Code of Practice for managing particulate matter emissions in the potash sector: chapter 4

Code of practice for the management of PM2.5 emissions in the potash sector in Canada

4. Recommended PM2.5 emission control practices

This section presents recommended best environmental practices and mitigation measures to control total particular matter (TPM) and PM2.5 emissions from processes in the potash sector. The recommendations in this Code should be applied where and when appropriate based on the particular circumstances of each facility. The recommendations are categorized as follows:

  • Emission control devices
    • Wet scrubbers
    • Baghouses
    • Electrostatic precipitators
  • Emission control devices - general
  • Dryers and compactors - maintenance
  • Material handling practices
  • Environmental management practices.

Each recommendation listed in Table S-1 is followed by a brief discussion of the sources of particulate matter listed in the table.

4.1 Emission control devices

Emission control devices are defined as equipment, other than inherent process equipment, that is used to destroy or remove air pollutants from emission prior to the discharge to the atmosphere. Control devices that are typically used in the potash industry include wet scrubbers, baghouses (fabric filters), and electrostatic precipitators (ESPs). Since cyclones can be used to separate coarse particulate from the exhaust stream, they are often installed as a control device in series with one of the other devices, namely a scrubber, baghouse, or an ESP.

4.1.1 Wet scrubbers

Wet scrubbers are classified as a wet particulate removal system. They remove particulates from an air stream by having them impinge on water droplets, or by becoming absorbed by the water. The water containing the particulate matter is then removed from the collector as a waste stream.

There are several types of wet scrubbers available using different operating principles; four of the more common types are described below:

  • Low-energy or gravity-spray-tower scrubbers are equipped with spray nozzles that atomize water and inject it into the rising exhaust gases. Dust particles are caught by the droplets through direct impaction, diffusion, or interception. Wastewater falls by gravity and is collected at the bottom of the scrubber. These types of scrubbers are moderately effective for particulate less than 10µ in size.
  • Low-to-medium energy scrubbers use centrifugal force to spin the particulate against the wetted walls of the collector, where the particles are carried away by water introduced from the top. Collection efficiency is good for particles 5µ in size and above.
  • Medium to high-energy scrubbers are of the packed-bed design. Beds of packing elements made from various materials break down the liquid flow into a high surface area film to achieve maximum contact with the air stream moving up through the bed. Particles from the air stream are deposited on the bed, which are absorbed or washed down by the water for discharge at the bottom. This type of scrubber provides good efficiency for 10µ particles
  • High-energy, or venturi scrubbers use a narrow throat (venturi) design to collect particulate under very high pressures. Water is injected with the exhaust gases and accelerated through the venturi section. The water is atomized and extreme turbulence promotes collisions between the water droplets and dust particulates in the throat. At the exit of the venturi section throat there is a high pressure drop, and particles are agglomerated with the droplets. The resulting gases then travel through a wetted elbow, a cyclonic section and finally a discharge hopper where the dirty water is discharged. Venturi scrubbers have a very high collection efficiency down to 1µ size particles.

Recommendation R01 - For venturi scrubbers, continuously monitor and record daily average gas flow rate and pressure drop, daily average brine/water flow rate, and daily average fan amperage; calculate liquid-to-gas (L/G) ratio daily.

Recommendation R02 - For non-venturi scrubbers, continuously monitor and record daily average gas flow rate, brine/water flow rate, daily brine nozzle pressure, and daily average fan amperage; calculate L/G ratio daily.

Sources targeted - S5, S6, S7, S8

As outlined in the Compliance Assurance Monitoring (CAM) provisions, first establish the normal operating ranges of these devices during initial equipment  installation and commissioning, periodic stack testing, performance testing, and/or periodic calibration as specified by the manufacturer.

Pressure drop and liquid flow rate are often required to be monitored continuously for wet scrubbers. Any situation that increases the resistance to air movement through a device will increase the pressure drop and any situation that decreases the resistance to air movement will decrease the pressure drop.

The flow rate of liquid (i.e., water or brine) to the wet scrubber is another simple operating parameter that can be monitored to ensure proper operation of the control device. An increased liquid flow rate may increase the size of droplets beyond the optimum size required to collect dust particles. It may also indicate the risk of erosion of the nozzle orifice. A decreased liquid flow rate may indicate a sub-optimum liquid contact, or sub-optimum droplet size. It may also indicate the risk of solids deposition or plugging in the liquid supply header or nozzles. Key attention should be paid to the nozzles and the water flow. Nozzles are typically set up for easy maintenance and they should be regularly checked because any plugging in the nozzles affects performance.

Measuring liquid flow rate is also required as it is directly related to the liquid-to-gas (L/G) ratio. Since the exhaust gas flow rate is typically a constant value set by the fan speed on the ventilation system, the liquid flow rate is the simplest parameter to monitor to ensure that the optimum L/G ratio is being achieved. For dynamic ventilation systems, where exhaust gas flow may change significantly, the monitoring of liquid flow rate alone is insufficient so monitoring of exhaust gas flow is also required to ensure that the ventilation system is achieving the optimum L/G ratio.

The fan current (fan amperage) should be monitored since it is proportional to power draw. In case the fan amperage decreases, it implies a blockage and in case the fan amperage increases, it indicates that there is cold air going through the system. If a wet fan is involved, the total fan power is a function of how much water is being applied to the fan and the specific gravity (SG) of the brine.

Recommendation R03 - Implement maintenance practices specific to wet scrubbers.

Sources targeted - S5, S6, S7, S8

To ensure that the scrubbers are operated at their optimum design conditions, create and implement a maintenance program and schedule specific to the equipment based on manufacturing recommendations and site-specific requirements. The program may include:

  • Inspecting for any possible air leak in the system and try to minimize the leakages;
  • Performing visual inspecting for possible corrosion and plugging (e.g. spray nozzle for plugging in spray scrubbers);
  • Cleaning filters on scrubber liquid inlet stream; and
  • Inspecting liquid recirculating pump, piping, and pressure gages for any abnormality including leakage and plugging.

Recommendation R04 - For those facilities equipped with wet scrubber recirculation monitoring systems, monitor and record changes in the brine specific gravity (SG) at regular intervals.

Sources targeted - S5, S6, S7, S8

Establish the normal operating range in brine specific gravity for the scrubber recirculation system.

Critical instrumentation to consider would be a SG monitor, which would indicate that the scrubber is maintaining a brine flow at the desired level. Changes in the SG indicate increasing concentration is occurring, which is an indicator of brine flow failure. A higher SG provides an early indication of reduced water flow and potential for plugging. It may also indicate potential reduced cyclone collection efficiency upstream (see 4.2.1).

4.1.2 Baghouses

A baghouse is a large filter housing filled with numerous long filter bags. Typically, the bags are cylindrical and made of fabric, although a flat bag or a pleated filter can also be used, and ceramic and sintered metal bags are available. In operation, dust-laden gases enter the chamber and pass through fabric bags that act as filter. A cake of solids is built up on the fabric surface, and it is this porous cake that conducts the particulate filtering. If the cake does not build up, the fine particulate present in the flue gas would penetrate into the fabric pores and quickly plug or blind the filter bag. With the cake, the blinding process is substantially slowed, and the bags may last from weeks to years, depending on the bag and particulate characteristics. The bags are usually cleaned by a reverse air, mechanical shaking, or a pulse jet. The pressure drop for a baghouse can range from 1 to 2.5 kPa (4 to 10 inches of water).

Recommendation R05 - Continuously monitor the daily average pressure drop and average fan amperage of all baghouses.

Sources targeted - S6, S7, S8

Establish and record the baseline pressure drop of the baghouse being monitored on initial equipment installation and commissioning, periodic stack testing, performance testing, and/or periodic calibration as specified by the manufacturer.

The static pressure drop establishes an indicator of the resistance provided by the fabric cloth and its collected layer of dust. It is also directly proportional to the exhaust gas volumetric flow rate. The ongoing operational pressure drop for each baghouse can be compared against baseline values (normally established during performance testing) to ensure proper baghouse operation. An increased static pressure drop generally indicates high gas flow rates, fabric blinding, or system cleaning problems.Footnote 5 Conversely, a decreased static pressure drop is generally caused by reduced gas flow rates, excessive cleaning intensities or frequencies, reduced inlet PM loading, or possibly bag leakage.Footnote 6

The continuous measurement of fan motor current through an ammeter is a method of determining the load that the fan must overcome to push (or pull) the exhaust gas through the fabric filter. This is an indicator of the resistance offered by the filters and the built-up dust. While increases in fan amperage generally indicate high exhaust gas flow, excessive cleaning, or possibly bag leakage, decreases in fan amperage generally imply reduced gas flow rates, or a higher degree of dust build-up.

Recommendation R06 - For those facilities equipped with Baghouse Leak Detection Systems (BLDS) monitor their voltage continuously.

Source targeted - S7

Leaking or broken filters can lead to safety risks, reduced process efficiency, housekeeping and maintenance issues, damaged ventilation equipment, and environmental compliance violations. A recent compliance trend in the U.S. is to require baghouses to have a BLDS installed in the clean-air exhaust gas outlet to monitor significant changes in dust levels. The BLDS operates based on the triboelectric effect (also known as particle impingement or frictional electrification), which is the electrical charge transfer that occurs between two materials when one rubs or is impacted against the other. In operation, dust particles flowing in the air stream in the duct collide with the probe, generating an electrical charge. The electronics converts this charge to a particle emission signal voltage and continuously monitors and analyzes the signal during the baghouse operation. When the signal exceeds a pre-set PM level for a specified time delay, an alarm notifies the operators that a filter bag is leaking or has failed.Footnote 7

Recommendation R07 - Implement maintenance practices specific to baghouses.

Sources targeted - S6, S7, S8

Baghouses should be subject to a prescribed operations/maintenance routine that includes several components at prescribed frequencies: inspection and maintenance of hopper dust removal, compressed air supply and distribution, proper operation of cleaning cycles, functioning of bag cleaning mechanisms, bag integrity, and physical integrity of baghouse. Furthermore, inspection of baghouses is critical in maintaining reliable long-term operation. There are several areas that require attention through routine inspection, including:

  • Daily check for indications of plugged pressure taps. The taps leading to the static pressure gauges need to be free of material and liquids to function properly. The gauge face should be free of water and deposits and the gauge should fluctuate slightly each time one of the diaphragm valves activates;
  • Monthly inspection of triboelectric probe for dust build-up. The triboelectric probe only generates voltage from direct impacts of dust on the probe’s metal surface. A visual inspection of the triboelectric probe is required to ensure that no dust has built up or caked on its surface;
  • Inspection of fans. Fans need to be inspected periodically for wear, material build-up, and for corrosion. Continuous monitoring of vibration can provide ongoing information and periodic visual inspections can assess fan integrity;
  • General inspection of equipment and maintenance. A baghouse inspection needs to cover all components of the system, including the compressed air equipment, bag cleaning mechanisms, and dust removal mechanisms from hoppers. On pulse-jet baghouses, inspections should focus on the conditions of the bags, cages, and compressed air delivery systems. On reverse air or shaker baghouses, inspections should focus on the bag tension and the status of the bag attachments at the tube sheet. For these baghouses, the majority of problems usually occur within the bottom 1-2 feet of the bags. Where possible, inspections should examine the clean side of the fabric filter to assess potential dust breakthrough. Fresh dust deposits on the clean side that are more than 1/8” deep indicate potential PM emission problems;
  • Internal bag inspection. An internal inspection of the bags should be done semi-annually to assess their condition. The bag connections and tension need to be examined; and
  • Bag replacement - Replacement schedules for bags should be determined based on manufacturer recommendations, and site-specific equipment and process conditions.
4.1.3 Electrostatic precipitators

Particulate removal in an electrostatic precipitator (ESP) involves the discharge electrodes and the collection electrodes. In the first step, particulate is given an electrical charge by means of a high voltage (up to 100 000 V) applied to the discharge electrodes. The particulate is then attracted to and precipitates on the collection electrodes by virtue of their opposite charge. For proper precipitation to occur, the drag force on the particles from the gas flow must be lower than the electrostatic force, and the residence time in the ESP must be sufficient for the particles to reach the collection electrodes. Gas velocity in an ESP typically ranges from 0.6 to 1.5 m/s, and gas residence time can be as high as 15 sec.

Recommendation R08 - Continuously monitor the secondary current and secondary voltage of all electrostatic precipitators. As needed, monitor the spark rate.

Source targeted - S5

Establish the baseline voltages and spark rates on initial equipment installation and commissioning, performance testing, and/or periodic calibration as specified by the manufacturer.

The secondary voltage provides an indication of the strength of the electrical field surrounding the discharge electrodes, which is related to the attraction force exerted on the particles in the exhaust gas. The secondary current is a measure of the quantity of dust that is diverted from its flow path to contact and adhere to the discharge electrode. This parameter is related to the overall dust load being captured by the ESP’s discharge electrode. These two parameters should be monitored continuously for dry ESPs under the Compliance Assurance Monitoring provisions in the United States Environmental Protection Agency (U.S. EPA) Maximum Achievable Control Technology (MACT) standards. Also, spark rate is a measure of how close to the maximum voltage at which an ESP is operating and provides an indicator of collection efficiency.Footnote 8

The ESP’s performance can be evaluated by comparing the secondary currents, secondary voltages, and spark rates against baseline values. If the unit does not have secondary voltage meters, similar analyses can be conducted using the primary currents, primary voltages, secondary currents, and spark rates. Having a large spacing between discharge and collecting electrodes allows higher electric fields to be used, which tends to improve dust collection. To generate larger electric fields, however, power supplies must produce higher operating voltages.Footnote 9

Recommendation R09 - Implement facility maintenance practices related to electrostatic precipitators.

Source targeted - S5

To ensure that ESPs are operated at their optimum design conditions, the following checklists can be applied:

  • Monitor electricity consumption, power voltage and amperes;
  • Check dust concentration at the exit of the ESP;
  • Perform visual inspections for the coated plates/tubes and wires with dust, and also possible broken plates/tubes and wires; and
  • Verify that the equipment is operated within the appropriate operating range

4.2 Emission control devices - general

Recommendation R10 - Implement recordkeeping of monitoring, excursion evaluation and excursion correction of all emission control devices at significant sources.

Sources targeted - S5, S7

Ongoing recordkeeping is important to determine the effectiveness of the Code, and to identify opportunities for improvement.

When an excursion occurs, it is good practice to monitor the control device performance at a regular frequency interval appropriate to the control device and specific situation in the period after the excursion until it is corrected. It is also recommended to evaluate and correct the problems that have affected the control equipment in a formal manner. A quality improvement plan (QIP) is a formalized written plan that outlines the procedures used to evaluate problems that affect the performance of control equipment, and has two basic components:Footnote 10

  • Initial Investigation procedures to evaluate and determine the cause of control device performance problems. These usually contain a list of inspections, system operation verifications, and a parameter-monitoring schedule that must be initiated within a specified number of days from the date of the last excursion; and
  • Modifications to enhance current CAM practices including the procedures that should be implemented to reduce the probability of a recurrence of the problem, and the schedule for making such improvements. Procedures might include: improved preventative maintenance practices; process operation changes; and appropriate improvements to control methods.
4.2.1 Cyclones

Cyclones separate solids from gas streams by centrifugal force. Cyclone separators are vertical, cylindrical vessels with a gas entrance designed to give a spiralling gas flow around and down the cyclone wall. Once the gas is in the cyclone, the downward spiralling flow of the gas stream imparts a centrifugal force on the particulate, which is thrown radically outward to the cyclone wall. When the particles hit the wall, much of their momentum is absorbed, and they fall to a cone-shaped section at the bottom of the cyclone. The particles are discharged out of the cone through a narrow neck, while the gas continues spinning along the wall inside the cylindrical vessel and exiting through a tube that is mounted in the center of the top of the cyclone.Footnote 11

Recommendation R11 - Implement maintenance practices specifically related to cyclones.

Sources targeted - S5, S6, S7, S8

The particle collection efficiency of cyclones depends on a number of factors, including the dimensions (length and diameter) of the cyclone, the inlet gas velocity, the particle size, and the dust concentration in the gas stream. Collection efficiency often rises when inlet gas velocity increases and when particle size and dust concentration increases. Also, smaller cyclones are usually more efficient than larger cyclones. The physical condition of the cyclone body also affects removal efficiency. Dents, riveted joints, and other surface irregularities can disrupt the vortex within the cyclone, thereby causing particles to bounce back into the centre of the cyclone instead of being concentrated near the cyclone wall. Air infiltration through the solids discharge valve, holes, or weld failures can also disrupt the vortex.Footnote 12

Cyclones are the simplest piece of equipment among the PM removal systems, but the following actions can be taken to ensure that they are operated at their optimum conditions:

  • Perform visual inspections of all elements associated with cyclones (including airlock and rotary valve device operation) as maintenance is being performed, and more frequently if needed due to equipment malfunction or process upsets.

4.3 Dryers and compactors

Recommendation R12 - Ensure there are no leaks in the dryer air discharge system that would allow dust to escape.

Source targeted - S5

As part of normal operations, regularly inspect the drying equipment, including sealing joints and other sections of the air ducting to prevent dust from escaping. Initiate corrective actions accordingly.

Recommendation R13 - Ensure that compactor hoods and ducting are fitted properly and have no cracks to prevent dust from escaping.

Source targeted - S7

As part of normal operations, regularly inspect the compacting equipment, including sealing joints and other sections of the air ducting to prevent dust from escaping. Initiate corrective actions accordingly.

4.4 Material handling practices

Material handling operations are commonly found in any solid fertilizer production operation, and these operations generally consist of storage (piles, silos, bins) and the transfer operations (conveyor belts, elevators, gravity drop, pneumatic transfer, etc.). Material handling operations generate PM2.5 emissions but are usually enclosed, and some points have control devices.

Recommendation R14 - Optimize material handling, storage, and conveying practices.

Source targeted - S8

Particulate matter emission controls for material handling involves physical practices. Weather monitoring and suspension of operations during severe/adverse weather conditions (mainly high wind speeds) can decrease particulate emissions. Point sources in the materials handling processes can be designed to reduce particulate emissions. More specifically, conveyors can be designed to reduce particulate emissions in various ways, such as:

  • Enclosed; side wind guards, covers;
  • Reduced drop heights at transfer points;
  • Suitable speed;
  • Loading of belt not to edges; and
  • Maintenance and operation of conventional conveyors.

Loading, unloading and transfer points can be optimized to minimize emissions by reducing drop heights. Minimizing drop heights and regular cleanings of front-end loaders would also reduce emissions.

4.5 Environmental management practices

Recommendation R15 - Implement broad-based best practices for general environmental management.

Sources targeted - S1 through S8

There are a number of reference materials that speak to good environmental practice, such as:

  • ISO 14000 Environment Management Systems,
  • Environment Canada’s Environmental Code of Practice for Metal Mines, 2009, and
  • Environmental Aspects of Phosphate and Potash Mining, United Nations Environment Program (UNEP), 2001.
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