GB1588078A - Direct ignition of pulverized coal - Google Patents

Description

(54) DIRECT IGNITION OF PULVERIZED COAL (71) We, COMBUSTION ENGINEERING, INC., a corporation organized and existing under the laws of the State of Delaware, United States of America, of 1000 Prospect Hill Road, Windsor, Connecticut, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to a burner designed for the combustion of pulverized coal and particularly to such burners as may be utilized in coal-fired boilers of steam generators. More specifically, the invention is directed to a method for igniting pulverized coal in the absence of a substantial energy input derived from the combustion of a liquid or gaseous fuel.Accordingly, the general objects of the invention are to provide apparatus and method of such character.

Because of cost and availability, it is becoming increasingly desirable to utilize coal rather than natural gas or oil in electricity generating facilities. Present coal-fired steam generators use premium liquid and gaseous fuels to provide both ignition and low-load flame-stabilizing energy. The required amount of these auxiliary premium fuels is significant. Accordingly, a need exists for a means of reducing the amount of auxiliary fuels needed in pulverized-coal boilers.

In accordance with the invention, there is provided a method for causing ignition and sustaining combustion of a pulverized coal in a combustion area which has not been preheated, comprising the steps of introducing into the combustion area a fuel stream consisting essentially of a mixture of pulverized coal and air, said mixture having an airto-coal transport weight ratio at or below substantially 1.0 prior to discharge into the combustion area; delivering energy to the mixture in the combustion area in order to ignite the coal in the said mixture; and causing recirculation of hot combustion products back toward the point of introduction of the fuel stream into the combustion area, the combustion products thereby contributing to the ignition energy in the combustion area.

The step of causing recirculation may comprise establishing a flow of secondary air generally coaxial with the fuel stream; and imparting a rotational moment to the secondary air to cause a swirling air flow to envelop the fuel stream in the region of delivery of ignition energy.

The step of establishing a flow of secondary air may further include verifying the presence of flame in the combustion area; and initiating the flow of secondary air when the presence of flame has been verified.

The step of delivering energy may comprise establishing an intermittent electrical discharge.

Preferably, the air to coal weight ratio of the fuel stream may be selected to be less than 0.5.

Apparatus embodying the method of the invention is hereinafter described, by way of example, with reference to the accompany drawings wherein like references numerals refer to like elements in the several figures, and in which: Figure 1 is a cross-sectional view of an arcignited pulverized coal burner; Figure 2 is a cross-sectional view of a pulverized coal feed system which may be associated with the burner of Figure 1; and Figure 3 is a front elevation view of the feeder system of Figure 2.

The apparatus shown in the accompanying drawings constitutes a representative means for accomplishing the direct ignition of a stream of pulverized coal and air in accordance with the invention and without any significant consumption of petroleum or natural gas. The invention relies on having a dense phase coal/air mixture wherein the transport-air stream-to-coal weight ratio, measured in a delivery conduit upstream of a combustion zone, is 1.0 or less. The ignition energy source is positioned so as to be either in or insertable in the flowing air/coal stream in the combustion zone. The energy delivered to the air/coal mixture by the ignition energy source ignites the coal particles.

Considering the case where the ignition energy source comprises a high energy electric arc, the ignitor thus being operated in a pulsed mode, a series of flame pockets are created.

A secondary air flow to the combustion zone is also established through burner secondary-air registers. The burner secondary-air registers are designed, in a manner known in the art, so as to establish a region of recirculating air and combustion products (hot gases) whereby the pockets of burning coal are recirculated back toward the point of initial coal injection and the energy on the recirculation region will increase until the flame becomes self-sustaining.

A method embodying the invention has been successfully carried out employing a high energy electric arc as the ignition energy source. However, the ignition energy source may also be a resistance heater or heaters or a hydrocarbon fuelled pilot torch.with minimal energy consumption. In a preferred embodiment, the ignitor will be removed from the flame region once the existence of flame has been verified. When a. high energy arc supplies the ignition energy, secondary air flow will be delayed until the existence of flame has been verified.

With reference now to FIGURE 1, a burner in accordance with an embodiment of the invention is shown. A coal pipe 16 is employed to convey coal pneumatically to the ignition zone in the burner. Accordingly, as the apparatus is shown in FIGURE 1, the left hand end of coal pipe 16 is in communication with the coal feeder of' FIGURES 2 and 3 while the right hand end of coal pipe 16 terminates at a hollow-cone diffuser 22 which is mounted from coal pipe 16 by means of supports 21. An ignitor 23 will be positioned immediately downstream of the discharge end of coal pipe 16. In the disclosed. embodiment of the invention the ignitor 23, enters through the side of the burner and comprises a- high-energy arc ignitor similar to the type presently used for igniting oil.It is to be noted that any ignition source which imparts sufficient energy to heat the reactants enough to ignite them may be used. Accordingly, a resistance heater or small pilot torch fuelled by natural gas could be. employed in place of the high energy arc ignitor. The high energy arc ignitor is, however; preferred because of its reliability and controllability; Ignitor 23, as shown in FIGURE 1,. will typically be retractably mounted so that it can be removed from the combustion zone into a protective area after the coal has been ignited.

The burner also includes a secondary air supply conduit 20 which is coaxial with coal pipe 16. Conduit 20 communicates with an air chamber 14 which will typically be a cylindrical chamber somewhat larger in diammeter than that of conduit 20. Air chamber 14 contains a plurality of vanes 12. Vanes 12 are arranged to impart a swirl to air entering conduit 20 from chamber 14. An air inle.t: duct 10 leads to air chamber 14 from. a pressurized air supply, not shown.Air con.- duit 20 terminates in a refractory-linediquar- rel 24 which defines a divergent nozzle In. a typical example of the apparatus the coal pipe. 16 had a one inch inner diameter, conduit 20 had a six inch inner diameter and quarrel 24 had a thirteen inch diameter at its open end and an angle of divergence of 35 .

FIGURES 2 and 3, which will be discussed simultaneously, show a pulverize.d-.

coal feed system for supplying a coal. air mixture to the coal pipe or fuel conduit 16.

The. feed: system consists of a pulverized-coal.

hopper 40 that can be supplied by any of a number of means known in the art. Prefer~ bly, hopper 40 should be sized to store sufficient pulverized coal 41 to supply the burner throughout the warm-up period of the furnace in which the burner is. to be used: Hopper 40' communicates with gravimeteric: feeder 43. Feeder 43 consists of a variable- speed feed device 42, a weight-sensitive conveyor 44 and appropriate control circuitry, not shown. The speed of rotation of variable-speed feeder 42. determines the amount of coal allowed to drop onto weightsensitive conveyor 44, and the weight sensed by weight-sensitive conveyor 44 controls the speed of this rotation.Gravimetric feeder 43 introduces coal into a rotary air-lock feeder 46 at a constant. rate; Rotary air-lock feeder 46 is a cylindrical chamber with blades 47 that approach an airtight fit with the chamber. At the bottom of the chamber are entrance. opening 48 and exit opening 49. The fit of blades 47 is such that there is almost no free air path between openings 48 or 49 and feeder 43. Accordingly, it is possible for an air stream entering opening 48 to continue out through opening 49 without being deflected into gravimetric feeder 43. The rotation of blades 47 carries pulverized coal dropped onto blades 47 by gravimetric feeder 43 into the air path between openings 48 and 49. Compressed air is supplied to feeder 46 by an appropriate source 50 at a controlled rate whereby a coalair mixture having a predetermined air-tocoal weight ratio will be supplied through conduit 16 to the. bumer.

The coal-air mixture measured in conduit 16 suitably has an air-to-coal weight ratio of 1.0 or less and preferably 0.5 or less.

Using sub-bituminous C rank coal that had been pulverized to 70 percent minus 200 mesh was employed, the upper limit on airto-coal weight ratio was 1.0. The optimum air-to-coal weight ratio will vary with coal type.

In order to operate the burner of Figure 1, ignitor 23 is moved to its inserted position and turned on. Employing an arc ignitor, sparks produced by the ignitor having an energy contact of approximately 25 joules, lasting about 10 microseconds each, and having. a repetition rate of 10 Hertz have been successfully employed. Compressor 50 is turned on once the ignitor has begun operation, and gravimetric feeder 43 is also started. The compressed air flowing through rotary-air-lock feeder 46 entrains measured amounts of pulverized coal and carries it through conduit 16 and hollow-cone diffuser 22. While it is possible to operate the burner of Figure 1 without hollow-cone diffuser 22, it is considered desirable to include diffuser 22 in older to introduce a minor amount of recirculation during the ignition stage of operation.The coal-air mixture brought into the vicinity of ignitor 23 is ignited by the energy imparted by the ignitor and the resultant flame propagates through the coal.

As a result, ignition occurs, and a relatively unsteady flame exists at the outlet of the burner. At this point, an observer 26 or an automatic flame-detection system determines that ignition has occurred and causes a secondary air (ambient or heated) flow through air inlet duct 10. Vanes 12 introduce a rotation into the air flow, and this results in a spiraling, or swirling, stream of air that flows down conduit 20 and through quarrel 24. Quarrel 24 is a divergent nozzle that enhances the recirculating effect that naturally occurs due to the vortical flow of air.

The swirling stream of air envelops the combustion zone, and as a result the hot combustion products are drawn back into the region of fresh coal injection. The observable effect of this recirculation is that the flame becomes steady, and the stability of the flame is such that ignitor 23 can be turned off and withdrawn. Thus retracted, ignitor 23 remains in a protective area, thereby preventing damage to the ignitor due to the intense heat of combustion.

It is thought that the direct ignition of pulverized coal provided by the invention is successful because it provides appropriate conditions for propagation of flame within the flowing coal stream after the ignitor has caused ignition of some of the coal particles; above a substantially 1.0 air-to-coal weight ratio, propagation 6f the flame is difficult with unheated air, and direct ignition of pulverized coal by an arc ignitor accordingly does not work effectively. On furnace startup, pre-heated air is not available and can be produced only through the expenditure of a significant quantity of energy and/or liquid or gaseous fuel. Combined with the provision of proper conditions for propagation of the flame is the provision of a recirculation zone that contributes to stability of the resultant coal flame.The recirculation causes hot products to be drawn back into the combustion zone, thereby causing the flame to provide its own ignition energy. Whatever the reasons for its success, however, the invention does provide a means for satisfactory direct ignition of flowing pulverized coal.

The burner of Figure 1 can be used as a warm-up burner for utility boilers. In utilityboiler operation, it is necessary for the boiler to be brought to an elevated temperature in order for its conventional coal burners to work properly. The burners of the invention can be used to bring the furnace up to a temperature high enough for stable combustion in conventional burners. The invention can also be used for both ignition and lowload stabilization.

WHAT WE CLAIM IS: 1. A method for causing ignition and sustaining combustion of a pulverized coal in a combustion area which has not been preheated, comprising the steps of: introducing into the combustion area a fuel stream consisting essentially of a mixture of pulverized coal and air, said mixture having an air-to-coal transport weight ratio at or below substantially 1.0 prior to discharge into the combustion area; delivering energy to the mixture in the combustion area in order to ignite the coal in the said mixture; and causing recirculation of hot combustion products back toward the point of introduction of the fuel stream into the combustion area, the combustion products thereby contributing to the ignition energy in the combustion area.

2. A method according to Claim 1, in which the step of causing recirculation comprises: establishing a flow of secondary air generally coaxial with the fuel stream; and imparting a rotational moment to the secondary air to cause a swirling air flow to envelop the fuel stream in the region of delivery of ignition energy.

3. A method according to Claim 2 in which the step of establishing a flow of secondary air includes: verifying the presence of flame in the combustion area; and initiating the flow of secondary air when the presence of flame has been verified.

4. A method according to any one of Claims 1 to 3, in which the step of delivering energy comprises: establishing an intermittent electrical discharge.

5. A method according to any one of Claims 1 to 4, in which the air-to-coal weight ratio of the fuel stream is selected to be less than 0.5.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. In order to operate the burner of Figure 1, ignitor 23 is moved to its inserted position and turned on. Employing an arc ignitor, sparks produced by the ignitor having an energy contact of approximately 25 joules, lasting about 10 microseconds each, and having. a repetition rate of 10 Hertz have been successfully employed. Compressor 50 is turned on once the ignitor has begun operation, and gravimetric feeder 43 is also started. The compressed air flowing through rotary-air-lock feeder 46 entrains measured amounts of pulverized coal and carries it through conduit 16 and hollow-cone diffuser 22. While it is possible to operate the burner of Figure 1 without hollow-cone diffuser 22, it is considered desirable to include diffuser 22 in older to introduce a minor amount of recirculation during the ignition stage of operation.The coal-air mixture brought into the vicinity of ignitor 23 is ignited by the energy imparted by the ignitor and the resultant flame propagates through the coal. As a result, ignition occurs, and a relatively unsteady flame exists at the outlet of the burner. At this point, an observer 26 or an automatic flame-detection system determines that ignition has occurred and causes a secondary air (ambient or heated) flow through air inlet duct 10. Vanes 12 introduce a rotation into the air flow, and this results in a spiraling, or swirling, stream of air that flows down conduit 20 and through quarrel 24. Quarrel 24 is a divergent nozzle that enhances the recirculating effect that naturally occurs due to the vortical flow of air. The swirling stream of air envelops the combustion zone, and as a result the hot combustion products are drawn back into the region of fresh coal injection. The observable effect of this recirculation is that the flame becomes steady, and the stability of the flame is such that ignitor 23 can be turned off and withdrawn. Thus retracted, ignitor 23 remains in a protective area, thereby preventing damage to the ignitor due to the intense heat of combustion. It is thought that the direct ignition of pulverized coal provided by the invention is successful because it provides appropriate conditions for propagation of flame within the flowing coal stream after the ignitor has caused ignition of some of the coal particles; above a substantially 1.0 air-to-coal weight ratio, propagation 6f the flame is difficult with unheated air, and direct ignition of pulverized coal by an arc ignitor accordingly does not work effectively. On furnace startup, pre-heated air is not available and can be produced only through the expenditure of a significant quantity of energy and/or liquid or gaseous fuel. Combined with the provision of proper conditions for propagation of the flame is the provision of a recirculation zone that contributes to stability of the resultant coal flame.The recirculation causes hot products to be drawn back into the combustion zone, thereby causing the flame to provide its own ignition energy. Whatever the reasons for its success, however, the invention does provide a means for satisfactory direct ignition of flowing pulverized coal. The burner of Figure 1 can be used as a warm-up burner for utility boilers. In utilityboiler operation, it is necessary for the boiler to be brought to an elevated temperature in order for its conventional coal burners to work properly. The burners of the invention can be used to bring the furnace up to a temperature high enough for stable combustion in conventional burners. The invention can also be used for both ignition and lowload stabilization. WHAT WE CLAIM IS:

1. A method for causing ignition and sustaining combustion of a pulverized coal in a combustion area which has not been preheated, comprising the steps of: introducing into the combustion area a fuel stream consisting essentially of a mixture of pulverized coal and air, said mixture having an air-to-coal transport weight ratio at or below substantially 1.0 prior to discharge into the combustion area; delivering energy to the mixture in the combustion area in order to ignite the coal in the said mixture; and causing recirculation of hot combustion products back toward the point of introduction of the fuel stream into the combustion area, the combustion products thereby contributing to the ignition energy in the combustion area.

2. A method according to Claim 1, in which the step of causing recirculation comprises: establishing a flow of secondary air generally coaxial with the fuel stream; and imparting a rotational moment to the secondary air to cause a swirling air flow to envelop the fuel stream in the region of delivery of ignition energy.

3. A method according to Claim 2 in which the step of establishing a flow of secondary air includes: verifying the presence of flame in the combustion area; and initiating the flow of secondary air when the presence of flame has been verified.

4. A method according to any one of Claims 1 to 3, in which the step of delivering energy comprises: establishing an intermittent electrical discharge.

5. A method according to any one of Claims 1 to 4, in which the air-to-coal weight ratio of the fuel stream is selected to be less than 0.5.

6. A method of igniting and sustaining

ignition of pulverized coal substantially as hereinbefore described with reference to and as shown in the accompanying drawings.