Systematic decision-making framework for evaluation of process alternatives for sustainable ammonia (NH3) production

Abstract

Ammonia (NH3) is the second highest produced chemical commodity worldwide, which commonly used in manufacturing of fertilisers, refrigerants, dyes and other chemicals. Haber Bosch process is the most established process to produce NH3 from hydrogen (H2) and nitrogen (N2). However, this process releases large amount of greenhouse gases (GHG) annually as this process consumes H2 which produces from steam methane reforming process that utilises methane (CH4) from natural gas. In addition, such process also consumes large quantity of energy to support the production of NH3. In order to enhance the sustainability of NH3 production, renewable energy (RE) such as solar can be used to replace the traditional energy system that consumes fossil fuels. Besides, production of H2 and N2 via water electrolysis and air separation technology which powered by RE can further enhance the sustainability of NH3 production. Such process is known as green NH3 production. One of the challenges of utilising RE is to have a stable supply of power. Therefore, concentrated solar power (CSP) is a good option to overcome this challenge, because it has capability of providing stable power supply. Based on literature review and market survey, a superstructure which consists of all possible and available technologies for production of green NH3 using CSP as an energy supply is presented. Next, the advantages and disadvantages of each alternative are evaluated and summarised in a comparison table. Ranking matrices are developed to evaluate and rank the alternatives quantitatively to determine the optimum pathway for green NH3 production. Based on the analysis, a conceptual integrated green NH3 production which involves a CSP tower plant with a two-tank molten salt thermal energy storage (TES) system is determined as the optimum pathway. Such process fulfils the electricity demand of a pressure swing adsorption (PSA) air separator, polymer electrolyte membrane (PEM) and NH3 synthesis plant. The results of this work are useful for feasibility studies on the commercialisation of green NH3 production.