The release and transformation of inorganic elements during grate-firing of bran was studied via experiments in a laboratory-scale reactor, analysis of fly ash from a grate-fired plant, and equilibrium modeling. It was found that K, P, S, and to a lesser extent Cl and Na were released to the gas phase during bran combustion. Laboratory-scale experiments showed that S was almost fully vaporized during pyrolysis below 700°C. Sixty to seventy percent of the K and P in bran was released during combustion, in the temperature range 900À1100°C. The release of K and P was presumably attributed to the vaporization of KPO 3 generated from thermal decomposition of inositol phosphates, which were considered to be a major source of P and K in bran. The influence of additives such as CaCO 3 , Ca(OH) 2 , and kaolinite on the release was also investigated. Ca-based additives generally increased the molar ratio of the released K/P, whereas kaolinite showed an opposite effect. Thermodynamic modeling indicated that the fly ash chemistry was sensitive to the molar ratio of the released K/P. When the molar ratio of the released K/P was below 1, KPO 3 and P 4 O 10 (g) were the main stable K and P species at temperatures higher than 500°C. Below 500°C, the KPO 3 and P 4 O 10 (g) may be converted to H 3 PO 4 (l), which may cause severe deposit build-up in the economizers of a grate-fired boiler. By increasing the molar ratio of the released K/P to above 2, the equilibrium distribution of the K and P species was significantly changed and the formation of H 3 PO 4 (l) was not predicted by thermodynamic modeling.
Discharge of waste in general, and food waste, in particular, is considered one of the major environmental problems today, as waste generation increases continuously, reaching values of 32% of all food produced worldwide. There are many different options that can be applied to the management and evaluation of waste treatment, and Anaerobic Digestion seems to be one of the most suitable solutions because of its benefits, including renewable energy generation in form of biogas. Moreover, if FW (food waste) is digested in anaerobic digesters from Waste Water Treatment Plants, a common solution is provided for both residues. Furthermore, co-digestion of food waste and sewage sludge provides benefits in terms of anaerobic process stability enhancing the buffer capacity of ammonia (for example) and biogas formation, which can be increased up to 80% when compared with monodigestion. The present paper reviews food waste anaerobic digestion from its generation, characteristics and different options for its management, and it does focus specifically on the anaerobic digestion and co-digestion process, stages, limiting rates and parameters, utilizing numerous experiences, strictly related to food waste. Pre-treatments are also considered as they are important and innovative for enhancing biogas production and its methane yield. The paper shows an extensive collection of pre-treatments, its basics, improving factors, and numerical data of biogas formation improvements that are related both to substrate modification and to the synergistic effect of co-digestion, which could lead to an increase of methane production from 11% to 180%.
This study presents a complete characterization of the residual materials found in fruit and vegetable markets and their adaptability to be treated by anaerobic digestion with the aim of generating biogas as a new and renewable energy source. It has been determined that these substrates are perfectly suitable to be treated by anaerobic digestion, being rich in simple carbohydrates, with a high content of moisture and solids (total and volatile), which makes it a substrate of easy solubilization and with a great amount of matter directly accessible to the microorganisms responsible for anaerobic degradation. The process develops smoothly, with a slight release of acidic elements, but without impact by the development of the buffer effect by ammonia. In addition, a phenomenon of digestion is observed in two phases, indicating that despite the particulateing of the substrate, it manages to digest the organic matter directly accessible and the inaccessible. In numerical terms, 100 g of residue V produce 913.282 NmL of biogas, of which 289.333 NmL correspond to methane. The disintegration constant is 0.200 days−1, with 16,045% of the substrate degraded. As an innovation, the hydrogen generated in the process has been used as an indicator of the stability and development of the process. Accompanied by a statistical analysis and mathematical adjustments, it is possible to characterize in depth the process and its evolution, determining that the degradation is fast, with a rapid and stable hydrolysis.
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