Membrane separations, extractions and precipitations in aqueous solutions for CO2 mineralisation

G5 Doktorsavhandling (artikel)


Interna författare/redaktörer


Publikationens författare: Evelina Koivisto
Förläggare: Åbo Akademi University
Förlagsort: Turku/Åbo
Publiceringsår: 2019
ISBN: 978-952-12-3857-4
eISBN: 978-952-12-3858-1


Abstrakt

The global carbon dioxide (CO2) emissions have been growing rapidly since the industrial revolution starting in the 18th century. This has led to increasing CO2 concentrations in the atmosphere, which in turn heats up the atmosphere as a result of CO2 being able to absorb heat. Heating of the atmosphere has naturally taken place many times, so it is not a new feature on Earth. The problem with the global heating today, however, is that it apparently goes much faster. The annual increase of CO2 in the atmosphere is about 100 times faster today than it was the last time when natural heating occurred, around 300 000 years ago.

Different commitments, like the Kyoto Protocol, have the aim to decrease the greenhouse gases to certain levels in different commitment periods. The target for EU by 2020 is to reduce the emissions by 20%, compared to the CO2-eq. emissions measured in 1990. On a longer-term basis, Finland has the target to reach a carbon neutral society. Finland is on its way, but this target will be challenging to reach. However, finding options to decrease emissions further or develop options for CO2 storage is of highest importance in this context.

The only option for carbon capture and storage, CCS, in Finland is through CO2 mineralisation. An option has been developed at Åbo Akademi University during the last years, often referred to as the ÅA route. The process extracts magnesium from magnesium silicates through a thermal reaction at ≈ 400 oC, in the presence of ammonium sulphate serving as flux salt. The extracted magnesium is thereafter precipitated as magnesium hydroxide by the addition of ammonia, which is released in the thermal reaction. Magnesium hydroxide will finally react with flue gas in a pressurized fluidized bed reactor at 500 oC and a pressure of 20 bar to obtain magnesium carbonate, a stable compound that enables storing CO2 safely. As an alternative ÅA route, the process could take place in aqueous solutions with an initial extraction step where ammonium bisulphate and/or ammonium sulphate are present. The final carbonation would also take place in aqueous solution inserting CO2 gas and ammonia.

The concept for the ÅA route is extensively studied but the streamlining and the generation of input chemicals must still be established. This thesis focused on several different methods, i.e. the removal of excess water after dissolution (reverse osmosis), precipitation and separation of precipitated products in one unit (inclined settlers), extraction of magnesium from overburden rock from mining activities (thermal solid/solid or aqueous extraction), separation of two salts from each other (electrodialysis with monovalent ion selective membranes), or producing acid and bases from input chemicals used, which could be used in the process again (bipolar membrane electrodialysis).

Two of the studied rocks, both containing significant amounts of serpentinite, resulted in CO2 binding capacities of 292 and 260 kg CO2/ton rock, respectively, in aqueous solution extraction. The capacities for the same rock types were somewhat lower when applying the thermal solid/solid extraction step; 240 and 207 kg CO2/ton rock, for Hitura serpentinite and Vammala serpentinite, respectively. This indicates that the aqueous extraction route seems to have a better potential than the thermal solid/solid extraction step, at least under the studied conditions.

Attempts to separate magnesium and iron precipitates in an inclined tube settler were made in order to make the ÅA route more continuous. Magnesium and iron must be stepwise precipitated and separated from the process stream in the conventional ÅA route. The particle size of both precipitates was found to be very small according to SEM analyses, which affected the settling rate negatively. The settling rate for magnesium hydrocarbonates, however, was more rapid. This was confirmed by SEM images showing that the magnesium hydrocarbonates had a larger particle size, which also affected the settling rate positively. Continuous sedimentation and separation was still hard to achieve with the chosen set-up, even at low pump speed as to avoid too turbulent conditions.

A reverse osmosis membrane was used in order to try to separate water from the process stream. Some excess water must be used in order get all reacted rock washed out from the kiln as the thermal solid/solid route is applied. Reverse osmosis might therefore be an option to control the water volumes. The osmotic pressure of typically 23 bar in a solution after the magnesium extraction step must be overcome before any separation can take place. No significant separations were achieved, even with adjustments of the permeate stream composition replaced by downstream aqueous solutions. At least with the available membrane, RO will not be an option for controlling the amount of water used in the ÅA route.

An in-house built set-up of monovalent ion selective membrane electrodialysis was used to see if the separation of ammonium sulphate from ammonium bisulphate could be achieved. Initially, the compartment width was 40 mm. Ion selective electrode analyses (ISE) were done to measure the change in ammonium ions during the tests. These measurements show that almost 100% of the ammonia was transported from the feed compartment to the compartment where ammonium sulphate was collected. By comparing the exergy of mixing with the input exergy, however, it was shown that the input exergy (i.e. electricity) needed compared to what extent the ions were separated was in a magnitude of several thousand larger. One change in the set-up was therefore to narrow the compartment distance to 18 mm to overcome part of the electrical resistance in the solutions. This distance was used for four different set-ups using bipolar membranes. The bipolar membranes were arranged in different ways together with monovalent membranes, with the aim to produce acidic and alkaline solutions that may be used for further extraction and carbonation steps, respectively.

The energy demands for different type of tests were ranging from 1.7 to 350 MJ/kg separated NH4+. The two best cases, either a set-up placed after the extraction step or after the carbonation step, respectively, should be further studied. The need for running experiments in a commercial lab-scale set-up with sufficiently narrow compartment width and more repeating units than was used here is now substantial. No funding could do far be made available for purchase of a commercial set-up at the time of writing this thesis. Using a commercial labscale set-up will give better indications of the suitability of the individual set-ups and could also give more accurate numbers of the exergy (electricity) input needed for the electrodialysis step. The separation in more and more narrow repeating units will still use two electrodes so the energy used for the water electrolysis will not increase. A significantly lower exergy input will most likely be the case where more repeating units are used.
The results from all methods used in this thesis indicate that the all-aqueous variety of the ÅA route seems to be more favorable also when it comes to the streamlining of the process and the regeneration of the flux salt used.

Senast uppdaterad 2020-29-01 vid 03:55