Production of Wood 

Pellets From AlaskaGrown White Spruce 

and Hemlock

Allen M. Brackley and Daniel J. Parrent

United States 

Department of 

Agriculture

Forest Service

Pacific Northwest 

Research Station

General Technical Report 

PNW-GTR-845

July 2011

DEPAR TMENT OF AGRICULT URE
Authors

Allen M. Brackley is a research forester, U.S. Department of Agriculture, Forest 

Service, Alaska Wood Utilization Research and Development Center, 204 Siginaka 

Way, Sitka, AK 99835; and Daniel J. Parrent is a wood utilization specialist, 

Juneau Economic Development Council, 204 Siginaka Way, Sitka, AK 99835. 

Cover photograph by Allen M. Brackley.

The Forest Service of the U.S. Department of Agriculture is dedicated to the principle of 

multiple use management of the Nation’s forest resources for sustained yields of wood, 

water, forage, wildlife, and recreation. Through forestry research, cooperation with the 

States and private forest owners, and management of the National Forests and National 

Grasslands, it strives—as directed by Congress—to provide increasingly greater service 

to a growing Nation.

The U.S. Department of Agriculture (USDA) prohibits discrimination in all its programs and 

activities on the basis of race, color, national origin, age, disability, and where applicable, 

sex, marital status, familial status, parental status, religion, sexual orientation, genetic 

information, political beliefs, reprisal, or because all or part of an individual’s income 

is derived from any public assistance program. (Not all prohibited bases apply to all 

programs.) Persons with disabilities who require alternative means for communication of 

program information (Braille, large print, audiotape, etc.) should contact USDA’s TARGET 

Center at (202) 720-2600 (voice and TDD). To file a complaint of discrimination, write 

USDA, Director, Office of Civil Rights, 1400 Independence Avenue, SW, Washington, DC 

20250-9410 or call (800) 795-3272 (voice) or (202) 720-6382 (TDD). USDA is an equal 

opportunity provider and employer.
Abstract

Brackley, Allen M.; Parrent, Daniel J. 2011. Production of wood pellets from 

Alaska-grown white spruce and hemlock. Gen. Tech. Rep. PNW-GTR-845. 

Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest 

Research Station. 28 p.

An extensive literature review failed to locate any information relative to the pelleting characteristics of hemlock species—western hemlock (Tsuga heterophylla 

(Raf.) Sarg.) and mountain hemlock (Tsuga mertensiana (Bong.) Carr.)—that 

grow in Alaska. To determine more about the pelletizing properties of the species, 

arrangements were made with a pelleting company to conduct test runs using mixtures of pure Alaska-grown hemlock and combinations with white spruce (Picea 

glauca (Moench) Voss). The test runs were conducted by using the same methods 

and procedures that the firm had been using in the production of white spruce 

pellets. The initial tests with Alaska hemlock resulted in pellets that were visually 

similar to those that the firm had been producing from spruce. Laboratory testing 

indicated that pellets made entirely of Alaska hemlock, or of Alaska hemlock and 

spruce mixtures, met most 2008 Pellet Fuels Institute (PFI) premium pellet and/

or standard pellet specifications, with a few exceptions. Given recent (October 17, 

2010) changes to the PFI pellet specifications, pellets made entirely of hemlock, or 

of hemlock and spruce mixtures, could meet all premium pellet specifications with 

minor adjustments to the manufacturing process. 

Keywords: Wood pellets, pellet production, pellet formation, Alaska hemlock, 

white spruce. 

1

Production of Wood Pellets From Alaska-Grown White Spruce and Hemlock

Introduction

The charter of the Alaska Wood Utilization Research and Development Center 

(AWURC) in Sitka, Alaska, states that the unit is responsible for conducting 

research projects that have the potential to help rebuild and maintain the forest 

products industry in Alaska. A second charge is to conduct research that improves 

conditions in the rural communities that are embedded in Alaska national forests.

During the past 10 years, there has been increasing interest in the use of 

biomass from the Nation’s forest lands as a source of renewable energy (Niebling et 

al. 2010, Patton-Mallory 2008, Perlack et al. 2005, PR.com 2009, Spelter and Toth 

2009). Given the national focus on renewable energy, AWURC has conducted a 

number of projects to evaluate biomass issues at the national, international (Nicholls 

et al. 2008, 2009), and regional levels (Brackley et al. 2010, Nicholls et al. 2010).

Most of the land in southeast Alaska is part of the Tongass National Forest 

(Nowacki et al. 2001, USDA FS 2008). Western hemlock (Tsuga heterophylla

(Raf.) Sarg.) and mountain hemlock (Tsuga mertensiana (Bong.) Carr.), collectively 

referred to as Alaska hemlock, are the major species growing on the forest lands 

of southeast Alaska, making up approximately 56 percent of total growing-stock 

volume (van Hees 2003). It is not possible to differentiate between the two hemlock 

species in the log form. The Sitka spruce (Picea sitchensis (Bong.) Carr.) component is approximately 28 percent of total growing-stock volume (van Hees 2003). 

The properties of this species are similar to white spruce (Picea glauca (Moench) 

Voss).

There are limited volumes of sawmill residues in the region (Brackley et al. 

2006). Given the limited sawmill residuals, the major source of biomass for renewable energy is in unutilized portions of harvested trees, and in rough and rotten 

trees that are unsuitable for use as saw logs. It is also possible that partial cutting 

of young-growth stands may result in additional volumes of material suitable for 

use in energy applications (TFRT 2008).

Use of local species for production of energy products in Alaska has been the 

topic of several state and regional conferences and many less formal discussions. 

Given the expressed interest in renewable energy products, an effort was made 

to locate any project reports that included specific references to species that are 

indigenous to southeast Alaska. The literature review failed to locate a specific 

reference relative to the pelleting characteristics of these species, but located firms 

in other regions of North America (CRE 2010) that advertised pellet products based 

on a similar species, eastern hemlock (Tsuga canadensis (L.) Carr.). The review 

also identified a firm in Canada (General Biofuels Canada [GBC]) that announced 

plans to build a large pellet production facility in the vicinity of Terrace, British 
2

GENERAL TECHNICAL REPORT PNW-GTR-845

Columbia, that may use western hemlock (EIN 2010), at least in part. In the news 

release, GBC reported that the plant would produce 500,000 metric tons per year of 

industrial wood pellets. The material would be exported to utilities and industrial 

firms in Asia and Europe. Raw material for the plant will be supplied by local 

British Columbia forest license holders. The source (EIN 2010) states, “The British Columbia Ministry of Forests has expressed a willingness to meet any supply 

shortfalls with additional license capacity if required.” 

Literature Review 

The original objective of this effort was to locate articles that specifically referenced 

use of eastern or western hemlock in the production of wood fuel pellets. A considerable search, however, failed to locate any articles that specifically referenced 

or documented tests of hemlock pelletization. The authors found that numerous 

factors affected the process of pellet production (Lever n.d., Tumuluru et al. 2010, 

Wilson 2010,.), including characteristics of the equipment processing variables, 

production parameters, and the physical and chemical bonding mechanisms that, in 

combination, are part of the process of producing pellets. Leaver (n.d.) stated: “Each 

wood used as feedstock has specific polymeric characteristics and thus specific 

temperature, pressure, and moisture characteristics at which it will begin to soften 

and form a pellet. Thus, the pelleting process is quite material specific and the more 

consistent the feedstock, the more stable the pelleting process.” The success or 

failure of any particular test for producing pellets may result from a combination of 

equipment-related variables and their impact on the physical characteristics of the 

furnish, some of which may be species-specific characteristics. Given these findings, the authors have included a short review of existing standards and production 

procedures to provide information that all pellet manufacturers must consider. 

Standards for Pellet Production

Since 1988, the U.S. Environmental Protection Agency (EPA) has been updating 

performance standards for wood heaters used in residential construction (Hager 

2010, PFI 2010). As part of this program, EPA is also requiring standardization 

of pelletized fuel. The EPA activity will result in changes to the Pellet Fuel Institute (PFI) voluntary standard that has existed since 2008. These changes were 

announced as this report was nearing final completion and have been incorporated 

into the text.
3

Production of Wood Pellets From Alaska-Grown White Spruce and Hemlock

The 2008 PFI standard that existed at the start of this project was a voluntary, 

uniform standard for use by member mills when labeling wood pellets for sale (PFI 

2008b). The standard defined pellet quality in terms of bulk density, acceptable 

diameter and length, allowable percentages of fines “at the mill gate,” levels of inorganic ash, moisture content wet basis [wb],1

 chloride content, and durability index. 

The durability index (PFI 2010c) is “...a standardized parameter for specifying the 

ability of the fuel pellets to resist degradation caused by shipping and handling.” 

Table 1 summarizes the 2008 PFI standards for various grades of fuel.

Table 1—Pellet Fuels Institute (PFI) 2008 residential/commercial densified fuel standards

Fuel property PFI super premium PFI premium PFI standard PFI utility

Pounds per cubic foot

Bulk density 40.0–46.0 40.0–46.0 38.0–46.0 38.0–46.0

Inches

Diameter 0.250–0.285 0.250–0.285 0.250–0.285 0.250–0.285

Millimeters

Diameter 6.35–7.25 6.35–7.25 6.35-7.25 6.35–7.25

Pellet durability index ≥97.5 ≥97.5 ≥95.0 ≥95.0

Percent

Fines (at mill gate) ≤0.50 ≤0.50 ≤0.50 ≤0.50

Inorganic ash ≥ 0.50 ≥ 1.0 ≥ 2.0 ≥ 6.0

Length (> 1.5 inches) ≤ 1.0 ≤ 1.0 ≤ 1.0 ≤ 1.0

Moisture (green basis) ≤ 6.0 ≤ 8.0 ≤ 8.0 ≤ 10.0

Parts per million

Chloride ≤ 300 ≤ 300 ≤ 300 ≤ 300

Ash fusion NA NA NA NA

Heating valuea As received As received As received As received

± 2SD ± 2SD ± 2SD ± 2SD

NA = Not available. a There is no required value or range for heating value. It is required that the mean higher heating value in British thermal units 

per pounds and ash content be printed on the label using a bar scale to represent the mean value ± 2SD.

Source: PFI 2008b. 

1 There are two methods for reporting the moisture content of forest products. Wood 

Technologists—scientists working with most solid forest products such as lumber, plywood, 

and particleboard—report moisture content in relation to the bone dry weight of the material, 

and the resulting values are referred to as dry basis (db). Engineers concerned with energy 

relationships or the pulp and paper industry, however, report moisture content in relation to 

the original wet weight of the material; the resulting values are referred to as wet basis (wb). 

Moisture content values expressed as percentage values can be converted from one basis to 

the other using the formula db = [(wb/100)/(1 – wb/100)] × 100 or wb = [(db/100)/(1 + db/100)] 

× 100. A moisture value of 50 percent wb converts to 100 percent db, and dry basis values can 

be in excess of 100 percent. In this report, the authors have followed moisture content values 

with (db) or (wb) to identify the basis (denominator of the moisture content formula) used to 

obtain the value. 

Engineers interested 

in energy values use 

moisture content as a 

percentage of wet or 

green weight.
4

GENERAL TECHNICAL REPORT PNW-GTR-845

On October 7, 2010, the PFI Board of Directors unanimously passed an updated 

version of the PFI North American Residential/Commercial Densified Fuel Standards (PFI 2010a). Important features of the new program include initial accreditation, certification of mills, monthly third-party inspection, sampling, testing, and 

overall program oversight. Table 2 compares the 2008 and 2010 board-approved 

changes that have been forwarded to EPA. Changes in specifications are shown 

in bold print. The PFI is also negotiating with the American Lumber Standards 

Committee (ALSC) to provide certification and enforcement regulations (ALSC 

2010). Under the new enforcement regulations, a mill that wants to use PFI marks 

must apply to the ALSC Board of Review for accreditation. Prior to awarding 

accreditation, the ALSC Board of Review will require that the applicant mill have 

a suitable quality control program that includes a third-party production auditing 

program. The auditing agency will initially certify product quality, and thereafter 

quality control sampling will be at a minimum rate of one sample per every 1,000 

tons of production (ALSC 2010). Under section 5, (ALSC 2010) the board of review 

field staff will be responsible for monitoring agency performance. The four types of 

monitoring will include (ALSC 2010):

Table 2—Comparison of Pellet Fuel Institiute (PFI) 2008 standards and 2010 PFI Board of Directors 

approved standards

Super premium grade Premium grade Standard grade Utility grade

Fuel property 6/18/2008 10/7/2010 6/18/2008 10/7/2010 6/18/2008 10/7/2010 6/18/2008 10/7/2010

Bulk density 40.0–46.0 This grade 40.0–46.0 40.0–46.0 38.0–46.0 38.0–46.0 38.0–46.0 38.0–46.0 eliminated

from 2010 standard

Diameter (inches) 0.250–0.285 0.250–0.285 0.230–0.285 0.250–0.285 0.230–0.285 0.250–0.285 0.230–0.285

Diameter (mm) 6.35–7.25 6.35–7.25 5.84–7.25 6.35–7.25 5.84–7.25 6.35–7.25 5.84–7.25

Pellet durability ≥97.5 ≥97.5 ≥96.5 ≥95.0 ≥95.0 ≥95.0 ≥95.0

index

Fines (percent at ≤0.50 ≤0.50 ≤1.0 ≤0.50 ≤1.00 ≤0.50 ≤1.00

mill gate)

Inorganic ash ≤0.50 ≤1.0 ≤1.0 ≤2.0 ≤2.00 ≤6.0 ≤6.0

percent

Length (percent ≤1.0 ≤1.0 ≤1.0 ≤1.0 ≤1.0 ≤1.0 ≤1.0

> 1.5 inches)

Moisture ≤6.0 ≤6.0 ≤6.0 ≤8.0 ≤10.0 ≤10.0 ≤10.0

percentage

(wet basis)

Chloride (ppm) ≤300 ≤300 ≤300 ≤300 ≤300 ≤300 ≤300

Not mandatory:

Ash fusion

Heating value

Changes in specifications are shown in bold print.

Source: PFI 2008b, PFI 2910c.

The American Lumber 

Standards Committee 

may certify and enforce 

pellet fuel standards.
5

Production of Wood Pellets From Alaska-Grown White Spruce and Hemlock

1. Random sample survey inspections (at manufacturer sites—section 5.1.1).

2. General inspections (see section 5.1.2).

3. Destination inspections (conducted in market areas where pellets are sold—

section 5.1.3).

4. Recall inspections, defined as inspections resulting from finding serious 

deficiencies during a random sample, general, or destination inspection. 

The objective of recall inspections is to determine that corrective action 

has been taken.

As previously noted, the above changes were in process as the final draft of 

this report was being prepared.

When this report was initially prepared, standard specifications for wood pellets in Canada (Peksa-Blanchard et al. 2007) did not exist. However, in early 2010, 

the PFI Web site (http://www.pelletheat.org/2/index/index.html) listed four Canadian firms as registered “manufacturer” members. A similar check of the Web site 

on October 25, 2010, indicated that five Canadian manufacturer members (one of 

which also operated a plant in Pennsylvania), one Canadian university, and several 

European organizations are listed as PFI associate members. 

As previously noted, major additions and changes were made to the PFI 

Web site on October 25, 2010. The title of one of the new documents was “North 

American Certification of Residential/Commercial Densified Fuel.” Note the use of 

“North American” in reference to certification. The Web site also announced that 

PFI was actively negotiating a contract with ALSC to assume responsibilities as 

the “certification body” for purposes of implementation of the PFI standards. The 

result of all of the PFI activities is that Canadian pellet producers have the option 

of adopting the PFI standards, and selected or designated Canadian agencies will 

become certifiers and accrediting agencies of the PFI standard in a manner similar 

to Canadian lumber producers under the American Lumber Standard (NIST 2010). 

The roots of the European Energy Charter (EEC) date back to the mid 1980s 

(ECS 2010). Under the terms of the EEC (ECS 2004), each signatory nation has 

agreed to pass energy legislation to reduce carbon emissions. Several of the countries have passed legislation to substitute renewable biomass energy sources for 

fossil fuels. In the early years of the EEC, these nations created their own standards 

for wood pellets. At this time, standards for production of pellets in Europe are in a 

state of flux. Alakangas (2010) stated:

The European Committee for Standardization, CEN/TC TC335, has 

published 27 technical specifications (pre-standards) for solid biofuels 

during the period 2003–2006. Now these technical specifications are being 
6

GENERAL TECHNICAL REPORT PNW-GTR-845

upgraded to full European standards (EN). Where EN-standards are in 

force the national standard of member countries has to be withdrawn or 

adapted to these EN-standards.

Prior to 2010, Hahn (2004) reported that the major pellet-using nations of 

Europe (Austria, Sweden, Germany, Italy, and Great Britain) had established 

standards for pellet products. These standards will now be replaced by three new 

standards. The first, EN 14961-1, covers pellets for general use that includes specific 

reference to pellets from different biomass raw materials. Second, EN 14961-2 

covers wood pellets for nonindustrial use. Finally, EN 14961-6 provides standards 

for nonwoody pellets for nonindustrial use. 

The pellet standards developed by the European Union (EU) are presented in 

table 3. Note that pellets for domestic use will be 6 or 8 mm (0.234 or 0.315 in) in 

diameter. The pellets greater than 8 mm (0.315 in) are used in industrial and commercial applications. Both the EN (Alakangas 2010) and PFI (2008b, 2010c) require 

that pellets meet specific conditions with respect to length, diameter, bulk density, 

percentage of fines, ash percentage, moisture content, and heating value. Neither 

the PFI or EN specify a standard for heating value, but both require reporting of the 

average heating value of the pellets on the package label. Both standards require 

reporting of chloride (parts per million [ppm] in the United States) or chlorine 

(percentage of weight/dry basis EU). The EU standards also require reporting 

percentages of nitrogen and sulphur. The most recent updates to the PFI standards 

(PFI 2010b) increase the amount of fines allowed in the standard grade of pellets 

from ≤ 0.5 percent to ≤ 1.0 percent, but this level must be maintained at the “market 

level” as opposed to the previous standards (PFI 2008b) “at the mill gate.”

Both PFI (2010b) and EN (Alakangas 2010) require reporting of a durability 

index that is indicative of degradation from handling and shipping. Several 

approaches and methods for durability testing are outlined in a report by 

Temmerman et al. (2006). In the PFI (2008b, 2010b) standards, a slightly modified 

version of the Kansas State University method as outlined in American Society of 

Agricultural and Biological Engineers (ASABE) S269.4 (ASABE 2007) is used to 

evaluate durability. In the testing process, a 500 g (1.1 lb) sample of wood pellets 

is tumbled for 10 minutes in a baffled box and sieved on a 1/8-in (3.175-mm) 

screen (PFI 2010b). The requirements outlined for feed pellets in standard 5.2.2 

(ASAE 2007) states that “Fines shall be determined by screening a sample on wire 

sieve having openings just smaller than the nominal pellet diameter.” Alakangas 

(2009) described the same baffled box testing device, built to the same dimensions 

as specified for durability testing in the EU standard. Duration of the test and 

Standards for pellets 

produced and sold in 

the European Union 

are in a state of flux.
7

Production of Wood Pellets From Alaska-Grown White Spruce and Hemlock

Origin of pellets:

N or I a Code Source

N 1 Wood biomass

N 2 Herbaceous biomass

N 3 Fruit biomass

N 4 Blends

Dimension:

N or I Code Diameter (mm) Code Length (mm)

N D 06 6 ± 1.0 3 15 ≤ L ≤ 40 

N D 08 8 ± 1.0 3 15 ≤ L ≤ 40 

N D 10 10 ± 1.0 3 15 ≤ L ≤ 40 

N D 12 12 ± 1.0 3 15 ≤ L ≤ 40 

N D 25 25 ± 1.0 10 10 ≤ L ≤ 50 

Moisture (as received):

N or I Code Moisture content

 Percent

N M10 ≤ 10

N M15 ≤ 15

Percentage ash (dry weight basis):

N or I Code Percentage of dry weight

N A0.5 ≤ 0.5

N A0.7 ≤ 0.7

N A1.0 ≤ 1.0

N A1.5 ≤ 1.5

N A2.0 ≤ 2.0

N A3.0 ≤ 3.0

N A5.0 ≤ 5.0

N A7.0 ≤ 7.0

N A10.0 ≤ 10.0

N A10.0+ > 10.0

Mechanical durability (DU fine weight as

 percentage of original sample weight):

N or I Code Percentage of original weight

N DU97.5 ≥ 97.5

N DU96.5 ≥ 96.5

N DU95.0 ≥ 95.0

N DU95.0 < 95

Amount of fines after production and packaging 

 (fine weight as percentage of original sample weight):

N or I Code Percentage of original weight

N F1.0 ≤ 1.0

N F2.0 ≤ 2.0

N F3.0 ≤ 3.0

N F5.0 ≤ 5.0

N F5.0+ > 5.0

Additives:

N Type and content of pressing aids, slagging 

 inhibitors, or any other additive must be 

 stated on label

Bulk density (BD as received kg/M3):

N or I Code kg/M3 (lbs/ft3)

N BD550 ≥ 550 ≥ 34.3

N BD600 ≥ 600 ≥ 37.4

N BD650 ≥ 650 ≥ 40.6

N BD700 ≥ 700 ≥ 43.7

N BD700+ > 700 (minimum > Stated value

 value stated)

N Q or net calorific value as received 

 = MJ/kg or EN = kWh/kg

Sulphur S (w-percent dry basis):

N or I Code Weight

 Percent

N or I S0.02 ≤ 0.02 N for chemically treated,

N or I S0.05 ≤ 0.05 I for not chemically treated

N or I S0.08 ≤ 0.08

N or I S0.10 ≤ 0.10

N or I S0.20 ≤ 0.20

N or I S0.20+ > 0.20 

 (maximum 

 value stated)

Nitrogen N (w-percent dry basis):

N or I Code Weight

 Percent

N or I N0.3 ≤ 0.3 N for chemically treated, 

N or I N0.5 ≤ 0.5 I for not chemically treated

N or I N1.0 ≤ 1.0

N or I N2.0 ≤ 2.0

N or I N3.0 ≤ 3.0

N or I N3.0+ > 3.0

 (maximum 

 value stated)

Chlorine Cl (w-percent dry basis):

N or I Code Weight

 Percent

N or I Cl0.02 ≤ 0.02 N for chemically treated, 

N or I Cl0.03 ≤ 0.03 I for not chemically treated

N or I Cl0.07 ≤ 0.07

N or I Cl0.10 ≤ 0.10

N or I Cl0.10+ > 0

 (maximum 

 value stated)

Table 3—Specification of properties for pellets (EN 14961-1) 

a Normative (N) items are required on the product label. Informative (I) items may be included at the option of the manufacturer.

Source: Alakangas 2010.
8

GENERAL TECHNICAL REPORT PNW-GTR-845

rotational speed specified in the EU standard (Alakangas 2009) are in agreement 

with those specified in ASABE (2007). The sieve size used in the EU procedures is 

slightly different, but the ultimate testing results should be comparable. 

In summary the PFI and EU standards are essentially the same. The EU standard recognizes pellet diameters of 6, 8, 10, 12, and 25 mm. Most of the pellets for 

domestic or home use will be 6 or 8 mm. The PFI (PFI 2010b) standard is focused 

on pellets that conform to the 6 mm EU product. Table 4 presents the information 

required on a label by the EU standards. The label information in the table is from 

test runs to produce pellets from hemlock grown in Alaska, as reported by Twin 

Ports Testing in Lake Superior, Wisconsin.

The Bonding Process

Several recent reports and theses (Jonsson 2009, Kaliyan and Morey 2006, 

Sokhansan et al. 2005, Wilson 2010), when reporting the various bonding 

mechanisms that are essential to the pelleting process, make reference to a 1962 

article by H. Rumpf, “The strength of granules and agglomerates,” which appeared 

in a book titled Agglomeration, edited by W.A. Knepper and published by Wiley 

Interscience, and which reported the proceedings of the 1961 Agglomeration 

Symposium. Reference is made to Agglomeration at the following Web site: 

http://www.agglomeration.org/Information/tabid/65/AgglomerationBooks.aspx. 

We were unable to locate a copy of the original Rumpf article, but did locate an 

Table 4—Example of label required by EN 14961-2 

Producer Alaska Pellet Company

Origin 1.0 wood biomass

Trade Form Pellets—Class A1 (a0.5)

Country of origin Alaska, United States

Normative (EN 14961-2)

Dimension D06 (diameter = 6 ± 0.5 mm) and length ≤ 5× diameter

Moisture M10 (≤ 10 percent)

Ash A0.5 (≤0.5 percent)

Mechanical durability DU96.5 (≥ 96.5 percent)

Amount of fines F1.0 (≤ 1.0 percent)

Net calorific value, Q Q ≥ 5.1 kWh/kg

Additives None

Bulk density BD700+ (< 755 kg/m3 or 47.21 lbs/ft3)

Chemical composition N 0.3 (≤ 0.3% nitrogen), S 0.02 (≤ 0.02% sulfur), Cl 0.02 (≤ 0.02% chlorine)

Ash melting behavior Not available

Minor elements Informative only, not required

Data in the above table taken from tests of hemlock pellets produced by Alaska Pellet Company. The data has been converted to the 

form required by EN 14961-2 as of the date of the testing of the Alaska product. EN standards are in a state of flux and may change 

after date of publication. 
9

Production of Wood Pellets From Alaska-Grown White Spruce and Hemlock

article by Sastry and Fuerstenau (1973) that appears to be very close to the original 

source (it cites Rumpf as a source) and in agreement with the more recent works. 

Quoting from Sastry and Fuerstenau (1973): 

Forces contributing to the formation of green pellets from particulate solids 

are of two kinds: natural (or physical) and applied (or mechanical). The 

natural forces responsible for the formation of agglomerates can result from 

a number of sources: (a) the attraction between solid particles due to van 

der Waals forces, magnetic forces, or electrostatic charges, (b) the interlocking effects between particles, depending on the shape of particles, (c) the 

adhesional and cohesional forces in bridging bonds, which are not freely 

moveable, and, very importantly, (d) the interfacial and capillary forces due 

to the presence of a liquid phase. The strength of green agglomerates results 

from the physical forces that hold the particles together; and the magnitudes 

of these physical forces are dependent on the particle size, surface charge, 

crystal structure, the proximity of the particles, the amount of additives, and 

other physical-chemical properties of the system.

Pellets can be produced from almost any biomass material. The preparation 

process focuses on reduction of material to form particles with optimum geometric 

configuration. The surface properties of the particles are extremely important, as 

are the forms and levels of moisture in the material. Chemical composition in terms 

of cellulose, hemicelluloses, and lignin are also important characteristics. Just prior 

to feeding the mixture or furnish into a pelleting machine it can be described in 

terms of bulk density, temperature, moisture content, and chemical components. 

Chemical components may be determined using procedures such as Klason lignin 

or acid detergent lignin procedures (Hatfield et al. 1994) or spectrographic analysis. 

The pelleting process itself subjects the prepared raw material to heat and 

pressure as the material is forced through a die. The interaction between the mix 

of materials and the production equipment produces compaction and mechanical 

interaction of the particles. Friction caused by forcing the material together and 

pressing against the die generates heat that is applied to the raw material. As heat 

and pressure are applied, the remaining water temporarily fills cavities that are 

decreasing in size. The additional pressure causes attraction of the particles by 

capillary action. Ultimately, much of this moisture exits the final product and there 

is uncertainty about the role of this phase on ultimate pellet quality. Van der Waals 

forces contribute to the process of pellet formation. If the critical temperature and 

pressure for melting (plasticizing) of native unaltered lignin is obtained, upon 

cooling, the lignin will function as an adhesive and maintain the structural integrity 

of pellets. 

Pellets can be 

produced from any 

biomass material.
10

GENERAL TECHNICAL REPORT PNW-GTR-845

Much of the research by the pulp and paper industry deals with lignin as 

an extracted product, and the extraction processes results in a modified form of 

the material. Kelley et al. (1987) cited research by Irvine (198) using differential 

scanning calorimetry (DAS) of whole wood that revealed a single glass transition 

point2 (Tg) between 50 and 120 °C (122 to 248 °F) which was assigned to lignin. 

Kelley et al. (1987) also reported that the Tg was affected differently by water. The 

research by Kelley et al. (1987) determined that wood material plasticized with 

ethyl formamide followed Williams-Landel-Ferry behavior in the temperature 

range Tg to Tg + 85 °C. It must be stated that Tg is lower than the melting point 

and that melting takes place at some temperature above Tg + 85 °C (Tg + 185 °F). 

By definition, the transition state between Tg and Tg + 85 °C is constant, but the 

above research deals with altered lignin as opposed to the material in the native 

unaltered form. Regardless, attaining the transitional state (Tg) creates an increase 

in the potential for binding and bonding in a process such as pelleting. At the 

melting point of lignin, fusion becomes a reality. The temperatures generated in 

the pelleting process are not sufficient to obtain fusion.

Recent research relative to welding of wood provides additional knowledge 

about the properties of lignin in the unaltered form. Numerous papers have been 

published about the process and resulting technology (Gerber and Gfeller 2000, 

Gfeller et al. 2004, Pizzi et al. 2003, Pizzi et al. 2004). The most relevant point 

reported by Pizzi et al. (2003) is the statement: “...the temperature of just the 

bondline reaches 170 °C (338 °F) or higher during the welding.” This temperature 

is much higher than the Tg of lignin and hemicelluloses, above which these materials are known to flow (Kelley et al. 1987). Melting of some of the major structural, 

polymeric wood constituents occurs and can be observed by scanning electron 

microscopy. If pressure is applied at these temperatures, lignin will glue cellulose 

together, which is solidified upon cooling. We note that the pelleting process as it 

exists generates temperatures slightly lower than the Tg and are not high enough to 

weld the particles together. 

In combination, the above factors contribute to the process of pellet formation. 

A more detailed explanation requires a knowledge and understanding of physical 

chemistry.

2 Glass transmission point or temperature is that temperature at which a polymer changes 

from a hard glassy form to an elastic one (Szczesniak et al. 2008).
11

Production of Wood Pellets From Alaska-Grown White Spruce and Hemlock

Pelletizing Material 

Wood (1987) reported that “...pelleting of animal feeds has become a firmly 

established process within the feed industry.” Research conducted by Wood (1987) 

tested pellet quality (durability) resulting from inclusions of denatured soy protein 

and native tapioca starch in mixtures of animal feeds. In this research, it was 

determined that improvements in durability were greatest for raw protein. Pellet 

hardness was improved with increasing levels of pregelatinized starch content. 

Lehtikangas (2000, 2001), employed by the Department of Forest Management 

and Products at the Swedish University of Agriculture Sciences, conducted research 

to evaluate the storage effects on wood pellets made from sawdust, logging residues, and bark. Pellets made from fresh logging residues were found to absorb the 

greatest amount of water and have the lowest durability. In general, all pellets stored 

for 5 months showed minor but notable reductions in quality. In the second study, a 

mixture of materials from Norway spruce (Picea abies (L.) Larst.) and Scots pines 

(Pinus sylvestris L.) were evaluated. In this project, nine pellet formulations from 

varied sources (fresh and stored sawdust, bark, and logging residues) were obtained 

from producers. The resulting pellets were evaluated based on moisture content, 

heating value, content of ash, sulphur, chlorine, and Klason lignin. Other factors 

reviewed included pellet dimensions, bulk density, individual pellet density, durability, and ash sintering as it relates to slag formation.3

 In this study, durability of 

wood-based pellets was determined to be significantly increased by greater lignin 

content and moisture content of wood particles delivered to the pelleting machine. 

Bark pellets demonstrated good durability regardless of moisture content. It should 

be noted that the maximum moisture content evaluated (20 percent, wb) was for 

an all-bark mixture. Based on an interpolation of the figures in the report, pellet 

moisture content from other stock types ranged between 6 and 15 percent (wb). 

Given the increasing interest in renewable energy and pellets during the past 

10 years, several reports (Kaliyan and Morey 2006, Peksa-Blanchard et al. 2007, 

Wilson 2010) have included extensive literature reviews to summarize information 

relative to material characteristics, processing methods, and production parameters 

for pellet production. These papers include research relative to pelletization of 

both agricultural feedstocks and forest-based material. Wilson (2010) and Kaliyan 

and Morey (2006) identified the factors that affect pellet strength and durability as 

moisture content, particle size, preheating and steam conditioning, and inclusion 

3 Sintering refers to the agglomeration of powders into solids. In this instance we are 

concerned with the agglomeration of ash into slag and the strength of the resulting slag 

formed during the burning process. Slag can affect the performance of pellet burners. A 

complete discussion of this problem in contained in Öhman et al. (2004).
12

GENERAL TECHNICAL REPORT PNW-GTR-845

of binders. Leaver (n.d.), in a report concerned with the pelleting of feedstock, 

stated that ambient conditions, both temperature and relative humidity, can affect 

pelletability. He noted that “northern installations routinely have a problem 

reaching as high a mash temperature in winter as in summer.” Kaliyan and Morey 

(2006) also reported the effect of mixing various raw materials and cited a report 

by Bradfield and Levi (1984). In the Bradfield and Levi (1984) project, furnish 

for pelleting came from wood and bark of six common southern hardwoods—red 

maple (Acer rubrum L.), southern red oak (Quercus falcata Michx.), sweetgum 

(Liquidambar L.), tupelo (Nyssa L.), white oak (Quercus alba L.), and yellow poplar 

(Liriodendron tulipifera L.)—as well as loblolly pine (Pinus taeda L.). Pellets were 

produced by using a laboratory system. In these tests, hardwood mixtures without 

bark did not pelletize. Furnish formulations that were a mixture of wood and bark 

mixed to the natural proportions produced pellets that were relatively durable. The 

pure pine wood pelleted easily and produced hard, durable pellets.

Both Wilson (2010) and Kaliyan and Morey (2006) reviewed the binding 

mechanisms or physical forces that take place in the process of pellet formation. 

This included solid bridges that are developed by diffusion of molecules from 

one particle to another at point of contact; attraction forces between solid particles (chemical bonds, hydrogen bridges, and van der Waals forces); mechanical 

interlocking of particles resulting during the compression process; adhesion and 

cohesion forces (noted only by Kaliyan and Morey [2006]), which occur when the 

mixtures include highly viscous binders such as molasses and tar that adhere to the 

surface of solid particles to generate strong bonds; and interfacial forces and capillary pressures that are created during the three stages of the densification process 

(pendular, funicular, and capillary states). Both sources noted the importance of 

moisture as essential to the development of interfacial and capillary pressure. Also, 

the bonds created by interfacial forces and capillary pressure disappear once the 

liquid evaporates, but they are critical to the cohesive forces in the final pellet.

As mentioned, the Kaliyan and Morey (2006) paper included a literature 

review or synthesis of factors that contribute to strength and durability of densified products. Wilson (2010), however, reasoned that the effect of tree species on 

pellet durability is through variation in lignin content. His study evaluated pellet 

durability from furnish mixtures that were characterized in terms of Klason lignin 

content, particle size distribution, triaxial compression testing, and particle shape. 

Wilson (2010) concluded that lignin content differs with tree species, but it did not 

appear to have an impact on pellet durability. There were significant differences 

between the bulk material properties that produced the highest and lowest levels 

of durability in this project. In summary, Wilson (2010) reported that the lowest 
13

Production of Wood Pellets From Alaska-Grown White Spruce and Hemlock

durability was in pure pine pellets with a furnish moisture content of 10 percent 

(wb). The highest durability was for red oak pellets with a furnish moisture content 

of 10 percent (wb). Wilson recommended that additional research be focused on 

furnish bulk material characteristics. Specific furnish characteristics recommended 

for review were the bulk modulus and elastic response, along with a review of the 

geometric and surface characteristics of the particle components on compaction 

behavior. Wilson noted that the techniques used in powder mechanics and the pharmaceuticals industry that characterize and predict the behavior of irregular particle 

distributions should be applied to the pelleting process.

Stahl et al. (2004) conducted a project to evaluate processes of biomass drying 

and effects on wood pellet quality. Pellets, green material, and dry furnish were 

supplied by producers. Moisture content (wb) of green material ranged from 41.23 to 

53.54 percent, dry furnish from 6.05 to 11.68 percent, and pellets from 6.00 to 7.79 

percent. This project also reported that most of the terpenes in the sawdust were lost 

in the drying process. High levels of terpenes in the finished product increase the 

energy content. Leaver (n.d.) reported that the optimum moisture content (wb) of 

feedstock for pelleting agricultural products was 10 percent ± 1 percent. 

The available literature is in general agreement relative to the impact of source 

material (solid wood, bark, and foliage), dimensional characteristics of particle size, 

and moisture content on the pelleting process. Wilson (2010) reported that for six 

southern hardwoods and one softwood species there was no relationship between 

Klason lignin and pellet durability. Lehtikangas (2001), working with Norway 

spruce and Scots pine, noted that pellets made of bark and logging residues had 

higher Klason lignin values than pellets produced from sawdust. He also noted that 

lignin content increased with storage time of material prior to production. Pellets 

made from material with these characteristics had greater durability. The results of 

Wilson (2010) and Lehtikangas (2001) are somewhat contradictory, but the project 

by Lehtikangas included bark and storage time as additional factors not fully 

considered by Wilson. In the production process, the geometry of the die (die length/

diameter [L/D] ratio) and gap between the die and the pressure head both have an 

impact on pellet durability. 

Pellet Production Equipment

Production variables also affect the pelleting process. Wilson (2010) and Kaliyan 

and Morey (2006) noted that die configuration, die speed (referenced in the tangential direction for vertical machines), and the gap between the roller and die are 

also of concern. The openings in the die matrix are characterized by the ratio of 

the diameter of the holes to their length. Both sources noted that pellet durability 

Important settings on 

pelleting machines 

include die hole 

configuration, die 

speed, and the gap 

between presuure 

member and die.
14

GENERAL TECHNICAL REPORT PNW-GTR-845

increases as the length of the die holes increases in relation to diameter (L/D ratio). 

Wilson (2010) stated that this increase in durability is due to increased friction 

between the feedstock and the die and a resulting increase in temperature. He also 

reported that too large an L/D ratio results in blockage in the die. Wilson stated that 

softwood pellet producers commonly use deeper dies (longer L) than hardwood producers. With respect to the gap between the compression source and the die, both 

authors made reference to a report by Robohm and Apelt 4

 that found the optimum 

gap in feed stock production between the die and compression member to be 2.0 to 

2.5 mm (0.008 to 0.10 in). Both authors reported that at a gap thickness of 4.0 to 4.5 

mm (0.016 to 0.018 in) there was a significant degradation of pellet quality. Kayliyan 

and Morey (2006) stated, “The initial increase in pellet quality was due to a dense 

layer of material compressed through the die as a result of increased shear and 

prolonged pre-compression. A further increase in gap size resulted in a decreased 

stability of the feed mash on the edge of the roller and die because of sideways 

leaking of the feed mash.” 

In summary, a review of a considerable volume of literature failed to locate 

any specific reference to the production of pellets from western or eastern hemlock. 

The literature, however, provides considerable information relative to the pelleting 

process and possible explanations why any specific attempt to produce pellets from 

a specific species or species mix might succeed or fail.

Methods

The literature review located one reference (Chow and Pickles 1971) that concerned 

properties of western hemlock that would affect the pelleting process. The focus 

of the Chow and Pickles article, however, was thermal softening and degradation 

of the wood and bark, and no reference to pelleting was included. The literature 

review also failed to locate any reference to the spruces that grow in Alaska (Sitka 

spruce (Picea sitchensis (Bong.) Carr.), white spruce (Picea glauca (Moench) 

Voss), Lutz spruce (Picea X lutzii (Little)), or black spruce (Picea mariana (Mill.) 

B.S.P.). Given this fact, co-author Dan Parrent reviewed feasibility for conducting 

a preliminary project to produce test batches of pellets from the species of interest. 

4 We have attempted, without success, to locate and reference the original publication 

by Robohm and Apelt. Based on a check using Google Scholar on December 22, 2010, 

it was determined that the authors have been cited 37 times. A check of the various 

citations determined that in fact several articles have been referenced. It appears that the 

original 1974 publication was in a German Trade Magazine that has not been converted 

to electronic form. The authors wish to recognize Robohm and Apelt as the source of the 

information, but are unable to cite a specific published article.
15

Production of Wood Pellets From Alaska-Grown White Spruce and Hemlock

Alaska Pellet Company,5 a unit of Logging and Milling Associates in Delta Junction, Alaska, was contacted and agreed to make trial runs of western hemlock and 

white spruce singularly and in combination. 

Alaska Pellet Company has been in business for 2 years. The parent company 

purchased the small pellet mill as a means of converting dry planer shavings and an 

ever-increasing pile of slabs and edgings into a more valuable product. The ability 

to produce a valuable product from otherwise value-less raw material created an 

additional income stream and further diversified the existing operation. The pellet 

mill requires two to three workers when operating, and may fill work hours when 

circumstances (such as severe weather conditions) do not allow production elsewhere in the production chain. 

Twin Ports Testing of Superior, Wisconsin, a PFI-recognized testing agency, 

was selected to evaluate the physical and chemical properties of the final products 

in accordance with PFI standards. Arrangements were made and the trial production runs were conducted in late January 2010.

In the original study plan, it was agreed that Alaska Pellet Company personnel 

would use their established procedures and equipment to produce the test runs. The 

production runs would produce pellets from three formulations:

1. 100 percent Alaska hemlock. 

2. 67 percent Alaska hemlock and 33 percent white spruce.

3. 33 percent Alaska hemlock and 67 percent white spruce.

During the study, 100 percent white spruce pellets were also produced for 

comparative purposes. Trial production runs were made during the week January 

18 to 22, 2010. Mixtures of feedstock were made on a volumetric basis.

Raw Material

Before the study, Logging and Milling Associates had purchased hemlock logs harvested in the vicinity of Haines, Alaska, and trucked to Delta Junction. This material had been purchased to fill a special order for heavy timbers. The jacket boards, 

slabs, and edgings from this run were saved for the hemlock pelleting tests. Most of 

the bark on the hemlock logs was lost during harvesting and subsequent handling.

The white spruce logs for the tests were procured locally in the Delta Junction 

area. Slabs, edgings, and shim cuts from the spruce were used to produce pellet 

material. The spruce logs had not been debarked, and it was estimated that the bark 

component was 15 percent by volume. Alaska Pellet Company routinely produces 

5 The use of trade or firm names in this publication is for reader information and does not 

imply endorsement buy the U.S. Department of Agriculture of any company, product or 

service.
16

GENERAL TECHNICAL REPORT PNW-GTR-845

pellets from a mixture of barky slabs, planer shavings, and Pendu shavings with 

an estimated moisture content of 12 to 15 percent dry basis (db) (10.7 to 13.0 

percent wb). It was estimated that shavings were 10 percent by volume of the 

spruce mixture used as raw material. 

Material Preparation

Hemlock feedstock consisted of hemlock slab wood, essentially free of bark, kiln 

dried approximately 2 weeks prior to testing to approximately 5 percent moisture 

content (db). In this instance, moisture content was determined by using a lumber 

moisture meter. After moisture content testing, the material was left uncovered 

outdoors in subzero (°F) temperatures. Moisture content at the time of testing was 

9.24 percent (wb) (10.18 percent db), as later determined using laboratory drying 

methods. 

Spruce feedstock consisted of two mixtures. When combined with hemlock, 

spruce feedstock consisted of spruce slabwood, edgings, and shim cuts, including 

bark (estimated to make up about 15 percent of the spruce feedstock volume). Slabs 

were removed from the dry kiln the first morning of the test, and moisture content 

ranged from 4.8 to 5.1 percent (db), as determined by a lumber moisture meter. In 

the test of pure spruce, the slabwood was combined with shavings from a Pendu 

resaw amounting to approximately 10 percent by volume. The moisture content of 

the Pendu shavings ranged from 12 to 15 percent (db). 

Slabs were ground in a Rotochopper Model MC266 horizontal grinder, and 

further reduced in a hammermill with a 0.190-inch screen. Temperature of the feedstock just prior to pelletizing was not recorded; however, ambient air temperature 

was approximately -15 to -20 oF (-26 to -28 oC).

Pelleting Equipment

Raw material furnish was converted to pellets by using a California Pellet Mill 

(CPM) Century 100 pellet machine, equipped with a 100 hp electric motor. The ring 

die is 16 inches in diameter and 2.75 inches thick. Hole diameter is 0.25 inches with 

an effective zone of 1.75 inches and a 1.00 inch taper zone. The gap between the 

die and the rollers was set at 1 mm (0.0039 inch). The tangential rotational speed of 

the compression member was calculated at 13.96 feet per second. As noted above, 

the outdoor air temperature during the tests was approximately -15 to -20 °F. The 

mill facilities were unheated, although heat produced by the equipment kept temperatures in the mill reasonably comfortable. During the off hours, heat lamps were 

used to keep the temperature of the die above freezing.

Pellets produced in the pellet machine are deposited onto an inclined belt 

conveyor, air cooled with fans, dumped into a trough, augered to a bucket conveyor, 
17

Production of Wood Pellets From Alaska-Grown White Spruce and Hemlock

and conveyed into a small storage hopper. Cooled pellets are dispensed directly 

from the hopper, metered, and bagged in plastic as 40-lb units with the Alaska Pellet Company label. The pellets were not screened to remove fines prior to bagging. 

At this time, the local market currently consumes all pellet products produced by 

Alaska Pellet Company. To date, none of the local consumers have required PFI 

certification 

A sample of pellets produced from each species mix was loaded into a 0.5-ft3

U.S. Postal Service “priority mail” flat rate box, lined with a plastic bag. After 

being filled, the plastic bag was closed to allow minimum interaction with outside 

moisture. The samples were shipped from North Pole, Alaska, to Twin Ports Testing, Inc., in Superior, Wisconsin, for evaluation. Twin Ports Testing, Inc., is listed 

on the PFI Web site as a certified testing facility.

Table 5—Chemical properties and energy values for Alaska hemlock and spruce pellets as reported by 

Twin Ports Testing, Inc., Superior, Wisconsin

Batch 1 Batch 2 Batch 3 Batch 4

Property (100% WS) (67% WS; 33% AH) (33% WS; 67% AH) (100% AH)

Percent

Moisture content of sample 7.84 5.39 6.83 5.79

Ash:

Moisture free 0.76 0.82 0.55 0.35

As received MC 0.70 0.78 0.51 0.33

Sulfur:

Moisture free 0.008 0.010 0.009 0.011

As received MC 0.008 0.009 0.008 0.010

Gigajoules per tonne

Net calories at constant pressure:

Moisture free 18.69 18.51 18.60 18.28

As received MC 17.03 17.38 17.17 17.08

Joules per gram

Net calories at constant pressure:

Moisture free 18,690 18,514 18,605 18,277

As received MC 17,033 17,384 17,168 17,078

Gross calories at constant volume:

Moisture free 19,968 19,779 19,875 19,551

As received MC 18,402 18,712 18,516 18,419

British thermal units per pound

Gross calories at constant volume:

Moisture free 8,585 8,504 8,545 8,406

As received MC 7,912 8,045 7,961 7,919

PFI grade Non-standard Non-standard Non-standard Non-standard

Limiting factor(s) Fines Fines Fines Fines

MC = moisture content, WS = white spruce, and AH = Alaska hemlock.

Pellets were not 

screened to remove 

fines prior to bagging.
18

GENERAL TECHNICAL REPORT PNW-GTR-845

Table 6—Chemical elements analysis for Alaska hemlock and spruce pellets as reported by Twin 

Ports Testing, Inc., Superior, Wisconsin

Batch 1 Batch 2 Batch 3 Batch 4

Property (100% WS) (67% WS; 33% AH) (33% WS; 67% AH) (100% AH)

Percent

Tested moisture content 7.84 5.39 6.83 5.79

Carbon:

Moisture free 51.64 51.31 51.48 51.67

As received MC 47.48 48.45 47.89 48.59

Hydrogen at tested MC:

Moisture free 5.87 5.81 5.83 5.85

As received MC 5.40 5.49 5.42 5.50

Nitrogen at tested MC:

Moisture free 0.09 0.09 0.09 0.07

As received MC 0.08 0.08 0.08 0.07

Oxygen at tested MC:

Moisture free 41.63 41.96 42.04 42.05

As received MC 38.49 39.80 39.26 39.71

Total at tested MC:

Moisture free 99.23 99.17 99.44 99.64

As received MC 91.45 93.82 92.65 93.87

Parts per million

Chlorine as received:

Moisture free 34 28 11 <10

As received MC 31 26 10 <10

MC = moisture content, WS = white spruce, and AH = Alaska hemlock.

Table 7—Physical properties analysis of Alaska hemlock and spruce pellets as reported by Twin Ports 

Testing, Inc., Superior, Wisconsin

Batch 1 Batch 2 Batch 3 Batch 4

Property (100% WS) (67% WS; 33% AH) (33% WS; 67% AH) (100% AH)

Pounds per cubic foot

Bulk density 44.29 47.41 46.34 47.21

Percent

Sample longer than 1.5 inches 0 0 0 0

Inches

Maximum length 0.911 1.364 0.964 1.638

Minimum diameter 0.249 0.248 0.247 0.246

Maximum diameter 0.257 0.250 0.249 0.248

Average diameter 0.254 0.249 0.248 0.247

Percent

Fines (by weight) 0.74 0.65 0.55 0.78

Durability index 93.6 96.5 96.7 96.8

WS = white spruce and WH = Alaska hemlock.
19

Production of Wood Pellets From Alaska-Grown White Spruce and Hemlock

Testing and Evaluation

Testing was completed at Twin Ports on February 3, 2010. A summary of chemical 

properties and energy values are presented in table 5. A complete report of basic 

chemical elements is included in table 6. Table 7 reports on the physical properties of 

the samples, including the durability test results.

Discussion of Results

The initial tests resulted in successful formation of pellets, regardless of the mix 

of raw material. However, a discussion of the results of this experiment is complicated by the fact that changes to the June 18, 2008, PFI grading specifications were 

approved by the PFI Board on October 7, 2010. Prior to those changes, none of the 

mixtures produced pellets that could be assigned a PFI grade. But subsequent to those 

changes, most of the specifications for “premium grade” were met. It should also be 

noted that the “super premium” grade has been dropped from the new standard.

Bulk Density

The bulk density specification was not changed. For premium grade, a bulk density 

value between 40.0 and 46.0 lbs/ft3

 is required. For standard and utility grades, the 

bulk density value is required to be within the range of 38.0 to 46.0 pounds lbs/

ft3

. All samples containing hemlock exceeded 46.0 lbs/ft 3, and therefore could not 

qualify for any PFI grade because the value falls outside the specified limits. Pure 

spruce had a bulk density value of 44.29, and meets the premium grade specification. 

Bulk density is an important fuel characteristic, as it affects the fuel-to-air ratio in, 

and the efficiency of, the pellet-burning appliance. Appliance manufacturers have 

determined that bulk density values above 46 lbs/ft3 can detrimentally affect the 

operation of such appliances. Bulk density is controlled by the “effective length” of 

the pellet die. A shorter effective length will reduce the bulk density (Wiberg 2010). 

Diameter

The old specification for all grades called for diameters to be in the range of 0.250 to 

0.285 inch. The new specification calls for diameters to fall within the range of 0.230 

to 0.285 inch across all grades. Samples containing hemlock had diameter values of 

0.246 to 0.248 inch, 0.247 to 0.249 inch, and 0.248 to 0.250 inch. Pure spruce had a 

diameter range of 0.249 to 0.257 inch. Under the old specification, pellets containing 

hemlock could not earn any PFI grade. However, under the new specification, all 

pellets met the premium grade specification.

The bulk density 

of hemlock pellets 

exceeded allowable 

levels.
20

GENERAL TECHNICAL REPORT PNW-GTR-845

Pellet Durability Index (PDI)

For premium grade, the old specification called for a pellet durability index (PDI) 

value of at least 97.5, whereas the new specifications call for a PDI value of at least 

96.5. For standard and utility grades, the PDI must be at least 95.0 (unchanged). The 

PDI values for samples containing hemlock ranged from 96.5 to 96.8, which did not 

meet the old premium grade specification (but did meet standard and utility specification). They would, however, meet the new premium grade specification. 

Fines

Under the old specifications, “fines” is defined as “the percentage of fuel material 

in the fuel sample passing through a 1/8-inch screen when the fuel is sampled after 

completion of production and bagging and before transportation, unloading, distribution, use, etc.” (PFI 2008b). Across all grades, the specification for fines was not 

more than 0.5 percent by weight.

Under the new specifications, “fines” is defined as “the percentage of fuel material in the fuel sample passing through a 1/8-inch screen when the fuel is sampled 

in accordance with the requirements in 6.1.4.” (PFI 2010c). The specification for 

premium grade calls for a maximum of 0.5 percent by weight (unchanged), and 

the specification for standard and utility calls for fines not to exceed 1.0 percent by 

weight.

It is worth noting that the old specification called for a fines determination, 

essentially “at the mill gate,” whereas the new specification calls for a measurement 

“as delivered.” 

Fines measurements in this experiment were all determined after transportation 

and delivery to the testing facility. An accurate determination “at the mill gate” 

was not made; therefore, no conclusions regarding conformance with the old 

standard can be made. The results for fines in all samples ranged from 0.55 to 

0.78 as delivered. None of these measurements would meet the specification for 

premium grade; however, all would meet the specification for standard and utility 

grade. At the time of the tests, Alaska Pellet Company did not have any means to 

remove fines between the pellet storage bin and the bagging machine. It is likely 

that additional fines could be removed with the installation of an additional screen 

for that purpose.

Inorganic Ash

The inorganic ash specification was not changed from 2008 to 2010. The specification for premium grade calls for the inorganic ash component not to exceed 1 

percent. All samples met the specification.
21

Production of Wood Pellets From Alaska-Grown White Spruce and Hemlock

Length

The length specification was not changed from 2008 to 2010. The specification for 

premium grade calls for the percentage of pellets longer than 1.50 inches not to 

exceed 1.0 percent. All samples met the specification.

Moisture

The specification for premium grade was unchanged at 8.0 percent (wb) or less, 

which all samples met. The specification for standard grade was changed from 

8.0 percent or less to 10.0 percent or less. The specification for utility grade was 

unchanged at 10.0 percent or less.

Chloride

The specification for chloride across all grades remained unchanged at 300 ppm or 

less. All samples easily met this specification with values ranging from less than 10 

to 31 ppm.

Recommendations and Conclusions

In this project, the personnel at Alaska Pellet Company applied the knowledge 

gained from 2 years of experience producing pellets from white spruce to a new 

species, Alaska hemlock. Processing procedures and equipment parameters were 

not changed or modified for the trial runs of the new species. Regardless, the process produced pellets that differed little in visual appearance from those made with 

the traditional white spruce material. The results of product testing in accordance 

with PFI standards, however, indicate that the process needs to be fine-tuned to 

create an Alaska hemlock pellet that fully meets PFI specification. 

It is noted that the Alaska Pellet Company had never submitted any samples for 

laboratory testing and was not aware that pellets were not meeting PFI standards. 

Furthermore, by relying on results derived from trial and error, the company was 

unaware of the potential deficiencies caused by particle size being too big or inconsistent, cold feedstock temperatures, or excessively dry feedstock.

Subsequent to these tests, the lower screen on the hammermill was reduced 

from one with a 0.190-inch mesh to one with a 0.109-inch mesh. It is reported that 

this has resulted in smaller furnish particle size and a notably visible increase in 

sheen and apparent increase in pellet hardness. 

Slight modifications to 

material preparation 

and adjustment to the 

pellet die should result 

in hemlock pellets 

that exceed Pellet Fuel 

Institute standards.
22

GENERAL TECHNICAL REPORT PNW-GTR-845

Acknowledgments

This project has been conducted in accordance with Joint Venture Agreement 

11261935-069 between the USDA Forest Service Alaska Wood Utilization Research 

and Development Center and the Juneau Economic Development Council. The 

assistance of Alaska Pellet Company and its parent company, Logging and Milling 

Associates of Delta Junction, Alaska, are appreciated.

Metric Equivalents

When you know: Multiply by: To find:

Inches (in) 25.4 Millimeters (mm)

Inches 2.54 Centimeters (cm)

Cubic feet (ft3) .0283 Cubic meters (m3)

Pounds (lb) 454 Grams (g)

Pounds per cubic foot (lbs/ft3

) 16.02 Kilograms per cubic meter

Tons .907 Metric tons or Megagrams

Degrees Fahrenheit .56(°F – 32) Degrees Celsius (°C)

Btu/lb 0.00064568 Kilowatt hours per kilogram 

 (kWh/kg)

British thermal units (BTU) 1,050 Joules (J)

Literature Cited

Alakangas, E. 2009. Current status of European pellets standardization procedure: 

EN 145961-1 and EN 14961-2. [PowerPoint presentation] Pellets@LAS Final 

Workshop. Brussels, Belgium. http://www.pelletsatlas.info/cms/site.aspx?p=9062. 

(May 13, 2010).

Alakangas, E. 2010. EUBIONET III—New European pellet standard. EN 14961-1. 

Jyvaskyla, Finland. 9 p. http://www.wsed.at/fileadmin/redakteure/WSED/2010/

download_presentations/Alakangas_paper_neu.pdf. (May 6, 2011).

American Lumber Standard Committee, Inc. [ALSC]. 2010. PFI residential/

commercial densified fuel enforcement regulations. October 25, 2010. 

Germantown, MD. http://pelletheat.org/wp-content/uploads/2010/01/ALSCdensified-wood-pellet-fuels-enforcement-regulations-10-25-10.pdf. (October 

26, 2010).
23

Production of Wood Pellets From Alaska-Grown White Spruce and Hemlock

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