Gregorio Marbán

Low-temperature SCR of NOx with NH3 over carbon-ceramic supported catalysts

A new method for preparing vanadium oxide supported on carbon-ceramic cellular monoliths is described. This includes a support oxidation step with HNO3, followed by ionic exchange with a NaOH solution, equilibrium adsorption impregnation of VO 2+ and thermal treatment. As a result an active catalyst for low-temperature selective catalytic reduction (SCR) reaction is obtained. The V-catalyst is more resistant to SO2 poisoning than the previously developed Mn-catalyst. Inhibition by water is reversible for both types of catalysts. Testing of the vanadium catalyst after subjecting it to the outlet gas stream of a power plant shows fast deactivation until constant residual activity is reached. Deactivation seems to be caused by arsenic poisoning and the formation of superficial sulphates.

Stainless steel wire mesh-supported ZnO for the catalytic photodegradation of methylene blue under ultraviolet irradiation

The aim of this study was to assess the activity of catalysts formed by nanostructured zinc oxide supported on stainless steel wire mesh for the photocatalytic degradation of methylene blue under UV irradiation. Catalysts prepared by means of different low temperature synthesis methods, as described in a previous work (Vu et al., Mater. Res. Bull. 47 (2012) 1577-1586) were tested. A new activity parameter was introduced in order to compare the catalytic activity of the different catalysts. The best catalyst showed a catalytic activity higher than that of the reference material TiO(2) P25 (Degussa-Evonik). This high activity is attributed to a higher quantum yield derived from the small particle length of the ZnO deposited on the wire mesh. The photocatalytic degradation kinetics of methylene blue fitted a potential model with n orders ranging from 0.5 to 6.9. Reaction orders over 1 were attributed to catalyst deactivation during the reaction resulting from the photocorrosion of ZnO.

SURFACE AREA AND PORE SIZE CHANGES DURING SINTERING OF CALCIUM OXIDE PARTICLES

Changes in surface area and pore size during the sintering of CaO samples between 0·71 to 1 mm, were studied. Typical operating conditions in fluidized bed combustors (temperature between 973 and 1173  K and CO2 concentrations up to 15%) were chosen. The longest sintering time was 90min. The decrease in the surface area with sintering time was described using an empirical equation deduced from the German-Munir model. Surface area reduction and increase in pore size was strongly affected by the presence of CO2. Shrinkage of the CaO particles was not observed during sintering. The pore volume and porosity remain unaffected by sintering. A qualitative model, based on SEM microphotographs, for describing the sintering process was proposed.

Decomposition of nitrous oxide over ZSM-5 catalysts

Abstract Comparative kinetic and in-situ DRIFT studies of the N 2 O decomposition over Co-, Fe- and Cu-ZSM-5 have been performed. The implications of the presence of O 2 , CO, NO, H 2 O and SO 2 on the catalyst activity and stability and on the mechanism are evaluated.

Mechanism of low temperature selective catalytic reduction of NO with NH3 over carbon-supported Mn3O4

In this work, the interactions at low temperature (125 °C) of gaseous NH3, NO and O2 with carbon-supported manganese oxide catalysts have been extensively studied, mainly via step–response experiments, with the objective of explaining the low-temperature mechanism of the SCR reaction on such catalysts. To achieve this aim, the present work analyses the active phase and the interaction of NO species with the active phase both in the absence and in the presence of O2. The catalyst was found to be Mn3O4 composed locally of octahedral Mn2O3 and a slightly oxidised (oxygen excess) tetrahedral MnO. Both local species have different though interrelated roles during the SCR process. On the octahedral Mn2O3 NO can be adsorbed as nitrosyls (on oxygen vacancies) and as linear and bridged nitrites on oxygen atoms (absent under steady state SCR conditions). On the tetrahedral MnO NO2 is adsorbed as nitrates on hydroxyl groups previously formed by the reaction of water with the oxygen excess of this phase. NO2 is produced via the reaction of nitrosyls with O2. These nitrates are responsible for the partial deactivation of the catalyst under SCR conditions.

Sulphur retention by ash during fluidized bed combustion of bituminous coals

Abstract The present work examines SO 2 retention by coal ashes during the fluidized bed combustion of four bituminous coals with high ash content (> 47 wt%) and moderate sulphur content (≈ 1 wt%).The combustion was carried out in a 0.14 m i.d. fluidized bed reactor, in a range of temperatures between 700 and 900 °C. A dependence of the emissions on combustion temperature was not observed. The ashes resulting from combustion were examined by X-ray diffraction and scanning electron microscopy with energy dispersive X-ray analysis. The study shows that the sulphur retention at low temperature (700 °C) takes place by direct sulphation of the calcite. When the temperature is increased, the phenomena observed are calcination and breakage of the calcite particles. This results in an increase in the calcium content and consequently in the retained sulphur, in the smallest particles ( μ m). In consequence, most of the calcium and the retained sulphur are carried away in the elutriated material.

Kinetics of the Low-Temperature Selective Catalytic Reduction of NO with NH3 Over Activated Carbon Fiber Composite-Supported Iron Oxides

A kinetic analysis in a variety of conditions (gas composition and temperature) has been conducted on catalysts of iron oxides supported on activated carbon fiber composites (ACFC). Additionally, experiments of temperature-programmed desorption (TPD) of NO were conducted on the catalysts in order to reveal mechanistic features of the low-temperature SCR reaction. In the light of current SCR literature and previous work, a qualitative picture of the catalytic behavior of low-temperature SCR catalysts is offered. Apparently, the strength of adsorption of NO during the low-temperature SCR reaction is responsible for the governing reaction mechanism. Thus, highly stable nitrates formed on the surface provoke catalyst deactivation and reaction through an ER mechanism, with NO reacting from the gas phase, whereas the absence of these nitrates permits reaction of less stable nitrites from the catalyst surface, following an LH-type mechanism. This is the case for the ACFC-supported iron oxide catalyst analyzed in this work.

Influence of percolation on the modification of overall particle properties during gasification of porous solids

A quantitative model for gas-solid reactions has been used in order to analyze the effect of percolation on the evolution of the overall particle properties (particle size, overall porosity and reactive surface area) during gasification/combustion reactions. Simulations of gasification of porous reactive solid spheres under different reaction regimes have been performed. Additionally, the influence of apparent reaction order and solid porous structure on the variation of overall particle properties has also been included in the simulation results. The initial porous structure of the solid has been introduced by the pore size distribution and initial porosity, thus offering a realistic description of the solid pore network. The results include the variation of the overall solid properties during gasification under different reaction regimes (from chemical control to external mass transfer control). The influence of pore size distribution and initial porosity on particle transformations are also analyzed. The model was validated by comparing the simulation results with experimental data found in the literature.

Characterizing fuels for atmospheric fluidized bed combustion

A complete methodology for characterizing coal combustion in atmospheric fluidized bed reactors is presented. The methodology comprises studies of fragmentation and particle size variations during combustion, necessary to allow an accurate determination of kinetic parameters and attrition rates. Samples of three different carbonaceous materials (a medium-ash lignite, a medium-ash anthracite and a graphite) were pyrolyzed in N2 and partially burned in air in a bench-scale fluidized bed reactor (5 cm i.d.) at different operating conditions (0.68 mm < d0 < 3.50 mm, 750°C < Tb < 950°C and U = 0.13 m/s). The samples were retrieved from the sand-bed by means of a basket. The particle size distribution, apparent density and number of particles were evaluated by Image Analysis. Thus, the evolution of these parameters with burnoff was determined for different materials. Additionally, the sphericity factors were calculated. Combustion studies were carried out in batch experiments in the laboratory-scale, fluidized bed reactor at the same operating conditions. The reactor outlet concentrations of O2, CO2, and CO were monitored continuously. The results indicate that only anthracite particles experienced both primary (due to devolatilization) and secondary (during char combustion) fragmentation. Graphite particles underwent secondary fragmentation, whereas lignite particles did not significantly vary in number during combustion. Size and density variations during combustion suggest that graphite particles burn under regime II, intraparticle diffusion being the rate controlling step. On the other hand, anthracite and lignite particles developed an ash layer, which may control combustion. The attrition constants of the medium-ash materials (lignite and anthracite) were found to be very low (0.3 × 10−7 and 0.8 × 10−7, respectively), whereas that of graphite was much higher (17.6 × 10−7), due mainly to peripheral percolation during combustion.

Simulation of secondary fragmentation during fluidized bed combustion of char particles

A computer program has been developed in order to simulate the secondary fragmentation process during fluidized bed combustion of a population of char particles. The simulation intends to predict the evolution of char particle size distribution during combustion. It is based on two statistical functions, one which accounts for the probability density that a particle of a given size breaks into fragments and the other for the size distribution on the basis of the number of fragments generated by the breaking up of the particle. The parameters involved in the simulation have been obtained by minimizing the difference between theoretical predictions and experimental results. The latter were obtained by image analysis of partially burnt samples of anthracite char (d0 = 1,98 and 3.50 mm, 750 < Tb < 950 °C) and graphite particles (d0 = 1.98 mm, 850 < Tb < 950 °C) in a laboratory-scale fluidized bed reactor (inner diameter 5 mm).