Pak Sui Lam

Bulk Density of Wet and Dry Wheat Straw and Switchgrass Particles

Bulk density is a major physical property in designing the logistic system for biomass handling. The size, shape, moisture content, individual particle density, and surface characteristics are few factors affecting the bulk density. This research investigates the effects of true particle lengths ranging from 6 to 50 mm and moisture contents ranging from 8% to 60% wet basis (wb) on the bulk density of wheat straw and switchgrass. Three types of particle densities of straw and switchgrass measured were: a hollow particle density assuming a hollow cylindrical geometry, a solid particle density assuming a solid cylindrical geometry, and a particle density measured using a gas pycnometer at a gas pressure of 40 kPa. The bulk density of both loose-fill and packed-fill biomass samples was examined. The calculated wet and dry bulk density ranged from 24 to 111 kg m-3 for straw and from 49 to 266 kg m-3 for switchgrass. The corresponding tapped bulk density ranged from 34 to 130 kg m-3 for straw and 68 to 323 kg m-3 for switchgrass. The increase in bulk density due to tapping the container was from 10% for short 6-mm particles to more than 50% for long 50-mm particles. An equation relating the bulk density of stems as a function of moisture content, dry bulk density, and particle size was developed. After the validation of this bulk density equation, the relationship would be highly useful in designing the logistics system for large-scale transport of biomass to a biorefinery. The bulk density and particle density data of uniform particles would be important, if straw and switchgrass is used for pulping and paper making.

Physical Properties for Flow Characterization of Ground Biomass from Douglas Fir Wood

The particle size distribution and packing (loose bulk and tapped density) of a mixture of ground biomass from Douglas fir wood particles was characterized by different practical methods: sieving, digital imaging and scanning electron microscopy. The ground mixture was analyzed using a set of 14 wire mesh sieves. The calculated mean diameter of mixture was 251 µm. The mixture was divided into four size fractions of mean size ranging from 74 to 781 µm. Particle length measured by imaging technique were 3–4 times larger than the mean diameter determined by sieve analysis. Similarly, particle width was 1.0–2.5 times larger than mean particle diameter. The sphericity of particles in each of the four fractions increased with decreasing size of the sieve indicating that smaller particles also have a smaller aspect ratio. Empirical power law equations were developed to correlate the packing and flow ability of ground particles (HR) to the mean diameter, with R2 values of 0.88 and 0.91, respectively. The HR values indicated good flow ability for the large particles and poor flow ability for the smallest particles and the entire mixture. HR and porosity ratio reached an asymptote for particles larger than 400 µm.

Drying characteristics and equilibrium moisture content of steam-treated Douglas fir (Pseudotsuga menziesii L.)

Douglas fir (Pseudotsuga menziesii L.) particles were exposed to high pressure saturated steam (200 and 220 °C for 5 and 10 min) to improve the durability and hydrophobicity of pellets produced from them. Depending on treatment severity, the moisture content of the particles increased from 10% to 36% (wet basis). Douglas fir particles steam-treated at 220 °C for 10 min had the fastest drying rate of 0.014 min(-1). The equilibrium moisture content (EMC) of steam-treated samples decreased with increasing steam temperature and treatment time. The Giggnheim-Anderson-deBoer (GAB) equilibrium model gave a good fit with the equilibrium data with R(2) = 0.99. The adsorption rate of untreated pellets exposed to humid air (30 °C, 90% RH) for 72 h was 0.0152 min(-1) while that of steam-treated pellets ranged from 0.0125 to 0.0135 min(-1) without a clear trend with steam treatment severity. These findings are critical to develop durable and less hygroscopic pellets.

Physical characterization of wet and dry wheat straw and switchgrass – bulk and specific density

Bulk density of biomass is a major factor in determining the cost and logistics requirements of handling and moving biomass from farm to biorefinery. Bulk density is a strong function of size and shape and individual particle density. In this research we study experimentally the effect of particle length, moisture content, and particle density on bulk density of wheat straw and switchgrass. Wheat straw and switchgrass stems were cut to exact nominal lengths of 6, 12, 25, and 50 mm. The moisture contents of biomass samples were adjusted upward from an original 8% to 20, 40, and 60%. Three particle densities were measured assuming two structural geometries for the stems; i.e a hollow cylinder and a solid cylinder. The particle densities were measured using a gas pycnometer at a gas

Colorimetry applied to steam-treated biomass and pellets made from western douglas fir (Pseudotsuga menziesii L.).

Wood pellets made from sawdust and shavings are white in color and low in ash content. Pellets made from a mix of bark and white wood are darker in color, and bark increases the ash content of pellets. Steam treatment of biomass prior to pelletization improves the durability of pellets. Both wet (steam) and dry thermal (torrefaction) treatments of biomass darken the pellets. The off-color pellets have a lower commercial value than white wood pellets in residential applications. In this research, ground white wood (western Douglas fir, Pseudotsuga menziesii L.) was treated with saturated steam at 200°C and 220°C for 5 and 10 min. The colors of the treated and untreated powders and pellets were evaluated. The Hunter color coordinates L*, a*, and b* were recorded using a Minolta CM-5 spectrophotometer. Compared to untreated samples, the steam-treated samples became darker; the hues shifted from red to green and from blue to yellow. Multi-linear regression models of three color coordinates with elemental composition of carbon and hydrogen were developed. The values of L*, a*, and b* showed good correlation with percentage of carbon, hydrogen, and oxygen of the samples, with R2 values of 0.97, 0.99, and 0.97, respectively. The developed equations can be useful tools for the bioenergy industry for a quick estimation of fuel properties by a simple color measurement.

Effect of Steam Treatment on Pellet Strength and the Energy Input in Pelleting of Softwood Particles

Three whitewood species (spruce, Douglas fir, and pine) and one sample of bark (Douglas fir) were treated with high-pressure steam at 220°C for 5 min. The steam treatment resulted in a reduction in average particle size by as much as 25%. Pine particles showed the largest reduction in size, while bark showed the least. Despite a slightly lower density, pellets made from treated particles had a higher mechanical strength (hardness) than untreated pellets. The mechanical energy required to compact steam-treated material was higher than the energy required to make pellets from untreated wood. Douglas fir required the least energy input among debarked samples. Spruce was the stickiest pellet to be pushed out of the cylindrical die. Bark pellets required the lowest energy to be compacted and pushed out of the cylindrical die. The overall conclusion is that steam treatment reduces particle size, reduces pellet density slightly, but increases the mechanical strength of the produced pellets. Steam treatment increases the energy input required to make pellets, and more energy is required to push pellets out of the die compared to pellets made from untreated biomass.

Effect of Particle Size and Shape on Physical Properties of Biomass Grinds

Dry switchgrass, wheat straw and corn stover were ground using a laboratory cutting mill with 2 mm square hole screen. The ground biomass was fractionated into 12 size fractions (2 mm down to 0.09 mm) using stacks of wire mesh sieves. The contents left on each sieve were subjected to bulk and specific density, and angle of repose measurements.The bulk density of wheat straw and corn stover decreased with increasing particle size while the bulk density of switchgrass increased with increasing particle size. The Hausner ratio (the ratio of tapped bulk density over loose filled density) decreased linearly with increasing particle size.The measured angle of repose characterized switchgrass as a free flowing particulate whereas it characterized wheat straw and corn stover as cohesive particles. Individual particle size analysis by microscopy on wheat straw and switchgrass showed the ground particles were thin rectangular slabs. Switchgrass particles had a higher aspect ratio (length over width) of 3.55 whereas the wheat straw particles with an aspect ratio of 2.99.

Engineering Properties of Biomass

Engineering properties of biomass are important for the design and operation of processing facilities for handling, storage, transportation, and conversion to fuels, heat, and power. These properties include bulk density, particle density, particle size, color, moisture content, ash content, heating value, and flowability. In this chapter, the characterization methods of these properties are reviewed. In particular, the recent development of the characterization techniques and progress in understanding these engineering properties of the biomass are discussed. The heterogeneous nature of biomass requires standardized characterization procedures and statistical models development to predict their physical properties for engineering design and operation.

Pretreatment and Pelletization of Woody Biomass

Pretreatment is a first crucial step to modify the structure of wood via physical, chemical, and biological treatment for cost effective and sustainable fuels and chemicals production. Different pretreatments would be selected to upgrade the characteristics of wood with respect to different applications and process efficiencies. High-temperature pretreatment (e.g., torrefaction) at the temperature range greater than 250 °C led to higher degradation rate of sugars and extractives, which is not preferable for fuel and chemicals production from lignocellulosic biomass. Instead, high-temperature pretreatment was used to upgrade the solid fuel for thermochemical conversion (e.g., combustion and gasification). It can remove the moisture and volatiles with a low-heating value of the native biomass, which favors for the ease of fuel combustion compared to the raw wood. In addition, it can increase the hydrophobicity of the biomass which improves their handling and storage performance. In this chapter, the production chain of the wood pellet production with incorporating recent novel pretreatment technologies (torrefaction, steam explosion, and hydrothermal carbonization) were discussed. The resulted pellets are a uniform feedstock for producing chemicals, heat, and energy via biochemical and thermochemical conversion, respectively.

Evaluation of Hybrid Poplar and Salix (Salix) Biomass for Pellet Production

The physical characteristics of 3 year old chipped Hybrid Poplar (HP) and Salix (Salix) stems and pellets made from these feedstocks were determined. Four species of HP and six species of Salix were tested. The moisture content of as–received biomass for HP ranged from 12.1 to 52% w.b. and for Salix ranged from 13.6 to 43.4 % w.b. The bulk density of chipped material ranged from a high of 178 kg/m3 for Salix to a low of 22.21 kg/m3 of HP (Hybrid poplar mix with twigs). The low bulk density corresponded to the low moisture content. Pellets were made on a 6.2 mm die California Pellet Mill. Pellets made from both Salix and HP were dense with a specific gravity of 1.1. to 1.3 and bulk density from 557 to 701 kg/m3. Salix pellets showed a wider range in density. Pellets made from HP and Salix made were durable on DURAL Scale ranging from 74.9 % to 91.1 %. The lowest value belonged to Salix Mix. Pure wood chip as feedstock produced better pellets than pellets made from mix of plant parts (branches and stem). Pellets were ranked using the following characteristics: heating value, durability, rate of production, bulk density, particle density, and color. Lighter color had a better grade than the darker color. For each property a mark between 1 to 10 was assigned to pellets. Pellets made from one species of Hybrid Poplar came on the top followed by two Salix species, two HP species, four Salix and one HP. It is concluded that both varieties make an overall acceptable pellets but the final pellet quality depends on the quality of initial raw material.