Excimer Laser Induced Fragmentation Fluorescence Research Articles

Continuous in situ measurements of alkali species in the gasification of biomass

This work describes the first on-line, in situ measurement of alkali species in biomass gasification using excimer laser induced fragmentation fluorescence (ELIF). Three pelletised biomass fuels were gasified under different operating conditions in a bubbling fluidized-bed reactor, using steam as the gasifying medium. Concentrations of potassium and sodium were measured in real time for several hours per measurement. Average concentrations obtained for potassium ranged from 140 to 350ppb, and those for sodium ranged from 1.7 to 60ppb. Also observed was a slow release of alkali after shutting off the fuel feed, which suggests that alkali species had been taken up in the bed material (olivine); this was confirmed by analysing the bed material before and after use. Consideration of the olivine structure/composition and possible interactions with alkali and other elements during gasification suggests that alkali-containing layers could have formed on the bed particles but appear not to form crystalline phases. This may allow for re-release of alkali while the bed is still hot.

An in situ optical gas phase analysis approach for TGA: Its assessment and application

Thermogravimetric analyzers (TGA) are widely applied for studies of coal conversion as a function of process parameters. However, weight change of fuel samples as a function of temperature is not always sufficient for detailed identification of volatilized or reacted species. Coupling TGA with various gas analyzers could provide useful additional information for understanding mechanisms of trace element release and reaction kinetics. Until now, coupling of TGA with gas analysis has mostly been carried out by extraction of gas samples from the hot reaction zone followed by time-delayed and external analysis in a separate measurement cell. Changes of sample composition during gas analysis often cannot be avoided due to temperature and pressure reduction and filtering of hot reaction gas. Additionally, effects of local gas phase change on sample surfaces due to displacement of purge gas by evolved components cannot be determined. Compared to external analysis, in situ measurement of gas composition immediately in the hot reaction zone would provide inherent advantages. In this paper, the application of three optical in situ measurement techniques for a high temperature TGA is shown. Therefore, a TGA standard setup was redesigned and equipped with sapphire windows and optical ports. Transmission behavior of sapphire against wavelength and temperature is presented. Coal and char gasification and alkali release were examined with thermogravimetric methods and complemented with optical techniques. The release of gaseous sodium compounds was investigated with Excimer Laser Induced Fragmentation Fluorescence Spectroscopy (ELIF). The main gasification products (H 2, CO 2, CO, H 2O) were detected with Raman and Fourier Transformation Infrared Spectroscopy (FTIR). Various experiments demonstrate that in situ spectroscopy is applicable up to 1300 °C even in an optically difficult accessible TGA and results give useful additional input to further evaluation of TGA data.

Simultaneous detection of nickel and potassium in the flue gas of a fluidised bed coal combustor by excimer laser-induced fragmentation fluorescence

Photofragmentation fluorescence is now well known as a sensitive, real-time technique for detection of metals and other species of environmental and industrial interest. This work aims to demonstrate the feasibility of real-time monitoring of nickel species in a realistic industrial flue gas using excimer laser-induced fragmentation fluorescence (ELIF), but at the same time, potassium species were monitored in a second detection channel. An ArF-excimer laser (193 nm) was used for photofragmentation and the fluorescence of Ni and K atom fragments was detected at 440 and 769 nm, respectively. Actual flue gas measurements were performed at the atmospheric pressure fluidised bed combustor (FBC) of DMT and first observations of the behaviour of these two species under flue gas conditions were made. Results suggest that the release of both metal species is affected by both the amount of chlorine and by the amount of clay minerals in the system. Estimating from present signal–noise levels, a sensitivity of ca. 50 ppb is achievable for Ni at the present wavelength.

Phase discrimination of alkali species in PCFB combustion flue gas using simultaneous monitoring by surface ionisation and photofragmentation fluorescence ☆

This paper is concerned with the on-line determination of alkali traces in solid fuel combustion using simultaneous surface ionisation (SI) and excimer laser induced fragmentation fluorescence (ELIF). Both methods measure in real-time but are complementary in that ELIF can discriminate towards gas phase alkali species, whereas SI detects alkali both in the gas phase and on aerosol particles. Measurements were made in the flue gas of a 10 MW PCFB combustor at 10 bar total pressure, whereby the measuring equipment was mounted onto the flue gas pipe downstream of the hot gas filter. The combustor was fed with a bituminous coal and operated in the temperature range 850–900 °C. Results show that the general response of the measured signals to parameters such as fuel load and to bed and flue gas temperatures was very similar for the two methods and reflected variations in these parameters. Concentrations determined by ELIF ranged from 6 to 63 ppb (Na) and 1 to 54 ppb (K), depending on conditions. In contrast, concentrations obtained by the SI technique were a factor of 1–10 higher than for ELIF, depending on the flue gas temperature and the total alkali (gas+particle phase) present in the flue gas. In this way it can be shown that information about the physical state of the alkali species can be obtained by direct comparison of the two methods.

In-situ study of the effect of operating conditions and additives on alkali emissions in fluidised bed combustion

In this work, alkali concentrations were determined in a realistic industrial flue gas by an in-situ laser spectroscopic method, excimer laser induced fragmentation fluorescence (ELIF). An excitation wavelength of 193 nm (ArF-excimer laser) was used to photofragment the alkali species and fluorescence signals from the atomic metal fragments were detected at 589 (Na) and 766 nm (K), respectively. These signals provide a measure of the alkali compounds in the flue gas. In the actual flue gas measurements, which were performed at the atmospheric pressure fluidised bed combustor of DMT, the flue gas alkali concentrations were observed to vary, e.g., with bed temperature and with the rate of fuel supply. Data taken following a hot gas filter in the flue gas line gave significantly reduced levels of alkali species compared to those before the filter. The addition of sodium or potassium acetate to the coal leads to increases of up to a factor of two in flue gas alkali concentrations. In some experiments, small quantities of heavy metal chlorides were added to the coal fired and this resulted in very strong increases in flue gas alkali concentrations. A possible explanation for this effect may lie in reactions with the mineral matter to form heavy metal aluminosilicates and consequent release of alkali.

The influence of fuel additives on the behaviour of gaseous alkali-metal compounds during pulverised coal combustion.

The alkali-metal vapour release during pulverised hard (bituminous) coal combustion was investigated in a semi-technical drop flow reactor in the temperature range 1100-1400 degrees C. Absolute concentrations of total gas-phase sodium and potassium species were determined using the in situ/on-line excimer laser induced fragmentation fluorescence technique (ELIF). Alkali-metal concentrations measured for the untreated coals were found to be in the range 0.1 to 4.7 ppm, depending on the temperature. As well as observing the temperature dependence, the effect of co-feeding defined amounts of silica and clay minerals was studied. In addition, to assist interpretation of ELIF measurements, ash samples were taken and analysed by scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX). The additives lead to a pronounced binding of the alkali-metal species and suppression of the sharp temperature dependence observed without co-feeding. Therefore, the use of such getter materials can be confirmed as an effective way to remove corrosive alkali-metal species from the flue gas in pulverised coal combustion.

Single-port optical access for spectroscopic measurements in industrial flue gas ducts

An in situ optical access to industrial-scale flue-gas ducts under high pressure has been designed and realised. The system is used here for the determination, by excimer-laser-induced fragmentation fluorescence (ELIF), of alkali species released from the combustion of coal in a pressurised circulating fluidised bed (PCFB) combustor. However, it could be applied to the detection of other species in a variety of industrial plants. The essential feature of the design is the access for the light source and collection of signal via the same, single window. The set-up is compact, robust and durable, having been tested under realistic flue-gas conditions for periods of up to two weeks in the PCFB pilot plant. The main features of the access are described and examples of data collected are presented to demonstrate its capabilities.

98/02280 In situ alkali concentration measurements in a pressurized, fluidized-bed coal combustor by excimer laser induced fragmentation fluorescence

98/02280 In situ alkali concentration measurements in a pressurized, fluidized-bed coal combustor by excimer laser induced fragmentation fluorescence

In situ alkali concentration measurements in a pressuried, fluidized-bed coal combustor by excimer laser induced fragmentation fluorescence

Absolute concentrations of total gas-phase sodium and potassium compounds were determined in the flue gas of a fluidized bed coal combustor at 10 bar using excimer laser-induced fragmentation fluorescence (ELIF). In this in situ and on-line technique, the alkali compound molecules are photodissociated by ArF-excimer laser light at 193 nm, whereby excited Na(32P) and K(42P) atoms are formed. Fluorescence from these atoms is simultaneously detected in two separate channels at 589 nm and 768 nm, respectively. The calibration system and data evaluation procedure to convert measured signals to absolute concentrations is described. Studies on lignite and hard coal under several different combustion conditions are reported. The measured concentrations span a dynamic range of more than three orders of magnitude, with a detection limit of <0.2 ppb for Na and K. Large differences between the coals in the release of alkalis were observed. For the lignite investigated, potassium-compound concentrations up to 800 ppb and sodium-compound concentrations up to 500 ppb were determined, whereas the concentrations for the hard coal were less than 10 ppb. These large variations in alkali release reflect the very different structures of the respective coals, emphasizing the influence of mineral coal components on release and recapturing of gas-phase alkali species. It is also clear that more detailed, systematic studies are needed for a proper understanding of alkali release in PFBC.

Determination of flue gas alkali concentrations in fluidized-bed coal combustion by excimer-laser-induced fragmentation fluorescence

Gas-phase sodium concentrations were measured for the first time in situ in the flue gas of a fluidizedbed reactor by the excimer-laser-induced fragmentation fluorescence (ELIF) technique. This method involves using ArF-excimer laser light at 193 nm to simultaneously photodissociate the alkali compounds of interest and excite electronically the alkali atoms formed. The resulting fluorescence from Na (32P) atoms can be readily detected at 589 nm. Measured signals were converted to absolute concentrations using a calibration system that monitors alkali compounds under known conditions of temperature, pressure, and composition and using the same optical setup as at the reactor. Several different coals were investigated under a specific set of reactor conditions at total pressures close to 1 bar. Sodium concentrations ranging from the sub-ppb region to 20 ppb were obtained, and a detection limit for sodium of 0.1 ppb under the present conditions was estimated. Over the course of the reactor programme, contrasting concentration histories were observed for the two lignites and the hard coal investigated. In particular, significantly higher sodium concentrations were found for the hard coal, consistent with both the higher chlorine and sodium contents determined in the corresponding coal analysis.

Determination of Alkali Traces in Coal Combustion by Excimer Laser Induced Fragmentation Fluorescence

AbstractGas phase sodium concentrations were measured in situ in the flue gas of a fluidised bed reactor using the excimer laser induced fragmentation fluorescence (ELIF) technique. Several different coals were investigated for a fixed set of reactor conditions at ∼ 1 bar total pressure. Measured signals were converted to absolute concentrations using a calibration system that monitors alkali compounds under well‐defined conditions. Concentrations ranging from the sub‐ppb region to 25 ppb were obtained. Contrasting concentration histories were observed for the brown and hard coals studied.