J.A. Menéndez

On the pyrolysis of sewage sludge: the influence of pyrolysis conditions on solid, liquid and gas fractions

Abstract The pyrolysis of a sewage sludge, produced by a Spanish urban wastewater treatment plant, was carried out in a laboratory furnace. Pyrolysis conditions, like heating rate and final pyrolysis temperature, were varied so that their influence on the characteristics of the resulting gases, liquids and solid residues could be studied. It was found that increasing the pyrolysis temperature decreases the solid fraction yield and increases the gas fraction yield while that of the liquid fraction remains almost constant. Furthermore, the effect of the heating rate was found to be important only at low final pyrolysis temperatures. Independently of the pyrolysis conditions, all the solid products obtained were of a basic nature and highly macroporous, the meso- and micro-pore volumes being relatively low. Both oils and gases produced in the pyrolysis showed relatively high overall heating values, comparable to some conventional fuels, revealing the potential application of these products as fuel.

Microwave-induced pyrolysis of sewage sludge

This paper describes a new method for pyrolyzing sewage sludge using a microwave furnace. It was found that if just the raw wet sludge is treated in the microwave, only drying of the sample takes place. However, if the sludge is mixed with a small amount of a suitable microwave absorber (such as the char produced in the pyrolysis itself) temperatures of up to 900 degrees C can be achieved, so that pyrolysis takes place rather than drying. Microwave treatments were also compared with those carried out in a conventional electric furnace, as well as the characteristics of their respective carbonaceous solid residues.

Thermal stability of oxygenated functions in activated carbons

Activated carbons with various degrees of oxidation were prepared by treatment with concentrated nitric acid at various temperatures. Selective removal of the oxygen complexes by heating under nitrogen flow at 573, 773 and 1073 K was also considered. The samples were characterised through temperature-programmed desorption (TPD), Fourier transformed infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) techniques, FTIR and XPS techniques were found to be very useful for relating the shape of CO2 desorption profiles during pyrolysis with the nature of the functional groups desorbed. Three steps in TPD profiles were observed: (i) the desorption of carboxylic acid groups in the form of CO2 in the lower temperature range, 400-623 K; (ii) the decomposition of lactone groups created during HNO3 treatment and, to some extent, originated during heating rearrangements, in the temperature range 623-823 K; and (iii) the elimination of the more stable ether groups upon treatment at temperatures above 823 K. The combination of these three techniques (TPD, FTIR and XPS) gives a reasonable picture of the surface chemistry of the oxidised activated carbons

Gas chromatographic-mass spectrometric study of the oil fractions produced by microwave-assisted pyrolysis of different sewage sludges.

The pyrolysis of sewage sludge was studied in a microwave oven using graphite as microwave absorber. The pyrolysis temperature ranged from 800 to 1000 degrees C depending on the type of sewage sludge. A conventional electrical furnace was also employed in order to compare the results obtained with both methods. The pyrolysis oils were trapped in a series of condensers and their characteristics such as elemental analysis and calorific value were determined and compared with those of the initial sludge. The oil composition was analyzed by GC-MS. The oils from the microwave oven had n-alkanes and 1-alkenes, aromatic compounds, ranging from benzene derivatives to polycyclic aromatic hydrocarbons (PAHs), nitrogenated compounds, long chain aliphatic carboxylic acids, ketones and esters and also monoterpenes and steroids. The oil from the electric oven was composed basically of PAHs such as naphthalene, acenapthylene, phenanthrene, fluoranthene, benzo[a]anthracene, benzofluoranthenes, benzopyrenes, indenepyrene, benzo[ghi]perylene, and anthanthrene. In contrast, these compounds were not produced in the case of microwave-assisted pyrolysis.

Modification of the surface chemistry of active carbons by means of microwave-induced treatments

The surface chemistry of two active carbons was modified using a microwave device as a heat source. When microwave treatments are conducted in a nitrogen flow, most oxygen-containing groups are removed from the surface of the carbons. This in turn gives rise to a significant increase in the pH of the carbons. Microwave treatment is less time-consuming than conventional heating. Only a few minutes are required to transform an acidic carbon into a basic carbon with a relatively low oxygen content. On the other hand, microwave-treated carbons undergo re-oxidation, to a greater or lesser extent, upon atmospheric exposure. It was observed that atmospheric re-oxidation of microwave-treated carbons tends to restore more acidic groups when the treatment is carried out on an acidic precursor than when the carbon is of a basic character. Nevertheless, carbons exposed to the atmosphere for up to 2 months still showed higher pHs than those of the corresponding precursors.

Microwave pyrolysis of microalgae for high syngas production

The microwave induced pyrolysis of the microalgae Scenedesmus almeriensis and its extraction residue was carried out at 400 and 800°C. The results show that it is possible to obtain a gas fraction with a high content (c.a. 50vol.%) in H2 from both materials, regardless of the pyrolysis temperature. Furthermore, an outstanding syngas production and high gas yields were achieved. The maximum syngas concentration obtained was c.a. 94 vol.%, in the case of the pyrolysis of the residue at 800°C, indicating that the production of CO2 and light hydrocarbons was minimized. The same experiments were carried out in a conventional electric furnace in order to compare the products and yields obtained. It was found that microwave induced pyrolysis gives rise not only to higher gas yields but also to greater syngas and H2 production.

Low temperature regeneration of activated carbons using microwaves: Revising conventional wisdom

The purpose of this work was to explore the application of microwaves for the low temperature regeneration of activated carbons saturated with a pharmaceutical compound (promethazine). Contrary to expectations, microwave-assisted regeneration did not lead to better results than those obtained under conventional electric heating. At low temperatures the regeneration was incomplete either under microwave and conventional heating, being this attributed to the insufficient input energy. At mild temperatures, a fall in the adsorption capacity upon cycling was obtained in both devices, although this was much more pronounced for the microwave. These results contrast with previous studies on the benefits of microwaves for the regeneration of carbon materials. The fall in the adsorption capacity after regeneration was due to the thermal cracking of the adsorbed molecules inside the carbon porous network, although this effect applies to both devices. When microwaves are used, along with the thermal heating of the carbon bed, a fraction of the microwave energy seemed to be directly used in the decomposition of promethazine through the excitation of the molecular bonds by microwaves (microwave-lysis). These results point out that the nature of the adsorbate and its ability to interact with microwave are key factors that control the application of microwaves for regeneration of exhausted activated carbons.

Reactivity of pyrolyzed sewage sludge in air and CO2

Abstract Pyrolysis is currently being considered as an alternative method of treating sewage sludge. It yields residual oils and gases, which can be used as fuels, and a solid which can either be burned or physically activated with air or CO 2 . The aim of this work was to study the influence of different pyrolysis conditions (e.g. temperature and heating rate) on the reactivity in air and in CO 2 of carbonaceous materials obtained from these types of residues. An anaerobic sewage sludge produced in a Spanish urban waste water treatment plant, containing 5 wt.% moisture after air-drying, was pyrolyzed in an electrical laboratory furnace under different pyrolysis conditions. Non-isothermal reactivities (up to 1100°C) in air and in CO 2 of the carbonaceous materials obtained after pyrolysis were performed in a thermobalance. The TG and DTG curves obtained from these experiments are discussed.

Fast microwave-assisted synthesis of tailored mesoporous carbon xerogels

Resorcinol-formaldehyde carbon xerogels with several initial pH were synthesized using two different heating methods (conventional and microwave heating). The effect of the pH of the precursor solution and the method of synthesis employed on the textural and chemical properties of the final materials was evaluated. It was found that both methods produce tailored carbon xerogels depending on the initial pH and that the pores of the carbon xerogels become larger as the initial pH decreases. High pHs result in exclusively microporous carbon xerogels, while a decrease in the amount of NaOH added, i.e. lower pH, causes the materials to evolve firstly into micro-mesoporous samples and then into micro-macroporous carbon xerogels. The main difference between the two heating methods studied, apart from the duration of the synthesis (i.e. approximately 5 h for the microwave-assisted synthesis as opposed to several days by conventional methods) lies in the meso-macroporosity of the resulting materials, since microwave radiation produces mainly mesoporous carbon xerogels with a specific mesopore size over a wider range of pH than conventional synthesis. For example, the pH range for mesoporous MW samples is 4.5-6.5 while equivalent samples that are conventionally synthesized require an initial pH of between 5.8 and 6.5. This work also illustrates a simple and precise method for determining the gelation point (t(g)) of different pH resorcinol-formaldehyde mixtures, based on varying the energy consumed by the microwave device during the synthesis of organic gels, without the need for other more complicated techniques.

On the distribution of oxygen-containing surface groups in carbons and their influence on the preparation of carbon-supported molydenum catalysts

Abstract Activated carbon was modified by thermal treatment in order to obtain carbon samples with a very different surface chemistry and similar textural properties. The distribution of oxygen-containing surface groups in these carbons was assessed. The difference between the point of zero charge and the isoelectric point (pHPZC−pHIEP) showed the same trend as the difference between the oxygen content on the external surface, determined by XPS, and that in the bulk, obtained by chemical analysis (%Osurface−%Obulk). Thus, pHPZC−pHIEP can be used as a parameter for indicating the radial distribution of oxygen-containing groups in carbons. Carbon samples were loaded with molybdenum, using the procedure of incipient-wetness impregnation. Hydrophilicity conferred by the oxygen functionalities to the carbon substrates promotes the formation of small, well dispersed crystals of the metal precursor on the support. Results seem to indicate that oxygen-containing surface groups work as anchorage centres for molybdenum complexes. The activated catalyst prepared with the most acidic carbon showed a higher amount of molybdenum than in the case of the other carbon supports. This may be due to the presence of metal sulphides that are bound to the oxygen groups of the support.