Online-measurement systems for agricultural and industrial AD plants – A review and practice test
(2014)
Online-measurement systems for AD plants in general are crucial to allow for detailed and comprehensive process monitoring and provide a basis for the development and practical application of process optimisation and control strategies.
Nevertheless, the online measurement of key process variables such as Volatile Fatty Acids (VFA) and Total Alkalinity (TA) has proven to be difficult due to extreme process conditions. High Total Solids (TS) concentrations and extraneous material often damage the sensors or have a strong negative impact on measurement quality and long-term behaviour.
Consequently, there is a need for new robust and accurate online-measurement systems.
The purpose of this paper is to give an overview of existing online-measurement systems, to present the current state of research and to show the results of practice tests at an agricultural and industrial AD plant. It becomes obvious that a broad variety of measurement solutions have been developed over the past few years, but that the main problem is the upscaling from lab-scale to practical application at full-scale AD plants. Results from the practice tests show that an online-measurement of pH, ORP, TS is possible.
Biogas, mit geringen Konzentrationen an Methan, entsteht unter anderem bei verschiedenen industriellen Prozessen. Wegen der Umweltschädlichkeit des Methans gilt es dessen Eintrag in die Umwelt zu vermeiden.
Das Ziel des vorgestellten Projektes war die Überprüfung eines umweltfreundlichen Verfahrens zur Reduzierung des Methans durch methanotrophe Bakterien. Die einzelnen Batchversuche liefen über 15 Stunden und zeigten eine starke Reduzierung des Methans von 18 auf 1 Vol.-%.
Mit Hilfe der Inline-ATR-FTIR-Spektroskopie im mittelinfraroten (MIR) Spektralbereich lassen sich gleich mehrere Prozessparameter für Biogasanlagen in Echtzeit und ohne Probenahme verfolgen. Die gemessenen Absorptionsspektren geben simultan Aufschluss über den Gehalt an flüchtigen organischen Säuren (FOS), die alkalische Pufferkapazität (TAC) und die Ammoniumstickstoff-Konzentration (NH4-N).
Dabei können unter Verwendung intelligenter Datenanalyseverfahren, wie z.B. Partial Least Squares (PLS), Regression oder Support Vector Regression (SVR) sowie in kontrollierter Laborumgebung, Vorhersagefehler (RMSECV) von 0.372 g/l (FOS: R2=0.971), 0.336 g/l (TAC: R2=0.996) und 0.171 g/l (NH4-N: R2=0.992) im Falle der PLS, bzw. 0.386, 0.259 und 0.110 g/l für die SVR erreicht werden.
Erste Inline-Messungen in einer Biomüllvergärungsanlage zeigen, dass die erwarteten Absorptionsbanden auch im Prozessbetrieb wiedergefunden werden können. Sie unterliegen jedoch einem ausgeprägten Temperatureinfluss, der bei der Quantifizierung dieser Prozessdaten berücksichtigt werden muss. Weiterführende Untersuchungen sind notwendig, um die Inline-Tauglichkeit des Messsystems unter Beweis zu stellen.
The introduction of Feed-in tariffs in the German Renewable Energy Act (EEG) fuelled the growth of anaerobic digestion (AD) industry making Germany the country with highest number of operational AD plants. However, the rapid expansion of AD industry resulted in some unwanted side-effects such as food vs fuel debate, increased prices for electricity and the temporal mismatch between supply and demand of electricity grid. Subsequent amendments in EEG has tried to address some of these issues by reduction in Feed-in tariffs, introduction of a cap on cereal based feedstocks and providing premium for energy production in accordance with market demand. Furthermore, the Feed-in tariffs which were introduced for 20 years are soon going to expire. The changes in legal and political discourse is soon going to introduce some new challanges to the AD industry. This paper has discussed some of these challanges and their potential solutions.
In this paper a closed-loop substrate feed control for agricultural biogas plants is proposed. In this case, multi-objective nonlinear model predictive control is used to control composition and amount of substrate feed to optimise the economic feasibility of a biogas plant whilst assuring process stability. The control algorithm relies on a detailed biogas plant simulation model using the Anaerobic
Digestion Model No. 1. The optimal control problem is solved using the state-of-the-art multi-objective optimization method SMS-EGO. Control performance is evaluated by means of a set point tracking problem in a noisy environment.
Results show, that the proposed control scheme is able to keep the produced electrical energy close to a set point with an RMSE of 0.9 %, thus maintaining optimal biogas plant operation.
The electricity network is undergoing a change due to reducing costs for renewable energy sources. Subsidy programs for renewable energy sources are changing and the funding available is being reduced. This will have an impact on anaerobic digestion which in some cases may struggle financially. Overfeeding is one of the most common mechanisms of inhibition in the process, and by shifting to intermittent feeding for on-demand production, this change will provide more information about the digestion process and could be used to detect the beginning of inhibition due to overfeeding. This paper discusses the shift towards intermittent production and how this change can be used to monitor the anaerobic digestion process.
Anaerobic Digestion of spent grains: Potential use in small-scale Biogas Digesters in Jos, Nigeria
(2014)
In order to ascertain biogas yield potential and applicability of spent grains (SG)1 in small-scale biogas production, laboratory batch fermentation was performed with various masses of dry and wet SG using sewage sludge (SS)2 and digested maize silage (DMs) 3 as inoculums. Different volumes of biogas and CH4 were measured with higher volumes observed for batch fermentation with DMs in com-parison to those produced by SS. Results from the study reveals minimum biogas yield of 118.10 L/kg
VS and maximum yields of 769.46 L/kg VS, which are indicative of the possible use of SG for domestic biogas production in Jos, Nigeria. The study established the fact that the use of both dry and wet SG results in the yield of a useful amount of biogas having 40 - 60 % CH4 content depending on the inoculum and amount of volatile solids present. Using the parameters of dry matter and volatile solids contents analysed for SG and DMs, it was estimated that a reactor volume of 6.47 m3 would be capable of meeting the daily cooking needs of rural households in Jos, Nigeria.