How Is Hydrogen Gas Generated?
Diatomic hydrogen gas (H2) has been an integral part of industrial chemistry, petrochemical engineering and scientific research for well more than a century. Several efficient techniques for generating hydrogen gas currently exist; the preferred technique ultimately depends on the volume of gas desired, the industry and the resources available to the chemist/engineer.
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Steam Methane Reforming (SMR)
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As of March 2010, SMR accounts for nearly 95 percent of commercial hydrogen gas production. Natural gas, which consists primarily of methane (CH4), is heated with steam (i.e., gaseous water) to temperatures between 700 and 1100 degrees Celsius while flowing through a nickel catalyst. The resulting gaseous mixture (a.k.a. "syngas") enters into a chemical equilibrium with the reactants CH4 and H2O on one side and the products carbon monoxide (CO) and hydrogen gas (H2) on the other.
Next, the syngas is diverted to a chamber where cooler steam (350°C) is added, causing the equilibrium to shift toward a higher H2 concentration. After being diverted to a new, cooler steam chamber (190°C), the syngas is rich with hydrogen and ready to be separated into pure H2 gas.
Gasification of Coal
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Coal gasification is essentially a form of steam reforming. The main difference between this method and SMR is the process for creating the syngas. In coal gasification, coal (i.e., elemental carbon) is heated to its spontaneous combustion temperature in an wet, oxygen-deficient environment. This causes the coal to burn incompletely; instead of forming carbon dioxide (CO2), the carbon partially oxidizes into CO. Likewise, some of the carbon even ends up reducing the H2O molecules, creating CO and H2 instead. The final result is a syngas equilibrium mixture of CO, H2 and water.
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Chlorine Electrolysis
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The electrolysis of a sodium chloride solution (a.k.a. brine) is the primary method for the commercial production of chlorine gas (Cl2). However, for every mole of chlorine produced, one mole of hydrogen gas is produced as a byproduct. In this reaction, a concentrated solution of NaCl is poured into a vessel with an electrical terminal at each end. Because the Na+ and Cl- electrolytes conduct electricity, electrical current can pass through the water between the two terminals. When sufficient electrical voltage is applied across the solution, H2 gas forms at one terminal while Cl2 forms at the other. The remaining Na+ and OH- ions in the solution form caustic soda (NaOH).
Water Electrolysis
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In water electrolysis, a small amount of electrolyte (e.g., H2SO4, KOH, NaOH) is added to water to allow for electrical conductivity. The apparatus is identical to that of chlorine electrolysis. However, instead of forming hydrogen and chlorine gases, water electrolysis forms hydrogen and oxygen (O2) gases.
Biomass Reactor
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Certain microorganisms like green algae and cyanobacteria naturally produce hydrogen gas as a metabolic waste. The idea behind a biomass reactor is to divert sewage and other organic waste into chambers filled with genetically engineered microorganisms that will gradually consume the waste and produce hydrogen gas.
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