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Microwave dielectric characterization of switchgrass for bioenergy and biofuel
Fuel 124 (2014) 151–157
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Microwave dielectric characterization of switchgrass for bioenergy and biofuel F. Motasemi a, Muhammad T. Afzal a,⇑, Arshad Adam Salema a, J. Mouris b, R.M. Hutcheon b a b
Department of Mechanical Engineering, University of New Brunswick, P.O. Box 4400, Fredericton, NB E3B 5A3, Canada Microwave Properties North, Deep River, ON K0J 1P0, Canada
h i g h l i g h t s Microwave dielectric properties of switchgrass were analyzed during pyrolysis. Dielectric properties are highly influenced by the pyrolysis temperature. The microwave absorbing property of the biomass increases in the char region. Some modeling work was presented on dielectric properties.
a r t i c l e
i n f o
Article history: Received 30 October 2013 Received in revised form 9 January 2014 Accepted 25 January 2014 Available online 7 February 2014 Keywords: Dielectric properties Pyrolysis Switchgrass Char
a b s t r a c t This study investigates the microwave dielectric properties of switchgrass at 915 and 2450 MHz under nitrogen (N2) environment, during the process of heating it to 700 °C. The heating process can be divided into three distinct stages, namely drying (from room temperature to 200 °C), pyrolysis (from 200 °C to 450 °C), and char region (from 450 °C to 700 °C). The dielectric properties decreased during the drying and pyrolysis stage, but increased dramatically in the char region, indicating the strong microwave absorption capability of the char. The maximum microwave penetration depth occurred at 392 °C, just at the high end of the pyrolysis zone. Dry biomass is a low loss material, but the microwave absorption improved significantly after the pyrolysis stage. The resulting char showed high microwave absorbing properties which can be used in many microwave absorbing applications. The experimental data were fitted using regression fit and based on this the dielectric properties model related to the temperature was developed. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Switchgrass is considered as one of the high potential energy crops and a renewable source for transportation fuel and/or electricity [1]. It shows high yield across a wide geographic range; require low water and nutrient requirements, can grow well in marginal quality land and have positive environmental benefits [2]. As a result of this, switchgrass has been selected as a model bioenergy species by Bioenergy Feedstock Development Program (BFDP) of US Department of Energy [3]. The energy produced from switchgrass primarily depends on the concentration of energy primarily derived from cell walls and particularly from lignin and cellulose [4]. Pyrolysis is one of the most effective thermochemical methods to convert biomass-based energy sources such as switchgrass into ⇑ Corresponding author. Tel.: +1 506 453 4880; fax: +1 506 453 5025. E-mail address: [email protected] (M.T. Afzal). http://dx.doi.org/10.1016/j.fuel.2014.01.085 0016-2361/Ó 2014 Elsevier Ltd. All rights reserved.
potential bioenergy and biofuels. The effectiveness of the pyrolysis process is that it can generate three different products in the form of solid, liquid, and gas depending on the feedstock characteristics and process conditions [5–8]. Therefore, pyrolysis has received considerable attention during the past decades [9]. Moreover, pyrolysis products can be further upgraded into fuels and chemicals [10,11]. Bio-oil, a new fuel coming into the market [12] with about half the heating value of petroleum [13,14] (17 MJ kg1; wet weight basis), is produced from biomass pyrolysis. Bio-oil cannot be used directly as a fuel in heating applications or as a transportation fuel; however, by using suitable upgrading techniques it can be converted into usable fuels. The gaseous fraction from pyrolysis process contains many components including syngas which can be used as fuel for combustion and gasification. On the other hand, biochar has a heating value of about 18 MJ kg1 which can also be used either to generate energy or heat [15]. Several technologies such as fixed bed, fluidized bed, transported bed, augur or screw, ablative, rotating cone, and chain gates
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[9] have been used to pyrolyze the biomass. Each of these technologies has its own advantages and disadvantages. However, all of these technologies use traditional type of heating methods in which the heat is transferred from the surface towards the center of the material by conduction and convection, inherently indirect and slow processes. Alternatively, in dielectric or microwave heating, the electromagnetic energy is transformed into heat directly within the material [16] and has been developed by numerous researchers [17]. The microwaves can penetrate materials and deposit energy, thus the heat can be produced throughout the volume of the materials rather than an external source (in core volumetric heating). Generally, microwave provides rapid and selective heating as compared to conventional heating techniques [18–20]. Depending on frequency, microwave systems have the overall efficiency of 80–85% which is considered reasonable [21]. Microwave pyrolysis process has attracted a considerable attention during the past decade because of several advantages it offers compare to other methods [22]. It has been proved as an energetically efficient way to convert different types of biomass into bioproducts. These include switchgrass [23], wheat straws [24,25], rice straws [26,27], corn stover [28,29], algae [30], coffee hulls [31,32], oil palm shell [33], and different types of wood [34–37] . Determining the fundamental microwave dielectric properties and the penetration depth of microwaves in the material [21] over the range of process parameters, is an important step before designing microwave processing systems [38]. As the material undergoes microwave pyrolysis, the dielectric properties of the material is expected to change according to the temperature. Hence, it is crucial to know the change in dielectric properties during actual pyrolysis process as performed by Peng et al. [39]. Although extensive research work has been carried out on microwave pyrolysis of biomass, but there is a clear lack of fundamental information on dielectric properties of new agricultural biomass crop such as switchgrass. Several research work regarding dielectric properties of rubber wood [40], oil palm fibers [41], rice husk, rice straw, kenaf, and resin [42], peanut hull pellets [43], woody biomass [44,45], hardwood [46], Aleppo pine, Holm oak and Thuja burl woods [47], fine residual waste electrical and electronic equipment [48], thin plastic films [49], coal [50], softwoods (black spruce, balsam fir, and tamarack) [51], oil palm shell carbon–polyester resin [52], oil palm biomass [53], empty fruit bunch [54], and wood and wood-based products [55] has been carried out. However, none has attempted to investigate the dielectric properties with varying pyrolysis temperature for switchgrass biomass. Based on the above review of literature, there is a strong need to investigate dielectric properties of materials during actual pyrolysis process. Thus, the main objective of this study was to investigate the microwave absorption capacity of switchgrass in addition to dielectric properties (dielectric constant, loss factor, and tangent loss) and penetration depth. The dielectric properties were determined from room temperature to 700 °C and at two commercial microwave frequencies (915 MHz and 2450 MHz). Lastly, the microwave absorption behavior during pyrolysis was fitted to a numerical function in order to develop dielectric properties model.
2. Material and methods 2.1. Materials In this study switchgrass was used as sample material. Table 1 shows its proximate and ultimate analysis. The sample was produced by grinding in a Wiley mill to particle size less that 2 mm, then drying the powder at 105 °C for 15 h in an oven. The dielectric properties were investigated on pelletized samples which were
Table 1 Proximate and ultimate analysis of switchgrass. Parameters
Value (wt.%, dry basis)
Ash Volatile Fixed carbon Moisture content
3.61 68.46 25.70 2.23
Carbon Nitrogen Hydrogen Sulfur Oxygen
47.20 0.15 6.00 0.04 46.61
formed by uniaxial pressing in a die lined with tungsten carbide at 135 MPa. Three pellets of switchgrass with similar dimension were stacked on top of each other to form one test sample, with diameter (3.65 ± 0.050 mm), length (14.38 ± 0.050 mm), mass (0.142 ± 0.002 g), initial density (0.94 ± 0.050 g/cc), and final density at the end of the experiment (0.34 ± 0.04 g/cc). The purpose of pressing it to pellet form was to reduce the air fraction in the pellet, thus increasing the accuracy of the data.
2.2. Measurement of dielectric properties The dielectric response of a substance is commonly presented as permittivity (e), which can be given by:
e ¼ e0 er ¼ e0 ðe0r je00r Þ
ð1Þ
where e0 is permittivity of free space ð8:854 1012 F=mÞ, er is complex relative permittivity, and j is imaginary unit (j2 = 1). As indicated by Eq. (1), the complex relative permittivity is comprised of two components: ðe0r Þ, the real part (which is generally known as the relative dielectric constant and it is a measure of the ability of the dielectrics to store electrical energy) and e00r Þ, the imaginary part (which is also called the relative dielectric loss factor and represents the loss of electrical energy in dielectrics). For common nonmagnetic materials, only e00r and e0r are needed to determine the heating rate under microwave irradiation [55]. The dielectric properties (e0r and e00r ) of the samples were measured during pyrolysis using the cavity perturbation technique. The details of equipment is published elsewhere [39] and on the website www.MicrowavePropertiesNorth.ca. The major components of the measurement system consisted of resistive heating furnace and a cylindrical TM0n0 resonant mode cavity (£ 580 mm 50 mm). In order to measure the permittivity at specific temperature, a hot sample and its holder are rapidly removed from a conventional furnace and inserted into a high electric field region of a thick-walled, well-cooled cavity. The resonant frequency and loaded Q of the cavity are measured by a Hewlett–Packard 8753 network analyzer and stored for off-line analysis which includes subtraction of hot empty sample holder effects. The sample and holder are either left in the cavity for further measurements at lower temperatures as the sample cools, or are quickly returned to the furnace for further processing. In the latter case, the sample can be out of the furnace for as little as 2 s for a measurement at a single frequency. In the cavity perturbation method, the differences in the microwave cavity response between a cavity with an empty sample-holder and the same cavity with a sample-holder plus the sample were measured. These differences were then used to calculate the permittivity [56]. In order to further minimize the error, the cavity perturbation method was first calibrated with high-purity sapphire solid sample and the second calibration point was measured with Teflon to check low dielectric constant values.
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Fig. 1. Dielectric properties of switchgrass vs. temperature under nitrogen environment at 915 MHz and 2450 MHz with initial density of 0.94 ± 0.050 g/cc; (a) relative dielectric constant, (b) relative loss factor, and (c) tangent loss.
The error of this calibration technique for the present sample length to diameter ratio (>3.5) was shown to be less than ±3%. The samples were heated to the desired temperatures with a ramp rate of 5 °C/min, starting at 30 °C and progressing in 25 °C steps up to 150 °C, then 50 °C steps to 700 °C. The pyrolysis reaction was done with the sample pellets mounted inside a 4 mm ID quartz tube with a 0.01 L/min flow of nitrogen (N2) in order to avoid the oxidation and combustion. The literature indicates that if the loss factor (e00 ) is caused by free electron conductivity, there is a relationship between the dc resistance and the microwave loss factor [57]. Making this assumption for the final ‘‘char’’ state of the sample, the final state loss factor at room temperature was estimated by measuring the final dc resistance of the final pellets. This relationship is expressed as follow (f is the microwave frequency, R is the resistance measured in the experiment, D is the diameter of the switchgrass pellet, and L is the length of the switchgrass pellet):
e00r ¼
1 2pf e0 R p4
D2 L
ð2Þ
The final dc resistance value was used to calculate a theoretical loss factor, which was then compared with the measured final loss factor as a test of the microwave loss mechanism.
2.3. Loss tangent The loss tangent (tan d = e00 /e0 ) is an estimate of a material’s ability to convert electromagnetic energy into heat at a specific temperature and frequency under general irradiation conditions. In any specific applicator system, the actual heating rate is also dependent on the oven geometry.
2.4. Penetration depth The penetration depth (Dp) is a very important factor in the design and scale-up of a microwave heating system. It is defined as the depth in the material at which the power carried by a forward-travelling electromagnetic wave of the specified frequency falls to 1/e of the value it had just inside the surface. The penetration depth can be calculated using the following equation [21] (k0 is the microwave wavelength in free space):
91=2 8" 00 2 #1=2 = < er Dp ¼ 1þ 0 1 1=2 : 0 ; e 2pð2e Þ r k0
ð3Þ
r
3. Results and discussions The thermal decomposition reaction of switchgrass is regarded as having three distinct stages, namely drying (from room temperature to 200 °C), pyrolysis (from 200 °C to 450 °C), and char region (from 450 °C to 700 °C). According to the derivative thermogravimetric (DTG) analysis of switchgrass, a significant loss of mass was found to occur in temperature range of 180–440 °C and that the thermal decomposition was completed at 550 °C [58]. The dielectric properties measured in drying and pyrolysis stages are discussed under Section 3.1, whereas Section 3.2 reports the dielectric properties in char region. 3.1. Drying and pyrolysis stages Fig. 1 shows that the dielectric properties of switchgrass were almost independent of the microwave frequency during drying and pyrolysis temperature range. During drying stage, the
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dielectric properties decreased sharply from room temperature to 75 °C and then remained roughly constant until 200 °C, reflecting the release of free and lightly bound water [59] which are strong absorbers. In the pyrolysis region, the dielectric properties drop between 200 °C and 350 °C (reflecting the breaking of bonds and the release of volatile matter) after which the constant values up to 450 °C indicate little reaction. It seems that, once the water is evaporated, any polar components produced by ‘‘cracking’’ in the biomass are immediately volatized and thus cannot contribute to an increase in dielectric properties. Thus switchgrass is poor absorber of microwave during these stages. A similar observation was reported by Salema and Ani [60] during microwave pyrolysis of oil palm biomass. The following phenomena might be responsible for the drastic drop in dielectric properties during pyrolysis reaction. First, the weight loss during the drying and pyrolysis reaction (70–90%) and the resultant density decrease which might have caused significant decrease in the number of atoms per unit volume, producing an automatic decrease in polarizability per unit volume. Second, any remaining bound or capillary water within the chemical structure of switchgrass will be released during the early pyrolysis stage, with bond-breaking occurring during the latter stages of formation of carbonaceous material [61]. The decreasing trend of dielectric properties during drying and pyrolysis stages was in total agreement with previous studies [39,62,63]. The penetration depth of microwaves in switchgrass biomass during drying and pyrolysis is illustrated in Fig. 2. The ratio of the penetration depth at 915 to that at 2450 MHz is largely a function of the ratio of the free space wavelengths [39,53]. It is observed that penetration depth almost remained constant during drying stage, while it increased significantly during pyrolysis stage. The highest penetration depths of about 30 and 10 m at 915 and 2450 MHz, respectively were found at 392 °C. The initial increase in microwave penetration depth was actually due to the removal of surface moisture from the sample which leads to decrease in
Fig. 2. Penetration depth of microwave in switchgrass biomass at varying temperature.
dielectric loss factor [39]. Further, the remarkable difference between the microwave penetration depth at frequencies 915 and 2450 MHz might be due to the difference in microwave wavelengths. Thus, the biomass sample was transparent to the microwaves in the pyrolysis region since the penetration depth exceeded the sample size. Therefore, the biomass will not be able to absorb the microwave in the pyrolysis region. 3.2. Char stage Fig. 3 illustrates the dielectric properties of switchgrass in char region. As can be seen from these figures, the dielectric properties increased significantly with the temperature. The loss of an electrically insulting barrier [61] due to the thermal decomposition of switchgrass and the phase transformation to solid char [62] could
Fig. 3. Dielectric properties of char vs. temperature under nitrogen environment at 915 MHz and 2450 MHz with initial density of 0.94 ± 0.050 g/cc; (a) relative dielectric constant, (b) relative loss factor, and (c) tangent loss.
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Fig. 4. Penetration depth of microwave in switchgrass char product at varying temperature.
be the main reasons behind this sudden growth of dielectric properties. Increase in dielectric properties of carbonaceous material such as char has encouraged researchers [22,60,62] to use it as a microwave absorber in many biomass pyrolysis applications. At 700 °C, the dielectric constant and loss factor at 915 MHz were about 2.5 times those at 2450 MHz at temperature. Theoretically, the dielectric loss of materials is closely related to the electrical conductivity (R, resistance measured in the experiment) as given in Eq. (2). If any change in the R due to the conduction electrons, then the value of dielectric constant and loss factor will vary accordingly. Thus, the pyrolyzed biomass i.e. char might consists of free electron conduction due to possible change in the structure ordering during pyrolysis which might have led to high dielectric constant and loss factor in char region compared to drying and pyrolysis. The results were in agreement with dielectric
155
properties of bituminous coal [39]. The complex permittivity was also highly dependent on pyrolysis temperature and microwave frequencies in case of activated carbon [63]. From these results, we may conclude that the ability of the char to convert electromagnetic energy into heat is higher than the original switchgrass biomass. Some of the researchers [62,64] have even carried out microwave pyrolysis of biomass without adding any microwave absorber such as char. However, this was possible because of using single mode MW system in which high MW power intensity and focused microwaves was applied. To our knowledge, at the initial stage (drying) the moisture content in the biomass can help to generate the heat by absorbing the microwave energy. At later stage of pyrolysis, the biomass starts transforming into carbonaceous material i.e. char and the microwave absorbing capability of the biomass material increases due to sudden change in the dielectric property. The char then starts to act as a microwave absorber and helps to pyrolyze the remaining biomass material. The penetration depth decreased dramatically as the temperature continued to increase in the char region as depicted in Fig. 4. The low penetration depths above 550 °C at both frequencies indicated the strong microwave absorption capability of the switchgrass char, which was in agreement with previous study [39]. The penetration depth was 0.5 cm and 0.3 cm at 915 and 2450 MHz respectively at 689 °C. Hence, the microwave penetration depth at this temperature for 2450 MHz frequency was less than the size of the switchgrass pellet which confirms that the microwaves are absorbed by the char material. The sudden change in the penetration depth at high temperatures might be associated with the complete loss of volatile matters, as confirmed by the weight loss in the sample. Furthermore, this will result in increase in the density of delocalized electrons in the char. This relates to the structure transformation of the biomass material during pyrolysis.
Fig. 5. Experimental and predicted values of dielectric constant (a) and (b); loss factor (c) and (d).
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Table 2 Regression coefficients and R2 values for relative dielectric constant model. Stage
915 MHz A1
A2
T0
dT
R2
Drying Pyrolysis Char region
2.64 2.31 1.51
2.27 1.41 4970.26
52.63 270.35 849.19
0.96 21.51 28.61
0.98 0.99 0.99
Drying Pyrolysis Char region
2.53 2.24 1.38
2450 MHz 2.21 1.40 29.21
55.91 271.91 728.98
3.47 21.67 36.52
0.98 0.99 0.99
Table 3 Regression coefficients and R2 values for relative dielectric loss factor model. Stage
Acknowledgment
915 MHz A1
A2
T0
dT
R2
Drying Pyrolysis Char region
0.17 0.09 0.11
0.08 0.01 2972.26
52.28 253.88 755.59
4.15 20.21 22.46
0.99 0.99 0.99
Drying Pyrolysis Char region
0.16 0.09 0.08
2450 MHz 0.09 0.03 2263.50
52.26 267.04 788.88
1.04 17.22 22.51
0.94 0.99 0.99
Fig. 5 shows the measured dielectric properties as a function of temperature at 915 and 2450 MHz frequency. The resulting best-fit generalized equation for dielectric properties was obtained based on a sigmoidal function using regression model which is as follow:
A1 A2 þ A2 1 þ eðTT 0 Þ=dT
The authors appreciate the financial assistance from New Brunswick Soil and Crop Improvement Association, New Brunswick Agricultural Council, and Agriculture and Agri-Food Canada. References
3.3. Dielectric properties model development
y¼
of microwave at high temperatures. The developed dielectric properties models can be helpful to design and simulate large scale microwave system for biofuel and bioproduct applications. The present data shows that temperature plays a significant role in defining the dielectric properties of biomass materials which vary greatly during pyrolysis. This suggests that proper adjustment of microwave operating conditions can achieve more efficient processes to produce biofuels and bioproducts from switchgrass. Switchgrass biomass as a low loss material is considered to be poor microwave absorber. However, the final product, char exhibits very strong microwave absorbing properties, with accompanying very small penetration depths, which may be useful in microwave heating and absorbing applications.
ð4Þ
where y is dielectric property (dielectric constant and loss factor), temperature, T, is the lone independent variable (°C) and Ai (i = 1, 2), T0, and dT are the regression coefficients. The regression coefficients for different stage of switchgrass thermal degradation are provided in Tables 2 and 3. For this, an exponential function was adopted to fit the experimental data in one-way nonlinear regression analysis based on the observed exponential decay trend shown in experimental data. Similar method was adopted to develop the dielectric property model for biological human tissues [65]. The developed fits describe the relationship between the temperature and relative dielectric properties of switchgrass during different stages of pyrolysis. The coefficient of determination (R2) was in the range of 0.949 and 0.999 which shows the fitness of the model [66]. 4. Conclusions The dielectric properties of switchgrass were measured at 915 and 2450 MHz from room temperature to 700 °C under nitrogen (N2) environment. The dielectric properties decreased during drying and pyrolysis stages; however, there was a sharp increase in char region. The variations in the dielectric properties during the pyrolysis process are due to presence of free and capillary water, decomposition of volatile matter, continuous change in weight and density, and transformation of biomass material into carbonaceous material. The penetration depth of microwaves in the biomass material almost remained constant in drying stage while it increased drastically in pyrolysis region. The sudden drop of penetration depth in char region confirms its strong absorption capacity
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Fuel journal homepage: www.elsevier.com/locate/fuel
Microwave dielectric characterization of switchgrass for bioenergy and biofuel F. Motasemi a, Muhammad T. Afzal a,⇑, Arshad Adam Salema a, J. Mouris b, R.M. Hutcheon b a b
Department of Mechanical Engineering, University of New Brunswick, P.O. Box 4400, Fredericton, NB E3B 5A3, Canada Microwave Properties North, Deep River, ON K0J 1P0, Canada
h i g h l i g h t s Microwave dielectric properties of switchgrass were analyzed during pyrolysis. Dielectric properties are highly influenced by the pyrolysis temperature. The microwave absorbing property of the biomass increases in the char region. Some modeling work was presented on dielectric properties.
a r t i c l e
i n f o
Article history: Received 30 October 2013 Received in revised form 9 January 2014 Accepted 25 January 2014 Available online 7 February 2014 Keywords: Dielectric properties Pyrolysis Switchgrass Char
a b s t r a c t This study investigates the microwave dielectric properties of switchgrass at 915 and 2450 MHz under nitrogen (N2) environment, during the process of heating it to 700 °C. The heating process can be divided into three distinct stages, namely drying (from room temperature to 200 °C), pyrolysis (from 200 °C to 450 °C), and char region (from 450 °C to 700 °C). The dielectric properties decreased during the drying and pyrolysis stage, but increased dramatically in the char region, indicating the strong microwave absorption capability of the char. The maximum microwave penetration depth occurred at 392 °C, just at the high end of the pyrolysis zone. Dry biomass is a low loss material, but the microwave absorption improved significantly after the pyrolysis stage. The resulting char showed high microwave absorbing properties which can be used in many microwave absorbing applications. The experimental data were fitted using regression fit and based on this the dielectric properties model related to the temperature was developed. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Switchgrass is considered as one of the high potential energy crops and a renewable source for transportation fuel and/or electricity [1]. It shows high yield across a wide geographic range; require low water and nutrient requirements, can grow well in marginal quality land and have positive environmental benefits [2]. As a result of this, switchgrass has been selected as a model bioenergy species by Bioenergy Feedstock Development Program (BFDP) of US Department of Energy [3]. The energy produced from switchgrass primarily depends on the concentration of energy primarily derived from cell walls and particularly from lignin and cellulose [4]. Pyrolysis is one of the most effective thermochemical methods to convert biomass-based energy sources such as switchgrass into ⇑ Corresponding author. Tel.: +1 506 453 4880; fax: +1 506 453 5025. E-mail address: [email protected] (M.T. Afzal). http://dx.doi.org/10.1016/j.fuel.2014.01.085 0016-2361/Ó 2014 Elsevier Ltd. All rights reserved.
potential bioenergy and biofuels. The effectiveness of the pyrolysis process is that it can generate three different products in the form of solid, liquid, and gas depending on the feedstock characteristics and process conditions [5–8]. Therefore, pyrolysis has received considerable attention during the past decades [9]. Moreover, pyrolysis products can be further upgraded into fuels and chemicals [10,11]. Bio-oil, a new fuel coming into the market [12] with about half the heating value of petroleum [13,14] (17 MJ kg1; wet weight basis), is produced from biomass pyrolysis. Bio-oil cannot be used directly as a fuel in heating applications or as a transportation fuel; however, by using suitable upgrading techniques it can be converted into usable fuels. The gaseous fraction from pyrolysis process contains many components including syngas which can be used as fuel for combustion and gasification. On the other hand, biochar has a heating value of about 18 MJ kg1 which can also be used either to generate energy or heat [15]. Several technologies such as fixed bed, fluidized bed, transported bed, augur or screw, ablative, rotating cone, and chain gates
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[9] have been used to pyrolyze the biomass. Each of these technologies has its own advantages and disadvantages. However, all of these technologies use traditional type of heating methods in which the heat is transferred from the surface towards the center of the material by conduction and convection, inherently indirect and slow processes. Alternatively, in dielectric or microwave heating, the electromagnetic energy is transformed into heat directly within the material [16] and has been developed by numerous researchers [17]. The microwaves can penetrate materials and deposit energy, thus the heat can be produced throughout the volume of the materials rather than an external source (in core volumetric heating). Generally, microwave provides rapid and selective heating as compared to conventional heating techniques [18–20]. Depending on frequency, microwave systems have the overall efficiency of 80–85% which is considered reasonable [21]. Microwave pyrolysis process has attracted a considerable attention during the past decade because of several advantages it offers compare to other methods [22]. It has been proved as an energetically efficient way to convert different types of biomass into bioproducts. These include switchgrass [23], wheat straws [24,25], rice straws [26,27], corn stover [28,29], algae [30], coffee hulls [31,32], oil palm shell [33], and different types of wood [34–37] . Determining the fundamental microwave dielectric properties and the penetration depth of microwaves in the material [21] over the range of process parameters, is an important step before designing microwave processing systems [38]. As the material undergoes microwave pyrolysis, the dielectric properties of the material is expected to change according to the temperature. Hence, it is crucial to know the change in dielectric properties during actual pyrolysis process as performed by Peng et al. [39]. Although extensive research work has been carried out on microwave pyrolysis of biomass, but there is a clear lack of fundamental information on dielectric properties of new agricultural biomass crop such as switchgrass. Several research work regarding dielectric properties of rubber wood [40], oil palm fibers [41], rice husk, rice straw, kenaf, and resin [42], peanut hull pellets [43], woody biomass [44,45], hardwood [46], Aleppo pine, Holm oak and Thuja burl woods [47], fine residual waste electrical and electronic equipment [48], thin plastic films [49], coal [50], softwoods (black spruce, balsam fir, and tamarack) [51], oil palm shell carbon–polyester resin [52], oil palm biomass [53], empty fruit bunch [54], and wood and wood-based products [55] has been carried out. However, none has attempted to investigate the dielectric properties with varying pyrolysis temperature for switchgrass biomass. Based on the above review of literature, there is a strong need to investigate dielectric properties of materials during actual pyrolysis process. Thus, the main objective of this study was to investigate the microwave absorption capacity of switchgrass in addition to dielectric properties (dielectric constant, loss factor, and tangent loss) and penetration depth. The dielectric properties were determined from room temperature to 700 °C and at two commercial microwave frequencies (915 MHz and 2450 MHz). Lastly, the microwave absorption behavior during pyrolysis was fitted to a numerical function in order to develop dielectric properties model.
2. Material and methods 2.1. Materials In this study switchgrass was used as sample material. Table 1 shows its proximate and ultimate analysis. The sample was produced by grinding in a Wiley mill to particle size less that 2 mm, then drying the powder at 105 °C for 15 h in an oven. The dielectric properties were investigated on pelletized samples which were
Table 1 Proximate and ultimate analysis of switchgrass. Parameters
Value (wt.%, dry basis)
Ash Volatile Fixed carbon Moisture content
3.61 68.46 25.70 2.23
Carbon Nitrogen Hydrogen Sulfur Oxygen
47.20 0.15 6.00 0.04 46.61
formed by uniaxial pressing in a die lined with tungsten carbide at 135 MPa. Three pellets of switchgrass with similar dimension were stacked on top of each other to form one test sample, with diameter (3.65 ± 0.050 mm), length (14.38 ± 0.050 mm), mass (0.142 ± 0.002 g), initial density (0.94 ± 0.050 g/cc), and final density at the end of the experiment (0.34 ± 0.04 g/cc). The purpose of pressing it to pellet form was to reduce the air fraction in the pellet, thus increasing the accuracy of the data.
2.2. Measurement of dielectric properties The dielectric response of a substance is commonly presented as permittivity (e), which can be given by:
e ¼ e0 er ¼ e0 ðe0r je00r Þ
ð1Þ
where e0 is permittivity of free space ð8:854 1012 F=mÞ, er is complex relative permittivity, and j is imaginary unit (j2 = 1). As indicated by Eq. (1), the complex relative permittivity is comprised of two components: ðe0r Þ, the real part (which is generally known as the relative dielectric constant and it is a measure of the ability of the dielectrics to store electrical energy) and e00r Þ, the imaginary part (which is also called the relative dielectric loss factor and represents the loss of electrical energy in dielectrics). For common nonmagnetic materials, only e00r and e0r are needed to determine the heating rate under microwave irradiation [55]. The dielectric properties (e0r and e00r ) of the samples were measured during pyrolysis using the cavity perturbation technique. The details of equipment is published elsewhere [39] and on the website www.MicrowavePropertiesNorth.ca. The major components of the measurement system consisted of resistive heating furnace and a cylindrical TM0n0 resonant mode cavity (£ 580 mm 50 mm). In order to measure the permittivity at specific temperature, a hot sample and its holder are rapidly removed from a conventional furnace and inserted into a high electric field region of a thick-walled, well-cooled cavity. The resonant frequency and loaded Q of the cavity are measured by a Hewlett–Packard 8753 network analyzer and stored for off-line analysis which includes subtraction of hot empty sample holder effects. The sample and holder are either left in the cavity for further measurements at lower temperatures as the sample cools, or are quickly returned to the furnace for further processing. In the latter case, the sample can be out of the furnace for as little as 2 s for a measurement at a single frequency. In the cavity perturbation method, the differences in the microwave cavity response between a cavity with an empty sample-holder and the same cavity with a sample-holder plus the sample were measured. These differences were then used to calculate the permittivity [56]. In order to further minimize the error, the cavity perturbation method was first calibrated with high-purity sapphire solid sample and the second calibration point was measured with Teflon to check low dielectric constant values.
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Fig. 1. Dielectric properties of switchgrass vs. temperature under nitrogen environment at 915 MHz and 2450 MHz with initial density of 0.94 ± 0.050 g/cc; (a) relative dielectric constant, (b) relative loss factor, and (c) tangent loss.
The error of this calibration technique for the present sample length to diameter ratio (>3.5) was shown to be less than ±3%. The samples were heated to the desired temperatures with a ramp rate of 5 °C/min, starting at 30 °C and progressing in 25 °C steps up to 150 °C, then 50 °C steps to 700 °C. The pyrolysis reaction was done with the sample pellets mounted inside a 4 mm ID quartz tube with a 0.01 L/min flow of nitrogen (N2) in order to avoid the oxidation and combustion. The literature indicates that if the loss factor (e00 ) is caused by free electron conductivity, there is a relationship between the dc resistance and the microwave loss factor [57]. Making this assumption for the final ‘‘char’’ state of the sample, the final state loss factor at room temperature was estimated by measuring the final dc resistance of the final pellets. This relationship is expressed as follow (f is the microwave frequency, R is the resistance measured in the experiment, D is the diameter of the switchgrass pellet, and L is the length of the switchgrass pellet):
e00r ¼
1 2pf e0 R p4
D2 L
ð2Þ
The final dc resistance value was used to calculate a theoretical loss factor, which was then compared with the measured final loss factor as a test of the microwave loss mechanism.
2.3. Loss tangent The loss tangent (tan d = e00 /e0 ) is an estimate of a material’s ability to convert electromagnetic energy into heat at a specific temperature and frequency under general irradiation conditions. In any specific applicator system, the actual heating rate is also dependent on the oven geometry.
2.4. Penetration depth The penetration depth (Dp) is a very important factor in the design and scale-up of a microwave heating system. It is defined as the depth in the material at which the power carried by a forward-travelling electromagnetic wave of the specified frequency falls to 1/e of the value it had just inside the surface. The penetration depth can be calculated using the following equation [21] (k0 is the microwave wavelength in free space):
91=2 8" 00 2 #1=2 = < er Dp ¼ 1þ 0 1 1=2 : 0 ; e 2pð2e Þ r k0
ð3Þ
r
3. Results and discussions The thermal decomposition reaction of switchgrass is regarded as having three distinct stages, namely drying (from room temperature to 200 °C), pyrolysis (from 200 °C to 450 °C), and char region (from 450 °C to 700 °C). According to the derivative thermogravimetric (DTG) analysis of switchgrass, a significant loss of mass was found to occur in temperature range of 180–440 °C and that the thermal decomposition was completed at 550 °C [58]. The dielectric properties measured in drying and pyrolysis stages are discussed under Section 3.1, whereas Section 3.2 reports the dielectric properties in char region. 3.1. Drying and pyrolysis stages Fig. 1 shows that the dielectric properties of switchgrass were almost independent of the microwave frequency during drying and pyrolysis temperature range. During drying stage, the
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dielectric properties decreased sharply from room temperature to 75 °C and then remained roughly constant until 200 °C, reflecting the release of free and lightly bound water [59] which are strong absorbers. In the pyrolysis region, the dielectric properties drop between 200 °C and 350 °C (reflecting the breaking of bonds and the release of volatile matter) after which the constant values up to 450 °C indicate little reaction. It seems that, once the water is evaporated, any polar components produced by ‘‘cracking’’ in the biomass are immediately volatized and thus cannot contribute to an increase in dielectric properties. Thus switchgrass is poor absorber of microwave during these stages. A similar observation was reported by Salema and Ani [60] during microwave pyrolysis of oil palm biomass. The following phenomena might be responsible for the drastic drop in dielectric properties during pyrolysis reaction. First, the weight loss during the drying and pyrolysis reaction (70–90%) and the resultant density decrease which might have caused significant decrease in the number of atoms per unit volume, producing an automatic decrease in polarizability per unit volume. Second, any remaining bound or capillary water within the chemical structure of switchgrass will be released during the early pyrolysis stage, with bond-breaking occurring during the latter stages of formation of carbonaceous material [61]. The decreasing trend of dielectric properties during drying and pyrolysis stages was in total agreement with previous studies [39,62,63]. The penetration depth of microwaves in switchgrass biomass during drying and pyrolysis is illustrated in Fig. 2. The ratio of the penetration depth at 915 to that at 2450 MHz is largely a function of the ratio of the free space wavelengths [39,53]. It is observed that penetration depth almost remained constant during drying stage, while it increased significantly during pyrolysis stage. The highest penetration depths of about 30 and 10 m at 915 and 2450 MHz, respectively were found at 392 °C. The initial increase in microwave penetration depth was actually due to the removal of surface moisture from the sample which leads to decrease in
Fig. 2. Penetration depth of microwave in switchgrass biomass at varying temperature.
dielectric loss factor [39]. Further, the remarkable difference between the microwave penetration depth at frequencies 915 and 2450 MHz might be due to the difference in microwave wavelengths. Thus, the biomass sample was transparent to the microwaves in the pyrolysis region since the penetration depth exceeded the sample size. Therefore, the biomass will not be able to absorb the microwave in the pyrolysis region. 3.2. Char stage Fig. 3 illustrates the dielectric properties of switchgrass in char region. As can be seen from these figures, the dielectric properties increased significantly with the temperature. The loss of an electrically insulting barrier [61] due to the thermal decomposition of switchgrass and the phase transformation to solid char [62] could
Fig. 3. Dielectric properties of char vs. temperature under nitrogen environment at 915 MHz and 2450 MHz with initial density of 0.94 ± 0.050 g/cc; (a) relative dielectric constant, (b) relative loss factor, and (c) tangent loss.
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Fig. 4. Penetration depth of microwave in switchgrass char product at varying temperature.
be the main reasons behind this sudden growth of dielectric properties. Increase in dielectric properties of carbonaceous material such as char has encouraged researchers [22,60,62] to use it as a microwave absorber in many biomass pyrolysis applications. At 700 °C, the dielectric constant and loss factor at 915 MHz were about 2.5 times those at 2450 MHz at temperature. Theoretically, the dielectric loss of materials is closely related to the electrical conductivity (R, resistance measured in the experiment) as given in Eq. (2). If any change in the R due to the conduction electrons, then the value of dielectric constant and loss factor will vary accordingly. Thus, the pyrolyzed biomass i.e. char might consists of free electron conduction due to possible change in the structure ordering during pyrolysis which might have led to high dielectric constant and loss factor in char region compared to drying and pyrolysis. The results were in agreement with dielectric
155
properties of bituminous coal [39]. The complex permittivity was also highly dependent on pyrolysis temperature and microwave frequencies in case of activated carbon [63]. From these results, we may conclude that the ability of the char to convert electromagnetic energy into heat is higher than the original switchgrass biomass. Some of the researchers [62,64] have even carried out microwave pyrolysis of biomass without adding any microwave absorber such as char. However, this was possible because of using single mode MW system in which high MW power intensity and focused microwaves was applied. To our knowledge, at the initial stage (drying) the moisture content in the biomass can help to generate the heat by absorbing the microwave energy. At later stage of pyrolysis, the biomass starts transforming into carbonaceous material i.e. char and the microwave absorbing capability of the biomass material increases due to sudden change in the dielectric property. The char then starts to act as a microwave absorber and helps to pyrolyze the remaining biomass material. The penetration depth decreased dramatically as the temperature continued to increase in the char region as depicted in Fig. 4. The low penetration depths above 550 °C at both frequencies indicated the strong microwave absorption capability of the switchgrass char, which was in agreement with previous study [39]. The penetration depth was 0.5 cm and 0.3 cm at 915 and 2450 MHz respectively at 689 °C. Hence, the microwave penetration depth at this temperature for 2450 MHz frequency was less than the size of the switchgrass pellet which confirms that the microwaves are absorbed by the char material. The sudden change in the penetration depth at high temperatures might be associated with the complete loss of volatile matters, as confirmed by the weight loss in the sample. Furthermore, this will result in increase in the density of delocalized electrons in the char. This relates to the structure transformation of the biomass material during pyrolysis.
Fig. 5. Experimental and predicted values of dielectric constant (a) and (b); loss factor (c) and (d).
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Table 2 Regression coefficients and R2 values for relative dielectric constant model. Stage
915 MHz A1
A2
T0
dT
R2
Drying Pyrolysis Char region
2.64 2.31 1.51
2.27 1.41 4970.26
52.63 270.35 849.19
0.96 21.51 28.61
0.98 0.99 0.99
Drying Pyrolysis Char region
2.53 2.24 1.38
2450 MHz 2.21 1.40 29.21
55.91 271.91 728.98
3.47 21.67 36.52
0.98 0.99 0.99
Table 3 Regression coefficients and R2 values for relative dielectric loss factor model. Stage
Acknowledgment
915 MHz A1
A2
T0
dT
R2
Drying Pyrolysis Char region
0.17 0.09 0.11
0.08 0.01 2972.26
52.28 253.88 755.59
4.15 20.21 22.46
0.99 0.99 0.99
Drying Pyrolysis Char region
0.16 0.09 0.08
2450 MHz 0.09 0.03 2263.50
52.26 267.04 788.88
1.04 17.22 22.51
0.94 0.99 0.99
Fig. 5 shows the measured dielectric properties as a function of temperature at 915 and 2450 MHz frequency. The resulting best-fit generalized equation for dielectric properties was obtained based on a sigmoidal function using regression model which is as follow:
A1 A2 þ A2 1 þ eðTT 0 Þ=dT
The authors appreciate the financial assistance from New Brunswick Soil and Crop Improvement Association, New Brunswick Agricultural Council, and Agriculture and Agri-Food Canada. References
3.3. Dielectric properties model development
y¼
of microwave at high temperatures. The developed dielectric properties models can be helpful to design and simulate large scale microwave system for biofuel and bioproduct applications. The present data shows that temperature plays a significant role in defining the dielectric properties of biomass materials which vary greatly during pyrolysis. This suggests that proper adjustment of microwave operating conditions can achieve more efficient processes to produce biofuels and bioproducts from switchgrass. Switchgrass biomass as a low loss material is considered to be poor microwave absorber. However, the final product, char exhibits very strong microwave absorbing properties, with accompanying very small penetration depths, which may be useful in microwave heating and absorbing applications.
ð4Þ
where y is dielectric property (dielectric constant and loss factor), temperature, T, is the lone independent variable (°C) and Ai (i = 1, 2), T0, and dT are the regression coefficients. The regression coefficients for different stage of switchgrass thermal degradation are provided in Tables 2 and 3. For this, an exponential function was adopted to fit the experimental data in one-way nonlinear regression analysis based on the observed exponential decay trend shown in experimental data. Similar method was adopted to develop the dielectric property model for biological human tissues [65]. The developed fits describe the relationship between the temperature and relative dielectric properties of switchgrass during different stages of pyrolysis. The coefficient of determination (R2) was in the range of 0.949 and 0.999 which shows the fitness of the model [66]. 4. Conclusions The dielectric properties of switchgrass were measured at 915 and 2450 MHz from room temperature to 700 °C under nitrogen (N2) environment. The dielectric properties decreased during drying and pyrolysis stages; however, there was a sharp increase in char region. The variations in the dielectric properties during the pyrolysis process are due to presence of free and capillary water, decomposition of volatile matter, continuous change in weight and density, and transformation of biomass material into carbonaceous material. The penetration depth of microwaves in the biomass material almost remained constant in drying stage while it increased drastically in pyrolysis region. The sudden drop of penetration depth in char region confirms its strong absorption capacity
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