Characterisation and prediction of deposits in biomass co-combustion

This PhD thesis deals with the theoretical, experimental and modeling work which was performed to study deposition during biomass and waste co-combustion in pulverised coal facilities. Fossil fuels dominate the current energy scenario. Increasing concerns about fossil fuels availability and about the impact of their extensive use on the environment, had led public and governments to focus their attention on the utilisation of sustainable forms of energy. Reducing CO2 emissions, increasing the shares of renewable energy consumption and improving energy efficiency have become world wide targets. In this context biomass and waste fuels are being increasingly used. An economical option to introduce biomass fuels is to co-fire them together with coal in existing pulverised coal facilities. But formation of ash deposits from solid fuels combustion influences operation of utility boilers. Many studies have been done to characterise deposition trends of coal fuels. The behaviour of biomass and waste fuels differs considerably from that of coals given the distinct composition and association of their organic matter. A better knowledge of the behaviour of biomass and waste fuels is required to predict deposits formation. Different and complementary approaches are explored in this PhD work in order to contribute to the current knowledge on deposition of ashes from co-firing biomass and waste fuels with coal. A set of fuels fired in full scale facilities were available for this research including two different coals, pine chips, B-wood, corn and rape straw, plastic and green house residue, biomass mix, palm kernels, olive residue, pepper plant, chicken litter and meat and bone meal. In the experimental part of this work, a wide range of biomass and waste fuels has been analysed by standard methods and their ashes have been obtained at laboratory. A leaching procedure for the ashes has been developed based on chemical fractionation techniques widely used for biomass fuels. Thermo Mechanical Analysis (TMA) has been applied to the ashes and their leached solid residues to investigate their fusion characteristics. The TMA traces of the ashes and their successively leached solid residues are a fuel ‘foot print’ and depend strongly on the chemical composition of the samples. The difference between the TMA traces of biomass ashes before and after leaching them, clearly relates with the reactivity of the inorganic compounds and can be used to rank the fuels depending on the reactivity of their inorganic compounds during combustion and their tendency to form deposits. In parallel to this work on the leached solid residues, chemical analysis of the liquid leachates has been done and the results used to classify the fuels as having more or less reactive inorganics. Both approaches convey to the same conclusion about the fuel reactivity ranking. Chicken litter, MBM, olive residue and plastic and green house residue are predicted to be more prone to deposition, while pine chips and biomass mix appear to have the smaller presence of ‘problematic’ inorganic species and therefore will give less deposition. Experimental work has also been conducted to investigate the effects of deposits build up on heat transfer surfaces, as this is one of the biggest impacts associated to deposits in plant operation. This effect can be monitored on line using appropriate probes and thermocouples to evaluate the heat transferred to the cooling medium. Two different air cooled metal probes have been developed and tested. Deposition experiments have been done at a 50kWel pulverized fuel flow combustion reactor (CR) designed, commissioned and started up during this research work. At the CR different deposition probes have been tested and different fuels and fuel blends have been co-fired with coal, such as MBM, chicken litter, olive residue, B-Wood or palm kernels. The decay in thermal flow from the hot flue gases to the cold probe cooling air has been measured but characterisation of the different behaviour of fuels and fuels blends based only on heat transfer measurements was not possible. Samples of deposits were collected from the cooled metal probes and from the specially designed ceramic probes. Some fuels are found to produce more sintered deposits –chicken litter, palm kernels, olive residue- while the others appear less sintered. The results of these observations compare well with the results from TMA. Samples of the deposits collected in the probes are analysed to obtain their chemical composition. In the modeling part of this work chemical equilibrium models are used to investigate the occurrence of ash deposits due to co-firing of biomass and coal. Traditional chemical equilibrium calculations combined with an existing combustion model are used to model the deposition experiments done in the CR facility. The enrichment of some elements found in the deposits and the amount of melt phase obtained by modeling compare well with the experimental values. All combustion experiments performed at 1300°C give a percentage of melt above the 15%, meaning that sintered deposits form. The model takes into account data about reactivity of the inorganic species in each fuel as obtained by ash leaching analysis and the availability of some inorganics existing in the outer layers of the fuel particle. It has been observed that the results of ash melting by equilibrium calculations strongly depend on the fraction of inorganic components of the fuel that are accounted for as reactive/non reactive. The model has been applied to predict the formation of condensed fases for the fuels studied. The combination of modeling results and the information obtained from the chemical analyses performed on the deposit samples gives a better insight on the ash formation and deposition phenomena