Engineered pellets from dry torrefied and HTC biochar blends

Dry torrefaction and hydrothermal carbonization (HTC) are two thermal pretreatment processes for making homogenized, carbon rich, hydrophobic, and energy dense solid fuel from lignocellulosic biomass. Pellets made from torrefied biochar have poor durability compared to pellets of raw biomass. Durability, mass density, and energy density of torrefied biochar pellets decrease with increasing dry torrefaction temperature. Durable pellets of torrefied biochar may be engineered for high durability using HTC biochar as a binder. In this study, biomass dry torrefied for 1 h at 250, 275, 300, and 350 °C was pelletized with various proportions of biomass HTC treated at 260 °C for 5 min. During the pelletization of biochar blends, HTC biochar fills the void spaces and makes solid bridges between torrefied biochar particles, thus increasing the durability of the blended pellets. The engineered pellets' durability is increased with increasing HTC biochar fraction. For instance, engineered pellets of 90% Dry 300 and 10% HTC 260 are 82.5% durable, which is 33% more durable than 100% Dry 300 biochar pellets, and also have 7% higher energy density than 100% Dry 300 biochar pellets.     

Gasification of torrefied oil palm biomass in a fixed-bed reactor: Effects of gasifying agents on product characteristics

Gasification represents an attractive pathway to generate fuel gas (i.e., syngas (H₂ and CO) and hydrocarbons) from oil palm biomass in Malaysia. Torrefaction is introduced here to enhance the oil palm biomass properties prior to gasification. In this work, the effect of torrefaction on the gasification of three oil palm biomass, i.e., empty fruit bunches (EFB), mesocarp fibres (MF), and palm kernel shells (PKS) are evaluated. Two gasifying agents were used, i.e., CO₂ and steam. The syngas lower heating values (LHV_syngas) for CO₂ gasification and steam gasification were in the range of 0.35–1.67 MJ m⁻³ and 1.61–2.22 MJ m⁻³, respectively. Compared with EFB and MF, PKS is more effective for fuel gas production as indicated by the more dominant emission of light hydrocarbons (CH₄, C₂H₄, and C₂H₆) in PKS case. Gasification efficiency was examined using carbon conversion efficiency (CCE) and cold gas efficiency (CGE). CCE ranges between 4% and 55.1% for CO₂ gasification while CGE varies between 4.8% and 46.2% and 27.6% and 62.9% for CO₂ gasification and steam gasification, respectively. Our results showed that higher concentration of gasifying agent promotes higher carbon conversion and that steam gasification provides higher thermal efficiency (CGE) compared to CO₂ gasification.     

Effects of torrefaction on the physiochemical properties of oil palm empty fruit bunches,mesocarp fiber and kernel shell

In this work, the effects of torrefaction on the physiochemical properties of empty fruit bunches (EFB), palm mesocarp fiber (PMF) and palm kernel shell (PKS) are investigated. The change of properties of these biomass residues such as CHNS mass fraction, gross calorific value (GCV), mass and energy yields and surface structure when subjected to torrefaction process are studied. In this work, these materials with particle size in the range of 355–500 μm are torrefied under light torrefaction conditions (200, 220 and 240 °C) and severe torrefaction conditions (260, 280 and 300 °C). TGA is used to monitor the mass loss during torrefaction while tube furnace is used to produce significant amount of products for chemical analyses. In general, the study reveals torrefaction process of palm oil biomass can be divided into two main stages through the observation on the mass loss distribution. The first stage is the dehydration process at the temperature below than 105 °C where the mass loss is in the range of 3–5%. In the second stage, the decomposition reaction takes place at temperature of 200–300 °C. Furthermore, the study reveals that carbon mass fraction and gross calorific value (GCV) increase with the increase of torrefaction temperature but the O/C ratio, hydrogen and oxygen mass fractions decrease for all biomass. Among the biomass, torrefied PKS has the highest carbon mass fraction of 55.6% when torrefied at 300 °C while PMF has the highest GCV of 23.73 MJ kg⁻¹ when torrefied at the same temperature. Both EFB and PMF produce lower mass fraction than PKS when subjected to same torrefaction temperature. In terms of energy yield, PKS produces 86–92% yield when torrefied at light to severe torrefaction conditions, until 280 °C. However, both EFB and PMF only produce 70–78% yield at light torrefaction conditions, until 240 °C. Overall, the mass loss of 45–55% of these biomasses is observed when subjected to torrefaction process. Moreover, SEM images reveal that torrefaction has more severe impact on surface structure of EFB and PMF than that of PKS especially under severe torrefaction conditions. The study concludes that the torrefaction process of these biomass has to be optimized based on the type of the biomass in order to offset the mass loss of these materials through the process and increase the energy value of the solid product.     

Characterisation of the torrefaction of beech wood using NIRS: Combined effects of temperature and duration

A new approach is proposed to retrace the combined effects of temperature and duration within the thickness of heat-treated Fagus sylvatica wood. Torrefaction is a mild pre-treatment of biomass carried out at 200-300 °C to improve its properties for pulverized systems such as gasification. The properties of wood treated at high temperature are closely related to chemical modifications induced by temperature levels and treatment duration. This study involved the spectral analysis of solid wood in the near infrared range with the aim of developing a predictive model for process assessment. Samples of beech wood were used for calibration under high temperature conditions of 220, 250 and 280 °C for 1 and 8 h. For prediction, a 50-mm thick solid piece of wood was treated at 250 °C for 3 h. It was demonstrated that it is possible not only to distinguish between wood samples that have undergone different heat treatments, but also to retrace the thermal history of a piece of wood. Statistical processing showed the compensatory effects of temperature and duration, along with the existence of an exothermal reaction in the solid piece of wood. It should thus be possible to ensure cheaper and faster quality control in continuous torrefaction processes.     

Experimental assessments of pyrolytic and fluid-dynamic interactions between pretreated residual biomasses and fluidized beds made up of oxygen carriers for chemical looping gasification

Oxygen carriers (OCs) and wheat straw pellets (raw or pretreated by combinations of torrefaction, CaCO₃-addition, water-washing) were experimentally investigated for Chemical Looping Gasification (CLG) in the Horizon2020 project CLARA (Chemical Looping gAsification foR sustAinable production of biofuels, grant agreement n.817841). Laboratory-scale experiments included: (i) pellet devolatilizations (700 °C-900 °C) in bubbling fluidized beds made of OCs or sand; (ii) study of pressure fluctuations within OC fluidized beds (700–1000 °C) containing 10 vol% of ashes from investigated pellets, to assess OC/ash interactions and fluidization quality. Those two campaigns – performed at relevant process conditions for CLG – allowed straightforwardly screening and selecting the most eligible biomasses and OCs for future CLG demonstrations in fluidized beds at higher scales. Gas yield, H₂/CO molar ratio, and carbon conversion increased as the devolatilization temperature increased from 700 °C to 900°C. Untreated and pretreated wheat straw pellets showed different pyrolytic behaviors. Analyses of pressure fluctuations and characterizations of spent beds (scanning electron microscopy, particle size distribution) suggested that biomass pretreatments limit agglomeration phenomena, depending on OCs nature (e.g., mechanical strength, composition). Ilmenite was the most mechanically and chemically stable OC. Among investigated materials, pretreated wheat straw pellets and ilmenite emerged as the most promising for future CLG demonstrations.     

The study of optimal parameters of oxygen-enriched combustion in fluidized bed with optimal torrefied woody waste

The study explored the oxygen-enriched combustion behavior of torrefied waste wood pellets in a fluidized bed. For biomass torrefaction, three indexes, namely energy yield index (EY), proximate analysis-based index (PA), and effective comprehensive combustion index (S_mix), are used to present the optimal conditions from each viewpoint. Four operating parameters, incorporating torrefaction temperature, residence time and nitrogen flow rate, were taken into consideration in this study. The signal-to-noise ratios of each parameter were evaluated to examine the influencing impact of different factors. The optimal results were employed in the investigation of biochar combustion using a laboratory-scale fluidized-bed reactor with oxygen lancing. Oxygen was injected into different zones of the fluidized bed to investigate its influence on combustion efficiency. The parameters of biochar combustion optimization include torrefied materials, fluidized-bed temperature, oxygen inlet position, and oxygen concentration. The total fluidized-bed efficiency and the volatile combustion ratio were evaluated.     

Analysis of syngas production rate in empty fruit bunch steam gasification with varying control factors

Biomass gasification is a prevailing approach for mitigating irreversible fossil fuel depletion. In this study, palm empty fruit bunch (EFB) was steam-gasified in a fixed-bed, batch-fed gasifier, and the effect of four control factors—namely torrefaction temperature for EFB pretreatment, gasification temperature, carrier-gas flow rate, and steam flow rate—on syngas production were investigated. The results showed that steam flow rate is the least influential control factor, with no effect on syngas composition or yield. The gasification temperature of biomass significantly affects the composition of syngas generated during steam gasification, and the H₂/CO ratio increases by approximately 50% with an increase in temperature ranging from 680 °C to 780 °C. The higher H₂/CO ratio at a lower gasification temperature increased the energy density of the combustible constituents of the syngas by 3.43%.     

Influence of torrefaction on the grindability and reactivity of woody biomass

The use of biomass to produce energy is becoming more and more frequent as it helps to achieve a sustainable environmental scenario. However the exploitation of this fuel source does have drawbacks that need to be solved. In this work, the torrefaction of woody biomass (eucalyptus) was studied in order to improve its properties for pulverised systems. The process consisted in a heating treatment at moderate temperature (240, 260, 280 °C) under an inert atmosphere. The grindability of raw biomass and the treated samples was compared and an improvement in the grindability characteristics was observed after the torrefaction process. Thermogravimetric analysis of the samples was carried out in order to study their reactivity in air. The DTG curves of the torrefied biomass showed a double peak nature. The kinetic parameters were calculated for each reaction stage. The torrefaction process was found to influence the parameters of the first stage, whereas those corresponding to the second remained unaffected.     

Techno-economic comparisons of hydrogen and synthetic fuel production using forest residue feedstock

Mobile distributed pyrolysis facilities have been proposed for delivery of a forest residue resource to bio-fuel facilities. This study examines the costs of producing hydrogen or synthetic petrol (gasoline) and diesel from feedstock produced by mobile facilities (bio-oil, bio-slurry, torrefied wood). Results show that using these feedstock can provide fuels at costs competitive to conventional bio-fuel production methods using gasification of a woodchip feedstock. Using a bio-oil feedstock in combination with bio-oil steam reforming or bio-oil upgrading can produce hydrogen or petrol and diesel at costs of 3.25 $ kg⁻¹ or 0.86 $ litre⁻¹, respectively, for optimally sized bio-fuel facilities. When compared on an energy basis ($ GJ⁻¹), hydrogen production costs tend to be lower than those for synthetic petrol or diesel production across a variety of bio-fuel production pathways.     

A generalized activation energy equation for torrefaction of hardwood biomasses based on isoconversional methods

In this work an extended kinetic analysis involving both experimental measurements and modelling procedures to describe the thermal degradation process of biomass is proposed. Three biomasses belonging to the hardwood family are investigated: ash-wood, beech-wood and hornbeam. The experiments are performed with a thermogravimetric balance working at four constant heating rate: 3, 5, 10 and 20 °C/min. This study is specifically dedicated to investigate the thermal behaviour of the selected biomasses when they undergo to torrefaction temperature conditions. The modelling analysis focuses on investigating the capability of consolidated models, usually applied to solids, in determining the activation energy (E_a) of the indicated biomasses within the torrefaction range. The adopted methods belong to the so-called isoconversional “model free” methods and, in this contest, both the differential ones as those of Friedman and Flynn and the integral ones as those of Kissinger-Akahira-Sunose, Doyle and Starink have been applied. The performances reached by adopting the integral methods widely satisfy the accepted accuracy level conventionally set at values lower than 10%. At the same time it is verified even that, when the methods are applied to biomasses belonging to the same family, the resulting E_a vs. α trends are very close for all the biomasses. This condition is exploited to propose a generalized predictive approach for the E_a calculation based on the knowledge of only the conversion fraction α. The presentation of the results includes also the investigation of the limits of the proposed methods in view of indicating their reliable application range when utilized for torrefaction design procedures.