EP2260236A2

EP2260236A2 - Coal-fired power station and method for operating the coal-fired power station

Info

Publication number: EP2260236A2
Application number: EP09710549A
Authority: EP
Inventors: Andreas Rohde; Christian Bergins; Friedrich Klauke; Martin Ehmann; Thorsten Buddenberg; Bernd Vollmer; Thomas Krause; Alfred Gwosdz
Original assignee: Hitachi Power Europe GmbH
Current assignee: Hitachi Power Europe GmbH
Priority date: 2008-02-14
Filing date: 2009-02-10
Publication date: 2010-12-15
Also published as: WO2009100881A2; WO2009100881A3; US20110014578A1; DE102008009129A1; CA2715625A1; WO2009100881A4; AU2009214317A1

Abstract

In a method for operating and controlling/regulating a power station comprising a coal-fired steam generator (11), the steam generator (11) of which is rated for the steam parameters achievable by the heat transfer onto the steam mass flow upon coal firing in the steam generator (11) carried out using combustion air, a solution is to be created, which enables the operation of coal-fired power stations rated for air operation utilizing a firing of the fuel carried out according to the oxy-fuel process in the firing chamber of the steam generator of the coal-fired power station. This is achieved in that a firing of the fuel containing coal is carried out in the steam generator (11) according to the oxy-fuel process utilizing approximately pure oxygen containing more than 95% by volume, and recirculated flue gas containing a high amount of CO2, such that the mass flows of all fuel flows supplied to the coal-fired burners (10) and to the steam generator (11), and the combustion gas, carrier gas, and process gas flows from the combustion oxygen and/or recirculated flue gas are configured and adjusted to each other with respect to the respective composition ratio thereof of oxygen and/or flue gas such that the heat transfer occurring in the steam generator by means of flame radiation, gas radiation, and convection onto the steam mass flow is maintained equal overall in the steam/water cycle as compared to air combustion, in particular, that the same steam parameters are obtained.

Description

Coal-fired power plant and method of operation of the coal-fired power plant

The invention is directed to a method for operating and for controlling a coal-fired steam generator comprising a power plant, the steam generator of which is designed for combustion with combustion coal combustion in the steam generator by the heat transfer to the steam mass flow achievable steam parameters.

Furthermore, the invention is directed to a coal-fired power plant with a coal-fired steam generator, the steam generator is designed for achievable with combustion air coal combustion in the steam generator by the heat transfer to the steam mass flow achievable steam parameters.

In order to be able to greatly reduce CO ₂ emissions in the generation of electricity from fossil fuels in the future, the following two concepts are currently being developed, which can be used in conventionally designed pulverized coal power plants (hard coal and lignite) with large steam generators and for future power plant generations to be commercially available:

1. Post-combustion CO ₂ Capture by downstream CO ₂ flue gas scrubbing

Here, the present in low concentration (~ 13 vol .-%) in the flue gas CO ₂ with washing solutions in one

Packed packed column with energy release and desorbed in a second column with energy. The additional energy required for desorption (usually in

Energy supplied from the mid-pressure turbine) leads to efficiency losses of 10-12% -points compared with power plants without such CO ₂ capture. One advantage of this technology is that the process is followed by the combustion process and has no repercussions on the steam generator, which makes it possible to retrofit existing power plants. The disadvantages include the high space requirement for the flue gas scrubbers and the high energy demand, which - in a possible retrofitting - brings a lot of change in the field of steam cycle and the turbine with it.

2. Oxy-fuel combustion with direct CO ₂ compression

In the oxyfuel process, the CO ₂ concentration in the flue gas is greatly increased by using a mixture of recirculated flue gas and almost pure oxygen instead of air to burn the coal. In particular, it is the tightness of the power plant with all its components which is still operated under light vacuum for safety reasons, the purity of the oxygen obtained from an air separation plant (LZA), the quality of the flue gas cleaning plants (denitrification, desulfurization, dedusting) and the process control, which is decisive the location of the flue gas recirculation (eg the locations / locations 1-6 in Fig. I) - on the purity of the process after a flue gas drying (condensation of the water by cooling) leaving CO ₂ . In any case, the CO ₂ concentration should be so high and the exposure to pollutants so low that the CO ₂ can be directly compressed and fed to storage. The advantage of this concept is that both steam generator and steam cycle and turbine design, which are designed for a conventional air operation, do not differ fundamentally from those that are designed for oxyfuel operation. In addition, the efficiency of this process compared to power plants without CC> 2 separation with 8-10 percentage points lower than with a downstream CO ₂ flue gas scrubber. Significantly contribute to this loss of electrical demand of the installed air separation plant (> 60% of the loss) and the additional flue gas compression (> 25% of the loss) at. The proportion of other aggregates required for conventional energy generation compared to conventional power plants is <15%.

In order to equip the value stability of investments already made in power plant new buildings of coal power plants with the greatest possible flexibility with regard to future developments in connection with a possibly still to be retrofitted CO ₂ separation, it is necessary that current power plant new buildings in the future by a conversion to the oxyfuel operation also operate as CO ₂ -powered power plants. However, it is also desirable to provide older, existing coal-fired power plants cost-effectively, ie with the least possible capital expenditure and the least possible loss of efficiency, with a CO ₂ separation and to be able to convert to an oxyfuel operation with CO ₂ separation.

The invention is therefore based on the object to provide a solution that makes it possible to operate for an air operation designed coal power plants with a successful after the oxyfuel process combustion of the fuel in the furnace chamber of the steam generator of the coal power plant.

In a method of the type described, this object is achieved in that in the steam generator combustion of the carbonaceous fuel after the oxyfuel process with approximately pure, more than 95 vol .-% O ₂ containing oxygen and recirculated, high CO ₂ -containing flue gas is carried out such that the mass flows of all the coal-fired burners and the steam generator supplied fuel streams and combustion gas, conveying gas and process gas streams of combustion oxygen and / or recirculated flue gas formed in their respective composition ratio of oxygen and / or flue gas and each other be agreed that the heat transfer in the steam generator by flame radiation, gas radiation and convection on the steam mass flow in the steam / water cycle of the steam generator compared to the air combustion is kept the same, in particular the same steam parameters are obtained.

In a power plant of the type described in more detail above, the above object is achieved analogously in that in the steam generator combustion of the carbonaceous fuel after the oxyfuel process with approximately pure, more than 95 vol .-% O ₂ containing oxygen and recirculated , high Cθ ₂ ~ containing flue gas such that the mass flows of all the coal-fired burners and the steam generator supplied fuel streams and combustion gas, conveying gas and process gas streams of combustion oxygen and / or recirculated flue gas formed in their respective composition ratio of oxygen and / or flue gas and are matched to one another that the heat transfer in the steam generator by flame radiation, gas radiation and convection on the steam mass flow in the steam / water cycle of the steam generator compared to the air combustion remains the same overall, in particular the D obtained ampfparameter are the same. Advantageous developments and expedient embodiments of the invention will become apparent from the respective dependent claims.

The invention achieves that a coal-fired power plant designed for combustion with air can also be operated without difficulty as a CO ₂ -free or CO ₂ -free power plant operating according to the oxyfuel process with recirculated flue gas and converted to the oxyfuel process or oxyfuel operation. or can be retrofitted. In such a power plant, the burners are then operated with a supply of pure oxygen> 95 vol .-% O ₂ or a mixture of pure oxygen and recirculated, highly CO ₂ -containing flue gas.

In order to operate a designed for air operation conventional power plant in oxyfuel operation, flue gas resulting from the combustion process is returned to the burner and the burner or combustion chamber, ie recirculated. In order to achieve the same steam parameters as in air operation in oxyfuel operation, treated and / or untreated flue gas can be recirculated to the steam generator in accordance with the invention. By "untreated" flue gas is meant that which is branched off in the flue gas path to the steam generator for recirculation, before in the flue gas flue gas treatment such as by an electrostatic precipitator, a flue gas desulfurization or a flue gas dryer.The passage of heat transfer systems or elements with which the flue gas only In the embodiment shown in Fig. 1, possible locations and locations for a branch of untreated flue gas are designated by the reference numerals 1, 2 and 3. Under the term "treated "Flue gas is understood a treatment that alters the flue gas composition. Possible locations or locations for a branch of treated flue gas for recirculation are indicated in FIG. 1 by the reference symbols 4, 5 and 6.

Since, in particular modern power plants are to be provided expediently for a subsequent conversion to the oxyfuel operation, the invention further provides that an existing, in particular a so-called 600 ⁰ C - power plant with the method according to claim 1 or 2 is retrofitted.

In order to achieve the same steam parameters in oxyfuel operation as in air operation, it is expedient according to a further embodiment of the invention that the recirculation rate of the flue gas is 65% to 80%, in particular 74% to 78%.

With a particularly low conversion or retooling a designed for air operation power plant can operate in oxyfuel operation when the flue gas behind a desulfurization or flue gas desulfurization or one, in particular additional and / or retrofitted, flue gas cooler is withdrawn for recirculation, which the Invention also provides.

It is particularly advantageous according to embodiment of the invention then, when the flue gas is withdrawn in the flow direction behind a flue gas condensation dryer.

Due to the high CO ₂ ~ content in the flue gas, it is

Oxyfuel operation continues to be appropriate when in the

Flue gas desulphurisation plant is used as absorbent fire lime (CaO). In an embodiment of the power plant according to the invention, the invention provides that between a induced draft and a desulfurization or desulfurization a heat transfer system is installed. As a result, the heat balance of the recirculating flue gas stream and the temperature of the recirculated flue gas can be influenced by energy dissipation.

In order to be able to effect an effect on the temperature of the flue gas, but also of the steam supplied to the steam generator after an air separation in an air separation plant, the power plant according to the invention finally characterized by the fact that the flue gas duct in the flow direction after a denitrification, one in particular parallel to a Air preheater (LUVO) guided bypass line having arranged therein gas-gas heat exchanger.

It goes without saying that the features mentioned above and those yet to be explained below can be used not only in the respectively specified combination but also in other combinations. The scope of the invention is defined only by the claims.

The invention is explained below with reference to a drawing. This shows in

1 is a schematic representation of a process and system diagram of the gas and fire side of a

Steam generator,

2 is a schematic diagram of a process scheme of the gas and fire side of a steam generator, 3 is a schematic diagram of another embodiment of a gas and fire side process scheme of a steam generator,

4 is a schematic principle and sectional view of the structure of a steam generator,

5 shows a comparison of the temperature profile over the furnace height in the radiation part of a steam generator in FIG

Comparison between air operation and oxyfuel operation,

6 shows a comparison of the temperature profile in the convective part of a steam generator in comparison between air and oxyfuel operation,

7 shows the temperature profile of the water / steam system in the convective part of a steam generator in comparison between air operation and oxyfuel operation,

8 shows a tabular representation of the difference between different characteristic values in comparison between oxyfuel and air operation, once in metric units and once in percentages,

9 shows in tabular representation the difference in the flue gas composition behind the electrostatic precipitator in comparison between the air operation and the oxyfuel process,

Fig. 10 in tabular table form a comparison between the air operation and the oxyfuel operation with respect to various physical properties and in Fig. 11 is a representation corresponding to FIG. 2 with additional indication of the gas temperatures in ° C and the mass flows in kg / s.

In the following, the plant supplements and modifications are described with reference to FIGS. 1-3 with which a conversion of a conventional coal-fired power plant can be carried out on the oxyfuel operation. In addition, a method is described according to which the oxyfuel operation can then be carried out in an optimal manner. A special feature of the conversion and the process is that even after the conversion normal air operation is still possible. Thus, the system can safely started in air operation, switched to the oxyfuel operation and before switching off or in case of malfunction or interruption of the power plant downstream CO ₂ storage option on the easier-to-control air operation (with higher efficiency) with CO ₂ emissions be switched.

2 shows a process diagram of the gas side of a coal power plant, according to and with which the coal power plant on the burner side can be operated both in air operation and in oxyfuel operation. In the air operation of the system, the combustion air is preheated after being sucked by the fresh ventilator 7 in a possibly existing heat displacement system (WVS) 35 (WÜ3). However, this is not realized today in all power plants. After further heating in the air preheater (LUVO) 8, the air in carrying air, other burner air, upper air and - if available - divided into a Mühlenkreislaufström. The carrying air is fed to the mill before a further blower (primary fan) 9, by its increase in pressure, the discharge of the coal is ensured to the burners 10. The Mühlenkreislaufström is used for heat displacement (WÜ1 and WÜ2) from the exhaust gas in the feedwater pre-heating. This has a positive effect on the overall efficiency of the system, because a larger heat flow of the exhaust gas is used and thus the exhaust gas losses are reduced. In addition, bleed steam for the feedwater pre-heating can be saved.

After combustion in the steam generator 11, in addition to the heat exchange with the combustion air (LUVO), the flue gas furthermore undergoes catalytic nitrogen removal 12, a dedusting 13 and a desulfurization 14 in order to comply with the respective emission limit values.

In the area of the steam generator 11 takes place in the combustion chamber, the combustion of the coal and a part of the heat transfer to the working medium steam / water in the Wandheizflächen of

Steam generator (mainly by radiation). In the subsequent Konvektivheizflachen carried out a convective

Heat transfer in / to superheater, reheater and economizer heating surfaces.

In the operation of such a system according to the oxyfuel process would now be a use of recirculation gas (flue gas) at a high temperature level and before completion of the flue gas cleaning, for example, at the points 1-4 taken as shown in FIG. 1 and the steam generator 11 recirculated flue gas due to high dust and / or SO ₂ / SO ₃ levels are problematic. The increased temperature level in comparison to the design of the system for air operation, the higher dust content and SO ₂ / SO ₃ content and the resulting erosion / corrosion hazard of flue gas and air ducts, induced draft blower, possibly existing mill air blowers and mill air heat exchangers, mills, burner installations and boiler materials would require a complete replacement of these aggregates due to increased wear hazard do. In addition, the heat engineering / interconnection of the heat exchanger would have to be completely revised due to the changed temperature levels to the burner. Although this type of conversion is possible in principle, it would then no longer permit any (simple) air operation of the system without further changes / additions. The realization of both modes and the creation / erection of all required in this case aggregates and air and Rezirkulationskanäle is for the retrofit case due to the lack of space in today's power plants but almost impossible or in any case associated with very high technical and financial cost. The risk of increased SO3 formation in the flue gas with appropriate enrichment and repeated contact with catalytically active surfaces in DeNOx reactor (denitrification reactor) 12 and LUVO (air preheater) 8 could not be avoided.

For the reasons described, the flue gas, which is enriched as recirculation gas with oxygen and used as a replacement for the combustion air, therefore, in a preferred embodiment behind the desulfurization 14, 15 at the point 5 or in the described here method of oxyfuel operation behind a additionally installed flue gas cooler 16 deducted at the point 6, which of course also admixtures of recycled at the points 1 to 4 or 1 to 5 or one of these points flue gas are possible. At these points 5 and 6, the flue gas has a very high purity (in terms of dust, SO ₂ / SO ₃ content) and a sufficiently low temperature. This ensures that when converting to oxyfuel operation, all existing units and air / flue gas ducts can continue to be used. Only the flue gas return duct 22, 23 must be connected to the fresh air intake 7 and a flap as close as possible to switch between Oxyfuel and air operation can be installed. The steam parameters of the air operation can then be achieved during oxyfuel operation, as can be seen from FIGS. 5 to 7. FIG. 5 shows that the temperature profile curve 43 for the oxyfuel operation 43 agrees well with the curve 44 for the air operation. 6 also shows in a comparison of the result shown in the left partial image in an oxyfuel operation with the air operation shown in the right partial image, that in oxyfuel operation essentially the same temperatures can be set on the firing side as well as on the steam side, as the respective specified temperature values over the height of a steam generator 11 show. This shows that the heat transfer taking place in the steam generator 11 by flame radiation, gas radiation and convection to the steam mass flow in the steam / water cycle of the steam generator is at least substantially equal compared to air combustion, in particular the same steam parameters are obtained. This is also shown in Fig. 7, from which it follows that the curve 45 for the air operation and the curve 46 for the oxyfuel operation, each of the heat transfer into the water / steam system over the height of the evaporator and thus the heat transfer in Represent convection, reproduce substantially the same temperature profile over the height of the steam generator 11.

It may be necessary to install a heat transfer system (WVS) 16, 35 between induced draft 17 and desulphurisation 14, 15 if the system is not already installed. Further changes relate to the area of oxidation in the desulfurization unit 15 and in the denitrification 12, the oxygen preheating 19, and the PM-blocking gas 40. In order to avoid major interventions in the heat engineering of the power plant, it is ensured that the heat transfer in the area of the combustion chamber 18 and in the area of the downstream convective heating surfaces according to the design for combustion with air is ensured even in oxyfuel operation. This is achieved in the inventive method, characterized in that at the same combustion chamber end temperature, the amount of flue gas flowing through the Konvektivheizflachen is determined so that despite the changed due to the combustion of O ₂ with recycled CO ₂ -containing flue gas heat transfer conditions (density, flue gas temperature profile, flow velocity and Heat transfer coefficient) almost the same amounts of heat are transferred to the working medium vapor.

Due to the changed flue gas properties, both in terms of heat content (density, heat capacity) and heat transfer (changed flow velocities, heat transfer coefficients), the firing side, i. H. adapted the flue gas / gas side of the steam generator 11. The water / steam cycle should remain at least substantially unchanged.

In the case of oxyfuel operation (oxy-fuel case), the size of the total recirculation amount of the flue gas, the division into different combustion gas,

Conveying gas and process gas streams (burners "air", mills "air",

Burnout "air", veil "air") and the oxygen contents of these gas streams and the total oxygen surplus new degrees of freedom, which are used for the adjustment of the furnace.

First, the total amount of recirculation gas is determined so that the heat transfer in the convective heating surfaces corresponds to the old design data. As FIG. 8 shows, which lists in terms of mean values the variables characterizing the heat transfer in the convective heating surfaces, this is achieved in the oxyfuel case with a recirculation gas quantity which has a smoke gas density which is 38.5% higher than that of the air, a higher heat transfer (alpha) Radiation (+ 23%) and convection (+ 6.8%), causes a 7.8% increase in flue gas mass flow and a reduction in the mean logarithmic temperature difference by 11.2% or brings. In this case, the recirculation rate of the flue gas is 75.7%. However, this value depends on the exact fuel characteristics and the basic design of the power plant and can assume values between 65 and 80%. The recirculation rate is the proportion of the recirculated flue gas to the total amount of flue gas.

Power plants are nowadays equipped as a primary measure against nitrogen oxide emissions with low NOx burners and firing systems, which in addition to the fuel carrying carrier "air" at least one mostly twisted - secondary "air" and the swirl stage burner also an outer tertiary "air" and a inner core "air" stream have. Optimum control of coal burnup and NOx emission by controlling oxygen-rich and oxygen-depleted zones in the flame is possible by appropriate selection of convolutions and pulse / momentum ratios of the individual streams. Therefore, such a burner is also able to work with different gas compositions and thus offers the possibility of burnup and temperature in a suitable manner in oxyfuel operation set so that the air operation analog heat quantities are transmitted in the combustion chamber and also a low-emission combustion takes place , Furthermore, virtually no thermal NO _{x is} formed in oxy-fuel operation due to the largely nitrogen-free flue gas, so that lower NO _x emissions (in comparison to air operation) occur.

The proportion of the recirculated flue gas, which is required in the oxyfuel operation for the burner, results in the first approximation from the requirement for maintaining the pulse currents at the burners in the different modes of operation.

For the momentum current Eq. 1

PG, SG ~ ^m PG, SG ^'W PG, SG Eq. 1

With

/ PG, SG Pulse flow of primary and secondary gas m PG, SG Mass flow of primary or secondary gas

^W _{PG SG} flow velocity of the primary or

Secondary gases.

With a constant cross section and unchanged pulse current

With

m _PG , s _G Mass flow of primary or secondary gas in

Oxyfuel operation m _PL , s _L Mass flow of primary and secondary gas in air operation

P _PL , _SL Density of primary and secondary air PPG, SG Density of the recirculated flue gas depending on the composition of the Primärbzw. secondary gas

The portion 47 of the burner "air", which is used as a partial flow of the combustion gas to discharge the coal from the mill 36 (carrier gas), must also be determined according to the equality of the impulse forces acting on the carbon particles.

Whether the carrier gas is capable of discharging the coal from the mill 36 with the momentum current maintenance at the burner depends on the flow resistance (equation 3), the buoyancy force (equation 4), and the weight force (equation 5).

Fs = ^■ g - Ps - V _s 61. 4

With

F ₃ W ~ flow force c _w - drag coefficient of a dust grain w - flow velocity of the fluid F ₃ - weight of the dust grain

A ₃ , V ₃ - surface area, volume of the dust grain p _F , ps - density of the fluid, of the dust grain g - gravitational acceleration

The weight forces are the same in both cases and are omitted upon further consideration. The buoyancy force is negligibly small, since the density difference between flue gas and dust grain is very large. It follows that only the flow force has to be compared. _CW - A _S - ^{PF I} * ^'WÜ * ² = C _W - A ₅ . ^PF *>^'W ° * Eq. 6

Thus Eq. 2 impulse current maintenance and it is ensured that the coal is discharged.

The oxygen content of the burner gas streams 38 or the burner "vent" is adjusted so that the adiabatic combustion temperature remains nearly constant.

Since the stoichiometry in the burner area in modern power plants on the one hand up by the then increased formation of NOx on the other down by the risk of the formation of strands reducing atmosphere in the combustion chamber 18 is limited with the result of possibly increased wall corrosion, must also in the oxyfuel case the design value for the stoichiometry of the combustion is maintained approximately. Since the risk of formation of thermal nitrogen oxide is reduced by the absence of atmospheric nitrogen, but also slightly increased stoichiometries can be used to adjust the level of furnace temperatures.

Depending on the fuel properties, in particular the ignitability of the coal, the oxygen contents of transport gas and the other gas streams should also differ by up to 15% by mass. As a result, the burnout speed in the flame can be controlled in order to make the temperature curves in the combustion chamber 25 of the steam generator 11 similar to those of the air combustion.

Temperature course and temperature level are finally responsible for the dominating in the combustion chamber 18 of the steam generator 11 heat transfer by radiation in this area, which is the relevant design criterion to to comply with the required Brennkammerendtemperatur or transferred to the working fluid water in the combustion chamber walls amount of heat.

Not shown in Figure 2 is the so-called veil or side gass injection used in some power plants to reduce the risk of reducing areas near the walls of the combustion chamber and to avoid the risk of increased wall corrosion. Their proportion of the total quantity of gas or "air" supplied in the area of the burners 10 remains unchanged in the exemplary embodiment, in order to continue to provide the momentum flow necessary for the penetration depth and coverage of the entire wall, but their oxygen content is compared in oxyfuel operation up to 20 percentage points of pure air operation to effectively protect against the higher levels of CO (in this part of the firebox) due to the Boudouard balance and high CO ₂ levels.

The proportion of the recirculated flue gas which is added as burnout gas (ABL) or overfire air (OFA) 37 in the upper part of the combustion chamber 25 of the steam generator 11 is determined by the determination of the other gas flows. The oxygen content set in the combustion air (ABL) is determined from the total stoichiometry, ie from the excess of oxygen necessary for the safe combustion of all coal constituents and minimizing the CO content - in the exemplary embodiment the stoichiometric factor is 1.17. However, depending on the design basis of the power plant, it may assume values between 1.1 and 1.25, but should be as low as possible in the oxyfuel case to avoid further dilution of CO ₂ by excess oxygen. This essentially completes the transmission of the process parameters from air operation to oxyfuel operation. In addition to the illustrated analogy of flow pulses, further thermal optimization of combustion for oxyfuel operation can be accomplished through the inclusion of detailed CFD (Computational Fluid Dynamics) results and experimental results for specific coals, which, however, only marginally affect the size of the adjusted process parameters. In any case - that is, even with a different division of the recirculation gas flows and deviating oxygen contents - a performance of both the conventional air operation and an oxyfuel operation with such a modified power plant is possible by the illustrated approach and the consideration of the available degrees of freedom in oxyfuel operation. This makes it possible to set the at least substantially the same vapor parameters in the steam generator 11 during the oxyfuel operation.

The rinsing and / or sealing gas 40 air used today in / at rotating parts of the coal mills associated with the power plant 36 is replaced by CO ₂ during the conversion to the oxyfuel operation.

As shown in FIG. 2, the LUVO (air preheater) 8 remains required in the illustrated embodiment. The recycled recirculated flue gas amount, however, can not absorb the entire amount of heat due to the lack of oxygen, since the oxygen is not mixed before the LUVO 8 in the flue gas. However, this is not desirable because conventional regenerative air preheater in the design case for an air operation between the two flowing in opposite directions gas streams (exhaust gas and recirculated flue gas) a have unavoidable leakage in the direction of the flue gas. Therefore, in oxyfuel operation, oxygen losses (and higher energy expenditure) and a lower CO ₂ purity would have to be expected. For this reason, a gas-gas heat exchanger 19 for preheating the oxygen is installed together with a control flap for dividing the flue gas to be cooled for the oxyfuel operation in the bypass to the LUVO 8. Thus, in the pure air operation by switching off the oxygen preheater by closing the bypass by closing the control valve, the old operating state can still be set.

Depending on the design of the LUVOS 8 (i.e., with rotating storage mass or with rotating hoods), the seal against the environment can be improved with more or less effort, since there is still a negative pressure on the flue gas side at this point.

Thus, in the area of the funnel 41 of the steam generator 11, no increased incursions of ambient air into the steam generator 11, it is in a case of conversion in any case of

Advantage that a wet ash removal of the steam generator 11 is retrofitted, if in the existing system a

Dry ash is installed. In wet scrubbing is by sealing the water-filled tub against the

Steam generator walls in each case a sufficient

Ensures tightness.

In the field of electrostatic precipitator 13, no significant changes must be made. However, it must be ensured that the shut - off valves are connected to the ash removal system of the

Electrostatic filters 13 are performed gas-tight (for example, gas-tight

Rotary valves). In order to prevent this leakage current to false air in

To avoid system completely, the shut-off should be acted upon with CO ₂ as a sealing gas barrier. Similarly, in the denitrification plant (SCR = Selective Catalytic Reduction) 12 structurally no significant change must be made. Mixed gas streams, for which air is used in normal operation, are replaced in the oxyfuel operation by CO ₂ (see arrow in FIG. 2), which can be taken after or in an intermediate stage of the compression. In principle, both ammonia and ammonia water can be used as reagent.

Since the chemical reactions within the flue gas desulphurisation plant (REA) 15 occur in an oxyfuel operation under an atmosphere mainly consisting of CO ₂ , the absorbent should be converted from limestone (CaCO 3) to quicklime (CaO), since the CO ₂ required for the solution of the limestone ~ Release the solution process by the saturation of the washing suspension with CO _{2 is} hindered. For this purpose, the systems of Absorbensanmischung be modified accordingly. Since in the flue gas desulphurization according to the method commonly used today, the calcium sulphide formed in the solution is oxidized by air injection to calcium sulphate, this process step is also modified during the conversion / conversion to the oxyfuel operation. Since in a system designed for air operation, the oxidation takes place for cost reasons by injecting air into the bottom of a spray tower, you would register without equipment change in oxyfuel operation when using air again undesirable nitrogen. The injection of pure oxygen prohibits, since on the one hand, the production of this oxygen would require a correspondingly greater energy demand of the air separation plant (LZA) 20. On the other hand, the need for an excess of oxygen leads to a lower purity of the CO ₂ intended for storage. For this reason, in the case of For example, desulfurization by the addition of an external stirred tank 39 (Figure 3) is switched to external oxidation, thereby avoiding N ₂ and (2+) entry into the system.

Since the desulphurisation plants designed today in Germany in accordance with the applicable laws achieve an SO ₂ value of <200 mg / Nm ³ (dry, with current oxygen content), an improvement in desulphurisation is necessary in order to prevent corrosion problems in downstream process steps (compaction, transport to the deposit). to avoid. In addition, in a conversion / conversion to oxyfuel operation and the fresh air blower 7 and the fuel / air ducts are not designed for increased SO ₂ / SO ₃ levels. Increasing the deposition is possible in two ways. On the one hand, the existing desulphurization can be improved by increasing the ratio of liquid circulation to flue gas flow, and, on the other hand, retrofitting a further spray level is possible if there is enough space in the head area of the spray tower. The retrofitting of a tray or the addition of solution-demanding acids also improves the desulfurization.

This high desulfurization can be achieved and SO ₂ / SO ₃ values of 20-40 200mg / Nm ³ (dry, at current oxygen content). Further cleaning takes place before the compression and after the flue gas recirculation in order to clean only the necessary flue gas flow to the required by the compression clean gas values.

For drying the flue gases after the REA process is a

Flue gas condensation dryer 21 of the REA 15 downstream.

This further cools the flue gases to reach the required water content of <3% (for recirculation). For this purpose, in addition to necessary cooling water - if available as a product - and the liquid nitrogen of the air separation plant 20 is used. After drying, the flue gas stream is divided and recirculated to a greater extent 22, 23. The partial flow to be supplied to the compression 24 is fed to another Purification.

Before the CO ₂ compressor station 24, corrosive components of the flue gases and water should be removed as far as possible.

For this purpose, a NaOH scrubber with a cooling of the

Rezirkulationsstroms used. While the NaOH wash causes a further significant reduction of the corrosive flue gas components (SO ₂ / SO ₃ "5 mg, HCl" 1 mg, HF "1 mg, dust" 1 mg), the cooling of the circulating fluid provides for another

Reduction of the water content. Optionally, another, downstream condensation cooler is used.

The thus purified flue gases are fed to the compressor station 24. After compression, the residues of O ₂ and N ₂ which are still present in the gas stream are removed from the liquefied CO ₂ by a phase separator, since these gases do not liquefy under these conditions. Now the CO _{2 is available} for storage and onward transport.

The above-described processes of advanced flue gas cleaning require large cooling capacities. For this purpose, liquid nitrogen produced in the LZA (air separation plant) 20 can be used in combination with a cooling water stream (if the products of the air separation oxygen and nitrogen are liquid). The nitrogen is initially used in the cooling system of the multi-stage CO ₂ compressor 24 in order to minimize the energy requirement of the compressor. Optionally, hereinafter, the energy transferred to the "superheated" nitrogen is removed by means of an expansion turbine partially recovered, the temperature of the nitrogen decreases again. Furthermore, the mass flow of nitrogen is then used via coupling heat exchanger both for cooling the NaOH recirculation as well as for cooling the arranged according to REA 15 flue gas condensation dryer 21 and there supplemented by cooling water streams. Thereafter, the nitrogen can optionally be routed again via expansion turbines for energy recovery and recycled through a chimney into the environment.

The described procedure for designing the firing of a oxyfuel steam generator can also be iteratively coupled with the normal design of the convective heating surfaces of a steam generator and used to reduce the Heizflächengröße while increasing the flue gas velocity by reducing the cross section of the combustion chamber for cost-optimized design of a new plant. In this case, a direct analogy of the momentum ratios must be deviated from on the fluid mechanics side: The case of air combustion can then be designed with correspondingly reduced power (partial load for startup / shutdown processes and malfunctions) (with the same flow velocity limiting under erosion aspects).

The following explanations also deal with a conversion concept for a coal-fired power station. It is discussed, which approaches to the realization of a CO ₂ ~ free power plant, in which the CO _{2 is} deposited and stored, exist and which advantages and disadvantages exist. Briefly, the state of research on the oxyfuel process is reproduced. After discussing possible variants of the oxyfuel process, two variants are selected. For these considerations are made to the necessary conversion measures of the power plant components Combustion gas system, flue gas cleaning plant, mills, burners and steam generator performed. Thermal engineering calculations for heat transfer in the steam generator show that the required steam parameters can be achieved without rebuilding the steam generator heating surfaces. Subsequent calculations with variation of the recirculation mass flow and the radiation exchange coefficient of the combustion chamber show possibilities to influence the heat transfer in the steam generator. The evaluation of the conversion effort for the two selected process variants and a final estimation of the overall efficiency show which variant considered is the most technically and economically advantageous.

The aim of the oxyfuel process is to achieve the highest possible CO ₂ - to achieve concentration in the flue gas to the energy-intensive CO can be saved ₂ scrubbing of the "post-combustion process" During the compression of CO _2, the energy consumption can be reduced. If the CO _{2 has} a high concentration, the compressor capacity is then only related to the CO ₂ and not to the impurities When burning with air, the nitrogen content of about 78% by volume prevents high CO ₂ enrichment in the flue gas By contrast, when burned with pure oxygen, significantly higher CO ₂ levels of up to 80% by volume can be achieved in the combustion of dry lignite and over 90% by volume for hard coal, which may vary depending on the firing conditions and coal composition nevertheless good conditions for a capture and storage of CO ₂ .

When burning coal with pure oxygen in the

Oxyfuel operation lacks the nitrogen, on the one hand as

Heat transfer for technically controllable flame temperatures acts and on the other hand for an increase in the Flue gas volume flow ensures. This Rauchgasvolumeήstrom carries the required heat flow in the convective part of the steam generator and there also ensures the necessary for heat transfer high flow velocities. In order to provide this volume flow, the flue gas is at least partially recirculated in the oxyfuel operation after combustion and fed again to the steam generator 11 after mixing with oxygen. The removal of the flue gas can be done at various points behind the steam generator 11. The choice of recirculation site 1 to 6 results in different concentrations of dust, SO _x and water in the flue gas.

The heat transfer in the steam generator 11 takes place convective or by radiation. With regard to the convective heat transfer, changed values with regard to heat capacity, viscosity and thermal conductivity as well as flue gas density result with an altered flue gas composition, as FIG. 8 shows. This also changes the flow velocity of the flue gas. However, it is possible to achieve similar heat flows in the convective heating surfaces with oxyfuel flue gas during oxyfuel operation. The initial assumption that under oxyfuel conditions larger heating surfaces would be necessary to transfer the heat flows is not confirmed. For example, brown coal with a calorific value of 22 MJ / kg (raw) and a moisture content of 19.95% is burned and the flue gas is dried before recirculation. With the same flue gas mass flow and a 46 K higher firing chamber outlet temperature, a 4 K lower flue gas temperature compared to the air process is measured at the outlet of the economizer heating surface. It is thus transferred in the convective part of a larger heat flow. When using the oxyfuel process in already existing or under construction facilities, which are responsible for the Air operations are designed, so no major modifications to the convection heating surfaces are necessary.

The heat transfer ratios can be adjusted by adjusting the molar recirculation ratio.

Optimum molar recirculation ratios of 3.25 are determined for the moist recirculation of the flue gas (removal at location 5) and 2.6 at a recirculation at location 6 after flue gas drying 21. As the recirculation ratio increases, heat transfer by radiation decreases due to lower flame temperatures.

The radiative heat transfer changes mainly depending on the composition and the temperature of the

Gas. There are different ways to set the

Flame and flue gas temperatures and the

Gas compositions. Among the most important are the

Oxygen content in the combustion gas and the proportion of the recirculated gas mass flow of the total produced

Flue gas mass flow.

The following flue gas components mainly influence the gas radiation behavior:

^■ CO ₂ content ■ H ₂ O content

^■ proportion of solid particles.

The high nitrogen content in the flue gas during air combustion is replaced by CO ₂ in the oxyfuel process. Depending on the Rezirkulationsort 1 to 6, the flue gas contains more or less water. However, CO ₂ and H ₂ O are not diathermanic like N ₂ and O ₂ , but they absorb and emit heat radiation as a function of the gas temperature. In addition, the increased heat capacity of the combustion gas, caused mainly by CO ₂ and water, changes important flame properties. The emissivity of the flame in oxyfuel operation and in air operation are similar. It depends mainly on the coal, the fly ash, soot particles in the flame, but not on the CO ₂ concentration.

For the same oxygen content in the combustion gas, it is generally observed in the oxyfuel process, as compared to an air operation, that • the flame propagation velocity decreases,

• the flame temperatures drop and

• the ignition delay increases.

Ignition delay is calculated by dividing the distance the coal particles travel before ignition by the particle velocity. The ignition delay increases

• sinking temperature,

• decreasing oxygen content in the combustion gas,

• increasing heat capacity of the combustion gas, • decreasing heat conductivity of the combustion gas and

• decreasing percentage of volatiles of coal.

The other parameters remained constant. In a CO ₂ rich atmosphere, the ignition delay is greater for the same oxygen contents than in a nitrogen-rich atmosphere (air operation). In order to achieve the same ignition delay as in air combustion, in oxyfuel operation the gas must consist of 30% by volume of oxygen and 70% by volume of CO ₂ .

The influence of the molar recirculation ratio shows

here too. For a recirculation RR = - -f of mRezi + mAbgas 0.58, similar flame temperature profiles and stabilities are observed in oxyfuel operation in addition to the same adiabatic combustion temperature as in air operation (R = recirculation _ratio , m _ReZ i ₌ mass flow of recirculated flue gas, m ^ g _as - mass flow of exhaust gas).

As shown in Fig. 4, 11 different heating surfaces exist in a steam generator, which differ with respect to the method of heat transfer. In the combustion chamber 25 and in the radiation spaces 26 and 27, the heat transfer is dominated by radiation, therefore they are referred to as Strahlungsheizfläche. Combustion chamber 25, radiation chambers 26 and 27 are referred to together as a combustion chamber 18. The heat transfer in superheater heating surfaces 28 and 29 and reheater heating surfaces 30 and 31 and the economizer heating surface 32 is mainly convective, so that these heating surfaces are referred to as Konvektivheizflachen. The convective part of the steam generator 11 represents the entirety of all Konvektivheizflachen.

The support tube bulkhead 49 has the peculiarity of having as Konvektivheizfläche also a large proportion of radiation. This can be explained by the position as the first bundle heating surface above the combustion chamber 18.

The power amplifiers of the HP (high pressure) and MD (medium pressure) section as well as the economizer heating surface are flowed through in the sense of a direct flow. This is used in the final stages to reduce the tendency to corrosion due to lower material temperatures and to protect the turbine from temperature fluctuations. In the Economizer Heizflache 32, the discharge of possibly resulting vapor bubbles should be guaranteed.

The goal is to convert a 600 ⁰ C or 700 ° C power plant designed for air operation to the oxyfuel process. The means that the steam generator 11 in air and in oxy fuel operation must be able to supply the turbine with the required steam parameters. This should succeed without changes to steam generator heating surfaces, mills and burners.

The decisive criterion for the economy of the oxyfuel process is the achievement of a high CO ₂ content in the exhaust gas. Only significant energy savings in CO ₂ concentration and compression justify the energy-intensive oxygen production as an additional process compared to the simple flue gas scrubbing with a detergent (eg monoethylamine). Penetrating air leaks in addition to supplied rinsing and sealing air counteract this endeavor and must therefore be reduced to a minimum.

As shown in FIG. 1, a branch from the flue gas to the flue gas circulation at the points

1, before the denitrification 12,

2, after denitrification 12,

3, after the regenerative air preheater 8,

4, after dedusting 13,

5, after desulfurization 15 or 6, after drying 21

respectively. It is also possible to combine several of these diversion possibilities.

The recirculation of the flue gas at the point 1 before the denitrification 12, which is located in modern hard coal power plants in Leerzug, has an enrichment of the flue gas with dust, sulfur oxides and water result. All pipes and components that come into contact with flue gas must be designed to be dust-proof be. This is especially true for the recirculation fan 48, 48a, 48b to be additionally installed. Due to the higher concentration of water and sulfur oxides results in an increase in the sulfuric acid dew point.

Neither existing regenerative air preheater 8 nor a heat transfer system 16 in front of the flue gas desulfurization system 14, 15 are necessary in oxyfuel operation, since at a diversion of a large part of the flue gas before the LUVO 8 the material flow to be cooled and the material to be heated are missing. An existing E-filter 13 and the flue gas desulfurization system 15 are oversized for oxyfuel operation. It may be possible to adjust the flow conditions for optimum dust separation or desulphurisation rate by shutting down individual e-filter lanes or laundry levels. The intended only for the oxyfuel operation dryer 21 can be designed in this case for small flow rates. The heating of the oxygen stream is then carried out in an additional heat exchanger 33, which is arranged in front of the E-filter 13. There, the flue gas has a temperature of about 380 ⁰ C.

On the other hand, with a recirculation of the flue gas from the point 2, only the conversion rate of SO ₂ to SO _{3 increases} since the flue passes through the area of the catalytic surfaces of the DeNO _x plant 12 and therefore sets a higher dew point temperature of the sulfuric acid.

In a diversion of flue gas at Rezirkulationsort 3 now the LUVO 8 is still used to cool the flue gas. The

Flue gas temperature behind the LUVO 8 is then above the

Schwefelsäuretaupunktes. The temperature of the flue gas after LUVO 8 is controlled by the inlet temperature of the heated, countercurrent medium. Here, however, the problem is that by heating of the recirculated flue gas in the recirculation fan, the mass flow to be heated at the LUVO re-entry is hotter than the mass flow to be cooled at the outlet. This problem can be solved by installing a heat sink in the form of a heat exchanger. The discrepancy between flue gas mass flow on the cooling and the heating side of the LUVO is solved by a LUVO bypass 34 with oxygen preheating 33.

In a recirculation of the flue gas from the point 4, behind the electrostatic precipitator 13, the flue gas enriches with sulfur oxides and water. The dust load for flue gas ducts and fans drops significantly. The E-filter 13 is then designed and used for both the air and the oxyfuel operation.

In a recirculation of the flue gas from the point 5, behind the desulfurization 14, 15 off, the flue gas enriches only further with water, since the sulfur is largely removed in the flue gas desulfurization 14,15. This reduces the risk of corrosion due to sulfuric acid. The quenching effect in the REA 15 cools the flue gas by partial evaporation of the absorber suspension. In this case, the water content and the outlet temperature of the flue gas in dependence on the saturation temperature.

In a recirculation of the flue gas from the point 6, behind the dryer 21, the flue gas is fully dedusted, desulfurized and dried sucked back or recycled (recirculated). With this quality of flue gas, the fresh air blower can be used as a recirculation blower. All used in air operation heat exchanger and flue gas treatment components can be operated unchanged in oxyfuel operation compared to air operation. However, the entire flue gas mass flow over the Dryer 21 passed so that it must be designed to be large enough to dissipate large heat flows can.

Due to the recirculation of certain flue gas substreams at various points 1 to 6, advantages and disadvantages of the respective recirculation sites can be combined.

A sub-stream routed through all flue gas treatment components is appropriately cleaned, thereby reducing the levels of pollutants such as dust, water and sulfur oxides. A second partial flow can then be recirculated very close to the steam generator 11, for example at the point 1, at a high energy level. This eliminates the cooling and subsequent reheating of this flue gas partial stream.

At a later recirculation, i. If more and more components of the flue gas treatment flow through the flue gas path after exiting from the economizer heating surface from the total flue gas mass flow, the result is an increasing number of points 1 to 6

• less enrichment of the flue gas with water, dust and sulfur oxides,

• better usability of the original components and components of this part of the coal-fired power plant in

Air and oxyfuel operation and

• less need to insert or install various additional components and / or plant components.

With the conversion or conversion to the oxyfuel process, the recirculation mass flow, which together with the mixed oxygen mass flow with unchanged flue gas temperature profile, primarily changes the flow rates in and on the heating surfaces affected. Due to the higher density of CO ₂ (oxyfuel operation) compared to the N ₂ (air operation) results in the same mass flow, a slower flow. The flow velocity of the flue gas, in addition to physical properties such as viscosity, thermal conductivity and heat capacity, plays an important role in the heat transfer from the flue gas to the heating surface. Despite the changed heat transfer conditions, the required steam parameters are achieved. For steam generators according to the Benson principle, the control introduces as much and as long as fuel until the high pressure (HP) mass flow, the temperature is controlled by the enthalpy control and injection, is reached. Influence on the medium pressure (MD) outlet temperature can be taken over the height, thus the quantity, of the recirculation mass flow, which ensures favorable flow velocities for the heat transfer. The oxygen excess λ ₀₂ is understood as meaning the ratio of the supplied oxygen _stream m _O 2 to the stoichiometrically required oxygen stream m _O 2, min. When burning air, it becomes the required one

100, Combustion _air _{mass flow} m _Air to ^m _Lufl ^{= m} o ₂ , _mm Λ _{? 2}

calculated. The excess air then corresponds to the oxygen surplus.

In the oxyfuel process, the use of an excess of air or the production of a reference to the burner gas does not make sense. It is possible to achieve almost any oxygen content in burner gas and flue gas. Therefore, the original term oxygen excess, which refers to the fuel mass flow, is reduced. This is calculated after oxyfuel combustion ₃ _ ^m LZA ^'X 02, LZA ^{+ m} FL ^{' X} O2, FL ^{+ 777} Re-I ^'X 02, Rezi λo2: mO2, mm with rh _LZA - Mass flow from air separation plant

^X _{O2, LZA} ^~ Oxygen content after air separation plant rh _FL - False air mass flow

^X _{O2, FL} ^~ Oxygen content of false air 7 ⁷⁷ _Re- , ^~ recirculation _{mass flow} xo _{2, Rtz} , ~ oxygen content of the recirculated

flue gas

^{/ 77} o _{2, m} i _n ^{~ s} töchiometriseher Sauerstoffström.

In the counter are all the combustion oxygen mass flows supplied (combustion gas, conveying gas, process gas). The denominator is the stoichiometric oxygen demand, which is calculated from the reaction of the carbon components C, H, 0 and S to CO ₂ , H ₂ O and SO ₂ .

A comparison of the emerging flue gas compositions between the air operation and the oxyfuel operation with flue gas circulation behind the E-filter 13 at the point 4 and behind the dryer 21 at the point 6 is shown in FIG. 9.

With changed gas composition also different material values result. Fig. 10 shows the higher density of the flue gas in the oxyfuel process (+23.9%, + 33.3%). Heat capacity, dynamic viscosity and thermal conductivity of the flue gases change little when taken at the point 6 compared to the air operation. In the oxyfuel process, the higher heat capacity and the higher thermal conductivity at point 4 compared to point 6 are due to the even higher water content of the flue gas there. In order to achieve the steam parameters upstream of the turbine also in the oxyfuel process, the mass flow of fuel and the recirculation mass flow are adjusted. Due to the higher density of the flue gas in the oxyfuel process, the flow in the convective part slows down despite higher flue gas mass flow.

The material values from FIG. 10 enter into the Reynolds and Prandtl numbers, which in turn lead to the Nusselt number.

λ _Rr ^■ Nu outside, convective »a outside ~ ® outside. convective ^" * ^" & outside. radiation

Q = k-A-AT

With

1 - characteristic dimension (e.g., pipe diameter)

Nu - Nusselt number δ _n - layer thickness λ _n - thermal conductivity coefficient of the layer (eg pipe)

A - Heat exchanger surface

Noteworthy is the improved heat transfer by radiation in the range of convective heating surfaces by more than 39%. In the case of the oxyfuel process with recirculation at points 4 and 6, this is due to the high CO ₂ concentration and at point 4 to the increased water concentration. Due to the greater proportion of radiation at the heat transfer in the convective part of the heat transfer coefficient k is increased. Overall, it can be seen from FIG. 10 that the heat flow transmitted in the convective part is increased. However, the heat transfer in the convective part also depends on the heat transfer in the radiant heating surfaces. This is due to the conditional flue gas and water / steam temperatures. The Strahlungsheizflächen are the Konvektivheizflachen 28 to 32, 49 along the flue gas path in the combustion chamber 25 and the radiation chambers 26, 27 upstream. The water flows through the Economizer Heizflache 32 first convective heating, then the Strahlungsheizflächen for evaporation and finally convective heating surfaces to overheat the steam.

With a total of the same transferred heat flow in the steam generator 11, the flue gas is significantly colder due to the higher heat capacity in the oxyfuel process in the combustion chamber 25 and slightly hotter at the end before LUVO than in the air process. The achievement of the adiabatic combustion temperature of the air process is therefore not absolutely necessary.

Attention is a change in the warm-up margins in the

Superheater and reheater heating surfaces. In all convective high-pressure heating surfaces, the warm-up margins are higher in the oxyfuel process with recirculation locations 4 and 6 than in the air process. In the medium pressure heating surfaces, however, only the heating surface 39 reaches a larger warm-up.

The most important conversion to oxyfuel operation is the recycling of some of the flue gases. The recirculation gases are branched off behind the induced draft 17 and returned. In Figures 2 and 11, the return is shown, in Fig. 11 in addition, the temperature and mass flow data of the recirculated flue gas stream, the oxygen supplied from the oxygen preheater and thermal energy flows are listed. The opposite one Air components to be converted or added are grayed out. The necessary pressure increase to overcome the flow resistance and to produce a pressure gradient in the LUVO 8 and in the gas channels is accomplished by a recirculation fan 48, 48 a, 48 b.

After heating of the flue gas by the induced draft 17 and then by one of the recirculation blower 48, 48 a, 48 b, the temperature recirculation side in the flow direction before the LUVO 8 is higher than flue gas side in the flow direction behind the LUVO 8. A cooling of the flue gas in the LUVO 8 before entering the E-filter 13 to a lower temperature is therefore not possible without further action. The flue gas side cooling in the LUVO 8 is therefore controlled via the heat exchanger 35 (WÜ3), 16 by setting a recirculation side temperature in front of the LUVO 8. Furthermore, the heat transfer in the LUVO 8 is determined by the mass flows. In order to achieve the same flue gas temperatures at the entrance of the REA 15 as in the air process, the flue gas is cooled further after the induced draft 17.

The designed for air operation flue gas desulphurisation plant 15 is usually oversized for oxyfuel operation and it may be necessary to adapt to the reduced flow and the higher sulfur concentration. This is made possible by a design in which a larger part of the scrubber can be shut down in oxyfuel operation. In the smaller part used then appropriate flow velocities are set. In air operation, the entire REA 15 would be used. The air supplied during the air process to improve the reaction must be replaced by external oxygen injection (external oxidation) 39 in oxyfuel operation, so that the CO ₂ concentration in the flue gas is not lowered. These two heat exchangers are used Feedwater. Due to the increased density of the flue gas, the flow velocity decreases during the oxyfuel operation compared to the air operation. In addition, the flue gas side heat transfer changes due to changed material values. It is transmitted a smaller heat flow. In order to change the intermediate temperatures of the feedwater pre-heating as little as possible, the Mühlenkreislaufström can be increased by about 60%. This results in negative feedbacks on the taps of the turbine and a positive increase in the heat-absorbing mass flow in the LUVO.

The temperature of the hot gas required for drying the coal can be determined. To control this temperature, cold air is added to the mill during the air process. For a high CO ₂ concentration in the exhaust gas recirculated CO ₂ -containing flue gas is used instead in the oxyfuel process. Correspondingly cold flue gas is branched off behind the flue gas dryer and condenser 21 with about 25 ⁰ C. After pressure increase and heating by an additional fan, it is mixed in front of the mill 36 with 30 ⁰ C in the required proportion.

The flue gas can absorb the fuel humidity and the usual sifter in the mills 36 of 9O ⁰ C need not be increased from the viewpoint of saturation.

The dew point temperature of the sulfuric acid is in the mills

36 and also in the oxygen preheater, in the heat exchangers and the Mühlenkreislauflüfter and the corresponding, zugerodneten gas channels. As a countermeasure, the surfaces of these components may optionally be coated with plastic. However, this additional layer has the

Disadvantages that it hinders the heat transfer and hardly

Resistance to erosion offers. The flue gas recirculation mass flow has a decisive influence on the flow rate of the flue gases in the convective part of the steam generator and on the adiabatic combustion temperature. A reduction of the flue gas mass flow at the burner has an effect on the processes in the combustion chamber 25 and the combustion chamber 18, such as an increase in the oxygen content.

Overall, a slightly larger heat flow in the Konvektivheizflachen and a lesser in the Strahlungsheizflächen is transmitted by increasing the recirculation mass flow.

The process scheme in a flue gas circulation at the point 6, the figures 1, 2 and 11. The flue gases are branched off behind the flue gas dryer 21 and returned. Since the flue gas cools in the dryer 21 to 25 ° C and is highly dedusted and desulfurized, the fresh ventilator 7 can also be used as a recirculation fan.

Behind the fresh ventilator the recirculated flue gas is warmed up to 107 ⁰ C in the heat transfer system (WÜ3) 35. This temperature has an effect on the cooling of the flue gas in the LUVO 8, which should be at most above the sulfuric acid dew point. The heat displacement system (WÜ3) 35 is no longer designed as a regenerative heat exchanger in oxyfuel operation, because it could not achieve the desired media temperatures. The heat transfer in the LUVO 8 is controlled by the distribution of the flue gas mass flow to the LUVO 8 and the LUVO bypass 34.

The inlet temperature into the REA 14, 15 is determined by the

Predetermined heat flow, which can be transferred in the heat transfer system (WÜ3) 35, 16 to the recirculated flue gas, without adversely affecting the cooling of the flue gas in the LUVO 8. The flue gas in the REA 14, 15 is 12% warmer than in the air process.

In order to investigate possibilities for influencing the heat transfer in the steam generator during oxyfuel operation, the processes in the steam generator are considered. It is assumed that a constant preheating of the combustion gases. The injections in the HP and MD part of the steam generator are kept constant. The results are determined with the optimum operating point of fuel, oxygen and recirculation mass flow to reach the steam parameters before turbine.

In oxyfuel operation, the adiabatic combustion temperature increases because a smaller flue gas mass flow must be heated at the burner. In the evaporator, a larger heat flow is thereby absorbed. The smaller flue gas mass flow cools down faster in the convective part, so that the flue gas behind the reheater heating surfaces 31 is colder than in air operation. The steam outlet temperature in the HP section is 17 K higher than in air mode. This is mainly due to the increased heat absorption in the evaporator. The MD part of the steam generator 11, consisting of pure Konvektivheizflachen, however, does not reach the required warm-up, so that the temperature before the turbine 17 K lower than in air operation.

While the heat absorption in the steam generator 11 decreases in the convective heating surfaces, it rises in the radiant heating surfaces.

If the recirculated mass flow is reduced by 20%, and the conditions for momentum conservation and oxygen content in the carrier gas are maintained, then the recirculated Gas mass flow beyond that not to the momentum conservation in the secondary gas. It is then out less flue gas on the secondary gas and no more over the top gas nozzles in the steam generator.

The fuel mass flow is now adjusted so that the required HD outlet mass flow is achieved. This is followed by an adaptation of the oxygen mass flow so that the desired oxygen excess of 1.17 is maintained. The injected for cooling water mass flow upstream of the reheater 31 is used to control the MD outlet temperature. As a result, the fuel mass flow is reduced by 2.5% compared to the optimal state during air operation. As a result, the adiabatic combustion temperature increases less and it is absorbed by the radiant heating a slightly lower heat flux. The flue gas cools faster due to the smaller heat flow introduced and is already behind the Überhitzerheizflächen 29 colder than in the optimal case of air operation. This, together with the lower flue gas mass flow, has an effect on the heat transfer in the MD part. Despite the reduction of the injected cooling mass flow by 100%, the required outlet temperature at the reheater heating surfaces 31 can not be achieved.

If, however, the recirculated flue gas mass flow is increased by 20% and the conditions for momentum conservation and oxygen content in the carrier gas and secondary gas are maintained, and also pass the upper gas nozzles excess sucked back flue gas into the steam generator, the required HD outlet mass flow can be achieved by adjusting the mass flow of fuel. The oxygen excess of 1.17 should be retained. The water injected for cooling Mass flow in front of the reheater heating surfaces 31 is used to control the MD outlet temperature.

In this case, the steam generator control reacts to the steam temperature not reached at the outlet of the HP section with an increase of the fuel mass flow by 2.5%. This has the consequence that the adiabatic combustion temperature increases and thus the heat absorption in the combustion chamber and Schottheizfläche. The slightly larger volume flow due to the increased flue gas temperatures ensures better heat transfer in the convective part of the steam generator. The MD part is negatively affected. The heat-up margins of the heating surfaces before the water injection increase, which can be compensated by quadrupling the injected Wassermassestroms.

When the radiation exchange coefficient of the combustion chamber 18 um

30% is reduced from 1.636 to 1.145 and the fuel, oxygen and recirculation mass flows are kept constant, decreases by lowering the radiation exchange coefficient, the heat absorption in the combustion chamber 18. The steam is at the separator by 25 K colder. The lower heat absorption in the evaporator is almost completely compensated by the convective heating surfaces of the HD section. This happens because the flue gas leaves the combustion chamber at a higher temperature. The heat transfer coefficient by radiation α _Strah i _un g is increased in the next heating surfaces to the heating surface 31. The final heating surfaces

Figures 31 and 29 show increased warm-up margins. As a result, the required outlet temperature is exceeded by 5 K in the MD section. The effects are similar to those of increased recirculation mass flow, albeit weaker. In both cases, the heat transfer in the convective part is improved, while it is lower at the radiant heating surfaces. When the radiation exchange coefficient of the combustion chamber 18 is increased by 30% from 1.636 to 2.127 and the fuel, oxygen and recirculation mass flows are kept constant, a larger heat flow is absorbed in the combustion chamber 18, whereby the flue gas leaves the combustion chamber more cooled. In the convective part, a lower heat flow is transferred overall. The exit temperature of the HD part is a bit too high and the MD part a bit too low. This is comparable with the effects of reducing the recirculation mass flow.

The effects of a changed radiation exchange coefficient C of the combustion chamber 18 by adjusting the fuel (HP outlet) and recirculation (MD) mass flows can be compensated. The excess oxygen is kept constant.

A smaller radiation exchange coefficient C can be achieved by minimizing the mass flow of fuel and increasing the

Rezirkulationsmassestroms be compensated. It is more difficult to balance the effects of a varied one

Rezirkulationsmassestroms. So the more important is the exact one

Adjustment of the recirculation mass flow in steam generator mode.

The required steam parameters can thus be achieved in the oxyfuel process without changes to the steam generator heating surfaces. The heat transfer can be adjusted by adjusting the fuel, oxygen and recirculation mass flows. Increasing the fuel mass flow, however, lowers the overall efficiency and should therefore be avoided. On the other hand, if variable combustion gas compositions are used to influence the heat transfer in the steam generator, this has an effect much cheaper. Due to the possibility of mixing the oxygen into the primary, secondary and upper gas, thereby adjusting the oxygen content as desired, the oxyfuel process has an additional degree of freedom for controlling the flame temperatures. With the distribution of the oxygen and recirculation mass flows on burner gas and top gas, both the radiation processes in the evaporator can be influenced via the flame temperature as well as the convective heat transfer via flue gas temperature and flow velocity. Thus, it is possible to achieve a corresponding distribution of heat flow absorption between Strahlungsheizflächen and Konvektivheizflachen. At the same oxygen excess, the adiabatic combustion temperature of the air process can not be achieved. The effects, in the form of a lower heat absorption in the combustion chamber due to the high temperature influence during radiant heat transfer, can be compensated in the convective part, so that the required steam parameters are achieved.

Due to the higher density and heat capacity of the flue gas flow rate or flue gas temperature drop at the same introduced into the steam generator 11 heat flow in the oxyfuel process. The resulting weaker convective heat transfer is compensated by the higher gas radiation, due to the high CO ₂ content, in the convective heating surfaces.

This development has a positive influence on the material temperatures of the HD part, since these depend mainly on the water or steam temperatures due to the higher heat transfer on the inside. In the HD part, high steam temperatures are due to the lower

Heat absorption in the evaporator reached late, so that the material temperatures compared to the air process something are lower. The slight shift of the heat transfer towards the convective part causes an increase of the steam temperatures in the first MD part of the steam generator 11 by a maximum of 10 K. Because of the large safety reserves in the material design and slightly elevated temperatures, this is not a problem.

In the air process, the excess air is controlled by the CO ₂ and O ₂ contents in the exhaust gas. This procedure can not be adopted for oxyfuel operation, because the recirculated with the flue gas oxygen and carbon dioxide mass flows must be included in the balance.

Increased NO _x concentrations occur during the uncontrolled air process due to the nitrogen in the air and high combustion temperatures. In the case of the oxyfuel process, by contrast, the nitrogen content in the combustion gas is less than 7%, which means that almost only fuel NO _x can form. Consequently, the formation of nitrogen oxides is mainly dependent on the burned coal mass flow and its composition. As a consequence, on a Luftbzw. Gas classification in the combustion chamber to avoid NO _{x be} waived. Instead, compositions and mass flows of the burner gases and the upper gas can be changed in order to optimally adapt the heat absorption in the combustion chamber and convective part of the steam generator to the operational conditions. During operation of the steam generator, the reduced heat absorption in the evaporator due to contamination can be compensated for example by adjusting the recirculation mass flow.

This procedure leads to an increased measurement and accounting effort, which, however, can be overcome with the help of boiler diagnosis programs. Advantageous in the diversion of flue gas to the flue gas circulation behind the E-filter 13 at the point 4 are the shorter flue gas lines and the higher temperature at which the flue gas is recirculated. On the other hand, the enrichment of the flue gas with sulfur compounds still present at this point and the resulting problem of sulfuric acid have a disadvantageous effect. Affected heat exchangers and flue gas lines must be converted in this case for oxyfuel operation accordingly corrosion resistant.

Furthermore, in the recirculation behind the E-filter 13, two additional heat exchangers are needed to reach the temperatures at the LUVO 8 and before the REA 15.

While the recirculation of the flue gas at the point 6 behind the dryer 21 while longer flue gas ducts are required, but most components of the system for air operation can be used unchanged. The changes are limited to the closure of some lanes of the e-filter 13, the adaptation of the REA 15 to higher flue gas temperatures and the construction of the LUVO bypass 34. The latter component must be interpreted as the only corrosion resistant.

By drying the entire flue gas mass flow in the dryer 21, the energy consumption increases. Nevertheless, it can be expected with lower conversion costs than at a recirculation at the point 4 behind the e-filter 13. Consequently, the variant with recirculation of the flue gas at the point 6 behind the flue gas dryer 21 is technically and economically more advantageous. The efficiency decreases in both variants by the additional electrical intrinsic demand of the air separation 20 and the CÜ ₂ -Verdichtung 24 by about 10 percentage points percentage. Compared to flue gas scrubbing (MEA), however, the efficiency losses are low.

Claims

claims

1. A method for operating and for controlling a coal-fired steam generator (11) comprising a power plant whose steam generator (11) is designed for combustion with coal combustion in the steam generator (11) achievable by the heat transfer to the steam mass flow steam parameters, characterized that in the steam generator (11) combustion of the carbonaceous fuel after the oxyfuel process with approximately pure, more than 95 vol .-% O ₂ containing oxygen and recirculated, highly CO ₂ -containing flue gas is carried out such that the mass flows of all the coal-fired burners (10) and the steam generator (11) supplied fuel streams and combustion gas, conveying gas and process gas streams of combustion oxygen and / or recirculated flue gas in their respective composition ratio of oxygen and / or flue gas are formed and matched to each other in the steam generator ( 11) by flame radiation, gas radiation and convection heat transfer to the steam mass flow in the steam / water cycle of the steam generator (11) compared to the air combustion is kept at least substantially the same, in particular the same steam parameters are obtained.

2. The method according to claim 1, characterized in that treated and / or untreated flue gas recirculating to the steam generator (11) is recycled.

3. The method according to claim 1 or 2, characterized in that an existing, in particular a so-called 600th ° C - power plant is retrofitted with the method according to claim 1 or 2.

4. The method according to any one of the preceding claims, characterized in that the recirculation rate of

Flue gas 65% to 80%, in particular 74% to 78%.

5. The method according to any one of the preceding claims, characterized in that the flue gas behind a

Desulfurization (14) or one

Flue gas desulfurization system (15) or one, in particular additional and / or retrofitted, flue gas cooler (16) is withdrawn for recirculation.

6. The method according to any one of the preceding claims, characterized in that the flue gas is withdrawn in the flow direction behind a flue gas condensation dryer (27).

7. The method according to any one of the preceding claims, characterized in that in the flue gas desulfurization plant (15) is used as the absorbent fire lime (CaO).

8. Power plant with a coal-fired steam generator (11), which can be reached by the heat transfer to the steam mass flow rate for coal combustion with combustion air in the steam generator (11)

Steam parameters is designed, characterized in that in the steam generator (11) combustion of the carbonaceous fuel after the oxyfuel process with approximately pure, more than 95 vol .-% O ₂ containing Oxygen and recirculated, CO ₂ -containing flue gas is such that the mass flows of all the coal-fired burners and the steam generator (11) supplied fuel flows and combustion gas, conveying gas and process gas streams of combustion oxygen and / or recirculated flue gas in their respective composition ratio of oxygen and / or flue gas are designed and matched to each other, that in the steam generator (11) by flame radiation, gas radiation and convection heat transfer to the steam mass flow in the steam / water cycle of the steam generator (11) compared to the total air combustion remains the same, especially the steam parameters obtained equal are.

9. Power plant according to claim 8, characterized in that between a induced draft (17) and a desulfurization (14) or a desulfurization plant (15), a heat transfer system (16) is installed.

10. Power plant according to claim 8 or 9, characterized in that the flue gas duct (42) in the flow direction after a denitrification (12), in particular parallel to an air preheater (LUVO) guided bypass line (34) with arranged therein gas-gas Heat exchanger (19, 33).