Pulverized Fuel Combustion Research Articles

Investigation of the High-Temperature Behavior of Excluded Siderite Grains during Pulverized Fuel Combustion

An investigation of the behavior of excluded siderite samples derived from both pure mineral species and by density separation of pulverized coal samples was conducted at combustion temperatures of 1100 and 1600 °C. Analysis of quenched combustion residues collected from the combustion gases under oxidizing conditions was obtained via scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) and X-ray fluorescence (XRF) spectroscopy. The FeO−MgO−CaO ternary phase diagram was used to predict the behavior of excluded siderite grains under combustion conditions and indicated that the siderite is unlikely to form sticky or completely melted particles at 1600 °C unless MgO 60wt %. The extent of melting observed for collected particles having different compositions agreed with this prediction. By comparison with published work for excluded pyrite particles, residues from excluded siderite particles are found to be sticky at higher temperatures (1380...

Advances in laboratory tests for predicting coal-related combustion problems

Problems in pulverized fuel combustion in steam-raising plant that can be attributed to the nature of the coal being fired include excessive slagging and fouling, poor char burnout, NOx emissions and flame stability. Coal performance in these areas can, to some extent, be related to the results from existing coal characterization tests but, particularly for coals from unfamiliar areas, these correlations may be unreliable and misleading. For example, attempts to relate slagging propensity to the chemical composition of the coal ash have met with only limited success. It now seems clear that empirical indices and viscosity measurements do not reflect the complex mineral interactions that occur during combustion. However, new microanalysis techniques, capable of characterizing the distribution of mineral matter in a pulverized fuel (PF), now appear likely to provide an improved assessment of the slagging propensity of a coal. Other factors, such as coal volatile yield and nitrogen release, are quite sensitive to the heating conditions. Laboratory tests which match PF combustion conditions much more closely than existing standard methods are now being developed. New approaches to predicting char burnout, by better identification of the small subset of the coal which is actually causing the problem, are also under investigation. While not a general panacea, the appropriate use of new tests is likely to offer engineers involved in coal procurement, who face an increasingly complex market, improved methods for evaluating coals before embarking on expensive pilot- or full-scale trials.

Pressurized Pulverized Fuel Combustion in Different Concentrations of Oxygen and Carbon Dioxide

Theoretical and experimental studies showed that increasing gas pressure at constant gas composition most strongly increases the combustion rate of less reactive coals, which are difficult to burn completely in atmospheric pulverized fuel boilers. The effect of pressure increase is greatest near 0.1 MPa and less at higher pressures. The limit at which increase in pressure has an effect varies from coal to coal, depending on the particle size. With less reactive coals and small particles, the effect can be seen at pressures greater than 1 MPa. The relative effect of pressure increases when the gas oxygen content is low, improving the burnout in furnaces. The effect of pressure is small for large particles and reactive fuels. At high pressures the rates of homogeneous and heterogeneous reactions increase raising the maximum particle temperature; the higher temperature may increase the extent of devolatilization and further decrease the total combustion time. Combustion rate and the temperature of burning coal particles were measured in experiments with a pressurized entrained flow reactor under the following conditions: gas temperature 1073−1473 K, pressure 0.2−0.8 MPa, oxygen partial pressure 0.025−0.1 MPa, and partial pressure of CO2 0.05−0.2 MPa. Measured and calculated results showed increased carbon dioxide concentration in the combustion environment to have an insignificant effect on the combustion rate in the studied temperature region, but it lowered the particle temperature to some extent, suggesting that the gasification reaction CO2−C takes place as well. Calculations indicated that in pressurized combustion the rate of gasification reaction is greater at higher temperatures.

Parametric sensitivity study of a CFD‐based coal combustion model

AbstractParametric sensitivity of a two‐dimensional pulverized‐fuel (PF) combustion model is studied extensively for the effect of parametric uncertainty on model predictions. Results show that error in coal devolatilization/oxidation parameters has the dominant effect on predicted burnout, NOx formation, local gas temperature, and coal‐gas mixture fraction. Uncertainty in the turbulent particle dispersion parameters appears to have a secondary effect, while error in the particle‐gas radiation parameters has little impact on model predictions. Regions of the computational domain exhibiting sensitivity to specific parameters are identified. Specific parameter sensitivity implies the relative importance of various mechanisms in the overall process. Turbulent particle dispersion seems to be important early in the reactor with kinetic processes dominating at and following the predicted ignition point. Radiation appears to be of minor importance. These results indicate the need for a better method of predicting the overall volatiles yield and further understanding of the devolatilization/oxidation mechanism and its role in the overall PF combustion process. The study provides fundamental direction for future comprehensive model development and focuses on experimental work to better quantify critical input parameters.

Pulverized Fuel Combustion in a Turbulent Round Jet Burner

Experiments were performed in the near-field of a combustion-driven, coaxial round jet burner to investigate the turbulent combustion of solid fuel particles. Pulverized coal of varying volatile content and size, ranging from submicron to 149 j*m, was employed as a fuel in this study. The flow configuration consisted of a central high-speed jet, comprising coal and air, exhausting into a coaxial shroud jet of hot combustion products. Concentration data obtained at distances of up to 20 initial jet diameters from the inlet to the combustor suggest that, for the conditions examined here, reaction of the solid fuel is confined to the interface between the two jets. Particles in excess of approximately 50 fim in diameter did not appear to react in significant quantity within the present range of particle residence times. Particle size data provide some evidence of a preferential outward dispersion or consumption of fine particulates. A well-defined optimum value of solid fuel loading at which combustion proceeds most vigorously was determined. This optimum value was only weakly influenced by coal size, injection velocity, and fuel volatile content.

Numerical Model for Predicting Performance of Three-Dimensional Pulverized-Fuel Fired Furnaces

A numerical furnace model (FURMO) was developed to model steady-state, three-dimensional pulverized-fuel combustion in practical furnace geometries. This model is based on a fundamental description of various interacting processes which occur during combustion: turbulent flow, heterogeneous and homogeneous chemical reaction, and heat transfer. The detailed analysis achieved by the method is useful for evaluating furnace performance and in the interpretation of laboratory, intermediate, and utility test data. In this paper, three-dimensional pulverized coal combustion is investigated for a 560 MW utility boiler. Numerical results for flow, heat transfer, and chemistry are presented. Contour maps are exhibited to describe these complex three-dimensional interactions. Model sensitivity is evaluated for changes in slag layer thickness, particle size distribution, and devolatilization and char oxidation rates.

Modern boiler plant

Modern boiler plant

Massachusetts Institute of Technology

In this chapter, technology options for solid biomass and waste fuel utilisation in large-scale combustion plants are presented with the focus on fuel flexible technologies allowing multi-fuel operation. Fuel flexibility (the ability to use various fuels) protects power plant investment from changes in competitiveness between different fuels, which can be caused, for example, by changes in legislation. In multi-fuel operation (burning various fuels at the same time), synergies between different types of fuels can be exploited allowing higher generation efficiency also from lower-quality fuels.The three main combustion technologies for solid fuels are grate, fluidised-bed and pulverised-fuel combustion. Grate boilers are adaptable to various fuels but are quite sensitive to changes in fuel quality and have limited multi-fuel operation capability. Fluidised-bed boilers provide the best fuel flexibility. With circulating fluidised-bed (CFB) boilers, biomass/waste fuels can be co-fired with coal on 0–100% share. Various technologies have been developed to enable co-firing biomass with coal in pulverised coal (PC) boilers as the enormous capacity of existing PC boilers offers huge potential for increasing biomass utilisation and economic benefits compared to new stand-alone biomass/waste plants. Capital costs are reduced markedly, and the high electrical efficiency of large PC boilers and the favourable properties of coal ash can be exploited. Three different routes can be taken to utilise co-firing in PC boilers: (1) direct co-firing, (2) indirect co-firing, and (3) parallel co-firing. In addition, a PC boiler can be converted into a bubbling fluidised-bed (BFB) boiler with attractive economics.