Resume of 1st Stage
Resume of 2nd Stage
Resume of 3rd Stage
Resume of 1st Stage
Contemporary world is facing numerous problems, among which, the most threatening are those related to energy generation. Oil and gas will be exhausted soon, non-conventional energy sources are not yet improved and nuclear energy is considered very dangerous. Coal power stations could be a good solution, for a period of several hundred years, but the major drawback is the high pollution. Worldwide have been adopted several national and international regulations which limit the environmental pollution. Achieving these objectives requires the emergence of new technologies, which, in turn, depend on new advanced material preparation. Earth's atmosphere is constantly polluted by human activities, particularly fossil-fuel energy production, which throw huge amounts of dust in the atmosphere (soot and ash), greenhouse gas (CO2) toxic gasses (SO2, NOx) and volatile organic compounds (VOCs). For example, the problems faced by residents near power stations are already known. Filtering hot gas flow and catalytic conversion of toxic compounds included in is cumbered by their high temperature. Thus, these processes require the use of thermo-resistant materials.
Gas cooling and remediation at low temperatures is not an economically advantageous method because of high energy loss. Therefore, a remediation of gasses, at high temperature, followed by heat recovery is necessary. in this manner corrosion or erosion of heat recovery equipment is avoided.
The main goal of this project, precisely, is obtaining of new thermo-resistant materials (ceramic foam). These materials are designed to promote low polluting energy technologies (particularly by limiting air pollution from coal-fired power plants) and new technologies for filtration and ultrafiltration of solids from gases and catalytic detoxification.
Literature study led to the following conclusions:
- Compliance with pollution rules set by the EU and assuring a proper energetic efficiency, using heat from hot gasses requires remediation at high temperatures. Thus, it is necessary to obtain new materials resistant in corrosion and erosion conditions. The most recommended filters for this purpose are porous ceramic materials (ceramic foams);
- There are several methods to obtain ceramic foams, each with advantages and drawbacks: the replication method of polymer foams (in particular of polyurethane), direct foaming method and sol-gel. The most appropriate method to obtain ceramic foam with high and controlled porosity is gelcasting (particularly the gel formation is obtained by polymerization);
- Gelcasting foam characteristics can be modified using various monomer types, ceramics oxides, dispersants or plasticizers mixed up in different concentrations. The final characteristics of inorganic-organic hybrid nanocomposite require selection of appropriate conditions for drying and burning of the green bodies. Raw, intermediate and final materials, involved in the process can be characterized by different techniques such as rheology, thermal analysis, nuclear magnetic resonance, image analysis, porosity analysis, measurements of pressure drop and solids retention effectiveness;
- The most common known ceramic oxide powders used to obtain ceramic materials, such as cellular ceramic foams, are Al2
, MgO, MgAl2
, or thereof mixtures. Nano-powders and nanoporous ceramic oxides are prepared using a high diversity of methods; most used methods mainly refers to hydrolytic processes (hydrothermal process, sol-gel);
- The mixed oxides, in the form of natural silicates, commonly used to obtain ceramic suspensions are kaolin and montmorilonite alone or in various mixtures with other inorganic oxides such as aluminium trioxide, magnesium oxide or silicon dioxide to improve performances and characteristics, depending on the application.
- Selecting a suitable dispersant and quantity depends mainly on the specific characteristics of the suspension, as well as subsequent processing conditions in accordance with proposed solutions;
- Filters based on porous materials have some advantages, compared with those derived from other types of materials, namely: high thermal and corrosion resistance, vibration resistance and the possibility of simultaneous gas cleaning. Porous ceramic materials with controlled pore shape and size are made using specific technologies;
- Porosity is one of the essential characteristics of this type of material. Therefore, methods have been reviewed for porosity evaluation in a wide range of sizes. in the literature, there are encountered several methods, conventional and unconventional, to obtain ceramic foams. The factors that influence the quality of this type of materials were much investigated.
After our preliminary researches, made in this direction, the following have been concluded:
- Polymerization reaction of the acrylic monomer took place in the presence of high concentrations (50% wt) of inorganic component. It was noted that KPS alone does not initiate the polymerization. Due to product characteristics obtained after polymerization of the two samples, we concluded the study with some recommendation for future research: the system requires the addition of a dispersant in the reaction mixture. It was observed a general pseudo-plastic character for the suspensions prepared with kaolin and water, with few exceptions (for 1: 1.2 = water: kaolin ratio at 25 °C);
- Preliminary experiments were conducted for stability study of ceramics in aqueous medium (double distilled water). Suspensions, having a solid concentration of 3% wt. and 20% wt., obtained with aluminium trioxide powder, kaolin and purified bentonite, were tested in the 3 -10 pH range of the dispersion medium. The increase of solid amount in the composition of the slurry contribute to a higher stability of the suspension;
- Physico-chemical proprieties and mineralogical phase structure were analyzed for both processed and unprocessed raw materials.
Resume of 2nd Stage
The main goal of this project, is the obtaining of new thermo-resistant materials (ceramic foam). These materials are designed to promote low polluting energy technologies (particularly by limiting air pollution from coal-fired power plants) and new technologies for filtration and ultrafiltration of solids from gases and catalytic detoxification.
In the second stage of the project, results were materialized as follows:
- Ceramic foams preparation was possible by gelcasting process using acrylic acid as monomer, N, N-metilenbisacrilamide as crosslinker and kaolin, alumina or silica as ceramic filler. This new approach avoided the use of acrylamide, a very toxic monomer (carcinogenic);
- The process of gel preparation is ecological because the polymerization reaction occurs in aqueous media and acrylic acid is not neuro-toxic;
- The addition of dispersing agent (PAA) in the reaction mixture led to lower Van-der-Waals forces between the inorganic particles, obtaining eventually a suspension with optimum viscosity;
- Dispersion time influences the polymerization process. For high dispersion times (up till 4 h), the polymerization didn't occur and at low dispersion time (1 h), the polymerization process was successful;
- Polymerization has been successful even in the presence of high concentrations of inorganic component (42% wt);
- Redox initiation system has proved to be very efficient because the polymerization took place at room temperature in a few minutes. The crosslinking agent influences the gel time. Gel time decreases with the addition of crosslinker at 5 min, compared with the gel time recorded for the samples without crosslinker, which was 8 to 10 min;
Based on a documentary study published in the literature in was firstly performed the nano-powder preparation from metal oxides, used in ceramics manufacture. These ceramic structures possessed high thermal stability and low expansion coefficient for surface deposition of some catalytic active phases in gas flows remediation. This was the theoretical base to establish a manufacturing protocol scheme for the final materials, in relation to the chosen starting reagents.
Selected preparation schemes for the synthesis of precursors containing aluminium trioxide, magnesium oxide, silica or iron trioxide, and thereof mixtures, included the techniques used to obtain powders by sol-gel and pyrolysis from inorganic salts (sulphates, nitrates, chlorides) in aqueous phase, citric acid and ammonium hydroxide. Through this process were synthesized six metal oxide powders mono-bi-and tri-phasic. For this purpose we used ordinary laboratory research equipment.
Metal oxide precursor powders were characterized by compositional analyses (chemical analyses) and morphological analyses (optical microscopy).
Microscopy study revealed a crystalline micro-aggregate formation at the ignition temperature of 600°C. The analyzed powder samples consisted from microcrystalline aggregates with sizes ranging from microns to hundreds of microns (Fig. 1, 3, 5, 7, 9, 11 from Scientific Report, Part B). The effects of anisotropy showed that aggregates contain submicron-sized crystallites (Fig. 2, 4, 6, 8, 10, 12 from Scientific Report, Part B).
Following characterization of polymer nanocomposite, by FTIR, it was found that all conducted tests showed a clear influence of monomer content upon sol-gel process. The best evidence was observed in the infrared spectra of nanocomposites containing alumina. For the sample containing alumina (cca.99%) the intensity of monomer and alumina characteristic bands increased and the specific polymer bands decreased. Thus, conversion was lower than 1Al-10 AA and 1Al-16 AA samples. Kaolin samples (which contain a smaller amount of alumina) showed a similar behaviour as previous alumina-containing samples.
The amount of alumina influenced too, the conversion and that was obvious in the spectra of samples containing crosslinker, where the typical polymer peaks are visible. The band attributed to -C=C- bond vibration is attenuated with the increase of alumina, sample 2Al-15 AA.
Experimental study, developed under this scientific research contract, regarding pressure drop across the ceramic filter (prepared by gel-casting) revealed the following conclusions:
- Pressure drop across all five types of ceramic filters, with different porosities (but in a relatively narrow range), in a turbulent regime of gas flow, was characterized by low values, below 0.02 at;
- Mechanical properties of ceramic filter material limits, for safe operation, the supply pressure range of the waste gas resulted from thermal plants;
- The friction coefficient is influenced by the porosity of ceramic filter material, regardless of gas flow rate, default Reynolds number;
- Experimental data processing allowed obtaining a dimensionless relationship for the calculation of the friction in the turbulent flow of gaseous phase. This was characterized by Reynolds number relative to the flow area before reaching the ceramic filter, detailed in a proposed new equation.
The study on ceramic foams, as support for catalysts for VOC destruction, was focused on the identification of the acid sites from the catalyst surface and determination of their strength and strength distribution. Another way to characterize the prepared catalysts aimed the identification of the temperature range where reduction of catalytic precursors to metal phase takes place. The latter being the active state of the catalyst in oxidation processes of organic compounds.
It was noted the presence of two types of acid centres: the weak acid centres, which presented a 100-230°C temperature range of diethyl amine desorption, in a 39% proportion relative to all acid centres; and centres of medium strength, which presented a 230-380°C temperature range of diethylamine desorption, in a 61% proportion relative to total acid centres.
XRD, FTIR, DTG / TG and porosity measurements, for samples K (containing kaolin), showed, that the crosslinking agent does not produce significant effects. For samples A (with alumina) its presence can be taken into account.
Dehydration occurred for kaolinite in the 400-600 °C temperatures range (with a maximum at 480°C), depending on the particle size of kaolinite components and its crystallinity degree (as crystallinity degree decreases water loss increases). Thus, all results indicate that the crystallinity of ceramics was low.
Some thermal treatments have been proposed depending on the intended application field. The presence of crosslinking agent does not produce significant structural changes (from XRD and FTIR analysis).
Resume of 3rd Stage
The experimental research conducted in this direction in the 3rd step consisted in the synthesis of new porous kaolin -based materials by gelcasting process, namely by redox copolymerization of the acrylic monomer with the crosslinker, in various ratios, in a concentrated suspension of kaolin.
The results showed that the density of bodies, the volumetric contraction, and porosity (open, closed and total) were not influenced by the monomer/crosslinker ratio or by the concentration of initiation system.
However the monomer concentration was found to have a great influence upon green bodies’ density, which decreased with the increase in monomer content; after drying, the same behaviour was noticed for the volumetric contraction - as the monomer concentration increased the volumetric shrinkage decreased.
Ceramic materials with the following properties were obtained:
- total porosity ranging from 40 - 80%;
- open porosity ranging from 35 - 60%;
- closed porosity ranging from 5 to 20%.
The rheological characterization revealed that concentrated kaolin suspension presented, at the beginning, higher values for viscosity module relative to elastic module and both parameters increased with time. Once the gel point was reached, the elastic modulus increased considerable, over the viscosity module.
The SEM images of the green bodies, showed a good coverage of kaolin platelets with polymer, and the sintered bodies, at 1100°C, presented a macroporous morphology attributed to the ceramic foam.
The process for ceramic foams synthesis, by gelcasting process was developed at laboratory scale.
From DTG analysis was noticed the presence of two degradation maximums for each material. The first peak at around 250°C corresponded to dehydration of gypsite and the peak value of 500°C was assigned to kaolin dehydroxylation.
The IR spectra did not reveal major differences between these 5 materials. The characteristic band around 1010 cm-1 was attributed to Si-O-Si vibration bond and the one at 3620 cm-1 was assigned to Si-OH vibration bond, both from kaolin.
Mechanical tests showed that compressive strength decreased when the kaolin/alumina balance was modified relative to the equilibrium mixture (50/50 wt/wt ratio).The only sample with good compressive strength was K-5Al2O3.
Selected laboratory technological schemes for the synthesis of precursors, containing aluminum trioxide, magnesium oxide, silica and thereof mixtures, included techniques used to obtain oxide powders by sol-gel and pyrolysis (ignition) from inorganic salts (sulphates, nitrates, chlorides) in aqueous phase, using various agents. The surface agents (citric acid, sodium citrate, magnesium stearate, various alcohols - methanol, ethanol and isopropyl) acted as stabilizers for the oxide phases and it also had an effect upon particle size and particle size distribution of the oxide powder when using various reaction media, for catalytic phase decantation. The main technological schemes included the following stages:
1. Solubilization/dissolution of predetermined quantities of inorganic compounds as salts of hydrochloric or sulphuric acid containing stoichiometric amounts of aluminum, magnesium, copper, nickel, titanium, iron, necessary for the synthesis of oxide precursors. These could be dissolved in distilled water separately or together to obtain solutions with proper concentrations;
2. Addition of different surface agents from citric acid or sodium citrate groups in various ratios ranging from 1:1=(Al, Mg, Fe)/acid to 0,1:1=(Al, Mg, Fe)/acid depending on final particle size distribution. Along these additives, various phase stabilizers, as metal chlorides diluted solutions, can be added;
3. Sol formation, under continue stirring, at room temperature for 2 hours in an alkaline medium, by adding the acid solution drop by drop;
4. Homogenizing the obtained mixture by stirring with a magnetic or mechanical stirrer;
5. Gel formation by pH control of the mixture and maturation by heating at temperatures ranging from 60 to 80°C;
6. Obtaining oxide powders by drying the gel in an electric oven, in air atmosphere at 70-115°C following a time program;
7. Grinding the dried solids;
8. Calcinating the solids, following a temperature program ranging from 400 to 1300°C; maintaining a 2-3 hours plateau when the temperature reaches 1300°C depending on the powders' nature.
After experimental research and determination of heat treatment technology (drying, calcinating and sintering), of nanocomposites precursors for ceramic foams, it was concluded that the thermal treatment proposed must take into account the results of thermal and structural analysis. It was proposed a differential heat treatment as follows: up to 500°C the heating rate should not exceed 5°C/min, and after this temperature until 1350°C the rate is maintained at 10°C/min with a 5 hours plateau after reaching the maximum. Heat treatment was achieved in air atmosphere and cooling took place simultaneous with the furnace cooling (laboratory processes).
The preliminary conclusions could define the following technological sequence to obtain porous ceramic materials:
- Grinding and homogenization of precursors. It started from kaolin and alumina (economic criteria must be taken into consideration); materials were grounded together to obtain an advanced fine powder. The particle size of materials was essential to improve the crystallization kinetics of mullite. Grinding can be divided in wet grinding or dry grinding, in ball mills; and the process can be continuous or discontinuous, depending on the amount of required material;
- Obtaining the composite material. The organic and the inorganic materials were mixed together and homogenized;
- Casting the slurries in the desired matrix. It is preferable to obtain less bulky object (eg 2D) to prevent cracks formation in the final ceramic bodies;
- Hardening and drying the slurries. This process was completed by polymerization. The process was controlled to prevent deformation of the bodies or cracks formation. It was recommended, drying the samples for two hours at 100°C after polymerization;
- Heat treatment. It took place in the oven, in air atmosphere respecting the conditions described above (heating rates and plateau). It could be adopted a continuous heat treatment process depending on the required amount. The cooling wasn't sudden.
Filtration experiments showed significant lower values for the friction coefficient of AS1-AS5 ceramic foams compared to the ones obtained for AS6-AS10 ceramic foams (stage II of the contract). This fact was due to large pore formation inside AS1-AS5 materials which enhanced the velocity of air flow.
AS6-AS10 filter materials are recommended for polluted air flows containing coarse solid particles due to similar values between the atmospheric supply pressure and atmospheric pressure and AS1-AS5 filter materials are recommended for polluted air flow with finer solid particles.
Decomposition catalytic research, of gasses containing VOC, on ceramic foams based catalysts has indicated the possibility of using this new catalytic support in the discussed purpose.
The catalytic combustion process on the tested catalysts was conducted at lower temperatures than thermal combustion process.
Platinum and copper-based catalysts showed a similar behavior in the combustion process of p-xylene, ensuring a high combustion of p-xylene.