Low Emission Combustion Research Articles

Non-Destructive Diagnostic Methods for Fire-Side Corrosion Risk Assessment of Industrial Scale Boilers, Burning Low Quality Solid Biofuels—A Mini Review

The use of low-emission combustion technologies in power boilers has contributed to a significant increase in the rate of high-temperature corrosion in boilers and increased risk of failure. The use of low quality biomass and waste, caused by the current policies pressing on the decarbonization of the energy generation sector, might exacerbate this problem. Additionally, all of the effects of the valorization techniques on the inorganic fraction of the solid fuel have become an additional uncertainty. As a result, fast and reliable corrosion diagnostic techniques are slowly becoming a necessity to maintain the security of the energy supply for the power grid. Non-destructive testing methods (NDT) are helpful in detecting these threats. The most important NDT methods, which can be used to assess the degree of corrosion of boiler tubes, detection of the tubes’ surface roughness and the internal structural defects, have been presented in the paper. The idea of the use of optical techniques in the initial diagnosis of boiler evaporators’ surface conditions has also been presented.

Autonomous Energy-Efficient Complexes Based on Crude Oil for Working in Arctic Conditions

The work is devoted to solving scientific and technical problems on the development of new generation Russian power equipment. Comprehensive theoretical and experimental studies of the effect of aerodynamic and heat-mass exchange processes on the ignition and combustion of crude oil in the straitened conditions of fire-tube boilers are reviewed and described. Based on the results of the research, design developments were carried out, prototypes of a fire-tube hot-water boiler with a remoted economizer and an automated low-emission burner device were manufactured and successfully tested. The efficiency of the boiler more than 94% and the reduced level of pollutants emissions in the operating load range are confirmed.

Describing the Mechanism of Instability Suppression Using a Central Pilot Flame With Coupled Experiments and Simulations

Abstract Pilot flames are commonly used to extend combustor operability limits and suppress combustion oscillations in low-emissions gas turbines. Combustion oscillations, a coupling between heat release rate oscillations and combustor acoustics, can arise at the operability limits of low-emissions combustors where the flame is more susceptible to perturbations. While the use of pilot flames is common in land-based gas turbine combustors, the mechanism by which they suppress instability is still unclear. In this study, we consider the impact of a central jet pilot on the stability of a swirl-stabilized flame in a variable-length, single-nozzle combustor. Previously, the pilot flame was found to suppress the instability for a range of equivalence ratios and combustor lengths. We hypothesize that combustion oscillation suppression by the pilot occurs because the pilot provides hot gases to the vortex breakdown region of the flow that recirculate and improve the static, and hence dynamic, stability of the main flame. This hypothesis is based on a series of experimental results that show that pilot efficacy is a strong function of pilot equivalence ratio but not pilot flow rate, which would indicate that the temperature of the pilot products as well as the combustion intensity of the pilot flame play more of a role in oscillation stabilization than the length of the pilot flame relative to the main flame. Further, the pilot-flame efficacy increases with pilot-flame equivalence ratio until it matches the main-flame equivalence ratio; at pilot equivalence ratios greater than the main equivalence ratio, the pilot-flame efficacy does not change significantly with pilot equivalence ratio. To understand these results, we use large-eddy simulation (LES) to provide a detailed analysis of the flow in the region of the pilot flame and the transport of radical species in the region between the main flame and pilot flame. The simulation, using a flamelet/progress variable-based chemistry tabulation approach and standard eddy viscosity/diffusivity turbulence closure models, provides detailed information that is inaccessible through experimental measurements.

Reducing Uncertainty in the Onset of Combustion Instabilities Using Dynamic Pressure Information and Bayesian Neural Networks

Abstract Modern low-emission combustion systems with improved fuel-air mixing are more prone to combustion instabilities and, therefore, use advanced control methods to balance minimum NOx emissions and the presence of thermoacoustic combustion instabilities. The exact operating conditions at which the system encounters an instability are uncertain because of sources of stochasticity, such as turbulent combustion, and the influence of hidden variables, such as unmeasured wall temperatures or differences in machine geometry within manufacturing tolerances. Practical systems tend to be more elaborate than laboratory systems and tend to have less instrumentation, meaning that they suffer more from uncertainty induced by hidden variables. In many commercial systems, the only direct measurement of the combustor comes from a dynamic pressure sensor. In this study, we train a Bayesain Neural Network to predict the probability of onset of thermoacoustic instability at various times in the future, using only dynamic pressure measurements and the current operating condition. We show that on a practical system, the error in the onset time predicted by the Bayesain Neural Networks is 45% lower than the error when using the operating condition alone and more informative than the warning provided by commonly used precursor detection methods. This is demonstrated on two systems: (i) a premixed hydrogen/methane annular combustor, where the hidden variables are wall temperatures that depend on the rate of change of operating condition, and (ii) full-scale prototype combustion system, where the hidden variables arise from differences between the systems.

Design, simulation, and validation of additively manufactured high-temperature combustion chambers for micro gas turbines

The demand for cleaner, more efficient, and durable sources of electricity is driving research into small-scale power generation. Micro gas turbines are especially suitable by virtue of their high power density and reliability, but a major drawback is their poor overall efficiency due to increasing parasitic energy losses relative to net power output as size decreases. Additive manufacturing offers design freedoms that could enable higher efficiency and lower emission combustors for micro gas turbine applications. A novel conical radial swirl-stabilized tubular combustor with internal vane fuel injection is designed and tested, and a validated reacting computational fluid dynamics model is used to design novel combustor features that can only be additively manufactured. Of the five different concepts tested, those benefitting from additively manufactured features outperform the traditional design in terms of peak temperature control and fuel–air mixing, translating to striking reductions in pollutant emissions, with up to 75% and 40% reductions in nitrogen oxides and carbon monoxides, respectively, while concepts incorporating upstream fuelling and a three-row lattice show a near 20% increase in mixture quality. As well as evaluating a number of novel and very promising additively manufactured combustor design features, this work provides guidance on the incorporation of additively manufactured features in combustors for any gas turbine application and demonstrates the clear benefits of additive manufacturing for low-emission combustor design.

The analysis of the level of emission characteristics depending on the aerosol structure

An attempt to combine the main verified recommendations for the organization of low-emission combustion of fuels (in relation to three main components of standardized harmful substances SN, NOx, and CO2) into one of the crucial modern problemsis implemented. Thepaper is devoted to the search for some general trends in the formation of emission characteristics based on the accumulated experience such as computational and experimental results obtained in CIAM.

Mathematical modeling of a swirling jet in applications to low-emission combustion of low-grade fuels

Abstract. The search for new solutions in the field of energy, preventing negative impact on the environment, is one of the priority tasks for modern society. Natural gas occupies a stable position in the demand of the UES of Russia for fossil fuel. Biogas is a possible alternative fuel from organic waste. Biogas has an increased content of carbon dioxide, which affects the speed of flame propagation, and a lower content of methane, which reduces its heat of combustion. However, the combined combustion of natural gas and biogas, provided that the mixture of fuel and oxidizer is well mixed, can, on the one hand, reduce the maximum adiabatic temperature in the combustion chamber of power boilers at TPPs, and, on the other, increase the stability of biogas combustion. For the combined combustion of natural gas and biogas in operating power boilers, it is necessary to reconstruct the existing burners. For a high-quality reconstruction of burners capable of providing stable and low-toxic combustion of fuel, it is important to have theoretical data on the combustion effect of combustion of combinations of organic fuels on the temperature distribution in the combustion zone and on its maximum value. In this paper, self-similar solutions of the energy equation for axisymmetric motion of a liquid (gas) in a model of a viscous incompressible medium are obtained. Basing on them, a stationary temperature field in swirling jets is constructed. A set of programs based on the ANSYS Fluent software solver has been developed for modeling and researching of thermal and gas-dynamic processes in the combustion chamber. On the basis of the k - ϵ (realizable) turbulence model, the combustion process of a swirling fuel-air mixture is simulated. The results of an analytical and numerical study of the temperature and carbon dioxide distribution in the jet are presented.

Experimental study on the stability and flame characteristics of LPG flames in O2/N2 and O2/CO2 environment

The current energy crisis and the escalating concern over pollution level necessitate the development of various low emission and clean technologies for efficient utilization of fossil fuels. These technologies offer the benefits of using fossil fuels continuously without harming the environment. Oxy fuel combustion technology is one of the promising low emission combustion technologies having an additional advantage of being a better retrofit option compared to other clean technologies. In the present study, the stability and flame characteristics of LPG flame in a tubular burner has been investigated with oxygen as the oxidizing gas and nitrogen or carbon dioxide as the balance gas. The experiments were conducted to study the effect of primary aeration and the effect of balance gas on the stability and flame characteristics of air and oxy flames. The experimental results from the preliminary experiments show that the flame height decreases with increase in primary aeration for both air and oxy cases and the premixed blue zone increases with primary aeration. The flames observed for oxy cases are dull and weak at all equivalence ratio (or primary aeration). The measured axial flame temperature indicated that the temperature of oxy flames is lower than that of the air flames for the same primary aeration. The data obtained from the stability tests show that the oxy flames can withstand relatively lower port velocity than its air counterpart. Therefore, the stable operating range of the given burner design is lower when CO2 is used as the balance gas. These differences in the stability and flame characteristics is attributed to the difference in the thermo physical properties like molar heat capacity, mass diffusivity and thermal conductivity of the balance gas CO2 and N2.

Use of EGR e-pump for Dual-Mode Dual-Fuel engines in mild hybrid architectures

Highway transport sector is still expected to be dominated by internal combustion engines in the future, especially due to limitations in the introduction and applicability of full electrification for heavy-duty propulsive systems. Nonetheless, this sector is still under the objectives imposed in the roadmap towards carbon neutrality. For this, engine efficiency and low-emission combustion modes must be improved, and the introduction of synthetic fuels, low electrification levels and devices for engine optimization and energy management have proven to be a great advance towards these objectives. This work studies the application of an EGR e-pump for energy recovery and combustion optimization on a powertrain running on Dual-Mode Dual-Fuel combustion mode as a substitute of a complex dual route EGR system. The results include the evaluation of the impact of using energy recovery devices on combustion performance and emissions levels, as well as a numerical evaluation of driving conditions considering a medium-duty application in the transport sector under different driving scenarios. The results point out that the inclusion of the EGR e-pump can contribute to mitigate the drawbacks of removing the complex dual route system in terms of equivalent fuel consumption without greater impact in terms of emissions. Additionally, its application in mild hybrid platforms can promote a significant improvement in CO2 emissions, especially for urban areas (20% compared to a conventional powertrain based on HP EGR).

Numerical investigation of the breakup mode and trajectory of liquid jet in a gaseous crossflow at elevated conditions

ABSTRACTThe commercial Computational Fluid Dynamics (CFD) software STAR-CCM+ was used to simulate the flow and breakup characteristics of a Liquid Jet Injected into the gaseous Crossflow (LJIC) under real engine operating conditions. The reasonable calculation domain geometry and flow boundary conditions were obtained based on a civil aviation engine performance model similar to the Leap-1B engine which was developed using the GasTurb software and the preliminary design results of its low-emission combustor. The Volume of Fluid (VOF) model was applied to simulate the breakup feature of the near field of LJIC. The numerical method was validated and calibrated through comparison with the public test data at atmospheric conditions. The results showed that the numerical method can capture most of the jet breakup structure and predict the jet trajectory with an error not exceeding ±5%. The verified numerical method was applied to simulate the breakup of LJIC at the real engine operating condition. The breakup mode of LJIC was shown to be surface shear breakup at elevated condition. The trajectory of the liquid jet showed good agreement with Ragucci’s empirical correlation.

Multi-bifurcation behaviors of stability regimes in a centrally staged swirl burner

Multi-bifurcation behaviors of staged swirl flames fueled with methane at atmospheric pressure are experimentally investigated by varying the global equivalence ratio (ϕglo). Based on the characteristics of measured pressure oscillations and the associated results of phase space reconstruction, recurrence plots, and synchrosqueezing-transform-based time-frequency analysis, the multi-bifurcation behaviors of this thermoacoustic system with four different stability regimes have been identified. With incremental increase in ϕglo from 0.55 to 0.79, these four stability regimes follow the sequence of a quiet mode (Regime I), the first limit cycle with moderate oscillations (Regime II), an intermediate state with intermittent bursts of multi-modes (Regime III), and the second limit cycle with much stronger oscillations (Regime IV). The flame dynamics in Regimes II and IV undergoing limit cycle oscillations are compared. The flame structure in Regime II displays an attached twin-flame structure, the same as that observed in Regime I. However, a large-scale periodic convective motion is found in Regime IV, which is identified to be the main thermoacoustic driving factor in the local Rayleigh index maps. Further experiments are carried out by continuously increasing ϕglo to examine the synchrosqueezing-transform spectra of transient processes during the two bifurcations. The present investigation is instrumental in obtaining a fundamental understanding of nonlinearity and multi-bifurcation of thermoacoustic instabilities in centrally staged swirl combustors, which is vital in guiding the early stage design and developing detection/control strategies in practical low-emission combustion systems.

Experimental investigation of flow field features and spark ignition process in a multi-swirl airblast injector

Lean combustion has been applied in aero gas turbine engines in response to rising concerns regarding the effects of pollutant emissions on the environment. As one of low emissions combustion technologies, stratified partially premixed combustion is capable of reducing NOx emissions while still providing robust pilot flames. In this work, the processes of liquid fuel atomization, fuel–air mixing and spark ignition in a multi-swirl staged combustor with airblast atomizer are experimentally investigated to further illustrate the effects of flow field features on flame propagation and the underlying reasons leading to successful and failed ignitions (misfires). A trajectory processing algorithm is developed to identify the detailed flame propagation behaviors in successful and misfire scenarios. PIV, PMie and High-speed imaging technologies are applied to provide the velocity distribution, droplets distribution and time-resolved flame images, respectively. Evolution of projected flames shows that the mechanism of flame motions in the early phase following spark can be explained by non-reacting flow structures and spray distributions. It is demonstrated that the early phase propagating trajectories play an important role in determining overall successful ignition. The variation regarding the flow field, droplets distribution, and flame evolution under pressure drops varying from 1% to 6% are analyzed. The ignition and combustion reach a relatively stable state when the pressure drop of injector exceeds 4%, which reflects in the minimum LLO FAR varies around 0.021 ± 0.001 and the time of flame development stabilizes at 40 ± 5 ms.

Parametrical investigation for the optimization of spherical jet-stirred reactors design using large eddy simulations

Due to the importance of gas-phase chemical reaction kinetics in low-emission combustion, stirred tank reactors have been used for decades as an experimental tool to study high- and low-temperature oxidation. A Jet-Stirred Reactor (JSR) setup is valuable to determine the evolution of species mole fractions. For the accuracy of the experimental results, it is important that a JSR is designed such that the concentration field is as homogeneous as possible in order to avoid disturbance of the chemical kinetics. In this work, numerical simulations were performed to investigate the mixing in a JSR chamber. The turbulent structures inside the JSR and the nozzles are captured using Large Eddy Simulations. We conducted numerically a parametric study to evaluate the effects of thermodynamic conditions and geometrical parameters on the mixing characteristics. More specifically, the diameter of the spherical chamber is modified together with the diameter of the nozzles through which fresh gases are fed. The characterization of the gas flow inside a typical spherical JSR layout and results derived by the normalized standard deviation of a tracer mass fraction show that a reduction of the JSR diameter at high pressures improves the homogeneity. Further, we propose a new optimized configuration consisting of six nozzles pointing to the center of the reactor which provides a more uniform composition compared to the standard JSR design.

The oxygen-deficient combustion and its effect on the NOx emission in a localized stratified vortex-tube combustor

The oxygen-deficient combustion characteristics of methane in a localized stratified vortex-tube combustor (LSVC) are studied by diluting combustion air with nitrogen. The influences of oxygen mole fraction (0.13–0.21) on flame configuration, combustion stability, combustion efficiency, and NOx emission characteristics are experimental investigated at the inlet temperature of 300 K. Combined with the numerical simulation method, the NOx generation, and emission mechanisms are analyzed in this combustor. Results show that the LSVC can achieve a wide stability limit, in which the global equivalence ratio can be as low as 0.22 at the lowest oxygen mole fraction (β) of 0.13. To ensure high combustion efficiency, the β should be kept above 0.16 since the oxygen-deficient condition reduces the reaction rate and flame temperature. The combustor can achieve ultra-low NOx emission of below 10 ppm (@ 15 vol% O2) due to low oxygen concentration and flame temperature. Furthermore, part of NOx entrained into the fuel-rich reduction zone by the swirl flow field is reduced by the reductive species (i.e., CO and H2) to further lowering NOx emissions. The results of this paper can guide the development of the LSVC in the high-efficiency and low-emission combustion fields.

Investigation of Low-Temperature Lean Combustion Characteristics in Power Plants with External Heating of Components

An integral part of any open-type gas turbine plant is a low-emission combustion chamber, which is usually two-zone and cooled. One of the ways to reduce emission of harmful substanc-es is organizing low-emission low-temperature lean combustion with external heating of compo-nents. This paper investigates the effect of external heating of air and fuel gas on expansion of the lower combustion limit and stable flame position in a single-zone uncooled combustion chamber of a microgas turbine power plant. Stable position of the flame front in combustion chambers of this type mainly depends on the ratio between the average flow rate of the combus-tible-air mixture and the rate of turbulent combustion. This ratio depends on thermal, gas-dynamic, thermochemical and geometric factors. The purpose of this work is to substantiate the possibility of using the relative flow rate as a generalized characteristic. This goal was achieved in processing a large amount of published experimental data and numerical modeling of low-temperature combustion of lean mixtures. The most significant research result is determination of the range of relative flow rate (gk = 0.3…3.5·10-4 kg⁄s∙N), at which it is possible to ensure sta-ble flame position in a single-zone combustion chamber. Significance of the obtained results lies in the fact that using the relative flow rate makes it possible to quickly determine and analyze the geometric and gas-dynamic parameters and characteristics of turbulent combustion in com-bustion chambers of micro-gas turbine power plants.

Investigations of the Emission Characteristics of a Dual-Fuel Gas Turbine Combustion Chamber Operating Simultaneously on Liquid and Gaseous Fuels

Abstract This study is dedicated to investigations of the working process in a dual-fuel low-emission combustion chamber for a floating vessel’s gas turbine. As the object of the research, a low-emission gas turbine combustion chamber with partial premixing of fuel and air inside the outer and inner radial-axial swirls was chosen. The method of the research is based on the numerical solution of the system of differential equations which represent the physical process of mass and energy conservation and transformations and species transport for a multi-component chemically reactive turbulent system, considering nitrogen oxides formation and a discrete ordinates model of radiation. The chemistry kinetics is presented by the 6-step mechanism of combustion. Seven fuel supply operating modes, varying from 100% gaseous fuel to 100% liquid fuel, have been analysed. This analysis has revealed the possibility of the application of computational fluid dynamics for problems of dual-fuel combustion chambers for the design of a floating vessel’s gas turbine. Moreover, the study has shown the possibility of working in different transitional gaseous and liquid fuel supply modes, as they satisfy modern ecological requirements. The dependencies of the averaged temperature, NO, and CO concentrations along the length of the low-emission gas turbine combustion chamber for different cases of fuel supply are presented. Depending on the different operating modes, the calculated emission of nitrogen oxides NO and carbon monoxide CO at the outlet cross-section of a flame tube are different, but, they lie in the ranges of 31‒50 and 23‒24 mg/nm3 on the peak of 100% liquid fuel supply mode. At operating modes where a gaseous fuel supply prevails, nitrogen oxide NO and carbon monoxide CO emissions lie in the ranges of 1.2‒4.0 and 0.04‒18 mg/nm3 respectively.

Efficient emission modelling in lean premixed flames with pre-tabulated formation characteristics

It is practically important to accurately predict NOx and CO emissions in lean premixed flames for the development of fuel-efficient and low-emission combustion systems. In this study, the efficient modelling with pre-tabulated formation characteristics of NOx and CO is systemically investigated. Their formation characteristics for turbulent flames in the flamelet and thin reaction zone regimes are first quantified with one-dimensional adiabatic premixed flames through the incorporation of turbulence induced diffusion. Results show that the formation of NOx and CO can be characterized by two different stages, i.e., a rapid increase of both in flame front and the linear growth for NOx and exponential decay for CO in post-flame zone. An efficient NOx and CO modelling approach is formulated and demonstrated in a full-scale methane gas turbine combustor, in which species NOx and CO are transported and solved, with the source terms being modelled by pre-tabulated formation characteristics from one-dimensional premixed flames with detailed chemical kinetics. Realizable k-epsilon model and finite rate/eddy dissipation model in conjunction with a two-step global mechanism are employed to primarily predict the flow and flame characteristics. Results show that the predicted NOx and CO agree with experimental measurements over a wide range of equivalence ratios. The discrepancy in NOx emission can be accounted by fuel/air unmixedness. For CO, the rapid increase due to the incomplete CO oxidation resulting from its increasing characteristic oxidation time near lean blow-out (LBO) is correctly captured. It is further shown that the predicted CO emission is sensitive to combustion/kinetics model parameters and the accurate prediction of flame shape and dynamics is crucial to capture the trend of CO formation near LBO.

Bayesian inference and uncertainty quantification for hydrogen-enriched and lean-premixed combustion systems

Development of probabilistic modelling tools to perform Bayesian inference and uncertainty quantification (UQ) is a challenging task for practical hydrogen-enriched and low-emission combustion systems due to the need to take into account simultaneously simulated fluid dynamics and detailed combustion chemistry. A large number of evaluations is required to calibrate models and estimate parameters using experimental data within the framework of Bayesian inference. This task is computationally prohibitive in high-fidelity and deterministic approaches such as large eddy simulation (LES) to design and optimize combustion systems. Therefore, there is a need to develop methods that: (a) are suitable for Bayesian inference studies and (b) characterize a range of solutions based on the uncertainty of modelling parameters and input conditions. This paper aims to develop a computationally-efficient toolchain to address these issues for probabilistic modelling of NOx emission in hydrogen-enriched and lean-premixed combustion systems. A novel method is implemented into the toolchain using a chemical reactor network (CRN) model, non-intrusive polynomial chaos expansion based on the point collocation method (NIPCE-PCM), and the Markov Chain Monte Carlo (MCMC) method. First, a CRN model is generated for a combustion system burning hydrogen-enriched methane/air mixtures at high-pressure lean-premixed conditions to compute NOx emission. A set of metamodels is then developed using NIPCE-PCM as a computationally efficient alternative to the physics-based CRN model. These surrogate models and experimental data are then implemented in the MCMC method to perform a two-step Bayesian calibration to maximize the agreement between model predictions and measurements. The average standard deviations for the prediction of exit temperature and NOx emission are reduced by almost 90% using this method. The calibrated model then used with confidence for global sensitivity and reliability analysis studies, which show that the volume of the main-flame zone is the most important parameter for NOx emission. The results show satisfactory performance for the developed toolchain to perform Bayesian inference and UQ studies, enabling a robust and consistent process for designing and optimising low-emission combustion systems.

An alternative approach to evaluate fuel/air mixing quality

ABSTRACTA practical method to evaluate quantitatively the uniformity of fuel/air mixing is essential for research and development of advanced low-emission combustion systems. Typically, this is characterised by measuring an unmixedness parameter or a uniformity index. An alternative approach, based on the fuel/air equivalence ratio distribution, is proposed and demonstrated in a simple methane/air venturi mixer. This approach has two main advantages: it is correlated with the fuel/air mixture combustion temperature, and the maximum temperature variation caused by fuel/air non-uniformity can be estimated. Because of these, it can be used as a criterion to check fuel/air mixing quality, or as a target for fuel/air mixer design with acceptable maximum temperature variation. For the situations where the fuel/air distribution non-uniqueness issue becomes important for fuel/air mixing check or mixer design, an additional statistical supplementary criterion should also be used.

Features of commissioning a gas turbine unit with a low-emission combustion chamber

Environmental safety issues are a priority in the modernization development of aviation or ground-based combustion chambers of gas turbine engines. In order to reduce the emission of harmful substances from combustion products, a low-emission combustion system, which is based on the concept of LPP (Lean-premixed and prevaporised), is being used today. This concept implements the principle of burning “ultra-poor” premixed air-fuel mixtures. The operational experience of combustion chambers developed in accordance with the indicated concept shows that one of the implementation problems is the difficulty in ensuring a good start. This paper presents the results of adjusting the operating process of a modern combustion chamber, launching a gas turbine unit, including drawing up a fuel supply control algorithm, experiments with igniting ignition devices and setting the gas turbine on idle mode. A feature of the commissioning work is the debugging of the operating process of the combustion chamber directly as part of the gas turbine unit, bypassing the traditional pilot-testing on the rig.