Effect of pyrolysis conditions on the char gasification with mixtures of CO2 and H2O

Two kinds of char were prepared from a lignite by fast pyrolysis using a drop tube furnace and by slow pyrolysis using a fixed-bed furnace at the temperature of 1273 K. Scanning electron microscopy, X-ray diffractometry and the BET method were employed to characterize char properties. The chars were gasified with CO₂, H₂O and their mixtures in a thermogravimetric analyzer (TGA) system to investigate gasification kinetics and derive the rate expression. To validate the gasification rate equation derived from TGA, a fluidized-bed gasification experiment was also carried out. The results showed that both fast-char and slow-char were mainly composed of dense char. The shrinking core model was applicable to predict both gasification of fast-char and slow-char. It was found that the char gasification rate in the mixtures of CO₂ and H₂O was obviously lower than the sum of the two rates of the char independently reacting with CO₂ and H₂O but higher than the rate of each independent reaction, for both the fast-char gasification and slow-char gasification. Both of the results from the TGA and the fluidized-bed reactor showed that char-H₂O reaction was independent on char-CO₂ reaction, while char-CO₂ reaction was inhibited by char-H₂O reaction.     

Effect of high CO2 concentration on char formation through mineral reaction during biomass pyrolysis

O₂/CO₂ combustion has attracted considerable attention as a promising technology for CO₂ capture. Using biomass for fuel is considered carbon neutral, and O₂/CO₂ biomass combustion can mitigate the deleterious environmental effect of greenhouse. In this study, the effect of CO₂, the main component gas in O₂/CO₂ combustion, on the pyrolysis characteristics of biomass is investigated. Cellulose, lignin, and metal-depleted lignin pyrolysis experiments were performed using a thermobalance. Information on the surface chemistry of the chars was obtained by Fourier transform infrared (FTIR) spectroscopy to investigate changes in the surface chemistry during pyrolysis under different surrounding gasses. When the temperature increased to 1073 K at heating rate of 1 K s^?1, the char yield of lignin in the presence of CO₂ increased by about 10% compared with that under Ar. However, for cellulose and metal-depleted lignin, no significant difference appeared between pyrolysis under CO₂ and that under Ar. FT-IR showed that a strong peak corresponding to carbonate ions appeared in the char derived from lignin under CO₂. Therefore, salts such as Na₂CO₃ or K₂CO₃ formed during the lignin pyrolysis under CO₂. At around 1650–1770 cm^?1, a significant difference appeared in the FTIR spectra of chars formed under CO₂ and those formed under Ar. C=O groups not associated with an aromatic ring were found only in chars formed under CO₂. It was suggested that these salts affected the char formation reaction, in that the char formed during lignin pyrolysis under CO₂ had unique chemical bands that did not appear in the lignin-derived char prepared under Ar.     

Effect of pyrolysis conditions on gasification reactivity of woody biomass-derived char

The effect of pyrolysis conditions on char reactivity has been studied using Raman spectroscopy. This paper reports on the relationship between the properties of biomass char and the gasification rate. The gasification kinetics of biomass char have been revealed by measuring the rate of weight loss during its reaction with CO₂ as a function of temperature. First-order kinetic rate constants are determined by fitting the weight loss data using a random pore model. The relationship between the char structure and CO₂ gasification reactivity was investigated in the range of 15–600 °C/min at a constant pyrolysis pressure (0.1 MPa), and 0.1–3.0 MPa at a constant heating rate (15 °C/min). The experimental results reveal that the reactivity of biomass char is determined by the pyrolysis condition. The CO₂ gasification rates in char generated at 0.1 MPa exhibited approximately twice the values as compared to those obtained at 3 MPa. This is because the uniformity of the carbonaceous structure increases with the pyrolysis pressure. The uniformity of carbonaceous structures would affect the CO₂ gasification reactivity, and the decreasing uniformity would lead to the progression of cavities on the char surface during the CO₂ gasification process. The gasification rate of biomass char increases with the heating rate at pyrolysis. This is due to the coarseness (surface morphology) of biomass char and rough texture, which increases with the heating rate.     

Modeling char surface area evolution during coal pyrolysis: Effect of swelling and gasification at high pressures

In this paper, a previous model for char surface area change during atmospheric coal pyrolysis was modified to include the effect of particle swelling and gasification to predict the N₂ adsorption specific surface area of high-volatile bituminous char generated at high pressures with or without gasification. A correlation between the change of char surface area and P₀ΔV_P/m_cm (the ratio of the expansion work for particle swelling to the mass of metaplast cross-linked with coal matrix) was developed and analyzed. The number of the defect regions generated by gasification was considered in calculating adsorption sites quantitatively. Particle swelling opens (at P₀ΔV_P/m_cm<1350 J/kg) and then compresses (at 1350 J/kg <P₀ΔV_P/m_cm<10,000 J/kg) the space between metaplast clusters, making the N₂ adsorption specific surface area of char increase first and then decrease. After the gaps between metaplast clusters are filled, the specific surface area changes are minimal. Gasification generates new defect regions in clusters and reduces the clusters in char, making the specific surface area of char first increase and then decrease.     

Quantifying the effect of CO2 gasification on pulverized coal char oxy-fuel combustion

Previous research has provided strong evidence that CO₂ and H₂O gasification reactions can provide non-negligible contributions to the consumption rates of pulverized coal (pc) char during combustion, particularly in oxy-fuel environments. Fully quantifying the contribution of these gasification reactions has proven to be difficult, due to the dearth of knowledge of gasification rates at the elevated particle temperatures associated with typical pc char combustion processes, as well as the complex interaction of oxidation and gasification reactions. Gasification reactions tend to become more important at higher char particle temperatures (because of their high activation energy) and they tend to reduce pc oxidation due to their endothermicity (i.e. cooling effect). The work reported here attempts to quantify the influence of the gasification reaction of CO₂ in a rigorous manner by combining experimental measurements of the particle temperatures and consumption rates of size-classified pc char particles in tailored oxy-fuel environments with simulations from a detailed reacting porous particle model. The results demonstrate that a specific gasification reaction rate relative to the oxidation rate (within an accuracy of approximately +/- 20% of the pre-exponential value), is consistent with the experimentally measured char particle temperatures and burnout rates in oxy-fuel combustion environments. Conversely, the results also show, in agreement with past calculations, that it is extremely difficult to construct a set of kinetics that does not substantially overpredict particle temperature increase in strongly oxygen-enriched N₂ environments. This latter result is believed to result from deficiencies in standard oxidation mechanisms that fail to account for falloff in char oxidation rates at high temperatures.     

The effect of CO on CO2-char gasification

The effect of CO on the gasification of a Polish coal-derived char was investigated in a fluidised bed from 1123 to 1248 K. Rate expressions developed from Ergun's mechanism or a modified three-step reaction mechanism, coupled with Cylindrical Pore Interpolation Model (CPIM) to account for the intra-particle mass transfer, were developed to predict gasification and the effect of CO. Compared to the Ergun's rate expression, the three-step expression has an extra term for p_co, making the inhibition effect of CO more pronounced. The agreement between experimental and numerical results was satisfactory for both models simulating gasification by CO₂/N₂. Then, when CO (1% or 3%) was intentionally introduced in the feed gas (CO₂/N₂), the gasification rates significantly decreased. It was found that the model based on Ergun's mechanism over-estimated the gasification rate, while the results from the model with the three-step mechanism agreed with the experimental data.     

Kinetics of coal char gasification in a carbon dioxide medium

Solid fuel samples with different carbon contents are gasified by successively subjecting to pyrolysis in argon and oxidation in carbon dioxide at various temperatures to determine the rate of the chemical reactions and the activation energy required for simulating and optimizing the operation of gas generators. The samples were prepared from bituminous coal, lignite, and anthracite of the Kuznetsk and Kansk-Achinsk coal basins. The gasification of coal char samples in a carbon dioxide medium at 900–1200°C is analyzed by thermogravimetry. The temperature dependences of the weight change rate and gasification time of coal char samples are measured and used to calculate the preexponential factor and activation energy of the carbon oxidation reaction. It is found that, with increasing oxidizing medium temperature from 900 to 1200°C, the gasification time of the coal char samples obtained from anthracite and bituminous coal decrease 8- and 22-fold, respectively. A physicomathematical model of coal char gasification in a fixed bed, with the oxidizing gas diffusing through the ash layer formed, is proposed.     

Mineral associations in coal and their transformation during gasification

The principle aim of this investigation was to determine the effect that minerals and mineral associations in dense medium coal fractions have on the ash fusion temperature (AFT) of coal, where the mineral matter associated with coal undergoes transformations during gasification. Samples from three coal sources used by Sasol for gasification were acquired and a comprehensive characterisation on all dense medium fractions was conducted, including proximate analyses, AFT, ash oxide analyses, XRD and Mössbauer spectroscopy. From the proximate analyses, the ash content was the highest for the higher density fractions, with an accompanying decrease in fixed carbon content, consistent with the XRD and Mössbauer analyses. From the ash oxide analyses, it was evident that at higher relative densities more Fe₂O₃ and SiO₂ were present than in the lower density samples. From XRD analyses, the low density fractions contained calcite and dolomite. Pyrite and quartz were found in the higher density fractions whereas kaolinite occurred in all density separated fractions. From the different techniques it follows that with the Fe-content high in high density fractions, the AFT was low. When Ca and other basic oxide levels were abundant in low density fractions, the AFT was low and when the SiO₂/Al₂O₃ ratio was high in high density fractions the AFT increased.     

Sulphur transformation during pyrolysis of an Australian lignite

Transformation of various sulphur forms, including inherent and added pyrite, sulphates (CaSO₄ and FeSO₄) and organic sulphur, during pyrolysis of an Australian lignite was studied using TGA, TGA-MS and a fixed bed reactor, supplemented by sulphur form analysis. It was shown that hydrogen sulphide (H₂S) and a small quantity of sulphur dioxide (SO₂) were released during the pyrolysis of the pyrite-lignite blend. However, only SO₂ was detected during the pyrolysis of the lignite with high pyrite content. Inorganic matter was found to help retaining some of the inorganic sulphur, including pyrite, in the char. Inherent sulphates decomposed at much lower temperatures than the added sulphates, releasing SO₂ rather than H₂S. The inherent sulphates in the lignite 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. The retention of organic sulphur in the high organic sulphur lignite was higher than in its acid washed lignite sample, due to the interaction between inherent inorganic matter with the organic sulphur retaining the organic sulphur in the solid phase. SO₂ was the only sulphur gas produced during pyrolysis of acid washed lignite. A comprehensive mechanism of sulphur transformation during pyrolysis of lignite was proposed.     

Transformation of organic and inorganic sulphur in a lignite during pyrolysis: Influence of inherent and added inorganic matter

The transformation of organic and inorganic sulphur during pyrolysis of a Western Australian lignite has been studied using several complimentary techniques. The lignite contains high moisture, oxygen, volatile matter, organic sulphur and inherent inorganic matter. FTIR and solid state ¹³C TOSS NMR spectroscopic studies revealed that the high oxygen content is attributed to the oxygenated functional groups, which is saturated with organically bonded inherent inorganic matter. Sodium and chlorine are the dominant inorganic matter and present in the form of sodium chloride. TGA-MS studies of the raw lignite showed that SO₂ is the dominant sulphur-bearing gases evolved during the pyrolysis of the raw lignite. The effect of inherent and added inorganics was studied by comparing the amounts of various forms of sulphur retained in the chars of the raw lignite, the acid washed lignite and the acid washed lignite doped with sodium and kaolinite following pyrolysis in a fixed-bed reactor in nitrogen. Inherent inorganic matter was shown to reduce the decomposition of organic sulphur but has little effect on the decomposition of inorganic sulphur. Sodium helps the retention of sulphur in the chars between 400 and 500 °C while kaolinite has a better ability to retain sulphur above 600 °C.     

Effect of CO2 gasification reaction on oxy-combustion of pulverized coal char

For oxy-combustion with flue gas recirculation, as is commonly employed, it is recognized that elevated CO₂ levels affect radiant transport, the heat capacity of the gas, and other gas transport properties. A topic of widespread speculation has concerned the effect of the CO₂ gasification reaction with coal char on the char burning rate. To give clarity to the likely impact of this reaction on the oxy-fuel combustion of pulverized coal char, the Surface Kinetics in Porous Particles (SKIPPY) code was employed for a range of potential CO₂ reaction rates for a high-volatile bituminous coal char particle (130 μm diameter) reacting in several O₂ concentration environments. The effects of boundary layer chemistry are also examined in this analysis. Under oxygen-enriched conditions, boundary layer reactions (converting CO to CO₂, with concomitant heat release) are shown to increase the char particle temperature and burning rate, while decreasing the O₂ concentration at the particle surface. The CO₂ gasification reaction acts to reduce the char particle temperature (because of the reaction endothermicity) and thereby reduces the rate of char oxidation. Interestingly, the presence of the CO₂ gasification reaction increases the char conversion rate for combustion at low O₂ concentrations, but decreases char conversion for combustion at high O₂ concentrations. These calculations give new insight into the complexity of the effects from the CO₂ gasification reaction and should help improve the understanding of experimentally measured oxy-fuel char combustion and burnout trends in the literature.     

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.     

High-temperature pyrolysis of petroleum coke and its correlation to in-situ char-CO2 gasification reactivity

The understanding of the pyrolysis behavior of petroleum coke (PC), a by-product of oil refining, is very critical to its energy utilization. Different from most previous work, this investigation particularly focused on PC pyrolysis at high temperatures > 1273 K. The CO₂ gasification reactivity of in-situ char, rather than quenched char, was evaluated and correlated to PC pyrolysis behavior at different temperatures. A Chinese PC with sizes below 100 μm was tested on a high-temperature thermogravimetric analyser (TGA). Three sets of tests were carried out for different purposes. The first set was to simultaneously obtain sample mass loss and gas evolution data during pyrolysis through thermogravimetry-mass spectrometry (TG-MS). The second set aimed to collect char samples for subsequent analyses by scanning electron microscopy (SEM) and Raman spectrometry. The third set was to evaluate in-situ char-CO₂ gasification reactivity through the TGA. The results showed that, in addition to the commonly-observed primary pyrolysis stage at low temperatures, there was a secondary PC pyrolysis stage at high temperatures > 1300 K. In this process, the gases such as HCN, CO₂ and SO₂ were significantly released. The observed changes of char morphology suggested a four-staged thermoplastic transformation of the PC during pyrolysis, which has little been discussed previously. At different stages, i.e. softening, plasticizing, resolidification and graphitization, the rate of carbon ordering was different. The in-situ char-CO₂ gasification reactivity was found to first increase, then decrease and finally increase again with increasing temperature. Such changes coincided with the thermoplastic state of the pyrolyzed char, but not with the changes of char surface area or carbon ordering. The obtained knowledge is new and highlights the potentially important roles of char thermoplastic state in determining its reactivity towards CO₂.     

Attrition of lignite char under fluidized bed gasification conditions: The effect of carbon conversion

The effect of carbon conversion on the attrition of lignite char particles during fluidized bed gasification by CO₂ was studied in a lab-scale apparatus. The influence of bed temperature and inlet CO₂ concentration on carbon conversion and, consequently, on attrition was studied. The mechanical resistance of the char particles was also characterized at different stages of char conversion by specific attrition experiments. A predictive kinetic model for CO₂ gasification of the lignite char was developed from the experimental results, that was able to correctly predict the evolution of carbon conversion versus time. On this basis a semi-empirical model was developed in order to simulate the evolution of carbon elutriation rate with carbon conversion degree, i.e. the gasification-assisted attrition enhancement effect.     

Effects of Ca and Na acetates on nitrogen transformation during sewage sludge pyrolysis

The study reports the effects of Ca and Na acetates on the transformation of nitrogen species during sewage sludge pyrolysis. Sludge samples, with or without acetates, were pyrolysed in a fix-bed reactor at 150–550 °C, and the nitrogen species in the pyrolysis products (char, tar, and gas) were characterised and quantified. Ca and Na acetates distinctly affect nitrogen transformation during sludge pyrolysis, which is ascribable to their different catalytic activities for the decomposition of nitrogen species in sludge. The addition of Ca acetate is found to increase nitrogen retention in char and reduce the formation of nitrogen species in tar, which is mainly due to the suppressed decomposition of protein-N as well as the promoted formation of stable nitrogen species in char. On the other hand, the addition of Na acetate enhances the decomposition of nitrogen species in sludge, such as protein- and inorganic-N. The levels of both Ca and Na acetates are significantly reduced in the nitrogen-containing gas emissions because acetone is produced when acetates are heated, and acetone readily reacts with NH₃ to produce binary clusters or amines. Our results show that acetate addition is an important strategy for the reduction of NH₃ emission during sludge pyrolysis.     

A novel single particle study of steam gasification kinetics of a coal-derived char at high temperatures and pressures

A novel single particle experiment was developed to allow for detailed char gasification measurements in pure steam at temperatures from 1000 °C to 1400  °C and pressures from 1  bar to 15  bar. A coal-derived activated carbon was thoroughly characterized with respect to its composition, physical structure, and reactivity revealing properties consistent with chars reported in the literature. The single particle approach allowed for the boundary and initial conditions to be well known and for the mass of the particle to be accurately measured before and after testing to provide high-quality conversion data. The resulting conversion data were analyzed using the random pore model and the shrinking core model, of which the random pore model provided the best fit. Apparent activation energies were calculated using the random pore model which provided values ranging from 57.1 kJ/mol to 129 kJ/mol which are nominally half of the magnitude of the values reported in the literature under kinetically controlled conditions, thus demonstrating that regime II conditions were present. Additionally, the activation energies decreased with increasing temperature further demonstrating the presence of regime II conditions. The calculated reaction order ranged from 0 to 0.5 and decreased with increasing pressure agreeing well with literature values and trends.     

Primary release and transformation of inorganic and organic sodium during fast pyrolysis of sodium-loaded lignin

This study employs a wire-mesh reactor (WMR) to understand the primary release and transformation of inorganic and organic sodium during fast pyrolysis of various sodium-loaded lignin samples at 300–800 °C. Due to the minimization of volatile-char interactions in WMR, the overall sodium release during lignin pyrolysis is relatively low, i.e., ∼9–11% and ∼7–14% for the inorganic and inorganic sodium loaded lignin, respectively. The presence of the inorganic sodium in the condensed volatiles (so-called oil) clearly indicates the important role of thermal ejection in the release of the inorganic sodium, since sodium salts are unlikely to evaporate under current conditions. While the release of the organic sodium into oil can be due to both thermal ejection of aerosols and evaporation of low carboxylates. Despite the low sodium release, significant transformation of the inorganic and organic sodium can take place during lignin pyrolysis. For the inorganic sodium loaded lignin, the inorganic sodium decreases continuously from ∼67% at 300 °C to ∼42% at 800 °C, accompanied by a steady increase in the organic sodium (i.e., the ion-exchangeable sodium) from ∼17% at 300 °C to ∼37% at 800 °C. While for the organic sodium loaded lignin, its transformation into the inorganic sodium is faster at higher temperatures, leading to a large increase in the inorganic sodium (i.e., carbonates) from ∼9% at 300 °C to ∼48% at 800 °C, as well as a reduction in the organic sodium from ∼79% at 300 °C to ∼28% at 800 °C. The data generated in this study will be important to understand the catalytic mechanism of sodium during thermochemical processing of alkali lignin for the production of bioenergy and biofuels.     

Chemical transformation of iron compounds during the process of coal pyrolysis in steam

When the content of sulphur in hard coal is higher than 1.5% weight, such coal cannot be used as fuel in thermal-electric power stations due to environmental protection. Such coal can be used as a raw product in various chemical processes in which many valuable substances are obtained. This study is a small part of a major project looking for improved ways of utilizing sulphur-rich coals. Mössbauer measurements were carried out for semi-cokes obtained from sulphur-rich coal in the pyrolysis process in a steam atmosphere in the temperature range of 300–800 °C. Results of these measurements showed that during the pyrolysis process, pyrite transformed completely into iron oxides (magnetite and hematite), whereas illite transformed only partly even at the highest temperatures. Unfortunately, it was not possible to identify some products of the illite transformation.     

The slagging propensity of Zn-rich scrap tyre ash in reducing environments consisting of CO and CO2 and its promotion of the slagging of Si-rich coal ash

This study includes a comprehensive investigation on the ash slagging characteristics of zinc (Zn)-rich tyre ash and its interaction with silica (Si)-rich coal ash blended at different ratios in a 5% CO in CO₂ reducing environment. Such an environment is expected to occur near the burner in a combustor, as well as serves as a fundamental study on the slagging propensity upon the effect of CO, which is a principal reducing agent in a gasifier. An approach consisting of experimental studies using modified inclined plane (M-IP) methodology, advanced analytical techniques including the scanning electron microscopy (SEM) and x-ray photoelectron (XPS) spectroscopy, and thermodynamic equilibrium calculations were used to elaborate the slagging propensities of the single and blended ashes. Furthermore, the fate of Zn within tyre ash and its potential implications on the slagging properties were evaluated. Tyre ash easily melts to liquid slag upon increasing temperature compared to the silica-rich coal ash. Zinc does not evaporate but rather remains within tyre slag mostly as a hemimorphite (Zn₄(Si₂O₇)(OH))-like coordination structure attributing to the characteristic blue colour of tyre slag. The low ionic potential value of 2.7 for Zn²⁺ is characteristic of basic ions that have a greater tendency to depolymerise the Si-rich ash slag matrix and acts as a fluxing agent in ash slag. Moreover, it was proven that Zn²⁺ plays a synergistic role with Ca²⁺ in enhancing the flowability of the Si-rich slag remarkably. Furthermore, for the pure tyre slag, Zn was found to preferentially deposit at the corundum (Al₂O₃) substrate interface and even penetrate/react with the porous corundum, causing a strong corrosion on the substrate. However, blending of tyre ash with coal ash alleviates this problem by a preferred scavenging of Zn into the entire Si matrix, which in turn protects the corundum plate.     

Iron-carbon nanoparticles prepared by CO2 laser pyrolysis of toluene and iron pentacarbonyl

CO₂ laser induced co-pyrolysis of toluene and iron pentacarbonyl in the presence of an ethylene sensitizer was used to produce iron-carbon nanostructures containing cementite Fe₃C as the major component. The passivated Fe-C nanocomposites were characterized by several complementary analytical methods. Good agreement is found between the results of X-ray diffraction, Mössbauer spectroscopy and high-resolution transmission electron microscopy techniques which show that besides cementite, iron, and iron oxides, traces of other carbides are also present. Specific morphological aspects of the nanograins encased in a mostly disordered and quasi-amorphous carbon matrix are revealed. The simultaneous presence of rather small crystallites (mean diameter between 3–6 nm), identified as possible Fe₃C/α-Fe and iron oxide (maghemite/magnetite) phases and of single-phase larger crystallites (10–13 nm mean diameter), identified as Fe₃C is illustrated. Raman spectroscopy seems to confirm maghemite as the iron oxide phase present in the iron-carbon nanopowders. The level of oxidation mainly induced by powder passivation is roughly estimated by FTIR spectroscopy and leads to iron oxide contents between 11–17 wt.?%. The catalytic role of iron nanoparticles in the pyrolyzed system is addressed in connection with nanocarbon samples obtained in the absence of an iron donor.