GB2141815A

GB2141815A - Method and apparatus for burning fuel

Info

Publication number: GB2141815A
Application number: GB08413620A
Authority: GB
Inventors: Colin Moore; Dr David Paul Jenkins
Original assignee: BOC Group Ltd
Current assignee: BOC Group Ltd
Priority date: 1983-06-16
Filing date: 1984-05-29
Publication date: 1985-01-03
Also published as: GB8413620D0; GB2141815B

Abstract

A liquid fuel medium, typically comprising pulverised coal in suspension in a liquid carrier such as a fuel oil or water is supplied by a pump 10 to burner 2. During operation of the burner, a combustible fluid such as propane is added by an injector 20 to a stream of the liquid fuel medium passing to the burner 2. The propane dissolves in the fuel oil. The liquid fuel medium issues from the burner and the dissolved propane flashes off preferentially, causing atomisation of the liquid and thus facilitates combustion of the fluid. To start the burner up, the propane is supplied alone, a stable flame created, and then liquid fuel medium is fed to the burner. The rate of feeding liquid fuel medium to the burner is increased to a maximum and the rate of supply of propane reduced to a minimum, e.g. merely that required for the purposes of atomisation. Oxygen, or an oxygen containing gas is supplied to the burner from a source 30. <IMAGE>

Description

SPECIFICATION Method and apparatus for burning fuel This invention relates to a method and apparatus for burning fuel. It is particularly but not exclusively concerned with the combusion of a fuel comprising a particulate carboniferous material in suspension in a liquid carrier which may itself be combustible (e.g. oil) or non-combustible (e.g. water) typically using substantially pure oxygen or a gas mixture rich in oxygen to support combustion of the fuel.

Potentially, pulverised coal or other such particulate combustible material offers a cheap source of fuel.

Our copending application No. 11130 (published as 2099132 A) provides a method of and burner for burning pulverised or particulate solid fuel, both of which use oxygen or a gas mixture relatively rich in oxygen (typically oxygen-enriched air) to support combustion of the fuel. This practice offers substantial advantages over the use of air to support such combustion and these advantages are discussed in our corresponding application No, 2099 132A. A burner for burning solid fuel in a gaseous carrier is particularly described in our copending application and the use of liquid carrier is also disclosed.

When a liquid carrier is used it is desirable to atomise the liquid so as to facilitate combustion of the fuel, just as it is desirable to atomise a liquid fuel in a conventional fuel burner. However, existing methods of atomising the liquid fuel in a conventional fuel burner are we believe unsuitable for use in a burner adapted to burn a particulate or pulverised solid fuel suspended in a liquid carrier using oxygen or oxygen-enriched air to support combustion of the fuel. For example, the use of a mechanical device in the burner, such as means for imparting a swirling motion to the liquid, is believed to be undesirable as the fuel particles may tend to clog in passages in the atomiser or abrade the atomiser.Moreover, difficulties may arise in achieving and maintaining ignition unless a conventionl burner is used to impart heat to the atomised liquid until a stable flame is obtained. Such a need for an auxiliary burner is likely to be particularly marked if the carrier comprises water. The use of steam or air as an atomising agent is also possible, but would require the use of a complex nozzle typically having six or more restricted non-axial outlet orifices for the liquid carrying the fuel. Moreover, there would still be the need to provide an auxiliary burner at start up, particularly in the instance of steam being used as the atomising agent as heat is required to cause the steam to dissociate into hydrogen and oxygen.

The invention thus aims at providing at least a partial solution to the problem of providing a simple and reliable means of atomisation for a burner particularly but not necessarily of particulate or pulverised fuel, which means also keeps to a minimum the requirements for the use of an auxiliary burner of conventional kind at start-up to obtain a stable flame.

According to the present invention there is provided a method of burning a liquid fuel medium (as herein defined), in which method a stream of the liquid fuel medium is fed to a burner which has an outlet nozzle from which the liquid issues and is atomised, is directed oxygen-containing gas (as herein defined) at the atomised liquid to support combustion of the fuel, and in which method a medium comprising at least one auxiliary combustible fluid is introduced into said medium so as to effect or facilitate atomisation of the stream or otherwise to assist in its combustion.

The invention also provides apparatus for use in such method comprising a burner having means defining a passage for a liquid fuel medium (as herein defined), said passage terminating in a nozzle from which the liquid fuel medium issues in operation of the burner, and one or more passages for directing oxygen containing gas (as herein defined) into atomised liquid fuel medium that has left the nozzle; means for feeding the liquid fuel medium to the burner; and means for introducing an atomising medium comprising at least one combustible fluid into the liquid fuel medium.

Suitable oxygen containing gas for use in the invention are substantially pure oxygen, oxygen-enriched air or other gas mixture (such as one of argon and oxygen) whose oxygen content is appreciably greater than the oxygen content of air. Air may be used but its use is not preferred.

The term "liquid fuel medium" as used herein encompasses conventional liquid fuels such as heavy fuel oil, and suspensions of pulverised or particulate solid fuel in a liquid carrier which may be combustible (e.g.

heavy fuel oil) or non-combustible (e.g. water) or a mixture of combustible and non-combustible liquids. The method and apparatus according to the invention are primarily directed at the combustion of solid fuel in a liquid carrier, but may if desired be used with a simple liquid fuel such as heavy fuel oil.

The combustible fluid is preferably selected from one of the following kinds: (a) A gas or liquid which dissolved in or is miscible with the liquid fuel medium and which is more volatile than the liquid fuel medium (or its liquid component). If the liquid fuel medium comprises a fuel oil, a gaseous hydrocarbon such as natural gas, methane, ethane, propane or butane may be employed. If the liquid fuel medium includes water as a carrierforthe pulverised or solid particulate material, the combustible fluid may be combustible compound of hydrogen, carbon and oxygen, for example an alcohol (eg, methanol or ethanol).

(b) A liquid which is immiscible with the liquid fuel medium and which is more volatile than the liquid fuel medium. For example, if the liquid fuel medium includes water as a carrier of solid pulverised or particulate material, the combustible fluid may be a liquid hydrocarbon, e.g. hexane or commercial 'paraffin'. The liquid hydrocarbon is typically introduced into the liquid fuel medium such that droplets of it are formed therein.

(c) A gas which is insoluble in the liquid fuel medium. For example, if the liquid fuel medium includes water as a carrier of solid pulverised or particulate material, the combustible fluid may be a gaseous hydrocarbon such as natural gas, methane, ethane, propane or butanes and bubbles or such gas are formed therein.

Generally, we prefer atomising agents of kind (a) and (b) to those of kind (c) as the former rely for their atomising effect on the expansion of the fluid (through its being flashed off or vaporised) as the liquid fuel medium leaves the burner, while the latter relies on the physical division of the stream of liquid fuel medium by gas bubbles. Therefore, comparatively less of the former kinds of atomising media (or agents) than the latter are required for the purpose ofatomising a given liquid fuel medium.

If the liquid fuel medium comprises a solid fuel in suspension in a carrier liquid, these of an atomising agent of the first kind offers the advantage that in being flashed off or vaporised as aforementioned it assists in vaporising carrier liquid surrounding solid fuel particles and thereby helps to expose these particles to the oxygen - containing gas.

The combustible fluid may be used in conjunction with one or more other atomising agents (for example air or steam).

The combustible fluid may be pre-dissolved in the liquid fuel medium but is preferably introduced into the aforesaid stream thereof.

In order to start operation of the burner, the combustible fluid is preferably supplied to the burner and a stable flame created and the liquid fuel medium supply started only once the stable flame has been created.

During such initial period, the combustible fluid is preferably supplied at a rate in excess of subsequent requirements for the purpose of atomisation only. Since the combustible fluid is typically relatively easy to ignite and burn, a stable flame may typically be established more or less immediately upon ignition. The liquid fuel medium may then be supplied to the flame and its rate of supply typically gradually increased to a chosen maximum while the rate of supply of combustible fluid is generally decreased to a chosen minimum (typically merely that, if any, needed for the purposes of atomisation). The chosen minimum is typically a rate such that the combustible fluid supplies up to 15% (e.g. 2 to 10%) of the total thermal energy supplied by the liquid fuel medium and combustible fluid to the burner.If the liquid fuel medium comprises a heavy fuel oil the combustible fluid may typically supply from 5 to 8% of the total thermal energy supplied by the liquid fuel medium and combustible fluid to the burner. Once a stable flame has been created, some of the combustible fluid may be replaced by a conventional non-combustible atomising agent such as steam or air.

Indeed, once the liquid fuel medium supply rate has been increased to the said maximum, the combustible fluid may be completely replaced by air or steam. Moreover, in some situations (e.g. in the heating of enclosures such as boiler, furnaces or kilms, once the enclosure has been raised to its full operating temperature it may be possible to discontinue all supply of atomising agent be it the combustible fluid, air or steam or mixtures thereof).

One potential disadvantage of substituting a conventional atomising agent for part of all of the combustible fluid is that the maximum attainable flame temperature is reduced.

If the combustible fluid is wholly replaced by a conventional atomising agent, as described above, it is to be appreciated that the combustible fluid will function as an atomising agent only for a fraction of the total operating time.

Variations in the flow rates of the combustible fluid, and any other atomising medium and the liquid fuel medium may be made manually or automatically. If desired, pneumatic, hydraulic, elecrical or electronic controls may be employed and may embody a computer or micro-processor.

The or each atomising medium is preferably introduced into the liquid fuel medium upstream of the burner. The combustible fluid may be premixed with the liquid fuel medium or introduced into the said stream of the liquid fuel medium at a supply pressure 2 to 15 psi above the supply pressure of the liquid fuel medium.

The liquid fuel medium preferably comprises a suspension of particles of coal, coke or other form of carbon in a liquid carrier. The particles are preferably of a size less than 100 mesh. Typically, solid particles constitute from 10 to 90% preferably from 60 to 80% by weight of the total weight of the liquid fuel medium.

These particles may consist entirely of fuel particles or may include a chosen proportion of one or more additives for the purpose of chemically modifying the ash produced as a result of the combustion of such fuel (examples of such additives are described in ourcopending application No. 2099132A) orforthe purpose of forming a stable suspension or assisting in pumping.

The carrier may be a fuel oil having a viscosity in the range 20 to 10,000 seconds Redwood (and preferably a viscosity in the range 100 to 5,000 seconds Redwood, e.g. 2500 or 3500 seconds Redwood) or may be water, or may be an emulsion of oil and water and may therefore include one or more emulsifying agents).

The nozzle of the burner preferably has a single outlet orifice or passage, whose axis typically extends parallel to the axis of the burner or is preferably coaxial therewith. The diameter of the passage can readily be selected in accordance with the thermal output of the burner to be such that there is substantially no risk of the passage or orifice becoming blocked in use. It is of course possible to use a great number of such passages (say from two to four) all of such size as generally to preclude blockages in use particularly in the example of a burner with a large thermal output (e.g. above 21/2 MW). Burners having such nozzles come within the scope of this invention.

The burner may, if desired, be provided with an incandescent spark ignitor or means for forming a pilot flame so as to effect ignition, or may be lit manually (e.g. by means of a taper). The burner will typically be water-cooled.

Preferably, substantially all the requirements for oxygen of the flame are met by the oxygen containing gas supplied to the burner.

The method and apparatus according to the invention will now be described by way of examples with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram illustrating apparatus according to the invention; Figure 2 is a prospective view, partly cut away, showing the burner illustrated in Figure 1.

Figure 3 is a side-elevation, partly in section, of an alternative form of nozzle for the burner shown in Figure 2; and Figure 4 is an end view of the nozzle shown in Figure 3.

Referring to Figure 1 of the drawings, the apparatus includes a burner 2 that extends through wall floor of an enclosure 6. The enclosure may be a boiler, kiln or furnace. A burner 2 is adapted to be supplied with a liquid fuel medium. The pipeline 8 is provided for this purpose and terminates in the burner 2 at one its ends, and at its other end in the outlet of a pump 10. The inlet or suction side of pump 10 is connected to a source 12 of the liquid fuel medium. Typically, this source 12 comprises a mixer of conventional type in which a stable suspension of pulverised coal is formed in a liquid such as fuel oil or water. Downsteam of the pump 10 there is provided means 20 for injecting atomising medium into the liquid being pumped along the pipeline 8 in operation of the apparatus shown in Figure 1.Typically, the injection means 20 comprises a pipeline 22 which terminates at one end in the pipeline 8 and at its other end in communication with a source 24 of atomising medium. The injection means may also include a metering system (not shown) of conventional kind, for example including an adjustable venturi (not shown). Where the atomising medium consists of a fuel gas the source 24 may comprise one or more cylinders of such fuel gas. If it is desired to mix the fuel gas with steam to form the atomising medium during part of the operation of the burner 2, then there may be means (not shown) provided for mixing the steam with the fuel gas upstream the of pipeline 8.

The burner 2 also has associated therewith a pipe 28 for feeding oxygen to it from a source 30. The source 30 may typically comprise a vacuum insulated container of liquid oxygen fitted with a vaporiser (both not shown). In operation of the burner, liquid oxygen passes through the vaporiser and is thereby vaporised to form gaseous oxygen at the desired pressure.

Flow control valves and the like are omitted from Figure 1 but are typically provided so as to control the flow of fluids into the burner 2. In addition, suitable shut-off and non return valves may be provided as is conventional in burner supply systems.

The burner 2 is illustrated in Figure 2. It includes an elongate passage 32 coaxial with the longitudinal axis of the burner 2. The passage 32 terminates in a head or nozzle 34 which is situated in an annular passage 36 for oxygen coaxial with the passage 32 and surrounding it.

Typically, the outer wall 40 of the passage 36 is defined by an outer shell 42 of the burner 2, this outer shell being provided with an annular passage 38 through which coolant (e.g. water can be circulated. As shown in Figure 2, the nozzle 34 is generally cylindrical in shape having a single inner passage or orifice 44 communicating with the passage 32 and coaxial therewith, and a ring of orifices 46 disposed around the passage 44 and communicating with the oxygen passage 36. The nozzle 34 engages the wall 40 and is in good heat-conductive relationship with it. The exposed face 48 of the nozzle 34 may be coplanar with the burner tip 50 or inset relatively thereto as shown in Figure 2.

An alternative embodiment of nozzle 34 is shown in Figures 3 and 4. The nozzle 34 shown in Figures 3 and 4 is of convergent- divergent section, having a frusto-conical first part 52 which diverges in the direction of the burner tip 50, a right cylindrical second part 54, and a frusto-conical third part 56 that converges in the direction of the burner tip 50. The part 54 has three integral lugs 58 that engage the wall 40 and are in good heat-conductive relationship therewith. The nozzle 34 is preferably monolithic, that is of one-piece construction, typically being formed of copper. The nozzle 34 has a single inner passage or orifice 44 formed there-through. The orifice 44 is coaxial with the passage 32 and communicates therewith.The orifice 44 can be formed of relatively large diameter (typically in the order of one-quarter of an inch) thereby precluding blockages of such orifice 44 by particles in the liquid fuel medium. In use, the oxygen passes through the passage 36 between the nozzle 34 and the wall 40.

During operation for the apparatus shown in Figure 1 and Figure 2, or in Figures 1 and 2 as modified by Figure 3 and 4, a liquid fuel medium comprising a solid fuel (typically, pulverised coal) and a liquid carrier (typically, a fuel oil) is supplied at a chosen pressure to the burner 2. A combustible fluid atomising medium, typically propane, is injected into the liquid fuel medium from the injection means 20 and dissolves in the liquid fuel medium. (In other examples of the method according to the intervention the combustible fluid may form droplets or bubbles in the liquid fuel medium as herein before described.) The liquid fuel medium passes into the passage 32 and then through the orifice 44 in the nozzle 34. After leaving the nozzle 34, the propane 'flashes' out of solution preferentially on encountering the relatively hot environment outside the burner 2. This causes the liquid fuel medium to be atomised into a fine divergent spray. Evaporation of liquid around individual solid fuel particles is also assisted by the propane flashing out of solution. The spray encounters oxygen issuing from the passage 36 of the burner 2 and issuing from its tip. Combustion of the combustible components of the fuel and propane is thus able to take place. Typically, the relative rates of supply of propane and, say, heavy fuel and oil-pulverised coal liquid fuel medium are such that the propane contributes from 5 to 8% of the total thermal energy output of the burner and the liquid fuel medium supplies the balance. The oxygen is typically supplied to the passage 36 at a rate of 90 to 110% of that required for stoichiometric oxidation of the fuel medium and propane.

The description in the preceding paragraph is of the 'steady-state' operations of the burner. To start operation of the burner, just propane is supplied to the passage 32 of the burner, while oxygen is suppied to the passage 38. For example, propane may be supplied to the passage 32 at three times its 'steady state' rate and oxygen supplied to the passage 36 at a rate sufficient for stoichiometric combustion. Ignition of such a mixture to form the burner flame can be effected without difficulty (e.g. by application of a glowing taper to the propane issuing from the burner 2).Once ignited, the liquid fuel medium may begin to be supplied to the passage 32 with the propane and the rate of its supply increased gradually, with the rate of oxygen supply made, until the 'steady state' operating conditions are achieved (typically at an enclosure temperature of 500 to 6000C) The invention is further illustrated by the following example. The burner employed in this example produced 0.485 MW of thermal energy and had a nozzle of the kind shown in Figures 3 and 4.

Example 4 tonnes of coal having a chemical composition as set out in Table 1 were pulverised until 90% of the particles so produced were less than 200 mesh (British sieve size). The pulverised coal was then sieved to produce a coarse fraction and a fine fraction having a size distributor as set out in Table 2. The fine fraction was taken and mixed with a heavy fuel oil of properties as set out in Table 3 to form a composition containing 40% by weight of coal particles. At 'stable state operation' the flow rates of propane, liquid fuel medium, and oxygen are set out in Table 4. Oxyen was supplied at the rate required for stoichiometric combustion, at 90% of this rate and at 110% for this rate.

TABLE 1 Chemical Composition of Coal Classification Bituminous Type 701 Calorific value (as received 32540 KJ/kg Volatiles 35.4% Ash Content 4.6% Moisture (D.A.F.) 0.8% TABLE 2 Size Distribution of Coal % in given range Sieze size 106 microns 75 microns 53 microns Coarse Fraction (% passing 73.6% 57.8% 40.7% through sieve) Fine Fraction (% passing 88.7% 76.8% 50.7% through sieve) TABLE 3 Properties of Heavy Fuel Oil Viscosity 3 500 R S C V (set) 39800 KJ/kg Sulphur content 2.8% (% by weight) TABLE 4 Composition ofliquid fuel medium % by weight of heavy fuel oil 60 % by weight of coal 40 Calorific value ofliquid fuel medium KJ/kg 36900 Flow rate of liquid fuel medium kg/min 0.735 Flow rate of propane/ls-l 0.4 % age of total thermal energy input provided by propane 7.25 Heat release/kw 485 Oxygen flow rate for stoichiometric combustion 1/s 27.5 m3 min-1 1.65 for 90% stoichiometric combustion 1/s 24.75 m3min-1 1.49 for 110% stoichiometric combustion 1/s 30.25 m3 min-1 1.8

Claims

1. A method of burning a liquid fuel medium (as hereinbefore defined), in which method a stream of the liquid fuel medium is fed to a burner which has an outlet from which the liquid issues and is atomised, oxygen-containing gas (as hereinbefore defined) is directed at the atomised liquid to support combustion of the fuel and a medium comprising at least one auxiliary combustible fluid is introduced into said medium so as to effect or facilitate atomisation of the stream or otherwise assist in its combustion.

2. A method as claimed in claim 1,which the oxygen - containing gas is substantially pure oxygen or oxygen - enriched air.

3. A method as claimed in claim 1 or claim 2, in which to start operation of the burner, the combustible fluid is supplied thereto and a stable flame created, and only then is the liquid fuel medium supplied to the burner.

4. A method is claimed in claim 3, in which the rate of supply of liquid fuel medium is increased to a maximum and the rate of supply of combustible fluid reduced to a minimum or stopped.

5. A method as claimed in claim 4, in which the combustible fluid at its minimum flow rate supplies up to 15% of the total thermal energy supplied to the burner.

6. A method as claimed in claim 5, in which the combustible fluid at its minimum flow rate supplies from 2 to 10% the total energy supplied to the burner.

7. A method as claimed in any one of claims 3 to 6, in which after a stable flame is formed, at least part of the combustible fluid is replaced by a non-combustible atomising agent.

8. A method as claimed in claim 7 in which the non-combustible atomising agent is steam.

9. A method as claimed in any one of the preceding claims in which the combustible fluid is introduced into the said stream upstream of the burner.

10. A method as claimed in any one of the preceding claims, in which the said outlet is formed in a burner nozzle and is defined by from one to four passages through the nozzle.

11. A method as claimed in any one of the preceding claims in which the liquid fuel medium comprises a particulate or pulverised solid fuel in suspension in a carrier liquid.

12. A method as claimed in any one of the preceding claim, in which the combustible fluid is a gas which dissolves in the liquid fuel medium or a liquid fuel medium which dissolves in the liquid fuel medium and which is more volatile than the liquid fuel medium.

13. A method as claimed in aay one of claims 1 to 11 in which the combustible fluid is a liquid which is immiscible with the liquid fuel medium, droplets ofthe liquid being formed in the liquid fuel medium.

14. A method as claimed in any one of claims 1 to 11, in which the combustible fluid is a gas which is insoluble in the liquid fuel medium, bubbles of the gas being formed in the liquid fuel medium.

15. A method as claimed in any one of claims 1 to 11, in which the carrier liquid is a fuel oil.

16. A method as claimed in claim 15, in which the combustible fluid is selected from natural gas, propane, methane, ethane and butane and is dissolved therein.

17. A method as claimed in claim 14, in which the carrier liquid is water.

18. A method as claimed in claim 17, in which the combustible fluid is a compound of carbon, hydrogen and oxygen.

19. A method as claimed in claim 18, in which said compound is an alcohol, the alcohol being dissolved in the water.

20. A method of burning a liquid fuel medium substantially as herein described with reference to the accompanying drawings.

21. A method of burning a liquid fuel medium substantially as herein described in the Example.

22. Apparatus for use in a method as claimed in any one of the preceding claims, comprising a burner having means defining a passage for a liquid fuel medium (as herein defined), said passage terminating in a nozzle from which the liquid fuel issues in operation of the burner, and one or more passages for directing oxygen - containing gas into atomised liquid fuel medium that has left the nozzle; means for feeding the liquid fuel medium to the burner; and means for introducing an atomising medium comprising at least one combustible fluid into the liquid fuel medium.

23. Apparatus for burning a liquid fuel medium substantially as herein described with reference to and as shown in Figures 1 and 2, or Figures 1,3 and 4 of the accompanying drawings.

24. A burner for use in a method as claimed in any one of claims 1 to 21, or for use in an apparatus as claimed in claim 22 or claim 23, said burner including a nozzle defining from one to four outlet passages for liquid fuel medium.

25. A burner substantially as described herein with reference to and as shown in Figure 2 or Figures 3 and 4 of the accompanying drawings.