Production of an iron-loaded carbonaceous material through pyrolyzing biomass impregnated with FeCl2

We propose a novel method for preparing iron-dispersed carbonaceous materials by utilizing low-grade materials and waste heat. Iron is loaded into the biomass through stirring it in a solution of FeCl₂ for 2 h and then pyrolyzed at 600–900 °C to prepare materials composed of iron and carbon. Further in order to investigate the reactivity of the sample, steam and CO₂ gasifications of the material was performed at 800–900 °C. Approximately 80% of the carbon in the biomass remained in the solid state during pyrolysis at 600 °C because of the effect of FeCl₂ in promoting the carbonization of the biomass. The prepared material displayed high reactivity during gasification due to the catalytic effect of loaded iron. This result indicated the possibility that the composite may be used as an iron and heat source for a steel converter. Furthermore, the high reactivity of the composite during steam gasification suggests its usefulness as a medium for hydrogen or carbon monoxide production.     

Steam-activated carbons from a bituminous coal in a continuous multistage fluidized bed pilot plant

A vertical three-stage fluidized bed pilot plant, with downcomers, was designed and built in order to study the continuous process of the production of activated carbons from a high-volatile bituminous coal from the Puertollano basin (Spain), by steam activation. The pilot plant can operate with a production of up to 40 kg per day. Very good activated carbons were produced at the selected operating conditions. The effect of the following operating conditions on the reactivity and adsorption characteristics of the activated carbons was analyzed: (1) carbonization conditions (one- and two-step activation), (2) activation temperature (800–850 °C), and (3) steam gas velocity (1.5–3 times the minimum fluidization velocity). Carbonization conditions considerably affect the reactivity of the chars obtained; the faster the carbonization process, the higher the reactivity. Nevertheless, the effect of this variable on the development of porosity is not very relevant, and consequently the direct activation process is an attractive alternative to the two-step (carbonization and activation) process. On the other hand, both temperature and steam flow rate (affecting the reaction rate) have a marked effect on the development of porosity.     

Microporosity and mesoporosity of PPTA-derived carbons. Effect of PPTA thermal pretreatment

Isothermal treatments of the polyaramid fiber, [poly(p-phenylene terephthalamide)] (PPTA) in an inert atmosphere below its decomposition temperature are known to induce an important increase in char yield and modify the chemical composition and some other properties of the resulting chars. The objective of this work was to study the effect of this isothermal stage on the porous texture of chars and activated carbon fibers (ACFs) produced from PPTA. To this end, chars and ACFs were prepared by PPTA pyrolysis to 850 °C followed by CO₂ activation at 800 °C to various burn-offs (BOs), introducing or not an intermediate isothermal pre-treatment under the conditions (500 °C, 200 min) known to lead to a maximum increase in char yield. The porosity characteristics of the resulting chars and ACFs were comparatively investigated by adsorption of CO₂ (0 °C), and N₂ (−196 °C). The isothermal stage led to a char with enhanced micropore volume and wider micropores. The ACFs prepared from this char exhibited larger amounts of wide micropores and mesopores than those prepared from PPTA pyrolyzed at a constant heating rate.     

Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part IX. Effects of volatile-char interactions on char-H2O and char-O2 reactivities

Volatile-char interactions are an important consideration in the design and operation of a gasifier. This study aims to investigate the effects of volatile-char interactions on the in situ char-steam reactivity at 800 °C and the ex-situ char-O₂ reactivity at 400 °C. A Victorian brown coal was gasified in 15% steam at 800 °C in a one-stage novel fluidised-bed/fixed-bed quartz reactor, in which the extent of volatile-char interactions could be controlled. The chars after varying extents of volatile-char interactions and/or varying extents of char conversion in steam were also collected for the measurement of their reactivity with air at 400 °C in a thermogravimetric analyser. Our results show that the char-steam gasification reactions were greatly inhibited by the volatile-char interactions. It is believed that the H radicals generated from the thermal cracking/reforming of volatiles slowed the char gasification in three ways: occupying the char reactive sites, causing the char structure to re-arrange/condense and enhancing the release of catalytic species inherently present in the brown coal. The importance of volatile-char interactions to char-steam reactivity was further confirmed by the char-air reactivity.     

Catalytic gasification of char from co-pyrolysis of coal and biomass

The catalytic gasification of char from co-pyrolysis of coal and wheat straw was studied. Alkali metal salts, especially potassium salts, are considered as effective catalysts for carbon gasification by steam and CO₂, while too expensive for industry application. The herbaceous type of biomass, which has a high content of potassium, may be used as an inexpensive source of catalyst by co-processing with coal. The reactivity of chars from co-pyrolysis of coal and straw was experimentally examined. The chars were prepared in a spout-entrained reactor with different ratios of coal to straw. The gasification characteristics of chars were measured by thermogravimetric analysis (TGA). The co-pyrolysis chars revealed higher gasification reactivity than that of char from coal, especially at high level of carbon conversion. The influence of the alkali in the char and the pyrolysis temperature on the reactivity of co-pyrolysis char was investigated. The experimental results show that the co-pyrolysis char prepared at 750 °C have the highest alkali concentration and reactivity.     

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).     

NH3 and HCN formation during the gasification of three rank-ordered coals in steam and oxygen

A Victorian brown coal (68.5% C), a Chinese high-volatile Shenmu bituminous coal (82.3% C) and a Chinese low-volatile Dongshan bituminous coal (90% C) were gasified in a fluidised-bed/fixed-bed reactor at 800 °C in atmospheres containing 15% H₂O, 2000 ppm O₂ or 15% H₂O + 2000 ppm O₂. While the gasification of these coals in 2000 ppm O₂ converted less than 27% of coal-N into NH₃, the introduction of steam played a vital role in converting a large proportion of coal-N into NH₃ by providing H on char surface. The importance of the roles of steam in the formation of NH₃ in atmospheres containing 15% H₂O + 2000 ppm O₂ decreased with increasing coal rank. This is largely due to the slow gasification of high-rank coal chars, resulting in low availability of H on char surface. The gasification of chars from the high-rank coal appears to produce higher yields of HCN than that of lower rank coals, probably as a result of the decomposition of partially hydrogenated/broken/activated char-N structures during gasification at high temperature. The alkali and alkaline earth metallic species in brown coal tend to favour the release of coal-N as tar-N but have limited effects on char-N conversion during gasification.     

Interstitial iron(II)-chloride compounds with graphite—I.: Formation by reduction of iron(III)-compounds with Fe(CO)5

A graphite intercalation compound of FeCl₂ with a composition of 1 Fe:4.7 C, closely approaching the theoretically limiting stoichiometry FeCl₂C_4.22, is obtained by the reduction of the intercalation compound FeCl₃C_7.1 with Fe(CO)₅ under 150 atm of CO at 150°C. The FeCl₂-compound is characterized by its X-ray powder pattern and Mössbauer spectra. The distance between carbon layers increases from 9.40 Å in FeCl₃C_7.1 to 9.56 in FeCl₂C_4.73 by this reduction reaction. Carbon monoxide reduces FeCl₃C_7.1 at 150°C in a sluggish reaction which occurs under removal of iron chlorides from the intercalation compound in contrast to the reduction by Fe(CO)₅, which increases the Fe:C ratio.     

Gasification studies of onakawana lignite

Onakawana lignite was gasified in air, steam and an air + steam mixture in a fixed bed reactor. The extent of devolatilization was determined by pyrolysis in nitrogen. The composition of products, expressed in terms of H₂/CO ratio, was temperature dependent. The ratio decreased with increasing temperature. During steam gasification the ratio decreased from 4.6 to 2.6 when temperature increased from 700° to 990°C. The addition of air to steam resulted in a marked decrease of this ratio. Steam gasification reactivity of chars prepared from Onakawana lignite at 500°C and 800°C were studied in the temperature range of 650°C to 1000°C. The carbon conversion results were fitted into equations describing the continuous and shrinking core models. The char prepared at 500°C was much more reactive than the one prepared at 800°C. Product distribution expressed as the H₂/CO ratio, was favourable in the temperature range. For comparison, the Kentucky #9 coal and chars derived from this coal were used as referee materials. The reactivity of these chars was markedly lower than that of chars derived from Onakawana lignite.     

Reactivity of heat-treated coals in steam

Reactivities of seventeen 40 × 100 mesh (U.S.) coals charred to 1000 °C have been measured at 910 °C in 0.1 MPa of a N₂H₂O mixture containing water vapour at a partial pressure of 2.27 kPa. Char reactivity decreases, in general, with increasing rank of the parent coal. The chars show a 250-fold difference in their reactivities. Results suggest that gasification of chars in air, CO₂ and steam involves essentially the same mechanism and that relative gasification rates are controlled by the same intermediate oxygen-transfer step. Removal of inorganic matter from raw coals prior to their charring or from chars produced from raw coals decreases the reactivities of lower-rank chars, whereas reactivities of higher-rank chars increase. Addition of H₂ to steam has a marked retarding effect on char reactivity in most cases. However, in a few cases H₂ acts as an accelerator for gasification. The effect of particle size, reaction temperature and water-vapour pressure on char reactivity is considered.     

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.     

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.     

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.     

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.     

Nickel catalysed steam gasification of chars obtained from coal-alkali reaction at 600 °C

Studies on the steam gasification of washed residual chars (obtained from coal-alkali reaction at 600 °C) were carried out at 500 °C and 100 kPa pressure in a fixed bed glass reactor with or without nickel (as nickel nitrate) as catalyst. The results when compared with the corresponding data on coal, revealed that under similar reaction conditions, the coals yielded more gas with higher H₂ and CO contents than their corresponding chars. It was concluded that presence of functional groups, especially oxygen containing is a requirement for nickel catalysed steam gasification of coals/lignites. The recovery of nickel achieved was about 80%.     

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.     

Study of heat-treated Spanish lignites: Characteristics and behaviour in co2 and o2 gasification reactions

Six Spanish lignites (raw and demineralized) have been charred to 1113 K in a N₂ atmosphere. The surface area, porosity and mineral matter content of the char coals so obtained have been studied, as well as their reactivity in CO₂ flow in the range 1073–1113 K, and in dry air in the temperature range 733–773 K. The reactivities of the raw chars in CO₂ may be explained according to the different inorganic matter content that may act as catalyst. The demineralization process brings about a lowering in reactivity and an increase, in general, in the apparent activation energy that may be interpreted as being due to a fall in mineral matter content and/or an increase in the amount of feeder pores. With regard to reactivity and apparent activation energy, in the case of dry air three groups of raw chars have been established. The differences between these three groups may be due to the different inorganic impurities present in the raw chars that catalyse the reaction of carbon with O₂ more than the porous texture parameters. Demineralization brings about a lowering in the reactivity values and a levelling off of apparent activation energies. The catalytic effect of iron has also been studied by adding different amounts of this metal to a demineralized char. The burn-off versus time curves of the different char coals have been adjusted by using the τ_0.5 parameter.     

Observation of alkali catalyst particles during gasification of carbonaceous materials in CO2 and steam

Results of a microscopical examination of catalysed carbon gasification are reported. Both in CO₂ and steam, alkali catalysts show evidence of mobility. In the steam gasification of coal chars, the catalysts irreversibly combine with indigenous mineral matter. This is less pronounced in C0₂. The catalysed CO₂ gasification was observed by hot stage microscopy, where alkali carbonate catalysts achieve an apparently molten state during incipient gasification. For single crystal graphite, circular pitting, hexagonal pitting and channelling were observed. For coal chars, irregular morphologies tend to obscure direct observation of surface/catalyst interactions, though subsequent scanning electron micrographs reveal the consequences of extensive catalyst mobility.     

Comparison of molecular sieve properties in microporous chars from low-rank bituminous coal activated by steam and carbon dioxide

A Polish high volatile bituminous coal was subjected to air oxidation, carbonization and gaseous activation. The activation with steam and carbon dioxide was performed to low levels of burn-off: 5-25%. Sorption measurements of CO₂, as well as of organic vapours with increasing molecular sizes (CH₂Cl₂, C₆H₆, C₆H₁₂, CCl₄) were applied to evaluate the porous structure of the activated chars. Steam and carbon dioxide develop the microporous system according to the same mechanism—opening (burn-off 5-10%) and then widening of the narrow micropores. For char from the oxidized coal mainly a widening of the narrow micropores takes place. Comparing both activating agents, it was stated that for steam greater micropore volumes were obtained. This was confirmed by other authors for chars from brown coal and coking coal, but was in disagreement with the results for olive stones and carbon fibres. This would indicate the importance of the carbon precursor in the formation of the porous structure of carbon materials by different activating agents. In the region of studied burn-offs, among the micropore sizes useful for separation of gases and vapours with small molecules, micropore volumes with widths close to 0.4-0.5 nm are dominating. At very low burn-offs (5-10%), steam activation renders greater micropore volumes within these sizes, than does activation with carbon dioxide. But with increasing burn-off (15-25%), this phenomenon becomes reversed. This effect is still more accentuated for the preoxidized coal.     

CO2 gasification of Thai coal chars: Kinetics and reactivity studies

Two sized fractions (<75 μm and 150–250 μm) of Ban Pu lignite A and Lampang subbituminous B coals were pyrolyzed in a drop tube fixed bed reactor under nitrogen atmosphere at 500–900 °C. Gasification of coal chars with excess carbon dioxide was then performed at 900–1,100 °C. The result was analyzed in terms of reactivity index, reaction rate and activation energy. It was found that chars at lower pyrolysis temperature had highest carbon conversion, and for chars of the same sized fraction and at the same pyrolysis temperature, reactivity indices increased with gasification temperature. The lower rank Ban Pu lignite A had higher R _s values than higher rank Lampang subbituminous B coals. Smaller chars from both coals had higher R _s values, due to the higher ash content. At present, it can be concluded that, within the gasification temperature range studied, gasification rates of chars obtained at various pyrolysis temperatures showed a linear correlation with temperature. However, additional experiment is needed to verify the correlation.