Integrated biomass gasification and fuel cell system

Details Integrated biomass gasification and fuel cell system Integrated biomass gasification and fuel cell system

2002-04-15

2004-01-20

Ryan, Patrick (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Having magnetic field feature

C429S010000, C429S010000, C422S139000, C048S1970FM

Reexamination Certificate

active

06680137

ABSTRACT:

FIELD OF INVENTION
This invention relates to a system and method for generating energy from a variety of biomass feedstocks, and more particularly to a system for generating energy which a biomass gasifier system in conjunction with an integrated fuel cell.
BACKGROUND OF THE INVENTION
Fuel cells have long been used in the space program to provide electricity and drinking water to astronauts. In the future, the electric power industry is expected to be an area where fuel cells will be widely commercialized. The electric power industry has generally been looking toward the use of fuel cells in relatively large electrical power generating applications. Power generation by fuel cells offers the advantages of high efficiency and low environmental emissions. Thus, fuel cells may offer a more economical means of power production than other existing power producing technologies.
Fuel cells produce electrical power by converting energy from the reaction of various products directly into electrical energy. An input fuel is chemically reacted in the fuel cell to create an electrical current. An electrolyte material is sandwiched between two electrodes, an anode and a cathode, making up the fuel cell. The input fuel passes over the anode, where it splits into ions and electrons. The electrons go through an external circuit to serve an electric load while the ions move through the electrolyte toward the oppositely charged electrode. At the electrode, ions combine to create by-products, primarily water and carbon dioxide. Depending on the type of electrolyte used in the fuel cell, different chemical reactions will occur.
For example, in some systems, hydrogen rich fuels and an oxidant gas, such as air are fed into a fuel cell stack, a series of electrode plates interconnected to produce a set voltage of electrical power. Typically, the hydrogen rich fuel gas is fed to the anode of the cell, while the cathode receives oxidant gas or air. Internal reforming of any hydrocarbons present in the fuel gas occurs at the anode. The reformed fuel gas in the anode compartment and the oxidant gas in the cathode compartment, in the presence of the electrolyte of the cell, undergo electrochemical conversion to generate electrical power.
There are several different types of fuel cells, the parameters of which can vary depending on what the cell will be used for, the structure of the cell and the materials used. These include proton exchange membrane fuel cells (PEFC), phosphoric acid fuel cells (PAFC), solid oxide fuel cells (SOFC) and molten carbonate fuel cells, among others.
Molten carbonate fuel cells (MCFC) use a molten carbonate salt mixture as an electrolyte. The composition of the electrolyte varies, but may consist of lithium carbonate and potassium carbonate. At the operating temperature of about 1200° F., the salt mixture is liquid and a good ionic conductor. The electrolyte is suspended in a porous, insulating and chemically inert ceramic (LiAlO
2
) matrix. The chemical reactions of the MCFC are as follows.
Anode reaction:
H
2
+ CO
3
−2
→ H
2
O + CO
2
+ 2e
−
CO + CO
3
−2
→ 2CO
2
+ 2e
−
Catode reaction:
O
2
→ 2CO
2
+ 4e
−
→ 2CO
3
−2
Solid oxide fuel cells (SOFC) use a ceramic, solid-phase electrolyte which reduces corrosion considerations and eliminates eletrolyte management problems sometimes associated with liquid electrolyte fuel cells. A preferred ceramic is yttria-stabilized zirconia, an excellent conductor of negatively charged oxide ions at high temperatures. The anode is preferably porous nickel/zirconia cement, while the cathode is preferably a magnesium-doped lanthanum manganate. The SOFC reactions are as follows.
Anode reaction:
H
2
+ O
−2
→ H
2
O + 2e
−
CO + O
−2
→ CO
2
+ 2e
−
CH
4
+ 4O
−2
→ 2H
2
O + CO
2
+ 8e
−
Cathode reaction:
O
2
+ 4e
−
→ 2O
−2
Phosphoric acid fuel cells (PAFC) uses liquid phosphoric acid as the electrolyte. The acid is contained in a TEFLON bonded silicone carbide matrix, the small pore structure of which keeps the acid in place through capillary action. Platinum catalyzed, porous carbon electrodes are used on both the anode and the cathode sides of the electrolyte. The PAFC reactions that occur are as follows.
Anode reaction:
H
2
→ 2H
+
+ 2e
−
Cathode reaction:
½ O
2
+ 2H
+
+ 2e
−
→ H
2
O
Proton exchange membrane fuel cells (PEFC) use a polymer membrane as the electrolyte. The membrane is an electronic insulator, but an excellant conductor of hydrogen ions. The PEFC membrane consists of fluorocarbon polymer materials, for example TEFLON, to which sulfonic acid groups are attached. The protons on these acid groups are free to migrate through the membrane. Platinum is used at both the anode and the cathode.
The electrode reactions in the PEFC are analogous to those in the PAFC, and are as follows.
Anode reaction:
H
2
→ 2H
+
+ 2e
−
Cathode reaction:
½ O
2
+ 2H
+
+ 2e
−
→ H
2
O
Molten carbonate fuel cells and solid oxide fuel cells are well suited for using heated gas streams and, thus, have shown the most promise in industrial power generation applications. There are several known sources for fuel gas suitable for use in these fuel cells. Natural gas may be used as a fuel, although it may be necessary to use a fuel processor to boost the concentration of hydrogen present in the natural gas. Fuel gas may also generated in coal gasifiers, which generate hydrogen, carbon monoxide and carbon dioxide has also been found suitable for use as a fuel gas to feed fuel cells. Additionally, biomass gasifiers are also known in the art and have been found useful for the production of fuel gases in remote areas or in areas wherein a large amount of agricultural biomass waste is produced.
Greater efficiency in conventional fuel cells may be obtained through integration with coal or biomass gasifiers. For example, U.S. Pat. No. 4,921,765 to Gmeindl et al. discloses a combined gasifier and fuel cell system wherein the gas stream travels from the gasifier through an external carbon dioxide separator. In the Gmeindl et al. fuel cell system, the anode reaction gases are recycled to provide the steam and heat needed to support the gasifier. The process disclosed in the Gmeindl process uses coal or coal char to feed the system.
U.S. Pat. No. 5,554,453 to Steinfeld et al. discloses a carbonate fuel cell system with thermally integrated gasification. The system disclosed by Steinfeld uses a portion of the output gas from a gasifier as the fuel gas for a molten carbonate fuel cell (MCFC). The remainder of the output gas is combusted to provide heat for driving the gasification reaction and to produce a CO
2
rich exhaust gas. The CO
2
rich exhaust gas is mixed with air and used as the oxidant gas at the cathode of the fuel cell. Steinfeld discloses system configurations, one wherein a catalytic combustor is situated within the gasifier and the other with a catalytic combustor situated externally to the gasifier. Each of Steinfeld's fuel cell systems require either hot or cold gas clean-up, followed by expansion to provide moisturization of the gas. The Steinfeld et al. fuel cell system may be suitable for use with either a coal gasifier or with some biomass gasifiers.
Biomass gasification systems known in the art generally rely on combustion of a portion of the biomass feedstock to provide the heat required for gasification of the remainder of the biomass feedstock. However, the combustion of a portion of the raw biomass stream for heat production can significantly reduce the overall efficiency of the gasifier system. As a result, these systems generally operate at an efficiency of less than 25% overall conversion efficiency to electrical power.
Higher efficiencies, approaching 60% have been achieved using the combustion of natural gas to provide heat for the

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