Plasma lysis

This article covers the chemical process. For a similar biological term, see plasmolysis .

Plasmalysis is an artificial word made up of plasma and lysis (Greek λύσις, "[dissolving]"). On the one hand, it describes the plasma-chemical dissociation of organic and inorganic compounds (e.g. CH and NH compounds) in interaction with a plasma in the absence of oxygen, and on the other hand the synthesis , i.e. the combination, of two or more elements into one new molecule (e.g. methane synthesis).

Thermal / non-thermal plasmas

Arcs and sparks are thermal plasmas; H. Electrons and ions are in thermodynamic equilibrium .

Pulsed coronal and dielectrically impeded discharges belong to the family of non-thermal plasmas. Here the electrons are much hotter (several eV) than the ions / neutral gas particles (room temperature). It is precisely the collisions between the hot electrons and the cold ions that determine and control plasma chemistry.

Atmospheric plasmas

Atmospheric pressure plasmas have been used for a variety of industrial applications, e.g. B. for the removal of volatile organic compounds (VOC), for the treatment of exhaust emissions and for the treatment of polymer surfaces. For decades, non-thermal plasmas have also been used to generate ozone for water purification. Atmospheric pressure plasmas can primarily be characterized by a large number of electrical discharges in which the majority of the electrical energy flows into the generation of energetic electrons. These energetic electrons produce chemically excited species - free radicals and ions - as well as additional electrons through dissociation, excitation and ionization of background gas molecules through electron impact. These excited species in turn oxidize , reduce or decompose the molecules such as B. also dirty water or bio-methane that are brought into contact with them.

principle

Plasmalysis requires a voltage source that supplies the electrical energy and drives the chemical reactions. Part of the electrical energy is converted into chemical energy . Plasma analyzes can thus be used to store energy, for example in the plasma analysis of ammonium from wastewater , which generates hydrogen and nitrogen . The hydrogen obtained in this way can serve as an energy carrier for a hydrogen economy. Through the hydrogen oxidation in a fuel cell , around 80% (compared to electrolysis 40%) of the electrical energy originally used can be recovered.

In the chemical reactions that take place during plasma lysis, electrons are transferred in the arc between the anode and cathode . There are therefore always redox reactions, whereby the oxidation and reduction do not take place spatially separated from each other:

• Voltage source
• Electrodes
• Overload
• Current density

Mechanisms of dissociation of gases and liquids

In the following, XH stands for any hydrogen compound, e.g. B. CH and NH compounds.

• Thermal dissociation: Gaseous water molecules are dissociated in plasmas at T> 3000 K, for example. Above 3500 K, H ₂ and O ₂ are also dissociated.

• electron impact dissociation:
  ${\displaystyle e+XH(s,l,g)\rightarrow H(g)+X(s,l,g)}$

The radical density scales with the electron density and with higher gas and electron temperatures (thermal dissociation and electron impact)

• ion impact dissociation:
  ${\displaystyle A^{+}+XH(g)\rightarrow A^{+}+H+X(s,l,g)}$

• dissociative electron attachment:
  ${\displaystyle e+XH^{*}\rightarrow X^{-}(s,l,g)+H(g)}$

This process creates both negative ions and neutral particles. The collision electron is captured, the capture taking place by shock excitation. The energy difference between the ground state and the excited state dissociates the molecule. The electron-induced dissociation of the water depends on the electron temperature, which has a decisive influence on the ratio of the OH density (n_OH) to the electron density (n_e). The maximum OH density is reached in the early afterglow when the electron temperature (T_e) is low.

• Photoionization:
  High-energy photons dissociate molecules

• Solvated electrons:
  reducing agent in the liquid

Dissociation efficiency of different hydrogen sources

Water electrolysis

Since the focus is always on the most energy-efficient dissociation of chemical compounds, the benchmark is the energy consumption of the electrolysis of distilled water (45 kWh / kg H ₂ ) analogous to the following reaction equation:

• ${\displaystyle \mathrm {2\ H_{2}O(f)\rightleftharpoons O_{2}(g)+2\ H_{2}(g)} \qquad \Delta H_{R\ 298}^{1bar(a)}=-286\ \mathrm {kJ/mol} }$

Methane plasma lysis

Methane plasma analysis is a particularly efficient way of generating hydrogen (10 kWh / kg H2). Methane (e.g. also from natural gas) is converted into the plasma in the absence of oxygen analogous to the following reaction equation:

• ${\displaystyle \mathrm {2\ CH_{4}(g)\rightleftharpoons C_{2}(f)+4\ H_{2}(g)} \qquad \Delta H_{R\ 298}^{1bar(a)}=-75\ \mathrm {kJ/mol} }$

decomposes, forming hydrogen and elemental carbon.

Methane plasma lysis offers u. a. the possibility of decentralized decarbonisation of natural gas or, if biogas is used, the implementation of an active process-related CO2 sink , whereby, contrary to the previously common CCS process, no gas has to be compressed and stored, but the elementary carbon that occurs can be bound in product form.

Dirty water plasma analysis

The plasma analysis of waste water and solid-free liquid manure enables hydrogen to be obtained from pollutants (ammonium (NH4) or hydrocarbon compounds (COD)) contained in the waste water. The plasmalytic cleavage of ammonia takes place analogously to the following reaction equation:

• ${\displaystyle \mathrm {2\ NH_{3}(f)\rightleftharpoons N_{2}(g)+3\ H_{2}(g)} \qquad \Delta H_{R\ 298}^{1bar(a)}=-92\ \mathrm {kJ/mol} }$

The treated dirty water is cleaned. The energy requirement for the production of green hydrogen is approx. 20 kWh / kg H ₂ .

Decomposition of hydrogen sulfide

Hydrogen sulphide - a component in petroleum and petroleum gas and a by-product in the digestion of biogenic substances - is also suitable for plasmalytic cleavage to generate hydrogen and elemental sulfur due to its weak binding energy.

${\displaystyle \mathrm {H_{2}S(g)\rightleftharpoons H_{2}(g)+S(s)} \qquad \Delta H_{R\ 298}^{1bar(a)}=-20,5\ \mathrm {kJ/mol} }$

The energy requirement for the production of such hydrogen is about 5 kWh / kg H ₂ .

Appropriate plasma processes

The following plasma processes are suitable for generating non-thermal plasmas:

• High frequency discharge
• Dielectric barrier discharge
• Coronal low frequency discharge

In non-thermal plasmas, the electrons are much hotter (several eV) than the ions / neutral gas particles (room temperature). It is precisely the collisions between the hot electrons and the cold ions that determine and control plasma chemistry.

Non-thermal plasmas are particularly suitable for the energy-efficient generation of hydrogen.

Reactor geometries

It turns out that both the reactor geometry and the method by which the plasma is generated have a strong influence on the performance of the system.

Individual evidence

1. ^ ^{A b} Claire Tendero, Christelle Tixier, Pascal Tristant, Jean Desmaison, Philippe Leprince: Atmospheric pressure plasmas: A review . In: Spectrochimica Acta Part B: Atomic Spectroscopy . tape 61 , no. 1 , January 2006, p. 2–30 , doi : 10.1016 / j.sab.2005.10.003 .
2. ↑ Alexander Fridman: Introduction to Theoretical and Applied Plasma Chemistry . In: Plasma Chemistry . Cambridge University Press, Cambridge, ISBN 978-0-511-54607-5 , pp. 1-11 , doi : 10.1017 / cbo9780511546075.003 .
3. ^ Steve Owen, Roger Woodward: Chemistry for the IB Diploma Coursebook with Free Online Material . Cambridge University Press, 2014, ISBN 978-1-107-62270-8 ( google.de [accessed April 22, 2020]).
4. ↑ Peter Kurzweil: Fuel Cell Technology: Basics, Materials, Applications, Gas Generation . Springer-Verlag, 2016, ISBN 978-3-658-14935-2 ( google.de [accessed on April 22, 2020]).