Download
1 / 45
1.76k likes | 3.74k Views
Lecture 15. Hydrogen Production . Hydrogen from water Hydrogen from biomass Using solar & nuclear energy . Hydrogen from Water. Hydrogen from Electrolysis Hydrogen from Photo-electrolysis Hydrogen from Nuclear Energy. Hydrogen from Water. Why from Water?
E N D
Lecture 15. Hydrogen Production • Hydrogen from water • Hydrogen from biomass • Using solar & nuclear energy
Hydrogen from Water • Hydrogen from Electrolysis • Hydrogen from Photo-electrolysis • Hydrogen from Nuclear Energy
Hydrogen from Water • Why from Water? • 1). Most of hydrogen stored in water; • 2). Water is abundant and renewable; • 3). No pollution generated in the process, only H2 and O2 produced; • 4). Could use renewable energy or nuclear energy to produce H2; • 5). All other advantages of using H2.
Hydrogen from Water • Major processes: • 1). Electrolysis of water using electricity; • 2). Photovoltaic-powered electrolysis; • 3). Methane steam reforming; • 4). Direct thermal water-splitting; • 5). Indirect thermochemical water-splitting; • 6). Other chemical and electrochemical methods.
Electrolysis of Water • Major components of an electrolyzer : • 1). Container; • 2). Electrolyte; • 3). Anode (positive electrode); • 4). Cathode (negative electrode); • 5). Separator. • Hydrogen flammable range: • 1). In air: 4-75% • 2). In pure O2: 4-94%
Electrolysis of Water • Electrolysis is the opposite of a fuel cell: • 1). At cathode: 4H+ + 4e- 2H2 • 2). At anode: 2 H2O O2 + 4H+ + 4e- • 3). H2 produced is very pure • 4). Pure water has high electric resistance (100 ohm/cm) • 5). Electrolyte is added to improve conductivity
Electrolysis of Water • Electrolyzer Polarization Curve
Electrolysis of Water • 6). Thermal efficiency of an electrolyzer is 30-35% • =Energy output/energy input • 7). Cell voltage efficiency (60-70%) • =Minimum V needed/Actual V • 8). Generating H2 at high pressure:
Electrolysis of Water • Photo-electrolysis: • 1). Electricity generated from photovoltaic cells; • 2). Electricity used for electrolysis; • 3). Solar to H2 efficiency of 12.4% using concentrated light; • 4). Solar to H2 efficiency of 7.8% under natural sunlight.
Thermolysis of Water • Direct thermal water-splitting: • H2O H2 + 1/2O2 • Water Dissociation at Various Temperatures
Thermolysis of Water • 1). High temperature reaction; • 2). Only 10% conversion at 2500 C; • 3). Reverse reaction of forming water is a serious issue; • 4). Solar energy or nuclear energy is used; • 5). A set of reaction is used to store solar energy: • Heat + CH4 + CO22CO + 2H2 (Dry reforming) • 2CO + 2H2 Heat + CH4+CO2 (Methanation)
Thermochemical Water-Splitting • Thermochemcial water-splitting is the conversion of water into hydrogen and oxygen by a series of thermally driven chemical reactions • More than 1000 reactions/cycles are available for hydrogen production from water-splitting reaction. The following Sulfur-Iodine cycle is an example of these reactions. This so-called advanced fuel cycle is being investigated at Sandia National Laboratory for potential hydrogen production.
Thermochemical Water-Splitting • Indirect thermochemcial water-splitting (S-I cycle): • H2SO4SO2 +H2O +1/2O2 (850C) • I2 +SO2 + 2H2O 2HI + H2SO4 (120) • 2HI I2 + H2 (450) • H2O H2 + 1/2O2 • The S-I cycle has significant conversion at much lower temperatures. With a suitable catalyst the high temperature decomposition of sulfuric acid achieves 10% conversion at 510C, and 83% at 850C. Moreover there is no need to perform high temperature separation as the reaction ceases when the stream leaves the catalyst.
Thermochemical Water-Splitting • Indirect thermochemcial water-splitting (S-I cycle): 1st reaction 2nd reaction 3rd reaction
Thermochemical Water-Splitting Energy, in the form of heat, is input to a thermochemical cycle via one or more endothermic high-temperature chemical reactions. Similar to the way that a heat engine must reject heat to a low temperature sink, a thermochemical cycle rejects heat via one or more exothermic low temperature chemical reactions. Finally, other thermally neutral chemical reaction may be required to complete the cycle so that all the reactants, other than water, are regenerated. In the case of the S-I, cycle most of the input heat goes into the oxygen generating reaction, the dissociation of sulfuric acid. Sulfuric acid and hydrogen iodine are formed in the endothermic reaction of the S-I cycle and the hydrogen is generated in the mildly endothermic decomposition of hydrogen iodine.
Thermochemical Water-Splitting • Gas separation needs in S-I cycles: • O2 & steam removal from H2SO4 decomposition reactants • Benefits of removing O2 and steam: • Increase H2SO4 conversion efficiency; • Concentrate SO2 for the 2nd reaction; • Reduce reactor size for the 2nd reaction.
Thermochemical Water-Splitting • Gas separation needs in S-I cycles: • H2SO4 /steam separation in a membrane process • Benefits: • Less energy consumption than distillation; • Concentrate H2SO4 for the 1st reaction; • Reduce the reactor size for the 1st reaction.
Thermochemical Water-Splitting • Gas separation needs in S-I cycles: • H2 separation and purification process • Benefits: • Increase HI conversion efficiency; • Concentrate I2 for the 2nd reaction; • Reduce reactor size for the 2nd reaction; • Purify H2 for down stream applications.
Thermochemical Water-Splitting • Gas separation needs in S-I cycles: O2-transport membrane layer H2SO4 SO2, H2O O2 Sulfuric acid decomposition catalyst
Hydrogen from Biomass • Hydrogen from Biofuels • Biomass Pyrolysis • Photosynthesis Production of H2
Biological Production of Hydrogen • Hydrogen from Biofuel • 1). Unit: Tonnes per hectare (t ha-1) • 2). Method of using biofuels: • Direct combustion • Conversion to biogas (pyrolysis, hydrogasification, anaerobic digestion) • Conversion to ethanol via fermentation • Conversion to syngas, and then methanol or ammonia • Conversion to liquid hydrocarbons by hydrogenation (Fischer–Tropsch) • Convert to biogas, then feed to a fuel cell (future application)
Biological Production of Hydrogen • 3). Biogas composition
Biological Production of Hydrogen • Biological extraction of hydrogen from fuels • 1). Photosynthesis using unicellular microorganims that use either • hydrogenase or nitrogenase reactions. • 2). Fermentation using bacteria to produce hydrogen • 3). Various stepwise processes that use a combination of bacteria to • predigest complex organic molecules to make less complex organic • matter that can be transformed using hydrogen-producing organisms. • So far the biological process is not very successful.
Biological Production of Hydrogen • Photosythesis: • 1). An improved process for H2 production is being developed at NETL; • 2). Researchers in Japan found that CO2 can be reduced to starch by • photosyntheis in the presence of light. The starch can further • decomposed to H2 and alcohol under anaerobic condition. • 3). Nitrogenase is an enzyme responsible for catalysing the fixation of • stmospheric nitrogen. It could also produce H2 and O2 simultaneously. • 4). Certain phoyosynthesis bacteria can utilize CO and H2O to produce H2 • and CO2 (water-gas shift reaction) at low temperature. • 5). All the photosynthesis schemes used so far have low efficiencies.
Biological Production of Hydrogen • Digestion process: • 1). H2 production from microbial digestion of organic matter without light; • 2). The process is very slow due to: inhabitation of hydrogenases growth at • high H2 partial pressure; and H2 will react with organics and CO2 in the • system. • 3). Increasing H2 production and preventing methane formation are the key. • 4). Fundamentals of biological process need to be understood.
