Beam burners

The above advantages, coupled with the development of techniques to improve penetration of the burning gas into the centre of the kiln, has enabled gas-fired shaft kilns with diameters of up to 5 m and outputs of over 200 t/d to remain competitive with modem designs of shaft kilns. Such techniques include the addition of beam burners, central burners and the adoption of the asymmetric wafting technique. 

The concept of the shaft kiln has been modernised in a number of designs, the characteristics of four (Nos. A to D) are summarised in Table 16.1. Some designs are more suitable for low outputs e.g. below 100 t/d, while others can be used for much higher outputs, e.g. up to 800 t/d for the beam-burner design. 

The design of a conventional atomic absorption spectrometer is relatively simple (Fig. 3.1), consisting of a lamp, a beam chopper, a burner, a grating monochromator, and a photomultiplier detector. The design of each of these is briefly considered. The figure shows both single and double beam operation, as explained below. 

Figure 3.1 Schematic diagram of an AAS spectrometer. A is the light source (hollow cathode lamp), B is the beam chopper (see Fig. 3.2), C is the burner, D the monochromator, E the photomultiplier detector, and F the computer for data analysis. In the single beam instrument, the beam from the lamp is modulated by the beam chopper (to reduce noise) and passes directly through the flame (solid light path). In a double beam instrument the beam chopper is angled and the rear surface reflective, so that part of the beam is passed along the reference beam path (dashed line), and is then recombined with the sample beam by a half-silvered mirror.

The burner head can be adjusted vertically, horizontally (toward and away from the operator), and rotationally. Initially, it should be adjusted vertically so that the light beam passes approximately 1 cm above the center slot and horizontally so that the light beam passes directly through the center of the flame, end to end. All three positions can be optimized by monitoring the absorbance of an analyte standard while making the adjustments. A maximum absorbance would indicate the optimum position. 

A water-cooled stainless-steel probe (4.1-millimeter internal diameter) with four inlet holes (0.50-millimeter diameter) was used to continuously sample combustion products 2 cm above the burner. The samples were drawn through an ice-bath-cooled water trap, a drying column, and a 5-micron filter to reduce the water mole-fraction and to remove particles. Temperature and static pressure in the absorption cell were monitored using a type-S thermocouple and a pressure gauge. The flow entered the cell on the same end as the optical beam and exited on the opposite end through 0.5-inch windows before... 

Instrumental methods have become more sophisticated to face these challenges. In particular, Westmoreland and Cool have developed a flame-sampling mass spectrometer that has provided several revelations in terms of relevant molecular intermediates in combustion. " Their setup couples a laminar flat-flame burner to a mass spectrometer. This burner can be moved along the axis of the molecular beam to obtain spatial and temporal profiles of common flame intermediates. By using a highly tunable synchrotron radiation source, isomeric information on selected mass peaks can be obtained. This experiment represents a huge step forward in the utility of MS in combustion studies lack of isomer characterization had previously prevented a full accounting of the reaction species and pathways. 

Figure 14.4—The diverse components of a single beam atomic absorption spectrophotometer. Model IL 157, built in the 1980s. 1, source 2, burner 3, monochromator 4, detector (design according to Thermo Jarrell Ash Corp.).

Instruments that have burners and require nebulisation of dilute aqueous sample solutions generally have low background noise in the signal. With graphite furnaces, incomplete atomisation of the solid sample at elevated temperatures can produce interfering absorptions. This matrix effect does not exist in an isolated state and thus cannot be eliminated by comparison with a reference beam. This is notably the case for solutions containing particles in suspension, ions that cannot be readily reduced and organic molecules, all of which create a constant absorbance in the interval covered by the monochromator. 

Experimental. The experimental set up consisted of a -pum-ped-dye laser (Molectron UV-14, DL-400), spatial filters to isolate the central part of the dye laser beam, a H2 02-Ar or N2 flame supported by a capillary burner with Ar or N2 sheath, and a fluorescence detection system at right angles (a JY-H-10 monochromator, a photomultiplier, and a PAR 162-164 boxcar averager). All measurements were taken 1 cm above the burner top the concentration of Ca, Sr, In, and Na was low ( 1 yg/ml). The fluorescence waveform was monitored with a 75 ps sampling head (PAR 163). The laser spectral bandwidth was also measured with a JY-HR-1000 monochromator (6AS < 0.1 ). 

The present work involves measurement of k in a 0.1 atmosphere, stoichiometric CH -Air flame. All experiments were conducted using 3 inch diameter water-cooled sintered copper burners. Data obtained in our study include (a) temperature profiles obtained by coated miniature thermocouples calibrated by sodium line reversal, (b) NO and composition profiles obtained using molecular beam sampling mass spectrometry and microprobe sampling with chemiluminescent analysis and (c) OH profiles obtained by absorption spectroscopy using an OH resonance lamp. Several flame studies (4) have demonstrated the applicability of partial equilibrium in the post reaction zone of low pressure flames and therefore the (OH) profile can be used to obtain the (0) profile with high accuracy. 

Signal and signal-to-noise ratio depend upon the passage of the light beam from the lamp through the flame centre at the optimum height. Thus the burner position must be optimized in three respects ... 

Both AAS and AFS with flame atomizers depend upon the stable and reproducible production of ground state atoms. Therefore atomizer parameters such as fuel-to-oxidant ratio or burner head position relative to the excitation beam will be equally important in both techniques. The major difference in AFS lies with the types of flames sometimes employed. 

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