Numerical study of H2S-H2O-air mixture conversion to hydrogen via activation of air by an electric discharge

The numerical analysis of H2 production during partial oxidation of H2S–H2O–air in a plug-flow reactor at a rather low temperature (T0=500 K) was conducted. For the reforming process promotion, the oxidizer (air) was activated by an electrical discharge with different values of reduced electric field E/N and input energy Es. It was shown that a significant hydrogen yield in a flow reactor can be obtained only after mixture ignition. The ignition delay length turned out to be minimal at E/N~4–10 and 120–150 Td, when O2(aΔg) mole fraction in the discharge products is maximal. If the H2S–H2O–air mixture ignites inside the flow reactor, the H2 mole fraction and its relative yield do not depend on E/N. The relative hydrogen yield increases monotonically with an increase of H2O amount. The specific energy requirement for H2 production in considered process was evaluated.


Introduction
Hydrogen sulfide is a part of associated petroleum and acid natural gases. It is also formed from the sulfur-containing fuels in petroleum refining industries. H2S is a toxic and environmentally hazardous compound. Therefore, up to now, issues concerning the development of methods for its utilization remain very topical. The promising way of H2S utilization is the H2 production during H2S partial oxidation [1][2][3]. It was shown earlier that the addition of water to H2S-O2(air) mixture allowed increasing the H2 yield [3]. However, the conversion of H2S to H2 during partial oxidation of H2S occurs only after H2S ignition [3,4]. In order to ignite the fuel-rich H2S-air mixture at a short residence time in the reactor, especially in the case with an admixture of water vapor, a significant gas heating is required. Plasma technologies can be used to enhance the ignition at a lower temperature [5], that allows decreasing the specific energy requirement and make the reforming process being more cost-effective. In this work, we analyze numerically the possibility of H2 production during partial oxidation of H2S in an atmospheric-pressure plug-flow reactor at a low initial temperature (T0=500 K) of the H2S-air mixture, when the air is preliminarily activated by an electrical discharge with different E/N values. The possibility of increasing the hydrogen yield in such a system via the addition of water vapor to H2S is also investigated.

Thermodynamic analysis
For the determination of the optimal composition of the initial mixture from the point of view of maximal H2 yield, we carried out thermodynamic calculations for H2S-air initial mixture at T0=500 K, P0=1 atm and different values of fuel-to-air equivalence ratio  under the conditions of constant pressure and enthalpy (Fig. 1). Maximal equilibrium mole fraction of H2 turned out to equal to ~5% at =3-6. A Significant amount of H2O, S2, sulfur oxides (SO and SO2), and H2S can present in conversion products. Their mole fractions depend on  value. At =2-3 there are mostly H2O, SO2 and S2 in the equilibrium products. At ≥4 the main products are H2O, S2, and unreacted H2S. Thermodynamic calculations have shown that mole fractions of S3-S8 species rise with the increase in  value. But even at =10, their total amount does not exceed 0.15%. A small amount of solid sulfur precursors is due to the fairly high temperature Te>1100 K of the conversion products when oxygen presents in the initial mixture.
Since the amount of H2S in the initial mixture varies with the change of  value, we will consider the relative amount of hydrogen molecules obtained from H2S, which was determined as denote corresponding molar flow rate of H2 and H2S). One can see from Fig. 1, that maximal equilibrium relative hydrogen yield at T0=500 K is observed at =1.5-2 and equals to ( e )max~0.2. Note that maximal value of ( e )max is obtained at the lower  value, than that of ( e 2 H  )max. Because the value ( e )max is realized at ~2, precisely such a value of  was chosen to study the conversion process in the flow reactor. The addition of water vapor can increase  e (see Fig. 1). At water fraction is the amount of H2O moles in the initial mixture, ( e )max increases by 10% at =2 and by 25% at =1.5. ( e )max is achieved at a lower  value when adding H2O than that in pure H2S-air mixture. At =3, a quite large additive of water vapor (=1) can even reduce the relative H2 yield. This is due to an essential decrease in temperature in the conversion products in the case of H2O addition. as a function of  value calculated for the initial H2S-air mixture.

Methodology
Numerical analysis of the H2 production during partial oxidation of H2S was conducted in an atmospheric-pressure plug-flow reactor. The air, activated in an electrical discharge and passed through a postdischarge reactor, and H2S-H2O mixture are supplied separately to mixing reactor, and the homogeneous mixture proceeds to the flow reactor. Both flows are preliminarily heated up to the temperature T0=500 K. An adiabatic plug-flow reactor was 1 m in length. The mixture velocity at the flow reactor inlet was U0=1 m/s that corresponds to the gas residence time of r≈1 s. Calculations were carried out with the use of CHEMKIN software package.
The temperature and gas composition of air plasma generated by the electric discharge were calculated according to the ideal discharge model [6] using the Boltsig+ code [7] to solve the electron Boltzmann equation. The model of air discharge includes 40 processes with electrons: elastic collisions, excitation of rotational, vibrational and electronic states of O2 and N2 molecules, dissociation and ionization of O2 and N2 molecules. The dependencies of processes cross sections on electron energy were taken from [8] in form of tables, suitable for the Boltsig+ code. Secondary plasma-chemical reactions, as well as quenching of electronically excited species and vibrational relaxation, were taken into account during passing the discharge products through a postdischarge channel with the residence time of 1 ms. Mechanism of plasma-chemical reactions was borrowed from [9].
One can see from Fig. 2 that, to the postdischarge reactor outlet, an essential part of the discharge energy transfers to heating the air. The air plasma temperature is in the range 880-893 K at E/N=1-150 Td and Es=0.5 J/ncm 3  For the numerical study of H2S-H2O-air mixture conversion in a flow reactor, the reaction mechanism [3] was taken as a basic one. This mechanism was supplemented by the reactions with participation of O2(a 1 Δg) and O2(b 1 Σg + ) molecules which were taken from [10]. Fig. 3 shows the dependencies of Lin(E/N) for two values of the supplied energy Es=0.3 and 0.5 J/ncm 3 . One can see that at Es=0.5 J/ncm 3 the H2S-H2O-air mixture ignites inside the reactor at any E/N value and =0-1 (Lin<1 m). But the shortest ignition length is observed for the discharge that produces more O2(a 1 g) molecules. This occurs at E/N~4-10 and 120-150 Td. Recall that, at E/N~60-70 Td, the mole fractions of O atoms and O2(a 1 g) molecules are equal (see Fig. 2). Analysis showed that the contribution of O atoms and O2(a 1 g) molecules to the ignition acceleration was almost the same. But since the total number of O and O2(a 1 g) in this case is two times smaller than the mole fraction of O2(a 1 g) molecules at E/N~6 Td, the ignition occurs later. At a lower energy input  H2 depending on water content  during partial oxidation of the H2S-H2O-air mixture with=2 in the flow reactor when air is exposed to the electric discharge with Es=0.3 and 0.5 J/ncm 3 (dashed and solid lines) at T0=500 K, P0=1 atm.

Results and discussion
If the mixture ignites, quantitative H2 yield does not depend on E/N. This occurs at Es=0.5 J/ncm 3 . At Es=0.3 J/ncm 3 hydrogen can be produced in the flow reactor in two ranges of E/N~4-15 and 120-150 Td if =0, and only at E/N~4.5-8 if =0.2. If the mixture ignites, then a small addition of water vapor (-0.3) slightly increases H2 mole fraction in the conversion products (Fig. 4). With further increase of H2O amount, 2 H  value decreases. The smaller H2 yield is obtained at Es=0.3 J/ncm 3 , which is due to a decrease in the conversion products temperature. The relative H2 yield, on the contrary, increases with an increase of H2O in the mixture, from =0.25 at =0 to =0.3 at =1. The main reaction responsible for the H2 production is the process H2S+H=HS+H2.
The formed H2 is consumed before the ignition in the reaction H2O+H=OH+H2 (R2) that occurs in the backward direction with the formation of H2O. However, just after the ignition event, the reaction (R2) starts to occur in the forward direction, resulting in the consumption of H2O and formation of the additional amount of H2. An admixture of H2O additionally shifts the equilibrium of this reaction towards the formation of H2, thus increasing hydrogen yield. The larger the amount of H2O in the initial mixture, the greater the role of the reaction (R2). Fig. 5 shows the H2 production rates for the case of =1.5 when, according to equilibrium calculations (Fig. 1), the most pronounced effect of H2O addition on the increase of relative H2 yield can be expected.  Fig. 5. Variation of H2 production rates during partial oxidation of the H2S-H2O-air mixture in the flow reactor at =1.5 and =0 and 1 when air is exposed to the electric discharge with Es=0.5 J/ncm 3 at T0=500 K, P0=1 atm.
For the evaluation of the energy efficiency of the process considered in this paper, we will use the parameter characterizing the specific energy requirement for production of H2 molecule: , where Q is the energy required to heat the H2S-H2O and air flows from room temperature to 500 K. The minimum value of SER(H2), obtained for the H2S-air mixture at =2, Es=0.3 J/ncm 3 and E/N~10 Td, turned out equal to 10.5 eV/(molecule H2). This value is essentially higher than the minimum SER(H2)=0.83 eV/(molecule H2) value for the H2S-O2 mixture at the same conditions, because, when using air as an oxidizer, a lot of energy is spent on heating and excitation of N2 molecules, which do not participate in the conversion process. Therefore, energy is wasted inefficiently. The use of air for H2S partial oxidation seems to be not so promising compared to the use of oxygen as an oxidizer. However, economic evaluation of the H2S conversion process should also include the oxygen cost.