Patent Description:
Nitrogen oxides (NOx) are gaseous pollutants emitted during the combustion process in a combustor or a furnace of a boiler. The flue gas or exhaust gas resulting from a combustor, such as a circulating fluidized bed (CFB) combustor is released to the atmosphere and, thus, it is an evident conclusion that the emissions of the exhaust gases including pollutants, such as NOx, SOx, etc., should be minimized, and the gases do not contain substantially any unburned material. The primary nitrogen pollutants produced by combustion are nitric oxide (NO) and nitrogen dioxide (NO<NUM>). NOx is formed from the fuel combustion in a combustor. Fuel nitrogen and nitrogen in air are the main sources of the pollutant formed at a temperature of <NUM>-<NUM> in a combustor. Since NOx is an atmospheric pollutant, regulations call for limiting its emission level into the atmosphere. They contribute to acid rain and ozone formation, visibility degradation and human health concerns. Emissions from the combustion sources are therefore regulated and monitored. As a trend the emission requirements are increasingly becoming more stringent.

With current and proposed emission regulations, it is a commonplace to use NOx controls to achieve the lowest emission levels possible. Post-combustion reduction techniques include selective catalytic reduction (SCR) and selective noncatalytic reduction (SNCR). In both technologies, NOx is reduced to nitrogen (N<NUM>) and water (H<NUM>O) through a series of reactions with one or more reagents injected into the flue gas. The most common reagents used are ammonia and urea for both SCR and SNCR systems.

A Selective Non-Catalytic Reduction (SNCR) procedure involves the use of a reducing agent, such as ammonia (NH<NUM>) or urea (CO(NH<NUM>)<NUM>) as an additive to be injected into the exhaust gases of a boiler system. These additives are injected into the hot exhaust gas stream at a temperature window of <NUM>-<NUM>, at a nominal NHiNO molar ratio in excess of one, while maintaining the NH<NUM> slip at reasonably low levels. The NOx reduction is achieved by the following reactions:.

A Selective Catalytic Reduction (SCR) process involves a catalytic reaction of NH<NUM> and NOx in the presence of a catalyst, such as vanadium pentoxide supported on a titanium dioxide substrate. NH<NUM> is injected upstream of the SCR catalyst and reduces NOx to nitrogen and water as the gases flow through the catalyst grid. The reaction temperature for NOx removal is much lower than the SNCR, in the range of <NUM>-<NUM> ° C. The SCR process is highly efficient in removing NOx, and over <NUM>% of the NOx may be removed through this process. The net reaction of the process is:.

NH<NUM> +NO+¼ <NUM><NUM> -> N<NUM> + <NUM>/<NUM><NUM>O.

It is currently the most effective method of post-combustion NOx reduction and has been applied to a variety of fuels (such as, natural gas, refinery gas, bituminous, subbituminous and lignite coals, fuel oils, petroleum coke, biomass and waste / waste wood).

In general, at steam boiler systems there are three basic SCR system configurations: high dust configuration with the SCR reactor between the economizer and air preheater, low dust configuration with the SCR reactor after particulate control devices, and tail-end configuration with the SCR reactor after the flue gas desulfurization (FGD) system. Generally an SCR reactor has to be operated in a certain temperature range to function properly and in an environment that does not deteriorate the catalyst material or its structure. In the hot side/high dust installation, the SCR system is typically installed at the economizer outlet, preceding air preheater. This enables conveniently the achievement of the ideal gas temperature for NOx reduction which is between <NUM> to <NUM>. In the cold side/low dust installation, the SCR system is typically installed after the air preheater and particulate collection. In such systems, the SCR system needs to include a method to increase the flue gas temperature.

A prior art document <CIT> discloses a circulating fluidized bed combustor arrangement that includes.

Several potential problems may be avoided with the above described arrangement. That arrangement provides a more reliable operation of the CFB combustor, e.g., because catalyst plugging due to a high dust load and alkalinity of ash may be eliminated or considerably reduced. With that arrangement, it is also possible to minimize ash deposits which react with ambient moisture and develop hard deposits and acidity, thereby damaging the base material of the catalyst grid. Also, permanent plugging of the channels and pores may be avoided and/or the lifetime of the SCR catalysts is considerably increased.

A prior art document <CIT> discloses a system for NOx reduction via SCR with variable boiler load. Since SCR reactions take place within a required temperature range, the SCR reactors are typically located downstream of the economizer flue gas outlet of a steam generator boiler and upstream (with respect to a direction of flue gas flow) of any air heater devices used to preheat the incoming combustion air. For economic reasons the desired gas temperature entering the SCR reactor should be maintained in the required range at all loads, from full load down to partial loads. Also, maintaining the desired flue gas temperature reduces the formation of ammonia and/or sulphate salts within or on the ammonia injection system and the catalyst. However, as boiler load decreases, the boiler exit gas temperature will drop below the optimal temperature range. To increase the gas temperature to the required temperature range while minimizing the impact on full load thermal efficiency, current practice has been to use an economizer gas bypass flue. The economizer gas bypass flue is used to remove some of the hotter flue gases upstream of the economizer, and then recombine the hotter flue gas with cooler flue gas that leaves the economizer thereby raising the overall flue gas temperature. By controlling the amount of gas that flows through the bypass system, the flue gas temperature entering the SCR reactor can be maintained within the required temperature range at the lower boiler loads. The document presents a system for removing waste heat and nitrogen oxides from boiler flue gas using a split economizer arrangement having a first economizer upstream of a selective catalytic reduction reactor and a second economizer downstream of the reactor. In one embodiment, the flue gas is preferably directed upwardly through the first economizer in a cross co-current heat exchange relationship with boiler feedwater flowing upwardly within the economizer tubes. The first economizer cools the flue gas to a temperature range needed for efficient removal of nitrogen oxides by catalyst in the reactor. A second economizer, downstream of the selective catalytic reduction reactor further heats boiler feedwater and cools the flue gas, thereby improving thermal efficiency.

A prior art document <CIT> discloses one embodiment of a combination of a CFB reactor or combustor and an SCR system. The combination comprises a CFB reactor enclosure for conveying a flow of flue gas/solids therethrough, primary particle separator means for separating solids particles from the flow of flue gas/solids, and means for returning the solids particles collected by the primary particle separator means to the reactor enclosure. At least one of superheater and reheater heat transfer surface is located downstream of the primary particle separator means with respect to the flow of flue gas/solids. Multiclone dust collector means, located downstream of the at least one of superheater and reheater heat transfer surface, are provided for further separating solids particles from the flow of flue gas/solids, together with means for returning the solids particles collected by the multiclone dust collector means to the reactor enclosure. An SCR system is located downstream of the multiclone dust collector means for removing NOx from the flow of flue gas/solids, and dry scrubber means is located downstream of the SCR system. Finally, means are provided for injecting ammonia into the flow of flue gas/solids upstream of the SCR system to cause the chemical reactions which reduce the NOx emissions.

Prior art documents <CIT>, <CIT>, <CIT>, <CIT> and <CIT> all disclose boiler systems and methods for operating them.

The present invention relates to a boiler plant comprising a furnace wherein fuel and air are introduced so as to combust the fuel with the air to form hot flue gas comprising NOx, a flue gas channel i.e. flow path for leading the flue gas to a stack, heat transfer surfaces in the flue gas channel and/or in the furnace for generating high-pressure steam, an air preheater in the flue gas channel that transfers heat from the flue gas to the air to be combusted, a flue gas desulfurization system and a final particulate control device downstream of the air preheater, and a tail-end SCR reactor arranged downstream of the flue gas desulfurization system and the final particulate control device. Thus, with the present configuration, the SCR reactor is essentially in a dust- and sulfur-free environment but its temperature is, without reheating of the flue gas, generally below the acceptable range. This means that reheating of the flue gas is required, which increases the operational costs of the SCR reactor. A tail-end configuration may be needed for certain fuels and/or, especially in some retrofit applications, due to space limitations. Because there is less fly ash, catalyst poisons, and SO<NUM> in the flue gas for tail-end units, the catalyst lifetime may be significantly increased, and less expensive catalyst may be used.

A known method to reheat the flue gas is to pass the flue gas over a high-pressure steam coil heat exchanger upstream of the SCR reactor, and arrange a gas-gas heat exchanger to transfer heat from flue gas after the SCR reactor to the flue gas upstream of the steam coil heat exchanger. This arrangement decreases the required heating with the stem coil heat exchanger and lowers the stack gas temperature to acceptable levels.

The above described configuration involves the problem of relatively high consumption of high-pressure steam for reheating the flue gas upstream of the SCR reactor. Moreover, the temperature of the flue gas conveyed from the gas-gas heat exchanger to the stack may still be relatively high.

Also a challenge that remains to be solved is that in the cold side/low dust installation, the temperature of the flue gas depends on load and fuel conditions as well as the environment temperature. Problems such as ammonia slip, high SO<NUM> -conversion rate and low efficiency of denitration of a SCR system are currently associated with too low temperature.

In the context of the present invention, the following terms are used to facilitate the description:.

be primary air or primary combustion air or secondary air or secondary combustion air.

An object of the invention is to improve the efficiency of a steam boiler system. Especially an object is to improve the efficiency in a system where an SCR system is applied as a tail-end SCR after a dry sorbent injection (DSI) and a filter baghouse to increase the lifetime of SCR catalysts in the SCR reactor. Further the system is configured to use steam to adjust the flue gases to a proper temperature of <NUM>-<NUM> for NOx reduction and depending on required temperature, the steam for flue gas heating would to be taken either from turbine extraction or from the main steam line, which would reduce the electric output and/or net heat rate of the plant comprising the steam boiler system.

A further object of the invention is to reduce the need of steam to heat the flue gas just before entering the SCR reactor to ensure that the temperature of the flue gas is maintained at an adequate temperature range suitable for SCR reduction for variety of boiler loads.

Objects of the invention can be met substantially as is disclosed in the independent claims and in the other claims describing more details of different embodiments of the invention. Especially with an arrangement for a steam boiler system according to independent claim <NUM> and with the method of operating a steam boiler system according to parallel independent claim <NUM>. According to the present invention a heat exchanger is arranged in the flue gas channel downstream of a SCR systems gas-gas heat exchanger, to transfer heat from the flue gas to a heat transfer medium in a heat transfer circuit so as to cool the flue gas conveyed from the gas-gas heat exchanger to the stack, wherein the heat transfer circuit comprises a second heat exchanger, a preliminary air heater, to transfer heat from the heat transfer medium to the air upstream of an air preheater.

Because the air is heated by the heat transfer medium before the air enters into the air preheater, i.e. before heat is transferred from the flue gas to the air, the temperature of the flue gas passing the air preheater decreases less, and the flue gas is conveyed to the SCR reactor at a higher temperature. Thus, the need for reheating the flue gas with high-pressure steam upstream of the SCR reactor is decreased. Thereby, the output of high-pressure steam and the efficiency of the boiler plant is increased.

One aspect of the present invention is to control the flue gas temperature in such a way that the temperature would be optimal for each phase along the flue gas path until release to the atmosphere. After combustion the heat energy is recovered from the flue gas at superheaters, economizers and air preheaters but after those heat recovery process steps the flue gas temperature is to be controlled from different perspective. Downstream from there on the emission control perspective is the decisive one. Another aspect of the invention is to have the flue gas temperature at a range of about <NUM> to <NUM> after those heat recovery processes to make sure that a dry sorbent injection (DSI) systems together with the fabric filter baghouse works efficiently regarding emission controls. In terms of materials / chemicals selection for the DSI (economical alternatives such as CaOH may be used) and fabric filter baghouse this <NUM> to <NUM> is a desirable temperature range. Further, this efficient removal of particulate matter and sulphur content from the flue gas before the SCR system both extend the lifetime of the SCR catalysts and the whole reactor significantly and it further improves the NOx reduction because the catalyst surfaces remain at a better condition.

Further, the flue gas temperature should not go too low since the minimum temperature for SCR is about <NUM>. Thus the optimum temperature required by the DSI and fabric filter baghouse is lower than the minimum temperature required for SCR. In some of the prior art configurations having a tail-end SCR-system the flue gas temperature adjustment for SCR have be done with steam coil heater using the high pressure and temperature steam either from turbine extraction or from the main steam line. If the required increase in temperature with the steam coil would be significant, that would decrease the calculated net efficiency of the steam boiler plant because the same steam could be used for the actual power generation, not for emission control systems.

On the combustion air inlet side, the conventional way is to use air preheaters that take the heat from the flue gas downstream next to economizers. Depending on the fuel, in general it is needed to keep the air inlet temperature before such air preheater high enough, at about <NUM>-<NUM>, to avoid air preheater corrosion and previously steam has been used for that heating purpose. To meet the above presented slightly contradictory demands, one aspect of the present invention is that the use of steam is reduced by providing at least one heat exchanger located in the flue gas channel after the SCR system, the heat exchanger is configured to transfer heat, when in use, from the flue gas downstream of the SCR system to a fluid medium in a fluid circuit;
the fluid circuit comprises at least one pump configured to lead the fluid medium to preliminary air heaters configured to heat the inlet air before entering to the air preheater. As a consequence of this arrangement, the air temperature entering the air preheater is higher and that causes that less heat is taken out from the flue gas at that point. That causes slightly higher temperature of the flue gas entering the DSI and fabric filter baghouse and further to the SCR system requiring less re-heating with the steam. So finally, as the heat has been taken with the heat exchanger from the flue gas just upstream of the stack, the temperature of flue gas exiting the stack is reduced, also the net heat ratio of the whole steam boiler plant is improved.

Thus with the present arrangement the flue gas temperature can be partially controlled with the temperature of inlet air by controlling the preliminary air heater. The inlet air upstream of furnace such as a fluidized bed is heated in two phases, in the first phase with the heat available from flue gas after the SCR system and then in the second phase with the heat available from flue gas after the economizers but before entering the SCR system, by the air preheater. Because the first phase already raises the temperature of inlet air, the relative portion of heat transferred to the inlet air from second phase decreases and the outcome is that less steam is needed to adjust the temperature of flue gas entering the SCR system. In other words, when the inlet air enters the air preheater in higher temperature (~<NUM>-<NUM>) than normally (<NUM>-<NUM>), also the air preheater flue gas exit temperature increases and thus less steam is required at tail-end SCR system. This means that less steam heating is required for flue gas entering the SCR system to have SCR-process working properly at a desirable temperature.

According to an embodiment of the invention the flue gas temperature after tail-end SCR is still reasonable high and flue gases are mainly free of dust and acid gases (SO<NUM>, HCl). The heat can be recovered to combustion air with indirect method i.e. by means that the fluid medium in the fluid circuit is pressurized water. The heat exchanger is configured to transfer the heat from the flue gas downstream the SCR system to a fluid medium in a fluid circuit, such as a closed circulation water loop. The fluid circuit transfers the heat to a water coil of a preliminary air heater. In some prior art configurations there has been a steam coil air preheater in the air channel to heat the air before it enters the actual air preheaters. For example a document <CIT> relating to a pulverized fuel fired steam generator discloses a steam air heater disposed in the combustion air intake duct between the forced draft fan and the main flue gas heated air preheater for raising the temperature of the combustion air entering the main air preheater. It is known in the prior art to utilize a steam air heater so disposed to raise the temperature of the combustion air entering the main air preheater on pulverized fuel steam generators during low load operation in order to limit the corrosion of the cold end heat transfer surfaces on the main air preheater. These conventional steam coil air preheaters are no longer needed in this present configuration to preheat the air before the flue gas operated combustion air preheater to a temperature (<NUM>-<NUM>) to avoid corrosion.

With the present method the temperature of the flue gas upstream of the SCR reactor is controlled by controlling the temperature difference of flue gas over the air preheaters by adjusting the temperature of the inlet air with preliminary air heaters prior to the air preheaters. As the air preheaters transfer heat from the flue gas to the inlet air for combustion, the temperature difference between these two flows determine how much heat may be transferred if the parameters such as physical construction and these two flows remain constant. Flue gas flow is the high temperature flow transferring heat to the cooler inlet air flow. If the temperature of the inlet air flow is already increased, the capability of the inlet air to cool down the flue gas temperature is smaller. Thus the temperature of the flue gas remains higher if the inlet air is hotter. This effect is now utilized here to reduce the usage of steam in steam coil heater to heat up again the flue gas temperature for optimal performance of NOx reduction in the SCR reactor.

There are clear practical advantages obtained by the specific elements:.

According to one aspect of the invention, the intake air comprises separate air streams, a primary air stream and a secondary air stream for different locations at the furnace. This depends on actual configuration of the boiler system in question, whether it is a circulating fluidized bed boiler or bubbling bed boiler or some other type of furnace. According to an embodiment of the invention the steam boiler system is a fluidized bed boiler system such as a circulating fluidized bed boiler system or a bubbling fluidized bed boiler system. The preliminary air heaters are configured to heat primary and/or secondary air upstream of primary and/or secondary air preheaters.

According to one aspect of the invention, when in operation, the heat exchanger transfers heat from the flue gas, such as in an exemplary set-up, by cooling the flue gas downstream of the SCR system from <NUM> to <NUM>. The fluid medium, such as pressurized water, is thereby heated from <NUM> to <NUM>. This heat is then further conveyed by the pump system to the preliminary air heaters that are configured to heat the inlet air, such as primary and/or secondary air, with the fluid medium. In this exemplary set-up, the primary air is heated from <NUM> to <NUM> and secondary air is heated from <NUM> to <NUM> while the fluid medium will be cooled from <NUM> to <NUM> due to the heat transfer in the preliminary air heaters.

According to one aspect of the invention, heating the inlet air i.e. the heated primary and/or secondary air has a consequence that the temperature of flue gases entering the fabric filter baghouse would increase. In an exemplary set-up the flue gas temperature upstream of the fabric filter baghouse would be at <NUM> and downstream the fabric filter baghouse the temperature would be at <NUM> when entering the SCR system. The <NUM> temperature is reasonably close to the effective operating temperature (<NUM>) of the SCR reactor and thus less steam from the boiler will be needed to heat the flue gas before it enters the SCR reactor.

According to the invention, the SCR system comprises in addition to an actual SCR reactor the gas-gas heat exchanger (internal to the SCR system) for heating the flue gas entering the SCR reactor with the flue gas exiting the SCR reactor and further a steam coil heater for heating the flue gas entering the SCR reactor with steam from the main steam line or from the auxiliary steam system. Advantageously, the steam coil heater is controlled such that steam is allowed at the steam inlet only in the amount which is required to heat the flue gas to a target temperature.

In the method of operating a boiler plant according to the one aspect of the invention, the heat exchanger located downstream the SCR system in the flue gas flow direction is used for transfer of heat from the flue gas exiting the SCR system to a fluid medium, and a fluid circuit for the fluid medium comprising at least one pump is used to lead the fluid medium to preliminary air heaters that are configured to heat the primary and/or secondary air with the fluid medium.

The exemplary embodiments of the invention presented in this patent application are not to be interpreted to pose limitations to the applicability of the appended claims. The verb "to comprise" is used in this patent application as an open limitation that does not exclude the existence of also unrecited features. The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims.

In the following, the arrangement for a power plant boiler system is explained in more detail in the following by way of the exemplary embodiments and as shown in the attached drawing in the sole <FIG> that shows components of a steam boiler system. The invention will be described with reference to the accompanying exemplary, schematic drawings, in which
<FIG> illustrates an arrangement for a power plant boiler system according to an embodiment of the invention.

<FIG> depicts schematically an arrangement for a steam boiler system <NUM>, which is illustrated as a circulating fluidized bed boiler system. The circulating fluidized bed boiler system <NUM> is operated preferably so that fuel material is combusted in a furnace <NUM> in a suspension of solid particles of a circulating fluidized bed. The exhaust gases resulting from the combustion and solid particles are passed from the furnace <NUM> to at least one solids separator <NUM> and a first fraction of solids particles are separated in the at least one solids separator <NUM> and are returned back into the furnace <NUM> via a solids return leg. An SNCR system <NUM> is configured as an early stage of NOx reduction downstream of furnace <NUM> and solids separator <NUM>. In other words, the figure presents an arrangement for a steam boiler system <NUM> comprising a furnace <NUM> and along a following flue gas channel <NUM> a number of superheaters <NUM>, 5a, 5b, a number of economizers <NUM>, 7a, 7b and at least one air preheater <NUM> located in the flue gas channel <NUM> downstream of the economizers <NUM>, 7a, 7b, a fabric filter baghouse <NUM> located in the flue gas channel <NUM> downstream of the air preheater <NUM>, 9a, 9b, and downstream of the fabric filter baghouse <NUM> is located a selective catalytic reduction (SCR) system <NUM> comprising an SCR reactor <NUM>, a high pressure steam coil heater <NUM> upstream of the SCR reactor <NUM> and a regenerative a gas-gas heat exchanger <NUM> connected upstream and downstream of the SCR reactor <NUM> to transfer heat from flue gas after the SCR reactor <NUM> to the flue gas upstream of the high pressure steam coil heater <NUM>. In other words, that one (inlet) end of the gas-gas heat exchanger is located upstream the SCR reactor <NUM> and other (outlet) end of the gas-gas heat exchanger <NUM> is located downstream the SCR reactor <NUM>.

In the <FIG> a section along the flue gas channel <NUM> downstream of air preheaters <NUM>, 9a, 9b comprises the flue gas emission control systems. The emission control system comprises a dry sorbent injection (DSI) system that provides additives, such as hydrated lime, bicarbonate and/or active carbon to the flue gas. As it is illustrated in the <FIG>, there are provided three dry sorbent silos <NUM>, <NUM>, <NUM> for the purpose. The actual configuration of the DSI and its silos <NUM>, <NUM>, <NUM> depends on number of parameters. In this case when temperature is low enough both hydrated lime or bicarbonate can be used as an additive. In case of higher temperature, hydrated lime is no more effective and bicarbonate is the additive that is effective. Then further in downstream direction a fabric filter baghouse <NUM> is arranged in the flue gas channel <NUM>. Because the flue gas emission control system is located after the air preheater <NUM>, 9a, 9b, the flue gas temperature has been dropped to a proper level for the fabric filter baghouse -type of filter and a need for a more expensive and less effective ESP (the electrostatic precipitator) is no longer there. Also, one advantage is that when the fabric filter baghouse <NUM> can be used due to the reasonable flue gas temperatures, the dry sorbent injection with hydrated lime, bicarbonate and/or active carbon is much more effective than in ESP and much lower emissions can be reached. No additional wet or semi-dry flue gas desulphurization (FGD) is needed after SCR. ESP itself is less expensive than fabric filter baghouse but when also some sort of FGD is needed, the combination is more expensive. With the present arrangement the flue gas is cleaned from the majority of the particulate matter of the combusted fuel and/or other particulate matter before the flue gas enters an SCR system. In some situations it is needed to recirculate part of the flue gases back to the furnace <NUM> for combustion and that is why from downstream of the fabric filter baghouse <NUM> there is configured a flue gas recirculating channel <NUM> with a flue gas fan <NUM> to enable this act if necessary, for example in start up phase or like.

The SCR system <NUM> requires quite precise flue gas temperature to function properly. That is why the embodiment of <FIG> is configured to have a steam-coil heater <NUM> with a steam inlet <NUM> and a gas-gas-heat exchanger <NUM> (preferably of a regenerative type of heat exchanger) located and connected in the flue gas channel both before and after the actual SCR reactor <NUM>. The temperature of the flue gas upstream of the SCR reactor is further controlled with the high pressure steam coil heater <NUM> and the gas-gas heat exchanger <NUM>. There is also an ammonia feeding system <NUM> comprising an ammonia inlet and an ammonia vaporized together with a gas blower to get the ammonia properly mixed into the flue gas before the SCR reactor. The ammonia feeding system <NUM> comprises also an SCR mix gas circulation channel <NUM> taking a fraction of flue gas from downstream of SCR reactor <NUM> to the ammonia feeding system <NUM> so that the mixing can be done in correct temperature and composition. In some situations it is needed to recirculate part of the flue gases back to the combustion and that is why there is configured a flue gas recirculating channel <NUM> with a flue gas recirculation fan <NUM> to enable this act if necessary, for example in start up phase or like. As an emission control overview a solids separator <NUM> and the fabric filter baghouse are configured upstream of SCR system <NUM> to remove the major part of particulate matter from the flue gas to improve the efficiency of the SCR system <NUM> in NOx reduction.

Still continuing with the <FIG> there is arranged at least one heat exchanger <NUM> located in the flue gas channel <NUM> after the SCR system <NUM>, the heat exchanger <NUM> is configured to transfer heat, when in use, from the flue gas downstream of the SCR system <NUM> to a fluid medium in a fluid circuit <NUM>;.

In a calculated example of a method of operating a power plant boiler system according to the invention, a heat exchanger <NUM> was located in the flue gas channel <NUM> downstream of the SCR system <NUM> and configured to collect heat to a fluid medium from the flue gas exiting the SCR system <NUM>. With a reduction of flue gas temperature of <NUM> and by transferring the heat with fluid medium via a fluid circuit <NUM> to the preliminary air heaters <NUM>, <NUM>, to get SCR operation temperature to the same level as would be done with a steam coil heater, the auxiliary steam consumption was reduced by almost <NUM>% and the power plant fuel consumption was reduced <NUM>,<NUM>%. This <NUM>,<NUM> % reduction in fuel consumption is about the same as an improvement in net efficiency.

Claim 1:
An arrangement for a steam boiler system (<NUM>) comprising a furnace (<NUM>), a following flue gas channel (<NUM>), a number of superheaters (<NUM>, 5a, 5b), a number of economizers (<NUM>, 7a, 7b) and at least one air preheater (<NUM>) located in the flue gas channel (<NUM>) downstream of the economizers (<NUM>, 7a, 7b), a fabric filter baghouse (<NUM>) located in the flue gas channel (<NUM>) downstream of the air preheater (<NUM>, 9a, 9b), and located downstream of the fabric filter baghouse (<NUM>) a selective catalytic reduction (SCR) system (<NUM>) comprising an SCR reactor (<NUM>), a high pressure steam coil heater (<NUM>) upstream of the SCR reactor (<NUM>) and a gas-gas heat exchanger (<NUM>) connected upstream and downstream of the SCR reactor (<NUM>) to transfer heat from flue gas after the SCR reactor (<NUM>) to the flue gas upstream of the high pressure steam coil heater (<NUM>), at least one heat exchanger (<NUM>) located in the flue gas channel (<NUM>) after the SCR system (<NUM>), the heat exchanger (<NUM>) being configured to transfer heat, when in use, from the flue gas downstream of the SCR system (<NUM>) to a fluid medium in a fluid circuit (<NUM>); wherein the fluid circuit (<NUM>) comprises at least one pump (<NUM>) configured to lead the fluid medium to preliminary air heaters (<NUM>, <NUM>) configured to heat inlet air before entering to the flue gas air preheater (<NUM>, 9a, 9b).