Mercury pollution from stationary combustion of fossil and biomass fuels

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Mercury pollution from stationary combustion of fossil and biomass fuels is a significant contributor to global Hg emissions, accounting for 24% of global Hg emissions as reported by the Global Mercury Assessment 2018, with no decrease in the sector's emissions since the last report in 2013. While most primary sources of Hg pollution through industrial uses are currently being phased out due to the international ban on Hg through the Minamata Convention on Mercury, the unintentional Hg emissions from energy generation have only started to be regulated for stationary coal power stations in recent years with the introduction of the US Mercury and Air Toxics Standards (MATS) in 2016 and in the EU by the implementing decision by the European Commission on the Best Available Technique decision for large combustion plants in 2017 and is still not regulated with emissions limits in many parts of the world. While common control strategies for other pollutants such as NOx and S may deliver a co-benefit with regards to reducing Hg emissions in stationary solid fuel plants, this effect is often not systematically studied and may vary on the specific operating conditions of the power plants. The demand for cheap and efficient sorbents to inhibit and/or lower Hg emissions will foreseeably rise as stricter Hg regulation moves from large-scale to smaller scale combustion units, and economies that are reliant on combustion for energy generation, such as Poland, China, India and Brazil, are seeking to combat their Hg emissions. Mercury is highly redox-active, with different species exhibiting very different redox behaviours and toxicity in the environment. Elemental Hg0, the single most important Hg species produced during combustion, is volatile and has a residence time of up to 1 year in the atmosphere, enabling it to redeposit on a global scale, contaminating ecosystems far removed from the point source of pollution. Gaseous Hg(II) species on the other hand exhibit a much higher propensity to interact with other particulates in the flue gas and traditional air pollution control units such as selective catalytic reduction units for NOx reduction commonly found in power plants. The fuel and additive chemistry significantly alter the reactions which determine the chemistry of flue gas Hg. While this is well established for coal-fired power stations, less is known about how components in biomass flue gas influence Hg speciation and subsequent emissions. To abate Hg from combustion, it is necessary to control its redox transformations within a gas stream. This PhD project will seek to apply insights from geochemical cycling of Hg, and adapt it to a power plant environment by engineering a biomaterial which is able to stabilise and trap Hg in its oxidised form within a flue gas stream. Biochar has frequently been discussed as a promising low-cost medium for Hg removal. However, especially in the available applied studies, a mechanistic understanding of the reaction by which the doped char captures and oxidises Hg is often lacking, hindering further optimisation of such materials. Hence, a detailed mechanistic understanding of the interactions of Hg with biochar engineered with Mn oxides will be developed as 1) an advanced sorbent for Hg capture and 2) catalyst for redox transformations of Hg so as to capture and store Hg in an amenable chemical form.