The Wood Pellet Process Explained

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The Wood Pellet Market

Two major factors are contributing to the growth of the wood pellet market.

First, the rise in cost of fossil fuels, and second, the effects of using fossil fuels on the environment.  Pellets can be made from practically any biomass material including wood, straw, grasses, energy crops, etc.

Wood pellets are now being used in power generation on a large scale as they have good thermal value, burn more efficiently than wood or other whole biomass, and are a cleaner source of energy than fossil fuels.

Wood pellets have moisture content typically below 10%. Wood pellets produce less ash, less smoke and more heat. Pellets have a uniform size, shape, density and moisture content. These consistent qualities are ideal for residential and commercial use, as well as generating power in the power industry.


The Pellet Making Process

Steps Of Pellet Production:
1. Raw Material: storage bunkers, buildings
2. Size Reduction: Grinders, hammer mills
3. Material Conveying: Fans, cyclone separators, screw, belt, and drag chain conveyors
4. Drying Solutions: Rotary drum dryers
5. Conditioning: Binders, Water and Steam
6. Pellet Production: Pellet Mill/press
7. Cooling: Counter flow air coolers
8. Pellet Conveying: Bucket elevators, conveyors, pneumatic conveying.
9. Bulk Storage: Bagging for residential use, or bins and silos for commercial use


Making Quality Pellets

Raw Material Preparation:
When making pellets, main ingredients are the raw material or a mixture of materials. There are several steps to preparing raw material. Initially raw material must to go through size reduction, which requires a hammer mill.

Raw material is processed into wood flour:
The hammer mill will reduce the particle size of the raw material, the size of particle is dictated by the size of screen used, depending on the end product requirements. The size of particle produced has impact on the quality of pellet produced.

Raw Material is conditioned:
Raw material moisture is measured and the percentage of water the material contains is adjusted.

Pellet Press:
Properly conditioned wood flour is fed into the pelletizer, or pellet press. Pellets are pressed or extruded through a spinning die at high pressure, creating heat and friction, as well as combustible dust called fines.  Pellets are conveyed to the cooler, and fines are picked up by the plant de-dusting, or dust collection system.

Pellet Cooler:
The wood pellets are conveyed to the counter flow air cooler for cooling for a period of time.

Storage Silo:
Wood pellets are then conveyed to storage bins or silos to await transport via, train, truck, or barge to the end user, often a power company for use as fuel in generating power.

Wood Pellet Process Safety

Awareness
The biggest concern in wood pellet production from a safety standpoint is a lack of awareness and ignorance of the hazards associated with the process.

A case can be made that a combustible dust explosion can be compared to a natural gas explosion. Both having similar potentially devastating and catastrophic effects.

Combustible dust hazard mitigation includes inherently safe design, hazard analysis, hazardous location identification, classifying dust as a hazardous material, as well as adequately designed engineering and administrative controls.


CSB Combustible Dust Study Findings
The 2006 U.S. Chemical Safety and Hazard Investigation Board Combustible Dust Hazard Study found “that industry and safety professionals often lacked awareness of combustible dust hazards, as MSDS’s ineffectively communicate to employees and workers the hazards of combustible dust explosions and ways to prevent them.” Pg. 78.

“The CSB and other agencies investigating serious dust explosions found a number of common causal
factors for dust incidents. First, the facilities failed to follow the widely recognized standards of good
engineering practice in the NFPA’s voluntary consensus standards. As a result, facilities did not
implement appropriate engineering controls, adequate maintenance and housekeeping, and other measures that could have prevented the explosions. These standards have been available for over a half century, and are updated for new technical and scientific knowledge regularly.”  Pg. 78.

http://www.csb.gov/assets/1/19/Dust_Final_Report_Website_11-17-06.pdf



Fire and Explosion Protection in Wood Pellet Production
At each step of the wood pellet production process, there is potential for fire. Pellet production is the process of making fuel.  The key to preventing fires and explosions, and thus catastrophic secondary explosions is mitigating combustible dust, and ignition sources.

Hazard Recognition is Essential
The first step is hazard recognition. Recognizing that at each step of the process, in each part of the facility, there is potential for creating fires. From storing and loading wood chips, to milling and grinding, to pelletizing, cooling and storage, combustible dust is created, and ignition sources are present.

Every time the biomass product is moved or manipulated, it creates combustible dust, creating a possible fire and explosion hazard.  Combustible dust from the process settles and migrates throughout the plant, creating a secondary explosion hazard.

Friction, Heat and Sparks are Created
Friction, heat and sparks are created in the processing of wood into pellets. The primary fire hazards are specifically in the drying, milling and grinding, and pelletizing of wood flour into pellets.

Combustible Dust Deflagration Hazards
The most dangerous parts of the process containing combustible dust in minimum explosible concentrations are found in the pellet cooler, dust collection, and storage bins and silos.
 
Risk Assessment
Risk can be quantified as potential for loss. Specifically the probability of occurrence, and potential magnitude of loss or consequences.

A risk assessment must be done to assess and quantify all risks associated with the production and storage of wood pellets, including production, business continuity and life safety.  All risks are quantified, prioritized, and addressed by risk rating.

Principles of Prevention
The keys to prevention are controlling the combustible dust emissions, and controlling ignition sources. 

Controlling Dust
Combustible dust emissions are primarily controlled through properly designed dust collection systems. Combustible dust migration from emissions can be controlled by utilizing oscillating fans to create an air barrier preventing migration of dust to high ceilings and rafters, or by using a misting system.

Hazardous Area Classification
Hazardous areas must be designated by class and division, or zone, depending on where, when and how dust emissions occur in production.  Operators must understand these hazardous areas. Only electrical equipment classified for a specific classified area or zone can be used.

Process Design
Inherent safe design of the equipment and process is one key to process safety. Where possible, various combustible dust generating systems as well as ignition sources shall be segregated, separated and isolated from each other.

Engineering and Administrative Controls
Engineering and administrative controls are used to limit combustible dust emissions and migrations, as well as ignition sources. A hierarchy of controls are applied based on the risk assessment.

Engineering controls
Engineering controls consist of many various types of mitigation equipment and systems.
Dust collection, oscillating fans and water mist systems may be used to control dust migration.


Preventing Fires
To prevent fires, spark detection and extinguishing systems are a primary control.  Bearing Ember, flame, temperature, CO and combustion gas, and emissions monitoring may also be used, as well as bearing temperature and bin temperature monitoring systems.

PreventingExplosions
Explosion Isolation, venting and suppression are used to control deflagrations.

Administrative Controls
Administrative controls such as housekeeping, change management, and preventative maintenance are used for hazard prevention and control.

By Jeffrey C. Nichols
Managing Partner
INDUSTRIAL FIRE PREVENTION, LLC 


Industrial Fire Prevention specializes in helping protect industrial manufacturing processes, pneumatic and mechanical conveying, and dust collection systems from combustible dust fires and explosions.

For requests for information or proposals, please contact info@industrialfireprevention.com



Industry Links


The NFPA standards on combustible dust:


OSHA Combustible Dust Resources:
Other Resources
Related Safety and Health Topics Pages
U.S. Chemical Safety and Hazard Investigation Board
Other Federal Resources
State Resources
  • Combustible Dust [PDF*]. Oregon Hazard Alert, OR-OSHA 2993, (2008, June).
  • Combustible Dust Explosion Hazards [PDF*] . Kentucky Department of Labor Hazard Alert.
  • Combustible Dust Poses a Dangerous, Explosive Threat in the Workplace [PDF*]. NCDOL Industry Alert.
Books
  • Development and Control of Dust Explosions (Occupational Safety and Health). John Nagy, Marcel Dekker Inc., ISBN 0-8247-7004-8.
  • Dust Explosions in the Process Industries. Eckhoff, Rolf K., Elsevier Science, ISBN 0-7506-7602-7.
  • Dust Explosion Prevention and Protection: A Practical Guide. Katherine Barton, Gulf Publishing Company, ISBN 0-7506-7519-3.



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