三种气氛热处理过程朔州煤中锂和镓及稀土元素的迁移规律

利用X射线衍射分析仪(XRD)和电感耦合等离子原子发射光谱仪(ICP-AES),分别研究了朔州煤中锂(Li)、镓(Ga)和稀土(REE)元素在空气、氮气(N_2)和二氧化碳气氛(CO_2)下从300℃升温至1100℃过程中的迁移规律。结果表明:在300℃～600℃范围内,煤中的Li,Ga和REE元素在热处理过程中逸出速率较大,主要与有机质的分解有关;当温度高于600℃时,Li,Ga和REE的逸出速率降低并逐渐趋于稳定。热处理气氛对微量元素的逸出有显著影响,空气气氛中Li,Ga和REE的挥发率最大,N_2和CO_2气氛下Li,Ga和REE的挥发率相当,均比空气气氛中挥发率小,煤中有机质的氧化能够促进微量元素在热处理过程中的逸出。     

Shell粉煤气化过程中微量元素的迁移

为研究煤气化过程中微量元素的迁移方向,采集了Shell粉煤气化过程中的煤样、渣和飞灰进行了微量元素检测,研究煤气化过程中不同微量元素迁移方向,并对迁移规律进行了探索。结果表明:Ba,Be,Co,Cr,Cu,Ga,Ge,Li,Mn,Pb,Sb,Sn,Sr,Ti,Tl,V,Y,Zr等18种微量元素气化过程中的迁移有不同的难易程度,其中Be,Sb,Sr 3种元素迁移率低于20%,属于难迁移元素,主要富集于煤气中;Co,Cu,Ga,Ge,Li,Mn,Pb,Tl和V等9种元素迁移率高于70%,为易迁移元素,主要富集于渣和飞灰中;Ba,Cr,Sn,Ti,Y,Zr 6种元素迁移率为30%～60%,为较易迁移元素,在各种废弃物中均有存在。使用统计学软件SPSS19.0进行相关性和聚类分析,结果与上述迁移方向基本一致。     

煤泥型煤燃烧特性的实验研究

以5种煤泥型煤为原料进行燃烧实验,研究煤泥型煤的燃烧特性及影响因素.结果表明,成型过程及黏结剂等对煤泥的燃烧性能基本无影响;煤泥型煤燃烧初期主要为挥发分的析出和燃烧,火焰旺盛火力强;型煤中后期燃烧为焦炭的燃烧,燃烧由型煤表面不断深入内部进行,氧气要扩散到焦炭表面会受到灰壳及其内部产生的挥发分和燃烧产物等扩散阻力.型煤挥发分越高,灰分越低,其燃烧速率越大,且易于燃尽.     

低阶煤筛分过程中汞及硫元素在不同粒度中的分布规律

燃煤是大气汞污染的主要排放源之一,针对目前煤炭洗选过程中重金属元素迁移脱除研究的不足,选用内蒙古低阶高硫煤和低硫煤样进行筛分试验,通过测汞仪、化学逐级提取、X射线衍射（XRD）、X射线荧光分析（XRF）及扫描电镜等测试方法研究汞元素在煤中的赋存形式及在不同粒度级煤中分布规律。试验结果表明,汞元素在煤中的分布不仅和赋存形式相关,还与矿物质在煤中的组成形式和矿物本身的矿物学性质相关。由逐级化学提取可知,高硫煤与低硫煤中汞主要以二硫化物结合态存在。两种煤中的汞和硫在不同粒度区间的富集和迁移趋势一致;高硫煤中汞含量与灰分在6～13mm粒度区间均为最高,说明汞主要与此粒度区间密度较大的黄铁矿伴生;低硫煤在粒度<0.074mm区域内,灰分及汞的含量较高,因为部分汞在以黏土矿为主的小颗粒中富集。     

淮南电厂煤燃烧过程中微量元素的迁移性研究

采用ICP-AES和HG-AFS法分析平圩电厂和田家庵电厂的飞灰和灰渣中微量元素含量,发现不同粒径的燃煤副产品中化学元素成分基本相同.但是,大多数微量元素的含量随粒径的减小而出现增加趋势.燃烧过程中微量元素的迁移规律与元素本身的地球化学性质、元素在煤中的存在形态以及电厂的燃煤设备等有关.对煤燃烧过程中微量元素的挥发程度和迁移规律进行了讨论和总结.     

燃煤过程中砷的赋存形态及其挥发特性

选取3个国内煤样,利用自制恒温热重装置研究了燃烧过程中砷的赋存特性及其挥发规律。通过测定不同停留时间下燃烧样品中砷的含量,拟合得到砷的挥发曲线和挥发速率曲线,并采用逐级化学提取的方法对原煤和不同停留时间下的燃烧样品进行形态分析。实验结果表明：温度是影响砷挥发的重要因素,700～1000℃是砷挥发的主要温度区间。煤粉燃烧过程中,砷的挥发速率与煤粉的失重速率具有同步性;伴随着煤中水分和挥发分的快速析出,砷也具有较高的挥发速率;随着燃烧过程的深入进行,砷的挥发速率变得缓慢。煤粉燃烧结束,3种煤（五里庄、红岩和梅花井）砷的挥发比例分别为49.5%、80.7%、65.0%,且在燃烧过程中煤中残渣态、硫化物结合态和可交换态砷相互作用迁移。     

煤中钠的赋存形态与迁移规律对煤沾污性的影响

为了研究高钠煤作为电站锅炉燃料时对受热面的沾污性,采用一步浸取和低温制灰相结合的方式,研究了新疆地区几种常见动力煤中钠的总量、赋存形态以及不同温度下的迁移特性,结合中试热态试验的沾污结果,分析了上述因素与煤沾污性之间的关系。结果表明,煤的沾污性与煤中活性Na2O含量有关;当煤中总Na2O含量、煤中水溶态钠含量和燃烧时煤中钠挥发比越高,灰分越低时,煤中活性Na2O越大,煤的沾污性越强。     

准东煤中钠的赋存形态及挥发特性

借助电感耦合等离子体原子发射光谱仪及管式炉实验系统对三种高钠准东煤进行实验研究.重点研究了煤中钠金属赋存形态及其在燃烧过程中的挥发特性.结果表明,三种准东煤中,金属钠以水溶钠和有机钠居多.钠的赋存形态对钠的挥发特性有一定影响,主要是可溶钠挥发到气相中,钠含量越高的煤,燃烧过程中钠的挥发量越高;煤中大部分钠的挥发发生在1 000℃之前,在此范围内,温度越高,钠金属的挥发率越高;在1 000℃前煤中钠会有大量集中挥发的现象,且水溶钠含量越高的煤,激增现象越明显.     

西部煤中环境敏感性痕量元素的燃烧迁移行为

应用仪器中子活化( INAA)、电感耦合等离子体原子发射光谱( ICP- AES)和原子吸收光谱( AAS)对我国西北部五个电厂原煤、底灰和飞灰中环境敏感性痕量元素的含量进行了系统测定,通过不同电厂原煤与燃烧产物中痕量元素的含量变化特征,揭示了痕量元素在不同燃烧产物中的相对富集规律.以痕量元素在不同燃烧产物中的相对富集系数为评价标准,建立了燃烧产物中痕量元素的分配模型.结合痕量元素的原始赋存状态,总结了痕量元素燃烧的迁移富集机理和环境效应.     

燃煤过程中砷的赋存形态及其挥发特性

选取3个国内煤样,利用自制恒温热重装置研究了燃烧过程中砷的赋存特性及其挥发规律。通过测定不同停留时间下燃烧样品中砷的含量,拟合得到砷的挥发曲线和挥发速率曲线,并采用逐级化学提取的方法对原煤和不同停留时间下的燃烧样品进行形态分析。实验结果表明:温度是影响砷挥发的重要因素,700~1000℃是砷挥发的主要温度区间。煤粉燃烧过程中,砷的挥发速率与煤粉的失重速率具有同步性;伴随着煤中水分和挥发分的快速析出,砷也具有较高的挥发速率;随着燃烧过程的深入进行,砷的挥发速率变得缓慢。煤粉燃烧结束,3种煤(五里庄、红岩和梅花井)砷的挥发比例分别为49.5%、80.7%、65.0%,且在燃烧过程中煤中残渣态、硫化物结合态和可交换态砷相互作用迁移。     

Trace elements (Mn,Cr, Pb,Se, Zn,Cd and Hg) in emissions from a pulverized coal boiler

The concentrations of seven trace elements (Mn, Cr, Pb, Se, Zn, Cd, Hg) in raw coal, bottom ash and fly ash were measured quantitatively in a 220 tons/h pulverized coal boiler. Factors affecting distribution of trace elements were investigated, including fly ash diameter, furnace temperature, oxygen concentration and trace elements' characteristics. Modified enrichment factors show more directly element enrichment in combustion products. The studied elements may be classified into three groups according to their emission features: Group 1: Hg, which is very volatile. Group 2: Pb, Zn, Cd, which are partially volatile. Group 3: Mn, which is hardly volatile. Se may be located between groups 1 and 2. Cr has properties of both Groups 1 and 3. The smaller the diameter of fly ash, the higher is the relative enrichment of trace elements (except Mn). Fly ash shows different adsorption mechanisms of trace elements and the volatilization of trace elements rises with furnace temperature. Relative enrichments of trace elements (except Mn and Cr) in fly ash are larger than that in bottom ash. Low oxygen concentration will not always improve the volatilization of trace elements. Pb forms chloride more easily than Cd during coal combustion.     

Status of trace element emission in a coal combustion process: a review

Several important aspects are described in this paper. The occurrences of trace elements (TEs) in coal are introduced. Four main groups of trace element content level, say, >50, 10–50, 1–10 and <1 ppm, can be drawn. Trace elements partitioning in emission streams; enrichment in submicron particles; vaporization and emission in flue gas; and the mobility and leaching behavior of trace elements in coal and combustion waste are summarized. The mechanisms of trace element transformation during combustion are illustrated as following: the vaporized metals at high temperature near the combustion flame will subsequently nucleate or condense at a lower temperature downstream. These metals form a suspended aerosol along with particles. The conversion of vaporized components into various solid and/or liquid forms is the key factor influencing the final trace elements' transformation/partitioning behavior. Finally, current trace element emission control technologies are briefly introduced. To control trace elements in particle phase, electrostatic precipitators and fabric filters are mainly used. To control trace elements in vapor phase, spray dryer absorbers, wet scrubbers, condensing wet scrubbers, wet scrubbers and solid sorbent injection should mainly be used. Research needs are identified and potentially promising research topics on trace elements emission are proposed as following: (1) trace element speciation and enrichment in coal and coal ash. (2) Trace elements partitioning in combustion process. (3) Mechanisms of transformation and control technologies for easily vaporized TEs during combustion.     

Volatilization of mercury,arsenic and selenium during underground coal gasification

Mercury, arsenic and selenium are trace elements well-known for their high volatility in underground coal gasification (UCG) which can lead to environmental and technical problems during gas utilization. In this paper, the volatilization of mercury, arsenic and selenium from coal in a seam during the process of UCG were investigated, based on comparison of their volatility during the transformation of coal to char and the conversion of char to ash. Three types of coal were involved in this study. The results indicate that the volatility of mercury, arsenic and selenium during UCG in the seam follows the sequence of Hg>Se>As. Mercury and selenium show volatility higher than 90% from coal to ash. The volatility of arsenic is lower than 60% as confirmed by arsenic enrichment in UCG ash. Arsenic volatilization during UCG is also enhanced by increasing temperature, which is different from the result during the combustion of crushed coal. Coal type has obvious effect on element volatilization. The higher the coal reactivity, the easier is the evaporation of the elements from coal in the seam. At the same time, thermodynamic equilibrium calculations using MTDATA program were performed to predict the possible species in UCG gas. With regard to UCG gas at the production well, mercury presents as Hg(g), H₂Se(g) is the main gaseous species of selenium, whereas arsenic occurs in condensed phase as As₂S₃ and As. The effect of the pressure on the equilibrium composition of the gas results in major changes of the proportions of the species. High pressure leads to the formation and enhancement of the reduced species and increases the condensation temperature of the volatile elements.     

煤灰基本特征及其微量元素的分布规律

对兖州矿区煤灰的化学成分、微量元素分布及灰的岩石学特征等方面进行了研究，并对灰产率与微量元素含量分布之间的关系进行了研究，同时对影响煤灰化学成分的因素方面进行了初步探讨。通过对兖州矿煤灰的采样分析可知,研究区煤灰由结晶物质、玻璃状物质和末燃尽有机质组成，其化学成分主要为SiO2,Al2O3,Fe2O3和CaO及少量的O3,P2O5,Na2O和TiO2,煤燃烧过程中微量元素发生了再分布，多数微量元素在煤灰中富集。同时它们在飞灰中富集的浓度明显高于底灰，即随着煤灰粒度的变小，它们在其中富集的浓度越高，其含量与煤灰的粒度成反比。微量元素Th,V,Zn,Cu,Pb与灰产率之间成正相关关系，而Cl与灰产率成负相关关系。     

Volatility of fly ash and coal

The volatilization of fly ash has been examined by a number of techniques including TGA—DTA, Knudsen cell mass spectrometry, volatilization of neutron-activated fly ash, and X-ray fluorescence analysis of sized fly ash, low-temperature ash, and the parent coal. At low temperatures, H₂O, CO₂, SO₂, and a number of organic compounds are the primary volatile species as determined by mass spectrometry. Analysis of the volatiles collected from activated fly ash heated to temperatures up to 1400 °C shows that Hg, Se, As, Br, and I are nearly completely volatilized. The analysis of the bulk and size fractions of fly ash, and parent coal, is consistent with this and provides evidence for volatilization of 15 elements during coal combustion. Comparison of coal and fly ash compositions also shows that significant amounts of Se are still present in the gas phase at the precipitators and more than 50 wt % of the Se is contained in the stack emissions. The results are consistent with present models for fly ash formation and trace element enrichment.     

Trace element partitioning during the firing of washed and untreated power station coals

The effect of coal washing on trace element content and combustion behaviour of four world-traded coals has been studied at rig scale. The inputs and process outputs from a 1 MW combustion test facility, including coal, bottom ash, suspended fly ash, retained ash and flue gas, have been analysed for a standard suite of 17 trace elements. The results suggest that although coal cleaning significantly reduces the total ash content of the coal, the concentrations of individual trace elements are not reduced proportionately. Combustion of the washed coals resulted in increased concentrations of trace elements in the fly ash, although total fly ash loadings were reduced. Cleaning appeared to have little effect on concentrations of gaseous trace elements in the flue gas. The partitioning of the more volatile trace elements such as mercury and selenium between the vapour and solid phase was influenced by the amount of excess oxygen in the furnace, presumably affecting carbon-in-ash levels.The results suggest that the coal cleaning undertaken for these experiments did not significantly reduce the emissions to atmosphere of trace elements. The ultimate emissions will be determined by the efficiency of the dust capture systems.     

Ash composition of hard-to-enrich coal

The composition of ash from coal that is hard to enrich is analyzed, for samples from an enterprise in the Kuznets Basin. Practically no enrichment of the coal is observed. However, after combustion, the ash from such coal may provide a source of valuable commercial products.     

Partitioning of trace elements during the combustion of coal and biomass in a suspension-firing reactor

A novel, quartz ‘suspension-firing’ reactor is described for monitoring trace element release during solid fuel combustion under conditions relevant to fluidised bed combustors. The new design allows the examination of fuel particle combustion in the absence of bed solids. Experiments have been conducted using two coals, a sample of wood bark and one of straw. Ash from the reactor walls and base have been analysed separately from ash collected on a sintered disc in the path of exit gas. Trace element concentrations in these samples were analysed by Inductively Coupled Plasma (ICP)-mass spectrometry and ICP-atomic emission spectrometry (AES). The fractions of original trace elements retained by the ash have been reported; relative enrichment in the ‘sinter-ash’ was calculated by comparing with ‘bottom ash’. Mercury was almost completely volatilised from all fuels, as was selenium for all except wood-bark. Chromium, manganese and thallium were partially volatilised and nickel mostly retained in all samples. The behaviour of beryllium, lead, molybdenum, vanadium and zinc varied, depending on the fuel sample. Beryllium was released to a greater extent from coal/straw than the other fuels. Vanadium was partially volatilised from wood-bark and coal/straw, while the largest proportion of the zinc released was from the wood-bark. Lead and molybdenum were retained to a greater extent by ‘Colombian coal’ and wood-bark, respectively. Evidence of the enrichment of certain trace elements on the finer ‘sinter-ash’ particles has also been observed, e.g. for As, Cd, Pb and Tl during the combustion of the ‘Colombian-coal’.     

Trace elements in coal: Associations with coal and minerals and their behavior during coal utilization - A review

Trace elements such as mercury, arsenic and selenium present in coal are known to be of concern for public health. Coal-fired power plants have resulted in emission of several tons of TEs in environment. These elements mostly evaporate during combustion and condense either homogeneously as sub-micron ash or heterogeneously onto already existing fine ash. The coal-mineral and mineral-mineral associations play an important role in the formation of fine particles and in subsequent condensation of trace elements in various phases. Any retention of these elements in fly ash particles is strongly influenced by their association with other minerals in individual coal and mineral grains. Clean coal technology development is, therefore, a priority area for research and needs continuous improvements in increased efficiency and decreased pollutant emission. The paper will include trace elements in different coals from around the world. It will consider different modes of occurrences present in coals, the ash formation and evaporation of trace elements and emissions. The typical emissions from typical power stations will be presented. The paper will also review different approaches adopted in estimating the deportment of these elements. The paper at the end would discuss control strategies for reducing emissions and future directions.     

Speciation of As, Cr, Se and Hg under coal fired power station conditions

Coal combustion from power stations is an important anthropogenic contributor of toxic trace elements to the environment. Some trace elements may be emitted in range of valencies, often with varying toxicity and bioavailability. Hence, determination of trace element speciation in coals and their combustion products is important for conducting comprehensive risk assessments of the emissions from coal-fired power stations. This study focuses on speciation of selected trace elements, As, Cr, and Se, in coal combustion products and Hg in flue gas, which were sampled at one Australian power station. Different analytical methods such as secondary ion mass spectrometry (SIMS), ion chromatography-inductively coupled plasma mass spectrometry (IC-ICPMS) and X-ray absorption near edge structure spectrometry (XANES) were used to determine trace element speciation in coal and ash samples. Results showed that As, Cr and Se are all present in a range of valency states in coal. Concentrations of As and Se in the bottom ash as well as the more toxic hexavalent chromium were less than the detection limits. The more toxic As³⁺ form in fly ash was at 10% of the total arsenic, while selenium was mainly found in Se⁴⁺ form. Hexavalent chromium (Cr⁶⁺) in fly ash was 2.7% of the total fly ash chromium. Mercury speciation in flue gas was determined using the Ontario Hydro sampling train and analysis technique. Approximately 58% of the total mercury in flue gas was released in the elemental form (Hg⁰), which, among all mercury species, has the highest residence time in the environment due to lower solubility. This work summarises the performance of the selected analytical techniques for speciation of trace elements.