Flue gas heat recovery system

Abstract

A flue gas heat recovery system includes a furnace, a combustion device, a flue gas duct, and a heat exchanging unit, and further includes a heat recovering unit disposed in the flue gas duct on the downstream side of the heat exchanging unit to recover heat from flue gas, and configured such that a path cross-sectional area of an outlet of the heat recovering unit is made smaller than a path cross-sectional area of an inlet of the heat recovering unit, and a first heat exchanger that heats combustion air by using heat recovered by the heat recovering unit.

1 . A flue gas heat recovery system for a boiler including a hollow furnace located in a vertical direction, a combustion burner that is disposed in a lower part of a wall constituting the furnace and blows fuel gas constituted by a mixture of fuel and combustion air toward the furnace, a flue gas path connected to an upper part of the furnace, in which a flue gas discharged from the furnace flows, and a heat exchanging unit disposed in the flue gas path to exchange heat between the flue gas and water from a condenser, comprising:
 a heat recovering unit disposed in the flue gas path on the downstream side of the heat exchanging unit to recover heat of flue gas and to heat a heat medium by the flue gas;   a first heat exchanger that heats the combustion air by using heat recovered by the heat recovering unit; and   a first heat medium circulation path, in which the heat medium circulates between the heat recovering unit and the first heat exchanger, wherein   a cross-sectional area of the flue gas pass at an outlet of the heat recovering unit is configured to be smaller than a cross-sectional area of the flue gas pass at an inlet of the heat recovering unit.   
     
     
         2 . The flue gas heat recovery system according to  claim 1 , wherein the heat recovering unit is configured such that an inner size of the outlet in the flue gas path is made smaller than an inner size of the inlet. 
     
     
         3 . The flue gas heat recovery system according to  claim 1 , wherein the heat recovering unit is configured such that density of a heat exchange tube or heat exchange fins at the outlet is made higher than density of the heat exchange tube or the heat exchange fins at the inlet. 
     
     
         4 . The flue gas heat recovery system according to  claim 1 , wherein the heat recovering unit includes a high temperature unit and a low temperature unit disposed at a downstream side of the high temperature unit, and is configured such that a path cross-sectional area of the low temperature unit is made smaller than a path cross-sectional area of the high temperature unit. 
     
     
         5 . The flue gas heat recovery system according to  claim 4 , wherein the flue gas path includes a vertical path extending in the vertical direction and a horizontal path connected with a lower part of the vertical path and extending in a horizontal direction, and the high temperature unit is disposed in the vertical path, and the low temperature unit is disposed in the horizontal path. 
     
     
         6 . The flue gas heat recovery system according to  claim 5 , wherein a NOx removal device is provided in the vertical path, and the high temperature unit is disposed below the NOx removal device. 
     
     
         7 . The flue gas heat recovery system according to  claim 5 , wherein a hopper is provided between the vertical path and the horizontal path and below the high temperature unit. 
     
     
         8 . The flue gas heat recovery system according to  claim 1 , further comprising a second heat exchanger that is disposed between the heat recovering unit and a stack on the flue gas path to reheat the flue gas prior to discharging from the stack by using heat recovered by the heat recovering unit. 
     
     
         9 . The flue gas heat recovery system according to  claim 8 , further comprising:
 a third heat exchanger that is disposed between the heat exchanging unit and the condenser to heat the water supplied from the condenser and supplied to the heat exchanging unit by using heat recovered by the heat recovering unit.   
     
     
         10 . The flue gas heat recovery system according to  claim 9 , further comprising a distribution amount control device including a flow rate control valve and a control device that controls the flow rate control valve for adjusting distribution amounts of a heat medium supplied from the heat recovering unit to the first heat exchanger, the second heat exchanger, and the third heat exchanger. 
     
     
         11 . The flue gas heat recovery system according to  claim 2 , wherein the heat recovering unit is configured such that density of a heat exchange tube or heat exchange fins at the outlet is made higher than density of the heat exchange tube or the heat exchange fins at the inlet. 
     
     
         12 . The flue gas heat recovery system according to  claim 2 , wherein the heat recovering unit includes a high temperature unit and a low temperature unit disposed at a downstream side of the high temperature unit, and is configured such that a path cross-sectional area of the low temperature unit is made smaller than a path cross-sectional area of the high temperature unit. 
     
     
         13 . The flue gas heat recovery system according to  claim 3 , wherein the heat recovering unit includes a high temperature unit and a low temperature unit disposed at a downstream side of the high temperature unit, and is configured such that a path cross-sectional area of the low temperature unit is made smaller than a path cross-sectional area of the high temperature unit. 
     
     
         14 . The flue gas heat recovery system according to  claim 6 , wherein a hopper is provided between the vertical path and the horizontal path and below the high temperature unit. 
     
     
         15 . The flue gas heat recovery system according to  claim 2 , further comprising a second heat exchanger that is disposed between the heat recovering unit and a stack on the flue gas path to reheat the flue gas prior to discharging from the stack by using heat recovered by the heat recovering unit. 
     
     
         16 . The flue gas heat recovery system according to  claim 3 , further comprising a second heat exchanger that is disposed between the heat recovering unit and a stack on the flue gas path to reheat the flue gas prior to discharging from the stack by using heat recovered by the heat recovering unit. 
     
     
         17 . The flue gas heat recovery system according to  claim 4 , further comprising a second heat exchanger that is disposed between the heat recovering unit and a stack on the flue gas path to reheat the flue gas prior to discharging from the stack by using heat recovered by the heat recovering unit. 
     
     
         18 . The flue gas heat recovery system according to  claim 5 , further comprising a second heat exchanger that is disposed between the heat recovering unit and a stack on the flue gas path to reheat the flue gas prior to discharging from the stack by using heat recovered by the heat recovering unit. 
     
     
         19 . The flue gas heat recovery system according to  claim 6 , further comprising a second heat exchanger that is disposed between the heat recovering unit and a stack on the flue gas path to reheat the flue gas prior to discharging from the stack by using heat recovered by the heat recovering unit. 
     
     
         20 . The flue gas heat recovery system according to  claim 7 , further comprising a second heat exchanger that is disposed between the heat recovering unit and a stack on the flue gas path to reheat the flue gas prior to discharging from the stack by using heat recovered by the heat recovering unit.