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
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.
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
BY IDROBASE Group srl
