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CHEMISTRY CAT 2: INVESTIGATION OF A CHEMICAL QUESTION Question: How does ceramic fuel cells work and is it going to be the future energy source? DEFINITION, KEY IDEAS AND CHEMICAL CONCEPTS: A ceramic fuel cell is an all solid-state energy conversion device that produces electricity by electrochemically combining fuel and oxidant gases across an ionic conducting oxide. This third generation fuel cell is the most versatile with the highest conversion efficiency rate and very flexible in choosing the fuel sources. In this project, my key ideas are:- The range of energy sources available to society and their future development.- The efficiency of energy conversion.and the chemical concepts would be:- Energy- Chemical reactions WHAT IS CERAMIC FUEL CELLS AND HOW DOES IT WORK? A fuel cell converts fuels such as hydrogen, natural gas gasified coal directly into electricity electrochemically without the limitation of the Carnot cycle. It operates like a battery except that it produces power continuously, when being supplied with fuel and oxidant and no need for recharging. Fig.1: Solid Oxide Fuel Cell – operating principleCeramic fuel cells, commonly referred to as solid-oxide fuel cells (SOFCs), are presently under development for a variety of power generation applications. Its operating principle is shown above. The SOFC is an all ceramic device. It operates at around 900-1,0000C and uses a ceramic membrane of zirconia (doped with another metal oxide), an excellent oxygen-ion conductor at high temperatures. It acts as a solid electrolyte between a pair of porous electrodes in contact with the air and the fuel. Oxygen is taken up at the air electrode (cathode) and converted to oxygen ions which diffuse through the zirconia and react with the fuel at the fuel electrode (anode). The electrons flow through the external load and reduce oxygen at the cathode. The charge flow in the external circuit is balanced by ionic current flow within the electrolyte and the equations represent the process are in (Fig.1). POWER OUTPUT AND TYPES OF DESIGN: Currently the voltage from a single cell under load conditions is around 0.6 to 1 V DC and current densities are ranging from 250 to 500 mAcm-2 . The open circuit or reversible voltage (Er) of the cell is given by the free energy (DG) of the fuel oxidation reaction: DG: the free energy of fuel oxidation reaction (DG = -nErF) n: number of electrons transferred F: Faraday constant Typical output from a cell is 2-3 kW/m2. Several of these cells connected in series though an interconnect material with high electronic conductivity from a fuel cell stack or module. A complete fuel cell power plant, in addition, consists of a fuel and air supply system, power conditioner (DC/DC or DC/AC converter) and waste heat processing or also known as the recovery system (Fig. 2). Fig. 2: A complete fuel cell power plant Three major design concepts of SOFC stacks currently under development are:- Tubular design has been built to 25 kW by Westinghouse, and is undergoing trials. This design is the safest and allows staged injection of reactants, but its disadvantages are low power density (+150-200 kW/m3) and requires exotic and expensive ceramic fabrication techniques.- The monolithic design demands more advanced materials but offer much higher power density (1.5 to 3 MW/m3). However, problems with seals between cells to avoid leakage and minimise interfacial resistance have to be resolved.- The planar design appears to be the ideal developments in the future due to the low fabrication costs. The power density forecast for planar cell design is + 1MW/m3 . EFFICIENCY BETWEEN VARIOUS FUEL CELLS AND THERMAL POWER PLANT: The efficiency of a fuel cell is defined in terms of the electric and fuel efficiencies. The fuel efficiency (jF) is the ratio of free energy (DG) and free enthalpy (DH) of the fuel oxidation reaction: jF(%) = (DG/DH) 100 = (1 – (TDS/DH) 100 = DS: entropy term (-nErF/DH) 100 Fuel efficiencies increase or decrease with temperature depending upon whether DS is positive or negative.In a fuel cell there are internal losses within the electrolyte (resistive, IR) and at both electrode/electrolyte interfaces (overpotential, h). The useful voltage, E, available from a fuel cell is given by: E = Er -IR – h The electric efficiency (jE) is defined as the ratio E/Er and the fuel/electric efficiency (jFE) equals the product of fuel and electric efficiencies (jF + jE). The remaining chemical energy is available as heat.In contrast, in a conventional thermal power plant, the chemical energy of the fuel oxidation reaction is first converted to heat by burning the fuel. Only part of this energy is converted to mechanical work (W) in the typical thermal generator. The theoretical limit is the Carnot cycle efficiency which depends on the initial (T) and final working temperature (To): jCarnot (%) = (W/DH) 100 = (1 – To/T) 100 As stated above, the major difference between a fuel cell and a thermal power plant is that in a fuel cell chemical energy of the fuel is converted directly to electric power without conversion first to heat. The efficiency of a coal fired thermal power plant is typically in the range of 30 – 35%. In a combined cycle gas turbine system running on natural gas as the maximum efficiency is in the range 45 – 50% . Since these combustion engine and gas turbine technologies are already fully developed, only small incremental efficiency improvements are likely to occur in the future.Similarly, SOFC’s efficiency is in the range 60 – 75% but future development will increase this percentage since new technologies are developed faster and more reliable. COMPARISON BETWEEN TYPES OF FUEL CELLS: The following table contains the characteristics of the six major types of fuel cells that are currently under development. They are, AFC, DMFC, PEMFC, PAFC, MCFC, and SOFC. The efficiency rate of solid oxide fuel cells is the highest in the pack but also revealing some disadvantages .
Fuel Cell Operating Temp. 0C Waste heat 0C Fuel (Direct) Oxidant Efficiency* % Capacity (kW)Alkaline Fuel Cell (AFC) 70-90 50-60 H2 Air, O2 35-40 10-100Direct Methanol Fuel Cell (DMFC) 70-100 50-60 CH3OH Air, O2 40-50 Proton Exchange Membrane Fuel Cell (PEMFC) 80-120 60-80 H2 Air, O2 35-40 5-100Phosphoric Acid Fuel Cell (PAFC) 190-215 80-120 H2 Air, O2 35-42 5, 40, 200, 1MW, 11MWMolten Carbonate Fuel Cell (MCFC) 650 500-600 H2, CO, CH4, CH3OH Air, O2 55-65 20Solid Oxide Fuel Cell (SOFC) 900-1000 700-900 H2, CO, CH4, CH3OH Air, O2 60 (70-75) 3, 5, 25* Except DMFC all other based on CH4 fuel Table 1: Major types of fuel cells and their characteristics. In the above fuel cells, particularly the second (MCFC) and third (SOFC) generation fuel cells with internal fuel reforming promise to become the most efficient power generation systems. Several low and medium temperature fuel cells (AFC, PEMFC, PAFC) only operate on hydrogen as fuel and thus result in their efficiency rate is below 50%. This is due to the low energy content of hydrogen and the inflexible of the fuel sources. However, total systems efficiencies have to account for the process of producing hydrogen from eg. natural gas, liquid hydrocarbons or methanol. Most commonly fuel processor units use steam reforming of hydrocarbon fuels or methanol followed by water gas shift reaction to convert CO to Hydrogen and CO2, one is endothermic process which requires energy input and the other is exothermic requiring heat removal. All these steps consume energy resulting in further efficiency losses. Electric efficiency (based on methane, methanol fuel) for PAFC system is in the range 35 to 42%, for AFC and PEMFC systems it is around 35% . FUEL CELL CATHODE (+) ANODE (-) AFC O2 + 2H2O + 4e- x 4OH- 4OH- + 2H2 x 2H2O + 4e-DMFC 3O2 + 12H+ + 12e- x 6H2O CH3OH + H2O x CO2 + 6H+ + 6e-PEMFC O2 + 4H+ + 4e- x 2H2O 2H2 x 4H+ + 4e-PAFC O2 + 4H+ + 4e- x 2H2O 2H2 x 4H+ + 4e-MCFC 2CO2 + O2 + 4e- x 2CO32- CO32- + CO x 2CO2 + 2e-CO32- + H2 x CO2 + H2O + 2e-SOFC O2 + 4e- x 2O2- H2- + O2- x H2O + 2e-CO + O2- x CO2 + 2e-CH4 + 4O2- x CO2 + 2H2O + 8e-Table 2: Operating equations for various Fuel Cells From the above table, it clearly shows that the SOFC is very flexible in term of fuel sources using in the cell. The three equations indicate a wider range of chemicals (fuels) can be used in SOFC, thus, it is a greatest advantage comparing to other cells that have limited fuel options. These fuels are also cheap and abundant in the future. They offer the same potential electrical supply to produce electricity and offering new technology developments with are easily accessed from the wide range of options as mentioned above. Besides the highest efficiency rate, SOFC requires very high temperature to operate and producing far too much heat than the other fuel cells. Moreover, the efficiency rate can be enhanced by the recovery of high quality heat. A number of heat recovery options were considered e.g. heating, cooling and hot water. The level of waste heat recovery is substantially lower when used for cooling compared to heating or hot water production. System efficiencies are improved if waste heat is recovered for heating and for hot water supply in the plant. Fig. 3 shows a plot comparing electric efficiencies of different power generation technologies as a function of power plant capacity. Fig.3: A comparison of fuel/electric efficiencies of different power generations High temperature fuel cells (MCFC and SOFC) are fuel flexible and the efficiencies for single power generation cycle of 50% and above are possible. With the combined internal fuel reforming, expander gas and steam turbines, and heat recovery system, the total system efficiencies up to 85% is achievable. TECHNOLOGICAL, ECONOMICAL AND ENVIRONMENTAL FACTORS: Fuel cells have the potential to reduce the environmental impact of conventional power generations. Emissions of environmental hazardous compounds from fuel cell power plants are dramatically lower compared to other technologies. Fig. 4 shows emissions of pollutants for different technologies. Fig. 4: Pollutant emissions for different power generation technologies The graph clearly shows the advantages between conventional power stations and the experimental fuel cells in term of emitting Nox and SOx gases into the atmosphere. Emissions of Nox from reciprocating engines range from around 1000-2500 ppm to 100 ppm (with emission control device). Gas turbines in the smaller capacity range produce 100-200 ppm uncontrolled and 20-60 ppm with pollution control. And as fuel cells require fuel poor in sulfur compounds, SOx emissions are negligible. The fuel cells simply out perform the current power station. The efficiencies and environmental friendliness of third generation fuel cells (SOFC) promise a bright future ahead comparing to conventional power generations. With more than twice the efficiency rate with other sources of electricity generations, SOFC can bring new developments for future industry. CONCLUSION: Fuel cells are expected to play an important role in the power generation industry in the future. Debates and actions in green-house gases and air pollution in general could severely restrict opportunities for expansion of capacity in the power generation industry. The development of fuel cell power plants, especially the third generation solid oxide fuel cell, offers for the first time the opportunity to break through the low efficiency barriers by the thermal generation technologies. In addition, SOFC is a new technology and currently under development by several major organisations and companies around the world. It shows the importance of research and development of new energy resources before fossil fuel runs out in 90 years or so. In future, dispersed fuel cell units running on natural gas or central power stations combining ‘clean’ and economic coal gasification with a fuel cell power plant will prove attractive options for electricity generation. From the report, I think that SOFC is not a good alternative for current power stations due to its wide range of disadvantages. Hopefully in the next ten years or so when new technologies allow a more efficient reheating system to be available and then SOFC would be the best option for future power plants.
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