f < M 2 ° ^ Cy COMMISSARIAT A L'ENERGIE ATOMIQUE

Centre fÊtrnJe» Nwdémkt* dt SocUy

DIVISION DE CHIMIE

DEPARTEMENT DE RECHERCHE ET ANALYSE

SERVICE D'ANALYSE ET D'ETUDES EN CHIMIE NUCLEAIRE ET ISOTOPIQUE

DRA/SAECNI/78-242/E.22/DL Laboratoire des I s o t o p e s Le 2 Mai 1978. S t a b l e s e t des Traitements

C a t a l y t i g u e s

ENERGY BALANCE CALCULATIONS AND ASSESMENT OF TWO THERMOCHEMICAL SULFUR

CYCLES (*)

D. LEGER, P. LESSART, J . P . MANAUD, R. BENIZRl and P. COURVOISIER

2. Wo& hydrogen energy conference. Zurich, Switzerlond, 1978, 21-24 August

CEA-CONF-4364

Visa du Chef de Serv i ce :

R. DARRAS

ENERGY BALANCE CALCULATIONS AND ASSESSMENT OF TWO THERMOCHEMICAL SULFUR

CYCLES. D. Léger, P. Lessart, J.P. Manaud, R. Fenizri

and P. Courvoisier. DRA/SAECNI,

CEN/Saclay, CEA France.

ABSTRACT Thermochemical cyclic processes which include the highly endo-thermal decomposition of sulphuric acid are promising for hydro­gen production by water-splitting. Our study is directed toward two cycles of this family, each involving the formation and de­composition of sulphuric acid and including other reactions using iron sulphide for the first and oxides and bronuaes "of cop­per and magnesium for the second. Thermochemical analyses of the two cycles are undertaken. Ther­modynamic studies of the reactions are carried out, taking into account possible side-reactions. The concentration of reactants, products and by-products resulting from simultaneous equilibria are calculated, tht problems of separation thoroughly studied and the flow-diagrams of the processes drawn up. Using as heat source the helium leaving a 3000 MWth high tempe­rature nuclear reactor and organizing internal heat exchange the enthalpy diagrams are drawn up and the net energy balances evalua­ted. The overall thermal efficiencies are about 28 %, a value corresponding to non-optimized process schemes. Possible impro­vements aiming at energy-saving and increased efficiency are indicated. INTRODUCTION Both the cycles studied in this paper include the sulphuric acid step which may be represented in terms of primary chemical reac­tions as follows : (1) 3 S0 2 • 2 H 20 *JH 2 S0 4 • S (2) 2 H 2 SO4 + 2 H 20 + 2 S0 2 • 0 2

this sequence, which corresponds to oxygen formation, is referred to as the oxygen loop and is common to both processes. The first cycle (cycle A) continues with three other reactions:

(3) 3 S • 2 H 2 0 - S 0 2 + 2 H 2S (4) 2 FeS • 2 H 2S * 2FeS 2 + 2H2

(5) 2 Fe S 2 - 2Feb + S 2

these three reactions (3,4,5) constitute the hydrogen loop of cycle A. The hydrogen loop of the second cycle (cycle B) consists of four chemical steps as follows :

1

(6) S • 2 Cu 70 + 4 Cu • SO? (7) 4 Cu* 4 HBr - 4 Cu Br~* 2 H 2

(8) 4 CuBr • 2 MgO * 2 Mg Br> • 2 Cu 20 (9) 2 MgBr 2 • 2 H 20 - 2 Mg 0"* 4 HBr

The cycle B (reactions 1, 2, 6, 7, 8, 9) is studied experimental­ly at the Los Alamos Laboratory and all the reactions have beer demonstrated [l]. The reactions (4) and (5) form the subject of experimental research in Aachen [2]. For the present study the process conditions are based on thermochemical considerations according to literature data [3,4,5,6], By calculation of the standard free reaction enthalpies as a function of temperature it is possible to determine in each case a working temperature zone in which the reaction approaches re­versibility. In this temperature zone a study of chemical equilibrium as a function of reagent intake concentration and pressure leads to a choice of working conditions corresponding to relatively low theoretical separation energies. Product recyclings and parasitic reactions due to incomplete separations are taken into account in setting up the mass and heat balance of each reactor. The net heat balance determines the overall thermal efficiency and for the assumed 3000 MWth heat source, the hydrogen produc­tion rate.

THERMODYNAMIC STUDY OF THE REACTIONS The thermodynamic quantities of the different reactions in both cycles are calculated as a function of temperature and pressure. To limit the theoretical separation energy we chose temperatures at which the reactions approach reversibility (AG-o) and. at gi­ven pressure and reagent composition, the degree of advancement is highest. As far as possible the pressure chosen for reactions in the gas phase was 10 bars, which offers the economic advanta­ge of using smaller reactors and heat enchangers. The working conditions finally adopted (see table I) account for parasitic reactions and temperatures available from a HTR.

Oxygen loop The overall decomposition of sulphur dioxide into sulphur and oxygen is obtained via sulphuric acid by the reactions : (1) 3 S0 2 • 2H20 + 2H 2S0 4 + S (2) 2 H2 S0 4 4 2 H20 + S0 2 • 0 2

Oxygen i s given off whereas the sulphur i s recycled in the next loop. Sulphuric acid i s synthesized in aqueous solut ion at a tem­pera ture and pressure where the sulphur i s s t i l l s o l i d . This acid must be concentrated then vapourised before pyro lys i . ; , which requi res the following three s teps :

( 2 ' ) 2 H 2S0 4 -> 2H?0 • 2S0-: (2") 2 SO3 Î 2 S02 + 0 2 (2" ' ) 2 SO; t 2 HzO •* 2 II2SO4

Reaction (2*) has a degree of advancement very close to 1 and is reversible. On the other hand the SO, decomposition is not com­plete at temperatures available from an HTR (1200 to 1300°K) and must be carried out adiabatically with a catalyst between 680 and 1100°K. Under these conditions the degree of advancement of reac­tion (2") is about 0.6 and the non-decomposed sulphur dioxide recombines with water and regenerates the sulphuric acid to be recycled.For simplification purposes the isothermal working con­ditions summed up in table I were adopted.

Hydrogen loop of cycle A The sulphur produced in the oxygen loop reacts on the incoming water to give sulphur dioxide and hydrogen sulphide as follows : (3) 3 S + 2 H 20 * S02 • 2 H 2S

The sulphur dioxide must be decomposed in the previous loop while the overall decomposition of the hydrogen sulphide into sulphur and hydrogen takes place here via iron sulphide by the reactions : (4) 2 FeS + 2 H 2S * 2 Fe S 2 • 2 H 2

(5) 2 FeS-j - 2 FeS + S 2

If the Sg and S 2 forms are considered as the most probable forms of the sulphur molecules in the gas state, reaction (3) is in fact the result of the following simultaneous equilibria : (3») 3/8 Sg • 2 H 20 - S0 2 + 2 H 2S (5") 3/2 S 2 + 2 H 20 + S0 2 • 2 H 2S (3"') 1/4 Sg - S 2

To begin with each of these equilibria was studied separatel) ,then a programme of simultaneous chemical equilibrium resolution by successive approximations was developed specifically for this set of three reactions. By means of this programme it was shown that the reversibility of the system lies around 820°K and that the overall equilibrium hardly depends on pressure. Hydrogen sulphide being available at 20 bars this pressure was chosen for reaction (4) where the equilibrium is not pressure-dependent. It should be noted moreover that the decomposition of pyritis at 1100°K produces sulphur at 1.9 bars.

Hydrogen loop of cycle B The sulphur produced in the oxygen loop reacts on cupric oxide to give sulphur dioxide and Cu bv the reaction : (6) S • 2 Cu 20 - 4 Cu • S0 2

The sulphur dioxide must be decomposed in the oxygen loop while the Cu reacts on the water to be decomposed, Riving hydrogen and regenerating cupric oxide by the formal reaction : 2 H 20 • 4 Cu - 2 Cu 20 + H 2

This reaction is in Fact obtained by the sequence : (7) 4 Cu • 4 H Br + 4 Cu Br • 2 H 2

(8) 4 Cu Br + 2 MgO * 2 Mg Br-» + 2 Cu20 (9) 2 MgBr2 • 2 H 20 - 2 MgO +"4HBr

Reaction (6) occurs in two steps. First a mixture of Cu2S and Cu->0 is formed in an exothermal stage :

** S * 2/3 Cu 2 0 -• 2/3 Cu 2S + 1/3 S 0 2

The second endothermal stage conpletes the reaction and fixes the SO2 pressure :

2/3 Cu 2 S + 4/3 Cu 2 0 + 4 Cu + 2/3 S 0 2

Reaction (/) is exothermal and nixtures of H?0 and HBr may be used. Reaction (8) cannot occur, considering the free enthalpy standard of the reaction, bxperimentally it was observed that this reaction is not possible by dry methods but takes place in aqueous solution, tests with other solvents such as 6 or 15 PÏ ammonia, ethyl alcohol, acetic acid and benzene proving negative. These various trials tend to shew that the reaction occurs by formation of the hexahydrate MgBr 2, 6 H2O in solution, CuO being insoluble in water. Reaction (8) is thus a two-step process : (3') 2n H->0+ 4CuBr • 2 MgO -• 2.[M5»Br2, 6H20] + 2 Cu20+ 2(n-6)H20 (8") 2[Mgêr2, 6H20] * 2 MgBr 2 + 12 H 20

the second being the drying step. The n value is determined by the solubility of the hexahydrate in water, assuming a saturated solution. The last reaction of the cycle is the high-temperature hydrolysis of Mg Br 2 where H 2

n i* supplied in the form of steam. FLOW DIAGRAM AND MASS BALANCE A detailed mass balance was established for the operation se­quence of each process, accounting for the degree of advancement of the reactions and the different separation devices. These lat­ter consist of condensers, flash drums, distillation columns or wash and flash systems. The simplified flow- sheets of process A and B are shown in fi­gures 1 and 2. Figures 3, 4 and 5 give the main results of the mass flow stu­dies for the oxygen loop, the cycle A hydrogen loop and the cycle B hydrogen loop respectively. The mass flows are expressed in moles and were calculated for the decomposition of two moles of water. The percentages and contents of the solutions are weighted percentages. Oxygen loop. The concentra t ion of the su lphur ic acid solut ion formed in reac tor R 1 a t 380°C under 10 bars pressure i s 84 3 %. Before d i s t i l l a t i o n t h i s so lu t ion i s sent into an expansion vesse l where the res idual sulphur dioxide is evaporated. The H2S04-H2O mixture is d i s t i l l e d in column D working at 1 atmosphere p r e s s u r e , with a b o i l e r temperature of 563 K. The sulphuric acid co l l ec t ed at the bottom ot tne column is eonccntra-

- 4 -

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for reactor R4. Hydrogen loop of cycle B. The water to be decomposed is fed into reactor R'8, where copper bromide from R 7 and magnesia from R9 react at 380°K to give insoluble cuprous oxide and a 5S.C9 % saturated magnesium bro­mide solution. The hexahydrate Mg Br 2, 6H2O is separated in the separator S at 303°K where crystals are deposited and the satu­rated solution, depleted to 44.69 %, is filtered and recycled to the head of R'8. Dehydration of the magnesium bromide by reaction (8") in reactor R"8 releases 12 moles of water, 10 being recycled in R*8 and the remaining 2 serving for hydrolysis in reactor RS with release of 98.4 % hydrobromic acid vapours. Such a concentration enables powdered copper to be attacked in reactor R 7, where hydrogen is liberated with traces of water and hydrobromic acid. The hydrogen is separated by condensation in C7 then sent to the production while the H7O-HBr mixture is vapourised in V7 before recycling in R9. The hydrogen leaving at 1 bar pressure carries a little water vapour which must be made up accordingly. Sulphur from the oxygen loop and cuprous oxide obtained in R'8 react in reactor R6 to give solid copper, sent to R7, and gaseous sulphur dioxide directed to the oxygen loop. Figure 5 shows the importance of the water recyclings.

ENERGY BALANCE AND EFFICIENCY The different operations were evaluated for a 2-mole hydrogen production in each of the two cycles. The compression energies are accounted for but the reagent circulation energy needs are neglected. The compression energy supply is obtained either from expansion energies involved in the cycle, with a 70?. efficiency, or from heat taken on helium with an efficiency of 25 %. The heat requirements are met either by recuperation of waste heat inside the cycle (exothermal reactions, cooling of the products..) or by the use as heat source of helium from a 3000 MWth high-temperature nuclear reactor. To organise the various heat exchanges for processes A and B the enthalpy diagrams were drawn up with a view to maximum recovery of waste heat discharges. For the heat exchanges the minimum temperature difference accep­ted is 50°K at high temperature and 20 K at low temperature. The efficiency is calculated with respect to the high heating value of hydrogen, from :

- A H liquid H20 p A H supplied by He

ratio of the standard enthalpy of liquid water formation to the total heat supplied by helium.

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Process A

The decomposition of 2 moles of water r equ i r e s 473 Kcal betheen 1272°K and 860°K taken from helium. Since the compression ener ­gies are supplied e n t i r e l y from expansions within the cycle the need for aux i l i a ry e l e c t r i c i t y production or outs ide energy sour­ces does not a r i s e . Under these condi t ions ( t ab l e I I ) the e f f i ­ciency cycle amounts to 28.3 %. which corresponds to a hydrogen production of about 244,500 Nn.-.h~1

Process B

To decompose 2 moles of water the amount of heat to be taken from helium i s 4 21.2 Kcal between 1205°K and 620°K. The sum of the compression energ ies needed for the var ious separa t ions being est imated at 16.8 Kcal the helium must con t r ibu te about 67.3 Kcal more which corresponds to a t o t a l of 488.5 Kcal. Under these con­d i t i ons ( t ab le I I I ) the ef f ic iency i s 27.9 %, represent ing a hy­drogen production of about 236,390 N m3.h~1.

CONCLUDING REMARKS Kith various improvements the efficiencies of both processes could be raised. The high solubility of oxygen in liquid sulphur dioxide is one of the difficult poi its encountered in this study. It should however be possible to reduce the compression energies in the separation of SO2 and O2 ; the saving could be about 2 Kcal with respect to the 19 Kcal needed in the scheme described here. For sulphuric acid concentration the fractional distillation could profitably be replaced by a flash distillation. The decom­position of SO3 at different temperature levels could reduce the energy requirements at all stages. A more energy-saving method of separating the SO2 and H2S gases produced in reaction (3) of cycle A could be to carry out instead of the direct reaction of hydrogen sulphide (containing about 10% SO2) on FeS, the high temperature reaction : S0 2 • 2 H? S * 3S + 2 H2O which would produce 3 moles of sulphur per mole of parasitic S0 2 releasing 26 Kcal/mole S0 2, i.e.7 Kcal at 8709K in the case of process A. The gas mixture containing H 2S and H 20, almost free from SO-, would then be sent to react on FeS. With the parasitic reaction eliminated as much FeS^ as hydrogen would be formed, while it would only be necessary to decompose 2 moles of pyritis instead of 2.8 in order to produce 2 moies of hydrogen, and this would represent a saving of 26 Kcal at 1100°K. Since the compression energy is supplied by expansion of super­heated steam in process A, better matching of enthalpy supply and demands should reduce the heat requirements from the nuclear reactor by about 20 Kcal at 584°K. Finally a variant of process A, using Bi,S, decomposition which

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gives sulphur.at 1 h;»r and 1025°K with an enthalpy demand of 54 Kcal per mole of sulphur and takes into account the improvements recommended for the iron sulphide cycle, could have an efficiency of about 34 \ . The most energy-consuming stage of cycle B is the dehydration of magnesium bromide hexahydrate. In the case of monohydrate formar tion the efficiency could reach 37 %. Various solvents have been laboratory tested to produce the reaction : 2 Cu Br + MgO •* Mg Br 2 • Cu 20

but the results are not encouraging. Unless an adequate complexing a^ent for magnesium bromide is discovered it is hardly worth pursuing a detailed study of this cycle, since other possible improvements to the process scheme would lead only to minor efficiency increases. To conclude, it seems that the schemes of each of the two pro­cesses are open to improvements which would considerably raise the thermal efficiencies. However a further technical and econo­mic estimation of these cycles would require an experimental investigation, especially measurements of the reaction rates. REFERENCES £l] M.G. BOWMAN ; "Chemistry of thermochemical cycles from U.S.A.

programs" ; at A.I.M. Congress on Hydrogen and its prospects Liège, Belgium, november 1976.

[2] K.F. KNOCHE, H. CREMER, G. STEINBORN, W. SCHNEIDER; Feasibility studies of chemical reactions for thermochemical water splitting cycles of the iron-chlorint^ron-sulfur and manganese-sulfur families ; First world Hydrogen Energv Conference Proceedings, 5A-37, Miami 1976.

[3] Selected values of chemical thermodynamic properties ; N.B.S. Technical Note n°270 Washinjton DC, 1970-1973.

[4] J. BARIN, 0. KNACKE; Thermochemical properties of inorganic substances, Springer Verlag, Berlin 1973.

[5] International Critical Tables, vol, I to VII, Mc Graw Hill 1933 [6] H. STEPHEN and T. STEPHEN ;

Solubilities of inorganic and organic compounds ; Vol I, 568 Pergamon New York 1963.

[7] R.W. DORNTE, C.W. FERGUSON; Ind. Eng. Chem., 32, 112, 1939.

8 -

TABLE I - Operating Conditions of processes A and B

Réaction n° T(K) P(bars) AH' (Kcal ) AG* (Kca l )

1 380 10 - 52.4 - 1.3 2» 900 10 • 59 .2 - 1 6 . 5 2" 1100 10 • 46 .2 - 1.9 2»» i 680 10 - 55.8 - 1.6 3 820 10 • 11.9 * 2 .3 4 500 20 - 29 .0 - 7 .6 S 1100 1.9 • 65.8 - 0 .7 6 975 • 4 .0 - 2 4 . 7 7 650 - 54 .9 - 2 3 . 5 8' 380 - 28.9 0 .0 8" 515 • 209.8 - 0 .2 9 950 • 28.3 - 9 .8

Table II - Production est imates .

Hydrogen production (N m\ h" 1) Nuclear reactor power (Mwth) Helium temperature variation (K) Amount of heat supplied by He (U'cal/2H2) Compression energy supplied by He (Kcal/2H 2) Compression energy recuperated in the cycle (Kcal/2ll2) Amount of heat recuperated in the cycle (Kcal/2H2) Overall ef f ic iency of the cycle (" with regard to H.H.V of H-,}

Process A Process B

244,500 236 ,390 3 000 3 000

1272-860 1205 -620 473 421

0 16.8

18.6 O

357 415

28.5 27.9

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62

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S0 2 : 0,055 H 20 : 0,017»

(+0,248)» ( H 2 S : 0 , ^

»(" 2° : °'3)

H 20 : 0,001

Sn2;0,0?2 Fly. 3 - Flow 'l.tanram of <ix"(»rn production loon (nolnr units) .

| H 2 0 : i . 946

H,0 : 1 1 , 8 2 2

0 , 3 2 5

0 , 0 0 4 S 0 2 : 0 , 3 2 5 | ' H 2 S

H 2 0 : 9 , 8 7 6

R3

so 2 : 0 , 3 2 5 HjS :O,004

1 , 0 7 8

t H 2 0 : 9 , 2 2 2

SO. 1 , 6 2 5 L-I-J H-0 : 9 , 2 2 » — | J

C.3

H 2 S : 2 , 6 0 4 S ; 3 , 8 0 7

S 0 2 : 1 , 6 2 5

S : 3,807

H,0 : 0 , 0 5 4

H 2 S : 2 , 5 9 ! S 0 2 : l , 2 9 ^

H 2 O : 0 , 2 0 (

* • - . - '

H 2 S:2 , !347 S O 2 : 0 , 2 7 4 H 2 0 : 0 , 0 0 6

F e S 2 : 2 , 8 2 1

S : 2 , 8 2 1

3 0 2 : 1 , 0 2 5 • 1 - 0 : 0 , 2 4 8

• Î 2 S : 0 , 0 5 2

H 2 n : n , 5 5 3

F i g . 4 - C y c l e P. F l o w d i a g r a m of h y d r o g e n p r o d u c t i o n

l o o p (piolar u n i t s ) .

R'e Mr?Br 2, 6 H 2 0 : 9 , 2 6 3

H 2 0 : 1 1 7 , 2 6 3

M g B r 2 , 6 H 2 0 : 1 1 , 2 6 3

H 2 0

t » ' >

1 - 5 , 2 6 3

M g B r , , 6 H 2 0 : 9 , 2 6 3

H , 0 : 1 0 5 , 2 6 3 Hfi.2

M g B r 2 , 6 H 2 0

MgC : 2

n=< i C5

H.,0 : 10

H 2 0 : 12

fctfnBr,: 2

CuBr

/?9 H B r : 4 , 0 2 3 H - O : 0 , 2 8 2

H 2 0 : 2 , 2 8 2

H B r : 0 , 0 2 3

282

fl2J

-K.0 : 7

Cu-0 : 2

R O H 2 0 : 0 ,

_ ^ " 2 -Cu : 4

SO, : 1 V / s : 1

V7 r-n H 2 0: 0,087

H22 H 2 0 : 0 , 0 8 7

Fig. 5 - Cycle B. Flow diagram of hydroftpr nroduction loon (molar units).

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