Material Characteristics and Bulk Handling Design – Biomass Engineering and Equipment
Despite the differences in raw materials, processors often do not take into account how variations affect bulk handling equipment.
The feedstocks of the granules are hardly uniform. The characteristics of a single raw material will differ as it passes through processing equipment. Yet despite the differences, processors often ignore how the variations affect bulk handling equipment. They apply a “one size fits all” approach to find that a machine that performs well at one point in the process is not performing as well at another. If pellet manufacturers are aiming for optimum performance and low expense, they must outfit their factories with conveyors and other machinery designed for the materials they will actually be handling at each step of the process.
Think about green biomass. Due to its high moisture content, acids in biomass escape more easily from the material than dry biomass. As a result, conveyors that handle green materials corrode faster than those that handle dry biomass. Thus, the divergent traits of the two materials – green and dry – affect equipment differently. If a manufacturer wants to reduce their maintenance costs, they have to understand it. Galvanizing their equipment can be a solution.
The fact that the green material is more acidic than dry does not mean that galvanizing is justified in all situations. Equipment that is not flooded with material may not need a protective finish, for example, a stacker. In addition, not all species of biomass are equally corrosive. Biomass contains various acids in varying amounts. White oak, for example, tends to be more acidic than red pine. In any case, protection is worth considering when it can reduce maintenance.
Acidity is only one quality that affects biomass handling machines. Density is also predominant among the characteristics that affect equipment and handling characteristics. Suppose a pellet manufacturer receives wood chips but later decides to accept chips as well. Suddenly, material is blocking the entrances and exits of the conveyor even though the operations team has accelerated the conveyors. What is happening? Although the equipment is often to blame when such problems arise, these problems are not due to bad equipment, but rather because it is not designed for the hardware operations personnel going through it.
To process the same volume of low-density material as a higher-density material, everything in a transport system has to grow. If you want to pass wood chips with a density of 22 pounds per cubic foot (lb / ft3) through a conveyor at a speed of 5 tons per hour. The conveyor must therefore move 455 cubic feet of material per hour. Later, you want to move the chips through the conveyor at the same rate. The chip density is 12 lb / ft3, which means the conveyor has to move 833 ft3 of material per hour to reach five tons – almost double the volume. Obviously, if the conveyor is sized for wood chips at five tonnes per hour, it is undersized for an equivalent chip flow.
In an attempt to overcome such a shortcoming, crews will sometimes increase the speed of a conveyor, but they may disregard the flow characteristics of the lighter material. With a belt conveyor, the material can move back at the tail because the belt slides underneath first. In a pallet conveyor, material can build up in the head because it cannot fall through the discharge opening as quickly as the denser material. For any conveyor, crews may also find that the chutes to and from it are too narrow and now clog easily. And if they are closed, they can learn that their conveyor needs a plenum to stabilize the air pressure so the material can settle.
Differences in density are not only due to changes in raw material. The density of a single raw material can vary. Green matter is denser than dry matter because it contains more water. If the green wood chips have a moisture content of 40%, when passed through a kiln, the humidity drops to 12%. This lowers the chip density to 17.8 lb / ft3 from 22 lb / ft3 previously. Their volume, however, does not change (they do not shrink), so the same size conveyor can be used to handle them as what handled them before the dryer. This post-dryer conveyor, however, requires less energy to operate, as the chips have less mass than before entering the dryer.
The characteristics of a material also change radically when it is densified. The granules, due to their cylindrical and smooth shape, flow freely like corn or soybeans. As a result, they can be handled with bucket elevators, which do not work well for non-granulated biomass due to the high friction of the material and its poor flow characteristics. Because the granules flow freely, they also cannot be elevated at steep angles without supports, such as pallets. Equipment specified for a material before densification is not ideal for handling it after it has been pressed.
Temperature is a third trait that affects material and the equipment that handles it. Pellets can come out of a baler at 200 degrees Fahrenheit and emit hot, acidic gases. The combination of heat and gas makes them extremely corrosive. A standard paint or powder coating will not protect the steel components of a conveyor under these conditions. We know that mild steel conveyors rust beyond use after a few months of handling hot pellets. Pellets require stainless steel handling equipment after pressing until they cool.
A conveyor designed for materials with different specifications will not perform well in this application – anything made of mild steel will rust quickly. The reality is that a machine, conveyor belt or whatever, cannot adapt to a lot of variation in the materials it handles. While it may seem practical to reuse a conveyor or match conveyor specifications to increase the number of common parts, a lot can go wrong when a conveyor is not designed for the job at hand. . Before making any modifications to your conveyor, consult the manufacturer for their recommendations. When your processes are running smoothly, you’ll be glad you did.