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The effect of hydrocarbon addition on tip opening of lean and stoichiometric hydrogen-air flames is studied computationally by performing two-dimensional numerical simulations. The numerical study reveals that the flame tip of the H2-air burner stabilized flame is open at lean and stoichiometric mixture conditions. The flame tip closes upon hydrocarbon addition. The tip closing is mainly affected by preferential diffusion of the multi-component mixture and the stretch effects. In the addition of light hydrocarbon (CH4), the tip closing starts at a higher percentage of hydrocarbon addition in H2-air flames. Whereas, upon the addition of heavy hydrocarbons such as propane and butane in H2-air flames, tip closing starts with a lesser amount of hydrocarbon addition. Temperature, OH mole fraction and heat release rate have been investigated, focusing on the flame structure at the tip. The tip opening regime diagram for H2–CH4-air, H2–C3H8-air and H2–C4H10-air mixtures are presented.

An experimental and numerical study on the combined effect of N2/CO2 dilution and hydrogen addition on the laminar burning velocity (LBV) of methane-oxygen mixtures was conducted. The experiments were performed at atmospheric conditions using the heat flux method for effective equivalence ratios (ϕF) varying from 0.7 to 1.3. The results show that the hydrogen addition causes an increase in LBV for all the mixture conditions. The variation in LBV based on hydrogen addition parameter (RH) for all N2 dilution conditions were following a linearly increasing trend. The strong effect of hydrogen addition on LBV is observed at lean and rich mixtures compared to that at near stoichiometric mixture conditions. The experimental results show that the percentage variation in LBV with RH at rich mixture is more substantial at 75% N2 dilution compared to that at 65% N2 dilution.

ispartof: Progress In Energy And Combustion Science vol:96 status: published

An empirical model based on machine learning is developed for predicting the variation of the laminar burning velocities of H2/CO/CH4/CO2/N2/air mixtures with volumetric fractions as the independent variables at different elevated mixture temperatures and pressures. The proposed model is derived partly based on the measured burning velocities of syngas-air mixtures at elevated temperatures and pressures using diverging channel method, and partly established from the predictions using the FFCM-1 detailed kinetic model. The experiments at elevated pressures and temperature strongly agree with the predictions of the FFCM-1 kinetic model for PG1 (H2/CO/CO2/N2 = 15/15/15/55) syngas composition. Based on the detailed analysis of the experimental results, a power-law correlation considering the α, β variations is proposed: Su = Su,o * (Tu/Tu,o)α0+α1 (1−Pu/Pu,o) * (Pu/Pu,o)β0+β1 (1−Tu/Tu,o). Machine learning model (multiple linear-regression) was trained for the variables (Su,o, αo, α1, β0, β1) in the power-law correlation to enable the prediction of laminar burning velocity at various pressure and temperature conditions. The empirical model was developed with mole fractions of various components (H2/CO/CH4/CO2/N2) in the syngas composition and equivalence ratio as independent variables. The developed model was intended for low-calorific value syngas mixtures, and it performs exceedingly well without solving detailed governing equations, detailed chemistry, and transport equations. The proposed model is accurate for a wide range of syngas-air mixtures reported in the literature. A detailed comparison showed that the empirical model accurately predicts the laminar burning velocity with error X H 2 0.70 , 0.25 X C H 4 X C O 2 X N 2

The laminar burning velocity of nine different syngas compositions has been measured at elevated temperatures (350–650 K) for various equivalence ratios using an externally heated diverging channel method. The measured values are compared with the predictions of two detailed kinetic models (GRI 3.0 and FFCM-1). The predictions of FFCM-1 for various syngas (H2/CO/CH4/CO2/N2) mixtures accurately capture the fundamental combustion characteristics, and GRI 3.0 mechanism overpredicts the laminar burning velocity for fuel rich syngas compositions. Temperature exponents mildly decrease with increase in equivalence ratio for mixtures investigated in the present work. Two correlations, one using linear regression, and another, a linear equation model is proposed to predict the laminar burning velocity accurately for various syngas compositional modifications. The developed linear model can be used for direct calculation of laminar burning velocities of different syngas compositions. Similarly, a temperature exponent correlation and its comparison with present measurements and simulations are also presented. The efficacy of the proposed models is evaluated through a detailed comparison with the literature. The model provides an excellent matching for following syngas compositions; 0.05 X H 2 0.50 , 0.08 X C H 4 X C O 2 X N 2

Research in chemical engineering requires experiments, which are often expensive, time-consuming, and labo-rious. Design of experiments (DoE) aims to extract maximal information from a minimum number of experi-ments. The combination of DoE with machine learning leads to the field of active learning, which results in a more flexible, multi-dimensional selection of experiments. Active learning has not yet been applied in reaction modeling, as most active learning techniques still require an excessive amount of data. In this work, a novel active learning framework called GandALF that combines Gaussian processes and clustering is proposed and validated for yield prediction. The performance of GandALF is compared to other active learning strategies in a virtual case study for hydrocracking. Compared to these active learning methods, the novel framework out-performs the state-of-the-art and achieves a 33%-reduction in experiments. The proposed active learning approach is the first to also perform well for data-scarce applications, which is demonstrated by selecting ex-periments to investigate the ex-situ catalytic pyrolysis of plastic waste. Both a common DoE-technique, and our methodology selected 18 experiments to study the effect of temperature, space time, and catalyst on the olefin yield for the catalytic pyrolysis of LDPE. The experiments selected with active learning were significantly more informative than the regular DoE-technique, proving the applicability of GandALF for reaction modeling and experimental campaigns.

Reliable data on fundamental combustion parameters are essential to validate kinetic schemes under a wide range of experimental conditions. Laminar burning velocity is one such fundamental intrinsic property of a flammable mixture. The challenges in determining the effect of simultaneous change in temperature and pressure on the burning velocity of low-calorific value syngas–air mixtures are presented. Comparing the present experiments with available kinetic schemes at elevated temperature and pressure reveals poor prediction capabilities. The major contributing factor to this discrepancy is the high sensitivity of key reactions at high temperatures and pressure conditions. The challenges of direct use of the experimental data to optimize kinetic schemes for the syngas–air mixtures are discussed.

The iron and steel industry is a carbon-intensive industry and one of the largest industrial sources of CO2 emissions. In this work, we show how the steel mill gases can be conditioned using three metal oxides to produce a CO/CO2 stream that can be used for the production of chemicals, thereby preventing the emission of carbon to the atmosphere as CO2. Abundant oxides of iron and manganese, characterised by their readiness to capture and release gaseous O2, and calcium oxide, characterised by its capacity to capture and release gaseous CO2 can be deployed in this process. Process analysis indicates that by fully utilising the chemical energy of the carbon-rich blast furnace gas (BFG) of the steel mill, 37% of the associated CO2 emissions can be eliminated. A techno-economic evaluation shows that further reduction of CO2 emissions is viable. Preliminary estimations indicate that the cost for processing BFG through the proposed process is 46 EUR2020 per tonneBFG. The sources of revenue are the product CO/CO2 stream (0.2 tonneproduct per tonneBFG) and electricity constituting 85% and 14% of the total revenue with the remaining 1% obtained by the sale of spent metal oxides used in the process. The technical feasibility of the process was experimentally proven in a fixed bed reactor to produce a CO/CO2 stream and an H2O-rich stream while the metal oxides were periodically regenerated in alternating redox conditions. Thirty executed cycles indicated stable performance of the process. The proposed process concept can be applied to any gas stream containing CO2 and fuel. Catalytic fast pyrolysis of polyolefinic waste streams was investigated to recover valuable base chemicals at high selectivity. HZSM-5 zeolite with different properties, affected by Si/Al, mesoporosity, phosphorus stabilization, and steaming, were tested and thoroughly characterized. Different feeds, catalyst/feed ratios and reaction temperatures were evaluated in a micropyrolysis reactor coupled to two-dimensional gas chromatography. While unmodified HZSM-5 rapidly deactivated, phosphorus-modified and steamtreated HZSM-5 showed almost no deactivation due to its lower coking propensity during 130 runs with stable conversion towards C5+ aliphatics and high C-2-C-4 olefins selectivity (-75%) using post-consumer mixed polyolefins. The performance of this direct olefins production route with unprecedented high olefin selectivity was further evaluated in a plantwide context. It was found that it requires-37% lower energy input than the plastics pyrolysis followed by pyrolytic oil steam cracking, while it results to at least a one order of magnitude lower environmental burden as compared to waste incineration.

The heated diverging channel technique for the measurement of laminar burning velocities was extended to incorporate measurements at elevated pressures. The evaluation of the combined influence of pressure and temperature on the fundamental combustion characteristics of CH4-air flames is realized through the new experimental facility. The laminar burning velocities of CH4-air mixtures at different equivalence ratios are reported for a pressure range (1–5 atm), and elevated temperatures of 350–650 K. The temperature exponent, α presents with the lowest value for marginally rich mixtures (ϕ = 1:1). The non-monotonic behavior of α is replicated at elevated pressures (2–5 atm). PREMIX calculations with commonly employed kinetic models (Aramco 2, GRI Mech 3.0) were compared with the present experiments. The predictions of Aramco 2 mechanism were in excellent agreement with the present measurements at high temperature and pressure conditions.

To date, the recycling of polyurethane (PU) and polyisocyanurate (PIR) waste still poses a significant problem. Within this contribution, the thermal degradation of high-resilient ether PU, semirigid PU, rigid PU, and (PIR) with and without TCPP flame retardant was studied at different temperatures to identify and quantify the main products. For this, a tandem micropyrolyzer coupled to GC × GC with FID and ToF-MS detectors was used, and the yield volatiles, light gases, and residue was quantified. The volatile pyrolysis vapours obtained from pyrolyzing high-resilient ether PU and semirigid PU at 600 °C were very similar, and the main products obtained were ethylene and propylene (combined yield of ~13 wt%), ~16 wt% of various oxygenates, and 4-6 wt% organic nitrogen compounds. The oxygenates included mostly poly-ether type compounds with varying molecular weight, and these are attributed to the decomposition of the polyol chains forming the soft segments of the polymer. The main semi-volatile nitrogen compound was 4,4'-methylenedianiline, attributed to the decomposition of the hard segments in the PU structure synthesized from MDI. The pyrolysis vapours from rigid PU contained several ether-type compounds and high yields of heteroatom-free monoaromatics—in particular styrene. The PIRs were more difficult to decompose and volatilize than the PUs, resulting in ~20 wt% higher residue yields. The charring propensity was even higher in the absence of a flame retardant in the formulation. Pyrolysis of TCPP-containing PIR produced Cl-containing pyrolysis vapours such as allyl chlorides. While the yield of H2O was comparable for the different PUs and PIR, pyrolysis of PIR produced higher yields of CO2 and less CO compared to the different PUs. The use of catalysts can help to convert PU waste pyrolysis vapours more efficiently to desirable chemicals.

The effect of H2/CO addition on flame dynamics of propane-air mixtures in a stepped microcombustor with heat recirculation was investigated experimentally. Unstable flame propagation regimes and their transition modes were observed for a range of mixture velocities (5���8 m/s) and equivalence ratios (0.6���0.95). Up to 60% H2 addition to lean propane-air mixtures helps improve the flame stability range in the combustor. The suppression of these flame instabilities with H2 addition is attributed to the increase in the overall mixture reactivity and burning velocity. The enhancement in OH intensity with H2 addition indicated a change in the elementary reactions, thereby affecting the flame dynamics. Contrary to H2 addition, CO addition does not show any noticeable effect on the formation of stable flame modes. The change in mixture properties and burning velocity leads to a change in the flame behavior and alters the heat release rate due to H2 addition to propane-air mixtures. A higher percentage of H2 addition led to the reappearance of the unstable (rotating) flames in the microcombustor. Intrinsic flame instabilities appeared at higher H2 percentages due to differential diffusion and increased thermal-wall coupling effects.

Syngas is a promising alternative fuel due to clean combustion with lower greenhouse gas emissions. The multi-component fuel mixture primarily consists of hydrogen, carbon monoxide, nitrogen, carbon dioxide and traces of moisture. The composition of syngas strongly depends on the feedstock and the choice of production method. The wide compositional variability of syngas poses hurdles in developing appliances such as burners and combustion chambers. This review summarizes the recent research on syngas' fundamental combustion characteristics, such as laminar and turbulent burning velocity. The burning velocity prediction capabilities of various reaction mechanisms were analyzed. Comparing the laminar burning velocity predictions using different kinetic schemes with available experimental data in the literature establishes the validity of kinetic schemes. A considerable discrepancy is observed between the experimental data and the present numerical predictions for elevated temperatures at different equivalence ratios. The recent developments in syngas burners, stability regimes, and the need for laminar/turbulent burning velocity data at high temperatures and pressure to improve computational modeling of industrial syngas burners are emphasized. The existing research gap in burners to accommodate syngas with the higher mole fraction of hydrogen is also explored.

Laminar burning velocities of low-calorific value syngas-air mixtures are measured at elevated temperature and pressure conditions using externally heated diverging channel method. The technique facilitates direct measurement of laminar burning velocities simultaneously at elevated pressures and temperatures. The mixture burning velocities are observed to increase with mixture enthalpy at elevated temperatures and decrease with an increase in the mixture pressure. The measurements are carried out for a mixture temperature range of 300–650 K and up to 5 bar pressure. Based on these measurements a new relation for the variation of laminar burning velocity with pressure and temperature is proposed. The power-exponents of temperature and pressure were expressed as linear functions of pressure and temperature ratios, respectively. The comparison of the measured burning velocities with the best performing kinetic schemes (FFCM-1 and Aramco 3) revealed that a significant deviation in the predictions is observed, when a simultaneous increase in pressure and temperature are considered.

A numerical investigation of steady laminar premixed propane–air flames is presented. A three-dimensional simulation has been performed to examine the impact of operating conditions on steady-state characteristics of a perforated burner flame. A numerical simulation has been carried out using a reduced propane–air reaction mechanism having 30 species and 192 reactions. The results are validated against the one-dimensional flat-flame result obtained using PREMIX. Effects of the equivalence ratio, inlet velocity, hole–hole distance, and plate thermal conductivity on flame stability are examined. The flame stand-off distance increases with the increase in the inlet velocity. As the equivalence ratio increases, the heat flux to the plate increases as the flame moves closer to the plate. When the plate is adiabatic, the conical flame rests on the plate. The flame stand-off distance increases as the plate thermal conductivity is increased. The flame moves downstream of the plate as the distance between the adja...