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Sewage sludge, the inevitable by-product of municipal wastewater treatment plant operation, is a key issue in many countries due to its increasing volume and the impacts associated with its disposal. Thermochemical processing offers a new way of managing sewage sludge, not only by providing effective volume reduction, but also enabling transformation of carbon-rich organic fraction into valuable energy and fuel. Owing to some unique properties, sewage sludge differs from other solid fuels such as lignocellulosic biomass and coal, making its thermochemical conversion application somewhat complicated and challenging. This paper reviews the options of converting sewage sludge to energy and fuel via three main thermochemical conversion processes namely pyrolysis, gasification and combustion. The fundamental aspects of sewage sludge and its behaviour in each of thermochemical process are summarised. The challenges in adopting thermochemical conversion technology in sewage sludge management are addressed, and various alternative approaches deserving further consideration, such as the incorporation of pre-processing and co-utilisation, are discussed. [ABSTRACT FROM AUTHOR]

• Char structure was investigated via characterization of its extraction and residue. • Effects of pellet sizes on char structure were weakened by temperatures. • Polymerisation of extract is account for the structural difference of pellet chars. • Aromatisation degree of pellet char is related to its surface-area-to-volume ratio. Char structure can reflect the pyrolysis characteristics and affect the efficiency of gasification and combustion processes, while biomass pellet size and shape are critical for the heat and mass transfer, which in turn influence greatly char structure. The aim of this study is to experimentally study and compare the effect of aspect ratio (diameter/length) on char structure during the pyrolysis of sawdust pellet. Cylindrical sawdust pellets with similar masses and volumes but different diameters (6, 8, 12, 16 mm) were pyrolysed at various temperatures. The chars were separated into two phases (liquid extraction, solid residue) via microwave-assisted extraction. Experimental results show that the pellet with a low aspect ratio had a large char yield because of intra-particle volatile-char interaction especially at low temperature, while there was an optimum aspect ratio due to the change of residence time. The pellet with a high aspect ratio had an intense polymerisation, resulting in the formation of fused aromatic rings in char and residue. By increasing aspect ratio, there was a lowest surface area. This means that less heat entered from surface, and low thermal conductivity caused a slow decomposition. Due to large heat and mass transfer residence, more aromatic molecules with small weight formed in extracts of char, especially at high temperature. Besides, an increase in temperature weakened the differences in structural features of the pellet char caused by different aspect ratios. [ABSTRACT FROM AUTHOR]

• Fe/activated char shows high performance on bio-oil upgrading. • The fresh catalysts favor the production of aromatic hydrocarbons. • The regenerated catalysts by active agents facilitate the production of phenols. • The catalyst regeneration promotes the formation of γ-Fe 2 O 3 on the char surface. Char-supported metals as catalysts has attracted much attention due to the renewability and structural controllability of char. However, such catalysts can not be regenerated by the conventional calcination method, which is widely used in industy. In this study, the catalytic performance of char-supported Fe on bio-oil upgrading during pyrolysis of pine wood was first investigated. It was found that the typical components (acids, ketones, furans, etc) in bio-oil were completely converted to aromatic hydrocarbons over char-supported Fe, in which char acted not only as a support but also as a catalyst for the converison of bio-oil to phenols. Then, the spent char-supported Fe catalyst was regenerated by CO 2 or steam gasification. The results showed that only phenols could be produced over both CO 2 and steam regenerated catalysts, which was caused by the changes in carbon structure and surface functional groups of char as well as the species of Fe. The CO 2 regenerated catalyst was rich in C O groups and α-Fe 2 O 3 , while the H 2 O regenerated one mainly contained O C O groups and Fe/FeO. [ABSTRACT FROM AUTHOR]

• Char activity in volatile-char interactions varies with biomass components. • Xylan char rich in C O group is the most active for (benzyloxy)benzene conversion. • Char C(sp2) O group promotes the breaking of C(sp3) O in (benzyloxy)benzene. • Char C(sp3) O group promotes the breaking of C(sp2) O in (benzyloxy)benzene. • Lignin char is highly selective to monomers due to the abundant C O group. Char activity is a critical factor for the interactions between char and volatile, which are recognized to determine the final product distribution from biomass pyrolysis. To clarify the effect of biomass composition on char activity, the interactions between (benzyloxy)benzene as a volatile model compound and char samples obtained from cellulose, xylan, and lignin were studied. The (benzyloxy)benzene conversion increased from 0.64 % for no char to 6.31 %, 6.98 % and 53.5 % for lignin char, cellulose char, and xylan char, respectively, which were mainly derived from the contribution of surface C O groups and surface defects in the carbon structure. Especially, surface C(sp2) O groups and C(sp3) O groups on char respectively favored the breakage of C(sp3) O and C(sp2) O bonds in (benzyloxy)benzene. Despite the low activity, lignin char showed high selectivity to monomeric products due to the high content of surface C O groups, which were considered to induce the production of hydrogen radicals to stabilize the resulting intermediates. The findings of this study prove that the volatile-char interaction could be adjusted facilely by changing the hemicellulose content during feedstock pretreatments. [ABSTRACT FROM AUTHOR]