An intrinsic kinetics model to predict complex ash effects (ash film,dilution, and vaporization) on pulverized coal char burnout in air (O2/N2) and oxy-fuel (O2/CO2) atmospheres

During coal combustion, char chemical reaction is the slowest step, particularly in the last burnout stage, where the char consists of small amounts of carbon in a predominant ash framework. Existing kinetics models tend to deviate from experimental measurements of late char burnout due to the incomplete treatment of ash effects. Ash can improve pore evolution through vaporization, hinder oxygen transport by forming an ash film, and reduce active carbon sites and available surface per unit volume by penetrating into the char matrix. In this work, a sophisticated kinetics model, focusing on these three ash evolution mechanisms (ash vaporization, ash film, and ash dilution) during pulverized coal (PC) char combustion, is developed by integrating them into a thorough mechanistic picture. Further, a detailed comparison of the three distinct ash effects on PC char conversion during air (O₂/N₂) and oxy-fuel (O₂/CO₂) combustion is performed. For the modeled coal, the mass of ash vaporization is approximate 3 orders less than the mass of ash remaining, which participates in ash dilution and ash film formation, both in O₂/N₂ and O₂/CO₂ atmospheres. The influence of these phenomena on burnout time follows the order: ash dilution > ash film > ash vaporization. The influence of ash vaporization on burnout time is minor, but through interactions with the ash dilution and ash film forming processes it can have an impact at high extents of burnout, particularly in O₂/CO₂ atmospheres. In O₂/N₂ atmospheres the residual ash predominately exists as an ash film, whereas it mainly exists as diluted ash in the char matrix in O₂/CO₂ atmospheres. The residual ash particle is encased by a thick film when the ash film forming fraction is high (low ash dilution fraction). These results provide in-depth insights into the conversion of PC char and further utilization of the residual ash.     

The influence of char surface oxidation on thermal annealing and loss of combustion reactivity

Thermal annealing associated with heat treatment of coal chars affects gasification reactivity and levels of unburned carbon in residual ash from coal-fired furnaces. The present study addresses the effect of char surface oxidation, occurring upon exposure to oxygen, on the course of thermal annealing, and related loss of combustion reactivity. This goal is pursued by comparing the extent of thermal annealing suffered by coal char upon heat treatment in a nitrogen atmosphere with that of chars that underwent oxidation prior to or during heat treatment. Oxidation of char was accomplished by supplying single or multiple pulses of air during the heat treatment, which were sufficient to oxidize the char surface but small enough to limit carbon gasification to less than 5%. The extent of thermal annealing was characterized both in terms of the loss of combustion reactivity and of the development of structural anisotropy of char samples, investigated by HRTEM. Results of the present study confirm that heat treatment reduces oxyreactivity of char samples, the effect being more pronounced at temperatures exceeding 1200 °C. Oxidation of samples mitigates the effects of heat treatment, as demonstrated by the smaller loss of gasification reactivity and by the more limited development of structural anisotropy of oxidized samples. Correspondingly, elemental analysis of samples indicates the formation of stable surface oxides upon oxidation, that are subsequently desorbed upon heat treatment. At temperatures exceeding 1200 °C, the effect of oxidation vanishes. Results are analysed and discussed in the light of the possible hindrance of thermal annealing due to the formation of stable surface oxides and of the parallel modifications occurring to the ash constituents.     

The influence of thermal annealing on oxygen uptake and combustion rates of a bituminous coal char

The effect of thermal annealing on the combustion reactivity of a bituminous coal char has been investigated with a focus on the role of the formation of surface oxides by oxygen chemisorption. The combined use of thermogravimetric analysis and of analysis of the off-gas during isothermal combustion of char samples enabled the determination of the rate and extent of oxygen uptake along burn-off. Combustion was carried out at temperatures between 350 and 510 °C. Char samples were prepared by controlled isothermal heat treatment of coal for different times (in the range between 1 s and 30 min) at different temperatures (in the range 900–2000 °C). Results indicate that oxygen uptake is extensive along burn off of chars prepared under mild heat treatment conditions. The maximum oxygen uptake is barely affected by the combustion temperature within the range of combustion conditions investigated. The severity of heat treatment has a pronounced effect on char combustion rate as well as on the extent and rate at which surface oxides are built up by oxygen chemisorption. Chars prepared under severe heat treatment conditions show negligible oxygen uptake and strongly reduced combustion rates. Altogether it appears that a close correlation can be established between the extent and the accessibility of active sites on the carbon surface and the combustion rate. Despite the investigation has been carried out at temperatures well below those of practical interest, results provide useful insight into the relationship existing between thermal annealing, formation of surface oxide and combustion reactivity which is relevant to the proper formulation of detailed kinetic models of char combustion.     

An improved model of fine particulate matter formation coupling the mechanism of mineral coalescence and char fragmentation during pulverized coal combustion

An improved model of fine particulate matter formation coupling the mechanism of mineral coalescence and char fragmentation under different pulverized coal combustion environments has been constructed. Firstly, based on the theoretical model of char fragmentation and percolation, the included minerals with different types and particle sizes are constructed in the model, and a three-dimensional char particle sub-model is established. And the type, content and particle size distribution of included minerals are introduced as input parameters by using computer controlled scanning electron microscopy (CCSEM) technology. All of the above makes it more in line with the actual distribution of the included minerals. Then a sub-model of char fragmentation is built based on the sub-model of the char particle. And considering the influence of char combustion reaction on the particle formation process and melting characteristics of included minerals, a sub-model of mineral melting coalescence under different combustion environments is established. Finally, based on this improved model, we compared the calculation results with the experimental data and the calculation results of the traditional model. Fully considering the process of mineral coalescence and char fragmentation, which contains the characteristics of different included minerals, the results show that the newly established model has a good fitting effect for the experiment and is closer to the actual process of char particle combustion to generate particles. By the new model, the influence of the factors (mineral content, particle size distribution and porosity) on the formation of particulate matter is preliminarily analyzed.     

The effect of bulk gas diffusivity on apparent pulverized coal char combustion kinetics

Apparent char kinetic rates are commonly used to predict pulverized coal char burning rates. These kinetic rates quantify the char burning rate based on the temperature of the particle and the oxygen concentration at the external particle surface, inherently neglecting the impact of variations in the internal diffusion rate and penetration of oxygen. To investigate the impact of bulk gas diffusivity on these phenomena during Zone II burning conditions, experimental measurements were performed of char particle combustion temperature and burnout for a subbituminous coal burning in an optical entrained flow reactor with helium and nitrogen diluents. The combination of much higher thermal conductivity and mass diffusivity in the helium environments resulted in cooler char combustion temperatures than in equivalent N₂ environments. Measured char burnout was similar in the two environments for a given bulk oxygen concentration but was approximately 60% higher in helium environments for a given char combustion temperature. To augment the experimental measurements, detailed particle simulations of the experimental conditions were conducted with the SKIPPY code. These simulations also showed a 60% higher burning rate in the helium environments for a given char particle combustion temperature. To differentiate the effect of enhanced diffusion through the external boundary layer from the effect of enhanced diffusion through the particle, additional SKIPPY simulations were conducted under selected conditions in N₂ and He environments for which the temperature and concentrations of reactants (oxygen and steam) were identical on the external char surface. Under these conditions, which yield matching apparent char burning rates, the computed char burning rate for He was 50% larger, demonstrating the potential for significant errors with the apparent kinetics approach. However, for specific application to oxy-fuel combustion in CO₂ environments, these results suggest the error to be as low as 3% when applying apparent char burning rates from nitrogen environments.     

多相燃烧分形模型及其实验研究

本文提出了多相燃烧的分形模型．模型中认为在多相燃烧中内部孔洞体积与表面积存在分维指数关系，而且反应面积的增长为两种分形增长模式的叠加．结合孔洞合并的因素得到了描述煤多相反应速率的分形模型．该模型描述的反应速率先增加后减少的规律与实验结果十分相符．对五个煤种和其中两个煤焦样品进行了试验研究，在两个不同升温速率下得出的试验数据与理论计算相符．     

Char characteristics and particulate matter formation during Chinese bituminous coal combustion

The characteristics of char particles and their effects on the emission of particulate matter (PM) from the combustion of a Chinese bituminous coal were studied in a laboratory-scale drop tube furnace. The raw coal was pulverized and divided into three sizes, <63, 63–100, and 100–200 μm. These coal samples were subjected to pyrolysis in N₂ and combusted in 20 and 50% O₂ at 1373, 1523, and 1673 K, respectively. Char samples were obtained by glass fiber filters with a pore size of 0.3 μm, and combustion-derived PM was size-segregated by a low pressure impactor (LPI) into different sizes ranging from 10.0 to 0.3 μm. The characteristics of char particles, including particle size distribution, surface area, pore size distribution, swelling behavior and morphology property, were studied. The results show that, coal particle size and pyrolysis temperature have significant influence on the char characteristics. The swelling ratios of char samples increase with temperature increasing from 1373 to 1523 K, then decrease when the temperature further increases to 1623 K. At the same temperature, the swelling ratios of the three size fractions are markedly different. The finer the particle size, the higher the swelling ratio. The decrease of swelling ratio at high temperature is mainly attributed to the high heating rate, but char fragmentation at high temperature may also account for the decrease of swelling ratio. The supermicron particles (1–10 μm) are primarily spherical, and most of them have smooth surfaces. Decreasing coal particle size and increasing the oxygen concentration lead to more supermicron-sized PM formation. The influence of combustion temperature on supermicron-sized PM emission greatly depends on the oxygen concentration.     

The effect of potassium bromide and sodium carbonate on coal char combustion reactivity

The addition of halogens, particularly iodine, to the gas during coal char oxidation has been used in previous studies to quench gas-phase chemistry, thereby allowing one to separate the effects of homogeneous and heterogeneous reactions. Halogen addition suppresses the gas-phase radicals to near-equilibrium levels. A similar effect can be expected from other compounds with high efficiency as fire suppressants, such as alkali metals. The effectiveness of the use of additives in distinguishing homogeneous and heterogeneous reactions during char oxidation relies on the assumption that radicals are suppressed while heterogeneous reactions occurring on the char surface are not affected. The present work tests this assumption for potassium bromide (KBr) and sodium carbonate (Na₂CO₃) reacting with a pulverized eastern bituminous coal char during oxidation. An increase in CO and a slight reduction in particle temperature were observed with the addition of KBr, consistent with known effects of halogens on gas-phase chemistry. An increase in particle size was also observed with the KBr addition. This observation and the results of model calculations suggest that there is significant incorporation of liquid KBr on the char surface under the conditions examined. With Na₂CO₃ addition, the particle temperature did not change, the particle size showed a slight decrease, and CO production increased. Although the mechanisms for Na interaction with radicals at combustion conditions are not well established, char oxidation modeling suggests that a decrease in OH concentration in the particle boundary layer is the cause for the observed increase in CO production. It is concluded that Na₂CO₃ has clear advantages over KBr for inhibiting gas-phase chemistry without affecting char oxidation for the conditions investigated here.     

O_2/CO_2气氛燃煤半焦孔隙特性分析

在沉降炉上制备了不同燃烧气氛、不同燃尽程度的半焦,采用低温氮吸附仪和扫描电子显微镜测定了其孔隙结构和表面形态.结果表明,所取的半焦试样均具有完整且连续的孔结构体系;但在相同的操作条件下,O_2/CO_2气氛下半焦试样的孔结构参数及其分形维数均小于相同O_2浓度的O_2/N_2气氛下的情况;两种气氛下煤焦的燃尽过程中,孔隙结构参数(S_(BET)、V_(BJH)和d_(pore))随燃尽率的增加均呈减小趋势;SEM图像的定性分析结果与N_2吸附的定量测量吻合较好.研究结果为深入认识O_2/CO_2气氛下煤粉的孔隙结构与其燃烧特性的关系提供了基础.     

A mechanistic study on kinetic compensation effect during low-temperature oxidation of coal chars

This paper provides mechanistic insights into the low-temperature oxidation of a range of carbon materials (graphite, a sub-bituminous coal char, and a brown coal char). Kinetic analysis was carried out on oxidation of the chars, prepared from fast-heating pyrolysis, under chemical-reaction-controlled regime. FT-Raman spectroscopic analysis was adopted to provide direct structural information on the carbon structure of reacting carbon materials throughout oxidation. The results demonstrate the significance of selective oxidation under the conditions, and parallel to this, the kinetic compensation effect of carbon oxidation reaction throughout conversion for all samples. Supported by the results from FT-Raman spectroscopy, the kinetic compensation effect seems to be a result of the selective oxidation of these carbon materials with heterogeneous carbon structures. Oxidation of all samples, with or without catalysts, appears to be similar in terms of the ‘nature’ of carbon structural condensation during low-temperature oxidation, suggesting a similar increase in apparent active sites population with respect to increase of apparent energy barrier. Under the current experimental conditions, a general kinetic compensation effect correlation has been deduced for various materials, requiring only the initial char kinetic parameters. The inherent inorganic species in chars also seem to alter the ‘degree/extent’ of carbon structural condensation as results of selective oxidation. In this case, the use of the compensation effect correlation will require more information on the catalysis during oxidation, apart from the initial char kinetic parameters.     

Mechanisms of the central mode particle formation during pulverized coal combustion

Ash particles produced from pulverized coal combustion are considered to be tri-modally distributed. These include the well-known ultrafine and coarse modes, and a central mode that is less reported but attracts increasing attention. This work presents a preliminary study on the formation mechanisms of the central mode particles during pulverized coal combustion. Experiments of four sized and density-separated coal samples were carried out in a laboratory drop-tube furnace under various controlled conditions. Experimental data show that the ash particle size distributions have an evident central mode at 4 μm for all coal samples. Increasing combustion temperature leads to an increase in the central mode particle formation, which is thought to be due to enhanced char fragmentation. The small-size coal sample produces a larger amount of the central mode particles, reasonably due to abundant fine particles in the parent coal sample. Under similar combustion conditions, both the Heavy (>2.0 g/cm³) and Light (<1.4 g/cm³) coal fractions produce a central mode, indicating that not only the included minerals but also the excluded minerals contribute to the formation of the central mode particles.     

煤焦燃烧过程中的亚观和微观形态及反应机理

通过FESEM和EBSP对煤粉在TGA、DTF和电站锅炉内燃烧过程中的亚观和微观形态及碳的含量变化的研究表明:炭的形态可分为5类:薄壁网架炭、厚壁网架炭、浅孔实体炭、实体炭和含碳矿物;同一煤样同一温度条件下炉内炭比TGA和DTF中的炭具有更大的反应比表面积;炉内炭的表面含碳量变化不大,不存在低温反应器的表面灰壳,因此,应用球形灰壳理论预报炉内煤粉的燃烧速率是值得怀疑的;亚观形态与微观形态间不存在几何分形上的自相似性.     

Optical Properties of Ag and Au Nanoparticles Dispersed within the Pores of Monolithic Mesoporous Silica

The optical absorption of silver and gold nanoparticles dispersed within the pores of monolithic mesoporous silica upon annealing at elevated temperatures has been investigated. With decreasing particle size, the surface plasmon resonance position of the particles blue-shifts first and then red-shifts for silver/silica samples, but only red-shifts for gold/silica samples. This size evolution of the resonance position is completely different from that previously reported for fully embedded particles. We assume a local porosity at the particle/matrix interface, such that free surface of particles within the pores may be in contact with ambient air, and present a two-layer core/shell model to calculate the optical properties. These calculations also consider deviations from the optical constants of bulk matter to account for corresponding effects below about 10 nm particle size. From the good agreement between experimental results and model calculations, we conclude a peculiar particle/ambience interaction dominating the size evolution of the resonance. Because of the difference of core electron structure, the relative importance of the effects of local porosity and free surface, respectively, are different for silver and gold. For silver, the effect of the local porosity is stronger, but for gold the opposite is found.     

Volatile–char interactions: Roles of in situ volatiles with distinctly-different chemistry in determining char structure and reactivity

This study reports the roles of volatiles with distinctly-different chemistry in determining char reactivity and char structure during in situ volatile–char interactions under non-catalytic conditions. Volatiles were generated in situ from polyethylene (PE), double-acid washed biosolid (DAWB), polyethylene glycol (PEG) or cellulose and interacted with char prepared from DAWB that is free of catalytically-active inorganic species in a two-stage reactor at 1000 °C. The experimental results show that both H- and O-containing reactive species play different roles during in situ volatile–char interactions. It has been found that char reactivity decreases substantially after in situ volatile–char interactions. Results from Raman analysis of the char after in situ interactions with the PE volatiles show H-containing reactive species substantially enhance the condensation of the aromatic ring systems within the char, thus slightly decreasing the H content in char and also making char carbon structure considerably less reactive. It has also been found that the reactivity of char after in situ volatile–char interactions increases with increasing O/H molar ratio of volatiles. The results indicate that O-containing reactive species in volatiles can react with char to form CO complex oxides that mitigate the carbon structure from condensing into large aromatic ring systems, thus increasing O and H contents in char and enhancing char reactivity.     

Positron beam studies on polyaniline and Ag-coated polyaniline

The effect of doping of the polyaniline emeraldine base (PEB), with Ni as well as Ni over layer coating has been investigated using variable low energy positron beam. Depth-resolved Doppler S-parameter measurements have been performed on undoped, Ni-doped polyaniline (PANI), and Ag (40 nm) film deposited PANI samples. Significant variation in S-parameter is observed for undoped and Ni-doped PANI. The size of the free volume hole has shifted to lower values upon doping with Ni as compared to that of undoped PANI, which is consistent with the conductivity measurements. For Ag-coated PANI systems, the S vs. E_p curves show distinct changes at the surface and interior regions. These results are discussed in the light of changes in free volume hole size distribution.     

煤焦燃烧反应动力学的通用规律研究

本文提出了有关煤焦燃烧反应动力学的新思想，即认为煤焦反应动力学参数Ｅ（活化能）与煤质无关，它只是煤焦温度的函数；但其反应频率因子ｋ０，ｃｈ与煤质有密切关系。本文中首次给出了煤焦燃烧反应动力学参数Ｅ／ｋ０，ｃｈ与煤的工业分析基值的通用关系。这样可以根据煤质的工业分析基值，就可获得一确定的Ｅ／ｋ０，ｃｈ值。从而避免了前人关于Ｅ／ｋ０，ｃｈ值与煤质无通用关系的状况。本文还揭示了反应频率因子不仅与煤质有关，而且还与其燃烧状态有关的物理本质，并给出了它们之间的定量关系。     

Mechanism on the contribution of coal/char fragmentation to fly ash formation during pulverized coal combustion

In this paper, the correlations between coal/char fragmentation and fly ash formation during pulverized coal combustion are investigated. We observed an explosion-like fragmentation of Zhundong coal in the early devolatilization stage by means of high-speed photography in the Hencken flat-flame burner. While high ash-fusion (HAF) bituminous and coal-derived char samples only undergo gentle perimeter fragmentation in the char burning stage. Simultaneously, combustion experiments of two kinds of coals were conducted in a 25?kW down-fired combustor. The particle size distributions (PSDs) of both fine particulates (PM_1-10) and bulk fly ash (PM₁₀₊) were measured by Electrical Low Pressure Impactor (ELPI) and Malvern Mastersizer 2000, respectively. The results show that the mass PSD of residual fly ash (PM₁₊) from Zhundong coal exhibits a bi-modal shape with two peaks located at 14?µm and 102?µm, whereas that from HAF coal only possesses a single peak at 74?µm. A hybrid model accounting for multiple-route ash formation processes is developed to predict the PSD of fly ash during coal combustion. By incorporating coal/char fragmentation sub-models, the simulation can quantitatively reproduce the measured PM₁₊ PSDs for different kinds of coals. The sensitivity analysis further reveals that the bi-modal mass distribution of PM₁₊ intrinsically results from the coal fragmentation during devolatilization.     

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.     

阶梯式循环冲击下煤中氢键红外光谱变化规律

煤结构中存在的大量氢键对其化学结构和性质具有重要影响。为了研究煤在冲击破坏过程中氢键的变化。设计落锤循环冲击系统,实现对煤样的循环定量冲击。利用红外光谱仪进行光谱分析。研究得出东庞1/3焦煤主要的氢键类型是free OH groups, OH…π, OH…OH, cyclic OH tetramers和OH…N以及它们随冲击破坏的变化规律。在冲击力的作用下,煤体分子间氢键吸收强度变化非常明显,自由羟基也在冲击作用下发生机械力化学反应。对红外光谱进行高斯函数拟合分峰,发现各类氢键吸收强度随着冲能量的增加而减小,但其趋势在放缓。统计两者关系,得到各类氢键的光谱吸收强度与冲击能量的拟合方程,两者符合幂函数关系。对比拟合方程的幂指数,得出氢键在受冲击破坏时发生变化的敏感性顺序：free OH groups>cyclic OH tetramers>OH…N>OH…π>OH…OH。     

燃烧过程中煤焦颗粒的逾渗特性变化

本论文根据分形理论及测定得到的准格尔褐煤在燃烧过程中颗粒的物性参数建立表征煤焦颗粒的分形体模型。 通过分形体上突入逾渗和普通逾渗模拟,研究在燃烧过程中由于颗粒内部分形体结构的改变造成的传输特性和反应特性 的变化情况。结果表明,建立的分形体模型能够很好的表征煤焦颗粒结构特征,逾渗模拟的结果与实际情况基本相符。