Abstract: A gas injection apparatus for injecting a quench gas. The gas injection apparatus provides a means for injecting a quench gas into a hot gas contained in a vessel having an inner liner. The apparatus is designed such that the independent thermal expansions and contractions of the inner liner do not create excess thermal stresses on an expansible supply duct. The expansible supply duct provides the means for transferring the gas from outside the vessel into the gas stream. A plenum assembly, creating an interface between the expansible supply duct and the inner liner is sealably attached to the inner liner allowing perpendicular injection of the quench gas while maintaining the liner's integrity.