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.     

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.     

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.     

The influence of pyrolysis conditions on the subsequent gasification of lignocellulosic chars

Prune pit chars prepared by pyrolysis at heating rates of 1 and 15°C/min to 500, 700 and 900°C were subsequently gasified by exposure to CO₂ at 900°C for various lengths of time. Gasification rate was found to be dependent on the conditions during pyrolysis: slow heating below 500°C and prolonged exposure to high temperatures (~900°C) during pyrolysis in an inert atmosphere lead to lower rate gasification. Despite differences in gasification rate, the pore structure developed for a given mass loss due to the gasification reactions was apparently independent of the char preparation conditions. Pore volume in the gasified char (expressed on an absolute basis) passed through a maximum at 40–50% burnoff, apparently due to mass loss from the exterior of the particles.     

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.     

Preparation of active carbons from coal Part II. Carbonisation of oxidised coal

Four coals differing in origin, volatile matter (VM) content, plastic properties and degree of preoxidation have been carbonised in nitrogen up to 1123 K. VM and oxygen contents of the chars obtained from unoxidised coals are very low. The VM content of the chars generally increases with an increase in the degree of coal preoxidation but the oxygen content increases only at lower degrees of preoxidation. While the mercury density of the chars decreases, the helium density increases with the degree of coal preoxidation and is related almost linearly to the helium densities of the oxidised coals. Preoxidation of coal also influences the pore size distribution of chars. The pore size distribution, which is more favourable to macropores in the case of chars obtained from unoxidised coal, becomes more and more in favour of micropores as the degree of coal preoxidation is enhanced. The percentage of micropores increases from 30% to more than 70% after coal preoxidation. Unoxidised coal chars adsorb an insignificant amount of nitrogen at 77 K while an appreciable amount of CO₂ is adsorbed at 273 K. The large difference between N₂ and CO₂ adsorption on chars prepared from coals with low oxidation degree becomes smaller as the degree of coal preoxidation increases. There is a linear relationship between the total pore volume of the char and that of the corresponding oxidised coal, indicating that the chars produced by carbonisation of oxidised coal retain fingerprints of the pore structure of the precursor oxidised coal.     

Reactivities of 34 coals under steam gasification

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

Reactivity of 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.     

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.     

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.     

Coal lump devolatilization and the resulting char structure and properties

Lumps of six bituminous coals, from 20 to 40 mm in size, were devolatilized in a laboratory oven in nitrogen atmosphere at different final temperatures ranging from 300 to 800 °C. The structure and morphology of the resulting chars with different degree of devolatilization have been examined under an optical microscope in order to better understand the formation mechanism of different types of char. The swelling of the caking coals and the fissuring of the non-caking coals were characterised by image analysis and some correspondences between the distribution of lithotypes within the initial coal lumps and the char structure obtained were revealed. The relation between chars structure and properties was also investigated. The char lumps obtained from caking coal exhibit better resistance to breakage than their parent coal lumps while non-caking coals show the opposite behaviour. For both caking and non-caking coals, a significant decrease of resistance is observed in the intensive devolatilization temperature range from 400 to 600 °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).     

On the difference in reactivity between high- and low- inertinite coals

Four high-inertinite and two low-inertinite coals have been studied with a view to establishing whether or not the reactivity of the two types of coal can be described in the same terms. It turned out that in most respects high- and low-inertinite coals behave similarly: they show the same correlations between VM content and total oxygen and aliphatic hydrogen content, and between char reactivity (air, 470°C) and VM content. Again, upon oxidation at 230°C in air the same surface groups form and in comparable amounts, while the effects of this (pre)oxidation step on VM content and char reactivity are also alike. Reduction with HI does not discriminate between these coals either. Yet, in one respect, the high- and low-inertinite coals differ: the extract yield obtained in basic organic solvents is much lower for the former than for the latter type of coals. After preoxidation, however, this difference is no longer noticeable.     

热解条件对煤焦气化活性影响的研究进展

简述了原煤性质与温度、压力和热解气氛等热解条件对煤焦结构和气化反应活性的影响;参考该领域的国内外研究成果,分析了热解条件影响煤焦气化反应活性的机理.由于实验设备和研究方法的差异,对温度和压力等热解条件对煤焦气化反应活性影响的评价不尽相同,但总体来讲,热解终温越高、停留时间越长、升温速率越快、热解压力越大,煤焦的气化反应活性越低;热解过程中,原煤性质的差异也会影响煤焦的结构和气化反应活性.煤焦的石墨化应该是导致煤焦气化反应活性下降的主要原因,因此,热解条件的改变,特别是温度和压力的改变对煤焦石墨化进程的影响值得进一步研究.     

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.     

The pyrolysis of a Wyoming coal in different nonreactive atmospheres

This study investigated how differing nonreactive atmospheres affected the properties of chars produced by coal pyrolysis. Samples of a Wyoming subbituminous coal were first heated at 586°C in helium for 6 hr. They were then heated for another 6 hr at 300°C higher in helium, argon or nitrogen. Weight losses in the chars during the second step were apparently unaffected by the choice of inert environment. Surface characteristics, pore structures, and reactivities of the resulting chars were, however, significantly different as shown by the results of scanning electron microscopy, nitrogen adsorption, carbon dioxide adsorption, and reactivity of the chars in carbon dioxide. Smoothness of the surface, adsorption capacities, N₂ and CO₂ surface areas, and reactivity during gasification all decreased in the direction (in order of inert atmospheres employed) He >Ar >N₂. In addition, there were strong indications that trace contaminants in the inert gases could alter the characteristics of the resulting char markedly.     

Influence of coal preoxidation and the relation between char structure and gasification potential

A study was carried out to ascertain the effects of coal preoxidation and carbonization conditions on the structure and relative gasification potential of a series of bituminous coal chars. Chars were prepared from two freshly mined bituminous coals and preoxidized samples derived from them. Carbonization conditions included a wide range of heating rate (0.2–10000K s^?1), temperature (1073–1273 K) and time (0.25–3600 s). Char properties were characterized in terms of analysis of char morphology, surface area, elemental composition, and gasification reactivity in air. Over the range of conditions used, preoxidation substantially reduced coal fluid behaviour and influenced macroscopic char properties (char morphology). Following slow heating (0.2 K s^?1), preoxidized coals yielded chars having higher total surface areas and higher reactivities toward gasification in air than did similar chars prepared from fresh coal. Following rapid heating (10000 K s^?1) and short residence times (0.25 s), chars prepared from preoxidized and fresh coals exhibited similar microstructural and chemical properties (surface area, $\text{C}\text{H}$ ratios, gasification rates). Carbonization time and temperature were found to be the critical parameters influencing char structure and gasification potential.     

Effects of Ca-based additives on desulfurization during coal pyrolysis

The effects of different Ca-based additives on the sulfur removal of coals during pyrolysis up to 900 °C have been studied in a fixed-bed reactor. It was found that Ca(OH)₂ and CaO were quite effective to capture the sulfur-containing gases, 95% of the sulfur evolved from untreated coal was retained in the char by the use of additives. Both the tar yield and the sulfur content of the tar decreased with addition of Ca-based additives. The effect of Ca(OH)₂ was better than that of CaO due to its higher activity, but CaCO₃ had little effect because of its higher decomposition temperature (−900 °C) than the peak temperature range (400–500 °C) of sulfur-containing gases emission. There is remarkable sulfur retention effect with Ca(OH)₂ prepared by impregnation and ultrasonic treatment due to the higher dispersion in coal particles than by simple mechanical mixing. The ultrasonic treatment is the best method with regard to the lowest SO₂ release during the char combustion. XRD results showed that the sulfur captured by Ca-based additives during pyrolysis turned into CaS. FeS detected in pyrolysis char without additives disappeared in chars with additives, which indicated that CaO could react with FeS through solid-solid reaction. When the chars with calcium-additives were burned in fixed bed reactor, they gave out less SO₂ than the raw coal added with same additives. The best total desulfurization efficiency could reach to about 85%.     

Effect of pre-oxidation on reactivity of chars in steam

The effects of pre-oxidation of char from Taiheiyo coal, a non-caking bituminous coal, in the 400–550 °C temperature range on its gasification reactivity with N₂-H₂O at 0.1 MPa (steam partial pressure of 13.2 kPa) have been investigated. The pre-oxidation of char markedly enhances gasification rates at temperatures between 800 and 900 °C. Reactivity is found to parallel the burn-off level during preoxidation at low temperatures (400–430 °C), whereas at relatively high temperatures (480–550 °C), the burn-off level only affects the reactivity slightly. The amount of CO and CO₂ evolved from the preoxidized char by heat treatment is proportional to the burn-off level at low temperatures (400–430 °C), being closely related to the enhancement of the gasification reactivity in steam.     

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.