Wave Channel Moisture Sensor for Biomass Materials Zdzisław Posyłek Częstochowa University of Technology Institute Telecomunications and Electromagnetic Compatibility Al. Armii Krajowej 17, 42-200 Częstochowa, Poland [email protected]

Marek Wróbel University of Agriculture in Kraków Faculty of Production and Power Engineering ul. Balicka 116B, 30-149 Kraków, Poland [email protected]

Aleksander Gąsiorski Krzysztof Mudryk University of Agriculture in Kraków Faculty of Production and Power Engineering ul. Balicka 116B, 30-149 Kraków, Poland [email protected]

Tomasz Dróżdż

Częstochowa University of Technology Institute of Industrial Electrical Engineering Al. Armii Krajowej 17, 42-200 Częstochowa, Poland alekg@ el.pcz.czest.pl

Paweł Kiełbasa

University of Agriculture in Kraków Faculty of Production and Power Engineering ul. Balicka 116B, 30-149 Kraków, Poland. [email protected]

University of Agriculture in Kraków Faculty of Production and Power Engineering ul. Balicka 116B, 30-149 Kraków, Poland [email protected]

Abstract—This paper presents results of the initial research into application of microwave techniques for measuring the moisture content in wood biomass. The goal of the research was to develop a control system for continuous monitoring of moisture control during the manufacturing process. We propose using a closed, low-energy microwave circuit, through a rectangular flow-through chamber with a unique profile. Results of the initial laboratory testing are promising. They indicate that this technique has the potential for an industrial application and may lead to a development of a simple and cost effective, continuous moisture control system. The research required designing a complex wave channel and high-precision manufacturing to build a laboratory prototype. The research team then conducted extensive testing of the prototype, including microwave leakage impact on the safety of the workplace. The paper also presents qualitative analysis.

process. As compared with coal, it contains less ash and chlorine. Disadvantages of biomass include low calorific value caused by high and highly variable moisture content that also depends on the type and duration of storage.

1

I. INTRODUCTION In many countries, renewable energy has entered a phase of fast growth, primarily due to wide adoption of green energy policies and regulatory frameworks to encourage investments in renewable energy generation. In Poland, the combustion of biomass is the largest renewable energy source, and the third largest source of fuel providing a substitute for fossil fuels, primarily coal. Biomass can be sourced as a by-product from the forestry industry, from growing plants specifically for this purpose, or by recycling biomass from farming, industrial or urban waste sources. The rules for assessing the quality of the fuel are the same for the biomass as for the other solid and liquid fuels. The most important criteria are: calorific value, moisture content, sulphur and ash content, ash melting point, volatile content, and granulation. An unquestionable advantage of the biofuel is the net zero carbon dioxide balance during the combustion

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Control of moisture content in the biomass is a very important issue both in the production process and the combustion process. II. IDENTIFICATION OF MOISTURE CONTENT IN THE BIOMASSE Identification of the moisture content (humidity) of the material plays an important role in many areas of industry and technology. The water content significantly affects the physical and chemical properties of various materials. Controlling the humidity in the process of production and processing of the material is necessary from the economic and technical perspective. One of the more advanced and highly accurate measurement methods is the application of microwave technology. In agriculture and agri-food industry measurements of moisture content mainly include loose materials, such as cereal grains, spices and herbs. It is particularly important to monitor the drying process. Dried food is characterized by lower risk of poisoning because with decreasing moisture content inhibits growth of pathogenic microorganisms. Testing moisture content is important in warehouses, since its presence is conducive to the development of fungi and can accelerate spoilage of food products. In the case of loose materials, measurement of the moisture content allows you to control the phenomenon of agglomeration. This parameter is also important in the assessment of the technical condition of buildings. In addition, in industrial and agricultural applications humidity is commonly monitored in paper, coal, cement,

soil and wood. Due to the specific characteristics of the above materials, testing of the moisture content requires the implementation of methods tailored to the application. Where other moisture content monitoring methods do not work, the measurements using the microwave technique play a significant role. Microwave frequencies of 3 to 30 GHz are used in hygrometers. Due to the nature of this method of measuring microwave moisture, the meters allow you to control the humidity of the test material non-invasively and in real time. This is an important advantage in industrial applications. III. ELECTRICAL PROPERTIES OF WOOD Dry wood is a poor conductor and its electrical permittivity ε can be expressed as

1 F     0 r  10 9  r   36 m

(1)

wherein ε0 – electric constant (permeability of vacuum). The value of the permittivity εr of wood is in the range of from 2 to 9 for the stationary field. Timber dielectric permittivity increases with the higher moisture content. Wood biomass placed between the capacitor plates in highfrequency, high-intensity electrical field heats evenly across its entire volume. This property is widely used in drying and gluing processes. Electrical conductivity along the grain of a solid wood is twice the conductivity compared to across the grain, thus the conductivity across the grain is usually relationship to electrical resistance. Changes in resistance can therefore be used to measure moisture content using electrical hygrometers. IV. EFFECTS OF MOISTURE CONTENT IN WOOD Trees growing in forests or plantations are periodically cut down to obtain materials for a wide range of applications. Wood is an anisotropic and hygroscopic material and therefore easily accumulates water, thereby changing its physical and chemical parameters. An important parameter in view of the use of wood in various forms is the moisture content, usually given as a percentage of the dry weight of H2O. Immediately after felling the trees, the moisture content (humidity) of wood is around 35%, but in some cases it is much higher. Air-dried wood or wood pulp has a relative humidity of about 15-20%. Storing in a dry place reduces the moisture content to about 8-13%. High humidity in wood or wood pulp creates favorable conditions for the development of various types of fungi. Wood with high moisture content is not suitable for combustion in furnaces due to the fact that during the combustion process, a lot of energy is lost while drying the material before a process of combustion can start. Electrical conductivity of wood across the grain is usually measured and reported. Its conductivity increases as the moisture content increases within the range of 0-30%, where 30% approaches the wood fiber saturation point. Further increase in moisture content has little impact on the conductivity and resistance of wood. Change of water content in wood or in the pulp results in a change of electrical conductivity and dielectric permittivity of the medium. Wood binds considerable amounts of water. The relationship of the dielectric permittivity and the water content is that increasing the amount of absorbed water increases the dielectric constant εr of the wood.

Hygrometers of different types are used to measure humidity of solid wood, sawdust, dust, shavings, wood chips, cut straw and baled straw, wood pellets, grain, husks, stalks, and shredded and compressed wood waste destined for various purposes, mainly for combustion. Moisture content of wood in various forms can be studied in several ways. The most accurate methods are used in laboratories because they require expensive equipment and a lengthy test procedure. Less accurate, industrial methods are used by pulp producers at various stages of the production process; from drying wood pulp, through timber depots and factories. The less accurate electrometric method is based of measuring the electrical conductivity of wood. It is widely used in wood or pulp processing both by large factories and small wood craft shops. The drying-weight method is a laboratory method and it takes a lot of time to complete the test. It involves taking a standard size sample, the precise weighing of the sample, and then placing it in a laboratory oven that maintains the desired temperature of the sample during the drying process, while it is repeatedly weighed. Comparing the wet sample weight and the weight of the sample after drying, moisture content can be calculated with error of less than 1%. In contrast, the electrometric method relies on the measurement of electrical resistance of wood, which varies with changes in moisture content of wood. There are different types of hygrometers on the market, some are equipped with sharp electrodes driven in a certain way into the wood, others have flat electrodes between which the compacted wood pulp is inserted. Typically, this type of equipment is not provided with a calibrated meter and the information about the moisture content of wood is given in ranges using simple signaling such as lighted LEDs. These devices also allow moisture measurement with automatic temperature compensation. Measurement of moisture content of wood and wood pulp using the electrometric method are carried out in accordance with the standard: PN-84/D-04150 Lumber-Determination of moisture content (publisher: Polish Committee for Standardization Measures and Quality). It has to be noted that continuous measurement of moisture content of wood and wood-based materials by this method presents a lot of technical difficulties. Permittivity of the material is related to its polarization vector. In the case of extremely low frequency fields the polarization vector is in phase with the vector of the electric field that causes it. Measured permittivity has then the actual value. In the case of high-frequency electromagnetic fields a delay of the polarization vector appears with respect to the electric field vector. Permittivity is then complex, with the time delay of polarization causing additional power losses. The real part of the complex permittivity describes the ability of the medium to store energy storage of the electromagnetic field. The imaginary part of the permittivity is a loss of heat energy field associated with the movement of the vibrating dipoles. The essence of microwave moisture measurement is the impact of the water content on dielectric properties of the material per unit volume. In the case of a wet sample it is a composite of the dielectric permittivity and permeability of the dry material and the water contained in the sample. V. WAVEGUIDES IN TECHNOLOGY The electromagnetic wave is an energy carrier, and today it is also practically used to transmit information. Energy of

high-frequency electromagnetic field is a cyclic exchange process between magnetic field energy and electric field energy. The propagation of the wave involves continuous conversion of electric energy into the magnetic and vice versa. In the electromagnetic wave there are regular periodic changes in the intensity of the field at points spaced apart by a distance that the wave travels during its interval or period. The distance between two adjacent points of the space, in which the intensity of the electric or magnetic field is the same length is called the wavelength. Wavelength [m] is closely related to the frequency [Hz], which specifies the number of vibrations per second of the electric or magnetic field. The relationship between the length λ and the frequency f of the wave is defined as follows (Six J., 2001):



c f

(2)

where: c - is the speed of light [m/s]. The electromagnetic wave can move in free space or may be conducted in the wave guides. Waveguide is a system of boundary surfaces forming a continuous path between two points in space, which allows directing the flow of the energy of the electromagnetic wave along this path. Keeping electromagnetic waves in the wave guides is carried out by matching its type to type of the associated electromagnetic field it is intended to propagate. Type of a field depends on the shape and dimensions of the waveguide, the filler material and the wave frequency. The set of possible types and kinds of electromagnetic fields occurring in wave guides can be determined from the corresponding solution of Maxwell's equations (Dobrowolski J., 1998a) and boundary conditions. Each waveguide has a characteristic cross-section. Assuming that the wave propagates in the direction of the axis is then classified into the following types (Dobrowolski, J., 1998; Morawski et al. 1998a): TEM-type wave is the Transverse Electric-Magnetic wave. Vectors of the electric component and the magnetic component are in a plane perpendicular to the propagation direction (Ez, Hz) and each of these vectors has at most two components. TEM waves of this type are present in the guides of which cross-sections are multi-coherent, (e.g., coaxial line, two-wire line, symmetrical and asymmetrical stripline) The E-wave, also referred to as TM (Transverse Magnetic) , has a non-zero component of the electric field intensity vector of the wave in the propagation direction (Ez) and zero vector component of the magnetic field strength in a plane perpendicular to the direction of wave propagation (Hz). H-wave, also referred to as TE (Transverse Electric), has a non-zero component of the magnetic field vector in the direction of wave propagation (Hz), and a zero electric field component lying in a plane perpendicular to the direction of wave propagation (Ez); TM and TE type waves are induced in guides with a single-coherent cross-sections (e.g., rectangular waveguide, circular, spine) and in the guides with multicoherent cross-sections at higher field frequencies.

Wave EH, has non-zero components of the electric field vector and the vector magnetic field in the direction of wave propagation (Ez, Hz) Each waveguide is bounded by surface which presents an abrupt change in one of the parameter characterizing the electrical properties (e.g., permittivity or conductivity). Waveguides are either closed or open. In closed waveguides (e.g. metal tube of rectangular, circular, oval, and other cross-section) the wave travels inside the waveguide. In open waveguides (e.g., a metal wire, rod or tube made of dielectric substance) the wave moves outside the waveguide. Propagation in the waveguide is possible when the transverse dimensions are comparable with the wavelength, or are larger. The waveguides are therefore primarily used for transferring electromagnetic wave lengths in the centimeter and millimeter range. The electromagnetic wave transmission characteristics in each depend on the geometry of the waveguide cross-section and types of materials, the dielectric filling of the working space, and the material of which the conductive walls are made. They also depend on the quality (smoothness) of finishing of the side walls. Phase velocity vf [m/s] in the waveguide is always greater than the speed of light, while the group velocity vg [m/s] is smaller than the speed of light. The product of the phase and group velocity is equal to the square of the speed of light (Rosłoniec S., 1999).

vf vg  c2

(3)

The study will consider a rectangular waveguide, wherein the energy in the electromagnetic field is propagating along the waveguide. For the simplified analytical calculations it is assumed that the waveguide path is homogeneous (it has a uniform cross section along its length), that the walls of the waveguide are made of an excellent conductor and that the waveguide is filled with a homogeneous, isotropic, linear and lossless substance. Field distribution in the rectangular waveguide is referred to as a type of wave or waveguide. There are many types of waves and their distinction described by the letter ‘E’ or ‘H’ and the two indices ‘m’ and ‘n’. Below the lower cut-off wavelength λc [m], also known as the wavelength cut-off frequency, the waveguide does not transfer energy. Wavelength cutoff frequency (minimum frequency that can move the waveguide) is defined by the following equation (Galwas B., 1985; Morawski et al., 1998b):

f 

 2

 1     c

(4)

Where: λ - the length of the wave generated by the microwave source. Wave phase velocity in the waveguide is determined by the formula (Galwas B., 1985a; Morawski et al. 1998c): vf 

c 2

 1     c

(5)

The lowest frequency of the rectangular waveguide is always type H. The basic type is then H01 or H10. The basic type has the lowest frequency, or the longest wavelength, at

which you can transfer electromagnetic energy in the waveguide. There is a range of frequencies at which the attenuation of waves in the waveguide with a rectangular cross section is smallest. For a rectangular waveguide which has a wider inner wall ‘a’ the frequency range is defined by the relation (Dobrowolski, J., 1998b):

(1.15  1.2)

c c  f  0.9  0.95 2a a

(6)

(6) we determined the actual length of wave propagating in the waveguide, which is 60 [mm]. This means that the length of standing waves in the waveguide is λf/2=30 [mm]. A slot of that size was cut on both sides of the waveguide, removing part of the walls of the short sides (Fig. 1). In the area of the recess waveguide essentially becomes a symmetrical transmission line with a slight drop in power propagation. With this solution, it was possible to place crushed and granulated pellets in the slot, which enabled the study of the effects of humidity of the wood on propagation of microwaves and power loss across the microwave path.

The equation above results from the properties of the electromagnetic field and its distribution in the rectangular waveguide. VI. INITIAL TESTS TO CONFIRM THE FEASIBILITY OF INDUSTRIAL APPLICATION OF THE METHOD Basic research and preliminary measurements were performed on a custom designed and built set at the HighFrequency Laboratory of the Faculty of Electrical Engineering Department Częstochowa University of Technology. Dielectric properties of the medium (relative permittivity) depend on the temperature and the concentration and activity of the electric dipoles due to their position in accordance with changes in the forcing electric field. Water molecules are dipoles, and in the absence of the external field due to thermal motions they remain disordered. When an external magnetic field is applied to a dielectric material, the energy of this field is dissipated, which is related to the internal polarization of water molecules in the wet dielectric. Preliminary studies have shown that by measuring the electromagnetic wave attenuation in the wood by-product and using a dependence of these parameters on water saturation, you can calculate its moisture content. If the length of the electromagnetic wave and the parameters of the sample during the measurements are constant, the attenuation depends only on the dielectric properties of the material inside of the waveguide. The moisture content can then be determined directly on the basis of knowledge of the relevant parameters of the electromagnetic wave after passing through the length of the waveguide. The use of novel methods for the identification of moisture in the wood biomass, is that a closed waveguide with a transverse slot is used. This measurement slot through which the measured wood based material can pass can easily be adapted to existing industrial installation applications where wood or pulp is used in power generation. VII. MODEL TESTING In the proposed method, the water content of the pulp is determined by measuring the ratio of the power P2 of the electromagnetic wave after passing through a modified waveguide P1 to the power supplied by the generator. On the basis of these measurements you can determine humidity of wood chips placed in a slot in the waveguide. The study used a standard copper waveguide with rectangular cross section of WR 159 (the number specified here is the width of the inner waveguide in hundredths of an inch) with internal dimensions of 40.4 × 20.2 [mm] and limit the operating frequency range of 4.9 -7.05 [GHz] (Fig. 1). The frequency of the generator device was 6.9 [GHz], corresponding to the wavelength λ=43 [mm]. Lower limit frequency (cutoff frequency) of 4.9 [GHz] corresponds to a wavelength of λ=61 [mm]. Substituting λ and λc in equations

Fig. 1. Photograph of rectangular waveguide with a transverse slot with dimensions measuring 30×20 [mm], built at the High-Frequency Lab of the Faculty of Electrical Engineering Częstochowa University of Technology.

Laboratory measurement system shown in Figure 2 was constructed from the following laboratory equipment: generator VGSH 100 A by Sintra-Alcatel operating in the frequency range from 0.5 [GHz] to 18 [GHz], which allows setting a fixed input power fed to the coaxial cable system waveguide. By matching circuit (transition waveguide) CWT-4880-0T company UTE Microvawe Inc.. (USA) containing an isolator dampening the reflected waves, the waveguide was fed through a cutout with dimensions 30×20[mm], corresponding to the length of the standing wave. The waveguide transition was completed by a twin UTE Microvawe Inc waveguide to N-type connector, connected to the power meter Boonton 4220, measuring output power P2 of the system. A sample of wood chips of known weight and moisture content was placed in the opening of the waveguide. It was slightly compressed to better fill the opening cross-section. The reference point was the power input P1=2 [mW] determined for the case of a waveguide filled with dry wood material. Accordingly, any decrease in output power was associated with the moisture of fine or granular pellets placed in the measurement slot. As a comparative tool to determine the moisture content of samples we used hygrometer BIO-1 by TANEL Electronics and Information Technology in Gliwice. It measures the relative humidity of sawdust, wood chips and shavings in the range of 8% to 30% at a concentration of about 0.2 [Mpa]. From the physical point of view, made research activities correspond to the introduction of the transverse hole of the dielectric waveguide having a relative dielectric constant greater than the relative dielectric constant of air filling the waveguide. It should be added that the introduction of fines of dry wood or dry piece of wood (with a moisture content of less than 8%) corresponds to the introduction of a dielectric waveguide having a relative dielectric constant

of order 3. Since the ε in the frequency range 1 [GHz] to approx. 22 [GHz] ( the first resonance frequency of water) is constantly increasing thereby increase the absorption of the microwave in the waveguide by the wet pulps. Therefore, the higher the relative humidity of the particulate granular material or pellets, the more attenuated the electromagnetic wave becomes. As a criterion for research we assumed transmission efficiency of the microwave power, i.e. power ratio P2/P1 (Fig. 2).

range of fmin=4.9 [GHz] to fmax=7.09 [GHz]. The frequency generator was set to 6.9 [GHz], which corresponds to H01 The results are presented graphically [9]. The test results are shown in Figures 3 (with pellet dust) and figure 4 (fine pellets).

Fig. 3. Efficiency of the measurement system for the relative humidity of fine pellets.

Fig. 2. Laboratory equipment configuration at the High-Frequency Lab of the Częstochowa University of Technology.

Preliminary studies have shown that the greater the moisture content of the pellet or the material the pellets were formed with, the lower the efficiency (or the more power is absorbed by the pellets). The amount of power at the input of the order of 2 [mW] which corresponds to the power level of 3 [dBm]. Using a low power level reduces the phenomenon of drying the pellet in the measurement slot while providing a safe environment for people. It should be added that most of the heat dissipation is within waveguide. The applied microwave power is relatively low and is completely safe for humans and animals. For comparison, a cell phone or a microwave oven radiate power of about 1000 [mW] which is 500 times greater. Recently, there is increased interest in the possible effects of exposure of the human body to microwaves in all areas of activity in which they are used. The electromagnetic waves in the frequency range used here are quite commonly used in mobile devices, broadcasting, satellite communications, microwave detectors and radar devices. Official exposure limits were set, beyond which there is a possibility of effects on human health. International institutions such as the ICNIRP (International Commission on Non-ionizing Protection - International Commission on Radiation Protection), whose activities in the field of nonionizing radiation is recognized by the World Health Organization (WHO), the International Labour Organisation (ILO) and the European Union (EU), sets limit on continuous exposure to the electromagnetic field in the microwave frequency range at power density of 1[mW/cm2], a value impossible to reach at power levels involved here. Frequency corresponding to H01 in the WR159 waveguide type with internal dimensions a×b=40.4×20.2 [mm] is in the

Fig. 4. The efficiency of the measuring system for the relative humidity of small pellets.

VIII. CONCLUSIONS With the increase in moisture content in the wood-based material, the power transmission efficiency in the waveguide decreases (this being the mean efficiency value of the measured output power to the reference power). Within the range of 15-16% humidity the waveforms of both characteristics (for dust and small pellets) as shown in Figures 2 and 3 show a slight disruption and change the characteristics form linear to hyperbolic. Since the dust contained more water particles in the same volume, testing for this material showed a steeper decrease of power output, which for 20% moisture content was about 0.44, while for small pellets it was 0.58. The study shows that it is possible to use this method of measurement for continuous control of moisture in a fine pellets or very small pellets, to avoid the use of excessively wet wood-based material as a fuel in industry, residential and technology applications. Due to the limited initial range of tests, further work is needed to validate the laboratory results and their applicability to the wide range of practical applications. Also a computer model of the phenomenon shown in laboratory tests is being created to verify and confirm the theoretical assumptions of the proposed solution.

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