Reduction of Energy Cost and CO
2Emission for the Boilers in a
Full-Scale Refinery Plant by Adding Waste Hydrogen-Rich Fuel Gas
Chih-Ju G. Jou,*
,†Chien-li Lee,
†Cheng-Hsien Tsai,
‡and H. Paul Wang
§Department of Safety, Health and EnVironmental Engineering, National Kaohsiung First UniVersity of Science and Technology, Kaohsiung, Taiwan, Department of Chemical and Material Engineering, National
Kaohsiung UniVersity of Applied Sciences, Kaohsiung, Taiwan, and Department of EnVironmental Engineering, National Cheng Kung UniVersity, Taiwan
ReceiVed April 11, 2007. ReVised Manuscript ReceiVed June 13, 2007
The use of hydrogen-containing fuel in gas turbines and industrial burners can benefit both losses of radiative heat and emission of pollutants. In this study, worthless waste hydrogen-rich fuel gas (RG) was led into a high-pressure boiler or a medium-pressure boiler in the fuel system to partially replace fuel oil (FO) and/or natural gas (NG) in a full-scale petrochemistry plant. Two sets of inlet fuel flow rate ratios were examined to evaluate the reductions of boiler energy consumption and CO2emission. The result showed that for the
high-pressure boiler at a loading of 75%, the usage of NG can be 13× 106m3/y less and the CO
2emission can be
2.2× 104tons/y lower by changing the inlet FO:NG:RG ratio from 1:1:0 to 1:0.65:0.35. Meanwhile, for the
medium-pressure boiler at a loading of 70%, the usage of NG can be cut by 8 × 106 m3/y and the CO 2
emission can be 3.0× 104tons/y lower by changing the inlet FO:RG ratio from 1:0.2 to 1:0.7. Therefore, the
addition of RG has practical benefits on both energy saving and the reduction of greenhouse gas emission.
Introduction
Today, about 90% of the total energy output worldwide is from the combustion of fossil fuels. Unfortunately, hydrocarbon combustion has a major impact on the global environment through the emission of CO2, which is a greenhouse gas, and
results in temperature rise, drought, flood, hunger, and eventually economic chaos. Furthermore, the emission of NOx, SOx, polycyclic aromatic hydrocarbons (PAHs), CO, and particles leads to air pollution, acid rain, and health hazards.1,2
Neverthe-less, the demand for fossil fuel continues to rise globally;1,3
therefore, techniques to achieve better combustion efficiency and the least amount of pollutant emission are necessary, and this goal can be reached through the control of combustion processes or adjustment of the fuel applied.
Steam boilers are a major source of heat and also serve as gas turbines for electricity generation. However, the boiler consumes a lot of fossil fuels and emits greenhouse gases, which have a serious impact on the environment.4,5 The use of
hydrogen in gas turbines and industrial burners can help reduce the emission of pollutants. This eliminates the formation of greenhouse gas CO2, CO, unburned hydrocarbons, and
particu-lates such as soot.6,7However, certain characteristics of hydrogen
prevent it from fully replacing fossil fuel as a major energy source, including its high flammability, fast reaction rate, low volumetric energy density, low flash point, and need for highly complicated storage techniques. The possibility of adding hydrogen to hydrocarbon fuel demonstrates a solution to the problems of hydrogen’s low flash point and complicated storage techniques. Burning hydrocarbon fuel with hydrogen added can improve ignitibility and flame holding, accelerate the relatively slower reaction rate of typical hydrocarbon fuel, and reduce the use of fossil oil.8–10Recently, more researchers have focused
on the radiative heat loss and emission of pollutants when applying different ratios of hydrogen into mixture fuels (NG, C3H8, and n-C7H16). Previous study results demonstrated
significant improvements on flame stability, degree of noise at the combustion room, radiative heat loss and emission of pollutants by adding an appropriate volume of hydrogen in mixture fuels.9–15
Recent studies indicate that excess air is a control variable affecting thermal efficiency and the operating reliability of
* Corresponding author. Tel.: 886-7-601-1000 ext 2316. Fax: 866-7-601-1061. E-mail: [email protected].
†National Kaohsiung First University of Science and Technology. ‡National Kaohsiung University of Applied Sciences.
§National Cheng Kung University.
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10.1021/ef070180z CCC: $40.75 2008 American Chemical Society Published on Web 12/19/2007
boilers. As the excess air ratio goes up, the O2concentration in
the main combustion area also increases, resulting in the rise of the flame temperature in the boiler. This then leads to a drop of the temperature in the boiler radiation area, and eventually affects the boiler efficiency.16,17For instance, Kuprianov studied
the effect of various excess air ratios (from 1.14 to 1.02) on the fuel oil-fired boiler, and the investigations showed that a low excess air ratio will result in the increase of boiler thermal efficiency and also help to reduce the fuel consumption.18
Although the literature is replete with explanations on the improvement of pollutant emission, there have been few papers focusing on applying different ratios of hydrogen in mixture fuels to bring down the excess air ratio and to improve energy efficiency. Among those few studies, even less were tested on boilers in a full-scale plant.
The authors previously studied the possibility of replacing natural gas (NG) and partial fuel oil (FO) with the hydrogen-rich fuel gas (RG) for the medium-pressure boiler under the fixed boiler loading (boiler loading 88–92 ton/h) and reducing the excess air ratio (boiler loading 70–72 ton/h).19The results
showed that the fuel cost could be decreased by $1.03 million USD/y. The worthless RG was the byproduct of production processes such as catalytic reforming and catalytic cracking. Due to its complex composition (H2, CH4, C2H6, C3H8, C3H6,
i-C4, i-C5, H2S, and H2O(g)), the waste gas is normally forwarded
to the waste gas combustion tower for burning. Here, RG was led to the fuel system as an alternative source of energy. In this study, RG was applied to both a full-scale high-pressure boiler and a full-scale medium-pressure boiler to replace the natural gas in full and part of the fuel oil. The study focused on the following points: (1) comparing two sets of inlet ratios of FO: NG:RG with different boiler loading for a high-pressure boiler; (2) comparing two sets of inlet ratios of FO:RG with different boiler loading for a medium-pressure boiler adjusting the volume of RG used; (3) adjusting the residual O2concentration in the
excess air based on the full-scale operation condition; and (4) furthermore, evaluating the impacts on the environment and its economic efficiency and pollutant emission.
Experimental Section
Experiment Equipment. The pieces of equipment used in this
study were a high-pressure boiler and a medium-pressure boiler from a full-scale petrochemistry plant; both are natural circulation boilers. The specifications of the boilers used in this study are as follows:
(1) The high-pressure boiler (Austrian Energy & Environment CO., 1996) had a size of 8.8 m× 6.8 m × 13.0 m. The designed loading of steam was 280 ton/h with an operating pressure of about 10.78 MPa. There were four burners inside the boiler, with two on the upper layer and two on the lower layer. The high-pressure steam generated from the high-pressure system is exclusively for electricity generation.
(2) The medium-pressure boiler (Cheng-Chen Machinery Co. Ltd., 1995) had a size of 6.7 m× 4.8 m × 8.9 m. The designed loading of steam was 130 ton/h with an operating pressure of about 1.96 MPa. There were six burners inside the boiler, with three layers inside the boiler and two burners on each layer. The medium-pressure steam yielded off the medium-medium-pressure boiler is for turbine operation and reboiler heating.
The burning system for these two boilers burns a mixture of fuel gas and fuel oil. They were originally designed to burn a mixture of NG and FO. In this study, we investigated the possibility to partially replace fuel oil (FO) and/or natural gas (NG) with RG for studying the effect of this replacement on the performance of the furnace and pollutant emission characteristics. A continuing emission monitor system (CEMS; E.S.A MIR 9000) was employed to detect and conduct the online analyses and to record the concentrations of the flue gas ingredients. Analyses of the flue gas were carried out according to the standard methods in NIEA A704.03C for CO, NIEA A411.72C for NOx, NIEA A442.70C for
O2, and NIEA A415.70A NIEA W448.50B for CO2. The instrument
was scheduled to be calibrated once every other month with an analytical grade gas. The temperature was measured with a thermocouple (K-type, error is 0.3% of full scale, Reotemp Co.); pressure measurement was done with a bourdon tube (error is 0.3% of full scale, Ashcroft Co.). The boiler efficiency was calculated using the formula provided by the boiler manufacturer (CPC Co. Ltd.); the formula was directly inserted in the distributed computer control system (DCS) as
Boiler efficiency % ) 102 - (0.0209Temp) + (0.24O2) -(0.0018Temp×O2) - 3 (1)
where Temp is the flue gas effluent temperature (deg C) and O2is
the flue gas effluent O2concentration (vol %).
Experimental Methods. These two boilers were operated by
an automatic combustion control system (ACC). The schematic diagram of the multiconsist fuel system is shown in Figure 1. The logic control point input adjusted the fuel oil and fuel gas input ratio based on the fuel oil and fuel gas volumetric flow rates calculated using eq 2.
Cacl (1) and (2) ) FIC (1) or (2) control valve actual scale× FIC (1) or (2) control valve maximum volumetric flow rate×
fuel heat value parameter×fuel ratio (2) where FIC is the volumetric flow rate that indicates control with the maximum flow rates of 12 m3/h for a gate control valve (the
FIC (1) and (4) control valve type is gate valve; error is 0.3% of full scale, Fisher Co.) and 10 m3/h for a butterfly control valve
(the FIC (2) and (3) control valve type is a butterfly control valve; error is 0.3% of full scale, Fisher Co.); the fuel heat value parameters were 1 (FO), 1.12 (NG), and 1.3 (RG).
FIC (3) and FIC (4) were used to control the volumetric flow rates of fuel oil and fuel gas entering the boiler. The maximum boiler stream pressure was set at 110 kg/cm2for the high-pressure
boiler and 20 kg/cm2for the medium-pressure boiler. Whenever
changes on the boiler loading or pressure occurred, ACC would automatically adjust the valve in the forced draft fan, the changing speed of the fan adjusted the fuel feeding rate. As a consequence, the air-fuel ratio would be kept constant to ensure a complete combustion. For the high-pressure boiler, a 6-in. pipeline was added to forward RG to the fuel system (Figure 1 dotted line). Here, RG was used as the main fuel gas and supplemented with natural gas. PIC-9046 and PV-9046 (made by K.O. Drum) were employed to control the quantity of RG; the flow rate of FO was kept constant, and the flow rate of NG was decreased as the flow rate of RG was (12) Hill, S. C.; Smoot, L. D. Modeling of nitrogen oxides formation
and destruction in combustion systems. Prog. Energy Combust. 2000, 26, 417–458.
(13) Naha, S.; Aggarwal, S. K. Fuel effect on NOxemission in partially premixed flames. Combust. Flame 2004, 139, 90–105.
(14) Konnov, A. A.; Colson, G.; Ruyck, J. D. NO formation rates for hydrogen combustion in stirred reactors. Fuel 2001, 80, 49–65.
(15) Choudhuria, A. R.; Gollahalli, S. R. Characteristics of hydrogen-hydrocarbon composite fuel turbulent jet flames. Int. J. Hydrogen Energy
2003, 28, 445–454.
(16) Bebar, L.; Kermes, V.; Stehlik, P.; Canek, J.; Oral, J. Low NOx burnerssprediction of emissions concentration based on design, measure-ments and modeling. Waste Manage. 2002, 22, 443–451.
(17) Yang, W.; Blasiak, W. Mathematical modelling of NO emissions from high- temperature air combustion with nitrous oxide mechanism. Fuel
Process. Technol. 2005, 86, 943–957.
(18) Kuprianov, V. I. Applications of a cost-based method of excess air optimization for the improvement of thermal efficiency and environ-mental performance of steam boilers. Renewable Sustainable Energy ReV.
2005, 9, 474–498.
(19) Lee, C. L.; Jou, C. J. G.; Tsai, C. H.; Wang, H. P. Improvement of a Medium-Pressure-Boiler Performance through Adjustments on the Hydrogen-Rich Fuel for Refinery Plant. Fuel 2007, 86, 625–631.
yielded from the production processes, such as catalytic reform-ing and catalytic crackreform-ing units, and changes in production processes will directly affect the composition and pressure of RG and may in turn have serious impacts on the operational safety of high-pressure boilers for the electricity generation. Therefore, considering the stability on electricity generation and operational safety, the use of RG on high-pressure boilers is limited to a certain degree.
Conclusion
This study focuses on the possibilities of replacing NG with the worthless waste hydrogen-rich fuel gas (RG) yielded from production processes, such as in the catalytic reforming and catalytic cracking. In the study the authors replaced part of NG with RG for the high-pressure boiler, with the inlet FO:NG:
RG ratio changed from 1:1:0 to 1:0.65:0.35. Under the boiler loading at 60% and 75%, the usage of natural gas can be 7.7× 106and 13× 106m3/y less, respectively. As for the
medium-pressure boiler, the inlet FO:RG ratio was elevated from 1:0.2 to 1:0.7 under the boiler loading at 50%, 60%, and 70%, and the usage of natural gas can be cut by 4.8× 106, 6.2× 106,
and 8 × 106 m3/y, respectively. Therefore, practical benefits
for both energy savings and reduction of greenhouse gas emissions can be achieved through replacing natural gas and part of the fuel oil with the hydrogen-rich fuel gas.
Acknowledgment. The authors are grateful for the support from
the National Science Council of Taiwan and to the Ta-Lin Refinery of CPC for providing the experimental apparatus.
EF070180Z
