Arrangement for and a method of operating a steam boiler system

1. An arrangement for a steam boiler system, the arrangement comprising:

a furnace;

a number of superheaters along a flue gas channel that follows the furnace;

a plurality of economizers;

at least one air preheater located in the flue gas channel downstream of the economizers;

a fabric filter baghouse located in the flue gas channel downstream of the at least one air preheater;

a selective catalytic reduction (SCR) system downstream of the fabric filter baghouse, the SCR system comprising an SCR reactor, a steam coil heater upstream of the SCR reactor and a gas-gas heat exchanger connected upstream and downstream of the SCR reactor to transfer heat from flue gas after the SCR reactor to the flue gas upstream of the steam coil heater; and

at least one heat exchanger located in the flue gas channel after the SCR system, the heat exchanger being 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 comprising at least one pump configured to lead the fluid medium to preliminary air heaters configured to heat inlet air before entering the flue gas air preheater.

2. The arrangement according to claim 1 , wherein the preliminary air heaters are configured to heat at least one of primary air and secondary air upstream of at least one of primary and secondary air preheaters.

3. The arrangement according to claim 1 , wherein the fluid medium in the fluid circuit is pressurized water.

4. The arrangement according to claim 1 , further comprising a control valve arranged in fluid circuit downstream of the pump to control the flow to the preliminary air preheaters.

5. The arrangement according to claim 1 , wherein the SCR system comprises the gas-gas heat exchanger for heating the flue gas entering the SCR reactor with the flue gas exiting the SCR reactor.

6. The arrangement according to claim 1 , wherein the steam coil heater is configured to heat the flue gas entering the SCR reactor with steam from the main steam line or from the auxiliary steam system.

7. The arrangement according to claim 1 , wherein a solids separator and the fabric filter baghouse are configured upstream of SCR system to remove a major portion of particulate matter from the flue gas to improve the efficiency of the SCR system in NOx re-duction.

8. A method of operating a steam boiler system, the method comprising:

providing a furnace;

providing a plurality of superheaters;

providing a flue gas channel following the plurality of superheaters;

providing a plurality of economizers and at least one air preheater located in the flue gas channel downstream of the economizers;

providing a fabric filter baghouse in the flue gas channel downstream of the air preheater;

locating a selective catalytic reduction (SCR) system downstream of the filter baghouse, the SCR system comprising an SCR reactor, a steam coil heater upstream of the SCR reactor;

connecting a gas-gas heat exchanger upstream and downstream of the SCR reactor to transfer heat from flue gas after the SCR reactor to the flue gas upstream of the steam coil heater;

locating at least one heat exchanger in the flue gas channel after the SCR system, the heat exchanger transferring heat, when in use, from the flue gas downstream of the SCR system to a fluid medium in a fluid circuit;

leading the fluid medium to preliminary air heaters by the fluid circuit; and

heating inlet air before entering to the flue gas air preheater.

9. The method of claim 8 , wherein the preliminary air heaters heat at least one of primary combustion air and secondary combustion air upstream of at least one of primary air preheaters and secondary air preheaters.

10. The method according to claim 8 , further comprising steps of heating primary combustion air in two steps, first with the preliminary air heater delivering heat from the flue gas after SCR system, raising the primary combustion air temperature to a range of 100±10° C.; and

then, in a second step, heating the primary air in the air preheater transferring the heat from the flue gas downstream of the economizers.

11. The method according to claim 8 , further comprising steps of heating secondary combustion air in two steps, first with the preliminary air heater delivering heat from the flue gas after SCR system, raising the secondary combustion air temperature to a range of 100±10° C.; and

then, in a second step, heating the secondary air in the preheater that transfers the heat from the flue gas downstream of the economizers.

12. The method according to claim 8 , further comprising controlling the temperature of the flue gas upstream of the SCR reactor by controlling the temperature difference of flue gas over the air preheaters by adjusting the temperature of the combustion air with the preliminary air heaters prior to the air preheaters.

13. The method according to claim 8 , wherein an SNCR system is configured as an early stage of NOx reduction downstream of the furnace and the solids separator.

14. The method according to claim 8 , wherein the fluid medium is pressurized water in a temperature range of 100 to 140° C.

15. The method according to claim 8 , further comprising controlling the temperature of the flue gas upstream of the SCR reactor with the steam coil heater and the gas-gas heat exchanger.