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While mercury can be removed from Oxy-Fuel flue gas, further work is required to understand its form and removal mechanisms. Gas quality impacts, assessment and control in oxy-fuel technology for CCS: Part 2. Mercury removal with SO 3 in the fabric filter and with NO x as liquids in CO 2 compression. March Oxy-Fuel involves firing a conventional pulverised fuel coal boiler with oxygen and recycled exhaust gases instead of regular air. This produces a concentrated stream of carbon dioxide that can be “captured” and safely stored deep underground indefinitely. While it was established mercury (along with other contaminants) could be removed from Oxy-Fuel flue gas in the presence of both SO 3 and NO x contaminants, further work is required to understand the nature of mercury removed and its exact removal mechanism. This will be important to reduce any potential health, environment and process risks involved with mercury. Also established in this study was the nature of the NO x speciation with the moist, CO 2 rich flue gas. The dominant species was nitrous acid, with the next most significant NO x species being nitric acid. Both nitrous acid and nitric acid will require controls to reduce any safety, environment or process issues. Oxy-Fuel Technology Carbon Dioxide gas quality, assessment and control (Part 2)

Mercury, SO x and NO x need to be cost effectively controlled within the Oxy-Fuel process. Oxy Fuel combustion produces approximately 75% less flue gas than air-fuelled combustion and produces exhaust consisting primarily of carbon dioxide and water. Unlike the other developing coal based low carbon dioxide emissions technologies, Oxy-Fuel does not have any inherent gas cleaning technologies built into the process. While existing controls are available – they add considerable expense and some complexity to a retrofit. In addition to carbon dioxide gas quality issues, Oxy-Fuel is also susceptible to the impacts of SO x in the power plant. Corrosion is expected to be enhanced under Oxy-Fuel conditions compared to air-firing due to higher SO x levels throughout the process. Image copyright CO2CRC

While impurities may be removed from Oxy-Fuel flue gas without significant additional process equipment, further work is required to establish the form of mercury removal to reduce health, environmental and process risks. What has the University of Newcastle established in this work? The level of carbon in the ash has an effect on the amount of mercury and sulfur captured within a fabric filter: Low ash carbon improves the capture of mercury and sulfur gases compared to air firing. High carbon ash results in high (>80%) mercury. It confirmed that acid attack is a practical issue in the Oxy-Fuel process – especially during transitions between air and oxygen environments. NO x, SO x, and Mercury can be removed from Oxy-Fuel flue gas during compression (before the CO 2 Processing Unit) without the need for additional removal equipment. The major NO x product associated with the volatile liquid condensates is Nitrous Acid (HONO) with the other dominant species in the gas phase being Nitric Acid (HNO 3 ). Some CO 2 compressor wastes are not stable and require further characterisation to determine suitable mitigation options. Oxy-Fuel Technology Carbon Dioxide gas quality, assessment and control (Part 2)

Carbon in ash has an impact on mercury and sulfur capture. Acid dew point is a practical problem that needs to be addressed. Practical Implications - Impurity Removal using a Fabric Filter There are distinct differences in the performance of fabric filters in an air and Oxy-Fuel environment, particularly when it comes to the sensitivity to carbon in ash. Low carbon in ash levels (<0.1%) results in greater mercury and sulphur gas capture in the Oxy-Fuel environment compared to the conventional air combustion environment - though the differences are coal specific. High carbon in ash levels (>5%) shows high mercury capture levels (> 80%). There is no indication of competition between mercury and SO 3 capture above 2% carbon in ash. Acid dew point sensitivity changes in Oxy-Fuel is related to an increase in SO 3 and H 2 O in an Oxy-Fuel environment. The Callide Oxy-Fuel test results emphasise that some process units may operate at temperatures below the acid dew point. This is a practical issue that needs to be addressed, particularly when transitioning between air and oxygen combustion conditions. Oxy-Fuel Technology Carbon Dioxide gas quality, assessment and control (Part 2) Image Source: Hitachi

NO x, SO x and Mercury can be removed during CO 2 compression. However, the form of the mercury removal remains unclear. Practical Implications - Impurity Removal During CO 2 Compression The removal of mercury from the Oxy-Fuel flue gas is required to eliminate the potential corrosion risk in the CO 2 Processing Unit. Mercury may cause embrittlement of the aluminium heat exchangers. This work found that: SO x and NO x can be removed as liquid acids in the condensates created by the reaction with the water vapour of the flue gases, to removal levels of ~100% SO x and 80% NO x during CO 2 compression. Mercury can be removed from the CO 2 rich gas, partially in the acidic liquids and partially retained in the compressor, to result in levels of less than the 0.01 μg/m3 set by the natural gas industry to avoid attack of the aluminium components in the CO 2 Processing Unit. However the form of the mercury removal product is currently unknown. These results indicate that NO x, SO x and Mercury can be removed from the CO 2 flue gas without the use of specific removal equipment. However, there are some processing and waste characterisation issues that still required clarification. Oxy-Fuel Technology Carbon Dioxide gas quality, assessment and control (Part 2) Image Source: NETL

The major nitrogen species in the flue gas were Nitrous Acid and Nitric Acid. Both require mitigation to reduce safety, environmental and process risks. Practical Implications - Nitrogen Speciation in Oxy-Fuel Flue Gas In many environments it is common for measured NO 2 to be used as the collective term for NO x other than NO. However, the addition of compression in the presence of oxygen and water may mean that this is no longer accurate. This is particularly relevant to Oxy-Fuel flue gas. This work concludes that: The major NO x product associated with the volatile liquid condensates is Nitrous Acid - HONO. In the liquid phase, HONO slowly converts to Nitric Acid - HNO 3. During the depressurisation of the compression condensate liquids, HONO release will require mitigation to reduce any safety or environmental issues. Other than HONO, HNO 3 was the dominant NO x species. HNO 3 is a likely cause of corrosion in downstream processing equipment. Oxy-Fuel Technology Carbon Dioxide gas quality, assessment and control (Part 2) 7 N Nitrogen N Nitrogen O Oxygen O Oxygen 16.00