Refractory Materials for Biofuel Boilers

The energy equipment usable for solid biofuel incineration usually operates upon aggressive conditions. The internal structures (lining) of the equipment are made of refractory materials that are affected by combined loads: thermal, mechanical and chemical (i.e. high temperature–up to 1200°C, chemical impact of alkaline compounds and slag, repeating thermal shocks, abrasive effect caused by solid particles and so on). A majority of traditional refractories usable for lining in such equipment are not durable. Upon certain conditions of use (such as high local temperatures, influence of alkaline biofuel combustion products and so on), durability of the traditional materials is 1–2 years only. The opportunities of new refractory materials application should be set upon taking into account the conditions of operation for biofuel boilers of specific types. In this section - the data on the peculiarities of using refractory materials in biofuel boilers are reviewed, and the impact of aggressive operating conditions of such thermal equipment on the properties of refractory materials is discussed. In addition, the investigations results of refractory castables alkali resistance and its explosive spalling are discussed. The recommendations for use of refractory materials in biofuel boilers are also presented

Investigation of influence of nano-reinforcement on the mechanical properties of composite materials

The present work studies the possibility to decrease the formation of micro and nano cracks around short fibres in fibre-reinforced concrete (FRC) composite with the help of nano-reinforcement, which is carbon nanotubes, or micro reinforcement, which is carbon short fibres and nano-fillers. Tensile and bending strength of FRC depends on the spatial distribution of fibres inside a material, type of fibre and cement matrix, as well as an effective micromechanical work of each fibre while pulling out of the concrete matrix. Shrinkage stresses, acting in the matrix in the vicinity of a fibre, lead to the formation of micro-cracks. Such micro-cracks were observed experimentally and were investigated numerically performing broad modelling based on the finite element method (FEM). The investigation was focused on the micromechanical behaviour of a single steel fibre in a cement matrix. Numerical modelling results demonstrated a high level of shrinkage overstresses around steel fibres in concrete. The role of nano and micro admixtures, nanotubes, short carbon fibres as well as the role of water/cement ratio in a high performance concrete matrix, changing (increasing or decreasing) the friction force between the matrix and the steel fibre, were investigated experimentally by way of per­forming a single fibre pull-out tests. The high scatters of experimental results were obtained in performed pull-out tests. At the same time, for the same series of samples, a positive role of micro and nano admixtures and carbon nanotubes in the increase of pull-out force was recognised

THE EFFECT OF FLUID CATALYTIC CRACKING CATALYST WASTE ON REFRACTORY CASTABLE PROPERTIES

The fluid catalytic cracking (FCC) catalyst waste (CW) is eroded catalyst debris from the cracking unit, which is collected by an electrostatic precipitator. CW is zeolite material, which unique properties are underused in refractory castable production technology. This work deals with untreated CW and thermally treated - CWt. The hydration and structure development of cementitous compositions with CW and CWt were investigated. In comparison with CW, CWt is a considerably softer accelerating additive of cement hydration and a certain amount of this additive (10 - 20 %) markedly increases the compressive strength of cementitious stone. The impact of CWt additive on refractory castable properties was investigated. It was established that the increase of CWt additive content causes the increase of castable compressive strength and decrease of shrinkage, ensures a more compact structure of castable and best thermal durability versus analogical castable without the additive

Properties of suspension and pastes of different types of microsilica with various deflocculants

Microsilica is currently being used in a wide range of refractory castables and its quality influences performance, particularly flow and set of castables. In the present work, two different types of microsilica “Elkem ASA Materials” (grade 983U) and polish “Silimic” have been investigated. It was established that the differences between the characteristics of paste and suspensions of microsilica (pH, electric conductivity, viscosity) depend on the quality of microsilica. The influence of type and amount of microsilica on the pH, electric conductivity and viscosity behavior of alumina cement Gorkal-40 and microsilica pastes has been investigated in details. Deflocculants Castament FS-20 and sodium tripolyphosphatum were used to control the characteristics of the investigated materials. The influence of their amount on the characteristics of various suspensions and pastes was estimated. It is shown that rheological characteristic of middle cement castable, which contains in its composition with microsilica of low quality, may be increased considerably if the right amount of the right deflocculant is added

Effect of Hollow Corundum Microspheres Additive on Physical and Mechanical Properties and Thermal Shock Resistance Behavior of Bauxite Based Refractory Castable

This paper analyses the effect of hollow corundum microspheres (HCM) on both physical-mechanical properties (density, ultrasonic pulse velocity, modulus of elasticity, and compressive strength) and thermal shock resistance behavior of refractory medium cement castable with bauxite aggregate. Moreover, the scanning electron microscopy (SEM) results of HCM and refractory castable samples are presented in the paper. It was found that the replacement of bauxite of 0–0.1 mm fraction by HCM (2.5%, 5%, and 10% by weight of dry mix) had no significant effect on the density and compressive strength of castable, while the modulus of elasticity decreased by 15%. Ultrasonic pulse velocity (Vup) values and the visual analysis of the samples after thermal cycling showed that a small amount of HCM in composition of refractory castable could reduce the formation and propagation of cracks and thus increase its thermal shock resistance

Effect of Thermochemical Boronizing of Alumina Surface on the Borate Crystals Growth and Interaction with Nickel and Nickel Alloy

Wettability at the metal-ceramic interface is highly important for the development of modern composite materials. Poor wettability by metal melts restricts the use of alumina in protective metal matrix composite (MMC) coatings. In the present experimental study, the possibility to modify wetting properties of alumina by thermochemical surface boronizing was investigated. The results of SEM, EDS, XRD and XPS characterisation of surfaces revealed the formation of oxygen containing Al–B compounds identified as aluminium borates (Al18B4O33/Al4B2O9); no signs of non-oxide Al–B compounds were observed. The shape of the single splats deposited on the boronized alumina surface by the thermal spray and re-melted in the furnace revealed that significant wetting improvement by self-fluxing nickel alloy did not occur. However, the improvement of adhesion between the nickel/nickel alloy and Al2O3 surface was obtained due to formation of an intermediate layer consisting of B, O, Al and Si between the metal and ceramic surfaces at the presence of some silicon at the modified surfaces. The presented study demonstrates that the thermochemical boronizing of alumina in amorphous boron medium is a simple method to obtain a thin aluminium borate layer consisting of oriented nano-rod-like crystals, whose growing direction is predetermined by the orientation of the alumina grains’ faces at surface

The effect of carbon and polypropylene fibers on thermal shock resistance of the refractory castable

 This work investigates medium cement refractory castable with additives of carbon and polypropylene fibers. The peculiarities of microstructure changes in the fiber and castable matrix contact zone, channel formation, and changes of cold crushing strength of fiber additives, which have a refractory castable matrix under temperature treatment, were investigated. The investigation results allowed to predict that using a mix of fibers more effective than using them indi­vidually. The influence of fiber additives on the mechanical characteristics and thermal shock resistance of the refractory castable with fiber additives was tested. It was found that the addition of carbon fiber has a positive impact on the ther­mal shock resistance of the investigated castable, which is confirmed by the results obtained by thermal cycling, as well as by the values calculated for thermal shock resistance R4 and Rst. In addition, the results of the investigation of thermal cycling show that the value of the thermal shock resistance was highest when a mixed fiber additive (CF+PP) was used

Kompozicinės rišamosios medžiagos struktūros formavimosi tyrimai

This study deals with the particularities of structure formation and hydration of the complex binder, which consists of liquid glass, metallurgical slag and high aluminate cement (Al2O3 > 70 %). According to the ultrasonic and exothermal temperature results, it was determined that due to the reaction between liquid glass and dicalcium silicate the highest hardening intensity was found in the first 20 minutes; however, the hydration of cement takes place much later, i.e. in the hardened structure. SEM investigations of the complex binder showed that its structure is dominated by areas where cement particles are joined by nanostructures (70%), struktūros formavimosi ir hidratacijos ypatumai kietėjimo metu. Ultragarso ir egzotermijos temperatūros tyrimais nustatyta, kad kompozicinis rišiklis dėl skystojo stiklo ir dikalcio silikato reakcijos sukietėja jau per pirmąsias 20 minučių, tačiau aliuminatinio cemento hidratacija vyksta daug vėliau, t.y. jau sukietėjusioje struktūroje. Tiriant kompozicinį rišiklį skenuojamuoju elektroniniu mikroskopu (SEM), nustatyta, kad struktūroje vyrauja plotai, kur cemento daleles sujungia skystojo stiklo ir kietiklio reakcijos produktų nanostruktūros (<50 nm), daugumos cemento dalelių paviršius padengtas 30 nm–100nm dydžio nanodalelių sluoksniais – cemento hidratacijos produktais

Carbonation of Alumina Cemen-bonded Conventional Refractory Castable in Fireplace

It was determined by verification by in situ analysis that combustion products of firewood in fireplaces cause carbonation of alumina cement-bonded conventional refractory castable and accelerate its destruction. The carbonation of hydrated cement (Gorkal 40) paste was modeled in a laboratory conditions: having been fired at various temperatures (300 ºC – 950 ºC) the specimens were curing in humid conditions for two days. It was determined by X-ray analysis that the hydrates CAH10, C2AH8 and C3AH6 change into a mineral C12A7 after the cement paste are fired at 300 ºC – 800 ºC temperatures. The amount of the mineral depends on the firing temperature. In humid conditions C12A7 hydrates and carbonizes, consequently calcium carboaluminate hydrate forms. In the composition of cement and microsilica (9 %) carbonation of cement hydrates noticeably slow down at the same conditions. Carbonized minerals are not identified when the specimens are fired at 500 ºC –850 ºC temperatures

Investigation of structure formation in complex binder mater

This study deals with the particularities of structure formation and hydration of the complex binder, which consists of liquid glass, metallurgical slag and high aluminate cement (Al2O3 > 70 %). According to the ultrasonic and exothermal temperature results, it was determined that due to the reaction between liquid glass and dicalcium silicate the highest hardening intensity was found in the first 20 minutes; however, the hydration of cement takes place much later, i.e. in the hardened structure. SEM investigations of the complex binder showed that its structure is dominated by areas where cement particles are joined by nanostructures (<50 nm) of the products formed during the reaction between liquid glass and hardener, and that the surface of most cement particles is coated by layers of nanoparticles sized from 30 nm up to 100 nm, i. e. the products of cement hydration