Thermogravimetric kinetics on catalytic combustion of bituminous coal

The catalytic combustion and non-isothermal kinetics of bituminous coal by CeO₂, Fe₂O₃, and NiO were investigated. The exothermic characteristics during catalytic combustion of bituminous coal were determined. Based on the Coats-Redfern method by introducing the function of kinetics mechanisms, the activation energies and pre-exponential factors of catalytic combustion of bituminous coal were estimated iteratively by regression. It is found that the catalysts promoted the transport of oxygen to the coal or char surface and effectively improved the combustion characteristics of bituminous coal. Under the same experimental conditions, the exothermic values were significantly increased and the catalysts of composite oxides exhibited higher exothermic values than pure metal oxide catalysts. The metal oxides significantly reduced the activation energies of bituminous coal combustion. SEM analysis presented that combustion residues became more porous with the addition of the catalyst.     

An investigation of thermal behaviour of biomass and coal during co‐combustion using thermogravimetric analysis (TGA)

The thermal behaviour and kinetic analysis of biomass (cypress wood chips and macadamia nut shells) and Australian bituminous coal during combustion were studies using the thermogravimetric technique with four different heating rates under an air atmosphere. Each type of biomass was blended with coal at mass ratios (biomass:coal) of 95:5, 90:10, 85:15 and 80:20 to investigate the effect of coal as a supplementary fuel on thermal behaviour during the combustion process. Combustion of the individual samples and the blends took place in three steps comprising dehydration, devolatilisation and char oxidation. During co‐combustion, the thermal decomposition behaviour of the blends followed that of the weighted average of the individual samples in the blends. In kinetic analysis, thermal decomposition of biomass and coal appeared to take place independently, and thus, the activation energy of the blends can be calculated from that of the two components. No evidence for any significant synergetic effects or thermal interaction was found between either type of biomass and the coal during co‐combustion based on the lack of deviation from expected behaviour of the blends. Copyright © 2013 John Wiley & Sons, Ltd.     

Combustion characteristics of Turkish lignites at oxygen-enriched and oxy-fuel combustion conditions

Combustion and oxy-fuel combustion characteristics of two Turkish lignites (Orhaneli and Soma) were investigated by Thermogravimetric Analysis (TGA) method. Experiments were carried out under oxygen-enriched air and oxy-fuel combustion conditions with 21, 30, 40% oxygen concentrations. Three heating rates of 5, 10, and 20 °C/min were considered and the isoconversional kinetic methods of FWO, KAS, and Friedman were employed to estimate activation energies. The uncertainty assessment in obtaining the activation energy values was also considered. The obtained results indicated that the combustion of volatiles at both air and oxy-fuel conditions were approximately identical. However, at air combustion conditions, the decomposition of CaCO₃ took place at temperatures above 700 °C. This decomposition process was independent of the oxygen concentration and took place when the temperature reached to a certain threshold. The decomposition of CaCO₃ did not accomplish in oxy-fuel conditions as far as the temperature was higher than 900 °C. Combustion in oxy-fuel conditions had higher activation energy values comparing to conventional combustion atmosphere. The activation energy values were approximately unchanged at the start of combustion regardless of oxygen concentration or combustion atmosphere at about 165 kJ/mol and 150 kJ/mol for Orhaneli and Soma lignites, respectively. The apparent activation energies were higher at elevated oxygen concentrations. The uncertainties values related to FWO method were lower than KAS and Friedman methods. The calculated average uncertainty values were found to be at the range of 5–15% for most of the cases.     

Experimental study on ash deposition of Zhundong coal in oxy-fuel combustion

To facilitate the large-scale utilization of high-alkali and -alkaline earth metals (AAEMs) coals in power generation, the ash deposition behaviors of a typical Zhundong coal in oxy-fuel combustion were experimentally investigated using a drop tube furnace. A wall-temperature-controlled ash deposition probe by which the bulk gas temperature could be measured simultaneously was designed and employed in the experiments. The deposition tendencies, ash morphologies, chemical compositions of deposited ash particles were studied respectively under various oxygen concentrations, bulk gas temperatures, probe surface temperatures and probe exposure times. The experimental results revealed that the oxygen concentration had a significant influence on the deposition behavior during oxy-fuel combustion of high-alkali coal. Compared with air case, more fine ash particles were generated during the combustion of Zhundong coal in 21% O₂/79% CO₂ atmosphere but the deposition tendency was weaker. However, a higher oxygen concentration could aggravate the tendency of ash deposition. The high contents of iron (Fe), calcium (Ca), sulfur (S), and sodium (Na) in Zhundong coal could result in the generations of low-melting point compounds. Calcium in flue gas existed as CaO and was captured prior to SO₃ by the probe surface during the ash deposition process. At the initial 30 min of the ash deposition process, the dark spherical fine ash particles rich in Fe, Na, oxygen (O), and S were largely produced, while in the range of 60–90 min the light spherical fine ash particles with high contents of Ca, barium (Ba), O, and S were generated on the other hand. The deposition mechanisms at different stages were different and the melted CaO (BaO)/CaSO₄ (BaSO₄) would give rise to a fast growth rate of ash deposit.     

Effects of oxy-fuel condition on morphology and mineral composition of ash deposit during combustion of Zhundong high-alkali coal

Zhundong coalfield is a super-large coal reserve, with high-alkali feature exacerbating ash deposition. Oxy-fuel combustion technology could propel the clean utilization of Zhundong high-alkali coal. While the ash deposition behavior of high-alkali coal under oxy-fuel condition has yet to be sufficiently investigated. The present study compared the differences of ash deposits between oxy-fuel and air combustion, and also examined the effects of oxygen content on ash deposition mechanism, employing a drop-tube furnace equipped with a specially designed sampling probe and some analysis methods, such as X—ray diffraction equipment, simultaneous thermal analyzer, etc. Experimental results indicated that ash deposition was weaker, with fewer contents of sodium chloride, calcium sulphate and less agglomeration ash in oxy-fuel atmosphere compared to the air case with same oxygen content. The content of the ash particle distributed in the range of 0–40 μm was up to 60% under oxy-fuel condition. The first weight loss of ash deposits, around 850 °C, was put down to the decomposition of carbonate and the second one, about 1150 °C, was ascribed to the decomposition of the sulphate minerals in the thermal process. Ash deposition worsened with more large particles (>120 μm), as the oxygen content rose. Sodium chloride content reached 9.7% with 50% oxygen content. The present study not only focuses on the morphology and chemical components, but also probes into the thermal volatility of ash deposits, which benefits the further understanding of the ash deposition mechanism and utilization of Zhundong high-alkali coal during oxy-fuel combustion.     

Modelling of the process of coal dust combustion in a cyclone furnace

This study presents the concept of a cyclone furnace for coal dust oxy-fuel combustion and gasification.The results of numerical calculations for the combustion and gasification processes were also presented.     

秸秆类生物质燃烧动力学特性实验研究

生物质能的利用越来越受到重视。直接燃烧技术由于其操作简单、取材方便、成本适宜等特点是一种符合我国国情的生物质能利用方式。采用热重分析的研究方法,对水稻秸秆、玉米秸秆和玉米芯三种秸秆类生物质的燃烧动力学特性进行了实验,研究了不同升温速率、氧浓度对不同种类的秸秆生物质燃料燃烧动力学特性的影响,并对着火温度、燃烧稳定性、挥发分析出特性、燃烧特性指数等相关特性参数进行定量分析,为设计秸秆工业锅炉燃烧设备,合理选择生物质种类、优化燃烧、提高锅炉效率提供了理论支撑。     

生物质与煤共燃的燃烧特性研究

采用热重分析法，对5种生物质与煤以相同比例掺混后在不同升温速率下进行了共燃试验。研究表明，生物质的加入改善了煤的燃烧性能，且随升温速率的升高，着火温度乃呈下降趋势；各试样的挥发分最大释放速率、固定炭最大燃烧速率、燃尽温度均呈增加趋势，它们的燃烧特性均随升温速率的升高而变好。     

Conversions of fuel-N to NO and N2O during devolatilization and char combustion stages of a single coal particle under oxy-fuel fluidized bed conditions

The conversions of fuel-N to NO and N₂O during devolatilization and char combustion stages of a single coal particle of 7 mm in diameter were investigated in a laboratory-scale flow tube reactor under oxy-fuel fluidized bed (FB) conditions. The method of isothermal thermo-gravimetric analysis (TGA) combing with the coal properties was proposed to distinguish the devolatilization and char combustion stages of coal combustion. The results show that the char combustion stage plays a dominant role in NO and N₂O emissions in oxy-fuel FB combustion. Temperature changes the trade-off between NO and N₂O during the two stages. With increasing temperature, the conversion ratios of fuel-N to NO during the two stages increase, and the opposite tendencies are observed for N₂O. CO₂ inhibits the fuel-N conversions to NO during the two stages but promotes those to N₂O. Compared with air combustion, the conversion ratios of fuel-N to NO during the two stages are lower in 21%O₂/79%CO₂, and those to N₂O are higher. At <O₂> = 21–50% by volume, the conversion ratios of fuel-N to NO during the two stages reach the maximum values at <O₂> = 30% by volume, and those to N₂O decrease with increasing O₂ concentration. H₂O suppresses the fuel-N conversions to NO and N₂O during the two stages. A higher coal rank has higher total conversion ratios of fuel-N to NO and N₂O. Fuel-N, volatile matter, and fixed carbon contents are the important factors on fuel-N conversions to NO and N₂O during the two stages. The results benefit the understanding of NO and N₂O emission mechanisms during oxy-fuel FB combustion of coal.     

TGA and kinetic study of different torrefaction conditions of wood biomass under air and oxy-fuel combustion atmospheres

Combustion and oxy-fuel combustion characteristics of torrefied pine wood chips were investigated by Thermogravimetric Analysis (TGA). Three torrefaction temperatures (250, 300, and 350 °C) and two residence times (15 and 30 min) were considered. Experiments were carried out at three heating rates of 10, 20, and 40 °C/min. The isoconversional kinetic methods of FWO, KAS, and Friedman were employed to estimate the activation energies. The assessment of uncertainty in obtaining the activation energy values was also considered. The obtained results indicated that due to torrefaction, the O/C and H/C atomic ratios decreased, resulting the 300ºC-30 min and 350ºC-15 min torrefied biomass to be completely embedded in lignite region in van-Krevelen's diagram. Oxy-fuel combustion affected the decomposition of cellulose and lignin components of biomass while the impact on the hemicellulose component was negligible. The kinetic analysis revealed that with the evolution of conversion degree, the activation energy values increased during hemicellulose degradation, remained approximately constant during cellulose decomposition and showed a sharp decrease for lignin decomposition. The activation energy trends were comparable in both air and oxy-fuel combustion conditions, however slight changes in activation energy values were noticed. The highest activation energy value was obtained for 250ºC-30 min torrefied biomass at 183.40 kJ/mol and the lowest value was 72.93 kJ/mol for 350ºC-15 min biomass. The uncertainty values related to FWO method were lower than KAS and Friedman methods. The uncertainty values for FWO and KAS methods were at the range of 5–15%.     

Comparative study on combustion and oxy-fuel combustion environments using mixtures of coal with sugarcane bagasse and biomass sorghum bagasse by the thermogravimetric analysis

Biomass and coal have different physicochemical properties and thermal behavior. During the co-combustion of coal-biomass mixtures, their thermal behavior varies according to the percentage of each fuel in the mixture. Thereby, this research aims to characterize the thermal behavior of mixtures of coal, sugarcane bagasse, and biomass sorghum bagasse as biomass in simulated combustion (O₂/N₂) and oxy-fuel combustion (O₂/CO₂) environments. Experiments have been performed in duplicate on a thermogravimetric analyzer at heating rate of 10 °C/min. A uniform granulometry was considered for all materials (63 μm) in order to ensure a homogeneous mixture. Four biomass percentages in the mixture (10, 25, 50 and 75%) have been studied. Based on thermogravimetric (TG) and thermogravimetric (DTG) analyses, parameters such as combustion index, synergism, and activation energy have been determined, as well as the combustion environment influence on these parameters. The results indicate that, although sugarcane bagasse has the lowest activation energy, the thermal behavior of both types of biomass is similar. Thus, biomass sorghum bagasse can be used as an alternative biomass to supply the power required during sugarcane off-season. For both mixtures, optimal results were obtained at 25% of biomass. By analyzing the environment influence on combustion behavior, the results indicate that when N₂ is replaced with CO₂, it is observed an increase in reaction reactivity, a higher oxidation rate of materials and an improvement in evaluated parameters.     

Thermal decomposition behavior of tobacco stem Part II: Kinetic analysis

As a continuation of the previous study on the thermal degradation behavior of tobacco stem, this work is focused on the kinetics of pyrolytic decomposition. Thermogravimetric analysis of tobacco stem samples was conducted under nitrogen atmosphere at different heating rates of 5, 10, 15, and 20°C/min at a temperature range of 25–1,000°C. The kinetic parameters, such as activation energy, pre-exponential factor, and reaction order, were determined by applying the Coats–Redfern method for the main pyrolysis occurred in the second zone by means of the decomposition of hemicellulose, cellulose, and lignin at a temperature range 180–540°C. In addition, the activation energy was calculated using various degradation models, including Kissinger, Friedman (FR), Flynn–Wall–Ozawa (FWO), and Kissinger–Akahira–Sunose (KAS). The average activation energy of tobacco stem was calculated to be 150.40, 230.76, 216.97, and 218.56 kJ/mol by the Kissinger, FR, FWO, and KAS models, respectively.     

关于影响煤燃烧固硫反应的主要因素及其机理的研究进展

燃烧过程中的脱硫是锅炉脱硫工艺的重要组成部分之一 ,已被广泛应用于各种流化床锅炉和煤粉炉中。为开发低成本、高效率的燃煤固硫技术 ,世界各国学者进行了大量的实验和机理性的研究。本文对这方面的研究进展做了总体回顾 ,并在前人研究的基础上提出了关于燃烧过程中固硫化学反应机理研究的发展趋势。     

Analysis of oxy-fuel combustion power cycle utilizing a pressurized coal combustor

Growing concerns over greenhouse gas emissions have driven extensive research into new power generation cycles that enable carbon dioxide capture and sequestration. In this regard, oxy-fuel combustion is a promising new technology in which fuels are burned in an environment of oxygen and recycled combustion gases. In this paper, an oxy-fuel combustion power cycle that utilizes a pressurized coal combustor is analyzed. We show that this approach recovers more thermal energy from the flue gases because the elevated flue gas pressure raises the dew point and the available latent enthalpy in the flue gases. The high-pressure water-condensing flue gas thermal energy recovery system reduces steam bleeding which is typically used in conventional steam cycles and enables the cycle to achieve higher efficiency. The pressurized combustion process provides the purification and compression unit with a concentrated carbon dioxide stream. For the purpose of our analysis, a flue gas purification and compression process including de-SO_x, de-NO_x, and low temperature flash unit is examined. We compare a case in which the combustor operates at 1.1 bars with a base case in which the combustor operates at 10 bars. Results show nearly 3% point increase in the net efficiency for the latter case.     

生物质水煤浆燃烧特性及动力学分析

采用热综合分析仪,利用TG-DTG热分析技术研究了以福建省无烟煤、水葫芦、添加剂制备的生物质煤浆的燃烧特性,并与煤粉的燃烧特性进行了对比。结果表明,生物质水煤浆的着火温度比煤粉低,并随着水葫芦添加量的增加呈降低趋势。动力学分析得出的表观活化能从大到小顺序为龙岩无烟煤、大田无烟煤、永安无烟煤、龙岩生物质水煤浆、大田生物质水煤浆、永安生物质水煤浆。     

Influence of biomass blends on the particle temperature and burnout characteristics during oxy-fuel co-combustion of coal

The difference in combustion performance between brown coal and black coal blended with Eucalyptus woodchip and woodchar in varying blending ratios were examined in the air and oxy firing conditions. On top of the experimental investigation using a drop tube furnace (DTF), a computational fluid dynamics (CFD) model was further developed to interpret these results, validated using the experimental data. The CFD model incorporates a comprehensive reaction for devolatilisation reaction to predict the gas release utilising predictions based on chemical percolation devolatilisation (CPD) model. The heterogeneous reactions are defined based on the intrinsic reaction model that accounts for the influence of char properties in chemical and pore diffusion reactions using a user-defined function (UDF). Moreover, the C–CO₂ gasification reaction rate which is critical in an oxy-firing mode was further studied using the CFD tool to determine how the role of gasification varied for various fuel blends. Based on carbon burnout and average particle temperature profiles, the blending of woodchips is highly beneficial to the overall combustion performance in particular for low reactive black coal while its effect on brown coal is marginal. Woodchar and black coal are comparable with similar temperature plots and relatively constant burnout but it behaves relatively inert with a highly reactive brown coal. During oxy firing, increasing the woodchip content enhanced the effect of C–CO₂ gasification due to its extremely large pre-exponential factor for the CO₂ gasification reactivity which explains the improved burnout. The blending of woodchar caused a gradual reduction in the gasification extent for both coals explained by the low heating rates under which woodchar was pyrolysed and also due to the decrease in the peak particle temperature. However, the observed gasification was found to be less than the expected value based on the linear addition of the two single fuels for both biomass blends.     

Effect of oxy-fuel combustion with steam addition on coal ignition and burnout in an entrained flow reactor

The ignition temperature and burnout of a semi-anthracite and a high-volatile bituminous coal were studied under oxy-fuel combustion conditions in an entrained flow reactor (EFR). The results obtained under oxy-fuel atmospheres (21%O₂-79%CO₂, 30%O₂-70% O₂ and 35%O₂-65%CO₂) were compared with those attained in air. The replacement of CO₂ by 5, 10 and 20% of steam in the oxy-fuel combustion atmospheres was also evaluated in order to study the wet recirculation of flue gas. For the 21%O₂-79%CO₂ atmosphere, the results indicated that the ignition temperature was higher and the coal burnout was lower than in air. However, when the O₂ concentration was increased to 30 and 35% in the oxy-fuel combustion atmosphere, the ignition temperature was lower and coal burnout was improved in comparison with air conditions. On the other hand, an increase in ignition temperature and a worsening of the coal burnout was observed when steam was added to the oxy-fuel combustion atmospheres though no relevant differences between the different steam concentrations were detected.     

煤气化置换燃烧分离CO2模拟分析与研究

基于Aspen plus软件,根据Gibbs自由能最小化原理对煤气化置换燃烧过程进行热力学平衡分析.对徐州烟煤以NiO作为载氧体,研究在常压时燃料反应器温度、水煤比对燃料反应器气体产率和载氧体循环倍率的影响.结果表明燃料反应器温度的降低有利于煤气化产物的氧化;水蒸气用量较少时可得到较高浓度的CO2燃料反应器温度的降低和水蒸气用量的减少将减少载氧体循环倍率.     

Sulphur impacts during pulverised coal combustion in oxy-fuel technology for carbon capture and storage

The oxy-fuel process is one of three carbon capture technologies which supply CO₂ ready for sequestration - the others being post-combustion capture and IGCC with carbon capture. As yet no technology has emerged as a clear winner in the race to commercial deployment. The oxy-fuel process relies on recycled flue gas as the main heat carrier through the boiler and results in significantly different flue gas compositions. Sulphur has been shown in the study to have impacts in the furnace, during ash collection, CO₂ compression and transport as well as storage, with many options for its removal or impact control. In particular, the effect of sulphur containing species can pose a risk for corrosion throughout the plant and transport pipelines. This paper presents a technical review of all laboratory and pilot work to identify impacts of sulphur impurities from throughout the oxy-fuel process, from combustion, gas cleaning, compression to sequestration with removal and remedial options. An economic assessment of the optimum removal is not considered. Recent oxy-fuel pilot trials performed in support of the Callide Oxy-fuel Project and other pilot scale data are interpreted and combined with thermodynamic simulations to develop a greater fundamental understanding of the changes incurred by recycling the flue gas. The simulations include a sensitivity analysis of process variables and comparisons between air fired and oxy-fuel fired conditions - such as combustion products, SO₃ conversion and limestone addition.     

Modeling of single coal particle combustion in O2/N2 and O2/CO2 atmospheres under fluidized bed condition

A one-dimensional transient single coal particle combustion model was proposed to investigate the characteristics of single coal particle combustion in both O₂/N₂ and O₂/CO₂ atmospheres under the fluidized bed combustion condition. The model accounted for the fuel devolatilization, moisture evaporation, heterogeneous reaction as well as homogeneous reactions integrated with the heat and mass transfer from the fluidized bed environment to the coal particle. This model was validated by comparing the model prediction with the experimental results in the literature, and a satisfactory agreement between modeling and experiments proved the reliability of the model. The modeling results demonstrated that the carbon conversion rate of a single coal particle (diameter 6 to 8 mm) under fluidized bed conditions (bed temperature 1088 K) in an O₂/CO₂ (30:70) atmosphere was promoted by the gasification reaction, which was considerably greater than that in the O₂/N₂ (30:70) atmosphere. In addition, the surface and center temperatures of the particle evolved similarly, no matter it is under the O₂/N₂ condition or the O₂/CO₂ condition. A further analysis indicated that similar trends of the temperature evolution under different atmospheres were caused by the fact that the strong heat transfer under the fluidized bed condition overwhelmingly dominated the temperature evolution rather than the heat release of the chemical reaction.