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Leistungen
This result consists of the methodology to use power plant data to produce comprehensive information on the boiler. Boiler instrumentation information is usually targeted on the water/steam circuit to control the steam properties to the turbine. The measurements on the flue gas path are more restricted due to the larger difficulties on measuring reliable flow rates and temperature in large flue gas cross-sections. The boiler exit gas temperature is usually registered as it affects directly heat losses and therefore boiler efficiency. For small boilers based on grate firing and fluidised bed systems, boiling is carried out not only at furnace walls as well as in tube banks subject to convection to allow for a flexible boiler operation. In these smaller plants flue gas temperature is also measured in some positions along the convection circuit that is built in independent blocks, providing further information on the boiler.
Two on-line assessment models were developed in parallel by Instituto Superior Tecnico (IST) and Technical University of Delft (TUD), both with a graphical interface to display and consult the values calculated based on the measured data. For both cases the calculation procedures implemented were specific for each plant analysed but the graphical interface from IST is more general as it can import or allows the configuration of the boiler.
The calculations from TUD are directly based on measured gas temperatures and convection heat transfer. This model generates calculated values of heat transfer resistances for the heat exchanger surfaces near the flue gas temperature measurement. The graphical application from TUD integrates further plant data such as the soot blowing operations. The evolution of calculated and measured properties can be represented along time to analyse the effects of soot blowing.
The calculations from IST are based on the boiler exit gas temperature, avoiding the consideration of flue gas temperature measurements in other sections in the boiler with higher temperature. The calculations consider the convective section of the boiler as well as the furnace with radiation heat transfer. Based on this model, temperature along the whole flue gas circuit is calculated and the comparisons performed in the project with plant data showed that calculated furnace exit gas temperature is in general higher than the correspondent plant data but is closer to specific measurement performed in test campaigns during the project. The IST model can therefore be applied to boilers with minimum plant instrumentation and may perform `virtual¿ instrumentation located in higher temperature sections. Based on the calculated and measured values, the heat transfer resistance in all heat exchangers are calculated and can be represented along time in the graphical interface.
The graphical interface from IST has three main menus to display data in sketches, tables or trend graphs. The boiler components for both gas and water/steam circuits are represented by icons where relevant parameters and properties can be made available. Tables and trend graphs can be configured by the user and saved in profiles that can be selected from a menu, allowing the users to configure different types of users, who see information in different levels of detail.
The purpose of the project was, to predict slagging and fouling behaviour in the boiler in order to minimise of the soot blowing effort.
This leads to:
- Optimisation of the boiler efficiency,
- Better knowledge of the fuel behaviour,
- Increase of boiler availability,
- Easy to use for operators at plants.
1) Prediction of plant behaviour and efficiency at load changes, fuel changes etc.
To achieve this goal, the following steps were undertaken:
- Use of easily accessible steam cycle parameters, such as temperature, pressure, mass flow and geometrical plant parameters, stored on-line in an access database by VAB,
- Modification of an existing Fortran programme for boiler modelling, including a model for deposit conductivity calculation, delivered by NTUA,
- Fuel characterisation of fuel and ash samples analysis to achieve more detailed information on the ash melting behaviour etc to feed to the NTUA model,
- Set-up of the plant environment (geometry) in the model, definition of boundary conditions, off-line and on-line testing,
- Calculation of discrete heat transfer coefficients for all boiler sections,
- Additional heat transfer degradation tests with a specially developed probe in a test facility at USTUTT,
2) Monitoring: Two models (USTUTT and TUD) for two different power plants (Uppsala and Nyköping) are ready and successfully tested. The USTUTT model was developed for the pulverised fuel boiler at Uppsala. The heat transfer degradation can be monitored with real time data, a slag thickness can be theoretically obtained and a soot blowing policy can be developed.
3) Prediction of heat transfer coefficient transience during desired variation of plant load or running conditions, as time and duration of soot blowing etc was realised. The model could follow the heat transfer degradation successfully. The USTUTT model makes slagging and fouling predictions possible with restrictions regarding unknown plant operation modes chosen by the operator.
Vattenfalls work has been detailed data sampling and advanced measurements of slagging and fouling in large CHP-plants. In addition to this CFD modelling for slagmodel evaluation.
Logging systems for collection of operational data has been set up at two plants in the project. The logging system has been logging about 100 signals every 30 seconds for two seasons in each plant. The data are used as an input to the slagmodels, both at development and as source when running on-line.
The goal with the measurements was to provide the partners with quantified data on slagging and fouling for better understanding. These data may then be used as a base for evaluations of the dynamic model, which is to be developed in the project. The measurements systems used in the project are developed previous to this project. Some of them patented. The measurements have included gas phase potassium and sodium chlorides, CO, O2, HCl and depositprobes.
CFD modelling.
Numerical modelling of the combustion in the Fyris boiler has been performed within the EU-project called SLAGMOD. The Fyris boiler is fired with biomass, mainly with pulverised peat and wood. The objectives were to simulate one operating case in order to improve input data to the on-line models and to study if streaks of carbon monoxide appear and if so define their locations. Measured data originates from the operating system at the plant. No measurements were made in the boiler and therefore no comparisons could be made e.g. in the upper part of the boiler, close to the super heater. For the case chosen, the boiler fired a fuel that consisted of 70 percent peat and 30 percent wood. The electrical output was 110 MW and the steam production was 430 tonnes per hour.
Conclusions drawn are that the simulation gave a non-even distribution of carbon monoxide and oxygen at the outlet boundary. Streaks of carbon monoxide are found in the right side of the boiler. In order to improve the quality of the simulations, improved input data are needed. A more accurate air velocity out from the OFA ports can be set if the pressure at different locations in the air distribution system can be measured. A more accurate determination of the total fuel flow is needed. The model can be used to find out how the air distribution between different levels and different airports can be made in order to minimise the risk of getting CO streaks.
NTUA provided simplified models for the heat transfer degradation evaluation due to the ash deposition on the heat exchange surfaces in boilers. Therefore, the heat transfer properties of the ash deposit layer and the heat transfer mechanisms were investigated and simulated in a manner to take into account, as far as possible, the influence of the operating conditions and the ash physical properties.
The interest was focused on the ash deposit layer thermal properties where, even though various experimental works have been executed, very poor information for theoretical formulas with widespread applicability is available. The target was to produce theoretical formulas for the estimation of the thermal properties of any ash deposit configuration. The above-mentioned target was achieved and the formulas were incorporated into the overall online dynamic models.
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