Green iron and syngas production IMAGE©S.Abanades and S. Chuayboon Green iron and syngas production via continuous solar-driven agricultural waste biomass gasification combined with iron(III) oxide reduction
Two international concentrated solar researchers have demonstrated using concentrated solar heat to produce green iron and syngas from renewable biomass, using no-oxygen conversion in a gasifier for a carbon-free process.
In their paper, Green iron and syngas production via continuous solar-driven agricultural waste biomass gasification combined with iron (III) oxide reduction Stéphane Abanades from PROMES-CNRS in France and Srirat Chuayboon from King Mongkut’s Institute of Technology Ladkrabang in Thailand detail their novel process, which was just published at Energy.
“This process has been developed before at PROMES, to produce green iron, but it used biomass from wood,” Chuayboon noted.
“That’s not the best choice: It’s not waste. So, in this work, we use only waste biomass.”
Agricultural waste is abundant in Thailand. After the harvest, much of it is burned in the open, creating severe air pollution.
Their demonstration leveraged the high heating content of fine bio-waste particles of betel nut waste. Waste biomass in Thailand also comes from palm oil, sugar cane, rice husks, and the Para tree (rubber) as well as betel nut.
Gasifiers use heat to convert carbon-rich materials like coal or waste biomass, producing synthetic gas mixtures of hydrogen (H₂) and carbon monoxide (CO). These mixtures can be used or refined into liquid fuels like aviation fuel, diesel, or gasoline.
However, in conventional autothermal gasification, partial combustion occurs by limiting the oxygen supply from the air, so 30-40% of biomass feedstock is lost from burning, thereby losing biomass feedstock and emitting CO₂, while contaminating the syngas product and lowering its heating content.
Instead, Chuayboon and Abanades investigated and trialed a two-meter-diameter solar dish concentrator that supplied heat up to 1400°C to pyrolyze the betel nut waste and reduce iron oxide in a gasifier they developed.
The main objective is to sustainably produce green iron using renewable solar energy and biomass to replace the current process of iron-making. The coal feedstock is replaced by bio-based feedstock for direct iron ore reduction while solar energy is used as the external heat source to eliminate all the CO₂ emissions.
“Moreover, it is a combined process,” said Abanades.”Because high-value solar fuels are also produced as syngas. So the process combines both solar biomass gasification and iron production at the same time. Therefore, co-production of iron and syngas can be achieved in a single process. Furthermore, we can use various wastes from agricultural residues, so this is a flexible process.”
This process is carbon neutral. Pyrolysis of agricultural waste also produces solid biochar, which is helpful in soil amendment.
Dish solar concentrators had a slow start because early CSP (concentrated solar power) was developed to produce power, not heat. Extracting the heat as power from a solar dish concentrator is more complicated.
Increasingly, however, concentrated solar R&D now leverages its core product – direct heat – because the unmet market need is high-temperature heat for industrial processes, to provide thermal energy storage, or to make solar fuels.
When concentrated solar heat can be used directly, dish concentrators excel. These circular parabolic mirrors with one central focal point can generate highly concentrated solar energy to generate temperatures as high as 1400°C from a relatively small land footprint.
Traditionally, synfuel is produced from reforming or gasification processes involving coal or natural gas while iron/steel is produced by ore reduction using coke and coal, heated by burning more fossil fuels in a blast furnace, so its production is carbon intensive.
Instead, the researchers used waste biomass and a two-meter solar dish concentrator to heat a pyrolysis process in a gasifier, resulting in a carbon-neutral process. The synfuel (hydrogen and carbon monoxide) is the co-product of the redox reaction that converts iron oxides to iron.
This initial demonstration was on a small scale to demonstrate its effectiveness, so it did not include storage. However, this process will include storage in practice, Chuayboon said.
“Because when we carry out the chemical reaction, we will want a stable temperature, and so we need to consider thermal storage to deal with the fact that the solar energy is unstable and intermittent,” he explained.
The researchers sourced betel nut biomass from the Asia Biomass Industry in Thailand, prepared with an average particle size of 1 mm.
“We chose the betel nut waste as a representative agricultural waste thanks to the benefits of its chemical and physical properties,” said Chuayboon.
Betel nut waste’s fine particle size, high energy content, high levels of carbon and hydrogen, and low ash content make it an effective bio-reducing agent for continuously reducing iron oxide.
The reaction mechanism was in 2 steps.
Step 1: Biomass undergoes pyrolysis, breaking into char (carbon), syngas (H₂, CO), methane (CH₄), and tars.
Step 2: Iron oxide (Fe₂O₃) reduces to iron (Fe) in three stages at temperatures above 570 °C:
The net reaction is equivalent to biomass gasification using iron oxide as a solid oxidizing agent. The iron oxides they tested were Hematite (Fe₂O₃) and Magnetite (Fe₃O₄), which are around 70wt% iron.
A two-meter dish-type solar concentrator supplied the solar heat with over 10,000 x concentration (the equivalent of 10,000 ’suns).
The researchers developed a novel 1.5kW-thermal prototype solar reactor to perform the gasification,
They tested different combinations of input materials and operating conditions in experimental trials, using solar heat as the energy input. They varied the biomass to iron oxide ratios and determined the performance at each increase in temperature from 900°C to 1200 °C.
They also studied the gasification process with conventional gaseous oxidants instead of iron oxide. At higher or lower temperatures and by adding oxygen either from water (H2O) or carbon dioxide (CO₂ ), more hydrogen-rich or carbon monoxide-rich syngas could be produced.
Therefore, the type of oxidants and solar concentration would differ depending on whether the gasifier primarily produces syngas or green iron.
Synfuel or hydrogen production would require temperatures in the range of 1200-1400°C, while green iron could be produced at lower temperatures: 900-1200°C.
They analyzed factors such as the molar ratio of biomass to iron oxide and the reaction temperature to determine the best combination of these variables.
The trial showed that their process efficiently produces high-quality syngas and green iron and can be optimized for either product.
Abanades explained:
“As a result, solar gasification of bio-waste combined with Fe₂O₃ reduction performed exceptionally well with continuous reactant particles feeding, demonstrating a feasible pathway for producing green iron and high-quality syngas. The maximum syngas yield reached 63.3 mmol/g of biomass, which is close to the theoretical maximum, and high-purity Fe was simultaneously produced.”
“The process demonstrated high efficiency, with maximum carbon conversion rates approaching 98 %, energy upgrade factor up to 1.26, and solar-to-fuel energy conversion efficiency up to 14.4 %, highlighting remarkable conversion performance of biomass and solar energy to chemicals.”
“Thus, the continuous solar-driven agricultural waste biomass gasification combined with Fe2O3 reduction could potentially become a sustainable pathway for co-production of iron and syngas based on two renewable energy sources (biomass and solar energy) in a single process.”
Further reading:
S Chuayboon, S Abanades – Energy, 2023
S Chuayboon, S Abanades – Chemical Engineering Science, 2023
