Hybrid sulfur flowsheets using PEM electrolysis and a bayonet decomposition reactor

Abstract

The specific hydrogen market determines the value of hydrogen from different sources.Each hydrogen production technology has its own distinct characteristics. For example, steam reforming of natural gas produces only hydrogen. In contrast, nuclear and solar hydrogen production facilities produce hydrogen together with oxygen as a by-product or co-product. For a user who needs both oxygen and hydrogen, the value of hydrogen from nuclear and solar plants is higher than that from a fossil plant because "free" oxygen is produced as a by-product. Six factors that impact the relative economics of fossil, nuclear, and solar hydrogen production to the customer are identified: oxygen by-product, avoidance of carbon dioxide emissions, hydrogen transport costs, storage costs, availability of low-cost heat, and institutional factors. These factors imply that different hydrogen production technologies will be competitive in different markets and that the first markets for nuclear and solar hydrogen will be those markets in which they have a unique competitive advantage. These secondary economic factors are described and quantified in terms of dollars per kilogram of hydrogen.

Energy Efficiency Limits for a Recuperative Bayonet Sulfuric Acid Decomposition Reactor for Sulfur Cycle Thermochemical Hydrogen Production

“…7 Similar limitations apply for sulfuric acid decomposition in the SI cycle. An efficiency analysis identified three different flowsheets for high-temperature sulfuric acid decomposition in the SI cycle that had energy requirements between 389 and 441.5 kJ/mol SO 2 , 8 while a fourth sulfuric acid decomposition flowsheet has been proposed for the SI cycle that requires only 373 kJ/mol SO 2 product.…”

“…7 The latter makes use of proven technology and could be deployed almost immediately. Water-splitting via electrolysis is conceptually simpler than any thermochemical cycle and, therefore, likely to require a smaller capital investment.…”

“…7 Choosing 45% as a representative value, pressurized alkaline electrolyzers coupled to an HTGR power plant could produce hydrogen at 30.6% efficiency, LHV basis, equivalent to a primary energy requirement of 791 kJ/mol H 2 product. Thus, alkaline electrolysis has a primary energy requirement of 800 to 1100 kJ/mol H 2 , depending on the efficiency of the power source.…”

“…7 simulation model used to determine the heating target for the bayonet reactor is available elsewhere. 7 The model tracks the sulfuric acid process fluid as it passes through the reactor, making several key assumptions: 1) operation is steady-state, 2) fluid moves through the reactor in plug (single-phase) or homogeneous (two-phase) flow, 3) the cross-section of the flow path is well-mixed, and 4) local thermodynamic and phase equilibrium is maintained throughout. In reality, there may well be localized disengagement between phases in the boiling and condensing regions as well as radial gradients.…”

“…7,9,18 Lower acid concentrations would allow more efficient electrolyzer operation in the hybrid sulfur cycle due to lower reversible potential, 19 so concentrations as low as 30 wt% (expressed as 0.068 mole fraction SO 3 , where 1 mole H 2 SO 4 is equivalent to 1 mole H 2 O + 1 mole SO 3 ) were considered. A value of 90 wt% (0.384 mole fraction SO 3 ) was used for the upper limit.…”