The_Catalyst_Review_August_2023 - 6

SPECIAL FEATURE
Energy, Economics,
Environment
State-of-the-Art Processes for Simultaneous Waste
Utilisation, CO2
PART TWO
Dawid Hanak
Part One of this article, including References 1 through 22, is available in the July 2023 edition
of The Catalyst Review.
The Energy Equation
The core premise for integrating CO2
capture and H2
production is to enhance the energy performance of the
gasification process while reducing the amount of CO2
released into the atmosphere. Higher H2
through the enhancement of the water gas shifts (WGS)
reaction will naturally result in syngas of a higher quality
that, in turn, requires less purification effort. Yet, the
actual energy performance of the process is highly
dependent on the gasification operating conditions,
such as the steam to municipal solid waste (MSW) ratio
that stands for the amount of steam needed to gasify a
unit amount of solid fuel.
The primary function of the gasification process is to
convert MSW into a high-purity hydrogen stream. The
energy performance of such a conversion process
is usually characterized by the ratio of the hydrogen
energy produced and the amount of energy input with
MSW. This performance metric is substantially influenced
by the amount of steam used in the gasification process.
For the conventional gasification process, the optimum
operating point is at the steam-to-MSW ratio of about 1.2, which results in the H2
further addition of steam would not improve the H2
yields achieved
Capture and Hydrogen Production:
Figure 5. Comparison of energy efficiency.15
production efficiency of 47.7%. This is because any
production efficiency, increasing the energy demand and water demand for the
process. On the contrary, the H2 production efficiency for the sorption-enhanced process has increased proportionally to the steam-toMSW
ratio, achieving 51.2% at the steam-to-MSW ratio of 2. This is because, in this case, MSW gasification takes place simultaneously
with the WGS reaction, which promotes steam conversion to hydrogen. Despite the 3.5%-point increase in hydrogen production
efficiency, the optimum steam-to-MSW ratio for sorption enhanced gas (SEG) is about 1.8-1.9. Operation under higher steam-to-MSW
ratios would result in an electricity deficit and SEG would need to draw electricity from the grid.
Given that both conventional gasification and SEG operate at elevated temperatures above 650°C, waste heat can be recovered to
generate electricity that can then offset some (or all) of the process electricity requirement. This usually takes place in a conventional
superheated steam cycle. Such a practice allows avoiding the need for electricity import from the grid and increases the overall energy
efficiency of the process.
6
The Catalyst Review
August 2023

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