Carbondioxide injection in thermal regenerators

IN HERr AND M%SS ~ Vol. 8, pp. 35-44, 1 9 8 1

0094-4548/81/010035-10502.00/0 ©Perc3mnm~Press Ltd. Printed in the thited States

CARBONDIOMIDE INJECTION IN THERMAL REGENERATORS

S.A.Aslf and A.I.Khandwawala Mechanical Engineering Department Shrl G.S.Instltute of Technology & Science, INDOR£, 452003, INDIA. (Cc~cated

by J.H. Whitelaw)

ABSTRACT The effects of Carbondioxide addition to combustion air on the counterflow regenerator thermal ratios are predicted. The alternating Direction Approximation Integration Scheme has been used for the purpose. The results are compared with those obtained by steam injection. It has been estimated that because of higher emissivity of carbondioxide substantial increase in thermal ratios are obtained by only 1.5 to 3~ carbondioxide. Introduction The use of fossil fuels - coal, oil and gas - is increasing day by day, and if this trend continues the coming generation will be faced with acute shortage of power.

This fact has attracted the attention of many

research workers to devise means for the most economical use of these resources.

One of the methods suggested is

to reduce the fuel consumption of the installed furnaces. Furnaces installed in many industrial units invariably employ thermal regenerators for preheating the combustion air, thereby beneficially utilising the heat of outgoing 35

36

S.A. Asif and A.I. Khandwawala

waste gases.

Vol. 8, No. 1

The proportion of waste heat that is thus

recovered depends upon the thermal ratios of regenerators. The industrial furnaces consumes such enormous volume of air that, if the cooling period thermal ratio is improved by even half a percent, it is possible to cut down the fuel cost by millions of rupees. Recently Chawla and Khandwawala ~-I_~ have proposed the use of waste-steam with combustion air for improving the thermal performance of regenerators.

This improvement

is

because of heat transfer taking place by radiation also during the cooling period, and also a substantial increase in radiation heat transfer coefficient during heating period. They have shown that by adding steam quantities, upto 15 percent by volume to combustion airt the cooling period thermal ratio is increased by about 25 percent.

The use of

steam has been proposed because steam is available almost free of cost from process industries or waste heat boilers. However~ one major objection to the use of steam is that it is corrosive, and hence reduces the life of chequer worker. The present paper investigates the use of carbondioxide for improving the thermal performance of counterflow regenerators. Carbondioxide is noncorrosive, has higher emissivity, and is cheaply and readily available. The work of Chawla and Khandwawala L-I_7

is restricted

to the regenerators of a limited range of thermal ratios. In the present work the effects of CO 2 injection on regenerator thermal ratios have been investigated and

Vol. 8, No. 1

CARBONDIOXIDE INIECTION IN ~ T O R S

compared with the effects of steam injection.

37

Regenerators

with a wide range of thermal ratios are considered. is no published literature

available

improving the regenerator

There

on the use of CO 2 for

thermal ratios.

Procedure of Analysis For the purpose of analysis,

a two dimensional

rator model is used wherein the temperature solid matrix are function of two heat-balance

equations

histories within

co-ordinates.

space

in non-dimensional

cross-section

.

.

~T 1

~L

(Z~x*)

are /-2_7:

Bi ( t *

2

bT

T; +1-Ti

~

( / ~ x*) 2

+ + .

In equations

- 71 )

~x*

(i)

'

Ti_ I - T i

~T m

(

.. =

(

/k .

Tin_ 1 -

and ~

channels of

.

T2 - 71

-

The

form for the

elements within the thickness of regenerator rectangular

regene-

~

x*)2 ...... i

= 2,3

, .....

(2)

,m-1

Tm

(3)

X.)2

(I) to (3), the suffix on T refers to the element

in x direction. A heat balance on the elementary volume of gas at any location along regenerator height is given by

•t*

*

*

The conditions

of constant inlet temperatures

of gas

!

and air are:

t* (0, ~ ' )

=

I

(Sa)

o

(Sb)

It

t* (o,

)

38

S.A. Asif and A.I. Khandwawala £or counter flow regenerators9

Vol. 8, No. 1

the flow reversal

conditions in between the periods are given by: I

II

( x* , ~ ,'

T*

D )'

=

r*

I1

T

1 - ~ '/n '

(x*

,/k"[

(x*

,AI-~

/

,0)

(6a)

,

(6b)

1

(

,~,n}

=

T

n"

%

The s o l u t i o n conditions

of e q u a t i o n s

(1) to ( 4 ) ,

s u b j e c t to

(5) and (6) has been c a r r i e d out by u s i n g s t e p - b y -

step i n t e g r a t i o n

process.

The method a l o n g w i t h a f l o w c h a r t

has been c o m p l e t e l y d e s c r i b e d i n Ref //-2_7 ,results and #iscussio n To compare the effects of injecting steam and of C02 into combustion air, a regenerator channel ~ith the following dimensions is considered:Channel cross-section:

.2286

Semithickness

.0331m

of ~all:

x

.1524m

The properties of solid matrix have been taken as: Es = 0.8, k = 1.5SI

%~/mK, ~

= 4 x 10-7m2/s

~urther the follovJing data have been assumed: !

t in =

I!

1723 K,

t in

=

323 K

Duration of heating and cooling periods

: 1200 s each.

,~aste gas volumetric composition when no radiating gas is introduced into combustion air, in percentage C02 = 12.33,

~O

is:

vapour = 11.36, N 2 = 74.51,

~atio of gas to air flo~, by volume ?or a fixed rate of

02 = 1.80

: 1.045

.01527 m3/s,

the heating

period thermal ratios and the cooling period thermal ratios are calculated for regenerator heights of 2.5m, 3.5m, 5.5m,

Vol. 8, No. 1

CARBONDIOXIDE INJECFIC~ IN ~ T O R S

7.5m and 9.0m.

These are p l o t t e d ,

and 2, b o t h f o r

CO2 and steam q u a n t i t i e s

respectively,

in Figs.

varying

p e r c e n t by volume added t o combustion a i r . it

39 1

from 0 t o 15

From these f i g u r e s

may be observed t h a t = 1)

With a d d i t i o n

the thermal

ratios

of a radiating

gas l i k e

f o r both the p e r i o d s

CO2 o r steam,

increase,

for

a fixed

regenerator height. 2)

By addition of steam upto about 6 percent,

thermal ratios increase almost uniformly.

the

Thus for a

regenerator height of 5.5m, the heating period thermal ratio increase from .4171 without steam to .4922 when six percent steam is mixed with combustion air.

The corresponding

increase in the cooling period thermal ratio is from .5194 to .6136.

This corresponds to a 18.01 percent increase in

I

0"8-1 t

....

o . G ~ . .

STEAM

_-,,,_ _ _ _

:xz= 7.5m. -- L ~ 5 " " m.

O'S-~1

O-S.

CA RBON-DI "OXIDE

_..

~ .-" .," -

O.I.

~/"

, 3

, , ,, 6 9 12 15 PERCENTAGE CAR SON--Ok-OXIDE o- PERCENTAGE STEAM

Fig.1. Effect on heating period thermal ratios for different regenerator heights.

.p~

2"

..""-

L.5~,..

-

O'S..~

. . . . .

C ARBOR-D! "OXIDE

/

I

[ 0"2 I 0

IE

0

....

STEAM

3 @ 9 II 18 PERCENTAGE CARBON-OI- OXIOE = PERCENTAGE STEAM

Fig.2. Effect on cooling period thermal ratloa for different regenerator heights.

40

S.A. Asif and A.I. Khandwawala

Vol. 8, No. 1

heating period thermal ratio and 18.13 percent increase in the cooling period thermal ratio. By further increase of steam injection from 6 upto about 10 percent,

there is a slow increase in both the thermal ratios,

and with increase of steam quantities more than about 10 percent, there is little improvement in the thermal ratios. The same trends are observed for other regenerator heights also, the results of which have been plotted in Fig. I and 2. Similar results have also been reported in Ref L-I_~ 3)

The addition of carbondioxide has shown a different

trend on the improvement of regenerator thermal ratios. For a given regenerator height,

an addition of only 1.5 percent CO 2

to combustion air is predicted to give a remarkable improvement in the heating and cooling period thermal ratios.

For a

regenerator height of 5.5m, the heating period and the cooling period thermal ratios increase respectively by 19.08 percent and 19.44 percent when air.

1.5 percent C02 is mixed with combustion

A further addition upto about 4.5 percent carbondioxide

gives a little improvement,

and beyond about 6 percent,

~here

is almost no further improvement in the thermal ratios for both the periods. The difference in the response to the increase in the regenerator thermal ratios by addition of CO 2 as against the addition of steam can be easily explained.

At a given tempera-

ture, for lower values of the product p x l, of partial pressure of the component and the mean beam length, the emissivity of CO 2

Vol. 8, No. 1

CARBONDIOXIEE INJECFI(~NIN ~

41

is much higher than that of steam.

But as the value

of p x 1 is increased,

o,

the

emissivities of the two constituents

approach

.~

closer,

a temperature

g For

~ ool

of 833 deg. K (1500R), this

_

has been shown in Fig. 3,

•

J

CARBON-DI-OXIDE

w

Ref L ~

. . . .

Thus for a p x 1

value of .0015m-atm, which corresponds to about 1.5

STEAM.

~oa PI

m . I l l no. - ~ . D . .

percent addition of steam or CO 2 in the considered examples, the emmissivity

Fig. 3. Emmissivity of CO 2 and water vapour at 833°K.

at 833 K is .0197 for CO 2 and .0068 for steam.

For p x i value of .021m-arm (about

15 percent addition of CO 2 or steam in the above examples) the respective emmissivities of CO 2 and steam are .071 and .057. Further, it is well established L--4 7 that the increased flow rates during the heating and the cooling periods in the regenerator have the adverse effect of decreasing the thermal ratios.

This adverse effect is insignificant when a small

quantity of a radiating component is added to the combustion air.

Hence when, say, 1.5 percent CO 2 is injected, the

increase in the regenerator thermal ratios because of increased heat transfer coefficient should be much large as compared

42

S.A. Asif and A.I. Khandwawala

Vol. 8, No. 1

to when same quantity of steam is added to combustion air. However, when a little more quantity of the radiating gas is introduced, the increased flow rates partly or wholely offsets the beneficial effects of increased heat transfer coefficients. This effect is more predominent for C02, as the increase in the value of emissivity of C02 wi~hincrease in the product p x 1 is relatively much less as compared to that for steam, Fig. 3.

This explains the results predicted above that

adding a small quantity of CO 2 to combustion air gives an appreciable rise in the thermal ratios, where mixing further more amount of C02 to combustion air does not give any additional benefit.

Conclusion From the results of computation presented in this paper, it may be concluded that additions of C02 to combustion air is more effective in improving regenerator thermal ratios as compared to steam addition.

By injecting as low as 1.5 per-

cent C02, the cooling period thermal ratio is increased by about 20 percent.

Because of noncorrosive property of C02,

the use of small quantities of C02 with combustion air in thermal regenerators may be recommended for increasing the thermal ratios and thereby effecting savings in fuel consumption of the furnaces.

The experimental verifications of the

findings of present paper is already in progress in an industry.

Vol. 8, No. i

CARBONDIOXIDE INTECYION IN ~

43

Nomenclature a

semithickness of wall, m

Bi

Blot number, ha/k

Hs

emissivity of matrix surface

h

total heat transfer coefficient, W/m 2 K

k

thermal conductivity of solid, V~/m K

L

regenerator height, m

l

mean beam length of regenerator channel, m

m

number of elements in x - direction

P

perimeter of regenerator channel, m

P

partial pressure of a radiating component, atm

S

volumetric specific heat of fluid, J/m 3 K

T

matrix temperature,

K I!

T-tln

dimensionless matrix temperature,

T

!

n

t in -

t

t in

fluid temperature, K I!

t*

dimensionless fluid temperature,

t

'I

t

t

in 'ii" '

in - t in

V

volumetric flow rate of fluid,mS/s

V

volume of fluid contained within regenerator channel, m

X

distance from channel surface perpendicular to fluid flow, X

g

m

dimensionless distance, x/a distance from regenerator entrance in the fluid flow direction,

m

3

44

S.A. Asif and A.I. Khandwawala

Vol. 8, No. 1

Greek Symbols thermal diffusivity of solid, m2/s dimensionless time,

~

(

hPy

dimensionless distance,

vs

time, [p

s

heating o~ cooling period,

S

regenerator thermal ratio A

reduced length,

hPL/VS

reduced period

~

(

"(p

V _ -..~)

a Subscript in

inlet to regenerator

.$upe!scripts '

refers to heating period

"

refers to cooling period

Refezences I .

O.P.Chawla and A.I.Khandwawala,

Letters Heat Mass Transfer,

~, 419 (1979)o .

A.I.Khandwawala and O.P.Chawla, Int. J.Heat Fluid Flow (to be published in June 1980 issue).