In 2014 and 2015, FIGENER carried out several studies to solve the problems presented in two coal-fired boilers, type CFB with 240 t/h of superheated steam @ 95.8 bar abs / 485 °C, installed in a large industry in the sector aluminum.
The boilers in question presented successive pipe breaks and production below the nominal, resulting in a very low availability of the steam generation system.
The scope of FIGENER included:
- Study of the origin of the corrosion process of in-bed evaparator tubes equipped with twisted-tapes
- Specification of rifled tubes instead of tubes with twisted tapes
- Study and project to increase natural circulation with the inclusion of new down-comers
- Analysis of flow in steady state and in load and pressure transients in in-bed evaporators
- (Non-implanted) project to change the modules of the in-bed evaporators for the option of complete renovation of the same
- Flexibility calculations of down-comers and solution with new in-bed evaporator modules
- suporte na discussão da alternativa de troca tubo-a-tubo dos in-bed evaporators
- Conceptual studies of alternatives for the treatment and recirculation of bottom ash (crushing for granulometric control of the bed) from the original solution proposed by the client and the previously acquired equipment (crusher, rotary valve and blowers)
- Basic and detailed design of the solution proposed by the client for the treatment and recirculation of bottom ash
- Basic and detailed design of the crushed ash pneumatic system
For the evaluation of the modifications to be made to the boiler, FIGENER developed three models, described below:
MODEL OF NATURAL CIRCULATION PERMANENTLY
For the permanent analysis of the boilers, FIGENER used its FNESS software, for, in different stationary loads:
- Identify the point-to-point flow regime (stratified, slug, froth, elongated bubbles, etc.),
- Identify a tendency for preferential flow in less heated tubes and
- Calculate resulting headings in each tube or group of tubes.
With FNESS it was possible to quantify the improvements in steam production with the new proposals for the arrangement of boiler tubes.
Model for the complete boiler.
Simplified model for in-bed evaporators.
NATURAL CIRCULATION MODEL IN TRANSITIONAL REGIME
To analyze the natural circulation for various transient conditions in the boilers (shutdown, hot start, cold start), the TRANSVAP software, also developed by FIGENER, was used.
The TRANSVAP software was developed for the analysis of disturbances in steam systems, such as, for example, the evolution of steam line pressures in the event of a turbine trip, and has a specific natural circulation analysis module whose purpose is to allow the calculation of the evolution of the level of a boiler during events such as cut or pull of consumption, variation of thermal load, etc. Thus, it is possible to identify how the natural circulation evolves during normal operation or during load variations.
The modeling in transient regime has the purpose of verifying the possible formation of flow in film in the tubes, conditions conducive to the overheating of the metal.
FLEXIBILITY ANALYSIS
A analise da flexibilidade das Caldeiras foi realizada no software CAEPIPE, específico para análises desta natureza verificando a existência de pontos críticos em termos de tensões e deslocamentos. A partir desta análise, determinaram-se melhores os encaminhamentos e suportações que se adequam à estrutura das Caldeiras.