Thermodynamic analysis of carbon deposition in the preparation of hydrogen-rich reducing gas for direct reduction processes

Hydrogen-rich reducing gas (H2RG) is a key fuel in the emerging carbon-neutral steel industry including gasbased direct reduction (DR) processes. If the preparation parameters in gas reforming are not properly controlled, carbon deposition can occur and it will affect the normal operation of this H2RG preparation process. Therefore, it is necessary to control the carbon deposition during the H2RG preparation process. In this paper, for the first time, a generic "Mass balance and chemical equilibrium diagram for H-C-O system" is proposed for determining the preparation parameters of the process and meanwhile controlling H2/CO ratio in the final gas composition. Then, by applying the phase rule and equilibrium principle of multi-reaction coexistence system, this diagram is used to determine the relationships between the carbon deposition curve and several key parameters including H2/CO ratio, preparation temperature, and total pressure and further determine the region of carbon deposition under the given conditions. For example, under the conditions of 1 atm, H2/CO = 2.0, 600 degrees C, the critical H2O/CH4 ratio is 2.0. That is, carbon deposition will occur if the H2O/CH4 ratio is less than 2.0 and otherwise not under such conditions. Then, the effect of the preparation parameters on the final gas composition is studied. This diagram provides an effective and handy tool for both proper preparation of H2RG without carbon deposition and meeting the requirements of the given DR process for the carbon-neutral steel industry.