Taking the air out of wood pellet cooling
How plate heat exchangers can help mills meet more stringent targets
Author: Gerald Marinitsch
The future is bright for wood pellets. As the push intensifies for more efficient and sustainable sources of energy, this increasingly popular biofuel is being hailed as a timely, eco-friendly alternative to fossil fuels for both domestic and industrial applications.
The equipment involved in manufacturing wood pellets is also playing a vital part in the overarching role of reducing global greenhouse gas emissions – notably at the cooling stage.
First, a bit of background . . .
A study released earlier in 2021 from the U.S.-based Biomass Thermal Energy Council and Life Cycle Associates found biomass fuels such as wood pellets can result in more than a 100 per cent reduction in GHG emissions when compared with heating oil.
Titled “Life Cycle Analysis of Renewable Fuel Standard Implementation for Thermal Pathways for Wood Pellets and Chips,” the study found that the range of life cycle GHG emissions were 0 to 23.5 g CO2e/MJ for wood pellets, compared with 90.45 g CO2e/MJ with heating oil. In many situation, the avoided emissions associated with burning waste biomass, avoided wildfire risk or composting were greater than the life cycle emissions from pellet fuel use.
Interestingly, heating oil currently exceeds the targeted 60% GHG reduction requirement for cellulosic biofuels replacing heating oil under the U.S. EPA’s Renewable Fuel Standard Program.
The wood pellet process
Wood pellets are created by, first, removing moisture from incoming wood fibre. That fibre is then ground into dust and compressed into small pellets measuring between six and eight millimetres in diameter and up to 40 millimetres in length. During the process, the pellets are heated up so the lignin in the wood acts as an adhesive to keep the compressed particles together.
Once discharged, these wood pellets range in temperatures from 70 to 90°C – making them too hot for storage and transport. At this temperature, they also pose a safety hazard that must be managed. The preferred option is to cool and dry the pellets to approximately 3 to 5°C above ambient and with a moisture content of around 10%.
The cooling stage
The technologies deployed at the cooling stage can be divided into two categories:
With air: Wood pellets enter a cooling bin, typically through a rotary valve, with the particles being levelled off by a distributor. Cold air is then injected into an isolated bin under the bed of pellets, thereby indirectly cooling the product from the bottom up. A level sensor inside the bin determines when the pellets are discharged.
Without air: The pellets enter a tower-like design and slowly pass between a parallel series of heat exchanger plates that contain a counter-current flow of water or other heat transfer fluid. Heat transfers from the pellets to the heat transfer fluid via a steel plate wall. The product then cools to temperatures between 3 to 5°C above ambient temperatures as, pulled by gravity, it slowly and uniformly moves downward, controlled by a discharge feeder.
Benefits to no air
One of the primary benefits to indirectly cooling wood pellets through a plate-based design is improved efficiency. Water is a more effective cooling medium than air, absorbing up to 24 times as much heat and resulting in lower power requirements to circulate the much lower volume of cooling medium.
In addition, cooling wood pellets with plate heat exchanger technology enables precise control of both the temperature and moisture content of the product. The combination of proprietary thermal modelling software and guaranteed mass flow design results in optimal cooling and drying of each individual pellet.
Related content | The Science Behind Moving Bed Heat Exchangers
The mass flow design also ensures uniform material flow is controlled at low velocities – typically less than 0.3 m/min. There is no mechanical handling and moving of the pellets, rendering the technology ideal for even the most friable grades of pellets. This gentle product handling prevents product abrasion, degradation and creation of additional fines.
And it does so while producing near-zero emissions while mitigating contamination and breakage, and ensures the safety of plant operations personnel.
Want to learn more? Contact a Solex team member today.
Solex is the global market leader and developer of high-efficiency, indirect heat exchange technology for the heating, cooling and drying/conditioning of free-flowing granular materials such as solid granules, pellets, beans, seeds and particles.
Over the past 30 years, our Canadian-headquartered company has installed more than 800 heat exchangers in more than 50 countries worldwide with applications ranging from industrial materials and fertilizer to oilseeds and sugar.
About the expert
Gerald Marinitsch, Global Director, Industrials, Solex Thermal Science
Gerald joined Solex in 2014 and now leads the company’s efforts within industrial applications such as chemicals, metals and minerals and sands. Contact Gerald.
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