Multi-parameter physico-chemical sensing principles for biogas-process monitoring

Renewable energies such as biogas are gaining steadily in importance. The use of methane gas obtained from fermentation processes in biogas plants takes on an added interest for energy supply. Overloading such biogas plants with excessive biomass may have considerable economic consequences including inactivation of the biomass resulting in a cost-intensive restart. On the other hand, adding too little biomass to the biogas reactor results in the generation of less electricity and heat and revenue is lost. All plant operators have therefore a crucial interest in running their biogas plant as efficiently as possible. To do this, reliable online analysis is needed, since better monitoring and control can improve process stability and enhance process performance for higher yield in terms of energy. The fundamentals of the biogas production process with the focus on monitoring and control are given in Chapter 1. The present thesis focuses on the design and characterization of miniaturized multi-parameter sensor chips for applications in biogas process monitoring. Within the frame of this work, sensors for the detection of five physico-chemical quantities were developed employing different transducer principles. These quantities included the dissolved hydrogen concentration, the metabolic activity, the electrolyte conductivity, the pH value and the temperature. General information of the sensing principles of the sensors used in this work are described in Chapter 2. An one-chip combined amperometric/field-effect sensor for the detection of the dissolved hydrogen concentration was developed. Two transduction principles were integrated at the microscale, enabling new electrochemical detection opportunities. Special emphasis was devoted to the independent functionality of the two transducers proving the indirect measurement of dissolved hydrogen (see Chapter 3). A cell-based sensor for monitoring the metabolic activity and thus, the vitality of relevant organisms was utilized. Metabolic responses of the model bacterium Escherichia coli in suspension as well as immobilized by gel entrapment on a capacitive field-effect structure were studied to pulses of glucose and acetate. Correlations between cell number, glucose and acetate concentration, acidification rate, and time of the acidification period due to the consumption of the carbon source were examined (see Chapter 4). An extensive study of the high-k material barium strontium titanate (BST) as passivation and protection layer of a miniaturized electrolyte-conductivity sensor, which is referred to a capacitively coupled contactless conductivity detection (C4D) sensor in the following, was examined. For better understanding, an equivalent circuit model of the C4D sensor chip was developed and discussed. For comparison, contact-mode electrolyte-conductivity (EC) sensors were addition ally fabricated. Both sensors (EC and C4D) were characterized in electrolyte solutions with various conductivities using two- and four-electrode operation modes to study the influence of the protective BST layer. The obtained results clearly demonstrate the benefits of the use of the BST-based C4D sensor in a four-electrode configuration for contactless conductivity measurements: A linear dependence between the measured conductance and the electrolyte conductivity was obtained in a wide range of electrolyte conductivity from 0.3mS/cm to 50mS/cm; in addition, no negative influence of the protective BST layer on the conductivity sensor performance was identified (see Chapter 5). A multi-parameter sensor chip using BST as multi-purpose material was developed to monitor electrolyte conductivity, pH value and temperature in buffer solutions and biogas slurry. This sensor united the capacitively coupled four-electrode electrolyte conductivity sensor with a capacitive field-effect pH sensor and a thin-film Pt-temperature sensor. Here, BST was utilized due to its multi-functional properties as final outermost coating layer of the processed multi-parameter sensor chip. It served as passivation and protection layer as well as pH-sensitive transducer material at the same time. Multi-parameter characterizations of the sensor chip in buffer solutions with different pH value and electrolyte conductivity were conducted. Finally, the sensor chip was exemplarily examined in biogas slurry to evaluate the sensor and the suitability of BST as multi-functional material under harsh environmental conditions. The experiments demonstrated that all three sensor parts exhibited a very stable sensor signal (see Chapter 6). Chapter 7 closes the current thesis with a final summary of the results, new experiences gained throughout this work and an outlook including potential ongoing strategies.

Jaar van publicatie:2014

Toegankelijkheid:Open