modelling of plasma supported coal combustion in fullscale...

Aliya S. Askarova1, Evgeni I. Karpenko2, Oleg A. Lavrichshev3,Vladimir E. Messerle3, Alexandr B. Ustimenko3

1 Department of Physics, al-Farabi Kazakh National University, Almaty, Kazakhstan2 Branch Centre of Plasma-Power Technologies of Russian J. S. Co. “UPS of Russia”, Gusinoozersk, Russia3 Research Institute of Experimental and Theoretical Physics, Research Department of Plasmotechnics, Almaty, Kazakhstan

Modelling of Plasma Supported CoalCombustion in Fullscale Boiler

Orig i nal sci en tific paperUDC: 662.62:66.088:66.011

Plasma ac ti va tion pro motes more ef fec tive and en vi ron men tal friendly low-rank coalcom bus tion. This work pres ents nu mer i cal mod el ling re sults of plasma thermo chem i calprep a ra tion of pul ver ized coal for ig ni tion and com bus tion in the fur nace of a util ityboiler. Two ki netic math e mat i cal mod els were used in the in ves ti ga tion of the pro cesses ofair-fuel mix ture plasma ac ti va tion, ig ni tion, and com bus tion. A 1-D ki netic code,PLASMA-COAL, cal cu lates the con cen tra tions of spe cies, tem per a tures, and ve loc i tiesof treated coal-air mix tures in a burner in cor po rat ing a plasma source. It gives ini tial datafor 3-D mod el ing of power boil ers fur naces by the code FLOREAN. A com pre hen sive im -age of plasma ac ti vated coal com bus tion pro cesses in a fur nace of pul ver ised coal firedboiler was ob tained. The ad van tages of the plasma tech nol ogy are clearly dem on strated.

Key words: coal, plasma-fuel sys tem, furnace, plasma aided com bus tion, ki netic sim u -la tion, three-di men sional mod el ling

In tro duc tion

The con ven tional com bus tion of solid fu els ac counts for higher pol lut ant lev elsthan emit ted by liq uid and gas fu els. The in crease in the solid fu els frac tion in the world -wide fuel bal ance de mands new, more ef fec tive, and eco log i cally ac cept able tech nol o -gies. One pos si ble so lu tion is “pul ver ised fuel thermo chem i cal prep a ra tion” of the coalus ing plasma [1]. Plasma ac ti va tion pro motes more ef fec tive and en vi ron men tal friendlypower coals com bus tion. The tech nol ogy uti lises air-fuel mix ture arc plasma to heat thecoal fuel to the tem per a ture of vol a tile re lease and char car bon par tial gasi fi ca tion. In this man ner, from an air-coal mix ture a highly re ac tive two-com po nent fuel is ob tained, re -gard less of whether the ini tial coal is of a high or low rank. When mixed with the sec ond -ary air in the fur nace it can be ig nited and caused to burn stably with out the need forsup ple men tary fu els (fuel oil or nat u ral gas) tra di tion ally used for start-up the boil ers andfor flame sta bili sa tion when the coal is out of de sign qual ity.

A. S. Askarova, et al.: Modelling of Plasma Supported Coal Combustion in Full-Scale BoilerTERMOTEHNIKA, 2009, XXXV, 2, 149‡162

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Fig ure 1 pres ents a sche matic of the plasmatron and its mount ing onto a pul ver -ized coal burner. The plasmatron is the main el e ment of the plasma-fuel sys tem (PFS). Itis a low-tem per a ture plasma gen er a tor with the plasma gas be ing formed from air blownthrough the elec trodes.

Ef fec tive de vel op ment of a new tech nol ogy is im pos si ble with out prior and ac -com pa ny ing pro cess mod el ling and nu mer i cal in ves ti ga tions. Two fluid dy nam ics ki neticmath e mat i cal mod els are con sid ered herein. Nu mer i cal sim u la tions of plasma thermochem i cal prep a ra tion of fuel for ig ni tion and com bus tion of a pul ver ized coal in util ityboil ers are per formed. Com par i son of the cal cu lated re sults with ex per i men tal data ob -tained un der fullscale in dus trial con di tions are ef fected.

One-di men sional mod el ling of pul ver ized coalplasma ac ti va tion in a plasma-fuel sys tem

The cal cu la tions of the coal thermo chem i cal plasma prep a ra tion pro cesses inthe PFS are per formed with the help of a spe cially con structed math e mat i cal model of the flow, heat trans fer and thermo chem i cal trans for ma tion of coal-dust fuel in the plasmade vice (fig. 1) [2]. The model de scribes two-phase (coal par ti cles and gas-oxi diser), chem -i cally re act ing flow, with an in ter nal heat source (elec tric arc, plasma torch or chem i cal re -ac tions). Gas and coal par ti cles are as sumed to en ter the PFS with equal tem per a tures.There is par ti cle-to-par ti cle, gas-to-par ti cle, and gas-to-elec tric arc heat and mass ex -change. Heat and mo men tum ex change be tween the flow and the wall of the PFS is ac -counted for. Also, some fuel chem i cal trans for ma tions are con sid ered. They are thefor ma tion of pri mary vol a tile prod ucts, con ver sion of evolved vol a tile prod ucts in the gasphase and the char car bon gasi fi ca tion re ac tions. An en trained flow re ac tor is con sid eredand plug flow is as sumed. The re sult ing set of or di nary dif fer en tial equa tions in cludesequa tions for spe cies con cen tra tions (chem i cal ki net ics equa tions) in cowunction withequa tions for gas and par ti cles ve loc i ties and tem per a tures, re spec tively. The model isde scribed in de tail in [2-4].

The model is dis tin guished by its de tailed de scrip tion of the ki net ics of the chem -i cal re ac tions whose gen eral scheme, along with the re ac tions of the evo lu tion of pri maryprod ucts, takes into ac count the re ac tions of their fur ther gas phase trans for ma tions andchar car bon gasi fi ca tion (tab. 1).

A. S. Askarova, et al.: Mod el ling of Plasma Sup ported Coal Com bus tion in Full-Scale BoilerTERMOTEHNIKA, 2009, XXXV, 2, 149–162

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Fig ure 1. Sche matic of theplasmatron and its mount -ing onto a pul ver ised coalburner (plasma-fuel sys -tem)


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Ta ble 1. Ki netic pa ram e ters of re ac tions in cluded in the math e mat i cal model

N Reaction(1) Lg A(2) n E

1 [H2]S = H2 18.2 0 88.8

2 [H2O]S = H2O 13.9 0 51.4

3 [CO]S = CO 12.3 0 44.4

4 [CO2]S = CO2 11.3 0 32.6

5 [CH4]S = CH4 14.2 0 51.6

6 [C6H6]S = C6H6 11.9 0 37.4

7 [C5H5N]S = C5H5N 11.9 0 22.8

8 [C4H5N]S = C4H5N 11.9 0 37.4

9 [CH3SH]S = CH3SH 11.9 0 37.4

10 [C4H4S]S = C4H4S 11.9 0 37.4

11 NS + NS + O2 = NO + NO 5.66 0 38.0

12 C + NO = CO + N 1.53 0 20.0

13 C + H2O = CO + H2 11.32 0 60.8

14 C + CO2=CO + CO 13.2 0 83.6

15 C + O2 = CO2 9.42 0 38.0

16 C + C + O2 = CO + CO 9.72 0 41.8

17 C+H2S=CSS+H2 7.0 0 32.0

18 CH4+H=CH3+H2 11.1 0 11.9

19 CH4+OH=CH3+H2O 0.54 3.1 2.0

20 CH4 + M = CH3 + H + M 14.15 0 88.4

21 CH4+O=CH3+OH 10.2 0 9.2

22 CH3 + H2O=CH4+OH 9.84 0 24.8

23 CH3 + H2 = CH4 + H 9.68 0 11.4

24 CH3 + M = CH2 + H + M 13.29 0 91.6

25 CH3 + O2 = CH3O + O 10.68 0 29.0

26 CH3 + OH = CH2O + H2 9.6 0 0

27 CH3 + O=CH2O + H 11.11 0 2.0

28 CH3O + M = CH2O + H + M 10.7 0 21.0

29 CH2O + M = HCO + H + M 13.52 0 81.0

30 HCO + M = H + CO + M 11.16 0 19.0

31 O2 + M = O + O + M 12.7 0 115.0

32 H2 + M = H + H + M 11.34 0 96.0

33 H + O2 = O + OH 11.27 0 16.8

34 H + H2O = H2 + OH 10.98 0 20.3

35 H2 + O = H + OH 7.26 1.0 8.9

36 H2O + M = H + OH + M 13.3 0 105.0

37 H2O + O = OH + OH 10.53 0 18.3

38 CO + OH = CO2 + H 4.11 1.3 –0.77

39 CO + O2 = CO2 + O 11.5 0 37.6

40 CO2 + H = CO + OH 6.15 1.3 21.6

41 CO + O + M = CO2 + M 12.77 0 4.1

42 C2H2 + M = C2H + H + M 11.0 0 114.0


43 C2H2 = C + C + H2 6.0 0 30.0

44 C2H2 + O2 = HCO + HCO 9.6 0 28.0

45 C2H2 + H = C2H+H2 11.3 0 19.0

46 C2H2 + OH = CH3 + CO 9.1 0 0.5

47 C2H2 + O = CH2 + CO 10.83 0 4.0

48 CH2 + H2O = CH2O + H2 11.0 0 3.7

49 CH2 + O2 = HCO + OH 11.0 0 3.7

50 C2H + O2 = HCO + CO 10.0 0 7.0

51 C2H + H2O = CH3 + CO 9.08 0 0.5

52 C6H6 = C2H2 + C2H2 + C2H2 12.0 0 85.0

53 OH + OH = H2O + O 9.5 0 1.1

54 H + OH + M = H2O + M 10.56 0 0.0

55 H + H + M = H2 + M 9.56 0 0.0

56 CH2O + OH = HCO + H2O 10.5 0 1.5

57 H + OH = H2 + O 9.84 0 7.04

58 H2 + OH = H2O + H 11.4 0 10.0

59 C5H5N = HCN + C2H2 + C2H2 12.57 0 70.4

60 C4H5N = NH3 + C2H + C2H 15.9 0 85.6

61 HCN + H = CN + H2 11.0 0 20.8

62 HCN + OH = CN + H2O 9.3 0.60 2.5

63 CH + N2 = HCN + N 10.3 0 48.0

64 CH2 + N2 = HCN + NH 9.45 0 22.5

65 NH3 + H = NH2 + H2 8.28 0.67 3.4

66 NH3 + OH = NH2 + H2O 7.6 0.68 1.1

67 NH3 + O = NH2 + OH 8.91 0.5 0.0

68 NH2 + H = NH + H2 8.15 0.67 4.3

69 NH2 + O = NH + OH 8.96 0.5 0.0

70 NH2 + OH = NH + H2O 7.48 0.68 1.3

71 NH + OH = N + H2O 9.2 0.56 1.5

72 NH + O = N + OH 9.93 0.7 0.1

73 NH + H = N + H2 9.0 0.68 1.9

74 NH + N = N2 + H 10.0 0 0.0

75 NH + NH = N2 + H2 8.6 0.55 1.9

76 CN + O = CO + N 10.07 0 0.0

77 NCO + H = NH + CO 8.3 0 0.0

78 CN + O2 = NCO + O 10.5 0 1.0

79 NH + O = NO + H 9.7 0 0.0

80 NH + OH = NO + H2 9.2 0.6 1.5

81 N + O2 = NO + O 6.8 1.0 6.3

82 N + OH = NO + H 8.8 0.5 0.0

83 N2 + O = NO + N 11.13 0 75.09

84 NO + NH = N2 + OH 9.4 0 0.0

The tem per a ture de pend ence of rate con stants is gov erned by the Arrheniusequa tion:

k AE

TTj j

j n= -æ

èçç

ö

ø÷÷exp

R

The coal com po si tion is rep re sented in the model by its or ganic and min eralparts. The or ganic mass of coal is spec i fied by the set of the func tional groups (CO, CO2,CH4, H2O, tar), fuel ni tro gen, sul phur and car bon, and the min eral part is rep re sented asash with ap pro pri ate thermo phys i cal prop er ties.

Ac cord ing to the as sumed scheme, the first chem i cal stage of the pro cess is coalther mal de struc tion (re ac tion 1-11 in tab. 1). The coal par ti cles gen er ate vol a tile and tarcom po nents. They are ben zol (C6H6), pyridine (C5H5N) and pyrrole (C4H5N), thiophene (CH9SH), and mercaptan (C4H4S).

The in ter ac tions be tween the char car bon and wa ter vapour, ox y gen, car bon di -ox ide, ni tro gen ox ide, and hy dro gen sul phide (re ac tions 12-17) are the rate-lim it ingstages of the pro cess. Each of the above re ac tions is a com plex pro cess, and in cludes el e -men tary stages: re agent ad sorp tion on the par ti cle sur face, dis so ci a tion, re ac tions in thegas eous phase, desorption, etc. The de tailed mech a nism of these re ac tions was not takeninto ac count in the model. Only the whole trans for ma tion, in ac cor dance with the grossre ac tions 12-17, was con sid ered.

For ni tro gen ox ide (NO) for ma tion, a model em body ing fuel NO, ther mal NOand prompt NO was con sid ered [5, 6]. The model com prises 36 chem i cal re ac tions (tab.


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Ta ble 1. (Con tin u a tion)


85 N + NO = N2 + O 10.3 0 0.0

86 NO + H = N + OH 10.5 0 48.0

87 NO + NH2 = N2 + H2O 9.9 0 0.0

88 N + N + M = N2 + M 8.7 0 0.0

89 CN + NO = CO + N2 11.0 0 7.6

90 N2 + M = N + N + M 14.3 0.5 225.0

91 CaCO3S + SO2 = CaSO3S + CO2 4.16 0 15.3

92 CaCO3S + H2S = CaS + CO2 + H2O 9.82 0 50.11

93 CaOS + H2S = CaSS + H2O 3.3 0 17.2

94 CaOS + SO2 = CaSO3S 8.0 0 32.0

95 FeS2S + H2 = FeSS + H2S 9.31 0 47.3

96 FeS2S + H2O = FeSS + H2 + SO 9.31 0 47.3

97 FeS2S + O2 = FeSS + SO2 0.0 0 0.0

98 FeSS + H2 = FeS + H2S 10.2 0 55.4

99 FeS + H2S = FeSS + H2 3.45 0 18.0

100 CaCO3S = CaOS + CO2 10.55 0 49.0

(1) Equa tions 1-6 are the devolatilization re ac tions(2) Di men sions of Aj are [s–1] for the first-or der re ac tions and [litre·mol–1s–1] for the sec ond-or der re ac tions


101 CH3SH = CH3 + HS 13.48 0 67.0

102 C4H4S = C2H + C2H + H2 S 12.0 0 85.0

103 HS + HS = H2S + S 10.0 0 0.0

104 H2S + M = HS + H + M 11.3 0 74.6

105 H + H2S = HS + H2 9.9 0 1.72

106 H + HS = H2 + S 10.3 0 0.0

107 S + H + M = HS + M 9.0 0 0.0

108 H2S + OH = H2O + HS 9.3 0 0.0

109 H2S + O = OH + HS 8.2 0 1.5

110 HS + O = SO + H 11.0 0 0.0

111 HS + O2 = SO + OH 7.8 0 0.0

112 SO + O2 = SO2 + O 8.54 0 6.5

113 SO + OH = SO2 + H 10.8 0 0.0

114 SO + O + M = SO2 + M 11.5 0 0.0

115 S + O2 = SO + O 9.1 0 0.0

116 SO2 + O + M = SO3 + M 11.38 0 2.5

1). Among them there are the re ac tions of C5H5N and C4H5N emis sion from coal, char ni -tro gen (N) ox i da tion, the fol low ing con ver sion of them through hy dro gen cy a nide (HCN) and am mo nia (NH3) to NO.

Con ver sions of the com po nents con tain ing sul phur are de scribed by re ac tions91-116. Note, ki netic scheme takes into ac count sul phur ox ides (SOx) for ma tion both inthe gas phase (re ac tions 100-106) and from gas eous spe cies in ter ac tion with com po nentsof coal min eral mass (re ac tions 107-116).

The model is real ised by the com puter code PLASMA-COAL which uses Gear’s method for the so lu tion of stiff sys tems of chem i cal ki net ics equa tions. Ex per i men tal val i -da tion of the one-di men sional model was per formed for dif fer ent coal-ox y gen flow ra -tions, plasma source power lev els and pro cesses (steam gasi fi ca tion, com bus tion and airgasi fi ca tion). Sat is fac tory re sults were achieved [4, 7].

The plasma thermo chem i cal prep a ra tion of coal for com bus tion is a com plexther mal pro cess which oc curs in sev eral stages. The plasma flame ini tially heats theair-coal mix ture, fol lowed by ig ni tion, and then by spon ta ne ous com bus tion and gasi fi ca -tion of the coal in the pri mary air of the mixture.

Cal cu la tions were per formed for a cy lin dri cal through flow PFS equipped with aplasmatron of 100 kW power. The di am e ter of the PFS is 0.73 m, its wall tem per a ture isas sumed to be 700 K. The PFS ther mal ef fi ciency was taken as 90%. The mean di am e terof the coal par ti cles is 60 mm. The tem per a ture of the air-coal mix ture at the in let of thePFS is 423 K. The PFS coal con sump tion rate was 7.3 t/h. The chem i cal anal y sis of thecoal, Ekibastuzski (Kazakhstan), is given in tab. 2.

The re sults of the nu mer i cal sim u la tions are pre sented in figs. 2 and 3. The gasspe cies con cen tra tions – fig. 2(a), at the PFS out let com prise com bus ti ble gas (CO + H2 ++ CH4 + C6H6) of 39.2%, ox i dants (CO2 + H2O) of 3.9% and ni tro gen (N2) of 56.8%.Fig ure 2(b) shows the vari a tion of the de gree of coal gasi fi ca tion along the PFS. The de -gree of gasi fi ca tion rises monotonically, at tain ing 81% by the end of the PFS.

Fig ure 3 dis plays the gas phase tem per a ture and gas and sol ids ve loc ity vari a tionalong the PFS. By the end of the PFS, the gas reaches a tem per a ture of 1328 K. The dif fer -ence be tween gas and sol ids tem per a tures is neg li gi bly small. The ve loc i ties of the gas and par ti cles in crease and are lit tle dif fer ent, the gas ve loc ity ex ceed ing that of the sol ids byless than 10%. By the end of the PFS (in let to the furnace) they are in equilibrium.

It is ev i dent that the gas eous prod ucts at the above tem per a tures will self ig niteon mix ing with the sec ond ary air in a fur nace. Note that the char car bon heated to suchhigh tem per a tures (~1300 K) will also ig nite. This two-com po nent highly re ac tive fuel will en hance ig ni tion and com bus tion of the main pul ver ised coal in a fur nace and will al lowstart-up of the boiler with out the need for sup ple men tary nat u ral gas or fuel oil.


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Ta ble 2. Chem i cal anal y sis of the bituminous coal (% dry mass ba sis)

Ash C O H N S SiO2 Al2O3 Fe2O3 CaO MgO

44 44.47 6.97 3.03 0.72 0.81 28.51 13.59 1.20 0.33 0.37

The coal mois ture con tent is 7%; vol a tile mat ter is 24%; lower cal o rific value is 16750 kJ/kg

It is seen from the ta ble that the NO con cen tra tion is quite low. This is due to theknown mech a nism of fuel ni tro gen ox ides sup pres sion in PFS [1]. In side the PFS, fuel ni -tro gen evolves to the gas phase to gether with the vol a tile mat ter of the coal. At this lo ca -tion, there is a def i cit of ox y gen for com plete com bus tion of the coal, so the fuel ni tro gentrans forms to its mo lec u lar form. The tem per a ture in side the PFS is not enough (fig. 3)for sig nif i cant ther mal NOx for ma tion. As re gards sul phur ox ides, their con cen tra tion atthe PFS out let is de ter mined mainly by the sul phur con tent of the coal (tab. 2).

Ta ble 3 pre sents the cal cu lated re sults at the PFS out let (x = 2 m)The data (tab. 3) were taken as ini tial con di tions for the 3-D sim u la tions of the

fur nace equipped with a plasma-fuel sys tem.


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Fig ure 3. Gas tem per a ture (a) and gas and sol ids ve loc ity (b) vari a tion along the PFS

Ta ble 3. 1-D cal cu la tion re sults of the coal plasma prep a ra tion for com bus tion

CO H2 CH4 C6H6 CO2 H2O N2 O2 NO SOx XC

[%]Vg

[ms–1]Tg

[K]tg[s]Volume [%] [mgNm–3]

25.3 13.3 0.3 0.3 3.8 0.1 56.8 0 7.5 2126.6 81.5 55.4 1328 0.023

Fig ure 2. Spe cies con cen tra tions (a) and coal gasi fi ca tion de gree (b) vari a tion along the PFS

Three-di men sional mod el ling of plasma ac ti vatedcoal com bus tion in a fur nace

The com puter sim u la tion was per formed with the help of the com puter codeFLOREAN [8, 9] ap pli ca ble to coal-fired fur naces. This code is based on the so lu tion ofthe con ser va tive equa tions of the flue gas mix ture by the fi nite vol ume method. It in -cludes a submodel of mo men tum and en ergy bal ances, the SIM PLE-method for pres sure cor rec tions, a k-e tur bu lence model, the six-flux model for ra di a tion heat trans fer, and abal ance equa tion for the com po nents of sub stance. This com puter code al lows to ex trac -tion of the ve loc ity com po nents {u, v, w}, the tem per a ture T, the pres sure P, the con cen -tra tion of com bus tion prod ucts, and other char ac ter is tics of the pro cess. Pul ver ised coalho mog e nous model was used. By the model sol ids ve loc i ties are in dy namic equi lib riumwith gas. The solid phase in flu ence to the tur bu lent trans fer was ac counted for with thehelp of a ra tio for the tur bu lent vis cos ity mPeff = mGeff(1 + rP/rG)–1/2 [10].

For the de scrip tion of the chem is try, sim pli fied mod els were used. They ac countonly for the chem i cal re ac tions of the main com po nents and the over all re ac tions of thefuel com po nents ox i diz ing to their sta ble end-prod ucts intermediate re ac tions, and alsothe for ma tion of un sta ble in ter me di ate prod ucts, are not taken into consideration.

The model from [12], which de mands min i mal com pu ta tional ex pense, wasused. More over, this model has been tested for a large num ber of lab o ra tory ex per i mentsand in large com bus tion cham bers. This model takes into ac count the re ac tions ofdevolatilization (wa ter steam, hy dro gen, car bon ox ides, meth ane) and also the trans for -ma tion re ac tions of the ini tial volatiles re leased from the coal as gas eous fi nal prod ucts(CO, CO2, H2O, H2, O, H). The re ac tions and their ki netic data are given in [8, 12].

The for ma tion of NO from the ni tro gen-con tain ing spe cies of fuel is cal cu latedby a sim pli fied scheme. Ac cord ing to this scheme the ni tro gen-con tain ing spe cies are de -com posed dur ing the com bus tion of the vol a tile mat ter to ac tive atomic ni tro gen, whichpar tially re com bines into mo lec u lar ni tro gen (N + N ® N2) and par tially oxi dises to amix ture of NOx (mN + O2 ® mNOx). For NOx for ma tion, a global mech a nism is used.The NOx-model con sid ers 31 el e men tary re ac tions with ac count of 15 spe cies. Zeldovichmech a nism is used for ther mal NO for ma tion de scrip tion. The mech a nism of fuel NOfor ma tion is taken from [15]. The model in volves NH3, HCN, NO, and N2 as ni tro -gen-con tain ing spe cies. The first re ac tion step is the con ver sion of HCN to NH3 by an at -tack of an ox i diz ing agent. The NH3 forms and de structs NO within a pair of com pet i tivepar al lel re ac tions. And the re cy cling of NO back to HCN through hy dro car bons is ac -counted [10]. The con cen tra tion of re sult ing mo lec u lar ni tro gen and mix ture of ni tric ox -ides dur ing gas com bus tion is found by the so lu tion of the equa tion of ho mo ge neouski net ics [11].

The code FLOREAN has been val i dated in de tails for a lot of ex per i ments inlab o ra tory and in dus trial fur naces [8, 9, 13, 14].The com puter sim u la tion ex per i mentswere con ducted for low-rank Ekibastuz bi tu mi nous coal (see tab. 2) sup plied to a boilerwith a steam pro duc tiv ity of 475 tonnes per hour. The boiler is in stalled at the ErmakovTher mal Power Plant in Kazakhstan. The fur nace size is as fol lows: 30 m height, 10 m inwidth and 7 m depth. There are twelve swirl burn ers. They are placed at two lay ers, six per


155

layer fac ing each other. A com pu ta tional grid of 27 ´ 61 ´ 60 was used for the nu mer i calsim u la tion. The cal cu la tions were car ried out for four, six, and twelve burn ers re placedwith, PFS. In the case of the four PFS they are placed in the fur nace cor ners at the lowerlayer. Next vari ant is six PFS placed in the lower layer. The last vari ant is twelve PFS in -stead of all main burn ers.

The model pre dic tions are pre sented in the fol low ing fig ures which show re sultsfor plasma ac ti vated coal com bus tion com pared with con ven tional coal combustion.

Fig ure 4 shows tem per a tures and NO con cen tra tions along the fur nace with andwith out plasmatron as sis tance. One can see that in gen eral the tem per a tures along thefur nace for plasma-ac ti vated coal com bus tion with four and six plasma sys tems are lessthan those for con ven tional coal com bus tion. How ever, a zone ex ists in the lower part ofthe fur nace where the tem per a ture of coal com bus tion with plasma ac ti va tion is morethan that with out it. It ex tends to the level of the up per tier of burn ers. It can be ex plainedby the in flu ence of the plasma-fuel sys tems. They cause ear lier heat ing and ig ni tion of theair mix ture and sub se quent dis place ment of the com bus tion front to the zone ofplasma-fuel sys tems. The ob served tem per a ture de crease in the up per part of the fur nace in di cates ear lier com ple tion of the coal com bus tion pro cess. Con cern ing NO for ma tionalong the fur nace we can see a de crease of NO con cen tra tion for the plasma ac ti vatedcom bus tion of mean, max i mal, and min i mal con cen tra tions along the full height of thefur nace. In creas ing the num ber of plasma-fuel sys tems the de creases in the tem per a tureand NO con cen tra tions be comes more pro nounced. When the coal com bus tion pro cessis ini ti ated by four plasma-fuel sys tems the dif fer ence be tween the tem per a tures forplasma activated and conventional combustion are noticeably less than is the case for sixplasma-fuel systems.

Fig ure 5 il lus trates the ox y gen and car bon di ox ide con cen tra tion dis tri bu tionsalong the fur nace height. The mean level of ox y gen along the full height of the fur nace islower for plasma ac ti vated com bus tion, but as con se quence the mean val ues of car bon di -


156

Fig ure 4. Tem per a ture (a) and NO con cen tra tion (b) vari a tion along the fur nace(0, 4, 6, 12) – re gimes of coal com bus tion: con ven tional, with four, six, and twelve PFS;

(1, 2, 3) – min i mal, mean, and max i mal val ues, cor re spond ingly

ox ide are higher. These data con firm more com plete com bus tion and un burned car bonde crease when the PFS are deployed.

Fig ures 6 and 7 vi su ally dem on strates the dif fer ence be tween con ven tional andplasma ac ti vated coal com bus tion in the fur nace by way of the tem per a ture field and NOformation.

Ex am in ing the dif fer ent re gimes of plasmatrons in stal la tion, the tem per a turefields (fig. 6) are no tice ably dif fer ent for the three con sid ered cases. In fur nace cross-sec -tion at the lower burner level, it can be seen that the tem per a tures in the zone ofplasma-fuel sys tems are higher. Con sid er ing the tem per a ture field in sec tion ZX, it is ev i -dent that im ple men ta tion of the plasmatrons causes ear lier heat ing and ig ni tion of theair-mix ture and initiation of combustion.

The PFS in flu ence re sults in NOx (fig. 7) for ma tion in the wall re gion of the fur -nace at the out let of the PFS. In con trast, for con ven tional coal com bus tion, NOx isformed in the cen tral area of the fur nace at the burner level. This is con firmed in the fig. 4by the pres ence of NO two peaks at the level of the burn ers. Com par i son of the up per andlower pan els re veals that the plasma-fuel sys tems in flu ence the NOx dis tri bu tion even atthis level. Re call that the burn ers of the up per tier are not equipped with plasmatrons.These ob ser va tions are con firmed by sig nif i cant dif fer ence in mean NO con cen tra tionsfor the cal cu lated cases. The mean NO con cen tra tion in vi cin ity of the sec ond tier of theburn ers is 1086.27 mg/Nm3 for the case of con ven tional coal com bus tion, 908.03 mg/Nm3

for coal com bus tion ac ti vated by the six plasma-fuel sys tems of the lower tier, and615.20 mg/Nm3 for coal com bus tion ac ti vated by twelve plasmatrons. The in flu ence ofthe plasma-fuel sys tems on NO for ma tion per sists along the full height of the fur nace asseen from the right up per and lower pic tures of the fig ure. In ter est ingly, the plasma-fuelsys tems de crease the NO con cen tra tion even in the lower part of the fur nace (be low thelevel of PFS). This is ex plained by the phe nom e non of sup pres sion of fuel NOx for ma tionin side PFS. Fuel ni tro gen is re leased with the coal vol a tile mat ter in side the PFS. It formsmo lec u lar ni tro gen due to a de fi ciency of ox y gen in the air-fuel mix ture treated by


157

Fig ure 5. Ox y gen (a) and CO2 con cen tra tion (b) vari a tion along the fur nace(0, 4, 6, 12) – re gimes of coal com bus tion: con ven tional, with four, six and twelve PFS;

(1, 2, 3) – min i mal, mean, and max i mal val ues, correspomdingly


158

Fig ure 6. Tem per a ture in cross-sec tion of the fur nace along the fur nace height, on the level oflower tiers of the burn ers, and on the fur nace out let (from left to right)


159

Fig ure 7. Ni tro gen ox ide in cross sec tions along the fur nace height, at the level of lower tiers ofthe burn ers and at the fur nace out let (from left to right)

plasma, a fact con firmed by the low NO con cen tra tion (7.5 mg/Nm3) at the out let of thePFS (see tab. 3). Fuel ni tro gen is a main source of ni tro gen ox ide emis sion at coal com -bus tion [5].

Ta ble 4 gives a com par i son of the two pre dicted cases, con ven tional and plasmaac ti vated coal com bus tion. Com par i son of the nu mer i cal and ex per i men tal data for con -ven tional coal com bus tion shows sat is fac tory agree ment. The dif fer ence in ex per i men taland cal cu lated pa ram e ters is less than 17%. These re sults dem on strate the im prove mentof the main char ac ter is tics of the coal com bus tion pro cess due to the use of plasma-fuelsys tems.

Con clu sions

Two fluid dy nam ics ki netic mod els have been used in this study. Com puter sim u -la tions of pul ver ised coal com bus tion as sisted by plasma-fuel sys tems have been per -formed through out the fur nace from the burner in let lo ca tions to the fur nace out let.Com par i son of the com puted re sults with the ex per i men tal data has proved the re li abil ity of the math e mat i cal mod els and their po ten tial for use in de sign ing commercialplasma-fuel systems.

Anal y sis of the re search re veals the eco log i cal ef fi ciency of the tech nol ogy. Thepres ent plasma as sisted com bus tion tech nol ogy has been shown to in crease the car bonburn out and re duce the emis sions of ni tro gen ox ides. For still fur ther en hanced en vi ron -men tal im pact, plasma fuel sys tems should be fit ted to all the burners of the power boiler.

Ref er ences

[1] Messerle, V. E., Ustimenko, A. B., Karpenko, E. I., Plasma-En ergy Tech nol o gies for Im prove -ment and Econ omy In dexes of Pul ver ized Coal In cin er a tion and Gasi fi ca tion, Pro ceed ings,28th In ter na tional Tech ni cal Con fer ence on Coal Uti li za tion and Fuel Sys tems, Clearwater,Fla., USA, Pub lished by U.S. De part ment of En ergy & Coal Tech nol ogy As so ci a tion of USA,2003, 255-266

[2] Karpenko, E. I., Messerle, V. E., Ustimenko, A. B., Math e mat i cal Model of the Pro cesses ofIg ni tion, Com bus tion, and Gasi fi ca tion of the Pul ver ized Coal Fuel in the Elec tric Arc De -vices, Thermophysics and Aero mech an ics, 2 (1995), 2, 151-165


160

Ta ble 4. Com par i son of flue gas char ac ter is tics at the out let of the fur nace

Conventional combustion(experiment)

Plasma activated combustion

Four PFS Six PFS Twelve PFS

T [°C] 1219.40 (1180) 1166.30 1110.10 1248.50

NOx [mg×Nm–3] 932.30 785.34 725.31 700.63

CO2 [kg×kg–1] 0.185 (0.17) 0.186 0.187 0.188

O2 [kg×kg–1] 3.2×10–2 (3.5×10–2) 2.78×10–2 2.75×10–2 1.74×10–2

[3] Goal, A., Gidaspow, D., Mod el ling of En trained Flow in Coal Hy dro ly sis Re ac tors, 1. Math e -mat i cal For mu la tion and Ex per i men tal Ver i fi ca tion, 2. Re ac tor De sign, Ind. and Eng. Chem.Pro cess Des. and De velop., 21 (1982), 4, 611-632

[4] Kalinenko, R. A., Kuznetsov, A. P., Levitsky, A. A., Messerle, V. E., Mirokhin, Yu. A., Polak,L. S., Sakipov, Z. B., Ustimenko, A. B., Pul ver ized Coal Plasma Gasi fi ca tion, Plasma Chem is -try and Plasma Pro cess ing, 13 (1993), 1, 141-167

[5] Tike, D. H., Slat er, S. M., Sarofim, A. F., Wil liams, J. C., Ni tro gen in Coal as a Sourse of Ni tro -gen Ox ide Emis sion from Fur nace, Fuel, 53 (1974), 2, 120-125

[6] Smoot, L. D., Pratt, D. J., Pul ver ized Coal Com bus tion and Gasi fi ca tion, Ple num Press, NewYork, USA, 1979

[7] Jankoski, Z., Lock wood, F. C., Messerle, V. E., Karpenko, E. I., Ustimenko, A. B., Mod el lingof the Pul ver ised Coal Prep a ra tion for Com bus tion in a Plasma Cham ber, Pro ceed ings on CD7th In ter na tional Con fer ence on En ergy for a Clean En vi ron ment, Lis bon, 2003, N. 04.4, 19

[8] Müller, H., Numerische Berechnung Dreidimensionaler Turbulenter Strömungen inDampferzeugern mit Wärmeübergang und Chemischen Reaktionen am Beispiel desSNCR-Verfahrens und der Kohleverbrennung, VDI-Verlag GmbH, Düsseldorf, Deutshlang,Reiche 6, 1992, 158

[9] Vockrodt, S., 3-dimensionale Sim u la tion der Kohleverbrennung in zirkulierendenatmosphärischen Wirbelschichtfeuerunge, VDI-Fortschrittberichte, VDI-Verlag GmbH,Düsseldorf, Deutshlang, Reiche 6, 1995, 334

[10] Smoot, L. D., Pul ver ized Coal Dif fu sion Flames: A Per spec tive Through Mod el ling, Pro ceed -ings, 18th In ter na tional Sym po sium on Com bus tion, The Com bus tion In sti tute, Pitts burgh,Penn., USA, 1981, 1185-1202

[11] Smart, J. P., Weber, R., Re duc tion of NOx and Op ti mi za tion of Burn out with an Aero dy nam i -cally Airstaged Burner and Airstaged Precombustor Burner, J. Inst. En ergy, (1989), 237-345

[12] Zel’dovich, Ya. B., Frank-Kameneckii, D. A., Sadovnikov, P. Ya., Ni tro gen Ox i da tion in Com -bus tion, Publ. House of the Acad emy of Sci ences of USSR, Moscow, 1947

[13] Müller. J., Dreidimensionale Sim u la tion der NO-Kinetik und SO2-Einbindung bei derKohleverbrennung in Zirkulierenden Atmosphärischen Wirbelschichtfeuerungen,VDI-Verlag GmbH, Düsseldorf, Deutshlang, Reihe 6, 1995, 323, 200

[14] Askarova, A. S., Messerle, V. E., Loktionova, I. V., Ustimenko, A. B., 3D Mod el ling of theTwo-Stage Com bus tion of Ekibastuz Coal in the Fur nace Cham ber of a PK-39 Boiler at theErmakovo Dis trict Power Sta tion, Ther mal En gi neer ing, 50 (2003), 8, 633-638

[15] Mitch ell, J. W., Tarbell, M., A Ki netic Model of Ni tric Ox ide For ma tion during Pul ver izedCoal Com bus tion, AIChE Jour nal, 28 (1982), 2, 302–311


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Apstrakt

Alija S. ASKAROVA1, Jevgenij I. KARPENKO2,Oleg A. LAVRI^[EV3, Vladi mir E. MESERLE3,Aleksandar B. USTIMENKO3

1 al-Farabi Kazahstanski Nacionalni Univerzitet, Odsek za fiziku, Alma-Ata, Kazahstan2 Centar za energetske plazma tehnologije Ruske J.S.Co ,,Ujediweni energetski sistem Rusije”, Gusinoozersk, Rusija3 Istraìva~ki institut za eksperimentalnu i teoretsku fiziku, Istraìva~ki odsek za plazmatehnologije, Alma-Ata, Kazahstan

Modelirawe plazmene podr{ke sagorevawuu kotlovima loènim ugqenim prahom

Tretman niskokaloricnih ugqeva termalnom plazmom omogua}va wihovo

efikasnije i ekolo{ki prihvatqivije sagorevawe. Ovaj rad predstavqa rezultate

numeri~kog modelirawa plazmenog tretmana spra{enog ugqa za pripalu i sagore-

vawe u loì{tu kotla. Dva kineti~ka matemati~ka modela su kori{}ena u izu~ava-

wu procesa plazmene aktivacije me{avine vazduha i spra{enog ugqa, wegove

pripale i sagorevawa. 1-D numeri~ki kod za simulaciju kinetike sagorevawa,

PLASMA-COAL, ra~una koncentracije sastojaka, temperaturu i brzinsko poqe u me-

{avini vazduha i spra{enog ugqa kod gorioni~kih paketa sa istaliranim plamatro-

nima. To daje po~etne podatke za 3-D modelirawe loì{ta kotla numeri~kim kodom

FLOREAN. Kompletna slika procesa plazmenog tretmana me{avine vazduha i spra-

{enog ugqa u loì{tima kotlova loènih ugqenim prahom je dobijena, i jasno de-

monstrirane prednosti upotrebe plazmene tehnologije u ovim postrojewima.

Kqu~ne re~i: ugaq, sistem za plazmenu pripalu, loì{te, plazmena stabili-zacija sagorevawa, simulacija kinetike sagorevawa, trodimenzio-nalno modelirawe

Odgovorni autor / Cor re spond ing au thor (A. B. Ustimenko)E-mail: ust@phys ics.kz


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modelling of plasma supported coal combustion in fullscale...

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