• Energy Research

AbstractEffects of CO2 on the yield and fuel properties of the solid product obtained from wet torrefaction of biomass were experimentally investigated. Norwegian forest residues were used as feedstock. CO2 and N2 were employed as purge gas, separately. The results show that, compared with wet torrefaction in N2, the process in CO2 is taking place faster, producing 4.6-6.0% less solid product with lower heating value in identical condition. A reduction of 6.5kWh/t in SGE and an increase of up to 1.4% in EMC were observed for the solid product obtained from WT in CO2 compared with that in N2. In addition, CO2 enhances the capacity of wet torrefaction to remove ash elements from solid biomass fuels.

Abstract In this study, CO2 was employed as an environment friendly catalyst for wet torrefaction of fresh Norwegian forest residues. The effects of CO2 addition on the yield and the properties of hydrochar were experimentally studied and compared with the base case, wet torrefaction in N2. The results showed that wet torrefaction in CO2 and N2 has similar effects on the improvement in the fuel properties of biomass. However, CO2 was found to improve the reactivity of forest residues during wet torrefaction, producing the same hydrochar yield at less severe conditions compared with N2. The hydrochars produced in CO2 had lower heating value and energy yield, but better grindability and hydrophobicity, compared with those in N2. More importantly, CO2-enriched wet torrefaction can remove up to 60–69% of the ash content in the forest residues, compared to only 14–26% for wet torrefaction in N2.

Abstract This work aims to build a comprehensive biomass torrefaction model, which can provide a wide range of information essential for industrialization and commercialization of the process. Norwegian forest residue (birch branches) was chosen as feedstock. The model is capable of presenting detailed distributions of main and by-products from the torrefaction process. In addition, important fuel properties (ultimate analysis and heating value) of the main solid product after torrefaction can be predicted. The model is validated and simulation results show good agreement with available experimental data in the literature. Reduction in mass and energy yields as well as improvement in heating value of torrefied biomass with increasing torrefaction temperature are observed. Trends for carbon, oxygen and hydrogen contents are also consistent with other experimental works. Moreover, overall energy consumption and process energy efficiency can be estimated from the model. It reveals that drying accounts for 76–80% of the total heat demand. Furthermore, the process energy efficiency reduces with increasing temperature, thus torrefaction at high temperatures is not advisable. More importantly, process optimization shows that optimal conditions for torrefaction of birch branches are 30 min holding time and a temperature between 275 and 278 °C.

Abstract In this work, thermal decompositions of wet torrefied microalga during pyrolysis and combustion are investigated. Microalga Chlorella vulgaris ESP-31 was first subjected to wet torrefaction with microwave-assisted heating. Then, the thermal decompositions of the raw and torrefied microalgae are studied by means of a thermogravimetric analyzer in nitrogen and synthetic air to simulate pyrolysis and combustion process, respectively. The results show that carbohydrate and protein are degraded to some extent during WT. In addition, thermal reactivity of lipid is also changed after WT. The changes in the microalgal composition affect the intensity and location of the corresponding peaks during pyrolysis and combustion. Moreover, the char produced from the raw microalga is more reactive and less stable than those from the torrefied microlagae.

AbstractFresh branches of Norway spruce and birch were torrefied in hot compressed water at varied temperatures(175, 200, or 225°C) and for 30minutes. The combustion of untreated and torrefied branchesin synthetic air (21% O2 and 79% N2) wasexperimentally studied by means ofa thermogravimetric analyzer, followed by a kinetic analysis adopting the distributed activation energy model. It appears that, wet torrefaction has significant effects on the combustion reactivity of forest residues. Compared with the raw materials, wet-torrefied branches are less reactive during devolatilization, but more reactive in the char combustion stage.

Abstract The pyrolysis of Norway spruce and birch woods under nitrogen atmosphere was studied by means of a thermogravimetric analyzer operated in the non-isothermal mode, followed by a kinetic analysis employing a three-pseudo-component model with n th-order reactions. Raw woods and the woods treated via wet torrefaction in the conditions of various temperatures (175, 200, 225 °C) and holding times (10, 30, 60 min) were included in this work. The study showed that wet torrefaction resulted in higher pyrolysis peaks for the woods, but less mass of volatiles was released during pyrolysis. The effects of wet torrefaction on pyrolysis of the lignocellulosic components are different. The activation energy of hemicellulose was significantly reduced by wet torrefaction. However, those for cellulose and lignin were slightly increased by wet torrefaction. In addition, a kinetic evaluation with assumption of common parameters was performed. The results confirmed that some kinetic parameters can be assumed to be common for pyrolysis kinetic modeling of different biomasses without substantial reductions in the fit quality. Wet torrefaction converts different biomasses into more homogeneous solid products, of which the pyrolysis kinetics could be modeled by assumption of common parameters.

This study aims at investigating the gasification behavior and kinetics of microalga Chlorella vulgaris ESP-31 before and after wet torrefaction. The raw and wet-torrefied microalgae were first gasified in a thermogravimetric analyzer under a continuous CO2 flow. Thereafter, the obtained thermogravimetric data were modeled for kinetic study, employing a seven-parallel-reaction mechanism. The decomposition of the microalgae in CO2 shows two reactive stages: devolatilization with two peaks and gasification with a peak accompanied by a shoulder, and the thermal decomposition of components in the samples can be clearly identified. Increasing wet torrefaction temperature lowers the height of the major devolatilization peak but enhances the height of the minor one. Moreover, the kinetic evaluation reveals that wet torrefaction affects most of the kinetic parameters of the microalgal components. Furthermore, wet torrefaction temperature influences the kinetic parameters of carbohydrate and lipid, but not on those of protein, "others", and chars.

Torrefaction of forest residues was studied under conditions relevant to oxy-fuel combustion flue gases. The results showed that the torrefaction in CO2 had a lower solid mass yield (81.36%) than that (83.06%) in N2. Addition of steam into CO2 (CO2/H2O=1/0.7 mole/mole) resulted in a higher mass yield (83.30%) compared to 81.36% in CO2. The energy yield was consistently increased from 79.17% to 84.12% or 88.32% for the torrefaction in N2, CO2, or the CO2 and steam mixture, respectively. On the other hand, additions of O2 into the mixture of steam and CO2 led to reductions in both mass yield (from 83.30% to 82.57% or 76.44%) and energy yield (from 88.32% to 84.65% or 79.16%, for the torrefaction in steam and CO2 without O2, with 5% v/v, or 10% v/v of O2, respectively).