|
|
| Fig. 1: Lower heating values and energy transport rates for natural gas (left), 80% natural gas and 20% hydrogen gas (center), and hydrogen gas (right). Pipe cross-sectional area and pressure gradient was chosen arbitrarily to make the natural gas energy flow rate 1 GJ/s and kept constant across different mixtures. [1,3,4] (Image Source: D. Merrell) |
Due to concerns over greenhouse gas emissions related to the burning of natural gas, alternative chemical fuels, namely hydrogen, have been considered as replacements. One popular suggestion to ease the transition to green energy is the adaptation and use of existing natural gas infrastructure to facilitate the transport of hydrogen across the United States. Although this idea may seem reasonable on its face, there are numerous real challenges to its implementation.
Before considering the difficulties associated with adapting infrastructure for the transport of hydrogen, its worth considering the sourcing of hydrogen itself. Hydrogen exists in extremely small amounts in the atmosphere, with most of Earths hydrogen residing in the crust. Currently, hydrogen gas is sourced from the reforming of natural gas and other hydrocarbons. Other potential sources of hydrogen are being investigated, such as via biological processes, but are still not developed enough for commercial use. Ultimately, many proposed alternative hydrogen sources would rely on solar or other renewables for its green generation. [1] Needless to say, there is little point in designing an economy around hydrogen in order to reduce greenhouse gas emissions if its sourced via carbon-intensive processes like coal gasification or just an energy carrier in the case of generating hydrogen via renewables.
Another consideration is the overall ownership of natural gas pipelines. Private natural gas companies in the United States provide around 90% of natural gas by volume and, on average, individually operate 3706 miles of mains each. Public services provide the remaining 10% with an average of 176 miles for each public distributor. [2] Therefore, the overwhelming majority of natural gas mains are under the purview of natural gas distributors, meaning natural gas companies would have to adapt their own pipelines to hydrogen. Unless these companies switch to hydrogen generation, it seems unlikely they would participate in an adaptation of natural gas pipelines that would be against their own interests.
When considering the potential implications of replacing natural gas with hydrogen, perhaps the most obvious issue is the difference in physical properties. Consider the ideal gas law in its mass specific form:
Where P is the pressure in Pascals, the Greek letter rho is the gas density in kilograms per cubic meter, R is the universal gas constant in Joules per mole Kelvin, M is the molecular weight of the gas in kilograms per mole, and T is the temperature in Kelvin. Although natural gas pipelines operate at pressures where real gas effects perturb the ideal gas law, it is still a good approximation of the equation of state. Assuming pipelines operate at the same pressure and temperature as they do now and using the molecular weights of hydrogen gas (2 g/mol) and the main constituent of natural gas, methane (16 g/mol), it can be shown that the equivalent mass flow rate in the hydrogen system would be much lower. For a slow gas or liquid, the volumetric flow rate due to a pressure differential can be derived from Bernoulli's principle to be proportional to the square root of the ratio of the pressure gradient to the gas density, where U is the gas velocity in meters per second. A quick look at the Bernoulli's principle below would indicate that for a pressure gradient along a pipe, the gas would continue to accelerate infinitely. This is because friction is neglected in Bernoulli's equation. When included, a finite velocity can be obtained for a given pressure differential. As the friction term is a function of the pipeline itself, its convenient to generalize the result in terms of proportionality rather than try to predict the exact friction force. The ∝ designates that the velocity is proportional to the square root of the ratio of the pressure gradient to the gas density, which holding the pressure gradient constant across both natural gas and hydrogen, results in an expression solely dependent on the molecular weights.
| P1 + 1/2 ρ U12 = P2 + 1/2 ρ U22 |
| U ∝ (ΔP/ρ)1/2 = (1/M)1/2 |
Plugging this into the equation for the mass flow rates, where m ̇ is the mass flow rate in kilograms per second and A is the cross-sectional area of the gas pipeline in square meters:
Now consider the lower-heating value (LHV), a measurement of the energy density of a fuel, for both hydrogen gas ( ~ 120 MJ/kg )and natural gas ( ~ 47.1 MJ/kg). [1,3] This means that the energy flow rate in existing pipelines would change by a proportionate amount, resulting in:
Thus, there is a 10% reduction in energy transport rate, assuming that current gas pipeline pressures are maintained. Its also worth noting that different pipelines operate with different types of natural gas, potentially changing this number between 2% and 20%, as well as different loadings of hydrogen. [4] As displayed in Fig. 1, a mixture of hydrogen and natural gas can have a lower energy transport rate than the individual constituents, complicating efforts to blend natural gas and hydrogen to avoid other negative effects. To overcome this, pipelines could operate at higher pressures, which, if the pipeline itself could withstand an increase in pressure, would require an increase in pumping station performance and thus an increase in energy expended in the distribution of fuel.
Even more so, hydrogen itself is more difficult to compress than natural gas. High performance pumps necessary to maintain 100s of atmospheres of pressure in natural gas pipelines operate via principles incompatible with efficient hydrogen compression due to hydrogen's high speed of sound. The speed of sound in a gas can be thought of as the speed at which pressure forces travel throughout the gas, or in other words, the reaction time of the gas to quick changes in density or pressure. When molecules are accelerated past this speed, they can be thought of as travelling faster than the pressure forces can react to their motion. Turbopumps operate by imparting momentum to particles through spinning blades. When the speed of sound of the pumped molecule is low, the motion of the particle due to the blade is overwhelming compared to the random motion of the gas and motion due to density gradients in the pump. When the speed of sound is high, such as for hydrogen, it becomes difficult to impart enough momentum to overcome these other factors and a different pumping technique must be used. This would necessitate the replacement of all pumping stations within the natural gas infrastructure in order to maintain the same operating pressures.
In addition to the difference in the physical properties, hydrogen can cause the weakening of certain metals, especially steels, known as hydrogen embrittlement. In a 2016 report, NASA noted that A106 Gr. B, a common carbon steel used in natural gas pipelines, was highly susceptible to hydrogen embrittlement at operating pressures of 6.9 MPa, well within the operating pressures of natural gas pipelines. Some mitigation strategies exist, but most of them rely on treating the metal during manufacturing. The only strategies that can be employed on existing pipelines would be surface treatments that, according to NASA, would likely need to be reapplied continually, or operating at significantly lower pressures or hydrogen fractions which would reduce the total hydrogen mass flow rate and introduce other problems. [5] Alternatively, pipe sections could be replaced with aluminum alloys which are known to be resistant to hydrogen embrittlement, but doing so would defeat the purpose of adapting existing infrastructure. There has been some investigation into ammonia as a hydrogen carrier to avoid embrittlement, but it is low energy density, toxic, sticky, and corrosive. [6]
All else held equal, hydrogen itself has serious safety risks compared to natural gas. Methane gas has narrow flammability limits in air with lower and upper limits in volume percentage of 5% and 15% respectively. Hydrogen, on the other hand, has limits of 4% and 75% in air. [3] In the case of leaks, this means there is a much larger dangerous area surrounding the source where a spark could cause a flame. Furthermore, the lack of carbon in hydrogen gas results in an invisible flame, making hydrogen flames due to leaks difficult to detect. As a result, much stricter safety standards would need to be enforced.
As many of the surrounding issues with using hydrogen have yet to be adequately addressed, its difficult to estimate the total amount of money that would be saved by adapting existing pipelines versus building new ones. One paper by Cerniauskas et al. found, in an analysis of German pipelines, that using existing pipelines could lower the transmission cost from €59/MWh for a new pipeline to €44/MWh if pipelines could be used in their existing state, assuming a relatively low hydrogen embrittlement rate. [7] However, this change in pricing is relatively small compared to the production and supply of hydrogen, which is estimated to be ~ 9.6 euro/kg using new pipelines and ~9.2 euro/kg using existing pipelines in 2030. [8] They also propose the use of chemical inhibitors in the form of additives to prevent embrittlement, but the proposed inhibitors are toxic and potential oxidizers, further reducing safety and likely resulting in heightened regulations.
In conclusion, its unlikely that adapting current natural gas infrastructure would significantly reduce electricity costs or availability in a new hydrogen economy without significant strides in hydrogen technology. Due to problems like embrittlement and safety concerns, utilizing existing pipelines without modification would likely result in a similar transport cost as that of new pipelines. Modifications to existing pipelines would require the excavation of large sections, replacing the entire pipeline with a different material or coating the inside. As hydrogen pricing currently stands, most of the cost is tied up in production and distribution to pipelines, not in delivery. If hydrogen production problems are addressed and green hydrogen is readily available in large quantities in the future, its possible that the transmission cost of hydrogen would become much more important in the overall pricing and some form of pipeline adaptation may be economical.
© Devin Merrell. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] S. Z. Baykara, " Hydrogen: A Brief Overview on Its Sources, Production and Environmental Impact," Int. J. Hydrogen Energy 43, 10605 (2018).
[2] R.P. Scott, T. A. Scott, and R. A. Greer, "Who Owns the Pipes? Utility Ownership, Infrastructure Conditions, and Methane Emissions in United States Natural Gas Distribution," Rev. Policy Res. 39, 170 (2022).
[3] J. M. Kuchta, "Investigation of Fire and Explosion Accidents in the Chemical, Mining, and Fuel-Related Industries - A Manual," U.S. Bureau of Mines, Bulletin. 680, 1986.
[4] D. Haeseldonckx and W. D;haeseleeer, "The Use of the Natural-Gas Pipeline Infrastructure For Hydrogen Transport in a Changing Market Structure," Int. J. Hydrogen Energy 32,,1381 (2007).
[5] J. A. Lee, "Hydrogen Embrittlement," U.S. National Aeronautics and Space Administration, NASA/TM-2016218602, April 2016.
[6] S. Chatterjee, R. K. Parsapur, and K.-W. Huang, "Limitations of Ammonia as a Hydrogen Energy Carrier for the Transportation Sector," ACS Energy Lett. 6, 4390 (2021).
[7] N. Nanninga et al., "A Review of Fatigue Crack Growth for Pipeline Steels Exposed to Hydrogen," J. Res. Natl. Inst. Stand. Technol. 115, 437 (2010).
[8] S. Cerniauskas et al., "Options of Natural Gas Pipeline Reassignment For Hydrogen: Cost Assessment For a Germany Case study," Int. J. Hydrogen Energy 45, 12095 (2020).