Improvement in the Performance of a Medium-Pressure-Boiler through the Adjustment of Inlet Fuels in a Refinery Plant

Review

Improvements in the performance of a medium-pressure-boiler

through the adjustment of inlet fuels in a refinery plant

Chien-li Lee

, Chih-Ju G. Jou

, Cheng-Hsien Tsai

, H. Paul Wang

a_{Department of Safety, Health and Environmental Engineering, National Kaohsiung First University of Science and Technology, 2, Juoyue Rd,}

Nantz District, Kaohsiung 811, Taiwan, ROC

b

Department of Chemical and Material Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung, Taiwan, ROC

c

Sustainable Environment Research Center, National Cheng Kung University, Tainan, Taiwan, ROC

d

Department of Environmental Engineering, National Cheng Kung University, Taiwan, ROC Received 7 May 2006; received in revised form 2 September 2006; accepted 13 September 2006

Available online 10 October 2006

Abstract

Hydrogen has been considered as a promising alternative for fossil fuel in recent years because it is very ‘‘clean’’. Fossil fuel generates

CO

, CO, SO

, unburned hydrocarbon and particles during combustion, while hydrogen only yields NO

. In this study, a

medium-pres-sure boiler with 130 ton/h boiler loading in a full-scale plant was studied with two inlet hydrogen-rich refinery gas (RG)/fuel oil (FO)

volumetric flow rate ratios (inlet RG/FO ratio) and two residual O

concentration (vol.%) in flue gases (2%, 4%) to evaluate their

influ-ence on the emissions of NO

and CO

, flue gas temperatures and boiler efficiencies. The result shows significant improvements in both

boiler efficiencies and emissions of air pollutants. By increasing the inlet RG/FO ratio from 1:5 to 1:1.5, the fuel cost was reduced by 11%,

NO

emission down by 12%, and the CO

emission 20,200 ton lower per year was achieved. Thus, better economic operating conditions

for the boiler are suggested at inlet RG/FO ratio = 1:1.5 with the residual O

concentration in flue gases = 2%.

 2006 Elsevier Ltd. All rights reserved.

Keywords: Nitrogen oxides; Hydrogen-rich fuel; Residual oxygen concentration

Contents

1.

Introduction . . . 626

2.

Experimental section . . . 626

3.

Results and discussion . . . 627

3.1.

Impacts of different inlet RG/FO ratios on the amount of NO

yielded. . . 627

3.2.

Impacts of different inlet RG/FO ratios on the boiler efficiency . . . 627

3.3.

Impacts of different inlet RG/FO ratios on the amount of CO

yielded . . . 628

3.4.

Impact of residual O

concentration in flue gases on the amount of NO

yielded and temperature of effluents. . . 629

3.5.

Impact of residual O

concentration in flue gases on the boiler efficiency . . . 629

3.6.

Analysis of advantages. . . 630

4.

Conclusion . . . 630

Acknowledgements . . . 630

References . . . 630

0016-2361/$ - see front matter  2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2006.09.001

*

Corresponding author. Tel.: +886 7 601 1000x2316; fax: +866 7 601 1061. E-mail address:george@ccms.nkfust.edu.tw(C.-J.G. Jou).

www.fuelfirst.com Fuel 86 (2007) 625–631

91.7% and 146.5

C, respectively. The decrease of residual

O

concentration in flue gases means the airflow rate in

the boiler chamber will go down, resulting in a decrease

in the speed of heat transfer from the radiation zone to

the convection heating surface zone. As a consequence,

more heat is absorbed by the boiler in the radiation zone,

thus improving the boiler efficiency and lowering the flue

dust temperature

[18]

. Hence, decrease of the residual O

concentration in flue gases can reduce the NO

emission,

improve boiler efficiency with less fuel required, and

decrease the temperatures of flue gases and flue ash.

More-over, most boilers are the positive pressure type, and boiler

efficiency can be improved by directly controlling the

resid-ual O

concentration in flue gases to show the rise in

tem-perature in the boiler. As for the negative pressure type of

boiler, the residual O

concentration in flue gases can be

controlled by adjusting the stack damper.

Table 1

shows that the actual/theoretical air volumetric

flow ratio m (A/A

) is between 1.38 and 1.61 in this study,

that is, about 12,700–21,000 N m

/h more than the

theoret-ical air amount. However, a boiler has been suggested to

operate at ‘‘m’’ being between 1.1 and 1.3 with the actual

air fed in the range of 45,400–53,700 N m

/h for the fuel

gas/fuel oil mixture combustion system

[19]

. Based on this

study, it may be feasible to further reduce the residual O

concentration in flue gases to 1.5% for the plant in this

paper.

3.6. Analysis of advantages

This study demonstrates outstanding improvements in

medium-pressure boiler efficiency and environmental

pro-tection. The advantages are listed as follows:

(1) Economic advantages: The result shows that, by

increasing the inlet RG/FO ratio from 1:5 to 1:1.5,

about US$1.03 million of fuel cost can be saved per

year in the plant studied. Meanwhile, as the residual

O

concentration in flue gases decreases from 4% to

2%, an extra US$50 thousand of fuel cost can be

saved per year.

(2) Environmental advantages: The average emissions of

NO

and CO

can be reduced by 81 ton/year and

20,200 ton/year, respectively, by increasing the inlet

RG/FO ratio from 1:5 to 1:1.5. In addition, if the

residual O

concentration in flue gases decreases from

4% to 2%, the emissions of NO

and CO

are reduced

by an extra 43 ton/year and 612 ton/year, respectively.

There are four similar boilers in the full-scale plant.

More cost savings and reduced air emission can be

achieved through the application of this study to all four

boilers.

4. Conclusion

This study evaluates the benefits for replacing natural

gas with the currently worthless hydrogen-rich refinery

gas yielded from the production processes, such as in the

catalytic reforming unit and catalytic cracking unit, which

should have been forwarded to the waste gas combustion

tower. By increasing the inlet RG/FO ratio from 1:5 to

1:1.5, the amount of NO

emission can be reduced by

81 ton/year, that is, a 12% drop. At the same time, the

CO

emission can be cut by 20,200 ton/year, and the fuel

cost can be decreased by US$1.03 million per year.

On the other hand, the theoretical amount of air volume

required decreases with increased H

concentration in the

RG. As the O

concentration in flue gases decreases from

4% to 2%, the mean emissions of NO

and CO

are reduced

by an extra 43 ton and 612 ton per year, respectively. At

the same time, the fuel cost can be cut by US$50 thousand

per year.

Hence, better economic efficiency for the

medium-pres-sure steam boilers can be achieved by operating at inlet

RG/FO ratio = 1:1.5 with 2% of residual O

concentration

in flue gases.

Acknowledgements

The authors are grateful for the support from the

National

Science

Council

of

Taiwan

under

Grant

NSC93-ET-7-327-001-ET, and to Talin Refinery of CPC

for providing the experimental apparatus.

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Table 1

The volumes of theoretical air and actual air required for RG/FO mixture combustion

Residual O2concentration in flue gases (vol.%) RG (m3/h) FO (m3/h) Theoretical air (N m3/h) Actual air (N m3/h) m (A/A0)

1.92 1.69 2.51 41,400 57,200 1.38

2.40 1.69 2.49 41,200 58,500 1.42

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