For a furnace burning solid wastes, one of the key areas in design is the sizing of the furnace which is often determined on the basis of empirically selected furnace heat release rate and grate burning rate. An independent estimation of these factors would require the knowledge of physical processes of refuse combustion such as drying and heating of the refuse bed, chemical reactions of waste combustion, supply and distribution of combustion air, and mixing of combustion air with the burning refuse bed. In principle, the gross combustion rate of solid refuse bed is related to the oxygen consumption rate of the burning refuse bed. In this paper an attempt is made to estimate the refuse burning rates in the furnaces by calculating the oxygen distribution in the combustion chamber. A model based on the fundamental principles of conservation of mass, energy and momentum balance is established for this purpose. For different combustion chemistry models, by using a reaction rate coefficient which could be affected by parameters like bed height, porosity of the fuel bed, and underfire air rates, the oxygen distribution in the fuel bed, and hence, the combustion rate can be determined. Comparisons of the theory with the laboratory test data and real plant operating data indicate that the theory can be tuned to fit the data reasonably well by adjusting the reaction rate coefficient.