OCR Text |
Show information which directly represents the combustion and heat transfer characteristics of the furnace. Among the techniques D I A L has deployed for measuring gas temperature and temperature profiles in different combustion facilities are: •Sodium line reversal • Multi-wavelength emission absorption spectroscopy •Multi-element thermocouple probe •Fourier-transform infrared spectroscopy •Spontaneous Raman spectroscopy probe •Coherent anti-Stokes Raman spectroscopy During the field measurement campaign reported here, spontaneous Raman spectroscopy (SRS) probe, multi-element thermocouple probe, and Fourier-transform infrared spectroscopy (FTIR) systems were employed. Spontaneous Raman Spectroscopy Probe Spontaneous Raman scattering can be described as an inelastic collision between a photon and a molecule during which the light undergoes a frequency change characteristic of the internal energy levels (vibrational and rotational) of the irradiated molecule. Of particular interest are the vibrational Raman transitions. If the molecules are initially in the ground vibrational level (v = 0) of the ground electronic state, then after the Raman interaction, some of the molecules will be transferred to higher vibrational levels. The resulting Raman transitions will be at lower energy (lower frequency) than that of the laser excitation; such transitions are referred to as Stokes Raman transitions. If the molecules are initially in an excited vibrational level of the ground electronic state, then after the Raman interaction, some of the molecules will be transferred to the ground vibrational level. For this case, the resulting Raman transitions will be at higher energy (higher frequency) than that of the laser excitation; such transitions are referred to as anti- Stokes Raman transitions. Both Stokes and anti-Stokes Raman signals can be used for temperature measurements. The advantages of S RS are: (1) it is a relatively simple and inexpensive technique compared with other laser-based techniques; (2) it is easier to implement as a fieldable system than many other laser-based techniques; and (3) it can be implemented using only a single port if the Raman signal is collected in the backward direction. The limitations of SRS are: (1) the isotropy of the signal leads to low signal collection efficiency and hence is most suited to clean gas streams; and (2) in a large industrial furnace, the technique must be interfaced with a probe. In order to measure temperature with SRS, one generally uses a major gas species (such as N2, 02, C O , C 0 2 , or Hj) present in the test medium as the probe molecule. N 2 anti-Stokes Raman transitions were used for these temperature profile measurements. T o reduce the data processing time, a library of computer-simulated spectra at different temperatures were generated prior to the field measurement. The temperature of the test medium was then inferred by fitting the observed Raman spectrum with the library of simulated spectra. The SRS apparatus is a modified version of DIAL'S laser-induced breakdown spectroscopy (LIBS) system^ for measuring the concentration of airborne or condensed phase metal species. A pulsed, frequency-doubled N d : Y AG laser beam at 532 n m is used to generate the S R S signal. The SRS signal is collected in the backward direction and focused into an optical fiber bundle (OFB). A 532-nm notch filter is placed in front of the O F B to reduce Rayleigh scattering. The other end of the O F B (which has a slit shape) is coupled to a spectrometer equipped with a gated intensified charge-couple device (ICCD) detector. A gate pulse to the detector is used to synchronize the measurement with the laser pulse. Gating the I C C D detector reduces the background emission from the furnace by a factor of 10° compared with continuous data collection. The Raman spectrum is recorded and averaged. T o assess the accuracy of the DIAL/SRS system, S R S measurements were performed prior to the field campaign at various temperatures in a laboratory furnace. A thermocouple (TC) -5 c m away from the laser focal point was used to monitor the furnace temperature. The laboratory furnace S R S data were analyzed and compared with the T C readings. Assuming the T C reading is close to the true gas temperature, a relative accuracy better than 1 % was determined from a series of S RS spectra recorded at temperatures near 1010°C. SRS temperature profile measurements were made inside the furnace using a water-cooled probe with a focusing lens mounted at the end of the probe. Since most combustion environments are particle rich, the lens was purged with nitrogen gas in order to keep it clean. The rest of the optics train and the small laser head were enclosed in an aluminum box [-10 kg (-25 lbs) total weight] and mounted at the end of the insertion probe. Depending on the lens used, the focal point is -24 to -44 c m from the end of the probe in order to minimize the influence of the probe |