Energy conversion in power plants

Conversion of chemically or nuclear bound energy into electrical and/or thermal energy (for heat use) takes place in power stations. When chemically bound energy is used, this primarily entails the burning of fossil fuels but also includes waste and biomass. A large proportion of electricity production in Germany comes from burning hard coal, brown coal and natural gas (for example, see: AG Energiebilanzen e.V.).

Within this research focus in the past the following research projects were associated:

Renewable Power Generation by Solar Particle Receiver Driven Sulphur Storage Cycle
(PEGASUS)
PEGASUS will investigate a novel power cycle for renewable electricity production combining a solar centrifugal particle receiver with a sulphur storage system for baseload operation. The proposed process combines streams of solid particles as heat transfer fluid that can also be used for direct thermal energy storage, with indirect thermochemical storage of solar energy in solid sulphur, rendering thus a solar power plant capable of round-the-clock renewable electricity production. Process scheme of the solar sulphur cycle / Image source: DLR The overall objective of PEGASUS is the development and demonstration of an innovative solar tower system based on solid particles combined with a novel thermochemical solar energy storage technology based on elemental sulphur, to achieve dispatchable and firm renewable electricity generation with a significant cost reduction with respect to current state-of-the-art concepts. The technology will be validated under real on- sun concentrated solar irradiation in the Solar Tower Jülich (STJ) thermal plant in Germany owned by the Project Coordinator, DLR. In this perspective, the project’s specific Technical Objectives of KIT are: To develop and realize a novel lab-scale sulphur burner able to modulate in a range of 10-50 kW with quantitative targets: sulphur combustion with >99 % combustion efficiency at power densities > 1,5 MW/m³ under atmospheric conditions (3 times higher than conventional sulphur combustion chambers) and flame temperatures > 1400 °C. To demonstrate the feasibility of the over-all proposed process, draft the complete flowsheet and analysis of optimized integrated process scaled-up to the 5MW_th power level, assess the technology vs. the targets set. More information is published in a press-release of KIT and on the public website of the project (link below)

AP2000 Klärung der Trennmechanismen
(METPORE II - Nanostrukturierte, metallgetragene Keramikmembranen für die Gastrennung in fossilen Kraftwerken)

Modular extension of an overall model for improved prediction of combustion process in liquid fuel/water emulsions
(CEC3H)
The research carried out withing the subproject 3H contributes to the fulfillment of the projects' goal "operation flexibility and fuel flexibility". Operation stability is mainly depending on the the stability limit of combustion, which is still difficult to predict. Fuel flexibility requires the thorough design of a combustor which is able to operate on gaseous and liquid fuels. The goals of the subproject 3H, which continues the successful work of the subproject 1F stem from these requirements and challenges. The liquid fuel and liquid fuel/water emulsion combustion model developed within in the framework of subproject 1F is able to predict the heat release during the combustion of liquid fuels with a given droplet diameter and velocity distribution. Hence, different important aspects which are important for the application of the model have not been accounted for. The first aspect concerns the specification of the droplet properties which is currently derived from experimental data. The second aspect is the neglect of heat losses which have a major impact on the calculation of flame stability and emissions. Furthermore, the model has only been validated for kerosene until now but not for diesel or diesel water emulsions. The subproject 3H addresses these questions and aims to the development of a tool which can be used in the design process of a gas turbine combustion chamber. To this end, the atomization of the liquid fuel shall be described by an empirical model. Moreover, the influence of heat losses on the heat release rate shall be captured. Further aspects, e.g. the droplet wall interaction and the role of the fuel-to-water ratio distribution which have a significant impact on the gas turbine combustion process are investigated.

Modellierung des Verbrennungsverlaufs bei der Verbrennung von flüssigen Brennstoffen und Flüssigbrennstoff/Wasser-Emulsionen.
(CEC_1F)
In the context of the objective of the German government and the European Union's energy policy, it is crucial to increase the share of renewable energy. However, due to the fect that renewable engergy production due fluctuating wind and sun energy does not correlate with the customer demand, it is neccesary to compensate this energy generation gap with flexible power plants. Such plants need to be operated in a flexible load range. In this context, gas power plants play an important role because they allow rapid load changes and provide energy at high efficiencies. Goal of the current project is the development of combustion technologies for climate-friendly energy conversion. The research that is to be done in subproject 1F serves to fulfill the subgoal "operational flexibility and fuel flexibility." The operational flexibility is critically dependent on the stability limits of the combustion, but their prediction is not adequately possible up-to-now. The fuel flexibility requires the safe design of burners that can be operated with both gaseous and liquid fuel. One important way to foster climate-friendly power generation is the increase in the efficiency of gas turbines. Since the increase in the efficiency is related with the increase of the pressure and temperature levels of the process, the main objective is based on the optimization of the cooling of the highly stressed parts. Such an optimization can not be done without the knowledge of the temperature distribution in the combustion chamber and the combustion chamber outlet, which is also a primary goal of the project. The calculation of the temperature distribution or the distribution of heat release depends on the following sub-processes: Detection of the droplet dispersion, which is dominated by the turbulent fluctuation movements Calculation of evaporation, which depends on the evaporation characteristics and the turbulent heat transfer from the gaseous to the liquid phase Analysis of the interaction between the turbulence and heat release The true representation of the realistic subprocesses represent the scientific part of the project goals 1F Calculated (LES) average temperature in the vicinity of an airblast nozzle using hexadecane as fuel (left: with - right without water addition).

TURBOmachinery REtrofits enabling FLEXible back-up capacity for the transition of the European energy system
(TURBO-REFLEX)

The energy sector accounts for two thirds of the global CO₂ emissions and is therefore crucial to ensure future green growth and to achieve the global emission reduction targets. Substantial reduction of CO₂ emissions can only be achieved by large scale deployment of renewable energy sources, including in particular the most abundant energy sources, wind and sun. Their intermittent nature however poses significant challenges for the energy system as peak demand from the system and peak production form those intermittent sources do not overlap. As there are no large scale storage solutions available yet, other backup capacities are needed. The installed fossil capacity is large enough to provide this back-up power. However, the plants were designed for baseload operation, which results in increased wear and costs through cyclic operation and unnecessarily high emissions in the start-up phase. Providing technology upgrades to retrofit the installed power plants to enable flexible operation without penalties on life, cost and emissions is an opportunity to quickly provide the necessary backup capacity to keep the energy system stable and resilient and at the same time enabling higher renewable shares.

The mission of TURBO-REFLEX is the development and optimisation of technologies, applicable to a selected set of turbomachinery engine components, which can be used to retrofit existing power plants as well as new machines in order to enable more flexible operation, providing the flexible back-up capacity needed for introducing a larger share of renewables in the energy system. TURBO-REFLEX will assess the impact of such technologies at plant level and prepare the transfer of component technology gains into reduction of both (unplanned and planned) outages and maintenance and operation costs.

The Lean Blow Off (LBO) limit is a significant hurdle to further reduce the part load of gas turbines as the operating zone of the combustor is restricted by the LBO limit. Jet stabilized premixed flames will be forecasted with blow off stability down to 1000°C–1200°C combustion temperature with or without using pilot flames. 1000°C–1200°C combustion temperature would equal emission compliant part load operation down to 20%-25%. Better blow-off stability in the combustor is a prerequisite to running higher load gradients. Therefore, it is expected that jet stabilized premixed flames with better LBO limits will allow also gradients faster than 40MW/min.

EBIvbt at KIT will model the blow-off of jet flames with advanced computational models. These models include basic geometrical parameters like duct diameter and dump ratio, but also the effect of neighboring pilot flames. A 3D simulation model will be developed and experimentally validated at conditions close to the application. The turbulence-chemistry interaction will be captured by two different combustion models. Within both models a transport equation of a reaction progress variable will be solved. The difference between the combustion models is in the source term modelling. In the first model the source term depends on mixture fraction and the progress variable itself. In the second model, which is based on the “turbulent flame speed closure” approach, the source term depends on the laminar flame velocity. So, one can calculate the influence of the stretch and heat loss on the laminar flame speed by simplified 1D modelling. The comparison of the two models to experimental data will show which model is more suitable for the applied boundary conditions.

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Within this research focus in the past the following research projects were associated: