Bo G Leckner

A fluidized bed combustion process with inherent CO2 separation; application of chemical looping combustion

For combustion with CO2 capture, chemical-looping combustion has the advantage that no energy is lost for the separation of CO2. In chemical-looping combustion oxygen is transferred from the combustion air to the gaseous fuel by means of an oxygen carrier. The fuel and the combustion air are never mixed, and the gases from the oxidation of the fuel, CO2 and H2O, leave the system as a separate stream. The H2O can easily be removed by condensation and pure CO2 is obtained without any loss of energy for separation. This makes chemical-looping combustion a most interesting alternative to other CO2 separation schemes, which have the drawback of a large energy consumption. A design of a boiler with chemical-looping combustion is proposed. The system involves two interconnected fluidized beds, a high-velocity riser and a low-velocity bed. Metal oxide particles are used as oxygen carrier. The reactivities needed for oxygen carriers to be suitable for such a process are estimated and compared to available experimental data for particles of Fe2O3 and NiO. The data available on oxygen carriers, although limited, indicate that the process outlined should be feasible.

The spectral distribution of solar radiation at the earth's surface—elements of a model

The attenuating properties of the atmosphere are presented in a form that permits the calculation of direct, diffuse and global spectral solar irradiance at ground level, using the extraterrestrial irradiance values of Thekaekara. Input data describing the state of the atmosphere are obtained from standard meteorological information or from tabulated average values. Clear sky diffuse spectral irradiance on a horizontal surface is estimated from the direct beam by means of an empirical simplification. Examples of the agreement between calculated and measured values are shown.

Characterization of fluidization regimes by time-series analysis of pressure fluctuations

Abstract This work compares time, frequency and state-space analyses of pressure measurements from fluidized beds. The experiments were carried out in a circulating fluidized bed, operated under ambient conditions and under different fluidization regimes. Interpretation of results in time domain, such as standard deviation of the pressure fluctuations, may lead to erroneous conclusions about the flow regime. The results from the frequency domain (power spectra) and state-space analyses (correlation dimension, D ML , and Kolmogorov entropy, K ML , together with a non-linearity test) of the pressure fluctuations are generally in agreement and can be used complementary to each other. The power spectra can be divided into three regions, a region corresponding to the macro-structure (due to the bubble flow) and, at higher frequencies, two regions representing finer structures that are not predominantly governed by the macro structure of the flow. In all fluidization regimes, the measured pressure fluctuations exhibited an intermittent structure, which is not revealed by power spectral analysis of the original signals. Fluctuations with pronounced peaks in the power spectrum and in the auto-correlation function, corresponding to passage of single bubbles through the bed, are non-linear with a low dimension ( D ML D ML D ML >5.5 both K ML (bits/cycle) and D ML are insensitive to changes in the distribution of energy in power spectra. Thus, the state-space analysis reflects that non-linearity is mostly found in the macro-structure of the flow. Fluidized bed time series treated in this work are available at http://www.entek.chalmers.se/∼fijo

Particle emissions from biomass combustion in small combustors

Literature data on particle emissions are compared with emissions from combustion of wood pellets and wood briquettes in commercial small-scale combustion devices: a pellet stove, two pellet burners and two smaller district heating boilers. The influence of operating parameters and fuel quality was investigated. Mass concentration, number concentration and number size distribution of particles were determined. The mass size distribution was analysed as well as the inorganic components. Gaseous compounds were recorded to give information about the combustion conditions. The mass concentrations of particles were between 34 and , increasing during unsatisfactory operation conditions. The number concentration was in the range of 107–108 particles per Ncm−3. The particle emission was dominated by submicron particles (size ), both from number and mass perspective. The main inorganic components of the submicron particles were potassium, sulphur, chlorine and oxygen. Small amounts of sodium, magnesium and zinc were also found. The contents of potassium, chlorine, and sulphur in the fuel are important for the composition of the emitted inorganic submicron particles.

Spectral and total emissivity of water vapor and carbon dioxide

Data on the infrared radiation characteristics of carbon dioxide and water vapor in the form of absorption coefficients and line spacings averaged over narrow spectral intervals have been compiled from various sources. These data are to be used in heat transfer calculations from hot gases. In order to investigate the accuracy of the data, the simplest case possible is chosen: a comparison with the total emissivity charts of water vapor and carbon dioxide. It appears however that the charts are not entirely reliable as standards for comparison: it seems probable that Hottels chart for water vapor gives too low values at temperatures above 900°C and that the partial pressure correction is temperature dependent. With the exception of some regions where judgment is difficult, the calculations using spectral data seem to represent total emissivities with a maximum error which is estimated to around 10%. Sources of error in the spectral data and in Hottels total emissivity charts are discussed. Total emissivity charts, pressure and overlap corrections based on calculations with spectral data are presented.

Fluidized bed combustion: mixing and pollutant limitation

Despite the lack of published information, this review concentrates on processes in large circulating fluidized bed combustors, and especially on mixing between fuel and air. Largeconcentration differences may be present in the combustion chamber, but the concentration cannot be properly predicted. Mixing is important for combustion but also for sulphur capture through the formation of reducing zones. This and other factors influencing sulphur capture are discussed. Measurement of gas concentration in a CFB combustion chamber are shown, and various methods to reduce the emissions of N2O emission are presented.

Fluidization regimes in non-slugging fluidized beds : the influence of pressure drop across the air distributor

The purpose of the present work is to study the influence of the pressure drop across the air distributor on the bubbling conditions of the bottom bed of circulating fluidized beds (CFB). The bottom bed is the dense bubbling zone just above the distributor. The experimental work was carried out in a 12 MWm CFB boiler and in a cold CFR Three different distributions of the bubble flow in time and space, termed fluidization regimes, were identified in the cold CFB: the multiple bubble regime with many small bubbles evenly distributed in the bed; the single bubble regime, characterized by the presence of only one bubble at a time in the bed; and the exploding bubble regime with large, single, irregular voids stretching from the air distributor to the bed surface. These bubbling conditions were observed during variations in the gas velocity and the distributor pressure drop. A comparison with the 2 mz cross-section CFB boiler showed that the boiler always operates in the single or in the exploding bubble regime, which indicates a bubble flow that is not continuous and not well distributed over the crosssection of the bed. The conditions in the boiler are influenced by the relatively large area of gas passage and the low pressure drop of the boiler air distributor.

Fundamentals of turbulent gas-solid flows applied to circulating fluidized bed combustion

A summary is made of the present state of knowledge of turbulent gas-solid flow modeling and in particular its application to circulating fluidized bed combustion chambers. Models are presented to close the set of equations describing isothermal non-reacting turbulent gas-particle flows applied to fluidization, and it is shown under which assumptions the models can be derived. With the kinetic theory of granular flow, transport equations for the velocity moments and closure laws for the stress tensor and the energy flux are derived for the particle phase. Closure equations for the drift velocity and for the fluid-particle velocity correlation tensor are presented, first based on algebraic models and, second, based on transport equations with the fluid-particle joint probability density function. An alternative derivation of the fluid-particle velocity covariance transport equation is compared to the formulation based on the fluid-particle joint probability density function. Two-way coupling is discussed, and a transport equation for the second-order velocity moments is used to derive a two-equation model accounting for the modulation of gas phase turbulence by particles. Boundary conditions for the set of equations describing a turbulent gas-solid flow are discussed. Provided that the domain of applicability of the models is known, a discussion on the usefulness of the models is given, as well as an application to fluidization and especially to circulating fluidized bed combustors. Prospects for improvement of the existing models are presented.

Expansion of a freely bubbling fluidized bed

Abstract The expansion of a freely bubbling fluidized bed is studied over a range of particle properties and gas velocities that applies to fluidized bed boilers. A bed expansion model is derived from a modified two-phase flow model. The results from the model are compared with measurements in both a cold two-dimensional bed and a 16 MW th fluidized bed boiler, as well as with data found in the literature. The model represents experimental results for sand particles of a diameter ranging from 0.15 mm to 4.0 mm and with gas velocities up to 3 m s −1 .

Combustion of wood particles—a particle model for eulerian calculations

A simplified model for the combustion of solid fuel particles is derived, relevant for particle sizes and shapes used in fluidized and fixed-bed combustors and gasifiers. The model operates with a small number of variables and treats the most essential features of the conversion of solid fuel particles, such as temperature gradients inside the particle, the release of volatiles, shrinkage, and swelling. Typical shapes (spheres, finite cylinders, and parallelepipeds) are also considered. The model treats the particle in one dimension, and to describe the conversion inside a fuel particle the model only needs the transfer of heat and mass to an element of its external surface. When modeling a large combustion system, it is a great advantage that the conversion is related to the external surface, because the model does not have to be limited to just a single particle. In fact, it can handle the conversion of a solid phase in a computational cell, where the conversion is related to surface area per unit volume, instead of the surface area of a single particle. The model divides the particle into four layers: moist (virgin) wood, dry wood, char residue, and ash. The development of these layers is computed as function of time. The model shows satisfactory agreement with measurements performed on more than 60 samples of particles of different sizes, wood species, and moisture contents. Comparison with the experiments shows that the simplifications made do not significantly influence the overall accuracy of the model. The model also demonstrates the great influence of shrinkage on the times of devolatilization and char combustion.