Plasma chemical systems

Modelling plasma chemical systems is a complex task, because these system are far from thennodynamical equilibrium. A complete model includes the external electric circuit, the various physical volume and surface reactions, the space charges and the internal electric fields, the electron kinetics, the homogeneous chemical reactions in the plasma volume as well as the heterogeneous reactions at the walls or electrodes. These reactions are initiated primarily by the electrons. In most cases, plasma chemical reactors work with a flowing gas so that the flow conditions, laminar or turbulent, must be taken into account. As discussed before, the electron gas is not in thennodynamic equilibrium... 

The main requirements of the plasma chemical system can be formulated as follows ... 

Plasma is not only a multi-component system, but often a very non-equihbrium one (see Section 1.3). Concentrations of the active species described earlier can exceed those of quasi-equilibrium systems by maiy orders of magnitude at the same gas temperature. The successful control of plasma permits chemical processes to be directed in a desired direction, selectively, and through an optimal mechanism. Control of a plasma-chemical system requires detailed understanding of elementary processes and the kinetics of the chemically active plasma. The major fundamentals of plasma physics, elementary processes in plasma, and plasma kinetics are to be discussed in Chapters 2 and 3 more details on the subject can be found in Fridman and Keimedy (2004). 

Mechanisms of ionization can be very different in different plasma-chemical systems and... 

Different mechanisms of destmction of negative ions releasing an electron are discnssed in special books by Massey (1976), McDaniel (1964), and Smirnov (1982). We are going to consider three detachment mechanisms most important in plasma-chemical systems. The first one, which is especially important in non-thermal discharges, is associative detachment ... 

However, wherever possible, the application of quasi-equilibrium statistical distributions is the easiest and clearest way to describe the kinetics and thermodynantics of plasma-chemical systems. 

The application of quasi-equilibrium statistics and thermodynamics to plasma-chemical systems requires a clear understanding and distinction between the concepts of complete thermodynamic equilibrium (CTE) and local thermodynamic equilibrium (LTE). CTE is related to uniform plasma, in which chemical equilibrium and all plasma properties are unambiguous functions of temperature. This temperature is supposed to be homogeneous and the same for all degrees of freedom, all components, and all possible reactions. In particular, the following five equilibrium statistical distributions should take place for the same temperature T ... 

A detailed description of non-equilibrium plasma-chemical systems and processes generally requires an application of kinetic models. The application of statistical models leads sometimes to significant errors (Slovetsky, 1980). In some specific systems, however, statistical approaches can be not only simple but also quite successful in describing non-equilibrium plasma, which is discussed next. 

The energy efficiency of the qtrasi-eqtrilibritrm plasma-chemical systems performed in thermal discharges is nstrally relatively low (less than 10-20%), which is dne to two major effects ... 

Mass and Energy Transfer Equations in Multi-Component Quasi-Equilibrium Plasma-Chemical Systems... 

The set of equations describing energy and mass transfer in quasi-equilibrium plasma-chemical systems are analyzed in the next section to determine the influence of transfer phenomena on energy efficiency. 

In some plasma-chemical systems, the destruction of negative ions by, for example, associative electron detachment is faster than ion-ion recombination ... 

Let us analyze the energy balanee of CO2 dissociation stimulated in plasma by vibrational excitation in the two-temperature approximation, assuming one-dimentional gas motion with density p through the plasma in the x-direction with velocity u. Such an energy balance can be illustrated in the framework of the following equations describing major energy transfer, relaxation, and chemical reaction processes separately for different individual vibrational modes in the plasma-chemical system, which includes CO2 and products of its dissociation (Rusanov Fridman, 1984) ... 

Here Too is the initial gas temperature in the tank before the supersonic nozzle. If the initial Mach number is not very close to unity, the critical heat release can be estimated as g cr CpToo. Numerical values of the critical heat release at different initial Mach numbers for the supersonic CO2 flow can also be found in Fig. 5 1. Further increase of the heat release in plasma over the critical value leads to the formation of non-steady-state flow perturbations like shock waves, which do no good to a non-equilibrium plasma-chemical system. Even taking into account the high energy efficiency of chemical reactions in supersonic flows, the critical heat release seriously restricts the specific energy input ... 

Figure 5—46. Evolution of static gas pressure along axis of supersonie plasma-chemical system (1) without discharge (2) in presence of microwave discharge.

DISSOCIATION OF WATER VAPOR AND HYDROGEN PRODUCTION IN PLASMA-CHEMICAL SYSTEMS... 

Water Dissociation and H2 Production in Plasma-Chemical System CO2-H2O... 

Heating of N2O in conventional chemical or thermal plasma-chemical systems leads mostly to N2O dissociation and reduction of molecular nitrogen. The selective oxidation of N2O (5-178) becomes possible in non-equilibrium plasma conditions by means of oxygen dissociation and by the following exothermic reactions of NO formation ... 

The activation energy of (6-58) is high therefore, its contribution is significant at high translational temperatures (To > 1500 K). When To > 1500 K, and the vibrational temperature of N2 is higher (7(, > Tq), the two-temperature qttasi-equilibriiun state of the plasma-chemical system can be established ... 

Formation of Intermediate N20 ( E+) Complex in Electronically Non-Adiabatic Channel of NO Synthesis. Estimate the maximum gas temperature To, when the formation of the intermediate N20 ( E+) complex proceeds as a multi-quantum transition and is determined by the vibrational energy of N2 molecules. Use relations (6-25) and (6-26) and estimate the probability of the multi-quantum transition. Explain why this mechanism is a preferred one in cold plasma-chemical systems. 

These processes were demonstrated in different non-equihbrium plasma-chemical systems (Hemptinne, 1897 Losanitch Jovitschitsch, 1897 Lob, 1906 Losanitch, 1911 Lind, 1923 Briner Hoefer, 1940). It is interesting to compare hydrogenization of carbon dioxide (9-59) with that of carbon disulfide (CS2), which has been done in non-thermal plasma conditions and leads to the production of acetylene and strlfur ... 

The most conventional non-equilibrium plasma-chemical systems that produce diamond films use H2-CH4 mixture as a feed gas. Plasma activation of this mixture leads to the gas-phase formation of hydrogen atoms, methyl radicals (CH3), and acetylene (C2H2), which play a major role in further film growth. Transport of the gas-phase active species to the substrate is mostly provided by diffusion. The substrate is usually made from metal, silicon, or ceramics and is specially treated to create diamond nucleation centers. The temperature of the substrate is sustained at the level of 1000-1300 K to provide effective diamond synthesis. The synthesis of diamond films is provided by numerous elementary surface reactions. Four chemical reactions in particular describe the most general kinetic features of the process. First of all, surface recombination of atomic lydrogen from the gas phase into molecular hydrogen returns back to the gas phase ... 

Methane Conversion into Aeetylene in Non-Equilibrium Plasma Conditions. Compare relations (9-14) and (9-15) to estimate the relative accuracy of logarithmic approximation (9-15) for calculations of the CH4 dissociation rate coefficient at Ty > Tq. Why can t the logarithmic approximation be applied to describe methane conversion into acetylene in strongly non-equilibrium plasma-chemical system, when the vibrational temperature significantly exceeds the translational one ... 

THERMAL AND NON-THERMAL PLASMA-CHEMICAL SYSTEMS FOR COAL CONVERSION... 

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