Research progress on pyrolysis of nitrogen-containing biomass for fuels, materials, and chemicals production.

Pyrolysis of nitrogen-containing biomass holds tremendous potential for producing varieties of high value-added products, alleviating energy depletion. Based on the research status about nitrogen-containing biomass pyrolysis, the effect of biomass feedstock composition on pyrolysis products is first introduced from the aspects of elemental analysis, proximate analysis, and biochemical composition. The properties of biomass with high and low nitrogen used in pyrolysis are briefly summarized. Then, with the pyrolysis of nitrogen-containing biomass as the core, biofuel characteristics, nitrogen migration during pyrolysis, the application prospects, unique advantages of nitrogen-doped carbon materials for catalysis, adsorption and energy storage are introduced, as well as their feasibility in producing nitrogen-containing chemicals (acetonitrile and nitrogen heterocyclic) are reviewed. The future outlook for the application of the pyrolysis of nitrogen-containing biomass, specifically, how to realize the denitrification and upgrading of bio-oil, performance improvement of nitrogen-doped carbon materials, as well as separation and purification of nitrogen-containing chemicals, are addressed.

Production of C5-C12 olefins by catalytic pyrolysis of low-density polyethylene with MCM-41 in CO2/N2.

The current high volume of plastic waste, but low recycling rate, has led to environmental pollution and wasted energy. Greenhouse gas CO2 can facilitate thermal cracking to dehydrogenate waste plastics, and has potential value for producing olefins. In this work, the pyrolysis properties of low-density polyethylene (LDPE) were studied by thermogravimetric analysis and Py-GC/MS. The effect of the pyrolysis atmosphere, using N2 or CO2, with various MCM-41 catalyst ratios on pyrolysis product distribution, were investigated. The experimental results show that the olefin selectivity under a N2 atmosphere was from 30.32 % to 44.66 % which increased as the MCM-41 catalyst was increased. Under a CO2 atmosphere, the olefin selectivity reached a maximum of 60.39 %. The Boudouard reaction was also enhanced by the introduction of CO2. The carbon content of the subdivided olefins showed that in CO2, the promotion of C5-C12 olefins was relatively weak when non-catalyzed or at low catalytic ratios, but increased significantly at higher MCM-41 catalyst ratios. With a ratio of LDPE: MCM-41 = 5:4, the CO2 atmosphere showed the greatest promotion of C5-C12 olefins over N2, with an increase of 14.66 % compared to N2, representing a 48.54 % yield of the liquid product. Producing C5-C12 olefins under these conditions maximized energy efficiency. These results show that catalytic pyrolysis of LDPE under a CO2 atmosphere has great potential to produce C5-C12 olefins, which can be used to produce high-value chemicals such as naphtha and gasoline. This opens new opportunities for the chemical recycling of plastic waste.

Enhanced sustainable integration of CO2 utilization and wastewater treatment using microalgae in circular economy concept.

Microalgae have been shown to have a promising potential for CO2 utilization and wastewater treatment which still faces the challenges of high resource and energy requirements. The implementation of the circular economy concept is able to address the issues that limit the application of microalgae-based technologies. In this review, a comprehensive discussion on microalgae-based CO2 utilization and wastewater treatment was provided, and the integration of this technology with the circular economy concept, for long-term economic and environmental benefits, was described. Furthermore, technological challenges and feasible strategies towards the improvement of microalgae cultivation were discussed. Finally, necessary regulations and effective policies favoring the implementation of microalgae cultivation into the circular economy were proposed. These are discussed to support sustainable development of microalgae-based bioremediation and bioproduction. This work provides new insights into the implementation of the circular economy concept into microalgae-based CO2 utilization and wastewater treatment to enhance sustainable production.

Application of Phosphate Materials as Constructed Wetland Fillers for Efficient Removal of Heavy Metals from Wastewater.

Constructed wetlands are an environmentally friendly and economically efficient sewage treatment technology. Heavy metals (HMs) removal is always regarded as one of the most important tasks in constructed wetlands, which have aroused increasing concern in the field of contamination control in recent times. The fillers of constructed wetlands play an important role in HMs removal. However, traditional wetland fillers (e.g., zeolite, sand, and gravel) are known to be imperfect because of their low adsorption capacity. Regarding HMs removal, our work involved the selection of prominent absorbents, the evaluation of adsorption stability for various treatments, and then the possibility of applying this HM removal technology to constructed wetlands. For this purpose, several phosphate materials were tested to remove the heavy metals Cu and Zn. Three good phosphates including hydroxyapatite (HAP), calcium phosphate (CP), and physic acid sodium salt hydrate (PAS) demonstrated fast removal efficiency of HMs (Cu2+, Zn2+) from aqueous solution. The maximum removal rates of Cu2+ and Zn2+ by HAP, CP, and PAS reached 81.6% and 95.8%; 66.9% and 70.4%; 98.8% and 1.99%, respectively. In addition, better adsorption stability of these heavy metals was found to occur with a wide variation of desorption time and pH range. The most remarkable efficiency for heavy metal removal among tested phosphates was PAS, followed by HAP and CP. This study can provide a basis for the application of HMs removal in manmade wetland systems.

Catalytic reforming of polyethylene pyrolysis vapors to naphtha range hydrocarbons with low aromatic content over a high silica ZSM-5 zeolite.

In this study, the microwave-assisted pyrolysis coupled with ex-situ catalytic reforming of polyethylene for naphtha range hydrocarbons, with low aromatic content, was investigated. Experimental results revealed that ZSM-5 zeolites with low SiO2/Al2O3 ratios led to high aromatic selectivity, while an extremely high SiO2/Al2O3 ratio significantly reduced the aromatic selectivity. The high selectivity of C5-C12 hydrocarbons (98.9 %) with low selectivity of C5-C12 aromatics (28.5 %) was obtained over a high silica ZSM-5 zeolite at a pyrolysis temperature of 500 °C, catalytic cracking temperature of 460 °C, and a weight hourly space velocity of 7 h-1. The liquid oil produced was mainly composed of C5-C12 olefins that can be easily converted into paraffin-rich naphtha by hydrogenation or hydrogen transfer reactions as the feedstock for new plastic manufacturing. 8 cycles of regeneration-reaction cycles were carried out successfully with little change on the product distribution, showing the great potential for continuous production of low-aromatic liquid oil. Catalyst characterization showed that the catalyst deactivation was primarily caused by coke deposition (approximately 16.0 wt%) on the surface of the catalysts, and oxidative regeneration was able to recover most of the pore structure and acidity of the zeolite by effectively removing coke. This study provides a better understanding for the plastic-to-naphtha process and even for scale-up studies.

Catalytic fast pyrolysis of low density polyethylene into naphtha with high selectivity by dual-catalyst tandem catalysis.

In this work, catalytic fast pyrolysis of low density polyethylene (LDPE) into highly valuable naphtha by the relay catalysis (Al2O3 followed by ZSM-5 zeolite) was conducted. Effects of different catalysts, pyrolysis temperatures, catalyst to plastic ratio, and Al2O3 to ZSM-5 ratio, on product distribution and selectivity were studied. Al2O3 shows an excellent performance for catalytic reforming of LDPE pyrolysis vapors, mainly producing C5-C23 olefins that are the important precursors to form aromatics via Diels-Alder, aromatization, and polymerization reactions in the pores of ZSM-5 catalyst. Experimental results also show that the selectivity of monoaromatics and C5-C12 alkanes/olefins can be up to 100% over Al2O3 followed by ZSM-5 relay catalysis at the temperature of 550 °C, the catalyst to plastic ratio of 4:1, and Al2O3 to ZSM-5 ratio of 1:1. The product (monoaromatics and C5-C12 alkanes/olefins), naphtha, could be a renewable feedstock for new plastic production in the petroleum industry so that this finding might provide a new insight for a circular economy.

Interaction of Chlorella vulgaris and bacteria when co-cultivated in anaerobically digested swine manure.

Mono-culture and co-culture of algae (Chlorella vulgaris) and bacteria (activated sludge) on anaerobically digested swine manure (ADSM) were investigated in this research. The results showed that during the co-cultivation biomass growth was promoted (2.43 ± 0.11 g/L) compared with the algae-only culture (1.09 ± 0.03 g/L), and the aerobic bacteria growth was initially promoted, then inhibited. The SEM (Scanning Electron Microscope) observation indicated that the amount of the C. vulgaris increased while bacteria 'disappeared' over time. After 30 min settlement, 95.5% of the biomass in co-cultivation group precipitated, while only 40.4% of the biomass settled for the algae-only group was. It is believed that the presence of bacteria enhanced the settling rate through the formation of algal consortium flocs. Bacterial community diversity and composition were measured and the results indicated that the bacterial diversity dropped and the bacterial active classes changed in the co-cultivation group.

Effect of lime mud on the reaction kinetics and thermodynamics of biomass pyrolysis.

The solid waste from papermaking factory, lime mud (LM), was previously demonstrated as an effective catalyst in biomass pyrolysis. Since understanding the kinetics and thermodynamics is the critical step for pyrolysis development, thereby the effect of LM on the kinetics and thermodynamics for biomass pyrolysis was systematically investigated in this study. More specifically, two representative biomasses, herbaceous corncob and woody aspen sawdust, were blended with LM at different mass ratio of 0:0, 0.5:1, 1:1, and 2:1. Based on this, pyrolysis was conducted through thermogravimetry under nitrogen atmosphere. The kinetic parameters of activation energy and pre-exponential factor were calculated by iso-conventional method. While the Avrami theory was used to determine the reaction order. Thermodynamic parameters were also calculated and compared with those of non-catalytic pyrolysis.

Catalytic oxidation of NO at ambient temperature over the chars from pyrolysis of sewage sludge.

Sludge char (SC) was prepared by pyrolysis of sewage sludge, then nitric acid washing, potassium hydroxide activation, and hydrogen reduction methods were used to seek for the optimum treatment for improving the catalytic oxidation of NO at 30 °C. The optimum NO conversion of 65.6% was achieved when SC was activated and hydrogen-reduced, indicating the promising prospect of NO oxidation catalyst preparation from sewage sludge. The prepared SCs showed an intensive specific pore volume peak at the micropore size of 0.89 nm which is beneficial for NO oxidation. SC characterization like temperature programmed desorption of CO2/NO/NO2, in-situ diffuse reflectance infrared Fourier transform spectroscopy, etc. were conducted to reveal the catalytic oxidation mechanisms of NO. The results indicated that the oxygen-containing functional groups, such as carboxylic acid, carboxylic anhydrites and lactones, were largely removed by hydrogen reduction, leading to marked increases of surface basicity, specific surface area, and catalytic activity of SCs. The NO oxidation over the SCs can be explained quite well by the Eley-Rideal reaction model.

Magnetic field intervention on growth of the filamentous microalgae Tribonema sp. in starch wastewater for algal biomass production and nutrients removal: Influence of ambient temperature and operational strategy.

This paper investigated the effects of temperature and cultivation methods (batch or semi-continuous culture) on the filamentous microalgae Tribonema sp. biomass production and nutrients removal in starch wastewater under low intensity magnetic field (MF) intervention. The MF significantly promoted algal growth in the late logarithmic-phase of batch cultivation, and the effect was even more obvious at lower temperatures. The MF treated group at 30 °C accumulated the highest biomass of 4.44 g/L of batch culture, an increase of 15.0% compared with the control group. The oil content of Tribonema sp. was enhanced with the MF intervention, especially for the batch culture. In the semi-continuous culture under MF intervention, Tribonema sp. reached the high biomass of 18.45 g/L after 25 days. When gradually reducing hydraulic retention time (HRT) to 1 day, the average removal rates for COD, TN, NH3-N and TP were all more than 90% in the semi-continuous cultivation.

Tribonema sp. and Chlorella zofingiensis co-culture to treat swine wastewater diluted with fishery wastewater to facilitate harvest.

Cultivating microalgae on wastewaters is an effective way to produce algal biomass whereas harvesting microalgae is a costly operation. This study we examined the feasibility of co-culturing a high-value microalga with an auto-flocculating strain to enable efficient recovery of biomass. Experiments were conducted to co-cultivate Chlorella zofingiensis with Tribonema sp. on swine wastewater diluted by fishery wastewater under different conditions. The result showed the optimal inoculum ratio of Tribonema sp. to Chlorella zofingiensis was 1:1. The removal efficiencies of pollutants (NH4+-N, TN, TP, and COD) and lipid content were high when the co-culture ratios of Tribonema sp. were high. Also, some larger chain fatty acids, specifically C20:5 and C22:6 were present when the two strains co-culture. The recovery efficiency increased with the increasing proportion of auto-flocculating Tribonema sp.. Algae co-culture has the potential to address limitations in substrate utilization by individual strains, also improve the recovery of biomass.

Microwave assisted flocculation for harvesting of Chlorella vulgaris.

Microalgae harvesting is a major hindrance for the development of the microalgae industry. In this paper, short microwave treatment was used to assist the flocculation of Chlorella vulgaris with three flocculants, Fe3+ (FeCl3), chitosan, and Ca2+ (CaCl2). A microwave irradiation time of 20 s, and a pH of 10 was found to be the optimum condition. The harvesting efficiency could be significantly increased by 43.2%, 49.5% and 39.6%, respectively for Fe3+, chitosan, and Ca2+ assisted by microwave under these conditions. Microwave treatment did not cause any damage to the algal cells, and had no obvious influence on the lipid extraction. Microwave treatment decreased the concentration of the flocculants in culture medium after flocculation; this treatment enabled the reuse of the supernatant. This study provides a new and promising method of improving the flocculation efficiency for microalgae harvesting, by using microwave energy.

Auto-flocculation microalgae species Tribonema sp. and Synechocystis sp. with T-IPL pretreatment to improve swine wastewater nutrient removal.

It is recognized coupling microalgae, which is rich in lipids or protein with wastewater treatment offers extra economic benefits that can potentially make microalgal production feasible by reducing production costs and providing environmental benefits. However, the pretreatment of high concentration nutrients such as ammonia nitrogen (NH3-N), total phosphorus (TP) and chemical oxygen demand (COD) in swine wastewater is the premise of application for microalgae in wastewater treatment. This study two auto-flocculation microalgae Tribonema sp. and Synechocystis sp. were selected for evaluation; they were cultivated in diluted swine wastewater together after it was pretreated with titanium dioxide (TiO2) plus intense pulsed light (T-IPL). The results showed that the growth of the two strains in the wastewater pretreated with T-IPL grew better than when grown without the pretreatment. The content of lipid in the two algae, cultured in the pretreated wastewater, was also higher than the lipid content from the un-pretreated wastewater; but protein content was lower. Overall, the removal efficiencies of pollutants NH3-N, TP, and COD by the two microalgae in anaerobic digestion of swine wastewater (ADSW) with T-IPL pretreatment, were higher than the removal efficiencies without pretreatment. This research also indicates that these two auto-flocculation microalgae have the potential to reduce harvesting costs. And, using T-IPL to pretreat wastewater could provide a promising method for the pretreatment of wastewater.

Cultivation of Chlorella vulgaris in sludge extracts: Nutrient removal and algal utilization.

In order to utilize the excess sludge and reduce the cost of algal cultivation, Chlorella vulgaris was cultivated in increasing proportions of sludge extracts for simultaneous nutrients removal and algal utilization. Results showed that C. vulgaris cultivated in the 100% sludge extract gained the highest total biomass (33.98 ±â€¯0.30 × 106 cells/mL) and showed good results in TOC (absolute value 175 mg/L) and nutrients (TN: 77.1%; TP: 95.0%) removals. According to the Excitation-emission matrix spectra (EEMs) analysis, the 8th day was suggested as the optimal time for biomass harvesting. Although the lipid contents showed a negative correlation with the proportion of sludge extract, the FAME analysis showed that the saturated fatty acids (SFA) contents decreased and the unsaturated fatty acids (UFA) content increased as the concentration of sludge extract increased. The 100% sludge extract could be a desirable alternative medium for the algae cultivation.

Integrated process for anaerobically digested swine manure treatment.

An integrated three-step process was proposed for the treatment of the anaerobically digested swine manure (ADSM). The flocculation and struvite precipitation were used as the pre-treatment to remove the particles and reduce phosphorus to balance the condition for the algae growth. In the biological step, the 40% group (2.5× dilution) represented the optimal cultivation condition for the A + B co-cultivation, with the highest biomass concentration of 2.325 ±â€¯0.16 g/L and performed well with nutrients removal (COD: 9770 ±â€¯184 mg/L; TN: 235 ±â€¯5.4 mg/L; TP: 25.3 ±â€¯0.8 mg/L). 94.8% of the biomass from the 40% group could naturally settle down in 30 min which is good for harvest. The activated carbon adsorption was applied as the advanced treatment to resolve the issues with the dark color and residual compounds. After these processes, the removal efficiencies of COD, TN, TP and NH4-N reached 97.2%, 94.0%, 99.7% and 99.9%, respectively.