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During handling and storage of conventional wood pellets, O2 depletion as well as CO and CO2 off-gassing can reach acutely hazardous levels and certain Volatile Organic Compounds (VOCs) may reach concerning levels from an occupational health and safety perspective. With new thermally pre-treated biomass commodities entering consumer markets, corresponding knowledge is needed on these assortments' off-gassing behaviour. In this study, relative concentrations of VOCs, CO, CO2, and O2 in the closed storage space of five different pilot-scale torrefied pine wood chip assortments were monitored over 12 days. The VOCs composition in the storage space differed between torrefaction treatment settings; terpenes decreased while furans and lignin degradation products peaked at narrow ranges with increased torrefaction severity, indicating that VOC off-gassing composition of individual compounds is highly specific. Generally, VOC amounts decreased with storage time, but for the mildest torrefied chips certain VOCs increased, predominantly compounds of higher volatility such as hexanal, acetone, and 2-pentylfuran. Also, the newly produced torrefied chips were cooled with two different post-process technologies: i) heat exchanging, and ii) heat exchanging with additional water spraying. Water spraying resulted in higher VOC concentrations, stronger O2 depletion, and factor four higher concentration of CO2 in the storage headspace.

A combined torrefaction and pelletization study was performed at industrially relevant settings using a factorial design. First, wood chips of Scots pine were torrefied at high temperatures (291-315 degrees C) and short residence times (6-12 min), facilitating high throughput in a continuous pilot-scale torrefaction process. Then the torrefied materials were pelletized, also in pilot-scale, using varying moisture contents (MCs) (10-14%), sieve sizes (4-6 mm), and press channel lengths (PCLs) (25 and 30 mm), in all 19 batches, each of 400 kg. The resulting so called black pellets exhibited bulk densities of 558-725 kg m(-3), durabilities of 46.3-86.5%, and fines contents of 3.8-85.8%. Through multiple linear regression modelling of all 11 responses, it was found that the parameter with the greatest influence on the responses was the torrefaction temperature, followed by torrefaction time, MC, and PCL. Longer PCL and higher MC resulted in higher pellet quality, with less fines and greater bulk density and durability. Furthermore, a low torrefaction degree decreased the amount of power required for pelletization. The energy required to grind pellets into a powder (<0.5 mm) decreased with increasing torrefaction degree as expected, but also with decreasing MC before pelletizing. Pyrolysis-GC/MS analysis of thermal degradation products from the pellets revealed correlations with the torrefaction temperature and time, but no correlations with the pelletization process. These results are useful for mapping chemical changes in torrefied materials and identifying complementary torrefaction and pelletization settings. Specifically of interest is adjustment of PCLs at low intervals to better match friction properties of torrefied materials.

An extensive evaluation program was carried out within the European SECTOR project to evaluate the feasibility of torrefied and densified biomass in available entrained flow gasifiers. Different entrained flow reactors (both atmospheric and pressurized) in different scales, from lab scale to a 240 MW industrial gasifier were used for evaluation of torrefied materials as feedstock. Total behaviours of the new fuel throughout the whole supply chains and the EFG systems were evaluated and documented, including process behaviours in terms of operation, gas quality, products of incomplete gasification, etc. Results showed a significant improvement in fuel properties in terms of storage, logistics, milling and feeding behaviour by torrefaction and densification. Entrained flow gasification of the torrefied biomass was also shown to be feasible without any major showstoppers, even improving the gasification processes. Production of tars and other products of incomplete gasification were often found significantly reduced during gasification of torrefied material.

As a solid energy carrier, biomass generally has a few disadvantages, which limits its use for coal replacement and as a feedstock for entrained flow gasification. The hydrophilic and fibrous nature, the low calorific value and low bulk energy content imply high accumulated costs in the whole supply chain and severe challenges in more advanced conversion systems. By thermally pretreating the biomass by torrefaction, these properties may be significantly improved. A continuous torrefaction rotary drum reactor was designed, constructed and evaluated to enable an accurate process control and allow a homogeneous well-defined high quality product to be produced. The combined effects of torrefaction temperature (260–310 °C) and residence time (8–25 min) on a large number of product properties (&gt; 25) were determined for Norway spruce. The resulting mass and energy yields were 46–97% and 62–99%, respectively. Exothermic reactions were evident both at low (260 °C) and high temperatures (310 °C) but with no thermal runaway observed. Increased torrefaction severity resulted in decreased milling energy consumption, angle of repose, mass and energy yield, content of volatile matter, hydrogen, cellulose and hemicellulose. Hydrophobicity, heating value, carbon and fixed carbon contents increased. For all responses, the effect of torrefaction temperature was larger than the effect of residence time. Substantial interaction effects were present for mass and energy yields, volatile matter and hydrogen content. Another correlation found was the relationship of hemicellulose degradation and the brittleness of the torrefied product. Data also suggest secondary char forming reactions during the torrefaction process, resulting in higher fixed carbon content in the torrefied material than expected. The results also suggest torrefaction temperature and residence time not to be totally interchangeable.

For small- to medium-sized streams of biogas (methane) produced at a biorefinery site where cost-efficient reforming by traditional methods are unavailable, combined biomass gasification and methane reforming could facilitate co-conversion and increase the H-2/CO ratio in the syngas from the gasification plant. In the present work, co-gasification of biomass with CH4 was evaluated by means of a parametric chemical equilibrium study for both wood/CH4 and black liquor/CH4 feedstocks and bench-scale fluidized-bed gasification experiments for a wood/peat/CH4 fuel mixture. The parametric study indicated that high-temperature, and steam and oxygen addition all facilitate a high conversion rate, i.e., methane reforming. Evaluating the influence of the gasification temperature on CH4 reforming and increasing the H-2/CO ratio experimentally demonstrated that high temperatures are required for efficient co-conversion.

In this work, four different fuels were gasified in a pressurized entrained flow pilot plant gasifier at approximately 270 kW(th). The different fuels were; two torrefied wood residues, one raw wood residue and one torrefied stem wood. The system pressure and oxygen equivalence ratio (lambda) were held constant for all four gasification experiments. It was found that the torrefaction pretreatment significantly reduced the milling energy consumption for fuel size reduction, which in turn contributed to increased gasification plant efficiency. Furthermore, the results indicate that the carbon conversion efficiency may be enhanced by an intermediate torrefaction pretreatment, whereas both less severe torrefaction and more severe torrefaction resulted in reduced carbon conversions. The results also indicate that the CH4 yield was significantly reduced for the most severely torrefied fuel.

When biomass is exposed to high temperatures in torrefaction, pyrolysis or gasification treatments, the enrichment of carbon in the remaining 'green coal' is correlated with the temperature. Various other properties, currently measured using wet chemical methods, which affect the materials' quality as a fuel, also change. The presented study investigated the possibility of using NIR spectrometry to estimate diverse variables of biomass originating from two sources (above-ground parts of reed canary grass and Norway spruce wood) carbonised at temperatures ranging from 240 to 850 C-circle. The results show that the spectra can provide excellent predictions of its energy, carbon, oxygen, hydrogen, ash, volatile matter and fixed carbon contents. Hence NIR spectrometry combined with multivariate calibration modeling has potential utility as a standardized method for rapidly characterising thermo-treated biomass, thus reducing requirements for more costly, laborious wet chemical analyses and consumables.

Co-firing biomass with coal is an efficient way of mitigating severe climate change by reducing the total atmospheric greenhouse gas burden. Although biomass is considered close to CO2-neutral, present biomass based energy carriers all however suffer from several logistical challenges. The high moisture content, hydrophobic and fibrous nature, low energy density and bulky characteristics are all challenges impeding large-scale industrial use. The thermal pre-treatment method torrefaction enhances the above biomass fuel qualities and significantly facilitate large scale industrial use of biomass raw materials with respect to, handling, storage, feeding as well as for final conversion. In the present work we evaluate the influence of torrefaction on some fuel powder quality aspects. Five different types of torrefied pellets was compared to a reference (regular white wood pellets) with respect to milling energy and different powder characteristics such as aerated and tapped bulk density, particle size distribution, angle of repose and image analysis. Milling energy decreased by between 77 and 93 % for torrefied pellets compared to wood pellets, and the resulting black powders consisted of considerably smaller particle sizes. Particles from torrefied fuels were also less elongated and more circular/less rough, properties which positively influence flowability. Powder bulk density increased by 90 % at most. Angle of repose, previously documented to be negatively correlated to flowability, decreased from 66° for the reference powder to 57° for the torrefied sample with the lowest angle. Torrefaction temperature affected the results considerably more than moisture content prior to pelletization. The total evidences found therefore suggest a positive effect of torrefaction on both powder production and characteristics.

Visible and near infrared imaging techniques for analysing characteristics of torrefied biomass were evaluated for possible use in future online process control. The goal of such a control system is to identify products with the desired properties and reject products outside the specification. Two pushbroom hyperspectral cameras with different wavelength regions and a commercial digital colour camera were evaluated. The hyperspectrat cameras, short wave infrared (SWIR) and visible-near infrared (VNIR), covered the ranges of 1000-2500 nm and 400-1000 nm, respectively. The biomass was produced according to an experimental design in a torrefaction pilot plant at different temperatures, residence times, and nitrogen and steam flow rates to obtain a wide range of different characteristics and qualities of torrefied material. Chemical characteristics, heating values and milling energy of the different torrefied materials were analysed or calculated using standardized procedures and were used for calibration. For the hyperspectral images, a principal-component analysis was performed on the absorbance spectra. The score plots and score images were used interactively to separate background, outlier pixels and shading effects from sample signal. Averaged spectra of individual torrefied woodchips were used. Partial least-squares regression was used to relate average spectra to heating values and chemical characteristics of the torrefied biomass. Owing to the small size of the data sets, cross-validation using leave-one-out validation was used for testing the models. The ratio of standard error of prediction to sample standard deviation (RPD) values were used for comparing the imaging techniques. For ROB images, all RPD values were 4 or lower. The RPD values for the VNIR technique were all below 5, while the SWIR images produced RPD values above 5 for eight of the 13 properties. The promising results of the SWIR technique strongly suggested that the torrefied biomass undergoes changes to chemical structures, which are not necessarily manifested as changes to the colour of the material.