Kinetic studies of steam gasification of char in the presence of H2, CO2 and CO

Kinetic studies have been carried out to elucidate the mechanisms of steam and CO₂ gasification of char and the interactions of these gasifying agents. The positive and negative influences of all product gases, excluding methane, were considered. An empirical approach was derived to describe the effects of the degree of carbon conversion on the reaction rate as a function of relevant test parameters (pressure range 1–70 bar, temperature 800–1000 °C). To assess the validity of the derived kinetic equation a model of a fluidized-bed gasifier is presented.     

CO2 and steam gasification of a grapefruit skin char

A kinetic study on the gasification of carbonised grapefruit (Citrus Aurantium) skin with CO₂ and with steam is presented. The chars from this agricultural waste show a comparatively high reactivity, which can be mostly attributed to the catalytic effect of the inorganic matter. The ash content of the carbonised substrate used in this work falls around 15% (db) potassium being the main metallic constituent. The reactivity for both, CO₂ and steam gasification, increases at increasing conversion and also does the reactivity per unit surface area, consistently with the aforementioned catalytic effect. Lowering the ash content of the char by acid washing leads to a decrease of reactivity thus confirming the catalytic activity of the inorganic matter present in the starting material. Saturation of this catalytic effect was not detected within the conversion range investigated covering in most cases up to 0.85-0.9. Apparent activation energy values within the range of 200-250 kJ/mol have been obtained for CO₂ gasification whereas the values obtained for steam gasification fall mostly between 130 and 170 kJ/mol. These values become comparable with the reported in the literature for other carbonaceous raw materials including chars from biomass residues and coals under chemical control conditions.     

Kinetics of catalytic steam gasification of bitumen coke

The catalytic steam gasification of coke from Athabasca bitumen was investigated by thermogravimetric analysis using K₂CO₃ and Na₂CO₃ as catalysts, both of which reduced the activation energy of the reaction considerably to 1.2 × 10⁵ J mol⁻¹ and 1.3 × 10⁵ J mol⁻¹, respectively, down from 2.1 × 10⁵ J mol⁻¹ for the uncatalyzed reaction. The reaction rates varied with the partial pressure of steam between 60 kPa and 85 kPa consistent with a Langmuir-Hinshelwood model, but a first order equation was also sufficient given the low partial pressures. The initial rate of gasification of the coke particles correlated linearly with the estimated external surface area of the particles, as expected from a surface reaction involving a non-porous solid. The initial reaction rate increased with increasing the catalyst loading up to 2.4 (mol potassium)/kg. A portion of the catalyst penetrated into the coke, as confirmed by secondary ion mass spectroscopy analysis, where it could not promote the reaction with steam. This result was consistent with a small increase observed in the reaction rate at low catalyst loading. The shrinking core model was successful in predicting the rates at higher conversions from the initial rate data, despite increases in BET surface area with conversion.     

Effect of mechanochemical treatment on petroleum coke-CO2 gasification

The effect of mechanochemical treatment during the grinding of petroleum coke on its gasification by CO₂ was studied. An additive derived by drying the black liquor in papermaking industry is adopted in grinding process. Results show that the gasification reactivity of petroleum coke is effectively improved by grinding, and the activation by wet grinding is more noticeable than that by dry grinding. Besides, by wet grinding petroleum coke and additive together, the active metal species in additive are not easily volatilized in gasification, and retain a high catalytic reactivity to the coke-CO₂ reaction throughout most of the conversion range. Changes in crystal structure of the petroleum coke induced by mechanochemical treatment is related to its gasification reactivity. In general, the crystalline-amorphous phase transition is the tendency of long time mechanical grinding, while a crystal structure re-formation stage is observed after wet grinding of petroleum coke with and without additive for some time. Similar phenomenon has also been found in the reported data, but not given attention. Some discussion is made in the paper, and more work should be undertaken to disclose the mechanism.     

Selective removal of H2S from coke oven gas

The catalytic performance of some metal oxides in the selective oxidation of H₂S in the stream containing water vapor and ammonia was investigated in this study. Among the catalysts tested, V₂O₅/SiO₂ and Fe₂O₃/SiO₂ catalyst showed good conversion of H₂S with very low selectivity to undesired SO₂. Hydrogen sulfide could be recovered as harmless solid products (elemental sulfur and various ammonium salts), and distribution of solid products was varied with types of catalyst and compositions of reactant. XRD and FT-IR analysis revealed that the salt was mixture of ammonium–sulfur–oxygen compounds. It was noteworthy that V₂O₅/SiO₂ catalyst produced elemental sulfur and ammonium thiosulfate, and that elemental sulfur was principal product on Fe₂O₃/SiO₂ catalyst. Small amount of ammonium sulfate was obtained with the Fe₂O₃/SiO₂ catalyst. In order to elucidate the reaction path, the effects of O₂/H₂S ratio and concentration of NH₃ and H₂O are also studied with the V₂O₅/SiO₂ catalyst.     

Chemical-looping with oxygen uncoupling using CuO/ZrO2 with petroleum coke

Oxygen carrier particles of CuO/ZrO₂ were reacted with petroleum coke using chemical-looping with oxygen uncoupling (CLOU). The fuel was burnt in gas-phase oxygen released from the oxygen carrier particles during the fuel oxidation. The particles were then regenerated in 5-21% oxygen. In this process, the carbon dioxide from the combustion is inherently separated from the rest of the flue gases without the need for an energy intensive air separation unit. Copper oxide has thermodynamic characteristics that make it suitable as an oxygen carrier in CLOU. Particles were prepared by freeze granulation and were exposed cyclically with petroleum coke and oxygen in a laboratory fluidized bed reactor of quartz. The reaction temperature and oxygen concentration during the oxidation were varied. The average conversion rate of petroleum coke was a function of temperature and varied between 0.5%/s and 5%/s in the set-point temperature interval 885-985 °C. The conversion rate is considerably higher than rates obtained with the same fuel using iron-based oxygen-carrier in chemical-looping combustion. As for the regeneration with oxygen, the reduced particles reacted at low oxygen concentrations, with a considerable part of the reaction occurring near the thermodynamic equilibrium.     

Kinetic studies of char gasification by steam and CO2 in the presence of H2 and CO

Char-CO₂ gasification reactions in the presence of CO and char-steam gasification reactions in the presence of H₂ were studied at the atmospheric condition using a thermogravimetric apparatus (TGA) at various reactant partial pressures and within a temperature range of 1123 K-1223 K. The char was prepared from a lignite coal. The partial pressure of H₂ and CO varied from 0.05 to 0.3 atm. The experimental results showed that Langmuir-Hinshelwood (L-H) kinetic equation was applicable to describe the inhibition effects of CO and H₂. The kinetic parameters in L-H equations were obtained. Interactions of char gasification by steam and CO₂ in the presence of H₂ and CO were discussed. It was found that the kinetic parameters determined from pure or binary gas mixtures can be used to predict multi-component gasification rates. The results confirmed that the char-steam and char-CO₂ reactions proceed on separate active sites rather than common active sites.     

The phenol steam reforming reaction towards H2 production on natural calcite

The steam reforming of phenol towards H₂ production was studied in the 650–800 °C range over a natural pre-calcined (air, 850 °C) calcite material. The effects of reaction temperature, water, hydrogen, and carbon dioxide feed concentrations, and gas hourly space velocity (GHSV, h⁻¹) were investigated. The increase of reaction temperature in the 650–800 °C range and water feed concentration in the 40–50 vol% range were found to be beneficial for catalyst activity and H₂-yield. A similar result was also obtained in the case of decreasing the GHSV from 85,000 to 30,000 h⁻¹. The effect of concentration of carbon dioxide and hydrogen in the phenol/water feed stream was found to significantly decrease the rate of phenol steam reforming reaction. The latter was probed to be related to the reduction in the rate of water dissociation as evidenced by the significant decrease in the concentration of adsorbed bicarbonate and OH species on the surface of CaO according to in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS)-CO₂ adsorption experiments in the presence of water and hydrogen in the feed stream. Details of the CO₂ adsorption on the CaO surface at different reaction temperatures and gas atmospheres using in situ DRIFTS and transient isothermal adsorption experiments with mass spectrometry were obtained. Bridged, bicarbonate and unidentate carbonate species were formed under CO₂/H₂O/He gas mixtures at 600 °C with the latter being the most populated. A substantial decrease in the surface concentration of bicarbonate and OH species was observed when the CaO surface was exposed to CO₂/H₂O/H₂/He gas mixtures at 600 °C, result that probes for the inhibiting effect of H₂ on the phenol steam reforming activity. Phenol steam reforming reaction followed by isothermal oxygen titration allowed the measurement of accumulated “carbonaceous” species formed during phenol steam reforming as a function of reaction temperature and short time on stream. An increase in the amount of “carbonaceous” species with reaction time (650–800 °C range) was evidenced, in particular at 800 °C (4.7 vs. 6.7 mg C/g solid after 5 and 20 min on stream, respectively).     

Influence of coal characteristics on CO2 gasification

The rate of CO₂ gasification of a coke is one of the most important qualities of metallurgical coke. Many workers are trying to estimate the rate of CO₂ gasification of coke by studying properties of coal, such as the reflectance of vitrinite and the amounts of inertinite and ash in coal. The specific-gravity separation method is used to prepare coals which possess almost the same reflectance, but contain different amounts of inertinite and ash. The relation between the rate of gasification and the properties of coal is quantitatively explained.     

Rapid preparation of CaB6 powders via induction heating from low-cost colemanite and petroleum coke

Calcium hexaboride (CaB₆) powders were successfully prepared at 1650?°C for 20?min by induction heating technology using low-cost natural colemanite and petroleum coke as starting materials. The effects of synthesis temperature, holding time, raw material ratio and milling duration on the formation of CaB₆ were investigated. The synthesis temperature and time for CaB₆ were reduced versus traditional carbothermal reduction methods. The final products obtained after repeated HCl pickling, washing, and drying have good crystallization with a uniform particle size distribution. This rapid preparation of CaB₆ can be ascribed to the rapid formation of the B₂O₃-CaO molten liquid phase that is conducive to the synthesis reaction under induced electromagnetic field. Thus, induction heating technology is a time-saving and energy efficient way to prepare CaB₆.     

一种石油焦与CO2高温气化原位反应特性

采用高温热台显微镜原位研究了气化温度和颗粒粒度对石油焦气化反应的影响,并对比了神府烟煤和石油焦的反应特性。研究发现,石油焦在气化过程中颗粒收缩,表面结构发生变化,反应速率的改变由低温下逐渐减小,到高温下1300℃时先增大后减小。相同转化率下,反应时间随温度的升高而减少,随颗粒粒度的减小而减少。对比神府煤焦和石油焦的气化反应发现,石油焦反应活性低,为神府烟煤焦平均气化反应速率的1/6。     

Low-temperature catalytic conversion of lignite: 2. Recovery and reuse of potassium carbonate supported on perovskite oxide in steam gasification

Catalytic recovery after repeated uses of unsupported K₂CO₃ or K₂CO₃ supported on 3 kinds of perovskites (LaMn_0.8Cu_0.2O₃, LaMn_0.8Cu_0.2O₃/γ-alumina, and La_0.9K_0.1MnO₃) was investigated during steam gasification of an Indonesian lignite (Adaro) at 700 °C. Perovskite supports effectively retained K₂CO₃ and maintained higher catalytic activity than K₂CO₃ alone. The supported catalysts were recovered from the ash after gasification based on their size and ferromagnetism. Quartz and alumina accumulation on the catalyst poisoned the ash due to reactivity with potassium. Catalytic activity as high as 90% carbon conversion was maintained up to seven cycles, and separation from the ash after gasification regenerated the activity.     

Yolk-shell structured Pt-CeO2@Ni-SiO2 as an efficient catalyst for enhanced hydrogen production from ethanol steam reforming

A novel Pt-CeO₂@Ni-SiO₂ yolk-shell catalyst was fabricated through modified Stöber method and was applied to the ethanol steam reforming reaction. The catalyst structure was characterized by scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectrometry, X-ray diffraction, nitrogen adsorption-desorption and X-ray photoelectron spectroscopy. Compared with the mechanically mixed catalyst, the yolk-shell catalyst exhibited excellent catalytic performance and stability in ethanol steam reforming. The selection of hydrogen reached as high as 66%, and the conversion reached nearly 100% at 400 °C for 28 h.     

H2 production through steam reforming of ethanol over Pt/ZrO2, Pt/CeO2 and Pt/CeZrO2 catalysts

The effect of the support nature and metal dispersion on the performance of Pt catalysts during steam reforming of ethanol was studied. H₂ and CO production was facilitated over Pt/CeO₂ and Pt/CeZrO₂, whereas the acetaldehyde and ethene formation was favored on Pt/ZrO₂. According to the reaction mechanism, determined by temperature-programmed desorption (TPD) and Diffuse Reflectance Infrared Spectroscopy (DRIFTS) analysis, some reaction pathways are favored depending on the support nature, which can explain the differences observed on the resulting product distribution.     

H2 rich syngas by selective CO2 removal from biomass gasification in a dual fluidized bed system — Process modelling approach

A process model of dual fluidized bed gasification is presented based on mass- and energy balances. The model further covers the evaluation of thermodynamic equilibrium states. The gasification is investigated for the special case that CaO/CaCO₃ is used as bed material allowing selective transport of CO₂ from the gasification reactor to the combustion reactor by repeated carbonation and calcination. Experimental data are used to determine the model parameters. An empirical approach towards the kinetics of fuel conversion allows prediction of process behaviour at varied fuel water content. The selective transport of CO₂ results in high H₂ contents in the produced syngas. The lower operating temperatures in the gasification reactor increase the efficiency of energy conversion. The results are in agreement with experimental data and show the thermodynamic limitations of the technology.     

Platinum,barium and platinum-barium catalysed gasification of graphite in steam and CO2

The concept of forming a bifunctional catalyst system for the gasification of carbon was investigated using controlled atmosphere electron microscopy. Platinum and barium were chosen as the catalyst components and their effect has been studied, both in separate and combined forms, on the gasification of graphite in steam and CO₂. Platinum was found to operate in both systems primarily by the channelling mode but was not very active and was susceptible to deactivation at temperatures 900 °C in a steam environment. This deactivation phenomenon is believed to arise from carbon deposition or precipitation during the reaction. In contrast, previous studies have shown that barium is a relatively active catalyst for both these reactions and operates predominantly by the edge recession mode. The mixed platinum-barium catalyst only exhibited channelling activity, but the intrinsic rate indicated that it probably functions as a true bifunctional catalyst with barium increasing the oxygen reactivity, and platinum the carbon reactivity. The results of this study demonstrate the potential of multicomponent catalysts in which the components are selected by their ability to enhance the rate of one particular step in the reaction scheme.     

10MWth高硫石油焦化学链气化制合成气耦合硫磺回收新系统模拟研究

基于化学链气化技术依靠气固反应定向调控气化产物中H₂S和SO₂摩尔比为2的优势，将化学链气化与Claus工艺中的催化转化单元相结合，提出了高硫石油焦化学链气化制合成气和回收硫磺的新系统。针对系统核心单元，即化学链气化过程，基于Aspen Plus，开展热输入10 MW_th的高硫石油焦化学链气化过程模拟，以赤铁矿石为载氧体，水蒸气为气化介质，重点考察了氧碳比、气化温度对化学链气化过程及硫转化过程的影响。结果发现，氧碳比的增大导致合成气产率显著降低，但系统从需要外部提供能量逐渐转变为对外部放热，在氧碳比0.8669～0.9535区间内，系统可以达到热量自平衡。同时，气化温度的提高对合成气产率是有利的，在975℃时达到2.15 m³/kg，主要是由于CO体积分数随气化温度增加而增加。氧碳比和气化温度的提高都会导致H₂S浓度的降低和SO₂浓度的提高。并且研究了当H₂S和SO₂摩尔比为2的最佳工况时，氧碳比和气化温度为反相关，其中氧碳比为0.8669，气化温度为900℃时，冷煤气效率为64.09%。