Methods and systems of the present invention use cellulose-containing materials, which may include post-consumer waste garments, scrap fabric and/or various biomass materials as a raw feed material to produce isolated cellulose molecules that can be used in the textile and apparel industries, and in other industries. A multi-stage process is provided, in which cellulose-containing feed material is subjected to one or more pretreatment stages, followed by a dissolution treatment and isolation of cellulose molecules. Isolated cellulose molecules may be used in a variety of downstream applications. Methods and systems for carbonizing precursor fibers to produce carbonized fibers having desired properties and three-dimensional configurations are provided.