燃煤灰渣活性差异及来源研究

粉煤灰、沸腾炉渣和固硫灰渣是三种具有代表性的燃煤灰渣,因生成方式与生成温度不同,火山灰活性存在较大差异。本文采用改进的28 d抗压强度比方法研究其活性差异,并在矿物组成及颗粒形貌等方面探讨活性差异来源。研究结果显示:沸腾炉渣和固硫灰渣活性相近,明显高于粉煤灰;不同种类燃煤灰渣的活性差异主要来源于其矿物组成及颗粒形貌差异。     

Effect of bed materials and additives on the sintering of coal ashes relevant to agglomeration in fluidized bed combustion

Agglomeration propensity of Thai low-rank coal ashes was determined by measuring the compressive strength of sintered ash pellets over the temperature range of fluidized bed combustion. Physical and chemical changes of the sintered products were ascertained from scanning electron microscope-energy dispersive X-ray detection (SEM-EDX) and X-ray diffractometry (XRD). A clear difference existed in the strength–temperature relationship between these ashes. This difference was attributed to the role and relative amounts of clays and anhydrite components that form the low temperature melting eutectics. The bed materials (sand, CaO, CaCO₃, and CaSO₄) and additives (gibbsite and andalusite) when combined with the ashes caused a strength reduction due to the inert dilution effect that prevented the interaction of anhydrite and clays. To comprehend the mechanism of sintering and bed agglomeration more clearly, modified ashes which produced extra amount of amorphous silicate materials were prepared and tested. The bed materials and additives, when sintered with these modified ashes, gave reduction of pellet strength by varying extents based on three possible mechanisms namely, a pure inert effect, an inert/reaction effect and an inert/adsorption effect, with gibbsite being the most effective. Of the four test ashes, Lanna ash was the only ash that exhibited almost no strength development under all conditions, due principally to its very low clays content and relatively stable forms of mineralogical compositions.     

Metal emissions from co-combustion of sewage sludge and coal/wood in fluidized bed

Co-combustion of sewage sludge with coal or wood has been investigated in the 12 MW_th circulating fluidized bed (CFB) boiler at Chalmers Technical University. The investigation focuses on emissions of trace metals from co-combustion compared to mono-combustion in CFB. The results show that co-combustion can be carried out in a CFB plant designed for the base fuel without exceeding EU emission limits for practically interesting sludge energy fractions. This was also the case for the mercury emission. In general, the ashes are enriched by trace elements with increasing share of sludge, especially in the case of wood when the ashes are dominated by the sludge properties also at small fractions of sludge addition. In the present tests the trace metals were trapped in the fly ashes regardless of base fuel.     

The effect of mechanically and chemically activated fly ashes on mortar properties

Fly ash is a waste material from coal-burning power plants that consume pulverized solid fuels. Two fly ashes from Asturias (Spain) were activated mechanically by wet milling and chemically by leaching with sulfuric acid. The activated fly ashes were characterized in terms of physico-chemical characterization, granulometry, density, blaine, BET, XRD and SEM.A comparative study was carried out of several mortars, in some cases using different additions of silica fume or activated fly ash. The influence that these additives have on the mechanical resistance of the mortars was studied. As well as the possible use of these activated fly ashes as a replacement for silica fume in producing high-strength mortar or concrete. It was found that mortars containing activated fly ash presented higher compressive strengths.A mercury intrusion porosimetry study was carried out on cement mortars made with mineral additives such as silica fume and activated fly ashes. In general, the porosities values of these mortars showed that mineral admixtures improved mechanical resistance due to the decrease in pore size.     

生物质流化床腐蚀特性及抑制方法综述

生物质燃料具有水分、挥发分、碱金属含量较高、热值偏低的特点,适合流化床燃烧应用,但在燃烧过程中挥发出的碱金属和Cl元素在一定温度下会对流化床锅炉安全经济运行造成威胁,如Cl元素的挥发容易导致受热面腐蚀。其腐蚀类型可分为气相腐蚀、液相腐蚀和固相腐蚀,腐蚀程度主要受到燃料成分、温度的影响。本文针对生物质循环流化床锅炉受热面发生腐蚀的关键因素Cl成分,阐述了生物质循环流化床锅炉发生腐蚀现象的机理,根据工程实践,指出了实际锅炉发生腐蚀的现象及工程应对方法,并提出防止腐蚀可以在生物质燃料预处理、炉膛密相区附近增加二次风管、改变过热器受热面布置方式、加入特定成分添加剂以及受热面材质选择等方面进行突破,以期为生物质循环流化床锅炉的高可靠性运行提供指导。     

A kinetic study of gaseous alkali capture by kaolin in the fixed bed reactor equipped with an alkali detector

In this study, the interaction between gas phase potassium species and kaolin was investigated in a fixed bed reactor equipped with a surface ionization detector, which is capable of detecting alkali metals in gas phase at ppb level. The effects of mass transport, space time, sorbent temperature and concentration of KCl on the rate of potassium adsorption on kaolin were studied in air. Kaolin, mainly composed of kaolinite—Al₂Si₂O₅(OH)₄, was found to be very efficient in removing gaseous alkali species from hot flue gases at fluidized bed combustion temperatures. The removal efficiency increased as temperature was decreased or KCl concentration was increased. The capture of potassium by kaolin was irreversible with formation of both water-soluble and water-insoluble products. Kaolin captured KOH almost as effectively as KCl, but K₂SO₄ was captured much less effectively than KCl.     

Application of chemical fractionation methods for characterisation of biofuels,waste derived fuels and CFB co-combustion fly ashes

In the important efforts to decrease the net CO₂ emissions to the atmosphere, new, alternative fuels are being included in the fuel mixes used in utility boilers. However, these fuels have ash properties that are different from those of the traditionally used fuels and in some cases technical problems, such as ash fouling and corrosion occur due to this. Therefore, diagnostic and predictive methods are developed and used to avoid such problems. Determination of the chemical association forms of important elements, such as potassium and sodium, in the fuel by chemical fractionation is a method well defined for coal and biofuels, such as wood pellets, bark and forest residues. Chemical fractionation is a step by step leaching method extracting water soluble salts in the first step, ion exchangeable elements, such as organically associated sodium, calcium and magnesium in the second step and acid soluble compounds such as carbonates and sulfates in the third step. The solid residue fraction consists of silicates, oxides, sulfides and other minerals. The compound extracted in the two first steps is considered reactive in the combustion with a few exceptions. In this work, it has been applied to some waste fuels, i.e. sewage sludge, straw and refuse derived fuel (RDF), as well as to coal and wood. The present work also includes results from combustion tests in a fluidised bed boiler where three blends of the investigated fuels were used. The fractionation results for the fuel blends are weighted results of the fractionations of the pure fuels discussed above which are compared with fractionations of their corresponding fly ashes. The co-combustion strategy gave very good results in reducing ash problems. Possible chemical mechanisms involved are discussed in the article.     

Characterisation of fly ashes. Potential reactivity as alkaline cements

A representative group of Spanish fly ashes has been characterised in order to determine its capacity for being alkali activated and give place to a material with cementitious properties. The characterisation studies have been carried out through chemical analysis, laser granulometry, Blaine, BET, particle size distribution, XRD and ²⁹Si MAS NMR. Compressive mechanical strength test was used to determine the reactivity of the fly ashes as alkaline binders. The results obtained have demonstrated that all investigated fly ashes are suitable to be alkali activated. Additionally it has also been demonstrated that the key factors of their potential reactivity are: the reactive silica content, the vitreous phase content and the particle size distribution.     

Cd speciation in biomass fly ash particles after size separation by centrifugal SPLITT

The finest particle size fractions (≤25 μm) in four fly ash samples from fluidised bed combustion of three biomass based fuels and a municipal solid waste fuel were size separated using a centrifugal SPLITT fractionation cell. The ashes were separated into different size fractions and the cadmium concentration, partition and speciation in each separated fraction were then investigated in relation to their possible leaching.The fractionation was evaluated by environmental scanning electron microscopy, which also provided indications of the associations between Cd and other main elements on the particles through the use of X-ray fluorescence mapping. The total concentration of Cd in each fraction was determined by AAS analysis which showed different dependences on the ash particle size in the case of biomass or waste fuels. In addition, the speciation of Cd in each fraction, investigated by sequential chemical extractions and X-ray powder diffraction analysis, showed Cd to be present mainly as non-easy leachable forms, i.e. oxide and silicates. A readily leachable fraction was found only in the municipal solid waste fly ashes.     

Measures to reduce chlorine in deposits: Application in a large-scale circulating fluidised bed boiler firing biomass

Combustion of biomass with a high content of alkali (mainly potassium, K) and chlorine (Cl) can result in operational problems including deposit formation and superheater corrosion. Among the measures applied to decrease such problems are co-combustion and the use of additives. The positive effects of these measures are to a large extent either sulphation of the alkali chlorides (KCl) to less corrosive alkali sulphates or capture of alkali from KCl during release of HCl. A test campaign was carried out in a large-scale circulating fluidised boiler fired with biomass where the measures applied were sulphation by ammonium sulphate and co-combustion with peat. Their performance was evaluated by means of several advanced measurement tools including: IACM (on-line measurements of gaseous KCl); a low-pressure impactor (size distribution and chemical composition of extracted fly ash particles) and deposit measurements (chemical composition in collected deposits). The overall performance was better for ammonium sulphate, which significantly lowered KCl in the flue gas. Meanwhile no chlorine was found in the deposits. Only a minor reduction of gaseous KCl was obtained during co-combustion with peat although the chlorine content in the deposits was greatly reduced. These findings were supported by the results from the impactor measurements.     

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.     

矿物掺合料对机制砂砂浆性能的影响

研究了在相同水灰比与流动度情况下,单掺或复掺硅灰及不同地区粉煤灰等矿物掺合料对机制砂砂浆流动性、强度的影响,并与标准砂砂浆进行了比较。结果表明、与标准砂相比,机制砂砂浆需水量大、保水性差、易泌水,但具有增强作用;硅灰引起机制砂砂浆流动性减小、减少泌水、增进强度。水灰比相同时,随掺量灰增大,机制砂砂浆流动性增大,不同龄期下砂浆抗压强度均下降。复掺硅灰与粉煤灰可提高机制砂的强度。上海与贵州两地的粉煤灰品质基本相同,对砂浆性能的影响也基本相同。     

The effects of fly ash composition on the chemistry of pore solution in hydrated cement pastes

This paper reports the findings of an investigation to determine the influence of fly ash composition on the evolution of the pore solution chemistry in Portland cement/fly ash systems. Twelve fly ashes, selected to represent the wide range of composition of North American ashes, were used in the study. In addition to pore solution expression and analysis, inner hydration products were analyzed using energy-dispersive X-ray analysis. The study shows that the alkalinity of pore solution increases as the calcium and alkali content of the fly ash increase, and decreases as the silica content of the ash increases. However, there is no consistent trend between the composition of the inner calcium-silicate hydrate and fly ash composition.     

The effect of grinding process on mechanical properties and alkali–silica reaction resistance of fly ash incorporated cement mortars

The effect of fineness of fly ash on mechanical properties and alkali–silica reaction resistance of cement mortar mixtures incorporating fly ash has been investigated within the scope of this study. Blaine fineness of fly ash has been increased to 907 m²/kg from its original 290 m²/kg value by a ball mill. Test samples were prepared by replacing cement 20, 40 and 60%, with finer and coarser fly ashes and kept under standard and steam curing conditions until testing. Test results showed that grinding process improved the mechanical properties of all samples significantly. The beneficial effect of grinding fly ash, may increase utilization of this by-product in precast and ready-mix concrete industries. Incorporation of fly ash with different fineness values and ratios also decreased the expansions to harmless levels of cement mortars due to alkali–silica reaction.     

Ash formation during pulverized coal combustion : 1: Aerodynamic influences

The elemental composition of pulverized fuel boiler deposits often differs markedly from that of the corresponding fly ash and coal ash. Evidence is given here to support the hypothesis that such chemical segregation could be induced by competition between aerodynamic drag and inertial forces on a particle in a curved streamline; this increases the probability of impacting the boiler walls for large particles of high density, leading to preferential deposition. A conventional air classifier was used to separate mono-sized samples of pulverized coal and char particles into aerodynamically different fractions, and to prepare vitrinite-char and inertinite-char concentrates. The relevance of such aerodynamic segregation is considered by comparing the enrichment (or depletion) of non-volatile elements in boiler ashes with that in ashes derived from air-classified char fractions. Common trends, particularly for iron (the major fluxing element) have been identified (i) between the highest density fractions and furnace deposits, and (ii) between the lower density fractions and cyclone ash.     

The influences of coal type on in-bed desulphurization in a PFBC demonstration plant

Desulphurization features of Blair Athol (BA), Witt Bank (WB), Nang Tong (NT) and Drayton (DT) coals were investigated in a 71 MW demonstration plant PFBC boiler operated at bed temperature of around 850 °C, Ca/S molar ratios in feeds of above three, partial pressure of CO₂ at combustion boiler outlet of around 0.9 kg/cm² and boiler load of 50%, to clarify the influences of coal type on the in-bed desulphurization achievement. Bed materials (BM) and fly ashes (FA) were characterized to describe their roles in the desulphurization. SO₂ emission in the stack was found 3, 2, 41 and 27 ppm in the respective combustion of BA, WB, NT and DT. The desulphurization efficiencies were determined by their Ca/S molar ratio of the fine sorbent formed by attrition. The ratios were 1.51, 2.98, 0.79, and 0.67 for BA, WB, NT and DT, respectively. Calcination in the bed increased the attrition rate of the sorbent to allow a better desulphurization in PFBC. However, the very high attrition rate yielded a large amount of unreacted sorbent which was entrained out from the bed, lowering the calcium utilization efficiency.     

Removal of mercury in aqueous solution by fluidized bed plant fly ash

Coal combustion in power plant produces fly ash. Fly ash may be used in water treatment to remove mercury (Hg²⁺) from water or to immobilize mercury mobile forms in silts and soils. Experiments were carried out on two kinds of fly ashes produced by two circulating fluidized bed plants with different chemical composition: silico-aluminous fly ashes and sulfo-calcic fly ashes. For the two kinds of fly ashes, adsorption equilibrium were reached in 3 days. Furthermore, removal of mercury was increased with increasing pH. Sulfo-calcic fly ashes allow us to remove mercury more efficiently and more steady. The chemical analysis of fly ash surface was carried out by electron spectroscopy. The results show that mercury is bound to ash surface thanks to several chemical reactions between mercury and various oxides (silicon, aluminium and calcium silicate) of the surface of the ashes.     

Comparison of Two Kinds of Bed Materials During CFB Combustion of Cotton Stalk

Silica and aluminous soil were used as bed materials for the combustion of cotton stalk in a Circulating Fluidized Bed (CFB) combustion pilot plant (0.2 MW). After a long period of operation, the silica bed material was found to have sintered and the aluminous soil bed material was found to have resisted sintering. The particles of both types of bed materials were sampled. Two instrumental approaches (XRF and SEM/EDS) were employed in order to identify structural reasons for the observed differences in sintering behavior between the two bed materials when they were used in the same experimental conditions. From these experiments it was proven that the aluminous soil particles eliminate the bed agglomeration. Although the aluminous soil particles were enriched with alkali metals and alkaline‐earth metals after burning for 38 h, they still did not form any agglomerates. Therefore, these particles should provide a good bed material to substitute for the traditional silica bed materials in the CFB combustion of biomass with alkali ashes.     

Alkali release characteristics of blended cements

This paper presents results of an experimental program conducted to investigate the capacity of hydration products of different cementing materials to retain “bound” alkalis when the alkalinity of the surrounding solution drops. The study covered paste samples containing high-alkali Portland cement and various levels of silica fume and/or fly ash. The results showed that the ability of the hydration products of cement-fly ash systems to bind alkalis is a function of the CaO content of the fly ash, the binding increasing as the calcium content decreases. High-alkali fly ashes (Na₂O_e > 5.0% and CaO in the range of 15% to 20%) showed considerable amounts of alkali contributed to the test solutions. Silica fume does not have a high capacity to retain alkalis in its hydration products; however, ternary blends containing silica fume and fly ash have excellent capacity to bind and retain alkalis.     

流化床燃煤固硫灰渣体积稳定性及影响因素研究

采用检测固硫灰渣净浆试件线性膨胀率方法表征其体积稳定性,并对影响因素进行分析。结果显示:固硫灰渣净浆试件的线性膨胀率在14～28d龄期前迅速增加,而后增长幅度明显变缓;固硫灰的膨胀特性明显低于固硫渣;固硫灰渣体积稳定性主要决定于游离氧化钙(CaO)含量而非三氧化硫(SO3)含量。