Improved energy recovery from yard waste by water-starved hydrothermal treatment: Effects of process water and pressure

This study investigated the feasibility of a high-loading process with less water consumption for the valorization of wet biomass waste through hydrothermal carbonization (HTC) with and without N2 pressurization from the views of water saving, carbon utilization, and energy recovery. The results revealed that reducing the liquid-to-solid ratio from 10 to 2.5 significantly improved carbon storage in hydrochar due to preferential carbon sequestration as the solid phase (59.9%) instead of being lost in the liquid phase (∼10%). The pressurized HTC process resulted in a higher stability hydrochar through the devolatilization of secondary char that was less stable, yet resulted in ∼10% 15% more carbon transformation to the gas phase. A cost-benefit analysis further demonstrated the potential of the high-loading HTC process for enhancing energy recovery while minimizing energy consumption during hydrochar production from high-moisture yard waste.

An integrated life-cycle greenhouse gas protocol accounting on oil palm trunk and empty fruit bunch biofuel production

Improper discard of oil palm trunk and empty fruit bunch renders massive greenhouse gases. Turning these palm wastes into solid biofuels could aid in carbon reduction. The embodied environmental impacts of the solid biofuel densification process are neglected in carbon emission quantification studies applying Greenhouse Gas Protocol while the significance of classifying the system's direct and indirect carbon emissions were overlooked in those utilising life cycle assessment. Despite the prospect of both methodologies to complement their limitations for carbon emissions quantification, no study integrates both methodologies to investigate direct and indirect emissions systematically from a life cycle perspective. An integrated framework of life cycle assessment and Greenhouse Gas Protocol is developed to quantify the direct and indirect carbon emissions of oil palm trunk and empty fruit bunch densification from cradle-to-gate for three pellet plants in Indonesia and Malaysia. The emissions are categorised into three emission scopes: Scope 1, Scope 2, and Scope 3 according to the Greenhouse Gas Protocol, integrated with avoided emissions which are quantified via life cycle assessment. The pellet plants generate 534.7-732.3 kg CO₂-eq/tonne_pellet per hour, in which Scope 1 (i.e., direct emissions) is the major emission scope due to high emissions from wastewater production and drying fuel combustion. Washing equipment (169.2-439.0 kg CO₂-eq/tonne_pellet per hour) and burners (87.1-214.5 kg CO₂-eq/tonne_pellet per hour) are the hotspots found in the pellet plants. Producing empty fruit bunch pellets could reduce 62.0-74.1 % of emissions than landfilling the empty fruit bunch. Empty fruit bunch pellet and oil palm trunk pellet are recommended to co-fire with coal to phase down coal usage in achieving COP26 pledge. This study provides data-driven insights for quantifying carbon emissions through the integrated framework and could be a reference in future life cycle carbon footprint studies of the biomass densification process.

Bioflocculation of cyanobacteria with pellets of Aspergillus niger: Effects of carbon supplementation, pellet diameter, and other factors in biomass densification

One of the hurdles of renewable energy production from photosynthetic microorganisms is separating the biomass from water in cultures. Bioflocculation with filamentous fungus Aspergillus niger, an alternative low-cost method used for such separation, was studied with four cyanobacteria. Cocultures with Spirulina maxima and Synechococcus subsalsus resulted in bioflocculation efficiencies up to 94%, while with Anabaena variabilis and Anabaena siamensis bioflocculation did not occur. S. subsalsus was selected to evaluate the effect of cyanobacterial initial concentration, fungal:cyanobacterial ratio, carbon supplementation, and pH on biomass densification. Bioflocculation efficiencies up to 98% in 48 h were obtained with fungal:cyanobacterial ratio 1:5 and carbon supplementation. Despite the lower efficiency (54%), the highest concentration factor of S. subsalsus suspension (62.8 - from 0.9 to 56.5 g/L) was obtained with ratio 1:5 without supplementation. This result was attributed to the smaller pellet diameter (2.5 mm) and indicated that lower pellet growth is better for biomass densification.

Energetic characterization and evaluation of briquettes produced from naturally colored cotton waste

Cotton crops generate millions of tons of lignocellulosic waste in Brazil that could be used in energy generation; however, the main destination of this raw material is soil incorporation. The aim of this work was to perform an energetic characterization and evaluation of briquettes produced from different agricultural waste of naturally colored cotton for power generation. The cultivars Brasil Sementes (BRS) Jade and Topazio were studied, with white cotton (BRS 286) as standard for comparison purposes. Two different parts of each species, stalk and cotton shell, were analyzed by bulk density, proximate analysis, higher heating value, cellulose, hemicellulose, protein, fat and lignin content, thermogravimetric analysis, and briquette mechanical strength. The results of the energetic characterization indicated a higher energetic potential of the colored species when compared with the white cotton, especially because of the volatile matter content, fixed carbon, and higher heating value. The briquette mechanical strength was higher in the samples formulated by a mixture of stalk and shell. Finally, it was concluded that the waste from colored cotton cultivars, Jade and Topazio, is capable to generate briquettes with good mechanical and physico-chemical characteristics, especially those formed by the mixture of stalk and shell.

Improving agricultural straw preparation logistics stream in bio-methane production: experimental studies and application analysis

Long-term production in commercial straw biogas plants has been rare in China due to inefficiencies in the logistics stream. Biomass densification could be a potential solution to this issue. Therefore, we conducted a study to evaluate whether biomass densification is a more efficient and sustainable option. We performed methane production experiments to investigate fermentation characteristics of briquettes (with a new pretreatment, model II) and rubs (with a common pretreatment, model I). A 3000-m³ biogas plant was used to conduct a comparative analysis with solar eMergy joules. Results showed that the methane yield of briquettes of corn stover was 66.74% higher than that of rubs, and the briquettes had better digestion performance in terms of CH₄ content, VFA, and alcohol. The two models required almost the same eMergy investment input, while model II obtained a greater quantity of net eMergy (16.5% higher) in comparison with model I. The net eMergy yield ratio (EYR) (biogas only) of model I and model II was 0.99 and 1.67, respectively, showing less market competitiveness for commercial operations with model I. Meanwhile, the logistic costs of model II could be reduced to approximately US $34,514 annually.

Responses of biomass briquetting and pelleting to water-involved pretreatments and subsequent enzymatic hydrolysis

Although lignocellulosic biomass has been extensively regarded as the most important resource for bioethanol, the wide application was seriously restricted by the high transportation cost of biomass. Currently, biomass densification is regarded as an acceptable solution to this issue. Herein, briquettes, pellets and their corresponding undensified biomass were pretreated by diluted-NaOH and hydrothermal method to investigate the responses of biomass densification to these typical water-involved pretreatments and subsequent enzymatic hydrolysis. The densified biomass auto-swelling was initially investigated before pretreatment. Results indicated pellets could be totally auto-swollen in an hour, while it took about 24 h for briquettes. When diluted-NaOH pretreatment was performed, biomass briquetting and pelleting improved sugar conversion rate by 20.1% and 5.5% comparing with their corresponding undensified biomass. Pelleting improved sugar conversion rate by 7.0% after hydrothermal pretreatment comparing with the undensified biomass. However, briquetting disturbed hydrothermal pretreatment resulting in the decrease of sugar conversion rate by 15.0%.