Page 128 - CW E-Magazine (2-1-2024)

Point of View

the syngas is put to use. Uses of syngas can be in gas engines, turbines, and fuel cells to produce power, and in chemical & biochemical
systems to synthesize other chemicals and liquid fuels.

Benefits of gasification
One of the advantages of gasification is that it can pretty much take in any kind of solid waste (though the engineering challenges
change). The reaction proceeds at high temperature and besides producing syngas also yields a slag that contains inerts that can find
use in the ceramic industry, in civil applications and metallurgical processing. Typically, about 80% of the waste fed is converted into
syngas and water.
In contrast to waste incineration, gasification reduces MSW into simpler molecules, and substances like dioxins, and furans –
typically formed in incineration systems – are destroyed. Gasification also outperforms incineration in terms of energy efficiency and
exhaust emissions. The capital cost of MSW gasification plants is similar to those of conventional waste-to-energy systems, but have
the added advantage of yielding molecules that lend themselves to well-known chemical conversions.
Gasification of MSW
As pointed out earlier, while gasification of fossil fuels has been practiced for several decades, that of MSW or solid industrial wastes
is just about 20 years old. Since then plants have been scaled up to a size of about 200-ktpa in terms of wastes handled. Using pure
oxygen (instead of air) does bring some savings in capital costs (as the gasifier can be sized smaller), but this has to be weighed against
the costs of building an air separation unit to produce the oxygen needed. For syngas used in chemical syntheses, pure O gasification
2
is typically employed to avoid nitrogen (N ) in the syngas.
2
Multiple chemical pathways
The syngas produced can go down one or more pathways:
 Ammonia synthesis to make nitrogenous fertilisers, of which urea is the most important. The current routes for making ammonia are
mostly based on steam reforming of natural gas.
 Methanol synthesis to make olefins (methanol-to-olefins), dimethyl ether (a substitute for LPG), and other C1 chemicals (acetic acid,
formaldehyde, etc.)
 Fischer-Tropsch (FT) synthesis to yield synthetic fuels, and paraffins that can be cracked to yield other high value chemicals.
 Fermentation to ethanol using recently developed technologies that are now commercially available for licensing. The ethanol can serve
as a platform chemical for making ethylene, polyethylene, ethylene oxide, monoethylene glycol, etc., or for fuel-blending in gasoline.
 Methanation to yield methane (synthetic natural gas), which can then be used as energy source.
 Membrane separation to yield circular hydrogen, which can be deployed directly or indirectly for energy purposes, or used as a
reducing agent in chemical transformations.

Back of the envelop calculations indicate that 100-tonnes of MSW can be converted into 52-tonnes of methanol or 32-tonnes of
ethanol or 8-tonnes of H , with 50% less CO emitted (and with high purity) compared to waste incineration. Carbon negative fuels can
2
2
also be produced by capturing the residual CO and utilising or sequestering it.
2
Several waste to chemical initiatives are ongoing currently, involving the production of methanol, ethanol, low carbon H , sustainable
2
aviation fuel (SAF), and green steel (where the syngas serves as a reducing agent, converting iron oxides to iron). Of these, methanol
production is currently most popular, for several reasons including the fact that this is the largest volume chemical produced and has
many chemical and fuel uses. The world’s first commercial scale waste-to-methanol facility was built by Enerkem in Canada, and another
planned in Tarragona, Spain will, when completed in 2025, process 400-ktpa of non-recyclable MSW to produce 220-ktpa of methanol.
There is also considerable interest in SAF – including in India, where some small mandates have been set – and current routes to it
involve the use of used cooking oils and/or animal/vegetable oils & fats. MSW to SAF can be an interesting option for India, which is as
much starved for cooking oil as it is for crude oil.

One among many, but integral
Gasification offers but one option to partially mitigate the waste management problem India and much of the world faces. It is time
to treat MSW as a resource that is just not in the right place, and make its valorisation an integral part of a broader strategy to not just
clean up our cities, but also enable the chemical industry meet its carbon mitigation goals!
Ravi Raghavan

128 Chemical Weekly January 2, 2024

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