Effects of mild acid pre-treatment on the co-pyrolysis behaviour of biosolids and wheat straw

Abstract

Co-pyrolysis of biosolids (BS) and wheat straw (WS), as well as their treated variants, was performed in a fluidised bed reactor at 500 ℃. Under mild acid pre-treatment conditions (3% H2SO4 and 25 ℃), demineralisation was the dominating effect of pre-treatment on BS, while it was hemicellulose hydrolysis for WS. Consequently, the co-pyrolysis of raw biosolids (RBS) or treated biosolids (TBS) with raw wheat straw (RWS) or treated wheat straw (TWS) had varying effects on product distribution and product compositions. The co-pyrolysis of RBS+RWS gave the lowest bio-oil yield (36.3 wt%) and highest gas yield (23.1 wt%). In contrast, the highest bio-oil yield (45.8 wt%) and lowest gas yield (14.4 wt%) occurred during the co-pyrolysis of TBS+TWS. The degree of synergy in product distribution was strongest in TBS+RWS followed by RBS+RWS and was weakest in TBS+TWS, demonstrating the catalytic effect of inherent minerals on organic matter devolatilisation and char cracking reactions during co-pyrolysis. Biosolids pre-treatment alone performed competitively with BS co-pyrolysis alone with respect to enhancement in biochar properties producing biochar having 15–18 MJ/kg calorific value, 43–51 wt% carbon content, 0.7–1.0 fuel ratio, ⁓30% organic matter retention, and 27–41 wt% ash content. However, combining BS co-pyrolysis and pre-treatment gave the lowest reduction in biochar total heavy metal concentration. Furthermore, bio-oil compositions were influenced by pre-treatment and/or co-pyrolysis, increasing the yield (area%) of phenols to 25.4%, furans to 24.8%, anhydrosugars to 7.8%, and aromatic hydrocarbons to 20.6%. Lastly, pre-treatment alone weakened gas production while co-pyrolysis combined with pre-treatment impacted the pyrolysis gas profile, switching between light hydrocarbons and carbon oxides (CO and CO2). Overall, co-pyrolysis and pre-treatment can each be a standalone strategy for enhancing BS pyrolysis; however, their combination produced greater synergy for generating high value products.