The role of MgO in the binding of SO<Subscript>2 </Subscript>by lime-containing materials

Summary The results of investigation of MgO participation in the binding of SO₂ with lime-containing materials as sorbents are presented. Experiments of SO₂ binding into solid phase using model samples of reactive grade MgO and CaO varying the mole ratio of MgO/CaO from 9:1 to 1:9 were carried out. Besides, dolomite and limestone samples with different MgO/CaO mole ratio (from 1.24 to 0.13) and samples of ashes formed at combustion of Estonian oil shale (containing 35-40% of carbonates) and its semicoke were studied Initial samples, intermediate and final products were subjected to chemical, IR-spectroscopy, X-ray and BET specific surface area analyses. The results of the present study confirmed the active participation of MgO in the binding of SO₂ into the solid phase. In addition to CaSO₄ the formation of Ca,Mg-double sulphate CaMg₃(SO₄)₄ and -MgSO₄ was observed. The presence of CaMg₃(SO₄)₄ was fixed in a large temperature range 400-900°C and that of -MgSO₄ in between 500-700°C. The optimum temperature range for formation and durability of CaMg₃(SO₄)₄ was 700-800°C.     

Reactivity of oil shale ashes in the binding of SO<Subscript>2</Subscript>

The extensive use of fossil fuels in energy production causes serious pollution of atmosphere with SO₂, CO₂, NO_x, etc. In Estonia the electricity production is based mainly on the pulverized firing (PF) of low-grade local fuel – Estonian oil shale (EOS) which is characterized by a low calorific value (~9 MJ kg^–1) and a high content of mineral matter (65–70%) from which approximately 50% are carbonates. Since 2004, also two boilers based on circulating fluidized bed combustion (CFBC) of EOS are in exploitation. The present study is focused on the comparative investigation of the efficiency of different ashes collected from different technological points of CFB and PF boilers as sorbents for SO₂. The influence of experimental temperature on the SO₂-binding characteristics of ashes as well as the possibilities of activation of ashes (grinding, hydration) were investigated. It was shown that the SO₂-binding capacity of initial ashes at 700°C and p(SO₂)=190 mm Hg was for CFBC ashes 24–30 mg and for PF ashes 10–23 mg SO₂ per 100 mg sample, the best binding capacities belonging to economizer ash (ECOA) and electrostatic precipitator ash from the 1st field (PESPA1f), respectively. However, during initial stage of binding the best results were obtained with air pre-heater ash (PHAA) and ESPA1f (both CFBC ashes). Grinding improved the SO₂-binding ability, being the most effective in the case of bottom ash (BA) from CFBC and cyclone ash (PCA) from PF – increase in binding capacity 2 and 2.3 times, respectively. As compared to initial CFBC ashes, the binding characteristics of PF ashes remained lower even after grinding. Hydration and previous calcination improved the binding characteristics only of PF ashes. Hereby, the SO₂-binding ability of CFBC ashes is better than of PF ashes and they are more promising sorbents for acidic gases, for example, for sulphur dioxide.     

Emission of Sulphur Dioxide During Thermal Treatment of Fossil Fuels

The dynamics of SO₂ emission during thermooxidation of Estonian oil shale, its semicoke, different samples of coal and their mixtures, as well as the influence of Estonian oil shale ash addition (for modelling the CFBC process) on the dynamics were studied. The experiments were carried out with thermogravimetric equipment under dynamic heating conditions (5 K min^-1) in the atmosphere of dried air, with simultaneous gastitrimetric EGA. It was established that SO₂ emission from the fuels started at 200-320°C. Depending on the form of sulphur (organic, pyritic, sulphate), the emission took place in two or three steps, and continued up to 580-650°C, during which 35-75% of the total sulphur was emitted into the gaseous phase. Regulating the mole ratio of free CaO/S in the mixtures of fuels with oil shale ash addition the emission of SO₂ ceased abruptly at 460-540°C and it was limited to the level of 7-30%. This revised version was published online in August 2006 with corrections to the Cover Date.     

SO2 binding into the solid phase during thermooxidation of blendsestonian oil shale semicoke

Approximately one million tons of semicoke (SC) is formed and stored in open air dumps every year in the production of shale oil by processing Estonian oil shale (OS). The content of different harmful compounds as sulphides, PAH, phenols, etc. in SC make these dumps one of the most serious sources of environmental contamination. The aim of this work was to study the behaviour of sulphur compounds in OS and its SC, formation of SO₂ and possibilities of binding it into the solid phase during thermooxidation of fuel blends based on SC. Blends modified with SC ash addition were studied as well. It was determined that SO₂ emission in thermooxidation of SC samples started at 280-300°C and proceeded with a steady speed up to 580-600°C and the amount of sulphur evolved was 5-10% from the total content of sulphur in the sample. The amount of SO₂ emitted decreased depending on the mass ratio of the composite fuels from 49-56 to 15-35% during thermooxidation of OS samples studied or their blends with SC, respectively, from 43-80% for coal samples to 13-60% for their blends with SC and to 2-13% during thermooxidation of these blends modified with SC ash addition. In the products of thermooxidation formed at 800-900°C the only sulphur containing phase was CaSO₄, at 650°C also traces of CaS and CaMg3(SO₄)₄ were fixed. This revised version was published online in July 2006 with corrections to the Cover Date.     

The isothermal sulphation reactions of natural sorbents

The chemical reaction between sulphur dioxide and calcined limestone and dolomite particles was investigated using a thermogravimetric analyser. The samples used in this study originated from different parts of Turkey. Sulphation reactions were conducted under isothermal conditions in a gaseous mixture consisting of 15 vol% CO₂, 0.35 vol% SO₂ and a balance of dry air by volume. Before sulphation all samples were completely calcined in a gaseous atmosphere of 15 vol% CO₂ and 85 vol% dry air. It was observed that the sulphation reactions of samples are very rapid in their initial stage but after some reaction time, which varies depending on sample properties and temperature, the reaction rates quickly slow down. Also, the conversion-time results of sulphation experiments showed variations depending on the sulphation temperature, sorbent type and the physical properties of calcines.     

Catalytic sulphation of limestone/lime with platinum

The effects of the presence of a Pt catalyst on the limestone/lime sulphation process were investigated by thermal analysis methods to provide a better understanding of the factors limiting gas desulphurization when Ca-based sorbents are used. It was found that for the Pt-catalysed sulphation of precalcined limestone the weight increase is above 100% higher under isothermal and dynamic conditions (up to 830°C). These results are direct evidence that Pt catalyses the CaO-SO₂-O₂ reaction. It can be presumed that the process proceeds through a gaseous intermediate, SO₃, a highly reactive gas, which explains the increased rate of sulphation. SO₃ then reacts with CaO to form CaSO₄ directly, in contrast with the non-catalysed oxidation of SO₂ to SO₃, where CaSO₃ formation is the most probable early stage of sulphation. The proposed mechanisms were supported by the phase identification of the products.     

CO<Subscript>2</Subscript> and SO<Subscript>2</Subscript> uptake by oil shale ashes

In the present research, CO₂ and SO₂ binding ability of different oil shale ashes and the effect of pre-treatment (grinding, preceding calcination) of these ashes on their binding properties and kinetics was studied using thermogravimetric, SEM, X-ray, and energy dispersive X-ray analysis methods. It was shown that at 700 °C, 0.03–0.28 mmol of CO₂ or 0.16–0.47 mmol of SO₂ was bound by 100 mg of ash in 30 min. Pre-treatment conditions influenced remarkably binding parameters. Grinding decreased CO₂ binding capacities, but enhanced SO₂ binding in the case of fluidized bed ashes. Grinding of pulverized firing ashes increased binding parameters with both gases. Calcination at higher temperatures decreased binding parameters of both types of ashes with both gases studied. Clarification of this phenomenon was given. Kinetic analysis of the binding process was carried out, mechanism of the reactions and respective kinetic constants were determined. It was shown that the binding process with both gases was controlled by diffusion. Activation energies in the temperature interval of 500–700 °C for CO₂ binding with circulating fluidized bed combustion ashes were in the range of 48–82 kJ mol⁻¹, for SO₂ binding 43–107 kJ mol⁻¹. The effect of pre-treatment on the kinetic parameters was estimated.     

Sulfation behavior of CaO from long-term carbonation/calcination cycles for CO2 capture at FBC temperatures

In this work, SO₂ capture behavior of CaO derived from the dolomite and the limestone during long-term carbonation/calcination cycles for CO₂ capture at fluidized bed combustion (FBC) temperatures was investigated. The cyclic carbonation/calcination of CaO was performed in a dual fixed-bed reactor and then the cycled CaO was sent for sulfation in a thermo-gravimetric analyzer. At the typical FBC temperatures (850–950 °C), SO₂ capture capacity of CaO from the different carbonation/calcination cycles increases with the increasing the temperature. The sulfation conversion of CaO derived from the dolomite (CaO-dolomite) decreases as the carbonation/calcination cycle number increases from 0 to 200. Although the sulfation conversion of CaO derived from the limestone (CaO-limestone) decreases with increasing the cycle number from 0 to 40, its conversion does not always decay with the number of cycles. The sulfation conversion of CaO-limestone shows a slight increase with increasing the cycle number from 40 to 150 and then exhibits a decrease with increasing the cycle number further. The sulfation conversions of CaO-limestone after different cycles are determined by the specific surface area and the volume of macropores >0.2 μm in diameter. The particle size and SO₂ concentration have important effect on sulfation behavior of CaO from various cycles.     

TG-FTIR study of gaseous compounds evolved at thermooxidation of oil shale

The combined thermogravimetric (TG) Fourier transform infrared (FTIR) techniques were used for studying the gaseous compounds evolved at thermooxidation of oil shale samples from different deposits (Estonia, Jordan, Israel). In addition to H₂O and CO₂as the major species, the formation and emission of CO, SO₂, HCl and a number of organic species as methane, ethane, ethylene, methanol, formic acid, formaldehyde, chlorobenzene, etc. was determined. Differences in the absorbance of respective bands in FTIR spectra depending on the origin of oil shale and on the heating rate used were established. This revised version was published online in July 2006 with corrections to the Cover Date.     

SO2 retention by highly cycled modified CaO-based sorbent in calcium looping process

The limestone modified by pyroligneous acid has been proved to have good CO₂ capture behavior in the calcium looping process. In this work, SO₂ retention of the highly cycled modified limestone in the carbonation/calcination cycles was investigated in a thermogravimetric analyzer (TG). The cyclic carbonation/calcination of the modified limestone was performed in a dual fixed-bed reactor and then the cycled modified limestone was sent for sulfation in TG. The effects of sulfation temperature, cycle number, and prolonged carbonation on SO₂ retention of the cycled modified limestone were discussed. The optimum temperature for sulfation of the cycled modified limestone should be in the range of 900–950 °C. The effect of sulfation temperature on SO₂ retention of the modified limestone drops with increasing cycle number. With increasing cycle number from 20 to 100, the sulfation conversion of the cycled modified limestone is stable and can reach ~0.4. The cycled modified limestone exhibits obviously higher SO₂ retention than the cycled raw one for the same number of cycles. The prolonged carbonation increases SO₂ retention of the modified limestone and the raw one after the subsequent cycles. The sulfation conversions of the modified limestone and the raw one at 118 min after 9-h carbonation in the 20th cycle increase 43 and 56 %, respectively. The cycled modified limestone shows a greater SO₂ retention than the cycled raw one after the same prolonged carbonation treatment. The prolonged carbonation increases the pores in 5–20 nm range which is considered the optimum pore size for sulfation of CaO-based sorbent, so it results in an improvement in SO₂ retention of the cycled sorbents.     

Physico-Chemistry of the Limestone Sulphation Process

Aspects of the mechanism of the overall reaction between CaCO₃/CaO and SO₂/SO₃ under oxidizing conditions are discussed. The limestone and lime sulphation processes were carried out in a thermobalance under conditions relevant to atmospheric fluidized bed combustion. Sulphated samples, prepared in the form of cross-section particles, were examined in a scanning electron microscope by energy-dispersive X-ray and back-scattered electron imaging. Photomicrographs are presented. The reaction proceeded from the outer surface of the particles and along the pores. Surface textural changes during the reaction were considered. The layer of products was identified as controlling both the rate and extent of limestone/lime sulphation. In the products, two sulphur-bearing solids (CaSO₄ and CaS) were identified. The presence of CaS, which may cause difficulties in practice, is attributed to CaSO₃ disproportionation. This revised version was published online in August 2006 with corrections to the Cover Date.     

Application of thermoanalytical methods to the study of limestone sulphation

The role that can be played in the elucidation of the limestone sulphation mechanism by thermal analysis methods with some specific procedures is discussed. Contrasting examples of applications of thermoanalytical techniques using the variable conditions are provided. These examples deal with the programmed thermal analysis using different gas sequences, the influence the calcination and sulphation conditions on the capture of SO₂, the effect of catalysts on limestone sulphation and the thermal stability of CaSO₃. Two proposed mechanisms were supported by the phase identification of the solid products.     

The effect of sintering on the maximum capture efficiency of CO<Subscript>2</Subscript> using a carbonation/calcination cycle of carbonate rocks

The effect of sintering on the maximum capture efficiency of CO₂ is studied, using a carbonation/calcination cycle for a series of samples with different stoichiometries of dolomite and calcite. For the materials that belong to the categories of limestone and dolomitic limestone, sintering decreases the extent of carbonation significantly at the two different highest temperatures studied. The extent of carbonation for the same maximum heating temperature depends mainly on the percentage of dolomite. Sintering is negligible in the dolomitic rocks, especially at the maximum heating temperature of 1005°C. The composition of the carrier gas does not seem to play a significant role. The reduction of the extent of carbonation at the second heating /cooling cycle in limestone, and the durability after enough successive cycles of calcination/carbonation in the dolomitic rocks, does not seem to be affected by the maximum temperatures of calcination that were used at the experiments.     

Influence of lime-containing additives on the thermal behaviour of ammonium nitrate

Ammonium nitrate (AN) is one of the main nitrogen fertilizers used in fertilization programs. However, AN has some serious disadvantages — being well soluble in water hardly 50% of the N-species contained are assimilated by plants. The second disadvantage of AN is associated with its explosive properties. The aim of this paper was to clarify the influence of different lime-containing substances — mainly Estonian limestone and dolomite — as internal additives on thermal behaviour of AN. Commercial fertilizer grade AN was under investigation. The amount of additives used was 5, 10 or 20 mass%, or calculated on the mole ratio of AN/(CaO, MgO)=2:1 in the blends. Experiments were carried out under dynamic heating condition up to 900°C (10°C min⁻¹) in a stream of dry air or N₂ by using Setaram Labsys 2000 equipment coupled to Fourier transform infrared spectrometer (FTIR). The results of analyses of the gaseous compounds evolved at thermal treatment of neat AN indicated some differences in the decomposition of AN in air or in N₂. At the thermal treatment of AN’s blends with CaCO₃, MgCO₃, limestone and dolomite samples the decomposition of AN proceeds through a completely different mechanism — depending on the origin and the content of additives, partially or completely, through the formation of Mg(NO₃)₂ and Ca(NO₃)₂.     

Influence of the post-granulation treatment on the thermal behaviour and leachability characteristics of Estonian oil shale ashes

The power and heat production in Estonia is based over 90% on the combustion of a local solid fossil fuel—Estonian oil shale (OS), and at that 7–8 million tons of OS ash are formed annually. One promising possibility for large-scale utilization of cheap alkali ashes is the liming of acidic soils. In Estonia, there is 350,000 ha of agricultural land that needs permanent liming. To eliminate possible environmental contamination at liming of soils the oil shale ashes should be granulated. Thermal analysis was used for determination of the relationships between physico-mechanical and physico-chemical characteristics of granulated products and the post-granulation treatment conditions. For determination of leachability of ash components, the granulated products as well as the origin ashes were tested using laboratory lysimetries. Depending on OS ash and different post-granulation treatment used, it was possible to obtain granulated product with compressive strength between 5 and 15 N per granule. The leaching of Ca²⁺ decreased up to 26 and 34%, SO₄ ^2? 70 and 53%, Mg²⁺ and K⁺ up to 7–12% for granulated CA and ESPA, respectively, comparing with original ashes. The results of soil analysis indicate that the use of OS ashes improved the pH level of soil significantly. pH increased equally with initial and granulated ash: from 4.7 up to 6.4 and 5.8, respectively, using for that CA and ESPA. Prolonged effect of soil neutralizing ability by granulated product (if compare with fine ashes) was proven by decrease in the content of leached ions in filtrate solutions as well as not mobile ions in soil.     

Heating rate effect on the thermal behavior of ammonium nitrate and its blends with limestone and dolomite

The effect of heating rate on the thermal behavior of ammonium nitrate (AN) and on the kinetic parameters of decomposition of AN and its blends with limestone and dolomite was studied on the basis of commercial fertilizer-grade AN and several Estonian limestone and dolomite samples. Experiments were carried out under dynamic heating conditions up to 900 °C at heating rates of 2, 5, 10 and 20 °C min⁻¹ in a stream of dry air using Setaram Labsys 2000 equipment. For calculation of kinetic parameters, the TG data were processed by differential isoconversional method of Friedman. The variation of the value of activation energy E along the reaction progress α showed a complex character of decomposition of AN—interaction of AN with limestone and dolomite additives with the formation of nitrates as well as decomposition of these nitrates at higher temperatures.     

Thermal behavior of some Estonian clays and their mixtures with oil shale ash additives

Thermal behavior of green clay samples from Kunda and Arumetsa deposits (Estonia) as potential raw materials for production of ceramics and the influence of previously fired clay and hydrated oil shale ash additives on it were the objectives of this research. Two different ashes were used as additives: the electrostatic precipitator ash from the first field and the cyclone ash formed, respectively, at circulating fluidized bed combustion (temperatures 750–830 °C) and pulverized firing (temperatures 1,200–1,400 °C) of Estonian oil shale at Estonian Power Plant. The experiments on a Setaram Labsys Evo 1600 thermoanalyzer coupled with Pfeiffer OmniStar Mass Spectrometer by a heated transfer line were carried out under non-isothermal conditions up to 1,050 °C at the heating rate of 5 °C min^?1 in an oxidizing atmosphere containing 79 % of Ar and 21 % of O₂. Standard 100 µL Pt crucibles were used, the mass of samples was 50 ± 0.5 mg, and the gas flow 60 mL min^?1. The results obtained indicate the complex character of transformations and show certain differences in the thermal behavior of Arumetsa and Kunda clays and their mixtures with oil shale ashes depending on the chemical and mineralogical composition of the clays as well as of the oil shale ashes studied.     

Thermal behaviour of ammonium nitrate prills coated with limestone and dolomite powder

The thermal behaviour of ammonium nitrate (AN) and its prills coated with limestone and dolomite powder was studied on the basis of commercial fertilizer-grade AN and six Estonian limestone and dolomite samples. Coating of AN prills was carried out on a plate granulator and a saturated solution of AN was used as a binding agent. The mass of AN prills and coating material was calculated based on the mole ratio of AN/(CaO + MgO) = 2:1. Thermal behaviour of AN and its coated prills was studied using combined TG-DTA-FTIR equipment. The experiments were carried out under dynamic heating conditions up to 900 °C at the heating rate of 10 °C min⁻¹ and for calculation of kinetic parameters, additionally, at 2, 5 and 20 °C min⁻¹ in a stream of dry air. A model-free kinetic analysis approach based on the differential isoconversional method of Friedman was used to calculate the kinetic parameters. The results of TG-DTA-FTIR analyses and the variation of the value of activation energy E along the reaction progress α indicate the complex character of the decomposition of neat AN as well as of the interactions occurring at thermal treatment of AN prills coated with limestone and dolomite powder.     

Sulfation behavior of white mud from paper manufacture as SO<Subscript>2</Subscript> sorbent at fluidized bed combustion temperatures

The calcination characteristics, sulfation conversion, and sulfation kinetics of a white mud from paper manufacture at fluidized bed combustion temperatures were investigated in a thermogravimetric analyzer. Also, the comparison between the white mud and the limestone in sulfation behavior and microstructure was made. Although the white mud and the limestone both contain lots of CaCO₃, they are different in the alkali metal ions content and microstructure. It results in a marked difference in sulfation behavior between the white mud and the limestone. The CaO derived from white mud achieves the maximum sulfation conversion of 83% at about 940 °C which is 1.7 times higher than that derived from limestone at about 880 °C. The shrinking unreacted core model is appropriate to analyze the sulfation kinetics of the white mud. The chemical reaction activation energy E _a and the activation energy for product layer diffusion E _p for the sulfation of the white mud are 44.94 and 55.61 kJ mol⁻¹, respectively. E _p for the limestone is 2.8 times greater than that for the white mud. The calcined white mud possesses higher surface area than the calcined limestone. Moreover, the calcined white mud has more abundant pores in 4–24 nm range which is almost optimum pore size for sulfation. It indicates that the microstructure of the white mud is beneficial for SO₂ removal.     

Pt/SO42??ZrO2: The state of platinum and its relation with catalytic activity inn-hexane isomerization

Pt/SO₄ ²⁻−ZrO₂ calcined at 873 K shows the same catalytic activity forn-hexane isomerization as the calcined and reduced sample. A platinum reduction peak did not appear in the TPR profile and the presence of Pt⁰ was detected by XPS on the only calcined Pt/SO₄ ²⁻−ZrO₂. Nevertheless, this calcined material does not show hydrogen chemisorption and cyclohexane dehydrogenation activity.