BACKGROUND Compliance with ever more stringent regulations on emission of particulate matter, S02, S03, NOx and other pollutants increases the cost of doing business for the power, refinery, chemical and other industries and increases the pressure on suppliers to develop and provide low cost simple to operate air pollution control processes and systems.

The art teaches various processes and technologies designed to reduce the emission of S02 and NOx from combustion gas. For S02 capture they typically include the use of alkaline absorbents, such as limestone, lime, hydrated lime, sodium bicarbonate and others, which react with S02 in liquid or gas phase to form sulfites or sulfates. Typical processes to reduce NOx emission include: (i) processes to minimize NOx formation during combustion; (ii) use of chemicals, such as ammonia, to reduce NOx to elemental nitrogen with or without a catalyst; and (iii) oxidation of NOx to nitrate by strong oxidants followed by capturing of the nitrate in alkaline sorbent. Unfortunately, the present technologies for NOx reduction are either expensive, involve the use of expensive chemicals and/or are not efficient enough to meet air emission regulations.

It was observed that low-level reduction of NOx is also achieved during the course of operating Dry Flue Gas Desulfurization (DFGD) systems when using lime or hydrated lime to capture S02 and operating at typical DFGD temperature of 140-170F. Further work showed that at temperature in the range of 185-300F, efficiency of NOx capture can improve compared to its capture efficiency at lower temperatures. However, the improved capture efficiency of NOx is accompanied by simultaneous deterioration in S02 capture efficiency.

Particular examples of such processes are disclosed in U. S. patent No 4,442, 079 where a conventional DFGD system includes an absorber vessel and a baghouse that is applied to capture S02 and NOx simultaneously. NOx capture efficiency was relatively low and various improvements were tested including injection of additives, such as sodium, and a scheme for gas reheat. Various configurations of fluidized bed absorber, by the same inventors, are disclosed in U. S. patent No. 4,442, 080. The fluidized bed substitutes the conventional DFGD absorber vessel and it follows by a baghouse downstream. In both configurations and their derivatives, the S02 has to be captured first followed by the NOx reduction in the absorber vessels and in the baghouse. The attempts to capture both S02 and NOx in the same vessel resulted in non-optimized conditions for the capture of both species, in relatively low removal efficiency, in the need for costly additives and in high consumption of lime or hydrated lime reagent.

Additional examples of sulfite-based DeNOx process are described in U. S. Patents No. 4,645, 652 and No. 4,645, 653. These systems include an absorber reactor and a baghouse or electrostatic precipitator down stream. The system is a conventional DFGD with addition of waste sorbent recycle and injection into the duct upstream of the DFGD. The induct NOx capture efficiency is low due to poor gas-solid mixing and contact, short residence time and poor temperature control.

In spite of the apparent attractiveness of using the DFGD waste to capture NOx the various processes failed to gain commercial acceptance due to one or more of the following: 1. Attempts to capture both NOx and S02 in one vessel under the same conditions resulted in relatively low removal efficiency due to un-optimized conditions for both species.

2. NOx reduction requires tightly controlled and uniform temperature, long residence time and intimate mixing which could not be achieved with the present systems.

3. Conversion of NO to N02, N204, N205 and the likes in the DFGD and in the baghouse resulted in the formation and emission of visible brown plum, which is unacceptable for the plant operation.

The present state of the art did not create the conditions for high efficiency NOx reduction, they compromise the high efficiency of S02 capture and in few cases they produced brown plume and thus they failed to be attractive to users. Accordingly, it would be considered an advance in the art to develop new systems and methods to overcome the current problems and shortcomings.

SUMMARY OF THE INVENTION The present invention is a method and system to efficiently and cost-effectively reduce the < emission of S02 and NOx from combustion gases in utilities and industrial facilities. It is an objective of this invention that such a process would be relatively uncomplicated, would utilize low cost reagent and would produce easily disposable single waste stream.

The present invention is a dry method and system whereby a dedicated entrained bed absorber is applied to reduce the emission of NOx from flue gas. The entrained bed absorber provides intimate contact between waste sorbent from the DFGD absorber and the flue gas containing NOx and S02. The waste sorbent, in dry powder form, contains calcium sulfite and sulfate, excess lime, ash and other species. The waste product is recycled from the downstream DFGD system. The NOx absorber is dedicated to the capture of NOx and is optimized to operate at uniform and narrow range temperature required to achieve best conditions for NOx reduction. In addition to NOx reduction the capture of some SOx also occurs due to its reaction with lime or hydrated lime and with NOx.

In accordance with the current invention the temperature in the NOx reactor is tightly and uniformly controlled in the range of 170-240F and preferably in the range of 180-220F for optimal NOx reduction conditions (optimally preferred temperature is about 200F). The temperature is controlled by the injection of water into the gas stream, upstream of the NOx absorber, or by injection of water into the bottom of the entrained bed NOx absorber or both. The water is injected in a form of fine droplets so that it can quickly evaporate to lower the gas and sorbent temperature to the desired range. An interesting observation was that both NO reduction to elemental nitrogen and oxidation of NO to N02 and other NOx species do occur.

Following the NOx reduction, the flue gas flows into a second absorber stage where the conditions are optimized for SOx capture. The SOx absorber is of conventional design for such applications with entrained bed or a reactor vessel with rotary atomizer or other means to atomize slurry. In addition to capturing S02, the reactor contains sufficient amount of alkalinity to react with N02, N204, N205 and the like to form solids phase nitrates for disposal with the balance of the solids waste.

The present invention has the advantage of high efficiency capture of NOx and S02, there are no efficiency promoting additives and no additional waste streams. It is a low cost high efficiency process and system integrated and added upstream of a conventional dry or semidry flue gas desulfurization system to achieve high efficiency reduction of NOx emission from flue gas. The NOx capture takes place in a dedicated entrained bed absorber installed upstream of the DFGD system, operating at higher temperature than the DFGD system and utilizing dry waste product containing calcium sulfite from the DFGD system as a reagent for the NOx control. The simple, low cost and efficient system has notable advantages over previous DeNOx processes.

BRIEF DESCRIPTION OF THE DRAWINGS The above and other advantages of this invention will become more apparent from the following description taken in conjunction with the accompanying drawings, in which: FIG. 1 is a schematic representation of the DeNOx system integrated with a conventional entrained bed DFGD. The system includes 2 entrained bed reactors installed in series where the first stage reactor is optimized for NOx removal and the second stage reactor is optimized for S02 capture. An electrostatic precipitator or a baghouse is installed downstream for capturing particulate matter.

FIG. 2 is a similar schematic representation of the process as shown in FIG. 1 except for the design of the two absorption stages. In FIG. 2 both DeNOx and DeSOx functions are merged into one entrained bed reactor vessel but with two distinct zones. The first zone is optimized for NOx removal and the second for S02 removal; and FIG. 3 is a schematic representation of the DeNOx system integrated with a conventional semidry FGD reactor. This system includes two separate vessels with the first vessel being an entrained bed DeNOx absorber, the second vessel being a conventional absorber for S02 capture equipped with lime slurry injection and means of atomizing the slurry and the third vessel is an electrostatic precipitator or a baghouse for capturing particulate matter.

DETAILED DESCRPTION OF THE INVENTION In accordance with the present invention, an improved DeNOx process and system are provided. The DeNOx system is integrated with conventional Dry Flue Gas Desulfurization (DFGD) system, whereby nitrogen oxides, sulfur dioxide, particulate matter and other lesser species are removed from the flue gas. The integrated system includes a series of dedicated reactors and vessels each optimized for its duty. The system includes an entrained bed reactor optimized to reduce nitrogen oxides emission followed by a conventional reactor optimized for dry sulfur oxides capture and further followed by a baghouse or electrostatic precipitator for particulate matter capture. The only reagent in the system is lime or hydrated lime and the only waste stream is dry powder.

FIG. 1 is a schematic view of the integrated process and system comprises of NOx removal step followed by S02 removal step and further followed by particulate matter capture. As shown in FIG. 1, gas-containing NOx, SOx, particulate matter and other combustion products flow from the boiler (not shown) to gas conduit 102. Hydrated lime powder, stored in silo 120, is fed through conduit 104 into the flue gas in conduit 102 or directly to the bottom of the entrained bed absorber 122. Recycled powder, the waste product from the baghouse 126, collected in the baghouse hoppers 116, is fed through conduit 108 into the gas stream in conduit 102 or directly to the bottom of the absorber 122, as shown. Typically the gas stream in conduit 102 is at 250-350F, the lime in lime silo 120 is at ambient or slightly above ambient temperature and the recycled waste product from the baghouse is at about 140-170F. The resultant mixed stream of gas and particulate matter is at temperature somewhat lower than the initial temperature of the gas entering the system.

High gas velocity in the entrained bed absorber and, optionally, internals are responsible for intimate mixing of the gas and the entrained finely divided particulate matter. The temperature in the entrained bed absorber 122 is controlled by injection of water, preferably in atomized form, to the bottom of the absorber 122 through conduit 106, or alternatively into the gas stream in conduit 102 as shown or both. The temperature in the entrained bed reactor 122 is controlled in the range of 170-240F and preferably in the range of 180-220F. Due to proper distribution of water and the intense mixing occurring in the entrained bed absorber, a close to uniform temperature can be achieved and maintained across the absorber. Most of the nitrogen oxide in the flue gas is in the NO form and it reacts in absorber 122 to form elemental nitrogen and higher oxides such as N02, N204 and N205.

Some S02 is also captured in reactor 122 by the reaction with lime or hydrated lime.

The partially treated flue gas and the powder of finely divided particulate matter flows from the entrained bed absorber 122 through connecting conduit 128 to entrained bed absorber 124, which is optimized for SOx capture. The flue gas temperature in absorber 124 is controlled to about 140-170F by water injection, preferably in atomized form, into the connecting conduit 128 or to the bottom of absorber 124 through conduit 110. Under the optimal conditions created in absorber 124 the S02 efficiently reacts with the lime or hydrated lime to form calcium sulfite and calcium sulfate. The oxidized form of nitrogen N02, N204, N205 or other nitrogen oxides also react with lime or hydrated lime in absorber 124 to form calcium nitrate or other form of solid phase nitrogen oxides products.

The treated flue gas containing entrained particulate matter flows further downstream to a particulate collector 126, which can be a baghouse or an electrostatic precipitator. Additional capture of SOx and NOx occurs in the particulate matter collector. Clean flue gas flows to the stack through conduit 112 and the particulate matter is collected at the bottom in silos 116. A portion of the particulate matter is rejected to waste disposal or for utilization elsewhere, through conduit 114. The balance of the particulate matter is recycled to the entrained bed 122 through conduit 108 as shown. FIG 2 is an alternative embodiment where entrained bed reactor 122 and entrained bed reactor 124 are merged into one reactor, as shown. However, the distinct operating conditions of first stage DeNOx and second stage DeSOx are maintained. An internal separating device 228 is installed, to separate the two zones and prevent back flow and mixing of gas and particulate matter between the two zones. Each zone operates at distinct and controlled temperatures optimized for the duty of the zone.

As shown in FIG. 2, gas-containing NOx, SOx, particulate matter and other combustion products flow from the boiler (not shown) to gas conduit 202. Hydrated lime powder, stored in silo 220, is fed through conduit 204 into the flue gas in conduit 202 or directly to the bottom of the entrained bed absorber 222. Recycled powder, the waste product from the baghouse 226, collected in the baghouse hoppers 216, is fed through conduit 208 into the gas stream in conduit 202 or directly to the bottom of the first stage DeNOx in the lower section of the absorber, section 222, as shown. Typically the gas stream in conduit 202 is at 250-350F, the lime in lime silo 220 is at ambient or slightly above ambient temperature and the recycled waste product from the baghouse is at about 140-170F. The resultant mixed stream of gas and particulate matter is at temperature somewhat lower than the initial temperature of the gas entering the system.

High gas velocity in the entrained bed absorber and, optionally, mixing supporting internals are responsible for intimate mixing of the gas and the entrained finely divided particulate matter. The temperature in the lower part of the entrained bed absorber 222 is controlled by injection of water, preferably in atomized form, to the bottom of the absorber section 222 through conduit 206, or alternatively into the gas stream in conduit 202 as shown, or both. The temperature in the entrained bed absorber section 222 is controlled in the range of 170-240F and preferably in the range of 180-220F (optimal preferred temperature is at 200F). Due to proper distribution of water and the intense mixing occurring in the entrained bed absorber, a close to uniform temperature can be achieved and maintained across the absorber. Most of the nitrogen oxide in the flue gas is in the NO form and it reacts in absorber section 222 to form elemental nitrogen and higher oxides such as N02, N204 and N205. Some S02 is also captured in absorber section 222 by the reaction with lime or hydrated lime.

The partially treated flue gas and the powder of finely divided particulate matter flows from the entrained bed absorber section 222 through a separation device 228 to entrained bed absorber section 224, which is optimized for SOx capture. The flue gas temperature in absorber section 224 is controlled to about 140-170F by water injection, preferably in atomized form, into the separation device 228 or to the bottom of absorber section 224 through conduit 210. Under the optimal conditions created in absorber section 224 the SOx efficiently reacts with the lime or hydrated lime to form calcium sulfite and calcium sulfate. The oxidized form of nitrogen N02, N204, N205 or other nitrogen oxides also react with lime or hydrated lime in absorber 224 to form calcium nitrate or other form of solid phase nitrogen oxides products.

As explained above, the separation device 228 is a perforated plate or another type of separating device with the function of allowing one directional flow, upwards, of both gas and entrained bed material and of preventing mixing between the two zones. The treated flue gas containing entrained particulate matter flows further downstream to a particulate collector 226, which can be a baghouse or an electrostatic precipitator.

Additional capture of SOx and NOx occurs in the particulate matter collector. Clean flue gas flows to the stack through conduit 212 and the particulate matter is collected at the bottom in silos 216. A portion of the particulate matter is rejected to waste disposal or for utilization elsewhere, through conduit 214. The balance of the particulate matter is recycled to the entrained bed NOx absorber section 222 through conduit 208 as shown.

FIG. 3 is yet another embodiment where the process concept of optimized capture of NOx, SOx and particulate matter in a series of reactors and vessels is maintained and waste product is recycled to the entrained bed deNOx absorber. The main difference between the embodiments described in FIGS. 1 and 2 and the system shown in FIG. 3 is the method and form of lime introduction into reactor 324. The lime in FIG. 3 is injected as water slurry through rotary atomizer, two fluid nozzles or other means designed to atomize the sluny. The lime can also be injected to reactor 324 in dry powdery form. The water in the slurry from conduit 304 evaporates in absorber 324 to form dry powder and to cool and control the flue gas at the desired temperature in the range of 140-170F. Stream 306 is a water stream to control the temperature in the entrained bed DeNOx absorber 322 and stream 318 is a water stream introduced into the lime preparation system 320 together with dry lime in stream 319. The lime slurry is delivered to the FGD vessel through conduit 304 and device 330.

As shown in FIG. 3, gas-containing NOx, SOx, particulate matter and other combustion products flow from the boiler (not shown) to gas conduit 302. Recycled dry powder, the waste product from the baghouse 326, collected in the baghouse hoppers 316, is fed through conduit 308 into the gas stream in conduit 302 or directly to the bottom of the absorber 322, as shown. Typically the gas stream in conduit 302 is at 250-350F and the recycled waste product from the baghouse is at about 140-170F. The resultant mixed stream of gas and particulate matter is at temperature somewhat lower than the initial temperature of the gas entering the system.

High gas velocity in the entrained bed absorber and, optionally, mixing supporting internals are responsible for intimate mixing of the gas and the entrained finely divided particulate matter. The temperature in the entrained bed absorber 322 is controlled by injection of water, preferably in atomized form, to the bottom of the absorber 322 through conduit 306, or alternatively into the gas stream in conduit 302 as shown or both. The temperature in the entrained bed reactor 322 is controlled in the range of 170-240F and preferably in the range of 180-220F. Due to proper distribution of water and the intense mixing occurring in the entrained bed absorber, a close to uniform temperature can be achieved and maintained across the absorber. Most of the nitrogen oxide in the flue gas is in the NO form and it reacts in absorber 322 to form elemental nitrogen and higher oxides such as N02, N204 and N205. Some SOx is also captured in reactor 322 by the reaction with lime or hydrated lime.

The partially treated flue gas and the powder of finely divided particulate matter flows from the entrained bed absorber 322 through connecting conduit 328 to the conventional SOx absorber vessel 324, which is optimized for SOx capture. The flue gas temperature in absorber 324 is controlled to about 140-170F by lime slurry injection, in atomized form, into the SOx absorber 324 through conduit 304 and device 330, which is a rotary atomizer, two fluid nozzles or other slurry atomizing devices. Under the conditions created in absorber 324 the water in the slurry evaporate and the SOx efficiently reacts with the lime slurry to form dry powder of calcium sulfite and calcium sulfate. The oxidized form of nitrogen N02, N204, N205 or other nitrogen oxides also react with the lime slurry in absorber 324 to form calcium nitrate or other form of solid phase nitrogen oxides products.

The treated flue gas containing entrained particulate matter flows further downstream to a particulate collector 326, which can be a baghouse or an electrostatic precipitator.

Additional capture of SOx and NOx occurs in the particulate matter collector. Clean flue gas flows to the stack through conduit 312 and the particulate matter is collected at the bottom in silos 316. A portion of the particulate matter is rejected to waste disposal or for utilization elsewhere, through conduit 314. The balance of the particulate matter is recycled to the entrained bed 322 through conduit 308 as shown.

A portion of the particulate matter is collected at the bottom of absorber 324 and is recycled through conduit 310 and through conduit 308 to the DeNOx entrained bed absorber 322 together with a portion of the particulate matter collected in the silos 316 of baghouse 326.

The present invention has now been described in accordance with several exemplary embodiments, which are intended to be illustrative in all aspects, rather than restrictive. Thus, the present invention is capable of many variations in detailed implementation, which may be derived from the description contained herein by a person of ordinary skill in the art. All such variations and other variations are considered to be within the scope and spirit of the present invention as defined by the following claims and their legal equivalents.