7. Conclusions
The paper presents a numerical simulation of a small solid-fuel
fired retort boiler. The simulation involved both coal combustion
process and water flow in the boiler jacket. Computational effort
has been reduced by decoupling combustion from the water flow.
Such approach required introducing of a coupling mechanism. Thecoupling of the simulations has been pursued through boundary
conditions of the first and second kind. Small number of iterations
was sufficient to obtain convergence of the coupling process. The
coal combustion involved two sub-models. First model has been
responsible for coal combustion and involved evaporation of
moisture, depolymerization of coal particle, resolidification and
emission of liquid hydrocarbons, gasification and char combustion.
Second model has been used for combustion of the gasification
products. The simulations have been accompanied by temperature
measurements in selected parts of boiler. As described in the result
section, the agreement of simulation and measurements was good.
Also the power of boiler has been nearly identical to the one
determined during experiment.
The analysis of the water flow inside the water jacket led to the
conclusion that the intensity of heat transfer in the water side
controls the overall heat transfer in the boiler. There are regions
where the water flow is very small e stagnation zones. In those
regions the temperature of the jacket increases while the heat
transferred between flue gas and the water decreases. The pipes
which cross the combustion chamber play very important role.
First they contribute strongly in the heat transfer. Piper transfer
about 30% of heat transferred in the combustion chamber which is
over 14% of overall boiler power. The circulation of water at the
front wall of the combustion chamber is very weak. Only tiny part
of water arrives there. Thus the contribution of this wall in the heat
transfer is only at the level of 2.5% of boiler power. This may lead to
local overheat problems especially at lower than nominal boiler
power. It is also important to notice that vertical fins mounted in
the flue gas convective zones significantly increase the heat exchange.
Local value of the heat flux is increased by about 20%. The
developed and validated model can be used to optimize the design
of the retort boiler. This is a topic of current research activities.