Component 21: Combustion Chamber with heat output

Specifications

General       Notes       User Input Values       Characteristic Lines       Physics Used       Displays       Example

General

Component 21 is a calculation module for combustion chambers and fluidized bed firing. The terminology of the specification values has been adjusted accordingly.

For the combustion chamber of a boiler, the temperature input corresponds to the temperature at the control surface i.e.:

• if subsequently added heat exchanger are modelled, it is the temperature upstream of the first heat exchanger in the direction of the flue gas downstream;
  
• if no additional components are modelled in the direction of the flue gas, but instead the heat exchanger is assumed within the combustion chamber (without considering
  its detailed structure), it can also be assumed as the waste gas temperature.
  
• So it is the temperature that represents the border to the neighbouring component or even to the surroundings. It can be changed on the basis of a characteristic line in
  relation to the load.

The module executes a combustion calculation. The net calorific value of the fuel and the fuel analysis must be known.

The calorific value of the flue gas is not calculated from the composition, but results from the calorific value of the fuel (which can also be specified independently of the fuel composition) and the combustion efficiency. In the case of gas the combustion efficiency is always 1, so in this case the calorific value in the exhaust gas is zero.

Depending upon the flag FALAM, the air ratio is selectively

• specified by the nominal value ALAMN and a characteristic line. In this case, the specification of the fuel mass flow or of the air mass flow suffices; the other quantities
  are calculated from the air ratio
• calculated from the specification of the fuel mass flow and the air mass flow.

The ash content of the coal is removed by the ash extraction. The specification value "flying ash content" defines the amount of ash contained in the flue gas, which will be
extracted by the boiler as the flying ash and hence will not be removed through the ash extraction.

Connection 3 passes the useful heat to the connected component. Together with component 5 (water-steam part of the boiler), this value must be equal to the value at the
connection 5 of component 5, in order to reach the desired steam parameters. This can be done through a control.

Radiation losses can be defined as per EN 12952 or through your own input.

Previously the specification value “C“ had a double function:
When C < 1, C was used for the value of the radiation constant; for C = 1, 2, or 3, the values for oil/gas, hard coal, or lignite were used according to DIN EN 12952.
A new switch FC has been implemented.

The NOX and CO concentration, respectively at the outlet can be specified via a kernel expression. This is controlled via the flags FNOCON and FCOCON:

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

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

When calculating volume shares, a change in one share always affects all other shares. To keep CO and NOx both exactly the same, a recursion procedure would have to be implemented in two variables, which would certainly lead to a significant increase in computing time. Since the proportions of NOx and CO are very small in practice, the effects on the other concentration are correspondingly small: if NOx is increased from 1% to 2%, CO (if 1% is specified) increases from 1.01% to 1.06%, i.e. by 5% in relative terms. If NOx is increased from 100 to 200 ppm, CO (with a default of 100 ppm) increases from 100.79 to 100.84 ppm, i.e. relatively by 0.05%.
In practice, the values should rather be in the range of 100 ppm, so that the inaccuracy caused by this effect is certainly negligible. Therefore, only a simple calculation is implemented here with regard to NOx and CO. Therefore, a change of NOCON results in a change of the CO content previously set with COCON.

If part of the carbon is not completely oxidized to CO2 but only to CO, the latent heat present in the carbon is not fully utilized. This loss is shown separately in the result value QLCO. The loss due to CO is included in the result value QLNB.

Note: How much CO is formed is determined by the default values COCON or ECOCON. However, the combustion efficiency ETABN must be selected small enough to allow the losses due to CO. Otherwise, a warning is issued and the calorific value of the flue gas is reduced accordingly in order to comply with the energy balance. This reduction is also included in the result value QLNB. However, the result value QLCO contains the unchanged value. When the warning occurs, ETABN must be reduced until QLNB >= QLCO.

With the mode FSPEC=T2, T2 is specified to be externally validatable. During validation, an externally specified temperature is otherwise treated as an invariant variable. With FSPEC=2 an equation is generated, in which this temperature enters, so that a statistical compensation can take place.

Line type "Humid air"

For Component 21 (Combustion chamber with heat output), the line type “Humid air“ can now be used instead of “Air” as well. To do so, however, the component must be switched over to the new calculation mode FCALC=3 (see User Input Values).

Note - cp correction factors

The treatment of the cp correction factors has been harmonized for Components 21 and 90. As with Component 90, Component 21 also allows to specify a correction factor CPSL
for the ash in the slag and a correction factor CPFL for the ash in the exhaust gas (fly ash). The cp value calculated by Ebsilon according to the FDBR formula is multiplied by
this factor.

The option to specify the cp correction factor at the fuel consumption and have it transferred to the slag line only existed for Component 21. Now this is possible for
Component 90 as well, by leaving the value CPSL empty . Analogously, in the case of an empty CPFL the correction factor is transferred from the air line to the
exhaust gas line.

Direct desulfurization

By adding lime into the combustion chamber it is possible to bond the SO2 emerging during the combustion directly as CaSO4. This process can be represented in Ebsilon as well. The following chemical reactions are considered here:
Calcination:

• Ca(OH)2  à  CaO+H2O
• CaCO3    à   CaO+CO2
• MgCO3   à   MgO+CO2

Bonding of sulfur:
• CaO + SO2 + 0.5 O2  à CaSO4

As these reactions do not take place completely as a rule, the reaction rates are to be specified by the user. This is done by means of the two specification values CALCR
and DESN as well as four kernel expressions ERCAOH2, ERCACO3, ERMGCO3, and ERSO2.

The flag FDES is used to control which values are used:

• For FDES=0 no reaction takes place.
  
• For FDES=1, the values CALCR and DESN are used. The value CALCR must be between 0 and 1; it specifies which fraction of Ca(OH)2, CaCO3,, and MgCO3 is converted.
  Here the same fraction is assumed for all substances. DESN specifies which fraction of the existing SO2 is converted.
  
• For FDES=2, the kernel expressions are used. For each reaction, the user can write an independent program for how the reaction rate is to be determined, e.g. depending on
  certain influencing factors. In the kernel expressions, the molar Ca/S-relation and the bed temperature can be used as internal variables. The results of the kernel expressions
  must be between 0 and 1.
  
• For FDES=3, the specification matrix MXRSO2 is used for the degree of desulfurization, depending on the molar Ca/S-relation and on the bed temperature.
     The specification value CALCR is used for the degree of calcination in this mode.

The solids remaining after the reaction are distributed on the waste gas and slag outlets according to the specifications, like all incombustible solids.

Elements Magnesium and Calcium 

Mg and Ca are metals that are not usually burned. As, however, the new Component Gibbs Reactor treats these elements in balance, they have now been included in the list
of available substances .

In the Ebsilon components that carry out a combustion, however, these elements are always burned completely. In the exhaust gas and in the slag, there will not be any unburnt Mg or Ca. The specification values in Component 21 and 90 concerning combustion efficiency and distribution of unburnt matters apply (as before) to the elements C, H, O, N, S, Cl only. When specifying unburned matter in Components 21 and 90, this still only refers to the elements C, H, O, N, S and Cl; unburned Mg and Ca do not occur in the exhaust gas.

In Components 21 and 90, the combustion products MgO and CaO are distributed to the slag and exhaust gas lines according to the specification value RFLAN, just like all other incombustible solids.

Note - Characteristic Lines Related to Nominal Temperature

For the component 21 there is a characteristic line CT2, which refers to a nominal value of temperature. This is the characteristic line CT2 for the outlet temperature of the
combustion chamber, which provides the ratio T2/TBEDN.

Unfortunately, such temperature ratios depend on the selected system of units. In contrast to other units where the conversion is effected only via a certain factor and therefore
has no effects on the quotient, in the temperature conversion there is an additive offset whereby the value of the quotient changes.

As Ebsilon internally calculates with the temperature unit °C, this characteristic line was  interpreted in °C at all times. Users who prefer other unit systems therefore
had to manually convert all these characteristic lines into °C before the input. 

There is the possibility to specify this characteristic line in other units (°F, K).

Users who prefer other unit systems for the temperature (e.g. ° F, K) must set the selected temperature unit in the new flag FTNI, because Ebsilon calculates internally with
the temperature unit ° C.

Second fuel connection

Another fuel inlet (Pin 6) has been added to this component to enable an operation with two different fuels (like e.g. oil and gas). A new mode of calculation (FCALC=3) has been implemented for this. In this mode, it is also possible to use the line type “Humid air“ as combustion air.

The reason for this new mode of calculation was the complexity when treating the unburned matter at FCALC=2 that would render the operation too confusing when adding another fuel line. At FCALC=3, a simplified treatment of the unburned matter is therefore carried out:

• Only variant FTYPUB=0 is supported, i.e. a specification of the combustion efficiency ETAB and the distribution UBASH of the unburned matter to slag line and exhaust gas line. In this mode, it is not possible to specify certain concentrations in the slag and in the exhaust gas.
  
• In this mode, the combustion efficiency ETAB refers to the mass fraction according to the specification of the composition of the unburned matter (Flag FUB):

o At FUB=1 (unburned matter is pure C), ETAB only refers to the C fraction of the fuel, i.e. a fraction (1-ETAB) of the C in the fuel is branched off prior to the combustion and is added to the slag and the exhaust gas respectively.

o At FUB=0 (unburned matter is fuel), ETAB only refers to the fraction of the fuel that is given by the elementary analysis (C,H,O,N,S,Cl). A fraction (1-ETAB) of the (C,H,O,N,S,Cl) in the fuel is branched off prior to the combustion and is added to the slag and the exhaust gas respectively.

• Further reactions (NOx and CO formation, desulfurization) are then carried out with the remaining fuel. For instance, this allows to specify a CO formation at ETAB=1.0 as well. ETAB therefore no longer equals the used fraction of the fuel heat.
  
• The combustion efficiency is only applied to the two fuel lines. If there are any combustible substances in the air line, these are always burned completely. In this case, the air ratio (FALAM=0) should not be specified because combustible substances in the air cannot be considered when ALAM is specified.
  
• The residual calorific value of the slag and of the exhaust gas is calculated directly from the composition.
  
• The air ratio ALAM refers only to the fraction that is actually burned, previously ALAM was related to complete combustion.
  
• ALAM<1 can be specified as well or an oxygen quantity that is too low can be specified in the case of FALAM=1. Then the unburned fraction will be increased accordingly and a warning is output.
  

Adiabatic outlet temperature

By default, in Component 21 a heat balancing is effected in such a way that the exhaust gas temperature is specified internally or externally and the remaining combustion heat is output on logic connection 3.

It is also possible to transfer the entire combustion heat to the exhaust gas. The heat quantity transferred to logic connection 3 will then become 0. This mode is activated by the setting FSPEC=3 (“Use adiabatic combustion temperature“).

Gilli furnace parabola

The Gilli furnace parabola (according to “Vorlesung Wärmetechnik II, Teil 6: Dampferzeuger“ [“Lecture on Thermal Engineering II, Part 6: Steam Generator“] by Prof. Dr.-Ing. Jürgen Karl, TU Graz, Institute of Thermal Engineering, 2010) is based on considerations of the energy balance in the furnace and allows to estimate the furnace outlet temperature. This can be used in Ebsilon as well, with the settings FSPEC=4 and 5.

Doing so, however, requires the specification of the emissivity EMISS. It depends on the fuel and is typically

• 0.55 – 0.80 for hard coal and lignite
• 0.45 – 0.85 for fuel oil
• 0.40 – 0.60 for natural gas
• 0.35 – 0.60 for blast furnace gas

At FSPEC=4, the specification of the furnace surface A is required as well. The furnace outlet temperature is then calculated according to the Gilli formula in all load cases.

At FSPEC=5, the furnace outlet temperature is specified in the specification value TBEDN in the design case, and the furnace surface is calculated from it by means of the Gilli formula. In off-design, the furnace outlet temperature is then calculated using the Gilli formula based on this surface.

In addition, an effectiveness factor EFFN is available that allows to simulate a fouling of the heating surface. It is used in all load cases both at FSPEC=4 and at FSPEC=5, in such a way that the effective furnace outlet temperature EFFN*A is used at all times. The Boltzmann number (also known as Konakov number) used for the calculation and the adiabatic combustion temperature are output as result values.

Combustion efficiency

While in the case of gases, fuel and air mix completely and therefore a complete combustion occurs at all times (always provided that there is enough oxygen), with solid and liquid fuel it may happen that due to the limited dwell time of the fuel particles and droplets respectively in the combustion chamber, a certain fraction of the fuel does not come into contact with the air at all but leaves the combustion chamber as unburnt matter. The combustion efficiency ETABN serves to model this effect.

The flag FUB (matter of unburnt) serves to define which constituents of the fuel ETABN refers to:

• At FUB = 0 (fuel) it is assumed that the unburnt particles have the same composition as the fuel. In this case, ETAN refers to the constituents C, H, O, N, S, and Cl 
                    of the fuel.

• At FUB = 1 (carbon) it is assumed that the unburnt particles consist of pure carbon and the other constituents of the fuel burn completely. In this case,
                     ETAN only refers to the C-fraction of the fuel.

Example: At ETABN = 99%, 1kg/s of unburnt matter (consisting of 95% C and 5% H) will remain of 100 kg/s of a fuel that consists of 95% C and 5% H at FUB=0; at FUB=1, however, only 0.95 kg/s of unburnt matter (pure C). The NCV of the unburnt matter is calculated from the composition in any case.

Depending on the size of the unburnt particles, these can either be carried along with the exhaust gas or deposit in the slag. This is the purpose of the specification value UBASH that specifies which proportion of the unburnt matter is routed into the slag outlet (Pin 5).

Caution: This has nothing to do with the specification value RFLAS for the fly ash. RFLAS refers to the incombustible fraction of the fuel, i.e. the ash fraction that had already been present in the coal before, and specifies which proportion of these incombustible substances is entrained into the exhaust gas. In contrast to this, the unburnt matter is combustible material which, however, has not been burned.

Please note: This explication refers to the new calculation mode FCALC=3, which is activated by default. Older calculation modes partly use other definitions not further explained here. The ‘middle-aged’ mode (FCALC=2) may be of interest if measurements of the concentration of unburnt constituents in the slag and in the fly ash exist. At FCALC=2 it is possible to specify these concentrations instead of the combustion efficiency.

Radiation and heat losses

In this component, radiation losses could only be considered according to
QLRA = C * QN^0.7
whereby the factor C could either be freely specified or determined according to fuel type according to EN12592. QN is the useful heat in the design case.

As this formula only depends on the nominal value QN, the losses are always constant and, in particular, also independent of the load case, of the composition and the NCV of the fuel, of the air ratio, and of the exhaust gas temperature. Thus it is a rather rough estimate.

In order to enable a more precise modeling, further variants for calculating radiation and heat losses respectively have been implemented that are set via the
new flag FRAD:

• FRAD=0 carries out the calculation unchanged according to EN12592, i.e. the radiation loss is considered constant:
  QLRA = C *QN^0.7
  
• FRAD=1 refers the losses to the current useful heat:
  QLRA = DQLR * Q3
  
• FRAD=2 refers the losses to the heat quantity that is transferred to the exhaust gas. The calculation is effected in analogy to Component 22 according to
  QLRA = DQLR * (M1*(H1+NCV1)+M4*(H4+NCV4)+M6*(H6+NCV6) – (M2*NCV2 + Q3 + M5*(H5+NCV5))
  
• FRAD=3 refers the losses to the total heat released in the combustion:
  QLRA = DQLR * (M1*NCV1+M4*NCV4+M6*NCV6 – M2*NCV2 - M5*NCV5)

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

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

Here the calculation of the equilibrium can be combined with the specification options of Component 21 (however, only in the new calculation mode FCALC=3):

• 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 21.

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

Characteristic Lines and Matrix

Physics Used

Equations

Component Displays

	Display Option 1		Display Option 2
	Display Option 3		Display Option 4
	Display Option 5

Example

Click here >> Component 21 Demo << to load an example.