31 An Agent-based System to Monitor an Energy Biomass Process Filippo Laganà Domenico De Carlo Salvatore Calcagno DICEAM Department TEC cooperative Spin-in with DICEAM Department DICEAM Department University Mediterranea University Mediterranea University Mediterranea Reggio Calabria, Italy Reggio Calabria, Italy Reggio Calabria, Italy Email: filippo.lagana@unirc.it Email: domenico.decarlo@unirc.it Email: salvatore.calcagno@unirc.it Abstract—This research is the study of a project, promoted by the Italian Ministry of Education, University and Research under the protection of the European Community, in a more complex “Smart Cities” project, devoted to the realization of an alternative system for green energy production. The system consists of an electrical power supply generated from the anaerobic digestion of biomass. It also includes the storage of electrical energy in a superconducting magnetic energy storage device in order to overcome energy blackout and meet the energy needs of the network when the demand rises in closed-cycle production Fig. 1. Percentage distribution among renewable energy sources in 2011. systems. Finally, an agent platform of remote control should monitor the whole system in the future. The actual prototype system furnishes 10 kW of power for I. I NTRODUCTION 1 minute but, currently, it does not take advantage from the The Earth has undergone over the century significant benefits coming from the adoption of the agent technology1 , changes due to Mankind. Because of this the global con- particularly in terms of mutual coordination among the many sumption of natural resources has exceeded their availability. components exploited to rule this complex process, of the Human population growth has given rise to a serious problem modular approach intrinsic into the agents and from the of energy supply. Most of the world energy requirement is possibility to exploit reliable and consolidated communication satisfied by fossil fuels, a non-renewable energy source. standards (very useful in presence of heterogeneous compo- Biomass is a renewable, perpetual energy resource, which nents). Therefore, the next step in the evolution of this project can supply human energy needs in a sustainable way. Biomass consists in monitoring, managing and coordinating all the is also a generic term for all vegetable materials storing solar system components by means of intelligent software agent energy through photosynthesis. During photosynthesis, the so- belonging to a multi-agent system appositely designed to this lar energy absorbed by plants is converted into carbohydrates aim. and oxygen through the utilization of the carbon dioxide The paper is organized as follows. In Section II the mon- already present in the air and the water, but the use of vegetable itoring activity of the realized by the software sensor agents material in the biomass transformation process can be consid- is described, while Section III provides a description of the ered an unlimited resource only if its rate of consumption does biomass plant structure. The sensor agent monitoring system not exceed its rate of biological regeneration. Therefore, for is introduced in Section IV. Finally, in Section V some every vegetable species used in biomass production, there is conclusions and future scenarios are drawn. an inherent limit which depends on the size of the field where it is grown, besides climatic and environmental variables. II. E NERGY AND R ENEWABLE S OURCES Therefore, biomass processing for fuel purposes requires large Only 13.3% of the energy produced on Earth comes from areas of land. renewable sources. Fossil products (such as petroleum, coal Environmental conditions, such as temperature and the and natural gas) plus nuclear sources meet 81.6% and 5.1% availability of water, influence biomass seasonality. The use of worldwide energy needs, respectively [9]. of biomass to produce energy can be considered advantageous About 75.2% of the total primary energy production from when the growth of the vegetable species is thick and vigorous renewable sources comes from biomass [10] as shown in Fig.1 and its availability during the year is sufficiently constant. Biomass solves the problem of waste disposal and reduces In the following, a biomass energy plant is presented. The net carbon dioxide emissions. Biomass is a raw material complete system, which includes an anaerobic digester, a 1 The interested reader might refer to an overwhelming number of surveys co-generator and a superconducting magnetic energy storage dealing with different scientific areas that take advantage from the agent system, is monitored by a sensors network [1]. technology [2]–[8]. 32 which can be converted into gas, liquid and solid energy, and then further processed in order to generate electricity and heat. Biomass-to-energy conversion technologies include di- rect combustion, co-combustion, cultivated biomass-to-liquid fuel conversion, and biogas production. Direct combustion of biomass produces heat for industrial and domestic use, electricity and gas which can be used as a driving force. In order to monitor the processes involved in the storage and digestion of the biomass that produces biogas, locally a software sensor agent should monitor the biomass cells in order to maintain the working conditions and communicate to its agency when the specified conditions become altered. Parameter monitoring is carried out in the liquid and gas phases, ensuring early detection of changes in the parameters Fig. 2. Production process of a biomass plant. that are indicators of the proper process of digestion. The sensor agent also calibrates the organic load in the digester based on the parameters. The basic characteristics of a sensor carbohydrate-rich biomass, has a positive effect on the pro- agent monitoring system should include the determination of duction of methane. Anaerobic digestion involves different the concentrations of alkalinity, pH, fatty acids, and ammonia. microbic groups interacting with each other which include Another very important parameter is that of Volatile Fatty hydrolytic bacteria, acidifying bacteria (acetogenic and ho- Acids (VFAs), the accumulation of which is indicative of some moacetogenic) and methanogenic bacteria that are responsible type of instability occurring in the process. These parameters for producing methane and CO2 . The microorganisms, which are Normally detected by Infrared Spectroscopy (NIRS), by cause the biological degradation of organic matter, according an electronic nose, by gas chromatography and by means of to the temperature range where they act, are divided into biosensors, which should be managed by the sensor agent. Psychrophilic (temperatures below 20◦ C); Mesophilic, (tem- In particular, the measurement of the concentration of bio- peratures ranging from 25◦ C up to 45◦ C) and Thermophilic, gas produced is a reliable method of monitoring the digestion (temperatures above 45◦ C). phase. Thermal conductivity sensors are used for separate mea- Due to the slowness of the anaerobic reactions in the surements of methane and carbon dioxide; these are composed psychrophilic field (temperatures < 20◦ C), the process is of two thin coils covered with platinum and are relatively normally performed in mesophilic (30◦ C – 35◦ C) or even fragile. thermophilic (efficient values around 55◦ C – 60◦ C) ranges. The infrared sensors are used to simultaneously assess the Methane-producing bacteria can only live in an anaerobic concentration of methane and carbon dioxide, even though environment with humidity content of at least 50% in the they are very expensive and require a complex electronic substrate, and they also require continuous monitoring and configuration [11]. In order to monitor a process of the control of the other parameters involved in the process. anaerobic digestion of waste of food origin, it is sufficient to Depending on the type of gas to be analyzed, there are provide for the evaluation of parameters such as temperature different kinds of sensor systems. For carbon monoxide and and alkalinity, besides the concentration of ammonia and nitrogen dioxide an electrochemical cell is normally used; VFAs. total organic carbon, however, is determined according to the presence of volatile organic compounds (VOCs) through the III. B IOMASS P LANT S TRUCTURE use of a photoionization detector (PID). Another relevant aspect to consider during the monitoring A. Production Chain of a biomass energy plant is the analysis of all the parameters Biogas plants comprise several technological components involved in the inhibition of bacterial growth, which can divided into operational units, see Fig. 2 limit the transformation of the substrate inside the anaerobic Anaerobic digester reactors are made of the reinforced digestion system into the final product. Substances such as concrete or steel, and include several components designed heavy metals, salts, residues of pesticides and pharmaceutical to re-create the ideal conditions for the biochemical reactions products, solvents, etc., can adversely affect the whole process that lead to the production of biogas. of anaerobic digestion. Moreover, the substrate itself can be a limiting factor because it is able to influence the successive B. Anaerobic Digestion Process stages of the digestion process. The degradation of organic substances and their subsequent Attention should be paid to the monitoring of some conversion into biogas can vary from 40% to over 95%, metabolic intermediates such as propionate, which is a quan- depending on the type of biomass used, the processing con- titatively important intermediate in anaerobic digesters. Al- ditions and the time necessary for the degradation process. though the concentration of propionate is usually quite low, an Biomass that is rich in fats and proteins, as compared to increase can prove to be toxic. The toxicity limit for propionate 33 Radius 1.13 mm Number of filaments 36 Composition Ni 70%, Cu 20% Critical Current @ 22 K 550 A TABLE I M AIN P ROPERTIES OF M gB2 C ONDUCTOR . In the presence of a magnetic field [12], this material can withstand higher temperatures without compromising the operation of the device while subjecting it to less stress. Fig. 3. Block diagram of a SMES system. D. Agent Smart Grid The realization of an agent smart grid currently is over appears to be around 3 g/l. The degradation of propionate is the aim of this paper, although it is the complement to a also influenced by hydrogen which can inhibit the microbial renewable energy system as that above described. Therefore, degradation of ethanol and, reversibly, the growth of abundant in this section, only a brief description of an agent smart grid anaerobic bacteria. Along more general lines, it has been approach is provided. reported in literature that high concentrations of volatile fatty More in detail, an agent smart grid is the evolution of the acids (VFAs) can also have toxic effects, mainly due to the traditional electrical network in which a set of specialized resulting decrease in pH. agents should monitor and coordinate sensors, communication, control and measurement systems allowing them to work together in order to monitor the flow of energy with the C. Storage System aim of overcoming the variability of consumption demands. The increasing demand for a high-quality power supply has The implementation of the new-generation agent smart grid resulted in a growing interest in the use of high-performance networks comes in conjunction with the growing demands energy storage technology. Superconducting magnetic energy for electricity that cannot be met by the enlargement of the storage (SMES) is able to store considerable amounts of old electrical networks, due to economic and environmental energy within the magnetic field created by an electric current problems. flowing through a superconducting coil maintained below the The technology that drives the agent smart grids should temperature of superconductivity by means of a cryogenic make the integration of renewable energy sources of differ- liquid, as shown in Fig.3. The stored energy is instantly ent origins possible, which, because of their dependence on available in the form of electricity and may be unloaded from variable phenomena, are discontinuous over time. the superconducting ring at an efficiency of over 95% of the A case in point is the management of a sudden drop in whole charge/discharge cycle. voltage by taking current from other districts that are having The two main blocks of the system are the superconducting a low absorption or that can store energy, such as in the case coil and the cryogenic cooling system. The coil requires of those with an SMES system. To this end, agent networks the presence of a magnet designed and built to work at connected to a remote monitoring agency play a role of cryogenic temperatures. In this way, the critical temperature primary importance, allowing the reduction and in some cases of the coil, that is, the temperature of transition between the elimination of the load losses of the network and also of the normal and the superconducting state can be maintained the possible interruption of the supply of energy. Moreover, within a controllable range. Also, the critical current, which in order to improve their reliability such grids could exploit defines the maximum current of transition from the normal the benefits of different technologies widely adopted in the to the superconducting state, can be raised to increase the multi-agent scenarios [13]–[17] performance of the storage system. IV. T HE S ENSOR AGENT M ONITORING S YSTEM The cryogenic cooling techniques are usually based on nitrogen- or liquid-helium-bath systems, even though the The necessary biological reactions take place inside di- experimental closed-cycle system under investigation allows gesters (anaerobic reactors), where the monitoring of specific the achievement of high performance with minimum energy parameters is required, in order to ensure the optimal condi- consumption, avoiding contact between the superconducting tions for the working of the entire system. Unfortunately, the material and the coolant. Finally, close attention is also paid different components often presents mutual coupling problems to the sensors and electronic control systems with a view so that the adoption of intelligent software agents could easily to reducing losses which, although minimal, may affect the overcame them [18]. Specifically, the main current process efficiency of the system. parameters that agent sensors should control are: The coil is made of Magnesium Diboride (M gB2 ), the i. Temperature has important effects both on the physical- characteristics of which are reported in the table I. chemical characteristics of the biomass in the digester and 34 Component Volume percentage on the microorganisms. For example, it affects the kinet- Methane (CH4 ) 50% – 80% ics of the process along with the selection of the bacteria Carbon Dioxide (CO2 ) 50% – 20% capable of operating in the temperature range selected. Nitrogen (N2 ) < 1% It is recommended that temperature fluctuations greater Hydrogen (H2 ) < 1% Ammonia (N H3 ) < 1% than ±1◦ C – 2◦ C within the chosen temperature range Hydrogen sulphide (H2 S) < 1% be avoided, because even small changes can significantly TABLE II affect the outcome of the process. B IOGAS C OMPOSITION . ii. Volatile Fatty Acids (VFAs) are organic acids produced during the degradation of organic matter. The concentra- tion of VFAs is expressed as the concentration of acetic (CO2 ) with small amounts of other gases, including hydrogen acid in the material volume (mg/L); it depends on the sulfide (H2 S), hydrogen (H2 ), nitrogen (N2 ), and low molec- quantity and the quality of the material loaded into the ular weight hydrocarbons. Typically, the bioreactor contains digester, as well as on the balance between acidogenic 50% – 75% methane and 25% – 50% carbon dioxide; the and methanogenic bacteria. remaining gases are present in very small quantities. The iii. Alkalinity represents the system’s ability to accept pro- composition of the biogas can vary in terms of concentra- tons, and it is expressed as the concentration of calcium tion depending on the raw material used and the operating carbonate. The alkalinity of an anaerobic digester is conditions. determined by the coexistence of ammonia, originating Since biogas is normally made up of a mixture of gasses, from protein degradation, and bicarbonate, derived from its characteristics must be evaluated in each individual case. the dissolution of carbon dioxide in the medium, forming However, in many instances, the physical characteristics of the a system able to buffer the lowering of the pH due to the three main constituents, namely, methane, carbon dioxide and accumulation volatile fatty acids. hydrogen sulfide, can be used to characterize the biogas. iv. Ratio VFA/Alkalinity. The concentration of VFAs and The temperature of biogas is measured by a stainless alkalinity are two parameters which are very sensitive to steel electrode sensor, which is installed on the wall of the changes in the system, and their ratio is a diagnostic pa- bioreactor, and measures a range of values between −40 ◦ C rameter indicating possible conditions of instability. Val- and 135 ◦ C. The probe consists of a 20 kΩ thermistor, a ues of around 0.3 indicate stable operation of the digester, variable resistor the resistance of which decreases nonlinearly while higher values indicate the accumulation of VFAs with increasing temperatures. and the onset of stability problems. The VFA/Alkalinity The interface measures the value of resistance (R) at a ratio has diagnostic significance because it describes the certain temperature and converts the resistance using the dynamics going on between material already digested Steinhart-Hart equation: (alkalinity represented by ash and ammonia) and new degradation (VFAs). High VFA/total alkalinity ratio val- h i−1 3 ues often indicate an overload of the digester [19]–[21]. T = A0 + A1 (ln 1000R) + A2 (1000R) − 273.15 (1) v. The Carbon / nitrogen ratio (C/N) in the biomass must be Where T is the temperature, R is the resistance and A0 , A1 between 20 and 40 in order to avoid deficiency or excess and A2 are constants. of nitrogen. The alkalinity of the substrate can be measured by the vi. Concentration of ammonia. Ammonia is produced during use of laboratory instrumentation, such as titration, infrared the degradation of proteins. A high concentration of spectroscopy and liquid or gas chromatography. Before being ammonia can inhibit both methanogenic and acidogenic sent to a laboratory for analysis, the sample is pre-treated bacteria. Concentration ranges: with reagents. There are also indirect methods to measure the • 200 – 1,500 mg/L (never toxic); value of alkalinity that make use of sensors and calculation • 1,500 – 3,000 mg/L (inhibitory if the pH is below 7.4); software, and measurements are obtained in real time. This • 3,000 mg/L (always inhibitory). monitoring system uses three types of sensors, pH, redox However, the presence of ammonia is important because potential, and electrical conductivity, which are installed on it buffers the system inside the digester and it compen- the wall of the bioreactor and monitore by a sensor agent. The sates for the accumulation of VFAs, maintaining a stable data are displayed, stored, and processed by the agent for the pH. calculation of the alkalinity level, according to the following vii. pH. This value depends on such parameters as the con- equation: centration of VFAs, ammonia and alkalinity. In a stable digester the pH value should be around 6.5 – 8. If the pH value falls below 6.5, then an accumulation of VFAs has alk = −8906 + (1678 · pH) + (1.998 · redox) + (384.2 · EC) occurred, often because the digester has been overloaded. (2) The probe that detects the pH is a sensor electrode char- The gas produced during anaerobic digestion consists acterized by a double junction and a polycarbonate body mainly of a mixture of methane (CH4 ) and carbon dioxide equipped with a flat glass membrane which makes it durable 35 Fig. 5. Tank level monitoring based on PVDF transducer. Fig. 4. Experimental digester of the future sensor agent monitoring system. and easy to clean. The pH probe measures values between 0 and 14. The probe which measures electrical conductivity in order to determine the ionic content of an aqueous solution is characterized by three ranges of work: • Low Range: 0 to 200 µS/cm (0 to 100 mg/L TDS); • Mid Range: 0 to 2,000 µS/cm (0 to 1,000 mg/L TDS); • High Range: 0 to 20,000 µS/cm (0 to 10,000 mg/L TDS). Fig. 6. Piezo-polymer film transducer obtained by curving a PVDF resonator. This probe measures the ability of a solution to conduct electric current between two electrodes expressed in Siemens. The characteristic equation of operation of the probe is: properly designed to work in hazardous environments and was fabricated in cooperation with the BATS Company, s.r.l. C = GKc (3) The resonance frequency is inversely proportional to the bending radius and can be easily controlled by varying it. where C is the electric conductivity, G is the conductance Neglecting the clamping effects, the resonance frequency is and the cell constant Kc is defined by the ratio between given by: (distance between the two electrodes)/(surface values of the electrodes). s 1 1 The oxidation reduction potential (ORP) probe is composed f= (4) of electrodes that measure the capacity of a solution to act 2πr ρsE 11 as a reductant or oxidant. The electrodes are composed of a platinum part immersed in the solution in which the oxidation- where r is the radius of the curvature and 1/ρsE 11 and ρ reduction reaction takes place, and another part in which the Young’s modulus and mass density of curved PVDF film platinum electrode is immersed in a solution of silver chloride material, respectively. The system includes an operational which is used as a reference. The probe can measure the power amplifier chosen to design a specific electronic cir- redox potential between -450 mV and 1100 mV. The probe cuit capable of driving the PVDF transducer over a wide that assesses the concentration of ammonia in the bioreactor band around the resonance easy assembled in portable in- is made of an ion-selective membrane electrode specific for strumentation or mounting on mobile robots. Because of the ammonium N H4 (ISE). When this membrane electrode comes ferroelectric polymer’s inherent noise, the correct modelling in contact with a solution containing specific ions, it develops of the transducer’s electrical impedance plays an important a voltage which depends on the concentration of ions in the role in designing the electronic circuits. The actual custom solution. transmitter concentrates all its energy in the frequency band The data measured by the sensors are sent to a computa- of the transducer. The sonar system makes use of a high tional unit, on which has to run the sensor agent that has to performance hemi-cylindrical PVDF transducer working at 60 perform all its required tasks of data processing. kHz for the evaluation of the time of flight (TOF) by using Tank-level monitoring was accomplished through the design the cross correlation algorhythm. and realization of a sonar system, which included the fabrica- The concentration of VFAs can be measured using spectro- tion of both transmitter and receiver made in our laboratory scopic or more innovative techniques, such as the electronic with a sheet of polyvinylidene- fluoride (PVDF) as shown in nose or biosensors. Fig. 5. The methods of on-line detection of VFAs are divided into By applying an alternating voltage between the two elec- three categories: the titrimetic method, the optical method and trodes, the semi-cylindrical geometry and its lateral constraint the sensor/biosensor. allows the conversion of longitudinal motion into radial vi- The more reliable methodologies are those which require bration. The PVDF transducer is shown in Fig. 6. It has been the use of laboratory instrumentation, such as spectroscopy, 36 which are able to determine the concentration of individual R EFERENCES acids besides that of the total concentration. [1] A. S. Dhoble and P. C. 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