Field of the Invention

The present invention relates to combustion, especially in coal-fired boilers.

Background of the Invention

Large international research efforts are currently being expended to develop technologies to capture and sequester C0 ₂ from utility boilers. One of the promising approaches is to use oxy-fuel firing in boilers to produce flue gas with a very high concentration of C0 ₂ which can be subsequently separated, compressed and stored in appropriate underground storage sites. The existing boilers, however, are designed to be fired with air and process modifications are required to be fired with oxygen in stead of air. Over a hundred years of power plant development has resulted in highly complex and optimized boiler designs for air firing to maximize the steam power cycle efficiency. These designs require careful attention to the heat absorption pattern and the heat flux distributions. This is particularly true in coal fired boilers where mineral matter in the coal can lead to extensive slagging, fouling, and submicron ash formation depending on the flame temperature. One of the most important design parameters is the heat absorption pattern in the boiler. The proportion of heat absorbed to preheat the boiler feedwater to the boiling temperature, evaporate the water, and superheat the steam must be carefully controlled to achieve the required steam characteristics. This heat absorption pattern, coupled with temperature and pressure limitations of tube materials, tend to define conventional boiler designs. For example, feedwater is heated first in the economizer which is an indirect heat exchanger placed in the low temperature section of the convective passage of the boiler. High radiative heat flux from the flame zone to the 'water walls" formed by boiler tubes in the furnace section, i.e, the combustion chamber in the boiler, is used to evaporate boiler feedwater from the economizer. The relatively low temperature of the preheated feedwater and the high heat transfer coefficients under boiling conditions for subcritical boilers keep the tube temperature within the allowable material limit. (A similar heat transfer arrangement is used in a supercritical steam boiler although the distinct phase change from liquid water to steam does not exist above the supercritical pressure of water, i.e., above 3208.2 psia. )

Steam generated in the water walls is then superheated in the superheater tubes placed in the upper furnace section ("radiant superheater") or in the high temperature section of the convective passage of the boiler. The high pressure superheated steam is fed into the high pressure steam turbine and expanded to an intermediate pressure at a reduced temperature while generating power. This steam is then re-introduced into the reheater section of the boiler to raise the steam temperature again. Reheater tubes are also placed in the upper furnace section ("radiant reheater") or in the high temperature section of the convective passage of the boiler. The reheated steam is then fed into the intermediate pressure turbine for expansion and power generation. The steam from the intermediate turbine is typically divided into three streams and fed to the low pressure turbine, to the feedwater heater, and to the turbine to drive the feed water pump.

The flue gas temperature at the exit of the furnace (FEGT) is typically in a range between 1800 F and 2200 F and cool enough that heat flux to the superheat tubes is reduced. The lower heat flux in the convective section allows tube temperature limits to be avoided, even with high steam temperatures and relatively low steam side heat transfer coefficients.

In a typical 660 MW coal fired boiler with a heat rate of 9,500 Btu/kWh the average heat flux to water walls ranges from approximately 44,000 Btu/hr/ft ² for severely slagging lignite to 68,000 Btu/hr/ft ² for low to medium slagging bituminous coal. The ratio of the furnace volume to the total water wall surface area for these large coal fired boilers is in a range between 13 ft to 15 ft. The furnace size is enlarged for lignite, which reduces the average heat flux per unit water wall surface area and the surface temperature of the ash deposit. By reducing the surface temperature slagging problems can be avoided. The critical slagging temperature mainly depends on the ash composition. The ash deposit surface temperatures ("slag temperature") and the boiler tube surface temperature increase approximately linearly with the local heat flux.

Oxy-fuel combustion produces much higher adiabatic flame temperatures than air- fuel combustion. In order to convert an existing air-coal fired boiler to oxy-coal firing flue gas recirculation (FGR) is required to modulate the heat flux in the furnace section and to provide a sufficient volume of hot flue gas at proper temperature to satisfy the convective duties. Typical air fired boiler requires about 11 lb of air per 1 lb of coal. For oxy-coal fired boiler about 2 lb of oxygen and about 7 lb of recycled flue gas per 1 lb of coal is required to match the original air fired boiler conditions. Many technical experts have stated that FGR is required even for newly designed oxy-fuel fired boilers due to high adiabatic flame temperature and the resulting high heat flux of direct oxy-fuel combustion. In fact all of the proposed oxy-coal boiler projects for C02 capture are based on FGR to control the flame temperature and to maintain proper balancing of heat transfer to different parts of the steam cycle. FGR reduces boiler efficiency and increases the costs of boiler construction and operation. A directly fired oxy-fuel boiler furnace with no FGR is ideally desired. For air-coal fired boilers typically 1 to 2 lb of heated "primary air" per lb of coal is required for grinding and transporting of coal to burners at a minimum velocity of 3000 ft/min. For oxy-coal fired boilers the coal transport gas to replace the primary air should be recycled flue gas (RFG) with optional addition of some oxygen. High purity oxygen should not be used as the transport gas for safety reasons. "Recycled flue gas" (RFG) means a portion of the flue gas exited from the boiler that is re-introduced into the boiler with or without downstream treatment to remove some of the flue gas components such as water vapor, particulates, sulfur oxides and nitrogen oxides. "Recycled flue gas" (RFG) includes a purified carbon dioxide (C0 ₂ ) stream produced in a downstream C0 ₂ separation unit. In order to minimize nitrogen concentration in the flue gas stream the oxy-fuel fired boiler and the down stream flue gas treatment units must be designed to prevent air leakage into the flue gas stream. Brief Summary of the Invention

One aspect of the present invention is a combustion unit comprising a burner zone, a top zone above the burner zone, a bottom zone below the burner zone, and a flue gas passage either located in the top zone or in the bottom zone wherein

said burner zone comprises,

(A) a burner wall, a facing wall, and side walls extending from the burner wall to the facing wall thereby defining a combustion enclosure, and wherein each of said walls is configured to contain passages for carrying fluid within the wall and to heat said fluid by indirect heat exchange with heat from combustion in the combustion space,

(B) burners located in said burner wall which are capable of combusting fuel and oxidant containing at least 50 vol.% oxygen, preferably at least 80 vol. % oxygen, in the space between said burner wall and said facing wall, wherein the burners are arrayed in at least 2 columns each of which contains at least 2 burners,

(C) one or more partition walls in said burner zone and extending from the burner wall to the facing wall, wherein each partition wall extends from the burner wall between adjacent columns of said burners and thereby defines combustion spaces each of which is directly fired by at least one column of burners, and wherein each partition wall is configured to contain passages for carrying fluid within the partition wall and to heat said fluid by indirect heat exchange with heat from combustion in the combustion spaces on each side of the partition wall.

Another aspect of the invention is a combustion device comprising (A) a combustion enclosure having a roof, a floor, and side walls extending from the roof to the floor and enclosing the enclosure,

(B) at least 2 burners located in said roof which are capable of combusting fuel and oxidant containing at least 50 vol.% oxygen, preferably at least 80 vol. % oxygen, in the space below said roof,

(C) partition walls in said combustion enclosure and extending downward from the roof, and defining combustion spaces each of which is directly fired by only one of said burners located in said roof, and wherein each partition wall is configured to contain passages for carrying fluid within the partition wall and to heat said fluid by indirect heat exchange with heat from combustion in the combustion spaces on each side of the partition wall. Another aspect of the present invention is a method of combustion, comprising combusting fuel and oxidant containing at least 50 vol.% oxygen, preferably at least 80 vol. % oxygen, in any of the foregoing combustion units.

Yet another aspect of the present invention is a method of modifying a combustion apparatus wherein the apparatus comprises a burner wall, a facing wall, and side walls extending from the burner wall to the facing wall thereby defining a combustion enclosure and the apparatus further comprises burners located in said burner wall which are capable of combusting fuel and oxidant containing at least 50 vol.% oxygen, preferably at least 80 vol. % oxygen, in the space between said burner wall and said facing wall, wherein the burners are arrayed in at least 2 columns each of which contains at least 2 burners, the method comprising

providing one or more partition walls in said combustion enclosure and extending from the burner wall to the facing wall, wherein each partition wall extends from the burner wall between adjacent columns of said burners and thereby defines combustion spaces each of which is directly fired by one column of burners, and wherein each partition wall is configured to contain passages for carrying fluid within the partition wall and to heat said fluid by indirect heat exchange with heat from combustion in the combustion spaces on each side of the partition wall.

In preferred embodiments of the foregoing, burners are also located or provided in said facing wall which are capable of combusting fuel and oxidant containing at least 50 vol.% oxygen, preferably at least 80 vol. % oxygen, in the space between said burner wall and said facing wall, wherein the burners are arrayed in the facing wall in at least 2 columns each of which contains at least 2 burners, and the partition wall is configured to enable said fluid to be heated also by indirect heat exchange with heat from combustion at the burners in said facing wall in the combustion spaces on each of side of the partition wall.

Brief Description of the Drawings

Figures la and lb are sectional elevation views of one embodiment of apparatus with which the present invention can be practiced.

Figure lc is a top view of the apparatus of Figure la.

Figure 2 is a cutaway perspective views of one embodiment of burner zone.

Figure 3a is a perspective view of a portion of a partition wall useful in the present invention.

Figures 3b, 3c and 3d depict sectional plan views of alternative partition wall designs.

Figures 4a and 4b are sectional elevation views of the embodiment of the invention employing roof-mounted burners.

Figure 4c is a sectional plan view of the embodiment of Figure 4a.

Detailed Description of the Invention

Figures la, lb, lc, and 2 depict one embodiment of apparatus with which the present invention can be practiced. However, many other embodiments are useful, varying for instance in the number of burners, or in the number of rows of burners, or in the number of columns of burners.

Figures la depicts an elevation view of a combustion unit 1 with vertical partition walls 6, 7 and 8, creating four segmented combustion spaces 141, 142,

143, and 144. Figure lb is a cross-sectional elevation view along the centerline of a column of burners in combustion space 144. Figure lc is a cross-sectional top plan view of combustion unit 1. Figure lc shows only the top most burners in each column, namely, the top row comprising burners 15, 25, 35 and 45. The burners in each wall are fed fuel and oxidant, to combust the fuel and oxidant in each combustion space. Partition walls 6, 7 and 8 are provided within combustion unit 1. The partition walls 6, 7, and 8 preferably extend from burner wall 2 to facing wall 3. Each partition wall separates combustion unit 1 into combustion spaces 141 through 144. More generally, partition walls should be provided so that, preferably, there are as many combustion spaces as there are columns of burners. Optionally two or three columns of burners can be placed between two adjacent partition walls. Since the heat flux received by each partition wall becomes essentially the same as that received by the side wall of combustion unit 1 in this invention, the design of the partition wall can be very similar to the conventional water wall.

Without partition walls 6, 7 and 8, and when the burners combust coal or other fuel with air, unit 1 could be very similar in geometry to a conventional combustion chamber found in an air- fired boiler useful (for example) in electric power generation. The apparatus and method of the present invention are also useful to produce steam for electric power generation by combustion of fuel. Combustion unit 1 includes, in addition to the combustion spaces, burner zone 91, unfired top zone 90, bottom zone 92 which optionally has bottom hopper zone 93, and flue gas passage 95. Columns of burners 10-14 and 15-19 are located in combustion space 144 in the combustion zone. Additional columns of burners are located in the other combustion spaces, but for simplicity the other columns of burners are not numbered. Preferably partition walls 6, 7 and 8 extend most of the height of burner zone 91 and optionally extend to top zone 90 and bottom zone 92.

Radiant superheater and reheater tubes (not shown) are placed in the top zone 90 as is typically designed in the conventional air fired boiler. Radiant superheater and reheater tubes (not shown) are preferably placed in bottom zone 92 as well in this invention with one or more optional burners 50 and 51 to control the bottom zone temperature and heat flux. Bottom zone 92 has an optional hopper zone 93 with tapered walls of conventional design to collect ash and slag from coal combustion. Optionally the bottom hopper zone is fired with one of more oxy-fuel burners (not shown). Optionally radiant superheater and reheater tubes are placed in the bottom hopper zone as well in this invention. Flue gas from the burner zone preferably passes the unfired top zone (or the transition zone) and exits through flue gas passage 95 before it enters a convective heat transfer unit (not shown) as is the case in a typical air fired boiler. Since the volume of flue gas generated under oxy-fuel firing is only 1/3 to ¼ of that under air-fuel firing, there is considerable design flexibility in locating the flue gas passage and the convective heat transfer unit. For example flue gas passage 95 can be optionally located in bottom zone 92.

Combustion unit 1 includes burner wall 2, facing wall 3 which faces burner wall 2, and side walls 4 and 5 which extend from burner wall 2 to facing wall 3. Walls 2, 3, 4 and 5 together define a combustion zone enclosure 9. Figure 2 depicts the burner zone of combustion unit 1 with partition walls 6, 7 and 8 and the enclosure 9. Burners 10 through 14, 20 through 24, 30 through 34 and 40 through 44 are situated in burner wall 2 so that they combust fuel and oxidant within the combustion enclosure defined by walls 2, 3, 4 and 5 (which are shown in Figure lc). The burners are arrayed in columns and rows. Each burner is fed fuel and oxidant. For simplicity, Figure 2 depicts only the feeding of fuel and oxidant to burners 10-14. Fuel can be fed from feed line 101 into each of feed lines 108 through 112, to burners 10 through 14 respectively. Oxidant can be fed from feed line 102 into each of feed lines 103 through 107, to burners 10 through 14, respectively.

Optionally, but preferably in large boilers, rows and columns of burners are also provided in facing wall 3, as indicated by Figures lb and lc. The fuel fed to the aforementioned burners is preferably finely divided solid carbonaceous fuel, such as coal, coke, biomass or petroleum coke. The fuel is preferably fed as a stream mixed with transport gas, such as transport air or recycled flue gas (RFG). Other conventional fuels such as oil and gas can be used.

In conventional operation the oxidant fed to the aforementioned burners is air. In the practice of this invention, the oxidant fed to each burner is a gaseous stream comprising at least 50 vol.% oxygen, preferably at least 80 vol. % oxygen, more preferably at least 90 vol.% oxygen. Such a stream can readily be obtained from commercial sources, or, if desired, it can be produced by an adjacent apparatus such as a cryogenic ASU or a VPSA that separates oxygen from air. When a transport gas containing oxygen is used to transport solid fuel such as coal, the transport gas becomes a part of the oxidant for combustion. The oxygen concentration defined above should be interpreted as the average oxygen concentration of all of the oxygen containing streams fed to each burner.

Referring now to Figure 3 a, partition wall 6 (and preferably all partition walls) is composed of a single row of tubes with open spaces between tubes. Each tube has an inlet 61 for fluid (typically feedwater) that is to be heated, outlet 62 for heated fluid, an inlet manifold (not shown) to supply and distribute the fluid to each tube and an outlet manifold to collect the heated fluid from each tube. Each partition wall acts as a heat exchanger. A heat exchanger is any unit that provides indirect heat exchange of heat generated by combustion in a combustion space to the fluid. It will be recognized that the fluid passing through the partition wall is typically H ₂ 0 in any state, i.e. liquid, steam, supercritical, or any combination thereof. It will also be recognized that the fluid can undergo partial or complete change of state while passing through the partition wall.

The inlet and outlet manifolds for fluid being heated as the fluid passes through partition walls can be connected to other inlet and outlet manifolds in other partition walls and outer walls of the combustion unit, and/or to other heat exchangers such as superheater sections, reheater sections, and economizer sections, in any desired circuit that is effective to convert incoming feedwater to steam, superheated steam, and/or supercritical H ₂ 0, as desired. Figure 3b depicts in a cross sectional view an alternative partition wall composed of a membrane wall which is made of a row of tightly spaced tubes with welded plates connecting tubes on the diameter. It is a commonly used design in the conventional boiler as the outer wall of a typical boiler, but in the present invention unlike the typical boiler each partition wall receives heat flux on both sides and the heat flux to each side has to be properly controlled to prevent overheating of the partition wall. Figure 3 c depicts in a cross sectional view an alternative partition wall composed of two membrane walls (each like the wall described in Figure 3b) closely spaced to each other. In this design each membrane wall receives heat flux only on one side, i.e., as with an outer wall. Figure 3d depicts in a cross sectional view an alternative partition wall composed of three rows of multiple tubes with open spaces between any two adjacent tubes. First row 71 and third row 73 of tubes carry feedwater preferably heated in the economizer and receive intense flame radiation from adjacent combustion spaces. They act as "screen tubes' to partially shield radiative heat flux to the superheater/reheater tubes that are located in the middle row 72 (i.e., the second row). Due to the open space between tubes flue gas generated in the adjacent combustion spaces can interact and mix in this partition wall design.

Figures 4a, 4b and 4c depict another embodiment, employing roof- mounted burners firing downward. Side walls 202 through 205 and roof 201 define a combustion enclosure. Partition walls 206, 207, 208 and 209 divide the combustion space into individual combustion spaces. Radiant superheater and reheater tubes (not shown) are preferably placed in bottom zone 292 which optionally has bottom hopper zone 293 with tapered walls. Flue gas generated in combustion spaces flows through bottom zone 292 and then flows into a convective section (not shown) through flue gas passage 295. Burners 210 through 217 are situated in the roof 201 and combust fuel and oxidant in the respective combustion spaces, preferably one burner in each combustion space. Optionally multiple burners can be placed in each combustion space. The burners are fed fuel and oxidant through feed lines in the same manner as described herein with respect to the embodiment of Figures la, lb, lc and 2. The partition walls are provided with inlets, outlets and passages for fluid to be heated within the partition walls, in the manner described above with respect to Figures 3a, 3b, 3c, and 3d.

The present invention provides numerous advantages.

The invention minimizes the FGR requirement to control the heat fluxes to water walls and SH/RH tubes on oxy-fuel fired boilers, and keeps the boiler thermal conditions, especially the peak boiler tube temperature, within the limits of conventional air-fired boilers. The invention reduces the boiler size and simplifies the operation. The total water wall area of the furnace with partition walls can be increased more than twice the original furnace while maintaining the same overall furnace volume. As a consequence, the average wall heat flux of the furnace with the partition walls can be reduced to less than half of the original furnace at the same total firing rate while keeping the FEGT in the same range as in air firing (typically between 1800 to 2400 °F).

There are other advantages. Each combustion space has a large height to width ratio which reduces the radiant heat fluxes to the unfired top and bottom zones and facilitates the placement of pendant type superheater and reheater tubes. Each flame is placed in the same thermal environment surrounded by furnace side walls or partition walls, which would reduce the peak flame temperature. Once a flame has been optimized for proper heat flux, high carbon burnout and low NOx emission, the same burner setting can be used for all other burners. In a conventional wall fired furnace tuning of individual burners is very difficult as middle flames are surrounded by other flames and operate at a higher flame temperature that the flame adjacent to a side wall. Since the sensible heat of flue gas after the furnace section of a boiler under the oxy-fuel firing with minimum FGR is roughly 1/3 of that of the corresponding air firing, most of superheater and reheater tubes are located in the furnace section. In order to control the heat flux some of the SH/RH tubes can be located behind screen tubes for heating or boiling feed water as depicted in Figure 5.

Placing each flame in the same thermal environment surrounded by furnace side walls and partition walls reduces the peak flame temperature. Once a flame has been optimized for uniform heat flux, high carbon burnout and low NOx emission, the same burner setting can be used for all other burners. In a conventional wall fired furnace tuning of individual burners is very difficult as middle flames are surrounded by other flames and operate at a higher flame temperature that the flame adjacent to a side wall. The use of the screen tube arrangement enables the placement of SH/RH tubes in the burner zone without exceeding the tube temperature limits.

For reliable commercial operation it is highly desirable to keep the boiler thermal conditions for minimum flue gas recycle oxy-coal combustion within the range of the well proven boiler designs for air firing. In a typical air-coal fired boiler, the peak heat flux to the water walls is about 85,000 Btu/hr/ft ² located near the upper burner zone and the average heat flux is about 50,000 Btu/hr/ft ² . The highest slag temperature occurs in the area of the peak heat flux. Thus, controlling the peak heat flux below the ash slagging temperature is an important design criterion. In the conventional air-coal fired boiler designs the specific heat input to the furnace per square foot of furnace plan area is typically limited 1.5 to 1.8 MMBtu/hr/ft ² to avoid slagging problems. If the same boiler furnace design were used under oxy-fuel firing without FGR the specific heat input has to be reduced to 60 to 70% of the air firing condition, i.e., to 0.9 to 1.3 MMBtu/hr/ft ² . With the present invention the specific heat input under oxy-fuel firing can be increased to as high as 3.0 MMBtu/hr/ft ² .

The key design parameters for minimum flue gas recycle oxy-coal combustion include the peak boiler tube surface temperature, the peak heat flux, and the peak flame and gas temperatures. Furnace exist gas temperature (FEGT) should also be kept similar to those in conventional air-fired boilers to control the thermal conditions of the convective pass. At FEGT=2000 °F, about 55 to 60 % of the total heat transferred to water/steam is absorbed in the furnace and transition sections and about 30 to 35% is absorbed in the convective section under typical hot air-firing. If similar boiler furnace design is used under oxy-firing at the same FEGT=2000 °F, about 80 to 85% is absorbed in the furnace and transition sections, only about 15 to 20%> is absorbed in the convective section. This large shift in the heat absorption pattern is caused by the sensible heat contained in the nitrogen under hot air firing, which is eliminated under oxy-firing.

Typical heat duties of a conventional subcritical 660 MW coal-air fired boiler with a heat rate of 9,500 Btu/kWh and an oxy-fuel fired boiler with minimum FGR are compared in Table 1. For the oxy-coal boiler systems, coal is pulverized and transported by preheated flue gas. A portion of the cooled flue gas after a SOx scrubber is recirculated and heated in a recirculated flue gas (RFG) heater in the convective section. The water vapor content of RFG is assumed to be saturated at 106 F. FEGTs for the air- fuel and oxy-fuel cases are assumed to be 2100 F and 1900 F respectively. The temperature of flue gas after the economizer is assumed to be 750 F. The temperature of the flue gas after the air heater for the air-fuel case is assumed to be 350 F. For oxy-coal fired cases the flue gas after the RFG heater is cooled further in an auxiliary feedwater heater to 350 F. Thus, a portion of the steam normally extracted from steam turbines for feedwater heaters is eliminated, which increases the overall steam turbine output. In order to normalize the boiler condition at the same net steam output the other boiler heat duties for oxy-fuel cases are reduced slightly to account for the auxiliary feedwater heating.

As shown in Table 1, 42.4% of the heat duties are absorbed in the convective zones under the air-coal firing, while under the oxy-coal firing only 14.8% of the heat duties are absorbed in the convective zones. As a result most of the SH/RH duties have to be relocated in the furnace/transition zones and heated by radiation under oxy-coal firing.

Table 1 Distribution of boiler duties in furnace and convective zones

Boiler Type Air Oxy

Boiler duties

Boiling (%) 46.0 44.7

Superheater (%) 32.0 31.1

Reheater (%) 15.0 14.6

Economizer (%) 7.0 6.8

Feedwater heater (%) 0.0 2.8

Total (%) 100.0 100.0

Furnace / Transition zones

(%) 57.6 82.4

Boiling (%) 46.0 44.7

SH (%) 7.9 25.6

RH (%) 3.7 12.0

Convective zones (%) 42.4 14.8

SH (%) 24.1 5.5

RH (%) 11.3 2.6

Eco (%) 7.0 6.8

FW (%) 0.0 2.8 A major technical problem in relocating the SH/RH tubes in the furnace section for the oxy-coal fired boiler is how to control the heat flux from flame radiation. The maximum heat flux to SH/RH tubes has to be controlled in a range of 20,000 to 30,000 Btu/hr/ft ² to prevent tube overheating or severe slagging and fouling in a typical coal fired boiler. Radiation heated pendant type superheaters and reheaters are commonly used in the transition section of the conventional air- fired boiler design. They are placed near the furnace exit above the nose section of the furnace in order to limit the heat flux to SH/RH steam tubes. Although the same arrangement could be considered in the oxy-coal fired boiler, the space available above the nose section, i.e., the transition zone, in the traditional boiler design is too small to accommodate for the large number of superheater and reheater tubes required to be placed in the furnace section under oxy-fuel firing. Even though the space could be enlarged to place a large number of SH/RH tubes, the small flue gas volume under oxy-fuel firing could not provide sufficient heat to satisfy the SH/RH duties in the transition zone.

To prevent overheating the volume of the burner zone also have to be enlarged. The average heat flux to the water walls of a conventional air-coal fired boiler is in a range of 40,000 to 60,000 Btu/hr/ft ² and the peak heat flux can be as high as 90,000 to 100,000 Btu/hr/ft ² without exceeding the tube material temperature limit or the slagging temperature limit. The relatively low

temperature of the feed water and the high heat transfer rate in the water side of the tube provides the necessary cooling for water walls. If the furnace volume of the oxy-coal fired boiler were kept the same as that for the air-coal fired boiler, the heat flux to water/steam tubes would increase substantially and tube surface temperature could exceed the material and slagging limits. One way to reduce the heat fluxes to the conventional air-coal fired levels is to increase the size of the furnace to provide sufficient surface areas to absorb heat. The major drawback of this concept, however, is a huge increase in the cost of the boiler.

Two furnace design embodiments of the present invention are useful without significantly increasing the size of the overall furnace volume. One embodiment is to locate superheater and reheater tubes in the high heat flux zones of the furnace such as the burner zone and to use 'screen tubes' to partially shade the SH/RH tubes from the intense radiation of oxy-fuel flames. The first row of tubes consists of traditional water-boiling tubes or feed water heating tubes that can withstand high heat fluxes. Superheater and reheater tubes are placed behind the screen tubes in the second row. The fraction of the flame radiation that passes through the screen tubes can be well controlled by adjusting the spacing of the screen tubes and the distance between the first row and the second row. The ratio of the heat flux to superheater/reheater tubes to screen tubes are proportionally turned down upon firing rate changes, which is an important benefit of this design. Thus, careful design of the screen tubes can control distribution of heat absorbed by superheater and reheater tubes located in the high heat flux zones of the furnace and keep the steam tube temperature within the allowable limit. The partition walls can be composed of SH/RH tubes in the middle protected by screen tubes on both sides as depicted in Figure 3d.

The other embodiment is to place some superheater/reheater tubes below the burner zone, in addition to the radiant superheater/reheater tubes typically placed above the burner zone. In a conventional boiler the heat flux to the tubes below the first row of burners, such as the hopper section, is often much lower than the heat flux in the burner region. By enlarging the bottom zone of the furnace below the burner zone a large number of superheater and reheater tubes could be placed in this section with controlled radiative heat fluxes from the burner zone above. The overall heat fluxes to the top and bottom zones of a boiler furnace can be further controlled by changing the aspect ratio of the furnace. For example a tall furnace with a narrow width would reduce the radiant heat flux from the middle burner zone by reducing the view factor. The furnace aspect ratio or the ratio of height to "average width" (defined as square root of the burner zone cross-sectional area) of typical air-coal fired utility boiler furnaces is in a range of 2.0 to 4.0. With three partition walls depicted in Figures la, lb and lc the aspect ratio is increased by a factor of two to a range between 4 to8. The aspect ratio of the partitioned combustion space is preferred to be in a range of 3.5 to 12, more preferably to be in the range of 4 to 8.