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| CFD Leads to Successful Design of Gas-Fired Preheater |
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Posted Mon April 29, 2002 @05:22PM
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by Serguei Nester, Principal Combustion Engineer, and
Joseph Rabovitser, Assistant Director
Gas Technology Institute
Des Plaines, Illinois
Computer simulation saved more than $100,000 dollars in the design of a new coal preheating technology for low NOx burners by making it possible to get the design right the first time. The Gas Technology Institute (GTI) is developing a NOx reduction process under a cooperative agreement with the Department of Energy to provide a cost effective, combustion-based alternative to selective catalytic reduction (SCR). Development targets include NOx reduction to below 0.15 pounds per million Btu, reduced CO2 emissions, and a 55% electricity cost reduction compared to SCR. The technology combines GTI’s Methane de-NOX reburn technology with a pulverized coal (PC) preheating approach developed for utility pulverized coal combustors by the All-Russian Thermal Engineering Institute (VTI). Many variations of the initial burner design were evaluated using computational fluid dynamics (CFD), making it possible for engineers to determine an optimized design concept. As a result, the initial prototype met the design objectives, eliminating the need to build and test additional prototypes at a cost that would have probably run into six figures each.
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Typically, older wall-fired PC burner units produce NOx emissions in the range of 0.8 to 1.6 lb per million Btu. Low-NOx burner systems, using combinations of fuel staging within the burner and air staging by introduction of overfire air in the boiler, can drop the NOx emissions by 50 to 60%. But this approach alone is not sufficient to meet the desired 0.15 lb per million Btu NOx standard with a range of coals and boiler loads. Post combustive techniques, such as SCR can provide much higher levels of NOx reduction, but they greatly increase the cost of producing steam. GTI proposes to improve this situation by incorporating a unique modification to conventional or low-NOx burners that uses gas fired direct coal preheating to destroy NOx precursors before introducing pulverized coal into the boiler furnace and prevent NOx formation. Secondary fuel consumption for the preheating process is estimated to be 3% to 5% of the boiler heat input.
Superior NOx reduction
In this system, natural gas is carefully introduced at selected points in the combustion process to preheat pulverized coal to elevated temperatures up to 1500°F prior to combustion. This approach releases coal volatiles, including fuel-bound nitrogen compounds in a reducing (oxygen lean) environment. Preferential conversion of coal-derived nitrogen compounds to molecular N2 occurs, making the nitrogen unavailable for NOx formation in the early stages of the combustion process. Other coal volatiles including H2, CO, and hydrocarbons remain in the fuel stream, thus promoting easy ignition of the coal as it enters the combustion zone. This approach lowers NOx emissions in three ways: 1) by releasing and reducing NOx precursors before they have a chance to react with oxygen to form NO or NO2; 2) by limiting NOx formation in the PC flame via combustion staging in the burner; and 3) by reducing NOx formation in the coal combustion products by use of low excess air, followed with overfire air to complete burnout at lower temperature.
In a development project sponsored by the U.S. DOE’s National Energy Technology Laboratory, Gas Research Institute, and GTI’s Sustaining Membership Program (SMP), the pulverized coal preheat concept is being developed and tested for commercial application with U.S. utility coals and U.S. PC firing methods. The traditional method of developing the burner design would be to create the initial design using hand calculations, then build and test a prototype. The primary problem with this approach is that building and testing the prototype is quite expensive, with the cost typically running into six figures, and takes a number of months. In addition, it is difficult to place sensors in a position to adequately determine flow patterns within the chamber because the combustor must be sealed in a pressure vessel. This means that, even after the test is run, there is no way to obtain details of the combustor flow field outside of a few sensor locations. The result is that engineers have difficulty in intelligently diagnosing any problems that may have arisen during the burner scale up process.
A powerful design tool
On the other hand, CFD offers a relatively quick, reliable, and cost-effective tool to investigate the performance of combustors. Design modifications may be rapidly evaluated and details of the flow patterns which are not available through testing may be predicted. In this application, a parametric modeling study of the preheated pulverized coal combustor performance was done by varying the geometry of the combustion chamber, injection arrangements for the gas/air mixture and pulverized coal/air mixture, and the composition of selected for testing pulverized coals. FLUENT CFD software from Fluent, Inc., Lebanon, New Hampshire, was used for these studies because of its powerful combustion modeling tools, which allow the key reactions and their governing parameters to be easily defined. The CFD simulation of the PC burner included the turbulent fluid flow, particle flow, heat transfer including radiation, homogenous and heterogeneous combustion reactions, and heat and mass transfer between solids and gas. Meshes with approximately 70,000 cells were generated for the modeled cases. The 3D computational domain was filled with an unstructured hybrid mesh of tetrahedral and hexahedral elements. The mesh was refined around the inlet ports to allow the cell size to grow from the areas with the high strain rates to the rest of the domain.
The model was configured with nine different particle sizes ranging between 30 µm and 150 µm in diameter. The size distribution was applied based on a typical coal size distribution analysis for PC burners. The injections for the coal particles were set up based on the chemical composition and size distribution of the coal, along with the design requirements. Constant temperature boundary conditions were used for the combustion chamber walls. The standard k- model was deployed for turbulence modeling. The discrete ordinates radiation model was used for radiative heat transfer, and included the interaction between the particles and furnace environment. Chemical reactions were modeled using the eddy break-up model. The reaction mechanism was based on a 4-step mechanism with carbon monoxide and hydrogen as intermediate combustion products. The stoichiometric coefficients were determined from the fuel analysis for each fuel (coal, volatiles, and methane) separately.
Iterating to a solution
The model was then solved for the different configurations and the results were compared based on the required operating parameters. These include the temperature level and temperature uniformity inside the combustion chamber; the uniformity of mixing of the combusted gas and solids; the uniformity of trajectory patterns for the injected particles inside the combustor; and sufficient residence time for the particles’ moisture release and devolatilization. One of the main requirements for the design was to avoid the interaction of the injected solids and the gas/air mixture before ignition of the natural gas flame, so the flame would not be extinguished by the cold particles. Another important requirement was to minimize the interaction of the solids inside the combustion chamber with the chamber walls by optimizing the flow pattern and trajectories of the particles.
The results of the analysis provided invaluable assistance to the engineers charged with creating the burner design. In an early design, shown in Figure 1, particles are entrained by the recirculation flow to the area of the gas/air inlets and into the natural gas flame ignition area. Mixing of the cold particles and the gas/air mixture prior to ignition can extinguish the flame and lead to unstable operation of the PC combustor. Particle trajectories also exhibit intensive interaction of the solids with the combustion chamber walls, which can lead to deposition of solids on the walls. The optimized design shown in Figure 2 presents a more uniform flow pattern for the particles. Here the trajectories of the pulverized coal particles do not interact with the walls, and devolatilization inside the combustion chamber is sufficiently high. Figure 3 presents more insight on the flame and flow inside the combustion chamber for the optimized design. The pattern of devolatilization is uniform and the walls are shielded from solids by combustion products. This design was built and tested and the results demonstrated stable, pulsation-free operation, uniform temperature distribution inside the burner, and stability of combustion at solids loads of 20% to 100% of the design load value. The desired PC preheat temperatures were achieved during the testing. Inspection of the internals of the PC preheat unit have shown no deposits on the internal walls. The CFD model developed during pilot testing will also be used to guide the scaleup of the preheater burner and will also provide a valuable design tool for future commercial installations.
Fig. 1 Coal particle tracks, colored by particle mass (kg) for one of the early designs.
Fig. 2 Coal particle tracks for the optimized design.
Fig. 3 Devolitalization contours for the optimized design. Shows rate of nozzle devolitilization along nozzle centerline.
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