The present invention relates to a new Bypass Over Fire Air (BOFA) system.
Field of Invention:
The present invention particularly relates to a system to minimize furnace flue gas outlet temperature imbalance, better flue gas flow distribution across furnace outlet in addition to minimize generation of NOx in the furnace.
In a tilting tangential coal, oil or gas fired boilers, there exists flue gas outlet temperature imbalance between left and right side of the boiler flimace outlet plane because of the spinning flow effect caused by the tangential firing of the fuel.
To bum completely a given quantity of fuel, certain quantity of air is theoretically required which is called stochiometric air. In practice, however, excess air is required in order to bum all the combustibles within a limited space, such as the confines of the fumace of a steam generator. The addition of excess air, particularly makes excess oxygen available in the fumace in the high temperature combustion zone which aids for the formation of the oxides of nitrogen commonly known as NOx , which is an undesired air pollutant.
The novel Bypass Over Fire Air (BOFA) system minimizes the left right flue gas temperature imbalance and minimizes the availability of excess oxygen in the high temperature combustion zone thereby minimizing the formation of NOx in the fiimace.
Background Art / Prior Art:
The operation of industrial boilers gets affected due to the quality of coal, causing undesirable slagging, fouling, corrosion, and fly ash erosion, etc which are difficult to predict because of limited understanding of combustion processes. However, coal properties are not the only factor influencing slagging or fouling in a particular situation. For example, the same coal can be fired in two. boilers with one experiencing slagging and the other does not.
Ash deposition is one of the greatest problems associated with the efficient utilization of low rank coals in utility boilers. The formation of ash deposit as result of combusting low rank coals is a complex phenomenon. Ash deposition in utility boilers can be manifested in two ways, as slagging and fouling. Hence, boiler design parameters are important variables with regard to deposit problems.

The analysis of boiler performance data consists of establishing the furnace exit gas temperature (FEGT) for a given time and set of conditions. From the furnace exist gas temperature, a furnace effectiveness factor (fs) is derived.
Bypass Over Fire Air (BOFA) is developed to study/solve problems imbalance of the flue gas temperature in left and right side of boiler, non uniformity of flue gas flow across furnace cross section at the outlet and convection pass and higher NOx emission levels.
An object of the present invention is to provide a device/equipment capable of altering the furnace outlet temperature at the boiler furnace.
A flirther object of the invention is to provide a device/equipment capable of rectifying or nullifying the left/right imbalance in flue gas temperature of the second pass of the boiler.
A still further object of the present invention is to provide a device/equipment capable of altering to the desired super heater temperature.
Another object of the present invention is to provide a device/equipment capable of reducing NOx emission upto 40%.
Summary of Invention:
Many coal quality effects, such as slagging, fouling, corrosion, and fly ash erosion etc are difficult to predict because of limited understanding of combustion processes. However, coal properties are not the only factor influencing whether slagging or fouling will occur in particular situation. For example, the same coal can be fired in two boilers with one experiencing slagging and the other without it.
Ash deposition is one of the greatest problems associated with the efficient utilization of low rank coals in utility boilers. The formation of ash deposit as result of combusting low rank coals is a complex phenomenon. Ash deposition in utility boilers can be manifested in two ways, as slagging and fouling. Hence, boiler design parameters can be important variable with regard to deposit problems.
The analysis of boiler performance data consists of establishing the furnace exit gas temperature (FEGT) for a given time and set of conditions. From the furnace exit gas temperature, a furnace effectiveness factor (f) is derived. .
Bypass Over Fire Air (BOFA) is developed to minimize the imbalance of the flue gas temperature in left and right side of boiler, non uniformity of flue gas flow across furnace cross section at the outlet and convection pass and to reduce NOx emission levels.

The present invention is directed to an Bypass Over Fire Air arrangement wherein hot air is injected tangentially to an imaginary circle in the center of the furnace well above the wind box level.
A tangentially fired pulverized coal burning furnace, fuel and air are introduced into the furnace at the burner level tangentially of an imaginary circle so that the resultant fire ball moves upwards within the furnace with a rotational spin which creates imbalance in the left/right flue gas outlet temperature, non-uniform gas flow across the furnace outlet plane.
In order to find solution for problems explained above, the applicant has developed an improved system, namely Bypass Over Fire Air (BOFA) system. In the BOFA system, suitable parts with nozzles are provided at all the four comers well above the top of the wind box.
Further, desired percentage of hot primary air or hot secondary air is bypassed and admitted into the furnace at predetermined location above furnace. The air is injected into the furnace at a velocity so that the air mixes thoroughly and maintain a uniform flue gas flow across the furnace exit plane.
BOFA of the invention is described hereinbelow with reference to the accompanying drawings, wherein -
Figure 1 - shows the arrangement of the Bypass Over Fire Air
system. Figure 2 - shows the wind box nozzle assembly of BOFA. Figures - shows the tilting arrangement of BOFA.
To facilitate better control of over fire air admission into the furnace, each compartment is divided suitably into smaller horizontal compartments. All bypass air compartments are suitably equipped with a horizontal angle adjustment and a vertical tilting arrangement facility, to enable to vary the air angle and direction. All the bypass air compartments are provided with suitable dampers for airflow controls. The wind box and the tilting arrangement are shown in Fig.2 and 3, respectively.
In order to find solution for problems explained above, the applicant has developed a novel Bypass Over Fire Air (BOFA) system. In the BOFA system, suitable ports with nozzles are provided at all the four comers well above the top of the wind box. Arrangement of the Bypass Over Fire Air system illustrated as shown in Figure 1.
Suitable percentage of hot primary air or hot secondary air is bypassed and admitted into the furnace at suitable location above furnace. To facilitate better control of over fire air admission into the furnace, each compartment is divided suitably into smaller horizontal compartments. The air is injected into the furnace at a suitable velocity so that the air mixes thoroughly and maintain a uniform flue gas flow across the furnace exit plane. The arrangement is as shown in Fig.2.
All the bypass air compartments are suitably equipped with a horizontal angle adjustment and a vertical tilting arrangement facility, to enable to vary the air injection angle and direction. All the bypass air compartments are provided with suitable dampers for airflow controls. The wind box arrangements and tilting arrangements are shown in Fig.3.
Description of Drawings:
The nature of invention will be clear firom the following descriptions with reference to the accompanying Figures 1, 2 and 3.
Figure 1:
Suitable proportion of hot air is tapped from Hot Air (Primary air or Secondary air) headersof the boiler. The air is taken to the furnace
through a suitably designed ducts (1). Required devices for hot air isolation (2) and flow control devices (3) with necessary instrumentation system to control the quantity of hot air are provided. Flow metering devices (3) are provided to ensure admission of the correct proportion of bypass air to the furnace. The individual ducts are provided with flow regulating devices (4) to obtain desired quantity of air through individual nozzles. The air is injected into the furnace well above the regular burners with suitable velocity through wind box assembly as shown in Figure 2.
Figure 2:
Suitable designed wind box assembly (1) is provided to inject the hot air into the furnace at a desired angle. Air is injected through a number of nozzles (2). Each nozzle is provided with air injection angle adjustment devices as shown in Figure 3.
Figure 3:
Nozzle position and locking devices are provided to inject hot air into the furnace at desired angle. The nozzles can be positioned at any desired angle both vertically and horizontally. The angle adjustment is as per details A and B respectively in Figure 3.
Novel Features:
1. This system is capable of controlling the quantity of bypass air (i..e.) from 0% to 15% of total combustion air.
2. The angle and velocity of the bypass air jet is adjustable to a wide range. The nozzle can be tilted vertically up and down and also horizontally.
3. Airflow controls to the individual nozzles are possible.
4. The hot air is tapped in such a way that no additional booster fan is required to inject the bypass air at the required velocity.

WE CLAIM:
1. BOFA system comprising hot air tapping being suitable located, ducts, dampers, flow nozzles and bypass air injection nozzles being capable of changing its position and housed in a suitably designed water wall panels with necessary air flow controls and instrumentations.
2. BOFA system as claimed in Claim 1, said system being capable of altering the fiimace outlet temperature at the boiler furnace.
3. BOFA system as claimed in Claim 1 or 2 capable of rectifying or nullifying the left/right imbalance in flue gas temperature of the second pass of the boiler.
4. BOFA system as claimed in any one of Claims 1 to 3, capable of uniform flue gas flow distribution across the furnace outlet.
5. BOFA system capable of altering to the desired super heater temperature and reducing NOx emission upto 40%.
6. A system as claimed in Claim 1 wherein hot air tapping points are located at a critical location to bypass the optimum quantity of hot air at optimum temperature so that no additional booster fans are required.
7. A system as claimed in Claim 1 wherein suitably designed ducts are provided to convey the air with flow measurement nozzles.
8. A system as claimed in Claim 1 wherein suitably designed air injection nozzles are provided to inject air at optimum velocity into the furnace.
9. A system as claimed in Claim 1 wherein said hot air injection ports are located in the optimum position above the burner elevation.
10. A system as claimed in Claim 1 wherein control dampers are provided to regulate the hot air flow for optimum result.
11. A system as claimed in Claim 1 wherein the location hot air injection is at an optimum elevations to NOx emission at an optimum level.