Internal Recirculation Research Articles

Numerical Simulation of NO x Formation in Non-Premixed Methane Combustion Using Internal Flue Gas Recirculation

ABSTRACT Internal flue gas recirculation (IFGR) is one of the most effective techniques for NO x reduction in boilers. Previous studies mainly focused on premixed combustion; however, the NO x formation principles in non-premixed methane/air flames are still unclear. This work aims to investigate the NO x formation mechanisms in non-premixed methane combustion at various IFGR rates. A series of three-dimensional computational fluid dynamics (CFD) simulations with detailed mechanisms was conducted. The results show that NO x is generated below 30 mg/m3 for IFGR rates higher than 17.7%, corresponding to the oxygen concentration in the oxidant mixture below 0.15. In high IFGR rates, the incompletely mixed oxidants and the presence of the central recirculation zones weakens the NO x reduction performance. Influenced by the decreased temperature by IFGR, the generation of thermal NO x is largely eliminated from 66 to 1 mg/m3. Prompt NO x contributes more than thermal NO x for the IFGR rates above 12.2%. The reaction range of thermal NO x is found moving upstream toward the nozzle with the axial distance of 0.1 m at most. IFGR gradually reduces prompt NO x but its spatial reaction ranges are hardly influenced. IFGR has no obvious effects both on the reaction rates and ranges on the minorities of the N2O and the NNH routes. This work connects the IFGR rates and the NO x formation pathways in non-premixed methane combustion, and highlights the importance of eliminating prompt NO x to achieve low-NO x combustion rather than sole NO x suppression method, which can provide foundation for designing low-NO x techniques.

Optimized cabinet parameters for drying lithium-ion batteries based on coupled fluid–thermal field analysis

Hot-airflow desiccation is a commonly applied technique for drying lithium-ion batteries. However, most drying cabinet designs currently suffer from poor efficiency because they evacuate steam by ejecting the hot air in the cabinet to the open air continuously. This can be addressed by closing and opening the cabinet periodically, where the temperature of the heating zone is increased as quickly as possible through internal air recirculation in the closed position, and the steam is ejected with the hot air only during the open period. Nonetheless, drying cabinet designs of this nature have been rarely subjected to numerical analysis based on computational fluid dynamics and heat transfer, and the design factors enhancing the rate of temperature increase during the closed period remain poorly understood. The present work addresses these issues by outlining a detailed numerical approach for simulating the airflow temperature of a drying cabinet during internal air recirculation in its closed position, and the characteristics of the airflow and the temperature distribution in the dryer are evaluated via transient fluid–thermal coupling analysis. The results of the numerical investigation indicate that the heating efficiency is substantially influenced by the inlet airflow velocity, the distance between the trays holding the batteries, and the size of the free space between the front door and the trays. The results demonstrate that the developed model provides a useful means of evaluating and optimizing the heating efficiency of these types of drying cabinets.

Characterizing Swirl Strength and Recirculation Zone Formation in Tangentially Injected Isothermal Flows

This paper presents a computational study characterizing the swirl intensity distribution and Internal Recirculation Zone (IRZ) formed in a cylindrical domain with tangential injections and isothermal flow. The range of inlet boundary conditions investigated is 5o to 25o for the injection angle and 7190 to 100711 for the bulk flow Reynolds number. The evolution of swirl intensity is presented with and without incorporating effects of the accompanying pressure variations. The Shear Stress Transport (SST) k-ω is used to model turbulence. Results show that the Swirl strength created by such tangential injections strongly depends on the injection angle but does not vary with bulk flow Reynolds number (Re), except for low Re values. The swirling flow is shown to result in IRZ formation at injection angles 6o and above or when asymptotic value of the maximum Swirl Number in the domain exceeds approximately 0.6, same as the transition value of inlet Swirl Number in swirling flows with axial injections. The IRZ length increases with injection angle and varies with Re for lower values of Re at a given injection angle but asymptotes for higher values above 40000. The conventional Swirl Number rises rapidly downstream of the injection plane followed by a slow decline. On the other hand, an alternative Swirl Number, which incorporates the gauge pressure variation, shows slow and consistent decay all the way downstream of the injection plane. The Swirl Number incorporated with gauge pressure term subsumes interconversions between the axial momentum and pressure in the regions of vortex breakdown and IRZ formation, thereby presenting an alternative picture of swirl intensity evolution in swirling flows.

Efficient organic carbon utilization for combined nutrient removal and biogas production in hybrid biofilm activated sludge system

Moving bed biofilm technology is a successful and promising biological treatment process considered for cost-effective management of wastewaters. In this study, a hybrid moving bed biofilm pilot plant was operated and obtained results were compared with simulation results of two biological processes: a conventional Biological Nutrient Removal (BNR) system and a novel hybrid activated sludge configuration implemented with Moving Bed Biofilm Reactor (MBBR) and a reactive primary clarifier (RPC). An efficient organic carbon entrapment (87%) was obtained in reactive primary clarifier where the major component (95%) of the total Chemical Oxygen Demand (COD) was determined as slowly hydrolysable (XS1), according to respirometric tests. Pilot plant and simulation studies exhibited that an improved nitrogen and phosphorus removal performance, as well as higher biogas production (47%) were obtained by organic carbon redirection in proposed hybrid configurations, securing a 40% less reactor volume. Investigation of the economic aspects indicated that hybrid system has a lower operating cost (i.e., 23% less air requirement, 40% less mixing energy requirement), does not require internal recirculation pumps, and produces more energy through anaerobic digestion compared to conventional BNR. Therefore, proposed novel hybrid configuration is more efficient for the treatment of municipal wastewater in terms of Capital Expenditures (CAPEX) and Operational Expenditures (OPEX) within the energy self-sufficient treatment plant approach and sustainable treatment plant operation.

Estimation of the degree of internal recirculation in Internal Combustion Engines

The concepts of residual gas and admission (or volumetric efficiency) coefficients are used exclusively by specialists in the field of piston Internal Combustion Engines. However, it is preferable to apply the concepts of volume fractions of components of the working mixture consisting of air, fuel, residual, and recirculating gases, for the evaluation of filling. This simplifies and makes it more easy-to-grasp the influence of individual factors on the results of gas exchange processes. The proposed approach makes it possible to take into account the impact on the engine indicators of the molecular weight of the fuel used and the degree of external recirculation, as well as to reduce the number of independent variables. At the same time, the displacement coefficient A proposed by the author characterizes a decrease in filling when an engine with external mixing is switched to a gaseous fuel with a lower molecular weight. A change in the valve timing made it possible to produce an effect on the composition of the working mixture and, thereby, the environmental characteristics of the engine. In the case of external recirculation, it becomes necessary to estimate the summary fraction of neutral combustion products in the working mixture, on which all engine operation depended. This "overall degree of recirculation" can also be determined using the proposed approach.

Nickel removal from an industrial effluent by electrocoagulation in semi-continuous operation: Hydrodynamic, kinetic and cost analysis

In the nickel and cobalt production plant located in Punta-Gorda Cuba, an effluent liquor is obtained with several pollutants species of Ni(II)–NH3–CO2–SO2–H2O system. In previous studies the performance of electrocoagulation (EC) for the nickel removal from this effluent was demonstrated on a lab-scale and bench-scale. In this research, geometric, electrical and equilibrium parameters used as criteria for scaling-up the EC process were evaluated through hydrodynamic, kinetic and operating cost analysis. For this purpose, four semi-continuous stirred-tank electrochemical reactors of equal sized with 200 L useful volume were design. Based on the residence time distribution (RTD) curve, the hydrodynamic behavior of EC systems followed multi-branch tanks-in-series model. Experimental mean residence time was of 92 min and hydraulic efficiency 92% for a liquor flow rate 2 L∙min−1, due to the presence of short-circuit, internal recirculation and 3% dead zone. Then, at the operating parameters: nickel concentration 277 ± 52 mg L−1, current density 6.26 mA cm−2, pH 8.40 ± 0.28 and temperature 50 ± 2 °C, the maximum nickel removal was 97.8% with an operating cost $1.008 kg−1 Ni. Combination of RTD model and the conversion time kinetic model (CTV) was presented to predict the removal efficiency with successful outcomes, and the time for complete conversion was 23.3 min under control of chemical reaction. The precipitate presented a nickel concentration of 35.71 ± 2.13%, aluminum 6.05 ± 0.83%, true density 2836 ± 449 kg m−3 and typical layered structure. These results suggest the opportunity for the projection of an industrial production unit of double lamellar hydroxide and reduce the adverse environmental impact.

E-POD investigations of turbulent premixed flame dynamics approaching lean blow-out conditions

Thanks to the continuous computational power increase, the use of high-fidelity computational fluid dynamics (CFD) simulations is nowadays customary, especially in the gas turbines design process. The extraordinary temporal and spatial detail of such analyses generate large datasets, which must be carefully studied to correlate different quantities and gain information to characterize the behavior of combustor designs. Several advanced post-processing tools have been proposed; however, the Extended-POD (E-POD) holds the greatest potential for turbulent combustion applications when the mutual influence of different quantities is the main goal of the investigation. The present work investigates the application of the E-POD to an LES model of a perfectly-premixed, swirl-stabilized, methane-air flame approaching Lean-Blow-Out. Leveraging the validation against the experimental data at two different operating conditions on a laboratory test case, the numerical model has been used to collect several quantities of interest for shedding light on the flow-flame interaction near the blow-out. The post-processing algorithm has been used to highlight the differences between two conditions approaching the extinction at distinct air-flow velocities. It has been found that, when the burner is operated with a higher velocity, the flame is subjected to a cyclic low-frequency breakdown around the internal recirculation zone, leading to an ingestion of cold products from the external parts of the combustor toward the center. Although other local effects acting on the flame brush have been found in both conditions, they are related mainly to higher order coherent structures with a lower energy content. As a result, their impact onto flame stability is found to be of secondary importance since their limited interaction with flame stabilization. The work shows that E-POD represents a powerful tool for investigating the key features of flame dynamics even at near-blow-out conditions, constituting a valid algorithm for interpreting the results of CFD analyses on gas turbines combustors.

High‐Speed and Low‐Energy Actuation for Pneumatic Soft Robots with Internal Exhaust Air Recirculation

Multichamber soft pneumatic actuators (m‐SPAs) are widely used in soft robotic systems to achieve versatile grasping and locomotion. However, existing m‐SPAs have slow actuation speed and are either limited by a finite air supply or require energy‐consuming hardware to continuously supply compressed air. Herein, these shortcomings by introducing an internal exhaust air recirculation (IEAR) mechanism for high‐speed and low‐energy actuation of m‐SPAs are addressed. This mechanism recirculates the exhaust compressed air and recovers the energy by harnessing the rhythmic actuation of multiple chambers. A theoretical model to guide the analysis of the IEAR mechanism, which agrees well with the experimental results, is developed. Comparative experimental results of several sets of m‐SPAs show that the IEAR mechanism significantly improves the actuation speed by more than 82.4% and reduces the energy consumption per cycle by more than 47.7% under typical conditions. The promising applications of the IEAR mechanism in various pneumatic soft machines and robots such as a robotic fin, fabric‐based finger, and quadruped robot are further demonstrated. An interactive preprint version of the article can be found at: https://doi.org/10.22541/au.166428178.80668101/v1.

Experimental investigation of simultaneous hydrogen production and desalination via electrodialysis process

The primary purpose of the electrodialysis (ED) process is to desalinate saline water; besides this, hydrogen, a valuable energy carrier gas, is generated at the cathode side of the ED cell in this study. However, hydrogen gas reacts with oxygen gas and reforms water due to the internal recirculation of the electrolyte solution from the anode to the cathode compartment. This study investigates the potential of hydrogen gas generation in an ED system that desalinates sodium sulfate (Na2SO4) solution. The maximum conductivity removal ratio is 98.1% when ion-exchange resins are placed into the diluate compartment at pH 8, and the ED system's hydrogen production rate is 76.8 mg H2/h·kg Na2SO4. The maximum hydrogen production rate is achieved as 118.8 mg H2/h·kg Na2SO4 solution under 14.06 mA/cm2 current density. The maximum energy and exergy efficiencies are calculated as 25.29% and 28.78%, respectively. The results indicate that the conventional ED process makes hydrogen production possible while desalinating saline water.

Improving the asymmetric phenomenon effects on the combustion characteristics in an opposed-fired pulverized coal boiler

Numerical investigations were performed on an opposed-fired boiler to better understand the formation of the asymmetric phenomenon and its effects on the combustion characteristics. It is revealed that with the increasing of the inlet Reynolds numbers, the in-furnace flow and temperature fields evolve from symmetry to asymmetry under the symmetric combustion conditions. Nonlinearity was elaborated to explain this phenomenon. Three asymmetric combustion modes were proposed to optimize the deflected flowfield. The final optimal combustion pattern was established via comprehensive consideration of the general air stoichiometric ratio and reduction zone heights. Compared to the original symmetric combustion mode, asymmetric air disturbance creates better internal flowfield recirculation and improves the combustion performance, which obtains a significant NOx reduction from 369.16 mg/Nm3 to 216.16 mg/Nm3 and lowers the carbon content in fly ash from 3.465% to 2.07%, respectively.

Numerical Simulation of the Combustion Characteristics in a Flue Gas Internal Recirculation Burner

The combustion characteristics and NOx emissions of a newly designed flue gas internal recirculation low-NOx burner (FIR) were studied. In the study, experimental and numerical simulations of the FIR low-NOx burner were conducted under natural inlet air conditions at three different powers. Results show that the fuel inlet strongly influences the jet effect, thus influencing the flue gas recirculation rate, flame stability, and NOx emissions. With a medium power of 20 kW, the NOx emission of a FIR low-NOx burner is lower than 30 mg/N m3. Higher or lower power will increase the NOx emissions or induce combustion instability. The swirling flow and bluff body structure can effectively improve the combustion stability. Three dominant frequencies of 54, 264, and 448 Hz can be observed from the power spectral densities of axial velocity, corresponding to the shedding vortex in the shear layer, the swirl frequency of processing vortex core (PVC), and the vortex induced by the PVC structure, respectively. The influences of vortex shedding and PVC structure are weak and inadequate to affect the overall flame stability.

Characteristics of Premixed Flames of Hot and Diluted Mixtures

ABSTRACT Internal recirculation of product gases and mixing with fresh reactants, to form a hot and diluted combustible mixture, has been demonstrated to result in lower pollutant emissions. With sufficient amount of product gas recirculation, the mixture can reach above its auto-ignition temperature with low oxygen mass fractions (<4 wt%), resulting in distributed reactions instead of a flame. Some of the technologies with this feature are Moderate or Intense Low oxygen Dilution (MILD) combustion, FLameless OXidation (FLOX) and Colorless Distributed Combustion (CDC). However, if the amount of product gas recirculation is not enough and the auto ignition condition is not reached, premixed reactants of the hot and diluted mixture can be established. In this study, characteristics of such hot and diluted mixtures is studied which include the ignition delay time and the flame attributes such as laminar flame speed, S L and thermal flame thickness, . A perfectly stirred reactor (PSR) using the hot and diluted mixture as the inlet was further examined to simulate the main combustor and combustion efficiency η, and CO and NOx emissions were obtained. The investigations are performed at different product gas recirculation amounts , equivalence ratios ϕ and operating pressures P, using chemical kinetic simulations. The thermo-chemical state of the recirculated product gases was obtained using a PSR with varying residence times (τ) to simulate different “burntness” levels (smaller τ simulates lower “burntness” level). The hot and diluted mixture temperature is higher, while oxygen and fuel mole fraction are lower for higher ξ, ϕ, P, and τ. Radical species’ mole fraction is higher for higher ξ, ϕ, and lower P, τ. Lower is observed for higher ξ, ϕ, P and smaller τ. Higher S L is observed for higher ξ, ϕ and lower P. Lower is observed for higher ξ, ϕ and P. S L and are found to be higher or lower with variation in τ, depending on ξ, ϕ and P, owing to variation in temperature and radicals’ concentration. Higher η is observed for higher ξ, P, lower ϕ, τ, and higher residence time of gases (τ reac ) inside the main combustor. Lower CO is observed with increase in ξ, P and decrease in τ, while exhaust NOx increases with ϕ and mostly remains unchanged with increase in ξ, while inconsistent trends are observed with change in P and τ.

MILD Combustion of Methanol, Ethanol and 1-Butanol binary blends with Ammonia

In the present energy transition scenario, ammonia is considered a valuable candidate as energy-dense carrier with neutral or even negative carbon balance. However, the potential high NOx emissions and the reduced oxidation process stability, at least when conventional combustion plants are used, can burden its wide utilization on large scales. In this context, MILD Combustion, due to its inherent characteristics, may greatly improve combustion stability and keep the NOx emissions at an acceptable level. On the other hand, the addition of low or no-carbon fuels from biomasses and wastes, more reactive than ammonia, may be beneficial in further improving its combustion performance and the global sustainability of the energy supply chain.In this respect, the present work analyzes the sustainability and combustion performance of binary mixtures of ammonia and low-molecular-weight alcohols in a cyclonic burner, where MILD conditions are attained by means of a strong internal recirculation, and compares them with those obtained with NH3/methane blends. Results highlighted that NH3/alcohols mixtures ensure a stable oxidation process in a wide range of operational parameters without compromising the system performance. Moreover, they showed a significant reduction of NOx emissions for NH3/alcohols mixtures, especially for fuel-lean conditions, when compared to NH3/methane blends.Experimental data were also corroborated by chemical kinetic modeling results to provide some insights on the peculiar NOx formation routes when blends of different nature are used, highlighting the interaction between carbon and nitrogen fuels kinetics.

Flamelet LES of a 40 kW[formula omitted] pulverized torrefied biomass furnace in air and oxy-fuel atmospheres

For the first time, large-eddy simulations are conducted for a 40 kWth self-sustained pulverized torrefied biomass furnace in air and oxy-fuel atmospheres using an extended flamelet model. A recently developed chemical reaction mechanism specifically for oxy-fuel combustion (117 species and 840 elementary reactions) is employed. The simulation results are compared to the experimental data, including the axial and tangential velocities and selected species mass fractions. In addition to this, certain characteristics are investigated: the flame structure, thermo-chemical variable distributions and the NOx emissions in different atmospheres. The flame in the oxy-fuel atmosphere is predicted to be longer and broader than that in the air atmosphere. The main reason for this is the significantly different flow dynamics in different atmospheres that are specifically designed to achieve the same oxidizer-to-fuel ratio near the burner. Two external recirculation zones (ERZs) and a central internal recirculation zone (IRZ) exist in the air atmosphere due to the designed boundary condition with a strongly swirled flow. In contrast, only a single ERZ and a weak IRZ can be observed in the oxy-fuel atmosphere. These different flow dynamics significantly influence the torrefied biomass combustion characteristics for the two atmospheres. Overall, the combustion characteristics in the air atmosphere exhibit features which are expected for a strongly swirling flow, while the thermo-chemical variable distributions in the oxy-fuel atmosphere are similar to those of a jet flame. It is also found that the atmospheres have significant effects on the NOx emissions, particularly for the furnace studied, which features staged oxidizer combustion.

Flame characterisation of gas-assisted pulverised coal combustion using FPV-LES

A multiphase flamelet/progress variable (FPV) model for the large eddy simulation (LES) of gas-assisted pulverised coal combustion (PCC) is developed. The target of the simulation is the Darmstadt turbulent gas-assisted swirling solid fuel combustion chamber. The coal particles are treated as Lagrangian point particles, the position, momentum and energy of which are tracked. The gas phase is described by the low-Mach Navier-Stokes equations alongside the Eulerian transport equations of the governing variables for the FPV model. The set of chemical states of the PCC flame is pre-tabulated in a six-dimensional flamelet table and determined by the mixing of the primary fuel stream, volatiles and char off-gases with the oxidising air, the progress of chemical reactions, the interphase heat transfer, as well as sub-grid scale variations. A presumed β-PDF approach for the total mixture fraction is applied to capture sub-grid scale effects. The discrete ordinate method (DOM) with the weighted sum of grey gases model (WSGGM) is employed to model radiation. The FPV-LES results are validated against the experimental evidence and a good agreement of the predicted mean and RMS velocities, as well as the mean gas temperature between experiments and simulations is obtained. The contributions of the pilot, volatile and char off-gas fuel streams to the coal flame are analysed. It is found that most regions of the furnace are dominated by either pilot or volatile combustion, while char conversion only occurs in the far downstream and outer furnace regions. The pilot gas dominates the near-wall region inside the quarl, whereas the volatile gas mainly released from small particles dominates a first volatile combustion zone in the interior of the internal recirculation zone. Larger particles heat up more slowly and release their volatile content further downstream, leading to a secondary volatile combustion zone.

Enhanced NOx reduction performance in the I-type radiant tube by combining a novel triple air-staging with flue gas recirculation

In this study, a novel triple air-staged combustion (ASC) with internal flue gas recirculation (FGR) was proposed to further achieve low NOx emissions and high combustion efficiency in the I-type radiant tube (RT). The effects of air vent structural parameters, including secondary air vent gap d (0, 1, 2, 3 mm) and diameter D (2, 4, 6, 8 mm) of tertiary air vents, on NOx emission and thermal efficiency were analyzed by numerical simulation. The results showed that NOx emissions increased from 33.7 to 66.0 mg/m3 as the secondary air vent gap changed from 0 to 3 mm. The thermal efficiency was higher than the original structure only at d=3 mm. When the tertiary air vent diameter increased from 2 to 8 mm, NOx increased from 49.3 to 180.4 mg/m3 and the thermal efficiency changed with a slight variation. On the premise of higher thermal efficiency than the original structure, the NOx at D=2 mm reached the minimum value of 49.3 mg/m3 under all optimized schemes, with a 64.0% reduction.

Combustion of lean ammonia-hydrogen fuel blends in a porous media burner

Ammonia offers a carbon-free fuel alternative for industrial heating, power generation and transportation. However, its low flame speed and high NOx emission represent significant challenges for combustor applications. The present study investigates the use of porous media burners (PMBs) to stabilize lean NH3/H2/air flames. By stabilizing the flame at the interface between two thermally conductive ceramic foams, internal heat recirculation preheats the reactants and increases the flame speed, which increases the combustion stability limits to very lean NH3/H2/air mixtures. Experimental studies are conducted to determine the stability limits of porous media combustion over a wide range of NH3/H2 volume ratios, equivalence ratios and mass fluxes. The NH3/H2 ratio is found to play a significant role with a typical dependency of the equivalence ratio at extinction in the form ϕext∝XH2−0.3. NOx emissions were shown to reach their minimum when the burner was operated near its extinction limit and to increase with increasing mass flux rates. Because unburnt NH3 emissions were also quite low (1400 ppm) at these very lean conditions, this operating regime offers similar or better emissions compared to the commonly employed fuel-rich operation which has been suggested in the literature.

The Influence of Flow on Cycle-to-Cycle Variations in a Spark-Ignition Engine: A Parametric Investigation of Increasing Exhaust Gas Recirculation Levels

Cyclic variability is investigated in an optically accessible single-cylinder spark-ignition research engine by introducing artificial exhaust gas in controlled amounts to the homogenous air–fuel mixture before ignition. A skip-fire scheme ensures the absence of internal exhaust gas recirculation (EGR) and allows the engine to be fired continuously for acquisition of large statistics. Four operating conditions ranging from a stable 0% EGR case up to a highly unstable extreme EGR case are analyzed to examine the increasing effects of homogeneous EGR on the cycle performance. To that end, high-speed measurements of the velocity field via particle image velocimetry and flame imaging in the tumble plane allow the determination of phenomena leading to various flame positions and sizes as well as faster and slower combustion cycles. Through extensive conditional statistical and multivariate correlation techniques, flames are found to be heavily influenced by large-scale velocity motion, especially with the presence of greater EGR which leads to lower flame speeds. The greater sensitivity of slower flames to variations in the velocity field manifests itself in an exponential increase in cyclic variability of the maximum in-cylinder pressure and causes misfire cycles where the flame is blown off or quenched at the cylinder roof. In the most extreme cycles at the highest EGR level, the state of the large-scale velocity structures at the time of ignition determines whether the flame propagates towards the center of the cylinder (and is blown off or quenched) or if the flame sustains growth by propagating within the lingering tumble vortex.

Production of hydrogen from beverage wastewater by dark fermentation in an internal circulation reactor: Effect on pH and hydraulic retention time

• Interaction and the RSM should be performed to determine the optimal zone for H 2 . • Interaction between factors established that the pH was main factor in the operation. • Distribution of VFAs and multivariate statistics allow an integral analysis. Energy demand currently is mostly satisfied by the use of fossil fuels that present not only problems for the environment, but also are not renewable. Dark fermentation (DF) is a biological process that can provide an alternative to meet energy needs. The use of industrial effluents from carbon-rich waters in the production of hydrogen and volatile fatty acids (VFAs) through the DF is a promising strategy. However, determining the optimal operating conditions to increase the production of the sub-products has been not being thoroughly studied. This study aims at determining the optimal condition of hydraulic retention time (HRT) and pH to improve hydrogen production in an internal recirculation (IC) reactor treating beverage wastewater using Response Surface Methodology and interaction between factors. The study also assessed VFAs concentrations when operating at optimal conditions. The results showed that the combination of an 8.0 h HRT and 5.5 produced 30% of hydrogen and the interaction between factors established that the pH was the main factor influencing the results. Also, it was observed that the concentration of lactic acid did not inhibit the production of H 2 for the optimal value of pH. However, it is evident that more studies using HRT of less than 8 h and of VFAs distribution are needed. Future optimization technique considerations are necessary to reduce uncertainties for biological hydrogen production purposes.

Biogas bioconversion into poly(3-hydroxybutyrate) by a mixed microbial culture in a novel Taylor flow bioreactor

Biogas-based biopolymer production represents an alternative biogas valorization route with potential to cut down plastic pollution and greenhouse gas emissions. This study investigated for the first time the continuous bioconversion of methane, contained in biogas, into poly(3-hydroxybutyrate) (PHB) by a mixed methanotrophic culture using an innovative high mass-transfer Taylor flow bioreactor. Following a hydrodynamic flow regime mapping, the influence of the gas residence time and the internal gas recirculation on CH4 abatement was assessed under non nutrient limiting conditions. Under optimal operational conditions (gas residence time of 60 min and internal gas recycling ratio of 17), the bioreactor was able to support a CH4 removal efficiency of 63.3%, a robust CH4 elimination capacity (17.2 g-CH4 m-3h-1) and a stable biomass concentration (1.0 g L-1). The simultaneous CH4 abatement and PHB synthesis was investigated under 24-h:24-h nitrogen feast/famine continuous operation. The cyclic nitrogen starvation and the Taylor flow imposed in the bioreactor resulted in a relatively constant biomass concentration of 0.6 g L-1 with PHB contents ranging from 11 to 32% w w-1 (on a dry weight basis), entailing an average PHB productivity of 5.9 g-PHB m-3 d-1 with an associated PHB yield of 19.8 mg-PHB g-CH4-1. Finally, the molecular analysis of the microbial population structure indicated that type II methanotrophs outcompeted non-PHB accumulating type I methanotrophs, with a heterotrophic-methanotrophic consortium enriched in Methylocystis, Hyphomicrobium, Rubinisphaeraceae SH PL14 and Pseudonocardia.