Mercury pollution from stationary combustion of fossil and biomass fuels

根据《2018年全球汞评估》的报告，化石燃料和生物质燃料静止燃烧产生的汞污染是全球汞排放的一个重要因素，占全球汞排放量的24%，自2013年的上一份报告以来，该行业的排放量没有减少。由于国际上通过《关于汞的水俣公约》禁止使用汞，目前通过工业用途产生的大多数主要汞污染源正在逐步淘汰，近年来，随着2016年美国汞和空气毒物标准（MATS）的引入，以及2017年欧盟委员会关于大型燃烧电厂最佳可用技术决策的实施决定，固定式燃煤电厂的非故意汞排放才开始受到监管，并且在世界许多地方仍未受到排放限制的监管。虽然对其他污染物（如氮氧化物和硫）的常见控制策略可能会在减少固定固体燃料厂的汞排放方面带来共同利益，但这种影响通常没有系统研究，并且可能因电厂的具体运行条件而异。随着更严格的汞监管从大规模燃烧装置转向小规模燃烧装置，以及波兰、中国、印度和巴西等依赖燃烧发电的经济体正在寻求遏制其汞排放，可以预见，对抑制和/或降低汞排放的廉价高效吸附剂的需求将会上升。汞具有高度的氧化还原活性，不同种类的汞在环境中表现出非常不同的氧化还原行为和毒性。单质汞是燃烧过程中产生的最重要的汞，具有挥发性，在大气中的停留时间长达1年，使其能够在全球范围内重新沉积，污染远离污染源的生态系统。另一方面，气态汞（II）与烟气中的其他颗粒和传统的空气污染控制装置（如发电厂中常见的用于减少氮氧化物的选择性催化还原装置）相互作用的倾向要高得多。燃料和添加剂的化学性质显著地改变了决定烟道气中汞化学性质的反应。虽然对于燃煤发电站来说，这一点已经确立，但对于生物质烟道气中的成分如何影响汞的形态和随后的排放，我们所知甚少。为了减少汞的燃烧，有必要控制其在气流中的氧化还原转化。这个博士项目将寻求应用汞的地球化学循环的见解，并通过设计一种生物材料使其适应电厂环境，这种生物材料能够稳定和捕获烟气流中氧化形式的汞。生物炭作为一种有前途的低成本脱汞介质经常被讨论。然而，特别是在现有的应用研究中，对掺杂碳捕获和氧化汞的反应机制的理解往往缺乏，阻碍了这种材料的进一步优化。因此，对汞与锰氧化物工程生物炭相互作用的详细机制理解将被开发为1)一种用于汞捕获的高级吸附剂和2)汞氧化还原转化的催化剂，以便以可接受的化学形式捕获和储存汞。

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.