Triboelectrostatic Separation of Fly Ash with Different Charging Materials

A combination of mechanical sieving and triboelectrostatic separation were used to separate fly ash. The results indicate that a simple separation of unburned carbon from fly ash is achievable at particle sizes of 74 and 44 microns. Subsequently, triboelectrostatic separations were conducted via a louvered plate separator. The results show that the final carbon content in the products, which can be as low as 1.5 % or as high as 60 % with different mineral components, can be further adjusted with the combination of sieving, louvered plate separator with a tribocharger made of different materials (copper and Teflon), and the location on the louvered plate where the fly ash particles were collected.     

粉煤灰脱炭研究进展及展望

为了提高粉煤灰质量,实现粉煤灰的高效脱炭技术发展,促进粉煤灰资源可回收利用,介绍了粉煤灰脱炭的3种方法,即浮选法、重选法和电选法,其中,浮选法主要依据矿物的表面性质对粉煤灰进行脱炭;重选法依据矿物密度性质的差异对粉煤灰进行脱炭分离;电选法则依据矿物摩擦带电性质的不同进行分选。同时,对粉煤灰脱炭方法的发展趋势进行了展望,建议未来应深入研究粉煤灰性质,将脱炭方法与新型选矿设备相结合进行工业化生产,创新结合方式,如采取活性油泡与旋流静态微泡浮选柱结合的方式对粉煤灰进行脱炭处理。     

Catalytic effects of inorganic compounds on the development of surface areas of fly ash carbon during steam activation

This study was to investigate the catalytic effect of inorganic compositions present in fly ash carbons, high-unburned-carbon content fly ashes from coal-fired power plants, on the development of surface area during steam activation. Through this work, the relationships between the concentrations of alkali metals (Na and K) and a mixture of metals, Na-K-Ca and Na-K-Fe, in fly ash carbon and the surface areas of the produced activated carbon were studied.Six fly ash carbons were selected as feedstocks for activated carbon. SEM/EDS studies showed that there are two groups of inorganic particles present in fly ash carbons, in terms of the way they are associated with carbon particles: inorganic particles that exist as free single particles, and inorganic particles that are combined with the carbon particles. A series of froth flotation tests was conducted to separate the individual inorganic particles from the fly ash carbons. The concentration of the inorganic particles was analyzed using ICP-AES before and after froth flotation separation. Studies showed that the Na-K, Na-K-Ca, and Na-K-Fe which are combined with carbon particles had catalytic effects on the development of the surface areas of activated carbons. The higher the concentration of these catalytic particles, the more significant effects they had on the development of the surface areas with increasing carbon burn-off levels.     

大型循环流化床粉煤灰性能研究

通过分析循环流化床锅炉粉煤灰的特点,得出需水量及烧失量过高是制约飞灰分选的关键。为了改善粉煤灰活性,降低飞灰含碳量,并控制额外投资成本,是实现粉煤灰综合利用的新途径。     

粉煤灰中残碳的特性和利用

介绍了粉煤灰中残碳和富碳颗粒的特性和分离技术, 论述了残碳特性对颗粒分离和利用的影响, 分析了残碳的利用途径和前景。     

The impact of fly ash resistivity and carbon content on electrostatic precipitator performance

Despite the wide use of electrostatic precipitators for particulate control in pulverised coal combustion, there are still some aspects surrounding the performance of these devices which are not fully understood, particularly with respect to high carbon ash. The introduction of low NO_x burners to meet more stringent emissions regulations has also resulted in an increase in the carbon present in fly ash. An investigation was carried to assess how the operating conditions of a pilot-scale single-stage electrostatic precipitator test facility (ESPTF) affected the collection efficiency of two high carbon fly ashes. Whilst our findings demonstrated the effectiveness of the ESPTF in collecting both fly ash and carbon particles, the performance of the high-resistivity fly ash was significantly better than expected, particularly at higher temperatures. It was concluded that ESP performance of any particular fly ash and the separation of the carbon fraction may not necessarily be predicted by its resistivity alone.     

粉煤灰脱炭的流态化实验研究

粉煤灰的利用价值与其残余炭含量密切有关 .在采用现代分析测试技术的基础上 ,提出了采用流态化干法脱炭技术降低粉煤灰中残余炭含量并提高粉煤灰活性的基本思路 .流态化分选实验表明 :这种干法脱炭的思路是有效的 ,这一思路将为解决我国粉煤灰的综合利用问题提供有益的借鉴     

粉煤灰显微结构与流态化脱炭实验

在分析粉煤灰显微结构的基础上，提出了采用流态化方法脱除粉煤灰中残余炭并提高粉煤灰活性的基本思路，流态化分选实验表明：这种思路是有效的，这一思路将为解决我国粉煤灰的综合利用问题提供有益的借鉴。     

双通道低氮煤粉燃烧器飞灰碳形成机理的研究

为克服燃煤发电过程排放的NOx对人体和环境的危害,低氮煤粉燃烧器得到广泛的使用.但如果低氮煤粉燃烧器利用不当,会造成飞灰碳的大幅增加.利用筛分法和烧失量法研究双通道低氮煤粉燃烧器飞灰碳的分布特性,利用压汞仪和XRD分析飞灰碳的失活程度,从而研究双通道低氮煤粉燃烧器飞灰碳升高的原因.结果表明,双通道低氮煤粉燃烧器飞灰碳为双峰分布,小粒径飞灰碳失活不明显,大粒径飞灰碳失活明显.分析原因后认为:小粒径煤粉颗粒在燃烧区域停留时间过短,没有完全燃烧;大粒径煤粉颗粒在高温乏氧区域停留时间过长导致的失活是双通道低氮煤粉燃烧器飞灰碳升高的主要原因.     

Impact of the conversion to low-NOx combustion on ash characteristics in a utility boiler burning Western US coal

Fly ash quality before and after conversion to low-NO_x combustion was investigated at a 150 MW unit in Kentucky burning a blend of western United States bituminous coal and Powder River Basin, WY, subbituminous coal. The fly ash collection system is divided into mechanical and baghouse collection systems. The mechanical collection hoppers, capturing fly ash from a hotter portion of the flue gas stream than the baghouse hoppers, tends to capture a coarser, carbon-rich fraction of the fly ash than the baghouse hoppers. The latter was particularly the case for the pre-conversion fly ash. The post-conversion fly ash had significantly more carbon than the pre-conversion ash. The post-conversion baghouse has abundant coarse, thin-walled carbon, suggesting that this particular carbon form is an artifact of the lower post-conversion combustion temperatures.     

Mechanisms of surfactant adsorption on non-polar, air-oxidized and ozone-treated carbon surfaces

Ozone treatment of fly ash carbon has recently been reported to inhibit the adsorption of commercial surfactants in concrete paste, thus mitigating the known negative effects of carbon on ash utilization. This paper examines the general mechanism of surfactant adsorption on carbon and its suppression by surface oxidation. Experimental results are presented for two carbon types (carbon black, fly ash carbon), both raw and surface oxidized (by ozone and molecular oxygen) and several commercial anionic and non-ionic surfactants (Darex II, SDS, Tergitol). The treated carbon surfaces were characterized with XPS, FT-IR, thermal desorption in N₂ and H₂/He, surface acidity, hygroscopic behavior, interfacial energy and its components through contact angle measurement involving standard liquid probes. Surface oxidation is found to decrease surfactant adsorption in each of the carbon/oxidant/surfactant systems examined, and its effect correlates with the amount of surface oxides by XPS. The combined results suggest that surfactant adsorption primarily occurs on non-polar carbon surface patches where it is driven by hydrophobic interactions. The main mechanism of oxidative suppression is the destruction of this non-polar surface, though micropore blockage and increased negative surface charge may also contribute for some systems.     

Scheme for density separation and identification of compound forms in size-fractionated fly ash

A scheme has been developed for the separation of compounds in size-fractionated fly ash by linear density gradients. Minor crystalline components such as calcium ferrite, haematite, magnetite, MgO, barite, anhydrite, rutile and mica were identified in addition to the major compounds mullite, quartz and CaO in a fly ash of pulverized subbituminous coal from western USA. The utility of the scheme in identifying minor crystalline components in fly ash has been demonstrated.     

Significance of surface moisture removal on triboelectrostatic beneficiation of fly ash

The gas transport, triboelectrostatic beneficiation of coal combustion fly ash into carbon-rich and ash-rich products was studied relative to the effect of ash surface moisture. Increasing the humidity to which the ashes from American and Italian coal-fired utilities were exposed under process and ambient conditions affected carbon and ash separability. The effect of humidity and particle surface moisture became more important as particle size decreased: particles greater than 75 μm in diameter were nearly unaffected whereas particles smaller than 45 μm experienced up to a four-fold change in their separability upon changing their surface moisture contents. Although particle size influences the moisture adsorption, which in turn affects tribocharging, the decrease in adhesive forces between carbon and ash from otherwise intractable clusters during drying also may be a factor influencing triboelectrostatic beneficiation performance.     

Separation of unburned carbon from fly ash using a concurrent flotation column

In this study, a new concurrent flotation column that simulates the plug flow reactor was designed with the use of a static mixer, a froth separator column and an optional additional bubble generator for fly ash beneficiation. The objective was to improve the efficiency and effectiveness of unburned carbon removal from fly ash by minimizing energy costs. Cleaning tests were performed with and without the additional bubble genarator. Without the additional bubble generator unburned carbon in the ash product could be reduced to only 2.53%. Incidental loss of carbon particles and insufficient bubble generation were the main causes of poor carbon separation performance. By turning on the additional bubble generator it was aimed to assist bubble generation, to compensate bubble rupture and to recapture the detached or free carbon particles leaving the froth phase. With the additional bubble generator and under optimized conditions a froth product with 95% carbon recovery and a cleaned ash product with less than 1% unburned carbon was obtained. The separation process through the static mixer (feeder) and the separator column and the energy consumption of the unit were analyzed. It was seen that around 80% energy could be saved with the concurrent flotation column compared to conventional flotation.     

Fly ash as an assemblage of model Ca–Mg–Na-aluminosilicate glasses

Model Ca–Mg–Na-aluminosilicate glasses based on compositions found in a calcareous fly ash were synthesised and their reactivity assessed in NaOH solution and in Portland cement paste. It was found that the reactivity followed the same trend in both systems and that the reaction of the glasses in pastes was very similar to that of the components of similar composition in the real fly ash. This finding indicates that the reactivity of glass in cement can be directly linked to the chemical composition of the glass. Further, when the reactivity of the glasses was normalized to their surface area, it was found that there exists a strong correlation with the NBO/T, the ratio of non-bridging oxygens and tetrahedral ions in the glass.     

循环流化床锅炉飞灰残碳生成机理研究

论述了近十年来循环流化床锅炉飞灰残碳生成机理的主要研究成果,对国内外飞灰残碳生成机理的数值模拟和实验研究结果作了分类介绍和分析,对循环流化床锅炉飞灰残碳研究中的问题进行了剖析,探讨了今后飞灰残碳生成机理的研究方向。     

成纤性粉煤灰的低熔区组成及分布

粉煤灰的熔融温度分布图对粉煤灰成纤的降熔配比具有指导意义。利用X荧光光谱仪和灰熔点测试仪,分别测得5种不同来源粉煤灰样品的组成和熔融特性温度。运用实验所测值与117种粉煤灰组成及熔融温度的文献值,以Al₂O₃+SiO₂、CaO+MgO、Fe₂O₃+TiO₂为三相坐标制成三元相图,按照熔融温度高低划分不同区域,得到粉煤灰熔融温度分布图,找出低熔区（流动温度FT<1350℃）的组成及其组成与熔融温度的分布规律。同时将次要组分通过等电量换算后标在CaO-Al₂O₃-SiO₂三元相图中,发现所研究粉煤灰在低熔区的组成与CaO-Al₂O₃-SiO₂三元相图中1350℃低温共熔区的组成具有良好的一致性,由此找出高熔融温度粉煤灰成纤的降熔调配方法。     

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.     

The effect of combustion conditions in a full-scale low-NOx coal fired unit on fly ash properties for its application in concrete mixtures

The wide implementation of low-NO_x combustion technologies in pulverized coal combustion can lead to higher levels of carbon in fly ash and increase the adsorptivity toward surfactants of the carbon. Consequently, the air entraining agent (AEA) requirements of the fly ash used for concrete production increases, which can complicate the stabilization of entrained air. In this study, a low-NO_x tangential fired 875 MW_th power plant burning bituminous coal have been operated under extreme conditions in order to test the impact of the operating conditions on fly ash adsorption behavior and NO_x formation. It was found that the AEA adsorption of the fly ash was reduced up to five times compared to reference operation, when the plant was operated with minimum furnace air staging, three levels of burners instead of four and without recycled flue gas. The lower AEA requirements of the fly ash at these conditions were primarily caused by a reduction in total carbon content, while the AEA adsorptivity of the residual carbon was lowered to about 60% of reference value. The tested operation mode, however, increased the NO_x level in the flue gas before the DeNO_x plant by 60% compared to reference operation.     

气流床气化炉飞灰理化性质的研究

利用SEM、XRD、N2吸附法和CO2吸附法,文章考察了在不同操作条件下的气流床飞灰的理化性质,结果表明：飞灰的矿物质熔融现象比高温煤焦明显;飞灰含有丰富的中孔及中大孔,但高温煤焦具有较多的微孔;飞灰的碳晶结构不及高温煤焦的规则。