Operation of the Grand Forks Slagging Fixed-Bed Gasifier over the past three years has established the limitations on production rate of gas per vessel cross-section within which stable, sustained operation is possible. Operations in 1982 and 1983, with an expanded inside diameter and improved taphole burner have demonstrated the ability to run on lignite continuously for as long as desired. Models of mechanical and thermochemical behavior of the upper and lower fuel bed have been developed, providing partial bases for expanded gasifier designs and capacity prediction. The primary limitation on fixed-bed gasifier capacity and operational stability is the ability of fuel to settle into the combustion zone as fast as the oxygen can consume it. Closer optimization of gasifier design and control with respect to fuel bed behavior may achieve over 50% increases in capacity, with correspondingly increased return on capital investment in the vessel and accessories. For a well-designed slagging gasifier, refractory layers are protected by a layer of frozen slag, so selection of refractories is far less critical than formerly believed. Refractory damage is the result of catastrophic upsets rather than gradual degradation during extended, normal operation. 14 refs., 26 figs., 7 tabs.