Page 217 - Materials Chemistry, Second Edition

EXERGY ANALYSIS AND ITS CONNECTION TO LIFE CYCLE ASSESSMENT 203

substances (Naterer et ah, 2008). Figure 8.5 shows a conceptual schematic of the
five-step Cu-Cl cycle.
The cycle considered here involves five main steps:

1. HC1 (g) production using such equipment as a fluidized bed. In

this step, CuCl 2 reacts with water to produce HC1 and CuOCuCl 2
at a temperature of around 450°C in a fluidized bed.
2. Oxygen production. In this step, oxygen and CuCl are produced
by splitting CuOCuCl 2. Cu-Cl which is the output of the second
step moves to step 3 (copper production)
3. Copper (Cu) production. In this step, copper is produced from
molten CuCl, at a reaction temperature as low as around 25°C.
This reaction requires electricity.

4. Drying. In this step, the aqueous CuCl 2 is dried to solid CuCl 2
which is used in HC1 production.
5. Hydrogen production. In this step, solid copper particles from
step 3 (Cu production) react with HC1 from step 1 (HC1 produc-
tion). The hydrogen production step is exothermic and occurs at a
temperature of 450°C. The outputs are hydrogen gas, which is the
desired product, and CuCl.

Note that there are three- and four-step variations of the Cu-Cl cycle. The
four-step copper-chlorine cycle combines step 3 and step 4 in the five-step
cycle to reduce the complexity and equipment requirements. In the three-step
Cu-Cl cycle, the hydrogen production step and the combined step in the four-
step cycle are integrated.
The heat requirements for each step of the five-step Cu-Cl thermochemical
cycle, as evaluated by Wang et ah (2010), are used to calculate total thermal
energy requirement of the cycle. It is noted in that study that the required heat
input to the system is 554.7 kj/mol H 2 and that the total heat output of the
system is 232 kj/mol H 2. Assuming only low grade heat, i.e., 163.3 kj/mol H 2
is recovered (equivalent to 70% heat recovery), the external thermal energy
requirement of the system is 391.4 kj/mol H 2. The electrical energy require-
ment of the copper production step is 62.6 kj/mol H 2. Also, it has been esti-
mated that 38 kj/mol H 2 of work is required for auxiliary equipment (Rosen
et ah, 2010). Hence, the net energy requirement of the system can be estimated
as 492 kj/mol H 2.
To calculate the thermal and total energy requirements of the five-step Cu-Cl
thermochemical cycle per kg hydrogen produced, the evaluated and estimated
values are modified using the molar mass of H 2 (2 g/mol) as follows:

Total thermal energy requirement of five-step Cu-Cl cycle =
[391.4 kj/(l mol H 2 )] x [(1 mol H 2 )/(2 g H 2 )] x (8.5)
[(1000 g)/(l kg)] = 195.7 MJ/kg H 2
and