Study of the accelerated carbonation of coal fly ash for carbon dioxide sequestration and soil amendment

2017-02-22T00:36:56Z (GMT) by Ukwattage, Nadeesha Lakmali
Elevated levels of carbon dioxide in the atmosphere have created numerous environmental and socio-economic problems, including climate change. The scientific community is experimenting with various emission reduction and carbon capture and storage strategies. Mineral sequestration of carbon with alkaline industrial residues is one such emerging emission reduction technology which is being researched for its ability to be integrated into industrial plants, where both carbon dioxide (CO₂)and alkaline solid residues are generated on site. This concept can be applied to the coal-fired power generation industry, which produces enormous quantities of coal fly ash as a solid by-product along with massive emissions of gaseous CO₂ with the flue gas stream. Therefore, the mineral trapping of CO₂ with coal fly ash can help to sustain coal-based power generation, while bringing added advantages to fly ash disposal due to the favourable chemical changes which occur in fly ash during the above carbonation process. However, mineral carbonation to date remains an immature technology due to its main drawbacks related to kinetics and extensive research is necessary to find acceleration to accelerate mineral sequestration. The main aim of the present thesis is to investigate the effect of operational parameters on the accelerated carbonation of coal combustion fly ash and to study the effect of carbonation on the final disposal of fly ash, especially in relation to agricultural soil amendment. The research work is based on experimental studies conducted in the laboratory and in a greenhouse facility. The accelerated carbonation tests for fly ash were conducted in a newly-developed reactor facility in the Deep Earth Energy Research Laboratory in the Civil Engineering Department at the Clayton campus of Monash University. The main component of this facility is a continuously stirred cylindrical tank equipped with adjustable temperature and pressure mechanisms and monitoring and data acquisition systems. The fly ash materials were collected from the collection ponds of three major power plants located in the Latrobe Valley in Victoria, Australia. The carbonation reactions were designed to test the effect of reaction temperature (in the range of 20 ⁰C to 80 ⁰C), initial CO₂ pressure inside the reactor (in the range of 1MPa to 10 MPa), water-to-solid ratio or solid dosage (in the range of 0.1 to 1) and the super-critical phase of CO₂. In addition, the effect of fly ash particle size was tested with five different particle size categories varying from <100 µm to bulk samples. The results from the experimental work were analysed to evaluate the effect of the above parameters on the degree and rate of mineral carbonation of Latrobe Valley fly ash. The study found that no effect from the pressure was observed on the overall degree of fly ash carbonation, and hence on the amount of CO₂ sequestration. The increased initial pressure could transfer the CO₂ into fly ash faster, which helped to complete the reaction within 3 to 4 hours, through increased dissolution of CO₂ in the solution. The temperature showed a dual effect on the carbonation rate, such that increased temperatures up to a critical value of 60 ⁰C increased the overall carbonation due to increased leaching out of Ca²⁺ ions into the solution. With further increase in temperature, the dissolution of the gas in the solution was hindered, as was the carbonation reaction.The application of temperature test results in a pseudo second-order kinetic model derived an important kinetic parameter: activation energy for three of the fly ash types tested. The values were in the range of 35-45 kJ/mole, which is favourable in terms of energy requirement compared to gas solid carbonation. The water-to-solid ratio of 0.2-0.3 was the optimum for the Latrobe Valley fly ash carbonation and there was no significant effect observed by changing the phase of CO₂ from gaseous to super-critical. A series of chemical tests was designed to observe any changes in the agronomically important chemical properties of coal fly ash after carbonation. Based on the knowledge gained from literature review, alkalinity, salinity and trace metal leachability were identified as those properties most affected by carbonation and the most crucial for the use of Latrobe Valley fly ash in agricultural applications. Extreme alkalinity in the pH range of 11.3 to 11.95 and salinity with electrical conductivity (EC) in the range of 6 to 7 dS/min the ash as received were shown to be lowered with the chemical stability gained during carbonation. Basically, the reduced alkalinity of fly ash after carbonation causes changes in the metal ion mobility which acts as the dominant mechanism of chemical stability. In addition, changes in speciation, co-precipitation and absorption also alter the status of metal ions, which ultimately reduces the salinity and trace metal leachability of the carbonated ash. The salinity of the ash after carbonation was reduced to the moderately saline range (3.2-3.8 dS/m), whereas the alkalinity and trace metal leachability were favourably reduced. The use of carbonated fly ash as a soil amendment or an amendment to crop-growing media was evaluated using greenhouse experiments. The experimental design was equivalent to a two-factor factorial experiment, with three types of fly ash (from three power plants in Latrobe Valley F1, F2 and F3), two fly carbonation statuses (carbonated and non- carbonated) and two application dosages (5% and 10%w/w). Two sets of consecutive experiments were conducted, first with sweet corn plants followed by snow pea plants. The application of carbonated fly ash at 5% dosage was found to be the best treatment in terms of plant growth, maturation and production. Non-carbonated Latrobe Valley fly ash cannot be recommended for agricultural applications, especially at high dosages, due to its excessive salinity, which is unfavourable for most agricultural crops. However, after carbonation, application at low dosages such as 5% by weight is recommended, due to the potential saline toxicity, especially for non-salt-tolerant plant species. Snow peas are comparatively salt-tolerant, performed better than the sweet corn plants in terms of plant growth. The conclusion of the overall study is that the use of coal fly ash in agricultural soil amendment after neutralizing through accelerated carbonation can be introduced as a potential re-use pathway for coal ash, which will provide additional benefits in terms of emission reduction and waste management.