Computational time to model radiative heat transfer in a cylindrical Pressurized Oxy-Coal (POC) combustor was reduced by incorporating the multi-dimensional characteristics of the combustion field. The Discrete Transfer Method (DTM) and the Discrete Ordinates Method (DOM) were modified to work with a computational mesh that transitions from 3D cells to axisymmetric and then 1D cells, also known as a dimensionally adaptive mesh. For the DTM, three methods were developed for selecting so-called transdimensional rays, the Single Unweighted Ray (SUR) technique, the Multiple Unweighted Ray (MUR) technique, and the Single Weighted Ray (SWR) technique. For the DOM, averaging methods for handling radiative intensity at dimensional boundaries were developed. Limitations of both solvers with adaptive meshes were identified by comparison with fully 3D results. For the DTM, the primary limit was numerical error associated with view factor calculations. For the DOM, treatment of dimensional boundaries led to step changes that created numerical oscillations, the severity of which was lessened by both increased angular resolution and increased optical thickness. Performance of dimensionally adaptive radiation calculations, uncoupled to any other physical calculation, was evaluated with a series of sensitivity studies including sensitivity to spatial and angular resolution, dimensional boundary placement, and reactor scaling. Runtime was most impacted by boundary layer placement. For the upstream case which had 3D cells over 40% of the reactor length, the speedup versus the fully 3D calculations were 743%, 18%, 220%, and 76% for the SUR, MUR, SWR, and DOM calculations, respectively. The downstream case which had 3D cells over the first 60% of the reactor length, had speedups of 209%, 3%, 109%, and 37%, respectively. For the DTM, accuracy was most sensitive to optical thickness, with the average percent difference in incident heat flux for SUR, MUR, and SWR calculations versus fully 3D calculations being 0.93%, 0.86%, and 1.18%, respectively, for a reactor half the size of the baseline case. The case with four times the reactor size had average percent differences of 0.28%, 0.41%, and 0.39% for the SUR, MUR, and SWR, respectively. Accuracy of the DOM was comparatively insensitive to the different changes studied. Performance of dimensionally adaptive radiation calculations coupled with thermochemistry was also investigated for both pilot and industrial scale systems. For pilot scale systems, flux and temperature differences from either solver were less than 5% and 6%, respectively, with speedups being between 200% - 600%. For industrial systems, temperature differences as high as 15% - 20% and flux differences as high as 50% - 75% were seen. In the case of the DTM, these differences between fully 3D and adaptive results come from a combination of high property gradients and comparatively few rays being drawn and could therefore be improved, at the cost of additional computation time, by using a more sophisticated ray selection method. For the DOM, these issues stem from poor performance of the 1D portion of the solver and could therefore be improved by using a more sophisticated equation to model the radiative transfer in the 1D region.

Lephalale Municipality is a predominantly rural Municipality with 38 villages, two townships (Marapong and Onverwacht) and one town, Lephalale. Lephalale, formerly known as Ellisras, is a town situated in the “heart of the Bushveld” in Limpopo province. The town is growing rapidly and more industries are becoming concentrated within this small town. The construction of Medupi power station which is underway and other projects such as the expansion of Grootegeluk mine (coal 3 and 4 projects), and road developments in the area; have led to concern about the ambient air quality of the area. Other possible future projects are the Coal to Liquid project by Sasol and the Coal Bed Methane project by Anglo American Thermal Coal. The purpose of this study is to determine the ambient air quality impact of the Matimba power station in the Lephalale area. The AERMOD model and ambient air quality data obtained from Eskom’s Grootstryd and Marapong monitoring stations were used to assess the ambient air quality of Lephalale. Sulphur dioxide and Nitrogen oxides were investigated. Both the model’s results and the ambient air quality monitoring data indicated that the power station contributes to high -ground level concentrations of Sulphur dioxide. AERMOD simulated the nitrogen oxides results as nitrogen dioxide. From the study it is concluded that the power station is not the only source of nitrogen oxides. Nitrogen oxides concentrations were associated with low-level sources. The relationship between the criteria pollutants in this study was assessed. The study found that there is no relationship between sulphur dioxide and nitrogen oxides. This finding was used to support the idea that sulphur dioxide and nitrogen oxides are from different sources. It was also established that seasonality has an influence on the ground level concentrations of pollutants in the area.

Computational fluid dynamics (CFD) modeling and analysis were used in three projects: solar CO2 conversion modeling, improved coal combustion modeling using STAR-CD, and premixed combustion modeling. Each project is described below. The solar CO2 conversion modeling project involved CFD simulations of a prototype solar CO2 converter that uses sunlight to dissociate CO2 into CO and O2. Modeling was used to predict the performance of this prototype converter using three CFD software packages, and involved predicting the flow, heat transfer, and chemical kinetics. Accuracy was determined by comparison of model predictions and experimental data. Parametric modeling studies were performed in order to better understand converter performance and limitations. Modeling analysis led to proposed operational and design changes meant to improve converter performance. Modeling was performed to quantify the effects of proposed design modifications and operational adjustments. Modeling was also used to study the effects of pressure, some geometric design changes, and changing from pure CO2 to a CO2/He mixture. The insights gained from these modeling studies have played a key role in improving the performance of this process. The second project involved the implementation of advanced coal models into STAR-CD, a commercial CFD program. These coal models were originally developed for PCGC-3, a code developed at Brigham Young University. This project involved modifying modern PCGC-3 coal combustion and gasification models so that they could be incorporated into STAR-CD. Models implemented included a coal set-up subroutine, and coal reactions models for devolatilization, char oxidation, and vaporization. Each implemented model was tested to verify its accuracy by comparison of model predictions with experimental data. All implemented coal submodels were validated by comparison between overall modeling predictions and experimental data. These implemented coal models increased the capability of STAR-CD to model coal combustion and gasification systems. The third project was to assemble previously obtained experimental data on lean, premixed natural gas combustion. Velocity, temperature, and species concentration measurements were previously taken throughout a laboratory-scale gas turbine combustor using advanced laser diagnostics. However, these data were taken by different investigators at BYU over the course of 10 years, and the data were scattered through several publications, theses, and dissertations. This third project was to compile these data into a central location for analysis and distribution. This data set is excellent for validation of any comprehensive combustion model, and is now accessible to the public.

This work produced the retrofit of an air-fired, 150 kW reactor for oxy-combustion which was then used in three oxy-combustion studies: strategic oxy-combustion design, oxy-combustion of petroleum coke, and air versus oxy-combustion radiative heat flux measurements. The oxy-combustion retrofit was accomplished using a system of mass flow controllers and automated pressure switches which allowed safe and convenient operation. The system was used successfully in the three studies reported here and was also used in an unrelated study. A study was completed where a novel high oxygen participation burner was investigated for performance while burning coal related to flame stability, NO, and burnout using a burner supplied by Air Liquide. Parameters investigated included oxygen (O2) injection location, burner swirl number and secondary carbon dioxide (CO2) flow rate. The data showed swirl can be used to stabilize the flame while reducing NO and improving burnout. Center O2 injection helped to stabilize the flame but increased NO formation and decreased burnout by reducing particle residence time. Additional CO2 flow lifted the flame and increased NO but was beneficial for burnout. High O2 concentrations up to 100% in the secondary were accomplished without damage to the burner. Petroleum coke was successfully burned using the Air Liquide burner. Swirl of the secondary air and O2 injection into the center tube of the burner were needed to stabilize the flame. Trends in the data similar to those reported for the coal study are apparent. Axial total radiant intensity profiles were obtained for air combustion and three oxy-combustion operating conditions that used hot recycled flue gas in the secondary stream. The oxygen concentration of the oxidizer stream was increased from 25 to 35% O2 by decreasing the flow rate of recycled flue gas. The decrease in secondary flow rate decreased the secondary velocity, overall swirl, and mixing which elongated the flame. Changing from air to neat CO2 as the coal carrier gas also decreased premixing which elongated the flame. Flame elongation caused increased total heat transfer from the flame. The air flame was short and had a higher intensity near the burner, while high O2 concentration conditions produced lower intensities near the burner but higher intensities and temperatures farther downstream. It was shown that oxycombustion can change flame shape, temperature and soot concentration all influencing heat transfer. Differences in gas emission appear negligible in comparison to changes in particle emission.

No description available.

One or the characteristics of all anthracite coal, with its low volatile content, is its ability to burn completely in a small volume. Another characteristic and disadvantage of local, semi-anthracite coal is its high ash content. It is the authors' belief that local, semi-anthracite coal can be burned most effectively for domestic heating if the furnace design allows for these characteristics. With these facts in mind, it was decided that the Anthratube had excellent possibilities as a domestic unit for burning local coal. The Anthratube, by its compactness, takes full advantage of the first characteristic; with its ash-removing grate, it overcomes to a great extent the disadvantage of the second characteristic. The purpose of this thesis was, then, to determine whether or not various sizes of local, semi-anthracite coal from the Merrimac seam could be successfully burned in the Anthratube. The coal used for this investigation was obtained from the Great Valley Anthracite Corporation located at McCoy, Virginia. 1. Pea size, local coal can be burned very successfully in the Anthratube. Overall boiler efficiencies of the unit with this size coal are high over a wide range of loads. Of the sizes of coal burned, pea size is most suitable for the Anthratube. 2. Buckwheat size, local coal canoe burned in the Anthratube with good results. The overall boiler efficiencies obtained with this size of coal are good, although not as high as those obtained with the pea coal. 3. The performance of the Anthratube with rice size, local coal is inferior to that achieved with pea and buckwheat sizes. The output of the unit is seriously limited when using this size. 4. Culm size, local coal cannot be burned in the Anthratube. / M.S.

A research report submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science in Engineering, 2016 / Spontaneous combustion of coal may occur when coal is mined, stored or transported and is influenced by a combination of intrinsic and /or extrinsic factors. While it is unusual for intact seams to burn in the highwall, the most common occurrence is when surface mines extract seams previously partially mined by underground bord and pillar operations. The aim of the study is to provide a predictive model (matrix) of the spontaneous combustion potential of remnant pillars at Colliery X. A number of different thermal, chemical and petrographic tests (coal factors) will be undertaken to determine their individual and collective impacts on the sponcom predictive model. The primary geology at the mine is conformable with that of the Witbank Coalfield. Battacharyya (1982) described 3 main factors in the spontaneous combustion of coal, mining factor, coal factor and geological factor which have an aggregate effect. Some of the main historical and present theories of sponcom are the pyrite theory, the bacterial theory, the oxidation theory and the humidity theory. It is important to note that no single factor is responsible for spontaneous combustion. The oxidation of coal occurs constantly. The temperature of the coal is a function of the rate of heat generation versus the rate of heat loss. Fires can start at outcrops and move through interconnected workings with heat transfer by conduction (into the overburden) or convection (between panels).The overburden can also insulate the burning coal seam. Geological factors such as depth of overburden, the degree of fracturing, and the nature of the overlying strata vary between coalfields. A coal seam fire or mine fire is the underground smouldering of a coal deposit, often in a coal mine. Such fires have economic, social and ecological impacts In order to extinguish a fire, one of three elements, fuel, oxygen, or energy, must be removed. The components of the fire triangle can be further subdivided into conventional mine control techniques and more or less unconventional or unproven mine fire control techniques. The thermal techniques discussed include the crossing point temperature, thermogravimetric analyses and oxygen absorption. Macerals, the microscopically identifiable organic constituents of coal, are one of the three basic parameters that define coal. The other two parameters are the coal rank and the mineral matter Vitrinite is the principal maceral group of the No.5 seam and inertinite dominates the No.2 and No.4 seams. The results obtained from the 22 drill-core samples and 2 ROM samples were matched to the existing borehole dataset (2296 boreholes) based on similarity of heat value (figure 3.11). A total of 24 test results (thermal, chemical and petrographic) from borehole A and borehole B were thus assigned to the borehole database which has approximately 1500 samples for each seam. By linking the laboratory datasets (borehole A and B) and the existing borehole database used for resource modelling, the sponcom variables could be modelled in a similar way to the coal resources. The overall risk matrix was calculated on a full seam basis by combining 15 variable scores, each variable having a score of 0, 1 or 2 (low-mod-high probability). The overall results from this research produced clear and unambiguous contour plans of different factors effecting sponcom of coal using single variable and combined variable datasets. In conclusion, it appears that the acceptability of a method for determining spontaneous heating characteristics of coal mainly depends upon how closely it predicts the spontaneous heating behaviour in the field conditions / CK2018

The Matimba and soon to be completed Medupi power stations located in close proximity to the town of Lephalale are a cause for environmental concern due to the known effects that coal combustion has on air, soil and water quality. The Medupi power station is currently being constructed, while the Matimba power station may have already negatively altered the water quality of the rivers especially those downwind of the power stations. The Lephalala (perennial river, upwind), the Mokolo (perennial river, upwind) and Matlabas (seasonal river, downwind) Rivers were selected due to the locations relative to the power stations. The concentrations and flux of cations and sulphate ions within the rivers in the Waterberg District Municipality were investigated for any seasonal or annual patterns using monthly data from a single sampling station along each river. Data for the concentrations of sodium, potassium, magnesium, calcium, ammonium and sulphate were analysed in conjunction with river discharge, rainfall and ambient temperature data available for each hydrological year from 1999 to 2010. The data were converted to seasonal and annual values in order to determine the influence of the quality and quantity of coal combusted as well as climatic variables (rainfall, temperature and discharge) on ion fluxes measured. Sodium was the dominant cation in all rivers, reaching a maximum concentration of 0.0015 mol.ℓ-1 (in 2007), 0.0007 mol.ℓ-1 (in 2007) and 0.0006 mol.ℓ-1 (in 2001) in the Lephalala, Mokolo and Matlabas Rivers, respectively. Other cation concentrations were four times lower in the Lephalala and Mokolo Rivers, while they were eight times lower in the Matlabas Rivers. Sulphate concentrations were approximately nine, five and 15 times lower than the cation concentrations measured within the Lephalala, Mokolo and Matlabas Rivers, respectively. The mean summed cation flux was highest in the Lephalala River (0.0015 ± 0.0010 Eq.ℓ-1), which was approximately 1.7 and 2.1 times higher than summed cation fluxes measured in the Mokolo (0.0009 ± 0.0002 Eq.ℓ-1) and Matlabas (0.0007 ± 0.0006 Eq.ℓ-1) Rivers. Cation fluxes were highest during the rainfall season (summer and spring) in the river closest to the Matimba power station (Mokolo Rivers) while summed cation flux in the Lephalala and Mokolo Rivers (located further away from the power station) showed no specific seasonality. It was, however, noted that the cation fluxes during spring and winter were elevated for both rivers, possibly indicating

A dissertation submitted to the Faculty of Engineering, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master in Science in Engineering, Johannesburg September 1998 / Combustion modelling of utility furnace chambers provides a cost efficient means to extrapolate the combustion behaviour of pulverised fuel (pf) as determined from drop tube furnace (DTF) experiments to full scale plant by making use of computational fluid dynamics (CFD). The combustion model will be used to assimilate essential information for the evaluation and prediction of the effect of • changing coal feedstocks • proposed operational changes • boiler modifications. TRI comrnlssloned a DTF in 1989 which has to date been primarily used for the comparative characterisation of coals in terms of combustion behaviour. An analysis of the DTF results allows the determination of certain combustion parameters used to define a mathematical model describing the rate at which the combustion reaction takes place. This model has been incorporated into a reactor model which can simulate the processes occurring in the furnace region of a boiler, thereby allowing the extrapolation of the DTF determined combustion assessment to the full scale. This provides information about combustion conditions in the boiler which in turn are used in the evaluation of the furnace performance. Extensive furnace testwork of one of Eskom's wall fired plant (Hendrina Unit 9) during 1996, intended to validate the model for the ar plications outlined above, included the measurement {If : • gas temperatures • O2, C02, CO, NOx and S02 concentrations • residence time distributions • combustible matter in combustion residues extracted from the furnace • furnace heat fluxes. The coal used during the tests was sampled and subjected to a series of chemical and other lab-scale analyses to determine the following: • physical properties • composition • devolatilisation properties " combustion properties The same furnace was modelled using the University of Stuttgart's AIOLOS combustion code, the results of Which are compared with the measured data. A DTF derived combustion assessment of a coal sampled from the same site but from a different part of the beneficiation plant, which was found to burn differently, was subsequently used in a further simulation to assess the sensitivity of the model to char combustion rate data. The results of these predictions are compared to the predictions of the validation simulation. It was found that the model produces results that compare well with the measured data. Furthermore. the model was found to be sufficiently sensitive to reactivity parameters of the coal. The model has thereby demonstrated that it can be used in the envisaged application of extrapolating DTF reactivity assessments to full scale plant. In using the model, it has become apparent that the evaluations of furnace modifications and assessments of boiler operation lie well within the capabilities of the model. / MT2017

The use of Computational Fluid Dynamics (CFD) tools is crucial for the development of novel and cost-effective combustion technologies and the minimization of environmental concerns at industrial scale. CFD simulations facilitate scaling-up procedures that otherwise would be complicated by strong interactions between reaction kinetics, turbulence and heat transfer. CFD calculations can be applied directly at the industrial scale of interest, thus avoiding scaling-up from lab-scale experiments. However, this advantage can only be obtained if CFD tools are quantitatively predictive and trusted as so. Despite the improvements in the computational capability, the implementation of detailed physical and chemical models in CFD simulations can still be prohibitive for real combustors, which require large computational grids and therefore significant computational efforts. Advanced simulation approaches like Large Eddy Simulation (LES) and Direct Numerical Simulation (DNS) guarantee higher fidelity in computational modeling of combustion at, unfortunately, increased computational cost. However, with adequate, reduced, and cost-effective modeling of physical phenomena, such as chemical kinetics and turbulence-chemistry interactions, and state of the art computing, LES will be the tool of choice to describe combustion processes at industrial scale accurately. Therefore, the development of reduced physics and chemistry models with quantified model-form uncertainty is needed to overcome the challenges of performing LES of industrial systems. Reduced-order models must reproduce the main features of the corresponding detailed models. They feature predictivity and capability of bridging scales when validated against a broad range of experiments and targeted by Validation and Uncertainty Quantification (V/UQ) procedures. In this work, V/UQ approaches are applied for reduced-order modeling of pulverized coal devolatilization and subsequent char oxidation, and furthermore for modeling NOx emissions in combustion systems.For coal devolatilization, a benchmark of the Single First-Order Reaction (SFOR) model was performed concerning the accuracy of the prediction of volatile yield. Different SFOR models were implemented and validated against experimental data coming from tests performed in an entrained flow reactor at oxy-conditions, to shed light on their drawbacks and benefits. SFOR models were chosen because of their simplicity: they can be easily included in CFD codes and are very appealing in the perspective of LES of pulverized coal combustion burners. The calibration of kinetic parameters was required to allow the investigated SFOR model to be predictive and reliable for different heating rates, hold temperatures and coal types. A comparison of several calibration approaches was performed to determine if one-step models can be adaptive and able to bridge scales, without losing accuracy, and to select the calibration method to employ for wider ranges of coal rank and operating conditions. The analysis pointed out that the main drawback of the SFOR models is the assumption of a constant ultimate volatile yield, equal to the value from the coal proximate analysis. To overcome this drawback, a yield model, i.e. a simple functional form that relates the ultimate volatile yield to the particle temperature, was proposed. The model depends on two parameters that have a certain degree of uncertainty. The performances of the yield model were assessed using a collaboration of experiments and simulations of a pilot-scale entrained flow reactor. A consistency analysis, based on the Bound-to-Bound Data Collaboration (B2B-DC) approach, and a Bayesian method, based on Gaussian Process Regression (GPR), were employed for the investigation of experiments and simulations. In Bound-to- Bound Data Collaboration the model output, evaluated at specified values of the model parameters, is compared with the experimental data: if the prediction of the model falls within the experimental uncertainty, the corresponding parameter values would be included in the so-called feasible set. The existence of a non-empty feasible set signifies consistency between the experiments and the simulations, i.e. model-data agreement. Consistency was indeed found when a relative error of 19% for all the experimental data was applied. Hence, a feasible set of the two SFOR model parameters was provided. A posterior state of knowledge, indicating potential model forms that could be explored in yield modeling, was obtained by Gaussian Process Regression. The model form evaluated through the consistency analysis is included within the posterior derived from GPR, indicating that it can satisfactorily match the experimental data and provide reliable estimation in almost every range of temperatures. CFD simulations were carried out using the proposed yield model with first-order kinetics, as in the SFOR model. Results showed promising agreement between predicted and experimental conversion for all the investigated cases.Regarding char combustion modeling, the consistency analysis has been applied to validate a reduced-order model and quantify the uncertainty in the prediction of char conversion. The model capability to address heterogeneous reaction between char carbon and O2, CO2 and H2O reagents, mass transport of species in the particle boundary layer, pore diffusion, and internal surface area changes was assessed by comparison with a large number of experiments performed in air and oxy-coal conditions. Different model forms had been considered, with an increasing degree of complexity, until consistency between model outputs and experimental results was reached. Rather than performing forward propagation of the model-form uncertainty on the predictions, the reduction of the parameter uncertainty of a selected model form was pursued and eventually achieved. The resulting 11-dimensional feasible set of model parameters allows the model to predict the experimental data within almost ±10% uncertainty. Due to the high dimensionality of the problem, the employed surrogate models resulted in considerable fitting errors, which led to a spoiled UQ inverse problem. Different strategies were taken to reduce the discrepancy between the surrogate outputs and the corresponding predictions of the simulation model, in the frameworks of constrained optimization and Bayesian inference. Both strategies succeeded in reducing the fitting errors and also resulted in a least-squares estimate for the simulation model. The variety of experimental gas environments ensured the validity of the consistent reduced model for both conventional and oxy-conditions, overcoming the differences in mass transport and kinetics observed in several experimental campaigns.The V/UQ-aided modeling of coal devolatilization and char combustion was done in the framework of the Predictive Science Academic Alliance Program II (PSAAP-II) funded by the US Department of Energy. One of the final goals of PSAAP-II is to develop high-fidelity simulation tools that ensure 5% uncertainty in the incident heat flux predictions inside a 1.2GW Ultra-Super-Critical (USC) coal-fired boiler. The 5% target refers to the expected predictivity of the full-scale simulation without considering the uncertainty in the scenario parameters. The data-driven approaches used in this Thesis helped to improve the predictivity of the investigated models and made them suitable for LES of the 1.2GW USC coal-fired boiler. Moreover, they are suitable for scale-bridging modeling of similar multi-phase processes involved in the conversion of solid renewable sources, such as biomass.In the final part of the Thesis, the sensitivity to finite-rate chemistry combustion models and kinetic mechanisms on the prediction of NO emissions was assessed. Moreover, the forward propagation of the uncertainty in the kinetics of the NNH route (included in the NOx chemistry) on the predictions of NO was investigated to reveal the current state of the art of kinetic modeling of NOx formation. The analysis was carried out on a case where NOx formation comes from various formation routes, both conventional (thermal and prompt) and unconventional ones. To this end, a lab-scale combustion system working in Moderate and Intense Low-oxygen Dilution (MILD) conditions was selected. The results showed considerable sensitivity of the NO emissions to the uncertain kinetic parameters of the rate-limiting reactions of the NNH pathway when a detailed kinetic mechanism is used. The analysis also pointed out that the use of one-step global rate schemes for the NO formation pathways, necessary when a skeletal kinetic mechanism is employed, lacks the required chemical accuracy and dims the importance of the NNH pathway in this combustion regime. An engineering modification of the finite-rate combustion model was proposed to account for the different chemical time scales of the fuel-oxidizer reactions and NOx formation pathways. It showed an equivalent impact on the emissions of NO than the uncertainty in the kinetics of the NNH route. At the cost of introducing a small mass imbalance (of the order of ppm), the adjustment led to improved predictions of NO. The investigation established a possibility for the engineering modeling of NO formation in MILD combustion with a finite-rate chemistry combustion model that can incorporate a detailed mechanism at affordable computational costs. / Doctorat en Sciences de l'ingénieur et technologie / info:eu-repo/semantics/nonPublished