A promising industrial process can turn crushed sugar cane waste into green hydrogen far more efficiently than previously thought, according to a SECLG process simulation from the University of Johannesburg.
The simulation indicates high energy efficiency for green hydrogen and produces a small fraction of the unwanted tar, carbon monoxide (CO), carbon dioxide (CO2), and nitrogen (N) compared to conventional biomass gasification plants.
The process may assist in decarbonising energy-intensive industries such as steel and cement in the future.
Large-scale gasification methods produce too much tar
The large-scale gasification methods used at present are not energy-efficient, do not yield high rates of green hydrogen, and yield high rates of tar and other noxious by-products.
Professor Bilainu Oboirien from the University of Johannesburg explained: “A typical syngas from biomass gasification has hydrogen (10-35%), carbon monoxide (20-30%), carbon dioxide (10-25%), tar (10-100 g/nm3), nitrogen (40-50%), and a balance of hydrocarbons.”
“Here, the carbon dioxide generated is not captured by the process. Also, the high tar yields require a lot of additional equipment for cleaning. This, in turn, increases operational costs significantly.”
A more efficient way to produce green hydrogen
A far more effective method for gasifying biomass, such as bagasse, is called Sorption-Enhanced Chemical Looping Gasification (SECLG). Various research groups have been developing SECLG over the past decade.
Compared to methods used in industry today, SECLG can produce much higher purity green hydrogen, at higher yields from biomass. It is also far more energy-efficient and can capture carbon inside the process itself.
Professor Oboirien and UJ Master’s candidate Lebohang Gerald Motsoeneng created a mathematical model of the SECLG process.
They followed this up with a comprehensive Aspen Plus simulation of the SECLG process at laboratory scale. They compared two known metal oxides used as oxygen carriers in the process to see how these would impact the hydrogen yield and other parameters.
High hydrogen and low tar yields
“For SECLG, our model estimates hydrogen (62-69%), carbon monoxide (5-10%), carbon dioxide (less than 1%), tar (less than 1 g/nm3), nitrogen (less than 5%), and a balance of hydrocarbons,” said Oboirien.
This means that the high green hydrogen yield, low tar concentration, and low nitrogen dilution in the gas could significantly reduce the economic costs by reducing the additional equipment required.
The hydrogen quality can be expected to be good. However, it would still require further purification to get to an industrial-grade gas that can be readily used for linked processes.
Scaling the method for real-world applications
Currently, the model does not address the degradation of the oxygen carrier and sorbent material over time in real-world applications.
In addition, solid material conveying and efficient separation of unwanted ash and char were not modelled or simulated, but these are required for a viable SECLG system.
Oboirien said: “We are presently developing further proof of concept, experimentally, in a lab-scale environment. Through these experiments, we hope to be able to validate these models against experimental data.”
Furthermore, SECLG requires temperatures of around 600°C, pressure of around 5 bar, and multiple cycles. It also requires conveyance systems for the metal oxide oxygen carriers and sorbent material, in this case.
These enable the continuous catalysis and carbon capture cycle ‘looping effect’ of the process.
“The research requires investment in infrastructure and collaboration between the industries to become sustainable, and hopefully, to realise the potential of SECLG technology for green hydrogen,” Oboirien concluded.
