Method of Increasing the Performance of a Carbonaceous Fuel Combusting Boiler System

Abstract

A method of increasing the power of a carbonaceous fuel combusting boiler system includes the steps of (a) feeding carbonaceous fuel into a furnace of the boiler system, (b) feeding oxidant gas into the furnace for combusting fuel to produce exhaust gas, (c) discharging the exhaust gas from the furnace via an exhaust gas channel, (d) conveying a stream of feedwater from a boiler 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 steam from the high-pressure steam turbine at a decreased rate for preheating the feedwater, (g) conveying steam from the high-pressure steam turbine at an increased rate to a reheater arranged in the exhaust gas channel for generating reheated steam, (h) expanding the reheated steam in an intermediate pressure steam turbine for generating power, and (i) conveying the exhaust gas in the exhaust gas channel from the reheater through a boiler economizer to a gas heater. The heat exchange surface area of at least one of the reheater and the boiler economizer is increased and / or a high pressure economizer is added downstream of the boiler economizer and upstream of the gas heater.

Description

BACKGROUND OF THE INVENTION[0001]1. Field of the Invention[0002]The present invention relates to a method of increasing the performance, i.e., the thermal efficiency and / or power of a carbonaceous fuel combusting boiler system.[0003]2. Description of the Related Art[0004]In a carbonaceous fuel combusting boiler system, the flue gas discharged from a furnace along a flue gas channel is cooled down in a heat recovery area (HRA), including superheater (SH) and reheater (RH) sections, downstream of which the heat is recovered by a boiler economizer (ECO) to heat up feedwater, followed by a gas heater (GH), such as an air heater, to heat up oxidant gas for the combustion. The superheated steam is expanded in a high-pressure steam turbine (HPST). At least a portion of the expanded steam is reheated in the RH section and conducted to an intermediate-pressure steam turbine (IPST), and further, to a low-pressure steam turbine (LPST). The expanded steam is cycled back to the boiler via a cond...

AI Technical Summary

Benefits of technology

[0009]An object of the present invention is to provide a method of increasing the power of a carbonaceous fuel combusting boiler system without substantially decreasing the thermal efficiency.

[0010]In one aspect, the present invention provides a method of increasing the power of a carbonaceous fuel combusting boiler system, the method comprising the steps of (a) feeding carbonaceous fuel into a furnace of the boiler plant 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 from a boiler 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 steam from the high-pressure steam turbine at a decreased rate for preheating the feedwater, (g) conveying steam from the high-pressure steam turbine at an increased rate to a reheater arranged in the exhaust gas channel for generating reheated steam, (h) expanding the reheated steam in an intermediate pressure steam turbine for generating power, and (i) conveying the exhaust gas in the exhaust gas channel from the reheater through a boiler economizer to a gas heater. The method also comprises increasing the heat exchange surface area of at least one of the reheater and the boiler economizer and / or adding a high pressure economizer downstream of the boiler economizer and upstream of the gas heater.

[0011]To improve the performance of a carbonaceous fuel combusting boiler system, an approach has been proposed wherein the recovery of heat from the high temperature flue gas is increased, and high-grade steam is saved for expansion in a steam turbine. According to the present invention, this is done by increasing the heat transfer surface area of the boiler economizer (ECO) and / or steam reheater (RH), to compensate for the reduced feedwater temperature and increased reheat steam flow, because of less, or even no, steam extraction from the high pressure steam turbine section (HPST). Alternatively, a high pressure economizer (HPECO) may be added upstream of the gas heater (GH). In this way, additional heat is recovered from the flue gas at a relatively high temperature, upstream of the GH. Simultaneously, high pressure (HP) steam is saved to generate more power.

Problems solved by technology

A difficulty with an LPECO is that to avoid condensation of acid compounds in the flue gas on the heat exchange surfaces of the LPECO, the feedwater, heated by steam extracted from the turbines, has to be initially at a certain temperature, for example, above 120° C. for high sulfur fuel.

Such a high water inlet temperature to the LPECO leads to a low heat transfer coefficient, and may result in a significant increase of the LPECO.

In this manner, such an arrangement generates more power at a reduced efficiency.

For the same feedwater temperature increment, the improvement of the steam cycle efficiency decreases with the increase of steam extraction pressure, due to losing more power by losing steam at high pressure.

Images