Topic :Woody Biomass, Direct Burn, Torrefaction, Pelletization
Woody Biomass & Pelletization
The utilization of heat is an essential component contributing to the success of mankind. There is a variety of ways in which heat can be utilized; one of the ways to heat is used is to heat residential homes. A technology that is being used as a heating source is the burning of wood pellets (7). Wood pellets are a fuel source made from compressed biomass such as sawdust that usually comes from sawmilling waste, thus they are considered a renewable energy source (7).
Wood pellet fuel is a considerably convenient alternative fuel; it is cost-effective, renewable, clean-burning. Wood pellet technology is popular in Europe, and is beginning to become a trend in North America (7). Wood pellets can be used for heat, furnaces, boilers, fire place inserts, and stoves (7). Wood pellets can also be used at much larger scales as fuel (7). Large scale applications for wood pellets include prisons and schools (7). Using renewable biomass fuels, such as woody biomass, can yield many advantages to society (7). The use of renewable fuels can mitigate various environmental issues such as global climate change and acid rain (7). Another advantage of using biomass fuels is their stability in price (7). Biomass fuels, like wood pellets, are convenient and easy to use, can be stored in bulk, have a decent energy content, are clean burning, are produced from using waste materials such as forestry residues and sawdust (7).
Wood pellets are made from wood waste materials that are condensed into pellets under heat and pressure (7). Natural plant lignin holds the pellets together without glues or additives (7). Wood pellets are of uniform size and shape (between 1-1½ inches by approximately 1/4-5/16 inches in diameter), making them as easy to store and use as traditional fossil heating fuels (7).
Wood pellets also take up much less space in storage than other biomass fuels because they have a higher energy content by weight (roughly 7,750 Btu per pound at six percent moisture content) due to their densified nature and low-moisture content (typically between 4-6 percent moisture by weight) (7).
While wood pellets are typically not differentiated between soft and hardwood sources, there are three distinct grades based on the amount of ash produced when they are burned (7). The grades are distinguished as premium, standard, and industrial. For the residential market, wood pellets are sold in 40-pound bags at farm or building supply stores (7). Small commercial- or institutional-scale applications of the type being discussed here, however, require bulk delivery and storage (7).
It is well known that the emissions from wood-burning boilers are different than emissions from such traditional heating fuels as heating oil, propane, or natural gas (7). All heating fuels—including wood—produce particulate matter (PM), carbon monoxide (CO), nitrogen oxides (NOx), and sulfur dioxide (SO2) in varying amounts (7). Burning wood in a modern and well-maintained woodchip boiler, for example, produces more particulate matter than burning any of the fossil fuels, but less SO2 than oil or propane. Carbon dioxide (CO2) is an air pollutant currently not regulated in many parts of the globe despite it being a major contributor to global climate change (7). Replacing fossil fuels with biomass for space heating results in a significant net reduction in CO2 emissions; it is a meaningful way to begin reducing environmental impacts (7).
Wood fuels are often referred to as “carbon neutral.” This refers to the natural carbon cycle where CO2 emitted when wood is burned continues to be a part of the overall flux of carbon, while burning fossil fuels releases new carbon to the atmosphere that had been locked away underground (7). Trees capture and store (sequester) carbon. Although the carbon is released when the wood is burned, if harvested and burned at the rate it grows in the forest, no net carbon is released (7). Thus, burning fossil fuels for space heating increases the net amount of carbon in the atmosphere, while burning wood does not (7).
The process for making wood pellets is called pelletization. Wood pelletization is a multi-step process that compresses woody biomass to yield a higher energy density so that it may be used as fuel. Wood pelletization begins by having woody mass dry by either leaving it in the open sun environment or heating in an oven (5). The intent of the drying is to lower the moisture content of the woody mass (5). The dried woody mass is then fed into a mixer which ensures an even distribution of the mass within the batch which allows the final product to have a uniform density (5). The woody mass is fed into a roller die assembly and press (5). The pellet making process consists of a die and a roller (5). A die is a piece of metal with holes drilled through it (5). A roller rolls across the die and presses the material through the holes which are the size of the finished pellets (5). The opening of the holes is tapered like a funnel so as material gets compressed as it gets squeezed into a smaller and smaller hole (5). The compression creates a dense pellet and also creates enough heat to melt the binders in the material that will act as a glue to hold the pellet together (5). There are two basic pellet mill types. Flat die and round die (5). With a flat die pellet mill the material falls onto the top of a flat die surface and a roller rolls across the material pressing material between the roller and die (5). A round die pellet mill has a round ring shaped die (5). The rollers are inside the ring and press the material against the inside of the die (5). The pellets are then extruded out of the holes on the outside of the ring (5). The pressure induced by press causes the temperature in the woody mass to increase (5). Lignin plasticizes due to the high temperatures; this plasticization forms a natural glue-like property (5).
There is a variety of factors that impact the formation of the pellets (4). Among those factors are the characteristics of the wood used as raw biomass (4). Wood is made of cellulose, hemicellulose and lignins, these are all essentially complex hydrocarbons which form chains and fibers with lignin acting as an adhesive binding them all together (4). Not all pellets form properly (4). Some will fall apart into small pieces (4). These pieces are sifted through a screen (6). Depending on the quality and consistency of the pellets you are producing the pellets may be sifted for size (6). When the finished pellets leave the mill they will be hot and releasing moisture vapor so they will need to cool and dry (6). The most common way is to spread them out and let them cool and dry naturally (6). Sometimes air is circulated through them with blowers or fans (6). Care must be taken to not dry them too quickly or they could develop stress cracks (6). At the end of the process, pellets have a 4% to 8% water, compared to 20% to 60% for raw woody biomass (1).
Wood is a complex fuel that undergoes dramatic changes as it burns (2). One by-product of burning wood pellets is ash, a non-combustible residue (2). When heated to 500-600°F, wood undergoes pyrolysis which liberates organic gases and leaves behind carbon rich charcoal (2). One by-product of burning wood pellets is ash, a non-combustible residue (2). Pyrolysis is an exothermic reaction that is self-sustaining (2). Up to 85% of the mass and 60% of the heating value from wood is contained in gases produced by pyrolysis(2).
CH4+2O2 àCO+2H2O+∆Heat : Sample Combustion Reaction
Complete combustion of wood produces only carbon dioxide and water; natural wood has no significant sulfur or metals content. Incomplete combustion results in the production carbon monoxide and many hydrocarbons (2). Complete combustion requires adequate oxygen and the “3 T’s”: Time, Temperature, and Turbulence. Sufficient time is required at the right conditions for combustion reactions to go to completion (8). Temperature must be high enough for fuel mixture to ignite (2). Turbulence is required to mix combustible gases with oxygen (2). The best conditions for good wood combustion consist of temperatures in 1100-1500°F range (8).
Wood pellet boiler technology is becoming well established. There is continuous research being done on improving pelletization processes, thus wood pelleting is becoming an established commercial chemical/mechanical process. Wood pellets have a great potential as a renewable fuel; wood pellets have the potential to replace other fuels and have a significant positive impact on the environment.
Torrefaction and Direct Burn
Torrefaction is a process in which biomass is treated through a thermochemical process. The thermal process, torrefaction modifies various properties of the woody biomass (10). Generally, torrefaction takes place at temperatures between 250°C and 350°C (10). The process lasts about 30 minutes so it does not burn the woody biomass (10). Torrefaction can be used for all organic by-products that, preferably, contain as little water as possible (10). By-products with high water content can be treated, but a lot of the heat is lost in the subsequent drying process (10). The properties for thermal conversion, which can be very different for each of the unprocessed fuels, become more and more similar during the process resulting in the homogenization of the different by-products (10). Once biomass, has been processed through torrefaction, it becomes hydrophobic, meaning that it can be stored for a long time without disintegrating because it repels water (10). Depending on the torrefaction circumstances, the material becomes brittle and is easy to grind into powder with little energy. The latter is significant for co-firing in powder coal heaters (10). Due to the evaporation of the organic components, the material loses some 30% of its weight while the incineration value only decreases by 10% (10).The torrefacted product can be pressed into pellets to create a fuel with homogenous feeding and conversion properties (10). Pressing it into pellets makes it easy and safe to transport (10). Today’s pellet furnaces have automatic feeding systems that are still based on sawdust and wood chips. The market for this product will grow providing torrefacted material can be made into pellets with properties that are similar to the current woody biomass product (10).
Some important things to remember about torrefaction process and products:
• A thermochemical treatment process, similar to pyrolysis (10)
• Torrefaction occurs at around 250°C and 350°C, at or near atmospheric pressure (10)
• Torrefied biomass is more homogeneous physically and chemically (10)
• Makes pelletization easier, Torrefied pellets have more strength (10)
• A competitive fuel when co-firing with coal in power plants (10)
Direct burn, or rather, direct combustion is probably the simplest conversion technology to understand because it is very common and has been used for many years. For example: burning wood at the fireplace. Through furnace or boiler, direct combustion is the process that converts biomass material to heat energy (9). One can use this form of biomass utilization for many purposes, such as space heating, heating through transfer liquids, and steam production for the generation of electricity or motive power (9). Direct Combustion of biomass can be for cogeneration, which produces both heat and electricity (9). Direct combustion systems burn biomass in boilers to produce high pressure steam (9). The steam turns a turbine connected to a generator-the same kind of steam-electric generator used in fossil fuel power plants (9). As the turbine rotates, the generator turns, and electricity is produced (9). This is the simplest and oldest way to generate electricity from biomass (9).
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- Energy Biosciences Institute. (2013).Forest Bioenergy: Is it sustainable? Bioenergy Connection V. 2.3 Retrieved May 5, 2014 from http://www.energybiosciencesinstitute.org/sites/default/files/publications/V23_EBI_mag2.pdLeaver, R. (2000).
- Wood pellet fuel and the residential market. Retrieved May 7, 2012 from http://www.nrbp.org/papers/032.pdf.
- Making Wood Pellets. (2013). Retrieved from World Forest Industries: http://worldforestindustries.com/forest-biofuel/wood-pellets/making-wood-pellets/
- Massachusetts Division of Energy Resources . (2007). Wood Pellet Heating. Biomass Center, 26.
- Öhman, M. (2014, May 16). Introduction to Combustion.
- “Biomass Conversion Technologies.” . N.p., n.d. Web. 1 June 2014. <http://www.epa.gov/chp/documents/biomass_chp_catalog_part5.pdf>.
- “Torrefaction: Pre- or Post-Pelletization.” Biomass Magazine. N.p., n.d. Web. 1 June 2014. <http://biomassmagazine.com/articles/9522/torrefaction-pre-or-post-pelletization/>.