Patent Publication Number: US-2012024206-A1

Title: Method for reducing nitrogen oxide emissions in oxyfuel combustion

Description:
The invention relates to a method for reducing nitrogen oxide emissions in oxyfuel combustion, in which method at least one primary gas flow and at least one secondary gas flow are supplied in a furnace of a circulating fluidised bed boiler, which primary gas and secondary gas have been produced by mixing oxygen and circulated flue gas together. 
     Concern about climate change has brought on seeking new means to reduce carbon dioxide emissions causing global warming in energy production. One of the means suggested for diminishing greenhouse emissions is oxyfuel combustion. When fuel is combusted by means of air, flue gas contains a considerable amount of nitrogen that originates from the air. Recovery of carbon dioxide from such flue gas is expensive and technically difficult. When air used in combustion is replaced by a mixture of oxygen and circulated flue gas, flue gas produced as the result of combustion mainly contains carbon dioxide, oxygen, water vapour, and some impurities. Oxyfuel combustion enables relatively simple recovery of carbon dioxide. After water carried along with fuel and developed in combustion reactions has been removed from flue gas by condensing, remaining carbon dioxide can be liquefied by cooling and compressing. Oxyfuel combustion can be utilised in both pulverised fuel combustion and fluidised bed combustion. 
     In circulating fluidised bed combustion, combustion takes place in solids suspension that is fluidised and circulated by means of a gas flow blown from below. The fluidised bed consists of particle-like fluidised material (e.g. sand), fuel, combustion gas, and flue gas and ash produced in combustion. In this context, combustion gas refers to primary and secondary gases, which usually comprise air or some other oxygenous gas mixture. The primary gas flow is supplied at the bottom of the furnace and the secondary gas flow is guided to the furnace via nozzles on its walls above the grate plane. In the circulating fluidised bed boiler, fluidised material is carried along with flue gas away from the fluidising space and, for providing a steady state, it is returned to the furnace via separating and circulating devices. 
     Circulating fluidised bed boiler utilises low combustion temperature (e.g. 700-900° C.) compared to pulverised fuel combustion, which together with staged air supply enables low nitrogen oxide emissions. Nitrogen oxides (NO x ) refer to nitric oxide (NO) and nitrogen dioxide (NO 2 ), which are mostly produced from nitrogen contained by fuel in fluidised bed combustion. Staging of air supply provides reducing conditions in the lower section of the bed, whereby less nitrogen oxides are produced. The rest of air required for perfect combustion is supplied as secondary and possibly tertiary air. Circulating fluidised bed technology also enables desulphurisation of flue gases already in the boiler by supplying limestone or dolomite directly to the furnace. 
     Specifications U.S. Pat. No. 4,704,084 and U.S. Pat. No. 4,962,711 disclose examples of circulating fluidised bed boilers according to prior art which aim at reducing NO x  emissions by staged supply of combustion air. In both specifications, in the lower section of the furnace is formed a reducing zone by adjusting the supply of primary, secondary and possible tertiary air to the furnace. 
     In oxyfuel combustion, combustion air is replaced by a mixture of oxygen and circulated flue gas. If the process is run with a standard oxygen concentration, as it is usual in air combustion, diminishing the quantity of primary gas to provide a reducing zone decreases the internal and external circulation of fluidised material, whereby heat transfer onto the furnace walls and into a possible external heat exchanger also weakens. Furthermore, the temperature of the fluidised bed may rise too high, which causes sintering of solid particles. 
     The object of the invention is to avoid the above problems and enhance the reduction of nitrogen oxides in an oxyfuel combusted circulating fluidised bed boiler. 
     The method according to the invention is characterised by what is presented in the characterising part of claim  1 . 
     In the method according to the invention, the oxygen content of primary gas is adjusted such that at the bottom of the furnace is formed a reducing zone in which nitrogen oxides carried to the furnace along with circulated flue gas reduce to nitrogen when reacting with carbon monoxide and coke. Simultaneously, the oxygen content of secondary gas is adjusted such that above the reducing zone is formed an oxidising zone in which combustion can be completed. 
     Fluidisation speed can be kept constant or it can be adjusted independently, when the oxygen contents of primary and secondary gases are separately adjustable in a wide range. When decreasing the oxygen content of the primary gas, the proportion of oxygen in the secondary gas can be equivalently increased in order to provide desired total oxygen content. When the oxygen content and volume flow of both gas flows are separately adjusted, it is easier than before to maintain a suitable temperature level in both the reducing and oxidising zones. 
     It is possible to deliver secondary gas onto several different height levels, and different oxygen contents can be used on different levels in order for unburnt gases carried from the reducing zone not to cause a large temperature peak at the height of the secondary gas injection. Thus, it is possible to prevent forming of a hot oxygenous section at the height of secondary gas injections, which could easily lead to production of nitrogen oxides. 
     The invention provides an easy method based on running mode for the reduction of nitrogen oxides in a circulating fluidised bed boiler. By varying the oxygen contents of the primary and secondary gas, it is also possible to adjust the temperatures in the furnace, which is important for sulphur reduction, among others. 
     Effective reduction of nitrogen oxides decreases the risk of NO x  reacting with water and oxygen, thus producing corroding nitric acid during the pressurisation of flue gas, which could cause problems in the carbon dioxide cleansing and pressurising facility. 
    
    
     
       The invention will now be described with reference to the FIGURE of the accompanying drawing, to which the invention is by no means intended to be narrowly restricted. 
       The FIGURE schematically shows circulating fluidised bed combustion with a mixture of oxygen and circulated flue gas. 
     
    
    
     A circulating fluidised bed boiler  10  shown in the FIGURE comprises a furnace  11  in which fuel is combusted in a circulating fluidised bed, a cyclone separator  12  in which fluidised material is separated from flue gas, and a return channel  13  via which fluidised material is circulated back to the furnace  11 . Fuel  14  is supplied to the furnace  11  to which is also supplied oxygenous fluidisation and combustion gas as a primary gas flow  15  and as a secondary gas flow  16 . Combustion takes place in the fluidised bed, which is put to fluidise and circulate by means of the primary gas flow  15  supplied from below. The fuel  14  can be e.g. solid fuel, such as coal. 
     The fluidised bed consists of solid inert bed material (usually sand), fuel supplied in it, fuel ash, possible limestone, combustion gas, and flue gas produced in combustion. The gas flows  15 ,  16  are arranged so great that a part of fluidised material exits along with flue gas from the upper section of the furnace to the cyclone separator  12 . The cyclone separator  12  separates solid particles from flue gas, which are returned to the furnace  11  via the return channel  13  and an external heat exchanger (not shown in the FIGURE) possibly connected to it. 
     After separating solid matter, the flue gas is guided from the cyclone separator  12  to heat recovery  17  and from there further to fly ash separation  18 , which can be implemented e.g. with electrostatic or bag filters. After the fly ash separation  18 , the flue gas is guided to a condenser  19 , in which water and gases are separated from it by condensing. After the condenser  19 , the flue gas  20  of oxyfuel combustion mainly contains carbon dioxide, which can be cleansed and pressurised with methods known as such. 
     The primary gas flow  15  is supplied at the bottom of the furnace  11  via a wind box (not shown in the FIGURE) or equivalent. One or more secondary gas flows  16  are supplied above the bottom via injection nozzles (not shown in the FIGURE) on the walls of the furnace  11 . Both gas flows  15 ,  16  contain oxygen and circulated flue gas, the main components of which are carbon dioxide and possibly water vapour. Furthermore, flue gas contains small amounts of, inter alia, nitrogen oxides, sulphur dioxide, oxygen, and carbon monoxide. In order to provide good fluidisation and circulation of the solids suspension, the proportion of the primary gas flow  15  is usually at least 60% of the total amount of the combustion gases  15 ,  16  supplied to the furnace  11 . 
     The primary gas flow  15  is produced by means of first mixing means  21  by mixing oxygen  24  and circulated flue gas  25  together in a desired ratio. Equivalently, the secondary gas flow  16  is produced by means of second mixing means  22  by mixing oxygen  24  and circulated flue gas  25  together in a desired ratio. The oxygen  24  can be produced e.g. by removing nitrogen from air by means of an oxygen plant  23  or by some other equivalent means. The circulated flue gas  25  can be taken from the furnace flue either before the condenser  19  or after the condenser  19  depending on the wish of using wet or dry flue gas. 
     The first mixing means  21  for producing the primary gas flow  15  and the second mixing means  22  for producing the secondary gas flow can be in connection with the injection nozzles supplying gas to the furnace  11  or they can be separate from the furnace  11 , whereby the gas nozzles are supplied with a ready-mixed gas mixture. The mixing means  21 ,  22  can consist of means known as such (valves, measuring sensors, adjusters etc.) for adjusting the oxygen content of the gas flow supplied to the furnace. 
     The oxygen content of the primary gas flow  15  is adjusted such that a reducing zone I is formed at the bottom of the furnace  11 , in which zone there is oxygen less than required for the perfect combustion of fuel. The speed of the primary gas flow is again adjusted such that a suitable level of internal and external circulation of fluidised material can be provided. 
     In the reducing zone I, substochiometric conditions prevail in which more carbon monoxide and unburnt coal, i.e. coke, are produced than in the normal stochiometric combustion. From the effect of coke and carbon monoxide, nitrogen oxides NO x , both those carried along with the primary gas and those produced from the fuel, are reduced to nitrogen gas N 2  in this zone. 
     The oxygen content of the secondary gas flow  16  is adjusted such that above the reducing zone I is formed an oxidising zone II in which there is oxygen more than required for the perfect combustion of fuel. In the oxidising zone II, the combustion of fuel is completed. 
     There can be secondary gas nozzles located on several different heights and they can be supplied with secondary gas having different oxygen content. Then, each secondary gas flow  16  can be provided with its own mixing means  22  for adjusting the oxygen content of the secondary gas flows. 
     Due to the circulation of flue gas and the reducing zone, flue gas exiting from the circulating fluidised bed boiler to the carbon dioxide recovery only contains a very small amount of nitrogen oxides. 
     Many different variations of the invention are possible within the scope defined by claims presented next.