Patent Application
20140065559
The present disclosure relates to an energy efficient power plant. More specifically, the present disclosure relates to a pressurized oxy-combustion power plant including one or more of a circulating fluidized bed boiler and a circulating moving bed boiler.
Atmospheric oxy-combustion plants need to compress the product carbon dioxide for sequestration or enhanced oil recovery applications. As a result, atmospheric oxy-combustion plants have high parasitic power consumption from the air separation unit and gas processing unit. This design results in high capital costs and plant thermal efficiencies up to 10% points less than an air-fired plant without carbon dioxide capture.
Rather than compress the carbon dioxide at the end of the process, a pressurized oxy-combustion boiler and power plant can be configured to provide the oxygen and fuel to the cycle already at elevated pressures. This prior to combustion reduces the size of all gas-touched equipment, and enables many process improvements—such as enhanced heat transfer, more effective waste heat utilization, and potentially integrated emissions control within the waste heat recovery process itself.
A disadvantage of known pressurized oxy-combustion technologies is that all of these technologies rely upon flue gas recirculation to control the combustion temperature. This requires additional power consumption for the flue gas recirculation fan and also increases the size of the flue gas ducts and pollution control equipment downstream of the combustor.
According to aspects illustrated herein, there is provided a pressurized oxy-combustion circulating fluidized bed power plant having a circulating fluidized bed boiler. The boiler includes a combustion chamber and a separator. The combustion chamber is in fluid communication with the separator and is configured so that solids produced during combustion in the combustion chamber enter the separator. The power plant further includes an air separation unit that is in fluid communication with the combustion chamber. The air separation unit is configured to supply substantially pure oxygen to the combustion chamber at a pressure greater than 1 bar. An external heat exchanger is in fluid communication with the separator and in fluid communication with the combustion chamber. The external heat exchanger is configured so that a portion of the solids received in the separator pass through the external heat exchanger and transfer heat to a working fluid, after which the solids are returned to the combustion chamber to provide the primary means of moderating or controlling the temperature in the combustion chamber. A portion of the product gas (mostly CO2 and H2O) can be recirculated to the combustion chamber for fluidization. Recirculation gas can also be used for fluidization in the external heat exchanger if necessary.
According to other aspects illustrated herein, there is provided a pressurized oxy-combustion circulating moving bed power plant. The plant includes a circulating moving bed boiler having a combustion chamber and moving bed heat exchanger. The boiler is configured so that the combustion chamber is located in a tower above the moving bed heat exchanger such that solids produced during combustion in the combustion chamber flow downward into the moving bed heat exchanger. An air separation unit is in fluid communication with the combustion chamber. The air separation unit is configured to supply substantially pure oxygen to the combustion chamber at a pressure greater than 1 bar. Solids produced during combustion enter the moving bed heat exchanger and transfer heat to a working fluid, after which the solids are returned to the combustion chamber to provide the primary means of moderating or controlling the temperature in the combustion chamber.
The above described and other features are exemplified by the following figures and detailed description.
Referring now to the figures, which are exemplary embodiments, and wherein the like elements are numbered alike:
In reference to
In the embodiment shown, the ASU 30 delivers the substantially pure oxygen to the combustion chamber 22 at a pressure greater than 1 bar. In yet further embodiments of the present invention, the ASU 30 delivers the substantially pure oxygen to the combustion chamber 22 at a pressure between 6 and 30 bar. Fuel and sorbent (limestone or dolomite) are dry feed to the combustion chamber 22 by lockhoppers 32 or a solids pump, for example a Stamet's design (not shown in
Solids are generated during combustion in the combustion chamber 22. The solids are separated by a separator 28, which can also be referred to as a cyclone. A portion of the collected solids are returned directly to the combustion chamber 22 via conduit 24. The remaining solids pass through an external heat exchanger 40 via a conduit 26. In the embodiment shown, the external heat exchanger 40 is either a fluidized bed heat exchanger or a moving bed heat exchanger. The external heat exchanger 40 is in fluid communication with the separator 28 (via conduit 26) and the combustion chamber 22 via conduit 41. The remaining solids pass through the external heat exchanger 40 where energy is transferred from the solids to a working fluid 43, which is typically steam.
The external heat exchanger 40 could be a fluid bed heat exchanger (FBHE) or a moving bed heat exchanger (MBHE). The FBHE is conventional technology but it behaves as a continuous stirred reactor. The mixed FBHE solids temperature is therefore the same as the FBHE solids discharge temperature. This limits the maximum working fluid temperature (e.g. steam) that can be achieved in the FBHE. It would be difficult to heat steam much beyond 600-650° C. at normal circulated fluidized combustion temperatures. The MBHE is a plug flow countercurrent heat transfer device. It has a much higher log mean temperature difference than a FBHE and can therefore achieve much higher temperatures. Temperatures as high as A-USC steam conditions (700° C.) can be achieved in the MBHE. Higher temperatures may be achieved with either heat exchanger if the circulating fluidized bed is operated at higher temperatures. This may be feasible to some extent with certain fuels. Further, pressurized operation may enable somewhat higher temperatures since the sulfur capture mechanism is different under pressure and may have a higher optimum temperature.
The working fluid 43 in boiler 20 and the external heat exchanger 40 is used to drive a steam turbine 58 or a series of steam turbines 58. The cooled solids leaving the external heat exchanger 40 via conduit 41 are returned to the combustion chamber 22 to provide the primary means of moderating or controlling the temperature of combustion in the combustion chamber 22 of the boiler 20. As should be appreciated, the flue gas 23 may include sulfur (often in the form of sulfur oxides, referred to as “SOx”), nitrogen compounds (often in the form of nitrogen oxides, referred to as “NOx”), carbon dioxide (CO2), water (H2O) and other trace elements and/or impurities. As shown in
Pollution control typically takes place in the combustion chamber 22. Sulfur dioxide (SO2) is at least partially removed in-furnace via the injection of the sorbent, e.g., limestone or dolomite, from lockhoppers or other pressurized feeder 32. The relatively low combustion temperature in the embodiment shown in
The disclosed embodiment uses the condensing heat exchanger 50 to recover latent energy (e.g., heat) in the flue gas. The water vapor dew point temperature is about 150° C. at 10 bar in the conduit 54. The recovered energy (e.g, heat) is quite useful at this temperature and can be used to replace much of the extraction steam for the feedwater heaters. The plant 10 includes a plurality of feedwater heaters 70. The plurality of feedwater heaters 70 are in communication (shown generally at 51) with the condensing heat exchanger 50 so that a working fluid 55 of the condensing heat exchanging 50 can be routed to the feedwater heaters 70 to supplement or replace steam extracted from the turbines 58.
The pressurized oxy-combustion CFB cycle (using a conventional Rankine cycle) has net plant efficiencies 3-5% points better than an atmospheric pressure oxy-combustion plant while firing bituminous coals. The efficiency advantage improves by at least another 2% points while firing subbituminous coals because the latent heat in the flue gas can be recovered in the condensing heat exchanger at high temperatures and thus be effectively used in the steam/water cycle. Thermal plant efficiencies will improve by an additional 2% points if the steam conditions are increased to 700° C.
In some embodiments in which a more advanced pressurized oxy-combustion circulating fluidized bed design is employed, supercritical carbon dioxide is heated in the external heat exchanger to drive a supercritical carbon dioxide turbine in a modified Brayton cycle. Steam is still generated in the convective pass to drive the steam turbine in a Rankine bottoming cycle. The cycle efficiency is improved by another 3 percentage points by using supercritical carbon dioxide to drive an S—CO2 turbine. Coupled with higher turbine inlet temperatures, advanced pressurized oxy-combustion circulating fluidized bed cycles can have thermal efficiencies that match or exceed a super critical power pulverized coal plant without carbon dioxide capture. These efficiency improvements apply also to the pressurized oxy-combustion circulating moving bed 110 shown in
In reference to
An air separation unit 230 may be in fluid communication with the combustion chamber 122. The air separation unit is configured to supply substantially pure oxygen to the combustion chamber at a pressure greater than 1 bar. In the present embodiment, the air separation unit provides substantially pure oxygen at between 6 bar and 30 bar. The oxygen, and resulting combustion products including flue gas 23, remain pressurized during the power plant cycle. The combustion process produces solids, which flow downward and enter the moving bed heat exchanger 140 and transfer heat to a working fluid 132. After the solids pass through the moving bed heat exchanger 140, the solids are returned via conduit 142 to the combustion chamber 122 to moderate or control a temperature of combustion in the combustion chamber 122.
The approach of the design disclosed in
The pressurized circulating moving bed-based oxy-combustion concept disclosed in
An alternative approach for emissions control would be to take advantage of lead chamber process reactions in a direct contact condensing heat exchanger. This would require considerable additional development and could be considered as a future improvement to the pressurized oxy-combustion cycle. However, successful development could potentially eliminate the need for low NOx burners, a mercury control system, and the WFGD/DFGD.
One advantage of the presently disclosed pressurized oxy-combustion power plants is that heat transfer rates are enhanced as the pressure is increased. The convective heat transfer coefficient increases by a factor of 4 at 10 bar and the overall heat transfer coefficient by about 3 times greater than at atmospheric pressure. This results in a significant reduction in pressure part material weight.
Another advantage of the presently disclosed pressurized oxy-combustion power plants is the latent heat of vaporization of water vapor can be recovered in a condensing heat exchanger. This is because the water vapor dew point increases significantly at higher pressures and the recovered energy becomes much more useful. The dew point for atmospheric air-fired and oxy-fired combustion is about 45° C. and 95° C., respectively. At 10 bar, the dew point temperature increases to about 150° C. for pressurized oxy-combustion. The energy recovered in the condensing heat exchanger can be used to replace much of the extraction steam for the feedwater heaters.
Another advantage of the presently disclosed pressurized oxy-combustion power plants is pollution control in the pressurized oxy-circulating fluidized bed (or circulating moving bed) takes place mostly in the combustion chamber. Sulfur dioxide is removed in-furnace via limestone (or dolomite) injection, while the relatively low combustion temperatures minimize NOx emissions. Final sulfur dioxide polishing and particulate cleanup (NID, ESP, Flowpac, etc) can be included downstream of the combustor. Candle filters are also an alternative for final particulate cleanup. Any residual SOx is substantially removed in the condensing heat exchanger.
Another advantage of the presently disclosed pressurized oxy-combustion power plants, specifically the pressurized oxy-combustion circulating fluidized bed power plant, is that the boiler can use recycle solids from a fluidized bed heat exchange or a moving bed heat exchanger to moderate the temperature of combustion in the combustion chamber of the boiler. Flue gas recirculation is only required for solids fluidization and transport in the combustor and for fluidization in the external heat exchanger if a fluid bed heat exchanger is used.
Another advantage of the presently disclosed pressurized oxy-combustion power plants, specifically those employing the Rankine cycle, is that they result in net plant efficiencies that are 3-5% better than an atmospheric pressure oxy-combustion plant while firing bituminous coals. The efficiency advantage improves by at least another 2% while firing sub-bituminous coals because the latent heat in the flue gas can be recovered in the condensing heat exchanger at high temperatures and thus be effectively used in the steam/water cycle.
Another advantage of the presently disclosed pressurized oxy-combustion power plants is that the external heat exchanger (fluidized bed heat exchange or a moving bed heat exchanger) in the pressurized oxy-circulating fluidized bed (or circulating moving bed) can be used to generate steam. In a more advanced cycle design, supercritical carbon dioxide is heated in the external heat exchanger to drive a supercritical carbon dioxide turbine in a modified Brayton cycle. Steam is still generated in the convective pass to drive the steam turbine in a Rankine bottoming cycle. The cycle efficiency is improved by another 3% by using supercritical carbon dioxide to drive a S—CO2 turbine. Coupled with higher turbine inlet temperatures, advanced pressurized oxy-combustion cycles can have thermal efficiencies that match or exceed a super critical pulverized coal (SCPC) plant without carbon dioxide capture.
Another advantage of the presently disclosed pressurized oxy-combustion power plants is that equipment size and cost is reduced because of the reduced gas volumetric through put. This provides the potential for significant capital and operating cost savings due to smaller sized equipment, reducing the requirements on the pollution control and GPU equipment.
While the invention has been described with reference to various exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.
