Biomass Combustion

Abstract

A splitter divides a flow of low heating value biomass into a central stream and an annular stream. A stable flame may be achieved by combusting the central stream with oxygen. This avoids the use of costly fossil fuels or biomass (that have higher heating values than the biomass fuel) as an auxiliary fuel for achieving a stable flame.

Description

BACKGROUND

1. Field of the Invention

The present invention relates to biomass burners and methods of combusting biomass.

2. Related Art

The emission of carbon dioxide (CO₂) as a cause of global warming is of current concern to the power industry. The potential role of biomass energy acquired a new dimension when it was suggested that planting large areas of new forest could slow the increase in atmospheric carbon dioxide by removing carbon dioxide from the atmosphere. Therefore, the electric power industry uses biomass in order to significantly reduce CO₂ emissions.

In a typical staged combustion burner firing biomass, the biomass fuel is combusted with primary combustion air in a first combustion zone. Any non-combusted fuel is more completely combusted in a second combustion zone downstream of the first combustion zone with secondary combustion air injected around the biomass. If tertiary combustion air is utilized, combustion is completed in a third combustion zone downstream of the second combustion zone with tertiary combustion air injected around the secondary combustion air.

The heating value of widely available biomass fuels is generally lower than that of fossil fuels. In order to establish a stable flame in the boiler, a fossil fuel having a higher heating value than the biomass is typically co-fired with the biomass at the burner to ignite the flame. The combustion of the fossil fuel with available oxidant provides the necessary energy to ignite the biomass fuel. Usually, the fossil fuel is coal, oil, or natural gas in order to ensure the flame ignition and stability. Instead of using fossil fuel, another option is to inject higher heating value biomass, such as rapeseed oil, with the lower heating value biomass fuel for purposes of flame ignition. Again, this additional biomass has a higher heating value than the main biomass fuel and is used in the same manner as the fossil fuel in the above-described processes.

If an existing furnace designed for combusting natural gas is retrofitted for firing biomass fuel, such a retrofit has the potential of limiting the apparent power of the burner. This is because the solid biomass particles are combusted more slowly than a gaseous fuel. At lower furnace loads, the biomass burner is able to inject a flow of biomass and satisfactorily burn out the biomass particles before they impinge a furnace wall opposite the burner. At higher furnace loads, however, a higher velocity of primary combustion air will become necessary to achieve satisfactory conveyance of the biomass particles so that they may be injected by the burner. If a higher velocity of primary combustion was not used, the otherwise low momentum of the biomass particles would cause them to settle and accumulate. The higher velocity of the combustion air and biomass particles results in a residence time for the particles in between the burner and the opposing furnace wall that is too short to allow satisfactory burn-out of the biomass particles. Thus, although the biomass fuel flow rate may be increased, the resultant steam power from the boiler may be limited due to less than complete combustion of the biomass particles and inefficient heat transfer from the combustion of the biomass particles to the boiler steam tubes. In other words, the apparent burner power may be limited.

There has been several biomass combustion processes proposed in the patent literature.

U.S. Pat. No. 5,107,777 describes combustion of a low BTU high moisture biomass such as wood (known as Hog fuel). Biomass is injected into the boiler 15-20 ft above the floor. The combustion air, which is supplied from the bottom of the furnace is enriched with oxygen to a level of between 0.1 to 7%. Additional oxygen is also injected from the side. Oil burners are fired from the top. It claims that a higher flame temperature is achieved with injection of oxygen.

US 2008/0261161 A1 describes a burner or furnace for the combustion of biomass using two or more fuel injection ports located at non-radial injection angles. The biomass is mixed with oxidizer and then injected into the furnace via a cyclonic combustion vortex.

U.S. Pat. No. 6,699,029 B2 describes a boiler system where a low rank fuel is burned to achieve energy generation rate similar to that achieved with conventional fuels such as coal. It proposes certain oxygen injection methods for reducing the formation of nitrogen oxides (NOx). Operations with typical US-origin coals are described.

The co-firing system described above has been adopted in many EU electric power plants to meet the tightening EU regulations. While the co-firing or central injection of a higher heating value fossil or higher heating value biomass in the combustion of the lower heating value main biomass fuel may keep the flame ignited and provide a stable flame, the cost of the higher heating value biomass fuel or fossil fuel is very expensive compared with generally available lower heating value biomass such as wood and straw. Also, the current conventional co-firing system is relatively complex because it includes two fuel feeding systems.

Thus, it is an object of the invention to provide a burner, combustion system and method of combustion that would avoid or reduce the usage of the fossil fuel/high quality biomass and reduce the capital cost and maintenance cost while achieving a stable flame and maintaining ignition of the flame.

It is also an object of the invention to provide a burner, combustion system, and method of combustion that would tend to remove the limitation on the apparent power of the burner as the biomass fuel flow rate is increased.

SUMMARY

There is disclosed a biomass burner, comprising: a burner block, a fuel conduit, an oxygen injector, and a tubular fuel flow splitter. The burner block has an injector passage extending between rear and front faces. The fuel conduit has inlet and outlet ends and is concentrically disposed within said bore at said front face, an annular combustion air flow space being defined between an inner surface of said outer conduit and an outer surface of said fuel conduit. The oxygen injector has inlet and outlet ends and is concentrically disposed within said outer conduit at said front face. The tubular fuel flow splitter is concentrically disposed within said fuel conduit at said front face. The splitter has an inlet end disposed upstream of said fuel conduit inlet end and also has an outlet end. The oxygen injector has either an annular cross-sectional shape and is adjacent to and surrounds said splitter, or a cylindrical cross-sectional shape and is concentrically disposed within said splitter.

There is also disclosed a biomass combustion system, comprising the above-disclosed biomass burner, a biomass hopper, a biomass fuel feeder, a source of oxygen, and one or more blowers. The biomass fuel feeder is operatively associated with said hopper and at least one of said one or more blowers to receive particulate biomass from said hopper, convey the particulate biomass with a flow of combustion air from said at least one blower to provide a flow of biomass fuel, and direct the flow of biomass fuel to said fuel conduit inlet end. At least one of said one or more blowers is in fluid communication with said combustion air flow space. The source of oxygen is in fluid communication with said oxygen injector inlet end.

There is also disclosed a biomass-fired boiler installation, comprising: a plurality of the above-disclosed biomass burner; one or more blowers; at least one biomass hopper; at least one biomass fuel feeder; a source of oxygen; and a boiler. Said at least one biomass fuel feeder is operatively associated with said at least one hopper and at least one of said one or more blowers to receive particulate biomass from said at least one hopper, convey the particulate biomass with a flow of air from said at least one blower to provide a flow of biomass fuel, and direct the flow of biomass fuel to said fuel conduit inlet ends. At least one of said at least one blower is in fluid communication with said combustion air flow spaces. Said source of oxygen is in fluid communication with said oxygen injector inlet ends. Said plurality of burners is mounted on walls of said boiler.

There is also disclosed a method of combusting biomass, comprising the following steps. A flow of particulate biomass conveyed with air from a fuel conduit of a biomass burner is injected into a combustion space. A flow of oxygen is injected into the flow of injected biomass from an oxygen injector concentrically disposed within said fuel conduit. The injected central flow of biomass is combusted with the oxygen in the combustion space. An annular flow of combustion air is injected from the burner around the annular flow of biomass. The injected annular flow of biomass is combusted with the combustion air in the combustion space. The fuel conduit has a tubular splitter concentrically disposed therein. The flow of biomass is split by the splitter into a central flow on the inside of the splitter and an annular flow on the outside of the splitter.

There is also disclosed a method of retrofitting a conventional biomass-fired boiler installation. The boiler installation comprises: a plurality of biomass burners designed for combusting biomass only with air; one or more blowers; at least one biomass hopper; at least one biomass fuel feeder; and a boiler. Said at least one biomass fuel feeder is operatively associated with said at least one hopper and at least one of said one or more blowers to receive particulate biomass from said at least one hopper, convey the particulate biomass with a flow of air from said at least one blower to provide a flow of biomass fuel, and direct the flow of biomass fuel to said fuel conduit inlet ends. At least one of said at least one blower is in fluid communication with said combustion air flow spaces. Said plurality of burners is mounted on walls of said boiler. Said method comprises the steps of: replacing one or more of the burners designed for air-combustion with a corresponding number of the above-disclosed inventive burners and placing a source of oxygen in fluid communication with said oxygen injector inlet ends.

Any of the above-disclosed burner, biomass combustion system, biomass-fired boiler installation, method of combusting biomass, and method of retrofitting a conventional biomass-fired boiler installation may include one or more of the following aspects:

• • the outlet ends of the fuel conduit, oxygen injector, and splitter are flush with said front face.
  • the outlet ends of said oxygen injector and splitter are recessed back from said fuel conduit outlet end.
  • said oxygen injector has an annular cross-sectional shape and is adjacent to and surrounds said splitter.
  • said oxygen injector outlet end is configured as a closed face with a plurality of radially distributed injection holes.
  • said oxygen injector outlet end is configured as an open face.
  • said oxygen injector has a cylindrical cross-sectional shape and is concentrically disposed within said splitter.
  • said oxygen injector outlet end is configured as a closed face with a plurality of radially distributed injection holes.
  • said oxygen injector outlet end is configured as an open tube.
  • the burner further comprises an outer conduit concentrically disposed within said bore having an inlet end disposed downstream of said burner block rear face and also having an outlet end, said annular combustion air flow space being split into a secondary combustion air flow space and a tertiary combustion flow space by said outer conduit, the secondary combustion air flow space being defined by an outer surface of said fuel conduit and an inner surface of said outer conduit, and the tertiary combustion air flow space being defined by an outer surface of said outer conduit and an inner surface of said bore.
  • the biomass burner further comprises a secondary combustion air swirler disposed within said secondary combustion air flow space upstream of said burner block front face, and a tertiary combustion air swirler disposed along an inner surface of said bore adjacent to said burner block front face.
  • said splitter further comprises a main section extending between said splitter inlet and outlet ends, the splitter inlet end having a diameter D1, the main body having a diameter D2, wherein D1<D2.
  • said fuel conduit has a diameter D4, said splitter inlet end has a diameter D1 and 0.05 D4≦D1≦0.25 D4.
  • said source of oxygen is selected from group consisting of a vacuum swing adsorption system, an oxygen pipeline, a cryogenic air separation unit, and a vaporizer connected to a tank of liquid oxygen.
  • said source of oxygen is selected from group consisting of a vacuum swing adsorption system, an oxygen pipeline, a cryogenic air separation unit, and a vaporizer connected to a tank of liquid oxygen.
  • the central biomass flow has a velocity V1, the annular biomass flow has a velocity V2, and the flow of oxygen has a velocity V3, where (V3−V2)<(V3−V1).
  • no fuel other than the particulate biomass is combusted.
  • the oxygen and the central flow of biomass begin to mix at a point upstream of said fuel conduit outlet end.
  • said oxygen is injected in a center of the central flow of biomass.
  • said oxygen is injected in an annulus surrounding said splitter.
  • the oxygen is swirled.
  • the oxygen has a concentration of >95%.
  • the combustion air is injected in two annular flows, a first of which is secondary combustion air adjacent the annular flow of biomass and a second of which is tertiary combustion air adjacent the secondary combustion air.
  • an overall oxygen enrichment of the combined biomass fuel, injected oxygen and combustion air achieved by injection of the oxygen is between 21% and 25%.

BRIEF DESCRIPTION OF THE DRAWINGS

For a further understanding of the nature and objects of the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein:

FIG. 1A is a schematic, cross-sectional view of an embodiment of the inventive burner.

FIG. 1B is a variation of the burner of FIG. 1A.

FIG. 1C is a variation of the burner of FIG. 1A.

FIG. 1D is a variation of the burner of FIG. 1C.

FIG. 2A is a front elevation view of the oxygen lance of the burners of FIG. 1A or FIG. 1B.

FIG. 2B is a front elevation view of an oxygen nozzle for use with the burners of FIG. 1C or FIG. 1D.

FIG. 2C is a front elevation view of another type of oxygen nozzle for use with the burners of FIG. 1C or FIG. 1D.

FIG. 3A is a schematic, front elevation view of a simulated burner of the Comparative Example.

FIG. 3B is a partial, cross-sectional view of the burner of FIG. 3A taken along axis X-X that illustrates streams of rapeseed oil, wood pellets, and secondary combustion air.

FIG. 4A is a schematic, front elevation view of a simulated burner of Examples 1-8.

FIG. 4B is a partial, cross-sectional view of the burner of FIG. 4A taken along axis Y-Y that illustrates streams of oxygen, wood pellets, and secondary combustion air.

FIG. 5 is a graph showing the temperature distribution of the simulated flame and combustion chamber yielded by the Comparative Example.

FIG. 6 is a graph showing the temperature distribution of the simulated flame and combustion chamber yielded by Example 1.

FIG. 7 is a graph showing the temperature distribution of the simulated flame and combustion chamber yielded by Example 2.

FIG. 8 is a graph showing the temperature distribution of the simulated flame and combustion chamber yielded by Example 3.

FIG. 9 is a graph showing the temperature distribution of the simulated flame and combustion chamber yielded by Example 4.

FIG. 10 is a graph showing the temperature distribution of the simulated flame and combustion chamber yielded by Example 5.

FIG. 11 is a graph showing the temperature distribution of the simulated flame and combustion chamber yielded by Example 6.

FIG. 12 is a graph showing the temperature distribution of the simulated flame and combustion chamber yielded by Example 7.

FIG. 13 is a graph showing the temperature distribution of the simulated flame and combustion chamber yielded by Example 8.

DESCRIPTION OF PREFERRED EMBODIMENTS

The proposed invention solves the problem experienced by conventional biomass combustion by instead combusting a portion of lower heating value biomass injected from a central portion of the burner with oxygen to establish a stable flame ignition. The invention avoids the necessity of having a second fuel feeding system. It also reduces operational costs by avoiding or at least reducing the use of the relatively expensive higher heating value fuels. It also improves the ability of the burner to be boosted to a higher apparent burner power.

The overall flow of lower heating value biomass fuel in a fuel conduit is split into an inner, central stream and an outer, annular stream by a splitter. Upstream of the splitter, the velocities of each portion of the flow of biomass fuel are generally uniform. Because the narrower diameter splitter inside the fuel conduit creates a pressure drop, the velocity of the portion of biomass fuel entering the stream is lowered. On the other hand, there is little to no change in velocity of the portion of biomass that flows on the outside of the splitter. Oxygen is injected into the inner biomass stream from an injector. The injector may be concentrically disposed within the splitter or may be adjacent to and surrounds the splitter. The velocity of the injected oxygen is higher than each of the inner and annular streams of biomass. Because the difference in velocities between the oxygen and inner stream is greater than the difference in velocities between the oxygen and the annular stream, there is relatively more mixing between the inner stream and the injected oxygen in comparison to the annular stream and oxygen.

A stable central flame rooted near the face of the burner is achieved because the locally high concentration of oxygen in the mixed oxygen/inner stream allows the biomass particles of the inner stream to be ignited more easily and at an earlier point than the biomass particles in the annular stream. Thus, a combustion reaction is commenced at an upstream, inner zone adjacent the burner face between the biomass particles of the annular stream and the oxygen from both the biomass conveying air and also the injected oxygen.

A stream of combustion air is injected by the burner around the annular flow of biomass. A combustion reaction is commenced between the stream of combustion air and the biomass particles of the annular stream at a downstream, annular zone farther away from the burner face. The biomass particles of the annular stream are ignited less easily and at a later point than those of the inner stream because of the relatively lower oxygen concentration of the combustion air in comparison to the mixed oxygen and inner stream of biomass. There is little to no oxygen from the injected stream of oxygen available to combust the biomass particles in the annular stream. This is because there is relatively less mixing between the oxygen and the biomass particles in the annular stream and the injected oxygen is mostly or entirely already consumed through combustion with the biomass particles in the inner stream.

The combustion air may be swirled or not. The combustion air may be a single stream or it may be split into a secondary combustion air stream surrounding the annular biomass stream and a tertiary combustion air stream surrounding the secondary combustion air stream. Either or both of the secondary and tertiary combustion air streams may be swirled.

While the biomass fuel may be any biomass fuel known in the art, typically it is wood pellets, straw, or so-called hog fuel.

The oxygen is industrially pure oxygen. The specific purity of the industrially pure oxygen depends upon the method of production and whether or not the produced oxygen is further purified. For example, the industrially pure oxygen may be gaseous oxygen from an air separation unit that cryogenically separates air gases into predominantly oxygen and nitrogen streams in which case the gaseous oxygen has a concentration exceeding 99% vol/vol. The industrially pure oxygen may be produced through vaporization of liquid oxygen (which was liquefied from oxygen from an air separation unit, in which case it, too, has a purity exceeding 99% vol/vol. The industrially pure oxygen may be also be produced by a vacuum swing adsorption (VSA) unit in which case it typically has a purity of about 92-93% vol/vol. The industrially pure oxygen may be sourced from any other type of oxygen production technology used in the industrial gas business.

Although high concentration oxygen is used in the center oxy-flame for ignition, the overall oxygen enrichment is typically between 21% and 25%, where enrichment is defined as:

where:

$\frac{V_{\mathrm{central}O2}+\left(V_{\mathrm{combustion}\mathrm{air}}·0.209\right)}{V_{\mathrm{central}O2}+V_{\mathrm{combustion}\mathrm{air}}}\times 100\%$

where:

• • V_{central O2} is the volumetric flow rate of oxygen in the centrally injected oxygen
  • V_{combustion air} is the volumetric flow rate of combustion airAlthough higher levels of oxygen enrichment may be used, extremely high levels may produce a flame with a temperature to melt the ash thereby producing slag. Generally, it is preferred to avoid slag formation.

The burner includes a burner block that is installed into an opening in the wall of the furnace, and at least a fuel conduit, a fuel splitter concentrically disposed within the fuel conduit, and an oxygen injector disposed within the fuel conduit. The burner block is typically made of refractory material and includes a bore through which the fuel conduit, splitter, and oxygen injector extend. If both secondary and tertiary combustion air streams are desired, an outer conduit may also be included in the burner in which case it is disposed concentrically within the bore. Each of the fuel conduit, splitter, oxygen injector, and optional outer conduit are typically made of any metal suitable for burner elements.

As best illustrated in FIG. 1A, biomass is conveyed by air through the fuel conduit 1 to provide a fuel stream 11. The fuel stream 11 is split into two streams, a central, inner stream 8 and an annular, outer stream 9, by the splitter 7. The splitter is a tubular structure that has a narrower inlet end with a diameter D1, a main section having a diameter D2>D1, and an outlet end that has a diameter D3≧D2.

High concentration oxygen is injected through the oxygen lance 2. The oxygen and the inner stream 8 of the biomass pellet are mixed and ignited in the center of the flame. The split ratio (the ratio of the mass flow rate of the outer biomass stream 9 to the mass flow rate of the inner biomass stream 8) may be adjusted by changing the angle of the throat of the splitter 7 or by increasing or decreasing D1 for a given fuel conduit 1 diameter D4. Typically, 95%-50% of the fuel stream 11 becomes stream 9 while 5-50% of the fuel stream 11 becomes stream 8. Thus, the diameter D1 of the splitter 7 is related to the diameter D4 of the fuel conduit 1 according to the equation: 0.05·D4<D1<0.25·D4.

A stream of combustion air 13 flows in the annular space between the fuel conduit 1 and an inner surface 14 of the bore of the burner block B. The stream of combustion air 13 is divided into two portions by a sliding air damper 4. Secondary combustion air 6 flows in the annular space in between the fuel conduit 1 and an outer conduit 12, while the tertiary combustion air 5 flows in the annular space between the outer conduit 12 and an inner surface 14 of the bore of the burner block B. Each or either of the secondary and tertiary combustion air streams 6, 5 may be swirled. Typically, the secondary combustion air stream 6 is swirled by a swirl generator 10 before enters into the combustion space C of the boiler, while the tertiary combustion air stream 5 is swirled at the face of the burner where a plane P divides the combustion space C from the furnace wall W and the burner. The overall air swirl intensity may be adjusted by manipulation of the sliding air damper 4.

The outer, annular stream 9 of the biomass and the secondary combustion air stream 6 are mixed and ignited by the center oxygen/biomass flame. Combustion of the biomass fuel is completed by the tertiary combustion air stream 5.

The oxygen lance 2 may be configured in any one of several ways known in the field of oxy-combustion. Thus, the oxygen lance 2 may be a straight tube with a constant diameter. Alternatively, the oxygen lance 2 may diverge at its outlet end where the oxygen mixes with the inner stream 8 of biomass fuel. As best shown in FIG. 2A, the oxygen lance 2 could instead include a group of small nozzles 0 evenly distributed at the outlet end in order to even distribute the oxygen in the surrounding inner biomass fuel stream 8 and enhance mixing of the two. Another possible variation is to swirl the oxygen stream from the oxygen lance 2. A swirled oxygen stream will increase the mixing between the inner stream of biomass fuel 8 and the oxygen from the lance 2 and help keep the flame ignited.

In a variation of the burner of FIG. 1A, and as best shown in FIG. 1B, the oxygen lance 2 and the splitter 7 may be recessed from the plane P by a distance L. Such a configuration will result in mixing of the oxygen and the biomass in the inner stream 8 to ignite a flame at a point upstream of the plane P. The outlet ends of the fuel conduit 1, outer conduit 12 and burner block B are still flush with the plane P.

In a variation of the burner of FIG. 1A, and as best illustrated in FIG. 1C, instead of injecting oxygen from a position inside the splitter 7, the oxygen could be injected from a nozzle 2″ having an annular cross-section. The nozzle 2″ is adjacent to, and surrounds, the splitter 7 so that the inner stream of biomass 8 is combusted with an annular stream of oxygen. The nozzle 2″ is fed by an oxygen conduit 2′.

In a variation of the burner of FIG. 1C, and as best shown in FIG. 1D, the oxygen nozzle 2″ and the splitter 7 may be recessed from the plane P by a distance L. Such a configuration will result in mixing of the oxygen and the biomass in the inner stream 8 to ignite a flame at a point upstream of the plane P. The outlet ends of the fuel conduit 1, outer conduit 12 and burner block B are still flush with the plane P.

In either of the burners of FIG. 1C or 1D, the oxygen nozzle 2″ may be configured in a couple of different ways. As best illustrated in FIG. 2B, the oxygen nozzle 2″ may be open at the outlet end. Alternatively and as best shown in FIG. 2C, oxygen may be injected from the outlet end of the oxygen nozzle 2″ from a plurality of radially distributed holes 2bis.

The furnace may be shut down at regular intervals (i.e., annually) for furnace maintenance. It may be desirable at the resumption of furnace operation to first heat the furnace by combusting a stream of atomized oil from an oil gun with secondary combustion air in the conventional manner. Once a predetermined furnace temperature is reached, injection of the biomass fuel is initiated. Normal operation of the furnace is then commenced upon discontinuance of the stream of atomized oil and removal of the oil gun from the furnace. With this in mind, one of ordinary skill in the art will recognize that, despite the use of such a conventional furnace pre-heating technique, during normal operation the burner and furnace only combusts a single fuel: biomass.

Regardless of whether the oxygen is injected according to the burner configurations of FIG. 1A, 1B, 1C, or 1D, the injection of oxygen into an inner flow of biomass fuel 8 helps increase burnout of the biomass fuel particles in comparison to conventional biomass burner where no such oxygen injection is employed. Burnout is increased because the local oxygen concentration surrounding the biomass particles in the injected inner stream 8 is increased. An oxygen-enriched atmosphere at this region not only starts combustion of volatile components in the biomass particles earlier but also starts combustion of char earlier. As a result, satisfactory burnout of the biomass particles is completed in the path line of the biomass particles inside the furnace at a point earlier in comparison to biomass particles from biomass burners where no such oxygen injection is performed.

Faster burnout of the biomass particles is advantageous for allowing satisfactory operation of the biomass burner at higher apparent powers. This will be clearly evident when compared to operation of a conventional biomass burner in which no central oxygen injection is performed. When conventional biomass burners are operated at lower powers, the flow rate of primary combustion air necessary for satisfactory conveyance of the biomass particles has a velocity sufficiently low that satisfactory burnout of the biomass particles may be achieved over the path line traveled by the particles through the furnace. At higher burner powers, the flow rate of primary combustion air that is necessary for satisfactory conveyance of the biomass particles must be increased because the total mass of solid biomass particles is increased. As the flow rate of the primary combustion air is increased, it will soon reach a velocity that is too high to allow satisfactory burnout of the solid biomass particles along the path line through the furnace and enter the superheater. In other words, the residence time of a combusting biomass particle is decreased when higher velocity combustion air is used (such as at higher burner powers). Such a situation creates several disadvantages.

One disadvantage is related to wear to the furnace. In comparison to the relatively lower combustion air velocities when the burner is operated at lower power, the relatively higher combustion air velocities at higher burner powers changes the pattern of heat transfer from the combusting particles to the furnace. More particularly and in comparison to lower burner powers, relatively less heat is transferred to portions of the furnace closer to the burners and relatively more heat is transferred to portions of the furnace relatively distant from the burners. This shift in the amount of heat transferred to portions of the furnace adjacent the superheater can result in damage to that portion of the furnace because it is not designed for excessive radiative heat transfer.

The second disadvantage is realized for biomass furnaces that were originally commissioned as coal-fired furnaces but which have been retrofitted for biomass combustion. Coal-fired furnaces are designed to be heated by a large number of burners. Together, those burners provide a nominal power at which the furnace is designed to operate. The nominal power is related to the heat flux from combustion of the coal to water or stream in the boiler steam tubes and which is realized in the form of mechanical or electrical power. If the furnace is retrofitted with conventional biomass burners, at relatively high biomass fuel firing rates the burners may fall well short of the nominal power due to unsatisfactory burnout of the biomass particles. Primarily, this is because typical biomass particles (with an average size of around 100 m) combust more slowly than typical pulverized coal particles (with an average size of around 60 m). Although the furnace may have been designed to achieve the nominal power with the more quickly combusting coal particles, the more slowly combusting biomass particles shifts the pattern of heat transfer from the combusting particles to the furnace. In particular, less heat is transferred to portions of the furnace adjacent to upstream portions of the path line and more heat is transferred to portions of the furnace adjacent to downstream portions of the path line. Typical furnaces are not designed for such a modified heat transfer pattern where much of the heat transfer is shifted downstream along the path line. So, as the flow rate of the biomass fuel from the burner is increased in an attempt to increase the power, the apparent power of the burner soon reaches a limit beyond which it is difficult to increase by increasing the flow rate of the biomass fuel.

In contrast, by injecting oxygen in an inner flow of biomass fuel according to the invention, the above disadvantages may be avoided. The higher oxygen concentrations surrounding the biomass particles tends to ignite the flame earlier and increases the rate at which the biomass particles combust. As a result, the impact of the downstream shift in heat transfer that would otherwise be experienced in furnaces fired with conventional biomass burners is reduced or nullified by the increase in the rate of combustion of the biomass particles afforded by the localized oxygen-enriched environment. Because the distribution of heat transfer from the combusting particles to the furnace more closely matches the distribution of heat transfer that the furnace was originally designed for when it was commissioned as a coal-fired furnace, the apparent power of the burner may still be increased through an increase in the flow rate of the biomass fuel from the burner. Also, the above-described increase in furnace wear caused by conventional biomass burners is either decreased or avoided.

Thus, the invention provides multiple benefits. The invention can improve the overall system efficiency with minimum modifications on the current boiler combustion system. It can reduce a power plant's CO₂ foot print. Oxygen enrichment will reduce the flue gas volume. The proposed burner and combustion system only has one fuel and one fuel feeding system, so it is reduced in complexity in comparison to conventional biomass combustion processes. The avoidance of, or reduction in use of, a higher heating value auxiliary fossil fuel or biomass fuel reduces the operational cost. Finally, the apparent burner power may be increased beyond levels achievable with conventional biomass burners. Excess furnace wear may be reduced or avoided.

PROPHETIC EXAMPLES

Conventional and inventive biomass combustion processes were simulated in two dimensions axi-symmetrically with Fluent™ computational fluid dynamics (CFD) software.

Comparative Example

As best illustrated in FIGS. 3A, 3B, a burner was simulated that included a central stream 22 of rapeseed oil droplets (mean diameter of 100 μm) conveyed with primary combustion air, an annular stream 24 of primary combustion air, an annular stream 26 of wood pellets (mean diameter of 100 μm) conveyed with primary combustion air, and an annular stream 28 of secondary combustion air. The outer edge 29 of the secondary combustion air stream 28 had a diameter of 6 inches (15.24 cm). The outer edge 27 of the outer stream 26 had a diameter of 3 inches (7.62 cm). The outer edge 25 of the inner stream 24 had a diameter of 1 inch (2.54 cm). The outer edge 23 of the rapeseed oil stream 22 had a diameter of 0.375 inches (0.9525 cm). Each of the outer stream 26 and the secondary combustion air stream 28 was swirled with a 45° swirl angle. The rapeseed oil had a heating value of 39,000 kJ/kg and an elemental composition of C_18.95H_35.3O₂, while the wood pellets had a heating value of 19,700 kJ/kg. The physical and elemental compositions of the wood pellets are listed in Tables I and II, respective. The mass flow rates of the various streams 22, 24, 26, 28 are listed in Table III.

TABLE I
physical composition of wood pellets
physical component weight fraction
Volatile           0.75
Fixed Carbon       0.13
Ash                0.025
Moisture           0.095

TABLE II
elemental composition of wood pellets
element weight fraction
C       0.53128
H       0.05846
O       0.40615
N       0.00380
S       0.00031

TABLE III
Mass flow rates of streams
component                        Mass Flow Rate (kg/h)
primary combustion air in central stream 22 3.97
rapeseed oil droplets in central stream 22 0.6923
primary combustion air in annular stream 24 3.97
wood pellets in annular stream 26 26.5
primary combustion air in annular stream 26 26.5
secondary combustion air in annular stream 28 143.7

A “slice” of the burner and streams 22, 24, 26, 28 was taken along axis X-X and simulated with CFD.

EXAMPLES

As best illustrated in FIGS. 4A, 4B, a burner was simulated that included a central stream of oxygen 22 from an oxygen lance 23. An inner, annular stream 24 of wood pellets (mean diameter of 100 μm) conveyed with primary combustion air was injected from the inside of a fuel splitter 25 surrounding the central stream 22. An outer, annular stream 26 of the wood pellets conveyed with primary combustion air was injected from in between the splitter 25 and a fuel conduit 27. Finally, an annular stream 28 of secondary combustion air was injected from in between the fuel conduit 27 and a bore 29 in a burner block. The bore 29 had a diameter of 6 inches (15.24 cm). The fuel conduit 27 had a diameter of 3 inches (7.62 cm). The splitter 25 had a diameter of 1 inch (2.54 cm). The oxygen lance 23 had a diameter of 0.375 inches (0.9525 cm). The rapeseed oil and wood pellets are the same as those used for the Comparative Example. The mass flow rates of the various streams 22, 24, 26, 28, the levels of oxygen enrichment, and swirl angles for streams 26, 28 for Examples 1-9 are listed in Table IV.

TABLE IV
mass flow rates and enrichment/swirl parameters for Examples 1-9
	Mass Flow Rate (kg/h)
	Ex 1	Ex 2	Ex 3	Ex 4	Ex 5	Ex 6	Ex 7	Ex 8
component
oxygen in 32	1.86	1.86	3.31	1.86	1.86	1.86	3.72	1.86
primary combustion	1.37	1.37	2.74	1.37	1.37	1.37	1.37	2.74
air in 34
wood pellets in 34	1.37	1.37	2.74	1.37	1.37	1.37	1.37	2.74
primary combustion	26.5	26.04	24.67	26.5	26.5	26.5	26.04	24.67
air in 36
wood pellets in 36	26.5	26.04	24.67	26.5	26.5	26.5	26.04	24.67
secondary	143.7	135.1	128	143.7	143.7	143.7	127.1	135.1
combustion air in 38
parameter
O2 enrichment (%)	21.76	21.81	22.49	21.76	21.76	21.76	22.68	21.81
swirl angle of 36	45	45	45	45	63.44	63.44	45	45
swirl angle of 38	45	45	45	26.56	45	26.56	45	45

A “slice” of the burner and streams 32, 34, 36, 38 was taken along axis Y-Y and simulated with CFD.

Results

The temperature distribution in the flames and combustion chambers for the Comparative Example and Examples 1-8 are displayed in FIGS. 5-13. The simulation results show that combustion of wood pellets with a relatively small amount of oxygen at the center of the burner can achieve combustion results similar to those achieved with rapeseed oil but no central oxygen. Also, in the oxy-combustion cases, the results show that the flame shape and temperature profiles can be manipulated either by changing the mass flow rates of the different streams or by changing the swirl angle of the secondary combustion air stream and/or the swirl angle of the outer, annular stream of wood pellets.

A comparison of Examples 2 and 7 shows that, if the velocity of the center O₂ injection is doubled and the mass flow rate of the stream of secondary combustion air is decreased by a corresponding amount, the oxygen enriched combustion zone is extended and the flame ignition point is pushed further away from the burner tip. We can then conclude that the oxygen injection velocity may be used to control the flame ignition location and the main flame location. Also, the temperature in the oxygen enriched combustion zone is higher when the center O₂ injection velocity is doubled. This is important because the relatively high temperature could increase the stability of the main flame. We can also conclude that the center O₂ injection velocity provides an adjustable parameter that can be tailored to the particular heating value of the biomass being combusted in order to establish a robust, stable main flame.

A comparison of Examples 2 and 8 show that if the velocity of the inner, annular wood pellet stream is doubled and the mass flow rate of the outer stream of wood pellets is decreased by a corresponding amount, the relationship between the velocities of the wood pellet and oxygen streams is described by the equation: center oxygen velocity>inner, annular wood pellets velocity>outer, annular wood pellets velocity. This has the effect of increasing the mixing between the inner and outer annular streams of wood pellets and decreasing the mixing between the inner, annular stream of wood pellets and the central stream of oxygen. However, this reduces the oxygen enriched combustion zone—a condition which is not preferred with respect to main flame ignition and stabilization.

Preferred processes and apparatus for practicing the present invention have been described. It will be understood and readily apparent to the skilled artisan that many changes and modifications may be made to the above-described embodiments without departing from the spirit and the scope of the present invention. The foregoing is illustrative only and that other embodiments of the integrated processes and apparatus may be employed without departing from the true scope of the invention defined in the following claims.

Claims

1. A biomass burner, comprising: a burner block having an injector passage extending between rear and front faces;a fuel conduit having inlet and outlet ends and being concentrically disposed within said bore at said front face, an annular combustion air flow space being defined between an inner surface of said outer conduit and an outer surface of said fuel conduit;an oxygen injector having inlet and outlet ends and being concentrically disposed within said outer conduit at said front face; anda tubular fuel flow splitter concentrically disposed within said fuel conduit at said front face, said splitter having an inlet end disposed upstream of said fuel conduit inlet end and also having an outlet end, wherein said oxygen injector has:an annular cross-sectional shape and is adjacent to and surrounds said splitter; ora cylindrical cross-sectional shape and is concentrically disposed within said splitter.

2. The biomass burner of claim 1, wherein the outlet ends of the fuel conduit, oxygen injector, and splitter are flush with said front face.

3. The biomass burner of claim 1, wherein the outlet ends of said oxygen injector and splitter are recessed back from said fuel conduit outlet end.

4. The biomass burner of claim 1, wherein said oxygen injector has an annular cross-sectional shape and is adjacent to and surrounds said splitter.

5. The biomass burner of claim 4, wherein said oxygen injector outlet end is configured as a closed face with a plurality of radially distributed injection holes.

6. The biomass burner of claim 4, wherein said oxygen injector outlet end is configured as an open face.

7. The biomass burner of claim 1, wherein said oxygen injector has a cylindrical cross-sectional shape and is concentrically disposed within said splitter.

8. The biomass burner of claim 7, wherein said oxygen injector outlet end is configured as a closed face with a plurality of radially distributed injection holes.

9. The biomass burner of claim 7, wherein said oxygen injector outlet end is configured as an open tube.

10. The biomass burner of claim 1, further comprising an outer conduit concentrically disposed within said bore having an inlet end disposed downstream of said burner block rear face and also having an outlet end, said annular combustion air flow space being split into a secondary combustion air flow space and a tertiary combustion flow space by said outer conduit, the secondary combustion air flow space being defined by an outer surface of said fuel conduit and an inner surface of said outer conduit, the tertiary combustion air flow space being defined by an outer surface of said outer conduit and an inner surface of said bore.

11. The biomass burner of claim 1, further comprising: a secondary combustion air swirler disposed within said secondary combustion air flow space upstream of said burner block front face; anda tertiary combustion air swirler disposed along an inner surface of said bore adjacent to said burner block front face.

12. The biomass burner of claim 1, wherein said splitter further comprises a main section extending between said splitter inlet and outlet ends, the splitter inlet end having a diameter D1, the main body having a diameter D2, wherein D1<D2.

13. The biomass burner of claim 1, wherein said fuel conduit has a diameter D4, said splitter inlet end has a diameter D1 and 0.05 D4≦D1≦0.25 D4.

14. A biomass combustion system, comprising the biomass burner of claim 1, a biomass hopper, a biomass fuel feeder, a source of oxygen, and one or more blowers, wherein: said biomass fuel feeder is operatively associated with said hopper and at least one of said one or more blowers to receive particulate biomass from said hopper, convey the particulate biomass with a flow of combustion air from said at least one blower to provide a flow of biomass fuel, and direct the flow of biomass fuel to said fuel conduit inlet end;at least one of said one or more blowers is in fluid communication with said combustion air flow space; andsaid source of oxygen is in fluid communication with said oxygen injector inlet end.

15. The biomass combustion system of claim 14, wherein said source of oxygen is selected from group consisting of a vacuum swing adsorption system, an oxygen pipeline, a cryogenic air separation unit, and a vaporizer connected to a tank of liquid oxygen.

16. A biomass-fired boiler installation, comprising: a plurality of the biomass burner of claim 1;one or more blowers;at least one biomass hopper;at least one biomass fuel feeder;a source of oxygen; anda boiler, wherein said at least one biomass fuel feeder is operatively associated with said at least one hopper and at least one of said one or more blowers to receive particulate biomass from said at least one hopper, convey the particulate biomass with a flow of air from said at least one blower to provide a flow of biomass fuel, and direct the flow of biomass fuel to said fuel conduit inlet ends;at least one of said at least one blower is in fluid communication with said combustion air flow spaces;said source of oxygen is in fluid communication with said oxygen injector inlet ends; andsaid plurality of burners are mounted on walls of said boiler.

17. The biomass-fired boiler installation of claim 15, wherein said source of oxygen is selected from group consisting of a vacuum swing adsorption system, an oxygen pipeline, a cryogenic air separation unit, and a vaporizer connected to a tank of liquid oxygen.

18. A method of combusting biomass, comprising the steps of: injecting a flow of particulate biomass conveyed with air from a fuel conduit of a biomass burner into a combustion space, the fuel conduit having a tubular splitter concentrically disposed therein, the flow of biomass being split by the splitter into a central flow on the inside of the splitter and an annular flow on the outside of the splitter;injecting a flow of oxygen into the flow of injected biomass from an oxygen injector concentrically disposed within said fuel conduit;combusting the injected central flow of biomass with the oxygen in the combustion space;injecting an annular flow of combustion air from the burner around the annular flow of biomass; andcombusting the injected annular flow of biomass with the combustion air in the combustion space.

19. The method of claim 18, wherein the central biomass flow has a velocity V1, the annular biomass flow has a velocity V2, and the flow of oxygen has a velocity V3, where (V3−V2)<(V3−V1).

20. The method of claim 19, wherein no fuel other than the particulate biomass is combusted.

21. The method of claim 19, wherein the oxygen and the central flow of biomass begin to mix at a point upstream of said fuel conduit outlet end.

22. The method of claim 19, wherein said oxygen is injected in a center of the central flow of biomass.

23. The method of claim 19, wherein said oxygen is injected in an annulus surrounding said splitter.

24. The method of claim 19, wherein the oxygen is swirled.

25. The method of claim 19, wherein the oxygen has a concentration of >95%.

26. The method of claim 19, wherein the combustion air is injected in two annular flows, a first of which is secondary combustion air adjacent the annular flow of biomass and a second of which is tertiary combustion air adjacent the secondary combustion air.

27. The method of claim 19, wherein an overall oxygen enrichment of the combined biomass fuel, injected oxygen and combustion air achieved by injection of the oxygen is between 21% and 25%.