Exergy analysis of a two-stage coal thermochemical conversion process

Abstract

Improving the efficiency of solid fuel utilization requires a reliable assessment of the thermodynamic performance of fuel conversion processes using an exergy-based approach. In contrast to conventional energy analysis, exergy analysis makes it possible to evaluate not only the amount of energy involved in a process, but also its quality and the degree of irre-versibility accompanying energy conversion. In this study, a two-stage thermochemical con-version process of coal is considered, namely air gasification followed by combustion of the produced gas under two different process configurations: without preliminary cooling before combustion and with preliminary gas cooling. The study is focused on the exergy efficiency of these two configurations under different compositions and temperatures of the produced gas in the range of 800-1800 °C. For both process schemes, the overall exergy efficiency as well as the efficiencies associated with chemical and physical exergy were determined. The calcu-lations made it possible to establish the temperature dependence of the exergy efficiency and to identify the operating conditions corresponding to the most favorable thermodynamic per-formance.
It was found that, in the case without preliminary cooling of the producer gas, the ex-ergy efficiency reaches its maximum in the temperature range of 1000-1100 °C, where its value is about 63,0 %. When gas cooling is introduced before combustion, the maximum ex-ergy efficiency shifts toward higher temperatures, namely 1200-1300 °C, and reaches about 65,0 %. This result indicates that the thermal state of the produced gas has a direct influence on the distribution of exergy within the system and on the overall thermodynamic effective-ness of the process. It is shown that preliminary cooling of the producer gas changes the structure of exergy flows due to the formation of an additional thermal potential that can be further utilized in associated thermal processes. Owing to this effect, the overall exergy effi-ciency increases by 1,5-7,0 %, depending on the initial temperature and composition of the gas. Thus, gas cooling should be considered not only as an auxiliary operation, but also as a factor affecting the thermodynamic performance of the whole two-stage conversion scheme. The obtained results may be used in the analysis and optimization of coal conversion systems, in particular for improving thermal process schemes and increasing the efficiency of utilizing the energy potential of gaseous products.