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ÂŚPELLET CONTRIBUTION

Torrefaction: Pre- or Post-Pelletization Oakridge National Laboratory tests identify differing physical and chemical characteristics and energy use of two torrefied pellet pathways. BY SHAHAB SOKHANSANJ

R

ecent developments in densification technologies, including pelletization, have substantially improved the economics of moving biomass around the globe. This is one of the reasons that the wood pellet industry has become an inevitable part of the bioeconomy in the U.S., Canada and elsewhere. To be an effective alternative to coal, increasing the energy density of pellets is as important as increasing bulk density and durability. The high heating value of currently marketed pellets is about 19 gigajoule (GJ) per metric ton, which limits the proportion that can be used in cofiring with coal, with a heating value as high as 28 to 30 GJ per metric ton. In this context, removing the low-heat content volatiles from biomass is a promising strategy to increase the overall heat value of pellets and the ability to store them outside brings storage and handling costs on par with coal. Torrefaction, also known as mild pyrolysis/carbonization, is a thermal pretreatment

process to upgrade lignocellulosic biomass into a higher-quality energy and carbon carrier, to augment coal. The process subjects the biomass to temperatures ranging from 200 to 300 degrees Celsius in the absence of oxygen for up to 30 minutes. The slow heating process roasts biomass, releasing volatile compounds and breaking down hemicelluloses. Torrefaction has been proposed as a robust strategy to overcome the heterogeneity among different types of cellulosic feedstocks, thus producing a uniform-quality energy commodity with improved energy/carbon content and grindability. The bulk density of the torrefied material, however, is generally lower than that of the raw biomass, making transport and storage economically challenging. Therefore, combining torrefaction and pelletization has great potential to upgrade raw biomass to a universal energy commodity.

Pellets to the Test In a research project for the Environmental Sciences Division of Oak Ridge Na-

tional Laboratory, two process pathways were investigated to make torrefied wood pellets from Douglas fir wood chips. Both pathways started with dry wood chips as the raw material and ended with torrefied wood pellets as the final product. For Pathway I, the chips were ground, pelletized and pellets were torrefied. For Pathway II, the chips were initially torrefied and then ground and pelletized. Since there are pros and cons associated with each of these process schemes, a detailed laboratory experiment was planned to evaluate the characteristics of each operation, along with the compositional and physical properties as well as energy and mass balance. For the experiments, Douglas fir wood chips with an average size of 30 to 50 millimeter and initial moisture content of 45 to 50 percent were collected from Fibreco Co. located in North Vancouver, British Columbia. The material was first dried to 15 percent moisture content for grinding or almost bonedry condition for torrefaction prior to grind. Both untreated wood chips and torrefied

Physical Property Comparison Pellet type

Diameter (mm)

MC (%)

Particle density (g/cm3)

Bulk density (kg/m3)

High heat value (MJ/kg)

Durability (% )

Pellets made from untreated wood chips

6.43

6.7

1.16

674

18.82

80.7

Pellets made from torrefied woodchips mixed with 7% wheat flour binder, at 260 degrees Celsius

6.47

8.6

1.21

-

-

85

Regular white pellets torrefied at 260 C

6.28

1.9

1.14

614

21.08

63.9

Regular white pellets torrefied at 280 C

6.12

1.7

1.04

579

21.97

62

Regular white pellets torrefied at 300 C

6.12

1.5

0.96

510

23

60.9

28 BIOMASS MAGAZINE | OCTOBER 2013

October 13 Biomass Magazine  

October 13 Biomass Magazine

October 13 Biomass Magazine  

October 13 Biomass Magazine