The effect of a magnesia based additive on fly ash deposition in a chain-grate boiler system

• Main Author: Hadjforoosh, Kambiz
• Published: Sheffield Hallam University 1993
• Subjects: 621.3121; Conventional power plants
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.359889

Unlike in oil firing, the effective role of additives to alleviate deposition in coal fired power generation is still regarded with much scepticism and controversy amongst the power generators and boiler operators. The objectives of this research study were principally to explore the mechanisms involved in formation of coal ash deposits and thus determine the effectiveness of a magnesium based additive, namely Lycal 93HS, in reducing the bonding strength of ash deposits on boiler tube surfaces, by making them friable and easily removed by sootblowers during normal operation of the boiler. The experimental techniques developed involved visual, optical and scanning electron microscopy examination of a wide range of matured deposit samples collected over a period of two years, with and without injection of Lycal 93HS into the boilers at West Belfast power station. Specimen deposit samples "fashioned" into the form of Seger cones and "reconstituted" from their crushed, powder form were tested for their softening behaviour at elevated temperatures with and without further additions of Lycal 93HS. This technique was further used to evaluate the effect of Lycal 93HS on the softening behaviour of a range of coal ash components separated by high temperature ashing of coarse particles of coal as well as the bulk ash from the coarse and fine sizes of coal particles. The softening behaviour on heating and crystallisation tendency behaviour on cooling for a selected range of the ash components was investigated without and with additions of Lycal 93HS, using Hot-Stage Microscopy. The possibility of surface adhesion between the fly ash and injected Lycal 93HS within the boiler environment was investigated through a series of laboratory based Surface leaching experiments of deposit and particulate samples with and without injection of Lycal 93HS. The variation in concentration profiles of silicon, iron and magnesium within the collected solutions over a period of time were analysed, using Atomic Absorption Flame Spectroscopy. The elemental chemical composition of bulk deposit samples, the average high temperature ash and its separated components was carried out using X-Ray Fluorescence. Qualitative study of the mineralogy of low temperature ash, selected ash components, as well as a range of deposit samples with Lycal injection was conducted using X-Ray Diffractometry. The results of Lycal injection into a boiler were clearly evident from inspections of the boiler where Lycal injection over different periods of time had resulted in significantly cleaner boiler tube surfaces. Examination of deposit samples with Lycal injection showed lightly sintered, porous, friable textures compared to the highly sintered, fused and dense structures for samples without Lycal injection. The effect of Lycal on the softening behaviour of reconstituted deposit samples and various components of ash was shown to be dependent on their chemical composition, with iron oxide playing an important role. For a number of highly acidic ash components, additions of 5 and 10 mass% Lycal promoted crystallisation of their fluid melt, when cooled to specific temperatures. For the more ferriferous ash components, additions of 1 and 3 mass% Lycal enhanced the surface formation of spikes when their melts were cooled to specific temperatures. The results of leaching experiments showed that the initial magnesia concentrations were generally much higher for the deposit samples and fly ash particles from ash hoppers and grit arrestors with Lycal injection than those without.