Using a Detailed Chemical-Kinetics Mechanism to
    Ensure Accurate Combustion Simulation



               By Reaction De...
Reaction Design                                                                White Paper


INTRODUCTION
         Today’s...
Reaction Design                                                               White Paper

       We can adjust this simpl...
Reaction Design                                                                White Paper

steps) and species that are kn...
Reaction Design                                                                  White Paper

designer to evaluate the mec...
Reaction Design                                                                 White Paper


MAINTAINING ACCURACY DURING ...
Reaction Design                                                               White Paper

strictly control the target acc...
Reaction Design                                                             White Paper

computational time. If you need t...
Reaction Design                                                                                  White Paper




         ...
Reaction Design                                                                  White Paper

mechanism will be treated as...
Reaction Design                                                              White Paper

was found to consistently under-...
Reaction Design                                                                                White Paper

low oxygen lev...
Reaction Design                                                             White Paper

detailed kinetics mechanisms can ...
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Using a Detailed Chemical-Kinetics Mechanism to Ensure Accurate Combustion Simulation

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Today’s market opportunities for combustion systems require focus on high-efficiency, low emissions and fuel-flexibility. In three previous white papers , we have discussed how use of detailed chemical kinetics in combustion simulation can provide accurate emissions predictions, simulate fuel effects and help gain insight into instability phenomena like Lean Blow Off (LBO). All of these topics focus on the use of high-fidelity chemistry simulation models for advanced combustion simulation by using highly accurate and detailed kinetics mechanisms. This white paper describes what a detailed kinetics mechanism is, how it is developed and validated and how it can be used in high-fidelity combustion simulation models to accelerate advanced combustion technology development.

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Using a Detailed Chemical-Kinetics Mechanism to Ensure Accurate Combustion Simulation

  1. 1. Using a Detailed Chemical-Kinetics Mechanism to Ensure Accurate Combustion Simulation By Reaction Design 6440 Lusk Blvd, Suite D205 San Diego, CA 92121 Phone: 858-550-1920 www.reactiondesign.com
  2. 2. Reaction Design White Paper INTRODUCTION Today’s market opportunities for combustion systems require focus on high- efficiency, low emissions and fuel-flexibility. In three previous white papers1 , we have discussed how use of detailed chemical kinetics in combustion simulation can provide accurate emissions predictions, simulate fuel effects and help gain insight into instability phenomena like Lean Blow Off (LBO). All of these topics focus on the use of high- fidelity chemistry simulation models for advanced combustion simulation by using highly accurate and detailed kinetics mechanisms. However, to use these high-fidelity models to your advantage, you need to find an appropriate mechanism for your application – just as you needed DVD’s to enjoy your first DVD player. Identifying the right detailed kinetics mechanism has great influence over the accuracy of your combustion simulation results. Selection of the right mechanism requires an understanding of kinetics, the goals of the simulation and some practical combustion design issues. The multitude of detailed kinetics mechanisms available in the literature makes it too easy to choose the wrong one. Using the wrong mechanism can hurt you both by wasting simulation resources and potentially missing valuable collection of data points. This white paper describes what a detailed kinetics mechanism is, how it is developed and validated and how it can be used in high-fidelity combustion simulation models to accelerate advanced combustion technology development. COMBUSTION REQUIRES A DETAILED KINETICS MECHANISM When we think of the chemical reaction for combustion of hydrocarbon fuels, we typically think of burning fuel with oxygen and producing combustion products in a single step, as shown below for methane (CH4): 1 See http://www.reactiondesign.com/products/open/energico_resources.html 2 of 13
  3. 3. Reaction Design White Paper We can adjust this simple, global reaction mechanism for methane in oxygen for the combustion of methane in air by adding the nitrogen species, as shown below: This single-step reaction mechanism contains five species and represents, in an ideal sense, combustion chemistry in its simplest form for one of the simplest hydrocarbon fuels. We understand that this is only a global representation of the actual reaction mechanism and that real life is much more complex. In reality, the decomposition of the fuel and the formation of reaction products occur in hundreds or thousands of steps that can involve more than a thousand short-lived chemical species and radicals – each of which may affect the combustion behavior and the formation of pollutant species. The set of elementary reactions and species that are involved in the global reaction process is typically called a “detailed kinetics mechanism.” The additional reactions and species represented in a detailed mechanism are required to predict important combustion phenomena, including ignition, flame propagation and formation of pollutants such as NOx, CO, unburned hydrocarbons and soot. In other words, to gain a thorough understanding of how real fuels burn in real applications, you will need a detailed kinetics mechanism for your combustion simulation. DEVELOPMENT AND REDUCTION OF A DETAILED KINETICS MECHANISM Just like the mapping of the human genome, developing a fully detailed chemical mechanism is a task best suited for experts. In-depth understanding and broad experience in chemistry, kinetics, fuel analysis and combustion application conditions are required to successfully develop an accurate chemical kinetics mechanism from scratch. Understanding of the state-of-the-art research results in the fields of complex kinetics and validation results is necessary to piece together the building blocks of reaction kinetics that embody a comprehensive detailed chemical mechanism – a “Master Mechanism” that represents a comprehensive set of chemical reactions (or 3 of 13
  4. 4. Reaction Design White Paper steps) and species that are known to be involved in the combustion of that fuel for a broad set of operating conditions. The first step in developing a Master Mechanism is to carefully define the application conditions and fuel components involved. Then, an exhaustive investigation is carried out to identify all the reactions that could be important in the decomposition of the fuel during combustion under those conditions. Thermodynamic data for the species involved in the reactions must be determined. Reaction rates for each reaction, with temperature and (in some cases) pressure dependency, will also need to be determined. For many applications, transport properties are also required for each species in the reaction system. Assembling all of these data for a new fuel-combustion system is a significant effort and involves poring over volumes of research results and peer-reviewed technical articles. Assembly of a Master Mechanism employs an understanding of complex organic and physical chemistry to fill in the gaps that exist in the available literature. Building on accurate thermodynamic data, the next step is to make sure that the mechanism is complete so that all applicable elementary reactions for the conditions of interest are included. For example, mechanisms for alkanes can be divided into several reaction classes containing similar types of reactions, for example, H-atom abstraction reactions. These classes of reaction tend to follow rate rules, based on observed trends related to several properties such as bond strength, number of abstractable hydrogen atoms, etc. The mechanism should follow such rules for known classes of reactions to maintain internal consistency. That way, a developer can merge different fuel components into a comprehensive fuel-surrogate mechanism that yields consistent predictions with different fuel components. Using this approach, the Master Mechanism developer can leverage knowledge of other molecules in the determination of which mechanism is best suited for a new fuel system. Many research groups all over the world work in development of detailed kinetics mechanisms. As a result, the combustion literature includes several detailed kinetics mechanisms for the same fuel components. This makes it crucial for the combustion 4 of 13
  5. 5. Reaction Design White Paper designer to evaluate the mechanisms carefully before applying them to specific conditions and also before merging different components from different sources. VALIDATION AGAINST FUNDAMENTAL KINETICS EXPERIMENTS Once a Master Mechanism has been assembled for a new fuel component, the developer should rigorously validate it against theoretical tests and fundamental experimental results. The most relevant experiments for this exercise can be represented well with a reduced-order model (for example, 1-D laminar flame or 0-D reactor models), so the developer can include the full Master Mechanism in the validation simulation without compromise. Such experiments include measurements in shock-tubes, laboratory flame experiments, stirred reactors or flow reactors. The selection of validation data will depend on the application, where the focus may be on prediction of ignition delay, laminar flame speed or trace species concentrations. To be reliable, a Master Mechanism is validated for the performance results of interest. In general, the more fundamental the underpinnings of the Master Mechanism, the more likely it will extrapolate well to conditions not explicitly validated in the development work. So, while we can map the full human genome based on what we know today, the Master mechanism must always be compared to and evolve with new information and understanding as it becomes available. If you think this approach to developing a Master Mechanism sounds complicated, involved and time-consuming, then you are right. That is why the development of a Master Mechanism is usually a costly process conducted by experts in the field. Fortunately, there are many experts in the field that have been developing foundation data for many families of fuel components over several decades. The benefit of identifying a Master Mechanism is that, once all the hard work by experts is complete, the resulting mechanism can be used by non-experts to achieve a high level of simulation accuracy. 5 of 13
  6. 6. Reaction Design White Paper MAINTAINING ACCURACY DURING MECHANISM REDUCTION A Master Mechanism typically represents a comprehensive set of chemical reactions (or steps) and species that are known to be involved in the combustion of that fuel for a broad set of operating conditions. These mechanisms can be quite large. For example, the latest diesel fuel mechanism developed by the Model Fuels Consortium, a group of companies that are working to enable the design of cleaner burning, more efficient engines and fuels, now contains over 3,500 species and 12,000 reactions or steps. Luckily, you often do not need all of that detail for your specific application results of interest. You can reduce the Master Mechanism in varying degrees to suit your simulation model while maintaining accuracy and optimizing your simulation run times. Mechanism reduction is a process that eliminates “excess baggage” in the Master Mechanism that is not needed for a specific result of interest (for example, major vs. trace species) and the application conditions of interest (for example, high- temperature conditions or near-stoichiometric fuel-air mixtures). You may choose from many different techniques to reduce a mechanism, but they all employ the same strategy: remove the specific reactions and species that are not of interest for the desired results and simplify groups of reactions using similar species into a smaller group of reactions. It is important to understand that error is introduced every time you employ a reduced mechanism as you are removing information from the fully most accurate form of the mechanism. You must balance the trade-offs between the size of the mechanism and the acceptable error. For example, Reaction Design has developed a suite of advanced mechanism- reduction methods, including both skeletal-reduction and more severe reduction methods. Skeletal methods systematically remove elementary reactions and species from the system, without changing any properties of the remaining species or rates of the remaining reactions. Our implementations of mechanism-reduction techniques are useful over a broad range of conditions and testing in any of CHEMKIN’s reactor models. Skeletal methods can typically achieve 50-80 percent reduction, depending on the application, with very good accuracy over a wide range of conditions. We can 6 of 13
  7. 7. Reaction Design White Paper strictly control the target accuracy and achieve the optimal size to meet the target application of the mechanism. You may further reduce mechanisms from their skeletal forms using lumping methods for species or reactions. For example, you may lump several isomers together and treat them as a single species. Rate constants are then adjusted to account for the combination of the various pathways and product branching represented by the original isomers. For high-temperature combustion, lumping techniques usually provide reasonably accurate results. Other severe-reduction methods use quasi-equilibrium assumptions and the process for achieving these can be automated to various degrees. You can combine these methods to get the smallest mechanism possible, while still achieving a targeted level of accuracy relative to the Master Mechanism. This approach of determining error using direct comparisons in reduced-order CHEMKIN simulations provides quantitative uncertainty information that is key to managing the speed vs. accuracy trade-off. DIFFERENT REACTION MECHANISMS ARE APPROPRIATE FOR DIFFERENT SIMULATIONS The type of combustion simulation models that you employ will also influence your choice of detailed kinetics mechanism. You must consider the practicality of the mechanism size when determining what degree of reduction, if any, is necessary from an appropriate Master Mechanism. Early on, in the concept definition phase of combustion equipment design, it may make sense to employ reduced-order models, such as the reactor models available in CHEMKIN, to scope out a general approach to achieve the overall system goals (see Figure 1). For example, you may decide to use 0-D Perfectly Stirred Reactors (PSR) to identify the range of fuel-air ratios and overall system residence times that are needed to achieve a certain level of NOx. Additionally, you may use a 1-D Plug Flow Reactor (PFR) model to determine the length of the dilution section of a gas turbine combustor that is required to oxidize CO down to an acceptable level. One benefit of using these reduced-order models is that you can use the fully detailed kinetics mechanism (i.e., a Master Mechanism) within a reasonable 7 of 13
  8. 8. Reaction Design White Paper computational time. If you need to explore many conditions, or the mechanism is extremely large, then you can employ some mechanism reduction techniques to optimize the Master Mechanism for your application. Combustion system designers typically move to more detailed system geometry as the design matures from a concept to hardware definition. Computational Fluid Dynamics (CFD) is commonly employed to provide 3-D simulation of the combustion process in the realistic complex geometry of the hardware. While CFD provides excellent accuracy of the complex geometry, it is typically unable to accommodate the use of a complex, detailed kinetics mechanism beyond a handful of reactions and species. In general there is always a tradeoff in the degree to which detailed chemistry can be included and the degree to which geometry details can be captured (as suggested in Figure 1). While CFD simulations may be limited to using Global, Single- Step or severely reduced reaction mechanisms, the value of the Master Mechanism is that you can at least test these global mechanisms against the full chemistry to understand what errors are being introduced under what conditions. The reduced-order models, such as those found in CHEMKIN, can be very helpful to make these comparisons and assessments. In most cases, there is a significant amount of error associated with these severely reduced mechanisms within the CFD simulation that prevents the accurate prediction of chemical phenomena, such as pollutant species production. 8 of 13
  9. 9. Reaction Design White Paper Geometric Complexity Full 3-D CFD Simulation Reduced-Order Kinetics Model (0-D or 1-D) Chemistry Complexity Figure 1: Spectrum of Combustion Simulation Models with Reasonable Simulation Times IDENTIFYING A DETAILED KINETICS MECHANISM FOR YOUR SIMULATION As described above, the development and validation of a detailed chemical mechanism for real fuels in real applications typically requires the undertaking of a complicated and costly process that is predicated by way of an expert understanding of combustion and fuel chemistry. Once complete, the mechanism provides the recipe or description of the fuel chemistry that you can use for competitive advantage. If the development and validation of the detailed mechanism is funded by public sources such as the US DOE, NASA, or other agencies in the business of providing technology for the public good, then these mechanisms will be made publicly available. On the other hand, if the funding source for the detailed chemical mechanism development and validation is from private, commercial entities, then it is likely that the 9 of 13
  10. 10. Reaction Design White Paper mechanism will be treated as a proprietary trade secret. This is similar to the proprietary recipes for Coca-Cola and Heinz Ketchup that are carefully protected from the competition. Fortunately, a wealth of key mechanisms is available in the public domain for typical fuels and applications. If your application and the fuel are both fairly common, it is likely you will find a mechanism that is suitable for your simulation needs. Reaction Design2 and other organizations provide online resources to help you find publicly available models for you to evaluate. However, if you are developing a novel or advanced combustion technology using non-standard fuels, then a suitable mechanism may not be publicly available. It is important to note that there might not be appropriate public mechanisms for standard applications using non-standard fuels – or standard fuels being used in non-standard applications. EXAMPLE MECHANISMS FOR NATURAL G AS TURBINES To illustrate, let us consider natural gas combustion in industrial, power- producing gas turbines. In the 1990s, the Gas Research Institute developed and validated a widely used mechanism called GRI-Mech 3.0 for use with natural-gas-fired, atmospheric-pressure burners for boilers and furnaces at conventional NOx levels. The mechanism is excellent for use with those applications, and is also the most commonly used natural gas mechanism in gas turbine combustion. However, there are two recent industrial gas turbine applications in which this mechanism has been shown to be lacking. Ultra-Low NOx Gas Turbine Combustion: The first application3 involves the use of natural gas in an ultra-low NOx (i.e., single-digit level) gas turbine. GRI-Mech 3.0 2 http://www.reactiondesign.com/support/open/datalinks.html 3 Drennan, S.D, et al. “Flow field Derived Equivalent Reactor Networks for Accurate Chemistry Simulation in Gas Turbine Combustors” GT2009-59861, Proceedings of the ASME 2009 Turbo Expo, 2009. 10 of 13
  11. 11. Reaction Design White Paper was found to consistently under-predict the NOx emissions from these applications because the mechanism did not include some of the pressure-dependent, middle-to- low-temperature reactions that are present in highly staged, ultra-low NO x gas turbine combustors (note the dashed line with triangle symbols in Figure 2). When a new mechanism was developed that incorporated the modern understanding of these additional NOx formation reactions, the resulting mechanism produced excellent agreement with ultra-low NO x experimental results. Figure 2: Comparison of NOx and CO simulation accuracy against experimental data using GRI-Mech 3.0 and an improved C2-NOx mechanism High CO2 Gas Turbine Combustion for Efficient Carbon Capture: There is great interest in reducing the greenhouse gas emissions from fossil fuel sources. One approach for low CO2 emissions from power-producing combustion turbines that fire natural gas is Carbon Capture and Sequestration (CCS). The CCS approach captures the CO2 from the exhaust and then sequesters it in a manner to prevent its release into the atmosphere. Carbon Capture technologies are most effective at extremely low oxygen levels in the exhaust, but conventional gas turbines are unable to produce such 11 of 13
  12. 12. Reaction Design White Paper low oxygen levels due to materials temperature limitations in the hot section of the turbine (i.e., the combustor, transition piece and high pressure turbine blades). In a new approach, the design method of Exhaust Gas Recirculation (EGR) can supply CO2 into the combustion air upstream of the turbine. Through this method, the increased CO2 in the combustion air acts to inhibit the temperatures of the combustion gases, allowing full consumption of the available oxygen without overheating section parts. There are a couple of key challenges to combustion stability and emissions when you employ such a dramatic shift in the composition of the combustion air and the conditions in the combustor. The first challenge is that the combustion is occurring at lower, or vitiated, oxygen levels than it does with air at 21 percent oxygen by volume. The second challenge is that both NOx and CO are also recirculated with the CO2 in the EGR gases and they can affect the NOx and CO formation processes that determine the exit NOx and CO emissions. Even though this is an application of natural gas in a power generation combustion turbine, the conditions of the application are substantially different than the GRI-Mech 3.0 validation conditions, raising questions on mechanism’s applicability. A recent investigation by Alstom4 determined that the GRI-Mech 3.0 mechanism was not appropriate for the reburn of NOx that was carried along with the EGR into the combustion air. Effort was taken to develop a modified, or improved, NOx mechanism specifically to address the high CO2 , low combustion temperature conditions in this application. SUMMARY Accuracy in combustion simulation can provide significant strategic advantages for combustion system designers by reducing expensive experimental testing, speeding new technologies to market and reducing costly field problems associated with poor combustion performance predictions. Understanding how fuel burns through the use of 4 Guethe, F., M. de la Cruz Garcia, et al. (2009). Flue Gas Recirculation in Gas Turbine: Investigation of Combustion Reactivity and NOx Emission. ASME Turbo Expo 2009. Orlando, FL, ASME. GT2009-50221. 12 of 13
  13. 13. Reaction Design White Paper detailed kinetics mechanisms can help designers use simulation models, such as reduced-order kinetic reactor and comprehensive 3-D CFD, to better emulate real world systems. The development of these mechanisms is a costly, time-consuming process that requires expert knowledge of detailed combustion and fuel chemistry. However, once these accurate Master Mechanisms have been developed, they can be used by designers who are not experts in detailed chemistry for accurate simulation results. Mechanism reduction techniques allow the model to use a smaller version of the Master Mechanism for increased simulation speed, however careful attention must be paid to the amount of error that is introduced through the mechanism reduction process. Detailed kinetics mechanisms are available for many common fuels and applications, but the use of these mechanisms must be carefully evaluated for use under your specific conditions. 13 of 13

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