A source apportioned mercury mass balance across a coal-based petrochemical complex

Coal-fired power generation facilities are the largest single anthropogenic source of mercury to the atmosphere. Global mapping of anthropogenic emissions has previously estimated the mercury emission from stationary combustion from South Africa at being 82.6 tonnes per annum, but preliminary in-house calculations have placed this at a significantly lower amount. A petrochemical facility in South Africa, utilising a significant amount of coal in its gasification and steam generation processes, undertook a project to determine a mercury mass balance across the complex, and to apportion the Hg emissions from the facility.Mercury associated with the coal gasified predominantly reports to the crude gas stream, with a small proportion reporting to the ash fraction, and less to the liquid hydrocarbon co-products fraction. The Rectisol gas cleaning process is a functional sink that removes mercury from the crude gas as a solid. It is removed during maintenance cleaning and disposed of as per requirements for mercury containing waste. Mercury present in the purified synthesis gas stream is below the detection limit of the analytical equipment. Mass balance calculations around the steam generation facilities, based on isokinetic sampling, revealed that about 80% of the Hg entering the system is emitted from the steam generation process (combustion). The mercury which reports to the steam generation and gasification ash fractions is encapsulated within the mineral matrix and is not considered to pose a risk to the environment as determined by environmental leaching tests. Gasification can be considered to be a more amenable coal utilization process with regards to limited (or zero) coal derived mercury emissions when compared to combustion.