1. Introduction - Firenze University Press

In the case of simultaneous sulphation and carbonation experiments, sulphate formation has been
found to dominate when Ca(OH)2 or CaO is exposed to a SO2 (1 ppm) and CO2 (6 ppm) containing
gas [47]. Although, the conditions were different from those studied here, it was also noted that,
SO2 readily reacts with Ca species at dry conditions, while some humidity was needed to form
carbonate. On the other hand, Stanmore and Gilot [50] commented in a review article that
carbonation is initially much faster than sulphation and only in the longer run does sulphation
become the principal reaction. In contrast to this study, however, it should be noted that the sorbents
considered were Ca-based and that the conditions did not incorporate elevated pressures.
Despite the fact that SO2 readily reacts with various Ca-based sorbents, the conversion levels
obtained are typically in the range of 30–50% [42,50] leaving room for considerable improvement.
The reason for the low conversion levels has been attributed to the closing of pores at the surface of
the reacting particles due to the larger molar volume of calcium sulphate than that of either calcium
oxide or –carbonate [50].
To date, most of the experiments using the ÅA mineral carbonation process have only considered
the use of pure pressurised CO2. However, in order for the process to become a realistic alternative,
it is apparent that it needs to work with diluted CO2 streams as well, such as industrial flue gases. In
this paper we present the results from a number of experiments using CO2 containing a small
amount of sulphur dioxide and oxygen, both of which are common components in typical industrial
flue gases. If successful, simultaneous carbonation and sulphation may motivate the removal of flue
gas desulphurisation (FGD) equipment from sulphur-containing fossil fuel-fired power plants.
7.2 – Materials and methods
The materials used for the gas-solid carbonation experiments consists of two different types of
Mg(OH)2, one commercially obtained (Dead Sea Periclase Ltd.) and one derived from Finnish
serpentinite according the ÅA process described briefly above. From here on, these will be referred
to as DSP-Mg(OH)2 and serp-Mg(OH)2 respectively.
DSP-Mg(OH)2 has already been studied extensively [27] and is also used for reference purposes in
this paper. However, the difference between serpentinite-derived Mg(OH)2 and DSP-Mg(OH)2 is
apparent from surface analysis and typically serp-Mg(OH)2 has a much higher specific surface area
(~50 vs. ~5 m 2 /g) and porosity (0.24 cm 3 /g vs. 0.024 cm 3 /g) than DSP-Mg(OH)2. For this reason,
serp-Mg(OH)2 offers a much greater potential in form of reactivity and reaction extent than DSP-
Mg(OH)2.
The gas used in the carbonation experiments has been a high purity (99.999%-vol) CO2 bottle and a
pre-mixed CO2-O2-SO2 bottle with 90%, 8% and 2%-vol of each component respectively. For some
experiments steam was added to the gas stream. The amount of SO2 in the gas-mixture was varied
between 0 and 20 000 ppmv (parts per million, volumetric).
A more thorough description of the methods used to carbonate Mg(OH)2 can be found elsewhere
[26,27], but for purposes of continuity, a short summary is also given here.
The experimental setup for gas/solid carbonation at ÅA, see Fig 5, consists of a small (height
~0.5 m, inner diameter ~1.5 cm) pressurised fluidised bed (PFB) that is operated by preheating the
incoming fluidisation gas and by maintaining the reactor at the target conditions during each
experiment. The PFB is operated as a bubbling fluidised bed and run in batch mode (max. temp.
~600 °C, max pressure ~100 bar). After each experiment the particles are blown out and collected
by a cyclone for easy removal.
94