Procedures for estimating pile volume, biomass, and emissions  (Back to help)

The Piled Fuels Biomass and Emissions Calculator implements the calculation methods of Hardy (1996) as implemented in CONSUME 3.0 for estimating volume, biomass, and emissions for machine-constructed piles and Wright et al. (2010) for hand-constructed piles. The steps necessary to estimate emissions from pile burning differ for machine and hand piles and are presented separately in the following sections.

Machine Piled Fuels (adapted from Hardy (1996) and the CONSUME 3.0 User’s Guide)

CONSUME 3.0 uses a model developed by Hardy (1996) to estimate piled fuel biomass and to calculate consumption and emissions from pile burns. Unlike other fuel types, consumption of piles is not directly dependent upon fuel particle size. Six steps are required to estimate emissions from a machine-constructed pile or piles of the same shape, size, and composition. The product from each step is both relevant in itself and a prerequisite parameter for completion of the next step. Determine:

• Total gross volume of the pile
• Net volume of the woody biomass
• Density or weighted-average density of wood
• Consumable (oven-dry) mass of wood
• Percentage of mass consumed
• Mass of emissions produced

Volume, biomass, consumption, and emissions for multiple piles of the same shape, size and type are calculated as the amount for a single pile multiplied by the number of piles.

1. Total Gross Volume of the Pile
The volume of a pile is dependent upon its shape. Piles are categorized into one of seven generalized shapes as shown in Figure 1.

Figure 1 . Geometric pile shapes and required dimensions. Figure 4 from Wright et al. (2010).

The volume for each shape is calculated using various lengths, heights and/or widths. The formulas for each shape are in Table 1.

Table 1. Volume formulas for geometric shapes.

Pile Volume Adjustment – Some piles may contain a significant percentage of soil within the pile or mounded beneath the pile. Reduce the gross pile volume using an estimate of the percentage of the pile occupied by soil (Formula 1).

(1) Corrected Pile Volume (ft³ or m³) = Gross Pile Volume (ft³ or m³) × (100 - % Soil)

2. Net Volume of the Woody Biomass

Air comprises much of the gross volume of a pile. The ratio of wood volume to the total pile volume is the packing ratio. From destructive measurements of 17 piles, Hardy (1996) found that packing ratio ranges from 0.06 to 0.26. Consume uses the following three default packing ratios:

• Piles with species content dominated by long-needled pines and/or broadleaf deciduous litter. Mean diameters of large woody fuels < 10 inches (25 cm). Packing ratio = 0.1.
• Piles dominated by short-needled conifers. Mean diameters of large woody fuels < 10 inches (25 cm). Packing ratio = 0.2.
• Highly compacted, clean piles with large logs (diameters > 10 inches (25 cm)), especially those built with a crane or loader. Packing ratio = 0.25.

The net wood volume is the gross pile volume multiplied by the appropriate packing ratio (Formula 2).

(2) Net Wood Volume (ft³ or m³) = Gross Pile Volume (ft³ or m³) × Packing Ratio (proportion).

3. Density or Weighted-Average Density of Wood

The oven-dry density of wood is used to calculate mass of wood for fuel loading, fuel consumption, and smoke production. If a pile is composed of more than one species, the calculator will determine a weighted average oven-dry wood density for the piled material to be used to calculate the consumable mass of wood in step four. In the case of a pile or piles of mixed species, select the two most common species based on the composition of the pile(s) and note the proportion of each species in the pile(s).

The tree species wood densities available are listed in the Appendix. Wood density is calculated from specific gravity values found in Miles and Smith (2009), which were compiled from the Wood Handbook (USFS 1999) assuming an ovendry weight and wood volume at 12% moisture content (Formula 3).

(3a) Wood Density (lb/ft³) = 62.4 × Specific Gravity × (1 + 12/100) (formula 3–6b in USFS 1999)

(3b) Wood Density (kg/m³) = 1000 × Specific Gravity × (1 + 12/100) (formula 3–6a in USFS 1999)

4. Consumable (Oven-Dry) Mass of Wood

The consumable mass of wood in a pile is the net wood volume multiplied by the wood density or weighted average wood density (Formula 3).

(3) Mass of Wood (lb or kg) = Net Wood Volume (ft³ or m³) × Wood Density (lb/ft³ or kg/m³)

Multiply consumable mass of wood in pounds or kilograms by 2000 or 1000, respectively to determine mass in tons or megagrams.

5. Percentage of Mass Consumed

The amount of woody mass consumed when piles are burned typically ranges from 75 to 95 percent (Hardy 1996). Several western states have smoke management-reporting programs that recommend values of 85 or 90 percent. CONSUME 3.0 assumes the percentage of mass consumed is 90 percent. Experience and expert knowledge must be used to determine the most appropriate value for percentage consumption.

6. Mass of Emissions Produced

The mass of emissions produced by a fire is calculated by multiplying the mass of fuel consumed by an appropriate emission factor for the emission of interest. Emission factors differ with the combustion efficiency of the fire. Cleaner piles burn more efficiently than dirty piles and produce less of the products of incomplete combustion, of which particulate matter is a major emission species. Expert judgment as well as agency policy must be considered when assessing the cleanliness of a pile or piles and applying the appropriate combustion efficiency.

The rate of smoke emissions produced also varies with the combustion phase of a fir. Less smoke is produced per dry mass of fuel consumed during the more efficient flaming stage of combustion than during the less efficient smoldering and residual stages. Consequently, fuel consumption is analyzed by combustion stage to produce the best estimates for total emissions.

The flaming, smoldering, and residual combustion emission factors used to calculate total emissions are listed in Tables 3 and 4. These emission factors are weighted according to the amount of fuel consumed in each combustion phase (Formula 4). CONSUME 3.0 assumes that 70 percent of the consumption occurs during the flaming phase of combustion, 15 percent occurs during the smoldering phase of combustion, and 15 percent occurs during the glowing phase of combustion. Total emissions are simply the sum of emissions during flaming, smoldering, and residual combustion.

(4) Emissions (lb or kg) = Emission Factor (lb/ton or kg/Mg) × (Fuel Consumed (tons or Mg) × Combustion Phase (proportion))

Multiply total emissions in pounds or kilograms by 2000 or 1000, respectively to determine total emissions in tons or megagrams.

Table 3. Emission factors for particulate matter (PM, PM₁₀, PM_2.5) for different levels of combustion efficiency after Hardy (1996). Clean piles burn with greater combustion efficiency.

Table 4. Emission factors for carbon dioxide (CO₂), carbon monoxide (CO), methane (CH₄), and non-methane hydrocarbons (NMHC) for different phases of combustion. Emission factors taken from the CONSUME 3.0 User’s Guide.

Recommendations and Guidance

The largest errors expected from using these guidelines will occur during the process of determining the gross pile volume(s). The seven stylized pile shapes do not provide an exhaustive choice of geometric shapes for piled slash. These seven are presented because they reflect general shapes observed by the author [Hardy 1996] and other experts, and also because their volumes can be calculated relatively easily from the formulae. When the dimensions for a pile are observed, care must be taken to account for irregularities in the pile's surfaces. Try to mentally "smooth" the lobes, ridges, and valleys into an average, smooth surface. Long logs and poles extending from the pile's nominal surface can be accounted for by increasing the dimensions of the pile appropriately. If a significant amount of soil is either entrained within the pile or mounded beneath it, the volume of the soil must be estimated and subtracted from the gross pile volume.

The packing ratios presented in these guidelines represent empirical field data from destructive sampling of 17 piles. Even though guidelines are provided to determine an appropriate packing ratio for specific piles, an agency or administrative unit may choose to specify packing ratios for applications under their jurisdiction.

A continuous range of emission factors for PM, PM₁₀, and PM_2.5 are possible. The values given for piles with different amounts of soil contamination are weighted means from eight in situ field tests of emissions from burning of piles of woody debris. Results from many other related tests were used to develop the relationship for predicting emission factors by using combustion efficiency. The values for PM 10 were not derived from actual field observations – only PM_2.5 and PM were measured in the field tests from which these data were prepared. PM₁₀ emission factors were estimated by using limited knowledge of the size distribution of particles.

Hand Piled Fuels (adapted from Wright et al. (2010))

Relationships developed by Wright et al. (2010) are used to estimate hand pile biomass and to calculate consumption and emissions from pile burns. Unlike other fuel categories, consumption of piles is not directly dependent upon fuel particle size. Five steps are required to estimate emissions from a hand-constructed pile or piles of the same shape, size, and composition. Determine:

• Geometric volume of the pile
• Corrected volume of the pile
• Consumable (oven-dry) mass of wood
• Percentage of mass consumed
• Mass of emissions produced

Volume, biomass, consumption, and emissions for multiple piles of the same shape, size and type are calculated as the amount for a single pile multiplied by the number of piles.

1. Geometric Volume of the Pile

The volume of a pile is dependent upon its shape. Piles are categorized into one of seven generalized shapes as shown in Figure 1. Wright et al. (2010) found hand piles to be predominantly paraboloid and ellipsoid in shape. The volume for each shape is calculated using various lengths, heights and/or widths. The equations for each shape are in Table 1.

2. Adjusted Volume of the Pile

Geometric volume is adjusted to reflect what Wright et al (2010) termed true volume (Formula 5 or 6), depending on the calculated geometric volume.

If geometric volume is less than 1 m³ or 35.3 ft³:

(5a) Adjusted Pile Volume (m³) = e^0.2106 × Geometric Volume (m³)

(5b) Adjusted Pile Volume (ft³) = 35.3 × e^0.2106 × (Geometric Volume (ft³)/35.3)

If geometric volume is greater than or equal to 1 m³ or 35.3 ft³:

(6a) Adjusted Pile Volume (m³) = e^{(0.2106 + 0.7691 × ln(Geometric Volume (m3)))}

(6b) Adjusted Pile Volume (ft³) = 35.3 × e^{(0.2106 + 0.7691 × ln(Geometric Volume (ft3)/35.3))}

3. Consumable (Oven-Dry) Mass of Wood

The mass of hand-piled fuels is determined by using regression relationships reported in Wright et al. (2010). Conifer-dominated hand piles tend to be heavier for a given pile volume than shrub/hardwood-dominated hand piles; the relationship between volume and mass is modeled with separate equations (Formula 7 or 8).

If the hand pile is composed of coniferous material:

(7a) Mass of Wood (kg) = e^{(4.4281 + 0.8028 × ln(Adjusted Volume (m3)))}

(7b) Mass of Wood (lb) = 2.2 × e^{(4.4281 + 0.8028 × ln(Adjusted Volume (ft3)/35.3))}

If the hand pile is composed of shrub and hardwood material:

(8a) Mass of Wood (kg) = e^{(3.0393 + 1.3129 × ln(Adjusted Volume (m3)))}

(8b) Mass of Wood (lb) = 2.2 × e^{(3.0393 + 1.3129 × ln(Adjusted Volume (ft3)/35.3))}

Multiply consumable mass of wood in pounds or kilograms by 2000 or 1000, respectively to determine mass in tons or megagrams.

4. Percentage of Mass Consumed

Hardy (1996) found that the amount of woody mass consumed when machine piles are burned ranges from 75 to 95 percent. It is assumed that consumption of hand piles is similar to consumption of machine piles. Several western states have smoke management-reporting programs that recommend values of 85 or 90 percent. CONSUME 3.0 assumes the percentage of mass consumed is 90 percent. Experience and expert knowledge must be used to determine the most appropriate value for percentage consumption.

5. Mass of Emissions Produced

As with machine piles, the mass of emissions produced when hand piles are burned is calculated by multiplying the mass of fuel consumed by an appropriate emission factor for the emission of interest. Emission factors differ with the combustion efficiency of the fire. Cleaner piles burn more efficiently than dirty piles and produce less of the products of incomplete combustion, of which particulate matter is a major emission species. By virtue of the manner in which they are constructed, hand piles contain very little, if any, soil contamination and are considered clean for the purposes of selecting the proper emission factor from Table 3.

The rate of smoke emissions produced also varies with the combustion phase of a fire. Less smoke is produced per dry mass of fuel consumed during the more efficient flaming stage of combustion than during the less efficient smoldering and residual stages. Consequently, fuel consumption is analyzed by combustion stage to produce the best estimates for total emissions.

The flaming, smoldering, and residual combustion emission factors used to calculate total emissions are listed in Tables 3 and 4. These emission factors are weighted according to the amount of fuel consumed in each combustion phase (Formula 4). Multiply total emissions in pounds or kilograms by 2000 or 1000, respectively to determine total emissions in tons or megagrams.

Literature Cited

CONSUME 3.0 user’s guide. no date. Seattle, WA: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 234 p. https://www.fs.fed.us/pnw/fera/research/smoke/consume/consume30_users_guide.pdf (10 Dec 2010).

Hardy, C.C. 1996. Guidelines for estimating volume, biomass and smoke production for piled slash. Gen. Tech. Rep. PNW-GTR-364. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 17 p.

Miles, P.D.; Smith, W.B. 2009. Specific gravity and other properties of wood and bark for 156 tree species found in North America. Res. Note NRS-38. Newtown Square, PA: U.S. Department of Agriculture, Forest Service, Northern Research Station. 35 p.

U.S. Department of Agriculture, Forest Service [USFS]. 1999. Wood handbook–wood as an engineering material. Gen. Tech. Rep. FPL-GTR-113. Madison, WI: Forest Products Laboratory. 463 p.

Wright, C.S.; Balog, C.S.; Kelly, J.W. 2010. Estimating volume, biomass, and potential emissions of hand-piled fuels. Gen. Tech. Rep. PNW-GTR-805. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 23 p.

Table A.2.  Wood density and specific gravity listed in alphabetical order by scientific name.