Understanding of carbon nanomaterials oxidation is useful in many different applications, e.g., for soot emission abatement, or in defect engineering aiming to improve material properties. In this work, the oxidative behavior of three substantially different qualities of carbon black, multiwall carbon-nanotubes, and few-layer graphene, was studied using a combination of macroscale quantification (using thermogravimetric analysis) and nanoscale imaging of their structural evolution (using environmental transmission electron microscopy, ETEM). The materials were investigated both with and without the addition of a nanoparticulate iron oxide catalyst. Catalyst addition clearly lowered the conversion temperature during oxidation. The ETEM revealed that the catalyst nanoparticles induced primary surface damages in the carbon nanostructure at relatively low temperatures. From there, oxidation could proceed more rapidly at recently exposed edge sites due to their higher propensity for oxidation. Thus, the enhanced oxidation was not solely linked to the interface between catalyst and carbon. 

The escalating global concern about the expansion of CO2 emissions and its profound consequences on climate change underscores the critical need for robust CO2 capture materials. The core objective of this review was to conduct a comprehensive survey of recent advancements in CO2 capture, with a focus on porous materials, including metal-organic frameworks, zeolitic imidazolate frameworks, zeolites, metal oxides/metalloids, porous polymers, derived carbons, and (biochar, sludge, ash), as documented in the reported studies from 2017 onwards. By considering the CO2 adsorption capacity as the most important property, an up-to-date database of CO2 capture capacities in various porous adsorbents was provided, and other properties, such as selectivity, surface area, pore size/volume, recyclability, etc., for the promising adsorbents were further discussed. Furthermore, the issues on the mechanism, commercial viability (adsorbents cost and upscaling), environmental concerns and future directions (3D printing, artificial intelligence) were discussed. This review serves as an invaluable resource, guiding future investigations in this field and contributing to ongoing efforts to mitigate CO2 emissions. 

In this experimental study, we investigated the influence of various gas atmospheres on the chemistry of the inorganics during black liquor conversion. The focus was placed on oxyfuel conditions, which are characterized by a mixture of O<inf>2</inf>and recycled flue gases (e.g., CO<inf>2</inf>) as oxidation medium for combustion, to facilitate efficient CO<inf>2</inf>capture. The conventional air-based conversion and pure CO<inf>2</inf>and N<inf>2</inf>atmospheres were also tested for comparison. The experiments were conducted in a drop-tube furnace (DTF) at 1000 °C. A solid and gas phase compositional analysis was carried out to be able to quantify the carbonate, sulfate, and sulfide content in the collected solids and mapping gaseous compositions. In addition, thermodynamic equilibrium calculations (TEC) were used to analyze the chemistry that governed the composition of the formed inorganics. We found that CO<inf>2</inf>had an oxidative effect on the inorganics in the conversion of black liquor, contrary to inert N<inf>2</inf>. Moreover, in a CO<inf>2</inf>atmosphere, significantly higher levels of SO<inf>2</inf>were formed than in a pure N<inf>2</inf>atmosphere. These findings have important implications for the Industrial chemical recovery of sodium and sulfur in the kraft pulp process. Predictions of the effect of oxyfuel mode at realistic conditions relevant to industrial recovery boiler operations were also made with TEC. We found support for the existence of fuel-to-gas ratios that can render similar levels of sulfur reduction to sulfide as for conventional air-based operation in the substoichiometric zone of the boiler. These results can guide future development toward retrofitted recovery boilers to oxyfuel mode.

A machine vision driven sensor for estimating the instantaneous feeding rate of pelletized fuels was developed and tested experimentally in combustion and gasification processes. The feeding rate was determined from images of the pellets sliding on a transfer chute into the reactor. From the images the apparent area and velocity of the pellets were extracted. Area was determined by a segmentation model created using a machine learning framework and velocities by image registration of two subsequent images. The measured weight of the pelletized fuel passed through the feeding system was in good agreement with the weight estimated by the sensor. The observed variations in the fuel feeding correlated with the variations in the gaseous species concentrations measured in the reactor core and in the exhaust. Since the developed sensor measures the ingoing fuel feeding rate prior to the reactor, its signal could therefore help improve process control. 

Changed utilization of black liquor in the pulp and paper industry has the potential to offer simplified carbon capture and, thus, negative net emissions from these large point sources. This can be achieved either by adapting existing recovery boilers to oxyfuel combustion or by replacing them with black liquor gasification technology. In this work, the chemistry during black liquor conversion was therefore studied in detail under different atmospheres relevant for pyrolysis, gasification, and oxyfuel combustion. Experiments were performed using environmental scanning transmission electron microscopy (ESTEM) and thermogravimetric analysis (TGA), supported with thermodynamic equilibrium calculations (TECs) to understand and interpret the results. Black liquor conversion was found to be generally similar in air and oxyfuel atmospheres containing approximately 20-25 mol % oxygen. The results however indicated that there was a higher probability of forming carbonates in the melt at higher carbon dioxide (CO2) partial pressures, which in addition was found to be associated with potentially higher sulfur loss during black liquor conversion. Both of these characteristics can negatively affect the chemical recycling at the pulp mill by increasing the need for lime and makeup chemicals.

The aim of this work was to investigate whether the use of individual tree components (i.e., stem wood, bark, branches, and needles of spruces) as feedstocks during oxygen blow gasification is more efficient than using mixtures of these components. Experiments were performed at three oxygen levels in an 18-kW oxygen blown fixed bed gasifier with both single and mixed component feedstocks. The composition of the resulting syngas and the cold gas efficiency based on CO and H2 (CGEfuel) were used as response variables to evaluate the influence of different feedstocks on gasification performance. Based on the experimental results and data on the composition of ∼26000 trees drawn from a national Swedish spruce database, multivariate models were developed to simulate gasifier performance under different operating conditions and with different feedstock compositions. The experimental results revealed that the optimal CGEfuel with respect to the oxygen supply differed markedly between the different spruce tree components. Additionally, the models showed that co-gasification of mixed components yielded a lower CGEfuel than separate gasification of pure components. Optimizing the oxygen supply for the average tree composition reduced the GCEfuel by 1.3–6.2% when compared to optimal gasification of single component feedstocks. Therefore, if single-component feedstocks are available, it may be preferable to gasify them separately because doing so provides a higher gasification efficiency than co-gasification of mixed components. © 2022 The Authors

Oxygen blown high-temperature gasification constitutes an opportunity for chemical recycling of plastic wastes. This article summarizes the results from comparative tests of combustion and gasification of two complex plastic wastes: a plastic reject (PR) from processing recycled paper and an automotive shredder residue (ASR). Calculated gasification efficiencies corresponded to about 80% and 60%, respectively. Gasification resulted in lower yields of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F) compared to direct combustion. A two-stage process, including gasification followed by syngas combustion, reduced the emissions of HCl and PCDD/F in the flue gas to <1.4% and <0.2%, respectively, compared to the levels from direct combustion of the PR feedstock. Most of the PCDD/F (>99%) was captured along with particulate matter (soot) during gasification. The contribution to the toxic concentration of PCDD/F was mainly from the PCDF congeners. Fly ash particulate matter from ASR combustion contained a significant proportion of zinc, which thus constitutes a great potential for use in zinc recycling. © 2021 Elsevier Ltd

Petrochemical products could be produced from circular feedstock, such as waste plastics. Most plants that utilize syngas in their production are today equipped with entrained flow gasifiers, as this type of gasifier generates the highest syngas quality. However, feeding of circular feedstocks to an entrained flow gasifier can be problematic. Therefore, in this work, a two-step process was studied, in which polypropylene was pre-treated by pyrolysis to produce a liquid intermediate that was easily fed to the gasifier. The products from both pyrolysis and gasification were thoroughly characterized. Moreover, the product yields from the individual steps, as well as from the entire process chain, are reported. It was estimated that the yields of CO and H2 from the two-step process were at least 0.95 and 0.06 kg per kg of polypropylene, respectively, assuming that the pyrolysis liquid and wax can be combined as feedstock to an entrained flow gasifier. On an energy basis, the energy content of CO and H2 in the produced syngas corresponded to approximately 40% of the energy content of the polypropylene raw material. This is, however, expected to be significantly improved on a larger scale where losses are proportionally smaller. © 2021 by the authors. 

Synthetic fuel production via gasification of residual biomass streams from the pulp and paper industry can be an opportunity for the mills to enable improved resource utilization and at the same time reduce the production of excess heat. This paper summarizes initial oxygen-blown gasification experiments with two bark residues from a European pulp and paper mill, i.e., a softwood bark and a hardwood bark. The gasification process was characterized by measuring syngas yields and process efficiency to find optimum operating conditions. In addition, impurities in the syngas and ash behavior were characterized. Maximum yields of CO and H2 were obtained from softwood bark and amounted to approximately 29 and 15 mol/kg fuel, respectively. Optimum cold gas efficiency was achieved at an oxygen stoichiometric ratio of λ = 0.40 and was approximately 76% and 70% for softwood bark and hardwood bark, respectively. Increased λ had a reducing effect on pollutants in the syngas, e.g., higher hydrocarbons, NH3, HCl, and soot. The situation for sulfur species was more complex. Evaluation of the bark ashes indicated that slag formation could start already from 800◦C. Furthermore, a non-intrusive laser diagnostics technique gave rapid feedback on the millisecond scale. Measured syngas temperature and water content were in good agreement with the applied reference methods. © 2021 by the authors. 

Integration of renewable raw materials in existing refineries is most likely the shortest way for the successful, large-scale introduction of biofuels in the transport sector in the short term and medium term. One possible renewable raw material for this application is fast pyrolysis bio-oil (FPBO), which in this study has been coprocessed (at 0 and 20 wt %) with vacuum residue (VR, 50 wt %) and vacuum gas oil (VGO, balance) in a continuous, as well as a semibatch, slurry hydrocracking process. Experiments both with and without FPBO were performed at 450°C and 150 bar with a continuous hydrogen flow through the reactor. Oil-soluble molybdenum hexacarbonyl and molybdenum 2-ethylhexanoate were used as catalyst precursors, to be sulfided in situ. The continuous trials resulted in reactor walls completely free of coking, and they resulted in a low overall coke yield (about 1 wt %). The hydrodeoxygenation reached almost 92%, and the total acid number was reduced by nearly 99% in the FPBO experiment A mass balance of the renewable carbon from FPBO, based on the performed experiments, showed that the fossil CO2 emissions can be lowered by 1.35 kg per kg of processed FPBO if all renewable carbon in gaseous and liquid hydrocarbons is used to replace its fossil counterparts, and all methane formed from FPBO is used to produce hydrogen. Semibatch experiments gave less successful results when upgrading FPBO-containing feedstock, with a high coke yield (8 wt %) as well as a high gas yield (24 wt %). The results of this study demonstrate that FPBO can be successfully coprocessed with heavy fossil oils in a continuous slurry hydrocracking process without neg. affecting the processing of the fossil components of the feed and that a continuous process is preferred over batch or semibatch processes when studying coprocessing of bio-oils.