NUMERICAL SIMULATION OF

ENERGY RECOVERY INCINERATORS c. A. KODRES

Mechanical Systems Division

Naval Civil Engineering Laboratory

Port Hueneme, California

ABSTRACT

A mathematical model is developed to simulate the dual combustion chamber energy recovery incinerator. The key to the model is the analysis of the incinerator by components; conservation of energy is applied to the flame and primary combustion chamber, secondary com­bustion chamber, and heat exchanger, in sequence, to pre­dict temperatures and heat transfer rates throughout the system. Application of the model is illustrated by using it to conduct a limited parametric examination of this type of incinerator. The importance of combustion air control and heat exchanger performance is demonstrated.

A BD

Cp (1) hCONV

NOMENCLATURE

- surface area

- blow down

- specific heat at temperature T - convection heat transfer fUm coeffi-

cient

t!.h (1) - enthalpy at temperature T relative to enthalpy at TREF

K = thermal conductance

k - coefficient of thermal conductivity

LMTD - logarithmic mean overall temperature difference

M - mass flow rate

nJ through n12 - molar coefficients •

q - heat flux

•

qFLAME •

qPCC

T t:.T

fJ. a

Subscripts

AIR

ASH

AVe

COND

CONY

DRY

FEED

FLAME

FUEL

178

- energy liberated to the flame

- energy liberated to the primary com-bustion chamber

- temperature

- temperature relative' to the reference temperature; t:.T = T - TREF

= overall heat transfer coefficient of heat exchanger

= emissivity at temperature T = efficiency

- viscosity

- Stefan-Boltzmann constant

- refers to airflows, combustion or leakage as applicable

- refers to incinerator ash

- average value - by conduction heat transfer

= by convection heat transfer

= refers to fuel (waste) conditions with all moisture removed

= refers to feed water entering the heat exchanger

- refers to incinerator flame

- refers to the waste fed into the incin-erator

F�W G�W

HTEXC

LEAK

MIX

PCC

RAD

REF

SCC

SHELL

STACK

STEAM

WALLS

W�OO

00

= from the flame to the combustion products (gases)

= from the flame to the incinerator walls

= from the combustion gases to the in­cinerator walls

- refers to heat exchanger

= air leakage, into the PCC or down the dump stack as applicable

= refers to products of combustion in

the PCC or SCC as applicable

= primary combustion chamber

= by radiation heat transfer

- reference

= secondary combustion chamber

- refers to outer skin of incinerator

walls

= refers to combustion products exiting

the heat exchanger

- refers to steam generated by the ener­

gy recovery heat exchanger

- refers to inner surface of incinerator walls

= from the outer skin to surrounding atmosphere

= ambient condition

INTRODUCTION

Energy reco\rery incinerators are a solution to two

problems. Landfill disposal requirements are reduced by

decreasing the volume of the refuse and, simultaneously, conventional fuels are conserved by utilizing the energy

liberated in the combustion of the solid waste.

There are some drawbacks. Incineration can be expen­

sive, both in terms of initial costs and operating and maintenance costs. Some environmental problems are in­herent in these devices. A potential for air pollution exists. Particulate emissions and/or undesirable products of combustion may have to be faced. The ash must be disposed of, a problem if it is not completely inert or if hazardous nonorganic components were present in the waste. Finally, energy recovery, the generation of steam,

for example, is often sporadic and unpredictable.

Nevertheless, events of the last decade have acceler­

.ated the construction of energy recovery incinerators.

Land available for mls has been neady exhausted in many areas. Thus, the cost of the service and the distance to an

179

available site have increased, pushing up the cost of land­ml disposal. The oil crises have created an awareness of energy limitations, spawning the development of uncon­ventional sources.

The physics of incineration is extremely complex, coupling heat, mass, and momentum transfer to the chemical kinetics of solid waste combustion. Energy re­covery further complicates the problem. Therefore, most incinerator designs are based on experience and em­pirical correlations rather than theoretical predictions.

Yet, with assumptions, it is possible to theoretically

examine an incinerator to the extent of, at least, getting

a valid "feel" for its operation. Questions such as "What

are the most important parameters?" and "What are the

ranges of these parameters?" can often be answered. For new construction, the answers help determine where the design effort should be concentrated. Some of the trial and error can usually be avoided. An approximate theoret­ical analysis can also be used for troubleshooting existing facilities.

The Navy has developed a theoretical model to use as a tool for troubleshooting the three 20 TPD energy recov­ery incinerators located at the Naval Air Station (NAS) in Jacksonville, Florida [1]. These incinerators are dual com­bustion chamber devices. In this paper, the model is de­scribed. Application of the model is illustrated by using

it to conduct a limited parametric examination of the Jacksonville incinerators.

DUAL COMBUSTION CHAMBER INCINERATORS

Figure 1 is a schematic of a dual combustion chamber

energy recovery incinerator. Tyipcally, they are designed

to operate with the first, or primary, combustion chamber

(PCC) in a starvelf air mode. In the PCC, the waste is pyro­lyzed, the carbon residue is burned, providing the energy for the pyrolysis, and, perhaps, some of the products of pyrolysis are consumed. Combustion is completed in a secondary chamber (SCC) where additional air is supplied. Energy is normally recovered as the enthalpy of steam.

In the incinerator shown on Fig. 1, the hot combustion products are used to generate steam by diverting these

gases through a heat exchanger located downstream from the SCC. Primary combustion chamber waterwalls are commonly included.

MATHEMATICAL SIMULATION

The numerical model of the energy recovery incinera­tor is based on the simplified combustion reaction,

natural stack draft

t dump

stack steam damper

t 0

0 0

0 water tube

0 boiler feedwater

C) secondary

combustion

chamber see air

{) solid fire

waste door

pee ram loader overfire

leakage

" underfire air �

ash

FIG.1 SCHEMATIC OF DUAL COMBUSTION CHAMBER ENERGY RECOVERY INCINERATOR

Cn! Hn2 On3 Nn4 + ns H20 + n6 O2 + 3.76 n6 N2 -+

n7 CO2 + (ns + ns) H20 + n9 CO

+ nlO O2 + (0.5 n4 + 3.76 n6) N2

+nUH2+n12C

In addition, it is assumed that: (a) steady state exists;

(1)

(b) kinetic and potential energy changes are negligible; (c) the reactions go to completion regardless of the

temperature;

(d) combustion is limited only by the mass flow rates of fuel and oxygen;

(e) the products of combustion are perfectly mixed in the flame and in both combustion chambers;

(f) therefore, all temperature gradients are normal to the incinerator walls; the individual components of the incinerator can be represented one-<iimensionally.

The molar coefficients n! through ns are acquired from

ultimate and proximate analyses of the fuel (waste); n6 is

from the air supplied for combustion. Equation 1 is then

balanced by applying conservation of elements and ai-

180

•

qRAD.W 100 • qCONV.W '00

•

qpcc ----t--..

•

MA1R,LEAK Cp,AIR(Too)t.Too+--.

•

MF UE L.DR yL1h F UE L (Too)

TSHELL TWALL

TpCC •

qRAD.G .... W •

qCONV.G .... W

•

MFLAME Cp,MIX(T FLAME)t.T FLAME

•

•

MpCC Cp.MIX(TpCC)t.TpCC

wall

. primary combustion chamber . . mterlor

_-------"!-- MASH t.hASH(TFLAME) flame

•

MAIR Cp.AIR(Too)t.Too •

LJFLAME

FIG.2 CONSERVATION OF ENERGY IN FLAME AND PRIMARY COMBUSTION CHAMBER

locating available oxygen in order of increasing activation

energy for combustion in air: first to hydrogen to form

water vapor, then to carbon to form carbon monoxide. Any oxygen remaining is assumed to oxidize the carbon monoxide to form carbon dioxide.

Energy absorbed or liberated in breaking down the waste, primarily a heat of pyrolysis, is determined by

balancing Equation 1 for excess air, then subtracting

the enthalpies of formation of the combustion products

from the measured heating value of the waste. Once the heat of pyrolysis is known, the energy liberated during

starved air combustion is back-calculated from the en­thalpies of formation of the combustion products in an analogous manner.

By applying conservation of energy to the flame, pri­mary combustion chamber, secondary combustion cham­ber, and heat exchanger, in sequence, temperatures throughout the incinerator and, finally; steam generation

are determined. Energy terms included in the flame and PCC analyses

are illustrated on Fig. 2. Applying conservation of energy to the flame,

181

•

MFLAME Cp,MIX (TFLAME) ATFLAME • •

+ MASH Cp,ASH ATFLAME +qRAD,F-+W • • •

+ qRAD,F-+G - qFLAME -MFUEL,DRY AhFUEL (Too) •

-MAIR Cp,AIR (Too) AT 00 =0

where

(2)

tlRAD,F-+W = radiation from flame to walls of PCC

•

- A FLAME a [1 - €MIX (TFLAME)] T4FLAME

- [1 - €MIX (TWALLS)] TWALLS (3)

qRAD,F-+G - radiation from flame to products of combustion inside the PCC

= AFLAME a[€MIX (TFLAME) T'FLAME (4) - €MIX (Tpcc) Tpcc

Both the flame and inside of the PCC walls are assumed

to act as black bodies. The products of combustion are

assumed gray. Emissivities of these gases, EMIX (T), are de­rived by curve fitting the data of Hottel [2] . Gas emis-

sivities are a function of both composition and tempera­ture. Specific heat is also considered to be a function of both composition and temperature; the relationships of Sweigert and Beardsley were used in this model [4] .

Applying conservation of energy to the interior of the primary combustion chamber,

where

• •

Mpcc Cp,MIX (Tpce) ATpcc + qRAD,G-?W • • •

+ qCONV,G-?W - qpcc - qRAD,F-?G •

-M FLAME Cp MIX (TFLAME) ATFLAME , •

-MAIR,LEAK Cp,AIR (T�) AT � = 0

(5)

qRAD,G-?W = radiation from combustion gases to PCC walls

- ApCC a [eMIX (TpCC) T�CC (6) - eMIX (TWALLS) TWALLsl

qCONV,G-?W - convection heat transfer to PCC wall interior

- hCONV,PCC Apcc (Tpcc -TWALLS) (7) Finally, applying conservation of energy to the walls,

where

• 0 •

qCOND - qRAD;F->W - qRAD,G->W •

- qCONV G-?W = 0 , (8)

qRAD,W-+� + qCONV,W->� - qCOND = 0 (9)

= conduction heat transfer through the walls

- K Apcc (TWALLS - TSHELL) (10)

qCONV,W-?� = convection heat transfer off outer sur­face of PCC walls

= hCONV,� Apcc (TsHELL-T�) (11)

- radiation off outer surface of PCC walls

= A pcc a ESHELL (T'lmELL -T� ) (12)

Equations 2 through 12, along with the relationships for emissivity and specific heat, are solved simultaneously for the temperatures TFLAME, TpcC, TWALLS, and TSHELL. A Newton-Raphson iteration, with relaxation, is employed.

Temperatures of the combustion products in the secondary combustion chamber are calculated in a similar manner. If combustion has not been completed in the PCC, secondary air will induce further chemical reactions

and require an additional heat source term in the energy equations governing the SCC interior.

The heat exchanger unknowns are the steam generated, the total heat transferred between the combustion prod­ucts and the feed water-steam, and the temperature of the combustion gases as they enter the stack. Temperature and pressure of the feed water and steam are assumed to be known.

Applying conservation of energy to the combustion gases, the feed water and steam, and to the overall heat recovery heat exchanger (individual terms are shown on Fig. 3),

• •

(Mscc +MAIR,LEAK) Cp,MIX (TSTACK) ATsTACK • •

+ qSTEAM -Mscc Cp MIX (Tscc) ATscc ,

- MAIR,LEAK Cp,AIR (T�) AT � = 0 MSTEAM AhSTEAM (TSTEAM)

• •

+ BDMSTEAM AhSTEAM (TSTEAM) - qSTEAM

- (1 + BD) MSTEAM AhFEED (TFEED) = 0

(13)

(14)

qSTEAM - UMEAN (M,D A HTExcLMTD = 0 (15)

The heat exchanger overall heat transfer coefficient, UMEAN, varies with both flow rate and temperature. The magnitude of this coefficient is determined by noting that the resistance to heat transfer from the combustion gases is the dominant resistance, and therefore, only gas proper­ties have an appreciable effect on UMEAN. For example, with a staggered tube configuration [3] ,

Mscc + MAIR,LEAK 0.6 UMEAN 0: k A VG (16) IlAVG Equations 13, 14 and 15 are solved simultaneously,

using the technique described previously.

182

PERFORMANCE CRITERIA

The efficiencies of both the heat exchanger alone and the overall energy recovery incinerator are defined using the heat loss method [5] ,

= 1 _ � LOSSES 1/ - � INPUT

For the heat exchanger:

� LOSSES - sensible heat in stack gases + steam loss to blowdown

� INPUT - sensible heat in products of combus-tion entering boiler + sensible heat of feed water

• MSTEAM( SO )LlhSTEAM

• MSCC Cp,MIX(TSCC)LlTSCC

• MAIR,LEAK Cp,AIR(Too)LlToo

•

• • MSCC + MLEAK) Cp,MIX (T STACK)LlT STACK

MSTEAM(1 + SO) LlhFEED

FIG.3 CONSERVATION OF ENERGY IN BOILER

For the overall energy recovery incinerator:

� LOSSES = heat lost vaporizing moisture with waste + heat lost vaporizing moisture generated by burning hydrogen in waste + carbon carried out as ash + sensible heat of ash + heat transfer through walls of pee and see + car­bon monoxide in stack gases + sensi­ble heat in stack gases + steam lost to blowdown

� INPUT - chemical energy in waste + sensible energy in waste, air, and feed water + external power requirements

Individual terms in the summations are mathematically described in Ref. [1] .

PARAMETRIC ANALYSIS

The model is employed to conduct a parametric ex­amination of the NAS Jacksonville energy recovery in­cinerators . The dual combustion chamber device shown on Fig. I , without waterwalls, is 'the Jacksonville con­figuration. Dimensions are summarized in Ref. [1] . For

most of this study, a composite waste is burned, This composite and other types of waste considered are de­fined in Ref. [1] . Energy is recovered as the enthalpy of 177°e (350°F) saturated steam. •

COMBUSTION AIR

Figure 4 shows temperature profiles through the NAS Jacksonville incinerators as a function of the combustion air distribution. The output of the air blowers of this in­cinerator is constant; the total of the pee underfire air plus the air to the see is always equal to 3.5 kg/s (460 Ib/min). There is no overfire air. The temperatures peak when the incinerator is operating under approximately stoichiometric conditions. Note the sensitivity of flame temperatures to changes in combustion air flows.

The effect of combustion air on the energy recovery potential of these incinerators is illustrated in Fig. 5. Gas temperature and mass flow rate remain roughly constant at the heat exchanger inlet regardless of how much burn­ing is allocated to each combustion chamber. Thus, the steam generation efficiency is only a weak function of the combustion air distribution . Incinerator losses are also included in Fig. 5. The dip in overall efficiency occur-

183

UNDERFIRE AIR ( LB/MIN ) 0 119 238 357 176

2400 NOTES:

4352

(1) SCC AIR - 3.5 KG/SEC - U/f AIR (2) STEAM GENERATED AT 178 DEG.C ..

2100 951.5 kPa 3812

ADIABATIC

1800 fLAME

3272

........

HOMOGENEOUS u

fLAME � 1500 2732 W 0

w � => f-! a: � w CL L: . W f-!

1200 PCC COMBUSTION 2192

PRODUCTS

900 1652

600 SCC COMB"USTION

1112

PRODUCTS

300

STACK GASES 7' 572

o I 32 0 0.9 1.8 2.7 3.6

UNDERFIRE AIR KG/SEC )

FIG.4 TYPICAL TEMPERATURE PROFILES THROUGH DUAL COMBUSTION CHAMBER ENERGY RECOVERY INCINERATOR

184

u....

.

c..9 W 0

w 0:::: => f-! a: a:::: w CL L: W f-!

0. 84- -N-0-T-E -:--�------�------�------�

0.7

0.6 -I

0.5

0.4 (j) w (j) 0.3 (j) a ....J 0.2

)-1 0.1-1 U Z W I I

u • I

O. OS -I

(1) SCC AIR - 3.SKG/SEC - U/f AIR •

OVERALL Eff Of ENERGY RECOVERY

LOSS DUE TO SENSIBLE HEAT IN STACK GASES

HEAT TRANSfER BURNING HYDROGEN THRU PCC WALLS IN THE WASTE

HEAT TRANSfER THRU SCC WALLS

CARBON LEfT IN ASH .

v .----------� �----�------� ASH SENSIBLE VAPORIZATION

IOf MOISTURE N WASTE

.---------,f---�---_+_----\

�E===��'====��� O.OO�------�--------�------�------�-

o 0.9 1.8 2.7 UNDERfIRE AIR ( KG/SEC )

3.6

FIG.5 TYPICAL PERFORMANCE OF DUAL COMBUSTION CHAMBER ENERGY RECOVERY INCINERATOR

185

- 00

0\

0.

95

0.

90

0.

85

>--t

U

Z

0.

80

W

•

•

u

i

I

lL..

0.

75

L

w

0.

70

0.

65

2H

O

22

00

,-...

u

196

0

•

c..9

W

o

172

0

�

W

0:::

.::J

E-<

li8

0

a:

0:::

W

CL B

124

0

E-<

100

0

I (3)

ST

OI

CH

IO

ME

TR

IC

A

IR

=1

.0

K

G/

SE

C

(4)

ST

EA

M

GE

NE

RA

TE

D

AT

17

8

DE

G.

C,

95

1.

5

kP

a

-+-.B

OI

LE

R

.r

'" E

FF

•

ST

EA

M

GE

N

EN

ER

GY

R

EC

OV

ER

Y .-

---v O

VE

RA

LL

E

FF

BO

IL

ER

H

EA

T

TR

AN

SF

ER

C

OE

FF

PC

C

GA

S

TE

MP

SC

C

GA

S

TE

MP

o 0

.2

5

0.

50

0

.7

5

1 1.

25

1.

50

1.

75

2

2

.2

5

2.

50

2

.7

5

3

3.

25

AI

R

TO

S

EC

ON

DA

RY

C

OM

BU

ST

IO

N·

CH

AM

BE

R

(KG

/S

EC

) --

FIG

.6

PE

RF

OR

MA

NC

E O

F D

UA

L C

OM

BU

ST

ION

CH

AM

BE

R E

NE

RG

Y R

EC

OV

ER

Y I

NC

INE

RA

TO

R

OP

ER

AT

ING

IN

A S

TA

RV

ED

AIR

MO

DE

40

00

,-...

0:::

100

�

u •

U>

90

W

o I 1::

:r:

. 3

85

0..

....... 80

a

c.

.9 if)

�

"-

�

z

3:

37

00

0

�70

�

I

I •

E-<

b

Z

W

a:

_

0:::

u

W

_

35

50

z

60

c.

.... W

c.

.... c.

.9 �

u

L

3

40

0 2:3

50

5

E-<

�

(f)

Z

cr.

a:::

1950�--------�--------�--------�------�

1750

u •

c..9 w 1550 o .. '

w a::::: :::J r-. cc 1350 a::::: w CL L W r-.

(f)

a: 1150 c..9

u U (L

950

I o

•

PCC LEAKAGE = 0.375 KG/SEC

0.9 UNDERFIRE

PCC LEAKAGE ¥3= 0.075 KG/SEC

1. 8 AIR

2.7 ( KG/SEC )

3.6

FIG.7 EFFECT OF OVERFIRE AIR LEAKAGE ON GAS TEMPERATURES IN THE PRIMARY

COMBUSTION CHAMBER

187

850 4-------'----------L--------+ '3iSO

u

c.9 W 0 • •

a:::: w ....J I I

0 CD

0 � z I I

(f) w (f) a: c.9

z 0 I I

� (f) � CD L: 0 u

lL o

w a:::: �

800

750

700

650

� 600 a:::: w CL L: W � I

NOTES:

(1) fEED RATE - 700 KG/HR WET

(2) PCC U/f AIR - 2.75 KG/SEC

(3) SCC AIR - 0.75 KG/SEC

MEAN TEMPERATURE Of COMBUSTION GASES AS THEY ENTER BOILER

STEAM

GENERATION

�------�-----�------�-I

a O.S 1 AIR LEAKAGE DOWN DUMP STACK

1.S (KG/SEC)

FIG.8 EFFECT OF AIR LEAKAGE DOWN THE DUMP STACK

188

'3'375

'3'300 ,......; Ck:: :r: ""-c..:> � �

z a t-1

'3225 � Ck:: W Z W c..:> 1:: a: w r<

'31 so (j)

3075

•

22504---------�------�--------�------�

u 1950

• c..9 W o

w a::: => b a::: a::: w CL L w b

1650

1350

Ul 1050 => o w z w c..9

o L o I.

750

PAPER PLASTICS

WOOD

COMPOSITE

fOOD WASTE

GRASS

�--------�------�--------�------�-I o 0.9

UNDERFIRE 1 . 8

AIR ( 2.7

KG/SEC ) FIG.9 EFFECT OF THE TYPE OF WASTE

189

3.6

20004---------L-------�--------�------�

u 1800

• C9 W

o

w 1600

n::: :::J b a: n::: w CL

L W b

1400

(J1 1200 :::J a w

z w C9 a L a :r::.

1000

•

o

<:Jf----FEED RATE

FEED RATE

800 KG/HR

FEED RATE

700 KG/HR

•

0.9

UNDERFIRE 1 . 8

AIR

900 KG/HR WET

2.7

( KG/SEC ) FIG.10 EFFECT OF WASTE FEED RATE

190

3.6

44oo4-------�------�------�------�----� NOTES�

(1) FEED RATE = 700 KG/HR

(2) PCC U/F AIR = 2.75 KG/SEC

(3) SCC AIR = 0.75 KG/SEC

3900 BOILER EFF

1

STEAM GEN

0:::: � 3400

(j) 0.8 (j)

o �

C9 � .. .

z a I I

a:-::: o

� 2900 ENERGY RECOVERY

OVERALL EFF

)-t 0.6 u 0:::: w Z w C9

L.

Z W I I U I I

5 2400 L-.

0.4 L-. w f--;

(J)

1900

LOSS DUE TO ENERGY

LEFT IN STACK GAS

0.2

�I--------�--------�--------�--------� O o 10 80 120 160

OVERALL HEAT TRANSFER COEFFICIENT(W/SO.M - DEG.C)

FIG.11 EFFECT OF BOILER PERFORMANCE ON THE OVERALL PERFORMANCE OF A DUAL

COMBUSTION CHAMBER ENERGY RECOVERY INCINERATOR

191

26004---------�--------�--------�------�

u •

c...9 W

� 2300 w 0:::: :::::> b cc 0:::: W

� 2000 w b

Ul

cc c...9

0:::: w 1700 CD

L CC I U

Z o I I 1400

)-1 1 100 0::::

cc L I I 0:::: (L

o

NOTE:

(1) fEED RATE - 700 KG/HR

ILLINOIS

BITUMINOUS

SOL I D WASTE ------A

0.9

UNDERFIRE 1 . 8

AIR

PENNSY LVANIA

ANTHRACITE

2.7

( KG/SEC ) 3.6

FIG. 12 COMPARISON OF THE THEORETICAL AIR REQUIRED FOR THE COMBUSTION

OF SOLID WASTE AND COAL

192

ring at about stoichiometric operation (approximately 1.0 kg/s of combustion air) corresponds to an increase in heat transfer losses out through the PCC walls as the temperature of the combustion products increases. Energy remaining in the stack gases is, by far, the major loss.

The energy recovery potential of the NAS Jackson­ville incinerators with a different combustion air dis­tribution is parametrically summarized by Fig. 6. Her-e, PCC underfire air is kept constant at about 0.5 kg/s, establishing starved air operation, while the air to the SCC is varied. Again, the overall efficiency of the device re­mains approximately constant, but for different reasons. As the total air flow is increased beyond its stoichiometric value, the combustion products are diluted, and the tem­perature of the gases entering the heat exchanger de­creases. If there is no stack gas leakage, this temperature is equal to the SCC gas temperature also shown in Fig. 6. This dilution decreases heat exchanger temperature gradients. At the same time, the flow rate of the hot gases over the heat exchanger heat transfer surfaces is increasing. This increases the heat exchanger overall heat transfer coefficient; examine Eq. (15) and Fig. 6. The net result is a small but gradual decrease in efficiency. (Of course, when the total air flow drops below its stoi­chiometric value, combustion is never completed, and the efficiency of this incinerator decreases significantly.)

LEAKAGE AIR

Incinerator temperatures are equally sensitive to air leaking into, or out of, the combustion chambers. Fig­ure 7 shows the effect of overfire leak!lge on PCC gas temperatures. If combustion air is not changed according­ly, temperature fluctuations can be severe. The 0.3 kg/s (40 lb/min) increase in overfire leakage illustrated on Fig. 7 results in a 260°C (500°F) increase in gas tempera­ture.

The effect of leakage down the dump stack is shown in Fig. 8. This type of leakage is not a major problem. Com­bustion temperatures are not affected. The gas flow into the heat exchanger is diluted somewhat by the cooler outside air, decreasing heat exchanger temperature gra­dients, but the heat transfer coefficient is increasing, in­duced by the increased flow rate. This trend is analogous to the effects of increasing SCC airflows discussed above.

TYPE AND FEED R ATE OF WASTE

Effects of waste type and feed rate are shown in Figs. 9 and 10, respectively. These figures can only be considered as examples. The results are typical; however, they il­lustrate the wide range of operating conditions experi­enced by incinerators.

HEAT EXCHA NGER PERFORMANCE

As shown in Fig. 5, heat exchanger losses, represented here as the energy remaining in the stack gases, are the major inefficiencies limiting the overall performance of this energy recovery incinerator. In Fig. 11, the heat ex­changer is "switched" by changing the heat transfer characteristics. The range of coefficients examined is apprOximately the range available among applicable com­mercial heat exchangers. Note the order of magnitude differences in steam generation.

COMMENTS

Accuracy of the simulation has not been thoroughly evaluated. However, for excess air operation, predicted temperatures correspond to within ±20°C of tempera­tures measured in the NAS Jacksonville incinerators.

The importance of two parameters is highlighted by this analysis. These parameters are combustion air mass flow rate and heat exchanger efficiency.

The importance of combustion air control is well understood. For solid waste incinerators, air control can be crucial. First, the solid waste combustion process is very sensitive to changes in air flow. This is indirectly attributable to the high oxygen content of most types of waste. Combustion air (oxygen) requirements are low compared with, for example, coal, as contrasted in Fig. 12. Yet, the heating values are roughly equivalent. Ex­amine the slopy of the solid waste curves on Fig. 4 or Fig. 12. The difference between 1900°C (3500°F) flames and extinguished flames is less than a 0.5 kg/s (70 lb/ min) difference in combustion air. Furthermore, solid waste incinerators are subjected to continually changing air requirements because of the heterogeneous nature of the fuel and, at times,· the variability of the feed rate. Re-examine Fig. 9, and consider the response of an in­cinerator to the addition of a load of tires.

For these same reasons, a solid waste incinerator is much more vulnerable to air leakage than a coal furnace.

The importance of the heat exchanger is also well understood. but, perhaps, not always fully appreciated. The performance of the heat exchanger is the dominant factor influencing the energy recovery efficiency of the Jacksonville incinerators. Re-examine Figs. 5 and 11. This emphasizes the need to match incinerator and heat ex­changer. It also partially explains the poor performance of some of the "add on" configurations.

This parametric analysis is necessarily incomplete. The NAS Jacksonville troubleshooting also included examina­tions of the effects of moisture and of oil as an auxiliary fuel. With new designs, the effects of different energy re­covery configurations such as waterwalls could be studied; preheaters and economizers could be compared.

193

REFERENCES

[1] Kodres, C. A., "Technical Note N-1659: A Parametric Examination of the Heat Recovery Incinerators at NAS Jackson­ville," U.S. Naval Civil Engineering Laboratory, Port Hueneme, California, March 1983.

[2] McAdams, W. H., Heat Transmission, 3rd ed., McGraw­Hill, New York, 1954, pp 82-99.

[3] Ibid., Chapter 10. [4] Sweigert, R. L. and Beardsley, M. W., "Bulletin No. 10:

Instantaneous Values of Specific Heat," Georgia Institute of Tech­no1ogy, Atlanta, 19 38.

[5 ] "Power Test Code 4. 1, Test Code for Steam Generating Units," American Society of Mechanical Engineers, New York, 1964.

Key Words: Incinerator . Mathematical Model • Waste

Heat

•

194