ANALYSIS AND OPTIMIZATION OF THE STEADY STATE PERFORMANCE OF A FLUIDIZED BED COAL GASIFIER

The performance of a fluidized bed coal gasifier (FBG) can be evaluated on the basis of a number of parameters characterizing its power output, efficiencies and properties of the syngas produced. Each of these depends on a large number of geometrical parameters, coal properties and independent and dependent process variables. There is a complex interdependency among all these which can be studied by concurrent modelling of the physical structure of the fluidized bed, the chemical kinetics of gasification, transfer phenomena and material and energy balances. A number of constraints are imposed by the phenomena of fluidization, performance limitations, emissions and limitations originating from the auxiliary or other machinery coupled to the gasifier; these constraints should be also satisfied.; Two computer programs were developed for an exclusively analytical prediction of the steady-state performance of a FBG. The programs incorporate Johnson's kinetic model and differ in the way of treatment of the gas flow through the fluid bed, one assuming backmixed and the other plug flow.; Two more computer programs were developed for the optimization of the performance of a FBG. The programs, which again differ in the gas mixing model, can accept various objective functions depending mainly on the objectives of the plant operation (max/min efficiencies or power output etc.) and a large number of equality and inequality constraints.; As case studies, the computer programs have been used in the performance analysis and optimization of an imaginary FBG of commercial scale. Three coals of different ranks were considered as feedstocks and various combinations of stream, air and oxygen as gasifying media. Three types of plants were studied, i.e. gasification for power generation in a local steam turbine plant, in a local combined cycle plant and gasification for production of syngas for distant transportation. The optimum performance, turndown ratio and operation of the FBG under non-optimum conditions were studied and conclusions were drawn.; Finally the use of the developed algorithms in the process of optimal design of a FBG was discussed and a design procedure was suggested.