Biomass Handbook

8/4/2019 Biomass Handbook

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5 Industrial Combustion

5.1 IntroductionThi s chap te r d es cr ibe s combus ti on sys tems o f a nominal t he rmal c apac it y exce ed ing]00 kW. The se f ur na ces a re gener al ly equ ipped wi th mechan ic o r pneumat ic f ue l- fe ed ingsystems. Manual fuel-feeding is no longer customary due to high personnel costs ands tr ict em is si on l im it s. Mor eove r, moder n i ndus tr ial c ombusti on p lant s a re equi pped w it hprocess control sys tems support ing ful ly automat ic sys tem opera tion .In princ ip le , the fol lowing combust ion technologies can be dis tinguished: f ix ed- bed combust ion,

fluidised bed combustion,dust combustion.

Fuel

The bas ic p ri nc ip le s of t he se t hr ee t echnol og ie s a re shown in F igu re 5 .1 and des cr ib edbelow [118, J 19]. The fundamentals of biomass combustion are outlined in Chapter 2. j .

\Lr Air

dust firing

F ixed -bed combus ti on sys tems i nc lude gr at e f ur na ces and under feed s toke rs . P rima ry a irp as se s th rough a fi xed bed, i n whi ch dr yi ng , g as ifi ca ti on , and char coal c ombusti on t ak esplace. The combustible gases produced are burned after secondary air addition has takenplace, usually in a combustion zone separated from the fuel bed.Within a fluidised bed furnace, biomass fuel is burned in a self-mixing suspension of gasand solid-bed material into which combustion air enters from below. Depending on thef lu id is at ion ve lo ci ty , bubbl ing f lu id is ed bed and ci rc ul at ing f lu id ised bed combus ti on c anbe distinguished.Dus t combust ion i s s ui tab le f or f ue ls ava il ab le as smal l p ar ti cl es ( av er age d iame te r sma ll er2 mm), A mix tu re o f f ue l a nd p rima ry combus ti on a ir i s i nj ec te d i nt o t he combus ti on

Fuel

Ash

'rAirFixed bed furnace(grate furnace)

bubbling fluidisedbed furnace circulating fluidisedbed furnace

Figure 5.1: Diagram of principle combustion technologies for biomass !18].

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Handbook of Biomass Combustion and Co-Firing 5 Industrial Combustion

chamber. Combustion takes place while the fuel is in suspension, and gas burnout isachieved after secondary air addit ion.Va ri at ions o f t he se t echnologi es a re avai la bl e. Examp le s a re combus ti on sy st ems withspreader s tokers and cyclone burners .

the flue gas reaches high velocities and where the secondary air is injected at high speedvia nozzles that are well distributed over the cross-section of this channel. Other means ofachieving a good mixture of flue gas and secondary air are combustion chambers with avor te x fl ow o r cyc lone f low.

Based on the tlow directions of fuel and the flue gas, there are various systems for gratecombust ion plants (Figure 5 .3): counter-current flow (flame in the opposite direction as the fuel), co-current flow (flame in the same direction as the fuel), cross-flow (flue gas removal in the middle of the furnace).

Figure 5.3: Classification of grate combustion technologies: co-current, cross-current andcounter-current {i2l].

5.2 Fixed-bed combust ionsuriace off ixed bed charcoal and

ash layerignition front

furnaceGrate furnaces.2.1

The re ar e var iou s gr at e f ur na ce t echno logi es ava ila bl e: f ix ed gr at es , mov ing g ra te s,t rav el li ng g ra tes , r ot at ing g rat es , and v ib ra ti ng g ra te s. A ll o f t he se t echnol og ie s havespe ci fi c advant ages and d is advan tage s, d epending on fue l p rope rt ie s, s o t ha t c ar ef ulselec tion and planning is necessary .Gr at e f ur na ce s a re appr op ri at e f or b iomass f uel s w it h a h igh mois tu re con ten t, va ry ingparticle sizes (with a downward limitation concerning the amount of fine particles in thefuel mixture), and high ash content. Mixtures of wood fuels can be used, but currenttechnology does not allow for mixtures of wood fuels and straw, cereals and grass, due totheir different combustion behaviour, low moisture content, and low ash-melting point. Agood and well controlled grate is designed to guarantee a homogeneous distribution of thefuel and the bed of embers over the whole grate surface. This is very important in order toguarantee an equal primary air supply over the various grate areas. Inhomogeneous airsupply may cause slagging, higher fly-ash amounts, and may increase the excess oxygenneeded for a complete combustion. Furthermore, the transport of the fuel over the gratehas to be as smooth and homogeneous as possible in order to keep the bed of embers calmand homogeneous, to avoid the formation of "holes" and to avoid the elutriation of fly ashand unbu rned par tic le s a s much as pos si bl e.The technology needed to achieve these aims includes continuously moving grates, ah ei gh t con tro l s ys tem o f t he bed o f ember s ( e.g . by i nfr ar ed beams ), a nd f requency -controlled primary air fans for the various grate sections. The primary air supply dividedi nt o s ec ti ons i s n ec es sa ry t o be abl e t o adj us t t he spec if ic a ir amount s t o t he r equi rement so f t he zones whe re d ry ing, g as if ic ati on , a nd cha rcoal c ombust ion p reva il ( se e F igur e 5 .2 ).Th is s epa ra te ly cont ro ll ab le p rima ry a ir s upply a ls o a ll ows smooth ope ra ti on o f g ra tefurnaces at partial loads of up to a minimum of about 25% of the nominal furnace load andcontrol of the primary air ratio needed (to secure a reducing atmosphere in the primarycombust ion chambe r nec es sa ry f or l ow NO,operation). Moreover, grate systems can bewat er -c oo led t o avo id s la gg ing and t o ext end t he l if et ime o f th e mat er ia ls .Ano th er impor ta nt a spe ct o f g ra te f ur na ce s i s t hat a s ta ged combus ti on shoul d be obt ai nedby separating the primary and the secondary combustion chambers in order to avoid back-mixing of the secondary air and to separate gasification and oxidation zones. Due to thefact that the mixing of air and flue gas in the primary combustion chamber is not optimalbecause of the low turbulence necessary for a calm bed of embers on the grate, thegeometry of the secondary combustion chamber and the secondary air injection have toguarantee a mixture of flue gas and air that is as complete as possible. The better themixing quality between flue gas and secondary combustion air, the lower, the lower theamount of excess oxygen that will be necessary for complete combustion and the higherthe efficiency. The mixing effect can be improved with relatively small channels where

/,primary air

fuel b ed ~(drying and subsequent

devolatilisation)grate

Figure 5.2: Diagram of the combustion process ill f ixed fue l beds [120] .

Co-current Cross-current=JJb

1

114

Counter-current

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116 117


Count er -c ur re nt c ombust ion i s most s ui ta bl e f or fu el s w it h l ow heat ing val ue s (we t b ar k,wood chips, or sawdust). Due to the fact that the hot flue gas passes over the fresh andwet biomass fuel entering the furnace, drying and water vapour transport from the fuel bedis increased by convection (in addition to the dominating radiant heat transfer to the fuelsurface). This system requires a good mixing of flue gas and secondary air in thecombus ti on chambe r i n o rd er t o avo id t he f ormat ion o f s tr ai ns en ri ch ed w it h unburnedgases enter ing the boi le r and increas ing emiss ions .Co-current combustion is applied for dry fuels like waste wood or straw or in systemswhere pre-heated primary air is used. This system increases the residence time ofunburned gases released from the fuel bed and can improve NO, reduction by enhancedcontact of the flue gas with the charcoal bed on the backward grate sections. Higher fly-a sh ent ra inment c an occur and shoul d be impeded by app ropr ia te f low condi ti ons ( fur na cedesign).C ro ss -f low sys tems a re a comb inat ion of co-cur re nt a nd count er -c ur re nt un it s a nd a re al soe spec ia ll y appl ie d in combus ti on p lant s w it h ver ti ca l s econdar y combust ion chambe rs .In order to achieve adequate temperature control in the furnace, flue gas recirculation andwat er -c oo led combus ti on chambe rs a re u sed. Comb inat ion s o f t he se t echnol og ie s ar e a ls opossible. Water-cooling has the advantage of reducing the flue gas volume, impeding ashs in te ri ng on t he fu rn ac e wal ls a nd u sual ly ext endi ng th e l if et ime o f i nsul at ion b ri cks . Ifonly dry biomass fuels are used, combustion chambers with steel walls can also be applied(wi thout i nsula ti on b ri ck s) . Wet b iomass fu el s n eed combus ti on chamber s w it h i nsul at ionb ri cks ope rat ing a s he at a ccumul at or s and buf fe ri ng moi st ur e con tent and combust iontemperature fluctuations in order to ensure a good burnout of the flue gas. Flue gasrecirculation can improve the mixing of combustible gases and air and can be regulatedmore accurately than water-cooled surfaces. However, it has the disadvantage ofi nc rea si ng t he fl ue gas vol ume in th e f ur na ce and boi ler s ec ti on . F lu e ga s r ec ir cu lat ionshould be performed after fly-ash precipitation in order to avoid dust depositions in ther ec ir cu la ti on channel s. Mor eove r, f lu e gas r ec ir cul at ion shoul d not b e ope ra ted in st op -and-go mode, to avoid condensation and corrosion in the channels or on the fan blades.

c ombust ion (wh ich imp li es a l ower NO, r educt ion pot en ti al by p rima ry measur es) .Mor eover , non -homogeneou s b iomas s f uel s imply t he danger o f b ridgi ng and unevendistribution among the grate surface because no mixing occurs. This disadvantage can beavoided by spreader-stokers because they cause a mixing of the fuel bed by the fuel-feeding mechanism applied .

Figure 5.4: Technological principle of 0 travelling grate r 123l.

Travelling grateTravelling grate furnaces are built of grate bars forming an endless band (like a movings ta irc as e) mov ing t hr ough t he combus ti on chamber (s ee F igure 5.4) . Fuel is supplied atone end of the combustion chamber onto the grate, by e.g. screw conveyors, or isd is tri bu te d over t he g ra te by spr eade r- st oker s i nj ec ti ng t he f uel i nt o t he combus ti onchamber (see Figure 5.5) . The fuel bed itself does not move, but is transported throughthe combustion chamber by the grate, contrary to moving grate furnaces where the fuelbed is moved over the grate. At the end of the combustion chamber the grate is cleaned ofash and dirt while the band turns around (automatic ash removal). On the way back, thegrate bars are cooled by primary air in order to avoid overheating and to minimise wear-out. The speed of the travelling grate is continuously adjustable in order to achievecomplete charcoal burnout .The advan tages of t rav el li ng g ra te s yst ems a re un i f orm combus ti on condi ti on s f or woodchips and pellets and low dust emissions, due to the stable and almost unmoving bed ofembers. Also the maintenance or replacement of grate bars is easy to handle.In comparison to moving grate furnaces, however, the fact that the bed of embers is notstoked results in a longer burn-out time. Higher primary air input is needed for complete

Figure 5.5: Diagram o]a travelling grate furnace fed by spreader-stokers r 122}.

Fixed grate systemsF ixed g ra te s ys tems a re only u sed i n sma ll -sc al e appl ic at ions . In these sys tems, fue ltransport is managed by fuel feeding and gravity (caused by the inclination of the grate).As fuel transport and fuel distribution among the grate cannot be controlled well; thist echnol ogy i s no l onge r app lie d i n mode rn combust ion p lant s.


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I nc li ned mop ing g ra te s and hor ii on tal lv mov ing g ra te sMoving g ra te f urn ac es u sua ll y have an i nc li ned gr at e con si st ing o f f ixed and moveabl erows of grate bars (see Figure 5.7). By alternating horizontal forward and backwardmovements of the moveable sections, the fuel is transported along the grate. Thusunburned and burned fuel particles are mixed, the surfaces of the fuel bed are renewed,and a more even distribution of the fuel over the grate surface can be achieved (which isimportant for an equal primary air distribution across the fuel bed). Usually. the wholegrate is divided into several grate sections, which can be moved at various speedsaccording to the different stages of combustion (see Figure 5.6). The movement of thegrate bar is achieved by hydraulic cylinders. The grate bars themselves are made of heat-resistant steel alloys. They are equipped with small channels in their side-walls forprimary air supply and should be as narrow as possible in order to distribute the primarya ir a cr os s t he f ue l bed a s wel l a s pos si bl e.In moving grate furnaces a wide variety of biofuels can be burned. Air-cooled movinggrate furnaces use primary air for cooling the grate and are suitable for wet bark, sawdust,and wood chips. For dry biofuels or biofuels with low ash-sintering temperatures, water-cooled moving gra te sys tems are recommended.In contrast to travelling grate systems, the correct adjustment of the moving frequency oft he g rat e ba rs i s mor e compl ex . If t he mov ing f requenc ie s a re t oo hi gh , h igh concent ra t-ions of unburned carbon in the ash or insufficient coverage of the grate will result.Infrared beams situated over the various grate sections allow for adequate control of themoving frequencies by checking the height of the bed.Ash removal takes place under the grate in dry or wet form. Fully automatic operation oft he whol e sys tem i s common .

Figure 5.6: Operoting princ ip le o f a ll inc lined moving grate r 123].

118

I

feed ofthepipe network

return ofthepipe network

~flue gas recirculation-

secondary tir fansII

I primary combustion zoneII secondary combustion zone

primary-airfans infrared control2- - _

moving grate

DFigure 5.7: Modem grate furnace with infrared control svs tem and secrion-separated grate andprimary air control [119). .Explallatiolls: I....drying prevailing; 2....gasification prevailing; 3....charcoal combustionprevailing.

D

Figure 5.8: TIIO 3,2 MWth inc lined moving grare furnaces for wood chips , usedfor d is tr ic theating ill Interlaken (Courtesv ojSchniid AG, Slt 'it; .erlalld).

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Horizontallv moving grates have a completely hor izontal fuel bed. This isachieved by thediagonal posit ion of the grate bars (see Figure 5.9). Advantages of this technology are thefact that uncontrolled fuel movements over the grate by gravity are impeded and that thestoking effect by the grate movements is increased, thus leading to a very homogeneousdis tr ibution of mater ial on the grate sur face and impeding slag formation as a result of hotspots . A fur ther advantage of the hor izontally moving grate is that the overall height canbe reduced. In order to avoid ash and fuel par ticles to fall through the grate bars,hor izontally moving grates should be preloaded so that there is no free space between thebars.

secundary air/~

secundary air~

fuel

120 121

~ r r . - - " "= = f lue gasFigure 5.10: Diagram of a v ibra ting grate fed by spreader s tokers [ / J 8J.

ooosecondary

air Cigar burners _ .InDenmark, cigar burners have been developed tor s traw and cereal bale combustion (seeFigure5.11).

fuel.._" c~~

[ - - - - - - - - - - - - - - - - - - 11 1t t

primary air

Figure 5.9: Diagram of a horirnualt, moving gratefurnace !06].Vibrating gratesVibrating grate furnaces consist of a declined f inned tube wall placed on spr ings (seeFigure 5.10) . Fuel isfed into the combustion chamber by spreaders , screw conveyors , orhydraulic feeders . Depending on the combustion process , two or more vibrators transportfuel and ash towards the ash r emova l. Primary a ir i s f ed through the fuel bed f rom belowthrough holes located in the ribs of the finned tube wal ls [118]. Due to the v ib ra tingmovement of the grate at short per iodic intervals , the formation of larger s lag par ticles isinhibited, which is the reason why this grate technology isespecially applied with fuelsshowing sintering and slagging tendencies (e.g. s traw, waste wood) . Disadvantages ofvibrating grates are the high f ly-ash emiss ions caused by the vibrations, the higher COemissions due to the per iodic dis turbances of the fuel bed, and an incomplete burnout ofthe bottom ash because fuel and ash transport are more dif ficult to control .

Figure 5.11: Diagram o f a c iga r burne r [ 1 J 8].

Straw and cereal bales (as a whole or s liced) are delivered ina continuous process by ahydraulic pis ton through a feeding tunnel on a water -cooled moving grate . Upon e~lteringthe combustion chamber , the fuel begins to gasify and combustion of the charcoal followswhile the unburned mater ial is moved over the grate . Grate and furnace temperaturecontrol are very impor tant for s traw and cereal combustion, due to their low ash sinteringand melting points and the high adiabatic temperature of combustion caused by their lowmoisture content. Therefore, the combustion chambers have to be cooled either by water -cooled walls or by f lue gas recirculation (combinations of these two techniques are alsopossible). Furnace temperatures should not exceed 900C for normal operation.Fur thermore, in s traw and cereal combustion very f ine and light f ly-ash par ticles as well as


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Handbook of Biomass Combustion and Co-Firing

a er os ols a re fo rm ed fr om c on de ns ed a lk ali v ap ou rs . A n a uto ma tic h ea l e xc ha ng erc le an in g s ys te m is r eq uir ed to p re ve nt a sh d ep os it f orm atio n a nd corrosion. S ys te ms f ors hr ed de d o r c ut s tra w a ls o e xis t a nd o pe ra te in a s im ila r w ay to th e te ch no lo gy d es cr ib ed -o nl y th e fu el p re pa ra tio n a nd fe ed in g a re d iff ere nt.In se mi-co ntinu ou s s ys tem s suc h as w ho le b ale co mb ustion fu rna ce s, in w hic h th e b ale sa re fe d in b atch -w ise o pe ra tio n into th e furn ac e, are n ot re co mm end ed du e to th etem pe rature an d C O pe ak s c aus ed w he n a new b ale is de liv ered. C urre nt p ro ces s co ntrols ys te ms a re n ot a ble t o p re ve nt t he se u ns te ad y c om bu stio n c on di tio ns .Underfeed rotating grateU nd er fe ed ro ta tin g g ra te c om bu stio n is a n ew F in nis h b io ma ss c om bu stio n t ec hn olo gy t ha tm ak es u se o f c on ic al g ra te s ec tio ns t ha t ro ta te i n o pp os ite d ir ec tio ns a nd a re s up plie d w ithp rim ary air fro m b elo w (see Figu re 5 .1 2). A s a resu lt, w et and b urn ing fu els a re w ellm ix ed , w hich m ake s th e s ys tem a de qu ate fo r b urn in g v ery w et fue ls su ch a s b ark, s aw du st,a nd w ood ch ip s (w ith a m oistu re c on ten t u p to 65 w t.% (w .b .)). T he co mb us tib le g ase sf orm ed a re b ur ne d o ut w ith s ec on da ry a ir in a s ep ara te h oriz on ta l o r v ert ic al c om bu stio nc ha mb er . T he h or iz on ta l v er sio n is s uita ble fo r g en era tin g h ot w at er o r s te am in b oile rsw it h a n om in al c ap ac ity b etw ee n I and 10 MW,h' T he v ertic al v er sio n is a pp li ed f or h otw ater b oile rs w ith a ca pa city of 1 -4 MW,h' The fuel is fed to the grate from b elow b ys cre w c on ve yo rs ( sim il ar t o u nd er fe ed s to ke rs ), w hi ch m ak es it n ec es sa ry to k ee p th ea ve rag e p an icle s ize b elow 50 m m.U nd er fe ed ro ta tin g g ra te c om bu stio n p la nts a re a ls o c ap ab le o f b urn in g m ix tu re s o f s oli dw oo d fu els a nd b io lo gic al s lu dg e. T he s ys te m is c om pu te r- co ntro ll ed a nd a llo ws fu llya u tom at ic o p er at io n .

GI ~ .

..-------------~-~--~~

Figure 5.12: Underfeed rotating grate [124].Explanations: A...fue! feed, B...primary combustion chamber, C. .secondarv combustion chamber,D...boiler, E.. .flue gas cleaner, F...ash removal, g.. .stack.

122


Rotating cone[u maceT he ro ta ti ng c on e fu rn ac e b as ic all y c on sis ts o f a s lo wly ro ta ti ng in ve rte d c on ic al g ra te (s eeF ig ure 5 .1 3) . T he ro ta ti ng c on e fo rm s a n e nd le ss a nd s elf -s to kin g g ra te e na blin g a de qu atem ix tu re a nd q uic k ig nit io n o f fu el s o f v ary in g p arti cle s iz e a nd m oi stu re c on te nt. R ota tin gc on e furna ces a re a G erm an d ev elo pm ent an d ha ve b ee n us ed fo r b urn in g w as te w oo d a ndc oa l u p to no w. T he y ca n b e sup plied for no mina l b oiler c ap acitie s va ry ing b etw ee n 0.4a nd 5 0 MW'h [125].F ue l is d um pe d fro m ab ov e th ro ugh a tw o-stag e airtig ht lock . Prim ary air en ters th e g ratethro ug h c arrying b ars on ly in grate se ction s c ov ere d w ith fu el. D ue to the ca reful m ix ingo f the b ed o f e mb ers, a p rim ary air ratio of), = 0.3 to 0 .6 is a ch ie ve d, a llo win g t heu ti lis atio n o f f ue ls w ith lo w a sh -m elti ng te mp era tu re s (in th e ro ta tin g c on e g as ifi ca tio n o ft he fu el o nly ta ke s p la ce a t te mp er atu re s b el ow 8 00 C ). S ec on da ry a ir is fe d t an ge ntia llya nd a t h ig h s pe ed in to the cy lin drica l s ec on da ry c om bustion c ham ber, im plying arotation al tlo w th at en su res a go od m ixture of flue g as a nd a ir a s w ell a s an e fficie nt fly-ash s epa ra tio n from the flue gas . T he furn ac e w alls a re w ate r-c oo led stee l w alls in o rd erto e ns ur e a de qu ate t em pe ra tu re c on tr ol in th e o xi dis in g z on e a nd to a vo id a sh d ep os itfo rm atio ns . T hu s, th e t ot al c om bu stio n a ir r ati o c an b e k ep t b etw ee n A= 1 .2 . a nd 1 .4 , w hi chis a v ery l ow v al ue fo r fi xe d-b ed f urn ac es a nd e ns ure s a h ig h c om bu st io n e ffic ie nc y.T he w ea k p oin ts o r d is ad va nta ge s o f th is in no va tiv e c om bu stio n t ec hn olo gy fo r b io ma ssf ue ls a re : th e li mi te d e xp eri en ce w ith th e u ti lis ati on o f v ario us b io ma ss b io fu el s a t d iffe re nt

loa ds as w ell as w ith th e w ear-ou t o f t he grate and the fu rn ac e; the n ece ss ary a ux iliary b urn er n ee de d fo r start-u p du e to th e w ater-co oled w alls; the n ece ss ity of sh uttin g d ow n th e furn ac e p eriod ica lly fo r rem ov in g large ash

p artic le s th at a cc um ula te i n t he c ore (th is o pe ra ti on is p erfo rm ed a uto ma tic all y b y ag ra pp le r i ns ta ll ed ); th e fre qu en cy d ep en ds o n th e a mo un t o f m in er al im pu ri tie s in th ef ue l) .

9I ~

Figure 5.13: Diagram ofthe rotating conefurnace [125].Explanations: l ...fuetfeeding, 2...rotat ing grate , 3 bOI/OIlI of the cone. 4 ..primaiv air . 5 . .a ircontrol, 6.. .ash disposal, 7.. .ash s e re l " co n vev o r, 8 /}///II out tone, 9...secoudarv air.

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124 125


5 .2.2 Underfeed s tokers Underfeed stokers are suitable for biomass fuels with low ash content (wood chips ,sawdust, pellets) and small par ticle s izes (particle dimension up to 50 mm). Ash-r ichbiomass fuels like bark, straw, and cereals need more efficient ash removal systems.Moreover, s intered or melted ash par ticles cover ing the upper sur face of the fuel bed cancause problems in under feed stokers due to unstable combustion conditions when the fueland the air are breaking through the ash-covered sur face. An advantage of under feedstokers is their good par tial-load behaviour and their s imple load control . Load changescan be achieved more easily and quickly than in grate combustion plants because the fuelsupply can be controlled more easily.

1jnderfeed stokers (see Figure 5.14) represent a cheap and operationally safe technologylor small- and medium-scale systems up to a nominal boiler capacity of 6 MWth. The fuelisfed into the combustion chamber by screw conveyors f rom below and is transpor tedupwards on an inner or outer grate . Outer grates are more common in modern combustionplants because they allow for more tlexible operation and an automatic ash removingsystem can be attained easier. Primary air is supplied through the grate , secondary airusually at the entrance to the secondary combustion chamber . A new Austr ian develop-ment is an under feed stoker with a rotational post-combustion, in which a strong vor textlow is achieved by a specially designed secondary air fan equipped with a rotating chain(see Figure 5.15). 5.3 Fluidised bed combust ion

Figure 5.15: Diagram of a pos t-combustion chamber with imposed vor texf low [126] .

Fluid-bed (FB) combustion systems have been applied since 1960 for combustion ofmunicipal and industr ial wastes. S ince then, over 300 commercial ins tallat ions have beenbuilt worldwide. Regarding technological applications, bubbling fluidised beds (BFB)and circulating fluidised beds (CFB) have to be distinguished.A f luidised bed consists of a cylindr ical vessel with a per forated bottom plate f il led with asuspension bed of hot, inert , and granular mater ial. The common bed mater ials are s il icasand and dolomite. The bed mater ial represents 90-98% of the mixture of fuel and bedmater ial. Primary combustion air enters the furnace f rom below through the airdis tr ibution plate and f luidises the bed so that i t becomes a seething mass of par ticles andbubbles . The intense heat transfer and mixing provides good conditions for a completecombustion with low excess air demand (A between 1 .1 and 1 .2 for CFB plant s andbetween 1.3 and 1.4 for BFB plants) . The combustion temperature has to be kept low(usually between 800-900C) in order to prevent ash sintering in the bed. This can beachieved by internal heat exchanger sur faces, by f lue gas recirculation, or by waterinjection (in fixed-bed combustion plants combustion temperatures are usually 100-200Chigher than in FB units).Due to the good mixing achieved, FB combustion plants can deal f lexibly with var iousfuel mixtures (e.g. mixtures of wood and straw can be burned) but are limited when itcomes to fuel par ticle s ize and impur it ies contained in the fuel. Therefore, appropr iatefuel pre-treatment system covering particle size reduction and separation of metals isnecessary for fail- safe operation. Usually a par ticle s ize below 40 mm isrecommendedfor CFB uni ts and below 80 rnm for BFB units . Moreover, par tial load operation of FBcombustion plants is l imited due to the need of bed f luidisation.Fluidised bed combustion systems need a relatively long start-up time (up to 15hours) forwhich o il o r gas burne rs a re used. With rega rd to emiss ions , low NO, emiss ions can beachieved owing to good air s taging, good mixing, and a low requirement of excess air .Moreover, the uti lisation of additives (e.g. l imestone addition for S capture) works welldue to the good mixing behaviour . The low excess air quantit ies necessary increasecombustion eff iciency and reduce the f lue gas volume f low. This makes FB combustionplants especially interesting for large-scale applications (normal boiler capacity above30 MW(th. For smaller combustion plants the investment and operation costs areusually too high in comparison to f ixed-bed systems. One disadvantage of FB combustionplants isposed by the high dust loads entrained with the f lue gas , which make eff icient

I ---j- I-s.'dCWI :

I

l-'~~.'

Figure 5.14: Diagram ofan underfeed stokerfurnace [106].


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dust precipitators and boiler cleaning systems necessary. Bed material is also lost with theash, making it necessary to periodically add new material to the plant.

5 .3 .1 Bubbl ing f lu id ised bed combust ion (BFB)For plants with a nominal boiler capacity of over 20 MWth, BFB furnaces start to be ofinterest. In BFB furnaces (see Figures 5.16 and 5.17), a bed material is located in thebottom part of the furnace. The primary air is supplied over a nozzle distributor plate andfluidises the bed. The bed material is usually silica sand of about 1.0 mm in diameter; thefluidisation velocity of the air varies between 1.0 and 2.5 rnls. The secondary air isintroduced through several inlets in the form of groups of horizontally arranged nozzles att he beg inning o f t he uppe r p ar t o f t he f ur na ce ( ca ll ed f re eboar d) t o en su re a s ta ged- ai rsupply to reduce NO, emissions. In contrast to coal-fired BFB furnaces, the biomass fuelshould not be fed onto, but into, the bed by inclined chutes from fuel hoppers because ofthe higher reactivity of biomass in comparison to coal. The fuel amounts only to I to 2%of the bed material and the bed has to be heated (internally or externally) before the fuel isintroduced. The advantage of BFB furnaces is their flexibility concerning particle size andmoisture content of the biomass fuels. Furthermore, it is also possible to use mixtures ofdifferent kinds of biomass or to co-fire them with other fuels. One big disadvantage ofBFB fur na ce s, t he d if fi cu lti es t hey have a t p ar ti al l oad oper at ion, i s s o lv ed i n moder nf ur na ce s by spl it ti ng o r s ta gi ng t he bed .

Flue gas ashi i.. - tertiary a ir

A A 2 3Fuel , ,- -- -- -- -- -- -- - F lu id is ed bed - -- -- -- -- -- -- -I 2 3 11


9/17

128 129


Cyclone

Primary superheaters

combus ti on t empe rat ur e, t he mu ff le s houl d be wat er -c oo led , Fue l g as if ic at ion and cha r-c oal c ombusti on t ak e p la ce a t t he s ame t ime bec au se of t he sma ll p ar ti cl e s iz e, The ref or e,quick l oad changes and an e ff ici en t l oad cont ro l c an be a ch ieved,Muffle dust furnaces are being used more and more for fine wood wastes originating fromthe chipboard industry, Figure 5,19 shows a muffle dust furnace in combination with awater tube steam boiler. This technology is available for thermal capacity between 2 and8 MW [118]. The outlet of the muffle forms a neck, where secondary air is added in orderto achieve a good mixture with the combustible gases, Due to the high flue gas velocities,t he a sh i s c ar ri ed wi th t he f lu e ga s and is p ar tl y p re cip it at ed i n t he pos t- combus ti onchamber. Low excess air amounts (ll = 1 .3 -1. 5) and l ow NO, emiss ions can be achievedby proper a ir s ta gi ng , Besi de s muf fl e f ur na ce s, c yc lone bu rn er s f or wood dust combus ti onare also in use (see Figure 5,20), Depending on the design of the cyclone and the locationof fuel injection, the residence time of the fuel particles in the furnace (their burnout) canbe control led wel l.

Fuel_ ..

Tertiary superheater

Secondary superheater

Economizer

=~=- Generator banksto baghouseFigure 5.18: Diagram o f a CFB fu rnace [12S}.

5.4 Dust combustionI n du st c ombusti on sys tems, f ue ls l ik e s awdust and f in e shavings a re pneumat ic al lyinjected into the furnace, The transportation air is used as primary air. Start-up of thefurnace is achieved by an auxiliary burner. When the combustion temperature reaches acertain value, biomass injection starts and the auxiliary burner is shut down, Fuel qualityin dust combustion systems has to be quite constant. A maximum fuel particle size of 10-20 mm has to be maintained and the fuel moisture content should normally not exceed20 wt% (w.b.). Due to t he exp lo si on -l ik e gas if ica ti on o f t he f ine and sma ll bi omas sparticles, the fuel feeding needs to be controlled very carefully and forms a keyt echnol og ic al uni t w ith in t he ove ra ll s yst em . Fue l/ ai r mixt ur es a re u sual ly i nj ec te dt angenti al ly i nt o t he cyl indri ca l f urn ac e mu ff le t o e st ab li sh a r ot at iona l f low (usua ll y avortex flow), The rotational motion can be supported by flue gas recirculation in thecombustion chamber. Due to the high energy density at the furnace walls and the high

Figure 5.19: Diagram of a dust combustion plant (ml(fflefumace) ill combination with a water-tube steam boileriI IS].Explallatiolls; l.. .primary ail ' supply, 2".jilel feed, 3."gasijicalioll and partial combustion, 4"f/uegas recirculation, 5",ash disposal, 6."secolldarr ail ' supplv, 7", tertiary ail ' supplv, S",\I 'aler tubeboiler,


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5 . 5 Other combustion concepts5.5.1 Whole Tree Energy conceptsecondarycombustionchamber O ne p ro mi sin g c om bu sti on c on ce pt u nd er d ev el op me nt in th e U SA is th e s o-c al le d WholeTree Energy Concept" ( se e F ig ure 5 .2 2) . T he c on ce pt . w hic h is p ate nt ed in 3 0 c ou ntri es ,is b as ed on an in no va tiv e fue l ha nd lin g, drying , a nd c om bus tio n s ystem . A pilot plan tw ith a no mina l b oiler ca pa city of 1 0 M W ,h has b ee n b uilt to te st an d ev alua te th is te ch no -lo gy . A 50 M Wc de mo ns tra tio n pla nt is u nd er de ve lop men t in c entral W is co nsin w ith ane le ct ri ca l e ff ic ie nc y o f 3 2. 59 '0 . T he t ot al c ap it al c os ts o f t he e qu ip me nt , e ng in ee ri ng ,p ro cu re me nt . a nd c on stru ct io n a mo un t to a pp ro xi ma te ly 1 01 m ill io n U S$ [1 30 ).

secondary air

primarycombustionchamber

drying barnexhaust fan loading and unloading crane- Ratcheting drag conveyor

- Volatile gascombustion zone- - - - - - - - '\ Combust ion ai r heaterSteamgenerator toutsideharge pit

isolation gate

stack

Figure 5.20: Diagram of a two-s tage cyc lone burner (129) .wholetrees -

A disad va nta ge o f m uffle furn ac es a nd c yc lon e b urne rs is th at ins ula tio n b rick s w ear ou tqu ick ly du e to th erm al stres s a nd e ros io n. T he re fore. othe r du st co mb us tio n s ys tem s areb ein g b uilt w itho ut rotation al flo w, w he re du st in jec tio n ta ke s p lac e a s in a fue l o il- orn at ur al g as -f ir ed f ur na ce ( se e F ig ur e 5 .2 1) .

f ur na ce -isolation gate

dryingair tobuilding char burnout zone drying air heat exchanger

Particulate scrubber

Fluegas recirculalion ---sawdusl

' f . . . . . .- - .r. ,-------


11/17

to achieve complete gas-phase combustion. Similar to any pile burning system, theper iodic dumping off resh wood logs onto the burning pile dis turbs the combustionprocess and causes increased emiss ions . This point has to be especially considered in thefurther development of this technology.


Table 5.1 (con/in lied).BFB furnaces

no moving parts inthe hot combust ionchamberNOx reduction by air staging works wellhigh flexibility concerning moisture contentand k ind o f b iomass fue ls usedlow excess oxygen (3 - 4 Vol%) raiseseff ic iency and decreases f lue gas f low

5.6 Summary of combustion technologiesTable 5.1 gives an overview of the advantages and disadvantages as well as the f ields ofapplication of various combustion technologies. Regarding gaseous and solid emissions,BFB and CFB furnaces normal ly show lower CO and NO, emiss ions due to more homo-geneous and therefore more controllable combustion conditions. Fixed-bed furnaces, inturn. usually emit fewer dust par ticles and show a better burnout of the f ly ash [131, 132,133].Table 5.1: Overview ofadvantages, disadvantages andfields of application of various biomasscombustion technologies

Advantages Disadvantages

CFB furnaces nomov ing par ts in the hot combust ion

chamber NOx reduc tion bya ir s taging works wel l h igh f lexibi li ty concerning moisture content

and k ind o fb iomass fue ls used homogeneous combust ion condi tions in the

furnace ifseveral fuel injectors are used h igh speci fi c hea t t rans fe r capac it y due to

high turbulence us e o f ad di ti ve s ea sy v ery l ow e xces s o xygen (1 - 2 vol%) rai ses

eff ic iency and decreases f lue gas f low

underfeed stokers low inves tmen t cos ts for p lants < 6 MW(th) s impl e an d good l oad co nt ro l due t o

continuos fuel feeding low emiss ions a tpa rt ia l load ope ra tion due to

good fuel dosinggrate furnaces low inves tmen t cos ts for p lants < 20 MW(th) l ow ope rat ing c os ts low dust load in t he flue gas less sensi ti ve tos lagging than f lu id ised bed

furnaces

dust combustion l ow ex ce ss ox yg en ( 4 - 6 Vol%) i ncr ea se s

efficiency h igh NOx reduc tion bye f fi cien t a ir s taging

and mixing possible ifcyclone orvor texburners are used

very good load con trol and fas t a lterna tion o fload possible

132

sui table only for b io fuels w ith low ash con tentand high ash-melting point (wood fuels)

low f lexibi li ty in rega rd to par ti cle s ize

no mix ing o fwood fue ls and herbaceousfuels possible

e ff ic ient NOx reduc tion requi res specialtechnologies

hi gh e xces s o xygen (5 - 8 Vol%) de cr ea se sefficiency

combust ion conditions not as homogeneousas in f lu id ised bed furnaces

low emiss ions level a t par tial load ope ra tion isdifficult to achieve


h igh inves tmen t cos ts , interes ting only forp lants> 20 MWth

h igh ope ra ting cos ts low f lexibi li ty w ith regard to par ti cle s ize 80

mm) hi gh d us t l oad in t he f lue gas ope ra tion a tpa rt ia l load requi res special

technology medium sensitivi ty concerning ash slagging l os s of be d ma teri al wi th t he a sh medi um e ro si on of hea t ex changer tu be s i n

the f lu id ised bed

h igh inves tmen t cos ts , interes ting only forplants> 30 MW(th)

h igh ope ra ting cos ts low f lexibi li ty w ith regard to par ti cle s ize 40

mm) h igh d us t l oad i n t he f lue ga s par tial -load ope ra tion requi res a second bed l oss o f b ed mat er ia l wi th th e ash high sensitivi ty concerning ash slagging l oss o f b ed mat er ial wi th th e ash medi um er os io n o f h eat e xc hange r t ub es i n

the furnace

5.7 Heat recovery systems and possibilities for increasing plantefficiency

Table 5.2 gives an overview of the potential of var ious options for increasing theefficiency of biomass combustion plants [134, 135]. Biomass drying is one interestingmethod, though the efficiency increase and cost savings are usually moderate. Advantagesthat can be achieved are the prevention of auto- ignitions in wet bark piles , the reduction ofdry-matter losses due to microbiological degradation processes during storage, and areduction of the necessary storage volume at the plant. However , biomass dryingprocesses must be carefully examined for their economic advantage, considering theadditional investment costs as well as the operating costs inthe form of electricityconsumption, and man and machine hours necessary to run the process . In most cases ,biomass drying isonly economic ifpre-heated air isavailable at low or no cost (examplesare solar air collectors and the utilisation of pre-heated air from flue gas condensationunits - see Chapter 3.3.5). Drying biomass piles for several months by natural convectionis not economic in most cases because the dry-matter losses by biological degradation

par ti cle s ize o f b io fuel i s l imi ted 10-20 mm) hi gh wear ou t of th e i ns ul at ion b ri ckwor k i f

cyclone or vortex burners are used an ext ra s ta rt -up burne r isnecessary

133


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134 135


(1.0-2.0 wt. % per month) are higher than the increase of efficiency obtained when outdoors to rage i s con si de red ( se e Chapt er 3 .4. 1) .

Measures Potential thermal eff ic iency improvement,as c ompare d t o t he NCV o f o ne d ry to nne

of fu el (% )

There are two technological possibilities for decreasing the excess air ratio and to ensure acomplete combustion at the same time. On the one hand, an oxygen sensor is coupledwith a CO sensor in the flue gas at the boiler outlet to optimise the secondary air supply(CO-A .- con tr ol ); on t he o th er h and, th er e a re improvement s o f t he mix ing qual it y o f f lu ecas and air in the furnace (as already explained). In addition, a lower excess oxygen~oncen tr at ion i n t he f lu e gas c an a ls o s ign if ic an tly improve t he ef fi ci en cy o f tl ue gascondens at ion uni ts , due t o th e f ac t t ha t i t i nc re as es t he c lew poi nt a ncl t he re fo re r ai se s t heamount of recoverable latent heat of the condensing water at a certain temperature (seeFigure 5.24).

Table 5.2: Influence of various measures all the thermal efficiency of biomass combustion plants.Exolanations: Calcula tions per formed for wood chips and bark asfue l; CCV = 20 M.IIkg (d.b.].Abbreviat ions: db . ..drv bas is ; w.b . ..wet bas is ; NCII. . .ne t calor if ic value; CCII . .gross calo ritir:willie: efficiency = heal output (boiler) Ienergv input [uel (NCII).

+0.8% /

70-

GB

66

E 64'E 62'0Q 60:;:Q 58~'0

66

54

52-50 10 12

02-concentration [Vol%]

7 90 0 -

7800

7700

.ci 7600ia; 7500E01 7400'":, 7300= ."iii 72000 7100

7000

6900 10 1202-concentration [Vol%]

dr yi ng f rom a moi st ure co nt en t o f 5 0 w t% t o 3 0 w t%(w.b.)

decreasing the O2- con tent in the f lue gas by 1 .0vol%

bark combust ion: r educ ing the Corg contenti n t he ash f rom 100 t o 5 .0 wt .% (d. b. )

dec reas ing the f lue gas tempera tu re a t boi le r out le tby 1 0 DC

f lue gas condensat ion(compared to convent ional combust ion units)

+8.7%

about +0.9%

+0.3%

average +17%maximum +30%

Reducing the excess oxygen content in the flue gas is an effective measure for increasingthe efficiency of the combustion plant, as shown in Figure 5.23.

90

88 -I~ 86 tc->- 84 ~0c: :Q'u 82iEQ " ' )8

763 4 5 6 7 8 9 10 11 12 13 14

O,-concentration [Vol%] Figure 5.24: Injluence of the oxygen content ill the f lue gas all the heat recoverable inj lue gascondensation plants.Expla llat io ll s: dew point calcu lu ted for water i ll the f lue gas of a wood chips and bark combust ionplant; moisture content of the bioniass juel 55.0 lVI. '7c(\I'. b). Fl-content 6.0 Ilt.o/C (d.b.): grosscalorific value of the fuel 20 MJlkg (d.b.); Q/o",1 = total recoverable heat from J. O kg biomassfuel(w.b. ) burned when the [lue gas i s c oo le d down to 55C.

Figure 5.23: Inf luence of/he oxvgen con/em in the flue gas all the plant e ff ic iency.Explallatiolls: moisture content of thefuel 55.0 IV/. % (II. b.) ; b iomass fuel used: wood chips Ibark; Hscontent 6.0 1 1' /. O /C ( d.b.); gross calorific value of thefuel 20300 kJlkg (d.b.}; flue gastemperature at boiler outlet 165C, efficiency related /0 the NCII o j the fue l; O2 concentrationrelated to divflue gas; efficiency = heat outputfrom boiler I energy input with the fuel (NCII ).


13/17


Furthermore, reducing the excess oxygen concentration in the flue gas also decreases theflue gas volume flow, which limits pressure drops and reduces the sizes of boilers and fluegas cleaning units. Of course, one has to take care that a reduction of the excess oxygenconcent ra non m the f lu e gas al so i nc re as es t he combusti on t empe ra tu re , whi ch under lin est he impor ta nc e o f an app ropr ia te f ur na ce temper at ur e con tr ol s ys tem .Low carbon-in-ash values are of minor importance for the efficiency of the plant but ofmaj or impor ta nce f or t he pos si bi lit y o f s us tai nabl e a sh u ti li sa ti on , b ec au se t he concen tr at -i on o f or gani c con taminan ts i n b iomas s a shes normal ly i nc re as es w it h hi gher c ar bonconcen tr at ions [ 132, 136 J ( se e a lso Chapt er 8 ).The most effective way to recover energy from the flue gas - and in many cases also aneconomically interesting technology - is flue gas condensation. In addition to the highenergy recovery potential of this process (up to 20% of the energy input by the biomassf ue l r el ate d t o t he NCV), dus t p re cip it at ion e ff ici en ci es o f 40-75% can be a chi ev ed .Furthermore, there is the possibi lity of preventing condensation of the flue gas at thechimney up t.o ambient temperatures of about -10C [137, 138J. In Denmark, the majorityof biomass district hearing plants are equipped with flue gas condensation units. InSweden, Fi nl and, a nd Aus tr ia , t he number o f i ns ta ll at ion s i s r ap id ly i nc re as ing; Italy,Germany, and Switzerland also have several plants already in operation. Figure 5.25shows the principle of a flue gas condensation unit. The whole plant normally consists oft hr ee par ts , th e e conomis er (r ecover y o f s en si bl e he at f rom the f lu e gas ), t he condens er( re cover y of s en si bl e and la te nt h eat fr om the f lu e gas ), a nd t he a ir p re -he at er ( pr e- he at ingthe combustion air and the air used to dilute the saturated flue zas before enterinz the ~Co Cochimney).

The amount of energy that can be recovered from the flue gas depends on the moisturecontent of the biomass fuel, the amount of excess oxygen in the flue gas (as alreadyexplained), and the temperature of the return of the network of pipes. The lower thetemperature of the return water, the higher the amount of latent heat that can be recoveredwhen the flue gas is cooled below the dew point (see Figure 5.26). Consequently, theenergy recovery potential strongly depends on the quality of the heat exchangers, thehyd raul ic i ns tal la ti on s and t he p ro ce ss con tr ol s yst ems i ns ta lle d by t he c li en ts , b ec au sethey determine the return tempera ture .

136 137

85 -preheated air

Flue gases fromcombustion 80

35 55 75 95 115 135 155 175 195

Flue gas temperature [0C]

Economizer . . .High temperature return . . .from thepipe network . . . . Air preheater. . . outside air

Condensor . . . . . .Low temperature return . . . .rom thepipe network

Figure 5.26: Efficiency of biomass combustion plants vcitli f lu e gas condensat ion uni ts a s afunct ion off lue gas temperature .Explanations: oxygen concentra tion of the f lue gas 9.5 1101%(d.b.); biomassjuel uscd: woodchips / bark; ll-content 6.0 \ l r . % (d.b. ); gross calor if ic value of the fuel 20 MJ/kg (d.b. ); depoint calculated for water; efficiency = heat output (boiler + f lue gas condensat ion uni t ) / etiergvinputfuel (Nell).

CondensateCondensation sludge

The dus t pr ec ip it at ion e ff ic ie ncy o f 40- 75% ment ioned c an be s ign if ic an tly in cr ea sed byp la ci ng a s impl e a ero so l e le ct ros ta ti c fi lt er b eh ind t he conden sat ion uni t. Ear ly t es t r un sshowed a dust precipitation efficiency of 99.0% at temperatures below 40C [140). Due tothe low flue gas temperature, the ESP unit can also be kept small and therefore economic-a lly a ccept ab le . Fu rt he rmor e, not onl y a ero so ls but a ls o wat er d rople ts ent ra in ed w it h t heflue gas achieve efficient precipitation, lowering the amount of dilution air that has to beadded t o t he s at ura te d fl ue gas l eavi ng t he condens at ion uni t. The condens at ion s ludgehas t o be s epar at ed f rom the condens at e ( by s ediment at ion un it s) b ec aus e i t c on ta in ssignificant amounts of heavy metals upgraded in this fine fly-ash fraction. It has to beFigure 5.25: Diag},({1IIof aflue gas condensation unit/or biomass combustion plants f / 39].


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d is po se d o f o r in du stri all y u tili se d (s ee S ec tio n 8 .3 ). M or eo ve r, r es ea rc h h as s ho wn th atthe s ep ara tio n o f slu dg e a nd c on den sa te s ho uld b e do ne a t p H va lu es> 7.5 in o rd er top re ve nt d is so lu tio n o f h ea vy m eta ls a nd t o m ee t th e lim iti ng v alu es f or a d ir ec t d is ch arg eo f th e c on de ns ate in to ri v ers [141].Flu e ga s con de ns ation s ystem s c an a lso b e o pe ra ted as qu en ch p ro ce sse s. T he dis -ad van ta ge of su ch s ystem s is th at q ue nc hin g sligh tly low ers the am ou nt o f he at tha t ca n b ere co ve re d a nd r eq uir es e ve n l ow er te mp er atu re s o f t he re tu rn in o rd er to b e e ne rg et ic allye ff ic ie nt . A n in te re sti ng o ptio n f or i nc re as in g th e h ea t re co ve ry p ote nt ia l o f flu e g asc on de ns ati on u ni ts is a S we dis h a pp ro ac h w it h a n a ir h um id ifi er in te gra te d in to th e s ys te m( se e F ig ur e 5 .2 7) .T his te ch no lo gy fo re se es t he p re -t re atin g a nd m ois te nin g o f c om bu sti on a ir b y in je ctin gcon de ns ate w ate r into the air. B y this m ean s, th e m oisture co nte nt o f the flue g as rise s a ndth e h ea t re co ve ry p ote nt ia l i nc re as es a cc ord in gly . W he n a pp ly in g th is a ir h um id ific ati onun it, on e ha s to tak e care that the sp ra y n oz zle s p rod uc e very fin e w ate r d ro ple ts (in orde rto giv e the m e no ug h tim e fo r e va po ration ) a nd to d es ig n the no zzles in a w ay that pre ve ntsm alfu nc tio ns d ue to i mp ur itie s in th e c on de ns ate [1 42 ].W ith r eg ard to e co no mic a sp ec ts . f lu e g as c on de ns ati on i s g en era lly re co mm en da ble fo rb io ma ss c om bu sti on p la nts . F lu e g as c on de ns atio n u nits a re o f i nte re st if w et b io ma ssf ue ls a re u ti lis ed ( av era ge m ois tu re c on te nt b etw ee n 4 0 a nc l5 5 w t% (w .b .)) , if t he re tu rnof the netw ork of pipes is below 60C and if the nom inal boiler capacity is above 2 M Wth.

'ijco.~~.cEoo

r t E'"'". . . . . . ."-'"t-. . . . . .

'" '"n :s'"~ ~: 1~

" "~~~(~: ; : ;- : : : :.~'"'%~'"" "";:;'" ''":i""'": : : ;r .; ~t .!!! : ; : ;e "--'5 I : ; : ; .ro '" 0'" z '" ~c Z 2Qr--Nr . r i'" ~'"iii ::lOlu :

138


15/17


S . B Techno-economic aspects concerning the design of biomasscombustion plants . .' . . boiler heat produced per yearAnnual utilisatIOnrate - biomess combustion plant {%} = ,::,,::,:::: :__:_:_::::::_:_


16/17

2 3 4 5

Handbook o f B iomass Combust ion and Co-Fi ri ng 5 Industr ial Combust ion

1009080

'" U ) o uu '"C ~ ~Q) .


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boiler nominal capacity: 6 Power Generation and Co-Generation025

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