Dust pollution matters (PM10) by Idrobase Group

DUST POLLUTION MATTERS (PM10) 19.04.08 Dust Pollution Matters (PM10)

INTRODUCTION The issue of air pollution is getting more and more highlighted at present days all over the world, in particular dust emission and control is one of the priority of policy in many countries, developed or not. As we will see later in this paper, misting as a method to fight air pollution and for dust reduction is widely recognized and open us great opportunities of business. Scope of the present paper is to make everybody aware of the opportunity and to give basic knowledge of the science behind PM10 pollution control, as a tool to groove our capability to give appropriate answers to customers. It is crucial that any customer contacting us feels that we are the specialists in this field and we can provide the right answer and the right solution to his needs.

ACTUAL SITUATION According to Hans Bruyninckx, European Environment Agency (an Agency of EU) Executive Director, “Air pollution harms human health and the environment. In Europe, emissions of many air pollutants have decreased substantially over the past decades, resulting in improved air quality across the region. However, air pollutant concentrations are still too high, and air quality problems persist. A significant proportion of Europe’s population live in areas, especially cities, where exceedances of air quality standards occur: ozone, nitrogen dioxide and particulate matter (PM) pollution pose serious health risks. Several countries have exceeded one or more of their 2010 emission limits for four important air pollutants. Reducing air pollution therefore remains important. Air pollution is a local, pan-European and hemispheric issue. Air pollutants released in one country may be transported in the atmosphere, contributing to or resulting in poor air quality elsewhere. Particulate matter, nitrogen dioxide and ground-level ozone, are now generally recognised as the three pollutants that most significantly affect human health. Long-term and peak exposures to these pollutants range in severity of impact, from impairing the respiratory system to premature death. Around 90 % of city dwellers in Europe are exposed to pollutants at concentrations higher than the air quality levels deemed harmful to health. For example, fine particulate matter (PM2.5) in air has been estimated to reduce life expectancy in the EU by more than eight months.”

WHAT IS DUST Atmospheric aerosol particles – also known as atmospheric particulate matter, particulate matter (PM), particulates, or suspended particulate matter (SPM) – are microscopic solid or liquid matter suspended in the atmosphere of Earth. The term aerosol commonly refers to the particulate/air mixture, as opposed to the particulate matter alone. Sources of particulate matter can be natural or anthropogenic. They have impacts on climate and precipitation that adversely affect human health. Subtypes of atmospheric particles include suspended particulate matter (SPM), thoracic and respirable particles, inhalable coarse particles, which are coarse particles with a diameter between 2.5 and 10 micrometres (μm) (PM10), fine particles with a diameter of 2.5 μm or less (PM2.5), ultrafine particles, and soot. Particles larger than 10 μm are also found in the atmosphere, rapid removal generally limits their lifetime to the order of hours, and as they are too large to be respirable, their health impacts are considered of minor importance. The IARC and WHO designate airborne particulates a Group 1 carcinogen. Particulates are the deadliest form of air pollution due to their ability to penetrate deep into the lungs and blood streams unfiltered, causing permanent DNA mutations, heart attacks, respiratory disease, and premature death. In 2013, a study involving 312,944 people in nine European countries revealed that there was no safe level of particulates and that for every increase of 10 μg/m3 in PM10, the lung cancer rate rose 22%. The smaller PM2.5 were particularly deadly, with a 36% increase in lung cancer per 10 μg/m3 as it can penetrate deeper into the lungs. Worldwide exposure to PM2.5 contributed to 4.1 million deaths from heart disease and stroke, lung cancer, chronic lung disease, and respiratory infections in 2016. Overall, ambient particulate matter ranks as the sixth leading risk factor for premature death globally. Primary PM sources are derived from both human and natural activities. A significant portion of PM sources is generated from a variety of human (anthropogenic) activity. These types of activities include agricultural operations, industrial processes, combustion of wood and fossil fuels, construction and demolition activities, and entrainment of road dust into the air. Natural (nonanthropogenic or biogenic) sources also contribute to the overall PM problem. These include windblown dust and wildfires. Traffic (25%), combustion and agriculture (22%), domestic fuel burning (20%), natural dust and salt (18%), and industrial activities (15%) are the main sources of particulate matter contributing

to citiesâ&#x20AC;&#x2122; air pollution. However, there are significant differences between various regions of the world.

This diagram shows types, and size distribution in micrometres, of atmospheric particulate matter

According to the WHO, in 2012 ambient air pollution contributed to 6.7 % of all deaths worldwide. In particular, 16% of lung cancer deaths, 11% of chronic obstructive pulmonary disease deaths, and more than 20% of ischaemic heart disease and stroke are associated with ambient fine particulate matter. The economic cost of the approximate 600 000 premature deaths and of the diseases caused by air pollution in the WHO European Region in 2010 has been estimated in Euro 1.5 trillion.

What has been said before suggest that the most important trait for particles of particulate might be their granulometry. Indeed this trait has a considerable impact on any dust control system devised. Regarding this, here below as reference page we provide another table that summarize the principal traits of the various types of dust, highlighting the specific types in which dust control systems are commonly used against.

ORIGIN OF DUST POLLUTION There is a large number of researches on the sources of dust pollution across the world, and each city or area has its own specifics, depending on location, industrial activities in the region, climatic and weather conditions. Anyhow, it is clear that the most relevant sources that contribute to PM10 emissions at European level, in a percentage higher than 2%, are: thermal power stations and other combustion installations (54.42%), production of pig iron or steel including continuous casting (9.15%), manufacture of ceramic products including tiles (7.02%), production of nonferrous crude metals from ore (metallurgical) (3.49%), metal ore (including sulphide ore) roasting or sintering installations (3.45%), mineral oil and gas refineries (3.44%) and production of cement clinker or lime in rotary kilns or other furnaces (2.12%). Beside this, dust coming from traffic is one of the most relevant sources (non-industrial) of PM 10 and PM 2,5, from exhaust, tyre consumption, etc.

IS MISTING A SOLUTION? The use of misting is widely recognized as one of the methods to achieve dust and PM suppression and containment. A project founded by EU, called AIRUSE, provides National Authorities of Southern European countries with appropriate measures to reduce PM2.5 and PM10 concentrations in air. A combination of diverse emission sources (dust intrusions) with a complex climatology (strong radiation, high photochemical conversion rates, low rainfall rate) significantly enhances particle levels in South European and Mediterranean countries. (www.airuse.eu) In one of its reports (REPORT ON MITIGATION MEASURES IN SOUTHERN EUROPE) AIRUSE mention the following: The techniques of water misting/water curtains: The moistening of bulk materials is a practically proven technique to prevent dust formation from loading/unloading activities. The misting can be carried out by using a permanent installation or mobile containers (e.g. tankers). Water curtains are, e.g. used to keep dust in the hopper when grabs are opened above the water curtain. Another

example is the tipping to stockpiles made through chutes equipped with misting suppression systems. The technique of water misting is simple, but application is limited to bulk material that is not sensitive to moisture. Misting is particularly suitable for existing plants where the space for installing extraction equipment is not sufficient and water resources are available. The technique of misting (spraying with fine water fog) prevents the material from getting too wet. Misting can be used on heaps, for the loading/unloading of heaps and tip bunkers, the loading of ships with telescopic hoppers and the loading of trucks from silos. Beside the field of application mentioned in the report, some other applications are: foundry ashes suppression, belt conveyors transfer point dust containment, stacker reclaimers bucket loading system dust suppression, rocks and ores crushers, open areas dust pollution containment, de-dusting barriers, and so on. In the report, in particular, misting is mentioned as a core measure for the following: 4.3.1 Thermal power stations and other contribution installations 4.3.1.1 Diffuse PM emissions The BATs(Best Available Technics) for preventing releases from the unloading, storage and handling of coal, and lignite, and also for additives such as lime, limestone, ammonia, etc. are:  The use of loading and unloading equipment that minimises the height of fuel drop to the stockpile, to reduce the generation of diffuse dust.  In countries where freezing does not occur, using water misting systems to reduce the formation of diffuse dust from coal stockpiles. In below zero °C climatic conditions special anti-freezing additives can be added to water, and equipment need to be manufactured with particular solutions. For localized dust pollution sources Light Coker Gasoil can be for instance used.  Covering stockpiles of petroleum coke. The “Dust Control Handbook for Industrial Minerals Mining and Processing” of the US DEPARTMENT OF HEALTH AND HUMAN SERVICES says that “Dust affect the safety of workers. The five areas that typically produce dust that must be controlled are as follows: 1. The transfer points of conveying systems, where material falls while being transferred to another piece of equipment. Examples include the discharge of one belt conveyor to another belt conveyor, storage bin, or bucket elevator. 2. Specific processes such as crushing, drying, screening, mixing, blending, bag unloading, and truck or railcar loading. 3. Operations involving the displacement of air such as bag filling, palletizing, or pneumatic filling of silos. 4. Outdoor areas where potential dust sources are uncontrolled, such as core and blast hole drilling. 5. Outdoor areas such as haul roads, stockpiles, and miscellaneous unpaved areas where potential dust-generating material is disturbed by various mining-related activities and high-wind events.

Probably the oldest and most often used method of dust control at mineral processing operations is the use of water misting (wet spray systems). In essence, as the fines are wetted each dust particle's weight increases, thus decreasing its ability to become airborne. As groups of particles become heavier, it becomes more difficult for the surrounding air to carry them off. The keys to effective mist dust control are proper application of moisture, careful nozzle location, controlling droplet size, choosing the best spray pattern and spray nozzle type, and proper maintenance of equipment. The following two methods are used to control dust using wet sprays at mineral processing operations:  

Airborne dust prevention, achieved by direct misting of the ore to prevent dust from becoming airborne. Airborne dust suppression, which involves knocking down dust already airborne by misting the dust cloud and causing the particles to collide, agglomerate, and fall out

In UK the “IAQM Guidance on the assessment of dust from demolition and construction”, issued from the Institute of Air Quality Management estimated that Urban works generates 11% of PM10 and 6% of PM2.5, and recommend:  Misting (Water spraying) during construction, operations (earthmoving, demolition, grading)  Washing of lorries (not only the wheels and the underside, but also the rest of the vehicle) before leaving the site access area. The lorries should be washed every time that they leave the site.

The above are just a few of the various documents recommending the use of misting for dust control, all of them reporting the need to increase the awareness of the need to act to decrease the air pollution from airborne particles to improve air cleanness and the ecological situation. HOW DOES IT WORK Misting systems, used in PM10 pollution suppression, behave themselves like a combination of a “wet scrubber” (i.e. a device that removes pollutants from gas streams through sprays or fluid streams interacting with the gas stream itself) and a filter. Indeed, the wall of nebulised water acts as a filter which makes impossible for the particle of dust to pass through it without colliding with any drop produced by the misting system. This phenomenon is known as agglomeration process. After the particle of particulate is absorbed by the water drop, the mass of the latter tends to increase, making it fall to the ground by effect of the gravity and with it the particle inside it. When using misting, one of the primary considerations is the droplet size. If the droplet diameter is much greater than the diameter of the dust particle, the dust particle simply follows the air

stream lines around the droplet. If the water droplet is of a size comparable to that of the dust particle, contact occurs as the dust particle follows the stream lines and collides with the droplet (Figure 2.2). For airborne dust suppression, where the goal is to knock down existing dust in the air, the water droplets should be in similar size ranges to the dust particles. The intent is to have the droplets collide and attach themselves (agglomerate) to the dust particles, causing them to fall from the air. In particular, controlling PM10 emission requires droplets of water not bigger than 30 microns, to be able to attach to the PM10 particles. For optimal agglomeration, the particle and water droplet sizes should be roughly equivalent. The probability of impaction also increases as the size of the water spray droplets decreases, because as the size of the droplets decreases, the number of droplets increases

Figure 2.2. Effect of droplet size on dust particle impingement.

Looking back to figure 2.2, is shown that a big droplet is not able to capture particles much smaller than the droplet itself, making it â&#x20AC;&#x153;flowingâ&#x20AC;? along without interactions. It is evident that to capture

particles of small dimensions, as PM10 and PM 2.5 pollutants, droplets of comparable dimensions are needed. Specifically, droplets dimensions, for an equivalent flow of water, are a function of:   

nozzles number: the higher the number of nozzles, the smaller the dimension of the droplet; water pressure at the nozzle: the higher the pressure, the smaller the dimension of the droplet; finally, from the flow (the quantity of water thrown per unit of time).

In the following chart is possible to compare the different droplet sizes generated from different pressure and number of nozzles at different flows: Flow rate (lt/m) Pressure (bars) High pressure fog droplet sizes 6,5 60 11 70 15 70 33 70 33 70 42 70 42 70 Low pressure fog droplet sizes Flow rate (lt/m) Pressure (bars) 42 15 70 15 120 15

N° of nozzles

Avg. Droplet size (µm)

30 30 30 60 156 60 156

12 18 22 45 17 48 18

N° of nozzles 60 30 60

Avg. Droplet size (µm) 120 150 300

As seen in the chart, fog cannons are able to achieve these dimensions, producing fine mist thanks to the high pressure and the large number of small orifice nozzles used in relation with the defined flow. Summarizing, is relevant to notice that for an equivalent flow of water, a smaller number of nozzles lead to an increase of the droplet dimensions, and this is true for low as for high pressure. UNDERSTANDING DROPLET SIZE The process of generating drops is called atomization. The process of atomization begins by forcing liquid through a nozzle. The potential energy of the liquid (measured as liquid pressure for hydraulic nozzles or liquid and air pressure for two-fluid nozzles) along with the geometry of the nozzle causes the liquid to emerge as small ligaments. These ligaments then break up further into very small “pieces”, which are usually called drops, droplets or liquid particles.

Each spray provides a range of drop sizes; this range is referred to as a drop size distribution. A simple explanation of this process is the breakup of a liquid as it emerges from an orifice. The drop size distribution will be dependent on the nozzle type and will vary significantly from one type to another. Other factors such as the liquid properties, nozzle capacity, spraying pressure and spray angle can affect drop size too.

To assess droplet size, different techniques are used, and different lab instruments are needed. The two techniques are called “spatial” and “flux”, the first using the measurement on volume, the other measuring the cross section. Without going into deep technical details, it is important to compare results and size coming from the same technique, so to compare apple to apples.

Anyhow, the most used testing is actually the so called “Phase Doppler particle analyser (PDPA)”. These are flux sampling instruments and fall into the non-imaging (single particle counter) category. A data analysis routine is used to convert the raw drop count into a meaningful drop size distribution, generally based on ASTM Standard E799-03. This standard is used to classify the drop

Phase Doppler Analizer counts/diameters and also to calculate the distribution and the characteristic or mean diameters. The PDPA measures sizes in the 0.5 to 10,000 μm range using various optical configurations. The PDPA is best suited for two-fluid, hydraulic and flat spray nozzles in every capacity. It is ideal for complete spray evaluation and where drop velocities are required. A schematic of the PDPA is shown in the figure. Nozzle quality and wear have a relevant effect on nozzle performance. Typically, the spray appearance deteriorates and flow rate and drop size increase. There will be a difference in drop size between a new nozzle and one that has been in service

LOCATING A MISTING SYSTEM

Due to the unique characteristics of each application, there are no hard and fast rules for specifically locating spray nozzles in PM10 pollution control applications; however, the following guidelines will contribute to misting system efficiency. For dust pollution prevention systems, nozzles should be located upstream of the transfer point where dust emissions, in most cases, are being created. For airborne dust prevention, the nozzles should be located at an optimum target distance from the material—far enough to provide the coverage required but close enough so that air currents do not carry the droplets away from their intended target. Droplet size also needs to be considered when setting the correct target distance. For airborne dust suppression, nozzles should be located to provide maximum time for the water droplets to interact with the airborne dust. Normative links (Europe):  Direttiva UE 2008/50/CE del Parlamento Europeo e del Consiglio del 21 maggio 2008 relativa alla qualità dell’aria ambiente e per un’aria più pulita in Europa, pubblicata nella Gazzetta Ufficiale dell’Unione Europea L 152 dell’11.6.2008, pag. 1, modificata dalla Direttiva (UE) 2015/1480 della Commissione del 28 agosto 2015, pubblicata nella Gazzetta Ufficiale dell’Unione Europea L 226 del 29.8.2015, pag. 4

Normative links (Italy):  Decreto Legislativo 3 aprile 2006, n. 152 "Norme in materia ambientale", pubblicato nella Gazzetta Ufficiale n. 88 del 14 aprile 2006 - Supplemento Ordinario n. 96  Decreto Legislativo 13 agosto 2010, n.155 "Attuazione della direttiva 2008/50/CE relativa alla qualità dell'aria ambiente e per un'aria più pulita in Europa", pubblicato nella Gazzetta Ufficiale n. 216 del 15 settembre 2010 - Supplemento Ordinario n. 217

Bibliography:  

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Rudolf J. Schick, Spray Technology Reference Guide: Understanding Drop Size, Spraying Systems Co., Bulletin No. 459C, 2008 Andrew B. Cecala, Andrew D. O’Brien, Joseph Schall, Jay F. Colinet, William R. Fox, Robert J. Franta, Jerry Joy, Wm. Randolph Reed, Patrick W. Reeser, John R. Rounds, Mark J. Schultz, Dust Control Handbook for Industrial Minerals Mining and Processing, DEPARTMENT OF HEALTH AND HUMAN SERVICES, Centers for Disease Control and Prevention National Institute for Occupational Safety and Health Office of Mine Safety and Health Research, 2012 European Environment Agency, Costs of air pollution from European industrial facilities 2008–2012 — an updated assessment, 2014 Dust Prevention & Suppression study, IdroBase Group, Xavier Querol, Fulvio Amato et al, Guidebook - Measures to improve urban air quality, CSIC, 2017 European Environment Agency, Air pollution harms human health and the environment, 2008 Xavier Querol et al, AIRUSE, Report 13: Report on mitigation measures in Southern Europe, 12/2016 Xavier Querol et a., AIRUSE, Report 26: Technical guide for industrial emissions reduction, 12/2016 Xavier Querol et al, AIRUSE, Report 29: Technical Guide for Mitigation Measures from the experience of Northern and Central European Countries, 12/2016 Wikipedia, https://en.wikipedia.org/wiki/Particulates, last accessed 2019/03/19 Holman et al, IAQM Guidance on the assessment of dust from demolition and construction, Institute of Air Quality Management, London, 2014

Copyrights© to this brochure belong to IDROBASE GROUP® Srl. Reading, reference and dissemination in paper or electronic formats for personal use are permitted. Use for commercial purposes is not permitted, nor is any type of alteration except with the written consent of IDROBASE GROUP® Srl. This brochure provides an in-depth study on air pollution, especially about fine particulate matter (PM10), and on nebulization, a method widely recognized worldwide for reducing such pollution. The specific analysis of the situation worldwide has been accompanied by research into the origins of the problem but overall attention has been paid to IDROBASE GROUP® Srl solutions to this phenomenon: the nebulization as a fundamental tool for reducing pollution by fine particulate matter. It is intended for retailers, system engineers, designers and more generally, all those who are already adequately familiar with said products, the materials from which they are made, their characteristics, efficiency and operation. The illustrations, technical data, tables and all other information displayed here on air pollution and problem solving are based on the current state of affairs. We reserve the right to introduce changes in all the products. In case of doubts regarding interpretation, the original Italian text will prevail.

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