For
more than half a century, steam methane reforming (SMR) ruled the ammonia (NH3)
and hydrogen (H2) industries. Every decade since, commercial
viability witnessed tremendous innovation leading to large capacity plants, new
generation catalysts and equipment, operation’s reliability, energy and
material efficiency, safety and environmental targets and process economics. Anthropogenic
emissions due to the use of fossil fuels and feedstocks significantly
contributes to global warming. Therefore, many nations around the world are
investing in ways to reduce greenhouse gas (GHG) emissions and foster
sustainable low-carbon economies.
H2
is slated to play a significant role in the transition from carbonaceous fuels
to clean energy resources without carbon dioxide (CO2) emissions. H2
is abundantly available in nature, but it is in the combined form requiring high
order energy transactions for its liberation to the free molecular form. Therefore,
most manufacturing processes involving H2, NH3, methanol,
power and chemicals use fossil fuels for its separation and downstream uses,
which invariably results in CO2 emissions. Around 90% of the H2
produced today is either through the SMR, autothermal reforming (ATR) or the gasification
processes, using methane (CH4 ), naphtha, fuel oil, petroleum coke or
coal.1 Each of these originates from fossils and releases around 9
tons (t)–10 t of CO2/t of H2.
Water
electrolysis using grid electricity has been around for decades and was used in
the industry to produce H2 during the 1960s. Today, if electricity
generated from renewable sources is used to split water (electrolysis) and
produce H2, the product would not emit CO2, thus producing
green H2. Green H2 can serve as green fuel in fuel cells
and internal combustion engines (ICEs). The biggest challenge for green H2
to take over as a viable alternative to fossil fuels and feedstocks are the processes’
technological maturity, economic viability and the scale of operations
considerations. Research and developments in these areas are progressing rapidly,
such that commercialization of different versions of the electrolytic process
and its economy of scale operations in large capacity plants are expected
within the next 5 yr. Abundant generation of renewable electricity (e.g., wind,
solar, geothermal, biomass) will also be accessible. During the transition
period, fossil fuel-based processes will continue with added provisions for capturing
CO2, using it for chemical synthesis, its sequestration and storage
so that the emissions will not result in global warming. Additional facilities must
be built with added investments for the treatment.
Critical challenges in the progression
of green H2 technologies include the development of cost-effective
and efficient electrolyzers and building the necessary infrastructure for
handling, storing and dispensing H2. The Green Hydrogen Catapult
(GHC)—a global
initiative organized with the support of the UN High Level Champions for Global
Climate Action and a coalition of industry leaders in developing the clean fuel—is committed to
commissioning electrolyzers from 25 gigawatts (GW)–45 GW by 2027. The coalition claims that this effort will help keep the price
to produce green H2 below $2/kilogram (kg) of green H2,
which will allow the clean fuel to be cost effective in the short term.
CH4 pyrolysis technologies. Water electrolysis technology developers are also
exploring other options for green H2 generation. Prominent among
them is the pyrolysis of CH4 to H2 and solid carbon,
which emits no CO2 emissions in the process. It is an endothermic
equilibrium reaction, so high temperatures shift the equilibrium toward H2
production. Therefore, higher temperatures lead to better CH4 conversions and H2
yields.
The
advantages of this process are that it is an alternate route to green H2
from readily available natural gas without CO2 emissions, and it
requires no oxidizer or carbon capture and sequestration (CCS). Disadvantages
include technology reliability and attaining economy of scale operations. The
major challenges involved in this process are the H2 production
costs, the reactor’s energy efficiency and carbon disposal. In the interim,
capital expenditures (CAPEX) will be higher, but H2 production costs
are likely to be lower than SMR with CCS or water electrolysis. As the
technology readiness is proven in large plants and the economy of scale
operations are achieved, CH4 pyrolysis will have an edge over other competing
technologies. Significant research is ongoing, but its commercialization
may take some time (FIG.
1).
Pyrolytic decomposition can be affected in
three ways: thermal, plasma and catalytic. The H2 produced is termed
as turquoise, meaning the color between blue and green.
Thermal pyrolysis. Pyrolysis
of CH4 in the absence of oxygen yields
carbon (solid) and H2 (gas) as per reaction given below (Eq. 1):
CH4 (gas) → C (solid) +
2H2 (gas) Δ H -76 KJ/mole
This is a highly endothermic reaction, requiring external heating and an
equilibrium reaction, which starts to
produce carbon and H2 around 300°C and finishes around 1,300°C. It is like the reduction of iron oxide (ore) to metallic
iron by CH4.
Three
types of designs of the indirectly heated pyrolysis reactor are being developed
over which CH4 splitting takes place: fluidized bed, moving carbon
bed or molten metal/salt type. The Australian Hazer Group has developed a
fluidized bed type of externally heated reactor which uses iron oxide granules
to catalyze the reaction operating at around 900°C and near
atmospheric pressure. The company built a $16-MM pilot plant in Western
Australia and are planning a commercial demonstration unit, producing 100 million
tons per year (MMtpy) of H2 and a 380 MMtpy graphite capacity.
The
BASF design of the pyrolytic reactor is a vertical moving carbon bed type in
which electrodes are used to heat the carbon bed to 1,000°C–1,400°C under near atmospheric pressure.1 External
heating is done to sustain the endothermic pyrolysis reaction with renewable
electricity.
The
TNO/C-Zero, a joint research project between TNO (a Netherlands-based
independent research organization) and C-Zero (a hard technology startup in
Santa Barbara, California), reactor design uses molten metal alloy/salt to
facilitate the carbon and H2 bond in CH4 split and frees up both elements. Here,
the gaseous CH4 is fed to the
bottom of a high-temperature reactor filled with a molten metal as lead (Pb) or a
molten metal alloy of nickel and bismuth (Ni/Bi
27:73) heated to 1,300°C.
The molten metal promotes the formation of
solid carbon and gaseous H2. The molten salt mixture of manganese and
potassium chlorides (67:33) is also found to catalyze the reaction. The carbon
formed rises through the molten medium and floats at the top from
where it is skimmed off and
transferred to a carbon tank. A one-third mole
of the H2 produced is
used to heat the reactor and maintain its temperature while the rest is cooled and
stored.
In the Karlsruhe Institute of
Technology (KIT), Germany developed another process using a liquid-metal bubble
column reactor. The column is filled with liquid metal that is heated up to 1,000°C. Fine CH4 bubbles enter the
column through a porous filling at the bottom. These bubbles rise to the
surface, and, at such high temperatures, the ascending CH4 bubbles
are increasingly decomposed into H2 and carbon.
Catalytic
pyrolysis. For
the thermal CH4 decomposition, a temperature of 1,300°C is required. Metals like Ni, iron, copper and cobalt are found to have demonstrated catalytic activity
for CH4 decomposition at lower temperatures, around 500°C–800°C. The reaction is carried out in nanostructures
like nanotubes, nanofibers or graphene. A main
limitation of metal catalysts is its fast
deactivation and the difficulty in separating them from the carbon produced. To overcome these limitations, carbonaceous
catalysts in the form of amorphous high-surface area carbons that are active in the range of 800°C–900°C are considered. These carbon catalysts are subject
to deactivation, as well, but may last longer
than metal catalysts. Strategies for solving
this issue such as continuous regeneration of catalytically active carbons from
catalytically inactive carbons are an active area of research being undertaken now.
Plasma pyrolysis.
The plasma-driven CH4 thermal decomposition in the
gas phase, yielding H2 and solid carbon, is another environmentally
friendly alternative to conventional H2 production methods from
natural gas. In this process, plasma torches are used to crack the CH4
molecule into H2 and solid carbon powder. It is like water
electrolysis, but the same scale of H2 production uses only one-fifth
of electricity than electrolysis, making the operational costs more than 50%
cheaper than electrolysis. The plasma reactor is a tubular one to which CH4
feed is introduced. Part of the length of the reactor tube lies in a horizontal
rectangular waveguide through which microwave power is supplied to excite the CH4
to the plasma state. H2 produced through CH4 decomposition
is accompanied by a combination of cracking, oligomerization and aromatization
reactions, which tend to minimize the formation of elemental carbon. Research
is underway to limit the side reactions to attain a better carbon and H2
yield. Plasma pyrolysis is more expensive because of its higher power
intensity. The thermal and catalytic pyrolysis methods use cheap natural gas
for heating and, therefore, are low-cost options.
Industry outlook. bp’s Statistical Review of World Energy 2022 revealed that
natural gas accounted for 24% market share in the global primary energy
consumption in 2022 and was growing at around 5.3%/yr.2 Natural gas
production in 2021 was 4,037 billion cubic meters (Bm3), of which 1,022
Bm3 are traded globally. Biogas produced from biomass is another input for producing renewable energy and reduces GHG emissions. Biomass is subjected to anaerobic degradation using
microbes to yield biogas. The CH4
separated out of biogas is called biomethane and
is abundantly available from municipal waste treatment sites and the
disposal of agricultural biomass. These can be used as a sustainable feedstock
for CH4 pyrolysis. CH4
pyrolysis is not a sustainable process because it results in the depletion of
natural gas reserves. However, it can become an interim solution until green H2
production becomes mature enough in scale and costs.3,4
Catalyzed
pyrolysis uses only non-toxic metals, and therefore, the disposal of spent
catalysts is not a problem. Moreover, the ongoing innovation and research about
the thermodynamics and pyrolysis reaction kinetics may open new avenues for
more active catalysts. Co-product carbon may find use in the industry for the
manufacture of various products (e.g., carbon black, fibres and nanotubes) for
soil amendment—where
there is carbon depletion due to intensive cropping—and for pollution control applications and
environmental remediation. The carbon-free H2 processes are
evaluated in terms of investment, energy efficiency, cost of production and
carbon footprint. In the near-term, CH4 pyrolysis gas has a competitive
edge over others.
According to the International
Energy Agency (IEA), green H2 capacity through water electrolysis
doubled in 2020 compared to the previous year, taking low-carbon H2
production to 300 kilotons.5 A similar increase was achieved in blue
H2 production via SMR and CCS. The current pace of the green H2
market expansion is much lower than what is required for meeting net-zero goals
by 2050. In 2023, the majority of
investments for green H2 production was from the oil and gas,
refining and chemicals (fertilizer, methanol and petrochemicals) industries.
Sectors like steel and cement have not caught up with the new paradigm. While
all technologies available to produce green H2 are getting optimized
through research and innovation, it is expected that by 2025 major sectors should
witness an accelerated shift toward green H2 TABLE 1.
Economics. Although
the CAPEX for CH4 pyrolysis is high, operational expenditures (OPEX)
are certainly lower and additional revenue comes from the sale of elemental
carbon produced in the process.6,7 While the disposal of oxygen poses
a challenge to water electrolysis, added costs incurred toward building CCS
infrastructure is a disadvantage for SMR.
The cost of green H2 production
varies because each of the processes use different designs, technologies, feedstocks
and utilities. Therefore, a levelized cost is generated to compare
competitiveness. The economics of
various H2 production processes without GHG emissions is also better
understood through the levelized cost of H2 (LCOH). It considers the costs to
produce 1 kg of green H2, considering its CAPEX and OPEX components.
The CAPEX for blue H2 includes
the complete SMR/ATR unit with all drives and associated CCS facility; for
green H2 this includes an electrolyzer. This does not include
storage and transportation costs.
Various researchers, technology
providers and process operators have studied the economics of blue, green and
turquoise H2 production. For SMR with CCS, the average LCOH is $2–$3,
for electrolysis, it is $5–$6 and for CH4 pyrolysis, it is around $2.80–$3.
If credit is given to the carbon byproduct, the latter comes down to $1.80. These figures are based on an average gas
price of $9/million British thermal units (MMbtu) and renewable power at $45/megawatt
hour (MWhr).
The levelized cost
of blue H2 is likely to reduce with the fall in oil and gas prices
and increase in the availability of independent CCS facilities. Similarly, the
LCOH of green H2 is expected to fall by 2030 due to reductions in
the cost of renewable power, increases in the efficiency of electrolyzers and
reductions in its costs. A full-scale offtake of carbon black produced in the
pyrolysis process will invariably reduce the cost of turquoise H2.
More
work must be done on reactor designs, modularization, catalysts development and
process control in perfecting the technology of CH4 pyrolysis. Once
these barriers are overcome, it becomes a CO2-free technology and a promising
alternative that can serve as an interim option toward the production of green H2.8,9
The ongoing technological research and innovation and
achieving economies of scale in operation of the processes will likely
contribute to the decline in H2 prices. HP
LITERATURE CITED