Some variations provide a process for impregnating a biomass feedstock with a reaction solution, comprising: providing a biomass feedstock that contains non-condensable gases within biomass pores; introducing a condensable vapor into the biomass pores to remove non-condensable gases out of the biomass pores, thereby generating an intermediate biomass material, wherein at least a portion of the condensable vapor remains within the biomass pores; exposing the intermediate biomass material to a liquid solution to infiltrate the liquid solution into the biomass pores and condense the vapor to form a condensed liquid contained within the biomass pores, thereby generating an impregnated biomass material containing a reaction solution; and recovering or further processing the impregnated biomass material. The non-condensable gases may be oxygen, nitrogen, or carbon dioxide, for example. The condensable vapor may be steam, for example. The reaction solution may contain a pretreatment chemical, such as a catalyst and/or a solvent.

In a process for oxidation of white liquor, in particular of white liquor used in a process of production of paper or cellulose, the oxygen required for the oxidation is supplied to the reactor or the reactors in which the oxidation is carried out at least partially in the form of oxygen-containing nanobubbles. Due to the relatively long lifetime of the nanobubbles this very efficiently provides oxygen also for oxidation reactions in the white liquor proceeding at different rates.

A method of producing high yield chemical cellulosic pulp includes: (a) chemically pulping wood chips to separate lignin and liberate cellulosic fibers from the wood chips to generate a cellulosic pulp; (b) washing and screening the pulp of step (a); (c) pre-treating the washed pulp with oxygen; (d) optionally washing the treated pulp of step (c); (e) bleaching the pre-treated pulp in an extended duration oxidative bleaching stage; (f) optionally washing the bleached pulp of step (e); and (g) optionally further oxidatively or reductively bleaching the bleached pulp in a shorter duration bleaching stage, wherein the bleached pulp is produced at a yield of greater than 60% based on the weight of the pulped wood chips (dry basis).