Steam Gasification Of Coal Char Research Articles

Multiple synergistic effects exerted by coexisting sodium and iron on catalytic steam gasification of coal char

Numerous studies have been concerned with inexpensive sodium- and iron-based catalysts for coal gasification. This work aims to investigate the steam gasification of coal char using a Na/Fe bimetallic catalyst. The bimetallic catalyst with an appropriate loading exhibited a higher activity for accelerating the gasification rate than the Na or Fe monometallic catalyst at 700–800 °C. Compared to the sodium catalyst, the bimetallic catalyst was favorable to a higher production of H2 by the reactions with steam. Mechanism study revealed that the coexistence of iron and sodium significantly suppressed the growth of a crystalline Na2CO3 phase and the evaporation of sodium during the gasification leading to the preserve of more sodium as an effective form. Meanwhile, the sodium compounds promoted the reduction of hematite to form α-Fe during the heat-up and the dispersion of iron on the char during the gasification. These interactions were responsible for the transformation of graphitic carbon to amorphous carbon. The bimetallic catalyst could alleviate some shortcomings of the single sodium or iron catalyst.

In situ generation of nickel/carbon catalysts by partial gasification of coal char and application for methane decomposition

Catalytic methane decomposition (CMD) has a good potential to develop environmentally friendly hydrogen economy, and the catalyst plays a vital role on its applications. In this work, a novel strategy was proposed to fabricate efficient and effective nickel/carbon catalysts for CMD by introducing some additional nickel and K2CO3 into partial steam gasification of coal char. The gasification process is conducive to in situ synthesize nickel crystallites with high reduction degree (the value of Ni0/(Ni0+Ni2+) up to 76%–81%) on the catalyst surface, and it is competent for co-generation of hydrogen-rich gas and nickel/carbon hybrids with large surface areas (around 86–149 m2/g after washing off the residual potassium salts). The nickel/carbon hybrid as the gasification residue could serve as the catalyst for CMD, showing high and stable methane conversion (up to 80%–87%) at 850 °C. It is observed that co-production of hydrogen and filamentous carbons can be achieved in the 600-min process of CMD, thanks to the positive effect of K2CO3 on formation and activity improvement of the nickel/carbon catalyst.

Methane decomposition over Ni/carbon catalysts prepared by selective gasification of coal char

Catalytic methane decomposition (CMD) can pave the route for co-production of COx free hydrogen and valuable carbon materials, and it is considered as a potential promising strategy to develop eco-friendly and low-carbon hydrogen economy. However, there are still some challenges for utilization of traditional metal and/or carbon catalysts. In this work, relatively dispersed and efficient Ni/carbon catalysts were handily prepared for CMD by selective steam gasification of coal char with addition of nickel and/or cerium compositions. Carbothermal reduction during the gasification process could enable in-situ formation of Ni0 crystallites and facilitate the gasification efficiency and thus achieving co-production of hydrogen-rich gas and Ni/carbon hybrids with large surface areas (217–265 m2/g). When the Ni/carbon hybrids as the selective gasification residues were used for CMD, high and stable methane conversion (up to about 90% at 850 °C) can be obtained in the 600-min CMD reaction, thanks to the relatively dispersed and active Ni0 crystallites, large surface area of the porous carbon support and formation and growth of the fibrous carbon deposits.

Catalytic steam gasification of coal char with alkali carbonates: A study on their synergic effects with calcium hydroxide

The order of catalytic activity of three alkali carbonates (Li2CO3, Na2CO3, and K2CO3) towards the steam gasification of coal char varies depending on the presence and absence of the acidic mineral species and their enrichment in coal. In the gasification of an ash-rich coal char, Li2CO3 exhibits the severest catalytic deactivation among three alkali carbonates, whereas Na2CO3 is the most resistant to the deactivation. The impregnation of Ca(OH)2 to the ash-rich coal substantially increases the rate of char gasification for all three alkali carbonates as a consequence of the inhibited catalytic deactivation. In the gasification of an ash-free coal char, however, only the binary K2CO3/Ca(OH)2 catalyst shows a synergic effect on promoting the gasification rate, whereas the Li2CO3/Ca(OH)2 catalyst and the Na2CO3/Ca(OH)2 catalyst have a worse catalytic activity than their respective alkali carbonates alone. The mechanisms of the concerted effects between alkali carbonates and Ca(OH)2 on the gasification of the ash-rich char and the ash-free char are discussed.

Investigation of K2CO3‐Catalyzed Pyrolysis and Steam Gasification of Coal Char

AbstractPyrolysis and steam gasification experiments of coal char with and without K2CO3 were performed using a thermogravimetric analyzer to investigate the effect of catalyst loading method on gasification reactivity at different temperatures (650–800 °C), and an additional redox cycle experiment of catalyzed char‐steam gasification was conducted in a fixed‐bed reactor. The results showed that, at low temperature, the solution impregnation method had a larger effect on the gasification reactivity than the physical blending method; at temperatures above 700 °C, the differences of two methods diminished due to the high mobility of potassium. In catalytic pyrolysis, the release of CO and CO2 indicated that K2CO3 reacted with carbon. The redox cycle experiments suggest that the potassium reacted with carbon to produce intermediates of the form K2O2C, K2OC, and K2C. The K2C intermediate may help to explain the release of more H2 during the initial period of catalytic coal char–steam gasification.

A novel development of mixed catalyst–sorbent pellets for steam gasification of coal chars with in situ CO2 capture

The main purpose of this research was the development of a mixed catalyst–sorbent for gasification and in situ CO2 capture; therefore, novel mixed catalyst–sorbent (composite) pellets were prepared with different contents of potassium carbonate and calcium oxide as the catalyst and sorbent, respectively. The pellets were used in steam gasification experiments of two different types of coal. The maximum hydrogen yield of 80% was obtained with 50% of the catalyst in the composite pellets and lignite Boundary Dam (BD) coal at 700°C. Use of the catalyst enhanced the gasification and water–gas shift (WGS) reaction rates, while the capture of carbon dioxide (CO2) shifted the WGS equilibrium forward, favoring hydrogen production. Trace amounts of CO2 and carbon monoxide were detected. In terms of maximum achievable hydrogen yield, the pellets containing 50% catalyst demonstrated the best performance. As expected, BD coal (a low-rank coal) showed a higher reactivity than the Genesee sub-bituminous (a medium-rank coal) samples. Ultimate analyses and burn-off tests were performed on the residues to determine the amounts of unconverted carbon, and the results of these tests also confirmed the best performance of the pellets with a catalyst content of 50%. Despite the decreasing trend of pore surface area with increasing catalyst percentage, increasing the catalyst content of composite pellets to a certain level had a positive effect on hydrogen production. This indicates that the performance of pellets is almost independent of the pore surface area. Indeed, the optimal ratio is the result of the trade-off among pore surface area, dispersion, formation of a bimetallic solid phase, and the concentration of active catalyst sites on the pellets.

Investigation into the kinetics of pressurized steam gasification of chars with different coal ranks

The steam gasification of coal chars derived from three different ranks of typical Chinese coals was studied in a pressurized fixed-bed differential reactor at elevated pressure (up to 2.0 MPa). Three mathematical models [volumetric model (VM), grain model (GM), and random pore model (RPM)] for the gasification kinetics of different chars were validated, through which the kinetic parameters were obtained and discussed. The results show that the evolution trend of the coal char gasification rate with carbon conversion differs from coal ranks and has little change with pressure and temperature. The pressurized gasification process of the Shenmu sub-bituminous coal char (SM char) and the Jingcheng anthracite char (JC char) can be well-predicted by the RPM, while that of the Huolinhe lignite char can be better described by the VM. The pressure has little effect on the options of the reaction kinetic models for the three chars. The kinetic parameter E is almost a constant independent of pressure, while k0 changes with pressure, and it seems that k0 would be almost a constant over 1.0 MPa for SM and JC chars. The reaction order decreases with increasing the total system pressure and differs from different coal types.

Enhancement of coal char gasification using a pyrolyzer–gasifier isolated circulating fluidized bed gasification system

In the present study, we attempted to promote the steam gasification of coal char at low temperatures (below 1173K). A novel circulating fluidized bed gasification reactor was developed. This reactor consisted of three units: a pyrolyzer, a gasifier, and a combustor. The main feature of the reactor was that the pyrolyzer was physically isolated from the gasifier. Such physical isolation of the pyrolysis zone (pyrolyzer) from the char gasification zone (gasifier) in the reactor can prevent char gasification from being inhibited by contact of the char with hydrogen gas or the tar vapor formed by pyrolysis. Therefore, char gasification was certainly promoted by the physical isolation of the pyrolyzer from the gasifier in comparison with the conventional gasification reactor, in which pyrolysis and subsequent char gasification proceed in one unit.

Calcium-promoted catalytic activity of potassium carbonate for steam gasification of coal char: Effect of hydrothermal pretreatment

Hydrothermal pretreatment of coal with the addition of Ca(OH)2 was found more effective for promoting the K2CO3-catalyzed char gasification than the physical addition way for both JY anthracite coal and HB bituminous coal used. The effect of hydrothermal pretreatment was more remarkable for JY coal which suffers significant catalyst deactivation due to its high mineral content. For this type of coal, employing more severe hydrothermal pretreatment conditions was proven to significantly enhance the gasification rate. It was observed that kaolinite and quartz in coal hydrothermally reacted with calcium forming hydrated calcium aluminosilicates, which, unlike the original minerals, was inactive for the deactivation reactions of potassium. Consequently, the hydrothermal pretreatment allowed more potassium to persist as a water-soluble entity during the gasification. This was a main mechanism underlying the promoted catalytic gasification by the hydrothermal treatment.

Calcium-promoted catalytic activity of potassium carbonate for steam gasification of coal char: Transformations of sulfur

K2CO3-catalyzed char gasification with steam was carried out on a laboratory fix-bed reactor and it featured in the preparation of char from a high-sulfur coal by the prior addition of Ca(OH)2 to the coal. This work was focused on the clarification of sulfur transformations during the coal pyrolysis and the char gasification. Sulfur K-edge X-ray absorption near edge structure (XANES) spectroscopy was employed to determine quantitatively the forms of sulfur in coal and their changes in process. Results showed that the addition of Ca(OH)2 to coal promoted the decomposition of both pyrite and organic sulfur during the charring. In the K2CO3-catalyzed char gasification, the calcium additive enabled more sulfur to be retained eventually as K2SO4 rather than CaSO4. The formation of K2SO4 would be conducive to the recycled use of potassium catalyst.

Calcium-promoted catalytic activity of potassium carbonate for steam gasification of coal char: Influences of calcium species

Experiments were carried out to investigate the K2CO3-catalyzed gasification performances for varying chars prepared from four coals with prior addition of calcium additive and without calcium additive. Three calcium species, Ca(OH)2, Ca(Ac)2 or Ca(CH3COO)2, and CaCO3, were used in the study. Experiments were also conducted in an attempt to achieve the understanding of the effect of calcium additive on the potassium deactivation, as well as the interactive effect between potassium, calcium and char. It was found that each calcium additive acted as a deterrent to the potassium deactivation and thus promoted the catalytic gasification, and the magnitudes of promotion varied depending on coal and calcium species. Ca(OH)2 and Ca(Ac)2 were more effective to inhibit the potassium deactivation than CaCO3. Furthermore, potassium together with calcium showed a mineral-unrelated synergy towards the char gasification. A bimetallic carbonate, K2Ca(CO3)2, which formed even when the chars prepared from coal with either Ca(OH)2 or Ca(Ac)2 was subsequently mixed with K2CO3, was likely to contribute to the synergistic effect.

Kinetic Parameters and Reactivity for the Steam Gasification of Coal Chars Obtained under Different Pyrolysis Temperatures and Pressures

The effect of devolatilization conditions (pressure and temperature) on the steam gasification kinetics and reactivity of coal chars was investigated. For this purpose, five different rank coals were selected: a lignite (LT), two high-volatile bituminous coals (DT and SA), a low-volatile bituminous coal (DI), and a semianthracite (HV). The coal chars were obtained by devolatilization in a novel pressurized fixed-bed reactor at temperatures of 800, 900, and 1000 °C and pressures of 1, 5, 10, and 20 atm. The reactivity of the chars was studied by isothermal thermogravimetric analysis in a kinetically controlled regime at different temperatures under a steam atmosphere. The reactive behavior of the chars was evaluated by means of the random pore model (RPM), which satisfactorily described the steam gasification of the coal chars. The gasification apparent reaction rate increased as the devolatilization pressure increased, whereas this parameter decreased as the devolatilization temperature increased.

Enhanced catalysis of K 2CO 3 for steam gasification of coal char by using Ca(OH) 2 in char preparation

A novel approach has been proposed for mitigating the potassium deactivation in the K 2CO 3-catalyzed steam gasification of coal char by addition of Ca(OH) 2 in the char preparation. It was experimentally found that the Ca(OH) 2-added char had higher reactivity for the catalytic gasification than the raw char. Ca(OH) 2 played a role in suppressing the interactions of K 2CO 3 with acidic minerals in coal during the gasification and also probably in forming more active oxygenated intermediate on the char surface. The distribution of gaseous products was examined during the catalytic gasification. An oxygen transfer and intermediate hybrid mechanism is applied for understanding of the rate and selectivity of the catalytic gasification.

Factors affecting steam gasification rate of low rank coal char in a pressurized fluidized bed

A high-pressure bubbling fluidized bed reactor was used to study the steam gasification of coal char under pressure. Indonesian sub-bituminous coal char (Adaro) and Australian lignite char (Loy Yang) were gasified with steam in the reactor at temperatures below 1173 K and at total pressures ranging from 0.1 to 0.5 MPa. The steam gasification rates of the coal chars were determined by analysis of the gaseous products. Activation energies for the steam gasification of the chars were as high as about 250 kJ/mol, which suggests that the temperature dependence of the gasification was substantial. The apparent gasification rates under the study conditions were described by a Langmuir–Hinshelwood (L–H)-type equation. Analysis of the reaction kinetics on the basis of the L–H equation indicated that increasing steam pressure effectively increased the gasification rate.

Steam gasification of coal char catalyzed by K 2CO 3 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.

Gasification Property of Direct Coal Liquefaction Residue with Steam

Steam gasification of char from Shenhua coal direct liquefaction residue (DLR) was investigated in a fixed bed reactor. The effect of temperature, mineral, liquefaction catalyst and heavy oil remained in the residue on the gasification property were examined. Compared with coal char, liquefaction residue char has lower gasification reactivity due to its main component of unreacted coal, heavy oil and condensation products during liquefaction process. The mineral matter has catalysis during the steam gasification of coal char and DLR char, the liquefaction catalyst remained in the residue is Fe1–xS and has no remarkable effect on the steam gasification reactivity, because during the steam gasification process large amount of H2S is produced by the remained liquefaction catalyst (Fe1–xS), which restrains the steam gasification of residue. After extraction of the heavy oil from the residue, the BET surface and pore structure of residue char increase but the steam gasification ability decreases.

1-16.石炭チャーの加圧水蒸気ガス化における反応挙動(Session(1)石炭・重質油)

1-16.石炭チャーの加圧水蒸気ガス化における反応挙動(Session(1)石炭・重質油)

96/03758 Steam gasification of coal chars

96/03758 Steam gasification of coal chars

Rate retardation by hydrogen in the calcium catalysed steam gasification of coal char

Calcium catalysed steam gasification of coal char was studied to clarify the effect of hydrogen on the activity. Calcium showed high catalytic activity when well dispersed by ion exchange. However, hydrogen retarded the activity of calcium, that is, the introduction of hydrogen into the reactant gas decreased the reaction rate. The catalytic activity of calcium decreased in proportion to the increase in hydrogen concentration.

Composite catalysts for carbon gasification

The activities of a variety of binary and a ternary catalysts for carbon gasification have been compared. Equimolar mixtures of various metal salts, 0.5 mol-% in total cation concentration, were loaded onto an activated carbon by simultaneous impregnation of their nitrates, and the samples were subjected to gasification at 800°C in 50% steam—helium and/or at 850°C in 50% steam—hydrogen. It was found that some composite catalysts such as Na + Ca, Na + Fe, Ca + Fe and Na + Ca + Fe showed higher activities than the sum of the individual constituents. The nature of the gasifying agent influenced the catalytic effects. The Na + Ca, Ca + Fe, Na + Fe and also Na + Ca + Fe catalysts are generally effective for gasification with oxygen-containing gases (steam, carbon dioxide and a mixture of the two); and Fe + Ca and Fe + Ca + Na also enhance hydrogasification. Some dual catalysts were successfully applied to steam gasification of coal char. The results are discussed in association with the catalytic features of sodium, calcium and iron group metals.