Blending & Segregation

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It’s not uncommon for facility operators to blend their biomass before processing it. For example, they may incorporate a ratio of hardwood and softwood shavings into their pellets. Or they co-fire biomass with coal or municipal solid waste. Maintaining the integrity of any mixture is necessary to ensure the final product meets the specifications you require, whether those are material properties of pellets or efficiencies in energy systems.

But suppose the material handling portion of a facility’s processing system isn’t set up correctly. Those blends can segregate, creating off-spec products or difficult-to-control combustion units that raise operating expenses.  And such issues are common.

Problems almost always occur when the operator attempts to blend the materials well before or outside the processing center. Systems, such as boilers or gasifiers, that handle and inject the materials separately don’t encounter the same issues because they can control each element independently. With blending, there’s always the possibility that the material stream may segregate in a way that causes unwanted results.

As a rule of thumb, the further the blending takes place from where the material will be processed, the greater the likelihood it will segregate. For example, if you mix materials before storing them, those materials will likely segregate, creating inconsistent blends in the resulting material stream. Or perhaps they’re mixed before being transported by a pneumatic conveyance system. Differing densities and other particle characteristics can result in one material going through the pneumatic system faster than the other, creating a different mix on the output end than what is entered on the input side.

If there is a surge bin or buffering system, that is another place where materials can separate. Screw conveyors can cause separation, as the different materials interact with the sidewalls and augers differently and can move at different speeds through the system, disrupting the mixture.

Mixtures can experience problems even when materials are combined right before they’re fed into a process. We looked at a system once that blended shredded tires with biomass waste into a bin at a pulp mill. Due to the differences in density and particle size, the tires tended to “float” and edge away to the sides of the bin and weren’t consistently entering into the boiler.

Note also that you cannot mix in a bin. Although seemingly stable, materials in silos or other containers separate while reclaimed.

Blending takes careful preparation and equipment selection to ensure integrity. If possible, handle each material separately. If this cannot be done, use a dedicated mixture as close as possible to where the mix gets processed.