Significance of char active surface area for appraising the reactivity of low- and high-temperature chars

Contemporary char reactivity studies have focussed primarily on coal chars prepared under severe (high-temperature) conditions. In this study, the reactivity of chars prepared under mild (low-temperature) conditions has been addressed. A thermogravimetric analysis system (TGA) was used to determine the reactivity of chars in oxidizing atmosphere using isothermal or non-isothermal techniques. Coal chars were prepared in a TGA or in a slow heating rate organic devolatilization reactor (SHRODR) at a temperature range between 500 ° and 950 °C. The chars prepared by mild pyrolysis of coal at 500 °C are shown to be highly reactive. Comparison of reactivities of low- and high-temperature chars shows that the low-temperature chars exhibit higher reactivity than either the parent coals or the high-temperature chars. Correlation between isothermal reactivity results (e.g. time) and non-isothermal reactivity data (e.g. temperature) has been obtained. Hydrogen contents of chars correlate well with the reactivity of the chars. The study confirms the importance of oxygen chemisorption capacity as a significant reactivity parameter for both low- and high-temperature chars. A new approach has been used for calculating the oxygen chemisorption capacity of chars by accounting for the carbon surface sites occupied by hydrogen (and, therefore, these sites were unavailable for oxygen chemisorption). The occupied sites are readily freed during reactivity measurements and thus were available for participation in carbon-oxygen reactions.     

Reactivity of heat-treated coals in air at 500 °C

Twenty one US coals, of widely ranging rank, have been carbonized under controlled conditions to 1000 °C, and the reactivity in air at 500 °C of the resulting chars or cokes has been measured by a gravimetric method. The reactivities lie within a well-defined band when plotted against rank of the parent coal. The lower-rank coal chars are more reactive than those prepared from high-rank coals. In extreme cases, the reactivity found for a Montana lignite char is some 100 times as great as that obtained for a char produced from a Pennsylvania low-volatile coal. Variation of reactivity with heat-treatment temperature (600 to 1000 °C) has been studied for three coals. As heat-treatment temperature increases, there is a decrease in reactivity. Some results are reported on the effects which mineral matter and pore structure have on the reactivity parameter. Chars containing high concentrations of magnesium and calcium impurities are most reactive. The amount of macro and transitional porosity in a char has a marked influence on reactivity.     

Influence of carbonization conditions on the gasification of acacia and eucalyptus wood chars by carbon dioxide

Gasification rates of cubic shaped acacia and eucalyptus wood chars were measured thermogravimetrically in a carbon dioxide atmosphere at temperatures in the range 810–960 °C. The effects of wood species and carbonization conditions, such as temperature, heating rate and soaking time, were determined. Both reactivity and the activation energy for the gasification of wood chars were found to be strongly influenced by the carbonization conditions employed during their preparation and wood type. The reactivities of both the acacia and eucalyptus wood chars decreased with increasing preparation temperature; while the activation energy for their gasification increased. Slow carbonization (heating rate: 4 °C min⁻¹) led to the production of wood chars having lower reactivities and higher activation energies than those of the wood chars prepared under rapid carbonization (heating rate: 30 °C min⁻¹) at the same temperature. With increasing soaking time, at carbonization temperatures of 800 and 1000 °C, the reactivity of resulting wood chars was reduced. The results also show that the reactivities of acacia wood chars are higher than those of similarly prepared eucalyptus wood chars.     

Gasification reactivity of Chilean coals

Gasification reactivities of raw and acid-washed coal chars obtained from the three most important coal-bearing regions in Chile have been determined in 0.1 MPa of oxygen using a thermobalance. Oxygen chemisorption capacities of the demineralized chars were also measured gravimetrically at 373 K in 0.1 MPa of oxygen. The subbituminous coals of Catamutun and Peckett are more reactive than the bituminous coals of Lota and Trongol due to the catalytic effect of their inorganic constituents. However, in the absence of catalytically significant mineral matter, coal rank is not an important parameter of char reactivity. The reactivity of chars based on carbon active surface areas, estimated from gravimetric chemisorption measurements, agreed very well with the previously reported value based on active surface areas obtained in a volumetric system. These results supply additional evidence that active surface area is the fundamental parameter that can explain most of the observed differences in the kinetic behaviour of coal chars and carbons in general.     

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.     

Thermogravimetric determination of the carbon dioxide reactivity of char from some New Zealand coals and its association with the inorganic geochemistry of the parent coal

Thermogravimetrically-determined carbon dioxide reactivities of chars formed from New Zealand coals, ranging in rank from lignite to high volatile bituminous, vary from 0.12 to 10.63 mg/h/mg on a dry, ash-free basis. The lowest rank subbituminous coal chars have similar reactivities to the lignite coal chars. Calcium content of the char shows the strongest correlation with reactivity, which increases as the calcium content increases. High calcium per se does not directly imply a high char reactivity. Organically-bound calcium catalyses the conversion of carbon to carbon monoxide in the presence of carbon dioxide, whereas calcium present as discrete minerals in the coal matrix, e.g., calcite, fails to significantly affect reactivity. Catalytic effects of magnesium, iron, sodium and phosphorous are not as obvious, but can be recognised for individual chars. The thermogravimetric technique provides a fast, reliable analysis that is able to distinguish char reactivity differences between coals, which may be due to any of the above effects.     

Effects of alkaline metal on coal gasification at pyrolysis and gasification phases

A Chinese high-rank coal was acid-washed and ion-exchanged with Na and K to prepare the H-form, Na-form and K-form coals. After pyrolysis, H-form, Na-form and K-form chars and two additional H-form chars (acid washed Na-form and K-form chars) were prepared to investigate the effects of alkaline metal (AM) on coal gasification at the pyrolysis and gasification phases. The H-form char had the highest pryolysis rate; the H-form char had a relative low gasification rate. The AM loaded coals exhibited relative low pyrolysis rate, while the corresponding chars had high gasification reactivity. Acid-washing reduced the reactivities of Na-form and K-form chars. AM inhibited the progress of graphitization of the base carbon resulting in a more reactive char of less ordered crystalline carbon structure. A kinetic model incorporating AM-catalyzed gasification and non-catalytic gasification was developed to describe the gasification rate changes in the char conversion for AM-catalyzed gasification of chars.     

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.     

Reactivity of heat-treated coals in hydrogen

Reactivities of eighteen 40 × 100 mesh U.S. coals charred to 1000 °C have been measured in H₂ at 2.7 MPa and 980 °C. The char-hydrogen reaction usually occurs in two stages: a slow induction period followed by a constant-rate region. Reactivities of various chars in the initial stage (R_i) decrease, in general, with increasing carbon content of the parent coals, whereas reactivities in the constant-rate region (R_c) are essentially independent of the rank of the parent coals. Reactivities of chars in H₂ differ markedly from those in air and CO₂. Results of surface-area measurements of chars and activation energies for the hydrogasification reaction suggest that during the induction period the reaction is diffusion-controlled whereas in the constant-rate region it is chemically controlled. Upon removal of mineral matter, R_i values generally decrease but R_c values show a random variation. Removal of mineral matter from coals prior to their carbonization brings about profound changes in surface area and porosity of chars. The effect of char particle size on reactivity is considered.     

Catalysis of carbon-steam gasification by ash components from two lignites

Ash transfer from a reactive to a less reactive coal is an interesting possibility for improving and equalizing gasification characteristics of coals. To assess the catalytic action of coal impurities in the steam gasification of carbon, three approaches were used. In the first series, the effects of different coal ashes on the gasification kinetics of graphite were compared. A parallel study was made by adding lignite ash to a coal of low reactivity. Finally, gasification rates of chars prepared from demineralized coals were measured. While it was found that ash from reactive coals can significantly enhance the gasification rates of chars derived from coals of lesser reactivity, it was not possible to distinguish clearly between a catalytic lowering of the activation energy and an increase in the number of gasification sites. The gasification enhancement by lignite ash may open practical possibilities for blending coals of different reactivity, and warrants further study to identify the constituents associated with this effect.     

Reactivity of heat-treated coals

Lignite and two bituminous coals were pyrolysed at 1023 and 1223 K at different rates and for different heating times, producing different chars. The extent of devolatilization, the evolution of the pore structure and the differences in the ignition characteristics and reactivities of the chars during pyrolysis were examined. The kinetic data on the coals and chars produced by pyrolysis of these coals were obtained under ignition conditions. The apparent reactivity varied by two orders of magnitude among the parent coals and chars at a given temperature, whereas the intrinsic reactivity was found to vary over four orders of magnitude. Low-rank parent coal, high heat-up rates, and moderate pyrolysis time and temperature produced the most reactive chars. The values of hydrogen, volatile matter and ash content or the pore surface area could not provide an explanation for the differences in reactivity.     

Pyrolysis of brown coals. 1. Decomposition of acid groups in coals containing carboxyl groups in the acid and cation forms

The effect of low-temperature pyrolysis (up to 300 °C) on the acid groups of two low-rank coals (a brown coal from Victoria, Australia, and a lignite from Texas, U.S.A.) has been studied for samples in both the acid and cation forms. A preliminary study at temperatures above 300 °C was made on the brown coal. The carboxyl groups of coals in the acid form decompose to give one mole of carbon dioxide for each equivalent of carboxyl content. Cation-form coals yield more carbon dioxide on pyrolysis than can be accounted for by the carboxyl groups present. Water is evolved in proportion to the carbon dioxide evolved from both acid- and cation-form coals, but the ratios differ. Findings have been interpreted as indicating that some other oxygen-containing group is associated with the carboxyl group. In the case of the acid-form coal this group decomposes to give water. When the carboxyl group is in the cation form, decomposition of the associated groups gives carbon dioxide as well as water. Phenolic groups appear to be stable, at least to 300 °C.     

Reactivity study of combustion for coals and their chars in relation to volatile content

To determine the effect of volatile matter on combustion reactivity, the pyrolysis and combustion behavior of a set of four (R, C, M and K coals) coals and their chars has been investigated in a TGA (SDT Q600). The maximum reaction temperatures and maximum reaction rates of the coals and their chars with different heating rates (5–20 °C/min) were analyzed and compared as well as their weight loss rates. The volatile matter had influence on decreasing the maximum reactivity temperature of low and medium rank coals (R, C and M coals), which have relatively high volatiles (9.5–43.0%), but for high rank coal (K coal) the maximum reactivity temperature was affected by reaction surface area rather than by its volatiles (3.9%). When the maximum reaction rates of a set of four coals were compared with those of their chars, the slopes of the maximum reaction rates for the medium rank coals (C and M coals) changed largely rather than those for the high and low rank coals (R and K coals) with increasing heating rates. This means that the fluidity of C and M coals was larger than that of their chars during combustion reaction. Consequently, for C and M coals, the activation energies are lower (24.5–28.1 kcal/mol) than their chars (29.3–35.9 kcal/mol), while the activation energies of R and K coals are higher (25.0-29.4 kcal/mol) than those of their chars (24.1–28.9 kcal/mol).     

Effect of demineralization and catalyst addition on N2 formation during coal pyrolysis and on char gasification

Six coals with different ranks and different ash contents have been used to study the effect of demineralization on N₂ formation during coal pyrolysis. Chars obtained after pyrolysis have been also gasified with carbon dioxide at 1000 °C to investigate the influence of the demineralization on char gasification reactivity. The pyrolysis results show that the demineralization by acid washing drastically changes N₂ formation profiles and decreases nitrogen conversion to N₂ for low rank coals; on the other hand, the demineralization has little effect on N₂ formation for high rank coals. Addition of 0.5 wt% Fe promotes N₂ formation from the demineralized coals, but the catalytic effect depends on the coal type. It is found that the Fe remarkably promotes N₂ formation from the demineralized low rank coals, but the effect is much smaller for high rank demineralized coals. These observations suggest that the existing state of Fe-containing minerals and added Fe catalyst is important for catalytic N₂ formation during coal pyrolysis. Gasification results show that the demineralization lowers char gasification reactivity not only for low rank coals but also for high rank coals.     

Catalytic activities of ion-exchanged nickel and iron in low temperature hydrogasification of raw and modified birch chars

Raw, HNO₃ oxidized and carboxymethylated birch woods loaded with nickel or iron by the ion-exchange method were carbonized at 500 °C in a flow of nitrogen, and the resulting chars were hydrogasified in a thermobalance to examine their reactivities below 700 °C. The amounts of ion-exchanged metals on raw char were too small to give high gasification reactivity. However, oxidized and carboxymethylated woods with increased ion-exchange capacity produced much more reactive chars. Both nickel and iron exhibited larger catalytic activities on carboxymethylated chars than on oxidized chars, because better metal dispersion could be achieved on carboxymethylated wood with its larger cation exchangeability. It was noteworthy that only 1 wt% loading of iron, as well as nickel, on carboxymethylated char was sufficient to attain a gasification of 90 wt% at 700 °C. It was also noted that the catalytic effect, up to 600 °C, of iron on the gasification of oxidized and carboxymethylated chars was larger than that of nickel. This is ascribed to two factors; greater catalytic activity of metallic iron formed during the gasification than that of nickel metal, and low ash level in the chars. Above 600 °C, however, serious loss of activity of the iron was observed in the absence of wood ash. This showed the different influence of wood ash on the catalysis of iron in the low and high temperature regions.     

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).     

Calcium catalysis in air gasification of cellulosic chars

Cellulosic chars containing calcium have been prepared from carboxymethyl cellulose in the calcium form and from pure cellulose containing sorbed calcium acetate, at several heat treatment temperatures (HTTs) in the range 400–900°C. The chars have been gasified in air at 300°C. The results indicated a general decrease in reactivity with increasing HTT. However, instead of a monotonic decrease in reactivity reported previously for coal chars containing indigenous or added calcium, the reactivity versus HTT curve consisted of three distinct regions which probably reflect the transformations being undergone by the catalyst species on increasing the HTT. Crystalline CaO was detected by XRD only in chars heated to 1000°C, at which temperature the catalyst was no longer effective. The relationships between gasification rate and catalyst concentration and also mode of addition have been determined using chars of HTT 600°C. As found by earlier workers with some coal chars, the rate reached a maximum with increasing calcium content and then declined. Chars containing sorbed calcium showed a relatively early decline of rate. They also showed less reactivity than the chars containing ion-exchanged calcium at all calcium concentrations.     

Nitrogen release during high temperature pyrolysis of coals and catalytic role of calcium in N2 formation

Pyrolysis of 10 coals with carbon contents of less than 80 wt%(daf) has been studied with a fixed bed quartz reactor to examine mainly nitrogen release from char-N without volatile matters. When temperature is raised from 1000 to 1350 °C, N₂ yield increases but char-N decreases for all the coals used. There is a strong reverse correlation between N₂ and char-N, which points out that most of N₂ arises from char-N via solid phase reactions. NH₃ is also formed from char-N at high temperatures of ≥1000 °C. In the pyrolysis of low rank coals, demineralization by HCl washing increases yields of tar-N, HCN and char-N, but decreases NH₃ and N₂. The addition of 3 wt% Ca to the demineralized coals shows almost the reverse effect. The XRD measurements after pyrolysis at 1000–1350 °C reveal that the Ca exists predominantly as CaO with the average crystallite size of 25–65 nm and promotes carbon crystallization. As the extent of crystallized carbon increases, N₂ yield increases remarkably. It is likely that the highly dispersed CaO catalyzes efficiently conversion reactions of char-N to N₂ in the process of carbon crystallization. The reaction mechanism is discussed in term of interactions between CaO particles and char-N.     

Combustion characteristics of coals using a drop-tube furnace

Fourteen coals were selected for char refiring tests using a drop-tube furnace (DTF) in order to compare with previous tests on a 1 MW combustion facility. Each coal was sieved into two size fractions (53-75 and 106-125 μm) and characterised using proximate analysis and conventional petrographic tests as well as a test for % unreactives using image-analysis. The coal fractions were then pyrolysed at 1300 °C for 200 ms in 1 vol% oxygen in nitrogen. The reactivity, morphology and surface area of the chars were evaluated using thermal, optical and adsorption techniques. Each char fraction was then passed through the DTF at 1300 °C using a residence time of 600 ms and a furnace atmosphere of 5 vol% oxygen in nitrogen to evaluate burnout propensity. The characteristics of the coals, the chars and the residues after refiring were compared to determine whether any links exist between burnout, intermediate char products and coal composition. The link between % unreactives and burnout was confirmed for high volatile bituminous coals. Results obtained from two low volatile coals confirmed that their burnout was better than predicted from their properties. For the S. American (Guasare) coal poorer than expected burnout was obtained, as in previous work, but only for the larger size fraction.     

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.