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How To Make Wood Waste Fuel Into Pellets?

Date: 08/26/2020 08:30:35 From: wood-pellet-plant.com Clicks:

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How to make wood waste fuel into pellets? The waste preparation process generally involves crushing, dewatering, screening, size reduction, bulk storage, blending and drying prior to combustion so as to ensure a reliable and consistent supply of quality fuel to the burners. An equal amount of care and attention needs to be paid to the state of the wood waste used, as would normally be the case with any other fuel. The use of waste that is decayed, too wet or containing an excessive amount of contraries is false economy, due to the difficulty in handling and storing the wet residues, undue wear-and-tear on equipment and the detrimental effect on the overall combustion efficiency.

How To Make Wood Waste Fuel Into Pellets
How To Make Wood Waste Fuel Into Pellets

I. Collection And Handling

Waste collection and handling does not necessarily need to be either labour intensive or. involve costly and sophisticated mechanical handling plant, which could otherwise render the use of residues uneconomical. In small-scale forest industries in developing countries, the collection and handling of waste is predominantly manual, aided by a tractor or bulldozer to both convey and push the residues to a belt conveyor: system, thus avoiding the need for an enormous capital outlay and with maximized use of available labour.

The handling systems should be so designed as to afford the highest degree of flexibility to the operator and to be able to cater for the full range of sizes and moisture contents of was. e expected from outside and within the mill. It is the failure to attend to such aspects of design that invariably give rise to fundamental problems in operation.

Waste brought to the mill-site in the form of forest residues, or as purchased industrial wood waste, to supplement the plant's own wood-based fuel, may be delivered by road or rail. Methods of unloading range from the use of manual labour or a knuckle boom loader fitted with a clam-shell bucket, to live bottom vans or hydraulically elevated dump trucks, all of which are determined by economic considerations.

Collection And Handling
Collection And Handling

Generally slabs, edgings, peeler cores, veneer waste and trimmings would be transported by mechanical conveyors or carried manually to a chipper and, after screening, conveyed to storage piles for use as either furnish for pulp or particleboard manufacture or as fuel. Bark, panel trim and waste from ply glue spreaders would be hogged and conveyed to the hog-fuel storage area. Sawdust and sanderdust, depending on the quantities produced, would be pneumatically extracted and conveyed to a separate storage area (preferably covered). Retrieval is normally achieved by the use of belt, drag link, flight or pneumatic conveyors, in conjunction with front-end loaders, which may also be used to build-up the piles.

In order to safeguard against damage to moving parts, stone traps and magnetic separators need to be incorporated in the handling system, ahead of the reduction plant so as to remove all stones and tramp iron. Depending on the proportion of contraries normally expected in the fuel supply and the type of burning equipment employed, air classification may need to be employed in order to remove rocks and sand from the smaller-sized fuel particles, but only if they are comparatively dry.

II. Storage

The type of wood waste storage will be largely determined by:

- the form and moisture content of the residues;

- the frequency and reliability of year-round deliveries to the mill and production of residues;

- the availability of land;

- climatic conditions;

- the need for air drying;

- the volume of wood waste fuel involved;

- the system of waste handling and treatment adopted.

In instances where a large variety of residue sizes are involved, it is always advisable to segregate according to size, either before or after storage, and, in most cases, it is preferable to reduce the larger-sized waste in hoggers or chippers at an early stage in order to facilitate handling. Mixing of wet and dry waste should be avoided, as such a practice will reduce the efficiency of combustion; it is far better to have dual storage and feed systems in order to segregate the feed to the burners according to moisture content.

III. Size Reduction And Screening

Whereas sawdust, planer shavings and sanderdust may be burnt directly without the need for further processing, other forms of wood waste have to be reduced in size in order to facilitate handling, storage and metering to the combustion chamber. By achieving a uniform particle size, combustion efficiency will be improved due to the uniform and controlled fuel feed rate and the ability to regulate the air supply. Additionally, in the case of fuels with a high moisture content, the reduction process exposes a greater surface area of the particle to the heated gases, thus releasing the moisture more rapidly, thereby enhancing its heating value.

Size reduction may be carried out in several stages in a hog or attrition mill, with screening before and in between.

IV. Fuel Drying

As previously mentioned, combustion efficiency, boiler control and the operator's ability to provide a quick response to changes in steam demand become seriously impaired by a combination of high and fluctuating moisture content of incoming fuel. This situation may be improved by drying the fuel, which will also effectively increase boiler capacity and lead to better emission control.

dryer

The moisture in residues may be reduced either by mechanical pressing, air-drying or the use of hot air dryers, or a combination of all three. It is common practice for mechanical presses to be used on bark and wood waste with moisture levels in excess of 70 percent in order to reduce it to 55 to 60 percent m.c., which would then enable the waste to be mixed with dryer incoming materials to produce a combustible fuel. However, in the event that sufficient supplies of wood waste are readily available to meet the plant's energy needs and the disposal of bark does not present a serious problem to the mill, then it is not considered economically justified for it to be pressed and dried in view of the maintenance, power demand and the high capital intensive plant involved.

Air-drying of logging residues, assuming the right climatic conditions prevail, can bring about a moisture loss of some 10 to 15 percent, and the level may even drop further to 25 percent (67) should the residues be left in clear-felled spaces open to the action of wind and sun. Air-drying of mill waste, time and space permitting; is preferable under covered well-ventilated areas, especially for the smaller-sized residues such as sawdust, which is more liable to absorb rainfall and takes longer to air-dry than say mixed wood waste.

Green whole chips and mixed waste, when stored outside in piles for several months, may lose up to 10 to 25 percent (105) of their moisture content by way of the drying effect of wind, sun and spontaneous internal heating due to bacteriological action on the materials in the interior of the pile (108).

The use of fuel dryers to dry fuel to approximately 30 percent m.c. using plant such as rotary drum, flash and cascade type dryers employing waste stack gases, direct combustion of residues, steam or hot water as heating sources, undoubtedly lead to better combustion efficiency and boiler utilization. Nonetheless, the use of fuel dryers in medium-sized installations is questionable as the heat energy gained would be off-set by that which would be needed to dry the fuel, added to which one must take into consideration the high capital and operating costs involved.

V. Pelletization Section

Hubener says pellet quality needs can vary. "We have a long list of questions that we ask all of our customers," he says. "Some customers plan to burn all of the fuel they make, in which case a low-quality pellet is perfect for them. [But] if they are planning on shipping it on a barge or shipping it overseas, they are going to have to produce a very hard pellet."

To make pellets, the biomass must first be cleaned to remove contaminants. The clean biomass is then ground in a hammer mill or chipped to a uniform size, which must be less than the thickness of the pellet that will be produced. Grinding down biomass helps to reduce the horsepower the pellet mill must produce. If the biomass is high in moisture, it must be dried to approximately 10 percent moisture content.

While the lignin content in wood is generally enough to bind pellets, other forms of biomass require special conditioning to strengthen them. Sometimes binders such as starch, sugars, paraffin oils, or lignin must be added to make the biomass malleable.

Before pelletizing, the mixture must be conditioned using water of varying temperatures or steam. "Corn stover has a lower glass-transitioning temperature than switchgrass," Doering says. "You can get a very durable corn stover pellet at 165 degrees Fahrenheit, whereas to get an equivalent pellet with switchgrass, you have to obtain temperatures greater than 200 degrees, oftentimes 210 or 220 degrees."

Once conditioned, the biomass is fed to a pellet mill. Inside the mill, rollers extrude the mix through a perforated flat or ring die, which effectively condenses the product into pellet form. The hot pellets must then be cooled to harden. They are then screened to separate residual fines, which can be reused.

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