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 service 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 and in the fundamental simulation of the various processes in steam and power generation plants.

These include materials handling, mass transfer, fluid flow (both single and multi-phase), heat transfer, erosion and chemical reactions like combustion, 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.

The 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.

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 postcombustion systems, as well as the actual combustion process itself.

It is desirable for steam production to achieve timeous 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 original-equipment manufacturers 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 complete boilers or power generation components such as superheaters, air preheaters, mills and classifiers. The mass and energy balance is performed for different scenarios across the entire system, using measured coal properties to determine 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, cost and time required to develop fully detailed CFD models for the entire system, the company believes that submodels can and should be used. Implementation of realistic boundary conditions, sufficiently far upstream and downstream of the region of interest for each submodel, is a prerequisite.

The conditions assure accurate solutions within the bounds of operation of each sub-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 subsystems that make up the entire combustion system. This includes mills, pulverised fuel (PF) pipes and PF distribution units, primary air systems, secondary air systems and windbox arrangements, as well as the burners, the furnace and possible overfire and sidewall 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, the PF pipes and the 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 which have been investigated.

CFD Analysis
An analysis of the CFD results of the ducts and windboxes gives 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 in case studies, some results confirmed observations regarding overall flame instability in the furnace.

Of the 16 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 was 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 and it was found that the core air velocities in all burners contributed to the observed flame patterns.

Concerns such as flame stability, flame impingement, combustion efficiency and high emissions can be overcome by applying CFD modelling. Overfire and sidewall air used in conjunction with primary measures such as low nitrogen oxide burners are often useful to further reduce nitrogen oxide emissions. CFD analyses of such systems have been highly successful and have shown growth in use, owing to the accuracy of the models to real-world situations.