Effects of calcium,strontium, and barium as catalysts and sulphur scavengers in the steam gasification of coal chars

Effects of alkaline earths on the steam gasification of two coal chars, one of low and one of high intrinsic reactivity, were evaluated gravimetrically. The chars were derived from coal powders which had been impregnated with Ca, Sr, or Ba, pelletized, and pyrolysed in nitrogen. The additives increased the gasification rates in the order Ca < Sr < Ba. It follows from reaction kinetics that catalysis is caused by a large increase in the density of reaction sites, not by a lowering of the true activation energy. As shown by electron micrographs and elemental maps obtained by X-ray analysis, the strong catalytic effect is closely associated with the ability of the alkaline-earth species to spread over the char and to preserve contact with a freshly formed carbon surface as the gasification proceeds. The alkaline-earth catalysts are severely poisoned by hydrogen sulphide or sulphur dioxide from an external source.     

Chemical reduction of sulphur dioxide to free sulphur with lignite and coal. 2. Kinetics and proposed mechanism

The reactivity of lignite and different ranks of coal with sulphur dioxide has been investigated in a corrosive-gas, thermogravimetric reactor system. With all coals, the reaction occurred in two distinct stages. A rapid initial stage was controlled primarily by the devolatilization rate of the coal. The second stage limited the overall rate and was controlled by surface properties of the coal char. The portion of lignite associated with the second stage of reaction exhibited a much higher rate of SO₂ reduction than the corresponding material from all other coals. Correlation of the data showed an inverse relation between the reactivity of coal chars and the relative rank of the parent coal. Activation energies associated with the reduction of SO₂ by the coal chars increased slightly from 134 kJ mol^?1 for lignite char to 150 kJ mol^?1 for HVB bituminous coal char. The higher reactivity of lignite or lower-rank coals was due in part to entropy factors or available catalytic sites on the surface of coal. Formation of a thermally stable CS complex on the surface of coal appeared to poison the surface and thus limit further reaction. Alkali and alkaline earth metals in lignite served as active sites for catalysing the reaction of SO₂ with the CS complex and thus enhanced the rate of SO₂ reduction with lignite.     

Exploring the possibilities of using Brazilian subbituminous coals for blast furnace pulverized fuel injection

The current study investigates the combustion and blast furnace injection performance of three Brazilian subbituminous coals (Mina do Recreio) and their beneficiation products using laboratory scale combustion tests. The coals have relative high ash yields (up to 40 wt%) that were reduced stepwise to levels as low as 12 wt%, dry basis. The reduction of ash yields is paralleled by a significant decrease in sulphur and inertinite contents.The combustion tests were performed in a drop tube reactor operating at 1300 °C using two different atmospheres (2.5 and 5% O₂). The chars exhibited preferentially rounded shapes with thick walls and abundant secondary porosity for the 2.5% O₂ chars, whereas the 5% O₂ chars showed very thin walls as a consequence of extensive burnout. The intrinsic reactivities of both set of chars were similar. The differences in conversion between the two working atmospheres were 24-37% and roughly tend to increase with increasing mineral matter content. Conversions as high as 76-81% were reached operating under 5% O₂ indicating that the coals are easy to burn. The small differences in burnout among the coals and their beneficiation products cannot be clearly attributed neither to mineral matter or inertinite content. A rough inverse relationship was found between the intrinsic reactivity of the chars and the inertinite content of the parent coal indicating that the char material derived from inertinite was intrinsically less reactive than that derived from vitrinite. These differences were no longer relevant at high temperature.Blast furnace injection performance was studied through thermobalance experiments using CO₂ atmosphere and 1050 °C temperature. It is apparent that the beneficiation process has no effect on the reactivity of the coals from Recreio Mine. The only exception is the low ash coal-2-LabB (11.5 wt%), for which a higher reactivity is indicated. The reactivity tests show also that the coals have adequate properties to be used together with imported coal blends in pulverized coal injection in the blast furnace (PCI).     

New method for sulphur concentration measurements in coal and char

A new method has been developed for determining the organic sulphur concentration and the stoichiometry of iron sulphide compounds in coals and chars. The method employs a scanning electron microprobe which separates the contribution from different sulphur forms by examining the differences in their spatial distribution. The method is rapid, requires only small samples, and is non-destructive; so results may be repeated. In addition, a knowledge of the sulphide stoichiometry in a char and the original sulphide concentration in the coal yields the sulphide concentration in the char. Application of this method for sulphur measurements in more than thirty coals has yielded results in good agreement with results obtained with ASTM methods.     

Effect of catalyst on coal char structure and its role in catalytic coal gasification

The structure of the coal chars with and without catalyst was characterized using X-ray diffraction and laser Raman techniques. The catalyst changes the structure of the organic unit in coal char. The mechanism by which the catalyst affects the structure of coal char in pyrolysis was investigated by X-ray photoelectron spectroscopy. For the first time, the direct evidence of electron transfer in catalyst–coal interactions was obtained, and a K-Char intermediate forms. This makes it easier for H₂O to attack the K-Char intermediate, thus increasing the gasification reactivity of coal char.     

Co-carbonization of coking coal with K2CO3: structure of the resulting chars

Oakdale coking coal was co-carbonized with up to 30 wt% of K₂CO₃ to 900 °C. The resulting chars were examined for optical texture and morphology by scanning electron microscopy. No changes in optical texture were observed with additions of < 1 wt% K₂CO₃. Increased additions created an isotropic, non-fusing layer of char around the particles and this prevented the formation of a coherent coke. The size of the remaining anisotropy was also reduced, some char fragments being composed of isotropic carbon. Severe fissuring occurred in the particles of char, causing fragmentation; this was presumably due to diffusion of potassium into the char structure. X-ray studies indicated increased peak half-widths of (002) diffractions for the isotropic carbon.     

Thermogravimetric study of the devolatilization of hydropyrolysis chars

The devolatilization of hydropyrolysis (HyPy) chars formed at 485–850 °C under 3 MPa and chars formed at 580 °C under 0–5 MPa of H₂ and 3 MPa He is investigated in a thermobalance coupled to two gas chromatographs. The H₂, CH₄ and CO₂ released are analysed every 4 min and all are analysed at the end of the experiment. The amount of residual volatile matter in the chars decreases rectilinearly with the HyPy temperature, whereas their decrease is asymptotic with the HyPy pressure. The char formed under He contains 45% more residual volatile matter than that formed in the same conditions under H₂. The HyPy temperature must be limited if the char is to be burned in a boiler. The CH₄ formation is strongly dependant of the HyPy temperature.     

Reactivities of 34 coals under steam gasification

The reactivities of 34 coal chars of varying rank with H₂O have been determined to examine the effect of coal rank on the gasification rate of coal char. The reactivities of chars derived from caking coals and anthracites (carbon content > 78 wt%, daf) were very small compared with those from non-caking (lower-rank) coals. The reactivities of low-rank chars do not correlate with the carbon content of the parent coals. To clarify which factor is more important in determining the reactivity, the evolution of CO and CO₂ from char, the moisture content of char and the amount of exchangeable cations were determined for these low-rank coals or their chars. These values were considered to represent the amount of active carbon sties, the porosity and the catalysis by inherent mineral matters, respectively. It was concluded that the amount of surface active sites and/or the amount of exchangeable Ca and Na control the reactivity of low-rank chars in H₂O.     

Reactivity of Australian coal-derived chars to carbon dioxide

The reactivities to CO₂ of four chars derived from Australian coals at 610 °C, were measured thermogravimetrically. Reaction rates in 100% CO₂ (total pressure, 101 kPa) varied from 0.026 mg h^?1 mg^?1 at 803 °C for char derived from a Lithgow coal to 6.3 mg h^?1 mg^?1 at 968 °C for a Millmerran coal char. Activation energies for the four chars were in the range 219–233 kJ mol^?1. The results show that for Lithgow (Hartley Vale) coal char, reactivity increases with CO₂ concentration and decreasing particle size. The apparent reaction order for this char with respect to CO₂ concentration was found to be 0.7. For different chars, reactivity is inversely proportional to the rank of the parent coal. No general correlation has been established between total mineral content (ash) and char reactivity.     

Importance of carbon active sites in the gasification of coal chars

A demineralized lignite has been used in a fundamental study of the role of carbon active sites in coal char gasification. The chars were prepared in N₂ under a wide variety of conditions of heating rate (10 K min^?1 to 10⁴ K s^?1), temperature (975–1475 K) and residence time (0.3 s–1 h). Both pyrolysis residence time and temperature have a significant effect on the reactivity of chars in 0.1 MPa air, determined by isothermal thermogravimetric analysis. The chars were characterized in terms of their elemental composition, micropore volume, total and active surface area, and carbon crystallite size. Total surface area, calculated from C0₂ adsorption isotherms at 298 K, was found not to be a relevant reactivity normalization parameter. Oxygen chemisorption capacity at 375 K and 0.1 MPa air was found to be a valid index of char reactivity and, therefore, gives an indication, at least from a relative standpoint, of the concentration of carbon active sites in a char. The commonly observed deactivation of coal chars with increasing severity of pyrolysis conditions was correlated with their active surface areas. The importance of the concept of active sites in gasification reactions is illustrated for carbons of increasing purity and crystallinity including a Saran char, a graphitized carbon black and a spectroscopically pure natural graphite.     

Investigation into the evolution of char structure using Raman spectroscopy in conjunction with coal petrography; Part 1

The evolution of char structure during heat treatment was investigated using coal petrography and micro Raman spectroscopy (MRS). The heat treatment was in the temperature range of 300-1000 °C using inertinite-rich South African coals. Char morphology analyses, determined petrographically showed a significant increase in the amount of dense/solid chars as compared to porous chars as temperature increases. MRS results were given in terms of the I_D/I_G intensity ratios, band positions and bandwidths as a function of temperature. It was found that sp²-sp³ bonding (reactive sites/crystallites) was created in dense chars (originating from inertinite particles) at the initial heat treatment temperature, and these sp²-sp³ bondings were consumed later at high temperature. Earlier consumption of sp²-sp³ bonding was observed in porous chars, since they were vitrinitic in origin and contained more reactive sites. The D1 and G bandwidths showed a significant change with heat treatment, which were further correlated with the amount of dense and porous char determined petrographically. Therefore, the use of MRS and petrography on chars enhances the understanding of char evolution on a structural level and may lead to enhanced understanding of coal combustion.     

Hydropyrolysis of a high-sulphur-high-calcite Italian Sulcis coal. 1. Hydropyrolysis yields and catalytic effect of the calcite

Hydropyrolysis (HyPy) of a high-sulphur (4.3 wt% mf) and high-calcite (7.3 wt% mf) subbituminous coal (Sulcis coal) has been studied in a semi-batch fixed-bed reactor under a pressure of 1 or 3 MPa from 580 to 850 °C. The maximum temperature attained is not necessarily the temperature that the reactor is set but depends on the pressure and nature (reactive or not) of the gas; this phenomenon is due to the heat from the exothermic HyPy reaction. There is a correlation between the amount of heat released during the hydrogénation and the amount of water formed. The maximum conversion obtained is 62.5 wt% maf under H₂ at 3 MPa and 850 °C. The char, oil, water, gas (CH₄, C₂H₄, C₂H₆, CO, C0₂) yields and the oil analysis are reported. A significant proportion of the C0₂ evolved during the reaction results from the decomposition of the mineral matter rich in carbonates. A proportion of the CO evolved results from the degradation of phenols, a reaction which is catalysed by calcite and/or lime, and as a consequence the oil yield is reduced.     

Effect of iron enrichment with GIC or FeCl3 on the pore structure and reactivity of coking coal

The effect of a Lewis acid addition to a coking coal on the porosity and reactivity towards steam of the resulting iron enriched coal chars are studied. GIC (FeCl₃ graphite intercalation compound) or free FeCl₃ are used as iron containing additives. Coal iron enrichment was performed using either directly FeCl₃ in vapour phase, or by mixing of coal and additives in decaline or by common grinding of coal and additives under argon. Iron enriched coals were carbonized at 750°C (heating RATE = 5°C min) and activation made with pure steam at 800°C to a burn-off off of 50 wt%. The pore structures of coal chars before and after activation were evaluated on the basis of CO₂ and C₆H₆ sorption at 25°C. A significant development of the microporosity is observed in the iron enriched char before activation and its steam reactivity is also increased. After activation, BET surface area values are increased in presence of iron, and porosity is mainly microporous.     

碱金属对煤热解和气化反应速率的影响

通过对原煤、酸洗原煤、负载碱金属的酸洗原煤在800～1050℃热解制得焦样,用X射线衍射技术考察了碱金属对煤焦微晶结构的影响,在加压热天平（PTGA）上考察了煤样的热解过程,以及焦样的二氧化碳气化活性。结果表明：碱金属对煤的热解和气化阶段都有影响。在热解阶段,碱金属的存在抑制了煤焦的石墨化进程,降低了热解反应活化能,促进了热解反应的进行;在气化阶段,作为催化剂的碱金属,降低了气化反应活化能,延长了反应速率达到最大值的时间。修正的随机孔模型可以较好地描述煤焦-CO₂的气化反应过程。     

Sulphur reduction evaluation of selected high-sulphur Chinese coals

Under a collaborative project between China and UK partners, investigation was carried out on high-sulphur coals from Beisu Coal Mine in Shandong Province and Dizong Coal Mine in Guizhou Province to evaluate the sulphur reduction potential by coal preparation. Extensive evaluation of raw coal samples was carried out for design of optimum sulphur reduction processes. This paper discusses the background to the project and describes the evaluation process. This involved selection and sampling of the coals, determination of their washability characteristics and application of process prediction models to the washability data to assess their sulphur reduction potential. This paper demonstrates the potential of model predictions for the development of optimum desulphurisation processes using latest separation processes and plant design. The study has shown that over 50 wt% of sulphur can be removed from Beisu coal (ash 17.9 wt%, sulphur 4.8 wt%) and over 60 wt% of sulphur from Dizong coal (ash 33.5 wt%, sulphur 5.1 wt%) using latest coal cleaning processes.     

Catalysis of gasification of coal-derived cokes and chars

Calcium is the most important in-situ catalyst for gasification of US coal chars in O₂, CO₂ and H₂O. It is a poor catalyst for gasification of chars by H₂. Potassium and sodium added to low-rank coals by ion exchange and high-rank coals by impregnation are excellent catalysts for char gasification in O₂, CO₂ and H₂O. Carbon monoxide inhibits catalysis of the CH₂O reaction by calcium, potassium and sodium; H₂ inhibits catalysis by calcium. Thus injection of synthesis gas into the gasifier will inhibit the CH₂O reaction. Iron is not an important catalyst for the gasification of chars in O₂, CO₂ and H₂O, because it is invariably in the oxidized state. Carbon monoxide disproportionates to deposit carbon from a dry synthesis gas mixture (3 vol H₂ + 1 vol CO) over potassium-, sodium- and iron-loaded lignite char and a raw bituminous coal char, high in pyrite, at 1123 K and 0.1 MPa pressure. The carbon is highly reactive, with the injection of 2.7 kPa H₂O to the synthesis gas resulting in net carbon gasification. The effect of traces of sulphur in the gas stream on catalysis of gasification or carbon-forming reactions by calcium, potassium, or sodium is not well understood at present. Traces of sulphur do, however, inhibit catalysis by iron.     

Sulfur transformation during rapid hydropyrolysis of coal under high pressure by using a continuous free fall pyrolyzer☆

The behavior of sulfur transformation during rapid hydropyrolysis of coal was investigated using a pressurized, continuous free fall pyrolyzer under the conditions of temperature ranging from 923 to 1123 K and hydrogen pressure up to 5 MPa. The yields of sulfur converted to gas, tar and char were determined, together with the analyses of sulfur form distributions in coals and chars. The results showed that the decomposition of inorganic sulfur species was affected only by the temperature, while the increases in temperature and hydrogen pressure obviously enhanced the removal of organic sulfur from coal. The extent of organic sulfur removal was proportional to the coal conversion, depending on coal type. A significant retention of gaseous sulfur products by the organic matrix of the char was observed during hydropyrolysis of a Chinese coal above 1023 K, even under the pressurized hydrogen atmosphere. The kinetic analysis indicates that the rate of organic sulfur removal from coal was 0.2th-order with respect to the hydrogen pressure, and the activation energy for total sulfur removal and organic sulfur removal is 17-26 and 13-55 kJ/mol, respectively. The low activation energies suggest that the transformation and removal of sulfur from coal might be controlled by the diffusion and/or thermodynamic equilibrium during hydropyrolysis under the pressurized conditions.     

Reaction studies of an Egyptian coal under combustion-related conditions by thermal analysis-mass spectrometry

Thermal analysis-mass spectrometry (TA-MS) has been used to study the reactions of Al Maghara coal and its low temperature ash (LTA) under combustion-related conditions. The TA-MS profile of the coal gives information on combustion performance (ignition, peak combustion and burnout temperatures) and on chemical changes to the volatile matter (H₂O, SO₂, CO₂ and NO₂), char and minerals. X-ray diffraction identified pyrite as the major mineral component in the coal. The major and only feature of the TA-MS profile of the LTA is SO₂ evolution associated with decomposition of pyrite and iron sulphates. The high sulphur content of Al Maghara coal is a severe obstacle to its use in combustion. Cleaning of the coal to reduce the sulphur to an acceptable level is considered to be essential.     

Changes in char reactivity and structure during the gasification of a Victorian brown coal: Comparison between gasification in O2 and CO2

Char reactivity is an important factor influencing the efficiency of a gasification process. As a low-rank fuel, Victorian brown coal with high gasification reactivity is especially suitable for use with gasification-based technologies. In this study, a Victorian brown coal was gasified at 800 °C in a fluidised-bed/fixed-bed reactor. Two different gasifying agents were used, which were 4000 ppm O₂ balanced with argon and pure CO₂. The chars produced at different gasification conversion levels were further analysed with a thermogravimetric analyser (TGA) at 400 °C in air for their reactivities. The structural features of these chars were also characterised with FT-Raman/IR spectroscopy. The contents of alkali and alkaline earth metallic species in these chars were quantified. The reactivities of the chars prepared from the gasification in pure CO₂ at 800 °C were of a much higher magnitude than those obtained for the chars prepared from the gasification in 4000 ppm O₂ also at 800 °C. Even though both atmospheres (i.e. 4000 ppm O₂ and pure CO₂) are oxidising conditions, the results indicate that the reaction mechanisms for the gasification of brown coal char at 800 °C in these two gasifying atmospheres are different. FT-Raman/IR results showed that the char structure has been changed drastically during the gasification process.     

Effect of lignite pyrolysis conditions on calcium oxide dispersion and subsequent char reactivity

A demineralized North Dakota lignite was loaded with 2.9 wt% Ca by ion exchange. Chars were prepared by pyrolysis in N₂ at 1275 K and residence times between 0.3 s and 1 h. Major differences were observed in their subsequent reactivities in 0.1 MPa air. X-ray diffraction analysis was carried out to obtain information on the state and dispersion of the Ca species on the various chars. The results clearly indicate that CaO is the predominant species responsible for catalysis of lignite char gasification. It is concluded that pyrolysis residence time also has a profound effect on CaO dispersion. Thus, a correlation was established between a fundamental physical property (catalyst dispersion) and the observed gasification behaviour of lignite chars prepared under different pyrolysis conditions.