Hydrothermal Liquefaction of Municipal Sludge and Biocrude Upgrading Using Carinata Oil

Abstract

Crude oil is a finite resource. Due to an advancement in technology, it is now possible to produce petroleum from very tight and hard reservoir formations. However, the global outcry towards the adoption of sustainable and renewable energy sources has led to research into feedstocks that could justify the investment. This will expectedly limit the role of crude oil in the global economy, and will favor the use of wet wastes such as municipal sludge. Municipal sludge is a product of waste treatment processes. Conventional sludge treatment methods include incineration, land application and composting. The release of toxic substances into the atmosphere coupled with demonstrated leaching of contaminants into the subsurface are making these treatment methods unpopular. There is, therefore, a need to carry out research into better ways of valorizing this waste resource. This study presents the comparison on the effect of temperature and solids content on the product yield and fuel quality of municipal sludge hydrothermal liquefaction (HTL) products with and without using red clay as a catalyst. HTL was carried out on three sample types namely: Pre-dried sludge (PD: dewatered secondary sludge that was further pre-dried prior to use), Secondary sludge (Sec: after aerobic digestion but before running through the belt press) and Thickened sludge (TK: secondary sludge that has gone through the mechanical press for solids content increase at the treatment station). HTL was carried out at 300ºC and 325ºC, with a reaction residence time of 1hour and a continuous stirring speed of 95RPM maintained in all experiments. The highest oil yield of 37.7±1.6 wt.% (dry basis, ash free) was obtained in the thickened sludge liquefied at 325ºC, while the highest char yield of 20.6±1.6 wt.% was obtained in the catalyzed secondary sludge liquefaction at 325ºC.The higher heating value of the produced biocrudes ranged from 24.2±0.3 MJ/kg (catalyzed secondary sludge oil produced at 300ºC) to 35.0±0.8 MJ/kg (pre-dried sludge oil produced at 300ºC). The total acid number (TAN) was lowest (13.1±0.3 mg KOH/g) for the thickened sludge biocrude produced at 300ºC, and was highest (19.8±1.6 mg KOH/g) for the secondary biocrude produced at 325ºC. The integration of the red clay catalyst significantly improved the TAN value of all produced biocrudes (Pre-dried sludge biocrude: 19.3±0.4 to 17.4±0.1 mg KOH/g at 300°C and 15.7±0.8 to 13.8±0.3 mg KOH/g at 325°C; Secondary sludge biocrude: 17.0±0.2 to 14.9±0.1 mg KOH/g at 300°C and 19.8±1.6 to 17.9±0.3 mg KOH/g at 325°C; Thickened sludge biocrude: 18.6±0.4 to 15.7±0.2 mg KOH/g at 325°C). The obtained char products had a low HHV due to high oxygen content. The produced aqueous phase was rich in nutrients with high concentration of ammonium ions observed. Catalytic upgrading of municipal sludge based biocrudes became necessary due to high amount of heteroatoms (N,S and O) present in the oil. The cost of catalysts could significantly impact the economics of the overall cost of the fuels, and research into low cost catalytic options has become necessary. The synergistic effect of hydrotreating this cracked oil with municipal biocrude over inexpensive Ni/SiO2-Al2O3 is explored in this study. Triglycerides such as waste cooking oil and non-edible oils have been studied for possible reintegration as liquid transportation fuels. Esterification processes have revealed that the produced biodiesel was much comparable with petroleum-based diesel, thereby making the co-processing of both fuels very effective. Catalytic cracking of these triglycerides helps to break the glyceride backbones, thereby producing fuels with improved viscosity and higher heating value. The catalytic upgraded oil yields at 250°C 3h, 300°C 1h, 350°C 1h and 350°C 30min did not vary significantly in terms of the liquid product yield; although longer processing times favored the production of more solids. The higher heating value of the produced liquids was also comparable to petroleum-based liquid fuels, with the highest value recorded being 45.1±0.6 MJ/kg from processing at 350ºC for an hour. The 350°C 30min processing condition was most efficient, and was studied further. The increase in the catalyst loading rate resulted in the production of liquid fractions with lower viscosity and acidity, and the production of higher carbon gases due to further cracking of heavier compounds. The use of uncracked oil as opposed to cracking resulted in significantly lower liquid yield with lesser hydrogen consumption, suggesting slower levels of hydrodeoxygenation. The liquid, however, had a significantly lower total acid number of 0.89±0.07 mg KOH/g.