Steam conditioning used at some large pellet mills & agricultural pellet conditioning

Steam conditioning used at some large pellet mills & agricultural pellet conditioning

Adding dry steam into the mixed material can heat and soften lignin, thereby helping compress the mixed material during pelletization. Consistent quality in the mixed material is maintained which in turn ensures consistent quality in the pellets. The process ultimately increases production capacity.

Please note: Steam conditioning is only used at some large pellet mills. If planning to undertake small-scale production, this process is not necessary and this section can be skipped.

In pellet making operations, the pellets that come out are only as good as the raw material that goes in. The process can be compared to cooking: a cake is only as good as its ingredients and preparation, as guided by the competence of a chef. A pellet mill is like the chef, applying heat and pressure to reshape the material.

Once the material has been mixed to achieve a proper consistency, the raw material may require other ingredients to produce pellets. Other additives can increase the productivity of the pellet mill. Additives will react with other conditions in the pelletization process, elaborated on below.

Moisture content: As stated previously, there are maximum and minimum moisture content tolerances for a mill to produce pellets. However those tolerances are specific to the material in question, and the type of pellet mill used. As a general rule the most common average moisture content required for is 15 percent.

Adjusting the moisture content: If the material has a moisture content which is too high, a dryer is required. In the absence of a dryer it is possible to mix the material with a drier raw material to reach the desired average moisture content. If the moisture content is too low, peristaltic pumps can be used to add water at variable controlled rates. A peristaltic pump works on the principle of a rotating central cam, driven by an electric motor. As the cam turns it compresses the pipe. Flow rates are dictated by the speed of the cam, and speed adjustment takes place by changing the voltage of the electric motor. Peristaltic pumps can also prime themselves from several meters away. Other pumps too are used, including diaphragm pumps.

Binding qualities: Binder is the glue that holds a pellet together and produces a smooth shine. Some materials already contain enough lignin, a naturally occurring binder present in biomass materials. If the material lacks lignin to bind the material together, supplementary binders can be added. One of the simplest binders is vegetable oil. The oil can be simply added via a peristaltic pump. Generally oily products such as rape cake and dried distillers grain act as suitable binders.

Material density: A pellet is formed through heat and pressure; therefore a material’s density dictates the heat and pressure within the pellet mill. Material density can influence pellet quality and pellet mill productivity. Materials with a high density—for example, hard woods—require more heat and pressure to form pellets. Pellet quality therefore becomes an issue, as does decreased productivity. On the other hand low- density oily materials such as rape cake and dried distillers grain produce smooth hard pellets with heightened productivity taking place in the pellet mill. Some materials with a low density but without oily properties struggle to compress into a pellet, as not enough pressure and heat is generated.

Steam conditioning: As described previously, materials such as wood contain enough naturally occurring lignin so as not to require an additional binder. Under the heat and pressure of a pellet mill the lignin melts. The wood may then be reshaped into a pellet. When the lignin cools the result is a durable pellet. The pellet’s lustrous surface is also attributable to lignin. To aid the pellet process and increase productivity, large-scale pellet mills have steam conditioners where the material is exposed to dry steam before the material enters the pellet mill chamber. The dry steam does not add any moisture to the material, but is purely used to increase the temperature to denature lignin. After steam conditioning, the material enters the pellet mill chamber. Quality, lustrous pellets are more efficiently produced as the material passes through the die in a softened state with less resistance. Steam conditioning is only used in large-scale pellet mills due to the added expense and safety risks.

Increasing productivity: Steam conditioning is one way to increase pellet mill productivity. Another way is to use low-density oily materials. Materials such as rape cake or dried distillers grain can aid pellet production by reducing resistance through the pellet mill die, while the oil acts as a secondary binder. By adding up to 20 percent of the low density oily material to the original raw feedstock, productivity can be increased by up to 30 percent.

Pellet mill tolerances: Adding an oily material to the original feedstock can also increase the tolerances of the pellet mill. The oily properties aid to the pelleting process and can reduce the frequency of blocked dies and poor pellet quality.

Agricultural pellet conditioning

The alfalfa dehydration industry commonly aids the pelleting process through the application of a constant quality steam at a predetermined pressure that supplies additional heat and moisture. This steam can help release and activate natural binders and lubricants in biomass sources. In the case of alfalfa, high temperature steam additions enhance pellet durability and reduce energy consumption in the pelleting process. Generally, pellet durability increases linearly as conditioning temperatures are raised from 65 to 95°C. This pretreatment also influences the final temperature in which the pellet leaves the die. Pellet durability increases by 30 to 35 percent when the conditioning temperature is upped from 55 to 85°C. Pellet power consumption also typically declines by nearly 30 percent when increasing pellet temperature from 65 to 95°C. One of the alfalfa dehydration producers currently producing in western Canada is currently using 110 °C for steam conditioning.

A phenomenon known as “glass transition temperature” occurs with herbaceous biomass feedstock. This is the temperature at which the material softens due to the onset of long-range coordinated molecular motion. The glass transition temperature for dry switchgrass is from 75 to 100°C. Reaching these temperatures appears critical for densification processes. The lignin and hemi-cellulose components of plants undergo plastic deformation in the range of their glass transition zones. Higher temperatures generally improve pellet durability. Conditioning is an important part of this process.


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