In this work we have developed a knowledge-based combustion control system for a municipal solid waste incinerator plant. A dynamic model for a grate furnace has been proposed to analyse the relationships between the variables and obtain the rules for the knowledge bases. The input variables used were: furnace temperature, furnace pressure drop, power generated and oxygen content in the combustion gases. On the other hand, the output variables were: the actions that the operator should perform for waste feeding, the flow and the combustion air temperature with the aim of controlling the furnace temperature. The support tools used to control the process were abstractors, ALCMEN (Automaticians Language for Causal Modelling for Expert Knowledge) and SimCEES (C++ Embedded Expert System). To validate them, the real values of the furnace temperature have been compared with those obtained using a simplified combustion mathematical model and those obtained using the expert system.