Enhancing the pyrolytic conversion of biosolids to value-added products via mild acid pre-treatment

High concentrations of inorganic matter such as silicates, alkali and alkaline earth metals (AAEMs), and heavy metals (HMs) in biosolids limit their pyrolysis conversion to high-value products. Therefore, the reduction or passivation of the deleterious pyrolytic activities of these native inorganics in biosolids can enhance the yield and quality of products obtained during pyrolysis. The pyrolysis of raw and 3% sulfuric acid pre-treated biosolids was carried out in a fluidised bed reactor at 300–700 ℃, and the influence of pre-treatment was examined on biochar properties, gas production, and bio-oil composition. At all temperatures, the selective removal of ash-forming elements (demineralisation) in biosolids by pre-treatment improved organic matter devolatilisation yielding higher bio-oil and lower biochar than untreated biosolids. Demineralisation weakened gas production, particularly at higher pyrolysis temperatures. At 700 ℃, the in-situ formation of acidic metal sulfate salts in sulfuric acid-infused biosolids facilitated H+ release, thereby increasing H2 yield to a maximum of 15 mol% compared to 8 mol% from untreated biosolids and 4 mol% from demineralised biosolids. Biochar produced from treated biosolids had considerably lower HMs concentration and higher organic matter retention compared to raw biosolids biochar. The effect of pre-treatment on biochar properties was profound at 700 ℃ pyrolysis temperature. Pre-treatment increased biochar fixed carbon by 57%, calorific value by 37%, fuel ratio by 44%, doubled the specific surface area from 55 to 107 m2/g, and enhanced porous structure formation. At 300 ℃, the major chemical compounds in the bio-oil were amides (20%), N-heterocyclics (25%), and ketones (30%), and higher temperatures favoured phenols and aromatic hydrocarbon production. Pre-treatment enhanced the selectivity of furans by 10-fold, anhydrosugars by 2-fold, and aromatic hydrocarbons by 1.5-fold relative to the raw biosolids bio-oil. Acid pre-treatment is a promising strategy for improving biosolids quality as feedstock for pyrolysis to generate high-value products.