Turning sewage sludge into sintering fuel based on the pyrolysis I: lipid content and residual metal.

The use of pyrolysis to produce oil from sludge by the evaporation-condensation process is a promising technique. However, the resulting lipids are prone to be acidized under exposure to oxygen, which can affect their quality and use. To eliminate the need for this oil separation process, the present work uses blended pyrolysis to preserve the oil in the char and to prevent it from deteriorating. At the same time, metals are eliminated to a secure level of combustion emissions. The sludge was pyrolyzed into a sintering fuel through blended pyrolysis with SiO2, Al2O3, and sand. These materials are the main components of the sintered ceramsite obtained. Therefore, the influence of these substances and residence time on lipid formation and metal residue in the char were investigated. Non-blended pyrolysis required a 40-min duration, whereas sand-pyrolysis required 10 min to achieve the same yield. The concentration of C16:0 produced by blended pyrolysis with sand reached 2177 mg kg-1, which is 57% higher than that of non-blended pyrolysis. Blended pyrolysis with SiO2 required at least 20 min to immobilize As metal. In summary, blended pyrolysis simplifies the process, reduces time, and produces char with lipid-rich and low metal leaching, which can be used as a sintering fuel.

Sludge char-to-fuel approaches based on the pyrolysis III: Adding protein without dehydration.

Urban expansion has led to the accumulation of sludge, and its disposal has to meet increasingly stringent requirements. Therefore, pyrolysis has become an alternative option. However, it was still unclear which part of the sludge could be pyrolyzed to generate the product with a higher heating value, and therefore we divided sludge into extracellular polymeric substances (EPS) and cell phase and measured their heating values respectively. The obtained results showed that the high heating value (HHV) of the pyrolysis cell phase accounted for 85% of the sludge pyrolysis, and the addition of protein significantly increased the heating value of each component. Although the HHV of the pyrolysis cell phase increased by 1.8 MJ kg-1 for every 1% increase in protein, the HHV of the pyrolysis sludge and EPS increased by only 1.2 MJ kg-1. It is therefore suggested that EPS may contain substances that inhibit heat release. Properly increasing the cellular or protein components in the sludge could significantly increase the HHV produced by pyrolysis. Based on the measurement of fatty acids (FAs) and alcohol content and FTIR results, the addition of protein could increase the saturated FAs and accelerate the replacement of oxygen with nitrogen in the pyrolysis product, resulting in higher HHV. If the sludge was not dehydrated, more volatile compounds were carbonized and the HHV increased from 12 MJ kg-1 to 19 MJ kg-1. In short, since the HHV of the sludge was mainly derived from the cell phase, the HHV generation could be improved by increasing the cell phase or protein content without dehydration.

Sludge char-to-fuel approaches based on the catalytic pyrolysis II: heat release.

The pyrolyzed sludge is concerned currently, while the produced higher heating value (HHV) is unclear yet. In this work, the effects of moisture content (MC), catalysts amount, and catalytic types on the HHV production were investigated. Based on the known fatty acids (FAs) and alcohol content, the heat release by catalytic and non-catalytic pyrolysis product was examined. A good correlation between the measured and calculated HHV in non-catalytic pyrolysis indicates that the method can effectively evaluate the pyrolysis effect. The results show that a higher HHV can be obtained by adding a catalyst when the MC was between 20 and 40% compared to the non-catalytic pyrolysis. In the catalytic pyrolysis, the maximum HHV produced by bentonite is 50.61 MJ kg-1. Bentonite can rapidly initiate the decarboxylation but sand was a potential efficient catalyst because of the enrichment of large amounts of FAs C16:0. If sand is used in combination with bentonite, C16:0 may be enriched and further decarboxylated, eventually releasing more heat. Since sand is composed of SiO2 and Al2O3, in the production of HHV, the addition of Al2O3 has a better catalytic effect than adding SiO2. For the evaluation of catalytic pyrolysis products and HHV, it is proposed that the possibility of adding two types of catalysts for pyrolysis is of great significance for realizing sludge to the fuel.