Apr-2022

The cost of hydrogen production

Selecting routes to hydrogen production calls for a balance of environmental impact and cost.

LORENZO MICUCCI, Siirtec Nigi
SAEID MOKHATAB, Gas processing consultant

Article Summary

Hydrogen is increasingly viewed by many as the energy carrier of the future due to its potential environmental advantages over fossil fuels. Indeed, when burned hydrogen reacts with oxygen to form water and no other greenhouse gases are released to the environment.

Hydrogen must be produced through the expenditure of energy. Figure 1 shows the sources, the industrial applications, and the potential consumption areas of hydrogen. Today, large volumes of it are used as feedstock in the petrochemical and chemical industries to produce ammonia, refined petroleum products, and other chemicals.

Depending upon the source, hydrogen is currently classified as:
- Grey, if it is extracted from fossil fuels (coal, naphtha, LPG, and natural gas)
- Blue, if it is produced from fossil fuel in combination with carbon capture, utilisation and storage
- Turquoise, if it is co-produced along with carbon black by natural gas pyrolysis
- Green if it comes from renewables (solar photovoltaic, hydropower, and wind)

In fact, hydrogen production modes coded by colour are related to the actual hydrogen production pathway in terms of carbon footprint. Turquoise hydrogen is the result of methane cracking at high temperature in the absence of oxygen, wherein carbon is captured as carbon black, a valuable solid used to produce tyres, outdoor plastics cables, and other goods. It is regarded as carbon-neutral when renewable electric energy is used. Green hydrogen is clean but not only relies on very low-cost renewable electricity to be competitive but also requires sufficient water availability. Blue hydrogen can be less expensive than green hydrogen without emitting carbon like grey hydrogen. As several new green hydrogen production technologies are under development, we will see in the long term green hydrogen at an increasingly affordable price when green hydrogen will start to capture more market share than grey and blue hydrogen.²

Pending the transition to green hydrogen, natural gas will continue to be used also to produce grey and blue hydrogen, notably in those regions having abundant low cost natural gas and the infrastructure developed. In fact, among fossil fuels natural gas is by far the most important source of hydrogen since methane, its major component, has the highest H/C ratio. Consequently, the carbon dioxide (CO2) footprint is comparatively the lowest.

Hydrogen gas (H2) can be produced on an industrial scale from natural gas through three basic technologies: methane steam reforming (MSR), partial oxidation (POX), and autothermal reforming (ATR). However, the most widespread and at the same time least expensive process used for extracting hydrogen from natural gas is MSR, which involves the reaction of natural gas and steam over a nickel based catalyst that results in breaking the methane component of the natural gas into carbon monoxide (CO) and H2. Since almost 50% of hydrogen consumed worldwide is produced via MSR, this article will outline the steam methane reforming for the production of both grey and blue hydrogen.

Grey hydrogen from natural gas
In a MSR, natural gas is reacted with steam in the highly endothermic reaction 1 over a Ni/Al2O3 catalyst, at high temperature (800-1000°C), and under pressure (15-40 barg) to form CO and H2. Part of the formed CO reacts thereafter with steam according to reaction 2, known as water gas shift (WGS), to yield more H2 and CO2. The overall result is a mixture of H2, CO, and CO2:

CH4 + H2O ν CO + 3H2   ΔH° = 206 kJ/mol  [1]
CO + H2O ν CO2 + H2   ΔH° = -41.2 kJ/mol   [2]

The composition of the gas at the reactor outlet is set by the equilibria of reactions 1 and 2. In some cases, CO2 is added to natural gas to improve the syngas yield through the dry methane reforming reaction:

CH4 + CO2 ν 2CO + 2H2  ΔH° = 247 kJ/mol [3]

The CO2 can be sourced from outside battery limits, or recycled from the acid gas removal unit, typically located downstream of the steam reformer.

The operation of a MSR can be adversely affected by the thermal coking process due to the soot formed by cracking of the heavies that may be present in the natural gas source. Soot deposits on the catalyst and on internal parts of the reformer, as well as downstream equipment, result in operational problems.

The carbon deposition on the MSR catalyst is particularly dangerous because, in the zones where soot builds up, the heat transferred through the catalytic tube walls is not absorbed by the steam reforming reaction 3 and hazardous superheating (hot spot formation) of the tubes materials jeopardises the process operation.

Soot formation is counteracted by deploying an adiabatic pre-reforming step upstream of the MSR. Steam reforming of the heavy hydrocarbons (see reaction 4) can take place at a relatively low temperature without soot formation:

CnHm + nH2O à nCO + (n+m/2)H2                [4]

In addition to the pre-reformer, soot formation is prevented by adopting a steam-to-feed ratio in between 2.5:1-5:1 (which is much greater than the ratio suggested by reaction 1) and adopting short residence times in the reactor.

It is worth noting that the WGS reaction 2 is exothermic. Increasing the operating temperature, the chemical equilibrium shifts to the left, and the residual CO2 is increasingly consumed by the endothermic reforming reaction 3, which, in turn, is favoured by high temperature.

Steam reforming (tubular reforming)
The chemistry described above is achieved in a steam reformer consisting of a convection section, where process streams are heated against the hot flue gas originated in the radiant section, and a radiant section, where heat is supplied to the chemical system, mainly by the radiation (see Figure 2).

The radiant section of the reformer is where the reforming reactions take place. It consists of a number of catalytic reaction tubes, made of high Cr and Ni alloy, arranged in rows in fire boxes fitted with a number of burners. The latter can be placed alternatively at the bottom, at the top, on the terrace wall, or on the side wall, as is the case in Figure 2.