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Energy Procedia 37 (2013) 7793 - 7801
GHGT-11
Technology Assessment of Oxy-Firing of Process Heater
Burners
Cliff Lowea, Nick Brancaccioa, Jamal Jamaluddin b -Jaime A. Erazo, Jr. c, Charles E. Baukal, Jr.c
a Chevron Energy Technology Company, Richmond, CA, USA b Shell Projects and Technology, Houston, TX, USA _John Zink Co., LLC, Tulsa, OK, USA_
Abstract
The objective of this development program is to assess the feasibility of retrofitting burners for oxy-firing in process heaters. A secondary objective is to confirm this feasibility assessment by conducting single burner oxy-fired testing with flue gas recycle.
The CO2 Capture Project commissioned the John Zink Company to conduct oxy-fired testing on two of their conventional process heater burners, a PSFG staged gas low NOx burner and a COOLstar* Ultra-Low NOx burner.
Keywords: Oxy-firing; refinery; heaters; burners;
1. Study Background
The CO2 Capture Project (CCP) is an international effort funded by six of the world's leading energy companies. For the past ten years, this project has been addressing the issue of reducing emissions in a manner that will contribute to an environmentally acceptable and competitively priced continuous energy supply for the world.
The project seeks to develop new technologies to reduce the cost of capturing CO2 from combustion sources and safely store it underground. This concept is commonly referred to as carbon capture and sequestration or CCS. These CCS technologies will be applicable to many of the large point source CO2 emissions around the world - such as power plants and other industrial processes. Implementing these new technologies during this decade will reduce the impact of continued fossil energy use while cleaner energy sources are being developed.
As the name implies, oxy-firing refers to the concept of combustion with oxygen instead of air [']. The benefit is that the flue gas is mainly water and CO2, and is nearly nitrogen free [11,111]. A CO2-rich stream suitable for sequestration can be obtained by simply cooling the flue gases and condensing out the water [iv]. Some purification may be necessary to increase the CO2 concentration level [v].
1876-6102 © 2013 The Employers. Published by Elsevier Ltd. All Rights Reserved. Selection and/or peer-review under responsibility of GHGT doi: 10.1016/j.egypro.2013.06.011
In the first phase of the CO2 Capture Project (2000-2003), it was determined that oxy-firing of refinery heaters and boilers showed significant potential for lower avoided CO2 costs when compared to postcombustion capture [vi,vii]. Although the capex requirements were higher for oxy-firing (primarily due to the addition of an air separation plant), the significantly lower energy requirements resulted in lower overall avoided CO2 costs. The feasibility of using an alternate method of generating O2 called ion transport membrane [viii] was also investigated. Based on these studies, the CCP capture team decided to pursue development of natural gas-fed oxy-fired technology in the third phase of the CCP (2009-2013). While previous studies have investigated the feasibility of using oxy-firing, no previous studies were found in the literature where this technology has been tested on a process heater.
The objectives of this development program were to:
• Assess the feasibility of utilizing conventional process heater burners for oxy-firing
• Confirm this feasibility assessment by conducting single burner oxy-fired testing with flue gas recycle.
The C02 Capture Project commissioned the John Zink Company to conduct oxy-fired testing on two of their conventional process heater burners, a PSFG staged gas low NOx burner and a COOLstar® Ultra.
2. Program Description
The program consists of the following five tasks:
1. Heater performance modeling - Determine changes in heater performance (efficiency), maximum film/tube temperatures, heater draft, and radiant/convection heat absorption ratio, and the flue gas recycle requirements.
2. Burner confirmation - Verify the feasibility of oxy-firing based on burner air-side pressure drop, adiabatic flame temperature, flame speed, flammability limits, and heat flux (convection and radiation).
3. Computational fluid dynamics evaluation - Evaluation of the combustion performance, flame shape, and heat flux distributions for the two burner designs in the test furnaces and in typical process heaters.
4. Single burner oxy-fired testing - Full range testing with air (base case) and oxygen with two different fuels.
5. Computational fluid dynamics re-evaluation - Based on the results from Task 4, the CFD model will be modified and rerun to confirm the results from Task 3.
The heart of the project, Task 4 - Single Burner Testing, took place at John Zink's state-of-the-art test facilities in Tulsa, Oklahoma. The following data were collected:
• Furnace and stack gas temperatures,
• Oxidant temperature and composition,
• Pressure drop across the burner,
• Draft at the top of the radiant section,
• Fuel composition, temperature, pressure and flow rate,
• Excess 02, CO, C02 and NOx in furnace exit gases,
• Flame length,
• Burner turndown,
• Temperature of burner components,
• Radiant flux at multiple vertical locations in the furnace at a normal firing rate,
• Qualitative assessment of flame stability.
3. Furnace Simulation
John Zink commissioned OnQuest to provide process heater modeling. This first round of heater modeling was conducted to identify feasible operating conditions in a typical process heater. The heater modeling focused on the following:
• Overall heater efficiency
• Maximum film temperature and/or tube metal temperature limitations
• Radiant/ convection heat absorption ratio
• Flue gas recycle requirements
Several oxy-firing conditions were evaluated and a base case with ambient air was used for comparison purposes. As a starting point, the first set of simulations varied the level of excess oxidant without any flue gas recirculation. The findings there indicated that high levels of excess oxidant would be required to meet the limitations of design allowable film temperatures. The subsequent round of simulations maintained the same range of excess oxidant while the amount of flue gas recirculation was varied until the process constraint on film temperature was satisfied. Of these simulations, two feasible operating conditions were identified. Both conditions demonstrated that the process design constraints could be met at ~ 1.3 - 5% (vol. dry) oxygen concentration in the flue gases and a high flue gas recirculation rate. Compared to the base ambient air case, these conditions provided an improvement in heater efficiency, and met the maximum film and tube metal temperature limitations with small changes to the radiant/convection section duty split. Additionally, these operating conditions required the minimal use of oxygen.
4. Test Program
Two different types of burners were tested [ix]. The PSFG is a low NOx diffusion flame burner incorporating staged fuel injection to reduce NOx. The COOLstar® burner [x] is an ultra-low NOx diffusion flame burner incorporating staged fuel injection and internal furnace gas recirculation to reduce NOx emissions. The burners were tested in a rectangular test furnace with internal dimensions 13 ft (4.0 m) wide, 7 ft (2.1 m) deep and 31 ft (9.4 m) tall. The furnace is cooled by single-pass water tubes and was insulated to provide a nominal, mid-furnace flue gas temperature of 1600°F (870°C). The flue gas was redirected back to the burner as shown in a simplified schematic diagram of the system, Figure 1.
The test furnace does not have a convection section as conventional process furnaces do, therefore the
* Arrow Indicates Direction of Flow
Stack Air Inlet
O2 Injection
Damper
Figure 1- Simplified schematic diagram of Oxy-fire setup
flue gases exiting the furnace were typically well above 1000°F (540°C). The fan, which was used to recirculate the flue gas through the system, had an upper design limit of 700°F (370°C). To cool the flue gases down to this temperature, an atmospheric boiler was used. For natural draft operation, all of the flue gas was routed through the boiler and out the stack downstream of the fan. An inline damper in the ductwork ensured flue gas was exhausted and ambient air was drawn in. During oxy-fire testing [i], the air inlet was closed and the inline damper was opened to control the amount of flue gas recirculation to the burner. The oxygen was injected into the ductwork of the burner inlet. The oxygen was stored in a large liquid vessel with a vaporizer. The flow of oxygen was measured and controlled through a flow skid.
Figure 2: Photographs of the test furnace and oxy-fire setup. View of the boiler, fan, inlet and outlet ductwork is shown.
Figure 3: Photographs of the COOLstar® burner (left) and the PSFG burner (right).
Table 1 summarizes the operating cases and test fuel properties. Three operating cases were tested for each burner. The ambient air, natural draft case was tested for baseline comparison purposes. Two oxy-fire cases were selected for testing. The higher oxygen concentration condition is termed Oxy-fire A, the lower Oxy-fire B. The flue gas recirculation rate is defined as the fraction of the flue gas recirculated to the burner divided by the total flue gas flow rate. Two fuels were tested, Tulsa natural gas and a simulated refinery fuel gas mixture. Both fuel gas mixtures and properties are shown in Table 1. The test points collected were similar to the test points described in API-560 Standard [x1]. A description of the test points is provided in Table 2. In addition to these points, flame length was verified via CO probing and the incident heat flux profile [x11] was measured with a calibrated radiometer. Emissions measurements of CO, NOx, wet and dry O2 were made at the exit of the test furnace. Measurements of oxygen on a wet basis were made before and after the injection of oxygen. Dry measurements of O2 and CO2 were made just upstream of the burner inlet which allowed for the determination of the nitrogen content in the oxidant stream.
Table 1: Summary of operating cases and test fuel properties.
Parameter / Units Natural Draft Oxy-Fire
Heat Release (MMBtu/hr)
Maximum 7.68 6.45
Normal 6.4 5.37
Minimum - PSFG 2.56 2.56
Minimum - COOLstar 1.92 1.92
Oxidant Oxygen Concentration (%,v) 20.9 22.2 / 20.6
Recirculated Flue Gas (%) --- ~ 71
Oxygen Concentration in Flue (%,vd) ~ 3 5.2 / 1.3
Oxidant Temperature (°F) Ambient ~ 500
Fuel Tested - Natural Gas & RFG Both Both
Fuel TNG Fuel RFG
Molecular Weight 17.15 Molecular Weight 20.11
LHV (BTU/scf) 913 LHV (BTU/scf) 1104
Component % vol Component % vol
Methane 93.4 TNG 50
Ethane 2.7 Propane 25
Propane 0.6 Hydrogen 25
Butane 0.2
Nitrogen 2.4
Carbon Dioxide 0.7
Total 100 Total 100
Table 2 - Description of test points
Test Point
Description
Design - Maximum CO Breakthrough Normal Minimum Minimum
Absolute Minimum
Design heat release
Increase flow rate until CO > 250 ppmvd or flame instability Normal heat release
Turndown per normal burner pressure drop oxidant flow
Turndown with controlled oxygen concentration in furnace
Fuel pressure < 0.5 psig if possible, flame instability or excessive CO
5. Study Results
General Results
John Zink staged gas low NOx burner and a COOLstar® ultra-low NOx burner performed satisfactorily during oxy-fire operation. The burners were demonstrated from maximum heat release to turndown heat release with no performance issues. The transition from ambient air to oxy-fire operation was successfully demonstrated several times with each burner.
All measurements specified in the Test Program section were successfully obtained. The operation of the heater and oxy-fire system presented a few challenges and those are discussed in the subsections below.
System Operation - Nitrogen (N2) Purge
Since a significant fraction of the flue gas was re-circulated back to the burner, the system was a partially closed loop with little mass exiting out the stack. As a result, purging the system of nitrogen was an important consideration during operation. Air leakage into the furnace was inevitable as parts of the system were operated under negative pressure relative to atmosphere. Every effort was made to seal the furnace to minimize air leakage. Since the boiler was required to cool the flue gas, the furnace draft at design conditions was negative 2-4 inH2O. This is much higher than what a conventional refinery furnace is designed for. On average, it took approximately an hour before the system reached minimum nitrogen levels. Under design conditions 15-20% (v) N2 was present in the system. Operating conditions which allowed for reduced draft levels in the system resulted in nitrogen concentrations as low as 11% (v).
Flue Gas Re-Circulation and Oxygen Concentration
Two important operating parameters were the flue gas re-circulation (FGR) rate and the oxygen concentration in the oxidant stream. The FGR rate suggested in the heater simulation was successfully demonstrated in the test furnace for both oxy-fire conditions tested. For normal and maximum heat release operation, the FGR was approximately 71% and the burners were observed to handle fluctuations in the FGR fraction. Changes in the FGR fraction can have a significant impact to the burner performance. Larger FGR flow rates can push the combustion process to the flammability limits, especially if operating under low oxygen conditions. For a set burner size, lower FGR flow rates result in a decrease in burner pressure drop which decreases the mixing with the oxidant and fuel. This elongates the flames in a process heater, especially in ultra-low NOx technology burners.
The oxygen concentration in the oxidant stream was varied during testing, although it was coupled to the FGR fraction. The digital controller used the oxygen concentration (wet basis) as an input. Therefore, changes in the re-circulated flue gas mass flow rate resulted in changes in the mass flow rate of oxygen to the system as the controller attempted to keep the oxygen concentration constant. Practical limitations of the system prevented large variations in the oxygen concentration in the oxidant stream at design conditions. High oxygen concentrations require higher flow rates of oxygen which depleted the holding
tank at a much higher rate. Lower oxygen concentrations were achievable with increased FGR rates, however these conditions start to approach the flammability limits of the oxidant/fuel mixture.
Flue Gas Temperature Control
As previously mentioned, an atmospheric boiler was used to cool the flue gas temperature below the design operating limit of the fan. To control the flue gas temperature, a boiler by-pass was used to divert flue gases around the boiler. During natural draft operation, all of the flue gas was routed through the boiler to achieve steaming conditions. Once this was reached, the by-pass was opened, allowing for the duct work downstream to be heated in preparation for oxy-fire operation. The warm-up period was required for two reasons. The first being that as the flue gas temperature changed the fan speed required to maintain flow through the system changed. Second is that the transition to oxy-fire operation requires that the air inlet eventually be closed. Sufficient pre-heat must be available otherwise the flue gas stream re-circulated back to the burner, now containing significant amounts of carbon dioxide and water vapor, could extinguish the flame during the transition process.
Flame Appearance
Flame appearance was documented with digital photography. The PSFG burner flame appearance with ambient air and Oxy-fire A operation is provided in Figure 4. Few differences were noticed between the two modes of operation. The flameholder appears to be glowing slightly brighter under oxy-fire operation. The COOLstar® burner comparison is provided in Figure 5. Some sections of the tile appear to be glowing more brightly under oxy-fire conditions than with ambient air operation. Under Oxy-fire A conditions, the concentration of oxygen in the oxidant stream is higher than that of air which results in more intense local combustion of the fuel gas.
Figure 4: Photographs of PSFG burner flame appearance at normal heat release under natural draft and Oxy-fire operation.
Figure 5: Photographs of COOLstar® burner flame appearance at normal heat release under natural draft and Oxy-fire operation.
6. Conclusions
A John Zink staged gas low NOx burner and a COOLstar® Ultra-Low NOx burner were tested under oxy-fire conditions at the John Zink Research and Development Test Center. Flame appearance, flame stability, stack emissions, flame length and incident heat flux profile measurements were conducted for both burners under natural draft, ambient air conditions and forced draft, oxy-fire conditions. Suitable oxy-fire conditions were identified in a heater simulation study by OnQuest. Film and tube metal temperature were the design constraints used to identify potential oxy-fire operating conditions. Parameters such as heater efficiency, heat duty split between radiant and convection section and heater draft were also evaluated.
Air leakage into the system is a significant concern as it is desirable to obtain a C02-rich stream for ease of sequestration. High nitrogen concentrations will result in more costly downstream C02 purification systems. Unlike boilers, process heaters operate under negative pressure relative to atmosphere [ix]. Since the test furnace is not equipped with a convection section, as most conventional furnaces are, an atmospheric boiler was required to cool the flue gases and control the flue gas temperature. The additional pressure drop incurred through the boiler and by-pass damper created operating situations where the system was operated under greater negative pressure which increased air leakage. In a commercial application, consideration will need to be given for the fan placement and sealing of the heater and convection section to minimize air leakage.
The process burners operated under oxy-fire conditions satisfactorily. Important performance variables such as flue gas re-circulation rate and oxygen concentration in the system were varied without adverse effects on burner performance. This was most evident during the transition between ambient air and oxy-fire operation where flow rate, oxygen concentration and flue gas temperature were all constantly changing.
7. Acknowledgements
Financial support from Phase 3 of the CO2 Capture Project is gratefully acknowledged. The CCP is an award-winning partnership of several major energy companies, working to advance the technologies that will underpin the deployment of industrial-scale CO2 capture and storage. Current Phase Three (CCP3) members are: BP, Chevron, Eni, Petrobras, Shell and Suncor.
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