Alkali vapor

Most tars in the product gas are destroyed by thermal cracking as they pass through the hot reaction zone, and the raw product then exits the gasifier at high temperature. The particulate levels in the product gas are typically low due to the absence of turbulence in the gasifier, but the gas may contain alkali vapors as it exits the hot reaction zone. 

The need to remove alkali material from the biogas stream depends on the end use for the gas. Problems associated with alkali vapor formation and deposition are critical in systems where the hot biogas is to be used without significant cool-down. Moderate gas cooling followed by removal of bulk particulates provides adequate cleaning for simple boiler systems that can tolerate some ash deposition. Other applications such as gas turbines require cleaner fuel gases. Since turbines operate at high rotational speeds, deposition can... 

In spite of the fact that in alkali vapors, which contain about 1 % diatomic alkali-molecules at a total vapor-pressure of 10 torr, the atoms cannot absorb laser lines (because there is no proper resonance transition), atomic fluorescence lines have been observed 04) upon irradiating the vapor cell with laser light. The atomic excited states can be produced either by collision-induced dissociation of excited molecules or by photodissociation from excited molecular states by a second photon. The latter process is not improbable, because of the large light intensities in the exciting laser beam. These questions will hopefully be solved by the investigations currently being performed in our laboratory. 

Plasma Types. Figure 1 (7—9) indicates the various types of plasmas according to their electron density and electron temperature. The colder or low electron energy regions contain cold plasmas such as interstellar and interplanetary space the earth s ionosphere, of which the aurora borealis would be a visible type alkali-vapor plasmas some flames and condensed-state plasmas, including semiconductors (qv). 

Fig. 4.4 Schematic diagram of a typical arrangment for laser induced fluorescence measurements of lifetimes. A pulsed laser beam (or beams) passes through a heated glass cell containing alkali vapor and the time and wavelength resolved fluorescence is detected...

Many water soluble analytes can be sampled by bubbling the air through water in an impinger. Acid vapors, alkali vapors, or their dusts can be collected in water and their aqueous solutions analyzed by wet methods. Often, water is made basic or acidic to trap acidic or basic analytes, respectively. Other solvents can be used in the impinger, depending on the solubility of the analytes and the vapor pressure of the solvents. Certain organics in the air can also be trapped in impingers if one uses the proper solvents. 

Let us first discuss a system which is traditional for optical pumping in the Kastler sense [106, 224, 226], namely an optically oriented alkali atom A (see Fig. 1.1) in a noble gas X buffer surrounding. It is important to take into account the fact that in alkali atoms, owing to hyperfine interaction, nuclear spins are also oriented. However, in a mixture of alkali vapor with a noble gas alkali dimers A2 which are in the 1SJ electronic ground state are always present. There exist two basic collisional mechanisms which lead to orientation transfer from the optically oriented (spin-polarized) atom A to the dimer A2 (a) creation and destruction of molecules in triple collisions A + A + X <—> A2 + X (6) exchange atom-dimer reaction... 

Gupta, R., Happer, W., Moe, G. and Park, W. (1974). Nuclear magnetic resonance of diatomic molecules in optically pumped alkali vapors, Phys. Rev. Lett., 32, 574-577. 

Dayton, D. C. and Frederick, W. J., Direct observation of alkali vapor release during biomass combustion and gasification.2. Black liquor combustion at 1100 degrees C. Energy Fuels 1996, 10 (2), 284-292. 

A typical arrangement of the apparatus used for fluorescent intensity measurements in mixtures of alkali vapors and quenching gases is shown in... 

The transition from the atom to the cluster to the bulk metal can best be understood in the alkali metals. For example, the ionization potential (IP) (and also the electron affinity (EA)) of sodium clusters Na must approach the metallic sodium work function in the limit N - . We previously displayed this (1) by showing these values from the beautiful experiments by Schumacher et al. (36, 37) (also described in this volume 38)) plotted versus N". The electron affinity values also shown are from (39), (40) and (34) for N = 1,2 and 3, respectively. A better plot still is versus the radius R of the N-mer, equivalent to a plot versus as shown in Figure 1. The slopes of the lines labelled "metal sphere" are slightly uncertain those shown are 4/3 times the slope of Wood ( j ) and assume a simple cubic lattice relation of R and N. It is clear that reasonably accurate interpolation between the bulk work function and the IP and EA values for small clusters is now possible. There are, of course, important quantum and statistical effects for small N, e.g. the trimer has an anomalously low IP and high EA, which can be readily understood in terms of molecular orbital theory (, ). The positive trimer ions may in fact be "ionization sinks" in alkali vapor discharges a possible explanation for the "violet bands" seen in sodium vapor (20) is the radiative recombination of Na. Csj may be the hypothetical negative ion corresponding to EA == 1.2 eV... 

Thermionic conversion is a technology that needs, and can immediately use, research on high temperature properties of alkali metals. Electron transport properties of alkali vapors and characteristics of atomic clusters are particularly Important. Improved understanding in these areas could lead to performance improvements that would more than double the output power density and efficiency of cesium ignited mode thermionic converters. 

As reported earlier (18), the QRLPP effect is a threshold process, requiring sufficiently high atomic density N and laser intensity I. The threshold values depend on various parameters like laser detuning and bandwidth, focusing optics, and the particular excited state involved also, the threshold N and threshold I values are correlated, with the increase of one allowing a decrease of the other. Typical threshold values in Cs are N=10i cm and I=10 W cm when a 6010A laser is used to excite the 5(803 2) state. Threshold values when other states are excited or when other alkali vapors are used can be quite different for example, Stwalley and co-workers (19) have reported that the threshold values in a sodium vapor are N lQt cm and I 10 W cm when a focused cw dye laser at 5688 or S683A is used to excite the Na(4D) state. 

Alkali Vapor Transport in Coal Conversion and Combustion Systems... 

Even a minor amount of alkali vapor transport can be significant, as revealed by the turbine tolerance level of 0.02 ppm alkali needed for corrosion control in pressurized fluidized bed combustors (12). If we consider only the alkali halide content of the dolomite component, this tolerance level would require an alkali-scrubbing efficiency of better than 99.9999 percent for PFBC. Even if corrosion (alkali) resistant materials were available, uncontrolled alkali vapor transport would still lead to unmanageable deposits on cool downstream components. For instance, under typical coal gasifier conditions, a species partial pressure as low as 10 atm would lead to vapor transport and deposition in metric ton quantities on an annual basis. 

The need for a basic understanding of alkali vapor transport in fossil energy systems can be appreciated when we consider the diversity of conditions such as temperature, pressure, chemical composition, and time scale, present in existing and developing fossil fuel technologies. Table 1 summarizes some typical process conditions. 

HASTiE ET AL. Alkali Vapor Transport Coal Mineral Characteristics... 

Alkali vapor transport and deposition is a well-known, though poorly understood, factor in the corrosion or fouling of alloys and ceramics, both in established and developing technologies. Problem areas include oil-fired glass melting operations (21), blast furnaces, boilers, turbines, coal gasification ( ), MHD (23, 24, 25) and coal-fired pressurized fluidized beds (26). 

Despite the incomplete state of a thermodynamic data base and limited mechanistic insight, several attempts to model alkali vapor transport in reactive atmospheres have been made. The increased sophistication of modeling efforts in recent years is demonstrated by the following examples ... 

In complex combustion systems, alkali vapor transport can occur as the metal or as molecular species, such as NaCl, (NaCl)2, NaOH, (NaOH)2, Na2S04, NaSO (x = 2,3), NaPO (x = 2,3) and,... 

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