Effects of raw material particle size distribution on the characteristics of Scots pine sawdust fuel pellets

In order to study the influence of raw material particle size distribution on the pelletizing process and the physical and thermomechanical characteristics of typical fuel pellets, saw dust of Scots pine was used as raw material for producing pellets in a semi industrial scaled mill (∼ 300 kg h^− 1). The raw materials were screened to a narrow particle size distribution and mixed into four different batches and then pelletized under controlled conditions. Physical pellet characteristics like compression strength, densities, moisture content, moisture absorption and abrasion resistance were determined. In addition, the thermochemical characteristics, i.e. drying and initial pyrolysis, flaming pyrolysis, char combustion and char yield were determined at different experimental conditions by using a laboratory-scaled furnace. The results indicate that the particle size distribution had some effect on current consumption and compression strength but no evident effect on single pellet and bulk density, moisture content, moisture absorption during storage and abrasion resistance. Differences in average total conversion time determined for pellet batches tested under the same combustion conditions was less than 5% and not significant.     

Comparison of particle size distribution of celestite mineral by machine vision ΣVolume approach and mechanical sieving

A reliable and accurate measurement of particle size and particle size distribution (PSD) is central to characterization of particulate minerals. Using mineral celestite (SrSO₄) as the test material, an inexpensive machine vision approach as an alternative to standard mechanical sieving was proposed and results were compared. The machine vision approach used a user-coded ImageJ plugin that processed the digital image in a sieveless manner and automated the PSD analysis. A new approach of employing sum of volumes (ΣVolume) as weighting factor was developed and utilized in the ASABE standard PSD analysis. The plugin also evaluated 22 significant dimensions characterizing samples and 21 PSD parameters. According to Folk and Ward's classification, the PSD of ball-milled celestite was “very finely skewed” and “leptokurtic”. The PSD of celestite followed a lognormal distribution, and the plot against particle size exhibited almost a linear trend for both machine vision and mechanical sieving methods. The cumulative undersize PSD characteristics of both methods matched closely when the width-based mechanical sieving results were transformed to lengths by applying the shape factor (width/length). Based on the study, this machine vision approach can be utilized for PSD analysis of particulate minerals and similar products.     

Effects of particle shape and size on devolatilization of biomass particle

Experimental and theoretical investigations indicate particle shape and size influence biomass particle dynamics, including drying, heating rate, and reaction rate. Experimental samples include disc/flake-like, cylindrical/cylinder-like, and equant (nearly spherical) shapes of wood particles with similar particle masses and volumes but different surface areas. Small samples (320 μm) passed through a laboratory entrained-flow reactor in a nitrogen atmosphere and a maximum reactor wall temperature of 1600 K. Large samples were suspended in the center of a single-particle reactor. Experimental data indicate that equant particles react more slowly than the other shapes, with the difference becoming more significant as particle mass or aspect ratio increases and reaching a factor of two or more for particles with sizes over 10 mm. A one-dimensional, time-dependent particle model simulates the rapid pyrolysis process of particles with different shapes. The model characterizes particles in three basic shapes (sphere, cylinder, and flat plate). With the particle geometric information (particle aspect ratio, volume, and surface area) included, this model simulates the devolatilization process of biomass particles of any shape. Model simulations of the three shapes show satisfactory agreement with the experimental data. Model predictions show that both particle shape and size affect the product yield distribution. Near-spherical particles exhibit lower volatile and higher tar yields relative to aspherical particles with the same mass under similar conditions. Volatile yields decrease with increasing particle size for particles of all shapes. Assuming spherical or isothermal conditions for biomass particles leads to large errors at most biomass particle sizes of practical interest.     

Solids concentration simulation of different size particles in a cyclone separator

To deepen our knowledge of the flow in cyclones, the solids concentrations of different size particles in a scroll cyclone separator were numerically simulated by using the Lagrange approach on the platform of commercial CFD software package, FLUENT 6.1. The numerical calculations visualize that there exists a spiral dust strand near the cyclone wall and a dust ring beneath the cyclone top plate. There are two regions in the radial solids concentration distribution, with which the solids concentration is low in the inner region (r/R(dimensionless radial position) ≤ 0.75) and increases greatly in the outer region (r/R > 0.75). Large particles generally have higher concentration in the wall region and small particles have higher concentration in inner vortex region. The axial distribution of the solids concentration in the inner vortex region (r/R ≤ 0.3) shows that serious fine particle re-entrainment exists within the height of 0.5 D (cyclone diameter) above the dust discharge port. We study the effect of solids particle on the gas flow field by two-way couple. The concepts of back-mixing rate, first escaping rate and second escaping rate are proposed for quantifying the local flow phenomena.     

Effect of moisture content, particle size and pine addition on quality parameters of barley straw pellets

Agripellets from barley straw wastes have been produced in an annular die pellet mill. The semi-industrial scale of the pilot plant allowed for measurement of the die temperature, and an accurate control of the straw moisture at the die inlet. The pellet mechanical durability, density, length and moisture were evaluated for pure straw and blended pellets, together with the heating value and the ash content. The composition of the raw material barely changed with the compaction process. Optimum moisture contents for dense barley straw pellets production proved to be in the range of 19-23%. A durability value of 95.5% was reached under these conditions, increasing to 97-98% when small quantities of pine sawdust were added (2, 7, and 12 wt.% of pine in straw). Agglomeration of the ground particles was improved by water and pine addition, while a coarser grinding did not show any negative effect on barley straw compaction.     

Micrometer-scale 3-D shape characterization of eight cements: Particle shape and cement chemistry, and the effect of particle shape on laser diffraction particle size measurement

Eight different portland cements were imaged on a synchrotron beam line at Brookhaven National Laboratory using X-ray microcomputed tomography at a voxel size of about 1 µm per cubic voxel edge. The particles ranged in size roughly between 10 µm and 100 µm. The shape and size of individual particles were computationally analyzed using spherical harmonic analysis. The particle shape difference between cements was small but significant, as judged by several different quantitative shape measures, including the particle length, width, and thickness distributions. It was found that the average shape of cement particles was closely correlated with the volume fraction of C₃S (alite) and C₂S (belite) making up the cement powder. It is shown that the non-spherical particle shape of the cements strongly influence laser diffraction results, at least in the sieve size range of 20 µm to 38 µm. Since laser diffraction particle size measurement is being increasingly used by the cement industry, while cement chemistry is always a main factor in cement production, these results could have important implications for how this kind of particle size measurement should be understood and used in the cement industry.     

Characterizing activated sludge process effluent by particle size distribution, respirometry and modelling

The particle size distribution (PSD) of the effluent from a typical activated sludge process was measured. The relation between the particle size distribution (PSD), residual biomass and conventional suspended solids (SS) and turbidity of a typical activated sludge process effluent was established with the help of laser scattering, oxygen uptake rate (OUR) measurement and the IWA model analysis. The laser scattering technique is capable of measuring particles between 20 nm and 2000 µm. The final effluent also consists of residual biomass which is unsettled in the secondary settling tank (SST) and the amount of this residual biomass was estimated using OUR measurement and IWA model. The final effluent was found to be dominated by the particles of size 1-10 µm, the size range for most of the bacterial cells (Ekama et al. 1997). The results showed that the total number of particles was correlated to the amount of residual biomass. Correlation determining the relation between laser particle size monitoring results and SS/turbidity measurements was also found. The results showed that the particle monitoring was more sensitive than the SS/turbidity measurement and with further development and improved control, it can used for more complex and sensitive data.     

Particle size design of digitoxin in supercritical fluids

Bioavailability of the pharmaceutical substances is very important for their activity. In case of necessity, bioavailability can be improved by reducing the particle size of the drugs. In this study, particle size of digitoxin was reduced by the Rapid Expansion of Supercritical Solutions (RESS). The effects of pre-expansion temperature (90-110 °C), flow rate (2.5-7.5 ml/min), spray distance (3-7 cm) on the size and size distribution of the precipitated digitoxin particles were carried out. The particles were characterized by dynamic light scattering (DLS), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and LC-MS analysis.While the particle size range of the original digitoxin was 0.2-8 μm, it was decreased to 68-458 nm and determined that 97% of the particles were below 200 nm depending on the different experimental conditions.Response surface method (RSM) was used to optimize the process parameters. The flow rate, 7 ml/min; spray distance, 7 cm; pre-expansion temperature, 95 °C were found to be the optimum conditions to achieve the minimum particle size of digitoxin.     

Density of softwood bark and softwood char: procedural calibration and measurement by water soaking and kerosene immersion method

Particle density plays an important role in the design and operation of thermoconversion reactors fed with bed of particles such as softwood bark (SB). Little information is available on the single particle density of SB and charcoal derived. As SB and softwood char (SC) particles are highly irregular in shape and size, conventional methods of particle density measurement cannot be applied. A method known as the water soaking and kerosene immersion procedure, used for density measurement of sugarcane bagasse, has been tested and adapted to the density measurement of individual particles of softwood (SW), SB and SC. The particle density has been determined to be 360, 482 and 299 kg m⁻³ for SW, SB and SC, respectively. The average particle density of a typical SB feedstock sample comprised of ca. 30% SW and 70% SB was calculated to be 438 kg m⁻³.     

Internal particle size distribution of biofuel pellets

Several methods for disintegration of biofuel pellets were tested and compared for their ability to break up the pellets into the original particles of the raw material. Analyses performed on softwood pellets and straw pellets concluded that wet disintegration in water at ambient temperature is insufficient for a determination of the internal particle size distribution of wood- and straw pellets. When the wet disintegration was performed with water heated to the boiling point and coupled with mechanical disintegration in terms of stirring a more complete disintegration of the pellets was obtained. Based on the results obtained in the initial study a round robin was set up including six European laboratories where the selected method was tested. In the round robin test the method combining heated water and stirring of the slurry was tested on solid biofuel pellets produced of comminute straw, deciduous wood and coniferous wood respectively. With the method a satisfactory disintegration was obtained of all three types of pellets.Further wet disintegration of coniferous pellets was compared to a dry disintegration using a hammer mill. The dry disintegration of the coniferous pellets resulted in smaller particle sizes compared to the wet disintegration using heated water and stirring of the slurry indicating a further disintegration of the original particles in the hammer mill process.Overall the wet disintegration combined with mechanical impact was found to be the most suitable method for disintegration of solid biofuel pellets. Combined with sieving analysis the method gives realistic image of the internal particle size distribution of solid biofuel pellets.     

Influence of catalytic particle size on the performance of fluidized-bed chemical vapor deposition synthesis of carbon nanotubes

In this paper, the impacts of catalytic particle size on the overall reactor performance for carbon nanotubes (CNTs) production using a fluidized-bed chemical vapor deposition (FBCVD) process have been studied. Six different particle size fractions (10-20 μm, 20-53 μm, 53-75 μm, 75-100 μm, 100-200 μm, and 200-300 μm) were selected. It was observed that the smaller the catalytic particle diameter, the greater the carbon deposition efficiency and the greater CNT synthesis selectivity. The 10-20 μm catalytic particles exhibited 30% higher carbon deposition efficiency than the 200-300 μm catalytic particles. The selectivity toward CNTs formation was also approximately 100%. These observations could be explained by the fact that when the diameter of the catalytic particle gets smaller, the breakthrough capacities during frontal diffusion will be bigger due to a shorter diffusion path length within the particle. Moreover, the fine particles ensured high interstitial velocity which subsequently enhances the heat and mass transfer, and consequently improves the CVD reaction.     

The effect of coal particle size on pyrolysis and steam gasification

For future power generation from coal, one preferred option in the UK is the air-blown gasification cycle (ABGC). In this system coal particles sized up to 3 mm, perhaps up to 6 mm in a commercial plant, are pyrolysed and then gasified in air/steam in a spouted bed reactor. As this range of coal particle sizes is large it is of interest to investigate the importance of particle size for those two processes. In particular the relation between the coal and the char particle size distribution was investigated to assess the error involved in assuming the coal size distribution at the on-set of gasification. Different coal size fractions underwent different changes on pyrolysis. Smaller coal particles were more likely to produce char particles larger than themselves, larger coal particles had a greater tendency to fragment. However, for the sizes investigated in this study ranging from 0.5 to 2.8 mm, the pyrolysis and gasification behaviour was found not to vary significantly with particle size. The coal size fractions showed similar char yields, irrespective of the different char size distributions resulting from pyrolysis. Testing the reactivity of the chars in air and CO₂ did not reveal significant differences between size fractions of the char, nor did partial gasification in steam in the spouted bed reactor. From the work undertaken, it can be concluded that pyrolysis and gasification within the range of particle sizes investigated are relatively insensitive to particle size.     

Effect of grain size and density of spray-pyrolyzed hydroxyapatite particles on the sinterability of hydroxyapatite disk

The effect of grain size and density of hydroxyapatite particles, which were prepared by different spray-pyrolysis temperatures, on the sinterability of hydroxyapatite disk was investigated. Calcium phosphate solution (Ca/P ratio of 1.67 and 0.1 M concentration) was prepared by reacting calcium nitrate tetrahydrate and diammonium hydrogen phosphate solutions, and adding nitric acid. Spray-pyrolysis was carried out at 900 °C, 1200 °C, and 1500 °C at a carrier gas flowing rate of 10 L/min. The particles synthesized at 900 °C were large, hollow spheres with a hole at the outer surface, a broad size distribution, but had small grain sizes. Conversely, the particles synthesized at 1500 °C were small, solid spheres with a narrow size distribution, but had large grain sizes. The particles synthesized at 1200 °C had intermediate properties. A sinterability test conducted at 1100 °C for 1 h demonstrated that small and dense particles with large grain sizes showed a higher relative sintered density compared with large and hollow particles with small grain sizes. The results were explained in terms of the grain size and density of a particle, which were inversely and proportionally affected to sinterability. The practical implication of these results is that highly sinterable hydroxyapatite powders can be synthesized through spray-pyrolysis at a high temperature under a fixed initial concentration of calcium phosphate solution and flow rate of carrier gas.     

Density measurement of size selected multiwalled carbon nanotubes by mobility-mass characterization

We employ a combination of gas phase particle mobility and mass methods to make the first absolute density measurement of gas phase grown carbon nanotubes (CNTs). The approach combines a tandem differential mobility analyzer and aerosol particle mass analyzer in series to achieve two steps of electrical mobility classifications of the CNTs and one of mass classification. In the first mobility classification step a stream of monodisperse catalytic particles was produced by pulsed laser ablation. These mobility-classified catalysts seeded the aerosol growth of CNTs, where were directly passed to a second electrical mobility classification step which allows classification of the diameter-controlled CNTs in length. These diameter- and length-classified CNTs were finally introduced into the aerosol particle mass analyzer to measure their mass distribution. We found that the condensed phase density of CNTs was 1.74 ± 0.16 g/cm³ for two different groups of CNTs with diameters of ∼15 and ∼22 nm. This value is lower (about 3 sigma) than for graphite, and about 1 sigma lower than the average value for density measurements for carbon black.     

Pressurized gasification of woody biomass—Variation of parameter

Pressurized gas produced from biomass is a renewable resource that is attracting a great deal of attention due to its wide range of industrial applications, such as the production of hydrogen, chemicals or high grade fuels. Therefore, the Vienna University of Technology in cooperation with BioEnergy 2020+ is operating a bubbling pressurized gasification plant. The pressurized research unit (PRU) is able to perform the gasification of wood chips, wood pellets, coal and other solid fuels with gasification agents air, steam, oxygen or carbon dioxide. This paper gives the results of parameter variation at this plant with regard to the producer gas composition. The feedstock was wood pellets and as bed material olivine was used with an average particle size of 0.5 mm. The parameters varied were temperature (720-900 °C), pressure (1-5 bar), air ratio (0.2-0.4), gasification agent (air, steam, oxygen), biomass feed input (4.5-8 kg/h) and the fluidization conditions of the reactor fluidized bed (fluidization number (3-7)).     

Density measurement of fine aerosol fractions from wood combustion sources using ELPI distributions and image processing techniques

Aerosols from combustion sources are of high concern since they present a risk for health and environment. Particle size distribution of aerosols and in particular number size distribution are easily and quickly obtained using an Electrical Low Pressure Impactor (ELPI). However, this technique is depending of aerosol density; ρ, which may lead to biased particle size distributions. Aerosol density from combustion sources is usually not well known and depends on several parameters. Aerosol density cannot be measured with usual methods since there is generally not enough matter collected on each stage of the ELPI. Our approach uses electronic microscopy to evaluate ρ at each impaction stage in order to increase the accuracy of the number size distributions resulting from the ELPI measurements.Particles were collected on glass substrates deposited on each impaction stages. Images were obtained using a scanning electron microscope and image processing tools were applied.This method was first tested with silica particles resulting from a combustion process which have a constant density found to be comprised between 2.2 and 2.4 g cm⁻³ for stages 2 (57 and 95 nm) and 3 (95 and 158 nm), respectively. Once validated, this method was used to determine the density of wood combustion aerosols. The results match well for fly ashes from wood combustion with densities varying from 1.1 to 3.0 g cm⁻³ for particles of mean equivalent diameter ranging from 69 to 157 nm, respectively.     

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.     

Control of particle size distribution for the synthesis of small particle size high solids content latexes

A polymerization process to synthesize bimodal latexes with maximum particle diameters below 350 nm and solids content above 65 wt% has been developed.The process is based on an iterative strategy to determine the optimal particle size distribution that gives the maximum packing factor for a given range of particle sizes and at a given solids content. The calculated optimal bimodal PSD was experimentally obtained in a seeded semi-continuous emulsion polymerization reaction as follows: in the first step, a polymer seed latex was loaded in the reactor and grown, under monomer starved conditions, until a given particle size. At this point a fraction of the same seed was added to the reactor and the feed was continued until the desired particle size distribution and solids content were achieved. The point at which the seed was added again to the reactor and the amount of seed required were determined by the iterative strategy and depended on the competitive growth rate ratio of large and small particles that is an input for the iterative strategy.Implementation of the solution obtained from the iterative strategy, and for the first time in the open literature, led to the production of a coagulum free and stable bimodal latex with 70 wt% of solids content and particle sizes below 350 nm.     

Determination of non-spherical particle size distribution from chord length measurements. Part 2: Experimental validation

In this paper, the theory on the translation of a measured chord length distribution (CLD) into its particle size distribution (PSD), which was developed in the first part of this study [Li and Wilkinson, 2005. Determination of non-spherical particle size distribution from chord length measurements. Part 1: theoretical analysis. Chemical Engineering Science 60, 3251-3265], has been validated using experimental results. CLDs were measured using the Lasentec focused beam reflectance measurement (FBRM) with three different materials, spherical ceramic beads and non-spherical plasma aluminium and zinc dust particles. Meanwhile, the particle shape and PSD of each material were also investigated by image analysis (IA). Comparison of the retrieved PSDs with the measured PSDs by IA shows that the PSD can be retrieved from a measured CLD successfully using the proposed iterative nonnegative least squares (NNLS) method based on the PSD-CLD model.     

Effects of particle size on the fast pyrolysis of oil mallee woody biomass

This study aims to investigate the effects of biomass particle size (0.18-5.6 mm) on the yield and composition of bio-oil from the pyrolysis of Australian oil mallee woody biomass in a fluidised-bed reactor at 500 °C. The yield of bio-oil decreased as the average biomass particle size was increased from 0.3 to about 1.5 mm. Further increases in biomass particle size did not result in any further decreases in the bio-oil yield. These results are mainly due to the impact of particle size in the production of lignin-derived compounds. Possible inter-particle interactions between bio-oil vapour and char particles or homogeneous reactions in vapour phases were not responsible for the decreases in the bio-oil yield. The bio-oil samples were characterised with thermogravimetric analysis, UV-fluorescence spectroscopy, Karl-Fischer titration as well as precipitation in cold water. It was found that the yields of light bio-oil fractions increased and those of heavy bio-oil fractions decreased with increasing biomass particle size. The formation of pyrolytic water at low temperatures (<500 °C) is not greatly affected by temperature or particle size. It is believed that decreased heating rates experienced by large particles are a major factor responsible for the lower bio-oil yields from large particles and for the changes in the overall composition of resulting oils. Changes in biomass cell structure during grinding may also influence the yield and composition of bio-oil.