Experimental Investigation Of The Properties Of Sintered Deposits On Heat Exchange Surfaces In Municipal Wastes Incineration

During municipal solid waste(MSW) incineration process, sintered ash-deposits on the heat transfer surfaces and some relative problems often make troubles to the incinerator's safe and effective performance. To achieve the aim of non-dangerous, most-reduction and highly efficient energy utilization in MSW incineration, it is necessary to study deeply and broadely on the deposits sintering characteristics and mechanisms.In this paper, the essential study routing is sampling and analysis of ashes and deposits from a commercial operation MSW incinerator, combined with the laboratory experiments. Various test methods are adopted in these studies, including chemical ash composition analysis, micro-analysis such as scanning electron microscope (SEM) and energy dispersive X-ray analysis (EDX), X-ray powder diffraction analysis (XRD), ion exchange chromatogram (IEC), nitrogen adsorption and surface area analysis, etc.Generally, calcium and alkali metals are considered as the main mineral elements which cause sintering of deposits in MSW incineration. The study in this paper has good representation on this aspect. This study can be devided into two parts:Firstly, ash and sintered deposits samples from different positions of the MSW incinerator are collected and analysed. The results show that MSW ashes have high content of calcium. Earth and cinder as well as kitchen residues are main originations of ashes in MSW incineration. Ash deposits are rich in calcium and sulfur, main mineral phase is CaSO₄. The microstructures analyses show that no fusion phenomena appears in fly ashes particles. The sinted deposits' fabric is made up by alternations of compacted sintered frameworks and loose agglomerated particles. The compact parts of sintered deposits are built up mainly by large number of conglomerated submicron particles with molten films. A little amount of silicates of calcium may be the main reason for low-melting-point minerals on deposits particles. The framworks of sintered deposits become more solid with the increase of flue gas temperature. Deposits in the temperature around 600℃have the highest Ca and S contents. The strength of deposits develop rapidly within 10 days. So, keeping prompt ash blowing to the heat transfer surfaces is necessary, especially in the initial stages of incinerator's start-up.Stress crack corrosion was proposed to be the essential mechanism of sintered ash deposits corrosion in MSW incineration. Stress impact of the sintered deposits and ion diffusion reaction of the chlorine in the deposits make the corrosion develop. The corrosive deposits are charactered by the frameworks of loose inner layer whereas compact outer layer. Stress impact due to the solid deposit and the diffusion process due to the loose fabric of the bottom layer make corrosion easier and quicker. The inner layer of the deposit has very high content of chlorine and alkali metals. The ion of chlorine plays important role in corrosion process. To prevent the happening of deposit corrosion in MSW incineration, it is important to avoid the formation of highly sintered deposits on the heat transfer surfaces, and the feeding of large amount of high- chlorine MSW into the incinerator in a short interval.Secondly, based on the mechanism studies in laboratory, experiments on the cold-condition ash deposit characteristic of tube cluster, the sulfation reaction of MSW incineration deposits, sintering strength of MSW incineration deposits are applied separately. The results show that interleaving arrangement of tube cluster can reduce the accumulating of ash deposit. It is fit for the forming of ash deposit when the horizontal space of tube cluster equal to double of the tube's diameter. And this space should be avoided in practice. The most suitable temperature for MSW deposits sulfation reaction is 600℃. The increase of SO₂ concentration in the flue gas can accelerate the sulfation of the deposits. The water vapour also has promotion effect on this sulfation process. But HCl in the flue gas is an obstruction to deposits' sulfation reaction when the SO₂ concentration is low. Otherwise, the Al₂O₃ in the deposits can restrain this sulfation reaction and the growth of sintered strength. The sintered strength is enhanced with the increase of CaO content in the deposits. Both HC1 and SO₂ in the flue gas can promote the sintered strength of MSW incineration deposits, although the effect of SO₂ is more distinct. Among 600-800℃, the sintered strength of MSW incineration deposits increased sharply. The remarkable influence of time on the sintered strength of MSW incineration deposits is over 15 houres. So, 12 houres is recommended as the appropriate interval of the ash blowing in practice.