The dissolution of seven natural limestones and calcareous rocks in hydrochloric acid solution was examined to investigate their capability for wet flue gas desulphurisation. All samples were crushed, ground and sieved to a size-fraction of 150-250 mm. Thereafter they were subjected to a dissolution experiment utilising stepwise titration with hydrochloric acid. The dissolution rates of three calcareous rocks were found to be controlled by reaction kinetics, while the limestones showed mass transfer control. The surface characterisation was implemented before and after dissolution experiments using X-ray photoelectron spectroscopy and scanning electron microscopy. Additional characterisation was carried out with X-ray diffraction, X-ray fluorescence and polarizing microscope. Initial reactivities have been shown to decrease in the order limestone, calcareous rock with high calcium concentration, calcareous rock with low calcium concentration. Coarse grain structure is proposed to decrease the initial reactivity.

No abstract

A continuously operated CO2 capture unit, based on absorption in a membrane contactor and lowtemperature vacuum desorption, is demonstrated. The major advantage of membrane contactors is their high specific interfacial area per unit volume. The unit is designed to be modular to allow different absorption membrane modules and stripping units to be tested, with the aim of capturing CO2 from simulated flue gases at concentrations down to the ambient concentration. In addition, desorption can be performed under vacuum to improve the desorption efficiency. The experimental unit incorporates comprehensive measurements and a high level of automation, with heat integration and continuous measurement of electricity consumption providing real-time estimates of the energy consumed in the capture process.In preliminary tests, the results of which are described herein, a 3M Liqui-Cel™ polypropylene hollow-fiber membrane module and a glass vacuum chamber were used for absorption and desorption, respectively, along with a potassium glycinate amino acid salt absorbent solution. This solution has high surface tension and is fully compatible with the polypropylene membrane unit used. In preliminary tests, the highest observed CO2 flux was 0.82 mol m -2 h -1 , with a CO2 product purity of above 80%. The calculated overall mass transfer coefficient was comparable to reference systems. The performance of the unit in its current setup was found to be limited by the desorption efficiency. Due to the low desorption rates, the measured specific energy consumption was exceedingly high, at 4.6 MJ/mol CO2 (29.0 MWh/t) and 0.8 MJ/mol CO2 (5.0 MWh/t) of heat and electricity, respectively. Higher desorption temperatures and lower vacuum pressures enhanced the desorption efficiency and reduced the specific energy consumption. The energy efficiency could be improved via several methods in the future, e.g., by applying ultrasound radiation or by replacing the current vacuum chamber stripping unit with a membrane module or some other type of desorption unit.

Sulfur dioxide (SO2) is one of the pollutant gases that result from energy conversion by coal and oil combustion. Limestone slurries are widely utilized in wet flue gas desulfurization (WFGD) processes. The evaluation of the reagent's reactivity is fundamental for process design and plant operation. The comparison of different limestone and dolomite rocks through dissolution experiments was realized by applying a model for second‐order kinetics. And a total of 12 different samples were tested, at three different size fractions. A simulation model was developed to identify each sample's impact on sizing the dissolution tank of a WFGD scrubber. A second‐order model was applied within the scrubber's dissolution tank range of operation at pH values between 4 and 5. The regression coefficients for the second‐order model were higher than 0.98 in all cases. The dissolution tank volume estimation was done based on the reagent's overall chemical reaction constant in dissolution experiments. The limestone rocks, where the prevailing mineral was calcite (CaCO3), demonstrated higher chemical reactivity yielding smaller dissolution tank volumes than the dolomite rocks, where the prevailing mineral was dolomite (CaMg(CO3)2). The CaO and MgO content of a sample can be used to predict the dissolution rate; nevertheless more factors influence the rate and the tank sizing. The knowledge of the sample's composition and its reactivity is necessary for the design of a desulfurization scrubber; the reagent used is determinant for the economy and optimal operation of such scrubbers. © 2012 American Institute of Chemical Engineers Environ Prog, 32: 663–672, 2013