Differences in reactivity of pulverised coal in air (O2/N2) and oxy-fuel (O2/CO2) conditions

The reactivity of four pulverised Australian coals were measured under simulated air (O₂/N₂) and oxy-fuel (O₂/CO₂) environments using a drop tube furnace (DTF) maintained at 1673 K and a thermogravimetric analyser (TGA) run under non-isothermal (heating) conditions at temperatures up to 1473 K. The oxygen concentration, covering a wide and practical range, was varied in mixtures of O₂/N₂ and O₂/CO₂ in the range of 3 to 21 vol.% and 5 to 30 vol.%, respectively. The apparent volatile yield measured in CO₂ in the DTF was greater than in N₂ for all the coals studied. Pyrolysis experiments in the TGA also revealed an additional mass loss in a CO₂ atmosphere, not observed in a N₂ atmosphere, at relatively high temperatures. The coal burnout measured in the DTF at several O₂ concentrations revealed significantly higher burnouts for two coals and similar burnouts for the other two coals in oxy-fuel conditions. TGA experiments with char also revealed higher reactivity at high temperatures and low O₂ concentration. The results are consistent with a char–CO₂ reaction during the volatile yield experiments, but additional experiments are necessary to resolve the mechanisms determining the differences in coal burnout.     

Reduction and oxidation properties of Fe2O3/ZrO2 oxygen carrier for hydrogen production

Fe₂O₃ is a promising oxygen carrier for hydrogen production in the chemical-looping process. A set of kinetic studies on reduction with CH₄, CO and H₂ respectively, oxidation with water and oxygen containing Ar for chemical-looping hydrogen production was conducted. Fe₂O₃ (20 wt.%)/ZrO₂ was prepared by a co-precipitation method. The main variables in the TGA (thermogravimetric analyzer) experiment were temperatures and gas concentrations. The reaction kinetics parameters were estimated based on the experimental data. In the reduction by CH₄, CO and H₂, the reaction rate changed near FeO. Changes in the reaction rate due to phase transformation were observed at low temperature and low gas concentration during the reduction by CH₄, but the phenomenon was not remarkable for the reduction by CO and H₂. The reduction rate achieved using CO and H₂ was relatively faster than achieved using CH₄. The Hancock and Sharp method of comparing the kinetics of isothermal solid-state reactions was applied. A phase boundary controlled model (contacting sphere) was applied to the reduction of Fe₂O₃ to FeO by CH₄, and a different phase boundary controlled model (contacting infinite slab) was fit well to the reduction of FeO to Fe by CH₄. The reduction of Fe₂O₃ to Fe by CO and H₂ can be described by the former phase boundary controlled model (contacting sphere). This phase boundary controlled model (contacting sphere) also fit well for the oxidation of Fe to Fe₃O₄ by water and FeO to Fe₂O₃ by oxygen containing Ar. These kinetics data could be used to design chemical-looping hydrogen production systems.     

Factors affecting coal particle ignition under oxyfuel combustion atmospheres

A set of 13 coals of different rank has been tested for ignition propensity in a 20-L explosion chamber simulating oxyfuel combustion gas conditions. Their char residues were also analysed thermogravimetrically. The effects of coal type, coal concentration (from 100 to 600 g/m³), O₂ in CO₂ atmospheres (up to 40% v/v) and particle size were investigated.The higher rank coals were significantly more difficult to ignite and mostly required higher energy chemical igniters (1000 or 2500 J) whereas the lower rank coals could be ignited with a 500 J igniter even at low coal dust concentrations.The minimum explosibility limit/ignition concentration in air varied slightly around a value of 200 g/m³, a little higher for low volatile coals and a little lower for high volatile coals.The ignition limit changed significantly, however, with O₂ concentration in CO₂, where coals required more oxygen to ignite. Most coals failed to ignite at all in 21% v/v O₂ in CO₂, but an increase to 30 or 35% v/v O₂ gave ignition patterns similar to those in air. In addition, the minimum ignition concentration decreased with increase in O₂. However, a further increase to 40% v/v O₂ did not generally affect the minimum ignition concentration.Particle size had a non-linear effect on coal ignition. The fine particles (<53 μm) behaved almost identical to the whole coal. However, the larger size fraction (>53 μm) was generally more difficult to ignite and exhibited a much lower weight loss.     

NH3 and HCN formation during the gasification of three rank-ordered coals in steam and oxygen

A Victorian brown coal (68.5% C), a Chinese high-volatile Shenmu bituminous coal (82.3% C) and a Chinese low-volatile Dongshan bituminous coal (90% C) were gasified in a fluidised-bed/fixed-bed reactor at 800 °C in atmospheres containing 15% H₂O, 2000 ppm O₂ or 15% H₂O + 2000 ppm O₂. While the gasification of these coals in 2000 ppm O₂ converted less than 27% of coal-N into NH₃, the introduction of steam played a vital role in converting a large proportion of coal-N into NH₃ by providing H on char surface. The importance of the roles of steam in the formation of NH₃ in atmospheres containing 15% H₂O + 2000 ppm O₂ decreased with increasing coal rank. This is largely due to the slow gasification of high-rank coal chars, resulting in low availability of H on char surface. The gasification of chars from the high-rank coal appears to produce higher yields of HCN than that of lower rank coals, probably as a result of the decomposition of partially hydrogenated/broken/activated char-N structures during gasification at high temperature. The alkali and alkaline earth metallic species in brown coal tend to favour the release of coal-N as tar-N but have limited effects on char-N conversion during gasification.     

Study of intrinsic sulfidation behavior of Fe2O3 for high temperature H2S removal

Iron-based sorbent was preferable for desulfurization from coal-derived gas due to economic consideration and favorable dynamic property. The intrinsic behavior of Fe-based sorbent should be primarily understood in the sulfidation process for improving its performance. A series of tests were carried out with Fe₂O₃, Fe and other compounds containing-Fe (FO) made from the same precursor FeC₂O₄·2H₂O in H₂S-N₂ mixture in this study. The formation of H₂ was observed with Fe and FO as sorbents. While SO₂ was detected with FO and Fe₂O₃ as sorbents, its concentration in outlet was gradually decreased. The crystal phase and surface chemical state of fresh and sulfided Fe₂O₃ with different reaction times were characterized by XRD and XPS measurements. The result suggested that the intrinsic H₂S removal by Fe₂O₃ would produce multi-phase of sulfides. The possible mechanism of sulfidation reaction was discussed.     

Synthesis of magnetic Pb/Fe3O4/SiO2 and its catalytic activity for propylene carbonate synthesis via urea and 1,2-propylene glycol

To facilitate the recovery of Pb/SiO₂ catalyst, magnetic Pb/Fe₃O₄/SiO₂ samples were prepared separately by emulsification, sol-gel and incipient impregnation methods. The catalyst samples were characterized by means of X-ray diffraction and N₂ adsorption-desorption, and their catalytic activity was investigated in the reaction for synthesizing propylene carbonate from urea and 1,2-propylene glycol. When the gelatin was applied in the preparation of Fe₃O₄ at 60°C and the pH value was controlled at 4 in the preparation of Fe₃O₄/SiO₂, the Pb/Fe₃O₄/SiO₂ sample shows good catalytic activity and magnetism. Under the reaction conditions of a reaction temperature of 180°C, reaction time of 2 h, catalyst percentage of 1.7 wt-% and a molar ratio of urea to PG of 1:4, the yield of propylene carbonate attained was 87.7%.     

Volatilization behavior of fluorine in coal during fluidized-bed pyrolysis and CO2-gasification

The volatilization behavior of fluorine in five Chinese coals was investigated during fluidized-bed pyrolysis and CO₂-gasification at a temperature range of 500-900 °C. The effect of co-existed and added calcium on fluorine volatility during pyrolysis was also determined. With increasing pyrolysis temperature, the volatility of fluorine increases. However, the volatility is greatly dependent on the fluorine chemical forms occurred in coal. Except for Datong and Zhungeer coal, more than 65% of fluorine in other three coals occurs as the steady forms. Fluorapatite is not the major carrier of fluorine in the coals studied. Fluorine volatility is retarded by coexisting calcium during coal pyrolysis, indicating that at least part of the stable forms of fluorine in coal might occur as calcium fluoride or calcium fluoride with complex compounds which are stable even at high pyrolysis temperature. The addition of CaO and limestone can suppress the release of fluorine during pyrolysis. The effect of CaO is better than that of limestone. The volatility of fluorine of coal during CO₂-gasification depends on not only the occurrence mode of fluorine, but also the gasification reactivity of the coal. Compared with N₂ atmosphere, CO₂ is more favorable to the release of fluorine from coal.     

Nanocomposite Fe2O3-Se as a new lithium storage material

The electrochemical properties of nanocomposite Fe₂O₃-Se thin film prepared by pulsed laser deposition (PLD) method have been investigated by cyclic voltammetry and charge/discharge measurements. A large reversible capacity of nanocomposite Fe₂O₃-Se thin film was found to be around 650 mAh g⁻¹. A new couple of reduction and oxidation peaks at 1.4 and 1.8 V were observed from cyclic voltammogram for the first time. Our data demonstrated that nanocomposite Fe₂O₃-Se exhibit larger capacity and better cycle performance than pure Fe₂O₃. The electrochemical reaction mechanisms of Fe₂O₃-Se with lithium were examined by X-ray photoelectron spectroscopy (XPS), high resolution transmission electron microscopy (HRTEM) and selected-area electron diffraction (SAED). The reversible conversions reaction of nanosized metal Fe with Li₂Se and Li₂O formed after initial discharge process into FeSe and Fe₂O₃ respectively were revealed.     

Corrosion mechanisms of Al2O3/MgAl2O4 by V2O5, NiO, Fe2O3 and vanadium slag

The effects of V₂O₅, NiO, Fe₂O₃ and vanadium slag on the corrosion of Al₂O₃ and MgAl₂O₄ have been investigated. The specimens of Al₂O₃ and MgAl₂O₄ with the respective oxides above mentioned were heated at 10 °C/min from room temperature up to three different temperatures: 1400, 1450 and 1500 °C. The corrosion mechanisms of each system were followed by XRD and SEM analyses. The results obtained showed that Al₂O₃ was less affected by the studied oxides than MgAl₂O₄. Alumina was only attacked by NiO forming NiAl₂O₄ spinel, while the MgAl₂O₄ spinel was attacked by V₂O₅ forming MgV₂O₆. It was also observed that Fe₂O₃ and Mg, Ni, V and Fe present in the vanadium slag diffused into Al₂O₃. On the other hand, the Fe₂O₃ and Ca, S, Si, Na, Mg, V and Fe diffused into the MgAl₂O₄ structure. Finally, the results obtained were compared with those predicted by the FactSage software.     

Dehydrogenation activities of coal liquefaction catalysts on various ranks of coals

The amount of hydrogen evolved from a coal during pyrolysis at atmospheric pressure in the presence of coal hydroliquefaction catalysts can be used to compare the activity of different catalysts for the highpressure hydrogenation of coals. This technique has now been used to compare the variation in the effectiveness of MoO₃-TiO₂, and Fe₂O₃-SO^2?₄, as hydroliquefaction catalysts with change in coal rank. It is indicated that coals with carbon contents of ≈ 82 wt% are the most amenable to hydroliquefaction at high pressure.     

Fe2O3对高变质程度脱灰煤热解反应性与半焦结构的影响

利用热天平研究了共混合法负载Fe₂O₃高变质程度脱灰煤的热解反应性，结果表明煤粉负载Fe₂O₃后热解反应性高于无负载的热解反应性。负载Fe₂O₃煤样在程序升温加热的马弗炉中制备出半焦，利用FTIR、XRD和RAMAN 等分析了半焦结构。由TG和FTIR可知，负载Fe₂O₃煤样热解时，热解转化率增加，热解后自由基增加。由XRD可知，Fe₂O₃没有使得煤样半焦的002峰衍射角发生明显变化，但使L_a和L_c参数明显降低，说明半焦的微晶结构石墨化程度降低。另外，在XRD分析谱图中发现部分Fe₂O₃被还原成FeO。由RAMAN可知，Fe₂O₃使半焦的G峰峰面积降低，D峰峰面积增加，说明半焦的有机结构有序化程度降低。     

NiO/Al2O3 oxygen carriers for chemical-looping combustion prepared by impregnation and deposition-precipitation methods

Ni-based oxygen carriers (OC) with different NiO content were prepared by incipient wet impregnation, at ambient (AI), and hot conditions (HI) and by deposition-precipitation (DP) methods using γ-Al₂O₃ and α-Al₂O₃ as supports. The OC were characterized by BET, Hg porosimetry, mechanical strength, TPR, XRD and SEM/EDX techniques. Reactivity of the OC was measured in a thermogravimetric analyzer and methane combustion selectivity towards CO₂ and H₂O, attrition rate, and agglomeration behavior were analyzed in a batch fluidized bed reactor during multicycle reduction-oxidation tests.XRD and TPR analysis showed the presence of both free NiO and NiAl₂O₄ phases in most of the OC. The interaction of the NiO with the alumina during OC preparation formed NiAl₂O₄ that affected negatively to the OC reactivity and methane combustion selectivity towards CO₂ and H₂O during the reduction reaction. The NiO-alumina interaction was more affected by the support type than by the preparation method used. The NiO-alumina interaction was stronger in the OC prepared on γ-Al₂O₃.The OC were evaluated in the fluidized bed reactor with respect to the agglomeration process. OC prepared by the AI and HI methods with NiO contents up to 25 wt%, OC prepared by the DP method on γ-Al₂O₃ with NiO content lower than 30 wt%, and OC prepared by the DP method on α-Al₂O₃ with a NiO content lower than 26 wt% did not agglomerated. OC that agglomerated showed an external layer of NiO over the particles. It seems that the most important factor affecting to the formation of the external NiO layer on the OC, and so to the agglomeration process, was the metal content of the OC. The attrition rates of the OC prepared using γ-Al₂O₃ as support were higher than the ones prepared using α-Al₂O₃ as support, and in general the attrition rates of all the OC were low.The OC prepared by AI, HI or DP methods on α-Al₂O₃ as support had appropriated characteristics to be used in the chemical-looping combustion process.     

Pulverized coal combustion in air and in O2/CO2 mixtures with NOx recycle

This paper presents experimental results of a 20 kW vertical combustor equipped with a single pf-burner on pulverised coal combustion in air and O₂/CO₂ mixtures with NO_x recycle. Experimental results on combustion performance and NO_x emissions of seven international bituminous coals in air and in O₂/CO₂ mixtures confirm the previous findings of the authors that the O₂ concentration in the O₂/CO₂ mixture has to be 30% or higher to produce matching temperature profiles to those of coal-air combustion while coal combustion in 30% O₂/70% CO₂ leads to better coal burnout and less NO_x emissions than coal combustion in air. Experimental results with NO_x recycle reveal that the reduction of the recycled NO depends on the combustion media, combustion mode (staging or non-staging) and recycling location. Generally, more NO is reduced with coal combustion in 30% O₂/70% CO₂ than with coal combustion in air. Up to 88 and 92% reductions of the recycled NO can be achieved with coal combustion in air and in 30% O₂/70% CO₂ respectively. More NO is reduced with oxidant staging than without oxidant staging when NO is recycled through the burner. Much more NO is reduced when NO recycled through the burner (from 65 to 92%) than when NO is recycled through the staging tertiary oxidant ports (from 33 to 54%). The concentration of the recycled NO has little influence on the reduction efficiency of the recycled NO with both combustion media—air and 30% O₂/70% CO₂.     

Integrated coal pyrolysis with CO2 reforming of methane over Ni/MgO catalyst for improving tar yield

A new process to integrate coal pyrolysis with CO₂ reforming of methane over Ni/MgO catalyst was put forward for improving tar yield. And several Chinese coals were used to confirm the validity of the process. The experiments were performed in an atmospheric fixed-bed reactor containing upper catalyst layer and lower coal layer to investigate the effect of pyrolysis temperature, coal properties, Ni loading and reduction temperature of Ni/MgO catalysts on tar, water and char yields and CH₄ conversion at fixed conditions of 400 ml/min CH₄ flow rate, 1:1 CH₄/CO₂ ratio, 30 min holding time. The results indicated that higher tar yield can be obtained in the pyrolysis of all four coals investigated when coal pyrolysis was integrated with CO₂ reforming of methane. For PS coal, the tar, water and char yield is 33.5, 25.8 and 69.5 wt.%, respectively and the CH₄ conversion is 16.8%, at the pyrolysis temperature of 750 °C over 10 wt.% Ni/MgO catalyst reduced at 850 °C. The tar yield is 1.6 and 1.8 times as that in coal pyrolysis under H₂ and N₂, respectively.     

Experimental study of ash formation during pulverized coal combustion in O2/CO2 mixtures

The present paper was addressed to mineral transformations and ash formation during O₂/CO₂ combustion of pulverized coal. Four Chinese thermal coals were burned in a drop tube furnace to generate ashes under various combustion conditions. The ash samples were characterized with XRD analysis and ⁵⁷Fe Mössbauer spectroscopy. The impacts of O₂/CO₂ combustion on mineral transformation and ash formation were explored through comparisons between O₂/CO₂ combustion and O₂/N₂ combustion. It was found that, O₂/CO₂ combustion did not significantly change the mineral phases formed in the residue ashes, but did affect the relative amounts of the mineral phases. The differences observed in the ashes formed in two atmospheres were attributed to the impact of the gas atmosphere on the combustion temperatures of coal char particles, which consequently influenced the ash formation behaviors of included minerals.     

Synthesis of a TiO2 ceramic membrane containing SrCo0.8Fe0.2O3 by the sol-gel method with a wet impregnation process for O2 and N2 permeation

A SrCo_0.8Fe_0.2O₃ impregnated TiO₂ membrane (TiO₂-SrCo_0.8Fe_0.2O₃ membrane) was successfully prepared using a sol-gel method in combination with a wet impregnation process. The membrane was subjected to a single gas permeance test using oxygen (O₂) and nitrogen (N₂). The TiO₂ membrane was immersed in the SrCo_0.8Fe_0.2O₃ solution, dried and then calcined to affix SrCo_0.8Fe_0.2O₃ into the membrane. The effect of the acid/alkoxide (H⁺/Ti⁴⁺) molar ratio of the TiO₂ sol on the TiO₂ phase transformation was investigated. The optimal molar ratio was found to be 0.5, which resulted in nanoparticles with a mean size of 5.30 nm after calcination at 400 °C. The effect of calcination temperature on the phase transformation of TiO₂ and SrCo_0.8Fe_0.2O₃ was investigated by varying the calcination temperature from 300 to 500 °C. X-ray diffraction spectroscopy (XRD) and Fourier transform infrared (FTIR) analysis confirmed that a calcination temperature of 400 °C was preferable for preparing a TiO₂-SrCo_0.8Fe_0.2O₃ membrane with fully crystallized anatase and SrCo_0.8Fe_0.2O₃ phases. The results also showed that polyvinyl alcohol (PVA) and hydroxypropyl cellulose (HPC) were completely removed. Field emission scanning electron microscopy (FESEM) analysis results showed that a crack-free and relatively dense TiO₂ membrane (∼0.75 μm thickness) was created with a multiple dip-coating process and calcination at 400 °C. The gas permeation results show that the TiO₂ and TiO₂-SrCo_0.8Fe_0.2O₃ membranes exhibited high permeances. The TiO₂-SrCo_0.8Fe_0.2O₃ membrane developed provided greater O₂/N₂ selectivity compared to the TiO₂ membrane alone.     

NOx precursors evolution during coal heating process in CO2 atmosphere

The pyrolysis/gasification experiments of Xuzhou bituminous coal (XZ) and Longyan anthracite (LY) were carried out in a tube furnace under Ar or CO₂ atmosphere, and the effect of CO₂ on the evolution of NO_x precursors, NH₃ and HCN, was studied using a Fourier transform infrared (FTIR) spectrometer. Results show that CO₂ influences NH₃ and HCN evolution process in two main ways: one is blocking the contact of the N-sites and the H-radicals by absorbed on the coal matrix surface at low temperature, and the other is opening the N-sites from the coal matrix by gasification at high temperature. For both XZ and LY coals, CO₂ atmosphere suppresses NH₃ yield and enhances HCN yield due to the gasification effect compared with that in Ar atmosphere. But the impact is not the same. The HCN/NH₃ ratio is elevated in CO₂ atmosphere compared with that in Ar atmosphere.     

Chemical zoning of calcium aluminoferrite formed during melt crystallization in CaO-SiO2-Al2O3-Fe2O3 pseudoquaternary system

In the CaO-SiO₂-Al₂O₃-Fe₂O₃ pseudoquaternary system, the solid solutions of Ca₂(Al_xFe_1−x)₂O₅, with x<0.7 (ferrite), Ca₂SiO₄ (belite), Ca₃Al₂O₆ (C₃A) and Ca₁₂Al₁₄O₃₃ (C₁₂A₇), were crystallized out of a complete melt during cooling at 8.3 °C/min. Upon cooling to 1370 °C, both the crystals of ferrite with x=0.41 and belite would start to nucleate from the melt. During further cooling, the x value of the precipitating ferrite would progressively increase and eventually approach 0.7. At ambient temperature, the ferrite crystals had a zonal structure, the x value of which successively increased from the cores toward the rims. The value of 0.45 was confirmed for the cores by EPMA. The chemical formula of the rims was determined to be Ca_2.03[Al_1.27Fe_0.68Si_0.02]_Σ1.97O₅ (x=0.65). As the crystallization of ferrite and belite proceeded, the coexisting melt would become progressively enriched in the aluminate components. After the termination of the ferrite crystallization, the C₃A and belite would immediately crystallize out of the melt, followed by the nucleation of C₁₂A₇. The C₁₂A₇ accommodated about 2.1 mass% Fe₂O₃ in the chemical formula Ca_12.03[Al_13.61Fe_0.37]_Σ13.98O₃₃, being free from the other foreign oxides (SiO₂ and P₂O₅).     

Effect of TiO2 supporting layer on Fe2O3 photoanode for efficient water splitting

For an electrochemical water splitting system, titanate nanotubular particles with a thickness of ∼700 nm produced by a hydrothermal process were repetitively coated on fluorine-doped tin oxide (FTO) glass via layer-by-layer self-assembly method. The obtained titanate/FTO films were dipped in aqueous Fe solution, followed by heat treatment for crystallization at 500 °C for 10 min in air. The UV–vis absorbance of the Fe-oxide/titanate/FTO film showed a red-shifted spectrum compared with the TiO₂/FTO coated film; this red shift was achieved by the formation of thin hematite-Fe₂O₃ and anatase-TiO₂ phases verified using X-ray diffraction and Raman results. The cyclic voltammetry results of the Fe₂O₃/TiO₂/FTO films showed distinct reversible cycle characteristics with large oxidation–reduction peaks with low onset voltage of I–V characteristics under UV–vis light illumination. The prepared Fe₂O₃/TiO₂/FTO film showed much higher photocurrent densities for more efficient water splitting under UV–vis light illumination than did the Fe₂O₃/FTO film. Its maximum photocurrent was almost 3.5 times higher than that obtained with Fe₂O₃/FTO film because of the easy electron collection in the current collector. The large current collection was due to the existence of a TiO₂ base layer beneath the Fe₂O₃ layer.     

Enhanced cycling performance of Fe3O4-graphene nanocomposite as an anode material for lithium-ion batteries

Fe₃O₄-graphene nanocomposite was prepared by a gas/liquid interface reaction. The structure and morphology of the Fe₃O₄-graphene nanocomposite were characterized by X-ray diffraction, scanning electron microscopy and high-resolution transmission electron microscopy. The electrochemical performances were evaluated in coin-type cells. Electrochemical tests show that the Fe₃O₄-22.7 wt.% graphene nanocomposite exhibits much higher capacity retention with a large reversible specific capacity of 1048 mAh g⁻¹ (99% of the initial reversible specific capacity) at the 90th cycle in comparison with that of the bare Fe₃O₄ nanoparticles (only 226 mAh g⁻¹ at the 34th cycle). The enhanced cycling performance can be attributed to the facts that the graphene sheets distributed between the Fe₃O₄ nanoparticles can prevent the aggregation of the Fe₃O₄ nanoparticles, and the Fe₃O₄-graphene nanocomposite can provide buffering spaces against the volume changes of Fe₃O₄ nanoparticles during electrochemical cycling.