GB2093366A - Improvements in or relating to fluidised bed combustion techniques - Google Patents

Abstract

The combustion process within a fluidised bed combustor is controlled by using the freeboard temperature as a control parameter.

Description

SPECIFiCATION Improvements in or relating to fluidised bed combustion techniques This invention concerns improvements in or relating to fluidised bed combustion techniques.

Fluidised bed combustion has gained some currency over recent years because of its ability to provide enhanced heat transfer compared with conventional combustion mechanisms. One of the problems attendant upon the utilisation of fluidised bed combustion is that of controlling the combustion taking place within the bed over the range of fuel and air flow rates required for obtaining variable output of a boiler with which the combustion is usuaily associated. If bed temperature is monitored as a control parameter, temperature changes may be used to generate error signals for the control of fuel feed to the bed.

However, bed temperature is not readily responsive to changes in combustion conditions due to the thermal inertia of the bed material and its combustible content, the latter being particularly significant when lump coal is being consumed. The consequence of the use of this temperature as a control parameter is a sluggish response, which either limits the allowable rate of change of boiler output, or results in departures from the preferred fuel: air operating ratio.

In some existing fluidised bed boiler installations designed and operated by the present Applicants, a technique for load following has been developed. However, this technique effectively requires three separate but interdependent control loops. ín particular, an increase or decrease in required heat output of the boiler is related to a falling or rising steam pressure in the case of steam boilers, or a falling or rising water temperature in the case of hot water boilers. The variations in the pressure or temperature parameter are monitored and an inverted proportionate control signal is transmitted to the positioner of the induced draught (I.D.) fan dampers A decrease, for example, in steam pressure therefore results in the induced draught fan damper moving to a more open position and vice versa.

Movements of the l.D. damper cause a change in the above bed combustion chamber pressure which is detected by a control device linked to the positioner of the forced draught (F.D.) fan damper.

Thus, the F.D. damper is automatically moved in order to reinstate the desired combustion chamber pressure condition. The speed of response of the F.D. damper positioner is generally sufficiently rapid, in comparison to the l.D. damper positioner, to maintain control of the combustion chamber pressure during periods of load following.

The variations in forced draught damper position create variations in fluidising air flow rate to the bed material, causing proportionate changes in the fluidising velocity and hence influencing the degree of bed expansion relative to the depth of the static bed. If, for example, there is an increase in air flow rate, the bed suffers a cooling effect for two reasons. Firstly a greater amount of air at ambient temperature passes through the bed material and removes additional heat in so doing, and secondly, increased bed expansion causes more bed cooling surface to be contacted by the bed material. Thus, a bed temperature sensor linked to the coal feed system compensates for changes in bed cooling by initiating appropriate changes of coal feed rate to recover bed temperature to a preset value.

Although this method of control maintains, in essence, a constant bed temperature and freeboard pressure, it does not necessarily maintain a constant fuel: air ratio. In addition, it suffers from the bed thermal inertia íag mentioned previously. Thus, although the l.D. and, consequently, the F.D. dampers may be moved quickly producing rapid changes in flue gas and combustion air flow rates, the resulting rate of change of bed temperature is much more sluggish.

Since maintenance of a constant fuel :air ratio is largely dependent upon the fuel and air changing at similar rates, it is necessary with the above control mode to inhibit the rate of movement of the fan dampers such as to match the speed of response of the bed temperature. This is especially important on a decreasing load when a rapid reduction in combustion air flowrate might produce an undesirable excursion into fuel rich conditions within the fluidised bed.

A further disadvantage of such a method of control can be that where cooling of the bed by expansion thereof to immerse large heat exchange surface in the bed or by other means, for example by adding material to the bed, the restriction of bed temperature to a preset value limits the range of load changes which can be followed, since the temperature controls combustion rates at a given bed carbon concentration and air input rate.

Additionally the bed temperature, as a parameter, reflects only the combustion occurring in the bed and the boiler output component resulting from this combustion. It is not indicative of other boiler output components such as those due to heat transfer from the freeboard above the bed and from exhausting gases passing through the usual heat exchange smoke tubes.

An object of the present invention is to provide a method of controlling fluidised bed combustion which improves the effectiveness of the control upon the combustion process.

Accordingly the invention provides a method of controlling fluidised bed combustion, wherein in a fluidised bed combustor containing a fluidised bed of particulate material with a freeboard thereabove, the temperature of the freeboard is monitored and is used as a control element for controlling combustion within the bed.

It has been found that there is a relationship between the temperature of the bed material and the freeboard temperature, that is the temperature of the gas within the freeboard. Indeed, the freeboard temperature gives a swift and more accurate indication as to the state of combustion taking place within and above the bed and thus affords a more effective control parameter.

The freeboard temperature can be used to control the rate of fuel feed to the fluidised bed thereby to control the bed temperature and thus the combustion process. For example, a change in the air supply to the fluidized bed results in a far swifter change in the freeboard temperature than it does in the bed temperature. This rapid response is due to the effects of the fuel: air ratio generating a change in the combustion taking place in the freeboard. A change in the coal feed in response to the change in the freeboard temperature swiftly corrects the air-fuel ratio. The freeboard flue gases have a lower thermal inertia than that of the bed and this results in a faster response referred to. A steadier performance is therefore, attainable, enabling operation nearer to the operational limit.

In this example, the bed temperature is not controlled to remain at a preset value but is allowed to vary within a range of temperature, i.e.

750 900 C. The invention also includes the step of monitoring the bed temperature as well as the freeboard temperature in order to measure the difference therebetween. This difference is a function of various operating parameters but mainly the air:fuel ratio. Thus by measuring this temperature difference, indications of the kind provided by a flue gas 2 or CO2 analyser are given without the cost and complexity of such apparatus.

The invention thus provides the potential for a íow cost control method for fluidised bed combustion coupled with the sophistication of air:fuel ratio monitoring.

Claims (6)

1. A method of controlling fluidised bed combustion wherein a fluidised bed combustor containing a fluidised bed of particulate material with a freeboard thereabove, the temperature of the freeboard is monitored and is used as a control element for controlling combustion within the bed.

2. A method according to claim 1 in which the temperature of the freeboard is used as a parameter to control the rate of fuel feed to the fluidised bed.

3. A method according to claim 1 in which the air:fuel ratio is controlled by using the temperature of the freeboard as a control parameter.

4. A method according to claim 1 in which the temperature of the fluidised bed is monitored.

5. A method according to claim 4 in which the difference between the bed temperature and freeboard temperature is monitored and the difference is used to control combustion within the fluidised bed.

6. A method of controlling fluidised bed combustion substantially as hereinbefore described.