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Elevated pressure secures the highest fixed-carbon yields of charcoal from corncob. Operating at a pressure of 0.8 MPa, a flash-carbonization reactor realizes fixed-carbon yields that range from 70 to 85% of the theoretical thermochemical equilibrium value from Waimanalo corncob. The fixed-carbon yield is reduced to a range from 68 to 75% of the theoretical value when whole Waimanalo corncobs are carbonized under nitrogen at atmospheric pressure in an electrically heated muffle furnace. The lowest fixed-carbon yields are obtained by the standard proximate analysis procedure for biomass feedstocks; this yield falls in a range from 49 to 54% of the theoretical value. A round-robin study of corncob charcoal and fixed-carbon yields involving three different thermogravimetric analyzers (TGAs) revealed the impact of vapor-phase reactions on the formation of charcoal. Deep crucibles that limit the egress of volatiles from the pyrolyzing solid greatly enhance charcoal and fixed-carbon yields. Likewise, capped crucibles with pinholes increase the charcoal and fixed-carbon yields compared to values obtained from open crucibles. Large corncob particles offer much higher yields than small particles. These findings show that secondary reactions involving vapor-phase species (or nascent vapor-phase species) are at least as influential as primary reactions in the formation of charcoal. Our results offer considerable guidance to industry for its development of efficient biomass carbonization technologies. Size reduction handling of biomass (e.g., tub grinders and chippers), which can be a necessity in the field, significantly reduces the fixed-carbon yield of charcoal. Fluidized-bed and transport reactors, which require small particles and minimize the interaction of pyrolytic volatiles with solid charcoal, cannot realize high yields of charcoal from biomass. When a high yield of corncob charcoal is desired, whole corncobs should be carbonized at elevated pressure. Under these circumstances, carbonization is both efficient and quick.

Biochar properties vary, and characterization of biochars is necessary for assessing their potential to sequester carbon and improve soil functions. This study aimed at assessing key surface properties of agronomic relevance for products from slow pyrolysis at 250-800 °C, hydrothermal carbonization (HTC), and flash carbonization. The study further aimed at relating surface properties to current characterization indicators. The results suggest that biochar chemical composition can be inferred from volatile matter (VM) and is consistent for corncob and miscanthus feedstocks and for the three tested production methods. High surface area was reached within a narrow temperature range around 600 °C, whereas cation exchange capacity (CEC) peaked at lower temperatures. CEC and pH values of HTC chars differed from those of slow pyrolysis biochars. Neither CEC nor surface area correlated well with VM or atomic ratios. These results suggest that VM and atomic ratios H/C and O/C are good indicators of the degree of carbonization but poor predictors of the agronomic properties of biochar.

The prosperity of Silicon Valley is built upon a foundation of wood charcoal that is the preferred reductant for the manufacture of pure silicon from quartz. Because ordinary pyrolysis processes offer low yields of charcoal from wood, the production of silicon makes heavy demands on the forest resource. The goal of this paper is to identify process conditions that improve the yield of charcoal from wood. To realize this goal, we first calculate the theoretical fixed-carbon yield of charcoal by use of the elemental composition of the wood feedstock. Next, we examine the influence of particle size, sample size, and pressure on experimental values of the fixed-carbon yields of the charcoal products and compare these values with the calculated theoretical limiting values. The carbonization by thermogravimetric analysis of small samples of small particles of wood in open crucibles delivers the lowest fixed-carbon yields, closely followed by standard proximate analysis procedures that employ a closed crucible and realize somewhat improved yields. The fixed-carbon yields (as determined by thermogravimetry) improve as the sample size increases and as the particle size increases. Further gains are realized when pyrolysis occurs in a closed crucible that hinders the egress of volatiles. At atmospheric pressure, high fixed-carbon yields are obtained from 30 mm wood cubes heated in a closed retort under nitrogen within a muffle furnace. The highest fixed-carbon yields are realized at elevated pressure by the flash carbonization process. Even at elevated pressure, gains are realized when large particles are carbonized. These findings reveal the key role that secondary reactions, involving the interaction of vapor-phase pyrolysis species with the solid substrate, play in the formation of charcoal. Models of biomass pyrolysis, which do not account for the impacts of sample size, particle size, and pressure on the interactions of volatiles with the solid substrate, cannot predict the yield of charcoal from biomass. These findings also offer important practical guidance to industry. Size reduction of wood feedstocks is not only energy and capital intensive; size reduction also reduces the yield of charcoal and exacerbates demands made on the forest resource.

ABSTRACT. Two different corncob samples from different continents and climates were studied by thermogravimetry at linear and nonlinear heating programs in inert gas flow. A distributed activation energy model (DAEM) with three and four pools of reactants (pseudocomponents) was used due to the complexity of the biomass samples of agricultural origin. The resulting models described well the experimental data. When the evaluation was based on a smaller number of experiments, similar model parameters were obtained which were suitable for predicting experiments at higher heating rates. This test indicates that the available experimental information was sufficient for the determination of the model parameters. The checks on the prediction capabilities were considered to be an essential part of the model verification. In another test the experiments of the two samples were evaluated together, 2 assuming more or less common kinetic parameters for both cobs. This test revealed that the reactivity differences between the two samples are due to the differences in their hemicelluloses and extractives.The kinetic parameter values from a similar earlier work on other biomasses (Várhegyi, G.; Bobály, B.; Jakab, E.; Chen, H. Energy Fuels, 2011, 25, 24-32.) could also been used, indicating the possibilities of a common kinetic model for the pyrolysis of a wide range of agricultural by-products.

In this work, Norway spruce stem wood, stump and bark were torrefied in a bench scale tubular reactor at 225, 275 and 300 °C with two residence times (30 and 60 minutes). The effects of torrefaction process conditions and feedstock type on the physical properties, chemical composition and grindability of torrefied biomass samples were investigated. Furthermore, information was also obtained by conducting scanning electron microscopy (SEM) analysis to gain insights into changes of microstructure and morphology of biomass samples upon torrefaction at different conditions. Higher heating value and fixed carbon content of torrefied biomass samples increased with increased torrefaction severity. Torrefaction caused decrease of hydrogen-to-carbon (H/C) and oxygen-tocarbon (O/C) atomic ratios with increasing temperature and residence time, which results in increase of energy density of torrefied biomass samples. Chemical compositions of torrefied biomass samples considerably changed with increase of torrefaction severity.For the stem wood and stump, the relative hemicellulose content significantly decreased from 42.3% and 29.8% to less than 1% after torrefaction at 300 °C for 60 minutes, *Manuscript Click here to view linked References respectively. The hemicellulose content of untreated bark decreased from 27.5% to 0.14% after torrefaction at the same conditions. Additionally, the cellulose content of the torrefied bark drastically decreased already to half the initial value at a torrefaction temperature of 275 °C, with only trace amounts left in the 300 °C torrefied products. The grindability of stem wood and stump were substantially improved after torrefaction treatment. The energy required for grinding stem wood and stump torrefied at 225 °C decreased to respectively 87 and 70 kwh/ton, which are less than 50% of the energy needed for grinding the untreated samples. For raw bark, much less grinding energy is required compared to those for raw stem wood and stump, and torrefaction has minor effects on the grindability of bark. The ground torrefied biomass samples have much smaller particles than those of the untreated ones. The improvement of grindability of torrefied biomass samples can be coupled to the weakening of the fibre bonds indicated by change in chemical compositions. SEM analysis results show that particles from ground torrefied samples lose their fibrous structure with decrease of length-to-diameter ratios, compared to untreated biomass samples. It explains the shift in particle size distribution curves towards smaller particles as obtained from the sieving tests.

A novel carbonization process that realizes near-theoretical fixed-carbon yields in ∼3 h is presented. Norwegian spruce and birch sawdusts were carbonized in a hermetically sealed reactor at an initial nitrogen pressure of 0.1 MPa. During a carbonization test, the reactor vessel retained all pyrolytic products inside the hot reaction zone invoking high pressures as the temperature was raised. Given the elevated partial pressures of volatiles and their extended residence times, secondary, heterogeneous, char-forming reactions between the hot solid and the tarry vapors appeared to be promoted. This resulted in charcoals with a remarkably high fixed-carbon yield, noncondensable gases mainly composed of CO2, and negligible amount of free tars. This work presents a reproducibility study on the experimental method and explores the effects of heat-treatment temperature, particle size, mass loading, and immersion time on product distributions and charcoal properties. Proximate and elemental analyses, heating values, and scanning electron microscopy images of charcoal are presented. Higher heat treatment temperatures (from 300 °C to 400 °C), smaller grains (from <2 mm to <0.2 mm), longer immersion times (from 30 min to 190 min), and greater mass loadings (from 130 g of biomass per liter of reactor to 165 g of biomass per liter of reactor) intensified wood devolatilization without losing charcoal fixed-carbon yields. Final charcoal products had lower volatile matter contents and improved fixed-carbon contents. Temperature produced the strongest effect, transforming the virgin spruce with a fixed-carbon content of 15% to charcoals with fixed carbon contents of 52% at 300 °C and 73% at 400 °C. The increase in temperature resulted in a transient plastic phase that changed the char appearance from loose, particulate matter to a smooth, shiny solid product with the appearance of coke.

The CO 2 gasification of chars prepared from Norway spruce and its forest residue was investigated in a thermogravimetric analyzer (TGA) at slow heating rates. The volatile content of the samples was negligible; hence the gasification reaction step could be studied alone, without the disturbance of the devolatilization reactions. Six TGA experiments were carried out for each sample with three different temperature programs in 60 and 100% CO 2 . Linear, modulated, and constant-reaction rate (CRR) temperature programs were employed to increase the information content available for the modeling. The temperatures at half of the mass loss were lower in the CRR experiments than in the other experiments by around 120 °C. A relatively simple, well-known reaction kinetic equation described the experiments. The dependence on the reacted fraction as well as the dependence on the CO 2 concentration were described by power functions (n-order reactions). The evaluations were also carried out by assuming a function of the reacted fraction that can mimic the various random pore/random capillary models. These attempts, however, did not result in an improved fit quality. Nearly identical activation energy values were obtained for the chars made from wood and forest residues (221 and 218 kJ/mol, respectively). Nevertheless, the forest residue char was more reactive; the temperatures at half of the mass loss showed 20−34 °C differences between the two chars at 10 °C/min heating rates. The assumption of a common activation energy, E, and a common reaction order, ν, on the CO 2 concentration for the two chars had only a negligible effect on the fit quality.