Browsing by Subject "Solar drying"

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Development and optimisation of a low-temperature drying schedule for Eucalyptus grandis (Hill) ex Maiden in a solar-assisted timber dryer
(2006) Bauer, Konrad; Mühlbauer, Werner
The Brazilian furniture industry consumes about 45 million m³ of sawnwood per year which is mainly supplied by deforestation of the tropical rainforest. At the same time, fast growing eucalypt species are produced on almost 3 million ha for the production of wood pulp and charcoal. Meanwhile, several Brazilian companies try to substitute the expensive natural woods by hardwood from eucalypt trees for the production of high quality sawnwood. However, eucalypt wood has to be dried very carefully under controlled conditions to prevent drying defects. Ambient air drying is not suitable since missing control causes high losses and long drying times. Beside this, the low wood moisture content required in the furniture industry cannot be achieved. Artificial drying technologies reduce the drying time, the timber can be dried to a low wood moisture content and the quality can be improved. However, sophisticated high-temperature dryers cause high investments. Locally manufactured timber dryers do not allow an adequate control of the drying process. Furthermore, the required slow drying process is increasing the thermal and electrical energy consumption causing high drying costs. To overcome the existing problems, the Institute of Agricultural Engineering in the Tropics and Subtropics of the University of Hohenheim (ATS) developed in close co-operation with the German company THERMO-SYSTEM Industrie- & Trocknungstechnik Ltd (THS), Alfdorf and the Brazilian forest company CAF Santa Barbara Ltda (CAF) a solar-assisted dryer for sawnwood with integrated solar collector and biomass backup heating system. Aim of this research work was to analyse the newly developed solar dryer and to develop a suitable drying schedule which allows the economical production of high quality sawnwood for the furniture industry. Therefore, a prototype of the greenhouse type dryer was installed under subtropical climate in Brazil. Due to the low thermal insulation of the solar dryer and missing experience with the extremely sensitive Brazilian varieties of Eucalyptus grandis, a new type of drying schedule had to be developed. The new schedule considers not only the general drying demands of eucalypt sawnwood but also the system immanent characteristics of the solar dryer and the ambient air conditions. An oscillation of the drying air temperature according to the ambient air allowed to speed up the drying process and reduces the condensation of water on the cover without a negative impact on the timber quality. In more than 80 drying tests with 16 000 m³ of eucalypt sawnwood the schedule was tested and improved. Based on experiments and information from literature, the course of the temperature, relative humidity and velocity of the drying air and the mode of remoistening was systematically optimised. Thereby, the influence of the changing drying conditions on the drying time, the timber quality, the energy consumption and the drying costs were analysed. With the final version of the drying schedule, 27 mm thick boards could be dried in the solar dryer from a medium wood moisture content of 60 to 12 % d.b. in 27 days. This drying time was about 20 % higher than in a high temperature dryer. However, a drying time of at least 60 days was required to reduce the moisture content to about 20 % d.b. at ambient air drying. The electric energy consumption in the solar dryer was reduced to about 20 kWh per m³ dried eucalypt sawnwood. This is only 20 % of the energy usually consumed in a high temperature dryer. The thermal energy consumption was 1.2 GJ per m³ which is about 60 % less than the energy required in conventional high temperature dryers. The low thermal and electrical energy consumption combined with the considerable lower investment costs for the solar-assisted timber resulted in average drying costs of 7.90 Euro per m³. This is only half of the costs caused by drying 27 mm thick eucalypt hardwood in a high temperature dryer. For an economic evaluation, a sensitivity analysis was done for the most important cost parameters. The electrical energy costs, the currency exchange rate and the interest rate for credits were found to be the main influencing parameters on the Brazilian market conditions. However, solar drying was generally more cost efficient than conventional high temperature drying. In the framework of this research work, it was proved that Brazilian eucalypt timber can be dried economically to a low moisture content of 10 to 12 % at a high quality level by applying the developed drying schedule in the optimised solar-assisted dryer. Meanwhile, approximately 35 000 m³ of eucalypt hardwood is dried annually in two solar-assisted drying plants contributing significantly to the protection of the natural rain forests.
Drying behavior and curcuminoids changes in turmeric slices during drying under simulated solar radiation as influenced by different transparent cover materials
(2022) Komonsing, Nilobon; Reyer, Sebastian; Khuwijitjaru, Pramote; Mahayothee, Busarakorn; Müller, Joachim
Dried turmeric is used as a spice and traditional medicine. The common drying methods for turmeric (Curcuma longa L.) are sun drying and solar drying. In this study, turmeric slices with a thickness of 2 mm were dried at 40, 50, 60, and 70 °C in a laboratory hot-air dryer with a simulated solar radiation applied through transparent polycarbonate cover (UV impermeable) and PMMA cover (UV permeable). Air velocity and relative humidity of drying air were fixed at 1.0 M·s−1 and 25 g H2O kg−1 dry air, respectively. Light significantly increased the sample temperature under both covers. Page was the best model to predict the drying characteristics of turmeric slices. Drying rate correlated with the effective moisture diffusivity, which increased at higher temperature. The hue angle (h°) of turmeric was distinctly lower at 70 °C under both covers. The dried products were of intensive orange color. Curcumin, demethoxycurcumin, and total curcuminoids were affected by the cumulated thermal load (CTL). The lowest curcumin content was found at 40 °C under PMMA (highest CTL). The optimum drying condition was 70 °C under polycarbonate cover due to shorter drying time and better preservation of color and curcuminoids in the dried product.
Influence of biogas-digestate processing on composition, N partitioning, and N₂O emissions after soil application
(2023) Petrova, Ioana; Pekrun, Carola
The ever-growing need for agricultural products represents a global issue, particularly with a view to the limited availability of cultivable land. According to the latest estimates, the arable land per capita decreases and, in 2050, is expected to account for about 60% less than in the 1960s. In order to meet the demand, agriculture has evolved into industrial-like structures. This development often goes along with nutrient surpluses (e.g., excess of nitrogen and phosphorus) and increased emissions, caused by mismanagement and inappropriate agricultural practices (e.g., over-fertilization). Biogas plants offer a possibility to valorize organic residues and wastes, but potentially aggravate this problem since additional organic residues (referred to as digestates) with considerable nutrient contents are generated as by-products. A simple approach to adjust nutrient levels in the affected regions is the transfer of manures and digestates. However, to make this feasible, a reduction of water content (and consequently of total mass/volume) of digestates is required. Up to now, various techniques for digestate downstream processing are available. Previous research mainly addressed single processing stages or differences between feedstock mixtures. Only limited information was found about the influence of a completed downstream processing on total mass reduction and nitrogen concentration in digestate. Studies about the (gaseous) N losses that occur after the application of the respective intermediate and final products to soils were equally scarce. Therefore, the aims of the current doctoral thesis were to determine (i) the mass reduction achieved by the gradual removal of water within competing processing chains, (ii) the nitrogen partitioning after every single processing step and its recovery in the end products, and (iii) the amount of greenhouse gases (especially N₂O) released after the application of intermediate and end products to soils in comparison to untreated, raw digestate. For that purpose, two commercial, full-scale biogas plants were examined, which completely processed either the solid or the liquid fraction after mechanical screwpress separation of raw digestate. The separated solid fraction was subsequently dried and pelletized, while the liquid fraction was treated by vacuum evaporation with partial NH₃ scrubbing. As final products, digestate pellets and N-enriched ammonium sulfate solution were generated. Calculation of a mass flow balance served as the basis for determining (total) mass reduction, the partitioning of fresh mass and nitrogen during digestate processing, and the recovery of initial N in the products. Additionally, the environmental impact of utilizing digestate as an organic fertilizer was studied by measuring the N₂O release after application to soil under field and laboratory conditions. A further in-depth analysis was performed to observe the main factors influencing the production and release of climate-relevant N₂O from digestate pellets. It was found that the mass reduction caused by water removal during subsequent processing accounted for 6% (solid chain) and 31% (liquid chain) of the total mass of raw digestate. Liquid processing required 40% less thermal energy per ton of water evaporated than solid processing. At the end of the downstream processing, the recovery of initial nitrogen in pellets was 33% lower than in ammonium sulfate solution. Regarding the environmental impact of digestate application to soil, mechanical solidliquid separation showed the potential to reduce N₂O emissions. Contrary to expectations, pelletizing of dry solid boosted the emissions, which was linked to the properties and composition of the pellet. Here, indigenous microbial activity triggered N₂O production and release from denitrification immediately after wetting. Overall, the present work has shown that the subsequent processing of separated solid or liquid digestate generates different products with individual benefits and challenges. Solid digestates are characterized by a high share of recalcitrant organic compounds and therefore can serve, e.g., as soil improver. After processing to pellets, they can be easily transported, stored, and commercialized. However, it is questionable whether the pelletizing process is advisable, since pellets emitted a considerable amount of GHGs during utilization. Liquid processing produces ammonium sulfate solution, which can be utilized as a valuable inorganic fertilizer rich in plant-available N. Besides the discussed advantages, a final decision for or against digestate processing always depends on individual factors, such as local situation and financial means. Smart decision-making must include fertilizer properties, technological performance, and economic feasibility. With a view to future research, additional aspects were identified, such as returning to a laboratory-scale biogas plant for more accurate digestate sampling and analysis, consideration of digestate storage and transport, and economic evaluation of the entire digestate value chain including the assessment of digestate fertilizer value (expressed as e.g., N use efficiency or N fertilizer replacement value).