Indian Journal of Chemical Technology Vol. 8, January 2001, pp. 54-61

Effect of combustion temperature on the gas-phase formation and destruction of nitrous oxide

S S Verma Department of Physics, Sant Longowallnstitutc of Engineering and Technology , Longowal , 148 106, India

Recei1•ed 23 May 2000; accepted 2 November 2000

N20 emission is significant with in a restricted temperature window of 800-1 100 °C. Formation and destruction of 20 mainl y depends on the combustion temperature and residence time. In the present work , a detailed effect of temperature profiles and residence time on the N20 emission is being hi ghlighted.

The impact of temperature on N20 emissions is one of the few trends, studied and agreed upon by all re­searchers. Many workers 1

-13 have reported their stud­

ies related to the effect of combustion temperature on the N20 formation and destruction. They have shown significant deet·ease in N20 emissions as temperature increases. Jt is being reported that, small quantities (-5-6 ppm) of N20 emissions at low temperature win­dow (i.e., 800- 1100 °C) can be attributed to direct ox idation of cbar nitrogen but large quantities such as - 70 ppm of N20 can only be exp lained with the help of coal-devo latilization and its gas-phase react ions.

Formation and destruction behaviour of N~O has been theoretically stud ied and by using temperature profiles representing the coal combustion in one­dimensional pulveri zed coal combustion furn ace, fo­cussing on the gas-phase (homogeneous) reactions. Experimental in vesti gations under these conditions related to N20 emissions have been reported in the earlier work 1 ~. Though, the temperature effect on N20 emission is the most studi ed subject and in the present work also a detailed analysis of N20 formation and dest ruction behaviour with respect to combustion temperature and residence time has been discussed. The effect of combust ion temperature is studied more closely on N20 formation by using the temperature profil es with Tmax of 800, 850, 870, 880, 890, 900, 1000 and 1100 °C. Temperature profiles for Tmax of 800, 900, I 000 and I I 00 °C are the experimental 14

temperature profiles whereas temperature profiles for Tmax of 850, 870, 880 and 890 °C are generated from the ex perimental temperature profile equations.

Experimental Pt·ocedure

Numerical simulation/modeling To study the characteristics of i'h0 formation and

destruction under fluidized bed combustion conditions (i.e., lower combustion temperature range) with re­spect to various combustion parameters numerical simulations were carried out. Rate equations for spe­cies densiti es and temperatures can normally be writ­ten as a set of first-order, coupled, non-linear ordinary differential eq uations. For a given set of initi al condi­tions and chemical reactions, the physical problem can be cast in the following form : dn;ldt = Qi - L; n; where n; is the density of the ith species, Q; is the pro­duction rate, and L; is the inverse loss time. Thus, the sets of non-linear, first-order differenti al equations, which describe the time behaviour of chemical sys­tems, are ordi narily integrated with numeri ca l tech­niques that utilize the analytic form of onl y the first derivative of species concentrations. In the present stud ies, numerical calculations were attempted by using a modified LOMAX-BAILEY ( 1967)15 single­step implicit method with time-step control to handle the problem of stiffness. This method uses the con­stant-volume combustion model and the governing equations describing the adiabatic homogeneous con­stant-volume gas-phase reactions. To investigate the effect of combustion temperature of the flam e (along the length of the furnace) temperature profiles have been introduced in the ca lcul:ltions as input and tem­perature is expressed as a function of ti me. Thermo­dynamical data calculations for the backward reac ti on rate constants are discussed in Ref. 16.

VERMA: EFFECT OF COMBUSTION TEMPE RATU RE ON GAS-PHASE FORMATIO 55

Table !-Forward and backward reaction rate constants (in the Arhenius fo rm) fo r various chemical reactions considered in the chemica l kinetics

K=A t ' ex p(-£/RT) R= I. 987 cal/g-mol K

Sr Chemi cal reaction Kr (forward rate consta nt) K" (backward rate co nstant)

No. A II E A II E

I) CH4+M=C H.1+H+M 1.995E+ 17 0.00 88000 4.049E+ II 1.00 - 19920 2) CH"+OH=C H3+H20 1.600E+06 2. 10 2460 2.652E+05 2.10 171 35 3) CH4+0=CH3+01-l 1.020E+09 1.50 8604 1.7 15E+07 1.50 6029 4) Cl-l"+l-l=CHJ+l-12 2.200E+O-I 3.00 8750 8.42 1 E+02 3.00 8270 5) CHJ+02=CH20+0H 5.200E+I3 0.00 34570 5.4 17E+I3 0.00 88085 6) CH3+0=C I-l 2+H 8.000E+ 13 0.00 0 1.055E+I5 0.00 69630 7) CHJ+O H=CH20+H2 4.000E+I2 0.00 0 1.1 95E+ I4 0.00 7 1725 8) Cl-l 20+0=CH0+01-l 5.0 12E+ 13 0.00 4600 1.758E+ I2 0.00 17170 9) CH20+0H=CHO+H20 7.586E+ 12 0.00 170 2.623E+ I2 0.00 30000 10) CH20 +H =CHO+l-l 2 3.3 11 E+ I4 0.00 10500 2.644E+ I3 0.00 25 172 II ) CH20+M=CHO+H +M 3.3 10E+ I6 0.00 8 1000 1.4 16E+ II 1.00 - 11770 12) CHO+O=CO+O H I.OOOE+ I4 0.00 0 2.880E+I 4 0.00 87900 13) CHO+ l-l=CO+ l-1 2 1.995E+ I4 0.00 0 1.308E+I5 0.00 90000 14) CH0+02=CO+H02 3.3 10E+ 12 0.00 7000 7.424E+ I2 0.00 39290 15) CH0+0H=CO+H 20 I.OOOE+ I4 0.00 0 2.839E+ I5 0.00 105150 16) CHO+M=CO+H+M 1.445E+ I4 0.00 19000 5.024E+ IO 1.00 1558 17) CO+O H=C02+H 1.5 10E+07 1.30 -770 1.69 1 E+09 1.30 21565 18) CO+O+M=C02+M 6. 170E+ I4 0.00 3000 5.724E+20 -1.00 130680 19) l-1+02=0 +01-l 5. 129E+ 16 -0.82 165 10 4.050E+ I5 -0.82 395 20) 0+1-1 2=1-l+O H 5.060E+04 2.67 6290 2.222E+04 2.67 4 195 2 1) 0+H20 =0 1-l+0 1-l 6 .760E+ 13 0.00 18300 6.858E+ 12 0.00 11 10 22) H+ H20=l-l z+OH 9.550E+ 13 0.00 20300 2.206E+ I3 0.00 5 145 23) l-l+OH+M=H20+M 1.600E+22 -2.00 0 1.306E+27 -3.00 122595 24) O+O+ M=02+M 1.890E+ I3 0.00 - 1788 1.983E+ I8 -1.00 119672 25) l-l+H+M=I-l2+M I.OOOE+ I8 - 1.00 0 1.886E+22 -2.00 107440 26) H+ H0 2=0H+OH 1.400E+I4 0.00 1073 1.420E+ I3 0.00 40571 27) H+02+M=H02+M 3.6 10E+I7 -0.72 0 2.329E+2 1 -1.72 49732 28) O+N2=NO+N 6.630E+ I3 0.00 75050 1.464E+ I3 0.00 -1-10 29) N+02=NO+O 6.400E+09 1.00 6280 1.367E+09 1.00 38230 30) N+OI-l=NO+ l-1 3.800E+ I3 0.00 0 1.030E+ I4 0.00 48065 31) CH3+H=CH2+H2 9.000E+I3 0.00 15 100 1.8 18E+ 13 0.00 10400 32) CHJ+OH=C H2+l-l 20 7.500E+06 2.00 5000 6.558E+06 2.00 15455 33) C l-l 2+0=Cl-l+OH 2.000E+ II 0.68 25000 4.894E+ IO 0.68 25371 34) CH2+H=CH+H 2 I.OOOE+ I8 - 1.56 0 5.572E+ I7 - 1.56 2466 35) CH2+0H=CH+H20 1. 130E+07 2.00 3000 2.726E+07 2.00 20621 36) CH+NO= HCN+O 1. 100E+I4 0.00 0 2.922E+ I5 0.00 71224 37) CH 2+ O=HCN+Ol-1 2.000E+I3 0.00 0 1.300E+ I4 0.00 71595 38) CHJ+NO=HCN+l-l20 I.OOOE+I I 0.00 15000 5.684E+ II 0.00 97050 39) CH+N2=1-l CN+N 3.000E+II 0.00 13600 1.759E+ I2 0.00 9634 40) CH2+N2=HC +NH 1.000E+I3 0.00 74000 6.839E+ I2 0.00 42920 4 1) CH+N l-1 2=1-lCN+l-l=H 3.000E+ 13 0.00 0 2.240E+ II 1.00 42232 42) CH+N l-l=HCN+ H 5.000E+ I3 0.00 0 7.547E+ I5 0.00 14677-1 43) CH2+N I-I=HCN+l-l+l-l 3.000E+I3 0.00 0 1.338E+ II 1.00 41800 44) CH+N=CN+H 1.300E+ I3 0.00 0 1.5 12E+ I4 0.00 9930-1 45) CH2+N= HCN+ H 5.000E+ I3 0.00 0 8.800E+ I4 0.00 119660 46) CH3+N= HCN+l-l+l-l 5.000E+ I3 0.00 0 9.423E+09 1.00 7520 47) CH4+N=N l-l +C H3 I.OOOE+I3 0.00 24000 8.0 10E+ IO 0.00 -6060 48) 1-l CO+I-l=NH2+CO 2.000E+I3 0.00 3000 1.460E+ I2 0.00 20237 49) HCN+M=CN+l-l+M 5.700E+I6 0.00 11 7034 1. 11 3E+ I2 1.00 -8296 50) c +H2=HC +H 2.950E+05 2.45 2237 8.014E+05 2.45 20 127 S I) HC +CN=C2N2+ l-l 2.000E+I3 0.00 0 4.371E+ I4 0.00 6718 52) HCN+O=CN+Ol-1 2.700E+09 1.58 26600 4.365E+08 1.58 6615 53) HCN+O=NCO+H 1.380E+04 2.64 4980 3.689E+04 2.64 5337 54) 1-lCN+O=Nl-l+CO 3.450E+03 2.64 4980 1.402E+03 2.64 34740

Co11td.

56 INDIAN J. CHEM. TECHNOL., JANUARY 2001

Table !-Forward and backward reaction rate constants (in the Arhenius form) for various chemical react ions considered in the chemical kinetics-Collld.

K =AT' exQ( -EIR1) R= 1.987 callg-mol K

Sr Chemical reaction Kr (forward rate constant) Kb (backward rate constant)

No. A ll £ A ll £

55) HCN+OH=H10+CN 1.450E+l 3 0.00 10929 2.3 11 E+ l 3 0.00 8194

56) HCN+OH=HNCO+H 4.800E+ ll 0.00 11000 6.526E+ l2 0.00 22720

57) NH 3+M=NH2+H+M 1.400E+ l6 0.00 90600 1.176E+ll 1.00 -14794

58) NH 3+0H=NH1+H 20 2.040E+06 2.04 566 1.400E+06 2.04 17767

59) NH 3+0=NH2+0H 2.100E+l3 0.00 9000 1.46 1E+l 2 0.00 895 1

60) NH3+H=NH2+H2 6.360E+05 2.39 10 171 1.008E+05 2.39 122 17

61) NH2+0H=NH+H20 4.000E+06 2.00 1000 1.615E+07 2.00 19053

62) NH2+0=NH+OH 6.750E+l2 0.00 0 2.765E+ l2 0.00 803

63) NH2+0=HNO+H 6.630E+l4 -0.50 0 4 .354E+ l5 -0.50 23021

64) NH1+H=NH+H 2 6.920E+l 3 0.00 3650 6.454E+ l3 0.00 6548

65) NH2+02=HNO+OH 4.500E+l2 0.00 25000 2.334E+ l2 0.00 31906

66) NH+OH=HNO+H 2.000E+l3 0.00 0 3.206E+l4 0.00 22218

67) NH+OH=N+H20 5.000E+l l 0.50 2000 1.035E+l3 0.50 46735

68) NH+O=NO+H 2.000E+l3 0.00 0 1.1 36E+l4 0.00 75550

69) NH+O=N+OH 6.300E+ l l 0.50 7948 1.322E+l2 0.50 35433

70) NH+ H=N+H2 l.OOOE+l4 0.00 0 4 .779E+ l4 0.00 29580

71) NH+02=HNO+O l.OOOE+l 3 0.00 12000 1.266E+ 13 0.00 18103

72) NH+NH2=N2H2+H 5.000E+l3 0.00 0 3.996E+l5 0.00 25907

73) HNO+OH=NO+H 20 3.600E+l3 0.00 0 1.258E+l4 0.00 70582

74) HNO+O=NO+OH 5.000E+ll 0.50 1987 1.772E+l l 0.50 55319

75) HNO+H=NO+H2 5.000E+l2 0.00 0 4.035E+l2 0.00 55427

76) HNO+M=NO+H+M 1.500E+l6 0.00 48680 6.4 18E+l l 1.00 -3333

77) NH2+NO=N2+H20 6.200E+ l5 - 1.25 0 2.380E+l7 - 1.25 120728

78) NH 2+NO=N2+H+OH 6.300E+l9 -2 .50 1888 2.962E+ l6 - 1.50 21

79) NO+H02=N02+0H 2. 11 0E+ l2 0.00 -479 8.720E+ 12 0.00 8318

80) N02+0=N0+02 l.OOOE+l3 0.00 600 3. 107E+ l2 0.00 47416

81) N02+ H=NO+OH 3.500E+ l4 0.00 1500 8.585E+ l2 0.00 32201

82) CN+O=CO+N 1.800E+ 13 0.00 0 9.499E+ l3 0.00 77230

83) CN+OH=NCO+H 6.000E+ l3 0.00 0 9.922E+l4 0.00 20342

84) CN+H2=HCN+H 2.950E+05 2.45 2237 8.0 14E+05 2.45 20127

85) CN+02=NC0+0 5.600E+ 12 0.00 0 7.311E+l2 0.00 4227

86) CN+N02=NCO+NO 3.000E+ l3 0.00 0 1.217E+ 13 0.00 51043

87) NCO+M=N+CO+M 3. 100E+l6 -0.50 48000 1.1 94E+ 12 0.50 -457

88) NCO+H=NH+CO 5.000E+l3 0.00 0 7.603E+l2 0.00 29403

89) NCO+O=NO+CO 2.000E+l3 0.00 0 1.728E+l3 0.00 104953

90) NCO+OH=NO+CO+H l.OOOE+l3 0.00 0 1.043E+09 1.00 -392

9 1) C2N2+0=NCO+CN 4.570E+l2 0.00 8880 5.59 1E+ ll 0.00 2519

92) NCO+NO=N20+CO l.OOOE+l 3 0.00 -390 4.12 1E+ l4 0.00 67326

93) NO+N H=N20+CO 2.400E+l5 -0.80 0 6.504E+ l7 -0.80 38313

94) NO+NH2=N20+H2 5.000E+l 3 0.00 24640 1.264E+l6 0.00 65851

95) N02+NH2=N20+H20 1.900E+20 -3.00 0 5. 101E+21 -3.00 87067

96) HNO+HNO=N 20+H20 3.950E+ 12 0.00 5000 2.333E+ 14 0.00 91677

97) HNO+N0=N20+0H 2.000E+ 12 0.00 26000 3.38 1E+l3 0.00 42095

98) N2H2+NO=N20+NH 2 3.000E+ l2 0.00 0 1.0 17E+ 13 0.00 12406

99) N20+M=N2+0+M 1.620E+l4 0.00 5 1600 6.859E+08 1.00 10617

100) N20+H=N2+0H 7.600E+l3 0.00 15200 2.666E+ l2 0.00 79562

101) N20+0H=N2+H02 2.000E+l2 0.00 10000 6.920E+ ll 0.00 34864

102) N20+0=NO+N0 l.OOOE+l4 0.00 28200 2.096E+ l2 0.00 65437

I 03) N20+0=N2+02 l.OOOE+l 4 0.00 28200 4.442E+l3 0.00 108677

I 04) N20+CO=N2+C02 5.000E+ l3 0.00 44000 1.964E+ l4 0.00 130697

105) N20+CN=NCO+N2 l .OOOE+ l3 0.00 0 5.800E+ l2 0.00 84704

I 06) C0+02=C02+0 2.5 12E+ 12 0.00 47690 2.221E+ l3 0.00 539 10

107) CO+H02=C02+0H 5.754E+l3 0.00 22930 6.533E+ l4 0.00 84763

108) CH4+0:=CH 3+H02 7.900E+l3 0.00 56000 1.034E+ l2 0.00 -2 188

M=N2

VERMA: EFFECT OF COMBUSTION T EMPE RATURE ON GAS-PHASE FORMATION 57

50 1000

- 40 BOO ,:: ::!: a. " a. 2 -c

30 600 ~ 0

!'> v ~

E

"-:;!

" 20 400 c 0 . ~

::!: ~ ~

D

10 200 E 0

u

0 2.0 3.0 4.0

Rosid<nce Ti m• ( s )

Fig. 1-Shows the formation and dest ruction of N20 with resi­

dence time for a temperature profile of Tmax = 800°C.

50 Tm o.- =85 0 °C

1000

- 4 0 HCN X 10 -80 ,u ::!: 0.. " a. ~

~

c 30 2

600 ~ 0.

" E 0 " t:: ....

" 20 0

4 00 c 0

"" ::!: ~

~ D E

10 20 0 0 u

NO 0 0

1.0 2.0 3 .0 4 .0

Res idenc12: Time (s }

Fig. 2-Gives the N20 charac teri stics with residence time for a

temperature pro fil e o f Trnax = 850°C.

Chemical Kinetics A chemical kinetic scheme consisting of over 108

elementary chemical react ions being represented by 30 chemical species given in Table 1 and Table 2 re­spectively, was applied to highlight the importance of homogeneous gas-phase reactions of coal­devolatilisation and coal-nitrogen species in terms of N20 formation and destruction for different operating parameters under Fluidized Bed Combustion (FBC) conditions. A detailed chemical kinetics scheme and parametric dependence of N20 formation and de­struction behav iour has been reported in the earlier workt 6 also.

Coal Devolatilization Coal devolatili zation is a very complex phenome­

non and depends on many parameters of combustion and coal quality. In the present studies, the devolatili­zat ion of coal has been considered to give rise to its

Table 2-lmportant chemical species involved in vanous che mical reactions

(I) 0 ( 16) H2 (2) OH ( 17) C H20 (3) H ( 18) H20 (4) N ( 19) C02 (5) CN (20) co (6) NH (2 1) N2 (7) CHO (22) C2N2 (8) H02 (23) N2H2 (9) CH (24) HNCO

( 10) CH 2 (25) HNO ( 11) CH 3 (26) NH3

( 12) CH4 (27) NH2 ( 13) 0 2 (28) NCO ( 14) HCN (29) N02 ( 15) NO (30) N20

volatile matter and volatile matter-nitrogen, which burn to produce hydrocarbon and HCN and NH 3 re­spectively. Because, char combustion takes place at sufficiently high temperatures and with long time in­tervals, therefore, in the present studies related to coal combustion under FBC (i .e., lower combustion tem­perature range) conditions char combustion is not taken into consideration. Different mechanisms for the devolatilization of volatile matter and volatile matter-nitrogen species of coal with in the range of combustion temperature have been listed and used 16

in the present studies. Though the reaction of coal devo latili zation in­

volving coal- volatile matter and coal-nitrogen are of both nature i.e. , homogeneous as well as heterogene­ous but significantly less contribution of heterogene­ous reactions and great importance of homogeneous gas-phase reactions of fue l-voltaile matter and fuel­nitrogen in the formation of N20 in FBC (i.e., lower combustion temperature) has been reported by a num­ber of researchersi.J.?.S. I0.12

'17

• Therefore, in the present studies only homogeneous gas-phase reactions repre­senting the coal devolatil ization as well as combusti on were considered. It was found that coal devol atili za­tion in the form of CH4, CO, H20 and 1-12, etc. given in Table 3 as input gaseous components helps in ex­plaining the experimentall/ 4 obtained N20 emission levels from coal combustion in the temperature range 800-1100 °C. Whereas N20 emission levels obtained from coal combustion in the temperature range of 800-1100 °C can not be explained from other routes of coal devolatilization as discussed in Ref. 15. The various other operating parameters were also found 16

to influence the N20 formation and destruction but the effect of combustion temperature and residence

58 INDIAN J. CHEM. TECHNOL., JANUARY 200 1

1000

800 -::!: .:: o._

~ ~

c 600 3 ~ ~ u ~

E a.

u. E

20 ~

~ 400 ,_ 0 c ::!: 0

;; :>

10 200 .0 E 0 u

R< sid<nco Tim• ( s )

Fig. 3-Shows the variation of N20 emissions with residence

time for a temperature profile of r"'"·' = 870°C.

500 1100

HCN Tmo' = 880 •c

i BOO .u o._ o._ ~ -c 3 0 ·;:; 600 ~ u ~

E a. u. f

~

~ 200 ....

0 NO 400 c ::!: 0

:;:; ~

:>

100 200 .0 E 0

N20 u

0 2.0 3.0

0 4.0

Rrz:sidencfZ Time (s )

Fig. 4-Shows the formation and destruction of N20 with resi­dence time for a temperature profile of 7~nax = 880°C.

time was found to be paramount. Therefore, in the present work, the sole effect of combustion tempera­ture along with residence time in the temperature window was studied and is reported in a detailed manner. In order to investigate the effect of combus­tion temperature more closely on the N20 formation and destruction behavior, temperature profi les were extrapolated for small changes in Tmax of 800, 850, 870, 890, 900, l 000 and 110 °C.

Results and Discussion It is concluded in the earlier work 15 that no signifi­

cant N20 formation takes place if coal devo latilization is taken in the form of CH4 and HCN + NH3 route. Under this route of devo latilization even combustion temperature does not show any effect on the N20 emissions and emissions are very low. When it is con­cluded that the devolatilization of coal-volatile matter

sao,-------------------------------~

Tmax = 890°C 1000

800 -.u -::!: "-

~

2 "- 600 ::'

~

g 0. E

u NO ,!!! 0

..:: c -~

.!'! 0

::!: 100

~

:> .0

200 E 0

N20 u

0 2.0 3.0 4.0

R~sidence TimiZ ( s }

Fig. 5-Gives the characteristics of N20 with res idence time for a

temperature profile ofT"'"'= 890°C.

Table 3-Devolatilization of coai 16

Com bus- Stage E Ko v* (wt% ti on (kcal/ (s·l) of coal products mol) VM)

C02 1 36.2 2.1xl0 11 6.70 2 64.3 5.1xl0 13 3.17 3 42.0 5 .6xl0 °6 1.28

co 44.4 1.8x l0 12 2.08 2 59.5 2.6x I 0 12 6.28 3 58.4 5.9xl 0°9 2.65

CH" 17.6 3.7xl0 °5 7.66 2 60.0 1.46x I 0 13 1.92

H20 51.4 7.9xl0u 11 .40 H2 88.8 1.6xl0 1x 0.59

K=Ko EXP(-E/RT)

has to be considered in the form of CH4, CO, J-1 20 and H2 then the N20 emission values calculated theoreti­call y very wel l match the experimentally reported values under similar operating conditions. The theo­retical results obtained in the presen t studies have been compared 16 in a good agreement with the ex­perimental results in the li terature, thus validating the au thenticity of thermodynamical input data, the cor­rectness of the simulation technique and justification of the chemical reaction scheme selected presently.

Figs 1-8 show the N20 formation and destruction behaviour for temperature profiles with Tmax of 800, 850, 870, 880, 890, 900, 1000 and 1100 °C with resi­dence time respectively. From these figures repre­senting the N20 emissions for the respective tem­perature profiles, it can be seen that N20 emissions are very small at a Tmax of 800 °C. Bu t as the tem­perature increases to 850-870 °C, the formation of

VERMA: EFFECT OF COMBUSTION TEMPERATURE ON GAS-PHASE FORMATION 59

500 Tmax=900°C 1000

400 800 ·:: ~ ~

3 00 600 ~ ~ ., E

NO ;! .. 200 40 0 c 0 ~ ::1: ,

D

100 E 200 0

u N,o

0 1.0 2.0 J.O

R~sidenc~ Tim~ (s )

Fi g. 6-Shows the dependence of N20 emissions with residence

time for a temperature profile of Tmax = 900°C.

600

i c.. ~ c 0

v 0 u: !1. 200 0 ::1:

.-------------------, 1200

Tma,= 1100 •c

NO

1-0 1. 5 2 -0

Rq. si d~nc~ Time ( s

-.:: ~

800 ,

~ ., E ,'!. c 0

400 -~ , D

E 0 u

Fig. 7-Shows the formati on and destruction of N20 with res i­

dence time for a tem perature profile o f Tmax= I 000°C.

N20 takes place and the concentration also sustains with the residence time for these temperatures. But, as the temperature increases for just 10 degree from T max

=890 °C to T max =900 °C, the N20 concentrations de­creases very fast with the residence time. Though, the formation of N20 is high in the combustion zone at temperatures of T111,, = 900- 1100 °C but the destruc­tion mechanism also become stronger and there is phenomenal decrease in the N20 concentrations even for a small residence time. Hence, N20 emiss ions are very sensitive to the combustion temperature and this sensitiveness stands with any other variable operating parameters.

At higher temperatures CTmax >900 °C), the strong destruction process in the gas phase becomes domi­nant and the effect of coal properties almost disap­pears. Thus, coal properties and operating parameters, other than combustion temperature are only important

60 0 Tmax=1000'C

1200

- HCN .}-' ::!: ~

800 ~ :: "' a.

c.. a. 400 -c 0

E "' .... c

u NO 0

u: .. .!?

40 0 ~ , D

0 200 I

E 0 u

Residence Timrz (s )

Fig. 8-Gives the characteristics o f N20 with residence time for a

temperature profile of Tmax = II 00°C.

at lower combustion temperature of the order of T max

>800 and < 900 °C. As the N20 formation in the gas­phase is considered to be clue to the reactions of NCO + NO and NH + NO, hence, there should exist some cotTelation between the concentration of N20 and NO. It is very clear from Figs 1-3 (i.e. , for Tmax of 800, 850 and 870°C) that when there is only increase in N20 formation, NO concentration are very less ( <5ppm) which may be attributed to some other route. Whereas, for temperature of Tmax =880°C, the N20 formation peaking in the combustion zone gets stabi­li zed at a slightly lower value at the exit. The process of destruction of N20 in the combustion zone gives the input for the NO formation and it is very clear from Fig. 4. Thus, from Figs 4-5, clear correlation that N20 decreases with the increase of NO have been obtai ned . This result implies the possibility of finding some rule to control the formation of NO from N20 by the reaction of NO with NCO and NH.

The combustion temperature has a remarkable ef­fect on the N20 concentration profiles as well as on its exit values . N20 destructi on during and after the combustion process occurs more strongly for the higher temperatures . N20 concentration level for T max

= 850°C may be lower than its value for higher tem­peratures but it is without destruction tendency for long residence times. N20 levels after increase for T max of 800-870°C almost become stable for longer residence time. The N20 concentration continues to increase even after the combustion process and reaches a constant value of -50 ppm to the ex it with­out any decreasing or destruction tendency. For the cases of Tmax of 880 and 890°C (Figs 4 and 5), the N20 concentration has a peak (>I 00 ppm) at the end

60 INDI AN J. CHEM. TECHNOL., JANUARY 200 1

of combustion process and then decreases slightly and then becomes stable. On the other hand for the cases of Tmax > 900°C (Fig. 6), the N20 concentration has a peak ( -150 ppm) in the combustion zone (which gets shifted earlier with the increase in temperature-2.0 s) and then rapidly decreases to the level of 50-60 ppm. Similarly for Tmax of 1000 °C (Fig. 7) and Tmax of 1100 °C (Fig. 8) temperature profiles, N20 formation and destruction behaviour is same. The concentration of N20 maximize at an earlier residence time to the level of > 150 ppm and then decreases very fast (i.e., resi­dence time of -1.5 s) to a very low ex it value.

At very low temperatures i.e., Tmax less than 850 °C, only NH 3 contributes towards the N20 formati on as can be seen from Fig. 1 for Tmax= 800 °C. At lower temperatures i.e., for Tmax =850-870 °C, NH3 is con­sumed faster than HCN and Figs 1-3 clearly show the emergence of N20 at the decreasing point of NH3 concentrations. Whereas, in Figs 4-5 it is seen that HCN is being consumed faster and N20 peak occurs at the point where HCN concentrations decrease. NH3 concentrations remain stable and almost disappear where N20 starts converting into NO. Same behaviour (i .e., spiking of N20 concentrations) can be seen for higher temperatures also. This can be explained as the temperature increases from Tmax=870 to Tmax=880 °C the formation of N20 from HCN is being competed by the N20 destruction reactions. Moreover, NH3 contribution in N20 formation vanishes at Tmax =880°C whereas HCN starts contributing significantly towards the NO and N20 formation. But simultane­ously N20 is being removed by the presence of NO also . All this is likely to happen through the following reaction scheme.

Formation of N20 NH3 ~ NHz ~ NH ~ NzO HCN ~ NCO ~ NzO

Formation of NO NH3 ~ NH2 ~ NH ~NO HCN~NH~NO

Destruction of N20 N20 + H ~ Nz + OH NzO + OH ~ N2 + HOz

The N20 formation is dominated by the presence of HCN and NH3. Hence, the distribution of fuel­nitrogen between HCN and NH3 species in coal de-

volatilization will play a great role in N20 formation and destruction along with the combustion tempera­ture as an important controlling parameter in the tem­perature window of 800-11 00°C.

Conclusion In the present studies with respect to the depend­

ence of N20 formation and destruction behaviour on the combustion temperature, the fo llowing conclu­sions can be drawn.

Coal devolatili zation route plays an important role in order to match the reported experimental emi s­sion levels of N20 from coal combustion in the temperature window of 800-1100 °C. Nitrous oxide formation is favoured by a low combustion temperature in agreement with the ex perimental data in the literature. The effect of various coal properties and operat­ing parameters viz., quality factor (fraction of volatile matter in coal pyrolysis changing into ul­timate volatile matter), frequency factor (fraction governing the conversion of coal-nitrogen into volatile-nitrogen), concentration of input pa­rameters, pressure, stoichiometry, HCN/NH3 ratio etc. has already been studied 15

• The coal proper­ties and operating parameters other than combus­tion temperature are only important at lower com­bustion temperatures of the order of Tmax >800 and <900 °C. These observations are also sup­ported by various theoretical and experimental studies reported in the literature on N20 formation and destruction under FBC conditions. The combustion temperature has a remarkable effect on N20 concentration profiles as well as on its exit values. N20 destruction during and after the combustion process occurs more strongly for the higher temperatures. Temperature range as close as Tmax of 850-890°C only gives significant N20 emissions with a longer residence time. The formation of N20 is higher at hi gher tem­peratures in the combustion zone, but it decreases very fast with residence time. It means the high temperature helps the N20 destruction as fast as it helps its formation.

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VERMA: EFFECT OF COMBUSTION TEMPERATURE ON GAS-PHASE FORMATION 61

European workshop on N20 emissions, Lisbon , Portugal, 1990.

3 Hiltunen M, Kilpinen P, Hupa M & Lee Y Y, N20 Emissions from CFBC-boilers: experimental resu/1s and chemical in­terpretalion, Proceeding of II'" International Conference on FBC, (1991) 687.

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14 Okazaki K, Niwa T & Verma S S, N20 Formation and De­struclion in Pulverized Coal Combus1ion at Low Tempera ­ture, 5'" workshop on N20 emissions, Tsukuba, Japan, ( 1992) 83.

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17 De Soete G G, Proceedings of the Joi111 Meeling of I he Brit­ish and French Sec/ions of the Combustion lnsti!Ute, Rauen (U .K.) (1989) 9.