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.     

Effects of volatile-char interactions on the reactivity of chars from NaCl-loaded Loy Yang brown coal

In a fluidised-bed gasifier, char particles are in constant contact with the volatiles and the products from the gasification and thermal cracking of volatiles. The highly reactive nature of volatiles as well as the vulnerable structure of char from brown coal means that there are strong interactions between volatiles and char. The purpose of this study is to investigate the effects of volatile-char interactions on the reactivities of chars from a Victorian brown coal. NaCl-loaded and acid-washed Loy Yang brown coal samples were pyrolysed in a novel fluidised-bed/fixed-bed reactor at 900 °C that has been specially designed to investigate the volatile-char interactions. Char reactivity in air was measured in a thermogravimetric analyser (TGA) under conditions minimising mass transfer limitations. The oxidation of char with air in the TGA showed apparent kinetic compensation effects between the apparent activation energies and pre-exponential factors. The presence of the apparent kinetic compensation effect is a reflection of the heterogeneous structure of char having sites of a wide range of energy levels. Our results have clearly shown that volatile-char interactions can lead to drastic decreases in char reactivity due to the volatilisation of Na and the changes in char structure. The reactivities of chars from the pyrolysis of the catalyst-free H-form Loy Yang brown coal provided unequivocal evidence for the changes in char structure after volatile-char interactions. For chars from the NaCl-loaded Loy Yang brown coal, it appears that the condensation of ring structures in char as a result of volatile-char interactions could have also led to changes in the dispersion of Na catalyst.     

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 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.     

Co-gasification behavior of meat and bone meal char and coal char

The co-gasification behavior of meat and bone meal (MBM) char and two types of coal (Jincheng anthracite (JC) and Huolinhe lignite (HLH)) char was investigated using a thermogravimetric analyzer (TGA). The effects of coal type, mineral matter in MBM, gasification temperatures and contacting conditions between MBM char and coal char on the gasification behavior were studied. The results show that the gasification behavior of MBM char and HLH char can be well described by ash diffusion controlled shrinking core model, while that of JC char can be described by chemical reaction controlled shrinking core model. The co-gasification rate of MBM/JC chars at 950 °C is approximately 1.5 times faster than that calculated from independent behavior. The mineral matter in MBM may play as a catalyst during co-gasification. However, the analogous effect observed in the blends of HLH/MBM chars is smaller, suggesting that the coal types play a great role. Furthermore, as the gasification temperature increased from 850 to 1000 °C, the maximum synergistic effect is observed at 900 °C. The lower temperature is not conducive to transferring the mineral matters of MBM to the coal char, while the higher temperature makes Na and Ca react with minerals of coal, leading to a loss of catalytic activity.     

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.     

Catalytic gasification of char from co-pyrolysis of coal and biomass

The catalytic gasification of char from co-pyrolysis of coal and wheat straw was studied. Alkali metal salts, especially potassium salts, are considered as effective catalysts for carbon gasification by steam and CO₂, while too expensive for industry application. The herbaceous type of biomass, which has a high content of potassium, may be used as an inexpensive source of catalyst by co-processing with coal. The reactivity of chars from co-pyrolysis of coal and straw was experimentally examined. The chars were prepared in a spout-entrained reactor with different ratios of coal to straw. The gasification characteristics of chars were measured by thermogravimetric analysis (TGA). The co-pyrolysis chars revealed higher gasification reactivity than that of char from coal, especially at high level of carbon conversion. The influence of the alkali in the char and the pyrolysis temperature on the reactivity of co-pyrolysis char was investigated. The experimental results show that the co-pyrolysis char prepared at 750 °C have the highest alkali concentration and reactivity.     

Fate of the chlorine and fluorine in a sub-bituminous coal during pyrolysis and gasification

The fate of the chlorine and fluorine present in a sub-bituminous coal from Indonesia during pyrolysis and gasification has been studied with fixed and entrained bed reactors. The rate profile for HCl evolved in the temperature programmed pyrolysis exhibits the main and shoulder peaks at 480 and 600 °C, respectively. Model experiments and subsequent Cl 2p XPS measurements show that HCl reacts with metal impurities and carbon active sites at 500 °C to be retained as inorganic and organic chlorine forms, from which HCl evolves again at elevated temperatures. It is suggested that the HCl observed in the coal pyrolysis may originate from the above-mentioned chlorine functionalities formed by secondary reactions involving the nascent char. In the CO₂ gasification of the 900 °C char at 1000 °C and 2.5 MPa, any measurable amounts of HCl and HF could not be detected even at a high conversion of 75 wt% (daf), suggesting the accumulation of these halogens in the residual char. When the coal is injected into an O₂-blown, entrained bed gasifier at 1200-1400 °C under 2.6 MPa, the partial oxidation proceeds to a larger extent at a higher O₂/coal ratio, whereas the chlorine and fluorine are enriched in the remaining char, and the extent of the enrichment at the latter stage of gasification is larger with the fluorine. The XPS measurements of the chars reveal the presence of the broad F 1 s peak, which can cover a wide range of binding energies attributable to inorganic and organic fluorine. The halogen enrichment during gasification is discussed in terms of secondary reactions of HCl and HF with char.     

Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part III. The importance of the interactions between volatiles and char at high temperature

A novel two-stage fluidised-bed/fixed-bed reactor was designed to investigate the effects of volatile-char interactions on the volatilisation of alkali and alkaline earth metallic (AAEM) species during the pyrolysis of Victorian brown coal at 900 °C. With the two-stage reactor configuration, the AAEM-free volatiles generated from the pyrolysis of the H-form coal in the fluidised bed came into direct contact with the char from NaCl-loaded or Na-form coals in the fixed bed. The results indicated that the interactions between the volatiles, especially free radicals in the volatiles, and the char particles enhanced the volatilisation of Na from the char drastically. However, such radical-char interactions resulted in little volatilisation of Mg and Ca, indicating the importance of valence of the AAEM species. The degree of the volatile-char interactions was also related to the ageing of the char and the chemical form of AAEM species in the coal substrate. The volatiles interacted more strongly with the nascent char than the aged char, indicating that the AAEM species existed in the aged char in more stable forms than in the nascent char.     

Decomposition and gasification of pyrolysis volatiles from pine wood through a bed of hot char

Pine wood was pyrolyzed in a fixed bed reactor at a heating rate of 10 °C and a final temperature of 700 °C, and the resultant volatiles were allowed to be secondarily cracked through a tubular reactor in a temperature range of 500-700 °C with and without packing a bed of char. The thermal effect and the catalytic effect of char on the cracking of tar were investigated. An attempt was made to deconvolute the intermingled contributions of the char-catalyzed tar cracking and the char gasification to the yields of gaseous and liquid products. It was found that the wood char (charcoal) was catalytically active for the tar cracking at 500-600 °C, while at 650-700 °C, the thermal effect became a dominant mode of the tar cracking. Above 600 °C, the autogenerated steam gasified the charcoal, resulting in a marked increase in the yield of gaseous product and a significant change in the gas composition. An anthracite char (A-char), a bituminous coal char (B-char), a lignite char (L-char) and graphite also behaved with catalytic activities towards the tar cracking at lower temperature, but only L-char showed reactivity for gasification at higher temperature.     

Steam gasification of coal char catalyzed by K2CO3 for enhanced production of hydrogen without formation of methane

Steam gasification of coal char catalyzed by potassium carbonate was investigated on a laboratory fixed-bed reactor to examine the catalytic effects not only on the reaction rate but also on the reaction selectivity, and non-catalytic gasification of coal char was performed by way of contrast. It was observed that the catalytic gasification of coal char with steam occurred significantly in a temperature range of 700-750 °C, producing a hydrogen-rich gas with slight formation of carbon monoxide and virtually no formation of methane. An oxygen transfer and intermediate hybrid mechanism of the catalytic char gasification with steam is proposed for understanding of the experimental data regarding both the kinetic behaviors and reaction selectivity. The study has highlighted the advantages of the catalytic gasification of coal char over the conventional coal gasification with respect to the reaction selectivity. The catalytic steam gasification of coal char makes it possible to eliminate or simplify the methane reforming and water-gas shift processes in the traditional gas-to-hydrogen purification system.     

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.     

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.     

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.     

Modeling of catalytic gasification kinetics of coal char and carbon

Calcium- and potassium-catalyzed gasification reactions of coal char and carbon by CO₂ are conducted, and the common theoretical kinetic models for gas-carbon (or char) reaction are reviewed. The obtained experimental reactivities as a function of conversion are compared with those calculated based on the random pore model (RPM), and great deviations are found at low or high conversion levels as predicted by theory. Namely, calcium-catalyzed gasification shows enhanced reactivity at low conversion levels of <0.4, whereas potassium-catalyzed gasification indicated a peculiarity that the reactivity increases with conversion. CO₂ chemisorption analysis received satisfactory successes in both interpreting catalytic effects and correlating the gasification reactivity with irreversible CO₂ chemical uptakes (CCU_ir) of char and carbon at 300 °C. In details, calcium and potassium additions led to significant increases in CCU_ir and correspondent high reactivities of the char and carbon. Furthermore, CCU_ir of char and carbon decreased with conversion for calcium-catalyzed reaction but increased for potassium-catalyzed one, corresponded to the tendency of their reactivity. The RPM is extended and applied to these catalytic gasification systems. It is found that the extended RPM predicts the experimental reactivity satisfactorily. The most important finding of this paper is that the empirical constants in the extended RPM correlate well with catalyst loadings on coal.     

Gasification rate analysis of coal char with a pressurized drop tube furnace

Two coal chars were gasified with carbon dioxide or steam using a Pressurized Drop Tube Furnace (PDTF) at high temperature and pressurized conditions to simulate the inside of an air-blown two-stage entrained flow coal gasifier. Chars were produced by rapid pyrolysis of pulverized coals using a DTF in a nitrogen gas flow at 1400°C. Gasification temperatures were from 1100 to 1500°C and pressures were from 0.2 to 2 MPa. As a result, the surface area of the gasified char increased rapidly with the progress of gasification up to about six times the size of initial surface area and peaked at about 40% of char gasification. These changes of surface area and reaction rate could be described with a random pore model and a gasification reaction rate equation was derived. Reaction order was 0.73 for gasification of the coal char with carbon dioxide and 0.86 for that with steam. Activation energy was 163 kJ/mol for gasification with carbon dioxide and 214 kJ/mol for that with steam. At high temperature as the reaction rate with carbon dioxide is about 0.03 s⁻¹, the reaction rate of the coal char was controlled by pore diffusion, while that of another coal char was controlled by surface reaction where reaction order was 0.49 and activation energy was 261 kJ/mol.     

Low-temperature gasification of a woody biomass under a nickel-loaded brown coal char

Catalytic gasification of a woody biomass, Japanese cypress, was investigated under a prepared nickel-loaded brown coal (LY-Ni) char in a two-stage fixed-bed reactor. The nickel-loaded brown coal was prepared by ion-exchange method with a nickel loading rate of 8.3 wt.%. Nickel species dispersed well in the brown coal, and the LY-Ni char via devolatilization at 600 °C showed a great porous property with a specific surface area of 382 m² g^− 1.The LY-Ni char was confirmed to be quite active for the Japanese cypress volatiles gasification at a relatively low-temperature range from 450 to 650 °C. For example, at 550 °C, 16.6 times hydrogen gas and 6.3 times total gases were yielded from the catalytic steam gasification of Japanese cypress volatiles under the LY-Ni char, compared with the case of non-catalyst. The biomass tar decomposition showed a dependence on catalyst temperatures. When the catalyst temperature was higher than 500 °C, Japanese cypress tar converted much efficiently, high gas yields and high carbon balances were obtained.     

Some recent advances in the understanding of the pyrolysis and gasification behaviour of Victorian brown coal

The brown coal in the Latrobe Valley, Victoria, Australia, has many unique structural features and properties. The brown coal has a very low ash yield and contains highly dispersed alkali and alkaline earth metallic (AAEM) species, either as carboxylates forming part of its organic matter or as NaCl dissolved in its moisture. Owing to its unique structural features and properties, the brown coal behaves very differently from many other solid fuels such as biomass, bituminous coals and anthracites. For example, the highly reactive nature of its volatiles, the vulnerable nature of its nascent char and the presence of finely distributed AAEM species mean that the volatile–char interactions, a common phenomenon in all gasifiers, especially in the fluidised-bed gasifiers, would influence almost every aspect of its pyrolysis and gasification behaviour. Some recent progress in the understanding of the pyrolysis and gasification behaviour of Victorian brown coal will be reviewed in this paper. After a brief account of the effects of AAEM species on the pyrolysis yields, the factors influencing the volatilisation of AAEM species will be summarised. This will be followed by the discussion of the factors influencing the reactivity of brown coal char and the catalytic reforming/cleaning of volatiles and gasification products by char-supported catalysts. The effects of dewatering/drying on the pyrolysis behaviour of Victorian brown coal and the conversion of pollutant-forming elements will be mentioned briefly. The progress in the fundamental understanding of the pyrolysis and gasification behaviour of Victorian brown coal has laid solid foundation for the further development of advanced gasification technologies for the clean and efficient utilisation of this cheap but important resource.     

Effect of pyrolysis pressure and heating rate on radiata pine char structure and apparent gasification reactivity

The knowledge of biomass char gasification kinetics has considerable importance in the design of advanced biomass gasifiers, some of which operate at high pressure. The char gasification kinetics themselves are influenced by char structure. In this study, the effects of pyrolysis pressure and heating rate on the char structure were investigated using scanning electron microscopy (SEM) analysis, digital cinematography, and surface area analysis. Char samples were prepared at pressures between 1 and 20 bar, temperatures ranging from 800 to 1000 °C, and heating rates between 20 and 500 °C/s. Our results indicate that pyrolysis conditions have a notable impact on the biomass char morphology. Pyrolysis pressure, in particular, was found to influence the size and the shape of char particles while high heating rates led to plastic deformation of particles (i.e. melting) resulting in smooth surfaces and large cavities. The global gasification reactivities of char samples were also determined using thermogravimetric analysis (TGA) technique. Char reactivities were found to increase with increasing pyrolysis heating rates and decreasing pyrolysis pressure.     

The effect of coal particle size on pyrolysis and steam gasification

For future power generation from coal, one preferred option in the UK is the air-blown gasification cycle (ABGC). In this system coal particles sized up to 3 mm, perhaps up to 6 mm in a commercial plant, are pyrolysed and then gasified in air/steam in a spouted bed reactor. As this range of coal particle sizes is large it is of interest to investigate the importance of particle size for those two processes. In particular the relation between the coal and the char particle size distribution was investigated to assess the error involved in assuming the coal size distribution at the on-set of gasification. Different coal size fractions underwent different changes on pyrolysis. Smaller coal particles were more likely to produce char particles larger than themselves, larger coal particles had a greater tendency to fragment. However, for the sizes investigated in this study ranging from 0.5 to 2.8 mm, the pyrolysis and gasification behaviour was found not to vary significantly with particle size. The coal size fractions showed similar char yields, irrespective of the different char size distributions resulting from pyrolysis. Testing the reactivity of the chars in air and CO₂ did not reveal significant differences between size fractions of the char, nor did partial gasification in steam in the spouted bed reactor. From the work undertaken, it can be concluded that pyrolysis and gasification within the range of particle sizes investigated are relatively insensitive to particle size.