1. Introduction - Firenze University Press

Abstract:
PROCEEDINGS OF ECOS 2012 - THE 25 TH INTERNATIONAL CONFERENCE ON
EFFICIENCY, COST, OPTIMIZATION, SIMULATION AND ENVIRONMENTAL IMPACT OF ENERGY SYSTEMS
June 26-29, 2012, Perugia, Italy
Production of Mg(OH)2 for CO2 emissions
removal applications: parametric and process
evaluation
Experience Nduagu a* , Inês Romão a,b , Ron Zevenhoven a
a Åbo Akademi University, Åbo/Turku, Finland, enduagu@abo.fi. CA
b University of Coimbra, Coimbra, Portugal
Technological processes that accelerate natural and geochemical weathering of abundantly available Mgsilicate
minerals have the potential for large-scale, safe and permanent storage of CO2. One of these CO2
sequestration routes involves as a first step the production of reactive Mg(OH)2 from Mg-silicates using
recoverable ammonium sulfate (AS) salt. This route avoids the very slow kinetics of carbonating magnesium
silicates. A recently identified Mg(OH)2 production process involves a closed loop, staged process of Mg
extraction followed by Mg(OH)2 precipitation and reagent (AS) recovery. This process has been applied to
different Mg-silicate (serpentinite and olivine rocks in particular) minerals from worldwide locations, having
varying physical and chemical properties. Experimental results showed some dependence of Mg extraction
and mass of the Mg(OH)2 product on the reaction parameters: mass ratio of Mg-silicate mineral (S) to AS
salt reacted, reaction temperature (T) and time (t). This paper statistically evaluates the contribution of these
effects and their interactions using a 2n-1 factorial experimental design. Both Mg(OH)2 production and
carbonation were simulated using Aspen Plus® software while process heat integration was done by pinch
analysis. Process energy evaluation, on an exergy basis, gives 3.88 GJ of energy requirement for 1t-CO2
sequestered (for Finnish serpentinite). This value is ~ 0.5 GJ/t-CO2 (10 % points) less than the energy
requirement of the process in a previous model. The results of this analysis would be beneficial for
optimization and pilot scale studies of this process.
Keywords:
Mg-silicates, Magnesium hydroxide, CO2 mineralization, Process evaluation.
1. Introduction
Weathering of alkaline silicate rocks plays a significant role in absorbing atmospheric CO2 [1].
Alkaline and alkaline-earth silicate mineral deposits are abundant and larger than fossil
resources[2]. A resource of this magnitude, over 300,000 Gt of Mg-based silicate minerals[3]
provides significant amounts of base ions for the natural process of neutralizing atmospheric CO2
emissions. However, natural weathering has very slow kinetics and occurs on geological
(multimillion-year) timescales [4]. So, it becomes foolhardy to rely on natural weathering in
reducing or stabilizing atmospheric CO2 emissions. The goal of meeting both current and future
energy demands in a “carbon neutral” manner has therefore spurred research that aims at
accelerating the kinetics of the reaction of mineral silicates and CO2. This geochemical option of
carbon (dioxide) capture and storage is known as CO2 mineralization or mineral carbonation.
The direct carbonation chemistry of Mg silicates is exothermic, and potentially allows for a process
with a zero or negative overall energy input [5]. Mg silicates, for example, serpentine and olivine
which are abundantly available (with a combined capacity of ~ 200,000 Gt[3]) reacts with CO2
according to (1) and (2)[6].
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