Co-pyrolysis of sewage sludge with lignocellulosic and algal biomass for sustainable liquid and gaseous fuel production: A life cycle assessment and techno-economic analysis

Abstract

Biowastes are marked sources of global greenhouse gas emissions, however, locally accessible biowaste could be a sustainable source for biofuel production to generate clean energy. This study examined the environmental sustainability and economic feasibility of using co-pyrolysed sewage sludge (SS) with lignocellulosic and algal biomass for liquid and gaseous fuel production from the perspective of life cycle assessment and techno-economic analysis. Wastewater-grown microalgae, sawdust, and wheat straw were selected as co-feed materials, and three mixing ratios (25, 50, and 75 wt%) of each feedstock with SS were examined. Wastewater-grown microalgae and SS were obtained from wastewater treatment plants (WWTPs), which were assumed to be near the pyrolysis plant facility, obviating the need for transportation. An Aspen simulation was used to estimate the amount of energy recovered from the pyrolysis as gaseous fuel for electricity and heat generation via gas turbines. A sensitivity analysis was conducted to identify future considerations for SS co-pyrolysis to make it a more environmentally and economically viable industrial process for producing liquid and gaseous fuels. The co-pyrolysis of SS with biomass reduced the overall environmental burden, with sawdust co-pyrolysis outperforming wheat straw and microalgal co-pyrolysis. All scenarios achieved a high net positive energy balance (from 2,595 to 6,323 kWh/1,000 kg bio-oil) and reduced global warming potential (GWP) (from −793 to −1,970 kg CO2 eq/1,000 kg bio-oil). Sawdust co-pyrolysis resulted in the highest GWP reduction (from −1,510 to −1,970 kg CO2 eq/1,000 kg bio-oil) and energy recovery (from 5,551 to 6,323 kWh/1,000 kg bio-oil). Energy recovery from gaseous fuel through gas turbine operation generated a process energy demand of 23 %–224 %. Sawdust co-pyrolysis exhibited the highest net present worth (NPW) of 6.16 million CAD. Sensitivity analysis demonstrated that the GWP and terrestrial ecotoxicity potential were affected by the transportation of sawdust and wheat straw. Overall, the co-pyrolysis of SS with both lignocellulosic and algal biomass is sustainable and economically feasible for producing liquid and gaseous fuels and treating hazardous waste originating from WWTPs, thereby facilitating environmental sustainability and a circular economy from the sale of pyrolysis products. Broader implication of this study is using effective catalysts to further strengthen the sustainability and increase the profitability of the co-pyrolysis process, which has not been yet investigated.