The disclosure describes a system for generating hydrogen gas from a hydrocarbon through pyrolysis with reduced soot formation and increased carbon loading. The system includes one or more pyrolysis reactors configured to generate the hydrogen gas from the hydrocarbon through pyrolysis. Each pyrolysis reactor of the one or more pyrolysis reactors includes one or more fibrous substrates and a concentration sensor downstream of at least one fibrous substrate of the one or more fibrous substrates. Each fibrous substrate of the one or more fibrous substrates defines a deposition surface for carbon generated from the pyrolysis of the hydrocarbon and includes a plurality of fibers configured to maintain chemical and structural stability between 850° C. and 1300° C. The concentration sensor is configured to measure a concentration of at least one of a hydrocarbon byproduct or a hydrocarbon soot precursor, such as acetylene.