Burning Rates in Incinerator

BURNING RATES IN INCINERATORS

PART I
A SIMPLE RELATION BETWEEN TOTAL, VOLUMETRIC

AND AREA FIRING RATES

R. H. ESS E NHIGH

Pennsylvania State University
University Par k, Pennsylvania

ABSTRACT firing rate or incinerator capacity (lb/hr)
area firing rate (lb/sq ft hr)
A simple relation is derived between the area firing rate, combustion chamber height (ft)
combustion intensity (Btu/cu ft hr atm)
FA (lb/sq ft hr), the total firing rate or incinerator capac­ waste factor (equation 2.12)
ity, F (lb/hr), and the average volumetric reaction rate, logarithmic waste factor (equation 1.1)
length of incinerator
Rv (lb/cu ft hr). It has the form P pressure in combustion chamber (atm)
R proportionality factor for (equation 2.13)
_ 2/3 1/3 1/3 RV = average reaction rate (lb/hr cu ft)
Vc combustion chamber volume (cu ft)
FA = c.Rv .F = K.F W combustion chamber width (ft)

where c is an incinerator factor that depends on the pro­ INTRODUCTION
portional dimensions of the combustion chamber and has
a value near unity. K is a waste factor that varies with the Of all the problems involved in incineration one of the
waste type and the incinerator factor. more obscure is that concerned with determining the maxi­
mum duty or capacity of an incinerator. The units in
This equation has essentially the same form as an em­ which duty is measured or specilled are the same as for
pirical semi-logarithmic equation used in practice. Com­ any other furnace or combustion chamber. They are cus­
parison of the two has enabled the calculation of com­ tomarily either combustion intensity, I (Btu/cu ft hr), or
bustion intensities for different waste types. The combus­ furnace firing rate, F (lb/hr: frequently known simply as
tion intensity for a waste with 10 percent moisture is about furnace capacity). In grate fired systems it is also and
38,500 Btu/cu ft hr, but this drops to 1,400 Btu/cu ft hr more frequently given on the basis of unit area of grate
(a factor of 20 difference) as the moisture rises to 85 per­ surface, either as Btu heat release rate per unit area, or as
cent. This influence of moisture is greater than was ex­ pounds-per-hour rate-of-Ioading of fuel per unit area, FA.
pected; possible reasons for this are discussed in Part II.

NOMENCLATURE

a ratio of W to H
A grate area (sq ft)
b ratio of L to H
B heating value of fuel (Btu/lb)

87

The problem is that there are limits to the maximum where KL is a "waste factor" of value 13 for Types 0 and
allowable values of I, F, or FA ' but for a solid fuel bed 1, 10 for Type2, and 8 for Type 3. The loading rates in
there is no method yet available for estimating these from Table 3 are of interest, ranging from 16 to 39 lb/sq ft hr
first principles, in contrast to pulverized coal and oil at which upper value they are comparable [3] with coal
flames where the requirement is that the largest coal par­ rates on stoker-fired furnaces (but with higher calorific
ticle or oil drop must burn out during its transit through value fuel).
the combustion chamber. This requires a match of the
transit and the burning times, and was used by Rosin [1] All this information, though quite comprehensive, is
as the basis for his combustion intensity equation which, still incomplete. The use either of lor of F has certain
as shown recently [2], gives correct order of magnitude value in different situations, but neither alone gives a
values for combustion intensities computed from burning complete picture of the incinerator boundary limitations,
times. and use of one without the other can be misleading. How­
ever, a simple relationship exists between the three quan­
For solid bed systems the maximum duty still has to be tities that is easily derived, and the purpose of this paper
determined empirically because of the lack of knowledge is to present the derivation together with some appro­
of burning times, particularly in the overbed combustion. priate comment.
For coal it was found after many years experience [3]
that combustion intensities in excess of 35,000 Btu per THE BURNING RATE EQUATION
cu ft hr generally caused trouble by flame impingement on
the boiler tubes because the combustion volume was then Combustion Chamb er Mod el
too small for the attempted firing rate. This value is seen
to be on the low side compared with intensities obtained To derive the burning rate equation that relates I, F,
with some other fuels (Table 1) though it is typical of and FA ' we assume that the combustion chamber of any
pulverized coal boilers. The same intensity is also sug­ incinerator can be represented as equivalent to a rectangu­
gested by the Building Research Advisory Board [4] as an lar box of height H, width W, and length L. The width
upper limit for incinerators on a semi-continuous rating, and length can then be represented as multiples of the
though a figure almost half that (18,000) is suggested for height, thus
intermittent operation. This is for refuse consisting of
80 percent rubbish and 20 percent garbage and given [4] W = aH (2)
somewhat confusingly both as 4500 Btu/lb and 6000 Btu/ L = bH (3)
lb [The former figure would be more appropriate to the
For the grate area A, and the combustion volume Vc
"classic" 50/50 ratio]. we can then write
Somewhat lower combustion intensities (25,000
(Area) A = W L= (a 2 (4)
Btu/hr cu ft) are suggested by the I.I.A. [5] but their (5)
recommendations are given in a more comprehensive form b)H
in terms of total and area capacity, F and FA ' They
divided all wastes into seven types (Table 2). The last (Volume) Vc=HW L = (a b )H 3
three are "special" and the first four, as argued elsewhere
[6], are in essence the same "basic" material of 10,000 The quantitiesa and b are ratios and their ranges of
Btu/lb on a dry inert-free (D.I.F.) basis but otherwise with values are of importance. The Building Research Ad­
increasing percentages of moisture. In domestic and visory Board [4] recommend the ratio of length to width
municipal incineration we are concerned primarily with ( b/a) to lie between one and two so (ab) lies betweena 2
waste types 0 to 3 alone. and 2a2• For a value of a, the true height may be about
equal to the width (a=l), but the effective height due to
Variations of capacity and loading rate for the four combustion in the flue above or in a secondary chamber
waste types of interest were extensively studied some alongside could be twice this (i.e., roughly equal to the
years ago [7, 8] and the results obtained reduced [5] to
tabular form (reproduced as Table 3) and the empirical length), so a would be about 1/2. The quantity (a b)
expression could therefore lie between 0.25 and 2.

FA = KL.log F (1)

88

TABLE 1
TABLE OF COMPARISONS OF COMBUSTION INTENSITIES

OBTAINED WITH DIFFERENT FUELS

Combustion Intensity Gas Fuel Type Solid
Btu/hr cu ft atm Liquid

4 x 109 Mullins theo-
retical upper
limit

109

108 Longwell bomb

(80% combustion)

(special research Liquid fuel

reactor) rockets

107 Premixed gas Ramjet

burners (intensity

defined on flame Gas turbines

volume). using pressure

atomized oil. Solid fuel rockets

106 Premixed or P.F. (pulverized.

turbulent dif- Medium fuel oils fuel) (Experimental

fusion gas flames (pressure and for M.H.D.). Also

with intensity air atomized). cyclone burners

defined on furnace alone (excluding

lOs volume. Heavy fuel oils radiant chamber).

(Air and steam

atomized).

Household oil
104 burners

P.F. and stoker
firing
(industrial).

103

102 A ll fuels - for drying and baking ovens.

NOTE: The industrial furnace operations can be taken as being at one atm. The gas turbines are
normally pressurized operation.

89

TABLE 2 (5)
CLASSIFICATION OF WASTES TO BE INCINERA TED

Btu Btu Recommended
of Aux. Fuel Min Btu/hr
Approximate Moisture Value/lb
Composition Content Per Lb Burner Input
Classification of Wastes % by Weight Incom bustible of Refuse as of Waste per lb
Type Description Principal Components % Waste
Solids % Fired to be
included in
Combustion

Calculations

*0 Trash Highly combustible Trash 100% 10% 5% 8500 o o
waste, paper, wood,
carboard cartons,
including up to 10%
treated papers,
plastic or rubber
scraps ; commercial
and industrial
sources

*1 Rubbish Combustible waste, Rubbish 80% 25% 10% 6500 o o
paper, cartons, rags, Garbage 20%
wood scraps, combustible
floor sweepings;
domestic, commercial,
and industrial sources

*2 Refuse Rubbish and garbage; Rubbish 50% 50% 7% 4300 o 1500
residential sources Garbage 50%

*3 Garbage Animal and vegetable Garbage 65% 70% 5% 2500 1500 3000
wastes, restaurants, Rubbish 35%
hotels, markets;
institutional,
commercial, and
club sources

4 Animal Carcasses, organs, 100% Animal 85% 5% 1000 3000 . 8000
solids and solid organic wastes; and Human
organi• c hospital, laboratory, Tissue ( 5000 Primary)
wastes abattoirs, animal
pounds, and similar (3000 Secondary)
sources

5 Gaseous, Industrial Variable Dependent Variable Variable Variable Variable
liquid or process wastes on pre- according according according according
semi-liquid dominant to wastes to wastes to wastes to wastes
wastes components survey survey survey survey

6 Semi-solid Combustibles requiring Variable Dependent Variable Variable Variable Variable
and solid hearth, retort, or grate on pre­ according according according according
wastes burning equipment dominant to wastes to wastes to wastes to wastes
components survey survey survey survey

*The above flgures on moisture content, ash, and Btu as fired have been determined by analysis of many samples. They are recommended for use in
computing heat release, burning rate, velocity, and other details of incinerator designs. Any design based on these calculations can accommodate
mm• or vari•ati• ons.

90

TABLE 3 [5)

MAXIMUM BURNING RATE IN LBs/sa FT/HR OF VARIOUS TYPE WASTES

Burning rates are calculated as follows:

Maximum burning rate in lbs per sq ft per hr for types #0, #1, #2 and #3 wastes, using factors as

noted in the formula

Br = Factor for type waste x log of capa�ity/hr

#0 Waste Factor 1 3
# 1 Waste Factor 1 3
#2 Waste Factor 1 0
#3 Waste Factor 8

Br = Max. burning rate in lbslsq ft/hr

I.E: - Assume incinerator capacity of 1 00 lbs/hr for type #0 waste
Br = 13 (Factor for #0 waste) x log 1 00 (capacity/hr) = 1 3x2 = 26 lbslsq ft/hr

--- --- --- . ---

Capacity #0 Waste* #1 Waste* #2 Waste #3 Waste #4 Waste**

lbs/hr Logarithm Factor 1 3 Factor 1 3 Factor 1 0 Factor 8 No Factor

1 00 2.00 26 26 20 1 6 1 0

200 2.30 30 30 23 1 8 1 2

300 2.48 32 32 25 20 1 4

400 2.60 34 34 26 21 15

500 2.70 35 35 27 22 1 6

600 2.78 36 36 28 22 1 7

700 2.85 37 37 28 23 18

•

800 2.90 38 38 29 23 18

900 2. 95 38 38 30 24 1 8

1000 3.00 39 39 30 24 18

*The density of the mixture and therefore the burning rate in lbslsq ft of Type 0 Waste, or Type 1 Waste is
affected if the trash or rubbish mixture contains more than 10% by weight of catalogues, magazines, or
packaged papers.

**The maximum burning rate in lbslsq ft/hr for Type 4 Waste depends to a great extent on the size of of the

largest animal to be incinerated. Therefore, whenever the largest animal to be incinerated exceeds 113 the
hourly capacity of the incinerator, use a rating of 10 lbslsq ftlhr for the design of the incinerator.

91

Definitions R.K R (IIB)213 /( a b )1/3 (13)

We now introduce the two definitions for I and FA.

Com bustion Intensity (I) is given by cR (l/B)2/3

I = FBI VeP Btu/cu ft hr atm (6) where c = ( a b fl /3 and can be called the incinerator factor.
Using the values of KL from Table 3, I can be estimated.
where B is Btu per lb; P is the absolute pressure in atmos­
pheres, and P = 1 for incinerators. The other symbols are The values obtained have some interesting features and
already defined (see Nomenclature). Substituting for Ve are discussed below. In particular the ratio (liB) has sig­
by equation (5), and rearranging (with P = 1) nificance as the average volumetric reaction rate R v'
given by

F = (I /B) (ab)H3 RV = FI Ve = (fiB) lblhr cu ft (14)

(7)

Area Firing Rate (FA) is given by APPROXIMATE VALUES OF
COMBUSTION INTENSITY

(8) Equation (13) can be used to estimate f for the dif­
ferent types of waste using the experimental values of KL
Substituting for A by equation (4) and rearranging (9) if we select appropriate values for R and c or (a b). For
F = (a b)H2 .FA R, the best value seems to be 3.0 according to the data

Burning Rate Equation given in the previous section. For (ab), we have already

The incinerator height, H, in equations (7) and (9) can concluded (under Combustion Chamber Model) that it
be eliminated giving the burning rate equation
should lie between 0.25 and 2. The value of [l/(ab) 1 can
(10) therefore range from 0.5 to 4 and its cube root (c) can lie
between 0.794 and 1.59. This generates an overall multi­
This can be written in the more convenient form plying factor for calculating the ratio (liB) of 0.42 to
0.21.
FA = K.F1/3 (11)
Applying these factors to the Type 0 waste (Trash) to
where K is a "waste factor" for a given incinerator. This
factor depends primarily on the calorific value of the par­ estimate I gives the two figures108,000 and 38,000
ticular waste and the allowable combustion intensity that Btulcu ft hr respectively. Comparing these with the
can be achieved with that waste.
values cited in the Introduction we may conclude that the
E mpirical Relation
0.21 multiplying factor is the appropriate one, giving a
The basic similarity between equations (1) and (11) is value of 1.59 for the Incinerator Factor (c), and implying
evident. This can be made more explicit by considering that the incinerator used to obtain the data in Table 3
the relation was equivalent to one of roughly square cross-section and

Fl/3 = R.log F (12) of height about twice its side.
Using these factors of R = 3, and of 0.21 for the over­
where R ranges from2.3 to3.3 as F rises from100 to 1000
(lb/hr), but it can be written as (3.0 ± 0.3) for the range all multiplying factor, Table 4 lists the estimated values
of the waste factor, K, and the combustion intensities, I.
in F of 300 to 1000. Since the combustion intensities are computed from
Substituting for F in equation (11) by equation (12)
values of KL given as integers, and in view of the other
gives equation (1) with the "logarithmic waste factor" approximations involved, the errors in I are probably at
given by
least 10 percent. In computing these values the heats of
combustion, B, have been adjusted to a common basis of
5 percent inert. Type 4 waste has also been included, with
a K waste factor of 2 obtained directly from the Table 3
data by plotting FA against the cube root of F.

The computed combustion intensities in Table 4 show
a much wider variation than is suggested by the values

quoted in the Introduction and they bring out more clear-

92

TABLE 4 This is o(RbtaloingaFb)lefforroFm1/t3h,e derived equation by substitu­
ESTIMATED VALUES OF WASTE FACTOR (K) tion of which substitution holds to

AND OF COMBUSTION INTENSITIES (I)

Waste Type 0 1 234 within ±10 percent over the range300 to 1000 lb/hr

Logarithmic Waste 13 13 10 8 capacity.
Factor (KL)
Ash % (A) 5 5 555 4) The theoretical waste factor, K, is then related to
10 25 50 70 85
Moisture % (M) 8500 7000 4500 2500 1000 the logarithmic waste factor K L, determined experimen­
tally, by the relation
Heat of Combustion
(B) Btu/lb cR(I/B)213

Auxiliary fuel 4.53 4.53 1500 3000 5) With appropriate values adopted for the constants
Btu/lb to c, R, and B, the experimental values of KL were used to
calculate the corresponding values of combustion intensity,
Average Volumetric 8000
reaction rate eii v) I, for the five waste types, 0 to 4. The values of I were
lb/hr cu ft 3.06 2.17 1.41 found to vary by a factor of 20 for a change in moisture
from 10 percent to 85 percent.
Waste Factor (K) 4.33 4.33 3.33 2.67 2.0
38,500 31,750 13,750 5,425 1,405 6) This range in combustion intensity is unexpectedly
Combustion wide and is only partly offset by the auxiliary fuel require­
Intensity (I) ments of the wastes types3 and 4. Moisture has long been
Btu/cu ft hr known to have a significant influence on the incinerator
capacity but the magnitude of its influence on combustion
ly than anything the overriding influence of moisture on intensity was not suspected. It is obviously of prime im­
the incinerator behavior. Probable reasons for this in­ portance to determine the reasons for this effect that are
fluence are 'discussed in Part II. considered in Part II of this paper.

REFERENCES

CONCLUSIONS [1] Rosin, P.O., Braunkohle, Vol. 24, 1925, p. 241; Proc.
Internat. Conf. on Bituminous Coal, Vol. 1, 1925, p. 838.
1) The simple analysis of incinerator behavior pre­
sented in this paper leads to the following relation be­ [2] Essenhigh, R. H., Ind. Eng. Chern., Vol. 59, 1967, p. 52.
tween incinerator capacity, F (lb/hr), and the grate load­ [3] de Lorenzi, Otto, Ed., Combustion Engineering, first
ing or area firing rate, FA (lb/sq ft hr) edition, Combustion Engineering Co., Inc., 1948, �h. 9, p. 20.
[4] Ziel, P. H., "Apartment House Incinerators,:' Building
2) The "waste factor" K is given by Research Advisory Board Technical Study for Federal Housing
Administration, National Academy of Sciences, National Research
/K = c(I/B)2/3 = c.R 3 Council Publication No. 1280, 1965.
[5] I.I.A. Incinerator Standards, May, 1966, Incinerator
1 Institute of America, New York.
[6] Essenhigh, R. H., and Gelernter, G., "Systematic Ap­
where I is the combustion intensity (Btu/cu ft hr), B is praisal of Incinerator Research Requirements," Preprint No. 37C.
Presented at A.!, Chern. E. National Meeting, November, 1967.
the calorific value of the waste (Btullb), Rv is the average [7] Rose, A. H., and Crabaugh, H. R., "Incinerator Design
Standards: Research Findings," publications of the Los Angeles
volumetric reaction rate (lb/hr cu ft), and c is a dimen­ County Air Pollution Control D istrict No. 60.
[8] Williamson, J. E., MacKnight, R. J., and Chass, R. L.,
sionless Incinerator Factor that depends on the relative "Multiple-Chamber Incinerator Design Standards for Los Angeles
County," Publications of the Los Angeles County Air Pollution
proportions of the incinerator. Control District,October, 1960.

3) The equation obtained between FA and F has a
similar form to the following semi-logarithmic equation

obtained empirically from experiment.

93

PART II
THE INFLUENCE OF MOISTURE ON THE

COMBUSTION INTENSITY

ABSTRACT 1 combustion intensity (Btu/cu ft hr)
10 combustion intensity when firing dry waste
The influence of moisture in reducing incinerator
capacity is attributed to the extra thermal load it pro­ (Btu/cu ft hr)
vides on the flame which so reduces the average flame ] factor defined by equation (10)
temperature that the average burning rate of the waste j' = j factor modified for auxiliary fuel (equation 11)
is significantly decreased. The actual change in flame k kinetic constant for waste (lb/hr cu ft)
temperature is quite small but the effect is greatly magni­ ko kinetic constant for combustible (lb/hr cu ft)
fied by the high temperature coefficient of the reaction. m moisture fraction = (M/100)
Analysis of published experimental data gave an activation
energy of 22 kcal, which is consistent with combustion M% moisture percentage
of smoke. When moisture reduces the flame temperature Rv average reaction rate (lb/hr cu ft)
the burning time increases, and the input of air and fuel Rvo average reaction rate of dry waste (lb/hr cu ft)
(waste) have to be reduced to increase the stay time to
match the burning time. Tb boiling temperature of water tR)
Tf flame temperature (OR)
NOMENCLATURE Tfo flame temperature of dry waste (R)
To surroundings temperature (R)
A % ash percentage (=5%)
B calorific value of waste (Btu/lb) V reaction rate factor in equation (2.2) (lb/hr cu ft)
Bo calorific value of dry waste (Btu/lb) Vo reaction rate factor firing dry waste (lb/hr cu ft)
C concentration W moisture weight ratio = M/(0.95-m)
W' moisture weight ratio adjusted for auxiliary fuel
cf = concentration of fuel
cg = concentration of oxidizer (oxygen) Po air density at s.t.p. (lb/cu ft)
cp specific heat (Btu/lb of)
E activation energy (kcal/mole) INTRODUCTION
E% excess air percentage
There are several ways in which moisture can influence
f factor defined by equation (24) the combustion intensity in an incinerator. The most
F = firing rate (lb/hr) obvious is by straight dilution. The presence of evapor­
Fo firing rate of dry waste (lb/hr) ated moisture increases the gas volume so that the con­
centrations of the fuel (smoke, volatiles, etc.) and the
hg evaporation enthalpy of water oxygen are reduced. At the same time the increased
Hw total wall loss (Btu/hr) volume of gas decreases the residence time in the combus­
Hwo wall loss when firing dry waste (Btu/hr) tion chamber so that, either combustion is completed

utside the chamber, or else the residence time is in-
94

creased again by reducing the air input which in turn where Ucontains the intrinsic reactivity factors and Tf is
must be balanced by reducing the overall combustion the average flame temperature.
rate. The presence of moisture also provides an extra
thermal load so that the flame temperature will drop. In The calorific value of the fuel, B, depends on th� mois­
general, as shown by calculations given elsewhere on ture content. If the calorific value of the D.I.F. material is
coal [ 1), the moisture has to be at a very high level before BoBtu/lb, then a waste of A% ash and containingM%
the extra thermal load becomes appreciable, but relatively moisture has a calorific value given by
small changes in flame temperature can have a big effect
on the rate of reaction if the activation energy of the B=Bo (1 - (A+M)/100) Btu/lb waste (3)
process is high; and this affects the burning time. Finally,
the moisture may interfere directly (chemically) with the Bo is quoted in the Part I Introduction as 10,000 Btu/lb
progress of the reaction, though this is thought to be A is taken in Table 4 (Part I) as 5 percent.
unlikely.
Using the above information, equation (1) can be
This summation of possibilities ignores the real split of
the reaction into a solid bed Zone (I) and an overbed rewrI•tten as

combustion Zone (II), as described in a previous paper l=Bo (0.95-m)Uexp(-E/RTf) (4)

(Part I: (6)). This is a split that will have to be taken where m =M/I00.
into account for more detailed computations in the The problem now is to determine Uand Tf as functions
future, but for our purposes here the continued use of
the approximation of an overall combustion intensity, of W or m.
also used for the equations and values discussed in Part I,
involves only a relatively small and still tolerable increase HEAT AND MASS BALANCE

in the overall error (+ 20 percent may be a fair estimate). The flame temperature, Tf, may be estimated by means
of a heat and mass balance on the incinerator. In prin­
With this proviso as a basis, this paper assesses the rela­ ciple this is quite simple to set up since all the heat of
tive importance of the four factors listed above in so far combustion must go either into the sensible heat of the
as their influence on the combustion intensity due to
moisture is concerned. stack gases or the wall loss, Hw (Btu/hr). Two com­
plications to be considered are the magnitude of the un­
THE INFLUENCE OF REACTIVITY known wall loss and the effect of supplementary fuel.

• Basic Balance

The factors listed in the Introduction can be quantified At a firing rate F lb/hr the total heat input is FB Btu/hr.
and separated by making use of the overall reactivity or Equating this to the sensible heat in the stack gases plus
the wall loss leads to an equation with the firing rate, F,
- appearing in every term but the wall loss. Dividing through
by F gives a balance on the basis of one lb of waste
average volumetric reaction rate, Rv (lb/hr cu ft). This is
defined by equation (14) of Part I as

- (1)

Rv = (l/B) (lb/hr cu ft) (12.13)

- B = [(I-m) + (po B/100) (1 + E/I00)) cp(Tr To)

The mean volumetric reaction rate, Rv' depends on a + [hg + 2cp(Tf -Tb) ) (m) + (Hw/F) (5)
variety of factors such as the concentrations of the fuel
and the oxidant, some intrinsic "reactivity factor", and a This simple balance is based on the common assumption
temperature function. The "fuel" involved is an immense­
ly complex mixture of gases, tars, volatile vapors, "smoke", that the specific heat (cp) of the total mass of fuel (with
carbon particles, etc., but as a first approximation we may
assumed that there exists some "global" activation energy, ash) and air is the same as that of the same mass of com­
bustion products. The heat in the moisture is the evap­
-
oration enthalpy (hg) plus the sensible heat above the
E, that can be assigned to the reaction system. Rv can
boiling point, Tb' with the specific heat of the moisture
therefore be written taken as twice that of the combustion products (= 0.5

- for cp = 0.25).

(2)

95

•

Substituting for B gives approximately [for the ratio where ] = 1/[l+(poBo/100)(1+E/100)) (10)
0.05/(0.95-m) assumed small)
These can be evaluated if Tfo is evaluated from equation
(6) (7) making some reasonable assumption about the ratio
+ [ hg+2cp( Tr Tb)) [m/(0.95-m))+Hw/F(0.95-m) (Hw o/0.95 FoBo)' This is considered below.

If the material is dry this becomes Effect of Supplementary Fuel

Bo = [1+(poBo/100)(1+E/100))cp( Tfo- To)+Hw/0.95Fo In the case of wastes of Types 3 and 4 considerable
(7)
supplementary fuel is supplied: Evaluation of this con­
Elimin ation of Wall Loss tribution is a little difficult as it alters a number of factors
simultaneously. Primarily it increases the Btu input, but
The wall loss can be taken into account in one of two it does so without increasing the total gas volume as much
ways. The first is by direct calculation. The second is by as would be the case if the increased Btu input came in the
its elimination. So far as direct calculation is concerned, waste. This is because the supplementary fuel is fired near
this would be very much a matter of guess work. The stoichiometric whereas the waste is fired with high excess
range of possibilities can be reduced somewhat by using arr.
known ranges of the heat utilization factor defined by
So far as the heat balances are concerned the effect of
Thring [2) and used in furnace analysis by MacLellan [3).
supplementary fuel is clear. In equations (6) and (7) a
The full details of this type of analysis are out of place quantity B' for the supplementary Btu input should be
here though available in the reference cited. However, atgrdaasdcevtdioolnutomtBoeoos.hbotTauhilnednebqoeunianttchioreenaR(s8.eH)d.tbSh.yeo(epfxoetBarca,h/B1e0' q0tue)a.rmtiBosnyotnshutebh-e
certain assumptions and conclusions of the analysis can
be utilized. One conclusion is that wall losses for refrac­ L.H.S. will cancel out again and the overall effect will be

tory lined furnaces are typically in the range of 10 to 20 replacement of the] term [equation 10] by a]' given by
percent. Use of this value in equation (7) gives a flame
temperature in the region of2000 F for300 percent This can be regarded as equivalent to a reduction of the
excess air (as suggested [4) of Part I). This seems realistic.
true moisture content to an effective content W', given by
In the absence of any other method of estimating the
wall loss, this way would be acceptable. However, a better W' = (J '/J).W (12)
way may be the elimination of the loss term. Inspection
Strictly, there should be a further correction to the maxi­
of the two loss terms in equations (6) and (7) shows that
Hw is divided by the rate of input of combustible alone. mum flame temperature faoprptehaersdorynlmy ainteariasml, aTllfoc'obrruetctthioen
correction is small and it
As this drops and the moisture rises the flame tempera­
term.
ture drops, and so must Hw' As a first approximation
Hw is likely to be roughly proportional to the actual Approximate Values
thermal input, BF, or [BoF(0.95-m)). (Hw/FB) is then
approximately constant so subtracting equation (7) from The equations given above can be substantially simpli­
(6) would eliminate the loss terms. Performing this sub­
fied by evaluating the constants. Values adopted for the
traction gives
coefficients are as follows cp = 0.25, Po = 0.05 lb/cu ft, of
(8) 250.
Bo = 10,000 Btu/lb, E% = These give a value for]
where W is the weight ratio of moisture, Ib/lb, fuel =
m/(0.95-m) 0.054.

Rewriting this in terms of Tf we have The boiling point of water, Tb, is 672 Rj the evapora­

!tion enthalpy, h , can be taken as 1150 Btu/lb. The flame

temperature, Tf ' can be calculated from equation (9) by
assuming 15 percent wall loss so Tfo = 2300 R. Inserting
these values equation (11) can be written

( TiTfo) = (1 - 0.0765W)/(1 + 0.108W) (13)

96

Substituting for W by the moisture fraction, m, then gives This is a factor that exactly corrects for the change in B
by equation (3) due to dilution by moisture.
(TfITfo) = (1 - 1.13m) /(I-0.94m) (14)
The two concentration ratios can be determined on the
REACTIVITY FACTORS
following basis. Assume that a certain fuel and oxidant
The reactivity factor is contained in V, given in equa­ concentration exists in unit volume of air and combustion
tion (2). This equation can be made more explicit by products at a temperature Tfo' If now moisture is intro­
taking V = Vo and I = 10 at m = 0 from which we get duced the volume will expand because of the extra volume

(15) occupied by the moisture, and simultaneously contract
because of the drop in temperature. If Co is the concen­
The only component of the R.H.S. of this equation not tration of either component in lblcu ft, then the equiva­
expressed or expressible in terms of m is the ratio (VIVo)' lent, dry, gas volume at Tf containing this weight of solid
would be (TfITfo) cu ft.
To elucidate this ratio we now have to make assump­
tions about the nature of the reaction. The most reason­ To this must be added the volume occupied by the
able assumption is that the component, V, is proportional moisture. For every lb of fuel containing M% of moisture
to the product of the fuel and oxidant concentrations, the cold volume of combustion and excess air is [(BII00)
cf and cg. (1 + EII00)] cu ft. At the same time a weight m lb of
moisture is evaporated. Treating this as a perfect gas then
(16 ) this would occupy 20 m cu ft. This gives a factor increase
of

1 + 20m/(BII00) (1 + EII00) (20)

and evaluating for Band E gives

so (17) 1 + 0.05m/(1 -1.05m) = (1- m)/(I -1.05m) (21)

The constant, k, is a basic kinetic constant of propor­ The concentration ratio is therefore given by
tionality for the reaction and, for concentrations ex­
clco = (1-1.05m)/(I- m) (TtfTfo) (22)
pressed in ratios, k has dimensions of lblhr cu ft. It should
be, strictly, the reactivity constant for the combustible If the concentration factor holds true for both fuel and
oxidant then the ratio has to be squared to give the correct
fraction of the waste. However, it is clear from equation factor in (VIVo). The final expression for ( IIIo) therefore
(1) that Rv' as computed, is an average reaction rate of becomes
total waste, not just of combustible, since this is the quan­
tity required experimentally. Similarly, equation (2) 1110 = f.exp [(-EIRTfo) (0.19m)/(1 -1.13m)] (23)
gives V for the waste, not the combustible, and equation
(16) does likewise for k. For zero moisture ko is the where
kinetic constant for the combustible. The kinetic con­
stant for the wet waste, k, is therefore computed for a 22
real quantity of combustible that is less than the real
quantity of waste. If, therefore, k was recalculated on j = [(I-1.05m) (1-0.94m)]/[(I-m) (1-1.13m)]
the basis of lblhr of combustible instead of total waste its (24)

value would be decreased. The correction factor is there­ EVALUATION
fore the opposite to that for B [equation (3)], thus

ko = k[I-(A+M)/I00] (18) In the evaluation of equation (23) using the data tabu­
lated in Table 4 of Part I the pre-exponential factor, j,
When ko is based on zero moisture and 5 percent ash, was found to be quite insensitive to the moisture fraction,
as for Vo and 10 defined above, equation (18) becomes
m. This was a conclusion anticipated from inspection of
(kolk) = (1 - 1.05m) (19) equation (24) but it was confirmed by calculation. The
factor ranged from 1 to 1.5 as m increased from 0 percent

97

•

to an effective value of 78 percent [after correction for of the reaction, E. According to equation (25) the slope
the effect of auxiliary fuel by equation (14)]. In com­ of Fig. 1 should be
parison,I dropped from a value of 45,000 for 10, obtained
by extrapolation, to 5,640 (after correction for auxiliary slope = 2.3Tfo/0.19E (26)
fuel), a factor of about 8. Unexpectedly, the two factors
are actually working in opposite directions. The slope has a value of 1.4. The flame temperature,
Tfo' was estimated above as 2300 R. This corresponds
It was clear from this assessment that the principal to about 1280 K. The calculation therefore gives us

variation in 1 was due to the exponential factor. To test
this, equation (23) was rewritten in the form

E = 22,000 cal/mole (27)

1/1n (jIoII) = (RTfoI0.19E) [(11m) -1.13] (25)

Fig. 1 shows the data plotted according to this form. The DISCUSSION
graph is consistent with this equation, even to the zero
ordinate at a positive fInite value of (l/m). There is some The results of the evaluation above are clear cut. It is
discrepancy in the value of the intercept. The predicted
value, from equation (25), is 1.13, and the value obtained obvious that the factor dominating the incinerator capacity
from the graph is about half this. However, a discrepancy
of a factor of 2 in an intercept is generally tolerable, par­ as the moisture increases is the highly temperature­
ticularly where so many approximations are involved. sensitive reactivity of the material, due to the high activa­

Since the form of the equation is clearly correct the tion energy of the reaction. The influence of the moisture
graph can be used to estimate the global activation energy
is parametric and is predominantly due to the "ballasting"

action that drops the flame temperature; this drop in

tflhaem'meatjeomr pcehraantguereininvatuluren drops the reactivity because of
of the Arrhenius factor (-EIRT).

Waste Type The influence of all the other factors listed in the Intro­

duction such as dilution, decreased stay time, etc, is quite

nl I l negligible.

II ,.. �43�2� �1� �O,, ____ In purely operational terms the reduced rate of reaction
____ ____________________

in the presence of moisture simply means that the gases,

vapors, tars, and smoke take longer to burn. To refit the

flame inside the combustion chamber when moisture is

24 present, the stay time or transit time has to be reduced

- and this requires reduction of the air supply. With this
-
reduced, the firing rate also has to be reduced to maintain
...
the required fuel air ratio.
0
This suggests one possible way in which moisture could
-
• be taken into account in incinerator design. If the waste
...
could be dried before charging into the combustion cham­
-
• ber in such a way that moisture was evaporated without
'"
signifIcant decomposition of the waste, the incinerator
0

-

...
...

'0
l!

::I:

-

.,

>

capacity could be directly increased because it would be

O���� � � � �____ burning drier waste. The moisture could then be charged

____ ____ ____ to the combustion chamber through an effluent burner,

o2 4 6 • 10

Values of (1/m) either over the bed or into the exit gases as they leave the

incinerator. It is even possible that they could be used

FIG. 1 PLOT OF RECIPROCAL OF LOG (flo/I) AGAINST for attemperating so that a higher combustion intensity

RECIPROCAL OF m, THE MOISTURE FRACTION, could be maintained but with attemperation of the gas
TO TEST EQUATION (25), AND TO DETERMINE
THE ACTIVATION ENERGY OF THE OVERALL temperature by the moisture so that any risk of damage
REACTION.
to the bri�kwork was not increased. This latter problem

presupposes substantially reduced excess air and better

ml• xm• g.

98

•

However, other than use of attemperation, it is now The second debatable element of design is the general
clear that incinerator capacity is dominated by the re­ method of use of supplementary fuel. This does two
activity of the smoke and other gases. The activation things. It raises the temperature but reduces the oxygen
energy obtained, of 22 kcal, is known to be typical of concentration. If the burner is properly designed this can
solid carbon and coal combustion when the particles are be advantageous in igniting gases because the reduced
small enough. This is equally true for pulverized coal oxygen concentration is generally more than offset by
particles [4] and for submicron carbon particles formed the greatly increased reactivity due to the higher tem­
in situ by cracking of the fuel gas [5]. There is currently perature. Nevertheless there are problems. If the excess
some argument as to whether the activation energy repre­ air goes through the burner itself then the burner flame
sents an adsorption or desorption step. This writer be­ temperature may be drastically reduced and there could
lieves, for reasons given elsewhere [6], that it is a desorp­ even be stability problems. If the extra air is already
tion energy and the reaction is therefore close to zero mixed with the smoke then there is a mixing problem
order. If that is so then, given good overfire mixing of involving how to get the smoke and flame gases to mix
secondary air and smoke, the principal factor controlling fast enough. This particular aspect of the problem was
the flame length or burning time is the flame temperature pointed up recently by Crouse and Waid [8) in a fume
and this is where reduction of the excess air is of greatest combustion problem.
value.
For igniting a solid bed, a direct flame is the most
This, in fact, points up one of two debatable elements common method, but this too suffers from a number of
of incinerator design. It is generally assumed, because disadvantages. The ideal is a highly concentrated flame
wastes are often of low calorific value, that it is desirable that will heat a small area to a high temperature so that
to have all refractory lining for the walls. Unfortunately, reaction will start when the flame is removed. The ignit­
this makes incinerator operation very inflexible. If the ing flame rarely has much excess air, and if it does this
flame temperature could be increased then the burning again militates against the high flame temperature that is
time would be reduced so the incinerator capacity could the ideal requirement. It is not always appreciated that
be increased. Unfortunately, a reasonably dry waste may without excess air, ignition of the solid starts at the
then produce so hot a flame that the refractories become periphery of the flame zone since the center is devoid of
endangered. However, if the walls are artificially cooled oxygen. When the ignition flame is swithced off, this
a small increase in wall loss will hardly affect the flame peripheral flame on the solid may be too distributed for
temperature, particularly if this can be offset by a further best maintenance and may extinguish. This is particuarly
reduction of excess air. probable if the center of the zone has been devolatilized
but has still not reached the ignition temperature of the
The point behind this is that an increase in combustion resultant char.
intensity does not necessarily mean a change in flame
temperature. If the combustion intensity is increased a A far better method of ignition would be by hot air,
reasonably constant flame temperature can be maintained particularly for the solid. The obvious difficulty with
if only the thermal load can be increased. This was first such a device would be the design of an effective, high­
demonstrated by an analysis of furnace behavior by Thring temperature (and low cost) heat exchanger. If continuous
and Reber [7] (see also Thring [2)) and extended recent­ operation is required then the temperature may be too
ly by MacLellan [3). The conclusion from this is that limited, but if the ignition time is fairly short a refractory
some form of variable load w•ould be advantageous as an regnerator working on a single cycle might be more ef­
alternative to moisture as a means of controlling flame fective.
temperatures. A simple way of doing this is a partly water
cooled wall. This could be disadvantageous for the bed However, the general problem of incinerator capacity
itself but is not necessarily so for the overbed flame. A and smoke elimination centers on maintaining and pos­
valid objection to this would be on grounds of .-cost, but sibly increasing the flame temperature. It may be that
this is overcome if the heated water can be put to use. In satisfactory methods will be developed to achieve this.
other words, the incinerator should also function as a In the meantime, the central problem has now been iden­
boiler. Such an idea is by no means new, as the long tified, and the analysis given here shows how moisture
European use of waste fuels in central power stations can can be taken into account if the basic kinetics of the dry
show. material can be established. This simplifies the overall
problem considerably. As a first approximation it will

99

be quite satisfactory to work with dry materials, and it of high moisture and thus maintains burn-out within a
also justifies further the decision already made on other reasonable time.
grounds to restrict the initial materials under investiga­
tion to simple cellulosic materials such as wood and [ 8 ] For the future, however, the problems to be
paper. Otherwise, it is now clear that a prime problem solved center, first, on maintenance of the combustion in
for investigation is the kinetics of smoke formation and the fuel bed, and, second, on increasing the rate of burn­
combustion. up of the gases, volatiles and smoke in the overfue volume
of the incinerator. These are the two problems being given
CONCLUSIONS principal attention in current research programs.

From the above analysis of the heat and mass balance ACKNOWLEDGMENT
on an incinerator it is concluded that:
This paper has been prepared as part of a research pro­
1) The reduction in incinerator capacity when burning gram on Incinerator Emissions, sponsored by the Depart­
waste of high moisture is directly due to the reduced re­ ment of Health, Education, and Welfare (Public Health
activity of the reactants (mostly smoke, volatiles, and Service) under Grant Number SR01AP00397-03 whose
similar gaseous combustibles) . financial support is gratefully acknowledged.

2) The presence of moisture reduces the flame tem­ RE F E RENCES
perature, and the reaction rate then drops substantially
because the activation energy of the reaction is high, in ( 1 ] Essenhigh, R. H., Colliery Engineering, Vol. 38, 1 9 6 1 ,
excess of 20 kcal. p. 534; Vol. 39, 1962, pp. 23, 6 5, 103.

3) The influence of other factors directly introduced [ 2 ] Thring, M. W., Science of Flames and Furnaces, 2nd ed.,
by the moisture such as dilution, decreased stay time, J ohn Wiley, N . Y., 1 962.
etc, is evidently negligible.
[ 3 ] MacLellan, D. E., "Thermal Efficiency of Industrial
4) The reduction in reactivity leads to a reduced Furnaces: A Study of the Effect of Firing Rate and Output,"
capacity because the burning time increases and, unless M. S . Thesis, Department of Fuel Science, The Pennsylvania State
the stay time is increased to match this, combustion will University, September, 1 96 5 .
be completed outside the combustion chamber . Since
the stay time is increased by cutting back the air, the [ 4] Beer, J . M . , and Essenhigh, R. H., Nature, Vol. 187, 1 960,
firing rate of the waste has to be reduced also to main­ p. 1 10 6 .
tain the same fuel!air ratio.
Beer, J . M., Lee, K . B . , Marsden, C., and Thring, M. W.,
S) The activation energy obtained for the overall re­ Fifth Journe, es Internationales de I'Institut Francais des Combus-
action, of 22 kcal, is typical of the carbon-oxygen re­ tibles et de l'Energie Paris, 19=23 May, 1964.
action when the particles are so small that boundary
layer diffusion is very rapid. It should seem then that [ 5 ] Lee, K. B . , Thring, M. W., and Beer, J . M., Combustion
smoke burn-up is the limiting factor to increasing in- and Flame, Vol. 6 , 1962, p. 137.
cm• erator capac•lty.
[ 6 ] Essenhigh, R. H., "Dominant Mechanisms in the Com­
6) Increase of incinerator capacity is therefore best bustion of Coal," accepted for publication by ASME.
achieved by increasing the flame temperature. This can
be achieved directly by reducing the excess air if the [ 7 ] Thring, M . W . , and Reber, J . , "The Effect of Output on
overfire air mixing can be improved. This is the objec­ the Thermal Efficiency of Heating Appliances," J. Inst. Fuel,
tive of one current research program. 1945.

( 7] The effect of auxiliary fuel seems to be somewhat [ 8 ] Crouse, L. F . , and Waid, D . E., "I ncineration of Industrial
similar. It maintains the flame temperature in the presence Fumes by Direct Gas Flame," presented at 1 9 6 7 Technical Meet­
ing of the Central States Section of the Combustion Institute,
Cleveland, Ohio, 1 967.

1 00