The development of efficient components to save energy plays an important role in designing of sustainable solutions. Based on the concept of green energy, gas burners based on porous ceramic structures are interesting technologies to supply heat and lighting by burning even low calorific fuels as biogas. In this work, perspectives on development of porous ceramic burners in Brazil are presented. For this study a mixture of rare earth oxides―yttria (YTR) was selected as raw material, considering the unique luminescence proprieties of rare earth elements. Ceramic nettings with homogeneous morphology were produced by colloidal processing of rare earth powders. The results highlighted the potentiality of these components to be applied as biogas burners.
In the present world the progress and welfare of society are directly associated with consumption of energy. According to the International Energy Agency (IEA), the demand on energy increases by 1.5% a year, where lighting corresponds to one fifth of this demand. Further, the most alarming thing is that fossil fuels still supply almost 80% of total energy, causing several known environmental damages[
The United Nations has established that the universal access to energy is one of the great global challenges in this century, since it is a basic requirement of citizenship [
Biogas derived from anaerobic decomposition of organic wastes (urban, rural and sewage) [
A reliable path to drive the present economic pattern to a green economy consists in using modern renewable sources, as well as advanced components to improve energetic efficiency [
By ONU’s Renewables Global Status Report [
Many authors have discussed that the characteristics of the component play a very important role in the combustion process of the fuel, so that components with a considerable porosity are more efficient than solids ones. Porous components are bodies constituted by gaps forming a net/strut structure, wherein a matter can flow through [
The combustion of a mixture of air and fuel (natural gas, ethanol, biogas) in a PB is detailed in (
Comparative of calorific power of some energy sources. LPG: Liquefied petroleum gas
. Average composition of biogas from different organic wastes [5]
Constituents | Percentage (%) |
---|---|
Methane (CH4) | 40 - 75 |
Carbon dioxide (CO2) | 25 - 40 |
Nitrogen (N) | 0.5 - 2.5 |
Oxygen (O) | 0.1 - 1 |
Hydrogen sulfide (H2S) | 0.1 - 0.5 |
Ammonia (NH3) | 0.1 - 0.5 |
Carbon monoxide (CO) | 0 - 0.1 |
Hydrogen (H) | 1 - 3 |
. The three fundamental aspects to use biogas as energy source [1] [4] [18] -[20]
Environmental | Reduction of greenhouse gases, where methane (CH4) is 21 times more harmful than CO2; diversification of energy matrix and gradual substitution of fossil fuels (natural gas, petroleum, charcoal); improvement of air quality. |
---|---|
Social | Diffusion of basic sanitary programs, where garbage is not a problem anymore and become an energy solution; increasing of energy supply, since energy is life quality; reduction of migration from rural areas to cities by production of local energy in a sustainable form. |
Economic | Creation of job opportunities; development of a new energy market; carbon credit (US$150 million a year, considering only bovine waste) [20] [25] . |
The process of heating circulation in a porous burner idealized as a tube [26]
where the ceramic structure conducts and irradiates the heating flow to the combustion zone; Heating of the gas (3), being the temperature of the ceramic structure higher than the gas, resulting in heat conduction by convection in solid-vapor system. In this way, the entrance gases are preheating up to the limit temperature in order to start the reaction and maintain the cycle. This system improves the velocity of the flame and also the efficiency of the combustion, resulting in lower emission of greenhouse gases [
Materials selection is an important consideration during the design of PB. Proprieties such as emissivity, corrosion resistance, thermal expansion, thermal conductivity and mechanical strength must be considered carefully. Among ceramic materials, rare earth oxides (REOs) present unique luminescent proprieties, being employed in high technological applications as LED panels, lasers, lens [
The processes commonly applied to produce porous ceramic components are sacrificial template, gel casting and replica method [
Even though REOs show unique luminescent proprieties, few studies about colloidal processing of these compounds have been done.
The Global Competitiveness Index (GCI) is a parameter to evaluate the economy of a country and its capacity of development. The World Economic Forum defines competitiveness as “the set of institutions, policies, and factors that determine the level of productivity of a country” [
Nowadays, Brazil places at 53th position in CGI index (total of 142 countries). Among those pillars, Innovation presents the lowest score, which means that deep structural reforms must be done in order to take Brazil to upper positions and consolidate itself as a solid player in the world market. These reforms can be summarized in four key challenges as political, ideological, economic and technological.
In political aspect, due to the increase of Brazilian population and their life expectation, the investment in renewable technologies competes with others national issues as forethought reform, minimum salary, education, public health and labor reform. In addition, these issues are usually used by opposition groups to wear off the government.
By ideological view, the acceptation confronts with sustainability. The welfare of society is driven by minority groups with political and financial power. If a project looks to supply the poorest population (with lack of information, culture, health, dignity), however eventually strikes any interest of that minority, they will do everything to block it and the project hardly will be done.
Overview of replica method
. Some studies about colloidal processing of REOs from literature
Compound | GDC [36] | Lu:Eu [37] | YAG [38] | Y2O3 [39] | Y:Ce [40] | |
---|---|---|---|---|---|---|
Processing | Tape casting | Slip casting | Slip casting | Replica | - | |
pH | 7.0 | 9 - 10 | 9 - 11 | 10.0 | - | |
ζ(|mv|) | 40.0 | - | - | 56.0 | - | |
%Vol | 23.0 | 5 - 10 | 23.0 | 30.0 | 40.0 | |
Disp. (wt%) | 1.0% (PAA) | 1.0% (PMA) | 1.5% (PAA) | - | - | |
Binder (wt%) | 5.0% (PVA) | - | - | 0.5% (CMC) | - | |
Plast. (wt%) | 5% (PEG) | - | - | - | - |
ζ: Zeta potential; %vol: solids load in volume percent; Disp.: dispersant dosage in weight percent; Plast.: plasticizer in weight percent; (-) parameter not evaluated, or not divulgated.
The 12 pillars of competitiveness
A great concern in industrial market consists in a lack of fiscal incentives for medium or small companies to invest in renewable technologies. In this scenario, only multinational companies are able to research and implement innovative and renewable technologies.
Nowadays Brazil does not have suitable technology to produce rare earth compounds to supply internal demand. Many oxides have been imported such as yttrium oxide (Y2O3), cerium oxide (CeO2), dysprosium oxide (Dy2O3) and also metal alloys as cerium-iron. The main owners of rare earth sources are China, United States, Australia, Malaysia and India, whereas Brazil has only 0.03% of the total. China has the biggest sources, the most producers and developed a competitive processing technology of rare earth compounds.
Even though Brazilian market of rare earth has not been consolidated yet, many efforts have been done by universities and institutes to develop competitive technology to design materials based on rare earth elements [
Face to a global challenge which is waste management, this work aims to present the perspectives on development of rare earth porous burners for lighting in Brazil. In addition, the use of biogas is an alternative to convert waste to energy solution and the welfare and comfort of the society can be maintained by a sustainable program.
The main world owners of rare earth sources
Brazilian publications in national and international scientific and technological periodicals about rare earth technology [45]
A mixture of yttrium rare earth carbonate (CaYTR), a raw material achieved from monazite processing supplied by Nuclemon (Nuclebrás de Monazita e Associados, Brazil). The thermal processing condition of CaYTR powders were determinate by Thermal Gravimetric and Differential Analyses (TGA/TDA, Setaram S60/38336), with a heating rate of 10˚C/min, up to 1400˚C in air atmosphere and having alumina as reference material.
As oxides condition, the powder characterization of yttria rare earth oxides (YTR) was performed by the following technics, Helium Pycnometry (Micrometrics 1330); Scanning Electronic Microscopy (SEM, Philips XL30); X-Ray Fluorescence (XRF, Rigaku RIX 3000); X-Ray Diffraction (XRD, Rigaku Multiflex), with scanning at 1˚/min, range from 10˚ - 80˚ (2θ), Cu-Kα radiation; Photon Correlation Spectroscopy (PCS, Zeta PALS Analyzer, Brookhaven Instruments). For PCS, diluted aqueous solutions with 0.01 vol% of particles were prepared at pH 10.5 by adding NaOH solution (0.5 M). Before measurements samples were homogenized in ball mill for 24 h (optimized time), using alumina spheres; Specific Surface Area, by BET method (SSA, Micrometrics ASAP 2010). In addition to SSA, theoretical mean particle diameter (dBET) was determinate by BET equation (Equation (1)), which considers particles having spherical and homogeny morphology. Besides, based on dBET result was calculated the agglomeration factor (Fag) as shown in Equation (2).
where: dBET = theoretical mean particle diameter (mm); rt = theoretical density (g∙cm−3); SM = Specific Surface Area (m2∙g−1).
where: d50 = Experimental mean particle diameter (mm); dBET = Theoretical mean particle diameter (mm); Rheological behavior of YTR suspensions was performed with a rheometer (Haake RS600, Thermo Scientific, Germany). The sensor system consisted on a double cone rotor and a stationary plate (DC60/1˚). Suspensions characterization carried on by flow curves in a control rate mode (CR). Measurements were performed at 25˚C by increasing the shear rate from 0 to 1000 s−1 in 5 min, holding at 1000 s−1 for 2 min and returning to 0 in 5 min. For each CR mode 200 points were measured.
In replica method, a cotton-nylon netting template (TNA) was coated with YTR suspension and after squeezing out the excess; the structure was dried and subjected to careful thermal treatment in a vertical furnace (Lindberg/Blue M), wherein conditions were based on thermal and gravimetrical analysis (TGA/TDA) results of TNA. Sintered samples surface and microstructure were evaluated by stereoscope (Jena GSZ, Carl Zeiss) and SEM.
Light emission by thermal stimulation (thermoluminescence) of YTR powders was performed by TL reader (Risø TL/OSL-DA-20). The samples were evaluated at a heating rate of 10˚C/s until 400˚C in environmental atmosphere.
Thermal decomposition of CaTR powders (from carbonate to oxide) until 1200˚C is shown in
X-ray fluorescence (XRF) result showed that YTR powders have a majority concentration of Y2O3 (56.6 wt% ± 0.1 wt%) and Dy2O3 (19.4 wt% ± 0.5 wt%). Others rare earths are Er2O3 (6.5 wt% ± 0.1 wt%), Lu2O3 (4.1 wt% ± 0.4 wt%), Ho2O3 (3.1 wt% ± 0.1 wt%),Yb2O3 (wt%). The sum of other compounds was around 13.4 wt% (each one concentration less than 1 wt%). Yttria is very used as matrix for luminescent phosphors as Eu3+, Tb3+, Er3+, Dy3+ due to its physical and chemical proprieties are quite similar to REOs. In addition, Dy2O3 shows yellow emission spectrum, which color according to International Commission on Illumination (CIE) [
Mean particle size diameter of YTR powders by PCS is shown in
Thermal decomposition of CaTR powders until 1200˚C by TGA/TDA
Comparative of diffraction patterns of rare earths carbonate as received with after calcination at 750˚C/3h yttria―rare earth oxide
density were 20.45 m2∙g−1 and 6.03 g∙cm−3 respectively. High density of powders was due to high atomic weight of rare earth elements (Y, Dy, Er, Lu). Comparing calculated mean particle diameter (dBET = 48.7 nm) from Equation (1) with experimental (d50 = 722.6 nm) from PCS the difference was 673.9 nm. As a consequence high agglomeration factor (Fag) of 14.8 was achieved, which means the particles were much agglomerated. Besides, the morphology of particles can contribute to agglomeration state as shown by Scanning Electronic Microscopy (SEM) of YTR powders (
The flow behavior of 25 vol% YTR suspensions prepared with 0 - 0.7 wt% of CMC is shown in
Particle size characterization of YTR powders after thermal processing. (a) Mean particle size distribution by PCS; (b) micrograph by SEM showing like board shape agglomerates. CR flow curve of SYTR with 25 vol%
Flow behavior of 25 vol% of YTR suspensions prepared with 0 - 0.7 wt% CMC. (a) Flow curves in CR mode; (b) Apparent viscosity from 0 - 1000 s−1
effect on flow behavior was clearly defined. At low shear rates (<200 Pa∙s−1) all suspensions based on CMC presented high viscosity values. However, from 200 Pa∙s−1 viscosity started decreasing until a minimum value at 1000 s−1. Furthermore, the viscosity parameter was dependent on shear stress/rate. Shear thinning suspensions are desired for replica, once viscosity is higher in static condition (good adhesion on template surface) and lower when an external force is applied (immersion of template in suspension).
The weight loss of TNA template as a function of temperature was evaluated by thermo gravimetric analyzes until 800˚C (
Furthermore, the control of the structure of ceramic suspension (stability) drives the forming step as well as the final microstructure. The advantage of the proposed method consists in faster forming, where only one immersion of template into ceramic suspension was required. Complementary methods to improve impregnation as recoating [
Thermogravimetric analysis of TNA template (10˚C/min, in air)
YTR nettings by replica impregnated with 25 vol% YTR suspensions, having from 0.2 - 0.7 wt% CMC and sintered at 1600˚C/1h. (a) CNT template; YTR porous netting architecture produced with (b) 0.7 wt% CMC; (c) 0.5 wt% CMC; (d) 0.2 wt% CMC; (e) micrograph of netting cell from (d); (f) zoom of an open porous from (e) micrograph
of their spectra. In addition, one rare earth ion can interfere with other rare earth ion emission. Consequently, the maximum intensity and spectra wavelength are damage as well as the light emission performance for gas burners applications.
As an emerging economy Brazil has been producing thousands of organic waste (biomass), which should be converted onto biogas. Besides, this fuel should be used for lighting by porous ceramic burners contributing to green economy. By technological view, colloidal processing is a viable alternative to produce porous ceramics with potential to be applied as biogas burners. Ceramic suspensions based on a mixture of rare earth oxides-yttria (25 vol%), pH = 10, 1 wt% of ammonium poly-acrylic (PAA), 0.2 wt% of cellulose carboxyl methyl (CMC) presented suitable viscosity to be applied in impregnation process. By replica method near net shape ceramic
Emission spectra of YTR powders heating at 10˚C/s until 400˚C in environmental atmosphere
nettings with homogeneous morphology of cells, struts and porous microstructure were produced by only one immersion step and sintering at 1600˚C/1h. Thermoluminescence evaluation showed that whole emission of YTR powders fell on infrared range (λ = 750 - 4300 nm), with λmax at 1000 nm at 400˚C behaving as black body. The interference between different rare earth ions results in no visible light emission, which damaged the performance for gas burners applications.
We authors are deeply thankful for Dr. Linda Caldas and Dr. Maira Tiemi Yoshizumi to help us with thermoluminescence characterization of yttria powders; MSc. Douglas Will Leite and MSc. William Naville for collaboration with materials characterization by Scanning Electron Microscopy; Dr. Ana Maria Segadães and Dr. Thomaz Restivo for revising this work; The State of São Paulo Research Foundation (FAPESP) and The National Council for Scientific and Technological Development (CNPq) for financial support; in addition, Coordination of High Degree People Improvement (CAPES) for scholarship support of the student Silas Cardoso dos Santos.