Jul 19 2008
Conventional Hydrogen
According to CAN Europe estimation, global hydrogen production is at around 45 million tons per year or 500 million cubic metres per year. Half of the global production is used in the manufacture of ammonia-based fertilisers and 1 % of global primary energy demand used for energy purposes. For example, U.S. demand for hydrogen currently is about 9 million tons per year and this always growth every year.
There are various of conventional hydrogen production, which are :
|
|
Fossil Fuel Reforming
Steam Reforming (SR) - natural gas reacts with steam at 3 - 25 bar pressure (1 bar = 14.5 psi) and temperature 700 - 1,000°C (endothermic/heat absorbing reaction) with the help of a catalyst, thus producing the synthetic gas (syngas). Syngas is a mixture of hydrogen with carbon monoxide. Then by water-gas shift reaction, syngas react with steam to turn CO into CO2 and release more of hydrogen and heat at temperature 200 - 500 °C (exothermic/heat releasing). After that, hydrogen-rich gas undergoes the purification, and carbon dioxide and trace amount of carbon monoxide removed, usually called by pressure-swing adsorption.
CnHm + n H2O ? n CO + (m/2+n) H2 (endothermic at 700 - 1,000 °C)
CO + H2O ? CO2 + H2 (exothermic at 200 - 500 °C)
According to DOE, there are about half of world hydrogen production used this method. But when we look about the cost of hydrogen fuel, this method still very expensive. The cost of producting and delivering hydrogen from small scale of natural gas reformer to fuel cell application could reach high as $40 per million BTU with current technology - about four times more expensive with gasoline at the pump untaxed (DOE, October 28, 2005).
Coal Gasification - coal reacting with oxygen and steam at high pressure and high temperature to form a syngas. Then syngas enter the water-gas shift reaction to release more hydrogen dan reducing the carbon monoxide.
Partial Oxidation (POX) - this reaction perform by limited amount of oxygen that make incomplete heavy hydrocarbons (diesel, oil) oxidation. This reaction release primarily hydrogen and carbon monoxide, nitrogen (when oxygen taken from the air) and small of carbon dioxide. POX is exothermal reaction and typically much faster reaction than SR and just requires a smal reactor vessel. But POX release hydrogen product less than SR with same amount of raw material.
CnHm + (n/2) O2 ? n CO + (m/2) H2 (exothermic)
CO + H2O ? CO2 + H2 (exothermic at 200 - 500 °C)
Autothermal Reforming (ATR) - combination of SR and POX that could achieve higher efficiencies at high temperature and can applied to produce hydrogen from methanol.
Biomass Gasification
Reforming gasified biomass with rich oxygen as gasifying agent can produce 12% - 17% hydrogen by weight of the dry biomass according to some sources. This method has high efficiency, but limited by the lower energy content of biomass compared to fossil fuels. This is the general chemical reaction for bio-oil steam reforming :
CnHmOk + (2n - k) H2O ? n CO2 + (2n + m/2 - k) H2
The mainly technical problems that faced by biomass gasification is have much higher concetrations of Chlorine, Nitrogen, Sulphur, and Alkalis - kinds of air pollutant.
Advantage Hydrogen Production via Steam Reforming
Compare with Internal Combustion Engine (ICE) vehicles that using gasoline, ICE vehicles powered by hydrogen from reforming of natural gas could reduce greenhouse gas emissions by 60% (EERE of U.S. Department of Energy). EERE also write, in order for hydrogen economy enter transportation market, hydrogen need to be available at $2 - $3 per gge (untaxed).
Read More the Related Posts: