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Hydrogen: How It Works
While much talk of a hydrogen economy focuses on starting to use hydrogen, roughly 42 million metric tons are produced and consumed annually. Nearly all of this is produced from fossil fuels, with natural gas accounting for 48%, 30% from oil and 18% from coal, with the remaining 4% from water electrolysis. Roughly 60% of produced hydrogen is used as a feedstock to produce ammonia, which is then used for fertilizer, 23% for petroleum refining, 9% to manufacture methanol and the remainder for other industrial purposes. Less than 1% is used for fuel. Hydrogen is produced through a number of methods, each of which have unique feedstocks, energy usage and outputs. The only methods listed below are those that can be processed without fossil fuels. Biomass Gasification - This method converts biomass into hydrogen by applying heat to pressurized steam and a controlled amount of oxygen. The heat, steam and oxygen break the biomass into syngas, which is a mixture of hydrogen, carbon monoxide and carbon dioxide. Further hydrogen is produced as the carbon monoxide reacts with the water to form more carbon dioxide and hydrogen. The hydrogen is then separated from this gas stream. This is similar to the first step of the Gasification process of producing Cellulosic Ethanol. Hydrogen can also be produced less efficiently through Biomass Gasification in an oxygen-free environment, but at the expense of creating additional hydrocarbon compounds which requires additional steps to clean the syngas stream. Electrolysis - This is the primary method for producing hydrogen in a small scale, such as on-board hydrogen generation. An electric charge is applied to water, which splits the water by breaking the hydrogen-water bond. Most power used for electrolysis (50kWh electricity : 1kg hydrogen) is obtained from fossil fuel sources, but can be used with renewable sources to reduce emissions. Electrolyzers can also be configured to pressurize the hydrogen as part of the production process. High-Temperature Electrolysis (HTE) - Under this method, sources of heat/steam are used to break the hydrogen-oxygen bond of water in an electrolyzer. High temperatures produce a more efficient break of this bond and does not require the added step of converting the heat/steam into electricity. Geothermal and possibly concentrated solar could be used, however nuclear is seen as the most efficient and consistent option for HTE. Partial Oxidation - This method burns methane in air, but not enough to convert the hydrocarbons into carbon dioxide and water. The reduced oxygen level reacts to produce hydrogen and carbon monoxide, as well as nitrogen if reacted in air instead of oxygen. To be fully renewable, the methane to use in this method could be obtained from anaerobic digesters. Photobiological - Different types of algae and cyanobacteria can be used in a bioreactor where these microbes will consume oxygen in the presence of sunlight and absence of sulfur and excrete hydrogen. Photoelectrochemical - A photovoltaic solar cell that powers an electrolyzer to produce hydrogen, making it the cheapest (on an ongoing basis) and cleanest hydrogen production method. Steam Methane Reforming - This method introduces high temperature, low pressure steam into a natural gas stream to extract hydrogen from the methane within the natural gas. This method accounts for 95% of the hydrogen produced in the U.S., according to the DOE. Carbon monoxide and carbon dioxide are also produced in this process. To be fully renewable, the methane to use in this method could be obtained from anaerobic digesters. Thermochemical - There are 353 chemical combinations that have been identified to split the hydrogen-oxygen bond of water without the use of electricity. Of these, only 12 are being explored as potentially feasible, but none have gone beyond small scale laboratory production. |
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