GLIDARC-PLASMA REACTORS FOR HYDROGEN RECOVERY FROM WASTE H2S A. CZERNICHOWSKI 1 and K. WESOLOWSKA 2 1
University of Orleans, Department of Physics, 45067 Orleans cedex 02, France
[email protected] 2 Etudes Chimiques et Physiques, 22 rue Denis Papin, 45240 La Ferté Saint Aubin, France,
[email protected] www.glidarc-tech.com
Key words: Hydrogen Sulfide, GlidArc, acid gas, SulfArc, Hydrogen, Synthesis Gas Abstract: Plasma process of Hydrogen extraction from Hydrogen Sulfide contained in waste gases is proposed as an alternative to the classical Claus process of H2 S oxidation to H2 O. Various concentrated mixtures of H2 S and CO2 are processed at atmospheric pressure in small GlidArc reactors, in a 3-steps GlidArc pilot at 60 m3 /h scale, and in a recent 60-L GlidArc-T pilot plant. Precise mass and energy balances show that such acid gas processing (called SulfArc) can reach the H2 S conversion rates of 99% providing the Hydrogen and Carbon Monoxide (Synthesis Gas) at very low energy consumption.
1. Introduction Hydrogen sulfide is frequently present in various fluids issuing from under the ground, such as geothermal fluids, natural gases, gases stored in natural reservoirs or fluids used for assisted oil recovery. The H2 S is also present in industrial gases (oil desulfurization, coking, rubber pyrolysis, metallurgy, viscose, paper mills, fermentation, etc.), and in tail gases issuing from various cleaning processes. Hydrogen sulfide is an unwanted compound that has to be removed in the most of processes. However H2 S is also an abundant source for potentially the cheapest Hydrogen. Despite that fact the petroleum industry spends a lot of money to produce Hydrogen from other sources for Sulfur removal from crude oils and refinery products. This creates more than 6 million tons per year of concentrated H2 S in which very weakly bonded Hydrogen is then stupidly burned to water via 113-years old Claus process. The Claus process is quite sensitive to CO2 ; generally the process cannot be sustained if CO2 concentration in the gas exceeds 60%. In most of cases CO2 is accompanying H2 S in natural gases as well as in other effluents and the both gases are hard to separate in order to satisfy such limitation. Also steam, hydrocarbons and NH3 in the raw gas stream tend to poison or to plug the Claus catalyst. Moreover, large plants are required for this process to be economically feasible, additional tail gas treatment is necessary, controlling high temperature gas reaction and maintenance of the catalysts make the Claus process not flexible enough to immediately adjust to changes in the load, etc. The economic advantages of recovering hydrogen from H2 S were recognized some 30 years ago and some thermal processes have been proposed. However the thermal dissociation of
H2 S is less than 13% at 1300 K under atmospheric pressure according to the thermodynamic equilibrium calculations based on [1]. Non-equilibrium plasma processes present much better hope … since 1876' works of Marcelin Berhelot (France). Various electrically processes for hydrogen recovering from H2 S were then studied in silent, glow, corona, short pulse, radio-frequency frequency as well as microwave discharges. Very interesting laboratory results were obtained under quite low specific energy consumption due to the presence of very active species (radicals, excited molecules) - but no one could arrive up to industrial applications because of low-pressure use necessity and nonexistence (or cost) of specific high power electric supplies. We are presenting that history in a review [2] where our previous experimental contribution is also described. It seems that the only way of industrial and totally upgrading of the hydrogen sulfide goes through a direct use of non-thermal electric arc and discharges under atmospheric pressures. We are proposing such a process called SulfArc. It is based on our very simple Gliding Arcs (GlidArc) devices.
2. SulfArc process 2.1 Chemistry It is worth to repeat that Hydrogen is very weakly bound in the H2 S molecule so it is so pity to convert it to water through the classical Claus process. In theory, the energetic requirement for 1 mole H2 production from such endothermic dissociation: H2 S → H2 + S (1) is only 20.6 kJ, which is 14 times less than that of H2 from an ideal water electrolysis. Such an "extraction" of Hydrogen from pure H2 S would be therefore worthwhile when H2 S concentration in a waste gas is high enough and its separation from other gas components is reasonable. But, as already mentioned, in most of cases CO2 is accompanying H2 S in natural gases as well as in other effluents and the both molecules are quite hard to separate. We propose therefore the SulfArc process of complete Hydrogen Sulfide upgrading also for H2 S + CO2 mixtures where the reaction (1) is followed schematically by the process H2 + CO2 = H2 Ovap + CO. (2) A parasitic process CO + S = COS (3) takes also place but its rate is low. The standard enthalpies of the reactions (2) and (3) are +41.1 and -31.5 kJ/mol, respectively. The initial H2 S + CO2 mixture can contain a certain amount of steam or moisture so that the "inverse water shift" reaction (2) is partially blocked by the exothermic water shift process (2a): H2 Ovap + CO = H2 + CO2 . (2a) The reactions (2) and (2a) show that the carbon monoxide is hydrogen-equivalent (and viceversa). The global process (1) + (2) + (3) can be described as follows: H2 S + x CO2 = (1-x) H2 + (1-y) S + x H2 Ovap + (x-y) CO + y COS (4) with the x and y values in the range of at y
