Investigation into role of CO2 in two-stage pyrolysis of spent coffee grounds

A Review of Recent Advances in Spent Coffee Grounds Upcycle Technologies and Practices

Coffee is the world’s second largest beverage only next to water. After coffee consumption, spent coffee grounds (SCGs) are usually thrown away and eventually end up in landfills. In recent years, technologies and policies are actively under development to change this century old practice, and develop SCGs into value added energy and materials. In this paper, technologies and practices are classified into two categories, those reuses SCGs entirely, and those breakdown SCGs and reuse by components. This article provided a brief review of various ways to reuse SCGs published after 2017, and provided more information on SCG quantity, SCG biochar development for pollutant removal and using SCG upcycle cases for education. SCG upcycle efforts align the best with the UN Sustainable Development Goals (SDG) #12 “ensure sustainable consumption and production patterns,” the resultant fuel products contribute to SDG #7 “affordable and clean energy,” and the resultant biochar products contribute to SDG #6, “clean water and sanitation.”

A state-of-the-art review on spent coffee ground (SCG) pyrolysis for future biorefinery

Coffee is a globally consumed beverage that produces a substantial amount of valuable organic waste known as spent coffee grounds (SCG). Although SCG is a non-edible biomass, research initiatives focused on valorizing/utilizing its organic content, protecting the environment, and reducing the high oxygen demand required for its natural degradation. The integration with biorefinery in general and with pyrolysis process in specific is considerered the most successful solid waste management strategy of SCG that produce energy and high-value products. This paper aims at providing a quantitative analysis and discussion of research work done over the last 20 years on SCG as a feedstock in the circular bioeconomy (CBE). Management stratigies of SCG have been thoroughly reviewed and pyrolysis process has been explored as a novel technology in CBE. Results revealed that explored articles belong to Chemical, physical., biological and environmental science branches, with Energy & Fuels as the most reporting themes. Published works correlate SCG to renewable energy, biofuel, and bio-oil, with pyrolysis as a potential valorization approach. Literature review showed that only one study focused on the pyrolysis of defatted spent coffee grounds (DSCG). The insightful conclusions of this paper could assist in proposing several paths to more economically valorization of SCG through biorefinery, where extracted oil can be converted to biofuels or value-added goods. It was highlighted the importance of focusing on the coupling of SCG with CBE as solid waste managment strategy.

Novel Correlations between Spectroscopic and Morphological Properties of Activated Carbons from Waste Coffee Grounds

Massive quantities of spent coffee grounds (SCGs) are generated by users around the world. Different processes have been proposed for SCG valorization, including pyrolytic processes to achieve carbonaceous materials. Here, we report the preparation of activated carbons through pyrolytic processes carried out under different experimental conditions and in the presence of various porosity activators. Textural and chemical characterization of the obtained carbons have been achieved through Brunauer–Emmett–Teller (BET), ESEM, 13C solid state NMR, XPS, XRD, thermogravimetric and spectroscopic determinations. The aim of the paper is to relate these data to the preparation method, evaluating the correlation between the spectroscopic data and the physical and textural properties, also in comparison with the corresponding data obtained for three commercial activated carbons used in industrial adsorption processes. Some correlations have been observed between the Raman and XPS data.

Highly efficient adsorptive removal of toluene using silicon-modified activated carbon with improved fire resistance

Activated carbons (ACs) are widely applied in the removal of volatile organic compounds (VOCs) emitted from industrial processes, because of their high adsorption capacity, low cost and reusability. Their poor thermal stability under oxidative conditions is a limiting factor and often leads to fire risk in real applications. Here, Si-modification was performed over a wood-derived AC material, and a series of modified ACs with different Si/C mass ratios (0.1-0.9) were prepared via a hydrothermal route. Physicochemical characteristics of Si/C samples was examined by XRD, SEM, TEM, XPS, FTIR and N₂-physisorption measurements. As compared to pristine AC, Si-modified ACs showed enhanced fire resistance, and an increase of ignition temperature by 79 ℃ was achieved at a Si/C mass ratio of 0.9. A combination of TEM, XPS and FTIR characterization suggests that the formation of amorphous SiO₂ nanoparticles and SiC species on the surface was responsible for the enhanced fire resistance of Si-modified ACs. By increasing microporosity, Si-modification also significantly improved the adsorption capacity of toluene as a model VOC molecule. Static and dynamic adsorption experiments were performed to understand the adsorption kinetics of the Si-modified ACs. Reusability tests showed that the desorption rate of the modified AC remained at nearly 80% even after five cycles of repeated adsorption-desorption, indicating that the modified AC has a great potential for industrial applications.

Recent progress on CO-rich syngas production via CO2 gasification of various wastes: A critical review on efficiency, challenges and outlook

Carbon monoxide (CO) is a highly valuable component of syngas which could be used to synthesize various chemicals and fuels. Conventionally, syngas is derived from fossil-based natural gas and coal which are non-renewable. To curb the problem, CO₂ gasification offers a win-win solution in which CO₂ is converted with wastes to CO, achieving carbon emission mitigation and addressing waste disposal issue simultaneously. In this review, gasification of various wastes by CO₂ with particular focus given to generation of CO-rich syngas is presented and critically discussed. This includes the effects of operating parameters (temperature, pressure and physicochemical properties of feedstocks) and advanced CO₂ gasification techniques (catalytic CO₂ gasification, CO₂ co-gasification and microwave-driven CO₂ gasification). Furthermore, associated technological challenges are highlighted and way forward in this field are proposed.

Progress in waste valorization using advanced pyrolysis techniques for hydrogen and gaseous fuel production

Hydrogen and gaseous fuel derived from wastes have opened up promising alternative pathways for the production of renewable and sustainable fuels to substitute classical fossil energy resources that cause global warming and pollution. Existing review articles focus mostly on gasification, reforming and pyrolysis processes, with limited information on particularly gaseous fuel production via pyrolysis of various waste products. This review provides an overview on the recent advanced pyrolysis technology used in hydrogen and gaseous fuel production. The key parameters to maximize the production of specific compounds were discussed. More studies are needed to optimize the process parameters and improve the understanding of reaction mechanisms and co-relationship between these advanced techniques. These advanced techniques provide novel environmentally sustainable and commercially procedures for waste-based production of hydrogen and gaseous fuels.

Effects of temperature, oxygen and steam on pore structure characteristics of coconut husk activated carbon powders prepared by one-step rapid pyrolysis activation process

Activated carbon powders made from coconut husk (CHCs) were prepared by one-step rapid pyrolysis activation process. Effects of temperature, oxygen and steam on the pore structure of CHCs were investigated. Results showed that high temperature, oxygen and steam all motivated the development of the CHCs pore structure. High temperature accelerated the evaporation of volatiles and led to more micropore structures. Oxygen promoted the development of both micropores and mesopores. CHCs' porosity separately presented a linear and a logarithmic growth with the increase of the preparation temperature and oxygen content. CHCs prepared under 1000 ℃ with activation agents of 6% oxygen and 20% steam exhibited the largest specific surface area and total pore volume of 415.85 m²/g and 0.1748 cm³/g. Steam can diffuse into the CHC matrix and enhance the formation of more mesopores. Steam over 20% would over-burn the substance and lead to the collapse of some pore structures.

Consequence of replacing nitrogen with carbon dioxide as atmosphere on suppressing the formation of polycyclic aromatic hydrocarbons in catalytic pyrolysis of sawdust

This study evaluates the effect of replacement of N₂ with CO₂ as atmosphere in catalytic pyrolysis of waste lignocellulosics with acidic and metal-modified zeolites, respectively, on the 16 EPA priority pollutant polycyclic aromatic hydrocarbons (PAHs) in bio-oils. By coupling solid phase extraction pretreatment with single ion monitoring detection, it is found that the replacement alleviates PAHs in bio-oil concerning synchronously abating the 16 PAHs with low, medium and high molecular weights, and the benzo[a]pyrene equivalent toxicity of bio-oil decreases. Meanwhile, CO₂ decreases the content of small oxygenates, e.g. furans, ketones, acids, and increases phenolics and aromatics affording more stable and valuable bio-oils. Moreover, CO₂ enhances carbon conversion efficiency, especially in combination with Fe-modified zeolite, which presents a synergistic effect. This study indicates the practical application of CO₂ as an atmosphere in catalytic pyrolysis to improve the bio-oil quality by suppressing PAHs formation and adjusting compound constituent.

The "COFFEE BIN" concept: centralized collection and torrefaction of spent coffee grounds

Spent coffee grounds are the moist solid residues of coffee brewing and in most cases, the disposal is done without any intermediate valorization actions for materials and energy recovery. State-of-the-art applications include extraction of the liquids and application of high-temperature pyrolysis. Both strategies have significant potential but have also some disadvantages (extensive pre-treatment, high costs) when applied on a large scale. This study highlights the lack of mild pyrolysis valorization strategies and presents the idea of the "COFFEE BIN." Separated spent coffee grounds are collected, dried, and thermally treated. The optimal pyrolysis conditions were identified and product characteristics and the mass balances were assessed. Elemental analysis, thermogravimetric analysis, physisorption analysis and higher heating value (HHV) determination was performed for the characterization of the carbonaceous products. The torrefied coffee grounds returned solid yields from 78 to 83%, which are significantly higher than in other cases of conventional biomass and heating values of 24-25 MJ/kg. Higher temperature pyrolysis did not sustain the advantage of increased returned mass yields and the adsorbance potential of all the carbonaceous products was lower than 25 cm³/g. The study highlighted that spent coffee grounds-due to the nature of their production process via roasting-can be suitable for torrefaction because of the high recovered solid yield and the high energy density. The results will be used for the development of a collection scheme for spent coffee grounds in a big municipality of Athens (Greece).

CO2-mediated chicken manure biochar manipulation for biodiesel production

This study employs chicken manure as a feedstock to produce different forms of energy to abate environmental burdens. To achieve ultimate carbon management, the possible utilization of CO₂ during pyrolysis of chicken manure was fundamentally investigated. The roles of CO₂ in pyrolysis of chicken manure include enhanced thermal cracking and shifting of the carbon distribution via reaction between volatile organic compounds and CO₂. The identified roles induced by CO₂ were catalytically enhanced because of the inorganic content in the feedstock. The morphology of biochar created from the chicken manure pyrolysis was significantly affected by CO₂. For example, a well-developed pore structure was observed in the biochar developed under a CO₂ environment; this biochar was used as an effective porous material for biodiesel synthesis.