Method for operating a steam generator

Abstract

A method for operating a steam generator comprising a combustion chamber having a plurality of evaporator heating surfaces which are connected in parallel on the flow medium side is provided. An object is to provide a steam generator which has a particularly long service life and which is particularly reliable. For this purpose, a flow medium is introduced into an inlet of a first evaporator heating surface at a temperature which is lower than the temperature of the flow medium introduced into the inlet of a second evaporator heating surface.

Claims

1. A method for operating a steam generator with a combustion chamber having a plurality of evaporator heating surfaces which are connected in a parallel manner on the flow medium side, comprising: forming a peripheral wall of the combustion chamber of steam generator pipes; forming an inner wall and a further inner wall at least partly from additional steam generator pipes; arranging the inner wall and the further inner wall inside the combustion chamber; connecting the further inner wall downstream from the inner wall on a flow medium side by an intermediate collector; supplying a flow medium to a first inlet of a first evaporator heating surface at a lower temperature than to a second inlet of a second evaporator heating surface, wherein a preheater is connected upstream of the first and second inlets on the flow medium side, wherein a bypass line is arranged so that a first part of the flow medium is conducted to bypass the preheater, wherein a branch point is provided upstream of the preheater on the flow medium side such that the first part originates at the branch point and bypasses the preheater, wherein the first part of the flow medium is mixed with a second part of the flow medium that is branched downstream of the preheater on the flow medium side, the mixture of the first part and the second part is supplied to the first inlet, and wherein the mass throughflow of the first part flow is regulated via a throughflow regulating valve disposed in the bypass line based on thermodynamic characteristics at a measurement point downstream of the first inlet of the first evaporator heating surface, wherein the measurement point is disposed in the intermediate collector connected downstream of the first evaporator heating surface.

2. The method as claimed in claim 1, wherein a mass throughflow of the second part flow has an upper limit.

3. The method as claimed in claim 1, wherein pressure and temperature are used as thermodynamic characteristics, with the saturated steam temperature being determined from the measured pressure and the actual subcooling value being determined based on the measured temperature.

4. The method as claimed in claim 3, wherein a setpoint value is predefined for subcooling, and wherein the mass throughflow of the first part flow is regulated based on the deviation between the actual and setpoint values for subcooling.

5. The method as claimed in claim 4, wherein when the actual value for subcooling is lower than the setpoint value, the mass throughflow of the first part flow is increased.

6. The method as claimed in claim 1, wherein the mass throughflow of the second part flow is regulated based on the mass throughflow of the flow medium supplied to the first evaporator heating surface.

7. The method as claimed in claim 1, wherein the flow of the medium supplied to the first evaporator heating surface is regulated based on the outlet enthalpy of a last evaporator heating surface connected downstream of the first evaporator heating surface on the flow medium side.

8. The method as claimed in claim 7, wherein the outlet enthalpy of the evaporator heating surface is determined based on the temperature at the outlet of the flow medium at the last evaporator heating surface connected downstream of the first evaporator heating surface on the flow medium side and the pressure in a water/steam separator connected downstream of the first, second, and last evaporator heating surfaces on the flow medium side.

9. A steam generator, comprising means for executing the method as claimed in claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) An exemplary embodiment of the invention is described in more detail below with reference to a drawing, in which:

(2) FIG. 1 shows a schematic diagram of the lower part of the combustion chamber of a forced-circulation steam generator with fluidized bed combustion with a partially bypassed preheating facility,

(3) FIG. 2 shows the circulation steam generator from FIG. 1 with regulation of the throughflow to the inner walls,

(4) FIG. 3 shows the circulation steam generator from FIG. 1 with regulation of the outlet enthalpy of the inner walls, and

(5) FIG. 4 shows a graph, illustrating specific enthalpy and pressure of the flow medium in different regions of the circulation steam generator with different loads.

(6) Identical parts are shown with the same reference characters in all the figures.

DETAILED DESCRIPTION OF INVENTION

(7) The steam generator 1 illustrated schematically in FIG. 1 is embodied as a forced-circulation steam generator. It comprises a number of tube walls, which are formed from steam generator tubes and contain an upward flow, specifically an enclosing wall 2 and symmetrically disposed, angled inner walls 4, connected downstream of which by way of an intermediate collector 6 on the flow medium side is a further inner wall 8. The circulation steam generator 1 is thus embodied with the so-called pant-leg design.

(8) Flow medium passes through the inlets 10, 12 assigned respectively to the enclosing wall 2 and inner walls 4 into the tube walls. In the interior 4 a solid fuel is combusted in the manner of fluidized bed combustion, as a result of which heat is input into the tube walls, bringing about heating and evaporation of the flow medium. If the medium enters all the tube walls with the same enthalpy, the steam content in the intermediate collector 6 can be so high that there is irregular distribution to the tubes of the inner wall 8 with the result that the tubes with a high steam content superheat.

(9) To avoid the disadvantages that would result, such as for example a shorter service life or a greater need for repair, flow medium is supplied to the inner walls 4 upstream of the intermediate collector 6 at a lower temperature than to the enclosing wall 2. Provision is therefore made first in the steam generator 1 for modifications to the preheater 16, which ensure different heat inputs into the different medium flows.

(10) To this end a branch point 18 is provided upstream of the preheater 16 on the flow medium side according to FIG. 1. A part of the flow medium is thus directed around the preheater 16 in a bypass line 20. A further branch point 22 is initially provided downstream of the preheater 16 in a flow medium side direction, with a line passing from it to the inlets 10 of the enclosing wall 2. A part of the preheated flow medium is thus supplied to the enclosing wall 2. Another part of the preheated flow medium is conveyed in a line 24, which meets the bypass line 20 at a mixing point 26. The mixing of the medium flows here produces a medium at lower temperature, which is then supplied to the inlets 12 of the inner walls 4.

(11) An non-return valve 30 is disposed in the line 24, to prevent undesirable cooling by a return flow into the branch point 22. A manual throughflow regulating valve 32 is also provided, which limits the branched mass flow of preheated medium upward. An automatic throughflow regulating valve 28 in the bypass line 20 allows the quantity of bypassed flow medium and therefore the temperature of the flow medium supplied to the inner walls 4 to be easily regulated.

(12) Pressure p and temperature T in the intermediate collector 6 are used as input variables for automatic regulation in the throughflow regulating valve 28. The saturated steam temperature is first determined from the determined pressure, its difference in respect of the determined temperature T giving the actual subcooling. In order to prevent separation of water and steam in the intermediate collector 6, a setpoint subcooling in the intermediate collector 6 is predefined. If the actual subcooling is below the setpoint subcooling, the automatic throughflow regulating valve 28 is closed further so that the temperature at the inlets 12 rises. Conversely the throughflow regulating valve 28 is opened further. If pressure and temperature are above the critical point of the flow medium, the throughflow regulating valve 28 is closed completely, since at supercritical pressures water and steam cannot occur simultaneously at any temperature and therefore separation can no longer occur in the intermediate collector 6.

(13) FIG. 2 shows an alternative embodiment of the invention. The steam generator 1 here is identical to FIG. 1 apart from the throughflow regulating valve 32. The throughflow regulating valve 32 here is automated like the regulating valve 28. This also allows the quantity of medium supplied to the inner walls 4 to be regulated. The input variable for regulation here is the overall flow F to the inlets 12, which is determined at a measurement point 34. The overall flow F here is conveyed based on a setpoint value determined by means of design calculations.

(14) A further embodiment of the invention is illustrated in FIG. 3. The steam generator 1 here is identical to FIG. 2 but further components are illustrated, specifically the outlet 36 of the inner wall 8 and the outlets 38 of the enclosing wall 2. The medium flows from the outlets 36, 38 are combined and conveyed to a water/steam separator 40. The main regulating circuit, which regulates the entire quantity of flow medium supplied to the steam generator 1 by means of a throughflow regulating valve 42, is also shown here. Pressure p and temperature T at the steam-side outlet of the water/steam separator 40 serve as input variables for regulating the overall medium flow here.

(15) In FIG. 3 the quantity of flow medium supplied to the inner walls 4 by way of the inlets 12 is regulated as a function of the outlet enthalpy of the inner wall 8. This is determined based on the temperature T at the outlet 36 of the inner wall 8 and the pressure p in the water/steam separator 40. Provision is made here for the mean fluid enthalpy in the water/steam separator 40 to be the setpoint value for the outlet enthalpy of the inner wall 8. The outlet temperature at the outlet 40 is also limited above the maximum permissible material temperature.

(16) FIG. 4 finally shows a state diagram for water/steam, in which the states of the flow medium are shown in different regions of the steam generator. The diagram shows the specific enthalpy h in kJ/kg against the pressure p in bar. Lines of identical temperature T, in other words isotherms 44, are shown first, their respective temperature values being indicated on the right axis of the graph in degrees Celsius. The bulge-like structure 46 on the left side of the graph shows the steam content of the water/steam mixture. Outside the structure 46 the medium is single-phase, in other words only medium in an aggregate state is present. The peak of the structure 46 at around 2100 kJ/kg and 221 bar here marks the critical point 48. When the pressure rises above 221 bar, water and steam do not occur simultaneously at any temperature.

(17) A water/steam mixture is present within the structure 46. The proportion of water and steam is shown here with characteristic lines 50 at 10 percent intervals, from 0% steam content at characteristic line 52 to 100% steam content at characteristic line 54. The characteristic lines 50, 52, 54 converge here at the critical point 48. Within the structure 46 the isotherms 44 run perpendicular to the pressure axis, so they are also isobars. An energy input into the medium at constant pressure therefore does not bring about a higher temperature but rather a displacement of the water/steam component toward more steam.

(18) Depending on the load state of the steam generator 1 the steam process within the steam generator 1 runs on different load characteristic lines 56, 58, 60, which are not isobars, as the pressure losses of the heating surfaces are shown. The load essentially determines the pressure within the system as a whole. Load characteristic line 56 represents the steam process at 100% load, load characteristic line 58 the steam process at 70% load and load characteristic line 60 the steam process at 40% load. Points A, B, C, D here respectively represent the state of the flow medium at different points of the steam generator 1, initially still without the inventive separate regulation of the temperature at the inlets 12 of the inner walls 4: point A the state at the inlet of the preheater 16, point B the state at the inlet 12 of the inner walls 4, point C the state in the intermediate collector 6 and point D the state at the outlet of the evaporator.

(19) As shown in FIG. 4 at 100% load the steam generator is operated completely in the supercritical region. At no point A, B, C, D on the load characteristic line 56 is it possible to distinguish water and steam, so separation cannot occur. At 70% load the subcritical region has already been reached but only a small part of the load characteristic line 58 lies within the structure 46. Points A, B, C of the load characteristic line 58 are still below the structure 46 and single-phase water is present. Separation cannot occur in the intermediate collector 6 here either.

(20) However at 40% load a significant part of the load characteristic line 60 lies within the structure 46. Points A and B on the load characteristic line 60 are still below the structure 46, so single-phase water is still present here. Point C on the load characteristic line 60 however lies within the structure 46 with a steam component of 10%. The described separation in the intermediate collector 6 can take place here. However if a part of the flow medium is conveyed past the preheater 16, which is achieved in pressure regions below the load characteristic line 62 by opening the throughflow regulating valve 28, the temperature and therefore the energy content of the flow medium are specifically reduced. On load characteristic line 60 point E then shows the state of the flow medium at the inlet 12 of the inner walls 4 with a reduced temperature. This also reduces the energy content in the intermediate collector 6, as shown by point F on load characteristic line 60. This point F is now outside the structure 46, so single-phase water is present here and separation is reliably prevented.