HK1120589B - Module-based oxy-fuel boiler - Google Patents

Abstract

A boiler system for producing steam from water includes a plurality of serially arranged oxy fuel boilers. Each boiler has an inlet in flow communication with a plurality of tubes. The tubes of each boiler form at least one water wall. Each of the boilers is configured to substantially prevent the introduction of air. Each boiler includes an oxy fuel combustion system including an oxygen supply for supplying oxygen having a purity of greater than 21 percent, a carbon based fuel supply for supplying a carbon based fuel and at least one oxy-fuel burner system for feeding the oxygen and the carbon based fuel into its respective boiler in a near stoichiometric proportion. The oxy fuel system is configured to limit an excess of either the oxygen or the carbon based fuel to a predetermined tolerance. The boiler tubes of each boiler are configured for direct, radiant energy exposure for energy transfer. Each of the boilers is independent of each of the other boilers.

Description

Module-based oxy-fuel boiler

Background

The present invention relates to an oxygen fed boiler (oxygen furnace). More particularly, the present invention relates to an oxy-fuel boiler (oxy-fuel boiler) based module having a flexible design.

The advantages of oxy-fuel combustion systems are recognized. For example, Gross U.S. patent nos. 6,436,337 and 6,596,220 provide certain advantages of oxy-fuel combustion systems: reduced environmental pollution (reduced NOx generation), high efficiency, high flame temperature, and a smaller package physical equipment design. The Gross patents, which are commonly owned with the present application, are incorporated herein by reference.

To extract energy from the fuel, boilers typically provide some manner in which energy is typically input into the fluid (via combustion of the fuel) to change the state of the fluid. Energy is then extracted from the fluid, usually in the form of mechanical (or kinetic) motion. Most boilers use water as the working fluid to extract energy from the fuel. The water passes through tubes that form one or more "walls" or tube bundles within the boiler.

Typically, the walls of a boiler are designed to transfer energy (in the form of heat) through the walls to the water in certain circuits and channels within the walls. As the water passes through the tubes, the water is heated under pressure and reaches a high energy level (and phase change) via superheating, reheating, and/or supercritical conditions. Other stages, such as an economizer unit through which water passes in the furnace wall section before the superheating channel, may also be used. The water is further heated (as in an economizer) by convective heat transfer from the heated gas flowing through the tube bundle.

The stages or zones of the boiler are individually designed to operate according to some type of heat transfer mechanism or phenomenon. For example, the lower furnace wall is designed for radiant heat transfer, while the upper tube bundle, superheating, reheating and economizer sections are designed to function by the convective heat transfer principle. It should be understood by those skilled in the art that the heat transfer mechanisms are not mutually exclusive when water is heated in a boiler.

While such boiler configurations continue to work well for their applications and purposes, such boiler configurations do not necessarily fully utilize the flame high temperature and low exhaust gas volume of an oxy-fuel combustion system. Therefore, a boiler using an oxyfuel combustion system is required to reduce environmental pollution. Desirably, such boiler designs achieve high efficiency (relative to a high ratio between the heat transferred to the working fluid and the heat extracted from the combustion products) and utilize the high temperature of the flame. Most desirably, such a boiler configuration may provide a smaller overall physical plant design.

Disclosure of Invention

The module-based boiler system uses a plurality of independent, serially arranged oxyfuel boilers for generating steam from water. Boilers are provided to perform energy transfer functions that are different from each other. The first or main boiler has a feedwater inlet in fluid communication (flow communication) with a plurality of furnace tubes for transporting water. The boiler is provided to substantially prevent the introduction of air.

The tubes of the main boiler form at least one waterwall. Each boiler includes an oxygen supply for supplying oxygen at a purity greater than 21% and preferably at least about 85%, a carbon-based fuel supply for supplying a carbon-based fuel, and at least one oxy-fuel burner system. The furnace system feeds oxygen and fuel into the (feed) boiler at approximately stoichiometric ratios to limit excess oxygen or carbon-based fuel to a predetermined tolerance. The tubes of each boiler are configured for direct exposure to radiant energy to transfer energy from the flame to the waterwall tubes. Following conventional nomenclature, reference to a water wall is intended to include all boiler tubes in the radiant section, and even tubes that may be used to transport steam.

In one embodiment of the boiler system, the second boiler is a superheating boiler, and the steam generated via the first boiler is fed directly into the superheating boiler. The steam leaves the superheating boiler and flows to the main steam turbine. Alternatively, the system can include a reheat boiler (which takes feed from the exhaust of the high pressure steam turbine) that reheats steam in an oxyfuel boiler similar to the main boiler and feeds the reheated steam to the reheat steam turbine. The energy transfer or heating function of each boiler is different from each other boiler. In other words, in the main boiler, water is heated to saturated steam from a relatively low energy (enthalpy) value. In the superheating boiler (if used), the steam is further heated to a superheated state. Then, in the reheater, the exhaust steam from the high-pressure turbine is reheated to be fed into the reheat steam turbine.

The boiler system can include a condenser positioned such that steam is discharged from the high pressure steam turbine to one or more reheat steam turbines, optionally to one or more low pressure turbines, and on to the condenser. The preferred boiler system includes an economizer. The economizer has a gas side that receives combustion products ("flue gas" or "flue gas") from the boiler and a feedwater side such that the combustion products preheat the boiler feedwater prior to introducing the feedwater to the main boiler. After exiting the economizer, the exhaust gas may be used to preheat the oxidant of the oxy-fuel combustion system, and is typically discharged into the exhaust system prior to any desired downstream exhaust gas processing. Increased power can be obtained by parallel grouping of modular boiler systems.

Oxy-fuel burners may be provided for many different types of fuel, such as natural gas, petroleum, coal, and other solid fuels. When a solid fuel is used, a portion of the off-gas (optionally mixed with oxygen) may be used to carry the solid fuel into the boiler. The fuel feed gas may be an off-gas from downstream of the economizer.

These and other features and advantages of the present invention will be apparent from the following detailed description, in conjunction with the appended claims.

Drawings

The benefits and advantages of the present invention will become more readily apparent to those of ordinary skill in the relevant art after reviewing the following detailed description and accompanying drawings, wherein:

FIG. 1 is a schematic flow diagram of a single reheat/subcritical boiler system having a modular based oxyfuel boiler embodying the principles of the present invention;

FIG. 2 is a schematic flow diagram of a non-reheat/subcritical boiler system having a module-based oxyfuel boiler embodying the principles of the present invention;

FIG. 3 is a schematic flow diagram of a single reheat/supercritical boiler system having a modular based oxyfuel boiler embodying the principles of the present invention; and

FIG. 4 is a schematic flow diagram of a saturated steam boiler system having a module-based oxy-fuel boiler embodying the principles of the present invention.

Detailed Description

While the present invention is susceptible of embodiment in various forms, there is shown in the drawings and will hereinafter be described a presently preferred embodiment with the understanding that the present disclosure is to be considered an exemplification of the invention and is not intended to limit the invention to the specific embodiment illustrated.

It should be further understood that the title of this section of this specification, namely, "detailed description of the invention", relates to a requirement of the United states patent office, and does not imply, nor should be inferred to limit the subject matter disclosed herein.

Oxy-fuel combustion systems use essentially pure oxygen in conjunction with a fuel source to generate heat by flame generation (i.e., combustion) in an efficient, environmentally friendly manner. Such combustion systems achieve high efficiency (relative to a high ratio of heat transferred to the working fluid to heat extracted from the combustion products) combustion and take advantage of the high temperature of the flame. Preferred combustion systems use relatively high purity oxygen (above about 21% and preferably at least about 85% oxygen) so that the total gas volume passing through the boiler is correspondingly reduced. By using oxy-fuel, flame temperatures in boilers above about 3000 ° F and up to about 5000 ° F can be expected.

Furthermore, one operating parameter of the present boiler system is the use of an oxyfuel combustion system, in which relatively pure oxygen is used as the oxidant, rather than air. As used herein, oxidant is used to refer to a gas that carries oxygen for combustion. For example, when pure oxygen (100%) is supplied to the system, oxygen contains 100% of the oxidant, and when air is used as the oxidant, oxygen contains about 21% of the oxidant. Thus, the required volume of oxidant is significantly reduced compared to conventional boilers (because essentially only oxygen is used instead of air), which results in a reduction in the volume of gas input (and hence throughput) to the boiler and a lower gas flow rate through the boiler than conventional boilers. The main advantages provided by the low flow rate and volume are: the overall size of the physical equipment system may be smaller than conventional boiler systems, and the capital expenditure for such boiler systems is expected to be correspondingly reduced accordingly.

One functional aspect or functionality of the present boiler system is aimed at harvesting the maximum amount of energy (in the form of heat transfer from the combustion products/exhaust gases) from the combustion process. The above combined with the low flow rate achieves lower energy losses at comparable stack temperatures.

Another aspect or functionality of the present invention is directed to utilizing higher flame temperatures to the greatest extent possible. Also, as will be described below, a substantial proportion of the heat transfer from the combustion products to the boiler tubes and hence to the working fluid (water or steam) is achieved by radiant heat transfer rather than convective heat transfer.

FIG. 1 shows a schematic view of one embodiment of a boiler system 10. The illustrated system 10 is a reheat/subcritical unit. The system includes three separate and distinct boilers, namely boiler No. 1 (main boiler 12) for generating steam from water, boiler No. 2 (superheating boiler 14) and boiler No. 3 (reheat boiler 16) for generating superheated steam. Oxygen is fed into the boiler separately from the fuel by oxidant and fuel feed systems 18, 20.

As illustrated, and as will be described below, the boilers 12, 14, 16 each include a separate oxy-fuel combustion system 22, 24, 26. In such oxy-fuel combustion systems, the water walls of each boiler 12-16 (see tubes T of boiler 12 in FIG. 1) are each sufficiently exposed to the flame that a substantial portion of the heat transfer is achieved by a radiative heat transfer mechanism rather than a convective heat transfer mechanism. In other words, the majority of the heat transfer is due to the direct exposure of the furnace tubes to the flame, rather than to the movement of the heated exhaust gases over the furnace tubes. This preferred radiant heat transfer mechanism is in clear contrast to conventional boilers that use a large number of long and complex exhaust gas flow paths (through-convection paths, through-convection superheat paths, economizer sections, etc.) to maximize heat transfer through the convection mechanism.

The present boiler system 10 also includes an economizer 28, the economizer 28 transferring energy from the boiler combustion gases (preferably in all boilers) to the main boiler feedwater (in a feedwater line 30) to preheat the feedwater prior to introduction into the main boiler 12. In the present system, oxygen is formed by separation from, for example, air in the oxygen generator 32. It will be appreciated by those skilled in the art that various methods are within the scope of the present invention in which oxygen may be provided to the boilers 12-16, for example, oxygen may be supplied from a source such as a storage tank, separated water, or the like. The fuel supply 20 may be any of various types of fuels and various types of supplies. For example, the fuel may be a gaseous fuel (such as natural gas), a fluid fuel (such as fuel oil, diesel fuel or other organic or inorganic based fluid fuels) or a solid fuel (such as coal, a by-product of agriculture or animal husbandry). All such oxygen production and supply apparatus 18 and all such fuel and fuel supply apparatus 20 are within the scope and spirit of the present invention.

Turning now to FIG. 1, a boiler system 10 is shown as a source of supply for a generator 34. Throughout, the system includes a turbine/generator set 36 including a generator 34, a high pressure or main steam turbine 38, an intermediate pressure steam turbine 40, a low pressure steam turbine 41, and a condenser 42.

The system 10 is arranged such that feedwater enters the main boiler through a feedwater conduit 30 and is heated as it flows through the water tubes T of the boiler 12. In a typical boiler plant, water enters the boiler 12 at a relatively low point in the boiler and is raised through the furnace tubes when heated. This serves to maintain the furnace tubes in an flooded condition and to maintain the fluid under pressure in the furnace tubes.

The heated fluid is separated and saturated steam exits the main boiler 12 through conduit 44 and enters the superheating boiler 14. In the superheating boiler 14, the steam is further heated to a superheated state and then flows through the wall tubes again. The superheated steam exits the superheating boiler 14 through a main steam line 46 and enters the high-pressure (main steam) turbine 38. The low pressure steam is discharged from main high pressure steam turbine 38 and returned to reheat boiler 16 through reheat steam line 48. Steam exits reheat boiler 16 through reheated steam flow line 50 and enters the intermediate pressure turbine. The steam discharged by the intermediate-pressure turbine 40 flows through the crossover duct 43 and enters the low-pressure turbine 41.

The steam exits low pressure turbine 41 through turbine exhaust conduit 52 and is sufficiently condensed in condenser 42 (typically at low pressure (sub-atmospheric) so as to extract the maximum amount of energy from the steam through turbine 40) and then returned (pumped back) to main boiler 12 through economizer 28, economizer 28 (as set forth above) preheating the water prior to introduction to boiler 12.

As for the fuel circuit, as described above, the fuel and the oxidant are fed into the boilers 12, 14, and 16, respectively, independently. The combustion gases each leave the respective boiler through lines 13, 15 and 17, respectively, and enter an economizer 28, where the gases preheat the main boiler feed water. The combustion gases exit the economizer 28 and can be used to preheat the oxidant in the oxidant preheater 60. The flue gas leaving the economizer 28 is sent to an oxidant preheater 60 (via conduit 61) and then returned (via conduit 63) to the inlet of any necessary downstream processing equipment (generally indicated at 54), such as a scrubber, precipitator or the like. Further, where desired, a portion of the fuel gas may be recirculated (via fuel gas recirculation conduit 56) to the boilers 12-16, typically followed by oxidant preheating. The secondary recycle line 56 may also be used as a carrier (by diverting to a fuel carrying line 58) for carrying fuel into the boilers 12-16, such as carrying coal dust into the boilers.

As will be appreciated by those skilled in the art, because the gas flow rate and total gas volume (substantially pure oxygen) entering the boiler is less than that of a conventional boiler, the flow rate and volume of exhaust gas or fuel gas is correspondingly less than that of a conventional boiler. Likewise, the downstream processing equipment 54 may be smaller and less expensive than conventional equipment for a same scale (power take off) power plant.

FIG. 2 shows a schematic view of a second embodiment of a boiler system 110. The illustrated boiler system 110 is a non-reheat/subcritical unit, and as such, the system includes two separate and distinct boilers, namely boiler No. 1 (main boiler 112) for generating steam from water and boiler No. 2 (superheat boiler 114) for generating superheated steam. There is no reheat boiler. The system 110 is otherwise similar to the embodiment of the system 10 of FIG. 1 and includes oxidant and fuel supply systems 118, 120 (in separate oxy-fuel combustion systems 122, 124) to feed the boilers 112, 114, respectively, independently. The boiler system 110 includes an economizer 128, the economizer 128 using combustion gases to preheat the feedwater prior to its introduction into the main boiler 112. The exhaust gas after the economizer 128 can be used to preheat the oxidant in the oxidant preheater 160.

Also provided for the boiler system 110 are a turbine/generator set 136 including a generator 134, a high pressure (or main steam) turbine 138, an intermediate pressure turbine 140, a low pressure turbine 141, and a condenser 142.

Feedwater enters the main boiler through a feedwater line 130 and is heated as it flows through the water tubes. The heated fluid is separated and saturated steam exits the main boiler 112 through a conduit 144 and enters the superheat boiler 114 where the steam is heated to a superheated state in the superheat boiler 114. The superheated steam exits the superheated boiler 114 through a main steam line 146 and enters the high pressure turbine 138. Unlike the previous embodiments, in this system 110, steam exiting the high pressure turbine 138 traverses the crossover conduit 143 and enters the intermediate pressure turbine 140 (e.g., no reheater is present). Steam exits the intermediate pressure turbine 140 and traverses the crossover conduit 148 and enters the low pressure turbine 141. The low pressure steam then passes through the low pressure turbine, is discharged by the low pressure turbine 141 to the condensing conduit 152, and is then returned (pumped back) to the main boiler 112 through the economizer 128.

As with the fuel circuit, as with the previous embodiments, fuel and oxidant are fed independently into the boilers 112, 114, respectively. The flue gases exit the respective boilers through conduits 113 and 115, respectively, and enter economizer 128 to preheat the main boiler feedwater. The combustion gases exit the economizer 128 and can be used to preheat the oxidant in the oxidant preheater 160. The flue gas leaving the economizer 128 is sent to an oxidant preheater 160 (via conduit 161) and then returned (via conduit 163) to the inlet of any necessary downstream processing equipment (generally indicated at 154) after leaving the economizer 128. The combustion gases may be recycled 156 and/or used as a carrier for carrying fuel (e.g., coal dust) into the boilers 112, 114.

FIG. 3 illustrates another embodiment of a boiler system 210 showing a single reheat/supercritical boiler plant. The system includes two separate and distinct boilers, namely boiler No. 1 (supercritical main boiler 212) and boiler No. 2 (reheat boiler 216) for generating supercritical steam from water. Oxygen and fuel (in separate oxy-fuel combustion systems 222, 226) are fed into the boilers 212, 216, respectively, by oxidant and fuel supply systems 218, 220. The boiler system 210 includes an economizer 228, the economizer 228 preheating feedwater using combustion gases prior to introduction into the main boiler 212.

Also provided for the boiler system 210 are a turbine/generator set 236 including a generator 234, a supercritical turbine 238, an intermediate pressure turbine 240, a low pressure turbine 241, and a condenser 242.

Feedwater enters main boiler 212 through feedwater line 230 and is heated as it flows through the water tubes. The heated fluid exits the supercritical boiler 212 through a supercritical steam conduit 246 and enters the supercritical turbine 238. Fluid (steam) exits supercritical turbine 238 and enters reheat boiler 216 through reheat line 248 and then flows to intermediate pressure turbine 240 through reheat steam line 250. Steam is discharged from the intermediate pressure turbine 240 into the low pressure turbine 241 through crossover conduit 243. The low pressure steam exits the low pressure turbine 241 and is condensed in the condenser 242. The condensate is then returned (pumped) to the supercritical boiler 212 through the economizer 228.

As for the fuel circuit, as in the previous embodiment, fuel and oxidant are fed independently into the boilers 212, 216, respectively. The flue gases exit the respective boilers through conduits 213 and 217, respectively, and enter economizer 228 to preheat the main boiler feedwater. The combustion gases exit the economizer 228 and can be used to preheat the oxidant in the oxidant preheater 260. The flue gas leaving the economizer 228 is sent to an oxidant preheater 260 (via conduit 261) and then returned (via conduit 263) to the inlet of any necessary downstream processing equipment 254 as needed after leaving the economizer 228. The fuel gas may be recycled 256 and/or used as a carrier to carry fuel (e.g., coal dust) into the boiler.

Fig. 4 illustrates yet another embodiment of a boiler system 310, which shows a saturated steam boiler plant. The system includes a saturated steam boiler 312 for generating saturated steam and an oxyfuel combustion system 322. The boiler system 310 may include an economizer 328, the economizer 328 preheating feedwater using combustion gases prior to the feedwater being introduced into the main boiler 312.

The boiler system 310 is arranged to supply saturated steam to a desired (currently unspecified) downstream processor 360. Throughout the system 310, shown with "steam demand" (downstream processors requiring steam) and a condenser 342, the demand for steam will depend on the equipment 360 requiring steam.

Feedwater enters main boiler 312 through feedwater line 330 and is heated as it flows through the water tubes. The heated fluid is separated into saturated steam and water in, for example, a steam drum 313. Saturated steam exits the boiler 312 from drum 313 through steam line 346 and flows to the equipment 360 that requires the steam. The fluid (vapor) may then (optionally) be condensed in a condenser 342, with the condensate then being returned (pumped back as feedwater) to the boiler 312 via an economizer 328.

As for the fuel circuit, as with the previous embodiment, fuel and oxidant are fed into the boiler 312 through an oxyfuel combustion system 322. The combustion gases exit the boiler 312 through conduit 313 and enter an economizer 328 to preheat the feedwater of the main boiler 312. The combustion gases exit the economizer 328 and can be used to preheat the oxidant in the oxidant preheater 370. The exhaust gas after leaving the economizer 328 is sent to an oxidant preheater 370 (via conduit 371) and then returned (via conduit 373) to the inlet of any necessary downstream processing equipment 354 as needed after leaving the economizer 328. The combustion gases may be recycled 356 and/or used as a carrier to carry fuel (e.g., coal dust) into the boiler 312. Oxygen is supplied by oxidant supply 318 and fuel is supplied by fuel supply 320.

In embodiments of the boiler systems 10, 110, 210, 310, respectively, the boiler must be a stand-alone unit that is constructed to operate to maximize the heat transfer via the radiant heat transfer mechanism. Likewise, the boiler is relatively small (to ensure effective exposure of the waterwalls/tubes T), or at least smaller than comparable conventional boilers that rely on convective heat transfer. It should be understood by those skilled in the art that although each of the boilers of each system (e.g., the main boiler 12, the superheater boiler 14, and the reheat boiler 16 of one reheat boiler system 10) are shown and described as one boiler plant, it is also contemplated that each of these boilers could be provided as a continuous plurality of units. Further, for example, the main boiler 12 may be provided as two or three continuous small boilers. Additionally, although each boiler is shown with one oxyfuel burner, it is contemplated that each boiler may have multiple burners, as desired. It should be understood that: the use of a boiler or boilers for each heating stage, and a furnace or furnaces for each boiler, will further improve the ability to control the heat input to a single boiler, and thus more effectively control the overall process and steam conditions.

Energy is input into the boiler by an oxy-fuel combustion system, as assumed in the above-mentioned patents to pertain to Gross. With such a configuration, the principle mode of heat transfer to the furnace is radiation, accompanied by some convective heat transfer. Oxy-fuel combustion systems provide this efficient radiant heat transfer because these burners (and typical oxy-fuel systems) produce a high temperature flame. The geometry of the boiler (e.g., direct exposure of the boiler tubes to the flame) further enhances heat flux by maximizing the metal surface area over which heat transfer from the flame to the metal occurs.

Advantageously, the present boiler utilizes radiant heat transfer to the maximum extent possible in conjunction with the use of oxyfuel combustion, which can allow the boiler to be physically smaller than a conventional boiler of about equal size (power output). In other words, because essentially pure oxygen (rather than air) is used as the oxidant, all of the oxidant is available for combustion, and when air is used as the oxidant to provide the oxygen needed for combustion, the volume of gas input to the boiler is about 21% of the required gas volume. Since essentially pure oxygen is used instead of air, the boiler can be considerably smaller.

Furthermore, the fuel/oxygen mixture (again, not the fuel/air mixture) results in a higher flame temperature in the boiler. A flame temperature of about 5000F can be achieved with oxy-fuel in the boiler. This is higher than the flame temperature of conventional boilers, which is about 1500 ° F to 2000 ° F. It has also been observed that the use of oxy-fuel in combination with these higher flame temperatures results in a more efficient process.

In current boiler systems using natural gas as a fuel, the ratio of oxygen to natural gas is about 2.36: 1. The ratio will vary depending on the purity of the oxygen supplied and the nature of the fuel. For example, in the ideal case of 100% pure oxygen, the ratio is theoretically calculated to be 2.056: 1. However, such variations are to be expected since the oxygen supplied may have a percentage of non-oxygen content (typically up to about 15%) and natural gas may not always be 100% pure. Likewise, those skilled in the art will appreciate and understand that the ratio may vary somewhat, but the algorithm used to calculate the ratio must remain accurate, where the ratio is close to the stoichiometric ratio of fuel and oxygen.

This ratio of oxygen to fuel provides a number of advantages, for example, a near stoichiometric ratio achieves complete combustion of the fuel, thus resulting in substantially less volume of NOx and other harmful exhaust gases being emitted.

It is worth noting that precise control of the oxygen to fuel ratio ensures complete combustion of the fuel. This is in stark contrast to conventional equipment (e.g., fossil fuel powered power plants) that combat LOI (loss on ignition). In essence, LOI is equivalent to incomplete combustion of the fuel. On the other hand, the present boiler systems 10, 110, 210, 310 use essentially pure oxygen tightly controlled close to the fuel stoichiometry (the boiler is "sealed", in other words, the sealed boiler is arranged to substantially prevent the introduction of air) with the aim of minimizing and possibly eliminating these losses. Furthermore, when using these burners (in oxy-fuel systems), the only theoretical NOx obtained comes from nitrogen in the fuel (fuel-borne nitrogen), rather than being produced otherwise by combustion using air. Thus, NOx is reduced (if not completely exhausted) to a negligible amount compared to conventional combustion systems.

Furthermore, because radiant heat transfer is a desirable heat transfer mechanism, there is little reliance on convective (gas) channels within the boiler. This also allows for a smaller, simpler boiler design. These design concepts allow the boiler to be configured as a stand-alone, modular unit. In other words, referring to fig. 1, the separate main boiler 12 may be combined with a separate superheating boiler 14, and the separate superheating boiler 14 may be combined with a separate reheating boiler 16. Likewise, referring to FIG. 3, a separate supercritical main boiler 212 can be combined with a separate reheat boiler 216 as the core of the boiler system 210. This separate configuration has a control advantage over conventional systems in which the temperature is controlled by desuperheating. The process of post-superheating cooling cools the superheated steam by adding water or steam (e.g., steam or water mist) and reduces the effectiveness of the system and can be eliminated through the use of separate boiling and superheating boilers. There are also advantages during turndown operation (operating at a lower power than the design power). In the turndown regime, the heat input to the boiling region can be controlled independently of the heat input to the superheat region or reheat region, and more efficient operation is achieved.

Studies of heat and mass balances according to different boiler configurations have shown that the expected boiler efficiency is rather high and significantly higher than known boiler systems. For example, in the first reheat/subcritical unit of the main boiler, the change in enthalpy of the water inlet to the steam outlet is about 1.95E9 BTU/hr, while the enthalpy of the fuel input is about 2.08E9 BTU/hr. In a superheating boiler, the change in enthalpy from the steam inlet to the steam outlet is about 7.30E8 BTU/hr with a fuel input enthalpy of about 8.32E8 BTU/hr, and in a reheat boiler, the change in enthalpy from the water inlet to the steam outlet is about 5.52E8 BTU/hr with a fuel input enthalpy of about 6.22E8 BTU/hr. This resulted in a main boiler efficiency of 93.8% (including economizer gain), 87.8% and 88.7% for the superheat boiler and reheat boiler, respectively.

Likewise, in the second non-reheat subcritical unit, the enthalpy change from the water inlet to the steam outlet in the main boiler is about 1.99E9 BTU/hr, while the fuel input enthalpy is about 1.97E9 BTU/hr. In a superheating boiler, the enthalpy change from the steam inlet to the steam outlet is about 1.22E9 BTU/hr, while the enthalpy of the fuel input is about 1.60E9 BTU/hr. This results in a main boiler efficiency of 101.0% (including economizer gain) and 76.2% for the superheat boiler, respectively. It is worth pointing out that the economiser is included in the calculation of the main boiler (the main boiler uses the exhaust gases of the boiler and the superheating boiler), and as such, it is believed that the energy of the flue gas comes from the superheating boiler, which makes the efficiency look more than 100% (but not).

In the third reheat supercritical boiler, the change in enthalpy of the water inlet to the steam outlet in the supercritical main boiler is about 2.37E9 BTU/hr, while the fuel input enthalpy is about 2.72E9 BTU/hr. In a reheat boiler, the enthalpy change from the steam inlet to the steam outlet is about 6.23E8 BTU/hr, while the enthalpy of the fuel input is about 7.24E8 BTU/hr. These conditions resulted in a supercritical main boiler and reheat boiler efficiency of 87.2% (including economizer gain) and 86.0%, respectively.

In a final or saturated steam boiler system, the enthalpy change from the water inlet to the steam outlet is about 3.42E9 BTU/hr, while the fuel input enthalpy is about 3.73E9 BTU/hr. There is an exhaust loss of about 0.13E8 BTU/hr. This loss makes the main boiler 91.7% efficient.

Table 1 below shows the partial mass and energy balance components for the reheat/subcritical unit classified by boiler, table 2 shows the partial mass and energy balance components for the non-reheat/subcritical unit classified by boiler, table 3 shows the partial mass and energy balance components for the reheat-supercritical unit classified by boiler, and table 4 shows the partial mass and energy components for the saturated steam boiler plant. It should be noted that the partial mass and energy balance values of the reheat supercritical boiler plant in table 3 show the portions of the first and second boilers, which are added together to determine the effectiveness and confirm the schematic of fig. 3. In each of the summary of partial mass and energy balance values of tables 1-3, the specific and total enthalpy values are the values of the respective first combustion section prior to the water inlet to the economizer.

TABLE 1 part Mass and energy balance of reheat/subcritical boiler System

TABLE 2 part Mass and energy balance of non-reheat/subcritical boiler systems

TABLE 3 part Mass and energy balance of reheat/supercritical boiler System

TABLE 4 partial mass and energy balance of saturated steam boiler System

As indicated above, each boiler system differs from conventional processes in two main respects. First, conventional combustion processes use air (as an oxidant to supply oxygen) for combustion rather than essentially pure oxygen. The oxygen component of air (about 21%) is used for combustion, while the remaining component (essentially nitrogen) is heated in and exhausted from the furnace. Second, the present process uses oxygen and fuel in proportions close to stoichiometric with each other (within about ± 5% of the allowable tolerance). In other words, only enough oxidant is added in proportion to the fuel to ensure complete combustion of the fuel within predetermined tolerances. In addition, the process is carried out in a plurality of boiler components or modules arranged as a coordinate system, each module heating in a respective desired state (e.g. main boiler, superheat zone, reheat zone).

Numerous advantages and benefits are obtained using the present combustion system. As will be described below, reduced combustion of fuel producing an equivalent amount of power or heat has been observed. Notably, this reduction can achieve a dramatic reduction in the amount of contamination that results. Furthermore, in some applications, NOx emissions may be substantially reduced to zero.

Furthermore, it has been observed that because the throughput of gas is significantly lower than in conventional boilers, the volume of exhaust gas discharged is also correspondingly reduced. In fact, because the input of oxidant (oxygen in this system compared to air in conventional systems) is about 21% of conventional systems, the exhaust gas is also about 21% of conventional systems (conventional systems may have, for example, 40% for solid fuel due to the large amount of pushing gas required to transport the solid fuel to the boiler). Also, it is contemplated that the main components of the exhaust gas will be water (e.g., water vapor) and CO that may be condensed or otherwise released₂. It is also contemplated to obtain CO in a concentrated form₂For other industrial and/or commercial applications and/or for carbon sequestration.

It has also been found that higher flame temperatures as discussed above are obtained using a fuel/oxygen mixture (again, not a fuel/air mixture). A flame temperature of about 5000F may be achieved with oxy-fuel. This is higher than flame temperatures of about 1500 ° F to 2000 ° F for other known boilers. It has also been observed that the use of oxy-fuel in combination with these higher flame temperatures results in a very efficient process.

In the context of the present invention, the terms "a" or "an" are to be taken to include both the singular and the plural. Conversely, any reference to plural items shall, where appropriate, include the singular.

From the foregoing it will be observed that numerous modifications and variations can be effectuated without departing from the true spirit and scope of the novel concepts of the present invention. It is to be understood that no limitation with respect to the specific embodiments illustrated is intended or should be inferred. It is intended that the following claims be interpreted to embrace all such modifications as fall within the scope of the claims.

Claims (25)

1. A module-based oxy-fuel boiler system for generating steam from water, comprising:

a first boiler having a feed water inlet in fluid communication with a plurality of furnace tubes for transporting water, the furnace tubes forming at least one water wall, the first boiler being configured to substantially prevent the introduction of air;

a first boiler oxygen supply for supplying oxygen with a purity higher than 21%;

a first boiler carbon-based fuel supply source for supplying a carbon-based fuel;

at least one first boiler oxy-fuel burner system that feeds oxygen and carbon-based fuel into the first boiler at approximately stoichiometric ratios to each other to limit excess oxygen or carbon-based fuel to a predetermined tolerance,

wherein the first boiler tube is configured for direct exposure of radiant energy to transfer energy to water to produce steam;

a second boiler having a plurality of tubes, the second boiler in series with the first boiler and arranged to perform a different energy transfer function than the first boiler, the tubes in the second boiler forming at least one tube wall, the second boiler arranged to substantially prevent the introduction of air;

a second boiler oxygen supply for supplying oxygen with a purity higher than 21%;

a second boiler carbon-based fuel supply source for supplying a carbon-based fuel;

at least one second boiler oxyfuel burner that feeds oxygen and carbon-based fuel into the second boiler in approximately stoichiometric ratios to each other to limit excess oxygen or carbon-based fuel to a predetermined tolerance,

wherein the second boiler tube is arranged for direct exposure of radiant energy for energy transfer to produce steam, and

wherein the first and second boilers are independent of each other and in series with each other.

2. The module-based oxy-fuel boiler system of claim 1, wherein the first boiler oxygen supply supplies oxygen having a purity of 85%.

3. The module-based oxy-fuel boiler system of claim 1, wherein the second boiler oxygen supply supplies oxygen having a purity of 85%.

4. The module-based oxyfuel boiler system of claim 1, wherein the first boiler is a main boiler and the second boiler is a superheating boiler, and wherein steam generated via the first boiler is fed directly into the superheating boiler.

5. The module-based oxyfuel boiler system of claim 4, comprising a steam turbine, wherein steam exiting the superheating boiler is fed into the steam turbine.

6. The module-based oxyfuel boiler system of claim 5, comprising a reheater boiler, wherein the reheater boiler has a plurality of furnace tubes, the reheater boiler is in series with the main boiler and the superheater boiler, and the reheater boiler is configured to perform a different energy transfer function than the main boiler and the superheater boiler, the furnace tubes in the reheater boiler form at least one tube wall, the reheater boiler is configured to substantially prevent the introduction of air, the reheater boiler system comprises an oxygen supply for supplying oxygen having a purity greater than 21%, a carbon-based fuel supply for supplying carbon-based fuel, and at least one reheat boiler oxyfuel burner that feeds oxygen and carbon-based fuel into the reheat boiler at approximately stoichiometric ratios to each other to limit excess oxygen or carbon-based fuel to a predetermined tolerance, wherein the reheat boiler tubes are configured for direct exposure to radiant energy for energy transfer to superheat steam, and wherein the reheat boiler is independent of the main boiler and the superheat boiler, the reheat boiler taking feed from exhaust of the steam turbine and configured to produce steam.

7. The module-based oxy-fuel boiler system of claim 6, wherein the reheater boiler oxygen supply supplies oxygen of 85% purity.

8. The module-based oxyfuel boiler system of claim 6, comprising an intermediate pressure turbine, wherein steam generated via the reheat boiler is fed into the intermediate pressure turbine.

9. The module-based oxyfuel boiler system of claim 8, comprising a low pressure turbine, wherein steam exhausted by the intermediate pressure turbine is fed into the low pressure turbine, and wherein steam exhausted by the low pressure turbine is fed into a condenser.

10. The module-based oxyfuel boiler system of claim 1, comprising an economizer having a gas side and a feedwater side, wherein flue gas from the first and second boilers flows into the gas side of the economizer, and wherein feedwater flows through the economizer into the feedwater inlet.

11. The module-based oxyfuel boiler system of claim 10, wherein the first and second boilers are solid fuel boilers, and wherein a portion of the exhaust gas is used to carry solid fuel into at least one of the boilers.

12. The module-based oxyfuel boiler system of claim 11, wherein a portion of the exhaust gas is used to carry solid fuel into the first boiler and the second boiler.

13. The module-based oxyfuel boiler system of claim 11, wherein a portion of the exhaust gas used to carry solid fuel into at least one of the boilers is discharged from an exhaust gas flow path downstream of the economizer.

14. The module-based oxyfuel boiler system of claim 10, wherein the oxygen supply of the first and second boiler oxygen supplies is preheated by exhaust gas discharged from a gas side of the economizer.

15. The module-based oxyfuel boiler system of claim 1, wherein the first boiler is a main boiler and the second boiler is a reheat boiler, and comprises a main steam turbine and an intermediate pressure turbine, wherein steam exiting the main boiler is fed into the main steam turbine, steam exhausted by the main turbine is fed into the reheat boiler, and steam exiting the reheat boiler is fed into the intermediate pressure turbine.

16. The module-based oxyfuel boiler system of claim 15, comprising a low pressure turbine, wherein steam is discharged from the intermediate pressure turbine to the low pressure turbine.

17. The module-based oxyfuel boiler system of claim 16, comprising a condenser, and wherein steam exhausted by the low pressure turbine is vented to the condenser.

18. The module-based oxyfuel boiler system of claim 15, comprising a condenser having a gas side and a feedwater side, wherein exhaust gases from the main boiler and the reheat boiler are exhausted through the economizer, and wherein feedwater from the condenser flows through the economizer into the feedwater inlet.

19. The module-based oxyfuel boiler system of claim 18, wherein the main boiler and the reheat boiler are solid fuel boilers, and wherein a portion of the exhaust gas is used to carry solid fuel into at least one of the boilers.

20. The module-based oxyfuel boiler system of claim 19, wherein a portion of the exhaust gas is used to carry solid fuel into the main boiler and the reheat boiler.

21. The module-based oxyfuel boiler system of claim 20, wherein a portion of the exhaust gas used to carry solid fuel into at least one of the boilers is discharged from an exhaust gas flow path downstream of the economizer.

22. The module-based oxyfuel boiler system of claim 18, wherein the exhaust gas discharged from the gas side of the economizer preheats the oxygen of the main boiler oxygen supply and the reheat boiler oxygen supply.

23. A boiler system for generating steam from water, comprising:

a plurality of serially arranged boilers including a first boiler and a second boiler,

the first and second boilers each having an inlet in fluid communication with a plurality of coils surrounding a water transport, the coils forming at least one tube wall, each of the boilers being configured to substantially prevent the introduction of air, each of the boilers including an oxygen supply for supplying oxygen having a purity greater than 21%, a carbon-based fuel supply for supplying a carbon-based fuel, and at least one oxy-fuel burner system for feeding oxygen and the carbon-based fuel into their respective boilers at approximately stoichiometric ratios to limit excess oxygen or carbon-based fuel to a predetermined tolerance, wherein the boiler coils of each boiler are configured for direct exposure to radiant energy for energy transfer, wherein each of the boilers is independent of each other boiler.

24. The boiler system according to claim 23, wherein each of said boiler oxygen supplies oxygen having a purity of 85%.

25. The boiler system according to claim 23, comprising a plurality of columns of said plurality of serially arranged boilers, each of said plurality of columns being parallel to each other.