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TRANSIENT MODELLING OF S-I CYCLE THERMOCHEMICAL HYDROGEN GENERATION COUPLED TO PEBBLE BED MODULAR REACTOR
Figure 3: Simplified HyS cycle schematic
thermochemical hydrogen generation plant, some idea of the behaviour of the coupled system is
attained. In this paper, a pebble bed modular reactor (PBMR) is considered as the primary heat source
for an S-I/HyS thermochemical hydrogen generation plant.
This paper describes previously developed models of the S-I/HyS cycle and a PBMR-268. A general
coupling methodology via the IHX is developed, and applied to these models. Finally, two nuclear
reactor driven transient scenarios are considered.
Simplified S-I/HyS cycle model
A simplified transient analysis model of the sulphur iodine and Westinghouse hybrid sulphur cycle was
presented by Brown, et al. (2009). This model is utilised in this paper via coupling to a PBMR-268 model
and a simple point kinetics model. Some of the key tenants of the analysis model are summarised;
however interested readers are referred to the original paper for greater detail. The S-I and HyS
analysis model is a control-volume model which treats the chemical plant as a closed system.
In this paper the chemical kinetics of the S-I cycle are assumed to be elementary. It is trivial to
write each of the reaction rate equations from the chemical reactions themselves. Each reaction rate
constant is calculated via an Arrhenius expression. In Section 1, the depletion rate of sulphur dioxide
is expressed as (Brown, 2009):
[ ]
d SO2
2
= k1
⋅ [ I2
] ⋅ [ H2O] ⋅ [ SO2
]
(1)
dt
The chemical kinetics for Section 2 is expressed as:
[ SO ] d[ SO ]
d H2
dt
4
3
[ SO ]
= − = k2
⋅ 3
(2)
dt
Thermodynamic calculations reveal that there is a significant reverse reaction rate for the
decomposition of HI (Brown, 2009). This reverse reaction rate requires an accurate consideration of
several coupled reaction rate equations. These expressions are:
[ 2 ]
2
= k3
⋅ [ HI] − k−3
⋅ [ H2
] ⋅ [ I2
]
dt
[ ] d[ I ]
d H
d H2
2
=
dt dt
1 d[ HI]
= −k3
⋅
2 dt
2
[ HI] + k ⋅ [ H ] ⋅ [ I ]
−3
2
2
(3)
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