Torrefaction of herbaceous biomass: A study of product, process and technology

Co-firing biomass with coal in pulverized fuel boilers is a readily implementable means for attaining renewable electricity generation targets. Even as utilities have gained considerable operational experience over the past years with co-firing small quantities (0-3% on energy basis) of assorted biomass and waste residues for power production, they rely on wood pellets when required to co-fire higher fractions (>10% on energy basis). However, wood pellets are 4-6 times as expensive as coal (on an energy basis), resulting in the viability of renewable electricity production from large scale co-firing of biomass being heavily reliant on subsidy. The use of cheaper herbaceous biomass (such as verge grass or bagasse) may be appealing in view of reducing fuel cost, but is hampered in practice on account of technical and logistical challenges emanating from its high moisture content, low bulk density, fibrous structure and seasonality. Torrefaction, alternatively referred to as roasting or mild pyrolysis, is a thermal pre-treatment that can be applied to biomass to improve its physicochemical properties for subsequent thermochemical conversion (including co-firing). The present work is a study of torrefaction as applied to herbaceous biomass in relation to its process, products and technology.