生物质燃烧过程中结焦、积灰及腐蚀形成机理及其制剂开发研究进展

大力发展生活垃圾及农林废弃物等生物质直接燃烧发电和煤炭掺烧生物质燃烧发电对缓解我国能源安全问题和实现“双碳”战略目标具有重要意义。然而生活垃圾和农林废弃物中较高含量的碱/碱土金属、硫、氯和硅等元素在高温燃烧过程中会发生复杂交互反应,导致锅炉结焦、积灰和腐蚀等一系列问题,严重影响锅炉的安全稳定运行。通过系统分析生活垃圾和农林废弃物等固体燃料燃烧过程中可能的结焦、积灰和腐蚀形成机理,探讨了原料灰分组成和结焦、积灰、腐蚀形成的关联关系和预测方法,在此基础上比较了不同类型结焦、积灰和腐蚀抑制剂的作用机制及其施加效果,并对未来高效抑制剂的开发进行了展望。     

Co-combustion of sludge with coal

Utilisation of waste-water sludge is one of the most difficult processes of environment protection because of the high moisture and contents of harmful substances. The waste-water treatment systems in Poland, especially in small towns, are often not separate from industrial sewage systems and this causes relatively large contents of heavy metals and other toxic substances in the sludge. Incineration is the only effective method of utilisation of such waste material. From the other side, coal is still the most popular fuel in Poland, and it is often used in many boilers producing hot water for the central heating of buildings or parts of the towns. Mostly they are stoker fired boilers of old construction, and they need urgent modernisation. The use of stoker-fired boilers for incineration of the sludge in small towns is the object of the analysis presented in this paper. There are two important points in the co-combustion of sludge with coal in a stoker-fired boilers: ecological and exploitation requirements. The important restriction of the sludge/coal ratio is the emission of harmful substances with the flue gas. There are substances originating from the components of the sludge: SO₂, NO_x, HCl, heavy metals and dust. Their concentrations in the flue gas should meet the environmental regulations. The other factor influencing the co-combustion process is the change of physical and thermal properties of the fuel: heating value, moisture content and ash composition. These influence the thermal output of the boiler, the amount of air required for combustion, the volume of flue gases and dust concentration and particle distribution. In this paper, the results of experiments performed with an experimental boiler are presented. The effects of the following parameters are considered: composition and thermal parameters of the sludge and their change during the year, emissions of SO₂, NO_x, CO and dust from the experimental boiler for various compositions of the fuel (sludge/coal ratio, moisture content). As a result of the analysis, the parameters limiting the amount of sludge in a mixture with coal are identified.     

Influence of phosphorus on ash fouling deposition of hydrothermal carbonization sewage sludge fuel via drop tube furnace combustion experiments

Phosphorus effect on ash fouling deposition produced during combustion process of sewage sludge solid fuel is a very important factor. Previous studies have only focused on decrease of the ash melting temperature and increase of slagging and sintering by phosphorus content. Therefore, research regarding combustion fouling formation and its effect on temperature reduction of deposit surface by phosphorus content is insufficient. Ash fouling is an important factor, because ash in the combustion boiler process deposits on the surface of heat exchanger and interferes with heat exchange efficiency. In particular, temperature reduction of heat exchanger surface via fouling should be considered together with fouling deposition, because this is related to the heat exchanger efficiency. Synthetic ash, phosphorus vaporization, and drop tube furnace experiments were performed to investigate effect of phosphorus on ash fouling formation and temperature reduction of deposit surface under combustion condition. Phosphorus was highly reactive and reacted with ash minerals to produce mineral phosphate, which promoted ash fouling deposition during the combustion experiments. In contrast, the occurrence of sintering on deposited fouling resulted in formation of a large hollow structure, which alleviated the temperature reduction on the deposit surface. Phosphorus content had a substantial correlation with fouling deposition behavior and influenced reduction in the surface temperature of the heat exchanger, because it led to generating low temperature mineral phases.     

Potential applications of renewable energy sources,biomass combustion problems in boiler power systems and combustion related environmental issues

This paper describes the potential applications of renewable energy sources to replace fossil fuel combustion as the prime energy sources in various countries, and discusses problems associated with biomass combustion in boiler power systems. Here, the term biomass includes organic matter produced as a result of photosynthesis as well as municipal, industrial and animal waste material. Brief summaries of the basic concepts involved in the combustion of biomass fuels are presented. Renewable energy sources (RES) supply 14% of the total world energy demand. RES are biomass, hydropower, geothermal, solar, wind and marine energies. The renewables are the primary, domestic and clean or inexhaustible energy resources. The percentage share of biomass was 62.1% of total renewable energy sources in 1995. Experimental results for a large variety of biomass fuels and conditions are presented. Numerical studies are also discussed. Biomass is an attractive renewable fuel in utility boilers. The compositions of biomass among fuel types are variable. Ash composition for the biomass is fundamentally different from ash composition for the coal. Especially inorganic constituents cause to critical problems of toxic emissions, fouling and slagging. Metals in ash, in combination with other fuel elements such as silica and sulfur, and facilitated by the presence of chlorine, are responsible for many undesirable reactions in combustion furnaces and power boilers. Elements including K, Na, S, Cl, P, Ca, Mg, Fe, Si are involved in reactions leading to ash fouling and slagging in biomass combustors. Chlorine in the biomass may affect operation by corrosion. Ash deposits reduce heat transfer and may also result in severe corrosion at high temperatures. Other influences of biomass composition are observed for the rates of combustion and pollutant emissions. Biomass combustion systems are non-polluting and offer significant protection of the environment. The reduction of greenhouse gases pollution is the main advantage of utilizing biomass energy.     

降低循环流化床锅炉结渣风险的准东煤配煤掺烧研究

针对循环流化床锅炉燃用准东煤带来的受热面结渣和沾污问题,从降低入炉煤碱金属着手,采用准东五彩湾煤掺烧不同比例的煤矸石、低钠煤、沙漠黄沙,探究不同掺烧物料及比例对锅炉受热面结渣和沾污的影响。研究结果表明:掺烧25%煤矸石时,炉内受热面沾污、结渣严重,且存在频发的爆管现象;掺烧38%艾维尔沟低钠煤时,各级受热面均比较干净,没有明显的结渣沾污现象,但锅炉运行成本显著增加;掺烧5%古尔班通特沙漠黄沙时,仅在一级蒸发管高温烟区出现了类似于沙子的大焦块,其余受热面均比较光滑、干净,没有结渣和沾污现象。     

Ash effects during combustion of lignite/biomass blends in fluidized bed

Aiming at investigating the role of minerals in evaluating co-firing applications of low rank coals and biomass materials, agricultural residues characteristic of the Mediterranean countries, one lignite and their blends with biomass proportions up to 20% wt, were burned in a lab-scale fluidized bed facility. Fly ashes and bed material were characterized in terms of mineralogical, chemical and morphological analyses and the slagging/fouling and agglomeration propensities were determined.The results showed that combustion of each fuel alone could provoke medium or high deposition problems. Combustion of raw fuels produced fly ashes rich in Ca, Si and Fe minerals, as well as K and Na minerals in the case of biomass samples. However, blending of the fuels resulted in a reduction of Ca, Fe, K and Na, while an increase of Si and Al elements in the fly ashes as compared to lignite combustion, suggesting lower deposition and corrosion problems in boilers firing these mixtures. The use of bauxite as an additive enriched bottom ash in calcium compounds. Under the conditions of the combustion tests, no signs of ash deposition or bed agglomeration were noticed.     

660 MW超临界直流锅炉大比例燃用准东煤燃烧调整试验分析

准东煤因其灰分低、易着火、燃尽性好等优点通常被作为优良的动力用煤,但因其煤灰中碱金属含量较高,容易出现受热面沾污、腐蚀、结渣等危害锅炉运行安全的问题.在大比例燃用准东煤的过程中,为了增大锅炉热效率、减小热偏差、降低污染物排放量、防止受热面结渣和提高锅炉燃烧经济性,首先通过摸底试验测定了某660 MW锅炉目前的运行情况及...     

生物质燃料成份对生物质流化床燃烧的影响

基于流化床的生物质燃烧技术应用日益广泛.生物质燃料流化床的缺点是容易产生床体结焦.灰的组分和生物质燃料中的硫、氯是影响流化床锅炉烧结倾向、锅炉污染速率、灰沉积过程、结焦和过热器腐蚀的主要因素.以灰成分为基础划分生物质燃料,可分为具有显著的不同燃烧特性的3类.在实践的基础上,阐述了各类生物质燃料及其灰分特性,以及在流化床...     

Co-gasification of wastewater sludge and different feedstock: Feasibility study

Gasification experiments were performed for several feedstocks alone (wastewater sludge, waste wood, reeds, olive pomace, solid recovered fuel, paper labels and plastic labels) using a fixed bed reactor operating in semi-batch conditions. In order to combine them in an optimal gasifying blend, the gasification behavior of each feedstock was compared with that of wastewater sludge through the following criteria: the raw feedstock proximate and ultimate composition, the solid conversion, the gas heating value, the pollutants release and the ashes melting. Operated alone, the conversion rate of the feedstocks after 58 min of solid residence time was over 77% of initial mass. The Syngas low heating value produced at 1123 K was in the range of 9.0 to 11.9 MJ m⁻³. The major concerns regarding the wastewater sludge were the pollutants precursors' release (NH₃, COS…) and the ash slagging and fouling. The calculated slagging and fouling indexes were high also for olive pomace and for waste wood. Finally, among the possible blends studied the paper labels and plastic labels can be co-gasified with secondary and digested wastewater sludge without any restriction, reeds and solid recovered fuel can be blinded with secondary wastewater sludge without any restriction, a specific attention have to be taken to fouling when they are blended with digested wastewater sludge. The blend based on waste wood and olives pomace should be avoided for instance due to their ash slagging and fouling tendency.     

稠油热采注汽锅炉弱爆炸除灰技术应用研究

本文针对稠油热力开发在用23t／h工业注汽锅炉燃烧渣油过程中出现的炉管结渣和积灰严重、导致热效率下降的状况,通过技术调研,根据弱爆炸吹灰原理,经过优化设计,首次在稠油油田注蒸汽锅炉上应用弱爆吹灰技术,确保注汽锅炉的高效运行,有明显的节能效果。     

Design of solid biofuels blends to minimize the risk of sintering in biomass boilers

Biomass with high concentration of alkali/alkaline and silica components can lead to slagging/fouling, and sintering of the ash deposits, causing corrosion and erosion of the boilers. There are several methods to predict bed agglomeration such as slagging/fouling indexes. However, these indexes are developed to be used for coal ashes, which shows a different behaviour than biomass fuels. The aim of this work was to determine the suitable percentage of different species found in biomass blends in order to reduce the risk of slagging and sintering. We studied the ash behaviour of 24 blended biomass samples using two slagging indexes: the alkali index, and the % of bases in ashes index, and we validated these two indexes with the Bioslag test, and with the Hardness Index (%D₁). There is low risk of slagging in most of the samples, as well as a low sintering risk. However, some samples present moderate risk of sintering possibly due to SiO₂. The Bioslag test and the %D₁ test support our results. Samples showing risk of sintering exceed 25% of the total accumulated ash weight percentage ac%_1P>25%, and show a hardness of %D₁ > 0.7. These validations can be considered as useful tools for estimating slagging and sintering of woody biomass fuels in domestic pellet boilers.     

Investigation on ash deposition characteristics during Zhundong coal combustion

The reserves of Zhundong (ZD) coal in China are huge. However, the high content of Na and Ca induces serious slagging and fouling problems. In this study, the ZD coal was burned in a DTF (drop tube furnace), and the ashes collected at different gas temperature with non-cooling probe were analyzed to obtain the ash particle properties and their combination mode. The results showed that Na, Ca and Fe are the main elements leading to slagging when the gas temperature is about 1000 °C during ZD coal combustion, but their mechanisms are quite different. Some sodium silicates and aluminosilicates and calcium sulfate keep molten state in the ashes collected at 1000 °C. These molten ash particles may impact and adhere on the bare tube surface, and then solidified quickly. With the growth of slag thickness, the depositing surface temperature is increased. The molten ash particles might form a layer of molten film, which could capture the other high fusion temperature particles. The Fe₂O₃ sphere were captured by the formed molten slag and then they blended together to form a new molten slag with lower melting temperature.     

Ash-related issues during biomass combustion: Alkali-induced slagging,silicate melt-induced slagging (ash fusion), agglomeration,corrosion, ash utilization,and related countermeasures

Biomass is available from many sources or can be mass-produced. Moreover, biomass has a high energy-generation potential, produces less toxic emissions than some other fuels, is mostly carbon neutrality, and burns easily. Biomass has been widely utilized as a raw material in thermal chemical conversion, replacing coal and oil, including power generation. Biomass firing and co-firing in pulverized coal boilers, fluidized bed boilers, and grate furnaces or stokerfed boilers have been developed around the world because of the worsening environmental problems and developing energy crisis. However, many issues hinder the efficient and clean utilization of biomass in energy applications. They include preparation, firing and co-firing, and ash-related issues during and after combustion. In particular, ash-related issues, including alkali-induced slagging, silicate melt-induced slagging (ash fusion), agglomeration, corrosion, and ash utilization, are among the most challenging problems. The current review provides a summary of knowledge and research developments concerning these ash-related issues. It also gives an in-depth analysis and discussion on the formation mechanisms, urgent requirements, and potential countermeasures including the use of additives, co-firing, leaching, and alloying.Alkali species, particularly alkali chlorides and sulfates, cause alkali-induced slagging during biomass combustion. Thus, the mechanisms of generation, transformation, and sequestration of alkali species and the formation and growth of alkali-induced slagging, formed as an alternating overlapping multi-layered structure, are discussed in detail. For silicate melt-induced slagging (ash fusion), the evolutions of chemical composition of both the elements and minerals in the ash during combustion and existing problems in testing are overviewed. Pseudo-4D phase diagrams of (Ma₂O)-MaeO-P₂O₅-Al₂O₃ and (Ma₂O)-MaeO-SiO₂-Al₂O₃ are proposed as effective tools to predict ash fusion characteristics and the properties of melt-induced slagging. Concerning agglomeration that typically occurs in fluidized bed furnaces, melt-induced and coating-induced agglomeration and coating-forming mechanisms are highlighted. Concerning corrosion, seven corrosion mechanisms associated with Cl₂, gaseous, solid/deposited, and molten alkali chlorides, molten alkali sulfates and carbonates, and the sulfation/silication of alkali chlorides are comprehensively reviewed. The effects of alloying, salt state (solid, molten, or gaseous), combustion atmosphere, and temperature are also discussed systematically. For ash utilization, potential approaches to the use of fly ash, bottom ash, and biomass/coal co-fired ash as construction and agricultural materials are explored.Several criteria or evaluation indexes are introduced for alkali-induced slagging and agglomeration, and chemical equilibrium calculation and multicomponent phase diagrams of silicate melt-induced slagging and agglomeration. Meanwhile, remedies, including the use of additives, co-firing, leaching, alloying, and the establishment of regulations, are discussed.It is suggested that considerable attention should be focused on an understanding of the kinetics of alkali chemistry, which is essential for the transformation and sequestration of alkali species. A combination of heterogeneous chemical kinetics and multiphase equilibrium modeling is critical to estimating the speciation, saturation levels, and the presence of melt of the ash-forming matter. Further practical evaluation and improvement of the existing criterion numbers of alkali-induced slagging and agglomeration should be improved. The pseudo-4D phase diagrams of (Ma₂O)-MaeO-P₂O₅-Al₂O₃ and (Ma₂O)-MaeO-SiO₂-Al₂O₃ should be constructed from the data derived from real biomass ashes rather than those of simulated ashes in order to provide the capability to predict the properties of silicate melt-induced slagging. Apart from Cr, research should be conducted to understand the effects of Si, Al, and Co, which exhibit high corrosion resistance, and heavy metals such as Zn and Pb, which may form low-melting chlorides that accelerate corrosion. Regulations, cooperation among biomass-fired power plants and other industries, potential technical research, and logistics should be strengthened to enable the extensive utilization of biomass ash. Finally, alkali-induced slagging, silicate melt-induced slagging, agglomeration, and corrosion occur concurrently, and thus, these issues should be investigated jointly rather than separately.     

Influence of storage time on the quality and combustion behaviour of pine woodchips

The current situation in the energy sector suggests the possibility of using biomass in co-combustion systems as an alternative to other fuels. In the case of the North of Spain the amount of forest residues that is generated guarantees it as a valuable source of energy for the future. However, an effective exploitation of these residues must first overcome a number of serious problems such as transport, storage, handling and pre-treatment, to meet the requirements of the power plants. The aim of this work is to study the influence of storage time on the moisture content and chemical and combustibility properties of pine woodchips. Their combustibility behaviour was evaluated by means of the following tests: heating value, ash composition, slagging/fouling indices, and the combustion profiles obtained from TG analysis. As a result of the weather conditions in the North of Spain open-air storage in the area under study is not suitable for dry pine woodchips, although their combustion behaviour remains practically unaltered.     

能源动力系统除灰新技术的机理及应用

我国能源动力系统尤其是电站锅炉燃用大量高灰煤,致使系统尾部受热面严重积灰,影响锅炉的效率和出力,中国科学院力学研究所采用了可燃气体快速燃烧,产生燃气脉冲清除积灰的原理,研究发展了一种适用于我国大型电站锅炉的新一代除灰技术,目前,燃气脉冲除灰技术已在全国几十台125MW-600MW机组上获得成功应用。图9参4     

稻秆水热碳化过程中碱（土）金属的迁移

以稻秆为原料,通过水热碳化实验结合电感耦合等离子体发射光谱仪分析及结渣、沾污指数计算,深入研究稻秆水热碳化过程中碱(土)金属的迁移特性。结果表明:在反应强度lg R₀为3.85～8.27区间,水热焦产率从60.8%降至35.6%,液体产物产率从32%增至53.2%,如以碳化为主,lg R₀不宜超过6.5;水热焦中碱(土)金属残留率逐渐减小,至lg R₀为8.27时,K、Na残留率分别为3.6%和6.6%,Mg、Ca残留率最低分别为49.7%和77.1%,而Si残留率从68.8%增加为74.6%,灰分残留率先增大后减小,最低为66.76%;稻秆水热焦结渣和沾污指数远低于参考标准,结渣和沾污行为得到有效改善。     

Ash deposition behaviors during combustion of raw and water washed biomass fuels

Biomass-fired boilers have the tendency to suffer from severe problems of fouling and slagging due to the high potassium content of biomass fuel. The troublesome potassium, however, can be removed efficiently by water washing pretreatment. In this study, the ash deposition behaviors during combustion of raw and water washed biomass fuels were investigated by a one-dimensional furnace and a deposition probe. Two biomass fuels (corn stalk and wheat straw) were used, and deposition mass, deposition efficiency, composition and morphology of the deposit were studied. The ash deposition while firing raw biomass exhibits a “fast?slow?fast?slow” trend with the sampling time. After water washing, the deposition mass decreases dramatically, and the deposition efficiency reduces gradually as the sampling time increases. The analyses of elemental composition, morphology and chemical composition on the deposit from raw biomass imply that the condensation/thermophoresis is quite significant in the earlier deposition stage, whereas the chemical reaction is remarkable in the later stage. After water washing, the potassium content of the deposit decreases significantly. Morphology and chemical composition analyses indicate that the deposit from water washed biomass ascribes to the physical accumulation of non-viscous fly ash particles. The deposition mass can easily approach a maximum value. The ash fusion temperatures of deposits increase remarkably after water washing. In addition, ash deposition mechanisms during biomass combustion are discussed.     

A novel method used to study growth of ash deposition and in situ measurement of effective thermal conductivity of ash deposit

Ash deposition always brings boilers some trouble due to fouling or slagging. In this paper, a completely controlled system was developed to study the growth of ash deposit. A novel sampling probe was designed to online measure the heat flux through ash deposit. Additionally, the thickness of ash deposit can be obtained by an online figure collecting system. The results of this research showed that as the thickness of ash deposit increased, the heat flux decreased. It was also found that at the initial stage of ash deposition when the thickness of ash deposit is approximately 1 mm, the heat flux through ash deposit had a sharp reduction. An effective method was attempted to situ measure the effective thermal conductivity of the ash deposit in the simulated combustion flue gas. It was found that temperature of the ash deposit layer had no obvious effect on its value. It was concluded that the structure of ash deposit had no obvious change in a short deposition time of 30 min with varied surface temperatures of the probe head between 400 °C and 600 °C.     

Effects of leaching and additives on the ash fusion characteristics of high-Na/Ca Zhundong coal

Considering the intensive application of Zhundong coal and its serious slagging during combustion in pulverized coal fired boilers, the effects of water leaching and different dosages of additives (Na₂CO₃, CaO, SiO₂, and kaolin) into the leached coal on ash fusion characteristics were studied using sintering tester, X-ray Fluorescence, and X Ray Powder Diffraction. After leaching, the contents of Na₂O, K₂O, CaO, MgO, SO₃, and Cl decreased, whereas the contents of SiO₂ and Fe₂O₃ increased. As a result, the slagging and fouling index of Basic/Acid ratio decreased from 1.6 in the raw coal ash to 1.1 in the leached coal ash, and alkali-induced slagging was reduced. However, the decreased ash fusion temperatures (AFTs) of the leached coal worsen the melt-induced slagging. Detailed analysis on the effects of ash compositions on the AFTs by additions of Na₂CO₃, CaO, SiO₂, and kaolin additives into the leached coal showed that Na₂CO₃ and SiO₂ improved the formation of low-melting Na₂SO₄ and Na₂SiO₃, and thus decreased the AFTs; However, overdosed Na₂CO₃ (for instance 9 wt%) improved the formation of relative high-melting Na₄Al₂Si₂O₉ and thus increased the AFTs. Overdosed SiO₂ as a high-melting contaminant also increased the AFTs. Similar with the function of overdosed SiO₂, CaO in the ash of leached coal was excess, and the AFTs increased with increased CaO dosage. Kaolin, which decomposed as high-melting mullite and cristobalite, caused an elevation of the AFTs. Therefore, water leaching decreased the AFTs of Zhundong coal and consequently worsen melt-induced slagging, lime and kaolin additives increased the AFTs effectively, but Na₂CO₃ and SiO₂ additives showed ‘V’ shaped functions on the AFTs. These general results are useful for the pretreatment of Zhundong coal during combustion and alleviating slagging.