Kyoto team develops two-stage process for direct liquefaction of low-rank coal and biomass under mild conditions
Researchers at Kyoto University in Japan have proposed a novel two-stage process to convert low-rank coals or biomass wastes under mild conditions to high-quality liquid fuel. A paper describing the process, which combines a degradative solvent extraction method they had developed earlier with the liquefaction of the resulting soluble, appears in the ACS journal Energy & Fuels.
One of the issues hampering the development of direct liquefaction of low-grade carbonaceous resources—such as low-rank coals and biomass wastes—to produce liquid fuel is their oxygen content. In low rank coals, cross-linking reactions among oxygen functional groups form large-molecular-weight compounds at temperatures lower than the liquefaction temperature; the oxygen-functional-group-derived cross-links may change to stronger carbon−carbon covalent linkages, suppressing the formation of light hydrocarbons.
On the biomass side, direct liquefaction of biomass far away from economic and technical feasibility. The core problem is also the high content of oxygen functional groups in biomass, which causes a series of problems during the liquefaction and produces oils with a high oxygen content.
Also, liquefying high-oxygen content low-ranking coal and biomass also consumes more hydrogen and produces more CO2, significantly reducing process efficiency. However, the team observed in the paper:
If low-rank coal and biomass can be deoxygenated largely and separated into several fractions having different molecular weights, it is expected that the low-molecular-weight fraction can be hydroliquefied readily under relatively mild conditions to produce low-oxygen-content liquid products by minimizing the cross-linking reactions.—Li et al.
The researchers earlier had devised a degradative solvent extraction method, which treats low-rank coals or biomass wastes in a non-hydrogen donor at around 350 °C that dewaters without phase change; removes oxygen functional groups; and separates them into several fractions having different molecular weights. The products formed during the extraction are then filtrated at the treatment temperature to obtain the extract and residue.
The extract is then separated to two fractions at room temperature: one is the fraction that precipitates as solid at room temperature (deposit), the other is the fraction that is soluble in the solvent at room temperature (soluble). The solid soluble is finally obtained after removing the solvent by distillation.
The carbon basis soluble yields were 19.4−31.2% for low-rank coals and were as high as 36.7−71.7% for biomass wastes. Solubles were almost free from water and ash. The carbon contents of solubles were as high as 81.5−84.8 wt %, and the oxygen contents were as low as 6.5−12.1 wt %. The solubles have the molecular weight around 300 and can soften and melt at less than 100 °C.
Furthermore, the team observed, the chemical structure and chemical and physical characteristics of the solubles obtained from both low-rank coals and various types of biomasses were almost independent of the raw materials and rather similar to each other.
In their latest work, the team liquefied the resulting soluble. Liquefaction was performed at 400 °C in a small-batch reactor (12 mL) using tetralin as the solvent and iron hydroxide (FeOOH) and sulfur as the catalyst (the molar ratio of Fe/sulfur was 1:2) in the presence of gaseous hydrogen. The reactor was charged with 0.5 g of soluble (dry basis), 2 g of tetralin, and catalyst.
The reactor was heated to 400 °C in 10 minutes and kept at that temperature for 30 minutes (rapid liquefaction). The researchers also performed direct liquefaction of the raw materials (single-stage liquefaction) by two methods for comparison purposes.
With the two-stage process, 62.1 wt % of rice straw soluble and 56.8 wt % of brown coal soluble were converted to the liquid fraction (oil), excluding H2O. Oxygen contents were as low as 2.2 and 3.9 wt %, respectively—much lower than those from the direct liquefaction of the raw materials via the single-stage processes. Both the amount of CO2 emitted and the amount of H2 consumed during the two-stage liquefaction were much less than those of the single-stage liquefaction.
The oils produced by the two-stage liquefaction have rather lower oxygen contents and mainly consist of aliphatic and aromatic hydrocarbons with a rather small amount of oxygen-containing compounds compared to the oils produced by the single-stage liquefactions of the raw materials … More oxygen in the raw materials was removed as H2O rather than CO2 during the two-stage liquefaction compared to the single-stage liquefactions. Most oxygen was removed during the degradative solvent extraction stage for the two-stage liquefaction.
The amount of H2 consumed and the amount of CO2 emitted to produce the same amount of oil through the two-stage liquefaction were all much less than those through the single-rapid liquefaction. Additionally, the solid products (residue and deposit) of the two-stage liquefaction had high possibilities as high-quality solid fuel or high-quality raw materials for gasification. … Thus, the possibility of combining the degradative solvent extraction method and the soluble liquefaction under mild conditions as a two-stage liquefaction method to produce high-quality liquid fuel from low-rank coals or biomass wastes was shown.—Li et al.
Xian Li, Dedy Eka Priyanto, Ryuichi Ashida, and Kouichi Miura (2015) “Two-Stage Conversion of Low-Rank Coal or Biomass into Liquid Fuel under Mild Conditions” Energy & Fuels Article doi: 10.1021/ef502574b