Hideyuki Matsumoto

Superheated-steam (SHS) torrefaction has emerged as a promising pretreatment for improving biomass gasification performance; however, the mechanisms responsible for tar suppression and enhanced high-temperature carbon conversion remain insufficiently understood. In this study, three low-quality woody biomass resources—pruned wood, driftwood, and street-tree residues—were subjected to SHS torrefaction at 300 °C and compared with inert-gas torrefaction and conventional drying. Structural evolution, tar-precursor behavior, gasification characteristics, carbon structure, and inorganic-element redistribution were systematically investigated. SHS torrefaction induced cell-wall swelling, partial collapse, and the formation of interconnected micro–mesoporous structures, resulting in improved grindability, enhanced gas accessibility, and reduced diffusion limitations. Major aromatic tar precursors decreased by 60–88%, and tar deposition during primary pyrolysis was substantially suppressed. In the secondary pyrolysis–gasification region (650–800 °C), SHS-treated samples exhibited higher syngas fractions (H2 + CO = 0.64–0.69), increased gas yield, accelerated fixed-carbon conversion, and markedly reduced residue fractions (~10–15 wt%) compared with dried and inert-gas-torrefied samples. SEM–EDX analysis revealed enrichment and retention of alkali and alkaline-earth metal species (K, Ca, and Mg), while Raman spectroscopy indicated the formation of defect-rich aromatic-char structures with enhanced gasification reactivity. The results demonstrate that SHS torrefaction promotes coupled structural and catalytic enhancement effects through pore-network development, improved gas diffusion, and AAEM-assisted heterogeneous gasification reactions. Consequently, low-quality woody residues can be upgraded into more suitable gasification feedstocks with enhanced carbon-conversion efficiency, increased syngas production, reduced tar formation, and lower residual-carbon generation.

This study proposes a nitrogen recycling system that collects and recycles nitrogen compounds from waste gases in the industrial sector, such as those from stationary sources, from industrially processed wastewater containing livestock effluent, and from household wastewater. Multiple scenarios are set, and the potential global flows of anthropogenic nitrogen in 2050 are estimated and compared to assess the effects on the largest planetary boundary problem. In contrast to the business-as-usual (BAU) scenario, in which environmental conditions are worsened through a 47% increase in nitrogen emissions by 2050 above the 2010 levels, the agricultural countermeasures scenario produced a reduction in emissions which was less than the 2010 levels. The industrial countermeasures scenario proposed in this study achieved comfortable reductions in nitrogen production by constructing a nitrogen recycling system that installs the nitrogen compounds to ammonia (NTA) technologies. Combining the agricultural and industrial countermeasures achieves a 66% reduction in nitrogen emissions compared with the BAU scenario in 2050. The combination of both countermeasures with a high installation rate of NTA technologies can achieve the reduction of nitrogen emissions beneath the planetary boundary.