¿Como de verde es el Hidrogeno azul?Robert W. Howarth, Mark Z. Jacobson

 

 El hidrógeno suele considerarse un importante vector energético en un futuro mundo descarbonizado. Actualmente, la mayor parte del hidrógeno se produce mediante el reformado al vapor del metano del gas natural ("hidrógeno gris"), con altas emisiones de dióxido de carbono. Cada vez son más los que proponen utilizar la captura y el almacenamiento de carbono para reducir estas emisiones, produciendo el llamado "hidrógeno azul", frecuentemente promocionado como de bajas emisiones. En un artículo revisado por expertos, se examina por primera vez el ciclo de vida de las emisiones de gases de efecto invernadero del hidrógeno azul, teniendo en cuenta las emisiones de dióxido de carbono y de metano fugitivo no quemado. Lejos de ser bajas en carbono, las emisiones de gases de efecto invernadero derivadas de la producción de hidrógeno azul son bastante elevadas, especialmente debido a la liberación de metano fugitivo. Para nuestros supuestos por defecto (tasa de emisión de metano del gas natural del 3,5% y un potencial de calentamiento global de 20 años), las emisiones totales de dióxido de carbono equivalente para el hidrógeno azul son sólo un 9%-12% menos que para el hidrógeno gris. Mientras que las emisiones de dióxido de carbono son menores, las emisiones fugitivas de metano del hidrógeno azul son mayores que las del hidrógeno gris debido al mayor uso de gas natural para alimentar la captura de carbono. Quizás sea sorprendente que la huella de gases de efecto invernadero del hidrógeno azul sea más de un 20% mayor que la quema de gas natural o carbón para calefacción y alrededor de un 60% mayor que la quema de gasóleo para calefacción, de nuevo con nuestras hipótesis por defecto. En un análisis de sensibilidad en el que la tasa de emisión de metano del gas natural se reduce a un valor bajo de 1,54%, las emisiones de gases de efecto invernadero del hidrógeno azul siguen siendo mayores que las de la simple combustión de gas natural, y sólo son un 18%-25% menores que las del hidrógeno gris. Nuestro análisis supone que el dióxido de carbono capturado puede almacenarse indefinidamente, una hipótesis optimista y no probada. Sin embargo, incluso si fuera cierto, el uso del hidrógeno azul parece difícil de justificar por motivos climático

How green is blue hydrogen?

Robert W. Howarth, Mark Z. Jacobson

First published: 12 August 2021

https://doi.org/10.1002/ese3.956

Funding information:

Funding was provided by the Park Foundation and by Cornell University

https://onlinelibrary.wiley.com/doi/full/10.1002/ese3.956

1 INTRODUCTION

Hydrogen is widely viewed as an important fuel for a future energy transition. Currently, hydrogen is used mostly by industry during oil-refining and synthetic nitrogen fertilizer production, and little is used for energy because it is expensive relative to fossil fuels.¹ However, hydrogen is increasingly being promoted as a way to address climate change, as indicated by a recent article in the New York Times.² In this view, hydrogen is to be used not only for hard to decarbonize sectors of the economy such as long-distance transportation by trucks and airplanes but also for heating and cooking, with hydrogen blended with natural gas and distributed to homes and business through existing pipeline systems.² Utilities are also exploring the use of hydrogen, again blended with natural gas, to power existing electric generating facilities.³ In Europe, a recent report from Gas for Climate, an association of natural gas pipeline companies, envisions large scale use of hydrogen in the future for heating and electricity generation.⁴ The Hydrogen Council, a group established in 2017 by British Petroleum, Shell, and other oil and gas majors, has called for heating all homes with hydrogen in the future.⁵

The vast majority of hydrogen (96%) is generated from fossil fuels, particularly from steam methane reforming (SMR) of natural gas but also from coal gasification.⁶ In SMR, which is responsible for approximately three quarters of all hydrogen production globally,⁷ heat and pressure are used to convert the methane in natural gas to hydrogen and carbon dioxide. The hydrogen so produced is often referred to as “gray hydrogen,” to contrast it with the “brown hydrogen” made from coal gasification.⁸ Production of gray hydrogen is responsible for 6% of all natural gas consumption globally.⁷ Hydrogen can also be generated by electrolysis of water. When such electricity is produced by a clean, renewable source, such as hydro, wind, or solar, the hydrogen is termed “green hydrogen.” In 2019, green hydrogen was not cost competitive with gray hydrogen,⁹ but that is changing as the cost of renewables is decreasing rapidly and electrolyzers are becoming more efficient. Still, the supply of green hydrogen in the future seems limited for at least the next several decades.^{2, 5}

Greenhouse gas emissions from gray hydrogen are high,^{10, 11} and so increasingly the natural gas industry and others are promoting “blue hydrogen”.^{5, 8, 9} Blue hydrogen is a relatively new concept and can refer to hydrogen made either through SMR of natural gas or coal gasification, but with carbon dioxide capture and storage. As of 2021, there were only two blue-hydrogen facilities globally that used natural gas to produce hydrogen at commercial scale, as far as we can ascertain, one operated by Shell in Alberta, Canada, and the other operated by Air Products in Texas, USA.¹² Often, blue hydrogen is described as having zero or low greenhouse gas emissions.^{8, 9} However, this is not true: not all of carbon dioxide emissions can be captured, and some carbon dioxide is emitted during the production of blue hydrogen.¹ Further, to date no peer-reviewed analysis has considered methane emissions associated with producing the natural gas needed to generate blue hydrogen.¹ Methane is a powerful greenhouse gas. Compared mass-to-mass, it is more than 100-times more powerful as a warming agent than carbon dioxide for the time both gases are in the atmosphere and causes 86-times the warming as carbon dioxide over an integrated 20-year time frame after a pulsed emission of the two gases. Approximately 25% of the net global warming that has occurred in recent decades is estimated to be due to methane.¹³ In a recent report, the United Nations Environment Programme concluded that methane emissions globally from all sources need to be reduced by 40%-45% by 2030 in order to achieve the least cost pathway for limiting the increase in the Earth's temperature to 1.5°C, the target set by COP 21 in Paris in December 2015.¹⁴

Here, we explore the full greenhouse gas footprint of both gray and blue hydrogen, accounting for emissions of both methane and carbon dioxide. For blue hydrogen, we focus on that made from natural gas rather than coal, that is gray hydrogen combined with carbon capture and storage. In China, brown hydrogen from coal now dominates over gray hydrogen from natural gas, due to the relative prices of natural gas and coal, but globally and particular in Europe and North America, gray hydrogen dominates.¹

^{El hidrógeno se considera un combustible importante para la futura transición energética. En la actualidad, el hidrógeno se utiliza sobre todo en la industria durante el refinado del petróleo y la producción de fertilizantes nitrogenados sintéticos, y se utiliza poco para la energía porque es caro en relación con los combustibles fósiles.1 Sin embargo, el hidrógeno se promueve cada vez más como una forma de abordar el cambio climático, como se indica en un artículo reciente en el New York Times.2 En esta opinión, el hidrógeno se va a utilizar no sólo para los sectores de la economía difíciles de descarbonizar, como el transporte de larga distancia en camiones y aviones, sino también para la calefacción y la cocina, con el hidrógeno mezclado con gas natural y distribuido a los hogares y las empresas a través de los sistemas de tuberías existentes.2 Las empresas de servicios públicos también están explorando el uso de hidrógeno, de nuevo mezclado con gas natural, para alimentar las instalaciones de generación eléctrica existentes.3 En Europa, un informe reciente de Gas for Climate, una asociación de empresas de gasoductos de gas natural, prevé el uso a gran escala del hidrógeno en el futuro para la calefacción y la generación de electricidad.4 El Consejo del Hidrógeno, un grupo creado en 2017 por British Petroleum, Shell y otras grandes empresas de petróleo y gas, ha pedido que se calienten todos los hogares con hidrógeno en el futuro.5}

^{La gran mayoría del hidrógeno (96%) se genera a partir de combustibles fósiles, en particular del reformado de metano por vapor (SMR) del gas natural, pero también de la gasificación del carbón.6 En el SMR, que es responsable de aproximadamente tres cuartas partes de toda la producción de hidrógeno a nivel mundial,7 se utiliza calor y presión para convertir el metano del gas natural en hidrógeno y dióxido de carbono. El hidrógeno así producido suele denominarse "hidrógeno gris", en contraste con el "hidrógeno marrón" que se obtiene de la gasificación del carbón.8 La producción de hidrógeno gris es responsable del 6% de todo el consumo de gas natural en el mundo.7 El hidrógeno también puede generarse por electrólisis del agua. Cuando dicha electricidad es producida por una fuente limpia y renovable, como la hidráulica, la eólica o la solar, el hidrógeno se denomina "hidrógeno verde." En 2019, el hidrógeno verde no era competitivo en cuanto a costes con el hidrógeno gris9 , pero eso está cambiando, ya que el coste de las energías renovables está disminuyendo rápidamente y los electrolizadores son cada vez más eficientes. Aun así, el suministro de hidrógeno verde en el futuro parece limitado al menos durante las próximas décadas.2, 5}

^{Las emisiones de gases de efecto invernadero del hidrógeno gris son elevadas,10, 11 por lo que cada vez más la industria del gas natural y otros están promoviendo el "hidrógeno azul".5, 8, 9 El hidrógeno azul es un concepto relativamente nuevo y puede referirse al hidrógeno fabricado mediante SMR de gas natural o gasificación de carbón, pero con captura y almacenamiento de dióxido de carbono. Hasta 2021, sólo había dos instalaciones de hidrógeno azul en todo el mundo que utilizaban gas natural para producir hidrógeno a escala comercial, por lo que hemos podido comprobar, una operada por Shell en Alberta (Canadá) y la otra por Air Products en Texas (EE.UU.).12 A menudo se describe el hidrógeno azul como un producto con cero o bajas emisiones de gases de efecto invernadero, 9 Sin embargo, esto no es cierto: no todas las emisiones de dióxido de carbono pueden capturarse, y se emite algo de dióxido de carbono durante la producción de hidrógeno azul.1 Además, hasta la fecha ningún análisis revisado por expertos ha considerado las emisiones de metano asociadas a la producción del gas natural necesario para generar hidrógeno azul.1 El metano es un potente gas de efecto invernadero. Comparado masa a masa, es más de 100 veces más potente como agente de calentamiento que el dióxido de carbono durante el tiempo que ambos gases están en la atmósfera y provoca un calentamiento 86 veces superior al del dióxido de carbono en un periodo integrado de 20 años tras una emisión pulsada de los dos gases. Se calcula que aproximadamente el 25% del calentamiento global neto que se ha producido en las últimas décadas se debe al metano.13 En un informe reciente, el Programa de las Naciones Unidas para el Medio Ambiente concluyó que las emisiones de metano a nivel mundial, procedentes de todas las fuentes, deben reducirse entre un 40% y un 45% para el año 2030, con el fin de lograr la vía de menor coste para limitar el aumento de la temperatura de la Tierra a 1,5°C, el objetivo fijado por la COP 21 de París en diciembre de 2015.14}

^{Aquí exploramos toda la huella de gases de efecto invernadero del hidrógeno gris y azul, teniendo en cuenta las emisiones de metano y de dióxido de carbono. En el caso del hidrógeno azul, nos centramos en el producido a partir de gas natural y no de carbón, es decir, el hidrógeno gris combinado con la captura y el almacenamiento de carbono. En China, el hidrógeno marrón procedente del carbón domina ahora sobre el hidrógeno gris procedente del gas natural, debido a los precios relativos del gas natural y del carbón, pero a nivel mundial, y en particular en Europa y Norteamérica, domina el hidrógeno gris.1}

2 ESTIMATING EMISSIONS FROM PRODUCING GRAY HYDROGEN

Greenhouse gas emissions from the production of gray hydrogen can be separated into two parts: (a) the SMR process in which methane is converted to carbon dioxide and hydrogen; and (b) the energy used to generate the heat and high pressure needed for the SMR process. For blue hydrogen, which we discuss later in this paper, emissions from the generation of electricity needed to run the carbon dioxide capture equipment must also be included. In this analysis, we consider emissions of only carbon dioxide and methane, and not of other greenhouse gases such as nitrous oxide that are likely to be much smaller. For methane, we consider the major components of its lifecycle emissions associated with the mining, transport, storage, and use of the natural gas needed to produce the hydrogen and power carbon capture. Emissions are expressed per unit energy produced when combusting the hydrogen, to aid in comparing the greenhouse gas footprint with other fuels.^{15, 16} In this paper, we use gross calorific values.

We start by estimating how much methane is consumed and how much carbon dioxide is produced in the two aspects of production of gray hydrogen. From this information, we can subsequently below estimate emissions of unburned methane.

Las emisiones de gases de efecto invernadero de la producción de hidrógeno gris pueden separarse en dos partes: (a) el proceso de SMR en el que el metano se convierte en dióxido de carbono e hidrógeno; y (b) la energía utilizada para generar el calor y la alta presión necesarios para el proceso de SMR. En el caso del hidrógeno azul, del que hablamos más adelante, también hay que incluir las emisiones procedentes de la generación de electricidad necesaria para el funcionamiento del equipo de captura de dióxido de carbono. En este análisis, sólo consideramos las emisiones de dióxido de carbono y de metano, y no las de otros gases de efecto invernadero, como el óxido nitroso, que probablemente sean mucho menores. En el caso del metano, tenemos en cuenta los principales componentes de las emisiones de su ciclo de vida asociados a la extracción, el transporte, el almacenamiento y el uso del gas natural necesario para producir el hidrógeno y alimentar la captura de carbono. Las emisiones se expresan por unidad de energía producida al quemar el hidrógeno, para facilitar la comparación de la huella de gases de efecto invernadero con otros combustibles.15, 16 En este trabajo, utilizamos valores caloríficos brutos.

Comenzamos estimando cuánto metano se consume y cuánto dióxido de carbono se produce en los dos aspectos de la producción de hidrógeno gris. A partir de esta información, podemos estimar a continuación las emisiones de metano no quemado.

2.1 Consumption of methane and production of carbon dioxide in SMR process

In the SMR process, 1 mole of carbon dioxide and 4 moles of hydrogen gas (H₂) are produced per mole of methane consumed, according to this overall reaction:

(1)

The gross caloric calorific heat content of hydrogen is 0.286 MJ per mole,¹⁷ or inverting this value, 3.5 moles H₂ per MJ. The carbon dioxide produced during the SMR process is given by:

(2)

With a molecular weight of 44.01 g per mole, the amount of carbon dioxide produced during the SMR process is 38.51 g CO₂ per MJ (Table 1). The amount of methane consumed is given by:

(3)

TABLE 1. Comparison of methane that is consumed, of carbon dioxide that is produced, and of emissions of both methane and carbon dioxide for each step in the processing of methane to hydrogen for gray hydrogen, blue hydrogen with carbon dioxide capture from the SMR process but not from the exhaust flue gases created from burning natural gas to run the SMR equipment, and blue hydrogen with carbon dioxide capture from both the SMR process and from the exhaust flue gases

	Gray H2	Blue H2 (w/o flue-gas capture)	Blue H2 (w/flue-gas capture)
SMR process
CH4 consumed (g CH4/MJ)	14.0	14.0	14.0
CO2 produced (g CO2/MJ)	38.5	38.5	38.5
Fugitive CH4 emissions (g CH4/MJ)	0.49	0.49	0.49
Fugitive CH4 emissions (g CO2eq/MJ)	42.1	42.1	42.1
Direct CO2 emissions (g CO2/MJ)	38.5	5.8	5.8
CO2 capture rate	0%	85%	85%
Energy to drive SMR
CH4 consumed (g CH4/MJ)	11.6	11.6	11.6
CO2 produced (g CO2/MJ)	31.8	31.8	31.8
Fugitive CH4 emissions (g CH4/MJ)	0.41	0.41	0.41
Fugitive CH4 emissions (g CO2eq/MJ)	35.3	35.3	35.3
Direct CO2 emissions (g CO2/MJ)	31.8	31.8	11.1
CO2 capture rate	0%	0%	65%
Energy to power carbon capture
CH4 consumed (g CH4/MJ)	0	3.0	6.0
CO2 produced (g CO2/MJ)	0	8.2	16.3
Fugitive CH4 emissions (g CH4/MJ)	0	0.11	0.21
Fugitive CH4 emissions (g CO2eq/MJ)	0	9.5	1
Direct CO2 emissions (g CO2/MJ)	0	8.2	16.0
Indirect upstream CO2 emissions (g CO2/MJ)	5.3	5.9	6.5
Total CH4 consumed (g CH4/MJ)	25.6	28.6	31.6
Total CO2 emitted (g CO2/MJ)	75.6	51.7	39.7
Total fugitive CH4 emissions (g CO2eq/MJ)	77.4	86.9	95.4
Total emissions (g CO2eq/MJ)	153	139	135

Note

• The methane leakage rate is 3.5%.

With a molecular weight of 16.04 g per mole, 14.04 g CH₄ per MJ is consumed during the SMR process (Table 1). There is essentially no uncertainty in these estimates of how much methane is consumed, and how much carbon dioxide is produced during the SMR process: the relationship is set by the chemical stoichiometry shown in Equation (1).

2.2 Consumption of methane and production of carbon dioxide from energy needed to drive SMR process

The production of hydrogen from methane is an endothermic reaction and requires significant input of energy, between 2.0 and 2.5 kWh per m³ of hydrogen, to provide the necessary heat and pressure.¹⁸ This energy comes almost entirely from natural gas when producing gray hydrogen, and therefore, also presumably when producing blue hydrogen proposed for Europe or North America.¹ Using a mean value of 2.25 kWh per m³ of hydrogen, we estimate the energy in natural gas (methane) required to produce a mole of hydrogen as follows:

(4)

That is, 0.1814 MJ of energy from burning methane is required per mole of hydrogen produced. When burning natural gas for heat, 50 g CO₂ per MJ in emissions are produced, using gross calorific values.¹⁹ Note that higher carbon dioxide emission values are reported when using net calorific values.

Therefore,

(5)

As noted above, the gross calorific heat content of hydrogen is equivalent to 3.5 moles H₂ per MJ. Therefore,

(6)

So 31.8 g of carbon dioxide are produced to generate the heat and pressure to drive the SMR process per MJ of hydrogen produced (Table 1). Since one mole of methane in natural gas is burned to produce one mole of carbon dioxide emissions, we can estimate the methane consumed as follows:

(7)

See Table 1.

2.3 Total carbon dioxide and methane emissions for gray hydrogen

The sum of the carbon dioxide from the SMR process (38.5 g CO₂ per MJ) and from the energy used to generate the heat and electricity for the SMR (31.8 g CO₂ per MJ) is 70.3 g CO₂ per MJ. Additionally, it takes energy to produce, process, and transport the natural gas used to generate the hydrogen. Using the analysis of Santoro et al.²⁰ as reported in Howarth et al,²¹ these indirect upstream emissions are approximately 7.5% of the direct carbon dioxide emissions for natural gas, or an additional 5.3 g CO₂ per MJ (7.5% of 70.3 g CO₂ per MJ). Therefore, the total quantity of carbon dioxide produced is 75.6 g CO₂ per MJ (Table 1).

The total quantity of methane in natural gas consumed to generate gray hydrogen is the sum of that used in the SMR process (14.04 g CH₄ per MJ) and the amount burned to generate the heat and high pressure needed for the process (11.6 g CH₄ per MJ) or 25.6 g CH₄ per MJ. It is not possible to produce and use natural gas without having some methane emitted unburned to the atmosphere, due both to leaks and to purposeful emissions including venting.^{21, 22} Below, we briefly discuss the recent literature that characterizes methane emissions from natural gas operations, and use a range of values in a sensitivity analysis. Here, for our default estimation of the greenhouse gas footprint of gray hydrogen, we rely on a recent synthesis on “top–down” emission studies.¹⁶ Top–down estimates use information such as from satellites or airplane flyovers that characterize an integrated flux. The mean value of estimates from 20 different studies in 10 major natural gas fields in the United States, normalized to gas production in those fields, indicates that 2.6% of gas production is emitted to the atmosphere.¹⁶ This is a good estimate for the upstream emissions that occur in the gas fields. Methane is also emitted from storage and transport to consumers, and the data in the top–down study of Plant et al²³ suggests this is an additional 0.8%.^{16, 24} Combined with the 2.6% for field-level emissions, we estimate a total of 3.4% of production is emitted to the atmosphere overall. Note that in addition to some methane being lost between production and consumption due to leaks, methane is also burned by the natural gas industry to power natural gas processing and transport. This is important to consider, since we want to evaluate how much methane is emitted for the methane in natural gas that is consumed in producing hydrogen. In 2015, natural gas production in the United States was 817 billion m³, while consumption was 771 billion m³,^{25, 26} (converting cubic feet to cubic meters). Using this information, we can estimate the methane emission as a percentage of gas consumption as follows:

(8)

With this value and the quantity of methane consumed to produce gray hydrogen, we can estimate the upstream emissions of methane:

(9)

To compare methane emissions with carbon dioxide emissions requires a specified time frame, since the half-life of methane in the atmosphere is only 12 years or so, far less than that of carbon dioxide.¹³ Greenhouse gas inventories often compare methane with carbon dioxide for an integrated period of 100 years following pulsed emissions of both gases. However, this underestimates the role of methane in global warming over shorter time periods. An increasing number of scientists have called for using a 20-year integrated time period instead of or in addition to the 100-year period.^{15, 21, 24, 27, 28} The 20-year time frame is now mandated by law in the State of New York, as part of the Climate Leadership and Community Protection Act of 2019.²⁴ And a 20-year period is more appropriate than a 100-year time frame given the urgency of reducing methane emissions globally over the coming decade.¹⁴ Here, we use the 20-year time frame using the Global Warming Potential (GWP) for 20 years of 86.¹³ We also consider other GWP values in a sensitivity analysis presented below. Using the 86 value, we estimate upstream methane emissions associated with the production of gray hydrogen in units of carbon dioxide equivalents (CO₂eq) thus:

(10)

The sum of emissions of carbon dioxide (75.6.0 g CO₂ per MJ) and unburned methane (77.4 g CO₂eq per MJ) for the production of gray hydrogen is 153 g CO₂eq per MJ (Table 1).

There are remarkably few published peer-reviewed papers with which to compare our estimate. Many non peer-reviewed reports give estimates for carbon dioxide emission from gray hydrogen that are in the range of 10 tons carbon dioxide per ton of hydrogen,^{1, 7} although data in support of these values are generally absent, perhaps because they are based on confidential information.¹¹ Since the gross calorific heat energy content of hydrogen is 0.286 MJ per mole,¹⁷ 10 tons of carbon dioxide per ton of hydrogen corresponds to 70 g CO₂ per MJ. This is similar to but somewhat lower than our value of 75.6 g CO₂ per MJ. Most of these non peer-reviewed reports do not include methane in their estimates,¹ or if they do, they provide no detail as to how they do so. The most thorough peer-reviewed analysis of carbon dioxide emissions for gray hydrogen is that of Sun et al¹¹ who obtained data on both rates of hydrogen production and emissions of carbon dioxide from many individual facilities across the United States. They concluded that on average, carbon dioxide emissions for gray hydrogen are 77.8 g CO₂ per MJ, remarkably close to our value of 75.6 g CO₂ per MJ. They did not estimate methane emissions.

3 ESTIMATING EMISSIONS FOR BLUE HYDROGEN

Blue hydrogen differs from gray hydrogen in that, with blue hydrogen, some of the carbon dioxide released by the SMR process is captured. In another version of the blue-hydrogen process, additional carbon dioxide is removed from the flue gases created from burning natural gas to provide the heat and high pressure needed to drive the SMR process. A third set of emissions, not usually captured, is the carbon dioxide and methane from the energy used to produce the electricity for the carbon-capture equipment.

3.1 How much carbon dioxide is emitted after carbon capture?

As noted above, only two facilities that produce blue hydrogen from natural gas are in commercial operation in 2021. Thus, only limited data are available on the percentage of carbon dioxide that can be captured. For the carbon dioxide generated during SMR, the reported capture efficiencies range from 53% to 90%.²⁹ Actual data from one of the two commercially operating facilities, the Shell plant in Alberta, show a capture a mean capture efficiency of 78.8%, with daily rates varying from 53% to 90% except for one outlier of 15%.³⁰ For our baseline analysis, we use a capture rate of 85%, roughly half way between the 78.8% for the Shell plan and the best-case of 90%. Applying 100% minus the capture efficiency to the carbon dioxide produced in SMR:

(11)

That is, 5.8 g CO₂ per MJ are emitted from the SMR process after emissions are treated for carbon capture (Table 1).

For the blue-hydrogen facilities so far in commercial operation, carbon capture has focused only on the SMR process, and no attempt has been made to capture the carbon dioxide generated from the combustion of natural gas used to provide the heat and high pressure. If these combustion emissions are captured, the carbon dioxide capture efficiency may be lower than that from the SMR process because the carbon dioxide is more dilute in the former case. We are aware of no data on carbon-capture efficiency from any plant, including any electric power plant, that combusts natural gas, but capture efficiencies of carbon dioxide from the exhaust stream of two coal-burning power plants are reported in the range of 55%-72%.^31-33 Note that efficiencies of up to 90% have been observed in one of the plants when running at full load. However, this does not reflect long-term performance, which is evaluated at average load. Load is less than full load either when the carbon-capture equipment is down for repair or when the demand for carbon dioxide is lower than it is at full load. In this analysis, we use a value of 65% capture efficiency from flue gases for our baseline analysis. Applying 100% minus this factor for emissions from the natural gas burned to produce the heat and pressure:

(12)

Therefore, total carbon dioxide emissions from the SMR process, including the energy used to drive the process, are in the range of 16.9 g CO₂ per MJ if the combustion flue is captured (5.8 g CO₂ per MJ plus 11.1 g CO₂ per MJ) to 37.6 g CO₂ per MJ (5.8 g CO₂ per MJ plus 31.8 g CO₂ per MJ) if the flue gases are not treated (Table 1).

3.2 Consumption of methane and production of carbon dioxide from electricity used to capture carbon dioxide

Energy is required to capture the carbon dioxide, and often this is provided by electricity generated from burning additional natural gas.⁷ The existing blue-hydrogen facilities make no effort to capture the carbon dioxide from the fuel burned to generate this electricity, nor has there been any effort to do so in the case of carbon capture from coal-burning power plants.³¹ Often, an energy penalty of 25% is assumed for this additional electricity.^34-36 However, this estimate is based on very little publicly available, verifiable information and may be optimistically low. A recent analysis of carbon capture from the flue gases of a coal-burning power plant, where the electricity for carbon capture came from a dedicated natural gas plant, found that the carbon dioxide emissions from the natural gas were 39% of the carbon dioxide captured from the coal-flue gases.³¹ Carbon dioxide is more concentrated in the gases produced through SMR than in the flue exhaust from combustion, suggesting that it can be captured more easily.

For this analysis, we assume that the energy used in the carbon-capture results in carbon dioxide emissions equal to 25% of the carbon dioxide captured from the stream reforming process, based on IPCC,³⁴ Jacobson,³⁵ and Sgouridi et al.³⁶ Therefore,

(13)

That is, emissions from the energy used to drive the carbon captured from the SMR process are themselves an additional 8.2 g CO₂ per MJ (Table 1).

If carbon dioxide is also captured from the flue gases used to generate heat and pressure, we assume the emissions from the energy cost is equal to 39% of the emissions captured, based on Jacobson.³¹ That is,

(14)

Therefore, the carbon dioxide emissions from the energy used to drive the carbon capture is between 8.2 g CO₂ per MJ if only emissions from the SMR process are captured or an additional 8.1 g CO₂ per MJ for a total of 16.3 g CO₂ per MJ if emissions from the energy source used for heat and pressure are also captured (Table 1).

As above for Equation 7, one mole of methane is burned for every mole of carbon dioxide emitted from the burning. Therefore, we can estimate the methane burned to produce the electricity required for the carbon dioxide capture as follows, for the case where only the SMR carbon is captured:

(15)

That is, 3.0 g CH₄ per MJ are consumed to generate the electricity used for carbon capture if only the reforming process emissions are captured (Table 1). Similarly, if the emissions from the energy used for the heat and pressure are also captured,

(16)

Therefore, the quantify of methane used to drive carbon capture when the flue gases from the combustion of the gas used to generate heat and pressure for the SMR process are 3.0 g CH₄ per MJ plus 3.0 g CH₄ per MJ, for a total of 6.0 g CH₄ per MJ when carbon capture is applied both to SMR and exhaust flue gases (Table 1).

If we again assume that 3.5% of the natural gas that is consumed is emitted unburned to the atmosphere (as in Equation 9), then for the case where only carbon dioxide emissions from SMR are captured, upstream methane emissions are:

(17)

For the case where flue gases are also treated for carbon capture, the upstream methane emissions are:

(18)

Converting these methane emissions to carbon dioxide equivalents:

(19)

And

(20)

Therefore, upstream emissions of unburned methane from the energy used to drive carbon capture are between 9.5 g CO₂eq per MJ if only the SMR carbon is captured and 18 g CO₂eq per MJ if the flue-gas emissions are also captured (Table 1).

3.3 Total carbon dioxide and methane emissions for blue hydrogen

The total emission of carbon dioxide for the production of blue hydrogen is the sum of the emissions from the SMR process after carbon capture, emissions from the energy used for heat and pressure to drive SMR, emissions from the energy used to power the carbon capture, and the indirect upstream emissions associated with producing and transporting natural gas. The indirect upstream carbon dioxide emissions result from the activity needed to provide the natural gas, and so should be applied as a percentage to the carbon dioxide produced from using natural gas, and not simply the carbon dioxide emitted after carbon capture. Using the approach of Howarth et al,²¹ this is 7.5% of the carbon dioxide produced in the SMR process plus energy needed to fuel that process as for gray hydrogen (70.3 g CO₂ per MJ) plus the emissions from the energy needed to drive the carbon capture (8.2-16.3 g CO₂ per MJ depending on whether or not the flue gases from the SMR-energy source is captured). Therefore, these indirect upstream carbon dioxide emissions are between 5.9 g CO₂ per MJ and 6.5 g CO₂ per MJ depending on whether or not the flue-gas emissions are captured (Table 1). For the case where only the emissions from the SMR processes are treated for carbon capture, total emissions of carbon dioxide are:

(21)

When the emissions from exhaust flue gases are also treated for carbon capture:

(22)

To summarize, when only the carbon from the SMR process itself is captured, total emissions of carbon dioxide are 51.7 g CO₂ per MJ. When efforts are also taken to capture the carbon dioxide from the flue exhaust from the energy driving the reforming process, total carbon dioxide emissions are 39.7 g CO₂ per MJ (Table 1). Treating the exhaust flue gases for carbon capture reduces total lifecycle emissions of carbon dioxide by 23%, less than might have been expected. This is due both to a relatively low efficiency for the carbon capture of flue gases³¹ and to the increased combustion of natural gas needed to provide the electricity for the carbon capture.

The methane emissions from blue hydrogen are the same as for gray hydrogen, except for those associated with the increased use of energy from natural gas to drive the carbon-capture process. The emissions for gray hydrogen are 77.4 g CO₂eq per MJ. The additional methane emissions from the gas used to drive carbon capture are given in Equations 19 and 20: 9.5 g CO₂eq per MJ when only SMR is treated for carbon capture and 18 g CO₂eq per MJ when the exhaust flue gases are also captured. Therefore, the total upstream methane emissions for the production of blue hydrogen are:

(23)

when only emissions from the SMR process are captured (Table 1). When flue gases are also treated, total upstream methane emissions are:

(24)

Total emissions for blue hydrogen when only the SMR process is treated are the sum of the carbon dioxide emissions and the upstream methane emissions:

(25)

See Table 1. When the exhaust flue gases are also treated for carbon dioxide capture, total emissions for producing blue hydrogen are:

(26)

We are aware of no previously published, peer-reviewed analyses on either total carbon dioxide or methane emissions associated with producing blue hydrogen. Several non peer-reviewed reports suggest that it may be possible to reduce carbon dioxide emissions for blue hydrogen by 56% (when only the SMR process is treated) to 90% (when exhaust flue gases are also treated) relative to gray hydrogen.^{1, 7} However, no data have been presented to support these estimates, and they apparently do not include emissions associated with the energy needed to drive carbon capture. Our results using a full lifecycle assessment show the 56% to 90% assumptions are too optimistic.

In Figure 1, we compare the greenhouse gas footprint of gray hydrogen with blue hydrogen where only the SMR process is captured and with blue hydrogen where carbon capture is also used for the exhaust flue gases. Because of the increased methane emissions from increased use of natural gas when flue gases are treated for carbon capture, total greenhouse gas emissions are only very slightly less than when just the carbon dioxide from the stream reforming process is treated, 135 vs 139 g CO₂eq per MJ. In both cases, total emissions from producing blue hydrogen are only 9% to 12% less than for gray hydrogen, 135 or 139 g CO₂eq per MJ compared with 153 g CO₂eq per MJ. Blue hydrogen is hardly “low emissions.” The lower, but nonzero, carbon dioxide emissions from blue hydrogen compared with gray hydrogen are partially offset by the higher methane emissions. We further note that blue hydrogen as a strategy only works to the extent it is possible to store carbon dioxide long term indefinitely into the future without leakage back to the atmosphere.

FIGURE 1

Open in figure viewerPowerPoint

Comparison of carbon dioxide equivalent emissions from gray hydrogen, blue hydrogen with carbon dioxide capture from the SMR process but not from the exhaust flue gases created from burning natural gas to run the SMR equipment, blue hydrogen with carbon dioxide capture from both the SMR process and from the exhaust flue gases, natural gas burned for heat generation, diesel oil burned for heat, and coal burned for heat. Carbon dioxide emissions, including emissions from developing, processing, and transporting the fuels, are shown in orange. Carbon dioxide equivalent emissions of fugitive, unburned methane are shown in red. The methane leakage rate is 3.5%. See text for detailed assumptions

4 COMPARISON OF EMISSIONS WITH OTHER FUELS AND SENSITIVITY ANALYSES

4.1 Emissions for fossil fuels

In Figure 1, we also compare greenhouse gas emissions from gray and blue hydrogen with those for other fuels per unit of energy produced when burned. The carbon dioxide emissions shown for coal, diesel oil, and natural gas include both direct and indirect emissions. The direct emissions are based on gross calorific values from EIA.¹⁹ Indirect emissions are those required to develop and process the fuels and are based on Howarth et al.²¹ These indirect carbon dioxide emissions are 4 g CO₂ per MJ for coal, 8 g CO₂ per MJ, and 3.8 g CO₂ per MJ for natural gas. Upstream fugitive emissions of unburned methane are assumed to be 3.5% for natural gas, as we have assumed for the hydrogen estimates. Methane emissions for coal and diesel oil are as presented in Howarth²⁴: 0.185 g CH₄ per MJ for coal and 0.093 g CH₄ per MJ for diesel oil, corresponding to 8.0 and 15.9 4 g CO₂eq per MJ respectively based on a 20-year GWP of 86.

Combined emissions of carbon dioxide and methane are greater for gray hydrogen and for blue hydrogen (whether or not exhaust flue gases are treated for carbon capture) than for any of the fossil fuels (Figure 1). Methane emissions are a major contributor to this, and methane emissions from both gray and blue hydrogen are larger than for any of the fossil fuels. This reflects the large quantities of natural gas consumed in the production of hydrogen. Carbon dioxide emissions are less from either gray or blue hydrogen than from coal or diesel oil. Carbon dioxide emissions from blue hydrogen are also less than from using natural gas directly as a fuel, but not substantially so. Carbon dioxide emissions from gray hydrogen are somewhat larger than from natural gas (Figure 1).

4.2 Sensitivity analyses for methane emissions

Given the importance of methane emissions to the greenhouse gas footprints of gray and blue hydrogen, we here present sensitivity analyses on our estimates. We separately consider different rates of fugitive methane emissions and different assigned GWP values.

Our default value for methane emissions used above for gray hydrogen, blue hydrogen, and natural gas is 3.5% of consumption. As noted above, this is based on top–down estimates for emissions from 20 different studies in 10 different gas fields plus a top–down estimate for emissions from gas transport and storage.¹⁶ This is very close to an independent estimate of emissions from shale gas production and consumption estimated from global trends in the ¹³C stable isotopic composition of methane in the atmosphere since 2005.³⁷ For the sensitivity analysis, we also evaluate one higher rate and two lower rates of methane emission. The higher rate is from the high-end sensitivity analysis for shale gas emissions based on the global ¹³C data, or 4.3% of consumption.³⁷ The lower rates we analyze are 2.54% and 1.45% of consumption. The 2.54% value is based on Alvarez et al²² who used “bottom–up” approaches to estimate the upstream and midstream methane emissions for natural gas in the United States as 12.7 Tg per year in 2015. This is 2.54% of consumption, based on annual gas consumption for 2015 of 771 billion m³ of natural gas in the United States,²⁶ assuming methane comprises 93% of the volume of gas.³⁸ The bottom–up approach presented by Alvarez et al²² likely underestimates methane emissions.^{24, 39, 40} We also consider an even lower estimate based on Maasakkers et al.⁴¹ Using an inverse model in combination with satellite data and the US EPA methane emissions inventory, they concluded that methane emissions from natural gas operations in the United States were 8.5 T per year in 2012. This is 1.45% of gas consumption, based on again assuming methane is 93% of gas and a national US consumption of gas of 723 billion m³ in 2012.²⁶

Our baseline analysis is based on a 20-year GWP value of 86.¹³ There is uncertainty in this estimate, so here we also explore the higher 20-year GWP value of 105 presented in Shindell et al.⁴² Most traditional greenhouse gas inventories use a 100-year GWP, so we explore that as well, using the latest value from the IPCC¹³ synthesis report of 34. However, the IPCC¹³ noted that the use of a 100-year time period is arbitrary. We prefer the use of 20-year GWP, since it better captures the role of methane as a driver of climate change over the time period of the next several decades, and the 100-year time frame discounts the importance of methane over these shorter time frames.^{15, 24}

In our sensitivity analyses, we substitute emission rates of 4.3%, 2.54%, and 1.54% for our baseline value of 3.5% in Equations 9, 17, and 18 for gray and blue hydrogen and in our estimate for natural gas presented in Figure 1. We also substitute a 20-year GWP value of 105 and a 100-year GWP value of 34 for the 20-year GWP of 86 used in Equations 10, 19, and 20. The sensitivity estimates are shown in Table 2. Across the full set of assumptions, both gray hydrogen and blue hydrogen without flue-gas capture (where only the carbon dioxide from SMR is captured) always have greater emissions than natural gas. The differences between the greenhouse gas footprint of blue hydrogen with or without the capture of carbon dioxide from the exhaust flue gases are generally small across all assumptions concerning fugitive methane emissions, with the total greenhouse gas emissions without the flue-gas treatment usually higher. The emissions from blue hydrogen with full carbon capture including the exhaust flue gases are higher than for natural gas across all set of assumptions except for the analysis with the 100-year GWP of 34 and low methane emissions, 2.54% or less (Table 2).

TABLE 2. Sensitivity analysis for total emissions of carbon dioxide and methane (g CO₂-equivalents per MJ of heat generated in combustion) for different upstream fugitive methane leakage rates and for either 20-year or 100-year global warming potentials (GWP20, GWP100)

	Gray H2	Blue H2 (w/o flue-gas capture)	Blue H2 (w/flue-gas capture)	Natural gas
Fugitive CH4 = 3.5%
GWP20 = 8	153	139	135	111
GWP20 = 105	170	158	155	123
GWP100 = 34	106	86	77	76
Fugitive CH4 = 4.3%
GWP20 = 86	171	159	156	124
GWP20 = 105	192	182	181	139
GWP100 = 34	113	94	86	81
Fugitive CH4 = 2.54%
GWP20 = 86	133	115	109	95
GWP20 = 105	144	129	124	104
GWP100 = 34	98	76	67	70
Fugitive CH4 = 1.54%
GWP20 = 86	110	90	82	79
GWP20 = 105	117	98	91	84
GWP100 = 34	89	67	57	64

We also evaluate the sensitivity of our conclusions to the percentage of carbon dioxide that is captured from SMR and from the flue exhaust from the natural gas burned to power the SMR process. Our default values presented above are for 85% capture from the SMR process and 65% capture from the flue gases, if an effort were made to capture those. Our sensitivity analysis includes a low estimate for SMR capture of 78.8% based on actual data from one commercial blue-hydrogen plant³⁰ and a high estimate of 90%, the highest yet reported.³¹ For capture of the flue gases, we explore carbon dioxide capture efficiencies of 55% at the low end and 90% at the high-end based on actual facility performance for flue gases from coal-burning electric plants.^31-33 Note that the 90% rate is the best ever observed and does not reflect likely actual performance under long-term commercial operations. We present the results of this sensitivity analysis in Table 3. Perhaps surprisingly, our conclusions are very insensitive to assumptions about carbon dioxide capture rates. This is because capture is very energy intensive: to capture more carbon dioxide takes more energy, and if this energy comes from natural gas, the emissions of both carbon dioxide and fugitive methane emissions from this increase in such proportion as to offset a significant amount of the reduction in carbon dioxide emission due to the carbon capture.

TABLE 3. Sensitivity analysis for combined emissions of carbon dioxide and methane (g CO₂-equivalents per MJ of heat generated in combustion) while producing blue hydrogen as a function of the percent carbon dioxide captured from the SMR process and from flue gases for the energy that drives the SMR process

	Total CO2	Total fugitive CH4	Total emissions
Blue H2 w/o flue-gas capture
85% SMR capture	51.7	86.9	139
90% SMR capture	50.2	86.9	137
78.8% SMR capture	53.5	85.7	139
Blue H2 w/flue-gas capture
85% SMR & 65% flue-gas capture	39.7	95.4	135
90% SMR & 90% flue-gas capture	33.3	98.9	132
78.8% SMR & 55% flue-gas capture	43.4	93.2	137

Note

• The methane leakage rate is 3.5%. The first row in each case is from the baseline case in Table 1.

These sensitivity analyses show that our overall conclusion is robust: the greenhouse gas footprint of blue hydrogen, even with capture of carbon dioxide from exhaust flue gases, is as large as or larger than that of natural gas.

5 IS THERE A PATH FOR TRULY “GREEN” BLUE HYDROGEN?

Some of the CO₂eq emissions from blue hydrogen are inherent in the extraction, processing, and use of natural gas as the feedstock source of methane for the SMR process: fugitive methane emissions and upstream emissions of carbon dioxide from the energy needed to produce, process, and transport the natural gas that is reformed into hydrogen are inescapable. On the other hand, the emissions of methane and carbon dioxide from using natural gas to produce the heat and high pressure needed for SMR and to capture carbon dioxide could be reduced if these processes were instead driven by renewable electricity from wind, solar, or hydro. If we assume essentially zero emissions from the renewable electricity, then carbon dioxide emissions from blue hydrogen could be reduced to the 5.8 g CO₂ per MJ that is not captured from the SMR process (Equation 11) plus the indirect emissions from extracting and processing the natural gas used as feedstock for the SMR process, estimated as 2.9 g CO₂ per M (7.5% of 38.5 g CO₂ per MJ; see section on “total carbon dioxide and methane emissions for gray hydrogen”), for a total of 8.7 g CO₂ per MJ. This is a substantial reduction compared with using natural gas to power the production of blue hydrogen. However, the fugitive methane emissions associated with the natural gas that is reformed to hydrogen would remain if the process is powered by 100% renewable energy. These emissions are substantial: 3.5% of 14 g CH₄ per MJ (Equation 3). Using the 20-year GWP value of 86, these methane emissions equal 43 g CO₂eq per MJ of hydrogen produced. The total greenhouse gas emissions, then, for this scenario of blue hydrogen produced with renewable electricity are 52 g (8.7 g plus 43 g) CO₂eq per MJ. This is not a low-emissions strategy, and emissions would still be 47% of the 111 g CO₂eq per MJ for burning natural gas as a fuel, using the same methane emission estimates and GWP value (Table 1). Seemingly, the renewable electricity would be better used to produce green hydrogen through electrolysis.

This best-case scenario for producing blue hydrogen, using renewable electricity instead of natural gas to power the processes, suggests to us that there really is no role for blue hydrogen in a carbon-free future. Greenhouse gas emissions remain high, and there would also be a substantial consumption of renewable electricity, which represents an opportunity cost. We believe the renewable electricity could be better used by society in other ways, replacing the use of fossil fuels.

Similarly, we see no advantage in using blue hydrogen powered by natural gas compared with simply using the natural gas directly for heat. As we have demonstrated, far from being low emissions, blue hydrogen has emissions as large as or larger than those of natural gas used for heat (Figure 1; Table 1; Table 2). The small reduction in carbon dioxide emissions for blue hydrogen compared with natural gas are more than made up for by the larger emissions of fugitive methane. Society needs to move away from all fossil fuels as quickly as possible, and the truly green hydrogen produced by electrolysis driven by renewable electricity can play a role. Blue hydrogen, though, provides no benefit. We suggest that blue hydrogen is best viewed as a distraction, something than may delay needed action to truly decarbonize the global energy economy, in the same way that has been described for shale gas as a bridge fuel and for carbon capture and storage in general.⁴³ We further note that much of the push for using hydrogen for energy since 2017 has come from the Hydrogen Council, a group established by the oil and gas industry specifically to promote hydrogen, with a major emphasis on blue hydrogen.⁵ From the industry perspective, switching from natural gas to blue hydrogen may be viewed as economically beneficial since even more natural gas is needed to generate the same amount of heat.

We emphasize that our analysis in this paper is a best-case scenario for blue hydrogen. It assumes that the carbon dioxide that is captured can indeed be stored indefinitely for decades and centuries into the future. In fact, there is no experience at commercial scale with storing carbon dioxide from carbon capture, and most carbon dioxide that is currently captured is used for enhanced oil recovery and is released back to the atmosphere.⁴⁴ Further, our analysis does not consider the energy cost and associated greenhouse gas emissions from transporting and storing the captured carbon dioxide. Even without these considerations, though, blue hydrogen has large climatic consequences. We see no way that blue hydrogen can be considered “green.” 

Algunas de las emisiones de CO2eq del hidrógeno azul son inherentes a la extracción, el procesamiento y el uso del gas natural como fuente de alimentación de metano para el proceso de SMR: las emisiones fugitivas de metano y las emisiones de dióxido de carbono procedentes de la energía necesaria para producir, procesar y transportar el gas natural que se transforma en hidrógeno son ineludibles. Por otro lado, las emisiones de metano y dióxido de carbono derivadas del uso de gas natural para producir el calor y la alta presión necesarios para la SMR y para capturar el dióxido de carbono podrían reducirse si estos procesos se impulsaran con electricidad renovable procedente de la energía eólica, solar o hidráulica. Si suponemos que las emisiones de la electricidad renovable son prácticamente nulas, las emisiones de dióxido de carbono del hidrógeno azul podrían reducirse a los 5,8 g de CO2 por MJ que no se capturan del proceso de SMR (ecuación 11) más las emisiones indirectas de la extracción y el procesamiento del gas natural utilizado como materia prima para el proceso de SMR, estimadas en 2,9 g de CO2 por M (7,5% de 38,5 g de CO2 por MJ; véase la sección sobre "emisiones totales de dióxido de carbono y metano para el hidrógeno gris"), lo que supone un total de 8,7 g de CO2 por MJ. Se trata de una reducción sustancial en comparación con el uso de gas natural para la producción de hidrógeno azul. Sin embargo, las emisiones fugitivas de metano asociadas al gas natural reformado en hidrógeno se mantendrían si el proceso se alimentara con energía 100% renovable. Estas emisiones son considerables: 3,5% de 14 g de CH4 por MJ (ecuación 3). Utilizando el valor de GWP de 86 en 20 años, estas emisiones de metano equivalen a 43 g de CO2eq por MJ de hidrógeno producido. Las emisiones totales de gases de efecto invernadero, por tanto, para este escenario de hidrógeno azul producido con electricidad renovable son de 52 g (8,7 g más 43 g) de CO2eq por MJ. Esta no es una estrategia de bajas emisiones, y las emisiones seguirían siendo el 47% de los 111 g de CO2eq por MJ de la quema de gas natural como combustible, utilizando las mismas estimaciones de emisiones de metano y el mismo valor de GWP (Tabla 1). Aparentemente, la electricidad renovable se utilizaría mejor para producir hidrógeno verde mediante electrólisis.

Este escenario óptimo para la producción de hidrógeno azul, utilizando electricidad renovable en lugar de gas natural para alimentar los procesos, nos sugiere que realmente no hay un papel para el hidrógeno azul en un futuro libre de carbono. Las emisiones de gases de efecto invernadero siguen siendo elevadas, y además habría un consumo importante de electricidad renovable, lo que representa un coste de oportunidad. Creemos que la sociedad podría utilizar mejor la electricidad renovable de otras maneras, sustituyendo el uso de combustibles fósiles.

Del mismo modo, no vemos ninguna ventaja en el uso de hidrógeno azul alimentado por gas natural en comparación con el simple uso del gas natural directamente para el calor. Como hemos demostrado, lejos de ser bajo en emisiones, el hidrógeno azul tiene unas emisiones tan o más grandes que las del gas natural utilizado para la calefacción (Figura 1; Tabla 1; Tabla 2). La pequeña reducción de las emisiones de dióxido de carbono del hidrógeno azul en comparación con el gas natural se compensa con creces por las mayores emisiones de metano fugitivo. La sociedad necesita alejarse de todos los combustibles fósiles lo antes posible, y el hidrógeno verdaderamente ecológico producido por electrólisis impulsada por electricidad renovable puede desempeñar un papel. Sin embargo, el hidrógeno azul no aporta ningún beneficio. Sugerimos que el hidrógeno azul se vea mejor como una distracción, algo que puede retrasar la acción necesaria para descarbonizar realmente la economía energética mundial, de la misma manera que se ha descrito para el gas de esquisto como combustible puente y para la captura y el almacenamiento de carbono en general.43 Además, observamos que gran parte del impulso al uso del hidrógeno para la energía desde 2017 ha venido del Consejo del Hidrógeno, un grupo creado por la industria del petróleo y el gas específicamente para promover el hidrógeno, con un gran énfasis en el hidrógeno azul.5 Desde la perspectiva de la industria, el cambio del gas natural al hidrógeno azul puede considerarse económicamente beneficioso, ya que se necesita incluso más gas natural para generar la misma cantidad de calor.

Insistimos en que nuestro análisis en este documento es el mejor escenario para el hidrógeno azul. Supone que el dióxido de carbono capturado puede almacenarse indefinidamente durante décadas y siglos en el futuro. De hecho, no hay experiencia a escala comercial con el almacenamiento de dióxido de carbono procedente de la captura de carbono, y la mayor parte del dióxido de carbono que se captura actualmente se utiliza para la recuperación mejorada de petróleo y se devuelve a la atmósfera.44 Además, nuestro análisis no tiene en cuenta el coste energético y las emisiones de gases de efecto invernadero asociadas al transporte y almacenamiento del dióxido de carbono capturado. Pero incluso sin estas consideraciones, el hidrógeno azul tiene grandes consecuencias climáticas. No vemos que el hidrógeno azul pueda considerarse "verde". 

ACKNOWLEDGMENTS

This research was supported by a grant from the Park Foundation and by an endowment given by David R. Atkinson to Cornell University that supports Robert Howarth. We thank Dominic Eagleton, Dan Miller, and two anonymous reviewers for their valuable feedback on earlier drafts of this paper.

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First published: 12 August 2021

https://doi.org/10.1002/ese3.956