CC BY
8Journal of Siberian Federal University. Engineering & Technologies 1 (2014 7) 83-95
УДК 662.612: 662.613: 66.088
Numerical Investigation of Influence Thermal Preparation Coal on Nitric Oxides Formation in Combustion Process
Nelya S. Chernetskayaa, Mikhail Yu. Chernetskiyab* and Alexander A. Dekterevab
aSiberian Federal University 79 Svobodny, Krasnoyarsk, 660041, Russia bKutateladze Institute of Thermophysics, SB RAS, 1 Lavrentev, Novosibirsk, 630090, Russia
Received 05.12.2013, received in revised form 21.01.2013, accepted 02.02.2014
Emissions of nitrogen oxides from coal combustion are a major environmental problem because they have been shown to contribute to the formation of acid rain and photochemical smog. Coal thermalpreparation before furnace delivery is effective method to reduce NOx emissions, shown by experiments in small-scale facilities [1]. This paper presents the mathematical model of burning thermal preparation coal. Validation of the model was carried out on laboratory-scale plant of All-Russia thermal engineering institute. Modeling of low-emissive burner with preliminary heating coal dust is made for the purpose of search of burner optimal constructions which provides low concentration of nitric oxides in the boiler. For modeling are used in-house CFD code «SigmaFlow» [2].
Keywords: NOx, boiler, thermalpreparation coal, CFD.
Introduction
Existing problems of using coal in the boiler units to a large extent can be overcome by the burning of coal dust to expose to heat treatment directly at the plant. Heat treatment of coal samples leads to significant changes in the composition and properties of solid residue, which is to reduce the yield of volatile substances and oxygen, to increase the caloric content of residual volatile matter and calorific solid residues. Good flammability of products of heat treatment, high caloric content and their reactivity, environmental cleanliness provide ample opportunities for use in thermal power plants the pre-thermal preparation of coal before burning.
In 1980-1983 at the experimental facility of the institute, detailed studies of the influence of preliminary heat of fuel preparation on the nitrogen oxides formation were carried out. Coal dust, with varying degrees of metamorphism: berezovskiy lignite coal (Vdaf = 44.7 %, Ndaf = 0.8 %), ekibustuzskiy
© Siberian Federal University. All rights reserved
* Corresponding author E-mail address: micch@yandex.ru
bituminouscoal (Vdaf = 31.2 %, NSedaf = 1.5 %% and l^uzii^istisl^iye l<a£in cokl (Vd!rf = 133,5 %, Ndaf = 2.2 %) were ored The studies werc cc.^ndluctiacJl in a wide range of temperature (until 820 °C) and heating rate ok asr into ehe durner (o = it was found that preheating of highly concentrated dust
suspension in a gaseous medium by a factor of the oxygen a < 0h05 until 600...820 °C outlet of fuel nitty.en oxides can be reducck by 22-55 timee. These studres were corfirmed on the demonstration plant ofheat power S.12 MW at combustion kuenerskiy low-caking cod. Wlien heaced ctust to 585 °C decrease (ei^issiicstt^ of niieogen oxidek is almost 2.5 times compared with conventional regime without hesiting tire duse reuched. io is e;se£lSerislleecl tlma"^ the healing nf pulcerized coal consiCerably improves c oieditions: iemperature increace in the axial zone of reverse flow at thy i nidirl part of u torch and in the core burning is reduced almort Sit1 half she distance from the mouCh of Che burrrrr to the zone of maximum tempecatures.
Thn dain oklained were nsed to develop a full-ecale pulvceizedccoar branei dnsign with pre-tnermal taeatmerct nf fuel . in -s984 tiiis .urner heat output of 60 MW lick I) e en implemented and terted on an industrial boiler TPP-klOA. Further lests were camed out on other boiler units using the syatem fon hectiing fu^k in burners. AlC oests coieítrn-e(s the fe,a!Зllfil:^1;fy oi- using these burners. in all cases it was postible to reduce nitrogen oxicte emissicns ^non^ a200 and 180(i to 500 ond 700 mg/m3 reapectinyly aa combu stion o0 koonetskiy laan and low-cakmn korl si [3 ].
ma for rther implement tdis te^ono logy on tlie boilers of various desiggns at burning coals of various grades is neceesnty do conduct additional studies, including by means of numerical simulations. In this paper ote maehematical model and some results of thermal preparation coal combustion calculation are examined.
Mathematical me odel
Tlie model er non-iroehetmal mcompressible multi-componene gas was assumed as a model of flow m combustion ctamber. The gks flow in the studied problem ia coneikered as established, thus all equations aire written in the strady-state form, it ia assumed that combustikn gasek consiet of N2, O2, CO2, H^O aind comple?i; 0^voiatiles VOL. TIk model includna tile fotlowing equetions: equrtion of continuity
f+v(H = 0; (1)
equation oc momentum balance
dvv
+ (p-p„ )g where tii_e viiscous stress tensor is
,ôu. du. 0 „ du
u V(pv-v) =-Vvu V(tm ut') (2)
T = f
(PL +1Z1. ) S^
dx j dxt 3 dxk
T is trhe; Reynolds stress tensor;
equatia n ok i-tHe componenk concentiation ^ass fraction) transfer
dpf,
dt ++7(pn-fl) = vUDm+-cyw,\+sl,
(3)
where D is the molecular diffusion constant, Sc - turbulent Schmidt number, S- source term describing reections;
equation of energy transfer
dph
(
o V(/rr-h) = V (/fo^e-Vr dt y ' ( Pr.
\
(4)
+ So, + Se>
Sr, in are source teems describing, cntrespondinnly, energy rfficr o:f renctiont and eadintion heat transfer.
The modified high-Rennolds k-e model t)lf tufSulence (Cgen k-e model) ir used to describe the turbulent characteristics of flow. The equations determining tne kinetic energy of turbulence and its dtssipation rare hcve a form (Cited ¡and Kim, ^87):
dpk
p +V(pv-k ) = v|(2f—)-Vk | + G-pe
(5)
dpe
+ a-V(pe-e) = 2(.Ve
e e2 G2
+ Cr— G - C2p--—
d k pk
wgeer G is the rate of turbulence gene+ation:
Gaged,
dxj
turnulerrt viscosity is determined as
C ri2
P,aCp C—.
P s
Reynolds stress tensor has a form
Z.. a P ij ft
. dut du 2 , ,
(—L +—-) —8 pr
dx- — 3
The empirical constants C|u = 0.09), crk = 0.8, cts = 1.15, Q = 1.15, C2 = 1.9, C3 = 0.25 are given in the was)tk (CPen and Kim, 1987), These constants are approved for a wide class of isothermal flows. The form or kee model is ndapted fo2 fully doveloped turbulent flows. Irs the ne2r-wall region wall function are used en rive computftionel resourcus.
Temperaluee of mixtu22 T in each point of flow fietd fs round usinf tbo known local values of enShalpe sired. mietuee componints content:
^Zmetd/ri
m=1
where the dependencies of component enthalpy on temperature hm(T) is described by polynomials of 5th degree.
The cadiation i n the domioant heat taanskeo mechanism. The modeling of radiant energy transfer ts honducted basing on the P1 approximation oar spherical harmonics for y grey medium (Siegel and Howell, 1992). The advantage of this method is easiness of its matching with methods of aerodynamics and heat transfer calculation realized in curvilinear meshes. The absorption coefficients were calculated using the weighted-sum-of-yray-gases model.
Calculatioa ol volatile fuel componenis combustion is hased on the ure of global irreversible reactions betwhen fuel and oxidant. To describe the neaction io turbulewt flows with large mixing time used a hybrid model using the kinetic model and the oddy break up model to determine the reaction rate. According to this model as the rosultanp veiocity is chosen the lowest Prom rates:
Coal Dusr Combnftion. The Lagrange meiliod wes used in tye ptesent woek eo model of coal dust motioa Dutihg the modelimg, tine main forces acting an a particle weee the force of phaset interaction taerodynamia resistance for<c(t)i and grtefty forte. Ae the conl particles mones, iit is heated up and it uidergoei a eumber of pracess: extractionoe residual moieture ood volatile components, combustion of volatile components and char. When the coal particle advances in the furnace, the reaction processes of vaporization coal pyrofycit and chao hombustion ase considered. Thn ton. paeticle consists of four komponents: water volaiiles. carbon tad ash. Vaporization of moisture from the coal particle is described by the diffusron-limited model. Coal postiysis is modelled by a nimple, one-step mechanism and the volttila composition is assumeb to be constont. The ceaeiiom rate of coal py rofy sis is ^aLken from experimental data.
Cliae combustion is controlled by hhe ctemical surface seaction and ttHnh ox^en diffusion to the partieln This model inchidts the factor r| whict ddis^ciilbes the transition betwten the char combustion regime iimiiedby the rate ob oxneen diУfusioIi and the regime is sufficiently limited by tiie chemical reaction raie. Chat pieticies ase considereh to bburn at constant density ond variable size.
The diameter change of a particle follows:
R^-MimR k4K bbu\i
(6)
kd _ 2 * Kc dn pK s
(7)
Kd = pc02p in Tg )*a
' K
(8)
■k
1 + 1 f akkrn< nak.d,ff
(9)
a.
'k.kin ak.dif_
a,
fK =a
'k-.diff n ak.km > nak.diff
a
NuDD ' = D
'kkifd 5
NUd = T+0.TT0V
,0.66
-EJRT
jfnt is thedensity of tlie chor particle (kg nr3); KSC is the char combustion rate (kg m-2*s-1); NuD is the diffusion Nusselt numOee; Hi is tho bulk molecular <d^:ffi:^s>:ic^ri coefficient lm2/si; ak, kin is the reaction-rate coeffitient for o cliemical ]leiгtc;tiiore (ms-1), аМУ° is ttie reaction-eate coefficient for c(effusion (ms-1).
The instantaneous burning rate of an individual particle is determined from temperature, velocity, and size i nformatiosby solving the enoegy balance for the ¡particle, assuming a spherical, homogeneous, reacting partiole surrounded b at chemically frozen boundary layer (i .e., single-film model). Heat losbes frbm convection and radiation are considered, as well as the effects Stefan flow:
fffb°1=SbfPduP)ua (T -T) (10)
Our2 dt Krai cawK Our2
i i
aconv is the convection heat transfer co efficient:
NuX
The correction to the heat-transfer equation due to Stefan flow is provided modification of Nusselt number:
ea e Pe 37 o 2 PePe Pe
Nu = 2 +---Pe*----
2 960 4 2
where Pe is a Peclet number, Pe is a modified version of the Peclet number (i.e., the ratio of the convective velocity of the net mass lebving the particle surface to the diffusive velocity of heat leaving the surface).
When char burn, heat losses from monvection is much larger than that is predicte d for not burning perticles. In the montel is used correlation ceefficient Kcomb:
coms _ K (11)
°rconv ~ ctsonvfrsmS l)
Kcomb=145e
5000
aCOnV, aconv - are convective heat transfer coefficients for burning and not burning coal particle respectively. Present model of coal particle combustion was validated in [4].
The influence of particles on the averaged gas motion, the gas components concentration and enthalpy was takeg into account on ehe bese of PSI-cell method proposed by Ceow ((Crow et al ., e977).
NOx Foemation. In thr process od NOx eormation simulation dhree mechanisms are taken into account: formation of thermal NOx according Zeldovich's model (Zeldovich, 1946), formation of prompt NOx according the Fenimore's model (Fenimore, 1979) and formation of fuel NOx (Magel et al., 1996). Addiaional er[uationb for NO and inteamediate Itydrogen cyanide HCN transfer are introduced:
P-V(bvare) = V(D V/wo)+b„a (12)
p^ + Vpfrcw) V(DVdggV)+.S,ffat
The source term in the equation of NO transfer describing thermal mechanism (Zeldovich, 1946) may be written as
s =m dlNOl
thermal-NOx NO
where d[NO]/dr is calculated as following:
d[NO] = 2O](Kk2 [O21-kAk2 [NOl2) (13)
O ~ kN[O2 ] + kk[NO 1
Assuming the partial equililnium for oxygen atom density [O], we obtain
=o] = 36.64rsi [O2]1/2 =xpN27f23/:r) Reaftion raie constints (m3/tmo6=)) are equd k7 =1.8-108exp(-38370/T) T - 3.8-107 eop(-42tS0) k2 = 1.8-104 •n-eo44(-0680/01] k-n = C.) • /03 • T • eop(-20820/0t]
The prompt NOx forms in the presence oC hyCdocarbon redPcale which prerril in huels with high molecular H:C aha. The mechamsm oC jo^om^) NrXi^xx formation was Cescr4bed 1r>9) Fenimore (1979). Source term in the equation of NO transfer may be written
S -m hM
prompt-NOx 1X NO :te '
diNO]/dt is calculaded pccording tho expression:
dlP = Kr [O2N][VOL]eop{-R-X (14)
dt Fr L ^ L iJL J * ^ R0, where
kpr. = 1.2-107 ((0 /P)+1 Ea = 60 kcal - mol-
Oxygen aeactio n 0 oder a depends on flame copCitions (de Soete, r975).
The fuel NOx is a result of reaction between oxygen end fuel niteogen. In the process of eoal fuel gasificrtion and char buae there 7akes place the transformation of nitrogen containing compounds to NH3 (ammonia) end HCN (nyaoopen a^nide). Depending on pcheme os chemical reactions between theae rompounds pnd ch^irTnlDiiSiils^oi'^ gjji^iiesi)^ eormation dl NO or N. takes pn^i^c^.
Modified de Soete nrroci^l (Magel er dle r9e6), consisting o4 thcee gloCal reactsons, is realized to calculate the fuel NOx :
<-„11,,,
NO
I
HON
dx
dXfN = -3.5 -1010 exp(-3370/ T) xHCNxa0i dt
dx
—~CN = -3 -1012 exp(-30200/ T) XhcnXno (15)
dt
dx
—JN° = -2.7 -106 exp(-9466 /T)xN0xC H dt n m
Numerical algorithms. Conservation equations for gas phase are written down in a generalized conservation law in a control volume. For the volume finite-difference analog of equation is written dow n. For calculation of diffusion flow on the face of control -volume centrally-difference scheme with second order paecision is employed.
At the approximation of convective terms Leonard's scheme is employed that is substantially minimizes the circuit viscosity. For connection velocity and pressure fields SIMPLE-C procedure is employed.
Results
Validation of the model was carried out on laboratory-scale plant of All-Russia thermal engineering institute. This plant consists of dust preheater in which coal is heated until specific temperature, burner consisting of two coaxial cylinder and combustor. Coal-gas mixture from a preheater moves on the internal flow path, air- on to the external. Original fuel compound is shown in Table 1. Coal and gas compound after preheater are shown in Table 1, 2. Experiments and numerical calculations are carried out with two coal sorts used: brown and black lean coals. The stationary plant is shown in Fig. 1.
The calculation results for black coal are shown in Fig.2, 3. Fig. 3 shows concentration NOx along the furnace. One can see satisfactory agreement with experimental data at volatile nitrogen to total nitrogen ratio equal to one. In Fig. 4, 5 the comparison of calculated and experiment results for brown
Table 1. Coal composition
Black lean Coal
W A C H O S N V
Until 1.8 22.7 67.8 2.79 2.72 0.45 1.66 13
After 0.3 25.1 70.77 1.12 0.67 0.45 1.49 4.4
Brown coal
Until 10.4 4.84 59.5 3.81 20.34 0.34 0.76 45.7
After 0.2 7.78 77.57 2.94 10.21 0.37 0.83 18
Table 2: Gas composition, %
CO2 CO H2 CH4 N2 O2 NOx
15.8 10.6 6.6 0.9 61.2 0.5 0,005 Black lean Coal
26.9 18.7 5.1 1.52 44.7 0.3 0.016 Brown coal
Fig. 1. The stationary plant
16CO (j 150Q —
|i-aS -
\
Toiipcralunu '
t ♦ # l-IMCr.niCTil
-I—I—I—I—I—I—I—I—I—I—I—s
0-] 0-2 fri tA 0.5 0.4
I.CI^Ilt PI
Fig. 2. Temperature (°C) Along the combustor (Black lean coal, Ttp=790 °C)
Black coal [Ttp=790 °C,alfa=l.1!|
N0,=750 mg/m1 O o—0 HCN(vol)iHCN(1otal|=C.7
4-*-+ HC NI yo l).'HC m total
- Etperirrent
1200 -»
1000 -
g 800 -
£, 600 -o"
Z 400 -|
200 ■ 0 ■
WilhoiH Ihermalpreparation
NQv-1400 mtii'ml
I 1 I 1 I 1 I 1 I
o. | «.2 0.3 0,4 0,S
enghl m
_CQ
Fig. 3. NOx concentration,(mg/m3). Along the combustor
Fig. 4. Temperature (°C) along the combustor (Brown coal, Ttp=612 °C)
000
nM -
c
S zoo H
rtHhOL-l (Hr"nij|pri'pwU|>n
Brown (WI i.T[p^! ®C, ill*"1.15| MJ,"*J T mgym'
I r I 1 [ 1 I r I
(J.l 4.! OJ 0.4 OJ L:n?ln, fit
u.e
Fig. 5. NOx concentration,(mg/m3). Along the combustor
♦
coal. The profiles of the temperature and NOx concentration are generally in good agreement with the measurements.
For research heating, combustion coal and also influence of application of thermo-preparation concentration for decrease of nitric oxides the geometrical model of a burner consisting from muffle and furnace extension lias been consOructed. For reseercO of initial development of a flame, and also, for the purpose of approach to real working conditions of a burner the limited area of combustion space waa considened. The fizes of a burner are shown in Fif. 6. ComputaOional grid of burner is presented in Fig. 7.
For calculatien Ohere was use. black coal tfe stoucture or which in nepresented in Table 3. The analysis of the used coal is given in Table 4.
Fuel with grinding 30 micrometer aod a part or primaiy air ad the temperature 25 °C is tangentially supplyed in to the an input 1 (Fig.6). In the cannel 2 supply of fuel (d=90 micrometer) and air at the temperature 120 °C. Secondary air at the temperature 300 °C is supplied into the cannel 3. Results of modeling in the form of distribution of aemperatuee and concentnation are resulted in Fig. 8.
Then, with the aim to stud. fie influence of une af bunner with fhermo-pneparation on decrease of nitric oxides concentration on an exit furnace chamber, modeling of boiler PK-39-IIM (see Fig.10) was carried out. As the initial data for a boiler results of modeling of burner were employed presents above. Comparison was made with a basic variant of boiler without thermo-preparation. The temperature on
Fig. 7. Computational grid of burner
Table 3. Coal composition
Characteristic, %
Cr 43
Hr 2,7
Or 7
Nr 0,8
Sr 0,6
Ar 39,6
Wr 6,3
ydaf 31,4
Qri, 3917
kcal/kg
Table 4. Operating conditions of the pulverized-coal combustion burner
Fuel rate, kg/s 2,08
Total air flow, m3/h 467640
Temperature secondary air, °C 300
Temperature dust air mixture 120
Excess air coefficient 0,812
Fig. 8. Module of velocity, m/s
Fig. 9. Temperature distribution, °C
Fig. 10. PK-39-IIM scheme
■ TClltMlHf f«TE9Q pcfpa ralki»
--T«ilhi £crmt> prrpa ration
is
--
I
----
r
f
0 J S 11 li 19 25 1£ 13 U AS Height of furuacc. in
Fig. 11. The nitric oxides concentration upheight of furnace chamber
an exit of furnace was 1167 °C. Unburned carbon 0.17 %. NOx concentration on an exit from furnace was 2755 mg/nm3 (see Fig. 11).
Conclusion
1. Applicability of the burning thermal preparation coal mathemotical mod«;l lias been validated by comparing its predictions with the experimental data of a laboratory-scale pulverized-coal combustion burner. The results of the calculation show good agreement with the measurements.
2. Numerical research of low emissive burner with step supply of air and preliminary heating coal dust was executed. As a results of calculation optimal sizes of a burner providing heating air-and-coal mixture have been found. The given warming up provides an intensive exit of flying that give the chance to lower nitric oxides concentration on an initial site of a flame.
3. On the basis of the data received at the modeling of low emissive burner modeling of boiler PK-39-IIM with thermally prepared fuel was carried out. Application of the given technology of preliminary heating coal dust provides to reduction of nitric oxides concentration in combustion products to level of 275 mg/nm3.
Acknowledgement
This work was supported by the Ministry of Education and Science of the Russian Federation. government contract № 14.A18.21.1962 and Russian Foundation for Basic Research (Grant № 14-0801079).
References
[1] Babiy V.I., Alaverdov P.I., Barbarash V.M. et. al. // Thermal Engineering. 1983. № 9. P. 10-13.
[2] Chernetskii, M. Yu. and Dekterev A.A. // Flame Combustion, Explosion, and Shock Waves. 2011. Vol. 47(3). P. 280-288.
[3] Babiy V.I., Verboveckiy E. Kh., Artem'evIu. P. // Thermal Engineering 2000. № 10. P. 21-28.
[4] Chernetskiy M., Dekterev A., Gavrilov A. // Proc. of ICCHMT09. 2009. P. 409-412.
[5] Crow C.T., Sharma M.P., Stock D.E. // J. Fluids Engg., Trans. of the ASME. 1977. Vol. 99. P. 25-332.
[6] Zeldovich Y.B., Sadovnikov P.Y., Frank-Kamenetckiy D.A. // 1947 AS USSR. P. 317.
[7] Fenimore C.P. // 17th Symp. (Int.) Comb., The Combustion Institute, Pittsburgh, 1979. P. 661.
[8] De Soete G.G. // In 15th Symp. (Int'l.) on Combustion, The Combustion Institute, 1975. P. 1093.
[9] Magel H.C., Greul U., Schnell U. et. al. // In Proc. Joint Meeting of the Portuguese, British, Spanish and Swedish Section of the Combustion Institute, Madeira, 1996. Vol 1. P. 123-130.
Численное исследование влияния термоподготовки угля на образование оксида азота в процессе горения
Н.С. Чернецкаяа, М.Ю. Чернецкийаб, А.А. Дектереваб
аСибирский федеральный университет, Россия, 660041, Красноярск, пр. Свободный, 79 бИнститут теплофизики им. С.С. Кутателадзе СО РАН, Россия, 630090, Новосибирск, пр. Академика Лаврентьева, 1
Выбросы оксидов азота при сжигании углей являются одной из основных экологических проблем, так как они служат причиной возникновения кислотных дождей и смога. Термоподготовка угля перед его сжиганием в топочной камере позволяет значительно снизить образование оксидов азота. В данной статье представлена математическая модель горения угольной пыли, прошедшая предварительную термоподготовку. Тестирование математической модели было выполнено с использованием экспериментальных данных, полученных на огневом стенде ВТИ. Проведены расчеты с целью оптимизации низкоэмиссионного горелочного устройства с использованием некоммерческого пакета программ вычислительной гидродинамики SigmaFlow.
Ключевые слова: оксиды азота, топочная камера, термоподготовка угля, вычислительная гидродинамика.
