A flow control apparatus for a coal pipe containing a stream of primary combustion air carrying pulverized solid fuel particles, includes a frame sealingly secured to an outside surface of the coal pipe around a window opening formed on one side of the pipe. An orifice plate having an orifice opening at the center is supported for pivotal movement in the coal pipe on a spindle extending radially outwardly from an edge portion on one side of the orifice plate and out through the window opening in the pipe. A bearing sleeve is provided for supporting the spindle for rotating movement about a spindle axis extending transversely of the coal pipe across the center thereof. A mounting plate structure for the sleeve is provided and is attached to the frame. A removable handle is attached on an outer end of the spindle for rotating the orifice plate in the coal pipe to vary the effective flow cross-section of the orifice opening for controlling the flow of coal and primary air to a burner.