Resistance of Wood Chips and Sawdust to Airflow

C. W. Suggs, Alicia Lanier FELLOW ASSOC. MEMBER

ASAE ASAE

ABSTRACT

A IRFLOW resistance through variable height columns of wood chips and sawdust was evaluated

by means of the pressure drop across an orifice plate. Input pressure to the bottom of the column was controlled by means of a sliding gate valve or damper on the supply fan air intake. Flow per unit of cross section plotted against input pressure per unit of bed depth yielded the expected straight line response on a log-log plot. The response for chips was similar in both actual value and slope to the flow characteristics of similar size products such as bean pods. The flow through sawdust was similar to the flow through fescue seed. Coefficients for the classical airflow equation were evaluated from the data.

INTRODUCTION

Because of the increase in the cost of coal and other fossil fuels since the mid 1970's there has been a renewed interest in biomass fuels. Many of these are cheap at the source but are relatively expensive to collect, process and haul. Wood, for example, depending on species, may be valued at only a few dollars per cord as standing trees but after cutting and splitting into fireplaces size pieces costs about $90 per delivered cord. The cost of feeding cordwood into the furnace would also need to be include in the cost of producing heat for a commercial operation.

Wood chips are attractive as fuel because they can be harvested mechanically, limbs and tops can be utilized, and the resulting material can be handled and fed into the furance automatically. Sawdust is a low-cost, low utility byproduct of the lumber industry suitable for fuel usage. Wood is normally chipped green because the chipper blades are rapidly dulled on dry wood and chipping is usually associated with timber harvesting. Moisture content of green chips and sawdust typically run from 35 to 55% wet basis (Riley et al., 1983). Both are difficult to burn without forced draft and the high moisture materials support fungal growth which causes allergic reactions in some people. Also, chips and sawdust with moisture contents above 40% freeze together and are very difficult to feed at low temperatures (Riley et al., 1983).

Article was submitted for publication in May, 1984; reviewed and approved for publication by the Electric Power and Processing Div. of ASAE in October, 1984.

Paper No. 9298 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh, NC.

The use of trade names in this publication does not imply endorsement by the North Carolina Agricultural Research Service of the products named, nor criticism of similar ones not mentioned.

The authors are: C.W. SUGGS, Professor and ALICIA LANIER, Student, Biological and Agricultural Engineering Dept., North Carolina State University, Raleigh, NC.

Piles of green wood chips and sawdust dry very slowly under natural ventilation and if not protected from the rain may actually show an increase in moisture content (Curtis, 1980). They can, of course, be dried with natural or heated air provided the air is forced through the materials with a suitable fan. If not properly dried, decomposition, heating and reduction in energy content can occur. For example, corncobs piled outside in Indiana and ventilated with forced air to keep the temperature below 30 °C (86F) lost 24% of their net energy content in nine months (Smith, et al., 1983). Moisture content decreased in the center of the pile but increased on the surface, yielding a significant increase in total moisture content, and therefore energy loss via microbial activity.

The objective of this study was to evaluate the air flow characteristics through various depths of wood chips and sawdust when exposed to several input air pressure levels. This information, which was not found in the literature, can be used in designing forced ventilation systems for drying thse products.

METHODS AND MATERIALS

Airflow was meaured with the same equipment, except that wall baffles were removed, which Abrams and Fish (1982) used for sweet potatoes. It consisted of an 89 cm x 89 cm x 58 cm high (35 x 35 x 23 in.) plenum onto which up to five 61 cm x 61 cm x 41 cm (24 x 24 x 16 in.) column sections could be added, Fig. 1. Expanded metal with a 1.3 cm x 1.3 cm (1/2 in. x 1/2 in.) mesh was used to cover the plenum outlet to keep chips and sawdust from falling into the plenum. Pressure drop across this screen was negligible. A 75 W (0.1 hp) 1570 rpm squirrel cage motor driven blower drawing air through a sliding gate valve with six approximately equal opening increments supplied air to the lower plenum. Airflows produced by this fan are in the range which will be useful in drying chips.

An empty column section fitted with a 3.2 mm (1/8 in.) thick plate containing a central orifice was used on the top of the column to produce a pressure drop from which flow could be determined. A 12.7 cm diameter (5 in.) orifice was used with the chips and 7.5 and 5 cm (3 in. and 2 in.) orifices were used with the sawdust. Pressure taps were located 5 cm (2 in.) below the mesh level in the plenum and 5 cm (2 in.) below the orifice level at the top of the column. The pressure difference measured between these two taps was the pressure drop across the column of chips or sawdust, and the pressure drop measured from the upper tap to the atmosphere was the pressure drop across the orifice.

Pressures were measured in terms of water column with a Dwyer Microtector Portable Electronic Gage which had an accuracy of ±0.062 Pa (0.00025 in H20).

1985—TRANSACTIONS of the ASAE © 1985 American Society of Agricultural Engineers 0001-2351/85/2801-0293$02.00 293

ORIFICE PLATE 3.2 mm Thick

PRESSURE TAP

CHIP DEPTH

Fig. 1—Apparatus used to measure air flow characteristics of wood chips, from Abrams and Fish, 1982.

(0.0469 in.) sieve and 23.8% through the last sieve into the pan.

Five duplicate readings were made of the pressure across the orifice and the column for each combination of fan input valve opening and column height. After making readings for all six valve openings the orifice section was removed, another 41 cm (16 in.) section of chips or sawdust was added and the orifice section was replaced so another set of readings could be made. This procedure was repeated until the column was four sections high for sawdust and five sections high for chips except for the third set of observations where only one or two sections were used.

As each 41 cm section was added it was filled with chips or sawdust. The material was allowed to fall into the empty section but the maximum drop for any of the material was approximately the 41 cm height of one section.

The effects of bed depth were normalized by dividing the pressure across the chips or sawdust by height (depth) of the column to obtain a pressure gradient. Flow was expressed in terms of volume per unit of time per unit of column cross section which is the superficial velocity of the air leaving the upper surface of the material. Superifical velocity was plotted against the pressure gradient on a log-log scale to produce a Shedd Chart (Shedd, 1953).

Plastic tubing and valves were used to connect the desired taps to the gage.

Airflow was evaluated from the pressure across the orifice plate by means of the classical orifice equation (Henderson and Perry, 1966).

Q = VA = AK yfTglS^ly

where Q :

A = K = g :

Ap = Y z

Airflow Orifice area Orifice coefficient (0.61) Acceleration of gravity Pressure drop across orifice Specific weight

The first set of observations were made on hardwood chips which had been left in a sheltered storage bin for about 10 months. These chips had a moisture content of 9.3% wet basis and a density in the test chamber of 221 kg/m.3 (13.8 lb/ft3). When separated by size, 28.4% of these chips passed over a 1.9 cm x 1.9 cm (3/4 in. x 3.4 in.) sieve (large), 29.5% passed over a 1.3 cm x 1.3 cm (1/2 in. x 1/2 in.) sieve (medium) and the remainder 42.1% passed through this sieve (small).

The second set of observations were made on fresh green hardwood chips which had a moisture content of about 43.2% and a density of 353 kg/m3 (22.03 lb/ft3). Size distribution of these chips was 21.3% large, 47.4% medium, and 31.3% small.

In a third set of observations the green chips from test two were divided into large (>1.9 cm), medium (>1.3 cm) and small (<1.3 cm) fractions and evaluated.

The fourth set of observations was made on fresh green pine sawdust with a moisture content of 53% and a density of 380 kg/m3 (23.7 lb/ft3). Size distribution was 11.5% over a 0.475 cm (0.187 in.) sieve, 25% over a 0.238 cm (0.0937 in.) sive, 40.1% over a 0.119 cm

RESULTS AND DISCUSSION

Flow through the dry chips, Fig. 2, exhibited the typical Shedd response. A linear regression of log Q = log a + b log P gave a slope, b of 0.6304, a unity intercept, a (when Pa/m equals one) of 0.01193 and a correlation coefficint (r value) of 0.9685, Table 1. Each point on the graph represents the average of five observations and each symbol refers to a specific bed depth. The depth effect was essentially normalized, as

E

>-t o o -J UJ > _ J < u u_ tr LU Q_ 3 C/)

1.0 .8

.6

.4

SOUTHERN k PEAS

.02

.01

DRY CHIPS-

SNAP BEANS-

GREEN CHIPS

DEPTHS Q-0.41 m O-0.82 m 0 - | . 2 3 m A - | . 6 4 m O-2.05m

I

LIMA BEANS

BEAN AND PEA DATA FROM WILHELM et al., 1983

OPEN SYMBOLS-DRY CHIPS CLOSED SYMBOLS-GREEN CHIPS

JL J _ _L J _ 8 10 20 4 0 60 80IOO 200 4 0 0

PRESSURE GRADIENT, Pa/m

Fig. 2—Air flow characteristics of green and dry wood chips. Different symbols represent different column depths. 1 in H20/ft = 75.85 Pa /m , 1 m/s = 196.85 ft/min.

294 TRANSACTIONS of the ASAE—1985

TABLE 1. SLOPE OF REGRESSION LINE, UNITY INTERCEPT AND CORRELATION COEFFICIENT, r

FOR THE EQUATION LOG Q = LOG a + b LOG P.

Dry chips

Green chips

Large chips Over 1.9 cm x

Medium chips Over 1.3 cm x

Small chips

1.9

1.3

Through 1.3 cm x

Sawdust

cm

cm

1.3 cm

Slope b

0.6304

0.6448

0.5204

0.5352

0.6169

1.2172

Intercept, a

0.0119

0.0082

0.0295

0.0220

0.0124

0.00005

r

0.9685

0.9957

0.9994

0.9998

0.9997

0.9778

4! p

ITY

, r

o

s UJ

> I

[FIC

IA

CL

CO

expected, by division of the pressure by column height. That is, flow resistance was proportional to bed depth.

The flow through green chips, Fig. 2, was similar to dry chips with a slope of 0.6448, a unity intercept of 0.008175 and a correlation coefficient of 0.9957. The point scatter (solid symbols) was less than for dry chips and the depth effect was again normalized by dividing the pressure by the bed depth. The flow for a given input air pressure gradient was slightly less for the green chips and indicated that these green chips had a higher resistance to airflow than the dry chips. The difference was not significant and might be also related to the differences in the size distributions.

Lines for pods of snap beans, southern peas and lima beans (Wilhelm et al., 1983) have also been drawn in on Fig. 2. The locations of these lines indicate that the airflow characteristics of these products are similar to wood chips.

Superficial velocity at a given input pressure gradient increased as the chip particle size increased, Fig. 3 and Table 1. This is consistent with the literature in which large particles such as ear corn have less resistance to flow than small particles such as shelled corn or clover seed (Shedd, 1953). The small chips had about the same flow resistance as the ungraded dry chips.

30 40 60 80 100 200 400 600800

PRESSURE GRADIENT, Pa/m

Fig. 4—Air flow characteristics of sawdust. 1 in H20/ft = 75.85 , 1 m/s = 196.85 ft/min.

Results of higher pressure airflow through wood chips from Kimball (1975) have been included in Fig. 3 for comparison with our results. His response line has almost the same slope as ours and if extended would pass through the same area indicating a similar flow resistance value.

Airflow through sawdust with respect to input pressure per unit depth also yielded a straight line on a log-log plot, Fig. 4. Superficial velocity for a given pressure was much less than for chips, as was expected, due to the small particle size. The response line was very close to the response line for fescue seed (Shedd, 1953). Slope of the regression line (1.217) was steeper than for chips, also the unity intercept was much lower, Table 1, indicating less flow for any given pressure. The correlation coefficient was 0.9778 indicating a good fit of the data to the regression line.

E

O

Q. Z> CO

1.0

. 8

.6

2 h

.08

06

0 4 LARGE-OVER 1.9x1.9cm SEIVE MEDIUM-OVER 1.3x1.3cm SEIVE SMALL-THROUGH 1.3x 1.3cm SEIVE

J I L 3 5 7 10 20 30 50 70 100 200 300 500 7001000

PRESSURE GRADIENT,Pa/m

Fig. 3—Air flow characteristics of small, medium and large wood chips. 1 in H20/ft = 75.85 Pa /m , 1 m/s = 196.85 ft/min.

SUMMARY AND CONCLUSIONS

Airflow through wood chips and sawdust was evaluated in a 61 cm x 61 cm (24 x 24 in.) column which could be varied in height by 41 cm (16 in.) steps to a maximum height of 5 steps or 205 cm (80 in.). Ambient air passed through a sliding gate valve to a 75 W fan and then to a plenum below the column. It exited through an orifice in an upper plenum. Orifice diameter was 12.7 cm (5 in.) for the wood chip observations and 7.5 cm or 5 cm (3 or 2 in.) for the sawdust observations.

Plots of flow per unit of column cross section (superficial velocity) versus the input pressure gave straight lines on a log-log plot in agreement with the literature on flow through porous materials. Flow was inversely proportional to bed depth or column height so that division of the input pressure by depth made all of the different depth results fall along the same line. Actual values fall in the same range as other products with similar particle sizes such as bean and pea pods for chips and fescue seed for sawdust. Consistent with the literature regarding particle size, large chips had less resistance to flow than small chips.

(continued on page 301)

1985—TRANSACTIONS of the ASAE 295

the drying air over the range of values tested. 2. The effect of drying air velocity is insignificant on

the drying constant of unsteamed rice but it appeared to increase with an increase in air velocity from 0.05 m3/s-m2 to 0.50 m3/s-m2 for steam treated rice.

3. Drying constant varied with temperature according to an Arrhenius relationship below 65°C for unsteamed rice and from 30°C to 75°C for steam treated rice for a constant air velocity. Drying constant values were in general higher for steam treated rice than unsteamed rice.

4. A two-term diffusion equation can describe the single layer drying of steamed and unsteamed rice.

LIST OF SYMBOLS

A dimensionless constant related to particle shape (shape factor) D hygroscopic diffusivity, cmVs F dimensionless coefficient L constant depending on shape (6/TT2 for sphere) M moisture content, % (d.b.) ME dynamic equilibrium moisture content, % (d.b.) Mj initial moisture contest, % (d.b.) MR moisture ratio N geometric factor related to particle shape (n2/r()

2 for sphere), cm-2

T absolute temperature, °K c material constant g second term drying constant, s1

k first term drying constant, s_1

k0 material constant r0 effective kernel radius, cm r correlation coefficient t time, s

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drying of maize and rice. Journal of Agricultural Engineering Research 5(4):363-385.

2. Becker, H. A. and H. R. Sallans, 1955. A study of the internal moisture movement in the drying of the wheat grain. Cereal Chemistry 32(3):212-226.

3. Bhattacharya, K. R., 1969. Breakage of rice during milling, and effect of parboiling. Cereal Chemistry 46(5):478-485.

4. Bhattacharya, K. R. and Y. M. I. Swamy, 1967. Conditions of drying parboiled paddy for optimum milling quality. Cereal Chemistry 44(6):592-600.

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6. Chen, C. S. and W. H. Johnson, 1969. Kinetics of moisture movement in hygroscopic materials, I. TRANSACTIONS OF THE ASAE 12(1):109-113.

7. Chittenden, D. H. and A. Hustmlid, 1966. Numerical solution of diffusion equations. TRANSACTIONS OF THE ASAE 9(l):52-55.

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characteristics of bulk piled sweet potatoes. TRANSACTIONS of the ASAE25(4):1103-1106.

2. Curtis, M. L. 1980. The effect of outside storage on the fuel potential of green hardwood residues. M.S. Thesis, Virginia Polytechnic Institute and State Univ., Blacksburg, VA.

3. Henderson, S. M. and R. L. Perry. 1966. Agricultural process engineering 3rd ed. pg. 61, 63, 65. AVI Publishing Co., Westport, CT.

4. Riley, J. G., N. Smith and C. S. Drechsel. 1983. Drying and

8. Chu, S. T. and A. Hustrulid, 1968. Numerical solution to diffusion equations. TRANSACTIONS OF THE ASAE ll(5):705-709.

9. Henderson, S. M. and S. Pabis, 1961. Grain drying theory, I. Temperature effect on drying coefficient. Journal of Agricultural Engineering Research 6(3): 169-174.

10. Hogan, J. T. and M. L. Karon, 1966. Hygroscopic equilibria of rough rice at elevated temperatures. Agricultural and Food Chemistry 3(10):855-860.

11. Hukill, W. V. and J. L. Schmidt, 1960. Drying rate of fully exposed grain kernels. TRANSACTIONS OF THE ASAE 3(2):71-77,80.

12. Hustrulid, A. and A. M. Flikke, 1959. Theoretical drying curve for shelled corn. TRANSACTIONS OF THE ASAE 2(1): 112-114.

13. Juliano, B. O., 1972. The rice caryopsis and its composition. In: Rice, chemistry and technology. Edited by D. F. Houston. American Association of Cereal Chemists, Inc. St. Paul, MN. pp 16-74.

14. Mannapperuma, J. D., 1975. Analysis of thermal and moisture stresses caused during drying of brown rice. Unpublished Masters thesis in Agricultural Engineering, Louisiana State University, Baton Rouge.

15. McEwen, E. and J. R. O'Callahan, 1955. The effect of air humidity on thorough drying of wheat grain. Transactions of the Institution of Chemical Engineers 33(2):135-154.

16. Newman, A. B. 1931. The drying of porous solids: Diffusion calculations. Transactions of the American Institute of Chemical Engineers 27:310-333.

17. O'Callahan, J. R., D. J. Menzies and P. H. Baily, 1971. Digital simulation of agricultural dryer performance. Journal of Agricultural Engineering Research 16(3):223-244.

18. Pabis, S. and S. M. Henderson, 1961. Grain drying theory, II. A critical analysis of the drying curve for shelled maize. Journal of Agricultural Engineering Research 6(4):272-277.

19. Salleh, H. B., 1975. The effect of short interval steam treatment of rough rice upon the mill yield, physical and chemical properties. Unpublished Masters Thesis in Agricultural Engineering, Louisiana State University, Baton Rouge. 20. Sharma, A. D., O. R. Kunze and H. D. Tolley. 1982. Rough

rice drying as a two compartment model. TRANSACTIONS OF THE ASAE 25(l):221-224.

21. Simmonds, W. H. C , F. T. Ward and E. McEwen, 1953. The drying of wheat grain. Part I: The mechanism of drying. Transactions of the Institute of Chemical Engineers 31(3):265-278.

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Resistance of Wood Chips to Airflow (continued from page 295)

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