What is biomass densification and upgrading?
Agricultural residues and energy crops usually have irregular shapes and a low bulk density, resulting in loose harvest formats and a corresponding energy density that is lower than coal or corn grain. They also have high moisture content which can accelerate degradation during storage. These properties lead to:
- problematic biomass logistics and storage.
- an increase in the frequency of biomass handling.
- transportation challenges.
- storage space requirements.
- processing inefficiencies.
- conversion equipment/reactor volumes that ultimately incur costs for biomass logistics and conversion.
Methods that increase biomass density or enhance its properties can improve the utilization of biomass and reduce associated logistics, storage, handling, transportation, and processing costs.
Biomass densification refers to increasing the bulk and energy densities of a biomass feedstock by reducing its bulk volume. Mechanical means are used to configure biomass into predetermined smaller, uniform sizes that facilitate better handling, transportation, and storage.
Biomass upgrading changes the properties of biomass to improve storage, transportation, pretreatment, and conversion. For example, upgrading can reduce moisture content and increase hydrophobicity, which reduces feedstock deterioration during storage. Upgrading can also increase feedstock flowability, which improves the handling of feedstocks and their grindability. These upgrades reduce energy requirements during the size-reduction process. The resulting smaller particles allow for a more efficient conversion of feedstocks. Ultimately, biomass densification and upgrading can reduce dry matter losses, improve biomass handling, and streamline transportation logistics for biorefineries.
Biomass densification and upgrading techniques
Different biomass densification and upgrading techniques are used based on the type of feedstock, available equipment, and the desired final product. Some common densification and upgrading techniques are reviewed below.
Baling, which is used to collect agricultural residues such as corn stover, is the most common method of biomass densification and is usually performed in the field during harvest. The round bales seen in Figure 1 are the most common format, and usually have a diameter of 1.2–1.8 m (4–6 ft.) and a width of 1.2–1.5 m (4–5 ft.) (Darr and Shah 2012). In recent years, with increasing demand for cellulosic feedstocks for biobased industries, biomass feedstocks are collected in the form of rectangular bales, commonly referred to as the square bales shown in Figure 1. These rectangular bales are typically 0.4–0.9 m (1.3–3.0 ft.) high, 0.45–1.2 m (1.5–4 ft.) wide and 0.9–2.4 m (3–8 ft.) long (Shah and Darr 2016; and Hayforks.com 2021). Round bales are usually cheaper to produce than rectangular ones and shed water more effectively when stored in open fields. However, stacking round bales for a long period of time can distort the shape and integrity of the bales leading to higher biomass losses (Darr and Shah 2012). For commercial applications, rectangular bales are easier to load and transport in a trailer, and are also easier to stack and store in high volumes. Rectangular bales provide the added benefit of a higher bulk density resulting in more efficient feedstock logistics. Currently, biomass bales are primarily used for either animal feeds and bedding or, more recently, for biofuel production.
Briquetting is a densification process in which feedstocks are compacted using a hydraulic, mechanical, or roller press under high pressure, such as a piston press or screw press/extruder (Wright et al. 2010). Briquettes, such as those shown in Figure 2, can be produced from biomass with a particle size of 6–30 mm (0.2–1.2 in.), although 25–30 mm (1.0–1.2 in.) are most common. Briquetting bonds materials by interlocking them (Wright et al. 2010; and Tumuluru et al. 2010). Also, briquettes can be made from feedstocks with a 6–18% range of moisture (C.F. Nielsen 2021) and may use binders, such as starch, molasses, and arabic gum. Binders allow for considerable flexibility in the production method and type of feedstock. Briquettes are primarily used for heating purposes.
A pellet mill is used to produce high-density pellets by compressing ground biomass between rollers and a die, and then extruding the compressed ground biomass through die holes. The most common pellet mills are flat die and ring die mills. The primary difference between briquettes and pellets are their bulk densities, the particle size of the biomass inputs, and their final product size. For pelletization, the biomass is ground to 0.18–3 mm (0.007–0.1 in.) in diameter (Sudhagar, Tabil, and Sokhansanj 2004; and Tumuluru et al. 2011). The production of high-quality pellets is enhanced by using steam to soften the biomass and entrained lignin, and by adding different binders. Pellets, such as those shown in Figure 3, usually have a moisture content between 4–6% (weight basis) and a uniform shape and size with a length of 25–40 mm (1.0–1.6 in.) and diameter of 6–8 mm (0.2–0.3 in.). Pellets usually have high bulk density and durability, good flowability (during loading and unloading, transportation, and conveyance between processes during conversion), and storability. Currently, pellets are primarily used for domestic space heating. They also have the potential for supplying feedstock to biobased industries with improved processability. .
Torrefaction, a thermal biomass upgrading process, occurs at 200–300°C (392–572°F) under inert conditions (no oxygen), with a reactor residence time of 10–30 minutes and longer (Medic et al. 2012). Figure 4 shows the primary product, torrefied biomass, which has reduced moisture content, improved hydrophobicity, enhanced brittleness, and greater energy content when compared to the raw biomass. It is also better-preserved during storage, and its enhanced brittleness improves downstream grindability (Tumuluru et al. 2011). Torrefied biomass can be further densified by pelletization, which increases its specific bulk and energy densities while enhancing its flowability by creating uniformity among the different biomass types and forms.
Chipping is a biomass upgrading method that reduces the size of woody biomass, including forest residues and short rotation woody crops (e.g. hybrid poplar, willow, etc.), into smaller, more uniform wood chips. The bulk density of chipped biomass is usually higher than forest residue, which is loosely piled, but is lower than stacked wood, which is cut to specific lengths and split or sorted by diameter before stacking. Chipping improves the handling, storage, and transportation of irregular-sized and shaped woody biomass and prepares it for use in biorefineries or combustion power plants. Wood chips used in biofuels plants and boilers are irregular in shape, such as those in Figure 5, and typically range in size from 3 to 80 mm (0.1–3.0 in.) (veic 2021).
Other feedstock densification and upgrading techniques
Biomass may also be densified into products, such as pucks and cubes that have bulk densities between 400 to 600 kg/m3 (25–37.5 lb/ft3) (Sokhansanj and Fenton 2006). Typically, pucks, such as those shown in Figure 6, are shaped similar to hockey pucks with a 75 mm (2.9 in.) diameter. Cubes are 33 mm (1.3 in.) × 33 mm (1.3 in.) in cross-section (Sokhansanj and Fenton 2006). A common biomass size reduction method includes chopping to around 25 mm (1.0 in.) or grinding to less than 2 mm (0.08 in.) (Sudhagar, Tabil, and Sokhansanj 2004; Tumuluru et al. 2011; and Sokhansanj and Fenton 2006). Reducing the biomass particle size improves flowability, handling, and bulk density.
Properties of biomass after densification and upgrading
As biomass is densified, further densification and upgrading via briquetting, pelletization, and torrefaction techniques produces biomass with higher uniformity and/or lower particle size and size range than the initial harvested bulk biomass, as shown in Table 1.
|Densified form for feedstocks||Bulk Density [kg/m3 (lb./ft3)]||Energy density [MJ/m3 (thousand BTU/ft3)]||Size range [mm (in.)]|
|Loose biomass (usually piled or stacked) after harvest|
|Wheat straw [4, 15]*||36.1 (2.3)||444 (11.9)||Variable (depends on harvesting practices
|Variable (depends on harvesting practices and needs)|
|Corn stover [4, 15]*||52.1 (3.3)||900 (24.2)|
|Switchgrass [4, 15]*||67.5 (4.2)||1,200 (32.2)|
|Miscanthus *||85 (5.3)||1,500 (40.3)|
|Willow *||170 (10.6)||3,100 (83.2)|
|Stacked wood – Norway spruce *||310 (19.4)||6,350 (170)|
|Forest wood residue [5, 10]*||150 (9.4)||2,735 (73)|
|Coal – presented here as a comparison *||750 (46.8)||13,600 (365)|
|Size reduced format (chopped, chipped, and ground biomass)|
|Miscanthus chopped *||90 (5.6)||1,600 (42.9)||Variable||30-40 (1)|
|Willow chip (25% moisture) *||150 (9.4)||2,700 (72.4)||3-80 (0.1-3.2)|
|Wood chips [5, 10]*||220-265 (13.7-16.5)||2,693 - 3,244 (72.3-87)||3-80 (0.1-3.2)|
|Ground particles loose fill *||120 (7.5)||2,200 (59)||< 2 (0.1)|
|Wheat straw bales – 1 m × 0.5 m × 0.38 m [12, 24]*||115-130 (7.2-8.1)||2,100-2,400 (56.3-64.4)||Variable (depends on harvesting practices and needs)||Variable (depends on harvesting practices and needs)|
|Baled miscanthus - square bales 0.9 m×1.2 m cross section *||140 (8.7)||2,500 (67.1)|
|Corn stover bale - square bales 0.9 m×1.2 m cross section *||150 (9.4)||2,700 (72.4)|
|Switchgrass bale - square bales 0.9 m×1.2 m cross section *||180 (11.2)||3,300 (88.6)|
|Briquettes [8, 20]*||350 (21.8)||6,400 (171.8)||6-30 (0.2-1.2)||Dia.: 50-100 Length: 60-150|
|Pellets [20, 21]*||450-700 (34.3-43.7)||8,200-12,700 (220.1-340.9)||0.2-3 (0.008-0.1)||Dia.: 6-8 Length: 18-32|
|Wood residue pellets *||675 (42.1)||12,300 (330.1)|
|Torrefied biomass *||230 (14.4)||4,600 (123.5)||Variable|
|Torrefied pellets *||750-850 (46.8-53.1)||13,600-15,500 (365-416)||0.2-3 (0.008-0.1)|
Biomass densification and upgrading can be used to improve the inherently high variability of biomass properties. Such improvements can reduce biomass loss, improve handling and transport logistics, streamline biomass conversion in a biorefinery, and result in overall reduced costs. However, biomass densification and upgrading techniques need to be selected based on the type of biomass, storage requirements, distance between the production and utilization sites, and the intended application. For example, while biomass bales can be transported and utilized at biorefinery facilities that are close to the production site, pelletization may be necessary for low bulk density biomass that requires transportation over longer distances or biomass that is sold in international markets. Thus, biomass densification and upgrading technologies can result in a reliable supply of biomass feedstock to biobased industries and contribute towards enhancing the bioeconomy.
The authors thank Dr. Katrina Cornish, Professor; Dr. John Fulton, Professor; and Mary Wicks, Program Coordinator, Department of Food, Agricultural and Biological Engineering, The Ohio State University, for their technical and editorial review of this factsheet.
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