Evaluation of structural features of chars from pyrolysis of biomass of different particle sizes

The structural features of chars derived from pyrolysis of mallee wood of different particle sizes in a novel fluidized-bed/fixed-bed reactor have been investigated. Raman spectroscopy was used for structural evaluation of chars. Spectra were curve-fitted with 10 Gaussian bands representing typical structural features of the chars. The temperature had a significant influence on the evolution of char structure and thus the total Raman peak area between 800 and 1800 cm^− 1 is seen to decrease significantly with increasing pyrolysis temperature for all chars. On the other hand, the ratio I_D/I_{(Gr + Vl + Vr)} between the band intensities of condensed aromatic ring systems (> 6 rings) and amorphous char structures with small aromatic ring (3-5 rings) systems is seen to increase with increasing temperature. The particle size of biomass has a great role in char structure at fast heating rate (> 1000 °C/s) pyrolysis although it has no effect on char structure at slow heating rate pyrolysis (0.17 °C/s). However, in the bigger biomass particle, the structure of char prepared under fast heating rate pyrolysis is similar to that of the structure of char prepared under slow heating rate pyrolysis.     

Investigation into the evolution of char structure using Raman spectroscopy in conjunction with coal petrography; Part 1

The evolution of char structure during heat treatment was investigated using coal petrography and micro Raman spectroscopy (MRS). The heat treatment was in the temperature range of 300-1000 °C using inertinite-rich South African coals. Char morphology analyses, determined petrographically showed a significant increase in the amount of dense/solid chars as compared to porous chars as temperature increases. MRS results were given in terms of the I_D/I_G intensity ratios, band positions and bandwidths as a function of temperature. It was found that sp²-sp³ bonding (reactive sites/crystallites) was created in dense chars (originating from inertinite particles) at the initial heat treatment temperature, and these sp²-sp³ bondings were consumed later at high temperature. Earlier consumption of sp²-sp³ bonding was observed in porous chars, since they were vitrinitic in origin and contained more reactive sites. The D1 and G bandwidths showed a significant change with heat treatment, which were further correlated with the amount of dense and porous char determined petrographically. Therefore, the use of MRS and petrography on chars enhances the understanding of char evolution on a structural level and may lead to enhanced understanding of coal combustion.     

Char structure characterised by Raman spectroscopy and its correlations with combustion reactivity

Raman spectroscopy was applied to characterise the microstructure of coal chars generated under various heat treatment conditions, which was correlated with the combustion reactivity measured by thermogravimetric analysis. The Raman spectra were fitted with the combination of 4 Lorentzian bands and 1 Gaussian band. It was found that the increase of char microstructural order under heat treatment can be characterised by Raman parameters, in particular the band area ratios, indicated by the increase in I_G/I_All and the decrease in I_D1/I_G, I_D2/I_G, I_D3/I_G and I_D4/I_G with increasing treatment temperature and/or time. The combustion reactivity of the chars from demineralised coals was found to have good correlations with the band area ratios, independent of coal type and heat treatment condition. It was found that the presence of inorganic matter in coal chars marginally affected the evolution of the average char microstructure. However, it did affect the char reactivity evolution. It was confirmed that the thermal deactivation of coal char during heat treatment was dependent not only on the ordering of char crystalline structure but also on the loss of catalytic activity of the inorganic matter.     

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.     

Raman spectroscopy studies of the high-temperature evolution of the free carbon phase in polycarbosilane derived SiC ceramics

The Raman spectra of a number of SiC ceramics synthesized from polycarbosilane at 1200 °C and annealed at 1400, 1600, 1800 and 2000 °C have been recorded using laser excitation wavelength of 532 nm. The peak positions, their intensities (I_D/I_G) and full width at half maximum (FWHM) were used to obtain information about the degree of disorder in the free carbon phases. The increasing ordering with annealing temperature was confirmed by lower FWHM values and G-peak positions obtained from the SiC ceramics annealed at higher temperature. However, the I_D/I_G has shown to be the highest point at 1600 °C, which illustrates that the temperature is one critical point of the microstructure evolution of the free carbon phase changing amorphous to turbostratic with increasing temperatures. Obviously, the oxidation behaviors of the SiC ceramics are significantly affected by the microstructures of the free carbon phases. In the SiC ceramics with above 1600 °C annealing, the oxidation temperatures of the SiC phases are postponed more than 100 °C, because they are surrounded by the free carbon phases.     

An experimental study of sulphate transformation during pyrolysis of an Australian lignite

The transformation of sulphate minerals during pyrolysis of an Australian lignite has been studied using pure sulphates (CaSO₄, FeSO₄ and Fe₂(SO₄)₃), a high mineral (HM) lignite sample and a low mineral (LM) lignite sample collected from different locations of the same deposit, and samples of acid-washed LM doped with sulphates (CaSO₄+ LM and FeSO₄+ LM), respectively. Thermogravimetric analysis and fixed-bed reactor techniques were used for the pyrolysis experimentation and the lignite samples and their chars were analysed using FTIR and XRD. The TGA experiments showed that CaSO₄ decomposes between 1400 and 1700 K in nitrogen and a 50/50 N₂/CO₂ mixture, while in air CaSO₄ decomposes between 1500 and 1700 K. Using a TGA-MS it was found that only a small fraction of CaSO₄ in CaSO₄+ LM decomposed at 653 K, releasing SO₂. CaSO₄ was still observed in the char recovered at 1073 K as confirmed by the FTIR and XRD analysis. FeSO₄·7H₂O released the bound water below 543 K and the remaining FeSO₄ decomposed between 813 and 953 K. FeSO₄ in FeSO₄+ LM decomposed at 500 K to release SO₂. The inherent sulphates in HM were dominated by iron sulphates which started to decompose and release SO₂ at around 500 K and all sulphate had been decomposed at 1073 K. It was observed that during the fixed-bed pyrolysis at 1073 K in nitrogen, approximately 36% of the total sulphur in the CaSO₄+ LM decomposed, 88% of the total sulphur in the FeSO₄+ LM decomposed and around 76% of the total sulphur in HM decomposed. It was also confirmed that FeSO₄+ LM produced more volatile sulphur than CaSO₄+ LM during pyrolysis.     

Effect of lignite pyrolysis conditions on calcium oxide dispersion and subsequent char reactivity

A demineralized North Dakota lignite was loaded with 2.9 wt% Ca by ion exchange. Chars were prepared by pyrolysis in N₂ at 1275 K and residence times between 0.3 s and 1 h. Major differences were observed in their subsequent reactivities in 0.1 MPa air. X-ray diffraction analysis was carried out to obtain information on the state and dispersion of the Ca species on the various chars. The results clearly indicate that CaO is the predominant species responsible for catalysis of lignite char gasification. It is concluded that pyrolysis residence time also has a profound effect on CaO dispersion. Thus, a correlation was established between a fundamental physical property (catalyst dispersion) and the observed gasification behaviour of lignite chars prepared under different pyrolysis conditions.     

Comparative study of first- and second-order Raman spectra of MWCNT at visible and infrared laser excitation

Comparative studies of first- and second-order Raman spectra of multi-walled carbon nanotubes (MWCNT) and three other graphitic materials - carbon fiber, powdered graphite and highly ordered pyrolytic graphite - are reported. Three laser excitation wavelengths were used: 514.5, 785 and 1064 nm. In first-order Raman spectra, the positions of the bands D, G and D′ (1100-1700 cm⁻¹) presented very similar behavior, however the intensity (I) ratio I_D/I_G ratio showed differed behaviors for each material which may be correlated to differences in their structural ordering. In the second-order spectra, the G′ band varied strongly according to structure with the infrared laser excitation.     

Co-pyrolysis of biomass and coal in a free fall reactor

An experimental study on co-pyrolysis of biomass and coal was performed in a free fall reactor under atmospheric pressure with nitrogen as balance gas. The coal sample selected was Dayan lignite, while the biomass used was legume straw. The operation temperature was over a range of 500-700 °C, and the blending ratio of biomass in mixtures was varied between 0 and 100 wt.%. The results indicated that there exist synergetic effects in the co-pyrolysis of biomass and coal. Under the higher blending ratio conditions, the char yields are lower than the theoretical values calculated on pyrolysis of each individual fuel, and consequently the liquid yields are higher. Moreover, the experimental results showed that the compositions of the gaseous products from blended samples are not all in accordance with those of their parent fuels. The CO₂ reactivities of the chars obtained from the co-pyrolysis under the higher blending ratio (around 70 wt.%) conditions are about twice as high as those of coal char alone, even higher than those of biomass alone.     

Effect of mineral matter on product yield in supercritical water extraction of lignite at different temperatures

The effect of demineralization on conversion of Soma Lignite in supercritical water extraction was studied using a batch autoclave operated at 400, 450 and 500 °C under nitrogen atmosphere. The experiments were carried out to investigate the effect of mineral matter and temperature on gaseous, liquid, residue yield and composition of gaseous products. According to the results, main product in gaseous state is CO₂. Temperature is key factor affecting product distribution when compared the effect of minerals in lignite. As temperature was increased, yield of gas and solid residue increased, while yields of liquid decreased for raw and demineralized lignite samples. The removal of mineral matter caused to decrease the conversion for all lignite samples and to increase the carbon content of solid residue in supercritical water extraction.     

Study of heat-treated Spanish lignites: Characteristics and behaviour in co2 and o2 gasification reactions

Six Spanish lignites (raw and demineralized) have been charred to 1113 K in a N₂ atmosphere. The surface area, porosity and mineral matter content of the char coals so obtained have been studied, as well as their reactivity in CO₂ flow in the range 1073–1113 K, and in dry air in the temperature range 733–773 K. The reactivities of the raw chars in CO₂ may be explained according to the different inorganic matter content that may act as catalyst. The demineralization process brings about a lowering in reactivity and an increase, in general, in the apparent activation energy that may be interpreted as being due to a fall in mineral matter content and/or an increase in the amount of feeder pores. With regard to reactivity and apparent activation energy, in the case of dry air three groups of raw chars have been established. The differences between these three groups may be due to the different inorganic impurities present in the raw chars that catalyse the reaction of carbon with O₂ more than the porous texture parameters. Demineralization brings about a lowering in the reactivity values and a levelling off of apparent activation energies. The catalytic effect of iron has also been studied by adding different amounts of this metal to a demineralized char. The burn-off versus time curves of the different char coals have been adjusted by using the τ_0.5 parameter.     

Orientation of graphitic planes during annealing of “dip deposited” amorphous carbon film: A carbon K-edge X-ray absorption near-edge study

Annealing effect of amorphous carbon thin films on Si(1 0 0) substrates is studied by normal incidence and angle dependent carbon K-edge X-ray absorption near-edge structure (XANES) spectroscopy. The angle dependence of the XANES signal shows that the graphitic basal planes are oriented perpendicular to the surface when the film is annealed at 1000 °C. Micro-Raman spectroscopy reveals two well-separated bands the D band at 1355 cm⁻¹ and G band at ∼1600 cm⁻¹, and their I_D/I_G intensity ratio indicates the formation of more graphitic film at higher annealing temperatures. X-ray diffraction pattern of 1000 °C temperature annealed film confirms the formation of graphite structure.     

Enhancement of lignite char reactivity to steam by cation addition

Chars produced from lignites typically have much higher reactivities to gasification than those produced from bituminous coals. This has been attributed previously to the presence of carboxylate salts of inorganic constituents on the lignites. Upon charring of the lignites, the carboxylate salts decompose leaving behind well dispersed inorganic constituents which act as catalysts for gasification. In this study, a raw lignite has been treated with HCl and HF to demineralize it and to increase its carboxyl content prior to exchanging selected cations with the hydrogen on the carboxyl groups. Up to 2.14 mmol of calcium per g of coal could be added using this procedure. Addition of varying amounts of calcium to the lignite resulted in the production of chars containing calcium contents ranging from 1.1 to 12.9 wt %. Such addition resulted in a rectilinear increase in reactivity of the char to steam with increasing amount of calcium added. Maximum reactivity attained was over ten times the reactivity found for the char produced from the raw lignite. At comparable molar loadings of metal cations onto the acid-treated lignite, the chars subsequently produced had reactivities in steam in the order: K >Na ≈ Ca >Fe >Mg. Char reactivity could also be enhanced by the addition of cations to nitric acid-treated char which had been produced, in turn, from demineralized lignite.     

Reactivities of 34 coals under steam gasification

The reactivities of 34 coal chars of varying rank with H₂O have been determined to examine the effect of coal rank on the gasification rate of coal char. The reactivities of chars derived from caking coals and anthracites (carbon content > 78 wt%, daf) were very small compared with those from non-caking (lower-rank) coals. The reactivities of low-rank chars do not correlate with the carbon content of the parent coals. To clarify which factor is more important in determining the reactivity, the evolution of CO and CO₂ from char, the moisture content of char and the amount of exchangeable cations were determined for these low-rank coals or their chars. These values were considered to represent the amount of active carbon sties, the porosity and the catalysis by inherent mineral matters, respectively. It was concluded that the amount of surface active sites and/or the amount of exchangeable Ca and Na control the reactivity of low-rank chars in H₂O.     

Exploring the possibilities of using Brazilian subbituminous coals for blast furnace pulverized fuel injection

The current study investigates the combustion and blast furnace injection performance of three Brazilian subbituminous coals (Mina do Recreio) and their beneficiation products using laboratory scale combustion tests. The coals have relative high ash yields (up to 40 wt%) that were reduced stepwise to levels as low as 12 wt%, dry basis. The reduction of ash yields is paralleled by a significant decrease in sulphur and inertinite contents.The combustion tests were performed in a drop tube reactor operating at 1300 °C using two different atmospheres (2.5 and 5% O₂). The chars exhibited preferentially rounded shapes with thick walls and abundant secondary porosity for the 2.5% O₂ chars, whereas the 5% O₂ chars showed very thin walls as a consequence of extensive burnout. The intrinsic reactivities of both set of chars were similar. The differences in conversion between the two working atmospheres were 24-37% and roughly tend to increase with increasing mineral matter content. Conversions as high as 76-81% were reached operating under 5% O₂ indicating that the coals are easy to burn. The small differences in burnout among the coals and their beneficiation products cannot be clearly attributed neither to mineral matter or inertinite content. A rough inverse relationship was found between the intrinsic reactivity of the chars and the inertinite content of the parent coal indicating that the char material derived from inertinite was intrinsically less reactive than that derived from vitrinite. These differences were no longer relevant at high temperature.Blast furnace injection performance was studied through thermobalance experiments using CO₂ atmosphere and 1050 °C temperature. It is apparent that the beneficiation process has no effect on the reactivity of the coals from Recreio Mine. The only exception is the low ash coal-2-LabB (11.5 wt%), for which a higher reactivity is indicated. The reactivity tests show also that the coals have adequate properties to be used together with imported coal blends in pulverized coal injection in the blast furnace (PCI).     

Co-liquefaction of lignite and sawdust under syngas

Individual and co-liquefaction of lignite and sawdust (CLLS) under syngas was performed in an autoclave and the effects of temperature, initial syngas pressure, reaction time and ratio of solvent to coal and biomass on the product distribution of CLLS were studied. Sawdust is easier to be liquefied than lignite and the addition of sawdust promotes the liquefaction of lignite. There is some positive synergetic effect during CLLS. In the range of the experimental conditions investigated, the oil yield of CLLS increases with the increase of temperature, reaction time (10-30 min) and the ratio of the solvent to the feedstock (0-3), but varies little with the increase of initial syngas pressure. Accordingly, the total conversion, the yield of preasphaltene and asphaltene (PA + A) and gas, changes by the difference in operation conditions of liquefaction. The gas products are mainly CO and CO₂ with a few C₁-C₄ components. The syngas can replace the pure hydrogen during CLLS. The optimized operation conditions in the present work for CLLS are as follows: syngas, temperature 360 °C, initial cold pressure 3.5 MPa, reaction time 30 min, the ratio of solvent to coal and sawdust 3:1. Water gas shift reaction occurs between CO in the syngas and H₂O from coal and sawdust moisture during the co-liquefaction, producing the active hydrogen which increases the conversion of liquefaction and decreases the hydrogen consumption.     

Raman spectroscopy studies of carbide derived carbons

The Raman spectra of a number of carbide derived carbons (CDCs) synthesised from TiC at 700, 800, 900, 1000, 1100 and 1200 °C and from VC, WC, TaC, NbC, HfC and ZrC made at 1000 °C have been recorded using laser excitation wavelengths of 514 and 785 nm. The spectra show two main features, the D- and G-peaks situated around 1350 cm⁻¹ and 1600 cm⁻¹, respectively. The peak positions, their intensities (I_D/I_G) and full width at half maximum (FWHM), as well as their wavelength dependent dispersion, were used to obtain information about the degree of disorder in CDCs. The increasing ordering with synthesis temperature was confirmed by lower FWHM values obtained from CDCs made at higher synthesis temperatures. However, this parameter was not very sensitive to variation in ordering in CDCs made at 1000 °C from different carbides. The I_D/I_G was used for determination of the in-plane correlation length, which has shown to be independent of synthesis temperature and more sensitive to the choice of the precursor carbide. However, the changes in in-plane correlation length were small and barely accounted for the size of one sixfold ring.     

An investigation of the causes of the difference in coal particle ignition temperature between combustion in air and in O2/CO2

The ignition temperatures of a Loy Yang brown coal and a Datong bituminous coal were investigated in a wire-mesh reactor where the secondary reactions of the evolved volatiles were minimised. An increase in the average particle ignition temperature of 21 °C was observed for the brown coal when air (21% O₂ + 79% N₂) was replaced with a mixture of 21% O₂ + 79% CO₂. Combustion was also carried out in the mixtures of 21% O₂ + 79% argon and 21%O₂ + 79% helium in order to determine the effects of heat transfer on the observed particle ignition temperature. It is concluded that the thermal conductivity of gas atmosphere surrounding the particles greatly influences the observed particle ignition temperature while the effects of the heat capacity of the gas atmosphere was very minor under our experimental conditions. The structure of char and the reactions involving the char (char-O₂ and char-CO₂) can greatly affect the observed particle ignition temperature. In particular, the char-CO₂ reactions were largely responsible for the observed difference in particle ignition temperature in air and in 21% O₂ + 79% CO₂. Alkali and alkaline earth metallic (AAEM) species in the brown coal also significantly affect the observed particle ignition temperature.     

Phase behavior of the poly(neopentyl methacrylate) + supercritical fluid solvents + neopentyl methacrylate system and CO2 + neopentyl methacrylate mixtures at high pressure

High-pressure phase behaviors are measured for the CO₂ + neopentyl methacrylate (NPMA) system at 40, 60, 80, 100, and 120 °C and pressure up to 160 bar. This system exhibits type-I phase behavior with a continuous mixture-critical curve. The experimental results for the CO₂ + NPMA system are modeled using the Peng-Robinson equation of state. Experimental cloud-point data up to the temperature of 180 °C and the pressure of 2000 bar are presented for ternary mixtures of poly(neopentyl methacrylate) [poly(NPMA)] + supercritical solvents + NPMA systems. Cloud-point pressures of poly(NPMA) + CO₂ + NPMA system are measured in the temperature range of 60-180 °C and to pressures as high as 2000 bar with NPMA concentration of 0.0, 5.2, 19.0, 28.1 and 40.2 wt%. It appears that adding 51.2 wt% NPMA to the poly(NPMA) + CO₂ mixture does significantly change the phase behavior. Cloud-point curves are obtained for the binary mixtures of poly(NPMA) in supercritical propane, propylene, butane, 1-butene, and dimethyl ether (DME). The impact of dimethyl ether concentration on the phase behavior of the poly(NPMA) + CO₂ + x wt% DME system is also measured at temperature of 180 °C and pressure range of 36-2000 bar. This system changes the pressure-temperature (P-T) slope of the phase behavior curves from upper critical solution temperature (UCST) region to lower critical solution temperature (LCST) region as the NPMA concentration increases.     

Experimental study on CO2 capture conditions of a fluidized bed limestone decomposition reactor

In this study, the decomposition conditions of limestone particles (0.25-0.50 mm) for CO₂ capture in a steam dilution atmosphere (20-100% steam in CO₂) were investigated by using a continuously operating fluidized bed reactor. The results show that the decomposition conversion of limestone increased with the steam dilution percentage in the CO₂ supply gas. At a bed temperature of 920 °C, the conversions were 72% without steam dilution and 98% with 60% steam dilution. The conversion was 99% with 100% steam dilution at 850 °C of the bed temperature. Steam dilution can decrease not only the decomposition temperature of limestone, but also the residence time required for nearly complete decomposition of CaCO₃. The hydration and carbonation reactivities of the CaO produced were also tested and the results show that both the reactivities increased with the steam dilution percentage for decomposing limestone.