Special features of wood pellets manufacturing process
One of critical stages of raw material pre-treatment before pelletization is its disintegration. This process is aimed at getting finer feedstock fraction, and afterwards – at better pelletizing conditions inside pelleting press chamber. The finer the particle size of raw material feeding the compression chamber, in particular entering the holes of rotating former block, the lower the load on both electric motor and pressing rolls. This effect, in its turn, prolongs service life period for all parts of the pelleting press. While in case of sunflower peelings the pelletizing train starts with hammermill, in case of sawdust pellets production the waste wood including branchwood, tree stumps, etc. are first disintegrated into wood chips in woodchippers. Usually, wood chips are first passed through cutting machine and then the first stuff is fed to the hammermill. It is also important to take sawdust humidity into account since dump groundwood is to be dried in dedicated drying unit, as required by the technology. This point should be considered in wood pelletizing line design engineering phase.
Powdered sawdust drying process
Powdered sawdust is fed to the fryer cylinder. Feedstock is dried by hot air generated by thermal generator. The use of direct-fired thermal generator is demonstrated as promising and recommended, wherein furnace gas is used for heat transfer agent. Heat transfer agent temperature reaches 400°C at the entry to the drier cylinder, and about 100°C at the exit from the cylinder. Raw material is dried until it reaches the target relative humidity of 15% or less. The pelleting press is equipped with damping system or steam injection system in order to prevent drying of raw material down to the final humidity of 10% or lower, and make it possible to control humidity directly in the blender chamber, just before pelleting. Dried sawdust is further transported via pneumatic conveyor system into the smoke exhauster coupled with cyclone cell system where the dried material is separated from heat transfer agent.
Sawdust pelletizing process
Modern pelletizing machines are typically equipped with integrated set of special equipment that makes it possible to operate the pelleter in the optimum way. Those support systems include steam and water injection systems, blenders, measuring feeders, and electric parameters control devices installed in control cabinets. Pellets are immediately formed upon penetrating of pre-conditioned material between the rotating female former block and fixed rollers. While the feedstock is squeezed through former block holes (draw dies), it is compressed to acquire desired geometry, size, and density.
Cooling down pellets
Pellets, even completely shaped, acquire desired hardness only on cooling; moreover, the more advanced the drying process is, the better will be the quality of finished product you get. In this regard, the design of the cooling tower must provide effective cooling of finished product. Among other things, when cooling tower is discharged, the finished pellets pass through shaking screen where crushed material (chips) is separated and transported back to the feeder for re-pelleting.
Fuel pellets are manufactured with no added bounding chemicals, under high pressure alone; they are 20 to 50 mm long, and 4 to 10 mm in diameter. 1 kg of fuel pellets provides a thermal equivalent of 0.97 kg of black coal, each of two emitting 4.8 kWh of heat. Combustion process is more efficient and complete in case of sawdust pellets compared to crude wood; the design solutions have been found to develop pellet-fired systems intended for use in both industrial and domestic boilers. Pellets are not prone to self-ignition in storage, and take up far less space than do wood chips and sawdust. “Releasable” carbon dioxide (CO2) in fuel pellets is referred to as “neutral”. In fact, wood releases as much CO2 as it had absorbed in the plant growth phase (so-called confined carbon turnover). During the combustion of fossil fuels, however, all carbon dioxide is released which has been accumulated over millions of years. This, in turn, results in increased atmospheric CO2 content and, therefore, induces anthropogenic greenhouse effect.