Preventing Dust Explosions in Pellet Production

Dust explosion prevention and mitigation systems must be tailored to the application and the specific equipment used.
By Clive Nixon | August 28, 2020

For ideal operation and combustion, biomass pellet systems require product of uniform size, shape, density and moisture content. To accomplish this, raw materials typically go through many processing steps that generate dust. Combined with many potential sources of ignition, present are all the ingredients for a dust explosion.

When a plant’s dust explosion potential is evaluated, three questions must be asked: How can a deflagration event be prevented from starting? How can pressure created from a deflagration be mitigated? How can a deflagration be prevented from propagating to another piece of equipment or into the environment around the equipment?

Relevant NFPA codes that apply to wood pellet processing are NFPA 68 Standard on Explosion Protection by Deflagration Venting, NFPA 69 Standard on Explosion Prevention Systems, and NFPA 654 Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing and Handling of Combustible Particulate Solids.

To protect process equipment and personnel, a hybrid of technical measures is often required. Among the options are passive devices like explosion vents, along with active devices such as explosion suppression and spark detection and extinguishing systems. In addition, chemical or mechanical isolation devices are required to protect connected equipment and piping from propagation that could lead to a secondary event, which can often be more dangerous and destructive than the initial event.

Dust Explosion Prevention
Explosions result from the ignition of dust when mixed with air during processing, handling or storage operations. A rapid rise in pressure occurs in the containing structure, and if it is not of adequate strength to withstand the pressure, extensive damage and injury to personnel can occur.

Equipment in which airborne dust can accumulate include mechanical conveyors. Whenever an enclosed conveyor is being filled or emptied, there is a potential dust cloud that could be ignited. Dust collection equipment such as baghouses are particularly subject to potential explosions since they typically handle the driest, finest dust in a process. Dust can also accumulate when conveyed in bucket elevators, or when pellets are discharged into silos.

The first step in mitigating the risk for a dust explosion is preventing an event from occurring.  Good housekeeping keeps the area free of dust and is a vital activity to protect both the building structure and personnel. Identifying and controlling potential ignition sources is also critical. While ignition sources cannot be totally eliminated, they can be significantly reduced. Techniques include spark detection along with monitoring temperature, belt alignment, slippage and motor drive overloading.

Dryers, which are used to reduce moisture content below 10% in pellet production, are one of the potential ignition sources. Temperature, flow and moisture monitoring are also important tools to help prevent ignition. Size reduction equipment such as hammer mills can also present a potential ignition source, either from foreign materials entering the mill or due to an internal failure.  The introduction of metal and other foreign materials must be avoided using solutions such as stone separators and magnetic separators.

Dust Explosion Venting
During the early stages of a dust or gas explosion, explosion vents quickly open at a predetermined burst pressure, allowing the rapidly expanding combustion gases to escape into the atmosphere and limit the pressure generated inside the process equipment to calculated safe limits.

Venting is the most widely adopted protection mechanism because it provides an economical solution and is often considered “fit-and-forget” solution. However, it is important to note that vents need to be regularly inspected per NFPA 68. For decades, explosion vents have traditionally been designed using a “composite” approach that sandwiches plastic film between more resistant stainless steel sheets with holes or slots cut into them.  These vents are designed to open at typically 1 to 1.5 PSI set pressure.

With this type of technology, over time, the holes and slots in the stainless steel sheets can admit particulates and debris. The buildup can eventually affect the functionality of the vent. A vent that becomes heavier in weight will open slowly and less efficiently. A better solution is a single-section explosion vent, comprised of a solitary sheet of stainless steel in a domed configuration. Perforations around the perimeter aid opening at the desired low-set pressure are protected with gasket materials. 
The single-section domed design produces a vent that is more robust, lighter in weight and largely eliminates the potential for buildup or contamination. 

Despite its popularity, explosion vents will not work for every application. With venting, the combustion process releases a large ball of flame into the atmosphere.

While this might be an acceptable consequence for outdoor equipment such as silos, for applications within a plant, it could endanger personnel or equipment, and even lead to a secondary explosion. In cases where a flame ball must be avoided, flameless venting can be deployed. Flameless vents are designed to absorb the pressure wave, flame and particulates that would normally be ejected by a vented explosion.

To address this need, companies like BS&B Pressure Safety Management provide a flameless system designed with the vent installed inside a housing that incorporates a flame arrestor.

Explosion Suppression Equipment
For processes where an explosion would ideally be prevented altogether, suppression systems are the optimal alternative.  Explosion suppression equipment detects a dust explosion in the first milliseconds of the event, signaling explosion suppressors to rapidly release a flame-quenching medium, such as sodium bicarbonate, into the process equipment. This effectively stops the explosion in its infancy and results in a reduced explosion pressure that is safe for the protected equipment.

For a 24/7 process, a suppression system can be desirable because the speed of cleanup and refit allows for a quick return to production. With venting or flameless venting, the explosion fully develops in the process equipment, requiring cleanup, fire-related damages and other consequences that take time to get the process back into operation.

A typical suppression system consists of sensors and several explosion suppression cannons that propel an extinguishing agent into the process equipment. Pressurized nitrogen is typically used to provide the motive power.

Explosion Isolation
Chemical isolation systems protect interconnected equipment in the event of an explosion. Ducting and piping connecting process equipment can propagate an explosion of even greater intensity, and this is why NFPA 64 calls for isolation. If unprotected, the ducting, piping and connected vessels and equipment are at risk.

Broadly, explosion isolation can be categorized as passive or active, per NFPA 69. A common example of passive isolation is a flap valve that is essentially a one-way valve installed on the inlet duct to a dust collector. The flap is open during normal operation, latching close against a seat in response to the cessation of air flow and a pressure wave traveling in the opposite direction. These valves must be mounted horizontally, and due to pressure piling, must be mounted on a duct having a strength twice that of the reduced explosion pressure for the equipment it is isolating.

Another example of a passive isolation device is a suitably designed airlock to isolate a hopper in the event of a deflagration. These devices must conform to the requirements of NFPA 69 to provide adequate isolation. Not all rotary airlocks are equal in this regard.

Chemical isolation systems overcome the application limitation of flap-style valves. Chemical isolation is an active isolation method that typically consists of an explosion pressure detector that triggers a chemical suppressor. Chemical isolation is not limited to horizontal ducts or air flow direction.

Furthermore, chemical isolation can used on rectangular ducts and casings with moving internals such as drag conveyors.  This method of isolation provides the more economical solution for large ducts.
Dust explosion prevention and mitigation systems must be tailored to the application and the specific equipment used. Careful attention to prevention, mitigation and isolation will ensure the protection of both personnel and plant while limiting the potential for preventable business interruptions.
Author: Clive Nixon
BS&B Pressure Safety Management LLC
(918) 622-5950
[email protected]