Reduction of dust emissions

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Dust refers to a complex mixture of small to tiny particles and liquid droplets suspended in air. Sizes of dust range from several nanometers (nm) up to 100 micrometers (µm).

Dust may be differentiated according to the aerodynamic diameter into:

large particles with an aerodynamic diameter of more than 10 µm,
coarse particles with an aerodynamic diameter of 2.5 to 10 µm,
fine particles with an aerodynamic diameter of less than 2.5 µm,
ultrafine particles with an aerodynamic diameter of less than 0.1 µm,

and more particularly into:

Total Suspended Particles (TSP) as the sum of fine, coarse and large particles,
PM₁₀: the mass of particulate matter that is measured after passing through a size-selective inlet with a 50 % efficiency cut-off at 10 μm aerodynamic diameter,
PM_2.5: the mass of particulate matter that is measured after passing through a size-selective inlet with a 50 % efficiency cut-off at 2.5 μm aerodynamic diameter;
PM₁: the mass of particulate matter that is measured after passing through a size-selective inlet with a 50 % efficiency cut-off at 1 μm aerodynamic diameter.

Besides this size dependent classification, dust is also differentiated according to its origin into primary and secondary dust. Dust can be natural (sea salts, volcanoes, soil erosion...) and anthropogenic (combustion, processes...).

According to its source, dust has different chemical compositions.

Primary dust is composed of salts (nitrates, sulphates, carbonates...), of black carbon (BC), of organic carbon (non-carbonate carbonaceous particles other than elemental carbon) (OC) and trace elements such as heavy metals.
Secondary PM is formed in the atmosphere of the precursors ammonia, sulphuric acid, nitric acid and NMVOC-related organic oxidation products.

BC means carbonaceous particulate matter that absorbs light. Absorption occurs at all wavelengths of solar radiation. The BC content of dust increases with incomplete combustion of various fossil fuels, biofuels and biomass. BC is part of a complex particle mixture called soot which primarily consists of BC (which is a warming agent) and OC (which is a cooling agent). There is a close relationship between the two compounds. They are always co-emitted, but in different proportions for different sources. Soot mixtures can vary in composition, having different ratios of OC to BC and usually include inorganic materials such as metals and sulphates.

BC forms during combustion, and is emitted when there is insufficient oxygen and heat available for the combustion process to burn the fuel completely. BC originates as tiny spherules, ranging in size from 0.001 to 0.005 micrometers (μm), which aggregate to form particles of larger sizes (0.1 to 1 μm). The characteristic particle size range, in which fresh BC is emitted, also makes it an important constituent of the ultrafine particles (

For dust, there are several natural and anthropogenic sources with differences in the size and the chemical composition of the generated dust. Dust formation may result from:

mechanical processing of solid matter (crushing, grinding, surface processing, abrasion etc.),
chemical and physical reactions (incomplete combustion, gas-to-particle conversion, condensation, deposition etc.),
exposure of solid matter (wind erosion etc.)
re-suspension of dust (from roads, stockpiles etc.).

In order to reduce dust formation and dust emissions different types of measures like energy efficiency improvements, fuel switching, fuel cleaning, better handling of materials as well as abatement measures are applied.

To achieve the most efficient dust reduction, beyond energy management measures, a combination of measures should be considered. To identify the best combination of measures a site-specific evaluation is needed.

General approaches to reduce dust emissions

Fuel switching (for combustion sources)

Fuel switching is an important option to reduce dust emissions from combustion but is governed by country specific conditions such as infrastructure and energy policy. Dust emissions are in general lower if the fuel allows a more homogenous combustion, contains less sulphur and less ash but more hydrogen. Therefore, combustion of natural gas is in general associated with low emissions whereas high dust emissions result from combustion of fuel oil, biomass and coal if no abatement measures are applied.

BC mainly results from incomplete combustion. The form of the fuel influences the likelihood of complete combustion and the reduction of BC can be achieved through fuels able to limit the occurrence of incomplete combustion:

Gas phase fuels (e.g., natural gas) can be readily mixed with oxygen, which reduces the emission of carbonaceous particles.
Liquid fuels (e.g., gasoline) generally must vaporize in order to fuel flaming combustion. If a liquid fuel contains heavy oils, vaporisation and thorough mixing with oxygen are difficult to achieve.
Solid fuels (e.g., wood) require preheating and then ignition before flaming combustion can occur. High fuel moisture can suppress full flaming combustion, contributing to the formation of Brown Carbon particles as well as BC.

The choice of the fuel may also have effects on other emissions like sulphur, NOx and greenhouse gas emissions as well as on applicability and need of abatement measures.

Fuel cleaning (for combustion sources)

Fuel cleaning is important for coal and fuel oil.

Conventional coal cleaning techniques rely on gravity-based separation of ash and sulphur compounds using jigs, dense-medium baths, cyclones or flotation of grinded coal. While 60 to 90% and 85 to 98% of the heating value of the coal is retained, ash removal can reach 60% and total sulphur removal 10 to 40%. Both sulphur and ash removal contributes to a reduction of dust emissions. Sulphur removal increases with the content of pyritic sulphur in the coal. Advanced techniques are mostly based on:

advanced physical cleaning (advanced froth floatation, electrostatic, heavy liquid cycloning),
aqueous phase pre-treatment (bioprocessing, hydrothermal, ion exchange),
selective agglomeration (Otisca, LICADO, spherical agglomeration Aglofloat),
organic phase pre-treatment (depolymerisation, alkylation, solvent swelling, catalyst addition (e.g., carbonyl), organic sulphur removal).

These advanced coal cleaning techniques are still in development or demonstration phase. Besides a reduction of sulphur and dust emissions, reported advantages are lower transportation costs if coal is cleaned already at the mine, higher boiler availability, less boiler slagging and fouling, less wear on equipment, lower dust load. Disadvantages are energy loss from cleaning (2-15%), energy costs for the processes and an increased moisture content of the coal if water-based processes are used. Fuel desulphurisation for fuel oil is common practice in order to achieve low sulphur fuels which are e.g. required in the EU by Directive 1999/32/EC (heavy and light fuel oil less than 1% resp. 0.1% wt). Removing of sulphur reduces sulphur based dust emissions.

Primary measures

Unloading, handling and storage of solids

During unloading, storage and handling, e.g. loading, of solids dust emissions might occur. In general particle size of dust from unloading, storage and handling of solid is larger than dust from e.g. combustion. The use of enclosed or housed systems, e.g. covered continuous conveying systems, and reducing of drop heights may reduce dust emissions from unloading and handling. Approaches to minimise dust from storage can be differentiated into primary measures which reduce emissions and secondary measures which aim at limiting the distribution of the dust. Primary measures can be further differentiated into organisational, constructional, and technical measures. Technical primary measures are wind protection, covering or avoidance of open storage, and moistening of the open storage, e.g. by a sprinkler systems. Secondary measures are spraying, water curtains and jet spraying as well as installation of filters in e.g. silos . Spraying water is also a measure to reduce dust emissions from construction sites.

Capture of emissions

A prerequisite for later dust abatement is the capture of fugitive dust emissions, e.g. in the iron and steel industry, and venting to dust control systems.

Combustion technique and optimisation

A smooth, continuous and complete combustion generates less dust (including PM₁₀, PM_2.5 and black carbon) emissions. An optimised air supply, mixing of fuel and air as well as burner/boiler design reduce the formation of soot and other substances resulting from incomplete combustion such as BC. Therefore good housekeeping of boilers as well as the use of new, more efficient boilers and stoves, especially in the residential and commercial sector, may reduce dust and BC emissions. In this way dust and BC emissions from wood stoves can be considerably reduced. Changing from batch to continuous operation of boilers allows for a better combustion control and reduces dust and BC emissions. Primary measures for NOx reduction may, however, increase soot formation. A lowering of the combustion temperature reduces ash volatisation. Fuel additives and sorbents are proposed to reduce the formation of fine particles and metals in the fine particles. In Integrated gasification combined cycle (IGCC) the fuel is gasified under reducing conditions to syngas. The syngas is then cleaned and burnt in either air or oxygen. This allows the achievement of very low dust emission levels. IGCC is seen as one of several key technologies in the framework of carbon capture and storage (CCS). So far its application is restricted to few, mostly demonstration plants. With CCS-IGCC could become commercially available around 2020.

Secondary measures

Secondary measures (add-on or end of pipe technologies) reduce the emissions of PM which is already in the flue gas. Several main principles are used for secondary measures:

inertia of particles,
sieving and adsorption,
electrostatic charging of particles and subsequent precipitation making use of an electric field,
scrubbing.

The following secondary measures are mainly in use, each with its specific advantages and disadvantages according to the size of particles. Because BC from incomplete combustion is mainly associated to particles with a diameter less than 1 µm, only reduction techniques able to remove fine particles will have a significant efficiency on BC emissions:

The performance data in the following paragraphs mainly refer to boiler installations

Gravity settling chamber

In gravity settling chambers the flow rate of the air is reduced so that larger particles sink and settle. Gravity settling chambers are only useful for removing the largest particles in terms of "pre-cleaning". The minimum particle size removed by gravity settling chambers is >20 µm. This equipment is not suitable for removal of fine particle and BC.

Cyclones

In cyclones inertia of particles are used for dust removal. In a cyclone the flue gas is forced (usually via a conical shaped chamber) into a circular motion where particles are forced by inertia to the cyclone walls where they are collected. Collection efficiency depends strongly on particle size and increases with the pollutant loading. For conventional single cyclones it is estimated to be 70-90% for TSP, 30-90% for PM₁₀ and 0-40% for PM_2.5. The minimum particle size removed by cyclones is 5-25 µm and 5 µm in multicyclones. Conventional cyclones are therefore referred to as "pre-cleaners". Conventional cyclones alone are not BAT for industrial installations but could be an option to reduce dust emissions from small combustion installations, e.g. in households or in the commercial sector. High efficiency cyclones removing 60-95% of PM₁₀ and 20-70% of PM_2.5 have been developed but at the expense of a high pressure drop leading to high energy and hence operation costs [6]. Achieving higher removal efficiencies in cyclones is mainly a problem of the resulting pressure drop. High throughput cyclones have been designed on purpose for removing just the larger dust fraction at the expense of only low pressure drop. In multicyclones many small cyclones operate in parallel achieving removal efficiencies similar or superior to high efficiency cyclones. Application of cyclones as a pre-cleaner to remove abrasive particles may increase the lifetime of other abatement equipment. Cyclones are also used to recover recycling products, process materials etc. from the flue gas, e.g. in the ferrous and non-ferrous metals industry. Advantages of cyclones are: low investments, low operating and maintenance costs relative to the amount of PM removed, temperature and pressure range only limited by material, collection of dry material, relatively small in size. Disadvantages include low removal efficiencies for fine PM (or alternatively high pressure drops) and non-applicability for sticky materials. The efficiency of cyclones on BC can be assumed similar to the efficiency obtained on PM_2.5.

Electrostatic precipitator (ESP)

The principle behind ESP is that particles of the flue gas stream are electrostatically charged when passing through a region with gaseous ions (corona) generated by electrodes at high voltage (around 20 to 100 kV). The charged particles are then redirected in an electric field and settle at the collector walls. As large particles absorb more ions than smaller ones, ESP removal efficiency is higher for larger particles. New ESP typically may achieve PM removal efficiencies of 99% to about 99.99% if perfectly dimensioned and in optimal operation conditions, in the range 0.01 to >100 µm, older ones 90 to 99.9%.

The minimum particle size removed by ESP is

Dust at the collectors can be removed either dry or wet by a spray of usually water (dry or wet ESP). Dry ESPs are more common as dry collected dust is easier to handle than slurry which requires after treatment. Wet ESPs need noncorrosive materials. However, removal of particles with extremely low or high resistivity is difficult in dry ESPs whereas wet ESPs can also collect particles with high resistivity as well as sticky particles, mists or explosive dusts. Wet ESPs show also higher efficiencies for smaller particles. Injection of conditioning gases, liquids or solids, in particular water and SO3, may improve removal efficiencies. Advantages of ESPs are in general very low pressure drops, very good removal efficiencies (but less pronounced for fine particles), low operating costs as well as wide applicability (sticky, glowing, high resistivity (wet ESP) particles, mists, acids, ammonia, exploding gases (wet ESP)). Disadvantages are high investments, high space demand, ozone formation due to high voltage, need for specialised personnel for high voltage, and limited applicability in case of varying flue gas conditions (flow rate, temperature, dust load, composition of dust) as well as necessary after treatment of slurry (wet ESPs), but almost closed water loops are achievable.

For combustion installations, ESPs can guarantee low dust and BC emissions when stable and good combustion conditions are achieved. On contrary, during transient conditions, dust (including PM₁₀, PM_2.5 and black carbon) emissions can be increased not only due to increased raw gas concentrations, but additionally due to reduced precipitation efficiency.

To achieve high efficiency of dust and BC removal with ESP, the following recommendations are provided by references for biomass combustion:

Optimum design and system integration of combustion and ESP enabling steady operation,
Process integrated control of ESP with specific information as indicators for the particle properties: flue gas temperature (as often carried our presently), excess air ratio, combustion temperature, water content of the fuel. This increases the range of conditions when the ESP is effective.
Measures to avoid re-entrainment: limitation of gas velocity to < 1.5 m/s, optimised shape of collecting plates, shorter dedusting interval during re-entrainment regimes.

The lower efficiency of ESPs on sub micrometer particles can be addressed by the use of an association of an ESP and a FF or the use of an agglomerator (see here after). These two techniques have been defined as emerging techniques by TFTEI.

Fabric filter (FF)

In a FF the flue gasses pass through a permeable fabric where larger particles are sieved or adsorbed. The filter cake made up of collected particles supports the collection of further particles. As pressure drop increases with filter cake thickness the fabric filter needs to be cleaned from time to time. Three main cleaning mechanisms are applied: pulse jet filters where filters are cleaned by a pulse of pressurized air from the other side, shaker mechanisms and reverse gas flow. Pulse jet filters are today the most common type as they demand less space, are less expensive and applicable for high dust loadings and cause constant pressure drop. Removal efficiencies are 99 to 99.99% for new and 95 to 99.9% for older installations and depend on filtration velocity, particle and fabrics characteristics and applied cleaning mechanism. FF is in particular able to remove fine and ultrafine dust and is consequently efficient to remove BC. Flue gas conditioning using mainly elemental sulphur, ammonia and SO3 is applied to achieve higher removal rates, reduce pressure drop, and reduce re-entrainment of particles. New developments are the addition of activated carbon or lime to achieve reactions in the filter cake as well as a catalytic filter material. Flue gas temperature depends on the filter material used and the dew point of the flue gas and is in general between 120-180°C. Advantages of FF are very low emission levels even down to ultrafine particles (depending on fabric) and achieved independent from dust loading, flow rate (e.g. start-ups) and dust type (except COC due to their sticky properties), simple operation and in general no corrosion problems. Disadvantages are relatively high maintenance and operating costs due to replacement of filter bags (lifetime depends on temperature and dust) and pressure drop and in particular limitations in applicability in moist environments and for hygroscopic, glowing and sticky particles as well as for acids and ammonia and exploding gases. Large particles need to be removed in advance. Bypassing is necessary during failure.

Wet scrubber

Injecting water into the flue gas stream leads to formation of water droplets which with dust, forms a slurry. Scrubbers are mainly used for SOx removal but reduce also dust. Removal efficiencies are up to 80% for spray towers as well as dynamic and collision scrubbers and up to 99 % for venturi scrubbers. The minimum particle size removed by spray towers is >10 µm, by dynamic and collision scrubbers > 2.5 µm and by venturi scrubbers >0.5 µm. Advantages of wet scrubbers are simultaneous removal of SOx and dust (and even other pollutants like HCl and HF), low maintenance, rather high removal efficiencies (in particular venturi scrubbers), few application limits (flow rate fluctuations, hot or cold, wet and corrosive gases, mists are uncritical) and reduced explosion risks from dust. Disadvantages are waste generation (slurry), high maintenance costs due to potentially high pressure drop, corrosion problems and rather low removal efficiency for very fine particles such as those to which BC is associated.

Oxidation techniques (secondary technique VOC)

Oxidation techniques used to abate VOC, PAH and odours can also be a useful technique to break down organic matter, including BC in some specific applications. Organic matter like BC can be incinerated indeed. This oxidation takes place in a thermal oxidation step in an off-gas burner, or in a catalytic oxidation installation. Oxidation techniques are used to abate anode plant emissions which partially consist of pitch and tar fumes and are rich in PAHs. Oxidation techniques abate pitch and tar fumes as well as condensed and volatile PAHs as well.

promising emerging dust control techniques include:

COHPAC^tm and TOXECON^tm technologies

COHPAC^tm (Compact Hybrid Particulate Collector) and TOXECON^tm are multi multi-pollutant control technologies for mercury, dioxins but also other pollutants including fine particles developed and applied in the U.S.A. In COHPAC^tm a FF is installed downstream of an existing ESP. As the ESP removes most of the dust, the filtration rate of the FF can be increased substantially while keeping a modest pressure drop. ESP might also lead to agglomeration of very fine particles which can be then removed in the FF. TOXECON^tm refers to the injection of a dry sorbent like activated carbon between the ESP and FF.

indigo Agglomerator

The Indigo Agglomerator forms large agglomerated particles by attaching fine particles to larger particles. The agglomerated particles can be easily removed using standard techniques like ESP. This technique allows also the reduction of mercury emissions and may be used in case of significant concentrations of BC associated to sub micrometer particles.

Main Characteristics of techniques

Information provided by industry and accepted by the Clearing House Evaluation Committee

Details: Last Updated: 10 September 2018