Pulverised coal combustion in a furnace

Authors: CERCHAR

Type: Experimental

Status: question

Contents

Description

Flow Parameters

Reference Publications

Results

Description

The main objectif of the present test case is to validate Code Saturne's pulverised coal combustion model. The combustion system under investigation is a cylindrical, horizontally arranged furnace. The geometrical dimensions are: 7,7 m in length and 1,5 m in diameter. The coal burners are located at the most left end of the combustion chamber. The latter is water cooled in order to avoid overheating and thus, severe material damages. The exhaust gases are passing a horizontally arranged tube of 0,9 m inner diameter before they are emitted into the environment. A schematic draw of the furnace is shown in figure 1.


four_h.png
Figure 1 : Schematic draw of the furnace CERCHAR.


The furnace is operated with a nominal thermal output power of 3 MW, whereas the latter can be varried between 1,5 MW and 6 MW. The periphery of the furnace is shown in figure 2. The most relevant elements are: combustion air supply system, air preheating device, pulverised coal supply system, cooling system of the exhaust gases, and the operating/control center.


install.png
Figure 2 : Periphery of the coal furnace CERCHAR.

More details about the burners used at CERCHAR are shown in figure 3. A single burner consists of:
1. A small, central arranged tube, serving as a gaseous combustibles injector. In this way, mulit-phase combustion processes can be realised.
2. The swirled injection of pulverised coal and primary combustion air.
3. The diverging mixing zone of coal/primary combustion air and secondary combustion air.
4. The injection zone of the tertiary combustion air.

By reducing the primary and secondary air flow rates, respectively, the mixture fractions approaches to stoichiometric conditions yielding in increased combustion temperatures. Those prevent the flame from extinction.

bruleur.png
Figure 3 : Detailed illustration of a burner used at CERCHAR.

The geometric dimensions of the burner as well as the different inlets in the vicinity of the mixing zones are shown in figure 4.
cerchar.png
Figure 4 : Burner geometry in the vicinity of the mixing zones.

The experimental data provided includes radial profiles of gas temperatures, CO2, CO, O2, and NO concentartions, thermal heat fluxes, and velocity fields of the streaming fluid.

Flow Parameters

Turbulence:

In this study, the simulations performed are based on the 2-equation $k-\varepsilon$; model.

Radiation:

The radiative heat transfer is calculated by means of the discrete ordinate method and the P1-method as well. Conductive heat transfer is not considered.

Physical parameters:

  • dynamic viscosity : μ = 0,2 10-4 kg/(m s)
  • refrence density : ρ = 1.17 kg/m3 (cold) and ρ = 0.235 kg/m3 (hot)
  • fluid : gas
  • reference velocity : Uref = 5 m/s


Boundary conditions


The single inlet parameters are given in the following table 1:
Inlet Value
Total coal mass flow 450 kg/h
Total primary air mass flow 700 kg/h
Total secondary air mass flow 2300 kg/h
Total tertiary air mass flow 2100 kg/h
Temperature of coal and air 37 °C
Swirl number Cswirl 1

Table 1 : Parameters of the coal and air inlets.

By means of the swirl number Cswirl the unit vectors (eu, ev, ew) of the swirled inlet velocity are given by:
1. eu = -Cswirl(y/r)
2. ev = Cswirl(x/r)
3. ew = 1

Fuel characteristics


The burned fuel is coal from Freyming, which is characterised by the following parameters:
Parameter %-mass
Humidity 1,25
Volatile matter of dry coke 34,84
Carbon of dry coke 58,95
Ash content of dry coke 6,21

Table 2 : Coke characteristics.

Element %-mass of dry coke
Carbon 76,65
Hydrogen 5,16
Oxygen 9,9
Sulfur 0,8
Nitrogen 1,28

Table 3 : Elemental analysis of the burned coke.

Parameter Value
PCI of dry coke 30 MJ/kg
Specific heat capacity 1800 J/(kg K)
Density 1200 kg/m3
Initial particle diameter 25 µm

Table 4: Additional, coke related, physical parameters.

Pyrolysis and Heterogeneous Combustion


One of the most common expressions for coal pyrolysis is the Kobayashi model, an empirical approach which assumes two competitive reaction steps (r1 and r2) occuring simultaneously. The corresponding, temperature dependent, reaction rate constants are formulated according to the Arrhenius approach. The stoichiometric coefficients of these reactions are represented by Y1 and Y2, respectively. Moreover, the Arrhenius approach is used to describe the reaction constant of the heterogenous combustion rc. The relevant kinetic parameters of these approaches are given in the following table 5.


Parameter Value
A1 3,7 105 1/s
E1 7,4 104 J/mol
A2 1,3 1013 1/s
E2 2,5 105 J/mol
Ac 1,79 10-4 kg/(m2 s Pa)
Ec 6,92 104 J/mol
Y1 0.37
Y2 0.74

Table 5 : Parameter of the pyrolysis process and the heterogeneous combustion reaction.

Reference Publications

[1] Dalsecco S., Simonin O., Modélisation de flammes dans le four d’étude du CERCHAR, Rapport EDF, HE-44/92/022, 1992.

[2] Gourichon L., Données expérimentales sur les flammes de grandes dimensions, Rapport CERCHAR, SPE/LGo 89-510-300, 1989.

[3] Gourichon L., Seconde série de données expérimentales sur les flammes de grandes dimensions,Rapport CERCHAR, SPE/LGo 90-510-380, 1990.

Results

Simulation results available for this case:
Code Version Author Restrictions
Code_Saturne 2.0-beta2 M. Hassanaly AccessEDFGroup
Number of topics: 1

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Topic revision: r7 - 2011-09-02 - 14:44:51 - DavidMonfort
 

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