Oxy boiler power plant oxygen feed system heat integration

TECHNICAL FIELD

The present disclosure relates generally to heat integration schemes applied to coal fired oxy boiler power plant, and more specifically to the heat integration of oxygen feed systems into such plants.

BACKGROUND INFORMATION

Coal contributes to a large percentage of the electricity generation in the world today and is expected to maintain its dominant share in the foreseeable future. Nonetheless, significant environmental pressures have led to increased environmental demands requiring, not only high efficiency but also reduced emission levels of CO2, SO2, NOx, and mercury to ultra-low levels.

A particular advantageous plant arrangement is the use of an Oxy-combustion steam plant with CO2 capture. Oxy-combustion systems use oxygen, usually produced in an air separation unit, instead of air, for the combustion of the primary fuel. The oxygen is often mixed with an inert gas, such as recirculated flue gas, in order to keep the combustion temperature at a suitable level. In addition, it may be preferable to separately or additionally preheat oxygen from the Air Separation Unit in a steam coil oxygen preheater.

Oxy-combustion processes produce flue gas having CO2, water and 02 as its main constituents, the CO2 concentration being typically greater than about 70% by volume. Therefore, CO2 capture from the flue gas of an oxy-combustion process can be done relatively simply in a Gas Processing Unit.

An example of a typical water steam cycle of a high efficiency oxy-combustion steam plants is shown in Fig. 1. The plant comprises a triple-pressure series of reheat steam turbines (HP,IP. LP) fed by steam from a boiler (142). Exhaust steam from the last low pressure steam turbine (LP) is condensed in a condenser (102) before being polished (104) and pumped via a condensate pump (103) successively through a series of low pressure heater (106,107,108,109,131), a feed water tank (136) and high pressure heaters (132) before returning to the boiler (142) in a closed loop. The heat source for the low and high pressure heaters is typically steam extracted from the low/ intermediate and high pressure steam turbines.

Due to the large benefit in ensuring the highest efficiency cycle there is a continuing need to find ways of better integrating the thermal needs and sinks of the oxy-combustion capture systems within the steam power plant. This requires an optimization of the heat needs and sinks of the capture system with the plant cycle to ensure no energy is wasted. In particular, this needs consideration of how to integrate the steam coil oxygen preheater into the thermal cycle.

SUMMARY

A coal fired Oxy boiler with post combustion flue gas CO2 capture system and a steam cycle power plant scheme is provided that integrates major heat generation sources of the systems in order to provide flexible plant operation and improved overall plant thermal efficiency.

The disclosure attempts to address this problem by means of the subject matter of the independent claim. Advantageous embodiments are given in the dependent claims.

The disclosure is based on the general idea of a novel scheme for thermally incorporating an Air Separation Unit into the condensate system of a coal fired oxy boiler power plant.

An aspect provides a coal fired Oxy boiler power plant comprising a Rankine steam cycle having a high pressure steam turbine, adapted to expand steam, having an exit, an intermediate pressure steam turbine adapted to expand steam from the high pressure steam turbine, and a low pressure steam turbine adapted to expand steam from the intermediate pressure steam turbine having a steam extraction port. A condensate system of the cycle further comprises a condenser adapted to condense steam exhausted from the low pressure steam turbine, a series of low pressure heaters adapted to receive and serially heat condensate from the condenser, a feed water tank configured and arranged to receive condensate from the series of low pressure heaters, and a series of high pressure heaters adapted to receive condensate from the feed water tank.

The oxy boiler power plant further comprises an Air Separation Unit having an oxygen line with a steam coil oxygen preheater wherein an extraction line connects the steam extraction port to the steam coil oxygen preheater. A drain line then fluidly connects the steam coil preheater to a point of the Rankine steam cycle fluidly between the series of low pressure preheater and feed water tank.

In an aspect the intermediate pressure steam turbine is a multi-stage intermediate pressure steam turbine and the steam extraction port is configured and arranged to extract steam from an intermediate stage of the intermediate pressure steam turbine.

In an aspect an emergency line is connected to the drain line and the condenser.

It is a further object of the invention to overcome or at least ameliorate the disadvantages and shortcomings of the prior art or provide a useful alternative.

Other aspects and advantages of the present disclosure will become apparent from the following description, taken in connection with the accompanying drawings which by way of example illustrate exemplary embodiments of the present invention

BRIEF DESCRIPTION OF THE DRAWINGS

By way of example, an embodiment of the present disclosure is described more fully hereinafter with reference to the accompanying drawings, in which:

• Figure 1 is a schematic of a coal fired oxy boiler power plant of the prior art to which exemplary embodiments may be applied;
• Figure 2 is a schematic of the heat integration of an Air Separation Unit steam coil oxygen pre-heater into a coal fired oxy boiler power plant;
• Figure 3 is a schematic of the heat integration system of Fig. 2 in which an alternate drain line routing into the condensate system is shown; and
• Figure 4 is a schematic of another the heat integration system of Fig. 2 in which a further alternate drain line routing into the condensate system is shown.

DETAILED DESCRIPTION

Exemplary embodiments of the present disclosure are now described with references to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth to provide a thorough understanding of the disclosure. However, the present disclosure may be practiced without these specific details, and is not limited to the exemplary embodiment disclosed herein.

Throughout this specification reference is made to serial units. In this context serial means arranged in a series starting from an upstream end as defined by the nominal flow of working fluid through the unit during it's normal operation.

An exemplary embodiment shown in Fig, 2, which may be applied to a coal fired oxy boiler power plant shown in Fig. 1, provides a steam extraction arrangement and condensate return scheme for heat supply to an oxygen feed line 3 emanating from an Air Separation Unit. As shown in Fig. 2 the coal fired oxy boiler power plant comprises a Rankine steam cycle having a high pressure steam turbine HP adapted to expand steam, an intermediate pressure steam turbine 1 adapted to expand steam from the high pressure steam turbine HP and a low pressure steam turbine LP adapted to expand steam from the intermediate pressure steam turbine 1 having a steam extraction port 2. A condenser 15 connected to the low pressure steam turbine LP exhaust condenses exhausted steam as a first element of a condensate system. From the condenser 15 condensate serially passes through a series of low pressure heaters 24, 25, 11, 12, 20 where the condensate is successively heated. From the low pressure heaters 24, 25, 11, 12, 20 condensate flow in a feed water tank 23 which forms the next element of the condensate system. Condensate from the feed water tank 23 is directed into the last element of the condensate system, a series of High Pressure heaters 22.

The oxy boiler power plant further comprises an Air Separation Unit and a steam coil oxygen preheater 5 downstream of the Air Separation Unit for preheating oxygen produced in the Air Separation Unit. An extraction line 4 connects the steam extraction port 2 to the steam coil oxygen preheater 5. A drain line 8 then fluidly connects the steam coil oxygen preheater 5 to the condensate system.

In an exemplary embodiment shown in Fig. 2 steam is extracted from an IP steam turbine 1, preferably from an extraction port 2 taken from an intermediate stage of a multi stage IP steam turbine, which is typically used as a heat source for at least one of a serial of Low Pressure Heaters 11, 12, 20 or the Feed water Tank 23. In an exemplary embodiment shown in Fig. 2, the extraction steam is routed via an extraction line 4 to a steam coil oxygen preheater 5 of the Air Separation Unit oxygen feed line 3. The steam pressure is controlled, typically to around 10 bar by means of an extraction control valve 6 located in the extraction line. Depending of the temperature of the extraction steam a de-superheater 7 may additional be located in the extraction line 4 upstream of steam coil oxygen preheater 5 to ensure oxygen is heated to about 140°C so as to improve the global heat rates and avoid condensation risk at injection ports of the flue gas or burner.

From the steam coil oxygen preheater 5 the drain line 8 directs condensate formed in the steam coil oxygen preheater 5 to a condensate tank 9 from where it is pumped by condensate pump 10 back into the condensate system wherein a drain control valve 13 provides level control of the condensate tank 9. In an exemplary embodiment shown in Fig. 2 the condensate is pumped back to the condensate system between the fourth serial LP heater 12 and the fifth serial LP heater 20. In an exemplary embodiment shown in Fig. 3, condensate is pumped back to the condensate system at a point between the fifth LP serial heater 11 and the feed water tank 23. In an exemplary embodiment shown in Fig. 4 condensate is pumped back to the feed water tank 23.

In an exemplary embodiment shown in Fig. 2, an emergency line 14 connects the drain line 8 downstream of the condenser 15. This line is normally closed.

Although the disclosure has been herein shown and described in what is conceived to be the most practical exemplary embodiment, it will be appreciated by those skilled in the art that the present disclosure can be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restricted. The scope of the disclosure is indicated by the appended claims rather that the foregoing description and all changes that come within the meaning and range and equivalences thereof are intended to be embraced therein.

REFERENCE NUMBERS

1

IP Turbine

2

Extraction port

3

Oxygen feed line

4

extraction line

5

steam coil oxygen preheater

6

control valve

7

de-superheater

8

drain line

9

condensate tank

10

condensate pump

11

Low Pressure Heater #3

12

Low Pressure Heater #4

13

drain control valve

14

emergency line

15

condenser

20

Low Pressure heater #5

22

Serial High Pressure heaters

23

Feed Water Tank

24

Low Pressure Heater #1

25

Low Pressure Heater #2

42

Boiler

101

Condenser Extraction pump first stage

102

Condenser

103

pump

104

Condensate Polishing plant

106

Serial Low Pressure heater #1

107

Serial Low Pressure heater #2

108

Serial Low Pressure heater #3

109

Serial Low Pressure heater #4

131

Serial Low Pressure heater #5

132

Serial High Pressure heater

136

Feed water tank

142

Boiler

HP

High Pressure steam turbine

IP

Intermediate pressure steam turbine

LP

Low pressure steam turbine

ASU

Air Separation Unit