• Energy Research

To explore the mechanism of the influence of Ni-Fe bimetallic catalyst for the producing high-value carbon nanotubes (CNTs) with clean hydrogen from waste plastic pyrolysis, the pyrolysis-catalysis of plastics were performed using a two stage fixed bed reaction system with Ni and Fe loading at variant molar ratios. The catalysts and produced carbon were analysed with various characterization method, including temperature-programed reduction/oxidation, X-ray diffraction, scanning electron microscopy or/and Raman spectroscopy. Both the H2 concentration and H2 yield reached maximum values of 73.93 vol.% and 84.72 mg g−1 plastic, respectively, as the ratio of Ni:Fe at 1:3. The amount and quality of CNTs were greatly influenced by the catalyst composition, and Ni and Fe display different roles to the overall reactivity of Ni-Fe catalyst for the pyrolysis-catalysis of waste plastics. Catalyst with more Fe loading produced more hydrogen and deposited carbon, due to higher cracking ability and the relatively lower interaction between active sites and support. The presence of Ni in Ni-Fe bimetallic catalyst enhanced the thermal stability and graphitization degree of produced carbons. The thermal quality of filamentous carbons might be associated with carbon defects.

Biochar is a promising catalyst/support for biomass gasification. Hydrogen production from biomass steam gasification with biochar or Ni-based biochar has been investigated using a two stage fixed bed reactor. Commercial activated carbon was also studied as a comparison. Catalyst was prepared with an impregnation method and characterized by X-ray diffraction, specific surface and porosity analysis, X-ray fluorescence and scanning electron micrograph. The effects of gasification temperature, steam to biomass ratio, Ni loading and bio-char properties on catalyst activity in terms of hydrogen production were explored. The Ni/AC catalyst showed the best performance at gasification temperature of 800°C, S/B=4, Ni loading of 15wt.%. Texture and composition characterization of the catalysts suggested the interaction between volatiles and biochar promoted the reforming of pyrolysis volatiles. Cotton-char supported Ni exhibited the highest activity of H2 production (64.02vol.%, 92.08mgg(-1) biomass) from biomass gasification, while rice-char showed the lowest H2 production.

Three Ni/Al2O3catalysts prepared by co-precipitation, impregnation and sol-gel methods were investigated for the pyrolysis-steam reforming of waste plastics. The influence of Ni loading method on the physicochemical properties and the catalytic activity towards hydrogen and carbon monoxide production were studied. Three different plastic feedstocks were used, high density polyethylene (HDPE), polypropylene (PP) and polystyrene (PS), and compared in relation to syngas production. Results showed that the overall performance of the Ni catalyst prepared by different synthesis method was found to be correlated with the porosity, metal dispersion and the type of coke deposits on the catalyst. The porosity of the catalyst and Ni dispersion were significantly improved using the sol-gel method, producing a catalyst surface area of 305.21 m2/g and average Ni particle size of 15.40 nm, leading to the highest activity among the three catalysts investigated. The least effective catalytic performance was found with the co-precipitation prepared catalyst which was due to the uniform Ni dispersion and the amorphous coke deposits on the catalyst. In regarding to the type of plastic, polypropylene experienced more decomposition reactions at the conditions investigated, resulting in higher hydrogen and coke yield. However, the catalytic steam reforming ability was more evident with polystyrene, producing more hydrogen from the feedstock and converting more carbon into carbon monoxide gases. Overall the maximum syngas production was achieved from polystyrene in the presence of the sol-gel prepared Ni/Al2O3catalyst, with production of 62.26 mmol H2g−1plasticand 36.10 mmol CO g−1plastic.

Hydrogen production from renewable resources has received extensive attention recently for a sustainable and renewable future. In this study, hydrogen was produced from catalytic steam reforming of the aqueous fraction of crude bio-oil, which was obtained from pyrolysis of biomass. Five Ni-Al catalysts modified with Ca, Ce, Mg, Mn and Zn were investigated using a fixed-bed reactor. Optimized process conditions were obtained with a steam reforming temperature of 800 °C and a steam to carbon ratio of 3.54. The life time of the catalysts in terms of stability of hydrogen production and prohibition of coke formation on the surface of the catalyst were carried out with continuous feeding of raw materials for 4 h. The results showed that the Ni-Mg-Al catalyst exhibited the highest stability of hydrogen production (56.46%) among the studied catalysts. In addition, the life-time test of catalytic experiments showed that all the catalysts suffered deactivation at the beginning of the experiment (reduction of hydrogen production), except for the Ni-Mg-Al catalyst; it is suggested that the observation of abundant amorphous carbon formed on the surface of reacted catalysts (temperature programmed oxidation results) may be responsible for the initial reduction of hydrogen production. In addition, the Ni-Ca-Al catalyst showed the lowest hydrogen production (46.58%) at both the early and stabilized stage of catalytic steam reforming of bio-oil.

The densification of bio-chars pyrolyzed at different temperatures were investigated to elucidate the effect of temperature on the properties of bio-char pellets and determine the bonding mechanism of pellets. Optimized process conditions were obtained with 128MPa compressive pressure and 35% water addition content. Results showed that both the volume density and compressive strength of bio-char pellets initially decreased and subsequently increased, while the energy consumption increased first and then decreased, with the increase of pyrolysis temperature. The moisture adsorption of bio-char pellets was noticeably lower than raw woody shavings but had elevated than the corresponding char particles. Hydrophilic functional groups, particle size and binder were the main factors that contributed to the cementation of bio-char particles at different temperatures. The result indicated that pyrolysis of woody shavings at 550-650°C and followed by densification was suitable to form bio-char pellets for application as renewable biofuels.

Bio-char, produced from biomass pyrolysis, can be pelletized to improve its undesirable characteristics, such as low bulk and energy densities, poor transportation and storage properties, and troublesome to co-fire with coal. In this study, rice husk char was compressed into pellets with four kinds of binders (lignin, starch, calcium hydroxide and sodium hydroxide). The compressive process, mechanical strength, basic fuel properties, and combustion characteristics were investigated to elucidate the effect of binders on the properties of bio-char pellets. Results showed that starch pellets had good hydrophobicity, but low volume density and poor mechanical strength. The softening and morphological transition of lignin during compression may account for the high elastic modulus and good bonding of lignin pellets. The low saturated moisture content of Ca(OH)2 pellets is mainly ascribed to its hydration. NaOH pellets showed the highest compressive strength among all pellets, and also exhibited the highest moisture uptake that may worsen the handling and storage treatment of bio-char. Compared with raw bio-char, bio-char pellets had a lower ignition temperature, wider temperature interval, and higher oxidation activity according to the thermogravimetric analysis. The lignin and Ca(OH)2 pellets showed lower compression energy consumption and moisture uptake, enhanced mechanical strength and promoted combustion performance, which demonstrated more desirable properties for utilization as biofuels.

Nitrogen, as a common element, is widely present in biomass. The effects of nitrogenous substances on the same origin pyrolysis of biomass and the consequences of N-containing biochar on the catalytic process of volatiles are important for further analyzing the pyrolysis mechanism of biomass. In this research, N-containing biochar was prepared under different conditions, and the interaction between N-containing biochar and biomass pyrolysis volatiles at 400-700 °C was studied. The results show that N-containing biochar can simultaneously participate in reactions as adsorbents, catalysts, and reactants. Its catalytic effect is obviously different for various N configurations. Pyridinic N and pyrrolic N can promote the cracking of lignin into methoxy phenol compounds and promote the further cracking of 5-hydroxymethylfurfural. Graphitic N and oxidized N can promote the further decomposition of phenol and the conversion of D-xylose into small-molecule ketones. In addition, oxidized N can also inhibit the cracking of lignin to produce guaiacol. In the long-term interaction, the highly active pyridinic N tends to convert to a more stable graphitic N.