Method of controlling a process of generating power by oxyfuel combustion

1. Field of the Invention

The present invention relates to a method of controlling a process of generating power by oxyfuel combustion. More particularly, the present invention relates to controlling oxyfuel combustion in different load conditions.

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 at least 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, without having to separate it from a gas stream having nitrogen as its main component, as when combusting the fuel with air.

The feeding rate of oxygen into a combustion system is regularly controlled together with the feeding rate of fuel, so that almost complete combustion of the fuel is obtained. In conventional air-firing at full load, typically, a relatively low level, say 3%, of excess oxygen in the flue gas is sufficient to keep the CO level of the flue gas sufficiently low, but at low loads, a higher level of excess air is needed to maximize steam superheating and to complete combustion. The increased excess air at low loads leads to reduced boiler efficiency due to increased thermal stack losses.

In conventional combustion with air, harmful effects caused by too high of combustion temperatures, such as increased NO_X emissions or corrosion, or material strength problems of the furnace walls, are often prevented by recirculating a portion of the flue gas back to the furnace. Thus, the oxygen content of the inlet gas is reduced from the about 21% of air to a lower value, and the combustion temperature is thereby lowered.

One of the advantages of oxyfuel combustion is the possibility to increase thermal efficiency of the process by using high combustion temperatures. However, combustion with nearly pure oxygen as the inlet gas would provide very high combustion temperatures. Therefore, in order to avoid harmful effects of too high of combustion temperatures, especially when repowering air-fired boilers to oxyfuel combustion, flue gas recirculation is advantageously used to lower the average oxygen content of the inlet gas.

U.S. Pat. No. 6,935,251 discloses a method of combusting fuel with an oxidant stream comprising an oxygen-rich gas stream mixed with recirculated flue gas. According to this method, the rate of the flue gas recirculation is adjusted, so that the resulting mass flow rate of the flue gas is less than the corresponding mass flow rate of the flue gas generated by using air as the oxidant stream. By using such a reduced flue gas mass flow, the size of the flue gas channel and the pollution control equipment therein can be minimized. U.S. Pat. No. 6,418,865 suggests repowering an air-combustion boiler to oxycombustion by adjusting the exhaust gas recirculation rate so as to maintain the heat transfer at the original specification.

One of the requirements of a power generation process is its applicability for use in different power demand conditions with high efficiency. According to conventional practice, reduced steam outputs are achieved by operating the boiler with reduced fuel and air feeding rates. Japanese patent publication No. 2007-147162 discloses a combustion control method of an oxygen burning boiler, wherein oxygen is supplied in an amount corresponding to the boiler load, and the exhaust gas recirculation rate is controlled so as to obtain the required absorption of heat for producing steam.

For oxyfuel combustion, especially when the flue gas mass flow is less than that in combustion with air, the operation at low loads may lead to a distorted flow pattern of the flue gas, increasing the risk of operational problems, for example, due to excessive fouling or dust accumulation in low-velocity regions of the exhaust gas channel. Thus, there is a need for an improved method of controlling oxyfuel combustion in different load conditions.

An object of the present invention is to provide a method of controlling a process of generating power by oxyfuel combustion at different load conditions.

According to an aspect of the present invention, a method of controlling a process of generating power in a power plant with a boiler by combusting carbonaceous fuel with substantially pure oxygen is provided, the method comprising, at full load conditions, the steps of (a1) introducing a first carbonaceous fuel feed stream into a furnace, (b1) introducing a first substantially pure oxygen feed stream into the furnace for combusting the first carbonaceous fuel feed stream with the oxygen, (c1) discharging exhaust gas via an exhaust gas channel from the furnace, (d1) recovering heat from the exhaust gas by heat exchange surfaces arranged in the exhaust gas channel, and (e1) recirculating a portion of the exhaust gas via an exhaust gas recirculating channel at a first recirculation flow rate to the furnace, to form, together with the first substantially pure oxygen feed stream, a first inlet gas stream having a predetermined average oxygen content, thereby discharging exhaust gas from the furnace at a first discharge flow rate, and in second load conditions, corresponding to 90% or less of the full load, the steps of (a2) introducing a second carbonaceous fuel feed stream into the furnace, (b2) introducing a second substantially pure oxygen feed stream into the furnace for combusting the second carbonaceous fuel feed stream with the oxygen, (c2) discharging exhaust gas via the exhaust gas channel from the furnace, (d2) recovering heat from the exhaust gas by the heat exchange surfaces arranged in the exhaust gas channel, and (e2) recirculating a portion of the exhaust gas via the exhaust gas recirculating channel at a second recirculation flow rate to the furnace, to form together with the second substantially pure oxygen feed stream a second inlet gas stream, so as to discharge exhaust gas from the furnace at a second discharge flow rate, and controlling the second recirculation flow rate to be from the first recirculation flow rate to a value providing the second discharge flow rate to be substantially as high as the first discharge flow rate.

Any reference to a gas flow rate, throughout this description, can be considered to mean a volume flow rate, unless otherwise stated. By a “substantially pure oxygen feed stream” is meant an oxygen-rich stream, usually, having a purity of at least 95%, from an oxygen supply, such as a cryogenic air separator. The substantially pure oxygen feed stream is at all loads, as usual, such that substantially all of the fuel feed stream will be combusted with the oxygen, which means that the exhaust gas stream comprises a small amount, for example, 3%, of residual oxygen. The process also regularly comprises conventional measures for cleaning the exhaust gas from impurities, such as sulfur dioxide. The portion of the exhaust gas, which is not recirculated to the furnace, may be discharged from the system by condensing water and recovering carbon dioxide for sequestration or further use.

According to the present invention, the second fuel feeding rate corresponds to reduced load conditions, i.e., 90% or less of the full load. The second load conditions may preferably be 80% or less of the full load, even more preferably, 70% or less of the full load. According to the present method, the exhaust gas recirculation rate is, at reduced load conditions, adjusted so that the flow rate of gas discharged from the furnace remains at a sufficiently high range. By having a high exhaust gas flow rate at all load conditions, the designed flow pattern of the exhaust gas, and the distribution of heat transfer in different heat transfer surfaces of the boiler, can be maintained. In practice, the exhaust gas flow rate may be fixed to a predetermined value or range, which depends on the conditions in question. The selected value or range is naturally such that it guarantees problem-free operation in all load conditions. Thus, the exhaust gas flow is sufficient to prevent, for example, unwanted excessive dust accumulation in low-velocity regions.

According to a conventional method, where the exhaust gas flow rate decreases at low load conditions, the distribution of heat absorbed in different heat transfer surfaces can be distorted, because of the varying relative amount of heat transferred with the exhaust gas. Thereby, for example, the amount of superheating of steam or the preheating of the feedwater in the exhaust gas channel may, at low load conditions, become too low. According to the present invention, the distribution of heat transfer in different heat transfer surfaces can be maintained even at low load conditions. Because the sufficient flow rate of gas discharged from the furnace is, according to the present invention, achieved by recirculating exhaust gas, not by discharging extra exhaust gas to the environment, the problem of reduced thermal efficiency due to thermal stack losses, is avoided.

According to the present invention, the flue gas recirculation is increased at low load conditions, so as to at least partially compensate for the decreased production of combustion gas. This method of controlling the flue gas recirculation clearly differs from the conventional method used in combustion with air, in which flue gas recirculation is used to avoid too high of temperatures in the furnace. At low load conditions, when the temperature in the furnace already decreases due to a reduced fuel feed rate, the need for conventional flue gas recirculation is minimized.

The prevention of harmful decreasing of the exhaust gas flow rate at low load conditions by increasing or at least maintaining the recirculating gas flow rate is especially beneficial in oxyfuel combustion, where the equipment for high exhaust gas recirculation is usually readily available, and the gas flow to be compensated for consists mainly of the decreased CO₂ production. This is in clear contrast with air-fired combustion, where exhaust gas recirculation is normally low or missing, and the change of the exhaust gas at low loads includes, in addition to a reduced flow of carbon dioxide, also, as a larger component, a decreased flow of nitrogen. Thus, the application of the present invention in air-fired combustion would bring about high costs, due to the arrangements needed for high additional exhaust gas recirculation at low loads.

When using the present invention, the recirculated gas flow rate may be increased at low loads by the same amount, in moles, as the substantially pure oxygen feed stream is decreased. Thereby, the volume flow rate of the exhaust gas remains constant. Alternatively, the recirculated gas flow rate may be increased at low loads by a smaller amount or, at least, the recirculated gas flow rate shall advantageously be maintained at a constant level. In all of these alternatives, the share of exhaust gas recirculated to the furnace is, at low loads, increased from that at full load. Thereby, the average oxygen content of the inlet gas is, at low loads, decreased.

When retrofitting an air-fired boiler for oxyfuel combustion, it is usually required to maintain, as much as possible, of the original combustion system, and, therefore, it is advantageous to at least partially keep the original furnace, flue gas channel and heat transfer surfaces. Thus, in order to obtain an average oxygen content of the inlet gas, which is close to that of air, the oxyfuel combustion process of a retrofitted boiler advantageously uses a high exhaust gas recirculation rate. Thereby, the fuel can be combusted by almost maintaining the original temperatures and gas flow rates. A similar construction is also advantageously used in dual-firing boilers, i.e., in boilers, which can be used both for combustion with air and for oxyfuel combustion. To obtain average oxygen contents of the inlet gas of 20% to 25%, typically, exhaust gas recirculation rates of about 81% to 75%, respectively, are required, the exact values depending on the level of impurities and residual oxygen in the flue gas.

For oxycombustion boilers in which the designed oxygen content of the inlet gas is relatively low, say 20% to 25%, it is especially advantageous to increase the recirculated gas flow rate at low loads, so that the flow rate of the exhaust gas remains substantially constant. The reason for this is that, for such a low oxygen boiler, even the maintaining of the exhaust gas flow rate increases the recirculated gas flow rate only by a relatively small amount. Alternatively, the exhaust gas flow rate can be allowed to slightly decrease, which means that the flow rate of the recirculated gas is increased even less.

For example, the maintaining of the exhaust gas flow rate, when changing the load from 100% to 70% in a boiler having, at full load, an inlet gas oxygen content of 25%, is obtained by increasing the flow rate of the recirculated exhaust gas by about 10%. Because the flow rate of the exhaust gas is constant, the share of exhaust gas recirculated back to the furnace changes as the flow rate of the recirculated gas, i.e., by about 10%, typically, from 75% to 82%. The oxygen content of the inlet gas is thereby decreased from 25% to about 18%, i.e., it is multiplied by 0.72. Correspondingly, for a boiler with a 20% full load inlet gas oxygen content, the exhaust gas flow rate can be maintained, when reducing the load to 70%, by increasing the recirculation gas flow rate by about 7%, whereby the oxygen content of the inlet gas decreases from 20% to about 15%, i.e., it is multiplied by 0.75.