Combustor particle size

Glicksman and Farrell (1995) constructed a scale model of the Tidd 70 MWe pressurized fluidized bed combustor. The scale model was fluidized with air at atmospheric pressure and temperature. They used the simplified set of scaling relationships to construct a one-quarter length scale model of a section of the Tidd combustor shown in Fig. 34. Based on the results of Glicksman and McAndrews (1985), the bubble characteristics within a bank of horizontal tubes should be independent of wall effects at locations at least three to five bubble diameters away from the wall. Low density polyurethane beads were used to obtain a close fit with the solid-to-gas density ratio for the combustor as well as the particle sphericity and particle size distribution (Table 6). 

Ackeskog et al. (1993) made the first heat transfer measurements in a scale model of a pressurized bubbling bed combustor. These results shed light on the influence of particle size, density and pressure levels on the fundamental mechanism of heat transfer, e.g., the increased importance of the gas convective component with increased pressure. 

Combustion of aluminum particle as fuel, and oxygen, air, or steam as oxidant provides an attractive propulsion strategy. In addition to hydrocarbon fuel combustion, research is focussed on determining the particle size and distribution and other relevant parameters for effectively combusting aluminum/oxygen and aluminum/steam in a laboratory-scale atmospheric dump combustor by John Foote at Engineering Research and Consulting, Inc. (Chapter 8). A Monte-Carlo numerical scheme was utilized to estimate the radiant heat loss rates from the combustion products, based on the measured radiation intensities and combustion temperatures. These results provide some of the basic information needed for realistic aluminum combustor development for underwater propulsion. 

Based on the initial studies of unforced flows described above, inflow was selectively perturbed to investigate if the amount of particle dispersion and the location of enhanced dispersion within the combustor can be shifted as desired. Calculations were performed in which an acoustic perturbation was imposed from the back wall of the combustor with an amplitude of 0.5% of the initial chamber pressure and a frequency of 1380 Hz, 690 Hz, or 145 Hz, the characteristic frequencies of the system under study. The vortex-shedding frequency was 1380 Hz, the first-merging frequency was 690 Hz, and 145 Hz was the quarter-wave mode of the inlet. In addition, simulations were also performed with forcing at a frequency unrelated to the system, 1000 Hz. The particle size chosen for these simulations was 15 pm in diameter (St = 0.97), since this size particles were found to be optimally dispersed in the unforced flow case. All other parameters remain unchanged from the unforced case discussed above. 

Results of an experimental program in which aluminum particles were burned with steam and mixtures of oxygen and argon in small-scale atmospheric dump combustor are presented. Measurements of combustion temperature, radiation intensity in the wavelength interval from 400 to 800 nm, and combustion products particle size distribution and composition were made. A combustion temperature of about 2900 K was measured for combustion of aluminum particles with a mixture of 20%(wt.) O2 and 80%(wt.) Ar, while a combustion temperature of about 2500 K was measured for combustion of aluminum particles with steam. Combustion efficiency for aluminum particles with a mean size of 17 yum burned in steam with O/F) / 0/F)st 1-10 and with residence time after ignition estimated at 22 ms was about 95%. A Monte Carlo numerical method was used to estimate the radiant heat loss rates from the combustion products, based on the measured radiation intensities and combustion temperatures. A peak heat loss rate of 9.5 W/cm was calculated for the 02/Ar oxidizer case, while a peak heat loss rate of 4.8 W/cm was calculated for the H2O oxidizer case. 

The goal of the present study is to provide the information needed for design of a practical underwater propulsion system utilizing powdered aluminum burned with steam. Experiments are being conducted in atmospheric pressure dump combustors using argon/oxygen mixtures and steam as oxidizers. Spectrometer measurements have been made to estimate combustion temperatures and radiant heat transfer rates, and samples of combustion products have been collected to determine the composition and particle size distribution of the products. 

Measurements of combustion temperatures, radiation intensity distributions in the range from 400 to 800 nm, and particle size distributions of combustion products have been made for the reaction of aluminum powder with both 02/Ar and H2O oxidizers in atmospheric dump combustors. The fraction of unburned aluminum in the combustion products was also determined for the H2O oxidizer case. An analytical study was performed to determine if the measurements are consistent with each other and with theory, and also to estimate the rate of heat loss from the combustion products. A Monte Carlo technique was used to determine the expected spectral energy distribution that would be emitted from a viewport located in the side of a combustion chamber containing products of aluminum combustion. 

In this study, measurements of the combustion temperature, radiation intensity, combustion products particle size distribution, and combustion efficiency have been made for combustion of aluminum particles with steam in a small-scale atmospheric dump combustor. This data will be useful for designers of combustion chambers for burning of aluminum powder with steam. 

Particle size distributions of smaller particles have been made using electrical mobility analyzers and diffusion batteries, (9-11) instruments which are not suited to chemical characterization of the aerosol. Nonetheless, these data have made major contributions to our understanding of particle formation mechanisms (1, 1 ). At least two distinct mechanisms make major contributions to the aerosols produced by pulverized coal combustors. The vast majority of the aerosol mass consists of the ash residue which is left after the coal is burned. At the high temperatures in these furnaces, the ash melts and coalesces to form large spherical particles. Their mean diameter is typically in the range 10-20 pm. The smallest particles produced by this process are expected to be the size of the mineral inclusions in the parent coal. Thus, we expect few residual ash particles smaller than a few tenths of a micrometer in diameter (12). 

The aerosol produced by a laboratory pulverized coal combustor was size classified in the range 0.03 to 4 ym Stokes equivalent diameter using a low-pressure cascade impactor. The samples thus collected were analyzed using a focussed beam particle induced X-ray emission technique. This combination of techniques was shown to be capable of resolving much of the structure of the submicron coal ash aerosol. Two distinct modes in the mass distribution were observed. The break between these modes was at a particle size of about 0.1... 

A fly ash sample from a fluidized bed coal combustor is analyzed to obtain particle size data. Table PI. 1 shows the distributions of the projected area equivalent diameter of the particle dA obtained by the image analysis and the volume diameter of the particle d obtained by the electrozone technique [Ghadiri et al., 1991]. 

The performance of a fluidized bed combustor is strongly influenced by the fluid mechanics and heat transfer in the bed, consideration of which must be part of any attempt to realistically model bed performance. The fluid mechanics and heat transfer in an AFBC must, however, be distinguished from those in fluidized catalytic reactors such as fluidized catalytic crackers (FCCs) because the particle size in an AFBC, typically about 1 mm in diameter, is more than an order of magnitude larger than that utilized in FCC s, typically about 50 ym. The consequences of this difference in particle size is illustrated in Table 1. Particle Reynolds number in an FCC is much smaller than unity so that viscous forces dominate whereas for an AFBC the particle Reynolds number is of order unity and the effect of inertial forces become noticeable. Minimum velocity of fluidization (u ) in an FCC is so low that the bubble-rise velocity exceeds the gas velocity in the dense phase (umf/cmf) over a bed s depth the FCC s operate in the so-called fast bubble regime to be elaborated on later. By contrast- the bubble-rise velocity in an AFBC may be slower or faster than the gas-phase velocity in the emulsion... 

Figure 19 shows the size distribution of solids sampled at the top, middle, and bottom of the fast fluidized bed combustor and the downcomer (Li et al., 1991). It can be seen that all particle size distribution curves are rather... 

The dynamic and steady-state characteristics of a shallow fluidized bed combustor have been simulated by using a dynamic model in which the lateral solids and gas dispersion are taken into account. The model is based on the two phase theory of fluidization and takes into consideration the effects of the coal particle size distribution, resistance due to diffusion, and reaction. The results of the simulation indicate that concentration gradients exist in the bed on the other hand, the temperature in the bed is quite uniform at any instant in all the cases studied. The results of the simulation also indicate that there exist a critical bubble size and carbon feed rate above which "concentration runaway" occurs, and the bed can never reach the steady state. 

Let us consider a shallow fluidized bed combustor with multiple coal feeders which are used to reduce the lateral concentration gradient of coal (11). For simplicity, let us assume that the bed can be divided into N similar cylinders of radius R, each with a single feed point in the center. The assumption allows us to use the symmetrical properties of a cylindrical coordinate system and thus greatly reduce the difficulty of computation. The model proposed is based on the two phase theory of fluidization. Both diffusion and reaction resistances in combustion are considered, and the particle size distribution of coal is taken into account also. The assumptions of the model are (a) The bed consists of two phases, namely, the bubble and emulsion phases. The voidage of emulsion phase remains constant and is equal to that at incipient fluidization, and the flow of gas through the bed in excess of minimum fluidization passes through the bed in the form of bubbles (12). (b) The emulsion phase is well mixed in the axial... 

Gas cleaning efficiencies of ca. 99.95% have been obtained, with typical filter outlet dust loads of 5-10 mg/Nm LCV gas. These values are acceptable for gas turbine (combustor) operation considering also the sub-micron particle size of solids permeating the filter, see e,g. [14], The values are also well below Dutch emission standards for power producing stations as well as waste incinerators, see e.g. Bergsma et al. [15]. 

The rate constants of the reactions (1 )-(4) increase from the bottom to the top of the combustor. This reflects the modeled specific area rise when the particle size decreases. In the absence of the direct measurements of the oxidation rate for the char formed from almond shells, different rate constants from the literature [6,7,8] have been attempted. 

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