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Woody biomass imports to countries in the European Union from North America are on the rise. Pellets are deemed safe due to the heat and pressure inherently required during the pelletizing process; however, European markets demand other forms of woody biomass as well. Pellets are great for the energy sector if the boiler is designed for pellets, but not all woody biomass destined for Europe is going to end up as energy. Other uses for the woody biomass — paper quality chips or strands for oriented strand board (OSB) — are applications where pellets are not only impractical but impossible to use.

In the past, chemical sanitation has been used to comply with import/export regulations, but chemical sanitation is being phased out. The elimination of chemical sanitation will not decrease the demand for European imports of woody biomass, however, so it is important to look at other options.

Heat-based phytosanitation is a technology that can fill the void that chemical sanitation leaves for non-pelleted forms of woody biomass. In the United States, a woody biomass producer must work with the U.S. Department of Agriculture (USDA), which then works with the governing body in the European Union, to approve a proposed phytosanitary system design. Elsewhere in the Americas, producers will need to work with their corresponding agriculture governing bodies in order to export to Europe.

While numerous methods of drying or heating biomass exist, a rotary-drum system offers the potential to provide the highest capacity and lowest operation, maintenance and energy costs versus the alternatives. Wood that is destined for pelletizing typically must be dried to below 10 percent moisture content wet basis (mcwb). But, the desired final moisture of non-pelleted biomass could be anywhere from below 10 percent mcwb to above 30 percent mcwb, depending on the end-use. This means that particle retention time in the drum — or the amount of time that the particle is subjected to heat — must be tightly controlled to eliminate unnecessary and potentially harmful overdrying.

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As with any dryer system that uses flue gas recycle (FGR), primary and secondary separation is crucial to keep the recycle gas as free from particulate as possible, especially if a heat exchanger is being used in a waste heat application. Image provided by Thompson Dryers

The amount of energy required for heating is less than the amount of energy required for evaporating water from a product. When talking about heating and drying organic matter, we are really talking about heating or evaporating the internal water of each chip, strand, particle, etc. So, we must talk about the energy requirements of heating the portion of the product that is water and not necessarily the dry solids. For a practical illustration of this, imagine how much more quickly a T-shirt will warm to body temperature when it is dry versus when it was just soaked with a water balloon.

The definition of a British thermal unit, or BTU, is the energy required to raise the temperature of a pound of water, at atmospheric pressure, by 1°F. (Defined another way, 1 BTU is equal to 1.055 kJ, or 1 BTU/lb is equal to 2.33-kJ/kg.) If we extrapolate that out, we would expect to use approximately 142 BTUs (150 kJ) to raise the temperature of a pound of water from 70 to 212°F (21 to 100°C).

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Energy conservation is key for high capacity drying and heating applications; therefore, air locking and heat retention through whole-system insulation are important. Image provided by Thompson Dryers

But, if we want to actually evaporate that pound of water once it has warmed up, it requires the latent heat of vaporization, which is approximately 970 BTU/lb (1023 kJ/lb or 2256 kJ/kg), depending on where you look up that information.

To take this analogy one step further, we want to heat the water in the pot on the stove, not boil it. We do not want to use enough energy to boil off all the water and start heating up the pot, which is how one would torrefy biomass. Whether the end-product needs to be dried or just heated will depend on the goals of the end-user.

Drying and pelleting make sense from a shipping perspective because this eliminates shipping the extra water weight and volume. Also, it is easier to package pellets than loose chips. Nonpelleted forms of woody biomass will have to be densified in some manner for shipping. Options include baling, wrapping and form-and-fill bagging; pallet hooding also may  be employed. It does no good to go through the effort of sanitizing the chips only to have them recontaminated before or during shipment.

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No matter the heat source, whether solid fuel, natural gas or waste heat, flue gas recycle can be employed. Image provided by Thompson Dryers

Other Applications for Phytosanitation

Phytosanitation could open up markets for waste-heat applications due to waste heat typically being a “low quality” energy and not functional for many applications. Many U.S. states are offering tax credits to producers that can find a way to use their waste heat. Based on available energy and feedstock, multiple producers in a geographic region may want to work together with a broker to work out offtake agreements.

As with any drying or heating process, the three essential elements for fires will be present — heat, fuel and oxygen — so any fire risk must be mitigated during the design process. If the desired end-use of the product will always be above 25 percent mcwb, and it can be guaranteed that nowhere in the system can the product go below this moisture, there will be less risk for fires. However, it is difficult to guarantee product moisture at all points in a process, especially if any feedstock nonuniformity is present.

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Rotary drums for drying or heating for applications such as phytosanitation can be fabricated and shipped worldwide. Image provided by Thompson Dryers

Using a special blend of a single-pass rotary-drum dryer, heat source, gas recycle loop and control system, the residence time for an individual particle can be optimized for uniform heating and minimal drying. If the particle stays in the drum too long, it can start to dry, which will require more energy. If the particle is not in the drum long enough, it will not heat up to the appropriate temperature. The more uniform the size and moisture level of the product entering the drum, the better the results. But, if you know anything about handling woody biomass, you know that getting uniformly sized particles is nearly impossible. So, the drum internals and associated heat and gas flow must be designed to encourage smaller particles to exit the drum faster than larger particles, which take different lengths of time to heat.

After exiting the drum, the product must be separated from the gas stream using primary and secondary separation methods, typically in the form of drop boxes and cyclones. At that point, the product must remain above the required temperature for more than 45 minutes. The producer must be able to prove that any single particle has been held at temperature for the required amount of time. Using a batch-style process will make proving this easier, but batch-style processes fall short of continuous processes on capacity and footprint.

Phytosanitation of woody biomass for European import will fill the void left by phasing out chemical sanitation. As with any new technology, there will be energy consumption, manpower, supply-chain management and policy hurdles to overcome. Planning for these eventualities at the front end will help smooth the transition.