O

Fig. 6.4 Simple coupling of a thermochemical water-splitting process and a nuclear reactor for hydrogen production.

6.3.2.1 Iodine-sulfur cycle

The IS process consists of the following three chemical reactions:

I2+SO2+2H2O^2HI+H2SO4 (6.1)

The principles for the IS process are illustrated in Fig. 6.5. The so-called Bunsen reaction, Eq. (6.1), is an exothermic sulfur dioxide (SO2) gas absorbing reaction, which proceeds spontaneously in the liquid phase at 20-100°C. Gaseous sulfur dioxide reacts with iodine and water producing an aqueous solution of hydriodic acid and sulfuric acid. The two kinds of acids are separated by liquid-liquid phase separation in the presence of excess iodine. The hydrogen iodide (HI) decomposition reaction, Eq. (6.2), produces hydrogen with a low endothermic heat of reaction at 300-500°C in the gas phase. It can also be carried out in the liquid phase. The sulfuric acid (H2SO4) decomposition reaction, Eq. (6.3), is an endothermic oxygen-producing reaction, which proceeds in two stages. First, gaseous H2SO4 decomposes spontaneously into H2O and SO3 at 400-500°C and then SO3 decom

, poses into SO2 and O2 at about 800°C in the presence of a solid catalyst. By carrying out these three reactions sequentially, the net result is that water is decomposed into hydrogen and oxygen (Xinxin and Kaoru, 2005).

6.3.2.2 ISPRA mark 9 cycle

The ISPRA Mark 9 thermochemical cycle (shown in Fig. 6.6), one of the several proposed thermochemical cycles reported in the literature, is a three-step cycle involving iron chlorides as shown below:

1. Decomposition of Fe (III) chloride: 6FeCl3 ^ 3Cl2 + 6FeCl2 (6.4)

2. Hydrolysis: 6FeCl2 + 8H2O ^2Fe3O4 + 12HCl + 2H2 (6.5)

3. Chlorination: 3Cl2 + 2Fe3O4 + 12HCl ^6FeCl3 + 6H2O + O2 (6.6)

The hydrolysis reaction is conducted at 650°C, which is the highest temperature in the cycle. The decomposition of Fe (III) chloride is conducted at 430°C and the chlorination at 150°C (Utgikar and Ward, 2006).

6.3.2.3 Hybrid sulfur cycle

In the Hybrid sulfur cycle (see Fig. 6.7), developed by Westinghouse, H2SO4 is decomposed into SO2 at high temperature, and the SO2 is converted back into H2SO4 in a PEM electrolyzer at 80°C. Overall only water and energy are consumed, and H2 and O2 are produced (Sivasubramanian et al., 2007).

H2SO4 Hl

Heat

Step 2: I2+SO2+2H2O ^ 2HI+H2SO4 Step 3: 2H I ^ H2+I2

Fig. 6.5 Iodine-sulfur process for thermochemical production of H2 (adapted from

Xinxin and Kaoru, 2005).

HCI, Fe3O4

Fig. 6.6 Flow diagram for the ISPRA Mark 9 hydrogen production process (adapted from Utgikar and Ward, 2006).

In the calcium-bromine cycle, as illustrated in Fig. 6.8, CaO and CaBr2 are recycled in high-temperature, solid-gas, fixed bed reactors, and HBr is converted into Br2 in a PEM electrolyzer at 80°C. Overall only water and energy are consumed, and H2 and O2 are produced.

6.3.2.5 Copper-chlorine cycle

Most thermochemical cycles require process heat at high temperatures, exceeding 850-900°C. However, existing nuclear power plants in the USA and elsewhere are typically water-cooled plants operating at 250-500°C. Recently, Atomic Energy of Canada Limited and Argonne National Laboratory in the USA have been developing low-temperature cycles, designed to accommodate heat sources around 500-550°C. Such cycles can be more readily integrated with nuclear reactors. For this temperature range, the copper-chlorine (Cu-Cl) cycle is one of the most promising. Several Cu-Cl cycles have been examined in the laboratory and various alternative configurations identified. Proof-of-principle experiments that demonstrate the feasibility of the processes have been undertaken and a preliminary assessment of the cycle efficiency has demonstrated its potential.

A conceptual layout of a Cu-Cl pilot plant is illustrated in Fig. 6.9. Ther-mochemical water decomposition, potentially driven by nuclear heat with a copper-chlorine cycle, would split water into hydrogen and oxygen through intermediate Cu and Cl compounds. This cycle consists of three thermal reactions and one electrochemical reaction. The cycle involves five steps: (1) the HCl(g) production step using such equipment as a fluidized bed, (2) the oxygen production step, (3) the copper (Cu) production step, (4) the drying step, and (5) the hydrogen production step. A chemical reaction takes place in each step, except the drying step. The chemical reactions form a closed internal loop that re-cycles all of the copper-chlorine compounds on a continuous basis, without emitting any greenhouse gases externally to the atmosphere. The five steps of the copper-chlorine cycle are described in Table 6.1. The Cu-Cl cycle is one of the most promising ways to produce hydrogen efficiently, without emitting greenhouse gases into the atmosphere.

Fig. 6.7 A schematic of the hybrid sulfur cycle (adapted from Sivasubramanian et al., 2007).

h2so4

Fig. 6.7 A schematic of the hybrid sulfur cycle (adapted from Sivasubramanian et al., 2007).

CaBr2

Electro

Electrolysis H2

Fig. 6.8 A schematic of the modified Ca-Br cycle (adapted from Sivasubramanian et al., 2007).

Table 6.1 Key steps of Cu-Cl cycle with their corresponding reaction (adapted from Lewis et al., 2003).

Table 6.1 Key steps of Cu-Cl cycle with their corresponding reaction (adapted from Lewis et al., 2003).

Step

Reaction

Temperature range (°C)

Pressure (kPa)

Feed/output

(note: Q-thermal energy, V-electrical energy)

1

2CuCl2(s)+H2O(g)^ CuO*CuCl2(s)+2HCl(g)

400

101

Feed: Output:

CuCl2(s)+H2O+ Q CuO*CuCl2(s)+HCl(g)

2

CuO*CuCl2(s)^ 2CuCl(l)+1/2O2(g)

500

101

Feed: Output:

CuO*CuCl2(s)+Q Molten CuCl salt + O2

3

4CuCl(s)+H2O^ 2CuCl2(aq)+2Cu(s)

25-S0

101

Feed: Output:

CuCl and H2O + V Cu and slurry

4

CuCl2(aq)^CuCl2(s)

>100

101

Feed: Output:

CuCl2(aq)+Q CuCl2+H2O vapors

5

2Cu(s)+2HCl(g)^ 2CuCl(l)+H2(g)

430-475

101

H2 + CuCl(l) salt+Q

OUTPUT

INPUT

7jSteam (400

HE: Heat Exchanger S: Step P: Compressor

HCl(g) production Fluidized bed

ion,

heat

H2 production

Heat

H2 production

500 °C S2 O2 production

CuCl2(s)

CuCl(I)IY00 Heat recovery T 500 C

Flash dryer

HE8 22

water heat

Cu production

+ Water

Fig. 6.9 Conceptual layout of a thermochemical Cu-Cl hydrogen production cycle.

The heat transfer for a chemical process involving no work interaction W is determined from the energy balance £in - EEout = ^£system applied to a system with W = 0. For a steady-state reaction process, the energy balance reduces to

Q = Hp - Hr = £ np (hf + h - h - £ nR ( + h - h ) (6.8)

The variations of the reaction heats for steps involving a chemical reaction (steps 1, 2, 3, and 5) with the temperatures of the reactions are illustrated in Fig. 6.10. As explained earlier, all steps are endothermic except the fifth. The reaction in fifth step, in which hydrogen production occurs, is exothermic and the heat produced is seen in Fig. 6.10 to be rejected from the system. As reaction temperature increases, the reaction heat for steps 1, 3, and 5 decreases while that for step 2 increases. In all cases, the relations are nearly linear.

Fig. 6.10 Variation of reaction heat with reaction temperature for several steps in the Cu-Cl cycle.

Fig. 6.10 Variation of reaction heat with reaction temperature for several steps in the Cu-Cl cycle.

An exergy balance can be used in formulating an exergy efficiency for the reacting system (for each step of Cu-Cl cycle, individually); at steady state, the rate at which exergy enters the reacting system equals the rate at which exergy exits plus the rate at which exergy is destroyed within the system. It is assumed that the reactor is well insulated, so that there is no exergy transfer accompanying heat transfer. There is also no work Wcv . Accordingly, exergy exits only with the reaction products. An exergy efficiency can be written as eX out m nx = — (6.9)

where exin is the exergy that enters with the reactants plus heat, in the case of en-

dothermic reaction, and exout is the exergy that exits the system with the products plus heat, in the case of exothermic reaction. Using the exergy balance for the reacting system, the exergy efficiency expression can be written alternatively as nex = i - (6.10)

Using Eq. (6.10) the exergy efficiency of each step of the Cu-Cl cycle is given in Table 6.2, based on the specified state. These efficiencies may change by changing the state (i.e., temperature, pressure) of the reaction/process. As illustrated in Table 6.2, the efficiency of each step seems to be high, however, the overall efficiency of the cycle is not that high, at below 50%.

Table 6.2 Exergy efficiencies of the steps associated with Cu-Cl cycle at speci-__fied temperature and pressure. __

Step

Name

(°C)

Pres. (kPa)

'Hex (%)

1

Fluidized bed

105,266

400

101.325

76

2

O2 production step

110,523

500

101.325

96

3

Cu production step

140,450

45

101.325

99

4

Evaporator

18,346

150

101.325

67

5

H2 production step

-55,493

450

101.325

99

As mentioned earlier, we are analyzing a hypothetical Cu-Cl plant and that has not been built yet. Thus, many parameters such as quantity, capacity, and material of equipment (pumps, heat exchangers, compressors, fluidized bed, evaporator, etc.) that we need for these analyses are unknown. Therefore, for simplicity, in overall efficiency calculations we consider only the five main steps of the Cu-Cl cycle and assume that there are no heat losses in these steps, individually. However, overall we assume a total heat loss (QiosS) from the Cu-Cl cycle as a percentage of total heat (Qi„) that enters the cycle.

The overall energy efficiency of the Cu-Cl cycle, ^overall, can be described as the fraction of energy supplied that can be recovered from the energy content of H2 based on its lower heating value:

LHV h2

Qin + Q loss where LHVH2 is the lower heating value per kmole of hydrogen and Qin is the total energy demand by the process to produce a unit amount of product hydrogen. This total energy demand of the Cu-Cl cycle is the summation of the reaction heats of the five main steps described above. Note that in this summation of reaction heats, the exothermic reaction heat (i.e., fifth step) is taken as negative, assuming this heat can be used for other endothermic reactions. The lower heating value of hydrogen is given as 242,400 kJ/kmol H2. As explained earlier energy loss from the cycle (Qloss) cannot be calculated, so in this study we presume Qloss as a percentage of Qin. Using these assumptions and Eq. (6.11), we obtain Fig. 6.11 showing the relation between energy efficiency and the cycle temperature (rcycle). In this figure, curves (a), (b), and (c) are obtained assuming Qloss is equal to 20, 30, and 40% of Qin, respectively.

100 200 300 400 500 600

Fig. 6.11 Variation of overall energy efficiency of the Cu-Cl cycle with the cycle temperature based on three different assumptions: (a) Qloss=0.2Qin, (b) &oss=O.30m, (c) &oss=0.40m.

The overall exergy efficiency of the Cu-Cl cycle, ^ex_ overall, can be described as the fraction of exergy supplied that can be recovered from the exergy content of hydrogen:

cycle where ex ^^ is the specific exergy content of the hydrogen produced, taken to be

236,098 kJ/kmol (Ertesvag, 2007). Figure 6.12 shows the relation between overall exergy efficiency of the Cu-Cl cycle with its temperature. In this figure, curves (a), (b), and (c) are obtained assuming Qloss is equal to 40, 30, and 20% of Qin, respectively.

Fig. 6.12 Relation between overall exergy efficiency and temperature of the Cu-Cl cycle. (a) aoSS=O.40m, (b) Soss=O.30m, (c) Soss=O.20m.

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