And Alkaline Earth Metallic Species Research Articles

Re-examination of Thermogravimetric Kinetic Analysis of Lignite Char Gasification

Kinetic analysis of CO2 gasification by thermogravimetry was examined employing chars from a Victorian lignite. The gasification of char from the rigorously acid-washed lignite was extremely slow. More than 600 min was required for the complete gasification as a result of a near absence of inherent alkali and alkaline earth metallic (AAEM) species. It was nonetheless necessary to strictly choose the following conditions for eliminating the mass transfer effect: an initial mass of char of <1.3 mg, a particle size of char of <125 μm, and a total gas flow rate of ≥1000 mL standard temperature and pressure min–1. Such requirement arose from the inhibition of non-catalytic gasification by CO within the char particle. On the other hand, the char from the original lignite was gasified much faster than the non-catalytic gasification and completed within 50 min by catalysis of the inherent AAEM species. The measured rate of catalytic gasification was, unexpectedly, independent of the initial char mass and particle size and gas flow rate. This was reasonably explained by no or little importance of CO inhibition in the catalytic gasification. This study has also drawn another important conclusion that the non-catalytic gasification obeyed first-order kinetics with respect to the char mass. The rate constant was steady over the entire range of char conversion, while both the specific surface area and carbon-type distribution of the char varied along with the conversion.

Solar-Thermal Pyrolysis of Mallee Wood at High Temperatures

This study reports solar-thermal pyrolysis of mallee wood powders driven by direct concentrated solar radiations at different temperatures (1540, 1740, and 1930 °C), heating rates (320, 800, and 3200 °C/min), and holding times (0 and 5 min). Under such severe pyrolysis conditions, solar-thermal pyrolysis of mallee wood produces predominantly volatiles (≥90 wt %), with very low char yields (≤10 wt %), as reported on a dry basis. Majorities of inherent alkali and alkaline earth metallic (AAEM) species are also released into the gaseous phase. The severe pyrolysis conditions also lead to the low reactivity of char products as a result of not only the char carbon structure becoming ordered and graphitized (as evidenced by the Raman data) but also significant losses of catalytic AAEM species in the char. For example, during the solar-thermal pyrolysis of mallee wood at 1930 °C, 800 °C min–1, and 5 min holding time, the char yield is only ∼5 wt %, the retentions of Na and Mg are ∼1%, the retentions of K and Ca ...

Adsorption Characteristics of Bio-oil on Biochar in Bioslurry Fuels

This study reports the adsorption characteristics of bio-oil on biochar in biochar/bio-oil slurry (i.e., bioslurry) systems at three biochar loading levels (2.4, 4.8, and 16.7%, respectively). At low biochar loading levels (2.4 or 4.8%), heavy organic compounds (particularly those containing fused aromatic systems) in bio-oil was selectively adsorbed onto the biochar. Such selective adsorption also resulted in reduced concentrations of alkali and alkaline earth metallic (AAEM) species in the adsorbed bio-oil phase compared to that in the bio-oil blank. A comparison between the experimental and calculated distributions of AAEM species in bio-oil indicates that at least part of the AAEM species adsorbed on biochar is associated with heavy organic compounds. Consequently, such selective adsorption results in changes in the unadsorbed bio-oil phase that has decreased viscosity and aromaticity and increased polarity. It is interesting to note that the selective adsorption was weakened with an increasing biocha...

Effects of char chemical structure and AAEM retention in char during the gasification at 900 °C on the changes in low-temperature char-O2 reactivity for Collie sub-bituminous coal

This study is focused on the effects of char structure and alkali and alkaline earth metallic (AAEM) species on the changes in char-O2 reactivity for a Western Australian Collie sub-bituminous coal. The char samples used in this study were acquired from the partial gasification of the sub-bituminous coal in a fluidised-bed/fixed-bed reactor in pure CO2, 15% H2O balanced with Ar and a mixture of 15% H2O and CO2 (balancing gas) at 900°C. The structural features of the chars were characterised by FT-Raman spectroscopy. The concentrations of AAEM species in the chars were determined by inductively coupled plasma – optical emission spectroscopy. The O-containing structures in char that can cause resonance effects to give enhanced Raman intensity tend to improve the retention of AAEM species in char during the gasification at 900°C. However, these O-containing structures do not necessarily ensue a high char-O2 reactivity at low temperature. While AAEM species in char would catalyse the char-O2 reactions, they are not sufficient to determine the char-O2 reactivity. The relative ratio of small to large aromatic ring systems in char, as reflected by the Raman spectroscopy, greatly influences the char-O2 reactivity. The sub-bituminous coal in this study does not behave in the same way as the brown coal in our previous studies in terms of the factors influencing the char-O2 reactivity.

Effects of gasification temperature and atmosphere on char structural evolution and AAEM retention during the gasification of Loy Yang brown coal

This study aims to investigate the influence of gasification temperature on the evolution of char structure and the retention of alkali and alkaline earth metallic (AAEM) species for a Victorian brown coal. The experiments were carried out at 800, 850 and 900°C in pure CO2, 15% H2O balanced with Ar and 15% H2O balanced with CO2 in a one-stage fluidised-bed/fixed-bed reactor. FT-Raman spectroscopy was used to characterise the structural features of the chars obtained at varying conversion levels during gasification. The concentrations of AAEM species in the chars left after gasification were quantified with inductively coupled plasma – optical emission spectroscopy. The specific char-O2 reactivity of the gasified char was measured in a thermogravimetric analyser. The results show that the CO2 and H2O partly compete for the active sites on the char. The increasing total Raman area caused by the formation of O-containing structures in char does not appear to be the only factor determining the char gasification rate in H2O-containing atmospheres. With increasing temperature, CO2 plays an increasingly more important role in the char evolution and subsequently influences the AAEM retention in char from the gas mixture. The gasification temperature does not change the individual reaction pathway for char-CO2 or char-H2O reaction within temperature range investigated.

Influence of AAEM species in coal and biomass on steam co-gasification of chars of blended coal and biomass

This study investigated effects of blending ratio and alkali and alkaline earth metallic (AAEM) species in the feedstock on char reactivity and producer gas composition in steam co-gasification of chars of blended coal and biomass. Experiments were conducted on a bench-scale fixed bed gasifier in which lignite was used as coal and radiata pine was used as biomass. The blending ratios of lignite to pine (L/P) were, respectively, 0:100 (pure pine), 20:80, 50:50, 80:20 and 100:0 (pure lignite). Lignite and radiata pine were first separately ground to fine particles which were then blended based on pre-set ratios. After this, the blends were pelletized and charred at 900 °C. In order to investigate the effect of AAEM in the coal, experiments were also performed using blended pine and acid-washed lignite from which most of AAEM species were effectively removed. The co-gasification operation temperature was 950 °C. From the experimental results, it was found that the ratios of H2/CO, H2/CO2 and CO/CO2 in the producer gas were nonlinearly related to L/P ratio in the lignite blended chars; however, these gaseous ratios were linearly correlated to the L/P ratio in co-gasification of acid-washed lignite blended chars. In addition, by removing the AAEM species in lignite, yields of H2 and CO2 were reduced while CO yield was increased. The char reactivity of acid-washed lignite and pine blends was decreased and this decrease became more significant with increase in coal to biomass blending ratio.

HCl Capture by Rice Straw Char and Its Influence on the Transformation of Alkali and Alkaline Earth Metallic Species during Pyrolysis

HCl capture experiments with rice straw char were conducted in a fixed-bed reactor at 500–900 °C. The capture capacities of rice straw char at different temperatures were determined. A significant amount of HCl was found to be captured by rice straw char at all temperatures, and the amount of captured HCl decreased gradually with increased temperature. Analysis of char samples obtained at different thermal treatment times and under different atmospheres showed the effect of HCl capture on the transformation of alkali and alkaline earth metallic (AAEM) species in char. Both the release and occurrence of AAEM species in char significantly changed with HCl capture, based on which the reaction between HCl and AAEM species was analyzed. Desorption experiments showed the importance of adsorption by char at each temperature. The reaction of HCl with AAEM species in char and the adsorption of HCl by the char matrix were proposed to be the main mechanisms of HCl capture by char, and the former was the dominant pat...

Effect of pyrolysis temperature on char structure and chemical speciation of alkali and alkaline earth metallic species in biochar

To investigate the effect of pyrolysis temperature on char structure and chemical speciation of alkali and alkaline earth metallic (AAEM) species in biochar, experiments were carried out in an entrained flow reactor with nitrogen to reduce volatile–char interactions. Pyrolysis was conducted at temperatures ranging from 500°C to 900°C. Raman, Fourier transform infrared and X-ray photoelectron spectroscopies were used to identify the char structure and functional groups. The assessment of the chemical speciation of AAEM species in the biochar was carried out by chemical fractionation analysis and inductively coupled plasma mass spectrometry. Pyrolysis between 500 and 900°C results in ring condensation, which produces large aromatic ring systems containing six or more fused benzene rings. In addition, C–O and C–C linkages form through thermal polycondensation and cyclization to produce stable aromatic rings, which shows CC stretching bands in the FTIR. Conjugated aromatic carbonyl/carboxyl CO groups disappear at temperatures higher than 700°C, and the relative concentration of carbonyl oxygen from quinines increases gradually with increasing temperature. More than 50% of K and Na in the sawdust and chars prepared at 500–900°C exists as ions, while most of the Ca (65%) and Mg (60%) are bound in organic compounds. The release of K and Na occurs mainly in the form of inorganic salts and hydrated ions, while volatilization is the main release mechanism for Ca and Mg in the organic compounds.

Kinetics and Mechanism of Steam Gasification of Char from Hydrothermally Treated Woody Biomass

Hydrothermal treatment (HTT) is a promising way of upgrading biomass as a solid fuel and precursor of carbon materials by eliminating or transforming carbohydrates and also leaching alkali and alkaline earth metallic (AAEM) species. This study investigated steam gasification of a woody biomass that had been upgraded by HTT at 250 °C. HTT removed 87–97% of AAEM species from the biomass. The char from the pyrolysis of the treated biomass underwent gasification, obeying first-order kinetics with respect to the mass of char over the entire range of conversion. This kinetics arose from non-catalytic gasification. AAEM species remaining in the char had no catalytic activity. The specific surface area of char increased monotonically with its conversion from 500 to well above 2000 m2/g. The non-catalytic nature of the gasification was responsible for such a significant surface area development. The surface area was, however, not a factor influencing the rate of gasification. The presence of the inherent AAEM cata...

Leaching of Alkali and Alkaline Earth Metallic Species from Rice Husk with Bio-oil from Its Pyrolysis

Removal of alkali and alkaline earth metallic (AAEM) species, in particular, that of potassium, is an effective way to upgrade rice husk, because its combustion and gasification often suffer from the formation of potassium silicate with a low softening or fusion temperature. This work has been investigating the leaching of AAEM species with a bio-oil (BO) from the pyrolysis of the parent rice husk. The leaching with BO, which had a pH of 2.59, gave removal rates of K and Na equivalent to or higher than those with an aqueous solution of hydrogen chloride (HCl aqueous) or acetic acid (AcOH aqueous) at pH of 2. The leaching abilities of BO in terms of the removal rates of Mg and Ca were equivalent to HCl aqueous at pH 0 and AcOH aqueous at pH < 2, respectively. Such performances of BO arose from the presence of not only AcOH with a concentration of 6 wt % but also phenolic compounds (phenol, alkylphenols, and methoxyphenols). The phenols permeated into the organic matrix of the rice husk, forming hydrophobic...

Effect of minerals on surface morphologies and competitive reactions during char gasification in mixtures of O2 and CO2

This study aims to investigate the effect of minerals on surface morphologies and competitive reactions during char gasification in mixtures of O2 and CO2. Char gasification experiments were performed in O2/Ar, Ar/CO2, and O2/CO2 environments to study char structure evolution and competitive reactions between char-O2 and -CO2 gasification. Chars gasified under Ar/CO2 or O2/CO2 were observed to have large pores on their surface, while chars gasified under O2/Ar did not have them. The reaction between catalyst/carbon interfaces was key in explaining the difference in surface morphologies of chars gasified under O2 and CO2. High specific surface area and large pores produced by gasification in CO2 increased char gasification rates in the internal diffusion-controlled regime. Then, two coals of different ash composition (including high alkali and alkaline earth metallic (AAEM) and low AAEM species) were used to study competitive reactions between O2 and CO2 on the active sites. When chars prepared from coals including a low AAEM species were used, the char gasification rate in O2/CO2 environments was almost 60% of the sum of those in O2/Ar and Ar/CO2 environments, regardless of specific surface area. This indicated that char-O2 and -CO2 reactions were competitive. However, when chars prepared from coals including a high AAEM species were used, higher Ca and Na concentrations were detected on the char surface, and the char gasification rate in O2/CO2 environments was almost the same as the sum of gasification rates in O2/Ar and Ar/CO2 environments, implying that Ca and Na played an important role as active sites. Unlike air environments, all surrounding gases were reactive in the O2/CO2 environments. Therefore, a sufficient active site, mainly consisting of catalyst in this study, was important not only to enhance char gasification reactions but also to prevent competitive reactions between char-O2 and CO2 gasification.

A Method for the Quantification of Alkali and Alkaline Earth Metallic Species in Bioslurry Fuels

This study reports a method for the quantification of alkali and alkaline earth metallic (AAEM) species in bioslurry fuels. The so-called evaporation–ashing–digestion–ion chromatography (IC) method consists of four steps, including evaporation that converts bioslurry into solid-like residue, ashing that converts the residue into ash, acid digestion that dissolves the ash into a solution, and IC analysis that quantifies the AAEM species in the solution. The novelty of the method is the combination of the evaporation step with the existing ashing–digestion–IC method that is developed for solid fuels but not suitable for bioslurry fuels. The evaporation step consists of multi-steps of slow heating and holding at various segment temperatures corresponding to the boiling points of the major compounds in bio-oil, resulting in progressive evaporation of bio-oil vapors with little carry-over of biochar particles. The method has been successfully applied for quantifying AAEM species in bioslurry fuels with various biochar loading levels (5–20 wt %), with small relative standard errors (within ±3%) and low limitations of quantification (0.4–3.0 ppm). It also overcomes the biochar incomplete oxidation issue associated with the microwave–digestion-based methods, which considerably underestimate the concentrations of AAEM species in bioslurry fuels.

Influence of the Atmosphere on the Transformation of Alkali and Alkaline Earth Metallic Species during Rice Straw Thermal Conversion

The influence of the atmosphere on the transformation of alkali and alkaline earth metallic (AAEM) species during rice straw thermal conversion was investigated by a fixed-bed reactor at 1073, 1173, and 1273 K. Two types of atmospheres were considered: 100% (v/v) N2 to simulate pyrolysis and 1% (v/v) O2 in N2 balance to simulate combustion. The release of the AAEM species during pyrolysis and combustion of the rice straw was quantified by a mass balance based on the weight measurements and inductively coupled plasma (ICP) analysis of the rice straw and the solid residue. Solid residues and released and condensed particles obtained in experiments were processed by water leaching and 1% HNO3 washing and analyzed by ion chromatography (IC) to determine the possible chemical form of the AAEM species contained in them. The experimental results showed that the oxidizing atmosphere facilitated the release of K at 1073, 1173, and 1273 K by enhancing the decomposition of K2CO3 and char K in the char during the char combustion stage of the rice straw combustion experiment. However, the oxidizing atmosphere suppressed the release of Na, Ca, and Mg at 1073, 1173, and 1273 K. For Na, the release suppression was caused by its promoted interaction with silicate in the oxidizing atmosphere. For Ca and Mg, such suppression was supposed to be caused by the suppressing effect of the oxidizing atmosphere on the release of Ca and Mg from char Ca and char Mg in the char of the rice straw.

Effects of Chemical Forms of Alkali and Alkaline Earth Metallic Species on the Char Ignition Temperature of a Loy Yang Coal under O2/N2 Atmosphere

Experiments were performed to determine the effects of alkali and alkaline earth metallic (AAEM) species on the ignition behavior of char under O2/N2 conditions. The char ignition temperatures of a Loy Yang brown coal were investigated using a wire-mesh reactor, where the secondary reactions of the evolved volatiles were minimized. Water, ammonium acetate, and sulfuric acid sequential extractions were employed to prepare the coal samples of different chemical forms of AAEM compounds from raw coal. In comparison to that of raw coal, the increases in the average char ignition temperature of 33 and 55 °C were observed for ammonium-acetate-extracted coal and sulfuric-acid-extracted coal (H-form coal), respectively, indicating the strong catalytic effects of various chemical forms of AAEM species on the char ignition behavior of brown coal. To find out which chemical form of sodium has a higher catalytic effect on the char ignition behavior, combustion experiments were also carried out for Na-exchanged coals and NaCl-loaded coals, which were prepared from H-form coal. The average char ignition temperature of 0.05 M Na-exchanged coal is 48 °C lower than that of 0.05 M NaCl-loaded coal in air, although those two samples have a close content but different chemical forms of sodium. It was found that sodium in char as carboxylates (−COONa) or NaCl can both improve the ignition characteristics of coal samples but carboxylates (−COONa) show a more significant catalytic effect on the average char ignition temperature in air combustion.

Mallee wood fast pyrolysis: Effects of alkali and alkaline earth metallic species on the yield and composition of bio-oil

The purpose of this study was to investigate the effects of inorganic species in biomass, especially the alkali and alkaline earth metallic (AAEM) species (K, Na, Mg and Ca), on the yield and properties of bio-oil from the pyrolysis of biomass. A mallee wood sample from Western Australia was washed with water and a dilute acid solution to remove its AAEM species. The water-washed and acid-washed mallee wood samples were then pyrolysed in a fluidised-bed reactor at 500°C under fast heating rate conditions. The removal of AAEM species did not result in significant changes in the yields of bio-oil and bio-char. However, the bio-oil properties, e.g. viscosity, were drastically affected by the removal of AAEM species. Our results indicate that the water-soluble AAEM species were not as important as the water-insoluble but acid-soluble AAEM species in influencing the bio-oil composition and properties. It is believed that the acid-soluble AAEM species (especially Ca) were more closely linked with the organic matter in biomass and thus were closely involved in the reactions during pyrolysis. The removal of AAEM species, especially the acid-soluble AAEM species, led to very significant increases in the yields of sugars and lignin-derived oligomers, accompanied by decreases in the yields of water and light organic compounds in the bio-oil.

Effect of Alkali and Alkaline Earth Metallic Species on Biochar Reactivity and Syngas Compositions during Steam Gasification†

Biochars were prepared from the pyrolysis of the wood, leaf, and bark components of mallee biomass in a fixed-bed reactor at 750 °C. The results show that the volatilization of inherent alkali and alkaline earth metallic (AAEM) species is 10−20% during the pyrolysis of raw wood, bark, and leaf samples. Acid treatment of the biochar samples was also carried out to prepare a set of acid-treated biochar samples. Although the majority of the inherent AAEM species were removed by acid-treatment, there are always some AAEM species that could not be removed for all biochars. Steam gasification experiments of both the raw and acid-treated biochar samples were carried out in a fixed-bed reactor at 750 °C and a steam concentration of 8.2 vol %. Data on the instantaneous syngas composition were obtained as a function of biochar conversion during steam gasification. Our data illustrated the importance of, in the study of steam gasification reaction mechanisms and kinetics of solid fuels such as biochars, optimizing t...

Evolution of Char Structure during the Steam Gasification of Biochars Produced from the Pyrolysis of Various Mallee Biomass Components

This study reports the evolution of char structure during the steam gasification of biochars under chemical-reaction-controlled conditions. Partially gasified samples were collected at various conversion levels during the steam gasification of both the raw biochars and the acid-treated biochars that had been prepared via acid-washing of the raw biochars. Results from FT-Raman spectroscopy show that the biochars have highly heterogeneous and disordered structures, which are selectively consumed with progress of steam gasification, leading to enrichment of larger aromatic ring systems, hence the so-called “selective gasification”. Selective gasification of biochar can be significantly influenced by the inherent alkali and alkaline earth metallic (AAEM) species in the biochars. The abundant catalysts present in the raw biochars can alter the gasification reaction pathway, but such an alteration appears to have little effect on the evolution of pore surface area, which increases significantly with conversion. While the wood biochar has too low a content of AAEM species to have an apparent effect on selective gasification, for the raw leaf and bark biochars with high contents of AAEM species, selective gasification is considerably less significant in comparison with the respective acid-treated biochars. For acid-treated biochars, gasification seems to take place slowly throughout the biochar on carbon active sites to consume the smaller rings selectively; the reactivity is controlled by the biochar carbon structure. However, for the raw leaf and bark biochars, gasification would be more focused or localized on the catalytic sites so that the activity of carbon active sites becomes less important. The catalytic effect of the inherent AAEM species seems to in turn depend on the carbon structure that probably affects the catalyst dispersion.

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.

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

Abstract 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.

Behavior of Inherent Metallic Species as a Crucial Factor for Kinetics of Steam Gasification of Char from Coal Pyrolysis

Four pairs of raw and acid-washed brown coals were subjected to pyrolysis and in-situ steam gasification at 900 °C in fixed-bed reactors with or without forced gas flow through the bed of char particles. The chars from the acid-washed coals were gasified obeying a first-order kinetics with respect to the fraction of unconverted char regardless of the presence/absence of forced gas flow. Its presence/absence considerably influenced the gasification kinetics of the chars from the raw coals, even altering the manner of change in the rate of gasification with time and the char conversion, such as the apparent order of reaction. It was found that alkali and alkaline earth metallic (AAEM) species were allowed to stay in/on the char in the absence of the forced gas flow but volatilized extensively in its presence. Thus, the catalysis of AAEM species diminished in the presence of forced gas flow much quicker than in its absence, in which the catalysis was lost exclusively by intraparticle deactivation of AAEM species. The char conversion was quantitatively described as a function of time by a kinetic model that considered the progress of catalytic and noncatalytic gasification in parallel and also the loss of catalysis along with the progress of gasification.