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Mechanical characteristics of raw and processed peat
Powder
Technology,
51 (1987) 273 - 275
273
Short Communication
Mechanical Characteristics of Raw and Processed Peat
S. REBOUILLAT, and M. PELEG
0. H. CAMPANELLA
Department of Food Engineering, Massachusetts, Amherst, MA 01003 (Received February 23,1987; 16,1987)
University (U.S.A.)
of
in revised form March
Peat is a product of decaying organic matter and is apparently one of the initial stages in the formation of coal. It is usually considered as a low-grade energy source, primarily because of its high moisture content. In certain areas though, notably in the U.S.S.R., Finland and Ireland, it is used to fire power stations and as a domestic and industrial fuel [l]. Currently, in the U.S.A., peat is mainly used in agriculture and horticulture [2]. It is, however, considered a potential energy resource and raw material for a variety of chemical and fermentation processes [2]. There is little in the literature on the mechanical properties of peat and the effect of dewatering and processing on its physical characteristics. The objective of this note is to demonstrate the applicability of methods previously used to characterize food and other powders [3] to raw and dewatered peat. Experimental The bulk density of raw (78% moisture) and industrially dewatered (49% moisture) Canadian peat was determined using a metal cell with a known volume. The cell was then mounted on an Instron Universal Testing Machine, model TM, and its contents compressed, at a crosshead speed of 1.0 cm min-’ , recording the force-displacement curves [3]. In other experiments, the peat was compressed, at the same crosshead speed, to a predetermined deformation where the crosshead was locked in position and the force relaxation recorded. 0032-5910/87/$3.50
The compressive force-deformation curves were transformed to bulk density (in)normal stress (oN) relationships. The latter, as has been the case with a variety of powders at this stress range [ 41, could be described by =a
l"&OPB
+ b log,OoN
(1)
where a and b are empirical constants. The constant b representing the change in bulk density by the applied stress has been referred to as the powder compressibility. The recorded force relaxation curves were normalized and linearized by [5]
FW
= k, + k,t J’(O) -F(t) where F(0) is the force at time zero, F(t) is the decaying force after time t and k, and kz are constants. The reciprocal of constant k, in eqn. (2) represents an asymptotic level of the stress decay and can be used to calculate an asymptotic residual modulus EA from [3]
where A is the cross-sectional area of the specimen and e the strain. Results and discussion The fit of eqns. (1) and (2) by which the peat compressibility and asymptotic modulus were calculated is demonstrated in Figs. 1 and 2, respectively. Statistical analysis of all the experimental data yielded the following regression coefficients. For the compression results, the range was 0.976 < r2 < 0.994, and for the relaxation data 0.996 < r2 < 0.999, with a corresponding significance level of 0.001 or better. Consequently, the methodology used for food powders under low normal stresses (i.e., up to lo5 Pa or 1 kgf cmm2) could also be used for peat. The mechanical parameters of raw and dewatered peat are listed in the Table. It shows that both the compression and relaxation tests were highly reproducible, thus 0 Elsevier Sequoia/Printed in The Netherlands
274 TABLE Mechanical characteristics of raw and dewatered peat
Type
Density
Compressibilitya
Mean C.O.V. (g cmW3) (%)
Mean (-)
C.O.V. (%)
Raw
0.63
1
0.11
10
Dewatered
0.53
3.5
0.06
12
Henky ‘s
Relaxation parameters
strain (-)
Density
Asymptotic modulusb
Mean (g cm-3)
C.O.V. (%)
0.27 0.49
0.81 1.02
1 0.6
21 53
3 5
0.23 0.39
0.66 0.79
0.9 0.7
50 173
13 4
Mean (kpa)
C.O.V. (%)
aThe parameter b in eqn. (1). bThe parameter EA calculated using eqn. (3).
3
240 -
2.95
2.8
2.75 3.5
4
4.5
LOG cr, IPal
0? 0
20
40 TIME
60
00
1
keel
Fig. 1. Typical linearized compression curves of raw (untreated) and dewatered (treated) peat. Pn is the density and oN the applied compressive stress. (See eqn. (1))
Fig. 2. Typical linearized relaxation curves of raw (untreated) and dewatered (treated) peat. F( 0) is the initial force and F(t) the force after time t in relaxation. (See eqn. (2))
strengthening the view that the reported parameters are useful to characterize the mechanical properties of the peat. The bulk density of the processed peat was lower than that of the raw peat. This was mainly a result of the differences in water content. Therefore, the initial density could not serve as a cohesiveness indicator, as is the case in proper powders [ 3,6]. The difference in compressibility, however, was of the same character that is observed in food powders, i.e., the more moist the powder, the larger was its compressibility. In peat though, since the moisture content is by far higher than that found even in very wet powders, the moisture-compressibility relation is most probably a measure of the plasticity of the material itself rather than the openness of the bed structure as a result
of attractive interactions between the particles. As could be expected, the relaxation pattern also indicated that dewatering reduced the material plasticity. This was evident from the higher asymptotic moduli of the processed peat. The magnitude of these moduli is an index of solidity, where zero means that the sample cannot sustain unrelaxed stress and is therefore a liquid by definition. It is therefore clear that dewatered peat particles, both individually and in bulk, have significantly more rigid structure than in their original wet state. Support for this conclusion also comes from the observation that the asymptotic residual modulus of the dewatered peat was higher than that of raw peat even when the differences in density are taken into account. This excludes the alternative explanation that
275
the compactness of the structure was the primary cause of the higher observed moduli. Conclusions It has been shown that the compressibility and stress relaxation pattern of peat can be described by simple relationships (eqns. (1) and (2)) whose applicability, as well as parameters, can be tested and determined by linear regression. No attempt was made to analyze peat of different sources and/or after different kinds of treatments. The good fit of the mathematical equations and the high reproducibility of the tests, however, suggest that the described methodology can be used as a convenient means to characterize different peats and the effects of dewatering and other processes on their mechanical properties.
Acknowledgement The work was supported by the Massachusetts Agricultural Experiment Station in Amherst. The authors express their thanks to Mr. A. B. Allen for contributing the peat for the study as well as background material.
References L. A. Kristoferson and V. Bokalders, Renewable Energy Technologies, Pergamon, Oxford, 1986. A. B. Allen, Private communication, 1987. A. J. Dobbs, M. Peleg, R. E. Mudgett and R. Rufner, Powder Technol., 32 (1982) 69. J. Malave, G. V. Barbosa-Canovas and M. Peleg, J. Food Sci., 50 (1985) 1473. M. Peleg and R. Moreyra, Powder Technol., 23 (1979) 277. A. H. Hollenbach, M. Peleg and R. Rufner, Powder Technol., 35 (1983) 51.
Technology,
51 (1987) 273 - 275
273
Short Communication
Mechanical Characteristics of Raw and Processed Peat
S. REBOUILLAT, and M. PELEG
0. H. CAMPANELLA
Department of Food Engineering, Massachusetts, Amherst, MA 01003 (Received February 23,1987; 16,1987)
University (U.S.A.)
of
in revised form March
Peat is a product of decaying organic matter and is apparently one of the initial stages in the formation of coal. It is usually considered as a low-grade energy source, primarily because of its high moisture content. In certain areas though, notably in the U.S.S.R., Finland and Ireland, it is used to fire power stations and as a domestic and industrial fuel [l]. Currently, in the U.S.A., peat is mainly used in agriculture and horticulture [2]. It is, however, considered a potential energy resource and raw material for a variety of chemical and fermentation processes [2]. There is little in the literature on the mechanical properties of peat and the effect of dewatering and processing on its physical characteristics. The objective of this note is to demonstrate the applicability of methods previously used to characterize food and other powders [3] to raw and dewatered peat. Experimental The bulk density of raw (78% moisture) and industrially dewatered (49% moisture) Canadian peat was determined using a metal cell with a known volume. The cell was then mounted on an Instron Universal Testing Machine, model TM, and its contents compressed, at a crosshead speed of 1.0 cm min-’ , recording the force-displacement curves [3]. In other experiments, the peat was compressed, at the same crosshead speed, to a predetermined deformation where the crosshead was locked in position and the force relaxation recorded. 0032-5910/87/$3.50
The compressive force-deformation curves were transformed to bulk density (in)normal stress (oN) relationships. The latter, as has been the case with a variety of powders at this stress range [ 41, could be described by =a
l"&OPB
+ b log,OoN
(1)
where a and b are empirical constants. The constant b representing the change in bulk density by the applied stress has been referred to as the powder compressibility. The recorded force relaxation curves were normalized and linearized by [5]
FW
= k, + k,t J’(O) -F(t) where F(0) is the force at time zero, F(t) is the decaying force after time t and k, and kz are constants. The reciprocal of constant k, in eqn. (2) represents an asymptotic level of the stress decay and can be used to calculate an asymptotic residual modulus EA from [3]
where A is the cross-sectional area of the specimen and e the strain. Results and discussion The fit of eqns. (1) and (2) by which the peat compressibility and asymptotic modulus were calculated is demonstrated in Figs. 1 and 2, respectively. Statistical analysis of all the experimental data yielded the following regression coefficients. For the compression results, the range was 0.976 < r2 < 0.994, and for the relaxation data 0.996 < r2 < 0.999, with a corresponding significance level of 0.001 or better. Consequently, the methodology used for food powders under low normal stresses (i.e., up to lo5 Pa or 1 kgf cmm2) could also be used for peat. The mechanical parameters of raw and dewatered peat are listed in the Table. It shows that both the compression and relaxation tests were highly reproducible, thus 0 Elsevier Sequoia/Printed in The Netherlands
274 TABLE Mechanical characteristics of raw and dewatered peat
Type
Density
Compressibilitya
Mean C.O.V. (g cmW3) (%)
Mean (-)
C.O.V. (%)
Raw
0.63
1
0.11
10
Dewatered
0.53
3.5
0.06
12
Henky ‘s
Relaxation parameters
strain (-)
Density
Asymptotic modulusb
Mean (g cm-3)
C.O.V. (%)
0.27 0.49
0.81 1.02
1 0.6
21 53
3 5
0.23 0.39
0.66 0.79
0.9 0.7
50 173
13 4
Mean (kpa)
C.O.V. (%)
aThe parameter b in eqn. (1). bThe parameter EA calculated using eqn. (3).
3
240 -
2.95
2.8
2.75 3.5
4
4.5
LOG cr, IPal
0? 0
20
40 TIME
60
00
1
keel
Fig. 1. Typical linearized compression curves of raw (untreated) and dewatered (treated) peat. Pn is the density and oN the applied compressive stress. (See eqn. (1))
Fig. 2. Typical linearized relaxation curves of raw (untreated) and dewatered (treated) peat. F( 0) is the initial force and F(t) the force after time t in relaxation. (See eqn. (2))
strengthening the view that the reported parameters are useful to characterize the mechanical properties of the peat. The bulk density of the processed peat was lower than that of the raw peat. This was mainly a result of the differences in water content. Therefore, the initial density could not serve as a cohesiveness indicator, as is the case in proper powders [ 3,6]. The difference in compressibility, however, was of the same character that is observed in food powders, i.e., the more moist the powder, the larger was its compressibility. In peat though, since the moisture content is by far higher than that found even in very wet powders, the moisture-compressibility relation is most probably a measure of the plasticity of the material itself rather than the openness of the bed structure as a result
of attractive interactions between the particles. As could be expected, the relaxation pattern also indicated that dewatering reduced the material plasticity. This was evident from the higher asymptotic moduli of the processed peat. The magnitude of these moduli is an index of solidity, where zero means that the sample cannot sustain unrelaxed stress and is therefore a liquid by definition. It is therefore clear that dewatered peat particles, both individually and in bulk, have significantly more rigid structure than in their original wet state. Support for this conclusion also comes from the observation that the asymptotic residual modulus of the dewatered peat was higher than that of raw peat even when the differences in density are taken into account. This excludes the alternative explanation that
275
the compactness of the structure was the primary cause of the higher observed moduli. Conclusions It has been shown that the compressibility and stress relaxation pattern of peat can be described by simple relationships (eqns. (1) and (2)) whose applicability, as well as parameters, can be tested and determined by linear regression. No attempt was made to analyze peat of different sources and/or after different kinds of treatments. The good fit of the mathematical equations and the high reproducibility of the tests, however, suggest that the described methodology can be used as a convenient means to characterize different peats and the effects of dewatering and other processes on their mechanical properties.
Acknowledgement The work was supported by the Massachusetts Agricultural Experiment Station in Amherst. The authors express their thanks to Mr. A. B. Allen for contributing the peat for the study as well as background material.
References L. A. Kristoferson and V. Bokalders, Renewable Energy Technologies, Pergamon, Oxford, 1986. A. B. Allen, Private communication, 1987. A. J. Dobbs, M. Peleg, R. E. Mudgett and R. Rufner, Powder Technol., 32 (1982) 69. J. Malave, G. V. Barbosa-Canovas and M. Peleg, J. Food Sci., 50 (1985) 1473. M. Peleg and R. Moreyra, Powder Technol., 23 (1979) 277. A. H. Hollenbach, M. Peleg and R. Rufner, Powder Technol., 35 (1983) 51.