Carbon dioxide capture and storage by mineralisation using recyclable ammonium salts

Wang, Xiaolong (2011) Carbon dioxide capture and storage by mineralisation using recyclable ammonium salts. PhD thesis, University of Nottingham.

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Carbon dioxide capture and storage by mineralisation (CCSM) is considered to be an alternative solution for reducing anthropogenic C0₂in some regions, where geological storage is not possible or considered uneconomically viable. However, low efficiency of mineral dissolution and use of unrecyclable additives are two key barriers for the development of CCSM.

A novel CCSM process with recyclable ammonium salts is proposed to overcome these barriers in this study. This process integrates mineral carbonation with C0₂capture by employing NH₃, NH₄HSO₄and C0₂containing ammonium salts in the capture step, mineral dissolution and carbonation steps, respectively. The NH₄HSO₄ and NH₃can then be regenerated by thermal decomposition of (NH₄)₂SO₄, which is the by-product from the process. The use of C0₂ containing ammonium salts as the source of C0₂can avoid desorption and compression of C0₂, which account for 70 % of the total energy consumption in the whole CCS chain.

In this work, a CCSM process route at low solid to liquid ratio (50 g/I) was experimentally investigated to validate the process concept. It was found that the dissolution efficiency of magnesium (Mg) can achieve 100 % by using NH₄HSO₄and the carbonation efficiency can reach 96.5 % by using CO₂containing ammonium salts from the capture step and addition of aqueous NH₃. Three products, including Si rich residue, Fe rich residue and pure hydromagnesite were obtained from the process. The TGA studies reported that the regeneration efficiency of NH₄HSO₄ and NH₃ in this process was 95 %. Both dissolution and carbonation efficiencies achieved in this work are higher than the values reported in previous work.

In order to reduce the water usage, a CCSM process at high solid to liquid ratio (200-300 g/I) was developed. It was found that the dissolution efficiency of Mg was 64 and 72 % at 200 and 300 g/l, respectively. The increase of dissolution efficiency with a solid to liquid ratio could be explained by the removal of passive product layer caused by particle-particle interaction. At a solid to liquid ratio of 300 g/l, the highest carbonation efficiency achieved was 65.4 %. Magnesite instead of hydromagnesite was found after carbonation due to the CO₂ pressure caused by the decomposition of ammonium salts above 70 °C. Additionally, the carbonation efficiency was doubled by using (NH₄)₂CO₃compared to that using NH₄HCO₃.

A preliminary evaluation was conducted to estimate the OPEX, including energy consumption, chemical costs and feedstock cost, based on the experimental results from the two process routes developed. In order to get low OPEX, the optimization process conditions, such as solid to liquid ratio and reaction time, were determined. Then, experiments at these optimized conditions were conducted. The dissolution efficiency of Mg from serpentine with particle size 75-150 pm using 2.8 M NH₄HSO₄at 100 g/l solid to liquid ratio for 1h was around 80 %. The carbonation efficiency was 96 % when the molar ratio of Mg: CO₂ containing NH⁴+ salts: NH₃was 1: 1.5: 2. Thus, the mass balance of the process showed that 3.0 t' of serpentine, 0.2 t of NH₄HSO₄and 0.1 t of NH₃ were required to sequester 1t of CO₂and produce 1.9 t of magnesite. Moreover, 1.7 t of high Si content (46.9 wt. %) and 0.3 t of high Fe content (60 wt. %) were produced.

Finally, a cost evaluation study including CAPEX and OPEX was made using Aspen plus software to simulate the optimized CCSM process with recyclable ammonium salts for a 100 MW coal-fired power plant. For the input of 60 t/h CO₂, 93 % of them can be sequestered by the process with 29.5 % energy consumption and the total carbon capture and storage costs was 71.8 US$/t CO₂sequestered, excluding the product sale.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Maroto-Valer, M.
Keywords: Carbon sequestration, ammonium salts
Subjects: T Technology > TD Environmental technology. Sanitary engineering
Faculties/Schools: UK Campuses > Faculty of Engineering > Department of Chemical and Environmental Engineering
Item ID: 12982
Depositing User: EP, Services
Date Deposited: 14 Dec 2012 11:14
Last Modified: 22 Dec 2017 08:07

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