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In the future, cement clinker formation is likely to take place in high temperatures and high carbon dioxide atmospheres in carbon-neutral production processes as part of, for example, electrified processes. The aim of this study was thus to compare the volatilisation of minor and trace elements during cement clinker formation in a high carbon dioxide atmosphere and a conventional combustion atmosphere. Raw meal samples were exposed, at high temperature, to the two different atmospheres, with elemental analysis performed before and after. For both atmospheres, the minor elements potassium and sulfur, and the trace elements rubidium, lead, thallium, caesium, cadmium and mercury were highly volatile. For most of the analysed elements, no difference was observed between the two atmospheres. However, volatilisation of potassium, sodium and sulfur was lower in the high carbon dioxide atmosphere. It is suggested that this should be further studied in relation to the molar ratio of sulfur to alkalis in the clinker and the effect on clinker quality.

Quicklime, rich in CaO(s), is generated by calcining limestone at high temperatures. Parallel-flow regenerative lime kilns are the most energy-effective industrial method available today. To prevent major disruptions in such kilns, a high raw material quality is necessary. Under some conditions, impurity-enriched material may adhere to limestone pebbles and enter the kiln. In this study, limestone and corresponding quicklime were analyzed to evaluate the extent and composition of surface impurities and assess the effect on quicklime product quality, here defined as free CaO. This was performed by sampling and analyzing limestone, quarry clay, laboratory-produced quicklime, and industrially produced quicklime with XRF, SEM/EDX, and XRD; interpretations were supported by thermodynamic equilibrium calculations. In the laboratory-produced quicklime, the surface impurities reacted with calcium forming Larnite, Gehlenite, Åkermanite and Merwinite, reducing the quicklime quality. The results showed that the limestone surface layer comprised 1.2 wt.-% of the total mass but possessed 4 wt.-% of the total impurities. The effect on industrially produced quicklime quality was lower; this indicated that the limestone surface impurities were removed while the material moved through the kiln. Multicomponent chemical equilibrium calculations showed that the quarry clay was expected to be fully melted at 1170 °C, possibly leading to operational problems.

The production of quicklime and cement clinker are energy-intensive processes resulting in significant CO₂ emissions. Fuel switching, electrified heating, and carbon capture have gained attention as means of addressing this. Conventional production processes are direct-fired, meaning that the impurities, which originate from either quarries or fuels, interact with the product, influencing process performance and product quality. The suggested strategies for addressing CO₂ emissions will alter the process conditions. For example, introducing electrified heating using plasma would shift the process atmosphere to primarily CO₂, possibly affecting volatilisation and recirculation. The overall aim of this thesis was to generate new knowledge on the impact of impurities under process conditions in the context of the shift towards more sustainable quicklime and cement clinker production. 

Limestone surface impurities and their effects on quicklime product quality were evaluated. Ash-quicklime interactions were studied both on a laboratory scale and using multicomponent chemical equilibrium calculations (MECs). The volatilisation of minor elements in cement clinker production was investigated on a laboratory scale, and using a counter-current MEC-model with both a conventional combustion atmosphere and high-CO₂ atmosphere. 

The detailed analysis of the limestone surface layer showed enrichment of impurities. However, quicklime sampled from a parallel flow generative kiln (PFR) showed low amounts of reactants from surface impurities, which were suggested to contribute to build-ups and increased levels of lime-kiln dust instead. Laboratory-scale studies of coal ash and quicklime interactions and MECs showed that typical cement clinker phases are thermodynamically stable at the coal ash-quicklime interface. Porosity and pore-size distribution were evaluated in pure quicklime samples and quicklime samples exposed to olive pomace, pine bark, and wheat straw ash. Olive pomace ash affected quicklime microstructure severely by increasing porosity and pore size. The laboratory study on the volatilisation of minor and trace elements in cement clinker formation showed higher retention of K, Na, and S in a high CO₂ atmosphere, likely explained by low H₂O partial pressure and high CO₂ partial pressure. Counter-current MECs showed lower enrichment of K, Na, and S in a high CO₂ atmosphere.

Future work is suggested to investigate the fate of surface impurities entering industrial PFR kilns. Further, the effect of biomass ash on quicklime microstructure should be evaluated in a complete combustion atmosphere, as should the effects of the rolling bed and recirculation of volatile elements in the rotary kiln. The effects of an altered process atmosphere on cement clinker quality and the volatilisation of minor and trace elements are interesting topics for further studies, e.g. in a pilot-scale rotary kiln.

Quicklime is produced through the thermal processing of limestone in industrial kilns. During quarry operations, fine particulate quarry dust adheres to limestone lump surfaces, increasing the bulk concentration of impurities in limestone products. During thermal processing in a kiln, impurities such as Si, Mg, Al, Fe, and Mn react with Ca, reducing quicklime product quality. Which reactant phases are formed, and the extent to which these result in a reduction in quality, has not been extensively investigated. The present study investigated as-received and manually washed limestone product samples from two operational quarries using elemental compositions and a developed predictive multi-component chemical equilibrium model to obtain global phase diagrams for 1000–1500 °C, corresponding to the high-temperature zone of a lime kiln, identifying phases expected to be formed in quicklime during thermal processing. The results suggest that impurities found on the surface of the lime kiln limestone feed reduce the main quality parameter of the quicklime products, i.e., calcium oxide, CaO (s), content by 0.8–1.5 wt.% for the investigated materials. The results also show that, in addition to the effect of impurities, the quantity of CaO (s) varies greatly with temperature. More impurities result in more variation and a greater need for accurate temperature control of the kiln, where keeping the temperature below approximately 1300 °C, that of Hatrurite formation, is necessary for a product with higher CaO (s).

Industrially produced quicklime from a coal-fired rotary kiln was analyzed and compared with laboratory-scale studies of ash-quicklime interaction using i) two coal ashes ii) a coal-olive pomace ash mixture iii) olive pomace ash, and iv) a case without any ash representing an electrically heated process. Multicomponent equilibrium calculations were performed in order to predict ash melting behavior.

The industrially produced quicklime showed intrusion of mainly Al and Si up to 800 μm into the quicklime in accordance with the expected ash composition of the coal fuel used.

Laboratory-scale studies were performed at 1,100°C and 1,350°C followed by SEM-EDX analyses of the ash-quicklime interface to determine changes in quicklime microstructure and depth of ash infiltration.

At 1,100°C, coal ashes appeared solid and no distinctive microstructure differences of quicklime was observed underneath the ash-quicklime interface. At 1,350°C, the ashes appeared molten resulting in densification of the quicklime microstructure.

Potassium-rich olive pomace ash resulted in a coarsening of the quicklime microstructure, most obvious at 1,350 °C and probably as a result of a carbonate melt intrusion. Coal-olive pomace ash mixture resulted in a less severe quicklime microstructure coarsening. Without ash, the quicklime showed enhanced densification at the higher temperature. The results show that the different elemental compositions in biomass ash and coal ash results in different melting behavior, impurities and quicklime microstructure. This is important knowledge in the transition to a more sustainable quicklime industry.

The effect on quicklime microstructure seen in this study is not necessarily translatable to industrial scale and more studies are needed, for example by introducing the complete fuels resulting in a flue gas atmosphere more comparable to industrial conditions.

Net CO2 emissions from the production of quicklime can be reduced by introducing renewable solid fuels or sustainably produced electricity for heating of the process. This paper reports the results of a study examining the effects of new heat sources on quicklime surface reaction products and on quicklime microstructure. Limestone was heated to 1100 °C and 1350 °C in high CO2 atmosphere under three conditions: i) an ash mixture representing conventional coal and a solid biofuel (olive pomace); ii) olive pomace ash, and iii) no ash representing an electrically heated process. The ash-quicklime interfaces of the samples were analyzed for elemental composition and microstructure using SEM-EDX. Multi-component chemical equilibrium calculations were used to assess the stable chemical phases in the interface. Coal-olive pomace ash mixture resulted in coarsening of the quicklime microstructure; this effect was less severe compared to that of pure olive pomace ash. The calculations indicated that the potassium in olive pomace ash was bound to Si- and Al-rich coal ash phases. Exposure to potassium-rich olive pomace ash resulted in severe coarsening of the quicklime microstructure. The difference was most obvious at 1,350 °C, and was probably the result of intrusion of a potassium-rich salt melt. For limestone without ash, the quicklime showed enhanced sintering and reduced porosity at the higher temperature, in agreement with previous studies. Interface reactions and microstructure coarsening, here most apparent for the case with olive pomace, could be problematic in industrial quicklime production since they may contribute to decreased available CaO and reactivity.

This paper reports on results from detailed studies on coal ash and limestone interactions during calcination. Industrially produced quicklime from a coal-fired rotary kiln was analyzed and compared with laboratory-scale studies of surface interactions between two coal ashes and limestone. Exposure tests were performed at 1,100 °C and 1,350 °C, in a high CO2 atmosphere. SEM-EDX analyses of the ash-quicklime interface were performed to detect and quantify changes in microstructure, as well as the depth of ash interaction into quicklime. Stable phases in the ash-quicklime interface were assessed by multi-component chemical equilibrium calculations based on local EDX analysis. The industrially produced quicklime showed intrusion by extraneous elements, mainly Al and Si, up to 800 µm into the quicklime, in accordance with expected ash composition, based on the ash analysis of coal fuel used. In laboratory-scale and 1,100 °C, ashes appeared solid to a large extent, and no distinctive microstructure difference of quicklime was observed underneath the ash-quicklime interface. At 1,350 °C, the ashes appeared molten to a large extent, and the quicklime microstructure was affected compared to at 1,100 °C, resulting in densification. For both temperatures and both coal ashes, the interface reactions reduced the amount of reactive CaO, thereby resulting in a decrease in product quality. The laboratory methodology was shown to be useful to increase mechanistic understanding of the ash-quicklime interactions. The method could be expanded to test other limestone qualities and fuels, e.g. renewable biofuels.