Direct liquefaction of lignocellulose : exploration, design and evaluation of conceptual processes

Abstract

Fossil resources are currently the major energy sources but are the main culprits for the emission of CO2, NOx, SOx and particulate matters that cause air pollution and global warming. This has led to exploration of alternative energy sources, which are CO2 neutral and renewable to meet the future energy demand in a sustainable manner. Lignocellulosic biomass is one of the promising renewable energy sources which can be converted into a liquid product (bio-crude) by using “direct thermal liquefaction process” which is a variation of pyrolysis. This thesis aims to explore, design and evaluate direct thermal liquefaction processes for converting lignocellulosic biomass to bio-crude that can be further processed in an existing refinery unit to produce biofuels. This thesis shows that a working direct liquefaction process can be realized. Production of high molecular weight components (heavies: MW > 1000 Da) during the liquefaction was one of the major issues encountered in development of earlier processes, as it did not allow recycling of the bio-crude as a liquefaction medium. The other major issue was the very high pressure in the process (~20 MPa), which required expensive materials and process equipment hampering the economics of the process. With regard to these two major issues, a significant progress has been made in this thesis. The latter issue is resolved by not using any reactive gases in the process. The absence of the reacting gases allowed a reactor pressure of ~5-9 MPa, significantly lower than the pressure applied in the earlier PERC/LBL/HTU processes of 20 MPa. The former issue about the production of the heavies is addressed through various ways: minimizing the formation of the heavies (chapter 3-4), stabilizing/cracking them prior to recycling (chapter 5), removing them prior to recycling (chapter 6-7) and, in the last resort, not recycling the bio-crude at all but using a refinery stream as a liquefaction solvent instead (chapter 8-9). Proof of concepts of three processes of biomass liquefaction are provided in this thesis. The direct liquefaction process are found to be realized to produce the bio-crude at a crude oil equivalent price of \$60/bbl, which is competitive with current crude oil price also at \$60/bbl (2nd quarter of 2015). The calculated price of the bio-cruse obtained through the liquefaction processes is competitive if not better than other technologies in this field such as pysolysis, gasification and fermentation, and shows potential of cost-effective production of bio-fuels.