Mercury Speciation Transformation During Coal Combustion - Mercury Emission and its Control in Chinese Coal-Fired Power Plants - page 73

During Coal Combustion

formation, transformation and emission during the combustion process in the fur-
nace as well.

Table 4.16 Parameters of air distribution in furnace for simulation as needed

Item Ratio (%) Temp. (K) Velocity (m/s) Standard height (mm)

Tertiary air 24.7 333 60 17,355

OFA 12 588 48 16,590

Updraft secondary air 8.76 588 48 15,892

Updraft primary air 8.34 433 30 15,378

Mid-draft secondary air 8.76 588 48 14,864

Mid-draft primary air 8.34 433 30 14,350

Mid-downdraft secondary air 8.76 588 48 13,836

Downdraft primary air 8.34 433 30 13,297

Downdraft secondary air 12 588 48 12,670

4.5.1.3 Meshing and Model Establishment

Generally, it's the first basic step to do good meshing for simulation work. Meshing
quality has a positive effect on the convergence and accuracy of simulation results.
In view of the complex multi-phase flow, heat transfer and combustion simulta-
neously happening in the furnace, an empirical unstructured grid is employed to do
the simulation work here.

Model set-up
Firstly, we utilized software-Gambit to establish a model for a full-size furnace. It
was so hard to take into account the detailed structure of whole furnace beyond
controversy so we simplified some of the detailed structure during the modeling
process. For instance, to meet the demand of model simplification, the membrane
wall was replaced by a plane, a pendant super-heater was omitted and a jet plane
was a proper substitute for a burner. Even the perimeter air of the furnace was
spread apart into secondary air.
Furthermore, in order to obtain reasonable meshing, the whole furnace was
considered as one volume and divided into a number of zones which comprised the
complex burner area, hopper, pendant super-heater etc. In particular, the burning
zone with the characteristics of a large flow gradient, was meshed rather more
densely than other parts, which was of significance for precise simulation work.
Therefore, as shown clearly in Fig. 4.72, the simulation domain of the furnace was
9,980 mm×9,980 mm×39,000 mm (width×depth×height), nearly equal to the real-
istic size.