The role of potassium in the corrosion of superheater materials in boilers firing biomass

Inhibition of global warming has become one of the major goals for the coming decades. A key strategy is to replace fossil fuels with more sustainable fuels, which has generated growing interest in the use of waste-derived fuels and of biomass fuels. However, from the chemical point of view, biomass is an inhomogeneous fuel, usually with a high concentration of water and considerable amounts of potassium and chlorine, all of which are known to affect the durability of superheater tubes. To slow down or reduce corrosion, power plants using biomass as fuel have been forced to operate at lower steam temperatures as compared to fossil fuel power plants. This reduces power production efficiency: every 10°C rise in the steam temperature results in an approximate increase of 2% in power production efficiency. More efficient ways to prevent corrosion are needed so that power plants using biomass and waste-derived fuels can operate at higher steam temperatures. The aim of this work was to shed more light on the alkali-induced corrosion of superheater steels at elevated temperatures, focusing on potassium chloride, the alkali salt most frequently encountered in biomass combustion, and on potassium carbonate, another potassium salt occasionally found in fly ash. The mechanisms of the reactions between various corrosive compounds and steels were investigated. Based on the results, the potassium-induced accelerated oxidation of chromia protected steels appears to occur in two consecutive stages. In the first, the protective chromium oxide layer is destroyed through a reaction with potassium leading to the formation of intermediates such as potassium chromate (K2CrO4) and depleting the chromium in the protective oxide layer. As the chromium is depleted, chromium from the bulk steel diffuses into the oxide layer to replenish it. In this stage, the ability of the material to withstand corrosion depends on the chromium content (which affects how long it takes the chromium in the oxide layer to be depleted) and on external factors such as temperature (which affects how fast the chromium diffuses into the protective oxide from the bulk steel). For accelerated oxidation to continue, the presence of chloride appears to be essential.

Sinkin stabiloiminen pilaantuneesta maa-aineksesta: 28 vuorokauden stabilointiajan riittävyys liukoisuustesteissä eri sinkkiyhdisteillä

Tässä kandidaatintyössä selvitettiin, onko 28 vuorokautta riittävä aika sinkillä pilaantuneen maaperän massastabilointiin, ja kuinka sinkin esiintymismuoto vaikuttaa sen stabilointiaikaan. Kokeellisessa osassa jäljiteltiin malmijätteessä, orgaanisessa aineksessa sekä liuenneena maaperässä esiintyvän sinkin stabiloitumista lisäämällä maanäytteeseen sinkkiä eri yhdisteinä; sinkkirakeina, -kloridina ja -asetaattina. Näytteet stabiloitiin sementti-lentotuhkaseoksella 1–28 vuorokauden pituisia ajanjaksoja, minkä jälkeen ne kuvattiin pyyhkäisyelektronimikroskoopilla (SEM) ja niille tehtiin liukoisuustestit. Liukoisuustestien tuloksista voidaan huomata sinkkikloridin stabiloituvan jo ensimmäisen vuorokauden aikana ja pysyvän samalla tasolla koko tarkasteluajan. Sinkkirakeiden ja -asetaatin stabiloituminen ei ole yhtä tasaista; alun sitoutumisen jälkeen niiden liukoisuuksissa on havaittavissa selkeät piikit 21 vuorokauden kohdalla. Tämän jälkeen ne alkavat sitoutua uudelleen. Tulosten perusteella sinkin esiintymismuoto vaikuttaa sen stabilointiaikaan, eikä 28:aa vuorokautta voida pitää riittävänä aikana sinkillä pilaantuneen maa-aineksen stabilointiin. Vaikka liukoinen sinkki stabiloituu jo yhdessä vuorokaudessa, ei malmijätteessä tai orgaanisessa aineksessa esiintyvä sinkki ehdi stabiloitua vakaalle tasolle vielä 28 vuorokaudenkaan aikana. Tämä tulisi ottaa huomioon suunniteltaessa ja toteutettaessa sinkkiä sisältävien maiden kunnostushankkeita.

Puu- ja turvetuhkan hyötykäyttökelpoisuuteen vaikuttavat tekijät pienillä polttolaitoksilla

Energiantuotannossa syntyvä tuhka voi olla laadultaan hyvin vaihtelevaa ja laadunvaihtelulle on haastavaa löytää yksiselitteistä syy-seuraussuhdetta. Ympäristönsuojelulainsäädäntö ja taloudelliset intressit ohjaavat tuhkantuottajia etsimään tuhkalle sopivia hyötykäyttökohteita, ja sen vuoksi tuhkan laatuun ja hyötykäyttökelpoisuuteen vaikuttavia tekijöitä on tarpeen selvittää. Tässä diplomityössä on tutkittu pienissä, alle 50 MW:n polttolaitoksissa syntyvää tuhkaa. Tavoitteena oli selvittää, kuinka tuhkan hyötykäyttökelpoisuuteen voidaan vaikuttaa. Tutkimuksen kohteena oli polttoainekoostumuksen, poltto-olosuhteiden ja tuhkan jälkikäsittelyn vaikutus tuhkassa olevien haitta-aineiden pitoisuuksiin ja liukoisuuksin. Työhön sisältyi myös aiemmin tehtyjen tuhka-analyysien tarkastelu sekä tuhkakokeet kahdella kohderyhmään kuuluvalla laitoksella. Työssä todettiin lentotuhkan haitta-ainepitoisuuksien ja -liukoisuuksien olevan keskimäärin korkeampia kuin pohjatuhkan vastaavien, ja että tyypillisesti arinakattilan tuhkien haitta-aineet ylittävät useammin hyötykäyttökelpoisuuden raja-arvoja kuin kuplaleijupetikattilan tuhkien. Lisäksi havaittiin metsätähdehaketuhkan kelpaavan useammin hyötykäyttöön kuin rankahaketuhkan.

Teollisuusjätteen käyttö kiinteytys-stabiloinnin sideaineena ja haitta-aineiden stabiloinnin tehostamisessa

Tässä kanditaatintyössä selvitettiin kuinka erilaiset sideaineseokset soveltuvat raskas-metallien sitomiseen 28 vuorokautta stabiloiduissa näytteistä. Työssä oletettiin teollisten jätefraktioiden käytön tehostavan eräiden metallien, kuten kupari ja sinkki, immobilisointia lievästi pilaantuneista maa-aineksista. Kokeellisessa osassa stabiloitiin Kokkolan satamasta ruopattua sedimenttiä, jonka sinkkipitoisuudet olivat ylittäneet saastuneen sedimentin ohjearvon (≥400 mg/kg). Sedimenttiin lisättiin eri sideaineseoksia ja näytteiden annettiin stabiloitua 28 vuorokautta, minkä jälkeen niistä testattiin liukenevat raskasmetallit muokatulla ravistelutestillä. Eri sideaineseoksilla saatuja tuloksia verrattiin pelkän yleissementin käyttöön. Lisäksi erillisistä näytteistä otettiin pyyhkäisyelektronimikros-koopilla (SEM) kuvia havainnollistamaan stabiloitumista. Näissä näytteissä käytettiin samoja sideaineita kuin tehdyissä kokeissa. Liukoisuustestien tuloksista voidaan huomata näytteissä ongelmalliseksi raskasmetalliksi identifioidun sinkin sitoutuvan parhaiten sementin ja kipsin sekoituksella. Myös tuhkaa sisältävät sideainesekoitukset pienensivät sinkin liukoisuutta verrattuna pelkkään yleis-sementtiin. Jatkotutkimuksissa voitaisiin testata erilaisia sideainesekoituksia betonira-kentamisessa, joilla saadaan ainakin 25 MPa lujuusarvo, pilaantunutta sedimenttiä tai maa-ainesta käyttäen.

Behaviour of laterised normal and self compacting concrete subjected to elevated temperatures

Concrete is a universal material in the construction industry. With natural resources like sand and aggregate, fast depleting, it is time to look for alternate materials to substitute these in the process of making concrete. There are instances like exposure to solar radiation, fire, furnaces, and nuclear reactor vessels, special applications like missile launching pads etc., where concrete is exposed to temperature variations In this research work, an attempt has been made to understand the behaviour of concrete when weathered laterite aggregate is used in both conventional and self compacting normal strength concrete. The study has been extended to understand the thermal behaviour of both types of laterised concretes and to check suitability as a fire protection material. A systematic study of laterised concrete considering parameters like source of laterite aggregate, grades of Ordinary Portland Cement (OPC) and types of supplementary cementitious materials (fly ash and GGBFS) has been carried out to arrive at a feasible combination of various ingredients in laterised concrete. A mix design methodology has been proposed for making normal strength laterised self compacting concrete based on trial mixes and the same has also been validated. The physical and mechanical properties of laterised concretes have been studied with respect to different variables like exposure temperature (200°C, 400°C and 600°C) and cooling environment (air cooled and water cooled). The behaviour of ferrocement elements with laterised self compacting concrete has also been studied by varying the cover to mesh reinforcement (10mm to 50mm at an interval of 10mm), exposure temperature and cooling environment.

Behavior of geopolymer concrete exposed to elevated temperatures

The research in the area of geopolymer is gaining momentum during the past 20 years. Studies confirm that geopolymer concrete has good compressive strength, tensile strength, flexural strength, modulus of elasticity and durability. These properties are comparable with OPC concrete.There are many occasions where concrete is exposed to elevated temperatures like fire exposure from thermal processor, exposure from furnaces, nuclear exposure, etc.. In such cases, understanding of the behaviour of concrete and structural members exposed to elevated temperatures is vital. Even though many research reports are available about the behaviour of OPC concrete at elevated temperatures, there is limited information available about the behaviour of geopolymer concrete after exposure to elevated temperatures. A preliminary study was carried out for the selection of a mix proportion. The important variable considered in the present study include alkali/fly ash ratio, percentage of total aggregate content, fine aggregate to total aggregate ratio, molarity of sodium hydroxide, sodium silicate to sodium hydroxide ratio, curing temperature and curing period. Influence of different variables on engineering properties of geopolymer concrete was investigated. The study on interface shear strength of reinforced and unreinforced geopolymer concrete as well as OPC concrete was also carried out. Engineering properties of fly ash based geopolymer concrete after exposure to elevated temperatures (ambient to 800 °C) were studied and the corresponding results were compared with those of conventional concrete. Scanning Electron Microscope analysis, Fourier Transform Infrared analysis, X-ray powder Diffractometer analysis and Thermogravimetric analysis of geopolymer mortar or paste at ambient temperature and after exposure to elevated temperature were also carried out in the present research work. Experimental study was conducted on geopolymer concrete beams after exposure to elevated temperatures (ambient to 800 °C). Load deflection characteristics, ductility and moment-curvature behaviour of the geopolymer concrete beams after exposure to elevated temperatures were investigated. Based on the present study, major conclusions derived could be summarized as follows. There is a definite proportion for various ingredients to achieve maximum strength properties. Geopolymer concrete with total aggregate content of 70% by volume, ratio of fine aggregate to total aggregate of 0.35, NaOH molarity 10, Na2SiO3/NaOH ratio of 2.5 and alkali to fly ash ratio of 0.55 gave maximum compressive strength in the present study. An early strength development in geopolymer concrete could be achieved by the proper selection of curing temperature and the period of curing. With 24 hours of curing at 100 °C, 96.4% of the 28th day cube compressive strength could be achieved in 7 days in the present study. The interface shear strength of geopolymer concrete is lower to that of OPC concrete. Compared to OPC concrete, a reduction in the interface shear strength by 33% and 29% was observed for unreinforced and reinforced geopolymer specimens respectively. The interface shear strength of geopolymer concrete is lower than ordinary Portland cement concrete. The interface shear strength of geopolymer concrete can be approximately estimated as 50% of the value obtained based on the available equations for the calculation of interface shear strength of ordinary portland cement concrete (method used in Mattock and ACI). Fly ash based geopolymer concrete undergoes a high rate of strength loss (compressive strength, tensile strength and modulus of elasticity) during its early heating period (up to 200 °C) compared to OPC concrete. At a temperature exposure beyond 600 °C, the unreacted crystalline materials in geopolymer concrete get transformed into amorphous state and undergo polymerization. As a result, there is no further strength loss (compressive strength, tensile strength and modulus of elasticity) in geopolymer concrete, whereas, OPC concrete continues to lose its strength properties at a faster rate beyond a temperature exposure of 600 °C. At present no equation is available to predict the strength properties of geopolymer concrete after exposure to elevated temperatures. Based on the study carried out, new equations have been proposed to predict the residual strengths (cube compressive strength, split tensile strength and modulus of elasticity) of geopolymer concrete after exposure to elevated temperatures (upto 800 °C). These equations could be used for material modelling until better refined equations are available. Compared to OPC concrete, geopolymer concrete shows better resistance against surface cracking when exposed to elevated temperatures. In the present study, while OPC concrete started developing cracks at 400 °C, geopolymer concrete did not show any visible cracks up to 600 °C and developed only minor cracks at an exposure temperatureof 800 °C. Geopolymer concrete beams develop crack at an early load stages if they are exposed to elevated temperatures. Even though the material strength of the geopolymer concrete does not decrease beyond 600 °C, the flexural strength of corresponding beam reduces rapidly after 600 °C temperature exposure, primarily due to the rapid loss of the strength of steel. With increase in temperature, the curvature at yield point of geopolymer concrete beam increases and thereby the ductility reduces. In the present study, compared to the ductility at ambient temperature, the ductility of geopolymer concrete beams reduces by 63.8% at 800 °C temperature exposure. Appropriate equations have been proposed to predict the service load crack width of geopolymer concrete beam exposed to elevated temperatures. These equations could be used to limit the service load on geopolymer concrete beams exposed to elevated temperatures (up to 800 °C) for a predefined crack width (between 0.1mm and 0.3 mm) or vice versa. The moment-curvature relationship of geopolymer concrete beams at ambient temperature is similar to that of RCC beams and this could be predicted using strain compatibility approach Once exposed to an elevated temperature, the strain compatibility approach underestimates the curvature of geopolymer concrete beams between the first cracking and yielding point.

Hardening of concrete with a planned delayed ettringite formation

Delayed ettringite formation (DEF) in cementitious materials is widely considered as a harmful chemical reaction that causes extensive damages in hardened concrete. However, preventative measures and possible improvements in general are not extensively studied and require further attention. In this study was presented an investigation into a type of controlled DEF in places of finely dispersed crystallisation nuclei and provide evidence that the process may improve compressive strength of cementitious materials. The Alkali-Silica Reaction (ASR) in hydrated concrete was achieved with the addition of fly ash and was further accelerated with the Duggan’s test. Achieved strengths and monitoring of microstructure development conducted with electronic microscopy revealed that growth of ettringite crystals in the nuclei led to harmless internal compressive stresses, expansion of hydrated concrete and overall strengthening of the concrete matrix.

General circulation model estimates of aerosol transport and radiative forcing during the Indian Ocean Experiment

Aerosol sources, transport, and sinks are simulated, and aerosol direct radiative effects are assessed over the Indian Ocean for the Indian Ocean Experiment (INDOEX) Intensive Field Phase during January to March 1999 using the Laboratoire de Me´te´orologie Dynamique (LMDZT) general circulation model. The model reproduces the latitudinal gradient in aerosol mass concentration and optical depth (AOD). The model-predicted aerosol concentrations and AODs agree reasonably well with measurements but are systematically underestimated during high-pollution episodes, especially in the month of March. The largest aerosol loads are found over southwestern China, the Bay of Bengal, and the Indian subcontinent. Aerosol emissions from the Indian subcontinent are transported into the Indian Ocean through either the west coast or the east coast of India. Over the INDOEX region, carbonaceous aerosols are the largest contributor to the estimated AOD, followed by sulfate, dust, sea salt, and fly ash. During the northeast winter monsoon, natural and anthropogenic aerosols reduce the solar flux reaching the surface by 25 W m�2, leading to 10–15% less insolation at the surface. A doubling of black carbon (BC) emissions from Asia results in an aerosol single-scattering albedo that is much smaller than in situ measurements, reflecting the fact that BC emissions are not underestimated in proportion to other (mostly scattering) aerosol types. South Asia is the dominant contributor to sulfate aerosols over the INDOEX region and accounts for 60–70% of the AOD by sulfate. It is also an important but not the dominant contributor to carbonaceous aerosols over the INDOEX region with a contribution of less than 40% to the AOD by this aerosol species. The presence of elevated plumes brings significant quantities of aerosols to the Indian Ocean that are generated over Africa and Southeast and east Asia.

Landscape change in central Latvia since the Iron Age: multi-proxy analysis of the vegetation impact of conflict, colonization and economic expansion during the last 2000 years

This study represents the first detailed multi-proxy palaeoenvironmental investigation associated with a Late Iron Age lake-dwelling site in the eastern Baltic. The main objective was to reconstruct the environmental and vegetation dynamics associated with the establishment of the lake-dwelling and land-use during the last 2,000 years. A lacustrine sediment core located adjacent to a Late Iron Age lake-dwelling, medieval castle and Post-medieval manor was sampled in Lake Āraiši. The core was dated using spheroidal fly-ash particles and radiocarbon dating, and analysed in terms of pollen, non-pollen palynomorphs, diatoms, loss-on-ignition, magnetic susceptibility and element geochemistry. Associations between pollen and other proxies were statistically tested. During ad 1–700, the vicinity of Lake Āraiši was covered by forests and human activities were only small-scale with the first appearance of cereal pollen (Triticum and Secale cereale) after ad 400. The most significant changes in vegetation and environment occurred with the establishment of the lake-dwelling around ad 780 when the immediate surroundings of the lake were cleared for agriculture, and within the lake there were increased nutrient levels. The highest accumulation rates of coprophilous fungi coincide with the occupation of the lake-dwelling from ad 780–1050, indicating that parts of the dwelling functioned as byres for livestock. The conquest of tribal lands during the crusades resulted in changes to the ownership, administration and organisation of the land, but our results indicate that the form and type of agriculture and land-use continued much as it had during the preceding Late Iron Age.

Occupational airborne contamination in South Brazil: 2. Oxidative stress detected in the blood of workers of incineration of hospital residues

One of the most useful methods for elimination of solid residues of health services (SRHS) is incineration. However, it also provokes the emission of several hazardous air pollutants such as heavy metals, furans and dioxins, which produce reactive oxygen species and oxidative stress. The present study, which is parallel to an accompanied paper (Avila Jr. et al., this issue), investigated several enzymatic and non-enzymatic biomarkers of oxidative stress in the blood (contents of vitamin E, lipoperoxidation = TBARS, reduced glutathione = GSH, oxidized glutathione = GSSG, and activities of glutathione S-transferase = GST, glutathione reductase = GR, glutathione peroxidase = GPx, catalase = CAT and superoxide dismutase = SOD), in three different groups (n = 20 each) exposed to airborne contamination associated with incineration of SRHS: workers directly (ca. 100 m from the incinerator) and indirectly exposed (residents living ca. 5 km the incineration site), and controls (non-exposed subjects). TBARS and GSSG levels were increased whilst GSH, TG and alpha-tocopherol contents were decreased in workers and residents compared to controls. Increased GST and CAT activities and decreased GPx activities were detected in exposed subjects compared to controls, while GR did not show any difference among the groups. In conclusion, subjects directly or indirectly exposed to SRHS are facing an oxidative insult and health risk regarding fly ashes contamination from SRHS incineration.

Antioxidant therapy attenuates oxidative stress in the blood of subjects exposed to occupational airborne contamination from coal mining extraction and incineration of hospital residues

Coal mining and incineration of solid residues of health services (SRHS) generate several contaminants that are delivered into the environment, such as heavy metals and dioxins. These xenobiotics can lead to oxidative stress overgeneration in organisms and cause different kinds of pathologies, including cancer. In the present study the concentrations of heavy metals such as lead, copper, iron, manganese and zinc in the urine, as well as several enzymatic and non-enzymatic biomarkers of oxidative stress in the blood (contents of lipoperoxidation = TBARS, protein carbonyls = PC, protein thiols = PT, alpha-tocopherol = AT, reduced glutathione = GSH, and the activities of glutathione S-transferase = GST, glutathione reductase = GR, glutathione peroxidase = GPx, catalase = CAT and superoxide dismutase = SOD), in the blood of six different groups (n = 20 each) of subjects exposed to airborne contamination related to coal mining as well as incineration of solid residues of health services (SRHS) after vitamin E (800 mg/day) and vitamin C (500 mg/day) supplementation during 6 months, which were compared to the situation before the antioxidant intervention (Avila et al., Ecotoxicology 18:1150-1157, 2009; Possamai et al., Ecotoxicology 18:1158-1164, 2009). Except for the decreased manganese contents, heavy metal concentrations were elevated in all groups exposed to both sources of airborne contamination when compared to controls. TBARS and PC concentrations, which were elevated before the antioxidant intervention decreased after the antioxidant supplementation. Similarly, the contents of PC, AT and GSH, which were decreased before the antioxidant intervention, reached values near those found in controls, GPx activity was reestablished in underground miners, and SOD, CAT and GST activities were reestablished in all groups. The results showed that the oxidative stress condition detected previously to the antioxidant supplementation in both directly and indirectly subjects exposed to the airborne contamination from coal dusts and SRHS incineration, was attenuated after the antioxidant intervention.

Estabilização pozolânica de cinzas volante e pesada com cal dolomítica hidratada e areia, curadas pelos processos de autoclavagem e câmara a temperatura constante

Este trabalho tem como objetivo verificar o comportamento mecânico e físico de pozolanas artifíciais estabilizadas química e granulometricamente, curadas por dois processos de cura denominados de : autoclave(ATC), que utiliza temperaturas na faixa de 149 a 188°C e câmara à temperatura constante(CTC) que utiliza uma temperatura de ± 21°C. Também fez-se análises estatísticas com a finalidade de se determinar o efeito da energia de moldagem, temperatura e tempo de cura sobre os resultados dos ensaios, para cada processo de cura, além de se determinar modelos matemáticos para previsão de resultados de resistência através de regressões múltiplas e simples. As pozolanas artificiais empregadas foram as cinzas volante e pesada da Usina de Candiota, as quais foram estabilizadas com cal dolomítica hidratada e areia do Rio Guaíba. Os ensaios de resistência à compressão simples , absorção e perda de massa basearam-se nas normas da ABNT e DNER e para os estudos de análise estatística, fez-se anteriormente aos ensaios, dois planejamentos experimentais denominados de Split-Splot e Quadrado Latino, que foram utilizados nos processos de autoclavagem e câmara à temperatura constante, representativamente. Os Corpos-de-Prova curados na câmara à temperatura constante, até os 28 dias de cura, apresentaram resultados de resistências inferiores aqueles curados pelo processo de autoclave. Aos 60 dias de cura suas resistências ficaram na faixa dos valores de Corpos-de-Provas curados pela autoclave nas temperaturas de 149 a 188°C, excessão feita na mistura utilizando areia, onde em todos os períodos de cura estudados,os valores de resistência dos Corpos-de-Prova curados pelo câmara à temperatura constante foram inferiores. A proporção da quantidade de cal e cinza na mistura, bem como o valor da superfície específica da cinza influenciam nos resultados de ensaios, independentemente da variação dos fatores principais. Em termos de análise estatística verificou-se que a energia de moldagem e o tempo de cura são os fatores que apresentam os maiores efeitos sobre os resultados da resistência, para os processos de cura ATC e CTC, respectivamente.

Viabilidade tecnica da producao de blocos silico-calcarios na regiao de Santa Maria, no Rio Grande do Sul

Este trabalho é camposto por dois estudos basicamente: um estudo teórico sobre os blocos sílico-calcários de areia-cal e um estudo prático sobre a viabilidade técnica de produção destes blocos em uma região determinada. A pesquisa teórica procurou elucidar o que vem a ser um bloco síilico-calcário, buscando a viabilidade da pesquisa prática e os pontos principais a serem abordados. Foram estudados, nesta parte, as matérias-primas e o processo produtivo empregado na confecção destes elementos, mostrando a importância de cada item para as características do produto final. Foi dada atenção ainda às características técnicas, aos fatores que determinam a produção e emprego e aos aspectos positivos e negativos dos elementos deste tipo produzido em outros locais. A pesquisa prática buscou a disponibilidade de matéria-prima conveniente para a produção de blocos sílicos-calcários, e o efeito das variações do processo produtivo sobre os produtos confeccionados com os materiais estudados. A matéria-prima empregada foi uma areia quartzosa da região de Santa Maria-RS, cal calcítica virgem de São Sepé-RS, e cinza volante da termoelétrica Presidente Médici, em Bagé-RS, fazendo-se variações nas percentagens destes componentes. As variações do processo produtivo foram efetuadas no tipo de moldagem dos corpos de prova, em que foi usada pressão, vibração e vibro-compressão, e na pressão de autoclavagem. Para todas as variações efetuadas foram obtidas as seguintes características: variações por autoclavagem, massa específica aparente, absorção d´água, resistência à compressão, resistência à tração e módulo de elasticidade à compressão, as quais foram relacionadas através de quadros e figuras às diversas variações.

Otimização e análise mecânica de pastas geopoliméricas para uso em poços sujeitos à injeção cíclica de vapor

Oil wells subjected to cyclic steam injection present important challenges for the development of well cementing systems, mainly due to tensile stresses caused by thermal gradients during its useful life. Cement sheath failures in wells using conventional high compressive strength systems lead to the use of cement systems that are more flexible and/or ductile, with emphasis on Portland cement systems with latex addition. Recent research efforts have presented geopolymeric systems as alternatives. These cementing systems are based on alkaline activation of amorphous aluminosilicates such as metakaolin or fly ash and display advantageous properties such as high compressive strength, fast setting and thermal stability. Basic geopolymeric formulations can be found in the literature, which meet basic oil industry specifications such as rheology, compressive strength and thickening time. In this work, new geopolymeric formulations were developed, based on metakaolin, potassium silicate, potassium hydroxide, silica fume and mineral fiber, using the state of the art in chemical composition, mixture modeling and additivation to optimize the most relevant properties for oil well cementing. Starting from molar ratios considered ideal in the literature (SiO2/Al2O3 = 3.8 e K2O/Al2O3 = 1.0), a study of dry mixtures was performed,based on the compressive packing model, resulting in an optimal volume of 6% for the added solid material. This material (silica fume and mineral fiber) works both as an additional silica source (in the case of silica fume) and as mechanical reinforcement, especially in the case of mineral fiber, which incremented the tensile strength. The first triaxial mechanical study of this class of materials was performed. For comparison, a mechanical study of conventional latex-based cementing systems was also carried out. Regardless of differences in the failure mode (brittle for geopolymers, ductile for latex-based systems), the superior uniaxial compressive strength (37 MPa for the geopolymeric slurry P5 versus 18 MPa for the conventional slurry P2), similar triaxial behavior (friction angle 21° for P5 and P2) and lower stifness (in the elastic region 5.1 GPa for P5 versus 6.8 GPa for P2) of the geopolymeric systems allowed them to withstand a similar amount of mechanical energy (155 kJ/m3 for P5 versus 208 kJ/m3 for P2), noting that geopolymers work in the elastic regime, without the microcracking present in the case of latex-based systems. Therefore, the geopolymers studied on this work must be designed for application in the elastic region to avoid brittle failure. Finally, the tensile strength of geopolymers is originally poor (1.3 MPa for the geopolymeric slurry P3) due to its brittle structure. However, after additivation with mineral fiber, the tensile strength became equivalent to that of latex-based systems (2.3 MPa for P5 and 2.1 MPa for P2). The technical viability of conventional and proposed formulations was evaluated for the whole well life, including stresses due to cyclic steam injection. This analysis was performed using finite element-based simulation software. It was verified that conventional slurries are viable up to 204ºF (400ºC) and geopolymeric slurries are viable above 500ºF (260ºC)