Component 90: Steam Generator - Reaction Zone

 Specifications

 

General       User Input Values       Characteristic Lines       Physics Used       Displays       Example

 

General

Module 90 together with component 89 (steam generator, main heating surface) or component 91 (steam generator, auxiliary heating surface) allows for the geometry-based representation of combustion and heat transfer in the combustion chamber of a steam generator.

Module 90 represent a flue gas zone with combustion, which can contain a main heating surface (component 89). For that purpose PIN 3 of the combustion zone (component 90) shall be connected with PIN 3 of a main heating surface (component 89).  In addition, the radiation flow from the flue gas volume in the zone to the upstream and / or downstream heating surface component 89 can be taken into account. For this purpose, PIN 7 can be connected with PIN 5 of the main heating surface (component 89) in the upstream flue gas zone and correspondingly PIN 6 with PIN 4 of component 89 in the downstream zone.

Unlike component 21 (combustion chamber), the outlet temperature can not be given using nominal value and characteristic curve, but is calculated from the heat input and the heat exchange with the main heating surface.

Concerning the calculation of heat transfer and radiation flow, component 90 behaves as module 88 (steam generator, flue gas zone), which does not take combustion into account.

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 is given as a volume fraction or as a standardized mass fraction (mg/Nm³).

 

MODEL

It takes into account

• 1 flue gas inlet
• 2 flue gas outlet
• 3 heat exchange with the connected main heating surface
• 4 ash inlet (usually from the downstream flue gas zone)
• 5 ash outlet
• 6,7 radiation flow to upstream and downstream heating surface
• 8 air inlet
• 9 fuel inlet
• 10 additional flue inlet

The component calculates

H1 or H2, M1 or M2, if in each case one of them is given.

H4 or H5, M4 or M5, if in each case one of them is given.

H8 and H9 (or/and H10) must be given as well as M8 and M9 (or/and M10) or M8 or M9 (or/and M10) and the air ratio ALAM

P2 = P1 - DP12 as well as P1=P5=P8=P9 (or/and P10) and P4=P2, i.e. one of the pressures P1, P2, P4, P5, P8, P9 (or/and P10) must be given.

Pressure Drop for “Boiler: reaction zone” DP12

The pressure drop in the furnace usually refers to the pressure difference between inlet and outlet of the flue gas. In the lowest stage, however, no flue gas is present at the inlet yet.
But the pressure had to be specified there in order to receive the correct outlet pressure.

Note : This has been changed: if no line is connected at the flue gas inlet (Pin 1), the pressure at the outlet (Pin 2) is calculated in comparison to the air inlet (Pin 8).

 

CALCULATION FUNDAMENTALS

At first, a combustion calculation is done for determining the flue gas composition and the adiabatic combustion temperature TAD (result value).

For this purpose, the fuel fractions of fuel (9) (or/and 10),  ash (4) and flue gas (1) are cumulated. The burnt part of the total fuel ETABN (combustion efficiency) completely reacts with the total air from air inlet (8) and flue gas inlet (1). The unburned fuel (1-ETABN) is distributed to ash outlet (5) and flue gas outlet (2) according to the parameter UBASH (portion of unburnt fuel in ash).

A partial combustion with CO formation can be mapped at the outlet by specifying the CO concentration. However, the specification of an air ratio < 1 is currently not implemented, because it will result in component errors.

The ash fractions from fuel feed (9) (or/and 10) and ash feed (4) are cumulated and distributed to the flue gas outlet and the ash extraction according to the parameter RFLAN (fly-ash portion in the total ash). The ash extraction is done with at least one of the temperatures defined by the parameter TASHE.

There are several variation possibilities for specifying the unburnt fuel:

The composition of the unburnt fuel can be selected either the same as the fuel composition or as that of pure carbon.

The quantity of the unburnt fuel can be specified as follows:

The flag FTYPUB basically defines, whether

          1. ETABN  and UBASH or
          2. UBSL and UBFL

are specified.

However, further possibilities result from the increase of the parameter RFLAN:

          1. Specification: ETABN / UBASH / RFLAN -Calculated: UBSL / UBFL  (as case 1 above)
          2. Specification: UBSL / UBFL / RFLAN - Calculated: UBASH / ETABN  (as case 2 above)
          3. Specification: UBSL / UBFL / ETABN - Calculated: UBASH / RFLAN
          4. Specification: UBSL / UBFL / UBASH - Calculated: ETABN / RFLAN

In other words: If one specifies UBSL and UBFL, there is no need to specify RFLAN; Instead of that, ETABN or UBASH can also be specified independently, however,
then RFLAN should not be specified, because it results.

The gasification of ash ASG can be taken into account. According to DIN EN 12952  5% is to be assumed for burner and stoker-firing and 0% for fluidized-bed firing.

The calorific value of the unburnt fuel can be specified. DIN EN 12952 recommends here 33000 kJ/kg for hard coal and 27200 kJ/kg for lignite. By default, however, EBSILON^®Professional calculates the value from the elemental analysis.

The cp-value of the ash can be specified. This is done through a correction factor, with which the calculated cp-value is multiplied. For ash in waste gas and in the slag different correction values can be specified. The DIN EN 12952 recommends

          cpSlag =       1,0 kJ/kgK in case of dry firing,

                               1,26 kJ/kgK in case of slag-tap firing

          cpFlydust =  0,84 kJ/kgK (between 25 and 200 °C)

This corresponds to correction factors of 1.02 or 1.28 for cpSlag and 0.86 for cpFlydust respectively (at 200 °C)

 

The balancing of the energy flows assigned to these material flows results in the heat content of the flows at the flue gas outlet and ash outlet as well as the adiabatic
combustion temperature TAD.

The heat transfer to the main heating surface (component 89), which is attached to the combustion zone is calculated in component 89 and copied to component 90 using the logic lines. In addition, module 90 calculates the radiation flow to the upstream or the downstream surfaces, which may be attached to it according to

QR_6 = PHI6_I*CS*EBS3_I*(1-EBS6_I)*LAMBDA*(TRADH6**4-TRADL6**4)*BEW3

QR_7 = PHI7_I*CS*EBS3_I*(1-EBS7_I)*LAMBDA*(TRADH7**4-TRADL7**4)*BEW3

Form factors PHI, emissivities EBS, efficiency factors are evaluated in the attached modules, and transferred by logic lines. The radiation temperature of the emitting zone is a weighted average of the adiabatic combustion temperature TAD and the flue gas outlet temperature T2

TRADH6=TRADH7= ALPHA_T *TAD + (1-ALPHA_T)*T2+273.15

The weight factor ALPHA_T is set as specification value under EXTRAS->MODEL OPTIONS->SIMULATION (Calculation).

The radiation flows are always positive i.e. there is no radiation flow into the flue gas originating from heating surfaces with a higher radiation temperature than the flue gas. The radiation temperature of the main heating surfaces is the average wall temperature:

TRADL6 = TWAND6

TRADL7 = TWAND7

Similar to component 88, the contributions from the gas radiation and the solid particles radiation, possibly containing ash and soot particles, are taken into account for calculating the emissivities, as well as a contribution of the unburnt fuel particles for solid fuels. For this purpose, a portion PERCOKE of the solid fuel with a particle distribution described by the specification values CSCOKE, DIACOKE and DISTCOKE is equally spread out over the volume of the zone and is calculated with the same formula as an emissivity fraction for the ash loading.

Radiation losses can be specified either through the relative loss (DQLR) or from the C-factor and the nominal useful heat is calculated (DQ = C * QNMAX ^0.7, QNMAX in MW).

 

COMPONENT IDENTIFICATION

There is no identification mode for component 90.

GEOMETRIC DISPLAY OF FLUE GAS ZONES

The arrangement of inlet and outlet area of the flue gas zone essentially influences the calculation of the form factor PHI. To represent the different types of steam generator design with or without direction changes in the flue gas path, the specifications values OUTH and FTYP can be used. Their use is illustrated in the diagram given below:

 Note about the air ratio:

The flag FALAM is used to switch whether the air ratio is determined by the specification value ALAMN or by both mass flows on the fuel and the air inlet. In each case, the air ratio is displayed as result value ALAM. This value is defined ALAM in EBSILON^®Professionalon the air flow that is inserted via the air pipe. If there is some air on the fuel line (this is the case when the mill is included in the model, e.g.), the result value ALAM does not match the value of the air ratio that is expected by the O2 concentration in the flue gas.

For this reason, there is the result value ALAMT (total air ratio). This value is determined by the O2 concentration and therefore independent from the pipe where the air supplied.

 

Note about the unburnt fuel:

The specification value ETABN (Combustion efficiency, nominal) always refers to solid or liquid fuels. If one connects gas or crude gas to connection 9 (or/and 10), this is then burnt completely. ETABN is then used only for the solid unburnt fractions, which could be available from a previous burner stage via the slag (4) - or the flue gas (1)-inlet (for instance, when one operates the previous stage with coal).

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. This is also 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.
 

With the specification value ASHSPL, Component 90 also allows to apportion the ash coming from above (Pin 4) among the exhaust gas outlet (Pin 2) and the slag outlet (Pin 5).

If the specification value ASHSPL=0, the ash coming from above will always be forwarded to the slag outlet in its entirety. 

 

The NOX and CO concentration, respectively (CO only in the case of components 21 and 90) at the outlet can be specified via a kernel expression. This is controlled via the
flags FNOCON and FCOCON, respectively:

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

As before, the flag FCON controls whether the concentration is given as a volume fraction (mol/mol) or as a standardized mass fraction (mg/Nm³).

 

The following additional results are calculated and displayed:

• Volume load (result value VLOAD)
• Cross-section load (result value ALOAD)
• Total input heat (result value QTHIN)

 

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 been included in the list
of available substances as well.

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.

The combustion products MgO and CaO are divided in the components 21 and 90 according to the default value RFLAN on slag pipe and exhaust pipe, just like all the other non-combustible solids.

Second fuel connection 

Another fuel inlet (Pin 10) has been added to this component to enable an operation with two different fuels (like e.g. oil and gas).  

 

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

As in Component 21, the possibility to carry out a calculation of the equilibrium instead of a combustion calculation has been created in Component 90 too.

The operation is analogous to Component 21

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.

However, Component 90 features more pins that are treated as follows:

• Exhaust gas inlet from the predecessor stage (Pin 1): the exhaust gas entering here is treated and included in the calculation of the equilibrium just like the combustion air (Pin 8).

• Slag inlet from the predecessor stage (Pin 4): the slag entering here is not included in the equilibrium but leaves the component unchanged; in doing so, it is distributed to Pin 2 (exhaust gas outlet) and Pin 5 (slag outlet) according to specification value ASHSPL. 

Flame temperature for radiation exchange

For the calculation of the flame temperature TF relevant for the radiation exchange, there are two theoretical limit values – the adiabatic combustion temperature Tad and the flue gas outlet temperature Tout.
The flame temperature relevant for the radiation fluxes Q6 and Q7 was calculated with a weighting factor Alpha_T from the model settings (model settings => simulation => calculation).

TF= Alpha_T*Tad+(1- Alpha_T)*Tout , 0<=Alpha_T<=1

For the radiation flow Q3, by contrast, the arithmetic mean value from Tad and Tout was used at all times, which corresponds to an Alpha_T value of 0.5.

In component 90 there are three new specification values – FTRAD, ALPHAT, and ETRAD – that are used for controlling the calculation of the flame temperature TF. It is possible to specify a local value ALPHAT for the component, which is either used only for the radiation flow Q3 or for all radiation flows (Q3 and Q6, Q7). Alternatively, the flame temperature can also be calculated for all radiation flows as geometrical mean value of the Tad and Tout or it can be calculated by the user by means of a Kernel expression ETRAD.

The flame temperature is displayed as result value TRAD.

 

Radiation losses

As Component 21 before, Component 90  allows to make the following settings with the flag FC for the parameter used for calculating the radiation losses:

• 0: manual specification of C in the specification value C
• 1: use of the EN12952 value for oil and gas boilers (C=0.0113)
• 2: use of the EN12952 value for hard coal boilers (C=0.0220)
• 3: use of the EN12952 value for lignite boilers (C=0.0315)
• -1: use of the EN12952 value according to the line and coal type respectively of the main fuel line (Pin 9) 

However, FC is only active if FLOSS has been set to 1 (“Calculation from C and QNMAX “).

 

 

User Input Values

  

 Degree of desulfurization; the percentage 1-DESN of sulfur in the fuel will be converted to SO2

The identification value marked in blue is the reference value for the off-design mode. The actual off-design values refer to the values used in the equations.

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

No characteristic lines are used.

 

Specification-matrix

MXRSO2 - is used for the degree of desulfurization

Physics Used

Equations

 

 

 

 

Component Displays

Display Option 1

Example

Click here >> Component_90 Demo << to load an example.