Session: 08-02: Solar Chemistry: Thermochemical Fuel Production II

Paper Number: 135601

135601 - Simulation Study of Thermochemical Water Splitting Using a Hot Blast Stove in a Steel Mill 

Abstract: 

A two-step thermochemical water splitting represents a theoretically efficient pathway of utilizing heat energy to produce hydrogen. Although water is relatively inexpensive feedstock and its splitting does not generate carbon emissions, the substantial energy required for this endothermic reaction may result in both a high cost of hydrogen production and potential greenhouse gas emissions, depending on the energy source used. Electrolysis, which utilizes electricity–a relatively expensive form of energy–for this reaction, contributes to the high cost of hydrogen production.

To reduce the cost, various research has consistently explored utilizing relatively inexpensive and abundant renewable thermal energy instead of electricity. Thermochemical redox cycles utilize metal oxides (MO) as an additional reactant to the water splitting reaction so that it can separate oxidation (MO → M+1/2O₂) and reduction (M+H₂O → MO+H₂) reactions. Introducing an additional reactant can lower the required temperature compared to that in a direct thermolysis reaction.

Previous studies investigated two-step thermochemical cycles for hydrogen production through the thermal reduction of Co-ferrite, Ni-Mn-ferrite, Ni-ferrite, etc., at temperatures ranging from 1000 to 1400 ℃. However, a significant drawback was that the amount of hydrogen produced by reduction of part of oxygen in M (Co or Ni, or Ni and Mn)-ferrite is very small, less than 20 cc/g, making it challenging for practical applications. Additionally, the repetitive temperature swings during the reduction and oxidation reactions result in substantial energy losses when reheating metal oxides, leading to overall low efficiency.

  This kind of repetitive temperature swing can also be found in hot blast stoves, which are essential facilities in steelmaking. In steel mills, the hot blast stove generates the high-temperature air that is supplied to the blast furnace, which works as a tall, thermal regenerative heat exchanger. It is for the continuous supply of hot air to a blast furnace. The air is preheated by passing it through a hot blast stove which is heated by combustion of the enriched blast furnace gas (BFG).

  The hot blast stove operates dividing into two modes namely, on-gas and on-blast. In on-gas mode, BFG is combusted to heat up the hot blast stove. In the on-blast mode, air is heated by flowing it through the hot blast stove, which consequently reduces the temperature of the hot blast stove. Once the hot blast stove in the on-blast mode completes, it is switched into on-gas mode, and another stove in the steel mill takes the turn to supply the hot blast air to the blast furnace. The temperature of a hot blast stove fluctuates as it cycles through two alternating phases: the heating phase (on-gas) and the cooling phase (on-blast).

  In this study, we propose a novel system that utilizes this repetitive temperature swing, generated in the hot stoves of a steel mill, in the thermochemical redox cycle. A proposed system produces hydrogen through water splitting while concurrently manufacturing iron by placing a thermochemical water-splitting reactor on the top of the hot blast stove. In the high-temperature on-gas mode, reduction reactions occur, leading to the generation of oxygen, while in the low-temperature on-blast mode, oxidation reactions produce hydrogen. Additionally, we aim to set an optimal temperature for system operation by using pure oxygen, generated from thermochemical redox cycle, in the combustion.

  The simulation model of the system was constructed utilizing MATLAB and Cantera thermodynamic tools. The water-splitting reactor is modeled as a 0D model considering mass and energy conservation, while the hot blast stove was represented as a 1D model considering heat transfer, mass, and energy conservation. Moreover, to estimate and evaluate the performance of the proposed new hydrogen generation system, we quantitatively characterized the levelized cost of hydrogen and greenhouse gas emissions of the produced hydrogen.

Presenting Author: Yehyeong Lim Ewha Womans University

Presenting Author Biography: Yehyeong Lim is M.S. student in Ewha Womans University of Mechanical and Biomedical Engineering.
Her research interests include developing device and system designs of thermochemical redox cycle to produce green hydrogen from renewable heat.

Authors:

Yehyeong Lim Ewha Womans University
Wonjae Choi Ewha Womans University