HYDROGEN GENERATION USING A CALCIUM-BROMINE THERMOCHEMICAL WATER-SPLITTING CYCLE

摘要

The Secure Transportable Autonomous (STAR-H2) project is part of the US Department of Energy's (DOE's) Nuclear Energy Research Initiative (NERI) to develop Generation Ⅳ (Gen Ⅳ) nuclear reactors that will supply high-temperature (over 1 100K; 800℃) heat. The goal of NERI is to develop an economical, proliferation-resistant, sustainable, nuclear-based energy supply system based on a modular-sized fast reactor that is passively safe and cooled with heavy liquid metal. STAR-H2 consists of the following: 1. A 400-MW_(Thermal) reactor with Pb as the primary coolant; 2. Exchange of primary Pb coolant for a salt heat transfer pipe; 3. Exchange of salt for steam; 4. A combined thermochemical water-splitting cycle to generate hydrogen; 5. A CO_2 Brayton cycle to generate electricity (η = 47%), and 6. An optional capability to produce potable water from brackish or salt water. Here we review the thermodynamic basis for a three-stage calcium-bromine (Ca-Br) water-splitting cycle. The research builds upon pioneering work on the four-stage University of Tokyo Cycle #3 (UT--3) process, but employs a plasma-chemical stage for the recovery of HBr as H_2 and Br_2 as a substitute for the final two stages of UT-3. A detailed process design, developed by using the ASPEN model, suggests that the practical efficiency is 39-45% for the STAR-H2 Ca-Br cycle. For each tonne of H_2 produced hourly (1 000 kg/h), the demand for electricity for the plasma-chemical stage (13.5 MW_e) is much lower than the demand (28.5 MW_e) for a steam-electrolysis system. At current power grid heat-to-electricity efficiencies (η=33%), there is a clear benefit for using the STAR-H2 Ca-Br cycle. Anticipating Brayton cycle performance (η = 47%), H_2 production will demand a total power of 74 MW_(Thermal) per tonne of H_2 from the Gen Ⅳ reactor. It is important to recognise that there are capital and operating cost tradeoffs that will depend on the market value of low-carbon electricity in the future. Steam-electrolysis is a far less complex cycle than Ca-Br, but that simplicity comes at a high - and potentially uneconomical - cost for electric power.