Calvo Carrascal, Miguel Angel
(2018)
Sustainable manufacturing of next generation building materials using microwave energy.
PhD thesis, University of Nottingham.
Abstract
Global warming and the high energy demands of fossil fuel in industries have led governments to implement legislation aimed towards developing more energy efficient and sustainable processes. In the brickwork industry, the burning of coal and natural gas provides the energy to fire clay bricks in the 900-1200 oC range into high quality building materials. Microwaves powered by renewable energy sources have been suggested as a sustainable alternative to fossil fuels. Microwave heating has been considered a promising technique for the processing of clays due to the potential energy consumption and carbon footprint reductions, and for its volumetric heating nature, which enables the fast and uniform heating of a load. This could result in improving the mechanical properties of the fired products.
The aims of this project were to develop an understanding of how microwaves interact with clays in order to show whether they could be used to fire clay-based building materials, and to understand how this could be achieved and the parameters that affect it.
The composition of Danish clays was quantified, i.e. quartz, calcite, albite, orthoclase, kaolinite, montmorillonite and muscovite, and their thermal evolution was studied across the firing range. The dielectric properties of clays were measured at 912-2470 MHz and 20-950 oC in order to investigate the microwave/clay interaction, assess the effects of changing composition, temperature, frequency and material’s density on their potential for microwave processing, and provide critical information on the design and scale up of this technology. Relating the mineralogy of a material and its evolution during heating to changes on the dielectric property trends, and thus microwave processability, was examined for the first time in this thesis. Insight into the influence of individual components on the potential for microwave heating was gained from an analogous study on clay constituents.
While the dielectric constants of clays were found to be relatively stable during heating, their loss factors fluctuated with temperature. Free and physically bound water were the dominant dielectric species near room temperature, while their removal halved the loss factors until 350 oC. Beyond this temperature, a steady increase in the loss factors concurred with the mineral dehydroxylations. The loss factors sharply rose beyond 800 oC due to sintering effects, while calcite decomposition partially counteracted this growth. Montmorillonite and muscovite were the most microwave absorbing mineral species due to their water affinity and interlayer cation content, enabling the microwave treatment of the whole clay. On the other hand, a frequency shift from 2470 MHz to 912 MHz resulted in a loss factors increase. This is mainly due to the frequency shifting towards the dipolar dispersion area of physically bound water and the zone in which ionic conductivity heating effects dominate.
Mixing rules were used to relate each single mineral to dielectric property variations, and thus rapidly gain knowledge of the microwave processability of any clay across the firing range based on its composition. Böttcher model provided accurate estimations when compared to experimental measurements, and with the same degree of uncertainties at the 912-2470 MHz frequencies and 0.56-0.37 void fraction ranges. The model was expanded for different compositions with clays from Spain, England and Netherlands. This was the first time that mixing rules were successful in estimating mixtures of more than three constituents.
A microwave system was developed with the aim of firing clay products of comparable quality to conventional specimens. The basis of design focused on maximising the thermal uniformity of the clay load. The process design steps involved remodelling the clay load, building heat transfer models of the load, carrying out trials to study whether clays behave as expected from their dielectric properties, i.e. volumetric or selective heating, minimising thermal gradients, and assessing alternative methods for the control of the holding stage. Microwave firing cycles manufactured clays with a thermal uniformity at the height of firing of 1050±55 oC and reduced processing times to <3 h. This is 92% faster than in brickworks, where conventional samples could not match the heating rates without cracking. High temperature (>800 oC) mineral reactions went unfinished due to the reduced holding time of the microwave treatment (30 min), which resulted in dimmer surface colorations. Enhanced thermal uniformity and reduced time for densification resulted in specimens with a 12% higher compressive strength, 38% larger water absorption and 7% higher void fraction.
Clay samples three times as big were fired to gain an insight into the scale up of the technique. A tighter process control and higher reproducibility were reported, which is promising for potentially allowing longer holding times in scaled up processes, but the product quality improvement did not change. Looking into an industrial scale up, further work would be required to assess possible design concepts, and an optimal microwave firing process may require complete redesign of the furnace configuration, where several challenges need to be considered, such as brick arrangement, power availability and applicator size and shape. For the purposes of assessing the possible economic and environmental impact of implementing microwave clay firing at industrial scale, one of the most straightforward designs, i.e. retrofitting of an industrial tunnel kiln for microwave processing, was considered. Although energy expenditures would decrease from 11.6 GJ fuel/h to 6.1 GJ electricity/h when using a microwave system for the same throughput, the higher cost of electricity and microwave equipment over conventional burners made the conventional technique more economically feasible. The substitution of natural gas by electricity powered by green energy sources resulted in carbon footprint reductions of >95%, and agreed with the energy policies of numerous countries and supranational organisations worldwide.
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