- Title
- Capture and utilization of gaseous emissions from coal-fired power stations
- Creator
- Yang, Chien-Ying Anna
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2021
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- The massive greenhouse gases (GHG) emission from coal-fired power plants is a severe concern all over the world, especially in Australia and those countries that use fossil fuel (e.g., coal) as the main energy source. For example, coal-fired power plants are one of the primary sources of GHG emissions, generating over 40% of global CO2 emissions. In addition to CO2, the flue gas from the coal-fired power plants also contain appreciable amounts of sulphur oxides (SOx) and nitrogen oxides (NOx). These two gases cause local air pollution via aerosol formation leading to smog and acid rain. Ammonia (NH3)-based post-combustion capture (PCC) processes is a promising suite of technologies to reduce CO2 emissions with respect to its high efficiency, practicality, and relatively economical applicability in coal-fired power stations. In addition, it can also be engineered for simultaneous multi-pollutant control of SOx and NOx. The overall objective of the research work reported in this thesis is to test an advanced NH3-based process for the integrated capture of CO2 and SO2 emissions and the subsequent production of ammonium sulphate for agriculture applications. The experimental program involving post-combustion CO2 and SO2 capture with aqueous ammonia (NH3) in this work was conducted in a pilot plant located at Vales Point coal-fired power station, New South Wales, Australia, to validate the process modification and parameter optimisation. The pilot plant has an advanced design in which the CO2 absorber overhead water-wash is integrated with the pretreatment column for SO2 removal. The wash liquid circuit is used to recover NH3 vapour in the wash section and SO2 in the pretreatment column. The pilot plant's operation was shown to be effective in capturing CO2 and SO2 from the flue gas and evaluated process modification of several features, including the flue gas cooling, combined SO2 removal and NH3 recovery and recycled absorption. These promising results include (i) the achievement of high CO2 capture and SO2 removal (ii) the effectiveness of wash water for NH3 recovery (iii) recycled absorption with the reduction in NH3 slip in the absorber. Following this experimental work, simulation studies were undertaken to better understand aspects of the process and identify improvements that could be made. Ammonia (NH3) slip, the unreacted NH3 emitted from the capture process, is a major challenge of NH3-based GHG capture technology and can lead to unacceptable NH3 process emissions. To help address this, further information on NH3 recovery has been investigated based on the wash process of the pilot-plant. The performance of NH3 removal and CO2 capture in a bubbling reactor as a function of various conditions, including NH3 solution composition, temperature (10-25℃) and CO2 partial pressure was studied in this work. The NH3 removal efficiency can reach 98.8% and can facilitate additional CO2 capture in the wash section. Over the temperature range studied, elevated temperature resulted in a reduction in NH3 removal efficiency. Also, it was found that higher CO2 partial pressure can mitigate NH3 evaporation and offset the loss of efficiency at a higher temperature. A simulation of the wash section was also developed using the Aspen process simulation software package (ASPEN Plus® V10). The simulation results showed a similar trend to the experimental work helping to validate the results. This provides further understanding of NH3 recovery along with CO2 capture in the wash process for optimising the most economically practical conditions in the NH3-based CO2 capture technology. The pilot plant experimental campaign demonstrated a high level of SO2 capture in the flue gas. The majority of captured SO2 in aqueous NH3 solution remains primarily as ammonium sulphite ((NH4)2SO3), which can be oxidised by oxygen (O2) in the flue gas to ammonium sulphate ((NH4)2SO4) for subsequent use as a fertiliser. The oxidation of (NH4)2SO3 was studied to understand the rate of conversion of sulphite to sulphate in the pretreatment column of the NH3-based CO2 capture process. The rate of this conversion is crucial to the production of ((NH4)2SO4). Little data exists on the uncatalysed rate of sulphite oxidation in the presence of NH3 and CO2. As such, the emphasis was placed on sulphite oxidation as a function of (NH4)2SO3 concentration and the mixtures of (NH4)2SO3, (NH4)2SO4 and ammonium bicarbonate (NH4HCO3) at various temperatures (20, 40 and 60oC). These represent the compositions and conditions that would be encountered in the pretreatment column during process operation. The rate-limiting step of the reaction between (NH4)2SO3 and O2 was found to be first-order in both sulphite and O2 and second-order overall. The presence of sulphate and bicarbonate was shown to hinder the rate of sulphite oxidation. In solutions of (NH4)2SO3, the rate of oxidation showed a positive correlation with temperature (enthalpy and entropy of activation are 69.2 kJ/mol and 250 J/mol, respectively). The magnitude of the enthalpy of activation highlights that temperature plays an important role in determining the oxidation rate. The presence of (NH4)2SO4 and/or NH4HCO3 reduced the oxidation rate and the temperature dependence. Thus to maintain high rates of sulphite oxidation, the elevated temperature is beneficial as is keeping the sulphate and bicarbonate concentrations minimised. To conclude, an advanced NH3 process for the integrated capture of CO2 and SO2 emissions has been confirmed of its practicality at pilot-plant scale. The dedicated studies on the wash process have revealed a better understanding of the processes of NH3 recovery and sulphite oxidation. The water chilled to 10oC before entering the wash column has been shown to reduce NH3 slip by NH3 absorption into the wash. The NH3 removal efficiency can be maintained acceptably in the presence of bicarbonate before becoming saturated with (NH4)2SO4. According to the high-temperature dependence of sulphite oxidation, the (NH4)2SO4 production can be optimised by controlling the temperature and minimising accumulated sulphate and bicarbonate concentrations in the wash. The results from this project have provided vital information for a better NH3 recovery process and further value for the byproduct of (NH4)2SO4.
- Subject
- NH3-based capture technology; PCC; CO2 capture; SO2 removal; NH3 recovery; sulphite oxidation
- Identifier
- http://hdl.handle.net/1959.13/1488994
- Identifier
- uon:52593
- Rights
- Copyright 2021 Chien-Ying Anna Yang
- Language
- eng
- Full Text
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