(684b) Efficacy of Waste-to-Energy Ash in CO-SCR for NOX Abatement

Nitrogen oxides (NOx) are among the most harmful pollutants emitted from combustion processes, contributing to acid rain, smog, and adverse health effects¹. Conventional NOx reduction technologies primarily include selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR). NH3-SCR and Urea-SCR, which employ ammonia and urea, respectively, as the reductant, are highly effective² but unsuitable for certain industrial applications, such as alternative fuel combustion, cement production, and steel refining, due to catalyst cost, vulnerability to poisoning, and flue gas temperatures³. SNCR is often used in these cases but is less efficient, leaving room for innovation. Carbon monoxide selective catalytic reduction (CO-SCR) presents a potential alternative, particularly in settings where NH3-SCR and Urea-SCR are impractical.

Municipal solid waste (MSW) combustor ash, a byproduct of waste-to-energy (WtE) processes, is a promising CO-SCR catalyst due to its abundance and the presence of metal oxides commonly used in industrial catalysis⁴. This study evaluates the feasibility of WtE ash in CO-SCR, examining its catalytic performance under simulated flue gas conditions and the influence of thermal pretreatment on its activity. Understanding the catalytic properties of WtE ash could facilitate its use as a cost-effective NOx abatement material in challenging industrial environments.

Experiments were conducted using a quartz, down-flow tubular reactor at atmospheric pressure with a gas composition of 2700 ppm NO, 850 ppm CO, 0.12% H2O, and Ar balance. The gas hourly space velocity (GHSV) was maintained at 3100 mL/g_cat*h. The study investigated two reductive thermal pretreatment methods: hydrogen (H2) and carbon monoxide (CO). Ash samples were heated to 200 ºC and exposed to a dilute stream of the respective gas (balance Ar) for two hours before cooling to ambient conditions.

Catalytic activity was assessed over a temperature range of 150–600 ºC. Pretreatment significantly enhanced NOx conversion, with distinct effects observed for each method. Hydrogen pretreatment improved NOx reduction at temperatures above 400 ºC, achieving up to a 35% increase in conversion relative to untreated ash. Conversely, CO pretreatment enhanced catalytic activity at lower temperatures (below 400 ºC). These results suggest that pretreatment alters the oxidation state and availability of active sites within the ash, leading to temperature-dependent performance variations.

To further understand these effects, H2-temperature programmed reduction (H2-TPR) analysis was conducted. The reducibility of WtE ash correlated with the equilibrium reduction potential of its metal constituents, primarily iron, aluminum, silicon, and calcium. The temperature of maximum reducibility increased after pretreatment and catalytic testing, with the greatest shift observed in CO-pretreated samples. Additionally, H2 consumption varied, with CO-pretreated and spent ash exhibiting the highest uptake, H2-pretreated and spent ash showing the lowest, and untreated ash falling in between. These findings indicate that CO pretreatment induces more extensive modifications to the redox-active species within the ash, potentially influencing its catalytic performance.

The composition of WtE ash used in this study is representative of typical waste-to-energy facilities. Notably, WtE ash composition remains relatively consistent across different facilities worldwide, irrespective of socioeconomic, geographic, or cultural variations in waste generation. This consistency suggests that the study’s findings could be broadly applicable, supporting the feasibility of WtE ash as a CO-SCR catalyst.

Despite its promise, a key limitation of WtE ash is its relatively low catalytic activity compared to conventional industrial catalysts. This limitation implies that significantly larger quantities of ash would be required to achieve comparable NOx conversion, posing challenges for industrial implementation. However, given its virtually zero material cost and potential role in reducing landfill waste, WtE ash remains an attractive research candidate. Future studies should focus on optimizing catalyst formulations, incorporating dopants or structural modifications to enhance activity, and conducting long-term stability assessments to determine its feasibility for real-world applications.

In conclusion, this study demonstrates that WtE ash exhibits catalytic activity for CO-SCR, with pretreatment playing a crucial role in modulating its performance. Hydrogen pretreatment enhances NOx reduction at high temperatures, while CO pretreatment improves activity at lower temperatures. H2-TPR analysis confirms that pretreatment alters the reducibility and redox properties of the ash, with iron oxides likely serving as primary active sites. While WtE ash’s activity is lower than that of conventional catalysts, its low cost and widespread availability make it a promising alternative for NOx abatement in industrial settings where conventional SCR technologies are impractical. Further research and industrial-scale validation will be necessary to fully assess its potential for long-term application.

References:

1. Gordon, Pollution, Air, in: Encyclopedia of Toxicology, Elsevier, 2005: pp. 468–475. https://doi.org/10.1016/B0-12-369400-0/00779-1.
2. Skalska, J.S. Miller, S. Ledakowicz, Trends in NO abatement: A review, Science of The Total Environment 408 (2010) 3976–3989. https://doi.org/10.1016/j.scitotenv.2010.06.001.
3. Szymaszek, B. Samojeden, M. Motak, The Deactivation of Industrial SCR Catalysts—A Short Review, Energies 13 (2020) 3870. https://doi.org/10.3390/en13153870.
4. C.S. Kirby, J.D. Rimstidt, Mineralogy and surface properties of municipal solid waste ash, Environ. Sci. Technol. 27 (1993) 652–660. https://doi.org/10.1021/es00041a008.