Research Outputs
Numerical and experimental development of integrated electrostatic precipitator concepts for small-scaled biomass furnaces
Emission limited model predictive control of a small-scale biomass furnace
This paper presents the application of an emission limiting model-based predictive controller for a small-scale biomass grate furnace. The furnace has a nominal power of 100 kW with wood pellets as fuel, but it can be operated with different fuels as well. The model predictive approach extends the existing static feedforward controller of the investigated furnace with a dynamic feedback controller that is able to improve the combustion performance. Simultaneously, the formation of carbon monoxide emissions is minimized within the prediction horizon based on an available emission estimation model for pellets. The results obtained from closed-loop measurements show that the control concept is able to reduce carbon monoxide emissions in partial load operation up to four times while the control error of the supply water temperature for heating is nearly halved during transient operation. This is achieved by incorporating the emission estimation model into the constrained optimization of the predictive controller. Additional results obtained from closed-loop experiments for different fuel types with varying water contents demonstrate the advantages of the proposed model-based approach and its robustness with respect to typical uncertainties of the combustion process.
Integration of broader impacts and international perspectives into a sustainable energy engineering course
Energy-economic assessment of reduced district heating system temperatures
It is in the DNA of district heating (DH) systems that low temperatures are crucial for efficiency, guaranteeing cost competitiveness and integrating alternative heat sources. The general conviction within the DH community is, that reduced temperatures have positive effects for the whole system and economic benefits can be expected. However, there is a lack in evidence-based data to evaluate these effects in monetary terms. The innovative approach of this work is to analyze key characteristics for different technologies by means of energy-economic assessments to show evidence-based energy-related and monetary benefits of reduced system temperatures. The proven benefits should increase the motivation and conviction of utilities and customers in low-temperature systems, both for reducing system temperatures in existing networks and for new networks. The key indicator cost reduction gradient (CRG), introduced in previous work, was applied for the energy-economic assessment of reduced system temperatures. In total, investigations of nine heat generation technologies, the DH network itself and four storage types are presented. The CRGs for the heat generation technologies varies from 0.08 to 0.67 €/(MWh·°C). In the case of alternative heat generation technologies such as heat pumps and solar thermal, a higher sensitivity of the monetary effects compared to traditional heat generation technologies can be observed. Here, higher economic benefits and monetary savings can be expected in future DH networks.
Comparative thermodynamic analysis of an improved ORC process with integrated injection of process fluid
In contrast to water-steam Rankine cycles, the ORC process uses organic working fluids. For working fluids of the dry class, a recuperator heat exchanger is frequently installed to increase the cycle efficiency. This paper analyses an improved ORC process with these features: A liquid working fluid stream is injected into the vapour flow between the high-pressure and the medium-pressure stage of the turbine. Furthermore, the recuperator is replaced by a spray condenser. The main objective is to increase efficiency with moderate changes in the process layout. A thermodynamic comparison of the improved process with a state-of-the-art ORC process is carried out by simulations and optimisations. A significant efficiency gain for the improved ORC process is obtained by a combination of the aforementioned features, mainly because of an increase of the mass flow in the economiser of the vapour generator (better heat utilization) and a corresponding mass flow in the medium stage of the turbine (additional power production). As a use case, waste heat utilization from clinker cooler at a temperature level of 275 °C was simulated. The improved process would lead to a significant increase in the overall net efficiency by up to 14%, compared to a state-of-the-art ORC process.
Bewertung und Optimierung der Energieeffizienz von Thermoprozessanlagen
Die vergangenen Jahre standen im Zeichen von stark schwankenden Energiepreisen und Unsicherheiten in der Energieversorgung. Prognosen zeigen eine drastische Differenz zwischen Angebot und Nachfrage an fossilen Energieträgern für die kommenden Jahre. Energieeffizienz ist die Basis, um den zukünftigen weltweiten Energiebedarf decken zu können. Die Sektoren Industrieöfen bzw. Prozesswärme nehmen in Österreich und Deutschland einen merklichen Anteil am Gesamtenergieaufkommen ein. Durch Primärmaßnahmen an den Anlagen und eine konsequente Verwertung von Abwärmeströmen kann es zu einer deutlichen Verbesserung von Anlagenwirkungsgraden in Verbindung mit einer Reduktion von CO²-Emissionen kommen. Am Beispiel einer Durchstoß-Gasaufkohlungsanlage werden interne und externe Energieeffizienzmaßnahmen aufgezeigt und deren Auswirkung auf den Gesamtprozess einer Bewertung unterzogen. Eine Wirkungsgradsteigerung um bis zu 19 % sowie eine Reduktion der CO²-Emissionen um 547 t/a kann erzielt werden. Für die optimale Integration in ein betriebliches Energiekonzept ist die interdisziplinäre Zusammenarbeit von Anlagenhersteller, Energieplaner und Anlagenbetreiber erforderlich.
Carbon monoxide emission models for small-scale biomass combustion of wooden pellets
Tighter legal emission limits require means to prevent releasing harmful substances into the atmosphere during the combustion of biomass. Economic considerations suggest to meet these restrictions by improving the ability to predict and therefore prevent emissions, which can be done by improved control algorithms. This work presents different methods to obtain models for the prediction of carbon monoxide emissions in a small-scale biomass combustion furnace for wooden pellets. The presented models are intended for an application in model based control, either as part of the underlying model or for carbon monoxide soft sensing and fault detection. The main focus is on simple structures which can be handled by the already existing hardware of the furnaces. Different black-box models and a kinetic process model are introduced and compared. The black-box models are based on the measured flue gas oxygen concentration and the combustion temperature, since these measurements are typically available even for smaller plants. The obtained models are validated with measured data in order to find the most suitable structures, of which combined fuzzy black-box models show the most promising results. The presented methodology can be readily applied to the investigated furnace. However, the model parameters have to be adapted for other plants.