GB2073910A - Controls for fluidised bed burners - Google Patents

Abstract

A boiler 10 has a fluidised bed 13, a combustion space 14, an inlet plenum 11 and a steam and water- filled space 16. At low bed temperatures, a start-up control unit 23 is activated to fire a bed pre-heating unit 21 and, through a 3-term controller 28, set dampers 12, 16. At a predetermined bed temperature the unit 23 is switched off and control taken over by a 3-step controller 27. With a low-demand for steam, a reponse limiter, of a speed controller 27 for the fuel feed, is regulated by the controller 27 so that the fuel feed is limited, whatever the bed temperature seems to demand in extra fuel; since it can be in excess of what is truly needed due to the effects of cooling air and the introduction of cold fuel. <IMAGE>

Description

SPECIFICATION Controls for fluidised bed burners The present invention concerns fluidised bed burner controls.

Fluidised bed burners consume solid fuels which "float" in a seething mass of a carrier such as sand or alumina kept agitated by the flow of combustion air. The control of such burners is complex and the cost of the control equipment can be as high as a fifth of the total burner price.

Whilst burners consuming pulverised fuel, oils or gases can be controlled by regulating the fuel flow either in an infinitely variable manner or by an on-off procedure (if the load has sufficient thermal inertia to smooth the heat output), solid fuels when burnt in lump form do not burn instantaneously but there has to be a bed of fuel. The conventional method of regulating the heat output from a lump fuel burner is therefore by control of the air supply. However, in burners controlled by regulating the fuel feed it is necessary to adjust the air flow to prevent either over-rich or over-lean air-fuel mixtures which would result in a dirty exhaust containing unburnt fuel. Similarly the fuel bed with regulated air flow has to be adjusted but due to time lags this is far more complicated.For example, the bed has to be controlled not for immediate heat output demand but for the heat output demand expected over a period. There is an additional complication with fluidised bed burners in that additional air flow does not increase the temperature of the bed by boosting the rate of combustion faster than the cooling effect of the increased air flow. When sensing bed condition by means of bed temperature which is the usual way and normally the only parameter available in a fluidised bed, it will be appreciated that, after a time of low demand, the bed will be relatively thin and that increased demand will burn up that bed calling for more fuel but at the same time the increased air flow will simulate an increased fuel demand condition. Thus the control system must cope with long time lags and not forecast the future too inaccurately.

A control for a fluidised bed burner according to the present invention comprises a combustion air regulator, a heat demand setter controlling that regulator, a bed temperature senser and a fuel feed mechanism regulated in response to bed temperature, characterised in that the demand setter is also connected to an input signal limiter controlling the signal to the fuel feed mechanism so that at times of low demand the maximum rate of fuel is reduced.

Apart from simplifying the control, there is a further advantage in that when lump fuel is added to a bed it is initially cold and so cools the bed and that the input signal limiter prevents the bed being over-cooled by extra fuel down to a temperature at which sintering or clogging of the lumps together is probable.

At times of high demand, the bed is thicker and more able to absorb the cooling effect of the fresh fuel. It has been found advantageous to have a stepped demand setter or some way of modifying the set demand to the input signal limiter into a number of steps.

The demand setter can be a means for sensing the demand such as a steam pressure transducer in a steam raising boiler, or a transducer responsive to throughput and possibly moisture content in a hot gas generator coupled to a crop or other drier or can be manually controlled.

Mention has been made of keeping the fuelair ratio within limits in fluent fuel burners to avoid dirty exhaust gases. In lump burners, the same need arises but the fuel part of the ratio is the burning component of the fuel bed. It is possible to feed secondary air into the combustion gases to eliminate over-lean- ness but it is more usual to rely on regulating the combustion air at the inlet and at the outlet and this by regulating the combustion air density or pressure through the bed varies the leanness or richness of the fuel-air ratio.

However it is important that the outlet regulator should be co-ordinated with the input regulator so that the outlet pressure is within limits such that there is a little danger of blow-backs or hot gases being Forced into the fuel feed mechanism.

The present invention also can be provided with a start-up provision and can incorporate safety arrangements.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawing.

THE DRAWING is a schematic illustration of a fluidised bed boiler.

The boiler 10 has an air inlet plenurn 11 with an inlet controlled by a damper 1 2 connecting a forced draught fan (not shown) to the plenum. Above the plenum, there is the usual distribution plate supporting a fluidised bed 1 3 of conventional de design with its associated freeboard or combustion space 14.

From the freeboard, there is a flue controlled by a damper 1 6 with an induced draught fan (not shown) drawing the exhaust gases along the flue. There is a steam-raising water-filled space 1 5 associated with fluidised bed with a demand transducer 1 9 sensing either temperature or pressure. A pressure transducer 20 senses the pressure in the freeboard and there are two temperature transducers such as thermocouples 1 7 and 1 8 for sensing bed temperature. One of the bed temperature transducers, 17, is connected to a start-up control unit 23 so that on activating the unit 23 it will respond to a bed temperature below a predetermined level as sensed by transducer 17 and fire a bed preheating unit 21 through an ignition control unit 22.When the bed temperature is sensed as exceeding the predetermined level, the preheating unit is switched off. During the preheating period, signals are fed from the start-up control unit to a three-term controller 28 which also receives a signal from the pressure transducer 20 to set the damper 12, to the damper 1 6 to set that, and to a fuel feed mechanism (not shown) through a speed controller 26 so that both dampers are set to a predetermined startup position and a metered amount of fuel is charged into the bed. The start-up control unit 23 also blocks during the preheating period a fuel feed regulating signal derived from the other 1 8 of the bed temperature sensing transducers.After the preheating period, the boiler control stems basically from the demand transducer 19, the output of which is stepped by a three-step controller 27 and used to control the damper 1 6. The setting of the damper 1 6 affects the freeboard pressure, and the transducer 20 feeds a corresponding signal to the three-term controller 26 to set the damper 1 2. The fuel feed mechanism is called into play when the bed temperature as sensed by transducer 18 through a bed temperature control unit 24 which passes its output signal through the start-up unit 23 (so that this control loop is disabled during preheating) and line 25.However the speed controller includes a response limiter regulated by the three-step controller 27 so that when demand is low the fuel feed is also limited whatever the bed temperature seems to demand in extra fuel, which can be in excess of what is truly needed due to the cooling effects of the cooling air and the introduction of cold fuel. If the bed temperature falls below the normal working range down to the preheating predetermined limit, the start-up unit can be left active so that it will respond to the abnormal fall and result in switching on the preheating unit and a charge of fuel but it is anticipated that such an abnormal fall could only result from some failure in the main control system.

Taking for example a boiler generating steam at 960 kilo-Newtons per square metre which can be allowed to rise at most to 1000 kilo-Newtons per square metre with a conventional bed temperature of 950do and a maximum of 1000"C, depending on the fuel, the bed could require pre-heating to say 700"C with the start-up unit being reactivated if the bed temperature fell to say 600"C. The normal control could work in bands of 30-40 kilo-Newtons per square metre.Between 960 and 1000, the boiler would be required to provide no further heat and the burner would be slumped (i.e. damped down as far as possible with the damper 1 6 almost fully closed and the damper 1 2 fully closed with the bed no longer fluidised by any air stream), when the pressure falls below 960 within the range 930 to 960 (or increases from a lower figure into that range) the boiler would be in a "low-fire" band, when the pressure is within the range 890 and 930, the boiler is in a medium fire band and below 930 in a "highfire" band.On start-up after preheating, the normal control would be in the high-fire band calling for maximum opening of the damper 1 6. In any band the transducer 1 9 calls for a corresponding opening of the damper 1 6 which sets the freeboard pressure and thus the opening of the damper 1 2. The fuel is supplied as demanded by the bed temperature except for a limitation on the maximum rate of fuel supply set by the band.When the transducer 1 9 senses an increasing pressure either on start-up or after a period of great demand, the bed temperature would tend to indicate past experience, in other words high demand and demand and a high fuel input but this demand would be modified when going through the various bands so limiting the amount of overshoot possible and maintaining the bed thickness within the desirable limits.

When the load has considerable thermal inertia as is the case with boilers, the control is simplified to a considerable extent by having stepped control of the damper 1 6. The stepped control of the damper 1 6 results in the burner "hunting" between predetermined states for which optimal maximum fuel feed rates and other conditions can be predetermined. The "hunting" is at a comparatively slow rate and does not amount to instability.

At very small demand when the fire is at low level, the hunting occurs between a predetermined low firing rate and off but a fluidised bed burner can be turned off or slumped for a short time without extinguishing since the hot fuels are covered and thermally lagged when the bed is slumped by a layer of the carrier and like an earthed-over camp fire can be resurrected easily (by blowing combustion air through it).

It will be seen that the careful use of the present invention facilitates the design of a control arrangement for a fluidised bed burner in which the effects of transients and time lags are minimised.

For the avoidance of any doubt, it is pointed out that the fuel feed mechanism can supply fuel albeit at a low rate even at times of zero heat demand if the fire conditions so demand and this is especially necessary if the damper 1 6 is not controlled in steps when the fuel mechanism must be capable of a minimum fuel delivery.

Claims (11)

1. A control for a fluidised bed burner comprising a combustion air regulator, a heat demand setter controlling that regulator, a bed temperature senser and a fuel feed mechanism regulated in response to bed temperature, characterised in that the demand setter is also connected to an input signal limiter controlling the signal to the fuel feed mechanism so that at times of low demand the maximum rate of fuel feed is reduced.

2. A control according to claim 1 wherein the demand setter is arranged so as to give stepped outputs.

3. A control according to claim 2 wherein the regulator comprises an induced draught control and wherein the stepped outputs of the demand setter is used to control the induced draught control.

4. A control according to claim 3 wherein the regulator also comprises a forced draught control which is regulated by a pressure transducer upstream of the induced draught control.

5. A control according to any one of the preceding claims wherein the demand setter is a transducer.

6. A control according to claim 5 for use with steam raising plant wherein the demand setter is a transducer sensing a steam or water parameter.

7. A control according to anyone of the preceding claims wherein an ignition or preheating provision comprises a unit for igniting a preheating unit, opening the combustion air regulator and feeding a charge of fuel into the burner, overriding the normal regulation of the fuel feed mechanism.

8. A method of controlling a fluidised bed burner comprising setting the fuel feed under the control of the bed conditions and setting the combustion air supply under the control of a demand setter, in which the demand setter imposes a limit on the fuel feed at times of other than maximum demand.

9. A method according to claim 8 wherein the demand setter imposes the limitation on the fuel feed in steps.

1 0. A method according to claim 9 wherein the combustion air supply is also set in steps.

11. A control substantially as herein described with reference to the accompanying drawing.

1 2. A method of controlling a fluidised bed burner substantially as herein described with reference to the accompanying drawing.