Low BTU fuel flow ratio duct burner for heating and heat recovery systems

Duct burner systems include a primary duct configured to receive a primary flow stream of exhaust gases from an exhaust duct and a secondary duct configured to receive a secondary flow stream of exhaust gases from the exhaust duct. Primary and secondary variable geometry diverters may be configured to allow and restrict the primary and secondary flow streams, respectively. A combustion system may receive the secondary flow stream, combine it with at least one low BTU fuel source, combust for heating the secondary flow stream, and re-inject the heated secondary flow stream into the primary duct. A blower may be used to blow the secondary flow stream through the combustion system. In addition, a system controller may be used to control the stoichiometric flow ratio between the low BTU fuel flow source and the secondary flow stream by actuating the primary and secondary variable geometry diverters.