Difficulties in measuring biomass explosibility

As pointed out in Chapter 1, explosion data available in the literature for biomass are scarce and inexistent for torrefied biomass. The reason behind the lack of data for these materials lays on the challenges that characterising the explosibility of these types of dusts pose to the current explosion characterisation methods (1 m3, 20 L sphere). Many biomass materials are fibrous and have low bulk density (~200 kg/m3) and these cause problems with the dispersion system and dust holder.

2.3.3.1. The dispersion system

Fibrous materials tend to choke the delivery system when the dust is placed in a dust holder external to the explosion chamber (Figure ‎2-15); this is recognised by the standards [30, 183] and examples of special dispersers are proposed. The so called “rebound nozzle” is similar to the one used in the Siwek 20 L sphere. An in-vessel dispersion cup is another proposed option (Figure ‎2-16). These are meant to yield identical results to the standard system. Previous work by the Leeds group was concerned with the calibration of new dispersers for biomass powders. This work tested the dispersers proposed in the standard plus a wall mounted spherical perforated grid nozzle.

A turbulence factor β, analogous to the one used in venting correlations, in order to account for turbulence induced by obstacles in the path of the flame, was determined to account for the turbulence induced by the dispersion of dust in the explosion vessel. Explosion tests using 10% methane in laminar and turbulent conditions were performed. Turbulence was introduced by dispersing air from the dust holder. The turbulence factor β was found as the ratio of KG in turbulent condition to KG in

laminar condition. Comparable ratios were also found for other reactivity parameters such as maximum pressures and flame speeds. All dispersers were tested with 10% methane gas in turbulent and laminar conditions at different ignition delays. This way the dispersers were calibrated to provide the same β factor as the standard C- ring injector at the standard ignition delay of 0.6 s. However it was found that the dispersion cup failed to provide spherical flame propagation due to non-uniform dispersion of dust within the vessel. The rebound nozzle provided higher MEC measurements. In addition at high dust loadings (500-1500 g/m3) a lot of dust remained in the dust holder undelivered. The spherical nozzle provided good agreement with the results from the standard system and therefore this design has

been used in the present work for the characterisation of fibrous biomass and torrefied biomass, more details are given in the Chapter 3.

Figure ‎2-15. Dust holder pressure traces with fibrous dust delivered and undelivered due to system choking

Figure ‎2-16. Special dispersers proposed in European standard: rebound nozzle (left) and hemispherical disperser (right). Source: BS14034

Other researchers have faced the problem of delivering fibrous dust into explosion chambers, and therefore different dispersion nozzles were tested [184] with the aim to achieve comparable results with the standard system. However, it was only ensured that KSt values were comparable whereas other parameters such as

maximum pressure or the most reactive concentrations failed to match the results with the standard system.

2.3.3.2. Dust holder for low bulk density dusts

In addition to the delivery problems with fibrous woody biomass, due to their low bulk density, the dust holder cannot hold enough quantities of biomass dust to allow full characterisation of materials. The standard dust holder consists of a volume of 5 L in which the dust sample shall not exceed ¾ of the dust container in order to allow correct pressurisation. According to the European standard if this cannot be achieved, two holders of 5.4 dm3 shall be fitted in parallel. In order to characterise samples of biomass materials of low bulk densities an extended dust holder is needed. The extended volume suggested by the standards could only hold 1215 g/m3 of a sample with a bulk density as low as 150 kg/m3. Authors encountering this problem when using the 20 L sphere method opted for placing part of the sample in the dust holder and placing the rest of the dust in the bottom of the vessel [82, 185]. Unfortunately, none of these solutions allows measurements of upper flammability of most dusts, which have been reported to be generally around 2000-3000 g/m3 but also as high as 13000 g/m3 [186].

When the dust holder volume, or pressure, suffer modifications the velocity of fuel delivery changes. Therefore the turbulence levels change as well. This was investigated by Sattar et al. [187] and concluded that if the new 10 L volume was pressurised at 20 bar (like the standard 5 L holder) the dispersion time increased, whereas, if the volume was pressurised to 10 bar then the delivery time was equal as the standard system. Ignition delays and the time at which the valve started closing were varied as well and it was found that the optimum ignition delay was 0.6 s (like the standard) whereas the valve needed to remain open for a longer time (0.65 s) in order to allow all dust sample to flow into the vessel. Sattar et al. recognised that the longer opening of the valve could result in explosion pressure going into the dust holder which could in turn result in pressure piling and violent explosions in the dust holder. Results for different dusts showed good agreement with the standard 5 L system. Barknecht [72] had previously used an extended dust holder, and recommended a longer ignition delay for the extended volume (900 ms), however, the maximum rate of pressure rise was found at different concentrations and therefore the results were not comparable to the standard.