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Hydrogen is the lightest and most abundant element in the universe. It can be used as a source of power, and it is an important feed stock for many petrochemical processes.
When hydrogen is combusted, it forms water. Therefore, hydrogen can be used as a low-carbon fuel source. Hydrogen can be combusted directly, or it can be used in a fuel cell to produce electricity.
Since hydrogen produces minimal pollutants when combusted, it is envisioned by many as a core component of a cleaner energy future. President George W. Bush touted the potential for a “hydrogen economy” in his 2003 State of the Union address. Billions of dollars have been invested in the attempt to realize this vision.
But hydrogen must first be produced. Over 95% of the world’s hydrogen is produced using the steam methane reforming process (SMR). In this reaction, natural gas is reacted with steam at an elevated temperature to produce carbon monoxide and hydrogen. A subsequent reaction — the water gas shift reaction — then reacts additional steam with the carbon monoxide to produce additional hydrogen and carbon dioxide.
The reforming reaction with methane as:
CH4 + H2O ? CO + 3 H2
The water-gas shift (WGS) reaction is:
CO + H2O ? CO2 + H2
Hydrogen’s “dirty secret” is that it is produced primarily from fossil fuels. Thus, whether hydrogen is really “clean” depends on the method of production.
Hydrogen can also be produced by the electrolysis of water, but this is generally a costlier approach than the SMR route. When electricity is used to produce hydrogen, thermodynamics dictate that you will always produce less energy than you consume.
In other words, the energy input in electricity will be greater than the energy output of hydrogen. Nevertheless, if a cheap source of electricity is available — such as excess grid electricity at certain times of the day — it may be economical to produce hydrogen in this way. This will be the topic of a future article.
In any case, even though hydrogen itself is essentially non-polluting when burned (some nitrogen oxides, or NOx, may be formed), there is a carbon footprint associated with it. So, let us examine the carbon footprint of hydrogen when produced via the common SMR process.
The Steam Methane Reforming (SMR) Process
A modern SMR plant consists of four systems: Desulfurization, Reforming, High-Temperature Shift (HTS), and Pressure Swing Absorption (PSA). These systems reflect sulfur removal, the reforming reaction, the WGS reaction, and hydrogen purification.
The desulfurization step consists of passing the natural gas over a catalyst. These systems are generally passive, and while there are some minor carbon emissions associated with them, they are insignificant compared to the rest of the system.
The natural gas entering the SMR is split before desulfurization, with a small amount of flow mixing with the PSA waste gas and being combusted to provide the high temperatures needed for the reaction.
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The bulk of the natural gas is desulfurized, mixed with steam, and then reacted in the reformer. The reforming reaction typically takes place over a nickel-based catalyst at elevated pressures and temperatures.
The hot gas exiting the SMR is cooled, which simultaneously generates steam in the process. Steam is then added to the cooled gas in the shift reactor to convert the carbon monoxide to carbon dioxide and more hydrogen.
Finally, the hydrogen is purified in the PSA unit. This involves a pressurization step that causes impurities to bind to an adsorbent, while hydrogen passes through. When the adsorbent is saturated, the pressure is dropped to remove the impurities, which can then be recycled as fuel gas.
The Carbon Footprint of Steam Methane Reforming
The carbon footprint of hydrogen production via SMR can be broken down into two parts.
First, as indicated by the SMR and WGS reactions, 100% of the carbon in the incoming methane is ultimately converted to CO2. In the process of producing one molecule of CO2, four molecules of hydrogen (H2) are produced, with the steam contributing the additional hydrogen.
Thus, per 1 million standard cubic feet (SCF) of hydrogen produced from methane, 250,000 SCF of CO2 will be produced. There are 19,253 SCF of carbon dioxide in one metric ton, so 1 million SCF of hydrogen will produce 13 metric tons of carbon dioxide. This will be by far the largest piece of carbon footprint associated with the SMR process.
The second part is the carbon footprint associated with the individual process units. Steam must be generated, the reactor must be heated, etc. But steam is also created when the SMR exit gas is cooled, so that helps offset the carbon burden.
PraxairLIN, one of the world’s largest producers of hydrogen, has broken down the carbon footprint associated with the individual process steps. I have converted their data to metric tons of carbon dioxide emitted by these process units per million SCF of hydrogen produced.
Adding this to the carbon dioxide produced from the natural gas reactions, the total becomes 19.3 metric tons of carbon dioxide produced per million SCF of hydrogen. However, the Praxair paper noted that this is the theoretical minimum. Due to heat losses and inefficiencies, the actual number in practice in a large hydrogen plant is 21.9 metric tons.
This converts to 9.3 kilograms (kg) of CO2 produced per kg of hydrogen production. One kilogram of hydrogen is the energy equivalent of one gallon of gasoline, which produces 9.1 kg of CO2 when combusted.
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Carbon footprints are often reported in terms of energy. For example, power plants usually report carbon footprints in terms of kilowatt-hours (kWh). One million SCF of hydrogen contains 79,100 kilowatt hours of energy.
This converts to 0.28 kg of carbon dioxide emissions associated with one kilowatt-hour of hydrogen production.
Of course, that is just the carbon footprint of hydrogen production. To use the hydrogen for power, it still must be compressed, transported, and either combusted or converted to electricity in a fuel cell. The fuel cells themselves must also be built, and there are carbon emissions associated with those construction processes.
For perspective the Energy Information Administration lists the carbon footprint of electricity production from coal and natural gas as 1.0 kg/kWh and 0.42 kg/kWh, respectively. But that is for the actual conversion into power, and not just the energy content of the fuel.
On an apples-to-apples basis, it depends on several factors but it is likely that the conversion of hydrogen into power will have a carbon footprint greater than that of natural gas-fired power, but less than that of coal-fired power. However, it is possible in theory to capture the carbon emissions generated in the SMR process.
It is also possible to produce hydrogen via lower-carbon routes which historically have been less economical. That will be the topic of a future article.
By Robert Rapier
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