Influence of mill type on densified biomass comminution

The impact of different mill fracture mechanisms were examined for a wide range of densified biomass pellets to provide a comprehensive analysis of biomass milling behaviour for pulverised fuel combustion. The milling behaviour of 7 woody, herbaceous, fruit, and thermally treated densified biomasses were investigated for four distinct types of comminution fracture mechanism using traditional milling indices and novel application of 3D imaging techniques. For the coal mill trials, a reference coal was used to provide a milling performance comparator. For the pre-milled samples, woody and herbaceous pellets have the least spherical particles (φ 0.324–0.404), followed by thermally treated pellets (φ 0.428), La Loma coal (φ 0.503), with olive cake having the most spherical particles (φ 0.562). This trend was noted for all the shape factors. Conventional comminution did not significantly impact biomass particle shape, even after a significant change in particle size. Therefore biomass pellet process history plays a key role in determining the comminuted particle shape. La Loma coal had significantly enhanced milling performance in comparison to the biomasses in the coal mills. Significant improvements in grindability and shape factors were observed for the thermally treated pellets. Mill choking was experienced for several of the woody and herbaceous samples, which resulted in a significant energy penalty. The mechanisms of mill choking were found to be intrinsically linked to the critical particle size of comminution through compression, particle shape factors, and the Stokes conditions set for the classifier and burners in pulverised fuel combustion systems. The study showed that for optimal milling performance, biomass pellets should be composed of particles which meet the Stokes requirements of the mill classifier. This would minimise the potential for mill choking and milling energy penalties, and ensure maximum mill throughput

Improving the design of industrial microwave processing systems through prediction of the dielectric properties of complex multi-layered materials

Rigorous design of industrial microwave processing systems requires in-depth knowledge of the dielectric properties of the materials to be processed. These values are not easy to measure, particularly when a material is multi-layered containing multiple phases, when one phase has a much higher loss than the other and the application is based on selective heating. This paper demonstrates the ability of the Clausius-Mossotti (CM) model to predict the dielectric constant of multi-layered materials. Furthermore, mixing rules and graphical extrapolation techniques were used to further evidence our conclusions and to estimate the loss factor. The material used for this study was vermiculite, a layered alumina-silicate mineral containing up to 10 % of an interlayer hydrated phase. It was measured at different bulk densities at two distinct microwave frequencies, namely 934 and 2143 MHz. The CM model, based on the ionic polarisability of the bulk material, gives only a prediction of the dielectric constant for experimental data with a deviation of less than 5 % at microwave frequencies. The complex refractive index model (CRIM), Landau, Lifshitz and Loyenga (LLL), Goldschmidt, Böttcher and Bruggeman-Hanai model equations are then shown to give a strong estimation of both dielectric constant and loss factor of the solid material compared to that of the measured powder with a deviation of less than 1 %. Results obtained from this work provide a basis for the design of further electromagnetic processing systems for multi-layered materials consisting of both high loss and low loss components

Microwave processing of cement and concrete materials - towards an industrial reality?

Each year a substantial body of literature is published on the use of microwaves to process cement and concrete materials. Yet to date, very few if any have lead the realisation of a commercial scale industrial system and is the context under which this review has been undertaken. The state-of the–art is evaluated for opportunities, and the key barriers to the development of new microwave-based processing techniques to enhance production, processing and recycling of cement and concrete materials. Applications reviewed include pyro-processing of cement clinker; accelerated curing, non-destructive testing and evaluation (NDT&E), and end-of-life processing including radionuclide decontamination

Full electromagnetic simulation of a scanning microwave microscope for quantitative estimation of material properties

An essential step for studying and efficiently exploiting microwave fields in materials processing is the dielectric characterization of the samples to be treated. The dielectric behavior of a material determines the degree of interaction with the electromagnetic field, and then the intrinsic efficiency of the processing. From this information it is possible to design and optimize a proper microwave applicator

Microwave treatment of oil-contaminated drill cuttings at pilot scale

This paper details a new technology for the continuous treatment of contaminated drill cuttings at a throughput of 500 kg/hr, and can be scaled-up for use in offshore locations. The change in legislation for oily cuttings discharges in the North Sea at the beginning of the century resulted in the introduction of a 1% residual oil limit for discharged cuttings. At the time, because nothing was capable of achieving a 1% level offshore, only two options existed to deal with oily cuttings: containment for onshore processing (skip and ship), and injection (CRI). With time, a thermomechanical cuttings cleaning process has produced cuttings with oil <1%, although with a significant deck space impact and restricted throughput. Previous studies have shown that microwave treatment is able to reduce oil levels to well below 1% in a laboratory environment, and this work has studied the scale-up of the system to a 500 kg/hr continuous process. The manufacture of a pilot-scale cuttings treatment system involved the collaboration between microwave and electromagnetic engineering specialists, bulk solids handling, and process engineering disciplines. The feed cuttings are conditioned in a solids mixer, before being fed by way of a conveyor to a specifically designed microwave cavity. The oil is removed and recovered using an extraction and condensation system, with the product oil being very similar in composition to the base oil in the drilling mud. Residual oil levels of <0.1% are obtainable, and cuttings throughputs of 500 kg/hr are possible using a 30-kW microwave source. The microwave process typically consumes 60 kWh of electrical energy per tonne of cuttings, and the trade-off between microwave power, residual oil content, cuttings throughput, and overall energy requirements are discussed in this paper. This is the first step in the development of a modular system, with low-deck impact, flexible processing rates, and reduced environmental signature

Dielectric Properties of Free Radical InitiatorsInvestigation of Thermal Decomposition Products

Studies into the phenomena that explain the dielectric properties of azo initiators (AMBN and V70) at high temperatures are reported in this paper. Previous studies have successfully related the variation in dielectric properties of these species below 110 °C to either phase changes or the thermal production of free radicals. In the latter case the marked increase in their response was attributed to the free radicals having more prominent dipoles than their initiator precursors. This study reports the results of experiments designed to explain their observed dielectric characteristics within the temperature range of 110–150 °C. At these elevated temperatures, the decomposition half-life of the initiators studied should be of the order of few seconds. However, in both this and the previous reports, the dielectric response is found to remain at a significant level for several hours. The two prime explanations for the unexpected duration of the increased dielectric properties are (i) the presence of microwave induced protected radicals or (ii) the dielectric properties of the initiator decomposition products. The observations made in this study were subsequently used to define that the latter of these is the key to the observed phenomenon

Mechanistic Investigation into the Accelerated Synthesis of Methacrylate Oligomers via the Application of Catalytic Chain Transfer Polymerization and Selective Microwave Heating

The synthesis of methyl methacrylate (MMA) oligomers by catalytic chain transfer polymerization (CCTP) is demonstrated to be significantly accelerated by the use of microwave heating. The CCTP reactions, which use a cobalt-based catalyst to very efficiently control the molecular weight of the final polymer, were conducted in both a conventional oil bath and a CEM Discover microwave reactor with a target set point of 80 °C. The required reaction time was shown to be reduced from 300 to 3 min, while also retaining control over the polymerization. Additionally, for the first time the bulk temperature of these catalyzed polymerizations was monitored in both heating methods by the use of internal optical fiber sensors. It was demonstrated that, to monitor the temperature of the reaction correctly, it is essential to use an optical fiber sensor rather than the external IR sensor supplied with the reactor. The acceleration in the synthesis during microwave heating was attributed to selective heating of the radical and oligomeric species within the reaction, which lead to both rapid heating of the reaction bulk to reaction temperature and average reaction temperatures that were higher than the chosen set point. However, comparative reactions carried out under conventional heating (CH) conditions at the true reaction temperature of the microwave experiments (MWH) showed that MWH was able to produce significantly greater yields than the CH experiments after only 3 min, indicating the existence of a real selective heating effect during the reaction. Three methods have been investigated to optimize the acceleration achieved in the MWH experiments while retaining control and yield levels within the MWH experiments. These were varying the; solvent concentration, initiator concentration and chain transfer agent concentration. It was demonstrated that by understanding the influence of the microwave heating that it was possible to retain control over the molecular structure of the product polymer at the accelerated rate