Techno-economic analysis and life-cycle assessment of bio-oil production from microwave-assisted pyrolysis of pine sawdust

Abstract

Pyrolysis is a promising method for producing bio-oil from biomass. However, bio-oil must be upgraded before it can be used as fuel in internal combustion engines. While biofuels are often considered renewable and eco-friendly, it is important to understand the complete economic and environmental impacts of biofuel production to make informed decisions about their use. This study aims to evaluate the process’s economic viability and the environmental sustainability of converting pine sawdust to crude bio-oil via microwave-assisted pyrolysis. The study used ASTM D 410-84, D3173-5 and ASTM D5373 standards to characterize the feedstock and pyrolysis products, and thermogravimetric analysis to study the thermal degradation behavior of pine sawdust. Gas chromatography-mass spectrometry (GC-MS) and Fourier transform-infra red (FTIR) were used to analyze the compositional properties of the organic phase and fatty acid methyl esters. The study found that the optimal operating conditions for producing the highest amount of bio-oil were achieved at 550℃ and 1 atm, yielding 42.28 wt.% of bio-oil, with phenolics contributing the greatest percentage of organic compounds. Transesterification improved the bio-oil properties by converting organic acids and oxygenated compounds to fatty acid methyl esters with a concentration of 510.05 mg/L. The study also evaluated the economic feasibility of the process, establishing the minimum selling price (MSP) of bio-oil, and predicted MSP for biodiesel. The MSP of bio-oil and biodiesel was established through the use of a discounted cashflow rate of return (DCFROR) analysis. The study found that the process was economically viable, with a MSP of $1.14/L of bio-oil and a predicted MSP for biodiesel of $2.31/L. The minimum selling price of biodiesel was consistent with the prices reported in previous studies, albeit with minor variations primarily attributed to variations in feedstock composition and the complexity of the thermochemical conversion process. The life cycle assessment (LCA) utilized a cradle-to-gate system boundary approach. To evaluate the environmental sustainability of the system, the Ecoinvent v3.7 database in openLCA v2.0 software. They conducted an analysis of 18 environmental impact categories using the ReCiPe 2016 (H) midpoint impact assessment methodology. However, the study found that the process had environmental impacts, including global warming potential, photochemical oxidant formation, and human toxicity, primarily due to the use of methanol in the biofuel synthesis stage. The study suggests that implementing sustainable practices, such as using organic fertilizers, optimizing transportation routes, implementing gas cleaning technologies, and effective waste management practices, could enhance the environmental performance of the biofuel production system.