Oxy-fuel combustion of solid fuels

Oxy-fuel combustion is suggested as one of the possible, promising technologies for capturing CO₂ from power plants. The concept of oxy-fuel combustion is removal of nitrogen from the oxidizer to carry out the combustion process in oxygen and, in most concepts, recycled flue gas to lower the flame temperature. The flue gas produced thus consists primarily of carbon dioxide and water. Much research on the different aspects of an oxy-fuel power plant has been performed during the last decade. Focus has mainly been on retrofits of existing pulverized-coal-fired power plant units. Green-field plants which provide additional options for improvement of process economics are however likewise investigated. Of particular interest is the change of the combustion process induced by the exchange of carbon dioxide and water vapor for nitrogen as diluent. This paper reviews the published knowledge on the oxy-fuel process and focuses particularly on the combustion fundamentals, i.e. flame temperatures and heat transfer, ignition and burnout, emissions, and fly ash characteristics. Knowledge is currently available regarding both an entire oxy-fuel power plant and the combustion fundamentals. However, several questions remain unanswered and more research and pilot plant testing of heat transfer profiles, emission levels, the optimum oxygen excess and inlet oxygen concentration levels, high and low-temperature fire-side corrosion, ash quality, plant operability, and models to predict NO_x and SO₃ formation is required.     

Numerical simulation of pulverized coal MILD-oxy combustion under different oxygen concentrations

As an emerging clean coal combustion technology, Moderate or Intense Low-Oxygen Dilution (MILD) combustion or oxy-fuel combustion, compared with traditional coal combustion, has many advantages. However, compared with MILD combustion and oxy-fuel combustion, MILD-oxy combustion is believed more attractive. In this work, MILD-oxy combustion characteristics with oxygen concentrations from 10% to 50% are studied numerically. The results show that within a certain range, increasing the oxygen concentration is in favor of MILD-oxy combustion performance close to that of MILD-air combustion. When the oxygen concentration is higher enough, the momentum reduced by the increase of oxygen concentration has a great influence on the furnace temperature. With the increase of oxygen concentration, the radiation heat transfer is enhanced and the convective heat transfer is weakened. The increase of oxygen concentration can promote the occurrence of char gasification reaction with CO₂. In addition, MILD-oxy combustion has a large impact on CO emission.     

CFD modeling of heat transfer and fluid flow inside a pent-roof type combustion chamber using dynamic model

A numerical work has been performed to analyze the heat transfer and fluid flow in a pent-roof type combustion chamber. Dynamic mesh model was used to simulation piston intake stroke. Revolution of piston (1000 ≤ n ≤ 5000) is the main governing parameter on heat and fluid flow. k–ε turbulence model was used to predict the flow in the cylinder of a non-compressing fluid. They were solved with finite volume method and FLUENT 12.0 commercial code. Velocity profiles, temperature distribution, pressure distribution and velocity vectors are presented. It is found that the inclined surface of pent-roof type of combustion chamber reduces the swirl effect and it can be a control parameter for heat and fluid flow.     

Oxy-fuel combustion of coal and biomass, the effect on radiative and convective heat transfer and burnout

This paper focuses on results of co-firing coal and biomass under oxy-fuel combustion conditions on the RWEn 0.5 MWt Combustion Test Facility (CTF). Results are presented of radiative and convective heat transfer and burnout measurements. Two coals were fired: a South African coal and a Russian Coal under air and oxy-fuel firing conditions. The two coals were also co-fired with Shea Meal at a co-firing mass fraction of 20%. Shea Meal was also co-fired at a mass fraction of 40% and sawdust at 20% with the Russian Coal. An IFRF Aerodynamically Air Staged Burner (AASB) was used. The thermal input was maintained at 0.5 MWt for all conditions studied. The test matrix comprised of varying the Recycle Ratio (RR) between 65% and 75% and furnace exit O₂ was maintained at 3%. Carbon-in-ash samples for burnout determination were also taken.Results show that the highest peak radiative heat flux and highest flame luminosity corresponded to the lowest recycle ratio. The effect of co-firing of biomass resulted in lower radiative heat fluxes for corresponding recycle ratios. Furthermore, the highest levels of radiative heat flux corresponded to the lowest convective heat flux. Results are compared to air firing and the air equivalent radiative and convective heat fluxes are fuel type dependent. Reasons for these differences are discussed in the main text. Burnout improves with biomass co-firing under both air and oxy-fuel firing conditions and burnout is also seen to improve under oxy-fuel firing conditions compared to air.     

The effect of wood biomass blending with pulverized coal on combustion characteristics under oxy-fuel condition

In this study, combustion from the co-firing of coal and wood biomass, and thermal characteristics such as ignition temperature, burn-out temperature, and activation energy were discussed using a thermogravimetric analyzer (TGA). We investigated the effects of biomass blending with two kinds of pulverized coal (bituminous Shenhua, and sub-bituminous Adaro) under air and oxy-fuel conditions. The coal fraction in the blended samples was set to 1, 0.8, and 0.5. The oxygen fraction in the oxidant was set to 0.21, 0.3, 0.5, and 0.8. The ignition temperature was governed by the fuel composition, particularly in the blended biomass which has a much higher content of volatile matter comparing to coal. However, the burnout temperature, which shows a strong relationship with char combustion, depended on the oxidant ingredients rather than on the fuel components. Thermal characteristics such as ignition, burnout temperature, reaction region, and heat flow were very similar between air and a 0.3 oxygen concentration under oxy-fuel conditions with Shenhua coal.     

Numerical investigation of influence of homogeneous/heterogeneous ignition/combustion mechanisms on ignition point position during pulverized coal combustion in oxygen enriched and recycled flue gases atmosphere

It is expected that pulverized coal combustion will continue to play a major role in electricity generation for the foreseeable future. Oxy-fuel coal combustion is actively being investigated, as alternative to conventional pulverized-coal combustion, due to its potential to easier carbon dioxide sequestration. This paper presents experimental and numerical analysis of ignition phenomena in oxy-fuel conditions. A modification of standard sequential coal combustion model is proposed. The new model is developed following the criteria for the particle ignition mechanism as the function of surrounding conditions. The implemented model was validated based on ignition point position obtained from the drop tube facility experiments in various O₂-N₂ and O₂-CO₂ conditions. The obtained numerical results showed a much better agreement with the experimental results when compared with the simulations performed with the default FLUENT sub-models for coal particle ignition/combustion, thus enabling a quantitative determination of pulverized coal flame ignition point position using numerical analysis.     

The study of co-combustion characteristics of coal and microalgae by single particle combustion and TGA methods

The co-combustion characteristics of coal and microalgae with different blending ratios and under different atmospheres are studied by single particle combustion and thermogravimetric analysis methods. The combustion processes of coal, microalgae and their blends in the single particle combustion experiment have two stages, while the combustion process of coal in the thermogravimetric analysis experiment only has one stage. With the increasing blending ratio of microalgae, flames of volatiles and char of fuels become dimmer and smaller, and the average flame temperature decreases from about 1400 °C to about 1200 °C. The ignition delay time decreases from 200 ms to 140 ms, and the experimental ignition delay time of blended fuels is lower than the theoretical ignition delay time, which demonstrates that the synthetic effect between coal and microalgae exists. To analyze the influence of oxy-fuel atmosphere on the combustion characteristics, the air is replaced by the O₂/CO₂ atmosphere. The replacement decreases the luminosity, size and average temperature of flames. The average flame temperature of volatiles decreases from 1449.4 °C to 1151.2 °C, and that of char decreases from 1240.0 °C to 1213.4 °C. The replacement increases the ignition delay time of fuel from 80 ms to 100 ms. Increasing mole fraction of O₂ in O₂/CO₂ atmosphere can offset these influences. With the increasing mole fraction of O₂, flames of volatiles and char of fuels become brighter and larger, the average flame temperature increases from about 1100 °C to about 1300 °C, while the ignition delay time decreases from 100 ms to 77 ms.     

Mathematical modeling of MILD combustion of pulverized coal

MILD (flameless) combustion is a new rapidly developing technology. The IFRF trials have demonstrated high potential of this technology also for N-containing fuels. In this work the IFRF experiments are analyzed using the CFD-based mathematical model. Both the Chemical Percolation Devolatilization (CPD) model and the char combustion intrinsic reactivity model have been adapted to Guasare coal combusted. The flow-field as well as the temperature and the oxygen fields have been accurately predicted by the CFD-based model. The predicted temperature and gas composition fields have been uniform demonstrating that slow combustion occurs in the entire furnace volume. The CFD-based predictions have highlighted the NO_x reduction potential of MILD combustion through the following mechanism. Before the coal devolatilization proceeds, the coal jet entrains a substantial amount of flue gas so that its oxygen content is typically not higher than 3-5%. The volatiles are given off in a highly sub-stoichiometric environment and their N-containing species are preferentially converted to molecular nitrogen rather than to NO. Furthermore, there exists a strong NO-reburning mechanism within the fuel jet and in the air jet downstream of the position where these two jets merge. In other words, less NO is formed from combustion of volatiles and stronger NO-reburning mechanisms exist in the MILD combustion if compared to conventional coal combustion technology.     

Flexibly using results of CFD and simplified heat transfer model for pulverized coal‐fired boilers

The efficient use of pulverized coal is crucial to the utility industries. The use of computational fluid dynamics (CFD)‐based numerical models has an important role in the design of new boiler furnaces or in retrofitting situations. The results of CFD simulations can be used to better understand the complex processes occurring within the boiler furnace. The use of these results to support boiler operation and training of operators requires that the CFD models can be easily accessed and the results are easily analysed. This paper discusses two ways to simulate the heat transfer process in boiler furnaces. The method directly applying CFD results is employed, in which the grid for solving the energy equation is the same as the flow grid in the CFD simulation while radiation heat transfer is solved in another relatively coarse grid. Comparison of the prediction results between CFD and Heat Transfer code (Simple model) is performed under boiler full load (100%) with one side wall fouling, as well as for different boiler loads (100, 98 and 95 per cent boiler full load, respectively). Finally, the flexible use of the results of CFD and the simple model for pulverized coal‐fired boilers is presented. To facilitate the use of the system, a user‐friendly interface was developed which enables the user to manipulate new calculations and to view results, namely performing ‘what–if’ analysis. Copyright © 2000 John Wiley & Sons, Ltd.     

A complete review based on various aspects of pulverized coal combustion

Coal is the most abundant energy source, and around 40% of the world's electricity is produced by coal combustion. The emission generated through it put a constraint on power production by coal combustion. There is a need to reduce the emissions generated through it to utilize the enormous energy of coal for power production. Detailed understanding of various aspects of coal combustion is required to reduce the emissions from coal‐fired furnaces. The aim of present paper is to review various aspects of pulverized coal combustion such as oxy‐fuel combustion, co‐combustion of coal and biomass, emissions from pulverized coal furnaces, ash formation and deposition, and carbon capture and sequestration (CCS) technologies to outline the progress made in these aspects. Both experimental and numerical aspects are included in this review. This review also discusses the thermodynamic aspects of the combustion process. Furthermore, the effect of various submodels such as devolatilization models, char combustion models, radiation models, and turbulent models on the process of pulverized coal combustion has been investigated in this paper.     

Flame stabilization mechanisms and shape transitions in a 3D printed,hydrogen enriched,methane/air low-swirl burner

Flame shapes and their transitions of premixed hydrogen enriched methane flames in a 3D-printed low-swirl burner are studied using simultaneous OH×CH₂O planar laser induced fluorescence and stereoscopic particle image velocimetry. Three different flame shapes are observed, namely bowl-shape, W-shape, and crown-shape. The bowl-shaped flame has its base stabilized through flame-flow velocity balance and its sides stabilized in the inner shear layer. While the bulges of the W-shaped flame rely on a similar stabilization mechanism in the central flow, its outer edges are stabilized by large-scale eddies in the outer shear layer. The crown-shaped flame is also aerodynamically stabilized in the center, but its outer edges are anchored to the burner hardware. At a fixed equivalence ratio, the statistical transitions between flame shapes across test conditions are jointly dominated by hydrogen fraction and bulk velocity. Dynamically, W-to-crown transition is attributed to the upstream propagation and attachment of the flame outer edges.     

Characteristics of non-premixed oxygen-enhanced combustion: I. The presence of appreciable oxygen at the location of maximum temperature

The presence of appreciable molecular oxygen at the location of maximum temperature has been observed in non-premixed oxygen-enhanced combustion (OEC) processes, specifically in flames having a high stoichiometric mixture fraction (Z_st) produced with diluted fuel and oxygen-enrichment. For conventional fuel-air flames, key features of the flame are consistent with the flame sheet approximation (FSA). In particular, the depletion of O₂ at the location of maximum temperature predicted by the FSA correlates well with the near-zero O₂ concentration measured at this location for conventional fuel-air flames. In contradistinction, computational analysis with detailed kinetics demonstrates that for OEC flames at high Z_st: (1) there is an appreciable concentration of O₂ at the location of maximum temperature and (2) the maximum temperature is not coincident with the location of global stoichiometry, O₂ depletion, or maximum heat release. We investigate these phenomena computationally in three non-premixed ethylene flames at low, moderate, and high Z_st, but with equivalent adiabatic flame temperatures. Results demonstrate that the location of O₂ depletion occurs in the vicinity of global stoichiometry for flames of any Z_st and that the presence of appreciable O₂ at the location of maximum temperature for high Z_st flames is caused by a shift in the location of maximum temperature relative to the location of O₂ depletion. This shifting is attributed to: (1) finite-rate multi-step chemistry resulting in exothermic heat release that is displaced from the location of O₂ depletion and (2) the relative location of the heat release region with respect to the fuel and oxidizer boundaries in mixture fraction space. A method of superposition involving a variation of the flame sheet approximation with two heat sources is shown to be sufficient in explaining this phenomenon.     

Analysis of oxy-fuel combustion power cycle utilizing a pressurized coal combustor

Growing concerns over greenhouse gas emissions have driven extensive research into new power generation cycles that enable carbon dioxide capture and sequestration. In this regard, oxy-fuel combustion is a promising new technology in which fuels are burned in an environment of oxygen and recycled combustion gases. In this paper, an oxy-fuel combustion power cycle that utilizes a pressurized coal combustor is analyzed. We show that this approach recovers more thermal energy from the flue gases because the elevated flue gas pressure raises the dew point and the available latent enthalpy in the flue gases. The high-pressure water-condensing flue gas thermal energy recovery system reduces steam bleeding which is typically used in conventional steam cycles and enables the cycle to achieve higher efficiency. The pressurized combustion process provides the purification and compression unit with a concentrated carbon dioxide stream. For the purpose of our analysis, a flue gas purification and compression process including de-SO_x, de-NO_x, and low temperature flash unit is examined. We compare a case in which the combustor operates at 1.1 bars with a base case in which the combustor operates at 10 bars. Results show nearly 3% point increase in the net efficiency for the latter case.     

Three dimensional modeling of pulverized coal combustion in a 600 MW corner fired boiler

IntroductionThe developments relative to coal combustion havebeen performed into a general purpose 3D code namedESTET under quality assurance, and used to modelcomplex turbulent reactive flows. In the case ofindustrial boilers we can assume a no-slip conditionbetween gas and particles which is the case for the mostpart of the furnace, except possibly in the near field ofthe burners. With such an assumption, the equations for apafticle-gas fixture with a mean density can be written.The combu…     

CFD simulations of premixed hydrogen combustion using the Eddy Dissipation and the Turbulent Flame Closure models

This paper presents a CFD simulation of premixed combustion tests, and centers around a comparison between the classical Eddy Dissipation Model (EDM) and the more sophisticated Turbulent Flame Closure (TFC) model. The chosen tests relate to hydrogen-air deflagration experiments in the THAI and ENACCEF facilities, featuring respectively slow and fast dynamics.Validation of the models is accomplished by comparing model predictions against important measured combustion parameters (flame velocity and spatial propagation, pressure history, spectra, etc.). We follow CFD Best Practice Guidelines, in particular by conducting systematic mesh and time-step sensitivity studies.Both default models predict combustion evolution reasonably well in all tests studied. For the ENACCEF dual compartment experiments, the flame propagation features several dynamical phases, and the TFC model using the progress variable approach reproduces better than the EDM the flame velocity evolution, which leads to better estimation of the temporal gradient of pressure. The better performance of the TFC model comes however at the expense of a larger computational effort, i.e. larger meshes and smaller time steps. This observed trend in 2D geometries is likely to be enhanced in 3D settings.     

Coke generation and conversion behavior of pulverized coal combustion

Preheating combustion is a promising novel low-nitrogen technology. The coke generation and conversion behavior in the preheater and combustion chamber are studied in this paper. The particle size distribution, apparent morphology, specific surface area, pore structure distribution and combustion reactivity of the coke can be analyzed respectively with the particle size analyzer, scanning electron microscope, automatic specific surface area analyzer and Raman spectrometer. The results show that the preheated char generated by the preheating process has some merits: the particle size is smaller, specific surface area is larger, the pore structure is more developed and combustion reactivity is higher. In the down-fired combustor, the degree of particle breakage, apparent morphology, specific surface area and pore structure are influenced by the combustion reaction intensity. Combining the analysis of the graphite stable structure and the active carbon defect structure, the combustion reactivity order of four samples is: 100 mm  >  400 mm > preheated char> 900 mm. This paper provides experimental support and theoretical analysis for this technology to achieve deep control of nitrogen.     

Novel conceptual design of a supercritical pulverized coal boiler utilizing high temperature air combustion (HTAC) technology

Technical and ecological aspects of implementation of High Temperature Air Combustion to power station boilers fired with pulverized coal have been considered. Several boiler concepts have been examined in the context of the following three key points: existence of an intensive in-furnace recirculation, homogeneity of both the temperature and the chemical species fields, and uniformity of heat fluxes. CFD-based numerical simulations have been performed in order to determine the shape of the boiler and its dimensions, to optimize both the distance between burners and location of the burner block. It was concluded that HTAC technology could be a realizable, efficient and clean technology for pulverized coal fired boilers.     

Influence of biomass blends on the particle temperature and burnout characteristics during oxy-fuel co-combustion of coal

The difference in combustion performance between brown coal and black coal blended with Eucalyptus woodchip and woodchar in varying blending ratios were examined in the air and oxy firing conditions. On top of the experimental investigation using a drop tube furnace (DTF), a computational fluid dynamics (CFD) model was further developed to interpret these results, validated using the experimental data. The CFD model incorporates a comprehensive reaction for devolatilisation reaction to predict the gas release utilising predictions based on chemical percolation devolatilisation (CPD) model. The heterogeneous reactions are defined based on the intrinsic reaction model that accounts for the influence of char properties in chemical and pore diffusion reactions using a user-defined function (UDF). Moreover, the C–CO₂ gasification reaction rate which is critical in an oxy-firing mode was further studied using the CFD tool to determine how the role of gasification varied for various fuel blends. Based on carbon burnout and average particle temperature profiles, the blending of woodchips is highly beneficial to the overall combustion performance in particular for low reactive black coal while its effect on brown coal is marginal. Woodchar and black coal are comparable with similar temperature plots and relatively constant burnout but it behaves relatively inert with a highly reactive brown coal. During oxy firing, increasing the woodchip content enhanced the effect of C–CO₂ gasification due to its extremely large pre-exponential factor for the CO₂ gasification reactivity which explains the improved burnout. The blending of woodchar caused a gradual reduction in the gasification extent for both coals explained by the low heating rates under which woodchar was pyrolysed and also due to the decrease in the peak particle temperature. However, the observed gasification was found to be less than the expected value based on the linear addition of the two single fuels for both biomass blends.