Biomass torrefaction and CO2 capture using mining wastes – A new approach for reducing greenhouse gas emissions of co-firing plants

Abstract

A novel combination of biomass torrefaction and CO2 sequestration was explored in which carbonation of mining residues was conducted near ambient temperatures. We report that CO2 as one of the main gases evolved during torrefaction of lignocellulosic biomass can be trapped using cheap, abundant and already size-reduced mining residues resulting in a carbon-negative torrefaction process. Functionalities of torrefaction as a prerequisite step for an efficient biomass/coal co-firing could thus be further enhanced by curbing the overall process CO2 emissions. To test this concept, two different Mg-rich ultramafic mining wastes were loaded in a carbonation reactor hyphenated downstream of a torrefaction reactor and operated under total gas reflux to capture the incipient CO2. The torrefaction unit was operated with birch wood according to conventional dry torrefaction as well as using ionic-liquid-impregnated torrefaction to increase birch wood constituents’ torrefaction rate. The effect of some important parameters such as carbonation temperature, residue pore water saturation, and carbon monoxide and methane concentrations on mining residues CO2 uptake was studied systematically to assess if such a tandem concept would enable displacement and storage of sizeable amounts of CO2 emitted by torrefaction.