Pellet Fuels Institute Standard Specification for Residential/Commercial Densified 

Fuel June 1, 2011 

 Page 1 of 10 

Pellet Fuel Institute (PFI) Standard Specification for Residential/Commercial Densified Fuel 

1. Scope 

1.1 This specification is applicable for the determination of fuel quality grade for Residential 

or Commercial Densified Fuel as shown in Table 1. 

1.2 Normative fuel properties included in the specification are fines, bulk density, diameter, 

length, heating value, chloride, moisture content, pellet durability index and inorganic 

ash content. Determination of these properties is mandatory for determining fuel quality 

grade. Ash fusion is the only informative fuel property. Determination of ash fusion is 

not mandatory for determining fuel quality grade. 

1.3 This specification is for the use of densified fuel producers to establish grade 

requirements for North American Residential/Commercial densified fuel. It is also for 

the use of pellet fuel appliance manufacturers for the purpose of designing appliances 

that meet air emission regulations that cite this standard specification and for users of 

residential/commercial densified fuel in selection of the grade most suitable to their 

appliance. 

1.4 Commercial users include commercial facilities that utilize densified fuel burning 

appliances or equipment that have the same fuel requirements as residential appliances. 

Commercial applications should not be confused with industrial applications, which can 

utilize a much wider array of materials and have vastly different fuel requirements. 

1.5 The values stated in inch-pound units are to be regarded as the standard. Any values 

given in parentheses are mathematical conversions to the International System of Units 

(SI units), which are provided for information only and are not considered standard. If 

values are stated in SI units only, they are to be regarded as the standard. 

1.6 This standard specification does not purport to address all of the safety concerns, if any, 

associated with its use. It is the responsibility of the user of this standard specification to 

establish appropriate safety and health practices and to determine the applicability of 

regulatory limitations prior to use. 

2. Referenced Documents 

2.1 Pellet Fuels Institute (PFI) Program Standards: 

2.1.1 PFI Residential/Commercial Densified Fuel QA/QC Handbook 

2.2 ASTM Standards1

: 

2.2.1 ASTM E 873 Standard Test Method for Bulk Density of Densified Particulate 

 Biomass Fuels.

2.2.2 ASTM E 871 Standard Test Method for Moisture Analysis of Particulate Wood Fuels 

2.2.3 ASTM D 1102 Standard Test Method for Ash in Wood 

2.2.4 ASTM E 791 Standard Test Method for Calculating Refuse-Derived Fuel Analysis 

 Data from As-Determined to Different Bases 

2.2.5 ASTM E 776 Standard Test Method for Forms of Chlorine in Refuse-Derived Fuel 

2.2.6 ASTM D 4208 Standard Test Method for Total Chlorine in Coal by the Oxygen Bomb 

 Combustion/Ion Selective Electrode Method 

1. For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org.
Pellet Fuels Institute Standard Specification for Residential/Commercial Densified 

Fuel June 1, 2011 

 Page 2 of 10 

2.2.7 ASTM D 6721 Standard Test Method for Determination of Chlorine in Coal by 

 Oxidative Hydrolysis Microcoulometry 

2.2.8 ASTM E 711 Standard Test Method for Gross Calorific Value of Refuse-Derived 

 Fuel by the Bomb Calorimeter 

2.2.9 ASTM E 29 Standard Practice for Using Significant Digits in Test Data to Determine 

 Conformance with Specifications

2.2.10 ASTM C 702 Standard Practice for Reducing Samples of Aggregate to Testing Size

2.2.11 ASTM D 1857 Standard Test Method for Fusibility of Coal and Coke Ash 

2.2.12 IEEE/ASTM SI10-2002 Standard for the Use of the International System of Units 

 (SI): The Modern Metric System 

2.3 Other Referenced Documents: 

2.3.1 Accreditation Body Residential/Commercial Densified Fuel Enforcement Regulations

2.3.2 Kansas State University - Mechanical Durability of Feed Pellets, Call Number: 

 LD2668 .T4 1962 Y68 

3. Terminology 

3.1 Definitions: General 

3.1.1 bulk density – the fuel mass per cubic foot of the fuel sample 

3.1.2 diameter – the average diameter of the fuel pellets in the fuel sample. 

3.1.3 Pellet Durability Index (PDI) – a standardized parameter for specifying the ability of 

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

3.1.4 fines – 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 8.1.4. 

3.1.5 inorganic ash – the percent inorganic material in the fuel sample. 

3.1.6 length – the weight percent of pellets exceeding 1.5 inches in length in the fuel 

sample. 

3.1.7 Moisture – the moisture content of the as-received fuel sample. 

3.1.8 heating value – The higher heating value of the fuel sample.

3.1.9 NIST - The National Institute of Standards and Technology (NIST) is a federal 

technology agency that develops and promotes measurement, standards, and 

technology. 
Pellet Fuels Institute Standard Specification for Residential/Commercial Densified 

Fuel June 1, 2011 

 Page 3 of 10 

TABLE 1 PFI Fuel Grade Requirements 

Table 1 Notes: 

1. The following applies to all limits in this table: For purposes of determining the fuel grade, 

all properties must fall at or within the specified limits listed for a particular grade. Observed or 

calculated values obtained from analysis shall be rounded to the nearest unit in the last righthand place of the figures used in expressing the limit in accordance with ASTM E 29 Standard 

Practice for Using Significant Digits in Test Data to Determine Conformance with 

Specifications. 

2. It is the intent of these fuel grade requirements that failure to meet any fuel property 

requirement of a given grade does not automatically place a fuel in the next lower grade unless it 

meets all requirements of the lower grade. 

3. There are no specific grade requirements for heating value, however densified fuel 

manufacturers must provide a minimum higher heating value guarantee on quality marked 

product when these standard specifications are used in conjunction with the PFI 

Residential/Commercial Densified Fuel QA/QC Handbook and the Accreditation Body 

Residential/Commercial Densified Fuel Enforcement Regulations. Quality marked product must 

also disclose the type of materials used as well as any additives as specified in section 8 of the 

PFI Residential/Commercial Densified Fuel QA/QC Handbook. Quality marked product must 

also exclude chemically treated materials as specified in section 4.5 of the PFI 

Residential/Commercial Densified Fuel QA/QC Handbook.4. NA – Not Applicable; No value 

has been established. 
Pellet Fuels Institute Standard Specification for Residential/Commercial Densified 

Fuel June 1, 2011 

 Page 4 of 10 

4. Detailed Requirements 

4.1 The various grades of densified fuel shall conform to the limiting requirements shown in 

Table 1. 

5. Equipment and Supplies 

5.1 Caliper – A vernier caliper capable of measuring fuel diameter and length to within 

0.001 in. (0.025 mm). Must meet the calibration requirements specified in 9.1. 

5.2 Analytical Balance – A balance with a resolution of 0.1 grams or better. Must meet the 

calibration requirements specified in 9.2. 

5.3 Measuring Block – A 1.5 inch (38 mm) long gauge block used for screening fuel pieces 

for length. Must meet the requirements specified in 9.3. 

5.4 Scale – A scale with resolution of 0.05 kilograms or better. Must meet the calibration 

requirements specified in 9.2. 

5.5 Balance – A balance with a resolution of 0.01 kilograms or better. Must meet the 

calibration requirements specified in 9.2 

 

6. Sampling and Sample Handling 

6.1 The reader is strongly advised to review all intended test methods and sampling 

requirements prior to sampling in order to understand the importance and effects of 

sampling technique and special handling required for each method. 

7. Sample Preparation 

7.1 A sample preparation schematic is shown in Annex B.1 to illustrate how a 40 lb bag of 

pelletized material should be subdivided to perform the analysis procedures. All sample 

subdividing shall be conducted utilizing a sample splitter with a slot width of 3.5 inches 

(89 mm) and meeting the requirements specified in ASTM C 702. 

8. Test Methods 

8.1 The requirements enumerated in this specification shall be determined in accordance 

with the referenced ASTM test methods or other referenced methods except where 

modifications are noted or in accordance with the test procedures specified.

8.1.1 Bulk Density – Determine in accordance with ASTM E 873 except this method shall 

be revised to utilize a 1/4 cubic foot container that is tapped 25 times from 1 inch. In 

order to insure that an adequate sample quantity is available for this revised method, a 

minimum sample size of 12 pounds (5.44 kilograms) is recommended. 

8.1.2 Diameter - Select 5 pellets randomly out of the pellet sample being evaluated and 

measure the diameter of each pellet with the caliper specified in 5.1. Each measured 

pellet diameter shall be recorded to the nearest 0.001 inch. The average pellet 

diameter as well as the range of all pellet diameters measured shall be calculated and 

reported to the nearest 0.001 inch.

8.1.3 Pellet Durability Index (PDI) – Pellet durability shall be determined by using the 

method specified in Annex A.1. It should be noted that the pellets remaining after 

performing the fines determination as specified in 8.1.4 can be used without further 

preparation to conduct the durability test. 
Pellet Fuels Institute Standard Specification for Residential/Commercial Densified 

Fuel June 1, 2011 

 Page 5 of 10 

8.1.4 Fines – Determined using the following procedure that incorporates the use of a 1/8-

inch (3.18 mm) wire screen sieve. All weight measurements shall be recorded to the 

nearest 0.1 gram. 

8.1.4.1 Secure a representative fuel sample. 

8.1.4.2 Reduce the sample size down to a minimum of 2.5 pounds (1,134 grams) using a 

sample splitter with 3.5-inch (89 mm) slots. Larger sample sizes may be used. 

8.1.4.3 Using the analytical balance specified in 5.2, weigh the sample and record as the 

initial sample weight to the nearest 0.1 grams. 

8.1.4.4 Weigh the receiving pan and record the weight to the nearest 0.1 grams. 

8.1.4.5 Attach a 1/8-inch (3.18 mm) screen to the receiving pan and place the pellet 

sample on the screen using care not to overload the screen. The maximum load 

on the screen should not exceed 1 pound (453 grams) of pellets per 100 square 

inches (645 square centimeters) of screen surface area. Smaller screens may 

require the sample to be screened in increments. 

8.1.4.6 Screen the sample by tilting the screen side to side 10 times. 

8.1.4.7 If the sample is being screened in increments, after the first portion has been 

screened remove the 1/8-inch (3.18 mm) screen from the base pan, and empty the 

pellets off the screen. 

8.1.4.8 Repeat 8.1.4.5 through 8.1.4.7 until the entire sample has been screened. 

8.1.4.9 Remove the 1/8-inch (3.18 mm) screen and weigh and record the weight of the 

base pan with the fines to the nearest 0.1 grams. 

8.1.4.10 Calculate and report the percent of fines to the nearest 0.01% as follows: 

% Fines = [(Weight of Base Pan + Fines) – (Weight of Base Pan)] x 100 

 Initial Sample Weight 

8.1.5 An alternative fines determination procedure is provided in Annex C.1.

8.1.6 Inorganic Ash – Determine in accordance with ASTM D 1102. 

8.1.7 Length - Starting with 2.5 pounds (1.13 kilograms) of pellets randomly selected from 

the sample being evaluated, hand sort to identify pellets over 1.50 inches (38 mm) in 

length. Use the caliper specified in 5.1 or a certified measuring block as specified in 

5.3 to confirm that a pellet exceeds the specified length. The weight percent of all 

pellets exceeding the specified length shall be reported. In addition, of the pellets 

exceeding the specified length, the longest pellet shall be identified, measured with 

the caliper specified in 5.1, and the length reported as the maximum pellet length. 

8.1.8 Moisture – Determine in accordance with ASTM E 871. 

8.1.9 Chloride – Determine in accordance with ASTM E 776 or ASTM D 4208 or ASTM 

D 6721.

8.1.10 Ash Fusion - Determine in accordance with ASTM D1857. 

8.1.11 Heating Value – Determine in accordance with ASTM E 711. 

9. Calibration and Standardization 

9.1 Caliper – Before each test, audit the caliper by measuring one NIST traceable gauge 

block that corresponds to between 80 percent and 120 percent of the anticipated fuel 

diameter and by measuring the length of the measuring block specified in 5.3. If the 
Pellet Fuels Institute Standard Specification for Residential/Commercial Densified 

Fuel June 1, 2011 

 Page 6 of 10 

caliper can not reproduce the calibration gauge dimensions within 1%, the caliper may 

not be used. 

9.2 Balances and Scales - Perform an initial calibration using NIST or other recognized 

national standards in accordance with the manufacturer’s instructions. Before each test, 

verify the balance or scale by using at least one calibration weight that corresponds to 50 

to 150 percent of the weight of the fuel sample to be measured. If the balance or scale 

cannot reproduce the value of the calibration weight to within 1% of the mass, 

recalibrate before use. 

9.3 Measuring Block – The length of the block must be traceable to NIST and demonstrate 

accuracy of ±0.01 in. from length specified in 5.3. The block manufacturer’s certification 

documents are sufficient for this purpose. 
Pellet Fuels Institute Standard Specification for Residential/Commercial Densified 

Fuel June 1, 2011 

 Page 7 of 10 

Annex A.1 

Pellet Fuels Institute 

Standard Operating Procedure for: 

Durability Testing – Residential/Commercial Pellet Fuels

Produced by: PFI Standards Committee 

The Pellet Fuels Institute has adopted the test procedure outlined by Kansas State University 

(See Kansas State University - Mechanical Durability of Feed Pellets, Call Number: LD2668 

.T4 1962 Y68) for assessing the durability of residential/commercial densified fuel products, 

with the exception that the screen size used in determining durability has been modified to be a 

1/8-inch (3.17 mm) wire screen sieve. 

The durability tester consists of a dust tight box. The box shall be made of a rigid material with 

smooth and flat surfaces (e.g., stainless steel plate). The inner dimensions of the box shall be 

305 ± 3 mm long by 140 ± 3 mm wide by 305 ± 3 mm deep. It shall rotate about an axis which 

is perpendicular to and centered in the nominal 305 mm by 305 mm sides. A 230 ±3 mm long 

baffle is affixed symmetrically to a diagonal of one nominal 305 mm by 305 mm side of the box. 

The baffle extends 50 ± 1 mm into the box and is securely fastened to the back of the box. The 

edges of the baffle shall not be sharp, but rounded to avoid any cutting effect. A door may be 

placed on any side of the box. Projections, such as rivets and screws, shall be kept to a minimum 

and well rounded (alternatively, flat head screws may be used). An illustration of the tester is 

shown in Fig 1. 

The procedure is outlined as follows: 

1. Secure a representative pellet fuel sample. The minimum recommended representative 

sample weight is 1100 grams. 

2. Screen the representative sample with a 1/8-inch (3.18 mm) wire screen sieve to remove 

fines. Obtain the initial test sample from the portion of the representative sample retained 

on the screen. 

3. Using the analytical balance specified in 5.2, weigh 500 ± 10 grams of the initial test 

sample and record as the initial weight to the nearest 0.1 grams (IW). 

4. Tumble the pre-weighed initial test sample in the durability tester at 50 ± 2 rotations per 

minute for a total of 500 rotations. 

5. Re-screen the tumbled test sample weigh the portion retained on the 1/8-inch (3.18 mm) 

sieve and record as the whole pellet sample weight to the nearest 0.1 grams (WPW). 
Pellet Fuels Institute Standard Specification for Residential/Commercial Densified 

Fuel June 1, 2011 

 Page 8 of 10 

6. Compute Pellet Durability Index (PDI) by dividing the whole pellet sample weight 

(WPW) by the initial weight (IW) of the sample and multiply by 100. Report to one 

decimal place. 

PDI = 100 X WPW

 IW 

7. Repeat steps 2-6 on a second portion of the representative fuel sample. 
Pellet Fuels Institute Standard Specification for Residential/Commercial Densified 

Fuel June 1, 2011 

 Page 9 of 10 
Pellet Fuels Institute Standard Specification for Residential/Commercial Densified 

Fuel June 1, 2011 

 Page 10 of 10 

Annex C.1 

Pellet Fuels Institute 

Alternative Fines Testing Procedure 

Produced by: PFI Standards Committee 

As part of this standard specification PFI has developed a standard testing procedure for the 

determination of fines in a sample of densified fuel. The standard testing procedure is outlined 

in Section 6.1.4. The standard test procedure calls for a sub-sample to be taken from a 40 lb bag 

of pellets by means of a sample splitting device. This alternative testing procedure is offered as a 

comparable alternative to the standard testing procedure and is intended for use when it is 

desired to determine the percentage of fines in a 40 pound bag of densified fuel. 

Using a Lucas Welding and Fabrication (270-579-2114) Pellet Speed ScreenerTM (apparatus 

consists of a screener, a flanged cubic foot steel container, and a fines collection cup), the 

following steps shall be followed to determine the percent fines in a 40 pound bag of densified 

fuel: 

1. Secure a 40 pound bag of densified fuel. 

2. Using the scale specified in 5.4, weigh the sample with the bag and record the initial 

weight to the nearest 0.05 kilograms (IW). 

3. Pre-weigh the fines collection cup and record the weight to the nearest 0.01 kilograms 

(CW). 

4. Assure the screener is clean, the flanged 1 cubic foot steel container is in place at the 

bottom of the screener, and the fines collection cup is in its holder. 

5. Turn on the table vibrator. 

6. Pour the contents of the 40 pound bag of densified fuel into the hopper at the top of the 

table assuring all material is out of the bag. 

7. Weigh the bag and record to the nearest 0.01 kilograms (BW). 

8. When the hopper is empty and all material has migrated to either the flanged 1 cubic foot 

steel container or the fines collection cup, turn off the table vibrator. 

9. Weigh the fines collection cup with the fines and record the weight to the nearest 0.01 

kilograms (CFW). 

10. Calculate the percentage of fines in the sample. 

% Fines = (CFW – CW) / (IW – BW) x 100