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’.     

Co-combustion of pulverized coal and solid recovered fuel in an entrained flow reactor - General combustion and ash behaviour

Co-combustion of a bituminous coal and a solid recovered fuel (SRF) was carried out in an entrained flow reactor, and the influence of additives such as NaCl, PVC, ammonium sulphate, and kaolinite on co-combustion was investigated. The co-combustion experiments were carried out with SRF shares of 7.9 wt.%, 14.8 wt.% and 25 wt.%, respectively. The effect of additives was evaluated by maintaining the share of secondary fuel (mixture of SRF and additive) at 14.8 wt.%. The experimental results showed that the fuel burnout, NO and SO₂ emission in co-combustion of coal and SRF were decreased with increasing share of SRF. The majority of the additives inhibited the burnout, except for NaCl which seemed to have a promoting effect. The impact of additives on NO emission was mostly insignificant, except for ammonium sulphate which greatly reduced the NO emission. For SO₂ emission, it was found that all of the additives increased the S-retention in ash. Analysis of the bulk composition of fly ash from different experiments indicated that the majority of S and Cl in the fuels were released to gas phase during combustion, whereas the K and Na in the fuels were mainly retained in ash. When co-firing coal and SRF, approximately 99 wt.% of the K and Na in fly ash was present in water insoluble form such as aluminosilicates or silicates. The addition of NaCl, PVC, and ammonium sulphate generally promoted the vaporization of Na and K, resulting in an increased formation of water soluble alkalis such as alkali chlorides or sulphates. The vaporization degree of Na and K was found to be correlated during the experiments, suggesting an interaction between the vaporization of Na and K during pulverized fuel combustion. By collecting deposits on an air-cooled probe during the experiments, it was found that the ash deposition propensity in co-combustion was decreased with increasing share of SRF. The addition of NaCl and PVC significantly increased the ash deposition propensity, whereas the addition of ammonium sulphate or kaolinite showed a slight reducing effect. The chlorine content in the deposits generally implied a low corrosion potential during co-combustion of coal and SRF, except for the experiments with NaCl or PVC addition.     

煤粉炉掺烧生活垃圾对灰渣特性的影响研究

利用小试规模煤粉炉,研究掺烧不同比例生活垃圾对燃煤灰渣特性的影响,主要包括飞灰元素组成、飞灰粒径分布、飞灰形貌、灰熔点和结渣特性等影响变化研究。结果表明,随着生活垃圾掺烧比例的增加,灰分中Ca、Fe、Cl和S元素含量增加,Al、Mg、K、Na、Ti和Si含量降低,飞灰球形颗粒分布减少,层状堆积结构增多;灰渣熔融特征温度呈平缓下降趋势,但变化范围小于2%,影响较小;掺烧量为25%时,飞灰表面发现少量褐色大颗粒。总之,生活垃圾掺烧对燃煤灰渣特性影响很小,该结果为实际煤粉炉开展掺烧生活垃圾试验提供了一定理论基础。     

涡旋式分解炉中煤及垃圾衍生燃料共燃烧耦合CaCO3分解的数值模拟

针对一实际尺寸的带下置涡流室的分解炉进行了数值模拟,探讨了煤、垃圾衍生燃料（RDF）两种燃料共燃与碳酸钙分解相耦合的化学反应过程。计算所得煤粉及RDF燃烬率分别为99%和100%,碳酸钙分解率为95%,与工程实际数据吻合较好。结果表明：煤粉自涡流室顶部入炉后,先向下俯冲,再在气流的携带下转而向上运动,在分解炉柱体部分螺旋上升,其燃烧时以焦炭燃烧占主导,在涡流室上方的锥体部分及锥体部分上方的下半柱体部分形成主燃区;RDF自分解炉柱体部分下部水平入炉后,先运动至中部,旋即与煤粉流交织在一起螺旋上升,其燃烧时以挥发分燃烧占主导,在分解炉下半柱体部分形成主燃区;CaCO₃自涡流室顶部入炉后,首先在涡流室及其下方的锥体部分做涡旋运动,一部分因吸收高温气流的热量而分解,剩余大部分上旋至燃料主燃烧区,因吸收燃烧所释放的热量而分解;燃料燃烧放热与CaCO₃吸热分解相耦合后,最终在分解炉柱体部分形成了均匀、稳定的温度场。     

Biomass-coal co-combustion: opportunity for affordable renewable energy

This investigation explores the reasons for and technical challenges associated with co-combustion of biomass and coal in boilers designed for coal (mainly pulverized coal) combustion. Biomass-coal co-combustion represents a near-term, low-risk, low-cost, sustainable, renewable energy option that promises reduction in net CO₂ emissions, reduction in SO_x and often NO_x emissions, and several societal benefits. Technical issues associated with cofiring include fuel supply, handling and storage challenges, potential increases in corrosion, decreases in overall efficiency, ash deposition issues, pollutant emissions, carbon burnout, impacts on ash marketing, impacts on SCR performance, and overall economics. Each of these issues has been investigated and this presentation summarizes the state-of-the-art in each area, both in the US and abroad. The focus is on fireside issues. While each of the issues can be significant, the conclusion is that biomass residues represent possibly the best (cheapest and lowest risk) renewable energy option for many power producers.     

Thermogravimetric assessment of combustion characteristics of blends of a coal with different biomass chars

In connection to future energy demand and fossil fuel crisis particularly in India, biomass is gaining its importance for possible use as co-fuel. In India varieties of biomass products are available which do have tremendous potentiality for co-combustion with pulverized coal. Based on the emerging need, detailed investigations are felt necessary to examine the compatibility of different kind of biomass with coal and to select suitable blend composition(s) before utilizing those biomass products in utility operation as co-fuels. This study elaborates the lab scale findings of combustion experiments in DSC-TGA apparatus with a typical Indian coal, two biomass samples and low temperature biomass chars (300 and 450 °C) as well as with ‘blends of low temperature chars and coal’. Conventional TGA parameters, activation energy and ignition index of different blends were estimated which provided elaborate information on their basic combustion features. Results of non-isothermal combustion studies in general depict that blends containing less than 50% biomass char are better performing as compared those with higher biomass char content. Lowering of activation energy and improvement of reactivity in major combustion zone were also observed in the coal/biomass-char blends. Improvement of ignition index of the blends of coal with 300 °C chars over expected weighted mean values was noticed. Such attempts may help to identify appropriate biomass-type, blend proportion for a given coal and to derive some specific advantages with respect to particular combustion practice.     

Transformation of mineral and emission of particulate matters during co-combustion of coal with sewage sludge

Co-combustion of coal with sewage sludge was carried out in laboratory-scaled drop tube furnace to understand the interaction between different fuels. The combustion conditions were selected as follows: the raw material feeding rate was 0.2-0.3 g/min, temperature was 1200 °C, the atmosphere of 10% O₂ and N₂ being balance was used to guarantee an air ratio of 1.5, and the residence time varied from 0.6 to 2.4 s. The coal/sewage sludge is kept at 50:50 (wt% to wt%), four fuel pairs were selected with respect to the mineral association within individual fuel. The results showed the obvious interaction between coal and sewage sludge during their co-combustion. For the carbon conversion, the devolatilization of mixing fuel occurred quickly; the combustion of both char and evolved volatile progressed almost completely. As a result, the unburnt carbon was almost zero in the fly ash. In addition, the evolution of both mineral and PM varied with the association of minerals in raw fuels. For both coal and sewage sludge rich in included minerals, they combusted separately in the furnace, less interaction occurred accordingly. Conversely, for both them rich in excluded minerals, the minerals reacted with each other to form much agglomeration, and therefore, the particle size of the fly ash was increased, while the amount of PM was decreased, which changed as the coarse fly ash particles. Finally, for the case of coal rich in excluded mineral and sludge rich in included mineral, their co-combustion led to the interaction of their minerals. As a result, more the fine particles were formed, which in part changed into PM. For the vaporized trace elements, they were adsorbed by the melt CaPO₄/Al-Si in the ash and accordingly, their contents in the particulate matter were reduced whereas their particle size distribution shifted to the large value.     

Pulverized coal combustion characteristics of high-fuel-ratio coals

It is strongly desired for coal-fired power plants in Japan to utilize not only low-rank coals with high moisture and high ash contents, but also high-rank coals with high fuel ratio for diversifying fuel sources and lowering cost. In this study, pulverized coal combustion characteristics of high-fuel-ratio coals are experimentally investigated using an approximately 100 kg-coal/h pulverized coal combustion test furnace. The combustion characteristics are compared to those for bituminous coal. The coals tested are six kinds of coal with fuel ratios ranging from 1.46 to 7.10. The results show that under the non-staged combustion condition, the minimum burner load for stable combustion rises as fuel ratio increases. To improve the stability, it is effective to lengthen the residence time of coal particles in the high gas temperature region close to the burner outlet by using a recirculation flow. The conversion ratio of fuel nitrogen to NOx and unburned carbon fraction increases with increasing the fuel ratio. In addition, as the fuel ratio increases, NOx reduction owing to the staged combustion becomes small, and unburned carbon fraction increment becomes significant. The numerical simulations conducted under the staged combustion condition show that although the numerical results are in general agreement with the experimental ones, there remains room for improvement in NOx reduction model for high-fuel-ratio coals.     

Combustion of a fuel mixture in a high-temperature cyclone reactor

The results of an experimental study of the combustion of pulverized fuel mixtures in a high-temperature cyclone reactor, a new type of furnace device, are presented. The temperature profiles in the reactor and the composition of the ash residue are shown. Thermograms of the thermolysis of pulverized coal and sawdust, as well as mixtures thereof, are displayed. It was demonstrated that in contrast to the combustion of pulverized coal alone, the combustion of a coal-sawdust mixture yields not only fine ash, but also slag.     

Correlating the effects of ash elements and their association in the fuel matrix with the ash release during pulverized fuel combustion

During pulverized fuel combustion, inorganic elements such as alkalis, sulfur, chlorine, calcium and magnesium, as well as a range of minor elements are partly released into the gas phase. These gas-borne species can nucleate, coagulate and condense to form either aerosol particles or sticky layers on ash particles, leading to ash deposition and corrosion problems in power utilities. Furthermore, the fine aerosols can lead to harmful gaseous and particulate emissions. It is well documented that the mode of occurrence and the chemical speciation of ash forming elements in the coal/biomass structure are important for the release behavior of mineral components. In the presented work, this is investigated by performing quantitative elemental investigations of ash releases for two different coals (a Polish and a UK coal) and six diverse biomass fuels (Wood bark, Wood chips, Waste wood, Olive residue, Saw dust and Straw). The tests are performed within the Lab-scale Combustion Simulator (LCS) of the Energy Research Centre of the Netherlands (ECN). The operating conditions applied were that of a typical pulverized fuel (PF) fired boiler i.e. atmospheric pressure, high temperatures of 1400-1650 °C, and high heating rate of 10⁵ K/s. Gas phase elemental release of alkalis, sulfur, chlorine, calcium and magnesium has been quantified at relevant high carbon conversion levels. With the performed set of experiments several of the past observations from the literature are reconfirmed. In addition to this, based on the extensive data pool at hand, a simple but reliable (R² > 0.95) set of linear correlations have been proposed to predict the elemental release of potassium, sodium, chlorine and sulfur. It is also concluded that such linear expressions can be particularly effective for the prediction of elemental release from the fuels of similar characteristics, such as woody biomass.     

Effects of biomass co-firing with coal on ash properties. Part I: Characterisation and PSD

The alterations of ash quality and utilisation aspects when co-firing coal with biomass were investigated. Co-combustion tests were performed in lab and semi-industrial scale facilities, using several coal-biomass blends. The collected ash samples were analysed for major elements and heavy metals content, loss on ignition (LOI), free CaO content and grain size distribution. Since a variety of co-combustion residues were tested, important implications concerning the ash composition and, consequently, its further use in potential applications came up. Results showed that properties of co-combustion residues are directly connected to the combustion conditions and the individual blend components. Biomass exploitation as secondary fuel in co-combustion processes is technically and economically feasible up to 20% w/w and the produced ash could be further utilised without any major treatment.     

Emissions of NOx and N2O during co-combustion of dried sewage sludge with coal in a bubbling fluidized bed combustor

Emissions of NO_x and N₂O were measured during mono-combustion of dried sewage sludge and co-combustion with coal in a bench-scale bubbling fluidized bed combustor. After starting the sludge feed, emissions of NO_x increased with time, but N₂O emissions changed only slightly. After a certain amount of sludge was burned, the fuel was switched from sludge to coal. Emissions of NO_x from coal combustion after sludge combustion were higher than those before sludge combustion. These results suggest that the accumulation of sludge ash influenced NO_x emissions. A simple model of ash accumulation and removal was proposed. The transient change in NO_x after starting co-combustion was explained using the model presented here.     

我国燃煤工业锅炉现状及分析

不同类型燃煤工业锅炉具有各自的技术优势及应用范围,为了给用户在项目立项、选择锅炉时提供正确参考,阐述了3种主流燃煤工业锅炉的技术特点、应用现状,并着重针对循环流化床锅炉和现代煤粉工业锅炉,从燃烧组织方式和技术特点两方面进行了系统的技术对比分析。经分析认为,流态化燃烧组织是循环流化床锅炉的技术基础,浓相室燃燃烧组织是现代煤粉工业锅炉的技术基础。依托密相床炉料的巨大热容量,循环流化床锅炉定位于处理高灰劣质燃料;依托低变质高活性清洁煤粉快响应着火喷燃,现代煤粉工业锅炉定位于油(气)锅炉的备份及互换。因此,二者非取舍而是互为补充的关系。     

Single droplet combustion of coal-methanol slurry

Coal-methanol slurry (CMS) has attracted special interest as a new coal slurry fuel. In this work, the combustion characteristics of CMS have been investigated experimentally by single droplet combustion with the following results. The scattering of pulverized coal particles was observed during the gas-phase combustion period. The combustion behaviour of CMS was very similar to that of pulverized coal. The overall burning rate coefficient for CMS was apparently increased compared to other coal slurry fuels, using residual heavy fuel oil as the dispersion medium. It was concluded that CMS has excellent properties which can mask the defects of pulverized coal and other coal slurry fuels for combustion, as well as in transportation and handling.     

PCDD/PCDF emissions from co-combustion of coal and PVC in a bubbling fluidised bed boiler

This paper reports on the results obtained in the study of the co-combustion of PVC with hard coal from South Africa in a 0,5 MW_th Bubbling Fluidised Bed Boiler. The research has included the study of the effect of combustion temperature, fluidisation velocity and PVC content. The addition of urea to the raw fuel, as a dioxin-preventing compound has also been evaluated. Results have been analysed in terms of combustion efficiency, major pollutants emission (NO_x, CO), and PCDD/Fs formation in the flue gas and in the fly ash. Under the experimental conditions tested, co-combustion of coal and PVC has proved to be feasible from the combustion efficiency and emission of PCDD/Fs points of view, whose levels remained below limits set by existing legislation on persistent organic pollutants. The addition of solid urea to the fuel blend reduces the amount of chlorinated compounds emitted. However, it has a negative impact on nitrogen pollutants formation     

Toxicologic and Physicochemical Characterization of High-Temperature Combustion Emissions

Particles and combustion gases produced by two different high-temperature combustors, which burned pulverized coal and a No. 2 fuel oil-fly ash slurry, respectively, at adiabatic flame temperatures greater than 2400 K, were characterized. Effluent samples were taken at locations along the product gas stream and within the stack. Measurements of the particle size distributions, number concentrations, and gas species concentrations were made. The toxicity and mutagenicity of the effluent particles were determined. A large number of submicrometer particles were found in both cases of high-temperature combustion. The product emissions differed significantly in their particle size distribution and final chemical composition from those of conventional combustion systems having lower combustion temperatures.     

煤粉大比例掺混不同生物质的混燃特性研究

可再生能源生物质清洁低碳、易于获取、利于着火,含硫、氮量少且属于碳中性物质,但其能量密度低。在煤粉中大比例掺混生物质(生物质/煤粉质量比大于5∶5)可有效改善煤粉着火特性,碳排放水平接近燃烧天然气,且污染物排放显著降低,进而达到节能减排目的。目前研究主要集中在低掺混比例(小于5∶5)下生物质与煤粉的混燃特性,针对北方常见的玉米秸秆、稻杆和玉米芯等生物质与煤粉在大掺混比例下的燃烧特性,尚有待深入。笔者利用热重分析技术分别研究了煤粉与不同生物质种类(玉米秸秆、稻杆及玉米芯)在不同掺混比例下(5∶5、6∶4、7∶3和8∶2)的混燃特性,分析生物质种类和掺混比例对混合燃料的着火温度、燃尽温度、交互反应以及燃烧特性指数等的影响,确定了不同生物质的最佳掺混比例。结果表明:掺混比例对混合样品失重曲线的影响从大到小依次为玉米秸秆、玉米芯和稻杆。随掺混比例增加,第1阶段最大质量变化速率逐渐增大且燃烧进程前移,第2阶段则逐渐减小,这是由于挥发分相对增加且焦炭相对减少的原因。混合样品的着火温度和燃尽温度比纯煤粉分别下降约100和60℃。随掺混比例的增加,玉米芯着火温度逐渐减小,玉米秸秆和稻杆则先减小后增大,且均在7∶3时达到最小;燃尽温度均呈现下降趋势,下降幅度由大到小分别为玉米芯、稻杆和玉米秸秆。玉米秸秆和稻杆在8∶2时燃尽性能较差。混合样品发生不同程度的交互作用,该交互作用正是生物质的促进和抑制的协同作用,使3种生物质均在5∶5时对煤粉燃烧抑制作用大;玉米秸秆和稻杆在7∶3时、玉米芯在6∶4、8∶2时促进作用大。同时,3种生物质的燃烧特性指数远大于煤粉,随掺混比例的增大,玉米芯的燃烧特性指数变化最大并在8∶2时达到最大值,6∶4和7∶3时几乎相同;稻杆的变化最小且在7∶3时达到最大值;玉米秸秆在7∶3和8∶2时几乎相同并达到最大值。小范围改变掺混比例时,燃烧特性指数变化不大。这可能是由于燃烧特性指数不仅与着火温度和燃尽温度有关,还与样品在其主要燃烧过程的反应速率有关,而煤粉在焦炭燃烧阶段的反应剧烈程度高于生物质挥发分析出阶段,使不同掺混比例的混合样品出现以上现象。     

神华石炭-侏罗纪煤掺烧的结渣性能

神华侏罗纪煤具有易燃、低硫、低灰等优点,但存在易结渣等不足之处;神华石炭纪煤结渣特性则明显优于前者;侏罗纪煤与石炭煤掺烧,可充分利用石炭煤灰的高熔融温度特性以降低混煤的结渣倾向,并保持侏罗纪煤的易燃特性以提高混煤的火焰稳定性,具有低硫、低灰、较高热值、较低钙含量、低结渣、易燃烧等优点,保证了神华配煤在以非神华煤设计的锅炉中不会带来运行上的困难。笔者通过试验提出的神华两类煤掺烧结渣趋势变化图可为神华配煤方案的设计提供依据。     

Estimation of trace element release and accumulation in the sand bed during bubbling fluidised bed co-combustion of biomass, peat, and refuse-derived fuels

The experimental study and thermodynamic equilibrium calculations were performed to investigate the interactions of fuel-based lead, copper, zinc, and manganese with the bed material of the bubbling fluidised bed boiler (BFB) during the co-combustion of sawdust, peat, and refuse-derived fuel. Flue gas trace element measurements, a chemical analysis of process streams, and mass balance calculations over the boiler reveal that the bed material captures substantial amounts of Pb, Cu, Zn and Mn, but also that these metals are released from the bed when the fuel characteristics or the process parameters are changed. The study shows that the trace metal emissions of a BFB boiler are not necessarily directly related to metal concentrations of the fuel but are rather a result of a complex process combining the release of trace metals from the fuel, the interaction between the fuel and fuel ash particles within the bed material, and the release of trace metals from the bed. The EDS analysis of the bed material particles shows that the original sand particles are covered with a Ca- and K- rich layer. In combustion temperatures, the layer is assumably in a melt form and has an important role in the trace metal accumulation/release process. The thermodynamic multi-phase multi-component equilibrium calculation for zinc suggests that the release of zinc from the bed material is strongly increased with an increasing Cl-concentration of the fuel, due to the conversion of the bed-bound zinc to ZnCl₂.     

The influence of biomass co-combustion on boiler fouling and efficiency

The paper presents an attempt to evaluate the influence of biomass co-combustion on the fouling of boiler convection surfaces. In order to show the influence of co-firing biomass with bituminous coal on boiler efficiency, the calculations of pulverized fuel (PF) OP 140 steam generator have been carried out. Typical Upper Silesian coal with medium fouling inclination has been chosen as a basic fuel. Three kinds of biomass have been taken into consideration: straw, wood and dried sewage sludge. The results confirm that the properties of additional fuels cause deterioration of the boiler efficiency as well as the changes in boilers operational parameters (amount of water injected in attemperators, ash stream, hot air temperature). The biomass during cofiring in fact replaces the coal, but always the additional fuel consumption is higher than that of the substituted coal. Therefore, the actual decrease of coal consumption is smaller than the thermal fraction of the biomass.