AAU Update



11.05.2021 kl. 13.00 - 16.00


Ayaz Ali Shah, Department of Energy Technology, will defend the thesis "Hydrothermal Liquefaction of Sewage Sludge. Investigation of The Effects of Process Parameters, Recycling of Aqueous Phase, and Co-liquefaction on Bio-crude Properties"


Hydrothermal Liquefaction of Sewage Sludge. Investigation of The Effects of Process Parameters, Recycling of Aqueous Phase, and Co-liquefaction on Bio-crude Properties


Ayaz Ali Shah


Associate Professor Saqib Sohail Toor


Associate Professor Thomas Condra


Associate Professor Jens Bo Holm-Nielsen, Aalborg University (Chairman)
Senior Research Scientist, Brajendra K. Sharma, Illinois Sustainable Technology Center. University of Illinois at Urbana-Champaign (UIUC).
Professor Shuguang Deng, Professor in School for Engineering of Matter, Transport and Energy, Arizona State University. 



The visible changes in climate change lead the world towards the usage of renewable fuels, particularly in the transport sector to control GHG emissions and counter the dependence on fossil fuels. 

Hydrothermal liquefaction (HTL) is an efficient technology that thermochemically converts a variety of feedstocks into high-energy bio-crude. Nowadays, sewage sludge is considered to be a suitable candidate for renewable fuel production due to its cheap cost and abundant production. Thus, HTL is proposed as alternative disposal and simultaneous energy extraction process in the form of bio-crude from sewage sludge in promoting the principle of the circular economy. This PhD project investigated the three aspects of the valorization of the sewage sludge through HTL, its processibility at different temperatures, the impact of aqueous phase recycling on bio-crude properties, and its compatibility of co-liquefaction with other biomass like swine manure. 

In the first study, the effects of process conditions (temperature and catalyst) on the conversion of sewage sludge via HTL were deeply investigated. Four experiments were conducted at the sub and supercritical conditions 350 and 400°C, with and without alkali catalyst K2CO3. The results showed that in all the experiments, sufficient bio-crude yields (40%) were obtained. Whereas alkali catalyst slightly increased the bio-crude yield with reduction of solid residue at both temperature conditions. Moreover, the addition of the catalyst resulted in lower nitrogen content in the bio-crude with the average HHVs of 35 to 36 MJ/kg. The TGA analysis showed that almost 60% mass of all the bio-crudes was converted into gasoline, diesel, and jet fuel fractions. The bio-crude was composed of mostly N-heterocyclics, amides, ketones, acids, alcohols, and some fatty acids. During experiments, a substantial amount of organic carbon (19 to 27 g/l) was dissolved in the aqueous phase, particularly at supercritical conditions. The majority of the ash elements (Al, Ca, Cd, Cr, Cu, Fe, Mg, Mn, Ni, P, Pb, Zn) were recovered in the solid phase. While potassium and sodium showed a different trend as they were accumulated 30 to 50% in the aqueous phase. 

As the first study led to the higher carbon loss to the aqueous phase. In this context, a sustainable approach of aqueous phase recycling was adopted in the second study to recover that dissolved carbon from the aqueous phase. Eight experiments of sewage sludge were conducted at subcritical temperature 350°C with the recycling of the aqueous phase. The results revealed that aqueous phase recycling increased the bio-crude yield and energy recovery by 50% after eight rounds of recycling. However, the quality of the bio-crude was affected by higher nitrogen content (double with respect to the baseline experiment). The aqueous phase was comprised of mostly N-heterocyclic, short-chain acids (acetic acid, etc), alcohols, and ketones. Therefore, acetic acid (2.5% of the total slurry) was also employed as a catalyst to explore its catalytic effect, but no significant effect on bio-crude yield and quality was noticed. Aqueous phase recycling did not impact the distribution of the inorganic elements, as more than 80% of the elements were shifted to the solids phase. 
Phosphorus was mainly recovered in the solid phase, this implies that the solid phase has a good potential to be used as an extracting material for phosphorus recovery in the form of struvites. 

The third study explored the co-liquefaction of swine manure and sewage sludge-based upon their pumpability analysis (wet received basis). The syringe test demonstrated that swine manure was not pumpable due to its fibrous nature but it was efficiently pumped by the addition of sewage sludge as a co-substrate. For the detailed examination, SEM analysis was conducted to unfold the morphology of both of the feedstocks. A maximum of 80% swine manure was pumped with sewage sludge. Subsequently, five co-liquefaction experiments were performed on samples SM, SS,   SM/SS (50:50), SM/SS (80:20), and SM/SS (20:80). The higher bio-crude yield was obtained from mixed samples. The bio-crudes from sewage sludge containing samples had higher carbon and HHVs as compared to Swine manure alone. Almost 65% of the bio-crude was formed of volatile components within the range of 350° C. The aqueous phase analysis showed the values of TOC in the range of (26 to 40 g/l) and TN (3 to 14 g/l), and half of the TN was comprised of inorganic nitrogen. ICP-AES measurements indicated that the transference of inorganic elements was not highly affected by different mixing ratios of the feedstocks. A similar pattern of the migration of inorganic elements in the solid phase was observed as in the first and second studies.


THE DEFENCE IN ENGLISH - all are welcome.

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Department of Energy Technology