An investigation of a two-step, temperature-staged, direct coal liquefaction process.
From its inception in the 1700’s, deriving fuel from the Direct Coal Liquefaction (DCL) process has spawned numerous pursuits. While coal is an abundant fossil fuel in many countries and represents approximately 70% of the world’s total energy reserves (Birol, 2004), the DCL process is synonymous with the use of severe operating conditions and catalysts of poor activity. This work is an investigation of a two-step, temperature-staged DCL process and aimed at producing a high value liquid hydrocarbon product at, relatively, mild operating conditions. This stepwise process was initially carried out in a batch reactor. In this first stage, the aim was to maximise on the liquid product (oil) yield by enhancing the thermal dissolution of high grade bituminous type coal in tetralin as the hydrogen donor solvent, using 2:1 and 3:1 solvent: coal ratios. The oil obtained was refined by hydrotreating in a catalytic fixed bed reactor. Both stages were carried out isobarically at 100 barg and, in the first stage, temperatures of 250 ℃ and 300 °C were used. Thereafter, operating temperatures were staged with a 50 °C increase in the second stage reactor. In the first stage, molybdenum doped magnetite was used as the catalyst. The performances of cobalt-molybdenum (Co-Mo) and nickel-molybdenum (Ni-Mo) were trialled in the second-stage reactor. In order to assess the potential value of the oil between the stages, the oil was analysed using Gas Chromatography –Mass Spectrometry (GC-MS). Within the actual experimental boundary; oil yield, alkane and cycloalkane selectivity response data was fitted to linear models. In the first stage the liquid yield was increased with the use of molybdenum doped magnetite catalyst and affected mainly by the temperature and solvent: coal ratios. An oil yield of approximately 51.26% was obtained for blank runs and up to 54.77% for catalysed runs. As a hydrodesulphurisation (HDS) performer and selectivity to the production of long and branched chain alkanes, Ni-Mo had an improved performance over Co-Mo. Co-Mo is selective to a higher concentration of cycloalkanes. For 16 days on stream each, Ni-Mo had a higher activity than Co-Mo. A comparison of the actual data with a literature baseline, showed similarities for the results obtained using 2:1 solvent: coal ratios for both the blank and catalysed runs. As literature made use of severe operating conditions, the performance of the experimental batch reactor system was superior to literature. While there remains room for improvement in the design of the two-stage system, evidence exists that the potential to cover the demand for low–sulphur, crude diesel and solvents from the production of high value hydrocarbon liquid in the said process, is demonstrated.