Page 41 - Energize February 2022

TECHNICAL

Figure 11: Plasma gasifier (NRG/C&EN)

the feedstock of the gasifier are converted to a synthetic gas
(comprising carbon monoxide and hydrogen). All inorganic
materials are vitrified into an inert glass-like “slag”.
A plasma torch is a device which transforms electricity into
heat by passing the current through a gas stream. In plasma
gasification, the high temperature required is created by the
formation of plasma. The temperature from the torch can reach
3500 to 4000˚C. At this temperature, most types of waste are
decomposed into gaseous elements, and complex molecules are
separated into atoms. Plasma gasification requires fuel of the
same quality and characteristics as RDF, and thus sorting and
pre-processing of MSW is a requirement. RDF is often used as a
feedstock for gasification.
Gasification is the high temperature cracking of hydrocarbons
in an oxygen reduced atmosphere to produce a syngas, mixture of
hydrogen, methane, and carbon dioxide, as well as other gaseous Figure 12: A 1 tonne/day transportable waste-to-hydrogen plant
components. (TODA Corporation / Japan Blue Energy)

Waste to hydrogen (WtH) plasma gasification 1440 tonnes/day from 24 tonnes of waste. Any water, or moisture,
The recent interest in hydrogen as a fuel has led to the in the feedstock contains hydrogen, which also contributes to the
development of processes involving further processing of the proportion of hydrogen produced. There are several WtH plants in
syngas to produce a high quality “green” hydrogen suitable for operation or construction worldwide.
use in fuel cells. The hydrogen concentration in the syngas can
be increased by passing through a water shift reactor which African projects
converts the CO to CO 2 and hydrogen, creating a mixture that is The challenge for Africa is designing smaller sized plants which can
about half hydrogen and half carbon dioxide. The hydrogen is be operated effectively.
then filtered out using a commercial pressure-swing absorption
system. Ethiopia
The cost of green hydrogen produced from MSW is estimated The construction of Reppie WtE plant in Addis Ababa began in
to be three to five times cheaper than green hydrogen produced 2014 at an estimated cost of $180 million, under an agreement
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by electrolysis. For every tonne of dry waste put into the system, signed between UK-based Cambridge Industries and the Ethiopian
an estimated 40 to 50 kg of hydrogen are produced although this government. The Reppie waste-to-energy plant includes two
can vary between 30 kg and 120 kg depending on the contents of combustion systems with two waste cranes, each with a capacity of
the waste stream. 700 tonne/day to 1400 tonne/day.
The production of commercial units is variously estimated The plant has an expected electricity production capacity of
at 11 tonnes of green H 2 per day from 120 tonnes of waste and 185 GWh per year and is able to supply power to more than 30% of

energize | February 2022 | 39