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.

Pyrolysis of poly(vinyl chloride) and-electric arc furnacedust mixtures.

An investigation into the pyrolysis kinetics of PVC mixed with electric arc furnace dust (EAFD) was performed. Mixtures of both materials with varying PVC ratios (1:1, 1:2, 1:3) were prepared and pyrolyzed in a nitrogen atmosphere under dynamic heating conditions at different heating rates (5, 10, 30 and 50 °C/min). The pyrolysis process proceeded through two main decomposition steps; the first step involved the release of HCl which reacted with the metal oxides present in the dust, subsequently forming metal chlorides and water vapor. Benzene was also found to release as detected by TGA-MS. The remaining hydrocarbons in the polymer backbone decomposed further in the second step releasing further volatile hydrocarbons. Different models were used to fit the kinetic data namely the integral, the Van Krevelen, and Coats and Red fern methods. The presence of EAFD during PVC decomposition resulted in a considerable decrease in the activation energy of the reaction occurring during the first decomposition region. Furthermore, iron oxides were retained in the pyrolysis residue, whilst other valuable metals, including Zn and Pb, were converted to chlorides that are recoverable by leaching in water. It is believed that EAFD can be utilized as an active catalyst to produce energy gases such as propyneas evident from the TGA-MS.

Microwave treatment of electric arc furnace dust with PVC: dielectric characterization and pyrolysis-leaching.

Microwave treatment of electric arc furnace dust (EAFD) with poly(vinyl chloride) (PVC) was studied in this work. A comprehensive characterization of the dust as well as assessing the suitability of using the thermal de-chlorination of the common plastic (PVC) under inert atmosphere was carried out to assess the possibility of Zn and other heavy metals extraction (Pb and Cd) from EAFD. The dielectric and thermal properties of EAFD, PVC and their mixtures were measured. Once combined and heated the metal oxides present in the dust reacted with HCl released from PVC during thermal de-chlorination, forming metal chlorides which were subsequently recovered by leaching with water. It was found that zinc chloride could be almost completely recovered in the leaching stage, with the overall recovery of Zn reaching 97% when the EAFD:PVC ratio was 1:2. The investigation highlighted that franklinite, the most refractory mineral to leaching, was completely destroyed. The leaching residue was found to compose mainly of magnetite and hematite.

Electromagnetic simulations of microwave heating experiments using reaction vessels made out of silicon carbide.

There is a growing body of literature which reports the use of silicon carbide vessels to shield reaction mixtures during microwave heating. In this paper we use electromagnetic simulations and microwave experiments to show that silicon carbide vessels do not exclude the electric field, and that dielectric heating of reaction mixtures will take place in addition to heat transfer from the silicon carbide. The contribution of dielectric heating and heat transfer depends on the dielectric properties of the mixture, and the temperature at which the reaction is carried out. Solvents which remain microwave absorbent at high temperatures, such as ionic liquids, will heat under the direct influence of the electric field from 30-250 degrees C. Solvents which are less microwave absorbent at higher temperatures will be heated by heat-transfer only at temperatures in excess of 150 degrees C. The results presented in this paper suggest that the influence of the electric field cannot be neglected when interpreting microwave assisted synthesis experiments in silicon carbide vessels.

Understanding microwave heating effects in single mode type cavities-theory and experiment.

This paper explains the phenomena which occur in commercially available laboratory microwave equipment, and highlights several situations where experimental observations are often misinterpreted as a 'microwave effect'. Electromagnetic simulations and heating experiments were used to show the quantitative effects of solvent type, solvent volume, vessel material, vessel internals and stirring rate on the distribution of the electric field, the power density and the rate of heating. The simulations and experiments show how significant temperature gradients can exist within the heated materials, and that very different results can be obtained depending on the method used to measure temperature. The overall energy balance is shown for a number of different solvents, and the interpretation and implications of using the results from commercially available microwave equipment are discussed.

Techno-economic considerations in the commercial microwave processing of mineral ores.

Microwave heating of mineral ores had previously been shown to result in process benefits such as reduced strength and improved mineral liberation, but the economics of the process were not attractive and no attention was given to feasible scale-up. This paper provides an overview of the multi-disciplinary approach that has been required to address these failings and develop the technology to pilot scale. Thermal stress simulations show that the operation at high power densities and short residence times is the optimal operating strategy. Experiments using high power densities (approximately 10(9) W/m3 absorbing phase) and short residence times (approximately 0.1 s) were used to confirm that the benefits can now be achieved at economically viable microwave energy inputs (approximately 1 kWh/t). In order to design applicators, reliable measurement of effective microwave properties of crushed ores is required. A method has been developed to extract dielectric properties when the sample thickness is a multiple of half a guide wavelength at some point in the measurement band. Finite difference time domain modeling has been used to design and simulate applicators. A transverse E field applicator with a reflection compensating step has been developed, and a unit with a capacity of > 10 tons/h is being tested. Preliminary economic analysis shows that the overall cost of the process will be between US $0.16 - 0.85 per ton of ore.

Ultra-rapid processing of refractory carbides; 20 s synthesis of molybdenum carbide, Mo2C.

The microwave synthesis of molybdenum carbide, Mo(2)C, from carbon and either molybdenum metal or the trioxide has been achieved on unprecedented timescales; Ex- and in-situ characterisation reveals key information as to how the reaction proceeds.