Hydrogen gas and its use in Iron and Steel industry
Hydrogen gas and its use in Iron and Steel industry
Hydrogen is a chemical element, ranking first in the periodic table with element symbol of ‘H’. The (atomic number of hydrogen element is 1 and atomic weight is 1.008. It is the smallest atom in the universe and the simplest element in nature. Its molecule consists of two hydrogen atoms. It is the lightest gas, being about 1/14 times as dense as air. It has three isotopes named (i) protium, (ii) deuterium, and (iii) tritium. Pure hydrogen is odourless, colourless and tasteless.
Hydrogen has lowest atomic weight of any substance and therefore has very low density both as a gas and a liquid. The vapour density of hydrogen at 20 deg C and 1 atmosphere pressure is 0.08376 kg/cum. The specific gravity of gaseous hydrogen is 0.0696 and hence, it has around 7 % the density of air. The density of liquid hydrogen at normal boiling point and 1 atmosphere pressure is 70.8 kg/cum. The specific gravity of liquid hydrogen is 0.0708 and is thus, it has around 7 % the density of water.
Hydrogen is a liquid below its boiling point of -253 deg C and a solid below its melting point of – 259 deg C at atmospheric pressure. It is non-toxic but can act as a simple asphyxiant by displacing the oxygen in the air. When hydrogen is stored as a high-pressure gas at 250 kg/cum and atmospheric temperature, its expansion ratio to atmospheric pressure is 1:240.
The molecules of hydrogen gas are smaller than all other gases, and it can diffuse through many materials considered airtight or impermeable to other gases. This property makes hydrogen more difficult to contain than other gases. Leaks of liquid hydrogen evaporate very quickly since the boiling point of liquid hydrogen is so extremely low. Hydrogen leaks are dangerous in that they pose a risk of fire where they mix with air. Hydrogen leaks pose a potential fire hazard.
Hydrogen is chemically stable at room temperature, which is mainly determined by the strong covalent bond between the hydrogen atoms consisting of hydrogen. Hydrogen molecule is a stable molecule with high bond energy (104 kcal/mol), but it reacts with many different kinds of elements to form compounds with them.
Hydrogen has reducing properties. It easily reacts (burns) with oxygen at a wide range of mixing ratios and forms water. This also makes it possible to use hydrogen as an energy medium.
The energy density of hydrogen is poor (since it has such low density) although its energy to weight ratio is the best of all fuels (because it is so light). The energy density (lower heating value, LHV) of hydrogen gas at 1 atmosphere pressure and 15 deg C is 2400 kcal/cum and that of liquid is 2030 Mcal/cum.
Hydrogen, as a flammable gas, mixes with oxygen whenever air is allowed to enter a hydrogen vessel, or when hydrogen leaks from any vessel into the air. Ignition sources take the form of sparks, flames, or high heat. Flash point of hydrogen is less than -253 deg C.
Hydrogen is flammable over a very wide range of concentrations in air (4 % to 75 %) and it is explosive over a wide range of concentrations (15 % to 59 %) at standard atmospheric temperature. Hydrogen gas can also explode in a mixture of chlorine (from 5 % to 95 %). The flammability limits increase with temperature. As a result, even small leaks of hydrogen have the potential to burn or explode. Leaked hydrogen can concentrate in an enclosed environment, thereby increasing the risk of combustion and explosion. The combustion of hydrogen is described by the equation H2 + O2 = 2H2O + 136 kcal.
The auto-ignition temperature of hydrogen is relatively high at 585 deg C. This makes it difficult to ignite a hydrogen/air mixture on the basis of heat alone without some additional ignition source. Pure hydrogen-oxygen flames emit ultraviolet light and are invisible to the naked eye. As such, the detection of a burning hydrogen leak is dangerous and requires a flame detector. Hydrogen has a very high research octane number (+130) and is therefore resistant to knock even when combusted under very lean conditions.
Hydrogen does form compounds with most elements despite its stability. When participating in reactions, hydrogen can have a partial positive charge when reacting with more electronegative elements such as the halogens or oxygen, but it can have a partial negative charge when reacting with more electropositive elements such as the alkali metals. When hydrogen bonds with fluorine, oxygen, or nitrogen, it can participate in a form of medium-strength non-covalent (intermolecular) bonding called hydrogen bonding, which is critical to the stability of many biological molecules. Compounds that have hydrogen bonding with metals and metalloids are known as hydrides. Oxidation of hydrogen removes its electron and yields the hydrogen ion having single positive charge. Often, the hydrogen ion in aqueous solutions is referred to as the hydronium ion. This species is essential in acid-base chemistry.
Production of hydrogen
Although hydrogen is the preferred reductant fuel from the environmental and reduction kinetics viewpoints, it is presently expensive. However, there are widespread expectations for the development of the hydrogen economy, and thus the availability of inexpensive hydrogen. A large amount of effort and many resources are being devoted toward this goal. Production of hydrogen presently uses either reforming of methane or electrolysis of water, both of which are energy intensive processes. Presently the dominant technology for direct production is steam reforming from hydrocarbons.
Bulk hydrogen is usually produced by the steam reforming of methane or natural gas. The production of hydrogen from natural gas is the cheapest source of hydrogen currently. This process consists of heating the gas to between 700 deg C to 1100 deg C in the presence of steam and a nickel catalyst. The resulting endothermic reaction breaks up the methane molecules and forms carbon monoxide and hydrogen. The carbon monoxide gas can then be passed with steam over iron oxide or other oxides and undergo a water gas shift reaction to obtain further quantities of hydrogen.
For this process high temperature (700 deg C to 1100 deg C) steam reacts with methane in an endothermic reaction to yield syngas. The reaction is described by equation CH4 + H2O = CO + 3H2. In the second stage, additional hydrogen is generated through the lower-temperature, exothermic, water gas shift reaction, performed at around 360 deg C. The reaction is described by equation CO + H2O = CO2 + H2. Essentially, the oxygen atom is stripped from the additional water (steam) to oxidize CO to CO2. This oxidation also provides energy to maintain the reaction. Additional heat required to drive the process is generally supplied by burning some portion of the methane.
However, there is a large research effort devoted to using solar energy for the production of hydrogen, for example through use of solar cells to provide the electrons necessary to electrolyze water, or through photo-catalytic water splitting, in which the action of sunlight on a semiconductor immersed in water is used to produce hydrogen directly.
Hydrogen gas as reductant of iron ore
The reduction of iron ore by hydrogen in combination with carbon mono oxide is being practiced during iron production by injection of hydrogen rich gases like natural gas, and coke oven gas, or materials like waste plastics in the blast furnace or during the production of direct reduced iron with natural gas. The basic chemical reactions underlying reduction of iron ore by hydrogen to produce pure iron and water are as follows.
Fe2O3 + 3H2 = 2Fe + 3H2O
Fe3O4 + H2 = 3FeO + H2O
FeO + H2 = Fe + H2O
The consumption of hydrogen per ton of iron is around 500 N cum.
The equilibria of iron ore reduction with the gases carbon mono oxide and hydrogen are well known. Above 850 deg C, hydrogen is even stronger in reducing power than carbon mono oxide. Hydrogen, atomically small and with high diffusivity, has been noted as the faster reductant and thus offers the prospect of rapid reduction processes with no greenhouse gas emission. The equilibrium diagram for reduction with carbon mono oxide and hydrogen is given in Fig 1.
From the equilibrium diagram in Fig 1 it can be seen that at low temperatures carbon mono oxide is more effective at reducing Fe, while at high temperatures H2 is more effective at reducing FeO.
Fig 1 Equilibrium diagram for reduction with carbon mono oxide and hydrogen
Fig 2 (a) displays how close the limits of equilibrium can be approached in the case of pellets. Gas utilization is a function of temperature and depends on the degree of reduction. The thermodynamic limit is not reached. With ore fines in fluid bed reactors, reaction kinetics is more complex. Figure 2 (b) shows reduction characteristics of a typical hematite ore fines when reduced by a 50 % hydrogen and 50 % nitrogen gas mixture between 450 deg C and 800 deg C in a laboratory furnace.
Fig 2 Gas utilization of hydrogen as function of temperature and reduction degree
Gas utilization depends on temperature and on the reduction degree. Initially, gas utilization is high, but drops after 50 % to 60 % reduction, in particular at temperatures around 700 deg C. The cause is the rate minimum effect, which is attributed to morphological changes of solid phases and typically occurs between 600 deg C and 750 deg C. The distance of measured curves from equilibrium is known as insufficient gas utilization. A reason is the retarding effect of water vapour on the reaction FeO + H2 = Fe + H2O.
Another limiting factor in fluidized beds is sticking which is the de-fluidization through adhesion between ore particles. It leads to the breakdown of the fluid bed and also depends on the type of ore fines and on the reduction degree. Hydrogen reduction in fluidized beds can only be realized when carried out in stages which are to be selected in dependence on the degree of reduction, specific for each fine ore. Similar diagrams have been established for several hematite and magnetite ores.
Hydrogen reduction processes
There are several hydrogen reduction processes for the iron ore as described below.
Shaft furnaces are used for the reduction of iron ore pallets with reformed natural gas which is a mixture of hydrogen and carbon mono oxide. The two most relevant direct reduction processes are Midrex and Energiron (HyL-III).
Midrex process (http://www.ispatguru.com/midrex-process-for-direct-reduction-of-iron-ore/) typically applies hydrogen/carbon mono oxide ratios between 1 and 1.5 but is capable of reducing iron ore with any combination of hydrogen and carbon mono oxide. Improved reaction kinetics with hydrogen is expected to be offset from the lower burden temperatures as a result of the endothermic reduction of FeO. At present, there are no Midrex plants using 100 % hydrogen which is only due to economic considerations.
Energiron direct reduction process (http://www.ispatguru.com/energiron-direct-reduction-technology/) is designed to convert iron pellet/lump ore into metallic iron by chemical reactions based on hydrogen and carbon mono oxide. The key aspect of the process is the independent control of metallization and product carbon. Energiron direct reduction process is based on the zero reformer (ZR) schemes.
The Circored process (http://www.ispatguru.com/circored-and-circofer-processes-of-ironmaking/) is a fluidized bed process which produces direct reduced iron briquettes from iron ore fines and uses pure hydrogen as a reduction agent. During the process, iron ore fines are dried and heated through the combustion of natural gas at temperatures of upto 850 deg C to 900 deg C. Finally, it is reduced by hydrogen produced via natural gas reforming. The required hydrogen gas for the process is produced from natural gas by an external steam reformer.
An alternative to carbo-thermic reduction is reduction using hydrogen plasma, which comprises vibrationally excited molecular, atomic, and ionic states of hydrogen, all of which can reduce iron oxides, even at low temperatures. Besides the thermodynamic and kinetic advantages of hydrogen plasma, the byproduct of the reaction is water, which does not pose any environmental problems. Hydrogen in the plasma state provides thermodynamic and kinetic advantages for reduction because of the presence of atomic, and ionic, as well as vibrationally excited, hydrogen species. The energy carried by these species can be released at the reduction interface, leading to local heating. Thus, reduction by hydrogen plasma does not require volumetric heating, as is required for molecular hydrogen. This allows the heat loss from the reactor to be reduced, with accompanying cost savings. Hydrogen plasma reduction of iron oxide can occur for different physical states of iron oxide. Depending on the physical state of the reacting iron oxide at the reaction interface, the hydrogen plasma reduction of iron oxide can be divided into two classes namely (i) heterogeneous processes, in which the reduction reactions occur at the interface between the HP and the molten or solid iron oxide, and (ii) homogeneous processes, in which the iron oxide is vapourized, so reactions occur in the gas phase. Homogeneous processes can also be referred to as dissociative reduction. The great majority of processes are heterogeneous, but the characteristics of homogeneous processes are instructive.
A flash ironmaking process is being developed by American Iron and Steel Institute where hydrogen gas is used as reductant. The energy requirement of the process is 2.6 giga calories per ton of hot metal. The flowsheet of the process is at Fig 3. In the flash ironmaking furnace the operating temperature is 1325 deg C and residence time is 2 seconds to 10 seconds. Residence time is a combination of speed of reaction due to temperature, size of the feed material and amount of excess gas/distance from equilibrium line.
Fig 3 Flow sheet for flash ironmaking process