Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part I. Volatilisation of Na and Cl from a set of NaCl-loaded samples

A set of NaCl-loaded coal samples was prepared by physically impregnating NaCl into a Victorian (Loy Yang) brown coal. This set of brown coal samples was pyrolysed in a thermogravimetric analyser and in a novel fluidised-bed/fixed-bed reactor. The latter reactor has some features of both a fluidised-bed reactor and a fixed-bed reactor. The reactor configuration allowed the volatilised Na to be swept away by carrier gas from the bed of char particles, avoiding the re-condensation of the volatilised Na on the char particles at lower temperatures. The volatilisation of Na and of Cl during pyrolysis was quantified simultaneously. The results indicated that a significant proportion of Cl could be volatilised at temperatures around 200°C. The volatilisation of Cl increased drastically with increasing temperature, from 200 to about 500°C. At higher temperatures with a fast heating rate, Cl could interact with the nascent char to be retained in the char. The volatilisation of Na followed a different trend from that of Cl and increased monotonically with increasing temperature. The loading of NaCl into the brown coal had negligible effects on the total volatile yields and on the volatilisation of Mg and Ca during pyrolysis. It is concluded that NaCl in the brown coal was mainly released as Na and Cl separately rather than as NaCl molecules. Reactions involving radicals play important roles in the volatilisation of Na and Cl.     

Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part II. Effects of chemical form and valence

Alkali and alkaline earth metallic (AAEM) species (Na, Mg and Ca) exist in Victorian brown coal mainly as carboxylates forming a part of the coal organic matter or as dissolved salt (NaCl) in the coal moisture. The experimental results in this paper show that the chemical and/or physical form of sodium in the brown coal is an important factor influencing the volatilisation of sodium during pyrolysis. Significant amounts of light species containing carboxyl or carboxylate groups such as formate, acetate and oxalate were found in the volatiles from the pyrolysis of the brown coal. It is believed that the release of AAEM carboxylates is an important mechanism for the volatilisation of AAEM species, particularly at low temperatures (<600°C). The carrier gas flow rate passing through the coal bed can greatly affect the volatilisation of AAEM species through this mechanism. Another mechanism for the volatilisation of AAEM species is the breakage of bonds between AAEM species and char matrix at high temperatures. Under our experimental conditions, the sodium in the form of NaCl in the coal substrate seems to volatilise more easily than the sodium in the form of carboxylate in the coal substrate. The monovalent species (Na) is volatilised much more easily that the divalent species (Mg and Ca) during pyrolysis.     

Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part V. Combined effects of Na concentration and char structure on char reactivity

A set of NaCl-loaded Loy Yang brown coal was pyrolysed in a thermogravimetric analyser between 600 and 900 °C. The char sample after pyrolysis was cooled down directly for in situ reactivity measurement with air. The results indicated that the volatilisation of Na during pyrolysis is an important reason for the existence of catalyst loading saturation level with Na as a catalyst in char because the char prepared at high temperature had a limited holding capacity for Na. Under the experimental conditions in this study, the char reactivity showed good linear correlation with the Na concentration in the reacting char. Peak pyrolysis temperature, affecting the release of Cl and distribution of Na in char, is an important factor governing the correlation between the char reactivity and Na concentration in char. The catalytic activity of Na is a result of the interaction between Na and char and thus is greatly dependent on the char/carbon structure. At high char conversion levels where the char structure is more inert and highly condensed, the catalytic activity of Na is reduced compared with its activity at low char conversion levels. The catalytic activity of Na depends on the structure of char.     

Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part IV. Catalytic effects of NaCl and ion-exchangeable Na in coal on char reactivity

The purpose of this study is to investigate the catalytic effects of Na as NaCl or as sodium carboxylates (-COONa) in Victorian brown coal on the char reactivity. A Na-exchanged coal and a set of NaCl-loaded coal samples prepared from a Loy Yang brown coal were pyrolysed in a fluidised-bed/fixed-bed reactor and in a thermogravimetric analyser (TGA). The reactivities of the chars were measured in air at 400 °C using the TGA. The experimental data indicate that the Na in coal as NaCl and as sodium carboxylates (-COONa) had very different catalytic effects on the char reactivity. It is the chemical form and dispersion of Na in char, not in coal, that govern the catalytic effects of Na. For the Na-form (Na-exchanged) coal, the char reactivity increased with increasing pyrolysis temperature from 500 to 700 °C and then decreased with pyrolysis temperature from 700 to 900 °C. The increase in reactivity with pyrolysis temperature (500-700 °C) is mainly due to the changes in the relative distribution of Na in the char matrix and on the pore surface. For the NaCl-loaded coals, when Cl was released during pyrolysis or gasification, the Na originally present in coal as NaCl showed good catalytic effects for the char gasification. Otherwise, Cl would combine with Na in the char to form NaCl during gasification, preventing Na from becoming an active catalyst. Controlling the pyrolysis conditions to favour the release of Cl can be a promising way to transform NaCl in coal into an active catalyst for char gasification.     

Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part VI. Further investigation into the effects of volatile-char interactions

The purpose of this study is to further investigate the effects of volatile-char interactions on the volatilisation and dispersion of alkali and alkaline metallic species and changes in char structure during pyrolysis. Ion-exchanged (H-form, Na-form and Ca-form) Loy Yang brown coal samples were pyrolysed in a novel two-stage fluidised-bed/fixed-bed reactor over a wide temperature range of 500-930 °C. Our results indicate that soot formation and destruction on char (pore) surface during volatile-char interactions could be catalysed by Na and, to a lesser extent, Ca on char. Volatile-char interactions caused additional volatilisation of Na at temperatures higher than 700 °C although there are no effects on the volatilisation of Ca. The formation and simultaneous (catalytic) destruction of soot on char surface are closely linked to the volatilisation of Na from the char. Volatile-char interactions have also caused changes in char structure and/or changes in Na/Ca dispersion, as is reflected by the reduction in char reactivity. These results indicate that the volatile-char interactions are not limited on the char surface. It appears that H radicals must have penetrated into the char structure during volatile-char interactions.     

Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part IX. Effects of volatile-char interactions on char-H2O and char-O2 reactivities

Volatile-char interactions are an important consideration in the design and operation of a gasifier. This study aims to investigate the effects of volatile-char interactions on the in situ char-steam reactivity at 800 °C and the ex-situ char-O₂ reactivity at 400 °C. A Victorian brown coal was gasified in 15% steam at 800 °C in a one-stage novel fluidised-bed/fixed-bed quartz reactor, in which the extent of volatile-char interactions could be controlled. The chars after varying extents of volatile-char interactions and/or varying extents of char conversion in steam were also collected for the measurement of their reactivity with air at 400 °C in a thermogravimetric analyser. Our results show that the char-steam gasification reactions were greatly inhibited by the volatile-char interactions. It is believed that the H radicals generated from the thermal cracking/reforming of volatiles slowed the char gasification in three ways: occupying the char reactive sites, causing the char structure to re-arrange/condense and enhancing the release of catalytic species inherently present in the brown coal. The importance of volatile-char interactions to char-steam reactivity was further confirmed by the char-air reactivity.     

Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part VIII. Catalysis and changes in char structure during gasification in steam

The purpose of this study is to investigate the major factors influencing the Na-catalysed and non-catalysed gasification reactivity of a Victorian brown coal in steam. An acid-washed (H-form) sample and a Na-exchanged (Na-form) sample prepared from the same Loy Yang brown coal were gasified in 15% steam in a novel two-stage fluidised-bed/fixed-bed reactor. All C-containing species in the gasification product gas were converted into CO₂ that was monitored with a mass spectrometer continuously to determine the in situ gasification reactivity. While the volatile-char interactions were responsible for the volatilisation of Na when the coal was continuously fed into the reactor, the physical entrainment by gas of agglomerated Na-containing crystalline species (likely to be Na₂CO₃ or Na₂O) from char surface was the main mechanism for the loss of Na during char gasification. The Raman spectroscopy of char showed the preferential release of smaller aromatic ring system to be more significant during the non-catalysed char gasification than the Na-catalysed gasification. The dispersion of Na in char appeared to deteriorate with the enrichment of large aromatic ring systems in char, greatly affecting the char gasification reactivity. The char gasification reactivity showed a maximum with increasing conversion with the maximum to shift towards lower conversion with increasing temperature. Increasing temperature does not always lead to increases in the in situ char gasification reactivity.     

Release of alkali and alkaline earth metallic species during rapid pyrolysis of a Victorian brown coal at elevated pressures

Possible reaction mechanisms responsible for the release of Na and Mg during pyrolysis at elevated pressures are described in this paper. In order to evaluate these mechanisms a Victorian brown coal, Loy Yang coal, was pyrolysed in a wire-mesh reactor at pressures up to 6.1 MPa at a heating rate of 1000 °C s⁻¹. Release of Na and Mg were quantified as functions of temperature and pressure. The results demonstrated that increasing pressure suppresses or promotes release of Na and Mg depending on the combination of pressure and temperature. The results obtained have been explained qualitatively by the proposed reaction mechanisms. At temperatures of 600 °C and lower, the release of Na and Mg from the pyrolysing coal/char particles, as light carboxylates, other organic salts and/or metals, was controlled by their diffusion through the pore system of the particles and, therefore, was suppressed by increasing pressure. At higher temperatures, the release of Na and Mg seems to be affected by the changes in intra-particle mass transfer mechanism due to increasing pressure as well as by chemical reactions responsible for the formation of volatile Na and Mg species.     

Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part VII. Raman spectroscopic study on the changes in char structure during the catalytic gasification in air

FT-IR/Raman spectroscopies have been used to identify the structural features of Victorian brown coal chars during the gasification in air at 400 °C. The deconvolution of the Raman spectra has allowed us to identify the main structural sites in char where preferential reaction with O₂ takes place. The presence of Na and Ca catalysts is shown to alter the reaction pathways between char and O₂. In the absence of a catalyst, the O-containing functional groups formed in char during gasification were closely associated with the aromatic structure and thus tended to loosen the aromatic structure. The non-catalysed gasification was slow and took place on some specific (especially sp³-rich or sp²–sp³ mixture) sites distributed throughout the char. In the presence of a catalyst (Na or Ca), the O-containing functional groups were not closely associated with the main aromatic structure throughout the char. The catalytic gasification reactions were localised on the sites associated with the catalysts. The preferential removal of smaller aromatic ring systems and the persistence of cross-linking structures in the presence of a catalyst mean that the large aromatic ring systems were increasingly concentrated with little flexibility, affecting the dispersion of catalyst.     

Effects of volatile-char interactions on the evolution of char structure during the gasification of Victorian brown coal in steam

The purpose of this study is to investigate the effects of volatile-char interactions on the evolution of char structure during the gasification of Victorian brown coal in steam. A novel one-stage fluidised-bed/fixed-bed quartz reactor was employed to carry out the experiments in the presence and absence of volatile-char interactions. The effects of thermal annealing on char structure were also investigated under similar conditions. The structural features of char were evaluated using FT-Raman spectroscopy. The results indicate that the char structural features were considerably affected by volatile-char interactions, which was shown from the Raman band area or the ratios between the band areas. H radicals from the thermal cracking/reforming of volatiles are believed to play a vital role in the changes in char structure due to the volatile-char interactions. H radicals could penetrate into char matrix and favour the condensation of aromatic rings, which was the main reason for the decrease in the ratio of small (less than 6 fused rings) to large aromatic rings during the volatile-char interactions. The volatile-char interactions also greatly affected the concentrations of O-containing groups in char and thus significantly altered the observed Raman intensity of the char.     

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.     

Effects of volatile-char interactions on the volatilisation of alkali and alkaline earth metallic species during the pyrolysis of biomass

An important feature of a fluidised-bed gasifier is the continuous contact between volatiles and char. The aim of this study is to experimentally investigate the effects of volatile-char interactions on the volatilisation of AAEM species during pyrolysis of two sugarcane industry wastes, bagasse and cane trash. A two-stage quartz fluidised-bed/fixed-bed reactor was used for this fundamental study. Our results indicate that the volatile-char interactions could lead to the additional volatilisation of alkali and alkaline earth metallic (AAEM) species, particularly if the volatile-char interactions have resulted in additional char weight losses. The monovalent Na and K behaved differently from the divalent Mg and Ca in biomass. Our results provide circumstantial but clear evidence that the AAEM species in biomass could behave distinctly differently from those in brown coal, largely due to the differences in the structure and composition between biomass and coal. The development of biomass gasification technologies must consider the special thermochemical characteristics of biomass. Furthermore, even the bagasse and cane trash grown in the same area behave drastically differently, at least partly due to the different microstructures of bagasse and cane trash.     

Inhibition of steam gasification of char by volatiles in a fluidized bed under continuous feeding of a brown coal

Steam gasification of a Victorian brown coal was performed in an atmospheric bubbling fluidized-bed reactor with continuous feeding of the coal. The gasification converted no more than 28, 51 and 71% of the nascent char (on a carbon basis) at 1120, 1173 and 1223 K, respectively. The char recovered from the fluidized bed was, nonetheless, gasified toward complete conversion when exposed to steam in another reactor, in which volatiles from the pyrolysis were absent while interaction between the char and products from the gasification was minimized. Atmosphere created in the fluidized bed thus prevented the char gasification from taking place beyond upper-limit conversion. In the absence of volatiles, nascent char underwent gasification catalyzed by inherent metallic species and non-catalytic gasification in parallel. The non-catalytic gasification was greatly decelerated by the presence of H₂ in the gas phase due to its dissociative chemisorption onto free carbon sites forming H-laden carbon. H₂ was, however, not a so strong inhibitor as to terminate the gasification. It was rather suggested that much more H-laden carbon was formed through dissociative chemisorption of volatiles and/or chemisorption of hydrogen radical from thermal cracking of volatiles in the gas-phase, which resulted in prevention of the non-catalytic gasification. It seemed that the char was converted in the fluidized-bed mainly by the catalytic gasification, while the conversion was limited due to deactivation of metallic species within the char matrix and their release from the char.     

Changes in char reactivity due to char-oxygen and char-steam reactions using Victorian brown coal in a fixed-bed reactor

This study was to examine the influence of reactions of char–O2 and char–steam on the char reactivity evolution. A newly-designed fixed-bed reactor was used to conduct gasification experiments using Victorian brown coal at 800 °C. The chars prepared from the gasification experiments were then collected and subjected to reactivity characterisation (ex-situ reactivity) using TGA (thermogravimetric analyser) in air. The results indicate that the char reactivity from TGA was generally high when the char experienced intensive gasification reactions in 0.3%O2 in the fixed-bed reactor. The addition of steam into the gasification not only enhanced the char conversion sig-nificantly but also reduced the char reactivity dramatical y. The curve shapes of the char reactivity with involve-ment of steam were very different from that with O2 gasification, implying the importance of gasifying agents to char properties.     

Combined effects of pressure and ion-exchangeable metallic species on pyrolysis of Victorian lignite

A set of ion-exchanged samples prepared from Loy Yang lignite was pyrolyzed in a wire-mesh reactor at elevated pressures from 1 to 36 bar. The tar yields from the pyrolysis of H-form (acid-washed) sample at a fast heating rate of 1000 °C s⁻¹ were drastically reduced by increasing pressure to 6 bar and then remained unchanged with further increase in pressure to 36 bar. This behavior of the tar yield was in sharp contrast to that from the raw lignite which showed a minimum with increasing pressure. The sensitivities of the tar yields to changes in the heating rate were also suppressed by increasing pressure. The tar yields from Ca-form and Na-form samples (prepared by ion-exchanging Ca and Na on the H-form sample, respectively) were not very sensitive to changes in the heating rate and pressure up to 11 bar. At 20 bar, the tar yields from the Na-from sample nearly doubled whereas from the Ca-form sample nearly halved compared to those respective values at 1 bar. Although increasing pressure is thought to cause changes in the intra-particle mass transfer processes of volatile precursors, the rate of formation of volatile precursors tends to dictate the kind of mass transfer process responsible for the release of volatiles. Therefore, depending on the pyrolysis condition, bulk diffusion or forced flow would dominate the mass transfer processes for the release of volatiles. The introduction of cations is thought to result in irreversible changes in the lignite structure and not only control the process of formation but also the amount of volatile precursors and in turn alter the effects of pressure. Valence and catalytic activity of cations seem to play important roles in determining pyrolysis products distribution at elevated pressures.     

Effect of alkali and alkaline earth metals on nitrogen release during temperature programmed pyrolysis of coal

Formation of HCN, NH₃, and N₂ during fixed-bed pyrolysis at 10K min⁻¹ has been studied using coal samples after partial demineralization followed by addition of metal hydroxides from aqueous systems. Without additives, NH₃ is the predominant product at ≤ 700°C, showing the two peaks in the formation rate profile, whereas N₂ is the only product at ≥ 800°C. The presence of NaOH, KOH and Ca(OH)₂ promotes considerable NH₃ formation between 450 and 600°C, but in contrast suppresses HCN formation in this region. The Ca shows the largest effect on both the promotion and suppression. It is likely that the NH₃ increased by Ca addition arises partly from HCN, but mainly from secondary reactions of tar-N. These hydroxides affect N₂ formation in quite different manners: the Na decreases the rate between 700 and 950°C, and the K changes it less significantly than the Na, but the Ca remarkably increases the rate in a low temperature region of 550–700°C. These different features are discussed in terms of solid-phase reactions of alkali metal carbonates with char-N and secondary decomposition reactions of tar-N on CaO particles. As a result, total conversion of coal-N to HCN, NH₃ and N₂ up to 1000°C increases in the sequence of Na < none < K < Ca.     

Effect of cellulose, lignin, alkali and alkaline earth metallic species on biomass pyrolysis and gasification

Fundamental pyrolysis/gasification characteristics of natural biomass and acid-washed biomass without alkali and alkaline earth metals (AAEM) were investigated by a thermogravimetric analyzer (TGA) and a fixed-bed reactor. In these experiments, six types of biomass were used and the contents of cellulose, lignin and AAEM species in the biomass were measured. It was observed that the characteristic of biomass pyrolysis and gasification was dependent on its components and AAEM species on the basis of TGA experiments. During biomass pyrolysis, the tar and gas yields increased with the growth of cellulose content, but the char yield decreased. There were two reactions indicating two major decomposition mechanisms. The first stage of decomposition showed rapid mass decrease due to the volatilization of cellulose, while the second stage became slow attributed to the lignin decomposition. The higher the cellulose content, the faster the pyrolysis rate. In contrast, the pyrolysis rate of biomass with higher lignin content became slower. In addition, the rises of cellulose content elevated the peak temperature of gasification and prolonged the gasification time. Meanwhile, the effect of AAEM species on gasification behavior was studied by comparing unwashed and acid-washed biomass. AAEM species increased the peak gasification value, whereas decreased initial gasification temperature. It revealed that the activity of biomass gasification was attributed to the interaction between AAEM-cellulose/lignin.     

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.     

Formation of NOx precursors during the pyrolysis of coal and biomass. Part VIII. Effects of pressure on the formation of NH3 and HCN during the pyrolysis and gasification of Victorian brown coal in steam

Effects of pressure on the formation of HCN and NH₃ during the pyrolysis and gasification of Loy Yang brown coal in steam were investigated using a pressurised drop-tube/fixed-bed reactor. The NH₃ yield increased with increasing pressure during both pyrolysis and gasification. Increasing pressure selectively favours the formation of NH₃ at the expenses of other N-containing species. The changes in the yield of NH₃ with increasing pressure were mainly observed in the feeding periods both during pyrolysis and gasification and were closely related to the formation and subsequent cracking of soot both as a result of intensified thermal cracking of volatile precursors inside the particles and as a result of volatile-char interactions after the release of volatiles. While the corresponding HCN yield during pyrolysis showed little sensitivity to changes in pressure, the HCN yield during gasification in steam showed some increases with increasing pressure. Our data indicate that the direct hydrogenation of char-N by H radicals, favoured by the presence of steam, is the main route of NH₃ formation during pyrolysis and gasification. The direct conversion, either through hydrogenation or hydrolysis, of HCN into NH₃ on char surface during the pyrolysis and gasification of brown coal is not an important route of NH₃ formation.     

高碱低阶煤热解及热解半焦研究进展

我国碱金属、碱土金属(AAEM)含量高的低阶煤储量丰富。高碱含量造成锅炉受热面结渣沾污及气化炉结块腐蚀等难题,低阶煤内水高、氧含量高、挥发分高、发热量低以及易氧化自燃等特性为其储、运、用带来极大的难题。热解可生产优质燃料和高附加值化工原料,也是燃烧、气化、直接液化等过程的起始阶段和/或伴随反应,煤在热解阶段发生的反应、经历的变化,对煤转化利用的效率和清洁程度起重要、甚至决定性作用。笔者对煤热解与热解半焦研究及进展进行综述性评价,着重探讨煤中AAEM对热解过程及半焦的影响。结果表明,热解研究装置模拟的工况与现代煤化工过程中煤热解所处环境相差甚远,半焦样的代表性不强使热解研究成果的指导意义不大;对煤中不同赋存形态AAEM的分离方法有待完善,还需筛选、尝试新的萃取试剂;基本掌握了煤热解过程中AAEM的变迁行为,但尚缺乏控制煤中AAEM危害的有效方法。高碱低阶煤的安全高效洁净转化利用技术仍待突破。