GB2053018A - Combustor - Google Patents

Abstract

A method of igniting a fluidised bed combustor when the bed is in a cold state comprises supplying a liquid fuel to nozzle outlets (5) within the bed, and supplying oxygen- containing fluidising gas to nozzle outlets (4) adjacent the fuel outlet nozzles through the inner wind- box (10) to fluidise the bed. The liquid fuel is then ignited and the fuel and fluidising air supplies regulated so as to obtain a controlled rise in bed temperature until the whole bed achieves the hot fluidised condition. <IMAGE>

Description

SPECIFICATION Combustor The present invention relates to fluidised bed combustors and more particularly relates to the start up of fluidised bed combustors.

At present there are several methods of starting up fluidised bed combustors. For example, (a) pre-mixed gaseous fuel and air are fed through the bed and ignited in the cold condition, the main fuel being introduced when the bed has reached the required fuel ignition temperature, (b) conventional oil fired burners may be played on to the surface of the fluidised bed, the main fuel again being introduced when the correct bed temperature has been attained or (c) hot gases from an external source are passed through the bed, the fuel and fluidising air being introduced when the correct bed temperature is attained.

These methods are described in whole or in part in our GB patent nos. 1159310 and 1459766 and GB patent application no.

79/09452. However, these methods have certain disadvantages. Thus, methods (a) and (b) require provision for storage and feed of a start-up fuel, e.g. propane or gas oil, which adds to the cost and complexity of the fluid bed plant, method (b) may be slow and unreliable and method (c) in addition to requiring provision of hot gas from an external combustion source requires not only the use of more heat resistant materials for supply lines and distributor plate but also requires allowances for expansion of the distributor plate to be made which adds to the cost and complexity of the plant.

The present invention is directed towards a method of start-up of a fluidised bed combustor which alleviates or overcomes some of the above disadvantages.

Thus according to the present invention there is provided a method of igniting a fluidised bed combustor when the bed is in a cold state comprising the steps of (a) supplying a liquid fuel at a pre-determined rate to one or more first nozzles having their outlet or outlets within the fluidised bed (b) supplying a fluidising gas containing oxygen to second nozzles so as to fluidise at least the region of the bed adjacent to the outlets of the first nozzles (c) igniting the liquid fuel (d) regulating the liquid fuel flow and fluidising gas containing oxygen fuel supply so as to obtain a controlled rise in the temperature of the fluidised bed until the whole bed achieves the hot fluidised condition.

The liquid fuel may be supplied to the bed by the techniques described in our UK patent numbers 1368352, 1487391, 1502764, or alternatively, the liquid fuel may be injected, for example, in the form of a fine spray into the fluidising gas upstream of the air distributing nozzles and the resulting combustible mixture then fed to the bed via these nozzles, or by any other suitable technique.

The fluidised bed combustor preferably comprises a distributor plate for supplying fuel and fluidising gas containing oxygen to the bed, the bed lying above the plate. The fluidising gas containing oxygen is generally a supply of air under pressure at ambient temperature. The liquid fuel is preferably a fuel oil such as gas oil or kerosene.

The distributor plate preferably comprises an air chamber supplying fluidising air to the second nozzles. The air chamber preferably comprises partitioning means adapted to allow air to be supplied to a pre-determined number of second nozzles so as to allow localised fluidisation of the bed adjacent to the outlets of the first nozzles. Preferably the distributor plate comprises a valve or valves, most preferably proportional electro-mechanical valves, which allow the fluidising air to be fed sequentially to an increasing number of the predetermined second nozzles as the bed temperature increases.

The nozzle outlets for liquid fuel are preferably of the type disclosed in our GB patent nos. 1368352 and 1487391. The outlet heads of the nozzles can be of any suitable shape. The area of the nozzle outlets are designed to give good distribution of fuel and air. The nozzle density is varied to meet specific combustor requirements. The nozzle outlets preferably comprise lateral passageways through the sides of the nozzle body.

The fluidised bed preferably comprises mineral particles of a size range and bulk density appropriate to the velocity of the fluidising gas. Suitable materials include sand, dolomite and limestone.

The liquid fuel is preferably ignited directly for example by means of a spark igniter.

Preferably the liquid fuel is not ignited until the liquid fuel supply has been switched on for a short period e.g. a few seconds. This pre-conditioning of the bed material with the liquid fuel appears to give a better start-up performance.

The fluidising air is generally supplied from one or more fans. When the bed is cold, a more powerful air supply is required than when the bed is at its operating temperature.

Thus, for the start-up procedure it is preferable that the air supply fans are sufficiently powerful to fluidise a cold bed. However, if this is not possible the bed may still be ignited by using the fans to fluidise at least that part of the bed adjacent to the liquid fuel outlets. This enables localised ignition to occur and heat is gradually transferred from the hot zone of the bed to the cold zones until the whole bed is fluidised.

The liquid fuel supply to the bed is preferably controlled by valves, the flow rate preferably being regulated automatically by sensors detecting the bed temperature and composi tion of the exhaust gases. The liquid fuel may be supplied to all the liquid fuel injection nozzles simultaneously provided there in sufficient air to fluidise the bed regions adjacent to the liquid fuel nozzle outlets or alternatively the liquid fuel may be fed to one nozzle at a time.

The rate of temperature rise required is dependent upon the application for which the fluidised bed combustor is being used. Thus, for heat transfer applications a temperature rise of 1cG/minute is suitable whereas a faster rate could be used for incinerator applications.

Also the rate of temperature rise should be consistent with operation over the desired excess air range. Thus, it is desirable on the one hand to avoid operation at sub-stoichiometric mixtures as incomplete combustion, agglomeration, smoke etc may occur and on the other hand use of too much excess air leads to cooling of the bed.

The invention will now be described by way of example only with reference to the accompanying drawing.

The drawing shows a vertical section through a fluid bed combustor having liquid fuel injection and having a divided wind box for the fluidising air supply to the bed.

A fluidised bed combustor 1 comprises a distributor plate 2 above which lies a bed of sand 3 having particle sizes ranging from 0.6 to 1.2 mms. The distributor plate 2 also has a number of nozzles 4 each having outlets in the bed which are connected to an air supply 8.

A fuel injection nozzle 5 passes through the distributor plate 2 into the bed and is connected to a kerosene or gas oil supply 6 (of 5 to 10 psi). The outlets of nozzle 5 are located above the level of the outlets of the fluidising air nozzles 4 in the bed 3.

The lower walls of the combustor are lagged with insulation (not shown) to allow faster warm up and efficient operation of the combustor.

The combustor is also equipped with a flue gas sampling probes and thermocouples for continuously determining the combustion conditions in the combustor.

The air supply chamber comprises inner and outer wind boxes 10, 11 which are connected to the air supply 8. By means of a proportional electro-mechanical valve 9 governed by temperature of bed 3, the air supply 8 which is initially connected to the inner wind box 10 may be connected also to the outer wind box 11 when the bed temperature after ignition of the liquid fuel is sufficiently high.

During use of the combustor, a cold air supply 8 was passed into inner and wind box 10 of the distributor plate 2 and the flow regulated to give a fluidising velocity in the region of the outlets of the fuel injection nozzle 5 of about 0.5 metres/sec at an ambient temperature of about 10"C. Subsequent to this fluidisation, liquid fuel (kerosene or gas oil) was supplied from tube 6 to the outlets of nozzle 5. The climbing oil film technique was used in which the liquid fuel is supplied by means of a valve to the interior of in injector nozzle 5. The flow of air from air supply 7 along the nozzle 5 causes a climbing oil film to pass up the tube and eventually pass in the form of a liquid fuel/air mixture into the bed 3. The technique is more fully described in our UK patent numbers 1 368352 and 1487391.Fuel (kerosene or gas oil) was supplied from the fuel injector 5 to the bed 1 at a rate equivalent to an air fuel ratio of about the stoichiometric value.

After an amount of fuel equivalent to 0.1 to 0.2% by weight of the total bed weight had been injected, the bed 1 was ignited by passing propane gas through nozzles (not shown) in the distributor plate 2 with the fluidising air and lighting the combustible propane/air mixture above the bed. The flame struck back into the mass of the bed under the prevailing flow conditions and ignited the main fuel supply which was coating bed particles subsequent to its entry into the bed 1 from injector 5.

The propane flow was then turned off when combustion of fuel had been established in the bed and the rate of temperature increase of the bed was regulated by adjustment of the air and fuel flow rates, the fluidising velocity of the air being maintained at about 1 metre/sec as the temperature increase.

When the temperature of the bed 1 in the region above the inner wind box 10 had attained a pre-determined value by heat transfer from the fluidised region of the bed 1 the valve 9 opened and the fluidising air supply 8 was connected to the outer wind box 11 and the remainder of the fluidising air nozzles then enabled the whole of the bed 1 to become fluidised.

Clearly more than one fuel injection nozzle may be used by appropriate arrangement of the fluidising air supplies. Also the use of propane gas for ignition of the main fuel supply is not essential and alternative ignition means may be used such as spark ignition etc.

Claims (14)

1. A method of igniting a fluidised bed combustor when the bed is in a cold state comprising the steps of (a) supplying a liquid fuel at a pre-determined rate to one or more first nozzles having their outlet or outlets within the fluidised bed (b) supplying a fluidising gas containing oxygen to second nozzles so as to fluidise at least the region of the bed adjacent to the outlets of the first nozzles (c) igniting the liquid fuel (d) regulating the liquid fuel flow and fluidising gas containing oxygen fuel supply so as to obtain a controlled rise in the temperature of the fluidised bed until the whole bed achieves the hot fluidised condition.

2. A method according to claim 1 in which the liquid fuel is supplied to the first nozzle by feeding the liquid fuel to the interior surface of the first nozzle and passing fluidising gas containing gas along the interior surface of the first nozzle so as to form a climbing film of liquid fuel which subsequently emerges from the first nozzle outlet into the fluidised bed.

3. A method according to claim 1 in which the liquid fuel is injected in the form of a fine spray into the fluidising gas containing oxygen supply upstream of the nozzle outlet and passing the mixture through the outlet and into the bed.

4. A method according to any of the claims 1 to 3 in which liquid fuel is fed to the bed for a short time prior to igniting the liquid fuel.

5. A method according to any of claims 1 or 4 in which the liquid fuel is kerosene or gas oil.

6. A method according to any of the preceding claims in which the fluidising gas containing oxygen is air.

7. A method according to any of claims 1 to 6 having means for passing fluidising gas containing oxygen to a pre-determined number of second nozzles, the pre-determined second nozzles being those immediately adjacent to the outlets of the first nozzles.

8. A method according to any of the preceding claims in which the fluidised bed combustor comprises a distributor plate for supplying fluidising gas containing oxygen to the bed, the bed lying above the distributor plate.

9. A method according to claim 8 in which the distributor plate comprises a fluidising gas containing oxygen chamber, the chamber having partitioning means adapted to allow the fluidising gas containing oxygen to be supplied to a pre-determined number of second nozzles so as to allow localised fluidisation of the bed adjacent to the outlets of the first nozzles.

1 0. A method according to claim 9 in which the distributor plate comprises a valve or valves allowing the fluidising gas containing oxygen to be fed sequentially to an increasing number of pre-determined second nozzles as the bed temperature rises.

11. A method according to claim 10 in which the valve or valves are proportional electro-mechanical valves.

1 2. A method according to any of the preceding claims in which the liquid fuel is ignited by means of an auxiliary burner, a spark igniter, or by passing propane gas and air through the nozzles and igniting the resultant combustile mixture at the bed surface.

1 3. A method according to any of the preceding claims in which the liquid fuel supply is regulated automatically by sensors responsive to bed temperature and/or composition of the exhaust gases.

14. A method of igniting a fluidised bed combustor as hereinbefore described and with reference to the accompanying drawings.