Co-pyrolysis of microalgae with low-density polyethylene (LDPE) for deoxygenation and denitrification

Research advances on production and application of algal biochar in environmental remediation

Algae, comprising microalgae and macroalgae, have emerged as a promising feedstock for the production of functional biochar. Recently, the application of algal biochar in environmental remediation gains increasing attention. This review summarizes research advancements in the synthesis and application of algal biochar, a versatile and sustainable material for environmental remediation ranging from wastewater treatment to soil improvement. Algal biochar can be prepared by pyrolysis, microwave-assisted pyrolysis, and hydrothermal carbonization. Physical and chemical modifications have proven to be effective for improving biochar properties. Algal biochar is promising for removing diverse pollutants including heavy metals, organic pollutants, and microplastics. The role in soil improvement signifies a sustainable approach to enhancing soil structure, nutrient retention, and microbial activity. Research gaps are identified based on current understanding, necessitating further exploration into variations in biochar characteristics, the performance improvement, large-scale applications, and the long-term evaluation for environmental application. This review provides a better understanding of algal biochar as a sustainable and effective tool in environmental remediation.

Optimization and screening of process parameters for the robust co-pyrolytic study of waste motor oil and rice stubble toward sustainable waste-to-fuel generation

The current study explores the co-pyrolysis of waste motor oil (WMO) and rice stubble in a designed lab-scale pyrolyzer to produce alternative energy fuels. The parameter screening was followed by optimization utilizing the Box-Behnken design (BBD). Reactor temperature (TR), mixing ratio (M), and holding time (t) affected the co-pyro-oil yield substantially. A maximum co-pyro-oil yield of 90.3% was achieved at a TR = 485 °C, t = 12.5 min, and M = 5% rice stubble to waste motor oil, which was further characterized and compared with the commercial diesel fuel properties. The highest research octane number of 76.15 was obtained for the co-pyro-oil (Co-PO), followed by the pyro-oil generated from only waste motor oil (POWMO). Consequently, the paraffin content increased to 64.34 wt% from 27.66 wt % for PO RS. The carbon number varied from C7-C17 for PO WMO and Co-Po, aligning with the diesel fuel requirements. Furthermore, a substantial enrichment in the physio-chemical properties of the produced Co-PO with reduced moisture content and enhancement in higher heating value (HHV) was also noticed. Hence, the generated Co-PO could be utilized as transport-grade fuel.

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.

Dry torrefaction and continuous thermochemical conversion for upgrading agroforestry waste into eco-friendly energy carriers: Current progress and future prospect

Agroforestry Waste (AW) is seen as a carbon neutral resource. However, the poor quality of AW reduced its potential application value. Even more unfortunately, chlorine in AW led to the formation of organic pollutants such as dioxins under higher temperatures. Alkali and alkaline earth metals (AAEMs) in ash may deepen the reaction degree. Co-pretreatment of dry torrefaction and de-ashing followed by thermochemical conversion is a promising technology, which can improve raw material quality, inhibit the release of organic pollutants and transform AW into eco-friendly energy carriers. In order to better understand the process, theoretical basis such as the structural characteristics, thermal properties and separation methods of structural components of AW are described in detail. In addition, dry torrefaction related reactors, process parameters, kinetic analysis models as well as the evaluation methods of torrefaction degree and environmental impact are systematically reviewed. The problem of ash accumulation caused by dry torrefaction can be well solved by de-ashing pretreatment. This paper provides a comprehensive discussion on the role of the two- and three-stage conversion technologies around dry torrefacion, de-ashing pretreatment and thermochemical conversion in products quality enhancement. Finally, the existing technical challenges, including suppression of gaseous pollutant release, harmless treatment and reuse of torrefaction liquid product (TPL) and reduction of torrefaction operating costs, are summarized and evaluated. The future research directions, such as vitrification of the reused TPL (after de-ashing or acid catalysis) and integration of oxidative torrefaction with thermochemical conversion technologies, are proposed.

Microwave catalytic co-pyrolysis of Chlorella vulgaris and high density polyethylene over activated carbon supported monometallic: Characteristics and bio-oil analysis

Activated carbon (AC) has attracted much attention owing to its low cost and abundant sources. In this paper, three monometallic supported catalysts were prepared using AC as support (Ce/AC, Fe/AC, Ni/AC), and the effects of three catalysts on the microwave co-pyrolysis of Chlorella vulgaris (C. vulgaris) with high density polyethylene (HDPE) were studied. The results showed that the co-pyrolysis characteristics of C. vulgaris/HDPE = 1:1 (C1HP1) were significantly improved by three catalysts at high additions (>20 %). Among them, the C1HP1 group with 50 % Fe/AC addition had the shortest co-pyrolysis reaction time (2901 s). Besides, Ce/AC and Fe/AC have a promoting effect on bio-oil yields, while Ni/AC has an inhibiting effect. The maximum bio-oil yield (25.6 %) was obtained under 40 % addition of Fe/AC. Moreover, Ce/AC obtained the highest hydrocarbons content (66.68 %), while Fe/AC obtained the highest aromatic hydrocarbons content (36.64 %). Additionally, Ce/AC had the highest deoxygenation efficiency (47.33 %) and denitrification efficiency (42.28 %).

Full utilization of marine microalgal hydrothermal liquefaction liquid products through a closed-loop route: towards enhanced bio-oil production and zero-waste approach

The present study aimed to evaluate the potential of aqueous phase after hydrothermal liquefaction of microalgae (Aq-P), enriched with seawater, as a growth medium coupled with crude bio-oil production by the halophyte Dunaliella salina. Results showed that Aq-P showed higher content of total organic carbon (TOC) and total nitrogen (10.24, and 5.11 g L-1, respectively), while seawater showed higher anions and cations content. At the 12th day of microalgae incubation, the Aq-P growth medium showed 15.9% higher dry weight than the control (f/2 medium), with enhanced lipid content by 21.2% over the control, and 5.7% significant reduction in carbohydrates. The bio-oil yields of microalgal biomass cultivated in f/2 and Aq-P were 28.74% and 29.54%, respectively. Using Aq-P enhanced the fatty acids/esters and hydrocarbons in the crude bio-oil by 12.6% and 1.7 times, respectively, comparing to f/2-derived bio-oil. However, nitrogen-containing compounds in the Aq-P-derived bio-oil reduced by 60.7% comparing to f/2 medium. Interestingly, diesel carbon-range represented the majority of the products in both f/2- and Aq-P-derived bio-oil (69.1% and 78.3%, respectively). The findings of the present study provide a new approach for development of sustainable microalgal cultivation system for crude bio-oil production through a closed-loop route using Aq-P and seawater.

In-situ and ex-situ co-pyrolysis studies of waste biomass with spent motor oil: Elucidating the role of physical inhibition and mixing ratio to enhance fuel quality

Simultaneous renewable energy generation is an imperative part of sustainable hazardous waste management. Therefore, the present work explicates the co-pyrolysis of rice stubble (RS) waste biomass and spent motor oil (SMO) to upgrade the obtained bio-oil. Moreover, two different modes, namely, in-situ and ex-situ, were implemented to analyze the effect of physical inhibition. Monothetic analysis approach was followed to determine optimum process conditions. A substantial increment of ∼ 85% was observed in bio-oil yield for RS: SMO (1:1) in-situ operation whilst the only RS biomass pyrolysis. Moreover, the HHV increased by ∼ 2.15 times after co-pyrolysis with a considerable reduction (62.70%) in water content. Consequently, the paraffin content increased to 79.14 vol% with an iso-paraffin index of 0.285. Subsequently, a possible reaction mechanism is also proposed to evaluate results comprehensively. Altogether, the co-pyrolysis of these feedstocks resulted in improved aliphatic content and reduced oxygenates, encouraging its adequacy as an alternate fuel.

Synergetic biofuel production from co-pyrolysis of food and plastic waste: reaction kinetics and product behavior

This study aimed to develop a process for producing bio-oil, char, and value-added chemicals from food waste and plastic waste blend using co-pyrolysis under controlled conditions. The food waste (rice, vegetables, and fish) was blended in definite ratios (70:30, 60:40, and 50:50 w/w) with polyethylene terephthalate (PET). Experiments were conducted at various temperatures (250, 300, and 350 °C) and reaction times (30, 60, 90, and 120 min). A kinetic analysis was performed to fit experimental data, and reaction kinetics were observed to follow Arrhenius behavior. Maximum yields of bio-oil and bio-char, 66 and 40 wt% respectively, were attained at 350 °C, with yields being strongly influenced by variations in temperature and weakly affected by variations in reaction time. Co-pyrolysis promoted the formation of carboxylic acid, hydrocarbons, and furan derivatives. Formation of carboxylic acid could be increased by increasing the ratio of plastic waste. A maximum carboxylic acid content of 42.01% was achieved at 50% of plastic waste. Meanwhile, a maximum aliphatic hydrocarbon content of 44.6% was obtained with a ratio of 70:30 of food waste to plastic waste at 350 °C. Overall, pyrolysis of food and plastic waste produced value-added compounds that can be used as biofuels and for a variety of other applications.

A state-of-the-art review on algae pyrolysis for bioenergy and biochar production

Algae, as a feedstock with minimum land footprint, is considered a promising biomass for sustainable fuels, chemicals, and materials. Unlike lignocellulosic biomass, algae consist mainly of lipids, carbohydrates, and proteins. This review focusses on the bio-oil and biochar co-products of algae-pyrolysis and presents the current state-of-the-art in the pyrolysis technologies and key applications of algal biochar. Algal biochar holds potential to be a cost-effective fertilizer, as it has high P, N and other nutrient contents. Beyond soil applications, algae-derived biochar has many other applications, such as wastewater-treatment, due to its porous structure and strong ion-exchange capacity. High specific capacitance and stability also make algal biochar a potential supercapacitor material. Furthermore, algal biochar can be great catalysts (or catalyst supports). This review sheds light on a wide range of algae-pyrolysis related topics, including advanced-pyrolysis techniques and the potential biochar applications in soil amendment, energy storage, catalysts, chemical industries, and wastewater-treatment plants.

Improving of Pyrolysis Oil from Macroalgae Cladophora glomerata with HDPE Pyrolysis Oil

The slow pyrolysis of macroalgae at moderate temperatures in the reactor used resulted in an oil with a slightly better calorific value than that of the literature, but the other properties were not convincing. Therefore, co-pyrolysis with HDPE offers a way out in this study. However, this did not improve the property profile as a fuel, as the co-pyrolysate was incombustible due to its high water content. Only a mixture of the pyrolysis oil from algae and of the HDPE wax from the initial pyrolysis of HDPE resulted in a diesel-like product: the density was from 807 kg m−3, the viscosity 3.39 mm2 s−1, the calorific value was 46 MJ kg−1, and the oxidation stability was 68 min. The isoparaffin index indicates only a low branching of the paraffins, and therefore a low research octane number of 80. The blend did not need any further stabilizing additives.

Co-pyrolysis of different torrefied Chinese herb residues and low-density polyethylene: Kinetic and products distribution

In present work, the synergistic effects during co-pyrolysis of low-density polyethylene (LDPE) and torrefied Chinese herb residues (CHR) have been investigated by thermogravimetric analysis. The kinetic parameters of co-pyrolysis were calculated by Coats-Redfern method, and the difference values of experiment and theoretical were also investigated for gas and oil compounds. The results show that the extent of synergistic or inhibitory effects of co-pyrolysis was connection with the severity of CHR torrefaction, and the activation energy depend on the blend ratio of LDPE and CHRs. In addition, co-pyrolysis tends to generate more small molecule products and reduce oil yield, and increase the CO content but decreases CH₄ in the gas product. The results also found that the liquid products have a significant interaction during the co-pyrolysis process, because the content of aliphatic hydrocarbons and alcohols in the blends pyrolysis oil has been greatly increased, and improving the quality of oil.

Sustainable valorization of algae biomass via thermochemical processing route: An overview

Biofuels have become an attractive energy source because of the growing energy demand and environmental issues faced by fossil fuel consumption. Algal biomass, particularly microalgae, has excellent potential as feedstock to be converted to bio-oil, biochar, and combustible syngas via thermochemical conversion processes. Third-generation biofuels from microalgal feedstock are the promising option, followed by the first-generation and second-generation biofuels. This paper provides a review of the applications of thermochemical conversion techniques for biofuel production from algal biomass, comprising pyrolysis, gasification, liquefaction, and combustion processes. The progress in the thermochemical conversion of algal biomass is summarized, emphasizing the application of pyrolysis for its benefits over other processes. The review also encompasses the challenges and perspectives associated with the valorization of microalgae to biofuels ascertaining the potential opportunities and possibilities of extending the research into this area.

Co-pyrolysis of microalgae and other biomass wastes for the production of high-quality bio-oil: Progress and prospective

Microalgae are the most prospective raw materials for the production of biofuels, pyrolysis is an effective method to convert biomass into bioenergy. However, biofuels derived from the pyrolysis of microalgae exhibit poor fuel properties due to high content of moisture and protein. Co-pyrolysis is a simple and efficient method to produce high-quality bio-oil from two or more materials. Tires, plastics, and bamboo waste are the optimal co-feedstocks based on the improvement of yield and quality of bio-oil. Moreover, adding catalysts, especially CaO and Cu/HZSM-5, can enhance the quality of bio-oil by increasing aromatics content and decreasing oxygenated and nitrogenous compounds. Consequently, this paper provides a critical review of the production of bio-oil from co-pyrolysis of microalgae with other biomass wastes. Meanwhile, the underlying mechanism of synergistic effects and the catalytic effect on co-pyrolysis are discussed. Finally, the economic viability and prospects of microalgae co-pyrolysis are summarized.

High-value products from ex-situ catalytic pyrolysis of polypropylene waste using iron-based catalysts: the influence of support materials

Catalytic pyrolysis is considered a promising strategy for the utilisation of plastic waste from the economic and environmental perspectives. As such, the supporting materials play a critical role in the properties of the catalyst. This study clarified this influence on the dispersion of the iron (Fe) within an experimental context. Four different types of typical supports with different physical structures were introduced and explored in a two-stage fixed-bed reactor; these included metallic oxides (Al₂O₃, TiO₂), a non-metallic oxide (SiO₂), and molecular sieves (ZSM-5). The results show that the liquid products were converted into carbon deposits and lighter gaseous products, such as hydrogen. The Al₂O₃-supported catalyst with a relatively moderate specific surface areas and average pore diameter exhibited improved metal distribution with higher catalytic activity. In comparison, the relatively low specific surface areas of TiO₂ and small average pore diameters of ZSM-5 had a negative impact on metal distribution and the subsequent catalytic reformation process; this was because of the inadequate reaction during the catalytic process. The Fe/Al₂O₃ catalyst produced a higher yield of carbon deposits (30.2 wt%), including over 65% high-value carbon nanotubes (CNTs) and hydrogen content (58.7 vol%). Additionally, more dispersive and uniform CNTs were obtained from the Fe/SiO₂ catalyst. The Fe/TiO₂ catalyst promoted the formation of carbon fibre twisted like fried dough twist. Notably, there was interesting correspondence between the size of the reduced Fe nanoparticles and the product distribution. Within certain limits, the smaller Fe particle size facilitates the catalytic activity. The smaller and better dispersed Fe particles over the support materials were observed to be essential for hydrocarbon cracking and the subsequent formation of carbon deposits. The findings from this study may provide specific guidance for the preparation of different forms of carbon materials.

Integrated approach for enhanced bio-oil recovery from disposed face masks through co-hydrothermal liquefaction with Spirulina platensis grown in wastewater

Currently, the enormous generation of contaminated disposed face masks raises many environmental concerns. The present study provides a novel route for efficient crude bio-oil production from disposed masks through co-hydrothermal liquefaction (Co-HTL) with Spirulina platensis grown in wastewater. Ultimate and proximate analysis confirmed that S. platensis contains relatively high nitrogen content (9.13%dw), which decreased by increasing the mask blend ratio. However, carbon and hydrogen contents were higher in masks (83.84 and 13.77%dw, respectively). In addition, masks showed 29.6% higher volatiles than S. platensis, which resulted in 94.2% lower ash content. Thermal decomposition of masks started at a higher temperature (≈330 °C) comparing to S. platensis (≈208 °C). The highest bio-oil yield was recorded by HTL of S. platensis and Co-HTL with 25% (w/w) masks at 300 °C, which showed insignificant differences with each other. GC/MS analysis of the bio-oil produced from HTL of algal biomass showed a high proportion of nitrogen- and oxygen-containing compounds (3.6% and 11.9%, respectively), with relatively low hydrocarbons (17.4%). Mask blend ratio at 25% reduced the nitrogen-containing compounds by 55.6% and enhanced the hydrocarbons by 43.7%. Moreover, blending of masks with S. platensis enhanced the compounds within the diesel range in favor of gasoline and heavy oil. Overall, the present study provides an innovative route for enhanced bio-oil production through mask recycling coupled with wastewater treatment.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13399-021-01891-2.

Thermal and Kinetic Behaviors during Co-Pyrolysis of Microcrystalline Cellulose and Styrene–Butadiene–Styrene Triblock Copolymer

The interaction between various municipal solid waste components is very important for the actual solid waste treatment process. Microcrystalline cellulose (MC) and styrene–butadiene–styrene triblock copolymer (SBS) are important components of municipal solid waste. In this paper, co-pyrolysis characteristics and kinetics of MC and SBS with different heating rates were investigated using a thermogravimetric analyzer. The overlap ratio was defined to evaluate the interaction between MC and SBS. The results showed that the decomposition temperature of MC was lower than that of SBS during pyrolysis. The interaction between MC and SBS, an inhibitory effect, was most significant when the MC mass fraction was 70% with an overlap ratio of 0.9764. SBS had almost no effect on the pyrolysis temperature of MC, while MC delayed the pyrolysis of SBS. Adding MC in SBS can significantly reduce the energy required for the reaction.

Products distribution and interaction mechanism during co-pyrolysis of rice husk and oily sludge by experiments and reaction force field simulation

In this work, the co-pyrolysis behavior of rice husk (RH) and oily sludge (OS) was investigated by combining experiments and simulation. The thermogravimetric-derivative thermogravimetric (TG-DTG) and Reaction force field (ReaxFF MD) results indicate that synergetic effects exist in co-pyrolysis. Compared with the single component pyrolysis, the activation energy of RH and OS in co-pyrolysis was decreased by 15.97% and 17.14% shown by kinetic analysis, respectively. The Pyrolysis-gas chromatography/mass spectrometry (PY-GC/MS) experiments, and simulation products analysis reveal that more bio-oil and molecules with low molecular weight were produced during the co-pyrolysis process. The synergetic effect mechanism was studied by detecting the variation of free radical intermediates. The results show that hydroxyl radicals from RH pyrolysis reduced cracking temperature of OS, and the hydrogen radicals from OS pyrolysis increased the degree of ring-splitting of RH. The study explores an approach to identify the synergetic effect and reveal the mechanism of co-pyrolysis.

Enhancing hydrocarbon production via ex-situ catalytic co-pyrolysis of biomass and high-density polyethylene: Study of synergistic effect and aromatics selectivity

We conducted ex-situ catalytic fast co-pyrolysis (co-CFP) of corn stalk (CS) and high-density polyethylene (HDPE) over HZSM-5 catalyst to enhance the production of hydrocarbons. The effect of pyrolysis temperature and CS-to-HDPE mass ratio (CS/HDPE) on the yield of condensable volatile organic products (CVOPs) and the relative content of hydrocarbons were studied. The synergisms between CS and HDPE were determined based on the difference between the experimental and theoretical CVOP and hydrocarbons content. The results showed that the addition of HDPE significantly promotes the production of CVOPs, reaching the maximum value at 750 °C. In the presence of HZSM-5, the CVOP and hydrocarbons production, especially aromatics, were enhanced further, and 650 °C and 700 °C were the preferable pyrolysis temperature for desired products. Benzene, toluene, and xylenes were the predominant aromatics during the CFP process due to the good shape-selectivity of HZSM-5, contributing to the highest selectivity of C5-C11 compounds in C5+ hydrocarbons. CS/HDPE mass ratio of 1:1 was a critical point for enhancing aromatics yield. CS/HDPE < 1 was the recommended mass ratio for increasing the relative aromatic hydrocarbons content, which increases as the CS/HDPE mass ratio decreases. Meanwhile, we presented the potential reaction pathways between CS and HDPE to explain the synergistic effects during the co-CFP process.