Bed Combustion Of Solid Fuels Research Articles

Environmental Friendly Fluidized Bed Combustion of Solid Fuels: A Review About Local Scale Modeling of Char Heterogeneous Combustion

Fluidized bed combustion is currently intensively developed throughout the world to produce energy from several types of solid fuels, while significantly reducing pollutant emissions with respect to conventional combustion units. Accurate models must be formulated at both bed and particle levels to operate efficiently such units, since local phenomena such as particle temperature and combustion rate are crucial aspects for process improvement and control. In this sense, this article proposes a classification of local scale models to represent the evolution of char heterogeneous combustion of any carbonaceous particles. Existing models are described and classified based on the characteristics of the governing equations, the thermal behavior of the gas and solid phases and the description of both the burning particle and the surrounding gas, under a heterogeneous or pseudo-continuous assumption. Criteria for choosing one model instead of others are also considered, depending on the case. The so-called Intrinsic Reactivity Models are described in detail for evaluating the pertinence of their simulated results. The use of CFD to build a simulation scheme of the solid combustion process at local scale is also presented and discussed. A complete description of the solid fuel burning process is given, along with useful information concerning the evolution of different variables, such as particle internal temperature that governs the reaction rate and gas composition. This comparative analysis gives a strong basis to select the appropriate modeling approach. Finally, recommendations are proposed for model application and future development.

Burning of volatiles in fluidized bed combustion of solid fuels

An experimental study of the dynamics of the devolatilization of solid fuels during fluidized bed combustion is reported. The kinetic constants are obtained by statistical analysis, along with an expression for calculating the time over which the volatiles are released. A formula is proposed for calculating the fraction of heat released by combustion of volatiles in the fluidized bed layer.

Unsteady one-dimensional model for a bed combustion of solid fuels

A transient one-dimensional combustion model of a solid fuel bed that can be applied to various types of combustors employing the solid bed is suggested. Configuration of solid beds can be characterized by combinations of solid and gas flow directions, ignition methods, and dominant reactions. These features can be reflected numerically to the boundary and initial conditions of the model, while the governing mechanism in each type of bed is similar. The mechanism is expressed as forms of conservation equations for the solid and the gas phase. Solid-gas reactions, gaseous reactions, heat transfer, geometrical changes of the particles, and physical properties are modelled and reflected to each term of the governing equations. Simulation results are shown for two cases: a packed bed combustor and an iron ore sintering bed. Two cases have distinctively different bed configurations and ignition methods. The propagation rates of the combustion zone are quantitatively discussed in terms of the flame propagation speed. Future development of the model is discussed for possible application to more complicated simulation cases.

Influence of air supply on the emissions of NO and N 2O from a circulating fluidized bed boiler

The nitrogen oxides formed during fluidized bed combustion of solid fuels orginate from fuel nitrogen in both char and volatiles. In the course of combustion these fuel constituents not only contribute to the production but also take part in the reduction of the emission.In order to study the role of the nitrogen-containing volatiles for the production and reduction of nitrogen oxides in fluidized bed boilers a series of tests has been carried out with fuels having different contents of volatiles in a range from anthracite to wood in two types of boiler, a circulating and a stationary fluidized bed boiler. It was found that the different designs of boilers result in different behaviours of the nitrogen-containing volatiles. In stationary fluidized bed boilers the volatiles tend to reduce the emissions of NO comparable to what takes place in a pulverized coal flame, whereas in circulating fluidized bed boilers the volatiles do not have the same beneficial effect. In circulating fluidized bed boilers the emission of NO tends to increase with an increasing content of volatiles in the fuel, whereas the emission decreases from the stationarry fluidized bed type of boiler burning fuels with a larger amount of volatiles.

The role of fuel volatiles for the emission of nitrogen oxides from fluidized bed boilers—A comparison between designs

The nitrogen oxides formed during fluidized bed combustion of solid fuels orginate from fuel nitrogen in both char and volatiles. In the course of combustion these fuel constituents not only contribute to the production but also take part in the reduction of the emission. In order to study the role of the nitrogen-containing volatiles for the production and reduction of nitrogen oxides in fluidized bed boilers a series of tests has been carried out with fuels having different contents of volatiles in a range from anthracite to wood in two types of boiler, a circulating and a stationary fluidized bed boiler. It was found that the different designs of boilers result in different behaviours of the nitrogen-containing volatiles. In stationary fluidized bed boilers the volatiles tend to reduce the emissions of NO comparable to what takes place in a pulverized coal flame, whereas in circulating fluidized bed boilers the volatiles do not have the same beneficial effect. In circulating fluidized bed boilers the emission of NO tends to increase with an increasing content of volatiles in the fuel, whereas the emission decreases from the stationarry fluidized bed type of boiler burning fuels with a larger amount of volatiles.