Alkali Emission Research Articles

Effects of Temperature, Operation Mode, and Steam Concentration on Alkali Release in Chemical Looping Conversion of Biomass-Experimental Investigation in a 10 kWth Pilot.

Alkali release was studied in a 10 kWth chemical looping pilot operated with a Linz–Donawitz (LD) slag oxygen carrier (OC) and three biomass fuels. Experiments were performed at three temperatures and in three operation modes: chemical looping combustion (CLC), chemical looping gasification (CLG), and oxygen-carrier-aided combustion (OCAC). Gas-phase alkali release was measured with a surface ionization detector (SID). Fuel reactor (FR) gas-phase alkali emissions increased with the temperature. This occurred as a result of increased evaporation of KCl and enhanced decomposition of alkali salts during char conversion. Air reactor (AR) alkali emissions were lower than in the FR and independent of the operating temperature. In comparison of operating modes, CLC and CLG modes resulted in similar gas-phase alkali emissions due to the similar extent of char conversion. In contrast, operation of the reactor system in OCAC mode resulted in significantly lower levels of gas-phase alkalis. The difference in alkali emission was attributed to the steam-rich atmosphere of CLC. The effect of steam was further investigated in CLC and OCAC tests. Lowering steam concentrations in CLC operation resulted in lower gas-phase alkali emissions, while introducing steam to the FR during OCAC operation resulted in higher alkali emissions. It was concluded that steam likely enhances gas-phase K release through a reaction of K2CO3 within the fuel char with steam to produce KOH(g). Solid sampling and analysis for K content was used along with SID measurements to develop a K mass balance for the reactor system. Mass balance results for the straw pellet fuel tests showed that LD slag OC absorbs approximately 15–51% of fuel K, 2.2% of fuel K is released to the gas phase, and up to 3.4% of fuel K is captured in the AR fly ash. The residual 40–80% of fuel K was determined to leave the FR as K-rich fly ash.

Flame temperature and alkali emission in combustion of sugarcane bagasse and straw

This work presents a numerical and experimental study in a laboratorial rig to measure local flame temperature and alkali emission during the combustion of sugarcane bagasse and straw. Tubular combustion chamber to simulate combustion in grate is used. A spectral analysis method in the visible spectrum was used for measurement of local temperatures in a sugarcane bagasse flame and sugarcane straw flame. The two-color method is used to calculate flame temperature. In flame reactive zone, the bagasse and straw flame temperatures in the range of 1420 to 1510 K and in the range of 1450 to 1550 K respectively are calculated. The intensity of alkali emission during the sugarcane bagasse and straw combustion is calculated using data of the spectrum in visible range. It was found that potassium emission is correlated with the instantaneous bagasse flame temperature in the reactive combustion zone. Emission of potassium and sodium during straw combustion is higher than in bagasse combustion. Temperature and alkali emissions along the flame height are measured.

Study of Alkali Emission and Control with Firing a High Alkali Coal

In this study, the sodium emission and control with firing a high-alkali coal was investigated in a high-temperature muffle furnace. Kaolin was chosen as an additive tested to clarify its sodium capture mechanism and assess its capture performance. The influence of some firing-related factors, such as kaolin dosage, temperature, and particle sizes, of both kaolin and coal on sodium release and sodium capture performance of kaolin was also studied. It was found that more sodium was released when the combustion temperature rose or fine fuel particles were combusted. Kaolin could efficiently capture sodium species to form high-melting sodium aluminosilicates, primarily nepheline or carnegieite. Increasing kaolin dosage led to the promoted sodium retention. However, the promotion was not that pronounced in high kaolin dosages. High temperature had a negative effect on sodium capture by kaolin, which could be ascribed to the increasing sodium emission with temperature. Fine kaolin particles exhibited a good sodium capture performance. Coal particle size had little impact on the interactions between kaolin and sodium species. More than 80% of the total sodium was retained in ash by kaolin when firing coal (30 μm) with 3 μm kaolin addition in the dosage of 6 wt% at 1100°C.

Precursor Luminescence near the Collapse of Laser-Induced Bubbles in Alkali-Salt Solutions

A precursor luminescence pulse consisting of atomic line emission is observed as much as 150 nanoseconds prior to the collapse point of laser-created bubbles in alkali-metal solutions. The timing of the emission from neutral Na, Li, and K atoms is strongly dependent on the salt concentration, which appears to result from resonant radiation trapping by the alkali atoms in the bubble. The alkali emission ends at the onset of the blackbody luminescence pulse at the bubble collapse point, and the duration of the blackbody pulse is found to be reduced by up to 30% as the alkali-salt concentration is increased.

Alkali Emission from Birchwood Particles during Rapid Pyrolysis

The emission of potassium- and sodium-containing compounds during rapid birchwood pyrolysis was studied. Birchwood particles (2−130 mg) were inserted into a preheated furnace at constant temperature (350−850 °C) and the alkali emission was measured. Particle mass, furnace temperature, and moisture were varied. At temperatures ≤ 500 °C, the alkali emission from birchwood particles took place solely during the pyrolysis phase. At temperatures ≥ 600 °C, alkali evaporation from the ash increased. The total alkali release increased with temperature in the range studied and the release during the pyrolysis was larger or equal to the release from the ash phase. Small particles (2−10 mg) emitted more alkali per unit mass than large ones (60−130 mg) and this tendency increased with temperature. At 800 °C the emission per unit mass from small particles was 10 times the one for large particles. Wet particles went through a drying phase, which delays the heating, and thereby the alkali emission. The present findings ...

The effects of fuel washing techniques on alkali release from biomass

The influence of different washing techniques on the alkali release during pyrolysis of biomass is studied. After washing and drying, samples of wheat straw, wood waste and cellulose are subjected to a constant heating rate in a N 2 atmosphere, and the release rate of alkali compounds from the sample is measured continuously by a surface ionization technique. Alkali is released from the untreated biomass in two temperature intervals: (1) in connection with the pyrolysis process taking place at 200–500°C, and (2) from the material remaining after pyrolysis at temperatures above 600°C. Separate vacuum pyrolysis experiments show that the alkali release is dominated by potassium-containing compounds, with minor contributions from sodium-containing compounds. The effect of water washing of the biomass is compared with a more thorough acid leaching technique. In the temperature range 200–500°C, washing with water reduces the alkali emission from wood waste and wheat straw by 5–30%, while acid leaching is more effective and reduces the emission by around 70%. Above 600°C where the vaporization of alkali compounds from untreated wheat straws increases sharply, the washing procedures are sufficient for a reduction in the measured alkali release by more than 90%. Experiments with pure cellulose (ash content 0.07%) indicate that the washing methods are ineffective in removing alkali bound to the organic structure of the biomass. The results support the conclusion from earlier studies that relatively simple washing techniques can improve the combustion properties of biomass fuels with a high ash content. For fuels with a lower ash content like woody biomass, the concentration of alkali bound to the organic structure limits the effect of the washing techniques.

Alkali retention in hot coal slag under controlled oxidizing gas atmospheres (air–CO 2)

Many technical high temperature processes including coal conversion face problems of alkali vaporization and subsequent condensation of alkali-rich phases. Most alkali-rich phases cause hazardous reactions with a subsequent corrosion of high temperature refractories or construction materials. A reduction of the alkali emission to flue gases would improve the durability of these materials. The alkali retention of coal slag in varying gas atmospheres was studied under controlled gas atmospheres. Samples of ashed coal were treated with gas atmospheres in the range from air to pure CO 2 for short time intervals of 45–50 min. The residue of the experiments was a homogeneous slag that was quenched. The chemical composition of the experimental slag was quantified using wavelength dispersive electron probe microanalysis. The experiments showed that the concentrations of Si, Ca and Fe in the slag were not affected by varying gas atmospheres and temperature. Alkali concentrations show a strong dependence on temperature. The potassium loss increases by 25% over a temperature range from 1350°C to 1450°C. The alkali retention is influenced by the gas atmosphere. The highest retention of alkalis is achieved in pure air. This effect can be attributed to an increased polymerization of the siliceous slag by ferrous iron. At pure CO 2 atmospheres the alkali retention displays a second maximum. This effect is most probably due to an accelerated melting already during the heating process of the ashed coal. The rapid melting decreases the time for vaporization and improves the fixing of alkalis into the slag. The results have implications for high temperature processes in coal conversion, ceramics industry and glass industry.

Flame Temperature Measurement by Monitoring an Alkali Emission Doublet Exposed to a Selectively Filtered Background Source

The traditional line reversal method for measuring flame temperatures has been modified to eliminate balancing of the background radiation source and facilitate real-time measurement. Two closely spaced emission lines of a Group IA element are simultaneously monitored and compared to the emission of a known source. The new technique uses optical filtering to reduce the apparent temperature of the source at the wavelength corresponding to one of the transitions. The ratio of the absorption/emission intensities of the two lines is directly related to the flame temperature. Flames as hot as 2800 K can readily be probed using the new technique. Experimental data for cool flat flames compare well with thermocouple measurements.

Alkali emission accompanying fracture of sodium silicate glasses

Measurements of atomic Na emission accompanying the fracture of sodium trisilicate glass and a soda lime glass in vacuum were made by quadrupole mass spectroscopy and surface ionization techniques. Peak Na° emission intensities occur some 3–6 ms after the fracture event and decay over tens of milliseconds. This behavior is attributed to the diffusion of Na+ ions into a layer of damaged material at the surface where the ions are subsequently neutralized and thermally emitted as Na°. Charge carriers generated during fracture and subsequently trapped at defect sites apparently play important roles in charge compensating Na+ diffusion and in neutralizing Na+. During the first 300 ms following fracture, we also observe intense, short lived (400 μs) bursts in Na° emission which may be associated with catastrophic relaxation of residual stresses. The kinetics of Na emission suggest that the relaxation of newly formed glass surfaces involves rather complex surface physical and chemical processes.

Threshold effects and time dependence in electron-and photon-stimulated desorption

The use of optical techniques to study the dynamics of neutral alkali emission from alkali halides in electron- and photon-stimulated desorption (ESD/PSD) has produced significant new insights into the ways in which incident electronic energy is converted into kinetic energy of desorbing neutral atoms. In this paper, we show that the threshold for efficient PSD of Li from LiF lies below the band-gap energy, lower than those obtained from earlier ESD threshold measurements on NaCl. Time-resolved measurements of ground-state and excited-state Li desorption from LiF under both electron and photon irradiation exhibit a clear distinction between the spatial and physical origins of these two desorption products. The data suggest a possible identification of excited-state neutral desorption with surface-specific processes; one the other hand, a defect-diffusion model for ground-state desorption is more consistent with the data than are earlier thermal drift models. We also discuss briefly the implications of this work for technological problems as diverse as high-power laser optical damage and the production of intense lithium ion beams for fusion applications.

Analysis of Alkali Surface Ionization on Porous Substrates

The evaporation of alkali atoms and ions from porous substrates is analyzed, making the basic assumption that the transport of alkali material occurs only by surface flow of densely packed alkali atoms close to the emitter surface. The alkali efflux velocity to the emitter surface depends on the flow rate per pore and the minimum pore radius within the surface-flow range. A concentration gradient exists radial to the pore exit which causes the neutral efflux and threshold temperature for surface ionization to be larger than the values characteristic of ionizers coated with vapor-deposited alkali. Alkali emission originates from an area surrounding and including the pore exit. The diameter of the emitting region depends on the alkali flow velocity from the pore and the atom and ion desorption energies. The emission region is divided into an inner zone that emits atoms and an outer zone which predominantly emits ions. This analysis predicts that the neutral efflux from the inner zone is inversely proportional to the pore density N and gives numerical values in good agreement with measured data. The neutral efflux from the outer zone follows the Saha-Langmuir equation, the total neutral efflux being the sum of both. The analysis leads to the conclusion that the threshold temperature depends on the minimum pore radius within the surface-flow range. The threshold temperature is also related to the pore density. This analysis may not be applicable to short straight capillaries because it depends on surface flow in the vicinity of the emitter.

Combination of three lonisation detectors in one gas chromatograph for special applications

A report on detailed tests with the HPD (thermionic detector', tandem FID) has been published by Jentzsch and collaborators [ 1]. Reference is made again to the literature cited in the above report, as a possible source of ideas for clarification of the probable operating principle of this detector. The arrangement used consists of a hollow cylinder, mounted above an FID, for operation in conjunction with the latter. This cylinder is provided with a gas inlet for an additional hydrogen flame and a mount for the electrode delivering the HP signal. A small, coiled platinum capillary, frilled with alkali, is mounted on the flame jet. The Pt lattice expands during the heating process, so that alkali ions diffuse to the surface of the cot. Under the influence of a mechanism which has not as yet been clarified, the alkali ions are detached from the surface and collected to form the signal current. The amount of alkali ions yielded depends on the amount of F, CI, Br, I or P elements present in the combustion products of the pre--connected FID. The signal is accordingly proportional to a halogenstimulated alkali emission.