CO2-free energy from natural gas via BrOx cycle

Abstract

In the present work a novel process for energy generation from natural gas without concomitant CO2 emissions, namely the Bromination-Oxidation (BrOx) cycle, is proposed and studied. This process consists of two exothermic reactions and an internal brominere cycle. Methane and bromine react in the first reaction step to yield solid carbon, that is separated by means of a cyclone or a filter, and hydrogen bromide. The latter is oxidised with oxygen in a second reactor yielding water and bromine. Bromine is separated and recycled to the first reaction step so that the energy is released in the process with solid carbon and water as only by-products. Firstly, the thermodynamics of the process were studied to assess its feasibility and simulations on both reaction steps in plug flow reactors were carried out in order to estimate suitable reaction conditions. A bench-scale plant was constructed to study experimentally methane bromination and determine temperatures and residence times that lead to complete reaction and carbon formation. Additionally, the carbon produced was characterised and its bromine content quantitatively determined. During methane bromination carbon deposition occurs, leading to inefficient operation. Two reactor concepts were proposed in this work, a vortex reactor that avoids deposition via the hydrodynamics of the system, and a sacrificial wall reactor in which deposition is not avoided but reactor cleaning is facilitated by means of a sacrificial coating on the inner walls. CFD simulation and optimisation of the vortex reactor was performed while sacrificial walls were studied experimentally. A catalyst for the second reaction step, hydrogen bromide oxidation, was synthesised and characterised, and experiments on both thermal and catalytic oxidation were carried out. Moreover, both operation modes were simulated and modelling was used to propose an optimal strategy for this reaction step. Finally, the flowsheeting of the BrOx cycle, including a preliminary economical analysis of the process, was performed and convenient separation units and operation parameters were selected based on the aforementioned data and simulation result.