The Combustion of Low Grade Fuels in Fluidised Bed Combustors

摘要

Global energy consumption is projected to increase the world over from 546 EJ in 2010 to an estimated 879 EJ in 2050 (Frei et al., 2013). Several factors contribute to this projected increase including growing global population, better quality of life globally, and continued electrification of services and products. Three serious issues arise from the increase in consumption and production, that is, fuel supply, and availability and increased anthropogenic emissions. To meet demand, developing countries, such as Pakistan, are investing in power generation research and technologies. Whilst a number of technologies are available, fluidised bed combustion (FBC) is an attractive technological option because of its ability to handle fuels with variable calorific content, moisture content, mineral content and high alkaline content. FBC offers reliable thermal output because of the large thermal mass (fluidised bed) associated with the method. This thesis set out to explore the possibility of using low grade fuels in FBC and investigate the impact the fuels have on agglomerate formation rates and combustion efficiencies.udTo explore the potential of FBC in the first experimental investigation presented in this thesis, a 350KW pilot scale FBC rig was used to perform a series of combustion experiments on ten Pakistani coal blends from the Northern Punjab salt rage coal seams. The coals had high sulphur and alkaline content and presented challenges in both combustion and emissions control. Operational variables including bed temperature, bed additives (limestone), sulphur: calcium fuel ratio, additive particle size and co-firing with wood biomass were employed to evaluate the effect of fuel blending, combustion and emissions optimisation. This thesis argues high SO2 emissions resulting from the combustion of high sulphur coals can be reduced in emission concentration when optimising operational variables. The high alkaline content, because of pyrite (FeS) concentrations in the fuel caused bed agglomeration and slagging in the beds. The investigation analyses the agglomerates and defines the mechanisms involved. This research allows for remedies and implementation choices when considering the coals application in full scale systems.udIt is not only coal which can be utilised. Further work investigated the effects of five different biomass fuels in FBC. Biomass can be classified as a CO2 neutral fuel as the CO2 released during combustion is relatively equal to the CO2 absorbed in the growth of the original plant. However, biomass is known to contain high concentrations of alkaline species such as potassium (K) and sodium (Na) which were shown in the literature to cause agglomeration. The biomasses were combusted in the FBC rig to evaluate the combustion, emissions, agglomerates, temperatures and pressure outputs associated with each fuel. Following tests the air distribution plate was modified to simulate both a uniform air distribution system and a non-uniform air distribution system. This allowed for comparisons of the fuels in a system with uniform air flows and non-uniform airflows/distribution which would be experienced in damaged systems. Thus, this thesis argues biomass is significant and relevant to industrial application and allowed for identification of significant chemical components in the agglomeration mechanisms of each fuel as well as establishing the performance of each fuel in variable systems.udIn order to understand the fundamental chemical and physiological makeup of the low-grade fuels it was necessary to conduct an extensive series of fuel characterisation. The fuel characterisation research undertaken yielded information as to the fuels energy content, chemical makeup, combustion characteristic and identify key components such as alkaline species associated with the negative mechanisms seen in pilot scale testing. In order to analyse the fuels x-ray fluorescence (XRF) was used. This technique identifies major and minor oxides in coal samples. However, as demonstrated in the fuel characterisation work, there were limitations, inaccuracies and repeatability issues when analysing low grade fuels with XRF. Thus, a significant effort was made to improve the sampling, ashing, XRF medium and normalisation process. The results of this research led to a more reliable XRF method for analysing low grade fuels and their bi-product of combustion which is applicable for any industry utilising these types of fuels and techniques.udThe final part of the investigations focused on the prediction of agglomeration and slagging tendencies of the fuels. This was done by applying the results seen in the pilot scale tests and the results of the fuel characterisation work with slagging indices and the application of a thermodynamic model (FACTSAGE). FACTSAGE can be used to predict slagging tendencies of the fuels by modelling chemical species released over temperature ranges. The results showed correlation between the theoretical results and the experimental resultsudTogether this research demonstrates the implications of using low-grade fuels in small scale FBC. This thesis explores how this research can then be used in full scale FBC operations. This thesis not only highlights the problems with using low grade fuels in FBC but suggests remedies and potential solutions to the problems based on the results from experimental data and FACTSAGE modelling. It also presents suggestions on how to continue development of the technology to reduce or avoid some of the difficulties in combusting low grade fuels.