Permeability of Bulk Wood Pellets in Storage

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• Permeability of Bulk Wood Pellets in Storage

Biomass Bioenergy Research Group
(BBRG) Department of Chemical and Biological
Engineering University of British Columbia (UBC)

50th Annual Conference/Conférence Annuelle
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OUTLINE

• Introduction
• Objective
• Experimental Apparatus
• Materials and Methods
• Results and Discussion
• Conclusion
• Acknowledgment

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Introduction

• Wood pellets definition and properties
• Compacted biomass to increase density
• making it more economical to store and
• transport.
• Made from sawdust and planer shavings
• Natural resins and lignin in wood bind the loose
  particles together
• Common diameter is 6 mm and the length varies up
  to 30 mm
• Particle density of about 1.2 g/cm3
• Bulk density of pellets may range from 600 to 750
  kg m-3
• Moisture content of pellets is in the range of
  5-7 wet mass basis (wb)

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Moist and warm wood pellets are prone to spoil
during storage.
Aeration and cooling is one of the best ways to
control wood pellets temperature and moisture
content during storage.
Data on resistance of wood pellets as bulk to
airflow are needed for design and control of
ventilation, cooling and drying of pellets.
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• Wood pellets storage
• Flat bottom warehouse to a large upright flat or
  hopper bottom steel or concrete silo
• Wood pellets are shipped as bulk in large trucks,
  railcars, or in holds of ocean-going vessels
• Break during successive handlings between storage
  and transport equipment
• The mixture made up of unbroken pellets, broken
  pellets and dust creates a non-homogenous media
  affects the shelf-life of pellets while in
  storage
• Maintain stored pellets cool and dry and uniform
  in temperature
• Forced ventilation is a management tool to
  prevent excessive concentration of toxic gases
  such as CO, CO2 and CH4.
• Self heating of wood pellets.

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• e is porosity of bulk solids (fraction)
• V is air velocity (m/s)
• µ is air viscosity (Pa.s)
• ? is air density (kg/m3)
• ? is a shape factor
• dv (m) is a characteristics dimension of the
  particle

• ?P is pressure drop (Pa)
• L is bed depth (m)
• V is airflow rate (m s-1)
• a, b are constants

• ?P is pressure drop (Pa)
• L is bed depth (m)
• V is airflow rate (m s-1)
• A and B are constants

OBJECTIVE
develop equations that relate differential static
pressures in a column of bulk wood pellets
subject to forced airflow (The tested airflows
range from a low velocity to near fluidization
velocity)
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Experimental Apparatus

• Cylindrical pellet container and instruments for
  measuring the flow rates and static pressure
• Introducing air from the bottom of the cylinder
• Uniform entry air to the container by the plenum
  that contained
• plastic rings of air into the container.
• Centralized compressed air generating station at
  UBC
• Two float in-line flow meters cover a wide range
  of flow rates.
• Low-range flow meter to measured the air flow
  rate
• between 0.0142 to 0.1072 ms-1
• High-range flow meter to measured air flow rates
  up to 0.7148 ms-1
• Using an inclined manometer (Model 26 Mark II,
  Dwyer Instrument Inc.) for preliminary tests and
  a digital manometer (Model HHP-103, Omega
  Engineering Inc.) for subsequent tests

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Materials and Methods

• Two different types of white wood pellets
• Sample 1 was clean wood pellets 6 mm in diameter
  on average
• vertical shaker (Model TM-3, Gilson Company,
  Inc.), we screened wood pellets to two different
  length categories
• The first batch had a length greater than 6.7 mm
  (Lgt6.7mm). The second batch had a length in the
  range of 4mmltLlt6.7mm

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• The single particle density of a wood pellet was
  determined using Multi Pycnometer Quantachrome
  (Model MVP-D160-E)
• Pellet dimensions such as length and diameter
  were measured using electronic verneir scale.
• Bulk density measurement of wood pellets was
  determined according to a slight modification of
  ASAE Standard S269.4 DEC 01 (ASAE 2007).
• Durability was measured using a DURAL tester
  developed at the University of Saskatchewan for
  testing alfalfa cubes and pellets

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RESULT AND DISSCUSSION

• The air flow resistance of agricultural products
  is presented in the form of pressure drop per
  unit depth of material vs. airflow rate

• Air flow ranges between 0.0142 to 0.7148 ms-1
• Larger size led to lower pressure drop.
• Pellets (Lgt6.7mm) the pressure drop did not
  exceed 955 Pa/m
• Shorter pellets (4 mmltLlt6.7 mm), the pressure
  drop increased to 2450 Pa/m
• Pressure drop for a mixture of lengths fell in
  between these two extremes
• For pellets with 4 mmltLlt6.7 mm, the bulk density
  is 6.5 greater than those with mixed sizes
  (10.86 mmltLlt 27.87mm).
• Wood pellets with Lgt6.7mm, the bulk density
  decreased by 19.4 in comparison with the mixed
  bulk.

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Resistance of white wood pellets with different
sizes to airflow

• For pellets (Lgt6.7mm) the pressure drop did not
  exceed 955 Pa/m
• For pellets (4 mmltLlt6.7 mm), the pressure drop
  increased to 2450 Pa/m
• For mixture of lengths the pressure drop fell in
  between these two extremes

The lower the bulk density is, the lower the
pressure drop we would have
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Resistance of two kinds of white wood pellets
with different bulk densities to airflow for
mixed sizes
sample with a larger bulk density, the maximum
pressure drop was 6500 Pa/m while for lower bulk
density the maximum pressure drop was 2400 Pa/m.
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Constants a and b in equation ASABE presented

• Constants a and b in the following equation were
  estimated using experimental data and EXCEL
  solver
• The average pressure drop of three replicates was
  used in the estimation

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Constants A and B in Brooker equation

• Constants A and B in Brooker equation were also
  estimated using experimental data and EXCEL
  solver.
• The average pressure drop of three replicates was
  used in the estimation.

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Verifying Ergun equation

• Plotting pressure drop against two airflow
  ranges. (low range and higher range)
• Left Figure shows the low range up to 0.15 m s-1
  and the right one shows up to 1 m s-1 range.
• The low range indicated a linear relation
  between flow and pressure drop.
• For high airflow rate the relation was not
  linear.
• This observation validates Erguns equation that
  the pressure drop is related to the air flow by a
  first order function for viscous laminar flow.
  The second order function of flow rate for higher
  air flow rates becomes non linear and turbulent.
• Once the fluid flow surpasses the fluidization
  point, the Ergun equation will not be applicable

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Verifying Ergun equation
Resistance of white wood pellets to low laminar
airflow
Resistance of white wood pellets to high
turbulent air flow
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CONCLUSION
The resistance of wood pellets was measured as a
function of wood pellet sizes and moisture
content. Smaller wood pellets had the highest
resistance. Increasing the size of wood pellets
or using a mixture of different sizes decreased
the resistance. Therefore the preliminary results
for wood chips showed the lower resistance for
low airflow rates than the wood pellets. However
the moisture content did not influence the
pressure drop significantly. The wood pellets
resistance should be measured as a function of
bulk density, filling method, and percentage of
fines as well.
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ACKNOWLEDGMENT

• NSERC ( Natutal Science and Engineering Research
  Council of Canada) and Wood Pellet Association
  of Canada for financial support and Fiberco Inc.
  for donating wood pellets for this project.