Assessment of energy use and carbon footprint for low-rank coal-based oxygen-thermal and electro-thermal calcium carbide manufacturing processes

Research of calcium carbide production is of great importance to the utilization of coal. In order to assess and compare two different calcium carbide processes, oxygen-thermal calcium carbide manufacturing process system (OTMP) and electro-thermal calcium carbide manufacturing process system (ETMP) were established in which processed middle coke (PMC) refined from low-rank coal was used as feedstock. Two systems that have significantly different process conditions and multi-product characteristics are difficult to assess comprehensively and objectively by using traditional energy and exergy analysis methods, thus new indexes were proposed. The two systems were analyzed by using three new multi-product system indexes, namely, multi-product comprehensive energy consumption, multi-product comprehensive exergy consumption, and multi-product comprehensive carbon emission. The analysis shows that the percentages of all three indexes of OTMP are lower than those of ETMP at 56.47%, 39.02%, and 79.08%, respectively. Unlike the assessment of energy and exergy consumption, carbon footprint analysis indicates that OTMP has significantly better environmental performance. The carbon footprint flow diagram analysis proposed in this study suggests that indirect emission caused by material processing and utilities' utilization accounts for most of the total carbon emission in both systems. Direct emissions from OTMP and ETMP are only 0.92% and 0.38%, respectively, of the total carbon emission. This study suggests that from the point of view of a multi-product chemical process, OTMP is characterized by low energy consumption, exergy consumption, and carbon emission. Thus, a key point in developing OTMP as an alternative technique for calcium carbide production is determining whether high-purity CO produced in OTMP can be used effectively as a product.