GB1592736A - Burner assemblies - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO BURNER ASSEMBLIES (71) We, DAVID ETCHELLS (FUR NACES) LIMITED, a British Company of 20 Waterloo Street, Birmingham B25TF, 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: This invention is concerned with improvements in or relating to the burning of liquid hydro-carbons.

In the specification of U.K. Patent No.

1,299,928 there is described a burner assembly comprising fluid fuel supply means, passage means to supply combustion air to mix with the fuel supplied by the fuel supply means, the passage means being arranged to impart a swirling motion to the air passing therethrough so that it mixes thoroughly with the fuel, a heating element of ceramic material having a central passage arranged with one end adjacent to but spaced from said fuel supply means and positioned so as to receive the mixture of fuel and combustion air and so that in use the initial combustion of said mixture may take place within said central passage, and auxiliary passages in said heating element positioned around the central passage and extending from adjacent said one end of the central passage to adjacent the other end thereof.

Such a burner assembly is hereinafter referred to as being of the kind specified.

It is known that in burning liquid hydrocarbon fuels, there are two chemical reactions involved. One of these involves the cracking of the higher molecular weight hydro-carbons into smaller molecular weight hydro-carbons, and the other involves oxidation of hydro-carbons, producing carbon dioxide and water.

Where a mixture of liquid hydro-carbon fuel and air is burned directly these two reactions occur simultaneously, involving some direct oxidation of higher molecular weight hydro-carbons, and the production of some particulate carbon, which is combusted to produce carbon dioxide. This combustion process is identified by the production of a long, luminescent flame profile. Such a long, luminescent flame requires a large combustion chamber to effect heat transfer, and being associated with a relatively high production of unburned carbon, and/or partially oxidised carbon products.

However, if cracking of the hydrocarbons into relatively low fractions can be performed predominantly before oxidation commences, a flame is produced which is non-luminescent, due to the absence of burning carbon, and shorter, enabling heat exchange to be carried out over a relatively small region. Additionally, in view of the high combustion density, a non-luminescent flame is associated with a relatively low production of unburned carbon, and/or partially oxidised carbon products.

We have found that in a burner assembly of the kind specified, during operation some of the partially burned gases existing from the central passage of the heating element are recirculated back towards the nozzle assembly through the auxiliary passages.

This increases the temperature of the liquid fuel/air mixture entering the heating element, which causes significant cracking to take place in preference to direct oxidation of the higher molecular weight hydrocarbons. This in turn may produce a superior reaction within the heating element, and in theory could produce a self-sustaining superior operation.

However, in conventional use of a burner assembly of the kind specified, soot deposits tend to occur in the auxiliary passages.

These in themselves are of no significant disadvantage, since they may be removed if and when the temperature of the heating element is increased, or may be removed periodically by a cleaning operation.

However, we have determined that the occurrence of soot deposits does have a further effect, the significance of which does not seem to have been appreciated.

Firstly, to appreciate this further effect, it is necessary to understand that there exists along the length of the auxiliary passages of the heating element a small temperature gradient, falling from the forward region (remote from the nozzle) towards the rearward region.

When soot deposits occur, they will initially or preferentially occur within the auxiliary passages close to the rearward region. This causes a restriction to the flow of gases along the auxiliary passages, which causes the volume of partially burned products re-entering the inlet zone to be reduced. This lowers slightly the temperature of gases leaving the central passage of the heating element, reducing the quantity of initial hydro-carbon fuel which is cracked, prior to oxidation, which in turn still further reduces the effect of the recirculating gases in the production of optimum cracking.

Attempts have been made to improve the burning characteristics of liquid hydrocarbon burners by utilising a rotating nozzle in an attempt to atomise the liquid fuel, to produce more effective mixing of the fuel and air. In general, constructions have been used in which the air itself rotates the nozzle, this producing not only the fine spray of liquid fuel, but also the swirling pattern of air to carry the mixture of fuel and air into the central passage of the heating element.

However, one of the effects of this expedient is to significantly reduce the flow rate of air at the nozzle. Thus, to achieve a compatible fuel/air ratio, the same quantity of air must be fed to the burner under higher pressure. However, not only does this require a more powerful fan at the primary air inlet, causing a greater production of noise and consumption of energy, but also difficulty is encountered in balancing the speed of rotation of the nozzle with the swirl pattern produced in the air flow. This difficulty is exacerbated by the use of liquid fuels of high viscosity, since the use of such fuels requires a much greater effort at the nozzle to produce the required atomisation of the liquid fuel.

According to this invention there is provided a method of burning liquid hydrocarbon fuels involving the use of a burner of the kind specified in which fuel is delivered to the nozzle at a pressure in excess of five atmospheres and at a sufficient flow rate to produce a self-sustaining, short nonluminescent flame.

In this manner, the temperature of the mixture of oil and air entering the central passage may be raised, by reverse flow of partially burned gases flowing conversely through the auxiliary passages, to a temperature at which separation of the combustion processes may be achieved and the selfsustaining short, non-luminescent flame produced.

Preferably fuel is delivered to the nozzle at a rate sufficient to maintain the temperature of the heating element in excess of 1000"C. In this manner, deposition of soot within the auxiliary passages may be eliminated.

The rate at which fuel is so delivered may be up to fifteen gallons per hour, or higher: advantageously, the pressure at which fuel is so delivered to the burner is preferably 10 atmospheres or more.

By the use of this invention, the following advantages maybe obtained.

Firstly, since the flow pattern into the central passage of the heating element can be more accurately controlled, and since the flow of air is not reduced by the requirement for it to rotate the nozzle, and/or to cause atomisation of the liquid fuel, both liquid fuel and air flow rates maybe significantly increased whilst maintaining satisfactory combustion conditions. For example, in a typical design of the kind described in Patent Specification No. 1299928, the maximum rate at which liquid fuel may be handled is in the order of 61/2 gallons per hour, whilst by the use of the present invention, flow rates up to 21 gallons per hour may be achieved.

Secondly, inasmuch as in previous constructions, the capability of achieving desired atomisation was dependent upon the viscocity of liquid fuel, where atomisation is achieved by virtue of the pressure at which liquid fuel is delivered to the nozzle, the maximum viscosity which can be handled satisfactorily may be increased.

Thirdly, in burner assemblies of the kind specified, gaseous hydro-carbons are required on start-up, to heat up the heating element to a temperature at which high quality combustion of liquid-hydro carbons may be carried out. In the absence of such pre-heating, poor quality combustion associated with the start-up continues for a considerable period of time,. However, production of good combustion may be achieved in a very short period of time by the use of this invention, allowing the use of gaseous hydro-carbon for start-up to be dispensed with.

Fourthly, by the achievement of a short, non-luminous flame very shortly after commencement of combustion, said flame being self-sustaining throughout the combustion period, the burner may be used in conjunction with a shorter heat exchange unit, and because of the high thermal density of the flame, the production of hydro-carbon pollutants is minimised.

There will now be given a detailed description, to be read with reference to the accompanying drawings, of a burner system, and a method of burning liquid hydrocarbon fuels by the use of the burner system, which are preferred embodiments of this invention and which have been selected to illustrate this invention by way of example.

In the accompanying drawings: Figure 1 is a schematic ross-sectional view of a burner assembly of the burner system; Figure 2 is an enlarged cross-sectional view of a nozzle of the burner assembly; and Figure 3 is a front elevation of an element of the nozzle.

The burner system which is the preferred embodiment of this invention comprises a burner assembly of the kind specified (Figure 1) comprising a supply body 10a and a combustion chamber 29a, the supply body being adapted to supply a mixture of hydrocarbon fuel and air, which is burned within the combustion chamber. The burner assembly also comprises a ceramic heating element lia located in the combustion chamber.

The burner system also comprises first delivery means (not shown) to deliver liquid hydro-carbon fuel to the supply body under a positive pressure in excess of 5 atmospheres, and preferably at a pressure of up to 50 atmospheres, and second delivery means (not shown) to deliver gaseous hydrocarbon fuel to the supply body. The system also comprises control valving (not shown) whereby either the first delivery means is so operative, or the second delivery means is so operative.

The supply body 10a comprises an axial supply conduit 12a which extends from an inlet fixing 42a to a nozzle 13a (shown in more detail in Figure 2). Air is supplied to an inlet 14a by an air delivery means (not shown) comprising a fan. Air supplied by the air delivery means in the use of the burner system flows through the inlet, and flows through an annular passage 15a surrounding the nozzle 13a, the passage 15a converging in a direction towards the heating element 11. A swirling motion is imparted to the air by vanes 16a, ensuring a desirable flow pattern in the mixture of air and liquid hydro-carbon fuel into the combustion chamber 29a.

The heating element ila is in the form of a block of ceramics material, and comprises a central passage 21a and a plurality (specifically eight) of auxiliary passages 22a extending parallel to the longitudinal axis, and being spaced uniformly around the central passage. End faces of the heating element are planar, but if desired the end face of the heating element remote from the nozzle may be frusto-conical in form, as illustrated in the specification of our U.K. patent No.

1,299,928.

The second delivery means is arranged to deliver gaseous hydro-carbon fuel, such as natural gas, through an inlet 19a, such gas being discharged through a ring of apertures 20a surrounding the nozzle 13a, being generally directed towards the longitudinal axis of the nozzle and towards the central passage of the heating element.

In operation, a mixture of fuel and air, whether the fuel is oil supplied via the inlet fixing 42a through the nozzle 13a, or gas supplied through the inlet 19a and through the apertures 20a, enters one end 27a of the central passage 21a. Ignition of the fuel/air mixture takes place in the central passage 21a, and the resulting flame and hot gases issue from the end 25a of the central passage. By use of the arrangement illustrated in the drawings, and in particular by the shape of the mixture of the fuel/air as it flows towards the central passage of the heating element, there is produced within the combustion chamber, specifically in the space between the heating element and the supply body 10a, an annular region (indicated by the letter X) of relatively low pressure.As will be seen from Figure 1, this region of low pressure is located close to the ends of the auxilliary passages 22a which are located adjacent to the nozzle 13a. Thus, in use, some of the mixture issueing from the central passage 21a is drawn backwardly along the auxilliary passages 22a, into the space between the heating element and the supply body. This material is mixed with fuel/air issuing from the supply body, producing a rise in temperature therein prior to ignition of said fuel/air mixture within the combustion chamber.

The nozzle 13a (Figure 2) comprises a body 50a provided with a screw-threaded portion 52a, enabling the nozzle to be screwed into the end of the supply conduit 12a. Extending rearwardly from the body is a hollow, cylindrical portion 54a provided with radial inlet ports 56a, covered from the exterior by a filter mesh 58a, allowing liquid hydro-carbon fuel to pass from the supply conduit 12a into the interior of the portion 54a, and to flow into the body.

Within the body there is located a generally cylindrical cup 60a, having a generally frusto-conical front face 62a, in which there are provided grooves 64a. The cup 60a is held with the face 62a in engagement with a cup seat 66a by a retaining member 68a, which additionally diverts oil flowing into the body around the outside of the cup 60a.

The liquid fuel then flows along the grooves 64a and issues from a delivery outlet 70a provided in the cup seat.

Specifically the nozzle 12a used in the burner system which is the preferred embodiment of this invention is a nozzle supplied by Danfoss (London) Limited: it will of course be appreciated that nozzles of other manufacture may be used, and whereas the Danfoss nozzle is a static nozzle, if desired a nozzle comprising a cup mounted within the body for rotation under the pressure of liquid hydro-carbon fuel flowing through the nozzle may be provided, to obtain a desired atomisation of the liquid hydrocarbon fuel as it issues from the delivery outlet.

The first delivery means of the burner system which is the preferred embodiment of this invention is arranged to deliver liquid hydro-carbon fuel to the nozzle under pressure, particularly under a pressure greater than 5 atmospheres, and desirably at a pressure of about 10 atmospheres. This permits liquid hydro-carbon fuel to be delivered to the supply body, and to issue from the nozzle, at a sufficiently high rate, as will hereafter be explained, notwithstanding high viscosity of the liquid hydro-carbon fuel itself.

Thus, in the use of the burner system illustrated in Figure 1, by virtue of the delivery of liquid hydro-carbon fuel to the nozzle 12a under pressure, highly superior atomisation of the liquid hydro-carbon may be obtained. Additionally, inasmuch as the air flowing into the supply body is not used to produce atomisation of the liquid hydrocarbon fuel, relatively small restraint to flow of air into and through the supply body is encountered, and large volumes of air may be provided, without requiring an unduly large capacity fan, or producing unduly high noise levels.

Thus, by the use of the preferred embodiment, it is possible to deliver liquid hydrocarbon fuels to the combustion chamber, appropriately mixed with a required volume of air, at such a rate as to ensure that the temperature within the auxiliary passages of the heating element does not, during continued use of the burner assembly, fall below 1000"C. In such circumstances, by constructing and arranging the burner assembly in a manner such as to ensure that between 17 and 33% of the gases flowing through the central passage of the heating element are returned along the auxiliary passages to the region of the burner assembly adjacent to the nozzle, the temperature of the incoming mixture of liquid hydrocarbon fuel and air is raised to a temperature which maximises the splitting of the combustion process into the two chemical reactions involved.Thus, within the heating element, there primarily takes place a cracking of the higher molecular weight hydrocarbons into smaller molecular weight hydro-carbons, sustained by the combustion of the smaller molecular weight hydro-carbons present in the initially supplied hydrocarbon fuel, whereas within the combustion chamber downstream of the heating element, there primarily takes place oxidation of the hydro-carbons. Thus, within the combustion chamber, a relatively short, non-luminescent flame is produced, with a relatively low production of unburned carbon and/or partially oxidised carbon products. The length of the combustion chamber, and any heat exchange system associated therewith, may thus be commensurately reduced.

Additionally, since the temperature of the auxiliary passages is maintained (during continued use) above 1000"C, no soot deposits tend to occur within said auxiliary passages, as would otherwise produce a gradual reduction in the efficiency of the burner assembly, ultimately requiring cleaning of the auxiliary passages.

Because of the superior atomisation of the liquid hydro-carbon fuel at the nozzle, and the capability of delivery of liquid hydro-carbon fuel into the combustion chamber at relatively high levels in a desirable swirl pattern, mixed with a required amount of air, it has been found that on start-up, the heating element quickly attains the temperature necessary to sustain the desirable combustion reaction, as hereinabove described. Thus, upon start-up, combustion of the liquid hydro-carbon fuels is direct, involving direct oxidation of the higher molecular weight hydro-carbon fractions. However, this combustion process is, by virtue of the use of this invention, relatively "clean", and relatively quickly heats the heating element up to its normal operating temperature. Thus, by the use of the invention, it is possible to dispense with the start-up operation normally employed, involving the initial use of gaseous hydrocarbons, whilst the heating element is being heating up to its normal operating temperature.

However, desirably the burner system is capable of burning gaseous hydro-carbon fuels, as an alternative to burning liquid hydro-carbon fuels, maintaining versatility of the burner assembly.

In a typical example of the use of the burner system which is the preferred embodiment of this invention, between 15 and 21 gallons per hour of liquid hydro-carbon fuel were pumped by the delivery means, at a pressure of about 10 atmospheres, along the supply conduit 12a, to be discharged into the combustion chamber through the nozzle 13a, the liquid hydro-carbon fuel having a relatively high viscosity (specifically, a viscosity of about 35 seconds).

The length of the flame produced in the combustion chamber downstream of the heating element was between 18 inches and 2 foot.

WHAT WE CLAIM IS: 1. A method of burning liquid hydrocarbon fuels involving the use of a burner assembly of the kind specified, in which fuel is delivered to the nozzle at a pressure in excess of 5 atmospheres and at a sufficient flow rate to produce a self-sustaining short non-luminescent flame.

2. A method according to Claim 1 wherein the nozzle is mounted for rotation under the action of fuel flowing therethrough.

3. A method according to Claim 1 wherein the nozzle is static.

4. A method according to anyone of the preceding claims wherein the flow rate is such as to maintain the temperature of the heating element in excess of 1,000"C.

5. A method according to any one of the preceding claims wherein the rate at which fuel is delivered to the nozzle is 15 gallons per hour or more.

6. A method according to any one of the preceding claims, wherein the pressure at which the fuel is delivered to the nozzle is less than 50 atmospheres.

7. A method according to one of the preceding Claims wherein a mixture of liquid fuel and air is directed towards the central passage of the heating element so as to cause, when the system is in operation, a pressure at the ends of the auxiliary passages adjacent to the nozzle which is lower than the pressure at the ends of the auxiliary passages remote from nozzle, whereby flow of partially-burned gases along the auxiliary passaged backwardly towards the nozzle is caused.

8. A method according to Claim 7 wherein the construction and arrangement is such that in use, between 17 and 33% of the gases burned through the central passage is returned along the auxiliary passages to the region of the burner assembly adjacent to the nozzle.

9. A method of burning liquid hydrocarbon fuels, when carried out substantially as hereinbefore described, with reference to and as shown in the accompanying draw ings.

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

Claims (9)

**WARNING** start of CLMS field may overlap end of DESC **. The length of the flame produced in the combustion chamber downstream of the heating element was between 18 inches and 2 foot. WHAT WE CLAIM IS:

1. A method of burning liquid hydrocarbon fuels involving the use of a burner assembly of the kind specified, in which fuel is delivered to the nozzle at a pressure in excess of 5 atmospheres and at a sufficient flow rate to produce a self-sustaining short non-luminescent flame.

2. A method according to Claim 1 wherein the nozzle is mounted for rotation under the action of fuel flowing therethrough.

3. A method according to Claim 1 wherein the nozzle is static.

4. A method according to anyone of the preceding claims wherein the flow rate is such as to maintain the temperature of the heating element in excess of 1,000"C.

5. A method according to any one of the preceding claims wherein the rate at which fuel is delivered to the nozzle is 15 gallons per hour or more.

6. A method according to any one of the preceding claims, wherein the pressure at which the fuel is delivered to the nozzle is less than 50 atmospheres.

7. A method according to one of the preceding Claims wherein a mixture of liquid fuel and air is directed towards the central passage of the heating element so as to cause, when the system is in operation, a pressure at the ends of the auxiliary passages adjacent to the nozzle which is lower than the pressure at the ends of the auxiliary passages remote from nozzle, whereby flow of partially-burned gases along the auxiliary passaged backwardly towards the nozzle is caused.

8. A method according to Claim 7 wherein the construction and arrangement is such that in use, between 17 and 33% of the gases burned through the central passage is returned along the auxiliary passages to the region of the burner assembly adjacent to the nozzle.

9. A method of burning liquid hydrocarbon fuels, when carried out substantially as hereinbefore described, with reference to and as shown in the accompanying draw ings.