Energy evaluation and techno-economic analysis of low-rank coal (Victorian brown coal) utilization for the production of multi-products in a drying-pyrolysis process

Abstract

In this paper, utilization of Victorian brown coal in a drying–pyrolysis process to make products is techno-economically assessed. The pyrolysis process is coupled with the drying and briquette making processes in order to improve the overall efficiency and quality of the products. The pyrolysis process led to the production of char, liquid oil, and hydrogen-rich noncondensable gases. A steady-state Aspen Plus simulation model was developed that provides estimated mass and energy balances for the overall system. The effect of a change in the heating mode and heating medium of the dryer on the overall energy and overall product yields was examined. Additionally, the effect of a change in the pyrolysis gas composition and coal initial moisture was studied. Results revealed that the rotary drum dryer with hot flue gas as a heating medium showed the best performance in terms of the final yields of the pyrolysis products and CO2 emission rate. In the best case scenario, when hot flue gas is used directly in a rotary drum dryer, approximately 55% of the total gas produced from the pyrolysis process is needed to burn in a separate boiler to provide heat for the whole system. The shorter residence time in the pyrolysis reactor results in a lower calorific value gas with less hydrogen but more CO2 produced, which in turn increases the consumption of gas to be burnt to provide heat for the whole system. The wet coal initial moisture is another important factor affecting the energy required for the dryer and hence the total energy consumption. A coal with a higher moisture content needs more coal gas to be burnt and releases more CO2. The cash flow analysis indicated the net present value (NPV) of $52.8 million for a plant with a capacity of 70.6 t/h raw coal based on the first quarter of 2015 pricing, with a internal rate of return of 25% and the payback period of 5.1 years under the best case scenario.