DESC:FIELD OF THE INVENTION

The present invention describes a process and a system for improving the energy efficiency of Power Generator through hybridization with Solid oxide Fuel cell.

BACKGROUND OF THE INVENTION
The major cause of low efficiency of power generator is the heterogeneous uncontrolled combustion reaction for conversion of chemical energy of the fuel into useful work. As reported, imperfect combustion in power generators results in lower operating efficiency and discharge of harmful exhaust emissions including Carbon Monoxide, unburned Hydrocarbons, Oxides of Nitrogen and Particulate matter in the atmosphere.

Solid Oxide fuel cell has its own demerits including the requirement of a fuel reformer upstream for producing hydrogen rich feed, long start-up time inhibiting the start and stop operation and auxiliary requirements like steam and heat source for achieving the start up temperature.

US20050022450A1 discloses a reformer system comprises a reformer catalyst capable of reforming a fuel to hydrogen and carbon monoxide, and a water gas shift catalyst in fluid communication with the reformer catalyst and in fluid communication with an exhaust gas source comprising water, wherein the water gas shift catalyst is capable of reacting carbon monoxide with the water to produce hydrogen and carbon dioxide.

US6655325 discloses a system and method relate to power generation utilizing an exhaust side solid oxide fuel cell. Fuel is burned in an engine in the presence of air. The engine exhaust passes through a solid oxide fuel cell where it is consumed in the production of electricity and ionization of oxygen in an air stream also introduced to the solid oxide fuel cell. The solid oxide fuel cell effluent fuel stream and/or air stream can be recycled through the engine, directed through a turbine to recover additional energy therefrom, and/or passed through a catalytic converter. The resulting system exhaust has negligible to zero amounts of nitric oxides, hydrocarbons, carbon monoxide, and particulates.

US20040177607 discloses an internal combustion engine with a fuel cell in an exhaust system, fuel for power generation is able to be supplied to the fuel cell without regard to the operating condition of the internal combustion engine. The fuel cell has a fuel electrode side thereof connected with an exhaust passage of the engine. A fuel supply system supplies the power generation fuel to the exhaust passage at a location downstream of the engine and upstream of the fuel cell. A supply amount control part controls an amount of the power generation fuel supplied by the fuel supply system. According to such a construction, the power generation fuel can be supplied to the fuel cell by the fuel supply system so as to generate electricity without depending on the operating condition of the engine.

SUMMARY OF THE INVENTION:
The present invention seeks to integrate the two devices viz. power generator and SOFC, so to achieve higher efficiency and reduced specific fuel consumption resulting from the hybridization. Also, SOFC can be operated on exhaust of power generator thereby reducing the auxiliary requirements and improving the start up time.

Accordingly, present invention provides a system for improving energy efficiency of Power Generator, comprising:
a power generator (PG) configured to receive a mixture of fuel and air and produce an exhaust gas stream;
a water-gas shift reactor (WGSR) provided downstream of the power generator for receiving the exhaust gas stream as a first input and steam as a second input and for producing an intermediate gas stream rich in hydrogen; and
a Solid Oxide Fuel Cell (SOFC) receiving the intermediate gas stream rich in hydrogen from the WSGR as a first input and a gas stream containing oxygen as a second input and for producing water, a spent gas stream and electrical current.

In one of the feature of the present invention the fuel is selected from diesel, gasoline, natural gas, and LPG.

In another feature of the present invention, in the system, an operation of the PG is controlled so as to produce the exhaust gas stream comprising 4 to 6 mole % of hydrogen and 4 to 6 mole % of carbon monoxide. In yet another feature of the present invention the exhaust gas stream optionally comprising one or more of water, carbon dioxide, hydrocarbon based compounds and nitrogen oxide.

In another feature of the present invention, in the system, the PG is operated at an air to fuel ratio in the range of 14 to 15 at a full load condition.

In yet another feature of the present invention, in the system, the water as produced by the SOFC is in the form of steam.

In yet another feature of the present invention, in the system, the steam thus produced by the SOFC is provided as the second input to the WGSR.

In yet another feature of the present invention, in the system, a ratio of steam to carbon in the WGSR is in the range of 1.2 to 1.6.

In yet another feature of the present invention, in the system, the intermediate gas stream produced by the WGSR comprises 8 to 11 mole % of hydrogen.

In yet another feature of the present invention, in the system, the WGSR is operated at a temperature in the range of 340o to 360oC.

In yet another feature of the present invention, in the system, the SOFC is operated at a temperature in the range of 650o to 750oC.

In still another feature of the present invention, the system comprises a first heat exchanger adapted to receive the gas stream containing oxygen from an air blower as a first input and the exhaust gas stream from the PG as a second input, the heat exchanger being further adapted to increase a temperature of the gas stream containing oxygen using the exhaust gas stream.

In still another feature of the present invention, in the system, a temperature of the gas stream containing oxygen is increased using at least one of the exhaust gas stream and the spent gas stream.

In still another feature of the present invention, in the system, a second heat exchanger adapted to receive the intermediate gas stream from the WGSR as a first input and the spent gas stream from the SOFC as a second input, the heat exchanger being further adapted to increase a temperature of the intermediate gas stream using the spent gas stream.

In yet another feature of the present invention the system comprises a third heat exchanger adapted to receive the gas stream containing oxygen as a first input and the spent gas stream from the SOFC as a second input, the heat exchanger being further adapted to increase a temperature of the gas stream containing oxygen using the spent gas stream.

In yet another feature of the present invention, in the system, equivalence ratio (?) is in the range of 1.3 to 1.5. More particularly equivalence ratio is in the range of 1.40 to 1.45.

Present invention also provides a process for improving energy efficiency of Power Generator, comprising:
providing a power generator (PG) configured to receive a mixture of fuel and air and produce an exhaust gas stream;
providing a water-gas shift reactor (WGSR) downstream of the power generator for receiving the exhaust gas stream as a first input and steam as a second input and for producing an intermediate gas stream rich in hydrogen; and
providing a Solid Oxide Fuel Cell (SOFC) such that SOFC receives the intermediate gas stream rich in hydrogen from the WSGR as a first input and a gas stream containing oxygen as a second input and produces water, a spent gas stream and electrical current.

BRIEF DESCIPTION OF DRAWINGS

Fig. 1: illustrates process flow diagram of Hybrid system.
Fig. 2: illustrates graph between equivalence ratio vs. temperature –Power generator operating at various equivalence ratio and temperature;
Fig. 3: illustrates graph between equivalence ratio vs. Mole (%) - Variation of mole fraction of exhaust gases and Power generator with equivalence ratio;
Fig. 4: illustrates graph between equivalence ratio vs. Power (variation of Power Generator and SOFC power with ?) – Hybrid system;
Fig. 5: illustrates graph between equivalence ratio vs. Mole (%) & Power (Variation of mole fraction of Exhaust gases and Hybrid power with equivalence ratio) – Hybrid system;
Fig. 6: illustrates graph between equivalence ratio vs. Power & Energy saving (Comparison of PG/DG Set and Hybrid Power and efficiency increased) – Hybrid system;
Fig. 7: illustrates graph between equivalence ratio vs. specific fuel consumption (Variation of Specific fuel consumption for Hybrid system) – Hybrid system; and
Fig.8: illustrates graph between Steam/Carbon ratio vs. Power & % Increased – Hybrid system.

DETAILED DESCRIPTION OF THE INVENTION

The present invention describes a process for optimizing the energy efficiency of Power Generator through hybridization with Solid oxide Fuel cell.

The Power Generator (PG Sets) operates on fuel selected from diesel fuel, gasoline, natural gas, LPG etc. In one of the feature of the present invention, the Power Generator operates on diesel fuel is called as DG Set. During recent times, the gensets are facing competition from cleaner sources of energy especially from solar energy and PEM fuel cells. In order to sustain in the market, the efficiency of the genset needs to be improved and the exhaust emissions shall reduce. The major cause of low efficiency of power generator is the heterogeneous uncontrolled combustion reaction for conversion of chemical energy of the fuel into useful work. As reported, imperfect combustion in the combustion chamber results in lower operating efficiency and discharge of harmful exhaust emissions including Carbon Monoxide, unburned Hydrocarbons, Oxides of Nitrogen and Particulate matter in the atmosphere.

Solid Oxide fuel cell (SOFC) is a clean system but has its own demerits including the requirement of a fuel reformer upstream for producing hydrogen rich feed, long start-up time inhibiting the start and stop operation and auxiliary requirements like steam and heat source for achieving the start up temperature.

Present invention provides a hybrid system in which these two devices viz. Power Generator and SOFC are integrated so that to achieve higher efficiency resulting from the hybridization. By operating genset upstream of SOFC at a rich air to fuel ratio, hydrogen and CO rich exhaust can be produced which can be utilized in a fuel cell for generating power. In accordance with the present invention the hybrid system comprises the Water Gas Shift Reactor (WGSR) which increases the efficiency of fuel cells by increasing hydrogen production.

In accordance with the present invention, a Power generator (PG) is configured and operated at high air to fuel ratio in the range of 14-15 at full load so that PG can produce power as well as H2 (4-6 mole %) & CO (4-6 mole %) rich exhaust.

The Water Gas Shift Reactor (WGSR) can aid in the efficiency of fuel cells by increasing hydrogen production. The exhaust temperature of a PG set suitable for backup power generation can range between 450 to 600 °C depending on operating loads. If the air to fuel ratio at full load is decreased, apart from production of hydrogen in the exhaust, the CO concentration present in exhaust also increases significantly which can be further converted into H2 at an optimum temperature range. Therefore, water gas shift reactor at high temperature is connected in downstream, wherein CO reacts with steam in range of 340- 360°C temperature at steam/carbon ratio 1.2-1.6 produces substantial amount of H2 (8-11 mole %). The steam required for maintaining the S/C ratio in water gas shift reactor can be obtained from the genset exhaust as well as from the exhaust of SOFC on the anode side as per the following reaction:
CO + H2O -----? H2 + CO2 (Temperature 340-360°C at Steam/Carbon = 1.2-1.6)

SOFC can utilize both H2 and CO in the electrochemical reaction. However to enhance the overall system efficiency, Ni/Ceria can be used as an anode for intermediate temperature SOFC wherein CO gas present in the feed can get further converted to H2.

In accordance with the present invention, a medium temperature operated SOFC is operated after High Temperature (HT) reactor. The operating temperature of SOFC is in range of 650-750 °C. Wherein H2 fuel and air is continuously fed into the anode and cathode respectively. At cathode, the Oxygen molecules react with electrons flowing through the external circuit from the anode side and produces O2- ions. These ions transport towards anode side through an electrolyte and react with fuel to form water and electrical current, which can be used to do useful work.

Power generator operating at various equivalence ratio & temperature.
Figure 2 shows the variation of the Combustion and Exhaust temperature with the equivalence ratio (?) in the power generator. As the Equivalence ratio increases from 0.5 the fuel quantity present in Air-fuel mixture increases which increases the combustion in the chamber helps in the increment of both the exhaust and the combustion temperature. This trend follows till the optimum point when Air-fuel mixture is present in same quantity (?=1). But as the concentration of fuel increases beyond the ?=1 then the exhaust and combustion temperature decreases due to inefficient combustion of highly rich mixture. Temperature of power generator at exhaust is shown in the range of temperature 450-600°C.

Variation of mole fraction of exhaust gases and Power generator with equivalence ratio:
Figure 3 shows the variation of mole percentage of exhaust gases and power generation from Power Generator Set with the equivalence ratio. It is evident from the graph that mole fraction of H2 and CO increases with the increase in equivalence ratio while mole fraction of H2O decreases with increase in equivalence ratio. Increase in H2 and CO (being the fuel for SOFC) at richer charge shows the potential of combining with Solid Oxide Fuel Cell (SOFC).

Beside this can also be inferred that Power generator decreases with increase in equivalence ratio as richer the charge gets lesser is the availability of air thus resulting in incomplete combustion.

The present method relates to Power generator configured and operated at high air to fuel ratio in the range of 14-15 at full load so that PG can produce power as well as H2 (4-6 mole %) & CO (4-6 mole %) rich exhaust.

Variation of Power Generator and SOFC power with ?:
Figure 4 shows the variation of the Power Generator (PG Set) and Solid Oxide Fuel Cell (SOFC) power output with the equivalence ratio (?). Power produced by PG Set decreases with increase in equivalence ratio while power produced by SOFC increases with increase in equivalence ratio. The graph is plotted and where these two curves meet a vertical line is drawn which cuts the equivalence ratio axis and this point is called optimum point. It can also be derived that the hybrid power is maximum at this point.

The optimum equivalence ratio is 1.45 and power produced at this point by SOFC and PG Set are 12.1 KW and 12.3 KW respectively.

Variation of mole fraction of Exhaust gases and Hybrid power with equivalence ratio
Figure 5 shows the variation of mole percentage of exhaust gases and hybrid power with the equivalence ratio. It can be inferred from the graph that mole fraction of H2 and CO increases with the increase in equivalence ratio , when this H2 and CO produced are fed to the SOFC it also produce the power. The power produced by the SOFC overcomes the decrease in the power of Power Generator set due to increase in the equivalence ratio. Hence, Hybrid Power (Power Generator + SOFC) increases with increase of equivalence ratio.

Due to rich mixture, de-rating of the PG set occurs after a certain equivalence ratio and same time power from SOFC is increased due to H2 generation at the outlet.

Comparison of PG Set and Hybrid Power and efficiency increased
Figure 6 shows the comparison of PG/DG Set and Hybrid Power and efficiency variation with the equivalence ratio. Power produced by PG Set decreases with increase in equivalence ratio. While the power of the hybrid shows opposite trends i.e. it increases with increase in the equivalence ratio. It can also be seen with increase in equivalence ratio the percentage of energy increased over thermal efficiency increases.

Variation of Specific fuel consumption for Hybrid system
Figure 7 shows the variation of specific fuel consumption for hybrid system. The graphs compare the specific fuel consumption at two different equivalence ratio i.e. 1 and 1.4 (optimum point). At ? = 1, where the power produced by SOFC is low due to lower concentration of H2 and CO in the exhaust. On the other hand, at ? = 1.4, where H2 and CO concentration is higher side thus more power generated.

Graph between Steam/Carbon ratio vs. Power & % Increased – Hybrid system
Figure 8 shows the graph between Steam/Carbon ratio vs. Power & % Increased. The WGSR can aid in the efficiency of fuel cell by increasing hydrogen production. The exhaust temperature of a PG set suitable for backup power generation can range between 450 to 600°C depending on operating loads. If the air to fuel ratio at full load is decreased, apart from production of hydrogen in the exhaust, the CO concentration present in exhaust also increases significantly which can be further converted into H2 at an optimum temperature range. Therefore, water gas shift reactor at high temperature is connected in downstream, wherein CO reacts with steam in range of 340- 360°C temperature at steam/carbon ratio 1.2-1.6 produces substantial amount of H2 (8-11 mole %). The steam required for maintaining the S/C ratio in water gas shift reactor can be obtained from the PG exhaust as well as from the exhaust of SOFC on the anode side as per the following reaction.
CO + H2O -------? H2 + CO2 (Temperature 340-360 °C at Steam/Carbon = 1.2-1.6)

SOFC can utilize both H2 and CO in the electrochemical reaction. However to enhance the overall system efficiency, Ni/Ceria can be used as an anode for intermediate temperature SOFC wherein CO gas present in the feed can get further converted to H2.

Technical Advantages
• Overall Hybrid system efficiency can be improved by 30 % i.e. energy saving of 30% based on LHV of diesel fuel.
• Specific fuel consumption (gram/kW.hr) dropped by 24% as compared to standalone genset.
• Quick startup of Hybrid system makes it suitable for backup power applications unlike conventional SOFC based system having high start up time.
• Continuous power supply can be obtained from the Hybrid system.
• Reduction in NOx emissions from the PG set as the hybrid system operates under rich environment.
• No water storage is required in Hybrid system for producing steam which is generally required in conventional SOFC system.
• Can be integrated with available gensets at existing sites without adding any significant footprints.
• Fuel vaporizer device is not required as needed in conventional SOFC for pre-heating of liquid fuels like diesel. ,CLAIMS:1. A system for improving energy efficiency of Power Generator, comprising:
a power generator (PG) configured to receive a mixture of fuel and air and produce an exhaust gas stream;
a water-gas shift reactor (WGSR) provided downstream of the power generator for receiving the exhaust gas stream as a first input and steam as a second input and for producing an intermediate gas stream rich in hydrogen; and
a Solid Oxide Fuel Cell (SOFC) receiving the intermediate gas stream rich in hydrogen from the WSGR as a first input and a gas stream containing oxygen as a second input and for producing water, a spent gas stream and electrical current.

2. The system as claimed in claim 1, wherein an operation of the PG is controlled so as to produce the exhaust gas stream comprising 4 to 6 mole % of hydrogen and 4 to 6 mole % of carbon monoxide.

3. The system as claimed in claim 1, wherein the PG is operated at an air to fuel ratio in the range of 14 to 15 at a full load condition.

4. The system as claimed in claim 1, wherein a ratio of steam to carbon in the WGSR is in the range of 1.2 to 1.6.

5. The system as claimed in claim 1, wherein the intermediate gas stream produced by the WGSR comprises 8 to 11 mole % of hydrogen.

6. The system as claimed in claim 1, wherein the WGSR is operated at a temperature in the range of 340o to 360oC.

7. The system as claimed in claim 1, wherein the SOFC is operated at a temperature in the range of 650o to 750oC.

8. The system as claimed in claim 1, wherein the system comprises a first heat exchanger adapted to receive the gas stream containing oxygen from an air blower as a first input and the exhaust gas stream from the PG as a second input, the heat exchanger being further adapted to increase a temperature of the gas stream containing oxygen using the exhaust gas stream.

9. The system as claimed in claim 1, wherein a second heat exchanger adapted to receive the intermediate gas stream from the WGSR as a first input and the spent gas stream from the SOFC as a second input, the heat exchanger being further adapted to increase a temperature of the intermediate gas stream using the spent gas stream.

10. The system as claimed in claim 1, wherein the system comprises a third heat exchanger adapted to receive the gas stream containing oxygen as a first input and the spent gas stream from the SOFC as a second input, the heat exchanger being further adapted to increase a temperature of the gas stream containing oxygen using the spent gas stream.

11. The system as claimed in claim 1, wherein equivalence ratio (?) is in the range of 1.3 to 1.5.

12. A process for improving energy efficiency of Power Generator, comprising:
providing a power generator (PG) configured to receive a mixture of fuel and air and produce an exhaust gas stream;
providing a water-gas shift reactor (WGSR) downstream of the power generator for receiving the exhaust gas stream as a first input and steam as a second input and for producing an intermediate gas stream rich in hydrogen; and
providing a Solid Oxide Fuel Cell (SOFC) such that SOFC receives the intermediate gas stream rich in hydrogen from the WSGR as a first input and a gas stream containing oxygen as a second input and produces water, a spent gas stream and electrical current.