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Content archived on 2024-05-21

Elaborated mgo products for efficient flue gas treatment with minimisation of solid residues for waste to energy plants ('MGOGAS')


Objectives and problems to be solved :This project aims to improve the efficiency of flue gas treatment for energy valorisation of waste by using and elaborated MgO product and by developing an adapted process to promote the high benefit of its utilisation (lower sorbet consumption, local CO2 reduction, high efficiency prospects for SO2, Cal, dioxin and heavy metals capture, less solid residues) comparing with the currently used sorbets (lime, sodium bicarbonate).The oxide of magnesium is a widespread product in Europe and its promotion will be open new markets for this product with important consequences for the competitiveness of magnesia European industry but also for the cost effective gaseous treatment. Description of work :This project includes four phases : Phase 1 : Thermo-chemical information on the principal possible reactions between MgO and the gaseous components of flue gas in laboratory tests. Improvement of physic-chemical properties of MgO in order to increase reactivity and sorption of pollutants. Phase 2 : Efficiency tests of gas emissions treatment with MgO and mixture with active carbon or other substances in a medium scale rotary kiln (100 kg/h). A mixture of waste will be prepared (plastics, wood, sludge with heavy metals) in order to compose a gas emission mixture of Six, Cal, dioxin and heavy metals. This mixture or its components separately would be burned in the kiln under waste incineration conditions. A process is expected to be developed in order to increase reactivity of MgO (injection, form, residence time). Tests with in site injection will be conducted in order to test possibilities for heat recovery enhancement. A special attention will be given in preparation of MgO cakes with different properties and thickness and directly deposed on the dust filter. A comparison of efficiency with lime and Nacho will be also realised. Gas analyses as well as analyses of ashes, solid deposits and cakes are expected to assess the performances of the abatement process. Phase 3 : The retained characteristics of MgO (and MgO/active carbon) and the developed abatement process will be applied in an industrial waste to energy plant which normally utilises the usual sorbets. Phase 4 :
The association of producers and potential users is an opportunity to carry out a economic-economic assessment of MgO utilisation and to compare with currently used sorbets. Expected results and exploitation plans :The study is expected to :
- provide a new efficient and competitive MgO based product for gas treatment industry market with specific characteristics and properties ready for application at the end of this project;
- provide a process for the improvement of MgO reactivity for both in-site and downstream capitation of pollutants in waste to energy plants;
- show in both pilot and industrial scale tests the efficiency and advantages of magnesium oxide compared with the currently used sorbets. The project links partners from three EC countries. They are chosen in a way to combine producers of MgO, industrial scale research facilities and potential users of the results. The results obtained could be immediately exploited at the end of this project.
The results obtained from laboratory scale tests gave thermo-chemical information on the principal possible reactions between Mg-based product and the acidic gaseous components of flue gas. The Mg-based product from the current production (without modification) showed a comparable efficiency with the conventional used sorbing agents, for the same quantity used. From this study, it was observed that the most important characteristics for this application are the high specific surface area (S.S.A) and the appropriate grains size distribution of the sorbing agents. In order to increase the reactivity of the Mg-based product, the physico-chemical properties of this product have been improved and samples were produced with a specific surface area higher than 60 m2/g for MgO and more than 40 m2/g for Mg(OH)2. This elaboration constitute an innovation for products derived from natural magnesite, concerning magnesia samples, while commercial Mg(OH)2 samples even synthetic products from sea water have much lower specific surface area (< 25 m2/g). The abatement results obtained at laboratory scale, concerning the reactivity and sorption of pollutant using the modified Mg-based product, were very promising.

Moreover, an MgO sample with a S.S.A of 227m2/g was developed and produced and gave very good abatement results, but the tests did not proceed further due to the fact that this sample is not possible to be produced with the existing industrial facilities. In order to validate the results obtained from the laboratory scale, efficiency tests of flue gas treatment using the different samples of Mg-based product (from the current production and the modified ones) were conducted on a pilot-scale rotary kiln. A mixture of synthetic wastes was formulated and prepared in order to compose a gas emission mixture containing acidic pollutants, dioxin and heavy metals. The results obtained, concerning the efficiency of flue gas treatment using the downstream dry abatement, confirmed the results obtained from laboratory scale. However, the best results for acidic pollutants abatement was obtained using the semi-wet treatment and the modified sample Mg(OH)2 thin slurries. The characteristics of Mg(OH)2 retained were a sample with 40m2/g of S.S.A fine grains size with lack in very fine fractions. This magnesium hydroxide was produced from reactive caustic magnesia under well-controlled hydration conditions. The retained characteristics of the modified Mg-based product and the developed abatement process were applied, during three days trials, in an industrial waste-to-energy plant, which utilises lime sorbent. The results obtained, did not confirm those obtained from the laboratory and pilot scale trials, which could be explained by a low hydration of MgO to Mg(OH)2 during the sorbent preparation and also the possible insufficient dimensions of the bag filters. During the industrial scale test, the sorbent preparation tank was heated above 60 C, in order to activate the hydration reaction, but no increase of the suspension temperature was observed, however, the hydration reaction is exothermic, which explain the low hydration of MgO. Furthermore, the industrial plant used for trials was designed for lime utilisation and so the bag filters could be too small in order to have a good Mg-based cake's formation, which involve a very short residence time and so, a bad pollutants neutralisation.

A technico-economic assessment of Mg-based product utilisation and its comparison with currently used sorbents has been carried out regarding the results obtained from this project. The main advantages from the use of Mg-based instead of Ca-based sorbing agents, which are considered as the mains competitive reagents, are: Lower quantities used to neutralize the same amount of pollutants (28% less compared to CaO) and so, lower quantities of flue gas treatment residues produced; Mg-based sorbents provide safer handling; they are less corrosive for the installation and more environmentally friendly in comparison with Ca-based sorbents; The pH of Mg based product, of around 10, coincides with the pH value at which heavy metals present the lowest solubility and they remain as insoluble hydroxides, permitting an easier stabilisation of the solid issued from flue gas treatment.

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