Investigation of biocoal production and combustion of coal-biocoal mixtures in circulating fluidised bed under oxygen enri̇ched environment

The thesis mainly consists of two main sections. The first part includes the bicoal production of the pine woods. While the second part involves the co-combustion of biocoal and coal mixtures in the fluidised bed combustion system. Torrefaction process occurs in oxygen free atmosphere, between temperatures of 250 °C and 350 °C and between residence times of 10 and 60 minutes. Biocoal obtained by this process can be burned in pulverized coal firing plants and cement mills. It also can be used as a fuel in the small and medium biomass and pellet burners. Although bench type torrefaction reactors have been used by many researches, there are limited experimental works that used continuous lab scale torrefaction systems. Many parameters affect the torrefaction process; such as the temperature and residence time in the reactor, the heating rate and the particle size and composition of the feedstock. In this thesis; Pine wood torrefaction process was investigated parametrically in a continuous feed torrefaction production system (screw type). The experiments were carried out at four torrefaction temperatures (250, 275, 300 and 350 °C) and three residence times (5, 15 and 30 minutes). It is concluded that, the reactor temperatures and residence times had the greatest effect on mass and energy yields. Also, the higher heating value of the torrefied biomass goes up with increasing the reactor temperature and residence time. Biocoals obtained after torrefaction at around 300 °C and 30 minutes have properties close to the selected Turkish lignites from the point of its higher heating value, H/C, O/C ratios and grindability. Produced biocoals under these conditions will be combusted together with the selected Turkish lignites in circulating fluidised bed combustion systems for further studies. In the second part of the thesis; co-combustion of biocoals producted at 300 °C and 30 minutes with the two different coal in a circulating fluidised bed combustion (CFBC) system under air and oxygen-enriched atmosphere was realized. In the combustion experiments, a modified 30 kWth CFBC system was used. The modified CFBC system basically consists of one fuel feeder, a combustor, a cyclone, a down comer, a bag filter, a FD fan and an ID fan. Biocoal share increased up to 50 %. The oxygen concentration in the experiments was kept between 21 and 27 %. In the oxygen- enriched atmosphere, the emissions were significantly reduced when co-combustion of biocoal with coal. Particularly high oxygen concentrations reduced CO emissions, while the high level of biocoal additive significantly reduced SO2 emissions