Hydrogen Fuel for Future Economy
Hydrogen Fuel for Future Economy
Hydrogen is the fuel of the future. It is an energy carrier which can transform the present fossil-fuel dependent economy into a hydrogen economy and which can provide an emission-free fuel. It can be used in internal combustion engines or fuel cells producing virtually no greenhouse gas (GHG) emissions when combusted with oxygen. The only significant emission is water vapour. Henry Cavendish identified in 1776, hydrogen as a distinct species. It was given the name ‘water maker’ by Antoine Lavoisier seven years later, who proved that water was composed of hydrogen and oxygen.
The fundamental issue underlying the use of hydrogen as a fuel is that from where it can be obtained. Despite its abundance in the universe, hydrogen does not occur in ‘free state’ on earth, as it reacts very readily with other elements. For this reason, the vast majority of hydrogen is bound into molecular compounds.
Hydrogen is the simplest of all elements. One can visualize a hydrogen atom as a dense central nucleus with a single orbiting electron. In most hydrogen atoms, the nucleus consists of a single proton, although a rare form (or ‘isotope’) of hydrogen contains both a proton and a neutron. This form of hydrogen is called deuterium or heavy hydrogen. Other isotopes of hydrogen also exist, such as tritium with two neutrons and one proton, but these isotopes are unstable and decay radioactively.
Hydrogen is the first element in the periodic table with the atomic number 1. It is the lightest and most abundant element in the universe representing 75 % by mass or 90 % by volume of all matter. On earth, it is mostly found in compounds with almost every other element. It also exists as a free element in the atmosphere, but only to the extent of less than 1 ppm (parts per million) by volume. Free ionic hydrogen is more reactive than molecular hydrogen, the non-polar covalent compound of two hydrogen atoms.
Hydrogen can be considered an ideal gas over a wide temperature range and even at high pressures. At standard temperature and pressure conditions, it is a colourless, odourless, tasteless, non-toxic, non-corrosive, non-metallic di-atomic gas, which is in principle physiologically not dangerous. One of its most important characteristics is its low density, which makes it necessary for any practical applications to either compress the hydrogen or liquefy it. It is positively buoyant above a temperature of -251 deg C, i.e., over (almost) the whole temperature range of its gaseous state.
The molecules of hydrogen gas are smaller than all other gases, and it can diffuse through many materials considered air-tight or impermeable to other gases. This property makes hydrogen more difficult to contain than other gases. Gaseous hydrogen, with a density of 0.08345 kg/cum (kilogram per cubic metre), has a specific gravity of 0.0696 and is thus around 7 % the density of air. Liquid hydrogen, with a density of 70.78 kg/cum, has a specific gravity of 0.0708 and is thus around (and coincidentally) 7 % the density of water. Fig 1 shows phase diagram of hydrogen.
Fig 1 Phase diagram of hydrogen
Hydrogen gas is highly diffusive and highly buoyant. It rapidly mixes with the ambient air upon release. The diffusion velocity is proportional to the diffusion coefficient and varies with temperature. Corresponding diffusion rates of hydrogen in air are larger by about a factor of 4 compared to those of air in air. The positive buoyancy of hydrogen is a favourable safety effect in unconfined areas, but it can cause a hazardous situation in (partially) confined spaces, where the hydrogen can accumulate, e.g., underneath a roof. Both diffusion and buoyancy determine the rate at which the gas mixes with the ambient air. The rapid mixing of hydrogen with the air is a safety concern, since it leads very soon to flammable mixtures.
Hydrogen leaks pose a potential fire hazard. For small hydrogen leaks, buoyancy and diffusion effects in air are frequently overshadowed by the presence of air currents from a slight ambient wind. In general, these currents serve to disperse leaked hydrogen even more quickly with a further reduction of any associated fire hazard.
Leaks of liquid hydrogen evaporate very quickly since the boiling point of liquid hydrogen is extremely low. Hydrogen leaks are dangerous in that they pose a risk of fire where they mix with air. However, the small molecule size which increases the likelihood of a leak also results in very high buoyancy and diffusivity, so leaked hydrogen rises and becomes diluted quickly, especially out-doors. This results in a much localized region of flammability which disperses quickly. As the hydrogen dilutes with distance from the leakage site, the buoyancy declines and the tendency for the hydrogen to continue to rise decreases. Very cold hydrogen, resulting from a liquid hydrogen leak, becomes buoyant soon after is evaporates.
Hydrogen gas dissolved in liquids permeates into adjoining vessel materials. At elevated temperatures and pressures, hydrogen attacks mild steels severely, causing decarburization and embrittlement. This is a serious concern in any situation involving storage or transfer of hydrogen gas under pressure. Proper material selection, e.g., special alloy steels, and technology is needed to prevent embrittlement.
Hydrogen reacts both with non-metals (high electro-negativity) and with metals (low electro-negativity) to form either ionic or covalent hydrides (e.g. HCl, H2O). The electro-negativity of hydrogen is 2.20 (Pauling scale). The electro-negativity is a measure for the attraction of electrons to the nucleus and its difference to the partner’s electro-negativity defines the character of the bond such as non-polar-covalent (difference = 0), polar-covalent, or ionic (difference high).
Hydrogen is able to react chemically with most other elements. In connection with oxygen, hydrogen is highly flammable over a wide range of concentrations. As a fuel, it represents a clean, environmentally friendly energy source. The mass-related energy density of hydrogen is very high. 1 kg of hydrogen contains 132.5 MJ (mega joule), which is around 2.5 times more energy than is contained in 1 kg of natural gas. The energy content of hydrogen is given either as lower heating value (LHV) of 242 kJ/mol or as higher heating value (HHV) of 286 kJ/mol. The difference is 15.4 %, which is large compared to other gases. It is due to the heat liberated upon condensation of the water vapour (which can be captured in a turbine, but not in a fuel cell).
A stoichiometric hydrogen-air mixture, where all fuel is consumed upon reaction, i.e., where maximum combustion energy is released, contains 29.5 % by volume of hydrogen. The combustion product of hydrogen is water vapour. It burns in a non-luminous, almost invisible pale blue, hot flame to water vapour liberating the chemically bound energy as heat (gross heat of combustion). The flame temperature of a burning (pre-mixed stoichiometric) hydrogen-air mixture is 2130 deg C maximum.
There is a wide flammability range of hydrogen (at room temperature) between 4 % and 75 % by volume of concentration in air and upto 95 % by volume in oxygen. The lower flammability limit (LFL) as the minimum amount of fuel which supports combustion, is normally the ‘more important’ limit, since it is reached first in a continuous leakage. The flammability range widens with higher temperatures.
The potential for an explosion of a flammable hydrogen-air mixture is very high. The auto-ignition temperature, which is the minimum temperature of a hot surface that can ignite a flammable mixture, is in the range of 525 deg C to 725 deg C for hydrogen depending on the experimental conditions. It is relatively high, but can be lowered by catalytic surfaces. Hydrogen gas does not have a flash point as it is already a gas at ambient conditions. It means that cryogenic hydrogen flashes at all temperatures above its boiling point of -253 deg C.
The minimum ignition energy, i.e., the spark energy needed to ignite the ‘most easily ignitable hydrogen concentration in air’ (which is normally not the stoichiometric mixture), is with 0.02 mJ (milli joule) very low, much lower than for hydrocarbon-air mixtures. A weak spark or the electrostatic discharge by a flow of pressurized hydrogen gas (around 10 mJ) is sufficient for an ignition. The minimum ignition energy further decreases with increasing temperature, pressure, or oxygen contents. The hot air jet ignition temperature is lowest for hydrogen compared to all hydro-carbons decreasing further with increasing jet diameter. It is also dependent on jet velocity and mixture composition.
Although hydrogen has a higher auto-ignition temperature than methane, propane or gasoline, its ignition energy at around 0.02 mJ is low and is hence more easily ignitable. Even an invisible spark or static electricity discharge from a human body (in dry conditions) can have enough energy to cause ignition. However, it is important to realize that the ignition energy for all of the fuels is very low so that conditions that ignite one fuel normally ignite any of the other fuels.
Hydrogen has the added property of low electro-conductivity so that the flow or agitation of hydrogen gas or liquid can generate electrostatic charges which result in sparks. For this reason, all hydrogen conveying equipment are to be carefully grounded.
Hydrogen has lowest atomic weight of any substance and hence has very low density both as a gas and as a liquid. The specific volume of hydrogen gas is 11.9 cum/kg at 20 deg C and 1 atmosphere, and the specific volume of liquid hydrogen is 0.014 cum/kg at -253 deg C and 1 atmosphere.
Hydrogen has the second lowest boiling point and melting point of all substances, second only to helium. It is a liquid below its boiling point of -253 deg C and a solid below its melting point of -259 deg C and atmospheric pressure. Hydrogen as a fuel can be stored either as a high-pressure gas or as a cryogenic liquid.
Pure hydrogen is odourless, colourless and tasteless. A stream of hydrogen from a leak is almost invisible in daylight. Compounds such as mercaptans and thiophanes which are used to scent natural gas are not added to hydrogen for fuel cell use since they contain sulphur which poisons the fuel cells. Hydrogen which is derived from reforming other fossil fuels is typically accompanied by nitrogen (N2), carbon dioxide (CO2), carbon monoxide (CO) and other trace gases. In general, all of these gases are also odourless, colourless and tasteless.
Hydrogen is non-toxic but can act as a simple asphyxiant by displacing the oxygen in the air. Inhaled hydrogen can result in a flammable mixture within the body. Inhaling hydrogen can lead to unconsciousness and asphyxiation.
When hydrogen is stored as a liquid, it vapourizes upon expansion to atmospheric conditions with a corresponding increase in volume. Hydrogen’s expansion ratio of 1:848 means that hydrogen in its gaseous state at atmospheric conditions occupies 848 times more volume than it does in its liquid state.
When hydrogen is stored as a high-pressure gas at 250 atmosphere pressure and atmospheric temperature, its expansion ratio to atmospheric pressure is 1:240. While a higher storage pressure increases the expansion ratio somewhat, gaseous hydrogen under any conditions cannot approach the expansion ratio of liquid hydrogen.
The water-forming reaction of hydrogen and oxygen is reversible. Thus, it is possible to convert water, at a low energy state, to hydrogen and oxygen, at a higher energy state, by adding energy slightly greater than which was previously released (the extra to cover losses). This is the principle behind hydrogen production through electrolysis.
Hydrogen is a nearly ideal fuel in terms of smog reduction when combusted. Hydrogen contains no carbon or sulphur, so no CO, CO2 or SOx or soot is produced during combustion (although the combustion of lubricating oil can result in trace amounts). Hydrogen allows for leaner combustion, resulting in lower combustion temperatures and very low NOx emissions. Hydrogen is non-toxic so uncombusted hydrogen does not pose a direct health risk.
Hydrogen is an ideal fuel in terms of smog reduction when used electro-chemically in a fuel cell, rather than combusted. Hydrogen in a fuel cell produces zero harmful emissions. Oxides of nitrogen are completely eliminated due to the low operating temperature (80 deg C) of the cells. Lubricating oil is not present and is hence not reacted. Hydrogen’s energy density 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 of hydrogen based on the LHV is 10.05 MJ/cum of gas at 1 atmosphere pressure and 15 deg C, 1,825 MJ/cum of gas at 200 atmosphere pressure and 15 deg C, 4,500 MJ/cum of gas at 690 atmosphere pressure and 15 deg C, and 8,491 MJ/cum of liquid hydrogen.
The amount of energy liberated during the reaction of hydrogen, on a mass basis, is specifically, around 2.5 times the heat of combustion of common hydro-carbon fuels (gasoline, diesel, methane, propane, etc.). Hence, for a given load duty, the mass of hydrogen needed is only around one third of the mass of hydro-carbon fuel needed. The high energy content of hydrogen also implies that the energy of a hydrogen gas explosion is around 2.5 times that of common hydro-carbon fuels. Thus, on an equal mass basis hydrogen gas explosions are more destructive and carry further. However, the duration of a fire tends to be inversely proportional to the combustive energy, and hence hydrogen fires subside much more quickly than hydrocarbon fires.
Flammable mixtures of hydrogen and air can be easily ignited. Hydrogen, as a flammable fuel, mixes with oxygen whenever air is allowed to enter a hydrogen container, or when hydrogen leaks from any vessel into the air. Ignition sources take the form of sparks, flames, or high heat. 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. The flammability limits increase with temperature as shown in Fig 2. 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.
Fig 2 Variation in hydrogen flammability limits with temperatures
The burning speed of hydrogen at 2.65 m/sec to 3.25 m/sec is nearly an order of magnitude higher than that of methane or gasoline (at stoichiometric conditions). Thus hydrogen fires burn quickly and, as a result, tends to be relatively short-lived.
The quenching gap of hydrogen at 0.64 mm is around 3 times less than that of other fuels, such as gasoline. Thus, hydrogen flames travel closer to the cylinder wall before they are extinguished making them more difficult to quench than gasoline flames. This smaller quenching distance can also increase the tendency for backfire since the flame from a hydrogen-air mixture can more readily get past a nearly closed intake valve than the flame from a hydrocarbon-air mixture.
Hydrogen flames are very pale blue and are almost invisible in daylight due to the absence of soot. Visibility is enhanced by the presence of moisture or impurities (such as sulphur) in the air. Hydrogen flames are readily visible in the dark or subdued light. A hydrogen fire can be indirectly visible by way of emanating ‘heat ripples’ and thermal radiation, particularly from large fires. In many instances, flames from a hydrogen fire can ignite surrounding materials which do produce smoke and soot during combustion. Hydrogen emits non-toxic combustion products when burned. Gasoline fires generate toxic smoke.
Hydrogen fires can only exist in the region of a leak where pure hydrogen mixes with air at sufficient concentrations. In many respects, hydrogen fires are safer than gasoline fires. Hydrogen gas rises quickly due to its high buoyancy and diffusivity. As a result hydrogen fires are vertical and highly localized. Corn brooms are sometimes used by emergency response people to detect hydrogen flames.
Constant exposure to hydrogen causes a phenomenon known as hydrogen embrittlement in many materials including steel. Hydrogen embrittlement can lead to leakage or catastrophic failures in metal and non-metallic components. The mechanisms which cause hydrogen embrittlement effects are not well defined. Factors known to influence the rate and severity of hydrogen embrittlement include hydrogen concentration, hydrogen pressure and temperature, hydrogen purity, type of impurity, stress level, stress rate, metal composition, metal tensile strength, grain size, microstructure, and heat treatment history. Moisture content in the hydrogen gas can lead to metal embrittlement through the acceleration of the formation of fatigue cracks. Materials in contact with hydrogen are subject to hydrogen embrittlement. Tab 1 gives main characteristic data of hydrogen.
Tab 1 Properties of hydrogen
|Stoichiometric fraction in air||vol%||29.53|
|Boiling point (BP)||deg C||-252.882|
|Melting point (MP)||deg C||-259.14|
|Triple point||Temperature||deg C||-259.35|
|Critical point||Temperature||deg C||-239.9|
|Density of gas @ NTP||kg/cum||0.08345|
|Density of gas @ STP||kg/cum||0.0899|
|Density of gas @ BP||kg/cum||1.338|
|Density of liquid @ BP||kg/cum||70.78|
|Density of solid @ -269 deg C||kg/cum||88|
|Expansion ratio liquid / ambient||845|
|Diffusion coefficient @ NTP||sqm/sec||0.000061|
|Diffusion velocity @ NTP||m/sec||less than 0.02|
|Buoyant velocity||m/sec||1.2 – 9|
|Specific heat (constant p) of gas @ NTP||kJ/kg K||14.85|
|Specific heat (constant p) of gas @ STP||kJ/kg K||14.304|
|Specific heat (constant p) of gas @ BP||kJ/kg K||12.15|
|Specific heat (constant p) of liquid @ BP||kJ/kg K||9.66|
|Thermal conductivity of gas @ NTP||W/m K||0.187|
|Thermal conductivity of gas @ BP||W/m K||0.01694|
|Thermal conductivity of liquid @ BP||W/m K||0.09892|
|Viscosity of gas @ NTP||Micro Poise||89.48|
|Viscosity of gas @ BP||Micro Poise||11.28|
|Viscosity of liquid @ BP||Micro Poise||132|
|Surface tension @ BP||N/m||0.00193|
|Heat of conversion from para to ortho||kJ/kg||708.8|
|Heat of melting (fusion) @ MP||kJ/kg||58.8|
|Heat of vapourization @ BP||kJ/kg||445.6|
|Vaporization index||K cucm/J||8.9|
|Vaporization rate of liquid hydrogen pool||mm/sec||4.2 -8.3|
|Heat of sublimation||kJ/kg||379.6|
|Speed of sound in gas @ NTP||m/sec||1,294|
|Speed of sound in gas @ BP||m/sec||355|
|Speed of sound in liquid @ BP||m/sec||1,093|
|Speed of sound in stoichiometric H2-air mixture||m/sec||404|
|Inversion temperature||deg C||-80|
|Flammability limits in air||vol%||4-75|
|Detonability limits in air||vol%||13-70|
|Minimum ignition energy||J||0.000019|
|Minimum ignition energy for detonation||J||Around 10,000|
|Auto-ignition temperature in air||deg C||520 -750|
|Hot air jet ignition temperature||deg C||670|
|Gross heat of combustion or HHV @ 15 deg C||kJ/mol||286.1|
|Net heat of combustion or LHV @ 15 deg C||kJ/mol||241.7|
|Flame temperature||deg C||2,045|
|Burning rate of liquid hydrogen pool||mm/sec||0.5 -1.1|
|Laminar burning velocity in air||m/sec||2.65-3.25|
|Visible laminar flame speed||m/sec||18.6|
|Deflagration pressure ratio||8.15|
|Quenching distance @ NTP||mm||0.64|
|Maximum experimental safe gap @ NTP||mm||0.08|
|Adiabatic flame temperature||deg C||2045|
|CJ (Chapman–Jouguet) velocity||m/sec||1,968|
|CJ detonation pressure ratio||pCJ/p0||15.6|
|Energy release||Mj/kg mixture||2.82|
|Detonation cell size||mm||15|
|Critical tube diameter||mm||0.2|
|Detonation initiation energy||g tetryl||1.1|
|Detonation induction distance @ NTP||Length/diameter||Around 100|
|TNT equivalent||g TNT/g||26.5|
|Note: STP -Standard temperature and pressure (0 deg C, 1.01325 Pa), NTP – Normal temperature and pressure (20 deg C, 101325 Pa), HHV – High heating value, LHV – Low heating value.|
Production of hydrogen
To obtain hydrogen means to remove it from the molecules of hydrogen compounds. With respect to the energy needed, it is easy to remove hydrogen from compounds which are at a higher energy state, such as fossil fuels. This process releases energy, reducing the amount of process energy needed. It takes more energy to extract hydrogen from compounds which are at a lower energy state, such as water, as energy is needed to be added to the process.
The process of extracting hydrogen from fossil fuels is called reforming. Today, this is the principal and least expensive method of producing hydrogen. Unfortunately, reforming emits pollutants and consumes non-renewable fuels.
The process of extracting hydrogen from water is called electrolysis. In principal, electrolysis can be entirely non-polluting and renewable, but it needs the input of large amounts of electrical energy. Hence, the total environmental impact of acquiring hydrogen through electrolysis is largely dependent on the impacts of the source power.
Alternative methods of hydrogen production include thermo-chemical water decomposition, photo-conversions, photo-biological processes, production from biomass, and industrial processes. Although some of these methods show promise for the future, they are still largely in experimental stage and are capable of supplying only small amounts of hydrogen.
Hydrogen can be produced on a large scale at dedicated hydrogen production plants, or on a small scale at local production facilities. Large-scale production benefits from economies of scale and plants can be located near power and water, but suffers from the difficulties of hydrogen transportation. Some methods of hydrogen production (for example from coal or biomass) can be undertaken only on a large scale.
Small scale production can reduce the problems of hydrogen transportation by using energy which can be easily brought to the facility, such as electricity, natural gas, or solar. On the downside, the amount of equipment needed for the amount of hydrogen produced is significantly higher than for large-scale facilities, due to the economy of scale.
Presently, the vast majority of all hydrogen produced worldwide originates from fossil fuels, as a byproduct in chemical industries, or crude oil refining processes. Hydrogen production from renewable energy is not yet feasible on a large scale.
Hydrogen can be produced with multiple processes and energy sources. A colour code nomenclature is now commonly used to facilitate discussion. But policy makers are to design policy using an objective measure of impact based on life-cycle greenhouse gas (GHG) emissions, especially since there can be cases which do not fully fall under one colour (for example mixed hydrogen sources, electrolysis with grid electricity).
There are three main ways to generate hydrogen, represented by the colours grey, blue, and green. An additional pathway which is sometimes referred as turquoise (greenish blue) hydrogen is still at a TRL (technology readiness level) stage. It consists of the pyrolysis of methane. There is another colour code. Hydrogen, when produced by electrolyzers supplied by electricity from nuclear power plants is known as yellow (or purple) hydrogen. Fig 3 shows identification of hydrogen generation pathways with colour representation.
Fig 3 Identification of hydrogen generation pathways with colour representation
Grey hydrogen is the most common process which uses either natural gas or coal as feedstock that reacts with steam at high temperatures and pressures to produce synthesis gas (syn gas), which consists primarily of hydrogen and CO gas. The synthesis gas is then reacted with additional water to produce pure hydrogen and CO2. These are well-established processes, but they generate significant CO2 emissions, which is why the resulting hydrogen gas is termed ‘grey hydrogen’.
Blue hydrogen is the second-most-common process. Blue hydrogen, relies on the same basic processes as grey hydrogen, but it traps upto 90 % of the GHG emissions through carbon-capture storage or utilization (CCS/U) technology. In some cases, this carbon is stored which needs considerable underground space or capital costs, or it is reused as a feedstock for industrial applications, in which CO2 is still ultimately released into the atmosphere.
Green hydrogen is the most promising process. Green hydrogen uses renewable energy to power the electrolysis which splits water molecules into hydrogen and oxygen. Electrolysis needs energy. That this energy comes from lower-cost renewable sources is what makes this form of hydrogen ‘green’.
There are three major electrolysis technologies with different levels of maturity. One technology, alkaline water, is the most basic and mature technology and has a market share of around 70 % of the presently very small green hydrogen market. It benefits from low cost, and this process has a long operational life. However alkaline water process needs to run continuously or the production equipment can get damaged. The intermittent nature of renewable energy, hence, rules it out as a single source of power for alkaline water process. Another technology is polymer electrolyte membrane (PEM) electrolysis, which has a market share of around 30 %, This technology is being adopted by most of the leading electrolyzer manufacturers. PEM yields higher-quality hydrogen and can be operated intermittently, but is also expensive and has lower production rates than alkaline water process. A third technology is a solid oxide electrolyzer cell, which is still in the research and development stage. It offers high efficiency at low cost. However, it needs a long start-up time and the components of this process have a short operational life.
In electrolysis, electricity is used to decompose water into its elemental components namely (i) hydrogen, and (ii) oxygen. Electrolysis is frequently considered as the preferred method of hydrogen production as it is the only process which need not rely on fossil fuels. It also has high product purity, and is feasible on small and large scales. Electrolysis can operate over a wide range of electrical energy capacities, for example, taking advantages of more abundant electricity at night.
At the heart of electrolysis is an electrolyzer. An electrolyzer is a series of cells each with a positive and negative electrode. The electrodes are immersed in water which has been made electrically conductive, achieved by adding hydrogen or hydroxyl ions, normally in the form of alkaline potassium hydroxide (KOH). The anode (positive electrode) is typically made of nickel and copper and is coated with oxides of metals such as manganese, tungsten and ruthenium. The anode metal allows quick pairing of atomic oxygen into oxygen pairs at the electrode surface.
The cathode (negative electrode) is typically made of nickel, coated with small quantities of platinum as a catalyst. The catalyst allows quick pairing of atomic hydrogen into pairs at the electrode surface and thereby increases the rate of hydrogen production. Without the catalyst, atomic hydrogen builds up on the electrode and blocks current flow.
A gas separator, or diaphragm, is used to prevent intermixing of the hydrogen and oxygen although it allows free passage of ions. It is normally made of an asbestos-based material, and tends to break apart above 80 deg C.
The three reactions which are taking place at the cathode are (i) K+ + e- = K in which a positively charged potassium ion is reduced, (ii) K + H2O = K+ + H + OH- in which the ion reacts with water to form a hydrogen atom and a hydroxyl ion, (iii) H + H = H2 in which the highly reactive hydrogen atom bonds to the metal of the cathode and combines with another bound hydrogen atom.
The three reactions which are taking place at the anode are (i) OH- = OH + e- where a negatively charged hydroxyl ion is oxidized, (ii) 2OH = H2O + O where the ion reacts to form water and an oxygen atom, and (iii) O + O = O2 where the highly reactive oxygen atom then bonds to the metal of the anode and com-bines with another bound oxygen atom to form an oxygen molecule which then leaves the anode as a gas. Fig 4 shows atypical electrolysis cell.
Fig 4 Typical electrolysis cell
The rate of hydrogen generation is related to the current density (the amount of current divided by the electrode area measured in amps per area). Normally, the higher is the current density, the higher is the source voltage needed, and the higher is the power cost per unit of hydrogen. However, higher voltages decrease the overall size of the electrolyzer and hence result in a lower capital cost. State of the art electrolyzers are reliable, have energy efficiencies of 65 % to 80 % and operate at current densities of around 2000 A/sqm.
Fuel cell reverses the process of electrolysis. Electrolysis adds electrical energy to low-energy water to release two high-energy gases. A fuel cell allows the gases to react and combine to form water, releasing electrical power. Both the processes release heat, which represents an energy loss.
For electrolysis, the amount of electrical energy needed can be somewhat offset by adding heat energy to the reaction. The minimum amount of voltage needed to decompose water is 1.23 V at 25 deg C. At this voltage, the reaction needs heat energy from the outside to proceed. At 1.47 V (and same temperature) no input heat is needed. At greater voltages (and same temperature) heat is released into the surroundings during water decomposition.
Operating the electrolyzer at lower voltages with added heat is advantageous, as heat energy is normally cheaper than electricity, and can be recirculated within the process. Also, the efficiency of the electrolysis increases with increased operating temperature.
When viewed together with fuel cells, hydrogen produced through electrolysis can be seen as a way of storing electrical energy as a gas until it is needed. Hydrogen produced by electrolysis is hence the energy carrier, not the energy source. The energy source derives from an external power generating plant. In this sense, the process of electrolysis is not very different from charging a battery, which also stores electrical energy. Viewed as electricity storage medium, hydrogen is competitive with batteries in terms of weight and cost.
To be truly clean, the electrical power stored during electrolysis is to be derived from non-polluting, renewable sources. If the power is derived from natural gas or coal, the pollution has not been eliminated, only pushed upstream. In addition, every energy transformation has an associate energy loss. As a result, fossil fuels can be used with greater efficiency by means other than by driving the electrolysis of hydrogen. Also, the cost of burning fossil fuels to generate electricity for electrolysis is three to five times that of reforming the hydrogen directly from the fossil fuel.
Although a renewable energy source in conjunction with electrolysis eliminates the dependence on fossil fuels, it does not reduce the number of energy transformations needed to produce mechanical work using hydrogen. If clean, renewable power is available, it can also be used in other ways which need fewer energy transformations, such as direct storage in batteries or to compress air for propulsion.
Another consideration associated with electrolysis is the source of water. Water is already a precious commodity, and is to be consumed in huge quantities in order to support a large hydrogen economy. The water also is to be purified prior to use, increasing its cost.
Reforming is a chemical process which converts hydrogen-containing fuels in the presence of steam, oxygen, or both into a hydrogen-rich gas stream. When applied to solid fuels, the reforming process is called gasification. The resulting hydrogen-rich gas mixture is called reformate. The equipment used to produce reformate is known as a reformer or fuel processor.
The specific composition of the reformate depends on the source fuel and the process used, but it always contains other compounds such as N2, CO2, CO and some of the unreacted source fuel. After hydrogen is removed from the reformate, the remaining gas mixture is known as raffinate.
In essence, reforming a fossil fuel consists of the these steps (i) feedstock purification (including sulphur removal), (ii) steam reforming or oxidation of feedstock to form hydrogen and carbon oxides, (iii) primary purification i.e. conversion of CO to CO2, and (iv) secondary purification which constitutes further reduction of CO.
The reforming reactions need the input of water and heat. Overall reformer thermal efficiency is calculated as the LHV of the product hydrogen divided by the LHV of the total input fuel. This thermal efficiency depends on (i) the efficiencies of the individual processes, (ii) the effectiveness to which heat can be transferred from one process to another, and (iii) the amount of energy which can be recovered through means such as turbo-chargers. In the end, high temperature reformer efficiencies are around 65 % and low temperature methanol reformers can achieve efficiencies of 70 % to 75 %.
The advantages of the reforming process for fossil fuels are that this process (i) uses existing fuel infrastructures, (ii) reduces the need to transport and store hydrogen (iii) does not need the input of large amounts of energy as in electrolysis, (iv) is less expensive than other hydrogen production methods. The disadvantages of the reforming process for fossil fuels are that this process (i) can have relatively long warm-up times, (ii) is difficult to apply to vehicle engines because of irregular demands for power (transient response), (iii) is complex, large and expensive, (iv) introduces additional losses into the energy conversion process, especially that it has small thermal mass, (v) uses non-renewable fossil fuels, and (vi) generates pollution. The pollution generated by reformers take three forms namely (i) CO2 emissions, (ii) incomplete reactions, leaving CO and some of the source fuel in the reformate, and (iii) production of pollutants through combustion, such as nitrous oxides.
Reforming fossil fuels only makes sense if the hydrogen is needed directly, as in direct reduction of iron ore or a fuel cell engine. For internal combustion engines, it is always more efficient to use the fossil fuel directly without passing it through a reformer first.
Reformers are of three basic types namely (i) steam reformers, (ii) partial oxidation reactors, and (iii) thermal decomposition reactors. A fourth type results from the combination of partial oxidation and steam reforming in a single reactor, called an auto-thermal reformer.
Medium size or large-size reformers can be used. At these scales, the equipment complexity, warm-up time and transient response are not issues, pollutants can be controlled more effectively, and existing power infrastructures can be used. The facility need to store only small amounts of hydrogen, and hydrogen transportation is avoided.
Small-size reformers can be installed in fuel cell vehicles to entirely eliminate the problems associated with fueling, storing and handling hydrogen directly. In fact, many fuel cell experts think that the true challenge in fuel cell engine design now lies in the development of an efficient, compact, reliable and highly integrated fuel processor. Other experts think that the use of on-board reformers never possesses a realistic solution due to their size, complexity and cost.
In theory, any hydrocarbon or alcohol fuel can serve as a feedstock to the reforming process. Naturally, fuels with existing distribution infrastructures are the most commonly used.
The most popular reforming process is the natural gas reforming process. Hydrogen can be produced from natural gas using high-temperature steam. This process, called steam methane reforming process, accounts for around 95 % of the hydrogen used today. Another method, called partial oxidation, produces hydrogen by burning methane in air. Both steam reforming and partial oxidation produce a ‘synthesis gas’ or ‘syn gas’, which is then reacted with additional steam to produce a higher hydrogen content gas stream.
Some of the hydrogen producing processes where the development activities are still continuing is described below.
Gasification is a process in which coal or biomass is converted into gaseous components by applying heat under pressure and in the presence of air / oxygen and steam. A subsequent series of chemical reactions produces a syn gas, which is then reacted with steam to produce a gas stream with an increased hydrogen concentration which then can be separated and purified. With CCS/U, hydrogen can be produced directly from coal with very small GHG emissions. Since growing biomass consumes CO2 from the atmosphere, producing hydrogen through biomass gasification results in near-zero net GHG emissions.
Biomass can also be processed to make renewable liquid fuels, such as ethanol or bio-oil, which are relatively convenient to transport and can be reacted with high-temperature steam to produce hydrogen at or near the point of use. A variation of this technology known as aqueous-phase reforming is presently being explored.
Electrolysis uses an electric current to split water into hydrogen and oxygen. The electricity needed can be generated using any a number of resources. However to minimize GHG emissions, energy technologies (such as wind, solar, geothermal, and hydroelectric power), nuclear energy, or natural gas and coal with CCS/U are preferred. Heat from a nuclear reactor can be used to improve the efficiency of water electrolysis to produce hydrogen. By increasing the temperature of the water, less electricity is needed to split it into hydrogen and oxygen, which reduces the total energy required.
Another water-splitting method uses high temperatures generated by solar concentrators (mirrors which focus and intensify sunlight) or nuclear reactors to drive a series of chemical reactions to split water into hydrogen and oxygen. All of the intermediate process chemicals are recycled within the process.
Certain microbes, such as green algae and cyano-bacteria, produce hydrogen by splitting water in the presence of sunlight as a byproduct of their natural metabolic processes. Other microbes can extract hydrogen directly from biomass.
In the photo-electro-chemical (PEC) process, hydrogen can be produced directly from water using sunlight and a special class of semi-conductor materials. These highly specialized semi-conductors absorb sunlight and use the light energy to directly split water molecules into hydrogen and oxygen.
The biggest challenge for hydrogen production, particularly from renewable resources, is providing hydrogen which is cost competitive. For transportation, a key driver for energy independence, hydrogen is to be cost-competitive with conventional fuels and technologies. This means that the cost of hydrogen, regardless of the production technology, and including the cost of delivery is to be less than the cost of present fuels being used. For reducing overall hydrogen cost, development activities are being focused on improving the efficiency of hydrogen production technologies as well as reducing the cost of capital equipment, operations, and maintenance.
If the greatest challenge in hydrogen use is to extract it, the second biggest challenge is how to store it. As described earlier, hydrogen has the lowest gas density and the second-lowest boiling point of all known substances, making it a challenge to store as either a gas or a liquid. As a gas, it needs very large storage volumes and pressures. As a liquid, it needs a cryogenic storage system.
Hydrogen’s low density, both as a gas and a liquid, also results in very low energy density. Stated otherwise, a given volume of hydrogen contains less energy than the same volume of other fuels. This also increases the relative storage tank size, as more hydrogen is needed to meet a given range requirements. The amount of hydrogen needed for fuel cells is offset somewhat by the fact that it is used more efficiently than when burned, so less fuel is needed to achieve the same result.
Despite its low volumetric energy density, hydrogen has the highest energy-to-weight ratio of any fuel. Unfortunately, this weight advantage is normally over-shadowed by the high weight of the hydrogen storage tanks and associated equipment. Thus, most hydrogen storage systems are considerably bulkier and / or heavier than those used for gasoline or diesel fuels.
For all practical purposes, hydrogen can be stored as either a high-pressure gas, a liquid in cryogenic containers, or a gas chemically bound to certain metals (hydrides). Ironically, the best way to store hydrogen is in the form of hydrocarbon fuels although it needs additional systems to extract it.
High-pressure hydrogen is stored in cylinders, similar to those used for compressed natural gas. Most cylinders have a cylindrically shaped sidewall section with hemispherical end domes, although new conformal designs use multiple cylinders in tandem and distort the cylindrical shape in order to increase the usable volume.
Gas compression is an energy intensive process. The higher the end pressure, the greater the amount of energy needed. However, the incremental energy needed to achieve higher and higher pressures decreases so that the initial compression is the most energy intensive part of the process.
The energy economy of higher compression levels is counter-balanced by an incremental decrease in gas density at higher pressures, so that further compression packs less hydrogen mass into the cylinders (even if suitable cylinders are to exist). A useful way of understanding the energy cost of compression is as a percentage of the total energy content (LHV) of the hydrogen being stored. In these terms, around 5 % of the LHV is needed to compress the gas to 350 atmospheres. Exact energy usage depends on the flow capacity and efficiency of the compressors used.
Liquid hydrogen storage systems overcome many of the weight and size issues associated with high-pressure gas storage systems, although at cryogenic temperatures. Liquid hydrogen can be stored just below its normal boiling point of -253 deg C at or close to ambient pressure in a double-walled, super-insulating tank (or ‘dewar’). This insulation takes the form of a vacuum jacket, much like in a thermos bottle. Liquid hydrogen tanks do not need to be as strong as high-pressure gas cylinders although they do need to be adequately robust for automotive use.
Hydrogen cannot be stored in liquid form indefinitely. All tanks, no matter how good the insulation, allow some heat to transfer from the ambient surroundings. The heat leakage rate depends on the design and size of tank, in this case, bigger is better. This heat causes some of the hydrogen to vapourize and the tank pressure to increase. Stationary liquid hydrogen storage tanks are frequently spherical since this shape offers the smallest surface area for a given volume, and hence presents the smallest heat transfer area.
Tanks have a maximum overpressure capacity of around 5 atmospheres. If the hydrogen is not consumed as quickly as it vapourizes, the pressure builds to a point where it vents through a pressure relief valve. This vented hydrogen is not only a direct loss of usable fuel, but it also poses a flammability hazard. Provision is needed to be made to vent the hydrogen safely without the potential for accumulation.
Hydrogen can be drawn from the tank either as a liquid or as a gas. Hydrogen is much more bulky than gasoline on an equivalent energy basis. Liquid hydrogen storage systems can be four to ten times larger and heavier than an equivalent gasoline tank.
Hydrogen liquefaction is a very energy intensive process due to the extremely low temperatures involved. Liquefaction involves several steps, including (i) compression of hydrogen gas using reciprocating compressors, (ii) pre-cooling of the compressed gas to liquid nitrogen temperatures -195 deg C, (iii) expansion through turbines, and (iv) catalytic conversion to its stable ‘para-hydrogen’ form.
In total, the energy needed for the liquefaction process is the equivalent of upto 40 % of the LHV of hydrogen. Once in liquid form, hydrogen is relatively efficient to transport and easy to use. Clearly, to maximize the energy investment paid during liquefaction, it is prudent to store and use the hydrogen directly as a liquid whenever possible.
The worst scenario in terms of energy investment is to liquefy the hydrogen, transport it in liquid form, reconvert it to a gas, and store it as a high pressure gas. This erodes the net available energy twice, once during liquefaction and again during compression, while still being left with the disadvantages of a bulky and heavy gaseous fuel storage system.
Metal hydride storage systems are based on the principle that some metals readily absorb gaseous hydrogen under conditions of high pressure and moderate temperature to form metal hydrides. These metal hydrides release the hydrogen gas when heated at low pressure and relatively high temperature. In essence, the metals soak up and release hydrogen like a sponge.
The advantages of metal hydride storage systems revolve around the fact that the hydrogen becomes part of the chemical structure of the metal itself and hence does not need high pressures or cryogenic temperatures for operation. Since hydrogen is released from the hydride for use at low pressure (and is to be released before it can burn rapidly), hydrides are the most intrinsically safe of all methods of storing hydrogen.
There are several types of specific metal hydrides, but they are primarily based on metal alloys of magnesium, nickel, iron, and titanium. Normally, metal hydrides can be divided into those with a low or high hydrogen desorption (release) temperature.
The high temperature hydrides can be less expensive and hold more hydrogen than the low temperature hydrides, but need significant amounts of heat in order to release the hydrogen. Low temperature hydrides can get sufficient heat from an engine, but high temperature hydrides need an external source of heat.
The low desorption temperatures associated with some hydrides can be a problem since the gas releases too readily at ambient conditions. To overcome this, low temperature hydrides need to be pressurized, increasing the complexity of the process.
The main disadvantage of metal hydride storage systems is not so much the temperatures and pressures needed to release the hydrogen, but rather their low mass energy density. Even the best metal hydrides contain only 8 % hydrogen by weight and hence tend to be very heavy and expensive. Metal hydride storage systems can be upto 30 times heavier and ten times larger than a gasoline tank with the same energy content.
Another disadvantage of metal hydride storage systems is that they are to be charged with only very pure hydrogen or they become contaminated with a corresponding loss of capacity. Oxygen and water are prime culprits as they chemically adsorb onto the metal surface displacing potential hydrogen bonds. The storage capacity lost through contamination can to some extent be reactivated with heat.