CFD Modeling Improves Burner Performance
The overall heat-transfer and combustion efficiencies in industrial processes, as well as the reduction of pollutant emissions can be improved either by modifying existing combustion systems during retrofits and new installations, or through innovative designs and applications. A key focal point of these improvements is optimizing process fluid dynamics, heat transfer, and combustion using Fluent Inc.'s (Lebanon, N.H.) computational fluid dynamics (CFD) software.
CFD is a powerful tool for analyzing fluid flow, using computers to solve fundamental nonlinear differential equations that describe fluid flow (i.e., Navier-Stokes and heat and mass-transport equations) for predefined geometries and a set of initial boundary conditions, process flow physics, and chemistry. Analytical results predict flow velocity, temperature, turbulent characteristics and species concentrations at any point of the object in question. Therefore, CFD is a virtual-modeling technique having powerful visualization capabilities. A more detailed description of CFD can be found in References 1 and 2.
One of the most difficult challenges in industrial heating is the reduction of NOx emissions from high-temperature furnaces. Increasingly stringent government environmental regulations demand lower NOx levels from industrial sources, forcing companies to look for ways to improve combustion efficiency, while simultaneously reducing emissions. Many high-temperature processes (> 1400 F, or 760 C) require maintaining these temperatures for long times-conditions that favor increased NOx formation.
Three primary NOx formation mechanisms are thermal (formed by the high temperature reaction of nitrogen with oxygen), prompt (formed by reactions between oxygen and hydrocarbon radicals) and fuel (formed by the direct oxidation of fuel-bound nitrogen compounds). A detailed list and description of NOx formation mechanisms can be found in Reference 3.
Fuel-bound NOx formation is not a problem with most clean fuels, such as natural gas. The radical chain reaction process of the extended Zeldovich (thermal) mechanism overwhelms the contribution of prompt NOx formation in a majority of high-temperature, natural gas-fired processes.
Primary factors affecting thermal NOx yields are time, temperature and oxygen availability. Common methods of in-flame burner NOx control are flue-gas recirculation (FGR), steam or water injection and staged combustion. One of the easiest techniques to reduce burner thermal NOx emissions is staged combustion, without adding costly hardware and controls. By suitably proportioning air and fuel inside the burner and/or furnace, it is possible to form a fuel-rich zone where pyrolysis proceeds with little NOx generation, followed by a lean burnout zone where additional NOx production also is low. Air staging is the conceptual basis for two of Hauck's ultralow NOx burner designs discussed here.
One of the major problems in combustion-equipment design is the application of subscale results to full-size industrial combustion systems. Scaling parameters are difficult to identify and apply in practice for a number of reasons. For example, fuel/air-mixing strategies that work well at a small scale often are difficult or impractical to apply to larger burners due to physical size and geometric constraints, and, therefore, require modification and testing to validate performance. In addition, quantitative emission results from very small burner models may not be applicable to larger burners even when they are similarly scaled. Hauck addressed these problems in its full-scale high-temperature furnace, which includes a cooling air load simulation system. The research program was designed to take advantage of both experimental full-scale burner testing including emissions and temperature measurements, stability observations and flame appearances, combined with state-of-the-art CFD analysis. Two series of Hauck burners investigated included a medium-velocity gas and/or oil baffle burner (Hauck Beta burner) and a super high-velocity gas burner (Hauck SVG).
The Beta burner using natural-gas fuel was chosen as the baseline burner for NOx-emission reduction studies using a combined experimental and numerical CFD-modeling approach. The burner is a conventional baffle-type burner with a convergent nozzle tile designed for low air pressure (8 osig) operation in furnaces at temperatures to 2700 F (1480 C). Capacities range from 3-90 million Btu/hr and include preheated-air versions. Figure 1 shows the computed temperature distribution along the flame of a CFD (Fluent 5.4) numerically modeled 8 in. BBG-2108 Beta burner (8 MMBtu/hr, 10% excess air). A peak flame temperature of 3020 F (1660 C) is located in the central region at an axial distance from the burner of 6 to 8 burner-exit diameters. The predicted NOx was 65 ppm (all values corrected to 3%O2) at a furnace temperature of 1800 F (980 C), which was validated via laboratory data. The shear rate between the burner air jets and furnace gases is high (Fig. 2), which maximizes mixing and furnace gas entrainment. Furnace flue gas entrainment into the main stream reduces peak flame temperatures and NOx emissions.
An air-staged burner was developed to further reduce peak flame temperatures, and, therefore, NOx emissions. However, this complex combustion process is inherently unstable at high staging ratios. Thus, another important design consideration was flame stability, which features central fuel injection surrounded by multiple levels of air staging. Air staging is carefully controlled to ensure complete CO burnout and flame stability during cold furnace startups, while simultaneously minimizing NOx emissions. With suitable proportioning of combustion air between stages, NOx reductions of over 70% have been realized from the standard Beta burner both experimentally and with CFD. The computed temperature distribution along the flame is depicted in Fig. 3. Optimal fuel and air inlet port geometries combined with controlled air staging causes the flame zone to stretch and causes the resultant temperature peak to drop by roughly 100 F (55 C) compared with the standard Beta burner (Fig. 1). The shear rate between the staged burner air jets, the burner central exit region and the furnace flue gases is high (Fig. 4). As a result, gradual mixing of the flue products, combustion air and raw and partially pyrolized fuel is completed along the length of the flame inside the furnace rather than very quickly in the burner tile proper. Figure 5 shows the CO2 profile along the flame obtained via CFD. Complete combustion was verified in the laboratory via emissions analysis.
Super-velocity gas burner
An SVG (super velocity gas) burner was investigated with the goal of lowering NOx emissions. The SVG is a nozzle-mix gas burner designed for use in industrial applications that benefit from combustion gas recirculation, increased efficiency and improved temperature uniformity resulting from the burner's high exit velocity. Hot spots in the furnace are avoided because the primary flame envelope is contained in the burner tile. However, the tile remains cool due to controlled air/fuel distribution and mixing. Burner capacities range from 140,000 Btu/hr to over 9 MMBtu/hr at 16 osig static air pressure, and operating conditions can include temperatures exceeding 2600 F and 800 F preheated air (1430 and 430 C). A wide operational range extends from several thousand percent excess air to 40% excess fuel, with relatively low NOx production.
The computed velocity distribution of the baseline burner (standard SVG-125, 1 MMBtu/hr at 10% excess air and 16osig) is shown in Fig. 6. The CFD computed mean burner nozzle exit velocity is 660 ft/s. Based on laboratory exit gas-temperature profile measurements, gas density calculations based on the ideal gas equation of state and subsequent exit velocity determination via conservation of mass, the average calculated exit velocity was 567 ft/s, which Hauck considers the highest in the industry. The high kinetic energy entrains large amounts of furnace flue gases, which reduces peak flame temperatures in the furnace. The computed temperature distribution along the flame is shown in Fig. 7. Partial combustion occurs inside the burner tile resulting in two hot spots with temperatures of about 2890 F and a peak temperature of 3200 F (1590 and 1760 C) located in the main burner flame envelope. To reduce the peak flame temperature, the burner nozzle was redesigned to create greater air staging inside the nozzle and burner tile. The final design reduced NOx emissions approximately 25 to 30% across the burner input range without affecting burner performance. Computed velocity and temperature distributions are shown in Figs. 8 and 9, respectively. The modified burner mean exit gas velocity increased by 11% to 738 ft/s, while the peak flame temperature was reduced by 110 F (60 C). The hot region (2790 F, or 1530 C) inside the tile shifted toward the burner exit, and occupied slightly more volume than the standard burner. However, the mean residence time in the hot spots was decreased by 17%. Thus, the reduced peak temperatures and residence times in the tile, as well as the increased tile exit velocity and flue gas entrainment rates, contributed to the overall NOx reduction obtained.
Single-ended recuperative ceramic radiant tube burner
The single-ended recuperative ceramic radiant tube burner (SERamic) incorporates an internal recuperator, combustor tube and outer radiant tube, all made of silicon carbide (SiC). Operating thermal efficiencies are as high as 70% with preheated air, and NOx emissions are low due to internal flue gas recirculation. Burner capacities range from 80,000 to 300,000 Btu/hr with very high heat flux rates through the outer SiC tube. The flame exits the burner combustor tube at a very high velocity, entraining large volumes of exhaust gases to promote recirculation through the inner tube. This process results in reduced NOx emissions and improved temperature uniformity of the outer radiant tube.
An optimal nozzle-to-inner firing tube gap exists, which maximizes the flue gas entrainment effect for a given burner size and capacity. CFD was used to determine the optimal gap for the SER-112 (150,000 Btu/hr), and results were verified with laboratory testing. The _-_ turbulent model and probability density function (PDF) approach for combustion were used to model fluid dynamics, heat transfer and combustion processes inside the radiant tube.