Studies of Cd, Pb and Cr distribution characteristics in bottom ash following agglomeration/defluidization in a fluidized bed boiler incinerating artificial waste

This study focused on the effects of agglomeration/defluidization on the Cd, Pb and Cr distribution in bottom ash particles of different sizes. In this study we have incinerated artificial waste, which was a mixture of sawdust, polypropylene, selected metal solutions, and polyethylene. The experimental parameters included Na concentration, addition of Ca and Mg and operating temperature. The results indicated that particle size decreased by attrition and thermal impact in the absence of added Na. When Na was added to the system, this metal reacted with silica sand to form eutectics, which increased particle size. Further addition of Ca and Mg was found to prolong the operating time, with greater amounts of liquid eutectic accumulating, leading to increase particle size.The heavy metal concentrations in coarse and fine particles were greater than those present in particles of intermediate sizes over a range of experimental conditions. As the particle size decreased below 0.59 mm or increased above 0.84 mm, the heavy metal concentrations increased. As Ca and Mg were added, the heavy metal concentrations in particles of all sizes increased relative to those present in identical particles when no metals or only Na were added. Additionally, the ratio of Cd sorption to silica sand decreased with increasing Na concentration, but Cr sorption had the opposite tendency. Therefore, while the addition of Na tended to increase agglomeration, it also increased the tendency for heavy metals to remain in the sand bed of fluidized bed incinerators. Addition of Ca and Mg not only inhibited the agglomeration/defluidization process, resulting in increased operating time, but also enhanced the removal of heavy metals by silica sand, decreasing the concentration of heavy metals in reactor exhaust.     

Neutralizing capacity of bottom ash from municipal solid waste incineration of different particle size

This study measures the neutralizing capacity of bottom ash from municipal solid waste incineration of different particle sizes. We examine the effect of particle size on the weathering process, a method popularly used for stabilization of heavy metals in incineration of bottom ash. The distribution of particle sizes in municipal solid waste incineration bottom ash is rather broad, ranging from fine powder to as large as 40 mm in diameter. Although considered a by-product highly suitable as a road construction material, the high level of heavy metal leaching is an obstacle to its reuse. Weathering, a method used to reduce heavy metal leaching, is a lengthy process taking over thee months to complete. The chief reaction involved in weathering is carbonation occurring between Ca(OH)2 in bottom ash and CO₂(g) in the atmosphere. During this process, CaCO₃ is produced, causing the pH level to drop from over 12 to about 8.2 and reducing heavy metal leaching. In this paper, we attempt to determine the particle size best suited for reducing the period required for weathering bottom ash by identifying characteristics of different particle sizes that affect heavy metal leaching and neutralization.     

不同粒径污泥热解制备生物炭及其特性分析

以3种不同粒径污泥为原料,采用固定床反应器在500℃下制备生物炭。考察了3种不同粒径污泥的热解特性及其生物炭中重金属的分布特征,并运用TCLP对污泥及其生物炭的重金属浸出毒性进行了系统研究。结果表明,随着污泥粒径的增大,热解生成的生物炭和热解气产率均有所降低,而焦油产率则逐渐升高;在3种不同粒径污泥热解过程中,重金属除As外主要富集在固体产物生物炭中,相对富集系数均高于90%。随着污泥粒径的增大,污泥中Cu、Zn和Ni的含量增加,而Cr和Pb的含量则减少;虽然3种不同粒径污泥制备的生物炭中重金属的浸出规律不一致,但是污泥热解可以有效抑制重金属的浸出。生物炭中除As和Zn外其他重金属元素的浸出率均低于3.0%。     

Effect of particle agglomeration on heavy metals adsorption by Al- and Ca-based sorbents during fluidized bed incineration

Al- and Ca-based materials can serve as metal sorbents or agglomeration inhibitors in fluidized beds. Although particle agglomeration could affect the adsorption efficiency of metal sorbents, Al- and Ca-based materials have been found to inhibit the particle agglomeration phenomenon during the incineration process. Accordingly, this study emphasizes the effect of particle agglomeration on heavy metal adsorption by Al- and Ca-based sorbents in a fluidized bed. The results show that Al- and Ca-based sorbents inhibit the tendency of particle agglomeration caused by Na. Thus, high concentrations of heavy metals exist mainly in fine particles compared to those present in the large particles when Na is added to the system. However, the addition of Na apparently increased the adsorption efficiency and followed the sequence of Cr > Cd > Pb with an increase in the operating temperature. A comparison of sorbents and Na additive for the relative enrichment factor showed that Na plays an important role in capturing Cr with Al- and Ca-based sorbents. Also, both sorbent adsorption and Na agglomeration are the dominant mechanisms for capturing Cd. The results of adding Na and an Al-based sorbent presented positive effects for Pb adsorption; however, a negative effect was observed with the addition of Na and a Ca-based sorbent at low temperatures.     

The effects of agglomeration/defluidization on emission of heavy metals for various fluidized parameters in fluidized-bed incineration

The agglomeration/defluidization may be produced to generate the secondary pollutant during incineration. However, the effects of agglomeration/defluidization on heavy metal distribution have rarely been examined. Therefore, the effects of the agglomeration/defluidization process on heavy metal emission in flue gas are studied. The artificial waste is employed to simulate municipal waste and to form agglomerates, which contain alkali metals, earth alkali metals, a mixture of metals (Pb, Cr and Cd) and sawdust. The fluidized parameters (including gas velocity, sand particle size and static bed height) are varied to determine their influences on heavy metal emission. The results indicate that addition of Na increases the risk of agglomeration/defluidization, but the emission concentration of heavy metals decreases during agglomeration/defluidization. The heavy metals may react with Na to form the eutectics or are covered and adhered by the liquid-phase eutectics of Na to stay in sand particle and lead to a decrease in the emission of heavy metals.The system was operated at a low gas velocity that not only easily resulted in agglomeration/defluidization but also increased the emission concentration of heavy metals. Large particles (920 μm), which have a poor fluidized quality, had the highest emission concentration. Small particles (645 μm) were uniformly fluidized to enhance the fluidization quality and to decrease the emission concentration. Additionally, adding Ca did not decrease the heavy metal emission concentration, but maintained the fluidization during eutectic accumulation. The Ca prevented the sand bed from quickly achieving defluidization and prolonged the increased emission of heavy metals after defluidization.     

不同粒径城市垃圾焚烧飞灰中重金属的浸出规律研究

集中焚烧法已经逐渐成为我国城市生活垃圾处理的主要方式,但其中的重金属在焚烧过程中不会被破坏,最终主要集中在垃圾焚烧飞灰中排出。在垃圾焚烧飞灰的安全填埋过程中由于受到环境因素的影响,例如酸雨,其中的重金属会逐渐的发生迁移,从而影响环境。文章对不同粒径的垃圾焚烧飞灰中的重金属浸出特性进行了研究,实验结果表明：小粒径垃圾焚烧飞灰所占比例较大。不同粒径对重金属的浸出特性比较复杂,浸出浓度均不相同,其中除尘器飞灰中的重金属浸出浓度远高于其他三种飞灰,属于有浸出毒性的危险废物。     

粒径对餐厨沼渣热解制备生物炭中磷和重金属的影响

以3种粒径餐厨沼渣为原料,在600℃下热解制备生物炭,研究粒径对沼渣（BR）及生物炭（BRC）中磷和重金属的影响,并采用TCLP浸出毒性和重金属潜在生态风险评估对其安全性进行系统研究。结果表明：BR及BRC中的磷主要以酸溶态磷（HCl-P）为主,残渣态磷（Res-P）次之,其余磷形态含量较低,总磷含量均呈现出随粒径增大而降低的趋势。热解促进H₂O-P、NaHCO₃-P和NaOH-P向HCl-P和Res-P转化。随着粒径的增大,BR中Cu、Zn总量增加,Cr减少,BRC中的Cr、As总量增加,Zn、Pb减少。并且BR中Cr、Zn、Pb和As中可氧化态和残渣态F3+F4随粒径增大而减少;BRC中Cr、Pb、As中F3+F4随粒径增大而减少,而Cu、Zn、Cd与之相反。TCLP浸出毒性和重金属潜在生态风险评估结果表明BR及BRC中重金属均属于低风险水平。     

Comparison of the characteristics of bottom ash and fly ash from a medium-size (32 MW) municipal district heating plant incinerating forest residues and peat in a fluidized-bed boiler

In this study, the physical and chemical properties of bottom ash and fly ash originating from the co-combustion of biomass-derived fuels (i.e. wood chips, sawdust, bark, and peat) from a 32 MW fluidized bed boiler at a municipal district heating plant were investigated. Silicate minerals were predominant in the bottom ash and calcium minerals in the fly ash, with most of the inorganic nutrients and heavy metals being enriched in the fly ash. The enrichment factors for heavy metals in the fly ash varied between 0.2 for silicon and 16.3 for lead, and for plant nutrients, between 1.5 for phosphorous and 108 for potassium. However, all heavy metal concentrations in both the bottom ash and fly ash were significantly lower than the current Finnish limit for maximum allowable heavy metal concentrations for forest fertilizers, which came into force in March 2007. According to the particle size distribution, the mass loadings of heavy metals in the fly ash were more than 90% contributed by the smallest particle size fraction lower than 0.074 mm. In the bottom ash, between 83.6 and 91.9% of the mass loadings of heavy metals were contributed by the particle size fraction between 0.5 and 2.0 mm.     

Influence of fuel ash composition on high temperature aerosol formation in fixed bed combustion of woody biomass pellets

In this work, the influence of fuel ash composition on high temperature aerosol formation during fixed bed combustion of woody biomass (two wood pellets and one bark pellets) were investigated experimentally in a laboratory reactor and theoretically through chemical equilibrium model calculations. For all fuels, the particle mass size distribution in the PM_2.5 region was bimodal, with one fine mode and one coarse mode. Early in the flame, the fine mode was dominated by particles from incomplete combustion and these particles were rapidly oxidised in the post flame zone. After the hot flame, the fine mode concentration and the particle diameter increases gradually when the temperature decreases due to condensation of vaporised inorganic matter, K, Na, S, Cl, and Zn. For two of the fuels also P could be found in the fine particles. The coarse mode consisted of carbon, refractory metals and considerable amount of alkali. Further, the initial fuel alkali concentration and the alkali to silicon ratio (K + Na)/Si influenced the amount of vaporised aerosol forming alkali matter. Finally, the present study shows that, combustion temperature and fuel ash composition is of major importance for the formation of high temperature aerosols in fixed bed combustion of woody biomass pellets.     

Monodisperse fine aerosol generation using fluidized bed

Monodisperse, fine aerosols are needed in many applications: filter testing, experiments for testing models, and aerosol instrument calibration, among others. Usually, monodisperse fine aerosols are generated in very low concentrations, or mass flow rates, in the laboratory scale. In this work, we needed to generate aerosols with higher mass flow rate than typically available by the laboratory-scale methods, such as atomizers, nebulizers, ultrasonic generators, vibrating orifice generators, and condensation generators. Therefore, we constructed a fluidized bed aerosol generator to achieve particle mass flow rates in the range of 15-100 g/h. Monodisperse, spherical SiO₂ particles of two sizes with geometrical diameters of 1.0 and 2.6 µm were used in the aerosol generator. The aerosol generator was used at both atmospheric pressure, and at high pressures up to 5 bar (abs).The particle size, mass concentration and the net average particle charge were measured after mixing the aerosol with nitrogen. The particle size distributions with both particle sizes were monodisperse, and no particle agglomerates were entrained from the fluidized bed. The behavior of the fluidized bed generator was found to be markedly different with the two particle sizes in regard to particle concentration, presumably due to different particle charging inside the generator. After determining the net average charge of the particles, an ion source Kr-85 was used to reduce the charge of the particles. This was found to be effective in neutralizing the particles.     

工业硫化砷渣的性质研究与环境风险分析

采用湖北某冶炼厂工业硫化砷渣作为试样,对其腐蚀性、酸度、粒径分布、基本元素组成、表面形态、沉降性能、重金属浸出毒性和元素形态及分布等物理化学性质进行了研究。研究表明,该硫化砷渣中的Cu主要以可氧化态和残余态形式存在,Pb主要以可氧化态形式存在,Zn、Cd主要以酸可提取态和残余态的形式存在。其浸出酸度达到62.55 mg/g,具有很强的腐蚀性。在酸雨条件下,工业硫化砷渣中的重金属形态可以发生转化,易对环境造成二次危害。但是,该废渣中w(As)接近30%,可作为砷原料再利用。     

酵母悬浮液进料时膨胀床空隙率和粒径分布的动态变化

引言 膨胀床吸附技术可以直接从细胞破碎液(未澄清液)中提取目标产物[1-3],而无需预先进行固液分离操作,具有快速、高效和集成化的特点.在实际应用中,料液中细胞(碎片)的存在将会影响到膨胀床的床层稳定性和操作性能[4-6].     

Model based prediction of required cut size diameter for fractionation of fly ash from a grate-fired wood chip incineration plant

Utilization of biomass ash as a soil improvement material is limited by the admissible input of heavy metals. It is well known that heavy metal concentrations are increased in fine ash fractions. In this study, two models are investigated to describe the distribution of various heavy metals in different size fractions of fly ash from a grate-fired biomass incineration plant incinerating wood chips. The second model assuming a dependence of the heavy metal concentration from the reciprocal particle diameter to the power of the variable N correlated well with measured concentrations. This model was then used in the calculation of the required cut size of a classifier for the production of a coarse fraction with heavy metal concentrations below limits from a fly ash sample exceeding the Austrian limits for a soil improvement material. The predicted concentrations of the critical heavy metals Cd and Pb and the total mass of the coarse fraction produced corresponded well with measured values, and although the concentration of Zn was considerably underestimated in the model, the produced coarse fraction was within the limits for utilization.     

Attrition behavior of fine particles in a fluidized bed with bimodal particles: Influence of particle density and size ratio

To process the solid particulates in fluidized bed and slurry phase reactors, attrition is an inevitable consequence and is therefore one of the preliminary parameters for the catalyst design. In this paper, the mechanical degradation propensity of the zeolite catalysts (particles) was investigated in a bimodal distribution environment using a Gas Jet Attrition — ASTM standard fluidized bed test (D-5757). The experimentation was conducted in order to explore parameters affecting attrition phenomena in a bimodal fluidization. In a bimodal fluidization system, two different types of particles are co-fluidized isothermally. The air jet attrition index (AJI) showed distinct increases in the attrition rate of small particles in a bimodal fluidization environment under standard operating conditions, in comparison with single particle. A series of experiments were conducted using particles of various sizes, with large particles of different densities and sizes. Experimental results suggest that the relative density and particle size ratio have a significant influence on attrition behavior during co-fluidization. Therefore a generalized relationship has been drawn using Gwyn constants; those defined material properties of small particles. Moreover, distinct attrition incremental phenomenon was observed during co-fluidization owing to the change in collision pattern and impact, which was associated with relative particle density and size ratios.     

电炉镍铁渣重金属浸出研究

通过分析电炉镍铁渣的矿物成分和物相组成,考察不同粒度电炉镍铁渣的重金属含量,采用BCR(Bureau Community of Reference)连续提取方法和调节浸提剂pH值进行电炉镍铁渣的浸出测试。结果表明:粉末状电炉镍铁渣的重金属含量最高,完整电炉镍铁渣的重金属含量小于破损的重金属含量;电炉镍铁渣属于一般工业废物,不属于危险废弃物;水溶态中重金属Cd更容易浸出,重金属Cu、Ni在弱酸环境下更容易浸出,在可还原状态下重金属Cr、Pb和Mn更容易浸出,在可氧化态下Zn和Cu更容易浸出;除Cu和Pb外,pH值的降低会导致电炉镍铁渣中的重金属浸出量增大,呈L型浸出曲线,Cu在碱性环境下会加速浸出,而Pb只有在强碱环境下才会大量浸出,因此在强碱环境下要注意Pb的浸出检测。     

Effects of chlorine and sulphur on particle formation in wood combustion performed in a laboratory scale reactor

Fine particle formation in wood combustion was studied in a laboratory scale laminar flow reactor at various flue gas chlorine and sulphur concentrations. Aerosol samples were quenched at around 850 °C using a porous tube diluter. Fine particle number concentrations, mass concentrations, size distributions and chemical compositions were measured. In addition, flue gas composition, including SO₂ and HCl, was monitored. Experimental results were interpreted by thermodynamic equilibrium calculations.Addition of HCl clearly raised fine particle mass concentration (PM1.0) which was because of increased release of ash-forming material to fine particles. Especially the release of K, Na, Zn and Cd to fine particles increased. These species form chlorides which apparently increases their volatilization from the fuel. When a sufficient amount of SO₂ was supplied in a chlorine rich combustion (S/Cl molar ratio from 4.7 to 7.5), most of the HCl stayed in the gas phase, release of ash-forming elements decreased and also fine particle concentrations dropped significantly. The sulphation of alkali metals is suggested to play a key role in the observed decrease in the fine particle concentration. It seems that the formation of sulphates leads to alkali metal retention in the coarse particle fraction.     

Cold flow experimental study and computer simulations of a compact spouted bed reactor

Spouted beds are a very interesting class of gas–solid contactors that possess excellent heat transfer and mixing characteristics, while they are particularly suited to process coarse particles. Proper design of such beds requires the prediction of various hydrodynamic characteristics, such as the minimum spouting velocity and maximum spoutable height. Contrary to their typical initial applications, spouted beds have been finding recently more frequent use on the one hand at endothermic processes and on the other hand using much finer particle sizes. In the current work, the hydrodynamic characteristics of a laboratory scale spouted bed of 0.05 m diameter have been investigated via cold flow studies using olivine particles of 3.55–5.00 × 10⁻⁴ m size. Hydrodynamic parameters have been measured at this compact geometry and fine particle size and were compared with common literature correlations. An empirical correlation was derived to predict the fountain height for the studied fine particle spouted bed. Computer simulations have been further used to investigate the heat transfer characteristics of the bed under endothermic reactive conditions, using methane reforming as a case study. Given sufficient external heat supply, a spouted bed operating at a well-mixed regime can efficiently drive even highly endothermic reactions.     

Combustion in bubbling fluidised bed with bed material of limestone to reduce the biomass ash agglomeration and sintering

A bed material of limestone was used in order to reduce/eliminate the tendency for bed material agglomeration and sintering that normally occurs in plants that operate with the traditional silica bed material. Combustion tests were carried out in a bubbling fluidised bed (BFB) combustion pilot plant (1 MWth). Mass balances of the inorganic elements and ash characterisation with respect to bed agglomeration, fouling and emissions were performed in the BFB combustion pilot plant.Limestone bed material with particle sizes between 0.25 and 2 mm, corresponding to a mean fluidisation velocity of 1.2 m/s and at a mean bed temperature of 775 °C, were chosen. It has been successfully proven that the limestone bed material eliminates the bed agglomeration. The calcium particles, which escape from the limestone bed material and are adhered on heat exchangers, reduce the sintering of ash deposits on the tubes.     

Numerical simulation of cohesive particle motion in vibrated fluidized bed

The effect of vibration on the cohesive particle motion in the bed was examined by using discrete element method(DEM). The bed of dried fine particles was treated as the objective of calculation. The van der Waals force was used as the cohesive force because the van der Waals force was considered to be the main cohesive force in this case. Since the actual calculation time was too long, the fine cohesive particle was difficult to be treated. So a relatively large particle (1.0 mm in diameter) was used in calculation and the van der Waals force was assumed that the ratio of gravity force to van der Waals force of particle used in this calculation was equal to that of a fine particle (6.0 μm in diameter), to express the effect of van der Waals force significantly. The calculation results were compared with that case of cohesionless particle.In the case with vibration, the cohesive particle motion in the bed is observed, though no fluidization state appears in the case without vibration, and there is no bubble in the bed even the fluidization state. In the case of cohesive particle, the collision energy between particle and wall caused by vibration gap propagates from the bottom to top of bed, and the particle moves vigorously at the top of bed in the case with vibration. As the vibration gap increases, the effect of vibration on the cohesive particle motion becomes larger, i.e., the low vibration frequency at the same vibration strength or the large vibration strength at the same vibration frequency promotes the fluidization of the bed.     

Fluidization Characteristics of a Fine Magnetite Powder Fluidized Bed for Density-Based Dry Separation of Coal

Gas-solid fluidized bed separation technique is very beneficial for saving water resources and for the clean utilization of coal resource. The hydrodynamics of 0.15–0.06 mm fine Geldart B magnetite powder were experimentally and numerically studied to decrease the lower size limit. The results show that the static bed height should be controlled near 300 mm (e.g., 300–350 mm). The bubble size, amount, and frequency of the fine particle bed are smaller than those of the bed containing 0.3–0.15 mm large Geldart B particles, thus leading to a higher bed activity. The pressure drop and density of the fine particle bed are uniform and stable, which indicates a good fluidization quality. Furthermore, simulated results are consistent with experimental data, which indicates the correctness and effectiveness of the simulations. The superficial gas velocity should be adjusted to not more than 1.8U _mf for the fine particle bed. Additionally, wide size range magnetite powder, which contains 94.23 wt% < 0.3 mm particles with a 0.3–0.06 mm particles content of 91.38 wt%, was used in an industrial scale modularized demonstration system for 50-6 mm coal density separation. The ash content of feed coal was reduced from 55.35% to 14.67% with a probable error, E, value of 0.06 g/cm^3.