T - Scientific Bulletin

U.P.B. Sci. Bull., Series D, Vol. 68, No. 1, 2006
THE REDUCTION OF NITROGEN MONOXIDE
CONCENTRATION TO THE COALS HAVING A HIGH
CONTENT OF ASH
I. PÎŞĂ, C. NEAGA ∗
În lucrare autorii şi-au propus studierea formării de NO X la cărbunii cu
conţinut ridicat de cenuşă. În primul rând s-a determinat timpul de uscare funcţie de
masa minerală externă şi de diametrul iniţial al particulei de cărbune. În al doilea
rând a fost cuantificată influenţa masei minerale asupra concentraţiei de oxizi de
azot.
In this paper, the authors analyse the generation of NOx in the coals having
a high content of ash. On the one hand, the drying time has been determined,
depending of the external mineral mass and the initial diameter of the coal particle.
On the other hand, the influence of the mineral mass over the concentration of
nitrogen oxides has been quantified.
Keywords: steam generators, combustion, nitrogen oxides, ballast.
Introduction
The authors study the burning of the coals having a high content of ash which
derives from the mineral mass. The mineral mass of the coal consists of:
• the internal mineral mass which derives from the mineral mass of the genetic
material (the combustion of the particles leads to the appearance of the
internal ash which is carried from the particles surface and included in the
corresponding gas phase.)
• the external mineral mass which derives from the sterile drawn in the coal
extraction (after the combustion it transforms into the external ash and it
represents about 90% from the total mineral mass.)
The methods of approaching the elementary analysis of a test carried from the
raw coal bunker do not make evident the existence of such fully inert particles.
The mineral mass of the initial particle having the diameter δ oc is composed of the
internal mineral mass and of the external mineral mass. The extraction technology
of lignite (by uncovering) leads to the idea that the external mineral mass consists
of separate mineral particles (after grinding). These accompany the rest of the
particle which is burning and which contains only the internal mineral mass. This
∗
Reader; Prof., Dept. of Classical and Nuclear Thermal Power Equipment, University
POLITEHNICA of Bucharest, Romania

54
I. Pîşă, C. Neaga
is uniformly distributed and after burning it passed into the combustion gases. If
the fraction corresponding to the external mineral mass is noted by x e (usually
x e ≈ 0.90 [1]) and those mentioned above are taken into consideration, one can
reach the conclusion that after the external mineral mass (the ash) is removed, the
coal is enriched and will have the following elementary analysis:
i 0
i
0
i 0
i
0
i
0
C = f 0 −iC
,%; H = f 0 −i
H , %; Sc
= f0−iSc
, %; N = f 0 −i
N , %; O = f 0 −iO
, %;
i
A = f0− i ( 1 − xe
) A
0
i
0
, %; Wt
= f0−iWt
, %, (1)
where the transformation factor f 0-I is given by the expression:
100
f0− i =
(2)
0
100 − xe
⋅ A
Taking into account the transformation factor, the values of the imbibition
and hygroscopic moistures (together form the total humidity) modify, as follows:
i
0
i
0
Wi
= f0−iWi
, %; Wh
= f0−iWh
, % (3)
The technical (immediate) analysis of the coal (the content of volatile and
of fix coal) is also influenced by the transformation factor f 0-i :
i 0
0
V = f 0 V , %; C = f C , %; (4)
−i
i
f
0−i
f
The removed mineral mass, after the combustion of the remaining
combustible, leads to the formation of n a particles of ash. The jet of dust
introduced into the furnace consists of plenty of such independent “units” (Fig. 1).
A “unit” is formed by a particle which burns accompanied by the “cloud” of ash
particles. If the combustion process includes the drying phase, the total burning
time is divided into four areas (Fig. 2), compared with three zones [6], as follows:
the drying area, the ignition area, the area of combustion of volatile substances
and the zone of finishing the combustion.
The drying area Z 0 corresponding to the interval 0

The reduction of nitrogen monoxide concentration to the coals having a high content of ash 55
also present the burning of base of coke, a process carried on both in parallel with
the combustion of the volatiles and after that. Z 1 and Z 2 zones unfold either
simultaneous or they succeed one another.
3
Φ
T g
1
2
T fl
Fig..1. Schematic representation of “one unit”.
1 – coal particles; 2 – cloud of ash particles; 3 – diameter of gas cover of “one unit”, Ø
The finishing zone of the combustion corresponding to the interval
τ 3 < τ < τ a . This area noted Z 3 is the area when the combustion process is finished.
The combustion air has been completely introduced. The particle burns fully up to
the ash particle which, practically, derives from the internal mineral mass.
1. The mathematical model for Z 0
The equations characterizing the area Z 0 are:
• for the drying dynamics:
dW
p
dτ
4
[ a σ ( T − T ) + α ( T − T )]
6 4
kgapa
= −
cb o fl
gc g ,
rδ
ρ
kg ⋅ s
0
ap
where W p is the current humidity of the coal dust, [kg humidity /kg coal ]; r – the latent
vaporizing heat, r = 2258 kJ/kg; ρ ap – the apparent density, ρ ap = 900 kg/m³;
δ 0 – the diameter of the burning particle, m (constant in this area); a cb – the
energetic coefficient of coal emission (it can be imposed a cb = 0.8); α gc – the gas
convection coefficient – particle, kW/(m²·K); the convection coefficient α gc is
calculated by using the relationship [2]:
2 −7
5
α ( 0.8805 10 0.124 10
−
gc
= ⋅ Tg
− ⋅ )
(6)
δ
0
At the initial moment,
0
i 0.01W
h
τ = 0. Wp = Wp = W
0 h
= , (7)
0
A
1−
xe
100
After τ = τ 1 (τ 1 is the drying time) W p = 0.
(5)

56
I. Pîşă, C. Neaga
λ s
r evs
second. air
dust, H 2O
Z 0
Z 1 Z 2 Z 3
λ p ; r evp
0
τ 1 τ 2 τ 3
λ second. air
s
r evs
τ a
Fig.2. The structure of the pulverized jet coal
• for the variation of the temperature of the gas phase:
0
dTg
⎧⎪
⎡
V dWp
⎤
aum
= ⎨− ⎢ri ( ξ + λi − 1) + 1.242 ⋅ ⎥( Tg
−273)
−
dτ ⎪⎩
⎣
τa
dτ
⎦
''
0
6α
gc i dWp Vaum
− ( Tg −T) − ⋅ ++ ri ( ξ + λi −1) ⋅( Tfl
δρ
0 apcg cg dτ τa
−273)
− (8)
2 2
naαgaδa Φ σ0a
⎫
g 4 4 ⎪
−6 ⋅ ( T ) 6
3 g
− Ta + ⋅
3 ( Tfl −Tg ) ⎬ / VgZ0
δ0ρapcg δ0ρapcg
⎪ ⎭
where VgZ 0
is the gas volume with reference to one kilogram of drying coal ; it is
calculated with the relation :
0
i
VgZ = λ 1.242 ( 1) ( 1)
0 pVaum + Wi + ⎡
⎣
rf ξ + λf − + revp ξ + λev
− +
3
τ ⎤
0
mN
+ ri ( ξ + λi − 1)
⎥Vaum + 1.242 Wp
,
τ
a ⎦
kg
(9)
where λ p is the excess coefficient of primary air; λ i – the coefficient of air excess
in the furnace at the level of burners; λλ ev – the coefficient of air excess at the
evacuation; r i ,r f – the recirculating degree from the burning inlet and from
furnace end-part, respectively; W i i – the imbibition humidity (it is calculated with
the relation 3); V 0 aum – the theoretical volume of wet air that is corrected with the

The reduction of nitrogen monoxide concentration to the coals having a high content of ash 57
transformation factor, f 0-i ; τ 0 – the total time of combustion, initially admitted
between 0.5÷1.5 seconds, lately its value being checked up; c g – the heat specific
3
to the burning gases (it is admitted constant, c g =, =1.35 kJ /( mN ⋅ K)
); i”- the
enthalpy of the saturated steam, i”=2676 kJ/kg; α ga -the convection coefficient
from gases to the ash particles; α ga =2λ g /δ a , where δ a =20·10 -6 m (the diameter of the
ash particle); T a – the temperature of the ash, K; a g – the energetic coefficient of
the gas emission inside the cover, at the temperature T g , a g = 0.3÷0.4 [5]; T – the
temperature of the particle (373 K); Ф – the diameter of the cover [3], m; this
diameter is calculated by the relation:
1/ 3
⎛ Tg
0 0 ⎟ ⎞
⎜ ρ ap VgZ
⋅
T0
⎠
Φ = δ , (10)
⎝
where T 0 is the reference temperature, T 0 =273 K and the number of ash particles,
n a is calculated by the relation:
0
3
ρc
⋅ xe
⋅ A ⎛ δ 0
100 ⎟ ⎞
= ⋅
⎜
c
na
, (11)
⋅ ρa
⎝ δ a ⎠
where ρ a is the ash density, ρ a = 1500 kg/m 3 .
• for the variation of the temperature of the ash particles:
dTa
6 4 4
6δ
0σ
0acaϕ
can 4 4
= [ σ 0aa
( T fl − Ta
) + α ga ( Tg
− Ta
)] +
( T − T ),[
K / s]
d c
3
a (12)
τ ρ δ
n δ ρ c
a
a
a
c a is the specific heat of ash, kJ/(kg·K); it is admitted the constant, c a = 1
kJ/(kg·K); a a – the energetic emission coefficient of ash, a a =0.7; a ca the blackness
coefficient of the system particle-ash [4], a ca ≈0.7; φ can – the mutual radiation
coefficient between the particle and the ash, φ can = 0.2 [5].
a
a
a
a
2. The mathematical model for Z 1
The equations characterizing this area are:
• for the dynamics of volatiles emission:
−
i
( V −V
) ⋅ e
T
8900
dV
= 150000 ⋅
dτ
• for the dynamics of volatile substances combustion:
4650
,[kg/(kg.s)]; (13)
−
dW
T dV
g
W gZ1
= 690 ⋅ ( V −W
) ⋅CZ
⋅ e + ⋅ ,[kg/(kg⋅s)], (14)
1
dτ VgZ
dτ
1
where the gases volume of this zone is given by the relation:

58
I. Pîşă, C. Neaga
0 i
⎡
τ
0
VgZ = λ ( ) ( ) ( ) ( )
1 p
⋅ Vaum + 1.242Wt + ⎢rf ξ+ λf − 1 + ri ξ+ λi − 1 + revp ξ+ λev − 1 ⎤Vaum V W vv
τ
⎦ + − +
⎣
a
(15)
3 3
⎛ δ ⎞ ⎡
0 0 i
⎛ δ ⎞ ⎤
3
+ ⎜1 − ( V 1.242 ) 1
3⎟ g
−Vaum− Wt −⎢⎜ − V W
3⎟
− ⎥( 5.6Hv + 0.8Nv + 0.7 Ov)
, [ mN
/ kg]
⎝ δ0 ⎠ ⎢⎣⎝ δ0
⎠ ⎥⎦
3
where v v is the specific volume of the volatiles, v v =1.307 m N
/ kg [6].
For the volatiles with medium composition (H v , N v , O v – mass fractions of
hydrogen, nitrogen and oxygen from volatiles), the factor 5.6H v + 0.8N v + 0.7O v
= 0.86 – represents the volume growth of the combustion gases at the volatiles
burning [3]. The value of derivate dVgZ 1
/ dτ
is given by the following relation:
dV
2
gZ
1
1
( ) 0 ⎛dV dW ⎞ 3δ ( 0 0
i dδ
= ri ζ + λi −1 ⋅ Vaum + ⎜ − ⎟⋅vv − ⋅ V 1.242
3 g
−Vaum − Wt
) −
dτ τa
⎝ dτ dτ ⎠ δ
0
dτ
,(16)
2 3
3
⎡ 3δ dδ ⎛ δ ⎞dV dW ⎤ mN
−0.86 ⎢− V + 1 ,
3 ⎜ −
3 ⎟ − ⎥
⎢⎣
δ0 d τ ⎝ δ0
⎠ d τ d τ ⎥⎦
kg ⋅ s
and the concentration of oxygen in this zone is :
⎧⎪
⎛ ⎞
CZ = ⎨λ 1 p
+ rg λf − + revp λevp − + ri λi
− −⎜ − ⎟+
⎪⎩
⎝ ⎠
3
τ δ
( 1) ( 1) ( 1)
1
3
τ
a
δ
0
⎤ M
O
⎫ M
2v
⎪ O kgO
2
2
V W M V m
⎡
3
⎛ δ ⎞ ⎡ ⎤
+ ⎢⎜1 − ,
3 ⎟ − ⎥ ⎬ ⋅ ⎢ 3 ⎥
⎣⎢⎝ δ
0 ⎠ ⎦⎥ O2 ⎭⎪
gZ1
⎣ N ⎦
where the mass of oxygen for burning one kilo of ennobled coal, M O2
mass of oxygen for burning the volatiles,
M O 2 V
transformation factor f 0-i.
dT
dV 3 i
= { ⎡cv ( T 273)
Qv {( V V) cv ( T 273)
Qv
dτ ⎣ − + ⎤⎦ − − ⎡
dτ δ ⎣ − + ⎤⎦+
i
i dδ
6
+ C ⎡
f
cc ( T − 273) + Q ⎤ ( 273)}
f
c
+ c
f aA T − + ⋅
⎣ ⎦ dτ ρ ⋅δ
(17)
and the
will be corrected with the
• for the variation of the temperature of the coal particle:
4 4 T0
44
⎤
⋅{ σ0ac ( Tfl − T ) + αgc ( Tg − T)
+ k⋅C ⋅⎡cO ( T 273) ( 273)
2 g
cO
T
2
pZ1
T ⎣
− − −
g
32
⎥ −
⎦
i
⎡c ( )
4 4
v
V − V + ⎤
⎡K
⎤
−σ0
acaϕcan ( T −Ta
)}} / ⎢
⎥,
i i
⎢
⎢
c s ⎥
+
c
C
f f
+ caA
⎥ ⎣ ⎦
⎣
⎦
ap
(18)

The reduction of nitrogen monoxide concentration to the coals having a high content of ash 59
where k is the constant of the kinetic coal combustion speed and the concentration
of oxygen on the surface of coke particle which burn in Z 1 is:
CZ
1
CpZ
=
, (19)
1 2
kδ
kδ
1+ −0.75
D D⋅ Φ
where D is the diffusion coefficient, [m 2 /s]; the diameter of the cover Ф in this
area results from the relation 10, in which VgZ
is replaced with V
0
gZ .
1
• for the variation of the temperature of the gas phase:
2
dTg
⎧⎪ dVgZ
1 ⎛dV dW ⎞ 6δαgc
= ⎨−cg ( Tg −273) −⎜ − ⎟Qv −
3 ( Tg
−T)
−
dτ ⎪⎩
dτ ⎝ dτ dτ ⎠ δ0
ρap
2 3 3 i
6δα
⎡
2
a ga ( δ0 − δ ) ρap
A ⎤
6δ
T0
− ⎢n 3 a
+ ⎥
3 ( Tg −Ta)
− kC
3 pZ
⋅
1
δ0ρap ⎢ δa ρa ⎥ δ0ρap
T (20)
g
⎣
⎦
0
⎡
44
⎤
Vaum
⋅
⎢
cO ( T 273) ( 273) ( 1)( 273)
2 g
− − cO T − + rc
2
i g
ξ + λi − Tfl
− +
⎣
32
⎥
⎦
τ
a
2
6Φ
4 4
⎫⎪
+ a
3 gσ
o ( Tf
l
− Tg ) ⎬ /( cgVgZ
),[ K / s]
1
δ0
ρap
⎪ ⎭
• for the variation of the temperature of external ash particles (around the
burning particle):
2
dTg
⎧⎪ dVgZ
1 ⎛dV dW ⎞ 6δαgc
= ⎨−cg ( Tg −273) −⎜ − ⎟Qv −
3 ( Tg
−T)
−
dτ ⎪⎩
dτ ⎝ dτ dτ ⎠ δ0
ρap
3 3
( − )
2 i
6δα
⎡ δ 2
0
δ ρap
A ⎤
a ga
6δ
T0
− ⎢n 3 a
+ ⎥
3 ( Tg −Ta)
− kC
3 pZ
⋅
1
δ0ρap ⎢ δa ρa ⎥ δ0ρap
T
g
⎣
⎦
0
⎡
44
⎤
Vaum
⋅
⎢
cO ( T 273) ( 273) ( 1)( 273)
2 g
− − cO T − + rc
2
i g
ξ + λi − Tfl
− +
⎣
32
⎥
⎦
τ
6Φ
⎫⎪
T ⎬ / c V ,[ K / s]
⎪ ⎭
2
4 4
+ a
3 gσ
o ( Tf
l
−
g ) ( g gZ )
1
δ0
ρap
a
(21)
• for the variation of the diameter of the coal particle:
dδ
⎡δ
dV 2 T ⎛
⎞⎤
0
⎢
kC ⎜
1
= −
⎟
pZ
⎥ /
1
dτ
⎢ 3 dτ
ρap
T ⎜
g M O vM ⎟
⎣
⎝
− O ⎥
2
2v
⎠⎦
i i
( C + V −V
+ A )
i
f
,[m/s], (22)

60
I. Pîşă, C. Neaga
3. The mathematical model for Z 2
This area is characterized by the introduction only of the secondary air or
the secondary air together with the recirculated gases from the end-part of the
steam generator (r evs ). This zone corresponds to the interval τ 2 ≤ τ ≤ τ 3 . The
secondary air introduction period is Δ τ ( τ
Δ = τ 3 - τ 2 ).
The dynamics of emission of volatiles, the dynamics of combustion of
volatiles and the variation of the temperature of coal particle have the same
expression as relations 13, 14, respectively 18. The values of the gas volume, of
the oxygen concentration in the gas stage and of the oxygen concentration on the
surface of the coke particle have the following calculation formulae:
- for the volume of the gas phase in Z 2 :
[ λ − λ + r ( ξ + λ −1)
]
0 τ −τ1
V gZ = V +
2 gZ1
i p evs ev Vaum
;
Δτ
(23)
- for the average concentration of oxygen in the gas cover:
⎪⎧
τ −τ1
⎫ 1
CZ
= ⎨C
+ [ − + ( − )]
2 Z V
1
1 gZ λ
1 i λp
revs
λev
MO2
⎬
⎪⎩
τ 2 −τ1
⎭VgZ
2
; (24)
- for the diameter of the cover Ф (which enters into the relation of calculation
C ):
of pZ 2
πΦ
3
πδ
=
3
0
ρ ap
V
gZ
T
2
6 6 T
Other relations characterizing this area are:
• for the variation of the temperature of the gas phase:
g
0
. (25)
2
dTg
⎧⎪ dVgZ2
⎛dV dW ⎞ 6δαgc
= ⎨−cg ( Tg −273) −⎜ − ⎟Qv −
3 ( Tg
−T)
−
dτ ⎪⎩
dτ ⎝ dτ dτ ⎠ δ0
ρap
3 3
( − )
2 i
6δα
⎡ δ 2
0
δ ρap
A ⎤
a ga
6δ
T0
− ⎢n 3 a
+ ⎥
3 ( Tg −Ta)
− kC
3 pZ
⋅
2
δ0ρap ⎢ δa ρa ⎥ δ0ρap
T
g
⎣
⎦
0
⎡
44
⎤
Vaum
⋅
⎢
cO ( T 273) ( 273) ( 1)( 273)
2 g
− − cO T − r
2
i
ξ λi Tfl cg
32
⎥
+ + − − +
⎣
⎦
τ
a
''
0 τ −τ
+ ⎡
1
( λi − λp)
caumtp
+ revs ( ξ λev 1)
ct ⎤
⎣
+ −
g ev ⎦
Vaum
+
τ −τ
6Φ
a σ
2
+
g o
3
δ0
ρap
− ⎬
⎪ ⎭
⎫
T T / c V ,[ K / s]
4 4 ⎪
( fl g ) ( g gZ )
3
2 1
(26)

The reduction of nitrogen monoxide concentration to the coals having a high content of ash 61
dV dV
0
V
2 aum
evs ev ⋅
(27)
dτ
dτ
Δτ
• for the variation of the temperature of the external ash the relation 21;
• for the variation of the diameter of the particle which burns the relation 22,
with the remark that C pZ is replaced with C
1
pZ and in the equation of the
2
kinetic of nitrogen volatilization is introduced V gZ . The rest of the
2
equations of the mathematical model remain unchanged.
gZ gZ1
where = + [ r ( ξ + λ −1)
]
4. The mathematical model for Z 3
This area is a zone where the finishing of combustion takes place At the
moment of start, the entire quantity of air for combustion has been introduced and
all the volatiles have been released and burnt. Under these circumstances, the
relations are much simplified in this area comparison with the previous zone,
because:
dV
= 0
dτ
; dW
=
dτ
0 τ −τ
; 1 = 1; V = W = V i (28)
τ −τ
2
1
The volume of the gas stage of area Z 3 , the concentration of oxygen in this zone
and the concentration of oxygen on the surface of the coke particle are given by
the relations:
⎡
τ
⎤
VgZ = ( 1) ( 1) ( )( 1)
3 ⎢λi + rg ξ + λf − + ri ξ + λf − + revp + revs ξ + λev
− ⎥⋅
⎣
τ
a
⎦
⎛ δ ⎞
⋅ V + 1.242W + V − W v + 1− V −V −1.242W
−
3
t
( ) ⎜ ⎟( )
0 i
0 0
aum t v 3 g aum i
⎝ δ
0 ⎠
⎡
3
⎛ δ ⎞ ⎤
3
0.86 ⎢⎜1 V W , [ m / ]
3 ⎟ ⎥ N
kg
δ
0
; (29)
− − −
⎢⎣⎝
⎠ ⎥⎦
3
⎪⎧ τ ⎛ δ ⎞
CZ = ⎨λ ( 1) ( ) ( 1) ( 1)
1
3 i
+ rf λf − + revp + revs ⋅ λev − + ri λi
− −⎜ −
3 ⎟+
⎪⎩
τ
a ⎝ δ
0 ⎠
; (30)
⎡
3
⎛δ
⎞ ⎤⎫⎪
M
O ⎡kgO
⎤
2
2
+ ⎢1 −⎜ V W ,
3 ⎟ − ⎥⎬ ⋅ ⎢ 3 ⎥
⎢⎣
⎝δ
0 ⎠ ⎥⎦⎭
⎪ VgZ
m
3 ⎣ N ⎦

62
I. Pîşă, C. Neaga
C
C
Z3
pZ
=
3 2
kδ
k⋅δ
1+ −0.75
D D⋅ Φ
, (31)
where the diameter of the gas cover in this area is calculated by the expression:
Φ =
T
g
δ 0
3 ρapVgZ
⋅
(32)
3
T 0
In these circumstances the equations of the mathematical model are:
• for the variation of the temperature of the gas phase:
2 2
dTg ⎧⎪
dVgZ
6δα
3
gc
6δα
a ga
= ⎨−cg ( Tg −273) −
3 ( Tg
−T)
− ⋅
3
dτ ⎪⎩
dτ δ0ρap
δ0ρap
3 3 i
⎡
2
( δ0 − δ ) ρap
A ⎤
6δ
T0
⋅ ⎢na + ⎥
3 ( Tg −Ta)
− kC
3 pZ
⋅
3
⎢ δaρa ⎥ δ0
ρap
T
g
⎣
⎦
⎡
⋅
⎢
c
⎣
+
O2
44
⎤
( T − 273) − c ( T − 273) + r ( ξ + λ −1)( T − 273)
g
32
O2
0
aum
2
''
0
6Φ
agσ
o 4 4 ⎪⎫
[ λ c t + r ( ξ + λ −1)
c t ] V + ( T − T ) /( c V )
s aum p
evs
ev
g ev
⎥
⎦
i
aum
i
3
0 ρap
δ
fl
fl
g
V
cg
τ
⎬
⎪⎭
a
+
g gZ 3
[K/s ] (33)
• for the variation of the temperature of the external ash, the relation 21;
• for the variation of the diameter of the coal particle which burns, the
relation 22 ( C pZ is replaced by C
1
pZ 3
and in the equation of the kinetic of nitrogen
volatilization is introduced V
gZ
).
3
We remark that for the values of the gas volume, for the oxygen concentration in
the gas stage and for the oxygen concentration on the surface of the coke particle
the relations 29, 30 and 31 are used.
The mathematical model is completed by the equations of generation of
NO (thermal and from combustible) as follows:
• the equation of the kinetic of nitrogen volatilization:
v
i
dN ⎛ N ⎞
4500
−
v
1500 ⎜
N ⎟ ⋅ e
T
3
= ⋅ γ − ,[kg/( m
dτ
⎜ 100 × V ⎟
N
⋅ s )] (34)
⎝
gz
⎠
• the dynamics of generation of molecular nitrogen:

The reduction of nitrogen monoxide concentration to the coals having a high content of ash 63
2
1000
−
T g
c
c
dN ⎛ N ⎞
2 13
1 10 ⎜ ⎟
3
= ⋅
⋅ e [kg/( m
dτ
T
N
⋅ s )] (35)
⎝ g ⎠
• the formation speed of the nitrogen monoxide from the nitrogen from
combustible:
1,5
3750
−
T g
c
dNO
⎛
c C ⎞
11
1 10 ⋅ N ⎜ ⎟
3
= ⋅
⋅ e , [kg/( m
dτ
T
N
⋅ s )] (36)
⎝ g ⎠
• the generation of the nitrogen monoxide from the combustion air
(according to Zeldovici):
0,5
64500
−
Tg
a
dNO
⎛ C ⎞
−
13
−11
a 2
0,5 Tg
= 3.34 ⋅10
N ⎜ ⎟
2
⋅ e −15.58
⋅10
( NO ) ( C ⋅Tg
) ⋅ e
dτ
T
⎝ g ⎠
KgNO a 3
/( m N
⋅ s )] (37)
The equations system formed by the four zones has been solved (in Turbo
Pascal) by the Runge – Kutta method.
Results and conclusions.
Based on the model from the area Z 0 there have been made calculation in
order to determine the drying time, obviously dependent on the initial diameter of
the coal particle and on x e . For x e =0.9 and diameters of the coal particle between
40÷160μm, the drying time alternates between 0.01÷0.030 seconds, according to
Fig. 3.
The dependence of NO c concentration on the main parameters is expressed
by a relation of the form:
NO c max = F(δ 0 , x e , λ p , Δτ , r evp , r evs ) , (38)
where r evp , r evs – recirculating degree of the combustion gas from the steam
generator end-part which are introduced with the primary air and with the
secondary air, respectively.
The system was solved using an inferior fuel (lignite) having the low
heating value, Q i i = 7.100 kJ/kg and the concentration of the atomic nitrogen
existing in fuel, N i = 0.9 %.
In the Figs 4-6 the variation of the concentration of NO x is presented,
depending on the initial diameter of the coal particle, on the coefficient of
excessive primary air and on the time interval in which the secondary air is
introduced (following the linear law of time presented in the paper) for the two
cases analyzed: for x e = 1 and x e = 0.9. The other variables interfering in the
mathematical model (for example, the degree of recirculating of the combustion
gases) have been kept constant for all cases taken into consideration.
43000

64
I. Pîşă, C. Neaga
Fig.3. The variation of the drying time as function of δ 0
Fig.4. The variation of the concentration of NOx as function of δ 0
Fig. 5. The variation of the concentration of NOx as function of λ p

The reduction of nitrogen monoxide concentration to the coals having a high content of ash 65
Fig.6. The variation of the concentration of NOx as function of ∆ τ
Conclusions
• In the proposed physical model the secondary air linear introduction
divides the combustion time into four areas: the drying area, the ignition
area, the area of combustion of volatile substance and the fourth area when
the combustion process is finished;
• The secondary air introduction by an infinite number of air injections
(continuous access) led to nitrogen monoxide from fuel concentration
decreases up to 40-50% comparing to initial variant (air divided in two
parts – primary and secondary air). If one takes into account the
heterogeneous reduction of NO, in presence of residual coke (due to
carbon monoxide reaction on coke basis surface) it is possible to obtain a
total reduction of the nitrogen monoxide concentration greater than 60% -
80%;
• NO value diminishes when the fuel particle initial diameter increases
while primary air excess coefficient decreases. It is underlined that this
dynamics has a minimum NO concentration value obtained for a λ p value
which is function of initial volatiles content from coal;
• The results obtained demonstrated that the influence of the mineral mass
over the nitrogen monoxide means, practically, a reducing of its total
concentration, by an average, of 7-15 percents.

66
I. Pîşă, C. Neaga
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2. Bloh, A. G., Teploobmen v topcah parovâh cotlov, - L.;Energoatomiozdat. Leningrad otd-nie
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Energetica, 2/1996, pp. 52-59;
4. Isachenko, V.P. and a., Heat Transfer ( Translated from the Russian ). Izd, Mir, 1977, 494 st.;
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