利用差示扫描量热法测定CFBC脱硫灰中残留碳的研究

借助差示扫描量热分析仪对煤与高硫石油焦混烧排放的CFBC(循环流化床)脱硫灰的残留碳含量进行了测定,研究表明:(1)CFBC脱硫灰的热反应过程主要分为水分和挥发分逸出,碳氧化和矿物分解及继续反应等三个阶段.(2)是否含有氢氧钙石对CFBC脱硫灰的热反应过程影响很大,含有氢氧钙石的CFBC脱硫灰在碳氧化和矿物分解阶段(约...     

The influence of carbon particle type in fly ashes on mercury adsorption

Recent research has shown that certain fly ash materials produced in coal combustion for power generation have an affinity for the mercury compounds present in flue gases. However, the exact nature of Hg-fly ash interactions is still unknown and the different variables that influence mercury adsorption need to be identified. In this work the microscopic components of fly ashes derived from the combustion of different types of feed blends of different coal rank and mercury adsorption were investigated. The aim of this research was to establish relationships between Hg retention and the type of unburned carbons present in various fly ashes. The fly ashes and fly ash fractions studied were used as sorbent beds for high mercury concentrations, conditions in which mercury retention is highly favored. From the results obtained it was confirmed that the role of the unburned carbon components in mercury capture may depend, among other factors, on the type of unburned carbon. Fly ashes capture different species of mercury depending on their nature and the type of anisotropic particles.     

Emission of Polycyclic Aromatic Hydrocarbons (PAH) from Solid Waste Incinerator Equipped with an After-Combustion Chamber

Abstract

Polycyclic aromatic hydrocarbons (PAH) are typical indicators of incomplete combustion during solid waste incineration. The PAH emissions caused by waste incineration vary according to waste composition and operating parameters such as furnace temperature, after-combustion conditions, excess air, carbon monoxide levels.

In this work, the content of PAH in the fly ashes (sampled upstream the flue gas treatment system) and bottom ashes produced by the combustion of a refuse derived fuel (RDF) in a rotary kiln incinerator equipped with an after-combustion chamber is determined. The emission levels of PAH are correlated with different operating conditions of the incinerator to evaluate the destruction efficiency of the after-combustion chamber for these compounds.     

Influence of iron species present in fly ashes on mercury retention and oxidation

The aim of this study was to assess the effect of iron species present in fly ashes produced from coal combustion on mercury retention and oxidation. To achieve this objective the work was divided into two parts. In the first part the relationship between the mercury and iron content in fly ashes of different origin and characteristics was evaluated. In the second, a series of fractions enriched in iron oxides were separated from the fly ashes to determine the effect of increasing iron content on mercury retention and oxidation. Special attention was paid to the influence of iron on mercury behavior in enriched carbon particles in fly ashes. From the results obtained it can be inferred that, in the range of fly ashes studied, iron species do not affect the retention of mercury and do not play any role in heterogeneous mercury oxidation.     

Burning and physico-chemical characteristics of carbon in ash from a coal fired power plant

The paper addresses the relationship between the chemico-physical properties and the residual combustion reactivity of fly ashes from a full-scale front fired PF coal boiler. Ashes collected at different rows of electrostatic precipitators (EP) have been characterized for their particle size distribution, morphology, chemical composition and combustion reactivity. The combustion time of carbon in ash has been estimated for a wide range of temperatures using a thermobalance and a heated strip reactor.Results showed the existence of marked differences in the content of both carbon and inorganic elements according to the row of EP and the granulometric size of the samples. In contrast with this, the combustion reactivity of all ash samples was similar regardless of their collection point and particle size. Ash reactivity resulted to be approximately 100 times lower than that of the parent coal.The role of thermal annealing on the low reactivity of fly ashes and their propensity to undergo additional thermodeactivation upon further heat treatment has also been investigated. To this end coal and fly ashes have been heated under inert conditions up to 2000 °C and then characterised for their residual combustion reactivity. These tests showed that heat treatment does reduce the reactivity of coal but does not reduce any further the already low reactivity of fly ashes.     

Characterization of the coal fly ash for the purpose of improvement of industrial on-line measurement of unburned carbon content

The influence of selected properties of fly ash on the measurements of an on-line analyser was described. Fly ash studied in the research originated from the pulverized coal fired boiler. The samples were taken using an inspection method within a period of 3 months. Systematic observation of the properties of the ash allowed monitoring of the work of the industrial analyzer during a relatively long period of the power plant work. Samples of coal fly ash were examined for their chemical and physical properties. Morphology was analysed by scanning electron microscopy. Unburned carbon content in fly ash was determined by using loss-on-ignition (LOI) and thermogravimetry analysis (TGA). The particle size distribution of fly ash was examined. Correlation between laboratory and on-line industrial measurements of the unburned carbon content of ash was discussed.     

Aggregation experiments on fine fly ash particles in uniform magnetic field

Aggregation experiments on three fly ash samples in the size range of 0.023-9.314 μm were conducted in a uniform magnetic field. The fly ash particles were produced from combustion of three different bituminous coals. The coals were originated Dongshen, Datong and Xuzhou of China, respectively. A fluidized bed aerosol generator was used to disperse the fly ash particles to generate a constant aerosol. The aerosol particles aggregated when passing through the magnetic field. The variation of particle number concentration caused by particle aggregation was measured in real time by an Electrical Low Pressure Impactor (ELPI). The effects of several parameters, such as particle size, magnetic flux density, particle residence time in the magnetic field, total particle mass concentration and average gas velocity, on particle aggregation were examined. Experimental results indicated that removal efficiencies are the highest for particles with sizes in the middle of the size ranges tested. Increasing magnetic flux density, total particle mass concentration, particle residence time in the magnetic field or by reducing average gas velocity can increase removal efficiencies of single-sized and total fly ash particles. When fly ash particle magnetization reached saturation state, further increase of the magnetic flux density will have no effect on particle aggregation. The single-sized and total particle removal efficiencies of the three fly ashes are different under the same operating conditions. The removal efficiency is the highest for fly ash generated from Dongshen coal, followed by fly ash from Datong coal, and then fly ash from Xuzhou coal. Particle number median diameters decreases with the increase in the total particle removal efficiencies. The model prediction of particle aggregation under high total particle mass concentrations conditions indicated that the single-sized and total particle removal efficiencies will increase greatly with the increase in total particle mass concentration. The model predicted total removal efficiencies of the three fly ash particles are 53%, 43% and 14%, for Dongshen, Datong and Xuzhou coals respectively when total particle mass concentration is 40 g/m³.     

Mercury and selenium retention in fly ashes: Influence of unburned particle content

Mercury and selenium are present as trace elements in coal and may be emitted to the environment in gas phase during coal conversion processes or be partially retained on the fly ashes. The present work explores the possibility that selenium may contribute to mercury capture in fly ashes in two different situations: firstly the power station itself, in order to evaluate the influence of typical working conditions, and secondly in a fixed bed of fly ashes enriched with Se, in order to study the capture of mercury in more severe conditions. It was found that the presence of selenium in fly ashes may improve their capacity to capture mercury. However, in the four fly ashes of different origin studied, selenium is not the most important component for mercury retention. In fact, the presence of selenium in fly ash samples enriched in unburned carbon does not have any significant effect on mercury retention.     

Investigation on the combustion possibility of dry sewage sludge as a pulverized fuel of thermal power plant

Combustion possibility of three dry sludges as pulverized fuel of coal power plant like sub-bituminous Minco coal was studied by thermogravimetric analysis (TGA) and Drop Tube Furnace (DTF). TGA results showed that the fixed carbon contained with minor content in dry sludge was slowly burned than it of Minco coal. The linear regression for the Arrhenius plot to the experimental data is very good, and activation energies for overall combustion of Minco coal and DDSS are 64.382 and 26.799 kJ/mol, respectively. But, combustion patterns of KDSS and SDSS divided into devolatilization and oxidation reaction. It was derived that activation energies for the devolatilization of KDSS and SDSS are 27.127 and 12.571 kJ/mol in reciprocal proportion to volatile matter content, the fixed carbon combustion derives to 45.289 kJ/mol for KDSS, 33.777 kJ/mol for SDSS. Test results show that the volatile content in sludge significantly improved the combustion reactivity whereas the time for the combustion completion delayed. The conversion behavior of the coals and sludge observed in DTF was similar to that reflected in TGA. DTF studies showed that the individual sludge was lower conversion than the Minco coal, but the combustion of most sludge was completed at residence time of around 1 s, set temperature range of 1200 °C similar to commercial coal fired plant. These high IDT of sludge ashes with minimum 1214 °C are not expected to be associated with slagging and fouling in pulverized coal fired systems.     

Effect of particle size distribution of fly ash-cement system on the fluidity of cement pastes

This paper investigates the effect of particle size distribution of fly ash-cement system on the fluidity of the cement pastes using class F fly ash collected from the hopper attached to an electrostatic precipitator when the burning conditions and types of coal are changed at a coal-fired power plant. The unburned carbon content of fly ashes used in the experiment is less than 1.5%. To prevent the unburned carbon in fly ashes from affecting the fluidity of the pastes, polycarboxylic acid plasticizer with more saturation amount is added into the pastes for experimentation. The particle size distribution of fly ash-cement system is analyzed using n value of Rosin-Rammler function and the n value is derived with a nonlinear least squares fitting method. As the result, it is shown that the fluidity increases as the particle size distribution becomes wider, i.e., as n value gets smaller.     

单颗粒煤焦燃烧反应动力学研究方法

采用热天平研究了福建龙岩和加福的两种煤焦的单颗粒燃烧过程。探讨其燃烧过程中的气体扩散、灰层以及反应阻力的影响,建立了单颗粒煤焦燃烧过程分析方法,并由此建立了单颗粒煤焦热天平测定化学反应本征动力学常数的新方法。研究发现,随着单颗粒煤焦燃烧的进行,燃烧总阻力逐渐减小,当反应趋于结束时,燃烧阻力不再随反应时间变化,而是趋于稳定,此时燃烧阻力即为化学反应本征动力学的阻力,由此测定化学反应本征动力学常数。通过对不同温度、不同粒径煤焦以及不同空气流量下的实验与分析,表明该测定方法稳定性好,且测得的煤焦燃烧本征动力学常数和活化能与文献报道一致。     

软测量技术在电站锅炉优化燃烧上的应用

针对阜新电厂200 MW机组燃煤锅炉进行了多工况热态测试,获得了飞灰含碳量的现场数据样本,运用Back Propagation(BP)神经网络和Levenberg Marquardt(LM)算法建立了电站锅炉飞灰含碳量的软测量模型,并构造了飞灰含碳量的测量系统. 在此基础上开发了电站锅炉燃烧优化系统,实现了阜新电厂200 MW机组锅炉燃烧优化控制.     

Role of active sites in the steam activation of high unburned carbon fly ashes

Previous studies on carbon gasification have not included high unburned carbon content fly ashes, and therefore it remains unclear why not all fly ash carbon samples are equally suitable for activation. The concentration of active sites is well known to influence carbon gasification reactions. Therefore, the objective of this work was to investigate the effect of the concentration of active sites on the behavior of fly ash carbon samples upon steam activation. Six fly ash carbons were selected to produce activated carbons using steam at 850 °C. The concentration of active sites was determined by non-dispersive infrared analysis (NDIR), thermogravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS). XRD analyses were also conducted to determine the crystallite size. It was observed that the concentration of active sites played a more significant effect on the surface areas of activated carbons in the carbon burn-off zone of >60%. Statistical analysis was used to relate the surface areas of activated carbon variances with carbon burn-off levels.     

Speciation of mercury in fly ashes by temperature programmed decomposition

Mercury (Hg) is a toxic trace element which is emitted mostly in gas phase during coal combustion, although some Hg compounds may be retained in the fly ashes depending on the characteristics of the ashes and process conditions. To improve the retention of Hg in the fly ashes a good knowledge of the capture mechanism and Hg species present in the fly ashes is essential. The temperature programmed decomposition technique was chosen to identify the Hg species present in fly ashes obtained from two Pulverized Coal Combustion (PCC) plants and a Fluidized Bed Combustion (FBC) plant. The fly ashes were then used as Hg sorbents in a simulated flue gas of coal combustion and gasification. The Hg compounds found in the fly ash from the FBC plant after elemental mercury retention were mainly HgCl₂ and HgSO₄. The Hg species present in the two fly ashes from the two PCC plants were HgCl₂ and Hg⁰. The Hg species formed in the coal gasification atmosphere was HgS for all three fly ashes. The only Hg compound identified in the fly ashes after the retention of mercury chloride was HgCl₂.     

Aggregation Experiments on Fine Fly Ash Particles in a Gradient Magnetic Field

Aggregation experiments were conducted on two kinds of fly ash particles in the size range of 0.023–9.314 μm in a gradient magnetic field produced by permanent magnetic rings. The two types of fly ash particles were obtained from Dongsheng and Datong coal combustion. The effect of particle size, total particle mass concentration, particle residence time in the magnetic field and gas velocity were examined. Experimental results showed that the removal efficiencies in a gradient magnetic field are much higher than those in a uniform magnetic field. The total and single‐sized particle removal efficiencies can be improved by increasing the total particle mass concentrations and the particle residence time in the magnetic field or reducing the gas velocity. Mid‐sized particle removal efficiencies are higher than those of the larger and smaller ones. With the increase in total particle removal efficiencies, the particle size corresponding to the maximum values of single‐sized particle removal efficiencies and the particle number median diameters both decrease. Both the single‐sized and total removal efficiencies for the particles from the Dongsheng coal combustion are higher than those from the Datong coal combustion.     

Characterisation of meat and bone mill for coal co-firing

A most interesting solution for the disposal of meat and bone meal (MBM) is co-feeding with coal in combustion plants. MBM, is however, quite different from any other traditional or alternative solid fuel in terms of chemical composition, ash content and microstructural properties. Its effects on the performance of a boiler are largely unexplored. The present paper addresses the characteristics of MBM as alternative solid fuel and the effects of co-feeding MBM (6%) and coal (94%) in a utility boiler.A first activity consisted in the characterisation of the physico-chemical properties and the reactivity of MBM. The experimental campaign included ultimate and proximate analysis, granulometric analysis, ICP, SEM, XRD. An extensive campaign of isothermal and non isothermal thermogravimetric experiments was carried out to assess the reactivity of MBM upon pyrolysis, combustion and gasification and to obtain appropriate kinetic expressions.A second activity focused on co-firing of MBM and coal. Bottom and fly ashes were collected from an industrial boiler operated with MBM and coal. Ash samples were characterised by SEM, XRD, ICP, TGA and granulometric analysis. Results of this activity showed that MBM contributes mostly to bottom ash, however also the fly ashes are different from those typically encountered when the boiler is operated with coal alone. Differences concern the chemical composition and particle size distribution of ashes, in particular a large population of very fine particles characterised by perfectly spherical shape and non negligible carbon content is observed.     

Characterisation of surface properties of various fly ashes

The paper reports a study of fly ashes obtained as a result of black and brown coal combustion by the following methods: chemical analysis, X-ray diffraction (XRD), infrared (IR) spectroscopy, differential thermal analysis, (DTA), low-temperature nitrogen and methylene blue adsorption and scanning electron microscopy (SEM). The samples of fly ashes of different types were characterised by different chemical compositions, structures, surface areas and porosities.     

水泥回转窑内NO生成的模拟

采用气固流动、煤粉燃烧和NO生成模型,结合物料烧成过程的物理化学反应热效应的一维热流函数,对采用4通道燃烧器的某3000 t·d^-1生产能力的全尺寸水泥回转窑内NO的生成进行了数值模拟研究,对水泥回转窑内NO生成机理及分布规律进行深入的分析.研究结果表明：水泥回转窑内NO生成主要为热力型NO和燃料型NO,并且以热力型NO为主要生成方式,燃料NO主要在窑头的燃烧带产生,热力NO主要产生于高温烧成带,并且燃料NO与热力NO的生成过程存在着相互抑制作用.     

Mercury transformations in the exhausts from lab-scale coal flames

A laboratory-scale pulverized coal flame generated exhausts from five coals which were processed at realistic quench rates and residence times with typical flyash loadings. Mercury levels were monitored in the coal feed, all particulate streams from the furnace, and in the gaseous effluent. Contributions from elemental and oxidized species to the mercury vapors were also determined. Reported extents of Hg oxidation were not proportional to coal-Cl levels. Only the coal with excessive Cl generated an abundance of oxidized Hg species. Extents of Hg oxidation did not increase for progressively longer residence times in the exhaust system, but were affected by the level of unburned carbon, suggesting an essential role for heterogeneous chemistry. The split between particulate and vapor Hg species shifted toward particulate-Hg for progressively cooler temperatures at the exhaust outlet. The levels of particulate-Hg were generally higher for the coals that generated less oxidized Hg vapors. Appreciable Hg sorption was observed at temperatures as hot as 500 °C.     

Kinetics of carbon oxidation from fly ash

Thermal gravimetric analysis (TGA) and evolved gas mass spectroscopy were used to study the kinetics of carbon oxidation from a Class-F fly ash. A multi-process ignition loss schema is presented wherein carbon combustion is modeled as a series or discrete independent reactions. These processes were studied at temperatures up to 1000 °C (1832 °F), for oxygen partial pressures between 0.05 and 0.50 and for heating rates between 5 and 40 °C/min (9-72 °F/min). The results show that carbon combustion can be modeled by a series of at least three processes; the weights (fractions) of which are a function of temperature and not a function of oxygen partial pressure. Such detailed combustion kinetics are relevant for the post processing of fly ash to produce materials suitable for use as concrete admixtures or in the manufacture of sintered artificial aggregate or similar densified structures based on fly ash. Such are low temperature, low heating rate processes relative to coal combustion power generation applications.