Investigation Of A Chemical Question Essay Research

Investigation Of A Chemical Question Essay, Research Paper

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 sm