Effects of equivalence ratios on the normal and inverse diffusion flame of acid gas combustion in the pure oxygen atmosphere

The experimental studies on the effect of equivalence ratios to the acid gas (H₂S and CO₂) combustion in the pure oxygen atmosphere was presented in a coaxial jet double channel burner. Three equivalence ratios (Φ = 0.8, 1.0 and 1.5) are examined to analyze the distribution of the flame temperature and gas composition in the normal and inverse diffusion flame along central axial (R = 0.0) and axial line at 3 mm (R = 0.75) in radial direction. The results revealed that acid gas combustion mainly occurred chemical decomposition of H₂S and oxidation of H₂S and H₂ at R = 0.0, while mainly occurred H₂S and H₂ oxidation at R = 0.75 in the normal diffusion flame. Reducing Φ increased the flame temperature and it is higher at R = 0 than that at R = 0.75 because of heat loss. It also increased the volume fraction of CO, H₂ and COS in the flame combustion area, while decreased downstream reactor because of occurring oxidation. CO was formed by the reaction of CO₂ and H, and H₂ primarily derived from chemical decomposition of H₂S. COS was generated by the reaction of CO₂ with SH, H₂S and S as well as the reaction of CO with SO and SH at R = 0.0, while was mainly formed by the reaction of CO with SO and SH at R = 0.75. H₂S mainly occurred the oxidation in the inverse diffusion flame. The temperature at R = 0.0 was still higher than that at R = 0.75, and it was higher than that in the normal diffusion flame in the combustion area. Increasing Φ promoted the formation of CO, H₂ and COS, and each gas under Φ of 1.5 was higher significantly. The Φ had no significant effect on the distribution of SO₂ compared to the normal diffusion flame, but changed the distribution of CO, H₂ and COS. It can be inferred that the content of CO, H₂ and COS will be more higher under Claus condition in the inverse diffusion flame.     

Dynamic Behaviors of Sulfur Evolved in the Gas Phase from Pyrolysis of Six Chinese Coals

Dynamic behaviors of gaseous sulfur-containing compounds evolved from pyrolysis of six Chinese coals were studied in the temperature range of up to 800°C under N₂ and H₂. The released amount of total sulfur-containing gases was traced by an online flame photometric detector (FPD). Simultaneously, the changes of different sulfur forms, including H₂S, SO₂, COS and CS₂, also were investigated using an online mass spectrometer (MS). FPD results show that the effect of H₂ on gaseous sulfur evolved is complex, which promoted certain peak and suppresses other peaks. Based on the data from MS, it is suggested that a series of competitive reactions between active sulfur and other active matters during pyrolysis may impact on the sulfur form in gas phase. Interactions between active sulfur-containing intermediates and the coal matrix are attributed to be the main factor determining the dynamic behavior.     

A comparison of sulfur and chlorine gas species in pulverized-coal,air- and oxy-combustion

Gas phase measurements of sulfur species (SO₂, H₂S, SO₃, and COS) and HCl were collected in the staged near-burner region and the post tertiary injection oxidizing region of a pulverized-coal flame under air- and oxy-fired conditions. The near burner region was characterized by a high CO region near the center of the reactor directly below the burner. This reducing region as indicated by high CO concentrations produced high H₂S and COS and lower SO₂ concentrations in comparison to the post tertiary injection oxidizing region for both air and oxy-fired conditions. The concentrations of HCl were in most cases insensitive to radial location. Concentrations of all measured sulfur gases were 2.4–2.9 times higher in oxy-fired conditions than air-fired conditions consistent with the increase expected by the removal of N₂ from the oxidizer. The concentration of H₂S were on average 5.8 times higher in oxy-fired reducing conditions than air-fired reducing conditions indicating that the oxy-fired near burner regions were higher in H₂S and lower in SO₂ than would be predicted by removal of N₂ alone from an air-fired system. The concentrations of SO₃ were less than 2% of the total sulfur for both air and oxy-fired conditions but the uncertainty of the measurement did not allow for a comparison between the two firing modes. The concentrations of HCl were 2.4–2.8 times higher in oxy-fired conditions consistent with the removal of N₂ from the oxidizer. The total measured sulfur, HCl, SO₃, and reducing zone H₂S were all found to increase linearly with increasing sulfur and chlorine concentrations in the coal.     

High-temperature corrosion of water-wall tubes in oxy-combustion atmosphere

Oxy-combustion is one of the most promising technology for CO₂ capture in coal-fired power plants. However, under oxy-combustion conditions, the concentrations of acid gas species are significantly increased due to the introduction of the flue gas recycle, which aggravates the high-temperature corrosion of heat exchanger materials in boilers. In this study, the early-stage high-temperature corrosion (0–16 h) of two representative water-wall tube materials (20G, 12Cr1MoV) is experimentally tested in a lab-scale furnace with the simulated oxy-combustion atmosphere. The effects of material, temperature, CO₂, H₂O, SO₂, H₂S and CO atmospheres on high-temperature corrosion behaviors is investigated. The micro-morphologies and compositions of corrosion layers are characterized by scanning electron microscope with energy dispersive X-ray spectra (SEM-EDS) and X-ray diffraction (XRD). Kinetic analysis shows that the high concentration of CO₂ accelerates high-temperature corrosion of water wall materials. In the simulated oxy-fuel combustion atmosphere (CO₂/O₂/SO₂), the mass gain rate can be enhanced by 10%–30% compared to the conventional air combustion atmosphere (N₂/O₂/SO₂), and the major composition of oxide scale is magnetite. In a reducing oxy-fuel atmosphere (CO₂/CO/SO₂/H₂S), the major components of oxide scale are magnetite and ferrous sulfide. The high concentration of moisture in the atmosphere accelerated the corrosion rate by 10–30%. For both model alloys, the corrosion kinetics obey the parabolic law. Water-wall tube material 12Cr1MoV appears superiority in corrosion resistance compared with 20G material.     

Influence of impregnated copper and zinc on the pyrolysis of rice husk in a micro-fluidized bed reactor: Characterization and kinetics

Catalytic performance of Cu and Zn catalysts was investigated during rice husk (RH) high-temperature pyrolysis under isothermal conditions in a micro-fluidized bed reactor. The results showed that the presence of Cu and Zn evidently influenced the release characteristics and conversion of the gas components. The impregnated Cu promoted the conversion of H₂, CH₄, CO and CO₂, while Zn showed positive catalytic effect on the conversion of H₂, CH₄ and CO₂ and negative effect on the conversion of CO. The X-ray diffraction patterns of the residue chars revealed that metallic copper nanoparticles (Cu⁰) were formed during Cu impregnated biomass pyrolysis. Textural characterization and SEM images showed that the impregnation of Cu and Zn, particularly Zn, promoted the generation of micropores and mesopores, with the pore sizes predominantly at around 1.3 nm and 3.9 nm. Reaction kinetics for generating these gases was studied based on model fitting method, and the most probable reaction mechanism was obtained based on the relative error between experimental and calculated conversion data. The resulting apparent activation energies were 85.08, 12.56, 49.72 and 38.37 kJ/mol for the formation of H₂, CO, CH₄ and CO₂ from pure RH pyrolysis. The presence of Cu decreased the forming activation energies of the four gases, and Zn decrease the forming activation energies of H₂, CH₄ and CO₂ while increased the value for the formation of CO.     

Syngas production by gasification of aquatic biomass with CO2/O2 and simultaneous removal of H2S and COS using char obtained in the gasification

Applicability of gulfweed as feedstock for a biomass-to-liquid (BTL) process was studied for both production of gas with high syngas (CO + H₂) content via gasification of gulfweed and removal of gaseous impurities using char obtained in the gasification. Gulfweed as aqueous biomass was gasified with He/CO₂/O₂ using a downdraft fixed-bed gasifier at ambient pressure and 900 °C at equivalence ratios (ER) of 0.1–0.3. The syngas content increased while the conversion to gas on a carbon basis decreased with decreasing ER. At an ER of 0.1 and He/CO₂/O₂ = 0/85/15%, the syngas content was maximized at 67.6% and conversion to gas on a carbon basis was 94.2%. The behavior of the desulfurization using char obtained during the gasification process at ER = 0.1 and He/CO₂/O₂ = 0/85/15% was investigated using a downdraft fixed-bed reactor at 250–550 °C under 3 atmospheres (H₂S/N₂, COS/N₂, and a mixture of gases composed of CO, CO₂, H₂, N₂, CH₄, H₂S, COS, and steam). The char had a higher COS removal capacity at 350 °C than commercial activated carbon because (Ca,Mg)S crystals were formed during desulfurization. The char simultaneously removed H₂S and COS from the mixture of gases at 450 °C more efficiently than did activated carbon. These results support this novel BTL process consisting of gasification of gulfweed with CO₂/O₂ and dry gas cleaning using self-supplied bed material.     

CO2 and H2O gas conversion into CO and H2 using highly exothermic reactions induced by mixed industrial slags

This communication reports conversion phenomena in which CO₂ and H₂O gases are transformed into CO and H₂, respectively, when exposed to a mixture of molten CaO-rich metallurgical slag and V₂O₃-rich gasifier slag. On reaction, CO₂ and H₂O are thermodynamically driven to become CO and H₂, respectively, by giving up oxygen over the formation of calcium orthovanadate in the slag. The concept was experimentally investigated with a synthetic slag heated to 1500 °C (an assumed slag tap-out temperature in the metallurgical industry) in a CO₂ saturated atmosphere. On heating, a rapid drop in oxygen partial pressure occurred between 1405 °C and 1460 °C, where 97% of CO₂ transformed to CO. Potential industrial applications with the H₂O-to-H₂ conversion are then explored using detailed process computations. If the process is made economically viable, CO₂ and H₂O could be converted into products that are environmentally and industrially attractive and that have the potential for energy savings and greenhouse gas reduction in a process.     

The effect of powder preparation method on the artificial photosynthesis activities of neodymium doped titania powders

The effects of nanostructure on the artificial photosynthesis activities of undoped and Nd doped titania (TiO₂) powders prepared by three different chemical co-precipitation methods were investigated. Substitutional/interstitial N and S doping was observed in powders due to the presence of high concentrations of HNO₃ (NP) and H₂SO₄ (SP) in the powder preparation media, respectively. Nd, N and S doping caused anatase/rutile phase transformation inhibition and crystallite size reduction in the nanostructure. Light absorption was significantly enhanced by Nd doping and the residual SO₄^2?/NO_x species in the nanostructure. Photocatalytic hydrogen production activity of Nd doped NP powder was 4 times greater than undoped NP powder at 700 °C and had a high purity (CO:H₂ ratio～0.00). CO was determined to be the main product in photocatalytic CO₂ reduction. NP powders had the highest CO yields and Nd doping enhanced CO production. The powders with high crystallite sizes and rutile weight fractions had the highest artificial photosynthesis activities.     

Effects of H2O and CO2 on the homogeneous conversion and heterogeneous reforming of biomass tar over biochar

The effects of H₂O and CO₂ reforming agents on the homogeneous conversion and heterogeneous reforming of biomass tar were studied in the presence of a biochar catalyst to better understand the transformation pathway between tar and biochar. Catalysis was performed in a two-stage fluidized bed/fixed bed reactor while Raman analysis and Gas Chromatograph-Mass Spectrometry were used to investigate biochar and tar characteristics. The results show temperatures of 700–900 °C are required for the homogeneous transformation of tar in the presence of H₂O/CO₂, which especially affect polycyclic aromatic hydrocarbons. The tar homogeneous reforming effect of 15 vol.% H₂O is significantly higher than that of 29 vol.% CO₂. During heterogeneous reforming of tar over biochar at 800 °C, the tar yield decreases in varying degrees with the H₂O and CO₂ concentration increasing. H₂O and CO₂ not only directly affect the tar transformation on biochar, but also indirectly influence the reforming of tar through changing the structure of biochar catalyst. The formation of additional oxygen-containing functional groups and transformation of small aromatic rings to larger aromatic rings in the biochar structure are promoted with the concentration of H₂O and CO₂ increasing. Under a H₂O/CO₂ atmosphere, a higher degree of aromatic ring heterogeneous reforming occurs over biochar than for non-aromatic tar components. Heterogeneous reforming reactivity of tar is promoted by the biomass tar structure (e.g the substituents, large aromatic ring size and five-carbon ring structures) over biochar under H₂O/CO₂ atmospheres. Further increasing H₂O and CO₂ concentration enhances this effect.     

Exergy analysis of hydrogen production from different sulfur-containing compounds based on H2S splitting cycle

There is a tremendous demand for hydrogen production worldwide but the current H₂ production routes from natural gas and other carbon fuels lead to large greenhouse gas emissions. Intentionally coupled with nuclear power, the sulfur–iodine (S–I) thermochemical water splitting cycle is one of the most widely studied cycles for the large-scale hydrogen production that has environmental benignity. Based on the inspiration of the S–I cycle, a novel chemical cycle called hydrogen sulfide splitting cycle has been proposed for hydrogen production. In addition to the SO₂ production from the reaction of H₂S and sulfuric acid, SO₂ can be produced from the burning (direct oxidation) of hydrogen sulfide or elemental sulfur. And it can also be provided by SO₂ capture from flue gas or other SO₂-containing waste gases. This paper performs exergy analysis on the various SO₂ provisions to the Bunsen reaction that make different routes for hydrogen production from waste sulfur-containing compounds as feedstock. It has been found that the route including SO₂ from direct H₂S oxidation potentially makes the best energy-efficient process of H₂ production. The heat that is generated from H₂S oxidation can be recovered and used to support the energy requirements for other steps of the cycle, making the entire hydrogen production cycle more energy-efficient.     

Thermal radiation by combustion gasesRayonnement thermique dans les gaz de combustionThermische strahlung von verbrennungsgasenTeплoвoe излyчeниe тoпoчныч гaзoв

Generalized expressions for the calculation of the emissivity, absorptivity, and other relevant radiation properties of molecular gases are given. New rational correlations for the properties of H₂O, CO₂, CO, NO, SO₂ and CH₄ are shown to be readily applicable to combustion gas radiation problems. Hand calculations are shown to be easily made for any arbitrary mixture of the above gases, and a simple computer routine for high-speed computation is described. Tabular and graphical aids giving the engineer physical insight into the radiation heat-transfer characteristics of the gases considered are presented and explained.     

Carbonyl sulphide (cos) in geothermal fluids: an example from the Larderello field (Italy)

The carbonyl sulphide (COS) content in the fluids of 12 wells in the Larderello geothermal field ranges from 0.005 to 0.1 μmol/mol. Measured data are comparable with the theoretical concentrations, considering a homogeneous gas phase at the temperature and pressure conditions of the reservoir. However, the low temperature dependence of equilibrium constants of reactions involving COS prevents us from using them as geothermometers. On the contrary, P_C02 estimates in the gas equilibration zone can be inferred from the H₂S/COS ratio. The calculated CO₂ partial pressures are comparable with those estimated by means of the H₂/CO ratio.     

Release and transformation characteristics of Na/Ca/S compounds of Zhundong coal during combustion/CO2 gasification

Zhundong (ZD) coal from northwest China is a high quality steam coal with reserves of more than 390 billion tons. However, the utilization of ZD coal is limited due to the high content of alkali and alkaline earth metals. This study aimed at revealing the release and transformation mechanism of Na/Ca/S compounds during combustion/gasification of ZD coal. The results demonstrate that Na was primarily influenced by temperature, mostly releases at 600–800 °C. The transformation of Ca compounds was affected by both temperature and atmosphere. The high temperature of the combustion process could accelerate the decomposition of CaCO₃ and CaSO₄, and the high content of CO₂ during gasification prolonged the decomposition of CaCO₃. The transformation of S was primarily influenced by atmosphere. SO₂ could react with CaO and form CaSO₄ during the combustion process. While S compounds were mainly released as S (g) and H₂S (g) during gasification process. There was a significant interaction among Na/Ca/S compounds during combustion, original CaSO₄ in coal could adsorb Na compounds with SO₂ at 600–800 °C and then reacted with aluminosilicates, by this reaction, Na could be fixed above 1000 °C.     

Effects of temperature and limestone on sulfur release behaviors during fluidized bed gasification

Gaseous sulfur is released during fluidized bed coal gasification, and control the yield of gaseous sulfur or the conversion between gaseous organic sulfur and inorganic sulfur at source is necessary, because it can economically satisfy the requirements of industrial production and protect the environments. In this study, sulfur release behaviors of a middle-sulfur coal called Guizhou coal were quantitatively determined through controlled experiments in a lab-scale fluidized bed during oxygen rich-steam gasification. The measured gaseous sulfur species were H₂S, SO₂, COS and CS₂. The effects of temperature (850^OC-950^OC) and limestone (Ca/S = 2) on the sulfur release behaviors were investigated. Among the above four gaseous sulfur, the yield of H₂S is the highest, followed by COS, while only less than 1.5% of sulfur in coal is released as SO₂ and CS₂. With the increase in temperature, the yield of H₂S increases while that of SO₂ decreases, and the change of COS yield and CS₂ yield is not obvious. The molar ratio of H₂S/COS increases with increasing temperature, which is qualitatively matched by thermodynamic analysis. The addition of limestone reduces the released sulfur but not change the distribution of gaseous sulfur forms. Meanwhile, the molar ratio of H₂S/COS increases after adding limestone, while the trend with temperature of H₂S/COS does not change. The removal rate of H₂S is between 23% and 28%, which increases with temperature. The distributions of sulfur in bottom char and fly ash are similar. The main sulfur species in the bottom char is organic sulfur, and thiophene dominates the organic sulfur. The increase of temperature and the addition of limestone will both promote the increase of inorganic sulfur content, and the decrease of organic sulfur content.     

Vanadium-ceria catalysts for H2S abatement from biogas to feed to MCFC

Vanadium-based catalysts supported on ceria were studied for the direct and selective oxidation of H₂S to sulphur and water at low temperature.Catalysts with two vanadium loading (20–50 wt% of V₂O₅) were prepared, characterized and tested at temperature of 150–200 °C in order to identify the best catalytic formulation. The most promising catalyst was the sample with the 20 wt% of V₂O₅ that showed 99% of sulphur selectivity and equilibrium H₂S conversion at 150 °C.The effect of the components of a typical biogas stream (CH₄, CO₂ and H₂O) was studied at 150 °C in order to investigate the possible formation of secondary products such COS, CS₂. No significant effect was observed in terms of H₂S conversion (99%) and selectivity to SO₂ (<1%) by adding CH₄ and CO₂ to the feed stream. Furthermore, the effect of the H₂S inlet concentration, temperature, contact time and molar feed ratio (O₂/H₂S) were also investigated at a reaction temperature of 80 °C.Finally, time on stream tests of 30 h were performed at 80 and 120 °C, in order to examine the catalyst stability.     

Performance of a sulfur-resistant commercial WGS catalyst employing industrial coal-derived syngas feed

Catalyst pretreatment and reaction conditions (reaction temperature, H₂O/CO molar ratio and space velocity) for the Water Gas Shift reaction were studied in a bench scale set-up, using a commercial catalyst and an industrial coal-derived syngas feed. Catalytic activity showed an important dependence on reaction temperature and space velocity although it remained almost constant with varying H₂O/CO molar ratio. The effect of reduction with H₂ or sulfide activation with H₂S or carbonyl sulfide (COS) was also studied, giving good catalytic results for 94 ppm S provided by either H₂S or COS as sulfide agents. Selectivity to hydrogen was close to 100% in all catalytic reaction tests.     

Study of Ce-Pt/γ-Al2O3 for the selective oxidation of CO in H2 for application to PEFCs: Effect of gases

In order to supply pure hydrogen to proton exchange membrane (PEM) fuel cells and avoid CO poisoning, selective CO oxidation in H₂ was studied over Ce-Pt/γ-Al₂O₃. Adding the Ce promoted the CO conversion and selectivity of Pt/γ-Al₂O₃ with changing loading weights of Pt and Ce, oxygen concentration, residence time, and the composition of gases (H₂O, CO₂, and N₂). At 250 °C, adding H₂O to the feed gas enhanced the CO conversion due to the water–gas shift reaction. While, adding CO₂ to the feed gas suppressed the CO conversion due to the reversible water–gas shift reaction. In situ BET and XRD tests showed that well-dispersed metallic Pt particles (−2 nm) existed on the Ce oxide over the alumina support, which helps to supply oxygen to the Pt for a high activity of CO oxidation and selectivity.     

Characterization of chemical looping combustion of coal in a 1 kWth reactor with a nickel-based oxygen carrier

Chemical looping combustion is a novel technology that can be used to meet the demand on energy production without CO₂ emission. To improve CO₂ capture efficiency in the process of chemical looping combustion of coal, a prototype configuration for chemical looping combustion of coal is made in this study. It comprises a fast fluidized bed as an air reactor, a cyclone, a spout-fluid bed as a fuel reactor and a loop-seal. The loop-seal connects the spout-fluid bed with the fast fluidized bed and is fluidized by steam to prevent the contamination of the flue gas between the two reactors. The performance of chemical looping combustion of coal is experimentally investigated with a NiO/Al₂O₃ oxygen carrier in a 1 kW_th prototype. The experimental results show that the configuration can minimize the amount of residual char entering into the air reactor from the fuel reactor with the external circulation of oxygen carrier particles giving up to 95% of CO₂ capture efficiency at a fuel reactor temperature of 985 °C. The effect of the fuel reactor temperature on the release of gaseous products of sulfur species in the air and fuel reactors is carried out. The fraction of gaseous sulfur product released in the fuel reactor increases with the fuel reactor temperature, whereas the one in the air reactor decreases correspondingly. The high fuel reactor temperature results in more SO₂ formation, and H₂S abatement in the fuel reactor. The increase of SO₂ in the fuel reactor accelerates the reaction of SO₂ with CO to form COS, and COS concentration in the fuel reactor exit gas increases with the fuel reactor temperature. The SO₂ in the air reactor exit gas is composed of the product of sulfur in residual char burnt with air and that of nickel sulfide oxidization with air in the air reactor. Due to the evident decrease of residual char in the fuel reactor with increasing fuel reactor temperature, it results in the decrease of residual char entering the air reactor from the fuel reactor, and the decrease of SO₂ from sulfur in the residual char burnt with air in the air reactor.     

An experimental and modeling study of the influence of flue gases recirculated on ethylene conversion

The influence of those gaseous compounds that can be typically present in combustion processes with flue gas recirculation (FGR) techniques: CO₂, H₂O, CO, NO, NO₂, N₂O and SO₂, on ethylene conversion was analyzed through an experimental and modeling study. Ethylene oxidation experiments in the presence of the different gaseous compounds were carried out in the 700–1400 K temperature range, at atmospheric pressure, from fuel-lean to fuel-rich conditions, using N₂ as bath gas. These experiments were modeled by means of a detailed gas-phase chemical kinetic mechanism, which was used to identify the implications of the different gaseous compounds recirculated for the ethylene oxidation scheme, as well as for their own conversion. Overall, good agreement was obtained between the experimental data and the modeling, and thus the proposed mechanism can be successfully used to model the ethylene oxidation in the presence of flue gases recirculated (CO₂, H₂O, CO, NO, NO₂, N₂O and SO₂) in a wide range of operating conditions.