NOx emission characteristics of counterflow syngas diffusion flames with airstream dilution

Syngas is produced through a gasification process using variety of fossil fuels, including coal, biomass, organic waste, and refinery residual. Although, its composition may vary significantly, it generally contains CO and H₂ as the dominant fuel components with varying amount of methane and diluents. Due to its wide flexibility in fuel sources and superior pollutants characteristics, the syngas is being recognized as a viable energy source worldwide, particularly for stationary power generation. There are, however, gaps in the fundamental understanding of syngas combustion and emissions, as most previous research has focused on flames burning individual fuel components such as H₂ and CH₄, rather than syngas mixtures. This paper reports a numerical investigation on the effects of syngas composition and diluents on the structure and emission characteristics of syngas nonpremixed flames. The counterflow syngas flames are simulated using two representative syngas mixtures, 50%H₂/50%CO and 45%H₂/45%CO/10%CH₄ by volume, and three diluents, N₂, H₂O, and CO₂. The effectiveness of these diluents is characterized in terms of their ability to reduce NO_x in syngas flames. Results indicate that syngas nonpremixed flames are characterized by relatively high temperatures and high NO_x concentrations and emission indices. The presence of methane in syngas decreases the peak flame temperature, but increases the formation of prompt NO significantly. Consequently, while the total NO formed is predominantly due to the thermal mechanism for the 50%H₂/50%CO mixture, it is due to the prompt mechanism for the 45%H₂/45%CO/10%CH₄ mixture. For both mixtures, CO₂ and H₂O are more effective than N₂ in reducing NO_x in syngas flames. H₂O is the most effective diluent on a mass basis, while CO₂ is more effective than N₂. The effectiveness of H₂O is due to its high specific heat that decreases the thermal NO, and its ability to significantly reduce the concentration of CH radicals, which decreases the prompt NO. The presence of methane in syngas reduces the effectiveness of all three diluents.     

Flashback propensity of syngas fuels

This paper presents experimental measurements of the critical velocity gradient and flashback behavior of H₂-CO and H₂-CH₄ mixtures. Effects of H₂ concentration, external excitation, and swirl on the flashback behavior for flames of these fuel mixtures are discussed. For H₂ concentration burner and scaling studies the critical velocity gradient (g_F), defined as the ratio of the square of the laminar burning velocity to the thermal diffusivity of the mixture , was used to quantify the flashback propensity of the flames. The critical velocity gradient of both H₂-CH₄ and H₂-CO flames changed nonlinearly with the increase in H₂ contents in the mixture. The critical velocity gradient (g_F) of 5-95% and 15-85% H₂-CO mixtures somewhat agreed with the scaling relation and yielded an average c value of 0.04. Similarly, values of a 25%H₂-75%CH₄ for different burner diameters were also fitted using the scaling relation yielding an average c value of 0.044. The g_F values of 25-75% H₂-CO mixture showed non-linear variation with the ratio (especially for ), and at a lower ratios burner diameter had small effect on critical velocity gradient measurements. The opposite trend was observed for a 25-75% H₂-CH₄ mixture showing non-linear variation at a lower ratios (for ) and having less effect at higher ratios. It was also determined that the effect of external excitation on the flashback propensity of H₂-CO flames with more than 5% H₂ was not significant. Flashback through two mechanisms and their dependence on combustor parameters were also identified for swirl stabilized H₂-CO flames.     

Laminar burning velocities and Markstein numbers of syngas-air mixtures

In the currently reported work, three typical mixtures of H₂, CO, CH₄, CO₂ and N₂ have been considered as representative of the producer gas coming from wood gasification. Laminar burning velocities have been determined from schlieren flame images at normal temperature and pressure, over a range of equivalence ratios within the flammability limits. The study of the effects of flame stretch rate was also performed. Combustion demonstrates a linear relationship between flame radius and time for syngas-air flames. The maximum value of syngas-air flame speeds is observed at the stoichiometric equivalence ratio, while lean or rich mixtures have lower flame speeds. The higher is the syngas heat value the higher is the laminar burning velocity of the syngas mixture. Markstein numbers show that typical syngas-air flames are generally unstable. Karlovitz numbers indicates that typical syngas-air flames are little influenced by stretch rate. Based on the experimental data, a formula for calculating the laminar burning velocities of syngas-air flames is proposed. The magnitude of laminar burning velocity for typical syngas compositions is comparable to that of a simulated mixture comprising 5% H₂/95% CO and proved to be similar to methane, although somewhat slower than propane.     

On mechanisms of formation of environmentally harmful compounds in homogeneous combustors

A kinetic model is developed for calculating the emission characteristics of homogeneous combustors using methane and synthesis gas (syngas) as a fuel. The model is validated over a large set of experimental data on concentrations of NO, CO, and OH in laminar flames and in the Bunsen burner and on concentrations of OH, NO, and CO in a homogeneous combustor operating on a mixture of syngas with air. At an identical temperature of combustion products, i.e., identical thermodynamic efficiency, the combustor operating on syngas is demonstrated to emit a greater amount of NO, CO, and CO₂, as compared with the combustor operating on methane. Though the use of syngas allows one to organize stable combustion of ultralean mixtures and to obtain extremely low concentrations of NO and CO at the combustor exit (≈1–3 ppm), the amount of CO₂ in the exhaust of even extremely lean mixtures (α ≈ 3) is appreciably greater than that in the case of using methane.     

Numerical investigation of the distribution of oxygen atoms in syngas combustion products

The distribution of air oxygen atoms in the oxidation products of rich mixtures of syngas with air in flame and the under autoignition conditions at constant volume has been investigated by numerical simulation using the tracer method. It has been found that in rich mixtures, the oxidation of hydrogen and carbon oxide has a stepwise nature, which is clearly visible in the profiles of the rates of production of H₂O and CO₂. The observed stepwise nature inevitably results in the heat-release rate occurring in steps. The reaction pathways and the role of the oxygen atom of the CO molecule in the heat release in these flames has been investigated.     

Innovative steam methane reforming for coproducing CO-free hydrogen and syngas in proton conducting membrane reactor

Steam methane reforming (SMR) is a commercial process to produce syngas. Normally, the as-produced syngas is characterized by a H₂/CO ratio of 3. However, such H₂/CO ratio is unsuitable for Fischer–Tropsch synthesis. The hydrogen obtained by subsequent upgrading of syngas usually contains residual CO, which readily deactivates Pt electrocatalysts in fuel cells. Here we report an innovative route by coupling SMR with H₂ removal in a proton conducting membrane reactor to coproduce syngas with a preferable H₂/CO ratio of 2 and CO-free H₂ on opposite sides of the membrane, which can be directly used for Fischer–Tropsch synthesis and fuel cells, respectively. Notably, H₂ is in-situ extracted by the membrane that only allows the permeation of H₂ as protons through the oxide lattice with infinite selectivity, and thus the obtained H₂ is CO-free. This work could provide an alternative option in one-step conversion of methane into two inherently separated valuable chemicals.     

A numerical and experimental study of counterflow syngas flames at different pressures

Synthesis gas or “Syngas” is being recognized as a viable energy source worldwide, particularly for stationary power generation due to its wide availability as a product of bio and fossil fuel gasification. There are, however, gaps in the fundamental understanding of syngas combustion and emissions characteristics, especially at elevated pressures that are relevant to practical combustors. This paper presents a numerical and experimental investigation of the combustion and NO_x characteristics of syngas fuel with varying composition, pressure and strain rate. Experiments were performed at atmospheric conditions, while the simulations considered different pressures. Both experiments and simulations indicate that stable non-premixed and partially premixed counterflow flames (PPFs) can be established for a wide range of syngas compositions and strain rates. Three chemical kinetic models, GRI 3.0, Davis et al., and Mueller et al. are examined. The Davis et al. mechanism is found to agree best with the experimental data, and hence used to simulate the PPF structure at different pressure and fuel composition. For the pressure range investigated, results indicate a typical double flame structure with a rich premixed reaction zone (RPZ) on the fuel side and a non-premixed reaction zone (NPZ) on the oxidizer side, with RPZ characterized by H₂ oxidation, and NPZ by both H₂ and CO oxidation. While thermal NO is found to be the dominant route for NO production, a reburn route, which consumes NO through NO + O + M→ NO₂ + M and H + NO + M → HNO + M reactions, becomes increasingly important at high pressures. The amount of NO formed in syngas PPFs first increases rapidly with pressure, but then levels off at higher pressures. At a given pressure, the peak NO mole fraction exhibits a non-monotonic variation with syngas composition, first decreasing to a minimum value, and then increasing as the amount of CO in syngas is increased. This implies the existence of an optimum syngas composition that yields the lowest amount of NO production in syngas PPFs, and can be attributed to the combined effects of thermal and reburn mechanisms.     

The effects of enrichment by H2 on propagation speeds in adiabatic flat and cellular premixed flames of CH4 + O2 + CO2

Experimental studies of adiabatic flat and cellular premixed flames of (CH₄ + H₂) + (O₂ + CO₂) are presented. The hydrogen content in the fuel was varied from 0% to 35% and the oxygen content in the oxidizer was 31.55%. These mixtures could be formed when oxy-fuel combustion technology is combined with hydrogen enrichment. Non-stretched flames were stabilized at atmospheric pressure on a perforated plate burner. A heat flux method was used to determine propagation speeds under conditions when the net heat loss of the flame is zero. Adiabatic burning velocities of methane + hydrogen + carbon dioxide + oxygen mixtures were found in satisfactory agreement with the detailed kinetic modeling employing the Konnov mechanism. Under specific experimental conditions the flames become cellular; this leads to significant modification of the flame propagation speed. The onset of cellularity was observed throughout the stoichiometric range of the mixtures studied. Visual and photographic observations of the flames were performed to quantify their cellular structure. The results obtained in the present work in (CH₄ + H₂) + O₂ + CO₂ mixtures are in good accordance with the previous observations for different fuels, CH₄, C₂H₆ and C₃H₈. The enrichment by hydrogen leads to: the increase of the laminar burning velocities; the increase of the number of cells observed; the decrease of the mean cell diameter. The flame acceleration due to cellularity was not affected by the hydrogen enrichment.     

Detonation burning of anthracite and lignite particles in a flow-type radial combustor

Regimes of continuous spin detonation of anthracite and lignite particles in an air flow in a radial vortex combustor 500 mm in diameter with a constant (along the radius) cross-sectional area are studied. Ground coal with a particle size of 1–12 μm is used. For transporting coal into the combustor and promoting the chemical reaction on the surface of solid particles, hydrogen or syngas is added in the ratio CO/H₂ = 1/1, 1/2, or 1/3. Continuous spin detonation of two-phase mixtures of fine anthracite and lignite particles and air with addition of hydrogen up to 4% of the coal consumption rate is obtained for the first time. The amount of syngas added to coal increases with decreasing fraction of hydrogen in the syngas: 14, 21, and 27% for anthracite and 11, 20, and 29% for lignite at CO/H₂ = 1/3, 1/2, and 1/1, respectively. The structure of detonation waves and the flow in their vicinity are not principally different from those observed previously for long-flame bituminous coal and charcoal. Higher detonation velocities are observed for more energy-intensive coal (anthracite). A higher pressure is obtained near the cylindrical wall of the combustor in cold runs as compared to detonation in the case with identical flow rates of the coal–air mixtures.     

Effects of temperature and composition on the laminar burning velocity of CH4 + H2 + O2 + N2 flames

This work summarises available measurements of laminar burning velocities in CH₄ + H₂ + O₂ + N₂ flames at atmospheric pressure performed using a heat flux method. Hydrogen content in the fuel was varied from 0% to 40%, amount of oxygen in the oxidiser was varied from 20.9% down to 16%, and initial temperature of the mixtures was varied from 298 to 418 K. These mixtures could be formed when enrichment by hydrogen is combined with flue gas recirculation. An empirical correlation for the laminar burning velocity covering a complete range of these measurements is derived and compared with experiments and other correlations from the literature.     

Increasing carbon utilization in Fischer-Tropsch synthesis using H2-deficient or CO2-rich syngas feeds

Fischer-Tropsch technology has become a topical issue in the energy industry in recent times. The synthesis of linear hydrocarbon that has high cetane number diesel fuel through the Fischer-Tropsch reaction requires syngas with high H₂/CO ratio. Nevertheless, the production of syngas from biomass and coal, which have low H₂/CO ratios or are CO₂ rich may be desirable for environmental and socio-political reasons. Efficient carbon utilization in such H₂-deficient and CO₂-rich syngas feeds has not been given the required attention. It is desirable to improve carbon utilization using such syngas feeds in the Fischer-Tropsch synthesis not only for process economy but also for sustainable development. Previous catalyst and process development efforts were directed toward maximising C₅₊ selectivity; they are not for achieving high carbon utilization with H₂-deficient and CO₂-rich syngas feeds. However, current trends in FTS catalyst design hold the potential of achieving high carbon utilization with wide option of selectivities. Highlights of the current trends in FTS catalyst design are presented and their prospect for achieving high carbon utilization in FTS using H₂-deficient and CO₂-rich syngas feeds is discussed.     

Syngas production from liquid fuels in a non-catalytic porous burner

This paper investigates rich combustion of n-heptane, diesel oil, kerosene and rapeseed-oil methyl ester (RME) bio-diesel for the purpose of producing syngas ready for the clean-up stages for fuel-cell applications or for traditional combustor enrichment. Rich flames have been stabilised in a two-layer inert porous medium combustor and a range of equivalence ratios and porous materials have been examined. n-heptane was successfully reformed up to an equivalence ratio of 3, reaching a conversion efficiency (based on the lower heating value of H₂ and CO over the fuel input) up to 75% for a packed bed of alumina beads. Similarly, diesel, kerosene and bio-diesel were reformed to syngas in a Zirconia foam burner with conversion efficiency over 60%. A preliminary attempt to reduce the content of CO and hydrocarbons in the reformate has been also conducted using commercial steam reforming and water-gas shift reaction catalysts, obtaining encouraging results. Finally, soot emission has been assessed, demonstrating particle formation for diesel oil above φ = 2, whereas bio-diesel showed the lowest soot formation tendency among all the fuels tested.     

Gasification of an Indonesian subbituminous coal in a pilot-scale coal gasification system

Indonesian Roto Middle subbituminous coal was gasified in a pilot-scale dry-feeding gasification system and the produced syngas was purified with hot gas filtering and by low temperature desulfurization to the quality that can be utilized as a feedstock for chemical conversion. Roto middle coal produced syngas that has a typical composition of 36–38% CO, 14–16% H₂, and 5–8% CO₂. Particulates in syngas were 99.8% removed by metal filters at the operating temperature condition of 200–250°C. Sulfur containing compounds of H₂S and COS in syngas were also desulfurized in the Fe chelate system to yield less than 0.5 ppm level. The full stream gasification and syngas purifying system has been successfully operated and thus can provide clean syngas for the research on the conversion of syngas to chemicals like DME and on the future IGFC using fuel cells. This work was presented at the 6 ^th Korea-China Workshop on Clean Energy Technology held at Busan, Korea, July 4–7, 2006.     

High yields of synthesis gas by millisecond partial oxidation of higher hydrocarbons

Cyclohexane, n-hexane, and isooctane were reacted with air in a Rh-monolith reactor and converted into synthesis gas (H₂+CO) in yields exceeding 90%, with >95% conversion of fuels and 100% conversion of oxygen. The best catalyst was an 80 ppi washcoated alumina monolith containing 5 wt% Rh. There was a small effect of catalyst contact time on syngas selectivity and conversions for gas space velocities from 3×10⁵ to 3×10⁶ h^–1. Preheating the feed enhances syngas selectivities slightly, but no reactor preheat is necessary provided the fuel remains vaporized. Addition of 25 mol% toluene to isooctane also produces syngas, olefins, and methane with 90% yield, including 70% yield to syngas. Partial oxidation of gasoline–air mixtures was attempted but the catalysts were poisoned after several hours, probably by sulfur and/or metals.     

Co-gasification of coal, plastic waste and wood in a bubbling fluidized bed reactor

Seven mixtures of coals, plastics and wood have been pelletized and fed into a pre-pilot scale fluidized bed gasifier in order to investigate the main aspects of the co-gasification of these materials. The main components of the obtained syngas (CO, H₂, CO₂, N₂, CH₄, C_nH_m) were measured by means of on-line analyzers and a gas cromatograph. The performance of the gasifier was evaluated on the basis of syngas composition, carbon conversion efficiency, energy content of syngas, cold gas efficiency and yield of undesired by-products (tar and soot-like particulate). The results of a first series of experimental tests showed the effect of gas fluidizing velocity and that of equivalence ratio on the main performance parameters for a specific coal-plastics mixture. A second series of tests has been carried out by changing the mixture composition keeping fixed the gas velocity and equivalence ratio. The presence of wood and coal in the mixture with plastics contributed to reduce the tar production even though it is accompanied by a lower syngas specific energy.     

The effect of oxygen and carbon dioxide concentration on soot formation in non-premixed flames

The influence of oxygen concentration and carbon dioxide as diluents in the oxidizer side on soot formation was studied by Time Resolved Laser Induced Incandescence (TIRE-LII) and TEM photography in non-premixed co-flowing flames. TIRE-LII method was used to measure the distribution of two-dimensional soot volume fraction and primary particle size. The soot was directly sampled by the thermophoretic method, and its diameter was examined by TEM photography. Two suitable delay times of the TIRE-LII method affecting measurable range and sensitivity were determined by comparing TEM photographs with the TIRE-LII signal. The effects of oxygen concentration and carbon dioxide as diluents in the oxidizer side on soot formation were investigated with these calibrated techniques. An O₂+(CO₂, N₂, and [Ar+CO₂]) mixtures in co-flow were used to isolate carbon dioxide effects systematically. The primary particle number concentration and soot volume fraction were abruptly decreased by the addition of carbon dioxide to co-flow. This suppression was resulted from the short residence time in inception region because of the late nucleation and the decrease of surface growth distance by the low flame temperature due to the higher thermal capacity and the chemical change of carbon dioxide. The increase of oxygen concentration in the co-flow caused an enhancement of soot nucleation and thus the residence time increase, but the specific growth rate showed almost the same value regardless of the co-flow mixture in the growth region. This result suggests that the specific growth rate has a weak dependence on the relative change of co-flow conditions in non-premixed co-flowing flames.     

Numerical study of the effect of CO2/H2O dilution on the laminar burning velocity of methane/air flames under elevated initial temperature and pressure

Diluents have an essential effect during combustion. Discovering the influence of CO₂ and H₂O as diluents on laminar burning velocity (LBV) is helpful for combustion control and optimization. In this study, CH₄/air/CO₂/H₂O mixtures were investigated and validated using the FFCM-Mech 1.0 over extensive boundary conditions. The chemical effects of the diluents CO₂ and H₂O were separated using a decoupling method. It was found that an increase in initial temperature promotes the chemical effects, while an increase in initial pressure does the opposite. In addition, the inhibiting effect of CO₂ on LBV is stronger than that of H₂O. Sensitivity, mole fraction, and rate of production (ROP) analyses were used to reveal that the sum of the chemical effects of adding CO₂ and H₂O separately was greater than the chemical effects of adding equal amounts of CO₂ and H₂O simultaneously. This paper not only investigates the effect of CO₂ and H₂O on the LBV under wide boundary conditions, but also offers a valuable guide for studying the operating conditions and intensity settings of exhaust gas recirculation (EGR) and theoretical guidance for further research on the combination of EGR and in-cylinder water injection technology.     

Syngas combustion in a 500 Wth Chemical-Looping Combustion system using an impregnated Cu-based oxygen carrier

Chemical-Looping Combustion (CLC) is an emerging technology for CO₂ capture because separation of this gas from the other flue gas components is inherent to the process and thus no energy is expended for the separation. For its use with coal as fuel in power plants, a process integrated by coal gasification and CLC would have important advantages for CO₂ capture. This paper presents the combustion results obtained with a Cu-based oxygen carrier in a continuous operation CLC plant (500 Wth) using syngas as fuel. For comparison purposes pure H₂ and CO were also used. Tests were performed at two temperatures (1073 and 1153 K), different solid circulation rates and power inputs. Full syngas combustion was reached at 1073 K working at f higher than 1.5. The syngas composition had small effect on the combustion efficiency. This result seems to indicate that the water gas shift reaction acts as an intermediate step in the global combustion reaction of the syngas. The results obtained after 40 h of operation showed that the copper-based oxygen carrier prepared by impregnation could be used in a CLC plant for syngas combustion without operational problems such as carbon deposition, attrition, or agglomeration.     

Gas hydrate formation from hydrogen/carbon dioxide and nitrogen/carbon dioxide gas mixtures

Gas hydrates from CO₂/N₂ and CO₂/H₂ gas mixtures were formed in a semi-batch stirred vessel at constant pressure and temperature of 273.7 K. These mixtures are of interest to CO₂ separation and recovery from flue gas and fuel gas, respectively. During hydrate formation the gas uptake was determined and the composition changes in the gas phase were obtained by gas chromatography. The rate of hydrate growth from CO₂/H₂ mixtures was found to be the fastest. In both mixtures CO₂ was found to be preferentially incorporated into the hydrate phase. The observed fractionation effect is desirable and provides the basis for CO₂ capture from flue gas or fuel gas mixtures. The separation from fuel gas is also a source of H₂. The impact of tetrahydrofuran (THF) on hydrate formation from the CO₂/N₂ mixture was also observed. THF is known to substantially reduce the equilibrium formation conditions enabling hydrate formation at much lower pressures. THF was found to reduce the induction time and the rate of hydrate growth.     

Experimental study on CH* chemiluminescence characteristics of impinging flames in an opposed multi‐burner gasifier

The bandpass filtered images of impinging flames in an opposed multi‐burner (OMB) gasifier was visualized by a CCD camera combined with a high temperature endoscope. A filtering and image processing method by use of three bandpass filters was applied to subtract soot and CO₂* contributions in the CH* band and obtain the CH* chemiluminescence of impinging flames. The results show that a clear reaction core is generated in the impinging zone of four‐burner impinging flames. The size of the reaction core is affected by the O/C equivalence ratio ([O/C]_e) and the impingement effect is relatively stronger at lower [O/C]_e. The flame lift‐off length in the gasifier is jointly controlled by the syngas concentration and the diesel atomization effect. The impingement effect shortens the flame lift‐off length. The relationship between the syngas concentration and the maximum CH* intensity makes it possible to evaluate the syngas concentration from CH* intensity. © 2016 American Institute of Chemical Engineers AIChE J, 63: 2007–2018, 2017