually adjusted manually via the control

14

K. Findlay, S. D. Probert

panel adjacent to the lock-hopper unit (see Fig. 6), but it could also be controlled automatically by a Texas PM 550 IPC microprocessor located in the main control room. A capacitance probe sensed the level of coal in the top hopper and the lower hopper was mounted on load cells. This system enabled the coal's feed-rate to be monitored closely and so helped provide control during the filling process. The lower coal hopper, of approximately 1 m 3 capacity, discharged coal directly into the variable-speed metering screw, the speed of which was adj usted according to the coal's required feed-rate. This metered coal-stream was then fed on to yet another screw, which operated at a fixed speed, its speed of discharge always exceeding that of the previous screw (thereby avoiding any possibility of blockages occurring due to the impaction of the coal in the screws). 3.4 Sand and limestone feeder (see Fig. 7) The top pressure-vessel, with a solids-containing capacity of ,--0.75 m 3, was filled with either sand or limestone using a vacuum pump, which was ~TO

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Limit&g N O x and S02 emissions ['rom an FB comhustor

15

powered by a Sturtevant Mobile Vacuum Machine. By this means, the feeding of limestone, or if necessary sand, could continue from the lower vessel while the top vessel was being re-filled. The valve settings were adjusted via a programmable sequence-controller and the sand or limestone feed-rate measured using weighing scales placed immediately beneath the whole system. The amount of sand or limestone present in the lower vessel was metered, by a variable-speed rotary valve, and the material fed into the lean-phase 6.35-mm nominal-bore conveyancing line, where an air blast, supplied by a Compair regenerative-blower, carried the limestone or sand to the FB. When 'making up' a bed, prior to the start-up, the top vessel could be used as a batch blower by means of the programmable sequence-controller in order to feed a known amount of material into the FB.

3.5 Bed-material extraction system A gradual increase in bed depth, resulting from the addition of S O 2 sorbent (or high-ash coal) to the FB, necessitated some means of achieving beddepth reductions if defluidisation was to be avoided. The systematic extraction of material from the bed helped provide the control required: thus, combustion trials of long duration, using limestone or high-ash coals, could be completed successfully. However, the removal of hot bed-particles (at ---90OC) from the bed did present a handling problem, because the extraction system had to be capable of transporting such a hot abrasive material to a suitable storage-vessel. For conveying the material by the dense-phase conveyor, the extracted bedmaterial was cooled to less than 150°C by means of a water-and-steam hairpin-tube heat-exchanger, mounted below the sparge pipes. Below this heat-exchanger, there was a stainless-steel-lined, steam-traced dischargehopper for the spent bed-material. Where the hopper tapered to a 20-cmdiameter base, the bed-material entered an inclined extraction screw, which had a variable-speed drive to control the bed-material's extraction rate. The bed's extraction screw, with a stainless-steel half-pitch flight of 150 mm diameter on a 75-mm flight tube, was 4.6 m in length and had a 200mln-diameter casing. In order to facilitate the discharge of the extracted bedmaterial from the base of the discharge-hopper to a 0"14-m3-capacity pressure-vessel (which was part of the dynamic air-conveyancing system), the bed's extraction-screw was inclined at about 35" to the horizontal. The system then pneumatically conveyed the bed-material to a 10-tonnecapacity hopper (see Fig. 8). Here it was weighed by means of load cells placed beneath the hoppers. The rate of extraction of the bed-material could therefore be monitored closely and thereby controlled, the maximum bed

K. Findlay, S. D. Probert

16

-1.5m LOAD CELLS LOCATED AT THE BASE OF THE SIL0

Fig. 8. Sideview of the lO-tonnecapacity ash-storage silo. This was used during the tests on the industrial-size rig, for storing the limestone and bed-ash, which had been removed by the bed's extraction system. extraction rate being ---250 kg/h. The extracted sand was then separated, from the spent limestone and coal ash, by means of a vibrating screen and returned to the FB for re-use.

3.6 Bed-sampler system Each sample of bed-material was extracted via an aperture in the refractory plug, which was, as mentioned earlier, on the opposite side of the FBC chamber to the plenum (see Fig. 4). During normal FB activity, the aperture remained filled by the overflow of material from the FB. As the bed-material flowed through the 50-mm-diameter sample arm, into the collection pot, a nitrogen-purge there served to quench any ongoing combustion reactions which otherwise may have taken place. This facilitated obtaining a bed sample which was truly representative of the conditions present in the FB at that point in time, and prevented any chemically unstable compounds in the sample, such as calcium sulphide, from being destroyed. Because of the very high temperature o f the bed-material being sampled, 'high-temperature' disc-gate valves were used to control the flow of the sample into the cooling-pot. Here cold nitrogen-gas was introduced at the pot's base, from where it passed through the hot bed-material to a vent at the top o f the vessel.

Limiting N O x and S 0 2 emissions./Hml an FB comhustm"

17

Once the bed sample had been cooled, it could be dropped into a metal container by first closing the top valve and then opening the other hightemperature disc-gate valve, which was situated immediately below the sample cooling-pot. The bottom valve was then closed and the top one opened in order to allow the next sample of bed-material to enter (from the sample arm) the sample pot, where it was cooled by the nitrogen-purge injected from the base. The two valves were interlocked pneumatically in order to prevent them from both being open simultaneously. Meanwhile, the aperture was refilled by a further overflow of material from the FB.

3.7 Gas analysis and instrumentation A flue-gas analysis was carried out systematically throughout the tests using a standard gas-sampling system and instrumentation. Flue-gas samples were drawn continuously through a heated filter and sample line to either the water cooler and silica-gel dryer or a refrigerated cooler/dryer (depending on the gases to be analysed) by means of two diaphragm pumps. When the sampled water-soluble gases, such as NO x, SO x and CO 2, were to be analysed, their temperatures were reduced quickly in a refrigerated cooler, thereby shortening the period of contact these gases had with the condensed water present in the cool flue-gas. For carbon monoxide and oxygen, both of which exhibit poor solubilities in water, this was not so critical, provided the water was removed (by silica-gel crystals) before it could contaminate the gas-analysis instruments. The clean, dry sample of flue-gas was then conveyed to the gas analysers, in the control room, by means of two 60-mlong nylon lines. The first of these nylon tubes carried the quick-dried samples from the refrigerated cooler to the gas line feeding the NO X chemiluminescent analyser and the two infra-red spectrophotometers, measuring SO 2 and COx, respectively. The second conveyed the sampled gas from the silica-gel line to the 0 2 paramagnetic analyser and the infra-red spectrophotometer, which was used for measuring the CO concentration. The outputs from the various gas-analysers plus the 16 bed-temperature indications, as well as the freeboard and above-bzd temperatures, together with the pressure and flow measurements associated with each combustion test, i.e. for the 48 measured variables, were recorded on two YEW multipoint recorders. Each recorder was equipped with a GP IB system, by which the data could be transferred to the Hewlett-Packard 85 micro-computer for display and analysis. The micro-computer was programmed to transform the data into various displays and print-outs in order to indicate the general operating conditions apertaining during each test. The presented data included the primary-and-

18

K. Findlay, S. D. Probert

secondary-air flow-rates, recycled-gas and fuel flow-rates, average bed and freeboard temperatures, and the flue-gas chemical analyses. The microcomputer was also capable of providing averaged data, which could then be used to perform heat-and-mass balances. Normally the data were displayed in digital forms and all the observations were updated at 30 s time-intervals. Every 10 minutes the incoming scan was stored on a floppy disc, so that detailed analyses of the data could be carried out subsequently. Some stored data were also transformed into graphical form in order to illustrate any trends (with respect to time) in the operating conditions, as well as in the temperatures and gaseous emissions experienced throughout the test. In addition, the gas-analysis measurements were recorded on a standard pen-and-chart recorder and, for cross-checking, readings were taken periodically from the visual displays on the gas analysers themselves.

3.8 Above-bed gas sampling The above-bed gas samples could be taken from four different locations, as shown in Fig. 9. The use of the 4m-long water-cooled probe provided sufficient cooling of the gas sample to allow it to be passed directly through the gas analysers. However, frequent blockages of this probe occurred, due to condensation of moisture as well as solids deposition, so permitting only short periods for sampling. Continuous analysis with respect to the concentrations of O2, CO 2 and CO in the case of both the above-bed samples and the flue-gas allowed comparisons to be made and the following conclusions to be drawn: (i) Locations 1 and 2, as shown in Fig. 9, were too close to the splash zone to give meaningful results regarding the composition of the combustion gases leaving the bed. (ii) Compositions of the gas samples, taken from locations 3 and 4, were identical with those for the flue-gas. Therefore, the combustion reactions must have been completed below location 3. For trial 6 the above-bed sampling probe was modified in order to overcome the blockage p~oblems experienced during trial 5. The probe was shortened, thereby reducing the total rate of heat loss from it, and its outer jacket was steam-cooled rather than water-cooled (to try to maintain the gas temperature at above its dew point). Further improvements in the above-bed gas-sampling process involved passing the gases through a miniature cyclone (for the capture of the coarser particulates) and subsequently through a filter (for the removal of even finer particles). The gas sample was then conveyed to the drying and cooling equipment used in the gas-sampling line for the flue-gas analysis operation. The NO x and SO Xconcentrations in

Limiting NOx and S O 2 emissions.lrom an FB combustr."

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2 L~vR ~CONBARY~AIR PORTS. 1176.2mmNB)

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u . . . . . . . .

( ~ ) ABOVE- BED GAS-SAHPLE POSITION Fig. 9.

As f o r Fig. 2, b u t i n d i c a t i n g the locations at w h i c h the ' a b o v e - b e d ' gas samples were taken.

the above-bed gas samples could therefore be measured. Above-bed gas analyses could be carried out for periods of up to 20min, the only disadvantage being that the ordinary flue-gas analysis had to be interrupted for this period of time. However, where values for only the CO 2, CO and 02 concentrations were required, a portable gas-analyser was available for use, so leaving the main gas-analysers to measure the concentrations of the constituents of the flue-gas simultaneously. Measurements of the compositions of the gas samples were checked by taking samples from the exhaust side of the gas-analysis equipment; these were sent to the British Coal Research Association, Stoke Orchard, Cheltenham, UK, for gas-chromatographic analysis.

20

K. Findlay, S. D. Probert

3.9 Dust sampling of the flue-gas The relatively clean exhaust-gases leaving the twin parallel cyclones were analysed for dust loadings just prior to their exit to the stack. The method employed was that recommended by Stairman (1951); it required the isokinetic sampling of the gases in the exhaust duct leading to the stack, downstream of a half-area mixing baffle and a flow straightener (see Fig. 10). The sampling involved the use of a steam-heated probe (with a right-angled nose, which was placed along the centre line of the 450-mm-diameter

FLOWSTRAG IHTENER TOSTAC . K~ (_RS IHTA -NSLE SAMPLE ~ ,~PROBE ~ ~ [~FLowH / A-L-~ FM -A SR X IA N IE }S [ABAFFLEi

i----~ -

~

i

STEAM

ili i CYCLONES

-- --sl

~[

PP I ESARE TRACEHEATED

,

',

i

'--

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• . . . . . .

,

i

SAMPLE ROTAMETER

i

"-

FILTER

I

....

OVEN (185°C}

~STEAMTOVENT WATEROUT

II II

W AD TE R ST EC PA RO AT AN C A H P TO SR Fig. 10. The isokineticdust-sampling system.

Limiting NO,. and S02 emissions ti'om an FB comhustor

21

exhaust-duct). The steam-heated line maintained the sample above its dewpoint temperature, and the dust present in the gas sample was removed using a 'Balston' type boro-silicate-glass filter-element, inside an oven, at a temperature of 185°C, in order to ensure that no condensation of the water vapour, present in the gas sample, would ensue. The sample was drawn through the probe and filter by means of a doubleheaded diaphragm pump; it then passed, via a water-cooled jacket, before being discharged through a rotameter, where the flow was measured at approximately 15~C. The flow was controlled, by valves downstream of the cooler, to be at a rate which had been calculated to permit isokinetic sampling from the exhaust gases in the duct. The mass of dust present within a known volume of sample gas was determined by measuring the weight of the total filter-element, before and after a sampling period of one hour. The average Balston filter is capable of removing 98% of dust particles with sizes _>0'1/Lm, but it removes finer particles less efficiently. Nevertheless, this method of dust sampling permitted a good indication of dust concentrations to be obtained.

3.10 Operating procedure Prior to the FB start-up, sand was added to the fluid bed from the recently filled top vessel of the sand/limestone feeder. Once a suitable depth of sand was achieved within the FBC, the fluid bed start-up was initiated. First, the bed temperature was raised to around 70OC by means of the high-intensity gas/oil-fired burner, situated at the head of the plenum. This heated the primary-combustion (i.e. fluidising) air prior to its entry, to the bed of sand, via the sparge pipes (see Fig. 3). Feeding of coal to the bed was then started, and once the bed reached 750:C (i.e. well above the normal ignition-temperature for the coal) the start-up burner switched off automatically. The high bed-temperature was then maintained solely by the combustion of the coal. Because of the risk of water corroding the gas pipework, as a result of condensation of the steam (present in the flue-gases~ on the relatively cold metal surfaces, the gas-recycling fans should not be started up until the tracing, fed by steam from the flue-gas cooler, had warmed sufficiently the pipework conveying the recycled gases. Until then the fluid-bed temperature had to be regulated by the coal's feed-rate and the excess of primary air. In order to do this, the coal's feed-rate was limited to approximately one-third of the maximum throughput. Once the temperatures in the flue-gas cooler were sufficiently high to be able to sustain the high-temperature steam-tracing process, i.e. when the outlet temperature of the gases from the waste-heat boiler had risen to approximately 200°C, then the gas-recycling fans were started and the bed

K. Findlay, S. D. Probert

22

temperature could then be controlled more effectively. This allowed the coal's feed-rate to be increased (up to ~ 290 kg/h depending on the quality of the coal), and secondary-air injection could also be started simultaneously in order to try to ensure the complete combustion of the fuel. The operating conditions were then left to attain a steady state, stabilisation usually occurring approximately six hours after the secondary-air injection commenced. Gas analysis was then undertaken. The values of the operating parameters were recorded on a HewlettPackard mini-computer (once every 10 min). The plotted data included the average bed-temperature; the coal feed-rate; the flow rates of the primary air, secondary air and recycled gas; the fluidising velocity; the above-bed temperature; the freeboard temperature; the freeboard's temperature-uplift (i.e. freeboard temperature minus average bed-temperature); as well as, for the flue-gas, the cooler-inlet temperature, the oxygen concentration and the air/fuel ratio (i.e. expressed as a percentage of the stoichiometric condition). For each combustion trial an appropriate set of operating conditions was selected. The conditions depended on the type of coal to be burnt and the rates of primary-and-secondary-air flows required. The programme of tests to measure the emissions of SO2 and NOx under various operating conditions (and also to compare them with the corresponding results obtained with the small FBC test-rig) required the investigation of the effects of the following variables: • • • • • • •

coal and limestone types, limestone particle-size, ratio of primary-to-secondary-air rates, secondary-air nozzle height, bed depth, freeboard temperature, and other operating conditions

on the compositions of the flue-gas and the above-bed gas.

4 RESULTS A N D DISCUSSION One of the most important objectives of this project was to investigate the effects of bed size on the SO2 and NO Xemissions. For experimental purposes, it is preferable to use a large-scale industrial bed of, say, ~ 1 m 2 horizontal cross-sectional area, because it would be nearer to what is adopted in industrial practice. However, such a unit would be relatively costly to construct and maintain, as well as expensive to run in terms of material costs and power required. The aim was, therefore, to establish whether a smaller

Limiting NOx and SO, emissions.l~'om an FB comhustor

23

experimental FBC (of horizontal area -~2"3 x 10 2m2) could provide matching values of pollutant emissions (using similar operating conditions) to those occurring with the industrial-size bed. Results from corresponding situations with both the large and small testrigs were corrected, in order to allow for dilution effects, and plotted on the same graph. If the emissions behaviours for the small test-rig bore no quantitative or even qualitative resemblances to those for the large test-rig (under otherwise nominally similar operating conditions), this would have far-reaching consequences because most of the reported studies, undertaken by the present and other investigators regarding pollutant emissions from fluidised beds, have been carried out on relatively small FBC units, e.g. see Furusawa et al. (1978), whose FBC unit was only 5 cm in diameter. 4.1 Effect of bed size

4.1.1 When burning South A/J'ican (SA) duff During trial 1, SA duff was burnt in the large FB under similar operating conditions to those that appertained when burning the same grade of coal in

./ 15( OXIDISING

10(] "

Z

TWO - STAGE

OXYGEN (%)

Fig. 11.

The reduction in NO xemission which arises when burning SA duffin the large FBC under two-stage combustion conditions compared with oxidising conditions.

K. Findlay, S. D. Probert

24

the small FB test-rig. This allowed comparisons to be made concerning the effects of bed size on the SO2 and NO x emissions. As in the case of run 8 with the small test-rig, the use of two-stage combustion resulted in much reduced rates of NO x emissions compared with when oxidising conditions were employed (see Fig. 11). However, when NO~ emissions from the large test-rig were compared with those obtained from the smaller test-rig (during run 8), both under oxidising conditions, the magnitudes of the results bore no resemblance to one another. Although both sets of results showed increases in NO x emissions as the oxygen-in-theflue concentration increased, the NO x emissions from the larger FBC were much lower than those from the smaller FBC test-rig at the same Oz-in-theflue concentration (see Fig. 12). However, the SO 2 emissions from the large FB, during trial 1, were much greater than those for SO 2 emitted from the small FBC test-rig, at the same values of the O2-in-the-flue concentration (see Fig. 13).

4.1.2 When burning Maryport smalls A qualitatively similar 'scaling effect' occurred for the NO Xemissions from the small test-rig, compared with emissions from the large test-rig when Maryport smalls was the fuel used, i.e. towards the end of trial 1. In this case, the NO Xemissions from the large FBC were always less than half those from the small FBC (see Fig. 14) under similar two-stage-combustion operating

./-

=

• SHALL RIG • LARGE Rift

mml •

z

2

v

i

i

--

~.

6

B

10

0XY~N (%)

Fig.

12.

Comparison of NOx emissLons from the large and small test-rigs under oxidising conditions, at a bed temperature of 900°C, when burning SA duff.

25

L i m i t i n g N O x a n d S O 2 emissions li'om an F B c o m h u s t o r 8OO

750

700

650

6OO

550

• LARGE • SMALL

SOO

RIG RIG

350

Fig. 13.

I

I

2

¢

i

6 OXYGEN (%)

L

I

I

B

10

12

Comparison of SO 2 emissions from the large and small test-rigs under o.\idishz~ conditions, a t a b e d temperature of 900C, when burning SA duff.

conditions, at a bed temperature of ---900°C, without limestone being added to the bed. (However, when traces of limestone were present within the FB, the magnitudes of the NO Xemissions, from both the large and small test-rigs, became much closer.) 4.1.3 When limestone was present

Figure 15 suggests that the NO x emissions from the small FBC (during run 25) became more comparable for the same 02 concentration with those from the large test-rig (throughout section lc of trial 1), during a period when limestone was present within the bed. In a manner similar to that which occurred with SA duff, when Maryport

100.

• SMALL • LARGE

80_

RIG Rift

6O_

oX

z

/+0.

20.

OXYGEN (%)

Fig. 14. NO X emissions from the small and large test-rigs under two-stage combustion conditions, when burning Maryport smalls at a bed temperature of 900°C, without limestone present in the bed.

250-

200

•

A•

150_

• •

SO

2

3

LARGE RIG SHALL RIG

/*

5

OXYGEN ( % )

Fig. 15.

The effect of the presence of LG8 limestone, even in small concentrations (during a period of interrupted limestone feed).

Limiting NO~ and SOz emissions,/~'om an FB ('ombu.s'tor

27

3000

• LARGE-RIG DATA • SMALL-RIG DATA~,,

25011

~2ooo.

1500.

m@

1000

o

•

~

~

- - ~

OXYGEN (%)

Fig. 16. SO, emissions from the small and large test-rigs trader two-stage combustion cmulitimls, when burning Mao'port smalls at a bed temperature of 900 C , without limestone present in the bed.

smalls was burnt (as in run 25), the SO 2 emissions from the large FBC during the initial period of no limestone addition were significantly higher than those from the small test-rig, under similar operating conditions (see Fig. 16). Examination of the two curves in Fig. 17 reveals that, when traces of limestone were present in the large FBC, there was a dramatic difference of the behaviour compared with that for when no limestone was present in the small FBC (i.e, as in Fig. 16). If limestone was present in the bed, a greater O= availability was required for the complete sulphation of the limestone and consequent reduction of the SO z emission. In the small FBC test-rig, the condition corresponding to a 2% Oz-in-the-flue concentration was sufficient to reduce the SO 2 emission to ~ l l 0 0 p p m without limestone addition. However, in the case of the large test-rig, the condition corresponding to a 4% O2-in-the-flue concentration was required in order to bring the SO2 emission down to approximately the same level, even with traces of limestone present in the bed.

4.1.4 Comparison q/ results There are anomalies between the results obtained for NO, and S O 2 emissions obtained from the small and large FBC test-rigs. The NO x emissions tended to be lower from the large test-rig, regardless of whether South African duff or Maryport smalls was the coal burnt, whereas the

28

K. Findlay, S. D. Probert

• LARGE RIO DATA • SMALL RIB DATA

I • • '~

• •It• 41'•i"

I I A!

3',, L

nl

•

",~

•l

OXYDEN 1%)

Fig. 17. As for Fig. 16; but, for the large rig, the data presented are for time section e of trial 1, during which a trace of limestone existed in the bed.

reverse applied for the SO 2 emissions. This suggests that the beds' local 02concentrations may have differed somewhat for similar values of the O2-inthe-flue, possibly due to the differing fluidisation behaviours of the two FBC units. For example, the small bed of sand was usually better mixed and more uniformly fluidised. For the large test-rig, the FB's upper-surface area was 64 times (although the number of sparge-pipe air distributors was only twice, i.e. eight sparge pipes as opposed to four) the corresponding value for the small test-rig. It is probable, therefore, that the large FBC was much less evenly fluidised than the small FBC, and that pockets of almost dead (i.e. relatively unfluidised) sand occurred, where the local O2-concentrations would have been much lower than those found in the small bed, even when identical average values of the O2-in-the-flue concentrations occurred for the two rigs. This would also explain why the required O2-in-the-flue concentration was much higher for the large test-rig in order to maintain the SO 2 emissions were maintained at acceptably low levels. With traces of limestone in the bed, an Oz-in-theflue concentration of at least 4% was required in order to maintain the SO2 emission from the burning Maryport smalls at around 1100 ppm, compared with an O2-in-the-flue concentration of 2% for the small test-rig. Excess levels of O z are known to be required for the sulphation of the limestone to occur (Ruth, 1978; Henttonen et al., 1991). It is perplexing why the presence of limestone so significantly reduced the

Limiting NOx and SO 2 em&sions.fi-om an FB combustor

29

NO x emissions from the small FBC but had little effect under similar circumstances on the large rig. Fuel-N in coal produces both NH 3 and HCN, which are precursors for forming NO and N 2 0 , respectively. It is possible that, for Oz-in-the-flue concentrations of 3-4%, the local oxygen concentrations in the large test-bed were so low that HCN was effectively hydrolysed to ammonia within the FB; it then oxidised to NO in the high oxygenconcentration of the secondary-combustion zone. The catalytic effect of CaO on N 20 reduction has been documented by others (Bolting et al., 1991 ) and can be described by CaO + 2HCN ~ Ca(CN)2 + H 2 0 Ca(CN) 2 + 3H20 --+ CaO + 2NH 3 + 2CO NzO reductions of up to 60% have been reported {Bolting et al., 1991) at a bed temperature of 830°C (as well as 50% at a bed temperature of 850C) and subsequent oxidation of the NH 3 to NO would undoubtedly lead to an increase in the overall rate of NO x emissions. Oxygen is required for the sulphation of the bed's limestone, so reducing the 02 concentration in the bed even further compared with what it would be without limestone being present. Within the small, well fluidised FBC, more uniform, slightly higher 02-concentrations in the bed would have induced the following reactions (Curlyurtlu et al., 1991b):

(HCN + O --+ NCO + H At low bed-temperatures ~NCO + NO -+ NzO + CO (i.e. below 900°C) ] N O + (char + N) -+ N 2 0 + char 1,2(char + N) + ½02 -+ N 2 0 + 2(char) / N 2 0 ---+ N 2 + 1 0 2 At high bed-temperatures ~N20 + char --+ N 2 + {char + O) (i.e. of 900°C and above) | N H 3 t.~ 1½H2 + NO + char L --+ ½N2 + {char + O)

In the presence of limestone, the HCN would be converted to NH 3 {rather than N 2 0 ) which would then be eilher oxidised to NO in the freeboard, so leading to an increase in NO Xemissions, as observed during certain trials on the large FBC test-rig, or reduced to N 2 in the presence of char. The increase in NOx emissions observed when using the small FBC test-rig is due to its well fluidised, uniformly higher local O2-concentration in the bed. Under oxidising conditions, the NO=-reducing capacity of char becomes diminished, so leading to greater emissions of NO and N20. At lower bedtemperatures, NaO was found to have a greater affinity to the char than NO (Curlyurtlu et al., 1991b). Thus, NO reduction by the char increased with temperature, so lowering the overall NO~ emission under reducing

K. Findlay, S. D. Probert

30 55(

x~

S0( 4.5C 4.0(]

x

25O

•

LARGE TEST-RIG : 1.2 m-SQUARE SECTION SMALL TEST-RIG : 0.15m - SQUARE SECTION

2O0 151 10(

~

},

02 (as %)

Fig. 18. Comparison of N O Xemissions from the large and small test-rigs under oxidising conditions, when burning Gedling coal at a bed temperature of 900'~C during trial 7 and run 24, respectively.

200

150

I 100

/

/

/

•j//

,~#~A~A~,• •

SYMBOL • •

TEST RUN 2/* TRIAL 7

50

OXYGEN (%)

Fig. 19. Comparison of NO x emissions from the large and small test-rigs under two-stage combustion conditions, using a primary-to-secondary-air ratio of 70/30, when burning Gedling coal at a bed temperature of 900°C.

Limiting NO~ and SO, emissions.l?om an FB uombustor

31

conditions in the bed and leading to high char-concentrations in the freeboard. It can thus be concluded that, although the results from the small FBC test-rig were valuable for establishing qualitative trends, they should not be used to predict absolute emission values for the large rig (Ruth, 1978; Bolting et al., 1991). 4.1.4(a) N O x emissions. When Gedling coal was burnt under oxidising conditions (at --~5% 02), in the large test-rig, the results approximated to those obtained with the small test-rig--see Fig. 18. However, under similar two-stage combustion conditions, there was a marked reduction in the NO xemission concentrations from the large test-rig compared with those from the small FBC (see Fig. 19). 1300

1200 ± -.-m--

LARGETEST-RIG : 1.2 m-SQUARE SEETION SMALL TEST-RIG: 0.15m- SQUARE SECTION

110(]

• "I-

>

l I

lm I

I' I

m~

60(

•

~lm

k •

%

\

. ~I- ." 50(

------

-

-m--

t+0( OXYGEN (%)

Fig. 20. Comparison of the SO2 emissions from the large and small test-rigs under two-stage combustion conditions, when burning Gedling coal at a bed temperature of 900'C, using a primary-to-secondary-air ratio of 70/30, during trial 7 and run 24, respectively.

K. Findlay, S. D. Probert

32

The fact that the NOx-emission behaviours of the large and small FBs, under oxidising conditions, approximated to one another supports the contention that the reduced NOx-emissions from the large FBC were a consequence of the lower O2-concentrations in parts of the large bed resulting from the less-effective fluidisation of the much greater mass of material. Although similar primary-to-secondary-air ratios were utilised in each case, the invariance of the 02 concentration throughout the bed could only be ensured provided the bed was fluidised uniformly--a condition which is not easily achieved in an industrial-size FB with few sparge pipes. Nevertheless, where a single size of bed was employed for the tests, approximately reproducible results for the NOx-emissions ensued. Provided the operating conditions, such as bed temperature, primary-to-secondaryair ratio and freeboard temperature, were held constant, almost consistent results for the NOx-emissions ensued during completely different combustion trials. However, it must be emphasised that achieving exact consistency of imposed conditions, when using the large test-rig, was almost impossible. Hence, to reduce the likelihood of errors in the deductions, a large number of separate experiments, each generating a vast amount of data, were undertaken in the present tests.

4.1.4(b) SO, emissions. Under two-stage conditions, the SO2 emissions when burning Gedling singles, in the large test-rig, were more than double 180C x

160(:

x

x

xX

140(2

x

x

Xx x

x

120(2

8OO 6OO 4OO

x

LARGE TEST-RIG : 1.2 m-SQUARE SECTION

•

SHALL TEST-RIG : 0.15m- SQUARESECTION

~ - - ~ . m

200 0

½

4

6

8 10 02 ( as O/o)

12

14

16

Fig. 21. Comparison of the SO2 emissions from the large and small test-rigs under oxidising conditions, when burning Gedling coal at a bed temperature of 900°C, during trial 7 and run 24, respectively.

L/m/tiny{ NOx and SO,_ emissions.lim*z an FB comlmstor

33

those from the small FBC (see Fig. 20). Under oxidishTgconditions, with an 02 concentration of 5%, the SO 2 emissions from the large test-rig were 1100 ppm higher than those from the small test-rig under similar operating conditions (see Fig. 21). This difference would again be explained by the presence of pockets of low O2-concentration within the imperfectly fluidised large bed. The internal consistency (obtainable within the limitations of the experimental procedures) of the SO2-emission results from the large FBC test-rig is good. Comparison of the SO2 emissions, taken under similar operating conditions during trial 6 (section a), trial 7 [section d) and trial 12 (section f), all showed consistent results to within the expected range of experimental errors(see Fig. 22). It should be noted that all the results were significantly higher than those obtained with the small test-rig (see Fig. 22). 160(

150(

lt.,.OC

•"

130C

A

A

sS

~ °

1200 1100_ • • • •

N1000

TRIAL TRIAL TRIAL RUN

12F 6Q 7d 24

ON LARI3E TEST-RIG ON LARGE TEST-RIG ON LARGE TEST-RIG ON SHALL TEST-RIG

900 80O

! 700.

600

@ira

m @w

500

oxYr_£N (%}

Fig. 22.

Values of

the SO_, emission in relation to O2-in-the-fluc concentration when burning Gedling coal.

34

K. Findlay, S. D. Probert

4.2 Effect of bed depth on the emissions from the FB

4.2.1 NOx emissions During the penultimate part of trial 6, the secondary-air nozzle's height above the sparge pipes was again increased to 2-6 m, as for sections 6a, b and c (and 6d in the case of the NO x measurements) earlier in the trial However, a deeper FB, of height 660 mm, was employed at the end of the trial and the results were compared with those obtained using a bed depth of 340 mm, as in the earlier part of the test. Because the freeboard temperatures differed somewhat during the final period when the deeper bed was employed, the test was divided into two sections, namely section 6j, with a freeboard temperature of 860°C rising to 930°C, and section 6k, with a freeboard temperature increasing from 960 to 990°C. The freeboard temperatures experienced during the latter part of test section 6j were similar to those observed during section 6a (when a --~340 mm-deep bed was used), i.e. at around 930°C. Thus, the corresponding graphs o f NO x emissions versus Oz-in-the-flue concentration were compared in order to ascertain the effect of the bed's depth on the emissions. Figure 23 shows that use o[the deeper bed led to sign(ficantly increased rates of NOx emissions, e.g. at an Oz-in-the-flue concentration of 5% there was a ,-~75% increase in N O x emissions when the bed depth was approximately doubled. Hence, in order to achieve low rates of N O x emissions, the bed should be kept relatively shallow. Also, at higher freeboard temperatures, the percentage increase in NO x emissions for the greater depth was less. For an Oz-in-the-flue concentration of 5%, it is likely that a high recycled 02 concentration would have been employed (e.g. under the conditions experienced during test section 6j). There would then be sufficient 02 to ensure that stoichiometric combustion of the coal ensued within the bed. U n d e r such conditions, the deeper the bed, the greater the in-bed bulk surface area available to catalyse the oxidising reactions, and hence the larger the a m o u n t of NO x formed. The use of a deeper bed has also been found to lead to a greater variation of temperature (by up to 100°C) across the bed; this would also result in increased NO2 emissions (Vickers & Milner, 1991). Similarly, when the results of NO~ emissions taken during section 6k of the series of tests were compared with those obtained during sections 6a-6d, taken over a similar range of freeboard temperatures, there was again seen to be an increase in NO x emissions. This amounted to ~ 50 ppm at a 5% Oz-in-the-flue concentration, which was equivalent to a 25% increase in NO x emissions using the greater bed height at the higher freeboard temperatures.

Limiting NOx and S02 emissions.]rom an FB comhustor

35

,6

2 ,'5

,.,.. C,

r--

1

e... ©

¢"~1

0 ::Z::OQ U,-" "~'

e~ • ,,..,-

o

I

.o "-&

.E

I

(,,,dd so) xON

=

©

,'5

,'4

E

K. Findlay, S. D. Probert

36

4.2.2 S O 2 emissions

The use of a greater bed depth .led to a small reduction in SO2 emissions (as shown in Fig. 24), especially for low 02 concentrations within the bed. The use of the deeper FB resulted in an even greater reduction of SO2 emissions if CaS was formed under low-to-medium concentrations of 0 2 within the bed, and conditions of reduced in-bed combustion applied. The residence time of the coal ash in the FB's reaction zone was greatly increased when a deeper bed was used: the better mixing qualities of the FB then ensured that a greater percentage of the coal ash became sulphated. However, under oxidising conditions, i.e. if sufficient 02 was present in the FB for the sulphation reaction to occur, then the increased residence times of the combustion gases and coal ash in the deeper bed did not prove beneficial. This indicates that the sulphation of the coal ash would appear to be more influenced by the availability of oxygen within the FB rather than by time dependence. However, it is possible, under these conditions of high in-bed combustion, that the effect of the extended residence time was offset by the influence of the larger temperature variations across the bed (Vickers & Milner, 1991). 1¢00

1300

"-..,.-

O'

120(

• •

•

mi

OXYGEN (%)

Fig. 24. Effect of bed depth on the SO2 emissions, for a secondary-air nozzle-height of 2.6 m above the sparge pipes. Results from test sections 6d and 6e, taken when using a bed depth of 600 mm, are plotted together with results taken from time test-sections 6a~l, for a bed depth of 340 ram, and show ~ 4 % reduction in SO2 emissions at the greater bed-height.

Limiting NO~ and SO 2 emissions l?om an FB comhustor

37

4.2.3 Using different coal qualities When experiments were carried out using SA duff (rather than Gedling singles as discussed earlier) during trial 9, increasing the bed depth again led to a rise in the NOI emissions. Values obtained when using a 715 mm-depth bed during section 1 gave a NOx emission of 210--230 ppm, compared with only ll0-130ppm during section 2, when a bed depth of 400mm was employed. This resulted in a net increase in NO x emissions of ~ 8 3 % for a 79% growth in the bed depth. However, in an attempt to maintain the fluidising velocity invariant, whilst reducing the coal-feed rate during section 1, the increase in O2-in-the-flue had to be compensated for by reducing the secondary-air flow. Thus, the primary-to-secondary-air ratio was somewhat greater during section 2, at 50//50 (compared with 37.'63 during section l), and this could account for some of the increase in the NO~ emissions observed. A direct comparison between these two sets of results was therefore inappropriate with respect to the effect of bed depth, because of the influence of the other variables. Nevertheless, the results obtained went some way towards confirming the conclusions obtained during trial 6 and certainly did not contradict them. Results for SO2 emissions, observed during trial 9 using SA duff, also showed a slight reduction in the SO 2 concentration in the flue-gas of around 20 30ppm as the bed depth was increased from 400 to 715mm. This occurred despite the higher primary-to-secondary-air ratio experienced, with the lower bed depth, which would normally tend to reduce the SO2 emissions. This showed that the increased bed depth had the overriding effect in reducing the SO 2 emissions. Therefore, in general, in order to reduce the SO,-emission concentrations the depth of the bed should be increased. 4.2.4 Summary It is concluded that, in order to achieve tolerable concentrations of NOx and SO2 emissions, a compromise must be found: reducing the bed's depth serves to lower the NO~ emissions significantly, whereas increasing the bed's depth would be necessary in order to decrease the SO 2 emissions.

4.3 Effects of freeboard temperature and secondary-air nozzle-height 4.3.l The above-bed gas composition Measurements of the 'above-bed' gas samples revealed that a reduction of NO x concentration ensued as the freeboard temperature was increased. At an above-bed O2 concentration of 1%, the above-bed NO x concentration was approximately 30ppm lower during test section 6f (for a freeboard temperature of 1001°C) than it was during test section 6e, with a freeboard temperature of 948°C (see Fig. 25). Other than that, the O2 and NO x

K. Findlay, S. D. Probert

38 25C

TF:%8°C~o~ •

•

.~ 20c o> QD

15C

x o

~ 0 1 ° [

PERCENTAGE OXYGEN

ABOVE THE BED

Fig. 25. Effect of freeboard temperature TF on the NO Xconcentration of the above-bed combustion gases, produced under two-stage combustion conditions, when burning Gedling coal at a bed temperature of ~900°C, without limestone present in the bed. The combustiongas samples were taken from just above the fluidising bed, but below where the secondary air was introduced into the combustion zone.

~200 I I I I I

-.

E BED

I

I

%

I

g

,

4100 I

I r

Z

I I I I L

61

i

i

i

i

65

i

i

i

i

70

J

i

i

i

i

75 2-MINUTE

i

J

J

i

i

i

80

i

~

~

i

i

85

i

~

,

'lO

90

TIME INTERVALS

Fig. 26. The almost in-phase variations of the 02 and NO x concentrations above the FB, during the latter part of time section 6e.

Limiting N O x a n d S O 2 emissions.from an FB combustor

39

concentration fluctuations followed qualitatively similar trends, the close correlation being shown in Fig. 26. This phenomenon could be explained if the CO or char concentrations within the above-bed zone were sufficiently high so as to promote NOx-reducing reactions at these high freeboardtemperatures. However, this would be the case only for low 0 2 concentrations in the above-bed zone prior to the secondary-air injection. Under conditions of high 02 concentration in the freeboard, these nitrogenous intermediates might well be immediately re-oxidised to NO x, so leading to a net increase in NOx emissions in the flue-gas. However, the opportunity for this reaction to occur in the oxidising conditions of the 350

SECONDARY- AIR NOZZLES AT 1.5m ABOVE THE SPARGE PIPES

300

F-

•

.~ mm

250

/

~m

00~ 0 • 20O x

uo • • 150 / /

/

'8'0 .oooo o~-

SECONDARY- AIR NOZZLES

AT__2firn ABOVE THE SPARGE PIPES

100

PERCENTAGE OXYGEN ABOVETHE BED

Fig. 27. Effect of the secondary-air's nozzle-height on the N O x concentration in the abovebed gases, using a 340-mm-deep bed and a freeboard temperature of 990°C, during the twostage combustion of Gedling coal (without limestone present in the bed) at a bed temperature of 900°C.

40

K. Findlay, S. D. Probert

freeboard would tend to be increasingly reduced, the higher the secondaryair nozzle above the bed. It therefore followed that, when the secondary-air injection occurred at 1"5 m above the sparge pipes, rather than at 2-6 m, the NO x emissions in the above-bed gas were much greater (see Fig. 25). Experimental measurements of the SO2 concentration in the above-bed gases showed considerable scatter (see Fig. 28) for nominally identical conditions. However, this compilation of results does show the increased SO 2 concentrations at the higher freeboard-temperature experienced during section 6f (at -,~1001°C) compared with those measured during section 6e, when the freeboard temperature was lower (at --~948°C). When the abovebed SO2 and 02 concentrations were plotted against time (see Fig. 29), this revealed some correlation between the two fluctuations. When the height of the secondary-air injection was reduced from 2-6 to 1"5 m above the sparge pipes, lower SO 2 emissions tended to ensue (see Fig. 30). This was presumably because the injection of the secondary air, nearer 1500

• TF = 1001"C • TF = % 8 " C

¢¢

1/,,00 •

,.o

is

•

w u

•

iP'"

1200. mI

1100_

1000 ABOVE- BED OXYGEN CONCENTRATION(%)

Fig. 28. Compilation of results taken from both time sections 6e and 6f showing the relationship between the above-bed SO 2 and 02 concentrations, under conditions of twostage combustion, using a secondary-air nozzle-height of 2.6 m (above the sparge pipes).

Limiting NO~ and SO 2 emissions Ji'om an FB comhustm"

41

-,1500 ',

sx SO2 ABOVE BED

I I

0z ABOVE BED

~' -',500 I I )

J

lb

is

20

~s

30

5-MINUTE TIME INTERVALS

Fig. 29. Time-plot showing the variations of above-bed SO 2 concentration with above-bed 02 concentration, for example, during time sections 6e and 6f of trial 6 (between 3 and 30 5rain time-intervals following the commencement of the trial), under conditions of two-stage combustion, using a secondary-air nozzle-height of 2-6 in (above the spargc pipes).

to the FB's surface, increased the residence time of the secondary air in the freeboard, so leading to a greater sulphation of the elutriated coal ash as it passed through the oxygen-rich freeboard to the stack. In this instance, therefore, the injection of secondary air at the lower level (i.e. 1.5 m above the sparge pipes) would be preferred in order to reduce the SO2 emissions. It is unfortunate that, by using this secondary-air injection at the lower level, the c o n c e n t r a t i o n of NO x in the above-bed zone increased significantly--i.e, at an 02 concentration of 1%, from ~ 160 ppm to well over 300 ppm, when the secondary-air injection level was decreased from 26 to 1.5 m above the sparge pipes (see Fig. 27). Clearly, in order to reduce the NO x emissions, the use of the higher secondary-air injection level would be preferred, i.e. at 2"6 m above the sparge pipes, because this would reduce the exposure of the reduced nitrogenous intermediates, in the above-bed zone, to oxidation by the secondary air to form NO x. The qualitative relationship between the NO x and 0 2 concentration fluctuations is further confirmed by Fig. 31, albeit the patterns being slightly out of phase.

4.3.2 NO,.-emission concentrations These increased during the early part of trial 6 as the freeboard temperature rose (see Fig. 32). Allowing for variations in the O2-in-the-flue concentration, there was an average increase of ,-- 13 ppm of N O x for every 10°C rise in the freeboard temperature. This indicated an increase in thermal NO x formation at these high temperatures. Reports by others (Buchtela &

K. Findlay, S. D. Probert

42 150(

• ~ i ~ ~ ~~

~ rSECONDARY-AIR NOZZLES SITUATED2.6m ABOVE THE SPARGE PIPES

I~0C

13oo ,,,:¢

RI N

1200

mm •A

!. i A

110C

1000

SECONDARY-AIR NOZZLES SITUATED 1.Sin ABOVE THE SPARGE PIPES 1 OXYGEN (%)

Fig. 30.

Effect of secondary-air nozzle-height on the SO z concentration in the above-bed

gases, using a 340-ram-deepbed and a freeboardtemperatureof 900°C, during the two-stage combustion of Gedling coal (without limestonepresent in the bed) at a bed temperatureof 900°C. Hofbauer, 1991) have concluded that the freeboard temperature has a greater influence on NO and N20 emissions than bed temperature. Greater reductions in NO emission, as the height of the secondary-air injection was increased, have been observed (Buchtela & Hofbauer, 1991). However, the presence of CO and char could promote the NOx-reducing reactions within the above-bed zone in order to reduce the formation of NOx as a result of burning the fuel: such reactions also occur within the substoichiometric FB. The secondary-air nozzle's height had a significant effect on the NO~-emission concentration: the nearer that the secondary-air injection occurred to the FB's surface, the greater the likelihood of any nitrogenous intermediates being oxidised to form NO~. Hence when, for sections 6g, h and i of the experimental series, the secondary-air injection was applied 1.5 m above the sparge pipes (as opposed to 2.6 m above, as in the earlier test section, 6d), then the average NO~ emission was increased by

43

Limiting NOx and SO z emissions /kom an FB combustor

2 i i i I

NOx ABOVE BED

I I

/

~"

1. c.

~300 m

:

<

I

-Z

~200 c~

g ×

02 ABOVE BED

-100

o . . . . . . . . . . . . . . . . . . . . . . . . .

6O

_L,

80

70

.....

o

2 -MINUTE TIME INTERVALS

Fig. 31. Time-plot showing the variations of the above-bed NO x and 02 concentrations during the later part of time section 61 (between 55 and 83 2-rain time-intervals from the commencement of the time section).

approximately 15% (see Fig. 33). This adjustment could make the difference between satisfying or exceeding current EC environmental-protection legislation. Results from trials 6d, g, h and i and 7d are shown in the figure because these were obtained for almost similar operating conditions, e.g. a freeboard temperature of approximately 1000°C. 300_

250

~

TF = I002"C

c~ 2OO

o_~o 150 3

o

~

~

TF:937"C

J

S

6

7

OXYfiEN (%)

Fig. 32. Effect of freeboard temperature T v on the N O ~ emission, under two-stage combustion conditions, when burning Gedling coal, at a bed temperature of 900"C without limestone present in the bed.

K. Findlay. S. D. Prohert

44

300

• I,,'T,• i 280

• ~

Bill

_~.. m" m

260

•

×

z

~•

•

A~,A •

. .,D¢~s "A •

24.0

220

SYMBOL

DISTANCEOF SECONDARY-AIR NOZZLES ABOVE SPARGE PIPES Ira)

•

--•20O

3.s

1.5 -

2.6

,Lo

d.s

OXYGEN (%)

Fig. 33.

Effect of secondary-air nozzle-height on the N O x emission's from the two-stage FB

when burning Gedling coal at a bed temperature of 900 C without limestone being present in the bed, under similar operating conditions. 4.3.3 SO,-emission concentrations hz general, these tended to increase with the freeboard temperature, over the range 937-984"C see Fig. 34. However, there was evidence in one trial of another reaction taking place when the freeboard temperature exceeded 9 8 4 C , again without the presence of limestone, possibly as a result of the increased reaction with the char at these high temperatures; this led to a re~h~ction in SO2-emission concentrations during section 6c. They fell from 1380 ppm during section 6b to around 1280 ppm during section 6c, at an 02in-the-flue concentration of 5 %. This occurred despite a 15:'C increase in the freeboard temperature to 999~'C. However, increasing the freeboard temperature usually affected adversely the sulphur retention at values above 9 5 0 C , so corroborating the views of others (Marshall & Melling, 1991). This high freeboard temperature continued through to section 6g, when the secondary air was injected through the lower nozzles, i.e. at a height of 1"5 m above the sparge pipes compared with 2"6 m previously during section

45

Limilin,g N O x and S O z emis'sions /J'om an FB ~omhu.~n,r

1500 F

i

I

X

TF

=

98L,'[

'

•

TF

:

937"[

140~

x

x

i

×x

I

X

•

•

X

X •

i

•

1300~

•

•

J i

OXYGEN(%) Fig. 34. Effect of freeboard temperature T~. on the SO, emission undeF t\~o-stage combustion conditions, when burning Gedling coal ui:hou: lime.~:om, in the bed. ~t a bed temperature of 900 C.

6c. Thus, whilst all the other operating conditions remained inwuiant, the effect of the secondary-air nozzle's height could be identified - see Fig. 35. The use of the lower secondary-air nozzles led to a marked reduction in the SO2-emission concentration of - 1 2 0 p p m at an (),-in-the-flue concentration of 4% without limestone in the bed. Use of the lower nozzles, i.e. at 1'5 m above the sparge pipes, would therefore be preferred as a means for 1300 x x x xX

x X

1200

x

X

x X

1100

:>,,,. X

1000 3

SECONDARY-AIR INLET 2.6m ABOVE SPARGE PIPES SECONDARY-AIR INLET 1.5m ABOVE SPARGE PIPES

L OXYGEN

(%I

Fig. 35. Effect of the secondary-air nozzle-height on the SO 2 emissions when burning Gedling coal, without limesnme, under similar operating conditions in both cases.

K. Findlay, S. D. Probert

46

reducing the S O 2 emissions. This reduction ensued due to prolonged residence times. Oxygen is required for the sulphation of the coal ash (or limestone if present) to be completed at a sufficiently high rate: the earlier the introduction of O2 into the reducing gases coming from the primarycombustion zone, the greater the ultimate sulphation of the coal ash (or limestone particles) achieved. 4.4 Effect of limestone addition 4.4.1 S O 2 emis'sions

The addition of limestone LG8 to the FB early in trial 1 led to a significant reduction in SO 2 emissions--see Fig. 36. When the limestone feed was interrupted, increased SO2 emissions ensued (see Fig. 37) as a direct result of the reduced concentration of unsulphated lime present in the FB. During the 'without limestone' sectionmsee Figs 36 and 37--there was some residual LG8 in the bed, even though no limestone was added during those sections.

300(

•

•

•

•

I~

IIItl I I B m

2506

• WI'I~i0UTLIMESTONE • WITH LIMESTONE

•I I

A

2~C

~iI

1500~

I 2

Fig.

36.

•

• • w-

IAA• 3 OXYGEN (%)

l

The effects of adding the limestone LG8 feed to the bed on the SO2 emissions.

L i m i t i n g N O x anti S O 2 enussions /i'om an F B {'ol#lhtt,slor

2500-

•

•

47

•

•

•

I

2000

g

g

l 1000

1

• WITHOUTLIHESTONE • WITHLIMESTONE

2

&i

3

•

•

•;•

OXYGEN ~%)

Fig. 37. SO2 emissions taken during test section l e {i.e. without limestone prescntl, during a period of interrupted limestone feed, are compared with SO, emissions taken during tesl sections l b. c and d after limestone LG8 had been added to the bed.

Measurements of the SO2-emission concentration were taken at tile beginning and end of trial 2. Although the operating conditions had been maintained relatively invariant, the SO2-emission concentration at the beginning of the trial, as seen in Fig. 38, was -,- 1700 ppm, compared with only ---1300ppm at the end of the trial (for the same O2-in-the-flue concentration). This was the result of limestone accumulation within the FB over the period of time elapsing between the two tests, and is a measure of how effective limestone addition to the FB can be, in reducing SO 2 emissions, when used on a continuous basis. When the hner limestone ( L G l l ) was added to the FB during trial 9, so producing a Ca:S ratio of 38:1, initially there was approximately a 32% reduction in the SO2-emission concentration (see Fig. 39). Comparison between emissions at the beginning and end of the trial show that a further estimated 25- 30% reduction in SO2-emission concentration was achieved as a result of limestone accumulation in the bed (see Fig. 40). Similarly, at the same primary-to-secondary-air ratio of --.307/70, the S O 2 emissions that occurred during trial 9, using LG11 limestone addition to give a Ca:S mole ratio of 30:1, were ~ 7 1 % less than in trial 8, i.e. without limestone present (see Fig. 41). However, the increased (by 35%) bed-depth was also a contributory factor. Any variations in the behavioural trends {e.g. see Figs

K. Findlay, S. D. Probert

48

e,i

. ,,.-,

zz c~c~ (Jl(~ 114

~A .xz

.o ~,

'44

>..

aim

e-,

44 t 4

E~ 44i 41

2~

e,.O ~.,

0

","

.=

,~

(14dd se) ~OS

8.o E

._~

Limiting NO~ and S02 emissions./?om an FB cmnhustor

49

700

650

WITHOUT LIMESTONE WITH LG11 LIMESTONE ADDED

o£ ~,5( •

•

•

•

• • %"

•

~0(

350 _ _ _ _ _ 50

• •

•

•

5'.5 OXYGEN

6'0 (%)

Fig. 39. Effect of adding the finer LOll limestone to the bed. Results are taken from the early part of trial 9 under oxidising conditions, before and after the finer LG 11 limestone was added, whilst the operating conditions were otherwise kept constant.

39 and 41 for two-stage and oxidising conditions, respectively) were due to changes in the oxygen concentration within the bed. Comparison of the results of trial 8 with those of trial 9 showed an even greater reduction in SO2-emission concentration, i.e. ~73%, at a slightly higher primary-to-secondary-air ratio of 37/63, again using a bed depth increased by 35% (see Fig. 42). This should be compared with the reduction in SO2-emission concentration of only ,-~59% during trial 9, when the coarser LG8 limestone was added under similar operating conditions (see Fig. 43) even when employing a higher Ca: S ratio of 38:1 (compared with a

K. Findlay, S. D. Probert

50

400

•

350

~

)3oo

x

•

/-WITH LITTLE LIMESTONE

\

x× " - - - - ~ , x

\

•

\

250

xx

x

•

~

• -

x

x-x-

/-WITH MORE LIMESTONE /AECUMULATED IN THE BED

m

~

mmm~• mm •

ram|

• •

200

x

•

2I

1J

•

3i

OXYfiEN (%)

Fig. 40. The effect of limestone accumulation on reducing the SO 2 emissions. Comparison of the SO 2 emissions, at the beginning and end of trial 9, shows a 25% reduction in the SO2 emissions as a result of the limestone accumulated in the bed.

30:1 Ca:S ratio when using the finer L G l l lime). The SOz-emission concentration was increased by ,-~82% when the coarse LG8 limestone was present, rather than the finer LG11 limestone (as for Fig. 44), although exact comparisons are not strictly possible because the bed heights differed somewhat. It can be concluded that limestone addition invariably led to a lowering of the SO2-emission concentration, the reduction depending on the type of limestone and the Ca:S mole ratio employed. During trials 1 and 2 when burning Maryport smalls, by utilising the coarse LG8 limestone to obtain a Ca:S mole ratio of 3"6:1, a 25% reduction in SO2-emission concentration was achieved. This should be compared with a 59% reduction in SO/emission concentration when burning SA duff, with the same LG8 limestone added to the bed to obtain a Ca: S mole ratio of 38:1 during trial 9. It follows, therefore, that an approximately 950% increase in Ca:S mole ratio, when burning SA duff during trial 9, led to a ~236% increase in the reduction of the SO2-emission concentration, compared with when Maryport smalls

Limiting NOx and SO, emissions ~?ore an FB comhustor

51

60C

50C

-.-,A-60(

•

TRIAL B, WITHOUT LIMESTONE TRIAL 9, WITH LGII LIMESTONE Ca : S : 30:1

A

r, 300

200

10' OXYGEN (%)

Fig. 41. As for Fig. 39 but under two stage conditions, the results are taken from section d of trial 9, with limestone LG 11 added and compared with those taken during section d of trial 8, without limestone, under similar operating conditions, but with a 35% greater bed-depth.

were burnt. This occurred despite the lower overall sulphur-content of the SA duff, compared with that for Maryport smalls, together with its greater ash-content, which would also lead to a significant reduction in the SO2emission concentrations observed. Thus, the reduced effect of increasing the Ca:S mole ratio was demonstrated. The addition of the finer limestone LG 11, employing a slightly reduced Ca:S ratio (of around 30:1) during section b of trial 9, led to a much greater reduction in SO2-emission concentration. Also, when using a Ca:S mole ratio of 30:1, a ~ 7 3 % reduction in SO: emissions was achieved, compared with ~ 6 0 % when using a Ca:S mole ratio of 38:1 during section a of trial 9.

K. Findlay, S. D. Probert

52

600

550

500

=E

•

TRIAL 8 - WITHOUT LIMESTONE

•

TRIAL 9 - WITH LG 11 LIMESTONE

200

•.

,;.,

•

•

• • im

•

"%

,mm

150

100 OXY6EN (%)

Fig. 42. Effectof the presence of limestone LGll in reducing the SO2 emissions. Results taken from section d of trial 8, without limestone, are compared with those taken from section b of trial 9, with limestone LG 11 added° using a slightly higher primary-to-secondary-air ratio of 37/63 (compared with 30/70 in Fig. 39) and a bed depth again increased by 35%. It is believed that this improved sulphur-retention by the bed is primarily due to the slightly higher value of the primary-to-secondary-air ratio and occurred irrespective o f the Ca:S ratio being somewhat smaller. This indicates that, at these high levels of the Ca:S mole ratio, the in-bed 0 2 concentration became a far more significant factor in reducing the SO2emission concentration than the Ca:S ratio. It would appear from these results, therefore, that the use of the finer limestone particles, e.g. as LG11, resulted in more substantial reductions in SO2 emissions. Although the bed depth was greater when L G l l limestone was present in the FBC, the primary-to-secondary-air ratio was slightly smaller at 37/63, compared with 40/60 when coarse LG8 limestone was present. In addition, the Ca:S ratio was 30:1 compared with 38:1 when results were taken during the period with LG8 limestone present. Because the reduction o f the bed depth from 715 to 400 m m (as during trial 6) only led

Limiting NOx and SOz emissions />'om an FB comhustor

53

600

500

SYMBOL

TRIAL

--A--

(},WITHOUTLIMESTONE 9,WITH Lfi8 LIMESTONE

z,o0

300

200

100

L OXYGEN (%)

Fig. 43. Effect of adding the coarser LG8 limestone to the bed. Results taken from section c of trial 8, without limestone, are compared with those taken from section d of trial 9, with limestone LG8 added, using similar operating conditions.

to a 4% (or 50 ppm) increase in SO2-emission concentration, this would not account for the difference in SO2 emissions when using the LG8 and LGI1 limestones. With the increase in SO2 emissions being ~ 1 2 0 p p m (or 70%) greater, when the coarser LG8 limestone rather than the finer L G l l limestone was present in the bed, the improved sulphur-retention achieved by the latter is clearly demonstrated. The effect of limestone particle size on sulphur-retention has been observed by others (Lyngfelt, A. & Leckner, B., pers. comm., 1989; Ford & Sage, 1991). By reducing the mean particle size from 2000 to 500~m, the sulphation increased from 8% to 22%, although this fell rapidly to 3% if a mean particle size of 100 ttm was employed due to elutriation of the particles

54

K. Findlay, S. D. Probert tO(]

300

•

•

•

_ •

U•

m

B-ram • • B •

•

l ;;u,,

200

A•

SYMBOL

100 i

• •

~,

•

TYPE OF LIMESTONE ADDED LG8 LGII

0 OXYGEN (%1

Fig. 44. The increase in SO 2 emission experienced when adding the coarser LG8 limestone, rather than LGI 1, to the FB. Results taken during section d of trial 9 (with limestone LG8 added) are compared with those taken during section b of trial 9 (with limestone LG 1! added), using similar operating conditions, but with a greater bed-depth and a primary-to-secondaryair ratio of 50/50 compared with 40/60 for section d.

and hence their reduced residence-times in the bed (Ford & Sage, 1991). Results from a solids-circulating FBC conflict with these findings (Marshall & Melling, 1991), the coarser limestone particles then providing better sulphur-retention than the fine particles.

4.4.2 NOx emissions The addition of limestone LG8 to the FB led to an increase in the NO Remission concentration when Maryport smalls were burnt during trial 1 (see Fig. 45). The emission concentration increased from 30-40ppm to 200250ppm, as a result of limestone being added to the FB: even when the

Limitin~ NO.~ and S O z emissions /?om an FB comhustm

55

26C dk=

24C

•

•

../I x

X~mxX x

• ~/'~x~

22O ~'=

xm

20(

/

18{

/

x

x

16(

A

~. 12c

----X----

WITH LIMESTONE LG 8 PRESENT IN THE BED WITH TRAEES OF LIMESTONE LG 8, ie WHEN THE LIMESTONE FEED HAD BEEN INTERRUPTED WITHOUT LIMESTONE IN THE BED

10(

80 60

40

• .;~"~':"-,;'*'~'"'•

2O

00 ~¥GEN (%)

Fig. 45. Effectof traces of limestone in the bed. NOxemissionstaken during test section le, during a period of interrupted limestone feed, are compared with the NO~ emissions taken during test sections l b, c and d, after limestone LG8 had been added, and NO~ emissions taken during test section la, without limestone added. limestone feed was interrupted, the NO x emissions remained at approximately the same relatively high levels. If the other operating conditions are maintained invariant, in the presence of limestone, then the relationship between NO x emission and the O2-in-theflue concentration is well defined. As Fig. 46 shows, a single line could be drawn through both sets of points, taken during the beginning-section 2a and the end-section 2b of the trial, so indicating that the NOx-emission concentration was not affected by the build-up of limestone that occurred

56

K. Findlay, S. D. Probert 500-

ION 2a • SECTION 2b

=m•. ~ • •

2511_

20O 3

I

I

t+

5

i

i

6

Bi

7

C0(YfiENf/o) Fig. 46. The accumulation o f limestone in the bed during trial 2 is shown to have no discernible effect upon the N O x emissions. Values o f the NOx emissions taken during the later

part of the trial (section 2b) follow a similar relationship with respect to the Oz-in-the-flue concentration to those taken at the beginning of the trial (i.e. during section 2a).

220 •

i

x

X X

200

•

x~x "x x,~xx

X

~,

! •X

•

. •

i

t.,.... =,, ,

•

,

=•

••

,,

X

x o z

x

WITH LIMESTONELOll

•

WITHOUTLIMESTONE

180

160

I

I

55

6

615

OXYGEN (%1 Fig. 47. Effect of adding a trace amount of the finer LG11 limestone to the bed. Results taken during the early part of trial 9, before and after limestone LG11 had been added to the bed, show no significant changes in the NO Xemissions as a result of the limestone addition, whilst operating conditions were otherwise kept constant.

2/4 III

lI

It-

220

=~--iI~.~'. 200

E 180 X X z

160

I

.x ~

X x~X I

WITH LIMESTONE (TRIAL 9)

"x~ X X XX

--x--

WITHOUT LIMESTONE (TRIAL 8)

1/-4

120

3'.s

h

..........

. . . . .

~s

OXYGEN (%)

Fig. 48. Effect of adding the finer LG 11 limestone to the bed. Results taken from section d of trial 8, without limestone present, are compared with those taken from section a of trial 9, with limestone LGll added to the bed, under similar operating conditions but for a greater bed-depth.

18C

J.

TRIAL 8 : WITHOUT LIMESTONE

--m--

TRIAL 9 : WITH L611 LIMESTONE

E & g,

i

114

•

i

//

II

/

z

/ / /

IJ

120 ,t

100

/11

/

II • •

BI •

3'.s

~ OXY6EN

d.s

(%)

Fig. 49. Effect of L G l l limestone on reducing the NO x emissions. Results taken from section d of trial 8 (without limestone) are compared with those taken from section c of trial 9 (with limestone LG11 added) using similar bed-depths, but higher primary-to-secondary-air ratios of around 50/50, compared with 30/70 in trial 8.

K. Findlay, S. D. Probert

58

within the fluidised bed. The N O x emission was clearly not dependent on the exact limestone concentration provided sufficient was present, but only on its presence. Thus, the limestone is clearly acting as a catalyst in this reaction! Interestingly, when SA duffwas burnt during trial 9, the initial addition of a small a m o u n t of limestone L G l l to the FB led to no apparent increase in the N O X emissions: a primary-to-secondary-air ratio of 30/70 was employed--see Fig. 47. Using a similar primary-to-secondary-air ratio, but a shallower bed (as during trial 8), it was observed that a small increase in NO xemission concentration occurred when a significant a m o u n t of limestone LG11 was added to the bed (see Fig. 48). The results for the N O Xemissions during section c of trial 9, when the bed depth was reduced to 400 mm, were compared with those obtained during section 9 of trial 8, for the same bed-depth. There was a significant reduction in NOx-emissions concentration when LG11 limestone was added, using a 240

220

It', ; " ~,, ,m,m,,,,,,~,, " " •

•

m, ,,

imm

•

200

IBO

160 Z

140

12(

10(

8(

I

I

3"5

4

I

45

CCqltN(%) 50. Effect of adding the coarser LG8 limestone rather than LG 11 limestone to the bed. Results taken from section 3 of trial 9 (with limestone LG8 added) are compared with those taken during section 2 of trial 9 (with limestone L G l l added), using similar operating conditions but a greater bed-depth and a primary-to-secondary-air ratio of 50/50, compared with 40/60 during section 3. Fig.

Limiting NOx and SO 2 emissions/i'om an FB combustor

59

Ca: S ratio of 30:1, despite the primary-to-secondary-air ratio being higher (at 50/50)--see Fig. 49. Knowing that the NO Xemission rose as the primaryto-secondary-air ratio was raised, even greater reductions in the NO X emission may have been achieved had the primary-to-secondary-air ratio remained constant at around 30/70, so giving results similar to those obtained with the small test-rig. When LG8 limestone was added to the bed during section 3 of trial 9, this led to a net increase in NO Xemissions, compared with that for section 2 when LG11 was present--see Fig. 50. This occurred despite the bed depth during section 2 of the trial being, at 400 ram, double that when the results were being taken during section 3. An ~ 83% increase in NOx emissions occurred when the LG8 limestone was added even though the shallower bed ( ~ 200 mm depth) would have tended to reduce the NO, emissions, as would a slightly lower primary-to-secondary-air ratio (at 40/60 rather than 50/50). Thus, the actual increase in NO,-emission concentration would have been even greater had the bed depths been similar in both cases. When the results of the NO, 300

200

XXx)O(X

~.~

~u~-"

%

x

100 I

--m--

LGB LIMESTONE 1

--x

LG11 LIMESTONE

I

I

OXYGEN (%)

Fig. 51. Effect of the presence of two different limestone types in the bed on the NO, emissions when using similar values of primary-to-secondary-air ratio. Results taken from section 3 of trial 9 (with limestone LG8 added) are compared with those taken during section 2 of trial 9 (with limestone L G l l added), both using a primary-to-secondary-air ratio of 40/60. The NO~ emissions were reduced slightly in the case of the finer limestone, LGI 1, because of its increased surface area/volume ratio and hence it could be considered to be the more reactive of the two limestones. During the period of LG 11 limestone addition, the bed depth became more than three times greater.

K. Findlay, S. D. Probert

60

emissions, taken during the period with LG8 limestone added (i.e. during section 3 of trial 9), are compared with results taken during a period with LG11 limestone added in section 1, i.e. when the bed depth was more than three times greater (i.e. 700-715 mm compared with 200 mm in section 3), the apparent increase in NO,-emission concentration observed as a result of adding the LG8 lime is ~ 7 0 p p m (see Fig. 51). Comparison of the NO Xemissions obtained during section 3 of trial 9, with LG8 limestone present in the bed, with the results from section 4 of trial 8, without limestone added, again showed an increase in the NO, emissions when LG8 limestone was added to the bed, as was observed during trial 1 (see Fig. 52). This occurred despite the bed depth during trial 8 being much greater than that during the period of LG8 limestone addition: in itself, this would have led to increased NO, emissions during trial 8. Had the bed depths been similar, then the increase in the NO, emissions observed when LG8 limestone was added (as opposed to no limestone addition) to the bed would have been even greater. 300

250

• °=/a ,?,

200 XX

IE

150

X~

x×

z

10(

•

L5 8 LIMESTONEADDED

- - x - - N O LIMESTONE ADDED

3

L

OXYGEN (%) Fig. 52. Effect of LG8 limestone on the NO, emissions. Results taken from section d of trial 8 (without limestone present) are compared with those taken from section 3 of trial 9 (with limestone LO8 added) under similar operating conditions but for a smaller bed-depth.

Limiting NOx and SO 2 emissions [rom an FB combustor

61

Variations in the NO~ emissions, which occurred throughout trials 8 and 9, could have been due to changes in the values of the other operating variables, such as bed depth and primary-to-secondary-air ratio. The NO X emissions are known to increase with the bed's depth. The average depth of the bed during section 1 of trial 9 was much greater than those during sections 2, 3 and 4. Thus, the apparent increase in NOx-emissions concentration observed when limestone was added during section 1 of trial 9, compared with the NOx-emission concentration observed without limestone addition during trial 8, was partly the result of the increased beddepth. The results shown in Fig. 49 were taken when the bed depths were similar (,~400mm) and therefore give a reasonable indication of the effect of limestone on the NO x emissions. The 30% reduction in NO x emissions shown is for results obtained under similar operating conditions to those achieved on the small test-rig, and agrees with expectations: the limestone acted either as a catalyst for enhancing NOx-reducing reactions or as an NO~ absorbant. The increase in NO x emissions, following the addition of LG8 limestone, relative to the emissions with LG 11 lime present could be the result of using a different limestone type, but may also be due to an increased freeboardtemperature or different bed-depth and Ca:S mole ratio. Addition of the coarser LG8 limestone increased the bed depth from 200 to 400 mm during the period that the NO x measurements were taken, despite the limestone feed-rate being greatly reduced, so giving a Ca: S mole ratio which was as low as 5"75:1 (compared with 35:1 previously). However, even then the bed depth of 400 mm was only half that during the period of limestone LG 11 addition of section 1. Thus, this smaller bed-depth would have tended to reduce the NO x emissions rather than increase them. Also, it was concluded from trial 2 that the further accumulation of limestone (and hence increased Ca: S mole ratio) within the FB led to no noticeable increases in NO x emissions. The freeboard temperatures were reduced dramatically during section 2 of trial 9, i.e. when a primary-to-secondary-air ratio of 50/50 was employed. The freeboard temperatures during sections 1 and 3 of trial 9 were almost identical, because the air ratios were almost similar at 37/63 and 40/60, respectively. Thus, direct comparisons could be made between these two sets of results in order to ascertain the effects of using the different limestone types. Comparison between the results, taken during sections 1 and 3 of trial 9--see Fig. 51--indicates that an increase in NO x emissions occurred when LG8 limestone was added instead of LG11 limestone, even when the bed depth was about half that which occurred when LG11 limestone was added to the bed in section 1. Similar discrepancies concerning the effect of limestone on NO x emissions

62

K. Findlay, S. D. Probert

have been noted elsewhere. Lyngfelt and Leckner (pers. comm., 1989) noted that in circulating FBC boilers NO x emissions would increase when limestone was present, under oxidising conditions. This was thought to be due to the large amount of free calcium-oxide (CaO) which is active as a catalyst for the oxidation of the nitrogen compounds H C N and NH 3 to form NO. Others have noted either a reduction in NOx emissions when limestone was added (Marshall & Melling, 1991) or no significant effect (Vickers & Milner, 1991). The presence of limestone also resulted in a reduction in NzO emissions, the amount of reduction being proportional to the quantity of limestone present, particularly at temperatures below 800°C (Bolting et aL, 1991). However, NzO reduction by limestone as observed by others (Gourichon et aL, 1991) showed no correlation between the N 2 0 reduction and CaO concentration or Ca:S ratio. Although the surface of limestonederived compounds were thought to be involved in the reduction o f N z O , no explanation was offered as to why the Ca:S ratio had no effect. The influence of limestone quality was more evident in the case of NO~ emissions. These were found to increase rapidly when the limestone desulphurisation capability exceeded 80%. This was thought to be the result of competition between NO~ and SO2 for catalytic sites on the lime. Hence, an increase in sulphur adsorption by the limestone could only be achieved at the cost of higher NOx-emissions (Gourichon et aL, 1991). The presence of certain trace elements, e.g. Fe and Fe203, in the limestone or coal ash could also lead to NO~ reductions under sub-stoichiometric fb conditions, by catalysing its reaction with CO to form inert nitrogen and CO 2, i.e. NO + CO ~ ½N 2 + C O 2 This reaction increases with bed temperature. Tests have shown that the bed's solids act as a catalyst in this reaction. Thus, in the case of coal ash or limestone, it would continue to function as a SO2 absorbant without being consumed fully (Allen & Hayhurst, 1991). At temperatures above 685°C, the following reactions take place: 2Fe + 3 C O 2 2Fe + 3NO ~ Fe203 + 3/2N 2

3CO + Fe20 3 ~

Thus, the composition of the limestone, which is added to the bed, can also influence the level of NOx reduction achieved. 4.4.3 S O 3 emissions

Irrespective of the Oz-in-the-flue concentration, a reduction in the SO 3concentration emission ensued when limestone LG8 was added to the FB. Using a Ca:S ratio of approximately 3.6:1 and an Oz-in-the-flue concentration of 3%, the SO3-emission concentration decreased from

Limiting NO,, and SO z emissions .from an FB combustor

63

10. 9-5. 9, 85_ 57.5. 7.

g

IWITHOUT LIMESTONE =,WITH TRAr..ES OF LIME PRESENT • WITH LIMESTONE ADDED

" 6.5. ••=` 0

•0 •

All

5"5.

• 0 o

5.

ml

...;,:.." •

~,-5.

im I

~5. 3 OXYSEN (%)

Fig. 53. Effect of limestone LG8 on reducing the SO 3 emissions. Results taken from trial 1 are compared before and after limestone LG8 was added to the bed. During time-section 1e, during a period of interrupted LG8 feed, there would have been traces of limestone still present in the bed.

approximately 9"3 to 4-6 ppm when limestone was added to the bed (see Fig. 53). Even the presence of traces of limestone in the FB during section le of trial 1 (i.e. when the limestone feed had been interrupted) still led to a marked reduction in SO 3 emissions, although to a lesser extent: at an O/-in-the-flue concentration o f 3%, this gave a reduction in the SO3-emission concentration o f ~ 3"5 ppm. 4.5 Effects of primary-to-secondary-air ratio and two-stage combustion

4.5.1 Under the condition o f high excess oxygen-in-the-flue concentration The primary-to-secondary-air ratio had a direct influence on the bed's 02 concentration and hence on the SO2 and N O x emissions from the FB. The

K. Findla>, S. D. Probert

64

NOx-emission concentrations, recorded during trial 7, were reduced by ,-~40% as a result of using a primary-to-secondary-air ratio of 70/30 rather than 32.5/67-5 (see Fig. 54). However, this trend is the opposite of that observed with the small test-rig during runs 8, 10 and 13, when the NO X emissions were reduced significantly for the primary-to-secondary-air ratio of 30/70, compared with those for an air ratio of 70/30. The difference may have arisen due to the increased O2-in-the-flue concentration, which occurred during sections 7b-d of the test (i.e. 5% compared with only 3"7% during section 7a). Under such conditions, the recycled flue-gas would have had a higher O2-concentration and thus the 02 levels within the FB would have been much greater, so leading to larger NO x emissions (and reduced freeboard-temperatures as observed). Because the coal feed-rate remained almost constant throughout the trial, it was the increased air/fuel ratio (see Fig. 55), due to the greater secondary-air flow, which led to the increased 02in-the-flue concentration. Therefore, although the primary-to-secondary-air ratio was reduced, the increased O2-content of the recycled gas led to an increase in the bed's 02 concentration, and thus to higher NOx-emission concentrations. 300

II~-m 250

200

z

10(

SYMBOL - -I-

- -

I.

Fig.54.

PRIMARY I SECONDARYAIR RATIO 32.5

:

67.5

70

:

30

,+

i

6

OXYSEN (%) Effect o f the p r i m a r y - t o - s e c o n d a r y - a i r ratio o n the N O x emissions d u r i n g trial 7.

Limiting NO,, and S O 2 emissions lrom an FB combustor

Q.

•

-1-

• NOx'N FLUECP , + AIR / FUEL RATIO (% STOICHIOMETRIC)

~1,'1~

÷

. ~

.,4",

• +

,,-,

o

65

÷

+

120

•.÷÷ * . ÷ t , ÷ t : ,~ +÷÷,,,. ÷ .~:a~*" ÷Z ~.. + + , ~.÷+ .~l~.÷÷g. 4 * I

.._1

+ +

.l.i. ÷

+ 4+

4

4

",~+÷

÷

÷

+

+

Fig. 55.

HGG

'

÷÷

~'~÷÷+*"¢" -÷

+

+ ÷

÷

901 ,

# *+**+

00.00

Trials

'

~

I:Z.00

'

'

NOON 18-03-86 i" SECTION7a Programme---trial

00 .00

'

I

7. V a r i a t i o n

emissions)

'

12 l,00

'

'

00 .00

'

~

NOON 19-03- 66 I SECTIONS 7b-7d I of

the

air/fuel

ratio

(and

NO,

with time.

The importance of the Oz-in-the-flue concentration, and hence the recycled-gas Oz concentration, in maintaining two-stage combustion conditions, has been observed. A high Oz-in-the-flue concentration of 5-1%, even using a low primary-to-secondary-air ratio of around 32"5/67.5, led to unacceptably high 02 concentrations within the FB, followed by stoichiometric combustion and relatively low freeboard-temperatures. Only by reducing the secondary-air flow could the Oz-in-the-flue concentration be reduced to acceptable levels, in order to produce substoichiometric conditions within the FB, and two-stage combustion. Under these conditions, and using a primary-to-secondary-air ratio of 30/70, the freeboard temperature actually increased to give an uplift of 150°C (compared with 75°C during section 7a for a primary-to-secondary-air ratio of 70/30), so verifying the conclusion that two-stage combustion was taking place. 4.5.2 Under the condition of high freeboard-temperature Under the oxidising conditions experienced at the beginning of trial 7, the NO x emissions were much higher than those observed during test section 7a, under two-stage conditions, with a primary-to-secondary-air ratio of 70/30, or during test sections 7b and c, when the primary-to-secondary-air ratio was approximately 30/70 (see Fig. 56). A plot of NO, emissions throughout trial 7 showed an increase in NOx-emission concentrations towards the end of the trial (see Fig. 57), despite a lower primary-to-secondary-air ratio and reduced Oz-in-the-flue concentration arising from a lower air/fuel ratio (see Fig. 55). However, this reversal of normal trends was caused by the higher freeboard-temperatures during this period (see Fig. 57), resulting from the increased combustion occurring in the freeboard as a consequence of low

K. Findlay, S. D. Probert

66

°

" /

°

/

UNDER OXIDISING

coNDITIONS

400 -

35( c~ z

30( UNDER

TWO- STAGE

.Ill ~ lll~FL~/ji.//' 25C--

20C

4

~

COMBUSTION CONDITIONS

~

i 5

_

[ 6

J__ 7

I B

.___l

9

OXYGEN(%) Fig. 56. Effect of two-stage combustion on reducing the NO x emissions. Results taken during trial 7, under oxidising conditions, are compared with those taken using a primary-tosecondary-air ratio of 30/70.

O2-concentrations in the bed. This confirms the overriding effect of the high freeboard-temperature leading to increased NO x emissions, despite the lower values for the Oz-in-the-flue concentration and the bed's 02concentration (resulting from the smaller primary-to-secondary-air ratio). 4.5.3 S02 reduction under two-stage combustion

Interestingly, the 802 emission was also reduced when two-stage operating conditions were employed, irrespective of whether a primary-to-secondaryair ratio of 30/70 (see Fig. 58) or 70/30 (see Fig. 59) was employed. However, the emissions of SOE obtained when using the higher primary-to-secondaryair ratio of 70/30 were much less than with an air ratio of 30/70. These results corroborate conclusions from previous investigations (Henttonen et al., 1991; Marshall & Melling, 1991). Under oxidising conditions, the SOz-emission concentration was between

Limiting NOx and SO 2 emissions./rom an FB combustor

67 300

20C

280

FREEBOARD TEMPERATURE- BED TEMPERATURE

10(

260 2t,,O _

.r

"•

220 200

..~

,,j'~

•

~,. - ~ f

. .

180 160 1~0

120 _Z_

~e

100

NOx IN FLUE GAS

%

N -lO(

80

n

i60 tO 20

k.÷. i

Fig. 57.

x

0().00

12100 ' NOON 18-03-86

'

00100

'

'

12'.00 ' NOON 19-03-86

'

0d00 --'

i0

HGG Trials Programme--trial 7. The variation of the NO x emissions with the freeboard's temperature-uplift.

1(~

15(1(

1to(

a.

130( X

X ~

120(

110(

X X

• x

OXIOISING PRIMARY I SECONDARYAIR RATIO 30170

100( OXYGEN (%) F i g . 58.

Effect of two-stage combustion

on reducing the SO: emissions. Results taken from

the beginning of trial 7, under oxidising conditions, are compared with those taken during test section 7b, using a primary-to-secondary-air ratio of 30/70.

68

K. Findlay, S. D. Probert

151X1

1400

120{

•

0XIDISING

x

PRIMARY/ ~CONDARYAIR RATIO 70130

xX X Xx

100~ OXYGEN (%)

Fig. 59. The effect of two-stage combustion on reducing the SO 2 emissions. Results taken from the beginning of trial 7, under oxidising conditions, are compared with those taken during time test-section 7a, using a primary-to-secondary-air ratio of 70/30.

600

m m~mkm•mmm)m~,,~, ml

• •

m~,,,,,,~. • 55(

50(

mm

mmm

m =m

m•--mm m ~ _ _ • mmmm: • mm mm

3',s

• mmm

h

Ls

OXYGEN I%1

Fig. 60. Compilation of data for various values of the primary-to-secondary-air ratio, when burning SA duffduring trial 8, without limestone in the bed, which was at a temperature of 900°C. The overall trend towards reduced SO2 emissions, as the O2-in-the-flue concentration was increased, is shown.

Limiting N O x and S O z emissions./rom an FB combustor

69

1350 and 1610 p p m when using an O2-in-the-flue concentration of between 5% and 9%: it fell to between 1090 and 1210ppm when two-stage combustion was employed (see Fig. 59). Although the use of a primary-tosecondary-air ratio of 30/70 at an 02-in-the-flue concentration of around 52/0 did lead to a slight increase in SO z emissions (to between 1200 and 1320 ppm) compared with when an air ratio of 70/30 was used (see Figs 58 and 59), this still corresponded to a reduction in SO2-emission levels, compared with when oxidising conditions were employed. The increase could have been partly the result of a lower Oz-concentration in the FB, coupled with higher freeboard-temperatures (which occurred as two-stage 260

/

2t~0

s /

¢/Q S ,o

/

•

sO

/ S

•

22C

/ SO

¢ / I

20(

E 18C

z

".

/

5"

/ PRIMARY / SECONDARYAIR RATIO

I/.0

70 : 30 40 : 60 30:70

120

100

3.S OXYGEN

Fig. 61.

(%}

T h e effect of p r i m a r y - t o - s e c o n d a r y - a i r ratio on the NO~ emissions. The results are t a k e n from sections d, e a n d f o f trial 8 when burning SA duff~

K. Findlay, S. D. Probert

70

combustion ensued). Nevertheless, the overall effect of two-stage combustion was to reduce the SO 2 emissions during trial 7, when burning Gedling smalls.

4.5.4 Effect of coal quahty When SA duff was burnt, during trial 8, an alteration of the primary-tosecondary-air ratio had no major effect on the magnitude of the SO 2emission concentration. There was only a slight increase in the SO 2 emission during the early part of this test, when the oxygen-in-the-flue concentration and primary-to-secondary-air ratio were both low, i.e. at less than ~3.5% and 30/70, respectively, and the freeboard temperatures were high. When the SO2 concentrations for primary-to-secondary-air ratios of 40/60 and 70/30 were compared, only small differences (within the experimental data scatter) were discernible. Figure 60 shows the overall plot of all the measured values: the SO2-emission concentration can be seen to fall as the O2-in-the-flue concentration increased. When burning SA duff, there was a significant quantitative increase in the NOx-emission concentration as the primary-to-secondary-air ratio was increased from 30/70 to 70/30 (see Fig. 61). When burning Blanzy coal, a qualitatively similar trend to that with SA duff was revealed (see Fig. 62). 280

x;,,

260

X

x

•o S

~

x

•

~s "~ X X~ (3w

. .*"

,-~ .v

•

i

•

^

X

...;C

. • m ~i .- ~

•

~

~ll~'~

m.JISm~I= ll~i=-m

==-

240

Z

iI

220

• •

r;

~I~Iwl

ill

-- - x - -

iI • •

-

AIR

•

3.5

RATIO

70/30

PRIMARY/ SECONDARY-

AIR RATIO 50/50

=

200

PRIMARY/SECONDARY-

I

t,

I

~..s

I

|

I

s

5.5

6

OXYGEN (%)

Fig. 62.

Effect o f p r i m a r y - t o - s e c o n d a r y - a i r ratio o n the N O x emissions, when burning

Blanzy coal.

Limiting NOx and

SO 2

emissions from an FB combustor

71

4.5.5 Effects of bed depth and &-bed 02 concentration During trial 9, a fall in SOz emissions occurred as the primary-to-secondaryair ratio was reduced from 50/50 to 37/63; however, these results were obtained during different parts of the test and so under different conditions. Thus, this reduction was due in part to an increased limestone concentration in the bed (i.e. 38:1 at 37/63 air ratio as opposed to 27.5:1 at 50/50 air ratio) coupled with an increase of 315 m m in bed depth (to 715 mm, compared with 400 m m when a primary-to-secondary-air ratio of 50/50 was used). Thus, a generalised conclusion could not be obtained from this set of data. In a more controlled experiment, reducing the primary-to-secondary-air ratio from 50/50 to 30/70 (following the initial introduction of limestone at the beginning of trial 9) led to almost a doubling of the SO2-emission concentration--see Fig. 63. The different trends exhibited by these two sets of results suggest that the oxygen concentration only became a limiting factor for the SO z formation at the primary-to-secondary-air ratio of 30/70.

~,50

X X ( xX

X X X

xxX~Xx

X x

XX

350

30C 0 o

25C 0

00~

0 0000 0 0

20(

o 0

SYMBOL

PRIMARY : SECONDARYAIR RATIO

30 : 70 50 : 50

15(

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Ls

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Fig. 63. Effect of the primary-to-secondary-air ratio on the SO2 emissions. Results taken at the beginning of trial 9 (following limestone addition), using an air ratio of 30/70, are compared with those taken during test section 7b, using a primary-to-secondary-air ratio of

50/50.

K. Findlay, S. D. Probert

72

Results from trial 7 appeared to show that increasing the primary-tosecondary-air ratio from 37/63 to 50/50 led to a significant fall (by approximately 30%) in the NOx-emission concentration. However, this is contrary to the general behaviour and again was due to the effect of the greater bed-depth, as encountered for the lower values of the primary-tosecondary-air ratio during section 1 of trial 9.

4.5.6 Under the condition of constant bed-depth Trial 12 was carried out at a constant depth of bed (without limestone addition) and an invariant bed temperature of around 907°C (except during section 12i when the average bed-temperature was 878°C). The fluidising velocity and recycled-gas flow were maintained invariant, so allowing the effect of the primary-to-secondary-air ratio to be determined. When the results for trial 12 (sections a, h, c and e), all carried out using a primary-tosecondary-air ratio of 30/70, were plotted (see Fig. 64), the data were very scattered, the SO2-emission concentration being almost independent of the Oz-in-the-flue concentration. The results for trial 12 (sections b, d, g and i), all carried out using a primary-to-secondary-air ratio of 70/30 (see Fig. 65), indicated that the SO2-emission concentration fell as the O2-in-the-flue concentration increased from just above zero to 4.5%. Under the conditions, the presence of excess O 2 within the FB favoured the sulphation of the coal ash, by the SO 2 gases present, to form calcium and ferric sulphates. Comparison of the SO z emissions, for the two values of primary-tosecondary-air ratio adopted, shows that the lower the air ratio, the larger the

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Fig. 64. The effect of excess O2-in-the-flue concentration on the SO2 emission. Results are plotted from test sections 12a, h, c and e, all carried out using a primary-to-secondary-airratio of 30/70 and under similar operating conditions,

Limiting NOx and SO, emissions .[t'om a FB combustor

73

1

1600

..-

.

, I

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Fig. 65. Effect of the O2-in-the-flue concentration on the SOz emission. The results, plotted from test sections 12b, d, g and i, all carried out using a primary-to-secondary-air ratio of 70/30 and under similar operating conditions, show a decrease in S02 emissions as the 02-inthe-flue concentration was increased.

SO2-emission concentration. This confirms that the use of higher in-bed 0 2 concentrations did lead to reduced SO2-emissions, as observed during trial 7 and during tests on the small FBC test-rig. Comparison of the SO2 emissions for the air ratios of 70/30 and 40/60 (see Fig. 66) also indicates that greater SOz emissions occurred at the lower air ratio.

4.5.7 Consisteno, of results Results gathered during test sections 12a, h, c and e, obtained when using a primary-to-secondary-air ratio of 30/70, compared with those of trial 7b and c, also using a primary-to-secondary-air ratio of 30/70, are shown in Fig. 67. Despite the large scatter, qualitatively similar trends can be discerned in the data. The magnitudes of the SO2-emission concentrations were consistently ~50 ppm higher than those obtained during trial 7: this slight difference probably arose because of the change of bed depth. Comparison of the SOz results for test sections 12b, d, g and i with those obtained for test section 7a, all at a primary-to-secondary-air ratio of 70/30, revealed an even closer agreement (see Fig. 68).

4.5.8 Summary The consistency of the trends leads us to conclude that observations taken with the small test-rig provide a good indication of the effect of the primary-

74

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K. Findlay, S. D. Probert 1600

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Fig. 68. The consistency of results obtained during different combustion trials. Results of SO 2 emissions obtained during test sections b, d, g and i of trial 12 are compared with results taken during test section 7a of trial 7, all using a primary-to-secondary-air ratio of 70/30 under similar operating conditions.

to-secondary-air ratio on the SO 2- and NOx-emission concentrations, as observed with the large test-rig, the other variables being kept constant. Because of the effects of varying the bed depth, the Ca:S ratio and the freeboard temperature, the results taken when burning SA duffduring trial 9 did not provide an indication of the effect of changing the primary-tosecondary-air ratio alone, although the results obtained during trial 8 proved to be more conclusive. 4.6 Effects of the oxygen-in-the-flue concentration and the freeboard temperature

4.6.1 S02 emissions There was an overall reduction in the SO2 emissions as the O2-in-the-flue concentration increased under the oxidising conditions appertaining during trial 7 (see Fig. 69). Qualitatively this type of relationship between the two variables ensued even under two-stage combustion conditions. However, increased freeboard-temperatures, which occurred towards the end of the

77

Limiting N O x and S 0 2 emissions.[rom an FB combustor

16~ i

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1~

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Fig. 69. Time-plot showing the variation of the SO z emission with the 02-in-the-flue concentration during the early part of trial 7 (i.e. between l and 14 of the 5-min time-intervals following the commencement of the trial) under oxidising conditions and with the use of recycled gas for bed attemperation.

trial (see Fig. 70), tended to obscure this trend. Nevertheless, when SA duff was burnt during trial 8, the high freeboard-temperatures experienced at the low primary-to-secondary-air ratios of 30/70 and 40/60 did not affect significantly the decrease in SO 2 emissions as the O2-in-the-flue concentration increased. When all the measured values for the SO2-emission concentration obtained during trial 8, whilst burning SA duff, were plotted on a single graph (e.g. see Fig. 60), the overriding trend was for a decrease in SO2-emission concentration as the O2-in-the-flue concentration increased. Nevertheless, this trend was n o t observed when SA duffwas burnt, during trial 9, under conditions of low primary-to-secondary-air ratio (~30/70). However, when the primary-to-secondary-air ratio was increased to 40/60, once again a reduction in SO2 emissions occurred as the O2-in-the-flue concentration was increased (see Fig. 71). Results taken during trial 12, with no limestone added to the FB, also revealed a reversal of the general trend during certain sections of the test. Whereas sections 12a and e (using a primary-to-secondary-air ratio of 30/70), and sections 12b, d, gi and i (using an air ratio of 70/30), all exhibited reductions in the SOl emissions as the O2-in-the-flue increased, sections 12c and h (using an air ratio of 30/70) and section 12f (at an air ratio of 40/60) both showed slightly opposite trends. It is concluded that the general trend is for reduced SO2 emissions to ensue as the O~-in-the-

78

K. Findlay, S. D. Probert 2iX 15C

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H G G TriaDs Programme--trial 7. The variation of freeboard's temperature-uplift with time.

181111

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Fig. 71. The variation of the SO 2 emission with excess O2-in-the-flue concentrations when burning SA duff under conditions of two-stage combustion, with the use of recycled exhaustgas for bed attemperation, and a primary-to-secondary-air ratio of 40/60.

79

Limiting NO, and SO 2 emis'sions /rom an FB comhustor

flue concentration was increased. However, at low primary-to-secondary-air ratios, the presence of only low concentrations of 02 in the bed can sometimes lead to a reversal of this trend. Under these conditions, 02 can become a limiting factor for SO2 formation, so resulting in an increase in the SO2 emission as the O2-in-the-flue concentration rose. 4.6.2 S O 3 emissions

Maryport smalls, burnt during trial 1, exhibited an increase in SO3 emissions as the Oz-in-the-flue concentration rose. This occurred regardless of whether limestone was, or was not, present in the bed. The close correlation between the two variables is to be expected, because excess O= is required tbr SO3 to be formed from S02 by the reaction S O 2 -1- 1()2 ~

SO 3

The reaction also requires the system to be at high temperatures, of 900 C or above, for it to proceed at a significant rate. However, other factors, such as limestone addition to the bed, do have greater effects on the SO3-emission concentration than in the O2-in-the-flue concentration. For example, see Fig. 72, where a sudden increase in the SO 3 emission occurred, despite a reduction in O2-in-the-flue concentration (e.g. intervals 24-26); this increase was due to an interruption in the limestone feed. 4.6.3 NO.,. emissions

Without exception, all the pertinent measurements taken during trials 1 and 2 showed an increased emission of N O x as the oxygen-in-the-flue concentration increased. This was true regardless of whether the FB was

l

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Fig. 72. Variations of the SO3 emission and O2-m-the-flue concentration when burning Maryport smalls, under two-stage combustion conditions, during the latter part of trial 1, when limestone LG8 was still being added intermittently to the FB.

80

K. Findlay, S. D. Probert

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Fig. 73.

Variations of Ihe NO, emission and O2-in-the-flue concentrations, as observed

during time section I a of trial 1 when burning Maryport smalls, under two-stage combustion conditions, with and without limestone LG8 added to the bed. operated under two-stage or oxidising conditions, with or without limestone present in the bed, and irrespective of the coal quality (e.g. whether SA duff or Maryport smalls was used). Although the addition of limestone to the FB did serve to increase the NO x emissions, the latter still tended to follow the fluctuations of the O2-in-the-flue concentration closely, so showing the intimate relationship between these two variables (see Fig. 73). The only variable whose influence on the NO x emissions was strong enough to override that of the O2-in-the-flue was the freeboard temperature, as was seen in trial 9. Raising the freeboard temperature to at least 1050c~C led to a marked increase in the rate of NOx emissions. During sections g, h and i of trial 6, the freeboard temperature remained relatively invariant throughout (i.e. contrary to that which happened in test sections 6a-d inclusive) at around 984"C, and, as a result, the relationship between NOx emissions and O2-in-the-flue concentration was more clearly identified (e.g. see Fig. 74). An increase in the NO Xemissions with rising 02in-the-flue concentration was clearly evident during section 6i, using the lower secondary-air nozzle-height (i.e. at 1 5 m above the sparge pipes). These results could then be compared directly with those for section 6d, for which the greater secondary-air nozzle-height of 2.6 m was used (together with a freeboard temperature of 984°C), in order to determine the effect of the lower nozzle-height on the NO~-emission concentration. A good correlation between NO~ emissions and O2-in-the-flue concentration was also shown when the two variables were plotted against time. Results taken

Limitin~ N O x and SOe emi.vwons lrom wl F B conlhu.stor

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Fig. 74. Variations o1" the NO~ emission and the O,-in-the-ftue concentrations during lhc later parl of time section 6i of trial 6 (i.e. between 31 and 61 of the 2-rain time-intervals following the commencement of the time scctiont.

during section 6i showed that an increase in NO Xemissions closely followed any increase in the O2-in-the-flue. Similarly, freeboard temperatures during section 6j were relatively constant at around 860' C, so allowing a clear correlation between the N() X emissions and the O2-in-the-flue concentration to be obtained (see Fig. 75). Interestingly, for trial 7, a comparison between the NO x emissions and the O2-in-the-flue concentration (see Fig. 76) revealed that a correlation ensued between the two variables except towards the end of the trial, when the freeboard's temperature-uplift rose to well over IOOC (see Fig. 57). Under these conditions of high freeboard-temperature, the NOx emissions rose regardless of the lower O2-in-the-flue concentrations encountered during 300

280

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Fig. 75. NOx-emission concentration as a function of the O2-m-fl3e-flue concentration, as observed during lime section 6j of trial 6, when using a secondary-air nozzle-height of 2 6 m and a bed depth of 660 ram.

K. Findlay, S. D. Probert

82

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this period, so confirming that the high freeboard-temperature has an overriding effect. Where freeboard temperature was constant the N O x emission tended to increase with the 02 concentration (as shown in Fig. 77). Trial 8 also indicated a consistent increase in the N O x emissions as the 02in-the-flue concentration rose, regardless of whether Blanzy coal (see Fig. 62) or SA duff (see Fig. 61) was the fuel employed, and irrespective of the primary-to-secondary-air ratio. The NO X results taken during trial 9, with limestone added to the FB when the primary-to-secondary-air ratio was low at the beginning of the test, also showed an increase as the O2-in-the-flue concentration rose (see Fig. 78). This represented the effect of low 0255C

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K. Findlay, S. D. Probert

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Limiting NO., and S02 emissions ]i'om an FB comhustor

87

regardless of the effect of the reduced O2-in-the-flue concentration tsee Fig. 80). The rate of NO, emissions increased as the recycled-gas flow and hence the in-bed combustion was reduced (see Fig. 81 ), and the degree of secondary combustion within the freeboard increased. 4.6.4 E{[bct o[" in-bed oxygen concentration Although the NOx-emission concentration increased with the large bed's freeboard temperature, in tests carried out on the small test-rig, the use of two-stage combustion and its consequent increase in the freeboard temperature did not reveal this trend. By comparison, however, tests carried out using the same coal, i.e. SA duff, in the large test-rig, showed an increase in NO X emissions as sub-stochiometric combustion (and consequently a lower recycled-gas flow requirement) led to increased freeboardtenqperatures under two-stage combustion conditions. This was expected, because the NO X emissions are known to increase the higher the temperature, but to decrease the lower the O 2 concentration. It was demonstrated, with the small test-rig, that the bed's 02 concentration had the overriding effect. By reducing the oxygen concentration in the primary reaction zone of the FB, a reduced oxidation of the nitrogen in the coal might also be expected. However, under the reducing conditions resulting from extremely low O~ concentrations within the FB, ammonia might be expected to be formed in preference to nitrogen, i.e. 2N + 3H 2 ~ 2NH~ as opposed to 2N + O + H 2 ~ N z + HzO for which some 02 would need to be present. Under the condition of secondary combustion, the ammonia would then be oxidised to NOx in the secondary-combustion zone, the rate of the oxidation increasing with the rise in the freeboard's temperature. In the small FBC rig, by comparison, the improved fluidisation of the smaller bed led to a more uniform Oz-concentration occurring throughout the FB, so the formation of inert Nz, rather than ammonia, tended to occur. In addition, NO,-reducing reactions may also have been induced by greater concentrations of carbon in the bed {and freeboard). Such conditions did not occur when relatively high primary-to-secondary-air ratios of 50/50 and above were employed, and low levels of carbon inventory in the bed were evident, as during section 2 of trial 9. Further, these NO,-reducing reactions were also favoured at high bed-temperatures (of around 1000 C), which, although used to great effect in the tests on the small test-rig, were not utilised on the large test-rig for fear of causing the bed to clinker.

K. Find/ay, S. D. Probert

88

4.7 Combustion efficiency and particulate emissions

4.7.1 Effect of primar)'-to-secondary-air ratio The combustion efficiency remained relatively unaffected by changes in the primary-to-secondary-air ratio during trial 7. Whereas the carbon-in-ash loss (see Fig. 82) was approximately 65% during test section 1 (when the primary-to-secondary-air ratio was 70/30), it increased to over 70% during test sections 2-4 when the primary-to-secondary-air ratio equalled 30/70, the flue-gas dust loss at this lower air-ratio being less due to the reduction in the fluidising velocity (see Fig. 83). Thus, although the percentage of carbon content in the cyclone ash actually increased during the period of low primary-to-secondary-air ratio, this was counterbalanced by a reduced elutriation of cyclone ash to the stack. The larger primary-to-secondary-air ratio led to a more complete combustion of the carbon due to the higher levels of 02 within the FB. However, the lower fluidising velocities experienced when smaller air-ratios were employed and the higher freeboard-temperatures resulting from increased combustion in the secondary-combustion zone led to an almost constant combustion efficiency (,-~91-5%), irrespective of the primary-tosecondary-air ratio. This occurred despite a higher carbon-content in the cyclone ash when a low air-ratio was employed, with the consequent low 02concentrations in the bed. Had the recycled-gas temperature been reduced to 220°C, as would normally be the case with an industrial FBC unit, instead of the 290cC as used during the present experimental trials on the large test-rig, then even lower fluidising velocities may have resulted in an improvement in the combustion efficiency, under low-air-ratio, two-stage conditions. 8C =

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Limiting NO~ and S O 2 emissions/?ore an FB combustor

89

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7: fluidising velocity v a r i a t i o n s o v e r t h r e e d a y s .

4.7.2 Eff'ect o/high 02-in-the-flue concentrations The combustion efficiency fell when higher O2-in-the-flue (and hence recycled-gas Oz) concentrations occurred. This resulted in an even higher recycled-gas flow requirement (so increasing the elutriation of carbon fines). Thus, despite the lower primary-to-secondary-air ratio (of 30//70), net reductions in both the combustion efficiency and thermal efficiency ensued.

4.7.3 Effect of.freeboard temperature The maximum thermal efficiency occurred during test section 4 when a thermal efficiency of 63-7% (corresponding to 1500kg/h of steam output) was achieved as a result of the higher freeboard-temperature as well as the lower fluidising velocity. This should be compared with a thermal efficiency of 60.9% during section 1, when the freeboard's temperature-uplift was half the value achieved during test section 4, and only 58.9-60.2% during sections 2 and 3 (when high O2-in-the-flue concentrations led to much lower freeboard-temperatures than those that ensued in either sections 1 or 4). Corresponding conclusions were obtained when burning SA duff during trial 8. Combustion efficiencies of 87.7%, 84.5% and 84.1% ensued when using primary-to-secondary-air ratios of 30/70, 40/60 and 70/30. respectively.

4.7.4 Effect q/" coal quality Interestingly, Blanzy smalls exhibited the opposite effect: the combustion efficiency rose from 87.9% to 90% as the primary-to-secondary-air ratio was

K. Findlay, S. D. Probert

90

increased from 50/50 to 70/30. This indicates that the more highly volatile coals experienced improved combustion when the 02 concentration in the bed was kept high. For a primary-to-secondary-air ratio of 50/50, the high freeboard-temperatures that occurred (corresponding to an uplift of around 100°C) were insufficient to counteract the adverse effects of the low O zconcentrations in the FB. It can be seen in Fig. 84 that the cyclone carbon catch-rate was much higher at the lower primary-to-secondary-air ratio, and this effect was not compensated for by higher freeboard-temperatures. By comparison, the extremely high temperature-uplift of around 200°C a result of the lower Oz-concentrations in the FB achieved when burning SA duff~was sufficient to override the effect of the reduced carbon-combustion in the bed. Under such two-stage combustion conditions, using a primaryto-secondary-air ratio of 30/70, there was no discernible increase in the cyclone's carbon catch-rate, whilst the combustion efficiency actually increased. (Such increases in combustion efficiency have also been noted when burning forestry biomass, under two-stage combustion conditions (Curlyurtlu et al., 1991a).) 4.7.5 Effect o/" fimestone addition to the bed When SA duff(see Table 1) was burnt in the presence of fine LG11 limestone, during trial 9, the unburnt carbon loss was much greater than under similar operating conditions (as during trial 8) but without limestone added. The unburnt carbon loss was 25.4% during section 1 of trial 9, compared with 15-5% when using similar fuel and heat inputs and primary-to-secondaryair ratios during trial 8. The reduction of unburnt losses (during sections 2 and 4) to 18 % was a direct consequence of the lower fuel and limestone feedrates. When the coarser LG8 limestone was used, during section 3, there was 5O

+

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+ ÷ ÷

~ ~-

2(

÷

+

÷

÷

÷

÷ ÷

÷

÷ + ÷

+

÷

1( I

12:00 NOON

o oo . . . .12:00 . . . . . 00~) . . NOON

TIME IP R I H A R Y - TO - S E C O N D A R Y AIR RATIO 70 1 30

Fig. 84.

i

12:00 NOON

oo'=oo'

[HOURS)

-

NOON

' o oo'

'12' :00

NOON

I- PRIMARY AIR - T O RATIO - S E C O N D A_R_Y~ 50150

H G G Trials P r o g r a m m e - - t r i a l 8: c a r b o n catch-rate variations over four days, when b u r n i n g Blanzy coal.

Limiting NOx and

SO 2

emissions l~'om an FB comhustor

91

TABLE I Analysis (% by weight) of the Coals Used in Trial 8

Carbon Hydrogen Nitrogen Sulphur Oxygen Ash H 20

Blan-v smalls

South A/iican duff

55"91 3"59 0'76 1"4 8-9 17"8 11"64

627 l 3-89 0'96 052 9"75 14"08 8"09

a further reduction in unburnt losses to 15%. This indicates that the unburnt carbon losses increased when larger quantities of the finer limestone were added. The increased elutriation of one material ii.e. unburnt carbon or ash) as a result of high elutriation of another, in this case unsulphated LGI 1 limestone, is a phenomenon noted previously. As a consequence, the combustion efficiency was at its highest value (--~85%) during section 3, when the coarser (and hence less likely to be elutriated) limestone was employed. However, this reduced elutriation of LG8 limestone (compared with when LGI1 limestone was used) coincided with a fall in the fluidising velocity (which also served to reduce the unburnt carbon loss), so preventing a direct comparison being made. The lowest combustion efficiency(= 75 %} occurred during section 1, when the primary-to-secondary-air ratio (and hence the bed's O2 concentrationt was least. This agrees with previous results, except that the high freeboardtemperatures experienced during the secondary stage of combustion were insufficient to counteract the negative effect of the low O2-concentration. These high temperatures were also unable to raise the combustion efficiency to the levels achieved with LG11 limestone present in the bed, when the use of a higher primary-to-secondary-air ratio of 50/50, and hence higher concentration in the bed of 70-80% of stoichiometric requirements, led to combustion efficiencies of 82%. Measurements of the flue-gas dust loading showed that a maximum dust loss to the stack of 2500 mg/Nm 3 occurred during section 1, when the largest LG 11 limestone feed-rate was used. The dust loss was reduced to 1800-2400 mg/Nm 3 during section 2 when the limestone feed-rate was lower and the primary-to-secondary-air ratio higher, and to 854 mg/Nm 3 during section 3 when LG8 limestone was employed. This should be compared with a dust loading of 1000 mg/Nm 3 during trial 8, under similar conditions to those of section 1, but without limestone being added to the FB. When the fluidising velocity was reduced during section 4, the flue-gas

K. Findlay, S. D. Probert

92

dust loading fell to 227mg/Nm 3, despite the finer L G l l limestone being used, so illustrating the effect that reducing the fluidised velocity has in lessening the particulate emissions. When the coarser LG8 limestone was added, as during section 3, the fluegas loading was also found to decrease from 1890 to 854 mg/Nm 3 despite the fluidising velocity being reduced only slightly. Thus, limestone particle size is also influential: the coarser the particles, the lower the particulate emission and the more the limestone stayed in the bed, so increasing the bed height. The addition of almost twice the rate of limestone to the FB at 138 kg/h during section 1 compared with 74 kg/h during section 2 almost doubled the cyclone catch-rate. However, at these high Ca:S ratios, the percentage of soft, unsulphated limestone was high, and this material easily abraded into fine particles, which were readily elutriated. In practice, much lower Ca:S ratios would be employed, so leading to the production of a higher percentage of the harder, sulphated limestone, which is more resistant to abrasion and therefore less likely to be elutriated. Nevertheless, the addition of limestone to the FB, as a SO 2 absorbent, would undoubtedly lead to an increase in particulate emissions from the stack, and thus an adequate means for the removal of such particulates should be provided.

4.8 Effects of coal quality 4.8.1 Eff~wts oJnitrogen and sulphur contents of the coal During trial 8, the changeover, from Blanzy smalls to SA duff, led to a reduction in SO z emissions (see Fig. 85) from around 950 to approximately 400 ppm. This was due solely to the differing sulphur contents of the coals, because all the other influential operating parameters were kept constant. 1100 1000 90O O--

BOO~ 70(

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. . . . . . . . .

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S.A.DUFF

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BLANZY . . . . . . . . . .

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1 NOON 09-04-86 TIME

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00

1 : NOON 11-0/,-86

(HOURS)

HGG Trials Programme--trial 8: variations of the SO 2 in the flue-gas with time and type of coal.

Limiting NOx and SO 2 emissions ['rom an FB combustor

93

Blanzy had a sulphur content of 1"4%, i.e. almost three times that of SA duff, with a sulphur content of 0.52%. However, using a primary-to-secondaryair ratio of 70/30, the NO x emissions from Blanzy coal, at between 240 and 260 ppm, were similar to those from SA duff, despite the nitrogen content of SA duff being slightly higher (see Table 1). Such anomalies have been noted by others (Marshall & Melling, 1991) and are thought to be due to the differences in the distribution of nitrogen in the coal. The operating conditions for trial 8 were kept similar to those employed during trials 6 and 7, when Gedling coal was burnt, in order to permit the N O X- and SO2-emission concentrations resulting from burning the various coals to be compared. The NOx-emission concentration from Blanzy coal was almost double that from Gedling coal under similar operating conditions, using a primary-to-secondary-air ratio of 70/30, despite the nitrogen content of Gedling coal being greater. Similarly, the SO2-emission concentration from Blanzy coal was at least 250 ppm higher than that from Gedling coal, although this could be explained partly by the higher sulphur content of the Blanzy coal (i.e. 1-40% as compared with 1.29% in the case of Gedling coal).

4.8.2 Lffect o/primao:-to-secondao,-air ratio When using an air ratio of 40/60, burning SA duff gave much lower NO xemission concentrations than those which occurred with Gedling coal. The differences ranged from 60 to 100ppm NO x and these ensued despite the nitrogen content of the SA duff being higher. However, when using a primary-to-secondary-air ratio of 70/30, the trends were reversed, and the NO x emissions from SA duffwere higher (at 220-240 ppm) than those from Gedling coal (at 140-160ppm). Clearly some NOx-reducing reaction was taking place when burning SA duff at low primary-to-secondary-air ratios: possibly the reduction of NO by CO, which was catalysed by a constituent of SA duff ash (e.g. FezO3, which is present in the ash of Gedling or Blanzy coals only in smaller quantities). The SOz emissions from Gedling coal were approximately double those from SA duffwhen a primary-to-secondary-air ratio of 40/60 was employed. However, for a primary-to-secondary-air ratio of 30/70, approximately identical magnitudes of SO2 emissions were obtained in both cases. The increase, for the higher air-ratio, was due to the Gedling coal having a higher sulphur content. Yet with a sulphur content of 1.29%, compared with 0"52% in the case of SA duff, the increases in SOz-emission concentrations were much less than expected. This agrees with the results obtained with the small FBC. At the lower air-ratio, oxygen becomes rate-limiting. Also the CaO utilisation increases proportionately with SO2 concentration and therefore with sulphur content in the coal (Ford & Sage, 1991).

K. Findlay, S. D. Probert

94

4.8.3 SO: emissions The combustion of Maryport smalls, with a sulphur content of 2"37%, gave much higher SO2 emissions than those f r o m a n y of the other three coals used. During trial 1, without limestone addition to the bed, the SO 2 emissions were approximately 2300 ppm at O2-in-the-flue concentrations of 1-3%. This was more than four times the SO2 emissions from SA duff, which had a sulphur content of only 0.52%--see Fig. 86. Also, the SO2 emissions from Maryport smalls were approximately double those from either Gedling or Blanzy coals, both the latter having just over half the sulphur concentration of the Maryport smalls. Thus, the increases in SO2 emissions when burning Maryport smalls (rather than Gedling or Blanzy coals) were undoubtedly a result of the high sulphur content of this coal. The results (see Fig. 86) are taken from trials 1, 7 and 8, the burning taking place under twostage combustion conditions without the addition of limestone to the bed. The revealed trend, which shows that Maryport smalls have the largest SOsemission concentrations and SA duff the least, agrees with the conclusion 300(

250( • MARYPORT a

i

i

I

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200(

)--

~- 150(

= ,,,,,ll~mm=,,~,,,B L A N Z Y

N

m~mmm~m GEDLING 10(X

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OXYGEN (%)

Fig. 86.

The effect of coal quality on the SO,, emissions.

Limiting NO~ and S02 emissions./rom an FB comhustor

95

obtained, when comparing SO 2 emissions from the same coal types, when using the small test-rig.

4.8.4 NOx emissions Similarly, the NOx-emission concentrations from Maryport smalls were much lower than those from Gedling coal, i.e. --~40ppm from Maryport coal, at an O2-in-the-flue concentration of 3%, compared with NO X emissions of around 130 ppm from the Gedling coal. This was due partly to the Maryport smalls having a much lower nitrogen content than Gedling coal, i.e. 0.34% compared with 0-87% in the case of Gedling. However.

:0o

UFF

2~!

i xx~

20( 18(

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16(

120 100

80

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MARYPORT

OXYGEN (%) Fig. 87. The effect o f coal quality on NOx emissions.

96

K. Findlay, S. D. Probert

although this would explain a major part of the rise in NO x emissions when Gedling coal was burnt, it may not account for the whole increase. When Blanzy coal and SA duff were burnt under similar operating conditions, during trial 8, they gave almost similar NOx-emission concentrations, although the SA duff had a much higher nitrogen-content (at 0"96%) than Blanzy coal (at 0.76%). With such a low nitrogen-content, Bianzy coal when combusted would have been expected to give a much lower NO~-emission than Gedling coal, but the NO Xemissions were actually around 75 ppm higher, so indicating that the nitrogen content of the coal was not the sole factor affecting the NO~ emissions. Comparison of the results obtained during trials 1, 7 and 8, under twostage conditions without limestone being added to the FB (see Fig. 87), again shows a similar trend to that which occurred when the small rig was used. When burning SA duff, Gedling or Maryport smalls, the Maryport smalls gave the lowest NOx-emissions and SA duff the highest, irrespective of whether the small or large test-rig was employed. 5 CONCLUSIONS Because of the difficulties in being able to reproduce exactly identical applied conditions, a vast amount of information was obtained experimentally: the analysis of these data has led to various trends being clearly established.

5.1 Comparison between results from the large and small FBCs The qualitative basic trends remained similar for many of the results from both the large and small test-rigs. As a rough guide, the conditions leading to an approximate halving of the NOx-emissions concentration corresponded to a doubling of the SO2 concentration in the flue-gas. However, results obtained from the small experimental FBC could not be used for predicting quantitatively the emission concentrations from the larger FBC. Significant variations in the local O2-concentrations in the large bed, resulting from its less-effective, less-uniform fluidisation, meant that the magnitude of NOx emissions were invariably smaller (as a result of the lower local 02concentrations in the bed) and the SOz emissions higher.

5.2 Improvement of fluidised-bed performance with regard to:

5.2.1 Reduced NOx-emissions These were generally achieved by using: (i) a reduced bed depth (e.g. to --~400mm); (ii) an increased secondary-air nozzle-height (e.g. to --~2.6 m);

Limiting NO~ and SO z emissions/iota an FB combustor

97

(iii) lower freeboard temperatures (~900°C); or (iv) a low primary-to-secondary-air ratio (~ 30/70) combined with a low Oz-in-the-flue concentration (---3%); and (v) a coal with a medium to low nitrogen-content, e.g. Maryport smalls. 5.2.2 Reduced S03-emissions These were achieved by (1) adding limestone to the fluidised bed; (2) using lower freeboard temperatures (below 1000°C); (3) using a low Oz-in-the-flue concentration. 5.2.3 Reduced SO,-emissions These were achieved by using: (i) (ii) (iii) (iv) (v) (vi)

an increased bed depth (e.g. to ~710mm); a reduced secondary-air nozzle-height (e.g. to ~ 1.5 m): a low freeboard-temperature (~850°C); finer L G l l limestone added to the FB; an increased Ca:S ratio; a high primary-to-secondary-air ratio (,,-70/30) combined with a relatively large Oz-in-the-flue concentration (--~6%); and/or (viil coal with a low sulphur-content (of <0"52%). 5.2.4 Reduced particulate-emissions These were achieved by using: (it a low fluidising-velocity; (ii) a reduced primary-to-secondary-air ratio ( ~ 30/70) with two stage combustion; (iii) a small limestone feed-rate; (iv) coarser LG8 limestone or no limestone at all; and/or (v) a reduced Ca:S ratio. 5.2.5 Increased combustion-efficiencies These were achieved by using: (i) a high primary-to-secondary-air ratio (and ensuring that more complete combustion occurred in the bed); or (ii) a low fluidising-velocity (resulting from low recycled-gas and primary-air flows); and (iii) a high freeboard-temperature (resulting from a low primary-tosecondary-air ratio); and/or (iv) a low Oz-in-the-flue concentration (and therefore a small rate of recycled-gas flow);

98

K. Findlay, S. D. Probert

(v) a pressurised fluidised-bed (this was based on an independent set of tests). Conflicts between point (i) in Section 5.2.5 and the other conditions in the section (all of which were favoured by a low primary-to-secondary-air ratio) meant that the highest combustion efficiency for the present tests occurred~t a low primary-to-secondary-air ratio of -,~30/70, so long as conditions (ii)-(iv) inclusive were also achieved. Because the conditions required to ensure that reduced SOz-emissions tend to conflict with those for reduced SO 3- and NOx-emissions, a compromise condition should normally be employed. From a knowledge of the type of coal and limestone which will be used, it is feasible that some worthwhile predictions can be made for the emission concentrations of the various pollutants. So, when SO2 emissions are likely to approach the limits indicated by the EC directive, operating conditions can then be adjusted appropriately. If the NOx-emission limits are likely to be exceeded, the FBC combustor should be designed accordingly and then the conditions for reduced NO~-emissions employed. Where both SO 2- and NO~-emission limits are likely to be exceeded, a compromise should be adopted, e.g. the use of two-stage combustion, at a primary-to-secondary-air ratio of 70/30, would reduce the NO x emissions to a moderate extent, whilst the SO2 emission could be lowered to acceptable levels by using an increased Ca:S mole ratio. Finally, in the happy event that both NO~ and SO 2 emissions are well below the required limits, conditions favouring increased combustionefficiency (and possibly reduced particulate-emission) could then be adopted.

A C K N O W L E D G E M ENTS The authors are grateful to the Science and Engineering Research Council and Fina plc, respectively, for technical and financial support of this project. Particular thanks are expressed to the Energy Equipment research team of Fina plc who helped with the operation of test-rigs.

REFERENCES Allen, D. & Hayhurst, A. N. (1991). The reduction of nitric oxide by carbon monoxide in a fluidised bed. In Proc. Inst. Energy's 5th Int. Fluidised-Bed Combustion Conf. Published under the Adam Hilger imprint by lOP Publishing Ltd, Bristol, pp. 221-30.

Limiting NO,. and SO 2 emis'sions [kom an FB rombustor

99

Bolting, A. J., Gavin, D. G. & Hughes, I. S. C. (1991). Emissions of nitrous oxide from coal-fired fluidised-bed boilers. In Proc. Inst. Energy's 5th Int. FluidisedBed Combustion Conf. Published under the Adam Hilger imprint by IOP Publishing Ltd, Bristol, pp. 23948. Buchtela, G. & Hofbauer, H. (1991). Mechanisms of NO ~md N20 formation and destruction in fluidised-bed combustion'? In Proe. hlst, Enel\~.v's 5th Int. Fluidised-Bed Combustion Con/. Published under the Adam Hilger imprint by IOP Publishing Ltd, Bristol, pp. 213 20. Curlyurtlu, I., Cabrita, I., Frarco, C., Mascarentias, F. & Monteiro, A. (1991a). The use of the forestry business for energy Combustion versus gasitication in fluidised beds. In Proc. Inst. Energy's 5th Int. Fluidised-Bed Combustion Cot~/~ Published under the Adam Hilger imprint by IOP Publishing Ltd, Bristol. pp. 319 29. Curlyurtlu, I,, Costa, M. R., Esparteiro, H. & Cabrita, 1. (1991h). The study of homogenous and heterogenous reactions involving NzO and NO~ during fluidised bed combustion of coal particles. In Proc. htst. Enere.v's 5th htt. Flui~fised-Bed Combustion Cot{/. Published under the Adam Hilger imprint by IOP Publishing Ltd, Bristol, pp. 201 12. Findlay, K. & Probert, D. (1992). How to reduce pollutant emissions from small fluidised-bed combustors. Applied Energy, 41, 1 94. Ford, N. W. J. & Sage, E W. (1991). The characterisation of limestone for use as SO, sorbants in coal combustion. In Proc. h~st. Ener~)"~ 5th Int. Fluidised-Bed Combustion Con/. Published under the Adam Hilger imprint by lOP Publishing Ltd, Bristol, pp. 159-70. Furusawa, T., Honda, T., Takano, J. & Kunii, D. (1978). Nitric oxide reduction in an experimental FB coal combustor. In Proe. 2ml Engineering Foumhttion ('ot![., ed. J. F. Davidson & D. C. Keairns. Cambridge University Press, ('ambridge, UK, pp. 314 19. Gourichon, L., Joly, A. & Queva, T. (1991). Influence of limestone injection on N_,O emissions in CFBC. In Proe. btst. Energy's 5th Int. Fluidised-Bed Combustion Cotll? Published under the Adam Hilger imprint by IOP Publishing Ltd, Bristol, pp. 231 7. Henttonen, J., Kojo, I. & Kortela, V. (1991). NO~ and SO 2 emission-control optimisation in the FBC process. In Proe. brst. Energ)"s 5th Int. Fluidised-Bed Combustion Con[. Published under the Adam Hilger imprint by IOP Publishing Ltd, Bristol, pp. 261-8. Marshall, A. R. & Melling, P. J. (1991). The effect of staged combustion on performance and emissions from a solids circulation boiler. In Proc. Inst. Enerey's 5th btt. Fluidised-Bed Combustion Cot!['. Published under the Adam Hilger imprint by IOP Publishing Ltd, Bristol, pp. 269 80. Ruth, k. A. (1978). Regeneration of the sulphur acceptor in fluidised-bed combustion. In Proc. 2nd Enghleering Foundation CotT/?, 'Fluidisation', ed. J. F. Davidson & D.C. Keairns. Cambridge University Press, Cambridge, UK, pp. 308 13. Stairman, C. J. (1951). The sampling of dust-laden gases. Trans. btst. Chemical Enghzeers, 29, 15-44. Vickers, M. A. & Milner, C. N. (1991). A commercial demonstration of reduced NO~ and SO2 emissions from a 1-8 MW industrial fluidised bed boiler. In Proc. Inst. Energy's 5th Int. Fluidised-Bed Combustion Conic Published under the Adam Hilger imprint by IOP Publishing Ltd, Bristol, pp. 249 60.