An experimental work was carried out to investigate the viability of energy recovery from the air-steam gasification of sewage sludge. The relative influence of different factors, as well as the effect of their possible interactions, has been determined by means of analysis of variance. Temperature was found to be the most influential factor for most of the variables analyzed. Solid yield (35-41 wt. %) and tar content (11-45 g/m 3 STP ) were largely reduced with temperature, whereas gas production (0.89-1.32 m 3 STP /kg sewage sludge dry and ash free), carbon yield to gas phase (62-90 wt. %), gasification efficiency (39-66 %), and H 2 and CO yields (20-52 and 137-414 g/kg sewage sludge dry and ash free, respectively) were improved at high temperature. Other important parameters for the end-use of the gas such as its heating value (4.12-6.20 MJ/m 3 STP ) and its H 2 /CO molar ratio (1.46-3.25) were greatly influenced by the composition of the gasification medium, since the increase in the steam to oxygen ratio was favourable for both. The comparison of experimental and theoretical results highlights that equilibrium was not reached during the experimental runs.

Unlike petroleum diesel, the chemical structure of biodiesel makes it prone to oxidation during long-term storage, thus involving fuel quality deterioration. Therefore, the addition of antioxidants is usually required to meet the quality standards for biodiesel commercialization. Synthetic sterically-hindered phenols have been usually employed for this purpose as free radical scavenging antioxidants. However, naturally occurring phenolics are also available, for example, in the bio-oil produced in the pyrolysis of lignocellulosic biomass. In this work, the antioxidant potential of extracted fractions of lignocellulosic bio-oil has been evaluated. Different organic solvents were tested as extraction agents, acetate esters being the best ones for incorporating bio-oil antioxidant compounds into biodiesel. In the best case, the incorporation of a small concentration of bio-oil compounds (< 4 wt. %) led to an improvement of the biodiesel oxidation stability of 475 % which, in our case, was enough to meet the European standard requirement.

A three-stage thermochemical process comprising torrefaction, pyrolysis, and char activation is proposed for the treatment of dry sewage sludge or biomass materials. To assess the feasibility of the process, lab-scale experiments were carried out with dried sewage sludge as feedstock, and mass and energy balances were calculated. In the process, 19.3% of the sewage sludge initial weight was transformed into a bio-oil with three distinct phases and reduced water content (66.1% of water content in the aqueous phase compared to 73.8% in a single-step fast pyrolysis). The product gases had a high H 2 S content but also enough heating value to be combusted. After being activated by the torrefaction vapors, the solid fraction (48.2% of the initial sludge weight) showed certain pore development and might be suitable for adsorption applications. Regarding the energy balance, it was found that the combustion of part of the product gas would provide the necessary heat to drive the process (1019 kJ/kg of dry sewage sludge).

Air-steam gasification of char derived from fast pyrolysis of sewage sludge has been experimentally evaluated in a fluidized bed as a route towards a full recovery of energy from sewage sludge. The results have been compared with those obtained from the direct gasification of sewage sludge in order to evaluate how the previous pyrolysis stage affects the subsequent gasification process. The fixed carbon content in the solid increased after the pyrolysis stage so that heterogeneous reactions of carbon with steam or CO 2 assumed greater importance during char gasification than during sewage sludge gasification. Furthermore, char gasification led to an improvement in the gas yieldcalculated on a dry and ash-free basis (daf)-due to the increased concentration of carbon in the organic fraction of the solid after the pyrolysis step, with an increase in the average CO yield of about 70% -in terms of g/kg solid daf-. The reduction in the fraction of carbon which forms tar is another advantage of char gasification over the direct gasification of sewage sludge, with an average decrease of about 45%. Regarding the influence of the operating conditions, the response variables were mainly controlled by the same factors in both processes.

a b s t r a c tThermo-chemical treatment of sewage sludge is an interesting option for recovering energy and/or valuable products from this waste. This work presents an energetic assessment of pyrolysis and gasification of sewage sludge, also considering the prior sewage sludge thermal drying and the gasification of the char derived from the pyrolysis stage. Experimental data obtained from pyrolysis of sewage sludge, gasification of sewage sludge and gasification of char (all of these performed in a lab-scale fluidized reactor) were used for the energetic calculations. The results show that the energy contained in the product gases from pyrolysis and char gasification is not enough to cover the high energy consumption for thermal drying of sewage sludge. Additional energy could be obtained from the calorific value of the pyrolysis liquid, but some of its properties must be improved facing towards its use as fuel. On the other hand, the energy contained in the product gas of sewage sludge gasification is enough to cover the energy demand for both the sewage sludge thermal drying and the gasification process itself. Furthermore, a theoretical study included in this work shows that the gasification efficiency is improved when the chemical equilibrium is reached in the process.

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Due to its metal content, sewage sludge ash appears as a potential sorbent material for H 2 S removal at high temperature. The desulphurization ability of the solid by-products of combustion and gasification of sewage sludge has been evaluated in this work. Ash characterization results revealed that metal fraction in sewage sludge did not remained completely inert during the combustion and gasification processes. Iron content was lower in the gasification ash and X-ray patterns showed different crystalline phases in the solids: Fe 2 O 3 in the combustion ash and Fe 3 O 4 in the gasification ash. These differences resulted in a lower sulphur capture capacity of the gasification ash.Desulphurization tests were carried out in a lab-scale fixed bed reactor operating at 600-800 ºC. Different gases containing 5000 ppmv H 2 S (H 2 S/N 2 mixture and synthetic gasification gas) were used. The H 2 S breakthrough curves were negatively affected by the reducing atmosphere created by the gasification gas and by the presence of steam in the reaction medium. However, H 2 S breakthrough curves alone do not provide enough information to evaluate the sulphur capture capacity of the sorbent materials. Ultimate analyses of the spent solid samples showed that the total amount of H 2 S removed from the gas was only partially captured in the ash. Thermodynamic data pointed to a significant fraction of sulphur forming part of other gases, such as SO 2 . In the best 2 operating conditions, an outlet gas with less than 100 ppmv of H 2 S was obtained during 300 min, thus resulting in a sulphur loading of 63 mg S·g -1 ash . This experimental sulphur content was 39% lower than the maximum theoretical value predicted by equilibrium simulations.