WO2014105736A1

WO2014105736A1 - Biomass combustion method and system

Info

Publication number: WO2014105736A1
Application number: PCT/US2013/077127
Authority: WO
Inventors: Yuan XUE; Remi Pierre Tsiava; Sebastian DE LIMON
Original assignee: L'air Liquide, Societe Anonyme Pour L'etude Et L'exploitation Des Procedes Georges Claude
Priority date: 2012-12-31
Filing date: 2013-12-20
Publication date: 2014-07-03
Also published as: US20140182491A1

Abstract

Improved combustion of biomass is achieved by injected first (8) and second (6) streams of biomass from a burner where the first stream (6) of biomass has a median particle size larger than the biomass of the second stream (6) and oxygen is injected with the first stream (8) to provide an oxygen-enriched environment around the larger median sized particles. The oxygen-enriched environment is achieved either by injecting the oxygen directly into the first stream (8) or by premixing the oxygen with the conveying air of the first stream (8).

Description

BIOMASS COMBUSTION METHOD AND SYSTEM

Background

Field of the Invention

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

Related Art

The emission of carbon dioxide (C0₂) 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 C0₂ 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 primary combustion air must have a velocity that is sufficiently high enough to convey the biomass particles from the burner and into the combustion space. Too low of a primary combustion air velocity will cause the particles to settle within the burner. Conversely, too high of a primary combustion air velocity will result in a residence time of the biomass particles that is too short to allow satisfactory burnout of the particles along the path line through the furnace. This might be no problem for relatively small biomass particles since they generally require shorter residence times for satisfactory burnout. However, this can be a problem for relatively larger biomass particles due to the higher residence times necessary for their satisfactory burnout along the path line.

In order to overcome the above-discussed problem presented by larger biomass particle sizes, the larger biomass particles produced by comminuting process can be separated from the smaller particles and then subjected to the comminuting process again. This milling and re-milling of the particles is complex and costly. It is complex due to the need for size-separation equipment. Moreover, the milling equipment must either be over-sized in order to thoroughly process all the biomass feed stock into adequately sized particles or it must necessarily result in a lower throughput in terms of adequately sized particles due to the presence of a side stream of larger particles needing to be recycled back to the milling device. The cost is in part due to the relatively poor grindability of biomass. While both biomass and coal can be ground to a desired size distribution, the grindability of biomass is significantly higher than that of coal due to its fibrous nature. Thus, for a given reduction from an initial particle size distribution (such as dso < 1000 ιημ) to a final particle size (such as dso < 200 ιημ), far more energy is consumed in achieving the particle size reduction in biomass as compared to coal. Additionally, the amount of energy necessary for size reduction increases exponentially as the final particle size decreases.

Even if the above-described problem associated with larger biomass particles

(i.e., the burnout time exceeds the residence time along the path line through the furnace) is not experienced when operating a burner at its nominal rated power, it can appear when the burner is operated at higher powers. The nature of the problem experienced at higher powers is described below.

In comparison to the relatively lower primary combustion air velocities when the burner is operated at lower power, the relatively higher primary 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.

A related 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 many biomass fuels have median particle sizes of 200-500 μιη. This may be contrasted with pulverized coal particles which typically have an average size of around 60 μιη and a dgo of < 100 μιη. 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. As a result of the foregoing, when a coal-fired boiler is retrofitted to combust biomass, more than 20 to 30% reductions in productivity can be experienced.

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

U.S. Patent Application No. 13/479,877, filed May 24, 2012, and entitled "Biomass Combustion". Application '877 discloses the splitting of an inner flow of biomass (conveyed with air) into a central flow and an annular flow with a splitter in a burner. Oxygen is injected either inside the central flow or in an annulus surrounding the central flow and secondary and optionally tertiary air is injected annularly around central and annular flows of biomass. Due to the difference in velocities between the oxygen flow and the central flow of biomass, enhanced mixing between these two flows results in relatively higher oxygen concentrations surrounding the biomass particles in the central flow. This tends to allow earlier flame ignition and increases the rate at which the biomass particles combust - all without requiring the biomass to be fired with a high cost, supplementary biofuel such as rapeseed oil. Application '877 does not disclose the use of separate flows of biomass fuel having different particle sizes. US 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 2008/0261 161 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.

US 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.

Thus, it is an object to provide biomass combustion systems and methods that allow complete burnout of relatively larger biomass particles. It is another object to provide biomass combustion systems and methods combusting relatively larger biomass particles that do not damage portions of the furnace that are not designed for excessive radiative heat transfer. It is yet another object to provide biomass combustion systems and methods combusting relatively larger biomass particles that do not limit the apparent power of the burner as the fuel flow rate is increased.

Summary

There is disclosed a biomass combustion method comprising the following steps. A first stream of fuel comprising particulate biomass, air, and oxygen is injected from a burner into a combustion chamber. A second stream of fuel comprising particulate biomass and air from the burner is injected into the

combustion chamber, the second stream not including any oxygen apart from the air present in the second stream. The biomass, air and oxygen of the first stream are combusted in the combustion chamber. The biomass and air of the second stream are combusted in the combustion chamber, wherein the particulate biomass in the first stream has a median particle size larger than that of the particulate biomass in the second stream. There is also disclosed a biomass combustion system, comprising a biomass burner, a biomass particle size separator, first and second biomass hoppers, first and second blowers, first and second fuel conduits, and a source of oxygen. The biomass particle size separator is adapted and configured to separate a biomass feed stock into first and second flows of biomass, the biomass in the first flow having a median particle size larger than that of the biomass in the second flow. The first and second biomass hoppers receive the first and second flows of biomass, respectively. The first blower is adapted and configured to direct a first stream of biomass from the first biomass hopper, conveyed with air from the first blower, to the biomass burner. The second blower is adapted and configured to direct a second stream of biomass from the second biomass hopper, conveyed with air from the second blower, to the biomass burner. The burner comprises a first injector receiving the first stream of biomass and a second injector receiving the second stream of biomass. The first fuel injector receives the first stream of biomass and is adapted and configured to inject it from the burner into a combustion chamber. The second fuel injector receives the second stream of biomass and is adapted and configured to inject it from the burner into a combustion chamber. The burner receives oxygen from the oxygen source and is adapted and configured to inject it with the first stream of biomass injected from the burner by the first fuel injector either premixed with the air of the first stream of biomass or not premixed with the air of the first stream of biomass.

There is also disclosed a biomass-fired boiler installation, comprising the above-disclosed biomass combustion system and a boiler, wherein the burner is oriented to inject the oxygen and first and second streams of biomass into a combustion chamber in an interior of the boiler.

The above-disclosed method, system and/or installation can include one or more of the following aspects:

- the biomass is selected from the group consisting of wood pellets, straw, hog fuel, crushed olive stones, dried sewage sludge, wood dust, and combinations thereof.

- the median particle size of the biomass of the first stream is less than 300 microns and the median particle size of the biomass of the second stream is greater than 400 microns. - the injected oxygen is no greater than 8% vol/vol of the total amount of oxidant injected from the burner.

- the injected oxygen is supplied by an oxygen source selected from the group consisting of an air separation unit, a vapor swing adsorption unit, a vaporizer fed with liquefied oxygen, an oxygen pipeline, and combinations thereof.

- a biomass feedstock is separated into a first flow of biomass having a

relatively larger median particle size and a second flow of biomass having a relatively smaller median particle size, wherein the first stream is derived from the first flow and the second stream is derived from the second flow.

- the first flow is fed to a first hopper, the second flow is fed to a second hopper, the first stream is drawn from the first hopper, and the second stream is drawn from the second hopper.

- the oxygen is premixed with the air of the first stream of biomass.

- 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 oxygen is injected with the first stream of biomass through injection of the oxygen by an oxygen injector disposed concentrically within the first fuel injector, wherein the first and second fuel injectors are annular, and wherein the second fuel injector is disposed concentrically around the first fuel injector.

- the oxygen is injected with the first stream of biomass through injection of the oxygen by a plurality of oxygen injectors radially distributed within the first fuel injector, wherein the first and second fuel injectors are annular.

- the oxygen is injected with the first stream of biomass through injection of the oxygen by an oxygen injector disposed within the first fuel injector, wherein the first fuel injector is disposed parallel and adjacent to the second fuel injector.

- there are a plurality of the burners.

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: Figure 1 is a schematic, cross-sectional view of an embodiment of a burner for use with the invention.

Figure 2 is a schematic, cross-sectional view of another embodiment of a burner for use with the invention.

Figure 3 is a schematic, cross-sectional view of another embodiment of a burner for use with the invention.

Figure 4A is an elevation, cross-sectional view of a boiler including side-fired burners for use with the invention.

Figure 4B is a top, cross-sectional view of the boiler of Figure 3A.

Figure 4C is an elevation, front-face view of an embodiment of one of the side-fired burners of Figures 3A and 3B.

Figure 4D is an elevation, front-face view of another embodiment of one of the side-fired burners of Figure 3A and 3B.

Figure 4E is an elevation, front-face view of yet another embodiment of one of the side-fired burners of Figure 3A and 3B.

Description of Preferred Embodiments

By injecting oxygen with a stream of relatively larger sized biomass particles conveyed with air (i.e., the first stream), the resultant oxygen-enriched air

surrounding the particles allows them to be ignited sooner. Satisfactory burnout is also achieved. A separate stream of relatively smaller sized biomass particles (i.e., the second stream) need only be combusted with air (no oxygen need be injected with the stream) because their smaller size already allows earlier ignition and shorter burnout times. Because the combustion air is not globally enriched with oxygen, the burner consumes far less oxygen. Because satisfactory burnout of the larger size biomass particles may be achieved, it is not necessary to expend significant amounts of additional energy in reducing the size of the larger size biomass particles in order to obtain the small size particles that enjoy earlier ignition and shorter burnout times. The oxygen may be injected with the relatively larger biomass particles by premixing the oxygen with the air conveying the relatively larger biomass particles or the oxygen may be injected into the stream of larger biomass particles from an oxygen injector inside the burner.

A stable flame rooted near the face of the burner is achieved because the locally high concentration of oxygen in the stream of larger biomass particles allows those particles to be ignited more easily and at an earlier point than they would otherwise without such a locally high concentration of oxygen. Thus, a combustion reaction between oxygen (from the oxygen and the air) and the larger biomass particles is commenced at an upstream zone adjacent the burner face. The greater surface area to volume or mass ratio of the relatively smaller biomass particles allows them to be combusted with only air (and not oxygen). Thus, there is no need to globally enrich the air injected from the burner from all sources of combustion air.

The type of burner is not critical to the invention.

A pipe-in-pipe burner may be utilized where one of the two streams of biomass is injected from the inner pipe while the other of the two streams of biomass is injected from the outer pipe. In this case, the oxygen is injected in one of a variety of ways. It may be injected in a premixed state with the air of the stream of larger biomass particles. It may instead be injected from an oxygen injector disposed within the inner pipe in which case the stream of larger biomass particles is injected from the inner pipe. It may instead be injected from a plurality of oxygen injectors radially distributed within the outer pipe in which case the stream of larger biomass particles is injected from the outer pipe.

The burner may instead be of the side wall fired type including two fuel ports (one for each stream of biomass) and an overfire air port. The oxygen may be injected in a premixed state with the air of the stream of larger biomass particles. It may instead by injected from an oxygen injector disposed within the fuel port from which the stream of larger biomass particles is injected. In either case, the fuel ports and/or oxygen injector may be of any configuration.

The combustion air may be swirled or not. The combustion air may be two streams each one of which conveys one of the streams of biomass or additional combustion air streams may be utilized including a secondary combustion air stream surrounding or adjacent to or spaced from the biomass streams and even a tertiary combustion air stream surrounding or adjacent to or spaced from 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.

As best illustrated in FIG 1 , a first type of pipe-in-pipe burner includes an inner first fuel injector 1 , an outer second fuel injector 12, and a secondary combustion air pipe 14 disposed within a burner block B which is mounted on a wall W of the combustion space C. The burner injects a first stream 8 of larger particle size biomass conveyed with air from the fuel injector 1 . Oxygen is injected into stream 8 from a single oxygen injector 2 which is disposed within the first fuel injector 1 . A second stream 6 of smaller particle size biomass conveyed with air is injected from the second fuel injector 12. Finally, a stream 5 of secondary combustion air is injected from the secondary air pipe 14 through swirler 10. The biomass from the first stream 8 combusts with the oxygen and conveying air in an inner core of the flame while the biomass from the second stream 6 combusts with conveying air in an outer portion of the flame.

As best shown in FIG 2, a second type of pipe-in-pipe burner again includes an inner first fuel injector 1 , an outer second fuel injector 12, and a secondary combustion air pipe 14 disposed within a burner block B which is mounted on a wall W of the combustion space C. The burner injects a first stream 8 of smaller particle size biomass conveyed with air from the fuel injector 1 . A second stream 6 of smaller particle size biomass conveyed with air is injected from the second fuel injector 12. Instead of injecting oxygen into one of the streams 8, 6, the oxygen is already premixed with the conveying air of one of the streams 8, 6, so there are no oxygen injectors per se within the burner. Finally, a stream 5 of secondary combustion air is injected from the secondary air pipe 14 through swirler 10. If the larger size biomass is injected as stream 8, it combusts with oxygen-enriched conveying air in an inner core of the flame while the biomass from the second stream 6 combusts with the conveying air in an outer portion of the flame. If the larger size biomass is injected as stream 6, it combusts with oxygen-enriched conveying air in an outer portion of the flame while the biomass from the first stream 8 combusts with the conveying air in the inner core of the flame.

In a variation of the burner of FIG 1 and as best illustrated in FIG 3, a third type of pipe-in-pipe burner includes an inner first fuel injector 1 , an outer second fuel injector 12, and a secondary combustion air pipe 14 disposed within a burner block B which is mounted on a wall W of the combustion space C. The burner injects a first stream 8 of smaller particle size biomass conveyed with air from the fuel injector 1 . A second stream 6 of smaller particle size biomass conveyed with air is injected from the second fuel injector 12. Oxygen is injected into stream 8 from a plurality of oxygen injectors 2 which is radially distributed within the second fuel injector 12. There are at least three oxygen injectors 2 in the burner of FIG 3 with an upper number only being limited by the available space and the pressure drop in the stream 6 created by the injectors 2. Finally, a stream 5 of secondary combustion air is injected from the secondary air pipe 14 through swirler 10. The biomass from the first stream 8 combusts with conveying air in an inner core of the flame while the biomass from the second stream 6 combusts with the oxygen and conveying air in an outer portion of the flame.

The oxygen injector 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 one of streams 6, 8.

As best shown in FIGS 4A-E, the burner may instead be configured as a side wall-fired burner 32 mounted on one of the walls of the interior of a boiler 31 .

FIG 4C illustrates a first variation of the burner 32 where the first stream of larger size biomass particles is injected from a first fuel port 42 and the second stream of smaller size biomass particles is injected from a second fuel port 41 .

Secondary combustion air is injected from air ports 44 above and below the fuel ports 41 , 42. The oxygen is injected from an oxygen channel (i.e., oxygen injector) 43 disposed vertically within the first fuel port 42. FIG 4D illustrates a second variation of the burner 32 similar to that of FIG 4E where the oxygen channel (i.e., oxygen injector) 43 is disposed horizontally within the first fuel port 42. Finally, FIG 4E illustrates a third variation of the burner 32 similar to those of FIGS 4D-E where the oxygen channel (i.e., oxygen injector) 43 has a cross-shape. In each of the burners of FIGS 4C-E, the surface area of the oxygen injector typically does not take over 30% of the surface area of the first fuel port 42 from which the larger size biomass stream is injected. Instead of the oxygen channel (i.e., oxygen injector) 43 configurations of FIGS 4C-E, the oxygen injector could comprise a plurality of pipes adjacent to one another instead of a channel (not shown).

Regardless of whether the oxygen is injected according to the burner configurations of FIGS 1 , 2, 3, or 4C-E, the presence of oxygen in a stream of larger size biomass (whether the oxygen is pre-mixed with the combustion air or injected into such stream) helps increase burnout of that fuel in comparison to a conventional biomass burner where no such oxygen injection is employed. Burnout is increased because the local oxygen concentration surrounding the larger biomass particles 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. There is no need to inject oxygen into the stream of smaller biomass particles or enrich its conveying air because the smaller size of those particles allows them to be satisfactory burned out more quickly.

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 oxygen injection or premixing of the conveying air 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 larger biomass particles. Primarily, this is because the larger biomass particles (with a median size of around 200-500 μιη) combust more slowly than typical pulverized coal particles (with an average size of around 60 μιη). Although the furnace may have been designed to achieve the nominal power with the more quickly combusting coal particles, the more slowly combusting larger 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.

When conventional biomass burners are instead fed only with relatively smaller size biomass particles in an effort to avoid the above two disadvantages, such an avoidance still results in a higher cost associated with more grinding/milling of all of the biomass and/or wasting of the larger biomass particles after separation from the smaller biomass particles. The energy consumed in milling/grinding biomass is a major component of its cost as a fuel.

In contrast, by enriching the larger biomass particles with oxygen (either through injection or premixing with the conveying of that stream), the above disadvantages may be avoided without having to increase the energy consumption and cost associated with conventional biomass burner operation. The higher oxygen concentrations surrounding the larger biomass particles tends to ignite the flame earlier and increases the rate at which the larger 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 (fired with larger size biomass particles or a mixture of larger and smaller biomass particles) 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 C0₂ foot print. Oxygen enrichment will reduce the flue gas volume. Oxygen enrichment of only the stream of larger biomass particles is less costly than global enrichment of the burner's combustion air. 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. 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

What is claimed is:

1 . A biomass combustion method, comprising the steps of:

injecting a first stream of fuel comprising particulate biomass, air, and oxygen from a burner into a combustion chamber;

injecting a second stream of fuel comprising particulate biomass and air from the burner into the combustion chamber, the second stream not including any oxygen apart from the air present in the second stream;

combusting the biomass, air and oxygen of the first stream in the combustion chamber; and

combusting the biomass and air of the second stream in the combustion chamber, wherein the particulate biomass in the first stream has a median particle size larger than that of the particulate biomass in the second stream.

2. The biomass combustion method of claim 1 , wherein the biomass is selected from the group consisting of wood pellets, straw, hog fuel, crushed olive stones, dried sewage sludge, wood dust, and combinations thereof.

3. The biomass combustion method of claim 1 , wherein the median particle size of the biomass of the first stream is less than 300 microns and the median particle size of the biomass of the second stream is greater than 400 microns.

4. The biomass combustion method of claim 1 , wherein the injected oxygen is no greater than 8% vol/vol of the total amount of oxidant injected from the burner.

5. The biomass combustion method of claim 1 , wherein the injected oxygen is supplied by an oxygen source selected from the group consisting of an air separation unit, a vapor swing adsorption unit, a vaporizer fed with liquefied oxygen, an oxygen pipeline, and combinations thereof.

6. The biomass combustion method of claim 1 , further comprising the step of separating a biomass feedstock into a first flow of biomass having a relatively larger median particle size and a second flow of biomass having a relatively smaller median particle size, wherein the first stream is derived from the first flow and the second stream is derived from the second flow.

7. The biomass combustion method of claim 6, wherein:

the first flow is fed to a first hopper;

the second flow is fed to a second hopper;

the first stream is drawn from the first hopper; and

the second stream is drawn from the second hopper.

8. The biomass combustion method of claim 6, wherein the oxygen is premixed with the air of the first stream of biomass.

9. A biomass combustion system, comprising a biomass burner, a biomass particle size separator, first and second biomass hoppers, first and second blowers, first and second fuel conduits, and a source of oxygen, wherein:

the biomass particle size separator is adapted and configured to separate a biomass feed stock into first and second flows of biomass, the biomass in the first flow having a median particle size larger than that of the biomass in the second flow; the first and second biomass hoppers receive the first and second flows of biomass, respectively;

the first blower is adapted and configured to direct a first stream of biomass from the first biomass hopper, conveyed with air from the first blower, to the biomass burner;

the second blower is adapted and configured to direct a second stream of biomass from the second biomass hopper, conveyed with air from the second blower, to the biomass burner;

the burner comprises a first injector receiving the first stream of biomass and a second injector receiving the second stream of biomass;

the first fuel injector receives the first stream of biomass and is adapted and configured to inject it from the burner into a combustion chamber; the second fuel injector receives the second stream of biomass and is adapted and configured to inject it from the burner into a combustion chamber;

the burner receives oxygen from the oxygen source and is adapted and configured to inject it with the first stream of biomass injected from the burner by the first fuel injector either premixed with the air of the first stream of biomass or not premixed with the air of the first stream of biomass.

10. The biomass combustion system of claim 9, 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.

1 1 . The biomass combustion system of claim 9, wherein:

the oxygen is injected with the first stream of biomass through injection of the oxygen by an oxygen injector disposed concentrically within the first fuel injector; the first and second fuel injectors are annular; and

the second fuel injector is disposed concentrically around the first fuel injector.

12. The biomass combustion system of claim 9, wherein:

the oxygen is injected with the first stream of biomass through injection of the oxygen by a plurality of oxygen injectors radially distributed within the first fuel injector; and

the first and second fuel injectors are annular.

13. The biomass combustion system of claim 9, wherein:

the oxygen is injected with the first stream of biomass through injection of the oxygen by an oxygen injector disposed within the first fuel injector; and

the first fuel injector is disposed parallel and adjacent to the second fuel injector.

14. A biomass-fired boiler installation, comprising the biomass combustion system of claim 9 and a boiler, wherein the burner is oriented to inject the oxygen and first and second streams of biomass into a combustion chamber in an interior of the boiler.

15. The biomass-fired boiler installation of claim 14, wherein there are a plurality of the burners.