DEVELOPMENT AND APPLICATION OF A CFD-BASED RECOVERY BOILER MODEL FOR OPTIMISATION OF THE COMBUSTION AND NOX ABATEMENT IN UTILITY KRAFT RECOVERY BOILERS

摘要

Kraft recovery boilers are of great importance to the paper and pulp industry because of the high cost in terms of their investment and operation. A kraft recovery boiler has two main functions: firstly to recover the inorganic cooking chemicals in the black liquor that is a by-product from the pulping process, and secondly to make use of the chemical energy in the organic portion of the black liquor to generate superheated steam for the plant. The twin objectives of recovering chemicals and energy makes the recovery boiler design and operation more complex than virtually any other boiler application. It is known that the design and operation of the combustion air system has critical impact on the performance of a kraft recovery boiler. However, it is very difficult to quantify black liquor combustion by direct in-situ measurement due to the harsh environment in a kraft recovery boiler. Thus there is a demand to develop an analytical tool that can be used to evaluate the thermal and chemical performance of a kraft recovery boiler with respect to alternative parameters for boiler design and operation. As a result, a specially tailored three-dimensional recovery boiler model (RBM) has been developed using a state-of- the-art computer program: FLUENT. To take into account some special features for kraft recovery boilers, such as the combustion of black liquor droplets in flight and in the char bed, particular modules for describing these physical and chemical processes are developed and linked to the FLUENT solver as a run-time library. This paper presents some results achieved from a project jointly financed by Swedish Energy Agency (STEM) and the pulp and paper industry (Iggesund Paperboard) in Sweden. In this work, the developed FLUENT-RBM has been applied to evaluate alternative air system designs with respect to the combustion and Nox abatement in a utility kraft recovery boiler at Iggesund Paperboard. The predicted results are presented and discussed. This work demonstrates that the developed FLUENT-RBM can be used as an attractive tool to improve the operation and design of kraft recovery boilers.