Comparative Evaluation of Thermochemical Liquefaction and Pyrolysis for Bio-Oil Production from Microalgae

TLDR

Bio‑oil is produced by thermochemical liquefaction or pyrolysis of biomass, with TCL operating at 250–350 °C and 5–20 MPa on wet feedstocks, while pyrolysis uses 400–600 °C at atmospheric pressure and requires drying. The study compares experimental results of TCL and slow pyrolysis for bio‑oil production from algae. TCL was carried out in a 1.8‑L Parr reactor with 80 % moisture algae slurry, whereas pyrolysis used an 8‑L cubical reactor with dried algal powder (~4 % moisture), and yields and compositions of bio‑oil, char, gases, and aqueous phase were evaluated. TCL achieved higher bio‑oil yields (~41 %) and lower char (~6.3 %) and energy consumption than pyrolysis (23–29 % bio‑oil, 28–40 % solids), and the TCL bio‑oil had higher energy density and better fuel properties.

Abstract

Bio-oil is the liquid product of thermochemical liquefaction or pyrolysis of biomass. Thermochemical liquefaction (TCL) is a low temperature (250–350 °C) and high pressure (5–20 MPa) process particularly suited for high moisture feedstocks, whereas pyrolysis is accomplished at moderate to high temperatures (400–600 °C) and atmospheric pressure and requires drying of the feedstock. In this paper, we present experimental results that provide a critical comparison of TCL and slow pyrolysis processes for producing bio-oil from algae. TCL experiments were performed in a 1.8-L Parr reactor using algae slurry (80% moisture) and pyrolysis runs were carried out in an 8-L mild steel cubical reactor, using dried algal powder as received (∼4% moisture). Yields and composition of bio-oil, char, gases, and aqueous phase were evaluated and compared for TCL and pyrolysis. TCL resulted in higher bio-oil yields (∼41%), lower char yields (∼6.3%), and lower energy consumption ratio compared to pyrolysis, which resulted in 23–29% bio-oil, and 28–40% solids yields. Bio-oil obtained from TCL was found to have higher energy density and superior fuel properties such as thermal and storage stabilities, compared to pyrolysis bio-oil.