Orla Williams

‘Dry bases’: carbon dioxide capture using alkaline dry water

An alkaline form of ‘dry water’—a ‘dry base’—is prepared by the high-speed mixing of aqueous solutions of metal carbonates or organic amines with hydrophobic silica nanoparticles. Despite being mostly water, the dry base looks and flows like a powder, and adsorbs CO2 rapidly without any mixing because of its high surface-to-volume ratio. Unlike normal aqueous base solutions, dry bases can be non-corrosive because they do not readily wet surfaces.

Experiments on torrefied wood pellet: study by gasification and characterization for waste biomass to energy applications.

Samples of torrefied wood pellet produced by low-temperature microwave pyrolysis were tested through a series of experiments relevant to present and near future waste to energy conversion technologies. Operational performance was assessed using a modern small-scale downdraft gasifier. Owing to the pellets shape and surface hardness, excellent flow characteristics were observed. The torrefied pellet had a high energy density, and although a beneficial property, this highlighted the present inflexibility of downdraft gasifiers in respect of feedstock tolerance due to the inability to contain very high temperatures inside the reactor during operation. Analyses indicated that the torrefaction process had not significantly altered inherent kinetic properties to a great extent; however, both activation energy and pre-exponential factor were slightly higher than virgin biomass from which the pellet was derived. Thermogravimetric analysis-derived reaction kinetics (CO2 gasification), bomb calorimetry, proximate and ultimate analyses, and the Bond Work Index grindability test provided a more comprehensive characterization of the torrefied pellets suitability as a fuel for gasification and also other combustion applications. It exhibited significant improvements in grindability energy demand and particle size control compared to other non-treated and thermally treated biomass pellets, along with a high calorific value, and excellent resistance to water.

Applicability of Mechanical Tests for Biomass Pellet Characterisation for Bioenergy Applications

In this paper, the applicability of mechanical tests for biomass pellet characterisation was investigated. Pellet durability, quasi-static (low strain rate), and dynamic (high strain rate) mechanical tests were applied to mixed wood, eucalyptus, sunflower, miscanthus, and steam exploded and microwaved pellets, and compared to their Hardgrove Grindability Index (HGI), and milling energies for knife and ring-roller mills. The dynamic mechanical response of biomass pellets was obtained using a novel application of the Split Hopkinson pressure bar. Similar mechanical properties were obtained for all pellets, apart from steam-exploded pellets, which were significantly higher. The quasi-static rigidity (Young’s modulus) was highest in the axial orientation and lowest in flexure. The dynamic mechanical strength and rigidity were highest in the diametral orientation. Pellet strength was found to be greater at high strain rates. The diametral Young’s Modulus was virtually identical at low and high strain rates for eucalyptus, mixed wood, sunflower, and microwave pellets, while the axial Young’s Modulus was lower at high strain rates. Correlations were derived between the milling energy in knife and ring roller mills for pellet durability, and quasi-static and dynamic pellet strength. Pellet durability and diametral quasi-static strain was correlated with HGI. In summary, pellet durability and mechanical tests at low and high strain rates can provide an indication of how a pellet will break down in a mill.

Impact of Mill Type on Biomass Mill Behavior

With increasing use of biomass in pulverized fuel coal fired power stations, the impact of mill type on biomass size and shape is fundamental in optimizing mill and burner performance. The impact of mill type on the energy consumption, particle size and particle shape of four different biomasses commonly combusted in pulverized fuel boilers was investigated in this paper. Miscanthus, mixed wood, and steam exploded pellets, along with powdered olive cake, were comminuted in a planetary ball mill, Bond ball mill and cutting mill. For pelletized miscanthus, milling showed little impact on the particle size and shape of the pellets, with the milling action only reducing the pellets back to their original particle size distribution. This was also observed for the steam exploded pellets and mixed wood pellets in the cutting mill. For non-densified biomasses, such as olive cake, fines below the screen size should be removed before milling in a hammer mill as they pass straight through the mill, resulting in wasted mill capacity and energy consumption. Pellets should be composed of particles close to the required size for conveyance and combustions, and sphericity and roundness are crucial in determining this size. Olive cake showed the most spherical and round particles, but a coarser milled product size than the steam exploded pellets. Miscanthus and mixed wood pellets showed needle like shape profiles, as well as similar particle size distributions. Optimization of the particle size based on the Stokes shape factor is key to optimizing mill, conveyance and burner performance.