Patent Application
20120222591
1. Field of the Invention
The present invention relates to a method of and a system for selective catalytic NOx reduction (SCR) in a solid or a liquid hydrocarbon fuel firing power boiler. More particularly, the present invention relates to controlling the temperature of a flue gas entering an NOx catalyst of the boiler.
2. Description of the Related Art
Oxides of nitrogen, also known as NOX, contribute to the generation of acid rain and smog. Due to environmental regulations demanding that NOX emissions be maintained at acceptable levels, the reduction of NOX both during and after the combustion process is of a major concern in the design and operation of modern power plants.
Oxides of nitrogen are a byproduct of the combustion of solid and liquid hydrocarbon fuels, such as pulverized coal or oil, and are found in two main forms. If the nitrogen originates from the combustion air, the NOX is referred to as “thermal NOX”. Thermal NOX forms when molecular nitrogen (N2) is subjected to temperatures above about 1500° C. causing it to break down into elemental nitrogen (N), which can then combine with elemental or molecular oxygen to form NO or NO2. If the nitrogen originates from organically bound nitrogen in the fuel, the NOX is referred to as “fuel NOX”.
Various methods are used to control nitrogen oxide emissions. One method is selective catalytic reduction (SCR), which uses a catalyst and a reductant, typically, gaseous ammonia, to dissociate NOX to nitrogen gas and water according to the following reactions:
4NO+4NH3+O2=>4N2+6H2O
2NO2+4NH3+O2=>3N2+6H2O
Since NOx is approximately ninety-five percent NO, the first reaction dominates the process. The ideal operating temperature range for SCR is generally from about 300 to about 400° C. When operating conditions fall much below 300° C., the potential for ammonium bisulfate formation and sulfur trioxide deposits on the catalyst surface increases. This can cause permanent catalyst activity loss. Above 400° C., ammonia may dissociate, reducing the effectiveness of the process. If temperatures exceed about 450° C., the catalyst activity might be permanently impaired due to sintering.
A typical power boiler utilizing SCR as an NOx reduction technique comprises a furnace in fluid communication with a flue gas channel. Combustion of hydrocarbon fuels occurs in the furnace generating hot flue gases that rise within the furnace, giving up a portion of their energy to generate steam in the evaporator surfaces at the walls of the furnace. The flue gases are then directed through a heat recovery area (HRA) of the flue gas channel, wherein they give up additional energy to superheat the steam and to heat feed water in the economizer surfaces. Flue gases exiting the economizer section are directed through an NOX catalyst, an air preheater and possible flue gas cleaning systems, and finally, via a stack to the atmosphere.
In a typical SCR system, at some point in the flue gas channel upstream of the catalyst section, a reactant, such as gaseous ammonia or a solution of urea in water, is introduced into and mixed with the flue gas stream. The mixture of the reactant and flue gas then enters the catalyst section wherein catalytic reduction of NOX takes place between the reactant and excess oxygen in the flue gas.
The catalyst typically includes multiple layers of solid catalytic material lying within the path of the flue gas stream. The most common types of catalytic material in use and the approximate temperature ranges of the flue gas over which they are effective as catalysts are: titanium dioxide (270-400° C.), zeolite (300-430° C.), iron oxide (380-430° C.) and activated coal/coke (100-150° C.).
U.S. Pat. No. 5,555,849 discloses a fossil fuel power plant with an economizer system upstream of an NOX catalyst, wherein the economizer system comprises a water-side bypass line in order to maintain a desired flue gas temperature in the NOX catalyst even at low load conditions.
European patent publication EP 0 753 701 A1 discloses a boiler with an NOX catalyst disposed in the flue gas channel between two economizers and having a flue gas by-pass channel for the economizer upstream of the NOX catalyst.
U.S. Pat. No. 6,405,791 discloses a tubular air heater with an inlet plenum which permits retrofit installation of a selective catalytic reduction (SCR) system upstream of the air heater in an existing boiler.
In addition to the problem addressed by U.S. Pat. No. 5,555,849, it has been observed that, especially in retrofit installations of an NOX catalyst in an existing power boiler, the flue gas temperature at the NOX catalyst may, especially at high loads, tend to be too high. Due to, for example, changes in the fuel or operation mode of the boiler, or even poor design of the boiler, the economizer outlet temperature may be in excess of 430° C., i.e., above the optimal temperature range of existing NOX catalysts.
Therefore, the adding of an SCR downstream of the economizer to reduce NOX may require the use of a special catalyst. Another solution to this problem is to install additional economizer surfaces in the heat recovery area (HRA) of the boiler. This method, however, increases the feed water temperature and, if the temperature rises close to the saturation temperature of the steam drum, it will have negative effects on the water circulation of the boiler and ultimately, reduce boiler performance.
An object of the present invention is to provide a method of selective catalytic NOX reduction in a power boiler by which problems of the prior art described above can be minimized.
Another object of the present invention is to provide an apparatus for selective catalytic NOX reduction in a power boiler by which problems of the prior art described above can be minimized.
According to one aspect, the present invention provides a method of selective catalytic NOX reduction in a power boiler, the method comprising the steps of (a) combusting fuel in a furnace of the boiler and generating a flue gas stream that includes NOX, (b) conducting the flue gas stream from the furnace along a flue gas channel to a stack, (c) cooling the flue gas stream in a heat recovery area, including an economizer section, arranged in the flue gas channel, (d) reducing at least a portion of the NOX to N2 in an NOX catalyst arranged in the flue gas channel downstream of the economizer section, and (e) further cooling the flue gas and generating heated air in a gas-to-air heater arranged in the flue gas channel downstream of the economizer section and upstream of the NOX catalyst.
According to another aspect, the present invention provides a power boiler, with selective catalytic NOX reduction, the boiler including (a) a combustor for combusting fuel in a furnace of the boiler so as to generate a flue gas stream including NOX, (b) a flue gas channel for conducting the flue gas stream from the furnace to a stack, (c) a heat recovery area, including an economizer section, arranged in the flue gas channel for cooling the flue gas stream, (d) an NOx catalyst arranged in the flue gas channel downstream of the economizer section for reducing at least a portion of the NOX to N2, and (e) a gas-to-air heater arranged in the flue gas channel downstream of the economizer section and upstream of the NOX catalyst for further cooling the flue gas and for generating heated air.
The present invention, i.e., the arranging of a gas-to-air heater upstream of the NOX catalyst to cool the flue gas, provides the advantage of rendering possible the installation of a conventional NOX catalyst using a standard catalyst material. The gas-to-air heater is preferably a tubular air heater, but in may in some cases also be of other types of heat exchangers which transfer heat from the flue gas to combustion air of the boiler.
Advantageously, the power boiler comprises a burner, or, in practice, a set of burners, for combusting the fuel carried to the burners by a stream of primary air. According to a first embodiment of the present invention, the combustion air heated in the gas-to-air heater is conducted as secondary air to the burners. Due to the use of the secondary air as the cooling medium, the method does not have a risk of overheating the cooling medium, as the case may be when using feed water as the cooling medium. Moreover, because the heat transferred to the secondary air can be fully recovered in the boiler, the method does not affect the operation or efficiency of the existing boiler. The combustion air heated in the gas-to-air heater can also alternatively be of other types of combustion air, for example, primary air, to be conducted to the furnace.
When using a method in accordance with the present invention in different load conditions of the boiler, the flow of air through the gas-to-air heater can be modulated, or shut off, to maintain a desired temperature of the flue gas entering into the catalyst. The flow of air can thus advantageously be controlled directly on the basis of the load conditions of the boiler, or on the basis of a measured temperature of the flue gas entering the NOX catalyst. Thus, the present invention provides a simple method to provide optimized operation of the catalyst in different load conditions, without, for example, a need to provide an economizer with a flue gas by-pass, or water side by-pass, for low load operation. The present invention thus provides a wide range of temperature control, without requiring any change in the flue gas or steam/water circuitry of the boiler. The invention is thus especially useful in retrofit applications, but it can be applied in new units as well, for example, to control the temperature of the flue gas entering the NOX catalyst.
The above brief description, as well as further objects, features, and advantages of the present invention will be more fully appreciated by reference to the following detailed description of the currently preferred, but nonetheless illustrative, embodiments of the present invention, taken in conjunction with the accompanying drawing.
The flue gases are then directed through a heat recovery area (HRA) 34 of the flue gas channel, wherein they give up additional energy in superheater surfaces 36 to superheat the evaporated steam and in economizer surfaces 38, to preheat feed water to be fed to the evaporator surfaces. Typically, the HRA comprises multiple superheater and reheater surfaces, but because they are not important for the present invention, only one superheater 36 is shown in
Flue gases exiting the economizer 38 are directed through an NOX catalyst 40, an air preheater 42, a flue gas cleaning system 44, and a stack 46 to the atmosphere. The flue gas channel 32 also comprises an injector 48 for injecting NOx reductant, such as ammonia, upstream of the catalyst 40. The catalyst 40 preferably comprises conventional catalyst material, such as titanium oxide or iron oxide. Typically, the flue gas cleaning system comprises several flue gas cleaning units, such as a dust separator and a desulfurizer, but because they are not important for the present invention, only one schematic gas cleaning system 44 is shown in
In accordance with the present invention, the flue gas channel comprises a gas-to-air heater, in this case, a tubular air heater 50, arranged upstream of the NOX catalyst 40. By the tubular air heater, it is possible to cool the flue gas, as desired, to an optimal temperature range for the catalyst, for example, to below about 400° C.
The tubular air heater 50 is advantageously connected so as to render possible additional heating of the secondary air 22. In some embodiments, it is also possible to use the tubular air heater to heat the primary air 18, or tertiary air or overtire air, not shown in
The ratio of the air flows through the tubular air heater 50 and the air heater 42 downstream of the catalyst 40 can advantageously be determined on the basis of the boiler load, or on the basis of the flue gas temperature upstream of the catalyst, as measured by a temperature measuring device, such as a thermometer 56. Thus, the system advantageously comprises a controller 58 for controlling the control valves 54, 54′ on the basis of the measured temperature.
Typically, at high loads, when the temperature of the flue gas upstream of the catalyst tends to rise above the optimal operating temperature of the catalyst, a larger portion of the secondary air is conducted through the tubular air heater 50 by at least partially closing the valve 54′ arranged in the branch of the secondary air line leading through the air heater 42 arranged downstream of the catalyst 40. Correspondingly, at low loads, a smaller portion of the secondary air is conducted through the tubular air heater by at least partially closing the valve 54 arranged in the branch of the secondary air feeding line leading through the tubular air heater 50. Thus, by controlling the division of the air stream between the tubular air heater 50 and the air heater 42 downstream of the NOX catalyst 40, it is possible to optimize the temperature of the flue gas entering the NOX catalyst at different load conditions.
While the invention has been described herein by way of examples in connection with what are at present considered to be the most preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but is intended to cover various combinations or modifications of its features and several other applications included within the scope of the invention as defined in the appended claims.
