Concentration Of Ash Particles Research Articles

Increasing the efficiency of burning solid fuel in the layer

he main directions of increasing the efficiency of solid fuel heat generators are analyzed. It is noted that in industrial designs of heat generating installations depending on their class technological schemes of burning of solid fuel are applied at various levels on capital and operational expenses. The most expensive are the technologies of combustion of pre-prepared high-quality fuel in heat generators with complex mechanization at all stages of the combustion process, which provide high thermal and environmental performance. Cheaper combustion technologies are widespread, according to which the normative performance of heat generators is achieved due to the rational ratio of design and mode parameters of the combustion zone with the thermophysical characteristics of the burned fuel, including low-grade. Examples of realization of such technological schemes of combustion on the basis of results of theoretical and experimental researches in designs of solid propellant heat generators are resulted. The paper proposes a constructive scheme of a small-capacity heat generator with a combined technology of two-chamber solid fuel combustion, in which a layered (or shaft-layered) combustion process with combustion in volume is combined: in the primary combustion chamber above the burning fuel layer on the grate, and in the secondary cyclone chamber (or cameras). Due to the vortex motion in the lined secondary chamber of high-temperature gases containing solid particles carried by the flow from the combustion zone of the primary chamber, there is both afterburning of combustible substances and separation of ash particles with their discharge into the ash generator. This combustion technology also allows increasing the forcing of the combustion layer on the grate, increasing the stability of the process and reducing the dimensions of the structure. Improving the efficiency of a solid fuel heat generator with a two-chamber furnace is achieved by reducing heat loss with mechanical incomplete combustion of carbon particles in the exhaust gases, as well as reducing the concentration of ash particles in the gas stream at the inlet of convective heat exchange surfaces and increase their thermal efficiency. This is confirmed by the calculated dependences of the efficiency of the mechanized solid fuel heat generator under different technological schemes of solid fuel combustion

Experimental Study on Pressure Drop and Heat Transfer Characteristics of Wet Air with Ash Particles across Circular Finned Tube Bundles

In this paper, in order to provide guidance on the design and operation of waste heat recovery heat exchangers for the dryer exhaust of lignite pre-drying, pressure drop and heat transfer characteristics of wet air with ash particles across circular finned tube bundles were experimentally studied under different conditions. The experimental water vapor mass fraction ranged from 5% to 15%, ash particle concentration ranged from 0 to 12.8 g/m3 and mainstream flow velocity ranged from 1.73 m/s to 3.46 m/s, respectively. The pressure drop coefficient generally increases and Nusselt number decreases with time due to the gradual ash deposition. The pressure drop coefficient is sensitive to ash particle concentration, while Nusselt number is sensitive to water vapor mass fraction. Especially, Nusselt number for the ash particle concentration of 1.5 − 5.4 g/m3 is a bit larger than that for the wet air without ash particles. Based on the experimental data, a normalized empirical formula was proposed to calculate the heat transfer characteristics of wet air with ash particles across circular finned tube bundles, and the deviation between experimental data and predicted results is −16.5% to +27.0% for more than 90% of the data.

Experimental study on ash deposition characteristics of dryer exhaust along horizontal tube bundles using digital imaging techniques

In this study, wet air with ash particles was used to simulate real dryer exhaust from lignite-drying process, and its ash deposition characteristics along horizontal tube bundles were experimentally studied to provide theoretical guidance for the design and operation of waste heat recovery devices. The ash deposition on the longitudinal section of the tube bundles was considered, and the ash deposition thickness under different experimental conditions was quantitatively measured using digital imaging techniques. The spatial and temporal distributions of the ash deposition thickness and its variation with mainstream inlet temperature, mainstream velocity, water vapor mass fraction, and ash particle concentration were presented. The ash accumulated on the windward side of the tube bundles and mainly concentrated on the first five rows in the inlet section, especially on the first two rows. The rapid growth of ash deposition mainly occurred in the first 3000 s and then gradually stabilized. The ash deposition thickness was insensitive to the mainstream inlet temperature and velocity, whereas the water vapor mass fraction and ash particle concentration had significant influences on the ash deposition thickness. When the water vapor mass fraction increased from 5% to 15%, the average ash deposition thickness decreased from 7.67 mm to 1.87 mm. As the ash particle concentration increased from 1.0 g·m−3 to 14.8 g·m−3, the average ash deposition thickness increased from 1.91 mm to 26.0 mm. The change in ash deposition thickness was significantly larger than that in heat transfer coefficient under different conditions.

УЛЬТРАЗВУКОВАЯ КОАГУЛЯЦИЯ В СКРУББЕРАХ ВЕНТУРИ: ОСОБЕННОСТИ РЕАЛИЗАЦИИ И ЭФФЕКТИВНОСТЬ ПРИМЕНЕНИЯ

Актуальностьисследованияобусловлена экологическими требованиями к дымовым газам предприятий ТЭЦ и ТЭС, работающих на угле. Обеспечение экологических требований осуществляется с помощью ультразвуковой коагуляции твердыхзоловых частиц с каплями жидкости, которая реализуется в устройствах мокрой очисткигазов – в скрубберах Вентури. Цель: увеличение эффективности скрубберов Вентури для мокрой очистки газов за счет улучшения условия поглощения золовых частиц каплями жидкости в акустических полях, которые создаются при ультразвуковой коагуляции. Объекты: процессы ультразвуковой коагуляции золовых частиц с каплями орошаемой жидкости в скруббере Вентури. Методы. Теоретические методы основывались на численном решении уравнений, описывающих движение газодисперсного потока в скруббере Вентури и коагуляцию золовых частиц в акустическо мполе. Экспериментальные результаты были получены при проведении промышленных испытаний на котлоагрегате БКЗ-220-100Ф ТЭЦ, работающем на угле Харанорского месторождения, по утвержденной стандартомметодике РД153-34.1-27.301-2001. Результаты. Разработана физико-математическая модель, описывающая движение газодисперсного потока вскруббереи взаимодействие дисперсных частиц между собой в акустическом поле. Модель позволила рассчитать эффективность газоочистки в различных условиях и определить оптимальные режимы ультразвукового воздействия: уровень звукового давления не менее 145 дБ, частота 22 кГц, зона воздействия во всем объеме скруббере Вентури, направление акустического поля перпендикулярно потоку газа. Критериями оптимальности являлись максимальная эффективность коагуляции золовых частиц вскруббере Вентури и максимальная степень очистки дымовых газов. Проведены испытанияс озданного ультразвукового оборудования на котлоагрегате БКЗ-220-100Ф ТЭЦ в золоулавливающей установке: в акустическом поле уменьшается концентрация золовых частиц в отводящих газах до четырех и более раз (эффективность не менее 98%), процентное содержание золовыхчастиц (размеромменее5мкм) снижаетсядо15раз.

Ash sedimentation by fingering and sediment thermals from wind-affected volcanic plumes

Ash fallout and volcanic plume dispersion represent critical hazards for local and global human populations for minutes to years after the onset of an eruption. Understanding the key processes governing the sedimentation of ash particles is a major challenge in modern volcanology from modeling and risk management perspectives. Recent experiments predict that sedimentation from eruption clouds rich in fine-grained ash can be driven by convective phenomena in the form of ∼100 m to km-scale ash fingers related to the intermittent formation and detaching of ash-rich particle boundary layer. Remote sensing observations of mammatus and cloud veils from numerous eruptions over recent years as well as field observations of spatially discontinuous ash deposits are consistent with this prediction. Indeed, this mode of ash sedimentation is predicted to be the predominant mechanism of fine ash removal for a significant fraction of eruptions in the geological records. Here, we use a novel combination of 3 millimeter-wavelength Doppler radar observations and time series of optical disdrometer data to characterize for the first time in real-time the time-varying structure and sedimentation properties of volcanic ash plumes under steady wind conditions. As a case study, we apply this new method to weak short-lived plumes from Stromboli Volcano. 96% of the disdrometer proximal sedimentation data highlight pulsatory phases of increased sedimentation rate that are 20–60 s apart and characterized by particle size distribution variation with bulk concentrations up to 681 mg/m3. Radar data also record intermittent periods of higher reflectivity (i.e. a factor of 3 in mass concentration) inside the ash sedimentation interspersed by 30 to 50 s and interpreted as ash fingers crossing the radar beam. From time series of radar signals and ground-based disdrometer measurements, together with simple analog experiments, we develop a conceptual model for intermittent sedimentation from wind-affected ash plumes. In particular, we show that when wind speeds are comparable to or greater than ash settling velocities, the dynamics of wind-driven rolls in ash clouds can govern the production of gravitational instabilities and the occurrence and timing of ash fingers that form descending sediment thermals in turn. This study suggests that ambient wind should affect the maximum size and minimum concentration of ash particles needed to form fingers but also control where and when fingers form at the base of wind-drifted ash plumes. These novel predictions highlight the need for future work aimed at refining our understanding of ash finger formation under windy conditions from the perspectives of in-situ characterization, numerical modeling and risk assessment of fine ash dispersal from the most frequent style of eruptions on Earth.

Method for determining blast furnace tuyere zone blackness degree on pulverized coal fuel injection

Pulverized coal fuel (PCF) supply is one of the modern methods of improving the cast iron production technology in blast furnaces (BF). It has a significant effect on the heat exchange in the tuyere zone (TZ). The known method of radiant heat transfer calculation in TZ accounts only for the ash particle effect on the TZ blackness degree. The aim of this work is to develop a method for determining TZ blackness degree and to investigate the effect of different factors on this value. The work was based on the elements of the proven method of calculating heat exchange in boiler furnaces. Parametric sensitivity was investigated by alternately simulating an error in each of the input values and determining its effect on the sought value. By analysing the processes, the authors have derived dependences for obtaining the concentration of ash particles and triatomic gases and determined the degrees of TZ blackness by Bouguer’s law simultaneously taking into account the radiation of the burning coke and ash particles, and triatomic gases. It has been established that an increase in the PCF consumption from 0 to 250 kg/t of pig iron changes the TZ blackness degree from 0,067 to 0,243; and a 1 % increase in the pressure, concentration of triatomic gases, ray effective length, TZ temperature and ash particle size changes the TZ blackness by 0,714 %, 0,151 %, 0,72 %, –1,557 % and –0,176 %, respectively. The calculations have shown that the effects of coke particles, triatomic gases and ash particles on the sought value are approximately equal. In contrast, when the conventional method is applied, it results in an approximately 2-time underestimation of the values, while even for evaluation calculations, the maximum permissible error is 10–15 %. The developed method accounting for all factors has the maximum error of 5 %. It is important for designing tuyeres and TZ refrigerators.

Experimental research on convection–condensation heat transfer characteristics and sootblowing operation strategy of waste heat recovery heat exchanger for dryer exhaust

In this study, air/water vapor mixture with ash particles was used to simulate actual dryer exhaust gas. An experimental study of the convection–condensation heat transfer characteristics of air/vapor mixtures with ash particles across horizontal tube-bundles was conducted under various water vapor mass fractions and concentrations of ash particles. The variation of convection–condensation heat transfer coefficient with time presented four different types, as follows: almost did not change with time; decreased first, then reached stable values; decreased continuously in the experimental time; and decreased rapidly, decreased slowly, and decreased rapidly, with potential to increase again. In general, the convection–condensation heat transfer coefficient decreased with increasing concentration of ash particles and increased with increasing water vapor mass fraction. The reducing effect on the convection–condensation heat transfer coefficient by ash deposition would be weakened by increasing the water vapor mass fraction, but the heat exchanger would be blocked further easily by increasing the concentration of ash particles. Then, a distribution diagram was drawn to present different types of convection–condensation heat transfer curves under various working parameters. Sootblowing operation strategies were proposed under different working conditions to provide reference for the actual operation of the waste heat recovery heat exchanger for dryer exhaust.

Ash Particles Detected in the Tropical Lower Stratosphere

AbstractWe report here on measurements of relatively large (up to ≃5‐μm maximum dimension) particles detected in the tropical western Pacific lower stratosphere. The particles were observed at low relative humidities and contain no detectable ice mass. We conclude that these particles are silicate ash injected by the Mount Kelud eruption in Indonesia a few weeks prior to the aircraft observations. The Kelud ash particles were detected by cloud probes on the National Aeronautics and Space Administration Global Hawk between the lapse rate tropopause (≃16.5 km) and the aircraft ceiling (18.5 km). The ash particles detected in the lowermost stratosphere by the aircraft extended well into the northern subtropics. The concentration of ash particles dropped off rapidly below the lapse rate tropopause, presumably because of rapid removal by ice nucleation scavenging. Although ash particles are very effective ice nuclei, the low particle concentrations detected in the upper troposphere precludes a significant impact on cirrus clouds.

Interactions of Fly Ash Particles with Mucin and Serum Albumin.

Fly ash particles can contribute to haze and adverse health outcomes. In this study, two mucins, one from bovine submaxillary glands (bovine submaxillary mucin, BSM) and one from porcine stomach (porcine gastric mucin), as well as bovine serum albumin (BSA), which served as the physical barriers against foreign substances entering the tissues and the blood protein, respectively, were chosen as models for the investigations of the interactions between the proteins and the fly ash particles. Their adsorption behaviors were studied using spectroscopy and a quartz crystal microbalance with a dissipation monitor (QCM-D). The results indicated that the fly ash particles can induce the loosening of mucins and BSA, probably via the formation of complexes. Further, the secondary structure of proteins changed in the presence of fly ash particles. The α-helix content decreased with an increasing fly ash particle concentration. The addition of fly ash particles into protein solutions led to fluorescence quenching, which suggested that there were interactions between these particles and the mucins and BSA. The association constants ( Ka) for BSM and BSA were 5.35 and 4.18 L/g, respectively. Furthermore, the results of QCM-D analyses showed that the amount decreased on the mucin surface but increased slightly on the BSA surface, which indicated that the fly ash particles disrupted the mucin layer upon adsorption. These findings provide clear evidence of the interactions between the fly ash particles and the mucins and BSA, which can lead to structural changes. This study contributes to a better understanding of the interactions and adsorptions of atmospheric particulate pollutants with the proteins in the human body.

Convection–condensation heat transfer characteristics of air/water vapor mixtures with ash particles along horizontal tube bundles

Lignite drying is an effective way of clean and efficient use of lignite. In the waste heat recovery processes of dryer exhausts, ash particles widely exist. In order to provide a reference for the designation of waste heat recovery heat exchangers of dryer exhausts, air/water vapor mixture with ash particles was used to simulate actual dryer exhausts in this paper, and its convection–condensation heat transfer characteristics along horizontal tube bundles were experimentally investigated under various conditions. The effects of concentration of ash particles, water vapor mass fraction, mainstream velocity and temperature were discussed. In the presented experimental conditions, as the convection–condensation proceeded, the convection–condensation heat transfer coefficients of air/water vapor mixture with ash particles reduced and then gradually became stable due to the ash deposition. It was found that the convection–condensation heat transfer coefficients increased with the increase in the water vapor mass fraction and the mainstream velocity, and decreased with the increase in the concentration of ash particles and the mainstream temperature. As the concentration of ash particles increased from 0 to 6.5 g/m3, the convection–condensation heat transfer coefficient decreased from 339.6 to 315.5 W/(m2·K) when the water vapor mass fraction was 10%, decreased 7.10%. And with increasing the water vapor mass fraction from 5% to 25%, the convection–condensation heat transfer coefficient increased from 201.8 to 960.0 W/(m2·K) when the concentration of ash particles was 5.4 g/m3, increased 4.76 times. A normalized correlation of convection–condensation heat transfer coefficient of air/water vapor mixture with ash particles were derived, with the deviation of −12.5% to +12.8%.

Inside Volcanic Clouds: Remote Sensing of Ash Plumes Using Microwave Weather Radars

Microphysical and dynamical features of volcanic tephra due to Plinian and sub-Plinian eruptions can be quantitatively monitored by using ground-based microwave weather radars. The methodological rationale and unique potential of this remote-sensing technique are illustrated and discussed. Volume data, acquired by ground-based weather radars, are processed to automatically classify and estimate ash particle concentration and fallout. The physical– statistical retrieval algorithm is based on a backscattering microphysical model of fine, coarse, and lapilli ash particles, used within a Bayesian classification and optimal estimation methodology. The experimental evidence of the usefulness and limitations of radar acquisitions for volcanic ash monitoring is supported by describing several case studies of volcanic eruptions all over the world. The radar sensitivity due to the distance and the system noise, as well as the various radar bands and configurations (i.e., Doppler and dual polarized), are taken into account. The discussed examples of radar-derived ash concentrations refer to the case studies of the Augustine volcano eruption in 2002, observed in Alaska by an S-band radar; the Grímsvötn volcano eruptions in 2004 and 2011, observed in Iceland by C- and X-band weather radars and compared with in situ samples; and the Mount Etna volcano eruption in 2011, observed by an X-band polarimetric radar. These applications demonstrate the variety of radar-based products that can be derived and exploited for the study of explosive volcanism.

Combustion Properties of Norwegian Biomass: Wood Chips and Forest Residues

Flue gas emissions and particle size distribution were investigated during combustion experiments of wood, forest residue and mixtures of these two. The combustion experiments were carried out in a grate fired multi-fuel reactor with and without air staging at stable operation conditions and constant temperature of 850 °C. The overall excess air ratio was held at 1.6, and the primary excess air ratio was 0.8 during air staged experiments. NOx emissions are reduced by air staging. Fly ash particle concentration of forest residues in the flue gas is lower than wood. Aerosols number increased in the staged experiments for fuel blends.

Simulations of tephra dispersal from the 1991 explosive eruptions of Hudson volcano, Chile

The 1991 explosive eruptions of Hudson volcano in southern Chile produced 2.7 km 3 (dense rock equivalent) of basalt and trachyandesite tephra during the period August 8–15. The initial basaltic phase (phase I, August 8–9) produced a maximum column height of 12 km above sea level (ASL) and tephra fallout was directed to the north and northeast by the prevailing winds. The paroxysmal trachyandesitic phase (phase II, August 12–15) involved at least three separate events with a maximum ∼ 18-km-high (ASL) eruption column inferred from satellite temperature data. During the initial 24 h of this phase the plume was advected almost directly south, before swinging towards the east as the wind changed direction. The plume was ultimately directed to the southeast and stayed relatively fixed at this bearing for the remainder of the eruption. These temporal variations in the main dispersal direction during the earlier stages of the phase II eruption produced a much wider overall deposit than would be expected from a plume with a relatively fixed transport direction (e.g., latter stages of phase II). The Lagrangian ash tracking model PUFF was utilized to simulate the 1991 explosive eruptions and was able to successfully reproduce the aerial distribution and temporal evolution of the plumes. The optimal agreement between the observed and simulated plumes occurs when the highest concentration of ash particles coincides with the tropopause, a height that is typically lower than the maximum observed column heights for the 1991 eruptions. Gravitational settling of the laterally spreading umbrella region (e.g., Pinatubo 1991) may result in the concentration of ash at this level. This may account for differences in column height estimates between ground- or satellite-based and lithic-based models. The plume associated with the paroxysmal phase (August 12–15, 1991) produced a multilayered deposit composed of alternating layers of fine ash and pumice lapilli. The highly stratified nature of the fall deposit is likely the result of multiple eruptive events coupled with a time varying wind field. A strongly changing wind direction that occurred during the earlier stages of the paroxysmal eruption could have produced variations in the dominant grain size being deposited between fine ash and pumice lapilli during individual eruptive sequences.

Applications of the PUFF model to forecasts of volcanic clouds dispersal from Etna and Vesuvio

PUFF is a numerical volcanic ash tracking model developed to simulate the behaviour of ash clouds in the atmosphere. The model uses wind field data provided by meteorological models and adds dispersion and sedimentation physics to predict the evolution of the cloud once it reaches thermodynamic equilibrium with the atmosphere. The software is intended for use in emergency response situations during an eruption to quickly forecast the position and trajectory of the ash cloud in the near (∼1–72 h) future. In this paper, we describe the first application of the PUFF model in forecasting volcanic ash dispersion from the Etna and Vesuvio volcanoes. We simulated the daily occurrence of an eruptive event of Etna utilizing ash cloud parameters describing the paroxysm of 22nd July 1998 and wind field data for the 1st September 2005–31st December 2005 time span from the Global Forecast System (GFS) model at the approximate location of the Etna volcano (38N 15E). The results show that volcanic ash particles are dispersed in a range of directions in response to changing wind field at various altitudes and that the ash clouds are mainly dispersed toward the east and southeast, although the exact trajectory is highly variable, and can change within a few hours. We tested the sensitivity of the model to the mean particle grain size and found that an increased concentration of ash particles in the atmosphere results when the mean grain size is decreased. Similarly, a dramatic variation in dispersion results when the logarithmic standard deviation of the particle-size distribution is changed. Additionally, we simulated the occurrence of an eruptive event at both Etna and Vesuvio, using the same parameters describing the initial volcanic plume, and wind field data recorded for 1st September 2005, at approximately 38N 15E for Etna and 41N 14E for Vesuvio. The comparison of the two simulations indicates that identical eruptions occurring at the same time at the two volcanic centres display significantly different dispersal axes as a consequence of the different local wind field acting at the respective eruptive vents. At the Vesuvio the volcano, a plinian eruptive event with the dynamical parameters of the 79 A.D. eruption was simulated daily for one year, from 1st July 2005 to 30th June 2006. The statistical processing of results points out that, although in most cases the ash cloud dispersal encompasses many different areas, generally the easterly southeasterly direction is preferred. Our results highlight the significant role of wind field trends in influencing the distribution of ash particles from eruptive columns and prove that the dynamical parameters that most influence the variability of plume dispersal are the duration of the eruption and the maximum column height. Finally, the possible use of cloud simulations for refining hazard maps of areas exposed to volcanic ash dispersal is proposed.

Characterization of respirable volcanic ash from the Soufrière Hills volcano, Montserrat, with implications for human health hazards

Volcanic ash, generated in the long-lived eruption of the Soufriere Hills volcano, Montserrat, is shown to contain respirable (sub-4 μm) particles and cristobalite, a crystalline silica polymorph. Respirable particles of cristobalite can cause silicosis, raising the possibility that volcanic ash is a respiratory health hazard. This study considers some of the main factors which affect human exposure to respirable volcanic ash, namely, the composition and proportions of respirable ash, and the composition and concentrations of airborne suspended particulates. The composition, size distribution and proportion (by weight) of respirable particles in representative samples of the Soufriere Hills tephra (dome-collapse ash-fall deposits, dome-collapse pyroclastic-flow matrix, Vulcanian explosion ash and mixed ash) have been characterized. Dome-collapse ash-fall deposits are significantly richer in respirable particles (12 wt%) than the other tephra samples, in particular the matrices of dome-collapse pyroclastic-flow deposits (3 wt%). Within the respirable fraction, dome-collapse ash contains the highest proportion of crystalline silica particles (20–27 number%, of which 97 wt% is cristobalite), compared with other primary tephra types (0.4–5.6 number%). This enrichment of crystalline silica in the dome-collapse ash is most pronounced in the very fine particle fraction (sub-2 μm). The results are explained as being due to significant size fractionation during fragmentation of pyroclastic flows, resulting in a fines-depleted dome-collapse matrix and a fines-rich dome-collapse ash deposit. For all sample types, the sub-4 μm fraction comprises 45–55 wt% of the sub-10 μm fraction. Aeolian deposit, lahar deposit and airborne samples of suspended ash, collected on filters, were characterized. These samples show enrichment of crystalline silica in the respirable fraction (10–18 number%). The results are consistent with ash in the environment having a mixed origin but originating predominantly from dome-collapse eruptions. The reworked ash, however, contains low proportions of respirable ash (∼3 wt%) compared to primary ash samples. The concentration of ash particles re-suspended by road vehicles on Montserrat is found to decrease exponentially with height above the ground, indicating higher exposure for children compared with adults: PM4 concentration at 0.9 m (height of two-year-old child) is 3 times that at 1.8 m (adult height). The composition of the re-suspended road particles is similar to that re-suspended by the wind.

Fly Ash Penetration through Electrostatic Precipitator and Flue Gas Condenser in a 6 MW Biomass Fired Boiler

The effects of an electrostatic precipitator and a flue gas condenser on size resolved fly ash particle concentration and composition were studied in a 6 MW biomass combustion unit, fired with moist forest residue. The boiler was of moving grate type. The fly ash particles were sampled upstream and downstream of the electrostatic precipitator and flue gas condenser, respectively. Fine particle number size distributions were measured using an electric mobility spectrometer (SMPS) and coarse particle number size distributions were measured using a time-of-flight instrument (APS). The mass size distributions were measured using a multi-jet low pressure cascade impactor (DLPI). For chemical analyses of the impactor substrates particle induced X-ray emission analysis (PIXE) was used. After the flue gas passed the electrostatic precipitator (ESP), the fly ash particle concentration was reduced by approximately 96% by number and 83% by mass. After the particles passed the flue gas condenser, particle number concentration was only marginally altered, whereas the mass concentration was reduced by half. Both the ESP and the condenser showed, size dependent particle separation efficiency. The main elements (Z > 12) in the fine fly ash fraction were K, S, and Cl, whereas the main elements in the coarse fraction were Ca, K, S, and Cl. After passing the ESP the mass ratio of Ca decreased in the coarse fraction, while the ratios of K, S, and Cl increased, indicating transference of fly ash material from the fine to the coarse particle fraction. There was no significant difference in the elemental composition for any particle size fraction sampled upstream or downstream of the condenser.

A model for generation of ash clouds by pyroclastic flows, with application to the 1980 eruptions at Mount St. Helens, Washington

Pyroclastic flows are defined as consisting of a dense basal avalanche underlying a turbulent ash cloud. The generation of ash clouds by the basal avalanche is commonly assumed to be closely related either to fluidization mechanisms within the avalanche or to airflow/flow front interactions. These mechanisms carry ash upward from the basal avalanche into the ash cloud in currents of gas or air, but so far no mechanisms have been proposed to keep the ash in turbulent suspension in the ash cloud. Here I consider the ash cloud turbulence that is generated in two ways: by temperature differences between the basal avalanche and overriding air, and by the motions of the blocks, lapilli, and ash on the outer surface of the basal avalanche. This turbulence would carry ash from saltation into turbulent suspension at the base of the ash cloud, and this transition would simultaneously occur from the upper surface and front of the basal avalanche. This approach quantifies one type of airflow/flow front interaction. The analysis incorporates studies of heat and mass transfer from rough surfaces during high Reynolds number flow. Equations for conservation of mass, momentum, heat energy, and ash particle concentration are formulated and presented in integral form. Model results show that for typical avalanche velocities and temperatues, temperature differences between the basal avalanche and the surrounding air are insignificant in generating the initial turbulence within the ash cloud. Instead, the initial distribution of particle sizes in the ash cloud is controlled by the turbulence generated by the saltating/cascading particles in the basal avalanche. Study of the morphology of observed ash clouds suggests that thermally driven convection often becomes significant hundreds of meters or more behind the flow front, and this may result from deposition of significant quantities of ash from the ash cloud onto the basal avalanche. The variations in ash cloud heights, morphologies, and particle sizes that are predicted by this model are consistent with published observations of small pyroclastic flows at Mount St. Helens in 1980.