Title : Theoretical investigation of ammonia decomposition to produce hydrogen-rich fuel
Abstract:
Hydrogen stands out as a promising fuel for the future, holding significant potential in cutting down carbon dioxide emissions. However, challenges related to its storage, transportation, and safety hinder its widespread adoption. To address this, ammonia has emerged as a potential carbon-free carrier for hydrogen due to its several advantages such as high volumetric energy density and simple storage. Yet, due to some limitations related to ammonia's net combustion like its low flame speed, the suggestion is to store hydrogen in the form of ammonia and convert it back into hydrogen or hydrogen-rich gas just before utilization, aiming to overcome these limitations. In this paper, the thermodynamic analysis of hydrogen-rich gas production by ammonia decomposition is performed using Aspen Plus V.12 considering the idealized Gibbs reactor. Such a reactor determines the decomposition products by employing the Gibbs free energy minimization method. The system's thermodynamic analysis gives the opportunity to assess how various operational factors, particularly temperature and pressure, impact the system's performance measures. This analysis provides insights into the system's maximum energy efficiency and conversion rates. These limit levels can be a guideline for the optimum design of the elements of the ammonia decomposition system and for evaluating the experimental results. Different criteria have been evaluated, especially Ammonia conversion rate, Hydrogen-rich gas energy ratio, and Enthalpy change. The results show that at a specific temperature, the conversion rate of ammonia decreases as the pressure rises so that the conversion rate of more than 50% occurred at temperatures of about 427 and 513 K for pressures of 1 and 10 bar respectively. Since the reaction is endothermic, the increase in temperature provides the necessary energy to overcome the activation energy barrier required for the reaction to occur. Therefore, at higher temperatures, decomposition of ammonia is favored, leading to an increase in the value of equilibrium constant. Regarding the enthalpy of reaction, as temperature rises, the reaction's enthalpy also rises at a specified pressure, while the pressure increasing leads to a decrease in the reaction enthalpy. Moreover, adiabatic flame temperature is investigated for different blending of ammonia and hydrogen based on equivalence ratios from 0.4 to 1.6 so that the results reveal that increasing the molar percentage of hydrogen from 0 to 100% in the fuel mixture leads to the increase of maximum adiabatic flame temperature from 2079 K to 2438 K.
Audience Take Away
- Understanding of the potential of using ammonia as a carbon-free carrier for hydrogen
- Ultimate level of different performance criteria of ammonia decomposition such as conversion rate
- The effect of temperature and pressure on energy-related measures of the ammonia decomposition such as enthalpy and gibbs-free energy
- The positive effect of hydrogen addition to ammonia on adiabatic flame temperature and lower heating value
