Abstract
•Kinetics of hot-water extraction is analogous to surface renewal.•Synergy of hot-water extraction stems from pretreatment and fractionation.•Hot-water extraction improves biomass conversion efficiency, rate and yield.•Hot-water extraction enables multiple products recovery and sequential deconstruction.
Conversion of woody biomass to chemicals, materials and energy requires at least three steps: pretreatment, cracking and conversion. Size reduction is the minimum required pretreatment step prior to further processing. Cracking step is the middle step in woody biomass conversion whereby solid woody biomass is reduced to small building blocks (molecules) or intermediates. The last step in woody biomass conversion is the generation of desired products from the building blocks or intermediates. Reactions involve woody biomass usually as a solid. The (solid–liquid or solid–gas) surface reactions have an apparent zeroth order owing to the complete renewal of surfaces during the dissolution. A vast array of products (chemicals, energy and materials) can be produced from woody biomass based on its four major components: extractives, hemicellulose, lignin and cellulose. Each of these four components has a different degree of resistance to chemical, thermal and biological degradation. The pretreatment step is designed to improve the efficiency of the cracking step. Waste products are commonly produced during pretreatment step for a sugar-based biorefinery due in part to the desire to maximize (or degrade cellulose to) glucose production. Non cellulose components in a sugar-based biorefinery were commonly discarded, starting from the pretreatment step. A synergetic approach is to eliminate the waste generation in pretreatment and inserting a step to turn the otherwise waste into value-added product(s). Hot-water extraction can serve this purpose. With a hot-water extraction process as a pretreatment step, size reduction can be enhanced after the high pressure operation. Value-added products can be produced from the hot-water extraction and the treated woody biomass can be more efficiently transformed in the cracking step: either be sugar-based, or gasification, or pyrolysis, or direct conversion to solid wood products. The synergy stems from the selective separation/removal of components from woody biomass. The reaction of solid component dissolutions from the woody biomass is of zeroth order, following a surface renewal mechanism during the bulk of the hot-water extraction process.