Effect of pre-oxidation on reactivity of chars in steam

The effects of pre-oxidation of char from Taiheiyo coal, a non-caking bituminous coal, in the 400–550 °C temperature range on its gasification reactivity with N₂-H₂O at 0.1 MPa (steam partial pressure of 13.2 kPa) have been investigated. The pre-oxidation of char markedly enhances gasification rates at temperatures between 800 and 900 °C. Reactivity is found to parallel the burn-off level during preoxidation at low temperatures (400–430 °C), whereas at relatively high temperatures (480–550 °C), the burn-off level only affects the reactivity slightly. The amount of CO and CO₂ evolved from the preoxidized char by heat treatment is proportional to the burn-off level at low temperatures (400–430 °C), being closely related to the enhancement of the gasification reactivity in steam.     

The kinetics of gasification of carbon deposited on nickel catalysts

The kinetics of gasification of carbon deposited on nickel foils and nickel-alumina catalysts by steam, carbon dioxide and hydrogen are reported for the temperature range 450–850°C. At atmospheric pressure steam is the most effective gasifying agent. In all cases, the kinetic data obtained at lower temperatures (below 600°C) is consistent with control of the process by the chemical reaction. As the temperature increases, the rates of gasification by steam and carbon dioxide are limited by mass transfer effects. The concentration of methane near reaction sites affects the rate of hydrogen gasification above 650°C.     

K2CO3-catalysed steam gasification of supercritical extracted chars

The K₂C0₃-catalysed steam gasification of coal chars, obtained by the Supercritical Gas Extraction (SGE) process, is studied. Kinetics experiments used a gravimetric technique at atmospheric pressure and at temperatures ranging from 700 to 800 °C. It was found that K₂C0₃ is an effective catalyst for steam gasification of solvent extracted residue. The catalytic effect was similar to that observed for gasification of the unextracted parent coal. The gasification-time curves exhibited a sigmoid shape, which reduced to a single master curve for the various reaction temperatures studied and fitted well the predictions of the random capillary model. Activation energies, calculated using this model, varied from 155 to 173 kJ mol^?1 for the various chars studied.     

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.     

Kinetics of coal char gasification at elevated pressures

The effects of temperature, pressure, steam flow rate and CO₂/H₂O ratio of gasifying agent on the pressurized gasification of Linnancang coal char were investigated. A correlation of kinetic data was developed for coal chars from coals of different ranks at 30 kg cm ⁻² and 950 °C. The catalytic effects of Ca, Na and Fe catalysts on the gasification activity, activation energy and methane recovery were studied.     

Catalytic activity of coal minerals in water vapour gasification of coal

Possible catalytic influences of coal minerals during water vapour gasification of coal have been studied by kinetic measurements and microanalytical methods. A bituminous coal without and with various pretreatments and also model chars synthesized from PVC and PVC-sulphur mixtures were used as raw materials. Kinetic measurements were performed in a fixed-bed flow reactor at pressures between 0.2 and 2 MPa and temperatures from 880 to 1010 °C using hydrogen/water vapour mixtures as gasification agents. It was found that coal gasification at and beyond 880 °C can be decisively catalysed by the iron as constituent of mineral matter. Preconditions are elimination of inorganic sulphur and reducing atmosphere to stabilize elemental iron. The optimum pressure is in the range of 0.5 to 1 MPa. Scanning electron microscopy and electron probe microanalysis confirm that catalytic gasification starts as soon as the iron is free of sulphur. The organic sulphur of coal does not prevent but lowers the catalytic activity of iron.     

Application of K2CO3 catalysts in the coal gasification process using nuclear heat

Conventional gasification processes use coal not only as feedstock to be gasified but also for supply of energy for reaction heat, steam production, and other purposes. With a nuclear high temperature reactor (HTR) as a source for process heat, it is possible to transform the whole of the coal feed into gas. This concept offers advantages over existing gasification processes: saving of coal, as more gas can be produced from coal; less emission of pollutants, as the HTR is used for the production of steam and electricity instead of a coal-fired boiler; and a lower production cost for the gas. However, the process has the disadvantage that the temperature is limited to the outlet temperature (950 °C max) of the helium cooling gas of the HTR. Therefore the possibility of catalytic steam gasification was examined. Model calculations based on experimental results show that use of 3–4 wt% relative to coal of K²CO³ catalyst increases the throughput of a large scale nuclear gasification plant by ≈65%, while gas production costs decrease by ≈15%. Corrosion by catalysts is not significant at low concentration (< 5 wt%) and low temperature (< 900 °C).     

Ash-agglomerating gasification of coal in a spouted bed reactor

Australian bituminous coal (Hoskisson) was gasified with oxygen and steam in a 0.4m diameter spouted bed reactor at atmospheric pressure and temperatures of 1050–1170 °C to produce medium calorific value gas. High-ash agglomerates fell through the throat of the spouted bed under restricted gasification conditions, with no simultaneous loss of coal. The effects of temperature, steam-oxygen ratio, coal feed rate and coal size on carbon conversion, production of ash agglomerates, gas composition and decompsition of steam were established.     

Theoretical work on reaction sequences in the gasification of coke by carbon dioxide and by steam in conditions remote from equilibrium

The theory of the kinetics of gasification at atmospheric pressure expounded principally by Ergun is extended to reinterpret the experimental results obtained at pressures up to 5 MPa and temperatures up to 870 °C by Blackwood and his co-workers. The reaction sequences that are proposed enable a rate equation to be put forward for the general case of gasification in any mixture of carbon dioxide, carbon monoxide, steam and hydrogen at any pressure and temperature within the range of the original data. An equation for the rate of formation of methane during gasification in mixutres of steam and hydrogen is offered and some other consequences of the theory are mentioned.     

Three‐stage well‐mixed reactor model for a pressurized coal gasifier

Three Canadian coals of different rank were gasified with air‐steam mixtures in a 0.1 m diameter spouted bed reactor at pressures to 292 kPa, average bed temperatures varying between 840 and 960°C, and steam‐to‐coal feed ratios between 0.0 and 2.88. In order to analyze gasifier performance and correlate data, a three‐stage model has been developed incorporating instantaneous devolatilization of coal, instantaneous combustion of carbon at the bottom of the bed, and steam/carbon gasification and water gas shift reaction in a single well mixed isothermal stage. The capture of H₂S by limestone sorbent injection is also treated. The effects of various assumptions and model parameters on the predictions were investigated. The present model indicates that gasifier performance is mainly controlled by the fast coal devolatilization and char combustion reactions, and the contribution to carbon conversion of the slow char gasification reactions is comparatively small. The incorporation of tar decomposition into the model provides significantly closer predictions of experimental gas composition than is obtained otherwise.     

Variation of rate during potassium-catalysed CO2 gasification of coal char

The instantaneous rate of catalysed CO₂ gasification of char at 800 °C was measured at various levels of conversion. One important reason for the change in rate during the gasification is the change in the solid surface area, measured in the present study by CO₂ adsorption at 25 °C. The models which have been successful in representing the char porous structure under noncatalytic conditions were found inadequate for catalytic gasification at low conversions. Other important factors contributing to the variation in rate during conversion are the catalyst loss and the change in the catalyst/carbon ratio. A model is presented which combines the effects of these contributing factors and gives a satisfactory representation of the experimental data.     

Steam gasification of a cellulose surrogate over a fluidizable Ni/α‐alumina catalyst: A kinetic model

Catalytic steam gasification of a cellulose surrogate using a fluidizable Ni/α‐alumina catalyst is presented. Experiments were carried out in the CREC fluidized riser simulator. On this basis, a reaction network and a kinetic model for biomass catalytic steam gasification were proposed. This kinetic model was developed using a sound reaction engineering approach where reaction rates for various species are the result of the algebraic addition of dominant reactions. The modeling procedure also included the decoupled determination of intrinsic kinetic parameters and adsorption constants as allowed in the CREC riser simulator. The implemented approach eliminates overparametrization with successfully parameter correlation. Numerical regression of the experimental data led to intrinsic kinetic parameters with narrow spans showing that the proposed kinetic model satisfactorily describe the catalytic conversion of glucose under the selected gasification conditions. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

The characteristics of steam gasification of biomass and waste filter carbon

The carbon in a waste filter for water purification may be a new source of energy. The char of waste filter carbon and the char of wood chip have been gasified with steam in a thermobalance reactor under atmospheric pressure. The effect of gasification temperature (700-850°C) and partial pressure of steam (0.3-0.9 atm) on the gasification rate has been investigated. Several gas-solid reaction models have been compared for their prediction ability of the gasification reaction behavior. The modified volumetric reaction model was used to evaluate kinetic data. The gasification rate of waste filter carbon has been compared with the rates of coal and wood chip biomass. The activation energies of filter carbon and wood chip were determined to be 89.1 and 171.4 kJ/mol, respectively. The apparent reaction rate equation for waste filter carbon has been presented.     

Effects of catalysis on gasification of Datong coal char

The effects of eleven catalysts on the steam gasification of Datong coal char were studied. The catalysts were oxides and chlorides of alkali and alkaline earth metals, separately or in combination. The catalytic effects of the NaCa composite on the reaction rate, methane conversion, steam decomposition and product gas composition and heating value were studied at reaction temperatures of 700–900 °C and pressures of 0.1, 1.1, 3.1 and 5.1 MPa. A kinetic equation of catalytic gasification was derived and the reaction rate constants and the activation energy at elevated pressure were determined.     

Analysis of chemical intermediates from low-temperature steam gasification of biomass

A reactor system was developed to study the process of lignin and biomass gasification at low temperatures (100 °C to 350 °C) and high pressure (up to 375 atm). The reactor allowed for withdrawal of samples from either the top or bottom of the reaction environment throughout the period of the experiment while maintaining the reaction temperature and pressure. An analytical method was developed for separating and standardizing the initial decomposition products formed during steam-alkali gasification of kraft pine lignin and Douglas fir wood flour.     

Steam gasification of German hard coal using alkaline catalysts: Effects of carbon burn-off and ash content

Laboratory-scale experiments were performed on chars from German hard coals with potassium carbonate addition. The steam gasification rate at 4 MPa and 700 °C as a function of the amount of catalyst added is described for a low-and high-ash char. From the burn-off behaviour the reaction order relative to carbon was determined. For the low-ash char a uniform reaction order was found but the high-ash char indicated a complex interaction of catalytic gasification, catalyst deactivation, and the development of the reacting surface.     

EXPERIMENTAL INVESTIGATIONS OF BEET PULP DRYING IN SUPERHEATED STEAM UNDER PRESSURE

ABSTRACT

Beet pulp drying in superheated steam under pressure makes it possible to save energy in sugar factories. A new concept of a two-stage convective steam drier is presented. To obtain kinetic data on beet pulp drying, an experimental setup was built. Beet pulp samples were dried at steam pressure up to 4 bar and temperature up to 220° C.     

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.     

纸浆黑液对福建无烟煤水蒸气催化气化的动力学和补偿效应

在常压热分析仪上,采用纸浆黑液催化剂对福建无烟粉煤的水蒸气气化反应动力学进行了研究,在850～950℃温度范围内测定了催化剂浓度由0增至10％时的碳转化率随气化时间的变化。结果表明,纸浆黑液具有显著提高碳转化率和气化速率的作用,进而确定了实验条件下的纸浆黑液催化剂加载饱和浓度;在此基础上采用缩芯模型关联出无烟粉煤水蒸气催化气化反应动力学参数,分析表明,该催化气化过程存在明显的补偿效应,最后给出纸浆黑液对无烟粉煤水蒸气催化气化包括补偿效应的动力学方程。     

Steam gasification of coal char catalyzed by K2CO3 for enhanced production of hydrogen without formation of methane

Steam gasification of coal char catalyzed by potassium carbonate was investigated on a laboratory fixed-bed reactor to examine the catalytic effects not only on the reaction rate but also on the reaction selectivity, and non-catalytic gasification of coal char was performed by way of contrast. It was observed that the catalytic gasification of coal char with steam occurred significantly in a temperature range of 700-750 °C, producing a hydrogen-rich gas with slight formation of carbon monoxide and virtually no formation of methane. An oxygen transfer and intermediate hybrid mechanism of the catalytic char gasification with steam is proposed for understanding of the experimental data regarding both the kinetic behaviors and reaction selectivity. The study has highlighted the advantages of the catalytic gasification of coal char over the conventional coal gasification with respect to the reaction selectivity. The catalytic steam gasification of coal char makes it possible to eliminate or simplify the methane reforming and water-gas shift processes in the traditional gas-to-hydrogen purification system.