Experimental and numerical simulation of lignite chemical looping gasification with phosphogypsum as oxygen carrier in a fluidized bed

Phosphogypsum (PG) is a solid waste produced in the wet process of producing phosphoric acid.Lignite is a kind of promising chemical raw material.However,the high sulfur of lignite limits the utilization of lig-nite as a resource.Based on fluidized bed experiments,the optimal reaction conditions for the production syngas by lignite chemical looping gasification (CLG) with PG as oxygen carrier were studied.The study found that the optimal reaction temperature should not exceed 1123 K;the mole ratio of water vapor to lignite should be about 0.2;the mole ratio of PG oxygen carrier to lignite should be about 0.6.Meanwhile,commercial software Comsol was used to establish a fuel reaction kinetics model.Through computational fluid dynamics (CFD) numerical simulation,the process of reaction in fluidized bed were well captured.The model was based on a two-fluid model and coupled mass transfer,heat transfer and chemical reac-tions.This study showed that the fluidized bed presents a flow structure in which gas and solid coexist.There was a high temperature zone in the middle and lower parts of the fluidized bed.It could be seen from the results of the flow field simulated that the fluidized bed was beneficial to the progress of the gasification reaction.     

天然气转化制取合成气工艺方法

天然气（ＣＨ４）转化制得ＣＯ＋Ｈ２合成气可以生产氨、甲醇及有机化工产品，需要按照转化原理进行工艺优化，以节省原料消耗，满足合成产品对组分要求．     

Low-temperature methanol synthesis in a circulating slurry bubble reactor

A circulating slurry bubble reactor was developed to synthesise methanol via methyl formate from the gas mixture of carbon monoxide and hydrogen at low temperature. The strategy for designing and scaling up the bubble reactor involved a preliminary understanding of fluid dynamics in a cold model, continuous operations under industrial conditions and a parallel experiment in an autoclave. Per-pass syngas conversion was investigated during 100-h operations. The axial profile of solid catalyst concentration was measured just before the shutdown and the composition of liquid product was analysed after the shutdown. These results show that the circulating slurry bubble column will become a potential reactor for the commercial process of low-temperature methanol synthesis after the catalyst system has been improved.     

Modification of catalytic performances due to the co-feeding of hydrogen or carbon dioxide in the partial oxidation of methane over a NiO/γ-Al2O3 catalyst

The influence of the addition of 1–10 vol.% of hydrogen or carbon dioxide to the feed during the partial oxidation of methane was studied over a NiO/γ-Al₂O₃ catalyst. The addition of H₂ decreases the conversion and syngas selectivity. This decrease of performance seems to be related to a higher reduction of the catalyst due to the H₂ co-feeding. The addition of CO₂ also appears unfavorable to the production of hydrogen but increases the CO yield. A combination of the dry reforming and the reverse water gas shift reactions is suggested to explain the observed modifications in the product yields.     

Characterization and activity in dry reforming of methane on NiMg/Al and Ni/MgO catalysts

Dry reforming of methane has been investigated on two series of catalysts either prepared by co-precipitation: n(Ni_xMg_y)/Al, Ni_xMg_y and Ni_xAl_y or prepared by impregnation: Ni/MgO (mol% Ni = 5, 10). The catalysts, calcined at 600–900 °C, were characterized by different techniques: BET, H₂-TPR, TPO, XRD, IR, and TEM-EDX analysis. The surface BET (30–182 m² g⁻¹) decreased with increasing the temperature of calcination, after reduction and in the presence of Mg element. The XRD analysis showed, for n(Ni_xMg_y)/Al catalysts, the presence of NiAl₂O₄ and NiO–MgO solid solutions. The catalyst reducibility decreased with increasing the temperature of pretreatment. The n(Ni_xMg_y)/Al catalysts were active for dry reforming of methane with a good resistance to coke formation. The bimetallic catalyst Ni_0.05Mg_0.95 (calcined at 750 °C and tested at 800 °C) presents a poor activity. In contrast, the 5% Ni/MgO catalyst, having the same composition but prepared by impregnation, presents a high activity for the same calcination and reaction conditions. For all the catalysts the activity decreased with increasing the temperature of calcination and a previous H₂-reduction of the catalyst improves the performances. The TPO profiles and TEM-EDX analysis showed mainly four types of coke: CH_x species, surface carbon, nickel carbide and carbon nanotubes.     

Effects of microreactor wall heat conduction on the reforming process of methane

In this paper, the effect of wall conduction of an autothermal tubular methane microreformer is investigated numerically. It is found that the axial wall conduction can strongly influence the performance of the microreactor and should not be neglected without a careful a priori investigation of its impact. By increasing the wall thermal conductivity, the maximum wall surface temperature is decreased. Due to the complex exothermic–endothermic nature of the chemistry of reforming, the axial variation of the wall temperature is not monotonic. Methane conversion and hydrogen yield are strongly dependent on the wall inner surface temperature, hence the heat conduction through the channel wall. The equivalence ratio and the wall thickness also significantly affect the reforming effectiveness and must be carefully considered in reactor optimization. Furthermore, it is found that exothermic oxidation reaction mechanisms, especially partial oxidation, are responsible for syngas (hydrogen and carbon monoxide) production near the inlet. Farther downstream, in the oxygen deficient region, endothermic steam reforming is the main hydrogen producing mechanism. By increasing the thermal conductivity, steam reforming becomes stronger and partial oxidation becomes weaker. For all investigated inlet conditions, the highest hydrogen yield is obtained for no or very low conductive walls.     

Dry reforming of methane with isotopic gas mixture over Ni-based pyrochlore catalyst

Dry reforming of methane was carried out using isotopic ¹³CO₂ and C¹⁸O₂ gases over a Ni-based pyrochlore catalyst that was synthesized using the modified Pechini method. In this method, 1 wt% Ni was doped into the La₂Zr₂O₇ pyrochlore structure. The catalyst was characterized by H₂-TPR, TPO, and XRD and tested for its methane reforming activity under CO₂ dry-reforming reaction conditions.The results of repeated TPR/TPO cycles up to 950 °C showed that the consecutive TPR profiles were nearly identical, indicating that the catalyst was stable at high temperatures, and that the nickel oxidation/reduction processes were reversible. The dry-reforming experimental results using labelled ¹³CO₂ gas showed the syngas production for this material proceeded through the activation of CH₄ with O that came from breaking one of the CO bonds of CO₂ with the latter reaction (CO₂ activation) likely occurring at oxygen vacancies at or near the Ni particle-pyrochlore interface. It was also found that only a small portion of the CO originated from CH₄. A variation of the same experimental test, but using ¹²C¹⁸O₂, revealed only ¹²C¹⁸O was formed and no ¹²C¹⁶O was detected, ruling out the possibility of reaction with the lattice oxygen in the catalyst structure with this material. Over this catalyst, the activated CH₄ appeared to dissociate to elemental carbon on the catalyst surface, which was determined to be the source of carbon from a post reaction TPO of the catalyst that was exposed to the ¹³CO₂¹²CH₄ mixture. No carbon deposition appeared to originate from ¹³CO₂.     

Influence of fuel composition on chemiluminescence emission in premixed flames of CH4/CO2/H2/CO blends

The growing concern about pollutant emissions and depletion of fossil fuels has been a strong motivator for the development of cleaner and more efficient combustion strategies, such as the gasification of coal, biomass or waste, which have increased the interest in using a new type of fuels, mainly composed of CH₄, H₂, CO and CO₂.These new fuels, commonly called syngas, display a wide range of compositions, which affects their combustion characteristics and, in some cases, are more prone to instabilities or flashback. Since flame properties have been demonstrated to be strongly related to equivalence ratio, a precise measurement of the flame stoichiometry is a key pre-requisite for combustion optimization and prevention of unstable regimes. In particular, chemiluminescence emission from flames has been largely tested for stoichiometry monitoring for methane flames, but its use in syngas flames has been far less studied. Consequently, the main goal of this work is analyzing the effect of fuel composition on the chemiluminescence vs. equivalence ratio curves for different fuel blends, as a first approach for a wide range of syngas compositions. The experimental results revealed that the ratio OH*/CH*, which had been widely demonstrated to be the best option for methane, may not be suitable for monitoring with certain fuels, such as those with a high percent of hydrogen. Alternatively, other signals, in particular the ratio OH*/CO₂*, appear as viable stoichiometry indicators in those cases.The analysis was also completed by numerical predictions with CHEMKIN. The comparisons of calculations with different flame models and experimental data reveals differences in the chemiluminescence vs. equivalence ratio curves for the different combustion regimes, depending on the range of the equivalence ratio ranges and fuel compositions. This finding, which confirms previous observations for a much narrower range of fuels, could have important practical consequences for the application of the technique in real combustors.     

Performance and stability of La0.5Sr0.5CoO3−δ perovskite as catalyst precursor for syngas production by partial oxidation of methane

The aim of this work was to investigate the performance and stability of the perovskite La_0.5Sr_0.5CoO_3−δ, as a potential catalyst precursor, for the synthesis gas production by partial oxidation of methane. For this purpose, the catalytic activity of La_0.5Sr_0.5CoO_3−δ was studied as a function of the temperature, flow rate and feed composition. In addition, its stability with the time-on-stream and redox cycles was also explored. Before and after testing, the catalyst precursor was characterized by X-ray diffraction, SEM-EDX and specific surface area (BET). The results evidenced a remarkable catalytic activity due to the stability of the cobalt, which is in a highly disperse state, in its reduced state. The CH₄ conversion and the CO and H₂ selectivities were enhanced with the increase of redox cycles. Finally, the precursor was totally regenerated to the initial perovskite structure under a specific thermal treatment.