On the possibility of the utilization of hydrogen sulfide from the Black Sea

natural environment. Especially harmful is the release of carbon dioxide and sulfur oxides during the exploitation of these resources. A significant energy raw material potential of non-traditional resources lies in the waters and bot - tom of the Black Sea, which is a natural geobiotechnological reactor, capable of producing a variety of energy raw resources. This paper discusses the use of hydrogen sulfide available in the Black Sea waters to produce energy and useful industrial products and proposes the respective. The technology also has an ecological effect in terms of the purification of the hydrogen sulfide pool. The paper also discusses some technologies for the separation of hydrogen sulfide to hydrogen and sulfur. An estimation of the heat value of hydrogen sulfide in the water of the Black Sea is also presented.


Introduction
In recent decades, unconventional methods of energy production have been undergoing research and industrial experimentation (Etarski 1994;Demirbas 2006;Ushakov 2018;Kostov 2020). A large proportion of these studies has focused on the utilization of various gases that constitute emissions in industry, agriculture and households, as well as gasses from some natural sources (Baul et al. 2018;Krystev 2021;Lansche et al. 2017;Nyrkov et al. 2016). In addition to producing energy, in many cases, valuable raw materials can be obtained while simultaneously Rys. 1. Morze Czarne having a significant environmental effect. A typical example in this regard is the hydrogen sulfide (H 2 S) released from various natural sources and, in particular, oil refineries (Ngene et al. 2016;Rubright et al. 2017;Myasnikova et al. 2019). The significant energy and raw material potential of non-traditional resources lies both in the waters and at the bottom of the Black Sea (Fig. 1), which is a natural geo-biotechnological reactor, capable of producing a variety of energy and raw material resources Nyrkov at al. 2016;Maksymova and Kostrytska 2018).

Hydrogen sulfide in the Black Sea
One of the hypotheses for the origin of the Black Sea is that 7500 years ago, it was the deepest lake on the earth. It is supposed to have been 100 m deeper than the present sea depth (Maksymova and Kostrytska 2018). With the end of the Glacial Era and the rise of the level of the world's oceans the Bosporus strait breached. A hundred thousand square meters of already cultivated lands were flooded. The hypothesis presumes that the formation of the Black Sea went along with mass mortality of life in the lake's fresh-water life. As a result of its decomposition, methane and hydrogen sulfide appeared as a by-product.
Today, the Black Sea is the largest H 2 S field in the world. A characteristic feature of its structure is the presence of a deep-water H 2 S layer at a depth of 130-200 m. This layer differs sharply from the upper surface layers in its properties. For these reasons, the phenomenon of upwelling is of particular importance for explaining the changes in the chemical and biogeochemical processes occurring in the water masses and at the bottom. This upwelling provokes the rise of waters from the H 2 S layer, which are significantly more saline and rich in biogenic components ( Fig. 2) (Demirbas 2006). Figure 3 shows the gradual increment of the H 2 S concentration from the surface of the Black Sea to the bottom (Dimitrov et al. 2003). In bottom sediments, H 2 S contents range from 12--16 mg/l to 240 mg/l. The dissolved H 2 S gas phase reaches 0.24 g/t at a depth of 300 m and 2.2 g/t at a depth of 2000 m. In seawater, H 2 S is found not only in the dissolved gas phase but also as sulfides and hydrosulfides. The annual production of H 2 S in the basin is 107-108 tons (Dimitrov et al. 2003).
The idea of using H 2 S as an energy resource is attractive from an environmental point of view as it is linked to a waste-free technology. Through pipeline 1, the water from the bottom of the Black Sea goes to vessel 2, where the water is separated from the hydrogen sulfide. The purified water from hydrogen sulfide returns to the Black Sea. Separation of hydrogen sulfide from water can be accomplished by any of the methods shown in Figure 8.
The separated hydrogen sulfide goes to vessel 3, where it is separated into hydrogen and sulfur by one of the following methods: thermal decomposition, thermal methods, Claus process or plasma dissociation. All these methods are described in details in (Velichkova et al. 2018). After the separation of hydrogen and sulfur, hydrogen would be used for energy production (vessel 5) and sulfur (vessel 6) for chemical or other industries. An important issue is the toxicity and corrosive behavior of hydrogen sulfide, which usually causes significant problems. Hydrogen sulfide is present in high concentrations in most natural gases and also in biogas and landfill gas.
Different methods for the removal of the hydrogen sulfide from Biogas installations are presented in Figure 8.

Conclusion
A new idea for the utilization of hydrogen and sulfide from hydrogen sulfide in water in the Black Sea is presented. This technological scheme can be beneficial and helpful for the practice.
Besides, it would be more efficient for electricity production (using the hydrogen from hydrogen sulfide). The sulfur could be used in different industrial technologies.
From the calculations, it is clearly seen that the available hydrogen sulfide in the Black Sea has a high calorific value, i.e. the hydrogen sulfide available in the Black Sea can be used for the production of hydrogen or electricity, which will lead to a reduction in the carbon footprint. This work was supported by the European Regional Development Fund within the Operational Programme "Science and Education for Smart Growth 2014-2020" under the Project CoE "National Centre of Mechatronics and Clean Technologies BG05M2OP001-1.001-0008".