Some variations provide a process to produce renewable biogas and cellulosic fiber from a cellulose-rich feedstock, comprising: obtaining a cellulose-rich feedstock that is reduced in concentration of impurities and has a length less than 5 inches; feeding the cellulose-rich feedstock and water to a vacuum steam-explosion vessel, thereby generating a first cellulose-rich intermediate stream; feeding the cellulose-rich intermediate stream to a secondary separation vessel to remove remaining impurities, thereby generating a second cellulose-rich intermediate stream; feeding the second cellulose-rich intermediate stream to an enzymatic-hydrolysis vessel to convert the second cellulose-rich intermediate stream to low-viscosity cellulosic fibers comprising cellulose oligomers; feeding the low-viscosity cellulosic fibers to an anaerobic digester, thereby generating a biogas stream and residual fiber; optionally, recycling residual fiber to the enzymatic-hydrolysis vessel and/or to the anaerobic digester; and recovering some or all of the residual cellulosic fiber from the solid stream as a co-product.

An electrochemical reactor for capturing carbon dioxide and producing bicarbonate and hydrogen is described herein. The electrochemical reactor is useful for, among other things, converting biogas to a bicarbonate and hydrogen feedstock for biomethanation. The reactor comprises at least one reactor unit comprising an electrolyzer cell and at least one alkaline water electrolysis (AWE) cell adjacent to the electrolyzer cell. The electrolyzer cell comprises an anode spaced from a cathode by an ion exchange membrane between the anode and the cathode; and the electrolyzer cell is adapted and arranged to allow a flow of a neutral liquid electrolyte to contact the anode and the cathode. The ion exchange membrane can be a cation exchange membrane (CEM), or an anion exchange membrane (AEM). The AWE cell comprises a second anode spaced from a second cathode by a porous diaphragm.