DEW POINT OF THE FLUE GAS OF BOILERS CO-FIRING

Chemical and Pocess Engineeing 2013, 34 (1), 101-108 DOI: 10.2478/cpe-2013-0009 DEW POINT OF THE FLUE GAS OF BOILERS CO-FIRING BIOMASS WITH COAL Szymon Ciukaj *, Maek Ponobis Silesian Univesity of Technology (SUT), Institute of Powe Engineeing and Tubomachiney, Konaskiego 20, 44-101 Gliwice, Poland The pape deals with the impact of co-fiing biomass with coal in boiles on the dew point of the flue gas. Co-fiing of biomass may have twofold implications on coosion and fouling, which ae the pocesses that detemine the lowest acceptable flue gas outlet tempeatue and as a esult, boile efficiency. Both phenomena may be educed by co-fiing of usually low sulphu biomasses o enhanced due to inceased moistue content of biomass leading to inceased wate dew point. The pesent study concens the poblem of low-tempeatue coosion in utility boiles. The pape gives (in the fom of diagams and equations) a elationship between wate dew point and moistue content of fuel mixtue when co-fiing coal and vaious biomasses. The egession analysis shows that despite significant diffeences in the chaacteistics of coals and these of additional fuels, which ae planned fo co-fiing in lage-scale powe boiles, the wate dew point can be descibed by a function given with the accuacy, which shall be satisfactoy fo engineeing puposes. The discussion of the popeties of biofuels indicates that the acid dew point suplus ove the wate dew point (Δt = t - t w ) is not likely to exceed 10 K when co-fiing biomass. The concluding emaks give ecommendations fo the appopiate opeation of boiles in ode to educe isks associated with biomass co-combustion. Keywods: acid-dew point, low-tempeatue coosion, biomass, co-fiing 1. INTRODUCTION The main facto limiting the eduction of flue gas exit tempeatue in powe boiles is the necessity of peventing dew condensation on cold end elements (ai heates, electostatic pecipitatos and connecting ducts). Paticulaly endangeed ae the otay ai heates (RAH) whee the liquid phase emeging on the sufaces of heating elements esults in sevee fouling and coosion. Both phenomena ae associated with condensation of vapous fom the flue gas. When fiing sulphu-containing fuels, sulphu patly oxidises to fom SO 3, which afte having eacted with wate vapou pesent in the flue gas foms vapous of sulphuic acid. When cooled, the mixtue of H 2 SO 4 and H 2 O condensates. The dew point tempeatue and composition of liquid mixtue ae detemined by the patial pessues of wate and sulphuic acid vapous in the flue gas. By loweing the faction of H 2 SO 4, the dew point dops and eaches, by zeo concentation of the acid, the wate dew point t w. Paticula emphasis on combustion of enewable fuels fo CO 2 emission eduction and simultaneous demand fo themal utilisation of wastes guaantee that co-fiing of such fuels is now and will be in the futue unavoidable in the ation of lage powe boiles. The shae of enegy to be geneated fom enewable souces is in Poland established in Decee of the Ministe of Economy (2012) which adapt Poland s national law accoding to espective EU Diectives (2001) and (2003). *Coesponding autho, e-mail: szymon.ciukaj@polsl.pl 101

Sz. Ciukaj, M. Ponobis, Chem. Pocess Eng., 2013, 34 (1), 101-108 The co-fiing of biomass with coal in boiles may have twofold implications on low-tempeatue coosion and fouling pocesses. Both phenomena may be educed by co-fiing of usually low sulphu biomasses o enhanced due to inceased moistue content of biomass and inceased wate dew point. Most biomass fuels, which ae now bunt fo powe geneation, ae chaacteised by much lowe sulphu content than those found in coal. This leads to an indisputable positive effect on SO x emissions. Nevetheless, a simila positive effect may not be diectly associated with coosion and fouling hazads. The EU Decee (2003) in its Aticle 4(3) equests a gadual incease of biomass shae oiginating fom enegy cops and agicultual esidues. Thus, the unavoidable boadening of biomass vaiety beyond the most used wood, whose co-fiing chaacteistics ae elatively advantageous in tems of low S and Cl contents and high ash fusion tempeatues, is expected. The lowest acceptable flue gas outlet tempeatue t min fg _ out has to be set on the level avoiding condensation. By inceasing dew point the t min fg _ out has to be also inceased to pevent condensation what, in consequence, deceases boile efficiency. It has to be mentioned that the necessay elevation of t min fg _ out exceeds by moe than two times the incease of t. A espective example is pesented in Fig. 1, which has been calculated accoding to (Ponobis, 2002), assuming that the last heating suface is a RAH with ai inlet tempeatue 20 C. Fig. 1. Lowest acceptable flue gas outlet tempeatue as a function of the dew point 2. EXPERIMENTAL 2.1. Diect measuement technique Although diect estimation of the dew point may be caied out by means of vaious methods (Deichs et al., 1991), the conductivity method is usually used fo boile flue gases. The method is based upon detection of apid conductivity ise between two electodes when condensation occus. In paallel, the tempeatue of the suface being exposed to condensation is measued. The pobes ae made out of glass o ceamics in which electodes and themocouples ae embedded. When measuement takes place, a contolled dop of pobe tempeatue is induced by a cooling agent (compessed ai) until conductivity steeply ises. 102

Dew point of the flue gas of boiles co-fiing biomass with coal It is likely that new biomasses such as DDGS, ceeal staw, vaiety of gass, meat and bone meal and sewage sludge ae to be used fo powe geneation. The following consideations concening co-fiing ae based on 2 coals highly divese in tems of moistue content and caloific value in combination with 7 biomasses with stongly diffeent popeties - Table 1. The data fo coals A and B, BCT, WC and RG ae obtained fom analyses caied out at Wocław Univesity of Technology. 2.2. Popeties of coal and biomass fo the coosion studies When a elatively low caloific value biomass eplaces coal, significant amounts of moistue o sometimes sulphu, ae intoduced into the funace. An estimation of this phenomenon in tems of moistue and sulphu fed with biomass (when biomass displaces coal by enegy input) is calculated accoding to the following equation (fo sulphu): kg S fom biomass S _ equiv = ( Qi C / Qi Bio) ( SBio / SC ) (1) kg S fom coal Calculation esults ae gouped in Table 2 and values exceeding 1 ae typed in bold. Those values indicate co-fiing cases when lowe-than-coal caloific value biomass contibutes to inceased steams of moistue and sulphu being fed to the funace. It means, that the emission in mg/m 3 n o ppm will also incease - see values of S equiv in the last ow of Table 2. The esults show that biomass co-fiing may eithe decease sulphu steam fed to the funace, which is the case fo majoity of vegetable biomasses, o incease it when co-fiing sewage sludge, ape o DDGS. It should be noted that when co-fiing aw sewage sludge and othe wet biomasses, a multiplied wate steam fed into the funace will esult in a significantly inceased wate dew point. Table 1. Popeties of coals and additional fuels Unit Coal A B BCT WW (Wandasz et al., 2006) DSS (Wandasz et al., 2000) Additional fuels WC RG DDGS (IChPC, 2007) WSS (Nowaczyk, 2007) kj/kg 17878 21034 6295 14460 10900 9706 17502 18569 1000 W t % 19.4 9.70 52.42 20.00 10.34 45.65 5.25 6.90 77.50 S % 0.70 0.66 0.11 0.10 1.83 0.177 0.678 0.72 0.29 Table 2. Popeties of additional fuels calculated as enegy equivalent of coal (eq. 1) ic ibio Additional fuels BCT WW DSS WC RG DDGS WSS A B A B A B A B A B A B A B Q / Q 2.84 3.34 1.23 1.45 1.64 1.93 1.84 2.16 1.02 1.20 0.96 1.13 17.87 21.03 W_equiv 7.67 18.1 1.26 3.0 0.87 2.06 4.32 10.16 0.27 0.64 0.34 0.80 71.38 167.9 S_equiv 0.44 0.55 0.17 0.22 4.28 5.35 0.46 0.57 0.98 1.23 0.98 1.23 7.40 9.24 Fo pue coal combustion and co-fiing with biomass (biomass pecentages Z D =2.0; 5.0; 10, 20 and 50% on an enegy base), the mass pecent, Y D, poximate and elemental analysis as well as concentation of H 2 O in the flue gas was calculated based on popeties of fuels shown in Table 1. Fo 103

Sz. Ciukaj, M. Ponobis, Chem. Pocess Eng., 2013, 34 (1), 101-108 WSS the 50% shae was omitted, because fuel of such a composition cannot be combusted autothemally. Additionally, fo the sewage sludge shae Z D = 1.0% was investigated due to the fact that accoding to EU Diective (2000) this is an uppe limit allowed fo co-fiing. Below the limit the same emission standads ae to be kept as fo basic fuel e.g. coal. 2.3. Calculation of wate dew point In ode to estimate the influence of co-fiing fuels listed in Table 1 on the wate dew point level of the flue gas, calculations wee caied out and the esults ae pesented in Figs. 2 and 3. A wate vapou satuation tempeatue fo a given patial pessue was calculated fom Antoine equation (Ivine and Lilley, 1980). It was assumed that flue gas composition coesponds to that, which is typically obseved fo boile outlet, e.g. O 2 = 4.0%. The esults shown in the figues indicate that when changing coal popeties, the vaiations in dew point t w ae limited to 4 K. An addition of biomass pactically always inceases the dew point. Usually, when ealistic biomass amounts ae co-fied, the dew point incease does not exceed 8 K. Only when co-fiing wet sewage sludge, t w is shifted much moe, when it is accompanied by lowe sulphuic acid concentations being the esult of its dilution in a huge flue gas steam. Thee ae biomasses (e.g. DDGS) which pactically do not incease t w, and whee sulphu content in the fuel in compaison to that in coal does not decease. Based on the data pesented in Figs. 2 and 3 a dependence t w = f( W Σ ) was elaboated and shown in Fig. 4. The calculations wee caied out using the polynomial egession model with assumed confidence level 95 %. The diagam depicts that despite significant diffeences in the chaacteistics of coal and these of additional fuels, the wate dew point can be descibed by a function given with the accuacy, which shall be satisfactoy fo engineeing puposes. t Fig. 2. Calculated wate dew points while co-fiing biomasses with coal A It can be seen that a bette fitting is obtained fo highe than fo lowe moistue content. It is clea that the total moistue content in the flue gas diectly detemines the level of the dew point tempeatue. Howeve, the amount of moistue is not only dependent on wate content in the fuel but also on the amount of hydogen in the fuel and moistue in the ai led to the combustion. Fo the accuate wate dew point detemination the total moistue content in the flue gas should be measued. In case of utility 104

Dew point of the flue gas of boiles co-fiing biomass with coal boiles, calculations based on the moistue content in the fuel give a satisfactoy accuacy of detemining the wate dew point. Fig. 3. Calculated wate dew points while co-fiing biomasses with coal B Fig. 4. A dependence of wate dew point vesus moistue content of fuel mixtue when co-fiing coal and biomass (the lines depict bodes of confidence intevals) 2.4. Acid dew point Fo estimation of an acid dew point fo co-fiing, the investigations epoted in (Ponobis, 2002) wee used. The esults of these investigations ae shown in Fig. 5, which depicts the diffeences between the measued acid dew point and the calculated wate dew point t = t - t w in elation to sulphu content in coal. Fig. 5 shows that the measued tempeatues of acid dew points at the boile outlet (downsteam ai heate) wee only slightly highe than those calculated fo wate dew points. The diffeence between t and t w do not exceed 10 K. Chemical analyses of the fly ashes poved that fo all the cases the alkali compounds wee pesent in the quantities sufficient fo neutalisation of sulphuic acid in the flue gas. 105

Sz. Ciukaj, M. Ponobis, Chem. Pocess Eng., 2013, 34 (1), 101-108 The sulphates pesent in the ash (0.5-2.2 %) might be the poducts of this neutalisation. It was even moe evident when analysing the atio of fee CaO to H2SO4, which was an ode highe than the stoichiometic one. Fig. 5. The diffeence between acid and wate dew points in the flue gas of boiles in vaious Polish and Czech powe plants; dak maks denote measuements taken upsteam ai heate The investigations caied out upsteam the RAH point out that t in that egion may be significantly highe than downsteam ai heate. This poves that conditions in RAH pomote self-desulphuisation of the flue gas. Nevetheless, upsteam RAH the acid dew point was often only slightly highe than the wate dew point. The phenomenon obseved poves that in moden pulveised-fuel, low-emission boiles, the isk of coosion of flue gas ducts is geatly educed. Consequently, a low level of acid dew point allows fo lowe tempeatues of the flue gases to be used at boile outlet. 3. DISCUSSION As pesented ealie, co-fiing of some sots of biomasses contibutes to inceased steams of sulphu being fed to the funace. It does not mean that the sulphu pecentage in the fuel mix must also incease. As an example the WSS can be pointed out. Its co-fiing inceases the sulphu steam many times, deceasing simultaneously S Σ Figs. 2 and 3. Even in cases when an incease of S Σ is obseved (DSS, DDGS), the esulting values of sulphu content do not exceed in sevee degee the S values fo Polish coals. Wet woody biomasses, although they incease t w when co-fied, do not contibute to significant diffeences between acid and wate dew points due to thei low sulphu content. In such cicumstances, the acid dew point may even be lowe than that noted fo coal. Additionally, the wood ash consists mainly of CaO (Ponobis, 2002) impoving the flue gas desulphuisation. The only poblematic fuels ae fom this point of view DSS and WSS. The sulphu contents in othe biomasses listed in Table 1 do not diffe significantly fom those noted fo Polish coals. Theefoe, it is justified to assume that the acid dew point suplus ove the wate dew point is not likely to exceed 10 K when co-fiing biomass. 106

Dew point of the flue gas of boiles co-fiing biomass with coal 4. CONCLUDING REMARKS A significantly highe wate dew point is noted fo co-fiing of wet biomass with coal. This can be a paticulaly poblematic issue fo boiles not equipped with steam ai heates. Theefoe, it is ecommended that befoe a decision is taken on co-fiing of wet and high-sulphu biomasses, an appopiate installation peventing a tempeatue dop at the ai heate inlet is built. It is ecommended to avoid high moistue content coals when co-fiing biomass. An additional theat shall be associated with the pesence of substances catalysing fomation of SO 3. They can be fed into a boile when heavy oils ae used fo stat-up o flame suppot. These oils contain V 2 O 5, which then deposits on heating sufaces and contibutes to the incease of diffeence between wate and acid dew points. The above pesented consideations may lead to the conclusion that biomass co-fiing can eithe limit low-tempeatue coosion due to usually low sulphu content of biomasses, o incease it (due to an incease in wate dew point level when wet biomass is used, o due to inceased sulphu steams fed into boile with high sulphu biomasses). Theefoe, fo estimation of actual coosion isk it is mandatoy to cay out dew point measuements fo given opeating conditions. Fom last ow of Table 2 it can be seen that the values of S equiv ae in most cases lowe than 1 - only the co-fiing of DSS and WSS leads to inceased SO 2 emissions. SYMBOLS A ash content, % Q i lowe heating value, kj/kg S_equiv equivalent sulphu content (Equation 1) min t fg _ out lowest acceptable flue gas outlet tempeatue, o C t acid dew point of the flue gas, o C t w wate dew point of the flue gas, o C W moistue content, % t W_equiv equivalent moistue of biomass (Equation 1) [H 2 O] concentation of H 2 O in the flue gas, % YD biomass co-fiing pecentage (mass base), % ZD biomass co-fiing pecentage (enegy base), % Abbeviations BCT biomass fom conifeous tees DDGS died distilles gains with solubles DSS dy sewage sludge RAH otay ai heate RG ape gains WC wood chips WSS wet sewage sludge WW waste wood Subscipts C Bio coal biomass 107

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