Table Of Content

990 Boiler Operation Engineering sary to seal between them while providing al- Q. How are the nozzles arranged? lowance for thermal expansion. Ans. They are usually arranged on a uniform Perforated plate distributor is another popular pitch of 75to 100mm over the baseplate. design. But they have the tendency to allow fall- Q. How is the hole dia of the nozzles designed through ofbed materials. Forasmuch asthe plates to minimize particle fall-through? operate atornear thebed temperature, they must Ans. Theholesizeiskeptintherangeof2-5 mm be manufactured from aheat-resistant alloy plus which is a compromise between having an ex- they must be designed to retain sufficient me- cessivenumber andallowing particle fall-through. chanical strength atthe operating temperature to Larger holes/slots are sometimes used but they support the weight of the bed. are always associated with bubblecaps to pre- Again there are distributors which receive hot vent fall-through. gases from oil or gas combustion and direct the Q. What should be the distributor pressure- combustion gases intothebed forfluidization and drop? heat transfer. Such distributors must be designed to accept the hot gas. One approach is to retain Ans. A distributor pressure-drop of about 12% the nozzle standpipe design and water cool the of that across the bed-depth (typically 1.5 mm baseplate, with insulation on the underside. The water gauge per mm of static depth) is usually plate can either be of sandwich construction or required to ensure uniform distribution for the an alternative design uses heat-resistant steel and beds used in fluidized combustion. However, the allows for thermal expansion. One method of distributor pressure-drop increases with the achieving this is to instal agrid of pipes (sparge square of air flowrate if the bed pressure-drop is pipes) with multiple air outlets to introduce the held constant. Therefore, it is necessary to pro- air above the solid base of combustion chamber. vide good distribution of the minimum air The sparge pipes may have drilled holes in the flowrate. On this basis, the distributor pressure- underside or be fitted with nozzle standpipe. drop, at maximum airflow, must be: Q. What is the advantage of sparge pipes? 48% of bed-drop for 2:1turndown Ans. They can expand from the fixed end to 108% of bed-drop for a 3:1 turndown. accommodate thermal expansion while maintain- This requires a distributor with an open area in ing a gastight seal. the range of 0.5 to 2%. Q. What is the disadvantage of sparge pipes? Q. How is coal fed to the fluidized bed com- Ans. They must be carefully designed. The de- bustors? sign must take into account circumferential tem- Ans. During the early development of FB boil- perature gradients which can give rise to pipe ers, combustion was carried out using crushed distortion. coal whereupon it was found necessary to intro- Q. What is the height of the nozzles above the duce the feed immediately above the distributor baseplate? to ensure good combustion. This was achieved by pneumatic injection using pipes with adiam- Ans. Typically 50-100 mm. eter 6 times that of the largest coal particle, to Q. What is the nozzle dia? prevent blockage. The number of feed-nozzles Ans. Typically 12to 25 mm. varied from 1to 2 per m2 of the bed area. Boiler Design 991 However, morerecently,many unitshavebeen not heat the bed to a temperature at which com- designed to fire coal as supplied, without crush- bustion occurs below the surface. ing and drying. Usually, the coal is metered by a Q. Why is a pilot burner provided above the rotary valve or screw on to achute, down which bed? it drops to the bed surface. This method relies Ans. To light up gas firing. entirely on the bed to distribute the coal, which being fed in large lumps, keeps entrainment of Q. Why isthe gas start-up unit not used in units fines at a minimum. Alternatively there is an- designed to operate at highfluidizing velocity? other design that harnesses aspreader to distrib- Ans.The object of gas firing isthat there should ute the oncoming coal over a wide bed cross- be a sheet of flame at the bed surface, giving section. However, this system increases the pos- rapid heating of the bed particles so that com- sibilityofrapid entrainment offines.This method bustion moves into the bed at a temperature of proves tobe satisfactory if thefeed coal isdamp. about 925°K. Therefore, it is very much essen- There is yet another successful technique: tial during start-up that the fluidizing velocity Feeding uncrushed coal below thebed-surface by should not exceed the flame speed of the gas a screw-feeder. being used. This isunlikely for units designed to operate athigh fluidizing velocity and therefore, This system requires Jess feedpoints as the start-up through gas firing becomes void inthese uncrushed coal has a lower rate of volatile re- cases. lease and elutriation. Q. How is the additive feeding carried out? Q. Why is gas firing usually incorporated to fluidized bed coalfired units? Ans. Limestone (CaCOJ) or dolomite (CaCOJ Ans. It is incorporated so that it can be used to MgCOJ) used for sulphur capture are crushed to the correct size to form the fluidized bed. This preheat the bed to about 925°K during start-up. additive can be blended with coal before firing Also itbestows the FBC unit with cofiring ca- or it can be charged separately into the furnace pability. by apneumatic conveying system. Q. How is it injected into thefurnace? Q. Why is only one additive-feedpoint usually Ans. It is always premixed with the fluidizing sufficient? air. Ans. The additives enjoy greater residence time For small installations this premixing is car- in thebed than fuelparticles. Sothe additive par- ried out inthe airplenum, but forlarge units con- ticles introduced to the bed from a single siderable care is required for adopting this ap- feedpoint get sufficient time to get more or less proach. Preferably the large plenum should be uniformly distributed throughout the bed inven- subdivided into multiple units. A preferred alter- tory. native istoinject the gasinto the standpipes usu- Q. How does the bed-inventory dictate the de- ally from a gas grid in or below the plenum. sign of FB combustor units? Q. Whyaregas andfluidizing air not introduced Ans. When commercial size grades of coal or separately into the bed? other inert-containing solid fuels are fired in FB Ans. If the gas and the fluidizing airstream are combustors, there is a gradual accumulation of introduced separately to the bed, it is generally oversize inerts inthe bed. Most ofthe coal-inerts observed that it gives rise to above-bed flame tied tothe ash areofrelatively low density caus- above each gas entrypoint. Obviously, this does ingtheashparticles to diffusethroughout thebed, 992 Boiler Operation Engineering thereby increasmg the mean particle size. This For vertically laid tubes, flue gasbubbles tend has the effect ofincreasing the minimum fluidiz- to flow preferentially upwards in the tubes. And ing velocity and therefore, for agiven operating the bubbles in their wake bring solid particles velocity, less gas will pass through the bed as that cause erosion of these tubes at bends from bubbles, reducing the intensity of turbulence. If vertical to horizontal within the bed. Similarly, the accumulated mass of oversize inerts exceeds bubbles tend to flow preferentially up the line of 20%, the reduced mixing rate will give rise to the horizontal tubes arranged above each other lateral temperature gradients, eventually leading and for this reason a triangular rather than a to clinker formation around the fuel feedpoints. square tube-pitch is preferred. When this might occur, it is imperative to pro- Low fusible components in the fuel may fuse vide abed-regrading system permitting periodic at bed temperature to form clinker on the tube- withdrawal of the bed material, separating the surface, thereby contributing to inbed fQuling. oversize by screening and returning the correct size to the bed. The fluidized bed is constantly in a state of high degree of turbulence of bed inventory con- Large, dense particles such as stones in coal taining highly abrasive silica. These particles in- segregate gradually and accumulate asalayer on flicterosion to the tube surface. Highbed temper- thedistributor. These heavier fractions deflect lat- ature accentuates mechanical wear out of met- erally and can be removed from the bed via a als. single off-take point provided that thefeed mate- rialcontains avery lowpercentage ofsuch dense Therefore, it is essential to make careful se- inerts. When this is not the case, it is necessary lection of materials for the inbed evaporator, su- to make special provision to facilitate withdraw- perheater tubes and the tube banks of air-heat- al of segregated materials. This is achieved by ing units. incorporating an inclined distributor with theoff- Low-chromium ferriticsteelsareagoodchoice take atthe lowest point. An alternative approach provided the higher-limit of bed temperature is is to use a spargepipe-distributor with a hopper restricted to about 775°K. Austenitic steels such base underneath, allowing slow withdrawal of astypes 304, 316, 321 and 347demonstrate good segregated mass of inerts from the bed. performance upto925°K with orwithout sorbent Q. How are the inbed heat transfer tubesar- (limestone) addition. For bed temperatures <lS ranged? high as 1175°K, high-strength Ni-based alloys Ans. Heat transfer tubes for raising steam are appear to be suitable, provided the bed is oper- normally laid horizontally for forced circulation ated without limestone or dolomite. This is be- ofBFW, i.e. if the BFW is circulated by apump. cause with limestone addition, nickel-base alloys can suffer from intercrystalline grain boundary For natural circulation boilers, the inbed corrosion. For metal temperatures exceeding evaporator tubes must be inclined 10-20° to the 925°K in beds of limestone, it is therefore nec- horizontal, depending on the particular design. essary to use an iron-base austenitic steel, par- Superheater tubes and airpreheaters are usually ticularly type 347, up to 1075°K; above this tem- arranged horizontally. perature, it is necessary to use a low-strength Q. Whatfactors determine the layout of inbed super alloy such as GE2541 as acladding mate- tubes and selection of tube materials? rial to protect a high-strength alloy base. Ans. I. Fouling Q.How isaPFBC (Pressurized Fluid Bed Com- 2. Erosion bustion) unit designed? 3. High temperature corrosion