Mallee Wood Research Articles

Rapid pyrolysis of biochar prepared from slow and fast pyrolysis: The effects of particle residence time on char properties

This paper investigates the evolution of char properties with particle residence time during rapid pyrolysis of biochar under conditions pertinent to pulverized fuel (PF) applications. Two biochar samples were considered, prepared via slow (S-BC) and fast (F-BC) pyrolysis of mallee wood (150–250 µm) at 500 °C and two different heating rates (10 °C/s and ∼400 °C/s), respectively. The biochar samples were then subjected to rapid pyrolysis at 1300 °C using a novel drop-tube furnace (DTF), which enables direct determination of char yield experimentally. The evolution of char yield, the release of alkali and alkaline earth metallic (AAEM) species, and particle size and shape during rapid pyrolysis are investigated as a function of particle residence time (0.45 s to 1.4 s). The results show that char yields decrease from ∼77% to 75% when particle residence time increases from 0.45 s to 1.4 s. Rapid pyrolysis of F-BC has slightly higher char yields, due to the higher ash content of F-BC. More Cl in F-BC facilitates the release of Na during rapid pyrolysis, leading to the lower retention of Na in FC than in SC. Nevertheless, the retentions of K (∼90%), Mg (∼85%), and Ca (∼90%) are higher in FC, which can be ascribed to its higher contents of oxygen after rapid pyrolysis. The investigation of particle size and shape shows that biochar particles exhibit little changes after rapid pyrolysis, indicating their strong resistance to shrinkage and deformation even at high temperature.

An integrated two-step process of reforming and adsorption using biochar for enhanced tar removal in syngas cleaning

Tar reforming processes conducted at typically high-temperatures for syngas cleaning using biochar in biomass gasification plants, fail to remove the thermally stable smaller aromatic compounds (such as naphthalene, indene, anthracene) thereby retaining the risk of clogging downstream equipment. In this context, integration of an additional step of adsorption at relatively lower temperature, targeting the lighter tar compounds is proposed. An integrated approach for removal of tar generated from mallee wood bio-oil, involving reforming at 750 °C and adsorption at 220 °C was systematicaly evaluated against conventional reforming process. The comparative assessment included tar analysis estimating tar yield and examining tar constituents through UV-fluorescence spectroscopy, biochar structural analysis based on Raman spectroscopy supported by reactivity studies through TGA, and biochar morphological analysis by FESEM imaging and BET analysis. Tar analysis reveals a drastic drop in the two-step process by ∼ 80%: with ∼ 60% tar reduction in reforming and ∼ 40 % of residual tar in adsorption. The removal of lower aromatics was confirmed through UV-fluorescence of residual tars. The Raman spectroscopy shows a growing predominance of higher and lower aromatics in catalyst and adsorbent respectively. These trends were explained based on cross-linking of aromatic compounds through a polymeric condensation in reforming and the penetration of light tars into the micropores in adsorption. The effects of aromaticity are reaffirmed in reactivity studies by TGA. The BET results show a manifold decrease in porosity, implying coverage of the pore surface by the adsorbing species, which is also evident in FESEM images.

Insights into the mechanism of tar reforming using biochar as a catalyst

Biochar is an efficient catalyst for tar removal from syngas during biomass gasification. The aim of this research is to investigate the mechanism of tar reforming using biochar as a catalyst. A series of in situ steam tar reforming experiments were carried out using a two-stage fluidized-bed/fixed-bed reactor at 800 °C. Mallee wood biochar (106–250 μm) was activated in 15 vol% H2O balanced with Ar for different times (0–50 min) and then used as a catalyst for tar reforming. The on-line gas composition, light tar composition and the pore structure of biochar were analysed using mass spectrometer (MS), GC–MS and synchrotron small angle X-ray scattering (SAXS) respectively. An increased ratio of H2/CO was observed after reforming with biochar compared to reforming without biochar. The destruction of light tar compounds, especially the non-oxygen-containing compounds, was significantly enhanced when activated biochars were used. Steam activation increased the specific surface area (SSA), micro- and mesopore volumes in biochar while the values stayed almost unchanged during tar reforming. Results indicate that the micro- and mesopores in biochar promote the diffusion of both small and large tar molecules into the internal surface of biochar. However, the catalytic activity of biochar for tar reforming mainly depends on the content of O-containing functional groups in biochar. The O-containing functional groups facilitate the dissociation of tar molecules to form tar radicals, giving rise to the enhanced tar removal efficiency. Moreover, the formation of tar radicals over O-containing functional groups appears as the rate-limiting step in the process of catalytic reforming of tar over biochar catalysts.

In situ SAXS studies of the pore development in biochar during gasification

This work investigates the pore development in biochar during gasification using synchrotron small angle X-ray scattering (SAXS) as an in situ characterization technique. The influence of the gasifying agents (H2O, CO2 or H2O/CO2) and temperature on the pore structure development in biochar was studied by carrying out the hour-long gasification of mallee wood biochar (106–250 μm) in: (i) H2O at 700, 800 and 900 °C respectively, (ii) CO2 at 700 and 800 °C, and (iii) a mixture of H2O and CO2 (H2O/CO2) at 800 °C. There was a minor increase in the micro- and mesopore volumes in biochar during gasification in H2O at 700 °C, in contrast to CO2 gasification at the same temperature where no measurable changes to the pore structure were observed. At 800 °C, biochar derived from H2O/CO2 gasification exhibited the highest specific surface area (SSA). CO2 tended to produce a highly microporous biochar with a mesopore network showing pore fractal features. Micropore enlargement was a major process in the presence of H2O. In this case, the pore structure evolved from being a porous network of branched micropore clusters (pore fractal) to being dominated by rough surfaced mesopores (surface fractal) during gasification in H2O and H2O/CO2. The evolution of pore structures result from the different ways in which carbon atoms were removed by either H2O or CO2. H2O is more reactive and less selective towards reacting with biochar, resulting in a less worm-like network of pores than CO2. Moreover, it was found that increasing temperatures can lead to faster rates of pore generation and pore enlargement, which is attributed to the increased reaction rate and the less selective removal of carbon atoms.

Rapid Pyrolysis of Pulverized Biomass at a High Temperature: The Effect of Particle Size on Char Yield, Retentions of Alkali and Alkaline Earth Metallic Species, and Char Particle Shape

Fast pyrolysis of two biomass materials (mallee wood and pine wood) at 1300 °C was conducted in a novel drop-tube furnace (DTF) with a double-tube configuration, which enables the direct determinat...

Difference in tar reforming activities between biochar catalysts activated in H2O and CO2

This study investigates the difference in the activities between biochar catalysts activated in H2O and CO2 for in situ tar reforming. The experiments were performed in a two-stage fluidized-bed/fixed-bed at 800 °C. Mallee wood biochar (106–250 μm) was activated in pure CO2 or 15 vol% H2O balanced with Ar (15 vol. %H2O/Ar) for different times (0–50 mins) before being used as a catalyst for tar reforming. Based on the analysis of tar samples, it was found that H2O-activated biochar showed higher catalytic activity than CO2-activated biochar in both steam reforming and dry reforming of tar. Moreover, under otherwise identical conditions, reforming in steam was always more rapid than that in CO2 regardless of the atmosphere in which biochar was activated. The characterisation of biochar showed that H2O activation resulted in a biochar with a higher content of O-containing functional groups and aromatic C-O structures than that from CO2 activation. In addition, the content of O-containing functional groups and aromatic C-O structures of biochar after the steam reforming were higher than those after the dry reforming of tar. The different catalytic activities between the biochars activated by H2O and CO2 could be attributed to the different amounts of O-containing functional groups and aromatic C-O structures in biochar generated by H2O or CO2 activation. An extra supply of H2O plays an important role in improving and maintaining the catalytic activity of biochar during tar reforming.

Mechanistic insights into the kinetic compensation effects during the gasification of biochar in H2O

This study aims to gain insight into the mechanism and kinetics during the gasification of biochar in steam, which was formed in situ in a fluidised-bed reactor using mallee wood in two particle size ranges of 0.80–1.0 mm and 2.0–3.3 mm. The overall biochar gasification rate and the formation rates of key product components were calculated by continuously monitoring the product gas stream with a quadrupole mass spectrometer. The kinetic compensation effects reveal that CO and CO2 are both formed from the heterogeneous reactions between the biochar surface and H2O. CO2 is formed either by the surface (biochar)-catalysed water-gas-shift reaction or directly from the carbon active sites involving the same intermediate for the formation of CO, as revealed by the apparent activation energies and apparent pre-exponential factors for CO and CO2 formation. The changes in the particle size of biomass substrate do not affect the extent of the kinetic compensation effects of biochar consumption and formation of CO, CO2 and H2 in the kinetics-controlled and mixed regimes. The similar extent of the kinetic compensation effects of H2 formation and biochar consumption for both particle sizes indicates that the formation of H2 also mainly involve the carbon active sites on the biochar surface instead of the gas-phase water-gas-shift reaction.

Role of O-containing functional groups in biochar during the catalytic steam reforming of tar using the biochar as a catalyst

Tar formation is practically unavoidable during gasification. Catalytic tar forming is one of the most promising techniques for producing high-quality syngas at a commercial scale. Biochar has great potential to be used as a catalyst for the removal of tar from the syngas produced from the pyrolysis/gasification of biomass. The structure of biochar is a critical factor affecting its catalytic performance. This study investigates the role of O-containing functional groups in biochar during steam reforming of tar using biochar as a catalyst. Mallee wood biochar (106–250 μm) was activated in H2O for different times (0–50 min) and then used as a catalyst for the steam reforming of tar at 800 °C. The chemical structural features of biochars were characterized with Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). It was found that H2O activation increased the concentration of O-containing functional groups, mainly the aromatic CO structures in biochar, which enhanced the catalytic destruction of tar. During the catalytic reforming of tar, the content of aromatic CO groups decreased while the catalyst activity decreased. It is believed that the aromatic CO functional groups in biochar play a vital role in the steam reforming of tar.

A New Method for Direct Determination of Char Yield during Solid Fuel Pyrolysis in Drop-Tube Furnace at High Temperature and Its Comparison with Ash Tracer Method

A drop-tube furnace with a novel double-tube configuration was successfully developed to directly determine char yields during the pyrolysis of a wide range of solid fuels (mallee wood; mallee leaf; rice husk; biosolid; and subbituminous, bituminous, and anthracite coal) at a gas temperature of 1573 K. The char yield from pyrolysis of mallee wood and mallee leaf is 85%. This study shows using total ash as ash tracer results in 45–220% overestimation of char yields for biomass fuels and 13–27% for coals due to partial evaporation of ash. Similarly, selecting Na and K results in overestimation of biomass char yields by at least 2.5 times and selecting P leads...

Changes in the Biochar Chemical Structure during the Low-Temperature Gasification of Mallee Biochar in Air as Revealed with Fourier Transform Infrared/Raman and X-ray Photoelectron Spectroscopies

Structural changes in partially gasified char during low-temperature gasification in air were investigated using Fourier transform infrared/Raman and X-ray photoelectron spectroscopies. Two kinds of chars prepared from the gasification of mallee wood in 15% H2O–Ar at 600 and 900 °C were gasified in air at 375 °C in a thermogravimetric analyzer, separately. The breakage of large aromatic ring systems, the consumption of cross-linking structures, and the formation of dangling structures indicated the destruction of the aromatic ring systems of char with the progress of gasification in air. In addition, the oxidation of char was very significant at the initial stage (char conversion of <10%) of gasification in air, which was mainly due to the newly formed aromatic C–O structures. The newly formed aromatic C–O structure was also mainly responsible for the continuously increased total oxygen content, the total Raman intensity, and the total infrared (IR) intensity of char with the progress of gasification in a...

An X-ray photoelectron spectroscopic perspective for the evolution of O-containing structures in char during gasification

The purpose of this study is to investigate the evolution of O-containing structures of char during gasification. Mallee wood (4.75–5.60mm) from Western Australia was gasified in a fluidised-bed reactor at 600–900°C in O-containing (pure CO2, 15% H2O-Ar) and non-O-containing atmospheres (15% H2-Ar). X-ray photoelectron spectroscopy (XPS) was applied to obtain detailed information about the nature of oxygen bonding with carbon as well as the content of oxygen species in char. The similar O/C ratio of char from XPS and elemental analysis indicated the relative chemical uniformity between char surface and char matrix. The deconvolution results of the O 1s spectra showed that the reactivity of the inherent aromatic CO structure was much higher than that of the aromatic CO structure during gasification. The amount of aromatic CO structure left in char during gasification in non-O-containing atmosphere was lower than that in O-containing atmosphere while the consumption of aromatic CO structure was proportional to the progress of gasification, regardless of the atmosphere. The newly formed CO structure in char during the gasification in the O-containing atmosphere was likely to be responsible for the high gasification reactivity. The well-dispersed alkali earth metallic species could be carbonated to form CaCO3 and MgCO3 on char surface once the char was exposed to CO2 at 900°C.

Oxidative pyrolysis of mallee wood biomass, cellulose and lignin

The oxidative pyrolysis of mallee wood, cellulose and lignin was performed and the bio-oil products were analysed to understand how the externally added oxygen react with the pyrolysis products. Both wood cylinders of diameter 8 mm and fine particles (90–300 μm) were pyrolysed in this study to understand the combined effects of biomass particle size and the presence of oxygen. The results revealed that, at a low oxygen concentration, the gas-phase oxidation of volatiles would improve the yields of levoglucosan and syringaldehyde, as well as unsaturated hydroxyl ketones/aldehydes for small wood particles through the oxygen-induced radical reactions. Although oxygen could facilitate the production of some compounds in bio-oil through the gas-phase reactions, it did lead to decreases in the heavy bio-oil yield due to the over-oxidation of some pyrolysis products (e. g. aromatics, lactones, unconjugated alkyl aldehydes/esters and carboxylic acids). The effects of oxygen on the pyrolysis of wood cylinders were more complicated than mallee wood particles due to the secondary reactions of volatiles and the reactions involving the pyrolysing particle surface. The interactions between the polysaccharide-derived and lignin-derived products in gas phase might affect the oxidation of volatiles, changing the formation of pyrolytic products (e.g. levoglucosan and syringaldehyde).

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 ...

Grinding pyrolysis of Mallee wood: Effects of pyrolysis conditions on the yields of bio-oil and biochar

A novel technology termed as ‘grinding pyrolysis’ has been developed in response to the limitations hindering the commercialisation of existing fast pyrolysis technologies. Some of the features of this novel technology include capability of feeding biomass having a wide range of particle sizes in the same feedstock, simultaneous pyrolysis and grinding, no requirement for a fluidising gas and simplified requirement for cooling and condensing the pyrolysis products. A pilot plant was developed to study the effects of various operating conditions on the yields of bio-oil and biochar. Experiments have been done using mallee biomass grown in Western Australia. Feed particle sizes of up to a few centimetres have been used successfully. It was proved that the simultaneous grinding and pyrolysis helps liberating volatiles from a pyrolysing biomass particle and is capable of producing higher organic yields than pyrolysis without grinding. The effective size of a pyrolysing particle was significantly decreased by the simultaneous grinding activity.

Effects of thermal pretreatment and ex-situ grinding on the pyrolysis of mallee wood cylinders

This study investigated the effects of thermal pretreatment and ex-situ grinding on the production of bio-oil and biochar from the pyrolysis of mallee wood cylinders in a fluidised-bed reactor. The wood cylinders were firstly pretreated at 150–380°C and were then crushed into small particles before further pyrolysis at 500°C. Thermal pretreatment alone for wood cylinders could not promote the bio-oil yield. Combined thermal pretreatment at low temperatures and subsequent grinding facilitated the formation of bio-oil and minimised the formation of biochar. This is because the thermal pretreatment and grinding partially destroyed the cell wall structure and improved the mass transfer of volatiles exiting from the particles during the subsequent pyrolysis. However, if the pretreatment temperature was above 260°C, the biochar yields increased due to the cross-linking and charring reactions. These charring reactions compromised the beneficial effects of grinding, leading to decreases in the formation of bio-oil. The balance between thermal pretreatment and grinding needed to be delicately managed to maximise the formation of bio-oil. The pretreatment temperature would significantly affect the composition of bio-oil from the ex-situ grinding pyrolysis. The controlled pretreatment temperature could somewhat “activate” lignin or cause cross-linked bonds in biomass which was responsible for the formation of aromatics.

Pyrolysis of large mallee wood particles: Temperature gradients within a pyrolysing particle and effects of moisture content

Temperature profiles inside a large pyrolysing particle were studied and are reported in this paper. Mallee trunks of similar diameter from the same tree were used to prepare cylindrical samples with 40mm length. A fluidised-bed reactor was used to pyrolyse the large particles. The temperature profiles inside the particles were recorded during pyrolysis to allow the calculation of corresponding heating rate profiles inside the particle. The effects of moisture were studied by pyrolysing some particles with 15 to 20% moisture content. The temperature profiles obtained from the pyrolysis of dry and wet samples have been compared to identify the possible effects of moisture on the temperature profiles. A possible change in the thermal conductivity of the wood was identified around 100°C, which caused a peak in the heating rate profile. Some possible exothermic peaks were observed at around 325°C and 425°C. A peak in the heating rate profile at around 200°C in the case of the pyrolysis of wet particles was believed to be related to the changed 3-D macromolecular structure of the biomass in the presence of moisture. Some yields of tar and char along with other analytical results were presented to support our observations on the temperature profiles. Our results indicate that moisture can potentially alter the overall pyrolysis reactions and product distribution, in particular through changes in the 3-D macromolecular structure of biomass.

Inorganic PM10 emission from the combustion of individual mallee components and whole-tree biomass

This contribution reports the emission of inorganic particulate matter (PM) with an aerodynamic diameter <10µm (PM10) from the combustion of both individual mallee components and whole-tree biomass. Three major components of a mallee tree, namely bark, leaf, and wood, were size-reduced to 75–150µm and mixed at a dry mass ratio of 15% bark:35% leaf:50% wood, which is close to the real mallee's composition, to prepare a whole-tree biomass. The three individual mallee components and the whole-tree biomass were combusted in a laboratory-scale drop-tube furnace at 1400°C in air to produce inorganic PM10 for further quantification and characterization. The results demonstrate that, whereas the particle size distributions of the PM10 from the combustion of the bark, leaf and wood components generally follow a bimodal distribution, the yields of PM0.1, PM0.1–1, PM1, PM1–10, PM2.5, and PM10 from the three mallee components are quite different. On the bases of dry biomass and useful energy input, the yields of the PM of various size fractions studied follow a sequence of the bark>the leaf>the wood, consistent with that of the ash contents in the three components. Oppositely, the ash-based yields of PM0.1, PM0.1–1, PM1, PM1–10, PM2.5, and PM10 from the wood are substantially higher than those from the bark and the leaf. No obvious synergetic effect among different mallee components in PM10 emission is observed during the whole-tree biomass combustion, enabling the prediction of the PM10 yield from the whole-tree biomass combustion based on that from the individual mallee components.

Formation of aromatic ring structures during the thermal treatment of mallee wood cylinders at low temperature

Aromatics are important components in the tars from biomass gasification or in bio-oil from pyrolysis. This study investigated the formation of polycyclic aromatics during the thermal treatment of mallee wood at low temperature. The formation of tar was negligible below 200°C but became significant above 230°C. At the same time, the abundance of aromatics also became significant above 230°C. In addition, it was found that some aromatics were formed even at temperatures as low as 150°C. These aromatics were trapped in the resultant chars but could be extracted with solvents (i.e. methanol/chloroform). The abundance of trapped aromatics was different between the outer surface and the centre of the char (cylinder shape). FT-IR and UV-fluorescence analysis revealed that the aromatics contained polar functionalities (e.g. carbonyl groups) on the benzene rings. Some unsaturated hydroxyl aldehyde/ketone intermediates were formed in parallel with the aromatics.

Coke formation during the hydrotreatment of bio-oil using NiMo and CoMo catalysts

This study aims to investigate the coke formation during the hydrotreatment of bio-oil at low temperature. The catalytic hydrotreatment of bio-oil produced from the pyrolysis of mallee wood was carried out using pre-sulphided NiMo and CoMo catalysts at a temperature range of 150–300°C. Our results show that the catalysts play an important role in reducing the coke formation. The transformation of light products during the hydrotreatment was investigated. The role of levoglucosan in the coke formation was investigated by adding additional levoglucosan into the bio-oil prior to the hydrotreatment. In the presence of the catalyst, the hydrotreatment of bio-oil with the added levoglucosan did not yield more coke than that of original bio-oil under identical conditions. However, in the absence of the hydrotreating catalyst, coke formation was intensified. Our data indicate that levoglucosan could cross-link with other compounds in bio-oil in the absence of a hydrotreatment catalyst.

An advanced biomass gasification technology with integrated catalytic hot gas cleaning. Part III: Effects of inorganic species in char on the reforming of tars from wood and agricultural wastes

Char is used directly as a catalyst for the catalytic reforming of tar during gasification. Experiments have been carried out to examine the effects of inorganics in char as a catalyst for the catalytic reforming of tar during the gasification of mallee wood, corn stalk and wheat straw in a pilot plant. The char catalyst was prepared from the pyrolysis of mallee wood at a fast heating rate. The catalytic activities of char and acid-washed char for tar reforming were compared under otherwise identical gasification conditions. For all biomass feedstocks tested for gasification, the tar contents in product gas could be drastically reduced by the catalyst, reaching a tar concentration level well below 100mg/Nm3. The acid-washed char also showed profound activity for tar reforming although its catalytic activity was definitely lower than the raw char. Both catalysts could effectively reform the aromatic ring systems (especially large aromatic ring systems with three or more fused benzene rings) in tars as is revealed using UV-fluorescence spectroscopy. The char itself was also partially gasified. After being used as a catalyst, the condensation of the aromatic rings and the accumulation of inorganic species led to drastic changes in char reactivity with O2 at 400°C. The inorganic species in char tended to enhance the formation of H2 and CO during the reforming reactions in the catalytic reactor.