THE BIOMASS-COAL CO-FIRING TECHNOLOGY FOR THE ENERGY PRODUCTION

Transcription

1 THE BIOMASS-COAL CO-FIRING TECHNOLOGY FOR THE ENERGY PRODUCTION Conf. dr. ng. Ionel PISA, Prof.dr.ng. Lucan MIHAESCU, Prof.dr.ng. Tudor PRISECARU, Prof. dr. ng. Ion OPREA, Conf. dr.ng. Gabrel NEGREANU, s.l.dr.ng. Malna PRISECARU, s.l. dr.ng. Elena POPA, as.ng. Crstna CIOBANU ABSTRACT The paper presents the results of the research relatng to the development of an effcent technology for bomass-coal co-combuston adapted to the types and sorts of bomass exstng n Romana. Ths target was dctated by the need to develop a new technology for usng mportant natonal energy sources (e.g. bomass). For ths the sources and types of zonal bomass and exstng quanttes and types o coal recommended to be used wth bomass have been dentfed. Dfferent co-combuston technologes, as well as the mathematcal modelng of the processes usng domestc bomass and coal characterstcs have also been evaluated on the bass of the results obtaned abroad. A number of canddate technologes have been selected and tested n a 2 MW thermal power plot nstallaton from ETCN -UPB The project s a contrbuton to the Romanan effort for fulfllng the commtment to reduce greenhouse gas emssons. Moreover, the co-combuston technologes dentfed n ths project wll become a strong tool for the reducton of the envronmental mpact of resdues from tmber ndustry and agrculture that are currently pollutng both the sol and water. 1. INTRODUCTION Bomass and coal co-combuston s known as a clean, effcent and economcal soluton for replacng the currently used fuels n the medum or large sze bolers. The most mportant reason for utlzng bomass for energy purposes s that bomass s neutral as to CO 2 emssons, whch means that t wll not contrbute to CO 2 emsson ncrease n the atmosphere. Co-combuston can be appled to any category of sold fuels from the coal class, especally to brown coal and hard coal, but pt coal and anthracte can also be used. Cocombuston can be appled to all wood or agrcultural bomass categores. Generatng electrcty from burnng a mxture of bomass and coal (co-combuston) represents for present day Romana one of the most adequate solutons, as the nvestments necessary for the development of new electrcty generatng unts from renewable energy sources are stll greater than those based on conventonal fuel burnng. In order to choose the most adequate burnng system the characterstcs of the fuel to be burnt, envronmental protecton legslaton, and the cost and performances of the avalable equpment have been consdered. Bomass can be burnt drectly (n the same way as wood for heatng), or smultaneously wth coal (cocombuston). The co-combuston technology rases the problem of adaptng the exstng capacty that burns exclusvely coal, to the combned burnng of coal and bomass. The ecologcal aspect ncludes CO 2 emsson reducton through bomass burnng, startng from the fact that the latter requres CO 2 n the course of ts bologc development. As concerns SO 2 emssons, the process of co-combuston enables, under certan crcumstances, the observance of the exstng envronmental protecton legslaton, even n the case of coals wth average sulfur content, due to the fact that any type of bomass s practcally sulfur-free

2 2 Utlzaton of bomass-coal co-combuston reduces slag and ash deposts, even then when the weght percentage of bomass mxed wth lgnte s hgher than 25%, f we consder that the sulfur content of the latter s % and the ash content represents 20-35%, so that the ecologcal effect s vsble (practcally bomass does not contan sulfur, and the ash percentage s usually below 5%) Co-combuston technology utlzaton ams at reducng fossl fuel consumpton, effcently utlzng the bomass avalable at present that would otherwse pollute the envronment, especally the sol and water, and reducng greenhouse gas emssons. 2. CO-COMBUSTION TECHNOLOGY RESEARCH CARRIED OUT IN A PILOT INSTALLATION. EXPERIMENTAL INSTALLATION Research on the burnng process of bomass resultng from forestry and agrculture mxed wth coal such as lgnte and hard coal has been carred out n a plot boler used for burnng coal wth the maxmum calorfc power of 2MW, fg.1, from the Conventonal and Nuclear Thermo-Mechancal Equpment Department (Char). Fg.1. The plot nstallaton for technologes testng ( 2 MW t ) The 2 MW t plot (testng) boler could be adjusted so that to enable the development of the expermental nstallaton for the combned coal bomass combuston technologes. The plot boler was equpped wth all the nstallatons necessary for the coal and bomass preparaton, nstallatons that are entrely ncluded n the ndustral bolers. From ths pont of vew the degree of smulaton s 100%. The same degree of smulaton has been attaned n connecton wth the grndng fneness, thermal loadng of the furnace and combuston effcency. The accuracy of the exhaust gas composton and pollutant emsson measurements has been obtaned by utlzng a number of hghly effcent analyzers. 3. UTILIZED FUELS Bomass s a general term used for a varety of resources, fgures 2, 3 and 4. Besdes wood, t ncludes wastes resultng from ndustral wood processng or agrcultural wastes.

3 3 Bomass burnng requres the modfcaton of the burnng process n any nstallaton due to the bomass composton, especally n regard of the content of volatles. The content of bomass volatles reaches 80%, whle the content of volatles n coal s of 30% for hard coal and 65-70% for lgnte. The calorfc power of bomass volatles s hgher than the Fg.2 Sawdust brquettes Fg. 3 Straw bales Fg. 4 Sunflower seed shells calorfc power of the coal volatles. A comparatve analyss of the energy characterstcs and physcal propertes ndcate that bomass dffers from coal, havng a lower carbon, alumnum and ron content and more oxygen, slca and potassum, a lower calorfc power and a hgher water content, lower densty and low relablty. Pyrolyss, the thermal decomposton, begns qucker n the case of bomass, and s slower n the case of coal. The lmt bomass mosture content for supportng burnng should not surpass 60% of ts weght. In Romana, sold fuels, manly brown coal (lgnte), hard coal (mxed) from the Ju Valley, and mport coal are utlzed for energy purposes. Although the qualty of ndgenous coal s satsfyng, Romana cannot cover the necessary amount from nternal producton and that s why hard coal from the neghborng countres such Ukrane, Poland and even South Afrca and Australa s mported. The qualty of coal represents the nput data of the analyss relatng to the selecton of the type of equpment chosen for the preparaton and burnng of coal n co-combuston wth bomass. A comparson of the fuel characterstcs s gven table 1. Table 1 Fuel characterstcs Propertes Coal Peat Wood Wood chps Hard coal Vegetal resdues Ash content, % Mosture content, % Heatng value, MJ/kg C, % H, % N 2, % O 2, % S, % < <0.05 <

4 4 Cl, % < K, % Ca, % TESTED TECHNOLOGIES The research conducted n a plot plant conssted n testng several technologes out of whch the followng have been selected: Pulverzed lgnte and saw-dust co-combuston Co-combuston of hard coal mxed wth sawdust max-brquettes on an ntegrated grate Co-combuston of hard coal wth corn kernels on an ntegrated grate Co-combuston of lgnte-sold agrcultural bomass: corn seeds, hay, sunflower seed shells 4.1 Research on the sawdust pulverzed co-combuston n a plot plant The scope of the research conssted n determnng the performances of the burnng process wth a vew to more effcently utlzng sawdust n a pulverzed state together wth lgnte. The effcency of the sawdust admsson procedure drectly fed nto the mll, the burnng speed (rate), flame heght of the lgnte-sawdust mxture for dfferent weght proportons of mxtures, burnng effcency and level of pollutant emssons have been envsaged. Fg.5. Constructon of the burner wth openngs (slots) (rght sde) and ts locaton n the furnace (left sde) The weght lmt for the sawdust chargng ranged between 20 and 30 %. The coal utlzed was lgnte from Muntena explotaton ste wth the followng element analyss C = 32%, H = 1.5%, S c = 1.1%, N = 0.5%, O = 7.9%, A = 26.3%, W t = 30.7%. The low calorfc power s kj/kg (Q c = kj/kg). Durng the experments the coal charge of 300 kg/h has been mantaned constant. The thermal power produced by coal burnng was: P tc = 882 kw Sawdust has a low calorfc power of kj/kg (Q r = kj/kg), wth the mosture content of W t =10.5% and ash A = 5.3%. Utlzaton of flue gas njecton at the end of the furnace for dryng coal n the predryng tower and burnng n the burner n Fg. 5 represented the nert barrer aganst the possblty of sawdust gnton n the mll. Ths had to be consdered due to the fact that prmary ar ntroduced n the mll had hgh values, smlar to those from ndustry of 220-

5 5 260ºC. Durng the entre test tme no gnton tendences on the mll separator route were notced. As n the case of ndustral processes the separator s equpped wth ant-exploson doors. The calorfc power of mxtures s: Sawdust n the weght proporton of 20%:Q m = kj/kg Sawdust n the weght proporton of 30%:Q m = kj/kg For burnng a slot burner, desgned for pulverzed lgnte burnng, was used. The temperature of the prmary agent comng out from the mll had low values as a result of the ncreased flow of the mll, surpassng the rated value, but wthn the normal (80-90ºC) lmts. For all the test regmes, burnng was stable and wthout varatons (the depresson at the end of the furnace vared between normal lmts, from -3 to -4 mmhg). The hgh content of volatle matter of both mxtures (values rangng between 54 to 64 %) enabled a fast and stable gnton. The thermal power for the two mxture qualtes was: For the sawdust weght percentage of 20%:P t1 = 1 MW For the sawdust weght percentage of 30%:P t2 = 1,04 MW The excess ar λ at the furnace ends ranged between the standard lmts for ths type of fuel (at the end of the furnace the excess ar expressed through the oxygen concentraton was of 4 6%). In the case of complete combuston the excess ar can be determned by means of the relaton λ = 21/(21-O 2 ). The values obtaned for the excess ar were: λ = The presence of bomass dd not pose any problems to the process of dust separaton n the nertal separator of the mll and the hydraulc transport of the sold phase through the ducts from the mll separator to the burner. The burnng materal n the slag amounted to about 1.7%, and n the ash to about 2.2%, values smlar to those obtaned n the energy sector for the lgnte post combuston grate burnng. The level of slag retenton n the furnace was of 0.13, wthn the normal lmts for the utlzed combuston procedure. The technology of co-combuston n pulverzed state of the sawdust and lgnte was a success. The burnng effcency of the operatonal data based on a proporton of 30% sawdust weght s: η a = 100 ( q m + qch + qrf ), % ; η a = 100 ( ) = 98.7% The pollutant emssons were wthn the lmts of the envronmental protecton legslaton n force for all the emsson categores (CO, SO 2, and NOx). The experment proved that the sawdust pulverzed coal co-combuston s a vable soluton. Implementaton of ths soluton requres only the development of the sawdust storage facltes and mll admsson nstallatons. Wthn the expermental research other burnng tests through the drect admsson of the sawdust wth ar n the burner were made but the research had only a general aspect. Nevertheless, the success of the second research work depends on the development of dedcated burners. 4.2 Research at a plot scale relatng to grate co combuston (max-brquettes of sawdust /corn wth brown coal) Technology testng n the laboratory nstallaton In analyzng the burnng process of bomass mxed wth coal, the frst step was to study the bomass burnng behavor. Bomass s a fuel that has been less analyzed as coal has had the greatest share so far. Coal burnng wth ts mplcatons represents a process that has been studed n detal by the energy specalsts. Bomass studes have ponted out that bomass effcency n generatng electrcty ranges between 20 to 25%. Durng combuston, a partcle of bomass passes through several stages, more or less dstnct. That s why we have

6 6 consdered t useful to develop and analyze exclusvely wood bomass burnng n the frst stage of our experments. In our approach the lab research on wood bomass represented the frst stage. From the lab analyses there resulted that the sawdust brquettes have a calorfc power between Kj/Kg, mosture content and ash The densty of the materal s kg/m 3 for the brquettes entrely made of sawdust and Kg/m 3 for the brquettes that also nclude sunflower n ther composton. In the lab, bomass burnng has been carred out n a HOTERM boler, fg. 6 for sold fuel burnng that ncludes a combuston chamber and a grate wth the walls cooled wth water desgned for the heatng crcut. Fg. 6 Laboratory boler Bomass burnng on the grate From the data analyss there resulted that the bomass burnng behavor was good. The sawdust brquettes burnng was stable. The small sze brquettes (chps) gnted more easly and the burnt completely. There were no problems n the supply process. The max brquettes utlzed can be handled and stored easly. Ths s one of the advantages of sawdust n compact form. The most effcent burnng was acheved wth the ar valve 100% (fully) open. At the same tme the brquettes that had sunflower shells n ther composton gnted and burnt better (wth a more powerful flame) than the brquettes made of sawdust. The sunflower shells represent hgher calorfc power bomass n themselves. When burnt n crushed form t was necessary to blow supplementary ar under the grate for ntensfyng the process and for completely burnng the sawdust partcles. 4.3 Research at a plot scale for testng the technology wth the grate ntegrated nto the plot nstallaton The second stage of the expermental research was the co-combuston of bomass and coal. Ths was carred out n the UPB plot nstallaton and enabled the mplementaton of the fxed grate burnng technology. The utlzed fuels were: A mxture of btumnous coal from Petrla and sawdust max brquettes (20x6 mm) A mxture of btumnous coal from Petrla and corn shells The btumnous coal bomass co-combuston conssted n burnng the mxture under the proportons recommended by the lterature n the feld. The composton of bomass depends on ts apparent densty. In order to fnd the optmum proporton, a mxture of the followng proportons has been developed: btumnous coal 50% wth 50% of sawdust max brquettes The combuston ar was njected under the grate bars by means of an ar blower wth 70% ar feed. The rest of the ar at 200 o C was njected from 1.5 m dstance over the grate. As

7 7 a general remark for the grate co-combuston, regardless of the used fuels, the NOx emsson was not hgh due to the gradual (n stages) ar njecton technology. The secondary ar njected over the grate reduces (lmts) the NOx emsson. Durng the entre operaton the flame was slghtly lumnous. The values of the general operatonal parameters were normal, comparable to the values obtaned durng conventonal fuel burnng. The burnng took place wthout parameter varatons or specal event occurrence. The sawdust brquettes gnted n 10 mnutes. Representatve data relatng to btumnous coal-sawdust max brquettes burnng: The burnng process was stable the furnace thermal testng (burnng process stablty) q v =84,2 kw/m 3 the grndng process n not requred, the fuels were crushed and burned as such the ar was njected n stages and ths led to a sub-stochometrc burnng wth low NOx emssons the relatvely hgh value of the unburned matter n slag of 14.5% the relatvely hgh value of unburned matter n ash of 18.2% 4.4 Plot scale research on the coal sold agrcultural bomass co-combuston The development of ntensve energy crops requres that certan techncal problems relatng to ther better utlzaton by effcent burnng be solved. Bomass n general, and agrcultural bomass, respectvely, n partcular, does not contan sulfur, so that the reducton n SO 2 emssons when burnng fossl fuels contanng sulfur wll be proportonal wth the bomass thermal proporton burned n co-combuston. From the pont of vew of NO x emssons, the values are hgher when burnng sold agrcultural bomass than when burnng coals, due to ther hgher ntrogen content. Consequently, the expermental research that has been carred out wthn ths project envsaged the NO x emssons, as well. The techncal aspects are stll related especally to the supply of the burnng nstallaton wth sold agrcultural bomass, whch, except for the corn seeds, have a very low apparent densty. Consderng the hgh calorfc power (30000 kj / kg) and the hgh apparent densty, the corn seeds represent the deal sold agrcultural fuel. The corn seeds can be also drectly and easly fed nto the mll. The coal mll wll grnd t approprately grnd both the coal and the corn seeds. The corn cobs can be stored more easly than the corn seeds, but t has to be crushed before feedng t nto the coal mll. The corn, both the cobs and the seeds, can be very easly co-burnt mxed wth coal by means of the fxed and moble grate technology. In our country, n the thermal power sector such nstallatons do no longer exst, so that the research should be drected towards pulverzed coal co-combuston. The presence of post-combuston grates located n the funnel of certan bolers runnng on lgnte makes co-combuston easer both n the case of corn or other agrcultural fuels from the wastes, such as seed shells or vegetaton waste. Burnng tests on corn seeds n pulverzed state resultng from mll grndng (100% proportonng) alone have also been performed. Nevertheless, durng these tests the tubes connectng the mll separator and the burner got clogged. The tube cloggng was manly caused by the mosture content and the hgh fneness of the corn partcles obtaned by grndng n the fan mll. Ths test dd not represent the objectve of ths research t was just a supplementary test. Menton should be made that, durng dry corn seed mllng explosons may occur n the mll separator as a result of the hgh content of volatles, that s why t s compulsory to use protecton water.

8 8 The lgnte sunflower shells co-combuston was performed by means of sunflower seeds the weght proporton of whch was 20%. The shells were ground n the mll at the same tme wth the coal and were only slghtly ground. In our opnon, to 30 mllon tones of coal that are annually utlzed n Romana at a weght proportonng of 10-20% the maxmum amount of necessary sold agrcultural bomass ranges between 3 and 6 mllon tones of sold agrcultural bomass. For the frst ndustral stage t s estmated that ths amount for all the types of sold agrcultural sold bomass t wll not be greater than mllon tones. 5. CONCLUSIONS The measurement analyss ndcated that burnng pulverzed sawdust (n suspenson) together wth coal, drectly fed nto the mll, was a success. As sawdust storage dd not pose any problems t was suppled n the form of pellets. For a sawdust producer t mght be much easer to send t to a thermal power plant than to more effcently utlzng for ts own purposes n a specally desgned boler. Corn grndng together wth coal n a fan mll dd not rase problems, the mll energy consumpton beng close to that necessary for coal. Nevertheless, a reducton n the mll upset overpressure of about 10 15% was notced. There were no notceable varatons of the mll upset temperature, as the lgnte and corn mosture contents were wthn close lmts. The temperature at the end of the furnace durng coal and corn pulverzed burnng dd not vary sgnfcantly as compared wth the smlar load of the boler when burnng only coal. NO x emssons ncreased by 15 20%, whch was to be expected as both sold agrcultural and wood bomass ntrogen content s hgher than that of lgnte. By utlzng step sub-stochometrc burnng NO x emssons could be mantaned wthn envronmental protecton lmts (400 mg/m N 3 ) as NO x emssons n NO 2 equvalent was: NO 2 = mg/m N 3. The slot burner for the pulverzed coal was desgned so that to operate under substochometrc workng condtons. The NO x.emsson was consdered. The SO 2 emsson was sgnfcantly reduced, rangng between 30 40%, as a result of the lack of sulfur n the composton of the sold agrcultural bomass. As concerns grate burnng, utlzaton of sawdust max brquettes, corn cobs, and coal co-combuston can be performed. When utlzng the grate burnng technology, burnng was stable, the thermal load of the furnace ndcatng the stablty of the burnng process. The condtons were smlar to those n the case of burnng n the ndustral bolers operatng n our country. The advantage of ths technology s that t does not requre a grndng system, the fuels beng crushed and burnt as such. Nevertheless, the ar that s ntroduced n steps leads to sub-stochometrc burnng. Ths s reason accountng for the hgh value of unburned matter n the slag and ash: n slag % and n ash %. The research on the pulverzed coal (lgnte) and sold agrcultural bomass co combuston (corn, etc.) were successful and represent an alternatve n the future for utlzng renewable sold fuels. Due to ts hgh densty and calorfc power, lack of sulfur from ts composton and low mneral materal (ash) resultng from burnng, corn represents the deal sold agrcultural bomass for co-combuston wth pulverzed coal. In our opnon, co-combuston can solve the problem of utlzng great amounts of renewable fuels. Besdes solvng certan economc problems, co-combuston technologes sgnfcantly contrbute to the thermal energy sector ntegraton nto the ncreasngly strct envronmental protecton laws.