Toward a green circular economy – improving small scale renewable ammonia using absorbent-enhanced Haber-Bosch

Abstract

With carbon emissions continuing to increase yearly including reaching record levels of 37.4 giga tons of CO2 in 2023, there is a global desire to reduce greenhouse gas emissions and mitigate its negative impact on our climate system. A major reason for the increase in CO2 emissions is our linear economic model – take, use and dispose. This model relies on the extraction and utilization of fossil fuels to drive almost every aspect of global economic activity, releasing large volumes of carbon emissions in the process. The availability of renewable resources like wind, hydro and solar provide much needed clean energy alternatives, however, they are intermittent in supply and are mostly abundant in uninhabited areas, making them difficult and expensive to store and transport to population centers. One solution is to store energy from these renewables as ammonia. Ammonia provides a cheaper medium for long-term energy storage compared to hydrogen which requires expensive compression. Ammonia can also be used as a fertilizer, feedstock for specialty chemicals, converted into hydrogen fuel or burned directly to produce energy for various applications, making it an ideal vehicle for a decarbonized and sustainable economy.Synthesizing carbon-free ammonia using renewables like air, water and renewable electricity on a small-scale (<100 tonnes per day), which is suited for the intermittent nature of renewables, is very expensive. One way of reducing this cost is by replacing the traditional ammonia separation by condensation with reactive absorption using metal halides like MgCl2. In this process, the incoming ammonia reacts with the salt particles to form amminated salt, and this complex salt breaks down upon heating to release the ammonia. Although this method of ammonia separation is more efficient, pure metal halides are unstable and show decreasing capacity with prolonged usage. Incorporating inert support like silica beads into the absorbent material helps to stabilize its capacity but the mechanism through which stability is achieved is unclear. Furthermore, the optimum operating conditions that will yield the most ammonia per time during a cyclic ammonia absorption and desorption process in a small ammonia plant is not known. In addition, because ammonia absorption is very exothermic, the salt particles heat up to high temperatures upon contacting ammonia. This inhibits further ammonia uptake and limits the ammonia capacity for these materials. Finding better thermally conductive support would improve heat transport in the absorbent bed. By designing an experimental method for studying the supported metal halides - MgCl2, the mechanism for stabilizing ammonia absorbents using inert support was revealed: mass transport is aided by support providing enough surface for dispersed small salt crystals to form. Using this stable performing material, 40 wt.% MgCl2-SiO2, process optimization studies were performed to determine the process conditions that will yield the most ammonia per time during a cyclic ammonia absorption and desorption process. Using a sufficiently high regeneration temperature (~200 ⁰C) allowed for a small amount of sweep gas to be used for absorbent regeneration. Of note, the ammonia product exceeded 72 mol% purity in a mixture of N2 and H2. High ammonia capacities (about 154 mg NH3/g absorbent) were achieved within 33% or less time than was previously studied. Insights from the MgCl2-SiO2 system, enabled the design of a thermally conductive composite absorbent, MgCl2-Al. With a bed thermal conductivity (4.6 ± 0.1 W/m.K) up to an order of magnitude higher than MgCl2-SiO2, the MgCl2-Al material enabled heat to be conducted more efficiently throughout the absorbent bed during ammonia absorption and desorption; thus resulting in higher ammonia production yield.