Oxyfuel Combusting Boiler System and a Method of Generating Power By Using the Boiler System

Abstract

Carbonaceous fuel is combusted with an oxidant gas in a furnace of a boiler system to generate power. Oxidant gas is fed into the furnace for combusting the fuel to produce exhaust gas, the exhaust gas is discharged from the furnace via an exhaust gas channel, a stream of feedwater is conveyed from a final economizer arranged in the exhaust gas channel to evaporating and superheating heat exchange surfaces arranged in the furnace and in the exhaust gas channel for converting the feedwater to superheated steam, the superheated steam is converted in a high-pressure steam turbine for generating power, a first portion of steam is extracted from the high-pressure steam turbine for preheating the feedwater, a second portion of steam is conveyed from the high-pressure steam turbine to reheating heat exchange surfaces arranged in the exhaust gas channel for generating reheated steam, and the reheated steam is expanded in an intermediate pressure steam turbine for generating power. The oxidant gas can be a mixture of substantially pure oxygen and recycled exhaust gas, and the ratio of the first and second portions of steam can be controlled to obtain a desired flue gas temperature in the exhaust gas channel downstream of the final economizer.

1 . A method of generating power by combusting carbonaceous fuel with an oxidant gas in a furnace of a boiler system, the method comprising the steps of:
 (a) feeding carbonaceous fuel into the furnace at a fuel feeding rate;   (b) feeding oxidant gas into the furnace for combusting the fuel to produce exhaust gas;   (c) discharging the exhaust gas from the furnace via an exhaust gas channel;   (d) conveying a stream of feedwater at a feedwater conveying rate from a final economizer arranged in the exhaust gas channel to evaporating and superheating heat exchange surfaces arranged in the furnace and in the exhaust gas channel for converting the feedwater to superheated steam;   (e) expanding the superheated steam in a high-pressure steam turbine for generating power;   (f) extracting a first portion of steam from the high-pressure steam turbine for preheating the feedwater;   (g) conveying a second portion of steam from the high-pressure steam turbine to reheating heat exchange surfaces arranged in the exhaust gas channel for generating reheated steam; and   (h) expanding the reheated steam in an intermediate pressure steam turbine for generating power,   wherein, in first operating conditions, the oxidant gas is a mixture of substantially pure oxygen and recycled exhaust gas and the ratio of the first and second portions of steam is controlled so as to obtain a desired flue gas temperature in the exhaust gas channel downstream of the final economizer.   
     
     
         2 . The method according to  claim 1 , wherein the fuel feeding rate and the feedwater conveying rate are adjusted so as to obtain a desired furnace temperature. 
     
     
         3 . The method according to  claim 2 , wherein, in second operating conditions, the oxidant gas is air and, when operating the combustion system at full load at the first and second operating conditions, the fuel feeding rate is, in the first operating conditions, higher than that in the second operating conditions. 
     
     
         4 . The method according to  claim 3 , wherein, when operating the combustion system at full load of the first and second operating conditions, the first portion of steam in the first operating conditions is less than that in the second operating conditions, and the second portion of steam is, in the first operating conditions, greater than that in the second operating conditions. 
     
     
         5 . The method according to  claim 3 , wherein the system comprises a gas-gas heat exchanger and heat is transferred in the gas-gas heat exchanger in the first and second operating conditions from the exhaust gas in the exhaust gas channel to at least a portion of the oxidant gas. 
     
     
         6 . The method according to  claim 1 , wherein the controlling comprises measuring the exhaust gas temperature. 
     
     
         7 . The method according to  claim 3 , wherein the feedwater conveying rate is greater in the first operating conditions than that in the second operating conditions. 
     
     
         8 . The method according to  claim 1 , wherein the method comprises, in the first operating conditions, a further step of pressurizing a portion of the exhaust gas in multiple exhaust gas compressors so as to produce liquid or supercritical carbon dioxide. 
     
     
         9 . The method according to  claim 1 , wherein the method comprises, in the first operating conditions, a further step of extracting a portion of steam from the intermediate-pressure steam turbine for driving a compressor. 
     
     
         10 . The method according to  claim 9 , wherein, in the first operating conditions, the oxygen is mixed with the recycled exhaust gas so as to produce oxidant gas having an average oxygen content, by volume, from about 18% to about 28%. 
     
     
         11 . A boiler system for generating power by combusting carbonaceous fuel in a furnace of the boiler system, the boiler system comprising:
 means for feeding carbonaceous fuel into the furnace;   means for feeding substantially pure oxygen and recycled exhaust gas as an oxidant gas into the furnace for combusting the fuel to produce exhaust gas;   an exhaust gas channel for discharging the exhaust gas from the furnace;   means for conveying a stream of feedwater from a final economizer arranged in the exhaust gas channel to evaporating and superheating heat exchange surfaces arranged in the furnace and in the exhaust gas channel for converting the feedwater to be superheated steam;   a high-pressure steam turbine for expanding the superheated steam for generating power;   means for extracting a first portion of steam from the high-pressure steam turbine for preheating the feedwater;   means for conveying a second portion of steam from the high-pressure steam turbine to reheating heat exchange surfaces arranged in the exhaust gas channel for generating reheated steam;   an intermediate-pressure steam turbine for expanding the reheated steam for generating power; and   means for controlling the ratio of the first and second portions of steam so as to obtain a desired flue gas temperature in the exhaust gas channel downstream of the final economizer.   
     
     
         12 . The boiler system according to  claim 11 , wherein the boiler system comprises means for feeding air as an oxidant gas into the furnace for combusting the fuel to produce exhaust gas. 
     
     
         13 . The boiler system according to  claim 11 , wherein the boiler system comprises a gas-gas heater exchanger for transferring heat from the exhaust gas in the exhaust gas channel to at least a portion of the oxidant gas. 
     
     
         14 . The boiler system according to  claim 11 , wherein the means for controlling comprise means for measuring the exhaust gas temperature. 
     
     
         15 . The boiler system according to  claim 11 , wherein the boiler system comprises multiple exhaust gas compressors for pressurizing a portion of the exhaust gas so as to produce liquid or supercritical carbon dioxide. 
     
     
         16 . The boiler system according to  claim 11 , wherein the boiler system comprises means for extracting a portion of steam from the intermediate-pressure steam turbine for driving a compressor.