Control biogas digester

Control and monitoring of the production process in the biogas digester

Introduction

Biogas is a mixture of methane, carbon dioxide, and small amounts of nitrogen, hydrogen, hydrogen sulfide and oxygen. There is produced during anaerobic fermentation of organic material. The fermentation process allows to take advantage of waste from agricultural production, agricultural processing, the production of biofuels, as well as municipal waste. Produced biogas is converted to electricity and heat, which in turn reduces global demand for fossil fuels. Intensive work currently underway on increasing the efficiency of biogas fermentation in order to maximise the benefits for both the environment and the economic optimisation of the production. One of the directions of this work is the use the electrical capacitance tomography.


Fig. 1. Heating scheme

Fermentation process

Holding a constant and uniform temperature throughout the volume of the biomass fermentation requires installation of the tank heating installation in piping form on the inner walls of the tank (fig. 1). Using the electrical capacitance tomography the heating installation must be build in the walls of the digester and will be put behind the electrodes so as to minimise its impact on the results of the measurements. Mixing of the contents of the fermentation vessel makes it easy for degassing, allows you to provide a constant temperature and uniform development of micro-organisms in the entire volume of the digester, as well as prevents scum on the mirror biomass. We can distinguish three main mixing method: mechanical, hydraulic and pneumatic. The most widespread is mixing with the use of vertical, horizontal or oblique mechanical agitators (fig. 2b). In the case of the ECT application to test the contents of the fermentation vessel has a limited use due to the investigation entered the environment heterogeneity, which represents a significant impediment to determine the state of biomass. The other more useful method is the mixing of hydraulic, in which to mix the contents of the fermentation vessel used by pumping biomass (fig. 2a). The last of the above methods is the method of mixing, pneumatic (fig. 2c), which uses pumped under pressure in the lower part of the biogas fermentation reactor. The mixing of hydraulic and pneumatic does not require mounting devices inside the fermentation vessel, and therefore does  not introduce distortions of point to the test object, what is beneficial for the quality of the measurements of the scans.


Fig. 2. Mixing of biomass and hydraulic a), mechanical b), pneumatic c)

Control system

Control System for biogas plant that can be built on the basis of any industrial controller PLC, respectively upgraded with analog and digital inputs and outputs. The inputs and outputs should provide the parameters required in the installation of the test equipment and measuring instruments for implementing, and in most cases both manufacturers drivers, sensors, and actuators ensure, to their devices implement basic ways of communication. Due to the fact that the installations and infrastructure of biogas plants are typically located on a fairly large area (several-hundred meters), used the drivers should allow dissipation of the system of monitoring and control, while ensuring compliance of the equipment, and the possibility of communication between different subsystems. When designing the system, you should also take care of the ability to communicate with external systems such as. accident notification systems or control or remote control. In the case of the ECT application we should also include the ability to automatically use the measurement results in the control process scans each of the elements of the system. A specific example of external integration is to connect the generator to the local distribution of energy, and hence the realization of synchronization systems energy network, recording the amount of electricity produced, as well as, if necessary exchange of such information with the receiving institution’s electricity. Delivery of raw materials can be carried out in a continuous or cyclical, every few hours. The amount of feedstock that should result from the chemical composition (proportion of C: N 100: 3), hydraulic retention time of from 20 to 60 days depending on the feedstock used and load the pollutant, which usually ranges from 3.5 to 6 kg s.m.o/m3/day.


Fig. 3. Measuring capacitance tomography system block diagram

ECT

The application of electrical capacitance tomography in the investigation of biomass (fig. 3) enables you to monitor the process in a way that is non-invasive. Comparison of the results obtained from testing scans with those obtained using traditional laboratory tests will determine the characteristics that indicate problems with the fermentation process. If the designation of such dependencies is to prepare a suitable tomography probes, the sensitive measuring equipment and the use of algorithms for reconstruction monitoring changes the contents of the fermentation reactor continuously and the designation of the relationship between the results of the measurements, and the facts determined on the basis of data collected in laboratory tests. Information on the distribution of permeability coefficient is obtained by multiple measurement of capacity on the surface of an object by changing the position of the electrodes to stimulate. The basic problem is to determine by experiment the permeability distribution depending on the specified internal construction of the test element. Measured capacity are the order of fF, which requires special measurement techniques. The measuring system consists of a sensor, specialized electronics to measure capacity and reconstruction. Now to reconstruct it is convenient to use a computer class PC. Electrical Capacitive Tomography allows observation of the physical and chemical phenomena occurring without the need to penetrate in their interior. The source of the information are the electrical capacity between the electrodes placed on the perimeter of the measuring sensor. On the basis of measurements of electrical capacity-solving inverse problem for the electric field, you can get a picture of the electric permittivity distribution inside the area of the measuring sensor. A very important feature of Electrical Capacitance Tomography for measurement is no physical interaction with the sensor during the medium. Another advantage of this technique is high speed data collection. Verification of the results obtained will be carried out by comparing the simulation results with the results of the analytical analysis for a set of test data. Measurable effect of the task will create a working model of simulation, which will allow for further studies towards practical applications.

Summary

The use of the electrical capacitance tomography in the investigation of the fermentation biomass should bring benefits in the form of more information on the current status of the fermentation process. The information is available on a continuous basis, will be able to be used in control systems and biogas plant that will boost its performance. Unfortunately due to problems in the document presented to implement a ECT in existing and working a biogas plant may be difficult or impossible to implement, so you should look for such technological solutions that will ensure that the use of electrical capacitance tomography in this area of the indystry will become possible. Implementation of ECT should start already at the design stage, biogas plants and the manufacturing process, however, due to the significant cost of such investment, you must first carry out the appropriate tests for your models, or experimental installations. The following work in several areas are possible to apply: research of industrial control systems integration capabilities with ECT, researching of the biomass at various stages of fermentation and study the possibility of building a customized probe size and construction for the industrial fermentation reactors.