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)

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.

3.1 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.