Mineral matter effects on the rapid pyrolysis and hydropyrolysis of a bituminous coal. 1. Effects on yields of char,tar and light gaseous volatiles

The effects of minerals on product compositions from rapid pyrolysis of a Pittsburgh Seam bituminous coal were investigated. Whole, demineralized, and mineral treated samples of pulverized coal were heated in 100 KPa helium or 6.9 MPa hydrogen at 1000 K s^?1 to temperatures of up to 1300 K. Yields of char, tar and individual gaseous products were determined as a function of time-temperature conditions. Clays, iron-sulphur minerals, and quartz had few effects on pyrolysis in helium. Calcium minerals decreased yields of hydrocarbon products and increased yields of CO in helium pyrolysis. Calcite and clays decreased yields of CH₄ from hydropyrolysis, whereas iron-sulphur minerals had little effect on pyrolysis at 6.9 MPa H₂. Whole coal results were similar to demineralized coal results under all conditions.     

The N2O–carbon reaction: The influence of CO and potassium on reactivity and populations of oxygen surface complexes

N₂O reduction on two different chars promoted with potassium was investigated using temperature programmed reaction, as well as isothermal reaction followed by temperature programmed desorption. It was found that potassium promotion significantly increased the N₂O reduction activity of the phenolic resin char, but not for the Wyodak coal char. Additional CO in the reactor feed gas had no discernible effect on the N₂O reduction rate of the phenolic resin char, but it did significantly increase the reactivity of the promoted, demineralized Wyodak coal char. The latter is attributed to residual mineral matter impurities in the coal that are not removed by the demineralization process used.     

Sulfur release from coal in fluidized-bed reactor through pyrolysis and partial oxidation with low concentration of oxygen

Yima (YM) and Datong (DT) raw coal were pyrolyzed in a fluidized bed reactor under 0.6%O₂-N₂, 1.1%O₂-N₂ and 2.1%O₂-N₂ atmosphere, and a flue gas analyzer was used to check the SO₂ in pyrolysis gas. The product of sulfur removal and char yield is suggested to measure the efficiency of sulfur removal. For YM coal, sulfur removal generally has increasing trend with the increase of oxygen concentration in atmosphere. The char yield of YM coal has no remarkable decrease when the oxygen content is lower than 1.1%. However, in 2.1%O₂-N₂ less char yield is obtained. For DT coal more sulfur is removed in 0.6%O₂-N₂ than in N₂, and at the same temperature more SO₂ is released with increasing oxygen content. It is suggested that the atmospheres used selectively break the C-S bonds other than C-C bonds. Pyrolysis of coal in fluidized-bed reactor under low concentration of oxygen atmosphere is a promising method to greatly remove sulfur, and not remarkably decrease the char yield.     

The influence of mineral matters in coal on NO-char reaction in the presence of SO2

The effect of mineral matter in char on NO-char reaction in the presence of SO₂ was studied by temperature programmed reaction and isothermal experiments. Three coals with different ranks and their demineralized samples were pyrolyzed in N₂ at 900 °C to prepare the chars. Different kinds of metals were loaded on the demineralized chars to compare their catalytic effect on NO conversion during NO-char reaction. The results show that the effect of mineral matter is closely related to the content of catalytically active components. More catalytically active components in mineral matter in the char, higher catalytic activity for NO-char reaction. While the inert components, such as Al₂O₃ and Si₂O₃, will abate the NO conversion. Besides the catalytic effect of active mineral matter, the reactivity of the char is another important factor to affect the NO conversion during NO-char reaction. With increasing coal rank, the resultant char shows lower activity for reduction of NO. The effect of SO₂ on the NO-char reaction is changed with temperature. At higher temperatures NO conversion is further enhanced by the reaction of NO-SO₂ and the increase in the amount of active sites due to the release of SO₂ chemisorbed on the char surface.     

Desulfurization of coal through pyrolysis in a fluidized-bed reactor under nitrogen and 0.6% O2-N2 atmosphere

The raw Yima (YM) and Datong (DT) coal, their demineralized (YM-ash, DT-ash) and de-pyrite (YM-p, DT-p) coals were pyrolyzed in a fluidized-bed reactor to examine the sulfur removal efficiency. The effect of process parameters such as temperature, residence time and atmosphere were investigated. The results show that there is an optimal temperature and residence time for the maximum desulfurization, varying with type of coal and the thermal stability of organic sulfur. The alkaline-earth mineral in the raw coal plays an important role for the fixation of sulfur and makes desulfurization decrease. The interaction of pyrite with the organic matrix of coal is the dominant reason that leads to organic sulfur accumulation in char. YM has higher desulfurization of organic sulfur than DT due to more aliphatic sulfur in the raw coal. YM and DT were pyrolyzed in 0.6%O₂-N₂ mixed atmosphere, aiming at examining the effect of reactive gas on the sulfur removal during pyrolysis. The results show that sulfur removal is improved without great decrease in char yield. This indicates that small amount of O₂ in inert atmosphere can improve desulfurization efficiency. In addition, TG-MS runs in 0.6%O₂-Ar and Ar were carried out to check the sulfur-containing compounds in pyrolysis gas and further understand the desulfurization process.     

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.     

Kinetic analysis and modelling of thermal degradation of perspex (PMMA) and perspex blend plastic waste

The thermal decomposition of pure perspex and a mixture of 50% perspex and 50% poly(ethylene terephthalate; PET) was carried out between 295 and 325°C using a thermogravimetric analyser (TGA) in air and nitrogen (N₂) atmosphere. The weight losses of decomposition products were measured during these experiments. The thermal degradation process is slower in inert atmosphere than air, where oxidation reaction expedites the decomposition process. Kinetic rate constants (k), pre‐exponential factor (A) and activation energy (E) for both pure prespex and a blend of perspex/PET were calculated for both air and N₂ conditions. The thermal degradation process followed a third‐order reaction in air and second‐order in N₂. A second‐order (n = 2) model for the pyrolytic process based on simultaneous reactions was developed using experimental data for pure and blend. The pyrolytic products are gases, liquids, waxes, aromatics and char, which can be ultimately used as raw material and fuel in various applications. It is important to note that the addition of PET to perspex was found to suppress/inhibit the decomposition of perspex compared with pure perspex. Pre‐exponential factor (A) and activation energy (E) values support such an observation. © 2012 Canadian Society for Chemical Engineering     

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.     

煤热解特性研究

对大雁、协庄和昔阳3个不同煤化程度的煤样，在N2，CO2和水蒸气3种不同气氛及不同温度下进行了热解研究，考察了煤化程度、热解气氛和热解温度对煤热解产物产率和热解气性质的影响规律．研究表明，对上述3个煤样，随煤化程度加深，焦产率增加，油和气产率一般随煤中挥发分增加而增加，但又与煤的大分子结构、热解温度和加热速率等有密切关系；干馏气组成H2和CH4含量协庄煤样最高，而(CO CO2)含量因煤中氧含量的降低而下降．与N2气氛相比，CO2和水蒸气气氛中半焦产率下降，气产率增加；油产率水蒸气气氛下最高．H2组分含量在水蒸气气氛下最高，而CO，CH4和烃类C2～C5组分则最低．LHV在N2，CO2和水蒸气气氛下逐次降低．     

Flash pyrolysis of coals. Devolatilization of bituminous coals in a small fluidized-bed reactor

The devolatilization behaviour of ten bituminous coals was investigated under rapid heating conditions using a small-scale fluidized-bed pyrolyser. The pyrolyser operated continuously, coal particles being injected at a rate of 1–3 g h^?1 directly into a heated bed of sand fluidized by nitrogen. Yields of tar, C₁–C₃ hydrocarbon gases, and total volatile-matter and an agglomeration index are reported for all coals. Maximum tar yields were obtained at about 600 °C and were always substantially higher than those from the Gray-King assay. Total volatile-matter yields were also substantially higher than the proximate analysis values. The maximum tar yields appear to be directly proportional to the coal atomic $\text{H}\text{C}$ ratio. The elemental analysis of the tar is strongly dependent on pyrolysis temperature. The tar atomic $\text{H}\text{C}$ ratio is proportional to that of the parent coal. The effect on the devolatilization behaviour of two coals produced by changes in the pyrolyser atmosphere and the nature of the fluidized-bed material were also investigated. Substituting an atmosphere of hydrogen, helium, carbon dioxide or steam for nitrogen, has no effect on tar yield and, with one exception, little effect on the hydrocarbon gas yields. In the presence of hydrogen the yield of methane was increased at temperatures above 600 °C. Tar yields were significantly reduced on substituting petroleum coke for sand as the fluid-bed material. A fluidized bed of active char virtually eliminated the tar yield.     

Comparative investigations of coal pyrolysis under inert gas and H2 at low and high heating rates and pressures up to 10 MPa

Five German hard coals of 6–36 wt% volatile matter yield (maf) were pyrolysed at pressures up to 10 MPa, using two different apparatuses, which mainly differ in the heating rates. One consists of a thermobalance where a coal sample of ≈ 1.5 g is heated at a rate of 3 K min ^?1 under a gas flow of 3 I min^?1. The other apparatus is constructed for rapid heating (10²?10³ K s^?1) of a small sample of ≈10 mg of finely-ground coal distributed as a layer between the folded halfs of a stainless-steel screen, heated by an electric current. The product gas composition was determined by quantitatively analysing for H₂, CH₄, C₂H₄, C₂H₆, CO, CO₂ and H₂O. The amounts of tar and char were measured by weighing. The heating rate, pressure and gas atmosphere were varied. Under an inert gas atmosphere, high heating rates result in slightly higher yields of liquid products, e.g. tar. The yields of light hydrocarbon gases remain the same. With increasing pressure, the thermal cracking of tar is intensified resulting in high yields of char and light hydrocarbon gases. Under H₂, pyrolysis is influenced strongly at elevated pressure. Additional amounts of highly aromatic products are released by hydrogenation of the coal itself, particularly between 500 and 700°C. This reaction is less effective at higher heating rates because of the shorter residence time and diffusion problems of H₂. The yield of light gaseous compounds CH₄ and C₂H₆ increases markedly under either heating condition owing to gasification of the reactive char.     

Roles of inherent metallic species in secondary reactions of tar and char during rapid pyrolysis of brown coals in a drop-tube reactor

Two pairs of raw and acid-washed coal samples were prepared from Yallourn and Loy Yang brown coals, and subjected to rapid pyrolysis in a drop-tube reactor at 1073-1173 K in a stream of N₂ or H₂O/N₂ mixture. Examinations were made on the roles of the inherent metallic species in the secondary reactions of nascent tar and char that were formed by the intraparticle primary reactions. The experimental results revealed that the inherent metallic species were essential for vary rapid steam reforming/gasification of the nascent tar/char and simultaneous suppression of soot formation. In the absence of the metallic species, the soot formation from the tar accounted as much as 15-19 and 6-13% of the carbon in coal in N₂ and H₂O/N₂, respectively. The metallic species reduced the yield of soot to 6-8% in N₂ by enhancing the reforming of tar by H₂O generated from the pyrolysis of coal. In the H₂O/N₂ stream, instead of soot formation, a net gasification conversion up to 17% within 4.3 s was observed in the presence of the metallic species as a result of catalytic gasification of the nascent char. Moreover, the metallic species catalyzed the steam reforming of the nascent tar, giving its conversion up to 99%. Over the range of the conditions employed, a one-to-one stoichiometry was established between the steam consumption and the yield of carbon oxides formed by the steam reforming/gasification and water-gas-shift reaction.     

The effect of steam on pyrolysis and char reactions behavior during rice straw gasification

Steam gasification of biomass can generate hydrogen-rich, medium heating value gas. We investigated pyrolysis and char reaction behavior during biomass gasification in detail to clarify the effect of steam presence. Rice straw was gasified in a laboratory scale, batch-type gasification reactor. Time-series data for the yields and compositions of gas, tar and char were examined under inert and steam atmosphere at the temperature range of 873-1173 K. Obtained experimental results were categorized into those of pyrolysis stage and char reaction stage. At the pyrolysis stage, low H₂, CO and aromatic tar yields were observed under steam atmosphere while total tar yield increased by steam. This result can be interpreted as the dominant, but incomplete steam reforming reactions of primary tar under steam atmosphere. During the char reaction stage, only H₂ and CO₂ were detected, which were originated from carbonization of char and char gasification with steam (C + H₂O→CO + H₂). It implies the catalytic effect of char on the water-gas shift reaction. Acceleration of char carbonization by steam was implied by faster hydrogen loss from solid residue.     

Effects of coal carbonization conditions on rate of steam gasification of char

Effects of carbonization conditions on char reactivity in steam gasification were evaluated by a gravimetric method, using 12 coals varying widely in rank, type and source. The carbonization variables examined were

• 1.(1) heating rate (5–420K min⁻¹) in steam atmosphere;
• 2.(2) gaseous atmosphere (N₂,H₂,H₂O andCO₂);
• 3.(3) incomplete devolatilization in N₂ (final temperature 200–800 °C);
• 4.(4) quenching of incompletely devolatilized char; and
• 5.(5) complete carbonization (900–1400 °C).

The char reactivity to steam depended on the kind of coal but was almost independent of the carbonization conditions of heating rate, gaseous atmosphere and quenching at temperatures below ≈ 1000 °C. Carbonization above 1100 °C reduced the char reactivity, for example by a factor of 7 to 10 at 1300 °C compared with 900–1000 °C, depending on the parent coal. The char deactivation brought about by increasing carbonization temperature could be correlated with a decrease in the micropore volume of the char, unless graphitization was significant.     

Synthesis gas and char production from Mongolian coals in the continuous devolatilization process

Devolatilization of Mongolian coal (Baganuur coal (BC), Shievee Ovoo coal (SOC), and Shievee Ovoo dried coal (SOC-D)) was investigated by using bench-sized fixed-bed and rotary kiln-type reactors. Devolatilization was assessed by comparing the coal’s type and dry basis, temperature, gaseous flux, tar formation/generation, devolatilization rate, char yield, heating value, and the components of the raw coal and char. In the fixed bed reactor, higher temperatures increased the rate of devolatilization but decreased char production. BC showed higher rates of devolatilization and char yields than SOC or SOC-D. Each coal showed inversely proportional devolatilization and char yields, though the relation was not maintained between the different coal samples because of their different contents of inherent moisture, ash, fixed carbon, and volatile matter. Higher temperatures led to the formation of less tar, though with more diverse components that had higher boiling points. The coal gas produced from all three samples contained more hydrogen and less carbon dioxide at higher temperatures. Cracking by multiple functional groups, steam gasification of char or volatiles, and reforming of light hydrocarbon gas increased with increasing temperature, resulting in more hydrogen. The water gas shift (WGS) reaction decreased with increasing temperature, reducing the concentration of carbon dioxide. BC and SOC, with retained inherent moisture, produced substantially higher amounts of hydrogen at high temperature, indicating that hydrogen production occurred under high-temperature steam. The continuous supply of steam from coal in the rotary kiln reactor allowed further exploration of coal gas production. Coal gas mainly comprising syngas was generated at 700–800 °C under a steam atmosphere, with production greatest at 800 °C. These results suggest that clean char and high value-added syngas can be produced simultaneously through the devolatilization of coal at lower temperature at atmospheric pressure than the entrained-bed type gasification temperature of 1,300–1,600 °C.     

A TPD study of coal chars in relation to the catalysis of mineral matter

The steam gasification mechanism of brown coal was studied by a temperature-programmed desorption (TPD)technique. A Morwell coal was devolatilized in N₂ and then gasified in steam at 1100 K. During the TPD of a partially gasified char, H₂O, CO₂ and CO evolved approximately at 640, 870 and 1020 K, respectively. The presence of mineral matter was found to be responsible for these gas evolutions, since essentially no gas evolution was observed during the TPD of the demineralized coal char. The comparison of the above TPD pattern with those determined for the cation-exchanged samples revealed which inorganic species is responsible for each TPD peak: H₂O evolution was due to Ca; CO₂ evolution to Ca and Mg; CO evolution to Na and/or Fe. The exchanged metal species like Ca and Na significantly catalysed the gasification reaction. The relation between the catalytic activity and TPD pattern was discussed in terms of surface oxygen complexes.     

In situ diagnostics of Victorian brown coal combustion in O2/N2 and O2/CO2 mixtures in drop-tube furnace

Experimental investigation of the combustion of an air-dried Victorian brown coal in O₂/N₂ and O₂/CO₂ mixtures was conducted in a lab-scale drop-tube furnace (DTF). In situ diagnostics of coal burning transient phenomena were carried out with the use of high-speed camera and two-colour pyrometer for photographic observation and particle temperature measurement, respectively. The results indicate that the use of CO₂ in place of N₂ affected brown coal combustion behaviour through both its physical influence and chemical interaction with char. Distinct changes in coal pyrolysis behaviour, ignition extent, and the temperatures of volatile flame and burning char particles were observed. The large specific heat capacity of CO₂ relative to N₂ is the principal factor affecting brown coal combustion, which greatly quenched the ignition of individual coal particles. As a result, a high O₂ fraction of at least 30% in CO₂ is required to match air. Moreover, due to the accumulation of unburnt volatiles in the coal particle vicinity, coal ignition in O₂/CO₂ occurred as a form of volatile cloud rather than individual particles that occurred in air. The temperatures of volatile flame and char particles were reduced by CO₂ quenching throughout coal oxidation. Nevertheless, this negative factor was greatly offset by char-CO₂ gasification reaction which even occurred rapidly during coal pyrolysis. Up to 25% of the nascent char may undergo gasification to yield extra CO to improve the reactivity of local fuel/O₂ mixture. The subsequent homogeneous oxidation of CO released extra heat for the oxidation of both volatiles and char. As a result, the optical intensity of volatile flame in ∼27% O₂ in CO₂ was raised to a level twice that in air at the furnace temperature of 1273 K. Similar temperatures were achieved for burning char particles in 27% O₂/73% CO₂ and air. As this O₂/CO₂ ratio is lower than that for bituminous coal, 30-35%, a low consumption of O₂ is desirable for the oxy-firing of Victorian brown coal. Nevertheless, the distinct emission of volatile cloud and formation of strong reducing gas environment on char surface may affect radiative heat transfer and ash formation, which should be cautioned during the oxy-fuel combustion of Victorian brown coal.     

Synthesis and characterization of carborane‐containing polyester with excellent thermal and ultrahigh char yield

A series of wholly carborane‐containing polyesters with high thermostability were successfully synthesized by the catalytic polycondensation of carborane diol monomers with carborane diacid chlorides. They can be used for the preparation of materials of high temperature resistant coatings and adhesive. The influence of solvent, reaction temperature, and reaction time on the molecular weight and yield of the polymers were studied. In comparison with the carborane‐free polyester, the carborane‐containing polyesters showed higher degradation temperature and char yield and lower degradation rate. The thermal gravimetric analyzer (TGA) curves indicate that the carborane group could effectively reduce the degradation rate of carborane‐containing polyesters, which give a char yield of exceeding 64% under air (47% under N₂) at 700 °C. Such data are superior to the carborane‐free polyester, which showed a low char yield of around 0.3% under air (5% under N₂) at the same condition. Moreover, the thermal transition mechanism of carborane‐containing polyesters was also studied. The FTIR spectra and TG‐FTIR analysis indicate that the carborane cage could react with oxygen to form BOB and BC linkages at elevated temperatures, which postpones the thermal decomposition of polyester and accounts for the high char yield. The newly prepared kind of high temperature polyesters have enormous technical and economic value, especially in the high temperature fields. They can be widely used as raw materials to prepare the high temperature resistant coatings or adhesives for automotive engine, aircraft and other equipments worked in high‐temperature environments. Under high environmental temperature, the good thermal stability is capable of keeping polyesters stable and expanding their service lives. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44202.     

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.     

Effect of inorganic matter on reactivity and kinetics of coal pyrolysis

Two Chinese coals, Shenfu subbituminous coal and Huolingele lignite, were used to investigate the effect of mineral matter in coal on reactivity and kinetic characteristics of coal pyrolysis. The experiments were carried out by using thermogravimetry to check the pyrolysis behavior of raw coal, HCl/HF demineralized coal and demineralized coal with inorganic matter (CaO, K₂CO₃ and Al₂O₃) added, respectively. The results showed that inherent mineral in coal had no evident effect on the reactivity and kinetics of coal pyrolysis. CaO, K₂CO₃ and Al₂O₃ all had a catalytic effect on the reactivity of coal pyrolysis, their effects were closely related to temperature region and coal types. The pyrolysis process of all the samples studied can be described by three independent first order kinetic model. Addition of inorganic matter the activation energy decreased and the characteristic temperature of coal changed.