The reactivity of South African limestone of variable quality as potential sorbents in wet flue gas desulphurisation

Abstract

According to the World Health Organisation (WHO) clean air is a basic requirement for the well-being and general health of every human being. The results of WHO assessments have shown that annually more than 2 million deaths can be classified as premature and can be linked to outdoor as well as indoor air pollution. Sulphur dioxide (SO₂) emissions from coal-fired power plants and other industrial operations form part of the potentially hazardous pollutants, and it is critical that these emissions be reduced where possible. In 2012 the South African government enforced stricter air quality standards and stipulated that the SO₂ emissions from all existing power plants must comply with an SO₂ emission limit of 3500 mg/Nm³ by 2015, while the SO? emissions limit for all new power plants would be limited to 500 mg/Nm³ . Therefore, the decision was made to implement the most widely used desulphurisation technology, namely Wet Flue Gas Desulphurisation (WFGD), in the newly built South African power stations. The removal of the SO₂ component of the flue gas from a coal-fired power station by means of a WFGD process, involves making use of a sorbent such as limestone to capture the SO₂ from the flue gas stream. Limestone consists mainly of CaCO₃, is mixed with water to form a slurry and is sprayed into the absorption tower where absorption of SO₂ from the flue gas takes place. Further reaction then occurs below the absorber in the reaction tank. Despite the high desulphurisation efficiency offered by WFGD, the continuous improvement of the process efficiency is still required considering environmental legislation that is becoming increasingly stringent, while power plants age and the quality of the coal being consumed degrades over time. A sound understanding of the process and the various steps involved is of significant importance with regards to process and design optimisation. In this regard, the rate of dissolution of the limestone that is used as sorbent in a WFGD is a key step in the process that needs to be further studied to gain an understanding of the influence it has on the overall desulphurisation process. Maintaining the pH of the aqueous slurry in the WFGD reaction tank is of paramount importance to maintain the optimal reaction conditions for the formation and precipitation of the main WFGD by-product, namely gypsum CaSO₄. This can be achieved by matching the rate of fresh limestone addition to the reactor tank with the rate of CaCO₃ dissolution (or CaSO₄ precipitation). As limestone is a mineral resource that can consist of different constituents in varying concentrations depending on the origin of the limestone, limestone sourced from different origins can have significant differences in CaCO₃ content (quality) and therefore the reactivity with respect to WFGD processes. Limestone deposits that are in close proximity of the majority of the South African power stations generally contain limestone of relatively low quality (CaCO₃ content of between 75 and 85 %). A high-quality limestone source is however located in the Northern Cape. This limestone would have to be transported to the power station where it would be utilised, resulting in an escalated cost. It is therefore valuable to investigate the possibility of utilising sources that are closer by although the quality is lower. It is therefore prudent to study the rate of dissolution of limestone with varying quality and mineralogical composition as a function of other WFGD process parameters such as pH and reaction temperature. Consequently, four South African limestone samples were selected for this study, namely Limestone A, B, C and D. The quality of these limestone samples based on CaCO₃ results determined from XRF analysis were 95 % for Limestone A, 82 % for Limestone B, 89 % for Limestone C, and 71% for Limestone D. Additionally, these limestone samples were crushed and ground and subsequently sieved to obtain three particle size fractions, namely dp < 106 μm, dp < 75 μm and dp < 45 μm. The pH stat method was used to study the dissolution kinetics of the different limestone samples as a function of the particle size, the slurry temperature and the solution pH. A first-order semi batch model was used to model the experimental measurement results. The rate constant was determined from this model and found to range between 0.07 x 10 ̄ ⁶ and 4.09 x 10 ̄ ⁶ m/s. It was further found that the rate of dissolution did not correlate with the quality of the limestone, since Limestone D, which had the lowest quality, exhibited the highest rate of dissolution, It was found that the rate of dissolution of the Limestone samples were, in descending order: Limestone D, Limestone A, Limestone B, and Limestone C. It is therefore concluded that other factors such as particle surface area, porosity, degree of crystallinity, and the presence of the dolomite mineral and other impurities also have a significant effect. However, further study is required to elucidate the most influential parameters.