Review of bio-conversion pathways of lignocellulose-to-ethanol: Sustainability assessment based on land footprint projections

A review of 55 pretreatment and bio-conversion methods for the production of lignocellulose–ethanol was carried out. Bio-conversion studies include enzymatic fermentation, simultaneous saccharification and fermentation, process modelling, supply chain simulations, Life Cycle Assessment and other experimental studies. A new sustainability indicator labeled Land Footprint, from farm-to-fuel, was introduced. The Land Footprint (LF) results are projected for bio-ethanol derived from stover and switchgrass from U.S., sugarcane bagasse from Brazil and India, and rice husk and straw from China and India. In order to produce 1 million L bio-ethanol, bagasse and rice straw are observed to have the highest potential to be sustainable resources that demand the least amount of agricultural land. The Land Footprints for both countries are 85 ha-yr for bagasse–ethanol and 80 ha-yr for China rice straw–ethanol. The Land Footprints per 1 million bio-ethanol from switchgrass and stover in the U.S. are 140 ha-yr and 366 ha-yr, respectively. Utilizing stover as a feedstock, an estimated LF of 14.7 million ha-yr is required to satisfy up to 50% of U.S.’s year 2022 bio-ethanol mandate. The more efficient switchgrass–ethanol would require 4.4–6.6 million ha-yr agricultural land to meet 40–60% of bio-ethanol demands for year 2022. As the most productive crop per hectare of land, sugarcane bagasse require about 2.4 million ha-yr land to supply sufficient amount of bagasse to meet up to 60% of bio-ethanol national demands.