Deposit formation in waste incinerators
A number of ash samples where collected at four Danish municipal solid waste incineration (MSWI) plants. Samples of bottom ash/slag, 2nd-3rd pass ashes and ESP/E-filter ash were collected at the plants. The ashes were analyzed by a number of standard chemical analyses, and a number of advanced analytical techniques.
Material science focusing on the development of new more corrosion resistant materials has solved some of the operational problems related to formation of ash deposits and corrosion in waste incinerators. Anyhow, experience from Danish and German waste incinerators reveal significant corrosion of evaporator walls, caused by a high content of chlorine in the deposits. Deposits collected from brick walls in these incinerators, may contain up to 5 %-wt Cl, while the Cl-concentration in deposits on panel walls may even be as high as 10 - 20 %-wt. The most important counter-ions to Cl- in the deposits are Zn"z"+ and K"+. Thus, it has been assumed that low-melting mixtures of KCl and ZnCl_2 are causing the corrosion. Boiler inspections have clearly demonstrated the presence of a running slag on evaporator walls. A litteratur survey indicates that the presence of K and Zn in the fly ash, as either simple water soluble salts (KCl and ZnCl_2) or water non-soluble salts are the major factor controlling the corrosive attacks on the evaporator walls. The heat-uptake in the furnace and convective path of the system may be significantly affected by the structure, thermal conductivity and emissivity of the deposits. Purpose: The purpose of this project is to: 1) investigate the possibility of using additives to minimize the deposit formation in waste incinerators, 2) map the chemical environment for formation of salts and silicates of K, Zn, and Pb during waste combustion, in order to provide means of solving the corrosion problems, 3) map the transfer of heat to and transport of heat through a deposit, in order to provide means of minimizing the influence of deposits on boiler characteristics. The work, being a combined continuation of a CHEC EFP-2000 project and an inititation of a new joint project on waste incineration, will contain: 1) systemizing the experience on deposit formation and corrosion in waste incinerators, 2) thermodynamic characterization of the chemical environment in waste incinerators, 3) application of thermochemistry for identification of possible additives for use in waste incinerators, 4) theoretical and experimental investigation of alkali - ash interactions in TGA-type reactors and in the CHEC Solid Fuel Combustor, 5) theoretical description of deposit formation and alkali interactions with additives
The wet chemical analyses of the different ash fractions have revealed that residual ash is formed on the grate by interaction of the main ash forming elements, Al, Ca, Fe, and Si. Some of this ash is entrained from the grate and carried with the flue gas along the flue gas duct, where volatile species of K, Na, Zn, Pb, Cl and S starts to condense heterogeneously on the fly ash, thereby causing a dilution of the main ash forming elements. When compared plant-by-plant, the ash chemical analyses showed that the plant with the highest S-content in the fly ash is the one with the most often operational problems in relation to deposition, while a high Cl-content is indicative of a high corrosive potential. An existing CCSEM algorithm was extended with chemical classes covering Pb- and Zn-rich phases. This has made it possible to analyse also MSW-derived ashes by use of CCSEM. Representative samples of 2nd-3rd pass and ESP/E-filter ashes from the four plants have been analyzed by QXRD analysis. Only few crystalline phases were identified: KCl, NaCl, CaSO4, SiO2, and CaCO3 being the main ones. No crystalline phases containing Pb and Zn were identified by QXRD. A comparison between CCSEM and QXRD revealed the expected surface nature of the CCSEM analysis. Samples of 2nd-3rd pass and ESP/E-filter ash from the four plants where investigated for melting behaviour in the STA. It was shown that it is possible to quantify the melting behaviour of these ashes, and that the melting goes on in two steps (salts followed by silicates/oxides). The release of heavy metals was investigated as a function of temperature, local stoichiometry and feedstock chemical composition. It was shown that extreme caution must be taken if trying to affect the heavy metal vaporization by use of additives applied directly to the fuel bed. Thermodynamic modelling was also applied in order to interpret the melting corves generated by analysis of the STA-output. A complete review of models for different heat transfer mechanisms of importance to deposit temperature profiles and thereby to boiler heat up-take has been performed and submitted as a review paper to an international journal
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