I.- Ping Chung, PhD, is a senior development engineer in the Technology and Commercial Development Group at
2.5 Burner Performance Predictions
Burner performance can be divided into two parame- ters, basic burner efficiency features and meeting emis- sions at low capital and operational costs.
Boiler burner and process burner emissions are closely coupled with the furnace environment and fuel proper- ties. A complete description of the furnace is required including dimensions, wall surface description, tube location and spacing, refractory locations, burner(s) loca- tion and spacing, and combustion air temperature.
A complete fuel analysis is required including ash, sul- fur, nitrogen, hydrogen, carbon, SG or API gravity, vis- cosity, Conradson carbon, asphaltene content, heating value, and sometimes a distillation curve.
Other performance parameters such as turndown, excess air, and noise are burner/site specific and are incorporated in the design. For example, low excess air requires uniform air distribution to the burner from the wind box. Low draft loss is desirable to reduce horsepower (HP) costs, but may not be achievable due to constraints to even air distribution or high turn- down. Minimal parasitic HP costs associated with external flue gas recirculating (FGR) fans are desirable.
2.5.1 Droplet and Carbon Burnout
As discussed earlier, the process of droplet burnout is heat up, evaporation, gaseous oxidation, and char burn- out. Extensive analysis has been done on droplet heat up and evaporation, for example, Chin and Lefebvre.4 Most of this work has been related to gas turbines where the rate of combustion is limited by evaporation rates. In furnaces, the evaporation rate is small compared to the available residence time.
A much more simple approach is to use the effective evaporation constant that includes heat up and evapora- tion as discussed by Lefebvre.5 It is defined as
λeff= D t
o e
2 (2.1)
where
Do is the initial droplet diameter in μm te is the time in seconds
λeff is the evaporation constant
For injection of droplets into hot flame zones, λeff is about 0.8 μm2/μs.
For usual liquid fuels used in furnaces, the total heat up and evaporation time ranges from 10 to 20 ms and is not significant compared to mixing and char burnout.
For oxidation of the gasified droplet gases, it is custom- ary to use two-step kinetics where the hydrocarbon fuel
is assumed to oxidize to CO and CO oxidizes to final products. In practice, the limiting chemistry is the final CO oxidation. Further large-scale eddy mixing is partially responsible for the mixing oxidizer and fuel. For HC oxi- dation, several sources are available such as Ref. [6].
In general, d
dt
P T e
a b
T a b
( )
. ( . )( ) ( , / )( ) (. ) C H
C H O mol
= −5 52 10× 8 −0 815 12 200 0 5 2 //cm2s (2.2) For CO destruction, several kinetic data are available such as Ref. [7]:
d
dt e P
RT [CO]= − . × ( , /RT)(CO O)( ) (. H O).
−
1 8 107 25 000 2 5
2 5
2
(2.3) Almost all published CO rates involve H2O because CO destruction requires the (OH)−1 radical to produce the reaction.
After, and partially during, gaseous oxidation, the original droplet becomes void of hydrocarbons and becomes a char “particle.” The particle can be a solid sphere or a hollow ceno-sphere depending on the orig- inal droplet size and fuel properties such as Conradson carbon or asphaltene content. In either case, the
resultant char size and mass has shown to be related to fuel Conradson carbon and asphaltene content via the coke formation index or CFI8 (see Figure 2.9).
For hollow ceno-spheres, the CFI computes the resid- ual char size from the original oil droplet as follows:
CFI=6D2 D
c o3
cs o
ρ
ρ δ (2.4)
And for a solid char the relationship is CFI=D
D
c o
c o 3 3
ρ
ρ (2.5)
where
Dc is the char diameter Do is the droplet diameter
ρcs is the density of ceno-sphere char surface ρo is the density of oil droplet
ρc is the density of char particle δ is the ceno-sphere shell thickness
Smaller droplets will form solid char and larger drop- lets will form ceno-spheres. After determining the char size and structure, a char burnout model can be selected. The char will then oxidize by diffusion of oxy- gen to the surface and surface reactivity. Two models
0 5 10 15 20 25
0 5 10 15 20
% fuel property
asph % CCR % RCR % Coen CCR % Princt CI Poly. (CCR %) Poly. (asph %) Linear (Princt CI)
%CFI Figure 2.9
CFI versus CCR, RCR, and asphaltenes.
can be used depending on the initial atomizer droplet size. One model is shrinking diameter constant den- sity for solid chars, and the other is constant diameter shrinking density for ceno-spheres. In either case, the oxidation is defined as follows9:
dm dt
C A
K K
= −
+ 12
1 1
og p
m r
( / ) ( / ) (2.6)
where
dm/dt is the grams carbon/second consumed Cog is the molar density of O2 gmol/cm3 Ap is the area of initial char in cm2
Kr is the surface reaction coefficient in cm/s Km is the diffusion coefficient in cm/s and
r p kcm/s
K K RT
= 32 (2.7)
where
Kp Ae E T Rk
2
is the reaction rate char data in g/cm s at
=
⋅ ⋅
− ( / ′)
m
m O2 (2.8)
where
E is the activation energy in cal/g-mol R′ is 1.98 cal/g-mol K
A is the preexponential factor g/cm2 · s · atm O2 R is 82.057 cm3 atm/(gmol °K)
Tk is the temperature in °K and
K D
m DO N
c
2 2 cm/s
= 2 (2.9)
where10
D T
MW MW
O N2 2≅ 0.00026 k3 O N diffusivity in cm
2 2
1 + 1 2
/s
(2.10) and MW = molecular weight O2 or N2.
Combining Equations 2.6 through 2.10 yields a final char loss formula:
dm dt
C A
D T K RT
= −
+
12
0 000116 1 5 32
og p
p k
( /( . ) / ) C/s
c k. g (2.11)
Combining the preceding equations, dm/dt can be integrated numerically or directly for the case of constant diameter/shrinking density with constant O2 and temperature. Since this is rarely the case, the normal solution will involve droplet size distribution, reducing O2, changing temperature, and a simple
“marching” solution that is simple enough to do in a spreadsheet. Either the constant diameter or con- stant density model can be used after defining the char size distribution. Further, this procedure will yield not only the carbon particulate, but the size distribution as well. From this distribution and mass loading, opacity can be predicted using simple light scattering equations.
2.5.2 emission Formation and Prediction
In oil burner applications, similar emission predic- tions are required as in gas fuel burners, carbon mon- oxide, UBHC, VOCs, and NOx. Kinetic rate equations such as (2.2) and (2.3) can be utilized when the flow and temperature field is known to predict reduc- tion in emissions. The equations for thermal NOx formation are:
d
dt Ae E RT
(NO)=2 (− / )(O2) (eq N2) (2.12)
and
( )
( ). ( ).
O eq o O
eq
2 = k 0 5 2 0 5
RT (2.13)
One generally accepted practice is to assume (O2) in equilibrium with (O) and (O2) concentration using the Westenberg11 results for ko for (O2) equilibrium and Zel- dovich constants, A and E, as measured by Bowman.12
Then in all cases, one can post-process thermal map data in some discrete volume form and/or insert into a computational fluid dynamics (CFD) code using the Rayleigh flux theorem as follows:
∂
∂t
∫
n dv=∫
n V da⋅Cv Cs
ρ ρ( ) (2.14)
where
n is the chemical in mass units t is the time
ρ is the density v is the volume a is the area
V is the velocity vector
where, described in words, the formation of (n) through the volume surface is equal to the integrated rate of for- mation over the control volume.
It is a simple extrapolation to extend this concept for even coarse volumes as follows:
dn
dt ρ∆ =v n V aρ ⋅
∑
( ) (2.15)In the case of NOx formation using liquid fuels, the amount of elemental fuel nitrogen is extremely impor- tant to the addition of total NOx. A portion of the ele- mental fuel nitrogen is converted to NOx generally during gaseous combustion. The range of this conver- sion is very dependent on the burner and furnace type as well as any NOx reduction techniques utilized. The range can vary from 15% to 80% and can be more or less than the thermal NOx contribution.
From a practical perspective, particulate and opacity can be calculated on every application as outlined with great accuracy using Equation 2.11. Very expensive or large applications will utilize CFD (see Volume 1, Chapter 13) to compute the total flow field and post-process emissions utilizing Equation 2.14 with the appropriate destruction or formation kinetics. The fuel contribution to NOx is a function of many factors and each burner manufacturer will have a set of algorithms used for prediction.
The oil-fired burner is a complex process involv- ing atomization of droplets, evaporation, kinetics of destruction, kinetic formation, char oxidation, diffu- sion, two-phase flow, heat transfer, and special sta- bilization techniques. In many cases, the flames will need to be formed to fit the furnace. The right atom- izer must be selected to produce the right droplet size and distribution together with the right burner for proper aerodynamics. Extreme care must be con- sidered in material selection depending on the liquid properties.
2.6 Oil Burner Maintenance