Lignin waste processing into solid, liquid, and gaseous fuels: a comprehensive review

The conversion of abundantly available lignocellulosic biomass into useful energy has been a topic of research for combating the energy insecurity and conventional energy resource shortages, faced throughout the world. Lignin, a prime component present in lignocellulosic biomass resources is found to be one of the abundant wastes produced by second-generation (2G) ethanol plants (30-40% lignin waste) and paper and pulp industries (as a by-product). It is challenging to depolymerize lignin, owing to its complex structure. Several techniques have been devised for the effective transformation of lignin into useful fuels through a variety of catalytic and non-catalytic treatments. This review uniquely focuses on the availability of lignin throughout the Indian sub-continent along with its conversion into solid, liquid, and gaseous fuels through different methods respectively. Pyrolysis and hydrothermal carbonization are emphasized and reviewed in the conversion of lignin to solid fuels. Various methodologies such as hydrothermal liquefaction, catalytic fast pyrolysis, hydrotreatment, and hydrodeoxygenation are broadly focused on liquid fuel conversion along with the importance of several catalysts in the treatment. Interestingly, the production of gaseous fuels from lignin has also been thoroughly reviewed and several methods such as pyrolysis, steam reforming, and supercritical gasification are effectively discussed.

Automated summary

This article introduces the sources of lignin and various methods for its conversion into solid, liquid, and gaseous fuels. Effective transformation of lignin into useful energy through different catalytic and non-catalytic treatments is a challenging yet important research topic.