Biomass Pellet Mill

Applications Of Biomass Pellet Mill

The Biomass Pellet Mill is mainly a kind of mechanical equipment that uses crop straw as raw material, which is crushed, pressurized, densified, shaped, and extruded into small rod-shaped solid pellet fuel. The volume after molding is 1/30 ~ 40 of the volume of the raw material, the specific gravity is 10 ~ 15 times (the density is 0.8 ~ 1.4), the calorific value can reach 3400 ~ 6000 kcal, and it is a high-volatile solid fuel.

Raw materials for RICHI biomass pellet machine:

Biomass raw materials for bio pellets can be categorized into forestry, agriculture and waste‐based materials, and they can be subdivided into primary sources (directly produced materials) or secondary sources (derived from other processes). There has been a considerable amount of publications within the last years, mainly dealing with optimization of pelletization processes.

The raw materials studied range from wood species, such as pine, spruce, beech, oak, poplar, aspen, salix, and fir, to agricultural residues, such as alfalfa, barely, canola, oat, wheat, rice, soybean, rye, reed canary grass, corn stover, corn cobs, switch grass, big blue stem, sugar cane bagasse, cotton, olive residues and peanut hulls and also mixed residues.

Technical Features Of Biomass Pellet Mill

Working Principle Of Biomass Pellet Mill

Pellets are produced in a biomass pellet mill that generally consists of a die with cylindrical press channels and rollers that force the biomass to flow into and through the channels. Due to the friction between the steel surface and the biomass in the press channel, a high back pressure is built up and heat is generated. The physical forces built up in the press channel of a biomass pellet machine are crucial for understanding and optimizing the pelletizing process. A ring die with press channels and roller(s) are the basic parts of a biomass pellet making machine.

Biomass Pellet Machine Specification

Energy content for various biomass (dry weight basis)

Pelleting plays a significant role in biomass production as the act of pelletizing breaks down plant based structures, facilitating optimal energy withdrawal while simultaneously reducing original structure size and densifying raw materials to reduce shipping and handling costs. The application of biomass pellets is increasingly widespread, but the application as fuel pellets is still dominant. Therefore, it is still necessary to discuss the raw materials of biomass pellets.

Complete Biomass Pellet Produciton Line Process Flow

Wood and agricultural biomass pellets boost the potential as bio-fuels toward power production in tertiary and residential sectors. The production of pellets, however, is a multi-stage process where the supply-processing phases and the overall energy input strongly depend on the characteristics of the input biomass.

Generally speaking, just using a biomass pelletizer cannot support efficient biomass pellet processing, and equipment such as good quality biomass sawdust hammer mill, high efficiency wood chips, high drying efficiency biomass rotary dryer, biomass pellet cooler machine, pellet screening machine, biomass pellet bagging machine, etc., sre also required. The complete biomass pellet manufacturing process includes:

Biomass materials supply

This section includes the collection, transport, and storage of biomass to the collection point; green wood is mechanically removed, and pruning residues are air-dried.

Biomass pre-treatments section

This section includes the mechanical processes such as converting wood into wood chips (except when starting from sawdust), the removal of ferrous material, drying, milling, and conditioning. The wood chips are typically dried in heaters fed with conventional fuel, although sometimes the heaters use wood chips as fuel. The most common driers use rotating drums with flow of air, in which the wet biomass does not stick to the drum surfaces and over-heating is minimized.

If the biomass can withstand contact with the combustion products, the simplest and cheapest system is a direct-heating wood drier, in which the wet biomass is in contact with hot combustion gases. Alternatively, the wet biomass can be dried using hot air. The dried biomass is further milled to obtain wood chips with average length of 3 mm and to homogenize the end product.

The most common biomass material hammer mills use rotating hammers, and the material is shifted through the machine using compressed air. Sometimes, the secondary milling is bypassed by a simple tilted-plane mechanical size selector. The pre-conditioning involves exposure of the biomass to an appropriate mix of environmental conditions (temperature, moisture, length of exposure time) to optimize its behavior in the subsequent pelleting.

A common pre-conditioning process exposes the biomass to rapid heating using hot water vapor, with the effect of softening the wood chips and obtaining a partial decomposition of starch and cellulose in simpler sugars, which allow easier compacting. The short time of exposure to hot vapor minimizes significant increases in the moisture content of the biomass.

Additives aimed at improving biomass quality and extrusion behavior can be added to the raw materials during this section.

Pelletizing section

This section involves the physical production of the biomass pellet by applying mechanical pressure on the biomass through a suitable holed plate, to obtain pellets with diameters in the 2–12 mm range and heights in the 12–18 mm range. The main technical parameters of the biomass pellet making machine are as follows: canal geometry, number and speed of pressurizing drums, ratio between diameter and length of canals, and distance between drum and holed plate. The devices may use a vertical cylindrical holed drum or a plane plate.

Post-Treatments section

These sections include cooling, screening (selection of pellet size), collection, and storage in silos or sacks for subsequent sale. The cooling section is critical for the stabilization of the product, since, during the biomass pelletizing section, the pellets reach comparatively high temperatures (90–95℃) and are typically obtained via forced exposure to air at room temperature. Pellets with a non-standard size are mechanically removed to minimize development of dust in the storage areas.

Mixing and conditioning needed before pelletizing by biomass pellet machine?

Not all raw materials require mixing. Nevertheless, if necessary, mixers are used after the raw material has been milled and dried to: get a more consistent material blend to be fed into the biomass pellet mill machine when the raw material for biomass pellets presents significant changes in moisture percentage, binding properties or material density; produce a homogeneous mixture of raw materials in the case of pellets made up of different raw materials.

Once the biomass making raw material has been mixed and its consistency is high enough, it may require the addition of other constituents able to increase the productivity of the pellet mill and enhance the final properties of the biomass pellets produced.

Additives play a major role in wood pellet characteristics. They are a subject of major interest: as binding agents for the biomass raw material, they serve to improve pellet durability and physical quality, reduce the dust, improve pelleting efficiency and reduce energy costs.

A maximum content of 2% of additives is permitted in woody pellets. No limitation exists for the non-woody pellets, though it is required to indicate the type and quantity used. To produce wood pellets with desired physical and thermal characteristics, the additive should be suited to the right biomass material. The most common additives are:

• Water, which is used if the moisture content of the mixed material is too low;
• Binders, which act as a glue between the particles if the lignin content of the material is not enough to hold a pellet together.

Lignin is a natural, optimal binder of biomass, because it melts under the heat of the pellet mill. If the lignin content of the biomass is low, it may be necessary to add other additives, however. One of the simplest binders is vegetable oil, which also aids the biomass pellet line production process by reducing the frequency of blocked dies, but the most widely used substance overall is starch.

Common additive for biomass pellet machine pelleting process

Each additive will produce unique physical and thermal characteristics when used with different biomass materials:

• Lignosulphonate additives result in the best mechanical durability values for wood pellets but do not display high particle density. As they do not affect the calorific value of the wood pellets, they significantly increase sodium and sulphur content, and consequently increase emissions.
• Starch additives are the most widely used because they reduce the final moisture content more than lignosulphonate additives. However, too much starch will make the final product extremely dry, which affects the mechanical durability of the wood pellets.
• Additives such as motor oil, corn starch, sodium carbonate, urea, vegetable oil and dolomite decrease the wood pellet particle density and contribute to the pellet process by reducing the frequency of blocked dies.

Corn starch and dolomite additives are the most effective in reducing the wood pellet particle density. All types of starch (native wheat starch, oxidized corn starch, native potato starch and oxidized potato starch) increase the mechanical durability of the wood pellets, the best results for mechanical durability being obtained by adding oxidized corn starch.

Motor and vegetable oil additives increase the calorific value minimally, while corn starch and dolomite additives reduce the calorific values of wood pellets. Wheat starch is an additive that significantly reduces ash formation, but dolomite additive increases ash formation as well as the ash melting point in wood pellet combustion. Both corn starch and dolomite additives significantly increase carbon monoxide emissions.

Many biomass wood pelletizing machines come with a built-in steam conditioning chamber. Super-heated steam, at temperatures above 100°C, is used to soften the wood before it is compacted. Steam conditioning is not necessary, but it does make the biomass raw material less abrasive to the biomass pelletizer equipment and this helps reduce the maintenance costs.

In general, the final choice of the type of biomass pellet granulator depends on the customer's raw materials and technology. What Richi Machinery has to do is to design the best biomass pellet making process design and the most durable equipment for the customers.