Control method for the operation of a combustion boiler

Abstract

The invention is in the field of boiler control and relates to a control method for the operation of a combustion boiler, comprising providing a predetermined upper limit (VF,max) for the flue gas velocity in at least one location of the boiler; monitoring the flue gas velocity (VF) during the combustion of fuel in said at least one location of the boiler; comparing the flue gas velocity (VF) with the predetermined upper limit (VF,max); decreasing the thermal load of the boiler if the flue gas velocity exceeds the predetermined upper limit (VF,max). The invention also relates to a control system configured to execute the control method.

Claims

1. A control method for the operation of a combustion boiler, comprising: a) providing a predetermined upper limit (V.sub.F,max) for the flue gas velocity in at least one location of the boiler; b) monitoring the flue gas velocity (V.sub.F) during the combustion of fuel in said at least one location of the boiler, wherein monitoring the flue gas velocity (V.sub.F) comprises monitoring a flue gas temperature in the at least one location of the boiler; c) comparing the flue gas velocity (V.sub.F) with the predetermined upper limit (V.sub.F,max); d) decreasing the thermal load of the boiler if the flue gas velocity exceeds the predetermined upper limit (V.sub.F,max), wherein the boiler is a fluidized bed boiler having bed material that comprises ilmenite particles; and wherein at least a portion of the bed material is carried away with a primary gas stream and subsequently separated from the primary gas stream by a cyclone; wherein flue gas velocity (V.sub.F) is determined according to the following formula:
V.sub.F=.sub.C/A; where: .sub.C=the volume flow of flue gas; A=the cross-sectional area of the flue gas duct; and wherein the boiler is a circulating fluidized bed (CFB) boiler and the flue gas velocity is determined for the region adjacent and downstream the cyclone and the volume flow of flue gas is determined according to the following formula: ${\stackrel{*}{V}}_C=\left({\stackrel{*}{V}}_{\mathrm{Total},\mathrm{stack}}+{\stackrel{*}{V}}_{\mathrm{FGR}}-{\stackrel{*}{V}}_{\mathrm{Air},\mathrm{FGT}}-{\stackrel{*}{V}}_{\mathrm{Water}\mathrm{vapour},\mathrm{FGT}}\right).Math.\frac{T_c}{273}.Math.\frac{1}{P_c}\left(\frac{m^3}{s}\right)$ $\mathrm{where}\text{:}$ ${\stackrel{*}{V}}_{\mathrm{Total},\mathrm{stack}}=\mathrm{total}\mathrm{gas}\mathrm{flow}\mathrm{in}\mathrm{the}\mathrm{stack}\left(\frac{m_n^3}{s}\right)$ ${\stackrel{*}{V}}_{\mathrm{FGR}}=\mathrm{flow}\mathrm{of}\mathrm{recirculated}\mathrm{flue}\mathrm{gas}\left(\frac{m_n^3}{s}\right)$ ${\stackrel{*}{V}}_{\mathrm{Air},\mathrm{FGT}}=\mathrm{air}\mathrm{flow}\mathrm{added}\mathrm{to}\mathrm{the}\mathrm{flue}\mathrm{gas}\mathrm{treatment}\mathrm{plant}\left(\frac{m_n^3}{s}\right)$ ${\stackrel{*}{V}}_{\mathrm{Water}\mathrm{vapour},\mathrm{FGT}}=\mathrm{flow}\mathrm{of}\mathrm{water}\mathrm{vapour}\mathrm{from}\mathrm{the}\mathrm{water}\mathrm{added}\mathrm{to}\mathrm{the}\mathrm{flue}\mathrm{gas}\mathrm{treatment}\mathrm{plant}\left(\frac{m_n^3}{s}\right)$ T.sub.c=temperature just downstream the cyclone (° C.) P.sub.c=pressure just downstream the cyclone (Pa) wherein the flow of water vapour in the flue gas treatment plant is determined as the mass flow of water added divided by the density of the water vapour.

2. The control method of claim 1, wherein the thermal load is decreased to reduce the flue gas velocity (V.sub.F) below the predetermined upper limit (V.sub.F,max).

3. The control method of claim 1, characterized in that the thermal load is decreased until the flue gas velocity (V.sub.F) is below the predetermined upper limit (V.sub.F,max).

4. The control method of claim 1, wherein the thermal load is decreased by decreasing the mass flow of the fuel into the furnace of the boiler.

5. The control method of claim 1, wherein the predetermined upper limit (V.sub.F,max) for the flue gas velocity is smaller than or equal to the design value (V.sub.F,design) for the flue gas velocity for the boiler.

6. The control method of claim 5, wherein the predetermined upper limit (V.sub.F,max) for the flue gas velocity is equal to the design value (V.sub.F,design) for the flue gas velocity for the boiler.

7. The control method of claim 1, further comprising: e) providing a predetermined relationship between the air flow and the fuel flow rate into the furnace of the boiler; and/or a predetermined relationship between the air flow into the furnace of the boiler and the thermal load; f) measuring the fuel flow rate into the furnace of the boiler and/or the thermal load; g) adjusting the air flow into the furnace based on the predetermined relationship provided in step e) and the measured fuel flow rate into the boiler and/or the measured thermal load.

8. The control method of claim 1, further comprising: h) setting a predetermined lower limit and a predetermined upper limit for the oxygen concentration in the flue gas; i) monitoring the oxygen concentration in the flue gas during combustion; j) comparing the oxygen concentration in the flue gas with the predetermined upper limit and the predetermined lower limit for the oxygen concentration in the flue gas; k) adjusting the air flow into the furnace by increasing the air flow into the furnace if the oxygen concentration in the flue gas is below the lower limit; and decreasing the air flow into the furnace if the oxygen concentration in the flue gas is above the upper limit.

9. A control system for a combustion boiler, characterized in that the boiler is a fluidized bed boiler having bed material comprising ilmenite, wherein the control system is configured to execute the control method of claim 1.

10. The control system of claim 9, wherein the fluidized bed boiler is selected from the group consisting of bubbling fluidized bed boilers and circulating fluidized bed boilers.

11. The control method of claim 3, wherein the decrease is a continuous decrease.

12. The control method of claim 3, wherein the decrease is an incremental decrease.

Description

(1) It is shown in

(2) FIG. 1: schematically a CFB boiler;

(3) FIG. 2: schematically a predetermined relationship between air flow into the furnace of the boiler and the thermal load for a given fuel type;

(4) FIG. 3: an example of a prior art control system;

(5) FIG. 4: an example of an inventive control system;

(6) FIG. 5: the measured flue gas velocity in m/s and pressure drop in kPa as a function of time for a CFB boiler.

A CFB BOILER

(7) By way of example, FIG. 1 shows a typical CFB boiler, which can be controlled by the inventive method. The reference numerals denote: 1 Fuel Bunker 2 Fuel Chute 3 Primary Combustion Air Fan 4 Nozzle Bottom 5 Primary Air Distributor 6 Secondary Air Ports 7 Fluidized Bed 8 Furnace 9 Cyclone 10 Loop seal 11 Immersed Superheater 12 Return Leg 13 Heat Exchangers 14 Flue Gas Treatment Plant 15 Flue Gas Recirculation Fan 16 Stack

(8) Fuel is stored in the fuel bunker (1) and can be fed to the furnace (8) via a fuel chute (2). The fluidization gas, in this case air, is fed to the furnace (8) as primary combustion air via the primary air distributor (5) from below the bed and passed through the bed material so that the majority of solid particles (bed material, fuel and ash particles) are carried away by the fluidization gas stream. The particles are then separated from the gas stream using a cyclone (9) and circulated back into the furnace (8) via a loop seal (10). Additional combustion air (so called secondary air) is fed into the furnace to enhance the mixing of oxygen and fuel. Secondary air refers to all oxygen containing gas fed into the furnace for the combustion of fuel which is not primary fluidizing gas. To this end, secondary air ports (6) are located throughout the furnace, in particular the freeboard (the part of the furnace above the dense bottom bed).

(9) The flue gas is passed through the flue gas treatment plant (14) for post treatment and the treated flue gas escapes through the stack (16). A portion of the flue gas may be recirculated to the furnace as indicated in FIG. 1.

Comparative Example

(10) A CFB boiler as shown in FIG. 1 is operated with silica sand particles as bed material and controlled by controlling the air to fuel ratio. To this end, a predetermined relationship between the oxygen flow (here air flow) into the furnace of the boiler and the thermal load is provided for the fuel type utilized as shown in FIG. 2. The thermal load produced by the boiler is measured and the air flow into the furnace is adjusted based on the predetermined relationship between the air flow and the thermal load as well as the actual oxygen concentration in the flue gas. To this end, a predetermined lower limit and a predetermined upper limit are set for the oxygen concentration in the flue gas and the oxygen concentration in the flue gas during combustion is monitored. The oxygen concentration in the flue gas is compared with the predetermined upper limit and the predetermined lower limit for the oxygen concentration and the flow of oxygen into the furnace is adjusted by increasing the flow of oxygen into the furnace if the oxygen concentration in the flue gas is below the lower limit; and decreasing the flow of oxygen into the furnace if the oxygen concentration in the flue gas is above the upper limit.

(11) The lower limit and the upper limit for the oxygen concentration in the flue gas may be set to the same value. In this case, the oxygen concentration can essentially be kept at a setpoint value. The above method provides no handle on the flue gas velocity.

(12) A control system implementing this prior art method is schematically shown in FIG. 3.

Example 1

(13) A CFB boiler as shown in FIG. 1 is operated with ilmenite particles as bed material and controlled by the inventive control method.

(14) This involves providing a predetermined upper limit (V.sub.F,max) for the flue gas velocity, monitoring the flue gas velocity (V.sub.F) during the combustion of fuel, comparing the flue gas velocity (V.sub.F) with the predetermined upper limit (V.sub.F,max) and decreasing the thermal load of the boiler if the flue gas velocity exceeds the predetermined upper limit (V.sub.F,max).

(15) V.sub.F,max is set to the design value (V.sub.F,design) for the flue gas velocity for the boiler, with V.sub.F,design taken from the design specifications.

(16) The flue gas velocity is determined at the region adjacent and downstream the cyclone, according to the following formula:
V.sub.F=.sub.C/A; where: .sub.C=the volume flow of flue gas; A=the cross-sectional area of the flue gas duct;
and wherein the volume flow of flue gas is determined according to the following formula:

(17) ${\stackrel{*}{V}}_C=\left({\stackrel{*}{V}}_{\mathrm{Total},\mathrm{stack}}+{\stackrel{*}{V}}_{\mathrm{FGR}}-{\stackrel{*}{V}}_{\mathrm{Air},\mathrm{FGT}}-{\stackrel{*}{V}}_{\mathrm{Water}\mathrm{vapour},\mathrm{FGT}}\right).Math.\frac{T_c}{273}.Math.\frac{1}{P_c}\left(\frac{m^3}{s}\right)$$\mathrm{where}\text{:}$${\stackrel{*}{V}}_{\mathrm{Total},\mathrm{stack}}=\mathrm{total}\mathrm{gas}\mathrm{flow}\mathrm{in}\mathrm{the}\mathrm{stack}\left(\frac{m_n^3}{s}\right)$${\stackrel{*}{V}}_{\mathrm{FGR}}=\mathrm{flow}\mathrm{of}\mathrm{recirculated}\mathrm{flue}\mathrm{gas}\left(\frac{m_n^3}{s}\right)$${\stackrel{*}{V}}_{\mathrm{Air},\mathrm{FGT}}=\mathrm{air}\mathrm{flow}\mathrm{added}\mathrm{to}\mathrm{the}\mathrm{flue}\mathrm{gas}\mathrm{treatment}\mathrm{plant}\left(\frac{m_n^3}{s}\right)$${\stackrel{*}{V}}_{\mathrm{Water}\mathrm{vapour},\mathrm{FGT}}=\mathrm{flow}\mathrm{of}\mathrm{water}\mathrm{vapour}\mathrm{from}\mathrm{the}\mathrm{water}\mathrm{added}\mathrm{to}\mathrm{the}\mathrm{flue}\mathrm{gas}\mathrm{treatment}\mathrm{plant}\left(\frac{m_n^3}{s}\right)$ T.sub.c=temperature just downstream the cyclone (° C.) P.sub.c=pressure just downstream the cyclone (Pa) wherein the flow of water vapor in the flue gas treatment plant is determined as the mass flow of water added divided by the density of the water vapor.

(18) A is taken from the design specifications or obtained by actual measurement of the cross section.

(19) The total gas flow is measured by differential pressure using a Prandtl tube located in the flue gas duct at the stack. The flow of recirculated flue gas is measured by differential pressure using a Prandtl tube located downstream the recirculation gas fan. The air flow to the flue gas cleaning equipment is measured by means of the fan curve, which describes the characteristics of the fan. The gas temperature Tc is measured in situ by a thermocouple. The pressure Pc, in the specified location, is measured by subtracting the pressure drop of the super-heater tube banks from the absolute pressure measured upstream of the economizer.

(20) In this example, the thermal load is decreased either continuously or in increments to reduce the flue gas velocity (V.sub.F) below the predetermined upper limit (V.sub.F,max). The thermal load is decreased by decreasing the mass flow of the fuel into the furnace of the boiler.

(21) In addition, a predetermined relationship between the oxygen flow (here air flow) into the furnace of the boiler and the thermal load is provided for the fuel type utilized as shown in FIG. 2. The thermal load produced by the boiler is measured and the air flow into the furnace is adjusted based on the predetermined relationship between the air flow and the thermal load as well as the actual oxygen concentration in the flue gas. To this end, a predetermined lower limit and a predetermined upper limit are set for the oxygen concentration in the flue gas and the oxygen concentration in the flue gas during combustion is monitored. The oxygen concentration in the flue gas is compared with the predetermined upper limit and the predetermined lower limit for the oxygen concentration and the air flow into the furnace is adjusted by increasing the air flow into the furnace if the oxygen concentration in the flue gas is below the lower limit; and decreasing the air flow into the furnace if the oxygen concentration in the flue gas is above the upper limit.

(22) The lower limit and the upper limit for the oxygen concentration in the flue gas may be set to the same value. In this case, the oxygen concentration can essentially be kept at a setpoint value.

(23) A control system implementing this inventive method is schematically shown in FIG. 4.

Example 2

(24) The flue gas velocity has been determined in a commercially fired CFB boiler operated with ilmenite particles as bed material.

(25) The flue gas velocity has been calculated from the volume flow of flue gas divided by the cross-sectional area of the flue gas duct in the location just downstream the cyclone, wherein the volume flow of the flue gas was determined according to the formula in Example 1.

(26) The measured flue gas velocity (in m/s) is shown in FIG. 5 together with the measured pressure drop (in kPa) as a function of time for the CFB boiler. The pressure drop is the total pressure drop from the furnace to the suction side of the induced draught fan (the flue gas fan). The flue gas velocity is a very good indicator on the pressure drop during normal operation, as can be seen from FIG. 5, where no lagging between the signals can be seen. If the boiler gets fouled the relationship between the pressure drop and the gas velocity gets affected. FIG. 5 proves that the flue gas velocity is a suitable control parameter.