Patrik Yrjas

Limestone and dolomite as sulfur absorbents under pressurized gasification conditions

Abstract The capture of H2S by both limestone and dolomite was studied with a pressurized thermogravimetric apparatus. The experimental conditions were chosen to correspond to the conditions typical in a pressurized fluidized bed gasifier. Depending on these conditions the limestone reacts either uncalcined (CaCO3) or calcined (CaO), and the dolomite either half-calcined (CaCO3 + MgO) or fully calcined (CaO + MgO), with H2S. The reaction product is calcium sulfide (CaS). MgO does not react with H2S under these circumstances, due to thermodynamic limitations. The H2S absorption capacities of three dolomites and two limestones of particle size 125–180 μm were compared at 950°C and 2 MPa. The capture of H2S by calcined limestone and fully calcined dolomite was fast and 80–90 wt% of the Ca was converted to CaS. H2S capture by uncalcined limestone was slow and after 2 h no more than 20 wt% of the Ca had reacted to CaS. Half-calcined dolomite was significantly more efficient than uncalcined limestone and 60–80 wt% of the Ca was converted to CaS within 60 min. Experiments were also carried out at lower temperatures. For calcined limestone and fully calcined dolomite the reaction rate was still too fast for kinetic studies at 750°C. However, for uncalcined limestone and half-calcined dolomite the reaction rate clearly decreased at lower temperatures. The observed activation energy for uncalcined limestone was ∼ 100 kJ mol−1 and for half-calcined dolomite 300–400 kJ mol−1.

Comparison of SO2 capture capacities of limestones and dolomites under pressure

Abstract The sulfur capture capacities of 11 limestones and six dolomites were determined by means of a pressurized thermogravimetric analyser. The determinations were made under conditions relevant to pressurized fluidized bed combustion. The actual experiments were performed at two different temperatures (850 and 950°C) and at a pressure of 1.5 MPa. Additionally, some experiments were performed at atmospheric pressure for comparison. The particle size of the samples was screened to 200–400 μm. By using a thin (1 mm) sample layer diluted with an inert material (quartz sand), the external mass transfer and the interparticle diffusion problems were excluded. For each experiment the conversion versus time curve was determined. The results showed great variations between different sorbent qualities. Conversions between 7 and 83% were measured. The sulfur absorption capacity order was approximately the same under both atmospheric and pressurized conditions. Higher temperature resulted in considerably higher conversion. The temperature effect differed between the absorbents and was clearly more pronounced for the dolomites than for the limestones. This resulted in one absorbent being more efficient at 850°C, while the other absorbent was more efficient at 950°C.

Chlorine in Deposits During Co-Firing of Biomass, Peat, and Coal in a Full-Scale CFBC Boiler

Co-combustion of coal with biomass or firing biomass alone is increasingly used as a first step to meet the Finnish commitments under the Kyoto agreement. Fluidized bed combustors are commonly used when co-firing, however, even if FBC’s have a wide tolerance for different fuel qualities, co-combustion of biomass or firing biomass alone may lead to unwanted ash-related problems. A deposit measurement campaign was done, at the 550 MWth biofuelled CFB in Jakobstad, Finland. During the campaign a total of 16 different fuel blends were burned. The deposits were sampled with air-cooled probes with detachable rings. The deposits were sampled at two different locations, one where the flue gas temperature was about 730°C (probe surface temp. 540°C) and the second where the flue gas temperature was about 530°C (probe surface temp. 350°C). From every deposit sample three elemental analyses were done — one from the wind side, one from the lee side, and one from an angle of about 50° from the wind side. The analyses were done with a SEM/EDX analyzer. The fuels used during the measurement campaigns were sampled and analyzed. In addition to proximate and ultimate fuel analysis so called fuel fractionation was applied. The fractionation method is based on selective leaching by water, ammonium acetate, and hydrochloric acid, consecutively. After each leaching step the solutions are analyzed for the most important elements. The method can be used to determine how the elements are bound in the fuel and how they may behave during combustion. The analysis results from the measurement campaign and from the advanced fuel analysis were combined and are reported in this paper, with emphasis on the fate of chlorine.Copyright

Cyclic Carbonation Calcination Studies of Limestone and Dolomite for CO2 Separation From Combustion Flue Gases

Naturally occurring limestone and dolomite samples, originating from different geographical locations, were tested as potential sorbents for carbonation/calcination based CO2 capture from combustion flue gases. Samples have been studied in a thermogravimetric analyzer under simulated flue gas conditions at three calcination temperatures,viz., 750textdegreeC, 875textdegreeC, and 930textdegreeC for four carbonation calcination reaction (CCR) cycles. The dolomite sample exhibited the highest rate of carbonation than the tested limestones.At the third cycle, its CO2 capture capacity per kilogram of the sample was nearly equal to that of Gotland, the highest reacting limestone tested. At the fourth cycle it surpassed Gotland, despite the fact that the CaCO3 content of the Sibbo dolomite was only 2/3 of that of the Gotland. Decay coefficients were calculated by a curve fitting exercise and its value is lowest for the Sibbo dolomite. That means, most probably its capture capacity per kilogram of the sample would remain higher, well beyond the fourth cycle. There was a strong correlation between the calcination temperature, the specific surface area of the calcined samples, and the degree of carbonation. It was observed that the higher the calcination temperature, the lower the sorbent reactivity. The Brunauer?Emmett?Teller measurements and scanning electron microscope images provided quantitative and qualitative evidences to prove this. For a given limestone/dolomite sample, sorbent?s CO2 capture capacity depended on the number of CCR cycles and the calcination temperature.In a CCR loop, if the sorbent is utilized only for a certain small number of cycles 20, the CO2 capture capacity could be increased by lowering the calcination temperature.According to the equilibrium thermodynamics, the CO2 partial pressure in the calciner should be lowered to lower the calcination temperature. This can be achieved by additional steam supply into the calciner. Steam could then be condensed in an external condenser to single out the CO2 stream from the exit gas mixture of the calciner. A calciner design based on this concept is illustrated.

Co-Firing in FBC: A Challenge for Fuel Characterization and Modeling

Co-combustion of coal with biomass or firing biomass alone is used more and more in a first step in meeting the Finnish commitments under the Kyoto protocol. A frequently used technique for firing mixtures of fuels is fluidized bed combustion (FBC). Firing coal, co-combustion with biomass or firing biomass alone may, however, lead to unwanted ash-related problems. Prediction of ash formation behavior can help to avoid these problems before taking new fuels into use. Standard fuel analyses have shown to provide insufficient information for proper prediction especially when considering fuel mixtures. In an attempt to minimize the number of lab scale and pilot scale combustion experiments an extensive database is under development. This database contains data used as input for prediction models such as standard fuel analyses, results from stepwise leaching experiments, SEM/EDS analyses of original and partly burned-out fuels and thermodynamic estimations of the melting behavior of the fuels. Today the database contains 51 fuels, i.e. 8 bark fuels, 10 wood fuels, 3 annual biomasses, 8 peats, 6 coals and 16 miscellaneous fuels, such as RDF, sludge, hulls and husks, bagasse and other residues. Standardized fuel analysis is available for all fuels; melting calculations have been carried out for some 33 fuels. SEM/EDS analysis has been carried out for 20 fuels. The extended utilization of these data with computational fluid dynamic modeling (CFD) has proven to be a useful tool in prediction of deposits in FBC boilers. An example of the prediction tool shows the ability of deposit formation prediction.Copyright

Product layer diffusion in the sulphation of calcium carbonate

In pressurised fluidised bed combustion, sulphur dioxide is bound directly to uncalcined limestone. Diffusion through the calcium sulphate product layer has been found to be important in limiting the sulphation rate. The product layer diffusion controlled regime of the sulphation was studied in a pressurised thermogravimetric apparatus. The apparent activation energy of the product layer diffusion has been earlier reported to be high, of the order of 100–200 kJ/mol. The purpose of the work was to study more closely the cause of the high observed apparent activation energy. Experiments were made to decouple the effect of temperature on the diffusion process itself from the possible effect of the temperature on the type and structure of the product layer. The high values of the apparent activation energy were found to be due to the diffusion process itself. The diffusion process was dependent on the gas phase SO2 concentration raised to the power of 0,49±0,04. Both of these results indicate that the diffusion through the product layer occurs via solid state diffusion. Scanning electron microscopy photographs taken of partly sulphated limestone particles supported the assumption that the sulphation of uncalcined limestone proceeds as a shrinking core process.

Studies on the Partial Reactions Between Potassium Chloride and Metallic Chromium Concerning Corrosion at Elevated Temperatures

Recovery of energy from biomass by combustion has become important due to reduction of detrimental CO2 emissions. It has been suggested that the reaction between KCl released during combustion and the protective Cr2O3-layer is the one responsible for starting the complex series of corrosion reactions. In this work, the overall reaction between KCl and Cr was studied through reactions with compounds such as Cr2O3, K2CrO4, and K2Cr2O7 known to participate in the overall reaction or to be formed during it. The reactions were studied in synthetic air with a DTA/TGA apparatus. Under the conditions studied, both KCl and K2CrO4 reacted with pure, metallic Cr as well as with Cr2O3. In the case of Cr, Cr2O3 was formed via the formation of K2CrO4. In reactions including Cr2O3 as reactant also K2Cr2O7 was detected. However, when used as a reagent, K2Cr2O7 reacted with neither Cr nor Cr2O3.

Co-Firing of Sewage Sludge with Bark in A Bench-Scale Bubbling Fluidized BED — A Study of Deposits and Emissions

It has been shown that addition of either sulfur and/or aluminosilicates such as kaolinite may reduce alkali induced deposit formation when firing biomass fuels. Sewage sludge is a fuel containing substantial amounts of sulfur and aluminosilicates, such as zeolites. In this work different amounts of sewage sludge (0, 2, 4, 6 and 8%en) were co-fired with bark in a bench-scale BFB. SO2 and HCl emissions were measured and deposits were sampled during 3 hrs with an air-cooled probe with a surface temperature of 500°C at two different locations with flue gas temperatures of 850°C and 650°C, respectively. The test results showed that an increase of the share of sewage sludge to the fuel mixture increased theformation of HCl and simultaneously decreased the Cl-content in the deposits. Usually this is considered to be a sign of sulfation of alkali chlorides. However, the increase of HCl canalso be caused by AI-silicates capturing alkali, thus releasing Cl as HCl to the gas phase. AIthough, sulfur increased in the fuel input with an increased share of sewage sludge, this was not reflected in the gaseous emissions as may be expected. Up to 4%en sewage sludge was fired together with bark without increasing the sulfur content in theemissions. At higher shares of sewage sludge the sulfur emissions increased linearly with an increase of sewage sludge. The amount of water soluble potassium fed into the boiler remained relatively constant in the different tests. This potassium is usually released as volatile salts. Nevertheless, the amount found in deposits decreased with an increase in sludge feeding. In this paper it was shown that interaction of potassium with AI-silicates in the bed is a probable cause for the decrease of potassium in the deposits, while both the sulfation of potassium chlorides and possibly also, the alkali capture by AI-silicates can weaken the deposition of Cl.

The Effect of Pretreatment on the Corrosion Resistance of Superheater Materials

In order to improve the power production efficiency of biomass-fired boilers, power plants must be operated at higher steam temperatures than nowadays. One of the main factors hindering the rise of the steam temperatures is the corrosive nature of the flue gases and fly ash towards the superheaters. In this study, the high-temperature corrosion resistance of three commercial superheater steels exposed to potassium chloride was compared. The focus was on the effect of pre-oxidation on the protective properties of different steels, whereupon various variables were used during the pre-oxidation.

Detailed Studies on the High Temperature Corrosion Reactions between Potassium Chloride and Metallic Chromium

Recovery of energy from biomass and various waste–derived fuels by combustion has become important due to reduction of detrimental CO2 emissions. Biomass does, however, release significant amounts of chlorine and alkali metals, as e.g. HCl(g), KCl(g), KOH(g) and NaCl(g), into the gas phase during combustion. The alkali chlorides may cause deposits on superheater tubes, which interfere with operation and can lead to corrosion and/or blockage of the gas path. To prevent and diminish the problems mentioned above, better and more detailed knowledge of the reactions between potassium chloride and the tube materials during combustion is needed. These materials commonly contain, among other metals, chromium, which is thought to protect the rest of the material since it forms a very dense but thin oxide layer on the surface of the tube material. It has been suggested that the reaction between solid or partly molten KCl and chromium oxide is the one responsible for starting the complex series of corrosion reactions. In this work, the overall reaction between potassium chloride and chromium was studied through partial reactions with compounds known to participate to the overall reaction or to be formed during it. The reactions were studied in synthetic air by heating sample mixtures in a DTA/TGA (Differential Thermal Analysis/ Thermogravimetric Analysis) apparatus. Selected samples were also studied and analyzed with a scanning electron microscope equipped with an energy dispersive x-ray analyzer (SEM/EDXA). Under the used conditions both potassium chloride and potassium chromate reacted with pure chromium and chromium oxide. In the case of chromium, chromium oxide was formed via the formation of potassium chromate. In reactions including chromium oxide as reactant also potassium dichromate was detected.