Biochar synthesis from mineral and ash-rich waste biomass, part 2: characterization of biochar and co-pyrolysis mechanism for carbon sequestration

Abstract

The increase in mineral and ash-rich waste biomass (MWB) generation in emerging economies poses critical environmental problems and bottlenecks the solid waste and wastewater treatment systems. Transforming these MWB such as sewage sludge from wastewater treatment (SSW) to biochar can be a sustainable method for their disposal and resource recovery. However, such biochar has limited applicability due to the relatively low organic content and possibly contaminated nature of SSW. This may be offset through combined pyrolysis with other MWB, which can also support municipal solid waste management. Studies on this MWB co-pyrolysis are lacking and have not yet seen successful long-term implementation. This work is the second part of authors’ research encompassing an analytical and lab-scale investigation of biochar production from MWB through pyrolysis for the case of Chennai city, India. Here, the physicochemical properties of biochar derived from lab-scale co-pyrolysis of SSW with other MWB such as anaerobic digestate from waste to energy plants of food, kitchen or market waste fermentation, and banana peduncles (BP) collected from vegetable markets and their thermolysis mechanism are comprehensively investigated for purpose of carbon sequestration. Also, a novel preliminary investigation of the effect of sample weight (scaling effect) on the analytical pyrolysis of biomass (BP as model substrate) is undertaken to elucidate its impact on the heat of pyrolysis and carbon distribution in resultant biochar. The maximum carbon sequestration potential of the derived biochar types is 0.22 kg CO2 kg−1 biomass. The co-pyrolysis of MWB is exothermic and governed by the synergetic effects of the components in blends with emission profiles following the order CO2 > CH4 > CO > NH3. Co-pyrolysis reduced the heavy metal enrichment in SSW biochar. The derived biochars can be an immediate source of N, P and S in nutrient-deficient acidic soils. The biochar has only up to 4-ring polyaromatic compounds and a residence time longer than 1 h at 500 °C is necessary to improve carbonization. The heat released during analytical pyrolysis of the model biomass and distribution of carbon in the resultant biochar are significantly influenced by scaling effects, drawing attention to the need for a more detailed scaling investigation of biomass pyrolysis.

The influence of sample mass (scaling effect) on the synthesis and structure of non-graphitizing carbon (biochar) during the analytical pyrolysis of biomass

“…A similar inflection point was also seen when a cellulose-rich banana peduncle (BP) was used as the substrate in the authors' preliminary study. 92 However, the BP was rich (6.53 dry wt%) in potassium, 93 an AAEM that catalyzes pyrolysis making it comparatively difficult to isolate scaling from AAEM influence as a cause of the scaling trend and the inflection point. Hence, such scaling studies are better performed with relatively pure (minimal AAEM and transition metals) biomass such as walnut shells 94 or wood after a comprehensive fiber and elemental analysis.…”

Critical review of the role of ash content and composition in biomass pyrolysis

“…Examples of MSW include paper waste, paper scraps, boxes, broken glassware, light bulbs, ash, leather elements, textiles, toxic substances (e.g., paints, batteries), and hazardous solid waste (e.g., sanitary napkins and used syringes) (Islam et al, 2021). The inappropriate treatment of MSW like incineration and a landfill results in a substantial amount of greenhouse gas emissions (Chen et al, 2022;Nair et al, 2023).…”