Oxyfuel combusting boiler system and a method of generating power by using the boiler system

1. Field of the Invention

The present invention relates to an oxyfuel combusting boiler system and a method of generating power by using the boiler system. The invention relates especially to a dual-firing boiler system, i.e., a boiler system which can be operated by using either air or a mixture of substantially pure oxygen and recycled exhaust gas as the oxidant gas, i.e., as the oxygen carrier gas.

2. Description of the Related Art

Oxyfuel combustion is one of the methods suggested for removing CO₂ from the exhaust gases of a power generating boiler, such as a pulverized coal (PC) boiler or a circulating fluidized bed (CFB) boiler. Oxyfuel combustion is based on combusting carbonaceous fuel with substantially pure oxygen, typically, of about 95% purity, so as to have carbon dioxide and water as the main components of the exhaust gas discharged from the boiler. Thereby, the carbon dioxide can be captured relatively easily from the exhaust gas, without having to separate it from a gas stream having nitrogen as its main component, as when combusting the fuel with air.

Generating power by oxyfuel combustion is more complicated than conventional combustion by air, because of the need of an oxygen supply, for example, a cryogenic or membrane based air separation unit (ASU), where oxygen is separated from other components of air, mainly, nitrogen. The produced exhaust gas is then ready for sequestration of CO₂ when water is removed therefrom and, possibly, the exhaust gas is purified in order to reduce inert gases originating from the oxidant, fuel or air-leakage. This purification is typically done by CO₂ condensation at a low temperature and/or a high pressure. CO₂ can be separated from the exhaust gas, for example, by cooling to a relatively low temperature, while compressing it to a pressure greater than 110 bar. Both the production of oxygen and the compression and purification of carbon dioxide increase the total production costs of the power generation process, for example, by decreasing the net power produced in the process.

Combustion using oxygen differs from combustion using air, mainly by having a higher combustion temperature and a smaller combustion volume. Because oxyfuel combustion is still a developing technology, it is considered to be advantageous to design so-called first generation oxyfuel combustion boilers, where the combustion conditions are arranged to be close to those of air-firing combustion. This can be done by recycling exhaust gas back to the furnace, so as to provide an average O₂ content of the oxidant of, for example, 20-28%. Such first-generation oxyfuel combustion boilers can advantageously be built by modifying existing air-firing boilers. Due to many uncertainties related to oxyfuel combustion with capture and storage of carbon dioxide, there is also a need for dual-firing boilers, i.e., boilers which can be changed from air-firing to oxyfuel combustion and back, as easily as possible, and preferably, without any changes in the actual construction. With such a dual-firing boiler, it is also possible to have a maximum power output, by using air-firing combustion, during high load demand, such as in the summer or during the daytime, and to apply oxyfuel combustion with CO₂ removal in other conditions. Also, it is possible to use a dual-firing boiler in an air-firing mode, for example, when the air separation unit or CO₂ sequestration unit is out of order.

U.S. Pat. No. 6,418,865 discloses a boiler for combusting fuel with oxygen-enriched air, which boiler can be made by retrofitting an air-firing boiler, wherein flue gas is re-circulated to the furnace so as to have a flame temperature and total mass flow approximately the same as that for combustion with air.

Patent publication number WO 2006/131283 discloses a retrofitted dual firing boiler, where fresh air exiting an air heater is either conveyed directly, in the air-firing mode, to the combustion chamber, or it is, in the oxyfuel combustion mode, cooled by feedwater of the boiler, compressed by utilizing steam extracted from a high pressure steam turbine and conveyed to an air separator unit for producing oxygen. The net power generated in the CO₂ capturing oxyfuel combustion mode of the process disclosed in WO 2006/131283 is considerably reduced from that of the air-firing mode.

In order to more economically generate power by an oxyfuel combusting boiler system, there is a need for an improved method and boiler system for minimizing the loss of produced power, especially, in a dual-firing boiler.

An object of the present invention is to provide an oxyfuel combusting boiler system and a method of using the boiler system, so as to minimize the loss of produced power.

In one aspect, the present invention provides a method of generating power by combusting carbonaceous fuel with an oxidant gas in a furnace of a boiler system, the method comprising the steps of feeding carbonaceous fuel into the furnace at a fuel feeding rate, feeding oxidant gas into the furnace for combusting the fuel to produce exhaust gas, discharging the exhaust gas from the furnace via an exhaust gas channel, conveying a stream of feedwater at a feedwater conveying rate from a final economizer arranged in the exhaust gas channel to evaporating and superheating heat exchange surfaces arranged in the furnace and in the exhaust gas channel, for converting the feedwater to superheated steam, expanding the superheated steam in a high-pressure steam turbine for generating power, extracting a first portion of steam from the high-pressure steam turbine for preheating the feedwater, conveying a second portion of steam from the high-pressure steam turbine to reheating heat exchange surfaces arranged in the exhaust gas channel for generating reheated steam, and expanding the reheated steam in an intermediate-pressure steam turbine for generating power, wherein, in first operating conditions, the oxidant gas is a mixture of substantially pure oxygen and recycled exhaust gas, and the ratio of the first and second portions of steam is controlled so as to obtain a desired flue gas temperature in the exhaust gas channel downstream of the final economizer.

In another aspect, the present invention provides a boiler system for generating power by combusting carbonaceous fuel in a furnace of the boiler system, the boiler system comprising means for feeding carbonaceous fuel into the furnace, means for feeding substantially pure oxygen and recycled exhaust gas as an oxidant gas into the furnace for combusting the fuel to produce exhaust gas, an exhaust gas channel for discharging the exhaust gas from the furnace, means for conveying a stream of feedwater from a final economizer arranged in the exhaust gas channel to evaporating and superheating heat exchange surfaces arranged in the furnace and in the exhaust gas channel, for converting the feedwater to superheated steam, a high-pressure steam turbine for expanding the superheated steam for generating power, means for extracting a first portion of steam from the high-pressure steam turbine for preheating the feedwater, means for conveying a second portion of steam from the high-pressure steam turbine to reheating heat exchange surfaces arranged in the exhaust gas channel for generating reheated steam, an intermediate-pressure steam turbine for expanding the reheated steam for generating power, and means for controlling the ratio of the first and second portions of steam, so as to obtain a desired flue gas temperature in the exhaust gas channel downstream of the final economizer.

The decreasing amount of steam extracted from the high-pressure steam turbine for preheating the feedwater naturally lowers the temperature of the feedwater entering a final economizer in the exhaust gas channel. Thus, the decreasing of this steam extraction increases the temperature difference between the feedwater and the exhaust gas in the final economizer. Thereby, the decreasing of the steam extraction indirectly increases the rate of heat exchange taking place in the final economizer. Correspondingly, the increasing of the amount of steam conveyed from the high-pressure steam turbine to the reheating heat exchange surfaces increases the heat exchange rate taking place at the reheating surfaces. In some cases, it may be useful to increase the heat transfer area of the reheating surfaces in order to obtain the desired, increased heat transfer rate. Both of the above-described measures enhances the cooling of the exhaust gas in the exhaust gas channel, and together, they provide an especially efficient method for controlling the temperature of the exhaust gas.

When using the present invention, the fuel feeding rate and the feedwater conveying rate are advantageously adjusted so as to obtain a desired furnace temperature. This, together with the above-discussed method for controlling the temperature of the exhaust gas, provides an efficient method of adjusting the temperature profile of an oxyfuel combustion boiler retrofitted from an air-firing boiler, to be nearly the same as that of the air-firing combustion, and to avoid, e.g., corrosion or material strength problems of the boiler walls. According to an advantageous embodiment of the present invention, the fuel feeding rate at full load is when modifying an air-firing boiler for oxyfuel combustion, increased by 20% and, correspondingly, the feedwater conveying rate is at the same time increased by 10%. Thus, as a consequence of the method of the present invention, due to the higher firing rate, more energy can be released from the fuel when using oxyfuel combustion, and thereby, the net power loss, caused by the oxycombustion process as a whole, is minimized.

According to an especially advantageous embodiment of the present invention, the oxyfuel combustion boiler is a dual-firing boiler, i.e., an oxyfuel combusting boiler, which can, in special operating conditions, for example, when the oxygen supply is not operational, be used for combustion with air. When comparing the combustion, at full load, in normal operating conditions, i.e., in the so-called first operation conditions, with a mixture of oxygen and recycled exhaust gas as the oxidant, to that in the so-called second operation conditions, by using air as the oxidant, the fuel feeding rate in the first operating conditions is advantageously higher than that in the second operating conditions. The fuel feeding rate in oxyfuel combustion is preferably at least 10% higher, even more preferably, at least 15% higher, than that in the air-firing combustion. Due to the higher fuel feeding rate, the total firing rate of the boiler is increased, and the loss of produced power is minimized.

The use of an increased fuel feeing rate in the oxyfuel combustion, while still maintaining the furnace temperature, is advantageously partly based on the increased heat exchange in the evaporation surfaces, due to decreased temperature, and possibly, also increased flow rate, of the feedwater. As discussed above, the feedwater temperature can advantageously be lowered, especially before the final economizer, but to some extent, also after the final economizer, in oxyfuel combustion, by decreasing the extraction of steam for preheating the feedwater from that in air-firing combustion.

The furnace temperature is naturally, also to a large extent, determined by the exhaust gas cycling rate, which affects both the rate of feeding relatively cold inlet gas to the furnace and the rate of convective heat flow, by the exhaust gas, from the furnace. The exhaust gas recycling rate may, in the oxyfuel combustion mode, advantageously be determined such that the average oxygen content, by volume, of the oxidant gas is at a desired level, typically, from about 18% to about 28%. The exhaust gas recycling rate in the oxyfuel combustion mode may alternatively be determined so as to maintain a desired gas flow velocity, usually, the same as that in air-firing combustion, in the furnace.

The increased convective heat flow from the furnace is partially based on the fact that the mass and heat capacity of the exhaust gas of oxyfuel combustion, having carbon dioxide as its main component, are larger than those of the exhaust gas of air-firing combustion, having nitrogen as its main component. The high heat flow brings about that the exhaust gas carries an increased amount of heat to the exhaust gas channel, where the heat is advantageously recovered by an increased heat exchange rate in the reheating surfaces and the final economizer, as discussed above.

According to a preferred embodiment of the present invention, the system comprises a gas-gas heat exchanger, where heat is transferred from the exhaust gas in the exhaust gas channel to at least a portion of the oxidant gas. Thus, the same gas-gas heat exchanger is advantageously used in air-firing combustion to transfer heat from the exhaust gas to the combustion air, and in oxyfuel combustion to transfer heat from the exhaust gas to at least a portion of the oxidant gas.

As is common in oxyfuel combustion, the substantially pure oxygen is advantageously produced in an air separation unit (ASU), for example, a cryogenic or membrane based air separation unit. Correspondingly, a portion of the exhaust gas is advantageously cooled and pressurized in multiple exhaust gas compressors, so as to sequestrate liquid or supercritical carbon dioxide. Due to this auxiliary equipment, the net power produced by an oxyfuel combusting boiler tends to be considerably less than that of a corresponding air-firing boiler. According to an advantageous embodiment of the present invention, at least a portion of the exhaust compressors is directly driven by mechanical energy of auxiliary steam turbines using steam extracted from the steam turbine system. This steam is advantageously generated by firing more and saved from reducing the extraction of steam used for feedwater heating. Thus, the need for auxiliary power for the compression of carbon dioxide is minimized. Correspondingly, in a case in which the oxygen supply comprises a cryogenic air separation unit having compressors for pressurizing air, one or more of these compressors can also be driven directly by the auxiliary steam turbines, so as to further decrease the need for auxiliary power.

According to the present invention, the substantially pure oxygen and recycled exhaust gas can be fed to the boiler as separate streams, or as a mixture of the two streams. It is also possible to feed to the boiler multiple streams, which can be identical mixture streams, or streams having different temperatures or compositions. The multiple streams can naturally have different purposes in the furnace, such as primary, secondary and overfire gas streams of a PC boiler, or streams of fluidizing gas and secondary gas of a CFB boiler.