Oxy-fuel plant with flue gas compression and method

Abstract

A method of and control apparatus for operation of a boiler plant are described. The boiler plant has a furnace volume, an oxyfuel firing system for oxyfuel combustion of fuel in the furnace volume, and a compression system for compression of gases exhausted from the furnace volume after combustion. The method and control apparatus are characterized by the step of controlling mass flow of gases through the compression system as a means to control pressure within the furnace volume. This invention relates to both single and multi unit arrangements.

Claims

1. A boiler plant apparatus comprising: a power generation system having a furnace volume; an oxyfuel firing system for oxyfuel combustion of fossil fuel in the furnace volume; an induced draft fan with modulated vent damper to remove gases from the furnace volume; a compression system, disposed downstream from the induced draft fan, configured to compress gases exhausted from the furnace volume after combustion, wherein the compression system includes at least one CO2 compressor; a control system adapted to control pressure within the furnace volume by controlling the compression system to cause modulation of the mass flow of exhaust gases through the at least one CO2 compressor of the compression system downstream from the furnace volume without continuously venting the exhaust gases to the atmosphere, and extracting a fractional amount of the exhaust gases to be recycled in the oxyfuel firing system; and a forced draft fan to force the flue gases into the furnace volume; wherein the pressure within the furnace volume is dynamically varied by the compression system modulating the mass flow of the exhaust gases through the at least one CO2 compressor based on changes in operating requirements of the oxyfuel combustion in the furnace volume in response to changes in load demand by controlling at least one of a speed of the CO2 compressor or a damper of the CO2 compressor to thereby vary the fractional amount of the exhaust gases to be recycled in the oxyfuel firing system based on variations of at least one of a fuel flow or a furnace leakage flow.

2. An apparatus in accordance with claim 1 wherein the control system is adapted to dynamically adjust the furnace pressure by real time determination of a desired mass flow rate of gas through the compression system being a mass flow rate which will achieve or maintain a desired furnace pressure and by dynamic adjustment of the mass flow rate to the desired mass flow rate to achieve or maintain the desired furnace pressure.

3. An apparatus in accordance with claim 1 provided as part of a thermal power plant.

4. An apparatus in accordance with claim 1 wherein the compression system comprises a bypass that allows all or some of the CO2 to bypass the compression system.

5. A method of operation of a boiler plant having a furnace volume, an oxyfuel firing system for oxyfuel combustion of fuel in the furnace volume, a forced draft fan to recycle flue gases into the furnace volume, an induced draft fan with modulated vent damper disposed downstream from the furnace volume to remove gases from the furnace volume, and a compression system disposed downstream from the induced draft fan for compression of gases exhausted from the furnace volume after combustion, comprising: conducting oxyfuel combustion of fuel in the furnace volume; controlling pressure within the furnace volume by controlling the compression system to cause modulation of mass flow of exhaust gases through at least one CO2 compressor of the compression system downstream from the furnace volume without continuously venting the exhaust gases to the atmosphere; downstream of the induced draft fan, extracting a fractional amount of the exhaust gases to be recycled in the oxyfuel firing system; and compressing the exhaust gases for storage by using the at least one CO2 compressor disposed downstream from the induced draft fan; wherein the pressure within the furnace volume is dynamically varied by the compression system modulating the mass flow of the exhaust gases through the at least one CO2 compressor based on changes in operating requirements of the oxyfuel combustion in the furnace volume in response to changes in load demand by controlling at least one of a speed of the CO2 compressor or a damper of the CO2 compressor to thereby vary the fractional amount of the exhaust gases to be recycled in the oxyfuel firing system based on variations of at least one of a fuel flow or a furnace leakage flow; controlling the induced draft fan vent to minimize positive pressure excursions resulting from a rapid increase in firing rate, loss or partial loss or rapid partial shutdown of the at least one CO2 compressor; and controlling the forced draft fan vent to minimize negative pressure excursions resulting from a rapid decrease in firing rate, start-up or rapid increase in speed or power of the at least one CO2 compressor.

6. A method in accordance with claim 5 wherein the step of modulating the mass flow of gas through the at least one CO2 compressor is effected by adjusting the compressor speed as a means to effect dynamic control and modulation of furnace pressure.

7. A method in accordance with claim 5 wherein the step of modulating the mass flow of gas through the at least one CO2 compressor is effected by adjusting an inlet flow control device.

8. A method in accordance with claim 5 further comprising dynamic adjustment of furnace pressure by real time determination of a mass flow rate to achieve or maintain a desired furnace pressure and by dynamic adjustment of the mass flow rate to the desired mass flow rate to achieve or maintain the desired furnace pressure.

9. A method in accordance with claim 8 comprising dynamic adjustment of furnace pressure by real time determination of being a mass flow rate which will achieve or maintain a desired furnace pressure and by dynamic adjustment of the mass flow rate to the desired mass flow rate to achieve or maintain the desired furnace pressure in conjunction with dynamic adjustment of one or more of the following process parameters, in any combination, to optimize dynamic firing performance: fuel firing rate; oxygen content of furnace exit gas; recycled gas flow to the combustion system; oxygen injection flow; or compressor supply pressure.

Description

(1) The invention will now be described by way of example only with reference to FIGS. 1 to 12 of the accompanying drawings in which:

(2) FIG. 1 is a simplified schematic of the oxyfuel cycle;

(3) FIG. 2 is a general schematic of an embodiment of overall control system for the oxyfuel cycle;

(4) FIG. 3 is a schematic of the principles of oxyfuel mode co-ordinating control;

(5) FIG. 4 is an outline schematic of the principles of oxyfuel mode furnace pressure control using the CO.sub.2 compressor(s);

(6) FIG. 5 is a schematic of the principles of a chimney vent control method for control of furnace pressure;

(7) FIG. 6 is a schematic of the principles of a FD fan air inlet supply vent control method for control of furnace pressure;

(8) FIG. 7 is a schematic of re-cycled gas flow control;

(9) FIG. 8 is a schematic of oxygen control;

(10) FIG. 9 is a schematic of compressor supply pressure control.

(11) FIG. 10 is a schematic of oxyfuel system with CO.sub.2 compression recycle

(12) FIG. 11 is a schematic of multi oxyfuel boiler unit arrangement connected with common flue gas collector.

(13) FIG. 12 is a schematic of rich in CO.sub.2 gas recycle and air intake for used for negative furnace pressure.

(14) An example embodiment of the invention is described that develops a set of integrated control schemes which overcome fundamental problems in the control of an oxyfuel cycle and are able to simultaneously meet the various process conditions required to operate the plant in a safe and efficient manner. The embodiment of the invention also recognises the important role played by air leakage in the overall performance and controllability of the process and describes example methods to deal with these factors.

(15) A simplified schematic of the oxyfuel cycle and processes is shown in FIG. 1.

(16) For safe and efficient operation of the Oxyfuel cycle the following process requirements must be satisfied:

(17) Mass flows into the furnace must simultaneously meet the following inter-related requirements: i) Fuel mass flow must meet Load and Boiler steam pressure requirements and may vary significantly over relatively short periods of time. ii) For a particular fuel flow the mass flow of re-cycled gas must be such as to maintain the correct mass flow through the Fuel Preparation and Supply system plant and for transportation of pulverised fuel into the furnace. The required flow is usually a non-linear function of fuel flow. iii) Oxygen mass flow must be sufficient to provide complete combustion of fuel entering the furnace. iv) For designs where oxygen is mixed with re-cycled gas upstream of the Fuel Preparation and Supply system plant the concentration of oxygen in the mixture must be maintained at an appropriate and safe level in order to avoid potential explosions.

(18) In addition to the mass flow requirements the system must also meet the following requirements: The Furnace pressure must be maintained at a value slightly below atmospheric (0.05 to 0.1 kPag is typical) in order to avoid leakage of unburned fuel and combustion products into the boiler house. The supply pressure of gas to the CO.sub.2 compressors must remain within an acceptable pressure range in order to ensure efficient and stable operation of the compressors. Controls must operate such that a loss rapid start-up or shut-down of CO.sub.2 compressors does not induce pressure excursions likely to cause safety issues or to damage the fabric of the furnace or associated ductwork. Controls must operate such that a loss or partial loss of ignition within the furnace does not induce pressure excursions likely to cause safety issues or to damage the fabric of the furnace or associated ductwork.

(19) In designing the oxyfuel plant and in the development of operating Procedures a convenient assumption is that that the compressors will take a fixed fraction () of gas mass flow from the furnace with the remaining flow being recycled to the combustion system. Depending on the design of the plant's oxygen injection system, this fraction may be set at around 30-35% of the total re-circulated mass flow before the CO.sub.2 compressor extraction point.

(20) In particular the invention recognises that the following variations all cause significant changes in the fraction of re-cycled gas flow extracted by the compressors necessary to maintain the correct value of furnace pressure: fuel flow Furnace leakage flow due to furnace pressure changes Air Heater leakage flow due to seal wear and seal to seal variations in a rotary type Air-Heater leakage flow into the furnace exit gas in plant areas such as the ducting, ESP, FGD, Direct Contact Coolers (DCC), Fans

(21) In particular the invention recognises that these leakage factors cannot be measured directly and describes the principles and design of control systems which overcome these issues whilst simultaneously meeting other operating requirements outlined in previous sections.

(22) These functional design factors are considered in Table 1, which makes a comparison of the possible control methodologies that may be applied for conventional air firing and oxyfuel mode of operation to assist in an understanding of the functional structures and principles of the invention.

(23) TABLE-US-00001 TABLE 1 Comparison of Controls Required for Conventional Air Firing and Oxyfuel mode of Operation: Process Variable to Control Method Item be Controlled Conventional Air Firing Mode Oxyfuel Mode 1 Fuel Firing rate Established fuel firing rate controls. Established fuel firing rate controls Control scheme calculates Combined with additional model based control required Total Combustion Air CO2 Compressor extraction fraction .sub.ff flow for use in FD fan controls used as Feedforward term in CO2 (Item 3) compressor control. (Item 2 in Table 1) FGR system re-cycled mass flow control compensation to setpoint (Item 5 in Table 1) O2 mass flow setpoint for Oxygen Injection flow control (Item 6 in Table 1) Compressor Supply Pressure Control compensation to setpoint (Item 7 in Table 1) 2 Furnace Pressure ID Fan Compressor Extraction (speed or speed and control (via speed or speed and control damper or control damper) damper or control damper) and optionally with Chimney vent control dampers and optionally with FD air inlet control dampers 3 Total Combustion FD Fan N/A Air Flow (via speed or speed and control (no direct measurements available) damper or control damper) 4 Oxygen content of Trim to FD Fan control Trim to Oxygen supply controls furnace exit gas (see below) 5 Recycled Gas Flow to N/A FGR damper and/or FD fan Combustion system (via speed or speed and control damper or control damper) 6 Oxygen Injection Flow N/A Oxygen supply control damper Optionally with integration into UK patent application no 1018227.7 7 Compressor Supply N/A ID fan Pressure P.sub.c (via speed or speed and control damper or control damper)

(24) A general schematic of an embodiment of overall control system applying as control parameters modulation of those process variables identified in table 1 is shown in FIG. 2. In the embodiment a range of possible control parameters based on a range of possible process variables is considered.

(25) As will be understood, the invention at its most fundamental makes use of the CO.sub.2 compressors for control of furnace pressure (item 2 in Table 1). At its broadest, the invention is a control method and system based at least on control of this process variable. Other process variables, such as but not limited to those additionally identified in FIG. 2, may additionally be used separately or in any combination to optimize dynamics of operation.

(26) The process control method exemplified in FIG. 2 and the more detailed discussion of some of the particular process controls below will be understood as an example of a possible implementation of those general principles.

(27) In particular the example recognises the value of additional de-coupling and co-ordination between control loops due to the increased level of interaction caused by the presence of gas re-cycling in oxyfuel mode.

(28) In particular the example recognises that the co-ordinating and de-coupling function may be achieved in practice by the use of the oxyfuel co-ordinating control or by partial devolvement of this function to the individual control loops.

(29) The function of each part of the overall control design exemplified in FIG. 2 is on that basis considered in detail below.

(30) Oxyfuel Mode Co-ordinating Control (Item 1 in Table 1)

(31) The main function of this part of the overall control design is to ensure that individual control loops for each specific function such as furnace pressure and re-cycled gas flow operate in a coherent and stable fashion producing fast, accurate response to changes in operating conditions.

(32) The main functions of this control are shown in FIG. 3.

(33) Furnace Pressure Control (Item 2 in Table 1)

(34) A generalised schematic showing the Principle of the Invention and use of the CO.sub.2 compressors for control of furnace pressure is outlined in FIG. 4.

(35) The following specific observations are made in relation to this aspect of the example embodiment: that use of the ID fan for Furnace Pressure control is no longer effective for operation in oxyfuel mode; that effective control of furnace pressure may be achieved by modulating the mass flow of gas through the compressors; that this fundamental principle could optionally be achieved as a simple pressure control loop adjusting the compressor speed or an inlet flow control device or by a number of functionally related methods; that dynamic performance may be improved by the optional inclusion of a manifestation of feedforward type control within the invention such as exemplified in FIG. 4; that dynamic performance may be improved by the optional inclusion of non-linear compensation terms exemplified in FIG. 4; that air leakage is of importance in determining both the value of fractional flow () and therefore extraction flow required to maintain the correct value of Furnace pressure and also its effect and importance in terms of pressure sensitivity and therefore tunings of the Furnace Pressure control system; that ID vent control, such as exemplified in FIG. 5, may be optionally incorporated to minimise positive pressure excursions resulting from events such as a rapid increase in firing rate, loss or partial loss or rapid partial shutdown of one or more CO.sub.2 compressors; that FD vent control, such as exemplified in FIG. 6, may be optionally incorporated to minimise negative pressure excursions resulting from events such as a rapid decrease in firing rate, start-up or rapid increase in speed or power of one or more CO.sub.2 compressors; that for negative furnace pressure excursions invention could be realized by an additional rich in CO2 gas injection instead of air intake vent modulation control.

(36) A value for the feedforward term may be estimated by one of two methods in particular and appropriate code developed within the control system.

(37) In a first alternative method a direct calculation of the feedforward term may be made using appropriate equations to model the contributing factors, for example including Oxygen injection upstream of Fuel Preparation and Supply system plant and downstream of Fuel Preparation and Supply system (i.e. burners or windbox).

(38) A suitable algorithm may be developed that uses estimates (or design values) for airheater and ducting, ESP, FGD, DCC leakage mass flow rates and for furnace leakage factor k in conjunction with the setpoint value for furnace pressure control which is typically 5 kPag

(39) Fuel mass flow rate may not always be measurable in which case an estimate based on demanded fuel value taking into account the dynamic response of the Fuel Preparation and Supply system, or pulverised fuel silo in the case of indirect firing, may be used

(40) The preferred implementation of this method is by direct coding of equations within the control system since this allows the terms within the calculation to be updated either through direct measurement or from off-line data obtained as part of plant performance investigations.

(41) In a second alternative method plant tests are conducted to identify the value feedforward required to maintain the setpoint pressure at various firing rates. These values are then entered into a characterisation block within the control system such that the feedforward term is set as a function of fuel flow determined by calculation or plant test data.

(42) Chimney and FD Fan Air Inlet Duct Vent Controls

(43) These options allow account to be taken of the following for example: that positive going excursions in furnace pressure may be reduced by venting gas flow to the chimney by controlled opening of the chimney isolation damper assembly; that negative going excursions in furnace pressure may be reduced by admitting air to the system by controlled opening of the FD inlet air supply ductwork isolation damper assembly; that, since relatively small values of mass flow are normally required to correct pressure excursion, improved control may be obtained by the (optional) use of a by-pass damper arrangement rather than use of the main isolating dampers. that for large scale deviations the use of split range control whereby the small vent dampers are opened first followed by the large isolation dampers when necessary.

(44) Outline schematics for the vents and associated control designs are shown in FIGS. 5 and 6, respectively showing an outline schematic of a chimney vent control method for control of furnace pressure and an outline schematic of FD fan air inlet supply vent control method for control of furnace pressure.

(45) Re-cycled Gas Mass Flow Control (Item 5 in Table 1)

(46) This option allows account to be taken of the following for example: that in order to minimise Works Power the FGR flow control damper and FGR Fan speed (or FD fan speed) are controlled to provide the necessary FGR mass flow rate at the minimum fan speed and maximum damper opening commensurate with maintaining acceptable levels of flow control by movement of the damper; that, where FD Fan inlet supply vent control is employed (see Item 2 in Table 1,) the design and control of the FGR system and damper must maintain a sub-atmospheric pressure at the junction between the external air supply duct and the FD fan inlet.

(47) An outline schematic for recycled gas flow control is shown in FIG. 7.

(48) Oxygen Injection Controls (Item 6 in Table 1)

(49) This option allows account to be taken of the following for example: that accurate control of oxygen injection and concentration is essential for both combustion and safety reasons; that the importance of process dynamics associated with the Fuel Preparation and Supply system and pulverised fuel transport system in achieving accurate control of Oxygen concentration within the system is recognised.

(50) An outline schematic for oxygen flow control is shown in FIG. 8.

(51) Compressor Supply Pressure Control (Item 7 in Table 1) This option allows account to be taken of the following for example:

(52) that control of supply pressure to the compressors is achieved by modulation of gas flow through the ID fan either by changing fan speed or by changing ID fan damper position or by a combination of both, or by changing position of an additional damper downstream of the ID fan; that the close coupling and interaction with other control elements requires effective co-ordination of control between schemes, this being achieved by the oxyfuel mode co-ordinating control described previously. that compensation to de-couple the interaction between elements may be applied to the pressure controller as shown in FIG. 9 or alternatively to the feedforward element of the design or alternatively to both the feedforward and pressure control elements.

(53) An outline schematic for compressor supply control is shown in FIG. 9.

(54) An outline schematic of an oxyfuel system with CO.sub.2 compression recycle is shown in FIG. 10.

(55) FIG. 11 is a schematic of schematic of a multi oxyfuel boiler unit arrangement connected with common flue gas collector.

(56) FIG. 12 is a schematic of rich in CO.sub.2 gas recycle and air intake for used for negative furnace pressure.