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•Thermal kinetics of pyrolysis and subsequent in-situ gasification of torrefied biomass pellets were studied.•Both torrefaction and densification treatments delayed the thermal ...decomposition of biomass pellets during pyrolysis.•In-situ gasification reactivity of torrefied pellet char decreased with the increase of torrefaction temperature.•Changed biomass properties by torrefaction and densification led to different pyrolysis/gasification kinetics.
Torrefaction followed by densification improves the heating value, grindability, and logistical treatment efficiency of biomass. Study of the pyrolysis and gasification of torrefied biomass pellets has great significance for the efficient conversion and utilization of biomass. In this study, the thermal behavior and reaction kinetics of pyrolysis and following in-situ CO2 gasification of torrefied corn stalk pellets were investigated in a macro-thermogravimetric analyzer. Torrefaction reduced the amounts of volatiles released during pyrolysis and the maximum pyrolysis rate of pellets decreased with the increase of torrefaction temperature. A three-pseudocomponent model applied for the pellet pyrolysis process suggested that the contribution of hemicellulose reduced as indicated by a decrease in activation energy, while lignin contributed more to the overall kinetics with the increased torrefaction temperature. The gasification of chars after the pyrolysis of torrefied pellets based on the nucleation and growth model indicated that as the torrefaction temperature increased, the gasification reactivity decreased, as implied by the evolutions of pore structures, ash compositions and graphitized crystal structures of the pellet pyrolysis chars. The higher activation energy increased the reaction resistance during the pellet gasification process. The results showed that torrefaction and densification together influenced the reaction behavior, reactivity and overall kinetics of biomass.
Handling, storage, and transportation of dried microalgae is challenging due to its low density. In addition, using pure microalgae as fuel is currently not economically feasible. In the present ...research, the pelletization of microalgae Chlorella vulgaris blends and sawdust was studied. Pelletization of pure Chlorella occurred in two distinct regions of particles' rearrangement and particles' deformation. However, there was no clear separation between the two regions when sawdust was added to Chlorella. Adding microalgae Chlorella to sawdust resulted in a decrease in densification energy and improvement in pellets' properties, i.e. higher durability, density, and heating value, lower porosity, moisture absorption, and pellets' expansion.
Testing densification temperatures of 50, 75, and 100 °C revealed that by increasing the temperature from 50 to 75 °C, the pellets' quality was improved. However, further temperature increase to 100 °C enhanced properties of the pellets that had a higher fraction of sawdust. Similarly, increasing the compressive force from 2500 to 3500 N improved properties of the pellets having more sawdust. The results showed that adding microalgae to sawdust not only eliminates the need for elevated densification temperature and force, but also results in the production of pelleted fuels with improved quality.
•Adding microalgae to woody biomass decreased densification energy.•Adding microalgae to woody biomass enhances pellets' durability and density.•Pellets dominant with microalgae experienced less moisture absorption and expansion.•It was unnecessary to increase temperature to 100 °C in microalgae-dominant pellets.•It was unnecessary to increase force to 3500 N in microalgae-dominant pellets.
Drying harvested microalgae from an average moisture content of 80% wet basis to a safe moisture content of 10% is challenging. Removing this high amount of water from microalgal biomass is ...time-consuming and is not as easy as agricultural crop dehydration. The long drying time results in large drying costs. Although drying is a suitable technique for algal-based fuel production, it has not been commercialized due to its associated challenges. This study was performed to fulfill the knowledge gap in the microalgae drying mechanism and to understand the reason for long drying times. For this purpose, the thin-layer drying of microalgae Chlorella vulgaris at the temperature range of 40 to 140°C was studied in a convective oven. The effect of drying air temperature on Chlorella elemental and chemical composition, surface color, and surface structure in the aforementioned temperature range was also analyzed. The results revealed that the dominant mechanism in Chlorella drying is diffusion, which is attributed to the collapse of microalgal cells structure with increased drying temperature. In fact, moisture is entrapped in the Chlorella cells and it takes a long time for them to reach the biomass surface and evaporate. The result was that both low and high drying temperatures have adverse effects on Chlorella surface color, structure, and carbohydrate and lipid composition. This suggests that microalgae should be dried at an optimum medium (60-80°C) temperature.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, GIS, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
► Pelletization of torrefied sawdust from a fluidized bed was investigated. ► Effect of torrefaction severity on energy consumption of pelletization was studied. ► Effect of torrefaction severity on ...mechanical properties of pellets were examined. ► Suitable torrefaction treatment for hydrophobicity improvement was evaluated.
Pelletization of torrefied sawdust from a fluidized bed reactor was investigated to quantify the energy consumption and pellet properties, including moisture adsorption, pellet density and Meyer hardness. Energy consumptions in compaction and extrusion for torrefied sawdust were significantly higher than those for untreated sawdust at the same compression temperature, while the moisture uptake rate of pellets decreased with increasing the severity of torrefaction. The densities of torrefied pellets were lower than the control pellet due to the loss of chemically bonded water and low-melting point compounds during torrefaction which act as a binding agent when softened at ∼100°C. The properties of pellets were affected by the removal of most low-melting or – softening point components.
•The behaviour of Cd and Pb are closely related to the presence of Cl−1.•The concentrations of Cd and Pb increased sharply after recycling of kiln dust.•Thermodynamic software can effectively ...simulate the fixation of heavy metals.•Using model can optimize kiln control strategy and ash recirculation ratio.
Fly ash produced from incineration of municipal solid wastes (MSW) contains heavy metals, such as Cd and Pb, that make this material difficult to manage and dispose of safely. Because the composition of fly ash is similar to cement raw meal, partial replacement of raw meal with fly ash may be a feasible way to reduce the health and environmental hazards of the ash, provided that the heavy metals can be effectively stabilized in the solid phase. This research employs proprietary thermochemical software to simulate the thermodynamic behavior and single-step fixation of Cd and Pb in industrial cement kilns. The effect of Cd, Pb and Cl loadings on the fixation and/or evaporation of Cd and Pb during the sintering process is analyzed using data from industrial cement kilns. A simplified model is created based on elemental mass balance to evaluate multi-step fixation of Cd and Pb with cement kiln dust recycle.The results indicate that Cd forms Cd(OH)2(g) in a highly alkaline environment, while nearly 90% Pb is volatilized as PbCl2(g). In the clinker, increased Cl−1 decreased the proportion of Pb and Cd, moreover, Pb and Cd increased in kiln dust with Cl−1 increased; Calculations using a kiln dust recycle model showed that, the concentrations of Pb and Cd in both kiln dust and clinker increased sharply after recycling of kiln dust in steady state. Under unstable conditions, the concentrations of Pb and Cd in kiln dust increased, as well as the heavy metals re-entering the cement kiln.
•A user-defined solver for biomass steam gasification in fluidized bed is built.•The solver is tested against experimental data in literature and works well.•Biomass steam gasification in the DFB at ...UBC is preliminary predicted.•Flue gas and syngas can be well separated by the U-bend and cyclone.•Dry N2-free syngas is composed of 55% H2, 20% CO, 20% CO2 and 5% CH4.
A user-defined solver integrating the solid-gas surface reactions and the multi-phase particle-in-cell (MP-PIC) approach is built based on the OpenFOAM software. The solver is tested against experiments. Then, biomass-steam gasification in a dual fluidized bed (DFB) gasifier is preliminarily predicted. It is found that the predictions agree well with the experimental results. The bed material circulation loop in the DFB can form automatically and the bed height is about 1m. The voidage gradually increases along the height of the bed zone in the bubbling fluidized bed (BFB) of the DFB. The U-bend and cyclone can separate the syngas in the BFB and the flue gas in the circulating fluidized bed. The concentration of the gasification products is relatively higher in the conical transition section, and the dry and nitrogen-free syngas at the BFB outlet is predicted to be composed of 55% H2, 20% CO, 20% CO2 and 5% CH4.
The energy-contained fraction of the municipal solid waste (MSW) is called refuse-derived fuel (RDF). RDF has a diverse blend of various materials and thus its physical properties and chemical ...composition are not predictable. Pelletization is one way to reduce the variability of RDF for applications like combustion. In this research, a typical RDF sample was constructed by blending four components of paper, plastic, wood and household organics. The entire blend was subjected to size reduction, drying and wetting. The influence of particles passed through 2, 4 and 6-mm grinder screen size and moisture contents 15, 20, 25 and 30% w.b. on the durability and density of pellets was investigated. The 4-mm RDF sample consumed higher energy and produced pellets with a lower durability than pellets from larger particles. The 6-mm grinder screen produced large pieces of paper and plastics that reduced the free flow of the blend into the pellet die. The RDF sample with an initial moisture content of 20% consumed the lowest pelletization energy. The force vs. deformation curve for compaction of the material to form pellets exhibited rigid material characteristics.
•Paper particles shift the PSD to larger particles and weakens the bulk flowability.•Smaller RDF particles consume higher energy and reduce the mechanical indexes.•Knife milling with a 4-mm screen produces the optimum PSD for RDF pelletization purposes.•20% moisture content is the optimum moisture content for RDF pelletization process.•RDF pellets deform under the normal force and do not disintegrate completely.
•The cost of drying microalgae chlorella in a conveyor belt dryer using exhaust flue gas of an industrial plat is estimated.•The dryer is mathematically modeled to obtain the moisture distribution on ...the belt.•The economics is compared to spray dryer and conveyor belt dryer utilizing natural gas.•Using conveyor belt dryer coupled with waste heat recovery system improved the drying costs.
The objective of this research is to estimate the cost of drying microalgae chlorella in a conveyor belt dryer using waste heat from an industrial source. The recycling system consists of a run around thermal fluid between two tube heat exchangers. The dryer is mathematically modeled and the mass transport coefficient is obtained from empirical thin-layer kinetic data.
The dryer and heat recovery system are designed for the production of 1000kg/h dried microalgae at a moisture content of 10% (wet basis, w.b.). The input moisture content can range from 35 to 75% (wet basis). Depending upon the applied Hand factor, the total cost to dry microalgae from 55% to 10% ranges from $46.13 to $109.64 per ton of dried product. Using natural gas assumed at $6.27/GJ, the drying cost increases to $83.47 per ton (using hand factor equal to 1). The drying cost using a commercial spray dryer is $109.05 per ton of dried product (using hand factor equal to 1). The paper discusses the sensitivity of drying costs to initial moisture content as well. The results show that integrating waste heat recovery with conveyor belt dryer decreases the drying costs of chlorella drying in comparison to two other drying methods.
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•Slot-rectangular spouted bed reactor was used to oxidatively torrefy sawdust.•Adding oxygen can result in a more uniform temperature distribution in the reactor.•Sawdust lost 15–46% ...mass and retained 61–99% energy at 240–300 °C and 3–9% O2.•Oxygen had a more significant effect on sawdust properties at higher temperature.
A slot-rectangular spouted bed (SRSB) reactor was employed to torrefy spruce-pine-fir sawdust in an oxidative atmosphere. The effects of torrefaction temperature (240–300 °C) and feed-gas oxygen concentration (3–9 vol.%) on torrefaction performance and solid product properties were investigated. Adding oxygen in the feed-gas led to more uniform temperature distributions in the SRSB reactor, and was a promising way to achieve stable continuous operation. The mass fraction of cyclone-caught torrefied biomass increased, whereas the mass fraction of torrefied biomass remaining in the reactor decreased, with increasing oxygen concentration and temperature. In oxidative torrefaction, biomass weight loss was 15.4–46.4%, while energy yield was 61.2–99.4%. The effects of oxygen concentration on the weight loss, energy yield and torrefied biomass properties were more significant at 300 °C than at 240 °C. The torrefied sawdust also had greater HHV and contained less volatiles, more carbon, less oxygen and less hydrogen than the raw biomass.
► The char yields of biomass and fossil fuels were almost independent of char ultimate temperature. ► Alkali and alkaline earth metals of biomass ash began to evaporate as the temperature rose. ► ...Biomass and fossil fuel pyrolyzed almost independently in blended mixtures. ► Pyrolysis of biomass and fossil fuels occurred in three stages. ► The reaction mechanism of each pyrolysis stage was inferred from the Coats–Redfern method.
It is not well understood how co-feeding of coal and biomass influences the reaction kinetics of gasification and pyrolysis. Co-pyrolysis of biomass and fossil fuels is investigated in this paper. After fuel characterization, the influences of temperature on the physical and chemical properties of char produced from biomass and non-biomass fuels were investigated, and the kinetics of atmospheric-pressure pyrolysis in a nitrogen environment were determined. The results show that product physical properties, such as surface area, depend on the pyrolysis temperature. For individual fuels, pine sawdust char prepared at 750°C had the highest CO2 and N2 uptake, while switchgrass had very low N2 uptake, but high CO2 uptake. The surface area of the fluid coke decreased with increasing temperature, but was almost constant for coal. Co-pyrolysis in a thermogravimetric analyzer exhibited three stages. Devolatilization of the biomass and coal portions of blended samples occurred independently, i.e. without significant synergy. The Coats–Redfern method was used to analyze the kinetics of solid fuel pyrolysis, indicating that it can be described by multi-step reactions. The model was able to identify likely reaction mechanisms and activation energies of each pyrolysis stage, giving predictions consistent with the experimental results.
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