Fast screening of catalyst performances in hydrothermal processes for biofuel production using differential scanning calorimetry.

Catalysts are usually employed in hydrothermal processes for different purposes, such as enhancing quality and yield of produced biofuels. However, assessing catalyst performances can be time consuming and expensive. For this reason, in this work, a technique based on high pressure differential scanning calorimetry was applied to study heterogeneous catalyst behavior under hydrothermal conditions at the micro-scale. Heterogeneous catalysts were mixed with distilled water and cellulose, selected as substrate, and tested at 250 °C. The heat release profiles obtained were deconvoluted in three Gaussian peaks, each associated with a set of reactions. Siralox and iron chloride showed the highest catalytic activities impacting the development and the enthalpy of the reactions. Selected samples were further characterized to investigate synergies among acid and basic sites and emphasize the importance of the spatial distribution of the components inside the catalysts. This study highlights the crucial role of advanced techniques in optimizing catalyst performance for more efficient biofuel production.

Uncovering the transition between hydrothermal carbonization and liquefaction via secondary char extraction: A case study using food waste.

Despite the ability to perform both processes in the same reactor, hydrothermal carbonization (HTC) and hydrothermal liquefaction (HTL) are considered two distinct processes differentiated by their reaction temperatures. As temperatures increase from the less severe HTC range into the HTL regime, the product distribution progressively favors an organic bio-oil phase relative to solid hydrochar. Solvents are commonly used to extract bio-oil from the solid residues produced during HTL, and to separate the amorphous secondary char from the coal-like primary char of HTC hydrochars. This suggests secondary char is a HTL biocrude precursor. Lipid-rich food waste was hydrothermally processed between 190 and 340 °C, spanning HTC to HTL conditions. Higher temperatures produce more gas, less liquid, and similar amounts of a progressively less oxygenated hydrochars, suggesting a gradual transition from HTC to HTL. However, analyses of ethanol-separated primary chars and secondary chars tell a different story. While the primary char is progressively more carbonized with temperature, the secondary char composition sharply changes at 250 °C. That is, lipid hydrolysis begins around 220 °C, but proceeds rather completely at 250 °C and above. A lower HTL temperature reduces the energy cost of the hydrothermal process, yet enables full lipid hydrolysis into long chain fatty acids while minimizing recondensation and repolymerization of fatty acids onto the primary char and their subsequent amidation. This maximizes the conversion of lipid-rich feedstocks into liquid fuel precursors with up to 70 % energy recovery.

Combustion kinetics of hydrochar from cow-manure digestate via thermogravimetric analysis and peak deconvolution.

Hydrothermal carbonization (HTC) can convert wet biomass into hydrochar (HC), a solid carbonaceous material exploitable as fuel. In this study, HTC was applied to anaerobic digestate from cow manure. HCs obtained at three HTC temperatures (180, 220, 250 °C) were characterized in detail and their combustion behavior was investigated by thermogravimetric analysis (TGA) coupled with peak deconvolution. Increasing HTC temperatures increased the fixed carbon content (17.9-20.7%), the ash content (27.2-32.5%) and the calorific value (14.3-18.2 MJ/kg), while decreased the hydrogen (5.01-4.54%) and oxygen content (24.09-12.35%) of HCs. DTG profiles peak deconvolution unveils the presence of five major components in the HCs. HCs combustion kinetics were studied applying the KAS method. Average apparent activation energy values of 100, 88, 67 kJ mol-1 were obtained for HC180, HC220, HC250, respectively. HTC at 250 °C produced the HC with the best fuel characteristics.

Effect of solvent and feedstock selection on primary and secondary chars produced via hydrothermal carbonization of food wastes.

Hydrothermal carbonization is a thermochemical process that converts wet waste biomass into hydrochar, a renewable solid fuel that comprises a coal-like primary phase and an oily secondary phase. The varying oxidation rates of these phases may result in an inefficient energy recovery when combusting the hydrochar, as secondary char is more reactive. Brewer's spent grain, dairy cheese whey and food waste were hydrothermally carbonized at 250 °C. The hydrochars were extracted using six solvents to evaluate the hydrochar partitioning between primary and secondary char phases. Feedstock nature and solvent selection impact the amount and composition of these phases detected. For lipid-rich feedstocks, ethanol extracts up to 50 wt% secondary char enriched in liquid fuel precursors from a solid primary char with enhanced coal-like characteristics. For substrates rich in carbohydrates, proteins, and lignocellulose, less secondary char is produced. Acetone and dichloromethane remove the oily secondary char and maximize primary char yield.

Subcritical water hydrolysis coupled with hydrothermal carbonization for apple pomace integrated cascade valorization.

This study evaluates an integrated biorefinery approach based on the waste hierarchy for the valorization of biodegradable waste, focusing on apple processing residues. Firstly, subcritical water hydrolysis was investigated at different experimental conditions (temperature 80 to 120 °C, dilution factor 10 to 30, residence time 10 to 30 min, initial pressure 10 to 30 bar) with the coincident aim of dissolving fermentable sugars and assess the effects of such treatment on the downstream solids. Secondly, spent solids were further processed by hydrothermal carbonization in the same reactor at fixed conditions (i.e., 180 °C, 3 h). The results showed that not only up to nearly 500 g kgdb-1 of sugars are dissolved but also lignocellulosic structure is amended, improving products valorization potential. Depending on pretreatment conditions, the proposed approach can deliver hydrochar with potential either as soil amendment or for long-term applications, sustainably valorizing food waste.

Evaluating the Aqueous Phase From Hydrothermal Carbonization of Cow Manure Digestate as Possible Fertilizer Solution for Plant Growth.

Improving the agronomic use of recycled nutrients derived from organic waste is one of the priorities within the measures adopted by the European community to reduce environmental issues but remains an unexplored area of research. This study focused on investigating the possibility of using innovative fertilizer solutions in hydroponic systems for the growth of agricultural plants. To this purpose, a liquid fraction [aqueous hydrothermal carbonization (HTC) liquid (AHL)] derived from HTC of cow manure digestate was chemically characterized (pH, electrical conductivity, mineral elements, and organic compounds such as phytotoxins), diluted with distilled water (1:30, 1:60, and 1:90, v/v) to reduce its potential phytotoxicity, and used to grow hydroponic maize (Zea mays L.) plants instead of the classical full-strength nutrient solution. The results indicated that the dilution ratio 1:30 of the AHL solution maintained a high level of toxicity for the plants (phytotoxic substances, especially Na and alkalinity), inducing the arrest of their growth. Differently, the two other dilution ratios (i.e., 1:60 and 1:90) seemed to considerably limit the levels of toxicity, since they allowed the plants to develop. However, these dilution ratios were poor in nutrient elements, inducing alteration in photosynthesis and an onset of deficiency symptoms such as pronounced leaf chlorosis. In view of an eco-friendly approach, future studies are, therefore, needed to identify the correct species-specific dilution ratio to supply both low levels of phytotoxins and adequate content of essential nutrients for appropriate plant growth and development. Furthermore, in order to lower specific Na phytotoxicity, treatments are of utmost importance before using AHL as a fertilizer solution.

Phytotoxicity of hydrochars obtained by hydrothermal carbonization of manure-based digestate.

The management of digestate, the main by-product of the anaerobic digestion (AD) process, is one of the most serious environmental issues. Although digestate is used on arable land as a fertilizer, it can have a negative impact on the environment due to nitrate leaching into the groundwater and ammonia volatilization into the atmosphere, with high economic and environmental disposal costs. Therefore, hydrothermal carbonization (HTC), a thermochemical biomass conversion process, could represent a sustainable and efficient alternative for digestate management. Hydrochar, the solid product of the HTC process, has been recently proposed as a plant growing medium in soilless culture systems (SCS). Here, using cow manure digestate as feedstock, we investigated the influence of the HTC process reaction temperature (180, 220 and 250 °C) and residence time (1 and 3 h) on the physical-chemical properties (pH, electrical conductivity, and mineral element concentrations) of the resulting hydrochars. Furthermore, in order to fully valorize hydrochar as a growing medium, their possible phytotoxic effects and those of their water extracts (prepared at two different concentrations and at different pHs) were tested in germination tests with cress seeds (Lepidium sativum L.). Concentrations of nutrients, heavy metals, organic acids, sugars and furan compounds were determined in the water extracts. Characterization analysis of these hydrochars revealed that they can be distinguished from each other by their physical-chemical properties, which were significantly affected by the two process parameters. Specifically, the HTC temperature had a greater effect on the composition of hydrochars than the residence time. Germination tests found hydrochar water extracts to show significantly lower phytotoxicity than the hydrochars themselves. Notably, the phytotoxic effect of the extracts decreased with increasing extraction ratio and decreasing pH. The chromatographic characterization of extracts identified the presence of potential phytotoxins, such as furan compounds (i.e., hydroxymethylfurfural and furfural). However, before using hydrochars as potential and innovative growing media for plants, their phytotoxicity should be limited, for example through their dilution with other substrates. Overall, AD-HTC coupling could represent a valuable eco-sustainable expedient in the field of biomasses, green economy and waste conversion and, therefore, further investigations in this direction are necessary.

Effects of woody biochar on dry thermophilic anaerobic digestion of organic fraction of municipal solid waste.

This study presents the results of semi-pilot scale anaerobic digestion tests conducted under dry thermophilic conditions with the addition of biochar (6% on fresh mass basis of inoculum), derived from an industrial gasification plant, for determining biogas and biomethane production from organic fraction of municipal solid waste. By using two types of inocula (from a full-scale dry anaerobic digestion plant and from lab-scale biomethanation tests), the obtained experimental results did not show significant increase in methane yield related to the presence of biochar (330.40 NL CH4 kgVS-1 using plant inoculum; 335.41 NL CH4 kgVS-1 using plant inoculum with biochar, 311.78 NL CH4 kgVS-1 using lab-inoculum and 366.43 NL CH4 kgVS-1 using lab-inoculum with biochar), but led to significant changes in the microbial community composition. These results are likely related with the specific biochar physical-chemical features and low adsorption potential. Resulting digestate quality was also investigated: biochar-enriched digestates were characterized by increased biological stability (809 ± 264 mg O2 kgVS-1 h-1 vs. 554 ± 76 mg O2 kgVS-1 h-1 for biochar-free and biochar-enriched digestates, respectively), lower heavy metals concentrations (with the exception of Cd), but higher polycyclic aromatic hydrocarbons content, with a reported maximum concentration of 8.9 mgPAH kgTS-1 for biochar-enriched digestate derived from AD test with lab-inoculum, which could trigger non-compliance with regulation limits for agricultural reuse of digestates. However, phytotoxicity assessments showed a decreased toxicity of biochar-containing digestates when compared to biochar-free digestates.

Techno-economic assessment of turning gasification-based waste char into energy: A case study in South-Tyrol.

In the South-Tyrol region (Italy), 46 gasifiers are currently operating and €200,000 are annually paid to dispose of as a waste 1300 tons of char. Therefore, there is a considerable interest in finding alternatives for the valorization of this solid by-product. The aim of this work is to assess the potential of char as energy source and to compare two scenarios. The first scenario considers the possibility of exploiting char in a dedicated burner integrated in the gasification plant. The second scenario assumes that all the char is collected from South-Tyrol and co-fired with biomass in an existing combustion-ORC plant. An economic analysis was performed evaluating the discounted payback time and both scenarios were modeled using Aspen Plus®. The results reveal that substantial savings in the operating costs of the plants can be achieved. In the first scenario the owners of the gasification plants could save from 50% to 94% of the char disposal costs with a payback time ranging between 3 and 7 years. In the second scenario, the owner of the plant could save approximately €235 k per year with a payback time of approximately 7 years. The present study provides a basis for further techno-economic studies on char combustion. The results can be helpful for the owners of the gasification plants in determining the most cost-effective way to dispose char and to avoid disposing it of as a waste. Furthermore, it is demonstrated how char could be used as a renewable fuel, with better performance than raw biomass.

Mesoporous Acidic Catalysts Synthesis from Dual-Stage and Rising Co-Current Gasification Char: Application for FAME Production from Waste Cooking Oil.

The main purpose of this work is to investigate the application options of the char produced from gasification plants. Two promising mesoporous acidic catalysts were synthesized using char as a support material. Two char samples were collected from either a dual-stage or a rising co-current biomass gasification plant. The catalysts produced from both gasification char samples were characterized for their physiochemical and morphological properties using N2 physorption measurement, total acidity evaluation through TPD-NH3, functional groups analysis by FT-IR, and morphology determination via FESEM. Results revealed that the dual-stage char-derived mesoporous catalyst (DSC-SO4) with higher specific surface area and acidic properties provided higher catalytic activity for fatty acid methyl esters (FAME) production from waste cooking oil (WCO) than the mesoporous catalyst obtained from char produced by rising co-current gasification (RCC-SO4). Furthermore, the effects of methanol/oil molar ratio (3:1-15:1), catalyst concentration (1-5 wt.% of oil), and reaction time (30-150 min) were studied while keeping the transesterification temperature constant at 65 °C. The optimal reaction conditions for the transesterification of WCO were 4 wt.% catalyst concentration, 12:1 methanol/oil molar ratio, and 90 min operating time. The optimized reaction conditions resulted in FAME conversions of 97% and 83% over DSC-SO4 and RCC-SO4 catalysts, respectively. The char-based catalysts show excellent reusability, since they could be reused six times without any modification.

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).

Valorization of Char From Biomass Gasification as Catalyst Support in Dry Reforming of Methane.

This study responds to the need of finding innovative routes for valorizing char derived from biomass gasification. Char is currently treated as a waste representing an energetic and economic loss for plant owners. However, it displays many similarities to activated carbon (AC) and could replace it in several applications. In this regard, the current work investigates the use of gasification derived char as catalyst support in dry reforming of methane (DRM) reactions. Char collected from a commercial biomass gasifier currently in operation was characterized and employed for the synthesis of cobalt catalysts. The catalysts were characterized and tested in an atmospheric pressure fixed bed reactor operating at 850°C with CH4:CO2 = 1 and a weight hourly space velocity of 6,500 mL g-1 h-1. The effectiveness of the synthesized catalysts was defined based on CO2 and CH4 conversions, the corresponding H2 and CO yields and their stability. Accordingly, catalysts were synthesized with cobalt loading of 10, 15 and 20 wt.% on untreated and HNO3 treated char, and the catalyst with optimum comparative performance was promoted with 2 wt.%MgO. Catalysts prepared using untreated char showed low average conversions of 23 and 17% for CO2 and CH4, yields of 1 and 14% for H2 and CO, and deactivated after few minutes of operation. Higher metal loadings corresponded to lower conversion and yields. Although HNO3 treatment slightly increased conversions and yields and enhanced the stability of the catalyst, the catalyst deactivated again after few minutes. On the contrary, MgO addition boosted the catalyst performances leading to conversions (95 and 94% for CO2 and CH4) and yields (44 and 53% for H2 and CO) similar to what obtained using conventional supports such as Al2O3. Moreover, MgO catalysts proved to be very stable during the whole duration of the test.

The case of Frictional Torrefaction and the effect of reflux condensation on the operation of the Rotary Compression Unit.

This work introduces the process of Frictional Torrefaction and comes as a continuation to the previous work done on Frictional Pyrolysis, which is a novel method of pyrolysis that does not utilize heat but only friction and pressure. Both processes (i.e. Frictional Torrefaction and Frictional Pyrolysis) take place in a Rotary Compression Unit without and with a reflux condenser respectively. Rotating augers are used for the development of friction and the simultaneous increase of pressure. The following types of analysis were performed: TGA, BET, CHNS and HHV. Both products have similar heating values, around 21 MJ/kg. The elemental compositions are comparable but lower hydrogen content (3.5%) was measured for Frictional Torrefaction. BET analysis showed differences on the surface areas and porous sizes of the materials. Frictional Torrefaction has higher fixed carbon (31.23% vs 28.31%), higher surface area (58.16 m2/g vs 36.88 m2/g) and higher absorbance (35 cm3/g vs 26 cm3/g).

Modeling the emissions of a dual fuel engine coupled with a biomass gasifier-supplementing the Wiebe function.

There is a growing market demand for small-scale biomass gasifiers that is driven by the economic incentives and the legislative framework. Small-scale gasifiers produce a gaseous fuel, commonly referred to as producer gas, with relatively low heating value. Thus, the most common energy conversion systems that are coupled with small-scale gasifiers are internal combustion engines. In order to increase the electrical efficiency, the operators choose dual fuel engines and mix the producer gas with diesel. The Wiebe function has been a valuable tool for assessing the efficiency of dual fuel internal combustion engines. This study introduces a thermodynamic model that works in parallel with the Wiebe function and calculates the emissions of the engines. This "vis-à-vis" approach takes into consideration the actual conditions inside the cylinders-as they are returned by the Wiebe function-and calculates the final thermodynamic equilibrium of the flue gases mixture. This approach aims to enhance the operation of the dual fuel internal combustion engines by identifying the optimal operating conditions and-at the same time-advance pollution control and minimize the environmental impact.

Thermodynamic modeling of small scale biomass gasifiers: Development and assessment of the ''Multi-Box'' approach.

Modeling can be a powerful tool for designing and optimizing gasification systems. Modeling applications for small scale/fixed bed biomass gasifiers have been interesting due to their increased commercial practices. Fixed bed gasifiers are characterized by a wide range of operational conditions and are multi-zoned processes. The reactants are distributed in different phases and the products from each zone influence the following process steps and thus the composition of the final products. The present study aims to improve the conventional 'Black-Box' thermodynamic modeling by means of developing multiple intermediate 'boxes' that calculate two phase (solid-vapor) equilibriums in small scale gasifiers. Therefore the model is named ''Multi-Box''. Experimental data from a small scale gasifier have been used for the validation of the model. The returned results are significantly closer with the actual case study measurements in comparison to single-stage thermodynamic modeling.

Agro-industrial waste to solid biofuel through hydrothermal carbonization.

In this paper, the use of grape marc for energy purposes was investigated. Grape marc is a residual lignocellulosic by-product from the winery industry, which is present in every world region where vine-making is addressed. Among the others, hydrothermal carbonization was chosen as a promising alternative thermochemical process, suitable for the treatment of this high moisture substrate. Through a 50 mL experimental apparatus, hydrothermal carbonization tests were performed at several temperatures (namely: 180, 220 and 250 °C) and residence times (1, 3, 8 h). Analyses on both the solid and the gaseous phases obtained downstream of the process were performed. In particular, solid and gas yields versus the process operational conditions were studied and the obtained hydrochar was evaluated in terms of calorific value, elemental analysis, and thermal stability. Data testify that hydrochar form grape marc presents interesting values of HHV (in the range 19.8-24.1 MJ/kg) and physical-chemical characteristics which make hydrochar exploitable as a solid biofuel. In the meanwhile, the amount of gases produced is very small, if compared to other thermochemical processes. This represents an interesting result when considering environmental issues. Statistical analysis of data allows to affirm that, in the chosen range of operational conditions, the process is influenced more by temperature than residence time. These preliminary results support the option of upgrading grape marc toward its energetic valorisation through hydrothermal carbonization.

Hydrothermal carbonization of off-specification compost: a byproduct of the organic municipal solid waste treatment.

The possibility to apply the hydrothermal carbonization (HTC) process to off-specification compost (EWC 19.05.03) at present landfilled was investigated in this work. The aim was to produce a carbonaceous solid fuel for energy valorization, with the perspective of using HTC as a complementary technology to common organic waste treatments. Thus, samples of EWC 19.05.03 produced by a composting plant were processed through HTC in a batch reactor. Analytical activities allowed to characterize the HTC products and their yields. The hydrochar was characterized in terms of heating value, thermal stability and C, H, O, N, S and ash content. The liquid phase was characterized in terms of total organic carbon and mineral content. The composition of the gas phase was measured. Results show that the produced hydrochar has a great potentiality for use as solid fuel.

Clay-sewage sludge co-pyrolysis. A TG-MS and Py-GC study on potential advantages afforded by the presence of clay in the pyrolysis of wastewater sewage sludge.

Wastewater sewage sludge was co-pyrolyzed with a well characterized clay sample, in order to evaluate possible advantages in the thermal disposal process of solid waste. Characterization of the co-pyrolysis process was carried out both by thermogravimetric-mass spectrometric (TG-MS) analysis, and by reactor tests, using a lab-scale batch reactor equipped with a gas chromatograph for analysis of the evolved gas phase (Py-GC). Due to the presence of clay, two main effects were observed in the instrumental characterization of the process. Firstly, the clay surface catalyzed the pyrolysis reaction of the sludge, and secondly, the release of water from the clay, at temperatures of approx. 450-500 °C, enhanced gasification of part of carbon residue of the organic component of sludge following pyrolysis. Moreover, the solid residue remaining after pyrolysis process, composed of the inorganic component of sludge blended with clay, is characterized by good features for possible disposal by vitrification, yielding a vitreous matrix that immobilizes the hazardous heavy metals present in the sludge.