Environmental Influential Factors on Wood Pellets Degradation and Dust Generation - Coursework Example

Environmental Influential Factors On Wood Pellets Degradation And Dust Generation Name Subject & Code Instructor Date of submission Table of Contents Table of Contents 2 Chapter 2. Literature Review 3 2.1 Pellets Degradation and dust 3 2.2Major influence factors on pellets 5 2.2.1 Impact velocity 5 2.2.2 Particle properties 5 2.2.3 Environmental factors 6 2.3 Assessment methods 7 2.4Advantages & disadvantages 7 2.5 Summary 9 References 9 Chapter 2. Literature Review 2.1 Pellets Degradation and dust Graham (2015) pointed out that when it comes to wood pellet degradation, focus is placed on micro-organisms that degrade wood, specifically mould and rot fungi. In his view, the organisms that decay wood are essentially divided into brown-, white-, and soft-rot causing fungi. The main cause of brown and white root is fungi of the Basidiomycotina variety. On the other hand, the soft rot is caused by the Ascomycotina variety. According to Kymalainen (2015), most of the decay-causing fungi require certain environmental conditions. For instance, the environment should have carbon source, sufficient mineral availability, moisture, and oxygen are essential for growth and reproduction. He suggested that Nitrogen is a very important nutrient in protein synthesis and is needed for example for the construction of degrading enzymes. Kymalainen (2015) further observes that the stress tolerance for nitrogen starvation varies among fungal species. Generally wood decay fungi are mesophilic, i.e. active at 0–45 °C, with an average optimum of 20–30 °C (Kymalainen 2015). Thermotolerant and thermophilic species thrive at higher temperatures, at about 20–50 °C (Kymalainen 2015). The fungi that decay are tend to readily accessible substrates, such as soluble sugars, lipids, and peptides, for sustenance. The carbon sources can be extracted directly from the substrate, but to access polymeric substances, extracellular enzymes are needed; cellulases for solubilisation of cellulose, and ligninolytic enzymes for lignin. As Gupta (2016) comments, there are a great number of fungal species that are specialised in using certain exclusive substrates, such as charred wood generated in forest fires. During wood decay the fungal hyphae preferentially travel via passages of least resistance. As Kymalainen (2015) observes, they enter through the cell lumina of axial elements – fibres and vessels – and move from cell to cell through pits. As cell walls are eroded progressively, bore holes are formed, and movement between cells becomes easier. The growth of the hyphae of fungi is also facilitated inside the secondary walls (Wilson 2010). In his review, Kymalainen (2015) suggested that white rot decay fungi causing white rot can degrade all the major structural components of wood, in spite of the fact that lignin is often eliminated in place of cellulose (Wilson 2010). These fungi prefer hardwoods, which may be related to the more refractory nature of softwood lignin (Vukmirovic et al. 2010). During decay, the wood substance appears like it has been bleached. With time, it becomes fibrous and elastic (Gupta 2016). The use of carbohydrates to generate is a crucial prerequisite for the degradation of lignin, although they can as well be used to synthesise ligninolytic enzymes. In respect to brown rot, Kymalainen (2015) observes that this is a type of decay that leads to far-reaching degradation of the cellulose and hemicellulose fractions, in spite of the fact that lignin is does not become eliminated. It is, however, changed to some considerable degree although this is contingent on the wood species and fungus. While a brown rot is linked to softwood, it has no definite preference given that in most cases, it is for white rot for hardwood (Wilson 2010). Hydrolisation of the amorphous carbohydrates occurs. At the same time, hydrolisation of the accessible crystalline component of cellulose causes degraded in the later stages (Kymalainen 2015). The issue of dust generation is also crucial for explaining how wood pellets should be treated. Indeed, in respect to dust generation, Schott et al. (2016) observes that it plays a crucial role all though the process of handling wood pellets. In his view, dust generation relates to the products’ durability, of the wear rate of particles of the pellets when subjected to forces. This is particular so while transporting, storing, and handling the pellets when it has to go through different forces in differing pieces of equipment. 2.2Major influence factors on pellets 2.2.1 Impact velocity The significance of impact velocity to the quality of the pellet tends to be apparent during the pelleting process. As Kymalainen (2015) explained, die speed denotes the rollers’ tangential velocity during the pelleting process. Accordingly, high die speeds of up to 10 m/s is suitable for pellets of small sizes of a diameter of 3 to 6 millimetres. This is in contrast to lower die speeds, which tend to create larger pellets of a diameter of about 6 millimetres. Kymalainen (2015) further observed that low die speed of nearly 4 to 5 m/s are appropriate for materials that have bulk densities that are generally low. 2.2.2 Particle properties The properties of the wood pellets also play a determinant role in substance degradation. Indeed, an additional critical factor that affects the quality of the pellet is the particle size. According to Garcia-Maraver and Perez-Jimenez (2015), when the particle sizes are fine, this tends to lead to the greater the strength and durability of the pellet. The reason for this is that large particles provide greater surface area for more fissure points. Recent studies attempted to examine the overall impact of the particle size. The optimal quality of the pellet is attained when the particle sizes are mixed in nature, because it increases inter-particle bonding and eliminated inter-particle spaces (Vukmirovic et al. 2010). As Hussein (2009) discussed, given that high heating values for cellulose and lignin have a direct relation with their carbon content, degradation of wood pellets by microbial has a potential to alter the energy content of stored biomass. It can as well cause loss of dry matter (Svanberg 2013). The act of grinding wood pellets into dust or chips also fosters the colonisation of microbial, as it increased the overall surface area (Obernberger & Thek 2004). Once the dust or chipped material is stored in form of piles, the resulting ecosystem build depends on varied environmental factors. At this level, the size of the chip, the moisture content, the size of compaction, the level of the material’s freshness and the season, differing heating values losses and dry matter is imminent (Chico-Santamarta et al. 2010). 2.2.3 Environmental factors According to Sultana and Kumar (2012), among the most influential environmental factor that affect wood storage is moisture. The researcher further argued that moisture is a critical factor for successful growth of fungi in metabolic growth, actions, as well as cell functions. In a related study, Vukmirovic et al. (2010) also highlighted the significance of moisture in wood pellets degradation. In their view, the adsorption of moisture from the air is a crucial factor. The occurrence of water in cell cavities is suitable for microbes, although it has the capability to use moisture that is bound on the cell walls. In the event of severe drought, the moisture the microbes become dormant, although their revival takes place whenever the material is rewetted. The varied sizes of the moisture content required to facilitate decay determines the level of wood pellets degradation. In literature review, the most widely mentioned growth limiting values range between 18 percent and 21 percent, although the moisture content that are as low as 15 percent have been reported in previous studies. For a majority of the fungi, the area that facilitates most rapid growth is between 25 percent and 60 percent moisture content. 2.3 Assessment methods Pirraglia et al (2012) observes that several ASTM standards can be used in the assessment of various properties of torrefied wood or to determine the properties of wood pellets that torrefied wood produces. In the assessment of the pellets to determine their moisture content, Pirraglia et al (2012) pointed out that this should be on the Test Methods for Direct Moisture Content Measurement, which consists of measuring the weights of torrefied wood prior and after they dry up. The samples are then dried inside an over to a level that there is no significant change in weight. In assessing the volatile matter, the volatiles are determined using the ASTM D 3175-07, Standard Test Method for Volatile Matter, which is then applied in analysing Sample of Coal and Coke. In assessing the ash content, the ASTM D 3174- 04, Standard Test Method for Ash is used to analyse sample of coal and coke from coal. The High Heating Value of the torrefied wood is determined using the ASTM Standard D 2015, Standard Test Method for Gross Calorific Value of Coal (Pirraglia et al 2012). 2.4Advantages & disadvantages Wood pellets provide reliable biomass fuel for co-firing. As Nunes et al. (2014) explains, white wood pellets that have been treated non-thermally are mostly made by compressing dry sawdust under high pressure. Afterwards, they are squeezed out through a die. This process does not take plenty of time and it makes the pellets to be considered as the most reliable yet readily available short-term biomass fuel that is essentially used for biomass combustion and co-firing (Roy et al. 2007). The wood pellets also have readily available market. As Graham (2015) explains, the market for wood pellets is readily available, and provides an opportunity for suppliers to ship large cargo of pellets as fuel across the globe. Their low moisture content is also an advantage. Fang (2013) explains that the wood pellets offer more relevant benefits compared to agriculture-based and energy crops biomass fuel in respect to their nature of having low moisture content, low content of ash, high value of calories, and convenience during milling (Lee et al. 2014). In additionally, they have a highly homogenous distribution of the sizes of their particles. Its key raw materials are readily available and include the wood residues that emanate from wood chips, sawdust and shavings (Graham et al. 2012). They are Wood pellets are easier to store and handle than raw woody biomass. Wood pellets also have several disadvantages. According to Brackley and Parrent (2011), wood pellets present several disadvantages including brittleness, and a tendency to form dusts while being transported, handled and conveyed. Additionally, there storage should be in totally enclosed environment. This is because exposing them to moisture and highly humid condition may contribute to severe degradation and growth of fungus (Graham et al. n.d.). Additionally, as wood tends to retain fibrous nature in the process of pelletisation, co-firing wood pellets is presents complex situations, particularly in existing coal plants, which demand adjustments to be made to the types of conveying and milling machinery. 2.5 Summary In the process of wood pellet degradation, micro-organisms that degrade wood, specifically mould and rot fungi. The organisms are divided into brown-, white-, and soft-rot causing fungi. On the other hand, dust generation plays a crucial role all though the process of handling wood pellets. The major factor influencing quality of pellets includes impact velocity, particle properties, and environmental factors. Regarding the properties of the wood pellets, when the particle sizes are fine, it leads to the greater the strength and durability of the pellet. On the other hand, among the most influential environmental factor that affect wood storage is moisture. It is a critical factor for successful growth of fungi in metabolic growth, actions, as well as cell functions. The ASTM standards can be used in the assessment of various properties of torrefied wood or to determine the properties of wood pellets that torrefied wood produces. Overall, wood pellets provide reliable biomass fuel for co-firing. They are also more beneficial compared to agriculture-based and energy crops biomass fuel in respect to their nature of having low moisture content, low content of ash, high value of calories, and convenience during milling. Wood pellets also have several disadvantages, such brittleness, and a tendency to form dusts while being transported, handled and conveyed. Additionally, there effective storage is subject to environmental changes. References Brackley, A & Parrent, D 2011, Production of Wood Pellets From AlaskaGrown White Spruce and Hemlock, . Gen. Tech. Rep., Portland Chico-Santamarta, L., Humphries, A., Chaney, K., White, D., Godwin, R 2010, “The effect of storage on the quality properties of Oilseed Rape straw pellets,” World Sustainable Energy Days 2010 Fang, Z 2013, Pretreatment Techniques for Biofuels and Biorefineries, Springer Science & Business Media, New York Garcia-Maraver, A & Perez-Jimenez, J 2015, Biomass Pelletization: Standards and Production, WIT Press, Southampton Graham, S 2015, Degradation Of Biomass Fuels During Long Term Storage In Indoor And Outdoor Environments, viewed 21 Jan 2017, Graham, S, Eastwick, C, Snape, C & Quick, W 2012, "Degradation of biomass fuels during artificial storage in a laboratory environment," Int. J. Low-Carbon Tech., vol 11 no 4 Graham, S, Eastwick, C, Snape, C & Quick, W n.d., Mechanical Degradation Of Woody Biomass Pellets During Storage In Stockpiles, viewed 20 Jan 2017, Gupta, V 2016, Microbial Enzymes in Bioconversions of Biomass, Springer, New York Hussein, O 2009, Using of crop biomass for energy purpose (Processing, pelleting and pellets properties), viewed 21 Jan 2017, Kymalainen, M 2015, Moisture sorption properties and fungal degradation of torrefied wood in storage, viewed 20 Jan 2017, Lee, J, Lau, A, Lim, C, Bi, T, Basset, V & Yazdanpanah, F 2014, The effects of long term storage on the net calorific value of wood pellets, viewed 21 Jan 2017, Nunes, L, Matias, J & Catalao, J 2014, A review on torrefied biomass pellets as a sustainable alternative to coal in power generation, viewed 21 Jan 2016, Obernberger , I & Thek G 2004, “Physical characterisation and chemical composition of densified biomass fuels with regard to their combustion behaviour,” Biomass and Bioenergy, vol 27, pp.653-669 Pirraglia, A, Gonzalez, R, Saloni, D, Wright, J & Denig, J 2012, “Fuel properties and suitability of eucalyptus Benthamii and Eucalyptus Macarthurii for torrefied wood and pellets," BioResources, vol 7 no 1, pp.217-235 Roy, M, Dutta, A, Corscadden, K & Havard, P 2007, "Co-combustion of Biosolids with Wood Pellets in a Wood Pellet Stove," International Journal of Engineering & Technology, vol 11 no 3, pp.7-15 Schott, D, Tan, R, Dafnomilis, I, Hancok, V & Lodewijks, G2016, "Assessing a Durability Test for Wood Pellets by Discrete Element Simulation," FME Transactions vol 44, pp.279-284 Sultana, A & Kumar, A 2012, "Ranking of biomass pellets by integration of economic, environmental and technical factors," Biomass and Bioenergy, 1-12 Svanberg, M 2013, A framework for supply chain configuration of a biomass-to-energy pretreatment process, 21 Jan 2017, Vukmirović, D, Ivanov, D, Colovic, R, Kokic, B, Levic, O, Duargic, O 2010, "Effect Of Steam Conditioning On Physical Properties Of Pellets And Energy Consumption In Pelleting Process," Journal on Processing and Energy in Agriculture, vol 14 no 2, pp.102-108 Wilson, T 2010, Factors Affecting Wood Pellet Durability, viewed 21 jan 2017, ,http://www.pelletheat.org/assets/docs/industry-data/tow_thesis_final_rev2.pdf> Read More

In his review, Kymalainen (2015) suggested that white rot decay fungi causing white rot can degrade all the major structural components of wood, in spite of the fact that lignin is often eliminated in place of cellulose (Wilson 2010). These fungi prefer hardwoods, which may be related to the more refractory nature of softwood lignin (Vukmirovic et al. 2010). During decay, the wood substance appears like it has been bleached. With time, it becomes fibrous and elastic (Gupta 2016). The use of carbohydrates to generate is a crucial prerequisite for the degradation of lignin, although they can as well be used to synthesise ligninolytic enzymes.

In respect to brown rot, Kymalainen (2015) observes that this is a type of decay that leads to far-reaching degradation of the cellulose and hemicellulose fractions, in spite of the fact that lignin is does not become eliminated. It is, however, changed to some considerable degree although this is contingent on the wood species and fungus. While a brown rot is linked to softwood, it has no definite preference given that in most cases, it is for white rot for hardwood (Wilson 2010). Hydrolisation of the amorphous carbohydrates occurs.

At the same time, hydrolisation of the accessible crystalline component of cellulose causes degraded in the later stages (Kymalainen 2015). The issue of dust generation is also crucial for explaining how wood pellets should be treated. Indeed, in respect to dust generation, Schott et al. (2016) observes that it plays a crucial role all though the process of handling wood pellets. In his view, dust generation relates to the products’ durability, of the wear rate of particles of the pellets when subjected to forces.

This is particular so while transporting, storing, and handling the pellets when it has to go through different forces in differing pieces of equipment. 2.2Major influence factors on pellets 2.2.1 Impact velocity The significance of impact velocity to the quality of the pellet tends to be apparent during the pelleting process. As Kymalainen (2015) explained, die speed denotes the rollers’ tangential velocity during the pelleting process. Accordingly, high die speeds of up to 10 m/s is suitable for pellets of small sizes of a diameter of 3 to 6 millimetres.

This is in contrast to lower die speeds, which tend to create larger pellets of a diameter of about 6 millimetres. Kymalainen (2015) further observed that low die speed of nearly 4 to 5 m/s are appropriate for materials that have bulk densities that are generally low. 2.2.2 Particle properties The properties of the wood pellets also play a determinant role in substance degradation. Indeed, an additional critical factor that affects the quality of the pellet is the particle size. According to Garcia-Maraver and Perez-Jimenez (2015), when the particle sizes are fine, this tends to lead to the greater the strength and durability of the pellet.

The reason for this is that large particles provide greater surface area for more fissure points. Recent studies attempted to examine the overall impact of the particle size. The optimal quality of the pellet is attained when the particle sizes are mixed in nature, because it increases inter-particle bonding and eliminated inter-particle spaces (Vukmirovic et al. 2010). As Hussein (2009) discussed, given that high heating values for cellulose and lignin have a direct relation with their carbon content, degradation of wood pellets by microbial has a potential to alter the energy content of stored biomass.

It can as well cause loss of dry matter (Svanberg 2013). The act of grinding wood pellets into dust or chips also fosters the colonisation of microbial, as it increased the overall surface area (Obernberger & Thek 2004). Once the dust or chipped material is stored in form of piles, the resulting ecosystem build depends on varied environmental factors. At this level, the size of the chip, the moisture content, the size of compaction, the level of the material’s freshness and the season, differing heating values losses and dry matter is imminent (Chico-Santamarta et al. 2010). 2.2.

Read More