Monday Study – The Green Hydrogen Balance
Green hydrogen: The only oxygen and water balanced fuel
Marcus Newborough and Graham Cooley, March 2021 (Science Direct)
The use of any fuel depletes the oxygen content of the atmosphere, with one exception: hydrogen produced from water. Water electrolysis liberates oxygen from water in the precise stoichiometric ratio required to oxidise (and hence release energy from) the co-produced hydrogen. As a commercial fuel production process, electrolysis is unique in providing the oxidant as well as the fuel; electrolytic oxygen can thereby replenish the consumption of atmospheric oxygen due to hydrogen use. Furthermore, the amount of water consumed during electrolysis is reproduced when the hydrogen is oxidised. So the use of electrolysers and electrolytic hydrogen does not affect global oxygen and water resources: ‘green’ hydrogen may thus be described as the only oxygen and water balanced fuel. Conversely, the use of hydrogen derived from fossil fuels (with or without carbon capture and storage, CCS) depletes the oxygen resource and increases water vapour emissions to the atmosphere, which enhances the rate of global warming. Therefore, a worldwide multi-TW deployment of electrolysers could provide very substantial amounts of hydrogen for the energy system, and oxygen for the global ecosystem. This should be done in combination with other measures for combatting oxygen depletion (such as reducing combustion, increasing forestation, and reducing nutrient inputs to the ocean from sewage and agriculture). In this way the long-term objective should be to stabilise, or even increase slightly, the concentrations of atmospheric and aquatic oxygen, and possibly speed up the decay of atmospheric methane. Clearly the production-and-use of hydrogen derived from fossil fuels contravenes this objective, and should cease without delay.
Oxygen depletion and water vapour addition
Oxygen is the second most abundant gas on Earth after nitrogen. It is produced primarily by photosynthesis and consumed mainly by combustion, respiration and fire (e.g. it has been estimated that fossil fuel combustion consumes over eight times more oxygen per annum than human respiration). There are also several industrial processes which of themselves consume oxygen (e.g. steel production, oil refining and wastewater treatment). Based on measurements taken since 1989, the atmospheric oxygen concentration has been decreasing slowly at an annual rate of about 19 molecules per million. This trend will continue, and for the business-as-usual RCP8.5 (Representative Concentration Pathway, 8.5 W/m2) global warming scenario, the concentration is predicted to decrease from 20.946% to 20.825% by 2100, or on average by about 0.0015% per annum. The overall rate of change (e.g. see Figure 1), which is influenced both by changes in oxygen production and consumption, may be parabolic rather than linear, and so result in the concentration falling to zero in about 4400 years. In the short term, the change is too gradual to impact human health; this will only occur when the concentration falls below about 19.5%. However, the current rate of oxygen depletion is sufficient to influence global warming.
Oxygen, nitrogen and hydrogen do not absorb infrared radiation (unlike carbon dioxide, methane and water vapour), but a lower oxygen concentration thins the atmosphere, reduces the scattering of incoming shortwave radiation, and so allows more solar energy to reach the Earth's surface. This causes more moisture to evaporate, increasing humidity and cloud formation, and the additional water vapour in the atmosphere traps longwave re-radiation from the surface, so temperatures rise and precipitation increases. Increasing the amount of water vapour in the atmosphere due to a declining oxygen concentration serves to amplify global warming, because water vapour is the most potent greenhouse gas.
In oceans and lakes, water deoxygenation is occurring due to global warming and the oversupply of nutrients from agriculture and human sewage. Warmer water simply cannot hold as much dissolved oxygen, and as the atmospheric partial pressure of oxygen declines, it is easier for oxygen to diffuse out of water (Henry's Law). Greater thermal stratification of the ocean, in combination with a declining dissolved oxygen content of the upper layers, inhibits the oxygenation of deeper waters. At the same time, increased nutrient inputs to the ocean from agriculture and human sewage act to increase organic biomass, which consumes oxygen and produces CO2, causing deoxygenation and acidification. When oxygen levels become too low for aerobic respiration, microbes conduct denitrification to obtain energy, and this produces nitrous oxide – a powerful greenhouse gas that forms nitric oxide in the stratosphere, which destroys the ozone layer. Furthermore, the deoxygenation of inland water may be resulting in greater dissolved methane concentrations in the lower reaches of lakes and reservoirs, thereby increasing their global warming potential.
Good quality water has an oxygen concentration of about 7 ppm, but the global average value fell by ~0.04% per annum between 1960 and 2010, and further deoxygenation of 4–7% by 2100 has been predicted (i.e. in the region of 0.05% per annum). Since 1960, oxygen levels have dropped by 40–50% in parts of the ocean at low latitudes, threatening marine life close to the surface where most species live and in deep water.10, 11, 12 The area of low oxygen levels in the open ocean has now increased by 4.5 million km2, and over 500 low-oxygen sites have been identified in estuaries and coastal waters. In general, when compared with the rate of oxygen attrition in the atmosphere, water deoxygenation is occurring much more rapidly and is having a more immediate impact on life. In addition, because photosynthesis by phytoplankton in the ocean is the main source of oxygen production on Earth, lower levels of dissolved oxygen not only suppress life in the oceans, but in the long term could lead to a catastrophic loss of oxygen production. Schaffer et al. concluded that reductions in fossil-fuel use are needed, if extensive oxygen depletion for thousands of years in the ocean is to be avoided.
The global warming impacts of oxygen depletion are additional to those associated with the ongoing rapid increases in trace gases. Since 2000, CO2 and methane concentrations have each been rising at an average annual rate of roughly 1%. Small changes in the concentrations of these greenhouse gases, in combination with very small changes in the oxygen concentration, result in significant changes in the atmospheric water vapour content, which amplifies global warming. For example, it has been estimated that in a scenario where the atmospheric CO2 concentration is doubled, radiative absorption would increase by 4 W/m2, but when the associated effects of increased water vapour are taken into account, this rises to almost 20 W/m2.
It is clear that we should prioritise actions that will counteract oxygen depletion and water vapour addition, as well as curtail emissions of CO2, methane and nitrous oxide. For the energy system this equates to minimising its greenhouse gas emissions and reducing its rate of oxygen consumption. When renewable electricity is produced and used, oxygen isn’t consumed and CO2, methane and water vapour aren’t emitted. The same environmental benefits are now required for molecular energy. In this context, a key question is: will switching away from fossil fuels to hydrogen help solve the problems of oxygen depletion and water vapour addition?
Deriving hydrogen from water versus methane
Hydrogen has long been advocated as a fuel for displacing fossil fuels and combatting emissions of greenhouse gases and air pollutants. Commercial hydrogen production processes are based either on extracting hydrogen from a hydrocarbon or water. Usually attention is focused on the reduction of CO2 emissions, without considering how hydrogen impacts the production or depletion of oxygen and water. Water electrolysis produces hydrogen and oxygen simultaneously (in a volume ratio of 2:1 and a mass ratio of 1:8):(1)2H2O → 2H2 + O2
The main method competing with electrolysis is the autothermal reformation (ATR) of natural gas (predominantly methane) combined with CCS for capturing the CO2 emissions, in order to produce so-called ‘blue’ hydrogen. Unlike electrolysis, the ATR process consumes rather than produces oxygen:(2)4CH4 + O2 + 2H2O → 10H2 + 4CO The carbon monoxide is then converted to CO2 via the water-gas shift reaction:(3)CO + H2O → CO2 + H2
From equations 2 and 3, it can be seen that the production of 14 mol of hydrogen and 4 mol of CO2 requires a total input of 4 mol of methane, 1 mol of oxygen and 6 mol of water. For comparison, from equation 1, to produce 14 mol of hydrogen by electrolysis requires 14 mol of water and no oxygen.
The hydrogen may require further purification to satisfy the gas quality requirements of the end-use application, but normally when it's used (e.g. combusted or converted to electricity by a fuel cell) it yields water by consuming atmospheric oxygen:(4)2H2 + O2 → 2H2O
It is therefore important to consider the overall effect on the Earth's water and oxygen resources of hydrogen production-and-use for each method of production. Figure 2 shows the relative effects, based on the simple example of producing and consuming 14 mol of hydrogen. Clearly the electrolysis pathway results in no overall depletion of water or oxygen, while the other pathways act to consume oxygen and emit water vapour. Hydrogen production-and-use via ATR results in a consumption of 9.1 kgO2/kgH2 and an emission of 5.1 kgH2O/kgH2 (assuming perfect reformation and combustion).
The utilisation of hydrogen from ATR thereby imposes a double negative on the environment, relative to electrolytic hydrogen. Indeed, had the methane been combusted rather than reformed into hydrogen, the same amount of water vapour would have been emitted and the same amount of oxygen consumed:(5)4CH4 + 8O2 → 4CO2 + 8H2O Clearly the electrolysis pathway ensures water and oxygen conservation, while the ATR pathway achieves neither. Switching from fossil fuels to blue hydrogen production-and-use will continue the depletion of atmospheric oxygen and continue to increase water vapour emissions, while simply consuming more fossil fuels (due to the parasitic energy requirements of the reformation and sequestration processes). The alternative of switching to ‘turquoise’ hydrogen produced by methane pyrolysis will achieve a similar outcome. When compared with blue hydrogen, the production-and-use of turquoise hydrogen will consume slightly less oxygen but emit considerably more water vapour (see Figure 2). As a fuel, green hydrogen stands alone because it avoids impacting oxygen, water and CO2 levels (see Figure 3).
Reducing oxygen depletion
Estimates vary, but there is approximately 1.2 × 1018 kg of oxygen in the atmosphere. The aforementioned annual depletion rate of 0.0015% equates to a loss of about 18 gigatonnes (Gt). It is interesting to estimate the electrolyser capacity that would be required to counteract this scale of oxygen consumption. Current electrolyser performance is characterised by an oxygen production rate of up to 1.3 Gt O2 per TW, so an installed capacity of about 14 TW would yield up to 18 Gt O2 per annum. The concurrent annual production of hydrogen would be up to 2.3 Gt H2 (~77 000 TWh, lower heating value LHV).
This scale of electrolyser implementation may be used to guide future scenarios for achieving both a climate-neutral energy system and a much reduced rate of oxygen depletion. To achieve these objectives, the 2030 deployment targets (such as those recently set by the European Commission of 2 × 40 GW, and Chile of 25 GW), will need to be succeeded by TW-scale targets for 2040 and 2050. Implementation will involve much more than just installing very large capacities of renewable power sources and electrolysers: it will require extensive use of subterranean hydrogen storage (to manage the temporal mismatch between renewable energy supply and demand), hydrogen transmission pipeline infrastructures (to convey hydrogen to storage and the relevant points of use), and hydrogen carriers for long-distance transfer of renewable energy by ship (e.g. green ammonia).
In general, the future energy system design needs to be reoriented around renewable energy capture, electrolyser deployment, and the storage, distribution and use of electrolytic hydrogen. Fortunately, venting electrolytic oxygen to atmosphere is standard practice for electrolyser installations, so abating oxygen depletion presents no additional complications or costs. Electrolysers can also be designed to produce electrolytic oxygen at pressure for oxygenating lakes, river basins and oceans at depth if required (e.g. an electrolyser generating oxygen at 20 bar would enable oxygenation at water depths of up to 200 m without requiring additional energy for compression).
For a future energy system that makes extensive use of hydrogen, leaks and releases into the atmosphere should be expected – the very low density of hydrogen simply makes it the most leaky gas. The addition of hydrogen to the atmosphere will influence ozone, methane and water vapour concentrations. It may cause a small degree of stratospheric cooling and so slow down the recovery of the ozone layer, while increasing the build-up of methane and ozone in the troposphere, which will promote global warming. Based on limited data and the indirect role that hydrogen plays in global warming, its Global Warming Potential (GWP) value has been estimated to be about 4.3 on a 100-year time base (versus 1.0 for CO2). Therefore estimates should be made of the extent to which hydrogen leaks and releases will occur in practice from various designs of energy system, and their environmental consequences predicted for a range of future scenarios, including minimum use of fossil fuels and maximum use of electrolytic hydrogen. Clearly, a slightly greater rate of hydrogen production will be required to compensate for any hydrogen losses, but it should be noted that the corresponding rate of electrolytic oxygen production is always theoretically sufficient to oxidise the excess hydrogen. Only electrolysis offers this self-compensating capability.
Further research should address the long-term possibility of increasing slightly the oxygen concentration, in order to provide a degree of global cooling and help accelerate the decay of methane in the atmosphere. However, electrolysis of itself will not achieve this; it is only able to redress the balance for oxygen depletion due to fuel use. Therefore an overarching environmental strategy is needed for oxygen, which should involve a mass deployment of electrolysers in combination with other measures (including direct removal of greenhouse gases, forestation, and solutions for slowing/reversing oxygen depletion in the oceans). >p> Conclusions
Implementing hydrogen as a zero-emission fuel is not enough to combat global warming; it is now essential that we switch to the only oxygen and water balanced fuel: green hydrogen. Using hydrogen derived from fossil fuels results in a net decrease in atmospheric oxygen and a net increase in water vapour, irrespective of whether CCS is applied to the production process. Conversely, hydrogen derived from water electrolysis neither results in oxygen depletion nor increases the atmospheric concentrations of water vapour and CO2. It is therefore fundamentally important to avoid viewing blue, turquoise and green hydrogen as one and the same. It is not valid to compare these types of hydrogen on the basis of CO2 footprint alone, when they have markedly different impacts on atmospheric oxygen and water vapour. Policy makers and governments should place a priority on producing and using green hydrogen, not blue or turquoise hydrogen.
Future efforts to combat climate change should include the deployment of very substantial installed capacities of electrolysers. This will slow down the rate of oxygen depletion and provide us with a fuel that has minimal impact on the environment, provided that the energy system is designed to minimise hydrogen leaks and releases. The solution to the O2 and CO2 concentration problems is to minimise fossil fuel combustion and avoid using fossil fuels to make hydrogen. The global ecosystem now requires us to extract hydrogen and oxygen from water. Achieving a multi-TW electrolyser capacity by mid-century would have a massive positive impact on combatting climate change.