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.     

Variation of the pore structure of coal chars during gasification

The variation of the pore structure of several coal chars during gasification in air and carbon dioxide was studied by argon adsorption at 87 K and CO₂ adsorption at 273 K. It is found that the surface area and volume of the small pores (<10 Å) do not change with carbon conversion when the coal char is gasified in air, while those of the larger pores (10-20 Å, 20-50 Å, 50-2500 Å) increase with increase of carbon conversion. However in CO₂ gasification, all the pores in different size ranges increase in surface area and volume with increase of carbon conversion. Simultaneously, the reaction rate normalized by the surface area of the pores >10 Å for air gasification is constant over a wide range of conversion (>20%), while for CO₂ gasification similar results are obtained using the total surface area. However, in the early stages of gasification (<20%) the normalized reaction rate is much higher than that in the later stage of gasification, due to existence of more inaccessible pores in the beginning of gasification. The inaccessibility of the micropores to adsorption at low and ambient temperatures is confirmed by the measurement of the helium density of the coal chars. The random pore model can fit the experimental data well and the fitted structural parameters match those obtained by physical gas adsorption for coal chars without closed pores.     

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.     

Behavior of mercury release during thermal decomposition of coals

The mercury release behavior during thermal decomposition of three Chinese coals with different types was studied under nitrogen, carbon dioxide and air at temperatures of 800, 900, 1,000 and 1,100 °C. The thermal treatment experiments were carried out in a quartz tube reactor. Results showed that the release ratio of total mercury during thermal decomposition of coals increases with the increasing temperature. The order of the amount of mercury released under the three atmospheres is nitrogen<carbon dioxide<air for all three coals during thermal decomposition. This indicates that air and carbon dioxide can promote the mercury release due to their reactivity with coal. However, the order of amount of elemental mercury released under the three atmospheres is nitrogen>carbon dioxide>air for all three coals. The release behavior of the total mercury under air is independent of the coal type. Under the other two atmospheres the release behavior is distinguished by the coal type. This work was presented at the 7 ^th China-Korea Workshop on Clean Energy Technology held at Taiyuan, Shanxi, China, June 26–28, 2008.     

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.     

Low-temperature air oxidation of caking coals. 1. Effect on subsequent reactivity of chars produced

The effect of preoxidation of two highly caking coals in the temperature range 120–250 °C on weight loss during pyrolysis in a N₂ atmosphere up to 1000 °C and reactivity of the resultant chars in 0.1 MPa air at 470 °C has been investigated. Preoxidation markedly enhances char reactivity (by a factor of up to 40); the effect on char reactivity is more pronounced for lower levels of preoxidation. For a given level of preoxidation, the oxidation temperature and the presence of water vapour in the air used during preoxidation have essentially no effect on weight loss during pyrolysis and char reactivity. An increase in particle size of the caking coals reduces the rate of preoxidation as well as subsequent char reactivity. Preoxidation of caking coals sharply increases the surface area of the chars produced. Compared to heat treatment in a N₂ atmosphere, pyrolysis in H₂ of either the as-received or preoxidized coal results in a further increase in weight loss and a decrease in subsequent char reactivity.     

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.     

Enlargement of the micropores of a caking bituminous coal by controlled oxidation

In a study of the enlargement of pores of coals it has been found that treatment of a bituminous coal (PSOC No. 371, from the Pennsylvania State University Coal Section) with a 5:95 O₂:N₂ stream 4 h at 400 °C increases the surface area as measured by nitrogen adsorption at 77K by a factor of at least 50 to a value 52 m² g^?1. The increase in pore size was accompanied by a 9.7% weight loss. Simultaneously, the area as measured by carbon dioxide at 195K increased from 61 to 136 m² g^?1 and that measured by carbon dioxide at room temperature increased from 125 to 237 m² g^?1. Attempts to enlarge the pores by oxidation with hydrogen peroxide or ozone were unsuccessful. A Pittsburgh coal subject to a small percentage of oxygen in nitrogen or steam at 300 to 400 °C showed a surface area as measured by nitrogen adsorption of less than 1 m² g^?1 both before and after such pretreatment. This same coal with a 5:95 O₂:N₂ stream for 4 h at 450 °C showed a surface area of 110 m² g^?1 measured by nitrogen adsorption at 77K.     

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.     

High‐temperature pyrolysis and CO2 gasification of Victorian brown coal and Rhenish lignite in an entrained flow reactor

The low rank coals from Victoria, Australia, and Rhineland, Germany are of interest for use in entrained flow gasification applications. Therefore, a high temperature, electrically heated, entrained flow apparatus has been designed to address the shortage of fundamental data. A Victorian brown coal and a Rhenish lignite were subjected to rapid, entrained flow pyrolysis between 1100 and 1400°C to generate high surface area chars, which were subsequently gasified at the same temperatures under CO₂ in N₂ between 10 and 80 vol %. The Victorian coal was more reactive than the Rhenish coal, and peak char reactivity was observed at 1200°C. Char conversion and syngas yield increased with increasing temperature and plateaued at high CO₂ concentration. Ammonia and tar species were negligible and HCN and H₂S were present in parts per million (volume) concentrations in the cooled, filtered syngas. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2101–2111, 2016     

Microstructural variations of lignite,subbituminous and bituminous coals and their high temperature chars

Microstructure of a North Dakota lignite, a Washington subbituminous and a New Mexico bituminous coal and their chars produced by devolatilization in nitrogen at 1000 to 1300°C was investigated in this work using the CO₂ adsorption method conducted at 25°C. For each coal and char, specific surface area, micropore volume, micropore surface area, mean equivalent radius of micropores and characteristic energy of adsorption, as well as micropore volume distribution, were determined, and their variations with devolatilization temperature studied and interpreted. It was found that, overall, specific surface areas, micropore volumes and micropore surface areas of chars decreased monotonically as devolatilization temperature was raised from 1000 to 1300°C, although most of these values were much larger than that of their parent coals. The micropore volume distributions of the three coals and their high temperature chars were interpreted and found to provide an interesting insight into the micro structural variations of these coals and chars.     

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.     

Effect of rank,temperatures and inherent minerals on nitrogen emissions during coal pyrolysis in a fixed bed reactor

Pyrolysis of 11 coals with carbon contents of 77–93 wt.% (daf) and corresponding demineralized samples has been studied in a fixed bed quartz reactor with a heating rate of 20 K/min to examine rank, demineralization, temperature and inherent mineral species dependences of nitrogen distribution. Nitrogen mass balances fall within 92.5–104.6%. The results indicate that the chars derived from the coals with higher rank show larger nitrogen retention. Demineralization suppresses volatile nitrogen emission during coal pyrolysis, especially for low rank coals. Coal-N conversion to tar-N reaches the asymptotic values at 600 °C. HCN yields are lower than NH₃ yields during coal pyrolysis. The trends in HCN and NH₃ emissions are very similar and the yields reach the asymptotic value at about 1200 °C. N₂ starts emitting at 600 °C, and as the temperature increases the conversion increases linearly with a corresponding reverse change of char-N. With the catalysts added, N₂ formation is prompted with the sequence of Fe>Ca>K>Ti≫Na≫Si≈Al, meanwhile, char-N decreases correspondingly. Fe, Ca, K, Na, Si and Al increase coal-N conversion to NH₃ with the sequence of Fe>Ca>K≈Na≫Si≈Al in the pyrolysis. Na addition prompts HCN formation; however, the presence of Ti and Ca decrease the HCN yields with small value. The other catalysts have no notable influence on HCN emission in the pyrolysis. Demineralization and Ti addition increase coal-N conversion to tar-N slightly whereas K, Ca, Mg, Na, Si and Al additions decrease tar-N yield weakly, other catalysts hardly influence tar nitrogen emission. N₂ emits mainly from char-N with slight contribution of volatile nitrogen. The mechanism of different N-containing species formation and catalysts influence in the pyrolysis is also discussed in the paper.     

Effect of inorganic matter removal from coals and chars on their surface areas

Removal of inorganic matter from coals by acid treatment brings about random and, in some cases, significant changes in surface areas measured by adsorption of N₂ and CO₂. Changes in surface areas of chars are generally more pronounced than those found in coals. However, the surface area changes in chars are markedly dependent upon whether the acid treatment is given to the coal precursor prior to charring or to the char produced from the raw coal. Changes in surface areas of raw coals and chars produced therefrom have been attributed to: (i) ‘physical’ removal of inorganic matter from the aperture-cavity system, (ii) bonding of HCl to basic nitrogen present in pyridine-like structures, and (iii) adsorption of acid. Results suggest that the removal of inorganic matter from coals prior to charring affects the carbonization process and, hence, the surface area of the resultant char.     

Investigation of alkali and alkaline earth coal gasification catalysts by EXAFS spectroscopy

EXAFS spectroscopy and several supplementary techniques have been used to investigate a variety of coal and polymer chars containing alkali and alkaline earth catalysts, including Ca, K and Rb. In-situ EXAFS measurements were performed on Rb-loaded chars during gasification. At 450 °C in 90% N₂-10% O₂, the Rb XANES exhibited a doublet structure similar to that of Rb₂O and Rb₂CO₃. EXAFS and other techniques show that the Ca in lignites, both naturally occurring and ion-exchanged, is essentially molecularly dispersed and bonded to carboxyl groups. Calcite-like features are induced in both the near-edge structure and radial structure functions of chars with increasing pyrolysis time and temperature. The structure of K is found to differ significantly in polymer, lignite and bituminous coal chars.     

Effects of catalyst addition to coal on CO2 adsorption kinetics of coal and char

The surface area of Illinois No. 6 coal impregnated with a series of carbon gasification catalysts was measured by carbon dioxide adsorption before and after pyrolysis. Chars were prepared by pyrolysing the coal in flowing nitrogen at a low controlled rate. The adsorbate uptake was described by a logarithmic time dependence. The amounts of CO₂ adsorbed at a fixed pressure within 1 minute and at maximum coverage were used to describe the surface area associated with micropores larger than 0.5 nm in diameter and with the total surface area, respectively. Catalyst addition decreased the surface area of coal accessible to CO₂ in all cases. After pyrolysis, some of the chars showed a considerable increase, others a massive decrease, in their adsorption capacity.     

Coal pyrolysis at high temperatures and pressures

At high temperatures (s> 1100°C), pyrolysis of coal plays an increasingly important role in the overall coal conversion process. This Paper presents experimental data on the extent of pyrolysis of coal at 800–1600°C. In addition, the effects of the following parameters are examined: gaseous environment (N₂, CO₂ and H₂O), pressure (1–20 atm), particle size, moisture content and type of coal. Previous data on some of these parameters are non-existent. A unique TGA apparatus constructed for this work allows high heating rates (10²–10³°Cs^?1) due to the direct radiation heating. In all the gaseous environments, a plateau in per cent pyrolysis is noticed at 1200–1400°C followed by a sharp increase in the amount of pyrolysis as the temperature is raised. This is found consistent with the three-stage mechanism proposed for the evolution of volatiles. In CO₂ and steam environments, there is slightly less pyrolysis than in pure nitrogen, while considerably more pyrolysis is noted for predried coal and for smaller particle sizes. The results suggest a strong influence of secondary volatile reactions on the extent of pyrolysis. Pyrolysis in steam at 800–900°C shows an increase with pressure similar to that reported for pyrolysis in hydrogen. Finally, gasification rates of chars immediately following the pyrolysis are found to be much higher than those of chars prepared separately and then reacted. These results suggest morphological rearrangements and crystallization effects.     

The low-temperature SCR of NO over rice straw and sewage sludge derived char

Rice straw char and sewage sludge char were applied as catalysts for selective catalytic reduction between 50 and 250 °C using ammonia as the reducing agent. Each char was activated physically, using water vapor, or chemically, using KOH. The characteristics of the prepared catalysts were analyzed through elemental analysis, N₂ adsorption–desorption, FT-IR, NO-TPD, NH₃-TPD, and NO_x removal efficiency. The physically activated chars showed characteristics similar to those of the non-activated chars, whereas the chemically activated chars exhibited increased specific surface areas, pore volumes, NO adsorption capacities, NH₃ adsorption capacities, and oxygen functional group amounts, leading to higher NO_x removal efficiency. When the catalysts were impregnated with 3 wt% manganese, NO_x removal efficiency significantly increased. In particular, the NO_x removal efficiency was highest when the chemically activated chars were impregnated with manganese.     

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.     

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.