The affect of ash chemistry and deposits from co-firing biomass and coal in power plant systems
thesisposted on 08.03.2018, 17:14 by Victoria F. Lay
Hemp, eucalyptus, coal, hemp and coal blended fuel, and eucalyptus and coal blended fuel were ashed and then heat treated for 1 hour at temperatures from 600-1100°C. X-ray diffraction analysis indicated reactions between the phases present after initial ashing of the fuel showed biomass-biomass, biomass-coal and coal-coal interactions. Two phase systems were identified as dominant in the biomass and coal ash blends, these were CaO-MgO-SiO2 and CaO-Al2O3-SiO2. The phases identified in these systems have also been identified in ceramics produced at high temperatures which have similar compositions to the ash matrix of the laboratory synthesised ash; this indicates that phase diagrams can be powerful tools in phase formation prediction. Structures identified as trichomes (phosphate-silicate structures with melting points above 1100°C) from the hemp fuel which had not decomposed were present in both the hemp ash and the hemp and coal ash. The composition determined by Energy-dispersive X-ray spectroscopy analysis of laboratory synthesised ashes was also in agreement with the phases identified through X-ray diffraction. Hemp and coal, eucalyptus and coal, and eucalyptus ash samples (deposited, quenched, cyclone, and bottom ash) removed from a full scale 1MWth combustion rig were analysed. Phase composition of the fly ash samples are similar to those identified in the analagous samples produced in the laboratory with several of the same phases present; confirming that laboratory testing is useful for the predictions of phases present on the industrial scale combustion rig. Particle morphology is one of the largest differences between the laboratory scale tests and combustion rig samples. The dominant particle shape of fly ash particles removed from the combustion rig is spherical. These particles of characteristic shape are often referred to as plerospheres and cenospheres and were first identified in coal fly ash. The presence of the spheres in the combustion rig when only biomass (eucalyptus) is present indicates the formation mechanism of the particles is similar to that of coal. There are similarities between the chemical composition of the spheres which are solely of biomass origin and co-fired; it is likely that phase composition of the sphere and not the fuel origin contributes to the formation of the spheres. Phases identified in the bottom ash are similar to those identified in the fly ash. High temperature phases such as (e.g. Ca9MgK(PO4)7) ocur in the bottom ash suggesting that higher temperatures are reached in the bottom of the rig than in the flue gas. Analysis of 15Mo3 alloy corrosion coupons with fly ash deposited onto the surface, alongside the interactions between gas phases and coupons, deposits and coupons, and gas phases and deposits, showed that some oxidation/reduction of the metal had occurred. The presence vi of metal oxide flakes indicated corrosion. Oxidation of 15Mo3 alloy was observed in hemp and coal, and eucalyptus and coal combustion trials, likely due to the observed deposition of potassium chloride which has caused detachment of several scales. Between the metal-deposit interface, hematite whiskers were observed; magnetite octahedra were also present on the surface of scales. The phases present in the coupon deposit ash differ from those observed in the laboratory and fly ash due to the length of time spent in the high temperature environment. This indicates that some phases will not form until the deposits have built up and are in the furnace for an extended period of time. When the coupon samples were coated, fewer metal scales were observed meaning that the coatings are an affective method of corrosion reduction leading to an increased lifetime of boiler components. The dominant particle morphology present in the combustion rig is the cenospheres and plerospheres. The phases formed can be broadly catergorised into CaO-MgO-SiO2, CaO-Al2O3-SiO2, and K2O-Al2O3-SiO2 phases. Potassium chloride is observed in the laboratory ash and combustion rig ash indicating, alongside the presence of metal oxide scales, that the fuel blends are likely to lead to corrosion during combustion.
- Aeronautical, Automotive, Chemical and Materials Engineering