A promising industrial process can turn crushed sugar cane waste into green hydrogen far more efficiently than previously thought, shows a SECLG process simulation from the University of Johannesburg. The study is published in Renewable Energy . The simulation indicates high energy efficiency 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 decarbonizing energy-intensive industries such as steel and cement in the future.
Sugar cane and power grids
About 1.4 billion metric tons of sugarcane are produced around the world every year. From that, about 540 million metric tons of crushed sugarcane waste biomass (known as bagasse) is produced. Countries such as India, China, Brazil, and Mauritius are already gasifying bagasse to produce power for their national electricity grids.
Gasification is a way of 'chemically burning' biomass and turning it into syngas, which is a clean mixture of hydrogen and other gases. However, there is no conventional fire involved.
Too much tar
The large-scale gasification methods used at present are not energy-efficient, do not yield high rates of hydrogen, and yield high rates of tar and other noxious by-products, says Prof Bilainu Oboirien from the University of Johannesburg. He is a researcher at the Department of Chemical Engineering Technology.
"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," says Oboirien.
"Here, carbon dioxide generated is not captured by the process. Also, the high tar yields require a lot of additional equipment for cleaning. For context, tar is like dirty engine oil in a car. This, in turn, increases operational costs significantly," he adds.
A better way to green hydrogen
A far more effective method to gasify biomass such as bagasse is called Sorption-Enhanced Chemical Looping Gasification (SECLG). Various research groups have been developing SECLG over the last 10 years.
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 better able to capture carbon inside the process itself, says Oboirien.
Low tar process simulation
Prof Oboirien and UJ Master's candidate Mr 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.
Higher hydrogen 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," says Oboirien.
This means that the high 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, he adds.
Existing infrastructure
Countries with existing biomass gasification infrastructure and ready access to biomass stand to benefit most from SECLG of bagasse for green hydrogen, says Oboirien. Examples are China, Brazil, and South Africa. This is because it would be much easier and cheaper to retrofit existing technologies rather than to acquire and build new, dedicated SECLG plants, he says.
Tuning with oxygen carriers
The Aspen Plus model compares the efficiency of high-performance oxygen carriers, the well-known metal oxides nickel oxide (NiO) and ferric oxide (Fe2O3). The study also examines the stability of the oxygen carriers and sorbent material, given the harsh conditions during SECLG caused by high temperatures, pressures, and material conveying systems, says Oboirien.
The model shows that the oxygen carrier nickel oxide produces higher purity hydrogen and captures carbon dioxide more effectively in the reactor during the process.
Meanwhile, the other oxygen carrier, ferric oxide, is better at producing a more combustible gas blend. It also indicates a possibility of a tunable SECLG process to yield transportation fuels such as diesel in addition to hydrogen.
Next steps
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.
Says Oboirien: "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."
Scaling up
SECLG is a proven concept using process simulation models but has its own challenges. It is not yet used in large-scale industrial biofuel-to-syngas operations.
Oboirien says SECLG requires temperatures of around 600 degrees Celcius, pressure of around 5 bar, and multiple cycles. SEGLG 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.
"Sorption-enhanced chemical looping gasification of biomass is a promising process to produce hydrogen and transportation fuels," says Oboirien.
"The research requires investment in infrastructure and collaboration between the industries to become sustainable, and hopefully, to realize the potential of this SECLG technology," he adds.
