Alternative fuels in cement manufacturing: Modeling for process optimization under direct and compound operation

Due to thermal, or exhaust air blower capacity limitations in cement manufacturing processes, the use of alternative fuels may necessitate production volume reductions from the nominal value achieved by fossil fuels, leading to considerable profit reductions. A novel linear algebraic model is presented herein, which isolates combustion stoichiometry invariants from fuel feed flowrates (decision variables). It allows rigorous, though explicit modeling of the integrated kiln-soil mill-fuel mill system, based on fundamental mass and energy balances. It enables assessment of the impact of key operational parameters and inner circuits on production level and kiln emissions, when alternative fuels (AFs) are used. Necessary conditions for balancing the integrated kiln – soil mill – fuel mill process are given in terms of a priori known physicochemical parameters and process operational data, both for compound and direct operation. A five-step procedure for process balancing is proposed in order to reap the potential benefits of AF use by selecting a non-inferior AF and conventional fuel mixture. It is shown and demonstrated by an actual case-study, that the enthalpy balance is the clinker production level limiting factor (binding constraint) while the kiln – soil mill offgas exhaust blower may be operated well below capacity.