Investigation of thermal decomposition behaviours of lignite and peach stone blends

The big part of the world energy requirement, which increases in parallel with population and industrialization, is provided from fossil fuels. Today, issues which are dealing with the environmental pollution resulting from production and consumption of fossil fuels and restricted reserves of fossil resources are considered as the most important problems. Coal has a very important share of world energy production. The value of coal seems to be increasing in the near future due to its worldwide distributed reserves. Lignite resources existing in our country are ranging from low quality lignites, which have quite a low calorific value, high moisture and mineral matter contents, to high quality lignites. But, the amount of low quality lignites in the total lignite reserves is extremely high. Thus, the development of appropriate technologies for the evaluation of low-quality lignite reserves must be one of the most important energy policy of our country. Biomass is the transformed form of solar energy into chemical energy in plants and as a renewable fuel it is outstanding with its much lower polluting properties with respect to coal. In Turkey, biomass has a big potential among other renewable energy sources and many utilization options as well. Combustion of coal-biomass blends or thermal decomposition of them in the processes such as carbonization or pyrolysis gains more importance in terms of the utilization of low quality coals and biomass economically and reducing the total pollutant emissions. Investigation of the thermal decomposition behaviour and kinetics of coal-biomass blends is essential for the modelling, design and operation of these systems at industrial scale. In this study, thermal decomposition behaviours of Tekirdağ Malkara coal, peach stone and their blends prepared in different ratios were investigated. The results of thermal decomposition experiments which have been carried out in the thermogravimetric analyser showed that the TG curves of coal and coal-biomass blend with the biomass ratio of 5% by weight have two major weight loss regions; biomass and coal-biomass blends with the biomass ratios of 25% and 50% by weight have three major weight loss regions. It was determined that although the additive effect is dominant in the thermal decomposition of coal-biomass blends, very little synergistic interaction was exist. Additionally, the effect of ZnO compound on the thermal decomposition of coal, biomass and the 50% coal and 50% biomass blend was investigated. The experimental results showed that the ZnO addition to the samples caused very little change on their thermal decomposition behaviour. In the kinetic calculations performed by using experimental results belong to thermal decomposition reactions carried out in different conditions, it was determined that the best fitting kinetic model equations were f(α) = (1- α) for the first and second thermal decomposition regions and f(α) = (1- α)2 for the third thermal decomposition region for biomass sample and coal-biomass samples with the biomass ratios of 25% and 50% by weight; and f(α) = (3/2). (1-α)2/3.[1-(1-α)1/3]-1 for the first and second thermal decomposition regions for coal sample and coal-biomass blend with the biomass ratio of 5% by weight. When kinetic parameters calculated for thermal composition of ZnO added and ZnO free samples were evaluated, it was concluded that ZnO had very little inhibitor effect on thermal decomposition reactions. The effects of blend ratio, heating rate and ZnO addition on volatile matter and solid product yields obtained as a result of thermal decomposition were evaluated statistically according to 23 factorial design and empirical equations were derived.