FIELD OF THE INVENTION
The present invention relates to a gas fired boiler for reduction of auxiliary power consumption where different types of fuels with different calorific values with varying sulphur concentrations are fired.
More particularly the present invention discloses gas bypass system to economizerto achieve optimum performance parameters.
BACKGROUND OF THE INVENTION:
In current existing oil & gas fired boiler, where different kinds of fuels are envisaged, boiler designed for a fuel with high sulphur concentrations considering the SO3 acid dew point temperature and hence exit gas temperature for the high sulphur content fuel is high. While firing low sulphur content fuel, the exit gas temperature is also high since the surface is already fixed, but there is possibility that exit flue gas temperature further can be lowered to achieve better thermal efficiency. If surface area is fixed for lower sulphur concentration fuel to get the thermal efficiency, while firing the high sulphur content fuel the auxiliary power consumption is increased and Acid dew point temperature is reached. These issues could not be addressed in present boiler design arrangement. A conventional arrangement of

typical oil & gas fired boiler fuel and air is admitted into the furnace (1). Heat carried by flue gas produced after combustion infurnace, is allowed to pass through a series of heat recovery surfaces which comprise of Platen SH (superheater) & Final SH (superheater) (2) in first pass and LTSH (Low temperature superheater) (3) & Economizer stage-I (4), economizer stage-II (5) in second pass and air heater (6) as the last stage of heat recovery system. The flue gas is discharged to atmosphere through chimney (7) as shown in figure 1.
Hence to address this drawback in existing oil & gas fired boiler, a new design is required.
The prior art document CN 202018050 U discloses flue gas bypass system for preventing low temperature corrosion of an air preheater.
A flue gas bypass pipeline is arranged between a boiler and the entrance of an air preheater inlet. The flue gas flow unbalanced control device is provided for regulating the flue gas through bypass channel.The flue gas temperature of the entrance of the airheater is regulated by controlling two streams (i.e. bypass stream & main stream) of flue gas flow at different temperatures so as to achieve the purpose of controlling the exhaust flue gas temperature at the exit of the Airheater, ultimately prevent low-temperature corrosion of the air preheater.

Another prior art document CN 203336572 U discloses bypass economizer used for comprehensive utilization of flue gas waste heat in which the utility model is bypassed economizer and an air preheater which are arranged in parallel from the condensate pump to a condensate exhaust heat absorption in the low pressure economizer to reduce exhaust gas temperature condensate itself is heated to elevated temperature and then returned to the low pressure turbine heater system, Low pressure condensate heater saved the heat recovery steam extraction from turbine section. In the case of turbine system steam inlet conditions unchanged and by saving the extraction steam from the turbine. As the steam is not extracted from the turbine more work output achieved for the same fuel feed rate. Thus by providing economizer gas bypass system generating capacity is increased, fuel consumption is reduced and energy is saved.
Therefore a flue gas bypass system across economizer is provided to reduce the power consumption where the sulphur concentration in fuel is high and calorific value is high. Similarly fuel with less sulphur concentration and low calorific value is admitted through economizer to achieve less exit flue gas temperature resulting in higher thermal efficiency. Thus an economizer bypass system is designed to meet the optimum performance parameters while firing different types of fuels with varying sulphur concentrations and calorific values.

OBJECTIVES OF THE INVENTION
An object of the present invention is to develop a gas bypass system to economizer to achieve optimum performance parameters that ensures lower auxiliary power consumption while firing higher sulphur content fuel.
Another object of the invention is to develop a gas bypass system to economizer to achieve optimum performance parameters that ensures higher thermal efficiency while firing low sulphur content fuel.
A still another object of the invention is to develop a gas bypass system to economizer to achieve optimum performance parameters that operates the boiler with flexibility.
A further object of the invention is to develop a gas bypass system to economizer to achieve optimum performance parameters that protects the heat transfersurfaces from acid dew point corrosion while firing high sulphur content fuels.
A still further object of the invention is to develop a gas bypass system to economizer to achieve optimum performance parameters that avoid cold end corrosion in the economizer.
A still most important object of the invention is to develop a gas bypass system to economizer to achieve optimum performance parameters that tire multiple range of fuel having various sulphur content.

SUMMARY OF THE INVENTION
This invention relates to an oil and gas fired boilers, where multi fuel firing is envisaged with varying sulphur concentrations. A boiler is designed for the three fuels namely two gaseous fuels and one liquid fuel. Between the two gaseous fuels one fuel contains no H2S concentration (fuel 1) and another gaseous fuel (fuel 2) contains H2S concentration from 0.0002% to 0.0005% and Liquid fuel (fuel 3) also contains sulphur concentration from 0.5% to 4.0%. For designing the boiler it is required that boiler shall operate at higher thermal efficiency with lower auxiliary power consumption. The boiler has initially designed for fuel 1 with higher thermal efficiency with lowest possible exit gas temperature as the fuel does not contain sulphur concentration, accordingly economizer area is sized to meet the exit gas requirement in two stages. An air heater is located at the downstream of economizer which is the last stage heat transfer section on the flue gas circuit. While firing fuel 2 with H2S concentration second stage of the economizer is gas bypassed to reduce the auxiliary power consumption and to avoid the sulphur dew point condensation. Also while firing fuel 3 with sulphur concentration second of economizer is gas bypassed and partly airheater is air bypassed to reduce the auxiliary power consumption and to avoid the sulphur dew point condensation. This arrangement can be effectively utilized in multi fuel firing with different sulphur concentrations.

The comparison of this invention with the existing design is given below in table 1.

Table 1: Comparison of performance data of new design with existing design
The below table (table 2) shows the percentage opening of bypass damper with respect to damper opening for a typical liquid and gaseous fuels with various sulphur concentrations. The function of gas bypass dampers is to regulate the flow of gas through the economizer & air bypass dampers is to regulate the flow of the air through Airheater in order to achieve the high exit gas temperature at chimney inlet while firing high sulphur & high H2S in fuel.


Table 2: Bypass damper opening with respect to sulphur concentration in fuel
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 – shows air and gas path of typical oil & gas fired boiler, Prior art
Figure 2 – shows air and gas path of typical oil & gas fired boiler, Present invention

DETAILED DESCRIPTION OF THE INVENTION
A newly invented oil & gas fired boiler, fuel and air is admitted into the furnace (1). Heat carried by flue gas produced after combustion in furnace, is allowed to pass through a series of heat recovery surfaces which comprise of Platen SH & Final SH (2) in first pass and LTSH (Low temperature superheater) (3) & Economizer stage-I (4) , economizer stage-II (5) in second pass and air heater(6) as the last stage of heat recovery system. The flue gas is discharged to atmosphere through chimney (7).
A Gas bypass system with Bypass damper to economizer (8), Gas Bypass damper (9), Air bypass system with Bypass damper (11) to Airheater & Air Bypass damper (10) to provide a solution for the prior art oil and gas fired boilers disadvantages. The modified oil and gas fired boiler can serve as an alternate for the prior art as shown figure 2
With this newly invented modification method it will be suitable for high sulphur content fuel with optimum performance parameters & avoids acid dew point condensation occurs due to cooling of flue gas in the economizer, which will cause the corrosion of equipments where flue gas is touched. As the flow the flue gas is flowing through the economizer bypass in the present arrangement pressure drop occurs across economizer will decrease, which will lead to decrease in the pressure head on flue gas handling fans, thereby decreasing the auxiliary power consumption of fans. The air side bypass arrangement is also provided to bypass

Airheater & have operating flexibility while operating very high sulphur content fuels in association with very low sulphur content fuels. By the new bypass circuit flue gas is bypassed to economizer partly or fully by dampers depending on the sulphur content in the fuel, thus by reducing the auxiliary power consumption by fan as the economizer drop is not there during the bypass system operation.
Thus economizer bypass & Airheater bypass combined arrangement gives freedom to the operator to fire various sulphur content fuels with different calorific values. Hence all the issue will be address by this design.
Typical flue gas temperatures across the heat transfer surfaces is shown in table 3:

Table 3: Comparison of flue gas temperature profile with gas bypass to without gas bypass system.

We Claim
1. An improved gas bypass system to economizer to achieve optimum performance parameters comprising:
an oil and gas fired boiler producing multiple range of fuels having various sulphur content in furnace (1) allowed to pass through various heat recovery surfaces comprising platen superheater (SH) and final superheater (SH) (2) in first pass, a low temperature superheater (LTSH) (3), an economizer stage-I (4) respectively through a common path before entering the economizer state-II (5) wherein when the boiler is fired with high sulphur fuel, then the gas bypass damper (8) is opened and regulated through damper (9) which allows the flue gas to bypass economizer (5) and through the airheater bypass damper (11) in open condition and regulated the airflow through the airheater in order to have high flue gas temperature in chimney (7) to avoid formation of sulphur dew temperature and when the boiler is fired with low sulphur fuel, then the gas bypass damper (8) is closed which allow the flue gas to pass totally through the economizer (5) and air bypass damper (11) is closed which allow the air to pass totally through the airheater in order to have low flue gas temperature to chimney (7) ultimately getting high boiler thermal efficiency as there is no chance of formation of sulphur dew temperature due to less sulphur in fuel.

2. The gas bypass system as claimed in claim 1 wherein the economizer bypass system along with airheater bypass system are provided with isolating and regulating dampers (8,9,10 & 11) that helps to regulate the gas flow at various loads to maintain the exit gas temperature above the acid due point temperature due to H2S and sulphur presence in fuel.
3. The gas bypass system as claimed in claim 1 wherein the boilers are designed for three types of fuels having two gaseous fuels in which fuel (1) is with no H2S concentration and fuel (2) with H2S concentration, and the third being liquid fuel (3) containing high sulphur concentration.