Production and testing of magnesium carbonatehydrates for thermal energy storage (TES) application

Rickard Erlund*

*Korresponderande författare för detta arbete

Forskningsoutput: Typer av avhandlingarDoktorsavhandlingSamling av artiklar

Sammanfattning

This thesis covers both research of carbon capture and storage by a mineralisation (CCSM) process and production and use of the end product, magnesium carbonate hydrate (MCH) for thermal energy storage (TES). The development of a process using MCH as a TES material includes thermal system modelling and simulations using the experimental results. To reach the goals of the Paris climate agreement on reduced CO2 emissions, all feasible measures are important, including renewable energy technologies and carbon capture and storage (CCS) technologies. Considering that geological CO2 storage in Finland or the Baltic Sea region is not possible, the only option in/near Finland is CCSM. CCS (and CCSM) is broadly investigated around the world, and a set of magnesium silicate -based CCSM processes have been developed at Åbo Akademi University (referred to as ÅA-routes). (Often CCSM is classified as carbon capture and utilization, CCU.) TES can expand the use of heat produced by renewable sources that do not meet the demand at the same moment. Developing a material out of the CCSM carbonate product that is useful in the future motivates CCSM from both economical and environmental points of view. The thesis is a continuation of the earlier research on the CCSM process (ÅA routes) developed at Åbo Akademi University. The first two papers study the possibility to find suitable rock (for example mining tailings or overburden) closer to the CO2 emitting industry (in Finland). It is obvious that the energy use and CO2-emissions for the CCSM process should be minimized. Magnesium extraction from two serpentine mineral containing rock, a Mg-hornblende and a diopside found in Finland are compared. The study compares solid/solid (440 °C) and aqueous/solid extraction (70- 100 °C) from these rocks and suggests new mixtures of extraction salts . (Once extracted, the downstream carbonation conversion of the magnesium can be assumed to be > 90%.) The best extraction results with an aqueous/solid reaction would have CO2 binding capacity of 292 kg CO2/ton (Serp-A 500km from the CO2-source) and 260 CO2/ton (Serp-B, 100km from the CO2 source), while the solid/solid reaction would allow for binding 240 kg CO2/ton and 207 CO2/ton, respectively. This study determines the advantages of both methods although the selection of the most feasible process alternative needs to be done depending on the CO2 emitting process (eg. waste heat temperatures) and its location. The CCSM process (ÅA route) produces magnesium carbonate, magnesite (via dry, pressured carbonation) and magnesium carbonate hydrate , MCH, (via wet carbonation). Depending on the conditions nesquehonite, lansfordite and hydromagnesite may be formed. Nesquehonite can desorb its crystal water and form magnesium carbonate according to the reaction below, giving a heat effect sufficient for significant thermal energy storage: MgCO3 + 3H2O(g) ↔ MgCO3∙3H2O ΔH = -1.0 MJ/kg MgCO3∙3H2O, T=298K Compared to most of the other chemical sorption compounds, its advantages are low operating temperatures for TES while it can in case of emergency act as a fire retardant. The desorption/dehydration temperatures are 60-70 °C, and adsorption/hydration is possible at the temperature range of 5-25 °C. However, our studies suggests that the material should be mixed with silica gel for sufficiently fast reaction kinetics. The basic procedure is using solar heat (or other heat source) for heating up the sorbent material during summertime after which energy can be discharged (hydration) during winter. Two concepts were presented, a closed TES system using geothermal heat for water vapour generation and an open TES system using water vapour from indoor air, respectively. The open TES system was chosen for further system studies. The composite material, a 50%/50% weight mixed nesquehonite (NQ) and silica gel (SG), efficiently chemisorbs water vapour at high (75%) relative humidity (RH) but sorption at low RH (25–50%)) RH is compromised. The samples being small (3-5mm) granules, and the best heat capacity obtained of the chemisorption reaction was 0.7 MJ/kg at 25 °C and 0.36 MJ/kg at around 5 °C. The open TES system suggested is based on an exhaust air heat pump decreasing the outlet (from indoors) the air temperature as to increase the RH. For this, a laboratory pilot for testing the concept performance was built. Using this larger system a heat capacity of 0.41 MJ/kg at 25 °C and 0.29 MJ/kg at around 5 °C were obtained. The reaction rate data obtained was used in simulations based on the concept. The performance of using the TES reactor to supply heat instead of electrical resistance heat (to support the heat pump) in the winter was analysed. Around 70% of the otherwise needed electrical resistance heat may be substituted for with TES using MCH + silica gel. As a side-benefit, the system also improves the performance of the ventilations heat exchanger by eliminating the freezing issue by drying the exhaust air.
OriginalspråkEngelska
UtgivningsortÅbo
Tryckta ISBN978-952-12-4028-7
StatusPublicerad - 2021
MoE-publikationstypG5 Doktorsavhandling (artikel)

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