(16d) The Effect of Ash On Oxygen Carriers for Chemical Looping Combustion of Solid Fuels

Chemical looping combustion (CLC) is a promising technology [1-4] being investigated at the University of Kentucky Center for Applied Energy Research (CAER) for the combustion of solid fuels. CLC's potential is two-fold in that it can provide a high-purity, sequestration-ready stream of CO2 and theoretically provide a substantial improvement in thermal conversion efficiency relative to conventional energy conversion technologies with other CO2 control processes such as amine scrubbing and oxy-combustion. CLC uses a solid oxygen carrier (OC), usually a metal oxide to transfer the oxygen from the air to the fuel in a reducing reactor without the direct contact between the fuel and air. The reduced OC is recycled by using air to oxidize the carrier to its original state. Air is not mixed with the fuel, and so the product CO2 is not diluted by the nitrogen in the flue gas. Nearly pure CO2 is obtained in the exit gas stream from the reducer following condensation of H2O. The pure CO2 is ready for subsequent sequestration without costly purification [5]. This technology was first proposed primarily for the combustion of gaseous fuels and only recently been considered for solid fuels such as coal without gasification of the coal first to syngas [6-9].

Previous work at CAER indicated that iron oxide powder and catalysts were the best candidates for use as OC for CLC of solid materials from the OCs tested. They did not agglomerate; are relatively inexpensive; retained activity; and remained durable through multiple oxidation ? reduction cycles. The results suggested that access of oxidizing ? reducing gases to the active sites of the OC through adequate porosity was important. It was found that increased H2, CO concentrations resulted in increased reduction rate for the iron oxides which may prove useful in a CLC process for solid materials as it provides a method to control the reduction rate during processing. Fe2O3 powder was used to directly combust a beneficiated gasification char which contained 60% carbon. A mixture of Fe2O3: char mixture (90:10) heated to 950oC in Argon resulted in 88% removal of the carbon with the OC as the only source of oxygen. This was encouraging for the application of CLC to the combustion of solid fuels [8, 9].

This work was to determine the effect of fly ash on the reactivity of two of the iron oxide oxygen carriers previously studied at the CAER since this is a very important consideration for their potential use for CLC of solid fuels.

Experimental

Two iron oxide catalysts, Wustite and fused iron, were studied. The two oxygen carriers and the fly ash were characterized by elemental analysis and XRD. A Netzsch Jupiter 449C thermal analyzer-differential scanning calorimeter-mass spectrometer (TG-DSC-MS) was used. The TG-DSC-MS was fitted with a multiple gas mixing/controlling system comprised of 5 mass flow controllers (valves and controllers), a Lab View control system (software and interfaces), and NResearch switching valves. This system could be operated unattended continuously through multiple oxidation ? reduction cycles. The OCs were mixed with fly ash obtained from a KY power plant in the following ratios of OC to Ash: 75:25, 50:50, and 25:75. Oxidation/reductions were all carried out to completion at 950oC for a minimum of 5 cycles. All samples were oxidized first followed by reduction. The gas flow rate through the TGA was 200 ml/min metered at standard conditions. The oxidizing gas was 20% O2 in a balance of Ar. The reducing gas composition was 10% H2, 15% CO, 20% CO2, balance Ar. A 5 minute purge of Ar was used between oxidation and reduction. Weight gain/loss and maximum rates during oxidation/reduction were determined.

Results and Discussion

XRD analysis of the ash indicated it contained mostly quartz and aluminum alumosilicate with a small amount of Fe2O3. The XRD analysis was qualitative so quantitative amounts were not known. The ash also contained 6.22% C, 0.05%H, 0.25%S and 1.3%O. The elemental analysis suggests that S is present as SO4.

First step in oxidation/reduction cycling was always oxidation. All carbon in the ash should be removed during this first oxidation so the first oxidation result was discounted. The weight gain/loss on oxidation/reduction in response to the ash was similar for both catalysts. Corrected for dilution of the OC by the ash, the amount of weight gain and loss increased as the amount of ash increased. This increase was most likely the result of the presence of Fe2O3 and SO4 in the ash contributing as OCs. The increase in weight gain/loss increased with the concentration of the ash in the samples. Additionally there appeared to be an increase in the porosity of the pellet formed by the oxidation/reduction cycling of both Wustite and fused iron. With increasing ash, the pellet appears more porous and was more fragile. This would result in increased access (penetration into the pellet) of reactive gases to the OC particles and more complete oxidation and reduction of the OC. This was also supported by the fact that the rates of oxidation and reduction increased 4 and 3 fold respectively as the ash increased from 0 to 75% of the OC. Again both OCs tested behaved very similarly.

Summary and conclusions

This work was very encouraging for the use of iron oxide OCs for the CLC of solid fuels. The ash was not detrimental to the reactivity of the two OCs studied and in fact it appeared that some components, Fe2O3 and SO4, in the ash may have functioned as OCs contributing to additional weight gain/loss on oxidation/reduction cycling. The increase in the rates of oxidation/reduction with increased concentration of ash again suggests the importance of access to active sites of the reactive gases.

References

[1] Croiset, E Thambimuthu, K. Coal Combustion with Flue Gas Recirculation for CO2 Recovery. In: Riemer, P, Eliasson, BWokauun, A, Editors. Greenhouse Gas Control Technologies, New York: Elsevier; 1999, p. 581-6.

[2] Marion, J, Naskala, Y N, Griffin, T. Controlling Power Plant CO2 Emissions: a Long Range Approach. In: The First National Conference on Carbon Sequestration, Washington, DC,2001; DOE/NETL-2001/1114

[3] Herzog, H, Eliasson, B, Kaarstad, O. Capturing Greenhouse Gases. Scientific American 2000; 282(2):72-9.

[4] Hatanaka, T, Matsuda, S, Hatano, H. A New-concept Gas?solid Combustion System ?MERIT' for High Combustion Efficiency and Low Emissions. In: Intersociety Energy Conversion Engineering Conference 1997:944-8.

[5] Freund, P. Abatement and Mitigation of Carbon Dioxide Emissions from Power Generation. Powergen 98 Conference. 1998 [cited; Available from: http://www.ieagreen.org.uk/pge98.htm.

[6] Richter, H Knoche, K. Reversibility of Combustion Processes. In: ACS,1983:71-86.

[7] Anthony, E J. Solid looping cycles: A new technology for coal conversion. Industrial and Engineering Chemistry Research 2008; 47(6):1747-1754.

[8] Rubel, A, Lui, K, Neathery, J, Taulbee, D. Evaluation of oxygen carriers for chemical looping combustion of coal and solid fuels. In: 24th Pittsburgh Coal Conference, Johannesburg, SA,2007:paper 21-1.

[9] Rubel, A, Liu, K, Neathery, J, Taulbee, D. Oxygen carriers for chemical looping combustion of solid fuels. Fuel 2009; In Press, Corrected Proof.