Method of and System For Generating Power By Oxyfuel Combustion

Abstract

An oxyfuel combustion system for generating power that includes a furnace for combusting carbonaceous fuel and substantially pure oxygen to produce exhaust gas including mainly carbon dioxide and water. An exhaust gas channel system discharges the exhaust gas from the furnace. The exhaust gas channel system has an upstream channel, an outlet channel and a gas recycling channel. The upstream channel recycles a recycling portion of the exhaust gas through the recycling channel to the furnace, and conveys an end portion of the exhaust gas through the outlet channel for final processing. The upstream channel is divided between a first divider piece and a connecting piece into a first exhaust gas channel portion and a second exhaust gas channel portion. A gas-gas heat exchanger arranged in the first exhaust gas channel portion transfers heat from exhaust gas in the first exhaust gas channel portion to gas in the gas recycling channel. A first economizer arranged in the second exhaust gas channel portion transfers heat from exhaust gas in the second exhaust gas channel portion to a flow of feedwater in a feedwater line, and a second economizer arranged in the exhaust gas channel system downstream of the connecting piece transfers heat from gas in the exhaust gas channel system to the flow of feedwater in the feedwater line.

1 . A method of generating power by oxyfuel combustion, the method comprising the steps of:
 (a) feeding carbonaceous fuel into a furnace;   (b) feeding oxidant gas into the furnace, wherein, in a first operating mode, the oxidant gas comprises a stream of substantially pure oxygen conveyed from an oxygen supply for combusting the fuel with the oxygen to produce exhaust gas comprising mainly carbon dioxide and water;   (c) discharging an exhaust gas stream from the furnace;   (d) dividing the exhaust gas stream in a final divider piece into a recycling portion and an end portion;   (e) recycling the recycling portion through a gas recycling channel to the furnace; and   (f) conveying the end portion through an outlet channel to final processing, wherein the method also comprises the steps of:   (g) dividing the exhaust gas stream in a first divider piece, arranged upstream of the final divider piece, into a first exhaust gas stream and a second exhaust gas stream;   (h) transferring heat from the first exhaust gas stream to gas in the gas recycling channel by a gas-gas heat exchanger to form a cooled first exhaust gas stream;   (i) transferring heat from the second exhaust gas stream to a flow of feedwater in a feedwater line by a first economizer to form a cooled second exhaust gas stream;   (j) combining the cooled first exhaust gas stream and the cooled second exhaust gas stream in a connecting piece arranged upstream of the final divider piece to form a combined exhaust gas stream; and   (k) conveying at least a portion of the combined exhaust gas stream through a second economizer arranged to transfer heat from the combined exhaust gas stream to the flow of feedwater in the feedwater line.   
   
   
       2 . The method according to  claim 1 , wherein the second economizer is arranged in the exhaust gas channel upstream of the final divider piece. 
   
   
       3 . The method of  claim 1 , further comprising a step of transferring heat from the end portion of the exhaust gas to the stream of substantially pure oxygen. 
   
   
       4 . The method of  claim 3 , further comprising a step of mixing the stream of substantially pure oxygen and the recycling portion as a combined oxidant gas in a mixer, and feeding the combined oxidant gas into the furnace. 
   
   
       5 . The method according to  claim 3 , wherein the second economizer is arranged in the recycling channel to transfer heat from the recycling portion of the exhaust gas. 
   
   
       6 . The method according to  claim 1 , wherein the first economizer is arranged in the feedwater line immediately downstream of the second economizer. 
   
   
       7 . The method according to  claim 1 , further comprising a step of controlling the division ratio of the first exhaust gas stream and the second exhaust gas stream. 
   
   
       8 . The method according to  claim 2 , wherein, in the first operating mode, the temperature of the exhaust gas decreases in the second economizer by at least 30° C. 
   
   
       9 . The method according to  claim 1 , further comprising performing the first operating mode alternatingly with a second operating mode, in which the recycling portion is minimized, and wherein the oxidant gas comprises a stream of air introduced into the gas recycling line upstream of the gas-gas heat exchanger. 
   
   
       10 . The method according to  claim 9 , wherein, in the second operation mode, the temperature of the exhaust gas changes in the second economizer by less than 10° C. 
   
   
       11 . A system for generating power by oxyfuel combustion, the system comprising:
 a furnace for combusting carbonaceous fuel;   an oxygen channel for feeding substantially pure oxygen from an oxygen supply into the furnace for combusting the fuel with the oxygen to produce exhaust gas comprising mainly carbon dioxide and water;   an exhaust gas channel system connected to the furnace for discharging the exhaust gas from the furnace, wherein the exhaust gas channel system comprises an upstream channel, an outlet channel and a gas recycling channel, wherein the upstream channel is connected by a final divider piece to the gas recycling channel and the outlet channel, for recycling a recycling portion of the exhaust gas through the recycling channel to the furnace, and for conveying an end portion of the exhaust gas through the outlet channel for final processing, wherein the upstream channel is divided between a first divider piece and a connecting piece into a first exhaust gas channel portion and a second exhaust gas channel portion;   a gas-gas heat exchanger arranged in the first exhaust gas channel portion to transfer heat from exhaust gas in the first exhaust gas channel portion to gas in the gas recycling channel;   a first economizer arranged in the second exhaust gas channel portion to transfer heat from exhaust gas in the second exhaust gas channel portion to a flow of feedwater in a feedwater line; and   a second economizer arranged in the exhaust gas channel system downstream of the connecting piece to transfer heat from gas in the exhaust gas channel system to the flow of feedwater in the feedwater line.   
   
   
       12 . The system according to  claim 11 , wherein the second economizer is arranged in the upstream channel. 
   
   
       13 . The system according to  claim 11 , further comprising an oxygen heater arranged in the oxygen channel, connected to a gas cooler arranged in the outlet channel for heating the substantially pure oxygen by heat obtained from the end portion of the exhaust gas. 
   
   
       14 . The system according to  claim 13 , further comprising a mixer to mix the substantially pure oxygen and the recycling portion as a combined oxidant gas and a channel for feeding the combined oxidant gas into the furnace. 
   
   
       15 . The system according to  claim 13 , wherein the second economizer is arranged in the gas recycling channel. 
   
   
       16 . The system according to  claim 11 , wherein the first economizer is arranged in the feedwater line immediately downstream of the second economizer. 
   
   
       17 . The system according to  claim 1   1 , further comprising a damper in one of the first exhaust gas channel portion and the second exhaust gas channel portion for controlling the division ratio of the exhaust gas. 
   
   
       18 . The system according to  claim 11 , further comprising:
 a damper arranged in the gas recycling channel for controlling the recycling portion; and   an air intake arranged in the gas recycling channel for introducing a stream of air as the oxidant gas, wherein the air intake is arranged upstream of the gas-gas heat exchanger so as to transfer heat from the exhaust gas to the air stream in the gas-gas heat exchanger.