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A special issue of Processes (ISSN 2227-9717). This special issue belongs to the section "Chemical Processes and Systems".

Deadline for manuscript submissions: closed (31 October 2025) | Viewed by 14818

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Dr. Carmen Branca

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Istituto di Scienze e Tecnologie per l’Energia e la Mobilità Sostenibili (STEMS), National Research Council of Italy (CNR), P.le Tecchio 80, 80125 Napoli，Italy
Interests: pyrolysis, gasification, and combustion of biomass; flammability of synthetic and natural polymers and composite materials; kinetic modelling of biomass pyrolysis and combustion; composition, properties, and reactivity of pyrolysis products (bio-oil and biochar); biomass torrefaction
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Prof. Dr. Antonio Galgano

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Dipartimento di Ingegneria Chimica, dei Materiali e della Produzione Industriale, Università degli Studi di Napoli “Federico II”, P.le Tecchio 80, 80125 Napoli, Italy
Interests: biomass; thermochemical conversion processes; transport phenomena; computational modeling; biorefinery; response to fire of polymers and composite materials
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

Biomass fuels are recognized as renewable sources that may be converted to biofuels, chemicals, and heat and power through chemical and thermal conversion. Feedstock variability is a serious barrier to scaling up and commercialization. Further serious adverse factors are related to various inherent biomass properties, such as high moisture level; low bulk density; irregular shape and size; hydrophilic nature; low calorific value; and significant contents of problematic constituents, including sulphur, chlorine, alkalis, nitrogen, and heavy metals. To overcome these drawbacks, physical, thermal, and chemical methods of pretreatment are proposed (grinding, densification, demineralization, dry and wet torrefaction, acid or alkali treatment, steam explosion, etc.).

This Special Issue aims to address the development/optimization and application of biomass pretreatment methods and their influences on thermochemical conversion processes and products.

Dr. Carmen Branca
Prof. Dr. Antonio Galgano
Guest Editors

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Keywords

biomass pretreatments
pyrolysis
gasification
combustion
bio-oil and biochar upgrading
emissions

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Published Papers (10 papers)

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29 pages, 4712 KB

Open AccessArticle

Experimental Identification of the Pyrolysis Stages of Carya illioinensis Woody Pruning Waste in a Batch Reactor Heated by a Solar Simulator

by Arturo Aspiazu Méndez, Heidi Isabel Villafán Vidales, Nidia Aracely Cisneros Cárdenas, Ernesto Anguera Romero, Aurora Margarita Pat Espadas, Fabio Manzini Poli and Claudio Alejandro Estrada Gasca

Processes 2026, 14(1), 67; https://doi.org/10.3390/pr14010067 - 24 Dec 2025

Abstract

This study examines the influence of physical biomass pretreatment on the pyrolysis behavior of woody pruning residues of Carya illinoinensis (pecan tree) processed in a stainless-steel batch reactor heated by concentrated radiative energy. Experiments were conducted with 25.5 g of biomass using a solar simulator equipped with a mirror concentrator, operating at three constant thermal power levels (234, 482, and 725 W). As a pretreatment strategy, the woody residues were deliberately processed without drying, while mechanical size reduction and sieving were applied to obtain a controlled particle size range of 1–4 mm. This approach enabled the isolated assessment of the effects of physical pretreatment, particularly particle size and bulk density, on heat transfer, thermal response, and pyrolysis behavior. The pyrolysis performance of the pretreated woody biomass was systematically compared with that of walnut shell biomass and inert volcanic stones subjected to the same particle size control. Two consecutive experimental cases were implemented: Case A (CA), comprising heating, pyrolysis of fresh biomass, and cooling; and Case B (CB), involving reheating of the resulting biochar under identical operating conditions. An improved analytical methodology integrating temperature–time profiles, their derivatives, and gas composition analysis was employed. The results demonstrated the apparently inert thermal behavior of biochar during reheating and enabled clear temporal identification of the main biomass conversion stages, including drying, active pyrolysis of hemicellulose and cellulose, and passive lignin degradation. However, relative to walnut shell biomass of equivalent volume, the woody pruning residues exhibited attenuated thermal and reaction signals, primarily attributed to their lower bulk density resulting from the selected pretreatment conditions. This reduced bulk density led to less distinct pyrolysis stages and a 4.66% underestimation of the maximum reaction temperature compared with thermogravimetric analysis, highlighting the critical role of physical pretreatment in governing heat transfer efficiency and temperature measurement accuracy during biomass pyrolysis. Full article

(This article belongs to the Special Issue Biomass Pretreatment for Thermochemical Conversion)

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21 pages, 6328 KB

Open AccessFeature PaperArticle

Modeling of Tar Removal in a Partial Oxidation Burner: Effect of Air Injection on Temperature, Tar Conversion, and Soot Formation

by Yongbin Wang, Guoqiang Cao, Sen Wang, Donghai Hu, Zhongren Ba, Chunyu Li, Jiantao Zhao and Yitian Fang

Processes 2025, 13(12), 3903; https://doi.org/10.3390/pr13123903 - 3 Dec 2025

Viewed by 252

Abstract

In this study, a three-dimensional computational fluid dynamic (CFD) model was constructed and validated against experimental data. The oxygen injection methods—specifically the primary air flow and secondary air flow—were investigated. The results demonstrate that primary air flow is the dominant factor in combustion. An increase of primary air from an φ of 0.20 to 0.75 lead to a rise in combustion peak temperature from 892.17 K to 1321.02 K, while simultaneously expending the flame combustion zone and enhancing the conversion of C₁₀H₈ and CH₄. Conversely, increasing the secondary air flow from 1 L/min to 7 L/min reduced the centrally measured temperatures form 886.09 K to 856.07 K due to irregular flow patterns, which expanded the central low-temperature region. While secondary air flow promoted more uniform reactant conversion and slightly suppressed intermediate products (e.g., soot, C₆H₆), its overall effect was secondary to that of the primary air. This research reveals a critical design insight: using primary air injection to introduce oxygen into the reactor is a reasonable approach. The findings provide valuable guidance for optimizing partial oxidation burner design and operating conditions to maximize tar conversion while maintaining reactor integrity. The study also establishes a rigorously validated CFD framework for analyzing complex reacting flows in tar thermochemical conversion reactors. Full article

(This article belongs to the Special Issue Biomass Pretreatment for Thermochemical Conversion)

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21 pages, 4157 KB

Open AccessArticle

Particulate Matter Characteristics from a Gasification Cookstove: Implications of Operating Conditions Using Densified Wood Biomass

by Jonatan Gutiérrez, Alexander Santamaría and Juan F. Pérez

Processes 2025, 13(11), 3683; https://doi.org/10.3390/pr13113683 - 14 Nov 2025

Viewed by 471

Abstract

Biomass is commonly used for cooking in developing countries, but traditional cookstoves emit pollutants (CO, NO_x, PM), which harm indoor air quality. Improvements and solutions are essential for achieving Sustainable Development Goal 7 (SDG 7). This study assesses the impact of the combustion chamber design, the combustion-air/gasification-air ratio (CA/GA = 2.8, 3.0, and 3.2), and the start type of water boiling test (WBT) protocol (cold and hot starts) on the chemical and morphological characteristics of the total suspended particulate matter (TSPM) emitted from a biomass gasification-based cookstove using densified biomass as feedstock. TSPM was characterized using Fourier-Transform Infrared Spectroscopy (FTIR), X-ray Photoelectron Spectroscopy (XPS), Raman Spectroscopy, and Transmission Electron Microscopy (TEM) to evaluate their chemical composition and morphological features under the above operational conditions. Under the modified WBT protocol, the cookstove achieved CO levels ranging from 1.52 to 2.13 g/MJ_d, and efficiency between 26.56% and 27.81%. TSPM emissions ranged between ~74 and 122.70 mg/MJ_d. The chemical characteristics of TSPM surface functional groups weren’t affected by the start condition, except for decreased intensities as CA/GA increased, promoting oxidation and removal as CO/CO₂. While cold start produced TSPM with higher structural order at higher CA/GA levels, no significant differences were observed among samples from both start conditions at CA/GA ≥ 3.0, indicating chemical and structural similarity. Morphology and particle size were mainly unaffected, with only slight increases in particle size during hot start due to higher biomass-to-air ratios. Full article

(This article belongs to the Special Issue Biomass Pretreatment for Thermochemical Conversion)

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16 pages, 1770 KB

Open AccessArticle

Process Design and Simulation of Biodimethyl Ether (Bio-DME) Production from Biomethane Derived from Agave sisalana Residues

by Rozenilton de J. Rodrigues, Carine T. Alves, Alison B. Vitor, Ednildo Andrade Torres and Felipe A. Torres

Processes 2025, 13(11), 3451; https://doi.org/10.3390/pr13113451 - 27 Oct 2025

Viewed by 495

Abstract

This study presents the design and simulation of an integrated pathway to produce Biodimethyl ether (Bio-DME) from biomethane derived from Agave sisalana residues, focusing on the downstream sections such as: (i) steam reforming of biogas and water-gas shift to generate syngas and (ii) indirect methanol synthesis followed by methanol dehydration to Bio-DME, including separation and recycle steps. The modeled scope excludes the anaerobic digestion stage. Benchmarking against the literature was used to validate model fidelity. The simulation delivered a single-pass methanol conversion of 81.8%, a Bio-DME reactor conversion of 44.6 mol%, and a Bio-DME yield/selectivity of ≈99 mol%; product purities reached ≈99.99 mol% Bio-DME at the first distillation column and ≈99.9 mol% MeOH in the recycle, indicating efficient separation. Compared to the literature, Bio-DME conversion in this study is slightly below the reported values (0.446 vs. 0.499, Δ = 0.053), while yield is very close to literature (0.99 vs. 0.9979, Δ = 0.0079). Incomplete methanol conversion emerges as the primary optimization lever, pointing to adjustments in operating conditions (T, p), recycle/purge strategy, and H₂/CO control. Overall, the results confirm the technical feasibility of the simulated sections and support the development of a sisal-based, low-carbon Bio-DME route relevant to Northeast Brazil. Full article

(This article belongs to the Special Issue Biomass Pretreatment for Thermochemical Conversion)

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17 pages, 3901 KB

Open AccessFeature PaperArticle

Hydrothermal Carbonization Treatment as a Pathway for Energy Utilization of Municipal Sludge and Agricultural Residues Through Co-Gasification

by Georgia Altiparmaki, Dimitrios Liakos, Andreas Artikopoulos and Stergios Vakalis

Processes 2025, 13(9), 2713; https://doi.org/10.3390/pr13092713 - 26 Aug 2025

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Abstract

Municipal sewage sludge (S.S.) and abundant olive-tree pruning on Lesvos Island present both a disposal challenge and an untapped energy resource. This study proposes and evaluates on a preliminary level an integrated system that utilizes both sewage sludge and pruning. The integrated system converts sewage sludge into Hydrochar (HC) via Hydrothermal Carbonization (HTC), removes the aqueous phase using passive solar distillation, and co-gasifies the dried HC with olive pruning in an autothermal downdraft gasifier. HTC experiments on anaerobically digested sludge produced HC with higher heating values exceeding 20 MJ kg⁻¹ while reducing the chemical oxygen demand of the process liquor. Gasification modelling, using the MAGSY equilibrium model, demonstrated that replacing up to 50% of lignocellulosic biomass with HC increased hydrogen content and the Lower Heating Value (LHV) of syngas. Mass and energy balances suggest that the system could provide approximately 590 kW of continuous power, contributing around 4720 MWh to the island’s annual electricity generation. These results indicate that combining HTC, solar distillation, and co-gasification offers a viable pathway to close waste loops, reduce landfill needs, and deliver renewable energy. Future work will focus on Aspen Plus design and optimization, along with a life-cycle assessment in order to assess the environmental benefits. Full article

(This article belongs to the Special Issue Biomass Pretreatment for Thermochemical Conversion)

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18 pages, 4473 KB

Open AccessArticle

Comparison of Dry and Wet Torrefaction for Biochar Production from Olive Leaves and Olive Pomace

by Rafail Isemin, Alexander Mikhalev, Sergey Kuzmin, Mathieu Brulé, Tarik Ainane, Oleg Milovanov, Dmitry Klimov and Kirill Milovanov

Processes 2025, 13(7), 2155; https://doi.org/10.3390/pr13072155 - 7 Jul 2025

Cited by 5 | Viewed by 1240

Abstract

This work investigated the effect of experimental conditions of dry and wet torrefaction on the properties of olive leaves and olive pomace. Torrefaction improved the fuel properties of olive waste. According to Van Krevelen parameters (O/C and H/C ratios), torrefied biomass, tested as solid biofuel, achieved a similar quality threshold to lignite. For example, dry torrefaction conducted at 230 °C for 80 min reduced the O/C and H/C ratios of olive leaves from 0.51 and 1.51 for raw biomass to 0.25 and 1.17 for torrefied biomass, respectively. Under the same conditions, the O/C and H/C ratios of olive pomace were also reduced from 0.34 and 1.60 to 0.27 and 1.36, respectively. Calorific values of raw olive leaves and olive pomace amounted to 18.0 and 23.2 MJ/kg, respectively. Following dry torrefaction and biomass conversion into biochar, calorific values of olive leaves and olive pomace increased by 24% and 14% up to 22.2 and 26.3 MJ/kg through dry torrefaction, compared with 17% and 23% increments up to 21.1 and 28.5 MJ/kg through wet torrefaction, respectively. Interestingly, biomass processing through wet torrefaction performed in a fluidized bed powered by superheated steam could be completed 8- to 12-fold more rapidly than dry torrefaction. SEM analysis indicated a breakdown of the surface structure of olive waste following the torrefaction process. According to the Brunauer–Emmett–Teller (BET) method, total pore surface areas of biochar obtained from wet torrefaction of olive pomace and olive leaves amounted to 3.6 m²/g and 0.8 m²/g, with total pore volumes amounting to 0.0225 cm³/g and 0.0103 cm³/g, respectively. Maximal contents of 5-hydroxymethylfurfural and furfural in liquid by-products from dry torrefaction amounted to 1930 and 1880 mg/1 kg, respectively. Alternately, in liquid by-products from wet torrefaction, concentrations of these high-value compounds remained very low. Full article

(This article belongs to the Special Issue Biomass Pretreatment for Thermochemical Conversion)

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13 pages, 2678 KB

Open AccessArticle

A Systematic Approach to Determining the Kinetics of the Combustion of Biomass Char in a Fluidised Bed Reactor

by S. G. Newman, K. Y. Kwong and E. J. Marek

Processes 2024, 12(10), 2103; https://doi.org/10.3390/pr12102103 - 27 Sep 2024

Cited by 1 | Viewed by 1544

Abstract

The aim of this work was to investigate the combustion of biochar in a fluidised bed and determine the intrinsic kinetic parameters for combustion: pre-exponential constant

A_{i}

and activation energy

E_{i}

. When analysing the rates of reaction, Regimes I, II [...] Read more.

The aim of this work was to investigate the combustion of biochar in a fluidised bed and determine the intrinsic kinetic parameters for combustion: pre-exponential constant

A_{i}

and activation energy

E_{i}

. When analysing the rates of reaction, Regimes I, II and III were demonstrated, with values for the activation energy of 155, 57 and 9 kJ/mol, respectively, when combustion was limited by different factors: intrinsic kinetics, intraparticle and external mass transport phenomena. These mass transport phenomena were decoupled from a set of ‘apparent’ kinetics incorporating effectiveness factors, which we used as a starting point in the determination of the intrinsic kinetic parameters. We also investigated a simple approach to model the evolution of the char structure over the course of oxidation using an empirical function,

f (X)

, fitted with an O(7) polynomial. We then reassessed the division into three combustion regimes by exploring the changes in

f (X)

and the intraparticle effectiveness factor that occurred upon increasing the combustion temperature. Overall, we demonstrate that experiments in a fluidised bed can be used to determine biochar kinetics in a simplified but trustworthy way. Full article

(This article belongs to the Special Issue Biomass Pretreatment for Thermochemical Conversion)

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17 pages, 1523 KB

Open AccessArticle

Energy Recovery Efficiency of Integrating Anaerobic Co-Digestion of Pig Slurry and Feedlot Cattle Manure and Hydrothermal Carbonization of Anaerobic Sludge Cake

by Jun-Hyeong Lee and Young-Man Yoon

Processes 2024, 12(1), 198; https://doi.org/10.3390/pr12010198 - 16 Jan 2024

Cited by 1 | Viewed by 2387

Abstract

Hydrothermal carbonization (HTC) is a technology designed to improve the efficiency of bioenergy recovery by subjecting biomass to high-temperature and high-pressure conditions. By integrating this technical feature with anaerobic digestion (AD), enhanced energy recovery efficiency is achieved in treating anaerobic digestate (AD-T). The study investigates enhancing bioenergy recovery efficiency through an integrated process, combining AD of livestock manure and HTC. The primary objective is to improve the energy conversion efficiency of biomass characterized by varying solid contents and chemical compositions. Shortening the hydraulic retention time (HRT) in AD of livestock manure resulted in decreased degradation rate efficiency within the AD-T. This led to increased solid material accumulation, which was crucial for the subsequent HTC reaction. The HTC reaction exhibited its maximum bioenergy recovery at 160 °C. The input energy of the livestock manure, obtained by mixing pig slurry and feedlot cattle manure in a 1:1 (w/w) ratio, was 171,167 MJ/day. Under different HRT conditions (40, 30, and 20 days), recoverable energy from AD of livestock manure ranged from 60,336 to 68,517 MJ/ton. Integration of HTC increased net bioenergy recovery to 106,493 to 130,491 MJ/day under corresponding HRT conditions, highlighting the potential of integrating HTC with AD from livestock manure for enhanced bioenergy recovery efficiency. Full article

(This article belongs to the Special Issue Biomass Pretreatment for Thermochemical Conversion)

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26 pages, 2658 KB

Open AccessFeature PaperReview

Microwave Pretreatment for Biomass Pyrolysis: A Systematic Review on Efficiency and Environmental Aspects

by Diego Venegas-Vásconez, Lourdes M. Orejuela-Escobar, Yanet Villasana, Andrea Salgado, Luis Tipanluisa-Sarchi, Romina Romero-Carrillo and Serguei Alejandro-Martín

Processes 2025, 13(10), 3194; https://doi.org/10.3390/pr13103194 - 8 Oct 2025

Cited by 1 | Viewed by 1666

Abstract

Microwave pretreatment (MWP) has emerged as a promising strategy to enhance the pyrolysis of lignocellulosic biomass due to its rapid, volumetric, and selective heating. By disrupting the recalcitrant structure of cellulose, hemicellulose, and lignin, MWP improves biomass deconstruction, increases carbohydrate accessibility, and enhances yields of bio-oil, syngas, and biochar. When combined with complementary pretreatments—such as alkali, acid, hydrothermal, ultrasonic, or ionic-liquid methods—MWP further reduces activation energies, facilitating more efficient saccharification and thermal conversion. This review systematically evaluates scientific progress in this field through bibliometric analysis, mapping research trends, evolution, and collaborative networks. Key research questions are addressed regarding the technical advantages of MWP, the physicochemical transformations induced in biomass, and associated environmental benefits. Findings indicate that microwave irradiation promotes hemicellulose depolymerization, reduces cellulose crystallinity, and weakens lignin–carbohydrate linkages, which facilitates subsequent thermal decomposition and contributes to improved pyrolysis efficiency and product quality. From an environmental perspective, MWP contributes to energy savings, mitigates greenhouse gas emissions, and supports the integration of renewable electricity in biomass conversion. Full article

(This article belongs to the Special Issue Biomass Pretreatment for Thermochemical Conversion)

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14 pages, 3717 KB

Open AccessReview

An Overview on Production of Lignocellulose-Derived Platform Chemicals Such as 5-Hydroxymethyl Furfural, Furfural, Protocatechuic Acid

by Pravin P. Upare, Rachel E. Clarence, Hyungsub Shin and Byung Gyu Park

Processes 2023, 11(10), 2912; https://doi.org/10.3390/pr11102912 - 4 Oct 2023

Cited by 19 | Viewed by 4334

Abstract

Furan derivatives such as 5-hydroxymethyl furfural (HMF) and furfural (FA) and aromatic acids such as protocatechuic acid (PCA) represent the most essential classes of intermediates derived from lignocellulosic biomass. These bio-based compounds are potential feedstocks for producing bio-based chemicals and fuels. However, the derivatives of these bio-based compounds are useful in their antioxidative, antibacterial, and anti-aging activities. Protocatechuic acid (PCA, 2,3-dihydroxybenzoic acid), derived from lignin biomass, is also one of the essential bio-derived aromatic intermediates with an active acid and hydroxyl group, which can elevate it into an important class of potential platform chemicals for the production of value-added chemicals, such as HMF and furfuryl alcohol (FAL). The platform compounds are indeed the most used furan-based feedstocks since their chemical structure allows the preparation of various high-value-added chemicals. The related catalytic techniques are well known for the upgradation of biomass into these platform chemicals and their conversion into value-added chemicals. In this short review, we aim to briefly discuss biomass conversion into FA, HMF, and PCA and related heterogeneous catalytic processes. In addition, a few potential ongoing research trends are also proposed to provide some ideas for the further preparation of bio-based innovative derivatives in a much more green, simple, efficient, and economical way. Full article

(This article belongs to the Special Issue Biomass Pretreatment for Thermochemical Conversion)

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