Performance of an afterburner chamber with a natural gas burner

PII: SOO16-2361@6)00109-3

ELSEVIER

Performance of an afterburner with a natural gas burner

V6clav

Veselq,

Miloslav

Hartman,

Otakar

Fuel Vol. 75, No. 11, pp. 1271-1273, 1996 Copyright 0 1996 Elsevier Science Ltd Printed-in Great Britain. All rights reserved 0016-2361/96 $15.00+0.00

chamber

Trnka and David

Fetsch

Institute of Chemical Process Fundamentals, Academy of Sciences of the Czech Republic, 165 02 Prague 6-Suchdol, Czech Republic (Received 18 October 1994; revised 23 February 1996)

temperature and concentration profiles of CH4, 02, CO1 and CO in a pilot-plant refractory afterburner chamber fitted with a natural gas burner were measured as functions of the throughput and excess air ratio. The performance of the afterburner is strongly related to the overall temperature regime, concentration of oxygen and gas residence time. Copyright 0 1996 Elsevier Science Ltd.

The axial

(Keywords:afterburning;natural gas combustion; performance)

Various systems used for the combustion of fuels and hazardous wastes will burn out solids and volatilize organics. Not all the organics will generally be incinerated completely in a combustor. For example, the residual combustibles cause smoke and odour emissions in waste-sludge incineration in a multiple-hearth furnace. Complete combustion requires that a high temperature be maintained for a specific time for destruction of the organics. This is the purpose of a secondary combustion chamber, or afterburner, which is usually placed immediately downstream of the combustor. The afterburner normally contains at least one burner to provide the supplementary heat needed for burnout of the residual organics present in the gas stream. The gas discharged from the afterburner is passed to a gaseous emissions control system. Not much is known about the efficient design of the afterburner. It is constructed as a refractory-lined cylinder that is usually sized to ensure a flue gas residence time of 2s at a temperature of 1200°C. However, it is apparent that as well as temperature and residence time, the composition of the inlet gas and the operating pressure also influence the composition of the exit gas. Experience suggests that the performance of the afterburner is strongly affected by the kinetics of chemical reactions occurring in the process. The fuel for the afterburner is most often natural gas or fuel oil. Natural gas is essentially methane with low concentrations of ethane, propane and other higher hydrocarbons. The mechanism and kinetics of methane combustion are described in the literature’-7. The aim of this work was to examine experimentally the course of the combustion of natural gas in an afterburner. New data are reported on the effect of throughput and excess air ratio on the axial temperature and concentration profiles in a pilot-plant unit.

EXPERIMENTAL Apparatus

The empty shaft of a high-temperature fluidized bed reactor was used for the measurements. The reactor is of rectangular cross-section 0.3 m x 0.3 m that is enlarged to 0.6 m x 0.6 m at a height of 2 m. The overall height of the reactor is 4.2m, and the total volume for reaction is 0.67 m3. A combustion chamber of these dimensions provides gas-phase residence times of the order of seconds. The refractory material of the reactor makes it possible to operate at temperatures up to 1200°C. The combustor is well insulated to provide relatively low heat loss. The reactor is equipped with a perforated plate distributor with a free area of 4.2%. A natural gas burner with an input of 100 kW is located in the centre of the distributor. A stoichiometric amount of air was introduced into the burner and the remaining (excess) air was brought into the reactor through the distributor. The reactor vessel was equipped with thermocouples which enabled the axial temperature profiles along the centreline of the reactor to be measured. The flow rate of air as well as natural gas was metered and controlled. Neither natural gas nor air was preheated. Gas samples were withdrawn from the reactor through a quartz tube at different levels into a glass flask. The samples were immediately analysed for CO, COZ, CH4 and 02 by gas chromatography. Continuous analysers were also used for 02, CO, CO2 and NO,. Every effort was taken to ensure complete heat-up of the reactor prior to the experimental measurements. It usually took 6-8 h to reach steady-state conditions. The composition (vol.%) of the natural gas was as follows: 94-96 CH4, l-2 C2H6, 0.5 C3Hs and l-2 Nz. An important operating parameter is the fraction of

Fuel 1996 Volume 75 Number 11

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Performance

of an afterburner

chamber:

V. Veself et al.

Table 1 Operating conditions of the combustion experiments Excess air ratio 4

Run A B C D

Total input, n, (mol mW3s-‘)

Paramenter ‘parm’” (mol mm3s-‘)

1.486 1.090 1.900 1.586

0.4128 0.5136 0.7503 1.012

1.800 1.061 1.266 0.7839

‘Defined by Equation (2)

Relative distance (-) Figure 3 Concentration profiles in the chamber for 4 = 1.06, n, = 1.09 mol m-3s-1. Symbols as in Figure 2

0.20

i

,$o

0.3

0.2 w

0.4

0.5

@. _

.

_I 2 ._

‘!

1; i

I

c--j

Figure 1 Steady axial profiles of temperature at the centreline of the reactor. Data points show measured values: 0, n, = 1.49, parm = 4.13; X, n, = 1.09, parm = 5.14; A, nt = 1.59, parm = 1.01; +, n, = 1.90, parm = 7.50 0.20

’

‘:

0.1

0

___n_________v_____________________v_.

, ;:’ vv

0.15

iz 7 z O.‘O I,

8

6 ‘__*_____.___________--_._

\ i

Relative distance (-)

0.15 ”

A c

2

1

‘I\\

1’

‘;~_~____-_~_-_-9_-_-V

zI- 0.10 ti

2

Figure 4 Concentration profiles in the chamber for 4 = 1.27, n, = 1.90 mol m-3s-1. Symbols as in Figure 2

‘0

a___

_CC__,____?

/

I

'V

” o.oj%-y__y___~~~~--;5

0.20

*

*

A

. 1;

0.1

0.3

0.2

.L

flow rate

Fuel 1996 Volume 75 Number 11

“t

O.lO-$

zg

t

i 1

, .

______-------------_---

. .

.

_‘;_____;__;__________~

(1)

The experimental programme included measurements of the temperature and concentration profiles along the centreline of the reactor at different excess air ratios and flow rates of methane (natural gas). The excess air ratio varied in the range 0.78 to 1.8. The flow rate of natural gas ranged between 2.3 and 5.1 ml h-l. The operational conditions of the combustion experiments are shown in Table 1. Replicate experimental runs indicated that the overall reproducibility of measurements was better than 5-7%.

1272

z 2

u

stoichiometric air or excess air ratio, 4, defined as (moles airkctuai flowrate

\I

8 ._

Figure 2 Concentration profiles in the chamber for (b = 1.8, n, = 1.486 mol m-3s-1: ?? , CH4; 0, 0s; V, CO*; 0, CO; v, H20

airhtoi~hio~etric

A r

0.4

Relative distance (-j

’ = (moles

‘\

0

r--

0.1

_I_

0.2

L

0.3

I

0.4

_

d

0.5

Relative distance (-) Figure 5 Concentration profiles in the chamber for d = 0.78, rrt = 1.59 mol m-3s-1. Symbols as in Figure 2

RESULTS AND DISCUSSION Preliminary experiments confirmed that the combustion of methane in the chamber was complete at temperatures

Performance

> 1000°C and when the concentration of oxygen in the exit flue gas was > 3 vol. %. Only traces of carbon monoxide were detected in the flue gas produced under these conditions. If the concentration of oxygen at the above temperature was reduced, the concentration of carbon monoxide increased but methane was not present in the flue gas. When the concentration of oxygen in the flue gas was close to zero, methane as well as hydrogen occurred in the combustion products. A decrease in temperature resulted in increased formation of carbon monoxide. At temperatures < 65O”C, incomplete explosive combustion occurred. Repeated chromatographic analyses revealed the presence of formaldehyde and acrolein in the flue gas under certain conditions. In this respect, the temperature of the reactor walls appeared to be an important factor. Axial temperature profiles

To examine how the temperature field developed during combustion, temperatures were measured at the centreline of the chamber at different distances above the distributor. The hottest zone near the burner (t > 9OO’T) was characterized by a virtually steady state after N 46 h of operation. However, the temperature field in the upper part of the reactor was not steady until > 8 h of firing. Therefore all further measurements were made after an 8 h heating-up period. Axial profiles of tempature at the centreline of the chamber measured after 8 h heat-up are shown in Figure 1. Useful parameters for the temperature curves were found to be the feed rate nt and the excess air ratio 4 or the parameter parm = nt~[CH4(0)l/[02(0)l

(2)

where nt is defined as FUp,IFH (mol me3 s-‘l, F being the cross-sectional area of the chamber (m ), H the distance above the inlet (m), U the superficial gas velocity (m s-l), and ps the gas density (molmA3). [CH,(O)] and [O,(O)] denote the concentrations (mole fractions) of methane and oxygen at the inlet of the reactor. All the curves exhibit a more or less distinct maximum of 810°C (parm = 0.41) to 1160°C (parm = 0.75) at a relative distance w = 0.09 to w = 0.16 (ratio of distance above inlet to height of chamber). The increase in temperature is steep at the reactor inlet. On reaching the maximum, the temperature curve decreases slowly towards the exit of the combustion chamber. Axial concentration projiles

Concentration profiles of CH4, 02, C02, Hz0 and CO were monitored under four different operating conditions at the centreline of the combustor. The experimental data are plotted in Figures 2-5. The most dramatic changes in concentration of the individual species take place within a relatively narrow zone (w = 0.05-o. 15) at the chamber inlet. The height of this zone depends on the excess air ratio and the total input. The experiments proved that the combustion of methane ceased at N 620°C. The unstable, explosive burning of methane took place at 620-650°C. Such

of an afterburner:

V. Vesel); et al.

products as formaldehyde and acrolein were formed at temperatures < 800°C. This indicates that the stable and rapid burning of methane occurs at temperatures > 800°C. This clearly confirms that as well as the excess air ratio, the whole temperature profile determines the combustion performance of the reactor. The combustion efficiency, CE, is defined as CE

= [CO,]

w21 + [CO]

where [CO,] and [CO] are the concentrations of the respective species, and expresses the effectiveness of a combustion process to completely oxidize an organic material. An effective process should be capable of achieving efficiencies of CE > 0.99. Provided that an adequate excess of oxygen is available, the combustion efficiency CE is a strong function of temperature, 800°C 5 t 5 1000°C and mean gas residence time, 1.5 s < 7 < 3.5 s, i.e. 800°C 5 t 5 1200°C

CE 0: t . ?,

(4)

The combustion performance of the chamber was also treated in terms of CE, mean temperature, 5, where H’ &_! tdw, 800°C 5 t 5 1070°C (5) woJ and mean gas residence time, 7, in the chamber. The results indicate that the combustion efficiency CE > 0.99 when the product t - 7 is >, 650°C s and the concentration of oxygen is > 3 vol.%.

CONCLUSIONS Complete combustion of natural gas requires a gas-phase residence time of at least 1.3 s under practical conditions of combustion (t > 800°C [02] > 3 vol.%).

ACKNOWLEDGEMENTS Financial support for this research through a grant from the Grant Agency of the Czech Republic (no. 101/94/ 0112) and a grant from the Grant Agency of the Academy of Sciences of the Czech Republic (no. 472 113) is gratefully appreciated.

REFERENCES Egolfopoulos, F. N., Cho, P. and Law, C. K. Combust. Flame 1989,76,315

Zahnthoff, H. and Baems, M. Ind. Eng. Chem. Res. 1990,29,2 Wendt, J. 0. L., Martinez, C. H., Lilley, D. G. and Corley, T. L. Chem. Eng. Sci. 1919,34,

521

Barresi, A. A., Hung, S. L. and Pfefferle, L. D. Chem. Eng. J. 1992,50,123

Markatau, P., Pfefferle, L. D. and Smooke, M. D. Cornbust. Flame 1993, 93, 185 Bilger, R. W., Stgmer, S. H. and Ku. R. J. Combust. Flame 1990. 80,-l 35 Bhui-Pham, M., Seshardi, K. and Williams, F. A. Combust. Flame 1992.89,343

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