Quality variation of pyrolytic products from anaerobic digestion crop waste: Influence of increasing inorganics through anaerobic digestion and different pyrolysis conditions

A variety of scenarios concerning the anaerobic digestion (AD) process, due to the amount of material remaining, have been the subject of studies to increase circularity and to generate further high-value products in addition to biogas. Digestate, originated from crop waste, was first utilized in slow, fast and microwave pyrolysis as an additional treatment post-AD for more green product generation. An alternative scenario was assessed whereby the original crop waste (the pre-AD crop) was pyrolyzed directly, without being first fed into an AD system, to evaluate and differentiate the pyrolytic products between this crop waste and the digestate. Around 95 wt% of pre-AD consists of mainly biopolymers: 16 wt% cellulose, 36 wt% hemicellulose, 15 wt% lignin, 29 wt% starch, and only 4 wt% of ash. Crop digestate, collected from the same crop waste AD system, was also analyzed more thoroughly. This analysis revealed that pre-AD changes primarily in the composition of starch and some hemicellulose, as well as an increase in ash content by almost double the amount. The pre-AD crop treated with slow pyrolysis at 355–530 °C yielded 50 wt% of bio-oil with a large amount of primary holocellulose derivatives. In contrast, the same pre-AD crop subjected to microwave pyrolysis for 5 min at a power input of 500 and 700 W generated between 17 wt% and around 26 wt% of bio-oil, characterized by a high acetic acid concentration and a small amount of sugars. Furthermore, biochars derived from the pyrolysis of pre-AD and digestate were evaluated for their capacity as sustainable agents for CO₂ adsorption for potential applications in reducing CO₂ emissions. The surface area of biochars derived from the pre-AD crop subjected to microwave and slow pyrolysis showed values ranging from 170 to 227 m²/g. Although the digestate is from the same pre-AD crop, the biochars derived using slow pyrolysis had less surface area, with values between 80 and 130 m²/g at around 400 °C, and an average of 175 m²/g at 500 °C; with fast pyrolysis 145–185 m²/g, and a range of 71–115 m²/g with microwave pyrolysis. The AD waste exhibited considerable promise for thermochemical conversion; however, the modification of ash content during the AD process hindered the enhancement of quality in pyrolytic products derived from high-cellulose and -lignin crop digestate.