STANDARD A generic industrial boiler from Cyclotherm South Africa
Numerical modelling of bulk processes using computer-based models provides a quicker and more beneficial method than manual calculations for complex steam generation systems, says engineering, construction and maintenance services provider Steinmüller Africa.
Recently, full computational fluid dynamic (CFD) modelling has matured to the extent that models include a high level of detail both geometrically as well as in the fundamental simulation of the various processes in steam and power generation plants.
These include fluid flow (both single and multi-phase), materials handling, combustion, heat transfer, mass transfer and erosion to mention a few. In cases of modification as well as new designs, CFD is beneficial as an engineering tool in assessing various operating scenarios, both within and outside the design envelope.
These rigorous simulations give a clearer and more concise understanding of the reasons for operational behaviour and reliability trends that cannot always be explained through a combination of manual calculation and intuition.
CFD Models for Thermal and Steam
The CFD models are based on axiomatic conservation equations of mass, energy, momentum and chemical species. The models can be used to simulate and predict, both qualitatively and quantitatively, fluid flow fields, temperatures, pressures, chemical species, particle tracks and many others for even the most complex of schemes such as pre- and post-combustion systems as well as the actual combustion process itself.
It is desirable for steam production to achieve proper and complete combustion of the fuel, incorporating the correct proportions of air and fuel, for total energy transfer, not only for steam generation but also to increase process efficiency and reduce pollutants. These modelling techniques and simulations also allow entities to determine the cost-effectiveness and environmental impact of various combustion technologies or different fuels.
Thermal models – mass and energy balances – are generally employed to assess the thermal performance of power generation components, such as superheaters, air preheaters, mills, classifiers and complete boilers. The mass and energy balance is performed for different scenarios across the entire system, using measured coal properties to identify if the equipment and its subcomponents can operate within the original design limits.
The thermal models are therefore used to provide input boundary condition data for the CFD simulation of the entire system as well as deviation conditions resulting from changing fuel qualities or any other “out of normal” operating conditions.
Owing to the complexity of the CFD models, implementation of achievable boundary conditions is a desirable starting and/or ending point and is regarded as a prerequisite. The application of boundary conditions minimises calculation time, as a complete CFD model of the entire system may not be required. Thus, the company believes that submodels can and should be used.
The conditions assure accurate solutions within the bounds of operation of each model and a complete picture can be assembled if the modelling cases are selected correctly. Thermal models are employed for this purpose and develop useful boundary condition data directly from the operating data or design basis of the plant.
Reference to a standard firing system in a coal-fired steam generator does not focus on the burners and furnace only, but also refers to the input systems that make the combustion process possible. This includes mills, pulverised fuel (PF) pipes and distribution units, primary air systems, secondary air systems, windbox arrangements as well as the burners, furnace and possible over fire and side wall air ports in the furnace.
Ideally, PF distribution units allocate PF evenly between each PF pipe leading to individual burners. Owing to biased PF stratifications at the exit of each mill, as well as the geometry of the PF distribution units and the different lengths and configurations of individual PF pipes, the PF mass and particle size distribution to burners may be uneven.
The CFD models have been developed to determine the effectiveness of the mill, PF pipes and dust distribution units upstream of the burners, with great success. These models have allowed us to better understand the interaction of the various components that make up the fuel delivery system and rectify undesirable operational flow conditions.
The associated flow and energy variation from the outlet of the air heaters through the secondary air flow ducts to the windboxes under various scenarios are important aspects to investigate.
An analysis of the CFD results of the ducts and windboxes give a comprehensive representation of the possible maldistributions of secondary air between individual burners. The aforementioned PF maldistribution is independent of the maldistribution of the secondary air with the result that the air to fuel ratio in each burner deviates, sometimes appreciably, from the mean.
The windbox CFD models have, on various occasions, been extremely useful to redesign the air path within the windboxes to minimise air flow variances to each burner in order to keep the individual burner air to fuel ratios within specified limits.
The robustness of the CFD analysis allows the user to modify each system individually and gives a flow-based outcome that will take place within the selected region. Based on the simulations performed on case studies, some results raised questions regarding overall flame stability in the furnace.
Of the sixteen burners in operation in the study, only seven showed the typical pattern of stable ignition with a well-established central recirculation zone pulling hot combustion products back from the furnace towards the burner throat. This observation appeared to be substantiated by the high unburnt carbon in ash developed within the furnace.
It was, however, interesting to note that investigation of the maximum coal flow case, showed that some burners improved remarkably in terms of their ability to pull hot gas back from the furnace towards the burner throat. Flame impingement was shown to be present in all cases of the study and it was surmised that the high core air velocities observed in all burners could be a contributory cause to the observed flame patterns.
Concerns such as flame stability, flame impingement, combustion efficiency and high emissions can be overcome by applying CFD modelling. Over fire and side wall air sometimes used in conjunction with primary measures such as low oxides of nitrogen (NOx) burners are often used as additional measures to further reduce NOx emissions. CFD analyses of such systems have been highly successful and have shown growth in use, owing to the relative accuracy of the models to real-world situations.