Thermogravimetric analysis of gasification reactivity of coal chars with steam and CO2 at moderate temperatures

Comparative study on the gasification reactivity of the three types of Chinese coal chars with steam and CO₂ at 850–1050 °C was conducted by isothermal thermogravimetric analysis. The effects of coal rank, pore structure, ash behavior, and gasification temperature on the gasification reactivity of coal chars were investigated. It is found that the gasification reactivity difference between different coal chars changes with reaction degree and gasification temperature, and has no immediate connection with coal rank and initial pore structure. Ash behavior plays an important role in the char reactivity, and changes with gasification temperature and reaction degree due to the variation in the compositions and relative amount. The influence of pore structure is more noticeable during a relatively moderate reaction process. The relative reactivity ratio of steam to CO₂ gasification generally decreases with the increasing temperature, and is related with the catalytic effect of inherent minerals. The characteristic parameters of the chars were analyzed, finding that the value of half reaction specific rate is approximate to the average specific rate under the same conditions. The nth-order distributed activation energy model is proposed to describe the coal char gasification process, and the results show that the activation energy increases with the increasing carbon conversion.     

烟煤焦在H2O和CO2气氛下的结构演变与气化反应性关联

本研究以烟煤在1000 ℃热解所制得的焦样为研究对象,考察了其在H₂O、CO₂及两者混合气氛下的结构演变,以及气化反应性的影响。为了探究焦样在气化过程中的结构演变,利用氮吸附、SEM和拉曼光谱等表征手段分析不同碳转化率下的焦样结构。结果表明,H₂O气氛对焦样结构的演变明显不同于CO₂气氛,揭示了焦样在两种气氛下的反应路径不同。因结构演变的不同,随碳转化率的增加,焦样在两种气氛下表现出不同的气化反应性能。在CO₂气氛下,焦样的气化反应速率随碳转化率的增加而逐渐降低,与H₂O气氛存在下变化趋势相反。在H₂O和CO₂共气化条件下,煤焦在H₂O和CO₂混合气氛下的反应速率高于单气氛下的反应速率的计算值,表现出一定的协同作用。这是因为焦样与H₂O反应能够产生较大的比表面积,为焦样与CO₂反应提供更多的反应场所,促进了焦样与CO₂的反应。     

Thermal stability of nanocrystalline ε-Fe2O3

Thermal behavior of highly crystalline ε-Fe₂O₃ nanoparticles of different apparent crystallite sizes was characterized using thermogravimetry, differential thermal analysis, and analysis of evolved gas by mass spectrometry. Phase composition of the samples was monitored ex situ by X-ray powder diffraction. The results show that the thermal stability of this metastable iron oxide polymorph decreases with increasing particle size. For the particle diameter of 19(2) nm, the transformation temperature was equal to 794(5) °C, while for 28(2) nm only 755(10) °C. Surface of the nanoparticles contained adsorbed water and carbon dioxide. Elimination of these species proceeds in two steps. Water is removed at temperatures below 200 °C and CO₂ in the temperature range between 200 and 450 °C.     

Inhibition of interaction between kaolinite and K2CO3 by pretreatment using calcium additive

The aim of this paper is to investigate an approach for inhibiting the interaction of kaolinite with K₂CO₃ via the pretreatment with calcium additive (Ca(OH)₂ or CaCO₃). The reactions between kaolinite, calcium additive, and K₂CO₃ were, therefore, examined by means of thermogravimetry, differential scanning calorimetry, X-ray diffraction analysis, and inductively coupled plasma atomic emission spectrometry. It was found that Ca(OH)₂ could interact with kaolinite at low temperature, although the bulk reaction indeed required a temperature of higher than 900 °C to afford a crystalline calcium aluminosilicate product. The addition of Ca(OH)₂ to kaolinite even without heat pretreatment was proved to be significantly resistant to the reaction of kaolinite with K₂CO₃, while CaCO₃ showed such a smaller effect. The high-temperature product, calcium aluminosilicate, exhibited an almost complete inactivity for reaction with K₂CO₃ at a typical temperature of catalytic coal gasification (750 °C).     

Thermogravimetric analysis of the water vapor addition during the CaO carbonation process at moderate temperatures (40–70 °C)

Experiments were performed on calcium oxide, using water vapor with N₂ or CO₂ as carrier gases, between 40 and 70 °C. A initial experiment was performed with water vapor in the presence of N₂ to elucidate the possible hydroxylation process produced by water vapor exclusively. On the other hand, when CO₂ was used as carrier gas the CaO reactivity changed, producing different hydrated, hydroxylated, and carbonated phases. On the basis of these results and the fact that under dry conditions CO₂ is not absorbed on CaO at T < 70 °C, a possible CaO–H₂O–CO₂ reaction mechanism was proposed, where CaO superficial hydroxylation process seems to play a very important role during the CO₂ capture. Finally, a kinetic analysis was produced to compare the temperature and humidity relative influence on the whole process.     

Gasification characteristics of coal char under CO2 atmosphere

The char gasification characteristics and the composition of evolved gases in a CO₂ environment have been studied using a thermogravimetric analyzer (TG) coupled with a mass spectrometer. Three types of coal char were studied: lignite (TXL), sub-bituminous (PRB), and bituminous (KYB). TG results showed that the reactivities of TXL and PRB were higher than that of KYB, and the reactivity of TXL was higher than that of PRB. The characterization of the chars implied that the mineral content in the char plays an important role in the reactivity and that the surface area and pore volume may accelerate the reactivity of chars. The evolved gases from three chars were mainly CO and SO₂. SO₂ was slightly delayed by CO during gasification of TXL and PRB chars, but for KYB, SO₂ and CO formed in the same temperature range, but at higher temperatures compared with TXL and PRB. The CO production of KYB was the best, 0.98 mg mg^?1; and SO₂ was the least, 0.031 mg mg^?1. PRB and TXL chars had similar CO production, but SO₂ in TXL was higher.     

Synthesis and properties of BaCe1−xYxO3−δ–BaWO4 composite protonic conductors

Barium cerate doped by trivalent rare earth metal ions is a potentially huge component of materials for electrochemical industry due to its high protonic conductivity. However, the poor chemical stability especially in the presence of CO₂, SO₂ or H₂O, resulting in decreasing the mechanical durability of obtained materials, limits their possible applications. The new approach towards stable ceramic protonic conductors with high electrical conductivity is presented. Thermal stability of yttrium doped (10 mol%) of BaCeO₃ was enhanced by forming the composite material BaCe_0.9Y_0.1O₃–BaWO₄ (10 mol% of BaWO₄). The synthesis was performed by solid-state reaction method. The detailed study of thermal decomposition of starting powders mixture was performed using thermogravimetry and differential thermal analysis (TG/DTA) techniques combined with Evolved Gas Analysis (EGA—mass spectrometry). Structure, phase composition and microstructure together with thermal stability of sintered materials were determined. The exposition tests were performed to characterise the stability of composites in carbon dioxide and water vapour-rich atmospheres. The samples were exposed to atmosphere containing CO₂/H₂O (7 % of CO₂ in air, 100 % RH) at temperature of 25 °C for 300 h. Thermal analysis supplied with mass spectrometry was applied to analyse the materials after the test. The results of this experiment showed better chemical resistance of composite material—BaCe_0.9Y_0.1O₃ with 10 mol% of BaWO₄ compared to single phase material.     

Pyrolysis of Hailar lignite in an autogenerated steam agent

An in situ pyrolysis process of high moisture content lignite in an autogenerated steam agent was proposed. The aim is to utilize steam autogenerated from lignite moisture as a reactant to produce fuel gas and additional hydrogen. Thermogravimetric analysis revealed that mass loss and maximum mass loss rate increased with the rise of heating rates. The in situ pyrolysis process was performed in a screw kiln reactor to investigate the effects of moisture content and reactor temperature on product yields, gas compositions, and pyrolysis performance. The results demonstrated that inherent moisture in lignite had a significant influence on the product yield. The pyrolysis of L _R (raw lignite with a moisture content of 36.9 %, wet basis) at 900 °C exhibited higher dry yield of 33.67 mL g^?1 and H₂ content of 50.3 vol% than those from the pyrolysis of the predried lignite. It was also shown that increasing reaction temperature led to a rising dry gas yield and H₂ yield. The pyrolysis of L _R showed the maximum dry yield of 33.7 mL g^?1 and H₂ content of 53.2 vol% at 1,000 °C. The LHV of fuel gas ranged from 18.45 to 14.38 MJ Nm^?3 when the reactor temperature increased from 600 to 1,000 °C.     

Dual mode model for mixed gas permeation of CO2, H2, and N2 through a dry chitosan membrane

Dry chitosan is an excellent candidate for facilitated transport membranes that can be utilized in industrial applications, such as fuel cell operations and other purification processes. This article is the first to report temperature effects on transport properties of CO₂, H₂, and N₂ in a gas mixture typical of such applications. At a feed pressure of 1.5 atm, CO₂ permeabilities increased (0.381–26.1 barrers) at temperatures of 20–150 °C with decreasing CO₂/N₂ (19.7–4.55) and CO₂/H₂ (3.14–1.71) separation factors. The pressure effect on solubilities and permeabilities were fitted to the extended dual mode model and its corresponding mixed gas permeation model. The dual mode and transport parameters, the sorption heats and the activation energies of Henry's and Langmuir's regimes and their pre‐exponential parameters were determined. The Langmuir's capacity constants were utilized to estimate chitosan's glass transition temperature (CO₂: 172 °C, N₂: 175 °C, and H₂: 171 °C). The activation energies of diffusion in the Henry's law and Langmuir regimes were dependent on the collision diameter of the gases. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2620–2631, 2007     

Pressurized CO<Subscript>2</Subscript>-enhanced gasification of coal

Carbon dioxide was considered as a co-gasifying agent in a coal gasification reactor. The work presented herein describes the simulation results for the process and the experimental data on coal char gasification with CO₂ addition as the rate-controlling step for the entire process. To study the potentially beneficial effect of the introduction of CO₂ into the gasification system, several simulations were conducted using the commercial process simulation software ChemCAD 6.3^®. The results of a Gibbs equilibrium reactor were evaluated. The Boudouard reaction is a critical path for the development of this process, and the kinetics were studied experimentally. Four chars derived from the pyrolysis of Polish coals of different origins were selected for the experiments. The kinetic characteristics of this system were examined using a custom-designed pressurized fixed-bed reactor. To determine the effect of pressure on the gasification rate, several preliminary studies on the gasification of coal chars were performed isothermally at the temperature of 950 °C and pressures of 1, 10, and 20 bars. In contrast to the thermodynamic calculations, the experimental data revealed that increasing the CO₂ pressure leads to a higher reaction rate for medium-rank coal chars and low-rank lignite coal char, resulting in higher efficiency for carbon monoxide production. The pressure influences the reactivity more strongly when varied from 1 to 10 bars; a further increase in pressure affects the rate almost insignificantly. The observed behavior representing the changes in carbon conversion degree during gasification is satisfactorily described by the grain model.     

The kinetics of gasification of char derived from sewage sludge

Gasification of char derived from sewage sludge was studied under different oxidizing atmospheres containing CO₂, O₂ or H₂O. The gasification tests were carried out in thermobalance at different temperatures and oxidizing reagent concentrations. The most efficient were the gaseous mixtures containing oxygen. The reaction took place at temperature 400–500 °C, whilst in the case of CO₂ and steam much higher temperatures (700–900 °C) were necessary to complete the conversion. Two rate models for gas–solid reaction were applied to describe the effect of char conversion on reaction rate. The shrinking core model for reaction-controlled regime was found to be the best for predicting the rate of char gasification in CO₂ and O₂ atmosphere. The experimental data for steam gasification of the char were fitted best by the first-order kinetics. The kinetic parameters estimated from the experimental data are in accordance with the literature for lignocellulosic char gasification and are the first published for sewage sludge char gasification.     

TG–MS analysis of thermal behavior and gaseous emissions during co-combustion of straw with municipal sewage sludge

The thermal behavior and gas product distribution during combustion of straw (wheat straw, corn stalks, and cotton stalks), municipal sewage sludge (MSS), and their blends were investigated by thermogravimetry–mass spectroscopy. The experiments were conducted with various blending ratios and temperatures ranging from 323 to 1,173 K. Addition of MSS decreased the combustion performance of the straw. The reactions between wheat straw and corn stalks with MSS proceeded more easily than that of cotton stalks. Significant interactions were observed between the straw and MSS at the char combustion stage. Gaseous species (CO₂, SO₂, NH₃, HCN, and NO) were mainly produced at temperatures of 523–873 K at which most of the mass loss occurred. Higher MSS proportions in the blends resulted in lower emissions peaks for CO₂, NH₃, HCN, and NO except for SO₂. To ensure combustion performance and mitigate problematic gaseous emissions, the proportion of MSS added to the blends should be <30 mass%.     

Fast pyrolysis of coals under N2 and CO2 atmospheres

Oxyfuel combustion represents one way for cleaner energy production using coal as combustible. The comparison between the oxycombustion and the conventional air combustion process starts with the investigation of the pyrolysis step. The aim of this contribution is to evaluate the impact of N₂ (for conventional air combustion) and CO₂ (for oxy-fuel combustion) atmospheres during pyrolysis of three different coals. The experiments are conducted in a drop tube furnace over a wide temperature range 800–1400 °C and for residence time ranging between 0.2 and 1.2 s. Coal devolatilized in N₂ and CO₂ atmospheres at low temperatures (?1200 °C) and longer residence times (>?0.5 s), the char-CO₂ reaction is clearly observed, whose intensity depends on the nature of the coal. Furthermore, the volatile yields are simulated using Kobayashi’s scheme and kinetic parameters are predicted for each coal. The char gasification under CO₂ is also accounted for by the model.

     

Effects of alumina phases on CO2 sorption and regeneration properties of potassium-based alumina sorbents

A range of potassium-based alumina sorbents were fabricated by impregnation of alumina with K₂CO₃ to examine the effects of the structural and textural properties of alumina on the CO₂ sorption and regeneration properties. Alumina materials, which were used as supports, were prepared by calcining alumina at various temperatures (300, 600, 950, and 1,200 °C). The CO₂ sorption and regeneration properties of these sorbents were examined during multiple tests in a fixed-bed reactor in the presence of 1 vol% CO₂ and 9 vol% H₂O. The regeneration capacities of the potassium-based alumina sorbents increased with increasing calcination temperature of alumina. The formation of KHCO₃ increased with increasing calcination temperature during CO₂ sorption, whereas the formation of KAl(CO₃)(OH)₂, which is an inactive material, decreased. These results is due to the fact that the structure of alumina by the calcination temperature is related directly to the formation of the by-product [KAl(CO₃)(OH)₂]. The structure of alumina plays an important role in enhancing the regeneration capacity of the potassium-based alumina sorbent. Based on these results, a new potassium-based sorbent using δ-Al₂O₃ as a support was developed for post-combustion CO₂ capture. This sorbent maintained a high CO₂ capture capacity of 88 mg CO₂/g sorbent after two cycles. In particular, it showed a faster sorption rate than the other potassium-based alumina sorbents examined.     

CO<Subscript>2</Subscript> Biofixation and Growth Kinetics of <Emphasis Type="Italic">Chlorella vulgaris</Emphasis> and <Emphasis Type="Italic">Nannochloropsis gaditana</Emphasis>

CO₂ biofixation was investigated using tubular bioreactors (15 and 1.5 l) either in the presence of green algae Chlorella vulgaris or Nannochloropsis gaditana. The cultivation was carried out in the following conditions: temperature of 25 °C, inlet-CO₂ of 4 and 8 vol%, and artificial light enhancing photosynthesis. Higher biofixation were observed in 8 vol% CO₂ concentration for both microalgae cultures than in 4 vol%. Characteristic process parameters such as productivity, CO₂ fixation, and kinetic rate coefficient were determined and discussed. Simplified and advanced methods for determination of CO₂ fixation were compared. In a simplified method, it is assumed that 1 kg of produced biomass equals 1.88 kg recycled CO₂. Advance method is based on empirical results of the present study (formula with carbon content in biomass). It was observed that application of the simplified method can generate large errors, especially if the biomass contains a relatively low amount of carbon. N. gaditana is the recommended species for CO₂ removal due to a high biofixation rate—more than 1.7 g/l/day. On day 10 of cultivation, the cell concentration was more than 1.7?×?10⁷ cells/ml. In the case of C. vulgaris, the maximal biofixation rate and cell concentration did not exceed 1.4 g/l/day and 1.3?×?10⁷ cells/ml, respectively.     

Preparation of carbon molecular sieves from palm shell: effect of benzene deposition conditions

Application of carbon molecular sieve (CMS) for gas separation has been found much attention recently. In this work, CMS was prepared from locally available palm shell through carbonization, steam activation and carbon vapour deposition (CVD) technique. After carbonization step, the char produced was subjected to steam activation at various activation times. The activated carbon obtained at 53.2% burn-off, which contain the highest amount of micropore volume was further used in CVD step by using benzene vapour at various deposition conditions. The performance of CMSs produced was examined by assessing the adsorption kinetics of O₂, N₂, CO₂ and CH₄ gases. All CMS samples showed a small N₂ and CH₄ uptake compared to the O₂ and CO₂. The suitable conditions for CVD were found at 800°C, 30 min and 30 vol% benzene of deposition temperature, time and benzene concentration, respectively. At this point the O₂/N₂ and CO₂/CH₄ uptake ratios arrived 7.1 and 16.0, respectively.     

Electrical conductivity of titanium pyrophosphate between 100 and 400 °C: effect of sintering temperature and phosphorus content

The synthesis of titanium pyrophosphate is carried out, and the material is sintered at different temperatures between 370 and 970 °C. Yttrium is added during the synthesis to act as acceptor dopant, but it is mainly present in the material in secondary phases. The conductivity is studied systematically as a function of sintering temperature, pH₂O, pO₂, and temperature (100–400 °C). Loss of phosphorus upon sintering above 580–600 °C is confirmed by energy dispersive spectroscopy and combined thermogravimetry and mass spectrometry. The conductivity decreases with increasing sintering temperature and decreasing phosphorus content. The highest conductivity is 5.3?×?10^?4 S cm^?1 at 140 °C in wet air (pH₂O?=?0.22 atm) after sintering at 370 °C. The conductivity is higher in wet atmospheres than in dry atmospheres. The proton conduction mechanism is discussed, and the conductivity is attributed to an amorphous secondary phase at the grain boundaries, associated with the presence of excess phosphorus in the samples. A contribution to the conductivity by point defects in the bulk may explain the conductivity trend in dry air and the difference in conductivity between oxidizing and reducing atmospheres at 300–390 °C. Slow loss of phosphorus by evaporation over time and changes in the distribution of the amorphous phase during testing are suggested as causes of conductivity degradation above 220 °C.     

Synthesis and thermal behaviour of silicon containing poly(ester imide)s

A series of silicon containing poly(ester imide)s [PEIs] were synthesized using novel vinyl silane diester anhydride (VSEA) and various aromatic and aliphatic dimines by two-step process includes ring-opening polyaddition reaction to form poly(amic acid) and thermal cyclo-dehydration process to obtain poly(ester imide)s. VSEA was synthesized by using dichloro methylvinylsilane and trimellitic anhydride in the presence of K₂CO₃ by nucleophilic substitution reaction. The PEIs were characterized by FTIR spectroscopy. The thermal properties of PEIs were investigated by using differential scanning calorimetry (DSC) and thermogravimetric analysis (TG) methods. The prepared PEIs showed glass transition temperatures in the range of 320–350°C and their 5% mass loss was recorded in the temperature range of 500–520°C in nitrogen atmosphere. These had char yield in the range of 45–55% at 800°C.     

Evolved gas analysis of coal-derived pyrite/marcasite

The products evolved during the thermal decomposition of the coal-derived pyrite/marcasite were studied using simultaneous thermogravimetry coupled with Fourier-transform infrared spectroscopy and mass spectrometry (TG-FTIR–MS) technique. The main gases and volatile products released during the thermal decomposition of the coal-derived pyrite/marcasite are water (H₂O), carbon dioxide (CO₂), and sulfur dioxide (SO₂). The results showed that the evolved products obtained were mainly divided into two processes: (1) the main evolved product H₂O is mainly released at below 300 °C; (2) under the temperature of 450–650 °C, the main evolved products are SO₂ and small amount of CO₂. It is worth mentioning that SO₃ was not observed as a product as no peak was observed in the m/z = 80 curve. The chemical substance SO₂ is present as the main gaseous product in the thermal decomposition for the sample. The coal-derived pyrite/marcasite is different from mineral pyrite in thermal decomposition temperature. The mass spectrometric analysis results are in good agreement with the infrared spectroscopic analysis of the evolved gases. These results give the evidence on the thermal decomposition products and make all explanations have the sufficient evidence. Therefore, TG–MS–IR is a powerful tool for the investigation of gas evolution from the thermal decomposition of materials.     

Solid-state synthesis of undoped and Sr-doped K<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>NbO<Subscript>3</Subscript>

The solid-state synthesis of undoped K_0.5Na_0.5NbO₃ (KNN) and KNN doped with 1, 2 and 6 mol% Sr, from potassium, sodium and strontium carbonates with niobium pentoxide, was studied using thermal analysis and in situ high-temperature X-ray diffraction (HT-XRD). The thermogravimetry and the differential thermal analyses with evolved-gas analyses showed that the carbonates, which were previously reacted with the moisture in the air to form hydrogen carbonates, partly decomposed when heated to 200 °C. In the temperature interval where the reaction was observed, i.e., between 200 and 750 °C, all the samples exhibited the main mass loss in two steps. The first step starts at around 400 °C and finishes at 540 °C, and the second step has an onset at 540 °C and finishes with the end of the reaction between 630 and 675 °C, depending on the particle size distribution of the Nb₂O₅ precursor. According to the HT-XRD analysis, the perovskite phase is formed at 450 °C for all the samples, regardless of the Sr content. The formation of a polyniobate phase with a tetragonal tungsten bronze structure was detected by HT-XRD in the KNN with the largest amount of Sr dopant, i.e., 6 mol% of Sr, at 600 °C.