Control system and method for power plant

Abstract

A method of operation of a thermal power plant having an air separation system with a plurality of air storage unit (ASU) compressors and a liquid oxygen/liquid air (LOX/LA) storage facility for oxyfuel firing of fossil fuel and a power plant having a control system to perform the same are described. The method is characterized by the step of controlling the net power output of the plant in response to short term variations in grid demanded net plant output by dynamically adjusting the works power of the ASU compressors preferably in conjunction with co-ordinated changes in firing demand. The method is in particular a method to produce an improved primary and secondary response to transient changes in grid demand and to provide accurate response to load dispatch ramps.

Claims

1. A method of operation of a thermal power plant having an air separation system with a plurality of air storage unit (ASU) compressors and a liquid oxygen/liquid air (LOX/LA) storage facility for oxyfuel firing of fuel, the method comprising: controlling a net power output of the thermal power plant in response to a varying grid demanded net plant output by dynamically adjusting a works power of the ASU compressors, wherein the dynamically adjusting the works power is performed by: setting a design works power level W.sub.i.sup.des for the works power of each compressor of the ASU compressors; modifying, for each compressor of the ASU compressors, the design works power level to a setpoint value for a compressor power controller according to a relationship:
W.sub.setpoint(t)=W.sub.i.sup.des+WP.sub.i(t) where WP.sub.i(t) is a change in the works power for each compressor of the ASU compressors, and (t) is a time value; adjusting a power correction for each compressor of the ASU compressors WP.sub.i(t) to reduce the total change in works power for each compressor of the ASU compressors required to balance grid demand as determined by a relationship: $\underset{i=1,N}{.Math.}{\mathrm{WP}}_i\left(t\right)=\left(W_{\mathrm{so}}\left(t\right)-W_{\mathrm{dem}}\left(t\right)\right),$ wherein W.sub.dem(t) is the grid demand and W.sub.so(t) is a sent out power from the thermal power plant, wherein the net power output of the thermal power plant is controlled by dynamically adjusting the works power for each compressor of the ASU compressors in response to short term variations in grid demanded net plant output occurring over timescales of less than 30 minutes so as to provide a predominant source of an overall primary response of the thermal power plant to primary changes in the net power output demand.

2. A method in accordance with claim 1 wherein the step of dynamically adjusting the works power for each compressor of the ASU compressors is performed to provide essentially a sole source of the primary response of the thermal power plant.

3. A method in accordance with claim 1 wherein the thermal power plant load is kept constant during the primary response.

4. A method in accordance with claim 1 wherein the step of controlling the net power output of the thermal power plant in response to a varying grid demanded net power plant output by dynamically adjusting the works power for each compressor of the ASU compressors is performed additionally to provide at least part of a secondary response to secondary changes in the net power output demand.

5. A method in accordance with claim 4 wherein a secondary response is achieved by a step of controlling the net power output of the thermal power plant in response to a varying grid demanded net plant output by dynamically adjusting the works power for each compressor of the ASU compressors in conjunction with coordinated changes in firing demand.

6. A method in accordance with claim 1 wherein the controlling the net power output of the thermal power plant is performed without changing a CO.sub.2 capture rate.

7. A method in accordance with claim 1 wherein the controlling the net power output of the thermal power plant comprises dynamically adjusting the works power for each compressor of the ASU compressors to meet the grid requirement in conjunction to optimization of a rest of the thermal power plant equipment to reduce works power and move more closely towards target cycle efficiency.

8. A method in accordance with claim 7 wherein the controlling the net power output of the thermal power plant comprises dynamically adjusting the works power for each compressor of the ASU compressors to reduce the sum total of the works power adjustment made across all compressors.

9. A method in accordance with claim 1 wherein adjustments are made across all ASU compressors.

10. A method in accordance with claim 1 comprising the step of setting the design works power level for each compressor in response to a change in grid load demand which is then modified by the difference between the power demanded by the grid and that supplied by a power generation unit to give the setpoint value for control of ASU compressor power.

11. A method in accordance with claim 1 wherein at least the power consumption of compressors of the air separation system is used as a control parameter for the net power output of the thermal power plant and the method comprises a step of making a dynamic adjustment of the same in response to changing net power output demand from the grid.

12. A method in accordance with claim 11 wherein a liquid oxygen storage level/storage pressure is used as a control parameter for the net power output of the thermal power plant and the method comprises a step of making a dynamic adjustment of the same in response to changing net power output demand from the grid.

13. A method in accordance with claim 1 wherein the step of adjusting the works power of one or more compressors of the ASU compressors in response to a change in grid demanded plant output comprises either: tending to reduce the works power for each compressor of the ASU compressors in response to an increased grid demand and balancing the same by unstoring liquid oxygen and/or liquid air from the LOX/LA storage to make up a required supply for oxyfuel firing; or tending to increase the works power for each compressor of the ASU compressors in response to a reduced grid demand and balancing the same by supplying a resultant excess liquid oxygen and/or liquid air to a LOX/LA storage.

14. A method in accordance with claim 1 wherein the thermal power plant is operated close to or at its design output in normal demand conditions and is not operated at part load with capacity reserve by means of an output restrictor.

15. A method in accordance with claim 1 wherein the power correction for each compressor is adjusted taking account of factors selected from: cost of power, compressor efficiency and turn-down, storage levels for LOX and/or LA, expected further changes in demand and consideration of mechanical factors including life usage.

16. A thermal power plant comprising a power generation unit having an oxyfuel firing system including an air separation system with a plurality of air storage unit (ASU) compressors and a liquid oxygen/liquid air (LOX/LA) storage facility, comprising: a control system adapted to control a net power output of the thermal power plant in response to a varying grid demanded net plant output by dynamically adjusting a works power of the ASU compressors, wherein the control system dynamically adjusts the works power of the ASU compressors, the control system configured to: set a design works power level W.sub.i.sup.des for each compressor of the ASU compressors; modify, for each of compressor of the ASU compressors, the design works power level to a setpoint value for a compressor power controller according to a relationship:
W.sub.setpoint(t)=W.sub.i.sup.des+WP.sub.i(t) wherein WP.sub.i(t) is a change in the works power for each compressor of the ASU compressors, and (t) is a time value; and adjust a power correction for each compressor WP.sub.i(t) to reduce the total change in works power for each compressor of the ASU compressors required to balance grid demand as determined by a relationship: $\underset{i=1,N}{.Math.}{\mathrm{WP}}_i\left(t\right)=\left(W_{\mathrm{so}}\left(t\right)-W_{\mathrm{dem}}\left(t\right)\right),$ wherein W.sub.dem(t) is the grid demand and W.sub.so(t) is a sent out power from the thermal power plant, wherein the net power output of the thermal power plant is controlled by dynamically adjusting the works power for each compressor of the ASU compressors in response to short term variations in grid demanded net plant output occurring over timescales of less than 30 minutes so as to provide a predominant source of an overall primary response of the thermal power plant to primary changes in the net power output demand.

17. A thermal power plant in accordance with claim 16 wherein the control system is adapted to dynamically adjust the works power for each compressor of the ASU compressors in such manner to reduce the sum total of the works power adjustment made across all ASU compressors.

18. A thermal power plant in accordance with claim 17 wherein the control system is adapted to adjust the works power for each compressor of the ASU compressors in conjunction with a control of the supply of LOX/LA to/from the liquid oxygen storage facility.

19. A thermal power plant in accordance with claim 18 wherein the control system is adapted: to tend to reduce the works power for each compressor of the ASU compressors in response to an increased grid demand and balancing the same by unstoring LOX/LA from a LOX/LA storage to make up the required supply for oxyfuel firing; and/or to tend to increase the works power for each compressor of the ASU compressors in response to a reduced grid demand and balancing the same by supplying a resultant excess LOX/LA to the LOX/LA storage.

20. A thermal power plant in accordance with claim 16 wherein the air separation system with the ASU compressors has a capacity which is bigger than required for steady state operation of the thermal power plant in order to have additional capacity to generate excess oxygen for storage at times of lower demand.

Description

(1) The principles of operation of the invention will be described in greater detail by way of exemplification with reference to FIGS. 1 and 2 of the accompanying drawings in which:

(2) FIG. 1 is a schematic flow chart of the process;

(3) FIG. 2 illustrates the improved dynamic response that may be achieved thereby.

(4) An outline schematic of the process is shown in FIG. 1 which shows an oxyfuel process, and an optional PCC stage for a part oxyfuel with PCC process.

(5) TABLE-US-00001 Reference numerals in FIG. 1 are as follows. 1 - ASU unit a - oxygen supply 2 - LOX/LA storage b - liquid CO.sub.2 to transportation 3 - Power plant i - electrical power to power the ASU 4 - Boiler unit compressors 5 - Turbine unit ii - electrical energy sent out to the grid 6 - optional PCC process 7 - CO.sub.2 compression system

(6) At any instant in time the Sent Out Power from the Power Plant Unit W.sub.so(t) is the difference between the power generated by the main plant W.sub.gen(t), which is a complex function of steam pressure and plant dynamics, and the total Works Power WP(t) used by the unit to generate the steam:
W.sub.so(t)=W.sub.gen(t)WP(t)(1)

(7) In the case of an oxyfuel plant, the ASU compressors form part of the overall works power used by the Power Plant. Proposals for dealing with rapid load increases by stopping one or more ASU compressors suffer from the problem that the additional power is only available in fairly large increments, which may or may not be appropriate in all situations. For smaller changes and where the available power changes does not exactly match the change in Grid demand these solutions produce significant disturbances to operating conditions in the main Power Plant which persist for some period of time after the event.

(8) The proposed design avoids these problems and is able to accurately follow both large and small changes for both increases and decreases in transient demand changes without attendant disturbances to the main Power Plant operating conditions.

(9) Primary Response Algorithm

(10) This algorithm provides the main, short term response for both large and small changes in Grid demand.

(11) Each compressor is controlled to a design power level W.sub.i.sup.des necessary to provide the correct flow of Oxygen required by the Main Power plant at each point in time. The design level is determined by the integrated control system and depends on the ASU compressor characteristics, the required flow and storage tank level together with plant operational and commercial objectives entered into the optimisation algorithm by plant operatives or management. The design power level for each compressor is then modified by the difference between the power demanded by the Grid and that supplied by the Power Plant unit to give the setpoint value for the ASU compressor power controller.
W.sub.setpoint(t)=W.sub.i.sup.des+WP.sub.i(t)(2)
where the total change in ASU compressor works power required to balance the Grid demand is

(12) $\begin{array}{cc}\underset{i=1,N}{.Math.}{\mathrm{WP}}_i\left(t\right)=\left(W_{\mathrm{so}}\left(t\right)-W_{\mathrm{dem}}\left(t\right)\right)& \left(3\right)\end{array}$

(13) The power correction for each ASU compressor WP.sub.i(t) is optimised taking account of factors including the cost of power, compressor efficiency and turn-down, storage levels for LOX and LA, likely further changes in demand and consideration of mechanical factors including life usage.

(14) For small changes in Grid demand the modulation of power to one or more ASU compressors may be the optimum solution whilst for larger changes it may be more desirable to start or stop one or more ASU compressors. This range of possible scenarios may be handled by a comprehensive optimisation algorithm which takes account of the relevant operational factors and plant constraints. The rules for this algorithm may be set and modified by plant management or operators.

(15) Where the change in Grid demand falls within the range for which equation 3 is applicable, the main unit firing controls will make only a relatively slow adjustment to the firing controls setpoint based on Grid demand W.sub.dem(t). Changes to the firing controls will be done in such a manner as to tend to minimize power plant works power, maximizing the cycle efficiency and reducing thermal stress on plant.

(16) In cases where this range is exceeded, larger and more rapid changes in firing control system parameters will automatically occur in addition to maximum permissible action being taken on ASU compressor power control.

(17) Secondary Response Algorithm

(18) Unit Secondary Response is met by the Primary Response algorithm used in conjunction with coordinated changes in firing demand.

(19) The principal objectives of Secondary Response control function are to: i) Reduce and in the ideal case minimise the total ASU compressor power correction factor

(20) $\underset{i=1,N}{.Math.}{\mathrm{WP}}_i\left(t\right)$ in the long term. This is achieved by modification of firing demand in a co-ordinated manner taking into account a Cost or Objective function which itself is dependent on management objectives and plant factors relevant at the time including the need to maintain an adequate reserve of liquefied Oxygen. ii) Maintain the storage levels of both LOX and LA within acceptable operating limits and/or according to operator manual setpoint. This is achieved by co-ordinated modification of the ASU compressor Works Power demand signals in conjunction with appropriate changes in firing level.

(21) Results from a detailed non-linear simulation of an Oxyfuel plant indicate that the expected open governor response of an oxyfuel power plant to a +7% change in load demand would be as shown in FIG. 2. FIG. 2 shows the open governor response of an oxyfuel plant embodying the principles of the invention to +7% step change in grid demand. MW sent out is plotted against time and graphical representations are shown of the change in load demand, the typical response of a conventional system without integrated control in accordance with the principles of the invention, and a suggested typical response with such integrated control in accordance with the principles of the invention. It can be seen that the invention offers a capability to match the primary response much more closely to the step change in grid demand.

(22) In a further preferred case the plant has a part oxyfuel and post-combustion carbon capture (PCC) facility. The method is applicable to this cycle in same fashion as in a pure oxyfuel plant.