Abstract
The purpose of the present work is to analyze and investigate various solutions for heat and hydrogen recovery and reuse, as well as for carbon capture in the manganese (Mn) metallurgical industry. The calciner and the pre-reduction reductor (PRR) are among the most important systems within the production of Mn and Mn alloys from Mn ores and Mn-containing waste. The top gas released from PRR is made up of steam and hydrogen at 750oC, while the calciner off-gas stream, which is composed of nitrogen, steam, and carbon dioxide (CO2), is at a temperature level of 300oC. Regarding the PRR top gas, two different scenarios are examined. In Scenario 1, a portion of the PRR top gas stream feeds the calciner, while in Scenario 2, methane is used in the calciner.
First, waste heat recovery solutions are examined. More specifically, the top gas stream can feed water-stream Rankine cycles for power generation, and hydrogen heat exchangers to preheat the incoming hydrogen stream to the PRR. Two cases of top gas heat recovery are investigated in each Scenario. In Case 1, the top gas stream first passes through the hydrogen heat exchanger and then through the water-steam Rankine cycle, whereas in Case 2, the opposite sequence is applied, the top gas first feeds the Rankine cycle and then the heat exchanger. For each configuration analyzed, the top gas stream must exit the hydrogen heat exchanger or the Rankine cycle at a temperature above the dew point. If the top gas stream has already passed through both the hydrogen heat exchanger and the Rankine cycle, and its temperature remains significantly higher than the dew point, a bottoming power cycle can be integrated. After implementing all the aforementioned cycles and heat exchangers, the top gas stream can enter a cooler to be cooled down below 100oC. A flash tank is integrated to separate the liquified water amount from the gaseous stream, which includes hydrogen and small amounts of steam. Thus, the hydrogen amount contained in the top gas stream is recovered.
On the other hand, the off-gas stream leaving the calciner can drive ORC systems. The ORC modules are studied parametrically, examining various working media. Furthermore, two configurations capable of utilizing heat sources at different temperature levels are analyzed: the dual-evaporator and the dual-source Rankine cycle. In these systems, both heat sources are exploited through a single power-to-heat unit, rather than separately as previously described. All the examined layouts are compared in terms of their net electricity production rate. Moreover, a carbon capture unit based on monoethanolamine (MEA) is investigated to capture the CO2 amount contained in the calciner off-gas. Finally, it is highlighted that the utilized software tool is Aspen Plus.