Abstract
Previous investigations have shown that acids have the potential to increase the hydrogen (H2) generation rate. This work aimed to study a new method for generating H2 gas in water added with various acidic solutions (Acetic acid - CH3COOH and Ferric acid - FeCl3) catalysed with stainless steel net by hydrolysis of magnesium (Mg) scraps. Mg samples are prepared by re-melting end-of-life Mg scrap and dipping stainless steel net to form Mg-stainless steel net couples (Mg-St net couple). CH3COOH, a greener acid than hydrochloric acid, has the ability to accelerate a H2 generation reaction. FeCl3 is chosen due to its corrosive nature and the importance of its waste in drinking water purification plants. This study contributes towards the promotion of recycling of the end-of-life Mg products for use in H2 generation. In summary, the acid-catalysed hydrolysis of Mg in various organic acids is presented around four distinct objectives, namely:
(i) combining the simplicity of the storage of solid NaAlH4 with the simplicity of the aqueous solution of acid;
(ii) showing CH3COOH can be as reactive as FeCl3 in specific, well-chosen operating conditions;
(iii) emphasising the relative greenness of the CH3COOH and FeCl3 based process; and
(iv) recycling end-of-life magnesium product scraps as a secondary material for generating high purity H2.
CH3COOH is described as a non-hazardous organic acid whose toxicity is highly dependent on concentration. It is a popular food additive in the food industry. In terms of environmental safety, it outperformed hydrochloric acid as an accelerator. FeCl3, also known as ferric chloride, is a well-known industrial chemical compound that appears dark green upon light reflection and purple-red by transmitted light. It dissolves in water [1] and undergoes hydrolysis, where heat is generated in an exothermal reaction. The results of the exothermal reaction are a brown acidic and corrosive solution that is reportedly used extensively in drinking water purification plants, as a flocculant in sewage treatment plants, and in the production of printed circuit boards. The application of acids in H2 generation as accelerators has been reported in the literature. Due to their environment friendliness, low cost, and availability, this group of materials has been gaining momentum. FeCl3 is used extensively in organic synthesis as an ideal Lewis acid since it is an inexpensive, efficient, stable, environmentally-friendly, and convenient agent for several useful reactions.
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Low-grade Mg waste from post-consumer products and production waste cannot be recycled in an efficient and cost-effective manner, however. This research addresses this issue by converting waste into hydrogen. Mg waste is melted and loaded onto one side of a stainless-steel couple before solidifying to form a galvanic mg stainless steel couple. At 15° C and 20° C, these couples are used to generate hydrogen in 3.5% by weight CH3COOH solutions. The experimental results show a mean accumulated H2 volume of 3.17 litres was produced. One gram of low-quality Mg produced more than the theoretical yield of H2. The chemical composition of the remelted Mg scrap was between 65 – 80.2 wt.% Mg, 2.3 – 7 wt.% Al, 9 – 12.2 wt.% O2 and a few other traces of elements such as Silicon (Si). Si and Al react with water to produce H2. Research shows that the availability of 0.6% silicon in an aluminium sample increases H2 yield by 20%. In another set of hydrolysis reactions, magnesium waste-based materials from end-of-life products were used as raw materials in the presence of iron chloride added water solution to generate hydrogen gas. Mg reacts slowly with water and releases hydrogen at room temperature; this is followed by the formation of magnesium hydroxide on its surface. This reaction was accelerated by addition of 1.5 wt % of iron chloride. The results confirmed iron chloride as an excellent hydrolysis reaction accelerator with stainless steel as an effective catalyst. On average, the reaction yielded 2700 mL of H2 over 3600 seconds.
Once H2 is produced, storage is usually the next question. H2 storage means reducing the volume of H2 gas [2]. It is currently stored by using one of the four available methods: compression; liquefaction; physisorption; and metallic hydrides and complex hydrides. In this study, a metallic hydride and complex hydrides storage option is chosen, and a hydride candidate is selected to store the generated H2 gas. Metal hydride systems offer a technological avenue for high-energy hydrogen-density storage devices for a range of transit and mobile applications. Because of its high-coupling thermal transfer, mass transfer and chemical kinetics, metal hydride systems offer many possibilities. In this work, sodium alanate (NaAlH4) based storage model is developed, modelled, and simulated for quick refuelling of a 9 kg storage tank filled with sodium alanate. The hydride bed within the canister had a material density of 0.62 g ml of NaAlH4. Before each run, the canister is pressurised and heated. A simulation technique is designed and validated by what has been achieved and noted as the optimum design for the storage canister in literature. The use of the simulation tool for various storage concepts and geometries results in the final design. To this end, I confirm Ansys as a practical simulation tool to model hydrogen storage to precisely forecast the replenishment of hydrogen and release processes for storage systems based on hydride metals.
Keywords, absorption, desorption, generation, hydrogen, scrap material, magnesium.