No abstract

Slow pyrolysis is a promising technology to convert sewage sludge into char: a stable solid product with high carbon and phosphorus content. However, due to its heavy metals content, char use in agriculture is avoided in many European Union (EU) countries. This study aimed to test a solution, based on integrating slow pyrolysis and chemical leaching, to separate phosphorus and other inorganics from char, obtaining an inorganic P-rich fertiliser and a C-rich solid usable for industrial purposes. The sludge was first characterized and then processed in a 3 kg/h slow pyrolysis reactor at 450 °C for 30 min. The resulting char was processed by chemical leaching with acid (HCl, HNO3) and alkali (KOH) reagents to extract inorganic compounds. To optimize the inorganic extraction, three case studies have been considered. The char obtained from sewage sludge pyrolysis contained around 78% d.b. (dry basis) of inorganics, 14% d.b. of C, 14% d.b. of Al, and almost 5% d.b. of P. The leaching tests enabled to extract 100% of P, Mg, and Ca from the char. The remaining char contained mainly carbon (27%) and silica (42%), with a surface area of up to 70 m2/g, usable as adsorbent or precursor of sustainable materials.

No abstract

Fast pyrolysis of biomass is a well-known opportunity for sustainable alternative fuel production for transport and energy. However, bio-oils from biomass pyrolysis are viscous, acidic bio-crudes that need further steps of upgrading before being used either as fuels or chemicals. A process that is complementary to bio-oil hydrotreatment or co-processing consists of optimizing and tuning the upstream condensation steps of fast pyrolysis to separate and concentrate selected classes of compounds. This can be implemented by varying the condensation temperatures in a multi-step condensation unit. In this study, fractional condensation of fast pyrolysis vapors from pinewood has been applied to a bubbling fluidized bed reactor of 1 kg h−1 feed. The reactor was operated at 500 °C and connected to a downstream interchangeable condensation unit. Tests were performed using two different condensing layouts: (1) a series of two spray condensers and a tube-in-tube water-jacketed condenser, referred to as an intensive cooler; (2) an electrostatic precipitator and the intensive cooler. Using the first configuration, which is the focus of this study, high boiling point compounds—such as sugars and lignin-derived oligomers—were condensed at higher temperatures in the first stage (100–170 °C), while water-soluble lighter compounds and most of the water was condensed at lower temperatures and thus largely removed from the bio-oil. In the first two condensing stages, the bio-oil water content remained below 7% in mass (and therefore, the oil’s high calorific content reached 22 MJ kg−1) while achieving about 43% liquid yield, compared to 55% from the single-step condensation runs. Results were finally elaborated to perform a preliminary energy assessment of the whole system toward the potential upscaling of this fractional condensation approach. The proposed layout showed a significant potential for the upstream condensation step, simplifying the downstream upgrading stages for alternative fuel production from fast pyrolysis bio-oil.