Chemical Reactor Network Research Articles

A chemical kinetic analysis: Influence of post-flame chemistry, combustion pressure, premixing degree (fully premixed to non-premixed), and secondary air supply on NOx emissions from NH3/CH4-air combustion

Numerous studies assessed the viability of ammonia blends for the wide-scale application of ammonia in carbon-free energy systems. NOx emissions from ammonia combustion are the primary concern constraining the adoption of NH3 as a fuel. The present study explores the effect of post-flame chemistry, combustion pressure, reactant-mixing (premixing degree), and secondary air supply on NOx emissions from NH3/CH4 swirl flame. Furthermore, an extensive chemical kinetic analysis is carried out using the rate of production and sensitivity analyses to study NOx formation and destruction with the above-discussed techniques. The present study adopts the NH3/CH4 reaction model developed by Okafor et al. and a PSR-PFR chemical reactor network for predicting the NOx formation chemistry in NH3/CH4-air combustion. The analyses are conducted for the values of ammonia heat fraction in the fuel mixture (ENH3) = 0.1–0.3 and equivalence ratio (ϕ) = 0.6–1.4. The present work suggests that providing a post-flame zone is advantageous in reducing both NOx and NH3 emissions, especially for rich NH3/CH4 flames. Increasing the combustion pressure to achieve low NOx without significant ammonia slip appears to be a viable method for NH3/CH4 flames with ϕ < 1.2. Adopting a non-premixed injection mode is a feasible low NOx combustion technique for 0.9 < ϕ < 1.2, especially for higher ammonia fractions in the fuel mixture. An excess air supply of 10% reduces the NOx emissions for fuel-lean conditions (ϕ < 0.9) with a negligible rise in unburnt NH3 emissions under atmospheric conditions.

Machine learning clustering algorithms for the automatic generation of chemical reactor networks from CFD simulations

Predicting the thermal and environmental performances of combustion systems can be difficult and computationally expensive. Chemical Reactor Networks (CRN) represent an appealing solution for performing faster numerical simulations since they can carry out calculations with detailed kinetics in a significantly reduced amount of time. However, the design of such models is challenging, as it usually requires a considerable amount of expertise from the user. In this work, we present a novel, automatic methodology for the design of CRN models, which consists of the post-processing of CFD data via a combination of unsupervised clustering and graph scanning algorithms. The methodology was tested on a semi-industrial furnace which can operate in Moderate or Intense Low oxygen Dilution (MILD) combustion. The furnace operates at a nominal power input of 15 kW and is fed with several CH4-H2 mixtures, with two different air injector diameters. First, RANS simulations of the different cases were performed and validated against experimental data. Subsequently, different CRNs were extracted singularly for each case and simulated using detailed kinetics mechanisms. The different CRNs showed good performances in predicting NO emissions for the entire range of cases, as the results were in reasonable agreement with experimental data. Last, the capability of a single CRN to extrapolate towards different operating conditions was also assessed by using one single CRN for the simulation of different fuel mixtures. The approach showed a good level of generalization since the NO predictions were close to the experimental values.

Chemical kinetic analysis to study the potential of fuel staging in reducing the emissions from NH3/CH4-air combustion at different pressures

The combustion characteristics of ammonia/methane blends to enhance the reactivity of ammonia are investigated successfully for carbon–neutral energy applications. However, commercializing ammonia blends demands techniques to retrench the superfluous NOX emissions under fuel-lean conditions. The present study is a chemical kinetic analysis investigating a new ammonia-methane combustion strategy of injecting ammonia into hot methane combustion gases (ammonia fuel staging) to reduce the NOX propensity without ammonia slip. The combustion kinetics of NH3/CH4 mixtures are studied in chemical reactor networks (CRNs), depicting typical burner flow conditions with and without fuel staging. A comparison of the emission chemistry in the fuel-staged CRN model with the single-stage CRN model is carried out for the ammonia mole fraction (XNH3) in the fuel mixture ranging from 20 to 60 % and the equivalence ratio ranges (ϕ) of 0.6–0.9 at both low and high-pressures of 1 and 10 atm, respectively. It is observed that in the single-stage reactor, the entire reactor is an active reaction zone due to the reduced chemical kinetics of methane in the presence of ammonia. Recognizing the need of raising the radical concentration and temperature in the flame zone to enhance ammonia chemistry, a fuel-staged reactor model is proposed. In the fuel-staged model, staged combustion is achieved by injecting NH3 into the lean methane combustion products for the constant combustor power output of 25 kW. Results show that fuel staging enhances the ammonia chemistry at low and high-pressure conditions of 1 and 10 atm. In the fuel-staged reactor, the flame zone of the second stage is observed to be the active reaction zone with maximum NH3 consumption. At low-pressure conditions, fuel staging leads to a reduction in both CO and NOX emissions without ammonia slip. The NOX reduction is due to the stable availability of NO-consuming NHi radicals for the entire duration of combustion in the flame zone via the reactions: 2NH2→NH3+NH and NH3+OH→NH2+H2O. The CO-consuming reactions: NH2+CO→HNCO+H and OH+CO→H+CO2 further reduces the CO emissions from the second stage. However, the augment in ammonia chemistry adversely affected NOX reduction at high-pressure operating conditions due to the lack of NHi radicals for NO consumption. The study contributed to the NOX reduction from ammonia combustion by providing an active high-temperature combustion zone with a self-sustaining NHi environment.

Humidification Towards Flashback Prevention in a Classical Micro Gas Turbine: Thermodynamic Performance Assessment

Combustion air humidification has proven to be effective to stabilize hydrogen combustion and to avoid flashback apparition in a typical micro Gas Turbine (mGT). However, both the fuel alteration and combustion air dilution will impact the cycle performance. A complete characterization of this thermodynamic impact is essential to ensure that the mGTs become cleaner, and fully flexible to fit with the expectation of future small-scale decentralized power production. Therefore, the objective of this work is twofold: the determination of the necessary dilution for combustion stabilization, depending on the type of fuel, as well as the impact assessment on the cycle performance. In this framework, a hybrid model of the Turbec T100 mGT combustor, combining a 0D Chemical Reactor Network and 1D Laminar flame calculations, is used to first assess the flashback limits. The laminar flame speed is evaluated to predetermine the necessary minimal water dilution of the combustion air to avoid flashback for several CH4/H2blends. Second, a thermodynamic analysis is performed to assess the impact of the flame stabilization measures on the cycle performance of the mGT using Aspen Plus. The 0D/1D simulation results show that the combustor of the Turbec T100 can operate with fuels containing up to 100% hydrogen. However, the thermodynamic analysis shows that the water dilution leads to a decreased electrical performance. Future work consists in the iterative coupling of both 0D/1D and the Aspen model to correctly predict the flashback limits, considering the altering operating conditions. To conclude, with this work, we provide a framework for future mGT operations with alternative fuels.

NOx formation from ammonia, and its effects on oxy-combustion of hydrocarbon fuels under supercritical-CO2 conditions

NOx formation from ammonia impurities and its effects on fuel oxidation is not well understood for high-pressure supercritical CO2 oxy-combustion conditions. This effect is investigated computationally and experimentally in the present work. A chemical kinetic analysis revealed that the reaction between NH radical and CO2 plays an important role in determining the rate of NOx formation at these conditions. It was also found that a significant reduction in the NOx formation from ammonia oxidation occurs with sCO2 oxy-combustion at 300 atm when compared to traditional gas turbine conditions. A chemical reactor network simulation of realistic sCO2 cycle conditions confirmed the reduced NOx emissions. Monte Carlo simulations used to study the sensitivity of model input variables on the emissions showed that CO2 cooling impacts the CO emissions while most of the NOx are generated in the flame zone. Uncertainty analysis showed that the reaction between NH and CO2 to form HNO radical is an important contributor to the model uncertainty under sCO2 oxy-combustion conditions.The presence of combustion-generated NOx in the recycled-CO2 can impact the fuel oxidation in the primary zone of the combustor. Hence, to understand the effect of NOx on ignition, high-pressure shock tube ignition delay time experiments were performed at conditions relevant to sCO2 oxy-combustion. The ignition delay time measurements were made for syngas and CH4 fuels with and without NO addition under supercritical conditions using CO2 as the bulk diluent at nominal pressures around 100 atm. Experimental data showed that the presence of NO promotes ignition at these conditions. The effect is more pronounced for CH4 compared to syngas. Nitric oxide acts as a chemical catalyst to promote ignition by increasing the combustion radical pool. The catalytic cycle involves the conversion of NO to NO2 which also contributes to CH4 oxidation by H-atom abstraction to generate CH3 radical.

A Comparative Study of NOx Emission Characteristics in a Fuel Staging and Air Staging Combustor Fueled with Partially Cracked Ammonia

Recently, ammonia is emerging as a potential source of energy in power generation and industrial sectors. One of the main concerns with ammonia combustion is the large amount of NO emission. Air staging is a conventional method of reducing NO emission which is similar to the Rich-Burn, Quick-Mix, Lean-Burn (RQL) concept. In air-staged combustion, a major reduction of NO emission is based on the near zero NO emission at fuel-rich combustion of NH3/Air mixture. A secondary air stream is injected for the oxidation of unburned hydrogen and NHx. On the other hand, in fuel-staged combustion, NO emission is reduced by splitting NH3 injection, which promotes the thermal DeNOx process. In this study, NOx emission characteristics of air-staged and fuel-staged combustion of partially cracked ammonia mixture are numerically investigated. First, the combustion system is modeled by a chemical reactor network of a perfectly stirred reactor and plug flow reactor with a detailed chemistry mechanism. Then, the effects of ammonia cracking, residence time, and staging scheme on NOx emission are numerically analyzed. Finally, the limitations and optimal conditions of each staging scheme are discussed.

Numerical Investigation of Rich-Lean Staging in a SGT-750 Scaled Dry Low Emission Burner With Partially Decomposed Ammonia

Abstract Ammonia is emerging as a very convenient hydrogen- and energy-carrier in the context of present efforts to curb carbon emissions from the power-generation and transport sectors. As opposed to hydrogen, the properties of ammonia make it significantly simpler to transport and store. Early exploratory work on the combustion of pure ammonia in laboratory-scale gas turbine combustors revealed that the adoption of a longitudinal rich-lean staging strategy in the operation of the device is a convenient approach to minimize NOx and N2O emissions from fuel-bound nitrogen oxidation. Moreover, recent experimental evidence acquired at SINTEF confirms that the low-emission performance achieved with rich-lean staging also applies to the combustion of partially decomposed ammonia. In this paper, we report a comprehensive numerical modeling study that exploits large eddy simulation (LES) in conjunction with detailed chemical kinetics and a chemical reactors network (CRN) model to assess a rich-lean staging strategy applied to the combustion of partially decomposed ammonia in the Siemens Energy fourth-generation dry low emission (DLE) burner. Data analysis performed from both numerical modeling approaches, LES and CRN, confirm that the rich-lean staging strategy tested in the present study indeed results in significantly lower emissions compared to the conventional operational profile of the burner. Furthermore, reaction pathways analysis performed on the CRN data reveals important details that characterize the different evolution of nitrogen species between the nonstaged and staged operation of the burner, ultimately leading to the observed difference in NOx and N2O emissions

Decarbonisation potential of dimethyl ether/hydrogen mixtures in a flameless furnace: Reactive structures and pollutant emissions

This article sheds light on the combustion characteristics of dimethyl ether and its mixtures with methane/hydrogen under flameless conditions at different equivalence ratios. It was found that combustion of 100% dimethyl ether in flameless conditions minimises the NO formation, keeping it less than 10 ppm with no CO or unburned hydrocarbons. Progressive addition of methane was found to reduce the NO, reaching up to zero value at 50% methane in molar fraction along with a marginal CO2 reduction. However, large amounts of CO were found for higher methane levels, greater than 60% CH4 in molar fraction. Reactive structures based on OH∗ chemiluminescence revealed that adding methane results in increased ignition delay times and, consequently, a more distributed reaction zone characterised by reduced temperature gradients. No visible flame was observed for pure dimethyl ether as well as dimethyl ether/methane mixtures. Furthermore, a more intense and narrower reaction zone, characterised by the presence of a visible flame, was formed upon hydrogen addition. Adding hydrogen by 50% in molar fraction did not cause a noticeable rise in NO levels; however, CO2 was lowered by about 18%. Further addition of hydrogen resulted in increased peak temperatures of about 1700 K and higher NO emissions of about 50 ppm. Additionally, a skeletal Chemical Reactor Network was built and simulated with the commercial software CHEMKIN Pro to investigate the effect of the different mixtures and operating conditions on NO formation from a chemical point of view. N2O pathway was observed to be the root source of NO emissions for pure DME and DME/CH4 mixtures, however; the thermal pathway became gradually more important as hydrogen concentration was increased in the mixture.

Combustion performances of premixed ammonia/hydrogen/air laminar and swirling flames for a wide range of equivalence ratios

Ammonia, a carbon-free source of hydrogen has recently gained considerable attention as energy solution towards a green future. Previous works have shown that adding 30VOL.% hydrogen with ammonia can eradicate the drawbacks of pure ammonia combustion but no study in the literature has investigated this blend across a wide range of equivalence ratios. The present work investigates 70/30VOL.% NH3/H2 blend from 0.55 ≤ Φ ≤ 1.4 for both premixed laminar spherically expanding flames and turbulent swirling flames at atmospheric conditions. A detailed chemistry analysis has been conducted in Ansys CHEMKIN-PRO platform using a chemical reactor network (CRN) model to simulate the swirling turbulent flames. NO and NO2 emissions have followed similar bell-shaped trends, peaking at around Φ = 0.8, while N2O emission rises at lean conditions (Φ ≤ 0.7). The results indicate that Φ = 1.2 is the optimum equivalence ratio with reduced NOX emissions and some ammonia slip.

A Stochastic and Bayesian Inference Toolchain for Uncertainty and Risk Quantification of Rare Autoignition Events in Dry Low-Emission Premixers

Abstract Quantification of aleatoric uncertainties due to the inherent variabilities in operating conditions and fuel composition is essential for designing and improving premixers in dry low-emissions (DLE) combustion systems. Advanced stochastic simulation tools require a large number of evaluations in order to perform this type of uncertainty quantification (UQ) analysis. This task is computationally prohibitive using high-fidelity computational fluid dynamic (CFD) approaches such as large eddy simulation (LES). In this paper, we describe a novel and computationally efficient toolchain for stochastic modeling using minimal input from LES, to perform uncertainty and risk quantification of a DLE system. More specially, high-fidelity LES, chemical reactor network (CRN) model, beta mixture model, Bayesian inference and sequential Monte Carlo (SMC) are integrated into the toolchain. The methodology is applied to a practical premixer of low-emission combustion system with dimethyl ether (DME)/methane–air mixtures to simulate auto-ignition events at different engine conditions. First, the benchmark premixer is simulated using a set of LESs for a methane/air mixture at elevated pressure and temperature conditions. A partitioning approach is employed to generate a set of deterministic chemical reactor network (CRN) models from LES results. These CRN models are then solved at the volume-average conditions and validated by LES results. A mixture modeling approach using the expectation-method of moment (E-MM) is carried out to generate a set of beta mixture models and characterize uncertainties for LES-predicted temperature distributions. These beta mixture models and a normal distribution for DME volume fraction are used to simulate a set of stochastic CRN models. The Bayesian inference approach through SMC method is then implemented on the results of temperature distributions from stochastic CRN models to simulate the probability of auto-ignition in the benchmark premixer. The results present a very satisfactory performance for the stochastic toolchain to compute the auto-ignition propensity for a few events with a particular combination of inlet temperature and DME volume fraction. Characterization of these rare events is computationally prohibitive in the conventional deterministic methods such as high-fidelity LES.

Nitrogen oxide emissions analyses in ammonia/hydrogen/air premixed swirling flames

Ammonia/hydrogen fuel blends have gathered interest as a promising solution for the development of a hydrogen economy, with advantages in storage cost or combustion properties compared to pure hydrogen or pure ammonia, respectively. In that pursuit, the present work reports the trends of nitrogen oxide emissions for ammonia/hydrogen blends at atmospheric conditions. NO, NO2 and N2O productions/consumptions are approached in detail in combination with unburnt ammonia. All cases are measured in a turbulent, swirl-stabilised flame configuration across hydrogen fuel fractions from 0% to 25% and equivalence ratios from 0.55 to 1.30. A detailed chemistry analysis was conducted using a chemical reactor network (CRN) employing detailed reaction chemistry. The results show that NO and NO2 emissions peaks around Φ = 0.8, whereas considerable amount of N2O is generated at very lean conditions, Φ ≤ 0.65. Availability of OH radicals and O/H pools in the flames contribute towards fuel NO formation, which in turn produces NO2 and N2O. However, very lean conditions lead to lower temperatures that ensure the survival of N2O. The results identified Φ = 1.05–1.2 as the optimum equivalence ratios for reduced NOX emissions in ammonia/hydrogen blends, with further understanding of the flame chemistry responsible behind these emissions.

Stabilization and Emission Characteristics of Gliding Arc-Assisted NH3/CH4/Air Premixed Flames in a Swirl Combustor

This paper investigates the effects of gliding arc (GA) discharge on the stabilization and emission characteristics of premixed NH3/CH4/air swirl flames under various ammonia contents, flow rates, and global equivalence ratios. First, the lean blowout (LBO) limits are measured. We find that although blending CH4 extends the LBO limit of the ammonia flame to 0.6–0.8, the GA discharge further remarkably extends it to 0.3–0.4. Then, the flow and flame structures are visualized by simultaneous OH planar laser-induced fluorescence and particle imaging velocimetry measurements. The results reveal that the discharge increases the OH radical concentration and expands the inner recirculation zone, leading to improved flame stability. Second, the NOx emissions are investigated over a wide range of global equivalence ratios and ammonia contents. It is seen that the GA discharge slightly increases the NOx emission by less than 7% at low NH3 contents (<0.6), which can be attributed to the thermal and OH-involved reaction pathway of NOx formation. However, as the NH3 content further increases (which is accompanied by the rapid growth of the NOx emission), the GA discharge effectively reduces the NOx emission by up to 30%. This effect might be due to the more intensive NOx-consuming reactions by plasma-induced NH2 radicals at a higher ammonia content, which is confirmed by the strengthened NH2* chemiluminescence under GA discharge conditions. Finally, a chemical reactor network analysis gives reasonable NOx predictions without GA discharge and highlights the NOx-reduction effects of NH2 radicals under high ammonia contents.

Effects of hydrogen addition on combustion characteristics of a methane fueled MILD model combustor

Combined with need of the carbon emissions, the feasibility of Moderate or Intense Low-oxygen Dilution (MILD) combustion fueled with hydrogen/methane blends needs to be investigated. This paper discusses the pollutant emissions, the stable operating range and the flame morphology for a jet-induced MILD model combustor. The hydrogen/methane volume ratios range 0:10 to 5:5. The NOx emissions are less than 5 ppm@15%O2 when the hydrogen content is less than 50% by volume in the atmospheric conditions. The calculation using chemical reactor network (CRN) model demonstrates that the effect of heat loss on NOx emissions increases as the adiabatic combustion temperature increases, which is consistent with the experimental results. The maximum OH∗ signal intensity increased at higher hydrogen content, especially when the hydrogen content exceeds 30% by volume. Due to the increase in turbulent burning velocity and the enhancement in the reaction intensity, the reaction zones shrink with increasing hydrogen content. In addition, with increasing hydrogen content, the stable operation range of the combustor becomes narrower, and the stable combustion is not maintained when the hydrogen content exceeds 50% by volume. The findings of the paper help to further understand the effect of hydrogen content on the formation of MILD combustion in the jet-induced combustor.

Non-Idealities in Lab-Scale Kinetic Testing: A Theoretical Study of a Modular Temkin Reactor

The Temkin reactor can be applied for industrial relevant catalyst testing with unmodified catalyst particles. It was assumed in the literature that this reactor behaves as a cascade of continuously stirred tank reactors (CSTR). However, this assumption was based only on outlet gas composition or inert residence time distribution measurements. The present work theoretically investigates the catalytic CO2 methanation as a test case on different catalyst geometries, a sphere, and a ring, inside a single Temkin reaction chamber under isothermal conditions. Axial gas-phase species profiles from detailed computational fluid dynamics (CFD) are compared with a CSTR and 1D plug-flow reactor (PFR) model using a sophisticated microkinetic model. In addition, a 1D chemical reactor network (CRN) model was developed, and model parameters were adjusted based on the CFD simulations. Whereas the ideal reactor models overpredict the axial product concentrations, the CRN model results agree well with the CFD simulations, especially under low to medium flow rates. This study shows that complex flow patterns greatly influence species fields inside the Temkin reactor. Although residence time measurements suggest CSTR-like behavior, the reactive flow cannot be described by either a CSTR or PFR model but with the developed CRN model.

Effects of inter-stage mixing on the NOx emission of staged ammonia combustion

Rich-lean staged combustion is a promising method to achieve low NOx emission for NH3 fuel. The NOx emission of the rich-lean staged combustion heavily relies on inter-stage mixing, which has not been explicitly explored for NH3 fuel. Therefore, this paper presents a modeling study concerning the effects of the inter-stage mixing on the NOx emission of NH3 fueled rich-lean staged combustion. The rich-lean staged combustor is modeled by chemical reactor networks. The non-uniform mixing is modeled by assuming Beta distribution, which is a suitable model for the random behavior of percentages and proportions that is featured in the jet-in-cross-flow mixing of the secondary air and the primary-stage combustion products. The NOx emission is obtained at different inter-stage mixedness, ηl, and rich-burn stage equivalence ratios, ∅r, with constant combustor outlet temperature (constant global equivalence ratio for lean stage, ∅g). The major impact of inter-stage mixing on NOx for staged NH3 combustion is strongly dependent on ∅r. The most significant NOx increase caused by poor inter-stage mixing occurs at around ∅r=1.25 and is due to the increase of thermal NOx, which is similar to hydrocarbon fuels. This finding has important implications for practical applications because the ∅r=1.25 point has the lowest NOx emission and is considered to be the desired operating point for staged gas turbine combustor. Therefore, complete mixing of the primary-stage combustion products and the secondary air needs to be done to ensure low NOx emission for rich-lean staged combustion of NH3. In contrast, at ∅r>1.4, poor mixing may lead to NOx reduction due to the reduction in fuel NOx. However, the NOx at this point is still high and should be avoided in practical operation.

Machine Learning Clustering Algorithms for the Automatic Generation of Chemical Reactor Networks from Cfd Simulations

Machine Learning Clustering Algorithms for the Automatic Generation of Chemical Reactor Networks from Cfd Simulations

Investigation of Oxy-Fuel Combustion through Reactor Network and Residence Time Data

Oxy-fuel combustion is a promising strategy to minimize the environmental impact of combustion-based energy conversion. Simple and flexible tools are required to facilitate the successful integration of such strategies at the industrial level. This study couples measured residence time distribution with chemical reactor network analysis in a close-to-reality combustor. This provides detailed knowledge about the various mixing and reactive characteristics arising from the use of the two different oxidizing streams.

Numerical investigation of a dual-stage off-gas burner to support high pressure and high temperature solid oxide fuel cell/gas turbine (SOFC/GT) hybrid systems

The objective of this study is to investigate the challenges associated with burning these off-gases and to develop a simple yet efficient combustor to support solid oxide fuel cell/gas turbine hybrid power systems. As a result of operating of the fuel cell under conditions of high fuel utilization, a solid oxide fuel cell produces high temperature exhaust gases on the anode side that are of low heating value, and depleted air with low oxygen content on the cathode side. Anode off-gases characterized by heating values less than 2 MJ/m3 and cathode off-gases with as little as 9 vol % oxygen concentration present a challenge for achieving stable combustion. In this study, the focus is on the development of a dual-stage combustor for low-calorific solid oxide fuel cell off-gases at combustor inlet temperatures up to 1100 K. In addition to taking advantage of high temperature off-gases to support the stable combustion of these low heating value off-gases, the influence of elevated-pressure combustor operation (set by the solid oxide fuel cell operating pressure) is investigated. Initially, a chemical reactor network is developed and used to simulate mixing and flow characteristics of a typical gas turbine combustor operating on the aforementioned fuel/air mixtures at the elevated operating conditions. These results are then used to inform a dual-stage burner design in the form of a computational fluid dynamics model for simulations incorporating both flow and combustion dynamics. The final parameters that are used to evaluate the final design are criteria pollutant emissions such as carbon monoxide and oxides of nitrogen, as well as suitable temperature profiles across the burner.

Experimental study and mechanism analysis of the NOx emissions in the NH3 MILD combustion by a novel burner

NH3 can be used as an H2 energy carrier or an alternative carbon-free fuel because of its high volumetric energy density and easy liquefaction, which leads to convenient and economic transportation and storage within the global energy network. However, the combustion of NH3 is associated with some practical challenges, such as weak reactivity and high NOx emission. In this study, Moderate & Intense Low Oxygen Dilution (MILD) combustion of NH3 was conducted experimentally in a novel burner. The heated and diluted air was produced by burning a premixed lean CH4 mixture using a metal fiber burner installed at the bottom of the combustion chamber and NH3 was injected into the furnace by 20 nozzles oppositely installed on the sidewalls of the combustion chamber at 40 mm above its bottom. A total of 20 cases were investigated in the present work, including premixed combustion of CH4 and CH4/NH3, and MILD combustion of NH3, under various conditions. The Chemical reactor network (CRN) in the ANSYS Chemkin 17.0 and the HUST mechanism were used to simulate and analyze the NO formation mechanism under several experimental conditions. The experimental results showed that the NH3 MILD combustion significantly reduces the NOx emissions. The NOx reduction kinetics and the impact of NH3 flow rate and the equivalence ratio on NO emission in NH3 MILD combustion were analyzed in detail.

Multi-objective optimization of a lean premixed laboratory combustor through CFD-CRN approach

In this paper, the effects of geometrical characteristics variations on the pollutant emissions in a laboratory lean premixed combustor are studied numerically and experimentally. For this purpose, the combustion chamber, premixing chamber, and equipment for measuring the flame temperature and combustion product concentrations are designed, fabricated, and assembled for a laboratory atmospheric test stand. An object-oriented optimization algorithm based on computational fluid dynamics (CFD), a chemical reactor network (CRN), and the response surface method (RSM) is presented for optimization of the combustion chamber geometry to obtain minimum pollutant emissions. The numerical approaches for modeling the flow field and predicting the detailed chemistry are validated against the experimental results. Parameters, such as minimizing NOx, CO, and NO emissions, are considered for optimizing the combustion chamber's geometry. Finally, it is shown that the hybrid approach of CFD-CRN can serve as a useful tool in the design process and the optimization of combustion chambers. The optimization process results show that minor changes in the combustor design can minimize the defined objectives to an acceptable level.