Lean Stability Research Articles

Early clinical drug product shelf-life setting using accelerated predictive stability and metabolite data for impurity qualification: A case study

This case study demonstrates how knowledge of degradation products together with predictions can establish a lean stability strategy using the accelerated predictive stability (APS) principles. Applying all available data for AZD4831, (R)-1-(2-(1-aminoethyl)-4-chlorobenzyl)-2-thioxo-2,3-dihydro-1H-pyrrolo[3,2-d]pyrimidin-4(5H)-one, a reliable predictive model was developed despite minor differences in technical batch tablet compositions. Early forced degradation studies were performed to map potential degradation pathways. The insights from these studies guided the design of an APS study, which in turn inform on a suitable clinical stability program, initial specification and shelf-life. The use of APS predictions of degradants as well as total impurities highlighted at an early stage, when designing the clinical stability program, the opportunity to identify which degradation product that would be shelf-life limiting. Hence, it was possible to guide the development stability activities and set an initial shelf-life of a tablet formulation. The presented study displays the importance of combining several sources of information in drug development, e.g., potential degradation pathways, accelerated stability, stability program design, metabolite data, and specification limits.

Combustion Intensification Mechanism in a Vortex-Tube Reactive Flow

The combustion intensification mechanism was revealed in a stratified vortex-tube combustor. The results show that a triple-flame structure is formed in this combustor and the peak heat release is located exactly at the triple-point position. The peak heat release rates in the five selected representative cases studied in this paper are all over . The stability limit is wide, and the lean stability limit is always lower than 0.2. The decreased mixing length and decreased velocity difference between the fuel and oxidant streams delay the mixing process and then bring about a stratified distribution of species. This is crucial for the generation of a triple-flame structure and the corresponding high species concentration in the vicinity of the reaction zone under lean conditions. The Damköhler numbers on the lines across the triple points were calculated with values above 1.0 in each of the cases, indicating that the species’ transport time is longer than the chemical reaction time. Therefore, the transport flux of species determines the final combustion strength. The synergistic coupling of the flowfield and species field gives rise to a large transport flux of the key species in the vicinity of the reaction zone. The results show that the corresponding fuel transport flux is large, with values all over . This is the principal reason for the high combustion intensity in this combustor. The different transport fluxes of the critical species are responsible for the different chemical reaction strengths on the triple point in each of the different cases.

Stability and structure of lean swirling spray flames with various degrees of prevaporization

Achieving full premixing and complete evaporation in lean prevaporized premixed combustors is challenging as it depends on the spray injector characteristics, prevaporisation strategy, and flow conditions. This article experimentally explores the stability and structure of a turbulent swirling n-heptane spray flame under various degrees of prevaporization. The results show that preheating the air to 343 K and 393 K has little effect on the lean blow-off velocity, while recessing the fuel injection significantly decreases the lean stability limit. To correlate these limits, various attempts to define a Damköhler number were made, but unlike previous studies with no prevaporisation, the difficulty in defining laminar flame speed in the present case does not allow a single correlation to work for all degrees of prevaporization. Four stable cases that differ in equivalence ratio, air preheat temperature, and fuel injection recess are investigated using one-dimensional PDA, OH* chemiluminescence and CH[Formula: see text]O-planar laser-induced fluorescence (PLIF). Cases without fuel injection recess or air preheat exhibit a conical-shaped heat release zone near the shear layers. Preheating the air to 393 K reduced the Sauter mean diameter, increased prevaporization, and enabled a second heat release zone downstream of the fuel injection. Recessing the fuel injection by 25 mm reduced droplet velocities and led to a semi-spherical instead of a conical heat release zone. The CH[Formula: see text]O-PLIF signal without injection recess was high along the central axis and its distribution resembled that observed for spray jet flames. In contrast, with recessed spray injection, CH[Formula: see text]O was mainly found outside the central recirculation zone and only appeared inside during lean blow-off; similar to previous work with premixed flames. These findings show that different methods of prevaporization, which only differ by subtle changes in droplet characteristics, strongly impact flame stability. The present data can be used for turbulent flame modelling focusing on sprays and finite-rate kinetics.

Aerodynamic and thermo-acoustic stability analysis in a super-steady reactive flow

Aerodynamic and thermo-acoustic stability were systematically analyzed to reveal the underlying cause of the super-steady performance found in a strong rotating reactive flow, which is achieved using a stratified vortex-tube combustor (SVC) employing methanol as fuel. Results show that the SVC possesses a wide stability limit and weak pressure fluctuation with a uniform flame. The lean stability limit is always smaller than 0.2 and the amplitude of pressure fluctuation is generally within 2 kPa, indicating a super-steady combustion process. The large tangential velocity produces a large centrifugal force that suppresses the turbulent motion to promote aero-dynamic stability, yielding a small Froude number. The delay in the mixing process is the crucial reason for the generation of the stratified distribution of species and the triple-flame structure. The large transport fluxes of the species and enthalpy promote the reaction and ignition processes to intensify the combustion, which produces a large density gradient to promote thermo-acoustic stability. The large tangential velocity and density gradient result in a large Richardson number, suggesting the laminarization of the fluid. The momentum flux and its fluctuation decrease distinctly downstream of the vortex-flow because of the strong momentum exchange and transfer in the vortex flow, which weakens the pressure fluctuation in the post-flame zone. The intensified combustion can also increase the burning velocity, and promotes its matching with the flow field. The reduced momentum flux and the increased burning velocity are responsible for good thermo-acoustic stability.

Multi-fuel combustion performance analysis and operating characteristics of a vortex-tube combustor

In this work, an ultra-steady combustion technique was attempted to achieve fuel adaptability combustion. The combustion performances of five representative gaseous fuels in a stratified vortex-tube combustor were investigated in terms of the stability limit, pressure fluctuation, and flame topology. Results show that the lean stability limit of the global equivalence ratio for the five fuels can always be less than 0.15 with a uniform flame front, whilst the amplitude of pressure fluctuations is always below 2300 Pa, indicating a super-steady combustion process. The non-premixed flame structure guarantees a high mass concentration near the reaction zone, whilst the vortex flow also decreases the local flow velocity, inhibiting flame blow-out, and suggesting good self-adjusting capacity under various global equivalence ratios. The synergistic action of the flow and flame structures transports the interior high-enthalpy burnt gas and exterior unburnt gas to the exterior unburnt gas and reaction zones to promote the ignition and reaction procedures, resulting in an intensified combustion. The large tangential velocity and density gradient result in the large values of Richardson number, which indicates that laminarization of the flow arises and results in good aero-dynamic and thermo-dynamic stabilities. The resultant good self-adjusting capacity and three types of dynamic stabilities are the intrinsic causes of the ultra-steady combustion process in this combustor. Ultimately, the generalized criterion of stabilization can be defined by the combination of Richardson and Rayleigh numbers, for which large Richardson and small Rayleigh numbers are required for a highly steady combustion process.

Hydrogen-enriched methane combustion in a swirl vortex-tube combustor

The hydrogen-enrichment combustion performance was investigated numerically by employing methane mixed with different volume proportions of hydrogen, wherein a vortex-tube combustor was employed to achieve a steady combustion process. Results show that this combustion technique indicates good adaptability to hydrogen-enrichment combustion together with an ultra-steady combustion process. The lean stability limit is always within 0.15, which decreases further with the increase of hydrogen content, whilst the amplitude of pressure fluctuation △p is always within 500 Pa with a uniform flame front. The generated non-premixed property flame structure can guarantee a high concentration of components in the reaction zone, which improves the local concentration of species and yields a large stability limit. The strong vortex flow can decrease the local flow velocity to inhibit flame blow-out and yield a large tangential velocity component. The high combustion intensity generates a large density gradient. The large density gradient and large tangential velocity promote the laminarization of the flow, resulting in a large Richardson number Ri* that is always much greater than 1.0. The laminarization of the flow field provides good aero-dynamic and thermo-dynamic stability. Further, the thermo-acoustic coupling is ultra-weak as well, and therein the Ra(x) is always smaller than 0.0024, indicating good flame-dynamic stability. In summary, the resultant good aero-dynamic, thermo-dynamic, and flame-dynamic stabilities of this vortex-tube combustor are the principal reasons for the super-steady hydrogen-enrichment combustion.

The Degradation Map Process – A Tool for Obtaining a Lean Stability Strategy in Drug Development

Stability is fundamental when exploring a drug candidate's potential as a drug product. During the pharmaceutical industry drug development process information regarding stability and degradation are captured in different departments, e.g. from discovery to operations, and will be included in the overall control strategy. With a profound understanding of a drug candidate's degradation chemistry, a science and risk based approach in progressing a lean stability strategy is possible. This case study present a clear and visible concept to facilitate a lean stability strategy by the use of degradation maps and describes a process for how these can be used during drug development. The understanding of possible and/or observed degradation pathways will guide the design of the drug product and stability studies in development. A degradation map displays degradation pathways with short comments on the reaction/mechanism involved. The degradation map process starts with a theoretical degradation map. The map is updated as the drug project progresses, preferably after forced degradation experiments, after compatibility studies and finally when the late stage formulation is set. The degradation map should be used to capture information of intrinsic chemical properties of the active pharmaceutical ingredient (API) and can thereby be used to mitigate stability issues. The map is foremost a cross-functionally available tool collecting and visualizing stability information throughout the development process, and as such a valuable tool to efficiently develop a lean stability strategy.

Design of experiments and multivariate analysis approach to study dissolution stability of a modified-release drug product to support lean design strategies

Objective The purpose of this paper is to present the use of Design of Experiments and multivariate analysis for evaluation of a modified-release drug product stability to support post-approval lean stability approaches. The focus of the paper was to investigate potential root-causes for acceleration of dissolution upon stability. Methods For statistical evaluation of stability data, multiple linear regression statistics was used. The design space of the stability study was modeled using MODDE 12.1 software. For experimental set-up, parameters such as Temperature, Time, Packaging, Batch, and Active Pharmaceutical Ingredient supplier were selected. Results With multiple linear regression modeling of the all generated stability data until six months, we were able to identify or confirm the Stability-related quality attributes and Shelf life limiting attributes. From the multiple linear regression correlation coefficients, we have evaluated that decrease of an antioxidant upon stability could cause potential shift in dissolution. However, main factors for accelerated dissolution can be attributed to other material and process variables. In the last part of the study, we have shown the usefulness of these methodologies for supporting lean stability approaches. With enhanced drug product knowledge, we designed two reduced long-term stability studies and showed that with ‘One-half’ reduced design, we would still be able to confirm 24-month shelf life. Conclusions Implementing Quality by design approaches on stability studies could reduce the need for excessive analytical testing, help to evaluate meaningfulness of the data and set a risk-based stability testing strategy.

Lean Stability Approaches in Pharmaceutical Product Development, Registration, and Post-marketing CMC Change Management; Industry Survey Insights

PurposeThis paper summarizes results from a recent pharmaceutical industry survey to understand the adoption status of science- and risk-based approaches in pharmaceutical stability (lean stability) in regulatory submissions across the full pharmaceutical product life cycle and to ascertain whether companies are deriving their stability strategies based on scientific risk or applying generic strategies.MethodsA comprehensive survey was distributed to pharmaceutical companies affiliated with the International Consortium for Innovation and Quality in Pharmaceutical Development (IQ Consortium) to solicit non-proprietary responses pertaining to regulatory strategies for stability monitoring across the product life cycle, perceived risks, and health authority feedback.ResultsAmong the 19 responding companies, utilization of lean stability approaches varied with respect to (1) phase of product life cycle, (2) specific product quality attributes cited in effective ICH guidances or which demonstrated shelf-life limiting stability trends, and (3) the degree of perceived risk of rejection by health authorities based upon companies’ degree of experience from previous regulatory submissions.ConclusionContinued collaboration between the pharmaceutical industry and health authorities—through case studies and revision/harmonization of regulatory guidances—will be essential for continued implementation of lean stability strategies to accelerate delivery of innovative medicines to the clinic and to the market.

Extended Aging and Marbling Class Effects on Color Stability of Beef Longissimus lumborum, Gluteus medius, and Biceps femoris Steaks

Postmortem aging improves palatability of various muscles, especially those from lower quality grades. This study evaluated postmortem aging and marbling class effects on the color stability of longissimus lumborum, gluteus medius, and biceps femoris steaks. Carcasses were selected at grading to have Lower Small (Small00 to Small50; n = 50) or Upper Slight (Slight50 to Slight90; n = 50) marbling scores. Strip loin and top sirloin subprimals from each carcass side were assigned to aging treatments (14, 21, 28, or 35 d) in an incomplete block arrangement. After aging, subprimals were cut into longissimus lumborum, gluteus medius, and biceps femoris steaks, respectively. Steaks were placed in a simulated retail display for 11 d. Changes in redness (a* and hue angle) were much slower and less extensive (P < 0.001) in longissimus lumborum steaks than in gluteus medius steaks, which had slightly slower and less extensive (P < 0.01) redness changes than biceps femoris. Increasing aging time increased (P < 0.001) the rate and extent of overall color change (ΔE) during simulated retail display. Steaks from Lower Small carcasses had higher (P < 0.01) L* values than steaks from Upper Slight carcasses at 14, 28, and 35 d postmortem. In steaks from Upper Slight carcasses, L* values were lower (P < 0.01) in steaks aged for 28 d compared to other aging times. In steaks from Lower Small carcasses, L* values were highest (P < 0.001) when aged for 14 d. Increased aging time generally decreased (P < 0.05) a*, b*, and chroma values. However, within each aging time, only b* values of steaks aged for 35 d differed (P = 0.01) with regard to marbling class. Results indicate that increasing aging time decreased color life of beef muscles, and that marbling class had minimal impact on lean color stability.

Science and Risked-Based Stability Testing Strategies-a Test Case on the Global Implementation and Regulatory Reception.

The global regulatory implementation and acceptance of a science and risk-based stability (SRB) strategies for a stable drug product is reviewed. This stability strategy may also be referred to as "lean stability" since it includes a stability protocol reduction in the number of tests and time points than might normally be applied. A product line extension (new solid dosage form) for a well-understood product was used as a test case for this stability strategy. The drug product line extension was filed globally with a proposed reduction to analytical stability tests and frequency. This stability protocol is aligned with the International Conference on Harmonization of Technical Requirements for Registration of Pharmaceuticals for Human Use (ICH) Q1A (R2) stability guidelines (ICH: Guideline Q1A(R2), 2003), the World Health Organization (WHO) (WHO INT. [Online], 2020), and most country regulatory guidelines. Although this strategy was not accepted universally, the strategy enabled opportunities to engage in productive dialogs with regulatory authorities in several countries on how the case leveraged product understanding and development knowledge to develop a fit for purpose stability protocol. A persistent and consistent effort from industry to move toward product-specific protocols based on product knowledge and stability risk assessments should lead to further acceptance of science and risk-based stability strategies in all regions.

Lean Stability Case Studies\u2014Leveraging Science- and Risk-Based Approaches to Enable Meaningful Phase Specific Pharmaceutical Stability Strategies

PurposeLean stability is a science- and risk-based initiative which utilizes the enhanced understanding of drug substance and drug product physical and chemical characteristics to (1) reduce and optimize the design of standard stability protocols; (2) expedite the generation of stability data without impact to safety, efficacy, or quality of the product; and (3) decrease time to market for innovative drugs. Lean stability was introduced in the early 2000s [ICH: Guideline Q1A(R2) (2003), ICH: Guideline Q1D (2002)] followed by reduced stability protocols, focusing on the critical quality attributes and critical time points, being reported in the literature [Skrdla et al. (J Pharm Biomed Anal 50: 794–796, 2009)]. While the concept of lean testing is not entirely new, and it is currently a part of several regulatory guidances, it continues to evolve and gain acceptance of the industry and regulators.MethodsIn this review, twelve case studies are presented where stability data was collected during clinical, registration, and post-approval phases of the product development.ResultsCase studies summarize the lean stability testing design, the strategies applied during the regulatory filing and the outcomes of the regulatory filings.ConclusionThe authors expect that the case studies presented in this review will increase the visibility of lean stability, facilitate overcoming of the existing challenges, and accelerate the global regulatory acceptance of lean stability practices in the industry.

Enhancement of the lean stability and blow-off limits of methane-air swirl flames at elevated pressures by nanosecond repetitively pulsed discharges

This paper examines the ability of nanosecond repetitively pulsed (NRP) plasma discharges to improve stabilization and extend the blow-off limit of lean premixed methane-air swirl flames at pressures up to 5 bar. The effect of two discharge regimes, NRP glows and NRP sparks, was investigated. The electrical characterization of the discharges was performed and direct images at 60 Hz of the flames, with and without NRP discharges, were collected to assess the effect of the discharges on flame stabilization. Results showed that NRP discharges efficiently extended the lean blow-off and stability limits of premixed methane-air swirl flames, at pressures up to 5 bar. These results were obtained for a ratio of NRP discharge power to flame thermal power of 0.7% or less. Moreover, the peak voltage necessary to maintain constant this power ratio did not increase linearly with increased pressure, even though the reduced electric field should linearly decrease with the pressure. It was also observed that the relative effectiveness of the NRP glows and NRP sparks changed by increasing the pressure. Based on discharge physics and current knowledge of the effect of pressure on the electrical properties of flames, explanations for these results are proposed.

Optimization of Lambda across the Engine Map for the Purpose of Maximizing Thermal Efficiency of a Jet Ignition Engine

<div class="section abstract"><div class="htmlview paragraph">Progressively more stringent efficiency and emissions regulations for internal combustion engines have led to growing interest in advanced combustion concepts for spark ignition engines. MAHLE Jet Ignition<sup>®</sup> (MJI) is one such concept which enables ultra-lean (λ > ~1.6) combustion via air dilution. This pre-chamber-based combustion system has demonstrated highly efficient lean operation, producing efficiencies competitive with those of advanced compression ignition concepts. Compared to a traditional spark ignition engine, the additional degrees of freedom associated with Jet Ignition introduce further complexity when optimizing the system for peak efficiency throughout the engine map. The relationship between operating condition and the lambda at which peak efficiency occurs for a Jet Ignition engine has been presented in prior work by the authors. This paper further explores this complex relationship, introducing new interdependencies which must be considered in order to improve brake thermal efficiency (BTE).</div><div class="htmlview paragraph">Experimental data is obtained from a 1.5L 3-cylinder gasoline-fueled Jet Ignition engine. An engine map of BTE is presented. An analysis at several key points throughout the engine map shows how certain parameters are adjusted to optimize BTE, providing a peak value >42%. Insight into the factors affecting efficiency is provided through parameter sweeps of pre-chamber fuel and analysis of high-speed pressure measured in both the pre-chamber and the main combustion chamber. The impact of precise fuel metering on BTE and combustion stability is highlighted. Finally, an analysis of trends in burn duration segments, pre-chamber combustion metrics, and cycle-by-cycle variation in these parameters qualifies the challenges associated with accurate prediction of lean stability limits in a jet ignition engine operating in the ultra-lean region, as well as the opportunities for further efficiency improvement through manipulation of in-pre-chamber conditions.</div></div>

Stabilization characteristics and mechanisms in a novel tubular flame burner with localized stratified property

The combustion characteristics in a localized stratified tubular flame burner (LSTFB) were experimentally studied under lean conditions. The stability limit and pressure fluctuation were obtained under various global equivalence ratios and fuel flow rates. The mechanisms driving the combustion instability were analyzed with the combination of numerical simulation. Results show the lean stability limit of the burner can be as low as 0.12 of the global equivalence ratio. On the basis of the local stratification of species, a kind of unique binary tubular flame is formed with both the premixed and non-premixed flame properties. Meanwhile, the internal recirculation heats the incomplete combustion gases distributed inside the tubular flame, thereby guaranteeing the balance between the flow velocity and the local flame speed. The amplitudes of pressure fluctuation are less than 4 kPa in the entire experimental range because of the flow laminarization caused by the large body force and density gradient. The pressure fluctuation characteristics depend on the relative magnitude of viscous force, inertial force, body force, and density gradient. The Taylor number Ta and stratification parameters Ri∗ are key parameters in evaluating the flame stability in the LSTFB.

Evaluating Ground Beef Formulated with Different Fat Sources

Objectives were to evaluate effects of fat source and formulated fat percentage on fatty acid composition, lean color stability, lipid oxidation, and aerobic microbial load during simulated retail display of ground beef patties. In Experiment 1 beef carcasses (n = 30) were chilled for 2 d and then fabricated. M. semimembranosus muscles were removed along with 2 fat sources (kidney and pelvic = KP and subcutaneous = S from the same carcass) and ground to achieve 75 and 95% lean. Fatty acid profile and thiobarbituric acid reactive substances (TBARS) were determined over a 7 d simulated retail display period. Saturated fat differed (P = 0.0004) by fat source, with KP having a higher percentage than S. Thiobarbituric acid reactive substances were higher for (P < 0.05) for patties made with S compared to KP. In Experiment 2 beef carcasses (n = 20), were fabricated and blended into ground beef as described in Experiment 1. After designated display time patties were removed and instrumental color measurement, myoglobin concentration, TBARS, and aerobic plate counts (APC) were collected. Oxymyoglobin (OMb) percentage decreased (P < 0.0001) by storage day and had source x formulated fat percentage interaction (P = 0.011). Inversely, storage day increased metmyoglobin (MMb) percentage (P < 0.0001) where d 1 < d 3 < d 5 < d 7, respectively. Changes in myoglobin form contributed to decreased a* values (P < 0.0001) over time. However, APC did not differ (P > 0.05) for d, fat source, or fat percentage. Discoloration in ground beef over 7 d of retail display was more a function of muscle pigment oxidation (OMb to MMb) than aerobic microbial spoilage.

Surface stabilized combustion technology: An experimental evaluation of the extent of its fuel-flexibility and pollutant emissions using low and high calorific value fuels

Surface-stabilized combustion (SSC) is a fuel-flexible technology that extends the stability limits of lean premixed systems, while achieving ultra-low emissions of NOx, CO and UHC (unburned hydrocarbons). To evaluate these attributes, the present study quantifies (1) operability characteristics i.e., lean blowoff limit and flashback behavior and (2) pollutant emissions (CO, N2O, NH3, NO and NO2) of a commercial SSC burner when operating at a fixed fire rate on a wide range of fuel compositions. The experimental burner is the Duratherm™ ceramic-fiber burner by Alzeta Corporation. Experiments were carried out with multiple gaseous fuel blends starting with low reactivity biogas blends containing up to 65% CO2 (by volume) balanced with natural gas, and high reactivity fuels such as hydrogen enriched natural gas (HENG) blends. The burner was able to stabilize multiple fuel compositions without requiring any modifications, which attests to the inherent fuel-flexibility of the technology. It was found that the modes of operation and flame stabilization of the SSC burner can be parametrized by the ratio of the speed of the mixture at the surface to its laminar burning velocity. The results affirm low emission operation is attainable on all fuels. In terms of nitrogen species other than oxides of nitrogen, the study found trace levels of N2O and NH3 (<0.1 ppm) emitted during steady state operation. However, the emission of those species can become significant during ignition and near the lean stability limit.

Catalytic Influence of Water Vapor on Lean Blow-Off and NOx Reduction for Pressurized Swirling Syngas Flames

It has become increasingly cost-effective for the steel industry to invest in the capture of heavily carbonaceous basic oxygen furnace or converter gas, and use it to support the intensive energy demands of the integrated facility, or for surplus energy conversion in power plants. As industry strives for greater efficiency via ever more complex technologies, increased attention is being paid to investigate the complex behavior of by-product syngases. Recent studies have described and evidenced the enhancement of fundamental combustion parameters such as laminar flame speed due to the catalytic influence of H2O on heavily carbonaceous syngas mixtures. Direct formation of CO2 from CO is slow due to its high activation energy, and the presence of disassociated radical hydrogen facilitates chain branching species (such as OH), changing the dominant path for oxidation. The observed catalytic effect is nonmonotonic, with the reduction in flame temperature eventually prevailing, and overall reaction rate quenched. The potential benefits of changes in water loading are explored in terms of delayed lean blow-off (LBO), and primary emission reduction in a premixed turbulent swirling flame, scaled for practical relevance at conditions of elevated temperature (423 K) and pressure (0.1–0.3 MPa). Chemical kinetic models are used initially to characterize the influence that H2O has on the burning characteristics of the fuel blend employed, modeling laminar flame speed and extinction strain rate across an experimental range with H2O vapor fraction increased to eventually diminish the catalytic effect. These modeled predictions are used as a foundation to investigate the experimental flame. OH* chemiluminescence and OH planar laser-induced fluorescence (PLIF) are employed as optical diagnostic techniques to analyze changes in heat release structure resulting from the experimental variation in water loading. A comparison is made with a CH4/air flame and changes in LBO stability limits are quantified, measuring the incremental increase in air flow and again compared against chemical models. The compound benefit of CO and NOx reduction is quantified also, with production first decreasing due to the thermal effect of H2O addition from a reduction in flame temperature, coupled with the potential for further reduction from the change in lean stability limit. Power law correlations have been derived for change in pressure, and equivalent water loading. Hence, the catalytic effect of H2O on reaction pathways and reaction rate predicted and observed for laminar flames are appraised within the challenging environment of turbulent, swirl-stabilized flames at elevated temperature and pressure, characteristic of practical systems.

Emissions and stability performance of a low-swirl burner operated on simulated biogas fuels in a boiler environment

This paper addresses the experimental and numerical modeling of NOx emissions and lean blow off (LBO) stability limits of natural gas and biogas fuels reactions stabilized with a low swirl burner (LSB). The paper presents the methodology to set up a chemical reactor network (CRN) based on experimental results and computational fluid dynamics (CFD) simulations. The CRN is a simplified representation of the fluid dynamics and energy balance of the reactive gases in the boiler environment. The CRN uses a combination of perfectly stirred reactors (PSRs) and plug flow reactors (PFRs) to couple the fluid dynamics with detailed reaction kinetics. By analyzing the CFD and CRN results, it is possible to gain insight into the relationship between internal flue gas recirculation, heat losses, geometric variables and fuel composition on the emission of NOx and the stability of the reactions. In this study, three injector configurations featuring different quarl expansion strategies were considered. Two nozzles were tested experimentally, whereas the third configuration represents an additional hypothetic design used to show the capabilities of the methodology. The numerical results are in good agreement with the experimentally observed trends and show direct relation between the emission of NOx and the lean stability limits. The quarl expansion plays a key role in the interaction of the premixed reactants and recirculated gases within the boiler environment. By varying this interaction, it is possible to attain acceptable tradeoffs between NOx emissions and burner stability. In general, the results demonstrate the ability of this design methodology to efficiently estimate burner performance and evaluate design modifications to accommodate varying fuel compositions.

Dissociative influence of H2O vapour/spray on lean blowoff and NOx reduction for heavily carbonaceous syngas swirling flames

Recent studies have described and evidenced the enhancement of fundamental combustion parameters such as laminar flame speed due to the catalytic influence of H2O with heavily carbonaceous syngas mixtures. In this study, the potential benefits of these subtle changes in water loading and hence reaction pathways are explored in terms of delayed lean blowoff, and primary emission reduction in a premixed turbulent swirling flame (Ø = 0.6–0.8), scaled for practical relevance. Chemical kinetic models initially confirm that H2O has a substantial impact on the employed fuel behaviour; increasing flame speed by up to 60% across an experimental range representative of fluctuation in atmospheric humidity (∼1.8 mol%). OH* chemiluminescence and OH planar laser induced fluorescence (PLIF) were employed to analyse the changes in heat release structure resulting from the experimental addition of H2O vapour to the combustor. Equivalent concentrations of liquid H2O were introduced into the central recirculation zone of the premixed flame as an atomised spray, to investigate the influence of phase changes on the catalytic effect. Near the lean stability limit, H2O addition compresses heat release to shorten the elongated flame structure. Whereas with a stable and well-defined flame structure, the addition triggers a change in axial heat release location, causing the flame front to retract upstream toward the burner outlet. Higher quantities of two-phase flow were combined to explore the possibility of employing the spray as a stabilising mechanism, effectively dampening the observed influence of humidity. The chemical enhancement induced by the controlled supply was shown to reduce the lean blowoff stability limit, enabling an increase in additional air flow of almost 10%. However, the catalytic effect of H2O diminishes with excessive supply and thermal quenching prevails. There is a compound benefit of NOx reduction from the use of H2O as a flame stabiliser with the practically-relevant syngas: First NOx production decreases due to thermal effect of H2O addition, with potential for further reduction from the change in lean stability limit; leanest experimental concentrations reduced by up to a factor of four with two-phase flow at the highest rates of supply. Hence, the catalytic effect of H2O on reaction pathways and reaction rate predicted and observed in the laminar environment, is shown to translate into practical benefits in the challenging environment of turbulent, swirl-stabilised flames.