Kinetic Modeling of Direct Liquefaction of Coal

Mehran Heydari
During the past few decades, petroleum has been the main source of liquid fuels. On one hand petroleum reserves are declining, and on the other hand coal reserve is the most abundant fossil fuel known in the world. Liquefaction aims to convert solid coal into liquid fuels. Coal can be converted to liquid and gaseous fuels and chemicals by two different processing methods, normally called direct and indirect. Direct coal liquefaction converts solid coal to liquid fuels at high temperature and
more » ... temperature and pressures in the presence or absence of catalyst. One of the advantages of direct liquefaction is that, this process can convert coal to liquid without the need for producing syngas (H 2 and CO) as in indirect process and also has higher efficiency than indirect liquefaction. In order to develop commercially practical processes for deriving liquid fuels from coal, a description of coal liquefaction kinetics is essential for design and scale-up of coal liquefaction experiments. The liquefaction of a Canadian lignite coal has been studied experimentally and modeled mathematically. Since conventional batch autoclave is not suitable for isothermal experiments, as the time required for the autoclave to reach the reaction temperature can be substantial, the liquefaction runs were investigated with a rapid injection reactor designed specifically for isothermal kinetic study. The liquefaction experiments were carried out in a tubular bomb reactor in presence of tetralin at temperature ranged from 350 to 450˚C and reaction time range of 15 to 120 min under nitrogen atmosphere. No catalyst was used in the experiments. The results show that Canadian lignite coal is readily liquefied, with its conversion exceeds 80% at 400 °C even without catalyst. Different kinetic models have been iii proposed and examined to describe distributions of products such as preasphaltene, asphaltene, oil and gas. Rate constants for each of the specified reaction network have been calculated by nonlinear regression analysis. A genetic algorithm (GA) optimization method was applied to find the optimal set of kinetic parameters. Arrhenius activation energies were calculated for each rate constant. The high activation energies seemed to indicate that direct coal liquefaction was kinetically controlled. The results showed that at high temperature and extended time undesirable retrograde reactions may take place. iv
doi:10.7939/r34x54t0q fatcat:ztf6ywrodbfhrei6w2dyxoe6iu