Component 41: Duct burner (for waste-heat boiler)

Specifications

General       User Input Values       Physics Used       Displays       Example      

General

The duct burner is intended as an additional combustion after the gas turbine and before the first superheated bundle of the waste heat boiler. The component calculates the adiabatic combustion temperature and the air ratio based on a given fuel and primary  air mass flow (exhaust gas flow of the gas turbine). The secondary air mass flow is not involved in the combustion but it constitutes a mixture with the primary combustion gases. The component works adiabatically.

It is recommended to control the O2-concentration in the exhaust gases coming from the duct burner. This value must not drop below 3%. Also, the air ratio must be controlled, because it is calculated. This value must not drop below 1.1.

The NOX concentration at the outlet can be specified via a kernel expression. This is controlled via the flag FNOCON:

• In the case of a value of 0, the specification value NOCON  is used
• In the case of a value of 1, the kernel expression ENOCON is used for the calculation.

The flag FCON controls whether the concentration (NOx) is given as a mol fraction or as a standardized mass fraction (mg/Nm³).

Note:

As of Release 9.00, there is a new flag FETAB. It serves to specify whether the heat loss is to be related to the total heat or only to the fuel heat. 

(Before, in the case of fuel mass flows < 10^-4 kg/s the loss was related to the fuel heat, but in the case of greater ones to the total heat.)

For this component, M3N and M4N have been added as nominal values to allow access in scripts or expressions. The Ebsilon calculation kernel does not use these values.

In this component, the new combustion algorithm is applied where not only the combustible substances contained at the fuel inlet (4) but also those contained in the air and flue
gas supply respectively (1) are burned. This enables a post-combustion of the CO contained in the inflowing exhaust gas in particular.

The component always performs a complete combustion.  

Model

The model is based on an assumption of a complete combustion between

• Fuel        (line 4), and
• Primary air (line 1)

and the subsequent mixing with the secondary air mass flow (line 3).

Mass balances and compositions on the output lines are calculated as follows

• M2 = M1 + M3 + M4 - M5
• M5 from the ash balance

The composition of the flue gas in the combustion calculation is based on the composition of the incoming gases (this is normally the waste gas of the gas turbine), the fuel composition and the additional air under consideration of the ash balance.

The outlet enthalpy of the hot gas results from a balance based on the first fundamental principle considering

• the ash extraction temperature
• the combustion efficiency, and
• the adiabatic conditions.

The outlet pressure P2 results from the inlet pressure and the pressure loss

• P1 = P4 = P5 = P2 + DP12

The pressure P3 (cooling air) must always be higher than or equal to P1.  

The combustion efficiency ETAB can optionally be related to the total heat or only to the fuel energy.

Chemical equilibrium

A calculation of the chemical equilibrium based on the NASA code used in the Gibbs reactor (Component 134) can be carried out instead of a combustion calculation. The new flag FOP is used to switch over:

•  FOP=0: conventional combustion calculation
•  FOP=1: chemical equilibrium 

• When specifying a combustion efficiency ETABN < 1, the corresponding fraction of the fuel (C fraction at FUB=1, (C,H,O,N,S,Cl) at FUB=0) is already branched off before the equilibrium is calculated, i.e. it is no longer included in the calculation of the equilibrium. As in the equilibrium, however, possibly not everything is burned either, the combustion efficiency may in total become smaller than the specification.
  
•  The concentrations for CO and NOx are set to the specified values after the calculation of the equilibrium (this also applies if the specified value is 0!). If, however, the values for CO and NOx calculated in the equilibrium are to be retained, the flags FCOCON and FNOCON respectively must be set to -1.
  
• The specification of the reaction rates for the direct desulfurization, however, cannot be combined with the calculation of the equilibrium.
  
• The distribution of the ash and of the unburned matter is carried out according to RFLAS and UBASH.
  

The equilibrium is calculated at the temperature and the pressure of the exhaust gas. However, it is possible to increase the reaction temperature by means of the specification value DTREACT (DTREACT>0) or to decrease it (DTREACT<0). A decrease can make sense if an equilibrium cannot be achieved because the sojourn time in the reactor is too low. However, this feature is only available if the exhaust gas temperature is specified (as a default value or from outside), not when the adiabatic combustion temperature is used (see section "Adiabatic outlet temperature"

Ionization is not considered in Component 41. 

Hiding of the Slag Outlet

As usually no slag accumulates in the operation with gas, the slag outlet (Pin 5) is now hidden by default. If needed, it can be shown again in the tab “Ports” of the Component Properties.

Second Fuel Pin

As previously with Components 21 and 90, there is now a second fuel pin in Components 22 and 41 too for e.g. displaying an oil firing. By default, however, it is hidden. If needed, it can be shown again in the tab “Ports” of the Component Properties.

Combustion of Cooling Air

Previously, the cooling air (Pin 3) was added to the exhaust gas only downstream of the combustion in order to decrease its temperature. It did not take part in the combustion as a rule. When it was impossible to provide enough oxygen for the combustion via Pin 1, an error message was output.

In Release 15, it is possible to have the cooling air participate in the combustion too. For this purpose, there is a flag FCOOLAIR:

• For FCOOLAIR=0, the components behave as previously, i.e. the cooling air does not take part in the combustion. This is the default setting.
• For FCOOLAIR=1, the cooling air takes part in the combustion.

The cooling water mass flow has to be specified externally in either case. 

Similar components:

Component 21  (Combustion chamber/fluidized bed):

For the given total air mass flow (or the quantity of fuel), the given air ratio and the given final temperature of hot gas, this component calculates the necessary mass flow of fuel (or the total air mass flow) and the ratio of the primary air mass flow of the total air mass flow. The component does not work adiabatically. The released quantity of heat is fed to a line connection.

Component 22  (combustion chamber of a gas turbine):

For the given fuel mass flow (or primary air mass flow) and the given air ratio, this component calculates the necessary primary air mass flow (or the fuel mass flow) and the final temperature of the hot gas. This is a mixture of the primary air, which has reached adiabatic combustion temperature, and the secondary air, which only serves the jacket cooling and is not involved in the combustion. The component works adiabatically.

In this component always a complete combustion takes place. However, it is possible by setting the specification value ETAB (combustion efficiency) that the generated heat is not utilized completely (i.e. a heat loss takes place). If one wants to take into account the unburnt portion in the slag or in the flue gas, the component 21 (combustion) or 90 (combustion chamber in geometric boiler mapping) are to be used.

User Input Values

The parameters marked in blue are reference quantities for the off-design mode. The actual off-design values refer to these quantities in the equations used.

Generally, all inputs that are visible are required. But, often default values are provided.

For more information on colour of the input fields and their descriptions see Edit Component\Specification values

For more information on design vs. off-design and nominal values see General\Accept Nominal values

Physics Used

Equations

Component Displays

Display Option 1

Example

Click here >> Component 41 Demo << to load an example.