- Title
- Reactions, transformations and impacts of sulfur oxides during oxy-fuel combustion
- Creator
- Belo, Lawrence Phoa
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2015
- Description
- Higher Doctorate - Doctor of Philosophy (Chemical Engineering)
- Description
- Oxy-fuel combustion is one of the three main CO₂ capture and storage (CCS) technologies for coal-fired power generation; the other two being post-combustion and pre-combustion. Oxy-fuel combustion uses oxygen diluted with recycled flue gas as the oxidant rather than air to regulate temperature within the boiler. As a result, flue gas compositions are more concentrated in CO₂ and up to 4 times higher in impurities such as SOₓ and mercury. Emissions of SOₓ as well as condensation and deposition of corrosive sulfates are detrimental to processes utilised in oxy-fuel flow sheets and also to the plant efficiency. An understanding of the role and impacts of sulfur during coal combustion will benefit control of sulfur emissions and potential corrosion throughout the plant. It is vital for the Callide Oxy-fuel Project (COP) as no SOₓ and Hg removal is used at this plant due to the low sulfur coal used, as expected in all Australian plants. This study aims to establish an understanding of the reactions, transformations and impacts of SOₓ in oxy-fuel combustion by an experimental and theoretical study comparing oxy-fuel sulfur species with those in air firing for which plant impacts are known, including: (1) pilot scale coal combustion experiments to evaluate the overall impacts of coal quality on sulfur release and capture; (2) ash decomposition experiments to test the capture and retention of sulfur in the fly ash and the different sulfur species in fly ash; (3) conversion experiments to establish effects of different flue gas cleaning configurations on the pathways of SO₃ formation and the catalytic effect of fly ash on the conversion of SO₂ to SO₃; and (4) mercury and SOₓ interaction experiments to investigate the speciation and competition between mercury and SOₓ species in the bag filter. Ash produced during oxy-fuel combustion is expected to differ to ash produced during air combustion due to the higher CO₂ and SO₂ atmosphere in which it is generated. For a quantitative understanding of the sulfation behaviour of fly ash in oxy-fuel combustion, fly ash from three commercial Australian sub-bituminous coals were tested and decomposed under an inert atmosphere. Thermal evolved gas analysis was completed for ash produced in both air and oxy-fuel environments. Pure salts were also tested under the same conditions to allow identification of the species in the ash which potentially capture sulfur, along with thermodynamic modelling using FactSage 6.0. Sulfur evolved during the decomposition of air and oxy-fuel fly ash was compared with the total sulfur in the ash to close the sulfur balance. Both total sulfur captured by the ash and sulfur evolved during decomposition were higher for oxy-fuel fly ash than their air counterparts. The extent of sulfation resulting from thermal decomposition of oxy-fuel fly ash was 2 to 3 times greater than air fly ash. Correlations of capture with ash chemistry were attempted to show extent of sulfur capture and to identify active species in the fly ash responsible for the capture. The reaction of SO₂ with fly ash in the presence of O₂ and H₂O may lead to the formation of SO₃ and eventually H₂SO₄. Homogeneous experiments were conducted to evaluate the effects of the procedural variables: temperature, gas concentrations and residence time on the post-combustion conversion of SO₂ to SO₃. Results were compared to those predicted by existing global kinetics and found to be dependent on SO₂, O₂, residence time and temperature and independent of H₂O content. For a residence time of 1 s, at least 900°C is needed to have an observable conversion of SO₂ to SO₃. Literature suggests that the conversion of SO₂ to SO₃ is dependent on the iron oxide content of the fly ash. Experiments using the same fly ash were used to investigate the catalytic effects of fly ash on SO₂ conversion to SO₃ at a temperature range of 400°C to 1000°C. It was observed that fly ash acts as a catalyst in the formation of SO₃, with the largest conversion occurring at 700°C. Homogeneous reaction at 700°C, without fly ash present, converted 0.10% of the available SO₂ to SO₃. When fly ash was present the conversion increased to 1.78%. The catalytic effect accounts for roughly 95% of the total conversion at 700°C. Average SO₃/SO₂ conversion values between fly ash derived from air and oxy-fuel firing and under different flue gas environments were found to be similar. Increased mercury concentration is of concern because mercury is known to attack aluminium heat exchangers required in the compression of CO₂ during oxy-fuel combustion. A recent study has indicated that interaction of Hg and SOₓ may occur at oxy-fuel concentrations and so the competition between SOₓ and Hg was investigated in this study. The effect of Hg, SOx, H₂O and temperature on the native capture of Hg by fly ash was assessed using a quartz flow reactor packed with fly ash to simulate a bag filter. Doubling Hg in the system from 5 to 10 μg/Nm³ doubled the amount of Hg captured in the fly ash from 1.6 to 2.8% and increases the amount of Hg unaccounted from 5.8 to 18.1%. Increased SO₂ decreased the proportion of Hg⁰ in the flue gas. Temperature in the bag filter was found to have a huge impact on the mercury capture by fly ash. As temperature was increased from 90 to 200°C, Hg⁰ in the flue gas was found to increase from 77.9 to 98.3%, indicating better capture of Hg at lower temperatures. The COP is the largest existing plant utilising oxy-fuel technology that combines retrofit options for existing power plants; electricity generation and CO₂ processing. As there is no dedicated SOₓ, NOₓ and Hg removal in place, removal is expected to occur at the fabric filter or during compression. With higher SO₂, SO₃ and H₂SO₄ during oxy-fuel combustion, sulfur capture by the ash is higher compared to oxy-fuel firing. Higher SO₂ forms sulfates responsible for fireside corrosion, SO₃ captured in the fabric filter causes corrosion in the metal components while H₂SO₄ causes fouling of the air pre-heater (APH) and low temperature corrosion due to condensation of H₂SO₄ on surfaces. Shifting from air to oxy-fuel with full flue gas recycling can easily increase the H₂SO₄ dew point by as much as 60°C. Apart from corrosion and fouling, maintaining the APH at high temperature results in efficiency losses. Native mercury capture by the bag filter is also decreased with increasing ADP since the BF must be operated at much higher temperatures. With these reductions in capture at the bag filters due to an increase in operating BF temperature, downstream impacts in the CO₂ processing requiring a need for secondary Hg removals could be created.
- Subject
- sulfur oxides; oxy-fuel; combustion; fly ash; ash; sulfation; sulfuric acid; mercury; sulfur
- Identifier
- http://hdl.handle.net/1959.13/1294244
- Identifier
- uon:18766
- Rights
- Copyright 2015 Lawrence Phoa Belo
- Language
- eng
- Full Text
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