An integrated furnace co-simulation methodology based on a reduced order CFD approach

An integrated thermofluid modelling methodology for pulverised fuel fired utility-scale boilers that is computationally inexpensive, fast, and sufficiently accurate would be valuable in an industrial setting. Such a model would enable boiler operators to investigate a range of off-design operating conditions, which includes flexible operation. The aims of this study was: to develop a reduced order computational fluid dynamics (CFD) model of the furnace and radiative heat exchangers that captures all the important particulate effects while using a Eulerian-Eulerian (EE) approach; using the reduced order CFD model to generate a database of results that covers a wide range of operating conditions; to develop a data-driven surrogate model using machine learning techniques; to integrate the surrogate model with a 1-D process model of the complete boiler; and finally to demonstrate the use of the integrated model to investigate flexible operation and off-design operating conditions. The validity of the CFD modelling approach was demonstrated via application to a 2.165 [MWth] lab-scale swirl pulverised fuel burner, as well as to a 620 [MWe] utility-scale subcritical two-pass boiler, both operating at various loads. The results were compared to measured data and a detailed CFD model using the conventional Eulerian-Lagrangian (EL) approach. A computational speed enhancement of 30% was achieved. The data-driven surrogate model uses a mixture density network (MDN) to predict the heat transfer in the furnace and radiative heat exchangers, together with the uncertainty in the predicted values. The integrated model was validated against applicable measured data and then applied to a utility-scale case study boiler to investigate the optimal burner firing positions for low-load operation, as well as to investigate the effects of fuel quality on the overall boiler performance. It was shown that the integrated data-driven surrogate model and 1-D process model can predict the overall thermofluid response of the boiler and the uncertainties associated with it with good accuracy, whilst maintaining a low computational effort when compared to a conventional CFD model coupled to 1-D process model

Numerical study of particle interaction in gas-particle and liquid particle flows: part I analysis and valdation

A detailed study into the turbulent behaviour of dilute particulate flow under the influence of two carrier phases namely gas and liquid has been carried out behind a sudden expansion geometry. The major endeavour of the study is to ascertain the response of the particles within the carrier (gas or liquid) phase. The main aim prompting the current study is the density difference between the carrier and the dispersed phases. While the ratio is quite high in terms of the dispersed phase for the gas-particle flows, the ratio is far more less in terms of the liquid-particle flows. Numerical simulations were carried out for both these classes of flows using an Eulerian two-fluid model with RNG based k-e model as the turbulent closure. An additional kinetic energy equation to better represent the combined fluid-particle behaviour is also employed in the current set of simulations. In the first part of this two part series, experimental results of Fessler and Eaton (1995) for Gas-Particle (GP) flow and that of Founti and Klipfel (1998) for Liquid-Particle (LP) flow have been compared and analysed. This forms the basis of the current study which aims to look at the particulate behaviour under the influence of two carrier phases. Further numerical simulations were carried out to test whether the current numerical formulation can used to simulate these varied type of flows and the same were validated against the experimental data of both GP as well LP flow. Qualitative results have been obtained for both these classes of flows with their respective experimental data both at the mean as well as at the turbulence level for carrier as well as the dispersed phase

Identification of clinker formation in power station boilers – CFD based approach

Pulverised coal combustion continues to be one of the main conventional methods of producing electricity over the last several decades. Mineral matter present in coal is usually present as free ions, salts, organically bound inorganic and hard minerals. During coal combustion these minerals partly vaporized, coalesce or fragment. The mineral matter in coal transforms into ash during combustion and deposition on wall surfaces causing problems such as fouling and slagging. The deposited lumps called clinkers, mainly in radiation zone directly exposed to flame radiation resulting to slagging, while sintered deposit in convection zone not directly exposed by flame radiation called fouling. The scope of this work encompasses identification of slagging and clinker formation areas in a typical 330 MW boiler using commercial code FLUENT and several available empirical indices. The propensity of the slagging with the used coal is calculated by several thermal indices. Temperature distributions, velocity profiles and particle trajectories were analysed and utilised to predict the most probable zones likely to experience clinker formation. Most probable spots for slagging were found in the radiation zone near to the nose of furnace and left-top side of superheater tube sections which agrees closely with the plant observations. However, the propensity of deposited ash obtained from the plant is seemed low to medium using several indices.Results from the current investigation demonstrate the usefulness of modelling approach in identifying the probable zones of clinker formation which can prove to be valuable for power utilities to adopt corrective measures for soot blowing to clean the ash deposits before it grows bigger in size

Boiler system modelling using Flownex®

The objective of this project is to develop a boiler modelling methodology, specifically using Flownex, which is capable of running transient simulations for a large variety of coal-fired boiler designs typically used in Eskom. Flownex has been identified as the key software to accomplish the global objective of the Centre for Energy Efficiency under EPPEI at the University of Cape Town, which is to develop a software model of a complete coal-fired power station which includes all the main systems required for independent transient simulation. The boiler model captures the true geometric layout and flow orientation with associated characteristics of a wide variety of boiler designs utilised by Eskom. In order to achieve this, boilers and heat exchangers are grouped according to common physical properties which simplify the modelling process and optimise results. This is preceded by an investigation into the types of boiler designs currently operational in Eskom including available associated geometrical and process characteristics. A study into heat transfer mechanisms applicable to coal-fired boiler heat exchangers was done to ensure fundamental theoretical principles are adhered to during the development of the analytical models, the first step in the modelling process. The Flownex solving methodology is evaluated against the analytical models in a simplified heat exchanger before full detail modelling of heat exchangers are done. The component and method used in Flownex requires convection and radiation heat transfer to be accounted for separately and thus heat exchangers are classified sequentially according to their location in the boiler, this process relies heavily on data obtained in the boiler study. Heat exchangers and auxiliary systems are then integrated into a single system used to obtain steady-state results. The steady-state boiler model is evaluated against actual boiler design data for various loads to prove applicability to various boiler designs and operating conditions

Analysis of thermal resistance evolution of ash deposits during co-firing of coal with biomass and coal mine waste residues

Co-firing biomass or waste fuels with coal in conventional thermal plants is a promising way to reduce environmental impact of human activities with an acceptable economic investment. One of the main issues to be addressed is the worsening in ash fouling and the reduction of heat transfer rate. In the present paper, the deposits thermal resistance during direct combustion of different blends of coal and various native fuels is investigated by using a deposition probe, kept at 550 °C in order to emulate the conditions of superheaters of conventional power units. Two energy crops (Cynara cardunculus L. and Populus spp.), a forest residue (Pinus pinaster) and a waste coal (coal mine waste residues) were successfully tested in a semi-industrial scale pilot plant. A thermal model of the probe is presented to estimate heat transfer rate and thermal resistance of ash deposits. After the validation with experimental data, a sensitivity analysis allows to identify the deposit surface emissivity and the flue gas temperature as the most influential parameters. The heat uptake in air flow decreases with time for all the experimental tests in spite of the increase in flue gas and walls temperatures. Except for poplar blends, under similar operation conditions, a rise in the substitution percentage means faster decreasing rates in heat transfer and higher thermal resistance due to the ash deposits, especially for cynara and coal mine waste residues. The present work demonstrates the usefulness of thermal models to estimate the thermal resistance of ash deposits without the need of sophisticated instrumentation. Dedicated thermal models, similar to the developed one, could serve to design smart cleaning sequences to improve efficiency in power generation processes