Investigating the adsorption behavior and quantitative contribution of Pb2+ adsorption mechanisms on biochars by different feedstocks from a fluidized bed pyrolysis system.

The aim of this study was to examine the qualitative and quantitative analysis of Pb2+ adsorption mechanisms performed with biochars derived from rice straw (RSBs), rice husk (RHBs) and saw dust (SDBs) at several pyrolysis temperatures (400-600 °C) in a fluidized bed system. Adsorption isotherms, kinetics, and desorption analysis were determined, and biochars were analyzed by X-ray Photoelectron Spectroscopy (XPS), Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscope with Energy Dispersive Spectrometer (SEM-EDS) and Boehm titration method. The effect of minerals on Pb2+ adsorption, including precipitation and cation exchange, revealed increasing contribution of precipitation from a range of 4.13%-38.83% to a range of 34.08%-79.94% and decreasing effect of cation exchange from a range of 50.17%-69.75% to a range of 9.57%-43.47% with increasing pyrolysis temperature. However, it remained the dominant adsorption mechanism of all biochars (accounted for 69.49-89.52%). Especially, RSBs with quite high maximum adsorption capacity (qm) values (116-127.57 mgg-1) were mainly due to precipitation mechanism of Pb2+ adsorption, which exhibited better adsorption capacities than RHBs (25.15-30.40 mgg-1) and SDBs (21.81-24.05 mgg-1). Only with the fluidized bed shown in this study, 2.00t RSBs could be produced and the corresponding Pb2+ adsorption may reach 255.50kg per year depending on its maximum adsorption capacity under 500 °C pyrolysis temperature. The results suggest that RSBs produced in a fluidized bed reactor is a promising, cost-effective, engineered biochar for application of Pb2+ remediation in aqueous solutions.

Simultaneous production of high-valued carbon nanotubes and hydrogen from catalytic pyrolysis of waste plastics: The role of cellulose impurity.

Upcycling waste plastics into valuable carbon nanotubes (CNTs) and hydrogen via catalytic pyrolysis is a sustainable strategy to mitigate white pollution. However, real-world plastics are complex and generally contain organic impurities, such as cellulose, which have a non-negligible impact on the catalytic pyrolysis process and product distribution. In this study, cellulose was chosen as a model compound to distinguish the effects of oxygen-containing components on the CNTs and hydrogen production during the catalytic pyrolysis of waste polypropylene. Different amounts of cellulose were mixed with polypropylene to regulate the O/C mass ratio of the feedstock, and the relationship between the O/C mass ratio and the yield of products has been built quantificationally. The results revealed that the relative content of CNTs increased to over 95%, and the stability and purity of carbon deposition increased accordingly when the O/C mass ratio is 0.05. This could be ascribed to the etching effects caused by small amounts of H2O and CO2 on amorphous carbon. However, further increasing the amount of cellulose caused the deactivation of the Fe-Ni catalyst. This not only decreased the carbon yield but had an adverse impact on its morphology and graphitization, leading to the increase of amorphous carbon. This study can provide fundamental guidance for the efficient utilization of waste plastics that take advantage of organic impurities in waste plastic to promote the formation of high-purity CNTs.

Thermal decomposition behavior and sulfur release characteristics for torrefied wheat straw during pyrolysis process.

Understanding the release characteristics of S for pyrolysis process is crucial to the development of biomass thermochemical conversion. The thermal decomposition behavior and S release characteristics for torrefaction and pyrolysis process as well as the impact of torrefaction on the S release during subsequent pyrolysis process of wheat straw were evaluated. In the case of torrefaction, high reaction temperature promoted the increase in S release percentage, which was linearly proportional to mass loss. For pyrolysis process, the release percentage of S increased rapidly up to 70.50% at 500 ℃, whereas the release percentage curve showed an unchanged trend for further increase of the pyrolysis temperature. Additionally, torrefaction pretreatment enhanced the pore properties of char, which promoted the physical resistance of released S during the diffusion process. Thus, torrefaction pretreatment effectively inhibited the release of S into the gas phase, and has a promoting effect on retaining more S in char.

The synergistic mechanism between coke depositions and gas for H2 production from co-pyrolysis of biomass and plastic wastes via char supported catalyst.

Co-pyrolysis of biomass and polyethylene(PE) wastes with different blending ratios were performed in a bench-scale fixed bed over Ni/char catalyst. The synergistic mechanism between coke depositions and gas products during co-pyrolysis was studied for better regulation of H2 production. The results showed that feedstock blending ratio played a decisive role in competitive growth of amorphous coke and multi-walled carbon nanotubes (CNTs) on the catalyst surface. For low PE ratio (≤50 wt%) a negative synergy on H2 yield was generated. It was ascribed to more oxygenates that were more inclined than hydrocarbons to be absorbed by porous char to form amorphous coke, which encapsulated Ni active sites and internal pore channel of catalyst, thus resulting in deactivation of catalyst. For higher PE content, Ni/char catalyst produced more than triple the amounts of H2 yield (42.28 mmol/gfeedstock) as compared to low PE ratio (11.3 mmol/gfeedstock). A maximum positive synergy on syngas quality was yielded at 75% PE. Despite the high yield (37.8 wt%) of deposited coke, more hydrocarbon gas from plastic pyrolysis condensed on catalyst and promoted CNTs growth via dehydrogenation and polymerization, simultaneously generating H2. The unique hollow tubular structure and tip-growth mode of CNTs exposed more Ni active sites and endowed catalyst with lower deactivation extent. The scission of more chain hydrocarbons was subsequently enhanced to interact with oxygenated compounds. Therefore, appropriate PE ratios (>50%) can exert a positive synergy on gaseous conversion by regulating coke nature during co-pyrolysis of biomass and plastics. Furthermore, coke structure rather than content seems to exert more significant effect.

Catalytic conversion of plastic wastes using cost-effective bauxite residue as catalyst into H2-rich syngas and magnetic nanocomposites for chrome(VI) detoxification.

Red mud (RM) as bauxite residue from aluminum plant was investigated as cost-effective catalyst for pyrolysis and ex-situ catalytic conversion of plastic wastes into H2-rich syngas and magnetic carbon nanocomposites. The results showed that the introduction of RM catalyst elevated gas yield from 23.8 to 60.3 wt% as a rise of catalytic temperature (700-850 °C), due to its high iron activity for scission of polymer chains. Furthermore, the endothermic nature of cracking reactions of hydrocarbons led to the maximum H2 production of 28.8 mmol gfeed-1 and 63 vol% at 850 °C. The carbon/RM nanocomposites were comprehensively evaluated by multiple characterizations. High-resolution TEM indicated considerable carbon nanotubes(CNTs) depositing on the RM surface that modified iron sites dispersion and diminished nanoparticle size of iron at higher temperature of ≥800 °C. XRD and XPS results confirmed that higher temperature provided carbon components surrounding iron species to form metallic iron. The carbon/RMs were initially applied to chromium(VI) removal in sewage. RM-800 delivered high-profile adsorption capacity of 193.8 mg g-1, mainly attributed to the synergistic effect of chemical reduction by sufficient Fe0 exposure and CNTs growth promoting electrostatic attraction and electron transfer capacity. Furthermore, the correlation mechanism between catalytic temperature and the evolution of products and was discussed.

Simultaneous production of aromatics-rich bio-oil and carbon nanomaterials from catalytic co-pyrolysis of biomass/plastic wastes and in-line catalytic upgrading of pyrolysis gas.

An integrated process that includes catalytic co-pyrolysis of biomass/plastic wastes and in-line catalytic upgrading of pyrolysis gas were conducted to simultaneously produce aromatics-rich bio-oil and carbon nanotubes (CNTs). The influences of feedstocks blending ratio on the characteristics of bio-oil and CNTs were established. The reaction mechanism of carbon deposition during the system was also probed. The results showed that co-feeding plastic to biomass siginificantly enhanced the selectivity of monoaromatics (benzene, toluene, and xylene) from 5.6% for pure biomass to the maximum yield of 44.4% for 75.0% plastic ratio, and decreased naphthalene and its derivates from 85.9 to 41.7% correspondingly. The most synergistic effect on BTX selectivity occurred at 25% of plastic ratio. The multi-walled CNTs were successfully synethsized on Ni catalyst by utilizing prolysis gas as feedstocks. For pure biomass, the least CNTs yield with ultrafine diameters of 3.9-8.5 nm was generated via disproportionation reaction of CO which was derived from decarboxylation and decarbonylation of oxygenates on the ZSM-5 acid sites. With the rise of plastic ratio, sufficient hydrocarbons were produced for CNTs growth, endowing CNTs with long and straight tube walls, along with uniform diameters (~16 nm). The CNTs yield increased as high as 139 mg/g-cata. In addition, the decreased CO2 inhibited dry reforming with C1-C4 hydrocarbons and deposited carbon, avoiding excessive etching of CNTs. Thereby, high-purity CNTs with less defects were fabricated when plastic ratio was beyond 50% in the feedstock. The strategy is expected to improve the sustainability and economic viability of biomass pyrolysis.

A green route for pyrolysis poly-generation of typical high ash biomass, rice husk: Effects on simultaneous production of carbonic oxide-rich syngas, phenol-abundant bio-oil, high-adsorption porous carbon and amorphous silicon dioxide.

Rice husk is a widespread agriculture waste in rice-farming country. High silica content in rice husk prevent its efficient utilization. So in this work, concept of poly-generation was introduced to improve its utilization value. This study provided CO-rich syngas, phenol-abundant bio-oil, high-adsorption porous carbon and amorphous SiO2 as four end products for first time via combination of acid washing and activated carbon catalyst. Specifically, acid washing effectively decreasedsoluble ash, which altered pyrolysis paths, increased volatiles release and reduced impurities in bio-char. After catalytic pyrolysis, phenol content of 65.56% and CO of 56.09 vol% were detected in bio-oil and syngas from AWRH. For solid products, acid washing benefited both bio-char and silica. A low-cost porous carbon with developed pores and rich surface functional groups was prepared for water absorption. And high purity amorphous SiO2 was recycled from alkali etching solution. Finally, a green process with no waste emission was proposed.

Adsorption characteristics and mechanism of Pb(II) by agricultural waste-derived biochars produced from a pilot-scale pyrolysis system.

The objective of this study was to investigate the feasibility of removing Pb2+ by pilot-scale fluidized bed biochar, and then to put forward an industrial-scale fluidized bed pyrolysis progress of cogeneration of biochar and high-temperature gas. Corn stalk biochars (CSBs) were prepared at 400-600 °C, in which the maximum Pb2+adsorption capacity (Qm) of CSB450 is 49.70 mg⋅g-1 at the optimal condition. Adsorption isotherms, kinetics, and thermodynamics were determined, and Pb2+-loaded biochar was analyzed by fourier transform infrared spectroscopy (FTIR), x-ray photoelectron spectroscopy (XPS), x-ray diffraction (XRD) and scanning electron microscope with energy dispersive spectrometer (SEM-EDS). Ion exchange, complexation and mineral precipitation together contributed to Pb2+ adsorption on CSBs. For high-temperature CSBs with fewer oxygen functional groups (OFGs) and stronger aromatization, Pb2+ adsorption by ion exchange and functional group complexation was reduced. The mineral precipitationwas formed during the adsorption process. Using the pilot-scale fluidized bed in this study, the carbon yield per year would achieve 31.79 t, and about 1.58 t of Pb2+ would be adsorbed according to the adsorption capacity at the pyrolytic temperature of 450 °C.The results are beneficial to screen for effective biochar as a cost-effective industrial adsorbent to remove Pb2+ in contaminated water.

Investigation of representative components of flue gas used as torrefaction pretreatment atmosphere and its effects on fast pyrolysis behaviors.

In this study, three torrefaction atmosphere (N2, CO2 and 2 vol% O2 with N2 balance) were used to study effects of representative main components of flue gas during torrefaction and subsequent pyrolysis. Torrefaction pretreatment was carried out in a fixed-bed reactor at 230 °C and 250 °C, respectively. Results showed after torrefaction, torrefied samples from oxygenated atmosphere presented severer hemicellulose decomposition. And its effects on fast pyrolysis were investigated in thermogravimetry analysis and bench-scale fixed-bed reactor. It was found that oxygenated atmosphere preferred to give higher relative content of phenols at 230 °C and furans at 250 °C. For CO2, higher relative content of ketones and lowest phenols were got. The result also indicated that it's the O2 in flue gas which significantly improved the char yield. These results will be beneficial reference to predict and interpret alterations of pyrolysis behaviors when flue gas constitution changes in industrial application.

Assessment of hydrothermal carbonization and coupling washing with torrefaction of bamboo sawdust for biofuels production.

Two kinds of biofuels were produced and compared from hydrothermal carbonization (HTC) and coupling washing with torrefaction (CWT) processes of bamboo sawdust in this study. The mass and energy yields, mass energy density, fuel properties, structural characterizations, combustion behavior and ash behavior during combustion process were investigated. Significant increases in the carbon contents resulted in the improvement of mass energy density and fuel properties of biofuels obtained. Both HTC and CWT improved the safety of the biofuels during the process of handling, storing and transportation. The ash-related issues of the biofuels were significantly mitigated and combustion behavior was remarkably improved after HTC and CWT processes of bamboo sawdust. In general, both HTC and CWT processes are suitable to produce biofuels with high fuel quality from bamboo sawdust.

Effect of inorganic species on torrefaction process and product properties of rice husk.

The objective of this study was to evaluate the effect of inorganic species on torrefaction process and product properties. Torrefaction process of raw and leached rice husk was performed at different temperatures between 210 and 270 °C. Inorganic species have significant effect on the torrefaction process and properties of torrefaction products. The results indicated that solid yield increased, gas yield decreased and liquid yield remained unchanged for leached rice husk when compared to raw rice husk. Gas products from torrefaction process mainly contained CO2 and CO, and leaching process slightly reduced the volume concentration of CO2. Removal of inorganic species slightly decreased water content and increased organic component content in liquid products. Acetic acid, furfural, 2,3-dihydrobenzofuran and levoglucosan were the dominant components in liquid product. Inorganic species enhanced the effect of deoxygenation and dehydrogenation during torrefaction process, resulting in the enrichment of C component in solid products.