Fahimeh Yazdanpanah, CSBE Conference , July 13-16 , 2008 North Vancouver1

Permeability of Bulk Wood Pellets in Storage

Biomass & Bioenergy Research Group (BBRG)Department of Chemical and Biological EngineeringUniversity of British Columbia (UBC)

50th Annual Conference/Conférence Annuelle

OUTLINE:

- Introduction

- Objective

- Experimental Apparatus

- Materials and Methods

- Results and Discussion

- Conclusion

- Acknowledgment

2Fahimeh Yazdanpanah, CSBE Conference , July 13-16 , 2008 North Vancouver

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)

Fahimeh Yazdanpanah, CSBE Conference , July 13-16 , 2008 North Vancouver3

Fahimeh Yazdanpanah, CSBE Conference , July 13-16 , 2008 North Vancouver 4

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.

- 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.

Fahimeh Yazdanpanah, CSBE Conference , July 13-16 , 2008 North Vancouver5

Fahimeh Yazdanpanah, CSBE Conference , July 13-16 , 2008 North Vancouver6

-ε is porosity of bulk solids (fraction)-V is air velocity (m/s)-µ is air viscosity (Pa.s)-ρ 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

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)

OBJECTIVE

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

Fahimeh Yazdanpanah, CSBE Conference , July 13-16 , 2008 North Vancouver7

Fahimeh Yazdanpanah, CSBE Conference , July 13-16 , 2008 North Vancouver8

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 (L>6.7mm). The second batch had a

length in the range of 4mm<L<6.7mm

Sample 1 Sample 2

Wood species Spruce saw dust Spruce saw dust

Diameter (mm) 6.47 (6.35-6.7 ) 6.14 ( 6.37-6.49)

Length (mm)14.77 (10.86-

27.87 )14.04 (10.59-20.42)

Moisture content (%) 4.67 4.80

Durability (%) 58.1 (57.2-59.5 ) 73.4Loose bulk density (kg

m-3)804.7 (clean)

761.57 (mixed with 14.6% fines)

Specific gravity (g/cm3) 1.17 1.13

- 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

Fahimeh Yazdanpanah, CSBE Conference , July 13-16 , 2008 North Vancouver9

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

Fahimeh Yazdanpanah, CSBE Conference , July 13-16 , 2008 North Vancouver10

- Air flow ranges between 0.0142 to 0.7148 ms-1

- Larger size led to lower pressure drop.

- Pellets (L>6.7mm) the pressure drop did not exceed 955 Pa/m

- Shorter pellets (4 mm<L<6.7 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 mm<L<6.7 mm, the bulk density is 6.5% greater than those with mixed sizes

(10.86 mm<L< 27.87mm).

- Wood pellets with L>6.7mm, the bulk density decreased by 19.4% in comparison with the

mixed bulk.

Fahimeh Yazdanpanah, CSBE Conference , July 13-16 , 2008 North Vancouver 11

The lower the bulk density is, the lower the pressure drop we would have

Resistance of white wood pellets with different sizes to airflow

- For pellets (L>6.7mm) the pressure

drop did not exceed 955 Pa/m

- For pellets (4 mm<L<6.7 mm), the

pressure drop increased to 2450 Pa/m

- For mixture of lengths the pressure

drop fell in between these two

extremes

Fahimeh Yazdanpanah, CSBE Conference , July 13-16 , 2008 North Vancouver 12

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.

Fahimeh Yazdanpanah, CSBE Conference , July 13-16 , 2008 North Vancouver13

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

- 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.

Fahimeh Yazdanpanah, CSBE Conference , July 13-16 , 2008 North Vancouver 14

Constants A and B in Brooker equation

Fahimeh Yazdanpanah, CSBE Conference , July 13-16 , 2008 North Vancouver15

-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 Ergun’s 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

Verifying Ergun equation

Fahimeh Yazdanpanah, CSBE Conference , July 13-16 , 2008 North Vancouver 16

Resistance of white wood pellets to low laminar airflow

Resistance of white wood pellets to high turbulent air flow

Verifying Ergun equation

CONCLUSION

Fahimeh Yazdanpanah, CSBE Conference , July 13-16 , 2008 North Vancouver17

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.

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.

Fahimeh Yazdanpanah, CSBE Conference , July 13-16 , 2008 North Vancouver18