Environmental, exergetic and economic tradeoffs of catalytic- and fast pyrolysis-to-renewable diesel

Abstract

This paper compares the cost and life cycle environmental performance of renewable diesel produced from upgraded catalytic fast pyrolysis (CFP) and fast pyrolysis (FP) bio-oils produced from forest residues. Integrating zeolite catalyst into the pyrolysis reactor in CFP requires upgrading in one catalytic hydrocracking step, whereas FP requires upgrading by multiple catalytic hydrotreating and hydrocracking steps, raising capital costs. The FP system considers hydrogen production from co-produced biochar, further raising capital costs but eliminating the large external hydrogen need. Despite lower capital costs, due to higher operating costs from catalyst replacement and purchased hydrogen, the estimated minimum selling price of CFP fuel ($2/L) is greater than FP fuel ($1.68/L). Utilities, hydrogen and catalyst contributions to greenhouse gas (GHG) emissions, exergy and annual cost are sizeable for CFP. However, GHG emissions for CFP (−72 to 32 g CO2e/MJ) are low and negative when considering credits for biochar due to a high biochar to fuel product ratio (2:1). In contrast, FP with catalytic upgrading maintains a high biofuel yield and low GHG emissions (8–13 g CO2e/MJ) that meet energy policy objectives with more favorable economics compared to CFP. Neither fuel is cost competitive with petroleum-diesel even with renewable fuel credits.