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.     

A reactivity study of chars obtained at different temperatures in relation to their petrographic characteristics

This study reports on the reactivity of chars obtained at 1000°C and 1300°C (within the range of temperatures reached by coal particles in the near-burner zone of pulverised fuel boilers) from three different coals. The coals were selected according to petrographic criteria: two of them are high volatile bituminous coals differing in maceral composition and the third one is a vitrinite-rich low volatile bituminous coal.The morphology and optical texture of the chars were studied by optical microscopy. The kinetic parameters for the combustion of the high temperature chars under Regime I (combustion controlled by chemical kinetics) have been obtained and related to the optical texture and reflectance of the chars. The intrinsic reactivity of the high temperature chars was found to be lower than that of the low temperature chars, whereas the enhanced porosity observed in the high temperature chars had a positive effect on their combustion reactivity under Regime II (combustion controlled by oxygen pore diffusion). The intrinsic reactivities of the chars decreased following the sequence: vitrinite-rich low rank char>inertinite-rich char>vitrinite-rich high rank char. As the combustion temperature increases, the reactivity of the inertinite-rich char approaches that of the low rank vitrinite-rich char, which justifies the good performance observed for high volatile bituminous inertinite-rich coals in power plants.     

Kinetic analysis of NO-Char reaction

Two Chinese coals were used to prepare chars in a flat flame flow reactor which can simulate the temperature and gas composition of a real pulverized coal combustion environment. Acid treatment on the YB and SH chars was applied to obtain demineralized chars. Kinetic characterization of NO-char reaction was performed by isothermal thermogravimetry in the temperature range of 973–1,573 K. Presence of catalytic metal matter can increase the reactivity of chars with NO, which indicates that the catalytic effects of inherent mineral matter play a significant role in the NO-char reaction. The discrete random pore model was applied to describe the NO-char reactions and obtain the intrinsic kinetics. The model can predict the data for all the chars at various temperatures well, but underestimate the reaction rates at high carbon conversions for the raw YB and SH chars, which can be attributed to the accumulation of metal catalyst on char surface. This work was presented at the 7 ^th China-Korea Workshop on Clean Energy Technology held at Taiyuan, Shanxi, China, June 26–28, 2008.     

The estimation of char reactivity from coal reflectogram

Measurements of the intrinsic reactivity of chars to oxygen are increasingly being sought as an indicator of the combustion potential of fuels. The coal reflectogram has been used to characterize the chemical properties of coal and its resultant char structure. In this study, six Australian coals varying in rank were separated using density separation technique to obtain vitrinite and inertinite rich fractions. Chars were obtained from these density fraction samples in a Drop Tube Furnace (DTF) at 1673 K. The reactivity of the chars was measured non-isothermally in a Thermal Gravimetric Analysis (TGA) in the temperature range of 573–1073 K. The results suggested that with the increase in the coal rank, the maximum reactivity of chars derived from vitrinite rich fractions decreases, while the reactivity of chars derived from inertinite rich fractions decreases with the increase in the inertinite content in samples and has no obvious relationship with rank. The kinetic parameters were derived using data from non-isothermal TGA after accounting for changing in surface area with conversion. The frequency factor is found to decrease with increasing coal FMR, defined as the summation of each reflectance value multiplied by its frequency, for a constant activation energy (E=146 kJ/mol). This suggests that the behavior of a maceral is characterized primarily by its reflectance distribution instead of the type of its parent coal.     

The kinetics of coal chars hydrogasification

Hydrogasification reaction of chars produced from two rank coals was investigated in temperature up to 1173 K and pressure up to 8 MPa. The reactivity of the lignite Szczerców char has been found to be slightly higher than of the subbituminous coal Janina char produced at the same conditions. A high value of the char reactivity was observed to certain carbon conversion, above which a sharp drop takes place. It has been shown that to achieve proper carbon conversion the hydrogasification reaction must proceed at temperature above 1200 K. Based on the active centres theory the kinetic equations of the hydrogasification process were developed and the kinetic constants at the maximum reaction rate evaluated for the analyzed chars.     

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.     

Reactivity of heat-treated coals in carbon dioxide at 900 °C

Reactivities of sixteen 40 × 100 (U.S.) mesh U.S. coals charred to 1000 °C were measured in carbon dioxide at 900 °C. Chars derived from coals with less than 80% carbon, on a dry-ash-free basis, were the most reactive. These chars also gave the widest spread in reactivity. Plots of inorganic element content in the chars versus reactivity showed that magnesium and calcium are important to char reactivity. Six coals were acid-washed with hydrochloric acid and four coals were further demineralized with hydrofluoric acid. Most acid-treated coals showed a decrease in reactivity; but two coals of high rank increased in reactivity. This increase in reactivity is attributed to the creation of additional porosity as a result of mineral matter removal and thus a reduction in resistance to carbon dioxide diffusion to reactive sites. Two demineralized and two original coals were divided into four size ranges and chars were produced from each size of each coal. Gasification rates increased monotonically with decreasing particle size reacted.     

Investigation of an iron-based additive on coal pyrolysis and char oxidation at high heating rates

Iron-based catalysts have been shown to enhance coal pyrolysis and char oxidation at low to moderate temperatures and heating rates (< 1250 K and 1–1000 K/s). Such catalytic activity has not been demonstrated at high heating rates and temperatures approaching pulverized coal combustion applications. The effect of an iron-based additive on coal pyrolysis and char combustion was studied in a flat-flame burner system at high particle heating rates using a Kentucky bituminous coal. Pyrolysis and char reactivity of two treated coals with different catalyst loadings were studied and compared with the untreated coal. The total volatiles yield for the treated coals increased between 14 and 18% (absolute) on a dry ash-free basis compared to the untreated coal in experiments conducted at 1300 K. A first-order char oxidation model was used to compare the apparent char reactivities of the treated and untreated coals measured at 1500 and 1700 K. An increase in apparent char reactivity was observed for both treated samples.     

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).     

Characterization of chars made of solvent extracted coals

Thermal extraction of a sub-bituminous coal (Roto south) using 1-methylnaphthalene solvent has produced ash-free coals successfully. The extracted (EC) and residual coal (RC) as well as its parent coal (PC) were pyrolyzed at 300–900 °C and then the carbonized products were characterized. The extracted coal (EC) contained lower molecular weight components than PC and RC, showing much higher fuel ratio after the pyrolysis. EC is expected to be advantageous over PC and RC when applied to coal gasification and reforming, because EC is readily decomposed and volatized. The heating value of EC chars (7,610–8,120 kcal/kg) was independent of the pyrolysis temperature and was higher than those of PC and RC chars, especially for the chars carbonized below 600 °C. The oxygen content of PC chars at T≤600 °C was mostly at least twice that of EC/RC chars, pointing out the difference in the chemical composition. 13CNMR and FT-IR spectra revealed the release of aliphatic hydrocarbons and reactive functional groups with increasing temperature, in agreement with ultimate/proximate analysis results.     

A comparative reactivity and kinetic study on the combustion of coal-biomass char blends

The combustion behavior and kinetics of various biomass chars, a lignite and a hard coal char and their blends were investigated. Pure fuel chars were compared to blended chars with respect to their performance during combustion. Non-isothermal thermogravimetry experiments were performed in air atmosphere, over a temperature range of 25-850 °C and at a heating rate of 10 °C/min. Kinetic evaluation was performed using a power law model. Reaction kinetic parameters were obtained by modeling the combustion of biomass and coal chars as a single reaction, with the exception of lignite and olive kernel chars, the combustion of which was modeled by two partial reactions. A single reaction model was used in the case of coal-wood char blends, while for the lignite-biomass char blends two partial reactions were used. Reactivity was assessed using the specific reaction rate, as a function of conversion. Biomass chars were generally more reactive than those of hard coal and lignite. The combustion behavior of the blends was greatly influenced by the rank of each coal (hard coal or lignite) and the proportion of each component in the blend. Combustion performance of the blends showed some deviation from the expected weighted average of the constituent chars. An attempt was made to estimate the kinetics of the blends using, as a basis, the parameters estimated for the individual components. In this case, because of the interactions between the components of the blends, the kinetic parameters needed to be slightly modified. Alteration in reactivity was more pronounced in the case of lignite-biomass chars than coal-wood chars.     

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.     

Gasification kinetics of five coal chars with CO<Subscript>2</Subscript> at elevated pressure

For five coals, the reactivity of char-CO₂ gasification was investigated with a pressurized thermogravimetric analyzer (PTGA) in the temperature range 850-1,000 C and the total pressure range 0.5-2.0 MPa. The effect of coal rank, initial char characteristics and pressure on the reaction rate were evaluated for five coal chars. The reactivity of low lank coal char was better than that of high rank coal char. It was found that Meso/macro-pores of char markedly affect char reactivity by way of providing channels for diffusion of reactant gas into the reactive surface area. Over the range of tested pressure, the reaction rate is proportional to CO₂ partial pressure and the reaction order ranges from about 0.4 to 0.7 for five chars. Kinetic parameters, based on the shrinking particle model, were obtained for five chars.     

Char characterization and DTF assays as tools to predict burnout of coal blends in power plants

The aim of this study is to predict efficiency deviations in the combustion of coal blends in power plants. Combustion of blends, as compared to its single coals, shows that for some blends the behavior is non-additive in nature. Samples of coal feed and fly ashes from combustion of blends at two power plants, plus chars of the parent coals generated in a drop-tube furnace (DTF) at temperatures and heating rates similar to those found in the industrial boilers were used. Intrinsic kinetic parameters, burning profiles and petrographic characteristics of these chars correlated well with the burnout in power plants and DTF experiments. The blend combustion in a DTF reproduces both positive and negative burnout deviations from the expected weighted average. These burnout deviations have been previously attributed to parallel or parallel-series pathways of competition for oxygen. No deviations were found for blends of low rank coals of similar characteristics yielding chars close in morphology, optical texture and reactivity. Negative deviations were found for blends of coals differing moderately in rank and were interpreted as associated with long periods of competition. In this case, fly-ashes were enriched in material derived from the least reactive char, but also unburnt material attributed to the most reactive char was identified. Improved burnout compared to the weighted average was observed for blends of coals very different in rank, and interpreted as the result of a short interaction period, followed by a period where the less reactive char burns under conditions that are more favorable to its combustion. In this case, only unburned material from the least reactive char was identified in the fly-ashes.     

Ambient-pressure thermogravimetric characterization of four different coals and their chars

Ambient-pressure thermogravimetric characterization of four different coals and their chars was performed to obtain fundamental information on pyrolysis and coal and char reactivity for these materials. Using a Perkin-Elmer TGS-1 thermobalance, weight loss as a function of temperature was systematically determined for each coal heated in helium at 40 and 160 °C/min under various experimental conditions, and for its derived char heated in air over a temperature range of 20 to 1000 °C. The results indicate that the temperature of maximum rate of devolatilization increases with increasing heating rate for all four coals. However, heating rate does not have a significant effect on the ultimate yield of total volatiles upon heating in helium to 1000 °C; furthermore, coupled with previous data⁹ for identical coal samples, this conclusion extends over a wide range of heating rate from 0.7 to 1.5 × 10⁴ °C/s. Using the temperature of maximum rate of devolatilization as an indication of relative reactivity, the devolatilization reactivity differences among the four coals tested that were suggested by this criterion are not large. For combustion in air, the overall coal/char reactivity sequence as determined by comparison of sample ignition temperature is: N. Dakota lignite coal ≈ Montana lignite coal > North Dakota lignite char > III. No. 6 bituminous coal ≈ Pittsburgh Seam bituminous coal > Montana lignite char > III. No. 6 bituminous char > Pittsburgh Seam bituminous char. The reactivity differences are significantly larger than those for devolatilization. The reactivity results obtained suggest that coal type appears to be the most important determinant of coal and char reactivity in air. The weight loss data were fitted to a distributed-activation-energy model for coal pyrolysis; the kinetic parameters so computed are consistent with the view that coal pyrolysis involves numerous parallel first-order organic decomposition reactions.     

Influence of the volatiles formation conditions on the combustion of wood fuels

The results of experimental investigation of pyrolysis, ignition, and combustion of biofuels (wood chips, date stones) and their processing products (charcoal, pellets) at temperatures of 250–1200°C are presented. The influence of the yield of volatiles on the ignition and the mode of combustion of a particle has been considered. The results are compared with published data on the combustion of pellets and brown coals and with the data calculated according to the classical diffusion-kinetic model.     

Coal char combustion under a CO2-rich atmosphere: Implications for pulverized coal injection in a blast furnace

Pulverized coal injection (PCI) is employed in blast furnace tuyeres attempting to maximize the injection rate without increasing the amount of unburned char inside the stack of the blast furnace. When coal is injected with air through the injection lance, the resolidified char will burn in an atmosphere with a progressively lower oxygen content and higher CO₂ concentration. In this study an experimental approach was followed to separate the combustion process into two distinct devolatilization and combustion steps. Initially coal was injected into a drop tube furnace (DTF) operating at 1300 °C in an atmosphere with a low oxygen concentration to ensure the combustion of volatiles and prevent the formation of soot. Then the char was refired into the DTF at the same temperature under two different atmospheres O₂/N₂ (typical combustion) and O₂/CO₂ (oxy-combustion) with the same oxygen concentration. Coal injection was also performed under a higher oxygen concentration in atmospheres typical for both combustion and oxy-combustion. The fuels tested comprised a petroleum coke and coals currently used for PCI injection ranging from high volatile to low volatile bituminous rank. Thermogravimetric analyses and microscopy techniques were used to establish the reactivity and appearance of the chars.     

Investigation of combustion reactivity and NO emission characteristics of chars obtained from the devolatilization of raw and partially dried lignite

In an attempt to achieve the clean and efficient utilization of lignite, drying pre-treatment was performed in this study before lignite combustion. The combustion reactivity and NO emission characteristics of the partially dried lignite samples in the char combustion stage were investigated by means of TG and isothermal combustion tests, and the reactivity could be summarized as the following order: L1>L0.5>raw>L3>LT>L5 (chars obtained from the devolatilization of the raw and partially dried coals at 378 K for 0.5, 1, 3, 5, and 120 minutes) and the NO conversion ratio of L1 was the lowest. When the moisture content in the coal particles was relatively high (19.68%-35.84%), the drying treatment could increase the combustion reactivity and inhibit NO emission in the char combustion stage; When the moisture content was within a relatively low range (0.17%-19.68%), the moisture removal had negative effects on the reactivity and NO emission in the char combustion stage. The surface behaviour and microstructure of the raw coal char and chars derived from the partially dried coals were clarified by temperature programmed desorption/reduction (TPD/TPR) and Raman spectroscopy. The results illustrated that L1 derived from L_c1 (19.68%) was the most reactive sample with the largest amount of C(O) on the particle surface. There were also more reactive aromatic structures in L1 than other samples. Compared with direct combustion or excessive drying treatment, lignite should be dried to a certain degree (19.68%) for optimized lignite combustion.     

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.     

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.